Time travel refers to the imaginary idea of moving to the past or future. It is a topic studied in philosophy, science, and especially in science fiction. In stories, time travel is often shown using a machine called a time machine. The idea of a time machine was first widely shared in H.G. Wells's 1895 novel The Time Machine.
It is not certain if traveling to the past could be physically possible. If it were, it might create problems with cause and effect. Moving forward in time, beyond how we normally experience it, is a common occurrence observed in science. This is explained by the theories of special relativity and general relativity. However, current technology cannot make one object move much faster or slower in time compared to another. For backward time travel, some solutions in general relativity, such as a rotating black hole, suggest it might be possible. However, traveling to any random point in time and space is not strongly supported by physics theories. This idea is usually linked to quantum mechanics or wormholes.
History of the concept
Some ancient stories include characters who seem to jump forward in time. In the Hindu story called the Vishnu Purana, Raivata Kakudmi visits the god Brahma in heaven and returns to Earth to find that many years have passed. In the Buddhist Pāli Canon, the Payasi Sutta describes how the disciple Kumara Kassapa explains that time moves differently in heaven. The Japanese story "Urashima Tarō," first written in the Manyoshu, tells of a fisherman who visits an undersea palace for three days and returns to find centuries have passed and his world is gone.
In one Jewish tradition, Moses is taken by God to the study hall of Rabbi Akiva, where he is confused by how Jewish law will change in the future. Another Jewish story, from the Talmud, describes Honi HaMe'agel, a 1st-century BCE miracle worker who sees a man planting a carob tree that takes 70 years to bear fruit. Honi falls asleep and wakes 70 years later to find the tree grown and its fruit being harvested by the man's grandson.
In Islam, the Quran tells the story of the Seven Sleepers, young men who hid in a cave to escape danger. Allah protected them for centuries, and when they woke, they found their world had changed. This story, in Surah Al-Kahf, shows divine protection and time stopping.
Time travel in science fiction and media can be divided into three types: a timeline that cannot be changed, a timeline that can be changed, and alternate histories, like the many-worlds theory. The word "timeline" often refers to all events in history, so changing events creates a new timeline.
Early science fiction stories include characters who sleep for years and wake in a changed world or are sent to the past by supernatural means. These include 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 way to travel through time in these stories.
The earliest work about backward time travel is unclear. A Chinese novel, A Supplement to the Journey to the West (c. 1640) by Dong Yue, includes magical mirrors and gates that connect different times. The hero Sun Wukong travels back to the Qin dynasty and forward to the Song dynasty in a dream. Samuel Madden’s Memoirs of the Twentieth Century (1733) is a collection of letters from the future, written by British ambassadors in 1997 and 1998. Paul Alkon suggests this may be the first time-travel story in English, as the letters are received by the narrator from his guardian angel.
An early short story about time travel, An Anachronism; or, Missing One's Coach (1838), appears in a magazine. The narrator is transported back in time to meet the Venerable Bede, but the story does not say if this is real or a dream. Another early work is The Forebears of Kalimeros: Alexander, son of Philip of Macedon (1836) by Alexander Veltman.
Charles Dickens’s A Christmas Carol (1843) shows time travel to the past and future, as the main character, Ebenezer Scrooge, visits Christmases from his life. Other stories use the idea of a character sleeping and waking in a different time. A clearer example of backward time travel is Paris before Men (1861) by Pierre Boitard, where the protagonist is taken to the prehistoric past by a "lame demon" and meets ancient creatures. Edward Everett Hale’s "Hands Off" (1881) describes a being who changes ancient Egyptian history by preventing Joseph’s enslavement, possibly the first story with an alternate history from time travel.
One of the first stories about time travel using a machine is The Clock that Went Backward (1881) by Edward Page Mitchell, where a clock winds backward and sends people to the past. The mechanism is unclear. Enrique Gaspar y Rimbau’s El Anacronópete (1887) may be the first story about a time-traveling machine. H. G. Wells’s The Time Machine (1895) made the idea of time travel through machines popular.
Time travel in physics
Some solutions to Einstein's equations for general relativity suggest that certain shapes of spacetime or specific movements in space might allow time travel to the past or future if these shapes or movements were possible. In scientific papers, physicists discuss the possibility of closed timelike curves, which are paths in spacetime that loop back on themselves, allowing objects to return to their own past. Known solutions to general relativity describe spacetimes with closed timelike curves, such as Gödel spacetime, but whether these solutions are physically possible is unclear.
Any theory that allows backward time travel could create problems with cause and effect. A well-known example is the "grandfather paradox," which involves traveling to the past and changing events that led to one's own existence, such as preventing an ancestor from being born. Some scientists, like Novikov and Deutsch, suggest that these paradoxes might be avoided through the Novikov self-consistency principle or a version of the many-worlds interpretation where different timelines exist.
Time travel to the past is theoretically possible in some general relativity scenarios, such as those involving faster-than-light travel through structures like cosmic strings, wormholes, or Alcubierre drives. General relativity provides a scientific basis for backward time travel in unusual cases, but arguments from semiclassical gravity suggest that including quantum effects might close these possibilities. These ideas led Stephen Hawking to propose the chronology protection conjecture, which claims that natural laws might prevent time travel. However, scientists cannot confirm this without a complete theory that unites quantum mechanics and general relativity.
General relativity describes the universe using equations that define the shape of spacetime. Some exact solutions to these equations include closed timelike curves, where paths in spacetime loop back on themselves, allowing time travel. One such solution, called the Gödel metric, was proposed by Kurt Gödel, but it requires the universe to have characteristics it does not seem to have, such as rotation and no expansion. Whether general relativity rules out closed timelike curves in realistic conditions is still being studied.
Wormholes are imagined structures allowed by general relativity's equations. A proposed time-travel machine using a wormhole might work by moving one end of the wormhole at high speeds or placing it in a strong gravitational field, causing time to pass more slowly for that end. This would create a difference in time between the two ends. An observer entering the "younger" end of the wormhole could exit the "older" end at a time that appears to be in the past from an outside perspective. However, such a machine could only travel back to the time it was created, not earlier than that.
Creating a wormhole that allows travel would require a type of matter with negative energy, called "exotic matter." This matter would violate standard energy rules, but quantum effects, like the Casimir effect, might allow small amounts of negative energy to exist. Early calculations suggested large amounts of negative energy would be needed, but later work showed smaller amounts might suffice.
In 1993, Matt Visser argued that bringing wormhole ends together could cause effects that collapse the wormhole or push the ends apart, preventing time travel. However, in 1997, he suggested a complex setup of multiple wormholes, called a "Roman ring," might still allow time travel, though he believed this might be a flaw in current theories rather than proof of time travel.
Another idea involves a dense, spinning cylinder called a Tipler cylinder, a solution discovered by scientists in the 1920s and 1930s. If such a cylinder were infinitely long and spun fast enough, a spaceship could travel around it and move backward in time. However, the required density and speed are impossible to achieve with ordinary matter.
Stephen Hawking argued that time travel using rotating cylinders or cosmic strings might be impossible due to a theorem he proved. His work, called the chronology protection conjecture, states that the laws of physics likely prevent closed timelike curves from forming. Hawking's 1992 paper concluded that time machines requiring only positive energy density (normal matter) cannot exist, and that negative energy would be needed to create them.
When a signal is sent from one place and received at another, it must move at or below the speed of light. The mathematics of how events are simultaneous in different locations depends on this rule.
Time dilation
There is strong evidence that time moves differently depending on speed and gravity, as shown by the example of muons in Earth's atmosphere. According to the theory of relativity, the speed of light is the same for all observers, no matter their motion. This leads to time dilation, where time appears to pass more slowly for objects moving at high speeds or near strong gravitational fields. In a limited way, time dilation can be thought of as "moving forward in time more slowly": for example, someone traveling at high speeds might experience less time passing compared to someone who stays still. This effect can occur through fast movement or near large gravitational sources like stars or black holes.
When two identical clocks move relative to each other without speeding up or slowing down, each clock measures the other as running slower. This happens because of the relativity of simultaneity, a concept that explains how events can appear to occur in different orders depending on the observer's motion. If one clock accelerates, the symmetry breaks, and the clock that accelerated experiences less time passing. This is demonstrated in the twin paradox: one twin stays on Earth, while the other travels at high speed into space, turns around, and returns. The traveling twin ages less because of time dilation during their journey. General relativity shows that gravity and acceleration have similar effects on time, causing clocks near strong gravity (like near Earth or a black hole) to tick more slowly. This is why GPS satellite clocks are adjusted to account for gravity's influence on time.
A theoretical time machine using this principle could involve a structure with the mass of Jupiter but the size of a small sphere. A person inside would age four times slower than someone far away. However, creating such a structure is not possible with current technology. Today, space travel can only cause humans to age slightly less than people on Earth—by a few milliseconds after several hundred days in space.
Philosophy
Philosophers have studied the ideas of space and time since ancient Greece. For example, Parmenides believed that time is not real. Later, Isaac Newton thought time was absolute, while his friend Gottfried Wilhelm Leibniz argued that time only exists as a relationship between events and cannot be separated from them. Leibniz’s view helped develop the idea of spacetime in relativity theory.
Many philosophers believe that relativity supports eternalism, the idea that the past and future exist as real things, not just changes that happened or will happen. Dean Rickles has some different ideas but agrees that most scientists think relativity does not support presentism, the belief that only the present is real. Some philosophers think time is like a dimension of space, with future events already existing in the same way different places exist. Others disagree with this view.
Presentism is the idea that the past and future only exist as changes to the present and are not real on their own. In this view, time travel is impossible because there is no real past or future to visit. Keller and Nelson argue that even if the past and future are not real, there can still be true facts about them. For example, a time traveler’s decision to return to the present might explain their appearance in the present. However, some people disagree with this idea.
A common problem with time travel is the grandfather paradox. If someone could travel back in time and change the past, it might create contradictions, like preventing their own birth. Philosophers debate whether these problems mean time travel is impossible. Some say time travel might be possible, but changing the past is not, an idea similar to the Novikov self-consistency principle in physics.
The theory of compossibility says that what can happen in situations like time travel depends on all other factors involved. If the past is a certain way, it cannot be different. Time travelers can only do things that already happened to avoid contradictions.
The Novikov self-consistency principle, named after Igor Novikov, states that time travelers cannot change the past because their actions were always part of history. Their actions might cause events in their own past, leading to circular causes, such as a predestination paradox or a bootstrap paradox. The term "bootstrap paradox" comes from a story by Robert A. Heinlein. This principle suggests that the laws of physics in areas with time travelers are the same as everywhere else.
Philosopher Kelley L. Ross argues that time travel involving closed loops, like a watch being passed back in time, might break the second law of thermodynamics. In the movie Somewhere in Time, a watch is given to a person and later returned to the same person in the past. Ross says the watch would wear down over time, increasing its entropy. However, scientists note that entropy usually increases in isolated systems, but objects interacting with the outside world might not always follow this rule.
In 2005, Daniel Greenberger and Karl Svozil proposed that quantum theory allows for a model of time travel where the past must remain self-consistent.