Time travel is the concept of moving backward or forward to different points in time, in a manner analogous to moving through space. Additionally, some interpretations of time travel suggest the possibility of travel between parallel realities or universes.
Origins of the concept
The short story The Clock That Went Backward by Edward Page Mitchell, which appeared in the New York Sun in 1881, was probably the first widely-published story to feature time travel.
This was followed by The Time Machine by HG Wells, published in 1895. It was the first novel to suggest the theme of time travel by machine, and helped to popularize the notion of time traveling.
Since that time, both science and fiction have expanded on the concept of time travel.
Time travel in theory
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.Concepts that aid such understanding include the closed timelike curve.
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 if one were to go back in time and kill one' own grandfather? 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 "presentist" view
Presentism holds that neither the future nor the past exist; that the matter of the universe only exists in the present moment, that time is merely a concept of man used to describe what is going on around him. This means that there is nowhere for a time traveller to go, thus rendering the whole topic of time travel null and void. This view argues that time does not flow, but that observations of time's apparent movement are simply the relation of old memory to the present, or instant.
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.
Time dilation
Time dilation is permitted by Albert Einstein's special theory of relativity. These theories state that, relative to a stationary observer, time appears to pass more slowly for faster-moving bodies, or bodies that are within a deep gravity well. For example, a moving clock will appear to run slow; as a clock approaches the speed of light it will appear to slow to a stop. This has given rise to the popular twin paradox. General relativity states that a similar effect would occur if the clock were to be close to a black hole.
Time can be apparently sped up for living organisms through hibernation, where the body temperature and metabolic rate of the creature is reduced. A more extreme version of this is suspended animation, where the rates of chemical processes in the subject would be severely reduced.
Time dilation and suspended animation only allow "travel" to the future, never the past, so they do not violate causality, and arguably should not be considered time travel.
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 his (and many others's) 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.
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 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.
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 densitynd "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 presume 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. 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.
Daniel Greenberger and Karl Svozil 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 tacitly assumes that there exists an arrow of time, the direction from the past to the future. However, there are only a few equations of physics which would give rise to such a direction of time, the main one being the second law of thermodynamics, which states that entropy increases with time. This means that the direction of time may not be a fundamental intrinsic property of the universe, which would mean that the notion of time travel is also not fundamental to the universe.
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.
Gradual and instantaneous
In literature, there are two (commonly used) methods of time travel:
Origins of the concept
The short story The Clock That Went Backward by Edward Page Mitchell, which appeared in the New York Sun in 1881, was probably the first widely-published story to feature time travel.
This was followed by The Time Machine by HG Wells, published in 1895. It was the first novel to suggest the theme of time travel by machine, and helped to popularize the notion of time traveling.
Since that time, both science and fiction have expanded on the concept of time travel.
Time travel in theory
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.Concepts that aid such understanding include the closed timelike curve.
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 if one were to go back in time and kill one' own grandfather? 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 "presentist" view
Presentism holds that neither the future nor the past exist; that the matter of the universe only exists in the present moment, that time is merely a concept of man used to describe what is going on around him. This means that there is nowhere for a time traveller to go, thus rendering the whole topic of time travel null and void. This view argues that time does not flow, but that observations of time's apparent movement are simply the relation of old memory to the present, or instant.
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.
Time dilation
Time dilation is permitted by Albert Einstein's special theory of relativity. These theories state that, relative to a stationary observer, time appears to pass more slowly for faster-moving bodies, or bodies that are within a deep gravity well. For example, a moving clock will appear to run slow; as a clock approaches the speed of light it will appear to slow to a stop. This has given rise to the popular twin paradox. General relativity states that a similar effect would occur if the clock were to be close to a black hole.
Time can be apparently sped up for living organisms through hibernation, where the body temperature and metabolic rate of the creature is reduced. A more extreme version of this is suspended animation, where the rates of chemical processes in the subject would be severely reduced.
Time dilation and suspended animation only allow "travel" to the future, never the past, so they do not violate causality, and arguably should not be considered time travel.
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 his (and many others's) 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.
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 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.
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 densitynd "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 presume 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. 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.
Daniel Greenberger and Karl Svozil 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 tacitly assumes that there exists an arrow of time, the direction from the past to the future. However, there are only a few equations of physics which would give rise to such a direction of time, the main one being the second law of thermodynamics, which states that entropy increases with time. This means that the direction of time may not be a fundamental intrinsic property of the universe, which would mean that the notion of time travel is also not fundamental to the universe.
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.
Gradual and instantaneous
In literature, there are two (commonly used) methods of time travel:
- 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; it's popular probably because it is more spectacular and makes time travel easier.
- 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.
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 him or her to "A.D. 1865" and step out into the exact same spot on Earth ignores the issue that Earth is moving through space around the Sun, which is moving in the galaxy, and so on. 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."
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.
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.
