has time travel been invented

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!

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).

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.

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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Can we time travel? A theoretical physicist provides some answers

Emeritus professor, Physics, Carleton University

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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.

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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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.

A history of time travel: the how, the why and the when of turning back the clock

Pop on Aqua's 'Turn Back Time' and settle in

For most of human history, the world didn’t change very quickly. Until the 1700s, kids could largely expect their lives to be similar to their parents, and that their children would have an experience very similar to their own, too. There were obviously changes in how humans lived over longer stretches of time, but nothing that even different generations could easily observe.

My first introduction to science fiction was Valérian and Laureline. I was ten years old. Every Wednesday there was a magazine called Pilote in France, and there was two pages of Valerian every week. It was the first time I’d seen a girl and a guy in space, agents travelling in time and space. That was amazing.

The past is written. The present? We have to deal with it. But the future is a white page. So I don’t understand why people on this white page are putting all this darkness.

God! Let’s have some color! Let’s have some fun! Let’s at least imagine a better world. Maybe we won’t be able to do it, but we have to try.

The industrial revolution changed all of this. For the first time in human history, the pace of technological change was visible within a human lifespan. 

It is not a coincidence that it was only after science and technological change became a normal part of the human experience, that time travel became something we dreamed of.

Time travel is actually somewhat unique in science fiction. Many core concepts have their origins earlier in history. 

The historical roots of the concept of a 'robot' can be seen in Jewish folklore for example: Golems were anthropomorphic beings sculpted from clay. In Greek mythology, characters would travel to other worlds, and it's no coincidence that The Matrix features a character called Persephone. But time travel is different.

The first real work to envisage travelling in time was The Time Machine by HG Wells, which was published in 1895. 

The book tells the story of a scientist who builds a machine that will take him to the year 802,701 - a world in which ape-like Morlocks are evolutionary descendants of humanity, and have regressed to a primitive lifestyle. 

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The book was a product of its time - both in terms of the science played upon (Charles Darwin had only published Origin of the Species 35 years earlier), and the racist attitudes: it is speculated that the Morlocks were inspired by the Morlachs, a real ethnic group in the Balkans who were often characterised as “primitive”.

Real science

But of course, this was science fiction - what about science fact? The two have always been closely linked, and during the early days it was no different. In 1907, the physicist Hermann Minkowski first argued that Einstein’s Special Relativity could be expressed in geometric terms as a fourth dimension (to add to our known three) - which is exactly how Wells visualised time travel in his work of fiction.

The development of Special and then General Relativity was significant as it provided the theoretical backbone for how time travel could be conceived in scientific terms. In 1949 Kurt Gödel took Einstein’s work and came up with a solution which as a mathematical necessity included what he called “closed timelike curves” - the idea that if you travel far enough, time will loop back around (like how if you keep flying East, you’ll eventually end up back where you started).

In other words, using what became known as the Gödel Metric, it is theoretically possible to travel between any one point in time and space and any other. 

There was just one problem: for Gödel’s theory to be right, the universe would have to be spinning - and scientists don’t believe that it is. So while the maths might make sense, Gödel’s universe does not appear to be the one we’re actually living in. Though he never gave up hope that he might be right: Apparently even on this deathbed, he would ask if anyone has found evidence of a spinning universe. And if he does ever turn out to be right, it means that time travel can happen, and is actually fairly straightforward (well, as far as physics goes anyway).

Since Gödel, scientists have continued to hypothesise about time travel, with perhaps the best known example being tachyons - or particles that move faster than the speed of light (therefore, effectively travelling in time). So far, despite one false alarm at CERN in 2011, there is no evidence that they actually exist.

Chancers and hoaxes

Of course, the lack of real science when it comes to time travel has not stopped some people from claiming to have done it. With the likes of Marty McFly and Doctor Who on the brain, chancers and hoaxers have realised that time travel is immediately a compelling prospect. Here’s a couple of amusing examples.

At the turn of the millennium, when the internet was still in its infancy, forums were captivated by the story of John Titor. Titor claimed he was from the year 2036, and had been sent back in time by the government to obtain an IBM 5100 computer. The thinking appeared to be that by obtaining the computer, the government could find a solution to the UNIX 2038 bug - in which clocks could be reset, Millennium Bug-style, leading to chaos everywhere.

Posting on the 'Time Travel Institute' forums, Titor went into details on how his time machine worked:  It was powered by “two top-spin, dual positive singularities”, and used an X-ray venting system. He also gave a potted history of what humanity could expect: A new American civil war in 2004, and World War III in 2015. He also claimed the “many worlds” interpretation of quantum physics was true, hence why he wasn’t violating the so-called “grandfather paradox”.

Titor claimed he was from the year 2036, and had been sent back in time by the government to obtain an IBM 5100 computer.

Okay, so he probably wasn’t a real time traveller, but in the early days of the internet, when anonymity was more commonplace, he truly captured the imaginations of nerdy early adopters who perhaps, just a little bit, hoped that he might be the real thing.

More recently, in 2013, an Iranian scientist named Ali Razeghi claimed to have invented a time machine of sorts. It was supposedly capable of predicting the next 5-8 years for an individual, with up to 98% accuracy. According to The Telegraph , Razeghi said the invention fits into the size of a standard PC case and “It will not take you into the future, it will bring the future to you”. The idea is that the Iranian government could use it to predict future security threats and military confrontations. So perhaps it is time to check in and see if he managed to predict Donald Trump?

So is this the best we can do? Will we ever manage to crack time travel? Some scientists are still sceptical that it could ever be possible. This includes Stephen Hawking, who proposed the 'Chronology Protection Conjecture' – which is what it sounds like. Essentially, he argues that the laws of physics are as they are to specifically make time travel impossible – on all but “submicroscopic” scales. Essentially, this is to protect how causality works, as if we are suddenly allowed to travel back and kill our grandfathers, it would create massive time paradoxes.

Hawking revealed to Ars Technica in 2012 how he had held a party for time travellers, but only sent out invitations after the date it was held. So did the party support his argument that time travel is impossible? Or did he end up spending the evening in the company of John Titor and Doctor Who?

“I sat there a long time, but no one came”, he said, much to our disappointment.

Huge thanks to Stephen Jorgenson-Murray for walking us through some of the more brain-mangling science for this article.

To celebrate the release of Valerian and the City of a Thousand Planets , Luc Besson is today behind the lens at TechRadar. Here’s what we’ve got in store for you:

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Valerian and the City of a Thousand Planets is released in UK cinemas August 2nd, and is out now in the US.

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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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Exploring the Reality of Time Travel: Science Fact vs. Science Fiction

By Adi Foord, University of Maryland, Baltimore County November 16, 2023

Time travel, a longstanding fascination in science fiction, remains a complex and unresolved concept in science. The second law of thermodynamics suggests time can only move forward, while Einstein’s theory of relativity shows time’s relativity to speed. Theoretical ideas like wormholes offer potential methods, but practical challenges and paradoxes, such as the “grandfather paradox,” complicate the feasibility of actual time travel.

Will it ever be possible for time travel to occur?

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 Science Behind Time Travel

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.

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 .

Theoretical Possibilities and Challenges

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.

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.”

Artist’s rendering of the James Webb Space Telescope. Credit: Northrop Grumman

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.

Written by Adi Foord, Assistant Professor of Astronomy and Astrophysics, University of Maryland, Baltimore County.

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8 comments on "exploring the reality of time travel: science fact vs. science fiction".

Until the problem of the second law of thermodynamics(entropy) is solved, the concept of time travel will remain the subject of science fiction. Since this is a basic law of our universe, there is no conceivable way that we know of to do this. The great thing about our knowledge of the universe is that it continues to grow and with that our view of what is possible continues to change. After all, at one time it was believed we could never leave earth!

The 7 planets are soul pollen in the space @ life has been the world has been prepared @ The pollen of the universe is hidden @ Around the axis of the galaxy, the universe is hidden@@ The pollen of the galaxies is a hidden cluster Itis made that we thought about what wisdom is in God’s work and how it is arranged in the form of words.The verse that is made is as follows @@ John, you are in time @ worlds, planets around the axis of the branches of galaxies @@ 8 Prophets, divine prophets, God-aware witnesses @ God’s words of revelation, they are aware @@ 48 What the words of revelation that every prophet has about @ sometimes Sometimes the message of God has become a verbal cliché in the head @@ 62 The message of God was given to every prophet @ The message was made around the power of God @@ 46 The truth of the religions of the cradle of time is said in the world @ Prophets always came for justice in time @@ 62 A warrior became brave in time @ Delaver Ghahrmani Boud Taarani@@ 43 Omar Noah never died, who knows!@ Imransan Is there an unseen world, immortality!?@@ 49 Men’s rights in the sign @ Human rights, the observance of world justice @@ 40 We have God’s love @ A love that is not patched, separation!!!: @@ 39 Take one word from the end of the first and second stanzas to the bottom of these eight verbal verses, and this sentence is made @@ God-aware, the world is born, you know the sign of God @ Agah,the beginning of time, the world, eternity, the world of separation @@ The meaning of this sentence is That God, who before us humans lived on the earth, formed the earth’s crusts with full knowledge, and we humans know the sign of God, which is on the earth, on the continents and countries, and some names have been shown by God for our knowledge since the end of time.In a later video, if I have a lifetime left, I will explain exactly about these poems, God willing @@ The word of the Prophet 17 is the 17th and Muhammad (PBUH) said that the Bedouin Arabs should pray 17 rakats so that theyperform ablution and be clean and not kill and loot.From the caravans, these were all God’s will, and he is good everywhere, in every nation, God does not like evildoers, sinners, and oppressors.Muhammad (PBUH) was God’s last messenger to the Arab people, he was God’s best prophet, and the third verse is because they do not accept Muhammad (PBUH).Some Iranians who were in contact with me, that’s why in the third verse of this surah, he said that his message was given to every prophet, the message is based on divine power, and the word truth, the first two letters of which istruth, is truth, and truth is the 43rd and forty-third word, and the word is time.It is exactly 46, and this song of the Prophets was revealed at the age of 46 ببخشید اگر قبلآ مطالبی فرستادم که به مذاهب مذاهم ارتباط داد شاید به درد شما نمیخورد من در کامنت های بعدی سعی میکنم از نجوم و حیات زمین سخن بگویم این چند بیت را به خارجی انگلیسی که تبدیل کردم معنی آن حیرت انگیز تعغیر کرد گفتم برای شما بفرستم و بداند که این کلام من نبوده کلام خداوند بوده شما نظرتان در مورد خداوند چیست من میگویم خداوند که پدر ومادر انسان بودند قادر به ساختن ما بودند اما آیا آنها قادر به ساختن ستاره ها هم بودند

Your comment has validity to God. But it surely has no place here, it is only fare if the hole comment were in english and has less of a convincing push in a belief a person either believes or not.

It’s too bad physics can’t come to a complete consensus about time, I would like to add some thought about discoveries it has been proven that time travels in only one direction forward, the experiment dealt with light thru glass and how it reacts in the middle and what change happens after light exits the other side, a simple explanation of this experiment. Brings me to theorize and start that time existed before the big bang and is outside of our universes influence, when time is acted on by gravity the ( Form ) of time is changed until the influence no longer has effect, this could go hand in hand with light photons the photon has a influence in the Form that time has. This can not be a observed difference unless we were to see beyond the speed of light. We do agree that physics changes at a subatomic state and also does some strange changing once the speed of light has been exceeded.

The experiment I referred to was posted on IFL in October 2023 headlined ( Solution to complex light problem shows that time can only go Forward ).

One of the problem with travel time is the one people keep forever. And, that is that the earth is always move through space. Matter of fact, the earth is not in the same place that it was 50 years ago. So you will have to move through space as well as time.

Ironically, the only science fiction that seems to handle this is the original story “The Time Machine”.

Time travel is happening now. It has been done since the 1950s. The method satisfies all the requirements. Traveler can’t change the past, but only observe. You can’t go farther back than when the machine was first invented (1950s). There’s one more limitation, you can only observe what the machine was directed. The time machine, the common video camera, and video tape recorder. Now it’s the camera, and file capture computer. Yes, viewing a video tape is effectively going back in time. It’s more than the video, it’s the sound too. There are working versions of smell, and touch which can be recorded too. If you record, and replicate all the senses, you have effectively complete time travel. The most primitive form is the picture. This technology has been around for thousands of years, and is manually intensive. Later many pictures were strung together to make a film. Using a camera to record film was the first example of complete visual time travel (back to when the film was made). Later sound was added, and we have the movies. A way of going back in time that included sight, and sound. Now we have video systems (YouTub) that can play back past events selected by you. Yes, video systems are virtual time travel machines we have now.

How Stellar Cannibalism Illuminates Cosmic Evolution

جزایر فیلیپین دایناسوری که توسط انسان خلق شده در بیش ده ها میلیون سال و بخاطر ریخته شدن دوهزار متر خاک غرق شده بخاطر بالا آمدن آب اقیانوس اما جزایر فیلیپین شبیه دایناسوریست

The address of the above comment on the site about a thief who was trained by a dinosaur bird who was trained by humans tens of millions of years ago and who arrested murderers and robbers.The police were arrested by big birds.don’t the scientists of the world think about this?They were buried in the bed of important cities, they were buried with all the tools and machines, the traces of humans tens of millions of years ago, they had a civilization and a history of hundreds of thousands of years, they built a base underthe earth, from the meteorites that explode from the planets when they hit the sun, and they knew that several thousand meters of soil is poured on the surface of the seas and islands, and they knew that the shapes they made of the islands in thecountry of Papua may go under the ocean, of course, the Philippine islands.The picture is of a baby dinosaur that went under the ocean, but the northern island of Australia, which is Papua, is quite clear.It is a big dinosaur whose tail is towards the east and its mouth is open towards the west.There is the Philippines, but it was more difficult to take the soil to the Philippine islands to create a dinosaur than the island of Papua, that is why the height of the soil in the Philippines is lower, and when the meteorites fell a few kilometerson the surface of the ocean, the image created by humans under the ocean in the shape of a dinosaur is hidden in the American continent The picture is of a bird in the shape of a dinosaur that is flying, and this bird was made to flyby the Indians of the tribe, that bird was talking to people, but its spirit might have heard something from the police because a thief while in the bird’s mouth He was handcuffed by the police and the weapon, which is a machine gun, is fromthe east of the American continent on the coast

The country of Florida is a machine gun.When you continue to New York City, you will reach the mouth of the dinosaur, where a thief is trapped in the mouth of a bird, and the little finger of the police handcuffed the thief’s hand, and a small colt is in the hand ofthe thief, who the police caught in the mouth of the dinosaur.put in the mouth of the bird dinosaur, you can clearly see that the thief fell on the ground and was shot in the head, and it is clear that his forehead was pierced, the bird’s mouth is open in flight, the head of the birdis from the east of the American continent, and a fish is placed in the bird’s mouth in the water of the ocean.The stretched glove of the police, which is in the form of a fist, with a handcuff attached to the left hand of the thief who fell on the ground in the sea and the mouth of the bird, the head of the thief and the killer, is located towards the southwest and west coast of America.All these images were created from the American continent and islands by Humans were created, but they didn’t have enough fuel and time to create more accurate images and meteorites ruined the beauty of the images, but it is clear that all these changes were createdby humans, but you have to consider that two thousand meters of soil from meteorites are fish.And they buried the whales under the beaches, and after a very long time, the bodies of the whales turned into oil under the two thousand meters of soil on the beaches, and on the other hand, the presence of two thousand meters of soil onthe surface of the seas and droughts could not make the created images disappear.

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Where Does the Concept of Time Travel Come From?

Time; he's waiting in the wings.

The dream of traveling through time is both ancient and universal. But where did humanity's fascination with time travel begin, and why is the idea so appealing?

The concept of time travel — moving through time the way we move through three-dimensional space — may in fact be hardwired into our perception of time . Linguists have recognized that we are essentially incapable of talking about temporal matters without referencing spatial ones. "In language — any language — no two domains are more intimately linked than space and time," wrote Israeli linguist Guy Deutscher in his 2005 book "The Unfolding of Language." "Even if we are not always aware of it, we invariably speak of time in terms of space, and this reflects the fact that we think of time in terms of space."

Deutscher reminds us that when we plan to meet a friend "around" lunchtime, we are using a metaphor, since lunchtime doesn't have any physical sides. He similarly points out that time can not literally be "long" or "short" like a stick, nor "pass" like a train, or even go "forward" or "backward" any more than it goes sideways, diagonal or down.

Related: Why Does Time Fly When You're Having Fun?

Perhaps because of this connection between space and time, the possibility that time can be experienced in different ways and traveled through has surprisingly early roots. One of the first known examples of time travel appears in the Mahabharata, an ancient Sanskrit epic poem compiled around 400 B.C., Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science 

In the Mahabharata is a story about King Kakudmi, who lived millions of years ago and sought a suitable husband for his beautiful and accomplished daughter, Revati. The two travel to the home of the creator god Brahma to ask for advice. But while in Brahma's plane of existence, they must wait as the god listens to a 20-minute song, after which Brahma explains that time moves differently in the heavens than on Earth. It turned out that "27 chatur-yugas" had passed, or more than 116 million years, according to an online summary , and so everyone Kakudmi and Revati had ever known, including family members and potential suitors, was dead. After this shock, the story closes on a somewhat happy ending in that Revati is betrothed to Balarama, twin brother of the deity Krishna. 

Time is fleeting

To Yaszek, the tale provides an example of what we now call time dilation , in which different observers measure different lengths of time based on their relative frames of reference, a part of Einstein's theory of relativity.

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Such time-slip stories are widespread throughout the world, Yaszek said, citing a Middle Eastern tale from the first century BCE about a Jewish miracle worker who sleeps beneath a newly-planted carob tree and wakes up 70 years later to find it has now matured and borne fruit (carob trees are notorious for how long they take to produce their first harvest). Another instance can be found in an eighth-century Japanese fable about a fisherman named Urashima Tarō who travels to an undersea palace and falls in love with a princess. Tarō finds that, when he returns home, 100 years have passed, according to a translation of the tale published online by the University of South Florida . 

In the early-modern era of the 1700 and 1800s, the sleep-story version of time travel grew more popular, Yaszek said. Examples include the classic tale of Rip Van Winkle, as well as books like Edward Belamy's utopian 1888 novel "Looking Backwards," in which a man wakes up in the year 2000, and the H.G. Wells 1899 novel "The Sleeper Awakes," about a man who slumbers for centuries and wakes to a completely transformed London. 

Related: Science Fiction or Fact: Is Time Travel Possible ?

In other stories from this period, people also start to be able to move backward in time. In Mark Twain’s 1889 satire "A Connecticut Yankee in King Arthur's Court," a blow to the head propels an engineer back to the reign of the legendary British monarch. Objects that can send someone through time begin to appear as well, mainly clocks, such as in Edward Page Mitchell's 1881 story "The Clock that Went Backwards" or Lewis Carrol's 1889 children's fantasy "Sylvie and Bruno," where the characters possess a watch that is a type of time machine . 

The explosion of such stories during this era might come from the fact that people were "beginning to standardize time, and orient themselves to clocks more frequently," Yaszek said. 

Time after time

Wells provided one of the most enduring time-travel plots in his 1895 novella "The Time Machine," which included the innovation of a craft that can move forward and backward through long spans of time. "This is when we’re getting steam engines and trains and the first automobiles," Yaszek said. "I think it’s no surprise that Wells suddenly thinks: 'Hey, maybe we can use a vehicle to travel through time.'"

Because it is such a rich visual icon, many beloved time-travel stories written after this have included a striking time machine, Yaszek said, referencing The Doctor's blue police box — the TARDIS — in the long-running BBC series "Doctor Who," and "Back to the Future"'s silver luxury speedster, the DeLorean . 

More recently, time travel has been used to examine our relationship with the past, Yaszek said, in particular in pieces written by women and people of color. Octavia Butler's 1979 novel "Kindred" about a modern woman who visits her pre-Civil-War ancestors is "a marvelous story that really asks us to rethink black and white relations through history," she said. And a contemporary web series called " Send Me " involves an African-American psychic who can guide people back to antebellum times and witness slavery. 

"I'm really excited about stories like that," Yaszek said. "They help us re-see history from new perspectives."

Time travel has found a home in a wide variety of genres and media, including comedies such as "Groundhog Day" and "Bill and Ted's Excellent Adventure" as well as video games like Nintendo's "The Legend of Zelda: Majora's Mask" and the indie game "Braid." 

Yaszek suggested that this malleability and ubiquity speaks to time travel tales' ability to offer an escape from our normal reality. "They let us imagine that we can break free from the grip of linear time," she said. "And somehow get a new perspective on the human experience, either our own or humanity as a whole, and I think that feels so exciting to us." 

That modern people are often drawn to time-machine stories in particular might reflect the fact that we live in a technological world, she added. Yet time travel's appeal certainly has deeper roots, interwoven into the very fabric of our language and appearing in some of our earliest imaginings. 

"I think it's a way to make sense of the otherwise intangible and inexplicable, because it's hard to grasp time," Yaszek said. "But this is one of the final frontiers, the frontier of time, of life and death. And we're all moving forward, we're all traveling through time."

• If There Were a Time Warp, How Would Physicists Find It?
• Can Animals Tell Time?
• Why Does Time Sometimes Fly When You're NOT Having Fun?

Originally published on Live Science .

Adam Mann is a freelance journalist with over a decade of experience, specializing in astronomy and physics stories. He has a bachelor's degree in astrophysics from UC Berkeley. His work has appeared in the New Yorker, New York Times, National Geographic, Wall Street Journal, Wired, Nature, Science, and many other places. He lives in Oakland, California, where he enjoys riding his bike. 

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Actor Rod Taylor tries to fast forward in the 1960 film The Time Machine .

Iranian Scientist Claims to Have Built "Time Machine"

Ali Razeqi says his time machine uses "complex algorithms" to see the future.

It's not quite Back to the Future, but a young Iranian inventor claims to have built a time machine that can predict a person's future with startling accuracy.

Ali Razeqi, who is 27 and the "managing director of Iran's Center for Strategic Inventions," claims his device will print out a report detailing an individual's future after using complex algorithms to predict his or her fate.

According to the Daily Telegraph , Razeqi told Iran's state-run Fars news agency that his device "easily fits into the size of a personal computer case and can predict details of the next 5-8 years of the life of its users. It will not take you into the future, it will bring the future to you."

Razeqi says Iran has decided to keep his prophetic time machine under wraps for now out of fear that "the Chinese will steal the idea and produce it in millions overnight."

FREE BONUS ISSUE

Iran's Deputy Minister of Science, Research, and Technology dismissed Razeqi's claims on Friday in an interview with Fars —a sign of just how much attention the story has received.

We talked to Thomas Roman, a theoretical physicist at Central Connecticut State University and a co-author of the book Time Travel and Warp Drives , to ask about the possibilities for a Razeqi-like time machine and to debunk popular misconceptions about time travel. Here's an edited version of our interview:

What do you think of Razeqi's claim that he's built a time machine that can predict a person's future?

It's completely nuts.

Does his alleged time machine break any laws of physics?

It's hard to know because it's so wacky.

What are some popular misconceptions about time travel?

One popular misconception is that you could go back to any time in the past. And that's not true. You can only go back as far as the time when the time machine was invented. So if I invent my time machine today and I wait 30 years and go back to the past, the farthest back in the past I can go to is today when I turned my time machine on.

Another major misconception—and you see this a lot in time travel movies—is the idea that you can go back in time and change the timeline. In these stories, the time traveler goes backward in time and does something that mucks up the future and subsequently has to do something to "restore the timeline." However, that can't be the case, since we can't have the same event both happen and not happen in the same universe. You can't change the past.

For example, suppose I go back in time and try to kill my grandfather. If I succeed, then of course I'm never born and I could never have made the trip back using the time machine.

Once again, we can't have the same event—the killing of my grandfather—both happen and not happen in the same universe.

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Is there any way of getting around this "grandfather paradox"?

There are two possibilities. One is what's sometimes called the self-consistency scenario, in which all events along the time loop that I make are adjusted to be self-consistent.

So for example, if I go backward in time and try to shoot my grandfather, something will always prevent me from doing so. The recoil on my shoulder makes me miss, or my grandfather ducks, or I change my mind. It's like the universe and the laws of physics are conspiring to make things consistent.

The other possibility is that when I shoot my grandfather the universe splits and there's one universe in which I shoot my grandfather and there's another universe in which I did not shoot my grandfather.

Didn't split timelines play a role in the latest Star Trek reboot by J. J. Abrams?

Yeah, there was something along those lines. In the movie, the Romulan bad guy Nero goes back to the past to get revenge against Spock, who he claims is responsible for the destruction of his home planet Romulus. So he's going to get even by going back into the past to destroy [the planet] Vulcan.

But since Vulcan wasn't destroyed in the original timeline—the one Nero came from—then upon going back into the past, he causes the universe to branch.

So the Vulcan he destroys is not the one in his original timeline, but the one in the new branch. So he's not really getting revenge on the original Vulcan from his timeline. But I suppose revenge is revenge.

That aside, I thought that [using the concept of a split timeline] was a clever way of rebooting the franchise because then you have the same characters but you don't have to slavishly follow the past history of the episodes since you're in a new timeline where everything can be different.

Okay, so you might not be able to travel to the past. But is future time travel possible?

There's no problem with that. In fact, we know how to do it in principle. If you travel very close to the speed of light, time slows down for the space traveler compared to someone on Earth.

Another way of traveling to the future is by orbiting very close to a black hole. For example, if you orbit around the black hole at the center of our galaxy, you could also have your time stretched relative to observers on the Earth.

If future time travel is possible, then could a time machine like the one the Iranian businessman claimed to have built actually work?

Going to the future is no problem. A mechanism for traveling into the future is afforded by [Einstein's] special theory of relativity. It's when you try to go backward that you run into the grandfather paradox. However, that said, what the businessman claims to have built is still nuts.

One thing that's rarely mentioned in time travel stories is that if you travel back only in time but stay in exactly the same point in space, the Earth won't be there anymore. So wouldn't time travel require traveling through space as well?

Yes, it would have to. The Earth is turning on its axis, and it's orbiting the sun. So the Earth isn't always in the same spot in its orbit. So if you're staying in the same place and traveling back to the past, the Earth is gone from underneath you. When you stop your time machine, you'll be in a bit of a pickle.

Why do you think time travel is so popular in books and movies?

You have to admit, it's a pretty tantalizing idea. Part of the appeal is that you can go back and see things for yourself that you only know through history books and the geological record. I think everybody would think it'd be really cool to go back and see dinosaurs or go back and visit ancient Greece.

I think another appeal is we all have things in our past that we wished that we hadn't done, or that we wished hadn't happened. And I think there's the desire to be able to go back and prevent those things from having happened.

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Time Machines

Recent years have seen a growing consensus in the philosophical community that the grandfather paradox and similar logical puzzles do not preclude the possibility of time travel scenarios that utilize spacetimes containing closed timelike curves. At the same time, physicists, who for half a century acknowledged that the general theory of relativity is compatible with such spacetimes, have intensely studied the question whether the operation of a time machine would be admissible in the context of the same theory and of its quantum cousins. A time machine is a device which brings about closed timelike curves—and thus enables time travel—where none would have existed otherwise. The physics literature contains various no-go theorems for time machines, i.e., theorems which purport to establish that, under physically plausible assumptions, the operation of a time machine is impossible. We conclude that for the time being there exists no conclusive no-go theorem against time machines. The character of the material covered in this article makes it inevitable that its content is of a rather technical nature. We contend, however, that philosophers should nevertheless be interested in this literature for at least two reasons. First, the topic of time machines leads to a number of interesting foundations issues in classical and quantum theories of gravity; and second, philosophers can contribute to the topic by clarifying what it means for a device to count as a time machine, by relating the debate to other concerns such as Penrose’s cosmic censorship conjecture and the fate of determinism in general relativity theory, and by eliminating a number of confusions regarding the status of the paradoxes of time travel. The present article addresses these ambitions in as non-technical a manner as possible, and the reader is referred to the relevant physics literature for details.

• 1. Introduction: time travel vs. time machines

2. What is a (Thornian) time machine? Preliminaries

3. when can a would-be time machine be held responsible for the emergence of ctcs, 4. no-go results for (thornian) time machines in classical general relativity theory, 5. no-go results in quantum gravity, 6. conclusion, other internet resources, related entries, 1. introduction: time travel vs. time machine.

The topic of time machines is the subject of a sizable and growing physics literature, some of which has filtered down to popular and semi-popular presentations. [ 1 ] The issues raised by this topic are largely oblique, if not orthogonal, to those treated in the philosophical literature on time travel. [ 2 ] Most significantly, the so-called paradoxes of time travel do not play a substantial role in the physics literature on time machines. This literature equates the possibility of time travel with the existence of closed timelike curves (CTCs) or worldlines for material particles that are smooth, future-directed timelike curves with self-intersections. [ 3 ] Since time machines designate devices which bring about the existence of CTCs and thus enable time travel, the paradoxes of time travel are irrelevant for attempted “no-go” results for time machines because these results concern what happens before the emergence of CTCs. [ 4 ] This, in our opinion, is fortunate since the paradoxes of time travel are nothing more than a crude way of bringing out the fact that the application of familiar local laws of relativistic physics to a spacetime background which contains CTCs typically requires that consistency constraints on initial data must be met in order for a local solution of the laws to be extendable to a global solution. The nature and status of these constraints is the subject of ongoing discussion. We will not try to advance the discussion of this issue here; [ 5 ] rather, our aim is to acquaint the reader with the issues addressed in the physics literature on time machines and to connect them with issues in the philosophy of space and time and, more generally, with issues in the foundations of physics.

Paradox mongers can be reassured in that if a paradox is lost in shifting the focus from time travel itself to time machines, then a paradox is also gained: if it is possible to operate a time machine device that produces CTCs, then it is possible to alter the structure of spacetime such that determinism fails; but by undercutting determinism, the time machine undercuts the claim that it is responsible for producing CTCs. But just as the grandfather paradox is a crude way of making a point, so this new paradox is a crude way of indicating that it is going to be difficult to specify what it means to be a time machine. This is a task that calls not for paradox mongering but for scientifically informed philosophizing. The present article will provide the initial steps of this task and will indicate what remains to be done. But aside from paradoxes, the main payoff of the topic of time machines is that it provides a quick route to the heart of a number of foundations problems in classical general relativity theory and in attempts to produce a quantum theory of gravity by combining general relativity and quantum mechanics. We will indicate the shape of some of these problems here, but will refer the interested reader elsewhere for technical details.

There are at least two distinct general notions of time machines, which we will call Wellsian and Thornian for short. In The Time Machine , H. G. Wells (1931) described what has become science fiction’s paradigmatic conception of a time machine: the intrepid operator fastens her seat belt, dials the target date—past or future—into the counter, throws a lever, and sits back while time rewinds or fast forwards until the target date is reached. We will not broach the issue of whether or not a Wellsian time machine can be implemented within a relativistic spacetime framework. For, as will soon become clear, the time machines which have recently come into prominence in the physics literature are of an utterly different kind. This second kind of time machine was originally proposed by Kip Thorne and his collaborators (see Morris and Thorne 1988; Morris, Thorne, and Yurtsever 1988). These articles considered the possibility that, without violating the laws of general relativistic physics, an advanced civilization might manipulate concentrations of matter-energy so as to produce CTCs where none would have existed otherwise. In their example, the production of “wormholes” was used to generate the required spacetime structure. But this is only one of the ways in which a time machine might operate, and in what follows any device which affects the spacetime structure in such a way that CTCs result will be dubbed a Thornian time machine . We will only be concerned with this variety of time machine, leaving the Wellsian variety to science fiction writers. This will disappoint the aficionados of science fiction since Thornian time machines do not have the magical ability to transport the would-be time traveler to the past of the events that constitute the operation of the time machine. For those more interested in science than in science fiction, this loss is balanced by the gain in realism and the connection to contemporary research in physics.

In Sections 2 and 3 we investigate the circumstances under which it is plausible to see a Thornian time machine at work. The main difficulty lies in specifying the conditions needed to make sense of the notion that the time machine “produces” or is “responsible for” the appearances of CTCs. We argue that at present there is no satisfactory resolution of this difficulty and, thus, that the topic of time machines in a general relativistic setting is somewhat ill-defined. This fact does not prevent progress from being made on the topic; for if one’s aim is to establish no-go results for time machines it suffices to identify necessary conditions for the operation of a time machine and then to prove, under suitable hypotheses about what is physically possible, that it is not physically possible to satisfy said necessary conditions. In Section 4 we review various no-go results which depend only on classical general relativity theory. Section 5 surveys results that appeal to quantum effects. Conclusions are presented in Section 6.

The setting for the discussion is a general relativistic spacetime \((\mathcal{M},g_{ab})\) where \(\mathcal{M}\) is a differentiable manifold and \(g_{ab}\) is a Lorentz signature metric defined on all of \(\mathcal{M}\). The central issue addressed in the physics literature on time machines is whether in this general setting it is physically possible to operate a Thornian time machine. This issue is to be settled by proving theorems about the solutions to the equations that represent what are taken to be physical laws operating in the general relativistic setting—or at least this is so once the notion of a Thornian time machine has been explicated. Unfortunately, no adequate and generally accepted explication that lends itself to the required mathematical proofs is to be found in the literature. This is neither surprising nor deplorable. Mathematical physicists do not wait until some concept has received its final explication before trying to prove theorems about it; indeed, the process of theorem proving is often an essential part of conceptual clarification. The moral is well illustrated by the history of the concept of a spacetime singularity in general relativity where this concept received its now canonical definition only in the process of proving the Penrose-Hawking-Geroch singularity theorems, which came at the end of a decades long dispute over the issue of whether spacetime singularities are a generic feature of solutions to Einstein’s gravitational field equations. [ 6 ] However, this is not to say that philosophers interested in time machines should simply wait until the dust has settled in the physics literature; indeed, the physics literature could benefit from deployment of the analytical skills that are the stock in trade of philosophy. For example, the paradoxes of time travel and the fate of time machines are not infrequently confused in the physics literature, and as will become evident below, subtler confusions abound as well.

The question of whether a Thornian time machine—a device that produces CTCs—can be seen to be at work only makes sense if the spacetime has at least three features: temporal orientability, a definite time orientation, and a causally innocuous past. In order to make the notion of a CTC meaningful, the spacetime must be temporally orientable (i.e., must admit a consistent time directionality), and one of the two possible time orientations has to be singled out as giving the direction of time. [ 7 ] Non-temporal orientability is not really an obstacle since if a given general relativistic spacetime is not temporally orientable, a spacetime that is everywhere locally the same as the given spacetime and is itself temporally orientable can be obtained by passing to a covering spacetime. [ 8 ] How to justify the singling out of one of the two possible orientations as future pointing requires a solution to the problem of the direction of time, a problem which is still subject to lively debate (see Callender 2001). But for present purposes we simply assume that a temporal orientation has been provided. A CTC is then (by definition) a parameterized closed spacetime curve whose tangent is everywhere a future-pointing timelike vector. A CTC can be thought of as the world line of some possible observer whose life history is linearly ordered in the small but not in the large: the observer has a consistent experience of the “next moment,” and the “next,” etc., but eventually the “next moment” brings her back to whatever event she regards as the starting point.

As for the third condition—a causally innocuous past—the question of the possibility of operating a device that produces CTCs presupposes that there is a time before which no CTCs existed. Thus, Gödel spacetime, so beloved of the time travel literature, is not a candidate for hosting a Thornian time machine since through every point in this spacetime there is a CTC. We make this third condition precise by requiring that the spacetime admits a global time slice \(\Sigma\) (i.e., a spacelike hypersurface without edges); [ 9 ] that \(\Sigma\) is two-sided and partitions \(\mathcal{M}\) into three parts—\(\Sigma\) itself, the part of \(\mathcal{M}\) on the past side of \(\Sigma\) and the part of \(\mathcal{M}\) on the future side of \(\Sigma\)—and that there are no CTCs that lie on the past side of \(\Sigma\). The first two clauses of this requirement together entail that the time slice \(\Sigma\) is a partial Cauchy surface , i.e., \(\Sigma\) is a time slice that is not intersected more than once by any future-directed timelike curve. [ 10 ]

Now suppose that the state on a partial Cauchy surface \(\Sigma_0\) with no CTCs to its past is to be thought of as giving a snapshot of the universe at a moment before the machine is turned on. The subsequent realization of a Thornian time machine scenario requires that the chronology violating region \(V \subseteq \mathcal{M}\), the region of spacetime traced out by CTCs, [ 11 ] is non-null and lies to the future of \(\Sigma_0\). The fact that \(V \ne \varnothing\) does not lead to any consistency constraints on initial data on \(\Sigma_0\) since, by hypothesis, \(\Sigma_0\) is not intersected more than once by any timelike curve, and thus, insofar as the so-called paradoxes of time travel are concerned with such constraints, the paradoxes do not arise with respect to \(\Sigma_0\). But by the same token, the option of traveling back into the past of \(\Sigma_0\) is ruled out by the set up as it has been sketched so far, since otherwise \(\Sigma_0\) would not be a partial Cauchy surface. This just goes to underscore the point made above that the fans of science fiction stories of time machines will not find the present context of discussion broad enough to encompass their vision of how time machines should operate; they may now stop reading this article and return to their novels.

Figure 1. Misner spacetime

As a concrete example of these concepts, consider the \((1 + 1)\)-dimensional Misner spacetime (see Figure 1 ) which exhibits some of the causal features of Taub-NUT spacetime, a vacuum solution to Einstein’s gravitational field equations. It satisfies all three of the conditions discussed above. It is temporally orientable, and a time orientation has been singled out—the shading in the figure indicates the future lobes of the light cones. To the past of the partial Cauchy surface \(\Sigma_0\) lies the Taub region where the causal structure of spacetime is as bland as can be desired. But to the future of \(\Sigma_0\) the light cones begin to “tip over,” and eventually the tipping results in CTCs in the NUT region.

The issue that must be faced now is what further conditions must be imposed in order that the appearance of CTCs to the future of \(\Sigma_0\) can be attributed to the operation of a time machine. Not surprisingly, the answer depends not just on the structure of the spacetime at issue but also on the physical laws that govern the evolution of the spacetime structure. If one adopts the attitude that the label “time machine” is to be reserved for devices that operate within a finite spatial range for a finite stretch of time, then one will want to impose requirements to assure that what happens in a compact region of spacetime lying on or to the future of \(\Sigma_0\) is responsible for the CTCs. Or one could be more liberal and allow the would-be time machine to be spread over an infinite space. We will adopt the more liberal stance since it avoids various complications while still sufficing to elicit key points. Again, one could reserve the label “time machine” for devices that manipulate concentrations of mass-energy in some specified ways. For example, based on Gödel spacetime—where matter is everywhere rotating and a CTC passes through every spacetime point—one might conjecture that setting into sufficiently rapid rotation a finite mass concentration of appropriate shape will eventuate in CTCs. But with the goal in mind of proving negative general results, it is better to proceed in a more abstract fashion. Think of the conditions on the partial Cauchy surface \(\Sigma_0\) as encoding the instructions for the operation of the time machine. The details of the operation of the device—whether it operates in a finite region of spacetime, whether it operates by setting matter into rotation, etc.—can be left to the side. What must be addressed, however, is whether the processes that evolve from the state on \(\Sigma_0\) can be deemed to be responsible for the subsequent appearance of CTCs.

The most obvious move is to construe “responsible for” in the sense of causal determinism. But in the present setting this move quickly runs into a dead end. For if CTCs exist to the future of \(\Sigma_0\) they are not causally determined by the state on \(\Sigma_0\) since the time travel region \(V\), if it is non-null, lies outside the future domain of dependence \(D^+ (\Sigma_0)\) of \(\Sigma_0\), the portion of spacetime where the field equations of relativistic physics uniquely determine the state of things from the state on \(\Sigma_0\). [ 12 ] The point is illustrated by the toy model of Figure 1 . The surface labeled \(H^+ (\Sigma_0)\) is called the future Cauchy horizon of \(\Sigma_0\). It is the future boundary of \(D^+ (\Sigma_0)\), [ 13 ] and it separates the portion of spacetime where conditions are causally determined by the state on \(\Sigma_0\) from the portion where conditions are not so determined. And, as advertised, the CTCs in the model of Figure 1 lie beyond \(H^+ (\Sigma_0)\).

Figure 2. Deutsch-Politzer spacetime

Thus, if the operation of a Thornian time machine is to be a live possibility, some condition weaker than causal determinism must be used to capture the sense in which the state on \(\Sigma_0\) can be deemed to be responsible for the subsequent development of CTCs. Given the failure of causal determinism, it seems the next best thing to demand that the region \(V\) is “adjacent” to the future domain of dependence \(D^+ (\Sigma_0)\). Here is an initial stab at such an adjacency condition. Consider causal curves which have a future endpoint in the time travel region \(V\) and no past endpoint. Such a curve may never leave \(V\); but if it does, require that it intersects \(\Sigma_0\). But this requirement is too strong because it rules out Thornian time machines altogether. For a curve of the type in question to reach \(\Sigma_0\) it must intersect \(H^+ (\Sigma_0)\), but once it reaches \(H^+ (\Sigma_0)\) it can be continued endlessly into the past without meeting \(\Sigma_0\) because the generators of \(H^+ (\Sigma_0)\) are past endless null geodesics that never meet \(\Sigma_0\). [ 14 ] This difficulty can be overcome by weakening the requirement at issue by rephrasing it in terms of timelike curves rather than causal curves. Now the set of candidate time machine spacetimes satisfying the weakened requirement is non-empty—as illustrated, once again, by the spacetime of Figure 1 . But the weakened requirement is too weak, as illustrated by the \((1 + 1)\)-dimensional version of Deutsch-Politzer spacetime [ 15 ] (see Figure 2 ), which is constructed from two-dimensional Minkowski spacetime by deleting the points \(p_1\)–\(p_4\) and then gluing together the strips as shown. Every past endless timelike curve that emerges from the time travel region \(V\) of Deutsch-Politzer spacetime does meet \(\Sigma_0\). But this spacetime is not a plausible candidate for a time machine spacetime. Up to and including the time \(\Sigma_0\) (which can be placed as close to \(V\) as desired) this spacetime is identical with empty Minkowski spacetime. If the state of the corresponding portion of Minkowski spacetime is not responsible for the development of CTCs—and it certainly is not since there are no CTCs in Minkowski spacetime—how can the state on the portion of Deutsch-Politzer spacetime up to and including the time \(\Sigma_0\) be held responsible for the CTCs that appear in the future?

The deletion of the points \(p_1\)–\(p_4\) means that the Deutsch-Politzer spacetime is singular in the sense that it is geodesically incomplete . [ 16 ] It would be too drastic to require of a time-machine hosting spacetime that it be geodesically complete. And in any case the offending feature of Deutsch-Politzer can be gotten rid of by the following trick. Multiplying the flat Lorentzian metric \(\eta_{ab}\) of Deutsch-Politzer spacetime by a scalar function \(j(x, t) \gt\) produces a new metric \(\eta '_{ab} :=\) j \(\eta_{ab}\) which is conformal to the original metric and, thus, has exactly the same causal features as the original metric. But if the conformal factor \(j\) is chosen to “blow up” as the missing points \(p_1\)–\(p_4\) are approached, the resulting spacetime is geodesically complete—intuitively, the singularities have been pushed off to infinity.

A more subtle way to exclude Deutsch-Politzer spacetime focuses on the generators of \(H^+ (\Sigma_0)\). The stipulations laid down so far for Thornian time machines imply that the generators of \(H^+ (\Sigma_0)\) cannot intersect \(\Sigma_0\). But in addition it can be required that these generators do not “emerge from a singularity” and do not “come from infinity,” and this would suffice to rule out Deutsch-Politzer spacetime and its conformal cousins as legitimate candidates for time machine spacetimes. More precisely, we can impose what Stephen Hawking (1992a,b) calls the requirement that \(H^+ (\Sigma_0)\) be compactly generated ; namely, the past endless null geodesics that generate \(H^+ (\Sigma_0)\) must, if extended far enough into past, fall into and remain in a compact subset of spacetime. Obviously the spacetime of Figure 1 fulfills Hawking’s requirement—since in this case \(H^+ (\Sigma_0)\) is itself compact—but just as obviously the spacetime of Figure 2 (conformally doctored or not) does not.

Imposing the requirement of a compactly generated future Cauchy horizon has not only the negative virtue of excluding some unsuited candidate time machine spacetimes but a positive virtue as well. It is easily proved that if \(H^+ (\Sigma_0)\) is compactly generated then the condition of strong causality is violated on \(H^+ (\Sigma_0)\), which means, intuitively, there are almost closed causal curves near \(H^+ (\Sigma_0)\). [ 17 ] This violation can be taken as an indication that the seeds of CTCs have been planted on \(\Sigma_0\) and that by the time \(H^+ (\Sigma_0)\) is reached they are ready to bloom.

However, we still have no guarantee that if CTCs do bloom to the future of \(\Sigma_0\), then the state on \(\Sigma_0\) is responsible for the blooming. Of course, we have already learned that we cannot have the iron clad guarantee of causal determinism that the state on \(\Sigma_0\) is responsible for the actual blooming in all of its particularity. But we might hope for a guarantee that the state on \(\Sigma_0\) is responsible for the blooming of some CTCs—the actual ones or others. The difference takes a bit of explaining. The failure of causal determinism is aptly pictured by the image of a future “branching” of world histories, with the different branches representing different alternative possible futures of (the domain of dependence of) \(\Sigma_0\) that are compatible with the actual past and the laws of physics. And so it is in the present setting: if \(H^+ (\Sigma_0) \ne \varnothing\), then there will generally be different ways to extend \(D^+ (\Sigma_0)\), all compatible with the laws of general relativistic physics. But if CTCs are present in all of these extensions, even through the details of the CTCs may vary from one extension to another, then the state on \(\Sigma_0\) can rightly be deemed to be responsible for the fact that subsequently CTCs did develop.

A theorem due to Krasnikov (2002, 2003 [Other Internet Resources], 2014a) might seem to demonstrate that no relativistic spacetime can count as embodying a Thornian time machine so understood. Following Krasnikov, let us say that a spacetime condition \(C\) is local just in case, for any open covering \(\{V_{\alpha}\}\) of an arbitrary spacetime \((\mathcal{M}, g_{ab}), C\) holds in \((\mathcal{M}, g_{ab})\) iff it holds in \((V_{\alpha}, g_{ab}|_{V_{\alpha}})\) for all \(\alpha\). Examples of local conditions one might want to impose on physically reasonable spacetimes are Einstein’s gravitational field equations and so-called energy conditions that restrict the form of the stress-energy tensor \(T_{ab}\). An example of the latter that will come into play below is the weak energy condition that says that the energy density is non-negative. [ 18 ] Einstein’s field equations (sans cosmological constant) require that \(T_{ab}\) is proportional to the Einstein tensor which is a functional of the metric and its derivatives. Call a \(C\)-spacetime \((\mathcal{M}', g'_{ab})\) a \(C\)- extension of a \(C\)-spacetime \((\mathcal{M}, g_{ab})\) spacetime if the latter is isometric to an open proper subset of the former; and call \((\mathcal{M}, g_{ab}) C\)- extensible if it admits a \(C\)-extension and \(C\)- maximal otherwise. (Of course, \(C\) might be the empty condition.) Krasnikov’s theorem shows that every \(C\)-spacetime \((\mathcal{M}, g_{ab})\) admits a \(C\)-maximal extension \((\mathcal{M}^{max}, g^{max}_{ab})\) such that all CTCs in \((\mathcal{M}^{max}, g^{max}_{ab})\) are to the chronological past of the image of \(\mathcal{M}\) in \((\mathcal{M}^{max}, g^{max}_{ab})\). So start with some candidate spacetime \((\mathcal{M}, g_{ab})\) for a Thornian time machine, and apply the theorem to \((D^+ (\Sigma_0), g_{ab}|_{D^+ (\Sigma_0)})\). Conclude that no matter what local conditions the candidate spacetime is required to satisfy, \(D^+ (\Sigma_0)\) has extensions that also satisfies said local conditions but does not contain CTCs to the future of \(\Sigma_0\). Thus, the candidate spacetime fails to exhibit the crucial feature identified above necessary for underwriting the contention that the conditions on \(\Sigma_0\) are responsible for the development of CTCs. Hence, it appears as if Krasnikov’s theorem effectively prohibits time machines.

The would-be time machine operator need not capitulate in the face of Krasnikov’s theorem. Recall that the main difficulty in specifying the conditions for the successful operation of Thornian time machines traces to the fact that the standard form of causal determinism does not apply to the production of CTCs. But causal determinism can fail for reasons that have nothing to do with CTCs or other acausal features of relativistic spacetimes, and it seems only fair to ensure that these modes of failure have been removed before proceeding to discuss the prospects for time machines. To zero in on the modes of failure at issue, consider vacuum solutions \((T_{ab} \equiv 0)\) to Einstein’s field equations. Let \((\mathcal{M}, g_{ab})\) and \((\mathcal{M}', g'_{ab})\) be two such solutions, and let \(\Sigma \subset \mathcal{M}\) and \(\Sigma ' \subset \mathcal{M}'\) be spacelike hypersurfaces of their respective spacetimes. Suppose that there is an isometry \(\Psi\) from some neighborhood \(N(\Sigma)\) of \(\Sigma\) onto a neighborhood \(N'(\Sigma ')\) of \(\Sigma '\). Does it follow, as we would want determinism to guarantee, that \(\Psi\) is extendible to an isometry from \(D^+ (\Sigma)\) onto \(D^+ (\Sigma ')\)? To see why the answer is negative, start with any solution \((\mathcal{M}, g_{ab})\) of the vacuum Einstein equations, and cut out a closed set of points lying to the future of \(N(\Sigma)\) and in \(D^+ (\Sigma)\). Denote the surgically altered manifold by \(\mathcal{M}^*\) and the restriction of \(g_{ab}\) to \(\mathcal{M}^*\) by \(g^*_{ab}\). Then \((\mathcal{M}^*, g^*_{ab})\) is also a solution of the vacuum Einstein equations. But obviously the pair of solutions \((\mathcal{M}, g_{ab})\) and \((\mathcal{M}^*, g^*_{ab})\) violates the condition that determinism was supposed to guarantee as \(\Psi\) is not extendible to an isometry from \(D^+ (\Sigma)\) onto \(D^+ (\Sigma^*)\). It might seem that the requirement, contemplated above, that the spacetimes under consideration be maximal, already rules out spacetimes that have “holes” in them. But while maximality does rule out the surgically mutilated spacetime just constructed, it does not guarantee hole freeness in the sense needed to make sure that determinism does not stumble before it gets to the starting gate. That \((\mathcal{M}, g_{ab})\) is hole free in the relevant sense requires that if \(\Sigma \subset \mathcal{M}\) is a spacelike hypersurface, there does not exist a spacetime \((\mathcal{M}', g'_{ab})\) and an isometric embedding \(\Phi\) of \(D^+ (\Sigma)\) into \(\mathcal{M}'\) such that \(\Phi(D^+ (\Sigma))\) is a proper subset of \(D^+ (\Phi(\Sigma))\). A theorem due to Robert Geroch (1977, 87), who is responsible for this definition, asserts that if \(\Sigma \subset \mathcal{M}\) and \(\Sigma ' \subset \mathcal{M}'\) are spacelike hypersurfaces in hole-free spacetimes \((\mathcal{M}, g_{ab})\) and \((\mathcal{M}', g'_{ab})\), respectively, and if there exists an isometry \(\Psi : \mathcal{M} \rightarrow \mathcal{M}'\), then \(\Psi\) is indeed extendible to an isometry between \(D^+ (\Sigma)\) and \(D^+ (\Sigma ')\). Thus, hole freeness precludes an important mode of failure of determinism which we wish to exclude in our discussion of time machines. It can be shown that hole freeness is not entailed by maximality. [ 19 ] And it is just this gap that gives the would-be time machine operator some hope, for the maximal CTC-free extensions produced by Krasnikov’s construction are not always hole free (Manchak 2009b). But Krasnikov (2009) has shown that the Geroch (1977) definition is too strong: Minkowski spacetime fails to satisfy it! For this reason, alternative formulations of the hole-freeness definition have been constructed which are more appropriate (Manchak 2009a, Minguzzi 2012).

Thus, we propose that one clear sense of what it would mean for a Thornian time machine to operate in the setting of general relativity theory is given by the following assertion: the laws of general relativistic physics allow solutions containing a partial Cauchy surface \(\Sigma_0\) such that no CTCs lie to the past of \(\Sigma_0\) but every extension of \(D^+ (\Sigma_0)\) satisfying ________ contains CTCs (where the blank is filled with some “no hole” condition). Correspondingly, a proof of the physical impossibility of time machines would take the form of showing that this assertion is false for the actual laws of physics, consisting, presumably, of Einstein’s field equations plus energy conditions and, perhaps, some additional restrictions as well. And a proof of the emptiness of the associated concept of a Thornian time machine would take the form of showing that the assertion is false independently of the details of the laws of physics, as long as they take the form of local conditions on \(T_{ab}\) and \(g_{ab}\).

Are there "no hole" conditions which show the proposed concept of a time machine is not empty? Let \(J^+(p)\) designate the causal future of \(p\), defined as the set of all points in \(\mathcal{M}\) which can be reached from \(p\) by a future-directed causal curve in \(\mathcal{M}\). The causal past \(J^-(p)\) is defined analogously. Now, we say a spacetime \((\mathcal{M},g_{ab})\) is J closed if, for each \(p\) in \(\mathcal{M}\), the sets \(J^+(p)\) and \(J^-(p)\) are topologically closed. One can verify that J closedness fails in many artificially mutilated examples (e.g. Minkowski spacetime with one point removed from the manifold). For some time, it was thought that a time machine existed under this no-hole condition (Manchak 2011a). But this turns out to be incorrect; indeed a recent result shows that any J closed spacetime \((\mathcal{M},g_{ab})\) of three dimensions or more with chronology violating region \(V \neq \mathcal{M}\) must be strongly causal and therefore fail to have CTCs (Hounnonkpe and Minguzzi 2019). Stepping back, perhaps there are other no-hole conditions which can be used instead to show that the proposed concept of a time machine is not empty. But even if such a project were successful, Manchak (2014a, 2019) has shown that the time machine existence results can be naturally reinterpreted as “hole machine” existence results if one is so inclined. Instead of assuming that spacetime is free of holes and then showing that certain initial conditions are responsible for the production of CTCs, one could just as well start with the assumption of no CTCs and then show that certain initial conditions are responsible for the production of holes. Given the importance of these no hole assumptions to the time machine advocate, much recent work has focused on whether such assumptions are physically reasonable in some sense (Manchak 2011b, 2014b). This is still an open question.

Another open question is whether physically more realistic spacetimes than Misner also permit the operation of time machines and how generic time-machine spacetimes are in particular spacetime theories, such as general relativity. If time-machine spacetimes turn out to be highly non-generic, the fan of time machines can retreat to a weaker concept of Thornian time machine by taking a page from probabilistic accounts of causation, the idea being that a time machine can be seen to be at work if its operation increases the probability of the appearance of CTCs. Since general relativity theory itself is innocent of probabilities, they have to be introduced by hand, either by inserting them into the models of the theory, i.e., by modifying the theory at the level of the object-language, or by defining measures on sets of models, i.e., by modifying the theory at the level of the meta-language. Since the former would change the character of the theory, only the latter will be considered. The project for making sense of the notion that a time machine as a probabilistic cause of the appearance of CTCs would then take the following form. First define a normalized measure on the set of models having a partial Cauchy surface to the past of which there are no CTCs. Then show that the subset of models that have CTCs to the future of the partial Cauchy surface has non-zero measure. Next, identify a range of conditions on or near the partial Cauchy surface that are naturally construed as settings of a device that is a would-be probabilistic cause of CTCs, and show that the subset of models satisfying these conditions has non-zero measure. Finally, show that conditionalizing on the latter subset increases the measure of the former subset. Assuming that this formal exercise can be successfully carried out, there remains the task of justifying these as measures of objective chance. This task is especially daunting in the cosmological setting since neither of the leading interpretations of objective chance seems applicable. The frequency interpretation is strained since the development of CTCs may be a non-repeated phenomenon; and the propensity interpretation is equally strained since, barring just-so stories about the Creator throwing darts at the Cosmic Dart Board, there is no chance mechanism for producing cosmological models.

We conclude that, even apart from general doubts about a probabilistic account of causation, the resort to a probabilistic conception of time machines is a desperate stretch, at least in the context of classical general relativity theory. In a quantum theory of gravity, a probabilistic conception of time machines may be appropriate if the theory itself provides the transition probabilities between the relevant states. But an evaluation of this prospect must wait until the theory of quantum gravity is available.

In order to appreciate the physics literature aimed at proving no-go results for time machines it is helpful to view these efforts as part of the broader project of proving chronology protections theorems , which in turn is part of a still larger project of proving cosmic censorship theorems . To explain, we start with cosmic censorship and work backwards.

Figure 3. A bad choice of initial value surface

For sake of simplicity, concentrate on the initial value problem for vacuum solutions \((T_{ab} \equiv 0)\) to Einstein’s field equations. Start with a three-manifold \(\Sigma\) equipped with quantities which, when \(\Sigma\) is embedded as a spacelike submanifold of spacetime, become initial data for the vacuum field equations. Corresponding to the initial data there exists a unique [ 20 ] maximal development \((\mathcal{M}, g_{ab})\) for which (the image of the embedded) \(\Sigma\) is a Cauchy surface. [ 21 ] This solution, however, may not be maximal simpliciter, i.e., it may be possible to isometrically embed it as a proper part of a larger spacetime, which itself may be a vacuum solution to the field equations; if so \(\Sigma\) will not be a Cauchy surface for the extended spacetime, which fails to be a globally hyperbolic spacetime. [ 22 ] This situation can arise because of a poor choice of initial value hypersurface, as illustrated in Figure 3 by taking \(\Sigma\) to be the indicated spacelike hyperboloid of \((1 + 1)\)-dimensional Minkowski spacetime. But, more interestingly, the situation can arise because the Einstein equations allow various pathologies, collectively referred to as “naked singularities,” to develop from regular initial data. The strong form of Penrose’s celebrated cosmic censorship conjecture proposes that, consistent with Einstein’s field equations, such pathologies do not arise under physically reasonable conditions or else that the conditions leading to the pathologies are highly non-generic within the space of all solutions to the field equations. A small amount of progress has been made on stating and proving precise versions of this conjecture. [ 23 ]

One way in which strong cosmic censorship can be violated is through the emergence of acausal features. Returning to the example of Misner spacetime ( Figure 1 ), the spacetime up to \(H^+ (\Sigma_0)\) is the unique maximal development of the vacuum Einstein equations for which \(\Sigma_0\) is a Cauchy surface. But this development is extendible, and in the extension illustrated in Figure 1 global hyperbolicity of the development is lost because of the presence of CTCs. The chronology protection conjecture then can be construed as a subconjecture of the cosmic censorship conjecture, saying, roughly, that consistent with Einstein field equations, CTCs do not arise under physically reasonable conditions or else that the conditions are highly non-generic within the space of all solutions to the field equations. No-go results for time machines are then special forms of chronology protection theorems that deal with cases where the CTCs are manufactured by time machines. In the other direction, a very general chronology protection theorem will automatically provide a no-go result for time machines, however that notion is understood, and a theorem establishing strong cosmic censorship will automatically impose chronology protection.

The most widely discussed chronology protection theorem/no-go result for time machines in the context of classical general relativity theory is due to Hawking (1992a). Before stating the result, note first that, independently of the Einstein field equations and energy conditions, a partial Cauchy surface \(\Sigma\) must be compact if its future Cauchy horizon \(H^+ (\Sigma)\) is compact (see Hawking 1992a and Chrusciel and Isenberg 1993). However, it is geometrically allowed that \(\Sigma\) is non-compact if \(H^+ (\Sigma)\) is required only to be compactly generated rather than compact. But what Hawking showed is that this geometrical possibility is ruled out by imposing Einstein’s field equations and the weak energy condition. Thus, if \(\Sigma_0\) is a partial Cauchy surface representing the situation just before or just as the would-be Thornian time machine is switched on, and if a necessary condition for seeing a Thornian time machine at work is that \(H^+ (\Sigma_0)\) is compactly generated, then consistently with Einstein’s field equations and the weak energy condition, a Thornian time machine cannot operate in a spatially open universe since \(\Sigma_0\) must be compact.

This no-go result does not touch the situation illustrated in Figure 1 . Taub-NUT spacetime is a vacuum solution to Einstein’s field equations so the weak energy condition is automatically satisfied, and \(H^+ (\Sigma_0)\) is compact and, a fortiori, compactly generated. Hawking’s theorem is not contradicted since \(\Sigma_0\) is compact. By the same token the theorem does not speak to the possibility of operating a Thornian time machine in a spatially closed universe. To help fill the gap, Hawking proved that when \(\Sigma_0\) is compact and \(H^+ (\Sigma_0)\) is compactly generated, the Einstein field equations and the weak energy condition together guarantee that both the convergence and shear of the null geodesic generators of \(H^+ (\Sigma_0)\) must vanish, which he interpreted to imply that no observers can cross over \(H^+ (\Sigma_0)\) to enter the chronology violating region \(V\). But rather than showing that it is physically impossible to operate a Thornian time machine in a closed universe, this result shows only that, given the correctness of Hawking’s interpretation, the observers who operate the time machine cannot take advantage of the CTCs it produces.

There are two sources of doubt about the effectiveness of Hawking’s no-go result even for open universes. The first stems from possible violations of the weak energy condition by stress-energy tensors arising from classical relativistic matter fields (see Vollick 1997 and Visser and Barcelo 2000). [ 24 ] The second stems from the fact that Hawking’s theorem functions as a chronology protection theorem only by way of serving as a potential no-go result for Thornian time machines since the crucial condition that \(H^+ (\Sigma_0)\) is compactly generated is supposedly justified by being a necessary condition for the operation of such machine. But in retrospect, the motivation for this condition seems frayed. As argued in the previous section, if the Einstein field equations and energy conditions entail that all hole free extensions of \(D^+ (\Sigma_0)\) contain CTCs, then it is plausible to see a Thornian time machine at work, quite regardless ofwhether or not \(H^+ (\Sigma_0)\) is compactly generated or not. Of course, it remains to establish the existence of cases where this entailment holds. If it should turn out that there are no such cases, then the prospects of Thornian time machines are dealt a severe blow, but the reasons are independent of Hawking’s theorem. On the other hand, if such cases do exist then our conjecture would be that they exist even when some of the generators of \(H^+ (\Sigma_0)\) come from singularities or infinity and, thus, \(H^+ (\Sigma_0)\) is not compactly generated. [ 25 ]

Three degrees of quantum involvement in gravity can be distinguished. The first degree, referred to as quantum field theory on curved spacetimes, simply takes off the shelf a spacetime provided by general relativity theory and then proceeds to study the behavior of quantum fields on this background spacetime. The Unruh effect, which predicts the thermalization of a free scalar quantum field near the horizon of a black hole, lies within this ambit. The second degree of involvement, referred to as semi-classical quantum gravity, attempts to calculate the backreaction of the quantum fields on spacetime metric by computing the expectation value \(\langle \Psi \mid T_{ab} \mid \Psi \rangle\) of the stress-energy tensor in some appropriate quantum state \(\lvert\Psi\rangle\) and then inserting the value into Einstein’s field equations in place of \(T_{ab}\). [ 26 ] Hawking’s celebrated prediction of black hole radiation belongs to this ambit. [ 27 ] The third degree of involvement attempts to produce a genuine quantum theory of gravity in the sense that the gravitational degrees of freedom are quantized. Currently loop quantum gravity and string theory are the main research programs aimed at this goal. [ 28 ]

The first degree of quantum involvement, if not opening the door to Thornian time machines, at least seemed to remove some obstacles since quantum fields are known to lead to violations of the energy conditions used in the setting of classical general relativity theory to prove chronology protection theorems and no-go results for time machines. However, the second degree of quantum involvement seemed, at least initially, to slam the door shut. The intuitive idea was this. Start with a general relativistic spacetime where CTCs develop to the future of \(H^+ (\Sigma)\) (often referred to as the “chronology horizon”) for some suitable partial Cauchy surface \(\Sigma\). Find that the propagation of a quantum field on this spacetime background is such that \(\langle \Psi \mid T_{ab} \mid\Psi \rangle\) “blows up” as \(H^+ (\Sigma)\) is approached from the past. Conclude that the backreaction on the spacetime metric creates unbounded curvature, which effectively cuts off the future development that would otherwise eventuate in CTCs. These intuitions were partly vindicated by detailed calculations in several models. But eventually a number of exceptions were found in which the backreaction remains arbitrarily small near \(H^+ (\Sigma)\). [ 29 ] This seemed to leave the door ajar for Thornian time machines.

But fortunes were reversed once again by a result of Kay, Radzikowski, and Wald (1997). The details of their theorem are too technical to review here, but the structure of the argument is easy to grasp. The naïve calculation of \(\langle \Psi \mid T_{ab}\mid\Psi \rangle\) results in infinities which have to be subtracted off to produce a renormalized expectation value \(\langle \Psi \mid T_{ab}\mid\Psi \rangle_R\) with a finite value. The standard renormalization procedure uses a limiting procedure that is mathematically well-defined if, and only if, a certain condition obtains. [ 30 ] The KRW theorem shows that this condition is violated for points on \(H^+ (\Sigma)\) and, thus, that the expectation value of the stress-energy tensor is not well-defined at the chronology horizon.

While the KRW theorem is undoubtedly of fundamental importance for semi-classical quantum gravity, it does not serve as an effective no-go result for Thornian time machines. In the first place, the theorem assumes, in concert with Hawking’s chronology protection theorem, that \(H^+ (\Sigma)\) is compactly generated, and we repeat that it is far from clear that this assumption is necessary for seeing a Thornian time machine in operation. A second, and more fundamental, reservation applies even if a compactly generated \(H^+ (\Sigma)\) is accepted as a necessary condition for time machines. The KRW theorem functions as a no-go result by providing a reductio ad absurdum with a dubious absurdity: roughly, if you try to operate a Thornian time machine, you will end up invalidating semi-classical quantum gravity. But semi-classical quantum gravity was never viewed as anything more than a stepping stone to a genuine quantum theory of gravity, and its breakdown is expected to be manifested when Planck-scale physics comes into play. This worry is underscored by Visser’s (1997, 2003) findings that in chronology violating models trans-Planckian physics can be expected to come into play before \(H^+ (\Sigma)\) is reached.

It thus seems that if some quantum mechanism is to serve as the basis for chronology protection, it must be found in the third degree of quantum involvement in gravity. Both loop quantum gravity and string theory have demonstrated the ability to cure some of the curvature singularities of classical general relativity theory. But as far as we are aware there are no demonstrations that either of these approaches to quantum gravity can get rid of the acausal features exhibited in various solutions to Einstein’s field equations. An alternative approach to formulate a fully-fledged quantum theory of gravity attempts to capture the Planck-scale structure of spacetime by constructing it from causal sets. [ 31 ] Since these sets must be acyclic, i.e., no element in a causal set can causally precede itself, CTCs are ruled out a priori. Actually, a theorem due to Malament (1977) suggests that any Planck-scale approach encoding only the causal structure of a spacetime cannot permit CTCs either in the smooth classical spacetimes or a corresponding phenomenon in their quantum counterparts. [ 32 ]

In sum, the existing no-go results that use the first two degrees of quantum involvement are not very convincing, and the third degree of involvement is not mature enough to allow useful pronouncements. There is, however, a rapidly growing literature on the possibility of time travel in lower-dimensional supersymmetric cousins of string theory. For a review of these recent results and a discussion of the fate of a time-traveller’s ambition in loop quantum gravity, see Smeenk and Wüthrich (2010).

Hawking opined that “[i]t seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians” (1992a, 603). He may be right, but to date there are no convincing arguments that such an Agency is housed in either classical general relativity theory or in semi-classical quantum gravity. And it is too early to tell whether this Agency is housed in loop quantum gravity or string theory. But even if it should turn out that Hawking is wrong in that the laws of physics do not support a Chronology Protection Agency, it could still be the case that the laws support an Anti-Time Machine Agency. For it could turn out that while the laws do not prevent the development of CTCs, they also do not make it possible to attribute the appearance of CTCs to the workings of any would-be time machine. We argued that a strong presumption in favor of the latter would be created in classical general relativity theory by the demonstration that for any model satisfying Einstein’s field equations and energy conditions as well as possessing a partial Cauchy surface \(\Sigma_0\) to the future of which there are CTCs, there are hole free extensions of \(D^+ (\Sigma_0)\) satisfying Einstein’s field equations and energy conditions but containing no CTCs to the future of \(\Sigma_0\). There are no doubt alternative approaches to understanding what it means for a device to be “responsible for” the development of CTCs. Exploring these alternatives is one place that philosophers can hope to make a contribution to an ongoing discussion that, to date, has been carried mainly by the physics community. Participating in this discussion means that philosophers have to forsake the activity of logical gymnastics with the paradoxes of time travel for the more arduous but (we believe) rewarding activity of digging into the foundations of physics.

Time machines may never see daylight, and perhaps so for principled reasons that stem from basic physical laws. But even if mathematical theorems in the various theories concerned succeed in establishing the impossibility of time machines, understanding why time machines cannot be constructed will shed light on central problems in the foundations of physics. As we have argued in Section 4, for instance, the hunt for time machines in general relativity theory should be interpreted as a core issue in studying the fortunes of Penrose’s cosmic censorship conjecture. This conjecture arguably constitutes the most important open problem in general relativity theory. Similarly, as discussed in Section 5, mathematical theorems related to various aspects of time machines offer results relevant for the search of a quantum theory of gravity. In sum, studying the possibilities for operating a time machine turns out to be not a scientifically peripheral or frivolous weekend activity but a useful way of probing the foundations of classical and quantum theories of gravity.

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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.

• Krasnikov, S., 2003, “ Time Machine (1988-2001) ,” a brief account of the time machine problem; talk given at 11th U.K. Conference on the Foundations of Physics, Oxford, England, 9-13, Sept. 2002.
• Rovelli, C., 2008, “ Loop Quantum Gravity ”, in Living Reviews in Relativity .

determinism: causal | space and time: the hole argument | time: thermodynamic asymmetry in | time travel | time travel: and modern physics

Acknowledgments

We thank Carlo Rovelli for discussions and John Norton for comments on an earlier draft. C.W. acknowledges support by the Swiss National Science Foundation (grant PBSK1-102693).

Copyright © 2020 by John Earman < jearman @ pitt . edu > Christian Wüthrich < christian . wuthrich @ unige . ch > JB Manchak < jmanchak @ uci . edu >

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Scientists have actually discovered how to time travel

Ready to be a little bit confused by complex science? Ok, here we go…

Until now, time travel has only been possible in fiction (and on TikTok, according to some ), but it might not be as far away as previously thought.

Scientists delving into subatomic structures and quantum physics have claimed that the concept of time travel might actually be possible.

The concept of humans hopping back and forth through time is still exactly that – a concept.

However, scientists from the Austrian Academy of Sciences (ÖAW) and University of Vienna believe they’ve proven it’s achievable in the world of quantum physics.

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In fact, in a series of papers over recent years, Miguel Navascués of ÖAW and Philip Walther of University of Vienna have been entertaining the idea of speeding up and reversing the flow of time at a subatomic level.

Speaking to the Spanish-language publication El País , Navascués attempted to explain the phenomenon by comparing it to watching a film.

“In a theater [classical physics], a movie is projected from beginning to end, regardless of what the audience wants,” he said. “But at home [the quantum world], we have a remote control to manipulate the movie. We can rewind to a previous scene or skip several scenes ahead.”

How do they do it? With a device called a “quantum switch”, which sounds more like something out of a Marvel movie than something in real life.

They use the device to ‘evolve’ a photon as it passes through a crystal. That way, the photon returns to its previous state before it completes the journey. The idea is to alter the states of quantum particles, which the scientists call “time translation”.

Humans going back and forth in time for now remains impossible, though, namely because human bodies contain so much information that it would take millions of years to process it all and make it work.

As you’d expect, the whole thing is incredibly complicated to track and observe even at a subatomic level. Just watching a system causes it to change. Once it changes, its progress through time can’t be recorded.

Explaining their method for going forward in evolutionary time, Navascués explained: “To make a system age 10 years in one year, you must get the other nine years from somewhere. In a year-long experiment with 10 systems, you can steal one year from each of the first nine systems and give them all to the tenth. At the end of the year, the tenth system will have aged 10 years; the other nine will remain the same as when the experiment began.”

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• The Inventory

All the Evidence that Time Travel is Happening All Around Us

This iconic footage of a person apparently talking on a cellphone in a Charlie Chaplin film is just one clue that time travel is happening all around us. People have seen the past, and the future — and there are tons of telltale photographs and films. Here are the clearest signs of real-life time travel.

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The mouberly-jourdain incident (or the ghosts of petit trianon) in the gardens of the petit trianon, a small château located on the grounds of the palace of versailles, france, august 10, 1901.

Two female academics, Eleanor Jourdain and Charlotte Anne Moberly allegedly experienced here a time slip, and saw Marie Antoinette, the Comte de Vaudreuil and some other people in the time of the French Revolution.

(via Myrabella and Wikimedia Commons )

An old woman using mobile phone in a short clip from the DVD extras of Charlie Chaplin's film The Circus, 1928, spotted only in 2010 by filmmaker George Clark

It could be a Siemens hearing instrument, patented in 1924 or a Western Electric Model 34A Audiphone Carbon Hearing Aid (pictured below).

And the same explanation for this video from 1938, showing a crowd exiting a factory in Massachusetts, 1938:

(via Hearing Aid Museum )

The time slip of Air Marshal Sir Robert Victor Goddard over the former Royal Air Force station Drem Airfield in 1935

"In 1935, while still a Wing Commander, he was sent to inspect a disused airfield near Edinburgh at a place called Drem. He found it in a very dilapidated state with cattle grazing on grass that had forced through cracks in the tarmac. Later that day, he ran into trouble while flying his biplane in heavy rain and decided to fly back to Drem to get his bearings. As he approached the airfield the torrential rain abruptly changed to bright sunlight. When he looked down he saw the airfield had been completely renovated and was now in use. There were mechanics in blue overalls walking around and four yellow planes parked on the runway. One of these was a model which, for all his aviation experience, he completely failed to recognize." – according to Time Travel: A New Perspective, by J. H. Brennan

Four years later, RAF began to paint their planes yellow and the mechanics uniforms were switched to blue.

(via Planes and Choppers and Scotlands Places )

The man often called Time Traveling Hipster from the reopening ceremony of South Forks Bridge in Gold Bridge, British Columbia, Canada, 1941

That type of sunglasses with leather side shields were used since the 1920s and he's wearing the sweater of a hockey team instead of a modern T-shirt, but it's still a cool photo!

(via Forgetomori )

The Philadelphia Experiment, 1943

There were no such experiments, of course, but some reports stated that U.S. Navy destroyer escort USS Eldrige travelled back in time for about 10 seconds on October 28, 1943.

(via Wikimedia Commons )

Henry Fonda and Shirley Temple checking their stagecoach route on an iPhone in the movie Fort Apache (1948)

We promise that they used Google Maps instead of Apple's Maps app.

The Fentz legend, 1950s

This urban legend is about a man in his early thirties named Rudolph Fentz, who was hit by a taxi and fatally injured at New York City's Time Square in mid-June 1950, dressed in the fashion of the late 1800s. In his pockets there were a copper token for a beer, a bill for the care of a horse and the washing of a carriage, a letter from 1876, 70 dollars and business cards, all without any signs of aging. A NYPD policeman found a person who was disappeared in 1876 in the age of 29.

The story originated in a 1951 sci-fi short story by Jack Finney, but the legend has been reported since the 1970s as an evidence for the existence of time travel.

(via Transpress NZ )

The Mountauk Project conspiration theory

The Montauk Air Force Station reportedly has a real time tunnel in its subterranean laboratory that allowed scientists to travel back to 1943. This story started with two men, the author Preston B. Nichols and Al Bielek in the 1980s, when they had begun to "recover repressed memories of working in the lab".

Time-traveler busted for insider trading, March 2003

The story originated with the Weekly World News, but appeared in some newspapers after Yahoo reprinted it two weeks later:

[…]"The fact is, with an initial investment of only $800, in two weeks' time he had a portfolio valued at over $350 million. Every trade he made capitalized on unexpected business developments, which simply can't be pure luck."

"The only way he could pull it off is with illegal inside information. He's going to sit in a jail cell on Rikers Island until he agrees to give up his sources." The past year of nose-diving stock prices has left most investors crying in their beer. So when Carlssin made a flurry of 126 high-risk trades and came out the winner every time, it raised the eyebrows of Wall Street watchdogs. […] Carlssin declared that he had traveled back in time from over 200 years in the future, when it is common knowledge that our era experienced one of the worst stock plunges in history. Yet anyone armed with knowledge of the handful of stocks destined to go through the roof could make a fortune. "It was just too tempting to resist," Carlssin allegedly said in his videotaped confession. "I had planned to make it look natural, you know, lose a little here and there so it doesn't look too perfect. But I just got caught in the moment." […] – according to a Yahoo Entertainment article, found in the Internet Archive here.

(via Engelcast and Snopes )

The story of John Titor

A man, appeared on some online bulletin boards in 2000 and 2001, and claiming to be a time traveler from 2036. He made numerous predictions about events after 2004 and often described his time machine .

None of those events have happened yet.

(via Stranger Dimensions )

The story of Håkan Nordkvist, who just met with an older version of himself, 2006

Nordkvist slipped through a wormhole in his kitchen and met an older man who had the same tattoo as Håkan. He just knew that no one's going to believe this — so he filmed the encounter.

As everyone later discovered, this was just part of an advertising campaign of the Swedish insurance company AMF, made by Forsman & Bodenfors.

(via Forsman&Bodenfors )

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• Q: Will time travel ever be invented?

Physicist : It already has!  Or rather, it already will be.

Once time travel has/will have been invented, you’d think that said inventor could just go back in time and show off their invention, or give it to some ancestor (or even become some ancestor) so they could be born into old money.

But despite being both common and world-changing, time travel is intrinsically very low-key.  In the 20th century the world population increased from 1.6 to 6 billion people, and even though time travelers account for about 3 of those 4.4 billion new folk, evidence for their presence is almost impossible to find.  It turns out that time machines are just like every other machine; they don’t exist if they’re not invented.  So whatever else anyone does with a time machine, it didn’t/won’t affect the invention of time machines themselves.

In 1992 Steven Hawking derived the “ chronology protection conjecture “, which posits that “closed time-like curves” are impossible.  Moving along a time-like path is what you (and every other chunk of matter in the universe) are doing right now; moving slower than light and experiencing time in the usual way.  Moving along a closed time-like path is like going for a walk in the woods and following a trail that returns you home yesterday.  Hawking showed that closed time-like curves produce “feed back” that destroys everything involved.  In other words: Timecop rules.

Ever empirical, on June 28th, 2009 Doc Hawk threw a party for time travelers and (to ensure only time travelers showed up) he kept it secret until June 29th, when he sent out invitations.  Save the date!

To his bemused shock, Hawking’s soiree was very well attended.  He claims to have met “people” from as far afield as 70189324233 AD, the year in which the invitation, as well as the spaciotemporal coordinates of Earth, were unceremoniously overwritten and forgotten during “The Great System Update”.

Left: Steven Hawking, blocking the photographer from getting into the party behind him. Right: The invitation he sent out the next day.  Hope you can/did make it!

At his party, the Hawk discovered three things.  First, time travel is not just possible, but easy.  Second, closed time-like curves are impossible, but that’s not how time travel works.  And third, time travelers don’t leave much evidence behind, because they couldn’t if they tried (and don’t when they do).

Hawking later wrote, “ Dear Diary, [I] wasn’t sure about actually buying champagne for the affair, since I knew (or thought I knew) that this was all a [silly stunt].  I’m glad I did!  Time travelers are a cagey lot and the evening didn’t really get into full swing until the 7th or 8th crate was opened.  A man (perhaps?) who introduced himself as the Designate Demithrall of the North Antarctic Seasteader Federation in 4372, mentioned that the key to time travel is my own work on imaginary time and that it’s ‘obvious really, if you think about it’.  This is remarkable!  But in the sober light of da [sic.] I can’t help wondering if the Designate Demithrall wasn’t drunk or sarcastic or both.  Forty-forth century humour is really hard to read. ”

When a time traveler intends to give instructions to someone in the past to help them be the first person to build a time machine, they inevitably and accidentally don’t.  The retro-self-cohesion principle of the time-line prevents grandfather paradoxes, so neither time travelers nor machines can change the logic of their own history.  In other words: not Back to the Future rules.  For example, if you go back in time to kill your own grandfather, then you won’t exist to go back in time and do said killing.  You have to come from somewhen.  Inescapably, you’ll either get the wrong guy or fail to get the right guy.  In other words: Bill and Ted rules.  Time travel is possible, and even common, but you can’t change things so much as confirm them.  In the archetypal example, Rufus goes back in time to ensure the Wild Stallions succeed in bringing about peace and enlightenment throughout the universe, and he knows they do because he was/will be there to help.

The “grandfather paradox”. Like all paradoxes, this only shows up on paper.  It can’t happen in reality.

In the same way that you don’t (presently) worry about your murderous unborn grandchildren, the inventor of time travel is immune to hints.  No matter how many time travelers they may incidentally meet, none of them will ever get past general pleasantries; the topic of time travel is logically verboten.  The same holds for common knowledge.  Presumably, the reason that you can’t go online and find the schematics for a (functional) time machine is that the future inventor of time travel doesn’t live in a cave.  The first thing they’re likely to do before getting down to work is a quick internet search to see if they’re reinventing the wheel (or flux capacitor), so all the universe must conspire to make that internet search fail.  Like time travelers themselves, the idea has to come from somewhen.  Being aware of this tautological time travel truth, and possibly having read Hawking’s published diary, the Designate Demithrall was most likely safeguarding the logical consistency (and existence) of the very conversation he was in by filling it with sarcasm and misdirection.

So if you ever meet anyone who claims to be a time traveler and makes no attempt to support their claim, then they’re probably telling you the truth.  Time machines are more common then cellphones, but they’re literally impossible to talk about.  And if you yourself are a time traveler, remember that we ran/will run out of prosecco about halfway through Hawking’s thing, so BYOB.

16 Responses to Q: Will time travel ever be invented?

LOVE IT!!! Happy AF to you as well 😉

The question of whether time travel will or will not ever be invented is much the same question as “Will my brain ever progress to the point that I will understand the futility of asking such a moronic question?” It reminds me of the days when everyone was asking Marconi “When are you going to invent a radio?” No it doesn’t.

Great 1st April joke!!!

There is of course a much simpler answer (from the perspective of FTL physics), time travel is impossible because above the speed of light Einstein was wrong. Below the speed of light and on its ‘mechanics’ side Special Relativity is one of the strongest and most accurate theories in physics. However at the speed of light and above Special Relativity is really no more than speculation.

Time travel as described by science fiction writers requires something called a general time dimension – but if such a thing exists then time travel should be easy and should be observable all the time. Steven Hawkings argument against closed time loops is actually a strong argument against such a time dimension. Another strong argument against it is the whole theory of quantum mechanics, because quantum uncertainty and a general time dimension at mutually exclusive.

In a universe without a general time dimension things like the grandfather paradox simply don’t exist. Paradox itself is an indication of an incomplete or wrong model.

Time travel of a sort is still at the very edge of being possible in a universe with no general time dimension but the time traveler destroys everything in the enclosing volume of the light cone of their jump, replacing it with a new version. The old future they leave behind is completely obliterated and the new past they enter is only some form of recreated copy of a previous past. A true time jump backwards is still at the verge of being possible but (like Steven Hawkings closed time loops) creates a reverse causality funnel that ends with the universe collapsing back in to the Big Bang. This kind of time loop very probably does exist though because it is a viable finite solution to the Anthropic / Tuned Laws of Physics problem. A universe containing life evolving through billions of cycles of universe evolution.

Some believe that the Bible implies God might exist in the form of light (do photons experience time?).

Yes it’s April 1! But what year?

You never cease to come up with bigger and bigger loads of garbage and absolute nonsense. How unscientific and ridiculous.

I attended an auditorium to watch a grandchild in a play yesterday. I filmed it, of course. Initially I made an error in selecting the definition mode and when I watched it, the video wasn’t quite as sharp as it could have been. Darn.

But then I realized a person behind me was taking cell video just as I was adjusting the camera and would have caught me in the edge of their video frame. I contacted them and got to see me do it.

So then in addition, I began to tell everyone that there was a video of me properly adjusting my camera until I now fully believe it myself because of the Often-ness Effect. Sure enough, when I just watched my video again before posting, the setting is now perfect and I don’t remember when it wasn’t.

I attribute it to the quantum effect. Me observing me making my proper adjustment made the result come out. I learned this trick from watching a video about Hawking, Bohr and quantum mechanics. It’s all in the power of being thee observer, which of course, can fool Mother Nature.

I’m surprised Einstein never realized this, but I guess nobody is perfect. Now I’m thinking about going back and taking some cool pictures of Albert and selling them in 2017. Of course I won’t be able to tell him what I’m doing.

It’s ironic that the main instinctive argument against time travel is causality because that is what makes time travel possible.

We used to think that a moving, read accelerating body moved through three dimensions. Now we know that it moves through four. We theorize that there are more dimensions but we don’t seem to presume that the accelerating body moves through them.

Think of the moving body progressing parallel universes from ‘now’ to all possible alternate futures that have a non-zero probability. An infinite number.

The link between a -t point in the ‘past’ and a t=0 point is effectively causality.

The relationship between a -t point and a t=0 point on the same causality line is 1:1 but the link between t=0 and any positive t value is one to many, or one to infinity.

The probability of my grandson popping into existance is non-zero as long as time travel is possible. If he were to kill me, the probability of his ‘future’ including his birth reduces to zero. But that doesn’t stop him from existing because in his -t, there occurred the causal event of him popping into existence.

Practical time travel depends on causality because travelling in a -t direction anywhere outside the direct causasality chain means ‘landing’ in a past that is impossible to know and has a non-zero probability of being inconsistent with the existence of matter.

It’s no longer 1 April

@Okieprof No, photons do not experience time, at least not time as we perceive it. From the perspective of a photon everything in its entire existence happens all at once in the same timeless instant.

The main issue with time travel is that we have such a limited and restrictive perception of time that we simply don’t know how time actually works, an unfortunate but predictable consequence of being limited to little more than watching time as it passes by us.

In short; we don’t have enough dimensions to understand this stuff without experimental data, which we can’t get because insufficient dimensions.

One day, someone may figure out how to loophole around this, until that day (or something along those lines) we are sadly restricted to hypothetical arguments on the internet and can go no further.

@ A Time Traveler… What do you mean, “It’s no longer 1 April”?

We can always go back to April 1, 2019 (or 2017). It’s a simple matter. All one need do is go back in time… like I’m doing now. Or did.

Now there are those that don’t believe we can violate the Arrow of Time; in other words go backwards in time instead of just slow it down by going fast. But it is a simple matter really. All one need do is be able to travel twice the speed of light.

For instance, first travel at the speed of light out into the friendly vacuum of space until one catches up with the light emitted from Ford Theater just before Lincoln was shot (or was/will be). Enjoy the view for a bit. Then quickly go back to earth at the speed of light again, to the theater before anything can happen, and stop Booth by dousing him and his gunpowder with a bucket of cold water.

Saving Honest Abe would be worth it. His next political rival would have at least needed to be somewhat honest to defeat him. After the popular trend was started, I think it likely all politicians would have necessarily been honest thereafter. Or we could keep going back, and do it again, until it finally worked anyway.

Of course we might lose the phrase, “Break a leg”, concerning theater. Others don’t believe Booth started this.

@Wes How exactly does one catch up to light by traveling at the speed of light?

… “How exactly does one catch up to light by traveling at the speed of light?”

@Neruz First… realize everything I write concerning the first day of April is intended in the spirit of humor.

Then slightly more seriously: Ok, you got me. A very sharp pointed question. Touché.

I think the Lorentz/Fitzgerald Contraction Ratio rules. Roughly, it seems we could possibly catch light in theory because the distance, to the “light-wavicles” we chase, becomes infinitely short if we leave at exactly the same time.

But since we are leaving well after the assassination, quite some years later, and the light from the Ford Theater is still moving under Einstein’s coordinate system even as we chase it, we must impossibly go even faster to beat the Ford wavicles to some appropriate distant rendezvous to view the living Lincoln.

Furthermore, this appropriate rendezvous must allow a bit of early recreational theater viewing, yet allow enough time to travel back to the theater at light-speed to stop (or at nearly light-speed to nearly stop) the theater managers clock in order to arrive in time to stop the dastardly assassination attempt.

Unfortunately, it is not possible for entire atoms (us) to even reach the speed of light because all generally known propellant forces reduce to electromagnetic in nature and, according to Maxwell, such electromagnetic thrust can only travel at the speed of light, max. Therefore I think any vessel mass at all precludes ever reaching any-speed if the thrust force isn’t pushing at least a hair faster than the desired said any-speed, and any-speed includes light-speed.

Obviously, and slightly less seriously, we would need to use something faster than light for thrust, something like Quantum Entanglement Drive (Bohr) or Spooky Repulsive Action at a Distance(Einstein).

Near April 1, there is the B.S. engineering factor, through and through.

As a close friend to the Designate Demithrall of the North Antarctic Seasteader Federation, I came back in time to say that I strongly resent the insinuation that he may or may not be drunk during the said party. He hasn’t been sober since the late 60s, for sure.

In the Temporal State, we are not just limited by time, we are also limited by sequence, i.e., the sequence of events. Even if you could physically step into the Sequential State, where there is not limitation of time, you will still be limited by sequence. Now in the Concurrent State, there is no limitation of time nor sequence. Jesus demonstrated that by saying, “Before Abraham was born, I am!” (John 8:56-58). That means he not only existed before Abraham but before his mother. Not limited by time nor sequence. If we had the ability to function in the Concurrent State, we could travel in time and not mess things up, unless we wanted to.

Surely there’s a simple answer to all this time travel stuff. The future does not yet exist and the past has been and gone. We can and do travel into the future whenever we move. We just don’t notice the difference. But if we travelled fast enough, we would notice, and we would see the future evolving.

Travelling into the past is a different problem. Without getting into the realms of parallel universes, it’s not at all obvious that ‘the past’ still exists. To genuinely travel backwards in time within our own universe, we would have to rearrange it into an earlier state, something that the second law of thermodynamics will not allow.

Note, however, that the second law does not preclude rearranging a subset of the universe into an earlier state. Given enough information, a technologically advanced entity (call it God if you like) could, in theory, rearrange a whole planet. Let’s hope it isn’t ours.

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• Q: If the Sun pulls things directly toward it, then why does everything move in circles around it?
• Q: Why is the area of a circle equal to πR 2 ?
• 0.999… revisited
• Q: Before you open the box, isn’t Schrödinger’s cat alive or dead, not alive and dead?
• Q: How many times do you need to roll dice before you know they’re loaded?
• Q: Since it involves limits, is calculus always an approximation?
• Q: How does Earth’s magnetic field protect us?
• Q: If a long hot streak is less likely than a short hot streak, then doesn’t that mean that the chance of success drops the more successes there are?
• Q: Where do the rules for “significant figures” come from?
• Q: If time slows down when you travel at high speeds, then couldn’t you travel across the galaxy within your lifetime by just accelerating continuously?
• Q: When something falls on your foot, how much force is involved?
• Q: If nothing can escape a black hole’s gravity, then how does the gravity itself escape?
• Q: Is there a formula for finding primes? Do primes follow a pattern?
• Q: If the number of ancestors you have doubles with each generation going back, you quickly get to a number bigger than the population of Earth. Does that mean we’re all a little inbred?
• Q: Why are many galaxies, our solar system, and Saturn’s rings all flat?
• Q: How do you define the derivatives of the Heaviside, Sign, Absolute Value, and Delta functions? How do they relate to one another?
• Q: What does “E=mc 2 ” mean?
• Q: Is it possible to have a completely original thought?
• Q: How can the universe expand faster than the speed of light?
• Q: How fast are we moving through space? Has anyone calculated it?
• Q: If you flip a coin forever, are you guaranteed to eventually flip an equal number of heads and tails?
• Q: What is radioactivity and why is it sometimes dangerous?
• Q: How do we know that π never repeats? If we find enough digits, isn’t it possible that it will eventually start repeating?
• Q: Why does carbon dating detect when things were alive? How are the atoms in living things any different from the atoms in dead things?
• Q: What role does Dark Matter play in the behavior of things inside the solar system?
• Q: Are some number patterns more or less likely? Are some betting schemes better than others?
• Q: Why does iron kill stars?
• Q: According to relativity, things get more massive the faster they move. If something were moving fast enough, would it become a black hole?
• Q: How do we know that atomic clocks are accurate?
• Q: “i” had to be made up to solve the square root of negative one. But doesn’t something new need to be made up for the square root of i?
• Q: Could the tidal forces of the Sun and Moon be used to generate power directly?
• Q: What would it be like if another planet just barely missed colliding with the Earth?
• Q: What are “delayed choice experiments”? Can “wave function collapse” be used to send information?
• Q: Why can some creatures walk on water yet I (a human) can’t?
• Q: What fair dice can be simulated by adding up other dice?
• Q: How do I encrypt/hide/protect my email?
• Q: Where do the weird rules for rational numbers come from? (Dealing with fractions)
• Dragon*Con 2013
• Q: Why doesn’t the air “sit still” while the Earth turns under it?
• Q: Can resonance be used to destroy anything? Is the “brown note” possible?
• Q: Are there examples of quantum mechanics that can be seen in every-day life, or do they only show up in the lab?
• Q: Why does it take thousands of years for light to escape the Sun?
• Q: What does it mean for light to be stopped or stored?
• Q: What are quasi-particles? Why do phonons and photons have such similar names?
• The nuptial effect
• Q: How do you prove that the spacetime interval is always the same?
• Q: Are numbers real?
• Q: If time were reversed would things fall up?
• Q: Why don’t “cheats” ever work on the uncertainty principle? What’s uncertain in the uncertainty principle?
• Q: Do the past and future exist? If they do, is the future determined and what does that mean for quantum randomness?
• Basic math with infinity
• Q: What is the Planck length? What is its relevance?
• Q: What causes friction? (and some other friction questions)
• Q: Is fire a plasma? What is plasma?
• Q: Why are determinants defined the weird way they are?
• Q: Are white holes real?
• Q: If a photon doesn’t experience time, then how can it travel?
• Q: What is energy? What is “pure energy” like?
• Q: Why is Schrodinger’s cat both dead and alive? Is this not a paradox?
• Q: What kind of telescope would be needed to see a person on a planet in a different solar system?
• Q: Is Murphy’s law real?
• Q: Why doesn’t life and evolution violate the second law of thermodynamics? Don’t living things reverse entropy?
• Q: Does quantum mechanics really say that there’s some probability that objects will suddenly start moving or that things can suddenly “shift” to the other side of the universe?
• Q: Using modern technology, are we any closer to turning lead into gold than alchemists were hundreds of years ago?
• Q: How do you turn/change directions in space?
• Q: If a man hangs on an un-insulated wire using both his hands what will happen and why?
• Learning intro number theory
• Q: Is the Alcubierre warp drive really possible? How close are we to actually building one and going faster than light?
• Q: Is darkness a wave the way light is a wave? What is the speed of dark?
• Q: Is it a coincidence that a circles circumference is the derivative of its area, as well as the volume of a sphere being the antiderivative of its surface area? What is the explanation for this?
• Q: If hot air rises, why is it generally colder at higher elevations?
• Q: What is quantum teleportation? Why can’t we use it to communicate faster than light?
• Q: Since all particles display wave-like characteristics, does that imply that one could use destructive wave interference to destroy or at least drastically change a particle?
• Q: How does the Oberth Effect work, and where does the extra energy come from? Why is it better for a rocket to fire at the lowest point in its orbit?
• Q: How do lenses that concentrate light not violate the second law of thermodynamics? If you use a magnifying glass to burn ants, aren’t you making a point hotter than the ambient temperature without losing energy?
• Q: What makes natural logarithms natural? What’s so special about the number e?
• Q: If the world were to stop spinning, would the people and everything on it be considered ‘lighter’ or ‘heavier’? Would any change take place? And does centrifugal force have an effect on gravity?
• Q: Two entangled particles approach a black hole, one falls in and the other escapes. Do they remain entangled? What about after the black hole evaporates?
• Q: If there are 10 dimensions, then why don’t we notice them?
• Q: Will the world end tomorrow?
• Q: In an infinite universe, does everything that’s possible have to happen somewhere?
• Q: Which of Earth’s life forms could survive on each planet of the Solar System?
• Q: What are fractional dimensions? Can space have a fractional dimension?
• Q: Are shadows 2-dimensional? Are there any real examples of 2-dimensional things in the universe?
• Q: Is it possible to experience different rates of time? If time were to speed up, slow down, or stop, what would you experience?
• Q: How many theorems are there?
• Q: How much of a direct effect do planets and stars have on us? Is astrology reasonable or plausable?
• Q: Why are scientists looking for life in space by looking for water? How can they be sure that all life uses water?
• Q: If energy is neither created nor destroyed, what happens to the energy within our bodies and brains when we die?
• Q: Could Kurt Vonnegut’s “Ice-9 catastrophe” happen?
• Q: How accurately do we need to know π? Is there a reason to know it out to billions of digits?
• My bad: If fusion in the Sun suddenly stopped, what would happen?
• Q: If fusion in the Sun suddenly stopped, what would happen?
• Q: Does opening a refrigerator cool down the room?
• Q: What is the probability of an outcome after it’s already happened?
• Q: How do you answer a question scientifically?
• Q: Why are the days still longer than nights, until a few days after the fall equinox?
• Q: What is a Fourier transform? What is it used for?
• Q: What are singularities? Do they exist in nature?
• Q: Is it likely that there are atoms in my body that have traveled from the other side of the planet, solar system, galaxy, or universe?
• Q: Is there a number set that is “above” complex numbers?
• Q: Are the brain and consciousness quantum mechanical in nature?
• Q: How are voltage and current related to battery life? What is the difference between batteries with the same voltage, but different shapes or sizes? What about capacitors?
• Q: What are virtual particles?
• Q: Would it be possible to create an antimatter weapon by “harvesting” enough antimatter, containing it in an electro-magnetic field and placing that in a projectile?
• Q: If Earth was flat, would there be a horizon? If so, what would it look like? If the Earth was flat and had infinite area, would that change the answer?
• Q: Is there an experiment which could provide conclusive evidence for either the Many Worlds or Copenhagen interpretations of quantum physics?
• Q: If you could drill a tunnel through the whole planet and then jumped down this tunnel, how would you fall?
• Q: How many people riding bicycle generators would be needed, in an 8-hour working day, to equal or surpass the energy generated by an average nuclear power plant?
• Q: Why is hitting water from a great height like hitting concrete?
• Q: How does instantaneous communication violate causality?
• Q: What is the “False Vacuum” and are we living in it?
• Q: How would the universe be different if π = 3?
• Q: Is it possible for an artificial black hole to be created, or something that has the same effects? If so, how small could it be made?
• Q: Do colors exist?
• Q: How can we see the early universe and the Big Bang? Shouldn’t the light have already passed us?
• Q: Are beautiful, elegant or simple equations more likely to be true?
• Q: If quantum mechanics says everything is random, then how can it also be the most accurate theory ever?
• Q: Why do wet stones look darker, more colorful, and polished?
• Q: What would the universe be like with additional temporal dimensions?
• The 2012 Venus transit
• Q: Why haven’t we discovered Earth-like planets yet?
• Q: Is quantum randomness ever large enough to be noticed?
• Q: How is radiometric dating reliable? Why is it that one random thing is unpredictable, but many random things together are predictable?
• Q: Is the final step in evolution an ascension into an energy-based lifeform?
• Q: What would life be like in higher dimensions?
• Q: How much does fire weigh?
• Q: Since the real-world does all kinds of crazy calculations in no time, can we use physics to calculate stuff?
• Q: Is there some way to actually play quidditch?
• Q: Can you poke something that’s far away with a stick faster than it would take light to get there?
• Q: Does how you deal cards affect how random they are?
• Q: Will CERN awaken the Elder Gods?
• Q: The information contained in a big system isn’t the same as the amount of information in its parts. Why?
• Q: Is the quantum zeno effect a real thing?
• Q: Is there an intuitive proof for the chain rule?
• Q: How do you write algorithms to enycrypt things?
• Q: Satellites experience less time because they’re moving fast, but more time because they’re so high. Is there an orbit where the effects cancel out? Is that useful?
• Q: Is it possible to objectively quantify the amount of information a sentence contains?
• Q: What would happen if a black hole passed through our solar system?
• Q: If you are talking to a distant alien, how would you tell them which way is left and which way is right?
• Q: Would it be possible in the distant future to directly convert matter into energy?
• Q: What’s the difference between anti-matter and negative-matter?
• Q: Why does gravity make some things orbit and some things fall?
• Q: Do you need faith to believe in science?
• Q: What keeps spinning tops upright?
• Q: Do time and distance exist in a completely empty universe?
• Q: Why is it that photographs of wire mesh things, like window screens and grates, have waves in them?
• Q: How does quantum physics affect electron configurations and spectral lines?
• Q: Is it possible for an atomic orbital to exist beyond the s, p, f and d orbitals they taught about in school? Like could there be a (other letter) orbital beyond that?
• Q: Will the world end in 2012?
• Q: How do you find the height of a rocket using trigonometry?
• Q: What are chaos and chaos theory? How can you talk about chaos?
• Q: What is the Riemann Hypothesis? Why is it so important?
• Q: Why does the entropy of the universe always increase, and what is the heat death of the universe?
• Q: Could God have existed forever? Is it actually feasibly possibly for some ‘being’ to have just existed, infinitely?
• Q: How can wormholes be used for time travel?
• Q: If gravity suddenly increased would airplanes fall out of the sky, or would it compress the air in such a way that airplanes could keep flying?
• Entanglement omnibus!
• Q: How are imaginary exponents defined?
• Q: Why do nuclear weapons cause EMPs (electromagnetic pulses)?
• Q: How does the expansion of space affect the things that inhabit that space? Are atoms, people, stars, and everything else getting bigger too?
• Q: What would Earth be like if it didn’t turn?
• Q: According to the Many Worlds Interpretation, every event creates new universes. Where does the energy and matter for the new universes come from?
• Q: Can wind chill make things “feel” colder than absolute zero?
• Q: What is “spin” in particle physics? Why is it different from just ordinary rotation?
• Q: What is Bayes’ rule and how do I use it to improve my life?
• Q: Are there universal truths?
• Q: What’s the difference between black holes and worm holes? Could black holes take you to other universes?
• Q: Is there an equation that determines whether a question gets answered on ask a mathematician/physicist?
• Q: If you could hear through space as though it were filled with air, what would you hear?
• Q: What is the three body problem?
• Q: How are fractals made?
• Q: CERN’s faster than light neutrino thing: WTF?
• Q: What’s the point of purely theoretical research?
• Q: Why does lightning flash, but thunder rolls?
• Q: Hyperspace, warp drives, and faster than light travel: why not?
• Burning Man 2011
• Q: If light slows down in different materials, then how can it be a universal speed?
• Q: What is mass?
• Q: How much of physics can be deduced from previous equations/axioms?
• Q: If God were all-seeing and all-knowing, the double-slit experiment wouldn’t work, would it? Wouldn’t God’s observation of the location of the photon collapse its probability wave function?
• Q: How do those “executive ball clicker” things work?
• Q: Why is cold fusion so difficult?
• Q: Why does light choose the “path of least time”?
• Q: Does light experience time?
• Q: Would it be possible for humans to terraform mars?
• Q: Can light be used to transfer energy instead of power lines?
• Particle physics, neutrinos, and chirality too!
• Q: What are integral transforms and how do they work?
• Q: How does reflection work?
• Q: What does a measurement in quantum mechanics do?
• Q: If you stood in the beam of a particle accelerator, what would happen?
• Q: What exactly is the vacuum catastrophe and what effects does this have upon our understanding of the universe?
• Q: What is a “measurement” in quantum mechanics?
• Q: How close is Jupiter to being a star? What would happen to us if it were?
• Q: Can you fix the “1/0 problem” by defining 1/0 as a new number?
• Q: How can we have any idea what a 4D hypercube or any n-D object “looks like”? What is the process of developing a picture of a higher dimensional object?
• Q: Is it possible to destroy a black hole?
• Q: Why does the Earth orbit the Sun?
• Q: If you suddenly replaced all the water drops in a rainbow with same-sized spheres of polished diamond, what would happen to the rainbow? How do you calculate the size of a rainbow?
• Q: If we meet aliens, will they have the same math and physics that we do?
• Q: Is 0.9999… repeating really equal to 1?
• Q: What would Earth be like to us if it were a cube instead of spherical? Is this even possible?
• Q: How do velocities add? If I’m riding a beam of light and I throw a ball, why doesn’t the ball go faster than light?
• Q: What is the universe expanding into? What’s outside the universe?
• Cheap experiments and demonstrations for kids.
• Q: How do I estimate the probability that God exists?
• Q: How do you calculate 6/2(1+2) or 48/2(9+3)? What’s the deal with this orders of operation business?
• Q: Is there a single equation that proves black holes are real?
• Q: Is the edge of a circle with an infinite radius curved or straight?
• Q: As a consequence of relativity, objects becomes more massive when they’re moving fast. What is it about matter that causes that to happen?
• Q: What is the evidence for the Big Bang?
• Q: Is there a formula to find the Nth term in the Fibonacci sequence?
• Q: Why is the integral/antiderivative the area under a function?
• Mathematical proof of the existence of God.
• Video: Getting Computers to Learn
• Q: What is going on in a nuclear reactor, and what happens during a meltdown?
• Q: How do I find the love of my life? (a Mathematician’s perspective)
• Q: Are all atoms radioactive?
• Q: How do you talk about the size of infinity? How can one infinity be bigger than another?
• Q: Why does E=MC 2 ?
• Q: What are the equations of electromagnetism? What all do they describe to us?
• Q: What is the entropy of nothing?
• Q: How can quantum computers break encryption?
• Q: How does quantum computing work?
• Q: What causes buoyancy?
• My bad: If atoms are mostly made up of empty space, why do things feel solid?
• Q: How many mathematicians/physicists does it take to screw in a light bulb?
• Q: Why is it that (if you exclude 2 & 3) the difference between the squares of any two prime numbers is divisible by 12?
• Q: Why does relativistic length contraction (Lorentz contraction) happen?
• Q: Why does Lorentz contraction only act in the direction of motion?
• Q: If atoms are mostly made up of empty space, why do things feel solid?
• Q: Can we build a planet?
• Q: How does a scientist turn ideas into math?
• Q: Is Santa real?
• Q: Why isn’t the shortest day of the year also the day with the earliest sunset?
• Q: Why does “curved space-time” cause gravity?: A better answer.
• Q: According to relativity, two moving observers always see the other moving through time slower. Isn’t that a contradiction? Doesn’t one have to be faster?
• Q: What does 0^0 (zero raised to the zeroth power) equal? Why do mathematicians and high school teachers disagree?
• Q: Can you do the double slit experiment with a cat cannon?
• Q: How is the “Weak nuclear force” a force? What does it do?
• Q: Does Gödel’s Incompleteness Theorem imply that it’s impossible to be logical?
• Q: If accelerating charges radiate, and everything is full of charges, then why don’t I radiate every time I move?
• Q: If you zoom in far enough, what do particles look like?
• Q: What would you experience if you were going the speed of light?
• Q: Why is pi not a definite number?
• Q: What came before the big bang?
• Q: How do “Numerology Math Tricks” work? (adding digits and tricks with nines)
• Q: What is a magnetic field?
• Q: What is the probability that two randomly chosen people will have been born on the same day?
• Q: Which is a better approach to quantum mechanics: Copenhagen or Many Worlds?
• Q: Why is our vision blurred underwater?
• Q: In the NEC “faster than light” experiment, did they really make something go faster than light?
• Q: How does a Tesla coil work?
• Q: What are Feynman diagrams, how are they used (theoretically & practically), and are there alternative/competing diagrams to Feynman’s?
• Q: Does the 2nd law of thermodynamics imply that everything must eventually die, regardless of the ultimate fate of the universe?
• Q: What is The Golden Ratio? How is it used in Mathematics?
• Q: Why can’t you have an atom made entirely out of neutrons?
• Q: What is the physical meaning of “symmetries”? Why is there one-to-one correspondence between laws of conservation and symmetries? Why is it important that there is such correspondence?
• Q: Why does energy have to be positive (and real)?
• Q: How does the Twin Paradox work?
• Q: How can photons have energy and momentum, but no mass?
• Q: If you were on the inside of the Sun falling in, the matter closer to the surface doesn’t affect your acceleration, but the matter closer to the core does. Why is that?
• Q: How do surge protectors work?
• Relativity and Quantum Mechanics: the elevator pitch
• Q: Why are orbits elliptical? Why is the Sun in one focus, and what’s in the other?
• Q: What would happen if everyone in the world jumped at the same time?
• Q: How can electrons “jump” between places without covering the intervening distance?
• Q: Why do we only see one rainbow at a time?
• Q: Why does putting spin on a ball change how it moves through the air?
• Quantum mech, choices, and time travel too!
• Q: Why is the speed of light finite?
• Q: Why is the speed of light the fastest speed? Why is light so special?
• Q: Will we ever overcome the Heisenberg uncertainty principle?
• Q: If gravity is the reaction matter has on space, in that it warps space, why do physicist’s look for a gravity particle? Wouldn’t gravity be just a bi-product of what matter does to space?
• Q: Is it possible to beat the laws of physics?
• Q: What’s the chance of getting a run of K or more successes (heads) in a row in N Bernoulli trials (coin flips)? Why use approximations when the exact answer is known?
• Q: Aren’t physicists just doing experiments to confirm their theories? Couldn’t they “prove” anything they want?
• Q: What’s up with that “bowling ball creates a dip in a sheet” analogy of spacetime? Isn’t it gravity that makes the dip in the first place?
• Q: If we find a “Theory of Everything” will we be done?
• Q: Is it possible to say if the Earth is moving or sitting still without going into space?
• Q: Will there always be things that will not or cannot be known?
• Q: If you could see through the Earth, how big would Australia look from the other side?
• Q: How is it that Bell’s Theorem proves that there are no “hidden variables” in quantum mechanics? How do we know that God really does play dice with the universe?
• Q: Does an electric field have mass? Does it take energy to move an electric field?
• Q: What would the consequenses for our universe be if the speed of light was only about one hundred miles per hour?
• Q: Do virtual particles violate the laws that energy can be created or destroyed? Have virtual particles ever been observed? In any other instance can energy ever be destroyed or created?
• Video: How do we know that 1+1=2? A journey into the foundations of math.
• Q: Would it be possible to generate power from artificial lightning?
• Q: What is the optimum spectrum to visualize things with? Theoretically, which type of vision would be the best to see things with?
• Q: What causes iron, nickel, and cobalt to be attracted to magnets, but not other metals?
• Q: Is it possible to fill a black hole? If you were to continuously throw galaxies worth of matter into a black hole, would it ever fill up? And what would theoretically happen if all the matter in the universe was thrown into a single black hole?
• Q: Can math and science make you better at gambling?
• Q: Spectroscopy?
• Q: Is it possible to breach the center of a nebula?
• Q: How does a gravitational sling shot actually speed things up?
• Q: If energy is quantized, what is the least amount of energy possible? And how did they measure it?
• Q: How did Lord Kelvin come up with the absolute temperature? I mean, how could he say surely that it was 273.15 C below zero?
• Q: What do complex numbers really mean or represent?
• Q: Is it odd that the universe’s constants are all so perfectly conducive to life?
• Q: How/when will the world end?
• Q: What would happen if an unstoppable force met with an unmovable, impenetrable object?
• My bad: Have aliens ever visited Earth?
• Q: How do Bell pairs (entangled particles) behave experimentally?
• Video: What your Spiritual Guru Never Told you about Quantum Mechanics
• Q: How big does an object have to be to gravitationally attract a Human or have a molten core?
• Video: The Scientific Investigation of Aliens – Evidence Examined
• Q: How do I count the number of ways of picking/choosing/taking k items from a list/group/set of n items when order does/doesn’t matter?
• Q: Who would win in a fight: Gödel or Feynman?
• Q: How hard would it be to make a list of products of primes that could beat public key encryption?
• Q: What are complex numbers used for?
• Q: Can one truly create something from nothing? If matter formed from energy (as in the Big Bang expansion), where did the energy come from?
• Q: Why does wind make you colder, but re-entry makes you hotter?
• Q: Are explosions more or less powerful in space?
• Q: What is infinity? (A brief introduction to infinite sets, infinite limits, and infinite numbers)
• Q: Are there physical limits in the universe other than the speed of light?
• Q: Is it of any coincidence that mathematics is able to describe physical reality – given that both are inventions of the human mind?
• Q: If you were to break down an average human body into its individual atoms, and then laid the atoms out in a single straight line, how far would it stretch?
• Q: What’s it like when you travel at the speed of light?
• Q: Is there a real life example where two negatives make a positive?
• Q: Do the “laws” of physics and math exist? If so, where? Are they discovered or invented/created by humans?
• Q: Do we have free will?
• Q: How did mathematicians calculate trig functions and numbers like pi before calculators?
• Q: How can planes fly upside-down?
• Q: A flurry of blackhole questions!
• Q: Why does going fast or being lower make time slow down?
• Q: What’s so special about the Gaussian distribution (i.e. the normal distribution / bell curve)??
• Q: Is the universe infinitely old?
• Q: Have aliens ever visited Earth?
• Q: Why is the sky blue?
• Q: Is there a formula for how much water will splash, most importantly how high, and in what direction from the toilet bowl when you *ehem* take a dump in it ?
• Q: What is the meaning of the term “random”? Can thinking affect the future?
• Q: Is it possible to choose an item from an infinite set of items such that each one has an equal chance of being selected?
• Q: Do aliens exist?
• Q: Is it true that all matter is simply condensed energy?
• Q: Which is better: Math or Physics?
• Q: Why is the number 1 not considered a prime number?
• Q: If the universe is expanding and all the galaxies are moving away from one another, how is it possible for galaxies to collide?
• Q: What happens when you fall into a blackhole?
• Q: Is the total complexity of the universe growing, shrinking or staying the same?
• Q: If two trains move towards each other at certain velocities, and a fly flies between them at a certain constant speed, how much distance will the fly cover before they crash?
• Q: Why does oxygen necessarily indicate the presence of life?
• Q: What’s the relationship between entropy in the information-theory sense and the thermodynamics sense?
• Q: Would it be possible to kill ALL of Earth’s life with nuclear bombs?
• Q: Will black holes ever release their energy and will we be able to tell what had gone into them?
• Q: What are the Intersecting Chord and Power of a Point Theorems?
• Q: How far away is the edge of the universe?
• Q: Why do superconductors have to be cold?
• Q: Why does the leading digit 1 appear more often than other digits in all sorts of numbers? What’s the deal with Benford’s Law?
• Q: How does the Monty Hall Problem work?
• Q: How/Why are Quantum Mechanics and Relativity incompatible?
• Q: What the heck are imaginary numbers, how are they useful, and do they really exist?
• Q: What’s that third hole in electrical outlets for?
• Q: Do physicists really believe in true randomness?
• Q: Could a simple cup of coffee be heated by a hand held device designed to not only mix but heat the water through friction, and is that more efficient than heating on a stove and then mixing?
• Q: What did Einstein mean by: “Do not worry about your difficulties in Mathematics. I can assure you mine are still greater.”
• Q: Why does saliva boil in the vacuum of space?
• Q: Can things really be in two places at the same time?
• Q: Why do weird things happen so much?
• Q: Will CERN create a black hole?
• Q: What’s the highest population growth rate that the Earth can support?
• Q: What is time?
• Q: What is “Dark Matter”?
• Q: Why do heavy objects bend space and what is it they are bending?
• Q: Why does math work so well at modeling the world around us?
• Q: Why is it that when you multiply a positive number with a negative number you get a negative number?
• Q: What is the best way to understand relativity theory? Why is it so counter intuitive?
• Q: Will we ever go faster than light?
• Q: If you were on a space station, would you be able to tell the difference between centrifugal force and normal gravity?
• Q: Is teleportation possible?
• Q: If black holes are “rips” in the fabric of our universe, does it mean they lead to other universes? If so, then did time begin in that universe at the inception of the black hole? Could we be in a black hole?
• Q: Since pi is infinite, do its digits contain all finite sequences of numbers?
• Q: What is the connection between quantum physics and consciousness?
• Q: What is the probability that in a group of 31 people, none of them have birthdays in February or August?
• Q: What is the meaning of life?
• Q: Why is e to the i pi equal to -1?
• Q: How does a refrigerator work?
• Q: How do I find the love of my life?
• Q: Why does “curved space-time” cause gravity?
• Q: What is monotony?
• Q: How do we know if science is right?
• Q: How plausible is it that the laws of physics may actually function differently in other parts of the universe?
• Q: Are there an infinite number of prime numbers?
• Q: How can we prove that 2+2 always equals 4?

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Breaking news, astrophysicist claims to have invented time machine.

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Time travel may soon be possible, according to an astrophysicist who believes he’s worked out a way to build a time machine.

Professor Ron Mallett from the University of Connecticut in the US claims to have written a scientific equation that could be used to create a device that takes people back in time.

The Physics expert told CNN that he’s already built a prototype to illustrate how the key component of his machine would work.

However, Mallett’s peers are said to be unconvinced by his proposals.

His machine is based around Albert Einstein’s theory of special relativity, which says that time accelerates or decelerates according to the speed at which an object is moving.

For example, if you were on a spacecraft traveling at the speed of light then the theory suggests time would be moving slower for you than people back on Earth.

If you were then to return to Earth having travelled in space for less than a week at this speed, Einstein theorizes that 10 years would have passed on Earth while you were gone.

This would technically mean the astronaut had travelled in time as they would be the same age but their loved ones would be older.

Most physicists accept that this form of time travel could be possible.

Time traveling to the past is faced with much more skepticism but Mallett thinks that lasers could be the solution.

His theory relies on an additional Einstein’s theory that states massive objects can bend space-time.

According to Einstein, the stronger the gravity, the slower time passes.

Mallett told CNN: “If you can bend space, there’s a possibility of you twisting space.

“In Einstein’s theory, what we call space also involves time — that’s why it’s called space time, whatever it is you do to space also happens to time. By studying the type of gravitational field that was produced by a ring laser, this could lead to a new way of looking at the possibility of a time machine based on a circulating beam of light.”

Mallett’s prototype shows how lasers could be used to facilitate his idea.

The astrophysicist has admitted that his idea is wholly theoretical right now and has some sever limitations.

He told CNN: “You can send information back but you can only send it back to the point at which you turn the machine on.”

Time travelling to the past is faced with much more skepticism but Mallett thinks that lasers could be the solution.

“In Einstein’s theory, what we call space also involves time — that’s why it’s called space time, whatever it is you do to space also happens to time.

“By studying the type of gravitational field that was produced by a ring laser, this could lead to a new way of looking at the possibility of a time machine based on a circulating beam of light.”

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