Wormholes



In physics, a wormhole is a hypothetical topological feature of spacetime that is fundamentally a 'shortcut' through space and time. Spacetime can be viewed as a 2D surface (to simplify understanding) that, when 'folded' over, allows the formation of a wormhole bridge. A wormhole has at least two mouths that are connected to a single throat or tube. If the wormhole is traversable, then matter can 'travel' from one mouth to the other by passing through the throat. While there is no observational evidence for wormholes, spacetimes containing wormholes are known to be valid solutions in general relativity.

The term wormhole was coined by the American theoretical physicist John Archibald Wheeler in 1957. However, the idea of wormholes had already been theorized in 1921 by the German mathematician Hermann Weyl in connection with his analysis of mass in terms 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

Definition
The basic notion of an intra-universe wormhole is that it is a compact region of spacetime whose boundary is topologically trivial but whose interior is not simply connected. Formalizing this idea leads to definitions such as the following, taken from Matt Visser's Lorentzian Wormholes.

"If a Minkowski spacetime contains a compact region Ω, and if the topology of Ω is of the form Ω ~ R x Σ, where Σ is a three-manifold of nontrivial topology, whose boundary has topology of the form dΣ ~ S2, and if, furthermore, the hypersurfaces Σ are all spacelike, then the region Ω contains a quasipermanent intra-universe wormhole."

Characterizing inter-universe wormholes is more difficult. For example, one can imagine a 'baby' universe connected to its 'parent' by a narrow 'umbilicus'. One might like to regard the umbilicus as the throat of a wormhole, but the spacetime is simply connected.

Schwarzschild wormholes
Lorentzian wormholes known as Schwarzschild wormholes or Einstein-Rosen bridges are bridges between areas of space that can be modeled as vacuum solutions to the Einstein field equations by combining models of a black hole and a white hole. This solution was discovered by Albert Einstein and his colleague Nathan Rosen, who first published the result in 1935. However, in 1962 John A. Wheeler and Robert W. Fuller published a paper showing that this type of wormhole is unstable, and that it will pinch off instantly as soon as it forms, preventing even light from making it through.

Before the stability problems of Schwarzschild wormholes were apparent, it was proposed that quasars were white holes forming the ends of wormholes of this type.

While Schwarzschild wormholes are not traversable, their existence inspired Kip Thorne to imagine traversable wormholes created by holding the 'throat' of a Schwarzschild wormhole open with exotic matter (material that has negative mass/energy).

Traversability
Lorentzian traversable wormholes would allow travel from one part of the universe to another part of that same universe very quickly or would allow travel from one universe to another. The possibility of traversable wormholes in general relativity was first demonstrated by Kip Thorne and his graduate student Mike Morris in a 1988 paper; for this reason, the type of traversable wormhole they proposed, held open by a spherical shell of exotic matter, is referred to as a Morris-Thorne wormhole. Later, other types of traversable wormholes were discovered as allowable solutions to the equations of general relativity, including a variety analyzed in a 1989 paper by Matt Visser, in which a path through the wormhole can be made in which the traversing path does not pass through a region of exotic matter. However in the pure Gauss-Bonnet theory exotic matter is not needed in order for wormholes to exist- they can exist even with no matter. A type held open by negative mass cosmic strings was put forth by Visser in collaboration with Cramer et al., in which it was proposed that such wormholes could have been naturally created in the early universe.

Wormholes connect two points in spacetime, which means that they would in principle allow travel in time, as well as in space. In 1988, Morris, Thorne and Yurtsever worked out explicitly how to convert a wormhole traversing space into one traversing time. However, it has been said a time traversing wormhole cannot take you back to before it was made but this is disputed.

Faster-than-light travel
Special relativity only applies locally. Wormholes allow superluminal (faster-than-light) travel by ensuring that the speed of light is not exceeded locally at any time. While traveling through a wormhole, subluminal (slower-than-light) speeds are used. If two points are connected by a wormhole, the time taken to traverse it would be less than the time it would take a light beam to make the journey if it took a path through the space outside the wormhole. However, a light beam traveling through the wormhole would always beat the traveler. As an analogy, running around to the opposite side of a mountain at maximum speed may take longer than walking through a tunnel crossing it. You can walk slowly while reaching your destination more quickly because the distance is smaller.

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.

Metrics
Theories of wormhole metrics describe the spacetime geometry of a wormhole and serve as theoretical models for time travel. An example of a (traversable) wormhole metric is the following:


 * $$ds^2= - c^2 dt^2 + dl^2 + (k^2 + l^2)(d \theta^2 + \sin^2 \theta \, d\phi^2).$$

One type of non-traversable wormhole metric is the Schwarzschild solution:


 * $$ds^2= - c^2 \left(1 - \frac{2GM}{rc^2}\right)dt^2 + \frac{dr^2}{1 - \frac{2GM}{rc^2}} + r^2(d \theta^2 + \sin^2 \theta \, d\phi^2).$$

In fiction
Wormholes are features of science fiction as they allow interstellar (and sometimes interuniversal) travel within human timescales. It is common for the creators of a fictional universe to decide that faster-than-light travel is either impossible or that the technology does not yet exist, but to use wormholes as a means of allowing humans to travel long distances in short periods. Military science fiction (such as the Wing Commander games) often uses a "jump drive" to propel a spacecraft between two fixed "jump points" connecting stellar systems. Connecting 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 used by Larry Niven and Jerry Pournelle in The Mote in God's Eye and related novels are an example, although the mechanism does not seem to describe actual wormhole physics. David Weber has also used the device in the Honorverse and other books such as those based upon the Starfire universe. Naturally occurring wormholes form the basis for interstellar travel in Lois McMaster Bujold's Vorkosigan Saga. They are also used to create an Interstellar Commonwealth in Peter F. Hamilton's Commonwealth Saga. In Jack L. Chalker's The Rings of the Master series, interstellar class spaceships are capable of calculating complex equations and punching Wormholes in the fabric of the Universe in order to enable rapid travel.

Concept of wormholes is used in The Wild Blue Yonder, a science fiction film by Werner Herzog.

The Mass Relays in the videogame Mass Effect can be perceived as stabilized wormholes that allow for near instantaneous, "faster-than-light" travel from one end to the other.

The Massively Multiplayer Online Game EVE Online utilizes wormholes extensively as they are created in the use of the stargate technology which allows for interstellar travel in the game world.

The Vega Strike first-person space trading and combat simulator features wormholes to travel through star systems. The engine is open-source and has various mods and total conversions which have wormholes too, like Vega Trek, a Vega Strike mod based on the Star Trek universe. Or the Privateer Remake, a remake of Wing Commander: Privateer.

Wormholes also play pivotal roles in science fiction where faster-than-light travel is possible though limited, allowing connections between regions that would be otherwise unreachable within conventional timelines. Several examples appear in the Star Trek franchise, including the Bajoran wormhole in the Deep Space Nine series. In 1979's Star Trek: The Motion Picture the USS Enterprise was trapped in an artificial wormhole caused by an imbalance in the calibration of the ship's warp engines when it first achieved faster-than-light speed. In the Star Trek: Voyager series, the cybernetic species the Borg use what, in the Star Trek universe, are referred to as transwarp conduits, allowing ships to move nearly instantaneously to any part of the galaxy in which an exit apeture exists. Although these conduits are never described as "wormholes", they appear to share several traits in common with them.

The 1979 Disney film The Black Hole's plot centers around a massive black hole, although it makes virtually no use of then-current worm-hole physics, with only one rather desultory mention of an Einstein-Rosen bridge. A trip through the black hole turns theological, abandoning scientific rationale.

In Carl Sagan's novel Contact and subsequent 1997 film starring Jodie Foster and Matthew McConaughey, Foster's character Ellie travels 26 light years through a series of wormholes to the star Vega. The round trip, which to Ellie lasts 18 hours, passes by in a fraction of a second on Earth, making it appear she went nowhere. In her defense, Foster mentions an Einstein-Rosen bridge and tells how she was able to travel faster than light and time. Analysis of the situation by Kip Thorne, on the request of Sagan, is quoted by Thorne as being his original impetus for analyzing the physics of wormholes.

Wormholes play major roles in the television series Farscape, where they are the cause of John Crichton's presence in the far reaches of our own galaxy, and in the Stargate series, where stargates create a stable artificial wormhole where matter is dematerialized, converted into energy, and is sent through to be rematerialized at the other side. In the latter series, the devices were discovered in Egypt by an archeologist, and were built by aliens known as the Ancients or the Alterans. In the science fiction series Sliders, a wormhole (or vortex, as it is usually called in the show) is used to travel between parallel worlds, and one is seen at least once or twice in every episode. In the pilot episode it was referred to as an "Einstein-Rosen-Podolsky bridge".

The central theme in the movie Donnie Darko revolves around Einstein-Rosen bridges.

It is possible that the Webway technology used by the Eldar of the fictional Warhammer 40,000 could be perceived as wormhole technology.

In Command & Conquer 3 and in its expansion the Scrin faction (an alien lifeform with unknown origins from outer solar system) uses artificial wormholes for military purposes to convey infantry and vehicles behind enemy lines.

In the Invader Zim episode, "A Room with a Moose" Zim utilizes a wormhole to send his classmates into a parallel universe that consists entirely of a room with a large moose inside it.

The television series Strange Days at Blake Holsey High is about a wormhole the science club found at their school.

In an episode called "wormhole" in the 13th season of the long running American series Power Rangers,called Power Rangers SPD the spd rangers go through a wormhole to team up with the previous team of Power Rangers Dino Thunder from year 2004, after their enemy Emperor Grumm goes through one.

In the video game "Supreme Commander" the UEF faction utilises aether-gates for long distance military strikes.

In the video game "Spore", the player can travel through various black holes, which act as wormholes for the player to go to its counterpart located usually on the other side of the galaxy; something that would take much longer to do by flying there manually.

In the 1995-1996 FOX military science fiction series SPACE: Above and Beyond, during the first several episodes, the United Earth Force travel through wormholes, called the "Kali Region" or "Galileo Region" to arrive at exo-solar destinations. This idea is abandoned after the second episode.

In the movie Race to Witch Mountain the 2 aliens from a planet which is 3000 light years away from Earth use wormholes to travel to Earth.

In the 2009 Doctor Who Easter special, Planet of the Dead, the Doctor and a group of passengers aboard a double-decker bus are transported to an alien world via a wormhole.