can zoom time travel

April 26, 2023

Is Time Travel Possible?

The laws of physics allow time travel. So why haven’t people become chronological hoppers?

By Sarah Scoles

yuanyuan yan/Getty Images

In the movies, time travelers typically step inside a machine and—poof—disappear. They then reappear instantaneously among cowboys, knights or dinosaurs. What these films show is basically time teleportation .

Scientists don’t think this conception is likely in the real world, but they also don’t relegate time travel to the crackpot realm. In fact, the laws of physics might allow chronological hopping, but the devil is in the details.

Time traveling to the near future is easy: you’re doing it right now at a rate of one second per second, and physicists say that rate can change. According to Einstein’s special theory of relativity, time’s flow depends on how fast you’re moving. The quicker you travel, the slower seconds pass. And according to Einstein’s general theory of relativity , gravity also affects clocks: the more forceful the gravity nearby, the slower time goes.

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“Near massive bodies—near the surface of neutron stars or even at the surface of the Earth, although it’s a tiny effect—time runs slower than it does far away,” says Dave Goldberg, a cosmologist at Drexel University.

If a person were to hang out near the edge of a black hole , where gravity is prodigious, Goldberg says, only a few hours might pass for them while 1,000 years went by for someone on Earth. If the person who was near the black hole returned to this planet, they would have effectively traveled to the future. “That is a real effect,” he says. “That is completely uncontroversial.”

Going backward in time gets thorny, though (thornier than getting ripped to shreds inside a black hole). Scientists have come up with a few ways it might be possible, and they have been aware of time travel paradoxes in general relativity for decades. Fabio Costa, a physicist at the Nordic Institute for Theoretical Physics, notes that an early solution with time travel began with a scenario written in the 1920s. That idea involved massive long cylinder that spun fast in the manner of straw rolled between your palms and that twisted spacetime along with it. The understanding that this object could act as a time machine allowing one to travel to the past only happened in the 1970s, a few decades after scientists had discovered a phenomenon called “closed timelike curves.”

“A closed timelike curve describes the trajectory of a hypothetical observer that, while always traveling forward in time from their own perspective, at some point finds themselves at the same place and time where they started, creating a loop,” Costa says. “This is possible in a region of spacetime that, warped by gravity, loops into itself.”

“Einstein read [about closed timelike curves] and was very disturbed by this idea,” he adds. The phenomenon nevertheless spurred later research.

Science began to take time travel seriously in the 1980s. In 1990, for instance, Russian physicist Igor Novikov and American physicist Kip Thorne collaborated on a research paper about closed time-like curves. “They started to study not only how one could try to build a time machine but also how it would work,” Costa says.

Just as importantly, though, they investigated the problems with time travel. What if, for instance, you tossed a billiard ball into a time machine, and it traveled to the past and then collided with its past self in a way that meant its present self could never enter the time machine? “That looks like a paradox,” Costa says.

Since the 1990s, he says, there’s been on-and-off interest in the topic yet no big breakthrough. The field isn’t very active today, in part because every proposed model of a time machine has problems. “It has some attractive features, possibly some potential, but then when one starts to sort of unravel the details, there ends up being some kind of a roadblock,” says Gaurav Khanna of the University of Rhode Island.

For instance, most time travel models require negative mass —and hence negative energy because, as Albert Einstein revealed when he discovered E = mc 2 , mass and energy are one and the same. In theory, at least, just as an electric charge can be positive or negative, so can mass—though no one’s ever found an example of negative mass. Why does time travel depend on such exotic matter? In many cases, it is needed to hold open a wormhole—a tunnel in spacetime predicted by general relativity that connects one point in the cosmos to another.

Without negative mass, gravity would cause this tunnel to collapse. “You can think of it as counteracting the positive mass or energy that wants to traverse the wormhole,” Goldberg says.

Khanna and Goldberg concur that it’s unlikely matter with negative mass even exists, although Khanna notes that some quantum phenomena show promise, for instance, for negative energy on very small scales. But that would be “nowhere close to the scale that would be needed” for a realistic time machine, he says.

These challenges explain why Khanna initially discouraged Caroline Mallary, then his graduate student at the University of Massachusetts Dartmouth, from doing a time travel project. Mallary and Khanna went forward anyway and came up with a theoretical time machine that didn’t require negative mass. In its simplistic form, Mallary’s idea involves two parallel cars, each made of regular matter. If you leave one parked and zoom the other with extreme acceleration, a closed timelike curve will form between them.

Easy, right? But while Mallary’s model gets rid of the need for negative matter, it adds another hurdle: it requires infinite density inside the cars for them to affect spacetime in a way that would be useful for time travel. Infinite density can be found inside a black hole, where gravity is so intense that it squishes matter into a mind-bogglingly small space called a singularity. In the model, each of the cars needs to contain such a singularity. “One of the reasons that there's not a lot of active research on this sort of thing is because of these constraints,” Mallary says.

Other researchers have created models of time travel that involve a wormhole, or a tunnel in spacetime from one point in the cosmos to another. “It's sort of a shortcut through the universe,” Goldberg says. Imagine accelerating one end of the wormhole to near the speed of light and then sending it back to where it came from. “Those two sides are no longer synced,” he says. “One is in the past; one is in the future.” Walk between them, and you’re time traveling.

You could accomplish something similar by moving one end of the wormhole near a big gravitational field—such as a black hole—while keeping the other end near a smaller gravitational force. In that way, time would slow down on the big gravity side, essentially allowing a particle or some other chunk of mass to reside in the past relative to the other side of the wormhole.

Making a wormhole requires pesky negative mass and energy, however. A wormhole created from normal mass would collapse because of gravity. “Most designs tend to have some similar sorts of issues,” Goldberg says. They’re theoretically possible, but there’s currently no feasible way to make them, kind of like a good-tasting pizza with no calories.

And maybe the problem is not just that we don’t know how to make time travel machines but also that it’s not possible to do so except on microscopic scales—a belief held by the late physicist Stephen Hawking. He proposed the chronology protection conjecture: The universe doesn’t allow time travel because it doesn’t allow alterations to the past. “It seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians,” Hawking wrote in a 1992 paper in Physical Review D .

Part of his reasoning involved the paradoxes time travel would create such as the aforementioned situation with a billiard ball and its more famous counterpart, the grandfather paradox : If you go back in time and kill your grandfather before he has children, you can’t be born, and therefore you can’t time travel, and therefore you couldn’t have killed your grandfather. And yet there you are.

Those complications are what interests Massachusetts Institute of Technology philosopher Agustin Rayo, however, because the paradoxes don’t just call causality and chronology into question. They also make free will seem suspect. If physics says you can go back in time, then why can’t you kill your grandfather? “What stops you?” he says. Are you not free?

Rayo suspects that time travel is consistent with free will, though. “What’s past is past,” he says. “So if, in fact, my grandfather survived long enough to have children, traveling back in time isn’t going to change that. Why will I fail if I try? I don’t know because I don’t have enough information about the past. What I do know is that I’ll fail somehow.”

If you went to kill your grandfather, in other words, you’d perhaps slip on a banana en route or miss the bus. “It's not like you would find some special force compelling you not to do it,” Costa says. “You would fail to do it for perfectly mundane reasons.”

In 2020 Costa worked with Germain Tobar, then his undergraduate student at the University of Queensland in Australia, on the math that would underlie a similar idea: that time travel is possible without paradoxes and with freedom of choice.

Goldberg agrees with them in a way. “I definitely fall into the category of [thinking that] if there is time travel, it will be constructed in such a way that it produces one self-consistent view of history,” he says. “Because that seems to be the way that all the rest of our physical laws are constructed.”

No one knows what the future of time travel to the past will hold. And so far, no time travelers have come to tell us about it.

Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

Image of galaxies, taken by the Hubble Space Telescope.

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

Illustration of GPS satellites orbiting around Earth

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

Illustration of a hand holding a phone with a maps application active.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

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Time travel is possible, but it’s a one-way ticket

Chenoa van den Boogaard , Physics and Astronomy editor

The ability to travel through time, whether it is to fix a mistake in the past or gain insight into the future, has long been embraced by science fiction and debated by theoretical physicists. While the debate continues over whether travelling into the past is possible, physicists have determined that travelling to the future most certainly is. And you don’t need a wormhole or a DeLorean to do it.

Real-life time travel occurs through time dilation, a property of Einstein’s special relativity . Einstein was the first to realize that time is not constant, as previously believed, but instead slows down as you move faster through space.

As part of his theory, Einstein re-envisioned space itself. He coined the phrase “spacetime,” fusing the three dimensions of space and one dimension of time into a single term. Instead of treating space as a flat and rigid place that holds all the objects in the universe, Einstein thought of it as curved and malleable, able to form gravitational dips around masses that pull other objects in, just as a bowling ball placed in the centre of a trampoline would cause any smaller object placed on the trampoline to slide towards the centre.

A computer-generated representation of Einstein’s curved spacetime. The Earth creates a gravitational dip in the fabric of spacetime which is deepest at its core. Courtesy and Â© of NASA

The closer an object gets to the centre of the dip, the faster it accelerates. The centre of the Earth’s gravitational dip is located at the Earth’s core, where gravitational acceleration is strongest. According to Einstein’s theory, because time moves more slowly as you move faster through space, the closer an object is to the centre of the Earth, the slower time moves for that object.

This effect can be seen in GPS satellites, which orbit 20,200 kilometres above the Earth’s surface. These satellites have highly precise clocks onboard that gain an average of 38 microseconds per day due to time dilation. While this time gain seems insignificant, GPS satellites rely on their onboard clocks to maintain precise global positioning. Running 38 microseconds fast would result in a positioning error of nearly 10 kilometres, an error that would increase daily if the time difference were not constantly corrected.

A more dramatic example of time dilation can be seen in the movie Interstellar when Matthew McConaughey and his crew land on a planet with an extreme gravitational field caused by a nearby black hole. Because of the black hole’s intense gravitational influence, time slows dramatically for the crew on the planet, making one hour on the surface equal to seven years on Earth. This is why, when the crew returns to Earth, Matthew McConaughey’s daughter is an old woman while he appears to be the same age as when he left.

So why hasn’t humanity succeeded in making such drastic leaps forward in time? The answer to this question comes down to velocity. In order for humanity to send a traveller years into the future, we would either have to take advantage of the intense gravitational acceleration caused by black holes or send the traveller rocketing into space at close to the speed of light (about 1 billion km/h). With our current technology , jumping a few microseconds into the future is all humans can manage.

But if technology one day allows us to send a human into the future by travelling close to the speed of light, would there be any way for the traveller to use time dilation to return to the past and report her findings? “Interstellar travel reaching close to the speed of light might be possible,” says Dr. Jaymie Matthews , professor of astrophysics at the University of British Columbia, “[but] this voyage is one way into the future, not back to the past.”

If we can’t use time dilation to return to the past, does this mean that the past is forever inaccessible? Perhaps not. Einstein proposed that time travel into the past could be achieved through an Einstein-Rosen bridge, a type of wormhole. Wormholes are theoretical areas of spacetime that are warped in a way that connects two distant points in space.

A visualization of a wormhole: The fabric of spacetime curves back upon itself, forming a bridge between two distant locations. Image by Panzi , CC-BY 3.0

Einstein’s equations suggested that this bridge in space could hypothetically connect two points in time instead if it were stable enough. “At the moment, even an Einstein-Rosen bridge cannot [be used to] go back in the past because it doesn’t live long enough – it is not stable,” Matthews explains.

“Even if it was stable, it [requires] other physics, which we don’t have. Hypothetical particles and states of matter that have “exotic” physical properties that would violate known laws of physics, such as a particle having a negative mass. That is why “wormholes” are only science fiction.”

While it would be fascinating to travel back in time to see the dinosaurs or to meet Albert Einstein and show him the reality of time travel, perhaps it is best if the past remains untouched. Travelling to the past invites the possibility of making an alteration that could destroy the future. For example, in Back to the Future , Marty McFly travels to the past and inadvertently prevents his parents from meeting each other, nearly preventing his own existence. But if he had undone his own existence, how could he have travelled back in time in the first place?

Marty’s adventures are a variation of the grandfather paradox: what happens if you go back in time and kill your grandfather before your father is conceived? If you are successful, how is it possible that you’re alive to kill your grandfather in the first place?

A recent study at the University of Queensland may have the answer to this baffling paradox. In this study, the researchers prove mathematically that paradox-free time travel is possible, showing that the universe will self-correct to avoid inconsistencies. If this is true, then even if we could travel back in time, we would never be able to alter events to create a different future.

While these new findings are enlightening, there appears to be more evidence that, although time dilation can allow us to glimpse the future, we will never be able to visit the past. As the late Stephen Hawking said in his book Black Holes and Baby Universes , “The best evidence we have that time travel [into the past] is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Banner image by Alex Lehner, CC BY 2.0

240 thoughts on “ Time travel is possible, but it’s a one-way ticket ”

How do I go about time travel? what do I need how do I get those required things?

Very large ring magnets and some mathematics and will to see it in reality.

How about a sphere magnet ship…

hoe about 3d time and hemi synch or portals augmented reality,power of suggestion..drugs pcp binural tones frequency amplitude .virtual computing ie.

I’m a time traveling tourist, Stephen Hawking was wrong.

Time is simply a measurement of space under the amount given its mass and the amount of light and dark in which governs its mass in a 4dimensional reality step outside of the force in which permenates its flow one would reside there would be no past present or future there be a fixed permance of a constant here and now and so ok then what is to come.

Very well explained article !!

But I think if physics says time travel can be possible then it’s definitely possible. Considering not to go back to your childhood and fix things but rather can go to the past but as invisible person to them. So that,

No actions by you would impact your future.

Regards, Kirankumar DR

Tell me more

Yes.. I wish I can do this too 🙂

We will understand it better, by and by…

I have a theory for warp speed, but nasa would have to put it to the test…check my Facebook

I am reading for this drive , i am ready , without think my life safe or not

@Ravi chandila English translation please?

Please someone help me I just want to send a message to myself in my past.,to get the love of my life, he never revealed to me his feelings now my life is ruined by the decision of my elders Please help me, it’s question of my life and death. Nazneen

Is time travel machine is their, if the time travel machine is true can it move to the past . To bring back my lost life

That’s the problem you know.. it is not there that’s why we aren’t able to travel time..and yes it it will be built then you will be able to do so…..

damn my life is also lost and broken but still no one can give a time machine for free

DO NOT change the future. That’s why people like you couldn’t go. One wrong person to ruin it for the rest of us

On the point of time reversal, it is evidently impossible. The Uncertainty Principle prohibits spacetime reversal. The Universe is unable to remember its past (as a consequence of the Uncertainty Principle), therefore the Universe cannot reorganise itself.

Can I have to go on my past with another time travel it is a possible when just tell me about one thing that can I have to go in my past one year

we dont need magnets.we need a strong gravitational force to warp spacetime allowing us to travel through with speed of sound or speed of light or faster.we need to learn how to control such force carefully or it could be lethal.gravity slows down time.but it can theoratically work both ways.if we can reverse the gravity’s natural reaction we could speed up a spacecraft faster than light(its all relative(and theoratical))

I WAS ACTUALLY JUST THINKING THE SAME THINFG BEFORE READING YOUR PIECE. VERY WELL EXPLAINED, AND IT DOES MAKE ALOT OF SENSE. WELL DONE.

oh and I forgot to add it can be the key to look into the universe and also travelling time(theoratical).speed and gravity are the key to the universe(theory not proved)

All you really need is a crystal diode with 16 sides, a large pain of glass, and a frequency transmitter near a bathtub full of ice cold water….if you reach the right frequency you can travel through time forward and reverse…

Magnetized metal(VCR Reading Head), to read time out of the Magnetosphere all around earth. The Magnetosphere kills 2 birds with one stone- it protects earth and it records human time:

Mystery solved and I will explain, I was in a coma 3 months and I experienced things, I traveled time forward and backward, it is not a one way ticket. Movies and songs are recorded on magnetic tape in a VCR tape Cartridge or Cassette tape,Â Â Magnetic tape recording works by converting electrical signals into magnetic energy, which imprints a record of the signal onto a moving tape covered in magnetic particles.Â Â 3D life on earth(a movie), and the Magnetosphere all around earth coming from the core of earth(MAGNETIC ACTIVITY) without Atom Made Tape, is like a movie on magnetic Atom made tape in a VCR tape cartridge. Revolution and Rotation is the motor(VCR).

This is why people have those freaky Deji’vuÂ feelings like they have lived this before, BECAUSE YOU HAVE, and how people can be psychic, and how there is Prophecy in the Bible. When a person dies, their Spirit- MIND(Thoughts, Feelings, Urges(Physical and mental personality)) breaks out of human body- a stopped heart is what releases the spirit from the human body. Then the Soul(Life) with the memory of your existence in it breaks out of spirit and goes back to your birthday with a erased memory, meanwhile your spirit goes back in time to when you were a teenager starting the mental puberty, maturity from that adult spirit you died with in last life.In that old movie Star Wars or maybe it was the Empire Strikes Back, there is a scene where Princess LayaÂ plays like a 3D movie, that is EXACTLY how its of life on earth.

Mr Snow, I believe you as I have seen it too. As humans we have deep knowledge of things we cannot rationally explain but you have done a great job here.

I thought that Analogy would be a better and easier way to explain, or in a picture of the earth from far out in space with the atmosphere around it looks like a DVD disk and the earth being the center sticker but is in 3D.

Actually you are on to several things here. I have also had the infusion of knowledge that also had to do with comparing life to recorded movies and music. I know you were using it to explain your theory, but I do think there is something there, I always have. When you watch a movie you are seeing the past. Why canâ€™t you somehow use a recording as a base to go back into? I agree with everything you said here, and itâ€™s worth looking into.

Jeffrey, very interesting idea!! Could be something to that. As far as your coma experiences, I think there are things we just do not understand and are nearly impossible to explain. Perhaps time IS like a video tape, or a DVD? Magnetism is one of the forces of nature. I too have had some odd experiences that suggest that we are able to perceive things beyond our five known senses.

I think if you have had a near death experience, such as being in a coma, then you have experienced the powerful hallucinations provided by the chemical substance DMT which your body creates naturally in times of extreme trauma, but also found in most plants and used recreationally by some who are brave enough and into that kind of thing. Your theory is interesting, but completely unproven and as far as I know untested. If things were so simple, I’m sure many scientists would have already thought of such an idea and tested it.

How do I travel through time

Be alive and live life to the fullest is the best way to travel through time ! OR Befriend grey aliens../ They may hold the key to the sum of all knowledge in the universe..

Sounds good will it work

Really log vaps mil sakte hau h kya

Can you plz explain I didn’t get it

You dont first all you are not experienced in the field of the space time continum and you could you upset the already fragile and multitude of alternate realitys that have looping due irresponsible ones who somehow gotten the technology causing another altered time frame there are a disarray multiple reality which are looping in earths 4dimensonal time frame time traveling is not for a vacation or just to get a joy ride its a serious and complex reality not be joked about it is a real thing and certain individual have are upset the balance of earths original time zone note now the gaurdians of this region of milky way the galatic order of the light keepers Angelic gaurdians of the (names with held)are working over time ooh nice pun (over TIME) ha wow to restore Earth back to a original time continum

Who said I want a joy ride, my life is devastated even my kids are suffering, I want to commit suicide but can’t leave my kids back, Being captive for most of my life, if my life is changed nothing will be disturbed, only thing happens is 3 life’s will be saved. And more so over I don’t want to travel I just want to send a message to myself in my past plz on the date of 30th May 1996. My life is ruined plz help me, it was my dad,brother, sister who pushed me into the dungeon and my husband and his family took over the charge of torturing me. Nazneen

I want to go back in time and tell my 5 year old self to burn the creepy dolls that my mom bought cause there is demons in it at the same time I will kidnap and torture my dad right now go back in time and show the younger version of my dad show him what will happen to his future self if he donâ€™t get rid of those possessed objects and keeps letting my mom buy those antiques Iâ€™m 18 now Iâ€™m single no girlfriend no friend alone nothing very depressed too and I try to remember the positive things that happened in my life which there arenâ€™t many tho but the demons keep squeezing my memory brain and my mom keeps on making so much loud noise including her damn mouth I have attempted to burn the demonic dolls but I only burned them for a minute or two with gas cause I was worried I might accidentally set my whole neighborhood on fire but then my mom threw it all in the recycle instead of the trash so the demons just keep bothering me its driving me nuts he he.

Access to a Quantum Computer Network on the web would be a good start. A series of ChatBots and webhook sites strategically placed in not only space, but in time. A series of algorithms and I think information can be transferred backwards to ones self…

How do we know that there are no horde of tourists among ourselves?

How do we know we’re all not tourists?

We’re all time travelers. We all travel into the future daily. 1 second at a time. Lol…

Agreed! I had the same thought!

Excellent question

If is possible, I would like to go back to: January the 1st 1975 & relive the 70â€™s as I prefer that decade to the awful one I am facing now, Back then We had more police our streets & left our front doors open, Those days were far much more better .

https://3netra.co.in/61-2/

Please do comment on my blog post regarding time travel

how about you ask the flash to help you

I need the time travel so I’m fails so many times i love time travel i have to go fast and future so i have no idea im travel is a my dream so my dream solution plz say me i have time travel so please help me someone please…..

I think you are over reacting

When we look at the stars now it is what they looked like years ago so what if we go to the stars and look down?

You cant go to the stars. It will just take billions and billions of years to go even to the next nearest star than our Sun- proxima centuri. Sorry to say, but do you think that you will be alive all those years??

You can do that without going to the stars… our planet reflects light as well thus making it visible from other parts of the universe…. has the word â€œreflectionâ€ crossed your mind ? 😉

Contact me on my hangout I will help you [email protected]

bro just time travel its not that hard

Please help me to time travel, can I see myself when I go back in time like Harmaini sees herself in Harry potter?? Or can I send messages to myself I know the particular date when to send. It’s not the mistake I had done in my past but it was done by my father and brother who are safe, happy enjoying their lives,my life is totally ruined Please help me. Nazneen

I want to go back in time to save my wife .it was a bad mastake she died .that could be changed i need to go back and save her. Please help me.yours gordon sutcliffe

Would love to hear more how itâ€™s possible, as I am really so desperate to go back in time. I lost my wife 6mons back because of COVID and I will do the impossible things to make it happen.

DMT Experience

what is that?

Dmt experience. Time travel, out of body and sometimes superhuman capabilities.

Jump into a black hole

We have to lose something(the past) to gain something(the future) in time travel.Time cannot be played with.Am I correct.

you need to have d e t e r m i n a t i o n

Time machine is possible

speeder than light LOL

speeder than light cuz if the light break it limits it will move backward in time

Don’t Just don’t disturb the past

I want to go back in time and see my dad. I miss him.

mee too raina I lost my father the day before you posted the comment 18th may, crap it hurts me so much. I would rather die to bring those moments back….

Everything is connected . Time isn’t real .

It is universe we travel to and not a time line in one universe

Ask trump….Mandela effect…. dmt 5th dimension

u need an X-WING starfighter and a lightsaber to fight the knights at past and a R2-B2 to track

The fact that no one has time travelled to the past is the proof that time travelling will NEVER exist.

Others have. Portals open most of the time. Example: Miami Fl. Magnetic Material gets bombarded by the sun. Which fractures and formed portals within that area. Ley lines can lead to the portals of travel within miami for just to start. One can laugh or wonder if. In my experience jumping for the better the word of it (Movie Jumper) can be done. You can either Teleport or Time Travel. Our sun open these portals everyday. The best time when Sun spots start to emerge. All that electrons traveling at light speed is enough to rupture our magnetic fields on Earth. You will return of course. Like water on a lake or an ocean time will corrects itself. Your inner clock is your ticket back home. With a little math,fourth dimensional thinking,a magnetic meter, the right location,history research and luck. You may get to expirence it. First clue….cold spots…it may not be a ghost.

Plz can you help me please help me you can save my life

I wish I could help you, I can sense your sufferings.

You need a bag of hyperlink modules to start, then nuclear beepbeep gatangas, when you have that come back here and I will tell you what you need next.

You need high voltage beepbeep gatangas and a large broonasic magnet of about 450 Gauss, come back here when you have these and I will tell you the rest.

you need an old fashioned police box

If you rotate the center of the earth in the opposite direction, then the whole earth can be moved back in time, on the other hand, if you move the center of the earth and change its position by separating it from the part of the earth, then you will be able to time correctly. Let’s reach the other side.

How I could time travel any time travel machines inverted

give audition in the flash series..

I think that to go back in time you’d to travel faster than the speed of light since time stops at the speed of light but if you wanted to go back to say mlk’s assassination you would need to go at least 10 times the speed of light

You don’t want to, the moment you wrote that message is a historical point in time.

When time travel is possible, you should dÍŒÌµÍ”Ì®Í‰Ì£Ì¯Ì³iÍ’ÌÌ†ÍŸÌ©Í…sÍ¬Ì½ÌŠÍ†Í¢ÍŽÌ²Ì–Í™ÌºrÍ†Í‚ÌšÍ˜Ì–Ì¹eÌ‚Í¬Ì›Ì«ÌªÌ±Í‡Ì˜Ì©gÍÍ£ÌŠÌŒÍœÌ–Í‰Ì¤ÍšaÍ‘Í‚Í’Í¡Ì¯Ì—ÍšÌ¬ÍÌ±Ì¦rÍ¦Í—Í˜Ì±ÌdÍ‹Ò‰ÌªÌ–Ì¥Í”ÌŸÌŸÍšÌ» Í§Ì”ÍÍŽÌ¬iÌ‡Í®Í«Ì†Ì§ÍšÌ«Ì»tÌ“Ì‚Ì•Ì©Ì»Í‰Ì©Ì˜Ì°Ì Ì« Ì‰Í†ÌŠÌ‡Í€Ì¦Ì»Ì³Ì¦iÍ£Í„Ì´Ì—ÍÌžÍ™Í‡mÍ¦ÌšÌ‘Í¦ÍÍšÌœÌ¬Ì¹Ì˜ÌŸÌmÍ«ÌŒÍ†Í¡Ì±Í•Ì»Í‡Ì®Ì Ì°Ì¼eÍ©Ì¢ÍˆÌœÌ±dÌƒÌ¿ÌˆÌšÌµÌ¦Í…Í™Í”ÌÌ¹iÍ¯Ì›Ì–Ì¬Í“ÍšÌ©ÌÌ—aÌ…ÌÍÌ¦ÍŽÌÌ£ÌÌ˜Í”Í™tÍ¥Ì€Í—Ì´Ì±ÌŸeÍ—Í¨Ì‚Í’ÌµÍŽÌÍ“ÌŸlÍ¦Í¦ÍœÌ¼Í•Í•yÍ£Ì¸Í™Ì¯ÌºÌ˜Í‰,Ì‘Í«ÍœÍˆÌ»Í™ÌÌºÌ˜Ìž Í’ÍœÍ”Ì—Ì£dÍ›ÌÌ¶Í‡ÍšÍ‰Ì¦ÌžÌ—oÌ’Í©Í„Í€ÌžÌ®Ì»Ì²ÌœÍ…Ì Ì‰ÌÍ‚ÌÍ€Ì²Ì™Ì¦Ì®ÌºnÌŒÍ ÌÌžÍ–oÍ›Í©ÌÍ›ÍœÌ¬Í•Ì¯Ì©Í“Ì®Ì«ÌtÌ¿ÍŠÍ†Ì•Ì¼Ì™ ÌÍ€Ì²ÍšÌ²Ì¬Ì¦Ì—mÍ—Ì†Í£Ì¢Ì¹ÌœÌÌ Ì¬aÍ„Ì‡Ì‰Í›Ì¨Ì²ÌºÌ»ÍˆÌ¹ÍŽkÌŒÍ€ÌœÌªÌ±eÍƒÍ—Í˜ÌœÌ³Í”Í‰Ì£Í“ Ì‰Ò‰Ì²Í…ÌžÌ˜ÍˆcÌ‹Í¥ÌÌ´Ì¦Ì£ÌÍ‡ÍˆÌ™oÍ¯Í€Ì«Í‡Í‡Ì˜Ì»Ì Ì¹ÍŽnÌ”Ì‡Ì¾Í¢ÌºÌ¹Ì£Ì¦tÌ„Í‚ÍœÍšÌ¹ÍšÌ™ÌžÌªÌ—ÌºaÌ‹Í›Ì†Í˜ÌžÌ—Ì–Ì»Ì©Í‰cÌŽÍ˜Í™Ì™tÌÍ§Í©Í„Ì•Ì»Ì Ì£Í‰Ì¹Ì Ì£Ì² Ì¿Ì¶Í•Ì—Ì¬wÍ¦ÍÍ¡Í“ÌžÍÌ¹Ì°Í–Í‰iÌ¾Í¦ÌƒÍ„Ì•ÍŽÌžtÌÍ¦ÍžÌœÌºÌ–ÌhÍ›Ì‡Í®Í«Í¡Í™Ì°Ì¬Ì–ÍŽÌ° Í£Í¯ÍÍ•Ì»ÍšÌ¹ÌºaÌ„Í¡Ì±Ì™ÌÌ¦Ì¤Ì¼Ì¥nÍ¥Í†ÌŒÌ‹Ì¶Í”ÌœyÍ¨Ì·Í“Ì»ÌºÌºÍ‰Í‡Ì»oÍ„Ì‰Í£Ì”ÍŸÌ£Ì±Í™ÌœnÌ’Í¬Ì§Ì¦ÌŸÍ”ÌœÌ«Ì—eÍƒÍ‚ÌšÌ¡Í•Ì® Ì‰ÍÍ‹Ì½Ì¡Í“Ì˜ÍšÌÌ¹Í”tÍŒÍ‹Í˜Ì–ÍÍšÌÌ¬ÍˆÍ…ÌhÍ†Ì“Ì¾ÍœÌ±Ì—Ì–ÍšÌ¼Í”Í•aÌ‹Í¦Í©ÍÍ¢ÍˆÌ£ÍÍ•ÍtÌÍ¡Ì–ÌªÌ¤Ì³ÍŽÌ± ÍƒÍ«Ì›Ì»Ì Ì¼Ì¬lÌ‚Ì¶ÌžÌ¤Ì£Í…Í”Ì—Í”oÍ„Í¯Í Ì¹ÌžÌ¦Ì–ÍšÌ«ÌœÌ±oÍƒÍ®Ì‹Ì§Ì¯Ì±ÌªkÌ“Í§Ì•Í‰ÍŽÌÌ»sÌÍŸÌ¤ÍˆÌª Ì”Ì‚Í¨ÍœÌ¤ÌžÌ³Í”ÌÌªÌŸÌ¹hÍ—Ì…ÌÌƒÌ›ÌÌ²Ì°Ì»uÍÌ‡ÌˆÍÌœÌ™mÌŠÌ§ÌžÌ®ÌŸÌ¦Ì³ÌŸaÍŠÍ›ÌÌ¸Í“ÌºÌ²Ì¼ÌœnÌ’Ì¶Ì³Ì®.Í‹Í¬Í—Í©Í¢Í‡Ì»ÍšÍ“Ì³ÌºÌœÌ±

It’s Close I can feel it

Yes it becomes a history but my life also in the past changes and the present also with it. The way I’m suffering from the pain and want to end my life I’m 100% sure at least sure no one around me is or was as hopeless and horrible as my hubby I’m devastated I really want to send a message to my past it may not start but it will definitely change. I was forced, not given any option, my father and brother gave me wrong information and had no concerns for me. It was just survival for me. I repent for not killing myself when I had time, but now if I have a chance why not. Now when I’m out of my marriage I come to know a guy then had feelings for me, was madly in love and wanted to ask for my hand, now I want to inform my self and change everything plz help me.

I too would like to go back in time. I just wish he lived a happy eternal life. I would just like to repeat to come back in 2020.

I heard from a guy in Idaho that time travel is possible. You’ll need to go online and purchase a pogo stick looking device and make sure not to forget the crystals.

I think u need a black-hole-proof spaceship, go to the centre, escape the black hole and viola! You are now in the past. If you can’t escape, then you’d travel to a time where that black hole didn’t exist.

Believe me you time travel! If not physically then you do mentally,like you through dreams.

Though they sale it online, it would not take the chance. It is as simple as beating the speed of light and having some system to send you to the time you want. Time however is not real, and were just traving universes. It will all be in the open in 2028 according to other travelers.

All you need base on how to travel to time is very simple but had to find firstly find a way to get to space through a space rocket secondly find a very perfect consifigration for traveling to tiTme then find a very fast rocket that could create a form of force reaction in space in order yo enable fast speed in space for the break through of non gravity in space and make sure that while doing all you activities is not far away from planet and not also to close to planet earth and make sure that you are with wristwatchs whose time is set disame then you can to the future

Man you can get all you need for too build a time machine in your local store man, man I sure wished Iâ€™d kept mine but it frightened the heck off me man, sometimes when I fart I find a grape in my pants

time travel is a fake, baseless and delusional idea. If you believe in that crap then tell us if we are living in the future or in the past. To travel backward the entire system has to return all along with nature and events, it won’t be for you alone except time travel only happens in the mind.

you would need to get about 1,000,000 pounds of silicon and then somehow conduct enough energy to make 500 cars run without an engine and then go to a nuqular power plant and somehow make a portal. but the whole world could go out of orbit if you do that so I wouldent sugest it.

Time machine is good and bad because,with the time machine you will know about your future which is not good.

Is time travel actually a real thing because if it is then I need it because I am trying to go back in time to fix all of my mistakes

So what if time travel is the reason that we now believe there are other realities in our own world.this could be that a Time traveler we could only go back and couldn’t come back, and on doing so if you do something to change the past in stead make a new reality.making other things are deferent and ours realty stays the same . sometimes reality gets mixed up make the mandela effect that we see today

Time in the future it is faster then now. The past is slower so you can travel . It is up to you. One way is to meditate. You can travel and see any body you want right now. You can fly faster then light. That is one way. You go to the future. To go to the past you sleep for a long time. Some time you go to the future or the past. Your heart well stop and your body gets cold. Sometimes you can control it sometimes you can’t.

but how do we know that is really true ? i mean i want to figure this out, i want to time travel, but how is it that simple ? so many people have been trying to figure this out for many years and its that simple ?

Yeah what if you get stuck in there what do you do than

You cant go there in the first place. Dont worry. With current technology, we will only end up messing some few microseconds. Highly doubtful, if we can end up getting the news of travelling hundreds of years in our lifetime.

wait what would happen if someone saw you while you where in past/future i’m curious

Time is an illusion based on perceived reality and is only relative to our limitations. Time isnâ€™t what it seems and all things canâ€™t be figured out

Im on a school computer looking this up and i found this article and scrolling trough it and ive not heard one statement here as good as yours bro

This is blowing my mind people, then I see the school boy on the post. Great stuff, whoever reads this is already capable of travelling through time. Think about all people who have posted on this thread, now think about who will read mine. Now think of those â‚¬opposite trolls $ who never ever bother posting on you tube thread etc. But ONE comment from one of the time travellers who wrote on this thread. So that opposite troll is me,I donâ€™t normally post.however because of previous comments Iâ€™m posting here. And I love the DMT shit I loved that and lived that one out in real life,,,,another day.

So my point is ifOne or two threads have made me write this….then what will my post make others write , think…..then I could travel back and not write this…. then what. Love the conception of time how can u travel something that doesnâ€™t YOU perceive to be time, like a train can only run on its train tracks, a car can only drive on a road etc Itâ€™s posibble I know it is. Sometimes when u have fun times moves swift but locked in jail it goes snail pace. U c me. I write letters to myself from past from future. Remember everything that happens in present becomes part the past. But the future is what you hold in your hands. Question is, now you know….what the f are u gonna do about it?.. 01/04 ==== 21

Hahahah only realised school boy is named BIG dick pissing myself laughing I gotta go pee. Respect certified

so not halal mode

True so were not traveling in time. It is just different universe (on what we call) different time, day, tears, etc.

You would be scared for life

you will desepear

Maybe it has happened before and we just don’t know that they’re from the future. If people in the future time traveled, the would know that it’s dangerous to mess with the past and would pretend to be part of the past.

I believe time travel is already possible, however we cannot fix past mistakes without altering future predicaments. Say we stop JFK’s assasination, that would completely change the future from that point forward to one none of us can know/guess or conclude the effects? Other time travel purposes go to the future I think that from now our world will die off before 2096 basdd on overpopulation, global warming & polution as such creating islands of plastic waste in our oceans. The best thing my opinion go back to the garden of Eden, kill that Serpent Satan before he tricks Eve into the forbidden fruit. Then let God raise, enlighten & teach us how to be humanly sustainable on his planet & I guarantee technology & smart phones? Ain’t no part of it!!

Time travel possible but one n only theory of Stephen hawking

How it is possible to jump in time …??

Many ways. The most used is creating a black hole which can be done in a few ways. 1) traveling forwards or backwords faster than the speed of light 2) been known during heavy lightning strikes. Each way is a fast movement that opens the black hole. It has been done by the Government since the 1980s though they claimed they never beet the speed of light until 2002. However, Time is a illusion and their for we are actually traveling different universe that are differnt than ours even if the difference is by 1 thing. Each universe may have (what we call) different time, days and years. And each time we change that time line we created a new one. It is belief as CERN has said they destroy 5 universe, that they can travel to them. Since 2012 it has seem we been shifting and is now belief they have possibly came together. The event is known as The Mandela Effect.

No one has the right theory in my thinking. Only a few things are wrong. It is universes with (what we call) different time, days and years we are traveling to and not time itself as it is a illusion. Their is no stop to how much we can do, or where we can go. No limit as such say.

There is no God. No magical serpent or Garden of Eden ever existed. Basing a scientific theory on archaic stories does no one any good.

You choose a hopeless eternity. I choose hope through the promise of salvation through Christ for those who believe. You see, I have child in heaven. Thankfully, have a hopeful reality that I can embrace. There is a God. Our known universe is only 14 or so billion years old… is it mathematically possible that random molecules out of the Big Bang mixed in just the right way from to form a complex cellular organism… with DNA… and result in humans and such diversity of life forms? Itâ€™s naive to accept this as a result of chance. Think about it. How is that remotely possible without a creator?

Hahaha. You make it seem as tho the big bang happened, and we just popped into existence? Naw it’s called evolution baby, we started out as microscopic organisms, seriously, when did you drop out of school? But that’s like saying a some guy writes a book to explain away natural phenomenons that they were to stupid (un-evolved) to grasp and the concept good and bad and the eternal damnation, And thus, the Bible, and boom, everyone now was made by God, hahaha. When you can prove he/she exists, and that the Bible was a autobiography, and not just some twisted piece of Fiction, that has no real basis in reality, and cannot be proved to be more that a work of Fiction. Rather than being used as the16th Century control tact, ‘be good or you’ll go to hell’. But I guess that’s what they mean when they say ignorance is bliss, (maybe if I was as ignorant as y’all believers I’d believe to). But I can’t see how a ‘GOD’ would ever ask one of its creations to kill another.. Genocide, Crusades, all the ethnic cleansing.. All In the name of God Almighty! Hahahahahhaaa. Aliens are more believable than this shit, and theirs no proof they exist either. Hahahahaha. Fug’n Bible thumpers. ‘Step out side your faith and see the world for what it really is, a complex organism, mad of gravity and dust, quite a unique specimen! And we, yes Bible bangers, this includes you, are destroying it like the bubonic plague.’. ‘The end is coming and it’s our fault’

Have you taken the time to read The Old Testament and the prophecies therein that came to be ?.

How do you explain that ?.

My last post should read GS not G

You have not had an encounter yet with God. Don’t be so certain on yuour theory of evolution. He came and shook my reality to it’s core. Made thing possibly that no one could ever explain.

What are you talking about? Ur so wrong and funny in every way.

BlissfullyInformed just told me his comment was all an April fools prank. He believes in Jesus and was just fooling.

Time travel is very much possible just as you decided to come existence in this century meaning one can decide to be in another time zone . life is all about numbers, you just have to work on numbers

I’m pretty sure ppl don’t decide to come into existence. If that were true I wouldn’t be replying to your comment.

Un like your other reply, I understand what you mean. Each timeline (or universe as some see it) can easily be traveled to at will. No different than traveling threw your time you want to visit.

Science has proven a few things from the Bible is true. God does exist. Christians are confused with time and what it says. For a example. God created the world, as science even belives it was God who created the big bang, yet the bang has happen itself creating the moon, planets and stars. Christians also fail to understand chapter 1 and 2 of gen. spoke of two different creations which can be why we see dinosaurs before humans as chapter 1 spoke of animals first and humans 2nd. Their also was different time than, as without the moon a full day is 6 hours. It would take 4 days back than to equal are 1 day. Time is lost and Christians are just confuse on that time. That does not proof their is no God. As they have already found the robes of Jesus and remains of Noah’s ark, it proves much did happen. The bible only has less than 50% of what was written.

Changing the past is impossible, because if we went back into the past, that means we were already there during the time you experienced it.

We all know how to get into time travel but how do we get out……..

You don’t need time travel – all you need is life. And what is life? Life is the evolution of the impossible into the inevitable over an infinite amount of time.

if it is shown that if something, such as a solution to a particular class of equations, were possible, then two mutually contradictory things would be true, such as a number being both even and odd. The contradiction implies that the original premise is impossible.

This is called proof by impossibility. Thus if some traveled back in time far enough to kill his grandfather, we have the contradiction and therefore it is impossible.

You could argue that he would be able to time travel, but not kill his grandfather. However almost anything a person does going back in time would cause the same contradiction, thererfore it is the traveling back in time that is impossible.

Actually, it probably is possible to travel back in time, however to do so, you would also have to travel so far in space that you cannot see anything that happened before your current time due to the speed of light, because this to could affect the future.

The reason I am here is that, i really want to go back the day when our matriculation exam was just finished. Everything around me is peaceful and happy. Currently, I am living in dire situation. People are dying outside on the streets. Smokes everywhere. Everything is in doom. Ah, yeah. I really miss my past. If you are reading this, you can judge me in anyways. I just want to live peacefully and happily.

You must live in Portland

I entirely know what you say and how you feel, Robin. I am totally convinced that future is no promise to offer a better place to live. World is becoming unnecessarily more complex and more horrible and more insecure. Therefore, travelling back in time to a point where things were still far away from such ordeals is what I aspire. But I think if it is possible to travel back in time without the possibility of carrying our lived experiences with us, it will be useless as we will be repeating the same mistakes over and over again. Now, this begs the questions “in what type of physique could we imagine ourselves back there if such time travel becomes possible? That is, becoming younger again in a physical regression (as I said this would be a torture without having learned from all these later years)? Or appearing at our desired times in our present physique and age? I believe the most ideal one would be if we appeared at our desired point in time at the same age that we were at that point of time with a good feeling of our later lived experiences.

Mam all u need to do is just run faster as much as u can or visit the black hole because in both condition time just slow it down ….

Time travel is simple. If you do happen to travel to the past you create a new time line not affecting the time line you left. In essence you going to the past is now your future. Even if you were able to return you may never know if you remained in your time-line or created a new one. So even if you changed something in your travels it would happen in the future not the past.

Sorry time traveling is not possible, there is no way you can go into the past or the future â€â™‚ï¸. You can only be in the time you are already in.

Incorrect. General relativity allows time travel into the future. You need a space ship that can travel extremely fast though, approaching the speed of light, or you need to get close to a supermassive black hole.

It is travel into the past that there is no known practical way to do, and is probably impossible.

So what happens when we Die? Where do we go? I want to go back in time so I can meet my childhood friends…

Simple question from a simple mind:

At what point, when a person says they are from the future, do we stop throwing them in the funny farm and actually start listening??

When they show actual proof. Not just some random prediction of the future.

I don’t believe that “glimpses into the future” could be possible. If it were so, we could glimpse blueprints of the future that we could bring back to the present and build before they were invented. My personal.beleif is in any time frame there is only one active time which is the present. The past no longer exists and the future hasn’t occurred yet, so there is no such thing as ‘time travel’ except for the frame we are in now.

First off time is not real we make time if you travel anywhere all you are doing is beating the Earth speed try this for a mathematical equation the Earth travels a thousand miles per hour you’re not beating human time that is your own equation the Earth travels a thousand miles per hour a space shuttle travel 17,000 mph you can beat time that you made so time is not real you are only beating the Earth speed if you go in a space shuttle and go around the earth 17,000 miles per hour the Earth only travels a thousand miles per hour plus it has all types of gravitational pull from the Moon Earth’s access on the til t you figure out the mathematical equation I cannot time travel is real if you can beat the Earth speed and we can it has nothing to do with its 12:00 it’s 1:00 that’s not real time is made up as a mathematical equation you can beat the Earth speed you can go back into the Earth’s time in a space shuttle but you’re not beating anything except the Earth’s speed think about that one time is not real at all all it is is a mathematical equation think about that one real long

What I’m trying to say is this a space shuttle travel 17,000 mph the Earth travels a thousand you beat it 16 times faster that’s all you did you’re not beating any time you’re not beating 1:00 you’re not beating 3:00 all you’re doing is beating the Earth’s time you can go in reverse around the Earth 17,000 mph okay you can go forward with the Earth’s centrifugal force 17,000 miles per hour you’re not beating anything you’re beating a mathematically equation that we we created astronauts been traveling time for instance for years and haven’t told us because of the space shuttle that does travel 17,000 mph it beats the Earth speed 16 times a boggles my mind you have the Earth access the moon gravitational pull but you can get in a space shuttle and travel 17,000 miles per hour and beat the Earth’s speed 17 times think about it

If any scientist or anybody can actually answer this question how do you set up this equation with the Earth spinning a thousand miles per hour you have the moon pulling gravity the Earth’s access on until I want to know tell me then wondering for a while this equation popped into my head about 2 years ago I’m not a math whiz or anything I just thought about it weird how the mind works I’m not into space or any space stuff at all I’m Samanthas boy friend John antos wrote this

I liked your post and the knowledge you given. I also written a post on Time Travel.

how would any of that stuff be true because e’*34+Em would stop all the forss of vissecs and how would we do it if you now what i mean??? also thanks for the scuff for my project

I would love it if I had a real life time machine here with me now which could take me to anytime I want, the past, present or future. If I had a time machine here with me now, I would go to the past in September 2004 when I was born and give myself to another family that is actually rich and not this horrible family that I have now.

that not nice

Close but not quite right scientists of the idiotic variety, yes, you don’t want people to travel back in time to mess with their own pasts, of course, but you say it’s impossible, but it’s not, and I’m always ignored with my crazed crackpot theories, so what’s the harm in telling the truth as I see it, while it could be possible to travel to the past, here in lies the problem with rewriting the future, while some believe it’s possible to travel back in time, but it’s very expensive and definitely a one-way trip to the future or to the past. Basically Doc Brown got the mechanism for time travel almost right but the energy out put needs to be quadrupled instead, allowing for the ‘physical item, being or vehicle’ to transport through time without killing the time traveler in question. Wormholes are unpredictable, until warp speed for spaceships are a thing, it is not possible for the space ships to achieve time travel, unless they want to enter a black hole, which I would not recommend. as you need warp speed to survive the emptiness of the black hole, without being ripped to shreds. Say for example, Back to the future 1, the timeline doesn’t erase it continues on without the ‘said time traveler’ in existence basically the Marty from Wimpy George’s timeline did time travel to the past and messed with his parent’s meeting so to speak, but never return to the same timeline therefore Marty A went known as a Missing Child in timeline A, while it continues on without him, however Marty A became Marty B/C, in the Successful George Timeline. So that is what I’m talking about. the timeline changes only for the time traveler themselves the ones who are left behind don’t experience a thing of timeline rewritten-ism, as it would never happen in the first place. The other thing is if you want to mess with your own childhood, to make a better life for the past self, the key thing to remember it’s not really you. It’s an alternative version of you, that you interfered with. creating a parallel timeline to it’s original, yet slightly different. Yes it would be awkward to raise yourself. but as long as you are staying in the past, nothing should happen until the age you traveled back in time, unless of course you touched your past self and suddenly de-aged and merged with your past self, is an option 1, option 2 the future self explodes spreading guts all over the place and therefore the past self, of you became a murderer of your future self, I am more inclined to believe option 1 as option 2 seems a little too out there. Basically you would have two memories one of the former timeline and one of the current different timeline. Still traveling through time is truly a one way trip and if you want to travel through time, you would need some time travel mechanism, the way you scientist talk is basically a dream version, or an OBE version (OUT-OF-BODY-EXPERIENCE) which is basically a vivid/lucid dream which is not true time travel, the true time travel is based on the BTTF Trilogy not the idiotic versions you preach about. I believe I’ve said enough.

Mystery solved and I will explain, I was in a coma 3 months and I experienced things, I traveled time forward and backward, it is not a one way ticket. Movies and songs are recorded on magnetic tape in a VCR tape Cartridge or Cassette tape, Magnetic tape recording works by converting electrical signals into magnetic energy, which imprints a record of the signal onto a moving tape covered in magnetic particles. 3D life on earth(a movie), and the Magnetosphere all around earth coming from the core of earth(MAGNETIC ACTIVITY) without Atom Made Tape, is like a movie on magnetic Atom made tape in a VCR tape cartridge. Revolution and Rotation is the motor(VCR).

This is why people have those freaky Dejiâ€™vu feelings like they have lived this before, BECAUSE YOU HAVE, and how people can be psychic, and how there is Prophecy in the Bible. When a person dies, their Spirit- MIND(Thoughts, Feelings, Urges(Physical and mental personality)) breaks out of human body- a stopped heart is what releases the spirit from the human body. Then the Soul(Life) with the memory of your existence in it breaks out of spirit and goes back to your birthday with a erased memory, meanwhile your spirit goes back in time to when you were a teenager starting the mental puberty, maturity from that adult spirit you died with in last life.In that old movie Star Wars or maybe it was the Empire Strikes Back, there is a scene where Princess Laya plays like a 3D movie, that is EXACTLY how its of life on earth.

If only wish I could undo everything what I’ve done wrong in the past, I’d be more happier

And that my friend is absolutely what you do not or would not know. Everyone focuses on what they don’t or haven’t had rather than what positives they do have around them. To change the ingredients of a past life only changes the flavour you have in this life, it does not make you happier.

No, travel to the future is not possible. Like, future is unpredictable and always have been so give up on that field

Already has been, and has been proven.

Time travel is not so possible for every one , but there are already time travelers on earth #@*

Who are these time travelers?

Depends if it is the Governments (they done it since the 80s), or if it was a Accidental travel, or a simple us creating our own machine. Either way, one can easily find storys, and other evidence with a good research. I have a website that shows the effects of change cause by time travel.

They are out their (done by the government since the 80s) but the future is open with time travel (told its open since 2028) so they travel back much.

Time travel 101-

Create a closed loop circuit around a full metal structure, hermetically seal it and bring O2, Use two tesla coils to create north and south poles. (Artificial Magneto sphere.) Make sure to pain the outside in lead to prevent any cosmic rays from penetrating the materials on the inside. (Radiation = bad). Connect a ball made of w/e with wires that alternate the current from the coils to w/e panel on the outside of the structure to make it move via inductive magnetic / electric Lorentzo (Lorentzo = ExMfield = Velocity. = Antigravity) Create Antigravity by using forces from the inside reactor. (Pressurized Mercury, and Tesla Turbine.) Then Move 10-100x faster than light depending on the charged field, Friction will be added to the electric field instead of the craft allowing the G-forces not to crush you inside. The field will take the pressures of outer space, The temperature of space will allow for super conductivity of the structure.

Eventually you will arrive in the future, if you stay in one place. but account for the movement of earth in your travel log. To see outside you will need a monitor / camera system, as any leaks through a viewing area will cause death by radiation from the cosmic rays from the field you have created.

The O2 can be used as a backup generator, through air pressure and the tesla turbine.

There are many different ways to make wormholes, but the curvature of space is really hard to calculate to send a machine far out to the end and create a link with the machine that wants to travel there. And leaving one behind to get back.

If you can imagine it, it can be done. You just need the knowledge of not dying to complete it.

U.S.S. Tourist, You’re a time traveler or just insanely smart.

You don’t need to go the speed of light. Human Time is recorded in the magnetospere as a movie is record, ed on magnet VCR Tape or a song on a record. A VCR or record does not have to go light speed to retrieve the recorded info. All of life is recorded in 3D by our Magnetosphere. My Analogy is imagine a VCR tape cartridge being the earth, imagine life on earth being the movie but in 3D with out adom made tape, imagine Rotation and Revolution of Earth being the VCR putting all in to motion- playing. That is how its done, the magnetosphere kills two birds with one stone, it protects earth and records time, human time is in a magnetic bubble that is why the Bible refers our time is different from gods time and this is how God the maker(PLANET OF UNITED SUPREME BEINGS) can flip through our time to know everything. By the way long before life on earth, he built the original 7 wonders of world(Pyramids) to Pump the Seven gasses into the atmosphere of this planet found in the goldilocks zone, so Life can live on it, and that life of all types is his technological cyborgs that grow and multiply on earth also he seeded it with plant, trees, sea creature and things that fly,. Anyway that above is how time is recorded.

Until recently, I thought my neighbor was a crackpot until he actually invented a time machine. He utilized an ordinary closet, and showed me the sophisticated (to me) instrumentation he had installed. I was very skeptical at first, until he offered a small demonstration and entered the time coordinates and energized his invention. To my amazement, when I opened the door, the clock on the wall was 30 minutes later than when we stepped into the machine. OMG!!! Destroy this thing before it destroys us!!!.

So happy to have my husband back after 6 months of separation. get any kind of relationship/marriage help you want from….Robinsonbuckler11 @gmail com………………………

I find it odd that people say time travel isn’t possible yet… If time travel is possible, it has always existed. Meaning, there is not past present it future, only our perception of time. What we know as past present and future have always been occurring simultaneously, so travel was invited the moment the universe wss formed. Dinosaurs are roaming the earth right now, and forever. A version of me is typing this and has always been typing this, within this perceived moment of “time” and time travel has always happened, whether or not we exist in that reality at the right “time” to observe time travel is the only question.

I find it odd that people say time travel isn’t possible yet… If time travel is possible, it has always existed. Meaning, there is no past present or future, only our perception of time. What we know as past present and future have always been occurring simultaneously, so travel was invited the moment the universe was formed. Dinosaurs are roaming the earth right now, and forever. A version of me is typing this has always been typing this, within this perceived moment of “time” and time travel has always happened, whether or not we exist in that reality at the right “time” to observe time travel is the only question.

Their had to be one point however, when it was created and started, and for that, there was nothing but the current time. Once it was created, than we had a pass, present and future to which we can go back to millions of years to see Adam and Eve with the dinosaurs or go millions of years in the future. However, given the events that changes, each time a new time line has been created. We also have destroyed the planet and repopulated many times in the last million years. Each event changed, or something we do different (without traveling) enters a new universe where some things may be different or the same. Today are universe are shifting a lot.

To be fair, even if it is a one way trip into the past, that doesn’t stop machines going back. We could send a machine back and order it to do anything we want and then tell it to meet us at a certain time in the future. We send it back, then go straight to the meeting point we agreed and then we’ll be able to prove if it worked or not.

I’m a girl who has read a book about seeing future through a box. So is it actually possible?

Time travel has been done on purpose by the Government since the late 1980s. From research, the mostly use kids, or future Presidents. Their are some cases where people have been struck by lightning or came across some tragically event that cause them to leave their timeline either forward or behind in time. The Mandela Effect is the current cause of how things go wrong when time travel is not done right. Click on my name to see the website.

Even as traveling to a location as a future or pass date is possible as what people here mean. However, as you said, it is numbers. Time is a illusion and we do not travel threw time, just universe that are different than ours. What we call time dates and months is what changes each universe. We are all from different universes today as they came together. The mandela effect is a fine example.

thx to eleon wont we soon be able to digitize our conscious being, then accelerate that data pass the speed of light some how then download it into some android or something…..i dunno…..just a thought

I want to go to my elementary school again. Someone help me out, I know its Idiotic but stil.. I am not good at science. As far I understood, 1) we can trace through time if we travel fast than speed of light.. I think memory os the only thing that is faster than light, Yeah I can go to Paris within 1 sec in my memory but yeah its illustion, i want in real 2) Through Blackhole – I think its Bermuda triangle

if you travel back in time you will still be your age now. That is how it worked with others. No one gets younger otherwise traveling to far back would kill you. No school would let you return to school as a adult so not possible.

Plz help me I just want to send a message to myself in my past and save my self from a beast plz help Nazneen

Would love to experience many moments in life again for the first time again!

I think that time traveling should be left alone, for the sake of humanity. There are some things we’re not ready for yet.

Well stephen hawking may be wrong. I mean, the study proved that the universe self corrects itself to prevent inaccuracies. So maybe tourists from past do visit us but we don’t remember them as the universe alters our memory. If you guys have read about Butterfly Effect, a simple mistake today may grow through years to become a giant disaster in future so if you think of it, oncoming tourists from future may cause giant inaccuracies. Imagine this, You have travelled to past. You brought two cakes for yourself, so you pay the shopkeeper 20$. The shopkeeper invests the 20$ in stocks, strikes gold there and becomes a rich businessman.His daughter goes to Cambridge and marries someone else than the person she was supposed to marry according to time. Can you imagine the magnitude of inaccuracy after 100 years? Therefore, whatever the tourists from future do, is corrected by the universe and we don’t remember it. Creepy, but food for thought.It also adds a special meaning to the word ‘Fate’.

How much wacky terbacky (i.e. weed) you be smokin’ JOE JOE?

Hmmmm…. As brilliant of a mind as Stephen Hawkins was, how is he so sure that he would even recognize hordes of tourists from the future? Almost everyone is aware of the warning of the Butterfly Effect. So I’m sure any future visitors Intelligent enough for Past-Time travel would be amply attuned to this.

Most future people coming to the pass (our time) seems careless and not intelligent. Most are taking FBI lie detector test and telling us what is happening in the future. That is a bad idea, because if you tell us (example) who is the next President, and the Government does not like the person they than can change that event to let someone else in (as seen in 2020) One should never acknowledge who he or she is or why they are their. Most traveling is to get knowing of the pass or to pick up certain things. Since are pass is changing, events are changing and are timelines are messed up, someone made a mistake. The Mandela Effect is a fine example.

Wow that’s great plz help me go to my past plz,I can’t do it by my own at least help me send a msg to myself in my past Nazneen

I think it is possible, but time traveling is really just changing universe created by different time lines. Our whole solar system is in a whole different place now and Earth is much smaller in this universe from the one I grew up end. Someone has already changed the timeline.

Roads? Where we’re going, you don’t need roads!

Youre wrong about your measurement of speed for traveling, in order for time to slow down, with inside an object compared to outside. Scientists proved that time with inside an object at an excelorated speed actually appeared to have slown down during the duration of time for the test. The speed was far less then the terminal speed of a rocket for NASA at 256,000 kms p/h.

In to the volicity of space. Generating a vacuum of space, could be no different the the actual transport of matter over frequency where in fact matter can be carried by sound. It is believed that an alien civilization harnessed this energy in the form of bolisks that where believed to carry the same properities and in consideration of harmonic resinance, the simularities could be used in order to carry large weight. In accordance with a documentry on theoretical science.

However the properties, present the fact that a working property controdicts your counter intuative theory of gravitational deceloration of matter to colide within itself to absorb all things into non existance as to the transfer of matter into energy, rather then your idiolisms of transfer between dimentional space to another destination that is not linked or the transfer between time that isnt, either.

However to reproduce the fabric of time within space in a practical measurement as I have mentioned, would put an end to all the lunacy of an unmeasureable field, which people fail to identify. Like running into a glass window. Only to not know what forcefield is present.

Time travel into the past can be achieved simply going faster than the speed of light.

The closer you get to the speed of light the slower time goes

If you reach the speed of light time stops

If you go faster than the speed of light it starts to reverse

Why does no one seem to know this?

Christopher Reeves did this in Superman 3 brah.

Any time travel, pass and future, is by going faster than the speed of light. It is said by reversing that that you can go back in time. However, I assume since the Government has done this since the 80s they have better ways (maybe tying in a date) and not having to go to a unknown date.

I want to send a message to myself in the past on a particular date plz can you help me, this means a lot lot lot to me,plz help me Nazneen

Why don’t we drop the declaratory statements that it “is or isn’t possible!” Until someone actually does so. Just say “maybe”.

People have and their are records both to the pass and future. The Government has done it since the 80s as part of the “star wars project” and are much better at it today. This explains the black holes in the sky of 2019, and the CERN destroying 5 parallel universes in 2013. We also see changes because of time travel events changing time. The Mandela Effect is a find example.

I want to send a msg to myself and my family in the past ,is it possible plz help me my life will be saved one who helps me saves me and my kids from a pack of beasts,

The worst idea ever. We all want to do this and where does it stop. A lottery win does not sound bad if you knew the actual location, time and place. After a while though, would you not want to write that hit song, become the author of the Harry Potter books, stop 9/11? The idea of giving your pass self (a time time travel was not proven) information of the future could change things in a major way. This would cause one small thing to change creating many others to change. This has already happen in simple ways of the The Berenstein Bears changing to The Berenstain Bears. This is a small event but this event “The Mandela Effect” now has over 3,000 changes.

What if you decided to give your pass self information about a lottery ticket that would be a winner, bought late at night and he was hit by a car on the way to get it. Changes the whole future. However, If detailed right, done right, with no large changes, it may not effect much, but to know your being given info from yourself in a future time (when that was not known much or provrn back than) You would either assume it is a joke or you gone crazy.

I don’t want to win a lottery, my decision about my career and studying was right but my family and their cruelty has put me into this worst condition I just want to go back complete my studies and live a life like a human not like a animal or slave,help me plz Nazneen

Can someone take me to 2013? i can pay later to all of you in bitcoins so its a win win and you dont need to do anything, just wait

LOL but still complicating on my side

You travel in your dreams where time and space colloids ..That’s y sometimes the dream which you dreamt might be a 10 mins reel time but you felt dreaming whole time like 6 to 8hrs .. Probably even traveling to parallel universe

I agree. Dreams as we know it is not a simple sleep. The part of the brain we do not use while awake, we use at night. This is the phenomenon part of the brain that can do thing we feel a human can not do. We of course use less than 30% of our brain. By the use of 100% of the brain we would use both sides and be able to do common things such as read thoughts, move things without touching them etc. The idea of using this side of the brain, would be the theory we can leave our bodies and visit different universe, see what could of happen shall we done something different, and even see future events. This may be why we notice different memories to some things as we could of held some from another reality.

It would be very weird, however, if we were trapped in that universe, or another body and fail to return to ours. Is that how people die in their sleep?

i just fell like going to late 70’s, where i can see majority of family.. i am willing to trade life for it…..

Time travel to the pass is just as common as the future. However, as both has been done it is NOT travel threw time. Time is a illusion we created. We are actually traveling threw different universe with (what we call) different time, dates, years, etc. The Mandela Effect is a find example how traveling threw different reality’s change the time lines.

As a add on to the above, Time travel is not a theory, has been proven, and has been done by the Government since the 1980s. Their is many residue in our history to even show some time travel storys to be real.

Where can one get a reverse watch, is it really possible to go back in past with its help, is it sooo easy ,plz help me ??????? Nazneen

US20060073976A1- search this patent number,this describes the process for time travelling,I really don’t think magnetic energy will work,maybe heat focused on a specific point could expand the fabric of space and make a hole in it.even then I will the hole take you to another time.it would be one thing to time travel but selecting a point in time would be impossible.you could only travel to the time you device was built?

Is there a watch which back travels in time or reverse time watch? Is it true? How to get one? But with that how can I send a message to myself in my past, plz help Nazneen

I don’t believe such a watch exist and their are plenty of smart minds with huge funds trying to travel.right now there are only theories.

Thank you very much for your response. I just want to send a message to myself in my past. Nothing much will be changed but 3 literally dying devastating lives will be saved. We are suffering for the mistakes and egoistic arrogance of others so if possible plz help me

Traveling back in time isn’t just a when problem, it’s a *where* problem. Where was the place you’re standing right now a thousand years ago, or a thousandth of a second ago? There is no useful answer to those questions, so there’s nowhere to travel back in time to.

Traveling forward in time? You’re doing it now.

when you step through a door is time lost when you come back through? lets say you return days Later how much time did you loose. what exactly is Time,.? is dialation a safe way to return ,. a Blackhole will assist you in in travel, the question is will you arrive safe,.

Traveling back in time is impossible. 2 reasons why that are never taken into account.

A) The stuff you are made of ( subatomic material) is being used by something else. It I not like you are a facsimile of the already existing material. What you are made of is exactly the same existing material. The problem is exact stuff can not exist in 2 different places in the same point in time. You will either : Decompile or fall out of phase with the universe. Both bad outcomes for the time traveler.

B) Lets look at it from logical commonsense. You have a bar of gold . You intend to send the bar back 1 second in time. Now you have 2 bars of gold . You send those 2 bars back one second . You have 4 bars …… do that 50 times . You have over 900 trillion bars of gold. All made of the exact subatomic particles. The more the bars back the more the existing mass of the universe increase. What are the consequences of changing the mass of the universe . Hence the paradox . Information can not be destroyed., It also can not be created.

At least this is the way my brain perceives going back in time.

Time is a function of change. None of the 4 forces The strong force , The weak force , Electromagnetism and Gravity can not work without time.

I will figure out time travel one day but only for the past.

I wish I could travel back to 18th of June to save my mom.

Is time travel really a one way ticket? Theoretically, if you can go one way, you should be able to go back.

Time is not one way. It’s consequences are however irreparable given certain circumstances and is not something that should be taken lightly or thought of in a manner of disregard. I’ve only very recently decided to take to your social platforms regarding space and time.

You can try finding me on Instagram. I’m not familiar with these platforms to better direct you there. My Instagram name is johnrvh

On Twitter it seems to be @_JohnRvH

If I go forward I will have to pay extra bills and taxes. I donâ€™t think I can afford it.

You’re the first person I’ve come across in this timeline that has a sense of humor. Thankfully, going forward is not possible if that future hasn’t been created yet.

timetraval is no joke if its created the whole universe could go out of orbit.

Cauchy problem converging to non minimal terraces as t â†’ +âˆž

Stephen Hawking may he rest in peace a genius but not all knowing. As far as he knows we haven’t been flocked by tourists, in the same maybe these UFO sightings are actually time travelers from the future coming to the past to view how we really lived why things really happened the way they did, etc. To limit the imagination of possible and impossible is wrong then you create fantasy. And we have learned from history that there is truth in fantasy. I.e. the different mythos of the different ancient cultures from around the world including those of the Norse. Improbable and probable should be more appropriate. It’s possible because it can be imagined improbable die to the right math or this or that not existing or matching up. I also believe that if time travel to the past were possible that the changing of something in the past would create a new timeline running current with your timeline at which will inevitably collide and will cause the collapse of the universe at which point a new universe will be born.

so i think the speed of light is only relative to deciding a point of destination -initially- as specific gravity of destination needs to be ascertained to calculate the frequency needed to run an alcubierre-white engine to bend space correctly to cross space ‘quickly’, the point of reference may well be jupiter in our solar system for the fact of the moons that orbit it, i surmise that by using a ‘dead end ‘ equation that usually puts notable mathematicians into the outer regions by trying to solve it may actually be the key as calculations end in a loop of 4-2-1 ie 3N+1; this process of calculation creates a sine wave over time/distance relative to specific gravity of chosen destination – as time is determined by gravity therefore if the speed of light to a destination can be used to ascertain the specific gravity of a ‘body’ to visit ie a star or sun due to receivable resonant frequencies emitted by the body, then the constrictions of the speed of light do not exist other than to give a constant, by using the 3N+1 method of calculation ,once the speed of light and returning resonant frequencies of a destination are determined the calculation can be extrapolated to match the distance giving the end point -in doing this the sine wave required can be ascertained and be condensed to create a wormhole and allow the alcubierre-white engine to ‘bend or distort space enough so that the bubble you are in matches the required specific gravity of the destination – the frequency of the body nearest to the destination point should be used and resonated inside the bubble to create synchronicity of frequency and cause attraction i also believe that travelling through space require the ability to see things from different perspectives and it requires the ability to navigate through a series of what may be described as “Aims Windows” where your point of view needs to change inherently with a given position at a given point in the galaxy

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Is time travel possible? An astrophysicist explains

Time travel is one of the most intriguing topics in science.

Will it ever be possible for time travel to occur? – Alana C., age 12, Queens, New York

Have you ever dreamed of traveling through time, like characters do in science fiction movies? For centuries, the concept of time travel has captivated people’s imaginations. Time travel is the concept of moving between different points in time, just like you move between different places. In movies, you might have seen characters using special machines, magical devices or even hopping into a futuristic car to travel backward or forward in time.

But is this just a fun idea for movies, or could it really happen?

The question of whether time is reversible remains one of the biggest unresolved questions in science. If the universe follows the laws of thermodynamics , it may not be possible. The second law of thermodynamics states that things in the universe can either remain the same or become more disordered over time.

It’s a bit like saying you can’t unscramble eggs once they’ve been cooked. According to this law, the universe can never go back exactly to how it was before. Time can only go forward, like a one-way street.

Time is relative

However, physicist Albert Einstein’s theory of special relativity suggests that time passes at different rates for different people. Someone speeding along on a spaceship moving close to the speed of light – 671 million miles per hour! – will experience time slower than a person on Earth.

Related: The speed of light, explained

People have yet to build spaceships that can move at speeds anywhere near as fast as light, but astronauts who visit the International Space Station orbit around the Earth at speeds close to 17,500 mph. Astronaut Scott Kelly has spent 520 days at the International Space Station, and as a result has aged a little more slowly than his twin brother – and fellow astronaut – Mark Kelly. Scott used to be 6 minutes younger than his twin brother. Now, because Scott was traveling so much faster than Mark and for so many days, he is 6 minutes and 5 milliseconds younger .

Some scientists are exploring other ideas that could theoretically allow time travel. One concept involves wormholes , or hypothetical tunnels in space that could create shortcuts for journeys across the universe. If someone could build a wormhole and then figure out a way to move one end at close to the speed of light – like the hypothetical spaceship mentioned above – the moving end would age more slowly than the stationary end. Someone who entered the moving end and exited the wormhole through the stationary end would come out in their past.

However, wormholes remain theoretical : Scientists have yet to spot one. It also looks like it would be incredibly challenging to send humans through a wormhole space tunnel.

Time travel paradoxes and failed dinner parties

There are also paradoxes associated with time travel. The famous “ grandfather paradox ” is a hypothetical problem that could arise if someone traveled back in time and accidentally prevented their grandparents from meeting. This would create a paradox where you were never born, which raises the question: How could you have traveled back in time in the first place? It’s a mind-boggling puzzle that adds to the mystery of time travel.

Famously, physicist Stephen Hawking tested the possibility of time travel by throwing a dinner party where invitations noting the date, time and coordinates were not sent out until after it had happened. His hope was that his invitation would be read by someone living in the future, who had capabilities to travel back in time. But no one showed up.

As he pointed out : “The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future.”

Telescopes are time machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel. As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago.

NASA’s newest space telescope, the James Webb Space Telescope , is peering at galaxies that were formed at the very beginning of the Big Bang, about 13.7 billion years ago.

While we aren’t likely to have time machines like the ones in movies anytime soon, scientists are actively researching and exploring new ideas. But for now, we’ll have to enjoy the idea of time travel in our favorite books, movies and dreams.

This article first appeared on the Conversation. You can read the original here .

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Everyone can travel in time . You do it whether you want to or not, at a steady rate of one second per second. You may think there's no similarity to traveling in one of the three spatial dimensions at, say, one foot per second. But according to Einstein 's theory of relativity , we live in a four-dimensional continuum — space-time — in which space and time are interchangeable.

Einstein found that the faster you move through space, the slower you move through time — you age more slowly, in other words. One of the key ideas in relativity is that nothing can travel faster than the speed of light — about 186,000 miles per second (300,000 kilometers per second), or one light-year per year). But you can get very close to it. If a spaceship were to fly at 99% of the speed of light, you'd see it travel a light-year of distance in just over a year of time.

That's obvious enough, but now comes the weird part. For astronauts onboard that spaceship, the journey would take a mere seven weeks. It's a consequence of relativity called time dilation , and in effect, it means the astronauts have jumped about 10 months into the future.

Traveling at high speed isn't the only way to produce time dilation. Einstein showed that gravitational fields produce a similar effect — even the relatively weak field here on the surface of Earth . We don't notice it, because we spend all our lives here, but more than 12,400 miles (20,000 kilometers) higher up gravity is measurably weaker— and time passes more quickly, by about 45 microseconds per day. That's more significant than you might think, because it's the altitude at which GPS satellites orbit Earth, and their clocks need to be precisely synchronized with ground-based ones for the system to work properly.

The satellites have to compensate for time dilation effects due both to their higher altitude and their faster speed. So whenever you use the GPS feature on your smartphone or your car's satnav, there's a tiny element of time travel involved. You and the satellites are traveling into the future at very slightly different rates.

But for more dramatic effects, we need to look at much stronger gravitational fields, such as those around black holes , which can distort space-time so much that it folds back on itself. The result is a so-called wormhole, a concept that's familiar from sci-fi movies, but actually originates in Einstein's theory of relativity. In effect, a wormhole is a shortcut from one point in space-time to another. You enter one black hole, and emerge from another one somewhere else. Unfortunately, it's not as practical a means of transport as Hollywood makes it look. That's because the black hole's gravity would tear you to pieces as you approached it, but it really is possible in theory. And because we're talking about space-time, not just space, the wormhole's exit could be at an earlier time than its entrance; that means you would end up in the past rather than the future.

Trajectories in space-time that loop back into the past are given the technical name "closed timelike curves." If you search through serious academic journals, you'll find plenty of references to them — far more than you'll find to "time travel." But in effect, that's exactly what closed timelike curves are all about — time travel

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There's another way to produce a closed timelike curve that doesn't involve anything quite so exotic as a black hole or wormhole: You just need a simple rotating cylinder made of super-dense material. This so-called Tipler cylinder is the closest that real-world physics can get to an actual, genuine time machine. But it will likely never be built in the real world, so like a wormhole, it's more of an academic curiosity than a viable engineering design.

Yet as far-fetched as these things are in practical terms, there's no fundamental scientific reason — that we currently know of — that says they are impossible. That's a thought-provoking situation, because as the physicist Michio Kaku is fond of saying, "Everything not forbidden is compulsory" (borrowed from T.H. White's novel, "The Once And Future King"). He doesn't mean time travel has to happen everywhere all the time, but Kaku is suggesting that the universe is so vast it ought to happen somewhere at least occasionally. Maybe some super-advanced civilization in another galaxy knows how to build a working time machine, or perhaps closed timelike curves can even occur naturally under certain rare conditions.

An artist's impression of a pair of neutron stars - a Tipler cylinder requires at least ten.

This raises problems of a different kind — not in science or engineering, but in basic logic. If time travel is allowed by the laws of physics, then it's possible to envision a whole range of paradoxical scenarios . Some of these appear so illogical that it's difficult to imagine that they could ever occur. But if they can't, what's stopping them?

Thoughts like these prompted Stephen Hawking , who was always skeptical about the idea of time travel into the past, to come up with his "chronology protection conjecture" — the notion that some as-yet-unknown law of physics prevents closed timelike curves from happening. But that conjecture is only an educated guess, and until it is supported by hard evidence, we can come to only one conclusion: Time travel is possible.

A party for time travelers

Hawking was skeptical about the feasibility of time travel into the past, not because he had disproved it, but because he was bothered by the logical paradoxes it created. In his chronology protection conjecture, he surmised that physicists would eventually discover a flaw in the theory of closed timelike curves that made them impossible.

In 2009, he came up with an amusing way to test this conjecture. Hawking held a champagne party (shown in his Discovery Channel program), but he only advertised it after it had happened. His reasoning was that, if time machines eventually become practical, someone in the future might read about the party and travel back to attend it. But no one did — Hawking sat through the whole evening on his own. This doesn't prove time travel is impossible, but it does suggest that it never becomes a commonplace occurrence here on Earth.

The arrow of time

One of the distinctive things about time is that it has a direction — from past to future. A cup of hot coffee left at room temperature always cools down; it never heats up. Your cellphone loses battery charge when you use it; it never gains charge. These are examples of entropy , essentially a measure of the amount of "useless" as opposed to "useful" energy. The entropy of a closed system always increases, and it's the key factor determining the arrow of time.

It turns out that entropy is the only thing that makes a distinction between past and future. In other branches of physics, like relativity or quantum theory, time doesn't have a preferred direction. No one knows where time's arrow comes from. It may be that it only applies to large, complex systems, in which case subatomic particles may not experience the arrow of time.

Time travel paradox

If it's possible to travel back into the past — even theoretically — it raises a number of brain-twisting paradoxes — such as the grandfather paradox — that even scientists and philosophers find extremely perplexing.

Killing Hitler

A time traveler might decide to go back and kill him in his infancy. If they succeeded, future history books wouldn't even mention Hitler — so what motivation would the time traveler have for going back in time and killing him?

Killing your grandfather

Instead of killing a young Hitler, you might, by accident, kill one of your own ancestors when they were very young. But then you would never be born, so you couldn't travel back in time to kill them, so you would be born after all, and so on …

A closed loop

Suppose the plans for a time machine suddenly appear from thin air on your desk. You spend a few days building it, then use it to send the plans back to your earlier self. But where did those plans originate? Nowhere — they are just looping round and round in time.

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Time Travel and Modern Physics

Time travel has been a staple of science fiction. With the advent of general relativity it has been entertained by serious physicists. But, especially in the philosophy literature, there have been arguments that time travel is inherently paradoxical. The most famous paradox is the grandfather paradox: you travel back in time and kill your grandfather, thereby preventing your own existence. To avoid inconsistency some circumstance will have to occur which makes you fail in this attempt to kill your grandfather. Doesn’t this require some implausible constraint on otherwise unrelated circumstances? We examine such worries in the context of modern physics.

1. Paradoxes Lost?

2. topology and constraints, 3. the general possibility of time travel in general relativity, 4. two toy models, 5. slightly more realistic models of time travel, 6. the possibility of time travel redux, 7. even if there are constraints, so what, 8. computational models, 9. quantum mechanics to the rescue, 10. conclusions, other internet resources, related entries.

Supplement: Remarks and Limitations on the Toy Models

Modern physics strips away many aspects of the manifest image of time. Time as it appears in the equations of classical mechanics has no need for a distinguished present moment, for example. Relativity theory leads to even sharper contrasts. It replaces absolute simultaneity, according to which it is possible to unambiguously determine the time order of distant events, with relative simultaneity: extending an “instant of time” throughout space is not unique, but depends on the state of motion of an observer. More dramatically, in general relativity the mathematical properties of time (or better, of spacetime)—its topology and geometry—depend upon how matter is arranged rather than being fixed once and for all. So physics can be, and indeed has to be, formulated without treating time as a universal, fixed background structure. Since general relativity represents gravity through spacetime geometry, the allowed geometries must be as varied as the ways in which matter can be arranged. Alongside geometrical models used to describe the solar system, black holes, and much else, the scope of variation extends to include some exotic structures unlike anything astrophysicists have observed. In particular, there are spacetime geometries with curves that loop back on themselves: closed timelike curves (CTCs), which describe the possible trajectory of an observer who returns exactly back to their earlier state—without any funny business, such as going faster than the speed of light. These geometries satisfy the relevant physical laws, the equations of general relativity, and in that sense time travel is physically possible.

Yet circular time generates paradoxes, familiar from science fiction stories featuring time travel: [ 1 ]

Consistency: Kurt plans to murder his own grandfather Adolph, by traveling along a CTC to an appropriate moment in the past. He is an able marksman, and waits until he has a clear shot at grandpa. Normally he would not miss. Yet if he succeeds, there is no way that he will then exist to plan and carry out the mission. Kurt pulls the trigger: what can happen?
Underdetermination: Suppose that Kurt first travels back in order to give his earlier self a copy of How to Build a Time Machine. This is the same book that allows him to build a time machine, which he then carries with him on his journey to the past. Who wrote the book?
Easy Knowledge: A fan of classical music enhances their computer with a circuit that exploits a CTC. This machine efficiently solves problems at a higher level of computational complexity than conventional computers, leading (among other things) to finding the smallest circuits that can generate Bach’s oeuvre—and to compose new pieces in the same style. Such easy knowledge is at odds with our understanding of our epistemic predicament. (This third paradox has not drawn as much attention.)

The first two paradoxes were once routinely taken to show that solutions with CTCs should be rejected—with charges varying from violating logic, to being “physically unreasonable”, to undermining the notion of free will. Closer analysis of the paradoxes has largely reversed this consensus. Physicists have discovered many solutions with CTCs and have explored their properties in pursuing foundational questions, such as whether physics is compatible with the idea of objective temporal passage (starting with Gödel 1949). Philosophers have also used time travel scenarios to probe questions about, among other things, causation, modality, free will, and identity (see, e.g., Earman 1972 and Lewis’s seminal 1976 paper).

We begin below with Consistency , turning to the other paradoxes in later sections. A standard, stone-walling response is to insist that the past cannot be changed, as a matter of logic, even by a time traveler (e.g., Gödel 1949, Clarke 1977, Horwich 1987). Adolph cannot both die and survive, as a matter of logic, so any scheme to alter the past must fail. In many of the best time travel fictions, the actions of a time traveler are constrained in novel and unexpected ways. Attempts to change the past fail, and they fail, often tragically, in just such a way that they set the stage for the time traveler’s self-defeating journey. The first question is whether there is an analog of the consistent story when it comes to physics in the presence of CTCs. As we will see, there is a remarkable general argument establishing the existence of consistent solutions. Yet a second question persists: why can’t time-traveling Kurt kill his own grandfather? Doesn’t the necessity of failures to change the past put unusual and unexpected constraints on time travelers, or objects that move along CTCs? The same argument shows that there are in fact no constraints imposed by the existence of CTCs, in some cases. After discussing this line of argument, we will turn to the palatability and further implications of such constraints if they are required, and then turn to the implications of quantum mechanics.

Wheeler and Feynman (1949) were the first to claim that the fact that nature is continuous could be used to argue that causal influences from later events to earlier events, as are made possible by time travel, will not lead to paradox without the need for any constraints. Maudlin (1990) showed how to make their argument precise and more general, and argued that nonetheless it was not completely general.

Imagine the following set-up. We start off having a camera with a black and white film ready to take a picture of whatever comes out of the time machine. An object, in fact a developed film, comes out of the time machine. We photograph it, and develop the film. The developed film is subsequently put in the time machine, and set to come out of the time machine at the time the picture is taken. This surely will create a paradox: the developed film will have the opposite distribution of black, white, and shades of gray, from the object that comes out of the time machine. For developed black and white films (i.e., negatives) have the opposite shades of gray from the objects they are pictures of. But since the object that comes out of the time machine is the developed film itself it we surely have a paradox.

However, it does not take much thought to realize that there is no paradox here. What will happen is that a uniformly gray picture will emerge, which produces a developed film that has exactly the same uniform shade of gray. No matter what the sensitivity of the film is, as long as the dependence of the brightness of the developed film depends in a continuous manner on the brightness of the object being photographed, there will be a shade of gray that, when photographed, will produce exactly the same shade of gray on the developed film. This is the essence of Wheeler and Feynman’s idea. Let us first be a bit more precise and then a bit more general.

For simplicity let us suppose that the film is always a uniform shade of gray (i.e., at any time the shade of gray does not vary by location on the film). The possible shades of gray of the film can then be represented by the (real) numbers from 0, representing pure black, to 1, representing pure white.

Let us now distinguish various stages in the chronological order of the life of the film. In stage $S_1$ the film is young; it has just been placed in the camera and is ready to be exposed. It is then exposed to the object that comes out of the time machine. (That object in fact is a later stage of the film itself). By the time we come to stage $S_2$ of the life of the film, it has been developed and is about to enter the time machine. Stage $S_3$ occurs just after it exits the time machine and just before it is photographed. Stage $S_4$ occurs after it has been photographed and before it starts fading away. Let us assume that the film starts out in stage $S_1$ in some uniform shade of gray, and that the only significant change in the shade of gray of the film occurs between stages $S_1$ and $S_2$. During that period it acquires a shade of gray that depends on the shade of gray of the object that was photographed. In other words, the shade of gray that the film acquires at stage $S_2$ depends on the shade of gray it has at stage $S_3$. The influence of the shade of gray of the film at stage $S_3$, on the shade of gray of the film at stage $S_2$, can be represented as a mapping, or function, from the real numbers between 0 and 1 (inclusive), to the real numbers between 0 and 1 (inclusive). Let us suppose that the process of photography is such that if one imagines varying the shade of gray of an object in a smooth, continuous manner then the shade of gray of the developed picture of that object will also vary in a smooth, continuous manner. This implies that the function in question will be a continuous function. Now any continuous function from the real numbers between 0 and 1 (inclusive) to the real numbers between 0 and 1 (inclusive) must map at least one number to itself. One can quickly convince oneself of this by graphing such functions. For one will quickly see that any continuous function $f$ from $[0,1]$ to $[0,1]$ must intersect the line $x=y$ somewhere, and thus there must be at least one point $x$ such that $f(x)=x$. Such points are called fixed points of the function. Now let us think about what such a fixed point represents. It represents a shade of gray such that, when photographed, it will produce a developed film with exactly that same shade of gray. The existence of such a fixed point implies a solution to the apparent paradox.

Let us now be more general and allow color photography. One can represent each possible color of an object (of uniform color) by the proportions of blue, green and red that make up that color. (This is why television screens can produce all possible colors.) Thus one can represent all possible colors of an object by three points on three orthogonal lines $x, y$ and $z$, that is to say, by a point in a three-dimensional cube. This cube is also known as the “Cartesian product” of the three line segments. Now, one can also show that any continuous map from such a cube to itself must have at least one fixed point. So color photography can not be used to create time travel paradoxes either!

Even more generally, consider some system $P$ which, as in the above example, has the following life. It starts in some state $S_1$, it interacts with an object that comes out of a time machine (which happens to be its older self), it travels back in time, it interacts with some object (which happens to be its younger self), and finally it grows old and dies. Let us assume that the set of possible states of $P$ can be represented by a Cartesian product of $n$ closed intervals of the reals, i.e., let us assume that the topology of the state-space of $P$ is isomorphic to a finite Cartesian product of closed intervals of the reals. Let us further assume that the development of $P$ in time, and the dependence of that development on the state of objects that it interacts with, is continuous. Then, by a well-known fixed point theorem in topology (see, e.g., Hocking & Young 1961: 273), no matter what the nature of the interaction is, and no matter what the initial state of the object is, there will be at least one state $S_3$ of the older system (as it emerges from the time travel machine) that will influence the initial state $S_1$ of the younger system (when it encounters the older system) so that, as the younger system becomes older, it develops exactly into state $S_3$. Thus without imposing any constraints on the initial state $S_1$ of the system $P$, we have shown that there will always be perfectly ordinary, non-paradoxical, solutions, in which everything that happens, happens according to the usual laws of development. Of course, there is looped causation, hence presumably also looped explanation, but what do you expect if there is looped time?

Unfortunately, for the fan of time travel, a little reflection suggests that there are systems for which the needed fixed point theorem does not hold. Imagine, for instance, that we have a dial that can only rotate in a plane. We are going to put the dial in the time machine. Indeed we have decided that if we see the later stage of the dial come out of the time machine set at angle $x$, then we will set the dial to $x+90$, and throw it into the time machine. Now it seems we have a paradox, since the mapping that consists of a rotation of all points in a circular state-space by 90 degrees does not have a fixed point. And why wouldn’t some state-spaces have the topology of a circle?

However, we have so far not used another continuity assumption which is also a reasonable assumption. So far we have only made the following demand: the state the dial is in at stage $S_2$ must be a continuous function of the state of the dial at stage $S_3$. But, the state of the dial at stage $S_2$ is arrived at by taking the state of the dial at stage $S_1$, and rotating it over some angle. It is not merely the case that the effect of the interaction, namely the state of the dial at stage $S_2$, should be a continuous function of the cause, namely the state of the dial at stage $S_3$. It is additionally the case that path taken to get there, the way the dial is rotated between stages $S_1$ and $S_2$ must be a continuous function of the state at stage $S_3$. And, rather surprisingly, it turns out that this can not be done. Let us illustrate what the problem is before going to a more general demonstration that there must be a fixed point solution in the dial case.

Forget time travel for the moment. Suppose that you and I each have a watch with a single dial neither of which is running. My watch is set at 12. You are going to announce what your watch is set at. My task is going to be to adjust my watch to yours no matter what announcement you make. And my actions should have a continuous (single valued) dependence on the time that you announce. Surprisingly, this is not possible! For instance, suppose that if you announce “12”, then I achieve that setting on my watch by doing nothing. Now imagine slowly and continuously increasing the announced times, starting at 12. By continuity, I must achieve each of those settings by rotating my dial to the right. If at some point I switch and achieve the announced goal by a rotation of my dial to the left, I will have introduced a discontinuity in my actions, a discontinuity in the actions that I take as a function of the announced angle. So I will be forced, by continuity, to achieve every announcement by rotating the dial to the right. But, this rotation to the right will have to be abruptly discontinued as the announcements grow larger and I eventually approach 12 again, since I achieved 12 by not rotating the dial at all. So, there will be a discontinuity at 12 at the latest. In general, continuity of my actions as a function of announced times can not be maintained throughout if I am to be able to replicate all possible settings. Another way to see the problem is that one can similarly reason that, as one starts with 12, and imagines continuously making the announced times earlier, one will be forced, by continuity, to achieve the announced times by rotating the dial to the left. But the conclusions drawn from the assumption of continuous increases and the assumption of continuous decreases are inconsistent. So we have an inconsistency following from the assumption of continuity and the assumption that I always manage to set my watch to your watch. So, a dial developing according to a continuous dynamics from a given initial state, can not be set up so as to react to a second dial, with which it interacts, in such a way that it is guaranteed to always end up set at the same angle as the second dial. Similarly, it can not be set up so that it is guaranteed to always end up set at 90 degrees to the setting of the second dial. All of this has nothing to do with time travel. However, the impossibility of such set ups is what prevents us from enacting the rotation by 90 degrees that would create paradox in the time travel setting.

Let us now give the positive result that with such dials there will always be fixed point solutions, as long as the dynamics is continuous. Let us call the state of the dial before it interacts with its older self the initial state of the dial. And let us call the state of the dial after it emerges from the time machine the final state of the dial. There is also an intermediate state of the dial, after it interacts with its older self and before it is put into the time machine. We can represent the initial or intermediate states of the dial, before it goes into the time machine, as an angle $x$ in the horizontal plane and the final state of the dial, after it comes out of the time machine, as an angle $y$ in the vertical plane. All possible $\langle x,y\rangle$ pairs can thus be visualized as a torus with each $x$ value picking out a vertical circular cross-section and each $y$ picking out a point on that cross-section. See figure 1 .

Figure 1 [An extended description of figure 1 is in the supplement.]

Suppose that the dial starts at angle $i$ which picks out vertical circle $I$ on the torus. The initial angle $i$ that the dial is at before it encounters its older self, and the set of all possible final angles that the dial can have when it emerges from the time machine is represented by the circle $I$ on the torus (see figure 1 ). Given any possible angle of the emerging dial, the dial initially at angle $i$ will develop to some other angle. One can picture this development by rotating each point on $I$ in the horizontal direction by the relevant amount. Since the rotation has to depend continuously on the angle of the emerging dial, circle $I$ during this development will deform into some loop $L$ on the torus. Loop $L$ thus represents all possible intermediate angles $x$ that the dial is at when it is thrown into the time machine, given that it started at angle $i$ and then encountered a dial (its older self) which was at angle $y$ when it emerged from the time machine. We therefore have consistency if $x=y$ for some $x$ and $y$ on loop $L$. Now, let loop $C$ be the loop which consists of all the points on the torus for which $x=y$. Ring $I$ intersects $C$ at point $\langle i,i\rangle$. Obviously any continuous deformation of $I$ must still intersect $C$ somewhere. So $L$ must intersect $C$ somewhere, say at $\langle j,j\rangle$. But that means that no matter how the development of the dial starting at $I$ depends on the angle of the emerging dial, there will be some angle for the emerging dial such that the dial will develop exactly into that angle (by the time it enters the time machine) under the influence of that emerging dial. This is so no matter what angle one starts with, and no matter how the development depends on the angle of the emerging dial. Thus even for a circular state-space there are no constraints needed other than continuity.

Unfortunately there are state-spaces that escape even this argument. Consider for instance a pointer that can be set to all values between 0 and 1, where 0 and 1 are not possible values. That is, suppose that we have a state-space that is isomorphic to an open set of real numbers. Now suppose that we have a machine that sets the pointer to half the value that the pointer is set at when it emerges from the time machine.

Figure 2 [An extended description of figure 2 is in the supplement.]

Suppose the pointer starts at value $I$. As before we can represent the combination of this initial position and all possible final positions by the line $I$. Under the influence of the pointer coming out of the time machine the pointer value will develop to a value that equals half the value of the final value that it encountered. We can represent this development as the continuous deformation of line $I$ into line $L$, which is indicated by the arrows in figure 2 . This development is fully continuous. Points $\langle x,y\rangle$ on line $I$ represent the initial position $x=I$ of the (young) pointer, and the position $y$ of the older pointer as it emerges from the time machine. Points $\langle x,y\rangle$ on line $L$ represent the position $x$ that the younger pointer should develop into, given that it encountered the older pointer emerging from the time machine set at position $y$. Since the pointer is designed to develop to half the value of the pointer that it encounters, the line $L$ corresponds to $x=1/2 y$. We have consistency if there is some point such that it develops into that point, if it encounters that point. Thus, we have consistency if there is some point $\langle x,y\rangle$ on line $L$ such that $x=y$. However, there is no such point: lines $L$ and $C$ do not intersect. Thus there is no consistent solution, despite the fact that the dynamics is fully continuous.

Of course if 0 were a possible value, $L$ and $C$ would intersect at 0. This is surprising and strange: adding one point to the set of possible values of a quantity here makes the difference between paradox and peace. One might be tempted to just add the extra point to the state-space in order to avoid problems. After all, one might say, surely no measurements could ever tell us whether the set of possible values includes that exact point or not. Unfortunately there can be good theoretical reasons for supposing that some quantity has a state-space that is open: the set of all possible speeds of massive objects in special relativity surely is an open set, since it includes all speeds up to, but not including, the speed of light. Quantities that have possible values that are not bounded also lead to counter examples to the presented fixed point argument. And it is not obvious to us why one should exclude such possibilities. So the argument that no constraints are needed is not fully general.

An interesting question of course is: exactly for which state-spaces must there be such fixed points? The arguments above depend on a well-known fixed point theorem (due to Schauder) that guarantees the existence of a fixed point for compact, convex state spaces. We do not know what subsequent extensions of this result imply regarding fixed points for a wider variety of systems, or whether there are other general results along these lines. (See Kutach 2003 for more on this issue.)

A further interesting question is whether this line of argument is sufficient to resolve Consistency (see also Dowe 2007). When they apply, these results establish the existence of a solution, such as the shade of uniform gray in the first example. But physicists routinely demand more than merely the existence of a solution, namely that solutions to the equations are stable—such that “small” changes of the initial state lead to “small” changes of the resulting trajectory. (Clarifying the two senses of “small” in this statement requires further work, specifying the relevant topology.) Stability in this sense underwrites the possibility of applying equations to real systems given our inability to fix initial states with indefinite precision. (See Fletcher 2020 for further discussion.) The fixed point theorems guarantee that for an initial state $S_1$ there is a solution, but this solution may not be “close” to the solution for a nearby initial state, $S'$. We are not aware of any proofs that the solutions guaranteed to exist by the fixed point theorems are also stable in this sense.

Time travel has recently been discussed quite extensively in the context of general relativity. General relativity places few constraints on the global structure of space and time. This flexibility leads to a possibility first described in print by Hermann Weyl:

Every world-point is the origin of the double-cone of the active future and the passive past [i.e., the two lobes of the light cone]. Whereas in the special theory of relativity these two portions are separated by an intervening region, it is certainly possible in the present case [i.e., general relativity] for the cone of the active future to overlap with that of the passive past; so that, in principle, it is possible to experience events now that will in part be an effect of my future resolves and actions. Moreover, it is not impossible for a world-line (in particular, that of my body), although it has a timelike direction at every point, to return to the neighborhood of a point which it has already once passed through. (Weyl 1918/1920 [1952: 274])

A time-like curve is simply a space-time trajectory such that the speed of light is never equaled or exceeded along this trajectory. Time-like curves represent possible trajectories of ordinary objects. In general relativity a curve that is everywhere timelike locally can nonetheless loop back on itself, forming a CTC. Weyl makes the point vividly in terms of the light cones: along such a curve, the future lobe of the light cone (the “active future”) intersects the past lobe of the light cone (the “passive past”). Traveling along such a curve one would never exceed the speed of light, and yet after a certain amount of (proper) time one would return to a point in space-time that one previously visited. Or, by staying close to such a CTC, one could come arbitrarily close to a point in space-time that one previously visited. General relativity, in a straightforward sense, allows time travel: there appear to be many space-times compatible with the fundamental equations of general relativity in which there are CTC’s. Space-time, for instance, could have a Minkowski metric everywhere, and yet have CTC’s everywhere by having the temporal dimension (topologically) rolled up as a circle. Or, one can have wormhole connections between different parts of space-time which allow one to enter “mouth $A$” of such a wormhole connection, travel through the wormhole, exit the wormhole at “mouth $B$” and re-enter “mouth $A$” again. CTCs can even arise when the spacetime is topologically $\mathbb{R}^4$, due to the “tilting” of light cones produced by rotating matter (as in Gödel 1949’s spacetime).

General relativity thus appears to provide ample opportunity for time travel. Note that just because there are CTC’s in a space-time, this does not mean that one can get from any point in the space-time to any other point by following some future directed timelike curve—there may be insurmountable practical obstacles. In Gödel’s spacetime, it is the case that there are CTCs passing through every point in the spacetime. Yet these CTCs are not geodesics, so traversing them requires acceleration. Calculations of the minimal fuel required to travel along the appropriate curve should discourage any would-be time travelers (Malament 1984, 1985; Manchak 2011). But more generally CTCs may be confined to smaller regions; some parts of space-time can have CTC’s while other parts do not. Let us call the part of a space-time that has CTC’s the “time travel region” of that space-time, while calling the rest of that space-time the “normal region”. More precisely, the “time travel region” consists of all the space-time points $p$ such that there exists a (non-zero length) timelike curve that starts at $p$ and returns to $p$. Now let us turn to examining space-times with CTC’s a bit more closely for potential problems.

In order to get a feeling for the sorts of implications that closed timelike curves can have, it may be useful to consider two simple models. In space-times with closed timelike curves the traditional initial value problem cannot be framed in the usual way. For it presupposes the existence of Cauchy surfaces, and if there are CTCs then no Cauchy surface exists. (A Cauchy surface is a spacelike surface such that every inextendable timelike curve crosses it exactly once. One normally specifies initial conditions by giving the conditions on such a surface.) Nonetheless, if the topological complexities of the manifold are appropriately localized, we can come quite close. Let us call an edgeless spacelike surface $S$ a quasi-Cauchy surface if it divides the rest of the manifold into two parts such that

every point in the manifold can be connected by a timelike curve to $S$, and
any timelike curve which connects a point in one region to a point in the other region intersects $S$ exactly once.

It is obvious that a quasi-Cauchy surface must entirely inhabit the normal region of the space-time; if any point $p$ of $S$ is in the time travel region, then any timelike curve which intersects $p$ can be extended to a timelike curve which intersects $S$ near $p$ again. In extreme cases of time travel, a model may have no normal region at all (e.g., Minkowski space-time rolled up like a cylinder in a time-like direction), in which case our usual notions of temporal precedence will not apply. But temporal anomalies like wormholes (and time machines) can be sufficiently localized to permit the existence of quasi-Cauchy surfaces.

Given a timelike orientation, a quasi-Cauchy surface unproblematically divides the manifold into its past (i.e., all points that can be reached by past-directed timelike curves from $S)$ and its future (ditto mutatis mutandis ). If the whole past of $S$ is in the normal region of the manifold, then $S$ is a partial Cauchy surface : every inextendable timelike curve which exists to the past of $S$ intersects $S$ exactly once, but (if there is time travel in the future) not every inextendable timelike curve which exists to the future of $S$ intersects $S$. Now we can ask a particularly clear question: consider a manifold which contains a time travel region, but also has a partial Cauchy surface $S$, such that all of the temporal funny business is to the future of $S$. If all you could see were $S$ and its past, you would not know that the space-time had any time travel at all. The question is: are there any constraints on the sort of data which can be put on $S$ and continued to a global solution of the dynamics which are different from the constraints (if any) on the data which can be put on a Cauchy surface in a simply connected manifold and continued to a global solution? If there is time travel to our future, might we we able to tell this now, because of some implied oddity in the arrangement of present things?

It is not at all surprising that there might be constraints on the data which can be put on a locally space-like surface which passes through the time travel region: after all, we never think we can freely specify what happens on a space-like surface and on another such surface to its future, but in this case the surface at issue lies to its own future. But if there were particular constraints for data on a partial Cauchy surface then we would apparently need to have to rule out some sorts of otherwise acceptable states on $S$ if there is to be time travel to the future of $S$. We then might be able to establish that there will be no time travel in the future by simple inspection of the present state of the universe. As we will see, there is reason to suspect that such constraints on the partial Cauchy surface are non-generic. But we are getting ahead of ourselves: first let’s consider the effect of time travel on a very simple dynamics.

The simplest possible example is the Newtonian theory of perfectly elastic collisions among equally massive particles in one spatial dimension. The space-time is two-dimensional, so we can represent it initially as the Euclidean plane, and the dynamics is completely specified by two conditions. When particles are traveling freely, their world lines are straight lines in the space-time, and when two particles collide, they exchange momenta, so the collision looks like an “$X$” in space-time, with each particle changing its momentum at the impact. [ 2 ] The dynamics is purely local, in that one can check that a set of world-lines constitutes a model of the dynamics by checking that the dynamics is obeyed in every arbitrarily small region. It is also trivial to generate solutions from arbitrary initial data if there are no CTCs: given the initial positions and momenta of a set of particles, one simply draws a straight line from each particle in the appropriate direction and continues it indefinitely. Once all the lines are drawn, the worldline of each particle can be traced from collision to collision. The boundary value problem for this dynamics is obviously well-posed: any set of data at an instant yields a unique global solution, constructed by the method sketched above.

What happens if we change the topology of the space-time by hand to produce CTCs? The simplest way to do this is depicted in figure 3 : we cut and paste the space-time so it is no longer simply connected by identifying the line $L-$ with the line $L+$. Particles “going in” to $L+$ from below “emerge” from $L-$ , and particles “going in” to $L-$ from below “emerge” from $L+$.

Figure 3: Inserting CTCs by Cut and Paste. [An extended description of figure 3 is in the supplement.]

How is the boundary-value problem changed by this alteration in the space-time? Before the cut and paste, we can put arbitrary data on the simultaneity slice $S$ and continue it to a unique solution. After the change in topology, $S$ is no longer a Cauchy surface, since a CTC will never intersect it, but it is a partial Cauchy surface. So we can ask two questions. First, can arbitrary data on $S$ always be continued to a global solution? Second, is that solution unique? If the answer to the first question is $no$, then we have a backward-temporal constraint: the existence of the region with CTCs places constraints on what can happen on $S$ even though that region lies completely to the future of $S$. If the answer to the second question is $no$, then we have an odd sort of indeterminism, analogous to the unwritten book: the complete physical state on $S$ does not determine the physical state in the future, even though the local dynamics is perfectly deterministic and even though there is no other past edge to the space-time region in $S$’s future (i.e., there is nowhere else for boundary values to come from which could influence the state of the region).

In this case the answer to the first question is yes and to the second is no : there are no constraints on the data which can be put on $S$, but those data are always consistent with an infinitude of different global solutions. The easy way to see that there always is a solution is to construct the minimal solution in the following way. Start drawing straight lines from $S$ as required by the initial data. If a line hits $L-$ from the bottom, just continue it coming out of the top of $L+$ in the appropriate place, and if a line hits $L+$ from the bottom, continue it emerging from $L-$ at the appropriate place. Figure 4 represents the minimal solution for a single particle which enters the time-travel region from the left:

Figure 4: The Minimal Solution. [An extended description of figure 4 is in the supplement.]

The particle “travels back in time” three times. It is obvious that this minimal solution is a global solution, since the particle always travels inertially.

But the same initial state on $S$ is also consistent with other global solutions. The new requirement imposed by the topology is just that the data going into $L+$ from the bottom match the data coming out of $L-$ from the top, and the data going into $L-$ from the bottom match the data coming out of $L+$ from the top. So we can add any number of vertical lines connecting $L-$ and $L+$ to a solution and still have a solution. For example, adding a few such lines to the minimal solution yields:

Figure 5: A Non-Minimal Solution. [An extended description of figure 5 is in the supplement.]

The particle now collides with itself twice: first before it reaches $L+$ for the first time, and again shortly before it exits the CTC region. From the particle’s point of view, it is traveling to the right at a constant speed until it hits an older version of itself and comes to rest. It remains at rest until it is hit from the right by a younger version of itself, and then continues moving off, and the same process repeats later. It is clear that this is a global model of the dynamics, and that any number of distinct models could be generating by varying the number and placement of vertical lines.

Knowing the data on $S$, then, gives us only incomplete information about how things will go for the particle. We know that the particle will enter the CTC region, and will reach $L+$, we know that it will be the only particle in the universe, we know exactly where and with what speed it will exit the CTC region. But we cannot determine how many collisions the particle will undergo (if any), nor how long (in proper time) it will stay in the CTC region. If the particle were a clock, we could not predict what time it would indicate when exiting the region. Furthermore, the dynamics gives us no handle on what to think of the various possibilities: there are no probabilities assigned to the various distinct possible outcomes.

Changing the topology has changed the mathematics of the situation in two ways, which tend to pull in opposite directions. On the one hand, $S$ is no longer a Cauchy surface, so it is perhaps not surprising that data on $S$ do not suffice to fix a unique global solution. But on the other hand, there is an added constraint: data “coming out” of $L-$ must exactly match data “going in” to $L+$, even though what comes out of $L-$ helps to determine what goes into $L+$. This added consistency constraint tends to cut down on solutions, although in this case the additional constraint is more than outweighed by the freedom to consider various sorts of data on ${L+}/{L-}$.

The fact that the extra freedom outweighs the extra constraint also points up one unexpected way that the supposed paradoxes of time travel may be overcome. Let’s try to set up a paradoxical situation using the little closed time loop above. If we send a single particle into the loop from the left and do nothing else, we know exactly where it will exit the right side of the time travel region. Now suppose we station someone at the other side of the region with the following charge: if the particle should come out on the right side, the person is to do something to prevent the particle from going in on the left in the first place. In fact, this is quite easy to do: if we send a particle in from the right, it seems that it can exit on the left and deflect the incoming left-hand particle.

Carrying on our reflection in this way, we further realize that if the particle comes out on the right, we might as well send it back in order to deflect itself from entering in the first place. So all we really need to do is the following: set up a perfectly reflecting particle mirror on the right-hand side of the time travel region, and launch the particle from the left so that— if nothing interferes with it —it will just barely hit $L+$. Our paradox is now apparently complete. If, on the one hand, nothing interferes with the particle it will enter the time-travel region on the left, exit on the right, be reflected from the mirror, re-enter from the right, and come out on the left to prevent itself from ever entering. So if it enters, it gets deflected and never enters. On the other hand, if it never enters then nothing goes in on the left, so nothing comes out on the right, so nothing is reflected back, and there is nothing to deflect it from entering. So if it doesn’t enter, then there is nothing to deflect it and it enters. If it enters, then it is deflected and doesn’t enter; if it doesn’t enter then there is nothing to deflect it and it enters: paradox complete.

But at least one solution to the supposed paradox is easy to construct: just follow the recipe for constructing the minimal solution, continuing the initial trajectory of the particle (reflecting it the mirror in the obvious way) and then read of the number and trajectories of the particles from the resulting diagram. We get the result of figure 6 :

Figure 6: Resolving the “Paradox”. [An extended description of figure 6 is in the supplement.]

As we can see, the particle approaching from the left never reaches $L+$: it is deflected first by a particle which emerges from $L-$. But it is not deflected by itself , as the paradox suggests, it is deflected by another particle. Indeed, there are now four particles in the diagram: the original particle and three particles which are confined to closed time-like curves. It is not the leftmost particle which is reflected by the mirror, nor even the particle which deflects the leftmost particle; it is another particle altogether.

The paradox gets it traction from an incorrect presupposition. If there is only one particle in the world at $S$ then there is only one particle which could participate in an interaction in the time travel region: the single particle would have to interact with its earlier (or later) self. But there is no telling what might come out of $L-$: the only requirement is that whatever comes out must match what goes in at $L+$. So if you go to the trouble of constructing a working time machine, you should be prepared for a different kind of disappointment when you attempt to go back and kill yourself: you may be prevented from entering the machine in the first place by some completely unpredictable entity which emerges from it. And once again a peculiar sort of indeterminism appears: if there are many self-consistent things which could prevent you from entering, there is no telling which is even likely to materialize. This is just like the case of the unwritten book: the book is never written, so nothing determines what fills its pages.

So when the freedom to put data on $L-$ outweighs the constraint that the same data go into $L+$, instead of paradox we get an embarrassment of riches: many solution consistent with the data on $S$, or many possible books. To see a case where the constraint “outweighs” the freedom, we need to construct a very particular, and frankly artificial, dynamics and topology. Consider the space of all linear dynamics for a scalar field on a lattice. (The lattice can be though of as a simple discrete space-time.) We will depict the space-time lattice as a directed graph. There is to be a scalar field defined at every node of the graph, whose value at a given node depends linearly on the values of the field at nodes which have arrows which lead to it. Each edge of the graph can be assigned a weighting factor which determines how much the field at the input node contributes to the field at the output node. If we name the nodes by the letters a , b , c , etc., and the edges by their endpoints in the obvious way, then we can label the weighting factors by the edges they are associated with in an equally obvious way.

Suppose that the graph of the space-time lattice is acyclic , as in figure 7 . (A graph is Acyclic if one can not travel in the direction of the arrows and go in a loop.)

Figure 7: An Acyclic Lattice. [An extended description of figure 7 is in the supplement.]

It is easy to regard a set of nodes as the analog of a Cauchy surface, e.g., the set $\{a, b, c\}$, and it is obvious if arbitrary data are put on those nodes the data will generate a unique solution in the future. [ 3 ] If the value of the field at node $a$ is 3 and at node $b$ is 7, then its value at node $d$ will be $3W_{ad}$ and its value at node $e$ will be $3W_{ae} + 7W_{be}$. By varying the weighting factors we can adjust the dynamics, but in an acyclic graph the future evolution of the field will always be unique.

Let us now again artificially alter the topology of the lattice to admit CTCs, so that the graph now is cyclic. One of the simplest such graphs is depicted in figure 8 : there are now paths which lead from $z$ back to itself, e.g., $z$ to $y$ to $z$.

Figure 8: Time Travel on a Lattice. [An extended description of figure 8 is in the supplement.]

Can we now put arbitrary data on $v$ and $w$, and continue that data to a global solution? Will the solution be unique?

In the generic case, there will be a solution and the solution will be unique. The equations for the value of the field at $x, y$, and $z$ are:

Solving these equations for $z$ yields

which gives a unique value for $z$ in the generic case. But looking at the space of all possible dynamics for this lattice (i.e., the space of all possible weighting factors), we find a singularity in the case where $1-W_{zx}W_{xz} - W_{zy}W_{yz} = 0$. If we choose weighting factors in just this way, then arbitrary data at $v$ and $w$ cannot be continued to a global solution. Indeed, if the scalar field is everywhere non-negative, then this particular choice of dynamics puts ironclad constraints on the value of the field at $v$ and $w$: the field there must be zero (assuming $W_{vx}$ and $W_{wy}$ to be non-zero), and similarly all nodes in their past must have field value zero. If the field can take negative values, then the values at $v$ and $w$ must be so chosen that $vW_{vx}W_{xz} = -wW_{wy}W_{yz}$. In either case, the field values at $v$ and $w$ are severely constrained by the existence of the CTC region even though these nodes lie completely to the past of that region. It is this sort of constraint which we find to be unlike anything which appears in standard physics.

Our toy models suggest three things. The first is that it may be impossible to prove in complete generality that arbitrary data on a partial Cauchy surface can always be continued to a global solution: our artificial case provides an example where it cannot. The second is that such odd constraints are not likely to be generic: we had to delicately fine-tune the dynamics to get a problem. The third is that the opposite problem, namely data on a partial Cauchy surface being consistent with many different global solutions, is likely to be generic: we did not have to do any fine-tuning to get this result.

This third point leads to a peculiar sort of indeterminism, illustrated by the case of the unwritten book: the entire state on $S$ does not determine what will happen in the future even though the local dynamics is deterministic and there are no other “edges” to space-time from which data could influence the result. What happens in the time travel region is constrained but not determined by what happens on $S$, and the dynamics does not even supply any probabilities for the various possibilities. The example of the photographic negative discussed in section 2, then, seems likely to be unusual, for in that case there is a unique fixed point for the dynamics, and the set-up plus the dynamical laws determine the outcome. In the generic case one would rather expect multiple fixed points, with no room for anything to influence, even probabilistically, which would be realized. (See the supplement on

Remarks and Limitations on the Toy Models .

It is ironic that time travel should lead generically not to contradictions or to constraints (in the normal region) but to underdetermination of what happens in the time travel region by what happens everywhere else (an underdetermination tied neither to a probabilistic dynamics nor to a free edge to space-time). The traditional objection to time travel is that it leads to contradictions: there is no consistent way to complete an arbitrarily constructed story about how the time traveler intends to act. Instead, though, it appears that the more significant problem is underdetermination: the story can be consistently completed in many different ways.

Echeverria, Klinkhammer, and Thorne (1991) considered the case of 3-dimensional single hard spherical ball that can go through a single time travel wormhole so as to collide with its younger self.

Figure 9 [An extended description of figure 9 is in the supplement.]

The threat of paradox in this case arises in the following form. Consider the initial trajectory of a ball as it approaches the time travel region. For some initial trajectories, the ball does not undergo a collision before reaching mouth 1, but upon exiting mouth 2 it will collide with its earlier self. This leads to a contradiction if the collision is strong enough to knock the ball off its trajectory and deflect it from entering mouth 1. Of course, the Wheeler-Feynman strategy is to look for a “glancing blow” solution: a collision which will produce exactly the (small) deviation in trajectory of the earlier ball that produces exactly that collision. Are there always such solutions? [ 4 ]

Echeverria, Klinkhammer & Thorne found a large class of initial trajectories that have consistent “glancing blow” continuations, and found none that do not (but their search was not completely general). They did not produce a rigorous proof that every initial trajectory has a consistent continuation, but suggested that it is very plausible that every initial trajectory has a consistent continuation. That is to say, they have made it very plausible that, in the billiard ball wormhole case, the time travel structure of such a wormhole space-time does not result in constraints on states on spacelike surfaces in the non-time travel region.

In fact, as one might expect from our discussion in the previous section, they found the opposite problem from that of inconsistency: they found underdetermination. For a large class of initial trajectories there are multiple different consistent “glancing blow” continuations of that trajectory (many of which involve multiple wormhole traversals). For example, if one initially has a ball that is traveling on a trajectory aimed straight between the two mouths, then one obvious solution is that the ball passes between the two mouths and never time travels. But another solution is that the younger ball gets knocked into mouth 1 exactly so as to come out of mouth 2 and produce that collision. Echeverria et al. do not note the possibility (which we pointed out in the previous section) of the existence of additional balls in the time travel region. We conjecture (but have no proof) that for every initial trajectory of $A$ there are some, and generically many, multiple-ball continuations.

Friedman, Morris, et al. (1990) examined the case of source-free non-self-interacting scalar fields traveling through such a time travel wormhole and found that no constraints on initial conditions in the non-time travel region are imposed by the existence of such time travel wormholes. In general there appear to be no known counter examples to the claim that in “somewhat realistic” time-travel space-times with a partial Cauchy surface there are no constraints imposed on the state on such a partial Cauchy surface by the existence of CTC’s. (See, e.g., Friedman & Morris 1991; Thorne 1994; Earman 1995; Earman, Smeenk, & Wüthrich 2009; and Dowe 2007.)

How about the issue of constraints in the time travel region $T$? Prima facie , constraints in such a region would not appear to be surprising. But one might still expect that there should be no constraints on states on a spacelike surface, provided one keeps the surface “small enough”. In the physics literature the following question has been asked: for any point $p$ in $T$, and any space-like surface $S$ that includes $p$ is there a neighborhood $E$ of $p$ in $S$ such that any solution on $E$ can be extended to a solution on the whole space-time? With respect to this question, there are some simple models in which one has this kind of extendability of local solutions to global ones, and some simple models in which one does not have such extendability, with no clear general pattern. The technical mathematical problems are amplified by the more conceptual problem of what it might mean to say that one could create a situation which forces the creation of closed timelike curves. (See, e.g., Yurtsever 1990; Friedman, Morris, et al. 1990; Novikov 1992; Earman 1995; and Earman, Smeenk, & Wüthrich 2009). What are we to think of all of this?

The toy models above all treat billiard balls, fields, and other objects propagating through a background spacetime with CTCs. Even if we can show that a consistent solution exists, there is a further question: what kind of matter and dynamics could generate CTCs to begin with? There are various solutions of Einstein’s equations with CTCs, but how do these exotic spacetimes relate to the models actually used in describing the world? In other words, what positive reasons might we have to take CTCs seriously as a feature of the actual universe, rather than an exotic possibility of primarily mathematical interest?

We should distinguish two different kinds of “possibility” that we might have in mind in posing such questions (following Stein 1970). First, we can consider a solution as a candidate cosmological model, describing the (large-scale gravitational degrees of freedom of the) entire universe. The case for ruling out spacetimes with CTCs as potential cosmological models strikes us as, surprisingly, fairly weak. Physicists used to simply rule out solutions with CTCs as unreasonable by fiat, due to the threat of paradoxes, which we have dismantled above. But it is also challenging to make an observational case. Observations tell us very little about global features, such as the existence of CTCs, because signals can only reach an observer from a limited region of spacetime, called the past light cone. Our past light cone—and indeed the collection of all the past light cones for possible observers in a given spacetime—can be embedded in spacetimes with quite different global features (Malament 1977, Manchak 2009). This undercuts the possibility of using observations to constrain global topology, including (among other things) ruling out the existence of CTCs.

Yet the case in favor of taking cosmological models with CTCs seriously is also not particularly strong. Some solutions used to describe black holes, which are clearly relevant in a variety of astrophysical contexts, include CTCs. But the question of whether the CTCs themselves play an essential representational role is subtle: the CTCs arise in the maximal extensions of these solutions, and can plausibly be regarded as extraneous to successful applications. Furthermore, many of the known solutions with CTCs have symmetries, raising the possibility that CTCs are not a stable or robust feature. Slight departures from symmetry may lead to a solution without CTCs, suggesting that the CTCs may be an artifact of an idealized model.

The second sense of possibility regards whether “reasonable” initial conditions can be shown to lead to, or not to lead to, the formation of CTCs. As with the toy models above, suppose that we have a partial Cauchy surface $S$, such that all the temporal funny business lies to the future. Rather than simply assuming that there is a region with CTCs to the future, we can ask instead whether it is possible to create CTCs by manipulating matter in the initial, well-behaved region—that is, whether it is possible to build a time machine. Several physicists have pursued “chronology protection theorems” aiming to show that the dynamics of general relativity (or some other aspects of physics) rules this out, and to clarify why this is the case. The proof of such a theorem would justify neglecting solutions with CTCs as a source of insight into the nature of time in the actual world. But as of yet there are several partial results that do not fully settle the question. One further intriguing possibility is that even if general relativity by itself does protect chronology, it may not be possible to formulate a sensible theory describing matter and fields in solutions with CTCs. (See SEP entry on Time Machines; Smeenk and Wüthrich 2011 for more.)

There is a different question regarding the limitations of these toy models. The toy models and related examples show that there are consistent solutions for simple systems in the presence of CTCs. As usual we have made the analysis tractable by building toy models, selecting only a few dynamical degrees of freedom and tracking their evolution. But there is a large gap between the systems we have described and the time travel stories they evoke, with Kurt traveling along a CTC with murderous intentions. In particular, many features of the manifest image of time are tied to the thermodynamical properties of macroscopic systems. Rovelli (unpublished) considers a extremely simple system to illustrate the problem: can a clock move along a CTC? A clock consists of something in periodic motion, such as a pendulum bob, and something that counts the oscillations, such as an escapement mechanism. The escapement mechanism cannot work without friction; this requires dissipation and increasing entropy. For a clock that counts oscillations as it moves along a time-like trajectory, the entropy must be a monotonically increasing function. But that is obviously incompatible with the clock returning to precisely the same state at some future time as it completes a loop. The point generalizes, obviously, to imply that anything like a human, with memory and agency, cannot move along a CTC.

Since it is not obvious that one can rid oneself of all constraints in realistic models, let us examine the argument that time travel is implausible, and we should think it unlikely to exist in our world, in so far as it implies such constraints. The argument goes something like the following. In order to satisfy such constraints one needs some pre-established divine harmony between the global (time travel) structure of space-time and the distribution of particles and fields on space-like surfaces in it. But it is not plausible that the actual world, or any world even remotely like ours, is constructed with divine harmony as part of the plan. In fact, one might argue, we have empirical evidence that conditions in any spatial region can vary quite arbitrarily. So we have evidence that such constraints, whatever they are, do not in fact exist in our world. So we have evidence that there are no closed time-like lines in our world or one remotely like it. We will now examine this argument in more detail by presenting four possible responses, with counterresponses, to this argument.

Response 1. There is nothing implausible or new about such constraints. For instance, if the universe is spatially closed, there has to be enough matter to produce the needed curvature, and this puts constraints on the matter distribution on a space-like hypersurface. Thus global space-time structure can quite unproblematically constrain matter distributions on space-like hypersurfaces in it. Moreover we have no realistic idea what these constraints look like, so we hardly can be said to have evidence that they do not obtain.

Counterresponse 1. Of course there are constraining relations between the global structure of space-time and the matter in it. The Einstein equations relate curvature of the manifold to the matter distribution in it. But what is so strange and implausible about the constraints imposed by the existence of closed time-like curves is that these constraints in essence have nothing to do with the Einstein equations. When investigating such constraints one typically treats the particles and/or field in question as test particles and/or fields in a given space-time, i.e., they are assumed not to affect the metric of space-time in any way. In typical space-times without closed time-like curves this means that one has, in essence, complete freedom of matter distribution on a space-like hypersurface. (See response 2 for some more discussion of this issue). The constraints imposed by the possibility of time travel have a quite different origin and are implausible. In the ordinary case there is a causal interaction between matter and space-time that results in relations between global structure of space-time and the matter distribution in it. In the time travel case there is no such causal story to be told: there simply has to be some pre-established harmony between the global space-time structure and the matter distribution on some space-like surfaces. This is implausible.

Response 2. Constraints upon matter distributions are nothing new. For instance, Maxwell’s equations constrain electric fields $\boldsymbol{E}$ on an initial surface to be related to the (simultaneous) charge density distribution $\varrho$ by the equation $\varrho = \text{div}(\boldsymbol{E})$. (If we assume that the $E$ field is generated solely by the charge distribution, this conditions amounts to requiring that the $E$ field at any point in space simply be the one generated by the charge distribution according to Coulomb’s inverse square law of electrostatics.) This is not implausible divine harmony. Such constraints can hold as a matter of physical law. Moreover, if we had inferred from the apparent free variation of conditions on spatial regions that there could be no such constraints we would have mistakenly inferred that $\varrho = \text{div}(\boldsymbol{E})$ could not be a law of nature.

Counterresponse 2. The constraints imposed by the existence of closed time-like lines are of quite a different character from the constraint imposed by $\varrho = \text{div}(\boldsymbol{E})$. The constraints imposed by $\varrho = \text{div}(\boldsymbol{E})$ on the state on a space-like hypersurface are:

local constraints (i.e., to check whether the constraint holds in a region you just need to see whether it holds at each point in the region),
quite independent of the global space-time structure,
quite independent of how the space-like surface in question is embedded in a given space-time, and
very simply and generally stateable.

On the other hand, the consistency constraints imposed by the existence of closed time-like curves (i) are not local, (ii) are dependent on the global structure of space-time, (iii) depend on the location of the space-like surface in question in a given space-time, and (iv) appear not to be simply stateable other than as the demand that the state on that space-like surface embedded in such and such a way in a given space-time, do not lead to inconsistency. On some views of laws (e.g., David Lewis’ view) this plausibly implies that such constraints, even if they hold, could not possibly be laws. But even if one does not accept such a view of laws, one could claim that the bizarre features of such constraints imply that it is implausible that such constraints hold in our world or in any world remotely like ours.

Response 3. It would be strange if there are constraints in the non-time travel region. It is not strange if there are constraints in the time travel region. They should be explained in terms of the strange, self-interactive, character of time travel regions. In this region there are time-like trajectories from points to themselves. Thus the state at such a point, in such a region, will, in a sense, interact with itself. It is a well-known fact that systems that interact with themselves will develop into an equilibrium state, if there is such an equilibrium state, or else will develop towards some singularity. Normally, of course, self-interaction isn’t true instantaneous self-interaction, but consists of a feed-back mechanism that takes time. But in time travel regions something like true instantaneous self-interaction occurs. This explains why constraints on states occur in such time travel regions: the states “ ab initio ” have to be “equilibrium states”. Indeed in a way this also provides some picture of why indeterminism occurs in time travel regions: at the onset of self-interaction states can fork into different equi-possible equilibrium states.

Counterresponse 3. This is explanation by woolly analogy. It all goes to show that time travel leads to such bizarre consequences that it is unlikely that it occurs in a world remotely like ours.

Response 4. All of the previous discussion completely misses the point. So far we have been taking the space-time structure as given, and asked the question whether a given time travel space-time structure imposes constraints on states on (parts of) space-like surfaces. However, space-time and matter interact. Suppose that one is in a space-time with closed time-like lines, such that certain counterfactual distributions of matter on some neighborhood of a point $p$ are ruled out if one holds that space-time structure fixed. One might then ask

Why does the actual state near $p$ in fact satisfy these constraints? By what divine luck or plan is this local state compatible with the global space-time structure? What if conditions near $p$ had been slightly different?

And one might take it that the lack of normal answers to these questions indicates that it is very implausible that our world, or any remotely like it, is such a time travel universe. However the proper response to these question is the following. There are no constraints in any significant sense. If they hold they hold as a matter of accidental fact, not of law. There is no more explanation of them possible than there is of any contingent fact. Had conditions in a neighborhood of $p$ been otherwise, the global structure of space-time would have been different. So what? The only question relevant to the issue of constraints is whether an arbitrary state on an arbitrary spatial surface $S$ can always be embedded into a space-time such that that state on $S$ consistently extends to a solution on the entire space-time.

But we know the answer to that question. A well-known theorem in general relativity says the following: any initial data set on a three dimensional manifold $S$ with positive definite metric has a unique embedding into a maximal space-time in which $S$ is a Cauchy surface (see, e.g., Geroch & Horowitz 1979: 284 for more detail), i.e., there is a unique largest space-time which has $S$ as a Cauchy surface and contains a consistent evolution of the initial value data on $S$. Now since $S$ is a Cauchy surface this space-time does not have closed time like curves. But it may have extensions (in which $S$ is not a Cauchy surface) which include closed timelike curves, indeed it may be that any maximal extension of it would include closed timelike curves. (This appears to be the case for extensions of states on certain surfaces of Taub-NUT space-times. See Earman, Smeenk, & Wüthrich 2009). But these extensions, of course, will be consistent. So properly speaking, there are no constraints on states on space-like surfaces. Nonetheless the space-time in which these are embedded may or may not include closed time-like curves.

Counterresponse 4. This, in essence, is the stonewalling answer which we indicated in section 1. However, whether or not you call the constraints imposed by a given space-time on distributions of matter on certain space-like surfaces “genuine constraints”, whether or not they can be considered lawlike, and whether or not they need to be explained, the existence of such constraints can still be used to argue that time travel worlds are so bizarre that it is implausible that our world or any world remotely like ours is a time travel world.

Suppose that one is in a time travel world. Suppose that given the global space-time structure of this world, there are constraints imposed upon, say, the state of motion of a ball on some space-like surface when it is treated as a test particle, i.e., when it is assumed that the ball does not affect the metric properties of the space-time it is in. (There is lots of other matter that, via the Einstein equation, corresponds exactly to the curvature that there is everywhere in this time travel worlds.) Now a real ball of course does have some effect on the metric of the space-time it is in. But let us consider a ball that is so small that its effect on the metric is negligible. Presumably it will still be the case that certain states of this ball on that space-like surface are not compatible with the global time travel structure of this universe.

This means that the actual distribution of matter on such a space-like surface can be extended into a space-time with closed time-like lines, but that certain counterfactual distributions of matter on this space-like surface can not be extended into the same space-time. But note that the changes made in the matter distribution (when going from the actual to the counterfactual distribution) do not in any non-negligible way affect the metric properties of the space-time. (Recall that the changes only effect test particles.) Thus the reason why the global time travel properties of the counterfactual space-time have to be significantly different from the actual space-time is not that there are problems with metric singularities or alterations in the metric that force significant global changes when we go to the counterfactual matter distribution. The reason that the counterfactual space-time has to be different is that in the counterfactual world the ball’s initial state of motion starting on the space-like surface, could not “meet up” in a consistent way with its earlier self (could not be consistently extended) if we were to let the global structure of the counterfactual space-time be the same as that of the actual space-time. Now, it is not bizarre or implausible that there is a counterfactual dependence of manifold structure, even of its topology, on matter distributions on spacelike surfaces. For instance, certain matter distributions may lead to singularities, others may not. We may indeed in some sense have causal power over the topology of the space-time we live in. But this power normally comes via the Einstein equations. But it is bizarre to think that there could be a counterfactual dependence of global space-time structure on the arrangement of certain tiny bits of matter on some space-like surface, where changes in that arrangement by assumption do not affect the metric anywhere in space-time in any significant way . It is implausible that we live in such a world, or that a world even remotely like ours is like that.

Let us illustrate this argument in a different way by assuming that wormhole time travel imposes constraints upon the states of people prior to such time travel, where the people have so little mass/energy that they have negligible effect, via the Einstein equation, on the local metric properties of space-time. Do you think it more plausible that we live in a world where wormhole time travel occurs but it only occurs when people’s states are such that these local states happen to combine with time travel in such a way that nobody ever succeeds in killing their younger self, or do you think it more plausible that we are not in a wormhole time travel world? [ 5 ]

An alternative approach to time travel (initiated by Deutsch 1991) abstracts away from the idealized toy models described above. [ 6 ] This computational approach considers instead the evolution of bits (simple physical systems with two discrete states) through a network of interactions, which can be represented by a circuit diagram with gates corresponding to the interactions. Motivated by the possibility of CTCs, Deutsch proposed adding a new kind of channel that connects the output of a given gate back to its input —in essence, a backwards-time step. More concretely, given a gate that takes $n$ bits as input, we can imagine taking some number $i \lt n$ of these bits through a channel that loops back and then do double-duty as inputs. Consistency requires that the state of these $i$ bits is the same for output and input. (We will consider an illustration of this kind of system in the next section.) Working through examples of circuit diagrams with a CTC channel leads to similar treatments of Consistency and Underdetermination as the discussion above (see, e.g., Wallace 2012: § 10.6). But the approach offers two new insights (both originally due to Deutsch): the Easy Knowledge paradox, and a particularly clear extension to time travel in quantum mechanics.

A computer equipped with a CTC channel can exploit the need to find consistent evolution to solve remarkably hard problems. (This is quite different than the first idea that comes to mind to enhance computational power: namely to just devote more time to a computation, and then send the result back on the CTC to an earlier state.) The gate in a circuit incorporating a CTC implements a function from the input bits to the output bits, under the constraint that the output and input match the i bits going through the CTC channel. This requires, in effect, finding the fixed point of the relevant function. Given the generality of the model, there are few limits on the functions that could be implemented on the CTC circuit. Nature has to solve a hard computational problem just to ensure consistent evolution. This can then be extended to other complex computational problems—leading, more precisely, to solutions of NP -complete problems in polynomial time (see Aaronson 2013: Chapter 20 for an overview and further references). The limits imposed by computational complexity are an essential part of our epistemic situation, and computers with CTCs would radically change this.

We now turn to the application of the computational approach to the quantum physics of time travel (see Deutsch 1991; Deutsch & Lockwood 1994). By contrast with the earlier discussions of constraints in classical systems, they claim to show that time travel never imposes any constraints on the pre-time travel state of quantum systems. The essence of this account is as follows. [ 7 ]

A quantum system starts in state $S_1$, interacts with its older self, after the interaction is in state $S_2$, time travels while developing into state $S_3$, then interacts with its younger self, and ends in state $S_4$ (see figure 10 ).

Figure 10 [An extended description of figure 10 is in the supplement.]

Deutsch assumes that the set of possible states of this system are the mixed states, i.e., are represented by the density matrices over the Hilbert space of that system. Deutsch then shows that for any initial state $S_1$, any unitary interaction between the older and younger self, and any unitary development during time travel, there is a consistent solution, i.e., there is at least one pair of states $S_2$ and $S_3$ such that when $S_1$ interacts with $S_3$ it will change to state $S_2$ and $S_2$ will then develop into $S_3$. The states $S_2, S_3$ and $S_4$ will typically be not be pure states, i.e., will be non-trivial mixed states, even if $S_1$ is pure. In order to understand how this leads to interpretational problems let us give an example. Consider a system that has a two dimensional Hilbert space with as a basis the states $\vc{+}$ and $\vc{-}$. Let us suppose that when state $\vc{+}$ of the young system encounters state $\vc{+}$ of the older system, they interact and the young system develops into state $\vc{-}$ and the old system remains in state $\vc{+}$. In obvious notation:

Similarly, suppose that:

Let us furthermore assume that there is no development of the state of the system during time travel, i.e., that $\vc{+}_2$ develops into $\vc{+}_3$, and that $\vc{-}_2$ develops into $\vc{-}_3$.

Now, if the only possible states of the system were $\vc{+}$ and $\vc{-}$ (i.e., if there were no superpositions or mixtures of these states), then there is a constraint on initial states: initial state $\vc{+}_1$ is impossible. For if $\vc{+}_1$ interacts with $\vc{+}_3$ then it will develop into $\vc{-}_2$, which, during time travel, will develop into $\vc{-}_3$, which inconsistent with the assumed state $\vc{+}_3$. Similarly if $\vc{+}_1$ interacts with $\vc{-}_3$ it will develop into $\vc{+}_2$, which will then develop into $\vc{+}_3$ which is also inconsistent. Thus the system can not start in state $\vc{+}_1$.

But, says Deutsch, in quantum mechanics such a system can also be in any mixture of the states $\vc{+}$ and $\vc{-}$. Suppose that the older system, prior to the interaction, is in a state $S_3$ which is an equal mixture of 50% $\vc{+}_3$ and 50% $\vc{-}_3$. Then the younger system during the interaction will develop into a mixture of 50% $\vc{+}_2$ and 50% $\vc{-}_2$, which will then develop into a mixture of 50% $\vc{+}_3$ and 50% $\vc{-}_3$, which is consistent! More generally Deutsch uses a fixed point theorem to show that no matter what the unitary development during interaction is, and no matter what the unitary development during time travel is, for any state $S_1$ there is always a state $S_3$ (which typically is not a pure state) which causes $S_1$ to develop into a state $S_2$ which develops into that state $S_3$. Thus quantum mechanics comes to the rescue: it shows in all generality that no constraints on initial states are needed!

One might wonder why Deutsch appeals to mixed states: will superpositions of states $\vc{+}$ and $\vc{-}$ not suffice? Unfortunately such an idea does not work. Suppose again that the initial state is $\vc{+}_1$. One might suggest that that if state $S_3$ is

one will obtain a consistent development. For one might think that when initial state $\vc{+}_1$ encounters the superposition

it will develop into superposition

and that this in turn will develop into

as desired. However this is not correct. For initial state $\vc{+}_1$ when it encounters

will develop into the entangled state

In so far as one can speak of the state of the young system after this interaction, it is in the mixture of 50% $\vc{+}_2$ and 50% $\vc{-}_2$, not in the superposition

So Deutsch does need his recourse to mixed states.

This clarification of why Deutsch needs his mixtures does however indicate a serious worry about the simplifications that are part of Deutsch’s account. After the interaction the old and young system will (typically) be in an entangled state. Although for purposes of a measurement on one of the two systems one can say that this system is in a mixed state, one can not represent the full state of the two systems by specifying the mixed state of each separate part, as there are correlations between observables of the two systems that are not represented by these two mixed states, but are represented in the joint entangled state. But if there really is an entangled state of the old and young systems directly after the interaction, how is one to represent the subsequent development of this entangled state? Will the state of the younger system remain entangled with the state of the older system as the younger system time travels and the older system moves on into the future? On what space-like surfaces are we to imagine this total entangled state to be? At this point it becomes clear that there is no obvious and simple way to extend elementary non-relativistic quantum mechanics to space-times with closed time-like curves: we apparently need to characterize not just the entanglement between two systems, but entanglement relative to specific spacetime descriptions.

How does Deutsch avoid these complications? Deutsch assumes a mixed state $S_3$ of the older system prior to the interaction with the younger system. He lets it interact with an arbitrary pure state $S_1$ younger system. After this interaction there is an entangled state $S'$ of the two systems. Deutsch computes the mixed state $S_2$ of the younger system which is implied by this entangled state $S'$. His demand for consistency then is just that this mixed state $S_2$ develops into the mixed state $S_3$. Now it is not at all clear that this is a legitimate way to simplify the problem of time travel in quantum mechanics. But even if we grant him this simplification there is a problem: how are we to understand these mixtures?

If we take an ignorance interpretation of mixtures we run into trouble. For suppose that we assume that in each individual case each older system is either in state $\vc{+}_3$ or in state $\vc{-}_3$ prior to the interaction. Then we regain our paradox. Deutsch instead recommends the following, many worlds, picture of mixtures. Suppose we start with state $\vc{+}_1$ in all worlds. In some of the many worlds the older system will be in the $\vc{+}_3$ state, let us call them A -worlds, and in some worlds, B -worlds, it will be in the $\vc{-}_3$ state. Thus in A -worlds after interaction we will have state $\vc{-}_2$ , and in B -worlds we will have state $\vc{+}_2$. During time travel the $\vc{-}_2$ state will remain the same, i.e., turn into state $\vc{-}_3$, but the systems in question will travel from A -worlds to B -worlds. Similarly the $\vc{+}$ $_2$ states will travel from the B -worlds to the A -worlds, thus preserving consistency.

Now whatever one thinks of the merits of many worlds interpretations, and of this understanding of it applied to mixtures, in the end one does not obtain genuine time travel in Deutsch’s account. The systems in question travel from one time in one world to another time in another world, but no system travels to an earlier time in the same world. (This is so at least in the normal sense of the word “world”, the sense that one means when, for instance, one says “there was, and will be, only one Elvis Presley in this world.”) Thus, even if it were a reasonable view, it is not quite as interesting as it may have initially seemed. (See Wallace 2012 for a more sympathetic treatment, that explores several further implications of accepting time travel in conjunction with the many worlds interpretation.)

We close by acknowledging that Deutsch’s starting point—the claim that this computational model captures the essential features of quantum systems in a spacetime with CTCs—has been the subject of some debate. Several physicists have pursued a quite different treatment of evolution of quantum systems through CTC’s, based on considering the “post-selected” state (see Lloyd et al. 2011). Their motivations for implementing the consistency condition in terms of the post-selected state reflects a different stance towards quantum foundations. A different line of argument aims to determine whether Deutsch’s treatment holds as an appropriate limiting case of a more rigorous treatment, such as quantum field theory in curved spacetimes. For example, Verch (2020) establishes several results challenging the assumption that Deutsch’s treatment is tied to the presence of CTC’s, or that it is compatible with the entanglement structure of quantum fields.

What remains of the grandfather paradox in general relativistic time travel worlds is the fact that in some cases the states on edgeless spacelike surfaces are “overconstrained”, so that one has less than the usual freedom in specifying conditions on such a surface, given the time-travel structure, and in some cases such states are “underconstrained”, so that states on edgeless space-like surfaces do not determine what happens elsewhere in the way that they usually do, given the time travel structure. There can also be mixtures of those two types of cases. The extent to which states are overconstrained and/or underconstrained in realistic models is as yet unclear, though it would be very surprising if neither obtained. The extant literature has primarily focused on the problem of overconstraint, since that, often, either is regarded as a metaphysical obstacle to the possibility time travel, or as an epistemological obstacle to the plausibility of time travel in our world. While it is true that our world would be quite different from the way we normally think it is if states were overconstrained, underconstraint seems at least as bizarre as overconstraint. Nonetheless, neither directly rules out the possibility of time travel.

If time travel entailed contradictions then the issue would be settled. And indeed, most of the stories employing time travel in popular culture are logically incoherent: one cannot “change” the past to be different from what it was, since the past (like the present and the future) only occurs once. But if the only requirement demanded is logical coherence, then it seems all too easy. A clever author can devise a coherent time-travel scenario in which everything happens just once and in a consistent way. This is just too cheap: logical coherence is a very weak condition, and many things we take to be metaphysically impossible are logically coherent. For example, it involves no logical contradiction to suppose that water is not molecular, but if both chemistry and Kripke are right it is a metaphysical impossibility. We have been interested not in logical possibility but in physical possibility. But even so, our conditions have been relatively weak: we have asked only whether time-travel is consistent with the universal validity of certain fundamental physical laws and with the notion that the physical state on a surface prior to the time travel region be unconstrained. It is perfectly possible that the physical laws obey this condition, but still that time travel is not metaphysically possible because of the nature of time itself. Consider an analogy. Aristotle believed that water is homoiomerous and infinitely divisible: any bit of water could be subdivided, in principle, into smaller bits of water. Aristotle’s view contains no logical contradiction. It was certainly consistent with Aristotle’s conception of water that it be homoiomerous, so this was, for him, a conceptual possibility. But if chemistry is right, Aristotle was wrong both about what water is like and what is possible for it. It can’t be infinitely divided, even though no logical or conceptual analysis would reveal that.

Similarly, even if all of our consistency conditions can be met, it does not follow that time travel is physically possible, only that some specific physical considerations cannot rule it out. The only serious proof of the possibility of time travel would be a demonstration of its actuality. For if we agree that there is no actual time travel in our universe, the supposition that there might have been involves postulating a substantial difference from actuality, a difference unlike in kind from anything we could know if firsthand. It is unclear to us exactly what the content of possible would be if one were to either maintain or deny the possibility of time travel in these circumstances, unless one merely meant that the possibility is not ruled out by some delineated set of constraints. As the example of Aristotle’s theory of water shows, conceptual and logical “possibility” do not entail possibility in a full-blooded sense. What exactly such a full-blooded sense would be in case of time travel, and whether one could have reason to believe it to obtain, remain to us obscure.

Aaronson, Scott, 2013, Quantum Computing since Democritus , Cambridge: Cambridge University Press. doi:10.1017/CBO9780511979309
Arntzenius, Frank, 2006, “Time Travel: Double Your Fun”, Philosophy Compass , 1(6): 599–616. doi:10.1111/j.1747-9991.2006.00045.x
Clarke, C.J.S., 1977, “Time in General Relativity” in Foundations of Space-Time Theory , Minnesota Studies in the Philosophy of Science , Vol VIII, Earman, J., Glymour, C., and Stachel, J. (eds), pp. 94–108. Minneapolis: University of Minnesota Press.
Deutsch, David, 1991, “Quantum Mechanics near Closed Timelike Lines”, Physical Review D , 44(10): 3197–3217. doi:10.1103/PhysRevD.44.3197
Deutsch, David and Michael Lockwood, 1994, “The Quantum Physics of Time Travel”, Scientific American , 270(3): 68–74. doi:10.1038/scientificamerican0394-68
Dowe, Phil, 2007, “Constraints on Data in Worlds with Closed Timelike Curves”, Philosophy of Science , 74(5): 724–735. doi:10.1086/525617
Earman, John, 1972, “Implications of Causal Propagation Outside the Null Cone”, Australasian Journal of Philosophy , 50(3): 222–237. doi:10.1080/00048407212341281
Earman, John, 1995, Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes , New York: Oxford University Press.
Earman, John, Christopher Smeenk, and Christian Wüthrich, 2009, “Do the Laws of Physics Forbid the Operation of Time Machines?”, Synthese , 169(1): 91–124. doi:10.1007/s11229-008-9338-2
Echeverria, Fernando, Gunnar Klinkhammer, and Kip S. Thorne, 1991, “Billiard Balls in Wormhole Spacetimes with Closed Timelike Curves: Classical Theory”, Physical Review D , 44(4): 1077–1099. doi:10.1103/PhysRevD.44.1077
Effingham, Nikk, 2020, Time Travel: Probability and Impossibility , Oxford: Oxford University Press. doi:10.1093/oso/9780198842507.001.0001
Fletcher, Samuel C., 2020, “The Principle of Stability”, Philosopher’s Imprint , 20: article 3. [ Fletcher 2020 available online ]
Friedman, John and Michael Morris, 1991, “The Cauchy Problem for the Scalar Wave Equation Is Well Defined on a Class of Spacetimes with Closed Timelike Curves”, Physical Review Letters , 66(4): 401–404. doi:10.1103/PhysRevLett.66.401
Friedman, John, Michael S. Morris, Igor D. Novikov, Fernando Echeverria, Gunnar Klinkhammer, Kip S. Thorne, and Ulvi Yurtsever, 1990, “Cauchy Problem in Spacetimes with Closed Timelike Curves”, Physical Review D , 42(6): 1915–1930. doi:10.1103/PhysRevD.42.1915
Geroch, Robert and Gary Horowitz, 1979, “Global Structures of Spacetimes”, in General Relativity: An Einstein Centenary Survey , Stephen Hawking and W. Israel (eds.), Cambridge/New York: Cambridge University Press, Chapter 5, pp. 212–293.
Gödel, Kurt, 1949, “A Remark About the Relationship Between Relativity Theory and Idealistic Philosophy”, in Albert Einstein, Philosopher-Scientist , Paul Arthur Schilpp (ed.), Evanston, IL: Library of Living Philosophers, 557–562.
Hocking, John G. and Gail S. Young, 1961, Topology , (Addison-Wesley Series in Mathematics), Reading, MA: Addison-Wesley.
Horwich, Paul, 1987, “Time Travel”, in his Asymmetries in Time: Problems in the Philosophy of Science , , Cambridge, MA: MIT Press, 111–128.
Kutach, Douglas N., 2003, “Time Travel and Consistency Constraints”, Philosophy of Science , 70(5): 1098–1113. doi:10.1086/377392
Lewis, David, 1976, “The Paradoxes of Time Travel”, American Philosophical Quarterly , 13(2): 145–152.
Lloyd, Seth, Lorenzo Maccone, Raul Garcia-Patron, Vittorio Giovannetti, and Yutaka Shikano, 2011, “Quantum Mechanics of Time Travel through Post-Selected Teleportation”, Physical Review D , 84(2): 025007. doi:10.1103/PhysRevD.84.025007
Malament, David B., 1977, “Observationally Indistinguishable Spacetimes: Comments on Glymour’s Paper”, in Foundations of Space-Time Theories , John Earman, Clark N. Glymour, and John J. Stachel (eds.), (Minnesota Studies in the Philosophy of Science 8), Minneapolis, MN: University of Minnesota Press, 61–80.
–––, 1984, “‘Time Travel’ in the Gödel Universe”, PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association , 1984(2): 91–100. doi:10.1086/psaprocbienmeetp.1984.2.192497
–––, 1985, “Minimal Acceleration Requirements for ‘Time Travel’, in Gödel Space‐time”, Journal of Mathematical Physics , 26(4): 774–777. doi:10.1063/1.526566
Manchak, John Byron, 2009, “Can We Know the Global Structure of Spacetime?”, Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics , 40(1): 53–56. doi:10.1016/j.shpsb.2008.07.004
–––, 2011, “On Efficient ‘Time Travel’ in Gödel Spacetime”, General Relativity and Gravitation , 43(1): 51–60. doi:10.1007/s10714-010-1068-3
Maudlin, Tim, 1990, “Time-Travel and Topology”, PSA: Proceedings of the Biennial Meeting of the Philosophy of Science Association , 1990(1): 303–315. doi:10.1086/psaprocbienmeetp.1990.1.192712
Novikov, I. D., 1992, “Time Machine and Self-Consistent Evolution in Problems with Self-Interaction”, Physical Review D , 45(6): 1989–1994. doi:10.1103/PhysRevD.45.1989
Smeenk, Chris and Christian Wüthrich, 2011, “Time Travel and Time Machines”, in the Oxford Handbook on Time , Craig Callender (ed.), Oxford: Oxford University Press, 577–630..
Stein, Howard, 1970, “On the Paradoxical Time-Structures of Gödel”, Philosophy of Science , 37(4): 589–601. doi:10.1086/288328
Thorne, Kip S., 1994, Black Holes and Time Warps: Einstein’s Outrageous Legacy , (Commonwealth Fund Book Program), New York: W.W. Norton.
Verch, Rainer, 2020, “The D-CTC Condition in Quantum Field Theory”, in Progress and Visions in Quantum Theory in View of Gravity , Felix Finster, Domenico Giulini, Johannes Kleiner, and Jürgen Tolksdorf (eds.), Cham: Springer International Publishing, 221–232. doi:10.1007/978-3-030-38941-3_9
Wallace, David, 2012, The Emergent Multiverse: Quantum Theory According to the Everett Interpretation , Oxford: Oxford University Press. doi:10.1093/acprof:oso/9780199546961.001.0001
Wasserman, Ryan, 2018, Paradoxes of Time Travel , Oxford: Oxford University Press. doi:10.1093/oso/9780198793335.001.0001
Weyl, Hermann, 1918/1920 [1922/1952], Raum, Zeit, Materie , Berlin: Springer; fourth edition 1920. Translated as Space—Time—Matter , Henry Leopold Brose (trans.), New York: Dutton, 1922. Reprinted 1952, New York: Dover Publications.
Wheeler, John Archibald and Richard Phillips Feynman, 1949, “Classical Electrodynamics in Terms of Direct Interparticle Action”, Reviews of Modern Physics , 21(3): 425–433. doi:10.1103/RevModPhys.21.425
Yurtsever, Ulvi, 1990, “Test Fields on Compact Space‐times”, Journal of Mathematical Physics , 31(12): 3064–3078. doi:10.1063/1.528960

How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

Adlam, Emily, unpublished, “ Is There Causation in Fundamental Physics? New Insights from Process Matrices and Quantum Causal Modelling ”, 2022, arXiv: 2208.02721. doi:10.48550/ARXIV.2208.02721
Rovelli, Carlo, unpublished, “ Can We Travel to the Past? Irreversible Physics along Closed Timelike Curves ”, arXiv: 1912.04702. doi:10.48550/ARXIV.1912.04702

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Is time travel possible? Why one scientist says we 'cannot ignore the possibility.'

A common theme in science-fiction media , time travel is captivating. It’s defined by the late philosopher David Lewis in his essay “The Paradoxes of Time Travel” as “[involving] a discrepancy between time and space time. Any traveler departs and then arrives at his destination; the time elapsed from departure to arrival … is the duration of the journey.”

Time travel is usually understood by most as going back to a bygone era or jumping forward to a point far in the future . But how much of the idea is based in reality? Is it possible to travel through time?

Is time travel possible?

According to NASA, time travel is possible , just not in the way you might expect. Albert Einstein’s theory of relativity says time and motion are relative to each other, and nothing can go faster than the speed of light , which is 186,000 miles per second. Time travel happens through what’s called “time dilation.”

Time dilation , according to Live Science, is how one’s perception of time is different to another's, depending on their motion or where they are. Hence, time being relative.

Learn more: Best travel insurance

Dr. Ana Alonso-Serrano, a postdoctoral researcher at the Max Planck Institute for Gravitational Physics in Germany, explained the possibility of time travel and how researchers test theories.

Space and time are not absolute values, Alonso-Serrano said. And what makes this all more complex is that you are able to carve space-time .

“In the moment that you carve the space-time, you can play with that curvature to make the time come in a circle and make a time machine,” Alonso-Serrano told USA TODAY.

She explained how, theoretically, time travel is possible. The mathematics behind creating curvature of space-time are solid, but trying to re-create the strict physical conditions needed to prove these theories can be challenging.

“The tricky point of that is if you can find a physical, realistic, way to do it,” she said.

Alonso-Serrano said wormholes and warp drives are tools that are used to create this curvature. The matter needed to achieve curving space-time via a wormhole is exotic matter , which hasn’t been done successfully. Researchers don’t even know if this type of matter exists, she said.

“It's something that we work on because it's theoretically possible, and because it's a very nice way to test our theory, to look for possible paradoxes,” Alonso-Serrano added.

“I could not say that nothing is possible, but I cannot ignore the possibility,” she said.

She also mentioned the anecdote of Stephen Hawking’s Champagne party for time travelers . Hawking had a GPS-specific location for the party. He didn’t send out invites until the party had already happened, so only people who could travel to the past would be able to attend. No one showed up, and Hawking referred to this event as "experimental evidence" that time travel wasn't possible.

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Time travel: Is it possible?

Science says time travel is possible, but probably not in the way you're thinking.

time travel graphic illustration of a tunnel with a clock face swirling through the tunnel.

Albert Einstein's theory

General relativity and GPS
Wormhole travel
Alternate theories

Science fiction

Is time travel possible? Short answer: Yes, and you're doing it right now — hurtling into the future at the impressive rate of one second per second.

You're pretty much always moving through time at the same speed, whether you're watching paint dry or wishing you had more hours to visit with a friend from out of town.

But this isn't the kind of time travel that's captivated countless science fiction writers, or spurred a genre so extensive that Wikipedia lists over 400 titles in the category "Movies about Time Travel." In franchises like " Doctor Who ," " Star Trek ," and "Back to the Future" characters climb into some wild vehicle to blast into the past or spin into the future. Once the characters have traveled through time, they grapple with what happens if you change the past or present based on information from the future (which is where time travel stories intersect with the idea of parallel universes or alternate timelines).

Related: The best sci-fi time machines ever

Although many people are fascinated by the idea of changing the past or seeing the future before it's due, no person has ever demonstrated the kind of back-and-forth time travel seen in science fiction or proposed a method of sending a person through significant periods of time that wouldn't destroy them on the way. And, as physicist Stephen Hawking pointed out in his book " Black Holes and Baby Universes" (Bantam, 1994), "The best evidence we have that time travel is not possible, and never will be, is that we have not been invaded by hordes of tourists from the future."

Science does support some amount of time-bending, though. For example, physicist Albert Einstein 's theory of special relativity proposes that time is an illusion that moves relative to an observer. An observer traveling near the speed of light will experience time, with all its aftereffects (boredom, aging, etc.) much more slowly than an observer at rest. That's why astronaut Scott Kelly aged ever so slightly less over the course of a year in orbit than his twin brother who stayed here on Earth.

Related: Controversially, physicist argues that time is real

There are other scientific theories about time travel, including some weird physics that arise around wormholes , black holes and string theory . For the most part, though, time travel remains the domain of an ever-growing array of science fiction books, movies, television shows, comics, video games and more.

Scott and Mark Kelly sit side by side wearing a blue NASA jacket and jeans

Einstein developed his theory of special relativity in 1905. Along with his later expansion, the theory of general relativity , it has become one of the foundational tenets of modern physics. Special relativity describes the relationship between space and time for objects moving at constant speeds in a straight line.

The short version of the theory is deceptively simple. First, all things are measured in relation to something else — that is to say, there is no "absolute" frame of reference. Second, the speed of light is constant. It stays the same no matter what, and no matter where it's measured from. And third, nothing can go faster than the speed of light.

From those simple tenets unfolds actual, real-life time travel. An observer traveling at high velocity will experience time at a slower rate than an observer who isn't speeding through space.

While we don't accelerate humans to near-light-speed, we do send them swinging around the planet at 17,500 mph (28,160 km/h) aboard the International Space Station . Astronaut Scott Kelly was born after his twin brother, and fellow astronaut, Mark Kelly . Scott Kelly spent 520 days in orbit, while Mark logged 54 days in space. The difference in the speed at which they experienced time over the course of their lifetimes has actually widened the age gap between the two men.

"So, where[as] I used to be just 6 minutes older, now I am 6 minutes and 5 milliseconds older," Mark Kelly said in a panel discussion on July 12, 2020, Space.com previously reported . "Now I've got that over his head."

General relativity and GPS time travel

Graphic showing the path of GPS satellites around Earth at the center of the image.

The difference that low earth orbit makes in an astronaut's life span may be negligible — better suited for jokes among siblings than actual life extension or visiting the distant future — but the dilation in time between people on Earth and GPS satellites flying through space does make a difference.

Can wormholes take us back in time?

General relativity might also provide scenarios that could allow travelers to go back in time, according to NASA . But the physical reality of those time-travel methods is no piece of cake.

Wormholes are theoretical "tunnels" through the fabric of space-time that could connect different moments or locations in reality to others. Also known as Einstein-Rosen bridges or white holes, as opposed to black holes, speculation about wormholes abounds. But despite taking up a lot of space (or space-time) in science fiction, no wormholes of any kind have been identified in real life.

Related: Best time travel movies

"The whole thing is very hypothetical at this point," Stephen Hsu, a professor of theoretical physics at the University of Oregon, told Space.com sister site Live Science . "No one thinks we're going to find a wormhole anytime soon."

Primordial wormholes are predicted to be just 10^-34 inches (10^-33 centimeters) at the tunnel's "mouth". Previously, they were expected to be too unstable for anything to be able to travel through them. However, a study claims that this is not the case, Live Science reported .

The theory, which suggests that wormholes could work as viable space-time shortcuts, was described by physicist Pascal Koiran. As part of the study, Koiran used the Eddington-Finkelstein metric, as opposed to the Schwarzschild metric which has been used in the majority of previous analyses.

In the past, the path of a particle could not be traced through a hypothetical wormhole. However, using the Eddington-Finkelstein metric, the physicist was able to achieve just that.

Koiran's paper was described in October 2021, in the preprint database arXiv , before being published in the Journal of Modern Physics D.

Alternate time travel theories

While Einstein's theories appear to make time travel difficult, some researchers have proposed other solutions that could allow jumps back and forth in time. These alternate theories share one major flaw: As far as scientists can tell, there's no way a person could survive the kind of gravitational pulling and pushing that each solution requires.

Infinite cylinder theory

Astronomer Frank Tipler proposed a mechanism (sometimes known as a Tipler Cylinder ) where one could take matter that is 10 times the sun's mass, then roll it into a very long, but very dense cylinder. The Anderson Institute , a time travel research organization, described the cylinder as "a black hole that has passed through a spaghetti factory."

After spinning this black hole spaghetti a few billion revolutions per minute, a spaceship nearby — following a very precise spiral around the cylinder — could travel backward in time on a "closed, time-like curve," according to the Anderson Institute.

The major problem is that in order for the Tipler Cylinder to become reality, the cylinder would need to be infinitely long or be made of some unknown kind of matter. At least for the foreseeable future, endless interstellar pasta is beyond our reach.

Time donuts

Theoretical physicist Amos Ori at the Technion-Israel Institute of Technology in Haifa, Israel, proposed a model for a time machine made out of curved space-time — a donut-shaped vacuum surrounded by a sphere of normal matter.

"The machine is space-time itself," Ori told Live Science . "If we were to create an area with a warp like this in space that would enable time lines to close on themselves, it might enable future generations to return to visit our time."

Amos Ori is a theoretical physicist at the Technion-Israel Institute of Technology in Haifa, Israel. His research interests and publications span the fields of general relativity, black holes, gravitational waves and closed time lines.

There are a few caveats to Ori's time machine. First, visitors to the past wouldn't be able to travel to times earlier than the invention and construction of the time donut. Second, and more importantly, the invention and construction of this machine would depend on our ability to manipulate gravitational fields at will — a feat that may be theoretically possible but is certainly beyond our immediate reach.

Graphic illustration of the TARDIS (Time and Relative Dimensions in Space) traveling through space, surrounded by stars.

Time travel has long occupied a significant place in fiction. Since as early as the "Mahabharata," an ancient Sanskrit epic poem compiled around 400 B.C., humans have dreamed of warping time, Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science .

Every work of time-travel fiction creates its own version of space-time, glossing over one or more scientific hurdles and paradoxes to achieve its plot requirements.

Some make a nod to research and physics, like " Interstellar ," a 2014 film directed by Christopher Nolan. In the movie, a character played by Matthew McConaughey spends a few hours on a planet orbiting a supermassive black hole, but because of time dilation, observers on Earth experience those hours as a matter of decades.

Others take a more whimsical approach, like the "Doctor Who" television series. The series features the Doctor, an extraterrestrial "Time Lord" who travels in a spaceship resembling a blue British police box. "People assume," the Doctor explained in the show, "that time is a strict progression from cause to effect, but actually from a non-linear, non-subjective viewpoint, it's more like a big ball of wibbly-wobbly, timey-wimey stuff."

Long-standing franchises like the "Star Trek" movies and television series, as well as comic universes like DC and Marvel Comics, revisit the idea of time travel over and over.

Related: Marvel movies in order: chronological & release order

Here is an incomplete (and deeply subjective) list of some influential or notable works of time travel fiction:

Books about time travel:

A sketch from the Christmas Carol shows a cloaked figure on the left and a person kneeling and clutching their head with their hands.

Rip Van Winkle (Cornelius S. Van Winkle, 1819) by Washington Irving
A Christmas Carol (Chapman & Hall, 1843) by Charles Dickens
The Time Machine (William Heinemann, 1895) by H. G. Wells
A Connecticut Yankee in King Arthur's Court (Charles L. Webster and Co., 1889) by Mark Twain
The Restaurant at the End of the Universe (Pan Books, 1980) by Douglas Adams
A Tale of Time City (Methuen, 1987) by Diana Wynn Jones
The Outlander series (Delacorte Press, 1991-present) by Diana Gabaldon
Harry Potter and the Prisoner of Azkaban (Bloomsbury/Scholastic, 1999) by J. K. Rowling
Thief of Time (Doubleday, 2001) by Terry Pratchett
The Time Traveler's Wife (MacAdam/Cage, 2003) by Audrey Niffenegger
All You Need is Kill (Shueisha, 2004) by Hiroshi Sakurazaka

Movies about time travel:

Planet of the Apes (1968)
Superman (1978)
Time Bandits (1981)
The Terminator (1984)
Back to the Future series (1985, 1989, 1990)
Star Trek IV: The Voyage Home (1986)
Bill & Ted's Excellent Adventure (1989)
Groundhog Day (1993)
Galaxy Quest (1999)
The Butterfly Effect (2004)
13 Going on 30 (2004)
The Lake House (2006)
Meet the Robinsons (2007)
Hot Tub Time Machine (2010)
Midnight in Paris (2011)
Looper (2012)
X-Men: Days of Future Past (2014)
Edge of Tomorrow (2014)
Interstellar (2014)
Doctor Strange (2016)
A Wrinkle in Time (2018)
The Last Sharknado: It's About Time (2018)
Avengers: Endgame (2019)
Tenet (2020)
Palm Springs (2020)
Zach Snyder's Justice League (2021)
The Tomorrow War (2021)

Television about time travel:

Image of the Star Trek spaceship USS Enterprise

Doctor Who (1963-present)
The Twilight Zone (1959-1964) (multiple episodes)
Star Trek (multiple series, multiple episodes)
Samurai Jack (2001-2004)
Lost (2004-2010)
Phil of the Future (2004-2006)
Steins;Gate (2011)
Outlander (2014-2023)
Loki (2021-present)

Games about time travel:

Chrono Trigger (1995)
TimeSplitters (2000-2005)
Kingdom Hearts (2002-2019)
Prince of Persia: Sands of Time (2003)
God of War II (2007)
Ratchet and Clank Future: A Crack In Time (2009)
Sly Cooper: Thieves in Time (2013)
Dishonored 2 (2016)
Titanfall 2 (2016)
Outer Wilds (2019)

Additional resources

Explore physicist Peter Millington's thoughts about Stephen Hawking's time travel theories at The Conversation . Check out a kid-friendly explanation of real-world time travel from NASA's Space Place . For an overview of time travel in fiction and the collective consciousness, read " Time Travel: A History " (Pantheon, 2016) by James Gleik.

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

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Ailsa is a staff writer for How It Works magazine, where she writes science, technology, space, history and environment features. Based in the U.K., she graduated from the University of Stirling with a BA (Hons) journalism degree. Previously, Ailsa has written for Cardiff Times magazine, Psychology Now and numerous science bookazines.

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Can we time travel? A theoretical physicist provides some answers

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Time travel makes regular appearances in popular culture, with innumerable time travel storylines in movies, television and literature. But it is a surprisingly old idea: one can argue that the Greek tragedy Oedipus Rex , written by Sophocles over 2,500 years ago, is the first time travel story .

But is time travel in fact possible? Given the popularity of the concept, this is a legitimate question. As a theoretical physicist, I find that there are several possible answers to this question, not all of which are contradictory.

The simplest answer is that time travel cannot be possible because if it was, we would already be doing it. One can argue that it is forbidden by the laws of physics, like the second law of thermodynamics or relativity . There are also technical challenges: it might be possible but would involve vast amounts of energy.

There is also the matter of time-travel paradoxes; we can — hypothetically — resolve these if free will is an illusion, if many worlds exist or if the past can only be witnessed but not experienced. Perhaps time travel is impossible simply because time must flow in a linear manner and we have no control over it, or perhaps time is an illusion and time travel is irrelevant.

a woman stands among a crowd of people moving around her

Laws of physics

Since Albert Einstein’s theory of relativity — which describes the nature of time, space and gravity — is our most profound theory of time, we would like to think that time travel is forbidden by relativity. Unfortunately, one of his colleagues from the Institute for Advanced Study, Kurt Gödel, invented a universe in which time travel was not just possible, but the past and future were inextricably tangled.

We can actually design time machines , but most of these (in principle) successful proposals require negative energy , or negative mass, which does not seem to exist in our universe. If you drop a tennis ball of negative mass, it will fall upwards. This argument is rather unsatisfactory, since it explains why we cannot time travel in practice only by involving another idea — that of negative energy or mass — that we do not really understand.

Mathematical physicist Frank Tipler conceptualized a time machine that does not involve negative mass, but requires more energy than exists in the universe .

Time travel also violates the second law of thermodynamics , which states that entropy or randomness must always increase. Time can only move in one direction — in other words, you cannot unscramble an egg. More specifically, by travelling into the past we are going from now (a high entropy state) into the past, which must have lower entropy.

This argument originated with the English cosmologist Arthur Eddington , and is at best incomplete. Perhaps it stops you travelling into the past, but it says nothing about time travel into the future. In practice, it is just as hard for me to travel to next Thursday as it is to travel to last Thursday.

Resolving paradoxes

There is no doubt that if we could time travel freely, we run into the paradoxes. The best known is the “ grandfather paradox ”: one could hypothetically use a time machine to travel to the past and murder their grandfather before their father’s conception, thereby eliminating the possibility of their own birth. Logically, you cannot both exist and not exist.

Read more: Time travel could be possible, but only with parallel timelines

Kurt Vonnegut’s anti-war novel Slaughterhouse-Five , published in 1969, describes how to evade the grandfather paradox. If free will simply does not exist, it is not possible to kill one’s grandfather in the past, since he was not killed in the past. The novel’s protagonist, Billy Pilgrim, can only travel to other points on his world line (the timeline he exists in), but not to any other point in space-time, so he could not even contemplate killing his grandfather.

The universe in Slaughterhouse-Five is consistent with everything we know. The second law of thermodynamics works perfectly well within it and there is no conflict with relativity. But it is inconsistent with some things we believe in, like free will — you can observe the past, like watching a movie, but you cannot interfere with the actions of people in it.

Could we allow for actual modifications of the past, so that we could go back and murder our grandfather — or Hitler ? There are several multiverse theories that suppose that there are many timelines for different universes. This is also an old idea: in Charles Dickens’ A Christmas Carol , Ebeneezer Scrooge experiences two alternative timelines, one of which leads to a shameful death and the other to happiness.

Time is a river

Roman emperor Marcus Aurelius wrote that:

“ Time is like a river made up of the events which happen , and a violent stream; for as soon as a thing has been seen, it is carried away, and another comes in its place, and this will be carried away too.”

We can imagine that time does flow past every point in the universe, like a river around a rock. But it is difficult to make the idea precise. A flow is a rate of change — the flow of a river is the amount of water that passes a specific length in a given time. Hence if time is a flow, it is at the rate of one second per second, which is not a very useful insight.

Theoretical physicist Stephen Hawking suggested that a “ chronology protection conjecture ” must exist, an as-yet-unknown physical principle that forbids time travel. Hawking’s concept originates from the idea that we cannot know what goes on inside a black hole, because we cannot get information out of it. But this argument is redundant: we cannot time travel because we cannot time travel!

Researchers are investigating a more fundamental theory, where time and space “emerge” from something else. This is referred to as quantum gravity , but unfortunately it does not exist yet.

So is time travel possible? Probably not, but we don’t know for sure!

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The Real Rules for Time Travelers

Time travel may in fact be possible, but it wouldn't work like in back to the future. (for one thing, you don't have worry about your parents failing to create you — you already exist.).

People all have their own ideas of what a time machine would look like. If you are a fan of the 1960 movie version of H. G. Wells’s classic novel, it would be a steampunk sled with a red velvet chair, flashing lights, and a giant spinning wheel on the back. For those whose notions of time travel were formed in the 1980s, it would be a souped-up stainless steel sports car . Details of operation vary from model to model, but they all have one thing in common: When someone actually travels through time, the machine ostentatiously dematerializes, only to reappear many years in the past or future. And most people could tell you that such a time machine would never work, even if it looked like a DeLorean.

They would be half right: That is not how time travel might work, but time travel in some other form is not necessarily off the table. Since time is kind of like space (the four dimensions go hand in hand), a working time machine would zoom off like a rocket rather than disappearing in a puff of smoke. Einstein described our universe in four dimensions: the three dimensions of space and one of time. So traveling back in time is nothing more or less than the fourth-dimensional version of walking in a circle. All you would have to do is use an extremely strong gravitational field, like that of a black hole, to bend space-time. From this point of view, time travel seems quite difficult but not obviously impossible.

These days, most people feel comfortable with the notion of curved space-time. What they trip up on is actually a more difficult conceptual problem, the time travel paradox. This is the worry that someone could go back in time and change the course of history. What would happen if you traveled into the past, to a time before you were born, and murdered your parents? Put more broadly, how do we avoid changing the past as we think we have already experienced it? At the moment, scientists don’t know enough about the laws of physics to say whether these laws would permit the time equivalent of walking in a circle — or, in the parlance of time travelers, a “closed timelike curve.” If they don’t permit it, there is obviously no need to worry about paradoxes. If physics is not an obstacle, however, the problem could still be constrained by logic. Do closed timelike curves necessarily lead to paradoxes?

If they do, then they cannot exist, simple as that. Logical contradictions cannot occur. More specifically, there is only one correct answer to the question “What happened at the vicinity of this particular event in space-time?” Something happens: You walk through a door, you are all by yourself, you meet someone else, you somehow never showed up, whatever it may be. And that something is whatever it is, and was whatever it was, and will be whatever it will be, once and forever. If, at a certain event, your grandfather and grandmother were getting it on, that’s what happened at that event. There is nothing you can do to change it, because it happened. You can no more change events in your past in a space-time with closed timelike curves than you can change events that already happened in ordinary space-time, with no closed timelike curves.

As we will see, the time travel paradox — the possibility of changing our past — seems intractable only because it conflicts with our notion of ourselves as beings with free will. Consistent stories are possible, even in space-times with closed timelike curves.

To illustrate this point, imagine that you stumble upon a time machine in the form of a gate. When you pass through it in one direction, it takes you exactly one day into the past; if you pass through in the other direction, it takes you exactly one day into the future. You walk up to the gate, where you see an older version of yourself waiting for you. The two of you exchange pleasantries. Then you leave your other self behind as you walk through the gate into yesterday. But instead of obstinately wandering off, you wait around a day to meet up with the younger version of yourself (you have now aged into the older version you saw the day before) with whom you exchange pleasantries before going on your way. Everyone’s version of every event would be completely consistent.

We can have much more dramatic stories that are nevertheless consistent. Imagine that we have been appointed Guardian of the Gate, and our job is to keep vigilant watch over who passes through. One day, as we are standing off to the side, we see a person walk out of the rear side of the gate, emerging from one day in the future. That’s no surprise; it just means that you will see that person enter the front side of the gate tomorrow. But as you keep watch, you notice that he simply loiters around for one day, and when precisely 24 hours have passed, the traveler walks calmly through the front of the gate. Nobody ever approached from elsewhere. That 24-hour period constitutes the entire life span of this time traveler. He experiences the same thing over and over again, although he doesn’t realize it himself, since he does not accumulate new memories along the way. Every trip through the gate is precisely the same to him. That may strike you as weird or unlikely, but there is nothing paradoxical or logically inconsistent about it.

The real question is this: What happens if we try to cause trouble? That is, what if we choose not to go along with the plan? Let’s say you meet a day-older version of yourself just before you cross through the front of the gate and jump backward in time, as if you will hang around for a day to greet yourself in the past. But once you actually do jump backward in time, you still seem to have a choice about what to do next. You can obediently fulfill your apparent destiny, or you can cause trouble by wandering off. What is to stop you from deciding to wander? That seems like it would create a paradox. Your younger self bumped into your older self, but your older self decides not to cooperate, apparently violating the consistency of the story.

We know what the answer is: That cannot happen. If you met up with an older version of yourself, we know with absolute certainty that once you age into that older self, you will be there to meet your younger self. That is because, from your personal point of view, that meet-up happened, and there is no way to make it un-happen, any more than we can change the past without any time travel complications. There may be more than one consistent set of things that could happen at the various events in space-time, but one and only one set of things actually does occur. Consistent stories happen; inconsistent ones do not. The vexing part is understanding what forces us to play along.

The issue that troubles us, when you get down to it, is free will. We have a strong feeling that we cannot be predestined to do something we choose not to do. That becomes a difficult feeling to sustain if we have already seen ourselves doing it.

Of course, there are some kinds of predestination we are willing to accept. If we get thrown out of a window on the top floor of a skyscraper, we expect to hurtle to the ground, no matter how much we would rather fly away and land safely elsewhere. The much more detailed kind of predestination implied by closed timelike curves, where it seems that we simply cannot make certain choices (like walking away after meeting a future version of ourselves), is bothersome.

The nub of the problem is that you cannot have a consistent “arrow of time” in the presence of closed timelike curves. The arrow of time is simply the distinction between the past and the future. We can turn an egg into an omelet, but not an omelet into an egg; we remember yesterday, but not tomorrow; we are born, grow older, and die, never the reverse. Scientists explain all of these manifestations of the arrow of time in terms of entropy — loosely, the “disorderliness” of a system. A neatly stacked collection of papers has a low entropy, while the same collection scattered across a desktop has a high entropy. The entropy of any system left to its own devices will either increase with time or stay constant; that is the celebrated second law of thermodynamics. The arrow of time comes down to the fact that entropy increases toward the future and was lower in the past.

A statement like “We remember the past and not the future” makes perfect sense to us under ordinary circumstances. But in the presence of closed timelike curves, some events are in our past and also in our future. So do we remember such events or not? In general, events along a closed timelike curve cannot be compatible with an uninterrupted increase of entropy along the curve. That’s a puzzle: On a closed curve, the entropy has to finish exactly where it started, but the arrow of time says that entropy tends to increase and never decrease. Something has to give.

To emphasize this point, think about the hypothetical traveler who emerges from the gate, only to enter it from the other side one day later, so that his entire life story is a one-day loop repeated ad infinitum. Take a moment to contemplate the exquisite level of precision required to pull this off, if we think about the loop as “starting” at one point. The traveler would have to ensure that, one day later, every single atom in his body was in precisely the right place to join up smoothly with his past self. He would have to make sure, for example, that his clothes did not accumulate a single extra speck of dust that was not there one day earlier. This seems incompatible with our experience of how entropy increases. If we merely shook hands with our former selves, rather than joining up with them, the required precision doesn’t seem quite so dramatic. In either case, though, the insistence that we be in the right place at the right time puts a very stringent constraint on our possible future actions.

Our concept of free will is intimately related to the idea that the past may be set in stone, but the future is up for grabs. Even if we believe that the laws of physics in principle determine the evolution of some particular state of the universe with perfect fidelity, we don’t know what that state is, and in the real world the increase of entropy is consistent with any number of possible futures. A closed timelike curve seems to imply predestination: We know what is going to happen to us in the future because we witnessed it in our past.

Closed timelike curves, in other words, make the future resemble the past. It is set in stone, not up for grabs at all. The reason we think the past is fixed once and for all is that there is a boundary condition at the beginning of time. The entropy of the universe started very small (at the time of the Big Bang) and has been growing ever since. Ordinarily we do not imagine that there is any analogous boundary condition in the future — entropy continues to grow, but we cannot use that information to draw any conclusions. If we use a closed timelike curve to observe something about our future actions, those actions become predestined. That’s extra information about the history of the universe, over and above what we normally glean from the laws of physics, and it makes us uncomfortable.

If closed timelike curves exist, ensuring that all events are consistent is just as strange and unnatural to us as a movie played backward, or any other example of evolution that decreases entropy. It’s not impossible; it’s just highly unlikely. So either closed timelike curves cannot exist, or big, macroscopic things cannot travel on truly closed paths through space-time — unless everything we think we know about entropy and the arrow of time is wrong.

Life on a closed timelike curve seems pretty drab. Once you start moving along such a curve, you are required to come back to precisely the point at which you started. An observer standing outside, however, has what is seemingly the opposite problem: What happens along such a curve cannot be uniquely predicted from the prior state of the universe. We have the strong constraint that evolution along a closed timelike curve must be consistent, but there will always be a large number of consistent evolutions that are possible, and the laws of physics seem powerless to predict which one will actually come to pass.

In the usual way of thinking, the laws of physics function like a computer. You give as input the present state, and the laws return as output what the state will be one instant later (or earlier, if we wish). By repeating this process many times, we can build up the entire history of the universe, from start to finish. In that sense, complete knowledge of the present implies complete knowledge of all of history.

Closed timelike curves would make such a program impossible, as a simple thought experiment reveals. Hark back to the stranger who appeared out of the gate into yesterday, then jumped back in the other side a day later to form a closed loop. There would be no way to predict the existence of such a stranger from the state of the universe at an earlier time. Let’s say we start in a universe that, at some particular moment, has no closed timelike curves. The laws of physics purportedly allow us to predict what happens in the future of that moment. This ability vanishes as soon as someone builds a time machine and creates a closed timelike curve. Mysterious strangers and other random objects can then appear out of thin air and disappear just as quickly.

We can insist all we like that what happens in the presence of closed timelike curves be consistent. But that requirement is not enough to make the events predictable, with the future determined by the laws of physics and the state of the universe at one moment in time. Indeed, closed timelike curves can make it impossible to de-fine “the universe at one moment in time.” Ordinarily we can imagine “slicing” our four-dimensional universe into three-dimensional “moments of time.” In the presence of closed timelike curves, though, we generally will not be able to slice space-time that way. Locally — in the near vicinity of any particular point in space-time — we can always divide events into the “past” and the “future.” But we might not be able to do this throughout the universe. The warping associated with the closed timelike curve could cause our slice to twist back on itself, making it impossible to divide all of space-time into distinct moments.

We would therefore have to abandon the concept of determinism, the idea that the state of the universe at any one time determines the state at all other times. We would also have to abandon free will — because witnessing part of our future history implies some amount of predestination.

Do we value determinism so highly that we should reject the possibility of closed timelike curves entirely? Not necessarily. We could imagine a different way in which the laws of physics could be formulated — not as a computer that calculates the next moment from the present moment but as a set of conditions that are imposed on the history of the universe as a whole. It is not clear what such conditions might be, but we have no way of excluding the idea on the basis of pure thought.

All this may sound like vacillation, but it provides an important lesson. Some of our understanding of time is based on logic and the known laws of physics, but some of it is based purely on convenience and reasonable-sounding assumptions. We think that the ability to uniquely determine the future from knowledge of our present state is important, but the real world might end up having other ideas. If physicists discover that closed timelike curves really can exist, we will have to dramatically rethink the way we understand time. In that case, the universe could not be nicely divided into a series of separate “moments” of time.

The ultimate answer to the puzzles raised by closed timelike curves is probably that they simply cannot exist. If that is true, though, it is because the laws of physics do not let you warp space-time enough to create them — not because they let you kill your grandfather before you are born.

This piece is adapted from Cosmic Variance blogger Sean Carroll ’s latest book, From Eternity to Here: The Quest for the Ultimate Theory of Time , which was published last month by Dutton.

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Time travel: five ways that we could do it

Cathal O’Connell

Cathal O'Connell is a science writer based in Melbourne.

In 2009 the British physicist Stephen Hawking held a party for time travellers – the twist was he sent out the invites a year later (No guests showed up). Time travel is probably impossible. Even if it were possible, Hawking and others have argued that you could never travel back before the moment your time machine was built.

But travel to the future? That’s a different story.

Of course, we are all time travellers as we are swept along in the current of time, from past to future, at a rate of one hour per hour.

But, as with a river, the current flows at different speeds in different places. Science as we know it allows for several methods to take the fast-track into the future. Here’s a rundown.

1. Time travel via speed

This is the easiest and most practical way to time travel into the far future – go really fast.

According to Einstein’s theory of special relativity, when you travel at speeds approaching the speed of light, time slows down for you relative to the outside world.

This is not a just a conjecture or thought experiment – it’s been measured. Using twin atomic clocks (one flown in a jet aircraft, the other stationary on Earth) physicists have shown that a flying clock ticks slower, because of its speed.

In the case of the aircraft, the effect is minuscule. But If you were in a spaceship travelling at 90% of the speed of light, you’d experience time passing about 2.6 times slower than it was back on Earth.

And the closer you get to the speed of light, the more extreme the time-travel.

Computer solves a major time travel problem

The highest speeds achieved through any human technology are probably the protons whizzing around the Large Hadron Collider at 99.9999991% of the speed of light. Using special relativity we can calculate one second for the proton is equivalent to 27,777,778 seconds, or about 11 months , for us.

Amazingly, particle physicists have to take this time dilation into account when they are dealing with particles that decay. In the lab, muon particles typically decay in 2.2 microseconds. But fast moving muons, such as those created when cosmic rays strike the upper atmosphere, take 10 times longer to disintegrate.

2. Time travel via gravity

The next method of time travel is also inspired by Einstein. According to his theory of general relativity, the stronger the gravity you feel, the slower time moves.

As you get closer to the centre of the Earth, for example, the strength of gravity increases. Time runs slower for your feet than your head.

Again, this effect has been measured. In 2010, physicists at the US National Institute of Standards and Technology (NIST) placed two atomic clocks on shelves, one 33 centimetres above the other, and measured the difference in their rate of ticking. The lower one ticked slower because it feels a slightly stronger gravity.

To travel to the far future, all we need is a region of extremely strong gravity, such as a black hole. The closer you get to the event horizon, the slower time moves – but it’s risky business, cross the boundary and you can never escape.

And anyway, the effect is not that strong so it’s probably not worth the trip.

Assuming you had the technology to travel the vast distances to reach a black hole (the nearest is about 3,000 light years away), the time dilation through travelling would be far greater than any time dilation through orbiting the black hole itself.

(The situation described in the movie Interstellar , where one hour on a planet near a black hole is the equivalent of seven years back on Earth, is so extreme as to be impossible in our Universe, according to Kip Thorne, the movie’s scientific advisor.)

The most mindblowing thing, perhaps, is that GPS systems have to account for time dilation effects (due to both the speed of the satellites and gravity they feel) in order to work. Without these corrections, your phones GPS capability wouldn’t be able to pinpoint your location on Earth to within even a few kilometres.

3. Time travel via suspended animation

Another way to time travel to the future may be to slow your perception of time by slowing down, or stopping, your bodily processes and then restarting them later.

Bacterial spores can live for millions of years in a state of suspended animation, until the right conditions of temperature, moisture, food kick start their metabolisms again. Some mammals, such as bears and squirrels, can slow down their metabolism during hibernation, dramatically reducing their cells’ requirement for food and oxygen.

Could humans ever do the same?

Though completely stopping your metabolism is probably far beyond our current technology, some scientists are working towards achieving inducing a short-term hibernation state lasting at least a few hours. This might be just enough time to get a person through a medical emergency, such as a cardiac arrest, before they can reach the hospital.

In 2005, American scientists demonstrated a way to slow the metabolism of mice (which do not hibernate) by exposing them to minute doses of hydrogen sulphide, which binds to the same cell receptors as oxygen. The core body temperature of the mice dropped to 13 °C and metabolism decreased 10-fold. After six hours the mice could be reanimated without ill effects.

Unfortunately, similar experiments on sheep and pigs were not successful, suggesting the method might not work for larger animals.

Another method, which induces a hypothermic hibernation by replacing the blood with a cold saline solution, has worked on pigs and is currently undergoing human clinical trials in Pittsburgh.

4. Time travel via wormholes

General relativity also allows for the possibility for shortcuts through spacetime, known as wormholes, which might be able to bridge distances of a billion light years or more, or different points in time.

Many physicists, including Stephen Hawking, believe wormholes are constantly popping in and out of existence at the quantum scale, far smaller than atoms. The trick would be to capture one, and inflate it to human scales – a feat that would require a huge amount of energy, but which might just be possible, in theory.

Attempts to prove this either way have failed, ultimately because of the incompatibility between general relativity and quantum mechanics.

5. Time travel using light

Another time travel idea, put forward by the American physicist Ron Mallet, is to use a rotating cylinder of light to twist spacetime. Anything dropped inside the swirling cylinder could theoretically be dragged around in space and in time, in a similar way to how a bubble runs around on top your coffee after you swirl it with a spoon.

According to Mallet, the right geometry could lead to time travel into either the past and the future.

Since publishing his theory in 2000, Mallet has been trying to raise the funds to pay for a proof of concept experiment, which involves dropping neutrons through a circular arrangement of spinning lasers.

His ideas have not grabbed the rest of the physics community however, with others arguing that one of the assumptions of his basic model is plagued by a singularity, which is physics-speak for “it’s impossible”.

The Royal Institution of Australia has an Education resource based on this article. You can access it here .

Related Reading: Computer solves a major time travel problem

Originally published by Cosmos as Time travel: five ways that we could do it

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Screen Rant

The flash: zoom's secret plan & timeline explained.

With The flash revealing the master plan of Zoom, we break it down so fans can grasp - and appreciate - every step along the way.

[WARNING: This article contains SPOILERS up to The Flash Season 2, Episode 18.]

The secret was out, but now it's really out: the big bad of The Flash 's second season has been in plain sight the entire time, delivering yet another speedster mentor who was really out to steal Barry Allen's speed for himself. Jay Garrick is Zoom-- well, actually, Hunter Zolomon is Zoom, and Jay Garrick is... nobody. The most recent episode of The Flash may have clarified the lingering questions and riddles of the season's earlier cliffhanger in the least elegant way - having the villain literally explain each step - but answers are answers, and fans got more than they bargained for.

Still, it's been a fairly complicated and time-travel-ish path to this point, so to make sure that each and every viewer can finally put things together, we've broken down the secret plot of the season from start to finish. Most fans have probably put together some, if not all of the twists and explanations, but the actual story is now simpler than we would have predicted. Well, as simple a plot involving time travel, parallel worlds and dual identity murderers really can be.

Hunter Zolomon's Origin Story

Ever wonder what makes a serial killer in the world of The CW's superheroes? According to Harrison Wells - upon learning that 'Jay Garrick' may be an alias, with 'Hunter Zolomon' his actual identity - it's the terrible childhood that befell Zolomon, forced to witness his father abusing his mother. Whatever the cause (trauma from his time at war overseas is implied), James Zolomon decided it was best to kill his wife in front of their son, Hunter. In a twisted bit of fate, it was the same act many (including Joe West) believed Barry Allen had witnessed himself.

Dispatched to the Central City Orphanage (which on Earth-2 seems to be built on the ruins of the Queen mansion?), Hunter was sent on his way to becoming a serial killer with odd notions of what 'being a hero' truly means... something he got from his father, we suppose. But after murdering a total of 23 people, becoming famous on the serial-killer-free Earth-2, Hunter was committed to an insane asylum, and forced to undergo electro-shock therapy as treatment.

Zolomon Becomes Zoom... & The Flash?

Although the actual origin story as shown in The Flash is a bit of a mystery - after all, we're just being shown what happened to Zolomon, when no character present could actually confirm what happened - but it seems to play out as seen here. Again, it's possible that Harrison Wells of Earth-2 was completely aware that the S.T.A.R. Labs particle accelerator explosion of his Earth turned Zolomon into the monster known as Zoom. But it would then be unclear how he would know that , but not... you know, what Zolomon looked liked without a beard.

Either way, it was during one of Zolomon's electricity-fueled 'therapy sessions' that the "dark matter" infiltrated the building, sending everyone in the room flying, and apparently flooding Zolomon's cells with the same Speed Force energy that turned Barry Allen into The Flash.

In one moment, giving birth to the speed demon known as Zoom, and at the same time, the heroic alter ego that Zolomon would create. Because playing Zoom and the plucky hero fighting him was in the words of the man himself, "fun." And Earth-2 was his playground.

Experimentation with Velocity 6 Begins

While it's a little surprising that such a deeply disturbed individual could keep up the role of a superhero for so long, Hunter couldn't stay in control forever. And when he discovered that Harrison Wells was concocting a formula dubbed 'Velocity' that would ramp up a person's speed, his next adventure was obvious. Whether Wells' experiments with the drug were made known to 'Jay' (as a potential gift that would allow him to defeat Zoom) or simply uncovered and stolen by his Zoom persona, Velocity 6 proved a successful batch.

Unfortunately, as Jay soon explained to Caitlin Snow, and the entire team got to see up close with the speedster Trajectory, the added velocity came at a price. It would boost the Speed Force in a person's cells, but eventually, start to kill the body on a cellular level.

Since Hunter actually seemed to develop a real connection to Caitlin (not all that surprising, given that the only other woman he loved was murdered in front of him), we can put some weight into his explanation. The drug proved addictive, and while it turned his lightning blue to show how it was changing his body chemistry, it was eating away at his cells so quickly, only the super-healing effects of the Speed Force could keep death at bay. And when Trajectory pushed those same limits with her own mix of Velocity, the Speed Force couldn't keep her from being turned to dust.

But something had to give, and as Hunter explained to Caitlin, his continued drug use severed his connection to the Speed Force, leaving the drug as his only source of power... while it would also kill him in due time.

The only solution, then, was more speed . But not from a bottle or electric chair - he'd need a legitimate speedster to tap into for himself.

The Singularity

What perfect timing, then, that just as his options seemed to be running out along with his days, that a hole should be torn in the sky above Central City, granting access to a brand new world. It's still unclear just how Hunter managed to traverse the singularity, but judging by his knowledge of breaches, quark matter and experimental physics, it's clear that he was a murderer, but not an idiot. With breaches all over the city, Hunter took the trip to see if his answer could be found on the other side.

What he found was an answer to his prayers: a parallel universe speedster who acquired his own connection to the Speed Force the same way he had, taking the same heroic name and mission. But he had yet to truly tap into the incredible speed that Hunter needed, which meant he had some work to do.

And so began the surveillance, learning his target's every motivation, acquaintance, and footstep, until he set the plan in motion. In short, fattening up his golden goose before he would slaughter it to gain even more power.

Hunter Makes His Move

It probably should have raised some suspicions at the time when Hunter-- sorry, 'Jay' arrived at S.T.A.R. Labs, having learned everything there was to know about this world's resident super speedster (and his family and friends). Potentially recognizing that Barry was still in need of a mentor, Hunter arrived to offer just that, perfectly camouflaged in the 'oh golly' personality that won him so many fans on Earth-2. So as not to arouse suspicion, he also decided not to use the speed that Velocity gave him - meaning the tests performed on him to determine any ulterior motives would come back negative, since he really wasn't connected to the Speed Force any more.

Everything seemed to be going perfectly to plan, as Hunter would blackmail Earth-2 criminals into murdering Barry Allen, pushing the hero's abilities farther and farther, accessing more and more of the Speed Force he so badly needed to one day steal. It would have worked without incident, too, if Caitlin hadn't started looking a bit closer at Hunter's test results, realizing that he wasn't as normal as he claimed. He was dying, and had to keep her from interfering with a plan that was proceeding beautifully.

Throwing Caitlin Off The Scent

When Caitlin revealed that she had already done some digging, and discovered that the name 'Jay Garrick' couldn't be found on this Earth, Hunter realized that he had to act to keep his secret (otherwise Caitlin could realize Hunter was using an alias). Whether he then found his doppelganger, or had discovered him earlier out of curiosity (who wouldn't do the same), Hunter escorted Caitlin to a park where she could see his doppelganger in the flesh. You see, Jay wasn't the alias, he claimed, it was this man who went by a made up name.

And, having already found his doppelganger with this crazy, 'Hunter Zolomon' title applied by his adoptive parents, he had already found that they were no longer genetic matches and therefore couldn't be turned to for healthy tissue. It was a clever move on Hunter's part, taking advantage of Caitlin's trust... and apparently, making the massive gamble that she would never share this information with the other Earth-2 resident in the next room. Or, in his defense, it's possible that Hunter Zolomon may not have known just how famous he had become. Whatever the case, things returned to normal, with Barry growing faster and faster. But when Barry decided to head through the breach to face Zoom on his own turf, Hunter suddenly has a new problem to solve.

He Can't Be Two Places At Once

Whether by plan or by prediction, Hunter knew, or suspected that Barry, Harrison Wells and Cisco traveling over to Earth-2 could cause the breach between the two worlds to collapse. And anyone with the smarts of your typical comic book move TV show viewer would be able to tell that Jay actually disappearing when Zoom was on Earth-2, or vice versa was suspicious. So, as he explained to the entire team in the most recent episode, "Versus Zoom," he needed to find a way to be in two places at once: the answer was a time remnant, retrieved from the past.

If you were wondering if or when Harrison Wells' explanation of why Eobard Thawne could still be alive would come back into play, it's here. Hunter saw the problem coming so he took a trip back in time (don't look too closely at that though, since we're guessing the writers don't want fans wondering how accessible time travel is to Hunter Zolomon) to cross paths with a former version of himself - just as Barry did when he recently sought help defeating a Time Wraith. Since his former self was as insane as he is, the beauty of the plan convinced him to come along for the ride - despite it ending in his own death.

"So," the science fiction buffs will no doubt ask, "how could Hunter kill himself in the past and still remain alive in the present?" It's a question that the writers smartly posed, and answered earlier in the show's second season, when Eobard Thawne arrived - for the first time, in his mind - after Eddie Thawne had killed himself, rendering his offspring nonexistent, and dissolving Eobard Thawne in an instant. Eddie killing himself should have erased every action Thawne ever took, right?

Wrong. As Harrison Wells explained, the past wasn't change by Eddie's actions since Eobard Thawne was, presumably, lost in the timestream to enough of a degree to go on existing. Although Thawne's past would never happen again, it already did for him,meaning his past actions - like arriving to discover Barry Allen's time period - would play out as it had. A man without past or future, he became, in the words of Wells, a "time remnant." The same logic is at work in Hunter's plan, presuming (correctly) that killing another version of himself wouldn't change his own existence. To put it simply: he didn't go back in time and kill himself, he killed another Hunter in the present.

The science here is obviously messy and unclear for that very reason, but Hunter's plan seemed to have been carried off perfectly. Travel back, tell your younger self of the plan, have them remain on Earth-1 with Caitlin and Iris as he wreaks havoc on Earth-2, and finally, tie up the last loose end in truly memorable fashion.

The Last Push Barry Needed

An arm through the chest is a hard thing to forget, but you have to hand it to Hunter: his master plan accomplished multiple tasks at once. Going back and partnering with his younger self allowed him to be on two Earths at once, but it also provided the perfect moment to guarantee his plan would go off perfectly. Not only would 'Jay's death confirm once and for all that Zoom had to be some other being, or person, but the loss of yet another mentor would drive Barry to become as fast as possible like nothing else could. Revenge is a powerful motivator, and as Hunter sat on Earth-2 and waited, Barry performed as if his strings were being pulled from another world.

Of course, the truth of Jay's death raises one major question: what did Hunter mean when he returned to his Earth-2 lair to state that "this complicates things"? While it now seems to be breaking the fourth wall and speaking directly to the audience, Hunter could have been referring to the closed breach, with absolutely nobody on his side as collateral, leaving only Barry's need to defeat him as the reason their paths should ever cross.

The real irony, of course, is that Hunter murdered his Earth-1 self before he could tell him Caitlin seemed well on her way to finding a permanent, non-harmful version of Velocity... oh well.

The Final Bargain

As expected, Barry seemed unable to just let Zoom go on terrorizing another parallel world - or, maybe, he actually did remember his promise to rescue the masked man in Zoom's dungeon, despite never mentioning it again. With the help of Cisco and Wells, Hunter got the breach he'd been waiting for, and headed to Earth-1 to reap the speed he helped sow. He was surprised to find that Wells and Allen had actually managed to amplify his speed well beyond his own, but simply kidnapping the newest member of the West Family took care of any imbalance.

And as he had planned from the very beginning, he stood opposite Barry, faster than ever, accessing even more of the Speed Force that would return Hunter's health and grant him nearly limitless speed. In exchange for a boy he hardly knew, it was his... for now, anyway.

The Mysteries That Remain

His plan seems completely reveal for now, or at least self-contained without dealing with the other mysteries that do remain. There looks to be more than just speed behind Hunter's experiments with Velocity, for one, going by his transformation just before escaping from the trap set by Barry and his friends. Claiming that "you can't lock up the darkness," , Hunter's eyes and voice became those of Zoom, before speeding away yet again. Assuming that it isn't merely a vocal trick for the sake of being 'extra spooky,' there are symptoms or illnesses - or even... beings? - Hunter is dealing with that we have yet to discover.

The mystery of the man in the iron mask also remains, with Barry asking Hunter who the man really is (but again keeping quiet about his promise to rescue him). And in true Flash fashion, the writers answer almost all of the lingering questions while raising entirely new ones. Hunter does offer an answer of sorts - that "you wouldn't believe me if I told you" - so it seems to be a question that absolutely will be answered, but may be a little too outlandish to heap onto the already ludicrous proceedings.

NEXT: Will Flash & Arrow Fans Ever Get a TV Justice League?

So there you have it: Hunter Zolomon's master plan, from the moment he decided it would be even more fun to play both a supervillain AND his heroic nemesis, to the jaw-dropping and completely confusing death of Jay Garrick at the hands of Zoom.

We hope that seeing the plan in a simple order helps dispel some of the confusion that may cause fans to feel that they've missed an important beat, and can get every fan on equal footing as the show launches into the final episodes of its second season.

The Flash airs Tuesdays @8pm on The CW.

Information Technology

What countries are restricted from using Zoom?

Users in certain countries or regions are unable to access Zoom for regulatory reasons. You can view a complete list of restricted countries here . ** Note: If VPNs are allowed in your country, this is an alternative way to access Zoom

If you are struggling with an issue not featured here, the answers to most commonly asked questions, including ones not listed below, can be found in our Zoom Cheatsheet (PDF).

How can I lock a meeting?

Meetings can be locked, which prevents other participants from joining the meeting in progress. You can learn more about locking meetings and managing participants here .

How can I upgrade my Zoom app?

As of Friday, April 10, 2020, all managed Windows computers’ Zoom app will be enabled for auto-upgrading. The next time users…

How do I add an attendee as an alternate host?

The alternative host option allows you to schedule meetings and designate another licensed user on the same account to…

How do I enable breakout rooms?

Visit the Zoom Help Center to learn more…

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Download the Zoom Microsoft Outlook Plugin MSI file from the Zoom Download Center…

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Changing time zone settings

By default, Zoom uses the time zone your computer is set to. Zoom also matches your scheduled meeting times with your time zone settings, even when your meetings are scheduled in another location.

You can manually correct or update your time zone settings to correspond to your location’s time zone. You can adjust your time zone settings on your account profile in the Zoom web portal. Editing the time zone on the Zoom web portal will apply it to the Zoom desktop client, mobile app, and provisioned desk phones . This allows your local time to be visible on your profile card. Additionally, you can change your scheduled meetings ’ time zone settings.

This article covers:

How to change your computer’s time zone

How to change your time zone on the web portal, how to change your scheduled meeting’s time zone, how to troubleshoot time zone issues, prerequisites for changing time zone settings.

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Zoom desktop client for Windows, macOS, or Linux: Global minimum version or higher
Open the Start Menu.
Click the Settings icon.
Click Time & Language .
Under Time zone , click the dropdown menu and select the time zone you want.

Your computer’s time zone will adjust to your selected time zone, and Zoom will automatically use your computer’s time zone settings.

At the top left corner of your screen, click the Apple icon.
Click System Preferences , then click Date & Time .
Automatically : Set your time zone automatically using your current location.
Manually : Manually set your time zone by selecting your location’s time zone.
Sign in to your Zoom account on the web portal.
In the navigation menu, click Profile .
Under Personal , to the right of Time Zone , click Edit .
Click the dropdown menu to select a different time zone.
Click Save .

Your time zone settings will update to your selected time zone.

Windows | macOS | Linux

Select your scheduled meeting’s time zone.

Sign in to the Zoom desktop client.
Schedule a Zoom meeting .
While creating your Zoom meeting, to the right of Time Zone , click the dropdown menu and select a different time zone.
Complete selecting your meeting’s settings.

Edit a scheduled meeting’s time zone

You can edit a meeting that you scheduled and adjust the time zone.

Click the scheduled meeting that you want to edit, then click Edit . An Edit Meeting pop-up window will appear.
In the pop-up window, to the right of Time Zone , click the dropdown menu and select a different time zone.
Sign in to the Zoom web portal.
In the navigation menu, click Meetings .
Hover your mouse over the scheduled meeting that you want to edit, then click Edit . You will be directed to an Edit page.
On the Edit page, to the right of Time Zone , click the dropdown menu and select a different time zone.

By default, Zoom will use the time zone set in your Zoom account profile . If Zoom is not syncing a meeting's scheduled time to your location time, reset your computer’s time zone settings, then reset Zoom’s time zone settings.

What Is Zoom Time Limit and How Can You Change It?

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Using Zoom for meetings, conference calls, and casual online gatherings has become commonplace since the pandemic hit in 2020. However, while the free app offers robust and user-friendly features, it also has its drawbacks. The biggest drawback is arguably the Zoom time limit. What is this time limit, and how can you extend it? Let’s find out.

What Is the Zoom Time Limit?

Is Zoom still limited to 40 minutes? If you’ve never had a Zoom meeting, you might not be familiar with the infamous Zoom time limit. Initially, the Zoom free time limit was a 40-minute time cap imposed by the app on free group calls.

The only way you could bypass this was by upgrading to a paid account. Otherwise, account holders using the free version could only host unlimited sessions between one-on-one calls. However, the Zoom time limit for 2 participants recently tightened and now, even one-on-one calls have a 40-minute cap.

Of course, the Zoom time limit only takes effect if the meeting host is using a free account. Hence, if the host is a paid account holder, the limit should extend regardless of the other caller’s account status.

This introduction has made the platform much less competitive overall, especially for folks who simply want to make personal calls. Nonetheless, the Zoom time limit exists, so if you want to extend the hours you spend on Zoom calls, you’ll need to either spend money or employ a few workarounds.

Why Does Zoom Have a Time Limit?

The Zoom meeting time limit exists for two reasons. Firstly, the imposition of a time limit is one way the company can encourage its free users to upgrade. After all, fewer people would pay for a subscription if there were no Zoom time limit on free accounts.

Secondly, the Zoom time limit is a good way to help the company manage its servers better. It helps the company prioritize the connections made by paid account users by limiting how much they serve free clients.

Overall, the Zoom time limit is simply a tactic that the app employs to gain more business. We honestly can’t blame Zoom, as it does need a source of funds to keep the app running. However, it is slightly disadvantageous for them because many competitor apps offer unlimited free calls.

What Affects the Zoom Time Limit?

Firstly, you should know that a Zoom subscription is not required to use the app. All you need is a free account, and you can make as many 40-minute calls as you want without hindrance. However, the only instance where the Zoom time limit is removed is when you have a subscription.

Of course, there are instances when Zoom removes the time limit. For example, Zoom can sometimes remove the 40-minute cap for a limited time. This happened during the holiday

season around Christmas and the New Year of 2020. It made many people wonder, “has Zoom removed the time limit?” as they hosted their online gatherings.

However, the occurrence didn’t repeat in 2021 despite the rise in Omicron cases. There’s no telling when Zoom will provide people with periodic unlimited sessions again. It’s truly up to the company at this point – and it likely won’t happen because there are fewer lockdowns.

Apart from occasional generosity on Zoom’s part, is there anything else that affects the Zoom time limit? In the past, Zoom only implemented the time limit when there were more than two people on a free call. However, the company has recently removed this feature in favor of limiting free calls altogether. As a result, even one-on-one calls can only last for 40 minutes at a time.

Zoom Time Limit on Paid Accounts

Paid accounts also have a Zoom time limit – although they don’t work the same as free accounts. That’s because the time limit for people with subscriptions is 30 hours instead of a measly 40 minutes. Furthermore, the limit is in effect regardless if the session is active or idle.

However, do note that there is one instance where Zoom imposes a 40-minute limit on sessions made by paid accounts. That is when only one person is left during a session after all the other accounts already left. Hence, if someone hosts a meeting and one or more participants join, and then either one host or one participant is left after everyone leaves, the meeting will end in 40 minutes.

How to Increase Your Zoom Time Limit?

Is there any way to increase your Zoom time limit? Here are the best methods you can use to make longer calls.

Upgrading to a Paid Account

How do you Zoom in longer than 40 minutes? If you’re learning how to extend the Zoom meeting time limit, know that the easiest way is through upgrading. After all, the limit only exists to encourage users to subscribe to the app’s paid plan.

How do you upgrade? Firstly, know that the paid plan costs $14.99 monthly. Hence, you’ll have to prepare to shell out at least that much to extend the Zoom time limit. Moreover, the Zoom time limit won’t be completely prevented as it will only extend to 30 hours. If you’re still up for upgrading, here are the steps you can take to do so:

Sign in to your Zoom account online
Go to “Account Management” and navigate to “Billing” in the navigation panel
Select “Current Plans” and choose “Upgrade Account”
Note: you can personalize your plan further by adding licenses and modifying the payment scheme
Press “Save & Continue”
Choose whether you want to buy other products or services from the company and then press “Go to Checkout”
Enter the details under “Sold To” and “Bill To” in the contact info sheet
Select the payment method
Agree to the Terms of Service that appears
Select “Upgrade Now” and then hit “Confirm”

Afterward, you can freely conduct meetings up to 30 hours long. In addition, you can add more participants to a meeting with a subscription and host larger events.

Without Upgrading Your Account

Upgrading to a paid account is the most straightforward method to eliminate the Zoom time limit. However, not everyone can afford to pay for a subscription, especially because it costs as much as a streaming service. With that said, is there any way to learn how to extend the Zoom meeting time limit for free?

You’re in luck because there happens to be an alternative path. The method does not exactly cheat the system – although it does abuse a few of the app’s technicalities. Simply follow these steps to extend your Zoom time limit without upgrading your account.

Step 1: Create a Scheduled Meeting

You can extend the Zoom time limit only with scheduled meetings. Take note that you can’t do this during spontaneous meetings so make sure to plan all your extended calls. To schedule a meeting on the desktop, simply follow these steps:

Click the “Schedule” (calendar) icon
Input the meeting details within the pop-up
Select “Schedule” at the lower-right

If you’re on a mobile device, you can create scheduled meetings through this process:

Open the Zoom app
Select the “Meet & Chat” homepage
Hit “Schedule”
Input the meeting settings and details
Input the names of the attendees and other information

Step 2: View the Zoom Time Limit

You’ll need to follow step three once the 40-minute timer is almost up. However, to do this, you’ll need to monitor the time that has already passed since the meeting began. If you don’t know how to check the Zoom time limit, simply follow these steps:

On a desktop, select your profile picture
Choose “Settings”
Check the option for “Show my meeting duration”

On Android/iOS

Tap “Settings”
Select “Meetings”
Toggle on the “Show My Connected Time” until it turns blue (Android) or green (iOS)

Step 3: Leave and Come Back to the Meeting

Leaving the meeting might be the last thing you think about if you want to extend the Zoom time limit. However, it is the only way free account hosts can extend the time when the first 40 minutes are up. Just follow these steps to extend your Zoom free meeting:

As the host, click the “End” button at the lower-right corner once the meeting timer is almost up
Note: don’t click the red “End Meeting for All” button or else none of the participants can come back
Restart the session using the same meeting ID
Tell your attendees to press the same meeting ID

Afterward, your previous meeting should refresh and have another 40 minutes left before it ends. You can repeat this method over and over anytime you want to extend the Zoom time limit. However, it will add a few short breaks to your sessions every 40 minutes. Moreover, it requires the cooperation of all the participants for it to be successful.

Benefits of Increasing Your Time Limit

There are many benefits to increasing your Zoom time limit. Firstly, if you use the workaround, you’ll no longer have to pay $14.99 monthly for a Zoom pro subscription. This will save you a lot of money and will extend your meetings at the same time. Granted, the method isn’t ideal and requires a lot of coordination. Nonetheless, it’s a good way to buy more time for the same meeting without rescheduling a new one.

Apart from this, you won’t have to download a separate app just to conduct video calls. Zoom will be the only app you need for conference calling – whether it’s for work or personal reasons.

On the other hand, if you decide on getting a subscription, extending your time limit this way will eliminate the constant workarounds you have to do during sessions. You won’t have to cut online conferences short – a crucial feature, especially for longer seminars and classes. Paid account users can say goodbye to forced breaks every 40 minutes.

How Much Is a Zoom Subscription?

The best way to get around the Zoom time limit is by subscribing to a paid plan. However, there’s more than one plan you can avail of to extend the time. Higher plans that cost more also offer additional features and benefits free users can’t avail of. To view a complete list of Zoom’s offerings, see the table below.

You can get around the Zoom time limit easily if you upgrade to any of the app’s paid plans. However, do note that higher tiers offer more features and add more value to its users. The number of participants increases with every tier jump. Moreover, there’s even a Large Meetings add-on you can avail of under lower tiers to increase that amount further.

In addition, paid plans come with at least 1GB of cloud capacity, with the Enterprise plan offering unlimited storage. It’s also worth noting that paid subscriptions offer you multiple licenses. The lowest Pro plan can provide up to 9 licenses while the Business tier can provide as much as 99. However, if your company is particularly large, the Enterprise plan is the best choice as it offers over 50 licenses.

Additional Features

Apart from extending the Zoom time limit and adding participants, licenses, and cloud storage, paid plans offer several useful features. These include ticketing, live chatting, and even phone calls to ask for technical support. Moreover, paid plans add polling, co-hosting/alternate hosting, meeting scheduling assignments, and even streaming to the list of features.

If you get the Business or Enterprise plan, you can even access transcript recording, domain management, and language interpretation. These are incredibly useful for seminars or live events that require a lot of administration to conduct. They’re also great for documentation and conducting more professional online conventions.

Finally, the highest Enterprise plan offers exclusive features such as executive business reviews, Webinar 500, and bundle discounts. It’s the best of the best if you want the most robust conference management tools within the Zoom app.

Add-On Services

Apart from Zoom’s standard features, the company also offers multiple optional add-on plans. For example, the Audio Conferencing tool starts at $1,200 yearly and provides global toll-free calling and local dial-in capabilities. On the other hand, if storage is your main concern, you can purchase more, starting at $480 yearly. This will help you with streaming, video recording downloads, and even access to as much as 3TB monthly.

For more conference-heavy companies, you can also avail of Premier Developer support for priority technical assistance and resources. The Large Meetings add-on is also great if all you want is to increase your max participant capacity. In addition, the Zoom Whiteboard only costs $24.90 yearly and provides unlimited concurrent whiteboards.

Alternatives to Zoom

Is there a time limit on Zoom? Unfortunately, yes. However, this doesn’t mean you don’t have a choice but to subscribe or use the workaround. That’s because there are numerous alternatives to Zoom, and you certainly aren’t limited to just the app. Here are a couple of alternatives you can try to make longer calls.

1. Cisco Webex

One lesser-known alternative to Zoom is Cisco Webex. Like the famous video conferencing tool, Cisco Webex allows users to conduct online meetings. It has all the essential features including webinar capability, screen sharing, and multi-platform support.

Like Zoom, Cisco Webex imposes a 50-minute time limit and only allows 100 people to join in conference calls. You can easily add participants and users while the meeting is live. Moreover, it provides both free and paid plans, starting at $13.50 monthly.

2. Google Meet

Google Meet is a great alternative if you want to get around the Zoom time limit. As the name implies, the video conferencing solution was made by Google. It served as the replacement for what was formerly Google Hangouts and Google Chats. The app’s main feature is, like Zoom, the ability to make one-on-one and group video calls.

However, unlike Zoom, the time limit within Google Meet is 60 minutes instead of 40 for free users. Moreover, it provides closed captions for calls by default and has a dynamic layout that highlights the current speaker. It’s also much cheaper compared to Zoom and starts at $7.99 monthly.

3. Microsoft Teams

If you’re already within Microsoft’s ecosystem, it makes sense to use Microsoft Teams within your company. It’s a powerful tool that comes with the Microsoft 365 office suite. The service’s key features are the ability to create unlimited conference calls with no time cap. You can host meetings and include up to 100 participants with a free account. Furthermore, the app can provide you with 10GB of cloud storage per account.

However, do note that Microsoft’s pricing scheme works differently compared to similar services like Zoom. That’s because, while its subscriptions start at $4 monthly, it’s charged on a per-user basis. Hence, each user who wants to utilize the app’s full feature set (including participating in calls) must pay $4 monthly.

Increase Your Zoom Time Limit Today

The Zoom time limit is arguably the most annoying feature within the app for most free users. Thankfully, there’s a way around it that doesn’t force you to subscribe to a paid plan. However, do note that the workaround takes some coordination and planning on the host’s part. If you want a hassle-free method to circumvent the Zoom time limit, we recommend simply getting a subscription.

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Can You Zoom On A Plane?

November 17, 2022

In the olden days if you went on a plane there were all those dire warnings about the use of electronics. Many believed if you happened to leave an iPad or Gameboy on, the whole plane would refuse to take off, or send the navigational tools in completely the wrong direction.

Nowadays, we’re a lot savvier and realize our devices can’t really do that much harm. So you can play Mario, watch Netflix and online shop in the sky to your heart’s content.

However, for many the question remains – can you use Zoom on a plane?

The answer is YES ! You can use Zoom on an airplane, however your connection quality will vary depending on factors such as your airline, flight route and in-flight Wi-Fi options. Exactly how reliable is Zoom video conferencing on a plane, you wonder? Well – if it’s really important you attend that meeting while in the sky, read on to learn more.

Attending a Zoom Call Mid-Flight: The Guide

Why would you want, or need, to zoom on an airplane.

It seems like an odd thing to want to do, but of course there may be times in your life when you simply need to use Zoom on an airplane.

Having an important Zoom meeting scheduled for the same time you’re flying is one of the most probable reasons you’ll want to video conference from the sky.

Or you might be traveling for work when you’re suddenly asked to join an emergency Zoom call. It happens.

Whatever the reason, if you find yourself needing to Zoom from 30,000 feet in the air, then don’t worry – it’s usually possible.

Which airlines allow you to use Wi-Fi & Zoom in the air?

Not all airlines offer inflight Wi-Fi, but an increasing number are starting to.

For example, American Airlines, Delta and United all offer some form of in-flight Wi-Fi.

Some airlines may even offer free Wi-Fi with Business and First Class tickets.

Of course, once you start using Zoom (or any other video conferencing app), you will quickly use up that free data allowance!

However, Wi-Fi on planes does still seem to be mostly restricted to long-haul flights as it’s not normally necessary on shorter flights.

Here is a list of airlines that have some sort of Wi-Fi available on board, including a few that offer it for free. This list isn’t exhaustive though, so please check with your airline before you travel.

Alaska Airlines
All Nippon Airways
American Airlines
British Airways
Delta Air Lines
Garuda Indonesia
GOL Linhas Aereas
HongKong Airlines
Philippine Airlines
Qatar Airways
Saudi Arabian Airlines
Singapore Airlines
Southwest Airlines
TAP Portugal
THAI Airways
Turkish Airlines
Vietnam Airlines
Virgin Atlantic

How much does airplane Wi-Fi cost?

The cost of using Wi-Fi on an airplane varies depending on the airline.

Some airlines, like United , have different tiers of pricing depending on how much data you want to use.

For United U.S. domestic and short-haul international flights, like those to Mexico and Canada, Wi-Fi is $8, or 1,600 miles and $10 for everyone else. There is also a Wi-Fi Day Pass for those who have a MileagePlus account. And finally a subscription service for frequent flyers offering Wi-Fi from $49, or 7,500 miles, a month.

For American Airlines , the price starts from around $10 per flight. If you are a frequent flyer the American Airlines Wi-Fi Subscription Plan for $49.95 for a monthly plan or $59.95 for a 2-device monthly plan.

The negatives of taking a Zoom call on a plane

First, it is important to note that while you may be pleased to find the answer to ‘Can you Zoom call on a plane?’ is YES – tt doesn’t mean it’s a great idea.

First off, the quality of the call may be lower than if you were on ground-based Wi-Fi. This is because the signal strength of Wi-Fi on an airplane is generally not as strong as on the ground.

Second, you may want to consider using a VPN when using Zoom on an airplane. This is because a VPN can help to improve the quality of your call by encrypting your data.

Third, you may want to consider using a calling app such as WhatsApp instead of Zoom on an airplane. This is because apps like WhatsApp use less data than Zoom and are therefore more likely to work well on airplane Wi-Fi.

Other things to be aware of though are to remember that:

You are on a plane!

And while they are not often the loudest of places, a person talking quite loudly (which is common if you are wearing headphones etc.) can be heard a mile away.

It’s important to take note of this if you are discussing private and sensitive information.

Also, be aware of the time difference, if it’s a night flight the people sitting around you may be trying to get some rest, so it might be best to use headphones or keep your volume down.

Or… you could just… not attend the meeting…

Don’t panic! We have a great solution!

But, just in case that’s not feasible…and you really HAVE to join a call…

How to use Zoom on an airplane

If you find yourself needing to join a Zoom call from an airplane, there are a few things you need to keep in mind.

First of all, you need to make sure that your airline does in fact offer inflight Wi-Fi.

As we mentioned before, not all airlines offer this service yet.

Once you’ve established that there is inflight Wi-Fi available, the next thing you need to do is sign up for the service.

You can usually do this by going to the airline’s website before your flight and entering your credit card information.

Once you’re signed up, you’ll be given a username and password that you can use to log in to the inflight Wi-Fi network.

Once you’re logged in, you should be able to connect to Zoom without any problems.

Just fire up the app and join your meeting as usual!

If, the Wi-Fi on a plane is being quite slow and unreliable, there is a chance that your Zoom call may drop out or be of poor quality.

If this happens, ask if you can try moving to a different seat on the plane (if there is one available) as sometimes the signal strength can vary depending on where you are sitting.

Do remember that you are not the only person trying to use the inflight Wi-Fi, so everyone else will be competing for bandwidth too!

What to do if you miss a Zoom call during travel?

If you have a Zoom call scheduled but the chance of getting decent Wi-Fi is looking slim to none, or even if you’re travelling for holiday, you don’t NEED to be in attendance at that Zoom call.

Using a tool such as tl;dv allows your colleagues to record, timestamp and pick out the most important bits of the meeting for you so that you can quickly get up to speed when you’re next able to check your email.

Get your colleagues to record the call, then you can either quickly fast forward through to the sections that are most relevant to you. Alternatively, you can read through the automated transcription that every tl;dv call generates, highlighted to show who was speaking when.

This way, you can easily catch up on what was discussed in the meeting, without needing to be there in person!

All you need to do is download the tl;dv meeting recorder app for Zoom , or alternatively the tl;dv Chrome Extension for Google Meet . It’s totally free, and used by remote and async teams all over the world.

A couple of clicks and you’re all ready to go!

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Happy Zooming while Zooming through the sky

However, we want to say that in this rapidly changing and evolving working world, at tl;dv we want to stress the importance of keeping boundaries and creating a healthy work-life balance.

There is already enough stress with catching a flight, remembering your passport, and ensuring that you get to the airport on time, before trying to panic about whether or not you can attend a meeting halfway through a flight to Bali.

While we understand that the great thing about online meeting tools is that they offer flexibility to all, a chance to explore the world and hold meetings anywhere, it doesn’t mean you have to be present all the time.

Unless you are conducting the meeting yourself (in which case, why not perhaps look at rescheduling?) we heavily recommend using tools such as tl;dv to make meetings work FOR you and not against you.

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That flight you take, it could be full of turbulence, or a crying baby, and the person next to you could spill a gin and tonic all over your laptop.

So, instead of worrying whether or not you can jump on that Zoom call, why not kick back? Enjoy the time unplugged and binge-watch all the Keeping Up With The Kardashians on the in-flight entertainment system?

tl;dv has got your back!

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Subscribe and stay up to date with the latest tips and news on Meetings, Sales, Customer Success, Productivity, and Work Culture.

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Are Zooms or Primes Better for Travel Photography? Let’s Find Out

Table of Contents

When it comes to travel photography, lens choice leads the way to stunning imagery. Juggling between zoom lenses with their versatile focal lengths and prime lenses, known for their crisp quality and generous maximum aperture, is a traveler’s silent debate. So, are zooms or primes better for travel photography?

While zooms whisper promises of adaptability on the go, primes boast an unwavering sharpness that can make your usual subjects pop against a full frame canvas.

It’s not just about what you capture—it’s how you tell the story through your lens.

Versatility and Convenience of Zooms

Zoom lenses are a game-changer for travel photography, offering the flexibility to capture a wide range of subjects without lugging around extra gear. They’re like a Swiss Army knife for photographers, ready to tackle any scenario with ease.

Multiple Focal Lengths

Zooming in on the action is key. Quality zoom lenses let you snap wide landscapes and tight portraits without swapping glass. Imagine standing before the Eiffel Tower one moment, capturing its grandeur with a wide angle, then zooming in on a street performer’s expressions the next – all with a twist of your lens barrel.

Dynamic Subjects Ease

Photographers know: life doesn’t stand still. With a 10x zoom range at your fingertips, you’re prepared for life’s unpredictability. Kids playing in the park or bustling city streets can be framed perfectly in an instant. No fiddling with camera bags , just pure focus on getting that shot.

Quick Framing Adjustments

Unexpected shots are travel photography’s bread and butter. A quality zoom lens lets you adapt faster than saying “cheese.” Spot an eagle soaring while hiking? Your telephoto reach brings it close without scaring it away. That’s the power of quick framing adjustments – they turn good photos into great ones.

Less Gear to Carry

Travel light and move freely – that’s every wanderer’s mantra. Instead of weighing down your camera bag with multiple lenses, one versatile zoom can cover most needs. This means more room for souvenirs or simply less strain on your shoulders as you explore.

Adaptable Shooting Scenarios

A full frame sensor camera equipped with a large aperture zoom lens is unstoppable. Whether it’s dimly lit cathedrals or sunny beach scenes , you’re covered. Photographers love this adaptability because it means no matter where their travels take them, they’re always ready to shoot.

Swiftly switch from landscape to portrait
Capture distant objects without moving
One lens solution reduces weight
Sometimes larger than prime lenses
Might not have the bokeh effect of primes

Photography isn’t just about what we see; it’s about sharing how we feel in that moment. Many photographers testify that having fewer choices actually frees them up creatively—less time digging through their camera system means more time composing beautiful images that tell stories.

Take Sarah, a travel blogger who switched to using only her trusty 24-70mm zoom lens during her six-month trek across Asia:

She captured vibrant market scenes and serene temples.
Her backpack was lighter by ditching extra lenses.
She never missed spontaneous moments thanks to quick adjustments.

Her blog exploded with comments praising the image quality and diversity she achieved with just one lens!

Quality and Portability of Primes

Primes shine with their superior optical quality, especially in dim lighting. They’re also compact, offering a lighter load for the travel photographer.

Superior Low Light Performance

Prime lenses are the go-to when you need to capture crisp shots as the sun dips. Their design allows more light to hit the sensor, making them ideal for those twilight cityscapes or starry night skies.

Perform exceptionally well in low-light environments.
Capture clear images without cranking up ISO too high.

Compact and Travel-Friendly

The beauty of primes lies in their simplicity. A single focal length means less glass and mechanics inside, leading to a smaller size that’s perfect for on-the-go photography.

Easier to pack than bulky zoom lenses.
Reduce strain on your shoulders during long explorations.

Sharper Focused Details

A prime lens is like a specialist; it knows its focal length and nails it every time. You’ll get pictures where every little detail pops, from the texture on an old building to the twinkle in a street performer’s eye.

Outperform zooms at their specific focal lengths.
Deliver edge-to-edge sharpness that can be lost with zooms.

Fast Aperture Mastery

With primes, you can play with shallow depth-of-field like a pro. This means you can make your subject stand out dramatically against a soft, blurred background—a storytelling tool that’s as powerful as words.

Offer wide apertures like f/1.4 or f/2.
Create beautiful bokeh that adds artistic flair to your photos.

Lightweight Equals Comfort

On long treks through new cities or hikes up mountains for that perfect sunrise shot, every ounce matters. Prime lenses won’t weigh you down like some heavy-duty zooms can.

Make walking around with your camera less tiring.
Allow for more flexibility and spontaneity in your travels.

Factors Affecting Lens Choice

Choosing the right lens for travel photography can be a puzzle. Your destination’s vibe and your personal style are huge clues to solving it.

dslr camera with different lens for photography

Destination Scenes Matter

Travel spots have their own magic. Think bustling city streets or sweeping mountain vistas. Each place begs for a lens that can capture its essence. In crowded cities, wide-angle lenses rock because they let you grab more of the scene. For vast landscapes, zoom lenses are kings, letting you snag details from afar.

But here’s the kicker: some destinations throw curveballs with varied scenes. One minute you’re shooting urban art, next it’s a sunset over the ocean. That’s when versatility becomes your best buddy.

Personal Style Speaks Loud

Your style is like your photography fingerprint – totally unique. Some folks love getting up close and personal with subjects, while others prefer to hang back and take it all in.

If you dig capturing every wrinkle on an old man’s face, prime lenses with their killer sharpness are your jam.
But if you’re about catching life as it unfolds without fussing over gear, zooms give you flexibility without missing a beat.

The right lens feels like an extension of your eye – seeing the world just how you like it.

Budget Calls The Shots

Let’s talk money – it matters big time. High-quality glass comes with hefty price tags; we know that drill.

Prime lenses often come cheaper than zooms but remember: one prime might not cut it.
Zoom lenses cost more dough but think about this: one solid zoom could replace several primes.

It’s about finding that sweet spot where your wallet doesn’t cry but your photos still pop!

Activities Shape Choices

Photography isn’t just standing still and clicking away; it’s an adventure! Your activities dictate what lens hops into your bag.

Hiking up mountains? A lightweight prime won’t drag you down. Roaming cities from dawn till dusk? A versatile zoom lets you cover all bases without swapping lenses. The gist is simple: match your lens to what you plan on doing so nothing slows down your groove.

a man taking a photo using a dslr camera on top of the mountain

Luggage Limits Decision

Packing light isn’t just nice, it’s necessary. Airlines have baggage limits tighter than skinny jeans after Thanksgiving dinner!

Every ounce counts when traveling:

Primes are featherweights compared to most zooms.
But packing multiple primes vs one do-it-all zoom? That math needs weighing out carefully.

Space-saver or heavyweight champ? Your luggage has the final say in this bout.

Advantages of Zoom Lenses Explored

Zoom lenses offer unmatched versatility and a cost-effective solution for travel photography. They allow photographers to quickly adapt to various scenes without switching gear.

Flexibility in Composition

Imagine standing atop a hill, gazing at a breathtaking landscape. With a zoom lens, you can capture the grandeur of the scene and then zoom in on specific details like distant mountains or wildlife without moving an inch. This flexibility is priceless when you find yourself in spots where moving around isn’t an option.

Snap wide landscapes or tight portraits.
No need to switch positions.

Diverse Landscapes Captured

Travel photographers often face changing environments. One minute you’re shooting urban architecture; the next, it’s a serene beach sunset. A zoom lens allows you to seamlessly transition between these varied subjects, ensuring that no matter the setting, you’re prepared.

Cityscape vistas and nature shots with one tool.
Adapt to different lighting and scales easily.

Safety and Accessibility

Sometimes getting closer isn’t just hard – it’s downright risky or forbidden. Wildlife photography is a prime example where safety must come first. Here, zoom lenses are invaluable as they let photographers respect boundaries while still nailing those close-up shots of animals in their natural habitat.

Keep safe distance from wild animals.
Overcome physical barriers effortlessly.

Cost-Effective Choice

Purchasing multiple prime lenses can be heavy on your wallet. A single zoom lens can cover a range of focal lengths that would otherwise require several primes. For travelers who prioritize both budget and bag space, this makes zooms an economically smart pick.

Replace several primes with one zoom.
Save money for more travel adventures!

Quick Photographic Responses

Travel often presents fleeting moments that make for stunning photos – think street performers in action or the flash of a smile from a local market vendor. With a zoom lens, you’re always ready to capture these transient moments before they slip away because you don’t waste time swapping lenses.

Catch spontaneous events instantly.
Always ready for unexpected photo ops.

Prime Lens Disadvantages and Benefits

Prime lenses have their quirks, like not zooming and needing swaps. But they shine with creamy bokeh and crisp images, pushing photographers to move around for the perfect shot.

Nikon DSLR camera on top of the table with bokeh effect in the background

Zoom Limitations

Prime lenses keep it simple; no zoom here. You’ve got your feet for that—literally. Want a closer look? You’ll need to scoot up to your subject or back away for a wider view. This can be a bit of a bummer when you’re trying to capture that perfect travel snapshot, and there’s just no room to back up. Or maybe there’s an epic scene unfolding right before you, but stepping forward isn’t an option because, well… cliff edge.

Changing Lenses

Got dust? Swapping prime lenses might invite unwanted specks onto your camera’s sensor , especially if you’re out in the wild or dusty streets during your travels. It’s like opening the door on a windy day; stuff just flies in uninvited! Each lens change is another roll of the dice, hoping your camera’s insides stay clean.

Wider Apertures

Now let’s talk about those dreamy backgrounds. Prime lenses are ace at making subjects pop with their wide apertures—hello beautiful bokeh! That blurred-out background isn’t just pretty; it makes whatever or whoever you’re photographing stand out big time. Whether it’s a street performer’s face or the delicate petals of a flower by the roadside, prime lenses give photos that professional touch.

Examples of bokeh in travel photography:
A street vendor’s cart in focus with bustling city life softened behind.
A single lantern glowing at night with everything else melting into shadows.

High Resolution Images

Less glass equals more clarity—it’s as simple as that with prime lenses. Fewer elements inside mean less chance for light distortion and sharper details. When you’re capturing the intricate textures of ancient architecture or vibrant market scenes abroad, every pixel counts!

Case study: Photographers often report noticing finer details in their shots when using primes compared to zooms.

Creative Composition

Prime lenses don’t let you get lazy—you have to hustle for that shot! They make you think outside the box and work those leg muscles to find fresh angles and perspectives. It’s all about positioning yourself rather than relying on a lens to do the heavy lifting.

Fun challenge: Try shooting only with a prime lens on your next trip; it could transform how you see and capture the world around you!

Guidance for Beginners on Lens Selection

Choosing the right lens for travel photography can be a game-changer. It’s crucial to consider what you’ll be shooting and your specific needs before making a decision.

Assess Photography Subjects

Before picking between zoom or prime lenses, think about what catches your eye. Are you into snapping bustling cityscapes or tranquil landscapes? Maybe capturing the local wildlife or street scenes gets you excited. Your subjects matter because they dictate the kind of lens that will serve you best.

Start with Zooms

If you’re just getting into travel photography and aren’t quite sure what tickles your fancy, start with a versatile zoom lens. They’re like the Swiss Army knife of lenses – good at a bit of everything.

Flexibility in composing shots
Less need to switch lenses
Can be bulkier than primes
Might not offer the sharpest image quality

Low-Light Priority?

Ask yourself if you often find yourself chasing sunsets or exploring cities by night. If dimly lit scenes are your jam, then low-light performance is key. Lenses with wider apertures (smaller f-number) let in more light, which is something prime lenses are known for.

Prime lens: Excellent for night photography due to wide apertures like f/1.4.
Zoom lens: May struggle unless it’s a high-end model with stable aperture ranges.

Ease vs Quality

There’s always a tug-of-war between convenience and quality. Zoom lenses mean less fussing around but might compromise sharpness and speed. Primes challenge you to move around more but reward you with crisper images.

Ease of Use:

Zooms win for their all-in-one solution.

Image Quality:

Primes often provide superior clarity and detail.

Long-Term Investment

Think long-term when choosing your gear; it’s an investment after all! A sturdy, high-quality lens can last years and adapt to different cameras—especially if you’re using a crop sensor now but plan to upgrade later.

Considerations:

Resale value
Compatibility with future camera upgrades

Crop Sensor Note: Lenses behave differently on crop sensors—your future full-frame dreams could affect today’s choices!

Immediate Needs vs Future Plans

Weigh up what matters most right now against where your photo journey might take you down the road:

For beginners:

A zoom could cover various situations as skills develop.
A prime might encourage mastering composition early on.

Making the Best Travel Choice

Your camera bag is a treasure chest, and the right lens is your golden ticket to capturing those breathtaking travel shots . Whether you’re trekking through bustling city streets or scaling serene mountain peaks, remember that the best gear is what gets the job done for you .

Zoom lenses offer a Swiss Army knife-like versatility, letting you switch from wide-angle to telephoto without missing a beat. On the flip side, prime lenses are like your trusty sword—sharp, fast, and lightweight—perfect for nailing those high-quality shots with character.

So what’s it gonna be? What’s your opinion on the question, “are zooms or primes better for travel photography?” Will you go for the all-in-one zoom to cover all bases or pick a prime to master one focal length at a time? Whichever path you choose, embrace it with confidence.

Photography is an adventure in itself; enjoy every step and click along the way.

And hey, if you ever feel stuck choosing your next lens companion, reach out to fellow shutterbugs or trusted experts—they’ll be stoked to help you make a snap decision!

When selecting between zoom and prime lenses for travel photography, consider factors such as weight and portability, image quality preferences (sharpness and low-light performance), versatility in different shooting scenarios (landscapes vs portraits), budget constraints, and personal style of shooting. Think about what subjects interest you most during travel.

Absolutely! Modern zoom lenses are engineered with advanced optics that can produce professional-quality images suitable for various types of photography. The key is understanding your lens’s strengths and optimizing its use within its best aperture ranges.

Not necessarily. Prime lenses can range from affordable to high-end prices depending on their maximum aperture size and build quality. Some primes are very cost-effective yet offer excellent image quality.

Lens speed refers to the maximum aperture of a lens—the larger it is (indicated by smaller f-numbers), the faster it can capture light. This becomes crucial in low-light conditions or when aiming for shallow depth-of-field effects. For travel photography where lighting conditions vary widely, having a fast lens can be quite beneficial.

This depends on your priorities: If minimizing gear weight while maintaining high image quality is essential, then carrying multiple primes might suit you better. However, if convenience and flexibility are paramount because of diverse shooting situations encountered while traveling, then one versatile zoom may be more practical.

Focal length determines how much of the scene will fit into your frame—it affects perspective and composition significantly. Wider focal lengths (lower numbers) capture more of the scene making them ideal for landscapes; longer focal lengths (higher numbers) bring distant subjects closer which can be great for isolating details or photographing wildlife.

I’m a professional travel photographer, and I’ve been living the digital nomad lifestyle since 2016. I make money by working on client assignments, selling stock photography and helping other photographers by sharing my experiences on this website. I move around at my own pace (I hate fast-paced travel) and like to spend a few months getting to know each place I base myself in.

My writing and photos have been featured on industry leading websites such as Digital Photography School , Atlas Obscura and the world’s leading underwater photography resource The Underwater Photography Guide . I authored an eBook called “ Breaking Into Travel Photography: The complete guide to carving out a career in travel photography ” that has been published on Amazon. My stock images have also appeared in ads promoting destinations and companies that sometimes has been a surprise, even to me. But I guess that’s the nature of stock photography, you never know who will license them!

I’m always happy to connect, so feel free to reach out!

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Could Trump Go to Prison? If He Does, the Secret Service Goes, Too

Officials have had preliminary discussions about how to protect the former president in the unlikely event that he is jailed for contempt during the trial.

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Several men and women wearing dark suits standing around an airport tarmac.

By William K. Rashbaum

The U.S. Secret Service is in the business of protecting the president, whether he’s inside the Oval Office or visiting a foreign war zone.

But protecting a former president in prison? The prospect is unprecedented. That would be the challenge if Donald J. Trump — whom the agency is required by law to protect around the clock — is convicted at his criminal trial in Manhattan and sentenced to serve time.

Even before the trial’s opening statements, the Secret Service was in some measure planning for the extraordinary possibility of a former president behind bars. Prosecutors had asked the judge in the case to remind Mr. Trump that attacks on witnesses and jurors could land him in jail even before a verdict is rendered.

(The judge, who held a hearing Tuesday morning to determine whether Mr. Trump should be held in contempt for violating a gag order, is far more likely to issue a warning or impose a fine before taking the extreme step of jailing the 77-year-old former president. It was not immediately clear when he would issue his ruling.)

Last week, as a result of the prosecution’s request, officials with federal, state and city agencies had an impromptu meeting about how to handle the situation, according to two people with knowledge of the matter.

That behind-the-scenes conversation — involving officials from the Secret Service and other relevant law enforcement agencies — focused only on how to move and protect Mr. Trump if the judge were to order him briefly jailed for contempt in a courthouse holding cell, the people said.

The far more substantial challenge — how to safely incarcerate a former president if the jury convicts him and the judge sentences him to prison rather than home confinement or probation — has yet to be addressed directly, according to some of a dozen current and former city, state and federal officials interviewed for this article.

That’s at least in part because if Mr. Trump is ultimately convicted, a drawn-out and hard-fought series of appeals, possibly all the way up to the U.S. Supreme Court, is almost a certainty. That would most likely delay any sentence for months if not longer, said several of the people, who noted that a prison sentence was unlikely.

But the daunting challenge remains. And not just for Secret Service and prison officials, who would face the logistical nightmare of safely incarcerating Mr. Trump, who is also the presumptive Republican nominee for President.

“Obviously, it’s uncharted territory,” said Martin F. Horn, who has worked at the highest levels of New York’s and Pennsylvania’s state prison agencies and served as commissioner of New York City’s correction and probation departments. “Certainly no state prison system has had to deal with this before, and no federal prison has had to either.”

Steven Cheung, the communications director for Mr. Trump’s campaign, said the case against the former president was “so spurious and so weak” that other prosecutors had refused to bring it, and called it “an unprecedented partisan witch hunt.”

“That the Democrat fever dream of incarcerating the nominee of the Republican Party has reached this level exposes their Stalinist roots and displays their utter contempt for American democracy,” he said.

Protecting Mr. Trump in a prison environment would involve keeping him separate from other inmates, as well as screening his food and other personal items, officials said. If he were to be imprisoned, a detail of agents would work 24 hours a day, seven days a week, rotating in and out of the facility, several officials said. While firearms are obviously strictly prohibited in prisons, the agents would nonetheless be armed.

Former corrections officials said there were several New York state prisons and city jails that have been closed or partly closed, leaving wings or large sections of their facilities empty and available. One of those buildings could serve to incarcerate the former president and accommodate his Secret Service protective detail

Anthony Guglielmi, the spokesman for the Secret Service in Washington, declined in a statement to discuss specific “protective operations.” But he said that federal law requires Secret Service agents to protect former presidents, adding that they use state-of-the-art technology, intelligence and tactics to do so.

Thomas J. Mailey, a spokesman for New York State’s prison agency, said his department couldn’t speculate about how it would treat someone who has not yet been sentenced, but that it has a system “to assess and provide for individuals’ medical, mental health and security needs.” Frank Dwyer, a spokesman for the New York City jails agency, said only that “the department would find appropriate housing” for the former president.

The trial in Manhattan, one of four criminal cases pending against Mr. Trump and possibly the only one that will go to a jury before the election, centers on accusations he falsified records to cover up a sex scandal involving a porn star. The former president is charged with 34 counts of felony falsifying business records. If convicted, the judge in the case, Juan M. Merchan, could sentence him to punishments ranging from probation to four years in state prison, though for a first-time offender of Mr. Trump’s age, such a term would be extreme.

If Mr. Trump is convicted, but elected president again, he could not pardon himself because the prosecution was brought by New York State.

Under normal circumstances, any sentence of one year or less, colloquially known as “city time,” would generally be served on New York City’s notorious Rikers Island, home to the Department of Correction’s seven jails. (That’s where Mr. Trump’s former chief financial officer, Allen H. Weisselberg, 76, is currently serving his second five-month sentence for crimes related to his work for his former boss.)

Any sentence of more than a year, known as state time, would generally be served in one of the 44 prisons run by New York State’s Department of Corrections and Community Supervision.

The former president could also be sentenced to a term of probation, raising the bizarre possibility of the former commander in chief reporting regularly to a civil servant at the city’s Probation Department.

He would have to follow the probation officer’s instructions and answer questions about his work and personal life until the term of probation ended. He would also be barred from associating with disreputable people, and if he committed any additional crimes, he could be jailed immediately.

Maggie Haberman contributed reporting.

William K. Rashbaum is a Times reporter covering municipal and political corruption, the courts and broader law enforcement topics in New York. More about William K. Rashbaum

Our Coverage of the Trump Hush-Money Trial

News and Analysis

The criminal trial of Trump featured vivid testimony about a plot to protect his first presidential campaign and the beginnings of a tough cross-examination of the prosecution’s initial witness, David Pecker , former publisher of The National Enquirer. Here are the takeaways .

Dozens of protesters calling for the justice system to punish Trump briefly blocked traffic on several streets near the Lower Manhattan courthouse where he is facing his first criminal trial.

Prosecutors accused Trump of violating a gag order four additional times , saying that he continues to defy the judge’s directions not to attack witnesses , prosecutors and jurors in his hush-money trial.

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