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Parallel universe or alternative reality is a self-contained separate reality coexisting with our own. A specific group of parallel universes is called a multiverse, although this term can also be used to describe the possible parallel universes that comprise physical reality. While the terms “parallel universe” and “alternative reality” are generally synonymous and can be used interchangeably in most cases, there is sometimes an additional connotation implied with the term “alternative reality” that implies that the reality is a variant of our own.

The term “parallel universe” is more general, without any connotations implying a relationship (or lack thereof) with our own universe. A universe where the very laws of nature are different (for example, it has no relativistic limitations and the speed of light can be exceeded) would in general count as a parallel universe but not an alternative reality.

Hyperspace
Main article: Hyperspace (science fiction)
Perhaps the most common use of the concept of a parallel universe in science fiction is the concept of hyperspace. Used in science fiction, the concept of “hyperspace” often refers to a parallel universe that can be used as a faster-than-light shortcut for interstellar travel. Rationales for this form of hyperspace vary from work to work, but the two common elements are:

- at least some (if not all) locations in the hyperspace universe map to locations in our universe, providing the “entry” and “exit” points for travelers.

- the travel time between two points in the hyperspace universe is much shorter than the time to travel to the analogous points in our universe. This can be because of a different speed of light, different speed at which time passes, or the analogous points in the hyperspace universe are just much closer to each other.

Sometimes “hyperspace” is used to refer to the concept of additional coordinate axes. In this model, the universe is thought to be “crumpled” in some higher spatial dimension and that traveling in this higher spatial dimension, a ship can move vast distances in the common spatial dimensions. An analogy is to crumple a newspaper into a ball and stick a needle straight through, the needle will make widely spaced holes in the two-dimensional surface of the paper. While this idea invokes a “new dimension”, it is not an example of a parallel universe. It is a more scientifically plausible use of hyperspace. (See wormhole.)

While use of hyperspace is common, it is mostly used as a plot device and thus of secondary importance. While a parallel universe may be invoked by the concept, the nature of the universe is not often explored. So, while stories involving hyperspace might be the most common use of the parallel universe concept in fiction, it is not the most common source of fiction about parallel universes.

http://en.wikipedia.org/wiki/Parallel_universe_(fiction)