is time travel possible essay

Time Travel: Is It Possible? Essay

Time is one of the most unique and uninvestigated phenomena in our world. Its unclear nature and peoples inability to manage it attracted our attention and created a solid basis for vigorous debates related to the interference in its structure. The brightest minds have been trying to answer this question and formulate the main regularities related to this process for years. Besides, the rapid evolution of science and technologies renewed interest in this topic and gave rise to vigorous debates around the possibility or impossibility of time travels. Nevertheless, at the moment, there is still no consensus as perspectives on the issue differ. The discovery of wormholes also preconditioned the undying interest in the topic. Thus, there are several modern theories that could be explored to prove either the possibility or impossibility of time travels.

When delving into the topic, it is crucial to understand the essence of time and its nature. One obviously knows the fact that it is one of the most powerful forces in the world. It could be defined as a continuous process of existence and numerous events that might occur in succession from the past through the present to the future. As comes from the definition, there are three states which are the past, present, and future. This subdivision contributes to the appearance of the debates related to the negotiation of the time barrier and moving in time from the past to the future and on the contrary, from the future to the past. The complexity of the problem gave rise to numerous speculations about the creation of the machine that could allow a person to move from one timeline to another.

However, the rise of spacefaring triggered the new wave of debates related to this sphere. Numerous scientists consider spaceships to be a sort of time machine that could be used to travel through time. When a person undergoes a serious acceleration, turns around, and comes back to earth, he/she might experience a time travel. In this regard, any spaceship that is able to reach a significant speed close to the light velocity could become a space machine. From this very perspective, every time machine has to travel through space. Additionally, the discovery of wormholes also provides numerous opportunities for travels through a higher-dimensional hyperspace (Tegmark 6).

In other words, we could speak about the existence of some alternative reality that might provide us with an opportunity to move in time and reach the needed destination point. This idea also correlates to the many-worlds interpretation of the universe. It means that there are numerous alternative worlds with similar histories and events that occurred during certain periods of time. If to accept this idea, we could state that a wormhole transfers us from our world to another, similar to ours but going through another timeline. Under these conditions, time travels become possible with the proviso that humanity will be able to build a spaceship that could experience a journey of this sort and understand the nature of wormholes. One realizes the fact that this task should be considered more than complex; however, the possibility to achieve this goal remains.

Additionally, the question of time travels comes close with the idea of time paradoxes that are expected to result in the collapse of the universe or some other significant problems. Besides, time paradox might appear when a time traveler interferes with the course of history and alters some events that conditioned the way the modern world looks. Moreover, this rude intervention is also suggested as one of the main reasons why time travels are impossible and time paradoxes serve as the guaranty that it will never happen.

Moreover, if to analyze the issue of time travels in terms of the single self-consistent timeline framework, it is possible to accept the possibility of the creation of a time machine that will move us to different eras. This theory states that if there is a certain event that might cause a time paradox that could change the history or the past, the probability of this event is zero and a time traveler is not able to create the time paradox. This theory becomes the key factor that preconditions the possibility of time travels and provides us with an opportunity to cogitate about the way we could explore the dimension of time. Besides, if to integrate the ideas of the multiverse and self-consistent timeline, we could obtain a solid basis for the further exploration of the given issue. For instance, in accordance with the first theory, using a spaceship and a wormhole we could move to some alternative universe where our alter-ego could be met.

However, we could easily communicate with him/her, exchange thoughts, feelings, emotions, and even give recommendations related to the future. These actions will not result in the appearance of the time paradox because of several reasons. Firsts, considering the fact that it is one of the dimensions that belong to the model of the multiverse, events that occur here are not necessarily the same as in our world and the future of this person will not alter because of our interference (Tegmark 4). Moreover, in accordance with the theory of a single self-consistent timeline, this meeting and its consequences could not be dangerous as it has already occurred. We could even kill our alter-ego, and this action will not have a significant impact on our own future. Resting on these theories, it is possible to assume the existence of at least theoretical possibility to create a time machine and travel through time. Hence, one should realize the fact that humanity does not possess technologies that might provide us with this very opportunity at the moment. The nature of wormholes remains unclear, and there is no spaceship that will be able to survive under the extreme conditions that are expected to be found in the heart of this unique phenomenon.

Altogether, the issue of time travels remains a topical question that gives rise to numerous debates. The evolution of science and significant progress in the exploration of space made this aspect especially exciting as these factors contributed to the appearance of the theoretical possibility to use a spaceship and a wormhole to move to another dimension and time. The undying interest to this question also conditioned the appearance of the many-world interpretation of our universe and the model of the multiverse that comprises all dimensions. Moreover, the single self-consistent timeline framework could also be used to support the idea of time travels and guarantee that it will not result in the appearance of numerous time paradoxes.

Works Cited

Tegmark, Max. The Multiverse Hierarchy . Edited by Bernard Carr, Cambridge University Press, 2007.

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April 26, 2023

Is Time Travel Possible?

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

By Sarah Scoles

yuanyuan yan/Getty Images

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Is Time Travel Possible?

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

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

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

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

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

How do we know that time travel is possible?

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

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

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

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

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

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

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

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

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

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

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

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

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

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

In Summary:

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

If you liked this, you may like:

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

Assistant Professor of Astronomy and Astrophysics, University of Maryland, Baltimore County

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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.

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.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to [email protected] . Please tell us your name, age and the city where you live.

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.

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

• Time Travel , entry by Joel Hunter (Truckee Meadows Community College) in the Internet Encyclopedia of Philosophy .

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered. Timothy A. Clary/AFP via Getty Images hide caption

A dog dressed as Marty McFly from Back to the Future attends the Tompkins Square Halloween Dog Parade in 2015. New research says time travel might be possible without the problems McFly encountered.

"The past is obdurate," Stephen King wrote in his book about a man who goes back in time to prevent the Kennedy assassination. "It doesn't want to be changed."

Turns out, King might have been on to something.

Countless science fiction tales have explored the paradox of what would happen if you went back in time and did something in the past that endangered the future. Perhaps one of the most famous pop culture examples is in Back to the Future , when Marty McFly goes back in time and accidentally stops his parents from meeting, putting his own existence in jeopardy.

But maybe McFly wasn't in much danger after all. According a new paper from researchers at the University of Queensland, even if time travel were possible, the paradox couldn't actually exist.

Researchers ran the numbers and determined that even if you made a change in the past, the timeline would essentially self-correct, ensuring that whatever happened to send you back in time would still happen.

"Say you traveled in time in an attempt to stop COVID-19's patient zero from being exposed to the virus," University of Queensland scientist Fabio Costa told the university's news service .

"However, if you stopped that individual from becoming infected, that would eliminate the motivation for you to go back and stop the pandemic in the first place," said Costa, who co-authored the paper with honors undergraduate student Germain Tobar.

"This is a paradox — an inconsistency that often leads people to think that time travel cannot occur in our universe."

A variation is known as the "grandfather paradox" — in which a time traveler kills their own grandfather, in the process preventing the time traveler's birth.

The logical paradox has given researchers a headache, in part because according to Einstein's theory of general relativity, "closed timelike curves" are possible, theoretically allowing an observer to travel back in time and interact with their past self — potentially endangering their own existence.

But these researchers say that such a paradox wouldn't necessarily exist, because events would adjust themselves.

Take the coronavirus patient zero example. "You might try and stop patient zero from becoming infected, but in doing so, you would catch the virus and become patient zero, or someone else would," Tobar told the university's news service.

In other words, a time traveler could make changes, but the original outcome would still find a way to happen — maybe not the same way it happened in the first timeline but close enough so that the time traveler would still exist and would still be motivated to go back in time.

"No matter what you did, the salient events would just recalibrate around you," Tobar said.

The paper, "Reversible dynamics with closed time-like curves and freedom of choice," was published last week in the peer-reviewed journal Classical and Quantum Gravity . The findings seem consistent with another time travel study published this summer in the peer-reviewed journal Physical Review Letters. That study found that changes made in the past won't drastically alter the future.

Bestselling science fiction author Blake Crouch, who has written extensively about time travel, said the new study seems to support what certain time travel tropes have posited all along.

"The universe is deterministic and attempts to alter Past Event X are destined to be the forces which bring Past Event X into being," Crouch told NPR via email. "So the future can affect the past. Or maybe time is just an illusion. But I guess it's cool that the math checks out."

• time travel
• grandfather paradox

Time Travel Probably Isn't Possible—Why Do We Wish It Were?

Time travel exerts an irresistible pull on our scientific and storytelling imagination.

Since H.G. Wells imagined that time was a fourth dimension —and Einstein confirmed it—the idea of time travel has captivated us. More than 50 scientific papers are published on time travel each year, and storytellers continually explore it—from Stephen King’s JFK assassination novel 11/22/63 to the steamy Outlander television series to Woody Allen’s comedy Midnight in Paris . What if we could travel back in time, we wonder, and change history? Assassinate Hitler or marry that high school sweetheart who dumped us? What if we could see what the future has in store?

These are some of the ideas that bestselling author James Gleick explores in his thought-provoking new book, Time Travel: A History. Speaking from his home in New York City, he recalls how Stephen Hawking once sent out invitations to a party that had already taken place ; why the Chinese government has branded time travel as “incorrect” and “frivolous” ; and how the idea of time travel is, ultimately, about our desire to defeat death.

Let’s cut right to the chase: What is time?

Oh, no, you didn’t! [ Laughs. ] In A.D. 400, St. Augustine said—and many people have said the same thing since, either quoting him consciously or unconsciously—“What, then, is time? If no one asks me, I know. If I wish to explain it to one that asks, I know not.” I think that is actually not a quip, but quite profound.

The best way to understand time is to recognize that we actually are very sophisticated about it. Over the past century-plus, we’ve learned a great deal. The physicist John Archibald Wheeler said, “Time is nature’s way to keep everything from happening all at once.” If you look it up in a dictionary, you get stuff like, “The general term for the experience of duration.” But that’s just completely punting because what is duration ?

I try to steer away from aphorisms and dictionary definitions, just to say two things. First, that we have a lot of contradictory ways of talking about time. We think of time as something we waste, spend, or save, as if it’s a quantity. We also think of time as a medium we are passing through every day, a river carrying us along. All of these notions are aspects of a complicated subject that has no bumper sticker answer.

When does the idea of time travel first appear in the West? And how did it impact popular culture?

I assumed, as a person who always read sci-fi a lot when I was a kid, that time travel is an obvious idea we’re born knowing and fantasizing about. And that it must always have been part of human culture, that there must be time travel Greek myths and Chinese legends. But there aren’t! Time travel turns out to be a very new idea that essentially starts with H.G. Wells’s 1895 novel, The Time Machine . Before that nobody thought of putting the words time and travel together. The closest you can come before that is people falling asleep, like Rip Van Winkle, or fantasies like Charles Dickens’s A Christmas Carol .

For Hungry Minds

The beginning of my book is an attempt to answer the question, “Why? Why not before? Why suddenly at the end of the 19 th century was it possible— necessary— for people to dream up this crazy fantasy?” Even though it’s H.G. Wells who does it, people pick up his ball very quickly and run with it. You find it in American science fiction that started appearing in pulp magazines in the 1920s and 1930s, or in the great new modernist literature of Marcel Proust’s In Search of Lost Time , James Joyce, and Virginia Woolf.

All these writers were suddenly making time their explicit subject, twisting time in new ways, inventing new narrative techniques to deal with time, to explore the vagaries of memory or the way our consciousness changes over time.

In 1991, Stephen Hawking wrote a paper called “Chronology Protection Conjecture , ” in which he asked: If time travel is possible, why are we not inundated with tourists from the future? He has a point, doesn’t he?

Yes! He even scheduled a party and sent out an invitation inviting time travelers to come to a party that had taken place in the past. Then he observed that none of them had shown up. [Laughs.] Hawking is one of these physicists who love playing with the idea of time travel. It’s irresistible because it’s so much fun! When he talks about the paradoxes of time travel it’s because he’s reading the same science fiction stories as the rest of us.

The paradoxes started appearing in magazines aimed mostly at young people in the 1920s. Somebody wrote in and said, “Time travel is a weird idea, because what if you go back in time and you kill your grandfather? Then your grandfather never meets your grandmother and you’re never born.” It’s an impossible loop.

Hawking, like other physicists, decided, “Time is my business. What if we take this seriously? Can we express this in physical terms?” I don’t think he succeeded but what he proposed was that the reason these paradoxes can’t happen is because the universe takes care of itself. It can’t happen because it didn’t happen. That’s the simple way of saying what the chronology protection conjecture is.

How have the Internet and other new technologies changed our perception and experience of time?

We are just beginning to see what the Internet is doing to our perception of time. We are living more and more in this networked world in which everything travels at light speed. We are multitasking and experiencing new forms of simultaneity, so the Internet appears to us as a kind of hall of mirrors. It feels as though we’re embedded in an ever expanding present.

Our sense of the past changes because in some ways the past becomes more vivid than ever. We’re looking at the past on our video screens and it’s just as vivid if the movie is about something that happened 20 years ago, as if it is a live stream. We can’t always tell the difference. On the other hand, the past that’s more distant—and isn’t available in video form—starts to seem more remote and fuzzier. Maybe we are forgetting how to visualize the past from reading histories. We’re entering a new period of time confusion, in which we suddenly find ourselves in what looks like an unending present.

In 2011, the Chinese government issued an extraordinary denunciation of the idea of time travel. What was their beef?

They thought it was corrupting and decadent. It’s a reminder that time travel is neither a simple nor innocent idea. It’s very powerful. It enables us to imagine alternative universes, and this is another line that science fiction writers have explored. What if someone was able to go back in time and kill Hitler?

Time travel is also a powerful way of allowing us to imagine what the future might bring. A lot of futurists nowadays tend to be dystopian. Time travel gives us ways of exploring how the worst tendencies of our current societies could grow even worse. That’s what George Orwell did in 1984 . I imagine the Chinese government doesn’t particularly want the equivalent of 1984 to be published in Beijing. [ Laughs. ]

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More than 50 scientific papers a year are now published on the idea of time travel. why are scientists drawn to the subject.

Scientists live in the same science fictional universe as all the rest of us. Time travel is a sexy and romantic idea that appeals to the physicist as much as it appeals to every teenager. I don’t think scientists are ever going to solve the problem of time travel for us but they still love to talk about wormholes and dark matter.

There’s a fascinating coincidence in the early history that when H.G. Wells needed to set the stage for his time machine hurtling into the future, he decided not to just jump right into his story but set the scene with a framing device—his time traveler lecturing a group of friends on the science of time—in order to justify the possibility of a time machine. His lecture introduces the idea that time is nothing more than a fourth dimension, that traveling through time is analogous to traveling through space. Since we have machines that can take us into any of the three special dimensions, including balloons and elevators, why shouldn’t we have a machine able to travel through the fourth dimension?

A decade later, Einstein burst onto the scene with his theory of relativity in which time is a fourth dimension , just like space. Soon after that, Hermann Minkowski pronounced that, henceforth, we were not going to talk about space and time as separate quantities but as a union of the two, spacetime , a four-dimensional continuum in which the future already exists and the past still exists.

I’m not claiming that Einstein read H.G. Wells 10 years before. But there was something in the air that both scientists and imaginative writers were empowered to visualize time in a new way. Today, that’s the way we visualize it. We’re comfortable talking about time as a fourth dimension.

You quote Ursula K. Le Guin , who writes, “Story is our only boat for sailing on the river of time.” Talk about storytelling and its relationship to time.

One of the things that has happened, along with our heightened awareness of time and its possibilities, is that people who invent narratives have learned very clever new techniques. Literal time travel is only one of them. You don’t actually need to send your hero into the future or into the past to write a story that plays with time in clever new ways. Narrative is also how everybody, not just writers, constructs a vision of our own relationship with time. We imagine the future. We remember the past. When we do that, we’re making up stories.

Psychologists are learning something that great storytellers have known for some time, which is that memory is not like computer retrieval. It’s an active process. Every time we remember something we are remembering it a little bit differently. We’re retelling the story to ourselves.

If time travel is impossible, why do we continue to be so fascinated with the idea?

One of the reasons is we want to go back and undo our mistakes. When you ask yourself, “If I had a time machine, what would I do?” sometimes the answer is, “I would go back to this particular day and do that thing over.” I think one of the great time travel movies is Groundhog Day , the Bill Murray movie where he wakes up every morning and has to live the same day over and over again. He gradually realizes that perhaps fate is telling him he needs to do it over, right. Regret is the time traveler’s energy bar. But that’s not the only motivation for time travel. We also have curiosity about the future and interest in our parents and our children. A lot of time travel fiction is a way of asking questions about what our parents were like, or what our children will be like.

At some point during the four years I worked on this book, I also realized that, in one way or another, every time travel story is about death. Death is either explicitly there in the foreground or lurking in the background because time is a bastard, right? Time is brutal. What does time do to us? It kills us. Time travel is our way of flirting with immortality. It’s the closest we’re going to come to it.

This interview was edited for length and clarity.

Simon Worrall curates Book Talk . Follow him on Twitter or at simonworrallauthor.com .

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Time Travel: Dream or Possible Reality?

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Time travel is a favorite plot device in science fiction stories and movies. Perhaps the most famous recent series is Dr. Who , with its traveling Time Lords who whisk throughout time as if traveling by jet. In other stories, the time travel is due to unexplainable circumstances such as a too-close approach to a very massive object like a black hole. In Star Trek: The Voyage Home , the plot device was a trip around the Sun that hurled Kirk and Spock back to 20th century Earth. In the popular movie series Back to the Future , the characters traveled both backward and forward in time. However it is described in stories, traveling through time seems to pique people's interest and ignite their imaginations. But, is such a thing possible? 

The Nature of Time

It's important to remember that we are always traveling into the future. That's the nature of space-time. This is why we remember the past (instead of "remembering" the future). The future is largely unpredictable because it hasn't happened yet, but everyone is headed into it all the time.

To speed up the process, to peer further into the future, to experience events more quickly than those around us, what would or could anyone do to make it happen? It's a good question without a definitive answer. Right now, no one has built a working time machine to travel temporally.

Traveling into the Future

While it's not possible (yet) to travel to the future fast than the rate at which we're doing it now, it is possible to speed up the passage of time. But, it only happens in small increments of time. And, it has only happened (so far) to very few people who have traveled off Earth's surface. For them, time moves at an infinitesimally different rate. Could it happen over longer time spans? 

It might, theoretically. According to Einstein's theory of special relativity , the passage of time is relative to an object's speed. The more quickly an object moves through space, the more slowly time passes for it compared to an observer traveling at a slower pace. 

The classic example of traveling into the future is the twin paradox . It works like this: take a pair of twins, each 20 years old. They live on Earth. One takes off on a spaceship on a five-year journey traveling at nearly the speed of light . The traveling twin ages five years while on the journey and returns to Earth at the age of 25. However, the twin who stayed behind is 95 years old! The twin on the ship experienced only five years of time passing, but returns to an Earth that is much farther into the future.

Using Gravity as a Means of Time Travel

In much the same way that traveling at speeds close to the speed of light can slow down perceived time, intense gravitational fields can have the same effect.

Gravity only affects the movement of space, but also the flow of time. Time passes more slowly for an observer inside a massive object's gravitational well. The stronger the gravity, the more it affects the flow of time. 

Astronauts on the  International Space Station experience a combination of these effects, though on a much smaller scale. Since they are traveling quite quickly and orbiting around Earth (a massive body with significant gravity), time slows down for them compared to people on Earth. The difference is much less than a second over the course of their time in space. But, it is measurable.

Could We Ever Travel into the Future?

Until we can figure out a way to approach the speed of light (and warp drive doesn't count , not that we know how to do that at this point, either), or travel near black holes (or travel to black holes for that matter) without falling in, we won't be able to do time travel any significant way into the future. 

Travel into the Past

Moving into the past is also impossible given our current technology. If it were possible, some peculiar effects could occur. These include the famous "go back in time and kill your grandfather" paradox. If you did do it, you couldn't do it, because you already killed him, so therefore you don't exist and can't go back in time to do the dastardly deed. Confusing, isn't it? 

Key Takeaways

• Time travel is a science fiction trope that may possibly be technically possible. But, no one has achieved it.
• We do travel into the future all our lives, at a second per second. To do it faster requires technology we don't have.
• Travel to the past is also impossible at the present time.
• Is Time Travel Possible?| Explore , www.physics.org/article-questions.asp?id=131.
• NASA , NASA, spaceplace.nasa.gov/review/dr-marc-space/time-travel.html.
• “Time Travel.”  TV Tropes , tvtropes.org/pmwiki/pmwiki.php/Main/TimeTravel.

Edited by Carolyn Collins Petersen .

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

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

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

Is time travel possible?

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

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

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

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

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

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

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

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

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

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

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

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This article originally appeared on USA TODAY: Is time travel possible? Why one scientist says we 'cannot ignore the possibility.'

Is time travel possible?

Will we ever get to rewind time, or fast forward through it? Sanden stops by to tell us all about how time works. He also has a couple ideas about how we might make time travel a reality! But one tricky thing about time travel is that it’s full of paradoxes — so we look into those, too. And science fiction expert Lisa Yaszek swings by to tell us just how long humans have been thinking about time travel (hint: a really long time).

Listen closely for the mystery sound, and the moment of um: how do clothes get dirty even though you can’t see it?

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Essay on Time Travel

Students are often asked to write an essay on Time Travel in their schools and colleges. And if you’re also looking for the same, we have created 100-word, 250-word, and 500-word essays on the topic.

Let’s take a look…

100 Words Essay on Time Travel

What is time travel.

Time travel means moving back or forward in time. It’s like skipping chapters in a book. Instead of reading in order, you jump to the past or future. Imagine going back to see dinosaurs or forward to meet robots. It’s a popular idea in stories and movies.

Time Machines

In stories, people use time machines to travel in time. These machines can be anything from cars to watches. The idea is to control time as easily as turning a page in a book. But in real life, we can’t do this yet.

Why Time Travel Fascinates Us

Time travel excites us because it offers endless possibilities. We could fix mistakes or see what the future holds. It’s about exploring unknowns and asking “what if?” This curiosity makes time travel a captivating concept for all ages.

Also check:

• Paragraph on Time Travel

250 Words Essay on Time Travel

What is time travel.

Time travel is the idea of being able to move forward or backward in time. It’s a popular concept in science fiction stories, but is it actually possible?

Is time travel possible?

We don’t know for sure if time travel is possible. Some scientists believe that it might be possible, but others think that it’s impossible. There are a lot of different theories about how time travel might work, but none of them have been proven.

How would time travel work?

There are a lot of different ideas about how time travel might work. One idea is that it might be possible to travel through a wormhole. A wormhole is a kind of shortcut through space-time. If you could find a wormhole, you could use it to travel to another time.

Would time travel be dangerous?

Time travel could be very dangerous. If you traveled to the past, you could accidentally change something that would have a big impact on the future. You could also end up in a time where you don’t know anyone or anything.

Time travel is a fascinating idea, but it’s important to remember that it’s just a theory. We don’t know for sure if it’s possible, and we don’t know how it would work. Even if it is possible, it would be very dangerous.

500 Words Essay on Time Travel

Time travel is a concept that has captured the imagination of humans for centuries. It is the idea of being able to move through time, either forward or backward, to a different point in history. While time travel remains a theoretical concept, it has been a popular theme in science fiction books, movies, and TV shows.

Theories of Time Travel

There are several different theories about how time travel might be possible. Some theories suggest that it could be possible to travel through wormholes, which are hypothetical tunnels through spacetime. Other theories suggest that it might be possible to travel through time by bending spacetime, or by creating a closed timelike curve.

The Grandfather Paradox

One of the biggest challenges to the idea of time travel is the grandfather paradox. This paradox states that if someone were to travel back in time and kill their own grandfather, it would create a logical contradiction. If the grandfather is killed, then the person who traveled back in time would never have been born. However, if the person was never born, then they would not have been able to travel back in time to kill their grandfather in the first place.

The Possibility of Time Travel

Despite the challenges, some scientists believe that time travel might be possible. They argue that the laws of physics do not explicitly rule out the possibility of time travel. However, they also admit that it is very unlikely that time travel will ever be possible for humans.

The Implications of Time Travel

If time travel were ever to become possible, it would have profound implications for our understanding of history, science, and the universe. It would allow us to learn about the past in a way that is not currently possible. It would also allow us to test scientific theories in a way that is not currently possible. And it would allow us to explore the universe in a way that is not currently possible.

That’s it! I hope the essay helped you.

If you’re looking for more, here are essays on other interesting topics:

• Essay on Time Is Gold
• Essay on Time In Life
• Essay on Tibet

Apart from these, you can look at all the essays by clicking here .

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Home — Essay Samples — Science — Scientific Theories — Time Travel

Essays on Time Travel

What makes a captivating time travel essay topic.

When it comes to selecting a topic for your time travel essay, it's crucial to consider various factors that will set your essay apart from the rest. Here are some recommendations on how to brainstorm and choose the perfect essay topic:

- Brainstorm: Start by generating ideas related to time travel. Explore different time periods, whether fictional or real, and consider how they can be examined in your essay.

- Research: Conduct extensive research on the chosen time period or concept to gather enough information to craft a compelling essay.

- Uniqueness: Look for topics that are extraordinary and avoid those that have been overdone. Select something that will captivate readers' attention and offer a fresh perspective.

- Relevance: Evaluate the relevance of the chosen time travel topic. Does it hold significance in history, literature, or popular culture?

- Controversy: Controversial topics often ignite interest and discussion. If your time travel topic touches upon controversial themes or theories, it can make your essay more intriguing.

- Personal interest: Choose a topic that genuinely interests you. This will help you stay motivated and engaged throughout the writing process.

- Creativity: Think outside the box and explore unconventional ideas related to time travel. Don't hesitate to push boundaries and present a unique perspective.

- Impact: Consider the potential impact of your chosen topic. Can it challenge existing beliefs or open up new discussions about time travel?

A good time travel essay topic combines elements of originality, relevance, and creativity, captivating readers and making them eager to delve into the essay's content.

The Best Time Travel Essay Topics

Looking for some extraordinary time travel essay topics to get your creative juices flowing? Here are 20 unique and thought-provoking ideas:

1. The Butterfly Effect: Exploring the consequences of altering small events in history through time travel.

2. Time Loops: Analyzing the concept of being trapped in a never-ending cycle of time.

3. The Ethics of Time Travel: Examining the moral implications and dilemmas faced by time travelers.

4. Time Travel in Literature: Analyzing the portrayal of time travel in famous novels and its impact on the story.

5. Time Travel and Paradoxes: Investigating the logical paradoxes that arise when contemplating time travel.

6. Time Travel and Ancient Civilizations: Imagining the influence of time travel on shaping ancient civilizations.

7. Time Travel and Quantum Physics: Exploring the connection between time travel theories and quantum physics concepts.

8. Time Travel and Popular Culture: Analyzing the portrayal of time travel in movies, TV shows, and music.

9. Time Travel and Historical Events: Examining how pivotal historical events could have unfolded differently with time travel intervention.

10. Time Travel and Future Predictions: Speculating on how time travel could be used to predict and shape the future.

11. Time Travel and Artificial Intelligence: Investigating the intersection of time travel and AI advancements.

12. Time Travel and Multiverse Theory: Exploring the idea of multiple universes and their relation to time travel.

13. Time Travel and Evolution: Examining the impact of time travel on the evolution of species.

14. Time Travel and Alternate Realities: Imagining how time travel could lead to the existence of alternate realities.

15. Time Travel and Philosophy: Analyzing the philosophical implications and theories surrounding time travel.

16. Time Travel and Time Perception: Examining how time perception changes when traveling through time.

17. Time Travel and Ancient Mysteries: Investigating how time travel could help unravel ancient mysteries and secrets.

18. Time Travel and Cultural Exchange: Imagining the cultural exchanges that could occur through time travel between different civilizations.

19. Time Travel and Technological Advances: Speculating on how future technological advancements might enable time travel.

20. Time Travel and Personal Transformation: Exploring how time travel experiences can transform an individual's perspective on life.

These unique time travel essay topics will surely make your writing stand out and captivate your readers' imagination.

Time Travel Essay Questions

Here are ten thought-provoking essay questions to explore your chosen time travel topic in greater depth:

1. How would altering a major historical event through time travel impact the present world?

2. Can time travel paradoxes be resolved, or are they inherent in the concept?

3. What ethical concerns arise when considering the potential consequences of time travel on future generations?

4. How does time travel in literature reflect the societal context in which the works were written?

5. Is it possible to change the course of history through time travel, or is everything predestined?

6. How does time travel challenge our understanding of cause and effect?

7. If time travel were possible, would you choose to visit the past or the future? Why?

8. How does time travel intersect with our perception of reality and the nature of existence?

9. Can time travel be reconciled with the laws of physics, or does it require new scientific principles?

10. What role does time travel play in shaping our collective imagination and popular culture?

These essay questions will encourage critical thinking and prompt in-depth analysis of your time travel topic.

Time Travel Essay Prompts

Here are five creative essay prompts to spark your imagination and inspire unique perspectives on time travel:

1. Write a fictional letter from a time traveler warning humanity about a future catastrophe.

2. Imagine a world where time travel tourism is a booming industry. Explore the positive and negative implications of this phenomenon.

3. Create a short story where a time traveler accidentally alters a minor event that has significant consequences for the present.

4. Write an essay exploring the psychological and emotional challenges faced by a time traveler who is unable to return to their original timeline.

5. Imagine a society where time travel is a common occurrence. Discuss the impact of this technology on social, economic, and cultural aspects of life.

These essay prompts will encourage imaginative thinking and allow you to explore time travel from unconventional angles.

Answering Your Time Travel Essay FAQs

Q: Can I write a time travel essay without any scientific background?

A: Absolutely! While scientific knowledge can enhance your understanding of time travel concepts, it is not a prerequisite. Focus on exploring the philosophical, ethical, and cultural aspects of time travel instead.

Q: Should I choose a specific time period for my time travel essay?

A: It depends on your topic and interests. While a specific time period can provide a focused approach, broader concepts such as alternate realities or time loops may not require a specific era.

Q: Can I mix fictional and real-life elements in my time travel essay?

A: Yes, blending fictional and real-life elements can add depth and intrigue to your essay. Just ensure that the narrative remains coherent and logical.

Q: How do I ensure my time travel essay stands out from others?

A: Choose a unique topic, conduct thorough research, present fresh perspectives, and maintain a captivating writing style. Incorporate personal insights and engage readers through clear arguments and imaginative storytelling.

Q: Can I propose new time travel theories in my essay?

A: Absolutely! Time travel is a subject that invites speculation and exploration. Feel free to propose new theories, but ensure you support them with logical reasoning and evidence.

Remember, writing a time travel essay should be an enjoyable experience where you can unleash your creativity and explore intriguing concepts. Embrace the adventure of time travel and let your imagination soar!

Argumentative About Time Travel

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The Issue of Time Travel Possibility

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The Metaphysical Possibility of Time Travel Fictions

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• Published: 16 June 2021
• Volume 88 , pages 1309–1329, ( 2023 )

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In some stories, time travellers cannot change the past. It is widely accepted that this is metaphysically possible. In some stories, time travellers can change the past. Many philosophers have explained how that, too, is metaphysically possible. This paper considers narratives where sometimes the past can change and sometimes it cannot, arguing that this is also something that is possible. Further, I argue that we can make sense of stories where some events appear to be ‘fixed points in time’.

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1 Introduction

In some time travel stories, the past can change e.g. Kleiser’s The Flight of the Navigator ( 1986 ), Curtis’s About Time ( 2013 ), or Landon’s Happy Death Day ( 2017 ). As an example, consider Happy Death Day . The protagonist, ‘Tree’, is murdered in a tunnel. She then awakes earlier that day, having travelled in time. Tree lives through that day again, but this time things play out differently and she isn’t murdered in a tunnel. The past has changed!

In other time travel stories, the past cannot change e.g. Moorcock’s Behold the Man ( 1969 ), Gilliam’s 12 Monkeys ( 1995 ), and Vigalondo’s Los Cronocrímenes ( 2007 ). As an example, consider Los Cronocrímenes . The protagonist, ‘Hector’, sees a woman, naked in the forest. Investigating, Hector finds her unconscious, whereupon he is attacked by a bandaged man. Managing to later travel in time, it transpires that it was Hector’s later self who forced the woman to strip and who knocked her unconscious, as well as being the bandaged man. Nothing plays out differently; whilst the viewer sees the same event multiple times, it always plays out the same way.

These two types of film mirror the two main philosophical approaches to time travel: ‘Ludovicianism’ (Lewis, 1976 ), the theory that the past cannot be changed, and the ‘non-Ludovician’ theories which allow for the past to change. For each, much work has already been done to show that they are metaphysically possible (for discussion, see Effingham, 2020 ).

But some stories don’t correspond to either. In those stories, the past is sometimes changeable whilst, on other occasions it cannot be changed. This paper argues that even these time travel scenarios are metaphysically possible.

There are two reasons to be interested in whether such fictions are metaphysically possible. First reason: It builds on the project David Lewis started. David Lewis asks whether any time travel narrative is consistent ( 1976 : 145). It’s only natural to further ask exactly which sorts of narratives are consistent i.e. whether a narrative according to which the past is only changeable on occasion is possible or not. Second reason: It’s independently interesting to map what logical space is like and what sort of time travel might be permitted, regardless of its connection with fiction.

Section  2 explains the Ludovician model and how probability works in that model. Section  3 explains the hypertemporal non-Ludovician model, arguing that it’s the best non-Ludovician model for understanding most fictional stories. Section  4 moves to those time travel fictions where the past is intermittently changeable, arguing that a model ‘mixing’ Ludovicianism with hypertime can account for such fictions. Section  5 discusses ‘fixed points in time’, whereby time travellers find they can change some events but not others; I discuss how the mixed model can allow for these fixed points.

This paper does not aim to explain all problematic elements in all time travel fictions. Many will still go unexplained. Nevertheless, in the spirit of speculation, I suggest that more advanced ‘mixed models’ might help with at least some of these issues. Section  6 gives an example of a narrative which can be explained by further mixing.

2 Ludovician Time Travel

2.1 examples in fiction.

Imagine I travel back to 1930 to assassinate Hitler. The Ludovician says that I would fail to succeed. Some event would inevitably get in my way, thwarting me. I might be unable to locate Hitler, or shoot the wrong person, or simply miss when I finally have him in my crosshairs. Taking this through to its natural conclusion, I’d be unable to change anything in the past—what once was, always will be. Were this how time travel worked, all time travellers will end up in the same situation as Hector from Los Cronocrímenes , unable to change events from being the way that they previously were.

Lewis ( 1976 ) is the most famous Ludovician [for a fuller exposition and list of supporters see Effingham ( 2020 : 67–73)]. It is a theory adopted by many fictions.

Star Trek . In ‘Captain’s Holiday’ (1990), aliens from the future attempt to rescue an artefact destroyed by Captain Picard. Partially due to the efforts of the aliens, Picard ends up destroying the artefact. Another example: In ‘Time’s Arrow’ (1992) the crew travel back in time, having found Data’s head buried at an archaeological dig. Their time travelling then leads to Data being decapitated in the past and his head being left for the future crew to find.

The Terminator ( 1984 ). A soldier from a dystopic future comes back to save the mother of an unborn child who will lead the resistance against the robots that have taken over the world. The mother is saved and the future stays the same (and the solider turns out to be the biological father of the unborn child).

Harry Potter . In ‘The Prisoner of Azkaban’ (Rowling, 1999 ) the protagonists, believing (but not having seen) a friendly hippogriff killed, return in time to save it. It turns out that the hippogriff never died in the first place, due to the actions of their future time travelling selves.

Doctor Who . In ‘The Aztecs’ (1964) the Doctor avers that history cannot be changed and that the Aztecs’ practice of human sacrifice cannot be stopped.

Red Dwarf . In ‘Future Echoes’ (1988) Lister witnesses an image from the future in which one of his companions loses a tooth. To demonstrate that he can change the future, Lister tries to prevent the tooth loss. In doing so, he breaks his companion’s tooth.

These examples also neatly describe ‘causal loops’ wherein an event causes another event which causes another event, and so on, until—via the miracle of time travel—they loop back to cause the original event. For instance, finding Data’s head brings about the time travel which results in him losing his head; it’s a causal loop. Similarly, in the other examples: the time travelling soldier in The Terminator is the father of the man who sends him back in time in the first place; Lister’s attempt to stop the future happening causes it to happen; and so on.

2.2 Ludovician Probability

In Red Dwarf ’s ‘Cassandra’ (1999) the protagonists meet an oracle who knows all future facts. The oracle reveals that one character, Rimmer, will imminently die before leaving the ship, whilst the other protagonists will live for some time yet. Another character, Kryten, notes that this means that the others are now invulnerable. Taking a gun, he points it at his own head, pulls the trigger, and—against the odds—it misfires. He then does the same to the other characters fated to survive, each time amazingly failing to discharge. Shooting it in Rimmer’s direction, it fires perfectly.

This fictional example is instructive (if not entirely representative!) of how probability would function in a Ludovician time travel case (Effingham, 2020 : 147–75). To see why, imagine that when I try and kill Hitler I am committed enough to make fifty attempts before giving up. Imagine also that I am so skilful an assassin that only one of two things can prevent my success: (i) a commonplace event, out of my control, occurring every time I try; (ii) a random heart attack killing me before I even make my first attempt. (In reality, of course, there’d be more outcomes than (i) and (ii), but they will do as heuristic placeholders.)

Let the chance of me being struck down by a heart attack be one in a billion. Let the chance of a commonplace event preventing an assassination attempt be 0.05; the chance of a chain of fifty of them occurring is then 8.9 × 10 –66 . Given those probabilities, were I to try and kill a regular, presently existing, dictator, I would almost certainly succeed. The chance would be 1 − 1 × 10 –9  − 8.9 × 10 –66 i.e. roughly 1.

But when I try to kill Hitler, things are different since something will stop me—indeed, given the assumption, one of (i) or (ii) will save Hitler. As a rational Ludovician, my credence of (i) and (ii) coming about should therefore proportionately increase given that I know I will fail (Effingham, 2020 : 152–54). Since the chance of (ii) is greater, by 57 orders of magnitude, than the chance of (i), the proportionate increase means that that my expectation of dying from a heart attack should be 57 orders of magnitude greater than my expectation of a chain of coincidental events coming about. So, were I to try and kill Hitler, I should expect to die of a heart attack.

In other time travel cases, similar thoughts will apply. If I try to change the past, something will stop me and it’ll be more likely that it’s a singular—quite possibly dangerous—event which does that preventative work. Time travel is dangerous for my health!

There are two objections the Ludovician might raise.

First objection. Lewis says that the events which play the preventative role are ‘commonplace’ (Lewis, 1976 : 150). Likewise, we might think he thought the probabilities of events in time travel cases would be normal and mundane, not strange and dangerous like I’ve indicated.

But Lewis’s talk of ‘commonplace’ events just means that the things which prevent me killing Hitler needn’t be outlandish events like supernatural agents or interventionist ‘time patrols’. For Lewis, the events which stop me can be ‘commonplace’ events like my gun misfiring, or me mistaking my target, and so on. Lewis says nothing about the likelihood of such ‘commonplace’ events occurring.

Second objection. The Ludovician says that in different contexts I have different abilities. For instance, I can’t kill Hitler in 1930 in a context which assumes that he survives 1930. Ludovicians are quick to point out that the same also applies in non-time travel scenarios e.g., assuming I will fail to assassinate a contemporary dictator, then I don’t have the ability to kill them. That said, the second objection is that the same parity between abilities in time travelling/mundane scenarios should apply to probabilities . Were that so, nothing strange would be going on. Rather, when we recognise that I have a high probability of having a heart attack when I try to kill Hitler, that’s no more unusual than recognising that, assuming I would fail , I’d have a high probability of having a heart attack were I to try and kill a contemporary dictator.

But something strange is going on and what is said about abilities cannot be said of probabilities. I agree that there’s some probability function according to which I have a high probability of a heart attack both when I try and kill Hitler and when I try and kill the contemporary dictator. But that’s because probability functions are cheap and plentiful; that probability function is not the salient probability function. The probability function we should care about is the ‘rational credence’ probability function of a (presumably non-actual) well-informed agent. When you’re trying to figure out what to expect (and what actions to take), it’s that function which you’re aiming for with your own probability judgements. And since the rational credence an agent assigns to a proposition varies depending only upon what that agent believes, not the context in which the agent finds themselves in, then the function we’re interested in isn’t context sensitive in the way that ability claims are.

Moreover, the rational credence function of a well-informed agent returns the result that it’s dangerous to use a time machine to kill Hitler but not for me to attempt the assassination of a contemporary dictator. Imagine a rational agent is gambling as to whether I’ll succeed in killing a contemporary dictator. Since they’re well-informed, they know that only one of (i) or (ii) could stop me. In this case, the gambler’s well-informed body of beliefs says nothing about whether—at the future time—I succeed or fail. Thus, the gambler will say I’ll likely succeed and bet accordingly. Next, imagine we ask the gambler to bet on me succeeding in killing Hitler. Being well-informed they know that I’m using a time machine, that Hitler survived 1930, and that only one of (i) or (ii) will stop me. Given all that knowledge, it’s now rational for them to predict that my assassination attempt will fail; indeed, the gambler will instead expect me to have a heart attack. Since we want our actions and expectations to mimic those of the well-informed rational agent, we should likewise expect me to have a heart attack when attempting historical assassinations but not contemporary ones.

The question then arises: Why are time travellers faced by these dangers whilst regular assassins are not? A bad answer is to think that contemporary assassins can change the future whilst time travellers cannot change the past. It’s a bad answer because, for the Ludovician, no facts can ever change, whether they’re past, present, or future (Horwich, 1987 : 116; Lewis, 1976 : 150; Putnam, 1962 : 669). Even contemporary assassins can’t change facts about the future—they can causally affect the future, but not change it.

The correct answer instead focuses on what ‘well-informed’ consists in. In the case of killing contemporary dictators, it’d be wrong to imagine that the rational gambler has an antecedent commitment to my succeeding or failing. But in the case of me using a time machine to kill Hitler, it is appropriate to imagine the rational gambler antecedently believing that I will fail. That is: In the former case, it’s unreasonable to think my succeeding or failing is part of being well-informed, whilst in the case of me attempting to kill Hitler, it’s the other way around (Hall, 1994 : 508–9; Lewis, 1986 : 94). This, in turn, is because when I try and kill Hitler I’m involved in (or potentially involved in) a causal loop, but I’m not involved in one when I try to kill the contemporary dictator. When killing Hitler, I’m involved in (or could be involved in) a causal loop because Hitler’s surviving 1930 plays a causal role in my personal history e.g. his survival, and later heinous acts, cause me to come back to kill him. (The ‘personal history’ of an agent is that web of events which has causally influenced—i.e. affected the intrinsic properties of—the person up until that point; this includes, say, events from long before the person is born e.g. those involving their ancestors.) Since causal loops appear only in time travel cases, killing contemporary dictators doesn’t involve causal loops and so won’t come hand-in-hand with weird expectations. Wannabe assassins of Hitler, from Helmut Hirsch to Fabian von Schlabrendorff, would not have been rational to expect to be definitely thwarted in their assassination attempts. Likewise, if I time travel to the past, and also travel to a place in space so far away that my activities can’t affect my personal history, no probabilistic peculiarities will arise. Similarly, if one time travels in a non-Ludovician fashion one avoids being in a causal loop (see Section 3 ) and, if loops are avoided, no probabilistic weirdness arises. But in the causal loop case, it’s reasonable for the well-informed rational gambler to know everything about my causal history—that is part and parcel of their being well-informed. In the case of me killing Hitler, this includes my failing in the assassination attempt. In the case of the contemporary dictator, even if it turns out that I actually fail and have a heart attack, that failure isn’t part of my causal history and so it’s illegitimate for it to feature in the beliefs of the imaginary well-informed agent.

In conclusion, time travellers caught up in causal loops (or liable to get caught up in causal loops) should expect weird events to happen. In particular, they should expect a single unlikely event to prevent their changing the past. It’s not hard to reach the conclusion that the singular event might well be deleterious to one’s health and well-being. Ludovician time travellers beware!

Even apparently inconsequential events may be part of my personal history, such that interacting with them is dangerous for me. Imagine that it’s 800 AD. Sigeburg is currently enamoured of Cuthbert. But tomorrow morning, Cuthbert will yawn when he opens his door. Wandering by, Sigeburg will find this repugnant and deem Cuthbert to be an unsuitable suitor. Years later, Sigeburg will go on to marry someone else. One of their descendants will be a man who, in 1915, arrives at the navy office slightly earlier than my great-grandfather. In turn, that descendent, rather than my great-grandfather, is assigned to a vessel destroyed by U-boats. Thus, if Cuthbert does not yawn at that exact moment in 800 AD, I will not be born. I can no more stop the yawn than I could assassinate Hitler. Were I to sit talking to Cuthbert—where I might accidentally cause him to sleep in a little longer by drinking just a tad more mead, in turn causing Sigeburg to miss his romantic faux pas—I am increasing the probability of some event thwarting me doing just that. Strange probabilistic occurrences would happen, thwarting my interacting with Cuthbert. Even if I were unaware of his role in my personal history, that’d make no difference to these probabilistic issues. Talking to Cuthbert could be deadly to me.

One last note. Even non-time travellers might be caught up in these cases if they—knowingly or otherwise—start interacting with time travellers. If, for instance, I go back in time and try and trick someone into killing Hitler for me, that will increase their chances of having a heart attack. Similarly, if I go back in time to Cuthbert’s village, those around me (who I might otherwise accidentally influence into stifling Cuthbert’s yawn) will likewise be threatened by unlikely events.

2.3 Probability and Fiction

In fictions portraying Ludovician time travel, there is a tendency to see either no strange coincidences or only a very limited number. But, given Section 2.2 , this isn’t representative of how it actually would be were Ludovicianism true. Indeed, elsewhere (2020: 168) I’ve argued that the practical upshot of the probabilistic concerns from Section 2.2 is that, since any time travel to within your past light cone will likely result in some interaction (even at a sub-atomic scale!) with your personal history, any attempt to activate a time machine would likely kill you. Very few fictions represent that probabilistic fact [for exceptions, see Niven’s ‘Rotating Cylinders and the Possibility of Global Causality Violation’ ( 1977 ) and Levinson’s ‘The Chronology Protection Case’ ( 1995 )].

Two things should be said about this. First, fictions often suppress truths about the world. In Die Hard II ( 1990 ) a plane blows up when John McClane lights its trail of fuel. In reality, aviation fuel has too high a flashpoint to ignite like that. In Point Break ( 1991 ) Keanu Reeve’s character conducts a lengthy conversation whilst skydiving. In reality, this would be impossible given the loud sound of rushing wind. In numerous TV shows and movies, chloroform soaked rags almost instantly render people unconscious, far from the real world truth. Fictional depictions of Ludovician time travel similarly involve such suppression.

Second, by failing to realistically depict how probability would work in a Ludovician time travel case, time travel fictions end up depicting unlikely narratives. But ‘unlikely’ doesn’t mean ‘impossible’. If we’re interested solely in their possibility, then possible they are! (And some narratives may lend themselves to the idea that the time travellers have supraphysical powers allowing them to warp probability, avoiding these problems entirely.)

3 Non-Ludovician Time Travel

3.1 examples in fiction.

Fiction contains many examples of time travellers changing the past:

Star Trek . In ‘Yesterday’s Enterprise’ (1990) a ship comes back from the past to the future, escaping a battle. That affects the result of the battle and the future is instantly changed, becoming dystopian. Eventually, the ship returns to the past and history reverts back to how it originally was. Another example: In ‘Time Squared’ (1989) Captain Picard returns from a future disaster and then dies. The crew then change events so that this never happens.

Terminator 2 ( 1991 ). A time travelling robot comes back to 1995 from 2029. It teams up with the protagonists and they try and prevent the end of the world. We discover, in Terminator 3: Rise of the Machines ( 2003 ), that they change the future by delaying the apocalypse eight years.

Harry Potter . In ‘Harry Potter and the Cursed Child’ ( 2016 ) the protagonists travel in time and change history so that Voldemort now lives and rules the world.

Red Dwarf . In ‘Tikka to Ride’ (1989) the crew of the ship return to the past for supplies, accidentally saving JFK which ultimately results in a nuclear war. Realising their error, they recruit JFK’s future self and return to 1963 where they get JFK to assassinate himself. The nuclear war is now averted.

3.2 Universe and Hypertemporal Indexing

Prima facie , changing the past seems to be impossible. Consider some instant, t 1930 , in 1930. At t 1930 , Hitler is alive. Later, I go back in time and change things so that Hitler is now dead at t 1930 . Where P is the proposition 〈Hitler is alive〉 the following contradiction would be true:

‘Non-Ludovician’ theories of time travel solve this problem by introducing extra entities to avoid the contradiction. One model introduces extra universes (Deutsch, 1991 ). Time travel takes you back to the past, but to a universe different from that which you left. In that new universe, the time traveller can change things however they want. Just as, without fear of contradiction, it can rain at one place and not rain at another, different things can be true at different universes (since a universe is simply a very large place). On this model, if I return to t 1930 to kill Hitler then there are universes \({\mathbb{U}}_{1}\) , \({\mathbb{U}}_{2}\) … whereby I leave universe \({\mathbb{U}}_{1}\) (at which Hitler was alive at t 1930 ) to arrive at t 1930 in universe \({\mathbb{U}}_{2}\) . There, I kill Hitler. The following proposition would be true:

That proposition is no more contradictory than it raining right now (in that it’s raining in Seattle) and it not raining right now (in that New York is clear skied). So universe non-Ludovicianism solves the problem.

A spin on this theory, ‘hypertemporal non-Ludovicianism’, introduces an extra dimension of time instead of extra universes (Bernstein, 2017 ; Goddu, 2003 ; Hudson & Wasserman, 2010 ; van Inwagen, 2010 ). Hypertemporal theories vary over their specifics (e.g. whether they accept growing block theory or eternalism, whether time is fundamentally tensed or tenseless, and so on). I assume an ‘eternalist’ hypertemporal theory (Chown, 2007 ; Effingham, 2020 : 76–79; Hudson & Wasserman, 2010 ). That model assumes there are two temporal dimensions, time and ‘hypertime’. They are temporal analogues to the two spatial dimensions one would find in a flatland. Just as, in a flatland, you can go left–right and backwards–forwards, in a world with two dimensions of time there is the pastwards–futurewards direction and a hyperpastwards-hyperfuturewards direction.

Use the variables t 1 , t 2 … to refer to regular temporal instants and the variables T 1 , T 2 … to refer to hypertemporal instants. Since the two-dimensional temporal world is analogous to the spatial dimensions of flatland, each hypertime has its own complement of times. For instance, at T 1 there exist instants t 1 , t 2 … whilst t 1 , t 2 … also all exist at T 2 and at T 3 , and so on for all hypertimes (analogous to how, in a two dimensional flatland consisting of x and y spatial axes, every ‘ x point’ exists along every ‘ y point’). Time travellers travel back in the regular temporal dimension but always move forwards in the hypertemporal dimension. If I go back to kill Hitler, I leave one hypertime (e.g. T 1 ) and arrive back in 1930 but at a hyperlater hypertime ( T 2 ). It’s at that hypertime at which I kill Hitler and make the following proposition true:

Again, that proposition isn’t contradictory.

Consider a second example. In Doctor Who ’s ‘Pyramids of Mars’ [1975] the Doctor and Sarah Jane go from 1980 to 1911. In 1911, the antagonist of the story, Sutekh, conspires to wipe out all life. Sarah Jane says not to worry for, having seen the future, she knows Sutekh fails. To disprove this, the Doctor takes Sarah Jane to 1980, where Earth is now a desolate wasteland orbiting a dead sun. Witnessing the results of inaction, they return to 1911 and defeat Sutekh. 1980 then changes back to how it once was.

See Fig.  1 . The Doctor starts at one time, t 1980 . Given the hypertemporal model, he’s also at a certain hypertime, in this case T 19 . Refer to that temporal/hypertemporal location using Cartesian co-ordinates i.e. ‘ t 1980 - T 19 ’. When the Doctor travels back to 1911, he moves forward in hypertime, arriving at t 1911 - T 20 . There, he shows Sarah Jane the future by travelling to 1980. Since he moves forwards in regular time, not backwards, he stays at the same hypertime, arriving at t 1980 - T 20 . Whilst t 1980 - T 19 was nice, t 1980 - T 20 is nasty. Travelling back in time again (and, therefore, ahead in hypertime, to T 21 ) the Doctor arrives at t 1911 - T 21 . There he frustrates Sutekh’s plans. Thus t 1980 - T 21 is as nice as t 1980 - T 19 .

A Hypertemporal understanding of ‘Pyramids of Mars’

3.3 In Favour of Understanding Fiction in Hypertemporal Terms

It’s curious that whilst there are fictions explicitly relying on universe indexing [e.g. Baxter’s The Time Ships ( 1995 ) and Hamilton’s The Saints of Salvation ( 2020 )] examples of fictions explicitly relying on hypertemporal indexing are absent. Footnote 1

Nevertheless, it is more natural to read fictions as presenting a hypertemporal non-Ludovicianism than a universe non-Ludovicianism. This is because hypertime bests captures the motives of fictional characters. Consider ‘Pyramids of Mars’. If universe non-Ludovicianism were true, the Doctor would have little reason to act to stop Sutekh. Imagine that time travellers create new universes when they travel back in time. In one universe, 1980 is nice. In a second universe, Sutekh’s actions make 1980 nasty. The Doctor then travels to the future of that universe, shows Sarah Jane that it’s nasty, and travels back in time to stop Sutekh. Thus, the Doctor creates a third universe (in which 1980 ends up again being nice). But what was the point? I don’t ameliorate the problem of global poverty and starving children in the world by fathering some non-starving children of my own. Similarly, if the Doctor is worried about the horrible universe Sutekh brings about, he doesn’t ameliorate that problem by creating a third universe at which Sutekh has not done terrible things—all he does is create a place which is nice, not eliminate the place which is nasty. Footnote 2

Given hypertemporal non-Ludovicianism, the Doctor has much more motive to act. Standardly, we favour how things presently are. I have been in pain in the past and my presently not being in pain is preferable; when in pain, I have a motive to make my pain a mere item of the past. Were there a second temporal dimension, we should likewise prefer how things hyperpresently are. When Sutekh makes 1980 a terrible place, he makes 1980 hyperpresently a terrible place. The Doctor—seeing that this is a bad thing—puts it right and makes it the case that, hyperpresently, 1980 is a nice place. Sutekh’s devastation is relegated to being a mere item of the hyperpast; whilst there’s nothing the Doctor can do about the hyperpast being the way that it is, at least the way the world hyperpresently ends up being is better in light of his actions. He is therefore clearly motivated to change time (in a way that he isn’t, given universe non-Ludovicianism).

I suspect that similar reasons apply to understanding other fictional narratives. We’re best advised to treat most fictional cases of the past changing as being cases of hypertemporal non-Ludovicianism.

4 The Mixed Model

For both Ludovician and non-Ludovician time travel, the same fictions have been used as examples i.e. Star Trek , Harry Potter , The Terminator , Red Dwarf , and Doctor Who . In those narratives, the characters can sometimes change time whilst, on other occasions, time travel results in a Ludovician causal loop with no change possible. This section deals with how to allow for the possibility of such narratives.

It can be allowed if we tinker with the hypertemporal model. The vanilla hypertemporal theory from Section 3 has it that time travellers always move forwards to hyperlater hypertimes when they travel back in time. Drop that stipulation. Whilst, in some cases, people ‘merely time travel’ (by going back in time and moving forward in hypertime), in other cases they can also ‘hypertime travel’ and go back to hyperearlier hypertimes (or, alternatively, manage to stay at the one they are hypercurrently at whilst nevertheless travelling back into the ‘regular’ past).

If hypertime travel is allowed, we run straight back into the original problem we were faced with. Imagine I ‘merely time travel’ from 2020 to 1930 and kill Hitler i.e. I travel from t 2020 - T 1 to t 1930 - T 2 and make it the case that Hitler’s dead at t 1930 - T 2 . If I have a hypertime machine, I can then travel from t 2020 - T 2 to t 1930 - T 1 . Can I then kill Hitler there?

One solution is to redux the same move the hypertemporal theorist made in the original case i.e. add an extra dimension of time. Adding in an ‘ultratemporal’ dimension, which hypertime machines always move forward in, it’d turn out that Hitler’s alive at t 1930 - T 1 at one ultratime and dead at t 1930 - T 1 at an ultralater ultratime.

But scotch that suggestion. The crux of this paper’s theory is that, in the example fictions, this isn’t what happens. Instead, when someone hypertime travels, that hypertemporal travel ends up being ‘Ludovician’. Were one to go back in hypertime to t 1930 - T 1 then one would fail to assassinate Hitler. Just as commonplace events like guns misfiring and mistaken identities save Hitler from assassination given regular Ludovicianism, the same sorts of events will occur to prevent Hitler from being assassinated at T 1 were someone to hypertime travel back to that point. Footnote 3

Call this the ‘mixed model’. If, like myself, you think both Ludovician and non-Ludovician time travel are metaphysically possible, there’s little reason to think that this mixed model is not also possible. And, given its possibility, we can allow for fictions wherein sometimes time is changed and sometimes it isn’t. Consider three examples.

Figure  2 depicts the narrative of The Terminator . At the first hypertime, T 1 , the protagonist, ‘Reese’, and the Terminator both travel in a Ludovician manner from 2029 back to 1984. Reese then becomes the father of the man who sends him back in time. Slightly later in 2029, t 2029+δ , (and at the same hypertime, T 1 ) another terminator is sent back in time. But in this case, it is in a non-Ludovician manner. Because the time travel is non-Ludovician, that terminator arrives in the past (in 1995) at a hyperlater hypertime, T 2 . That terminator can make time different from how it hyperwas at T 1 . The events of Terminator 2 then play out and the protagonists delay the apocalypse occurring by eight years.

The mixed model understanding of The Terminator/Terminator 2

As another example, consider (some of) the narrative of Star Trek , namely the events of ‘Yesterday’s Enterprise’ followed by the events of ‘Time’s Arrow’. See Fig.  3 . This time, the non-Ludovician time travel comes first and the Ludovician time travel comes second. At T 1 , the Enterprise-C is involved in a battle in 2344 and is destroyed. Later, at that same hypertime, the Enterprise-D (led by Captain Picard) somehow disturbs a portal into the past, affecting the past in a non-Ludovician manner. This allows the Enterprise-C through to the future, t 2367 , at a hyperlater hyperinstant, T 2 . The history of T 2 is changed by this event to be different than that of T 1 and the world, from t 2344 onwards, is more dystopian. The events of ‘Yesterday’s Enterprise’ then take place and the crew of the future convince the crew of the past to return back to the past and put right what once went wrong. So there is yet more non-Ludovician time travel as the Enterprise-C returns to the past (at t 2367+ δ ) at another, hyperlater, hypertime, T 3 . The past is now put aright and 2367 is no longer dystopian at T 3 .

The mixed model understanding of Star Trek

Later on, at T 3 , the crew—at t 2369 —come across Data’s head. Using a time machine, they travel back to 1893, but this time in a Ludovician manner—that is, they time travel back within the same hyperinstant. There, Data loses his head. Since that takes place at the same hyperinstant, the head can later be discovered by the crew at t 2369 - T 3 , creating the causal loop depicted in ‘Time’s Arrow’.

Both these examples involve causal loops arising from hypertime travel within the same hyperinstant. I’ve argued elsewhere (2020: 22–24) that travelling within the same instant is a type of time travel; similarly, to travel within the same hypertemporal instant is to hypertime travel. But there are also clearer cases of hypertime travel where causal loops stretch between hypertimes. I’ll use Red Dwarf as an example, although similar loops appear elsewhere, e.g. in Harry Harrison’s The Stainless Steel Rat Saves the World ( 1972 ), Baxter’s The Time Ships ( 1995 ), and various episodes of Doctor Who (e.g. ‘The Name of the Doctor’ ( 2013 ) and, indeed, mostly any episode where multiple Doctors meet one another).

See Fig.  4 , which depicts three cases of time travel in the Red Dwarf narrative. The first is the Ludovician causal loop from the episode ‘Future Echoes’. At one time, t 3m , Lister sees an event from the future and tries to stop it, which then causes it to happen later at t 3m+1 .

The mixed model understanding of Red Dwarf

The second case is the events of ‘Tikka to Ride’. Marooned three millions years into deep space (at t 3m+7 - T 21 ) the crew decide to return to the past for supplies. Accidentally arriving in 1963 (at t 1963 - T 22 ) they prevent Lee Harvey Oswald from killing JFK. Trying to escape the police, they travel forwards in time (to t 1966 - T 22 ) where they discover JFK’s survival has led to a nuclear war. Realising their mistake, they recruit JFK’s future self and travel back to 1963 (to t 1963 - T 23 ) where they get JFK to assassinate his past self. History is, more or less, put back to how it hyperwas (except for the fact that Oswald no longer killed JFK and instead JFK killed JFK). Red Dwarf is thus a mixed model.

The third case is a causal loop stretching back in hypertime. In a later episode, ‘Ourorobos’ (1997), Lister has a child. It transpires that the child is Lister [so Lister is a ‘bootstrapped’ person ( cf Effingham, 2020 : 59–65)]. Baby Lister is then returned to the past to become the adult Lister. The clear implication of the narrative is that these events have not changed time i.e. it is not that, hyperpreviously, Lister was a regular person born to regular parents but that, from T 22 onwards, he is instead his own father. Rather, the implication is that throughout the entire show he has always been his own father. Thus, Lister must have returned back to the original hypertime he was born at, t 2155 - T 1 , and left his baby self to be found by his adoptive parents. That requires going back to a hyperprevious hyperinstant. (And note that, since it involves a causal loop, we get the same probabilistic issues discussed in Section 2.3 ; we should, again, appreciate that whilst the narrative of ‘Ouroboros’ is possible, it’s therefore nevertheless very unlikely.)

5 Fixed Points

5.1 examples in fiction.

With the mixed model in place, turn to consider another feature common to time travel narratives: ‘fixed points’. In Simon Wells’ The Time Machine ( 2002 ), Dr. Alexander Hartdegen’s fiancé is killed by a mugger. To save her, Hartdegen invents a time machine, changes the past, and saves her. However, she is then run over by a carriage. Hartdegen realises that no matter what he does, he’ll be unable to stop her from being killed—every time she is saved, she’ll simply die in a different manner. Her dying can be changed in certain respects (e.g. from her being killed in a mugging to being killed in an accident) but not others (e.g. she must always die at roughly that time). Her death is a ‘fixed point in time’.

At first glance, there seems to be no room for fixed points in the mixed model. When Hartdegen travels back in time, he travels forwards in hypertime, so what’s stopping Hartdegen from preventing his fiancé’s death at that hyperlater hypertime? If Hartdegen can change time so his love isn’t shot but run over, why can’t he change time such that she lives to an old age? Why are events fixed in some respects but not others? This problem is compounded later in the film. Hartdegen travels to 802,701 AD and, further again, to the cataclysmic future of 635,427,810 AD. He then returns to 802,701, changing the future so the apocalypse is averted. The questions arises: Why can Hartdegen change that future but not that of his fiancé? How come some events are ‘fixed’ whilst other events are ‘unfixed’?

That some events are ‘fixed’ and others are ‘unfixed’ is a common trope in time travel fiction. Usually ‘major historical’ events are fixed and only more minor events are ‘unfixed’. Examples include:

Various episodes of The Twilight Zone . In ‘Back There’ (1961) the protagonist is unable to stop Lincoln’s assassination, whilst still finding themselves able to nevertheless change certain things. In ‘The Time Element’ (1958) the protagonist tries to stop the bombing of Pearl Harbour, finding himself unable to do so, but does manage to change history and cause himself to cease to exist. In ‘Memphis’ (2003) the protagonist tries to stop Martin Luther King’s assassination, failing to do so, but managing to change the past in other respects.

Fixed points routinely feature in Doctor Who . In ‘Earthshock’ (1982), Adric dies and the Doctor refuses to go back in time and save him. Even though the Doctor regularly changes historical events in other stories, there is something about this event which means he won’t try. In ‘The Visitation’ (1982), the Doctor accidentally starts the Great Fire of London but recognises that it must be let to run its course. Given the fire causes large amounts of property damage and kills six people—outcomes which the Doctor causes and intentionally allows to happen—that’s somewhat uncharacteristic of the Doctor. Presumably, he puts out other fires he accidentally starts, so what’s so special about this fire? In ‘The Water of Mars’ (2009) the Doctor appears on Mars in 2059, meeting the first astronauts on the red planet. He knows that the astronauts he meets there are going to die. When talking to one of the astronauts, Adelaide, he is quite explicit, saying that ‘Certain moments in time are fixed. […] those certain moments, they have to stand […] What happens here must always happen.’ When the Doctor tries to prevent Adelaide’s death, he still fails for she ends up unexpectedly committing suicide.

In Supernatural ’s ‘In the Beginning’, Dean is sent back in time to 1973. Whilst history can be changed in some respects, Dean is unable to save his mother from being murdered. It is explained to him ‘Destiny can’t be changed […] All roads lead to the same destination.’

Fixed points play a crucial role in the plot of Connie Willis’s To Say Nothing of the Dog ( 1997 ).

5.2 Fixed Points and Ludovician Probability

There is a way to make sense of fixed points within the mixed model. In the fixed point cases, unlikely things are happening. In The Time Machine the fiancé is—against the odds—run over by a carriage. And, taking Hartdegen at his word, the suggestion is that whatever action he takes to save her, some event will nevertheless kill her anyhow. In Doctor Who , Adelaide’s suicide is something which is unexpected and prima facie unlikely to have happened. Similarly, random occurrences prevent the derailing of history in The Twilight Zone episodes. These unlikely events sound similar to the unlikely events from Section 2 ’s regular Ludovician cases. In the one-dimensional Ludovican case, when I go to kill Hitler, a low probability event occurs to stop me i.e. a heart attack. This unlikely event comes about because I am interacting with things in my personal history. In the mixed model, the same interactions can arise and in such cases we should expect strange issues with probability to likewise arise on the mixed model. It is these issues with probability which explain why certain things seem to be ‘fixed’.

To understand what’s going on, we first must introduce ‘immanent causation’. There are two types of causation (Zimmerman, 1997 : 433ff). The first is the regular ‘transeunt’ causation we are well acquainted with e.g. I flick a switch which then causes a light to turn on. The second is immanent causation, which concerns something being a way at one time causing how it is at a later time. For instance, an electron being stationary and unmoving will, if it remains undisturbed, immanently cause itself to be stationary and unmoving at later times. Or if I prod an indelible mark on myself, it will remain on my body because of how my earlier body was.

Immanent causation plays an important role in hypertemporal models. Hyperlater hypertimes are how they are because they have been immanently caused to be that way by how hyperearlier hypertimes hyperwere. In a hypertemporal world in which no time travel takes place then, for every time t n , whatever is true at t n at one hypertime is true of it at every hyperlater hypertime. And in a hypertemporal world where I travel back in time to kill Hitler at t 1930 - T 2 then, whilst t 1930 is different from how it was at T 1 , it’s still mainly the same. For instance, how things are in the Andromeda galaxy are unaffected—a similarity explained by immanent causation i.e. how the Andromeda galaxy hyperwas at t 1930 - T 1 immanently causes it being that way at t 1930 - T 2 .

Immanent causation between hypertimes can feature as one of the links in a causal loop. Imagine someone travels back in both time and hypertime, going from t 2500 - T 10 to t 2020 - T 9 and causally interacting with what they find there. The qualitative features of t 2020 - T 9 immanently cause t 2020 - T 10 to be a certain way. Assuming that t 2020 - T 10 ′s changed features saliently affect the time traveller at t 2500 - T 10 then the time traveller’s travelling to the earlier/hyperearlier point results in a causal loop, with one of the links of that loop resulting from that inter-hypertemporal immanent causation. And, just as regular Ludovician travellers should expect weird probabilistic things to happen in causal loops, our imagined hypertime traveller should expect likewise. We can take this insight and use it to explain what’s going on with fixed points, although it does require some embellishment to the fictional narratives we are presented with.

Consider Simon Wells’ The Time Machine . If we thought it took place in a world of hypertime, then, given the narrative as it is explicitly presented, there would be no causal loop. See Fig.  5 a. Hartdegen is in 1903 at one hypertime. He travels back in time to 1899 to save his fiancé, moving forward in hypertime. He fails to save her and then travels onwards to 802,701 and then on, again, to 635,427,810. He then travels back in time (and, therefore, forward in hypertime) in order to prevent the apocalyptic future he’s witnessed. As Fig.  5 a makes clear, there is no causal loop.

a The Time machine: The unembellished narrative. b The time machine: The embellished narrative

But if we embellish the narrative, we get a causal loop—and once we get the causal loop, we can explain the fixed point. Imagine an agent, ‘Agent’, travels from a time and hypertime both later and hyperlater than that explicitly shown in Hartdegen’s story. As an example, imagine that Agent starts all life on Earth, four billion years ago and at the earliest hypertime. See Fig.  5 b. Agent only exists at t 900m -T 5 because, five billion years earlier at t 3.7 Billion BC - T 5 , life formed on the planet. But life formed at that point only because it formed at that point in time at earlier hypertimes e.g. at t 3.7 Billion BC - T 4 (which is in turn because it formed at t 3.7 Billion BC - T 3 , t 3.7 Billion BC - T 2 , and t 3.7 Billion BC - T 1 ). So, ultimately, Agent only exists because of a causal loop they started! Notice, also, that Agent also needs Hartdegen to time travel as well. Hartdegen prevents the future from being apocalyptic; had it not been for Hartdegen, Agent would never have been born either.

Given this causal loop, we can explain why points in time appear ‘fixed’. Hartdegen would never have travelled in time if not for the death of his fiancé. Indeed, even when he saves her from the mugger, had she not then been killed in the carriage accident, he would’ve remained in the twentieth century at T 2 and never ended up travelling to the future (and, thus, never have prevented the apocalyptic future of 635,427,810). So some unlikely events will transpire to ensure that Hartdegen keeps time travelling until he ultimately prevents the apocalyptic future, in turn allowing Agent to travel back to t 3.7 Billion BC - T 1 (an event which itself ensures Hartdegen comes into being). So whilst Hartdegen can change the circumstances of his fiancé’s death, because her dying spurs him on to change the future, she will always end up dying. Her dying no matter what Hartdegen does, and the unlikelihood of events which bring that about, are exactly the same as Section 2 ’s example where Hitler always survives my concerted attempts to kill him.

Consider another example. Return to the fiction of Doctor Who . Again, embellish the narrative such that the Doctor is part of a causal loop. The Doctor comes from the race of Time Lords. Imagine that the Time Lords came into existence at some point early in both time and hypertime e.g. t 999 - T 1 . Now embellish the narrative. Imagine that Time Lords from the future/hyperfuture, e.g. t 4000 - T 500 , come back in time/hypertime and interact with their earlier/hyperearlier ancestors e.g. travelling to t 1000 - T 1 to help their ancestors invent time travel. To distinguish the Time Lords of t 4000 - T 500 from their ancestors of t 1000 - T 1 , call them ‘Future Lords’. Footnote 4

Having interfered with events at t 1000 - T 1 , the Future Lords have affected the personal history of every Time Lord from t 1000 - T 1 onwards. Since causation is transitive, events which causally influence the Future Lords coming back from t 4000 - T 500 are likewise in the personal history of every Time Lord. As with Section 2 ’s regular Ludovicianism, strange issues with probability only arise when time travellers are interacting with their own personal history. So when the Doctor is interacting with events which don’t form part of his personal history—and, therefore, don’t form part of the personal history of the Future Lords—unlikely events are not to be expected. But, scattered throughout space, time, and hypertime, there will presumably be events which are in the personal history of the Future Lords. And any Time Lord who would interact with such events threatens (possibly deadly!) events befalling them (in just the same way that if I go back in time to Sigeburg and Cuthbert I risk deadly events befalling me). Thus, Time Lords must be careful when travelling through history and hyperhistory, ensuring that they avoid these events and stick just to influencing those events outside of their personal histories.

Personal historical events are the ‘fixed points’ which the Doctor (usually) tries to avoid interacting with. To interact with them would put him in danger, as well as those around him. Add further that the Time Lords are bestowed with some ability to forewarn them when they’re in the periphery of their personal history (a useful ability to have if you’re going to time travel willy-nilly!) and we would then have what we see on screen: the Doctor would have a preternatural awareness that some events should not be interacted with and he would stay well away from them. He would avoid trying to alter Adric’s death, avoid interacting with his own past, sense that some events (e.g. the Great Fire of London) must be let to run their course, and so on.

So we can make sense of fixed points in the mixed model, as long as we suitably embellish a narrative. Fixed points are events in one’s personal history where interaction with them brings about unlikely events. In Hartdegen’s case, they are events which transpire to keep his fiancé dead in order to ensure that he travels to the future and prevent an apocalypse. These events can be compared to similar cases in the regular Ludovician model, whereby unlikely events thwart my every effort to kill Hitler. In the Doctor’s case, he presumes that they are events which are dangerous and so he seeks to avoid them; he won’t try to stop fixed points because he fears what might happen if he did. They can be compared to the worry in the regular Ludovician model whereby I should fear that I would die were I to try and kill Hitler in 1930 (and that, therefore, I should not attempt to assassinate him). In the case of other fictions, I suggest that similar thoughts would apply (given, of course, similar embellishments to the narratives).

6 Extensions of the Programme

The mixed model helps make sense of some time travel fictions which have otherwise been thought to be metaphysically impossible. The model, though, doesn’t make sense of every fiction. For instance, in some fictions time changes ‘gradually’ and people pop in or out of existence as ‘the timeline changes’ (see, e.g., Red Dwarf ’s ‘Timeslides’). Or the past may change but it might take time for the ‘changes to catch up to the present’ (see Effingham [Forthcoming] for discussion). Such phenomena won’t be explained by the mixed model.

But some time travel fictions which are not be explained by the mixed model can nevertheless be explained by a suitable extension of the model. Return to Section 3.2 ’s explanation of ‘Pyramids of Mars’. Given there are fixed points, that explanation now looks flawed. Sutekh changes the future so that 1980 is a wasteland—if the future is a wasteland then (in 2059) Adelaide will never be alive in order to die. If the Doctor has to worry about fixed points, why doesn’t Sutekh? Isn’t it impossible for Sutekh to change the future such that Adelaide doesn’t exist? Moreover, the Doctor seems to think that the Time Lords are threatened by Sutekh, which makes little sense given that the Future Lords are definitely going to exist in the future/hyperfuture. If the Doctor knows about the Future Lords (and the fixed points which arise because of what they’ve done), wouldn’t the Doctor know that Sutekh’s efforts were futile?

By introducing a more complicated mixed model, these problems can be solved. Take the mixed model and mix it again with the non-Ludovician hypertemporal theory, adding in a third dimension of time, ‘ultratime’. Whilst travel through time and hypertime is easy, imagine that travelling forwards in ultratime is either very difficult or generally proscribed; ultratime travel is very rare indeed. (And assume that backwards ultratime travel, i.e. travelling back to ultraearlier ultrainstants, is impossible.) Sutekh either has resources that make moving forward in ultratime a mere trifle or he obeys no proscription against moving forward in ultratime. Using ‘ \({\mathscr{T}}\) s’ to represent different ultratemporal instants, imagine that the narrative of ‘Pyramids of Mars’ starts at ultratime \({\mathscr{T}}_{1}\) . At \({\mathscr{T}}_{1}\) Sutekh knows full well what history, and hyperhistory, is like i.e. it’s a history/hyperhistory which favours his enemies, the Time Lords. Upon escaping, Sutekh moves forward in ultratime to \({\mathscr{T}}_{2}\) . Now Sutekh can change history/hyperhistory however he wants, making it into a wasteland devoid of life. At \({\mathscr{T}}_{2}\) , there are no Time Lords intervening with their earlier selves and no fixed points like Adelaide’s death. Sutekh has free reign to do whatever he wants with no fear of unlikely events thwarting his actions.

Either ignoring the general proscription on ultratime travel (because of the severity of the situation) or somehow utilising the fact that Sutekh is moving forward in ultratime to also allow himself to do similar, the Doctor travels to join Sutekh at \({\mathscr{T}}_{2}\) . There he arrives (at some hypertime) at 1980, showing Sarah Jane the desolate solar system. Then, when the Doctor returns to 1911, he again travels forward in ultratime, to \({\mathscr{T}}_{3}\) . At \({\mathscr{T}}_{3}\) , he changes things so that history/hyperhistory goes back to much the same way it was at \({\mathscr{T}}_{1}\) (i.e. the Time Lords are back to being supreme, life isn’t extinguished from the twentieth century onwards at the appropriate times/hypertimes, etc.).

This also explains why the Doctor is at such loggerheads with Sutekh. Sutekh threatens the Time Lord’s history in a way that a more ordinary time traveller could not hope to. An ordinary time traveller, who travels through merely time and hypertime, cannot wipe the Time Lords from existence (in the same way that, given regular Ludovicianism, Hitler should have no fear of me killing him in 1930). But ultratemporal time travellers can wipe the Time Lords from existence.

It’s also worth noting why the Time Lords might not want to ultratime travel (and either proscribe it or, purposefully, make it difficult). How the world is, right now, immanently causes how it is later. If nothing acts to change it, it will forever remain the same. Similarly, if all of history is a certain way and no agents have used time machines to move forward in hypertime, then for the rest of hypereternity, all of history will be the same way. If not for time travellers, every hypertemporal instant would be the same as the hyperearlier hyperinstant. Finally: If all of history/hyperhistory is a certain way, and no agents have used machines to travel forward in ultratime, then at every subsequent ultratime it’ll turn out that history/hyperhistory is exactly the same. Having tweaked history/hyperhistory to be exactly how they like it, the Time Lords will want to make sure no-one travels forward to the next ultratime to muck it up. If you want to ensure that all of history/hyperhistory remains exactly how you prefer, both ultranow and ultraforevermore (i.e. at every subsequent ultratime from the one you’re ultrapresently at), then you’ll make efforts to prevent people using ultratime machines. For instance, you’d track down scurrilous rogues like Sutekh looking to make history/hyperhistory different by ultratime travelling. Only by doing this can you make the world ultraeternally how you want it.

Thus, another iteration of mixing the mixed model with non-Ludovicianism allows us to make sense of this narrative. More generally, there will be other fictional narratives which might not be captured by the mixed model but could be captured by more sophisticated models including yet more dimensions of time. (Indeed, we might ‘mix again’ with something other than the hypertemporal theory i.e. mix the mixed model with universe non-Ludovicianism, or the theory I discuss in Effingham [Forthcoming].)

DC Comics features ‘hypertime’. However, it’s a mere MacGuffin, rather than anything related to the theory described above.

Alternatively, universe non-Ludovicianism may not involve the creation of universes but the navigation of a pre-existing set of universes. But in the same way that you don’t solve the problem of starving children by booking a flight to somewhere where there aren’t any starving children, the Doctor again fails to ameliorate Sutekh’s actions by travelling back to 1911. All he does is move himself to a place where it’s not his problem.

That time machines may have both Ludovician and non-Ludovician ‘settings’ is something which was suggested to me by Sara Bernstein when discussing her MOP view of time travel [Bernstein 2017 ].

Such interactions are very dangerous, as we learnt in Section 2.2 . But assume that on this occasion they either ‘got lucky’ or otherwise developed some method to warp probability and purposefully avoid such dangers.

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Acknowledgements

I am particularly grateful to the two anonymous referees for Erkenntnis . Their comments were both extensive and exceedingly helpful. Further thanks goes to Sara Bernstein, Tyler Collins, Isabel Finn, Iain Law, and the attendees of my presentation of this paper at the University of Birmingham.

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Effingham, N. The Metaphysical Possibility of Time Travel Fictions. Erkenn 88 , 1309–1329 (2023). https://doi.org/10.1007/s10670-021-00403-y

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