Time machine
A time machine is a fictional or hypothetical device that allows time travel.Concept
A time machine is a device that makes time travel possible.
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. 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 distinguish between this and time slip, where the time travel occurs without technological aid. The Encyclopedia of Science Fiction additionally distinguishes the concept of an immobile time gate or portal.
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
Time travel has appeared in fiction since at least the 1700s, but the first known instance of a machine being used for the purpose was Edward Page Mitchell's 1881 short story "The Clock that Went Backward". The term "time machine" itself was coined by H. G. Wells in the 1895 novel The Time Machine. 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. Nahin comments that since travel along the CTCs in Gödel's scenario requires some form of accelerating force 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 wormholes were described in theoretical research by Michael S. Morris, Kip Thorne, and that was published in 1988.