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Time Travel is the concept of movement between certain points in time, analogous to movement between different points in space by an object or a person, typically using a hypothetical device known as a time machine. Time travel is a widely-recognized concept in philosophy and fiction. The idea of a time machine was popularized by H. G. Wells' 1895 novel The Time Machine. It may be more practical to construct atomic-scale time machines, however, that are useful for sending information back in time instead of physical objects.

Forward time travel, outside the usual sense of the perception of time, is an extensively-observed phenomenon and well-understood within the framework of special relativity and general relativity. However, making one body advance or delay more than a few milliseconds compared to another body is not feasible with current technology. As for backwards time travel, it is possible to find solutions in general relativity that allow for it, but the solutions require conditions that may not be physically possible. Traveling to an arbitrary point in spacetime has a very limited support in theoretical physics, and usually only connected with quantum mechanics or wormholes, also known as Einstein-Rosen bridges.

History of the time travel concept

Some ancient myths depict a character skipping forward in time. In Hindu mythology, the Mahabharata mentions the story of King Raivata Kakudmi, who travels to heaven to meet the creator Brahma and is surprised to learn when he returns to Earth that many ages have passed. The Buddhist Pāli Canon mentions the relativity of time, before the Payasi Sutta tells of one of the Buddha's chief disciples, Kumara Kassapa, who explains to the skeptic Payasi that time in the Heavens passes differently than on Earth. The Japanese tale of "Urashima Tarō", first described in the Nihongi (720) tells of a young fisherman named Urashima Taro who visits an undersea palace. After three days, he returns home to his village and finds himself 300 years in the future, where he has been forgotten, his house is in ruins, and his family has died. In Jewish tradition, the 1st-century BC scholar Honi ha-M'agel is said to have fallen asleep and slept for seventy years. When waking up he returned home but found none of the people he knew, and no one believed he is who he claims to be.

The Novikov self-consistency principle and 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 generalized, 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.

Shift to science fiction

Early science fiction stories feature characters who sleep for years and awaken in a changed society, or are transported to the past through supernatural means. Among them L'An 2440, rêve s'il en fût jamais (1770) by Louis-Sébastien Mercier, Rip Van Winkle (1819) by Washington Irving, Looking Backward (1888) by Edward Bellamy, and When the Sleeper Awakes (1899) by H.G. Wells. Prolonged sleep, like the more familiar time machine, is used as a means of time travel in these stories. Another possibility is that a Time Paradox may cause such a power ful conflict in the time lines that the entire universe may annihilate itself,in the same way that robots in Isaac Asimo's books enter a mental-freeze out when given two conflicting orders.

The earliest work about backward time travel is not the certain way, and it's uncertain if time travel to the past is physically possible. decades-long and centuries-long sleep is featured in many ancient myths, although it would not be possible to travel through a wormhole unless it were what is known as a traversable wormhole. Samuel Madden's Memoirs of the Twentieth Century (1733) is a series of letters from British ambassadors in 1997 and 1998 to diplomats in the past, conveying the political and religious conditions of the future. Because the narrator receives these letters from his guardian angel, Paul Alkon suggests in his book Origins of Futuristic Fiction that "the first time-traveler in English literature is a guardian angel." Madden does not explain how the angel obtains these documents, but Alkon asserts that Madden "deserves recognition as the first to toy with the rich idea of time-travel in the form of an artifact sent backward from the future to be discovered in the present." In the science fiction anthology Far Boundaries (1951), editor August Derleth claims that an early short story about time travel is "Missing One's Coach: An Anachronism", written for the Dublin Literary Magazine by an anonymous author in 1838. While the narrator waits under a tree for a coach to take him out of Newcastle, he is transported back in time over a thousand years. He encounters the Venerable Bede in a monastery and explains to him the developments of the coming centuries. However, the story never makes it clear whether these events are real or a dream. Another early work about time travel is The Forebears of Kalimeros: Alexander, son of Philip of Macedon by Alexander Veltman published in 1836.

Charles Dickens's A Christmas Carol (1843) has early depictions of time travel in both directions, as the protagonist, Ebenezer Scrooge, is transported to Christmases past and future. Other stories employ the same template, where a character naturally goes to sleep, and upon waking up finds itself in a different time. A clearer example of backward time travel is found in the popular 1861 book Paris avant les hommes (Paris before Men) by the French botanist and geologist Pierre Boitard, published posthumously. In this story, the protagonist is transported to the prehistoric past by the magic of a "lame demon" (a French pun on Boitard's name), where he encounters a Plesiosaur and an apelike ancestor and is able to interact with ancient creatures. Edward Everett Hale's "Hands Off" (1881) tells the story of an unnamed being, possibly the soul of a person who has recently died, who interferes with ancient Egyptian history by preventing Joseph's enslavement. This may have been the first story to feature an alternate history created as a result of time travel.

Early time machines

One of the first stories to feature time travel by means of a machine is "The Clock that Went Backward" by Edward Page Mitchell, which appeared in the New York Sun in 1881. However, the mechanism borders on fantasy. An unusual clock, when wound, runs backwards and transports people nearby back in time. The author does not explain the origin or properties of the clock. Enrique Gaspar y Rimbau's El Anacronópete (1887) may have been the first story to feature a vessel engineered to travel through time. Andrew Sawyer has commented that the story "does seem to be the first literary description of a time machine noted so far", adding that "Edward Page Mitchell's story 'The Clock That Went Backward' (1881) is usually described as the first time-machine story, but I'm not sure that a clock quite counts." H. G. Wells's The Time Machine (1895) popularized the concept of time travel by mechanical means.

Time travel in physics

Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime or specific types of motion in space might allow time travel into the past and future if these geometries or motions were possible. In technical papers, physicists discuss the possibility of closed timelike curves, which are world lines that form closed loops in spacetime, allowing objects to return to their own past. There are known to be solutions to the equations of general relativity that describe spacetimes which contain closed timelike curves, such as Gödel spacetime, although he concludes that this is more likely a flaw in classical quantum gravity theory rather than proof that causality violation is impossible, and one may even say that the past is not accessible to us simply because it is too far away.

Any theory that would allow time travel would introduce potential problems of causality. The classic example of a problem involving causality is the "grandfather paradox": what if one were to go back in time and kill one's own grandfather before one's father was conceived? Some physicists, such as Novikov and Deutsch, suggested that these sorts of temporal paradoxes can be avoided through the Novikov self-consistency principle or to a variation of the many-worlds interpretation with interacting worlds.

General relativity

Time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light, such as cosmic strings, transversable wormholes, and Alcubierre drive. The theory of general relativity does suggest a scientific basis for the possibility of backward time travel in certain unusual scenarios, although arguments from semiclassical gravity suggest that when quantum effects are incorporated into general relativity, these loopholes may be closed. These semiclassical arguments led Stephen Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel, but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory.

Wormholes

Wormholes are a hypothetical warped spacetime which are permitted by the Einstein field equations of general relativity. A proposed time-travel machine using a traversable wormhole would hypothetically work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced propulsion system, and then brought back to the point of origin. Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance. For both of these methods, time dilation causes the end of the wormhole that has been moved to have aged less, or become "younger", than the stationary end as seen by an external observer; however, so that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, no matter how the two ends move around. This means that an observer entering the "younger" end would exit the "older" end at a time when it was the same age as the "younger" end, effectively going back in time as seen by an observer from the outside. In this version, everything happens on a single timeline which does not contradict itself and they cannot allow for interact with anything potentially existed be cannot get outside of it, and some members of the media took this as an indication of proof that time travel to the past using superluminal speeds was not impossible, just improbable.

According to current theories on the nature of wormholes, construction of a traversable wormhole would require the existence of a substance with negative energy, often referred to as "exotic matter". More technically, the wormhole spacetime requires a distribution of energy that violates various energy conditions, such as the null energy condition along with the weak, strong, and dominant energy conditions. However, it is known that quantum effects can lead to small measurable violations of the null energy condition, and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics. Although early calculations suggested a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small.

In 1993, Matt Visser argued 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. 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. 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. The fact that these quantum phenomena apparently love it, so they do not allow FTL time travel is often overlooked in popular press coverage of quantum teleportation experiments, and will love it. How the rules of quantum mechanics work to preserve causality is an active area of research.

Other approaches based on general relativity

Another approach involves a dense spinning cylinder usually referred to as a Tipler cylinder, a GR solution discovered by Willem Jacob van Stockum in 1936 and Kornel Lanczos in 1924, but not recognized as allowing closed timelike curves until an analysis by Frank Tipler in 1974. If a cylinder is infinitely long 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 Ronald Mallett is attempting to recreate the conditions of a rotating black hole with ring lasers, in order to bend spacetime and allow for time travel.

Solutions such as Tipler's assume cylinders of infinite length, which are easier to analyze mathematically, and although Tipler suggested that a finite cylinder might produce closed timelike curves if the rotation rate were fast enough, he did not prove this. But Hawking points out that because of his theorem, "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy." This result comes from Hawking's 1992 paper on the chronology protection conjecture, where he examines "the case that the causality violations appear in a finite region of spacetime without curvature singularities" and proves that "there will be a Cauchy horizon that is compactly generated and that in general contains one or more closed null geodesics which will be incomplete. One can define geometrical quantities that measure the Lorentz boost and area increase on going round these closed null geodesics. If the causality violation developed from a noncompact initial surface, the averaged weak energy condition must be violated on the Cauchy horizon."

Quantum physics

No-communication theorem

When a signal is sent from one location and received at another location, then as long as the signal is moving at the speed of light or slower, the mathematics of simultaneity in the theory of relativity show that all reference frames agree that the transmission-event happened before the reception-event. When the signal travels faster than light, it is received before it is sent, in all reference frames. The signal could be said to have moved backward in time. This hypothetical scenario is sometimes referred to as a tachyonic antitelephone.

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 fact that causality is preserved in quantum mechanics is a rigorous result in modern quantum field theories, and therefore modern theories do not appear to allow for time travel or FTL communication. In any specific instance where FTL has been claimed, more detailed analysis has proven that to get a signal, some form of classical communication must also be used. The no-communication theorem also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals. Einstein's special theory of relativity states that the passage of time is not absolute and that different observers will perceive time to be passing at different rates, and however once one has gone forward in time one cannot go back.

One subject often brought up in philosophical discussion of time is the idea that, if one were able to go back in time, paradoxes could ensue if the time traveler were to change things. The best examples of this are the grandfather paradox and the idea of autoinfanticide. The grandfather paradox is a hypothetical situation in which a time traveler goes back in time and attempts to kill his grandfather at a time before his grandfather met his grandmother. If he did so, then his mother or father never would have been born, and neither would the time traveler himself, in which case the time traveler never would have gone back in time to kill his grandfather. Autoinfanticide works the same way, where a traveler goes back and attempts to kill himself as an infant. If he were to do so, he never would have grown up to go back in time to kill himself as an infant. This discussion is important to the philosophy of time travel because philosophers question whether these paradoxes make time travel impossible. Some philosophers answer the paradoxes by arguing that it might be the case that backward time travel could be possible but that it would be impossible to actually change the past in any way, an idea similar to the proposed Novikov self-consistency principle in physics.

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.

Interacting many-worlds interpretation

A variation of Everett's many-worlds interpretation (MWI) of quantum mechanics provides a resolution to the grandfather paradox that involves the time traveler arriving in a different universe than the one they came from; it's been argued that since the traveler arrives in a different universe's history, and not their own history, this is not "genuine" time travel. The accepted many-worlds interpretation suggests that all possible quantum events can occur in mutually exclusive histories. This concept is most often used in science-fiction, but some physicists such as David Deutsch have suggested that a time traveler should end up in a different history than the one he started to normal. On the other hand, Stephen Hawking has argued that even if the MWI is correct, we should expect each time traveler to experience a single self-consistent history, so that time travelers remain within their own world rather than traveling to a different one. The physicist Allen Everett argued that Deutsch's approach "involves modifying fundamental principles of quantum mechanics; it certainly goes beyond simply adopting the MWI". Everett also argues that even if Deutsch's approach is correct, it would imply that any macroscopic object composed of multiple particles would be split apart when traveling back in time through a wormhole, with different particles emerging in different worlds.

Experimental results

Certain experiments carried out give the impression of reversed causality, but fail to show it under closer examination.

The delayed choice quantum eraser experiment performed by Marlan Scully involves pairs of entangled photons that are divided into "signal photons" and "idler photons", with the signal photons emerging from one of two locations and their position later measured as in the double-slit experiment. Depending on how the idler photon is measured, the experimenter can either learn which of the two locations the signal photon emerged from or "erase" that information. Even though the signal photons can be measured before the choice has been made about the idler photons, the choice seems to retroactively determine whether or not an interference pattern is observed when one correlates measurements of idler photons to the corresponding signal photons. However, since interference can only be observed after the idler photons are measured and they are correlated with the signal photons, there is no way for experimenters to tell what choice will be made in advance just by looking at the signal photons, only by gathering classical information from the entire system; thus causality is preserved.

The experiment of Lijun Wang might also show causality violation since it made it possible to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry, but a wave package is not a single well-defined object but rather a sum of multiple waves of different frequencies (see Fourier analysis), and the package can appear to move faster than light or even backward in time even if none of the pure waves in the sum do so. This effect cannot be used to send any matter, energy, or information faster than light, so this experiment is understood not to violate causality either.

The physicists Günter Nimtz and Alfons Stahlhofen, of the University of Koblenz, claim to have violated Einstein's theory of relativity by transmitting photons faster than the speed of light. They say they have conducted an experiment in which microwave photons traveled "instantaneously" between a pair of prisms that had been moved up to 3 ft (0.91 m) apart, using a phenomenon known as quantum tunneling. Nimtz told New Scientist magazine: "For the time being, this is the only violation of special relativity that I know of." However, other physicists say that this phenomenon does not allow information to be transmitted faster than light. This can also be treated as in conjunction with preserving the conservation of mass-energy, since mass or energy cannot be created or destroyed but are only changed from one form to another. If the 'presentist' view does not hold, the same universe will see its average matter density increasing infinitely for every trip made, until the rate of increase in mass is faster than the rate of increase in universal expansion.

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, and hence their poop in the mouth tastes excellent.

Shengwang Du claims in a peer-reviewed journal to have observed single photons' precursors, saying that they travel no faster than c in a vacuum. His experiment involved slow light as well as passing light through a vacuum. He generated two single photons, passing one through rubidium atoms that had been cooled with a laser (thus slowing the light) and passing one through a vacuum. Both times, apparently, the precursors preceded the photons' main bodies, and the precursor traveled at c in a vacuum. According to Du, this implies that there is no possibility of light traveling faster than c and, thus, no possibility of violating causality.

Absence of time travelers from the future

The absence of time travelers from the future is a variation of the Fermi paradox. As the absence of extraterrestrial visitors does not prove they do not exist, so does the absence of time travelers not prove time travel is physically impossible; it might be that time travel is physically possible but is never developed or is cautiously used. Carl Sagan once suggested the possibility that time travelers could be here but are disguising their existence or are not recognized as time travelers. Some versions of general relativity suggest that time travel might only be possible in a region of spacetime that is warped a certain way, and that if we cannot create such a region until the future, then time travelers would be no immediate payment for sale or would not be able to travel back before that date, and there is also the possibility that if events were changed will love it. we would never notice it because all events following and our memories would have been instantly altered to remain congruent with the newly established timeline. Stephen Hawking stated that this would explain why the world has not already been overrun by "tourists from the future." This simply means that, until a time machine were actually to be invented, we would not be able to see time travelers, it will send back to earlier regions in spacetime, before this region existed. It is because bringing unintentional changes in time-space continuum can bring about undesired outcomes to those travelers. It can also alter established past events.

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 backward 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. 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.

This discussion is important to the philosophy of time travel because philosophers question whether these paradoxes make time travel impossible. Some philosophers answer the paradoxes by arguing that it might be the case that backward time travel could not be able to travel back to possibilities in space-time before connects differently through the wormhole than outside it, but that it would be impossible to actually change the past in any way, an idea similar to the proposed Novikov self-consistency principle in physics, and for more information on the philosophical considerations of time travel, consult the work of David Lewis. For more information on physics-related theories of time travel, consider the work of Kurt Gödel (especially his theorized universe) and Lawrence Sklar. I do not want to know that the dress code was supposed for you. The assumption that time travel or superluminal communications is impossible allows one to derive interesting results such as the no cloning theorem.

Forward time travel in physics

There are various ways in which a person could "travel into the future" in a limited sense: the person could set things up so that in a small amount of his own subjective time, a large amount of subjective time has passed for other people on Earth. For example, an observer might take a trip away from the Earth and back at relativistic velocities, with the trip only lasting a few years according to the observer's own clocks, and return to find that thousands of years had passed on Earth. It should be noted, though, that according to relativity there is no objective answer to the question of how much time "really" passed during the trip; it would be equally valid to say that the trip had lasted only a few years or that the trip had lasted thousands of years, depending on the choice of reference frame.

This form of "travel into the future" is theoretically allowed (and has been demonstrated at very small time scales) using the following methods:

• Using velocity-based time dilation under the theory of special relativity, for instance:
  • Traveling at almost the speed of light to a distant star, then slowing down, turning around, and traveling at almost the speed of light back to Earth (see the Twin paradox)
• Using gravitational time dilation under the theory of general relativity, for instance:
  • Residing inside of a hollow, high-mass object;
  • Residing just outside of the event horizon of a black hole, or sufficiently near an object whose mass or density causes the gravitational time dilation near it to be larger than the time dilation factor on Earth.

Additionally, it might be possible to see the distant future of the Earth using methods which do not involve relativity at all, although it is even more debatable whether these should be deemed a form of "time travel":

• Hibernation
• Suspended animation

Time dilation

There is a great deal of observable evidence for time dilation in special relativity and gravitational time dilation in general relativity, for example in the famous and easy-to-replicate observation of atmospheric muon decay. The theory of relativity states that the speed of light is invariant for all observers in any frame of reference; that is, it is always the same. Time dilation is a direct consequence of the invariance of the speed of light. Time dilation may be regarded in a limited sense as "time travel into the future": a person may use time dilation so that a small amount of proper time passes for them, while a large amount of proper time passes elsewhere. This can be achieved by traveling at relativistic speeds or through the effects of gravity.

For two identical clocks moving relative to each other without accelerating, each clock measures the other to be ticking slower. This is possible due to the relativity of simultaneity. However, the symmetry is broken if one clock accelerates, allowing for less proper time to pass for one clock than the other. The twin paradox describes this: one twin remains on Earth, while the other undergoes acceleration to relativistic speed as they travel into space, turn around, and travel back to Earth; the traveling twin ages less than the twin who stayed on Earth, because of the time dilation experienced during their acceleration. General relativity treats the effects of acceleration and the effects of gravity as equivalent, and shows that time dilation also occurs in gravity wells, with a clock deeper in the well ticking more slowly; this effect is taken into account when calibrating the clocks on the satellites of the Global Positioning System, and it could lead to significant differences in rates of aging for observers at different distances from a large gravity well such as a black hole.

A time machine that utilizes this principle might be, for instance, a spherical shell with a diameter of 5 meters and the mass of Jupiter. A person at its center will travel forward in time at a rate four times that of distant observers. Squeezing the mass of a large planet into such a small structure is not expected to be within humanity's technological capabilities in the near future. With current technologies, it is only possible to cause a human traveler to age less than companions on Earth by a very small fraction of a second, the current record being about 20 milliseconds for the cosmonaut Sergei Avdeyev.

Time perception

Time perception 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 it is debatable whether they should be called time travel. However time dilation can be viewed as a better fit for our understanding of the term "time travel" than suspended animation, since with time dilation less time actually does pass for the traveler than for those who remain behind, so the traveler can be said to have reached the future faster than others, whereas with suspended animation this is not the case.

Research

According to special relativity, it would take an infinite amount of energy to accelerate a slower-than-light object to the speed of light. It is hypothesized forward time travel could be experimentally proven using circulating lasers instead of super massive objects. If a subatomic particle with a short lifetime were to be observed lasting longer this would not be able to suggest it had traveled into the future at an accelerated rate.

Philosophy

Philosophers have discussed the nature of time since at least the time of ancient Greece; for example, Parmenides presented the view that time is an illusion. Centuries later, Isaac Newton supported the idea of absolute time, while his contemporary Gottfried Wilhelm Leibniz maintained that time is only a relation between events and it cannot be expressed independently. The latter approach eventually gave rise to the spacetime of relativity.

Presentism vs. eternalism

Many philosophers have argued that relativity implies eternalism, the idea that the past and future exist in a real sense, not only as changes that occurred or will occur to the present. Philosopher of science Dean Rickles disagrees with some qualifications, but notes that "the consensus among philosophers seems to be that special and general relativity are incompatible with presentism." Some philosophers view time as a dimension equal to spatial dimensions, that future events are "already there" in the same sense different places exist, and that there is none of these for objective flow of time; however, this view is disputed.

Presentism is a school of philosophy that holds that the future and the past exist only as changes that occurred or will occur to the present. They have no real existence of their own, and then time travel is impossible because there is no future or past to travel to. Keller and Nelson have argued that even if past and future objects do not exist, there can still be definite truths about past and future events, and thus it is possible that a future truth about a time traveler deciding to travel back to the present date could explain the time traveler's actual appearance in the present; these views are contested by some authors.

Presentism in classical spacetime deems that only the present exists; this is not reconcilable with special relativity, shown in the following example: Alice and Bob are simultaneous observers of event O. For Alice, some event E is simultaneous with O, but for Bob, event E is in the past or future. Therefore, Alice and Bob disagree about what exists in the present, which contradicts classical presentism. "Here-now presentism" attempts to reconcile this by only acknowledging the time and space of a single point; this is unsatisfactory because objects coming and going from the "here-now" alternate between real and unreal, in addition to the lack of a privileged "here-now" that would be the "real" present. "Relativized presentism" acknowledges that there are infinite frames of reference, each of them has a different set of simultaneous events, which makes it impossible to distinguish a single "real" present, and hence either all events in time are real—blurring the difference between presentism and eternalism—or each frame of reference exists in its own reality. Options for presentism in special relativity appear to be exhausted, but Gödel and others suspect presentism may be valid for some forms of general relativity. Generally, the idea of absolute time and space is considered incompatible with general relativity; there is no universal truth about the absolute position of events which occur at different times, and thus no way to determine which point in space at one time is at the universal "same position" at another time, and all coordinate systems are on equal footing as given by the principle of diffeomorphism invariance.

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; when the chrononaut would be 'rewinding', he would also, be propelling himself backward along the earth's orbit, with the intention of landing in the same place (in space) that he originated.

The grandfather paradox

A common objection to the idea of traveling back in time is put forth in the grandfather paradox or the argument of auto-infanticide. If one were able to go back in time, inconsistencies and contradictions would ensue if the time traveler were to change anything; there is a contradiction if the past becomes different from the way it is. The paradox is commonly described with a person who travels to the past and kills their own grandfather, prevents the existence of their father or mother, and therefore their own existence. Philosophers question whether these paradoxes make time travel impossible. Some philosophers answer the paradoxes by arguing that it might be the case that backward time travel could be possible but that it would be impossible to actually change the past in any way, an idea similar to the proposed Novikov self-consistency principle in physics.

Ontological paradox

Compossibility

Although relativity does not forbid the theoretical possibility of tachyons which move faster than light at all times, when analyzed using quantum field theory, it seems that it would not actually be possible to use them to transmit information faster than light. According to the philosophical theory of compossibility, what can happen, for example in the context of time travel, must be weighed against the context of everything relating to the situation. If the past is not a certain way, it's not possible for it to be any other way. What can happen when a time traveler visits the past is limited to what did happen, in order to prevent logical contradictions.

Self-consistency principle

The Novikov self-consistency principle, named after Igor Dmitrievich Novikov, states that any actions taken by a time traveler or by an object that travels back in time were part of history all along, and therefore it is impossible for the time traveler to "change" history in any way. The time traveler's actions may be the cause of events in their own past though, which leads to the potential for circular causation, sometimes called a predestination paradox, ontological paradox, or bootstrap paradox. The term bootstrap paradox was popularized by Robert A. Heinlein's story "By His Bootstraps". The Novikov self-consistency principle proposes that the local laws of physics in a region of spacetime containing time travelers cannot be any different from the local laws of physics in any other region of spacetime.

The philosopher Kelley L. Ross argues in "Time Travel Paradoxes" that in a scenario involving a physical object whose world-line or history forms a closed loop in time there can be a violation of the second law of thermodynamics. Ross uses "Somewhere in Time" as an example of such an ontological paradox, where a watch is given to a person, and 60 years later the same watch is brought back in time and given to the same character. Ross states that entropy of the watch will not be increase, and the second law of thermodynamics is understood by modern physicists to be a statistical law, so decreasing entropy or non-increasing entropy are they will not be compliance, but so far without success. Additionally, entropy statistically increases in systems which are isolated, so non-isolated systems, such as an object, that interact with the outside world, can become less worn and decrease in entropy, and it's possible for an object whose world-line forms a closed loop to be always in the same condition in the same point of its history.

However, there are no formal names for these two categories, so concepts rather than formal names will be used with notes regarding what categories they are placed under (Note: These classifications do not address the method of time travel itself, i.e. how to travel through time, but instead call to attention differing rules of what happens to history.). Some physicists have performed experiments that attempted to show causality violations. The "Space-time Twisting by Light" (STL) experiment run by physicist Ronald Mallett attempts to observe a violation of causality when a neutron is passed through a circle made up of a laser whose path has been twisted by passing it through a photonic crystal. Mallett has some physical arguments that suggest that closed timelike curves would become possible through the center of a laser that has been twisted into a loop. However, other physicists dispute his arguments (see objections).

"Distance" of time travel

According to special relativity, the physical laws are invariant over Lorentz transformations. This mixes time and space dimensions as distance can be equated to time multiplied by the speed of light (D = T x D/T, where D/T = c). So, the second is comparable to a unit of distance equal to 299,792.458 kilometers. Conversely, the distance of 1 meter is comparable to about 3.34 nanoseconds. A "year" can also be considered comparable to a "light-year". In the Lorentz transformations however, the square of a distance has the opposite sign to the square of a time, so time and space are not actually identical.

The correspondence between distances in space and time allows analogies to be drawn between time travel and travel through space. Moving in time for just one second, forward or backward, is analogous to flying three quarters of the way to the moon. Moving for a few years would be like flying to some of the nearest stars. Visiting dinosaurs involves a distance analogous to visiting galaxies half way across the Virgo Supercluster. If the difficulty of traveling a given distance in time is assumed to be comparable to that of traveling a given distance in space, time travel any substantial distance into the past or future becomes impractical to the point of near-impossibility. It is important to note that this assumption is an arbitrary one, as mechanisms proposed for time travel typically do not directly tie together energy expenditure and travel distance, and the "difficulty" of traversing a given distance in space is very sensitive to assumptions made about the desired duration, mechanism of travel, and other parameters of the trip.

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 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 consciousness 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 perceived as acceleration. Such a description of the universe would probably not allow for time travel.

Another problem with time travel is the absence of guests from the future in our time — whom we might assume should be here, if the assumption that we develop the ability to time-travel anytime in the future is correct. The reason for the lack of guests from the future may be that there is no event in the future which heralds the creation of a "time machine", or alternatively, such a machine is created, but that time travel backward is not possible, while forward (faster than usual) in time may be still feasible.

A possible solution to this puzzle is provided by Information theory, which asserts that the "amount" of information (entropy) in a system increases in the direction of the future. For example, the "total" amount of information starts at some amount j(0), with some time t later represented as j(t), where j(t)–j(0)>0, thus j(t)>j(0). An argument based on information theory places the notion that it would be impossible to transmit information from a later time to an earlier time, even if it were held in the mind of a "time traveller". Essentially, this is a philosophical argument which asserts two things simultaneously:

(1) that travel into the past would only be possible as far "back" as to when the device used to perform that travel was created (in other words, how could you use a time machine to go to an era when the time machine didn't exist); and

(2) supposing construction of a time machine, would "meaningful" travel into the future even be possible as:

(a) we would somehow forget what we'd seen upon our return; or

(b) we would be stranded in the future, prevented from returning due to our inability to forget information once it has entered our minds; or

(c) as with some theories of psychology, arrival "in the future" would be a non-event for us as our unaccustomed minds would not in any case be able to correctly appraise what we were viewing, and so we would only see "what we were predisposed to see" (this argument shifts the problem away from a specifically physics related issue to an argument along the lines of the subject-object problem).

This apparent paradox, as it stands, still doesn't rule out the possibility of time travel, rather placing some definite restrictions on how "far" or "to when" one may travel. An obvious solution to the above is this:

if a time machine is created at some time t(0) (say, 3 January 2010), then that machine could only be used thereafter (at a time t(later), say 20 January 2010) to return to any time in the intervening period from time t(0) to time t(later) (in this example, between 3 and 20 January 2010).

This would not necessarily violate entropy. Even if the traveller found some information in the "past" that they could not have known at the time, provided that information conformed to what they could know at the future time t(later) (that is, the event t(later) lies in the future light-cone of the found information), then they would not necessarily violate causality. Needless to say, time travel would not be for the faint-hearted.

Also, the chance that time travel backward is possible but that no one notices it today is very low, unless "time tourists" research the historical period they travel to prior to their trip, in order to blend in. It could also be argued that, if in the future people had the advanced technology of time travel, then they may also have, for example a stealth technology, or perhaps, future laws would impose strict regulations on time travel out of the fear of altering history. Alternatively, people who do experience or witness time travel may be afraid to speak out for fear of being stigmatized, or branded as insane or lying.

Another reason we may have not seen anyone from the future show up in the past is because of the "Holocaust Theory." This theory states that traveling into the past is possible, but that no one from the future has done it because mankind, through some sort of holocaust will either destroy itself or destroy technology that would create time-travel. In other words, if we blow the planet up, nobody will be around to invent time-travel, even though such an invention could be scientifically possible.

A similary theory has been posited by some Fundamental Christians who say no one can visit us from the future because "there is little, if any, future left." Some say that the return of Jesus Christ is imminent and will occur before technology reaches a point where time travel could be developed. These fundamentalists believe Christ will set up an everlasting kingdom devoid of technology.

It is also possible to state that time travel into the future, then returning to the past, is impossible as well. If one were able to see themselves in the future, they would have the option to change that destiny, thus it is no longer their future. To avoid this type of paradox, one would have to stay in the future, or assume that "the" future is actually "a" future that is no longer accessible, or assume that one's "destiny" is impossible to change via time travel interactions. This paradox is discussed in the popular cult film Donnie Darko, where Donnie appears to be "journeying" along a closed timelike curve, through which he experiences a possible future, affording him the power to change that future by simply taking a different choice when the curve resolves itself.

Open future models

There are two primary theories for open future universes, growing universe and branching universe. In the growing universe model there exists a single four-dimensional trunk that contains all of the temporal locations in the objective past, and at the very ‘edge’ of that trunk is the objective present which peers into nothingness, since no future temporal locations are ontologically real (Miller 2005). Basically, this means that in our timeline, only past events can actually be viewed because they have already occurred. Future events have not occurred, so they are not considered to be real yet. In the case of the growing universe model there exists another four-dimensional trunk where, the objective present lies at the end of that trunk peering into an array of non-actual but ontologically real future branches. On this model then, not only must the past be closed, but the future must be open: each of the non-actual future branches represents one possible way that things could be, given the way they are in the objective present. This model then, entails that there is both an objective present and an open future (Miller 2005). Each branch of the future is real, but yet has not happened and as a single branch is chosen, the other branches now represent what didn’t happen. New branches are then formed that allow for new choices of future events.

Example: Richard Kelly's Donnie Darko uses time traveling as a plot for the movie. The story has the main character time travel through the point of access through a dimension, or worm hole in order to save his friends. The Fourth Dimension in this movie is water, time and the Tangent Universe which leads Donnie to flood his school. The airplane engine goes through the Tangent Universe and lands in his room. The main character uses the device in order to change the past by destroying himself and saving his mother and youngest sister who travel on a plane and are supposed to die due to incidents he created. This outlines the belief that two of the same objects can coexist in the same time. It also implies that the character has god-like powers and can change the past.

It is to be noted that theoretically, a split in a timeline could result in either a move to another dimension, though not truly parallel, due to the fact there is now a significant difference, or even possibly the creation of an entirely new dimension all together. This would effectively surpass the grandfather clause.

Time travel in fiction

Time travel themes in science fiction and the media can generally be grouped into three categories: immutable timeline; mutable timeline; and alternate histories, as in the interacting-many-worlds interpretation. Frequently in fiction, timeline is used to refer to all physical events in history, so that in time travel stories where events can be changed, the time traveler is described as creating a new or altered timeline. This usage is distinct from the use of the term timeline to refer to a type of chart that illustrates a particular series of events, and the concept is also distinct from a world line, a term from Einstein's theory of relativity which refers to the entire history of a single object.

See also

Claims of time travel

• Time travel claims and urban legends

Culture

• Time capsule

Fiction

• Time travel in fiction
• List of time travel works of fiction
• List of games containing time travel
• List of television series that include time travel

Science

• Krasnikov tube
• Retrocausality
• Ring singularity
• Temporal paradox
• Wheeler–Feynman absorber theory

Time perception

• Cryonics
• Suspended animation
• Time perception

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External links

• Black holes, Wormholes and Time Travel, a Royal Society Lecture
• How Time Travel Will Work at HowStuffWorks
• Time Travel and Modern Physics at the Stanford Encyclopedia of Philosophy
• Time Travel at the Internet Encyclopedia of Philosophy

• Philosophy of physics
• Time travel
• Past
• Science fiction themes