best time travel paradoxes

[ May 9, 2024 ] Moon Phases: What They Are & How They Work Solar System
[ April 3, 2024 ] Giordano Bruno Quotes About Astronomy Astronomy Lists
[ March 28, 2024 ] Shakespeare Quotes: Comets, Meteors and Shooting Stars Astronomy Lists
[ March 28, 2024 ] Shakespearean Quotes About The Moon Astronomy Lists
[ November 30, 2022 ] The Night Sky This Month: December 2022 Night Sky

5 Bizarre Paradoxes Of Time Travel Explained

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

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

Types of Temporal Paradoxes

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

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

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

1: Predestination Paradox

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

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

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

– Cinema Treatment

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

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

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

2: Bootstrap Paradox

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

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

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

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

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

3: Grandfather Paradox

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

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

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

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

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

4: Let’s Kill Hitler Paradox

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

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

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

5: Polchinski’s Paradox

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

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

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

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

Are Self-Fulfilling Prophecies Paradoxes?

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

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

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

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

Are Time Paradoxes Inevitable?

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

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

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

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

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

How Time Travel Works

Share Content on Facebook
Share Content on LinkedIn
Share Content on Flipboard
Share Content on Reddit
Share Content via Email

Time Travel Paradoxes

As we mentioned before, the concept of traveling into the past becomes a bit murky the second causality rears its head. Cause comes before effect, at least in this universe, which manages to muck up even the best-laid time traveling plans.

For starters, if you traveled back in time 200 years, you'd emerge in a time before you were born. Think about that for a second. In the flow of time, the effect (you) would exist before the cause (your birth).

To better understand what we're dealing with here, consider the famous grandfather paradox . You're a time-traveling assassin, and your target just happens to be your own grandfather. So you pop through the nearest wormhole and walk up to a spry 18-year-old version of your father's father. You raise your laser blaster , but just what happens when you pull the trigger?

Think about it. You haven't been born yet. Neither has your father. If you kill your own grandfather in the past, he'll never have a son. That son will never have you, and you'll never happen to take that job as a time-traveling assassin. You wouldn't exist to pull the trigger, thus negating the entire string of events. We call this an inconsistent causal loop .

On the other hand, we have to consider the idea of a consistent causal loop . While equally thought-provoking, this theoretical model of time travel is paradox free. According to physicist Paul Davies, such a loop might play out like this: A math professor travels into the future and steals a groundbreaking math theorem. The professor then gives the theorem to a promising student. Then, that promising student grows up to be the very person from whom the professor stole the theorem to begin with.

Then there's the post-selected model of time travel, which involves distorted probability close to any paradoxical situation [source: Sanders]. What does this mean? Well, put yourself in the shoes of the time-traveling assassin again. This time travel model would make your grandfather virtually death proof. You can pull the trigger, but the laser will malfunction. Perhaps a bird will poop at just the right moment, but some quantum fluctuation will occur to prevent a paradoxical situation from taking place.

But then there's another possibility: The quantum theory that the future or past you travel into might just be a parallel universe . Think of it as a separate sandbox: You can build or destroy all the castles you want in it, but it doesn't affect your home sandbox in the slightest. So if the past you travel into exists in a separate timeline, killing your grandfather in cold blood is no big whoop. Of course, this might mean that every time jaunt would land you in a new parallel universe and you might never return to your original sandbox.

Confused yet? Welcome to the world of time travel.

Explore the links below for even more mind-blowing cosmology.

How Time Works
How Special Relativity Works
What is relativity?
Is Time Travel Possible?
How Black Holes Work
How would time travel affect life as we know it?

The real time-travel paradox was the friends we made along the way

Rodrigo Culagovski 0

Rodrigo is a Chilean architect, designer and web developer. He currently heads a web development agency and is a researcher and professor at Universidad Católica in Chile. He has published in Dark Matter Presents: Monstrous Futures , Solarpunk Magazine and Future Science Fiction Digest . On Mastodon as @[email protected] . He misses his Commodore 64. Pronouns he/him/él.

You can also search for this author in PubMed Google Scholar

Illustration: Jacey

She was taller than me. Prettier and with better muscle tone. Shinier hair and perfect skin and teeth. Which was odd because she claimed she was me — from the future.

Access options

Access Nature and 54 other Nature Portfolio journals

Get Nature+, our best-value online-access subscription

24,99 € / 30 days

cancel any time

Subscribe to this journal

Receive 51 print issues and online access

185,98 € per year

only 3,65 € per issue

Rent or buy this article

Prices vary by article type

Prices may be subject to local taxes which are calculated during checkout

doi: https://doi.org/10.1038/d41586-024-00897-w

1,001 best hikes on Mars: The Peterson Historic Trail (‘Peterson’s folly’)

Futures 05 JUN 24

It’s time for your performance review

Futures 29 MAY 24

Time’s restless ocean

Futures 22 MAY 24

Postdoctoral Fellow in Immunology at Northwestern University

Our T cell biology lab is looking for fellows interested in improving immunotherapies for cancer and autoimmune disease through immune engineering.

Chicago, Illinois

Choi Laboratory at Northwestern

Faculty Positions in School of Engineering, Westlake University

The School of Engineering (SOE) at Westlake University is seeking to fill multiple tenured or tenure-track faculty positions in all ranks.

Hangzhou, Zhejiang, China

Westlake University

High-Level Talents at the First Affiliated Hospital of Nanchang University

For clinical medicine and basic medicine; basic research of emerging inter-disciplines and medical big data.

Nanchang, Jiangxi, China

The First Affiliated Hospital of Nanchang University

Professor/Associate Professor/Assistant Professor/Senior Lecturer/Lecturer

The School of Science and Engineering (SSE) at The Chinese University of Hong Kong, Shenzhen (CUHK-Shenzhen) sincerely invites applications for mul...

Shenzhen, China

The Chinese University of Hong Kong, Shenzhen (CUHK Shenzhen)

Quick links

Explore articles by subject
Guide to authors
Editorial policies

Table of Contents
Random Entry
Chronological
Editorial Information
About the SEP
Editorial Board
How to Cite the SEP
Special Characters
Advanced Tools
Support the SEP
PDFs for SEP Friends
Make a Donation
SEPIA for Libraries
Entry Contents

Bibliography

Academic tools.

Friends PDF Preview
Author and Citation Info
Back to Top

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 ]

Adams, Robert Merrihew, 1997, “Thisness and time travel”, Philosophia , 25: 407–15.
Arntzenius, Frank, 2006, “Time travel: Double your fun”, Philosophy Compass , 1: 599–616. doi:10.1111/j.1747-9991.2006.00045.x
Asimov, Isaac, 1995 [2003], Gold: The Final Science Fiction Collection , New York: Harper Collins.
Baron, Sam and Colyvan, Mark, 2016, “Time enough for explanation”, Journal of Philosophy , 113: 61–88.
Belot, Gordon, 2005, “Dust, time and symmetry”, British Journal for the Philosophy of Science , 56: 255–91.
Benovsky, Jiri, 2011, “Endurance and time travel”, Kriterion , 24: 65–72.
Berkovitz, Joseph, 2001, “On chance in causal loops”, Mind , 110: 1–23.
Black, Max, 1956, “Why cannot an effect precede its cause?”, Analysis , 16: 49–58.
Brier, Bob, 1973, “Magicians, alarm clocks, and backward causation”, Southern Journal of Philosophy , 11: 359–64.
Carlson, Erik, 2005, “A new time travel paradox resolved”, Philosophia , 33: 263–73.
Carroll, John W., 2010, “Context, conditionals, fatalism, time travel, and freedom”, in Time and Identity , Joseph Keim Campbell, Michael O’Rourke, and Harry S. Silverstein, eds., Cambridge MA: MIT Press, 79–93.
Craig, William L., 1997, “Adams on actualism and presentism”, Philosophia , 25: 401–5.
Daniels, Paul R., 2012, “Back to the present: Defending presentist time travel”, Disputatio , 4: 469–84.
Deutsch, David and Lockwood, Michael, 1994, “The quantum physics of time travel”, Scientific American , 270(3): 50–6.
Dowe, Phil, 2000, “The case for time travel”, Philosophy , 75: 441–51.
–––, 2001, “Causal loops and the independence of causal facts”, Philosophy of Science , 68: S89–S97.
–––, 2003, “The coincidences of time travel”, Philosophy of Science , 70: 574–89.
Dummett, Michael, 1964, “Bringing about the past”, Philosophical Review , 73: 338–59.
Dwyer, Larry, 1977, “How to affect, but not change, the past”, Southern Journal of Philosophy , 15: 383–5.
Earman, John, 1995, Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes , New York: Oxford University Press.
Effingham, Nikk, 2020, Time Travel: Probability and Impossibility , Oxford: Oxford University Press.
Effingham, Nikk and Robson, Jon, 2007, “A mereological challenge to endurantism”, Australasian Journal of Philosophy , 85: 633–40.
Ehring, Douglas, 1997, “Personal identity and time travel”, Philosophical Studies , 52: 427–33.
Elliott, Katrina, 2019, “How to Know That Time Travel Is Unlikely Without Knowing Why”, Pacific Philosophical Quarterly , 100: 90–113.
Fulmer, Gilbert, 1980, “Understanding time travel”, Southwestern Journal of Philosophy , 11: 151–6.
Gilmore, Cody, 2007, “Time travel, coinciding objects, and persistence”, in Oxford Studies in Metaphysics , Dean W. Zimmerman, ed., Oxford: Clarendon Press, vol. 3, 177–98.
Goddu, G.C., 2003, “Time travel and changing the past (or how to kill yourself and live to tell the tale)”, Ratio , 16: 16–32.
–––, 2007, “Banana peels and time travel”, Dialectica , 61: 559–72.
Gödel, Kurt, 1949a [1990a], “An example of a new type of cosmological solutions of Einstein’s field equations of gravitation”, in Kurt Gödel: Collected Works (Volume II), Solomon Feferman, et al. (eds.), New York: Oxford University Press, 190–8; originally published in Reviews of Modern Physics , 21 (1949): 447–450.
–––, 1949b [1990b], “A remark about the relationship between relativity theory and idealistic philosophy”, in Kurt Gödel: Collected Works (Volume II), Solomon Feferman, et al. (eds.), New York: Oxford University Press, 202–7; originally published in P. Schilpp (ed.), Albert Einstein: Philosopher-Scientist , La Salle: Open Court, 1949, 555–562.
Godfrey-Smith, William, 1980, “Travelling in time”, Analysis , 40: 72–3.
Gorovitz, Samuel, 1964, “Leaving the past alone”, Philosophical Review , 73: 360–71.
Grey, William, 1999, “Troubles with time travel”, Philosophy , 74: 55–70.
Hafele, J. C. and Keating, Richard E., 1972a, “Around-the-world atomic clocks: Observed relativistic time gains”, Science , 177: 168–70.
–––, 1972b, “Around-the-world atomic clocks: Predicted relativistic time gains”, Science , 177: 166–8.
Hales, Steven D., 2010, “No time travel for presentists”, Logos & Episteme , 1: 353–60.
Hall, Thomas, 2014, “In Defense of the Compossibility of Presentism and Time Travel”, Logos & Episteme , 2: 141–59.
Hanley, Richard, 2004, “No end in sight: Causal loops in philosophy, physics and fiction”, Synthese , 141: 123–52.
Harrison, Jonathan, 1980, “Report on analysis ‘problem’ no. 18”, Analysis , 40: 65–9.
Hawking, S.W., 1992, “Chronology protection conjecture”, Physical Review D , 46: 603–11.
Holt, Dennis Charles, 1981, “Time travel: The time discrepancy paradox”, Philosophical Investigations , 4: 1–16.
Horacek, David, 2005, “Time travel in indeterministic worlds”, Monist (Special Issue on Time Travel), 88: 423–36.
Horwich, Paul, 1975, “On some alleged paradoxes of time travel”, Journal of Philosophy , 72: 432–44.
–––, 1987, Asymmetries in Time: Problems in the Philosophy of Science , Cambridge MA: MIT Press.
Ismael, J., 2003, “Closed causal loops and the bilking argument”, Synthese , 136: 305–20.
Keller, Simon and Nelson, Michael, 2001, “Presentists should believe in time-travel”, Australasian Journal of Philosophy , 79: 333–45.
Kiourti, Ira, 2008, “Killing baby Suzy”, Philosophical Studies , 139: 343–52.
Le Poidevin, Robin, 2003, Travels in Four Dimensions: The Enigmas of Space and Time , Oxford: Oxford University Press.
–––, 2005, “The Cheshire Cat problem and other spatial obstacles to backwards time travel”, Monist (Special Issue on Time Travel), 88: 336–52.
Lewis, David, 1976, “The paradoxes of time travel”, American Philosophical Quarterly , 13: 145–52.
Loss, Roberto, 2015, “How to Change the Past in One-Dimensional Time”, Pacific Philosophical Quarterly , 96: 1–11.
Luminet, Jean-Pierre, 2011, “Time, topology, and the twin paradox”, in The Oxford Handbook of Philosophy of Time , Craig Callender (ed.), Oxford: Oxford University Press. doi:10.1093/oxfordhb/9780199298204.003.0018
Markosian, Ned, 2004, “Two arguments from Sider’s Four-Dimensionalism ”, Philosophy and Phenomenological Research , 68: 665–73.
Markosian, Ned, 2020, “The Dynamic Theory of Time and Time Travel to the Past”, Disputatio , 12: 137–65.
Maudlin, Tim, 2012, Philosophy of Physics: Space and Time , Princeton: Princeton University Press.
Meiland, Jack W., 1974, “A two-dimensional passage model of time for time travel”, Philosophical Studies , 26: 153–73.
Mellor, D.H., 1998, Real Time II , London: Routledge.
Meyer, Ulrich, 2012, “Explaining causal loops”, Analysis , 72: 259–64.
Miller, Kristie, 2005, “Time travel and the open future”, Disputatio , 1: 223–32.
–––, 2006, “Travelling in time: How to wholly exist in two places at the same time”, Canadian Journal of Philosophy , 36: 309–34.
–––, 2008, “Backwards causation, time, and the open future”, Metaphysica , 9: 173–91.
Monton, Bradley, 2003, “Presentists can believe in closed timelike curves”, Analysis , 63: 199–202.
–––, 2009, “Time travel without causal loops”, Philosophical Quarterly , 59: 54–67.
Nerlich, Graham, 1981, “Can time be finite?”, Pacific Philosophical Quarterly , 62: 227–39.
Ney, S.E., 2000, “Are grandfathers an endangered species?”, Journal of Philosophical Research , 25: 311–21.
Price, Huw, 1996, Time’s Arrow & Archimedes’ Point: New Directions for the Physics of Time , New York: Oxford University Press.
Putnam, Hilary, 1975, “It ain’t necessarily so”, in Mathematics, Matter and Method , Cambridge: Cambridge University Press, vol. 1 of Philosophical Papers , 237–49.
Reinganum, Marc R., 1986, “Is time travel impossible? A financial proof”, Journal of Portfolio Management , 13: 10–2.
Riggs, Peter J., 1991, “A critique of Mellor’s argument against ‘backwards’ causation”, British Journal for the Philosophy of Science , 42: 75–86.
–––, 1997, “The principal paradox of time travel”, Ratio , 10: 48–64.
Savitt, Steven, 1994, “The replacement of time”, Australasian Journal of Philosophy , 74: 463–73.
–––, 2005, “Time travel and becoming”, Monist (Special Issue on Time Travel), 88: 413–22.
Sider, Theodore, 2001, Four-Dimensionalism: An Ontology of Persistence and Time , Oxford: Clarendon Press.
–––, 2002, “Time travel, coincidences and counterfactuals”, Philosophical Studies , 110: 115–38.
–––, 2004, “Replies to Gallois, Hirsch and Markosian”, Philosophy and Phenomenological Research , 68: 674–87.
–––, 2005, “Traveling in A- and B- time”, Monist (Special Issue on Time Travel), 88: 329–35.
Simon, Jonathan, 2005, “Is time travel a problem for the three-dimensionalist?”, Monist (Special Issue on Time Travel), 88: 353–61.
Slater, Matthew H., 2005, “The necessity of time travel (on pain of indeterminacy)”, Monist (Special Issue on Time Travel), 88: 362–9.
Smart, J.J.C., 1963, “Is time travel possible?”, Journal of Philosophy , 60: 237–41.
Smeenk, Chris and Wüthrich, Christian, 2011, “Time travel and time machines”, in The Oxford Handbook of Philosophy of Time , Craig Callender (ed.), Oxford: Oxford University Press, online ed. doi:10.1093/oxfordhb/9780199298204.003.0021
Smith, Joseph Wayne, 1985, “Time travel and backward causation”, Cogito , 3: 57–67.
Smith, Nicholas J.J., 1997, “Bananas enough for time travel?”, British Journal for the Philosophy of Science , 48: 363–89.
–––, 1998, “The problems of backward time travel”, Endeavour , 22(4): 156–8.
–––, 2004, “Review of Robin Le Poidevin Travels in Four Dimensions: The Enigmas of Space and Time ”, Australasian Journal of Philosophy , 82: 527–30.
–––, 2005, “Why would time travellers try to kill their younger selves?”, Monist (Special Issue on Time Travel), 88: 388–95.
–––, 2011, “Inconsistency in the A-theory”, Philosophical Studies , 156: 231–47.
–––, 2015, “Why time travellers (still) cannot change the past”, Revista Portuguesa de Filosofia , 71: 677–94.
–––, 2017, “I’d do anything to change the past (but I can’t do ‘that’)”, American Philosophical Quarterly , 54: 153–68.
van Inwagen, Peter, 2010, “Changing the past”, in Oxford Studies in Metaphysics (Volume 5), Dean W. Zimmerman (ed.), Oxford: Oxford University Press, 3–28.
Vihvelin, Kadri, 1996, “What time travelers cannot do”, Philosophical Studies , 81: 315–30.
Vranas, Peter B.M., 2005, “Do cry over spilt milk: Possibly you can change the past”, Monist (Special Issue on Time Travel), 88: 370–87.
–––, 2009, “Can I kill my younger self? Time travel and the retrosuicide paradox”, Pacific Philosophical Quarterly , 90: 520–34.
–––, 2010, “What time travelers may be able to do”, Philosophical Studies , 150: 115–21.
Wasserman, Ryan, 2018, Paradoxes of Time Travel , Oxford: Oxford University Press.
Williams, Donald C., 1951, “The myth of passage”, Journal of Philosophy , 48: 457–72.
Wright, John, 2006, “Personal identity, fission and time travel”, Philosophia , 34: 129–42.
Yourgrau, Palle, 1999, Gödel Meets Einstein: Time Travel in the Gödel Universe , Chicago: Open Court.

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 .

Accessibility

Support SEP

Mirror sites.

View this site from another server:

Info about mirror sites

Library of Congress Catalog Data: ISSN 1095-5054

The Time-Travel Paradoxes

What happens if a time traveler kills his or her grandfather? What is a time loop? How do you stop a time machine from just appearing somewhere in space, millions of kilometers from home? And is there such a thing as free will?

Congratulations! You have a time machine! You can pop over to see the dinosaurs, be in London for the Beatles’ rooftop concert, hear Jesus deliver his Sermon on the Mount, save the books of the Library of Alexandria, or kill Hitler. Past and future are in your hands. All you have to do is step inside and press the red button.

Wait! Don’t do it!

Seriously, if you value your lives, if you want to protect the fabric of reality – run for the hills! Physics and logical paradoxes will be your undoing. From the grandfather paradox to laws of classic mechanics, we have prepared a comprehensive guide to the hazards of time travel. Beware the dangers that lie ahead.

The machine from H. G. Wells’ “The Time Machine”. Credit: Shutterstock.

The Grandfather Paradox

Want to change reality? First think carefully about your grandparents’ contribution to your lives.

The grandfather paradox basically describes the following situation: For some reason or another, you have decided to go back in time and kill your grandfather in his youth. Yeah, sure, of course you love him – but this is a scientific experiment; you don’t have a choice. So your grandmother will never give birth to your parent – and therefore you will never be born, which means that you cannot kill your grandfather. Oh boy! This is quite a contradiction!

The extended version of the paradox touches upon practically every single change that our hypothetical time traveler will make in the past. In a chaotic reality, there is no telling what the consequences of each step will be on the reality you came from. Just as a butterfly flapping its wings in the Amazon could cause a tornado in Texas, there is no way of predicting what one wrong move on your part might do to all of history, let alone a drastic move like killing someone.

There is a possible solution to this paradox – but it cancels out free will: Our time traveler can only do what has already been done. So don’t worry – everything you did in the past has already happened, so it’s impossible for you to kill grandpa, or create any sort of a contradiction in any other way. Another solution is that the time traveler's actions led to a splitting of the universe into two universes – one in which the time traveler was born, and the other in which he murdered his grandfather and was not born.

Information passage from the future to the past causes a similar paradox. Let’s say someone from the future who has my best interests in mind tries to warn me that a grand piano is about to fall on my head in the street, or that I have a type of cancer that is curable if it’s discovered early enough. Because of this warning, I could take steps to prevent the event – but then, there is no reason to send back the information from the future that saves my life. Another contradiction!

Marty finds himself in hot water with the grandfather paradox, from ‘Back to the Future’ 1985

Let’s now assume the information is different: A richer future me builds a time machine to let the late-90s me know that I should buy stock of a small company called “Google”, so that I can make a fortune. If I have free will, that means I can refuse. But future me knows I already did it. Do I have a choice but to do what I ask of myself?

The Time Loop

In the book All You Zombies by science fiction writer Robert A. Heinlein the Hero is sent back in time in order to impregnate a young woman who is later revealed to be him, following a sex change operation. The offspring of this coupling is the young man himself, who will meet himself at a younger age and take him back to the past to impregnate you know whom.

Confused? This is just one extreme example of a time loop – a situation where a past event is the cause of an event at another time and also the result of it. A simpler example could be a time traveler giving the young William Shakespeare a copy of the complete works of Shakespeare so that he can copy them. If that happens, then who is the genius author of Macbeth?

This phenomenon is also known as the Bootstrap Paradox , based on another story by Heinlein, who likened it to a person trying to pull himself up by his bootstraps (a phrase which, in turn, comes from the classic book The Surprising Adventures of Baron Munchausen). The word ‘paradox’ here is a bit misleading, since there is no contradiction in the loop – it exists in a sequence of events and feeds itself. The only contradiction is in the order of things that we are acquainted with, where cause leads to effect and nothing further, and there is meaning to the question “how did it all begin?”

Terminator 2 (1991). The shapeshifting android (Arnold Schwarzenegger) destroys himself in order to break the time loop in which his mere presence in the present enabled his production in the future

Time travelers – where have all they gone?

In 1950, over lunch physicist Enrico Fermi famously asked: “If there is intelligent extraterrestrial life in the Universe – then where are they?” indicating that we have never met aliens or came across evidence of their existence, such as radio signals which would be proof of a technological society. We could pose that same question about time travelers: “If time travel is possible, where are all the time travelers?”

The question, known as the Fermi Paradox, is an important one. After all, if it were possible to travel through time, would we not have bumped into a bunch of observers from the future at critical junctures in history? It is unlikely to assume that they all managed to perfectly disguise themselves, without making any errors in the design of the clothes they wore, their accents, their vocabulary, etc. Another option is that time travel is possible, but it is used with the utmost care and tight control, due to all the dangers we discuss here.

But where is everybody? A painting of the Italian physicist Enrico Fermi – Emilio Segrè Visual Archives SPL

On June 28, 2009, physicist Stephen Hawking carried out a scientific experiment which was meant to answer this question once and for all. He brought snacks, balloons and champagne and hosted a secret party for time travelers only – but sent out the invitations only on the next day. If no one showed up, he argued, that would be proof that time travel to the past is not possible. The invitees failed to arrive. “I sat and waited for a while, but nobody came,” he reported at the Seattle Science Festival in 2012.

Multiple time travelers also undermine the possibility of a fixed and consistent timeline, assuming that the past can indeed be changed. Imagine, for example, a nail-biting derby between the top clubs, Hapoel Jericho and Maccabi Jericho. Originally Maccabi won, so a Hapoel fan traveled back in time and managed to lead to his team’s victory. Maccabi fans would not give up and did the same. Soon, the whole stadium is filled with time travelers and paradoxes.

One way or round trip?

When considering travel, it is always continuous – from point A to point B, through all the points in between. Time travel should supposedly be the same: travelers get into their machine, push the button, and go from time A to time B, through all the times in between. But there’s a catch, if we are only travelling through time, then to the casual observer, the time machine continuously exists in the same space between the points in time. The result is that our journey is one-way and the time travelers will stay stuck in the future or the past because the machine itself will block the time-path back. And that is before we even start wondering how to even build this thing in the first place if it already exists in the place where we want to build it.

If that’s the case, then there’s no choice but to assume that there is some way to jump from time to time or place to place and materialize at the destination. How will our machine “know” to jump to an empty area, and to avoid materializing into a wall or a living creature unlucky enough to occupy that same spot? The passengers will undoubtedly need effective navigation and observation equipment to prevent unfortunate accidents at the point of entry.

While travelling from one point in time to another are passengers passing through all the moments in between? Good question! Photo: Shutterstock

Advanced time travel

In addition to the problems that time travel poses for anyone trying to keep the notion of cause and effect in order, time travelers may also face – or already have faced – other challenges from physics, even classical physics.

One issue you have to consider during time travel, and which science fiction writers usually prefer to ignore for convenience sake, is the question of arrival at the specified time destination and what would happen to us there.

It is usually assumed, with no good reason, that if someone is travelling through time, he or she will land in the same place, but at a different time – past or future. But this is where we hit a snag: the Earth rotates around the sun at a speed of 110,000 kph, and the Solar System itself is moving in its trajectory around the galaxy at a speed of 750,000 kph. If we time-travel for even a few seconds and stay in the same coordinates of space, we will probably find ourselves floating in outer space and perhaps even manage a quick glance around before we die. Our time machine will have to take into account this movement of the heavenly bodies and place us at exactly the right spot in space.

This alone may be resolved, since time travel, in any case, takes place between two points in the four-dimensional space-time continuum. According to the theory of general relativity, the theoretical foundation for time travel, space and time are a single physical entity, known as space-time. This entity can be bent and distorted – in fact gravity itself is an external manifestation of space-time distortion.

The Time Lord ,Doctor Who explains what “time” is exactly (Doctor Who, Season 3, Chapter 10: Blink).

Time travel would be possible if we could create a closed space-time loop, or if we could go from one point to another through a shortcut called a “Wormhole”. This would, in any case, not be just moving from one point in time to another, but would also include moving through space. Thus, from the outset, the journey is not only in time, but necessarily includes a destination point in space, which we will need to pre-program on our machine, of course .

In practice, the situation is more complicated – especially if we want to go into the distant past or distant future. The speed of the celestial bodies, and even the Earth’s shape and the structure of the continents, the seas, and mountains on the face of the Earth, change over the years. And because even a tiny deviation in our knowledge of the past can land us in the core of the Earth, in outer space or somewhere else that immediately reduces life expectancy to zero – time travel becomes a Russian roulette.

How to travel in time and stay alive

Let’s assume we coped with this problem and managed to get to the exact point in space-time that can sustain life. Careful – we’re not there yet; we still have to deal with momentum.

Momentum is a conserved quantity, which basically represents the potential of a body to continue moving at the speed and direction in which it is already travelling. If we were to jump out of a moving car (heaven forbid!), conservation of momentum is what would cause us to roll on the ground and probably get injured (in the best-case scenario). And so, if we leap in time – say, a month back – and land at the exact same point on Earth – we would discover, much to our dismay, that even if we started motionless in relation to the ground, now the ground underneath us is moving quickly at one angle or another towards us . Thus, even if we were lucky enough not to crash immediately on impact, we’re likely to hit some obstacle. And if by some miracle we were to survive, we would quickly find ourselves burning up in the atmosphere or gasping for air in space – because we have far exceeded the escape velocity from Earth.

We still have to deal with the issue of momentum in our time travels / Illustrative picture, Shutterstock

A possible solution to this problem is to plan our landing point ahead, so that the ground speed will be equal in size and direction to our exit speed, but this places many constraints on our journey. We could always leap into space, where there are hardly any moving objects to be bumped into, and only then land again at our point of destination on Earth.

Having said all that, this problem arises chiefly when we assume that time hopping is immediate – that we disappear from one point in time and immediately appear in another, without losing mass, energy, or momentum. But since a “realistic” journey in time is not instantaneous, rather it involves travelling along space-time, it is no different from other types of journeys. This being the case, we can hope that we could adjust our speed to the desired value and direction prior to landing, just like a spacecraft slowing down before landing on a planet.

We should also keep in mind that thankfully, we will have access to a powerful technology that would enable us to cope with such problems: Time-travel technology itself. For example, we might decide to send thousands of tiny probes ahead of us, each to a slightly different point in space-time. Some of them, maybe even most, will be destroyed for one of the reasons already mentioned. The others will wait patiently until the present and then feed their programmed coordinates into the time machine. Thus by definition, the destination entered will be safe for us, except, perhaps for the annoying probe shower hitting the travellers. For the travellers themselves, the entire process will be immediate.

Time Travelling Grammar

Finally, we come to the question: How do you actually talk about time travel? The three tenses – past, present, and future – are insufficient to discuss a future event that happened some time in the past with someone who is in the present, which is another’s past and yet another’s future. And what is the correct grammatical tense to use when we talk about an alternative future that would have been created after we killed our grandfather? Or how do we express the future-past tense (or past-future, or past-future-past?), when we get stuck in a time loop where what will happen leads to what had already taken place, and so on? And of course the biggest question that Hebrew editors and translators have faced for years – is there really such a thing as present continuous?

It’s complicated.

Arguing about tenses and a time machine, The Big Bang Theory, Season 8, Episode 5, 2014

In his book, The Restaurant at the End of the Universe, science fiction writer Douglas Adams suggests to his readers to consult (by Dr. Dan Streetmentioner) Time Traveler's Handbook of 1001 Tense Formations (by Dr. Dan Streetmentioner) to find the answers to these questions. That’s all very well, but, Adams tells us, “most readers get as far as the Future Semi-Conditionally Modified Subinverted Plagal Past Subjunctive Intentional before giving up; and in fact in later editions of the book all pages beyond this point have been left blank to save on printing costs.”

If, despite all of the above, you’re still intent on travelling back to Mount Sinai or the Apollo 11 moon landing – let us then wish you bon voyage, and please give our regards to Neil Armstrong!

a graphic of a tunnel in outer space with eight analog alarm clocks appearing to tumb;e through the tunnel

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

Disclosure statement

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

University of Maryland, Baltimore County provides funding as a member of The Conversation US.

View all partners

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.

Time travel
Special Relativity
Thermodynamics
Stephen Hawking
Curious Kids
Curious Kids US
Time travel paradox

Head of School, School of Arts & Social Sciences, Monash University Malaysia

Chief Operating Officer (COO)

Clinical Teaching Fellow

Data Manager

Director, Social Policy

University of Notre Dame

Notre Dame Philosophical Reviews

Home ›
Reviews ›

Paradoxes of Time Travel

Ryan Wasserman, Paradoxes of Time Travel , Oxford University Press, 2018, 240pp., $60.00, ISBN 9780198793335.

Reviewed by John W. Carroll, North Carolina State

Wasserman's book fills a gap in the academic literature on time travel. The gap was hidden among the journal articles on time travel written by physicists for physicists, the popular books on time travel by physicists for the curious folk, the books on the history of time travel in science fiction intended for a range of scholarly audiences, and the journal articles on time travel written for and by metaphysicians and philosophers of science. There are metaphysics books on time that give some attention to time travel, but, as far as I know, this is the first book length work devoted to the topic of time travel by a metaphysician homed in on the most important metaphysical issues. Wasserman addresses these issues while still managing to include pertinent scientific discussion and enjoyable time-travel snippets from science fiction. The book is well organized and is suitable for good undergraduate metaphysics students, for philosophy graduate students, and for professional philosophers. It reads like a sophisticated and excellent textbook even though it includes many novel ideas.

The research Wasserman has done is impressive. It reminds the reader that time travel as a topic of metaphysics did not start with David Lewis (1976). Wasserman (p. 2 n 4) identifies Walter B. Pitkin's 1914 journal article as (probably) the first academic discussion of time travel. The article includes a description of what has come to be called the double-occupancy problem, a puzzle about spatial location and time machines that trace a continuous path through space. The same note also includes a lovely passage, which anticipates paradoxes about changing the past, from Enrique Gaspar's 1887 book:

We may unwrap time but we don't know how to nullify it. If today is a consequence of yesterday and we are living examples of the present, we cannot unless we destroy ourselves, wipe out a cause of which we are the actual effects.

These are just two of the many useful bits of Wasserman's research.

Chapter 1 usefully introduces examples of time travel and some examples one might think would involve time travel, but do not (e.g., changing time zones). There is good discussion of Lewis's definition of time travel as a discrepancy between personal and external time, including a brief passage (p. 13) from a previously unpublished letter from Lewis to Jonathan Bennett on whether freezing and thawing is time travel. I had often wonder what Lewis would have said; now I know what he did say!

Chapter 2 dives into temporal paradoxes deriving from discussions of the status of tense and the ontology of time (presentism vs. eternalism vs. growing block vs. . . . ). Here, Wasserman also includes the double-occupancy problem as a problem for eternalism -- though it is not clear that it is only a problem for eternalism. Then he turns to the question of the compatibility of presentism and time travel, the compatibility of time travel and a version of growing block that accepts that there are no future-tensed truths, and finally to a section on relativity and time travel. The section on relativity is solid and seems to me to pull the rug out from under some earlier discussions. For example, Lewis's definition of time travel is shown not to work. It also becomes clear that presentism and the growing block are consistent with both time-dilation-style forward time travel and traveling-in-a-curved-spacetime "backwards" time travel.

Chapters 3 and 4 cover the granddaddies of all the time-travel paradoxes: the freedom paradoxes that include the grandfather paradox, the possibility of changing the past, and the prospects of such changes given models of branching time, models that invoke parallel worlds, and hyper time models. Chapter 4 gets serious about Lewis's treatment of the grandfather paradox and Kadri Vihvelin's treatment of the autoinfanticide paradox (about which I will have more to say).

Chapter 4 also includes discussion of "mechanical" paradoxes that, as stated, do not require modal premises about what something can and cannot do, and no notion of freedom or free will. (See Earman's bilking argument on p. 139 and the Polchinski paradox on p. 141.) Wasserman introduces modality to these paradoxes, but I would have liked them to be addressed on their own terms. As I see it, these paradoxes are introduced to show that backwards time travel or backwards causation in a certain situation validly lead to a contradiction. On their own terms, for these arguments to be valid, the premises of the arguments themselves must be inconsistent. How can one make trouble for backwards time travel if the argument is thus bound to be unsound?

Chapter 5 takes on the paradoxes generated by causal loops or more generally backwards causation including bilking arguments, the boot-strapping paradox (based on a presumption that self-causation is impossible), and the ex nihilo paradox with causal loops and object loops (i.e., jinn) that seem to have no cause or explanation.

Chapter 6 deals with paradoxes that arise from considerations regarding identity, with a focus on the self-visitation paradox from both perdurantist and endurantist perspectives. I was surprised to learn that Wasserman had defended an endurantist-friendly property compatibilism -- similar to my own -- to resolve the self-visitation paradox. I was then delighted to find out that he cleverly extends this sort of compatibilism to the time-travel-free problem of change (i.e., the so-called, temporary-intrinsics argument).

The outstanding scientific issue regarding backwards time travel is whether it is physically possible. There is no question that forwards time travel is actual, or even whether it is ubiquitous. There is also not much question that backwards time travel is consistent with general relativity. Still, we await more scientific progress before we will know whether backwards time travel really is consistent with the actual laws of nature. In the meantime, there is still much to be said about Lewis's treatment of the grandfather paradox and Vihvelin's stated challenge to that treatment in terms of the autoinfanticide paradox.

I will start by being somewhat critical of Lewis's approach. For his part (pp. 108-114), Wasserman does a terrific job of laying out Lewis's position as a metatheoretic discussion of the context sensitivity of 'can' and 'can't'. My concern is that not enough attention is given to the 'can' and 'can't' sentences that turn out true on the semantics. The semantics works only by a contextual restriction of possible worlds based on relevant facts -- the modal base -- associated with a conversational context. In meager contexts, false 'can' sentences will turn out true too easily. For example, suppose two people are having a conversation about Roger. Maybe all the two know about Roger is his name and that he is moving into the neighborhood. So, the proposition that Roger doesn't play the piano is not in the modal base. So, according to Lewis's semantics applied to 'can', 'Roger can play the piano' is true in this context. That seems wrong. This would be an unwarranted assertion for either of the participants in the conversation to make. Notice it is also true relative to the same meager context that Roger can play the harpsichord, the sousaphone, and the nyatiti. Quite a musician that Roger! [1]

Interestingly, though this problem arises for 'can', it does not arise for other "possibility" modals. For example, notice that, with the meager context described above, there is a big difference regarding the assertability of 'Roger could play the piano' and of 'Roger can play the piano'. Similarly, there is also no serious issue with regard to 'Roger might play the piano'. 'Could' and 'might' add tentativeness to the assertion that seems called for. There also seems to be no problem for the semantics insofar as it applies to 'is possible'. 'It is possible that Roger plays the piano' rings true relative to the context. But 'Roger can play the piano'? That shouldn't turn out true, especially if Roger is physically or psychologically unsuited for piano playing.

This issue has been frustrating for me, but Wasserman's book has me leaning toward the idea that what is needed is a contextual semantics that includes a distinguishing conditional treatment of 'can' of the sort Wasserman suggests:

(P1**) Necessarily, if someone would fail to do something no matter what she tried, then she cannot do it (p. 122).

This is a suggestion made by Wasserman on behalf of Vihvelin. I find (P1**) as a promising place to start in terms of the conditional treatment.

Speaking of Vihvelin, her thesis is "that no time traveler can kill the baby that in fact is her younger self, given what we ordinarily mean by 'can'" (1996, pp. 316-317). Vihvelin cites Paul Horwich as a defender of a can-kill solution, what she calls the standard reply :

The standard reply . . . goes something like this: Of course the time traveler . . . will not kill the baby who is her younger self . . . But that doesn't mean she can't . (Vihvelin 1996, p. 315)

Vihvelin's doing so is appropriate given what Horwich says about Charles attending the Battle of Hastings: "From the fact that someone did not do something it does not follow that he was not free to do it" (1975, 435). In contrast, it strikes me as odd that Vihvelin (1996, p. 329, fn. 1) also attributes the standard reply to Lewis. I presume that she does so based on some comments by Lewis. He says, "By any ordinary standards of ability , Tim can kill Grandfather," (1976, p. 150, my emphasis) and especially "what, in an ordinary sense , I can do" (1976, p. 151, my emphasis). So, admittedly, Vihvelin fairly highlights an aspect of Lewis's view as holding that, in the ordinary sense of 'can', Tim can kill Gramps. And I can see how this is a useful presentation of Lewis's position for her argumentative purposes.

Nevertheless, I take Lewis's talk of ordinary standards or an ordinary sense to just be a way to identify the ordinary contexts that arise with uses of 'can' in day-to-day dealings, where the possibility of time travel is not even on the table. Simple stuff like:

Hey, can you reach the pencil that fell on the floor?

Sure I can; here it is.

More importantly, we have to keep in mind that the basic semantics only has consequences about the truth of 'can' sentences once a modal base is in place. To me, the fact that Baby Suzy grows up to be Suzy is exactly the kind of fact that we do not ordinarily hold fixed. Lewis's commitment to the semantics does not make him either a can-kill guy or a can't-kill guy.

What is the upshot of this? There is a bit of underappreciation of Lewis's approach in Wasserman's discussion of Vihvelin's views. The pinching case on p. 119 provides a way to make the point. Consider:

(a) If Suzy were to try to kill Baby Suzy, then she would fail.

(b) If Suzy were to try to pinch Baby Suzy, then she would fail.

According to Wasserman, Vihvelin thinks that even in ordinary contexts (a) and (b) come apart (p. 119, note 32) -- (a) is true and (b) is false. As I see it, a natural context for (a) includes the fact that Baby Suzy grows up normally to be Suzy. That is a supposition that is crucial to the description of the scenario and so is likely to be part of the modal base. No canonical story or suppositions are tied to (b), though Vihvelin stipulates that Suzy travels back in time in both cases. We are not, however, told a story of Baby Suzy living a pinch-free life all the way to adulthood. We are not told whether Suzy decided go back in time because Baby Suzy deserved a pinch for some past transgression. My point is that the stories affect the context. So, with parallel background stories, (a) and (b) need not come apart.

I am not sure whether Wasserman was speaking for himself or for Vihvelin when he says about (a) and (b), "Self-defeating acts are paradoxical in a way other past-altering acts are not" (p. 120). Either way, I disagree. Lewis gives a more general way to resolve the past-alteration paradoxes that is not obviously in any serious conflict with Vihvelin's many utterances that turn out true relative to the contexts in which she asserts them. Wasserman also says, "The only disagreement between Lewis and Vihvelin is over whether Suzy's killing Baby Suzy is compatible with the kinds of facts we normally take as relevant in determining what someone can do" (p. 117). That is an odd thing for him to say. Lewis sketches a semantic theory that provides a framework for the truth conditions of 'can' and 'can't' sentences. He is not in disagreement with Vihvelin. For Lewis, there is one specification of truth conditions for 'can' that gives rise to both 'can kill' and 'can't kill' sentences turning out true relative to different contexts. Indeed, it is tempting to think that Vihvelin takes the fact that Baby Suzy grows up to be Adult Suzy as part of the modal base of the contexts from which she asserts the compelling 'can't-kill' sentences.

That all said, Wasserman's book is a significant contribution. There are those of us who focus a good chunk of our research on the paradoxes of time travel for their intrinsic interest, and especially because they are fun to teach. That is contribution enough for me. But, ultimately, from this somewhat esoteric, fun puzzle solving, we also learn more about the rest of metaphysics. The traditional issues of metaphysics: identity-over-time, freedom and determinism, causation, time and space, counterfactuals, personhood, mereology, and so on, all take on a new look when framed by the questions of whether time travel is possible and what time travel is or would be like. Wasserman's book is a wonderful source that spotlights these connections between the paradoxes of time travel and more traditional metaphysical issues.

Cargile, J., 1996. "Some Comments on Fatalism" The Philosophical Quarterly 46, No. 182 January 1996, 1-11.

Gaspar, E., 1887/2012. The Time-Ship: A Chronological Journey . Wesleyan University Press.

Horwich, P., 1975. "On Some Alleged Paradoxes of Time Travel" The Journal of Philosophy 72, 432-444.

Lewis, D., 1976 "The Paradoxes of Time Travel" American Philosophical Quarterly 13, 145-152.

Pitkin, W., 1914. "Time and Pure Activity" Journal of Philosophy, Psychology and Scientific Methods 11, 521-526.

Vihvelin, K., 1996. "What a Time Traveler Cannot Do" Philosophical Studies 81, 315-330.

[1] This criticism was first presented to me by Natalja Deng in the question-and-answer period for a presentation at the 2014 Philosophy of Time Society Conference. Later on, I found a parallel challenge in work by James Cargile (1996, 10-11) about Lewis's iconic, 'The ape can't speak Finnish, but I can'.

Search Menu

Sign in through your institution

Browse content in Arts and Humanities
Browse content in Archaeology
Anglo-Saxon and Medieval Archaeology
Archaeological Methodology and Techniques
Archaeology by Region
Archaeology of Religion
Archaeology of Trade and Exchange
Biblical Archaeology
Contemporary and Public Archaeology
Environmental Archaeology
Historical Archaeology
History and Theory of Archaeology
Industrial Archaeology
Landscape Archaeology
Mortuary Archaeology
Prehistoric Archaeology
Underwater Archaeology
Zooarchaeology
Browse content in Architecture
Architectural Structure and Design
History of Architecture
Residential and Domestic Buildings
Theory of Architecture
Browse content in Art
Art Subjects and Themes
History of Art
Industrial and Commercial Art
Theory of Art
Biographical Studies
Byzantine Studies
Browse content in Classical Studies
Classical History
Classical Philosophy
Classical Mythology
Classical Literature
Classical Reception
Classical Art and Architecture
Classical Oratory and Rhetoric
Greek and Roman Papyrology
Greek and Roman Epigraphy
Greek and Roman Law
Greek and Roman Archaeology
Late Antiquity
Religion in the Ancient World
Digital Humanities
Browse content in History
Colonialism and Imperialism
Diplomatic History
Environmental History
Genealogy, Heraldry, Names, and Honours
Genocide and Ethnic Cleansing
Historical Geography
History by Period
History of Emotions
History of Agriculture
History of Education
History of Gender and Sexuality
Industrial History
Intellectual History
International History
Labour History
Legal and Constitutional History
Local and Family History
Maritime History
Military History
National Liberation and Post-Colonialism
Oral History
Political History
Public History
Regional and National History
Revolutions and Rebellions
Slavery and Abolition of Slavery
Social and Cultural History
Theory, Methods, and Historiography
Urban History
World History
Browse content in Language Teaching and Learning
Language Learning (Specific Skills)
Language Teaching Theory and Methods
Browse content in Linguistics
Applied Linguistics
Cognitive Linguistics
Computational Linguistics
Forensic Linguistics
Grammar, Syntax and Morphology
Historical and Diachronic Linguistics
History of English
Language Evolution
Language Reference
Language Acquisition
Language Variation
Language Families
Lexicography
Linguistic Anthropology
Linguistic Theories
Linguistic Typology
Phonetics and Phonology
Psycholinguistics
Sociolinguistics
Translation and Interpretation
Writing Systems
Browse content in Literature
Bibliography
Children's Literature Studies
Literary Studies (Romanticism)
Literary Studies (American)
Literary Studies (Asian)
Literary Studies (European)
Literary Studies (Eco-criticism)
Literary Studies (Modernism)
Literary Studies - World
Literary Studies (1500 to 1800)
Literary Studies (19th Century)
Literary Studies (20th Century onwards)
Literary Studies (African American Literature)
Literary Studies (British and Irish)
Literary Studies (Early and Medieval)
Literary Studies (Fiction, Novelists, and Prose Writers)
Literary Studies (Gender Studies)
Literary Studies (Graphic Novels)
Literary Studies (History of the Book)
Literary Studies (Plays and Playwrights)
Literary Studies (Poetry and Poets)
Literary Studies (Postcolonial Literature)
Literary Studies (Queer Studies)
Literary Studies (Science Fiction)
Literary Studies (Travel Literature)
Literary Studies (War Literature)
Literary Studies (Women's Writing)
Literary Theory and Cultural Studies
Mythology and Folklore
Shakespeare Studies and Criticism
Browse content in Media Studies
Browse content in Music
Applied Music
Dance and Music
Ethics in Music
Ethnomusicology
Gender and Sexuality in Music
Medicine and Music
Music Cultures
Music and Media
Music and Religion
Music and Culture
Music Education and Pedagogy
Music Theory and Analysis
Musical Scores, Lyrics, and Libretti
Musical Structures, Styles, and Techniques
Musicology and Music History
Performance Practice and Studies
Race and Ethnicity in Music
Sound Studies
Browse content in Performing Arts
Browse content in Philosophy
Aesthetics and Philosophy of Art
Epistemology
Feminist Philosophy
History of Western Philosophy
Metaphysics
Moral Philosophy
Non-Western Philosophy
Philosophy of Language
Philosophy of Mind
Philosophy of Perception
Philosophy of Science
Philosophy of Action
Philosophy of Law
Philosophy of Religion
Philosophy of Mathematics and Logic
Practical Ethics
Social and Political Philosophy
Browse content in Religion
Biblical Studies
Christianity
East Asian Religions
History of Religion
Judaism and Jewish Studies
Qumran Studies
Religion and Education
Religion and Health
Religion and Politics
Religion and Science
Religion and Law
Religion and Art, Literature, and Music
Religious Studies
Browse content in Society and Culture
Cookery, Food, and Drink
Cultural Studies
Customs and Traditions
Ethical Issues and Debates
Hobbies, Games, Arts and Crafts
Natural world, Country Life, and Pets
Popular Beliefs and Controversial Knowledge
Sports and Outdoor Recreation
Technology and Society
Travel and Holiday
Visual Culture
Browse content in Law
Arbitration
Browse content in Company and Commercial Law
Commercial Law
Company Law
Browse content in Comparative Law
Systems of Law
Competition Law
Browse content in Constitutional and Administrative Law
Government Powers
Judicial Review
Local Government Law
Military and Defence Law
Parliamentary and Legislative Practice
Construction Law
Contract Law
Browse content in Criminal Law
Criminal Procedure
Criminal Evidence Law
Sentencing and Punishment
Employment and Labour Law
Environment and Energy Law
Browse content in Financial Law
Banking Law
Insolvency Law
History of Law
Human Rights and Immigration
Intellectual Property Law
Browse content in International Law
Private International Law and Conflict of Laws
Public International Law
IT and Communications Law
Jurisprudence and Philosophy of Law
Law and Politics
Law and Society
Browse content in Legal System and Practice
Courts and Procedure
Legal Skills and Practice
Primary Sources of Law
Regulation of Legal Profession
Medical and Healthcare Law
Browse content in Policing
Criminal Investigation and Detection
Police and Security Services
Police Procedure and Law
Police Regional Planning
Browse content in Property Law
Personal Property Law
Study and Revision
Terrorism and National Security Law
Browse content in Trusts Law
Wills and Probate or Succession
Browse content in Medicine and Health
Browse content in Allied Health Professions
Arts Therapies
Clinical Science
Dietetics and Nutrition
Occupational Therapy
Operating Department Practice
Physiotherapy
Radiography
Speech and Language Therapy
Browse content in Anaesthetics
General Anaesthesia
Neuroanaesthesia
Clinical Neuroscience
Browse content in Clinical Medicine
Acute Medicine
Cardiovascular Medicine
Clinical Genetics
Clinical Pharmacology and Therapeutics
Dermatology
Endocrinology and Diabetes
Gastroenterology
Genito-urinary Medicine
Geriatric Medicine
Infectious Diseases
Medical Toxicology
Medical Oncology
Pain Medicine
Palliative Medicine
Rehabilitation Medicine
Respiratory Medicine and Pulmonology
Rheumatology
Sleep Medicine
Sports and Exercise Medicine
Community Medical Services
Critical Care
Emergency Medicine
Forensic Medicine
Haematology
History of Medicine
Browse content in Medical Skills
Clinical Skills
Communication Skills
Nursing Skills
Surgical Skills
Browse content in Medical Dentistry
Oral and Maxillofacial Surgery
Paediatric Dentistry
Restorative Dentistry and Orthodontics
Surgical Dentistry
Medical Ethics
Medical Statistics and Methodology
Browse content in Neurology
Clinical Neurophysiology
Neuropathology
Nursing Studies
Browse content in Obstetrics and Gynaecology
Gynaecology
Occupational Medicine
Ophthalmology
Otolaryngology (ENT)
Browse content in Paediatrics
Neonatology
Browse content in Pathology
Chemical Pathology
Clinical Cytogenetics and Molecular Genetics
Histopathology
Medical Microbiology and Virology
Patient Education and Information
Browse content in Pharmacology
Psychopharmacology
Browse content in Popular Health
Caring for Others
Complementary and Alternative Medicine
Self-help and Personal Development
Browse content in Preclinical Medicine
Cell Biology
Molecular Biology and Genetics
Reproduction, Growth and Development
Primary Care
Professional Development in Medicine
Browse content in Psychiatry
Addiction Medicine
Child and Adolescent Psychiatry
Forensic Psychiatry
Learning Disabilities
Old Age Psychiatry
Psychotherapy
Browse content in Public Health and Epidemiology
Epidemiology
Public Health
Browse content in Radiology
Clinical Radiology
Interventional Radiology
Nuclear Medicine
Radiation Oncology
Reproductive Medicine
Browse content in Surgery
Cardiothoracic Surgery
Gastro-intestinal and Colorectal Surgery
General Surgery
Neurosurgery
Paediatric Surgery
Peri-operative Care
Plastic and Reconstructive Surgery
Surgical Oncology
Transplant Surgery
Trauma and Orthopaedic Surgery
Vascular Surgery
Browse content in Science and Mathematics
Browse content in Biological Sciences
Aquatic Biology
Biochemistry
Bioinformatics and Computational Biology
Developmental Biology
Ecology and Conservation
Evolutionary Biology
Genetics and Genomics
Microbiology
Molecular and Cell Biology
Natural History
Plant Sciences and Forestry
Research Methods in Life Sciences
Structural Biology
Systems Biology
Zoology and Animal Sciences
Browse content in Chemistry
Analytical Chemistry
Computational Chemistry
Crystallography
Environmental Chemistry
Industrial Chemistry
Inorganic Chemistry
Materials Chemistry
Medicinal Chemistry
Mineralogy and Gems
Organic Chemistry
Physical Chemistry
Polymer Chemistry
Study and Communication Skills in Chemistry
Theoretical Chemistry
Browse content in Computer Science
Artificial Intelligence
Computer Architecture and Logic Design
Game Studies
Human-Computer Interaction
Mathematical Theory of Computation
Programming Languages
Software Engineering
Systems Analysis and Design
Virtual Reality
Browse content in Computing
Business Applications
Computer Security
Computer Games
Computer Networking and Communications
Digital Lifestyle
Graphical and Digital Media Applications
Operating Systems
Browse content in Earth Sciences and Geography
Atmospheric Sciences
Environmental Geography
Geology and the Lithosphere
Maps and Map-making
Meteorology and Climatology
Oceanography and Hydrology
Palaeontology
Physical Geography and Topography
Regional Geography
Soil Science
Urban Geography
Browse content in Engineering and Technology
Agriculture and Farming
Biological Engineering
Civil Engineering, Surveying, and Building
Electronics and Communications Engineering
Energy Technology
Engineering (General)
Environmental Science, Engineering, and Technology
History of Engineering and Technology
Mechanical Engineering and Materials
Technology of Industrial Chemistry
Transport Technology and Trades
Browse content in Environmental Science
Applied Ecology (Environmental Science)
Conservation of the Environment (Environmental Science)
Environmental Sustainability
Environmentalist Thought and Ideology (Environmental Science)
Management of Land and Natural Resources (Environmental Science)
Natural Disasters (Environmental Science)
Nuclear Issues (Environmental Science)
Pollution and Threats to the Environment (Environmental Science)
Social Impact of Environmental Issues (Environmental Science)
History of Science and Technology
Browse content in Materials Science
Ceramics and Glasses
Composite Materials
Metals, Alloying, and Corrosion
Nanotechnology
Browse content in Mathematics
Applied Mathematics
Biomathematics and Statistics
History of Mathematics
Mathematical Education
Mathematical Finance
Mathematical Analysis
Numerical and Computational Mathematics
Probability and Statistics
Pure Mathematics
Browse content in Neuroscience
Cognition and Behavioural Neuroscience
Development of the Nervous System
Disorders of the Nervous System
History of Neuroscience
Invertebrate Neurobiology
Molecular and Cellular Systems
Neuroendocrinology and Autonomic Nervous System
Neuroscientific Techniques
Sensory and Motor Systems
Browse content in Physics
Astronomy and Astrophysics
Atomic, Molecular, and Optical Physics
Biological and Medical Physics
Classical Mechanics
Computational Physics
Condensed Matter Physics
Electromagnetism, Optics, and Acoustics
History of Physics
Mathematical and Statistical Physics
Measurement Science
Nuclear Physics
Particles and Fields
Plasma Physics
Quantum Physics
Relativity and Gravitation
Semiconductor and Mesoscopic Physics
Browse content in Psychology
Affective Sciences
Clinical Psychology
Cognitive Psychology
Cognitive Neuroscience
Criminal and Forensic Psychology
Developmental Psychology
Educational Psychology
Evolutionary Psychology
Health Psychology
History and Systems in Psychology
Music Psychology
Neuropsychology
Organizational Psychology
Psychological Assessment and Testing
Psychology of Human-Technology Interaction
Psychology Professional Development and Training
Research Methods in Psychology
Social Psychology
Browse content in Social Sciences
Browse content in Anthropology
Anthropology of Religion
Human Evolution
Medical Anthropology
Physical Anthropology
Regional Anthropology
Social and Cultural Anthropology
Theory and Practice of Anthropology
Browse content in Business and Management
Business Ethics
Business Strategy
Business History
Business and Technology
Business and Government
Business and the Environment
Comparative Management
Corporate Governance
Corporate Social Responsibility
Entrepreneurship
Health Management
Human Resource Management
Industrial and Employment Relations
Industry Studies
Information and Communication Technologies
International Business
Knowledge Management
Management and Management Techniques
Operations Management
Organizational Theory and Behaviour
Pensions and Pension Management
Public and Nonprofit Management
Strategic Management
Supply Chain Management
Browse content in Criminology and Criminal Justice
Criminal Justice
Criminology
Forms of Crime
International and Comparative Criminology
Youth Violence and Juvenile Justice
Development Studies
Browse content in Economics
Agricultural, Environmental, and Natural Resource Economics
Asian Economics
Behavioural Finance
Behavioural Economics and Neuroeconomics
Econometrics and Mathematical Economics
Economic History
Economic Systems
Economic Methodology
Economic Development and Growth
Financial Markets
Financial Institutions and Services
General Economics and Teaching
Health, Education, and Welfare
History of Economic Thought
International Economics
Labour and Demographic Economics
Law and Economics
Macroeconomics and Monetary Economics
Microeconomics
Public Economics
Urban, Rural, and Regional Economics
Welfare Economics
Browse content in Education
Adult Education and Continuous Learning
Care and Counselling of Students
Early Childhood and Elementary Education
Educational Equipment and Technology
Educational Strategies and Policy
Higher and Further Education
Organization and Management of Education
Philosophy and Theory of Education
Schools Studies
Secondary Education
Teaching of a Specific Subject
Teaching of Specific Groups and Special Educational Needs
Teaching Skills and Techniques
Browse content in Environment
Applied Ecology (Social Science)
Climate Change
Conservation of the Environment (Social Science)
Environmentalist Thought and Ideology (Social Science)
Natural Disasters (Environment)
Social Impact of Environmental Issues (Social Science)
Browse content in Human Geography
Cultural Geography
Economic Geography
Political Geography
Browse content in Interdisciplinary Studies
Communication Studies
Museums, Libraries, and Information Sciences
Browse content in Politics
African Politics
Asian Politics
Chinese Politics
Comparative Politics
Conflict Politics
Elections and Electoral Studies
Environmental Politics
Ethnic Politics
European Union
Foreign Policy
Gender and Politics
Human Rights and Politics
Indian Politics
International Relations
International Organization (Politics)
International Political Economy
Irish Politics
Latin American Politics
Middle Eastern Politics
Political Behaviour
Political Economy
Political Institutions
Political Methodology
Political Communication
Political Philosophy
Political Sociology
Political Theory
Politics and Law
Politics of Development
Public Policy
Public Administration
Quantitative Political Methodology
Regional Political Studies
Russian Politics
Security Studies
State and Local Government
UK Politics
US Politics
Browse content in Regional and Area Studies
African Studies
Asian Studies
East Asian Studies
Japanese Studies
Latin American Studies
Middle Eastern Studies
Native American Studies
Scottish Studies
Browse content in Research and Information
Research Methods
Browse content in Social Work
Addictions and Substance Misuse
Adoption and Fostering
Care of the Elderly
Child and Adolescent Social Work
Couple and Family Social Work
Direct Practice and Clinical Social Work
Emergency Services
Human Behaviour and the Social Environment
International and Global Issues in Social Work
Mental and Behavioural Health
Social Justice and Human Rights
Social Policy and Advocacy
Social Work and Crime and Justice
Social Work Macro Practice
Social Work Practice Settings
Social Work Research and Evidence-based Practice
Welfare and Benefit Systems
Browse content in Sociology
Childhood Studies
Community Development
Comparative and Historical Sociology
Economic Sociology
Gender and Sexuality
Gerontology and Ageing
Health, Illness, and Medicine
Marriage and the Family
Migration Studies
Occupations, Professions, and Work
Organizations
Population and Demography
Race and Ethnicity
Social Theory
Social Movements and Social Change
Social Research and Statistics
Social Stratification, Inequality, and Mobility
Sociology of Religion
Sociology of Education
Sport and Leisure
Urban and Rural Studies
Browse content in Warfare and Defence
Defence Strategy, Planning, and Research
Land Forces and Warfare
Military Administration
Military Life and Institutions
Naval Forces and Warfare
Other Warfare and Defence Issues
Peace Studies and Conflict Resolution
Weapons and Equipment

< Previous chapter
Next chapter >

2 Temporal Paradoxes

Published: November 2017
Cite Icon Cite
Permissions Icon Permissions

Chapter 2 surveys the various theories of time and explores their consequences for the possibility of time travel. Section 1 introduces the traditional debates over tense and distinguishes between three different views of temporal ontology: eternalism, presentism, and the growing block theory. Section 2 discusses eternalism and the double-occupancy paradox. Section 3 focuses on presentism and various versions of the “no destination” objection. Section 4 looks at the growing block theory and the worry that time travel would allow for future indeterminacy to creep back into the past. Finally, sections 5 and 6 look at the special and general theories of relativity and consider their implications for our understanding of time travel.

Signed in as

Institutional accounts.

Google Scholar Indexing
GoogleCrawler [DO NOT DELETE]

Personal account

Sign in with email/username & password
Get email alerts
Save searches
Purchase content
Activate your purchase/trial code
Add your ORCID iD

Institutional access

Sign in with username/password
Recommend to your librarian
Institutional account management
Get help with access

Access to content on Oxford Academic is often provided through institutional subscriptions and purchases. If you are a member of an institution with an active account, you may be able to access content in one of the following ways:

IP based access

Typically, access is provided across an institutional network to a range of IP addresses. This authentication occurs automatically, and it is not possible to sign out of an IP authenticated account.

Choose this option to get remote access when outside your institution. Shibboleth/Open Athens technology is used to provide single sign-on between your institutionâ€™s website and Oxford Academic.

Click Sign in through your institution.
Select your institution from the list provided, which will take you to your institution's website to sign in.
When on the institution site, please use the credentials provided by your institution. Do not use an Oxford Academic personal account.
Following successful sign in, you will be returned to Oxford Academic.

If your institution is not listed or you cannot sign in to your institutionâ€™s website, please contact your librarian or administrator.

Enter your library card number to sign in. If you cannot sign in, please contact your librarian.

Society Members

Society member access to a journal is achieved in one of the following ways:

Sign in through society site

Many societies offer single sign-on between the society website and Oxford Academic. If you see â€˜Sign in through society siteâ€™ in the sign in pane within a journal:

Click Sign in through society site.
When on the society site, please use the credentials provided by that society. Do not use an Oxford Academic personal account.

If you do not have a society account or have forgotten your username or password, please contact your society.

Sign in using a personal account

Some societies use Oxford Academic personal accounts to provide access to their members. See below.

A personal account can be used to get email alerts, save searches, purchase content, and activate subscriptions.

Some societies use Oxford Academic personal accounts to provide access to their members.

Viewing your signed in accounts

Click the account icon in the top right to:

View your signed in personal account and access account management features.
View the institutional accounts that are providing access.

Signed in but can't access content

Oxford Academic is home to a wide variety of products. The institutional subscription may not cover the content that you are trying to access. If you believe you should have access to that content, please contact your librarian.

For librarians and administrators, your personal account also provides access to institutional account management. Here you will find options to view and activate subscriptions, manage institutional settings and access options, access usage statistics, and more.

Our books are available by subscription or purchase to libraries and institutions.

About Oxford Academic
Publish journals with us
University press partners
What we publish
New features
Open access
Rights and permissions
Accessibility
Advertising
Media enquiries
Oxford University Press
Oxford Languages
University of Oxford

Oxford University Press is a department of the University of Oxford. It furthers the University's objective of excellence in research, scholarship, and education by publishing worldwide

Copyright © 2024 Oxford University Press
Cookie settings
Cookie policy
Privacy policy
Legal notice

This Feature Is Available To Subscribers Only

This PDF is available to Subscribers Only

For full access to this pdf, sign in to an existing account, or purchase an annual subscription.

Screen Rant

10 best movies about time travel paradoxes.

Your changes have been saved

Email Is sent

Please verify your email address.

You’ve reached your account maximum for followed topics.

The One Problem Sci-Fi Time Travel Movies NEVER Resolve

Back to the future writer explains marty’s parents plot hole, dave bautista's new assassin movie is the anti-john wick the world needs right now.

Time travel movies often contain paradoxes, creating confusion for viewers. Each film's unique rules and characters' reactions to those paradoxes shape the plot.
Different types of paradoxes exist, such as bootstrap, predestination, and temporal paradoxes, which add depth and complexity to time travel movies.
Despite the presence of paradoxes, time travel movies can still be entertaining and thought-provoking, providing great storytelling and exploration of love, fate, and the concept of free will.

Time travel can't exist without paradoxes, and neither can science fiction movies about time travel. The way time travel works in each individual film is the first thing a director needs to think through to make sure their movie is consistent and viewers aren't left scratching their heads afterward. Unfortunately, time travel is a tricky subject, and specific rules do not guarantee the absence of paradoxes. The only difference between all time travel movies is that some characters acknowledge the paradox and try to do something about it, and others just ignore its existence and proceed with their goal no matter what.

There are quite a few types of time paradoxes. For instance, a bootstrap paradox is about information or objects that seemingly have no starting point in their timeline; a predestination paradox centers on the cause of someone's time travel being of their own doing in the past; and a temporal paradox revolves around someone's actions in the past that remove the necessity to time travel in the first place. From Interstellar to About Time , time travel movies are riddled with paradoxes , sometimes for the better, providing a great story, and sometimes for the worse, confusing anyone who tries to follow the plot.

Almost every Sci-Fi time travel movie runs into the causal loop paradox, but not all films deal with the logic of time travel in the same way.

10 Interstellar

Cooper gives himself the idea of contacting murth, interstellar.

Christopher Nolan's movies are largely regarded as sci-fi masterpieces, and Interstellar is no exception. The movie's main mystery, the identity of the ghost, was based on a time paradox. At the beginning of the movie, a book fell out of a shelf on its own, and Interstellar 's surprising ending revealed that it was Cooper who made the book fall out via the Tesseract mechanism to send his past self a message . However, Cooper just did what he'd already seen happen, so the concept raises the question of who originally thought of sending the message in this way. Still, this mind-bending time loop worked against all odds.

9 The Terminator

Kyle reese is john connor's father.

James Cameron's epic sci-fi tale is a classic example of a predestination paradox. In The Terminator , Kyle Reese arrived from the future to stop the Terminator from killing Sarah Connor , the mother of his colleague John. Unknowingly, Kyle ended up fathering John when he developed a romantic relationship with Sarah. If the Terminator hadn't been sent to kill Sarah, and Kyle hadn't followed him, John wouldn't have been born, since his father wouldn't have traveled to the past and met his mother. The Terminator 's paradox ending was controversial, and yet the movie managed to make the story entertaining enough to look past it.

8 The Time Traveler's Wife

Henry & clare meet out of order.

The Time Traveler's Wife explores a beautiful notion that love can transcend any boundaries — apparently, that includes the boundaries of time. The movie didn't pretend to be a serious sci-fi title, but it was essentially based on a paradox, specifically, the incorrect order, in which Henry met Clare. Henry first saw Clare when he time traveled to 1991 , but she already knew him because she had met Henry when she was but a child. That is a confusing concept that raises two questions: when their first meeting took place and how they ended up together at all. The Time Traveler's Wife' s paradoxical love story is endearing nevertheless.

7 The Butterfly Effect

Evan causes his own blackouts, the butterfly effect.

The Butterfly Effect is one of the most mind-blowing time travel movies out there, partly because the rules are very specific, and yet they make no sense whatsoever. The movie featured quite a few time paradoxes, but the biggest one was probably the existence of Evan's blackouts. Young Evan experienced blackouts, caused by his adult self's time travel ; adult Evan had to travel to his past because he knew that he was supposed to cause these blackouts. It is unclear how blackouts appeared in the first place. This plot detail makes The Butterfly Effect 's understanding of time circular rather than linear, but the paradox is still there.

6 About Time

Tim prevents the car crash.

About Time 's central point was Tim going back to the past to prevent the car crash, as this resulted in the erasure of his daughter Posy from existence. Although the film is full of inconsistencies, this event in particular showcases the classic paradox of time travel movies — if the car crash motivated Tim to go back in time to prevent it , then in doing so, he erased the very reason for him to travel to the past. Still, Domhnall Gleeson and accidental time-travel expert Rachel McAdams make up a dynamic duo, and it is impossible not to feel for Tim's struggle to help everyone through his gift.

The Protagonist Founds Tenet

The ending of Tenet , Christopher Nolan's sci-fi follow-up to Interstellar , turned out to be even more confusing than that of its predecessor, and not just because of the inverted entropy concept. The entire plot wouldn't exist if the Protagonist hadn't founded the mysterious organization Tenet that helped him in the first place and led him to create it in the future. Tenet explores the notion of a person's future and past intertwining and being part of the same time loop, with no one able to tell what the original cause of the event was. The Protagonist's survival in the film depended solely on himself from the future, who apparently knew that his past self once needed saving.

4 Back to the Future

Marty mcfly has to bring his parents together, back to the future.

Back to the Future is the movie that started the time travel film craze in the first place, and it features one of the most well-known paradoxes in the genre. When Marty traveled to the past, he saved George's life by preventing a car accident, but in the aftermath of the events, he accidentally jeopardized his own existence and had to make his parents fall in love with each other all over again. However, since there was a possibility that Marty would never be born , he should have disintegrated right then and there in the 1950s before he had a chance to fix his own timeline.

Back to the Future co-writer Bob Gale explains one of the movie's perceived plot holes, concerning Marty's parents not recognizing him at the end.

3 Donnie Darko

The plane comes out of nowhere, donnie darko.

Donnie Darko is a stunning dark tale with a timeless message about a person forging their own fate. In the film, the protagonist sent the engine of the plane that would kill his mother and sister back in time and allowed it to fall on him to prevent the catastrophe. However, Donnie Darko 's timeline created a paradox in the fact that the plane existed in the first place. If the future had rewound and the plane had never started to crash, the engine couldn't have been there in the past to kill Donnie in his bedroom. The movie is incredibly thought-provoking in a way that more lighthearted time travel films never are.

2 12 Monkeys

James cole originates the virus.

12 Monkeys' post-apocalyptic nature paints its deterministic narrative in dark colors, adding to the eerie atmosphere of the story at hand. Bruce Willis' James Cole traveled to the past to prevent humanity's extinction , but his every action just led to the devastating virus scenario taking place in the end. 12 Monkeys' predestination paradox lied in the fact that if Cole hadn't planted the idea of the viral outbreak in the past, it wouldn't have happened at all. The protagonist's desperate attempts to stop the apocalypse explore the notion that there is no such thing as free will and that everything in life is already determined.

Aaron & Abe Create A Causal Loop

Primer is decidedly the time-travel movie that has the most rules on the subject and surprisingly follows them through with the help of extremely complicated tech jargon. Aaron and Abe discovered how to create a causal loop and use it to their own advantage. Unfortunately, their actions unraveled in a heap of consequences, and each attempt to fix the problem just made it a lot worse. Primer requires at least two or three watches to fully understand its core concepts and follow the characters' decisions with ease, but it is worth every minute of the time spent.

The Quirk E. Newsletter

We promise to send you only the coolest stuff we have to offer every month, like information on new releases, preorder campaigns, giveaways, and discounts. Right now, you only need 3 referrals to get a free e-book!

Or subscribe and set genre preferences

By clicking subscribe, I acknowledge that I have read and agree to Quirk Books’ Privacy Policy and Terms of Use . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.

Classic Time Travel Paradoxes (And How To Avoid Them)

[Movie still from Time Machine , Warner Bros. and Dreamworks]

Editor's Note: We're bringing back one of our most loved posts because hey, time travel is always a relevant topic of discussion. Originally published 11/30/12.

Author's Note: I assume that some day, this article will serve as an invaluable guide and warning for our time traveling ancestors-to-be (who will of course be unable to read books and learn these lessons for themselves, either because [a] all the books will have been burned, or [b] kids will have stopped reading books entirely, because grumble grumble, god damn kids, when I was your age, video games, blah blah, detriment to society, buncha hooligans, kids these days, no respect, etc). In the meantime, just enjoy it for all of its delightfully entertaining/convoluted/paradoxical pleasures.

As anyone who’s anyone who’s read any time travel story ever could easily tell you, time travel is a tricky subject. Temporal paradoxes might seem simple and straightforward at the start (no they don’t), but they always devolve quite quickly (linear time-wise) into some sort of trippy, philosophically complicated, timey-wimey conundrum that makes even the most convoluted middle school relationship make sense by comparison. Come to think of it, maybe the reason that all those cool kids in middle school suffer from impossibly complicated and melodramatic romances to begin with is because they’re all too “cool” to read time travel stories in the first place, which would obviously teach them the benefits of temporally linear dating, if nothing else.

I’m looking at you, River Song.

For the most part, any paradox related to time travel can generally be resolved or avoided by the Novikov self-consistency principle, which essentially asserts that for any scenario in which a paradox might arise, the probability of that event actually occurring is zero — or, to quote from LOST, “whatever happened, happened,” meaning that no matter what anyone does, they can’t actually create a paradox, because the laws of quantum physics will self-correct to avoid such a situation. Still, I’m wary of such a loose explanation for things, and so below, I’ve compiled a list of a few of the more popular time travel paradoxes — and what to do to avoid them.

ONTOLOGICAL PARADOX : Also known as the “Bootstraps Paradox,” an ontological paradox arises when a person or object is sent through time and recovered by another person, whose actions then lead to the original person or object back to the time from when it came in the first place, thus creating an endless loop with no discernible point of origin. Thus, the original person or object is essentially “pulling itself up by its own bootstraps,” hence the nickname (thanks in no small part to the Robert Heinlein story “By His Bootstraps”).

Example : The Terminator films are a prime and popular example of the Ontological Paradox. In the future, a Terminator is sent back in time to kill the mother of resistance leader John Connor before he is born. While the original T-800 is ultimately destroyed, the leftover pieces are found by scientists who use the technological to…develop and create Skynet, and the Terminator-series robots. Skynet would have never been created if Skynet hadn’t taken over the world and then sent a Terminator back in time to get destroyed and ultimately lead to the creation of Skynet. Trippy, right?

There's also the fact that Future John Connor sends his buddy Kyle Reese back in time to protect his mother from the T-800, only Kyle ends up totally bangin' John's mom (dude high five! I mean, not cool, man) and impregnates her with his buddy John Connor. So to top it all off, if John hadn't sent his friend back in time, his friend would never have had sex with John's mom, and John would never have been born (meaning that Kyle Reese is either the best or worst friend, ever).

How to Avoid : No one’s really sure if a real-life ontological paradox would lead to some massive hemorrhaging of spacetime, or if the closed loop is kind of automatically self-corrected since it all works itself out evenly in the end anyway. Still, better to avoid these kind of complicated situations, and the best way to do that would simply be to stop taking candy from strangers — “candy” in this case being mysterious or alien artifacts with questionable origins, possibly given to you by mysterious people who may or may not come from the future. See? Maybe all those warnings that your Mom gave you when you were a little kid still mean something today. Or maybe all along she was just trying to prevent you from sending your friends back in time to sleep with her. Or perhaps encourage it…

PREDESTINATION PARADOX : The predestination paradox is similar to the ontological paradox in that the Cause leads to an Effect which then leads back to the initial Cause. The basic tenant of the predestination paradox is similar to that of a self-fulfilling prophecy: the motivation for the time traveler to travel in time is ultimately realized to have been the time traveler’s fault, due to his or her decision to time travel in the first place, or else otherwise unavoidable. Stories involving predestination paradoxes often involve a heavy sense of irony — the time traveler might go back in time in order to change something, for example, but his or her actions inadvertently lead to the exact situation that inspired the time traveler to have gone back and changed things. Thus, nothing ultimately changes. Determinism is a bleak friend.

Example : In Twelve Monkeys, James Cole is sent back in time to prevent a mysterious disaster involving the “Army of the Twelve Monkeys.” His wild rantings in the past about the terrible future from which he came are overheard by Jeffrey Goines, a mental patient who is remembered in the future as the leader of Army of the Twelve Monkeys. Ultimately, Cole’s efforts to prevent his future from happening inspire the actions that lead to his future coming to be. And in a cruel twist of irony, James Cole’s childhood memory of a man in a airport being shot and falling into the arms of a beautiful blonde — the memory that haunts him for the rest of his life — turns out that the guy who was shot was actually him, in the future, dooming young James Cole to grow up and repeat the cycle all over again.

How to Avoid : This one’s tricky, because philosophically, it’s all about free will (or lack thereof). So in fact, by trying to teach you to how to avoid falling victim to the tenants of the predestination paradox, I’m probably going to inspire you to go back in time and create the French film La jetée, which in turn inspires Terry Gilliam to make Twelve Monkeys, which in turn inspires me to use it as an example in this article, et cetera et cetera. Basically we’re all screwed, unless we avoid time travel and time travelers all together. Even a many worlds theory/alternate timeline thing can’t prevent this, because your actions wouldn’t even create a divergent timeline — they would just result in your present situation. So, sorry dude, nothing you can do is going to change anything. Again, unless you don’t do anything at all, although that still doesn’t guarantee anything.

GRANDFATHER PARADOX : This one perfectly demonstrates the aforementioned Novikov self-consistency principle. The basic idea is that, no matter how hard you try, you can’t go back in time and kill your grandfather, because if you did, your mother or father would never have been born, which means that you would never have been born, which means you couldn’t have gone back in time and killed your grandfather, which means that you didn’t go back in time and kill your grandfather, because you can’t go back in time and kill your grandfather, because if you did, you wouldn’t be born, which you obviously have already been born because if you were never born then you couldn’t have gone back in time and tried (and failed) to kill your grandfather in the first place.

That’s just a simple and straightforward summary though. You know, in Layman’s terms.

Basically, the Grandfather paradox conveys the idea of a self-correcting universe and/or fixed points in time. Even if you were able to go back in time and, I don’t know, shoot your Grandpa in the head before he ever meets your Grandma (jeez, you must really hate that guy, huh?), your Grandfather would turn out to be an early sperm donor or something, who would still manage even posthumously to impregnate your Grandmother, because you would have to exist in order to have shot him in the head in the first place. So you might be able to fudge a few temporal details here and there, but no matter what you do, the end result stays the same.

Example : Let’s just say that when you're LOST on a magical tropical island somewhere in the Pacific Ocean (ish?) and you end up skipping through time and decide to try to kill that evil guy while he’s still a kid and/or stop a nuclear bomb you've so affectionately nicknamed “The Jughead” from exploding and causing all kinds of electromagnetic problems and inconsistencies on your already-mystical island home, the best that’s going to happen is you get some kind of weird Hindu sideways limbo reality that works as a parallel narrative to the entire last season of your television show. Oh, and that little kid you shot still turns out to be pretty evil, and it’s all your fault.

How to Avoid : Uhh, don’t try to kill your grandfather in the past before the birth of your father? Take that as a metaphor all you’d like.

HITLER'S MURDER PARADOX : This is similar to the Grandfather Paradox, in that the time traveller goes back in time to change something significant that has already happened. Unlike the Grandfather Paradox (which we assume would self-correct despite our best efforts), the change that one wishes to affect in the Hitler’s Murder Paradox is one that is more technically feasible — as in not intrinsically paradoxical — but still ultimately problematic.

The name comes from the idea that one could theoretically go back in time and kill Adolf Hitler before the Holocaust happened, thus preventing the systematic annihilation of some six million Jews and other minorities. Which, ya know, all sounds good and well, except that it tends to lead to some kind of downward spiraling domino effect with plenty of other consequences that the well-intentioned time traveler probably didn’t consider, and which ultimately might lead to a worse situation than that which the time traveler had hoped to prevent.

Example : This kind of stuff is rampant in comic books, especially X-Men, but the best example of it was the early 90s Age of Apocalypse storyline, in which Professor Xavier’s schizophrenic mutant son, Legion, decides to make daddy proud by helping his dream of mutant-human co-existence come true. Legion concludes that the best way to do this is to go back in time and kill Magneto before he becomes, ya know, Magneto. The only problem is, Magneto and Xavier were like totally BFF back then, so Xavier ends up taking the bullet for Magneto and dies (so yes, Legion does technically end up killing his own father, but that’s not the point).

As a result of there being no Charles Xavier, the psycho evil Darwinist uber-mutant Apocalypse ends up taking over the world before Magneto’s team of X-Men (named in honor of his deceased friend) are able to stop him, which leads to all kinds of crazy situations like evil Hank McCoy aka Dark Beast, who works alongside the evil versions of Cyclops and Havok, or a Sabretooth who is actually a pretty likeable superhero and a member of the X-Men. Oh, also, Magneto and Rogue totally have the sex, and humans are being systematically slaughtered in concentration camps by Apocalypse and his cronies. So basically, in his attempt to kill a perceived “Hitler” in the form of Magneto, Legion caused a real and even more twisted Holocaust to happen. WHOOPS.

How to Avoid : In addition to the whole alternate-reality-that-is-ironically-worse-than-the-world-as-it-used-to-be problem, there’s also the moral compromise of killing an innocent child, even though you know that child is going to grow up to become pretty much the worst (greatest?) mass murderer in history. The best way to avoid it is simply and sadly to accept that you cannot change the past and shouldn’t even try. That is, unless you’re smart enough to have eliminated any possibility of negative domino effect resulting out of your actions.

For example, if you went back in time and eliminated M. Night Shyamalan shortly before the release of Signs, there would be nothing but positive results; the world would mourn the tragic and mysterious loss of a gifted young filmmaker taken before his time, we would all be so blinded by the shock of his death that we’d be able to ignore how bad the aliens looked in that movie (and the fact that seeing them at all was completely unnecessary), and the rest of us wouldn’t have been forced to endure such awful schlock as The Happening or Lady in the Water. See? That way everyone wins!

BUTTERFLY EFFECT : Similar to the cascading domino effect of the Hitler’s Murder Paradox, but on a different level. Whereas killing Hitler would obviously be a landmark event with quite a significant historical impact, something like, say, accidentally stepping on a bug in the past probably wouldn’t have as big of an effect, right?

Have you even been paying attention? Of course it will! That’s the whole point of a time travel paradox! Just like the way that a butterfly flapping its wings in Brazil can affect a weather system in Texas, one tiny change in the past can lead to all kinds of Rube Goldbergian complications that can subtly — or seriously — affect the present. The term “Butterfly Effect” is actually derived from “A Sound of Thunder,” a short story by Ray Bradbury, in which a character accidentally steps on a butterfly in prehistoric times and causes catastrophic changes in the future from which he came.

Example : In Orpheus With Clay Feet by Philip K. Dick, the main character, Jesse Slade, enlists in the services of a time travel tourism agency, who set him up with a trip that allows him to go back in time and act as a muse for some significant historical figure. Slade chooses to go back and inspire his favorite science fiction writer Jack Dowland (which was also Dick’s pen name). Unfortunately, in his efforts to inspire Dowland’s monumental science fiction work, Slade directly reveals to Dowland that he is a time traveler hoping to inspire his work. Dowland takes this as an insulting ruse, and as a result, never becomes the great science fiction writer that he is meant to be. He does, however, publish a single science short story, under the pen name Philip K. Dick: a story called Orpheus With Clay Feet, about a time traveler that goes back in time to inspire his favorite science fiction writer, a man named Jack Dowland.

How to Avoid : Watch your step

Like What You Just Read? We Suggest The Following Blog Posts.

Privacy Overview

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.

On supporting science journalism

If you're enjoying this article, consider supporting our award-winning journalism by subscribing . By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

“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.

Skip to main content
Keyboard shortcuts for audio player

LISTEN & FOLLOW
Apple Podcasts
Google Podcasts
Amazon Music
Amazon Alexa

Your support helps make our show possible and unlocks access to our sponsor-free feed.

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

Things you buy through our links may earn Vox Media a commission.

The 25 Greatest Time-Travel Movies Ever Made

It must say something, surely, about humans, how often time-travel movies are about returning to the past rather than jumping to the future. As Mark Duplass’s forlorn character says in Safety Not Guaranteed , “The mission has to do with regret.” With all the potential to explore the unknown world of the future, so often when our minds conspire to bend the rules of time it’s instead to rehash the old. It’s compelling to watch a character in a movie do what we cannot — right past wrongs or uncover the reason for or meaning behind the events in their lives, whether they be emotionally catastrophic or merely geopolitically motivated.

So absent is the future from the canon, in fact, that when it is involved, typically future dwellers are leaving their own time to come back to the present. Back to the Future Part II aside, it seems as if there’s something about going forward in time that just doesn’t track for humans. (Of course, you could argue that this is because the present-day concept of bidirectional time travel would infinitely multiply or change beyond recognition any future that may occur, but that’s a knot for another article.)

In any case, the time-travel stories deemed worthy of Hollywood budgets aren’t always straightforward in their mechanics. Some films on this list barely qualify as time-travel movies at all; others could hardly qualify as anything else. There are movies about trips through time but also ones about the bending and fracturing and muddying thereof; then there are those about, as Andy Samberg aptly puts it in Palm Springs , “one of those infinite time-loop situations you might have heard about.” There’s even a movie in which we get only 13 seconds’ worth of time travel, when it functions more like a joke whose punch line hits at the film’s climax.

What these films all do have in common is a fascination with changing the way time works. That being said, the list leaves out movies in larger, more extended franchises in which time meddling is a one-off dalliance thrown into a sequel with little by way of foreshadowing: think Harry Potter and the Prisoner of Azkaban , Avengers: Endgame , and Men in Black III . (It also leaves off perhaps the Ur-time-travel movie, Primer , and the quite good Midnight in Paris because their directors don’t deserve the column inches.) We’re looking at self-contained stories using time mechanics from the start, with preference given to those that involve themselves more intently with the ins and outs of time travel; that ask questions about time, aging, memory and so forth; and that try to succeed at it in new and interesting ways. So let’s get to it.

25. Galaxy Quest (1999)

Does Galaxy Quest really count as a time-travel movie? Some compelling reasons argue that it doesn’t: Time travel isn’t a major factor in the plot, and the time traveling that does occur is, yes, only a 13-second jump. But its use of time travel is meaningful insofar as the movie itself is a loving spoof of Star Trek , which makes use of time travel in three films ( one of which made this list ), not to mention dozens of episodes across its various TV iterations. Tacking on time travel as a deus ex machina for the actors in a Star Trek– like show pressed into service as an actual space crew by an endangered alien race is the exact right amount of ribbing in a movie that’s as on point as it is hilarious.

Galaxy Quest is available to rent on Amazon .

24. Happy Death Day (2017)

Pick away at the surface of a time-loop movie and you find a horror movie. Most of the entries on this list are covered in enough feel-good spin to land as comedies, but Happy Death Day stares the horror of the time-loop phenomenon right in the face. (It’s also quite funny.) Reliving the same day over and over is an unimaginably potent form of psychological torture, and adding murder to the equation does little to dull that edge. The film follows a college-age protagonist struggling to escape from a masked slasher hell-bent on killing her again and again while she tries to solve the mystery of how she got stuck in a time loop.

Happy Death Day is available to rent on Amazon .

23. Back to the Future Part II (1989)

Seriously, this may be the only good movie in which the film’s whole focus is using a time machine to travel into the future. The fact that it’s a sequel is telling — the characters already traveled into the past in the first movie , and the filmmakers decided to save “traveling even further into the past“ for the third film in the trilogy. Still, Back to the Future Part II is a fun time that makes great use of sight gags and references, recasting scenes from the first film in the distant future year of 2015 with all its hoverboards and self-lacing Nikes.

Back to the Future Part II is available to rent on Amazon .

22. See You Yesterday (2019)

It’s a dirty little secret of time-travel movies that they tend to be, well, pretty white. Tenet ’s Protagonist aside, if Hollywood’s sending someone through time, they’re almost certainly not a Black person, and for obvious reasons: Most of post-contact North American history is deeply unfriendly to people of color, and the problems a person running around out of time and place is going to encounter are deeply compounded if they’ll likely be the target of racist abuse or violence — which makes See You Yesterday all the more compelling. Produced by Spike Lee and featuring one of filmdom’s most famous time travelers in a cameo role, it follows a Black teenage science prodigy who uses a time machine to try to save her brother from being killed by a police officer.

See You Yesterday is streaming on Netflix .

21. Bill & Ted’s Excellent Adventure (1989)

No offense to the Back to the Future franchise, but time travel never looks more fun on film than it does in the first Bill & Ted movie. It’s a concept that feels distinctly of a different era, so pure is its zaniness, that it’s hard to imagine anyone concocting it today. The titular duo, Californian high-school students in the ’80s, travel through the past looking for historical figures in order to ace a history project, then bring them all back to the present. High jinks ensue! We get Genghis Khan in a sporting-goods store and Mozart on an electric keyboard. What more could you want?

Bill & Ted’s Excellent Adventure is streaming on HBO Max .

20. Source Code (2011)

Time-travel-film aficionados know this won’t be Jake Gyllenhaal’s only stop on this list, but no matter. Source Code finds him repeating the same eight minutes over and over as he struggles to find the culprit in a train bombing — with each replay ending in his own death by explosion. For some reason, a romantic subplot is shoehorned into this, along with a bunch of frankly unnecessary technical mumbo-jumbo, but the core idea is a compelling mix of the time-loop movie and the train whodunit that Gyllenhaal is a perfect fit for.

Source Code is available to rent on Amazon .

19. 12 Monkeys (1995)

Some sort of law of nature dictates that every genuinely good idea and/or piece of true art has to at some point be turned into a Hollywood movie. Thank God La Jetée was adapted into something that can stand on its own feet artistically. 12 Monkeys may not retain its source material’s black-and-white look or stripped-down, static-image presentation, but it is a rollicking good time nonetheless. That’s in no small part due to director Terry Gilliam getting the best out of Bruce Willis and a young Brad Pitt, and recasting World War III as a planet-decimating virus. Which, like at least one other movie on this list , “speaks to the present moment,” or whatever.

12 Monkeys is available to rent on Amazon .

18. Run Lola Run (1998)

Unlike almost all of the other films on this list, the terms time travel and time machine don’t show up anywhere in Run Lola Run . Rather, it’s a sort of de facto time-loop scenario in which the protagonist tries repeatedly to pay a ransom to save her boyfriend’s life. In fact, if not for a few key details, it could easily be characterized (and often has been) as an alternate-endings movie rather than a time-travel film. But the fact that Lola seems to be learning from her past attempts with each successive one suggests that she is, indeed, using knowledge gained from previous loops to bring a satisfactory end to this situation.

Run Lola Run is available to rent on Amazon .

17. Edge of Tomorrow (2014)

One of the most striking things about Groundhog Day is the mutability and replicability of its core conceit. Perhaps the best case in point is Edge of Tomorrow , sometimes known as Live. Die. Repeat. after its original tagline. It’s the kind of physically grueling movie only an actor as genuinely unhinged as Tom Cruise could pull off. A noncombatant thrust into a war against invading aliens, Cruise’s character finds himself reliving day one of combat over and over, slowly but surely refining his techniques in order to survive the extraterrestrial onslaught. Like the central twosome in the much less violent Palm Springs , he winds up with a partner in (war) crime, teaming up with the similarly time-trapped Emily Blunt, and the explanation for the replay glitch here is actually pretty satisfying.

Edge of Tomorrow is streaming on Fubo TV .

16. Star Trek (2009)

If you could create some sort of an advanced stat to measure controversy generated per unit of interesting filmmaking decisions, J.J. Abrams would have to be near the top in terms of his ability to rig up movie drama from almost nothing. This is a guy whose filmography is like Godzilla rip-off, Spielberg homage, safe reboot of cherished IP, repeat. Star Trek may be his best film, though, a sure-footed reinvention of a dorky sci-fi franchise that made it, well, cool. Somehow, the beauty of Spock and Kirk’s bromance being woven through chance encounters with future selves kind of … works?

Star Trek is available to rent on Amazon .

15. The Girl Who Leapt Through Time (2006)

There’s a relative dearth of time travel in animated film, which perhaps is a function simply of the fact that it’s less impressive to stage in a world that’s already unreal. If you can Looney Tunes your way through physics, what’s so special about grabbing the flow of time and tying it into a bow? Still, the original Girl Who Leapt Through Time deserves mention here. It’s a beautiful story that interlaces the complexity of time leaping with the intensity of teenage emotion and the thorny process of growing up where the opportunity to redo things leads, over time, to growth — a less shitty Groundhog Day , in a way.

The Girl Who Leapt Through Time is available to rent on Amazon .

14. Safety Not Guaranteed (2012)

She may not be the most famous, decorated, or emulated actress of her generation, but Aubrey Plaza is someone whose personality spoke to the irony-soaked 2010s in a way that simply could not be denied. Her character on Parks and Recreation , April Ludgate, was, by all accounts, created specifically to channel Plaza’s real-life personality to the screen, and she plays essentially the same character in Safety Not Guaranteed . Here, she’s a sarcastic intern at a magazine working on a story about a would-be time traveler and using her feminine wiles to slowly gain his trust. The chemistry between Plaza and Mark Duplass is probably the film’s high point; the subplot about the FBI feels like it was clipped out of a bad X-Files episode.

Safety Not Guaranteed is streaming on Tubi .

13. La Jetée (1962)

At only a 28-minute run time, La Jetée is arguably too short to merit inclusion on this list. However, what it lacks in content (and in, well, moving images; it’s almost exclusively a collection of static black-and-white shots set to voice-over), it more than makes up for in inventiveness and influence, and it would be a travesty to leave it out in favor of more recent by-the-book fare. Tracing the tale of a man held prisoner in post-WWIII Paris being used in time-travel experiments as his captors seek to remedy the postapocalyptic state of the world, he’s sent into both the future and the past and ends up unraveling a lifelong personal mystery while he’s at it.

La Jetée is streaming on the Criterion Channel .

12. Planet of the Apes (1968)

Unlike the worse but more straightforwardly time-traveling Tim Burton remake, the relationship between the original Planet of the Apes and time travel is inexact — technically, the astronaut crew that lands on the titular planet does travel forward 2,000 years, but it’s not done via a time machine. The travel isn’t instantaneous: It literally does take them 2,000 years to get there; they’re just unconscious and on life support. Still, the way the film’s ending handles the iconic reveal is exactly in line with the best of the time-travel canon, the telescoping, mise en abyme feeling of the world shifting in front of your very eyes without your moving an inch.

Planet of the Apes is available to rent on Amazon .

11. Groundhog Day (1993)

The famous Bill Murray vehicle essentially invented the infinite-time-loop genre (and it’s hardly a movie that succeeds on the strength of its concept alone), but the idea at its core is so steeped in the casual misogyny of late-’80s and early-’90s cinema that it’s hard to watch today without cringing. Murray’s character employing what amounts to PUA-style techniques over and over and over in a desperate bid to fuck his hapless co-worker just doesn’t hit the way it did back then. If the story arc didn’t present a guy detoxifying himself of the worst aspects of masculinity in order to be worthy of a woman’s love as the primary way for a 20th-century white man to achieve full personhood, this would be much higher on the list.

Groundhog Day is streaming on Starz .

10. Predestination (2014)

This is probably the most complicated film on the list. Following a “temporal agent” (played by Ethan Hawke) who’s trying to prevent a bombing in 1970s New York, it’s based on a Robert A. Heinlein short story and features Shiv Roy herself, Sarah Snook, in a star-making turn as someone with a complicated backstory and a secret. Like the best sci-fi, the film’s premise raises all kinds of fascinating questions about the titular concept and throws in some interesting musings on sex, gender, and the self in the process.

Predestination is streaming on Tubi .

9. Looper (2012)

Wes Anderson gets a lot of flak for his overwrought twee visuals, but Rian Johnson has a knack for making movies that feel and function like dioramas even if they don’t look it. Narratively speaking, everything here is constructed just so — and there’s a certain beauty in that — but who ever had a profound experience of art by looking at a diorama? Looper was probably Johnson’s least precious pre– Star Wars film, which is nice because the temptation to drastically overmaneuver the mechanics of a time-travel story can lead to disaster. The tech used to Bruce Willis–ify Joseph Gordon-Levitt’s face is distracting, and the third act’s retreat from the postapocalyptic city of the future to the postapocalyptic corn farm of the future is a brave choice that the film struggles to land. Still, Johnson’s vision of a future in which organized crime runs time travel is compelling and well worth a watch.

Looper is streaming on Netflix .

8. Donnie Darko (2001)

Donnie Darko is a bit of a genre mash-up. Part high-school movie, part sci-fi flick, part bleak meditation on the soullessness of late-’80s America, it’s nevertheless a weirdly successful piece of filmmaking that makes fantastic use of a young Jake Gyllenhaal, a great supporting cast (Maggie Gyllenhaal, Drew Barrymore, Jena Malone, and Patrick Swayze among others), and an absolutely iconic haunting cover of Tears for Fears’ “Mad World.” Watching high schoolers navigate parallel universes, wormholes, and time travel is a dicey proposition, but director Richard Kelly makes it work, somehow.

Donnie Darko is streaming on HBO Max .

7. Back to the Future (1984)

While it’s clearly superior to the sequel (and leagues ahead of the final film in the trilogy), the original Back to the Future is a bit of a mess (John Mulaney was right , to be honest). Its racial and gender politics are cringey, and the incest subplot is weird (“It’s your cousin Marvin. Marvin Pornhub . You know that new plot element you’ve been looking for?”), but there’s a clear interest in time travel beyond its shimmering surface: the very real addressing of the “grandfather problem” in time travel via the slow disappearance of Marty from his family photo, the accidental invention of rock music, and a genuine curiosity about the nuts-and-bolts mechanics of time machines. Ahh, what the hell. It’s a romp.

Back to the Future is available to rent on Amazon .

6. Palm Springs (2020)

No offense to Gen-Xers and boomers, but the best time-loop movie of all time is Palm Springs . The film isn’t without its missteps, but it’s much more curious about life than Groundhog Day was through the eyes of Murray’s misanthrope. Cristin Milioti and Andy Samberg‘s characters, stuck in the loop together, are a perfect comedic match, and their shared humanity makes for a beautiful arc. The film raises questions about what’s worth doing in life when nothing lasts and how to stay sane when every day is the same. Of course, as a sort of polar opposite of Tenet , it benefited from coming out during the pandemic by speaking, as it does, to the experience of lockdown.

Palm Springs is streaming on Hulu .

5. Tenet (2020)

Interstellar wasn’t enough for Chris Nolan, apparently. Tenet ’s legacy may end up being little more than that of the COVID action movie no one saw — a bloated thriller that Nolan fought to get into theaters and bar from home viewing reportedly to swell the size of his own pockets. It really did suffer from bad timing, though, because this is genuinely a quintessential big-screen popcorn movie whose absurdity is all the more palatable when it’s given the audiovisual bombast it deserves. Ambitious in scope as it traces a war on the past by the future (yes, you read that right), Tenet is as enamored of action tropes as it is in bucking them, and its investment in rendering visible the brain-bendingly knotty mechanics of moving through time is laudable, even when the movie itself remains opaque — as impenetrable as the future, as hazy as the past.

Tenet is streaming on HBO Max .

4. The Terminator (1984)

A partner to Blade Runner in the mid-’80s invention of sci-fi noir, The Terminator is a stunning film in many ways, despite the third act’s now-iffy visual effects. While it’s not James Cameron’s debut, and it would go on to be bested by its sequel , it functions as an incredible showcase for an emerging young director who would exclusively make big stories for the rest of his career. Arnold Schwarzenegger is perfectly cast as the relentless, unemotional killer cyborg sent back from the future to terminate the mother of the eventual resistance leader, and the film’s romantic subplot has just the perfect amount of time-travel-induced cheesiness for it to work.

The Terminator is streaming on Amazon Prime Video .

3. Interstellar (2014)

It’s not inaccurate to say Christopher Nolan is a director who’s more interested in scale and scope than in expressing the minutiae of the human experience in its purest form. But in Interstellar, a Nolan movie in its titular ambitions, there’s a core element of time travel wrought not as sci-fi fireworks but as a paean to the sheer force and will of the power of love. It both does and doesn’t work, depending on your capacity for cheese in space, but even besides that, Nolan’s use of time as story arc — the way Miller’s planet functions, in particular — is conceptually masterful in the best kind of time-travel-movie way.

Interstellar is streaming on Paramount+ .

2. Terminator 2: Judgment Day (1991)

Whereas the franchise’s first movie spends more time on the question of time travel, in the second it takes a bit of a back seat to the action itself. It’s hard to fault director James Cameron for this decision; T2 remains one of the best action movies of the ’90s and — along with Jurassic Park and The Matrix — one of the decade’s best when for special effects. The groundbreaking T-1000 would honestly be enough to get this movie on the list; a tween John Connor grappling with questions of predestination and the fact that he is vicariously responsible for his own conception feel almost like icing on the time-travel cake. Much as in 12 Monkeys , time travel here is mistaken for delusion, as valiant Sarah Connor, in a Cassandra-esque nightmare, has to battle against the future only she knows is coming. Of course, Cassandra never had access to any firepower stored in underground desert arsenals.

Terminator 2: Judgment Day is streaming on Netflix .

1. Arrival (2016)

It’s fair to wonder whether Arrival really is, in fact, a time-travel movie. The Ted Chiang short story it’s based on isn’t about time travel per se; rather, it’s an exploration of alternate forms of temporal understanding. The linguist protagonist, played by Amy Adams, doesn’t travel through time so much as come to experience it differently. Still, the plot ends up hinging on foreknowledge that she is granted not via visions but by actually experiencing her future simultaneously with her present and past. For our purposes, though, that’s time fuckery enough to merit inclusion, and boy howdy does the film deliver in overall quality. Partly, that’s simply a question of the source material. Chiang is arguably the most talented (and possibly the most decorated) American sci-fi writer of his generation. But the source story is not especially Hollywood friendly, and director Denis Villeneuve has adopted it lovingly, borrowing a plot device from another of Chiang’s stories, the more straightforwardly time-travel-based “The Merchant and the Alchemist’s Gate,” in order to add some third-act blockbuster flavor. The result is a beautiful meditation on love, choice, and courage that packs art-film ethos into a genuine sci-fi blockbuster.

Arrival is streaming on Hulu and Paramount+ .

vulture homepage lede
timey-wimey
vulture lists
time travel
vulture picks

Editor’s Picks

Most Popular

What is your email.

This email will be used to sign into all New York sites. By submitting your email, you agree to our Terms and Privacy Policy and to receive email correspondence from us.

Sign In To Continue Reading

Create your free account.

Password must be at least 8 characters and contain:

Lower case letters (a-z)
Upper case letters (A-Z)
Numbers (0-9)
Special Characters (!@#$%^&*)

As part of your account, you’ll receive occasional updates and offers from New York , which you can opt out of anytime.

Den of Geek

Five Greatest Films with Time Travel Paradoxes

Our list of the Top 5 films with time travel paradoxes and a discussion of how each one makes those paradoxes work.

Share on Facebook (opens in a new tab)
Share on Twitter (opens in a new tab)
Share on Linkedin (opens in a new tab)
Share on email (opens in a new tab)

Time travel is a sticky subject for a plot device in that it comes with a set of built-in contradictions. Could a time traveler bring back objects or information from the future if those objects or information only exist because they were brought back from the future? (The “Bootstrap Paradox .”) Could a time traveler truly influence time if their presence in the past means they’ve already time traveled? (The “Predestination Paradox.”) Could a time traveler change something in the past that would prevent them from time traveling in the first place? (The “Temporal Paradox.”)

Some films get around this metaphysical grey area by simply ignoring these messy repercussions, but the most interesting films are those that hinge on them, exploring time travel not as a device to spur conflict but rather as the conflict itself. Here are five of cinema’s best time travel paradoxes, in all their bewildering, inscrutable glory.

5. Donnie Darko

Paradoxes: bootstrap, predestination.

Writer-director Richard Kelly has made so many missteps in recent years (a losing streak consisting of the triple threat of Domino , Southland Tales and The Box ) it’s easy to forget his debut feature was one of the most original, satisfying and atmospheric puzzles ever put to film.

Though Donnie Darko ‘ s circular plot revolves around time travel, there’s no time machine and the eponymous hero never goes back in time himself. Instead, Donnie (played with striking vulnerability by Jake Gyllenhaal) learns over the course of the film that he can manipulate time, altering events that have already occurred. But its a skill he realizes only because his future self has already used it, setting into motion a spiral of destruction that must be prevented – by learning to manipulate time and setting it in motion all over again.

Ad – content continues below

It’s a paradox of a plot, but one more concerned with the intricate workings of a generic small town than the mechanics behind time travel. Backed by a nostalgic period setting (and a fantastic soundtrack to match) as well as a host of great actors, including Gyllenhaal’s on and off-screen sister Maggie, a young Seth Rogen and Patrick Swayze in perhaps his greatest, most ironic role, Donnie Darko is a code worth the multiple-viewings needed to decipher it. Hopefully, Kelly begins to show that initial ingenuity again.

4. 12 Monkeys

Based on Chris Marker’s short La Jetée , but expanding that film’s lyrical meditation on premonition to a sci-fi saga involving bio-terrorists, a humanity destroying virus and a post-apocalyptic future spent underground, Terry Gilliam’s 12 Monkeys centers around a time traveler, played by Bruce Willis, sent to the past to prevent a catastrophe from nearly destroying the entire human race.

Things don’t go as planned, however, as things so rarely do, and Willis’ mission to save the world is hindered by the general perception that he’s a paranoid schizophrenic, an explanation so much more plausible than the reality, Willis begins to believe it himself.

In the end, even with his knowledge of future events, Willis was never truly able to keep them from happening. The paradox of a time traveler sent back to change the event leading him to become a time traveler is inherent, but the real stumbling block is memory itself, which blurs and distorts the past enough to make the same mistakes an inevitability.

3. Back to the Future

Paradoxes: bootstrap, temporal .

Easily the lightest, most popcorn-friendly film on this list, Back to the Future is, nevertheless, serious about the implications of time travel. When he’s accidentally transported to 1955, Marty McFly (Michael J. Fox) inadvertently interferes with his own parents meeting, an event which leads to Marty’s own mother falling for him and, potentially, his ceasing to exist entirely.

Luckily, Marty and his scientist pal Doc Brown (Christopher Lloyd) devise a plan to get her to fall for the senior McFly, though their adventures in the past have interesting effects on the future. Yet unlike most other time travel films, Marty’s interferences almost universally make things better, saving his parents’ relationship, making his father more successful and putting the bully Biff (Thomas F. Wilson) in his place. In fact, the biggest paradox in Back to the Future isn’t that Marty nearly prevented his own birth, it’s that his parents seem to have forgotten that the man who introduced them looked exactly like their teenaged son.

2. Los Cronocrímenes

Paradoxes: predestination, temporal.

Writer-director Nacho Vigalondo’s Los Cronocrímenes ( Timecrimes ) opens with Héctor, a seemingly average middle-age husband, peering into the forest behind his new home with binoculars and finding not birds or foxes but a young woman undressing. Naturally, he walks into the woods to investigate, only to be stabbed by a man whose face is menacingly obscured by apparently blood soaked bandages.

Get the best of Den of Geek delivered right to your inbox!

Describing what happens next, and how time travel becomes involved, would risk spoiling Vigalondo’s meticulously constructed script, which adds a few new mysteries for each one it solves and piles overlapping timelines upon overlapping timelines. It’s a testament to Vigalondo that he manages to keep so many plates spinning, and in such a precisely choreographed fashion, transforming a taut thriller into an expertly crafted exercise in metaphysics.

Paradoxes: Bootstrap, Predestination,Temporal

Primer is a decidedly small film – it has essentially no special effects, no big action set pieces and it isn’t interested in holding the audience’s hand through its increasingly knotty structure – yet it seems to offer the most fully realized look at the problems inherent in time travel despite of (or, more likely, because of) its impossibly low budget.

Written, directed and starring Mathematics major and engineer Shane Carruth for only $7,000, Primer isn’t a flashy film, but it uses its nuances to great effect, creating a time machine that seems real and plausible and populating its minuscule world with telling clues that hint at the consequences such a device would cause.

Carruth and co-star David Sullivan speak like real engineers, act like real people and react plausibly to an increasingly implausible scenario. Their time machine has rigidly defined rules that, rather than simplify its use, instead make its application that much more complex, leading to a hilariously impenetrable infographic that doesn’t explain the film as much as it complicates it even further.

But Primer ‘s difficulty isn’t a demerit, it’s the reason the film works as well as it does. It rewards and practically demands repeat viewings, each revealing a new thread the viewer hadn’t noticed before. Ostensibly, Carruth and Sullivan are merely experimenting with their incredible discovery (and trying to make some money on the side), but the machine quickly leads to repercussions the protagonists (and the audience) don’t understand and each attempt to set things right only creates an ever more baffling mess to clean up.

It’s Primer ‘s low-key (and low-budget) charm that sells the fantastic premise as a credible reality, but that credibility ultimately makes Primer ‘s paradoxes even more terrifying to consider. It’s a film that eschews the larger question of “How would time travel change the world?” in favor of one even grimmer: “How would time travel change you?”

Kyle Phaneuf

Kyle Phaneuf likes art rock, genre films and magical realism. He lives in Brooklyn, New York. Music: Can, Jason Molina, Sufjan Stevens Film: Eternal Sunshine of…

Quantum time travel: The experiment to 'send a particle into the past'

Time loops have long been the stuff of science fiction. Now, using the rules of quantum mechanics, we have a way to effectively transport a particle back in time – here’s how

By Miriam Frankel

29 May 2024

Ryan wills/istock/Amtitus

When Seth Lloyd first published his ideas about quantum time loops, he hadn’t considered all the consequences. For one thing, he hadn’t anticipated the countless emails he would get from would-be time travellers asking for his help. If he could have his time over again, he jokes, he “probably wouldn’t have done it”.

Sadly, Lloyd, a physicist at the Massachusetts Institute of Technology, won’t be revisiting years gone by. Spoiler alert: no one will go back in time during the course of this article. But particles? That is another matter.

Theoretical routes to the past called time loops have long been hypothesised by physicists. But because they are plagued by impracticalities and paradoxes, they have been dismissed as impossible for just as long. But now Lloyd and other physicists have begun to show that in the quantum realm, these loops to the past are not only possible, but even experimentally feasible. In other words, we will soon effectively try to send a particle back in time.

Rethinking reality: Is the entire universe a single quantum object?

If that succeeds, it raises the possibility of being able to dispatch, if not people, then at least messages in the form of quantum signals, back in time. More importantly, studying this phenomenon takes us to the heart of how cause and effect really work, what quantum theory means and perhaps even how we can create a successor theory that more fully captures the true nature of reality.

In physics, time loops are more properly known as closed time-like curves (CTCs). They first arose in Albert Einstein’s theory of general…

Sign up to our weekly newsletter

Receive a weekly dose of discovery in your inbox! We'll also keep you up to date with New Scientist events and special offers.

To continue reading, subscribe today with our introductory offers

No commitment, cancel anytime*

Offer ends 2nd of July 2024.

*Cancel anytime within 14 days of payment to receive a refund on unserved issues.

Inclusive of applicable taxes (VAT)

Existing subscribers

More from New Scientist

Explore the latest news, articles and features

Atoms at temperatures beyond absolute zero may be a new form of matter

Subscriber-only

Glitching radio waves from dead stars explained by swirling superfluid

How materials that rewind light can test physics' most extreme ideas, time may be an illusion created by quantum entanglement, popular articles.

The 10 Most MIND-TWISTING PARADOXES of All Time!

Posted: June 8, 2024 | Last updated: June 8, 2024

Part 2: <a href="https://youtu.be/e41iPruvydcMy">https://youtu.be/e41iPruvydcMy</a> Discord: <a href="http://bit.ly/SantoroLounge">http://bit.ly/SantoroLounge</a> | My Twitch: <a href="http://bit.ly/MattsTwitchSubscribe:">http://bit.ly/MattsTwitchSubscribe:</a> <a href="http://bit.ly/SubToMattSantoro">http://bit.ly/SubToMattSantoro</a> | Website: <a href="http://MatthewSantoro.comCatch">http://MatthewSantoro.comCatch</a> up on my latest uploads! <a href="https://youtube.com/playlist?list=PLe7K6kDBf74EgSlmhy6Wavip716_-JRPG&playnext=1&index=1Watch">https://youtube.com/playlist?list=PLe7K6kDBf74EgSlmhy6Wavip716_-JRPG&playnext=1&index=1Watch</a> more:Creepy, Scary, and Chilling Topics! <a href="https://youtube.com/playlist?list=PLe7K6kDBf74HK4A_zHEB-aQLNEErSikq5&playnext=1&index=250">https://youtube.com/playlist?list=PLe7K6kDBf74HK4A_zHEB-aQLNEErSikq5&playnext=1&index=250</a> Amazing Facts to Blow Your Mind! <a href="https://youtube.com/playlist?list=PLe7K6kDBf74Hp_q_-bs6g5Ulju0Lp__Ss&playnext=1&index=2Mysterious">https://youtube.com/playlist?list=PLe7K6kDBf74Hp_q_-bs6g5Ulju0Lp__Ss&playnext=1&index=2Mysterious</a>, Cryptic, and Mystical Topics! <a href="https://youtube.com/playlist?list=PLe7K6kDBf74F_S6Jt4inbGZM6kfZ4Y2ar&playnext=1&index=2Fascinating">https://youtube.com/playlist?list=PLe7K6kDBf74F_S6Jt4inbGZM6kfZ4Y2ar&playnext=1&index=2Fascinating</a>, Amazing, and Funny Topics! <a href="https://youtube.com/playlist?list=PLe7K6kDBf74GbkIhmSYCfb0Ctc1sl8-v5&playnext=1&index=2All-Time">https://youtube.com/playlist?list=PLe7K6kDBf74GbkIhmSYCfb0Ctc1sl8-v5&playnext=1&index=2All-Time</a> Most Popular Videos! <a href="https://youtube.com/playlist?list=PLe7K6kDBf74GcJABXEUfqDHSIlBojZZQO&playnext=1&index=2Sources:">https://youtube.com/playlist?list=PLe7K6kDBf74GcJABXEUfqDHSIlBojZZQO&playnext=1&index=2Sources:</a> <a href="https://pastebin.com/0U9bWV3t">https://pastebin.com/0U9bWV3t</a>

More for You

Fashion Nova's 'body positive' campaign goes viral for all the wrong reasons

Silicon Valley's elite are moralizing about the future of AI

Norway's trillion-dollar sovereign wealth fund to vote against Musk's $56 billion Tesla pay package

Jimmy Carter Health Update Issued by Grandson

Vanna White says emotional goodbye to Pat Sajak for final show

I’ve Lived In The South All My Life—And These Are The 10 Most Underrated Destinations

NEW YORK, NEW YORK - DECEMBER 12: Lynda Carter attends The 15th Annual CNN Heroes: All-Star Tribute at American Museum of Natural History on December 12, 2021 in New York City. (Photo by Dominik Bindl/Getty Images)

Wonder Woman's Lynda Carter, 72, wows in silver swimsuit to promote new music

Scientists change the number of steps needed to stay healthy

Brittney Griner, Diana Taurasi Make Strong Statements After Controversial Team USA Selections

Southwest sees leisure-travel weakness, American provides downbeat loss outlook

Elliott reportedly builds big stake in Southwest Airlines, will seek changes

Donald Trump to Las Vegas Rally Attendees: 'I Don't Care About You'

Eagles sign veteran after All-Pro’s sudden offseason injury

Stars of ‘Happy Days’ Look Back at the Show 50 Years Later

12 Uncomfortable Truths About Christianity That People Aren't Ready to Hear

Here's How Many Aircraft Carriers Each Country Has

18 Outdated Laws in America That Remain in Effect

Samuel Alito May Have Made 'Grave' Move in Letter to Congress—Legal Analyst

Diddy Sells Majority Shares In His Media Company Revolt

Cam Sutton Defends Steelers Signing, Talks 'Morals' Amid Domestic Violence Controversy

Pittsburgh CB Cam Sutton Punishment: Steelers 'Know What's Coming'

104-year-old who swims 45 minutes every day shares simple tips for long life

A Father-and-Son Construction Team Try Their Hand at Tiny Homes

IMAGES

Three Amazing Time Travel Paradoxes
[THEORY] Time Travel Metaphysics/Paradoxes Infographic : timetravel
What Are Some Famous Time Paradoxes?
3 Amazing Time Travel Paradoxes
Time Travel Paradoxes : What Are They?
Top 5 Time Travel Paradoxes and How to Fix Them!

VIDEO

Paradoxes Unraveled A Time Travel Journey #space #science #universe #cosmicmystery
Time Travel Paradoxes: A Mind Bending
The Paradoxes of Time Travel #science #popularscience
Unveiling The Most Terrifying Time Paradox! #shorts #timetravel
20 Time Travelers That Will Convince You It's Real
Paradoxes of Time Travel

COMMENTS

5 Bizarre Paradoxes Of Time Travel Explained
1: Predestination Paradox. A Predestination Paradox occurs when the actions of a person traveling back in time become part of past events, and may ultimately cause the event he is trying to prevent to take place. The result is a 'temporal causality loop' in which Event 1 in the past influences Event 2 in the future (time travel to the past ...
Temporal paradox
A temporal paradox, time paradox, or time travel paradox, is a paradox, an apparent contradiction, or logical contradiction associated with the idea of time travel or other foreknowledge of the future. While the notion of time travel to the future complies with the current understanding of physics via relativistic time dilation, temporal paradoxes arise from circumstances involving ...
Can we time travel? A theoretical physicist provides some answers
The best known is the "grandfather paradox": one could hypothetically use a time machine to travel to the past and murder their grandfather before their father's conception, thereby ...
The Physics of Time Travel: Examining the Possibilities and Paradoxes
The bootstrap paradox is a type of time travel paradox in which an object or piece of information exists without a clear point of origin. The paradox is named after the idiom "pulling oneself up ...
Time Travel and Modern Physics
Adolph cannot both die and survive, as a matter of logic, so any scheme to alter the past must fail. In many of the best time travel fictions, the actions of a time traveler are constrained in novel and unexpected ways. Attempts to change the past fail, and they fail, often tragically, in just such a way that they set the stage for the time ...
Time Travel Paradoxes
Time Travel Paradoxes - Time travel problems include the grandfather paradox and parallel universes. Learn about some if the different kinds of time travel problems. ... Cause comes before effect, at least in this universe, which manages to muck up even the best-laid time traveling plans. For starters, if you traveled back in time 200 years ...
Time travel could be possible, but only with parallel timelines
Time travel and parallel timelines almost always go hand-in-hand in science fiction, but now we have proof that they must go hand-in-hand in real science as well. General relativity and quantum ...
The real time-travel paradox was the friends we made along the way
The real time-travel paradox was the friends we made along the way. Life at the cutting edge. ... 1,001 best hikes on Mars: The Peterson Historic Trail ('Peterson's folly')
Time Travel
Time Travel. First published Thu Nov 14, 2013; substantive revision Fri Mar 22, 2024. There is an extensive literature on time travel in both philosophy and physics. Part of the great interest of the topic stems from the fact that reasons have been given both for thinking that time travel is physically possible—and for thinking that it is ...
The Time-Travel Paradoxes
Congratulations! You have a time machine! You can pop over to see the dinosaurs, be in London for the Beatles' rooftop concert, hear Jesus deliver his Sermon on the Mount, save the books of the Library of Alexandria, or kill Hitler. Past and future are in your hands. All you have to do is step inside and press the red button.Wait! Don't do it!Seriously, if you value your lives, if you want ...
Is time travel even possible? An astrophysicist explains the science
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 ...
Paradoxes of Time Travel
Ryan Wasserman, Paradoxes of Time Travel, Oxford University Press, 2018, 240pp., $60.00, ISBN 9780198793335. Wasserman's book fills a gap in the academic literature on time travel. The gap was hidden among the journal articles on time travel written by physicists for physicists, the popular books on time travel by physicists for the curious ...
Temporal Paradoxes
Abstract. Chapter 2 surveys the various theories of time and explores their consequences for the possibility of time travel. Section 1 introduces the traditional debates over tense and distinguishes between three different views of temporal ontology: eternalism, presentism, and the growing block theory. Section 2 discusses eternalism and the ...
10 Best Movies About Time Travel Paradoxes
Runtime. 113 minutes. The Butterfly Effect is one of the most mind-blowing time travel movies out there, partly because the rules are very specific, and yet they make no sense whatsoever. The movie featured quite a few time paradoxes, but the biggest one was probably the existence of Evan's blackouts. Young Evan experienced blackouts, caused by ...
Classic Time Travel Paradoxes (And How To Avoid Them)
In the future, a Terminator is sent back in time to kill the mother of resistance leader John Connor before he is born. While the original T-800 is ultimately destroyed, the leftover pieces are found by scientists who use the technological to…develop and create Skynet, and the Terminator-series robots. Skynet would have never been created if ...
Is Time Travel Possible?
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 ...
Paradox-Free Time Travel Is Theoretically Possible, Researchers Say
Time Travel Theoretically Possible Without Leading To Paradoxes, Researchers Say In a peer-reviewed journal article, University of Queensland physicists say time is essentially self-healing ...
How many types of paradoxes are there in time travel and what ...
in causal loop paradox, the cause of a past event is a future event. heinlein's "by his bootstraps" story is an excellent example of this (and hence the name). the paradox being, if the cause precedes the effect how did it all begun? words like "beginning" and "starting" implies a temporal order and the possibility of time travel undermines it ...
The 25 Greatest Time-Travel Movies Ever Made
24. Happy Death Day (2017) Pick away at the surface of a time-loop movie and you find a horror movie. Most of the entries on this list are covered in enough feel-good spin to land as comedies, but ...
The best novels about time travel and the paradoxes that arise
This is a novel that explores all the complexities and paradoxes and oddities associated with time travel. It takes place in a future world in which time travel is a part of life, and so must be monitored and regulated like any other technology. Because time travel holds such a huge potential for disaster, strict rules must be established and ...
The best time travel novels that respect the paradoxes
But 300-year-old cabins come with 300 problems. Dormice & Moonshine is the true story of an Englishman seduced by Slovenia. In the wake of a breakup, he seeks temporary refuge in his hinterland house, but what was meant as a pitstop becomes life-changing when he decides to stay.
Five Greatest Films with Time Travel Paradoxes
Here are five of cinema's best time travel paradoxes, in all their bewildering, inscrutable glory. 5. Donnie Darko. Paradoxes: Bootstrap, Predestination. Writer-director Richard Kelly has made ...
Quantum time travel: The experiment to 'send a particle into the past
Time loops have long been the stuff of science fiction. Now, using the rules of quantum mechanics, we have a way to effectively transport a particle back in time - here's how. When Seth Lloyd ...
Simple Tips for Overcoming Time Travel Paradoxes : r/DMAcademy
Self Healing Timeline: My favorite mechanism for overcoming "causal loops" in campaigns containing time travel is the "self-healing" timeline. Every action has an equal and opposite reaction. If you travel to the past and stop yourself from inventing a time machine so it cannot be used for nefarious purposes in the present, an equal reaction ...
The 10 Most MIND-TWISTING PARADOXES of All Time!
The 10 Most MIND-TWISTING PARADOXES of All Time! ... The 25 best roles of Kevin Costner's career. My soon-to-be ex-husband placed his money in a trust before we married, and used it to buy ...