Time travel
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.
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.
[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] One story in Judaism concerns Honi HaMe'agel, a miracle-working sage of the 1st century BC, who was a historical character to whom various myths were attached.
Later that day, Honi sat down to rest but fell asleep for 70 years; when he awoke, he saw a man picking fruit from a fully mature carob tree.
[20] 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.
The Chinese novel A Supplement to the Journey to the West (c. 1640) by Dong Yue features magical mirrors and jade gateways that connect various points in time.
However, the time travel is taking place inside an illusory dream world created by the villain to distract and entrap him.
[22] 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.
[23]: 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[24] by an anonymous author in the June 1838 issue.
[25]: 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.
[26] 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.
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.
[28] Edward Everett Hale's "Hands Off" (1881)[29] 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.
[30]: 54 One of the first stories to feature time travel by means of a machine is "The Clock that Went Backward" by Edward Page Mitchell,[31] which appeared in the New York Sun in 1881.
[42] These semiclassical arguments led Stephen Hawking to formulate the chronology protection conjecture, suggesting that the fundamental laws of nature prevent time travel,[43] but physicists cannot come to a definitive judgment on the issue without a theory of quantum gravity to join quantum mechanics and general relativity into a completely unified theory.
[44][45]: 150 The theory of general relativity describes the universe under a system of field equations that determine the metric, or distance function, of spacetime.
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.
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.
[60] The no-communication theorem also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals.
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.
[65] The physicist Allen Everett argued that Deutsch's approach "involves modifying fundamental principles of quantum mechanics; it certainly goes beyond simply adopting the MWI".
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.
[68] 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.
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,[93] an idea similar to the proposed Novikov self-consistency principle in physics.
[41]: 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.
