Time travel is the idea of moving to the past or future. It is a topic in philosophy, science, and stories, especially science fiction. In stories, time travel usually happens using a machine called a time machine. The idea of a time machine became famous because of H.G. Wells's book, The Time Machine, published in 1895.
It is not known if traveling to the past could happen in real life. If possible, this could create problems with cause and effect. Moving forward in time, beyond how we normally experience it, is something scientists have studied a lot. It is explained by theories called special relativity and general relativity.
But today's technology cannot make one object move much faster or slower in time than another, even by a few thousandths of a second. Traveling backward in time might be possible according to general relativity, like near a spinning black hole. Going to any random point in time and space is not widely supported by physics theories. It is usually linked to quantum mechanics or wormholes.
History of the concept
Some old stories have characters who seem to jump forward in time. The Vishnu Purana, a Hindu text, tells the story of Raivata Kakudmi, who visits the god Brahma in heaven and returns to Earth to find that many ages have passed. In the Buddhist Pāli Canon, the Payasi Sutta describes how the disciple Kumara Kassapa explains to a skeptic that time moves differently in heavenly realms. The Japanese tale of "Urashima Tarō," first written in the Manyoshu, tells of a fisherman who visits an undersea palace for three days, only to return and 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 Talmudic story features Honi HaMe'agel, a 1st-century BCE miracle worker who sees a man planting a carob tree that will take 70 years to bear fruit. Honi falls asleep and wakes 70 years later to find the tree fully 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 persecution. While they slept, Allah protected them for centuries, and when they awoke, they found the world had changed. This story, in Surah Al-Kahf, describes divine protection and time stopping.
Time travel themes in science fiction and media can be divided into three types: an unchangeable timeline, a timeline that can be changed, and alternate histories, like the many-worlds idea. The word "timeline" is sometimes used to describe all events in history, so if a time traveler changes events, it is said they created a new timeline.
Early science fiction stories include characters who sleep for years and awaken in a changed society or are sent to the past through supernatural means. Examples 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 date of the earliest work about backward time travel is unclear. The Chinese novel A Supplement to the Journey to the West (c. 1640) by Dong Yue includes magical mirrors and gateways that connect different times. The main character, Sun Wukong, travels back to the "World of the Ancients" (Qin dynasty) to retrieve a bell and then moves forward to the "World of the Future" (Song dynasty) to find an emperor. However, the time travel occurs in a dream created by a villain. Samuel Madden’s Memoirs of the Twentieth Century (1733) is a collection of letters from 1997 and 1998 written to the past. Paul Alkon suggests that Madden’s work includes the first time-traveler in English literature, a guardian angel. Madden does not explain how the angel gets the letters, but Alkon says Madden deserves credit for exploring the idea of time travel through an artifact from the future. In Far Boundaries (1951), editor August Derleth claims an early time-travel story is An Anachronism; or, Missing One's Coach (1838), where a man is transported back in time to meet the Venerable Bede. The story does not clarify if the events are 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) includes early examples of time travel in both directions, as the main character, Ebenezer Scrooge, visits Christmases past and future. Other stories use a similar idea, where a character sleeps and wakes in a different time. A clearer example of backward time travel is in Paris avant les hommes (1881) by Pierre Boitard, where the protagonist is sent to the prehistoric past by a "lame demon" and interacts with ancient creatures. Edward Everett Hale’s "Hands Off" (1881) tells of a being who changes ancient Egyptian history by preventing Joseph’s enslavement, possibly the first story to show an alternate history caused by time travel.
One of the first stories to use a machine for time travel is "The Clock That Went Backward" (1881) by Edward Page Mitchell, published in the New York Sun. The story describes a clock that, when wound, moves backward in time. The author does not explain how the clock works. Enrique Gaspar y Rimbau’s El Anacronópete (1887) may have been the first story to describe a vehicle built to travel through time. Andrew Sawyer notes that this story is the first known literary description of a time machine, even though Mitchell’s clock is often cited as the first time-machine story. 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 could allow time travel to the past or future if these conditions were possible. In scientific papers, physicists discuss the idea of closed timelike curves, which are paths in spacetime that loop back on themselves, allowing objects to return to their own past. There are known solutions to the equations of general relativity that describe spacetimes containing closed timelike curves, such as Gödel spacetime. However, it is unclear whether these solutions are physically possible in the real universe.
Theories that allow backward time travel could create problems with causality, or the order of cause and effect. A well-known example is the "grandfather paradox," which involves traveling to the past and changing events that would prevent one’s own existence, such as causing an ancestor to die before having children. 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 that allows different timelines to interact.
Time travel to the past is theoretically possible in some general relativity scenarios where faster-than-light movement is allowed, such as through cosmic strings, wormholes, or Alcubierre drives. General relativity provides a scientific basis for backward time travel in unusual situations, but arguments from semiclassical gravity suggest that quantum effects might close these possibilities. These ideas led Stephen Hawking to propose the chronology protection conjecture, which suggests 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 field equations that define the metric, or distance measurements, of spacetime. Some exact solutions to these equations include closed timelike curves, where paths in spacetime intersect themselves, allowing time travel. A solution called the Gödel metric, proposed by Kurt Gödel, includes such curves, but it requires the universe to have characteristics it does not seem to have, like rotation and no expansion. Whether general relativity rules out closed timelike curves in realistic conditions is still being studied.
Wormholes are hypothetical structures in spacetime that general relativity allows. A time-travel machine using a wormhole might work by moving one end of the wormhole at a high speed or placing it in a strong gravitational field, causing time to pass differently for the two ends. This time difference could allow someone to travel back in time, but the machine could only take them as far back as when it was created. It would not allow the machine itself to move backward in time.
Creating a wormhole that can be traveled through would require a substance with negative energy, called "exotic matter." This type of matter violates certain 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 research showed it could be much smaller.
In 1993, Matt Visser argued that bringing the two ends of a wormhole with a time difference together would cause effects that might collapse the wormhole or push the ends apart, preventing time travel. However, in 1997, he suggested a complex setup 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 to general relativity equations. 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 far beyond what ordinary matter can achieve.
Stephen Hawking argued that certain time travel methods, like those involving rotating cylinders or cosmic strings, are impossible due to a theorem he proved. His work shows that general relativity prevents the creation of a specific type of time machine in regions with positive energy density. He concluded that negative energy is necessary for time travel, but no complete theory yet confirms this.
Time dilation
There is clear evidence that time changes depending on speed and gravity, as shown by the way particles called muons survive longer when moving through Earth's atmosphere. According to the theory of relativity, the speed of light remains the same for everyone, no matter how fast they move. This rule leads to time dilation, where time appears to move slower for someone moving very fast or near a strong gravitational field. In a simple way, time dilation can be thought of as "traveling into the future" because less time passes for someone moving quickly or near a massive object compared to someone who is not.
When two clocks move relative to each other without speeding up or slowing down, each clock shows the other as running slower. This happens because events that seem to happen at the same time for one observer might not for another. However, if one clock speeds up or slows down, the symmetry breaks, and one clock will show less time passing than the other. This is seen in the twin paradox: one twin stays on Earth while the other travels at a high speed into space, turns around, and returns. The traveling twin ages less because of time dilation during their journey. General relativity explains that gravity and acceleration have similar effects on time, causing clocks closer to a massive object, like Earth or a black hole, to run slower. This is why GPS satellite clocks must be adjusted to stay accurate.
A hypothetical time machine based on this principle could involve a structure with the mass of Jupiter but only five meters wide. 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 discussed 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, but his contemporary, Gottfried Wilhelm Leibniz, argued that time only exists as a relationship between events and cannot be separated from them. Leibniz's view helped lead to the concept of spacetime in relativity.
Many philosophers believe that relativity supports eternalism, the idea that the past and future exist as real things, not just changes that happen to the present. Dean Rickles, a philosopher of science, says that most philosophers agree special and general relativity do not support presentism, the belief that only the present exists. Some philosophers see time as a dimension like space, meaning future events are "already there" just as other places exist, and time does not flow objectively. However, this idea is debated.
Presentism is the belief that the past and future only exist as changes that affect the present and do not have real existence on their own. In this view, time travel is impossible because there is no past or future to travel to. Keller and Nelson argue that even if the past and future do not exist, there can still be true facts about past and future events. For example, a time traveler’s future decision to return to the present might explain their actual arrival. These ideas are not accepted by all philosophers.
A common argument against time travel is the grandfather paradox, which suggests that changing the past would create contradictions. For example, if someone traveled back in time and killed their grandfather, they might prevent their own birth. Philosophers debate whether these paradoxes prove time travel is impossible. Some say backward time travel might be possible, but changing the past would not be allowed, an idea similar to the Novikov self-consistency principle in physics.
The philosophical theory of compossibility says that what can happen in situations like time travel must fit with all other events in the situation. If the past is a certain way, it cannot be different. A time traveler’s actions in the past can only match what actually happened to avoid contradictions.
The Novikov self-consistency principle, named after Igor Dmitrievich Novikov, states that time travelers cannot change history because their actions were always part of history. Their actions might cause events in their own past, leading to circular causation, also called predestination or bootstrap paradoxes. 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 elsewhere in spacetime.
Kelley L. Ross, a philosopher, argues in "Time Travel Paradoxes" that if an object’s history forms a closed loop in time, it might break the second law of thermodynamics. For example, in the film Somewhere in Time, a watch is given to a person and later returned to the same person 60 years later. Ross says the watch would become more worn each time it repeats its history, increasing entropy. However, modern physics explains that entropy usually increases in isolated systems, but non-isolated systems, like the watch, can sometimes decrease in entropy. An object with a closed loop in time might stay the same over time.
In 2005, Daniel Greenberger and Karl Svozil proposed that quantum theory provides a model where time travel must be self-consistent.