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A parallel universe, also known as a parallel dimension, alternate universe, or alternate reality, is a hypothetical self-contained plane of existence, co-existing with one's own. The sum of all potential parallel universes that constitute reality is often called a "multiverse".
While the four 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. 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 the parallel realities, resulting in Platonism, in which the upper reality is perfect while the lower earthly reality is an imperfect shadow of the heavenly. The lower reality is similar but with flaws.
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.[page needed]
One of the first science fiction examples is Murray Leinster's "Sidewise in Time", in which portions of alternative universes replace corresponding geographical regions in this universe. "Sidewise in Time" describes it in the manner that similar to requiring both longitude and latitude coordinates in order to mark your location on Earth, so too does time: traveling along latitude is akin to time travel moving through past, present and future, while traveling along longitude is to travel perpendicular to time and to other realities, hence the name of the short 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, a parallel universe can be roughly divided into two categories: to allow for stories where elements that would ordinarily violate the laws of nature; and to serve as a starting point for speculative fiction, asking oneself "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 the 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. The aforementioned Discworld, for example, only very rarely mentions our world or any other worlds, as Pratchett set the books in a parallel universe instead of "our" reality to allow for magic on the Disc. The Chronicles of Narnia also utilizes this to a lesser extent - the idea of parallel universes is brought up but only briefly mentioned in the introduction and ending, its main purpose to bring the protagonist from "our" reality to the setting of the books.