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, popularised 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 might 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 has relatively little support for travelling to any point in spacetime, and is typically only associated with quantum mechanics or wormholes

History of the concept

• Mythical time travel

In certain myths from antiquity, a character is shown travelling through time. The Vishnu Purana, a Hindu epic, tells the tale of King Raivata Kakudmi, who ascends to heaven to meet 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 disciples, Kumara Kassapa, explains to the doubting Payasi that time moves differently in the heavens than it does on Earth. In the Manyoshu, a young fisherman named Urashima-no-ko (浦嶋子) pays a visit to an underwater palace in the Japanese legend "Urashima Tarō". 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.

• Abrahamic religion

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 while 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.

Shift to science fiction

Three categories can 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 travellers are said to be generating new timelines in cases where events are altered.

Characters from early science fiction stories either travel back in time by paranormal means or wake up in a civilization that has changed after years of sleep. 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.

Early time machines

The story "The Clock that Went Backward" by Edward Page Mitchell , which first appeared in the New York Sun in 1881, is among the earliest to depict time travel via mechanical methods. 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 might have been the first tale to depict a time-traveling vessel. Andrew Sawyer said the narrative "does seem to be the first literary description of a time machine noted so far", 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, popularised 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 motions 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 objects 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 backward time travel. Any hypothesis that permits time travel would raise possible issues of causality

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. Because of these semiclassical arguments, Stephen Hawking developed the chronology protection conjecture, which postulates that time travel is prohibited by the fundamental laws of nature. However, physicists are unable to reach a conclusive conclusion on this matter in the absence of a theory of quantum gravity that unifies general relativity and quantum mechanics.

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 propose such a solution, which is referred to as the Gödel metric. However, his (and others') solution necessitates the universe having physical properties that it does not seem to have, 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.

Wormholes

According to general relativity's Einstein field equations, wormholes are hypothetical distorted spacetimes. Theoretically, a traversable wormhole-based time-travel device could 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 higher than that of the other entrance, and then move 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 synchronised clocks at either end of the wormhole will always remain synchronised 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 travelling 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 essentially a path through time rather than a device that travels through time; additionally, it would not permit the advancement of technology.

.    .    .