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{{Short description|Hypothetical travel into the past or future}}
{{Other uses|Time Travel (disambiguation)}}
{{Pp-semi-indef}}
{{Use American English|date=May 2023}}
{{Use mdy dates|date=May 2023}}
[[File:The Time Machine – Frontpage Heinemann.png|thumb|The first page of ''[[The Time Machine]]'' published by [[Heinemann (publisher)|Heinemann]]]]
'''Time travel''' is the hypothetical activity of traveling into the [[past]] or [[future]]. Time travel is a concept in [[philosophy]] and [[fiction]], particularly [[science fiction]]. [[Time travel in fiction|In fiction]], time travel is typically achieved through the use of a device known as a [[Time machine (device)|time machine]]. The idea of a time machine was popularized by [[H. G. Wells]]'s 1895 novel ''[[The Time Machine]]''.<ref>{{cite book|title=Astounding Wonder: Imagining Science and Science Fiction in Interwar America|edition=illustrated|first1=John|last1=Cheng|publisher=University of Pennsylvania Press|year=2012|isbn=978-0-8122-0667-8|page=180|url=https://books.google.com/books?id=GoyzFw9UR8IC|access-date=2019-11-18|archive-date=2023-03-24|archive-url=https://web.archive.org/web/20230324181727/https://books.google.com/books?id=GoyzFw9UR8IC|url-status=live}} [https://books.google.com/books?id=GoyzFw9UR8IC&pg=PA180 Extract of page 180] {{Webarchive|url=https://web.archive.org/web/20230324181739/https://books.google.com/books?id=GoyzFw9UR8IC&pg=PA180 |date=2023-03-24 }}</ref>
It is uncertain whether time travel to the past would be physically possible. Such travel, if at all feasible, may give rise to questions of [[causality]]. Forward time travel, outside the usual sense of the [[Time perception|perception of time]], is an extensively observed phenomenon and is well understood within the framework of [[special relativity]] and [[general relativity]]. 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 [[Exact solutions in general relativity|solutions in general relativity]] that allow for it, such as a rotating [[black hole]]. Traveling to an arbitrary point in spacetime has very limited support in [[theoretical physics]], and is usually connected only with [[Quantum mechanics of time travel|quantum mechanics]] or [[wormhole]]s.
{{anchor|History}}
== History of the concept ==
=== In religion ===
[[File:Irvington statue of Rip van Winkle.jpg|thumb|Statue of [[Rip Van Winkle]] in [[Irvington, New York]]]]
Some ancient stories feature characters who appear to leap forward in time. ''[[Vishnu Purana]]'', in Hindu mythology narrate the myth of [[Kakudmi|Raivata Kakudmi]], who visits the god [[Brahma]] in heaven and returns to Earth to find that many ages have passed.<ref>{{citation|section-url=http://www.mythfolklore.net/india/encyclopedia/revati.htm|last1=Dowson|first1=John|section=Revati|title=A classical dictionary of Hindu mythology and religion, geography, history, and literature|year=1879|publisher=[[Routledge]]|access-date=2009-08-20|archive-date=2017-09-07|archive-url=https://web.archive.org/web/20170907064810/http://www.mythfolklore.net/india/encyclopedia/revati.htm|url-status=live}}</ref><ref>{{citation|url=https://www.sacred-texts.com/hin/vp/vp093.htm#page_355|title=The Vishnu Purana: Book IV: Chapter I|access-date=2022-01-08|archive-date=2022-05-27|archive-url=https://web.archive.org/web/20220527102631/https://www.sacred-texts.com/hin/vp/vp093.htm#page_355|url-status=live}}</ref> In the Buddhist [[Pāli Canon]], the [[Payasi]] Sutta describes how the disciple Kumara [[Mahākāśyapa|Kassapa]] explains to a skeptic that time flows differently in the heavenly realms.<ref>{{citation|title=Indian Philosophy|edition=7|author=Debiprasad Chattopadhyaya|publisher=People's Publishing House, New Delhi|year=1964|author-link=Debiprasad Chattopadhyaya}}</ref> The Japanese legend of "[[Urashima Tarō]]", first recorded in the ''[[Manyoshu]]'', tells of a fisherman who visits an undersea palace for three days, only to return and find that centuries have passed and his world is gone.<ref>{{cite book|title=Folklore, myths, and legends: a world perspective|first=Donna|last=Rosenberg|publisher=[[McGraw-Hill]]|year=1997|isbn=978-0-8442-5780-8|page=421}}</ref>
In one tradition in [[Judaism]], [[Moses]] is [[Moses sees Rabbi Akiva (Menachot 29b)|transported by God to the study hall of Rabbi Akiva]], where he is confused by the future evolution of Jewish law. Another Talmudic story features [[Honi HaMe'agel]], a 1st-century BCE miracle worker who sees a man planting a carob tree that will take 70 years to bear fruit. Honi falls asleep and awakens 70 years later to find the tree fully grown and its fruit being harvested by the man’s grandson.<ref name="Talmud">Babylonian Talmud Taanit 23a [http://www.mechon-mamre.org/b/l/l2803.htm Hebrew/Aramaic text at Mechon-Mamre] {{Webarchive|url=https://web.archive.org/web/20200809103228/http://www.mechon-mamre.org/b/l/l2803.htm|date=2020-08-09}}</ref><ref name="ChoniLATimes2000">{{cite news |author=Margaret Snyder |date=August 29, 2000 |title=Community Commentary |url=https://www.latimes.com/socal/glendale-news-press/news/tn-gnp-xpm-2000-08-29-export45571-story.html |access-date=November 10, 2022 |newspaper=[[The Los Angeles Times]]}}</ref>
In [[Islam]], the [[Quran]] narrates the story of the [[Seven Sleepers]], a group of [[Monotheism|monotheistic]] young men who sought refuge in a cave to escape persecution. As they slept, [[Allah]] preserved them for centuries, and when they awoke, they discovered that the world around them had changed. This narrative, found in the Quranic Surah [[Al-Kahf]], describes divine protection and time suspension.<ref>{{cite book |last1=Yahya |first1=Farouk |title=Chapter 8 Talismans with the Names of the Seven Sleepers of Ephesus/Aṣḥāb al-Kahf in Muslim Southeast Asia |chapter=Talismans with the Names of the Seven Sleepers of Ephesus/Aṣḥāb al-Kahf in Muslim Southeast Asia |url=https://brill.com/edcollchap/book/9789004529397/BP000009.xml?language=en |website=Brill |publisher=Malay-Indonesian Islamic Studies |access-date=December 7, 2023 |date=December 5, 2022|pages=209–265 |doi=10.1163/9789004529397_010 |isbn=978-90-04-52939-7 }}</ref><ref>{{cite web |title=Cave of the Seven Sleepers |url=https://madainproject.com/cave_of_the_seven_sleepers |website=Madain Project |access-date=December 7, 2023}}</ref><ref>{{cite web |last1=Blakeley |first1=Sasha |title=The Seven Sleepers |url=https://study.com/academy/lesson/seven-sleepers-story-significance.html |website=Study.com |access-date=December 7, 2023 |date=April 24, 2023}}</ref>
=== Science fiction ===
{{further|Time travel in fiction}}
Time travel themes in [[science fiction]] and the media can be grouped into three categories: immutable timeline; mutable timeline; and alternate histories, as in the interacting-[[many-worlds interpretation]].<ref>{{cite journal |last=Grey |first=William |title=Troubles with Time Travel |journal=Philosophy |volume=74 |issue=1 |pages=55–70 |year=1999 |publisher=Cambridge University Press |doi=10.1017/S0031819199001047|s2cid=170218026 |issn = 0031-8191}}</ref><ref>{{cite book |last=Rickman |first=Gregg |title=The Science Fiction Film Reader |publisher=Limelight Editions |year=2004 |isbn=978-0-87910-994-3 |url=https://archive.org/details/isbn_9780879109943}}</ref><ref>{{cite book |last=Schneider |first=Susan |title=Science Fiction and Philosophy: From Time Travel to Superintelligence |publisher=Wiley-Blackwell |year=2009 |isbn=978-1-4051-4907-5}}</ref> 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.<ref name="Prucher">{{Cite book|last=Prucher|first=Jeff|url=https://books.google.com/books?id=iYzi8m8FbEsC&pg=PA230|title=Brave new words|isbn=978-0-19-530567-8|language=en|access-date=2022-12-29|archive-date=2023-03-24|archive-url=https://web.archive.org/web/20230324181802/https://books.google.com/books?id=iYzi8m8FbEsC&pg=PA230|url-status=live}}</ref>
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 ''[[The Year 2440|L'An 2440, rêve s'il en fût jamais]]'' (''The Year 2440: A Dream If Ever There Was One'', 1770) by [[Louis-Sébastien Mercier]], ''[[Rip Van Winkle]]'' (1819) by [[Washington Irving]], ''[[Looking Backward]]'' (1888) by [[Edward Bellamy]], and ''[[The Sleeper Awakes|When the Sleeper Awakes]]'' (1899) by H. G. Wells. Prolonged sleep is used as a means of time travel in these stories.<ref>{{citation|title=The Cambridge Companion to Utopian Literature|chapter=Utopia, dystopia, and science fiction|author=Peter Fitting|editor=Gregory Claeys|publisher=Cambridge University Press|year=2010|pages=138–139}}</ref>
The date of the earliest work about backwards time travel is uncertain. The Chinese novel ''[[A Supplement to the Journey to the West]]'' ({{Circa|1640}}) by Dong Yue features magical mirrors and jade gateways that connect various points in time. The protagonist [[Sun Wukong]] travels back in time to the "World of the Ancients" ([[Qin dynasty]]) to retrieve a magical bell and then travels forward to the "World of the Future" ([[Song dynasty]]) to find an emperor who has been exiled in time. However, the time travel is taking place inside an illusory dream world created by the villain to distract and entrap him.<ref>{{cite book |last1=Dong |first1=Yue |translator-first1=Shuen-fu |translator-last1=Lin |translator-first2=Larry James |translator-last2=Schulz |first2=Chengẻn |last2=Wu| title=The Tower of Myriad Mirrors: A Supplement to Journey to the West |series=Michigan classics in Chinese studies |___location=Ann Arbor |publisher=Center for Chinese Studies, The University of Michigan |isbn=9780892641420 |year=2000 |edition=2nd }}</ref> [[Samuel Madden (author)|Samuel Madden]]'s ''[[Memoirs of the Twentieth Century]]'' (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.<ref name="madden">{{cite book|last=Alkon|first=Paul K.|title=Origins of Futuristic Fiction|publisher=The University of Georgia Press|year=1987|isbn=978-0-8203-0932-3|url=https://archive.org/details/originsoffuturis00alko}}</ref>{{rp|95–96}} Because the narrator receives these letters from his [[guardian angel]], Paul Alkon suggests in his book ''Origins of Futuristic Fiction'' that "the first time-traveler in English literature is a guardian angel".<ref name="madden" />{{rp|85}} 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".<ref name="madden"/>{{rp|95–96}} In the science fiction anthology ''Far Boundaries'' (1951), editor [[August Derleth]] claims that an early short story about time travel is ''An Anachronism; or, Missing One's Coach'', written for the ''Dublin Literary Magazine''<ref>{{cite journal|title=An Anachronism; or, Missing One's Coach|url=https://books.google.com/books?id=jfPAwAnj9JUC&pg=RA4-PA701|date=June 1838|journal=Dublin University Magazine|volume=11|access-date=2022-05-11|archive-date=2023-03-24|archive-url=https://web.archive.org/web/20230324181751/https://books.google.com/books?id=jfPAwAnj9JUC&pg=RA4-PA701|url-status=live}}</ref> by an anonymous author in the [https://books.google.com/books?id=jfPAwAnj9JUC&pg=RA4-PA665 June 1838 issue].<ref name="derleth">{{cite book|last=Derleth|first=August|author-link=August Derleth|title=Far Boundaries|publisher=Pellegrini & Cudahy|year=1951}}</ref>{{rp|3}} While the narrator waits under a tree for a [[Stagecoach|coach]] to take him out of [[Newcastle upon Tyne]], he is transported back in time over a thousand years. He encounters the Venerable [[Bede]] in a [[monastery]] 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.<ref name="derleth" />{{rp|11–38}} Another early work about time travel is ''The Forebears of Kalimeros: Alexander, son of Philip of Macedon'' by [[Alexander Veltman]] published in 1836.<ref>{{Cite web|title=Lib.ru/Классика: Акутин Юрий. Александр Вельтман и его роман "Странник"|url=http://az.lib.ru/w/welxtman_a_f/text_0090.shtml|access-date=2022-12-29|website=az.lib.ru|archive-date=2011-06-06|archive-url=https://web.archive.org/web/20110606034550/http://az.lib.ru/w/welxtman_a_f/text_0090.shtml|url-status=live}}</ref>
[[File:A Christmas Carol - Mr. Fezziwig's Ball.jpg|thumb|Mr. and Mrs. Fezziwig dance in a vision shown to Scrooge by the [[Ghost of Christmas Past]].]] [[Charles Dickens]]'s ''[[A Christmas Carol]]'' (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.<ref name="Scrooge">{{cite encyclopedia|url=http://www.towson.edu/~flynn/timetv.html|title=Time Travel Literature|access-date=October 28, 2006|author=Flynn, John L.|archive-url=https://web.archive.org/web/20060929071327/http://www.towson.edu/~flynn/timetv.html <!-- Bot retrieved archive -->|archive-date=September 29, 2006|year=1995|encyclopedia=The Encyclopedia Galactica}}</ref> A clearer example of backward time travel is found in the 1861 book ''Paris avant les hommes'' (''Paris before Men'') by the French botanist and geologist [[Pierre Boitard]], 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 [[Plesiosauria|Plesiosaur]] and an apelike ancestor and is able to interact with ancient creatures.<ref name="boitard">{{cite book|last=Rudwick|first=Martin J. S.|title=Scenes From Deep Time|publisher=The University of Chicago Press|year=1992|pages=166–169|isbn=978-0-226-73105-6}}</ref> [[Edward Everett Hale]]'s "Hands Off" (1881)<ref>{{cite book|title=Hands Off|last=Hale|first=Edward Everett|publisher=J. Stilman Smith & Co.|year=1895|url=https://archive.org/details/handsoff00halegoog/mode/2up}}</ref> 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 (Genesis)|Joseph]]'s enslavement. This may have been the first story to feature an [[alternate history]] created as a result of time travel.<ref name="Nahin2001" >{{cite book|last=Nahin|first=Paul J.|title=Time machines: time travel in physics, metaphysics, and science fiction|url=https://books.google.com/books?id=39KQY1FnSfkC&pg=PA54|publisher=Springer|year=2001|isbn=978-0-387-98571-8|access-date=2020-10-20|archive-date=2023-03-24|archive-url=https://web.archive.org/web/20230324181758/https://books.google.com/books?id=39KQY1FnSfkC&pg=PA54|url-status=live}}</ref>{{rp|54}}
=== Early time machines ===
{{main| Time machine (device)}}
One of the first stories to feature time travel by means of a machine is "[[The Clock that Went Backward]]" by [[Edward Page Mitchell]],<ref>{{cite web|last=Page Mitchell|first=Edward|title=The Clock That Went Backward|url=http://www.horrormasters.com/Text/a2221.pdf|access-date=December 4, 2011|url-status=dead|archive-url=https://web.archive.org/web/20111015110200/http://horrormasters.com/Text/a2221.pdf|archive-date=October 15, 2011}}</ref> which appeared in the ''[[The Sun (New York)|New York Sun]]'' 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.<ref name="Nahin2001" />{{rp|55}} [[Enrique Gaspar y Rimbau]]'s ''El Anacronópete'' (1887) may have been the first story to feature a vessel engineered to travel through time.<ref name="firsttimemachine">{{cite journal|last=Uribe|first=Augusto|title=The First Time Machine: Enrique Gaspar's Anacronópete|journal=[[The New York Review of Science Fiction]]|volume=11, no. 10|issue=130|page=12|date=June 1999}}</ref><ref>{{Cite book|last=Gaspar|first=Enrique|url=https://books.google.com/books?id=yAUiSP_Mr6wC&pg=PT17|title=The Time Ship: A Chrononautical Journey|date=2012-06-26|publisher=Wesleyan University Press|isbn=978-0-8195-7239-4|language=en|access-date=2022-12-29|archive-date=2023-03-24|archive-url=https://web.archive.org/web/20230324181757/https://books.google.com/books?id=yAUiSP_Mr6wC&pg=PT17|url-status=live}}</ref> [[Andrew Sawyer]] 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".<ref>{{cite news|last=Westcott|first=Kathryn|title=HG Wells or Enrique Gaspar: Whose time machine was first?|work=BBC News|date=9 April 2011|url=https://www.bbc.co.uk/news/world-europe-12900390|access-date=August 1, 2014|archive-url=https://web.archive.org/web/20140329161914/http://www.bbc.co.uk/news/world-europe-12900390|archive-date=March 29, 2014}}</ref> [[H. G. Wells]]' ''[[The Time Machine]]'' (1895) popularized the concept of time travel by mechanical means.<ref name="britannica1">{{cite encyclopedia|last=Sterling|first=Bruce|url=http://www.britannica.com/art/science-fiction/Major-science-fiction-themes#toc235731|title=science fiction | literature and performance :: Major science fiction themes|publisher=Britannica.com|date=August 27, 2014|access-date=November 27, 2015|archive-date=October 5, 2015|archive-url=https://web.archive.org/web/20151005152142/http://www.britannica.com/art/science-fiction/Major-science-fiction-themes#toc235731|url-status=live}}</ref>
== Time travel in physics ==
Some solutions to Einstein's equations for [[general relativity]] suggest that suitable geometries of [[spacetime]] or specific types of motion in [[space]] might allow time travel into the past and future if these geometries or motions were possible.<ref name="Thorne1994">{{cite book|last=Thorne|first=Kip S.|author-link=Kip Thorne|title=Black Holes and Time Warps|publisher=W. W. Norton|year=1994|isbn=978-0-393-31276-8|title-link=Black Holes and Time Warps}}</ref>{{rp|499}} In technical papers, [[physicist]]s discuss the possibility of [[closed timelike curve]]s, which are [[world line]]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 metric#Closed timelike curves|Gödel spacetime]], but the physical plausibility of these solutions is uncertain.<ref>{{Cite journal |last1=Ringbauer |first1=Martin |last2=Broome |first2=Matthew A. |last3=Myers |first3=Casey R. |last4=White |first4=Andrew G. |last5=Ralph |first5=Timothy C. |date=2014-06-19 |title=Experimental simulation of closed timelike curves |url=https://www.nature.com/articles/ncomms5145 |journal=Nature Communications |language=en |volume=5 |issue=1 |pages=4145 |doi=10.1038/ncomms5145 |pmid=24942489 |issn=2041-1723|arxiv=1501.05014 |bibcode=2014NatCo...5.4145R }}</ref>
Any theory that would allow backward time travel would introduce potential problems of [[Causality (physics)|causality]].<ref name="Bolonkin">{{cite book|title=Universe, Human Immortality and Future Human Evaluation|first1=Alexander|last1=Bolonkin|publisher=Elsevier|year=2011|isbn=978-0-12-415810-8|page=32|url=https://books.google.com/books?id=IFZWTf93KwgC|access-date=2017-03-26|archive-date=2023-03-24|archive-url=https://web.archive.org/web/20230324181808/https://books.google.com/books?id=IFZWTf93KwgC|url-status=live}} [https://books.google.com/books?id=IFZWTf93KwgC&pg=PA32 Extract of page 32] {{Webarchive|url=https://web.archive.org/web/20230324181801/https://books.google.com/books?id=IFZWTf93KwgC&pg=PA32 |date=2023-03-24 }}</ref> The classic example of a problem involving causality is the "[[grandfather paradox]]," which postulates travelling to the past and intervening in the conception of one's ancestors (causing the death of an ancestor before conception being frequently cited). Some physicists, such as Novikov and Deutsch, suggested that these sorts of [[temporal paradox]]es can be avoided through the [[Novikov self-consistency principle]] or a variation of the [[many-worlds interpretation]] with interacting worlds.<ref name="Everett MWI" />
=== General relativity ===
Time travel to the past is theoretically possible in certain general relativity spacetime geometries that permit traveling [[faster-than-light|faster than the speed of light]], such as [[cosmic string]]s, traversable [[wormhole]]s, and [[Alcubierre drive]]s.<ref>{{cite web|url=http://ccrg.rit.edu/files/FasterThanLight.pdf|title=Warp Drives, Wormholes, and Black Holes|author=Miguel Alcubierre|date=June 29, 2012|access-date=January 25, 2017|archive-date=March 18, 2016|archive-url=https://web.archive.org/web/20160318223348/http://ccrg.rit.edu/files/FasterThanLight.pdf|url-status=dead}}</ref><ref name="Gott">{{cite book|author=J. Richard Gott|title=Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time|url=https://books.google.com/books?id=3QBgCgAAQBAJ&pg=PT45|date=25 August 2015|publisher=HMH|isbn=978-0-547-52657-7|page=33|access-date=3 February 2018|archive-date=24 March 2023|archive-url=https://web.archive.org/web/20230324181808/https://books.google.com/books?id=3QBgCgAAQBAJ&pg=PT45|url-status=live}}</ref>{{rp|33–130}} The theory of [[general relativity]] does suggest a scientific basis for the possibility of backward time travel in certain unusual scenarios, although arguments from [[semiclassical gravity]] suggest that when [[quantum mechanics|quantum]] effects are incorporated into general relativity, these loopholes may be closed.<ref>{{cite book|arxiv=gr-qc/0204022|last=Visser|first=Matt|title=The quantum physics of chronology protection|url=https://archive.org/details/arxiv-gr-qc0204022|year=2002|bibcode=2003ftpc.book..161V}}</ref> These semiclassical arguments led [[Stephen Hawking]] to formulate the [[chronology protection conjecture]], suggesting that the fundamental laws of nature prevent time travel,<ref name="chronology protection">{{cite journal|first=Stephen|last=Hawking|author-link=Stephen Hawking|title=Chronology protection conjecture|journal=Physical Review D|volume=46|year=1992|issue=2|pages=603–611|doi=10.1103/PhysRevD.46.603|pmid=10014972|bibcode=1992PhRvD..46..603H|url=http://thelifeofpsi.com/wp-content/uploads/2014/09/Hawking-1992.pdf|archive-url=https://web.archive.org/web/20150227141021/http://thelifeofpsi.com/wp-content/uploads/2014/09/Hawking-1992.pdf|archive-date=2015-02-27}}</ref> but physicists cannot come to a definitive judgment on the issue without a theory of [[quantum gravity]] to join quantum mechanics and general relativity into a completely unified theory.<ref name="sagan-nova">{{cite web|url=https://www.pbs.org/wgbh/nova/time/sagan.html|title=Carl Sagan Ponders Time Travel|work=NOVA|date=December 10, 1999|publisher=[[PBS]]|access-date=April 26, 2017|archive-date=July 15, 2019|archive-url=https://web.archive.org/web/20190715000440/https://www.pbs.org/wgbh/nova/time/sagan.html|url-status=live}}</ref><ref name="futureofspacetime">{{cite book|last1=Hawking|first1=Stephen|author-link=Stephen Hawking|last2=Thorne|first2=Kip|author-link2=Kip Thorne|last3=Novikov|first3=Igor|author-link3=Igor Dmitriyevich Novikov|last4=Ferris|first4=Timothy|author-link4=Timothy Ferris|last5=Lightman|first5=Alan|author-link5=Alan Lightman|title=The Future of Spacetime|publisher=W. W. Norton|year=2002|isbn=978-0-393-02022-9|url=https://books.google.com/books?id=LlVcB7rz4mkC&pg=PA750}}</ref>{{rp|150}}
==== Different spacetime geometries ====
The theory of [[general relativity]] describes the universe under a system of [[Einstein field equations|field equations]] that determine the [[Metric (general relativity)|metric]], or distance function, of spacetime. There exist exact solutions to these equations that include [[closed time-like curve]]s, which are [[world line]]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]], a solution known as the [[Gödel metric]], but his (and others') solution requires the universe to have physical characteristics that it does not appear to have,<ref name="Thorne1994"/>{{rp|499}} such as [[Mach's principle|rotation]] and lack of [[Hubble expansion]]. Whether general relativity forbids closed time-like curves for all realistic conditions is still being researched.<ref name="Hawking">S. W. Hawking, ''Introductory note to 1949 and 1952'' in Kurt Gödel, ''Collected works'', Volume II (S. Feferman et al., eds).</ref>
==== Wormholes ====
{{
Wormholes are a hypothetical warped spacetime permitted by the [[Einstein field equations]] of general relativity.<ref name="Visser1996">{{cite book|last=Visser|first=Matt|author-link=Matt Visser|title=Lorentzian Wormholes|publisher=Springer-Verlag|year=1996|isbn=978-1-56396-653-8}}</ref>{{rp|100}} A proposed time-travel machine using a [[Wormhole#Traversable wormholes|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 [[Vehicle propulsion|propulsion system]], 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]] 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 [[Synchronization|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.<ref name="Thorne1994" />{{rp|502}} 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;<ref name="Thorne1994" />{{rp|503}} 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]]". More technically, the wormhole spacetime requires a distribution of energy that violates various [[energy condition]]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,<ref name="Visser1996" />{{rp|101}} and many physicists believe that the required negative energy may actually be possible due to the [[Casimir effect]] in quantum physics.<ref name="casimir">{{cite web|url=http://www.npl.washington.edu/av/altvw69.html|title=NASA Goes FTL Part 1: Wormhole Physics|work=Analog Science Fiction & Fact Magazine|year=1994|access-date=December 2, 2006|last1=Cramer|first1=John G.|author-link=John G. Cramer|archive-url=https://web.archive.org/web/20060627211046/http://www.npl.washington.edu/av/altvw69.html <!-- Bot retrieved archive -->|archive-date=June 27, 2006}}</ref> 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.<ref name="negative energy">{{cite journal|first=Matt|last=Visser|author-link=Matt Visser|author2=Sayan Kar|author3=Naresh Dadhich|title=Traversable wormholes with arbitrarily small energy condition violations|journal=[[Physical Review Letters]]|volume=90|year=2003|issue=20|pages=201102.1–201102.4|doi=10.1103/PhysRevLett.90.201102|arxiv=gr-qc/0301003|bibcode=2003PhRvL..90t1102V|pmid=12785880|s2cid=8813962}}</ref>
In 1993, [[Matt Visser]] 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.<ref name="visser_1">{{cite journal|first=Matt|last=Visser|author-link=Matt Visser|title=From wormhole to time machine: Comments on Hawking's Chronology Protection Conjecture|journal=Physical Review D|volume=47|year=1993|issue=2|pages=554–565|doi=10.1103/PhysRevD.47.554|pmid=10015609|arxiv=hep-th/9202090|bibcode=1993PhRvD..47..554V|s2cid=16830951}}</ref> Because of this, the two mouths could not be brought close enough for [[Causality (physics)|causality]] violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "[[Roman ring]]" (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.<ref name="visser_2">{{cite journal|first=Matt|last=Visser|author-link=Matt Visser|title=Traversable wormholes: the Roman ring|journal=Physical Review D|volume=55|year=1997|issue=8|pages=5212–5214|doi=10.1103/PhysRevD.55.5212|arxiv=gr-qc/9702043|bibcode=1997PhRvD..55.5212V|s2cid=2869291}}</ref>
==== Other approaches based on general relativity ====
Another approach involves a dense spinning cylinder usually referred to as a [[Tipler cylinder]], a GR solution discovered by [[Willem Jacob van Stockum]]<ref name="stockum">{{cite journal|first=Willem Jacob|last=van Stockum|author-link=Willem Jacob van Stockum|url=http://www-lorentz.leidenuniv.nl/history/stockum/Proc_R_Soc_Edinb_57_135_1937.jpg|title=The Gravitational Field of a Distribution of Particles Rotating about an Axis of Symmetry|year=1936|journal=Proceedings of the Royal Society of Edinburgh|url-status=dead|archive-url=https://web.archive.org/web/20080819215608/http://www-lorentz.leidenuniv.nl/history/stockum/Proc_R_Soc_Edinb_57_135_1937.jpg|archive-date=2008-08-19}}</ref> in 1936 and [[Kornel Lanczos]]<ref name="lanczos">{{cite journal|first=Kornel|last=Lanczos|author-link=Kornel Lanczos|doi=10.1023/A:1010277120072|title=On a Stationary Cosmology in the Sense of Einstein's Theory of Gravitation|year=1924 <!--republished in 1997-->|journal=General Relativity and Gravitation|publisher=Springland Netherlands|volume=29|issue=3|pages=363–399|s2cid=116891680}}</ref> in 1924, but not recognized as allowing closed timelike curves<ref name="Earman">{{cite book|last=Earman|first=John|title=Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes|publisher=Oxford University Press|year=1995|isbn=978-0-19-509591-3|bibcode=1995bcws.book.....E}}</ref>{{rp|21}} until an analysis by [[Frank Tipler]] in 1974.<ref name="tipler">{{cite journal|first=Frank J|last=Tipler|author-link=Frank J. Tipler|title=Rotating Cylinders and the Possibility of Global Causality Violation|journal=Physical Review D|volume=9|year=1974|issue=8|page=2203|doi=10.1103/PhysRevD.9.2203|bibcode=1974PhRvD...9.2203T|s2cid=17524515}}</ref> 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 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]]). 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,<ref name="Earman" />{{rp|169}} 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."<ref name="futureofspacetime" />{{rp|96}} This result comes from Hawking's 1992 paper on the [[chronology protection conjecture]], which Hawking states as "The laws of physics do not allow the appearance of closed timelike curves."<ref name="chronology protection" />
=== Quantum physics ===
{{main|Quantum mechanics of time travel}}
==== 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 [[Relativity of simultaneity|simultaneity]] 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.<ref name="Jarrell">{{cite web|url=http://www.physics.uc.edu/~jarrell/COURSES/ELECTRODYNAMICS/Chap11/chap11.pdf|title=The Special Theory of Relativity|access-date=October 27, 2006|last1=Jarrell|first1=Mark|pages=7–11|archive-url=https://web.archive.org/web/20060913173236/http://www.physics.uc.edu/~jarrell/COURSES/ELECTRODYNAMICS/Chap11/chap11.pdf <!-- Bot retrieved archive -->|archive-date=September 13, 2006}}</ref> The signal could be said to have moved backward in time. This hypothetical scenario is sometimes referred to as a [[tachyonic antitelephone]].<ref>{{cite journal|last=Kowalczyński|first=Jerzy|date=January 1984|title=Critical comments on the discussion about tachyonic causal paradoxes and on the concept of superluminal reference frame|journal=[[International Journal of Theoretical Physics]]|publisher=[[Springer Science+Business Media]]|volume=23|issue=1|pages=27–60|doi=10.1007/BF02080670|bibcode=1984IJTP...23...27K|s2cid=121316135}}</ref>
Quantum-mechanical phenomena such as [[quantum teleportation]], the [[EPR paradox]], or [[quantum entanglement]] 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]] presume that some information is being exchanged between particles instantaneously in order to maintain correlations between particles.<ref name="Bohm">{{cite web|url=http://plato.stanford.edu/entries/qm-bohm/|title=Bohmian Mechanics|date=March 27, 2017|access-date=April 26, 2017|last1=Goldstein|first1=Sheldon|archive-date=January 12, 2012|archive-url=https://web.archive.org/web/20120112030926/http://plato.stanford.edu/entries/qm-bohm/|url-status=live}}</ref> This effect was referred to as "[[action at a distance (physics)#spooky action at a distance|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 theory|quantum field theories]], and therefore modern theories do not allow for time travel or [[Superluminal communication|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.<ref name="Nielsen and Chuang">{{cite book|last1=Nielsen|last2=Chuang|first1=Michael|first2=Isaac|title=Quantum Computation and Quantum Information|url=https://archive.org/details/quantumcomputati00niel_056|url-access=limited|publisher=Cambridge|year=2000|page=[https://archive.org/details/quantumcomputati00niel_056/page/n55 28]|isbn=978-0-521-63235-5}}</ref> The [[no-communication theorem]] 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]]'s [[many-worlds interpretation]] (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.<ref>{{citation|title=Time Travel and Modern Physics|date=December 23, 2009|author1=Frank Arntzenius|author2=Tim Maudlin|url=http://plato.stanford.edu/entries/time-travel-phys/|encyclopedia=Stanford Encyclopedia of Philosophy|access-date=August 9, 2005|archive-date=May 25, 2011|archive-url=https://web.archive.org/web/20110525025650/http://plato.stanford.edu/entries/time-travel-phys/|url-status=live}}</ref> The accepted many-worlds interpretation suggests that all possible quantum events can occur in mutually exclusive histories.<ref name="many-worlds">{{cite web|url=http://plato.stanford.edu/entries/qm-manyworlds/|title=Many-Worlds Interpretation of Quantum Mechanics|date=January 17, 2014|access-date=April 26, 2017|last1=Vaidman|first1=Lev|archive-date=December 9, 2019|archive-url=https://web.archive.org/web/20191209220612/http://plato.stanford.edu/entries/qm-manyworlds/|url-status=live}}</ref> However, some variations allow different universes to interact. This concept is most often used in science-fiction, but some physicists such as [[David Deutsch]] have suggested that a time traveler should end up in a different history than the one he started from.<ref name="deutsch">{{cite journal|last=Deutsch|first=David|author-link=David Deutsch|year=1991|title=Quantum mechanics near closed timelike lines|journal=Physical Review D|volume=44|issue=10|pages=3197–3217|doi=10.1103/PhysRevD.44.3197|pmid=10013776|bibcode=1991PhRvD..44.3197D|s2cid=38691795|url=http://pdfs.semanticscholar.org/8e99/3e3e9b0952198a51ed99c9c0af3a31f433df.pdf|archive-url=https://web.archive.org/web/20190228075930/http://pdfs.semanticscholar.org/8e99/3e3e9b0952198a51ed99c9c0af3a31f433df.pdf|url-status=dead|archive-date=2019-02-28}}</ref><ref>{{citation|author=Pieter Kok|title=Time Travel Explained: Quantum Mechanics to the Rescue?|url=https://www.youtube.com/watch?v=uz9eLjO2BrA |archive-url=https://ghostarchive.org/varchive/youtube/20211211/uz9eLjO2BrA| archive-date=2021-12-11 |url-status=live|date=February 3, 2013}}{{cbignore}}</ref> 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.<ref name="Hawking warp"/> 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.<ref name="Everett MWI">{{cite journal|last=Everett|first=Allen|title=Time travel paradoxes, path integrals, and the many worlds interpretation of quantum mechanics|journal=Physical Review D|volume=69|issue=124023|pages=124023|year=2004|doi=10.1103/PhysRevD.69.124023|arxiv=gr-qc/0410035|bibcode=2004PhRvD..69l4023E|s2cid=18597824}}</ref>
=== Experimental results ===
Certain experiments carried out give the impression of reversed [[causality]], but fail to show it under closer examination.{{Citation needed|date=July 2025}}
The [[delayed-choice quantum eraser]] experiment performed by [[Marlan Scully]] involves pairs of [[Quantum entanglement|entangled]] [[photon]]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]]. 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 (wave propagation)|interference pattern]] 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.<ref name=Greene2004>{{cite book|last=Greene|first=Brian|title=The Fabric of the Cosmos|url=https://archive.org/details/fabricofcosmossp00gree|url-access=registration|year=2004|publisher=Alfred A. Knopf|isbn=978-0-375-41288-2|pages=[https://archive.org/details/fabricofcosmossp00gree/page/197 197–199]}}</ref>
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 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,<ref name="gauthier">{{cite news|last=Wright|first=Laura|title=Score Another Win for Albert Einstein|magazine=[[Discover (magazine)|Discover]]|date=November 6, 2003|url=http://discovermagazine.com/2003/nov/score-another-win-for-einstein1106|access-date=October 21, 2009|archive-date=June 12, 2018|archive-url=https://web.archive.org/web/20180612193137/http://discovermagazine.com/2003/nov/score-another-win-for-einstein1106|url-status=live}}</ref> so this experiment is understood not to violate causality either.
The physicists [[Günter Nimtz]] 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 traveled "instantaneously" between a pair of prisms that had been moved up to {{convert|3|ft|m|abbr=on}} 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 M. 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 speed of the center of the train exceeds the speed of any of the individual cars.<ref name="nimtz">{{cite news|last=Anderson|first=Mark|title=Light seems to defy its own speed limit|magazine=[[New Scientist]]|volume=195|issue=2617|page=10|date=August 18–24, 2007|url=https://www.eurekalert.org/pub_releases/2007-08/ns-lst081607.php|access-date=2018-09-18|archive-date=2018-06-12|archive-url=https://web.archive.org/web/20180612142609/https://www.eurekalert.org/pub_releases/2007-08/ns-lst081607.php|url-status=live}}</ref>
[[Shengwang Du]] claims in a peer-reviewed journal to have observed single photons' [[Precursor (physics)|precursor]]s, saying that they travel no faster than ''[[Speed of light|c]]'' in a vacuum. His experiment involved [[slow light]] as well as passing light through a vacuum. He generated two single [[photon]]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'' and, thus, no possibility of violating causality.<ref>{{citation|url=http://www.ust.hk/eng/news/press_20110719-893.html|publisher=The Hong Kong University of Science & Technology|access-date=September 5, 2011|title=HKUST Professors Prove Single Photons Do Not Exceed the Speed of Light|date=July 17, 2011|archive-date=February 25, 2012|archive-url=https://web.archive.org/web/20120225022608/http://www.ust.hk/eng/news/press_20110719-893.html|url-status=live}}</ref>
=== 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]] 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]] once suggested the possibility that time travelers could be here but are disguising their existence or are not recognized as time travelers.<ref name="sagan-nova" /> Some interpretations of general relativity propose that time travel could only occur within specific conditions such as regions of [[spacetime]] affected by extreme [[Gravitational field|gravitational fields]] or other distortions. In such scenarios, time travelers would be limited to moving within the period after such a region came into existence, and could not travel to a time before it formed. [[Stephen Hawking]] stated that this would explain why the world has not already been overrun by "tourists from the future".<ref name="Hawking warp">{{cite web|url=https://www.hawking.org.uk/in-words/lectures/space-and-time-warps|title=Space and Time Warps|year=1999|access-date=September 23, 2020|last1=Hawking|first1=Stephen|archive-date=October 31, 2020|archive-url=https://web.archive.org/web/20201031050328/https://www.hawking.org.uk/in-words/lectures/space-and-time-warps|url-status=live}}</ref>
[[File:WelcomeKrononauts Artforum Jan1980 p.90 800x600.png|thumb|Advertisement placed in a 1980 edition of ''[[Artforum]]'', advertising the Krononauts event]]
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]], [[Massachusetts Institute of Technology|MIT]]'s [[Time Traveler Convention]] and Stephen Hawking's [[Hawking's time traveller party|Reception For Time Travellers]] heavily publicized permanent "advertisements" of a meeting time and place for future time travelers to meet.<ref>{{citation|url=https://www.wired.com/2005/05/time-travelers-welcome-at-mit/|title=Time Travelers Welcome at MIT|author=Mark Baard|date=September 5, 2005|publisher=[[Wired (magazine)|Wired]]|access-date=June 18, 2018|archive-date=June 18, 2018|archive-url=https://web.archive.org/web/20180618125948/https://www.wired.com/2005/05/time-travelers-welcome-at-mit/|url-status=live}}</ref><ref>{{cite news |title=Stephen Hawking service: Possibility of time travellers 'can't be excluded' |url=https://www.bbc.co.uk/news/uk-england-cambridgeshire-44073903 |access-date=18 October 2024 |work=BBC News |date=12 May 2018}}</ref> In 1982, a group in [[Baltimore]], [[Maryland]], identifying itself as the Krononauts, hosted an event of this type welcoming visitors from the future.<ref>{{cite news|last=Franklin|first=Ben A.|date=March 11, 1982|url=https://select.nytimes.com/gst/abstract.html?res=F70E13FD395F0C728DDDAA0894DA484D81|title=The night the planets were aligned with Baltimore lunacy|archive-url=https://web.archive.org/web/20081206170526/http://select.nytimes.com/gst/abstract.html?res=F70E13FD395F0C728DDDAA0894DA484D81|archive-date=2008-12-06|work=[[The New York Times]]}}</ref><ref>"Welcome the People from the Future. March 9, 1982". Ad in ''[[Artforum]]'' p. 90.</ref>
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]] 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]].<ref>{{cite journal|author1=Jaume Garriga|author2=Alexander Vilenkin|doi=10.1103/PhysRevD.64.043511|year=2001|volume=64|issue=4|page=043511|journal=Phys. Rev. D|arxiv=gr-qc/0102010|bibcode=2001PhRvD..64d3511G|title=Many worlds in one|s2cid=119000743}}</ref>
==
{{main|Time
[[File:Time dilation02.gif|thumb|Transversal time dilation. The blue dots represent a pulse of light. Each pair of dots with light "bouncing" between them is a clock. For each group of clocks, the other group appears to be ticking more slowly, because the moving clock's light pulse has to travel a larger distance than the stationary clock's light pulse. That is so, even though the clocks are identical and their relative motion is perfectly reciprocal.]]
There is a great deal of observable evidence for time dilation in special relativity<ref name="tomroberts">{{cite web|last=Roberts|first=Tom|title=What is the experimental basis of Special Relativity?|date=October 2007|url=http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#Tests_of_time_dilation|access-date=April 26, 2017|archive-date=May 1, 2013|archive-url=https://web.archive.org/web/20130501002220/http://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html#Tests_of_time_dilation|url-status=live}}</ref> and gravitational time dilation in general relativity,<ref name="scoutrocket">{{cite web|title=Scout Rocket Experiment|url=http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/gratim.html#c3|last1=Nave|first1=Carl Rod|work=HyperPhysics|year=2012|access-date=April 26, 2017|archive-date=April 26, 2017|archive-url=https://web.archive.org/web/20170426195700/http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/gratim.html#c3|url-status=live}}</ref><ref name="hafelekeating">{{cite web|title=Hafele-Keating Experiment|url=http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/airtim.html#c3|last1=Nave|first1=Carl Rod|work=HyperPhysics|year=2012|access-date=April 26, 2017|archive-date=April 18, 2017|archive-url=https://web.archive.org/web/20170418005731/http://hyperphysics.phy-astr.gsu.edu/hbase/Relativ/airtim.html#c3|url-status=live}}</ref><ref name="GPS">{{cite web|last=Pogge|first=Richard W.|title=GPS and Relativity|date=April 26, 2017|url=http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html|access-date=April 26, 2017|archive-date=November 14, 2015|archive-url=https://web.archive.org/web/20151114135709/http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html|url-status=live}}</ref> for example in the famous and easy-to-replicate observation of [[Experimental testing of time dilation|atmospheric muon decay]].<ref name=easwar>{{cite journal|author1=Easwar, Nalini|author2=Macintire, Douglas A.|title=Study of the effect of relativistic time dilation on cosmic ray muon flux – An undergraduate modern physics experiment|journal=American Journal of Physics|volume=59|issue=7|year=1991|pages=589–592|doi=10.1119/1.16841|bibcode=1991AmJPh..59..589E|url=https://scholarworks.smith.edu/cgi/viewcontent.cgi?article=1041&context=phy_facpubs|access-date=2020-09-08|archive-date=2020-11-04|archive-url=https://web.archive.org/web/20201104084110/https://scholarworks.smith.edu/cgi/viewcontent.cgi?article=1041&context=phy_facpubs|url-status=live}}</ref><ref>{{cite journal|author1=Coan, Thomas|author2=Liu, Tiankuan|author3=Ye, Jingbo|title=A Compact Apparatus for Muon Lifetime Measurement and Time Dilation Demonstration in the Undergraduate Laboratory|journal=American Journal of Physics|volume=74|issue=2|pages=161–164|year=2006|doi=10.1119/1.2135319|arxiv=physics/0502103|bibcode=2006AmJPh..74..161C|s2cid=30481535}}</ref><ref name="Ferraro" /> 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.<ref name="Ferraro">{{citation|title=Einstein's Space-Time: An Introduction to Special and General Relativity|first1=Rafael|last1=Ferraro|pages=52–53|publisher=Springer Science & Business Media|date=2007|isbn=9780387699462|bibcode=2007esti.book.....F}}</ref> 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 speed]]s or through the effects of [[gravity]].<ref>Serway, Raymond A. (2000) ''Physics for Scientists and Engineers with Modern Physics'', Fifth Edition, Brooks/Cole, p. 1258, {{ISBN|0030226570}}.</ref>
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 well]]s, 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]].<ref name="Gott"/>{{rp|33–130}}
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.<ref name="Gott" />{{rp|76–140}} 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.<ref>{{cite magazine|last=Mowbray|first=Scott|title=Let's Do the Time Warp Again|url=http://www.popsci.com/scitech/article/2002-02/lets-do-time-warp-again?page=2|magazine=Popular Science|access-date=8 July 2011|date=19 February 2002|quote=Spending just over two years in Mir's Earth orbit, going 17,500 miles per hour, put Sergei Avdeyev 1/50th of a second into the future{{nbsp}}... 'he's the greatest time traveler we have so far.'|archive-date=28 June 2010|archive-url=https://web.archive.org/web/20100628190931/http://www.popsci.com/scitech/article/2002-02/lets-do-time-warp-again?page=2|url-status=live}}</ref>
== Philosophy ==
{{main|Philosophy of space and time}}
Philosophers have discussed the [[philosophy of space and 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]] of [[Theory of relativity|relativity]].<ref>{{citation|title=The Dictionary of Philosophy|editor=Dagobert D. Runes|page=318|section=Time|year=1942|publisher=Philosophical Library}}</ref>
=== 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.<ref name="Crisp">{{citation|chapter=Presentism, Eternalism, and Relativity Physics|title=Einstein, Relativity and Absolute Simultaneity|author=Thomas M. Crisp|editor1=William Lane Craig|editor2=Quentin Smith|page=footnote 1|chapter-url=https://thomasmcrisp.files.wordpress.com/2017/07/presentism-eternalism-and-relativity-physics.pdf|year=2007|access-date=2018-02-01|archive-date=2018-02-02|archive-url=https://web.archive.org/web/20180202012816/https://thomasmcrisp.files.wordpress.com/2017/07/presentism-eternalism-and-relativity-physics.pdf|url-status=live}}</ref> 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".<ref>{{citation|author=Dean Rickles|year=2007|title=Symmetry, Structure, and Spacetime|url=https://books.google.com/books?id=gDwJYtfoCh8C&pg=PA158|page=158|publisher=Elsevier |access-date=July 9, 2016|isbn=9780444531162|archive-date=March 24, 2023|archive-url=https://web.archive.org/web/20230324181809/https://books.google.com/books?id=gDwJYtfoCh8C&pg=PA158|url-status=live}}</ref> 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.<ref>{{citation|author=Tim Maudlin|title=The Metaphysics Within Physics|isbn=9780199575374|year=2010|chapter=On the Passing of Time|publisher=Oxford University Press |chapter-url=https://philocosmology.rutgers.edu/images/uploads/TimDavidClass/05-maudlin-chap04.pdf|author-link=Tim Maudlin|access-date=2018-02-01|archive-date=2021-03-08|archive-url=https://web.archive.org/web/20210308030324/https://philocosmology.rutgers.edu/images/uploads/TimDavidClass/05-maudlin-chap04.pdf|url-status=live}}</ref>
[[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.<ref name="Crisp" /> 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;<ref name="Presentism">{{cite journal|first=Simon|last=Keller|author2=Michael Nelson|title=Presentists should believe in time-travel|url=http://people.bu.edu/stk/Papers/Timetravel.pdf|journal= Australasian Journal of Philosophy|volume=79|issue=3|pages=333–345|date=September 2001|doi=10.1080/713931204|s2cid=170920718|url-status=dead|archive-url=https://web.archive.org/web/20081028211537/http://people.bu.edu/stk/Papers/Timetravel.pdf|archive-date=October 28, 2008}}</ref> these views are contested by some authors.<ref name="Bourne">{{cite book|author=Craig Bourne|title=A Future for Presentism|url=https://books.google.com/books?id=DJMSDAAAQBAJ|date=7 December 2006|publisher=Clarendon Press|isbn=978-0-19-921280-4}}</ref>
=== The grandfather paradox ===
{{main|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.<ref name="horwich">{{cite book|last1=Horwich|first1=Paul|title=Asymmetries in Time: Problems in the Philosophy of Science|date=1987|publisher=MIT Press|___location=Cambridge, Massachusetts|isbn=978-0262580885|page=116|edition=2nd}}</ref> 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''.<ref name="NicholasSmith">{{cite encyclopedia|url=http://plato.stanford.edu/entries/time-travel/index.html#CauLoo|author=Nicholas J.J. Smith|date=2013|title=Time Travel|encyclopedia=Stanford Encyclopedia of Philosophy|access-date=November 2, 2015|archive-date=August 18, 2018|archive-url=https://web.archive.org/web/20180818152007/https://plato.stanford.edu/entries/time-travel/index.html#CauLoo|url-status=live}}</ref><ref name="lobo">{{cite journal|title=Time, Closed Timelike Curves and Causality|journal=The Nature of Time: Geometry|volume=95|pages=289–296|author=Francisco Lobo|year=2003|arxiv=gr-qc/0206078v2|bibcode=2003ntgp.conf..289L}}</ref> 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.<ref name="sagan-nova" /> Philosophers question whether these paradoxes prove time travel impossible. Some philosophers answer these 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,<ref name="unchangeable">{{cite web|author=Norman Swartz|title=Time Travel: Visiting the Past|url=https://www.sfu.ca/~swartz/time_travel1.htm|date=1993|access-date=February 20, 2016|archive-date=August 18, 2018|archive-url=https://web.archive.org/web/20180818151754/https://www.sfu.ca/~swartz/time_travel1.htm|url-status=live}}</ref> an idea similar to the proposed [[Novikov self-consistency principle]] 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.<ref>{{cite journal|last=Lewis|first=David|title=The paradoxes of time travel|journal=[[American Philosophical Quarterly]]|volume=13|pages=145–52|year=1976|url=http://www.csus.edu/indiv/m/merlinos/Paradoxes%20of%20Time%20Travel.pdf|bibcode=1996gr.qc.....3042K|arxiv=gr-qc/9603042|access-date=2010-09-06|archive-date=2017-08-28|archive-url=https://web.archive.org/web/20170828174937/http://www.csus.edu/indiv/m/merlinos/paradoxes%20of%20time%20travel.pdf|url-status=live}}</ref>
==== Self-consistency principle ====
The [[Novikov self-consistency principle]], 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,<ref>{{cite book|last1=Erdmann|first1=Terry J.|last2=Hutzel|first2=Gary|title=Star Trek: The Magic of Tribbles|date=2001|publisher=Pocket Books|isbn=978-0-7434-4623-5|page=31}}</ref> ontological paradox,<ref name="smeenk">{{citation|last1=Smeenk|first1=Chris|last2=Wüthrich|first2=Christian|editor-last=Callender|editor-first=Craig|contribution=Time Travel and Time Machines|title=The Oxford Handbook of Philosophy of Time|year=2011|publisher=Oxford University Press|isbn=978-0-19-929820-4|page=581}}</ref> or bootstrap paradox.<ref name="smeenk" /><ref>{{citation|last=Krasnikov|first=S.|year=2001|title=The time travel paradox|journal=Phys. Rev. D|volume=65|issue=6|page=06401|arxiv=gr-qc/0109029|bibcode=2002PhRvD..65f4013K|doi=10.1103/PhysRevD.65.064013|s2cid=18460829}}</ref> The term bootstrap paradox was popularized by [[Robert A. Heinlein]]'s story "[[By His Bootstraps]]".<ref name="Klosterman">{{cite book|last1=Klosterman|first1=Chuck|title=Eating the Dinosaur|date=2009|publisher=Scribner|___location=New York|isbn=9781439168486|edition=1st Scribner hardcover|pages=[https://books.google.com/books?id=lZurDFJtAWwC&pg=PA60 60–62]}}</ref> 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.<ref>{{cite journal|first=John|last=Friedman|author2=Michael Morris|author3=Igor Novikov|author4=Fernando Echeverria|author5=Gunnar Klinkhammer|author6=Kip Thorne|author7=Ulvi Yurtsever|url=http://authors.library.caltech.edu/3737/|title=Cauchy problem in spacetimes with closed timelike curves|journal=Physical Review D|volume=42|year=1990|issue=6|pages=1915–1930|doi=10.1103/PhysRevD.42.1915|pmid=10013039|bibcode=1990PhRvD..42.1915F|access-date=2009-01-10|archive-date=2011-09-28|archive-url=https://web.archive.org/web/20110928215902/http://authors.library.caltech.edu/3737/|url-status=live}}</ref>
The philosopher Kelley L. Ross argues in "Time Travel Paradoxes"<ref>{{citation|first1=Kelley L.|last1=Ross|url=http://www.friesian.com/paradox.htm|title=Time Travel Paradoxes|year=2016|access-date=April 26, 2017|archive-date=January 18, 1998|archive-url=https://web.archive.org/web/19980118212457/http://www.friesian.com/paradox.htm|url-status=live}}</ref> 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.<ref name="Gott" />{{rp|23}}
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.<ref name="greenberger">{{cite book|doi=10.1007/3-540-26669-0_4|title=Quo Vadis Quantum Mechanics?|year=2005|arxiv=quant-ph/0506027|bibcode=2005qvqm.book...63G|chapter=Quantum Theory Looks at Time Travel|series=The Frontiers Collection|last1=Greenberger|first1=Daniel M.|last2=Svozil|first2=Karl|isbn=978-3-540-22188-3|page=63|s2cid=119468684}}</ref><ref>{{cite news|last=Kettlewell|first=Julianna|url=http://news.bbc.co.uk/2/hi/4097258.stm|title=New model 'permits time travel'|work=BBC News|date=June 17, 2005|access-date=April 26, 2017|archive-date=April 14, 2017|archive-url=https://web.archive.org/web/20170414111240/http://news.bbc.co.uk/2/hi/4097258.stm|url-status=live}}</ref>
== See also ==
{{col-begin}}
{{col-break|width=20%}}
;Claims of time travel
* [[Time travel claims and urban legends]]
* [[Parapsychology]]
;Culture
* [[Time capsule]]
;Fiction
* [[Time travel in fiction]]
* [[List of time travel works of fiction]]
* [[Time viewer]]
;Meetings
* [[Time Traveler Convention]]
* [[Hawking's time traveller party]]
{{col-break|width=20%}}
;Science
* [[Krasnikov tube]]
* [[Retrocausality]]
* [[Ring singularity]]
* [[Temporal paradox]]
* [[Wheeler–Feynman absorber theory]]
;Time perception
* [[Cryonics]]
* [[Suspended animation]]
* [[Time perception]]
{{col-end}}
== Further reading ==
* ''[[Time Travel: A History]]'' – book by [[James Gleick]]
==
{{reflist}}
{{Sister project links|auto=y|wikt=y|commonscat=y}}
* {{cite web | title=NOVA - s27e03 - Time Travel | website=[[Nova (American TV program)|NOVA]] | date=October 12, 1999 | url=https://archive.org/details/Nova.Time.Travel | access-date=June 9, 2023}} ([https://www.pbs.org/wgbh/nova/transcripts/2612time.html Transcript])
* [http://www.vega.org.uk/video/programme/61 Black holes, Wormholes and Time Travel], a Royal Society Lecture
* {{HowStuffWorks|time-travel|How Time Travel Will Work}}
* [http://plato.stanford.edu/entries/time-travel-phys/ Time Travel and Modern Physics] at the Stanford Encyclopedia of Philosophy
* [http://www.iep.utm.edu/t/timetrav.htm Time Travel] at the Internet Encyclopedia of Philosophy
{{Time travel}}
{{Time topics}}
{{Authority control}}
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[[Category:Time travel| ]]
[[Category:Philosophy of physics]]
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