The hypothetical act of going into the past or the future is known as time travel. The idea of time travel is well-known in philosophy and science fiction in particular. In fiction, time travel is usually accomplished by means of an imaginary apparatus called a time machine. The Time Machine, written by H. G. Wells in 1895, popularized the concept of a time machine.
Whether time travel to the past is physically feasible is debatable. If such travel is possible at all, it could raise causality issues. Outside of the conventional notion of seeing time, forward time travel is a widely seen phenomenon that is well understood in the context of general and special relativity. But with existing technology, it is not possible to have one body advance or delay more than a few milliseconds in relation to another body. In terms of backward time travel, whirling black holes are among the general relativity solutions that make it conceivable.
Theoretical physics offers relatively little support for traveling to any point in spacetime; the concepts are typically associated with wormholes or quantum mechanics.
The concept’s past
Mythical time travel
In several myths from antiquity, a figure is shown traveling across time. The Vishnu Purana, a Hindu epic, tells the tale of King Raivata Kakudmi, who ascends to heaven to see the creator Brahma and is shocked to discover that a great deal of time has passed upon his return to Earth. The relativity of time is mentioned in the Buddhist Pāli Canon.
According to the Payasi Sutta, one of the Buddha’s most important followers, Kumara Kassapa, explains to the doubting Payasi that time moves differently in the heavens than it does on Earth. A young fisherman named Urashima-no-ko (浦嶋子) pays a visit to an underwater castle in the Japanese folktale “Urashima Tarō,” which was initially recounted in the Manyoshu.
After three days, he travels back to his hamlet only to discover that it has advanced 300 years; his family has passed away, he has been forgotten, and his house is in ruins. According to Jewish tradition, the scholar Honi ha-M’agel slept for seventy years after falling asleep in the first century BC. Upon awakening, he went back to his house and discovered that nobody remembered him, and nobody took him seriously.
Abrahamic religions
The Quran, the sacred book of Islam, tells the tale of the Seven Sleepers, a group of young men who practiced monotheism and sought safety from persecution in a cave. Allah kept them safe as they slept for ages, and when they woke up, they found that the world had changed. This story, which is included in Surah Al-Kahf of the Quran, talks about divine protection and time suspension. A Christian myth recounts the tale of the Seven Sleepers, a group of young men who were trying to flee Roman oppression. At the time, Rome was a polytheistic empire.
In a different Islamic tale, Uzair—often mistaken for the biblical Ezra—felt intense sorrow over Jerusalem’s devastation by the Babylonians. It is stated that once the city was rebuilt, God took his soul and brought him back to life. When he returned to his hometown, no one in his household knew who he was, save for an old woman who had been blind but had become seen again as a result of his prayers. Uzair met his son too, during this reunion, who was older than he was but still knew who his father was.
Shift to science fiction
Three categories may be used to classify time travel topics in science fiction and the media: malleable timelines, interacting-many-worlds interpretations, and alternate histories. Since all historical physical events are frequently referred to by the non-scientific term “timeline,” time travelers are said to be generating new timelines in cases when events are altered.
Characters from early science fiction novels either travel back in time by paranormal methods or wake up in a civilization that has altered after years of slumber. Among these are Louis-Sébastien Mercier’s L’An 2440, rêve s’il en fût jamais (The Year 2440: A Dream If Ever There Was One, 1770), Washington Irving’s Rip Van Winkle (1819), Edward Bellamy’s Looking Backward (1888), and H. G. Wells’ When the Sleeper Awakes (1899). In these stories, time travel is accomplished by prolonged slumber, much like the later and more widely known time machine.
It’s unclear when the first writings about reverse time travel were published. Dong Yue’s Supplement to the Journey to the West, published in China in 1640, is replete with miraculous mirrors and jade portals that span many eras. The main character, Sun Wukong, goes back in time to the Qin dynasty’s “World of the Ancients” to reclaim a magical bell. He then goes ahead to the Song dynasty’s “World of the Future” to locate an emperor who has been banished in time. But the time travel is happening inside a false dream world that the villain has constructed to divert and imprison him.
The Memoirs of the Twentieth Century (1733) by Samuel Madden is a collection of letters sent by British ambassadors in 1997 and 1998 to their predecessors, outlining the political and religious landscape of the future. Paul Alkon claims in his book Origins of Futuristic Fiction that “the first time-traveler in English literature is a guardian angel” since the narrator gets these letters from his angel. Alkon claims 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,” albeit Madden does not explain how the angel receives these papers.
Editor August Derleth asserts in the science fiction collection Far Boundaries (1951) that an early time-travel short tale is An Anachronism; or, Missing One’s Coach, which was written for the Dublin Literary Magazine in the June 1838 edition by an unidentified author. The narrator is taken back in time more than a millennium as he waits under a tree for a carriage to take him out of Newcastle upon Tyne. In a monastery, he meets the Venerable Bede, who tells him about the events of the ensuing centuries. It is never made clear in the novel, though, if they are actual happenings or just dreams.
The Forebears of Kalimeros: Alexander, son of Philip of Macedon by Alexander Veltman, published in 1836, is another early essay regarding time travel.
Early examples of magical time travel in both directions may be found in Charles Dickens’ A Christmas Carol (1843), in which Ebenezer Scrooge, the main character, is taken to both past and future Christmases. Similar plot points are used in other works, in which a character falls asleep of their own volition and wakes up to discover they are in a different period. The well-known book Paris avant les hommes (Paris before Men), written and published posthumously in 1861 by the French botanist and geologist Pierre Boitard, provides a more lucid illustration of traveling backward in time.
The protagonist of this tale is magically thrust into prehistoric times by a “lame demon” (a play on Boitard’s name in French), where he gets to engage with extinct animals and meets a Plesiosaur and an apelike ancestor. In “Hands Off” (1881) by Edward Everett Hale, an unidentified entity—possibly the spirit of a recently deceased person—interferes with the history of ancient Egypt by averting Joseph’s captivity. This tale could have been the first to depict a different past produced by time travel.
Earliest Timepieces
The 1881 New York Sun story “The Clock that Went Backward” by Edward Page Mitchell is among the earliest to depict a machine-assisted time travel narrative. But the method is almost fantastical. When an odd clock is wound, it travels backward in time, taking adjacent individuals with it. The origin and characteristics of the clock are not explained by the author. El Anacronópete (1887) by Enrique Gaspar y Rimbau could have been the first tale to depict a time-traveling vessel.
The narrative “does seem to be the first literary description of a time machine noted so far,” according to Andrew Sawyer’s comments, stating 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”. The Time Machine by H. G. Wells, published in 1895, popularized the idea of mechanical time travel.
Time travel in physics
Certain theories, most famously special and general relativity, propose that if certain spacetime geometries or movements were feasible, they may permit time travel into the past and future. Physicists speculate about the idea of closed timelike curves in technical publications. These are world lines that create closed loops in spacetime and enable things to travel back in time. Although the physical validity of these solutions is disputed, solutions to the general relativity equations that describe spacetimes with closed timelike curves, like Gödel spacetime, are known to exist.
The scientific community as a whole is divided on the possibility of reverse time travel. Any theory that permits time travel might lead to possible causality issues. The “grandfather paradox,” which suggests traveling back in time and changing one’s ancestors’ conception (causing an ancestor to die before conception being usually stated), is a famous example of difficulty regarding causality. Some physicists, including Novikov and Deutsch, proposed that a modification of the many-worlds interpretation with interacting worlds or the Novikov self-consistency principle can prevent these kinds of temporal paradoxes.
General relativity
In general relativity, spacetime geometries like cosmic strings, traversable wormholes, and Alcubierre drives, which allow for faster-than-light travel, time travel to the past is potentially feasible. In some exceptional cases, the theory of general relativity does provide a rationale for the theoretical plausibility of backward time travel; however, semiclassical gravity provides evidence that these gaps in the theory may be filled in when quantum effects are included.
Stephen Hawking developed the chronology protection conjecture—which holds that time travel is prohibited by the fundamental laws of nature—as a result of these semiclassical arguments, but physicists are unable to reach a consensus on the matter in the absence of a theory of quantum gravity that would combine general relativity and quantum mechanics into a single, cohesive theory.
Different spacetime geometries
The cosmos is described by the theory of general relativity using a set of field equations that establish the metric, or spacetime distance function. Closed time-like curves, or world lines that overlap one another, are accurate solutions to these equations; time travel is possible when a point in the world line’s causal past also occurs at a point in its causal future. Kurt Gödel was the first to suggest such a solution, which became known as the Gödel metric.
However, his (and others) solution necessitates the existence of physical properties for the universe that do not seem to exist, such as rotation and the absence of Hubble expansion. It is still up for debate whether closed time-like curves are forbidden by general relativity under all plausible circumstances.
Porous structures
According to general relativity’s Einstein field equations, wormholes are hypothetical distorted spacetimes. Theoretically, a traversable wormhole-based time-travel device might function as follows: Perhaps using a sophisticated propulsion system, one end of the wormhole is propelled to a considerable fraction of the speed of light before being returned to its original location. One method is to take one wormhole entry, relocate it inside the gravitational field of an object whose gravity is stronger than that of the other entrance, and then transfer it back to a location close to the other entrance.
The shifted end of the wormhole appears “younger” to an external observer in both of these methods due to time dilation; however, time connects differently through the wormhole than outside of it, meaning that synchronized clocks at either end of the wormhole will always remain synchronized as seen by an observer passing through the wormhole, regardless of how the two ends move. This implies that, as viewed from the outside, an observer entering the “younger” end would leave the “older” end at a point when it was the same age as the “younger” end, essentially traveling back in time.
One major drawback of such a time machine is that it can only travel back in time to the moment the machine was created; in other words, it is not a time machine per se, but rather a path through time, and it would not permit the advancement of technology.
As per the prevailing ideas of wormholes, the creation of a traversable wormhole necessitates the presence of a material known as “exotic matter” that has negative energy. Technically speaking, wormhole spacetime necessitates an energy distribution that defies a number of energy requirements, including the null energy condition and the dominant, strong, and weak energy conditions. The null energy criterion can, however, be somewhat violated by quantum processes, and many scientists think that the Casimir effect in quantum physics may make the necessary negative energy genuinely achievable.
Subsequent studies revealed that the quantity of negative energy may be made arbitrarily minimal, despite initial estimates indicating that a relatively big amount would be needed.
A wormhole with two mouths and an induced clock difference could not be brought together, according to Matt Visser’s 1993 argument, without causing quantum field and gravitational effects that would either cause the wormhole to collapse or cause the two mouths to reject one another. As a result, it was impossible to get the two mouths near enough to violate causality. A complex “Roman ring” configuration of N wormholes arranged in a symmetric polygon, named after Tom Roman, may still be able to function as a time machine, according to a 1997 paper by Visser. However, he concludes that this is more likely a defect in classical quantum gravity theory than evidence for causality violation.
Other approaches based on general relativity
An alternative method employs a dense spinning cylinder, commonly known as a Tipler cylinder. This GR solution was identified by Willem Jacob van Stockum and Kornel Lanczos in 1936, but it wasn’t acknowledged as permitting closed timelike curves until Frank Tipler’s study in 1974. A spacecraft orbiting a cylinder on a spiral route has the potential to travel back in time if the cylinder is indefinitely long and rotates quickly enough around its long axis (or forward, depending on its spiral orientation). However, conventional matter is not strong enough to create it since the needed speed and density are so high.
In an effort to bend spacetime and enable time travel, physicist Ronald Mallett is using ring lasers to simulate the circumstances of a revolving black hole.
Stephen Hawking developed a theorem demonstrating that, in accordance with general relativity, it is impossible to construct 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, which means that the region contains no matter with negative energy density (exotic matter). This theorem provides a more fundamental argument against time travel schemes based on rotating cylinders or cosmic strings. Solutions like Tipler’s imply infinite-length cylinders, which are simpler to examine analytically. Tipler hypothesized that if the rotation rate were high enough, a finite cylinder might create closed timelike curves, but he did not provide any evidence for this.
However, Hawking notes that due to his thesis, “positive energy density cannot be used everywhere! I can demonstrate that negative energy is required to construct a limited time machine.” This conclusion stems from Hawking’s statement that “the laws of physics do not allow the appearance of closed timelike curves” in his 1992 study on the chronology protection conjecture.
Physics at the quantum level
Theorem of no communication
The theory of relativity’s mathematics of simultaneity demonstrates that when a signal is sent from one place and received at another, all reference frames concur that the transmission event occurred before the reception event, provided the signal is traveling at the speed of light or slower. In all reference frames, a signal that travels faster than light is received before it is delivered. It is possible to say that the signal went backward in time. This imaginary situation is called a tachyonic antitelephone at times.
Faster-than-light (FTL) communication or time travel may seem possible thanks to quantum-mechanical phenomena like quantum teleportation, the EPR paradox, or quantum entanglement. In fact, some interpretations of quantum mechanics, like the Bohm interpretation, assume that some information is exchanged between particles instantaneously in order to maintain correlations between particles. Einstein described this phenomenon as “spooky action at a distance”.
However, present quantum field theories are rigorous in their claim that causality is conserved in quantum mechanics; as a result, time travel and FTL communication are not permitted by current theories. A more thorough investigation has shown that in each particular case where FTL has been asserted, some kind of classical communication must also be utilized in order to receive a signal. Additionally, a broader argument that quantum entanglement cannot be utilized to transfer information faster than classical signals is provided by the no-communication theorem.
Interpretation interacting with several realms
The grandfather paradox can be resolved by applying a variant of Hugh Everett’s many-worlds interpretation (MWI) of quantum mechanics, in which the time traveler arrives in a different universe than the one from which they originated. However, some argue that this is not “genuine” time travel because they arrive in the history of a different universe rather than their own.
According to the widely recognized many-worlds interpretation, any conceivable quantum event may transpire in a history that is mutually exclusive. Some modifications, nevertheless, permit interaction between several realities. Though scientists like David Deutsch have proposed that a time traveler should wind up in a different history from the one he began from, this idea is most frequently employed in science fiction.
However, Stephen Hawking has contended that even in the event that the MWI is accurate, we should anticipate that every time traveler would have a single self-consistent history, meaning that time travelers will stay in their own reality as opposed to visiting another one. Deutsch’s method, according to scientist Allen Everett, “involves modifying fundamental principles of quantum mechanics; it certainly goes beyond simply adopting the MWI”. Everett further contends that even if Deutsch’s method is accurate, it would mean that, when a wormhole was used to go back in time, any macroscopic entity made up of many particles would be broken apart, with different particles appearing in various universes.
Outcomes of an Experiment
Some tests appear to demonstrate reversed causation, however careful inspection does not confirm this.
Pairs of entangled photons are separated into “signal photons” and “idler photons” in Marlan Scully’s delayed-choice quantum eraser experiment. The signal photons emerge from one of two places, and their position is subsequently monitored as in the double-slit experiment. The experimenter can either “erase” or discover which of the two sites the signal photon originated from, depending on how the idler photon is monitored. When one links measurements of idler photons to matching signal photons, one finds that although the signal photons may be detected prior to the decision about the idler photons, the decision appears to affect whether or not an interference pattern is noticed retroactively.
Causality is preserved because experimenters cannot predict the decision that will be made in advance by observing the signal photons alone; instead, they must gather classical information from the entire system and observe interference only after the idler photons have been measured and correlated with the signal photons.
Since Lijun Wang’s experiment allowed packages of waves to be sent through a cesium gas bulb in a way that made the package appear to exit the bulb 62 nanoseconds before it entered, it may also demonstrate a violation of causality. However, a wave package is not a single, well-defined object, but rather the sum of multiple waves of various frequencies (see Fourier analysis), and it is possible for the package to appear to move faster than light or even backward in time even though none of the individual waves in the sum do. This experiment is thought to not break causality either, as this effect cannot be exploited to transfer any matter, energy, or information faster than light.
By sending photons faster than the speed of light, scientists Günter Nimtz and Alfons Stahlhofen of the University of Koblenz assert that they have defied Einstein’s theory of relativity. They claim to have carried out an experiment employing the quantum tunneling phenomenon in which microwave photons moved up to 3 feet (0.91 meters) apart went “instantaneously” between a pair of prisms. “As of right now, this is the only violation of special relativity that I am aware of,” Nimtz stated in an interview with New Scientist magazine. Nonetheless, some scientists assert that this phenomena prevents information from traveling at a speed greater than light.
The University of Toronto’s Aephraim M. Steinberg is an expert in quantum optics. He compares the concept to that of a train that travels from Chicago to New York, picking up and dropping off cars at each station along the way. This allows the train’s center to advance at each stop, surpassing the individual cars’ speeds.
Shengwang Du states that single photon precursors travel no faster than c in a vacuum and claims to have witnessed them in a peer-reviewed journal. He used both slow light and light traveling through a vacuum in his experiment. He produced two single photons, one of which he passed through laser-cooled rubidium atoms, delaying the light, and the other through a vacuum. Evidently, the predecessors traveled at c in a vacuum both times, ahead of the main bodies of the photons. This suggests, in Du’s opinion, that there is no way for light to move faster than c and, hence, no way for causality to be broken.
The absence of future time travelers
Many have said that the lack of time travelers from the future proves that this kind of technology is unattainable and that it will never be produced. The Fermi conundrum, which has to deal with the lack of evidence for alien life, is comparable to this. Though the lack of time travelers does not establish that time travel is technically impossible, the absence of alien visitors does not prove that they do not exist. It is possible that time travel is physically viable but is never developed or utilized cautiously. It was originally proposed by Carl Sagan that time travelers may exist here but be hiding their presence or not being acknowledged as such.
According to some interpretations of general relativity, time travelers might not be able to return to previous periods of spacetime before this area existed. Instead, they might only be able to travel inside a specific, distorted zone of spacetime. This, according to Stephen Hawking, would explain why “tourists from the future” haven’t taken over the planet already.
In an attempt to persuade future humans who could develop time travel technology to return and show it to people in the present, a number of tests have been conducted. Permanent “advertisements” indicating a meeting time and location for future time travelers to meet were widely publicized during events like Perth’s Destination Day or MIT’s Time Traveler Convention. Something similar was held in 1982 in Baltimore, Maryland, by a group calling itself the Krononauts, who welcomed guests from the future to their gathering. Though no time travelers are known to have attended either event, both tests have failed thus far, despite the potential to produce a favorable outcome proving the reality of time travel.
Certain interpretations of the many-worlds theory imply that future humanity has returned to a parallel reality and journeyed back in time to the rendezvous point.
Dilation of time
Time dilation in special relativity and gravitational time dilation in general relativity are strongly supported by observables, such as the well-known and simple-to-replicate observation of atmospheric muon decay. According to the theory of relativity, the speed of light is constant for all observers regardless of their frame of reference. One immediate result of the speed of light’s invariance is time dilation. In a restricted sense, time dilation may be thought of as “time travel into the future”: someone can utilize it to make it such that a significant portion of appropriate time passes somewhere while just a little portion passes for them.
You may do this by using gravity’s pull or by traveling at relativistic speeds.
When two identical clocks are moving in relation to one another without increasing in speed, one clock determines that the other is ticking more slowly. Because of simultaneity’s relativity, this is feasible. If one clock accelerates, however, the symmetry is disrupted, allowing one clock to pass more correct time than the other. The twin paradox states that one twin stays on Earth while the other accelerates to relativistic speed, travels into space, and then returns to Earth. Because of the time dilation that occurs during the acceleration, the traveling twin ages less than the twin who stayed on Earth.
When calibrating the clocks on the Global Positioning System satellites, general relativity takes into account the fact that time dilation also occurs in gravity wells, with a clock located deeper in the well ticking more slowly. This could result in notable differences in aging rates for observers located at different distances from a large gravity well, such as a black hole. General relativity treats the effects of acceleration and gravity as equivalent.
A spherical shell with the mass of Jupiter and a diameter of five meters may serve as a time machine based on this theory. At its center, time will pass four times more slowly than it does for spectators who are farther away. In the near future, it is not anticipated that human technology will be able to fit the mass of a big planet into such a compact structure. After a few hundred days in space, existing technology can only make a human traveler age less quickly than their Earthly colleagues by a few milliseconds.
Philosophies
Since at least the time of ancient Greece, philosophers have debated the philosophy of space and time; Parmenides, for instance, put out the idea that time is an illusion. Centuries later, Gottfried Wilhelm Leibniz, a contemporary of Isaac Newton, argued that time is merely a connection between events and cannot be articulated independently, whereas Newton backed the concept of absolute time. The relativistic spacetime was ultimately the result of the latter strategy.
Eternalism against presentism
Many philosophers have contended that the concept of relativity entails eternalism or the belief that the past and future are real and do not merely exist as modifications to the present that have happened or will happen. Science philosopher Dean Rickles disagrees with some of the limitations, but he observes that “special and general relativity seem to be incompatible with presentism among philosophers.” Although this theory is debatable, some philosophers believe that time is a dimension equivalent to spatial dimensions, that future occurrences are “already there” in the same way that various locations exist, and that time does not objectively flow.
According to the philosophy of presentism, the past, present, and future only really exist as modifications that have happened or will happen to the present; they do not truly exist in and of themselves. According to this theory, there is no past or future to travel to, making time travel impossible. Some authors disagree with Keller and Nelson’s assertion that there can be definitive truths about past and future events even in the absence of past and future objects. As a result, it is plausible that a future truth regarding a time traveler choosing to return to the present day could account for the time traveler’s actual appearance in the present.
The paradox of the grandfather
The grandfather paradox or the auto-infanticide argument is a popular refutation of the notion of time travel. Inconsistencies and contradictions would result from changing anything in the past if one were able to travel back in time; a modification in the past would contradict itself. A typical description of the paradox involves a person going back in time to kill their own grandpa, stopping their mother or father from being and, hence, themselves from existing. Philosophers argue about whether or not these paradoxes rule out time travel.
In response to these paradoxes, some philosophers argue—a position akin to the proposed Novikov self-consistency principle in physics—that it may be feasible for backward time travel to occur but that it would be impossible to really alter the past.
Paradox of ontology
Ability to compose
The philosophical idea of compossibility states that one must consider the entirety of the circumstance while evaluating what may occur, say, in the event of time travel. It is impossible for the past to have been anything other than what it was. To avoid logical conflicts, the events that can transpire during a time traveler’s journey to the past are restricted to what really occurred.
Principle of self-consistency
It is impossible for a time traveler to “change” history in any manner since all acts made by a time traveler or an item that goes back in time were already part of history, according to the Novikov self-consistency principle, which bears Igor Dmitrievich Novikov’s name. However, there is a chance that the time traveler’s activities might have caused events in their own history, creating the possibility of circular causation, also known as an ontological conundrum, bootstrap paradox, or predestination paradox. The tale “By His Bootstraps” by Robert A. Heinlein popularized the phrase bootstrap paradox.
The philosopher Kelley L. Ross contends in “Time Travel Paradoxes” that the second law of thermodynamics may be broken in the case of a physical item whose world line or history forms a closed loop in time. A watch is handed to a person in the movie Somewhere in Time, and then 60 years later, the identical watch is transported back in time and given to the same character. Ross presents this scene as an example of an ontological contradiction. According to Ross, every time a watch is taken back in time, its entropy will rise and it will get more worn.
Modern physicists understand the second rule of thermodynamics to be a statistical law, meaning that non-increasing entropy and decreasing entropy are not impossible, only unlikely. Furthermore, non-isolated systems, like an object that interacts with the outside world, may become less worn and have a decrease in entropy because entropy statistically increases in isolated systems. Additionally, an object whose world-line forms a closed loop may always be in the same condition at the same point in its history.
Daniel Greenberger and Karl Svozil suggested in 2005 that time travel using quantum theory requires the past to be self-consistent.
