Parallel universes in fiction

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A parallel universe, also known as an alternate universe, parallel world, parallel dimension, alternate reality, or alternative dimension, is a hypothetical self-contained layer or plane of existence, co-existing with one's own. While the six terms are generally synonymous and can be used interchangeably in most cases, there is sometimes an additional connotation implied with the term "alternate universe/reality" that implies that the reality is a variant of our own, with some overlap with the similarly named alternate history.

The sum of all potential parallel universes that constitute reality is often called a/the "multiverse".

Fiction has long borrowed an idea of "another world" from myth, legend and religion. Heaven, Hell, Olympus, and Valhalla are all "alternative universes" different from the familiar material realm. Plato reflected deeply on parallel realities, resulting in the worlds of Platonism, in which the upper reality is perfect while the lower (earthly) reality is an imperfect shadow of the heavenly equivalent.

The concept is also found in ancient Hindu mythology, in texts such as the Puranas, which expressed an infinite number of universes, each with its own gods. Similarly in Persian literature, "The Adventures of Bulukiya" (a tale in the One Thousand and One Nights) describes the protagonist Bulukiya learning of alternative worlds/universes that are similar to but still distinct from his own.^[1]

One of the first science-fiction examples is Murray Leinster's short story Sidewise in Time, published in 1934, in which portions of alternative universes replace corresponding geographical regions in this universe. Sidewise in Time analogizes time to the geographic coordinate system, with travel along latitude corresponding to time travel moving through past, present and future, and travel along longitude corresponding to travel perpendicular to time and to other realities - hence the name of the story. Thus, another common term for a parallel universe is "another dimension", stemming from the idea that if the 4th dimension is time, the 5th dimension—a direction at a right angle to the fourth—is an alternate reality.

In modern literature, parallel universes can serve two main purposes: to allow stories with elements that would ordinarily violate the laws of nature; and to serve as a starting point for speculative fiction, asking the question "What if [event] turned out differently?". Examples of the former include Terry Pratchett's Discworld and C. S. Lewis's The Chronicles of Narnia, while examples of the latter include Harry Turtledove's Worldwar series.

A parallel universe—or more specifically, continued interaction between a parallel universe and our own—may serve as a central plot-point, or it may simply be mentioned and quickly dismissed, having served its purpose of establishing a realm unconstrained by realism.^{[clarification needed]} Discworld, for example, only very rarely mentions our world or any other worlds, as Pratchett set the books in a parallel universe instead of in "our" reality to allow for magic on the Disc. The Chronicles of Narnia also uses to a lesser extent the idea of parallel universes; this is brought up but only briefly mentioned in the introduction and ending,^{[citation needed]} its main purpose to move the protagonists from and to "our" reality and the principal setting of the books.