Time travel

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 is uncertain if time travel to the past is physically possible. 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.[1] The Buddhist Pāli Canon mentions the relativity of time. 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.[2] The Japanese tale of "Urashima Tarō",[3] 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.[4] 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 his claims of who he was.[5]

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.[6]

The earliest work about backwards time travel is uncertain. 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.[7]:95–96 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."[7]:85 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."[7]:95–96 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[8] by an anonymous author in 1838.[9]:3 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.[9]:11–38 Another early work about time travel is The Forebears of Kalimeros: Alexander, son of Philip of Macedon by Alexander Veltman published in 1836.[10]

A Christmas Carol - Mr. Fezziwig's Ball
Mr. and Mrs. Fezziwig dance in a vision shown to Scrooge by the Ghost of Christmas Past.

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 find themselves in a different time.[11] 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.[12] 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.[13]:54

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,[14] 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.[13]:55 Enrique Gaspar y Rimbau's El Anacronópete (1887) may have been the first story to feature a vessel engineered to travel through time.[15][16] 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."[17] H. G. Wells's The Time Machine (1895) popularized the concept of time travel by mechanical means.[18]

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.[19]:499 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, but the physical plausibility of these solutions is uncertain.

Many in the scientific community believe that backward time travel is highly unlikely. Any theory that would allow time travel would introduce potential problems of causality.[20] 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.[21]

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.[22][23]:33–130 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.[24] These semiclassical arguments led Stephen Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel,[25] 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.[26][27]:150

Different spacetime geometries

The theory of general relativity describes the universe under a system of field equations that determine the metric, or distance function, of spacetime. There exist exact solutions to these equations that include closed time-like curves, which are world lines that intersect themselves; some point in the causal future of the world line is also in its causal past, a situation which is akin to time travel. Such a solution was first proposed by Kurt Gödel, a solution known as the Gödel metric, but his (and others') solution requires the universe to have physical characteristics that it does not appear to have,[19]:499 such as rotation and lack of Hubble expansion. Whether general relativity forbids closed time-like curves for all realistic conditions is still being researched.[28]

Wormholes

Wormholes are a hypothetical warped spacetime which are permitted by the Einstein field equations of general relativity.[29]:100 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, time connects differently through the wormhole than outside it, 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.[19]:502 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. 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;[19]:503 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.

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,[29]:101 and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics.[30] 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.[31]

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.[32] 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 a flaw in classical quantum gravity theory rather than proof that causality violation is possible.[33]

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[34] in 1936 and Kornel Lanczos[35] in 1924, but not recognized as allowing closed timelike curves[36]:21 until an analysis by Frank Tipler[37] 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.[38]

A more fundamental objection to time travel schemes based on rotating cylinders or cosmic strings has been put forward by Stephen Hawking, who proved a theorem showing that according to general relativity it is impossible to build a time machine of a special type (a "time machine with the compactly generated Cauchy horizon") in a region where the weak energy condition is satisfied, meaning that the region contains no matter with negative energy density (exotic matter). 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,[36]:169 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."[27]:96 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."[25] This theorem does not rule out the possibility of time travel by means of time machines with the non-compactly generated Cauchy horizons (such as the Deutsch-Politzer time machine) or in regions which contain exotic matter, which would be used for traversable wormholes or the Alcubierre drive.

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.[39] The signal could be said to have moved backward in time. This hypothetical scenario is sometimes referred to as a tachyonic antitelephone.[40]

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.[41] 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 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.[42] The no-communication theorem also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals.

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.[43] The accepted many-worlds interpretation suggests that all possible quantum events can occur in mutually exclusive histories.[44] However, some variations allow different universes to interact. 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 from.[45][46] 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.[47] 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.[21]

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.[48]

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,[49] 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. Aephraim Steinberg, a quantum optics expert at the University of Toronto, Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the speed of the center of the train exceeds the speed of any of the individual cars.[50]

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.[51]

Absence of time travelers from the future

WelcomeKrononauts Artforum Jan1980 p.90 800x600
Krononauts

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 the absence of time travelers fails to 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.[26] 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 hence time travelers would not be able to travel back to earlier regions in spacetime, before this region existed. Stephen Hawking stated that this would explain why the world has not already been overrun by "tourists from the future."[47]

Several experiments have been carried out to try to entice future humans, who might invent time travel technology, to come back and demonstrate it to people of the present time. Events such as Perth's Destination Day or MIT's Time Traveler Convention heavily publicized permanent "advertisements" of a meeting time and place for future time travelers to meet.[52] In 1982, a group in Baltimore, Maryland, identifying itself as the Krononauts, hosted an event of this type welcoming visitors from the future.[53][54] These experiments only stood the possibility of generating a positive result demonstrating the existence of time travel, but have failed so far—no time travelers are known to have attended either event. Some versions of the many-worlds interpretation can be used to suggest that future humans have traveled back in time, but have traveled back to the meeting time and place in a parallel universe.[55]

Forward time travel in physics

Time dilation

Time dilation02
Transversal time dilation. The blue dots represent a pulse of light. Each pair of dots with light "bouncing" between them is a clock. For each group of clocks, the other group appears to be ticking more slowly, because the moving clock's light pulse has to travel a larger distance than the stationary clock's light pulse. That is so, even though the clocks are identical and their relative motion is perfectly symmetric.

There is a great deal of observable evidence for time dilation in special relativity[56] and gravitational time dilation in general relativity,[57][58][59] for example in the famous and easy-to-replicate observation of atmospheric muon decay.[60][61][62] 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.[62] 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.[63]

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.[23]:33–130

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.[23]:76–140 With current technologies, it is only possible to cause a human traveler to age less than companions on Earth by a few milliseconds, the current record being about 20 milliseconds for the cosmonaut Sergei Krikalev.[64]

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.[65]

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.[66] 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."[67] 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 no objective flow of time; however, this view is disputed.[68]

BarAndRing
The bar and ring paradox is an example of the relativity of simultaneity. Both ends of the bar pass through the ring simultaneously in the rest frame of the ring (left), but the ends of the bar pass one after the other in the rest frame of the bar (right).

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, and they have no real existence of their own. In this view, time travel is impossible because there is no future or past to travel to.[66] 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;[69] these views are contested by some authors.[70]

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.[71] 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,[72] and all coordinate systems are on equal footing as given by the principle of diffeomorphism invariance.[73]

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.[74] 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.[75][76] 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.[26] 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,[77] an idea similar to the proposed Novikov self-consistency principle in physics.

Ontological paradox

Compossibility

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 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.[78]

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,[79] ontological paradox,[80] or bootstrap paradox.[80][81] The term bootstrap paradox was popularized by Robert A. Heinlein's story "By His Bootstraps".[82] 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.[83]

The philosopher Kelley L. Ross argues in "Time Travel Paradoxes"[84] 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 increase, and the watch carried back in time will be more worn with each repetition of its history. The second law of thermodynamics is understood by modern physicists to be a statistical law, so decreasing entropy or non-increasing entropy are not impossible, just improbable. 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.[23]:23

Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel where the past must be self-consistent.[85][86]

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.[87][88][89] 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.[90] 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

Culture

Fiction

Science

Time perception

References

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External links

Causal loop

A causal loop in the context of time travel or the causal structure of spacetime, is a sequence of events (actions, information, objects, people) in which an event is among the causes of another event, which in turn is among the causes of the first-mentioned event. Such causally-looped events then exist in spacetime, but their origin cannot be determined. A hypothetical example of a causality loop is given of a billiard ball striking its past self: the billiard ball moves in a path towards a time machine, and the future self of the billiard ball emerges from the time machine before its past self enters it, giving its past self a glancing blow, altering the past ball's path and causing it to enter the time machine at an angle that would cause its future self to strike its past self the very glancing blow that altered its path.

Code Lyoko

Code Lyoko is a French animated television series created by Thomas Romain and Tania Palumbo and produced by the MoonScoop Group. The series centers on a group of teenagers who travel to the virtual world of Lyoko to battle against a malignant artificial intelligence known as X.A.N.A. who threatens Earth. The scenes in the real world are presented in 2D hand-drawn animation, while the scenes in Lyoko are presented in CGI animation.

The series began its first, ninety-five episode run on 3 September 2003, on France's France 3, and ended on 10 November 2007. It started airing in the United States on 19 April 2004 on Cartoon Network.

Code Lyoko aired every day on Cartoon Network, in the Miguzi programming block, at 5:30 P.M. U.S. Eastern Time, sometimes showing two episodes consecutively, in the cases of season finales.

On 31 May 2011, the MoonScoop Group announced on its Facebook page that the show would return for a fifth season. This project would later become a follow-up series named "Code Lyoko: Evolution," which began airing at the end of 2012. This "reboot" to the series featured live action sequences for scenes taking place in the real world, but retained the iconic CGI style for scenes taking place in Lyoko. The show consisted of 26 episodes with the final episode airing in 2013.

DeLorean time machine

The DeLorean time machine is a fictional automobile-based time travel vehicle device featured in the Back to the Future franchise. In the feature film series, Dr. Emmett L. Brown builds a time machine based on a DeLorean car, to gain insights into history and the future. Instead, he ends up using it to travel over 130 years of Hill Valley history (from 1885 to 2015) with Marty McFly to change the past for the better and to undo the negative effects of time travel. One of the cars used in filming is on display in Paarl South Africa and the official Back to the Future DeLorean can be viewed at the Petersen Automotive Museum.

Dinosaur Train

Dinosaur Train is an American/Canadian/Singaporean children's animated series created by Craig Bartlett, who also created Hey Arnold! and Ready Jet Go!. The series features a curious young Tyrannosaurus rex named Buddy who, together with his adopted Pteranodon family, takes the Dinosaur Train to explore his time period, and have adventures with all kinds of dinosaurs. It is produced by The Jim Henson Company in association with Media Development Authority, Sparky Animation, FableVision, and Snee-Oosh, Inc. PBS Kids has ordered 11 more episodes, taking the total number of episodes to 100 which were planned to be released 2018, but has since been postponed to summer 2019.

Donnie Darko

Donnie Darko is a 2001 American science fiction film written and directed by Richard Kelly. It stars Jake Gyllenhaal, Jena Malone, Drew Barrymore, Mary McDonnell, Katharine Ross, Patrick Swayze, Noah Wyle, Stu Stone, Daveigh Chase and Maggie Gyllenhaal. The film follows the adventures of the troubled title character as he seeks the meaning behind his doomsday-related visions.

Filmed over the course of 28 days (coincidentally mirroring the time gone by in the movie), the film was almost released straight-to-video. Donnie Darko was screened at the Sundance Film Festival on January 19, 2001, before receiving a limited theatrical release on October 26, 2001, by Flower Films. Due to the film's advertising featuring a crashing plane and the September 11 attacks that transpired a month before, the film was scarcely advertised. In the end, Donnie Darko grossed just over $7.5 million worldwide on a budget of $4.5 million. Despite its lackluster box office performance, Donnie Darko received critical acclaim. Critics lauded the film's story, acting, and tone. The film was listed number two in Empire's 50 Greatest Independent Films of All Time, as well as number 63 in Empire's 500 Greatest Movies of All Time.It was released on VHS and DVD in March 2002. The film became a surprising success on the home video market, reportedly grossing over $10 million in home video sales. The film has developed a cult following. The film's soundtrack is also famous for its cover of Tears for Fears's "Mad World" by Gary Jules and Michael Andrews. The cover reached number one on the UK Singles Chart and stayed there for three consecutive weeks. The song achieved lukewarm success in the United States, reaching number 30 on the Billboard Modern Rock Tracks chart. A director's cut was released in 2004, on a two-disc special edition DVD. A stage adaptation appeared in 2007, and a sequel, S. Darko, in 2009.

Grandfather paradox

The grandfather paradox is a paradox of time travel in which inconsistencies emerge through changing the past. The name comes from the paradox's common description: a person travels to the past and kills their own grandfather before the conception of their father or mother, which prevents the time traveller's existence. Despite its title, the grandfather paradox does not exclusively regard the contradiction of killing one's own grandfather to prevent one's birth. Rather, the paradox regards any action that alters the past, since there is a contradiction whenever the past becomes different from the way it was.

Immortus

Immortus () (Nathaniel Richards) is a fictional character appearing in American comic books published by Marvel Comics. He is the future (and later, alternate) self of Kang the Conqueror.

Looper (film)

Looper is a 2012 American science fiction thriller film written and directed by Rian Johnson, and produced by Ram Bergman and James D. Stern. It stars Bruce Willis, Joseph Gordon-Levitt, and Emily Blunt. It revolves around "present-day" contract killers called "loopers" hired by criminal syndicates from the future to terminate victims that they send back through time.

Looper was selected as the opening film of the 2012 Toronto International Film Festival and was released in the United States on September 28, 2012. The film grossed $176 million on a $30 million budget, while receiving critical acclaim.

Quantum Leap

Quantum Leap is an American science-fiction television series that originally aired on NBC for five seasons, from March 1989 through May 1993. Created by Donald P. Bellisario, it starred Scott Bakula as Dr. Sam Beckett, a physicist who leaps through spacetime during an experiment in time travel, by temporarily taking the place of other people to correct historical mistakes. Dean Stockwell co-stars as Admiral Al Calavicci, Sam's womanizing, cigar-smoking companion and best friend, who appears to him as a hologram.

The series features a mix of humor, drama, romance, social commentary, and science fiction. The show was ranked #19 on TV Guide's "Top Cult Shows Ever".

Russian Doll (TV series)

Russian Doll is an American comedy-drama web television series, created by Natasha Lyonne, Amy Poehler, and Leslye Headland, that premiered on February 1, 2019, on Netflix. The series follows a woman who repeatedly dies and relives the same night in an ongoing loop. It stars Lyonne, Greta Lee, Yul Vazquez, Charlie Barnett, and Elizabeth Ashley.

Science fiction film

Science fiction film (or sci-fi film) is a genre that uses speculative, fictional science-based depictions of phenomena that are not fully accepted by mainstream science, such as extraterrestrial lifeforms, alien worlds, extrasensory perception and time travel, along with futuristic elements such as spacecraft, robots, cyborgs, interstellar travel or other technologies. Science fiction films have often been used to focus on political or social issues, and to explore philosophical issues like the human condition. In many cases, tropes derived from written science fiction may be used by filmmakers ignorant of or at best indifferent to the standards of scientific plausibility and plot logic to which written science fiction is traditionally held.The genre has existed since the early years of silent cinema, when Georges Melies' A Trip to the Moon (1902) employed trick photography effects. The next major example in the genre was the film Metropolis (1927) - being the first feature length science fiction movie. From the 1930s to the 1950s, the genre consisted mainly of low-budget B movies. After Stanley Kubrick's landmark 2001: A Space Odyssey (1968), the science fiction film genre was taken more seriously. In the late 1970s, big-budget science fiction films filled with special effects became popular with audiences after the success of Star Wars and paved the way for the blockbuster hits of subsequent decades.

Seven Days (TV series)

Seven Days (also written as 7 Days) is a science fiction television series based on the premise of time travel. It was created by Christopher and Zachary Crowe and produced by UPN from 1998 to 2001.

Temporal paradox

A temporal paradox, time paradox, or time travel paradox is a paradox, an apparent contradiction, or a logical contradiction that is associated with the idea of time and time travel. In physics, temporal paradoxes fall into two broad groups: consistency paradoxes exemplified by the grandfather paradox; and causal loops. More broadly, a variation of the Fermi paradox also applies to time travel.

The Butterfly Effect

The Butterfly Effect is a 2004 American science fiction thriller film written and directed by Eric Bress and J. Mackye Gruber, starring Ashton Kutcher and Amy Smart. The title refers to the butterfly effect, a popular hypothetical situation which illustrates how small initial differences may lead to large unforeseen consequences over time.

Kutcher plays 20-year-old college student Evan Treborn, with Amy Smart as his childhood sweetheart, Kayleigh Miller, William Lee Scott as her sadistic brother, Tommy, and Elden Henson as their neighbor, Lenny. Evan finds he has the ability to travel back in time to inhabit his former self (that is, his adult mind inhabits his younger body) and to change the present by changing his past behaviors. Having been the victim of several childhood traumas aggravated by stress-induced memory losses, he attempts to set things right for himself and his friends, but there are unintended consequences for all. The film draws heavily on flashbacks of the characters' lives at ages 7 and 13, and presents several alternative present-day outcomes as Evan attempts to change the past, before settling on a final outcome.

The film had a poor critical reception. It was a commercial success, producing fairly large earnings of $96 million from a budget of $13 million. The film won the Pegasus Audience Award at the Brussels International Fantastic Film Festival, and was nominated for Best Science Fiction Film at the Saturn Awards and Choice Movie: Thriller in the Teen Choice Awards, but lost to Eternal Sunshine of the Spotless Mind and The Texas Chainsaw Massacre, another film from New Line Cinema, respectively.

Time loop

A time loop or temporal loop is a plot device in which periods of time are repeated and re-experienced by the characters, and there is often some hope of breaking out of the cycle of repetition. Time loop is sometimes used to refer to a causal loop; although they appear similar, causal loops are unchanging and self-originating, whereas time loops are constantly resetting: when a certain condition is met, such as a death of a character or a clock reaches a certain time, the loop starts again, with one or more characters retaining the memories from the previous loop.An early example of a time loop is used in the short story "Doubled and Redoubled" by Malcolm Jameson that appeared in the February 1941 Unknown. The story tells of a person accidentally cursed to repeat a "perfect" day, including a lucky bet, a promotion, a heroically foiled bank robbery, and a successful wedding proposal. This story was a precedent to the films Groundhog Day (1993) and 12:01 PM (1990).

Time travel claims and urban legends

There have been various accounts of persons who allegedly travelled through time reported by the press or circulated on the Internet. These reports have generally turned out either to be hoaxes or to be based on incorrect assumptions, incomplete information, or interpretation of fiction as fact, many being now recognized as urban legends.

Time travel in fiction

Time travel is a common theme in fiction and has been depicted in a variety of media, such as literature, television, film, and advertisements.The concept of time travel by mechanical means was popularized in H. G. Wells' 1895 story, The Time Machine. In general, time travel stories focus on the consequences of traveling into the past or the future. The central premise for these stories oftentimes involves changing history, either intentionally or by accident, and the ways by which altering the past changes the future and creates an altered present or future for the time traveler when they return home. Some stories focus solely on the paradoxes and alternate timelines that come with time travel, rather than time traveling itself. They often provide some sort of social commentary, as time travel provides a "necessary distancing effect" that allows science fiction to address contemporary issues in metaphorical ways.Time travel in modern fiction is sometimes achieved by space and time warps, stemming from the scientific theory of general relativity. Stories from antiquity often featured time travel into the future through a time slip brought on by traveling or sleeping, or in other cases, time travel into the past through supernatural means, for example brought on by angels or spirits.

Wormhole

A wormhole (or Einstein–Rosen bridge) is a speculative structure linking disparate points in spacetime, and is based on a special solution of the Einstein field equations solved using a Jacobian matrix and determinant. A wormhole can be visualized as a tunnel with two ends, each at separate points in spacetime (i.e., different locations or different points of time). More precisely it is a transcendental bijection of the spacetime continuum, an asymptotic projection of the Calabi–Yau manifold manifesting itself in Anti-de Sitter space.

Wormholes are consistent with the general theory of relativity, but whether wormholes actually exist remains to be seen.

A wormhole could connect extremely long distances such as a billion light years or more, short distances such as a few meters, different universes, or different points in time.

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