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A parallel universe is a theory of a self-contained separate reality co-existing with one's 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 constitute 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 of relationship, with our own universe. A universe where the very laws of nature are different – for example, one in which there are no Laws of Motion – would in general count as a parallel universe but not an alternative reality and a concept between both fantasy world and earth. The actual quantum-mechanical hypothesis of parallel universes is "universes that are separated from each other by a single quantum event." == Introduction == Fantasy 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 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. Modern fantasy often presents the concept as a series of planes of existence where the laws of nature differ, allowing magical phenomena of some sort on some planes. This concept was 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. In other cases, in both fantasy and science fiction, a parallel universe is a single other material reality, and its co-existence with ours is a rationale to bring a protagonist from the author's reality into the fantasy's reality, such as in ''The Chronicles of Narnia'' by C. S. Lewis or even the beyond-the-reflection travel in the two main works of Lewis Carroll. Or this single other reality can invade our own, as when Margaret Cavendish's English heroine sends submarines and "birdmen" armed with "fire stones" back through the portal from ''The Blazing World'' to Earth and wreaks havoc on England's enemies. In dark fantasy or horror the parallel world is often a hiding place for unpleasant things, and often the protagonist is forced to confront effects of this other world leaking into his own, as in most of the work of H. P. Lovecraft and the ''Doom'' computer game series, or Warhammer/40K miniature and computer games. In such stories, the nature of this other reality is often left mysterious, known only by its effect on our own world. The concept also arises outside the framework of quantum mechanics, as is found in Jorge Luis Borges short story ''El jardín de senderos que se bifurcan'' ("The Garden of Forking Paths"), published in 1941 before the many-worlds interpretation had been invented. In the story, a Sinologist discovers a manuscript by a Chinese writer where the same tale is recounted in several ways, often contradictory, and then explains to his visitor (the writer's grandson) that his relative conceived time as a "garden of forking paths", where things happen in parallel in infinitely branching ways. One of the first Sci-Fi examples is Murray Leinster's Sidewise in Time, in which portions of alternative universes replace corresponding geographical regions in this universe. While this is a common treatment in Sci-Fi, it is by no means the only presentation of the idea, even in hard science fiction. Sometimes the parallel universe bears no historical relationship to any other world; instead, the laws of nature are simply different from those in our own, as in the novel ''Raft'' by Stephen Baxter, which posits a reality where the gravitational constant is much larger than in our universe. (Note, however, that Baxter explains later in ''Vacuum Diagrams'' that the protagonists in ''Raft'' are descended from people who came from the Xeelee Sequence universe.) One motif is that the way time flows in a parallel universe may be very different, so that a character returning to one might find the time passed very differently for those he left behind. This is found in folklore: King Herla visited Fairy and returned three centuries later; although only some of his men crumbled to dust on dismounting, Herla and his men who did not dismount were trapped on horseback, this being one folkloric account of the origin of the Wild Hunt.〔Briggs (1967) p.50-1〕 C. S. Lewis made use of this in ''The Chronicles of Narnia''; indeed, a character points out to two skeptics that there is no need for the time between the worlds to match up, but it would be very odd for the girl who claims to have visited a parallel universe to have dreamed up such a different time flow. The division between science fiction and fantasy becomes fuzzier than usual when dealing with stories that explicitly leave the universe we are familiar with, especially when our familiar universe is portrayed as a subset of a multiverse. Picking a genre becomes less a matter of setting, and more a matter of theme and emphasis; the parts of the story the author wishes to explain and how they are explained. ''Narnia'' is clearly a fantasy, and the TV series ''Sliders'' is clearly science fiction, but works like the ''World of Tiers'' series or ''Glory Road'' tend to occupy a much broader middle ground. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Parallel universe (fiction)」の詳細全文を読む スポンサード リンク
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