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Hyperspace is a notion from science fiction of a superluminal method of travel. It is typically described as an alternative "sub-region" of space co-existing with our own universe which may be entered using an energy field or other device.  In most fiction, hyperspace is described as a physical place that can be entered and exited. Once in hyperspace, the laws of general and special relativity do not behave in the same way when compared to normal outer space, allowing travelers though hyperspace to go great distances without being physically present in normal space and taking less time, measured from normal outer space, to travel said distance. Because of these properties of hyperspace, matter accelerating in hyperspace can exceed the speed of light without being converted into energy allowing hyperspace travelers to travel faster than light. "Through hyper-space, that unimagineable region that was neither space nor time, matter nor energy, something nor nothing, one could traverse the length of the Galaxy in the interval between two neighboring instants of time."  Hyperspace is a part of the universe where time can be traveled just like normal space's distance. This allows faster-than-light travel which is necessary to have practical outer space travel.
Astronomical distances and the impossibility of faster-than-light travel pose a challenge to most science-fiction authors. They can be dealt with in several ways: accept them as such (hibernation, slow boats, generation ships, time dilation – the crew will perceive the distance as much shorter and thus flight time will be short from their perspective), find a way to move faster than light (warp drive), "fold" space to achieve instantaneous translation (e.g. the Dune universe's Holtzman effect), access some sort of shortcut (wormholes), utilize a closed timelike curve (e.g. Stross' Singularity Sky), or sidestep the problem in an alternate space: hyperspace, with spacecraft able to use hyperspace sometimes said to have a hyperdrive.
Though the concept of hyperspace did not emerge until the 20th century, stories of an unseen realm outside our normal world are part of earliest oral tradition. Some stories, before the development of the science fiction genre, feature space travel using a fictional existence outside what humans normally observe. In Somnium (published 1634), Johannes Kepler tells of travel to the moon with the help of demons. From the 1930s through to the 1950s, many stories in the science fiction magazines, Amazing Stories and Astounding Science Fiction introduced readers to hyperspace as a fourth spatial dimension. Kirk Meadowcroft's "The Invisible Bubble" (1928) and John Campbell's Islands of Space (1931) features an early reference to hyperspace. In John Buchan's Ruritanian romance novel The House of the Four Winds (1935), the young Scotsman John "Jaikie" Galt is said to know "...less about women than he knew about the physics of hyperspace."
Writers of stories in magazines used the hyperspace concept in various ways. In The Mystery of Element 117 (1949) by Milton Smith, a window is opened into a new "hyperplane of hyperspace" containing those who have already died on Earth. In Arthur C. Clarke's Technical Error (1950), a man is laterally reversed by a brief accidental encounter with "hyperspace".
Hyperspace travel became widespread in science fiction, because of the perceived limitations of FTL travel in ordinary space. In E.E. Smith's Gray Lensman (1939), a "5th order drive" allows travel to anywhere in the universe while hyperspace weapons are used to attack spaceships. In Nelson Bond's The Scientific Pioneer Returns (1940), the hyperspace concept is described. Isaac Asimov's Foundation series, first published between 1942 and 1944 in Astounding, featured a Galactic Empire traversed through hyperspace. Asimov's short story Little Lost Robot (1947) features a "Hyperatomic Drive" shortened to "Hyperdrive" and observes that "fooling around with hyper-space isn't fun". In the 1955 classic Forbidden Planet, the crew is in a hyperspace suspended state during interstellar travel.
By the 1950s, hyperspace travel was established as a typical means for traveling. Hyperspace is often depicted as blue, pulsing with Cherenkov radiation. Many stories feature hyperspace as a dangerous place, and others require a ship to follow set hyperspatial "highways". Hyperspace is often described as being an unnavigable dimension where straying from a preset course can be disastrous.
In some science fiction, the danger of hyperspace travel is due to the chance that the route through hyperspace may take a ship too close to a celestial body with a large gravitational field, such as a star. In such scenarios, if a starship passes too close to a large gravitational field while in hyperspace, the ship is forcibly pulled out of hyperspace and reverts to normal space. Therefore, certain hyperspace "routes" may be mapped out that are safe, not passing too close to stars or other dangers.
Starships in hyperspace are sometimes depicted isolated from the normal universe; they cannot communicate with nor perceive things in real space until they emerge. Often there can be no interaction between two ships even when both are in hyperspace. This effect can be used as a plot device; because they are invisible to each other while in hyperspace, ships will encounter each other most often around contested planets or space stations. Hyperdrive may also allow for dramatic escapes as the pilot "jumps" to hyperspace in the midst of battle to avoid destruction.
In many stories, for various reasons, a starship cannot enter or leave hyperspace too close to a large concentration of mass, such as a planet or star; this means that hyperspace can only be used after a starship gets to the outside edge of a solar system, so the starship must use other means of propulsion to get to and from planets. The reasons given for such restrictions are usually technobabble, but their existence is just a plot device allowing for interstellar policies to actually form and exist. Science fiction author Larry Niven published his opinions to that effect in N-Space. According to him, such an unrestricted technology would give no limits to what heroes and villains could do. In fact, every criminal would have the ability to destroy colonies, settlements and indeed whole worlds without any chance of stopping him.
Other writers have limited access to hyperspace by requiring a very large expenditure of energy in order to open a link (sometimes called a jump point) between hyperspace and normal space; this effectively limits access to hyperspace to very large starships, or to large stationary jump gates that can open jump points for smaller vessels. These restrictions are often plot devices to prevent starships from easily escaping by slipping into hyperspace, thus ensuring epic space battles. An example of this is the "jump" technology as seen in Babylon 5.
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- The Invisible Bubble title listing at the Internet Speculative Fiction Database
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- Hyperspace by Michio Kaku (Anchor)
- Surfing through Hyperspace: Understanding Higher Universes in Six Easy Lessons (Oxford University Press) by Clifford A. Pickover
- The Fabric of the Cosmos: Space, Time, and the Texture of Reality (Knopf) by Brian Greene
- Hyperspace A Vanishing Act by P. Hoiland
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- Hyperspace in Science Fiction : The Astronomy Cafe - Dr. Sten Odenwald at www.astronomycafe.net
- "Hyperspace". Dr Curtis Saxton. Star Wars technical commentaries.