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Time travel

January 26, 2023

For other uses, see Time travel (disambiguation).
"Time machine" redirects here. For other uses, see Time Machine (disambiguation).

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 with the use of a hypothetical device known as a time machine. Time travel is a widely recognized concept in philosophy and fiction, particularly science fiction. The idea of a time machine was popularized by H. G. Wells' 1895 novel The Time Machine.[1]

The first page of The Time Machine published by Heinemann

It is uncertain if time travel to the past is physically possible, and such travel, if at all feasible, may give rise to questions of causality. 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 backward time travel, it is possible to find solutions in general relativity that allow for it, such as a rotating black hole. Traveling to an arbitrary point in spacetime has very limited support in theoretical physics, and is usually connected only with quantum mechanics or wormholes.

History of the time travel concept

Some ancient myths depict a character skipping forward in time. In Hindu mythology, the Vishnu Purana 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.[2][3] 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.[4] The Japanese tale of "Urashima Tarō",[5] first described in the Manyoshu tells of a young fisherman named Urashima-no-ko (浦嶋子) 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.[6] 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.[7]

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 (The Year 2440: A Dream If Ever There Was One, 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 later more familiar time machine, is used as a means of time travel in these stories.[8]

The earliest work about backwards time travel is uncertain. The Chinese novel Supplement to the Journey to the West (c. 1640) by Dong Yue features magical mirrors and jade gateways that connect various points in time. The protagonist Sun Wukong travels back in time to the "World of the Ancients" (Qin Dynasty) to retrieve a magical bell and then travels forward to the "World of the Future" (Song Dynasty) to find an emperor who has been exiled in time. However, the time travel takes place inside an illusory dream world created by the villain to entrap and distract him.[9] 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.[10]: 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".[10]: 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".[10]: 95–96  In the science fiction anthology Far Boundaries (1951), editor August Derleth claims that an early short story about time travel is An Anachronism; or, Missing One's Coach, written for the Dublin Literary Magazine[11] by an anonymous author in the June 1838 issue.[12]: 3  While the narrator waits under a tree for a coach to take him out of Newcastle upon Tyne, 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.[12]: 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.[13]

 
Mr. and Mrs. Fezziwig dance in a vision shown to Scrooge by the Ghost of Christmas Past.

Charles Dickens' A Christmas Carol (1843) has early depictions of mystical 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 themself in a different time.[14] 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.[15] Edward Everett Hale's "Hands Off" (1881)[16] 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.[17]: 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,[18] 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.[17]: 55  Enrique Gaspar y Rimbau's El Anacronópete (1887) may have been the first story to feature a vessel engineered to travel through time.[19][20] 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".[21] H. G. Wells' The Time Machine (1895) popularized the concept of time travel by mechanical means.[22]

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.[23]: 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.[24] The classic example of a problem involving causality is the "grandfather paradox," which involves travelling to the past and intervening in the conception of one's ancestors (causing the death of an ancestor before said conception being frequently cited). Some physicists, such as Novikov and Deutsch, suggested that these sorts of temporal paradoxes can be avoided through the Novikov self-consistency principle or a variation of the many-worlds interpretation with interacting worlds.[25]

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, traversable wormholes, and Alcubierre drives.[26][27]: 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.[28] These semiclassical arguments led Stephen Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel,[29] 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.[30][31]: 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 that can be described as 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,[23]: 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.[32]

Wormholes

Main article: Wormhole

Wormholes are a hypothetical warped spacetime permitted by the Einstein field equations of general relativity.[33]: 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 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.[23]: 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;[23]: 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,[33]: 101  and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics.[34] Although early calculations suggested that a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small.[35]

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.[36] 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.[37]

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[38] in 1936 and Kornel Lanczos[39] in 1924, but not recognized as allowing closed timelike curves[40]: 21  until an analysis by Frank Tipler[41] 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.[42]

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,[40]: 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."[31]: 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."[29] 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 and black hole.

Quantum physics

Main article: Quantum mechanics of time travel

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.[43] The signal could be said to have moved backward in time. This hypothetical scenario is sometimes referred to as a tachyonic antitelephone.[44]

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.[45] 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.[46] 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 Hugh 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.[47] The accepted many-worlds interpretation suggests that all possible quantum events can occur in mutually exclusive histories.[48] 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.[49][50] 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.[51] 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.[25]

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 be observed only 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.[52]

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,[53] 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.[54]

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.[55]

Absence of time travelers from the future

Many have argued that the absence of time travelers from the future demonstrates that such technology will never be developed, suggesting that it is impossible. This is analogous to the Fermi paradox related to the absence of evidence of extraterrestrial life. As the absence of extraterrestrial visitors does not categorically 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.[30] Some versions of general relativity suggest that time travel might only be possible in a region of spacetime that is warped a certain way,[clarification needed] 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".[51]

 
Advertisement placed in a 1980 edition of Artforum, advertising the Krononauts event

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.[56] In 1982, a group in Baltimore, Maryland, identifying itself as the Krononauts, hosted an event of this type welcoming visitors from the future.[57][58] 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.[59]

Time dilation

Main article: Time dilation
 
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 reciprocal.

There is a great deal of observable evidence for time dilation in special relativity[60] and gravitational time dilation in general relativity,[61][62][63] for example in the famous and easy-to-replicate observation of atmospheric muon decay.[64][65][66] 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.[66] 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.[67]

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.[27]: 33–130 

A time machine that utilizes this principle might be, for instance, a spherical shell with a diameter of five meters and the mass of Jupiter. A person at its center will travel forward in time at a rate four times slower than 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.[27]: 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 after a few hundred days of space travel.[68]

Philosophy

Main article: Philosophy of space and time

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.[69]

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.[70] 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".[71] 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.[72]

 
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.[70] 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;[73] these views are contested by some authors.[74]

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 having 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.[75] 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,[76] and all coordinate systems are on equal footing as given by the principle of diffeomorphism invariance.[77]

The grandfather paradox

Main article: 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.[78] 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.[79][80] 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.[30] Philosophers question whether these paradoxes prove time travel impossible. Some philosophers answer these 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,[81] 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.[82]

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,[83] ontological paradox,[84] or bootstrap paradox.[84][85] The term bootstrap paradox was popularized by Robert A. Heinlein's story "By His Bootstraps".[86] 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.[87]

The philosopher Kelley L. Ross argues in "Time Travel Paradoxes"[88] 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 the film 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 and 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.[27]: 23 

In 2005, Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel where the past must be self-consistent.[89][90]

In fiction

Further information: Time travel in fiction

Time travel themes in science fiction and the media can be grouped into three categories: immutable timeline; mutable timeline; and alternate histories, as in the interacting-many-worlds interpretation.[91][92][93] The non-scientific term timeline is often used to refer to all physical events in history, so that where events are changed, the time traveler is described as creating a new timeline.[94]

See also

Further reading

References

  1. ^ Cheng, John (2012). Astounding Wonder: Imagining Science and Science Fiction in Interwar America (illustrated ed.). University of Pennsylvania Press. p. 180. ISBN 978-0-8122-0667-8. Extract of page 180
  2. ^ Dowson, John (1879), "Revati", A classical dictionary of Hindu mythology and religion, geography, history, and literature, Routledge
  3. ^ The Vishnu Purana: Book IV: Chapter I
  4. ^ Debiprasad Chattopadhyaya (1964), Indian Philosophy (7 ed.), People's Publishing House, New Delhi
  5. ^ Yorke, Christopher (February 2006). "Malchronia: Cryonics and Bionics as Primitive Weapons in the War on Time". Journal of Evolution and Technology. 15 (1): 73–85. Retrieved August 29, 2009.
  6. ^ Rosenberg, Donna (1997). Folklore, myths, and legends: a world perspective. McGraw-Hill. p. 421. ISBN 978-0-8442-5780-8.
  7. ^ "בבלי - מסכת תענית פרק ג". mechon-mamre.org. Retrieved 2022-12-29.
  8. ^ Peter Fitting (2010), "Utopia, dystopia, and science fiction", in Gregory Claeys (ed.), The Cambridge Companion to Utopian Literature, Cambridge University Press, pp. 138–139
  9. ^ Dong, Yue; Wu, Chengẻn (2000). The Tower of Myriad Mirrors: A Supplement to Journey to the West. Michigan classics in Chinese studies. Translated by Lin, Shuen-fu; Schulz, Larry James (2nd ed.). Ann Arbor: Center for Chinese Studies, The University of Michigan. ISBN 9780892641420.
  10. ^ a b c Alkon, Paul K. (1987). Origins of Futuristic Fiction. The University of Georgia Press. ISBN 978-0-8203-0932-3.
  11. ^ "An Anachronism; or, Missing One's Coach". Dublin University Magazine. 11. June 1838.
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External links

 
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Look up time travel in Wiktionary, the free dictionary.

Overviews and encyclopedic coverage

  • 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

January 26, 2023
time, travel, other, uses, disambiguation, time, machine, redirects, here, other, uses, time, machine, disambiguation, concept, movement, between, certain, points, time, analogous, movement, between, different, points, space, object, person, typically, with, h. For other uses see Time travel disambiguation Time machine redirects here For other uses see Time Machine disambiguation 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 with the use of a hypothetical device known as a time machine Time travel is a widely recognized concept in philosophy and fiction particularly science fiction The idea of a time machine was popularized by H G Wells 1895 novel The Time Machine 1 The first page of The Time Machine published by Heinemann It is uncertain if time travel to the past is physically possible and such travel if at all feasible may give rise to questions of causality 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 backward time travel it is possible to find solutions in general relativity that allow for it such as a rotating black hole Traveling to an arbitrary point in spacetime has very limited support in theoretical physics and is usually connected only with quantum mechanics or wormholes Contents 1 History of the time travel concept 1 1 Shift to science fiction 1 2 Early time machines 2 Time travel in physics 2 1 General relativity 2 1 1 Different spacetime geometries 2 1 2 Wormholes 2 1 3 Other approaches based on general relativity 2 2 Quantum physics 2 2 1 No communication theorem 2 2 2 Interacting many worlds interpretation 2 3 Experimental results 2 4 Absence of time travelers from the future 2 5 Time dilation 3 Philosophy 3 1 Presentism vs eternalism 3 2 The grandfather paradox 3 3 Ontological paradox 3 3 1 Compossibility 3 3 2 Self consistency principle 4 In fiction 5 See also 6 Further reading 7 References 8 External links 8 1 Overviews and encyclopedic coverageHistory of the time travel concept Statue of Rip Van Winkle in Irvington New York Some ancient myths depict a character skipping forward in time In Hindu mythology the Vishnu Purana 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 2 3 The Buddhist Pali 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 4 The Japanese tale of Urashima Tarō 5 first described in the Manyoshu tells of a young fisherman named Urashima no ko 浦嶋子 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 6 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 7 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 reve s il en fut jamais The Year 2440 A Dream If Ever There Was One 1770 by Louis Sebastien 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 later more familiar time machine is used as a means of time travel in these stories 8 The earliest work about backwards time travel is uncertain The Chinese novel Supplement to the Journey to the West c 1640 by Dong Yue features magical mirrors and jade gateways that connect various points in time The protagonist Sun Wukong travels back in time to the World of the Ancients Qin Dynasty to retrieve a magical bell and then travels forward to the World of the Future Song Dynasty to find an emperor who has been exiled in time However the time travel takes place inside an illusory dream world created by the villain to entrap and distract him 9 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 10 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 10 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 10 95 96 In the science fiction anthology Far Boundaries 1951 editor August Derleth claims that an early short story about time travel is An Anachronism or Missing One s Coach written for the Dublin Literary Magazine 11 by an anonymous author in the June 1838 issue 12 3 While the narrator waits under a tree for a coach to take him out of Newcastle upon Tyne 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 12 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 13 Mr and Mrs Fezziwig dance in a vision shown to Scrooge by the Ghost of Christmas Past Charles Dickens A Christmas Carol 1843 has early depictions of mystical 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 themself in a different time 14 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 15 Edward Everett Hale s Hands Off 1881 16 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 17 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 18 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 17 55 Enrique Gaspar y Rimbau s El Anacronopete 1887 may have been the first story to feature a vessel engineered to travel through time 19 20 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 21 H G Wells The Time Machine 1895 popularized the concept of time travel by mechanical means 22 Time travel in physicsSome 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 23 499 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 Godel 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 24 The classic example of a problem involving causality is the grandfather paradox which involves travelling to the past and intervening in the conception of one s ancestors causing the death of an ancestor before said conception being frequently cited Some physicists such as Novikov and Deutsch suggested that these sorts of temporal paradoxes can be avoided through the Novikov self consistency principle or a variation of the many worlds interpretation with interacting worlds 25 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 traversable wormholes and Alcubierre drives 26 27 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 28 These semiclassical arguments led Stephen Hawking to formulate the chronology protection conjecture suggesting that the fundamental laws of nature prevent time travel 29 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 30 31 150 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 that can be described as time travel Such a solution was first proposed by Kurt Godel a solution known as the Godel metric but his and others solution requires the universe to have physical characteristics that it does not appear to have 23 499 such as rotation and lack of Hubble expansion Whether general relativity forbids closed time like curves for all realistic conditions is still being researched 32 Wormholes Main article Wormhole Wormholes are a hypothetical warped spacetime permitted by the Einstein field equations of general relativity 33 100 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 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 23 502 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 23 503 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 33 101 and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics 34 Although early calculations suggested that a very large amount of negative energy would be required later calculations showed that the amount of negative energy can be made arbitrarily small 35 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 36 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 37 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 38 in 1936 and Kornel Lanczos 39 in 1924 but not recognized as allowing closed timelike curves 40 21 until an analysis by Frank Tipler 41 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 42 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 40 169 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 31 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 29 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 and black hole Quantum physics Main article Quantum mechanics of time travel 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 43 The signal could be said to have moved backward in time This hypothetical scenario is sometimes referred to as a tachyonic antitelephone 44 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 45 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 46 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 Hugh 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 47 The accepted many worlds interpretation suggests that all possible quantum events can occur in mutually exclusive histories 48 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 49 50 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 51 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 25 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 be observed only 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 52 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 53 so this experiment is understood not to violate causality either The physicists Gunter 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 54 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 55 Absence of time travelers from the future Many have argued that the absence of time travelers from the future demonstrates that such technology will never be developed suggesting that it is impossible This is analogous to the Fermi paradox related to the absence of evidence of extraterrestrial life As the absence of extraterrestrial visitors does not categorically 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 30 Some versions of general relativity suggest that time travel might only be possible in a region of spacetime that is warped a certain way clarification needed 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 51 Advertisement placed in a 1980 edition of Artforum advertising the Krononauts event 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 56 In 1982 a group in Baltimore Maryland identifying itself as the Krononauts hosted an event of this type welcoming visitors from the future 57 58 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 59 Time dilation Main article Time dilation 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 reciprocal There is a great deal of observable evidence for time dilation in special relativity 60 and gravitational time dilation in general relativity 61 62 63 for example in the famous and easy to replicate observation of atmospheric muon decay 64 65 66 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 66 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 67 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 27 33 130 A time machine that utilizes this principle might be for instance a spherical shell with a diameter of five meters and the mass of Jupiter A person at its center will travel forward in time at a rate four times slower than 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 27 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 after a few hundred days of space travel 68 PhilosophyMain article Philosophy of space and time 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 69 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 70 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 71 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 72 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 70 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 73 these views are contested by some authors 74 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 having 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 Godel and others suspect presentism may be valid for some forms of general relativity 75 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 76 and all coordinate systems are on equal footing as given by the principle of diffeomorphism invariance 77 The grandfather paradox Main article 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 78 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 79 80 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 30 Philosophers question whether these paradoxes prove time travel impossible Some philosophers answer these 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 81 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 82 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 83 ontological paradox 84 or bootstrap paradox 84 85 The term bootstrap paradox was popularized by Robert A Heinlein s story By His Bootstraps 86 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 87 The philosopher Kelley L Ross argues in Time Travel Paradoxes 88 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 the film 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 and 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 27 23 In 2005 Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel where the past must be self consistent 89 90 In fictionFurther information Time travel in fiction Time travel themes in science fiction and the media can be grouped into three categories immutable timeline mutable timeline and alternate histories as in the interacting many worlds interpretation 91 92 93 The non scientific term timeline is often used to refer to all physical events in history so that where events are changed the time traveler is described as creating a new timeline 94 See alsoClaims of time travel Time travel claims and urban legendsCulture Time capsuleFiction Time travel in fiction Time travel in The Lord of the Rings List of time travel works of fiction List of games containing time travel Science Krasnikov tube Retrocausality Ring singularity Temporal paradox Wheeler Feynman absorber theoryTime perception Cryonics Suspended animation Time perceptionFurther readingTime Travel A History book by James GleickReferences Cheng John 2012 Astounding Wonder Imagining Science and Science Fiction in Interwar America illustrated ed University of Pennsylvania Press p 180 ISBN 978 0 8122 0667 8 Extract of page 180 Dowson John 1879 Revati A classical dictionary of Hindu mythology and religion geography history and literature Routledge The Vishnu Purana Book IV Chapter I Debiprasad Chattopadhyaya 1964 Indian Philosophy 7 ed People s Publishing House New Delhi Yorke Christopher February 2006 Malchronia Cryonics and Bionics as Primitive Weapons in the War on Time Journal of Evolution and Technology 15 1 73 85 Retrieved August 29 2009 Rosenberg Donna 1997 Folklore myths and legends a world perspective McGraw Hill p 421 ISBN 978 0 8442 5780 8 בבלי מסכת תענית פרק ג mechon mamre org Retrieved 2022 12 29 Peter Fitting 2010 Utopia dystopia and science fiction in Gregory Claeys ed The Cambridge Companion to Utopian Literature Cambridge University Press pp 138 139 Dong Yue Wu Chengẻn 2000 The Tower of Myriad Mirrors A Supplement to Journey to the West Michigan classics in Chinese studies Translated by Lin Shuen fu Schulz Larry James 2nd ed Ann Arbor Center for Chinese Studies The University of Michigan ISBN 9780892641420 a b c Alkon Paul K 1987 Origins of Futuristic Fiction The University of Georgia Press ISBN 978 0 8203 0932 3 An Anachronism or Missing One s Coach Dublin University Magazine 11 June 1838 a b Derleth August 1951 Far Boundaries Pellegrini amp Cudahy Lib ru Klassika Akutin Yurij Aleksandr Veltman i ego roman Strannik az lib ru Retrieved 2022 12 29 Flynn John L 1995 Time Travel Literature The Encyclopedia Galactica Archived from the original on September 29 2006 Retrieved October 28 2006 Rudwick Martin J S 1992 Scenes From Deep Time The University of Chicago Press pp 166 169 ISBN 978 0 226 73105 6 Hale Edward Everett 1895 Hands Off J Stilman Smith amp Co a b Nahin Paul J 2001 Time machines time travel in physics metaphysics and science fiction Springer ISBN 978 0 387 98571 8 Page Mitchell Edward The Clock That Went Backward PDF Archived from the original PDF on October 15 2011 Retrieved December 4 2011 Uribe Augusto June 1999 The First Time Machine Enrique Gaspar s Anacronopete The New York Review of Science Fiction 11 no 10 130 12 Gaspar Enrique 2012 06 26 The Time Ship A Chrononautical Journey Wesleyan University Press ISBN 978 0 8195 7239 4 Westcott Kathryn 9 April 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time travel PDF American Philosophical Quarterly 13 145 52 arXiv gr qc 9603042 Bibcode 1996gr qc 3042K Erdmann Terry J Hutzel Gary 2001 Star Trek The Magic of Tribbles Pocket Books p 31 ISBN 978 0 7434 4623 5 a b Smeenk Chris Wuthrich Christian 2011 Time Travel and Time Machines in Callender Craig ed The Oxford Handbook of Philosophy of Time Oxford University Press p 581 ISBN 978 0 19 929820 4 Krasnikov S 2001 The time travel paradox Phys Rev D 65 6 06401 arXiv gr qc 0109029 Bibcode 2002PhRvD 65f4013K doi 10 1103 PhysRevD 65 064013 S2CID 18460829 Klosterman Chuck 2009 Eating the Dinosaur 1st Scribner hardcover ed New York Scribner pp 60 62 ISBN 9781439168486 Friedman John Michael Morris Igor Novikov Fernando Echeverria Gunnar Klinkhammer Kip Thorne Ulvi Yurtsever 1990 Cauchy problem in spacetimes with closed timelike curves Physical Review D 42 6 1915 1930 Bibcode 1990PhRvD 42 1915F doi 10 1103 PhysRevD 42 1915 PMID 10013039 Ross Kelley L 2016 Time Travel Paradoxes retrieved April 26 2017 Greenberger Daniel M Svozil Karl 2005 Quantum Theory Looks at Time Travel Quo Vadis Quantum Mechanics The Frontiers Collection p 63 arXiv quant ph 0506027 Bibcode 2005qvqm book 63G doi 10 1007 3 540 26669 0 4 ISBN 978 3 540 22188 3 S2CID 119468684 Kettlewell Julianna June 17 2005 New model permits time travel BBC News Retrieved April 26 2017 Grey William 1999 Troubles with Time Travel Philosophy Cambridge University Press 74 1 55 70 doi 10 1017 S0031819199001047 S2CID 170218026 Rickman Gregg 2004 The Science Fiction Film Reader Limelight Editions ISBN 978 0 87910 994 3 Schneider Susan 2009 Science Fiction and Philosophy From Time Travel to Superintelligence Wiley Blackwell ISBN 978 1 4051 4907 5 Prucher Jeff Brave new words ISBN 978 0 19 530567 8 External links Wikimedia Commons has media related to Time travel Look up time travel in Wiktionary the free dictionary Overviews and encyclopedic coverage 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 Retrieved from https en wikipedia org w index php title Time travel amp oldid 1130244956, wikipedia, wiki, book, books, library,

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