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Time travel

Time travel

Overview
Time travel is the concept of moving between different points in time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

 in a manner analogous to moving between different points in space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

. Time travel could hypothetically involve moving backward in time to a moment earlier than the starting point, or forward to the future of that point without the need for the traveler to experience the intervening period (at least not at the normal rate). Any technological device – whether fictional or hypothetical – that would be used to achieve time travel is commonly known as a time machine.

Although time travel has been a common plot device
Plot device
A plot device is an object or character in a story whose sole purpose is to advance the plot of the story, or alternatively to overcome some difficulty in the plot....

 in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

 since the late 19th century, and the theories of special
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

 and general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

 suggest methods for forms of one-way travel into the future via time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...

, it is currently unknown whether the laws of physics would allow time travel into the past.
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Encyclopedia
Time travel is the concept of moving between different points in time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

 in a manner analogous to moving between different points in space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

. Time travel could hypothetically involve moving backward in time to a moment earlier than the starting point, or forward to the future of that point without the need for the traveler to experience the intervening period (at least not at the normal rate). Any technological device – whether fictional or hypothetical – that would be used to achieve time travel is commonly known as a time machine.

Although time travel has been a common plot device
Plot device
A plot device is an object or character in a story whose sole purpose is to advance the plot of the story, or alternatively to overcome some difficulty in the plot....

 in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

 since the late 19th century, and the theories of special
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

 and general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

 suggest methods for forms of one-way travel into the future via time dilation
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...

, it is currently unknown whether the laws of physics would allow time travel into the past. Such backward time travel would have the potential to introduce paradoxes
Physical paradox
A physical paradox is an apparent contradiction in physical descriptions of the universe. While many physical paradoxes have accepted resolutions, others defy resolution and may indicate flaws in theory...

 related to causality
Causality
Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

, and a variety of hypotheses
Hypothesis
A hypothesis is a proposed explanation for a phenomenon. The term derives from the Greek, ὑποτιθέναι – hypotithenai meaning "to put under" or "to suppose". For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...

 have been proposed to resolve them.

Literature timeline

  • 700s BCE to 300s CE – Mahabharata
    Mahabharata
    The Mahabharata is one of the two major Sanskrit epics of ancient India and Nepal, the other being the Ramayana. The epic is part of itihasa....

  • 200s to 400s CE – Talmud
    Talmud
    The Talmud is a central text of mainstream Judaism. It takes the form of a record of rabbinic discussions pertaining to Jewish law, ethics, philosophy, customs and history....

  • 720 CE – "Urashima Tarō
    Urashima Taro
    is a Japanese legend about a fisherman who rescues a turtle and is rewarded for this with a visit to Ryūgū-jō, the palace of Ryūjin, the Dragon God, under the sea...

    "
  • 1733 – Samuel Madden
    Samuel Madden
    Samuel Madden was an Irish author. His works include Themistocles; The Lover of His Country, Reflections and Resolutions Proper for the Gentlemen of Ireland, and Memoirs of the Twentieth Century. Dr...

    's Memoirs of the Twentieth Century
  • 1771 – Louis-Sébastien Mercier
    Louis-Sébastien Mercier
    Louis-Sébastien Mercier was a French dramatist and writer.-Early life and education:He was born in Paris to a humble family: his father was a skilled artisan who polished swords and metal arms. Mercier nevertheless received a decent education.-Literary career:Mercier began his literary career by...

    's L'An 2440, rêve s'il en fût jamais
  • 1781 – Johan Herman Wessel
    Johan Herman Wessel
    thumb|Johan Herman WesselJohan Herman Wessel was a Norwegian-Danish poet. Some of his satirical poems are still popular.-Biography:...

    's Anno 7603
  • 1819 – Washington Irving
    Washington Irving
    Washington Irving was an American author, essayist, biographer and historian of the early 19th century. He was best known for his short stories "The Legend of Sleepy Hollow" and "Rip Van Winkle", both of which appear in his book The Sketch Book of Geoffrey Crayon, Gent. His historical works...

    's "Rip Van Winkle
    Rip Van Winkle
    "Rip Van Winkle" is a short story by the American author Washington Irving published in 1819, as well as the name of the story's fictional protagonist. Written while Irving was living in Birmingham, England, it was part of a collection entitled The Sketch Book of Geoffrey Crayon...

    "
  • 1824 – Faddey Bulgarin's "Pravdopodobnie Nebylitsi"
  • 1827 – Goethe Faust fragment
  • 1828 – Hans Christian Andersen
    Hans Christian Andersen
    Hans Christian Andersen was a Danish author, fairy tale writer, and poet noted for his children's stories. These include "The Steadfast Tin Soldier," "The Snow Queen," "The Little Mermaid," "Thumbelina," "The Little Match Girl," and "The Ugly Duckling."...

    's Journey on Foot from Holmen's Canal to the East Point of Amager
  • 1832 – Goethe's Faust: The Second Part of the Tragedy
  • 1836 – Alexander Veltman
    Alexander Veltman
    Alexander Fomich Veltman was one of the most successful Russian prose writers of the 1830s and 1840s, "popular for various modes of Romantic fiction—historical, Gothic, fantastic, and folkloristic." He was one of the pioneers of Russian science fiction....

    's Predki Kalimerosa
  • 1838 – Hans Christian Andersen
    Hans Christian Andersen
    Hans Christian Andersen was a Danish author, fairy tale writer, and poet noted for his children's stories. These include "The Steadfast Tin Soldier," "The Snow Queen," "The Little Mermaid," "Thumbelina," "The Little Match Girl," and "The Ugly Duckling."...

    's The Goloshes of Fortune
  • 1838 – Missing One's Coach: An Anachronism
  • 1843 – Charles Dickens
    Charles Dickens
    Charles John Huffam Dickens was an English novelist, generally considered the greatest of the Victorian period. Dickens enjoyed a wider popularity and fame than had any previous author during his lifetime, and he remains popular, having been responsible for some of English literature's most iconic...

    ' A Christmas Carol
    A Christmas Carol
    A Christmas Carol is a novella by English author Charles Dickens first published by Chapman & Hall on 17 December 1843. The story tells of sour and stingy Ebenezer Scrooge's ideological, ethical, and emotional transformation after the supernatural visits of Jacob Marley and the Ghosts of...

  • 1861 – Pierre Boitard
    Pierre Boitard
    Pierre Boitard was a French botanist and geologist. As well as describing and classifying the Tasmanian Devil, he is notable for his fictional natural history Paris avant les hommes , published posthumously in 1861, which described a prehistoric ape-like human ancestor living in the region of Paris...

    's Paris avant les hommes
  • 1881 – Edward Page Mitchell
    Edward Page Mitchell
    Edward Page Mitchell was an American editorial and short story writer for the New York Sun, a daily newspaper. He became that newspaper's editor in 1897, succeeding Charles Anderson Dana. Mitchell retired in 1926, a year before dying of a cerebral hemorrhage...

    's The Clock That Went Backward
  • 1887 – Enrique Gaspar y Rimbau
    Enrique Gaspar y Rimbau
    Enrique Lucio Eugenio Gaspar y Rimbau was a Spanish diplomat and writer, who wrote plays, zarzuelas , and novels.-Biography:...

    's El anacronópete
  • 1888 – H. G. Wells
    H. G. Wells
    Herbert George Wells was an English author, now best known for his work in the science fiction genre. He was also a prolific writer in many other genres, including contemporary novels, history, politics and social commentary, even writing text books and rules for war games...

    ' The Chronic Argonauts
    The Chronic Argonauts
    "The Chronic Argonauts" is a short story written by H. G. Wells. First published by the Royal College of Science in 1888, it is the first well-developed use of a machine constructed to travel through time in science fiction, as it predates Wells's more famous time traveling novel, The Time...

  • 1889 – Mark Twain
    Mark Twain
    Samuel Langhorne Clemens , better known by his pen name Mark Twain, was an American author and humorist...

    's A Connecticut Yankee in King Arthur's Court
    A Connecticut Yankee in King Arthur's Court
    A Connecticut Yankee in King Arthur's Court is an 1889 novel by American humorist and writer Mark Twain. The book was originally titled A Yankee in King Arthur's Court...

  • 1895 – H. G. Wells
    H. G. Wells
    Herbert George Wells was an English author, now best known for his work in the science fiction genre. He was also a prolific writer in many other genres, including contemporary novels, history, politics and social commentary, even writing text books and rules for war games...

    ' The Time Machine
    The Time Machine
    The Time Machine is a science fiction novella by H. G. Wells, published in 1895 for the first time and later adapted into at least two feature films of the same name, as well as two television versions, and a large number of comic book adaptations. It indirectly inspired many more works of fiction...


Forwards 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 travelling forward in time; for example, in Hindu mythology
Hindu mythology
Hindu religious literature is the large body of traditional narratives related to Hinduism, notably as contained in Sanskrit literature, such as the Sanskrit epics and the Puranas. As such, it is a subset of Nepali and Indian culture...

, the Mahabharata
Mahabharata
The Mahabharata is one of the two major Sanskrit epics of ancient India and Nepal, the other being the Ramayana. The epic is part of itihasa....

mentions the story of the King Revaita
Kakudmi
Kakudmi was the King of Kusasthali, a kingdom beneath the ocean. He was the father of Revati, who married Balarama...

, who travels to heaven to meet the creator Brahma
Brahma
Brahma is the Hindu god of creation and one of the Trimurti, the others being Vishnu and Shiva. According to the Brahma Purana, he is the father of Mānu, and from Mānu all human beings are descended. In the Ramayana and the...

 and is shocked to learn that many ages have passed when he returns to Earth. Another one of the earliest known stories to involve traveling forwards in time to a distant future was the Japanese tale
Japanese literature
Early works of Japanese literature were heavily influenced by cultural contact with China and Chinese literature, often written in Classical Chinese. Indian literature also had an influence through the diffusion of Buddhism in Japan...

 of "Urashima Tarō
Urashima Taro
is a Japanese legend about a fisherman who rescues a turtle and is rewarded for this with a visit to Ryūgū-jō, the palace of Ryūjin, the Dragon God, under the sea...

", first described in the Nihongi
Nihon Shoki
The , sometimes translated as The Chronicles of Japan, is the second oldest book of classical Japanese history. It is more elaborate and detailed than the Kojiki, the oldest, and has proven to be an important tool for historians and archaeologists as it includes the most complete extant historical...

(720). 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
Talmud
The Talmud is a central text of mainstream Judaism. It takes the form of a record of rabbinic discussions pertaining to Jewish law, ethics, philosophy, customs and history....

 with the story of Honi HaM'agel
Honi HaM'agel
Honi Ha-Ma'agel , was a Jewish scholar prior to the age of the tannaim, the scholars from whose teachings the Mishnah was derived....

 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. More recently, Washington Irving
Washington Irving
Washington Irving was an American author, essayist, biographer and historian of the early 19th century. He was best known for his short stories "The Legend of Sleepy Hollow" and "Rip Van Winkle", both of which appear in his book The Sketch Book of Geoffrey Crayon, Gent. His historical works...

's famous 1819 story "Rip Van Winkle
Rip Van Winkle
"Rip Van Winkle" is a short story by the American author Washington Irving published in 1819, as well as the name of the story's fictional protagonist. Written while Irving was living in Birmingham, England, it was part of a collection entitled The Sketch Book of Geoffrey Crayon...

" 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. Sleep was also used for time travel in Faddey Bulgarin's story "Pravdopodobnie Nebylitsi" in which the protagonist wakes up in the 29th century.

Another more recent story involving travel to the future is Louis-Sébastien Mercier
Louis-Sébastien Mercier
Louis-Sébastien Mercier was a French dramatist and writer.-Early life and education:He was born in Paris to a humble family: his father was a skilled artisan who polished swords and metal arms. Mercier nevertheless received a decent education.-Literary career:Mercier began his literary career by...

's L'An 2440, rêve s'il 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...L'An 2440 demanded to be read as a serious guidebook to the future."

Backwards time travel


Backwards time travel seems to be a more modern idea, but its origin is also somewhat ambiguous. One early story with hints of backwards time travel is Memoirs of the Twentieth Century (1733) by Samuel Madden
Samuel Madden
Samuel Madden was an Irish author. His works include Themistocles; The Lover of His Country, Reflections and Resolutions Proper for the Gentlemen of Ireland, and Memoirs of the Twentieth Century. Dr...

, which is mainly a series of letters from English 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. However, the framing story is that these letters were actual documents given to the narrator by his guardian angel
Guardian angel
A guardian angel is an angel assigned to protect and guide a particular person or group. Belief in guardian angels can be traced throughout all antiquity...

 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", 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 backwards from the future to be discovered in the present."

In 1836 Alexander Veltman
Alexander Veltman
Alexander Fomich Veltman was one of the most successful Russian prose writers of the 1830s and 1840s, "popular for various modes of Romantic fiction—historical, Gothic, fantastic, and folkloristic." He was one of the pioneers of Russian science fiction....

 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. In it the narrator rides to ancient Greece on a hippogriff
Hippogriff
A Hippogriff is a legendary creature, supposedly the offspring of a griffin and a mare.- Early references :...

, meets Aristotle
Aristotle
Aristotle was a Greek philosopher and polymath, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, linguistics, politics, government, ethics, biology, and zoology...

, 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
August Derleth
August William Derleth was an American writer and anthologist. Though best remembered as the first publisher of the writings of H. P...

 identifies the short story "Missing One's Coach: An Anachronism", written for the Dublin Literary Magazine by an anonymous
Anonymity
Anonymity is derived from the Greek word ἀνωνυμία, anonymia, meaning "without a name" or "namelessness". In colloquial use, anonymity typically refers to the state of an individual's personal identity, or personally identifiable information, being publicly unknown.There are many reasons why a...

 author in 1838, as a very early time travel story. In this story, the narrator is waiting under a tree to be picked up by a coach
Stagecoach
A stagecoach is a type of covered wagon for passengers and goods, strongly sprung and drawn by four horses, usually four-in-hand. Widely used before the introduction of railway transport, it made regular trips between stages or stations, which were places of rest provided for stagecoach travelers...

 which will take him out of Newcastle
Newcastle upon Tyne
Newcastle upon Tyne is a city and metropolitan borough of Tyne and Wear, in North East England. Historically a part of Northumberland, it is situated on the north bank of the River Tyne...

, when he suddenly finds himself transported back over a thousand years. He encounters the Venerable Bede
Bede
Bede , also referred to as Saint Bede or the Venerable Bede , was a monk at the Northumbrian monastery of Saint Peter at Monkwearmouth, today part of Sunderland, England, and of its companion monastery, Saint Paul's, in modern Jarrow , both in the Kingdom of Northumbria...

 in a monastery
Monastery
Monastery denotes the building, or complex of buildings, that houses a room reserved for prayer as well as the domestic quarters and workplace of monastics, whether monks or nuns, and whether living in community or alone .Monasteries may vary greatly in size – a small dwelling accommodating only...

, and gives him somewhat ironic explanations of the developments of the coming centuries. It is never entirely 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.

Charles Dickens
Charles Dickens
Charles John Huffam Dickens was an English novelist, generally considered the greatest of the Victorian period. Dickens enjoyed a wider popularity and fame than had any previous author during his lifetime, and he remains popular, having been responsible for some of English literature's most iconic...

' 1843 book A Christmas Carol
A Christmas Carol
A Christmas Carol is a novella by English author Charles Dickens first published by Chapman & Hall on 17 December 1843. The story tells of sour and stingy Ebenezer Scrooge's ideological, ethical, and emotional transformation after the supernatural visits of Jacob Marley and the Ghosts of...

is considered by some to be one of the first depictions of time travel, as the main character, Ebenezer Scrooge, is transported to Christmases past, present and yet to come. These might be considered mere visions rather than actual time travel, though, since Scrooge only viewed each time period passively, unable to interact with them.

A clearer example of time travel is found in the popular 1861 book Paris avant les hommes (Paris before Men) by the French botanist and geologist Pierre Boitard
Pierre Boitard
Pierre Boitard was a French botanist and geologist. As well as describing and classifying the Tasmanian Devil, he is notable for his fictional natural history Paris avant les hommes , published posthumously in 1861, which described a prehistoric ape-like human ancestor living in the region of Paris...

, 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 Boitard's name), where he encounters such extinct animals as a Plesiosaur
Plesiosaur
Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...

, as well as Boitard's imagined version of an apelike human ancestor, and is able to actively interact with some of them.

Another clear early example of time travel in fiction is the short story by Edward Page Mitchell
Edward Page Mitchell
Edward Page Mitchell was an American editorial and short story writer for the New York Sun, a daily newspaper. He became that newspaper's editor in 1897, succeeding Charles Anderson Dana. Mitchell retired in 1926, a year before dying of a cerebral hemorrhage...

, which appeared in the New York Sun
New York Sun (historical)
The Sun was a New York newspaper that was published from 1833 until 1950. It was considered a serious paper, like the city's two more successful broadsheets, The New York Times and the New York Herald Tribune...

in 1881.

Mark Twain
Mark Twain
Samuel Langhorne Clemens , better known by his pen name Mark Twain, was an American author and humorist...

's A Connecticut Yankee in King Arthur's Court
A Connecticut Yankee in King Arthur's Court
A Connecticut Yankee in King Arthur's Court is an 1889 novel by American humorist and writer Mark Twain. The book was originally titled A Yankee in King Arthur's Court...

(1889), in which the protagonist finds himself in the time of King Arthur
King Arthur
King Arthur is a legendary British leader of the late 5th and early 6th centuries, who, according to Medieval histories and romances, led the defence of Britain against Saxon invaders in the early 6th century. The details of Arthur's story are mainly composed of folklore and literary invention, and...

 after a fight in which he is hit with a sledge hammer, 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 the time traveler's actions.

The first time travel story to feature time travel by means of a time machine was Enrique Gaspar y Rimbau
Enrique Gaspar y Rimbau
Enrique Lucio Eugenio Gaspar y Rimbau was a Spanish diplomat and writer, who wrote plays, zarzuelas , and novels.-Biography:...

's 1887 book El Anacronópete. This idea gained popularity with the H. G. Wells
H. G. Wells
Herbert George Wells was an English author, now best known for his work in the science fiction genre. He was also a prolific writer in many other genres, including contemporary novels, history, politics and social commentary, even writing text books and rules for war games...

 story The Time Machine
The Time Machine
The Time Machine is a science fiction novella by H. G. Wells, published in 1895 for the first time and later adapted into at least two feature films of the same name, as well as two television versions, and a large number of comic book adaptations. It indirectly inspired many more works of fiction...

, published in 1895 (preceded by a less influential story of time travel Wells wrote in 1888, titled The Chronic Argonauts
The Chronic Argonauts
"The Chronic Argonauts" is a short story written by H. G. Wells. First published by the Royal College of Science in 1888, it is the first well-developed use of a machine constructed to travel through time in science fiction, as it predates Wells's more famous time traveling novel, The Time...

), which also featured a time machine and which is often seen as an inspiration for all later science fiction stories featuring time travel using a vehicle that allows an operator to travel purposefully and selectively. The term "time machine", coined by Wells, is now universally used to refer to such a vehicle.

Since that time, both science and fiction (see Time travel in fiction
Time travel in fiction
Time travel is a common theme in science fiction and is depicted in a variety of media. It simply means either going forward in time or backward, to experience the future, or the past.-Literature:...

) have expanded on the concept of time travel.

Theory


Some theories, most notably special
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

 and general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

, suggest that suitable geometries of spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...

, or specific types of motion in space
Space
Space is the boundless, three-dimensional extent in which objects and events occur and have relative position and direction. Physical space is often conceived in three linear dimensions, although modern physicists usually consider it, with time, to be part of a boundless four-dimensional continuum...

, might allow time travel into the past and future if these geometries or motions are possible. In technical papers, physicist
Physicist
A physicist is a scientist who studies or practices physics. Physicists study a wide range of physical phenomena in many branches of physics spanning all length scales: from sub-atomic particles of which all ordinary matter is made to the behavior of the material Universe as a whole...

s 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 curve
Closed timelike curve
In mathematical physics, a closed timelike curve is a worldline in a Lorentzian manifold, of a material particle in spacetime that is "closed," returning to its starting point...

s, 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
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

 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
Proper time
In relativity, proper time is the elapsed time between two events as measured by a clock that passes through both events. The proper time depends not only on the events but also on the motion of the clock between the events. An accelerated clock will measure a smaller elapsed time between two...

 has been experienced by both the Earth and the traveler, i.e., how much each has aged; see twin paradox
Twin paradox
In physics, the twin paradox is a thought experiment in special relativity, in which a twin makes a journey into space in a high-speed rocket and returns home to find he has aged less than his identical twin who stayed on Earth...

). On the other hand, many in the scientific community believe that backwards time travel is highly unlikely. Any theory that would allow time travel would require that problems of causality
Causality (physics)
Causality is the relationship between causes and effects. It is considered to be fundamental to all natural science, especially physics. Causality is also a topic studied from the perspectives of philosophy and statistics....

 be resolved. The classic example of a problem involving causality is the "grandfather paradox
Grandfather paradox
The grandfather paradox is a proposed paradox of time travel first described by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent . The paradox is this: suppose a man traveled back in time and killed his biological grandfather before the latter met the traveler's...

": 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
Novikov self-consistency principle
The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a principle developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general...

 or to the notion of branching parallel universes
Multiverse
The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...

 (see the 'Paradoxes' section below).

Tourism in time


Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...

 once suggested that the absence of tourists from the future is an argument against the existence of time travel — a variant of the Fermi paradox
Fermi paradox
The Fermi paradox is the apparent contradiction between high estimates of the probability of the existence of extraterrestrial civilizations and the lack of evidence for, or contact with, such civilizations....

. 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 is 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 "This picture would explain why we haven't been over run by tourists from the future." Carl Sagan
Carl Sagan
Carl Edward Sagan was an American astronomer, astrophysicist, cosmologist, author, science popularizer and science communicator in astronomy and natural sciences. He published more than 600 scientific papers and articles and was author, co-author or editor of more than 20 books...

 also once suggested the possibility that time travelers could be here, but are disguising their existence or are not recognized as time travelers.

General relativity


However, the theory of general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

 does suggest a scientific basis for the possibility of backwards time travel in certain unusual scenarios, although arguments from semiclassical gravity
Semiclassical gravity
Semiclassical gravity is the approximation to the theory of quantum gravity in which one treats matter fields as being quantum and the gravitational field as being classical....

 suggest that when quantum
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

 effects are incorporated into general relativity, these loopholes may be closed. These semiclassical arguments led Hawking to formulate the chronology protection conjecture
Chronology protection conjecture
The chronology protection conjecture is a conjecture by the physicist Professor Stephen Hawking that the laws of physics are such as to prevent time travel on all but sub-microscopic scales. Mathematically, the permissibility of time travel is represented by the existence of closed timelike curves...

, suggesting that the fundamental laws of nature prevent time travel, but physicists cannot come to a definite judgment on the issue without a theory of quantum gravity
Quantum gravity
Quantum gravity is the field of theoretical physics which attempts to develop scientific models that unify quantum mechanics with general relativity...

 to join quantum mechanics and general relativity into a completely unified theory.

In physics


Time travel to the past is theoretically allowed using the following methods:
  • Travelling faster than the speed of light
    Faster-than-light
    Faster-than-light communications and travel refer to the propagation of information or matter faster than the speed of light....

  • The use of cosmic string
    Cosmic string
    Cosmic strings are hypothetical 1-dimensional topological defects which may have formed during a symmetry breaking phase transition in the early universe when the topology of the vacuum manifold associated to this symmetry breaking is not simply connected. It is expected that at least one string...

    s and black holes
  • Wormhole
    Wormhole
    In physics, a wormhole is a hypothetical topological feature of spacetime that would be, fundamentally, a "shortcut" through spacetime. For a simple visual explanation of a wormhole, consider spacetime visualized as a two-dimensional surface. If this surface is folded along a third dimension, it...

    s and Alcubierre drive
    Alcubierre drive
    The Alcubierre drive, also known as the Alcubierre metric, is a speculative, but valid solution of the Einstein field equations. It is a mathematical model of a spacetime exhibiting features reminiscent of the fictional "warp drive" from Star Trek, which can travel "faster than light", although...


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 special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

, there would be some inertial frame of reference
Inertial frame of reference
In physics, an inertial frame of reference is a frame of reference that describes time homogeneously and space homogeneously, isotropically, and in a time-independent manner.All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not...

 in which the signal or object was moving backward in time. This is a consequence of the relativity of simultaneity
Relativity of simultaneity
In physics, the relativity of simultaneity is the concept that simultaneity–whether two events occur at the same time–is not absolute, but depends on the observer's reference frame. According to the special theory of relativity, it is impossible to say in an absolute sense whether two events occur...

 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
Light cone
A light cone is the path that a flash of light, emanating from a single event and traveling in all directions, would take through spacetime...

 of the other). 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.

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 backwards in time. And since one of the two fundamental postulates of special relativity
Postulates of special relativity
- Postulates of special relativity :1. First postulate 2. Second postulate - Alternate Derivations 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 backwards 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 A's frame but backwards in time in B's frame, and then B sends a reply which moves FTL in B's frame but backwards in time in A's frame, it could work out that A receives the reply before sending the original signal, a clear violation of causality
Causality (physics)
Causality is the relationship between causes and effects. It is considered to be fundamental to all natural science, especially physics. Causality is also a topic studied from the perspectives of philosophy and statistics....

 in every frame. An illustration of such a scenario using spacetime diagrams
Minkowski diagram
The Minkowski diagram was developed in 1908 by Hermann Minkowski and provides an illustration of the properties of space and time in the special theory of relativity. It allows a quantitative understanding of the corresponding phenomena like time dilation and length contraction without mathematical...

 can be found here. The scenario is sometimes referred to as a tachyonic antitelephone
Tachyonic antitelephone
A tachyonic antitelephone is a hypothetical device in theoretical physics that could be used to send signals into one's own past. Albert Einstein in 1907...

.

According to special relativity
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

, 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
Quantum field theory
Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...

, it seems that it would not actually be possible to use them to transmit information faster than light. There is also no widely agreed-upon evidence for the existence of tachyons; the OPERA neutrino anomaly
OPERA neutrino anomaly
The OPERA neutrino anomaly is the detection of apparently faster-than-light neutrinos by the OPERA experiment as publicly announced in September 2011. The detection is anomalous because speeds exceeding that of light in a vacuum are generally thought to violate special relativity, a prevailing...

 could possibly indicate that neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...

s are tachyons, but the results of the experiment have not been independently confirmed and another group of experimenters argue that the lack of Cherenkov radiation
Cherenkov radiation
Cherenkov radiation is electromagnetic radiation emitted when a charged particle passes through a dielectric medium at a speed greater than the phase velocity of light in that medium...

 indicates the neutrinos cannot have really been traveling faster than light, and the OPERA
OPERA Experiment
The Oscillation Project with Emulsion-tRacking Apparatus is a scientific experiment for detecting tau neutrinos from muon neutrino oscillations. It is a collaboration between CERN in Geneva, Switzerland, and the Laboratori Nazionali del Gran Sasso in Gran Sasso, Italy and uses the CERN Neutrinos...

 group must have just made a mistake in timekeeping.

Special spacetime geometries


The general theory of relativity extends the special theory
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

 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
Einstein field equations
The Einstein field equations or Einstein's equations are a set of ten equations in Albert Einstein's general theory of relativity which describe the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy...

, and there exist solutions to these equations that permit what are called "closed time-like curves," and hence time travel into the past. The first of these was proposed by Kurt Gödel
Kurt Gödel
Kurt Friedrich Gödel was an Austrian logician, mathematician and philosopher. Later in his life he emigrated to the United States to escape the effects of World War II. One of the most significant logicians of all time, Gödel made an immense impact upon scientific and philosophical thinking in the...

, a solution known as the Gödel metric
Gödel metric
The Gödel metric is an exact solution of the Einstein field equations in which the stress-energy tensor contains two terms, the first representing the matter density of a homogeneous distribution of swirling dust particles, and the second associated with a nonzero cosmological constant...

, but his (and many others') example requires the universe to have physical characteristics that it does not appear to have. Whether general relativity forbids closed time-like curves for all realistic conditions is unknown.

Using wormholes


Wormholes are a hypothetical warped spacetime which are also permitted by the Einstein field equations
Einstein field equations
The Einstein field equations or Einstein's equations are a set of ten equations in Albert Einstein's general theory of relativity which describe the fundamental interaction of gravitation as a result of spacetime being curved by matter and energy...

 of general relativity, although it would be impossible 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
Vehicle propulsion
Vehicle propulsion refers to the act of moving an artificial carrier of people or goods over any distance. The power plant used to drive the vehicles can vary widely. Originally, humans or animals would have provided the propulsion system, later being supplemented by wind power...

, 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
Time dilation
In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...

 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
Synchronization
Synchronization is timekeeping which requires the coordination of events to operate a system in unison. The familiar conductor of an orchestra serves to keep the orchestra in time....

 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. 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 backwards 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; 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 backwards in time. This could provide an alternative explanation for Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...

's observation: a time machine will be built someday, but has not yet been built, so the tourists from the future cannot reach this far back 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
Exotic matter
In physics, exotic matter is a term which refers to matter which would somehow deviate from the norm and have "exotic" properties. There are several uses of the term....

") . More technically, the wormhole spacetime requires a distribution of energy that violates various energy condition
Energy condition
In relativistic classical field theories of gravitation, particularly general relativity, an energy condition is one of various alternative conditions which can be applied to the matter content of the theory, when it is either not possible or desirable to specify this content explicitly...

s, such as the null energy condition along with the weak, strong, and dominant energy conditions. However, it is known that quantum effects can lead to small measurable violations of the null energy condition, and many physicists believe that the required negative energy may actually be possible due to the Casimir effect
Casimir effect
In quantum field theory, the Casimir effect and the Casimir–Polder force are physical forces arising from a quantized field. The typical example is of two uncharged metallic plates in a vacuum, like capacitors placed a few micrometers apart, without any external electromagnetic field...

 in quantum physics. 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.

In 1993, Matt Visser
Matt Visser
Matt Visser is a mathematics Professor at Victoria University of Wellington.Some of his research interests include General Relativity, Quantum Field Theory and Cosmology....

 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. Because of this, the two mouths could not be brought close enough for causality
Causality (physics)
Causality is the relationship between causes and effects. It is considered to be fundamental to all natural science, especially physics. Causality is also a topic studied from the perspectives of philosophy and statistics....

 violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "Roman ring
Roman ring
A Roman ring, in theoretical physics, is a configuration of wormholes where for each individual wormhole the time difference across its mouths is such that it may not allow a closed timelike curve , or 'closed-time loop'...

" (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.

Other approaches based on general relativity


Another approach involves a dense spinning cylinder usually referred to as a Tipler cylinder
Tipler Cylinder
A Tipler cylinder is a cylinder of dense matter of infinite length, rotating about its longitudinal axis. This hypothetical object is theorized to allow time travel and is also called a Tipler time machine....

, a GR solution discovered by Willem Jacob van Stockum
Willem Jacob van Stockum
Willem Jacob van Stockum was a mathematician who made an important contribution to the early development of general relativity....

 in 1936 and Kornel Lanczos in 1924, but not recognized as allowing closed timelike curves until an analysis by Frank Tipler 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
Cosmic string
Cosmic strings are hypothetical 1-dimensional topological defects which may have formed during a symmetry breaking phase transition in the early universe when the topology of the vacuum manifold associated to this symmetry breaking is not simply connected. It is expected that at least one string...

, but none are known to exist, and it does not seem to be possible to create a new cosmic string.

Physicist Robert Forward
Robert Forward
Robert Lull Forward — known as Robert L. Forward — was an American physicist and science fiction writer...

 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
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 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.

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
Exotic matter
In physics, exotic matter is a term which refers to matter which would somehow deviate from the norm and have "exotic" properties. There are several uses of the term....

). Solutions such as Tipler's 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, he did not prove this. But Hawking points out that because of his theorem, "it can't be done with positive energy density everywhere! I can prove that to build a finite time machine, you need negative energy." This result comes from Hawking's 1992 paper on the chronology protection conjecture
Chronology protection conjecture
The chronology protection conjecture is a conjecture by the physicist Professor Stephen Hawking that the laws of physics are such as to prevent time travel on all but sub-microscopic scales. Mathematically, the permissibility of time travel is represented by the existence of closed timelike curves...

, 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
Cauchy horizon
In physics, a Cauchy horizon is a light-like boundary of the domain of validity of a Cauchy problem...

 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." 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
Alcubierre drive
The Alcubierre drive, also known as the Alcubierre metric, is a speculative, but valid solution of the Einstein field equations. It is a mathematical model of a spacetime exhibiting features reminiscent of the fictional "warp drive" from Star Trek, which can travel "faster than light", although...

). 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 Hawking's chronology protection conjecture).

Experiments carried out


Certain experiments carried out give the impression of reversed causality
Causality
Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

 but are interpreted in a different way by the scientific community. For example, in the delayed choice quantum eraser
Delayed choice quantum eraser
A delayed choice quantum eraser, first performed by Yoon-Ho Kim, R. Yu, S.P. Kulik, Y.H. Shih, and Marlan O. Scully, is an elaboration on a quantum eraser experiment involving the concepts considered in Wheeler's delayed choice experiment...

 experiment performed by Marlan Scully
Marlan Scully
Marlan Orvil Scully is a physicist best known for his work in theoretical quantum optics. He is currently a professor at Texas A&M University and Princeton University. He has authored over 700 scientific articles, as well as standard textbooks such as “Laser Physics” and “Quantum Optics” ...

, pairs of entangled
Quantum entanglement
Quantum entanglement occurs when electrons, molecules even as large as "buckyballs", photons, etc., interact physically and then become separated; the type of interaction is such that each resulting member of a pair is properly described by the same quantum mechanical description , which is...

 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 backwards 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
Günter Nimtz
Günter Nimtz is a German physicist, working at the 2nd Physics Institute at the University of Cologne in Germany. He has investigated narrow-gap semiconductors and liquid crystals and was engaged in several interdisciplinary studies on the effect of non-ionizing electromagnetic radiation in...

 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 – energetic packets of light – traveled "instantaneously" between a pair of prisms that had been moved up to 3 ft (0.9144 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 center of the train exceeds the speed of any of the individual cars.

Some physicists have performed experiments which attempted to show causality violations, but so far without success. The "Space-time Twisting by Light" (STL) experiment run by physicist Ronald Mallett
Ronald Mallett
Ronald Lawrence Mallett is an American theoretical physicist, academic, and author. He has taught physics at the University of Connecticut since 1975. He is best known for his scientific position on the possibility of time travel....

 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
Photonic crystal
Photonic crystals are periodic optical nanostructures that are designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons...

. Mallett has some physical arguments that suggest that closed timelike curves would become possible through the center of a laser which has been twisted into a loop. However, other physicists dispute his arguments (see objections).

Shengwang Du
Shengwang Du
Shengwang Du is an assistant professor in the department of physics at Hong Kong University of Science and Technology.He is noted for having led a team that performed an experiment showing individual photons cannot travel faster than the speed of light in a vacuum, thus apparently removing one...

 claims in a peer reviewed journal to have observed single photons' precursors, saying that they travel no faster than c
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

 in a vacuum. His experiment involved slow light
Slow light
Slow light is the propagation of an optical pulse or other modulation of an optical carrier at a very low group velocity. Slow light occurs when a propagating pulse is substantially slowed down by the interaction with the medium in which the propagation take place.Researchers at the Rowland...

 as well as passing light through a vacuum. He generated two single photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

s, 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
C
Ĉ or ĉ is a consonant in Esperanto orthography, representing the sound .Esperanto orthography uses a diacritic for all four of its postalveolar consonants, as do the Latin-based Slavic alphabets...

 (and, thus, violating causality). (Some members of the media took this as an indication of proof that time travel was impossible
Faulty generalization
A fallacy of defective induction reaches a conclusion from weak premises. Unlike fallacies of relevance, in fallacies of defective induction, the premises are related to the conclusions yet only weakly buttress the conclusions. A faulty generalization is thus produced...

.)

Non-physics-based experiments


Several experiments have been carried out to try to entice future humans, who might invent time travel technology
Technology
Technology is the making, usage, and knowledge of tools, machines, techniques, crafts, systems or methods of organization in order to solve a problem or perform a specific function. It can also refer to the collection of such tools, machinery, and procedures. The word technology comes ;...

, to come back and demonstrate it to people of the present time. Events such as Perth's Destination Day (2005) or MIT's 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, MD., identifying itself as the Krononauts, hosted an event of this type welcoming visitors from the future. 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
Multiverse
The multiverse is the hypothetical set of multiple possible universes that together comprise all of reality.Multiverse may also refer to:-In fiction:* Multiverse , the fictional multiverse used by DC Comics...

. Another factor is that for all the time travel devices considered under current physics (such as those that operate using wormhole
Wormhole
In physics, a wormhole is a hypothetical topological feature of spacetime that would be, fundamentally, a "shortcut" through spacetime. For a simple visual explanation of a wormhole, consider spacetime visualized as a two-dimensional surface. If this surface is folded along a third dimension, it...

s), it is impossible to travel back to before the time machine was actually made.

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 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
Relativistic speed
A Relativistic speed is a speed which is a significant proportion of the speed of light. Therefore scientific analysis must take the consequences of special relativity into account...

, 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. It should be noted, though, that according to relativity there is 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 your choice of reference frame
Frame of reference
A frame of reference in physics, may refer to a coordinate system or set of axes within which to measure the position, orientation, and other properties of objects in it, or it may refer to an observational reference frame tied to the state of motion of an observer.It may also refer to both an...

.

This form of "travel into the future" is theoretically allowed (and has been demonstrated at very small time scales) using the following methods:
  • Using velocity-based time dilation
    Time dilation
    In the theory of relativity, time dilation is an observed difference of elapsed time between two events as measured by observers either moving relative to each other or differently situated from gravitational masses. An accurate clock at rest with respect to one observer may be measured to tick at...

     under the theory of special relativity
    Special relativity
    Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

    , for instance:
    • Traveling at almost the speed of light
      Speed of light
      The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...

       to a distant star, then slowing down, turning around, and traveling at almost the speed of light back to Earth
      Earth
      Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

       (see the Twin paradox
      Twin paradox
      In physics, the twin paradox is a thought experiment in special relativity, in which a twin makes a journey into space in a high-speed rocket and returns home to find he has aged less than his identical twin who stayed on Earth...

      )
  • Using gravitational time dilation
    Gravitational time dilation
    Gravitational time dilation is the effect of time passing at different rates in regions of different gravitational potential; the lower the gravitational potential, the more slowly time passes...

     under the theory of general relativity
    General relativity
    General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

    , for instance:
    • Residing inside of a hollow, high-mass object;
    • Residing just outside of the event horizon of a black hole
      Black hole
      A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

      , 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.


Additionally, it might be possible to see the distant future of the Earth using methods which do not involve relativity at all, although it is even more debatable whether these should be deemed a form of "time travel":
  • Hibernation
    Hibernation
    Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...

  • Suspended animation
    Suspended animation
    Suspended animation is the slowing of life processes by external means without termination. Breathing, heartbeat, and other involuntary functions may still occur, but they can only be detected by artificial means. Extreme cold can be used to precipitate the slowing of an individual's functions; use...


Time dilation



Time dilation is permitted by Albert Einstein
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...

's special
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

 and general
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

 theories of relativity. These theories state that, relative to a given observer, time passes more slowly for bodies moving quickly relative to that observer, or bodies that are deeper within a gravity well
Gravity well
A gravity well or gravitational well is a conceptual model of the gravitational field surrounding a body in space. The more massive the body the deeper and more extensive the gravity well associated with it. The Sun has a far-reaching and deep gravity well. Asteroids and small moons have much...

. For example, a clock which is moving relative to the observer will be measured to run slow in that observer's rest frame
Rest frame
In special relativity the rest frame of a particle is the coordinate system in which the particle is at rest.The rest frame of compound objects is taken to be the frame of reference in which the average momentum of the particles which make up the substance is zero In special relativity the rest...

; as a clock approaches the speed of light it will almost slow to a stop, although it can never quite reach light speed so it will never completely stop. For two clocks moving inertially
Inertial frame of reference
In physics, an inertial frame of reference is a frame of reference that describes time homogeneously and space homogeneously, isotropically, and in a time-independent manner.All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not...

 (not accelerating) relative to one another, this effect is reciprocal, with each clock measuring the other to be ticking slower. However, the symmetry is broken if one clock accelerates, as in the twin paradox
Twin paradox
In physics, the twin paradox is a thought experiment in special relativity, in which a twin makes a journey into space in a high-speed rocket and returns home to find he has aged less than his identical twin who stayed on Earth...

 where one twin stays on Earth while the other travels into space, turns around (which involves acceleration), and returns—in this case both agree the traveling twin has aged less. General relativity states that time dilation effects also occur if one clock is deeper in a gravity well than the other, with the clock deeper in the well ticking more slowly; this effect must be taken into account when calibrating the clocks on the satellites of the Global Positioning System
Global Positioning System
The Global Positioning System is a space-based global navigation satellite system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites...

, and it could lead to significant differences in rates of aging for observers at different distances from a black hole
Black hole
A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

.

It has been calculated that, under general relativity, a person could travel forward in time at a rate four times that of distant observers by residing inside a spherical shell with a diameter of 5 meters and the mass of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...

. For such a person, every one second of their "personal" time would correspond to four seconds for distant observers. Of course, squeezing the mass of a large planet into such a structure is not expected to be within our technological capabilities in the near future.

There is a great deal of experimental evidence supporting the validity of equations for velocity-based time dilation in special relativity and gravitational time dilation in general relativity. However, with current technologies it is only possible to cause a human traveller to age less than companions on Earth by a very small fraction of a second, the current record being about 20 milliseconds for the cosmonaut Sergei Avdeyev
Sergei Avdeyev
Sergei Avdeyev is a Russian engineer and cosmonaut.Avdeyev was born in Chapayevsk, Samara Oblast , Russian SFSR. He graduated from in 1979 as an engineer-physicist. From 1979 to 1987 he worked as an engineer for NPO Energiya...

.

Time perception


Time perception
Time perception
Time perception is a field of study within psychology and neuroscience. It refers to the sense of time, which differs from other senses since time cannot be directly perceived but must be reconstructed by the brain. Humans can perceive relatively short periods of time, in the order of milliseconds,...

 can be apparently sped up for living organisms
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

 through hibernation
Hibernation
Hibernation is a state of inactivity and metabolic depression in animals, characterized by lower body temperature, slower breathing, and lower metabolic rate. Hibernating animals conserve food, especially during winter when food supplies are limited, tapping energy reserves, body fat, at a slow rate...

, where the body temperature
Thermoregulation
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different...

 and metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 rate of the creature is reduced. A more extreme version of this is suspended animation
Suspended animation
Suspended animation is the slowing of life processes by external means without termination. Breathing, heartbeat, and other involuntary functions may still occur, but they can only be detected by artificial means. Extreme cold can be used to precipitate the slowing of an individual's functions; use...

, where the rates of chemical processes in the subject would be severely reduced.

Time dilation and suspended animation only allow "travel" to the future, never the past, so they do not violate causality
Causality
Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

, and it's debatable whether they should be called time travel. However time dilation can be viewed as a better fit for our understanding of the term "time travel" than suspended animation, since with time dilation less time actually does pass for the traveler than for those who remain behind, so the traveler can be said to have reached the future faster than others, whereas with suspended animation this is not the case.

Paradoxes


The Novikov self-consistency principle
Novikov self-consistency principle
The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a principle developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general...

 and calculations by Kip S. Thorne indicate that simple masses passing through time travel wormholes could never engender paradoxes—there are no initial conditions that lead to paradox once time travel is introduced. If his results can be generalized, they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that any situation you can set up in a time travel story turns out to permit many consistent solutions. The circumstances might, however, turn out to be almost unbelievably strange.

Parallel universes might provide a way out of paradoxes. Everett's
Hugh Everett
Hugh Everett III was an American physicist who first proposed the many-worlds interpretation of quantum physics, which he termed his "relative state" formulation....

 many-worlds interpretation
Many-worlds interpretation
The many-worlds interpretation is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction, but denies the actuality of wavefunction collapse. Many-worlds implies that all possible alternative histories and futures are real, each representing an...

 (MWI) of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories. These alternate, or parallel, histories would form a branching tree symbolizing all possible outcomes of any interaction. If all possibilities exist, any paradoxes could be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction, but some physicists such as David Deutsch
David Deutsch
David Elieser Deutsch, FRS is an Israeli-British physicist at the University of Oxford. He is a non-stipendiary Visiting Professor in the Department of Atomic and Laser Physics at the Centre for Quantum Computation in the Clarendon Laboratory of the University of Oxford...

 have suggested that if time travel is possible and the MWI is correct, then a time traveler should indeed end up in a different history than the one he started from. On the other hand, Stephen Hawking
Stephen Hawking
Stephen William Hawking, CH, CBE, FRS, FRSA is an English theoretical physicist and cosmologist, whose scientific books and public appearances have made him an academic celebrity...

 has argued that even if the MWI is correct, we should expect each time traveler to experience a single self-consistent history, so that time travelers remain within their own world rather than traveling to a different one. And the physicist Allen Everett argued that Deutsch's approach "involves modifying fundamental principles of quantum mechanics; it certainly goes beyond simply adopting the MWI." Everett also argues that even if Deutsch's approach is correct, it would imply that any macroscopic object composed of multiple particles would be split apart when traveling back in time through a wormhole, with different particles emerging in different worlds.

Daniel Greenberger and Karl Svozil
Karl Svozil
Karl Svozil is an Austrian physicist educated at the University of Vienna and Heidelberg University. Visiting scholar at the Lawrence Berkeley Laboratory of the University of California at Berkeley, U.S.A. ,...

 proposed that quantum theory
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

 gives a model for time travel without paradoxes. In quantum theory observation causes possible states to 'collapse' into one measured state; hence, the past observed from the present is deterministic (it has only one possible state), but the present observed from the past has many possible states until our actions cause it to collapse into one state. Our actions will then be seen to have been inevitable.

Using quantum entanglement


Quantum-mechanical phenomena such as quantum teleportation
Quantum teleportation
Quantum teleportation, or entanglement-assisted teleportation, is a process by which a qubit can be transmitted exactly from one location to another, without the qubit being transmitted through the intervening space...

, the EPR paradox
EPR paradox
The EPR paradox is a topic in quantum physics and the philosophy of science concerning the measurement and description of microscopic systems by the methods of quantum physics...

, or quantum entanglement
Quantum entanglement
Quantum entanglement occurs when electrons, molecules even as large as "buckyballs", photons, etc., interact physically and then become separated; the type of interaction is such that each resulting member of a pair is properly described by the same quantum mechanical description , which is...

 might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation
Bohm interpretation
The de Broglie–Bohm theory, also called the pilot-wave theory, Bohmian mechanics, and the causal interpretation, is an interpretation of quantum theory. In addition to a wavefunction on the space of all possible configurations, it also includes an actual configuration, even in situations where...

 presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles. This effect was referred to as "spooky action at a distance" by Einstein.

Nevertheless, the fact that causality is preserved in quantum mechanics is a rigorous result in modern quantum field theories
Quantum field theory
Quantum field theory provides a theoretical framework for constructing quantum mechanical models of systems classically parametrized by an infinite number of dynamical degrees of freedom, that is, fields and many-body systems. It is the natural and quantitative language of particle physics and...

, and therefore modern theories do not allow for time travel or FTL communication. In any specific instance where FTL has been claimed, more detailed analysis has proven that to get a signal, some form of classical communication must also be used. The no-communication theorem
No-communication theorem
In quantum information theory, a no-communication theorem is a result which gives conditions under which instantaneous transfer of information between two observers is impossible. These results can be applied to understand the so-called paradoxes in quantum mechanics such as the EPR paradox or...

 also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals. The fact that these quantum phenomena apparently do not allow FTL time travel is often overlooked in popular press coverage of quantum teleportation experiments. How the rules of quantum mechanics work to preserve causality is an active area of research.

Philosophical understandings of time travel


Theories of time travel are riddled with questions about causality and paradoxes. Compared to other fundamental concepts in modern physics, time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....

 is still not understood very well. Philosophers have been theorizing about the nature of time since the era of the ancient Greek philosophers and earlier. Some philosophers and physicists who study the nature of time also study the possibility of time travel and its logical implications. The probability of paradox
Paradox
Similar to Circular reasoning, A paradox is a seemingly true statement or group of statements that lead to a contradiction or a situation which seems to defy logic or intuition...

es and their possible solutions are often considered.

For more information on the philosophical considerations of time travel, consult the work of David Lewis
David Kellogg Lewis
David Kellogg Lewis was an American philosopher. Lewis taught briefly at UCLA and then at Princeton from 1970 until his death. He is also closely associated with Australia, whose philosophical community he visited almost annually for more than thirty years...

 or Ted Sider. For more information on physics-related theories of time travel, consider the work of Kurt Gödel
Kurt Gödel
Kurt Friedrich Gödel was an Austrian logician, mathematician and philosopher. Later in his life he emigrated to the United States to escape the effects of World War II. One of the most significant logicians of all time, Gödel made an immense impact upon scientific and philosophical thinking in the...

 (especially his theorized universe
Gödel metric
The Gödel metric is an exact solution of the Einstein field equations in which the stress-energy tensor contains two terms, the first representing the matter density of a homogeneous distribution of swirling dust particles, and the second associated with a nonzero cosmological constant...

) and Lawrence Sklar
Lawrence Sklar
Lawrence Sklar is an American philosopher. He is the Carl G. Hempel and William K. Frankena Distinguished UniversityProfessor at the University of Michigan. He specialises in the Philosophy of Physics, approaching a wide range of issues from a position best described as highly sceptical of many of...

.

Presentism vs. eternalism


The relativity of simultaneity
Relativity of simultaneity
In physics, the relativity of simultaneity is the concept that simultaneity–whether two events occur at the same time–is not absolute, but depends on the observer's reference frame. According to the special theory of relativity, it is impossible to say in an absolute sense whether two events occur...

 in modern physics favors the philosophical
Philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...

 view known as eternalism or four-dimensionalism (Sider, 2001), in which physical objects are either temporally extended spacetime worms, or spacetime worm stages, and this view would be favored further by the possibility of time travel (Sider, 2001). Eternalism, also sometimes known as "block universe theory", builds on a standard method of modeling time as a dimension in physics, to give time a similar ontology to that of space (Sider, 2001). This would mean that time is just another dimension, that future events are "already there", and that there is no objective flow of time. This view is disputed by Tim Maudlin in his The Metaphysics Within Physics.

Presentism
Presentism (philosophy of time)
Saint Augustine proposed that the present is a knife edge between the past and the future and could not contain any extended period of time. This seems evident because, if the present is extended, it must have separate parts - but these must be simultaneous if they are truly part of the present...

 is a school of philosophy that holds that neither the future
Future
The future is the indefinite time period after the present. Its arrival is considered inevitable due to the existence of time and the laws of physics. Due to the nature of the reality and the unavoidability of the future, everything that currently exists and will exist is temporary and will come...

 nor the past
Past
Most generally, the past is a term used to indicate the totality of events which occurred before a given point in time. The past is contrasted with and defined by the present and the future. The concept of the past is derived from the linear fashion in which human observers experience time, and is...

 exist, and there are no non-present objects. In this view, time travel is impossible because there is no future or past to travel to. However, some 21st century presentists have argued that although past and future objects do not exist, there can still be definite truths about past and future events, and thus it is possible that a future truth about a time traveler deciding to travel back to the present date could explain the time traveler's actual appearance in the present.

The grandfather paradox


One subject often brought up in philosophical discussion of time is the idea that, if one were to go back in time, paradoxes could ensue if the time traveler were to change things. The best examples of this are the grandfather paradox and the idea of autoinfanticide. The grandfather paradox is a hypothetical situation in which a time traveler goes back in time and attempts to kill his grandfather at a time before his grandfather met his grandmother. If he did so, then his mother or father never would have been born, and neither would the time traveler himself, in which case the time traveler never would have gone back in time to kill his grandfather.

Autoinfanticide works the same way, where a traveler goes back and attempts to kill himself as an infant. If he were to do so, he never would have grown up to go back in time to kill himself as an infant.

This discussion is important to the philosophy of time travel because philosophers question whether these paradoxes make time travel impossible. Some philosophers answer the paradoxes by arguing that it might be the case that backwards time travel could be possible but that it would be impossible to actually change the past in any way, an idea similar to the proposed Novikov self-consistency principle
Novikov self-consistency principle
The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a principle developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general...

 in physics.

Theory of compossibility


David Lewis
David Kellogg Lewis
David Kellogg Lewis was an American philosopher. Lewis taught briefly at UCLA and then at Princeton from 1970 until his death. He is also closely associated with Australia, whose philosophical community he visited almost annually for more than thirty years...

's analysis of compossibility
Compossibility
Compossibility is a philosophical concept from Leibniz. According to Leibniz a complete individual thing is characterized by all its properties, and these determine its relations with other individuals. The existence of one individual may contradict the existence of another...

 and the implications of changing the past is meant to account for the possibilities of time travel in a one-dimensional conception of time without creating logical paradox
Paradox
Similar to Circular reasoning, A paradox is a seemingly true statement or group of statements that lead to a contradiction or a situation which seems to defy logic or intuition...

es. Consider Lewis’ example of Tim. Tim hates his grandfather and would like nothing more than to kill him. The only problem for Tim is that his grandfather died years ago. Tim wants so badly to kill his grandfather himself that he constructs a time machine to travel back to 1955 when his grandfather was young and kill him then. Assuming that Tim can travel to a time when his grandfather is still alive, the question must then be raised; Can Tim kill his grandfather?

For Lewis, the answer lies within the context of the usage of the word "can". Lewis explains that the word "can" must be viewed against the context of pertinent facts relating to the situation. Suppose that Tim has a rifle, years of rifle training, a straight shot on a clear day and no outside force to restrain Tim’s trigger finger. Can Tim shoot his grandfather? Considering these facts, it would appear that Tim can in fact kill his grandfather. In other words, all of the contextual facts are compossible with Tim killing his grandfather. However, when reflecting on the compossibility of a given situation, we must gather the most inclusive set of facts that we are able to.

Consider now the fact that Tim’s grandfather died in 1993 and not in 1955. This new fact about Tim’s situation reveals that him killing his grandfather is not compossible with the current set of facts. Tim cannot kill his grandfather because his grandfather died in 1993 and not when he was young. Thus, Lewis concludes, the statements "Tim doesn’t but can, because he has what it takes," and, "Tim doesn’t, and can’t, because it is logically impossible to change the past," are not contradictions; they are both true given the relevant set of facts. The usage of the word "can" is equivocal: he "can" and "can not" under different relevant facts. So what must happen to Tim as he takes aim? Lewis believes that his gun will jam, a bird will fly in the way, or Tim simply slips on a banana peel. Either way, there will be some logical force of the universe that will prevent Tim every time from killing his grandfather.

Rules of time travel


Time travel themes in science fiction
Science fiction
Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

 and the media can generally be grouped into two general categories (based on effect—methods are extremely varied and numerous), each of which can be further subdivided. However, there are no formal names for these two categories, so concepts rather than formal names will be used with notes regarding what categories they are placed under (Note: These classifications do not address the method of time travel itself, i.e. how to travel through time, but instead call to attention differing rules of what happens to history.). As used in this section, timeline refers to all physical events in history, so that in time travel stories where events can be changed, the time traveler can create a new or altered timeline. This usage of "timeline" is fairly common in time travel fiction, and is distinct from the usage of "timeline" to refer to a type of chart created by humans to illustrate a particular series of events (see timeline
Timeline
A timeline is a way of displaying a list of events in chronological order, sometimes described as a project artifact . It is typically a graphic design showing a long bar labeled with dates alongside itself and events labeled on points where they would have happened.-Uses of timelines:Timelines...

). This concept is also distinct from the concept of a world line
World line
In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional spacetime. The concept of "world line" is distinguished from the concept of "orbit" or "trajectory" by the time dimension, and typically encompasses a large area of spacetime wherein...

, a term from Einstein's theory of relativity
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....

 which refers to the entire history of a single object (usually idealized as a point particle
Point particle
A point particle is an idealization of particles heavily used in physics. Its defining feature is that it lacks spatial extension: being zero-dimensional, it does not take up space...

) that forms a distinct path through 4-dimension
Dimension
In physics and mathematics, the dimension of a space or object is informally defined as the minimum number of coordinates needed to specify any point within it. Thus a line has a dimension of one because only one coordinate is needed to specify a point on it...

al spacetime
Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...

.
1. There is a single fixed history, which is self-consistent and unchangeable. In this version, everything happens on a single timeline which does not contradict itself and cannot interact with anything potentially existing outside of it.

1.1 This can be simply achieved by applying the Novikov self-consistency principle
Novikov self-consistency principle
The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture, is a principle developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s to solve the problem of paradoxes in time travel, which is theoretically permitted in certain solutions of general...

, named after Dr. Igor Dmitrievich Novikov, Professor of Astrophysics at Copenhagen University. The principle states that the timeline is totally fixed, and any actions taken by a time traveler were part of history all along, so it is impossible for the time traveler to "change" history in any way. The time traveler's actions may be the cause of events in their own past though, which leads to the potential for circular causation and the predestination paradox
Predestination paradox
A predestination paradox is a paradox of time travel that is often used as a convention in science fiction. It exists when a time traveller is caught in a loop of events that "predestines" or "predates" them to travel back in time...

; for examples of circular causation, see Robert A. Heinlein
Robert A. Heinlein
Robert Anson Heinlein was an American science fiction writer. Often called the "dean of science fiction writers", he was one of the most influential and controversial authors of the genre. He set a standard for science and engineering plausibility and helped to raise the genre's standards of...

's story "By His Bootstraps
By His Bootstraps
"By His Bootstraps" is a science fiction short story by Robert A. Heinlein that plays with some of the inherent paradoxes that would be caused by time travel. It was originally published in the October 1941 issue of Astounding Science Fiction under the pen name Anson MacDonald...

". The Novikov self-consistency principle proposes that the local laws of physics in a region of spacetime containing time travelers cannot be any different from the local laws of physics in any other region of spacetime.

1.2 Alternatively, new physical laws take effect regarding time travel that thwarts attempts to change the past (contradicting the assumption mentioned in 1.1 above that the laws that apply to time travelers are the same ones that apply to everyone else). These new physical laws can be as unsubtle as to reject time travelers who travel to the past to change it by pulling them back to the point from when they came as Michael Moorcock
Michael Moorcock
Michael John Moorcock is an English writer, primarily of science fiction and fantasy, who has also published a number of literary novels....

's The Dancers at the End of Time
The Dancers at the End of Time
The title of this volume comes from the poem "The Last Word" by Ernest Dowson.Reunited at the end of Time, Jherek and the other inhabitants of the End of Time have returned to their preferred amusements of parties and games. They are interrupted by a ship of alien musician/pirates, the Lat...

or where the traveler is rendered a noncorporeal phantom unable to physically interact with the past such as in some Pre-Crisis Superman stories and Michael Garrett's "Brief Encounter" in Twilight Zone Magazine May 1981.

2. History is flexible and is subject to change (Plastic Time)

2.1 Changes to history are easy and can impact the traveler, the world, or both
Examples include Doctor Who
Doctor Who
Doctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a time-travelling humanoid alien known as the Doctor who explores the universe in a sentient time machine called the TARDIS that flies through time and space, whose exterior...

and the Back to the Future trilogy. In some cases, any resulting paradoxes can be devastating, threatening the very existence of the universe. In other cases the traveler simply cannot return home. The extreme version of this (Chaotic Time) is that history is very sensitive to changes with even small changes having large impacts such as in Ray Bradbury's "A Sound of Thunder
A Sound of Thunder
“A Sound of Thunder” is a science fiction short story by Ray Bradbury, first published in Collier’s magazine in 1952. As of 1984 it was the most re-published science fiction story up to the present time...

".

2.2 History is change resistant in direct relationship to the importance of the event ie. small trivial events can be readily changed but large ones take great effort.
In the Twilight Zone episode "Back There
Back There
"Back There" is an episode of the American television anthology series The Twilight Zone.- Synopsis :On April 14, 1961, Peter Corrigan feels faint after a discussion of time travel with his friends at the Potomac Club, and finds it's April 14, 1865, the day of Abraham Lincoln's assassination. He...

" a traveler tries to prevent the assassination of President Lincoln and fails, but his actions have made subtle changes to the status quo in his own time (e.g. a man who had been the butler of his gentleman's club is now a rich tycoon).
In the 2002 remake of The Time Machine
The Time Machine (2002 film)
The Time Machine is a 2002 American science fiction film loosely adapted from the 1895 novel of the same name by H. G. Wells, and the 1960 film screenplay by David Duncan...

, it is explained via a vision why Hartdegen could not save his sweetheart Emma—doing so would have resulted in his never developing the time machine he used to try and save her.
In The Saga of Darren Shan
The Saga of Darren Shan
The Saga of Darren Shan is a young adult 12 part book series written by Darren Shan about the struggle of a boy who has become involved in the world of vampires. As of October 2008, the book is published in 37 countries around the world, in 30 different languages...

, major events in the past cannot be changed, but their details can alter while providing the same outcome. Under this model, if a time traveler were to go back in time and kill Hitler
Adolf Hitler
Adolf Hitler was an Austrian-born German politician and the leader of the National Socialist German Workers Party , commonly referred to as the Nazi Party). He was Chancellor of Germany from 1933 to 1945, and head of state from 1934 to 1945...

, another Nazi would simply take his place and commit his same actions, leaving the broader course of history unchanged.
In the Doctor Who
Doctor Who
Doctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a time-travelling humanoid alien known as the Doctor who explores the universe in a sentient time machine called the TARDIS that flies through time and space, whose exterior...

episode The Waters of Mars
The Waters of Mars
"The Waters of Mars" is the second 2009 special of British science fiction television series Doctor Who, broadcast on BBC One on 15 November 2009. It aired on BBC America on 19 December 2009 and was released on DVD and Blu-ray in the UK on 11 January 2010 and in the US on 2 February 2010...

, Captain Adelaide Brooke's death on Mars is the most singular catalyst of human travel outside the solar system. At first, the Doctor realizes her death is a "fixed point in time" and does not intervene, but later defies this rule and transports her and her crew to Earth. Rather than allow human history to change, Captain Brooke commits suicide on Earth, leaving history mostly unchanged.

3. Alternate timelines. In this version of time travel, there are multiple coexisting alternate histories, so that when the traveler goes back in time, he/she ends up in a new timeline where historical events can differ from the timeline he/she came from, but her original timeline does not cease to exist (this means the grandfather paradox
Grandfather paradox
The grandfather paradox is a proposed paradox of time travel first described by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent . The paradox is this: suppose a man traveled back in time and killed his biological grandfather before the latter met the traveler's...

 can be avoided since even if the time traveler's grandfather is killed at a young age in the new timeline, he still survived to have children in the original timeline, so there is still a causal explanation for the traveler's existence). Time travel may actually create a new timeline that diverges from the original timeline at the moment the time traveler appears in the past, or the traveler may arrive in an already existing parallel universe
Parallel universe (fiction)
A parallel universe or alternative reality is a hypothetical self-contained separate reality coexisting with one's own. A specific group of parallel universes is called a "multiverse", although this term can also be used to describe the possible parallel universes that constitute reality...

 (though unless the parallel universe's history was identical to the time traveler's history up until the point where the time traveler appeared, it is questionable whether the latter version qualifies as 'time travel').

James P. Hogan's The Proteus Operation
The Proteus Operation
The Proteus Operation is a science fiction novel which was written by James P. Hogan and published in 1985. Alternate history, time travel, and parallel universes form the basis of its plot, in which a group of military commandos, diplomats, and scientists travel back to 1939...

fully explains parallel universe time travel in chapter 20 where it has Einstein explaining that all the outcomes already exist and all time travel does is change which already existing branch you will experience.

Though Star Trek has a long tradition of using the 2.1 mechanic, as seen in "The City on the Edge of Forever", "Tomorrow is Yesterday", "Time and Again", "Future's End", "Before and After
Before and After (Star Trek: Voyager)
"Before and After" is an episode of the science-fiction television series Star Trek: Voyager, and the 21st episode of the third season. It has an average fan rating of 4.1/5 on the official Star Trek website as of September 2009 and foreshadows events seen in a later episode, "Year of...

", "Endgame" and as late as Enterprise's Temporal Cold War
Temporal Cold War
The Temporal Cold War is a fictional conflict waged throughout history in the Star Trek universe, predominantly during the 22nd century AD...

, "Parallels" had an example of what Data
Data (Star Trek)
Lieutenant Commander Data is a character in the fictional Star Trek universe portrayed by actor Brent Spiner. He appears in the television series Star Trek: The Next Generation and the feature films Star Trek Generations, Star Trek: First Contact, Star Trek: Insurrection, and Star Trek...

 called "quantum realities." His exact words on the matter were "But there is a theory in quantum physics that all possibilities that can happen do happen in alternate quantum realities," suggesting the writers were thinking of the many-worlds interpretation
Many-worlds interpretation
The many-worlds interpretation is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction, but denies the actuality of wavefunction collapse. Many-worlds implies that all possible alternative histories and futures are real, each representing an...

 of quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...

.

Michael Crichton's novel Timeline
Timeline (novel)
Timeline is a science fiction novel by Michael Crichton that was published in November 1999. It tells the story of a group of history students who travel to 14th Century France to rescue their professor...

takes the approach that all time travel really is travel to an already existing parallel universe where time passes at a slower rate than our own but actions in any of these parallel universes may have already occurred in our past. It is unclear from the novel if any sizable change in events of these parallel universe can be made.

In the Homeline setting of GURPS Infinite Worlds
GURPS Infinite Worlds
GURPS Infinite Worlds is a supplement for the Fourth Edition of the GURPS role-playing game, published by Steve Jackson Games in 2005 and written by Kenneth Hite, Steve Jackson, and John M. Ford...

there are echos—parallel universes at an early part of Homeline's history but changes to their history do not affect Homeline's history. However tampering with their history can cause them to shift quanta making access harder if not impossible.

A type of story which could be placed in this category is one where the alternative version of the past lies not in some other dimension, but simply at a distant location in space or a future period of time that replicates conditions in the traveler's past. For example, in a Futurama
Futurama
Futurama is an American animated science fiction sitcom created by Matt Groening and developed by Groening and David X. Cohen for the Fox Broadcasting Company. The series follows the adventures of a late 20th-century New York City pizza delivery boy, Philip J...

 episode called The Late Philip J. Fry
The Late Philip J. Fry
"The Late Philip J. Fry" is the seventh episode of the sixth season of the animated series Futurama. It originally aired on Comedy Central on July 29, 2010. In the episode, Fry attempts to make it on time to a birthday dinner date for Leela. He is sidetracked by Professor Farnsworth and Bender,...

, the professor designed a forward-only time travel device. Trapped in the future, he and two colleagues travel forward all the way to the end of the universe, at which point they witness a new Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...

 which gives rise to a new universe whose history mirrors their own history. Then they continue to go forward until they reach the exact time of their initial departure. Although this journey is not exactly a backward time travel, the final result is the same.

In the Japanese manga, Dragon Ball Z, the character Trunks travels back in time to warn the characters of their deaths soon to come. This does not change his time line, only creates a new one in which they do not die. Soon two of the characters destroy the lab where the monster Cell is being created, stopping him from absorbing the androids, creating a third time line. Later it is revealed that Trunks is killed by Cell in the future, then travels to three years before any of the events occurs, which creates a fourth time line. No matter what any character does in the past, their own original time line is unchanged.

Immutable timelines


Time travel in a type 1 universe does not allow paradox
Paradox
Similar to Circular reasoning, A paradox is a seemingly true statement or group of statements that lead to a contradiction or a situation which seems to defy logic or intuition...

es such as the grandfather paradox
Grandfather paradox
The grandfather paradox is a proposed paradox of time travel first described by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent . The paradox is this: suppose a man traveled back in time and killed his biological grandfather before the latter met the traveler's...

 to occur, where one deduces both a conclusion and its opposite (in the case of the grandfather paradox, one can start with the premise of the time traveler killing his grandfather, and reach the conclusion that the time traveler will not be able to kill his grandfather since he was never born) though it can allow other paradoxes to occur.

In 1.1, the Novikov self-consistency principle asserts that the existence of a method of time travel constrains events to remain self-consistent. This will cause any attempt to violate such consistency to fail, even if seemingly extremely improbable events are required.
Example: You have a device that can send a single bit of information back to itself at a precise moment in time. You receive a bit at 10:00:00 p.m., then no bits for thirty seconds after that. If you send a bit back to 10:00:00 p.m., everything works fine. However, if you try to send a bit to 10:00:15 p.m. (a time at which no bit was received), your transmitter will mysteriously fail. Or your dog will distract you for fifteen seconds. Or your transmitter will appear to work, but as it turns out your receiver failed at exactly 10:00:15 p.m., etc. Examples of this kind of universe are found in Robert Forward
Robert Forward
Robert Lull Forward — known as Robert L. Forward — was an American physicist and science fiction writer...

's novel Timemaster, the Twilight Zone
Twilight zone
-Television series and spinoffs:*The Twilight Zone, the anthology television series and its franchise:**The Twilight Zone , the 1959–1964 original television series***Twilight Zone: The Movie, a 1983 film based on the original series...

episode "No Time Like the Past
No Time Like the Past
"No Time Like the Past" is an episode of the American television anthology series The Twilight Zone.-Synopsis:Paul Driscoll uses a time machine with the noble intention to go back in time and alter past events...

", and the 1980 Jeannot Szwarc
Jeannot Szwarc
Jeannot Szwarc is a French Film/TV Director.Szwarc was born in Paris. He began working as a director in American television during the 1960s, in particular on Ironside...

 film Somewhere In Time
Somewhere in Time (film)
Somewhere in Time is a 1980 romantic science fiction film directed by Jeannot Szwarc. It is a film adaptation of the 1975 novel Bid Time Return by Richard Matheson, who also wrote the screenplay...

(based on Richard Matheson
Richard Matheson
Richard Burton Matheson is an American author and screenwriter, primarily in the fantasy, horror, and science fiction genres. He is perhaps best known as the author of What Dreams May Come, Bid Time Return, A Stir of Echoes, The Incredible Shrinking Man, and I Am Legend, all of which have been...

's novel Bid Time Return
Bid Time Return
Bid Time Return is a 1975 science fiction novel by Richard Matheson. It concerns a man from the 1970s who travels back in time to court a 19th century stage actress whose photograph has captivated him...

).


In 1.2, time travel is constrained to prevent paradox. How this occurs is dependent on whether interaction with the past is possible.

If interaction with the past is possible and one attempts to make a paradox, one undergoes involuntary or uncontrolled time travel. In the time-travel stories of Connie Willis
Connie Willis
Constance Elaine Trimmer Willis is an American science fiction writer. She has won eleven Hugo Awards and seven Nebula Awards. Willis most recently won a Hugo Award for Blackout/All Clear...

, time travelers encounter "slippage" which prevents them from either reaching the intended time or translates them a sufficient distance from their destination at the intended time, as to prevent any paradox from occurring.
Example: A man who travels into the past with intentions to kill Hitler finds himself on a Montana farm in late April 1945.


In the The Dancers at the End of Time
The Dancers at the End of Time
The title of this volume comes from the poem "The Last Word" by Ernest Dowson.Reunited at the end of Time, Jherek and the other inhabitants of the End of Time have returned to their preferred amusements of parties and games. They are interrupted by a ship of alien musician/pirates, the Lat...

 series, Michael Moorcock
Michael Moorcock
Michael John Moorcock is an English writer, primarily of science fiction and fantasy, who has also published a number of literary novels....

 invented a plot device called the Morphail Effect. This causes a time traveler to be ejected from the time in which he or she is about to cause a paradox.
Example 1: a man from the End of Time period travels to the past and is executed. Instead of dying (which would cause a paradox), he experiences a return to the End of Time
Example 2: time travelers sometimes visit the End of Time from their own epochs in the past. Those that attempt to return to their own period are likely to reappear inadvertently at the End of Time.


The general consequences are that time travel to the traveler's past is difficult, and many time travelers find themselves adventuring deeper and deeper into their future.

If interaction with the past is not possible then the traveler simply becomes an invisible insubstantial phantom unable to interact with the past as in the case of James Harrigan in Michael Garrett's "Brief Encounter".

While a Type 1 universe will prevent a grandfather paradox
Grandfather paradox
The grandfather paradox is a proposed paradox of time travel first described by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent . The paradox is this: suppose a man traveled back in time and killed his biological grandfather before the latter met the traveler's...

 it doesn't prevent paradoxes in other aspects of physics such as the predestination paradox
Predestination paradox
A predestination paradox is a paradox of time travel that is often used as a convention in science fiction. It exists when a time traveller is caught in a loop of events that "predestines" or "predates" them to travel back in time...

 and the ontological paradox (GURPS Infinite Worlds
GURPS Infinite Worlds
GURPS Infinite Worlds is a supplement for the Fourth Edition of the GURPS role-playing game, published by Steve Jackson Games in 2005 and written by Kenneth Hite, Steve Jackson, and John M. Ford...

calls this "Free Lunch Paradox").

The predestination paradox
Predestination paradox
A predestination paradox is a paradox of time travel that is often used as a convention in science fiction. It exists when a time traveller is caught in a loop of events that "predestines" or "predates" them to travel back in time...

 is where the traveler's actions create some type of causal loop, in which some event A in the future helps cause event B in the past via time travel, and the event B in turn is one of the causes of A. For instance, a time traveler might go back to investigate a specific historical event like the Great Fire of London
Great Fire of London
The Great Fire of London was a major conflagration that swept through the central parts of the English city of London, from Sunday, 2 September to Wednesday, 5 September 1666. The fire gutted the medieval City of London inside the old Roman City Wall...

, and their actions in the past could then inadvertently end up being the original cause of that very event.

Examples of this kind of causal loop are found in Robert Forward
Robert Forward
Robert Lull Forward — known as Robert L. Forward — was an American physicist and science fiction writer...

's novel Timemaster, the Twilight Zone
Twilight zone
-Television series and spinoffs:*The Twilight Zone, the anthology television series and its franchise:**The Twilight Zone , the 1959–1964 original television series***Twilight Zone: The Movie, a 1983 film based on the original series...

episode "No Time Like the Past
No Time Like the Past
"No Time Like the Past" is an episode of the American television anthology series The Twilight Zone.-Synopsis:Paul Driscoll uses a time machine with the noble intention to go back in time and alter past events...

", EC Comics
EC Comics
Entertaining Comics, more commonly known as EC Comics, was an American publisher of comic books specializing in horror fiction, crime fiction, satire, military fiction and science fiction from the 1940s through the mid-1950s, notably the Tales from the Crypt series...

 stories like "Man who was Killed in Time" (Weird Science #5), "Why Papa Left Home" (Weird Science #11), "Only Time will Tell" (Weird Fantasy #1), "The Connection" (Weird Fantasy #9), "Skeleton Key" (Weird Fantasy #16), and "Counter Clockwise" (Weird Fantasy #18), the 1980 Jeannot Szwarc
Jeannot Szwarc
Jeannot Szwarc is a French Film/TV Director.Szwarc was born in Paris. He began working as a director in American television during the 1960s, in particular on Ironside...

 film Somewhere In Time
Somewhere in Time (film)
Somewhere in Time is a 1980 romantic science fiction film directed by Jeannot Szwarc. It is a film adaptation of the 1975 novel Bid Time Return by Richard Matheson, who also wrote the screenplay...

(based on Richard Matheson
Richard Matheson
Richard Burton Matheson is an American author and screenwriter, primarily in the fantasy, horror, and science fiction genres. He is perhaps best known as the author of What Dreams May Come, Bid Time Return, A Stir of Echoes, The Incredible Shrinking Man, and I Am Legend, all of which have been...

's novel Bid Time Return
Bid Time Return
Bid Time Return is a 1975 science fiction novel by Richard Matheson. It concerns a man from the 1970s who travels back in time to court a 19th century stage actress whose photograph has captivated him...

), the Michael Moorcock
Michael Moorcock
Michael John Moorcock is an English writer, primarily of science fiction and fantasy, who has also published a number of literary novels....

 novel Behold the Man
Behold the Man
Behold the Man is a science fiction novel by Michael Moorcock. It originally appeared as a novella in a 1966 issue of New Worlds; later, Moorcock produced an expanded version which was first published in 1969 by Allison & Busby.. The title derives from the Gospel of John, Chapter 19, Verse 5:...

, and Harry Potter and the Prisoner of Azkaban
Harry Potter and the Prisoner of Azkaban
Harry Potter and the Prisoner of Azkaban is the third novel in the Harry Potter series written by J. K. Rowling. The book was published on 8 July 1999. The novel won the 1999 Whitbread Book Award, the Bram Stoker Award, the 2000 Locus Award for Best Fantasy Novel, and was short-listed for other...

. It is also featured in 1972's Doctor Who
Doctor Who
Doctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a time-travelling humanoid alien known as the Doctor who explores the universe in a sentient time machine called the TARDIS that flies through time and space, whose exterior...

, in the three part The Day of the Daleks, where three freedom fighters from the future attempt to kill a British diplomat they believe responsible for World War Three, and the subsequent easy conquest of Earth by the Daleks. In the future they were taught an explosion at the diplomat's (Sir Reginald Styles) mansion with foreign delegates inside caused the nations of the world to attack each other. The Doctor (Jon Pertwee
Jon Pertwee
John Devon Roland Pertwee , was an English actor. Pertwee is best known for his role in the BBC science fiction television series Doctor Who, in which he played the third incarnation of the Doctor from 1970 to 1974, and as the title character in the series Worzel Gummidge...

), figures out that they caused the explosion all along by way of a temporal paradox.

In the 2006 crime thriller Déjà Vu there appears to be causal loops, as Agent Doug Carlin decides to send a message back in time to save his partner's life, but this will eventually cause his death. Later in the movie, tough, Carlin is able to change events and create an alternate reality. This apparent paradox can be explained by multiple previous unseen time travels in a type 3 universe.

In the videogame Escape from Monkey Island
Escape from Monkey Island
Escape from Monkey Island is a computer adventure game developed and released by LucasArts in 2000. It is the fourth game in the Monkey Island series....

there's a section in which the player, controlling Guybrush Threepwood
Guybrush Threepwood
Guybrush Ulysses Threepwood is the main character of the Monkey Island series of computer adventure games by LucasArts. The voice of Guybrush is actor Dominic Armato in the third, fourth and fifth games, as well as the enhanced remakes of The Secret of Monkey Island and Monkey Island 2: LeChuck's...

, gets some items from his future self in the Swamp of Time. Soon after that, he will become the future Guybrush and will have to give the items to his past self in the same order. This is an example of causal loop because those items were created purely from the time travel. Anyway if the player doesn't repeat every action properly, will cause a paradox that sends Guybrush back to the entrance of the swamp, implying a type 1.2 universe.
The Novikov self-consistency principle can also result in an ontological paradox (also known as the knowledge or information paradox) where the very existence of some object or information is a time loop
Time loop
A time loop or temporal loop is a common plot device in science fiction in which time runs normally for a set period but then skips back like a broken record. When the time loop "resets", the memories of most characters are reset...

. GURPS Infinite Worlds
GURPS Infinite Worlds
GURPS Infinite Worlds is a supplement for the Fourth Edition of the GURPS role-playing game, published by Steve Jackson Games in 2005 and written by Kenneth Hite, Steve Jackson, and John M. Ford...

gives the example (from The Eyre Affair
The Eyre Affair
The Eyre Affair is the first published novel by English author Jasper Fforde, released by Hodder and Stoughton in 2001. It takes place in alternative 1985, where literary detective Thursday Next pursues a master criminal through the world of Charlotte Brontë's Jane Eyre.-Plot summary:In a parallel...

) of a time traveler going to Shakespeare's time with a book of all his works. Shakespeare pressed for time simply copies the information in the book from the future. The paradox is that nobody actually writes the plays.

The philosopher Kelley L. Ross argues in "Time Travel Paradoxes" that in an ontological paradox scenario involving a physical object, there can be a violation of the second law of thermodynamics
Second law of thermodynamics
The second law of thermodynamics is an expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system. From the state of thermodynamic equilibrium, the law deduced the principle of the increase of entropy and...

. Ross uses Somewhere in Time as an example where Jane Seymour's character gives Christopher Reeve's character a watch she has owned for many years, and when he travels back in time he gives the same watch to Jane Seymour's character 60 years in the past. As Ross states


"The watch is an impossible object. It violates the Second Law of Thermodynamics, the Law of Entropy. If time travel makes that watch possible, then time travel itself is impossible. The watch, indeed, must be absolutely identical to itself in the 19th and 20th centuries, since Reeve carries it with him from the future instantaneously into the past and bestows it on Seymour. The watch, however, cannot be identical to itself, since all the years in which it is in the possession of Seymour and then Reeve it will wear in the normal manner. It's entropy will increase. The watch carried back by Reeve will be more worn that the watch that would have been acquired by Seymour."


On the other hand, the second law of thermodynamics is understood by modern physicists to be a statistical law rather than an absolute one, so spontaneous reversals of entropy or failure to increase in entropy are not impossible, just improbable (see for example the fluctuation theorem
Fluctuation theorem
The fluctuation theorem , which originated from statistical mechanics, deals with the relative probability that the entropy of a system which is currently away from thermodynamic equilibrium will increase or decrease over a given amount of time...

). In addition, the second law of thermodynamics only states that entropy should increase in systems which are isolated from interactions with the external world, so Igor Novikov
Igor Dmitriyevich Novikov
Igor Dmitriyevich Novikov is a Russian theoretical astrophysicist and cosmologist.Novikov formulated the Novikov self-consistency principle in the mid-1980s, an important contribution to the theory of time travel.Novikov gained his Ph.D. degree in astrophysics in 1965 and the Russian D.Sc....

 (creator of the Novikov self-consistency principle) has argued that in the case of macroscopic objects like the watch whose worldlines form closed loops, the outside world can expend energy to repair wear/entropy that the object acquires over the course of its history, so that it will be back in its original condition when it closes the loop.

Mutable timelines


Time travel in a Type 2 universe is much more complex. The biggest problem is how to explain changes in the past. One method of explanation is that once the past changes, so do the memories of all observers. This would mean that no observer would ever observe the changing of the past (because they will not remember changing the past). This would make it hard to tell whether you are in a Type 1 universe or a Type 2 universe. You could, however, infer such information by knowing if a) communication with the past were possible or b) it appeared that the time line had never been changed as a result of an action someone remembers taking, although evidence exists that other people are changing their time lines fairly often.

An example of this kind of universe is presented in Thrice Upon a Time
Thrice Upon a Time
Thrice Upon A Time is a science fiction novel by James P. Hogan, first published in 1980. Unlike most other time travel stories, Thrice Upon A Time considers the ramifications of sending messages into the past and/or receiving messages from the future, rather than the sending of physical objects...

, a novel by James P. Hogan
James P. Hogan (writer)
James Patrick Hogan was a British science fiction author.-Biography:Hogan was born in London, England. He was raised in the Portobello Road area on the west side of London...

. The Back to the Future trilogy
Back to the Future trilogy
The Back to the Future trilogy is a comedic science fiction adventure film series written by Bob Gale and Robert Zemeckis, directed by Zemeckis, produced by Amblin Entertainment and distributed by Universal Pictures. The main plot follows the adventures of a high school student Marty McFly and...

 films also seem to feature a single mutable timeline (see the "Back to the Future FAQ" for details on how the writers imagined time travel worked in the movies' world). By contrast, the short story "Brooklyn Project" by William Tenn
William Tenn
William Tenn was the pseudonym of Philip Klass , a British-born American science fiction author, notable for many stories with satirical elements.-Early life:...

 provides a sketch of life in a Type 2 world where no one even notices as the timeline changes repeatedly.

In type 2.1, attempts are being made at changing the timeline, however, all that is accomplished in the first tries is that the method in which decisive events occur is changed; final conclusions in the bigger scheme cannot be brought to a different outcome.

As an example, the movie Déjà Vu depicts a paper note sent to the past with vital information to prevent a terrorist attack. However, the vital information results in the killing of an ATF agent, but does not prevent the terrorist attack; the very same agent died in the previous version of the timeline as well, albeit under different circumstances. Finally, the timeline is changed by sending a human into the past, arguably a "stronger" measure than simply sending back a paper note, which results in preventing both a murder and the terrorist attack. As in the Back to the Future movie trilogy
Back to the Future trilogy
The Back to the Future trilogy is a comedic science fiction adventure film series written by Bob Gale and Robert Zemeckis, directed by Zemeckis, produced by Amblin Entertainment and distributed by Universal Pictures. The main plot follows the adventures of a high school student Marty McFly and...

, there seems to be a ripple effect
Ripple effect
The ripple effect is a term used to describe a situation where, like the ever expanding ripples across water when an object is dropped into it, an effect from an initial state can be followed outwards incrementally....

 too as changes from the past "propagate" into the present, and people in the present have altered memory of events that occurred after the changes made to the timeline.

The science fiction writer Larry Niven
Larry Niven
Laurence van Cott Niven / ˈlæri ˈnɪvən/ is an American science fiction author. His best-known work is Ringworld , which received Hugo, Locus, Ditmar, and Nebula awards. His work is primarily hard science fiction, using big science concepts and theoretical physics...

 suggests in his essay "The Theory and Practice of Time Travel" that in a type 2.1 universe, the most efficient way for the universe to "correct" a change is for time travel to never be discovered, and that in a type 2.2 universe, the very large (or infinite) number of time travelers from the endless future will cause the timeline to change wildly until it reaches a history in which time travel is never discovered. However, many other "stable" situations might also exist in which time travel occurs but no paradoxes are created; if the changeable-timeline universe finds itself in such a state no further changes will occur, and to the inhabitants of the universe it will appear identical to the type 1.1 scenario. This is sometimes referred to as the "Time Dilution Effect".

Few if any physicists or philosophers have taken seriously the possibility of "changing" the past except in the case of multiple universes, and in fact many have argued that this idea is logically incoherent, so the mutable timeline idea is rarely considered outside of science fiction.

Also, deciding whether a given universe is of Type 2.1 or 2.2 can not be done objectively, as the categorization of timeline-invasive measures as "strong" or "weak" is arbitrary, and up to interpretation: An observer can disagree about a measure being "weak", and might, in the lack of context, argue instead that simply a mishap occurred which then led to no effective change.

An example would be the paper note sent back to the past in the film Déjà Vu, as described above. Was it a "too weak" change, or was it just a local-time alteration which had no extended effect on the larger timeline? As the universe in Déjà Vu seems not entirely immune to paradoxes (some arguably minute paradoxes do occur), both versions seem to be equally possible.

Alternate histories


In Type 3, any event that appears to have caused a paradox has instead created a new time line. The old time line remains unchanged, with the time traveler or information sent simply having vanished, never to return. A difficulty with this explanation, however, is that conservation of mass-energy would be violated for the origin timeline and the destination timeline. A possible solution to this is to have the mechanics of time travel require that mass-energy be exchanged in precise balance between past and future at the moment of travel, or to simply expand the scope of the conservation law to encompass all timelines. Some examples of this kind of time travel can be found in David Gerrold
David Gerrold
Jerrold David Friedman , better known by his pen name David Gerrold, is an American science fiction author who started his career in 1966 while a college student by submitting an unsolicited story outline for the television series Star Trek. He was invited to submit several premises, and the one...

's book The Man Who Folded Himself
The Man Who Folded Himself
The Man Who Folded Himself is a 1973 science fiction novel by David Gerrold that deals with time travel. It was nominated for the Nebula Award for Best Novel in 1973 and the Hugo Award for Best Novel in 1974...

and The Time Ships
The Time Ships
The Time Ships is a 1995 science fiction novel by Stephen Baxter. A sequel to The Time Machine by H. G. Wells, it was officially authorized by the Wells estate to mark the centenary of the original's publication. It won the John W. Campbell Memorial Award and the Philip K. Dick Award in 1996, as...

by Stephen Baxter
Stephen Baxter
Stephen Baxter is a prolific British hard science fiction author. He has degrees in mathematics and engineering.- Writing style :...

, plus several episodes of the TV show Star Trek: The Next Generation
Star Trek: The Next Generation
Star Trek: The Next Generation is an American science fiction television series created by Gene Roddenberry as part of the Star Trek franchise. Roddenberry, Rick Berman, and Michael Piller served as executive producers at different times throughout the production...

and the android saga in the Japanese TV series Dragon Ball Z.

Gradual and instantaneous


In literature, there are two methods of time travel:
  1. The most commonly used method of time travel in science fiction is the instantaneous movement from one point in time to another, like using the controls on a CD player to skip to a previous or next song, though in most cases, there is a machine of some sort, and some energy expended in order to make this happen (like the time-traveling De Lorean
    De Lorean DMC-12
    The DeLorean DMC-12 is a sports car originally manufactured in Dunmurry, a suburb west of Belfast, Northern Ireland by John DeLorean's DeLorean Motor Company for the American market in 1981-82. Most commonly known as the DeLorean, it was the only model produced by the company which would go into...

     in Back to the Future
    Back to the Future
    Back to the Future is a 1985 American science-fiction adventure film. It was directed by Robert Zemeckis, written by Zemeckis and Bob Gale, produced by Steven Spielberg, and starred Michael J. Fox, Christopher Lloyd, Lea Thompson, Crispin Glover and Thomas F. Wilson. The film tells the story of...

    or the TARDIS
    TARDIS
    The TARDISGenerally, TARDIS is written in all upper case letters—this convention was popularised by the Target novelisations of the 1970s...

     (Time and Relative Dimension in Space) that traveled through time in Doctor Who
    Doctor Who
    Doctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a time-travelling humanoid alien known as the Doctor who explores the universe in a sentient time machine called the TARDIS that flies through time and space, whose exterior...

    ). In some cases, there is not even the beginning of a scientific explanation for this kind of time travel; it's popular probably because it is more spectacular and makes time travel easier. The "Universal Remote" used by Adam Sandler
    Adam Sandler
    Adam Richard Sandler is an American actor, comedian, screenwriter, musician, and film producer.After becoming a Saturday Night Live cast member, Sandler went on to star in several Hollywood feature films that grossed over $100 million at the box office...

     in the movie Click works in the same manner, although only in one direction, the future. While his character Michael Newman can travel back to a previous point it is merely a playback with which he cannot interact.
  2. In The Time Machine
    The Time Machine
    The Time Machine is a science fiction novella by H. G. Wells, published in 1895 for the first time and later adapted into at least two feature films of the same name, as well as two television versions, and a large number of comic book adaptations. It indirectly inspired many more works of fiction...

    , H.G. Wells explains that we are moving through time with a constant speed. Time travel then is, in Wells' words, "stopping or accelerating one's drift along the time-dimension, or even turning about and traveling the other way." George Pal
    George Pál
    George Pal , born György Pál Marczincsak, was a Hungarian-born American animator and film producer, principally associated with the science fiction genre...

    , director of the 1960 adaptation
    The Time Machine (1960 film)
    The Time Machine is a 1960 American science fiction film based on the 1895 novel of the same name by H. G. Wells in which a man in Victorian England constructs a time-travelling machine which he uses to travel to the future...

     based on Wells's classic, accordingly chose to depict time travel by employing time-lapse photography
    Time-lapse
    Time-lapse photography is a cinematography technique whereby the frequency at which film frames are captured is much lower than that which will be used to play the sequence back. When replayed at normal speed, time appears to be moving faster and thus lapsing...

    . To expand on the audio playback analogy used above, this would be like rewinding or fast forwarding an analogue audio cassette and playing the tape at a chosen point. Perhaps the oldest example of this method of time travel is in Lewis Carroll
    Lewis Carroll
    Charles Lutwidge Dodgson , better known by the pseudonym Lewis Carroll , was an English author, mathematician, logician, Anglican deacon and photographer. His most famous writings are Alice's Adventures in Wonderland and its sequel Through the Looking-Glass, as well as the poems "The Hunting of the...

    's Through the Looking-Glass
    Through the Looking-Glass
    Through the Looking-Glass, and What Alice Found There is a work of literature by Lewis Carroll . It is the sequel to Alice's Adventures in Wonderland...

    (1871): the White Queen
    White Queen (Through the Looking Glass)
    The White Queen is a fictional character who appears in Lewis Carroll's fantasy novella Through the Looking-Glass.-Plot:Along with her husband the White King, she is one of the first characters to be seen in the story. She first appears in the drawing room just beyond the titular looking-glass as...

     is living backwards, hence her memory is working both ways. Her kind of time travel is uncontrolled: she moves through time with a constant speed of −1 and she cannot change it. T.H. White, in the first part of his Arthurian novel The Once and Future King
    The Once and Future King
    The Once and Future King is an Arthurian fantasy novel written by T. H. White. It was first published in 1958 and is mostly a composite of earlier works written in a period between 1938 and 1941....

    , The Sword in the Stone
    The Sword in the Stone
    The Sword in the Stone is a novel by T. H. White, published in 1939, initially a stand-alone work but now the first part of a tetralogy The Once and Future King. A fantasy of the boyhood of King Arthur, it is a sui generis work which combines elements of legend, history, fantasy and comedy...

    (1938) used the same idea: the wizard Merlyn lives backward in time, because he was born "at the wrong end of time" and has to live backwards from the front. "Some people call it having second sight", he says. This method of gradual time travel is not as popular in modern science fiction, though a form of it does occur in the film Primer
    Primer (film)
    Primer is a 2004 American science fiction drama film about the accidental discovery of a means of time travel. The film was written, directed, and produced by Shane Carruth and was completed on a budget of $7,000...

    .

Time travel or spacetime travel


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 the exact 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
Theory of relativity
The theory of relativity, or simply relativity, encompasses two theories of Albert Einstein: special relativity and general relativity. However, the word relativity is sometimes used in reference to Galilean invariance....

 rejects the idea of absolute time and space
Absolute time and space
Originally introduced by Sir Isaac Newton in the Philosophiæ Naturalis Principia Mathematica, the concepts of absolute time and space provided a theoretical foundation that facilitated Newtonian mechanics...

; 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
Special relativity
Special relativity is the physical theory of measurement in an inertial frame of reference proposed in 1905 by Albert Einstein in the paper "On the Electrodynamics of Moving Bodies".It generalizes Galileo's...

, which deals with situations where gravity is negligible, the laws of physics work the same way in every inertial frame of reference
Inertial frame of reference
In physics, an inertial frame of reference is a frame of reference that describes time homogeneously and space homogeneously, isotropically, and in a time-independent manner.All inertial frames are in a state of constant, rectilinear motion with respect to one another; they are not...

 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
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...

, which incorporates the effects of gravity, all coordinate systems are on equal footing because of a feature known as "diffeomorphism invariance".

Nevertheless, the idea that the Earth moves away from the time traveler when he takes a trip through time has been used in a few science fiction stories, such as the 2000 AD
2000 AD (comic)
2000 AD is a weekly British science fiction-oriented comic. As a comics anthology it serialises a number of separate stories each issue and was first published by IPC Magazines in 1977, the first issue dated 26 February. IPC then shifted the title to its Fleetway comics subsidiary which was sold...

 comic Strontium Dog, in which Johnny Alpha uses "Time Bombs" to propel an enemy several seconds into the future, during which time the movement of the Earth causes the unfortunate victim to re-appear in space. Much earlier, Clark Ashton Smith
Clark Ashton Smith
Clark Ashton Smith was a self-educated American poet, sculptor, painter and author of fantasy, horror and science fiction short stories. He achieved early local recognition, largely through the enthusiasm of George Sterling, for traditional verse in the vein of Swinburne...

 used this form of time travel in several stories such as "The Letter from Mohaun Los" (1932) where the protagonist ends up on a planet millions of years in the future which "happened to occupy the same space through which Earth had passed". Other science fiction stories try to anticipate this objection and offer a rationale for the fact that the traveler remains on Earth, such as the 1957 Robert Heinlein novel The Door into Summer
The Door into Summer
The Door into Summer is a science fiction novel by Robert A. Heinlein, originally serialized in The Magazine of Fantasy & Science Fiction and published in hardcover in 1957. It is a fast-paced hard science fiction novel, with a key fantastic element, and romantic elements...

where Heinlein essentially handwaved
Handwaving
Handwaving is a pejorative label applied to the action of displaying the appearance of doing something, when actually doing little, or nothing. For example, it is applied to debate techniques that involve logical fallacies. It is also used in working situations where productive work is expected,...

 the issue with a single sentence: "You stay on the world line
World line
In physics, the world line of an object is the unique path of that object as it travels through 4-dimensional spacetime. The concept of "world line" is distinguished from the concept of "orbit" or "trajectory" by the time dimension, and typically encompasses a large area of spacetime wherein...

 you were on." In his 1980 novel The Number of the Beast
The Number of the Beast (novel)
The Number of the Beast is a science fiction novel by Robert A. Heinlein published in 1980. The first edition featured a cover and interior illustrations by Richard M. Powers...

a "continua device" allows the protagonists to dial in the coordinates of space and time and it instantly moves them there—without explaining how such a device might work.

The television series Seven Days also dealt with this problem; when the chrononaut would be 'rewinding', he would also be propelling himself backwards around the Earth's orbit, with the intention of landing at some chosen spatial location, though seldom hitting the mark precisely. In Piers Anthony
Piers Anthony
Piers Anthony Dillingham Jacob is an English American writer in the science fiction and fantasy genres, publishing under the name Piers Anthony. He is most famous for his long-running novel series set in the fictional realm of Xanth.Many of his books have appeared on the New York Times Best...

's Bearing an Hourglass
Incarnations of Immortality
Incarnations of Immortality is the name of an eight-book fantasy series by Piers Anthony. The first seven books each focus on one of seven supernatural "offices" in a fictional reality and history parallel to ours, with the exception that society has advanced both magic and modern technology...

, the potent Hourglass of the Incarnation of Time naturally moves the Incarnation in space according to the numerous movements of the globe through the solar system, the solar system through the galaxy, etc.; but by carefully negating some of the movements he can also travel in space within the limits of the planet. The television series Doctor Who
Doctor Who
Doctor Who is a British science fiction television programme produced by the BBC. The programme depicts the adventures of a time-travelling humanoid alien known as the Doctor who explores the universe in a sentient time machine called the TARDIS that flies through time and space, whose exterior...

avoided this issue by establishing early on in the series that the Doctor's TARDIS
TARDIS
The TARDISGenerally, TARDIS is written in all upper case letters—this convention was popularised by the Target novelisations of the 1970s...

 is able to move about in space in addition to traveling in time.

Speculations

  • Grandfather paradox
    Grandfather paradox
    The grandfather paradox is a proposed paradox of time travel first described by the science fiction writer René Barjavel in his 1943 book Le Voyageur Imprudent . The paradox is this: suppose a man traveled back in time and killed his biological grandfather before the latter met the traveler's...

  • Krasnikov tube
    Krasnikov Tube
    A Krasnikov tube is a speculative mechanism for space travel involving the warping of spacetime into permanent superluminal tunnels. The resulting structure is analogous to a wormhole with the endpoints displaced in time as well as space...

  • Ronald Mallett
    Ronald Mallett
    Ronald Lawrence Mallett is an American theoretical physicist, academic, and author. He has taught physics at the University of Connecticut since 1975. He is best known for his scientific position on the possibility of time travel....

  • Ontological paradox
  • Predestination paradox
    Predestination paradox
    A predestination paradox is a paradox of time travel that is often used as a convention in science fiction. It exists when a time traveller is caught in a loop of events that "predestines" or "predates" them to travel back in time...

  • Ring singularity
    Ring singularity
    Ring singularity is a term used in general relativity to describe the altering gravitational singularity of a rotating black hole, or a Kerr black hole, so that the gravitational singularity becomes shaped like a ring.-Description of a ring-singularity:...

  • Retrocausality
    Retrocausality
    Retrocausality is any of several hypothetical phenomena or processes that reverse causality, allowing an effect to occur before its cause....

  • Temporal paradox
    Temporal paradox
    Temporal paradox is a theoretical paradoxical situation that happens because of time travel. A time traveler goes to the past, and does something that would prevent him from time travel in the first place...

  • Tipler Cylinder
    Tipler Cylinder
    A Tipler cylinder is a cylinder of dense matter of infinite length, rotating about its longitudinal axis. This hypothetical object is theorized to allow time travel and is also called a Tipler time machine....

  • Wheeler-Feynman time-symmetric theory


Claims of time travel

  • Chronovisor
  • Billy Meier
    Billy Meier
    "Billy" Eduard Albert Meier is a citizen of Switzerland who claims to be a UFO contactee and prophet. He is also the source of many controversial UFO photographs, which he states are evidence of his encounters...

  • Moberly-Jourdain incident
    Moberly-Jourdain incident
    The Moberly–Jourdain incident, or the Ghosts of Petit Trianon or Versailles was an event that occurred on 10 August 1901 in the gardens of the Petit Trianon, involving two female academics, Charlotte Anne Moberly and Eleanor Jourdain...

  • Montauk Project
    Montauk Project
    The Montauk Project was alleged to be a series of secret United States government projects conducted at Camp Hero or Montauk Air Force Station on Montauk, Long Island for the purpose of developing psychological warfare techniques and exotic research including time travel...

  • Philadelphia Experiment
    Philadelphia Experiment
    The Philadelphia Experiment is the name of an alleged naval military experiment said to have been carried out at the Philadelphia Naval Shipyard in Philadelphia, Pennsylvania, USA sometime around October 28, 1943. It is alleged that the U.S. Navy destroyer escort USS Eldridge was to be rendered...

  • Time slip
    Time slip
    A time slip is an alleged paranormal phenomenon in which a person, or group of people, travel through time via unknown means...

  • John Titor
    John Titor
    John Titor is the name used on several bulletin boards during 2000 and 2001 by a poster claiming to be a time traveler from the year 2036. In these posts he made numerous predictions about events in the near future, starting with events in 2004...

  • Time travel urban legends
    Time travel urban legends
    Time travel urban legends are accounts of persons who allegedly traveled through time, reported by the press or circulated on the Internet. All of these reports have turned out either to be hoaxes or to be based on incorrect assumptions, incomplete information, or interpretation of fiction as...



Fiction, humor

  • Chronodynamics
    Chronodynamics
    Chronodynamics is an invented system of time travel found in the fiction of James Boudreaux II. By adhering to the principle of self-consistency, put forth by Igor D. Novikov, Chronodynamics offers a reasonable and logical alternative to paradoxes....

  • Thiotimoline
    Thiotimoline
    Thiotimoline is a fictitious chemical compound conceived by science fiction author Isaac Asimov and first described in a spoof scientific paper titled "The Endochronic Properties of Resublimated Thiotimoline" in 1948...

  • Time loop
    Time loop
    A time loop or temporal loop is a common plot device in science fiction in which time runs normally for a set period but then skips back like a broken record. When the time loop "resets", the memories of most characters are reset...

  • Time travel in fiction
    Time travel in fiction
    Time travel is a common theme in science fiction and is depicted in a variety of media. It simply means either going forward in time or backward, to experience the future, or the past.-Literature:...



External links