Time travel

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.

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. 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.

Related Topics:
Albert Einstein - Special theory of relativity - General theory

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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.

Related Topics:
Occam's Razor - Causality - Grandfather paradox - Stephen Hawking - Fermi paradox

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The equivalence of time travel and faster-than-light travel

First of all, if one is able to move information from one point to another faster than light, then according to special relativity, there will be an observer who sees this information transfer as allowing information to travel into the past.

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The general theory of relativity extends the special theory to cover gravity. It does this by postulating that matter "curves" the space in its vicinity. But under relativity, properties of space are fairly interchangeable with properties of time, depending on one's perspective, so that a curved path through space can wind up being a curved path through time. In moderate degrees, this allows two straight lines of different length to connect the same points in space; in extreme degrees, theoretically, it could allow timelines to curve around in a circle and reconnect with their own past. General relativity describes the universe under a complex 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.

Related Topics:
General theory of relativity - Special theory - Closed time-like curve - Kurt Gödel

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Using wormholes

A proposed time-travel machine using a wormhole would (hypothetically) work something like this:

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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.

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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," or "negative 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.

Related Topics:
Sun - Exotic matter - Negative matter - Casimir effect

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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. http://arxiv.org/abs/hep-th/9202090 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. http://xxx.lanl.gov/abs/gr-qc/9702043

Related Topics:
Matt Visser - 1993 - Causality - 1997 - Roman ring - Tom Roman

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Another approach - attributed to Frank Tipler, but invented independently by W.J. van Stockum http://www-lorentz.leidenuniv.nl/history/stockum/Proc_R_Soc_Edinb_57_135_1937.jpg in 1936 and Kornel Lanczos http://dx.doi.org/10.1023/A:1010277120072 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.

Related Topics:
Frank Tipler - W.J. van Stockum - 1936 - Kornel Lanczos - 1924 - Spinning cylinder - Cosmic string

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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.

Related Topics:
Robert Forward - Magnetic field

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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.

Related Topics:
Quantum teleportation - EPR paradox - Quantum entanglement - Bohm interpretation - Action at a distance

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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.

Related Topics:
Information - FTL communication - No cloning theorem

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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.

Related Topics:
Novikov self-consistency principle - Kip S. Thorne

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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.

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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).

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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.

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Perhaps the simplest resolution of the paradoxes is to consider the space-time symmetry seriously, i.e. to treat time on an equal footing with space. Clearly, as there are no space-travel paradoxes, there can be no time-travel paradoxes. The Novikov self-consistency principle must be equally valid in both space and time. Unfortunately, such a resolution seems to be in contrast with an intuitive notion of time that differs significantly from that of space, because time, in contrast with space, is viewed as something that lapses. However, most physicists agree that the lapse of time is an illusion emerging from the existence of the arrow of time.

Related Topics:
Space-time - Novikov self-consistency principle - Time - Space - Arrow of time

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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 where our history tells of significantly less than multitudes being there.

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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.

Related Topics:
Max Tegmark - Causality - Anthropic principle

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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.

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