The Time Travel History
History of the time travel concept
Forward time travel
There is no widespread agreement as to which written work should be recognized as the earliest example of a time travel story, since a number of early works feature elements ambiguously suggestive of time travel. Ancient folk tales and myths sometimes involved something akin to traveling forward in time; for example, in Hindu mythology, the Mahabharata mentions the story of the King Raivata Kakudmi, who travels to heaven to meet the creator Brahma and is shocked to learn that many ages have passed when he returns to Earth.[4][5]
The Buddhist Pali Canon also mentions time moving at different paces, and in the Payasi Sutta, one of the Buddhas chief disciples, Kumara Kassapa, explains to the skeptic Payasi that "In the Heaven of the Thirty Three Devas, time passes at a different pace, and people live much longer. "In the period of our century; one hundred years, only a single day; twenty four hours would have passed for them."[6]
Another of the earliest known stories to involve traveling forward in time to a distant future was the Japanese tale of "Urashima Taro",[7] first described in the Nihongi (720).[8] It was about a young fisherman named Urashima Taro who visits an undersea palace and stays there for three days. After returning home to his village, he finds himself 300 years in the future, when he is long forgotten, his house in ruins, and his family long dead. Another very old example of this type of story can be found in the Talmud with the story of Honi HaMagel who went to sleep for 70 years and woke up to a world where his grandchildren were grandparents and where all his friends and family were dead.[9]
Statue of Rip Van Winkle in Irvington, New York
A more recent story involving travel to the future is Louis-Sébastien Merciers LAn 2440, rêve sil en fût jamais ("The Year 2440: A Dream If Ever There Were One"), a utopian novel in which the main character is transported to the year 2440. An extremely popular work (it went through 25 editions after its first appearance in 1771), it describes the adventures of an unnamed man who, after engaging in a heated discussion with a philosopher friend about the injustices of Paris, falls asleep and finds himself in a Paris of the future. Robert Darnton writes that "despite its self-proclaimed character of fantasy...LAn 2440 demanded to be read as a serious guidebook to the future."[10]
More recently, Washington Irvings 1819 story "Rip Van Winkle" tells of a man named Rip Van Winkle who takes a nap on a mountain and wakes up 20 years in the future, when he has been forgotten, his wife dead, and his daughter grown up.[7] Sleep was also used for time travel in H.G. Wells The Sleeper Awakes, about a man who wakes 200 years in the future after falling into a state of sleep resembling hibernation or suspended animation.
Backward time travel
Backward time travel seems to be a more modern idea, but its origin is also somewhat ambiguous. One early story with hints of backward time travel is Memoirs of the Twentieth Century (1733) by Samuel Madden, which is mainly a series of letters from British ambassadors in various countries to the British Lord High Treasurer, along with a few replies from the British Foreign Office, all purportedly written in 1997 and 1998 and describing the conditions of that era.[11] However, the framing story is that these letters were actual documents given to the narrator by his guardian angel one night in 1728; for this reason, Paul Alkon suggests in his book Origins of Futuristic Fiction that "the first time-traveler in English literature is a guardian angel who returns with state documents from 1998 to the year 1728",[12] although the book does not explicitly show how the angel obtained these documents. Alkon later qualifies this by writing, "It would be stretching our generosity to praise Madden for being the first to show a traveler arriving from the future", but also says 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."[11]
Mr. and Mrs. Fezziwig dance in a vision the Ghost of Christmas Past shows Scrooge.
In 1836 Alexander Veltman published Predki Kalimerosa: Aleksandr Filippovich Makedonskii (The Forebears of Kalimeros: Alexander, son of Philip of Macedon), which has been called the first original Russian science fiction novel and the first novel to use time travel.[13] In it, the narrator rides to ancient Greece on a hippogriff, meets Aristotle, and goes on a voyage with Alexander the Great before returning to the 19th century.
In the science fiction anthology Far Boundaries (1951), the editor August Derleth identifies the short story "Missing Ones Coach: An Anachronism", written for the Dublin Literary Magazine[14] by an anonymous author in 1838, as a very early time travel story.[15] In this story, the narrator is waiting under a tree to be picked up by a coach which will take him out of Newcastle, when he suddenly finds himself transported back over a thousand years. He encounters the Venerable Bede in a monastery, and gives him somewhat ironic explanations of the developments of the coming centuries. However, the story never makes it clear whether these events actually occurred or were merely a dream: the narrator says that when he initially found a comfortable-looking spot in the roots of the tree, he sat down, "and as my sceptical reader will tell me, nodded and slept", but then says that he is "resolved not to admit" this explanation. A number of dreamlike elements of the story may suggest otherwise to the reader, such as the fact that none of the members of the monastery seem to be able to see him at first, and the abrupt ending in which Bede has been delayed talking to the narrator and so the other monks burst in thinking that some harm has come to him and suddenly the narrator finds himself back under the tree in the present (August 1837), with his coach having just passed his spot on the road leaving him stranded in Newcastle for another night.[16]
Charles Dickens 1843 book A Christmas Carol is considered by some[17] to be one of the first depictions of time travel in both directions, as the main character, Ebenezer Scrooge, is transported to Christmases past, present and yet to come. However, these might be considered mere visions rather than actual time travel, since Scrooge only viewed each time-period passively, unable to interact with them.
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 main character is transported into the prehistoric past by the magic of a "lame demon" (a French pun on Boitards name), where he encounters such extinct animals as a Plesiosaur, as well as Boitards imagined version of an apelike human ancestor, and is able to actively interact with some of them.[18]
In 1881, Edward Everett Hale published "Hands Off", about an unnamed being (possibly the soul of a person who had recently died) free to travel through time and space, who interferes with Earth history in Ancient Egypt, preventing Joseph from being sold into slavery. This was the first known story to feature an alternate history being created as a result of time travel.[19]
The first time travel story to feature time travel by means of a machine of some kind was the short story "The Clock that Went Backward" by Edward Page Mitchell,[20] which appeared in the New York Sun in 1881. However, the mechanism is borderline fantasy in this case—a clock that, when wound, begins to run backward and transports people in the vicinity backward in time, with no explanation as to where the clock came from or how it gained this ability.[21]
Enrique Gaspar y Rimbaus 1887 book El Anacronópete[22] was the first story to feature a vessel that had been engineered by an inventor to transport its riders through time.[23] 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 Mitchells story The Clock That Went Backward (1881) is usually described as the first time-machine story, but Im not sure that a clock quite counts."[24]
In 1889 Svatopluk Cech published the novel Nový epochální výlet pana Broucka, tentokráte do XV. století ("The Epoch-making Excursion of Mr. Broucek, this time to the 15th Century").
Mark Twains A Connecticut Yankee in King Arthurs Court (1889), in which the protagonist finds himself in the time of King Arthur after a fight in which he is hit with a sledgehammer, was another early time travel story which helped bring the concept to a wide audience, and was also one of the first stories to show history being changed by a time travelers actions.[citation needed]
The notion of a vehicle designed for time travel gained popularity with the H. G. Wells story The Time Machine, published in 1895 (preceded by a less influential story of time travel which Wells wrote in 1888, titled "The Chronic Argonauts"). The term "time machine", coined by Wells, is now universally used to refer to such a vehicle.[citation needed]
Since that time, both science and fiction (see Time travel in fiction) have expanded on the concept of time travel.
Theory
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.[25] In technical papers, physicists generally avoid the commonplace language of "moving" or "traveling" through time ("movement" normally refers only to a change in spatial position as the time coordinate is varied), and instead discuss the possibility of closed timelike curves, which are worldlines 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.
Relativity predicts that if one were to move away from the Earth at relativistic velocities and return, more time would have passed on Earth than for the traveler, so in this sense it is accepted that relativity allows "travel into the future" (according to relativity there is no single objective answer to how much time has really passed between the departure and the return, but there is an objective answer to how much proper time has been experienced by both the Earth and the traveler, i.e., how much each has aged; see twin paradox). On the other hand, 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. The classic example of a problem involving causality is the "grandfather paradox": what if one were to go back in time and kill ones own grandfather before ones father was conceived? But some scientists believe that paradoxes can be avoided, by appealing either to the Novikov self-consistency principle or to the notion of branching parallel universes.
Tourism in time
Stephen Hawking has suggested that the absence of tourists from the future is an argument against the existence of time travel: this is a variant of the Fermi paradox. Of course, this would not prove that time travel is physically impossible, since it might be that time travel is physically possible but that it is never developed (or is cautiously never used); and even if it were developed, Hawking notes elsewhere that time travel might only be possible in a region of spacetime that is warped in the correct way, and that if we cannot create such a region until the future, then time travelers would not be able to travel back before that date, so "[t]his picture would explain why" the world hasnt already been overrun by "tourists from the future."[26] This simply means that, until a time machine were actually to be invented, we would not be able to see time travelers. Carl Sagan also once suggested the possibility that time travelers could be here, but are disguising their existence or are not recognized as time travelers, because bringing unintentional changes to the time-space continuum might bring about undesired outcomes to those travelers. It might also alter established past events.[27] There is also the possibility that if events were changed, we would never notice it because all events following and our memories would have been instantly altered to remain congruent with the newly established timeline.[citation needed]
General relativity
However, 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 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]:150
Time travel to the past in physics
Time travel to the past is theoretically allowed using the following methods:[31]
Traveling faster than the speed of light
The use of cosmic strings and black holes
Wormholes and Alcubierre drive
Via faster-than-light (FTL) travel
If one were able to move information or matter from one point to another faster than light, then according to the theory of relativity, there would be some inertial frame of reference in which the signal or object was moving backward in time. This is a consequence of the relativity of simultaneity in special relativity, which says that in some cases different reference frames will disagree on whether two events at different locations happened "at the same time" or not, and they can also disagree on the order of the two events. Technically, these disagreements occur when the spacetime interval between the events is space-like, meaning that neither event lies in the future light cone of the other.[32] If one of the two events represents the sending of a signal from one location and the second event represents the reception of the same signal at another location, then as long as the signal is moving at the speed of light or slower, the mathematics of simultaneity ensures that all reference frames agree that the transmission-event happened before the reception-event.[32]
However, in the case of a hypothetical signal moving faster than light, there would always be some frames in which the signal was received before it was sent, so that the signal could be said to have moved backward in time. And since one of the two fundamental postulates of special relativity says that the laws of physics should work the same way in every inertial frame, then if it is possible for signals to move backward in time in any one frame, it must be possible in all frames. This means that if observer A sends a signal to observer B which moves FTL (faster than light) in As frame but backward in time in Bs frame, and then B sends a reply which moves FTL in Bs frame but backward in time in As frame, it could work out that A receives the reply before sending the original signal, a clear violation of causality in every frame. An illustration of such a scenario using spacetime diagrams can be found here.[33] The scenario is sometimes referred to as a tachyonic antitelephone.
According to special relativity, it would take an infinite amount of energy to accelerate a slower-than-light object to the speed of light. Although relativity does not forbid the theoretical possibility of tachyons which move faster than light at all times, when analyzed using quantum field theory, it seems that it would not actually be possible to use them to transmit information faster than light.[34] There is also no widely agreed-upon evidence for the existence of tachyons; the faster-than-light neutrino anomaly had opened the possibility that neutrinos might be tachyons, but the results of the experiment were found to be invalid upon further analysis.
Special spacetime geometries
The general theory of relativity extends the special theory to cover gravity, illustrating it in terms of curvature in spacetime caused by mass-energy and the flow of momentum. General relativity describes the universe under a system of field equations, and there exist solutions to these equations that permit what are called "closed time-like curves", and hence time travel into the past.[25] The first of these was proposed by Kurt Gödel, a solution known as the Gödel metric, but his (and many others) example requires the universe to have physical characteristics that it does not appear to have.[25] Whether general relativity forbids closed time-like curves for all realistic conditions is unknown.
Using wormholes
Main article: Wormhole
Wormholes are a hypothetical warped spacetime which are also permitted by the Einstein field equations of general relativity,[35] although it would not be possible to travel through a wormhole unless it were what is known as a traversable wormhole.
A proposed time-travel machine using a traversable wormhole would (hypothetically) work in the following way: One end of the wormhole is accelerated to some significant fraction of the speed of light, perhaps with some advanced propulsion system, and then brought back to the point of origin. Alternatively, another way is to take one entrance of the wormhole and move it to within the gravitational field of an object that has higher gravity than the other entrance, and then return it to a position near the other entrance. For both of these methods, time dilation causes the end of the wormhole that has been moved to have aged less 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.[36] This means that an observer entering the accelerated end would exit the stationary end when the stationary end was the same age that the accelerated end had been at the moment before entry; for example, if prior to entering the wormhole the observer noted that a clock at the accelerated end read a date of 2007 while a clock at the stationary end read 2012, then the observer would exit the stationary end when its clock also read 2007, a trip backward in time as seen by other observers 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;[37] 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.[38] However, it is known that quantum effects can lead to small measurable violations of the null energy condition,[38] and many physicists believe that the required negative energy may actually be possible due to the Casimir effect in quantum physics.[39] Although early calculations suggested a very large amount of negative energy would be required, later calculations showed that the amount of negative energy can be made arbitrarily small.[40]
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.[41] 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.[42]
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[43] in 1936 and Kornel Lanczos[44] in 1924, but not recognized as allowing closed timelike curves[45] until an analysis by Frank Tipler[46] 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 Robert Forward noted that a naïve application of general relativity to quantum mechanics suggests another way to build a time machine. A heavy atomic nucleus in a strong magnetic field would elongate into a cylinder, whose density and "spin" are enough to build a time machine. Gamma rays projected at it might allow information (not matter) to be sent back in time; however, he pointed out that until we have a single theory combining relativity and quantum mechanics, we will have no idea whether such speculations are nonsense.[citation needed]
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 Tiplers 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,[47] he did not prove this. But Hawking points out that because of his theorem, "it cant be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy."[30]:96 This result comes from Hawkings 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 "[t]here 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."[48] However, this theorem does not rule out the possibility of time travel 1) by means of time machines with the non-compactly generated Cauchy horizons (such as the Deutsch-Politzer time machine) and 2) in regions which contain exotic matter (which would be necessary for traversable wormholes or the Alcubierre drive). Because the theorem is based on general relativity, it is also conceivable a future theory of quantum gravity which replaced general relativity would allow time travel even without exotic matter (though it is also possible such a theory would place even more restrictions on time travel, or rule it out completely as postulated by Hawkings chronology protection conjecture).[citation needed]
Experiments carried out
Certain experiments carried out give the impression of reversed causality but are subject to interpretation. For example, in the delayed choice quantum eraser experiment performed by Marlan Scully, pairs of entangled photons 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, and depending on how the idler photon is measured, the experimenter can either learn which of the two locations the signal photon emerged from or "erase" that information. Even though the signal photons can be measured before the choice has been made about the idler photons, the choice seems to retroactively determine whether or not an interference pattern is observed when one correlates measurements of idler photons to the corresponding signal photons. However, since interference can only be observed after the idler photons are measured and they are correlated with the signal photons, there is no way for experimenters to tell what choice will be made in advance just by looking at the signal photons, and under most interpretations of quantum mechanics the results can be explained in a way that does not violate causality.[citation needed]
The experiment of Lijun Wang might also show causality violation since it made it possible to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry. But a wave package is not a single well-defined object but rather a sum of multiple waves of different frequencies (see Fourier analysis), and the package can appear to move faster than light or even backward in time even if none of the pure waves in the sum do so. This effect cannot be used to send any matter, energy, or information faster than light,[49] so this experiment is understood not to violate causality either.
The physicists Günter Nimtz and Alfons Stahlhofen, of the University of Koblenz, claim to have violated Einsteins theory of relativity by transmitting photons faster than the speed of light. They say they have conducted an experiment in which microwave photons traveled "instantaneously" between a pair of prisms that had been moved up to 3 ft (0.91 m) apart, using a phenomenon known as quantum tunneling. Nimtz told New Scientist magazine: "For the time being, this is the only violation of special relativity that I know of." However, other physicists say that this phenomenon does not allow information to be transmitted faster than light. Aephraim Steinberg, a quantum optics expert at the University of Toronto, Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the speed of the center of the train exceeds the speed of any of the individual cars.[50]
Some physicists have performed experiments that attempted to show causality violations, but so far without success. The "Space-time Twisting by Light" (STL) experiment run by physicist Ronald Mallett attempts to observe a violation of causality when a neutron is passed through a circle made up of a laser whose path has been twisted by passing it through a photonic crystal. Mallett has some physical arguments that suggest that closed timelike curves would become possible through the center of a laser that has been twisted into a loop. However, other physicists dispute his arguments (see objections).
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, violating causality).[51] Some members of the media took this as an indication of proof that time travel to the past using superluminal speeds was impossible.[52][53]
Non-physics-based experiments
Krononauts
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 Perths Destination Day (2005) or MITs Time Traveler Convention heavily publicized permanent "advertisements" of a meeting time and place for future time travelers to meet. Back in 1982, a group in Baltimore, Maryland, identifying itself as the Krononauts, hosted an event of this type welcoming visitors from the future.[54][55][56] 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. It is hypothetically possible that future humans have traveled back in time, but have traveled back to the meeting time and place in a parallel universe.[57]
Another factor is that for all the time travel devices considered under current physics (such as those that operate using wormholes), it is impossible to travel back to before the time machine was actually made.[58][59]
Time travel to the future in physics
Twin paradox diagram
There are various ways in which a person could "travel into the future" in a limited sense: the person could set things up so that in a small amount of his own subjective time, a large amount of subjective time has passed for other people on Earth. For example, an observer might take a trip away from the Earth and back at relativistic velocities, with the trip only lasting a few years according to the observers own clocks, and return to find that thousands of years had passed on Earth. According to relativity, there would be no objective answer to the question of how much time "really" passed during the trip; it would be equally valid to say that the trip had lasted only a few years or that the trip had lasted thousands of years, depending on the choice of reference frame.
This form of "travel into the future" is theoretically allowed (and has been demonstrated at very small time scales) using the following methods:[31]
Using velocity-based time dilation under the theory of special relativity, for instance:
Traveling at almost the speed of light to a distant star, then slowing down, turning around, and traveling at almost the speed of light back to Earth[60] (see the Twin paradox)
Using gravitational time dilation under the theory of general relativity, for instance:
Residing inside of a hollow, high-mass object;
Residing just outside the event horizon of a black hole, or sufficiently near an object whose mass or density causes the gravitational time dilation near it to be larger than the time dilation factor on Earth.
Time dilation
Transversal time dilation
Main article: Time dilation
Time dilation is permitted by Albert Einsteins special and general theories of relativity. These theories state that, relative to a gi
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