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In physics and fiction, a wormhole is a hypothetical topological feature of spacetime that would be, fundamentally, a 'shortcut' through space and time; although they are very popular in science fiction, there is no actual evidence that they exist. For a simple visual explanation of a wormhole, consider spacetime visualized as a two-dimensional (2-D) surface (see illustration, right). Now, if we 'fold' this surface along a (non-existant) 3rd dimension, it allows us to picture a wormhole 'bridge'. (Please note, though, that this image is merely a visualization, to allow the limited human brain to grasp an essentially unimaginable structure existing in 4 or more dimensions.) A wormhole has at least two 'mouths' that are connected via a 'throat', or tube. If the wormhole is traversable, then matter can 'travel' from one mouth to the other via the throat.

There is no observational evidence for wormholes, and, although wormholes are valid solutions in general relativity, this is only true if exotic matter can be used to stabilize them. Even if the wormhole is stabilized, the slightest fluctuation in space would collapse it. If such exotic matter, matter with negative mass, does not exist, all wormhole-containing solutions to Einstein's field equations are vacuum solutions, which require an impossible vacuum, free of all matter and energy. There is no evidence or experimental suggestion that wormholes do exist, except as predictions of certain (exotic) physical models. Wormholes allowed by current physical theories might arise spontaneously, but would vanish nearly instantaneously, and would likely be undetectable.

The American theoretical physicist John Archibald Wheeler coined the term wormhole in 1957; however, in 1921, the German mathematician Hermann Weyl already had proposed the wormhole theory, in connection with mass analysis of electromagnetic field energy.

Wormholes and Time Travel

A wormhole could allow time travel.This could be accomplished by accelerating one end of the wormhole to a high velocity relative to the other, and then sometime later bringing it back; relativistic time dilation would result in the accelerated wormhole mouth aging less than the stationary one as seen by an external observer, similar to what is seen in the twin paradox. However, time connects differently through the wormhole than outside it, so that synchronized clocks at each mouth will remain synchronized to someone traveling through the wormhole itself, no matter how the mouths move around. This means that anything which entered the accelerated wormhole mouth would exit the stationary one at a point in time prior to its entry.

For example, consider two clocks at both mouths both showing the date as 2000. After being taken on a trip at relativistic velocities, the accelerated mouth is brought back to the same region as the stationary mouth with the accelerated mouth's clock reading 2005 while the stationary mouth's clock read 2010. A traveller who entered the accelerated mouth at this moment would exit the stationary mouth when its clock also read 2005, in the same region but now five years in the past. Such a configuration of wormholes would allow for a particle's world line to form a closed loop in spacetime, known as a closed timelike curve.

It is thought that it may not be possible to convert a wormhole into a time machine in this manner; some analyses using the semiclassical approach to incorporating quantum effects into general relativity indicate that a feedback loop of virtual particles would circulate through the wormhole with ever-increasing intensity, destroying it before any information could be passed through it, in keeping with the chronology protection conjecture. This has been called into question by the suggestion that radiation would disperse after traveling through the wormhole, therefore preventing infinite accumulation. The debate on this matter is described by Kip S. Thorne in the book Black Holes and Time Warps. There is also the Roman ring, which is a configuration of more than one wormhole. This ring seems to allow a closed time loop with stable wormholes when analyzed using semiclassical gravity, although without a full theory of quantum gravity it is uncertain whether the semiclassical approach is reliable in this case.

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