Time travel is the concept of moving forward and backward to different points in time, in a manner analogous to moving through space. Additionally, some interpretations of time travel result in travel between parallel realities or universes.

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.

Physics

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. The effects of this sort of time dilation are discussed further in the popular "twin paradox".

A second, similar type of time travel is permitted by general relativity, where a distant observer sees time passing more slowly for a clock at the bottom of a deep gravity well, and a clock lowered into a deep gravity well and pulled back up will indicate that less time has passed a stationary clock that stayed with the distant.

These effects allow "time travel" only toward the future: never backward. This is not typical of the "time travel" featured in 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 travelers 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. Additionally, 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.

Using wormholes

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.

According to current theories on the nature of wormholes, creating a wormhole of a size useful for a person or 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 hypothesized 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 Willem Jacob 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

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. The fact that these quantum phenomena apparently do not allow FTL/time travel is often overlooked 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.

In a 2005 paper, Professor Daniel Greenberger of City University of New York and Karl Svozil of the Vienna University of Technology proposed that quantum theory gives a model for time travel without paradoxes. In quantum theory observation causes possible states to 'collapse' into one measured state; hence, the past observed from the present is deterministic (it has only one possible state), but the present observed from the past has many possible states until our actions cause it to collapse into one state. Our actions will then be seen to have been inevitable.

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 will have already destabilised itself out of the timeline by the time we would have reached it.

Time travel and the direction of time

The notion of time travel (either towards the future or towards the past) tacitly assumes that there exists a direction of time, the direction from the past to the future. On the other hand, the direction of time (or the arrow of time) may not be a fundamental intrinsic property of time, but rather could be viewed as an emergent property traceable to the fact that we live in a universe in which the entropy increases with time. In this view, as the direction of time is not fundamental, the notion of time travel is also not fundamental. Without a fundamental notion of time travel there can be no fundamental problems with time travel. Without an intrinsic direction of time, time can be viewed as a "static" coordinate similar to other spacetime coordinates. From this point of view, the Novikov self-consistency principle is a tautology, a demand that hardly needs to be questioned, which automatically prevents causal paradoxes.

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.

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 main types and a third, less common type (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 (example: Doctor Who, where the time line is fluid and changes often naturally).

3. The time line is consistent, but only insofar as its consistency can be verified.

3.1 The Novikov self-consistency principle applies, but if and only if it is verified to apply. Attempts to travel into the past to change events are possible, but provided that:

-They do not interfere with the occurrence of such an attempt in the present (as would be the case in the Grandfather Paradox), and

-The change is never ultimately verified to occur by the traveller (e.g. there is no possibility of returning to the present to witness the change).

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.

Immutable timelines

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.

Example: A man who travels into the past and intends to shoot his grandfather as a young boy finds himself snapped back to the present as he's about to pull the trigger.

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: 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).

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 realizing 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 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 1950-2000, hoping to use it for financial gain. The book is stolen by the aged Biff Tannen, who takes the time-traveling DeLorean back in time to give the almanac to his 1955 younger self. When McFly and Doc Brown use the DeLorean to go back to 1985, they soon discover what Tannen had done; the younger Tannen has used the almanac for financial gain and changed the timeline. The alternate 1985 that McFly and Brown have returned to is now the future of a tangent that started in 1955, where Hill Valley is now corrupt and its citizens' lives changed because of Tannen.

Mutable timelines

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 travelers 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. This is sometimes referred to as the "Time Dillution Effect."

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 using the controls on a CD player to skip to a previous or next song. 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 traveling the other way." To expand on the audio playback analogy used above, this would be like rewinding or fast forwarding an analogue audio cassette and playing the tape at a chosen point. 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 a moving platform with a highly complicated trajectory. A vessel that moves "ahead" 5 seconds might materialize in the air, or inside solid rock, depending on where Earth was "before" and "after." In the 2000AD Comic Mutant Bounty Hunter Johnny Alpha uses "Time Bombs" to propel an enemy several seconds into the future, during which time the movement of the Earth causes the unfortunate victim to re-materialize in space. To really do what filmmakers make look so easy in films such as the Back to the Future series and The Time Machine, the device might have to be a very powerful spacecraft which could move across large distances in space to compensate for the offset of position associated with the change in time.

A possible rebuttal to this criticism is the fact that cars and airplanes manage to move around the surface of the Earth with it, despite the surface itself moving with an astronomical speed. One could postulate that a time traveller experiences a combination of spatial temporal inertia that makes him move along with the Earth.

In the 1980 Robert Heinlein novel The Number of the Beast a "continua device" allows the protagonists to dial in the six (not four!) co-ordinates of space and time and it instantly moves them there—without explaining how such a device might work. The television series Seven Days also dealt with this problem; when the chrononaut would be 'rewinding', he would also be propelling himself backwards along the earth's orbit, with the intention of landing in the same place (in space) that he originated.

References

Entertainment & fictional media

See Time travel in fiction

Scientific references

• 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 Schrödinger'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, , 1924, republished in 1997 by Springer Science+Business Media B.V.
• Willem Jacob van Stockum, , 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,
• Miller, Kristie. Time travel and the open future. Disputatio Vol 1. Issue 19 (2005): 223-232.

Literary references

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
• Empire Star by Samuel R. Delany
• The Anubis Gates by Tim Powers
• 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
• The Hounds of Tindalos by Frank Belknap Long
• Les Seigneurs de la guerre (The Overlords of War) by Gérard Klein
• Mastadonia by Cliford Simak
• October the First is Too Late by Hoyle
• Paratime series by H. Beam Piper
• Rotating Cylinders and the Possibility of Global Causality Violation by Larry Niven
• Scape Scope by Stith
• Time and Again by Jack Finney
• The Coming of the Quantum Cats by Frederik Pohl
• The End of Eternity by Isaac Asimov
• The Hitchhiker's Guide to the Galaxy series by Douglas Adams
• 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 Simon 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
• Night Watch by Terry Pratchett
• Timeline by Michael Crichton
• Slaughterhouse-Five by Kurt Vonnegut

Philosophical references

• Richard M Gale, The Philosophy of Time
• Fred Alan Wolf, The Yoga of Time Travel (2004) ISBN 083560828X

See also

• Tipler Cylinder
• Anachronism
• The mysterious "Chronovisor" device
• Grandfather paradox
• Predestination paradox
• Ronald Mallett
• Temporal mechanics
• Thiotimoline
• UFO Phil
• Montauk Project
• Time Traveler Convention
• Time loop

Tidsrejse Zeitreise Viaje a través del tiempo Aikakone Voyage dans le temps מסע בזמן Időutazás Perjalanan waktu Voyajo en tempo Viaggio nel tempo タイムトラベル Tijdreizen Viagem no tempo Машина времени (устройство) Tidsresa 时间旅行