Time travel is the concept of moving forward and backward to different points in time, much as we do through space. It also includes traveling sideways in time between parallel realities or universes.
Humans are in fact always travelling in time — in a linear fashion, from the present to the immediate future, inexorably, until death. Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime, or certain types of motion in space, may allow time travel into the past and future if these geometries or motions are possible. It has been confirmed that the effects of relativistic and gravitational time dilation can cause a traveller who starts at and returns to a point of origin that remains stationary, to arrive at a time farther in the future in that reference frame than their subjective elapsed time would indicate (a constrained form of time travel into the future).
In physics, the concept of time travel has been often used to examine the consequences of physical theories such as special relativity, general relativity and quantum mechanics. There is no experimental evidence of time travel, and it is not even well understood whether (let alone how) the current physical theories permit any kind of time travel. However, theories do exist about the possibility of folding time to hop from one point to another.
In science fiction it is a recurring plot device, used to set a character in a particular time not their own, and explore the character's interaction with the people and technology of that time—as a kind of culture shock. Other ramifications explored are change and reactions to it, such as alternate histories.
- 1 Physics
- 2 Time travel and religion
- 3 Time travel in fiction
- 4 Time travel, or space-time travel?
- 5 "Distance" of time travel
- 6 Fundamental problems with time "travel"
- 7 References
- 8 See also
- 9 External links
Albert Einstein's special theory of relativity (and, by extension, the general theory) very explicitly permits a kind of time dilation that would ordinarily be called time travel. The theory holds that, relative to a stationary observer, time appears to pass more slowly for faster-moving bodies: for example, a moving clock will appear to run slow; as a clock approaches the speed of light its hands will appear to nearly stop moving. Einstein referred to the effects of this sort of time dilation as the "twin paradox." However, this effect allows "time travel" only toward the future: never backward. It is not typical of science fiction, and there is little doubt surrounding its existence; "time travel" will hereafter refer to travel with some degree of freedom into the past or future.
Many in the scientific community believe that time travel is highly unlikely. This belief is largely due to Occam's Razor. Any theory which would allow time travel would require that issues of causality be resolved. What happens if you try to go back in time and kill your grandfather?—see grandfather paradox. Also, in the absence of any experimental evidence that time travel exists, it is theoretically simpler to assume that it does not happen. Indeed, Stephen Hawking once suggested that the absence of tourists from the future constitutes a strong argument against the existence of time travel—a variant of the Fermi paradox, with time travellers instead of alien visitors. However, assuming that time travel cannot happen is also interesting to physicists because it opens up the question of why and what physical laws exist to prevent time travel from occurring.
The equivalence of time travel and faster-than-light travel
If one were able to move information or matter from one point to another faster than light, then according to special relativity, there would be an observer who sees this transfer as allowing information or matter to travel into the past. Further, faster than light travel along suitable paths would correspond to travel backward in time as seen by all observers. This results simply from the geometry of spacetime and the role of the speed of light in that geometry.
Special spacetime geometries
The general theory of relativity extends the special theory to cover gravity, describing it in terms of curvature in spacetime caused by mass-energy and the flow of momentum. General relativity describes the universe under a system of "field equations," and there exist solutions to these equations that permit what are called "closed time-like curves," and hence time travel into the past. The first and most famous of these was proposed by Kurt Gödel, but all known current examples require the universe to have physical characteristics that it does not appear to have. Whether general relativity forbids closed time-like curves for all realistic conditions is unknown. Most physicists believe that it does, largely because assuming some principle against time travel prevents paradoxical situations from occurring.
A proposed time-travel machine using a wormhole would (hypothetically) work something like this: A wormhole is created somehow. One end of the wormhole is accelerated to nearly the speed of light, perhaps with an advanced spaceship, and then brought back to the point of origin. Due to time dilation, the accelerated end of the wormhole has now experienced less subjective passage of time than the stationary end. An object that goes into the stationary end would come out of the other end in the past relative to the time when it enters. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine; in essence, it is more of a path through time than it is a device that itself moves through time, and it would not allow the technology itself to be moved backwards in time. This could provide an alternative explanation for Hawking's observation: a time machine will be built someday, but has not yet been built, so the tourists from the future cannot reach this far back in time.
Creating a wormhole of a size useful for macroscopic spacecraft, keeping it stable, and moving one end of it around would require significant energy, many orders of magnitude more than the Sun can produce in its lifetime. Construction of a wormhole would also require the existence of a substance known as "exotic matter", which, while not known to be impossible, is also not known to exist in forms useful for wormhole construction (but see for example the Casimir effect). Therefore it is unlikely such a device will ever be constructed, even with highly advanced technology. On the other hand, microscopic wormholes could still be useful for sending information back in time.
Matt Visser argued in 1993 that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other.  Because of this, the two mouths could not be brought close enough for causality violation to take place. However, in a 1997 paper, Visser hypothesised that a complex "Roman ring" (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely than not a flaw in classical quantum gravity theory rather than proof that causality violation is possible. 
Another approach - attributed to Frank Tipler, but invented independently by W.J. van Stockum  in 1936 and Kornel Lanczos  in 1924 - involves a spinning cylinder. If a cylinder is long, and dense, and spins fast enough about its long axis, then a spaceship flying around the cylinder on a spiral path could travel back in time (or forward, depending on the direction of its spiral). However, the density and speed required is so great that ordinary matter is not strong enough to construct it. A similar device might be built from a cosmic string, but none are known to exist, and it does not seem to be possible to create a new cosmic string. *Note that as of September 2005, a cosmic string was observed. It's prescence was indicated by a distortion of light similar to that of a black hole, however, the distortion is different from the characteristic distortion of a black hole due to the fact that it only created distortion along 2 dimensions, not 3 as a normal black hole would have done.*
Physicist Robert Forward noted that a naïve application of general relativity to quantum mechanics suggests another way to build a time machine. A heavy atomic nucleus in a strong magnetic field would elongate into a cylinder, whose density and "spin" are enough to build a time machine. Gamma rays projected at it might allow information (not matter) to be sent back in time. However, he pointed out that until we have a single theory combining relativity and quantum mechanics, we will have no idea whether such speculations are nonsense.
Using Quantum Entanglement
Quantum-mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation presumes that some information is being exchanged between particles instantaneously in order to maintain correlations between particles. This effect was referred to as "spooky action at a distance" by Einstein.
Nevertheless, the rules of quantum mechanics curiously appear to prevent an outsider from using these methods to actually transmit useful information, and therefore do not appear to allow for time travel or FTL communication. This misunderstanding seems to be widespread in popular press coverage of quantum teleportation experiments. The assumption that time travel or superluminal communications is impossible allows one to derive interesting results such as the no cloning theorem, and how the rules of quantum mechanics work to preserve causality is an active area of research.
The possibility of paradoxes
The Novikov self-consistency principle and recent calculations by Kip S. Thorne indicate that simple masses passing through time travel wormholes could never engender paradoxes—there are no initial conditions that lead to paradox once time travel is introduced. If his results can be generalised they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that any situation you can set up in a time travel story turns out to permit many consistent solutions. The circumstances might, however, turn out to be almost unbelievably strange.
Parallel universes might provide a way out of paradoxes. Everett's many-worlds interpretation of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories. These alternate, or parallel histories would form a branching tree symbolizing all possible outcomes of any interaction.
Another possibility is that a Time Paradox may cause such a powerful conflict in the time lines that the entire universe may annihilate itself, in the same way that robots in Isaac Asimov's books enter a mental freeze-out when given two conflicting orders.
Since all possibilities exist, any paradoxes can be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction. However, in actuality, physicists believe that such interaction or interference between these histories is not possible (see Chronology protection conjecture).
A further suggestion related to paradoxes suggests that time travel will never exist, even if theoretically possible. The reasoning is that as long as time travel exists, history will change, and will only become static when a timeline is reached in which no time travel exists and thus no further changes can be made. Assuming there is only a single dimension of time, the timeline we perceive must be the one that exists after all changes (if any) are made, and thus we will never perceive the invention of time travel, since it has already destabilised itself out of the timeline by the time we reach it.
If time travel was practical, and affordable, then masses of people would want to be spectators at great events of history, leading to the Cumulative Audience Paradox; our history tells of significantly less than multitudes watching at great events (but then perhaps audiences would remain hidden).
“History would become an experimental science.” says Carl Sagan.
Time travel and the anthropic principle
It has been suggested by physicists such as Max Tegmark that the absence of time travel and the existence of causality may be due to the anthropic principle. The argument is that a universe which allows for time travel and closed time-like loops is one in which intelligence could not evolve because it would be impossible for a being to sort events into a past and future or to make predictions or comprehend the world around them.
Note that this imposes no restriction on supernatural agents (i.e. gods) which are not confined by the bounds of space-time. See the next section for details.
Time travel and religion
Prophecy and theology
It is interesting to note that any religion which postulates the existence of fulfilled prophecy requires, at the very least, an agent which can move information from the future into the past.
In Christian theology, for example, God is assumed to exist unbound by space or time. Doctrinally, God is held to be omniscient, omnipresent, and omnipotent. Statements in the Bible such as Jesus's claim "before Abraham was born, I am" (John 8:58) and Peter's claim "[Jesus] was chosen before the creation of the world" (1 Peter 1:20) (assuming the creation of the world began at t = 0) imply that God does not occupy the same timeline that we do. This is further supported by the assertion "I the LORD do not change" (Malachi 3:6), since change requires movement along, and constrained by, a temporal continuum.
Two popular interpretations of these statements are that God (1) exists outside the space-time continuum; or (2) exists at every point in space-time simultaneously. In either case, God can transfer information from one point in space-time to any other point without restriction because of his great overwhelming power.
Transcending time through ancient wisdom
The Yoga Sutras of Patanjali have been considered by some, such as physicist Fred Alan Wolf in his book, The Yoga of Time Travel, to describe an inner process by which we can access knowledge of the past and future in the present. This form of time travel can be acquired by transcending the five earthly anchors of the ego mind which otherwise leave us locked into the illusory self.
Time travel in fiction
- Main article: Time travel in fiction
Types of time travel
Time travel themes in science fiction and the media can generally be grouped into two types (based on effect—methods are extremely varied and numerous), each of which is further subdivided. These type classifications do not address the issue of time travel itself, i.e. how to travel through time, but instead call to attention differing rules of the time line.
- 1. The time line is consistent and can never be changed.
- 1.1 One does not have full control of the time travel. One example of this is The Morphail Effect.
- 1.2 The Novikov self-consistency principle applies (named after Dr. Igor Dmitrievich Novikov, Professor of Astrophysics at Copenhagen University). The principle states that if you travel in time, you cannot act in such a way so as to create a paradox.
- 1.3 Any event that appears to have changed a time line has instead created a new one.
- 1.3.1 Such an event can be the life line existence of a human (or other intelligence) such that manipulation of history ends up with there being more than one of the same individual, sometimes called time clones.
- 1.3.2 The new time line may be a copy of the old one with changes caused by the time traveler. For example there is the Accumulative Audience Paradox where multitudes of time traveler tourists wish to attend some event in the life of Jesus or some other historical figure, where history tells us there were no such multitudes. Each tourist arrives in a reality that is a copy of the original with the added people, and no way for the tourist to travel back to the original time line.
- 2. The time line is flexible and is subject to change.
- 2.1 The time line is extremely change resistant and requires great effort to change it. Small changes will only alter the immediate future and events will conspire to maintain constant events in the far future; only large changes will alter events in the distant future.
- 2.2 The time line is easily changed.
There are also numerous science fiction stories allegedly about time travel that are not internally consistent, where the traveler makes all kinds of changes to some historical time, but we do not get to see any consequences of this in our present day. For example in SPI's time travel wargame a player's traveler machine gunned a roman legion has no effect on same traveler's subsequent feudal era adventures.
Time travel in a type 1 universe does not allow any paradoxes, although in 1.3, events can appear to be paradoxical.
In 1.1, time travel is constrained to prevent paradox. If one attempts to make a paradox, one undergoes involuntary or uncontrolled time travel. Michael Moorcock uses a form of this principle and calls it The Morphail Effect. In the time-travel stories of Connie Willis, time travelers encounter "slippage" which prevents them from either reaching the intended time or translates them a sufficient distance from their destination at the intended time, as to prevent any paradox from occurring.
In 1.2, the Novikov self-consistency principle asserts that the existence of a method of time travel constrains events to remain self-consistent (i.e. no paradoxes). This will cause any attempt to violate such consistency to fail, even if extremely improbable events are required.
- Example #1: You have a device that can send a single bit of information back to itself at a precise moment in time. You receive a bit at 10:00:00 PM, then no bits for thirty seconds after that. If you send a bit back to 10:00:00 PM, everything works fine. However, if you try to send a bit to 10:00:15 PM (a time at which no bit was received), your transmitter will mysteriously fail. Or your dog will distract you for fifteen seconds. Or your transmitter will appear to work, but as it turns out your receiver failed at exactly 10:00:15 PM. Etc, etc. Two excellent examples of this kind of universe is found in Timemaster, a novel by Dr. Robert Forward, and the 1980 Jeannot Szwarc film Somewhere In Time (based on Richard Matheson's novel Bid Time Return).
- Example #2: In the case of Somewhere In Time, the film deals with events that have already or about to happen which the lead character Richard Collier (played by Christopher Reeve) could not control. Here, Collier is given a watch by a lady he has not yet known (but who already knew him in the past). Sometime later, Collier is fascinated by a picture taken in 1912 of a young actress. Eventually he learns that the woman in the picture is the old lady who gave him the watch, and that he was actually there in 1912 to meet her. Collier chooses to willfully go back in time 68 years in the past to fulfill what was written in the history books. He meets her and falls in love with her, but one day finds a penny in his pocket that he had brought back in time accidentally; the minting date on it is 68 years in the future. Holding tangible proof that he does not "belong" in the past hurls him back to the present day, and so everything that will be/was written in history has happened and Collier could not do anything to change that history. Had he remained in 1912, history would have been altered, and everything that happened at the beginning of the film would not have come true.
An example which could conceivably fall into either 1.1 or 1.2 can be seen in book and film versions of Harry Potter and the Prisoner of Azkaban. Harry and Hermione go back in time to change history. As they do so it becomes apparent that they are simply performing actions that were previously seen in the story, although neither the characters nor the reader were aware of the causes of those actions at the time. This is another example of the predestination paradox. It is arguable, however, that the mechanics of time travel actually prevented any paradoxes, firstly, by preventing them from realising a priori that time travel was occurring and secondly, by enabling them to recall the precise action to take at the precise time and keep history consistent.
In a universe that allows retrograde time travel but no paradoxes, any present moment is the past for a future observer, thus all history/events are fixed. History can be thought of as a filmstrip where everything is already fixed. See block time for a detailed examination of this way of considering the nature of time.
In 1.3, any event that appears to have caused a paradox has instead created a new time line. The old time line remains unchanged, with the time traveller or information sent simply having vanished, never to return. A difficulty with this explanation, however, is that conservation of mass-energy would be violated for the origin timeline and the destination timeline. A possible solution to this is to have the mechanics of time travel require that mass-energy be exchanged in precise balance between past and future at the moment of travel, or to simply expand the scope of the conservation law to encompass all timelines. Some examples of this kind of time travel can be found in David Gerrold's book The Man Who Folded Himself, the Robert Zemeckis film Back to the Future Part II (1989), and the (1994) film Star Trek: Generations.
- Example: In Back to the Future Part II, Marty McFly and Doc Brown decide (after Doc returns from the 21st century to 1985) to travel to 2015 to save McFly's future son. While there, McFly buys an almanac of sporting events from 1951 forward, and decides to use it for financial gain via time travel. Doc Brown forbids him to take the book with him, and inadvertently leaves it lying around for the aged Biff Tannen to take with him. That night, without McFly and Doc Brown knowing it, Tannen takes the time-traveling DeLorean with the book and goes back in time to change history (using the sports almanac for his own financial success). By the movie audience's point of view, Tannen shortly after returns to 2015 and leaves the DeLorean, and McFly and Doc Brown again use the car in an attempt to go back to 1985. But soon the two discover what Tannen had done: Tannen went back to a certain point in 1955, met up with his younger self, and gave the younger Tannen the almanac for him to use for personal and financial gain, so the 1985 that McFly and Brown returned to was the future of a tangent that started in the now alternate 1955, with Hill Valley now corrupt and its citizens' lives changed because of Tannen. McFly and Brown could not just go back to 2015-A (A for alternate) to nab Tannen because whatever they would have done there would have been the future of that particular tangent. In simple words, once you go back in time to change history in this particular instance, whatever happens next will be the future of that particular tangent you just altered (so, for example, if you went back in time to prevent the assassination of President Kennedy in 1963, which in the Twilight Zone episode Profile in Silver saved Kennedy but simulatneously led to Khruschev assassinated instead, and nuclear war in Europe, or, in the case of Star Trek: Generations, change the fate of a planet and thus saving the crew of the Starship Enterprise, the future after that will be the future based on whatever you altered). Back to the Future Part II is inconsistent in allowing Tannen himself to return to his own original 2015. (However, in scenes deleted from the film, an older Biff Tannen dies once arriving in the future, having been erased from existance in that time period. Marty's mother, Lorraine, shot Biff before 2015; therefore he would not exist in that time period.)
Time travel in a Type 2 universe is much more difficult to explain. The biggest problem is how to explain changes in the past. One method of explanation is that once the past changes, so too do the memories of all observers. This would mean that no observer would ever observe the changing of the past (because they will not remember changing the past). This would make it hard to tell whether you are in a Type 1 universe or a Type 2 universe. You could, however, infer such information by knowing if a) communication with the past were possible or b) it appeared that the time line had never been changed as a result of an action someone remembers taking, although evidence exists that other people are changing their time lines fairly often. An example of this kind of universe is presented in Thrice Upon a Time, a novel by James P. Hogan.
Larry Niven suggests that in a type 2.1 universe, the most efficient way for the universe to "correct" a change is for time travel to never be discovered, and that in a type 2.2 universe, the very large (or infinite) number of time travellers from the endless future will cause the timeline to change wildly until it reaches a history in which time travel is never discovered. However, many other "stable" situations may also exist in which time travel occurs but no paradoxes are created; if the changeable-timeline universe finds itself in such a state no further changes will occur, and to the inhabitants of the universe it will appear identical to the type 1.2 scenario.
Gradual and instantaneous
In literature, there are two (commonly used) methods of time travel:
1. The most commonly used method of time travel in science fiction is the instantaneous movement from one point in time to another, like the hand of a boy lifting a toy train from the rails with the wheels still turning, and putting it back at a different place. There is not even the beginning of a scientific explanation for this kind of time travel; its popularity is probably due to the fact that it is more spectacular and makes time travel easier.
2. In The Time Machine H.G. Wells explains that we are moving through time with a constant speed. Time travel then is, in Wells' words, "stopping or accelerating one's drift along the time-dimension, or even turning about and travelling the other way." This method of gradual time travel fits best in quantum physics, but is not popular in modern science fiction. Perhaps the oldest example of this method of time travel is in Lewis Carroll's Through the Looking-Glass (1871): the White Queen is living backwards, hence her memory is working both ways. Her kind of time travel is uncontrolled: she moves through time with a constant speed of –1 and she cannot change it. This would make Lewis Carroll the inventor of time travel. T.H. White, in the first part of his Arthurian novel The Once and Future King, The Sword in the Stone (1938) used the same idea: the wizard Merlyn lives back in time, because he was born "at the wrong end of time" and has to live backwards from in front. "Some people call it having second sight".
Time travel, or space-time travel?
The classic problem with the concept of "time travel ships" in science fiction is that it invariably treats Earth as the frame of reference in space. The idea that a traveller can go into a machine that sends you to "A.D. 1865" and leave through a door into the same spot in Poughkeepsie ignores the issue that Earth is moving through space around the Sun, which is moving in the galaxy, etc. So, given space-time as four dimensions, and "time travel" referring to just "moving" along one of them, a traveller could not stay in the same place with respect to the surface of Earth, because Earth is an accelerating platform with a highly complicated trajectory! A vessel that moves "ahead" 5 seconds might materialise in the air, or inside solid rock, depending on where Earth was "before" and "after." (As seen in the 2000AD Comic, in which Mutant Bounty Hunter Johnny Alpha uses "Time Bombs" to propel an enemy several seconds into the future, during which time the planets passage through space causes the unfortunate victim to re-materialise in vacuum) If you moved "behind" a year, you'd end up in cold outer space, where Earth was a year earlier—in the same part of the Sun's orbit, yes, but where has the sun gone over that year? So, to really do what filmmakers make look so easy in films such as the Back to the Future series and The Time Machine, a time machine might have to be a very powerful spacecraft that could move you large distances and that kept track of Earth's motion through space as part of the solar system, galaxy, etc.
But how can you decouple the ship from momentum? If you try to move forward in time, is your ship automatically going to be propelled by the momentum gained by riding Earth? Or does it decouple? But does not that bring back the idea of an absolute reference frame? Again, even to move one millisecond forward or backward in time, the ship would have to be far beyond anything humans can build, not to mention that the acceleration and deceleration in space-time would challenge the structural integrity not only of the vessel but also of the passengers' bodies. A theorist might even use this to argue in the style of Zeno's paradoxes, for the impossibility of time machines.
A possible rebuttal to this criticism, of course, is the fact that cars and airplanes built by humans manage to move around the surface of the Earth with it, despite the surface itself moving with an astronomical speed. It is reasonable to assume that a time traveller experiences a combination of spatial temporal inertia that makes him move along with the Earth.
In 1980 Robert Heinlein published a novel The Number of the Beast about a ship that lets you dial in the six (not four!) co-ordinates of space and time and it instantly moves you there—without explaining how such a device might work. The television series Seven Days also dealt with this problem; the chrononaut would pilot the time machine away from the earth's surface, and then back to it, by means of a joystick-like device.
"Distance" of time travel
According to special relativity, the physical laws may be invariant over Lorentz transformations. This mixes time and space dimensions as time can be compared to a distance times the speed of light. So, the second is comparable to a unit of distance equal to 299,792.458 kilometres. Conversely, the distance of 1 metre is comparable to about 3.34 nanoseconds. You can also compare a "year" to a "light-year" (since the square of a distance has the opposite sign to the square of a time, time and space are not actually identical).
Now, if we suppose that the same distances in space and time present the same level of technical difficulty, then moving in time for just one second, forward or backward, would be like flying to the Moon. Moving for a few years would be like flying to some of the nearest stars. And if you want to go visiting dinosaurs, perhaps it would be like flying to a far-off galaxy. On the basis of the above argument, some people think that time travel will require a lot of energy (unless we use something like teleportation).
Fundamental problems with time "travel"
Perhaps there is a more fundamental problem with time travel. That is, the concept of "travel" through time may be inherently flawed. "Travel" through space is a time dependent event. For example, you drive 50 miles in 1 hour. Your rate of travel is 50 miles through space per hour of time. To make the analogy between space travel and time travel then becomes problematic. What if I travel 50 years into the future? What then is my rate of travel? 50 years (or 50 lightyears) per what? In other words, travel through time would require another dimension that would assume the role that the time dimension plays in normal "space travel".
Another issue with time travel arises from our definition of the passage of time. It is often said that we all travel at a constant rate through time. This statement, however, may be flawed because of the reason described above about "travel" through time. The passage of time then could instead be rationalized as the way our conciousness observes the four dimensional world, rather than our travel through it. We see our four dimensional universe as a series of three dimensional cross-sections in succession. Einstein referred to a "world line" as the path a point traces out through time. This "point" is not "moving" through time but instead it is just a cross-section of the "world line". This applies to planets, rocks, and people. What we see in the present is just a cross-section of a higher dimensional world. Any type of "movement" in space is just how we observe objects of four dimensions that have shapes that deviate from a straight "world line". Bends in the world line, for example, would be percieved as movement. Such a description of the universe would probably not allow for time travel.
- Paul Davies, About Time ISBN 0684818221
- How to Build a Time Machine ISBN 0142001864
- J. Richard Gott, Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time ISBN 0618257357
- Gribbin, In Search of Schrodinger's Cat
- Paul J. Nahin, Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction ISBN 0387985719
- Pagels, Perfect Symmetry, the Search for the Beginning of Time
- Clifford A. Pickover, Time: A Traveler's Guide ISBN 0195130960
- Kornel Lanczos, On a Stationary Cosmology in the Sense of Einsteins Theory of Gravitation, 1924, republished in 1997 by Springer Science+Business Media B.V.
- W.J. van Stockum, The Gravitational Field of a Distribution of Particles Rotating about an Axis of Symmetry, 1936, Proceedings of the Royal Society of Edinburgh.
- Frank J. Tipler, Rotating Cylinders and the Possibility of Global Causality Violation, Physical Review D 9 (1974), 2203
- H. Nikolic, Causal paradoxes: a conflict between relativity and the arrow of time
- Main article: Time travel in fiction
- A Christmas Carol by Charles Dickens
- A Connecticut Yankee in King Arthur's Court by Mark Twain
- All the Myriad Ways by Larry Niven
- "All You Zombies", By His Bootstraps, The Cat Who Walks Through Walls, Farnham's Freehold, and Time Enough For Love by Robert A. Heinlein
- Richard Mgrdechian's 3000 Years
- The Time Machine by H. G. Wells
- The Time Ships, an authorized sequel to Wells' novel, by Stephen Baxter
- Timelike Infinity, also by Stephen Baxter
- A Bridge of Years and The Chronoliths by Robert Charles Wilson
- Bones of the Earth by Michael Swanwick
- Cascade Point by Timothy Zahn
- Harry Potter and the Prisoner of Azkaban and Harry Potter and the Order of the Phoenix by J.K. Rowling
- Mastadonia by Cliford Simak
- October the First is Too Late by Hoyle
- Paratime series by H. B. Piper
- Rotating Cylinders and the Possibility of Global Causality Violation by Larry Niven
- Scape Scope by Stith
- The Coming of the Quantum Cats by Frederik Pohl
- The End of Eternity by Isaac Asimov
- The Other Side of Time by Keith Laumer
- The Time Traveler's Wife by Audrey Niffenegger
- Thrice Upon a Time by James Hogan
- Time Wars series by S. Hawke
- Twice Upon a Time by Allen Appel
- Doomsday Book (ISBN: 0553562738) and To Say Nothing of the Dog by Connie Willis
- Lost in a Good Book by Jasper Fforde
- The House on the Strand by Daphne du Maurier
- Household Gods (1999) by Judith Tarr and Harry Turtledove
- "Thus We Frustrate Charlemagne" by R. A. Lafferty published in Galaxy in 1966 and also appeared in Alpha 1 (edited by Robert Silverberg) and World's Best SF 1968 (edited by Donald A. Wollheim and Terry Carr).
- Up the Line by Silverberg
- Wrinkle in Time by L'Engle, with several sequels
- Richard M Gale, The Philosophy of Time
- Fred Alan Wolf, The Yoga of Time Travel (2004) ISBN 083560828X
- Tipler Cylinder
- The mysterious "Chronovisor" device
- Grandfather paradox
- Predestination paradox
- Ronald Mallett
- Temporal mechanics
- John Titor
- UFO Phil
- Montauk Project
- Time Traveler Convention
- Time Travel's message forum
- Time, Time Travel & Traversable Wormholes and other time travel related science & technology topics
- SF Chronophysics, a discussion of Time Travel as it relates to science fiction
- On the Net: Time Travel by James Patrick Kelly in Asimov's Science Fiction
- Howstuffworks' article on "How Time Travel Will Work"
- Time Travel in Flatland?
- NOVA Online: Time Travel
- Time Traveler Convention, at MIT - "Technically, you would only need one..."
- Stanford Encyclopedia of Philosophy:
da:Tidsrejse de:Zeitreise es:Viaje a través del tiempo fr:Voyage dans le temps he:מסע בזמן id:Perjalanan waktu io:Voyajo en tempo it:Viaggio nel tempo nl:Tijdreizen ja:タイムトラベル pt:Viagem no tempo ru:Машина времени (устройство) fi:Aikakone sv:Tidsresa zh:时间旅行