tim travel

Time travel: Is it possible?

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

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

Albert Einstein's theory

General relativity and GPS
Wormhole travel
Alternate theories

Science fiction

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

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

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

Related: The best sci-fi time machines ever

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

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

Related: Controversially, physicist argues that time is real

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

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

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

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

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

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

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

General relativity and GPS time travel

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

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

Can wormholes take us back in time?

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

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

Related: Best time travel movies

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

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

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

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

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

Alternate time travel theories

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

Infinite cylinder theory

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

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

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

Time donuts

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

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

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

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

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

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

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

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

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

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

Related: Marvel movies in order: chronological & release order

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

Books about time travel:

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

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

Movies about time travel:

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

Television about time travel:

Image of the Star Trek spaceship USS Enterprise

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

Games about time travel:

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

Additional resources

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

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

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

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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.

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Is Time Travel Possible?

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

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

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

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

Image of galaxies, taken by the Hubble Space Telescope.

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

How do we know that time travel is possible?

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

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

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

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

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

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

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

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

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

Illustration of GPS satellites orbiting around Earth

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

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

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

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

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

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

In Summary:

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

If you liked this, you may like:

April 26, 2023

Is Time Travel Possible?

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

By Sarah Scoles

yuanyuan yan/Getty Images

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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Paradox-Free Time Travel Is Theoretically Possible, Researchers Say

Matthew S. Schwartz

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

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

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

Turns out, King might have been on to something.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

time travel
grandfather paradox

A beginner's guide to time travel

Learn exactly how Einstein's theory of relativity works, and discover how there's nothing in science that says time travel is impossible.

Actor Rod Taylor tests his time machine in a still from the film 'The Time Machine', directed by George Pal, 1960.

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

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

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

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

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

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

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

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

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

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

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

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

A party for time travelers

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

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

The arrow of time

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

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

Time travel paradox

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

Killing Hitler

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

Killing your grandfather

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

A closed loop

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

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Andrew May holds a Ph.D. in astrophysics from Manchester University, U.K. For 30 years, he worked in the academic, government and private sectors, before becoming a science writer where he has written for Fortean Times, How It Works, All About Space, BBC Science Focus, among others. He has also written a selection of books including Cosmic Impact and Astrobiology: The Search for Life Elsewhere in the Universe, published by Icon Books.

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Tutustu unohtumattomiin lomamuistoihin

Suosituin lomakokemus, myöhästynyt synttärimatka pärnun rantakaupunkiin, surffia ja enduroa azoreilla, kaveriporukalla krakovassa – yllätys vaimoille paljastui vasta lentokentällä, maltan keskiaikaisten kaupunkien ja turkoosin meren lumoissa, koe parhaat elämykset timin seurassa, tim on digitaalinen matka-assistenttisi joka auttaa sinua löytämään ja kokemaan unohtumattomia lomamuistoja. treffaa tim – hän on täällä sinua varten , inspiroidu euroopan matkakohteista, uusin lomakohde, ota yhteyttä.

Time travel could be possible, but only with parallel timelines

Assistant Professor, Physics, Brock University

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

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

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

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

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

Arguments against time travel

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

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

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

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

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

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

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

Eliminating the paradoxes

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

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

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

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

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

Introducing multiple histories

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

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

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

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

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

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

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

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The Ministry of Time

By Kaliane Bradley Avid Reader Press: 352 pages, $28.99 If you buy books linked on our site, The Times may earn a commission from Bookshop.org , whose fees support independent bookstores.

“People aren’t history,” scoffs Adela, vice secretary of the Ministry, whose work is shrouded in secrecy and subterfuge. This retort comes late in Kaliane Bradley’s debut novel, “The Ministry of Time,” but it’s a telling line. Its dismissal of individual lives reveals the novel’s stakes. If people aren’t history, what is? This is a disturbing statement to come out of the mouth of a high-ranking British bureaucrat. For a book that could also be easily described as witty, sexy escapist fiction, “The Ministry of Time” packs a substantial punch.

Of late, many critically acclaimed books embrace mystery and absurdity in a way that both suspends and expands conventionally held notions of time. Hilary Leichter’s “Terrace Story,” National Book Critics Circle award winner Lorrie Moore’s “I Am Not Homeless If This Is Not My Home,” Ali Smith’s “Companion Piece,” National Book Award winner Justin Torres’ “Blackouts,” and Marie-Helene Bertino’s “Beautyland,” among others, forge poignant, bracing emotional connections. Their playfulness reveals possibilities and perspectives that might be lost in a novel bound by fact-checked 21st century reality. After all, in a world where nothing feels normal, fiction that embraces a disregard for physics and convention mirrors our new upside-down quotidian life.

The Ministry of Time cover with the title in 3-D in many colors against a dark background

To this end, Kaliane Bradley proves that it’s possible to address imperialism, the scourge of bureaucracy, cross-cultural conflict and the paranoia inherent in a surveillance state through her utterly entertaining novel. “The Ministry of Time” begins with a sixth-round job interview for an undisclosed position. The unnamed narrator is caught off guard when “the interviewer said my name, which made my thoughts clip. I don’t say my name, not even in my head. She’d said it correctly, which people generally don’t.” For the narrator, who “plateaued” as a “translator-consultant” in the Languages department of the Ministry of Defense, this top-secret job that pays three times her current salary is worth the mystery.

Soon the work is disclosed. The narrator, whose mother immigrated to the United Kingdom from Cambodia, will be working closely with people who might bristle at the term “refugees.” She’s now part of the Ministry of Expatriation working with one of five “expats” scavenged from the past. Confident in her storytelling, Bradley sweeps away the details of how and why time travel exists in the novel.

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“All you need to know is that in your near future, the British government developed the means to travel through time but had not yet experimented with doing it.” With that, from the jump, readers and characters alike are asked to take a leap of faith as the narrator assumes the role of a “bridge” between an “expat” and modern life. The larger purpose of her work is elusive to her as well, but, in short, the narrator is tasked to be the roommate of an explorer named Commander Graham Gore who died on a doomed Royal Navy Arctic exhibition in 1847. He’s mannered, understandably jumpy, but also rather sexy for someone who died close to 200 years ago. Will this be an odd sort of meet-cute, or is something more chaotic afoot?

Buckle up, the ride has just begun. Juggling notions of “hereness” (the present) and “thereness” (the past), the novel’s five expats come to grips — or not — with the fact that they have been snatched from the past. The ministry selected individuals who were on the point of death so that their departures from the past would in no way rupture their historic timelines. But how would they affect the present? The bridges take notes, and medical examinations are de rigueur, but these collected data are merely passed along without much in the way of analysis.

The expats and bridges adjust to life together in a largely amusing fashion, sharing lovely homes provided by the ministry, visiting pubs, learning about the very existence of cinema. Music streaming services are a hit, but, generally speaking, the expats find it hard to accept the scale of modern life. Whether this is due to some physical mutation created by the process of time travel or if it’s merely the challenge of cultural displacement for people “loose as dust in narrative time,” true fissures begin to surface.

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In a manner that feels wholly unsurprising to the reader, suspicions arise as to the nature of the project. Why exactly is there a need for secrecy and what are the particulars about this ability to time travel that we’re asked to tacitly accept? Tensions also flare between the narrator and another bridge, Simellia, who are the two people of color in this tightly knit circle. The specter of imperialism looms and informs a certain tension between the narrator and Gore. Yet, as a green bureaucrat, happy to rise in the ranks, she confronts Simellia, saying, “You signed up for this job… [knowing], as much as I did, that what we were doing was world-changing. That’s what you wanted, remember? Do you think the world changes by being asked politely? Or do you think there has to be risk?”

But this brash confidence begins to waver. The narrator recognizes, “Every time I gave Graham a book, I was trying to shunt him along a story I’d been telling myself all my life.” Then she notes that the ministry “fed us all poison from a bottle marked ‘prestige’ and we developed a high tolerance for bitterness.” While the book does assume some obvious postures of university level post-colonial theory and language, it moves past these more cliched moments by focusing its attention on the characters. A tight narrative rich with witty banter, cutting observations and interspersed passages from Gore’s doomed expedition also keep the novel taut.

“Maybe I was tired of stories, telling them and hearing them,” muses the narrator. Curiously, while she made languages her profession (hence her tendency to consider that “the great project of Empire was to categorize: owned and owner, coloniser and colonised, evolué and barbarian, mine and yours.”), her antagonistic younger sister became a writer. Finding herself falling in love with Gore, the narrator becomes the story, upending history.

As the story’s momentum builds into that of a spy thriller, Bradley pulls off a rare feat. “The Ministry of Time” is a novel that doesn’t stoop to easy answers and doesn’t devolve into polemic. It’s a smart, gripping work that’s also a feast for the senses. An assassination, moles, questions of identity and violence wreak havoc on our happy lovers and the bubble they create in London. Yet our affection for them is as fresh and thrilling as theirs is for one another, two explorers of a kind, caught in a brilliant discovery. Bradley’s written an edgy, playful and provocative book that’s likely to be the most thought-provoking romance novel of the summer. Check your history: That’s no small feat.

Lauren LeBlanc is a board member of the National Book Critics Circle.

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Na prvom prstu poluostrva Halkidiki, Kasandri, 107 km jugoistočno od Soluna nalazi se popularno letovalište Pefkohori , smešteno u kristalno čistom zalivu Toroneos, Okruženo bujnom mediteranskom vegetacijom. Krase ga prelepa crkva, stari trg, uličice sa rascvetalim limunovim stablima, duga peščana plaža sa postepenim ulaskom u more, šetalište pored mora koje u večernjim časovima postaje centar svih zbivanja sa brojnim tavernama, restoranima, picerijama i barovima. Osim letovanja u klimatizovanim apartmanima, možete Grčku upoznati i na drugačiji način. Fakultativni izleti organizuju se svakoga dana, tako da možete obići Svetu Goru, osetiti neverovatnu lepotu Meteora ili krstariti zalivom Toroneos (poluostrvo Halkidiki). Dođite i uživajte u Pefkohoriju a svakako je ovo mesto i naša preporuka za leto 2024.

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The newer, the better: How wheelchair users can travel smoothly

Chris Ricci can walk only short distances at a time, but he has traveled the world on his scooter.

The 57-year-old U.S. Army veteran has limited mobility stemming from a foot injury he sustained in the 1980s and related complications has found cruises an accessible way to explore destinations like Alaska or the Caribbean. Just this month, he, his wife and friends took a week-long Royal Caribbean International cruise to Belize, Honduras and Mexico.

Ricci, who is based in Lakeland, Florida, simply had to drive to Tampa to board the Enchantment of the Seas. “I think it’s pretty easy,” he told USA TODAY. They booked an accessible stateroom for the first time, too.

“It was really big,” said Ricci, who chronicles his cruises and other travels on his YouTube channel, Scootin Around the World. “I had no problem getting the scooter into the room.”

Cruise ships can offer guests who use mobility devices an accessible, streamlined way to travel. But some extra planning can go a long way toward ensuring the trip goes smoothly. Here’s what to know.

How this adult-only cruise line integrated accessibility into its experiences

Do cruise ships have accessible staterooms for mobility devices?

Cruise lines do offer accessible staterooms, but they’re not all the same. “Make sure that you have the accessible room that's best for you,” said Joy Burns, organization and PR manager at accessible travel booking platform Wheel the World .

For instance, some staterooms may be better suited to motorized wheelchair users, but the setup may not be conducive for passengers who use other kinds of devices, according to her husband, Bert, who has been paralyzed since 1982 and has taken more than 20 cruises.

“If I got a wheelchair-accessible room that's made for a power wheelchair with a Hoyer lift, the bed’s up real high,” he said. “I can’t transfer with a high bed. I use a manual wheelchair, so I’m down low.”

Standard staterooms may also be able to accommodate mobility devices, but guests should familiarize themselves with cruise lines’ requirements.

Passengers booking a standard cabin on Carnival Cruise Line ships must have devices that can fit through a 22-inch-wide doorway, according to its website . The devices must also fold and collapse to allow for safe exit from the room (passengers cannot store mobility devices in corridors or public spaces).

“Guests who bring scooters that are larger than 21" (53.34 cm) wide, or travel with multiple scooters in the same stateroom, must purchase a fully accessible stateroom with a wider doorway (32”, 81.3 cm), or rent a smaller scooter appropriately suited for their stateroom,” the line said on its website. “Guests may be required to have their scooters sized at the time of check-in to ensure it fits in the stateroom.”

Major cruise lines typically have accessibility departments that can help guests navigate the process, according to Sylvia Longmire, an accessible travel writer who runs the blog and travel brand Spin the Globe .

“Talk to the accessibility department to ask the questions and let them know what kind of equipment you're bringing if you're bringing your own stuff,” said Longmire, who has multiple sclerosis and has taken over 30 cruises, most of which have been with a scooter or wheelchair. “Let them know what your ability or disability is, what your level is, what your accessibility requirements are. And that way, they can tell you what kind of cabin you might need, what location, (information) about the ship, and everything before you even call to make the reservation.”

For those booking through a travel agent, she said she “highly” recommends working with someone who has a disability or is an expert in accessible travel.

What cruises are best for guests with mobility devices?

Some destinations may also be better suited to guests with mobility devices than others.

Once, during a New Zealand cruise, the Burnses’ ship hit some rough water, causing Bert to roll out of the casino and crash into the bar (though no one was hurt). Joy recommended researching the conditions of the seas. ( Click here for USA TODAY’S guide to the best times to cruise by region.)

She added that expedition cruises , which often feature activities that require guests to get on and off Zodiac boats, may not be as accessible.

What cruise ship is best for guests with mobility devices?

New ships tend to have more features that can make a trip easier for guests with mobility devices, said the Burnses and Longmire. Those include automatic sliding doors in public spaces, stateroom locks that allow passengers to wave their key to open the door and pool lifts.

“The newer the ship, the better the accessibility,” said Longmire.

Can I get a mobility device through a cruise line?

Guests can arrange to rent mobility devices through vendors like Scootaround or Special Needs at Sea (Longmire is a contract employee for Scootaround).

Rentals may also be available at the cruise terminal, and some lines have a limited number of scooters and wheelchairs available on the ship, according to Longmire. She estimated rentals can run from roughly $80 to $250 depending on the type of equipment, the length of time guests need it and other factors.

Do cruise lines have accessible excursions?

Yes, but options may be limited. Joy said accessible shore excursions available through cruise lines are “few and far between,” and she and Bert have taken cruises where there were none available through the cruise line at all.

Longmire noted that “cruise lines are entirely dependent on the availability of excursions with whoever they're contracting with.” If a given destination doesn’t have wheelchair taxis, for example, that limits cruise lines’ offerings.

Are there blind scuba divers? Here's the accessible way anyone can explore the ocean

When they are available through the line, Longmire said many of them are what she calls windshield tours – bus tours with a group of other passengers that might include one or two 15-minute stops to take photos. “And the rest of it is just seeing everything out the window,” she said.

However, she added that cruise lines “get a little bit better” each year about offering those.

Joy said passengers can also book accessible shore excursions through companies like Wheel the World. Longmire has had luck booking shuttles and tours directly through providers in various ports as well, though guests should take care to be back in time before the ship leaves.

Are tender ports accessible?

When it comes to accessibility at tender ports – where guests have to take a small vessel from the ship to shore – Longmire said it’s “not very black and white.” Cruise ships may let guests board tenders and some have technology to help passengers with wheelchairs do so, but the port’s capabilities may be different.

“Just because you're able to get from the ship to the tender doesn't mean that you'll be able to get from the tender to the dock,” said Longmire.

Rules may differ as well, with some lines allowing guests in manual wheelchairs to use tenders but prohibiting motorized devices. She recommended checking with the accessibility department about those policies.

The Burnses, for their part, have generally found tender ports to be inaccessible, and Joy suggested travelers review itineraries to see how many they include.

Overall, Longmire said her experience on cruises has been “really good,” both for their accessibility and convenience (you can visit multiple places but unpack once, for instance). “I always recommend to wheelchair users for their first travel experience, like, if you don't get seasick and you’re okay with cruises, a cruise is the way to go.”

Nathan Diller is a consumer travel reporter for USA TODAY based in Nashville. You can reach him at [email protected].

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I quit a comfortable tech job to launch a startup. Moving back in with my mom was humbling, but now I make $400,000 and travel all the time.

Gene Caballero left his stable job at Dell to cofound GreenPal, an on-demand lawn care service.
He had to make financial sacrifices in the beginning, but now he earns more and travels often.
He's happy as an entrepreneur and will never return to 9-5 life despite the challenges.

This as-told-to essay is based on a conversation with Gene Caballero, a 44-year-old cofounder of GreenPal, based in Nashville. It's been edited for length and clarity.

I'm a cofounder of GreenPal, a platform that connects homeowners with lawn care professionals.

After purchasing my first home, finding reliable lawn care for myself and my mom was a daunting task. Working in tech on the West Coast and watching companies like Uber, Lyft, and Airbnb get started opened my eyes to one of my cofounders Bryan Clayton's initial idea of on-demand lawn care.

Before working full-time at GreenPal, I spent nine years at Dell , where I first held a sales role. Five years in, I transitioned into a management role, where I stayed until 2017. I learned invaluable lessons in leadership, strategic planning, and team management, all of which have been crucial in steering GreenPal toward success.

I left my stable job at Dell to find a more meaningful career

I had a six-figure salary and was comfortable at Dell, but launching GreenPal was fueled by a quest for greater purpose. I was driven by the urge to solve a common problem. This venture into entrepreneurship was not just a career change but a commitment to innovating how lawn care services are accessed.

Financing GreenPal's inception demanded significant personal sacrifices. Because of my belief in GreenPal, I sold my house and cashed out my 401(k) . To make this work, I moved home with my mom for a year, which was humbling, but I knew it was the right choice. I then got a small apartment with Bryan.

We're now making over $40 million in revenue

Since our founding, we've hired 20 employees. The financial health of the company affords me personal freedoms that were once unimaginable — especially in the corporate world.

My compensation of $400,000 annually supports my lifestyle of travel and exploration. In 2023, I visited 15 countries, and I've already planned five international trips, including summiting Kilimanjaro in September, for this year.

I work every day, but I've streamlined my responsibilities so I never exceed 20 hours a week. Every morning, I look at the number of transactions and revenue we booked the previous day. I then tackle emails, prioritizing our customer care app to promptly address all inquiries and issues. I'm usually done working by noon, if not earlier.

My entrepreneurial journey was both exhilarating and challenging

One of our most impactful mistakes was hiring an external firm to develop our website and apps. Entrusting this critical component to an outside company for a six-figure fee, we expected a seamless launch and a robust platform. The firm failed to deliver a product that met our specifications and went out of business shortly after. This left us without a functional website and with no recourse to recover our investment.

The setback delayed our market entry and forced us to reconsider our strategy. This experience taught us a crucial lesson about the importance of having technical expertise within our founding team. My other cofounder, Zach Hendrix, went to Nashville Software School and learned how to build the current website and apps we use today.

I absolutely do not miss my 9-5 at all

One piece of advice I'd share with other entrepreneurs is to brace yourself for a ride that can be lonely and seemingly without reward for a long stretch. Success in entrepreneurship often comes after enduring periods of uncertainty and solitude. Stay resilient and keep pushing forward, even when the immediate outcomes aren't visible.

I consider myself an entrepreneur, but I will never start another company. If you do it right once, you don't need to do it again.

Watch: Marketing leaders from Amazon, LinkedIn, Lego Group and more tell Insider what pandemic-fueled business changes are likely to stick around

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Can we time travel? A theoretical physicist provides some answers
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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 ...
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In Summary: Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.
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Time travel is 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 ...
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Digitaalinen matka-assistenttisi
Koe parhaat elämykset Timin seurassa. Tim on digitaalinen matka-assistenttisi joka auttaa sinua löytämään ja kokemaan unohtumattomia lomamuistoja. Treffaa Tim - hän on täällä sinua varten! Vastaamalla muutamaan kysymykseen, jätät matkakohteen, lennon sekä majoituksen valinnan Timin huoleksi. Luota Timiin, hän on täällä sinua ...
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Time travel appears to contradict logic. (Shutterstock) The other main issue is less practical, but more significant: it is the observation that time travel seems to contradict logic, in the form ...
Death From Time Travel : r/comics
Ehh, not really, if time is treated like dimension. By moving something in past from future you can change the outcome and even the future that moved the past, but by that point the event is already happend. It doesn't need to be connected because the time traveler cease to be connected to future in the moment they travel to past.
A Journey to Moscow
If you are to visit Moscow as a tourist or on business, or you are planning to meet your partners in Moscow and need accommodation, we are glad to offer you Moscow hotels reservation at discounted rates. You can reserve a hotel of any level - from tourist class to 5-star luxurious hotels. It is an easy on-line reservation with individual ...
21 Things to Know Before You Go to Moscow
1: Off-kilter genius at Delicatessen: Brain pâté with kefir butter and young radishes served mezze-style, and the caviar and tartare pizza. Head for Food City. You might think that calling Food City (Фуд Сити), an agriculture depot on the outskirts of Moscow, a "city" would be some kind of hyperbole. It is not.
Interview with Tim Kirby (RT) in Moscow. "Real Russia" ep.127 (4K)
Inteview with Tim Kirby from RT channel in Moscow, Russia on board of "Valery Brysov". It is located in the city center of Moscow, on the enbankment of Moskv...
Cheap Flights from Honolulu International to Moscow
Compare flight deals to Moscow from Honolulu International from over 1,000 providers. Then choose the cheapest plane tickets or fastest journeys. Flex your dates to find the best Honolulu International-Moscow ticket prices. If you're flexible when it comes to your travel dates, use Skyscanner's "Whole month" tool to find the cheapest month ...
Time travel and surveillance state paranoia collide in a witty, thought
Book Review. The Ministry of Time. By Kaliane Bradley Avid Reader Press: 352 pages, $28.99 If you buy books linked on our site, The Times may earn a commission from Bookshop.org, whose fees ...
- Letovanje 2024, Grčka leto 2024,
Ako tražite apartman u Grčkoj za leto 2024, posetite nasu ponudu i izaberite između različitih destinacija i cenova. Tim Travel je turistička agencija sa višegodišnjim iskustvom i profesionalnim pristupom.
Book at these times to save money on summer flights for 2024
Mid-to-late August is the best time for Americans to fly this summer, according to travel company Expedia. It's expected to be a less busy time, and Expedia says travelers can save up to $265 on ...
Pefkohori
Pefkohori je popularno letovalište u poluostavu Halkidiki, blizu Svete Gorge, Meteora i zaliva Toroneos. Osim letovanja u klimatizovanim apartmanima, možete se vratiti u Pefkohori i obići Svetu Gorge, Meteora i zaliv Toroneos, koji su zanimljivi turistički atrakciji u blizu.
MBTA warns of longer travel times during May Red Line closures
The Red Line is next to go through closures as part of the MBTA's Track Improvement Plan, and the transit agency is telling riders to plan for extra travel time for several weeks in May ...
Can I take a mobility device on a cruise? What you should know.
Chris Ricci can walk only short distances at a time, but he has traveled the world on his scooter. The 57-year-old U.S. Army veteran has limited mobility stemming from a foot injury he sustained ...
News releases
Gov. Tim Walz has proclaimed April 28, 2024, as Worker Memorial Day in Minnesota and the Interstate 35W Bridge in Minneapolis was lit orange that night to honor the men and women who have died while working in work zones. ... Expect delays, especially during peak travel times. Do the zipper merge. Avoid unnecessary lane changes. ...
Women's Tennis Receives NCAA Tournament Selection; Will Travel to
The Blue Devils will travel to Knoxville, Tenn., for the first and second rounds May 4-5. No. 21 Duke is set to face 63rd-ranked East Tennessee State (17-7) in first-round action on Saturday, May ...
I Quit My Stable Tech Job to Launch a Startup and Now I Make $400,000
Moving back in with my mom was humbling, but now I make $400,000 and travel all the time. As told to Perri Ormont Blumberg. 2024-04-29T09:05:01Z An curved arrow pointing right. Share. The ...

Time travel: Is it possible?

Albert Einstein's theory

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Can wormholes take us back in time?

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Time travel could be possible, but only with parallel timelines

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Journey to Moscow

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A History of Moscow in 13 Dishes

Time travel and surveillance state paranoia collide in a witty, thought-provoking romance novel

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I quit a comfortable tech job to launch a startup. Moving back in with my mom was humbling, but now I make $400,000 and travel all the time.