Time travel is the concept of movement between certain points in
time
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, to ...
,
analogous
Analogy (from Greek ''analogia'', "proportion", from ''ana-'' "upon, according to" lso "against", "anew"+ ''logos'' "ratio" lso "word, speech, reckoning" is a cognitive process of transferring information or meaning from a particular subject ( ...
to movement between different points in
space
Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consider ...
by an object or a person, typically with the use of a hypothetical device known as a time machine. Time travel is a widely recognized concept in
philosophy
Philosophy (from , ) is the systematized study of general and fundamental questions, such as those about existence, reason, knowledge, values, mind, and language. Such questions are often posed as problems to be studied or resolved. Some ...
and
fiction
Fiction is any creative work, chiefly any narrative work, portraying individuals, events, or places that are imaginary, or in ways that are imaginary. Fictional portrayals are thus inconsistent with history, fact, or plausibility. In a traditi ...
, particularly
science fiction
Science fiction (sometimes shortened to Sci-Fi or SF) is a genre of speculative fiction which typically deals with imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, parallel unive ...
The Time Machine
''The Time Machine'' is a science fiction novella by H. G. Wells, published in 1895. The work is generally credited with the popularization of the concept of time travel by using a vehicle or device to travel purposely and selectively for ...
''.
It is uncertain if time travel to the past is physically possible, and such travel, if at all feasible, may give rise to questions of
causality
Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the cau ...
. Forward time travel, outside the usual sense of the perception of time, is an extensively observed phenomenon and well-understood within the framework of
special relativity
In physics, the special theory of relativity, or special relativity for short, is a scientific theory regarding the relationship between space and time. In Albert Einstein's original treatment, the theory is based on two postulates:
# The laws o ...
and
general relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
. However, making one body advance or delay more than a few milliseconds compared to another body is not feasible with current technology. As for backward time travel, it is possible to find solutions in general relativity that allow for it, such as a rotating
black hole
A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...
. Traveling to an arbitrary point in spacetime has very limited support in
theoretical physics
Theoretical physics is a branch of physics that employs mathematical models and abstractions of physical objects and systems to rationalize, explain and predict natural phenomena. This is in contrast to experimental physics, which uses experim ...
, and is usually connected only with
quantum mechanics
Quantum mechanics is a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...
or
wormhole
A wormhole (Einstein-Rosen bridge) is a hypothetical structure connecting disparate points in spacetime, and is based on a special Solutions of the Einstein field equations, solution of the Einstein field equations.
A wormhole can be visualize ...
Vishnu Purana
The Vishnu Purana (IAST:, sa, विष्णुपुराण) is one of the eighteen Puranas#Mahapuranas, Mahapuranas, a genre of ancient and medieval texts of Hinduism. It is an important Pancharatra text in the Vaishnavism literature c ...
'' mentions the story of King Raivata
Kakudmi
Kakudmi (nominative case, stem Kakudmin), sometimes also called Raivata, is a character within Hindu Mythology. Kakudmi was the King of Kusasthali, son of Revata, and father of Revati, who married Balarama. Within Hinduism, his account is giv ...
, who travels to heaven to meet the creator
Brahma
Brahma ( sa, ब्रह्मा, Brahmā) is a Hindu god, referred to as "the Creator" within the Trimurti, the trinity of supreme divinity that includes Vishnu, and Shiva.Jan Gonda (1969)The Hindu Trinity Anthropos, Bd 63/64, H 1/2, pp. 21 ...
and is surprised to learn when he returns to Earth that many ages have passed. The Buddhist
Pāli Canon
The Pāli Canon is the standard collection of scriptures in the Theravada Buddhist tradition, as preserved in the Pāli language. It is the most complete extant early Buddhist canon. It derives mainly from the Tamrashatiya school.
During th ...
mentions the relativity of time. The
Payasi Pāyāsi or Paesi is a legendary king mentioned in the ancient Buddhist and Jain texts of India. In these legends, he appears as a materialist who orders several experiments to conclude that the soul does not exist. A Jain or Buddhist monk proves hi ...
Sutta tells of one of the
Buddha
Siddhartha Gautama, most commonly referred to as the Buddha, was a śramaṇa, wandering ascetic and religious teacher who lived in South Asia during the 6th or 5th century BCE and founded Buddhism.
According to Buddhist tradition, he was ...
's chief disciples, Kumara Kassapa, who explains to the skeptic Payasi that time in the Heavens passes differently than on Earth. The Japanese tale of "
Urashima Tarō
is the protagonist of a Japanese fairy tale (''otogi banashi''), who in a typical modern version is a fisherman rewarded for rescuing a turtle, and carried on its back to the Dragon Palace (Ryūgū-jō) beneath the sea. There he is entertained ...
", first described in the Manyoshu tells of a young fisherman named Urashima-no-ko (浦嶋子) who visits an undersea palace. After three days, he returns home to his village and finds himself 300 years in the future, where he has been forgotten, his house is in ruins, and his family has died. In
Jew
Jews ( he, יְהוּדִים, , ) or Jewish people are an ethnoreligious group and nation originating from the Israelites Israelite origins and kingdom: "The first act in the long drama of Jewish history is the age of the Israelites""Th ...
ish tradition, the 1st-century BC scholar
Honi ha-M'agel
Honi HaMe'agel (חוני המעגל Khoni, Choni, or Ḥoni; lit. Honi the Circle-drawer) was a Jewish scholar of the 1st-century BCE, during the age of the ''tannaim'', the scholars from whose teachings the Mishnah was derived.
During the 1st ce ...
is said to have fallen asleep and slept for seventy years. When waking up he returned home but found none of the people he knew, and no one believed his claims of who he was.Taanit 23 Hebrew/Aramaic text at Mechon-Mamre /ref>
Shift to science fiction
Early science fiction stories feature characters who sleep for years and awaken in a changed society, or are transported to the past through supernatural means. Among them '' L'An 2440, rêve s'il en fût jamais'' (''The Year 2440: A Dream If Ever There Was One'', 1770) by
Louis-Sébastien Mercier
Louis-Sébastien Mercier (6 June 1740 – 25 April 1814) was a French dramatist and writer, whose 1771 novel ''L'An 2440'' is an example of proto-science fiction.
Early life and education
He was born in Paris to a humble family: his father was a ...
, ''
Rip Van Winkle
"Rip Van Winkle" is a short story by the American author Washington Irving, first published in 1819. It follows a Dutch-American villager in colonial America named Rip Van Winkle who meets mysterious Dutchmen, imbibes their liquor and falls aslee ...
'' (1819) by
Washington Irving
Washington Irving (April 3, 1783 – November 28, 1859) was an American short-story writer, essayist, biographer, historian, and diplomat of the early 19th century. He is best known for his short stories "Rip Van Winkle" (1819) and " The Legen ...
, ''
Looking Backward
''Looking Backward: 2000–1887'' is a utopian science fiction novel by Edward Bellamy, a journalist and writer from Chicopee Falls, Massachusetts; it was first published in 1888.
The book was translated into several languages, and in short or ...
'' (1888) by
Edward Bellamy
Edward Bellamy (March 26, 1850 – May 22, 1898) was an American author, journalist, and political activist most famous for his utopian novel ''Looking Backward''. Bellamy's vision of a harmonious future world inspired the formation of numerou ...
, and '' When the Sleeper Awakes'' (1899) by H.G. Wells. Prolonged sleep, like the later more familiar
time machine
Time travel is the concept of movement between certain points in time, analogous to movement between different points in space by an object or a person, typically with the use of a hypothetical device known as a time machine. Time travel is a w ...
, is used as a means of time travel in these stories.
The earliest work about backwards time travel is uncertain. The Chinese novel Supplement to the Journey to the West (c. 1640) by Dong Yue features magical mirrors and jade gateways that connect various points in time. The protagonist
Sun Wukong
The Monkey King, also known as Sun Wukong ( zh, t=孫悟空, s=孙悟空, first=t) in Mandarin Chinese, is a legendary mythical figure best known as one of the main characters in the 16th-century Chinese novel ''Journey to the West'' ( zh, ...
travels back in time to the "World of the Ancients" (
Qin Dynasty
The Qin dynasty ( ; zh, c=秦朝, p=Qín cháo, w=), or Ch'in dynasty in Wade–Giles romanization ( zh, c=, p=, w=Ch'in ch'ao), was the first Dynasties in Chinese history, dynasty of Imperial China. Named for its heartland in Qin (state), ...
) to retrieve a magical bell and then travels forward to the "World of the Future" (
Song Dynasty
The Song dynasty (; ; 960–1279) was an imperial dynasty of China that began in 960 and lasted until 1279. The dynasty was founded by Emperor Taizu of Song following his usurpation of the throne of the Later Zhou. The Song conquered the rest ...
) to find an emperor who has been exiled in time. However, the time travel takes place inside an illusory dream world created by the villain to entrap and distract him.
Samuel Madden
Samuel Madden (23 December 1686 – 31 December 1765) 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' ...
's ''
Memoirs of the Twentieth Century
''Memoirs of the Twentieth Century'' is an early work of speculative fiction by Irish writer Samuel Madden. This 1733 epistolary novel takes the form of a series of diplomatic letters written in 1997 and 1998. The work is a satire perhaps model ...
'' (1733) is a series of letters from British ambassadors in 1997 and 1998 to diplomats in the past, conveying the political and religious conditions of the future. Because the narrator receives these letters from his
guardian angel
A guardian angel is a type of angel that is assigned to protect and guide a particular person, group or nation. Belief in tutelary beings can be traced throughout all antiquity. The idea of angels that guard over people played a major role in A ...
, Paul Alkon suggests in his book ''Origins of Futuristic Fiction'' that "the first time-traveler in English literature is a guardian angel". Madden does not explain how the angel obtains these documents, but Alkon asserts that Madden "deserves recognition as the first to toy with the rich idea of time-travel in the form of an artifact sent backward from the future to be discovered in the present". In the science fiction anthology ''Far Boundaries'' (1951), editor
August Derleth
August William Derleth (February 24, 1909 – July 4, 1971) was an American writer and anthologist. Though best remembered as the first book publisher of the writings of H. P. Lovecraft, and for his own contributions to the Cthulhu Mythos and the ...
claims that an early short story about time travel is ''An Anachronism; or, Missing One's Coach'', written for the ''Dublin Literary Magazine'' by an anonymous author in th June 1838 issue While the narrator waits under a tree for a
coach
Coach may refer to:
Guidance/instruction
* Coach (sport), a director of athletes' training and activities
* Coaching, the practice of guiding an individual through a process
** Acting coach, a teacher who trains performers
Transportation
* Co ...
to take him out of
Newcastle upon Tyne
Newcastle upon Tyne ( RP: , ), or simply Newcastle, is a city and metropolitan borough in Tyne and Wear, England. The city is located on the River Tyne's northern bank and forms the largest part of the Tyneside built-up area. Newcastle is ...
, he is transported back in time over a thousand years. He encounters the Venerable
Bede
Bede ( ; ang, Bǣda , ; 672/326 May 735), also known as Saint Bede, The Venerable Bede, and Bede the Venerable ( la, Beda Venerabilis), was an English monk at the monastery of St Peter and its companion monastery of St Paul in the Kingdom o ...
in a
monastery
A monastery is a building or complex of buildings comprising the domestic quarters and workplaces of monastics, monks or nuns, whether living in communities or alone (hermits). A monastery generally includes a place reserved for prayer which ...
and explains to him the developments of the coming centuries. However, the story never makes it clear whether these events are real or a dream. Another early work about time travel is ''The Forebears of Kalimeros: Alexander, son of Philip of Macedon'' by
Alexander Veltman
Alexander Fomich Veltman (russian: Алекса́ндр Фоми́ч Ве́льтман) ( — ) was one of the most successful Russian prose writers of the 1830s and 1840s, "popular for various modes of Romantic fiction — historical, Gothic ...
published in 1836.
Charles Dickens
Charles John Huffam Dickens (; 7 February 1812 – 9 June 1870) was an English writer and social critic. He created some of the world's best-known fictional characters and is regarded by many as the greatest novelist of the Victorian e ...
' ''
A Christmas Carol
''A Christmas Carol. In Prose. Being a Ghost Story of Christmas'', commonly known as ''A Christmas Carol'', is a novella by Charles Dickens, first published in London by Chapman & Hall in 1843 and illustrated by John Leech. ''A Christmas C ...
'' (1843) has early depictions of mystical time travel in both directions, as the protagonist, Ebenezer Scrooge, is transported to Christmases past and future. Other stories employ the same template, where a character naturally goes to sleep, and upon waking up finds themself in a different time. A clearer example of backward 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 (27 April 1787 Mâcon, Saône-et-Loire – 25 August 1859) 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 this story, the protagonist is transported to the prehistoric past by the magic of a "lame demon" (a French pun on Boitard's name), where he encounters a
Plesiosaur
The Plesiosauria (; Greek: πλησίος, ''plesios'', meaning "near to" and ''sauros'', meaning "lizard") or plesiosaurs are an order or clade of extinct Mesozoic marine reptiles, belonging to the Sauropterygia.
Plesiosaurs first appeared ...
and an apelike ancestor and is able to interact with ancient creatures.
Edward Everett Hale
Edward Everett Hale (April 3, 1822 – June 10, 1909) was an American author, historian, and Unitarian minister, best known for his writings such as "The Man Without a Country", published in ''Atlantic Monthly'', in support of the Union dur ...
's "Hands Off" (1881) tells the story of an unnamed being, possibly the soul of a person who has recently died, who interferes with ancient Egyptian history by preventing
Joseph
Joseph is a common male given name, derived from the Hebrew Yosef (יוֹסֵף). "Joseph" is used, along with "Josef", mostly in English, French and partially German languages. This spelling is also found as a variant in the languages of the mo ...
's enslavement. This may have been the first story to feature an
alternate history
Alternate history (also alternative history, althist, AH) is a genre of speculative fiction of stories in which one or more historical events occur and are resolved differently than in real life. As conjecture based upon historical fact, altern ...
created as a result of time travel.
Early time machines
One of the first stories to feature time travel by means of a machine is "
The Clock that Went Backward
"The Clock That Went Backward" is a fantasy short story by American writer Edward Page Mitchell.
Plot
The narrator recalls his visiting his great-aunt Gertrude in Maine, alongside his cousin Harry. Gertrude frequently related her family his ...
" by
Edward Page Mitchell
Edward Page Mitchell (1852–1927) was an American editorial and short story writer for '' The Sun'', a daily newspaper in New York City. He became that newspaper's editor in 1897, succeeding Charles Anderson Dana. Mitchell was recognized as a ...
, which appeared in the ''
New York Sun
''The New York Sun'' is an American online newspaper published in Manhattan; from 2002 to 2008 it was a daily newspaper distributed in New York City. It debuted on April 16, 2002, adopting the name, motto, and masthead of the earlier New York ...
'' in 1881. However, the mechanism borders on fantasy. An unusual clock, when wound, runs backwards and transports people nearby back in time. The author does not explain the origin or properties of the clock.
Enrique Gaspar y Rimbau
Enrique Lucio Eugenio Gaspar y Rimbau (2 March 1842 in Madrid – 7 September 1902 in Oloron) was a Spanish diplomat and writer, who wrote many plays (''zarzuelas''), and one of the first novels involving time travel in fiction, time travel ...
's ''El Anacronópete'' (1887) may have been the first story to feature a vessel engineered to travel through time.
Andrew Sawyer
Andrew "Andy" Sawyer (born 1952) is a librarian, critic and editor, as well as an active part of science fiction fandom (although he himself has not written much science fiction). He was educated at the Duke of York's Royal Military School, Dover ...
has commented that the story "does seem to be the first literary description of a time machine noted so far", adding that "Edward Page Mitchell's story ''The Clock That Went Backward'' (1881) is usually described as the first time-machine story, but I'm not sure that a clock quite counts".
The Time Machine
''The Time Machine'' is a science fiction novella by H. G. Wells, published in 1895. The work is generally credited with the popularization of the concept of time travel by using a vehicle or device to travel purposely and selectively for ...
'' (1895) popularized the concept of time travel by mechanical means.
Time travel in physics
Some theories, most notably
special
Special or specials may refer to:
Policing
* Specials, Ulster Special Constabulary, the Northern Ireland police force
* Specials, Special Constable, an auxiliary, volunteer, or temporary; police worker or police officer
Literature
* ''Specia ...
and
general relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
, suggest that suitable geometries of
spacetime
In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differen ...
or specific types of motion in
space
Space is the boundless three-dimensional extent in which objects and events have relative position and direction. In classical physics, physical space is often conceived in three linear dimensions, although modern physicists usually consider ...
might allow time travel into the past and future if these geometries or motions were possible. In technical papers,
physicist
A physicist is a scientist who specializes in the field of physics, which encompasses the interactions of matter and energy at all length and time scales in the physical universe.
Physicists generally are interested in the root or ultimate caus ...
s discuss the possibility of
closed timelike curve
In mathematical physics, a closed timelike curve (CTC) is a world line in a Lorentzian manifold, of a material particle in spacetime, that is "closed", returning to its starting point. This possibility was first discovered by Willem Jacob van Sto ...
s, which are
world line
The world line (or worldline) of an object is the path that an object traces in 4-dimensional spacetime. It is an important concept in modern physics, and particularly theoretical physics.
The concept of a "world line" is distinguished from con ...
s 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.
Many in the scientific community believe that backward time travel is highly unlikely. Any theory that would allow time travel would introduce potential problems of
causality
Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the cau ...
.Extract of page 32 /ref> The classic example of a problem involving causality is the "
grandfather paradox
A temporal paradox, time paradox, or time travel paradox is a paradox, an apparent contradiction, or logical contradiction associated with the idea of time and time travel. The notion of time travel to the future complies with current understanding ...
," which involves travelling to the past and intervening in the conception of one's ancestors (causing the death of an ancestor before said conception being frequently cited). Some physicists, such as Novikov and Deutsch, suggested that these sorts of
temporal paradox
A temporal paradox, time paradox, or time travel paradox is a paradox, an apparent contradiction, or logical contradiction associated with the idea of time and time travel. The notion of time travel to the future complies with current understanding ...
es can be avoided through the
Novikov self-consistency principle
The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture and Larry Niven's law of conservation of history, is a principle developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s. Novikov inten ...
or a variation of the
many-worlds interpretation
The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wave function collapse. This implies that all possible outcomes of quantum me ...
with interacting worlds.
General relativity
Time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling faster than the speed of light, such as
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 was not simp ...
s, traversable
wormhole
A wormhole (Einstein-Rosen bridge) is a hypothetical structure connecting disparate points in spacetime, and is based on a special Solutions of the Einstein field equations, solution of the Einstein field equations.
A wormhole can be visualize ...
s, and
Alcubierre drive
The Alcubierre drive () is a speculative warp drive idea according to which a spacecraft could achieve apparent faster-than-light travel by contracting space in front of it and expanding space behind it, under the assumption that a configurable ...
s. The theory of
general relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
does suggest a scientific basis for the possibility of backward time travel in certain unusual scenarios, although arguments from
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.
In semiclassical gravity, matter is represented by quantum matter field ...
suggest that when
quantum
In physics, a quantum (plural quanta) is the minimum amount of any physical entity (physical property) involved in an interaction. The fundamental notion that a physical property can be "quantized" is referred to as "the hypothesis of quantizati ...
effects are incorporated into general relativity, these loopholes may be closed. These semiclassical arguments led Stephen Hawking to formulate the
chronology protection conjecture
The chronology protection conjecture is a hypothesis first proposed by Stephen Hawking that laws of physics beyond those of standard general relativity prevent time travel on all but microscopic scales - even when the latter theory states that it ...
, 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 (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics; it deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vi ...
to join quantum mechanics and general relativity into a completely unified theory.
Different spacetime geometries
The theory of
general relativity
General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...
describes the universe under a system of
field equations
A classical field theory is a physical theory that predicts how one or more field (physics), physical fields interact with matter through field equations, without considering Quantum mechanics, effects of quantization; theories that incorporate qua ...
that determine the
metric
Metric or metrical may refer to:
* Metric system, an internationally adopted decimal system of measurement
* An adjective indicating relation to measurement in general, or a noun describing a specific type of measurement
Mathematics
In mathema ...
, or distance function, of spacetime. There exist exact solutions to these equations that include
closed time-like curve
In mathematical physics, a closed timelike curve (CTC) is a world line in a Lorentzian manifold, of a material particle in spacetime, that is "closed", returning to its starting point. This possibility was first discovered by Willem Jacob van St ...
s, which are
world line
The world line (or worldline) of an object is the path that an object traces in 4-dimensional spacetime. It is an important concept in modern physics, and particularly theoretical physics.
The concept of a "world line" is distinguished from con ...
s that intersect themselves; some point in the causal future of the world line is also in its causal past, a situation that can be described as time travel. Such a solution was first proposed by
Kurt Gödel
Kurt Friedrich Gödel ( , ; April 28, 1906 – January 14, 1978) was a logician, mathematician, and philosopher. Considered along with Aristotle and Gottlob Frege to be one of the most significant logicians in history, Gödel had an imme ...
, a solution known as the
Gödel metric
The Gödel metric, also known as the Gödel solution or Gödel universe, 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 di ...
, but his (and others') solution requires the universe to have physical characteristics that it does not appear to have, such as
rotation
Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
and lack of
Hubble expansion
Hubble's law, also known as the Hubble–Lemaître law, is the observation in physical cosmology that galaxies are moving away from Earth at speeds proportional to their distance. In other words, the farther they are, the faster they are moving ...
. Whether general relativity forbids closed time-like curves for all realistic conditions is still being researched.S. W. Hawking, ''Introductory note to 1949 and 1952'' in Kurt Gödel, ''Collected works'', Volume II (S. Feferman et al., eds).
Wormholes
Wormholes are a hypothetical warped spacetime permitted by the
Einstein field equations
In the general theory of relativity, the Einstein field equations (EFE; also known as Einstein's equations) relate the geometry of spacetime to the distribution of matter within it.
The equations were published by Einstein in 1915 in the form ...
of general relativity. A proposed time-travel machine using a
traversable wormhole
A wormhole (Einstein-Rosen bridge) is a hypothetical structure connecting disparate points in spacetime, and is based on a special solution of the Einstein field equations.
A wormhole can be visualized as a tunnel with two ends at separate po ...
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
Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived fro ...
, 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 these methods,
time dilation
In physics and relativity, time dilation is the difference in the elapsed time as measured by two clocks. It is either due to a relative velocity between them ( special relativistic "kinetic" time dilation) or to a difference in gravitational ...
causes the end of the wormhole that has been moved to have aged less, or become "younger", than the stationary end as seen by an external observer; however, time connects differently ''through'' the wormhole than ''outside'' it, so that synchronized 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 "younger" end would exit the "older" end at a time when it was the same age as the "younger" end, effectively going back in time as seen by an observer from the 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 backward 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
There are several proposed types of exotic matter:
* Hypothetical particles and states of matter that have "exotic" physical properties that would violate known laws of physics, such as a particle having a negative mass.
* Hypothetical particl ...
". More technically, the wormhole spacetime requires a distribution of energy that violates various
energy condition
In relativistic classical field theories of gravitation, particularly general relativity, an energy condition is a generalization of the statement "the energy density of a region of space cannot be negative" in a relativistically-phrased mathe ...
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
In quantum field theory, the Casimir effect is a physical force acting on the macroscopic boundaries of a confined space which arises from the quantum fluctuations of the field. It is named after the Dutch physicist Hendrik Casimir, who pr ...
in quantum physics. Although early calculations suggested that 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 is a mathematics Professor at Victoria University of Wellington, in New Zealand.
Work
Visser's research interests include general relativity, quantum field theory and cosmology.
Visser has produced a large number of research papers ...
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 (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the cau ...
violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "
Roman ring
In general relativity, a Roman ring (proposed by Matt Visser in 1997 and named after the Roman arch, a concept proposed by Mike Morris and Kip Thorne in 1988 and named after physicist Tom Roman) is a configuration of wormholes where no subset of ...
" (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
A Tipler cylinder, also called a Tipler time machine, is a hypothetical object Theory, theorized to be a potential mode of time travel—although results have shown that a Tipler cylinder could only allow time travel if its length were infinite ...
, a GR solution discovered by
Willem Jacob van Stockum
Willem Jacob van Stockum (20 November 1910 – 10 June 1944) was a Dutch mathematician who made an important contribution to the early development of general relativity.
Biography
Van Stockum was born in Hattem in the Netherlands. His father wa ...
in 1936 and Kornel Lanczos in 1924, but not recognized as allowing closed timelike curves until an analysis by
Frank Tipler
Frank Jennings Tipler (born February 1, 1947) is an American mathematical physicist and cosmologist, holding a joint appointment in the Departments of Mathematics and Physics at Tulane University. Tipler has written books and papers on the Omeg ...
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 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 was not simp ...
, but none are known to exist, and it does not seem to be possible to create a new cosmic string. Physicist
Ronald Mallett
Ronald Lawrence Mallett (born March 30, 1945) is an American theoretical physicist, academic and author. He has been a faculty member of the University of Connecticut since 1975 and is best known for his scientific position on the possibility of ...
is attempting to recreate the conditions of a rotating black hole with ring lasers, in order to bend spacetime and allow for time travel.
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
In theory of relativity, relativistic classical field theory, classical field theories of gravitation, particularly general relativity, an energy condition is a generalization of the statement "the energy density of a region of space cannot be ne ...
is satisfied, meaning that the region contains no matter with negative energy density (
exotic matter
There are several proposed types of exotic matter:
* Hypothetical particles and states of matter that have "exotic" physical properties that would violate known laws of physics, such as a particle having a negative mass.
* Hypothetical particl ...
). 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
The chronology protection conjecture is a hypothesis first proposed by Stephen Hawking that laws of physics beyond those of standard general relativity prevent time travel on all but microscopic scales - even when the latter theory states that it ...
, where he examines "the case that the causality violations appear in a finite region of spacetime without curvature singularities" and proves that "there will be a
Cauchy horizon
In physics, a Cauchy horizon is a light-like boundary of the domain of validity of a Cauchy problem (a particular boundary value problem of the theory of partial differential equations). One side of the horizon contains closed space-like geodesics ...
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." This theorem does not rule out the possibility of time travel by means of time machines with the non-compactly generated Cauchy horizons (such as the Deutsch-Politzer time machine) or in regions which contain exotic matter, which would be used for traversable wormholes or the
Alcubierre drive
The Alcubierre drive () is a speculative warp drive idea according to which a spacecraft could achieve apparent faster-than-light travel by contracting space in front of it and expanding space behind it, under the assumption that a configurable ...
and
black hole
A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...
.
Quantum physics
No-communication theorem
When a signal is sent from one location and received at another location, then as long as the signal is moving at the speed of light or slower, the mathematics of
simultaneity
Simultaneity may refer to:
* Relativity of simultaneity, a concept in special relativity.
* Simultaneity (music), more than one complete musical texture occurring at the same time, rather than in succession
* Simultaneity, a concept in Endogeneit ...
in the theory of relativity show that all reference frames agree that the transmission-event happened before the reception-event. When the signal travels faster than light, it is received ''before'' it is sent, in all reference frames. The signal could be said to have moved backward in time. This hypothetical scenario is sometimes referred to as a
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
presented a thought experiment of how faster-than-light signals can lead to a paradox of caus ...
EPR paradox
EPR may refer to:
Science and technology
* EPR (nuclear reactor), European Pressurised-Water Reactor
* EPR paradox (Einstein–Podolsky–Rosen paradox), in physics
* Earth potential rise, in electrical engineering
* East Pacific Rise, a mid-ocea ...
, or
quantum entanglement
Quantum entanglement is the phenomenon that occurs when a group of particles are generated, interact, or share spatial proximity in a way such that the quantum state of each particle of the group cannot be described independently of the state of ...
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 may refer to:
* Bohm (surname)
* Bohm Dialogue, free-flowing group conversation
Physics
* Aharonov–Bohm effect of electromagnetic potential on a particle
* Bohm sheath criterion for a Debye sheath plasma layer
* Bohm diffusion of plasma in ...
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
In physics, action at a distance is the concept that an object can be affected without being physically touched (as in mechanical contact) by another object. That is, it is the non-local interaction of objects that are separated in space.
Non-c ...
" by Einstein.
Nevertheless, the fact that causality is preserved in quantum mechanics is a rigorous result in modern
quantum field theories
In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles ...
, 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
In physics, the no-communication theorem or no-signaling principle is a no-go theorem from quantum information theory which states that, during measurement of an entangled quantum state, it is not possible for one observer, by making a measureme ...
also gives a general proof that quantum entanglement cannot be used to transmit information faster than classical signals.
Interacting many-worlds interpretation
A variation of
Hugh Everett
Hugh Everett III (; November 11, 1930 – July 19, 1982) was an American physicist who first proposed the many-worlds interpretation (MWI) of quantum physics, which he termed his "relative state" formulation. In contrast to the then-dominant Cope ...
's
many-worlds interpretation
The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wave function collapse. This implies that all possible outcomes of quantum me ...
(MWI) of quantum mechanics provides a resolution to the grandfather paradox that involves the time traveler arriving in a different universe than the one they came from; it's been argued that since the traveler arrives in a different universe's history and not their own history, this is not "genuine" time travel. The accepted many-worlds interpretation suggests that all possible quantum events can occur in mutually exclusive histories. However, some variations allow different universes to interact. This concept is most often used in science-fiction, but some physicists such as
David Deutsch
David Elieser Deutsch ( ; born 18 May 1953) is a British physicist at the University of Oxford. He is a Visiting Professor in the Department of Atomic and Laser Physics at the Centre for Quantum Computation (CQC) in the Clarendon Laboratory of ...
have suggested that a time traveler should end up in a different history than the one he started from. On the other hand, Stephen Hawking 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. 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.
Experimental results
Certain experiments carried out give the impression of reversed
causality
Causality (also referred to as causation, or cause and effect) is influence by which one event, process, state, or object (''a'' ''cause'') contributes to the production of another event, process, state, or object (an ''effect'') where the cau ...
, but fail to show it under closer examination.
The
delayed choice quantum eraser
A delayed-choice quantum eraser experiment, first performed by Yoon-Ho Kim, R. Yu, S. P. Kulik, Y. H. Shih and Marlan O. Scully, and reported in early 1998, is an elaboration on the quantum eraser experiment that incorporates concepts considered ...
experiment performed by
Marlan Scully
Marlan Orvil Scully (born August 3, 1939) is an American physicist best known for his work in theoretical quantum optics. He is a professor at Texas A&M University and Princeton University. Additionally, in 2012 he developed a lab at the Baylor ...
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
s that 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
In modern physics, the double-slit experiment is a demonstration that light and matter can display characteristics of both classically defined waves and particles; moreover, it displays the fundamentally probabilistic nature of quantum mechanics ...
. 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
In physics, interference is a phenomenon in which two waves combine by adding their displacement together at every single point in space and time, to form a resultant wave of greater, lower, or the same amplitude. Constructive and destructive ...
is observed when one correlates measurements of idler photons to the corresponding signal photons. However, since interference can be observed only 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, only by gathering classical information from the entire system; thus causality is preserved.
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
In mathematics, Fourier analysis () is the study of the way general functions may be represented or approximated by sums of simpler trigonometric functions. Fourier analysis grew from the study of Fourier series, and is named after Josep ...
), and the package can appear to move faster than light or even backward in time even if none of the pure waves in the sum do so. This effect cannot be used to send any matter, energy, or information faster than light, so this experiment is understood not to violate causality either.
The physicists
Günter Nimtz
Günter Nimtz (born 22 September 1936) 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. His claims show that particles may tra ...
and Alfons Stahlhofen, of the
University of Koblenz
The University of Koblenz and Landau (German ''Universität Koblenz-Landau'') is a public university located in Koblenz and Landau, Germany, founded in 1990.
History and profile
The University of Koblenz and Landau is one of the youngest univer ...
, 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
Microwave is a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter corresponding to frequencies between 300 MHz and 300 GHz respectively. Different sources define different frequency ran ...
photons traveled "instantaneously" between a pair of prisms that had been moved up to apart, using a phenomenon known as
quantum tunneling
In physics, a quantum (plural quanta) is the minimum amount of any physical entity (physical property) involved in an interaction. The fundamental notion that a physical property can be "quantized" is referred to as "the hypothesis of quantizati ...
. Nimtz told ''
New Scientist
''New Scientist'' is a magazine covering all aspects of science and technology. Based in London, it publishes weekly English-language editions in the United Kingdom, the United States and Australia. An editorially separate organisation publishe ...
'' 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
The University of Toronto (UToronto or U of T) is a public research university in Toronto, Ontario, Canada, located on the grounds that surround Queen's Park. It was founded by royal charter in 1827 as King's College, the first institution ...
, Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the speed of the center of the train exceeds the speed of any of the individual cars.
Shengwang Du Shengwang Du is a professor in the department of physics at The University of Texas at Dallas.
He is noted for having led a team that performed an experiment showing individual photons cannot travel faster than the speed of light (''c'') in a vacuu ...
claims in a peer-reviewed journal to have observed single photons'
precursor
Precursor or Precursors may refer to:
*Precursor (religion), a forerunner, predecessor
** The Precursor, John the Baptist
Science and technology
* Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of unr ...
s, saying that they travel no faster than '' c'' in a vacuum. His experiment involved
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 by the interaction with the medium in which the propagatio ...
as well as passing light through a vacuum. He generated two single
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
s, passing one through
rubidium
Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density higher ...
atoms that had been cooled with a laser (thus slowing the light) and passing one through a vacuum. Both times, apparently, the precursors preceded the photons' main bodies, and the precursor traveled at ''c'' in a vacuum. According to Du, this implies that there is no possibility of light traveling faster than ''c'' and, thus, no possibility of violating causality.
Absence of time travelers from the future
Many have argued that the absence of time travelers from the future demonstrates that such technology will never be developed, suggesting that it is impossible. This is analogous to the
Fermi paradox
The Fermi paradox is the discrepancy between the lack of conclusive evidence of advanced extraterrestrial life and the apparently high a priori likelihood of its existence, and by extension of obtaining such evidence. As a 2015 article put it, ...
related to the absence of evidence of extraterrestrial life. As the absence of extraterrestrial visitors does not categorically ''prove'' they do not exist, so the absence of time travelers fails to prove time travel is physically impossible; it might be that time travel is physically possible but is never developed or is cautiously used.
Carl Sagan
Carl Edward Sagan (; ; November 9, 1934December 20, 1996) was an American astronomer, planetary scientist, cosmologist, astrophysicist, astrobiologist, author, and science communicator. His best known scientific contribution is research on ext ...
once suggested the possibility that time travelers could be here but are disguising their existence or are not recognized as time travelers. Some versions of general relativity suggest that time travel might only be possible in a region of
spacetime
In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differen ...
that is warped a certain way, and hence time travelers would not be able to travel back to earlier regions in spacetime, before this region existed. Stephen Hawking stated that this would explain why the world has not already been overrun by "tourists from the future".
Several experiments have been carried out to try to entice future humans, who might invent time travel technology, to come back and demonstrate it to people of the present time. Events such as Perth's Destination Day or Massachusetts Institute of Technology, MIT's Time Traveler Convention heavily publicized permanent "advertisements" of a meeting time and place for future time travelers to meet. In 1982, a group in Baltimore, Maryland, 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. Some versions of the
many-worlds interpretation
The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wave function collapse. This implies that all possible outcomes of quantum me ...
can be used to suggest that future humans have traveled back in time, but have traveled back to the meeting time and place in a multiverse, parallel universe.
Time dilation
There is a great deal of observable evidence for time dilation in special relativity and gravitational time dilation in general relativity, for example in the famous and easy-to-replicate observation of experimental testing of time dilation, atmospheric muon decay. The theory of relativity states that the speed of light is speed of light#Fundamental role in physics, invariant for all observers in any frame of reference; that is, it is always the same. Time dilation is a direct consequence of the invariance of the speed of light. Time dilation may be regarded in a limited sense as "time travel into the future": a person may use time dilation so that a small amount of proper time passes for them, while a large amount of proper time passes elsewhere. This can be achieved by traveling at relativistic speeds or through the effects of gravity.
For two identical clocks moving relative to each other without accelerating, each clock measures the other to be ticking slower. This is possible due to the relativity of simultaneity. However, the symmetry is broken if one clock accelerates, allowing for less proper time to pass for one clock than the other. The twin paradox describes this: one twin remains on Earth, while the other undergoes acceleration to relativistic speed as they travel into space, turn around, and travel back to Earth; the traveling twin ages less than the twin who stayed on Earth, because of the time dilation experienced during their acceleration. General relativity treats the effects of acceleration and the effects of gravity as equivalence principle, equivalent, and shows that time dilation also occurs in gravity wells, with a clock deeper in the well ticking more slowly; this effect is taken into account when calibrating the clocks on the satellites of the Global Positioning System, and it could lead to significant differences in rates of aging for observers at different distances from a large gravity well such as a
black hole
A black hole is a region of spacetime where gravitation, gravity is so strong that nothing, including light or other Electromagnetic radiation, electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts t ...
.
A time machine that utilizes this principle might be, for instance, a spherical shell with a diameter of five meters and the Jupiter mass, mass of Jupiter. A person at its center will travel forward in time at a rate four times slower than that of distant observers. Squeezing the mass of a large planet into such a small structure is not expected to be within humanity's technological capabilities in the near future. With current technologies, it is only possible to cause a human traveler to age less than companions on Earth by a few milliseconds after a few hundred days of space travel.
Philosophy
Philosophers have discussed the nature of time since at least the time of ancient Greece; for example, Parmenides presented the view that time is an illusion. Centuries later, Isaac Newton supported the idea of absolute time and space, absolute time, while his contemporary Gottfried Wilhelm Leibniz maintained that time is only a relation between events and it cannot be expressed independently. The latter approach eventually gave rise to the
spacetime
In physics, spacetime is a mathematical model that combines the three dimensions of space and one dimension of time into a single four-dimensional manifold. Spacetime diagrams can be used to visualize relativistic effects, such as why differen ...
of theory of relativity, relativity.
Presentism vs. eternalism
Many philosophers have argued that relativity implies eternalism (philosophy of time), eternalism, the idea that the past and future exist in a real sense, not only as changes that occurred or will occur to the present. Philosopher of science Dean Rickles disagrees with some qualifications, but notes that "the consensus among philosophers seems to be that special and general relativity are incompatible with presentism". Some philosophers view time as a dimension equal to spatial dimensions, that future events are "already there" in the same sense different places exist, and that there is no objective flow of time; however, this view is disputed.
Presentism (philosophy of time), Presentism is a school of philosophy that holds that the future and the past exist only as changes that occurred or will occur to the present, and they have no real existence of their own. In this view, time travel is impossible because there is no future or past to travel to. Keller and Nelson have argued that even if 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; these views are contested by some authors.
Presentism in classical spacetime deems that only the present exists; this is not reconcilable with special relativity, shown in the following example: Alice and Bob are simultaneous observers of event''O''. For Alice, some event''E'' is simultaneous with''O'', but for Bob, event''E'' is in the past or future. Therefore, Alice and Bob disagree about what exists in the present, which contradicts classical presentism. "Here-now presentism" attempts to reconcile this by only acknowledging the time and space of a single point; this is unsatisfactory because objects coming and going from the "here-now" alternate between real and unreal, in addition to the lack of a preferred frame, privileged "here-now" that would be the "real" present. "Relativized presentism" acknowledges that there are infinite frames of reference, each of them having a different set of simultaneous events, which makes it impossible to distinguish a single "real" present, and hence either all events in time are real—blurring the difference between presentism and eternalism—or each frame of reference exists in its own reality. Options for presentism in special relativity appear to be exhausted, but Gödel and others suspect presentism may be valid for some forms of general relativity. Generally, the idea of absolute time and space is considered incompatible with general relativity; there is no universal truth about the absolute position of events which occur at different times, and thus no way to determine which point in space at one time is at the universal "same position" at another time, and all coordinate systems are on equal footing as given by the principle of general covariance, diffeomorphism invariance.
The grandfather paradox
A common objection to the idea of traveling back in time is put forth in the grandfather paradox or the argument of auto-infanticide. If one were able to go back in time, inconsistencies and contradictions would ensue if the time traveler were to change anything; there is a contradiction if the past becomes different from the way it ''is''. The paradox is commonly described with a person who travels to the past and kills their own grandfather, prevents the existence of their father or mother, and therefore their own existence. Philosophers question whether these paradoxes prove time travel impossible. Some philosophers answer the paradoxes by arguing that it might be the case that backward 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
The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture and Larry Niven's law of conservation of history, is a principle developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s. Novikov inten ...
in physics.
Ontological paradox
Compossibility
According to the philosophical theory of compossibility, what ''can'' happen, for example in the context of time travel, must be weighed against the context of everything relating to the situation. If the past ''is'' a certain way, it's not possible for it to be any other way. What ''can'' happen when a time traveler visits the past is limited to what ''did'' happen, in order to prevent logical contradictions.
Self-consistency principle
The
Novikov self-consistency principle
The Novikov self-consistency principle, also known as the Novikov self-consistency conjecture and Larry Niven's law of conservation of history, is a principle developed by Russian physicist Igor Dmitriyevich Novikov in the mid-1980s. Novikov inten ...
, named after Igor Dmitrievich Novikov, states that any actions taken by a time traveler or by an object that travels back in time were part of history all along, and therefore 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 causal loop, circular causation, sometimes called a predestination paradox, ontological paradox, or bootstrap paradox. The term bootstrap paradox was popularized by Robert A. Heinlein's story "By His Bootstraps". 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.
The philosopher Kelley L. Ross argues in "Time Travel Paradoxes" that in a scenario involving a physical object whose world-line or history forms a closed loop in time there can be a violation of the second law of thermodynamics. Ross uses the film ''Somewhere in Time (film), Somewhere in Time'' as an example of such an ontological paradox, where a watch is given to a person, and 60 years later the same watch is brought back in time and given to the same character. Ross states that entropy of the watch will increase, and the watch carried back in time will be more worn with each repetition of its history. The second law of thermodynamics is understood by modern physicists to be a statistical mechanics, statistical law, so fluctuation theorem, decreasing entropy and non-increasing entropy are not impossible, just improbable. Additionally, entropy statistically increases in systems which are isolated, so non-isolated systems, such as an object, that interact with the outside world, can become less worn and decrease in entropy, and it's possible for an object whose world-line forms a closed loop to be always in the same condition in the same point of its history.
In 2005, Daniel Greenberger and Karl Svozil proposed that quantum mechanics, quantum theory gives a model for time travel where the past must be self-consistent.
In fiction
Time travel themes in
science fiction
Science fiction (sometimes shortened to Sci-Fi or SF) is a genre of speculative fiction which typically deals with imaginative and futuristic concepts such as advanced science and technology, space exploration, time travel, parallel unive ...
and the media can be grouped into three categories: immutable timeline; mutable timeline; and alternate histories, as in the interacting-
many-worlds interpretation
The many-worlds interpretation (MWI) is an interpretation of quantum mechanics that asserts that the universal wavefunction is objectively real, and that there is no wave function collapse. This implies that all possible outcomes of quantum me ...
. The non-scientific term ''timeline'' is often used to refer to all physical events in history, so that where events are changed, the time traveler is described as creating a new timeline.Prucher, Jeff (2007) ''Brave New Words: The Oxford Dictionary of Science Fiction'' p. 230
See also
;Claims of time travel
:*Time travel claims and urban legends
;Culture
:*Time capsule
;Fiction
:*Time travel in fiction
:*Time travel in The Lord of the Rings, Time travel in ''The Lord of the Rings''
:*List of time travel works of fiction
:*List of games containing time travel
;Science
:*Krasnikov tube
:*Retrocausality
:*Ring singularity
:*Temporal paradox
:*Wheeler–Feynman absorber theory
;Time perception
:*Cryonics
:*Suspended animation
:*Time perception
Further reading
:*''Time Travel: A History'' - book by James Gleick