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time travel

3 Popular Time Travel Theory Concepts Explained

Time travel theory. It's one of the most popular themes in fiction. But every plotline falls into one of these three Time Travel Theories.

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Time travel is one of the most popular themes in cinema . Although most time travel movies are in the sci-fi genre, every genre, even comedy, horror, and drama, have tackled complicated storylines involving time travel theory. Chances are, you’ve seen at least a few of the movies listed below:

  • But what about...

The Possibility Of Time Travel

Time travel theory.

  • Bill & Ted’s Excellent Adventure (1989)
  • The Time Machine  (2002)
  • Timeline (2003)
  • Time Cop (2004)
  • Back to the Future  (1985)
  • 12 Monkeys  (1995)
  • Terminator Series (1984)
  • Star Trek (2009)
  • Harry Potter and the Prisoner of Azkaban (2004)
  • Freejack (1992)
  • Looper (2012)

But one thing you might not have realized, even if you’ve seen hundreds of time travel-related films, is that there are only  3 different theories of time travel. That’s it. Every time travel movie or book that you’ve ever enjoyed falls into one of these time travel theories.

Fixed Timeline: Time Travel Theory

Want to change the future on Earth by modifying the past or present? Don’t even bother according to this time travel theory. In a fixed timeline, there’s a single history that is unchangeable. Whatever you are attempting to change by time-traveling is what created the problems in the present that you’re trying to fix ( 12 Monkeys ). Or you’re just wasting your time because the events you are trying to prevent will happen anyway ( Donnie Darko ).

Dynamic Timeline: Time Travel Theory

History is fragile and even the smallest changes can have a huge impact. After traveling back in time, your actions may impact your own timeline. The result is a paradox. Your changes to the past might result in you never being born, like in Back to the Future (1985), or never traveling in time in the first place. In The Time Machine (2002), Hartdegen goes back in time to save his sweetheart Emma but can’t. Doing so would have resulted in his never developing the time machine that he used to try and save her.

One common way to explore this paradox theory is by killing your own grandfather. The grandfather paradox is when a time traveler attempts to kill their grandfather before the grandfather meets their grandmother. This prevents the time travel’s parents from being born and thus the time traveler himself from being born. But if the time traveler was never born, then the traveler would never have traveled back in time, therefore erasing his or her actions involving the death of their grandfather.

Multiverse: Time Travel Theory

Travel all over time and do whatever you want. It doesn’t matter because there are multiple universes and your actions only create new timelines. This is a common theory used by the science fiction TV series, Doctor Who . Using the multiverse theory of time travel, it’s assumed that there are multiple coexisting alternate timelines.

Therefore, when the traveler goes back in time, they end up in a new timeline where historical events can differ from the timeline they came from, but their original timeline does not cease to exist. This means the grandfather paradox can be avoided. Even if the time traveler’s grandparent is killed at a young age in the new timeline, he/she still survived to have children in the original timeline, so there is still a causal explanation for the traveler’s existence.

Time travel may actually create a new timeline that diverges from the original timeline at the moment the time traveler appears in the past, or the traveler may arrive in an already existing parallel universe. There’s just one problem… you can’t go back ( The One , 2002).

But what about…

Some may argue that people who are “trapped” in time are time travelers as well. This happens in countless time travel movies including Robin Williams ‘ character in the 1995 film Jumanji who gets trapped inside a board game. The list of “people who are cryogenically frozen and then successfully thawed out in the future” is even longer and includes Austin Powers: The Spy Who Shagged Me  (1999), Planet of the Apes (1968) and so on.

Although these characters are “moving” through time, they are doing so by pausing and then rejoining the current timeline. The lack of a time machine device disqualifies them from technically being “time travelers” and included in this list of theories on time travel.

So will time travel ever be possible? All we know for sure is that the experts don’t agree. According to the Albert Einstein theory of relativity, time is relative, not constant and the bending of spacetime could be possible. But according to  Stephen Hawking , time travel is not possible. The Stephen Hawking time travel theory suggests that the absence of present-day time travelers from the future is an argument against the existence of time travel — a variant of the Fermi paradox (aka where the hell is everybody?). But it’s fun to think about.

Theories Of Time Travel - Time Travel Theory

NERD NOTE:  What happens to time in a black hole? We don’t know for sure, but according to both Stephen Hawking and Albert Einstein’s theory, time near a black hole slows down. This is because a black hole’s gravitational pull is so strong that even light can’t escape. Since gravity also affects light, time would also slow down.

If you could successfully travel into the future, or back in time, what would you do? Warn people about natural disasters? Buy a winning lottery ticket ? Try to prevent your own death? What do you think about these time travel theory ideas or the time travel movies that we included in this article? Please tell us in the comments below.

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J. Frank Wilson

Frank Wilson is a retired teacher with over 30 years of combined experience in the education, small business technology, and real estate business. He now blogs as a hobby and spends most days tinkering with old computers. Wilson is passionate about tech, enjoys fishing, and loves drinking beer.

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three time travel theories

Mar 24, 2015 at 11:24 PM

are there really only 3 theories? i feel like there are more but i cant think of any besides the movies listed here. hummmmmmmmm

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three time travel theories

Inconceivable Paradoxes: 3 Theories of Time Travel

3 Theories of Time Travel

Time travel is one of those things that seems like it should be possible. After all, it’s been a plot device in countless works of fiction and pop culture . But serious physicists have never been able to come up with a way to do it. These theories explain why we’re not zipping around through time as much as we’d like to—and why you shouldn’t get your hopes up about ever going on a Star Trek-style adventure through history.

The Fixed Timeline

The Fixed Timeline is the theory of time travel where you can only travel to a point in your own future. It’s like time is like a train track and everything that happens has always happened and will always happen. The train cannot go backwards, it cannot go off the track and if you’re on the train you can only move forward with it. This means that if we were to travel back in time then our actions would already have been done before they could even happen in front of us again! This theory also suggests that there are parallel universes out there where every single event that ever happened or will ever happen has already happened somewhere else (and probably more than once).

The Dynamic Timeline

The Dynamic Timeline is a theory that says that the timeline is dynamic and can be changed. The theory is that the timeline is not fixed and can be changed by time travelers or other events in one’s life. This means that it’s possible for you to change your past, present and future if you travel back in time like in Back to the Future or 12 Monkeys.

The Multiverse

The multiverse theory , also known as the many-worlds interpretation (MWI), is a theory that states that our universe is just one of many universes. In this model, all possible alternative histories and futures actually exist; it’s just that we can’t see most of them. The MWI suggests that time travel is possible because we are constantly traveling through time—just in different directions than we normally experience in our everyday lives. The MWI has its roots in quantum mechanics, where it was first developed by Hugh Everett III during his PhD work at Princeton University. Basically, according to this theory you could go back in time if there were infinite universes: You’d simply hop across one of these other universes—and therefore a different timeline—when you tried to change something about your life or world line. It sounds confusing but it makes sense when explained more clearly:

Time travel is possible . You’ve probably heard of three different theories about time travel: the fixed timeline, dynamic timeline and multiverse. The first theory is that time travel isn’t possible. The second theory states that it is possible, but only in one direction (from future to past). The third says you can go back and forth between past and future in both directions. In the end, I think we’re going to have to accept that time travel is possible in some form. Whether it’s in the future or right now, the question isn’t whether there are people who can get around through time. It’s how they do it and why we haven’t seen them yet. They may have been there all along without us noticing—or maybe they’re still working on figuring out their own theories about what time really means in order for us humans (and our brains) to understand!

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Image that reads Space Place and links to spaceplace.nasa.gov.

Is Time Travel Possible?

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

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

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

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

Image of galaxies, taken by the Hubble Space Telescope.

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

How do we know that time travel is possible?

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

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

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

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

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

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

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

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

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

Illustration of GPS satellites orbiting around Earth

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

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

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

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

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

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

In Summary:

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

If you liked this, you may like:

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Is time travel even possible? An astrophysicist explains the science behind the science fiction

Published: Nov 13, 2023

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By: Magazine Editor

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Written by Adi Foord , assistant professor of physics , UMBC

Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] .

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

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

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

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

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

Time is relative

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

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

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

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

Paradoxes and failed dinner parties

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

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

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

Telescopes are time machines

Interestingly, astrophysicists armed with powerful telescopes possess a unique form of time travel. As they peer into the vast expanse of the cosmos, they gaze into the past universe. Light from all galaxies and stars takes time to travel, and these beams of light carry information from the distant past. When astrophysicists observe a star or a galaxy through a telescope, they are not seeing it as it is in the present, but as it existed when the light began its journey to Earth millions to billions of years ago. https://www.youtube.com/embed/QeRtcJi3V38?wmode=transparent&start=0 Telescopes are a kind of time machine – they let you peer into the past.

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

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

This article is republished from The Conversation under a Creative Commons license. Read the original article and see more than 250 UMBC articles available in The Conversation.

Tags: CNMS , Physics , The Conversation

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Time travel could be possible, but only with parallel timelines

three time travel theories

Assistant Professor, Physics, Brock University

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Barak Shoshany does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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Have you ever made a mistake that you wish you could undo? Correcting past mistakes is one of the reasons we find the concept of time travel so fascinating. As often portrayed in science fiction, with a time machine, nothing is permanent anymore — you can always go back and change it. But is time travel really possible in our universe , or is it just science fiction?

Read more: Curious Kids: is time travel possible for humans?

Our modern understanding of time and causality comes from general relativity . Theoretical physicist Albert Einstein’s theory combines space and time into a single entity — “spacetime” — and provides a remarkably intricate explanation of how they both work, at a level unmatched by any other established theory. This theory has existed for more than 100 years, and has been experimentally verified to extremely high precision, so physicists are fairly certain it provides an accurate description of the causal structure of our universe.

For decades, physicists have been trying to use general relativity to figure out if time travel is possible . It turns out that you can write down equations that describe time travel and are fully compatible and consistent with relativity. But physics is not mathematics, and equations are meaningless if they do not correspond to anything in reality.

Arguments against time travel

There are two main issues which make us think these equations may be unrealistic. The first issue is a practical one: building a time machine seems to require exotic matter , which is matter with negative energy. All the matter we see in our daily lives has positive energy — matter with negative energy is not something you can just find lying around. From quantum mechanics, we know that such matter can theoretically be created, but in too small quantities and for too short times .

However, there is no proof that it is impossible to create exotic matter in sufficient quantities. Furthermore, other equations may be discovered that allow time travel without requiring exotic matter. Therefore, this issue may just be a limitation of our current technology or understanding of quantum mechanics.

an illustration of a person standing in a barren landscape underneath a clock

The other main issue is less practical, but more significant: it is the observation that time travel seems to contradict logic, in the form of time travel paradoxes . There are several types of such paradoxes, but the most problematic are consistency paradoxes .

A popular trope in science fiction, consistency paradoxes happen whenever there is a certain event that leads to changing the past, but the change itself prevents this event from happening in the first place.

For example, consider a scenario where I enter my time machine, use it to go back in time five minutes, and destroy the machine as soon as I get to the past. Now that I destroyed the time machine, it would be impossible for me to use it five minutes later.

But if I cannot use the time machine, then I cannot go back in time and destroy it. Therefore, it is not destroyed, so I can go back in time and destroy it. In other words, the time machine is destroyed if and only if it is not destroyed. Since it cannot be both destroyed and not destroyed simultaneously, this scenario is inconsistent and paradoxical.

Eliminating the paradoxes

There’s a common misconception in science fiction that paradoxes can be “created.” Time travellers are usually warned not to make significant changes to the past and to avoid meeting their past selves for this exact reason. Examples of this may be found in many time travel movies, such as the Back to the Future trilogy.

But in physics, a paradox is not an event that can actually happen — it is a purely theoretical concept that points towards an inconsistency in the theory itself. In other words, consistency paradoxes don’t merely imply time travel is a dangerous endeavour, they imply it simply cannot be possible.

This was one of the motivations for theoretical physicist Stephen Hawking to formulate his chronology protection conjecture , which states that time travel should be impossible. However, this conjecture so far remains unproven. Furthermore, the universe would be a much more interesting place if instead of eliminating time travel due to paradoxes, we could just eliminate the paradoxes themselves.

One attempt at resolving time travel paradoxes is theoretical physicist Igor Dmitriyevich Novikov’s self-consistency conjecture , which essentially states that you can travel to the past, but you cannot change it.

According to Novikov, if I tried to destroy my time machine five minutes in the past, I would find that it is impossible to do so. The laws of physics would somehow conspire to preserve consistency.

Introducing multiple histories

But what’s the point of going back in time if you cannot change the past? My recent work, together with my students Jacob Hauser and Jared Wogan, shows that there are time travel paradoxes that Novikov’s conjecture cannot resolve. This takes us back to square one, since if even just one paradox cannot be eliminated, time travel remains logically impossible.

So, is this the final nail in the coffin of time travel? Not quite. We showed that allowing for multiple histories (or in more familiar terms, parallel timelines) can resolve the paradoxes that Novikov’s conjecture cannot. In fact, it can resolve any paradox you throw at it.

The idea is very simple. When I exit the time machine, I exit into a different timeline. In that timeline, I can do whatever I want, including destroying the time machine, without changing anything in the original timeline I came from. Since I cannot destroy the time machine in the original timeline, which is the one I actually used to travel back in time, there is no paradox.

After working on time travel paradoxes for the last three years , I have become increasingly convinced that time travel could be possible, but only if our universe can allow multiple histories to coexist. So, can it?

Quantum mechanics certainly seems to imply so, at least if you subscribe to Everett’s “many-worlds” interpretation , where one history can “split” into multiple histories, one for each possible measurement outcome – for example, whether Schrödinger’s cat is alive or dead, or whether or not I arrived in the past.

But these are just speculations. My students and I are currently working on finding a concrete theory of time travel with multiple histories that is fully compatible with general relativity. Of course, even if we manage to find such a theory, this would not be sufficient to prove that time travel is possible, but it would at least mean that time travel is not ruled out by consistency paradoxes.

Time travel and parallel timelines almost always go hand-in-hand in science fiction, but now we have proof that they must go hand-in-hand in real science as well. General relativity and quantum mechanics tell us that time travel might be possible, but if it is, then multiple histories must also be possible.

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Time Travel

There is an extensive literature on time travel in both philosophy and physics. Part of the great interest of the topic stems from the fact that reasons have been given both for thinking that time travel is physically possible—and for thinking that it is logically impossible! This entry deals primarily with philosophical issues; issues related to the physics of time travel are covered in the separate entries on time travel and modern physics and time machines . We begin with the definitional question: what is time travel? We then turn to the major objection to the possibility of backwards time travel: the Grandfather paradox. Next, issues concerning causation are discussed—and then, issues in the metaphysics of time and change. We end with a discussion of the question why, if backwards time travel will ever occur, we have not been visited by time travellers from the future.

1.1 Time Discrepancy

1.2 changing the past, 2.1 can and cannot, 2.2 improbable coincidences, 2.3 inexplicable occurrences, 3.1 backwards causation, 3.2 causal loops, 4.1 time travel and time, 4.2 time travel and change, 5. where are the time travellers, other internet resources, related entries, 1. what is time travel.

There is a number of rather different scenarios which would seem, intuitively, to count as ‘time travel’—and a number of scenarios which, while sharing certain features with some of the time travel cases, seem nevertheless not to count as genuine time travel: [ 1 ]

Time travel Doctor . Doctor Who steps into a machine in 2024. Observers outside the machine see it disappear. Inside the machine, time seems to Doctor Who to pass for ten minutes. Observers in 1984 (or 3072) see the machine appear out of nowhere. Doctor Who steps out. [ 2 ] Leap . The time traveller takes hold of a special device (or steps into a machine) and suddenly disappears; she appears at an earlier (or later) time. Unlike in Doctor , the time traveller experiences no lapse of time between her departure and arrival: from her point of view, she instantaneously appears at the destination time. [ 3 ] Putnam . Oscar Smith steps into a machine in 2024. From his point of view, things proceed much as in Doctor : time seems to Oscar Smith to pass for a while; then he steps out in 1984. For observers outside the machine, things proceed differently. Observers of Oscar’s arrival in the past see a time machine suddenly appear out of nowhere and immediately divide into two copies of itself: Oscar Smith steps out of one; and (through the window) they see inside the other something that looks just like what they would see if a film of Oscar Smith were played backwards (his hair gets shorter; food comes out of his mouth and goes back into his lunch box in a pristine, uneaten state; etc.). Observers of Oscar’s departure from the future do not simply see his time machine disappear after he gets into it: they see it collide with the apparently backwards-running machine just described, in such a way that both are simultaneously annihilated. [ 4 ] Gödel . The time traveller steps into an ordinary rocket ship (not a special time machine) and flies off on a certain course. At no point does she disappear (as in Leap ) or ‘turn back in time’ (as in Putnam )—yet thanks to the overall structure of spacetime (as conceived in the General Theory of Relativity), the traveller arrives at a point in the past (or future) of her departure. (Compare the way in which someone can travel continuously westwards, and arrive to the east of her departure point, thanks to the overall curved structure of the surface of the earth.) [ 5 ] Einstein . The time traveller steps into an ordinary rocket ship and flies off at high speed on a round trip. When he returns to Earth, thanks to certain effects predicted by the Special Theory of Relativity, only a very small amount of time has elapsed for him—he has aged only a few months—while a great deal of time has passed on Earth: it is now hundreds of years in the future of his time of departure. [ 6 ] Not time travel Sleep . One is very tired, and falls into a deep sleep. When one awakes twelve hours later, it seems from one’s own point of view that hardly any time has passed. Coma . One is in a coma for a number of years and then awakes, at which point it seems from one’s own point of view that hardly any time has passed. Cryogenics . One is cryogenically frozen for hundreds of years. Upon being woken, it seems from one’s own point of view that hardly any time has passed. Virtual . One enters a highly realistic, interactive virtual reality simulator in which some past era has been recreated down to the finest detail. Crystal . One looks into a crystal ball and sees what happened at some past time, or will happen at some future time. (Imagine that the crystal ball really works—like a closed-circuit security monitor, except that the vision genuinely comes from some past or future time. Even so, the person looking at the crystal ball is not thereby a time traveller.) Waiting . One enters one’s closet and stays there for seven hours. When one emerges, one has ‘arrived’ seven hours in the future of one’s ‘departure’. Dateline . One departs at 8pm on Monday, flies for fourteen hours, and arrives at 10pm on Monday.

A satisfactory definition of time travel would, at least, need to classify the cases in the right way. There might be some surprises—perhaps, on the best definition of ‘time travel’, Cryogenics turns out to be time travel after all—but it should certainly be the case, for example, that Gödel counts as time travel and that Sleep and Waiting do not. [ 7 ]

In fact there is no entirely satisfactory definition of ‘time travel’ in the literature. The most popular definition is the one given by Lewis (1976, 145–6):

What is time travel? Inevitably, it involves a discrepancy between time and time. Any traveller departs and then arrives at his destination; the time elapsed from departure to arrival…is the duration of the journey. But if he is a time traveller, the separation in time between departure and arrival does not equal the duration of his journey.…How can it be that the same two events, his departure and his arrival, are separated by two unequal amounts of time?…I reply by distinguishing time itself, external time as I shall also call it, from the personal time of a particular time traveller: roughly, that which is measured by his wristwatch. His journey takes an hour of his personal time, let us say…But the arrival is more than an hour after the departure in external time, if he travels toward the future; or the arrival is before the departure in external time…if he travels toward the past.

This correctly excludes Waiting —where the length of the ‘journey’ precisely matches the separation between ‘arrival’ and ‘departure’—and Crystal , where there is no journey at all—and it includes Doctor . It has trouble with Gödel , however—because when the overall structure of spacetime is as twisted as it is in the sort of case Gödel imagined, the notion of external time (“time itself”) loses its grip.

Another definition of time travel that one sometimes encounters in the literature (Arntzenius, 2006, 602) (Smeenk and Wüthrich, 2011, 5, 26) equates time travel with the existence of CTC’s: closed timelike curves. A curve in this context is a line in spacetime; it is timelike if it could represent the career of a material object; and it is closed if it returns to its starting point (i.e. in spacetime—not merely in space). This now includes Gödel —but it excludes Einstein .

The lack of an adequate definition of ‘time travel’ does not matter for our purposes here. [ 8 ] It suffices that we have clear cases of (what would count as) time travel—and that these cases give rise to all the problems that we shall wish to discuss.

Some authors (in philosophy, physics and science fiction) consider ‘time travel’ scenarios in which there are two temporal dimensions (e.g. Meiland (1974)), and others consider scenarios in which there are multiple ‘parallel’ universes—each one with its own four-dimensional spacetime (e.g. Deutsch and Lockwood (1994)). There is a question whether travelling to another version of 2001 (i.e. not the very same version one experienced in the past)—a version at a different point on the second time dimension, or in a different parallel universe—is really time travel, or whether it is more akin to Virtual . In any case, this kind of scenario does not give rise to many of the problems thrown up by the idea of travelling to the very same past one experienced in one’s younger days. It is these problems that form the primary focus of the present entry, and so we shall not have much to say about other kinds of ‘time travel’ scenario in what follows.

One objection to the possibility of time travel flows directly from attempts to define it in anything like Lewis’s way. The worry is that because time travel involves “a discrepancy between time and time”, time travel scenarios are simply incoherent. The time traveller traverses thirty years in one year; she is 51 years old 21 years after her birth; she dies at the age of 100, 200 years before her birth; and so on. The objection is that these are straightforward contradictions: the basic description of what time travel involves is inconsistent; therefore time travel is logically impossible. [ 9 ]

There must be something wrong with this objection, because it would show Einstein to be logically impossible—whereas this sort of future-directed time travel has actually been observed (albeit on a much smaller scale—but that does not affect the present point) (Hafele and Keating, 1972b,a). The most common response to the objection is that there is no contradiction because the interval of time traversed by the time traveller and the duration of her journey are measured with respect to different frames of reference: there is thus no reason why they should coincide. A similar point applies to the discrepancy between the time elapsed since the time traveller’s birth and her age upon arrival. There is no more of a contradiction here than in the fact that Melbourne is both 800 kilometres away from Sydney—along the main highway—and 1200 kilometres away—along the coast road. [ 10 ]

Before leaving the question ‘What is time travel?’ we should note the crucial distinction between changing the past and participating in (aka affecting or influencing) the past. [ 11 ] In the popular imagination, backwards time travel would allow one to change the past: to right the wrongs of history, to prevent one’s younger self doing things one later regretted, and so on. In a model with a single past, however, this idea is incoherent: the very description of the case involves a contradiction (e.g. the time traveller burns all her diaries at midnight on her fortieth birthday in 1976, and does not burn all her diaries at midnight on her fortieth birthday in 1976). It is not as if there are two versions of the past: the original one, without the time traveller present, and then a second version, with the time traveller playing a role. There is just one past—and two perspectives on it: the perspective of the younger self, and the perspective of the older time travelling self. If these perspectives are inconsistent (e.g. an event occurs in one but not the other) then the time travel scenario is incoherent.

This means that time travellers can do less than we might have hoped: they cannot right the wrongs of history; they cannot even stir a speck of dust on a certain day in the past if, on that day, the speck was in fact unmoved. But this does not mean that time travellers must be entirely powerless in the past: while they cannot do anything that did not actually happen, they can (in principle) do anything that did happen. Time travellers cannot change the past: they cannot make it different from the way it was—but they can participate in it: they can be amongst the people who did make the past the way it was. [ 12 ]

What about models involving two temporal dimensions, or parallel universes—do they allow for coherent scenarios in which the past is changed? [ 13 ] There is certainly no contradiction in saying that the time traveller burns all her diaries at midnight on her fortieth birthday in 1976 in universe 1 (or at hypertime A ), and does not burn all her diaries at midnight on her fortieth birthday in 1976 in universe 2 (or at hypertime B ). The question is whether this kind of story involves changing the past in the sense originally envisaged: righting the wrongs of history, preventing subsequently regretted actions, and so on. Goddu (2003) and van Inwagen (2010) argue that it does (in the context of particular hypertime models), while Smith (1997, 365–6; 2015) argues that it does not: that it involves avoiding the past—leaving it untouched while travelling to a different version of the past in which things proceed differently.

2. The Grandfather Paradox

The most important objection to the logical possibility of backwards time travel is the so-called Grandfather paradox. This paradox has actually convinced many people that backwards time travel is impossible:

The dead giveaway that true time-travel is flatly impossible arises from the well-known “paradoxes” it entails. The classic example is “What if you go back into the past and kill your grandfather when he was still a little boy?”…So complex and hopeless are the paradoxes…that the easiest way out of the irrational chaos that results is to suppose that true time-travel is, and forever will be, impossible. (Asimov 1995 [2003, 276–7]) travel into one’s past…would seem to give rise to all sorts of logical problems, if you were able to change history. For example, what would happen if you killed your parents before you were born. It might be that one could avoid such paradoxes by some modification of the concept of free will. But this will not be necessary if what I call the chronology protection conjecture is correct: The laws of physics prevent closed timelike curves from appearing . (Hawking, 1992, 604) [ 14 ]

The paradox comes in different forms. Here’s one version:

If time travel was logically possible then the time traveller could return to the past and in a suicidal rage destroy his time machine before it was completed and murder his younger self. But if this was so a necessary condition for the time trip to have occurred at all is removed, and we should then conclude that the time trip did not occur. Hence if the time trip did occur, then it did not occur. Hence it did not occur, and it is necessary that it did not occur. To reply, as it is standardly done, that our time traveller cannot change the past in this way, is a petitio principii . Why is it that the time traveller is constrained in this way? What mysterious force stills his sudden suicidal rage? (Smith, 1985, 58)

The idea is that backwards time travel is impossible because if it occurred, time travellers would attempt to do things such as kill their younger selves (or their grandfathers etc.). We know that doing these things—indeed, changing the past in any way—is impossible. But were there time travel, there would then be nothing left to stop these things happening. If we let things get to the stage where the time traveller is facing Grandfather with a loaded weapon, then there is nothing left to prevent the impossible from occurring. So we must draw the line earlier: it must be impossible for someone to get into this situation at all; that is, backwards time travel must be impossible.

In order to defend the possibility of time travel in the face of this argument we need to show that time travel is not a sure route to doing the impossible. So, given that a time traveller has gone to the past and is facing Grandfather, what could stop her killing Grandfather? Some science fiction authors resort to the idea of chaperones or time guardians who prevent time travellers from changing the past—or to mysterious forces of logic. But it is hard to take these ideas seriously—and more importantly, it is hard to make them work in detail when we remember that changing the past is impossible. (The chaperone is acting to ensure that the past remains as it was—but the only reason it ever was that way is because of his very actions.) [ 15 ] Fortunately there is a better response—also to be found in the science fiction literature, and brought to the attention of philosophers by Lewis (1976). What would stop the time traveller doing the impossible? She would fail “for some commonplace reason”, as Lewis (1976, 150) puts it. Her gun might jam, a noise might distract her, she might slip on a banana peel, etc. Nothing more than such ordinary occurrences is required to stop the time traveller killing Grandfather. Hence backwards time travel does not entail the occurrence of impossible events—and so the above objection is defused.

A problem remains. Suppose Tim, a time-traveller, is facing his grandfather with a loaded gun. Can Tim kill Grandfather? On the one hand, yes he can. He is an excellent shot; there is no chaperone to stop him; the laws of logic will not magically stay his hand; he hates Grandfather and will not hesitate to pull the trigger; etc. On the other hand, no he can’t. To kill Grandfather would be to change the past, and no-one can do that (not to mention the fact that if Grandfather died, then Tim would not have been born). So we have a contradiction: Tim can kill Grandfather and Tim cannot kill Grandfather. Time travel thus leads to a contradiction: so it is impossible.

Note the difference between this version of the Grandfather paradox and the version considered above. In the earlier version, the contradiction happens if Tim kills Grandfather. The solution was to say that Tim can go into the past without killing Grandfather—hence time travel does not entail a contradiction. In the new version, the contradiction happens as soon as Tim gets to the past. Of course Tim does not kill Grandfather—but we still have a contradiction anyway: for he both can do it, and cannot do it. As Lewis puts it:

Could a time traveler change the past? It seems not: the events of a past moment could no more change than numbers could. Yet it seems that he would be as able as anyone to do things that would change the past if he did them. If a time traveler visiting the past both could and couldn’t do something that would change it, then there cannot possibly be such a time traveler. (Lewis, 1976, 149)

Lewis’s own solution to this problem has been widely accepted. [ 16 ] It turns on the idea that to say that something can happen is to say that its occurrence is compossible with certain facts, where context determines (more or less) which facts are the relevant ones. Tim’s killing Grandfather in 1921 is compossible with the facts about his weapon, training, state of mind, and so on. It is not compossible with further facts, such as the fact that Grandfather did not die in 1921. Thus ‘Tim can kill Grandfather’ is true in one sense (relative to one set of facts) and false in another sense (relative to another set of facts)—but there is no single sense in which it is both true and false. So there is no contradiction here—merely an equivocation.

Another response is that of Vihvelin (1996), who argues that there is no contradiction here because ‘Tim can kill Grandfather’ is simply false (i.e. contra Lewis, there is no legitimate sense in which it is true). According to Vihvelin, for ‘Tim can kill Grandfather’ to be true, there must be at least some occasions on which ‘If Tim had tried to kill Grandfather, he would or at least might have succeeded’ is true—but, Vihvelin argues, at any world remotely like ours, the latter counterfactual is always false. [ 17 ]

Return to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a new objection—due to Horwich (1987)—not to the possibility but to the probability of backwards time travel.

Think about correlated events in general. Whenever we see two things frequently occurring together, this is because one of them causes the other, or some third thing causes both. Horwich calls this the Principle of V-Correlation:

if events of type A and B are associated with one another, then either there is always a chain of events between them…or else we find an earlier event of type C that links up with A and B by two such chains of events. What we do not see is…an inverse fork—in which A and B are connected only with a characteristic subsequent event, but no preceding one. (Horwich, 1987, 97–8)

For example, suppose that two students turn up to class wearing the same outfits. That could just be a coincidence (i.e. there is no common cause, and no direct causal link between the two events). If it happens every week for the whole semester, it is possible that it is a coincidence, but this is extremely unlikely . Normally, we see this sort of extensive correlation only if either there is a common cause (e.g. both students have product endorsement deals with the same clothing company, or both slavishly copy the same influencer) or a direct causal link (e.g. one student is copying the other).

Now consider the time traveller setting off to kill her younger self. As discussed, no contradiction need ensue—this is prevented not by chaperones or mysterious forces, but by a run of ordinary occurrences in which the trigger falls off the time traveller’s gun, a gust of wind pushes her bullet off course, she slips on a banana peel, and so on. But now consider this run of ordinary occurrences. Whenever the time traveller contemplates auto-infanticide, someone nearby will drop a banana peel ready for her to slip on, or a bird will begin to fly so that it will be in the path of the time traveller’s bullet by the time she fires, and so on. In general, there will be a correlation between auto-infanticide attempts and foiling occurrences such as the presence of banana peels—and this correlation will be of the type that does not involve a direct causal connection between the correlated events or a common cause of both. But extensive correlations of this sort are, as we saw, extremely rare—so backwards time travel will happen about as often as you will see two people wear the same outfits to class every day of semester, without there being any causal connection between what one wears and what the other wears.

We can set out Horwich’s argument this way:

  • If time travel were ever to occur, we should see extensive uncaused correlations.
  • It is extremely unlikely that we should ever see extensive uncaused correlations.
  • Therefore time travel is extremely unlikely to occur.

The conclusion is not that time travel is impossible, but that we should treat it the way we treat the possibility of, say, tossing a fair coin and getting heads one thousand times in a row. As Price (1996, 278 n.7) puts it—in the context of endorsing Horwich’s conclusion: “the hypothesis of time travel can be made to imply propositions of arbitrarily low probability. This is not a classical reductio, but it is as close as science ever gets.”

Smith (1997) attacks both premisses of Horwich’s argument. Against the first premise, he argues that backwards time travel, in itself, does not entail extensive uncaused correlations. Rather, when we look more closely, we see that time travel scenarios involving extensive uncaused correlations always build in prior coincidences which are themselves highly unlikely. Against the second premise, he argues that, from the fact that we have never seen extensive uncaused correlations, it does not follow that we never shall. This is not inductive scepticism: let us assume (contra the inductive sceptic) that in the absence of any specific reason for thinking things should be different in the future, we are entitled to assume they will continue being the same; still we cannot dismiss a specific reason for thinking the future will be a certain way simply on the basis that things have never been that way in the past. You might reassure an anxious friend that the sun will certainly rise tomorrow because it always has in the past—but you cannot similarly refute an astronomer who claims to have discovered a specific reason for thinking that the earth will stop rotating overnight.

Sider (2002, 119–20) endorses Smith’s second objection. Dowe (2003) criticises Smith’s first objection, but agrees with the second, concluding overall that time travel has not been shown to be improbable. Ismael (2003) reaches a similar conclusion. Goddu (2007) criticises Smith’s first objection to Horwich. Further contributions to the debate include Arntzenius (2006), Smeenk and Wüthrich (2011, §2.2) and Elliott (2018). For other arguments to the same conclusion as Horwich’s—that time travel is improbable—see Ney (2000) and Effingham (2020).

Return again to the original version of the Grandfather paradox and Lewis’s ‘commonplace reasons’ response to it. This response engenders a further objection. The autoinfanticidal time traveller is attempting to do something impossible (render herself permanently dead from an age younger than her age at the time of the attempts). Suppose we accept that she will not succeed and that what will stop her is a succession of commonplace occurrences. The previous objection was that such a succession is improbable . The new objection is that the exclusion of the time traveler from successfully committing auto-infanticide is mysteriously inexplicable . The worry is as follows. Each particular event that foils the time traveller is explicable in a perfectly ordinary way; but the inevitable combination of these events amounts to a ring-fencing of the forbidden zone of autoinfanticide—and this ring-fencing is mystifying. It’s like a grand conspiracy to stop the time traveler from doing what she wants to do—and yet there are no conspirators: no time lords, no magical forces of logic. This is profoundly perplexing. Riggs (1997, 52) writes: “Lewis’s account may do for a once only attempt, but is untenable as a general explanation of Tim’s continual lack of success if he keeps on trying.” Ismael (2003, 308) writes: “Considered individually, there will be nothing anomalous in the explanations…It is almost irresistible to suppose, however, that there is something anomalous in the cases considered collectively, i.e., in our unfailing lack of success.” See also Gorovitz (1964, 366–7), Horwich (1987, 119–21) and Carroll (2010, 86).

There have been two different kinds of defense of time travel against the objection that it involves mysteriously inexplicable occurrences. Baron and Colyvan (2016, 70) agree with the objectors that a purely causal explanation of failure—e.g. Tim fails to kill Grandfather because first he slips on a banana peel, then his gun jams, and so on—is insufficient. However they argue that, in addition, Lewis offers a non-causal—a logical —explanation of failure: “What explains Tim’s failure to kill his grandfather, then, is something about logic; specifically: Tim fails to kill his grandfather because the law of non-contradiction holds.” Smith (2017) argues that the appearance of inexplicability is illusory. There are no scenarios satisfying the description ‘a time traveller commits autoinfanticide’ (or changes the past in any other way) because the description is self-contradictory (e.g. it involves the time traveller permanently dying at 20 and also being alive at 40). So whatever happens it will not be ‘that’. There is literally no way for the time traveller not to fail. Hence there is no need for—or even possibility of—a substantive explanation of why failure invariably occurs, and such failure is not perplexing.

3. Causation

Backwards time travel scenarios give rise to interesting issues concerning causation. In this section we examine two such issues.

Earlier we distinguished changing the past and affecting the past, and argued that while the former is impossible, backwards time travel need involve only the latter. Affecting the past would be an example of backwards causation (i.e. causation where the effect precedes its cause)—and it has been argued that this too is impossible, or at least problematic. [ 18 ] The classic argument against backwards causation is the bilking argument . [ 19 ] Faced with the claim that some event A causes an earlier event B , the proponent of the bilking objection recommends an attempt to decorrelate A and B —that is, to bring about A in cases in which B has not occurred, and to prevent A in cases in which B has occurred. If the attempt is successful, then B often occurs despite the subsequent nonoccurrence of A , and A often occurs without B occurring, and so A cannot be the cause of B . If, on the other hand, the attempt is unsuccessful—if, that is, A cannot be prevented when B has occurred, nor brought about when B has not occurred—then, it is argued, it must be B that is the cause of A , rather than vice versa.

The bilking procedure requires repeated manipulation of event A . Thus, it cannot get under way in cases in which A is either unrepeatable or unmanipulable. Furthermore, the procedure requires us to know whether or not B has occurred, prior to manipulating A —and thus, it cannot get under way in cases in which it cannot be known whether or not B has occurred until after the occurrence or nonoccurrence of A (Dummett, 1964). These three loopholes allow room for many claims of backwards causation that cannot be touched by the bilking argument, because the bilking procedure cannot be performed at all. But what about those cases in which it can be performed? If the procedure succeeds—that is, A and B are decorrelated—then the claim that A causes B is refuted, or at least weakened (depending upon the details of the case). But if the bilking attempt fails, it does not follow that it must be B that is the cause of A , rather than vice versa. Depending upon the situation, that B causes A might become a viable alternative to the hypothesis that A causes B —but there is no reason to think that this alternative must always be the superior one. For example, suppose that I see a photo of you in a paper dated well before your birth, accompanied by a report of your arrival from the future. I now try to bilk your upcoming time trip—but I slip on a banana peel while rushing to push you away from your time machine, my time travel horror stories only inspire you further, and so on. Or again, suppose that I know that you were not in Sydney yesterday. I now try to get you to go there in your time machine—but first I am struck by lightning, then I fall down a manhole, and so on. What does all this prove? Surely not that your arrival in the past causes your departure from the future. Depending upon the details of the case, it seems that we might well be entitled to describe it as involving backwards time travel and backwards causation. At least, if we are not so entitled, this must be because of other facts about the case: it would not follow simply from the repeated coincidental failures of my bilking attempts.

Backwards time travel would apparently allow for the possibility of causal loops, in which things come from nowhere. The things in question might be objects—imagine a time traveller who steals a time machine from the local museum in order to make his time trip and then donates the time machine to the same museum at the end of the trip (i.e. in the past). In this case the machine itself is never built by anyone—it simply exists. The things in question might be information—imagine a time traveller who explains the theory behind time travel to her younger self: theory that she herself knows only because it was explained to her in her youth by her time travelling older self. The things in question might be actions. Imagine a time traveller who visits his younger self. When he encounters his younger self, he suddenly has a vivid memory of being punched on the nose by a strange visitor. He realises that this is that very encounter—and resignedly proceeds to punch his younger self. Why did he do it? Because he knew that it would happen and so felt that he had to do it—but he only knew it would happen because he in fact did it. [ 20 ]

One might think that causal loops are impossible—and hence that insofar as backwards time travel entails such loops, it too is impossible. [ 21 ] There are two issues to consider here. First, does backwards time travel entail causal loops? Lewis (1976, 148) raises the question whether there must be causal loops whenever there is backwards causation; in response to the question, he says simply “I am not sure.” Mellor (1998, 131) appears to claim a positive answer to the question. [ 22 ] Hanley (2004, 130) defends a negative answer by telling a time travel story in which there is backwards time travel and backwards causation, but no causal loops. [ 23 ] Monton (2009) criticises Hanley’s counterexample, but also defends a negative answer via different counterexamples. Effingham (2020) too argues for a negative answer.

Second, are causal loops impossible, or in some other way objectionable? One objection is that causal loops are inexplicable . There have been two main kinds of response to this objection. One is to agree but deny that this is a problem. Lewis (1976, 149) accepts that a loop (as a whole) would be inexplicable—but thinks that this inexplicability (like that of the Big Bang or the decay of a tritium atom) is merely strange, not impossible. In a similar vein, Meyer (2012, 263) argues that if someone asked for an explanation of a loop (as a whole), “the blame would fall on the person asking the question, not on our inability to answer it.” The second kind of response (Hanley, 2004, §5) is to deny that (all) causal loops are inexplicable. A second objection to causal loops, due to Mellor (1998, ch.12), is that in such loops the chances of events would fail to be related to their frequencies in accordance with the law of large numbers. Berkovitz (2001) and Dowe (2001) both argue that Mellor’s objection fails to establish the impossibility of causal loops. [ 24 ] Effingham (2020) considers—and rebuts—some additional objections to the possibility of causal loops.

4. Time and Change

Gödel (1949a [1990a])—in which Gödel presents models of Einstein’s General Theory of Relativity in which there exist CTC’s—can well be regarded as initiating the modern academic literature on time travel, in both philosophy and physics. In a companion paper, Gödel discusses the significance of his results for more general issues in the philosophy of time (Gödel 1949b [1990b]). For the succeeding half century, the time travel literature focussed predominantly on objections to the possibility (or probability) of time travel. More recently, however, there has been renewed interest in the connections between time travel and more general issues in the metaphysics of time and change. We examine some of these in the present section. [ 25 ]

The first thing that we need to do is set up the various metaphysical positions whose relationships with time travel will then be discussed. Consider two metaphysical questions:

  • Are the past, present and future equally real?
  • Is there an objective flow or passage of time, and an objective now?

We can label some views on the first question as follows. Eternalism is the view that past and future times, objects and events are just as real as the present time and present events and objects. Nowism is the view that only the present time and present events and objects exist. Now-and-then-ism is the view that the past and present exist but the future does not. We can also label some views on the second question. The A-theory answers in the affirmative: the flow of time and division of events into past (before now), present (now) and future (after now) are objective features of reality (as opposed to mere features of our experience). Furthermore, they are linked: the objective flow of time arises from the movement, through time, of the objective now (from the past towards the future). The B-theory answers in the negative: while we certainly experience now as special, and time as flowing, the B-theory denies that what is going on here is that we are detecting objective features of reality in a way that corresponds transparently to how those features are in themselves. The flow of time and the now are not objective features of reality; they are merely features of our experience. By combining answers to our first and second questions we arrive at positions on the metaphysics of time such as: [ 26 ]

  • the block universe view: eternalism + B-theory
  • the moving spotlight view: eternalism + A-theory
  • the presentist view: nowism + A-theory
  • the growing block view: now-and-then-ism + A-theory.

So much for positions on time itself. Now for some views on temporal objects: objects that exist in (and, in general, change over) time. Three-dimensionalism is the view that persons, tables and other temporal objects are three-dimensional entities. On this view, what you see in the mirror is a whole person. [ 27 ] Tomorrow, when you look again, you will see the whole person again. On this view, persons and other temporal objects are wholly present at every time at which they exist. Four-dimensionalism is the view that persons, tables and other temporal objects are four-dimensional entities, extending through three dimensions of space and one dimension of time. On this view, what you see in the mirror is not a whole person: it is just a three-dimensional temporal part of a person. Tomorrow, when you look again, you will see a different such temporal part. Say that an object persists through time if it is around at some time and still around at a later time. Three- and four-dimensionalists agree that (some) objects persist, but they differ over how objects persist. According to three-dimensionalists, objects persist by enduring : an object persists from t 1 to t 2 by being wholly present at t 1 and t 2 and every instant in between. According to four-dimensionalists, objects persist by perduring : an object persists from t 1 to t 2 by having temporal parts at t 1 and t 2 and every instant in between. Perduring can be usefully compared with being extended in space: a road extends from Melbourne to Sydney not by being wholly located at every point in between, but by having a spatial part at every point in between.

It is natural to combine three-dimensionalism with presentism and four-dimensionalism with the block universe view—but other combinations of views are certainly possible.

Gödel (1949b [1990b]) argues from the possibility of time travel (more precisely, from the existence of solutions to the field equations of General Relativity in which there exist CTC’s) to the B-theory: that is, to the conclusion that there is no objective flow or passage of time and no objective now. Gödel begins by reviewing an argument from Special Relativity to the B-theory: because the notion of simultaneity becomes a relative one in Special Relativity, there is no room for the idea of an objective succession of “nows”. He then notes that this argument is disrupted in the context of General Relativity, because in models of the latter theory to date, the presence of matter does allow recovery of an objectively distinguished series of “nows”. Gödel then proposes a new model (Gödel 1949a [1990a]) in which no such recovery is possible. (This is the model that contains CTC’s.) Finally, he addresses the issue of how one can infer anything about the nonexistence of an objective flow of time in our universe from the existence of a merely possible universe in which there is no objectively distinguished series of “nows”. His main response is that while it would not be straightforwardly contradictory to suppose that the existence of an objective flow of time depends on the particular, contingent arrangement and motion of matter in the world, this would nevertheless be unsatisfactory. Responses to Gödel have been of two main kinds. Some have objected to the claim that there is no objective flow of time in his model universe (e.g. Savitt (2005); see also Savitt (1994)). Others have objected to the attempt to transfer conclusions about that model universe to our own universe (e.g. Earman (1995, 197–200); for a partial response to Earman see Belot (2005, §3.4)). [ 28 ]

Earlier we posed two questions:

Gödel’s argument is related to the second question. Let’s turn now to the first question. Godfrey-Smith (1980, 72) writes “The metaphysical picture which underlies time travel talk is that of the block universe [i.e. eternalism, in the terminology of the present entry], in which the world is conceived as extended in time as it is in space.” In his report on the Analysis problem to which Godfrey-Smith’s paper is a response, Harrison (1980, 67) replies that he would like an argument in support of this assertion. Here is an argument: [ 29 ]

A fundamental requirement for the possibility of time travel is the existence of the destination of the journey. That is, a journey into the past or the future would have to presuppose that the past or future were somehow real. (Grey, 1999, 56)

Dowe (2000, 442–5) responds that the destination does not have to exist at the time of departure: it only has to exist at the time of arrival—and this is quite compatible with non-eternalist views. And Keller and Nelson (2001, 338) argue that time travel is compatible with presentism:

There is four-dimensional [i.e. eternalist, in the terminology of the present entry] time-travel if the appropriate sorts of events occur at the appropriate sorts of times; events like people hopping into time-machines and disappearing, people reappearing with the right sorts of memories, and so on. But the presentist can have just the same patterns of events happening at just the same times. Or at least, it can be the case on the presentist model that the right sorts of events will happen, or did happen, or are happening, at the rights sorts of times. If it suffices for four-dimensionalist time-travel that Jennifer disappears in 2054 and appears in 1985 with the right sorts of memories, then why shouldn’t it suffice for presentist time-travel that Jennifer will disappear in 2054, and that she did appear in 1985 with the right sorts of memories?

Sider (2005) responds that there is still a problem reconciling presentism with time travel conceived in Lewis’s way: that conception of time travel requires that personal time is similar to external time—but presentists have trouble allowing this. Further contributions to the debate whether presentism—and other versions of the A-theory—are compatible with time travel include Monton (2003), Daniels (2012), Hall (2014) and Wasserman (2018) on the side of compatibility, and Miller (2005), Slater (2005), Miller (2008), Hales (2010) and Markosian (2020) on the side of incompatibility.

Leibniz’s Law says that if x = y (i.e. x and y are identical—one and the same entity) then x and y have exactly the same properties. There is a superficial conflict between this principle of logic and the fact that things change. If Bill is at one time thin and at another time not so—and yet it is the very same person both times—it looks as though the very same entity (Bill) both possesses and fails to possess the property of being thin. Three-dimensionalists and four-dimensionalists respond to this problem in different ways. According to the four-dimensionalist, what is thin is not Bill (who is a four-dimensional entity) but certain temporal parts of Bill; and what is not thin are other temporal parts of Bill. So there is no single entity that both possesses and fails to possess the property of being thin. Three-dimensionalists have several options. One is to deny that there are such properties as ‘thin’ (simpliciter): there are only temporally relativised properties such as ‘thin at time t ’. In that case, while Bill at t 1 and Bill at t 2 are the very same entity—Bill is wholly present at each time—there is no single property that this one entity both possesses and fails to possess: Bill possesses the property ‘thin at t 1 ’ and lacks the property ‘thin at t 2 ’. [ 30 ]

Now consider the case of a time traveller Ben who encounters his younger self at time t . Suppose that the younger self is thin and the older self not so. The four-dimensionalist can accommodate this scenario easily. Just as before, what we have are two different three-dimensional parts of the same four-dimensional entity, one of which possesses the property ‘thin’ and the other of which does not. The three-dimensionalist, however, faces a problem. Even if we relativise properties to times, we still get the contradiction that Ben possesses the property ‘thin at t ’ and also lacks that very same property. [ 31 ] There are several possible options for the three-dimensionalist here. One is to relativise properties not to external times but to personal times (Horwich, 1975, 434–5); another is to relativise properties to spatial locations as well as to times (or simply to spacetime points). Sider (2001, 101–6) criticises both options (and others besides), concluding that time travel is incompatible with three-dimensionalism. Markosian (2004) responds to Sider’s argument; [ 32 ] Miller (2006) also responds to Sider and argues for the compatibility of time travel and endurantism; Gilmore (2007) seeks to weaken the case against endurantism by constructing analogous arguments against perdurantism. Simon (2005) finds problems with Sider’s arguments, but presents different arguments for the same conclusion; Effingham and Robson (2007) and Benovsky (2011) also offer new arguments for this conclusion. For further discussion see Wasserman (2018) and Effingham (2020). [ 33 ]

We have seen arguments to the conclusions that time travel is impossible, improbable and inexplicable. Here’s an argument to the conclusion that backwards time travel simply will not occur. If backwards time travel is ever going to occur, we would already have seen the time travellers—but we have seen none such. [ 34 ] The argument is a weak one. [ 35 ] For a start, it is perhaps conceivable that time travellers have already visited the Earth [ 36 ] —but even granting that they have not, this is still compatible with the future actuality of backwards time travel. First, it may be that time travel is very expensive, difficult or dangerous—or for some other reason quite rare—and that by the time it is available, our present period of history is insufficiently high on the list of interesting destinations. Second, it may be—and indeed existing proposals in the physics literature have this feature—that backwards time travel works by creating a CTC that lies entirely in the future: in this case, backwards time travel becomes possible after the creation of the CTC, but travel to a time earlier than the time at which the CTC is created is not possible. [ 37 ]

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Life's Little Mysteries

Where Does the Concept of Time Travel Come From?

Time; he's waiting in the wings.

Wormholes have been proposed as one possible means of traveling through time.

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?
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  • Why Does Time Sometimes Fly When You're NOT Having Fun?

Originally published on Live Science .

Adam Mann

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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Time Travel: Theories, Possibilities, and Paradoxes Explained

By Neha Rastogi

Time Travel has been a matter of great interest for Science fiction since ages. Whether it’s the movies like Planet of the Apes (1968) or modern franchises like “Doctor Who” and “Star Trek” ; the concept is grabbing a lot of eyeballs. Not only movies and shows but even some mythological tales like Mahabharata and the Japanese story of Urashima Taro support the evidence that time travel exists. We often see stories where characters use time machines to jaunt through the years but the reality is far more complex and inexplicable.

Understanding the Concept of Time Travel

Time Travel is defined as the phenomenon of moving between different points in time through a hypothetical device called “Time Machine”. Despite being predominantly related to the field of philosophy and fiction, it’s somehow supported to a small extent by physics in conjunction with quantum mechanics. However, before getting into the argument of how real it is, let’s comprehend the fundamental meaning of time.

Basically, the whole idea of Time Travel is administered by the concept of time. Usually, people believe that time is constant but the famous Physicist Albert Einstein introduced the “Theory of Relativity” as per which, time is relative. In other words, time slows down or speeds up depending on how fast the observer moves relative to something else. According to him, a person traveling inside a spaceship at the speed of light would age much slower than his/her twin back at home.

Time is Relative

Time is Relative

After Einstein’s Theory of Relativity, his teacher Herman Minkowski emphasized on space-time, a mathematical model that joins both space and time in a continuum. This implies that time and space cannot exist without each other. Space is a 3-dimensional arena consisting of length, width, and height. This is joined by Time with the fourth dimension called direction. So anything that happens in the universe takes place in this space-time continuum. Although this validates that space travelers are slightly younger than their twins when they return to earth, yet a huge leap in the past or future is not possible with the current technology.

Time Machines

It is believed that in order to travel back or forward in time, one would require a device called Time Machine . The research on such a device would involve bending space-time to such an extent that time lines turn back on themselves to form a loop, which is termed as “closed time-like curve.” Such an action demands the use of an exotic form of matter with “negative energy density” that has a unique property of moving in the opposite direction of the normal matter when pushed. Even if it exists, the quantity would be too small to construct a machine.

Pictorial Representation of Time Travel through closed time-like curve

Pictorial Representation of Time Travel through closed time-like curve

However, some another research suggests that time machines can also be constructed by building a doughnut-shaped hole enveloped within a sphere of normal matter. Inside this doughnut-shaped hole filled with vacuum, gravitational force can be used to bend the space-time so as to form a closed time-like curve. After racing around inside this doughnut a traveler would be able to go back in time with each lap. But in reality, it’s quite complex because the gravitational fields have to be very strong and would demand precise manipulation.

Time Travel Approaches in Physics

After studying and researching about Time Travel, various physicists have come up with approaches that may support its possibility, at least theoretically. Let’s take a look at these concepts so as to understand how Time Travel could actually work someday.

Time Dilation

Time Dilation Explanation

Time Dilation Explanation

An important aspect of Einstein’s relativity theory is the term “time dilation” , which is defined as the difference of elapsed time between two events as measured by observers who are either moving relative to each other or are situated at different locations from the gravitational mass. As per the theory, time dilation can be summarized as a phenomenon which occurs due to the difference in either gravity or relative velocity.

In special relativity the time dilation effect is reciprocal i.e. when two clocks are in motion with respect to each other, for both the observers, the other one will be time dilated or the other clock will move slower. However, in general relativity, an observer at the top of the tower will find the clock closer to the ground to be slower and the other observer would agree about the direction and magnitude of this difference.

Due to the concept of time dilation, the current human time travel record is held by Russian cosmonaut Sergei Krikalev . Owing to the high-speed (7.66 km/s) of ISS and the length of time spent in space, it is believed that the cosmonaut actually arrived 0.02 seconds in the future while returning to the earth.

Cosmic String

Diagram Depicting Cosmic Strings

Diagram Depicting Cosmic Strings

In 1991 J Richard Gott gave the idea of Cosmic Strings , which are believed to be left over from the early cosmos. These are defined as string-like objects or narrow tubes of energy that are stretched across the entire length of the universe. Owing to the huge amount of mass and massive gravitational pull, it would allow objects attached to the Cosmic Strings to travel at the speed of light.

So if two strings are pulled close to each other or one of them is stretched near the black hole, it might warp space-time to such an extent that would lead to creating a closed time-like curve and hence leading to the possibility of time travel. Theoretically, the gravity generated by these two Cosmic strings would help in propelling a spaceship into the past.

However, coming to the reality, the loop of strings is required to contain half the mass-energy of an entire galaxy so as to travel one year back in time. This implies that powering a time machine would require splitting half the atoms present in the whole galaxy.

Black holes

Illustration of Kerr Hole

Illustration of Kerr Hole

When stars (having a mass of more than four times our sun) reach their end of life and all their fuel is burned up, they collapse under the pressure of their own weight creating “Black Holes” . The boundary of a Black Hole, called Event Horizon , has such a strong gravitational pull that it doesn’t even allow light to pass through it. Since light travels at the fastest speed, everything else traveling through a black hole is also dragged back. Such a non-rotating black hole is named as Schwarzschild black hole .

However, traveling to a parallel universe is possible through a rotating black hole named Kerr Hole . It was proposed in 1963 by a mathematician named Roy Kerr . As per his theory, if dying stars collapse into a rotating ring of neutron stars, that would produce enough centrifugal force to prevent the formation of singularity.

Note: Singularity can be perceived as the point into which the black hole tapers much like an ice-cream cone. At this point, the laws of Physics cease to exist and all the matter is crushed beyond recognition.

Since there will be no singularity, it would be safe to pass through a black hole without being crushed and exit out of a “White Hole” . A white hole is believed to be the exhaust end of a black hole which pushes everything away from it. Hence we may travel into another time or even another universe.

Although Kerr Holes are just theoretical, if they exist then we may find our way to a one-way trip to the past or future. However, physicist Kip Thorne believes that such a black hole doesn’t exist and it would suck everything before someone even reaches the Singularity.

Diagrammatic Representation of Wormhole

Diagrammatic Representation of Wormhole 

Wormholes, also known as Einstein-Rosen Bridges , are believed to be the most potential means for time travel. It could allow us to travel several light years from earth and in much less time as compared to the conventional space travel methods. The possibility of wormholes is based on Einstein’s theory of relativity which says that any mass curves space-time. The following example is used to explain this curvature.

If two persons are holding a bed sheet stretching it tight and a baseball is placed on the sheet, its weight will make it roll to the middle of the sheet creating a curve at that point. Now if a marble is placed on the sheet, it would travel towards the baseball because of the curve. Here space is depicted as a two-dimensional plane than the four dimensions that actually makes up space-time.

Now if this sheet is folded over leaving a space at the top and bottom, placing the baseball on the top would form a curvature. If an equal mass is placed at the bottom part at a point corresponding to the location of the baseball, the second mass would eventually meet with the baseball. Similarly, wormholes might develop.

In space, masses that place pressure on different parts of the universe combine together to form a tunnel. Theoretically, this tunnel joins two separate times and allows passage between them. However, it’s possible that certain unforeseen physical properties may prevent the occurrence of wormholes and even if they exist, these might be really unstable.

Possibly someday human may learn to capture, stabilize and enlarge these tunnels but according to Dr. Hawking, prolonging the life of a tunnel through folded space-time may lead to a radiation feedback loop destroying the time tunnel.

Time Travel Paradoxes

If we ever work out a theory for time travel, we would give way to certain complexities known as paradoxes. A paradox is something that contradicts itself. In other words, time travel is not believed to be a practical concept because of certain situations that are likely to arise as the after-effects. These are broadly classified as -:

1. Closed Casual Loops: The cause and effect run in a circle causing a loop and is also internally consistent with the timeline’s history.

Diagram depicting time loop

Diagram depicting time loop

• Predestination Paradox

It is defined as a situation when a traveler going back in time causes the event which he is trying to prevent from happening. It implies that any attempt to stop any event from occurring in the past would simply lead to the cause itself. The paradox suggests that things are destined to turn out the way they have happened and anyone attempting to change the past would find himself trapped in the repeating loop of time. For example, if you travel in the past to prevent your lover from dying in a road accident, you will find out that you were the one who accidentally ran over her.

• Bootstrap Paradox

A bootstrap paradox, also known as an Ontological Paradox where an object, person, or piece of information sent back in time leads to an infinite loop where the object has no discernible origin and is believed to exist without ever being created. It implies that the past, present and future and not defined, thus making it complicated to pinpoint the origin of anything. It raises questions like how were the objects created and by whom.

2. Consistency Paradox: It generates a number of timeline inconsistencies related to the possibility of altering the past. It can be further divided into the following categories.

• The Grandfather Paradox

Grandfather Paradox

Grandfather Paradox

This paradox talks about a hypothetical situation where a person travels back in time and kills his paternal grandfather at the time when his grandfather didn’t even meet his grandmother. In such a situation, his father would never have been born and neither would the traveler himself. So if he was never born, how would he travel to the past to kill his grandfather?

The paradox also talks about auto-infanticide where a time traveler goes into the past to kill himself when he was an infant. Now if he killed himself when he was a kid, how would he exist in the future to come back in time? Some physicists say that you would be able to go back in time but you won’t be able to change it, while others suggest that you would be born in one universe but unborn in another universe.

• The Hitler Paradox

Similar to the grandfather paradox, the killing Hitler paradox erases the reason for which you would want to go into the past and kill Hitler. Moreover, killing grandfather might have a “butterfly effect” but killing Hitler would have a far-reaching impact on the History as it would change the whole course of events. If you were successful in killing Hitler, there’d be no reason that would make you want to go back in time and kill him.

This paradox has been explained very well in a Twilight Zone episode called “Cradle of Darkness” as well as an episode “Let’s Kill Her” from Dr. Who.

• Polchinski’s Paradox

American physicist Joseph Polchinski proposed a paradox where a billiard ball enters a wormhole and emerges out of the other end in the past just in time to collide with its younger version and prevents it from entering the wormhole in the first place. While proposing this scenario, Joseph had Novikov’s Self Consistency Principle in his mind which states that time travel is possible but time paradoxes are forbidden.

A number of solutions have been suggested to avoid these inconsistencies like the billiard ball will deliver a blow which changes the course of the younger version of the ball but it would not stop it from entering the wormhole. This also explains that if you go back in time to kill your grandfather then something or the other will happen to prevent you from making it happen thus preserving the consistency of the History.

Solutions for the Paradoxes

In order to come up with a solution for these above-mentioned paradoxes, scientists have proposed some explanations which are enlisted below

The Solution: Time Travel is impossible because of the paradoxes that it creates.

Self-Healing Hypothesis: If we succeed to change the events in the past, it will set off another set of events that will keep the present unchanged.

The Multiverse: Every time an event in the past is altered, an alternate parallel universe or timeline is created.

Erased Timeline Hypothesis: A person traveling to the past would exist in the new timeline but their own timeline would be erased.

Is Time Travel Possible?

Is Time Travel Possible?

Nobody seems to have a definite answer in support or against the existence of Time Travel. On one hand, Einstein suggested to traveling at the speed of light in order to jaunt through the future but this would mean an unimaginable amount of energy would be required. Moreover, the centrifugal force on the body would prove to be fatal. Although it has been observed that space travelers age a little slower as compared to their identical twin on earth but some believe that there is no definite answer to travel back in space.

Theoretical physicist Brian Greene of Columbia University says that “No one has given a definite proof that you can’t travel to the past. But every time we look at the proposals and detail it seems kind of clear that they’re right at the edge of the known laws of physics.” Besides, Prof. Hawking feels that “Today’s science fiction is tomorrow’s science fact.”

However, the paradoxes, especially the grandfather paradox, have imposed a big question mark on the possibility of Time Travel. Basically, with the present laws and knowledge of Physics, the human won’t be able to survive in the process of Time Travel. So, we need certain developments in the quantum theories till we are sure as to how the paradoxes can be solved.

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three time travel theories

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A Revolutionary New Physics Hypothesis: Three Time Dimensions, One Space Dimension

By University of Warsaw, Faculty of Physics February 2, 2023

Clock Time Relativity Concept Art

The researchers hope that their findings will contribute to a better understanding of the phenomenon of spontaneous symmetry breaking associated with the mass of the Higgs particle and other particles in the Standard Model, particularly in the early universe.

How would our world be perceived by observers moving faster than light in a vacuum? According to theorists from Warsaw and Oxford universities, such a view would differ from what we encounter daily, with the presence of not only spontaneous phenomena but also particles traveling multiple paths simultaneously.

Futhermore, the very concept of time would be completely transformed — a superluminal world would have to be characterized with three time dimensions and one spatial dimension and it would have to be described in the familiar language of field theory. It turns out that the presence of such superluminal observers does not lead to anything logically inconsistent, moreover, it is quite possible that superluminal objects really exist.

“In the early 20th century, Albert Einstein completely redefined the way we perceive time and space. Three-dimensional space gained a fourth dimension – time, and the concepts of time and space, so far separate, began to be treated as a whole. In the special theory of relativity formulated in 1905 by Albert Einstein, time and space differ only in the sign in some of the equations” explains Professor Andrzej Dragan, a physicist from the Faculty of Physics of the University of Warsaw and Center for Quantum Technologies of the National University of Singapore .

Einstein based his special theory of relativity on two assumptions – Galileo’s principle of relativity and the constancy of the speed of light. As Andrzej Dragan argues, the first principle is crucial, which assumes that in every inertial system, the laws of physics are the same, and all inertial observers are equal.

Typically, this principle applies to observers who are moving relative to each other at speeds less than the speed of light (c). However, there is no fundamental reason why observers moving in relation to the described physical systems with speeds greater than the speed of light should not be subject to it, argues Dragan.

What happens when we assume – at least theoretically – that the world could be observable from superluminal frames of reference? There is a chance that this would allow the incorporation of the basic principles of quantum mechanics into the special theory of relativity. This revolutionary hypothesis by Professor Andrzej Dragan and Professor Artur Ekert from the University of Oxford was presented for the first time in the article “Quantum principle of relativity” published two years ago in the New Journal of Physics .

There they considered the simplified case of both families of observers in a space-time consisting of two dimensions: one spatial and one time dimension. In their latest publication “Relativity of superluminal observers in 1 + 3 spacetime,” a group of 5 physicists goes a step further – presenting conclusions about the full four-dimensional spacetime. The authors start from the concept of space-time corresponding to our physical reality: with three spatial dimensions and one time dimension.

However, from the point of view of the superluminal observer, only one dimension of this world retains a spatial character, the one along which the particles can move.

“The other three dimensions are time dimensions,” explains Professor Andrzej Dragan.

“From the point of view of such an observer, the particle “ages” independently in each of the three times. But from our perspective – illuminated bread eaters – it looks like a simultaneous movement in all directions of space, i.e. the propagation of a quantum-mechanical spherical wave associated with a particle,” comments Professor Krzysztof Turzyński, co-author of the paper.

It is, as explained by Professor Andrzej Dragan, in accordance with Huygens’ principle formulated already in the 18th century, according to which every point reached by a wave becomes the source of a new spherical wave. This principle initially applied only to the light wave, but quantum mechanics extended this principle to all other forms of matter.

As the authors of the publication prove, the inclusion of superluminal observers in the description requires the creation of a new definition of velocity and kinematics. – This new definition preserves Einstein’s postulate of the constancy of the speed of light in a vacuum even for superluminal observers – prove the authors of the paper. “Therefore, our extended special relativity does not seem like a particularly extravagant idea” adds Dragan.

How does the description of the world to which we introduce superluminal observers change? After taking into account superluminal solutions, the world becomes nondeterministic, particles – instead of one at a time – begin to move along many trajectories at once, in accordance with the quantum principle of superposition.

“For a superluminal observer, the classical Newtonian point particle ceases to make sense, and the field becomes the only quantity that can be used to describe the physical world,” notes Andrzej Dragan.

“Until recently it was generally believed that postulates underlying quantum theory are fundamental and cannot be derived from anything more basic. In this work, we showed that the justification of quantum theory using extended relativity, can be naturally generalized to 1 + 3 spacetime and such an extension leads to conclusions postulated by quantum field theory” – write the authors of the publication.

All particles, therefore, seem to have extraordinary – quantum! – properties in the extended special relativity. Does it work the other way around? Can we detect particles that are normal for superluminal observers, i.e. particles moving relative to us at superluminal speeds?

“It’s not that simple,” says Professor Krzysztof Turzyński.

“The mere experimental discovery of a new fundamental particle is a feat worthy of the Nobel Prize and feasible in a large research team using the latest experimental techniques. However, we hope to apply our results to a better understanding of the phenomenon of spontaneous symmetry breaking associated with the mass of the Higgs particle and other particles in the Standard Model, especially in the early universe.”

Andrzej Dragan adds that the crucial ingredient of any spontaneous symmetry-breaking mechanism is a tachyonic field. It seems that superluminal phenomena may play a key role in the Higgs mechanism.

Reference: “Relativity of superluminal observers in 1 + 3 spacetime” by Andrzej Dragan, Kacper Dębski, Szymon Charzyński, Krzysztof Turzyński and Artur Ekert, 30 December 2022, Classical and Quantum Gravity . DOI: 10.1088/1361-6382/acad60

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20 comments on "a revolutionary new physics hypothesis: three time dimensions, one space dimension".

three time travel theories

This theory feels like a true breakthrough that could lead to interesting new predictions. I’m looking forward to the YouTube videos that will give me a way to visualize the theory and help us regular folk to play along. Thanks SciTechDaily!

You have a video made by the paper’s author Andrzej Dragan on the two dimensional case presented in their previous paper: https://www.youtube.com/watch?v=cfaeyGWTDqk

three time travel theories

“The researchers hope that their findings will contribute to a better understanding of the phenomenon of spontaneous symmetry breaking associated with the mass of the Higgs particle and other particles in the Standard Model, particularly in the early universe.” According to the topological vortex gravitational field theory, the correct term should be symmetry changing associated with the mass, not symmetry breaking associated with the mass.

three time travel theories

Hello What is topological vortex gravitational field theory?

If you are interested, you can browse https://zhuanlan.zhihu.com/c_1278787135349633024 . Enjoy your every day and colorful life!

Many physical phenomena observed in scientific experiments are often not the whole picture, let alone the essence. Scientific research is inseparable from seeking truth from facts. We should see the essence through the phenomenon and act according to the laws of nature, rather than distorting the facts and making arbitrary inference.

Hidden variables that cannot be observed in scientific research are closely related to symmetry. Symmetries are power. For centuries, symmetries have allowed physicists to find underlying connections and fundamental relationships throughout the universe.

three time travel theories

Infinity is not a ‘wrong result’ in maths, it is a basis for everything. Look at it from another angle.

three time travel theories

This reminds me of the concepts in Flatland; describing a two-dimensional observer’s perspective of an intersecting three-dimensional object.

Could the blinking in and out of quantum particles be associated with our three-dimensional observations of intersections with a different dimension?

Is there a dimension where my boss is not an idiot?

Observation/measurement is the assymmetricizing of the symmetric 2:2 dimensions of space and time into 3:1 or 1:3. The former representing the macrocosm that we call space/time and the latter representing the reciprocal of three time dimensions and one space—which presents the quantum field attributes of charge and spin direction correlations of space and time within the ‘microcosm’. Similar to ‘whirlpools’, the dimensional exchange of linear to rotational is further tiered by the rotation of its axis through the other two perpendicular dimensions of rotation. This is why, in the double slit experiment, when one slit is closed, the interference wave pattern goes from lateral (spatial) to a longitudinal (time) wave interference pattern, which can only be recognized by a constant rotating screen which shows the pattern does not ‘collapse’ but rather changes its dimension. So the dimensions of measurement/observation determine the dimensions of ‘known’ and the dimensions of ‘unknown’. In this way, the system state of measures and measured relation remains a physical constant/conservation, which has only assymetricized (exchanged its dimensions of ‘collapsed’ and ‘uncollapsed’).

three time travel theories

It turns out from gravity that space has 9 isotropic dimensions while time has 4 isotropic dimensions.

It follows from the lower approximation of GR and the FRW universe model that the rotational velocity and the cosmic time is 2/3 the Newtonian velocity and the Hubble velocity respectively.

However, the acceleration obtained from the lower approximation of GR is equal to the Newtonian acceleration.

One needs to introduce an additional potential and higher dimensional space and time to account for the rotation of planets in the solar system region.

It is found following this approach that space has 9 isotropic dimensions and time has 4 isotropic dimensions.

This seems in agreement with our daily experiences that space and time are isotropic.

It follows from the lower approximation of GR and the FRW universe model that the rotational velocity and the cosmic time is 2/3 the Newtonian velocity and the Hubble time respectively.

three time travel theories

Close, it’s actually two 1+1 (1 spacial + 1 time) dimensions that overlap. The universe is a Clifford torus. Each dimension completes a thermodynamic action, Mass coalesces, energy separates that mass, then there is a rest period (gravity) which causes the mass to recoalesce and repeat the pattern. Black holes are areas where the dimensions LEAST overlap (hence the high gravity).

just a hypothetical, but what if quantum principles like entanglement and uncertainty also occur through different areas of time?

three time travel theories

Every married man knows this. There’s the time it takes her to get ready, the time i start complaining about the time it takes her to get ready and the time to get to where we’re going, depending on who drives. We’re both in the same space, so it makes perfect sense. T

Is this tina

Title: Blackholes do not exist.One page of logic demonstration with Weinberg Steven book chapter 8.2


1) Black holes do not exist? 2) Yes. Black holes do not exist. 1) Why? Show the demonstration. 2)This is the demonstration. 2) Get the book of author Steven Weinberg, “Gravitation and Cosmology” 1972, at chapter 8.2 and follow the wrong logic of Schwarzschild. So it says at chapter 8.2 page 179, that “the field equations for empty space are” Rμν=0. But if it’s an empty space it means that the mass of any astrophysical object is zero, (for example stars which mass is zero M=0); so even the gravitational potential φ=-GM/r = 0, (see (3.4.4), Weinberg book, where r is the radius of the star); and so even the energy-momentum tensor is zero, Tij=0(see chapter 2.8 and 5.3 and formula (5.3.4), Weinberg book). So when in the book of Weinberg says on page 180 writing about the formula: rB(r)=r+constant1 (8.2.9) where B(r) = gij is the metric tensor: “To fix the constant1 of integration we recall that at great distances, r=”long distance”, from a central mass M…” that for hypothesis of “empty space” (that has an infinite extension without boundaries), the mass is zero M=0, the gravitational potential of that distant star is zero, φ=-GM/r = 0 implies B=1 and so constant1=0 (and not constant1=2GM). That means curvature with flat space when Rμν=0. So the formula of Schwarzschild is wrong: B(R)=[1-2MG/r] (see wrong logic and formula (8.2.10), Weinberg book). And this means, even that it’s failed the use of Schwarzwild radius R that satisfies a singularity as 2MG/c^2=R. So black holes horizon doesn’t exist. Further considerations in future papers will be made introducing the “fifth force” of GibbonsG.W.-Fischbach.E 1986, in the theory of general relativity together with the energy-momentum tensor Tij. So if we found that the metric tensor B=1 in empty space, that means that black holes do not exist for radius r going to 0, r–>0. So stars do not become black holes. And black holes do not exist in the universe.

three time travel theories


1) Black holes do not exist? 2) Yes. Black holes do not exist. 1) Why? Show the demonstration. 2)This is the demonstration. 2) Get the book of author Steven Weinberg, “Gravitation and Cosmology” 1972, at chapter 8.2 and follow the wrong logic of Schwarzschild. So it says at chapter 8.2 page 179, that “the field equations for empty space are” Rμν=0. But if it’s an empty space for its infinite extension, means that the mass of any astrophysical object is zero, (for example stars which mass is zero M=0); so even the gravitational potential φ=-GM/r = 0, (see (3.4.4), Weinberg book, where r is the radius of the star); and so even the energy-momentum tensor is zero, Tij=0 (see chapter 2.8 and 5.3 and formula (5.3.4), Weinberg book). So when in the book of Weinberg says on page 180 writing about the formula: rB(r)=r+constant1 (8.2.9) where the time metric tensor gtt = -B(r) is found in the following way: “To fix the constant1 of integration we recall that at great distances, r=”long distance”, from a central mass M…(in empty space): where -B(r)=-1-2f, where f is the Newtonian potential -MG/r*c^2. (see section3.4. Wrinberg S. book).” But using correct logics for hypothesis of “empty space” (that has an infinite extension without boundaries), the mass is zero M=0, the gravitational potential of that distant star is zero, f=-GM/r = 0 implies B(r)=1, for any distance r, and so constant1=0 (and not constant1=2*G*M/(c^2): it’s a wrong deduction and result of Scwarzschild). That means curvature with flat space when Rμν=0. So the formula of Schwarzschild is wrong: B(R)=[1-2*M*G/(r*c^2)] (see wrong logic and formula (8.2.10), Weinberg book). And this means, even that it’s failed the use of Schwarzwild radius R that satisfies a singularity as 2MG/(c^2)=R. So black holes horizon doesn’t exist. Further considerations in future papers will be made introducing the “fifth force” of GibbonsG.W.-Fischbach.E 1986, in the theory of general relativity together with the energy-momentum tensor Tij. So if we found that the metric tensor B=1 in empty space, that means that black holes do not exist for radius r going to 0, r–>0. So stars do not become black holes. And black holes do not exist in the universe.

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Time Travel: The Popular Philosophy of Narrative

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Introduction: Time Travel and the Mechanics of Narrative

  • Published: December 2012
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The Introduction begins the theoretical discussion of time travel and narrative theory with interpretations of three representative texts from the late 1960s: Robert Silverberg's Up the Line , Michael Moorcock's Behold the Man , and Larry Niven's “All the Myriad Ways.” In each case, time travel narrative exhibits essential problems of narrative theory, historiography, and the philosophy of time, all in the guise of literal plot events and mechanical devices. The Introduction concludes with a discussion of the contemporary context of time travel studies in literary criticism, media and film studies, philosophy, and physics. Finally, the structure of the book as a whole is briefly described, laying out what Wittenberg identifies as three historical phases of time travel fiction: the “evolutionary utopian travel” or “macrologue” phase (approximately 1880s to 1905), the “paradox” phase (approximately early 1920s to 1940s), and the “multiverse/filmic” phase (approximately mid-twentieth century to the present). Overall, Wittenberg argues that time travel fictions are simultaneously a minor or idiosyncratic subgenre of popular literature, and a paradigmatic instance of narrative structure and literary form—in short, a “narratological laboratory” for studying and testing fundamental principles of storytelling.

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three time travel theories

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5 Bizarre Paradoxes Of Time Travel Explained

December 20, 2014 James Miller Astronomy Lists , Time Travel 58

time, clock, alarm clock

There is nothing in Einstein’s theories of relativity to rule out time travel , although the very notion of traveling to the past violates one of the most fundamental premises of physics, that of causality. With the laws of cause and effect out the window, there naturally arises a number of inconsistencies associated with time travel, and listed here are some of those paradoxes which have given both scientists and time travel movie buffs alike more than a few sleepless nights over the years.

Types of Temporal Paradoxes

The time travel paradoxes that follow fall into two broad categories:

1) Closed Causal Loops , such as the Predestination Paradox and the Bootstrap Paradox, which involve a self-existing time loop in which cause and effect run in a repeating circle, but is also internally consistent with the timeline’s history.

2) Consistency Paradoxes , such as the Grandfather Paradox and other similar variants such as The Hitler paradox, and Polchinski’s Paradox, which generate a number of timeline inconsistencies related to the possibility of altering the past.

1: Predestination Paradox

A Predestination Paradox occurs when the actions of a person traveling back in time become part of past events, and may ultimately cause the event he is trying to prevent to take place. The result is a ‘temporal causality loop’ in which Event 1 in the past influences Event 2 in the future (time travel to the past) which then causes Event 1 to occur.

This circular loop of events ensures that history is not altered by the time traveler, and that any attempts to stop something from happening in the past will simply lead to the cause itself, instead of stopping it. Predestination paradoxes suggest that things are always destined to turn out the same way and that whatever has happened must happen.

Sound complicated? Imagine that your lover dies in a hit-and-run car accident, and you travel back in time to save her from her fate, only to find that on your way to the accident you are the one who accidentally runs her over. Your attempt to change the past has therefore resulted in a predestination paradox. One way of dealing with this type of paradox is to assume that the version of events you have experienced are already built into a self-consistent version of reality, and that by trying to alter the past you will only end up fulfilling your role in creating an event in history, not altering it.

– Cinema Treatment

In The Time Machine (2002) movie, for instance, Dr. Alexander Hartdegen witnesses his fiancee being killed by a mugger, leading him to build a time machine to travel back in time to save her from her fate. His subsequent attempts to save her fail, though, leading him to conclude that “I could come back a thousand times… and see her die a thousand ways.” After then traveling centuries into the future to see if a solution has been found to the temporal problem, Hartdegen is told by the Über-Morlock:

“You built your time machine because of Emma’s death. If she had lived, it would never have existed, so how could you use your machine to go back and save her? You are the inescapable result of your tragedy, just as I am the inescapable result of you .”

  • Movies : Examples of predestination paradoxes in the movies include 12 Monkeys (1995), TimeCrimes (2007), The Time Traveler’s Wife (2009), and Predestination (2014).
  • Books : An example of a predestination paradox in a book is Phoebe Fortune and the Pre-destination Paradox by M.S. Crook.

2: Bootstrap Paradox

A Bootstrap Paradox is a type of paradox in which an object, person, or piece of information sent back in time results in an infinite loop where the object has no discernible origin, and exists without ever being created. It is also known as an Ontological Paradox, as ontology is a branch of philosophy concerned with the nature of being or existence.

– Information : George Lucas traveling back in time and giving himself the scripts for the Star War movies which he then goes on to direct and gain great fame for would create a bootstrap paradox involving information, as the scripts have no true point of creation or origin.

– Person : A bootstrap paradox involving a person could be, say, a 20-year-old male time traveler who goes back 21 years, meets a woman, has an affair, and returns home three months later without knowing the woman was pregnant. Her child grows up to be the 20-year-old time traveler, who travels back 21 years through time, meets a woman, and so on. American science fiction writer Robert Heinlein wrote a strange short story involving a sexual paradox in his 1959 classic “All You Zombies.”

These ontological paradoxes imply that the future, present, and past are not defined, thus giving scientists an obvious problem on how to then pinpoint the “origin” of anything, a word customarily referring to the past, but now rendered meaningless. Further questions arise as to how the object/data was created, and by whom. Nevertheless, Einstein’s field equations allow for the possibility of closed time loops, with Kip Thorne the first theoretical physicist to recognize traversable wormholes and backward time travel as being theoretically possible under certain conditions.

  • Movies : Examples of bootstrap paradoxes in the movies include Somewhere in Time (1980), Bill and Ted’s Excellent Adventure (1989), the Terminator movies, and Time Lapse (2014). The Netflix series Dark (2017-19) also features a book called ‘A Journey Through Time’ which presents another classic example of a bootstrap paradox.
  • Books : Examples of bootstrap paradoxes in books include Michael Moorcock’s ‘Behold The Man’, Tim Powers’ The Anubis Gates, and Heinlein’s “By His Bootstraps”

3: Grandfather Paradox

The Grandfather Paradox concerns ‘self-inconsistent solutions’ to a timeline’s history caused by traveling back in time. For example, if you traveled to the past and killed your grandfather, you would never have been born and would not have been able to travel to the past – a paradox.

Let’s say you did decide to kill your grandfather because he created a dynasty that ruined the world. You figure if you knock him off before he meets your grandmother then the whole family line (including you) will vanish and the world will be a better place. According to theoretical physicists, the situation could play out as follows:

– Timeline protection hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger but the gun fails to fire. Click! Click! Click! The bullets in the chamber have dents in the firing caps. You point the gun elsewhere and pull the trigger. Bang! Point it at your grandfather.. Click! Click! Click! So you try another method to kill him, but that only leads to scars that in later life he attributed to the world’s worst mugger. You can do many things as long as they’re not fatal until you are chased off by a policeman.

– Multiple universes hypothesis: You pop back in time, walk up to him, and point a revolver at his head. You pull the trigger and Boom! The deed is done. You return to the “present,” but you never existed here. Everything about you has been erased, including your family, friends, home, possessions, bank account, and history. You’ve entered a timeline where you never existed. Scientists entertain the possibility that you have now created an alternate timeline or entered a parallel universe.

  • Movies : Example of the Grandfather Paradox in movies include Back to the Future (1985), Back to the Future Part II (1989), and Back to the Future Part III (1990).
  • Books : Example of the Grandfather Paradox in books include Dr. Quantum in the Grandfather Paradox by Fred Alan Wolf , The Grandfather Paradox by Steven Burgauer, and Future Times Three (1944) by René Barjavel, the very first treatment of a grandfather paradox in a novel.

4: Let’s Kill Hitler Paradox

Similar to the Grandfather Paradox which paradoxically prevents your own birth, the Killing Hitler paradox erases your own reason for going back in time to kill him. Furthermore, while killing Grandpa might have a limited “butterfly effect,” killing Hitler would have far-reaching consequences for everyone in the world, even if only for the fact you studied him in school.

The paradox itself arises from the idea that if you were successful, then there would be no reason to time travel in the first place. If you killed Hitler then none of his actions would trickle down through history and cause you to want to make the attempt.

  • Movies/Shows : By far the best treatment for this notion occurred in a Twilight Zone episode called Cradle of Darkness which sums up the difficulties involved in trying to change history, with another being an episode of Dr Who called ‘Let’s Kill Hitler’.
  • Books : Examples of the Let’s Kill Hitler Paradox in books include How to Kill Hitler: A Guide For Time Travelers by Andrew Stanek, and the graphic novel I Killed Adolf Hitler by Jason.

5: Polchinski’s Paradox

American theoretical physicist Joseph Polchinski proposed a time paradox scenario in which a billiard ball enters a wormhole, and emerges out the other end in the past just in time to collide with its younger version and stop it from going into the wormhole in the first place.

Polchinski’s paradox is taken seriously by physicists, as there is nothing in Einstein’s General Relativity to rule out the possibility of time travel, closed time-like curves (CTCs), or tunnels through space-time. Furthermore, it has the advantage of being based upon the laws of motion, without having to refer to the indeterministic concept of free will, and so presents a better research method for scientists to think about the paradox. When Joseph Polchinski proposed the paradox, he had Novikov’s Self-Consistency Principle in mind, which basically states that while time travel is possible, time paradoxes are forbidden.

However, a number of solutions have been formulated to avoid the inconsistencies Polchinski suggested, which essentially involves the billiard ball delivering a blow that changes its younger version’s course, but not enough to stop it from entering the wormhole. This solution is related to the ‘timeline-protection hypothesis’ which states that a probability distortion would occur in order to prevent a paradox from happening. This also helps explain why if you tried to time travel and murder your grandfather, something will always happen to make that impossible, thus preserving a consistent version of history.

  • Books:  Paradoxes of Time Travel by Ryan Wasserman is a wide-ranging exploration of time and time travel, including Polchinski’s Paradox.

Are Self-Fulfilling Prophecies Paradoxes?

A self-fulfilling prophecy is only a causality loop when the prophecy is truly known to happen and events in the future cause effects in the past, otherwise the phenomenon is not so much a paradox as a case of cause and effect.  Say,  for instance, an authority figure states that something is inevitable, proper, and true, convincing everyone through persuasive style. People, completely convinced through rhetoric, begin to behave as if the prediction were already true, and consequently bring it about through their actions. This might be seen best by an example where someone convincingly states:

“High-speed Magnetic Levitation Trains will dominate as the best form of transportation from the 21st Century onward.”

Jet travel, relying on diminishing fuel supplies, will be reserved for ocean crossing, and local flights will be a thing of the past. People now start planning on building networks of high-speed trains that run on electricity. Infrastructure gears up to supply the needed parts and the prediction becomes true not because it was truly inevitable (though it is a smart idea), but because people behaved as if it were true.

It even works on a smaller scale – the scale of individuals. The basic methodology for all those “self-help” books out in the world is that if you modify your thinking that you are successful (money, career, dating, etc.), then with the strengthening of that belief you start to behave like a successful person. People begin to notice and start to treat you like a successful person; it is a reinforcement/feedback loop and you actually become successful by behaving as if you were.

Are Time Paradoxes Inevitable?

The Butterfly Effect is a reference to Chaos Theory where seemingly trivial changes can have huge cascade reactions over long periods of time. Consequently, the Timeline corruption hypothesis states that time paradoxes are an unavoidable consequence of time travel, and even insignificant changes may be enough to alter history completely.

In one story, a paleontologist, with the help of a time travel device, travels back to the Jurassic Period to get photographs of Stegosaurus, Brachiosaurus, Ceratosaurus, and Allosaurus amongst other dinosaurs. He knows he can’t take samples so he just takes magnificent pictures from the fixed platform that is positioned precisely to not change anything about the environment. His assistant is about to pick a long blade of grass, but he stops him and explains how nothing must change because of their presence. They finish what they are doing and return to the present, but everything is gone. They reappear in a wild world with no humans and no signs that they ever existed. They fall to the floor of their platform, the only man-made thing in the whole world, and lament “Why? We didn’t change anything!” And there on the heel of the scientist’s shoe is a crushed butterfly.

The Butterfly Effect is also a movie, starring Ashton Kutcher as Evan Treborn and Amy Smart as Kayleigh Miller, where a troubled man has had blackouts during his youth, later explained by him traveling back into his own past and taking charge of his younger body briefly. The movie explores the issue of changing the timeline and how unintended consequences can propagate.

Scientists eager to avoid the paradoxes presented by time travel have come up with a number of ingenious ways in which to present a more consistent version of reality, some of which have been touched upon here,  including:

  • The Solution: time travel is impossible because of the very paradox it creates.
  • Self-healing hypothesis: successfully altering events in the past will set off another set of events which will cause the present to remain the same.
  • The Multiverse or “many-worlds” hypothesis: an alternate parallel universe or timeline is created each time an event is altered in the past.
  • Erased timeline hypothesis : a person traveling to the past would exist in the new timeline, but have their own timeline erased.

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Largest 3D map of our universe could hint that dark energy evolves with time

"If this is true, this just turns cosmology upside down."

It has been over two decades since the discovery of dark energy.

Scientists have therefore had more than 20 years to decode the secrets of this invisible substance that appears to be pulling the universe apart. Yet, they still know close to nothing about it. Dark energy, in fact, may not even be a substance. It could be a force or even an intrinsic property of space itself. 

For instance, the standard model of cosmology — our leading theory of cosmic evolution — does suggest dark energy is unwavering across the universe and throughout time, making it a fundamental property of space . If constant, the mysterious dark energy that makes up a whopping 70 percent of the universe would push away all stars and galaxies. However, the biggest survey of the universe’s cosmic history could indicate that dark energy, also known as a hypothetical "anti-gravity" force , may evolve with time rather than remain constant, hinting at a less lonely future for residents of the universe.

Related: 25 years after its discovery, dark energy remains frustratingly elusive

If this early result holds with future observations, cosmologists may have to, at the very least, explore systematic uncertainties in the prevailing Lambda CDM (LCDM) model, a mathematical model of the universe in which lambda represents dark energy. They may also need to start  sifting through dozens of other models of our universe to find the true best fit. Still, the evidence is tentative — it does not reach what's known as the "5-sigma threshold," which determines whether a signal can be celebrated as an official discovery. So, continuously emerging interpretations about dark energy's evolution could change with more data scheduled to come within the next few years.

"If this is true, this just turns cosmology upside down," said Dillon Brout of Boston University, who measures the acceleration of the universe with supernovas . Such a discovery would be a "paradigm shift in our thinking of what our best understanding of our universe is."

Streetlights of the universe 

Perched atop the Nicholas U. Mayall 4-meter telescope at Arizona's Kitt Peak National Observatory , the Dark Energy Spectroscopic Instrument, or DESI , pinpoints positions of a million galaxies each month. Through these observations, cosmologists can measure the universe's expansion rate as it increased over the past 11 billion years. These faraway galaxies, which can be likened to the " streetlights of the universe ," are thus helping cosmologists study the universe-permeating enigma of dark energy.

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And, on Thursday (April 4), the DESI collaboration shared the largest-ever 3D map of the universe. It includes high-precision measurements of the universe's expansion rate over the past 11 billion years as well. In its first year of operations alone, DESI has proven to be twice as powerful at measuring the expansion history of the early universe as its predecessor, the Sloan Digital Sky Survey, which took more than a decade to build a similar 3D map.

This "is the next generation of data we've been waiting a long time for, so it's really nice to see it having arrived," said Brout, who is not involved with the DESI collaboration. 

In addition to countless galaxies clustered together like knotted threads, DESI's new 3D map spotlights a faint pattern in the early universe known as Baryon Acoustic Oscillations, or BAO. These subtle, 3D wrinkles had flown through matter that existed during the first 380,000 years of our universe's history, freezing with time and turning into relics of an infant cosmos. By mapping the sizes of those frozen BAOs, researchers managed to estimate the distances to galaxies and infer how fast the universe was expanding at various points in time.

Because light from typical galaxies is too faint to see, as those galaxies sit very far away from us and the light they emit is relatively low-intensity, the DESI collaboration also studied over 400,000 intensely bright objects called quasars . As light from these objects glides through interstellar space , it gets absorbed by clouds of gas and dust, helping cosmologists map pockets of dense matter in a similar way to mapping galaxies. 

"It lets us look out further to when the universe was very young," Andreu Font-Ribera, a scientist at the Institute for High Energy Physics in Spain and a member of the DESI collaboration, said in a statement . "It's a really hard measurement to do, and very cool to see it succeed."

'If this is real, we're in uncharted territory' 

The preliminary conclusion that dark energy could be evolving with time comes from an early analysis of DESI data combined with data from other cosmological data. The researchers found a varying dark energy model agreed better with the data compared to the standard model. To be clear, no single dataset by itself convincingly reveals the time-evolving nature of dark energy, but the signal becomes slightly stronger when all datasets are combined. 

"It is not a strong enough preference that I would say Lambda CDM is wrong," Kyle Dawson, the co-spokesperson for DESI at the University of Utah, told Space.com. "We've actually never found deviations from that model before with any real meaning."

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From the early analysis, however, it appears dark energy is transitioning from being a strong driver of acceleration of our universe to tapering off to some degree, said Dawson. 

"If this is real, we're in uncharted territory," said Brout. The DESI collaboration used the second simplest model of our universe after Lambda CDM, which is unremarkable except for its ability to help cosmologists check for deviations from the standard model. If future observations in the pipeline indeed find dark energy is evolving with time, dozens of other models too would become viable, and cosmologists would have to start testing all of them individually, said Brout.

"If it's not Lambda CDM, who knows?"

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

Sharmila Kuthunur

Sharmila Kuthunur is a Seattle-based science journalist covering astronomy, astrophysics and space exploration. Follow her on X @skuthunur.

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three time travel theories

Screen Rant

X-men theory reveals how marvel's latest heartbreaking deaths can be undone.

X-Men '97 episode 5, "Remember It," ended in tragedy for Genosha's residents and the X-Men, but the series already revealed how this can be reversed.

Warning: This article contains spoilers for X-Men '97 episode 5, "Remember It."

  • X-Men '97 episode 5's heartbreaking ending saw members of the X-Men sacrifice themselves during an attack on the mutant nation of Genosha.
  • Cable's appearance in X-Men '97 episode 5 could hint at the perfect way for these tragic deaths to be reversed.
  • Cable's reappearance in future X-Men '97 episodes would also help to provide Scott Summers with closure regarding after his recent experiences.

Marvel Studios Animation has already revealed how the heartbreaking ending of X-Men '97 episode 5, "Remember It," can be reversed, thanks to one time-traveling cameo. X-Men '97 , set outside the main timeline of the MCU , brought back many fan-favorite mutant heroes from X-Men: The Animated Series , continuing their story roughly a year after Professor X's departure in the original show's finale. X-Men '97 has seen Magneto assume control of the X-Men, stripped Storm of her abilities, pitted the X-Men against Madelyne Pryor and Mister Sinister, and marked Jubilee's 18th birthday, but X-Men '97 episode 5 packed an even bigger punch.

X-Men '97 episode 5 released on April 10, and saw X-Men members Magneto, Rogue, and Gambit journey to Genosha to celebrate the mutant nation's integration into the United Nations. In the midst of the celebration, however, Genosha was attacked by a giant, three-headed Sentinel, dead-set on eliminating the island's mutant inhabitants . Taking inspiration from Marvel Comics' tragic E is for Extinction storyline from 2001, which saw Cassandra Nova use Sentinels to attack Genosha and eradicate 16 million mutants, and real world events including 9/11 and the Pulse nightclub shooting, X-Men '97 episode 5's heartbreaking ending marked tragedy for the X-Men.

X-Men '97 Creator Explains THAT Devastating Ending So Perfectly It Makes The Tragedy Even Better

X-men ’97 episode 5 killed off gambit & magneto.

While there were many mutants taken out by the looming Sentinel in X-Men '97 episode 5, the episode also marked the end of the line for two prominent members of the X-Men. Both Magneto and Gambit were killed during the attack on Genosha, though both died making a final stand against the Sentinel, and protecting others around them . Magneto used his magnetism to hold back the Sentinel's attack for as long as possible, though ultimately wasn't strong enough. Gambit had more success, using his ability to supercharge the Sentinel and blow it up, at the cost of his own life.

Since Magneto assumed control of the X-Men, per Professor X's last will and testament, in X-Men '97's premiere episode , he has proven his desire for change time and again, and has become a steadfast commander of the team. This makes his untimely death even more tragic, but it was Gambit's demise that packed a hugely emotional punch. Gambit used his last moments to destroy the Sentinel, despite knowing it would mean certain death, and only moments after Rogue chose him over Magneto. These deaths marked a dark turn for X-Men '97 , but may not be the end of the story.

Cable’s Appearance In X-Men ’97 Episode 5 Foreshadows Him Changing The Past

Seconds before the attack on Genosha commenced, Cable made his first X-Men '97 appearance , having traveled back in time to Genosha. Meeting his biological mother, Madelyne Pryor, Cable presumably wanted to warn the mutant inhabitants of Genosha about the impending attack, suggesting that he is set on reversing the tragedy. While Cable was too late this time, it could be assumed that he will simply go back in time again, warning the X-Men earlier, and enabling Genosha to perhaps be evacuated . This would save hundreds of mutants, including Magneto and Gambit themselves, meaning their deaths may not be the end.

Cable made his X-Men: The Animated Series debut in season 1, episode 7, "Slave Island," back in 1992. This episode saw X-Men members Storm, Gambit and Jubilee head to Genosha for the first time.

There would have been very little reason for Cable to appear in the moments before the Sentinel's attack on Genosha if he wasn't trying to change the past. Cable's ability to travel through time makes this possible, and since X-Men '97 has already been putting attention on Cable's origin story, being the child of Scott Summers and Madelyne Pryor, though also being connected to Jean Grey, it's likely he'll have a bigger role in X-Men '97's upcoming episodes . This could easily see Magneto and Gambit returning from the dead, but could also help to solve other issues within the X-Men.

Cable's Return Can Resolve Cyclops' X-Men '97 Arc

X-Men '97 episode 3 revealed Jean Grey to actually be a clone of the original X-Men member, created by Mister Sinister to birth a son with Scott Summers' Cyclops. Once the truth was revealed, Jean's clone became the Goblin Queen, while Sinister conducted experiments on the newborn Nathan Summers, inadvertently infecting him with a techno-organic virus. Once the day had been saved, Scott and the real Jean Grey sent Nathan into the future with Bishop to find a cure, leaving them to pick up the pieces of their unusual relationship while Scott's son would ultimately grow up to be Cable.

Scott Summers was shown to be suffering with his recent experiences in X-Men '97 episode 5 , particularly when being interviewed, but Cable's return in the series' upcoming episodes could help to calm him down. Currently, the only character who knows Cable's true identity is Madelyne Pryor, who was presumably killed during the attack on Genosha. When the other X-Men members realize the truth, a stronger connection to Cable will be established, and Scott and Jean may fall into their father and mother roles easily. X-Men '97 episode 5 may have marked tragedy, but good things may be on the way.

X-Men '97

X-Men '97 is the direct continuation of the popular 1990s animated series X-Men: The Animated Series. Taking up where the third season left off, Marvel's revival brings back famous mutants such as Wolverine, Storm, Rogue, Gambit, Cyclops, Beast, Magneto, and Nightcrawler, who fight villains like Mr. Sinister, the Sentinels, and the Hellfire Club.

Key Release Dates

Deadpool & wolverine, marvel's thunderbolts, the fantastic four (2025), blade (2025), avengers: the kang dynasty, avengers: secret wars.

April 8 solar eclipse totality will last for minutes, not cause 'days of darkness' | Fact check

three time travel theories

The claim: April 8 solar eclipse will cause 3-5 days of darkness

A March 18 Facebook post ( direct link , archive link ) shows a TikTok video that opens with footage of a total solar eclipse. Text superimposed on the video reads, “3 to 5 days of darkness” and “April 8, 2024” – the date of the 2024 eclipse .

"Three days of darkness will occur," says a narrator, who later states, "There will be no sunlight nor moonlight on the Earth's surface."

It was shared more than 30,000 times in two weeks. A similar version received thousands of likes on Instagram , and the original TikTok video was shared thousands of times .

Fact check roundup :  Total solar eclipse sparks many misleading flat Earth claims online. Here's what's true and

More from the Fact-Check Team: How we pick and research claims | Email newsletter | Facebook page

Our rating: False

The claim is nonsense. The entire planet will not go dark for days. Only those located in the narrow path of totality will experience any significant darkness, and experts said that will last for only about four minutes.

‘Nothing in this video is even adjacent to reality,' says expert

The claim in the TikTok video is baseless, experts said, because it conflicts with centuries of scientific and astronomical research that led to a clear understanding of the predictable movements of Earth, the moon and other celestial bodies.

“Nothing in this video is even adjacent to reality,” said Katie Mack , a theoretical astrophysicist, author and chair of cosmology and science communication at the Perimeter Institute for Theoretical Physics .

Fact check : Solar eclipse image is digital art, not from International Space Station

An eclipse occurs when the moon moves between Earth and the sun and casts a shadow on the planet. Their precise timing, location and length are determined by the calculations of the speeds, positions and movements of the sun, moon and Earth in three-dimensional space, according to NASA. Hundreds of years ago, scientists discovered that eclipses occur in predictable patterns known as the Saros Cycle , and improvements in computing and technology have fine-tuned predictive models to the accuracy of less than a minute over the span of hundreds of years, explains NASA on its website .

While the April 8 eclipse will affect some places located in a narrow geographic area of North America for minutes at a time, experts are certain it will not leave the entire planet shrouded in darkness for days.

“The eclipse happens at different times for everyone,” said Dan McGlaun , an expert who has observed and documented 15 eclipses and publishes the website Eclipse2024.org . “Just like someone walking in front of the TV blocks it momentarily for you, but not for the person on the couch next to you.”

A succession of cities in the narrow path of totality – a 115-mile-wide swath stretching from Mexico across the U.S. and into Canada – will experience a few minutes of darkness during the day when the moon completely blocks the sun. The moon travels at a speed of just over 2,000 miles each hour , McGlaun said, which means only about four minutes of totality for most places in that belt .

“No way (the moon) will stop, so there’s only so long that it can block the sun,” he said.

North American and Central American locations outside that path will experience a few hours of a partial solar eclipse that will not be accompanied by any significant darkness. Outdoors will “ be no darker than on a bright overcast day ” with shadows appearing sharper and crisper, according to Space.com.

The video gets a few other things wrong. At one point it states that light from the sun or stars will be unable to reach Earth for “72 hours or five days,” but 72 hours works out to three days, not five. Mack called its reference to a photon belt in space a “made-up concept” and said its claim that photons – the fundamental particles of light – will somehow block light from reaching Earth “a wild misunderstanding.”

“It’s just total nonsense,” Mack said.

USA TODAY reached out to the Facebook and Instagram users who shared the post but did not immediately receive responses. The TikTok user who shared the video could not be reached.

Our fact-check sources:

  • Katie Mack , April 3, Email exchange with USA TODAY
  • Dan McGlaun , April 2, Email exchange with USA TODAY
  • Detroit Free Press, March 23, Math, science, history and observation: How we know when, where eclipses will occur
  • NASA, accessed April 3, 2024 Total Eclipse: Where & When
  • NASA, accessed April 3, Eclipses: Frequently Asked Questions
  • NASA, accessed April 3, Fun Facts About The Moon
  • Space.com, April 2, Total solar eclipse April 8, 2024: What you'll see if you're outside the path of totality
  • California Institute of Technology, accessed April 3, How fast does the Moon travel around the Earth?

Thank you for supporting our journalism. You can subscribe to our print edition, ad-free app or e-newspaper here .

USA TODAY is a verified signatory of the International Fact-Checking Network, which requires a demonstrated commitment to nonpartisanship, fairness and transparency. Our fact-check work is supported in part by a grant from Meta .

three time travel theories

Solar Eclipse Will Pass Over Every US City Named Nineveh on April 8, 2024?

A total solar eclipse is caused by the moon and the sun being in exactly the right place at exactly the right time., published april 6, 2024.


About this rating

For a couple of minutes on April 8, 2024 , in a narrow, curved band across North America, one of the greatest spectacles in nature will occur: a total solar eclipse. Being in that path at exactly the right time is the only way people on the continent will be able to look directly at the sun without damaging their eyeballs until the next North American eclipse in 2044, and so millions of people from around the world will flock to cities in the path of totality, including Dallas and Indianapolis.

Eclipses do not discriminate, so anyone in the path of totality will be able to see the sun fully obstructed by the moon. However, some people  have claimed online that there's one interesting coincidence about the eclipse's path of totality: It will pass through every city in the United States named Nineveh. That name is shared by an ancient city in modern-day Iraq that was described in the bible as "evil." 

Snopes received an email from a reader who requested that we check the claim about cities named Nineveh in the eclipse path. In our research, we discovered that many of the people making the claim were Christians who were interpreting the eclipse as a bad omen . 

Contrary to the claims, Snopes discovered that the path of totality in the eclipse does not pass through seven cities in the United States named Nineveh — it passes through just two. But before counting places named Nineveh, we must first briefly clarify how eclipses work.

How Eclipses Work

A total solar eclipse is caused by the moon and the sun being in exactly the right place at exactly the right time. The moon fully blocks the light from the sun, casting a large shadow on the earth.

Those completely inside the moon's shadow, called the umbra, are the only ones who will be able to look directly at the sun without eye protection. It's the small path of the umbra that people travel to in order to see the total solar eclipse. The website GreatAmericanEclipse.com created a visualization of the shadow's path across North America.

Outside the umbra, in a much larger area where the moon blocks only some of the sun, viewers will experience a partial solar eclipse, where the sun looks like it has a giant bite taken out of it. You cannot view a partial solar eclipse without special eclipse glasses. The entirety of the continental United States will be able to see a partial solar eclipse on April 8, just as the entirety of the United States (even Alaska and Hawaii) was able to see a partial solar eclipse in  2017 . 

The cool part (partial) of an eclipse can be seen from a very large area, as long as you wear eclipse glasses. The really cool part (total) of an eclipse can be seen only in a small area. It is the total eclipse that people have thought held religious significance since practically as long as humans have had eyes to see and religions to follow.

To quote the essayist Annie Dillard :

A partial eclipse is very interesting. It bears almost no relation to a total eclipse. Seeing a partial eclipse bears the same relation to seeing a total eclipse as kissing a man does to marrying him, or as flying in an airplane does to falling out of an airplane. Although the one experience precedes the other, it in no way prepares you for it. 

Places Named Nineveh

We started with Wikipedia's list of places named Nineveh to get a general idea of where to look. Of course, we cross checked those results with more-reliable sources of knowledge, including Google Maps and data from the U.S. Census Bureau.

Wikipedia listed just six places in the U.S. named Nineveh, which made our claim of seven dubious to begin with. Checking the locations of those places on Google Maps, we found that three were actually townships, a term used for county subdivisons in some states. 

The first was the largest, Indiana's Nineveh Township (south of Indianapolis), which contains a small hamlet of the same name. Both the township and the hamlet will indeed fall in the path of the total eclipse.

Next, Wikipedia listed two townships in Missouri — one in Adair County (about halfway between Kansas City and Davenport, Iowa) and one in Lincoln County (about an hour northwest of St. Louis). But neither of the two townships contained a village named Nineveh on any of the maps we looked at. Furthermore, neither of the townships fell in the path of the total eclipse. 

The fourth place on Wikipedia's list, Nineveh, New York, is about 30 minutes east of Binghamton. We found it marked on maps but, again, it did not lie in the path of totality.

Fifth: Nineveh, Pennsylvania, roughly halfway between Pittsburgh and Morgantown, West Virginia. This Nineveh was marked on maps, but it was also outside of the total eclipse. It was also the last Nineveh listed by the U.S. Census Bureau.

Sixth, we found Nineveh, Virginia, an hour and a half west of Washington, D.C. This was the easiest to check: Nobody in the state of Virginia will be able to see full totality during the eclipse. We did not find a label for Nineveh on maps, and buildings located in the area had their postal addresses listed as White Post, Virginia.

That completed the Wikipedia list, but various posts about the supposed line-up listed two more Ninevehs located in the U.S.: one in Texas and one in Ohio.

Nineveh, Texas, was not marked on maps, nor did it have a post office. It was located not far off of Interstate 45 halfway between Houston and Dallas. This one was close, but we eventually confirmed that it would be outside of the zone of totality by referencing nearby cities that also were outside of totality.

Nineveh, Ohio, was a similar story: not found on maps, no post office, no Census data. But this Nineveh, 30 minutes northwest of Dayton, was finally our second hit.

In total, we counted two places named Nineveh in the United States that could be found in the path of totality.

2024 Total Eclipse . https://science.nasa.gov/eclipses/future-eclipses/eclipse-2024/. Accessed 28 Mar. 2024.

"A Total Eclipse Is near. For Some, It's Evidence of Higher Power. For Others It's a Warning." USA TODAY , https://www.usatoday.com/story/news/nation/2024/03/23/2024-total-solar-exclipse-religious-implications/72869724007/. Accessed 28 Mar. 2024.

April 8, 2024 Eclipse Will Pass Over 7 United States Cities Named Nineveh . www.youtube.com , https://www.youtube.com/watch?v=3n6dp85XynY. Accessed 28 Mar. 2024.

April 8 Eclipse and Third-Day Events in Scripture . https://www.biblejournalclasses.com/blog/april-8-eclipse-and-third-day-events-in-scripture-2. Accessed 28 Mar. 2024.

Dawson, Brandon. "THE JONAH ECLIPSE - 40 DAYS - GODS URGENT WARNING TO AMERICA!" Tribe of Christians , 2 Mar. 2024, https://www.tribeofchristians.com/single-post/the-jonah-eclipse-god-s-great-warning-to-america-april-8th-2024.

Dillard, Annie. "Total Eclipse." The Atlantic , 8 Aug. 2017, https://www.theatlantic.com/science/archive/2017/08/annie-dillards-total-eclipse/536148/.

Eclipse 2017 . https://eclipse2017.nasa.gov/. Accessed 28 Mar. 2024.

Mark, Joshua J. "Nineveh." World History Encyclopedia , https://www.worldhistory.org/nineveh/. Accessed 28 Mar. 2024.

"Nineveh (Disambiguation)." Wikipedia , 29 Oct. 2023. Wikipedia , https://en.wikipedia.org/w/index.php?title=Nineveh_(disambiguation)&oldid=1182408744.

Noah. "The Upcoming U.S. Eclipse Just Got Even Stranger!" WLT Report , 4 Mar. 2024, https://wltreport.com/2024/03/04/upcoming-u-s-eclipse-just-got-even-stranger/.

The APRIL 8, 2024 ECLIPSE & The 7 Cities Named Nineveh | The APRIL 8, 2024 ECLIPSE & The 7 Cities Named Nineveh | By Messiah GuguFacebook . www.facebook.com , https://www.facebook.com/100067092253715/videos/the-april-8-2024-eclipse-the-7-cities-named-nineveh/397509926249711/. Accessed 28 Mar. 2024.

The April 8 2024 Eclipse and the 7 Cities Named Nineveh . www.youtube.com , https://www.youtube.com/watch?v=eLkxKT65IFc. Accessed 28 Mar. 2024.

"Total Solar Eclipse 2024 US." Great American Eclipse , https://www.greatamericaneclipse.com/april-8-2024. Accessed 28 Mar. 2024.

By Jack Izzo

Jack Izzo is a Chicago-based journalist and two-time "Jeopardy!" alumnus.

Article Tags

  • Solar Eclipse 2024

10 Surprising Facts About the 2024 Solar Eclipse

A total solar eclipse will sweep across North America on Monday, April 8, offering a spectacle for tens of millions of people who live in its path and others who will travel to see it.

A solar eclipse occurs during the new moon phase, when the moon passes between Earth and the sun, casting a shadow on Earth and totally or partially blocking our view of the sun. While an average of two solar eclipses happen every year, a particular spot on Earth is only in the path of totality every 375 years on average, Astronomy reported .

“Eclipses themselves aren't rare, it's just eclipses at your house are pretty rare,” John Gianforte, director of the University of New Hampshire Observatory, tells TIME. If you stay in your hometown, you may never spot one, but if you’re willing to travel, you can witness multiple. Gianforte has seen five eclipses and intends to travel to Texas this year, where the weather prospects are better.

One fun part of experiencing an eclipse can be watching the people around you. “They may yell, they scream, they cry, they hug each other, and that’s because it’s such an amazingly beautiful event,” Gianforte, who also serves as an extension associate professor of space science education, notes. “Everyone should see at least one in their life, because they’re just so spectacular. They are emotion-evoking natural events.”

Here are 10 surprising facts about the science behind the phenomenon, what makes 2024’s solar eclipse unique, and what to expect.

The total eclipse starts in the Pacific Ocean and ends in the Atlantic 

The darker, inner shadow the moon casts is called the umbra , in which you can see a rarer total eclipse. The outer, lighter second shadow is called the penumbra, under which you will see a partial eclipse visible in more locations.

The total eclipse starts at 12:39 p.m. Eastern Time, a bit more than 620 miles south of the Republic of Kiribati in the Pacific Ocean, according to Astronomy . The umbra remains in contact with Earth’s surface for three hours and 16 minutes until 3:55 p.m. when it ends in the Atlantic Ocean, roughly 340 miles southwest of Ireland.

The umbra enters the U.S. at the Mexican border just south of Eagle Pass, Texas, and leaves just north of Houlton, Maine, with one hour and eight minutes between entry and exit, the National Aeronautics and Space Administration (NASA) tells TIME in an email.

Mexico will see the longest totality during the eclipse 

The longest totality will extend for four minutes and 28 seconds on a 350-mile-long swath near the centerline of the eclipse, including west of Torreón, Mexico, according to NASA.

In the U.S., some areas of Texas will catch nearly equally long total eclipses. For example, in Fredericksburg, totality will last four minutes and 23 seconds—and that gets slightly longer if you travel west, the agency tells TIME. Most places along the centerline will see totality lasting between three and a half minutes and four minutes.

three time travel theories

More people currently live in the path of totality compared to the last eclipse 

An estimated 31.6 million people live in the path of totality for 2024’s solar eclipse, compared to 12 million during the last solar eclipse that crossed the U.S. in 2017, per NASA .

The path of totality is much wider than in 2017, and this year’s eclipse is also passing over more cities and densely populated areas than last time. 

A part of the sun which is typically hidden will reveal itself

Solar eclipses allow for a glimpse of the sun’s corona —the outermost atmosphere of the star that is normally not visible to humans because of the sun’s brightness.

The corona consists of wispy, white streamers of plasma—charged gas—that radiate from the sun. The corona is much hotter than the sun's surface —about 1 million degrees Celsius (1.8 million degrees Fahrenheit) compared to 5,500 degrees Celsius (9,940 degrees Fahrenheit).

The sun will be near its more dramatic solar maximum 

During the 2024 eclipse, the sun will be near “solar maximum.” This is the most active phase of a roughly 11-year solar cycle, which might lead to more prominent and evident sun activity, Gianforte tells TIME.  

“We're in a very active state of the sun, which makes eclipses more exciting, and [means there is] more to look forward to during the total phase of the eclipse,” he explains. 

People should look for an extended, active corona with more spikes and maybe some curls in it, keeping an eye out for prominences , pink explosions of plasma that leap off the sun’s surface and are pulled back by the sun’s magnetic field, and streamers coming off the sun.

Streamers “are a beautiful, beautiful shade of pink, and silhouetted against the black, new moon that's passing across the disk of the sun, it makes them stand out very well. So it's really just a beautiful sight to look up at the totally eclipsed sun,” Gianforte says.

Solar Eclipse

Two planets—and maybe a comet—could also be spotted

Venus will be visible 15 degrees west-southwest of the sun 10 minutes before totality, according to Astronomy. Jupiter will also appear 30 degrees to the east-northeast of the sun during totality, or perhaps a few minutes before. Venus is expected to shine more than five times as bright as Jupiter. 

Another celestial object that may be visible is Comet 12P/Pons-Brooks , about six degrees to the right of Jupiter. Gianforte says the comet, with its distinctive circular cloud of gas and a long tail, has been “really putting on a great show in the sky” ahead of the eclipse.

The eclipse can cause a “360-degree sunset” 

A solar eclipse can cause a sunset-like glow in every direction—called a “360-degree sunset”—which you might notice during the 2024 eclipse, NASA said . The effect is caused by light from the sun in areas outside of the path of totality and only lasts as long as totality.

The temperature will drop 

When the sun is blocked out, the temperature drops noticeably. During the last total solar eclipse in the U.S. in 2017, the National Weather Service recorded that temperature dropped as much as 10 degrees Fahrenheit. In Carbondale, Ill. for example, the temperature dropped from a peak of 90 degrees Fahrenheit just before totality to 84 degrees during totality.

Wildlife may act differently 

When the sky suddenly becomes black as though nighttime, confused “animals, dogs, cats, birds do act very differently ,” Gianforte says.

In the 2017 eclipse, scientists tracked that many flying creatures began returning to the ground or other perches up to 50 minutes before totality. Seeking shelter is a natural response to a storm or weather conditions that can prove deadly for small flying creatures, the report said. Then right before totality, a group of flying creatures changed their behavior again—suddenly taking flight before quickly settling back into their perches again.

There will be a long wait for the next total eclipse in the U.S.

The next total eclipse in the U.S. won’t happen until March 30, 2033, when totality will reportedly only cross parts of Alaska . The next eclipse in the 48 contiguous states is expected to occur on Aug. 12, 2044, with parts of Montana and North Dakota experiencing totality.

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Watch CBS News

What time the 2024 solar eclipse started, reached peak totality and ended

By Sarah Maddox

Updated on: April 9, 2024 / 5:04 AM EDT / CBS News

The 2024 solar eclipse will be visible across North America today. As the moon's position between the Earth and sun casts a shadow on North America, that shadow, or umbra, will travel along the surface from west to east at more than 1,500 miles per hour along the path of totality . 

That means the eclipse will start, peak and end at different times — as will the moments of total darkness along the path of totality — and the best time to view the eclipse depends on where you are located. Some places along the path will have more totality time than others.

In Texas, the south-central region had clouds in the forecast , but it was better to the northeast, according to the National Weather Service. The best eclipse viewing weather was expected in New Hampshire, Vermont and Maine, as well as in Canada's New Brunswick and Newfoundland.

What time does the 2024 total solar eclipse start?

Eclipse map of totality

The total solar eclipse will emerge over the South Pacific Ocean before the shadow falls across North America, beginning in parts of Mexico. The path of totality , where onlookers can witness the moon fully blocking the sun (through eclipse viewing glasses for safety ), is expected to first make landfall near the city of Mazatlán around 9:51 a.m. MT. 

The total solar eclipse will cross over the U.S.-Mexico border into Texas, where it will emerge over Eagle Pass at 12:10 p.m. CT and then peak at about 1:27 p.m. CT.

In Dallas, NASA data shows the partial eclipse will first become visible at 12:23 p.m. CT and peak at 1:40 p.m. CT. The next states in the path of totality are Oklahoma and Arkansas, where the eclipse begins in Little Rock at 12:33 p.m. CT. 

Cleveland will see the beginning of the eclipse at 1:59 p.m. ET. Darkness will start spreading over the sky in Buffalo, New York, at 2:04 p.m. ET. Then, the eclipse will reach northwestern Vermont, including Burlington, at 2:14 p.m. ET. Parts of New Hampshire and Maine will also follow in the path of totality before the eclipse first reaches the Canadian mainland  at 3:13 p.m. ET.

Although the experience won't be exactly the same, viewers in all the contiguous U.S. states outside the path of totality will still be able to see a partial eclipse. Some places will see most of the sun blocked by the moon, including Washington, D.C., where the partial eclipse will start at 2:04 p.m. ET and peak at about 3:20 p.m. ET.

In Chicago, viewers can start viewing the partial eclipse at 12:51 p.m. CT, with the peak arriving at 2:07 p.m. CT.  In Detroit, viewers will be able to enjoy a near-total eclipse beginning at 1:58 p.m. ET and peaking at 3:14 p.m. ET.

New York City will also see a substantial partial eclipse, beginning at 2:10 p.m. ET and peaking around 3:25 p.m. ET.

In Boston it will begin at 2:16 p.m. ET and peak at about 3:29 p.m. ET.

The below table by NASA shows when the eclipse will start, peak and end in 13 cities along the eclipse's path.

What time will the solar eclipse reach peak totality?

Millions more people will have the chance to witness the total solar eclipse this year than during the last total solar eclipse , which was visible from the U.S. in 2017. 

The eclipse's peak will mean something different for cities within the path of totality and for those outside. Within the path of totality, darkness will fall for a few minutes. The longest will last more than 4 minutes, but most places will see between 3.5 and 4 minutes of totality. In cities experiencing a partial eclipse, a percentage of the sun will be obscured for more than two hours.

Mazatlán is set to experience totality at 11:07 am PT. Dallas will be able to see the moon fully cover the sun at 1:40 p.m. CT. Little Rock will start to see the full eclipse at 1:51 p.m. CT, Cleveland at 3:13 p.m. ET and Buffalo at 3:18 p.m. ET. Totality will reach Burlington at 3:26 p.m. ET before moving into the remaining states and reaching Canada around 4:25 p.m.

Outside the path of totality, 87.4% of the sun will be eclipsed in Washington, D.C. at 3:20 p.m. ET, and Chicago will have maximum coverage of 93.9% at 2:07 p.m. CT. New York City is much closer to the path of totality this year than it was in 2017; it will see 89.6% coverage at 3:25 p.m. EDT. 

Detroit is another city that will encounter a near-total eclipse, with 99.2% maximum coverage at 3:14 p.m. ET. Boston will see 92.4% coverage at 3:29 p.m. ET.

What time will the solar eclipse end?

The eclipse will leave continental North America from Newfoundland, Canada, at 5:16 p.m. NT, according to NASA.

At the beginning of the path of totality in Mazatlán, the eclipse will be over by 12:32 p.m. PT, and it will leave Dallas at 3:02 p.m. CT. The eclipse will end in Little Rock at 3:11 p.m. CT, Cleveland at 4:29 p.m. CDT and Buffalo at 4:32 p.m. ET. Burlington won't be far behind, with the eclipse concluding at 4:37 p.m. ET.

Meanwhile, the viewing will end in Chicago at 3:21 p.m. CT, Washington, D.C. at 4:32 p.m. ET, and New York City at 4:36 p.m. ET. 

In Detroit, the partial eclipse will disappear at 4:27 p.m. ET, and in Boston, it will be over at 4:39 p.m. ET.

How long will the eclipse last in total?

The total solar eclipse will begin in Mexico at 11:07 a.m. PT and leave continental North America at 5:16 p.m. NT. From the time the partial eclipse first appears on Earth to its final glimpses before disappearing thousands of miles away, the celestial show will dazzle viewers for about 5 hours, according to timeanddate.com . 

The length of the total solar eclipse at points along the path depends on the viewing location. The longest will be 4 minutes and 28 seconds, northwest of Torreón, Mexico. Near the center of the path, totality takes place for the longest periods of time, according to NASA.

Spectators will observe totality for much longer today than during the 2017 eclipse , when the longest stretch of totality was 2 minutes and 32 seconds.

The moon's shadow seen on Earth today, called the umbra, travels at more than 1,500 miles per hour, according to NASA. It would move even more quickly if the Earth rotated in the opposite direction.

What is the longest a solar eclipse has ever lasted?

The longest known totality was 7 minutes and 28 seconds in 743 B.C. However, NASA says this record will be broken in 2186 with a 7 minute, 29 second total solar eclipse. The next total solar eclipse visible from parts of the U.S. won't happen until Aug. 23, 2044.

Sarah Maddox has been with CBS News since 2019. She works as an associate producer for CBS News Live.

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    But every plotline falls into one of these three Time Travel Theories. J. Frank Wilson on May 8, 2014 (Updated: Nov 8, 2022) Facebook Tweet Pin LinkedIn. Reading Time: 4 minutes . Time travel is one of the most popular themes in cinema. Although most time travel movies are in the sci-fi genre, every genre, even comedy, horror, and drama, have ...

  2. 4 Time Travel Theories and the Physics Behind Them

    Experts have calculated the speed of light at 186,282 miles per second. This equates to 299,792 kilometres per second or an incredible 670,616,629 mph. In theory, there is nothing that travels faster than light. But if we turn to Einstein's special theory again, we know that time is not a single construct for everyone.

  3. Inconceivable Paradoxes: 3 Theories of Time Travel

    Time travel is possible. You've probably heard of three different theories about time travel: the fixed timeline, dynamic timeline and multiverse. The first theory is that time travel isn't possible. The second theory states that it is possible, but only in one direction (from future to past). The third says you can go back and forth ...

  4. Time Travel and Modern Physics

    Time Travel and Modern Physics. First published Thu Feb 17, 2000; substantive revision Mon Mar 6, 2023. Time travel has been a staple of science fiction. With the advent of general relativity it has been entertained by serious physicists. But, especially in the philosophy literature, there have been arguments that time travel is inherently ...

  5. Is Time Travel Possible?

    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 ...

  6. Can we time travel? A theoretical physicist provides some answers

    Some time travel theories suggest that one can observe the past like watching a movie, but cannot interfere with the actions of people in it. (Rodrigo Gonzales/Unsplash) Laws of physics.

  7. Is time travel even possible? An astrophysicist explains the science

    And since curiosity has no age limit - adults, let us know what you're wondering, too. We won't be able to answer every question, but we will do our best. Scientists are trying to figure out ...

  8. Time travel

    Alternate time travel theories. While Einstein's theories appear to make time travel difficult, some researchers have proposed other solutions that could allow jumps back and forth in time. These ...

  9. Is Time Travel Even Possible? An Astrophysicist Explains The Science

    Time isn't the same everywhere. 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 ...

  10. Time travel could be possible, but only with parallel timelines

    Time travel and parallel timelines almost always go hand-in-hand in science fiction, but now we have proof that they must go hand-in-hand in real science as well. General relativity and quantum ...

  11. PDF Can we time travel? A theoretical physicist provides some answers

    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 ...

  12. Time Travel

    Time Travel. First published Thu Nov 14, 2013; substantive revision Fri Mar 22, 2024. There is an extensive literature on time travel in both philosophy and physics. Part of the great interest of the topic stems from the fact that reasons have been given both for thinking that time travel is physically possible—and for thinking that it is ...

  13. Time travel

    The first page of The Time Machine published by Heinemann. Time travel is the hypothetical activity of traveling into the past or future.Time travel is a widely recognized concept in philosophy and fiction, particularly science fiction. In fiction, time travel is typically achieved through the use of a hypothetical device known as a time machine.The idea of a time machine was popularized by H ...

  14. Where Does the Concept of Time Travel Come From?

    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 ...

  15. Time Travel: Theories, Possibilities, and Paradoxes Explained

    Time Travel is defined as the phenomenon of moving between different points in time through a hypothetical device called "Time Machine". Despite being predominantly related to the field of philosophy and fiction, it's somehow supported to a small extent by physics in conjunction with quantum mechanics. However, before getting into the ...

  16. There's One Way Time Travel Could Be Possible, According to This

    But these are just speculations. My students and I are currently working on finding a concrete theory of time travel with multiple histories that is fully compatible with general relativity.Of course, even if we manage to find such a theory, this would not be sufficient to prove that time travel is possible, but it would at least mean that time travel is not ruled out by consistency paradoxes.

  17. A Revolutionary New Physics Hypothesis: Three Time ...

    The authors start from the concept of space-time corresponding to our physical reality: with three spatial dimensions and one time dimension. However, from the point of view of the superluminal observer, only one dimension of this world retains a spatial character, the one along which the particles can move. "The other three dimensions are ...


    a single variable. To understand better the time travel argument, it's useful to bring to light the difference between time (t) inside M and time (T) outside it, namely the difference between private time (internal time) and public time (external time). 8 There are at least three types of time travel in the wellsian meaning, depending on

  19. Introduction: Time Travel and the Mechanics of Narrative

    Abstract. The Introduction begins the theoretical discussion of time travel and narrative theory with interpretations of three representative texts from the late 1960s: Robert Silverberg's Up the Line, Michael Moorcock's Behold the Man, and Larry Niven's "All the Myriad Ways."In each case, time travel narrative exhibits essential problems of narrative theory, historiography, and the ...

  20. 3 Theories of Time Travel : r/coolguides

    3 Theories of Time Travel. I was running a sci-fi campaign with a lot of time travel in it, and this was my way of dealing with paradoxes. A paradox collapses the entire timeline and everything in it. Because all time is happening at once (from a time travel perspective) all paradoxes that will ever be possible have already been activated and ...

  21. 3 Time Travel Theories : r/Timeless

    So, the Dynamic timeline theory is a little more complex than that. For example in Terminator, you have a paradox where Arnold comes back to kill John Connors mom. If he hadn't, John Connor wouldn't exist. The most logical explanation is that something else caused John Connors father to travel back in time and changes the future.

  22. 5 Bizarre Paradoxes Of Time Travel Explained

    1: Predestination Paradox. A Predestination Paradox occurs when the actions of a person traveling back in time become part of past events, and may ultimately cause the event he is trying to prevent to take place. The result is a 'temporal causality loop' in which Event 1 in the past influences Event 2 in the future (time travel to the past ...

  23. Largest 3D map of our universe could hint that dark energy evolves with

    The preliminary conclusion that dark energy could be evolving with time comes from an early analysis of DESI data combined with data from other cosmological data. The researchers found a varying ...

  24. X-Men Theory Reveals How Marvel's Latest Heartbreaking Deaths Can Be Undone

    Marvel Studios Animation has already revealed how the heartbreaking ending of X-Men '97 episode 5, "Remember It," can be reversed, thanks to one time-traveling cameo. X-Men '97, set outside the main timeline of the MCU, brought back many fan-favorite mutant heroes from X-Men: The Animated Series, continuing their story roughly a year after ...

  25. No, April 8 eclipse will not cause days of darkness

    Text superimposed on the video reads, "3 to 5 days of darkness" and "April 8, 2024" - the date of the 2024 eclipse. "Three days of darkness will occur," says a narrator, who later states ...

  26. Chasing the eclipse with sounding rockets and high-altitude planes

    Soaring above the clouds. Three different experiments will fly aboard NASA's high-altitude research planes known as WB-57s. The WB-57s can carry almost 9,000 pounds (4,082 kilograms) of ...

  27. Solar Eclipse Will Pass Over Every US City Named Nineveh on April 8

    Jack Izzo. The United States has seven towns named Nineveh and all of them will fall in the path of totality during the April 8, 2024, solar eclipse. For a couple of minutes on April 8, 2024, in a ...

  28. Solar Eclipse 2024: 10 Surprising Facts

    By Mallory Moench. April 6, 2024 8:31 AM EDT. A total solar eclipse will sweep across North America on Monday, April 8, offering a spectacle for tens of millions of people who live in its path and ...

  29. What time the 2024 solar eclipse started, reached peak totality and

    New York City will also see a substantial partial eclipse, beginning at 2:10 p.m. ET and peaking around 3:25 p.m. ET. In Boston it will begin at 2:16 p.m. ET and peak at about 3:29 p.m. ET. The ...

  30. What to know for the total solar eclipse: Time, path of totality

    A total solar eclipse occurs when the moon passes between Earth and the sun, completely blocking the sun's face. Those within the path of totality will see a total solar eclipse. People outside ...