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Time travel is the hypothetical activity of traveling into the past or future. Time travel is a concept in philosophy, space, time and fiction, particularly science fiction. In fiction, time travel is typically achieved through the use of a device known as a time machine. The idea of a time machine was popularized by H. G. Wells's 1895 novel The Time Machine. Extract of page 180
It is uncertain whether time travel to the past would be physically possible. Such travel, if at all feasible, may give rise to questions of causality. Forward time travel, outside the usual sense of the Time perception, is an extensively observed phenomenon and is 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 .
In one tradition in Judaism, Moses is transported by God to the study hall of Rabbi Akiva, where he is confused by the future evolution of Jewish law. Another Talmudic story features Honi HaMe'agel, a 1st-century BCE miracle worker who sees a man planting a carob tree that will take 70 years to bear fruit. Honi falls asleep and awakens 70 years later to find the tree fully grown and its fruit being harvested by the man's grandson.Babylonian Talmud Taanit 23a Hebrew/Aramaic text at Mechon-Mamre
In Islam, the Quran narrates the story of the Seven Sleepers, a group of Monotheism young men who sought refuge in a cave to escape persecution. As they slept, Allah preserved them for centuries, and when they awoke, they discovered that the world around them had changed. This narrative, found in the Quranic Surah Al-Kahf, describes divine protection and time suspension.
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 is used as a means of time travel in these stories.
The date of the earliest work about backwards time travel is uncertain. The Chinese novel A Supplement to the Journey to the West () 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 is taking place inside an illusory dream world created by the villain to distract and entrap him. (2026). 9780892641420, Center for Chinese Studies, The University of Michigan. ISBN 9780892641420 Samuel Madden's Memoirs of the Twentieth Century (1733) is a series of letters from British ambassadors in 1997 and 1998 to diplomats in the past, conveying the political and religious conditions of the future. Because the narrator receives these letters from his guardian angel, Paul Alkon suggests in his book Origins of Futuristic Fiction that "the first time-traveler in English literature is a guardian angel". Madden does not explain how the angel obtains these documents, but Alkon asserts that Madden "deserves recognition as the first to toy with the rich idea of time-travel in the form of an artifact sent backward from the future to be discovered in the present". In the science fiction anthology Far Boundaries (1951), editor August Derleth claims that an early short story about time travel is An Anachronism; or, Missing One's Coach, written for the Dublin Literary Magazine by an anonymous author in the June 1838 issue. While the narrator waits under a tree for a Stagecoach 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. Another early work about time travel is The Forebears of Kalimeros: Alexander, son of Philip of Macedon by Alexander Veltman published in 1836.
.]] Charles Dickens's A Christmas Carol (1843) has early depictions of 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. A clearer example of backward time travel is found in the 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 Plesiosauria and an apelike ancestor and is able to interact with ancient creatures. Edward Everett Hale's "Hands Off" (1881) tells the story of an unnamed being, possibly the soul of a person who has recently died, who interferes with ancient Egyptian history by preventing Joseph's enslavement. This may have been the first story to feature an alternate history created as a result of time travel.
Any theory that would allow backward time travel would introduce potential problems of causality. Extract of page 32 The classic example of a problem involving causality is the "grandfather paradox," which postulates travelling to the past and intervening in the conception of one's ancestors (causing the death of an ancestor before conception being frequently cited). Some physicists, such as Novikov and Deutsch, suggested that these sorts of can be avoided through the Novikov self-consistency principle or a variation of the many-worlds interpretation with interacting worlds.
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 , such as the null energy condition along with the weak, strong, and dominant energy conditions. However, it is known that quantum effects can lead to small measurable violations of the null energy condition, and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics. Although early calculations suggested 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.
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. 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.
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, he did not prove this. But Hawking points out that because of his theorem, "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy." This result comes from Hawking's 1992 paper on the chronology protection conjecture, which Hawking states as "The laws of physics do not allow the appearance of closed timelike curves."
Quantum-mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles. This effect was referred to as "spooky action at a distance" by Einstein.
Nevertheless, the fact that causality is preserved in quantum mechanics is a rigorous result in modern quantum field theories, and therefore modern theories do not allow for time travel or FTL communication. In any specific instance where FTL has been claimed, more detailed analysis has proven that to get a signal, some form of classical communication must also be used. The no-communication theorem also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals.
The delayed-choice quantum eraser experiment performed by Marlan Scully involves pairs of entangled 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.
The experiment of Lijun Wang might also show causality violation since it made it possible to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry, but a wave package is not a single well-defined object but rather a sum of multiple waves of different frequencies (see Fourier analysis), and the package can appear to move faster than light or even backward in time even if none of the pure waves in the sum do so. This effect cannot be used to send any matter, energy, or information faster than light, so this experiment is understood not to violate causality either.
The physicists Günter Nimtz and Alfons Stahlhofen, of the University of Koblenz, claim to have violated Einstein's theory of relativity by transmitting photons faster than the speed of light. They say they have conducted an experiment in which microwave photons traveled "instantaneously" between a pair of prisms that had been moved up to 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 M. 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.
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 , 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.
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, MIT's Time Traveler Convention and Stephen Hawking's Reception For Time Travellers heavily publicized permanent "advertisements" of a meeting time and place for future time travelers to meet. In 1982, a group in Baltimore, Maryland, identifying itself as the Krononauts, hosted an event of this type welcoming visitors from the future."Welcome the People from the Future. March 9, 1982". Ad in Artforum p. 90.
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 Multiverse.
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 , with a clock deeper in the well ticking more slowly; this effect is taken into account when calibrating the clocks on the satellites of the Global Positioning System, and it could lead to significant differences in rates of aging for observers at different distances from a large gravity well such as a black hole.
A time machine that utilizes this principle might be, for instance, a spherical shell with a diameter of five meters and the Jupiter mass. 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. 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.
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. Keller and Nelson have argued that even if past and future objects do not exist, there can still be definite truths about past and future events, and thus it is possible that a future truth about a time traveler deciding to travel back to the present date could explain the time traveler's actual appearance in the present; these views are contested by some authors.
The philosopher Kelley L. Ross argues in "Time Travel Paradoxes" that in a scenario involving a physical object whose world-line or history forms a closed loop in time there can be a violation of the second law of thermodynamics. Ross uses 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.
In 2005, Daniel Greenberger and Karl Svozil proposed that quantum theory gives a model for time travel where the past must be self-consistent. (2026). 9783540221883 ISBN 9783540221883
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