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 universe co-existing with one's own, typically distinct in some way.^[1] The sum of all potential parallel universes that constitute reality is often called the "multiverse." 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 a direction into any of the alternate spacetime realities.

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.

One of the first science-fiction examples of a parallel universe is Murray Leinster's short story Sidewise in Time, published in 1934, 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.

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.