A **wormhole**, also known as an **Einstein–Rosen bridge**, is a hypothetical topological feature of spacetime that would be fundamentally a “shortcut” through spacetime. A wormhole is much like a tunnel with two ends each in separate points in spacetime.

For a simplified notion of a wormhole, visualize space as a two-dimensional (2D) surface. In this case, a wormhole can be pictured as the 2D surface of a tube that connects different parts of the surface. The mouths of a wormhole are analogous to the holes at either end of the tube in a 2D plane. An actual wormhole would be analogous to this but with the spatial dimensions raised by one, which can be modeled mathematically even if we find it impossible to visualize. For example, instead of circular holes on a 2D plane, a real wormhole’s mouths could be spheres in 3D space.

Researchers have no observational evidence for wormholes, but the equations of the theory of general relativity have valid solutions which contain wormholes. Because of its robust theoretical strength, a wormhole is one of the great physics metaphors for teaching general relativity. The first type of wormhole solution discovered was the *Schwarzschild wormhole*, which would be present in the Schwarzschild metric describing an eternalblack hole, but it was found that this type of wormhole would collapse too quickly for anything to cross from one end to the other. Wormholes which could actually be crossed in both directions, known as *traversable wormholes*, would only be possible if exotic matter with negative energy densitycould be used to stabilize them.

The Casimir effect shows that quantum field theory allows the energy density in certain regions of space to be negative relative to the ordinary vacuum energy, and it has been shown theoretically that quantum field theory allows states where the energy can be *arbitrarily* negative at a given point. Many physicists such as Stephen Hawking, Kip Thorne, and others therefore argue that such effects might make it possible to stabilize traversable wormhole. Physicists have not found any natural process which would be predicted to form a wormhole naturally in the context of general relativity, although the quantum foam hypothesis is sometimes used to suggest that tiny wormholes might appear and disappear spontaneously at the Planck scale, and stable versions of such wormholes have been suggested as dark matter candidates. It has also been proposed that if a tiny wormhole held open by a negative-mass cosmic string had appeared around the time of the Big Bang, it could have been inflated to macroscopic size by cosmic inflation.

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

This analysis forces one to consider situations…where there is a net flux of lines of force, through what topologists would call “a handle” of the multiply-connected space, and what physicists might perhaps be excused for more vividly terming a “wormhole”.

—John Wheeler in Annals of Physics

Excellent summary! Minor quibble: “Einstein-Rosen bridge” is not synonymous with “wormhole”. The bridge is a particular type of wormhole, specifically one that is not traversable, has an event horizons, and has a non-rotating black hole at either end. [Source: The Physics of Stargates, by E. Rodrigo (2010)]