A time machine is a fictional or hypothetical device that allows

time travel 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. In fiction, time travel is typically achieved through the use of a device known a ...

Concept

A time machine is a device that makes

possible. The ''

Stanford Encyclopedia of Philosophy The ''Stanford Encyclopedia of Philosophy'' (''SEP'') is a freely available online philosophy resource published and maintained by Stanford University, encompassing both an online encyclopedia of philosophy and peer-reviewed original publication ...

'' distinguishes between two different types of time machines: a

science fiction Science fiction (often shortened to sci-fi or abbreviated SF) is a genre of speculative fiction that deals with imaginative and futuristic concepts. These concepts may include information technology and robotics, biological manipulations, space ...

variety that is not bound by the laws of physics and a hypothetical variety discussed 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 List of natural phenomena, natural phenomena. This is in contrast to experimental p ...

that relies on the creation 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 St ...

s (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.

Related concepts

'' Brave New Words: The Oxford Dictionary of Science Fiction'' and ''

The Encyclopedia of Science Fiction ''The Encyclopedia of Science Fiction'' (''SFE'') is an English language reference work on science fiction, first published in 1979. It has won the Hugo Award, Hugo, Locus Award, Locus and BSFA Award, British SF Awards. Two print editions appea ...

'' distinguish between this and

time slip A time slip is a plot device in fantasy and science fiction in which a person, or group of people, seem to accidentally travel through time by unknown means, or by a means unknown to the character(s). Time slip is one of the main plot devices ...

, where the time travel occurs without technological aid. ''The Encyclopedia of Science Fiction'' additionally distinguishes the concept of an immobile time gate or portal. ''

'' 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

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 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 m ...

's 1881 short story "

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 hist ...

". The term "time machine" itself was coined by

H. G. Wells Herbert George Wells (21 September 1866 – 13 August 1946) was an English writer, prolific in many genres. He wrote more than fifty novels and dozens of short stories. His non-fiction output included works of social commentary, politics, hist ...

in the 1895 novel ''

The Time Machine ''The Time Machine'' is an 1895 dystopian post-apocalyptic science fiction novella by H. G. Wells about a Victorian scientist known as the Time Traveller who travels to the year 802,701. The work is generally credited with the popularizati ...

''. 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 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 profoundly ...

demonstrated that CTCs are in principle consistent with

Albert Einstein Albert Einstein (14 March 187918 April 1955) was a German-born theoretical physicist who is best known for developing the theory of relativity. Einstein also made important contributions to quantum mechanics. His mass–energy equivalence f ...

's theory of

general relativity General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...

—although the so-called Gödel solution to the

Einstein field equations In the General relativity, 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#In general relativity and cosmology, matter within it. ...

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 In geometry, a geodesic () is a curve representing in some sense the locally shortest path ( arc) between two points in a surface, or more generally in a Riemannian manifold. The term also has meaning in any differentiable manifold with a conn ...

) 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. Several different theoretically possible—even if impracticable—time machine variants have been conceived of and described since. One of these is the Tipler cylinder, first described as a source of CTCs by Frank J. Tipler in 1974. Time machines that create CTCs by way of

wormhole A wormhole is a hypothetical structure that connects disparate points in spacetime. It can be visualized as a tunnel with two ends at separate points in spacetime (i.e., different locations, different points in time, or both). Wormholes are base ...

s were described in theoretical research by Michael S. Morris,

Kip Thorne Kip Stephen Thorne (born June 1, 1940) is an American theoretical physicist and writer known for his contributions in gravitational physics and astrophysics. Along with Rainer Weiss and Barry C. Barish, he was awarded the 2017 Nobel Pri ...

, and that was published in 1988.

References

{{reflist Time travel Time travel devices

Concept

Related concepts

In fiction

In physics

See also

References