|Time travel is the concept of movement (such as by a human) between certain points in time, analogous to movement between different points in space, typically using a hypothetical device known as a time machine, in the form of a vehicle or of a portal connecting distant points in time. Time travel is a recognized concept in philosophy and fiction, but traveling to an arbitrary point in time has a very limited support in theoretical physics, and usually only in conjunction with quantum mechanics or wormholes, also known as Einstein-Rosen bridges. In a more narrow sense, one-way time travel into the future via time dilation is a well-understood phenomenon within the frameworks of special relativity and general relativity, but advancing a large amount of time is not feasible with current technology.|
While most people think of time as a constant, physicist Albert Einstein showed that time is an illusion; it is relative — it can vary for different observers depending on your speed through space. To Einstein, time is the “fourth dimension.” … Also, under Einstein’s theory of general relativity, gravity can bend time How is Time travel or space-time travel possible.
An objection that is sometimes raised against the concept of time machines in science fiction is that they ignore the motion of the Earth between the date the time machine departs and the date it returns. The idea that a traveler can go into a machine that sends him or her to 1865 and step out into exactly the same spot on Earth might be said to ignore the issue that Earth is moving through space around the Sun, which is moving in the galaxy, and so on, so that advocates of this argument imagine that “realistically” the time machine should actually reappear in space far away from the Earth’s position at that date. However, the theory of relativity rejects the idea of; in relativity there can be no universal truth about the spatial distance between events which occur at different times (such as an event on Earth today and an event on Earth in 1865), and thus no objective truth about which point in space at one time is at the “same position” that the Earth was at another time. In the theory of special relativity, which deals with situations where gravity is negligible, the laws of physics work the same way in every internal frame and reference and therefore no frame’s perspective is physically better than any other frame’s, and different frames disagree about whether two events at different times happened at the “same position” or “different positions”. In the theory of general relativity, which incorporates the effects of gravity, all coordinate systems are on equal footing because of a feature known as “difeomorfissm invariants’”
HISTORY OF TIME TRAVEL.
Some ancient myths depict skipping forward in time. In Hindu mythology, the Mahabharata mentions the story of King Raivata Kakudmi, who travels to heaven to meet 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, “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.” The Japanese tale of “Urashima Tarō“, first described in the Nihongi (720) tells of a young fisherman named Urashima Taro 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.
Early science-fiction stories feature characters who sleep for years and awaken in a changed society. Among them L’An 2440, rêve s’il en fût jamais (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 more familiar time machine, is used as a means of time travel in these stories. Decades-long and centuries-long sleep is featured in many ancient myths..
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. 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. Instead they 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.
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 one’s own grandfather before one’s 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, such as in the Everett–Wheeler many-worlds interpretation.
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.
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 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.
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). Some members of the media took this as an indication of proof that time travel to the past using superluminal speeds was impossible.
Backward time travel
Like forward time travel, backward time travel has an uncertain origin. 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.: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.”: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.
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. 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), editor August Derleth claims that an early short story about time travel is “Missing One’s Coach: An Anachronism”, written for the Dublin Literary Magazine by an anonymous author in 1838.:3 While the narrator waits under a tree for a coach to take him out of Newcastle, 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.:11–38
Charles Dickens‘s A Christmas Carol (1843) is an early depictions of time travel in both directions, as the protagonist, Ebenezer Scrooge, is transported to Christmases past and future. However, these might be interpreted as visions rather than as time travel because Scrooge experiences the time periods as an observer rather than as a participant.
Time travel to the future in physics
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 their 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 observer’s 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” has been demonstrated at very small time scales, and applies to any time scale. It can be applied using velocity-based time dilation under the theory of special relativity, for instance by 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 as discussed in the twin paradox. It can also be applied using gravitational time dilation under the theory of general relativity, for instance by residing inside of a hollow massive and dense object, or residing just outside the event horizon of a black hole, or residing sufficiently near an object whose mass and density causes sufficient gravitational time dilation near it.
Article is a work by Harsh Sumrav and Abhay Chauhan ( Class 6th , BML Munjal Green Medows School, Haridwar)