A wormhole, also known as an Einstein-Rosen bridge, is a hypothetical topological feature of spacetime that is essentially a "shortcut" from one point in the universe to another point in the universe, allowing travel between them that is faster than it would take light to make the journey through normal space.
The name "wormhole" comes from the following analogy used to explain the phenomenon: imagine that the universe is the skin of an apple, and a worm is travelling over its surface. The distance from one side of the apple to the other is equal to half the apple's circumference if the worm stays on the apple's surface, but if it instead burrows a wormhole directly through the apple the distance it has to travel is considerably less.
Theoretical basis
It is unknown whether wormholes are possible or not within the framework of general relativity. All known solutions of general relativity which allow for wormholes require the existence of exotic matter, a theoretical substance which has negative energy density. However, it has not been mathematically proven that this is an absolute requirement for wormholes, nor has it been established that exotic matter cannot exist. Furthermore, since there is no established theory of quantum gravity, it is impossible to say with any certainty whether they are possible or not within that theoretical framework.
Many physicists, including Stephen Hawking (see Hawking's Chronology Protection Conjecture), believe that due to the problems a wormhole would theoretically create, including allowing time travel, that something fundamental in the laws of physics would prohibit them. However, this remains speculation, and the notion that nature would censor inconvenient objects has already failed in the case of the cosmic censorship principle.
Timeholes
A wormhole could potentially allow time travel. This could be accomplished by accelerating one end of the wormhole relative to the other, and then sometime later bringing it back; relativistic time dilation would result in less time having passed for the accelerated wormhole mouth compared to the stationary one, meaning that anything which entered the stationary wormhole mouth would exit the accelerated one at a point in time prior to its entry. The path through such a wormhole is called a closed timelike curve, and a wormhole with this property is sometimes referred to as a "timehole."
It is thought that it may not be possible to convert a wormhole into a time machine in this manner, however; some mathematical models indicate that a feedback loop of virtual particles would circulate through the timehole with ever-increasing intensity, destroying it before any information could be passed through it. This has been called into question by the suggestion that radiation would disperse after traveling through the wormhole, therefore preventing infinite accumulation. There is also the Roman ring, which is a very stable configuration of more than one wormhole. This ring allows a closed time loop with stable wormholes. The debate on this matter is described by Kip S. Thorne in the book Black Holes and Time Warps [1], and will likely require a theory of quantum gravity to resolve.
Wormholes in fiction
Wormholes are also a feature of science fiction.
They are a centerpiece of Carl Sagan's novel Contact, for which Kip Thorne consulted Sagan on the possibilities of wormholes.
The setting of the television series Star Trek: Deep Space Nine is a space station, Deep Space Nine, located near the Bajoran wormhole. Wormholes are also the principle means of space travel in the Stargate movie and the spin-off television series, Stargate SG-1 and Stargate Atlantis. The central plot device of the programs is a transportation network consisting of the ring-shaped devices known as Stargates, which generate wormholes that allow one-way matter transmission between gates when the correct spatial coordinates are "dialed". The television series Farscape features an American astronaut who accidentally gets shot through a wormhole and ends up in a distant part of the universe, and also features the use of wormholes to reach other universes (or "unrealized realities") and as weapons of mass destruction.
In the FOX/Sci-Fi series Sliders, a method is found to create a wormhole that allows travel not between distant points but between different universes; objects or people that travel through the wormhole begin and end in the same location geographically (e.g. if one leaves San Francisco, one will arrive in an alternate San Francisco) and chronologically (if it is 1999 at the origin point, so it is at the destination, at least by the currently-accepted calendar on our Earth.) This series presumes that we exist as part of a multiverse and asks what might have resulted had major or minor events in history occurred differently; it is these choices that give rise to the alternate universes in which the series is set. The same premise is used in the Star Trek: The Next Generation episode Parallels.
In Star Trek: The Motion Picture, Willard Decker recalls that "Voyager 6" (aka V'ger) disappeared into what they used to call a "black hole". At one time, black holes in science fiction were often incorrectly endowed with the traits of wormholes. This has, for the most part disappeared as a black hole isn't really a hole in space but a dense mass and the visible vortex effect often associated with black holes is merely the accretion disk of visible matter being drawn toward it. Decker's line is most likely to inform that it was probably a wormhole that Voyager 6 entered.
In 2000, Arthur C. Clarke and Stephen Baxter co-wrote a science fiction novel, The Light of Other Days, which discusses the problems which arise when a wormhole is used for faster than light communication.
A related method of faster-than-light travel that often arises in science fiction, especially military science fiction, is a "jump drive" that can propel a spacecraft between two fixed "jump points" connecting solar systems. Connecting solar systems in a network like this results in a fixed "terrain" with choke points that can be useful for constructing plots related to military campaigns. The Alderson points postulated by Larry Niven and Jerry Pournelle in Mote in God's Eye and related novels is an especially well thought out example. The development process is described by Niven in N-Space, a volume of collected works. David Weber has also used the device in the Honorverse and other books and has described a 'history' of development and exploitation in several essays in collections of related short stories.
The Commonwealth Saga by Peter F. Hamilton describes how wormhole technology could be used to explore, colonize and connect to other worlds without having to resort to traditional travel via starships. This technology is the basis of the formation of the titular Intersolar Commonwealth, and is used so extensively that it is possible to ride trains between the planets of the Commonwealth.
See also
