Time machine
{{Short description|Device for time travel}}
{{Other uses}}
A time machine is a fictional or hypothetical device that allows time travel.
Concept
{{Quote box
| quote = A time machine is a device which brings about closed timelike curves—and thus enables time travel—where none would have existed otherwise.
| source = Stanford Encyclopedia of Philosophy (2024)
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A time machine is a device that makes time travel possible.{{Citation |last1=Earman |first1=John |title=Time Machines |date=2024 |editor-last=Zalta |editor-first=Edward N. |url=https://plato.stanford.edu/entries/time-machine/ |access-date=2025-03-06 |edition=Summer 2024 |publisher=Metaphysics Research Lab, Stanford University |last2=Wüthrich |first2=Christian |last3=Manchak |first3=JB |author-link=John Earman |editor2-last=Nodelman |editor2-first=Uri |encyclopedia=Stanford Encyclopedia of Philosophy}}{{Cite encyclopedia |year=2017 |title=Time Machine |encyclopedia=The Encyclopedia of Science Fiction |url=https://sf-encyclopedia.com/entry/time_machine |access-date=2025-03-13 |edition=4th |author1-last=Nicholls |author1-first=Peter|author1-link=Peter Nicholls (writer) |author2-last=Langford |author2-first=David |author2-link=David Langford |editor1-last=Clute |editor1-first=John |editor1-link=John Clute |editor2-last=Langford |editor2-first=David |editor2-link=David Langford |editor3-last=Sleight |editor3-first=Graham |editor3-link=Graham Sleight}}{{Rp|pages=137}}
The Stanford Encyclopedia of Philosophy distinguishes between two different types of time machines: a science fiction variety that is not bound by the laws of physics and a hypothetical variety discussed in theoretical physics that relies on the creation of closed timelike curves (CTCs). Paul J. Nahin distinguishes between "weak" time machines that exploit existing CTCs and "strong" time machines that are themselves responsible for the creation of the CTCs.{{Cite book |last=Nahin |first=Paul J. |author-link=Paul J. Nahin |url=https://books.google.com/books?id=4NfJDQAAQBAJ |title=Time Machine Tales: The Science Fiction Adventures and Philosophical Puzzles of Time Travel |date=2016 |publisher=Springer |isbn=978-3-319-48864-6 |language=en}}{{Rp|pages=xxxiii}}{{Cite book |last=Nahin |first=Paul J. |author-link=Paul J. Nahin |url=https://archive.org/details/timemachinestime0000nahi_m8y6 |title=Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction |date=1999 |publisher=Springer |others=Foreword by Kip S. Thorne |isbn=978-0-387-98571-8 |edition=Second |language=en |orig-date=1993}}{{Rp|pages=555}}
= Related concepts =
Brave New Words: The Oxford Dictionary of Science Fiction and The Encyclopedia of Science Fiction distinguish between this and time slip, where the time travel occurs without technological aid.{{Cite book |title=Brave New Words: The Oxford Dictionary of Science Fiction |date=2007 |others=with an introduction by Gene Wolfe |isbn=978-0-19-530567-8 |editor-last=Prucher |editor-first=Jeff |pages=230 |language=en |chapter=Time Travel |chapter-url=https://books.google.com/books?id=iYzi8m8FbEsC&pg=PA230}}{{Cite encyclopedia |year=2018 |title=Timeslip |encyclopedia=The Encyclopedia of Science Fiction |url=https://sf-encyclopedia.com/entry/timeslip |access-date=2025-03-06 |last=David |first=Langford |author1-link=David Langford |editor1-last=Clute |editor1-first=John |edition=4th |last2=Clements |first2=Jonathan |author1-last= |author1-first= |editor1-link=John Clute |editor2-last=Langford |editor2-first=David |editor2-link=David Langford |editor3-last=Sleight |editor3-first=Graham |editor3-link=Graham Sleight}}{{Rp|pages=230}} The Encyclopedia of Science Fiction additionally distinguishes the concept of an immobile time gate or portal.{{Cite encyclopedia |year=2018 |title=Time Gate |encyclopedia=The Encyclopedia of Science Fiction |url=https://sf-encyclopedia.com/entry/time_gate |access-date=2025-03-06 |author1-link=David Langford |editor1-last=Clute |editor1-first=John |edition=4th |author1-last=Langford |author1-first=David |editor1-link=John Clute |editor2-last=Langford |editor2-first=David |editor2-link=David Langford |editor3-last=Sleight |editor3-first=Graham |editor3-link=Graham Sleight}}
The Encyclopedia of Science Fiction lists a number of related concepts that allow for more limited interactions with other points in time, but not transportation: a time viewer allows for observation, while a time radio enables communication.
In fiction
{{Further|Time travel in fiction}}
Time travel has appeared in fiction since at least the 1700s, but the first instance of a machine being used for the purpose was Edward Page Mitchell's 1881 short story "The Clock that Went Backward".{{Rp|pages=137}}{{Cite encyclopedia |year=2024 |title=Time Travel |encyclopedia=The Encyclopedia of Science Fiction |url=https://sf-encyclopedia.com/entry/time_travel |access-date=2025-03-13 |edition=4th |author1-last=Edwards |author1-first=Malcolm |author1-link=Malcolm Edwards |author2-last=Stableford |author2-first=Brian |author2-link=Brian Stableford |author3-last=Langford |author3-first=David |author3-link=David Langford |editor1-last=Clute |editor1-first=John |editor1-link=John Clute |editor2-last=Langford |editor2-first=David |editor2-link=David Langford |editor3-last=Sleight |editor3-first=Graham |editor3-link=Graham Sleight}} The term "time machine" itself was coined by H. G. Wells in the 1895 novel The Time Machine.{{Cite book |last=Pilkington |first=Ace G. |author-link= |title=Science Fiction and Futurism: Their Terms and Ideas |date=2017 |publisher=McFarland |isbn=978-0-7864-9856-7 |series=Critical Explorations in Science Fiction and Fantasy |volume=58 |pages=137–139 |language=en |chapter=Time Machine |chapter-url=https://books.google.com/books?id=a2VEDgAAQBAJ&pg=PA137}}{{Rp|pages=137}} Later in the history of science fiction, time machines have often been wearable.
In physics
The first theoretical basis for a time machine based on our scientific understanding of the universe came in 1949, when Kurt Gödel demonstrated that CTCs are in principle consistent with Albert Einstein's theory of general relativity—although the so-called Gödel solution to the Einstein field equations requires a universe with properties different than ours (in particular, it would have to be rotating). Nahin comments that since travel along the CTCs in Gödel's scenario requires some form of accelerating force (as opposed to being a free-fall path, or geodesic) it is an example of a proper time machine, although since it does not create the CTCs but rather uses CTCs inherent in the structure of the universe it is only a time machine in the weaker sense.{{Rp|pages=xxxiii–xxxiv}}{{Rp|pages=18–19}}{{Cite book |last=Broderick |first=Damien |author-link=Damien Broderick |url=https://books.google.com/books?id=5RKiDwAAQBAJ |title=The Time Machine Hypothesis: Extreme Science Meets Science Fiction |date=2019 |publisher=Springer |isbn=978-3-030-16178-1 |series=Science and Fiction |language=en}}{{Rp|pages=10}} Several different theoretically possible—even if impracticable—time machine variants have been conceived of and described since.{{Cite book |last=Nahin |first=Paul J. |author-link=Paul J. Nahin |title=Holy Sci-Fi! |date=2014 |publisher=Springer Science & Business Media |isbn=978-1-4939-0618-5 |series=Science and Fiction |pages=149 |language=en |chapter=Time Travel: Fact or Fantasy? |doi=10.1007/978-1-4939-0618-5_7 |chapter-url=https://books.google.com/books?id=Hl64BAAAQBAJ&pg=PA149}}{{Rp|pages=149}} One of these is the Tipler cylinder, first described as a source of CTCs by Frank J. Tipler in 1974.{{Rp|pages=38–40}}{{Rp|pages=149}}{{Cite book |last=Webb |first=Stephen |author-link=Stephen Webb (scientist) |title=All the Wonder that Would Be: Exploring Past Notions of the Future |date=2017 |publisher=Springer |isbn=978-3-319-51759-9 |series=Science and Fiction |pages=139–146 |language=en |chapter=Closed Timelike Curves |doi=10.1007/978-3-319-51759-9_5 |chapter-url=https://books.google.com/books?id=TVPJDgAAQBAJ&pg=PA140}}{{Rp|pages=140–141}} Time machines that create CTCs by way of wormholes were described in theoretical research by Michael S. Morris, Kip Thorne, and {{Interlanguage link|Ulvi Yurtsever|qid=Q102357883}} that was published in 1988.