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A wormhole is a connection between two distant places in space that enables one to get from one place to another without passing through the space in between.

Science

A wormhole is one theoretical way to bypass the restriction of Einstein's Special Theory of Relativity that no physical object can move faster than the speed of light. Since an object passing through a wormhole does not actually traverse the distance between the two locations A (at one end of the wormhole) and B (at the other end of the wormhole), it can leave point A and arrive at point B in a time shorter than the time it would take light to move from A to B directly, without ever moving faster than light. Thus, using a wormhole could be a way to accomplish faster-than-light travel (FTL).

Such wormholes are allowed by Einstein's theory of General Relativity, although General Relativity does not require that they exist. In most theoretical models of wormholes in General Relativity, in order for a wormhole to remain open, it must contain "Exotic matter," which is matter (or stress-energy) that has so-called "negative energy density" in some frame of reference. Such matter has not been proven to exist in the real world, but may be theoretically possible. For practical purposes, it is desirable that a wormhole should be traversable, which is to say, it should remain open for at least long enough for an object to pass through it, and it should not subject an object passing through to tidal forces that would tear it apart. Although Einstein and his student Rosen theorized that such wormholes might exist, a detailed theory of traversable wormholes did not exist until a paper by Michael Morris and Kip Thorne laid out the basic theory of a simple wormhole in American Journal of Physics in 1987.

The two ends of a wormhole are conventionally called mouths. Thus, one enters the entry mouth of the wormhole, and comes out via the exit mouth.

In a paper in 1995, "Natural Wormholes as Gravitational Lenses" (Physical Review D, 15 March 1995, 3124–3127), authors J. Cramer, R. L. Forward, M. Morris, M. Visser, G. Benford, and G. Landis, argued that wormholes may have been formed naturally in the early universe, and could have been stabilized by negative mass cosmic string. The paper also pointed out that since wormholes obey the laws of conservation of mass, the "entry" mouth of a wormhole gains mass equal to the mass of all the objects which enter, while the "exit" mouth loses an equal amount of mass. This paper is noteworthy in that four of the authors (Cramer, Forward, Benford and Landis) are also science fiction writers.

Science Fiction

Wormholes have been featured extensively in science fiction. The physics paper by Morris and Thorne which first laid out the theory of a traversable wormhole, for example, was written after the writer and scientist Carl Sagan asked Kip Thorne whether such a wormhole would be allowed by the theory of relativity, because he wanted to use it in his science fiction novel Contact. Wormholes have featured notably in the television series Star Trek: Deep Space Nine, where the space station of the title is situated at one end of a wormhole; and in the series Farscape, where a wormhole is used to send an astronaut to a distant part of the galaxy. Likewise, the "Stargate" in the movie (Stargate) and television series (Stargate SG-1) features artificially created wormholes.

Wormholes have featured significantly in written science fiction, as well. For example, in Joe Haldeman's classic novel The Forever War, interstellar travel was accomplished via "collapsars" (short for "collapsed stars"). This makes use of an early hypothesis that the collapse of a star into a black hole would form a wormhole.

The basic idea behind a wormhole is explained in simplified form in Madeline L'Engle's classic book A Wrinkle in Time, in which she shows how an ant, crawling on a two-dimensional surface (a piece of cloth), can get from one point on the cloth to another more quickly by jumping off the cloth from one fold to another.

In general relativity, a wormhole may connect two places at different locations, or it may also connect two locations at different times. As shown in a classic paper by Yurtsever, Morris, and Thorne, any wormhole can be turned into a time machine. A wormhole may also connect to a different universe. In the television series Sliders, for example, the method used by the sliders to move from one universe to another was referred to as an "Einstein-Rosen bridge," which is to say, a wormhole.

Visualizations

A wormhole is often visualized in the form of a pair of funnels joined at the narrowest point, like an hourglass. This visualization, called an "embedding diagram," shows (in two-dimensional form) some of the geometrical features of a wormhole, but it is important to note that this is only a model. A real wormhole, if one existed, would not look at all like a funnel. The Morris-Thorne wormhole would be perfectly spherical. In appearance, it would look like a reflective sphere, with the interesting property that the apparent "reflection" would be an image of the scene from the other mouth of the wormhole. A variant form of a wormhole, the Visser portal, could appear as either a polyhedron, with a frame of exotic matter (in Visser's original formulation), or as a planar gateway, surrounded by a loop of negative-mass cosmic string (in the Cramer et al. paper, 1995.)

Other words used include "Einstein-Rosen Bridge," "portal," and "space warp."