A parallel universe, also sometimes called an alternate universe, is a hypothetical universe which exists separately from our own. Some theories of physics postulate the existence of many parallel universes, possibly even an infinite number. Depending on the details of the theory, these universes may or may not interact with each other. The word "multiverse" has been proposed to refer to the collection of all universes.
Everett's "many-worlds" theory
Main article: Many-worlds interpretation
Of the theories of parallel universes, that taken most seriously by modern physicists is the many worlds interpretation of quantum mechanics, proposed by Hugh Everett III in 1956, or some minor modification of it. A champion of this theory is the Oxford-based physicist David Deutsch.
Parallel universes as an explanation of "fine-tuning"
Parallel universes in fiction
Many science fiction stories feature parallel universes. Two early examples are Isaac Asimov's novel The Gods Themselves, and the Star Trek episode Mirror, Mirror, in which four principals are translated into the "Mirror Universe" in which the same characters are present, albeit evil. Other fictional instances of parallel universes:
- The television series Otherworld is premised around a family from our world transported to an alternate Earth.
- The television series Sliders is entirely premised around alternate universes in which the Sliders visit, typically, one new alternate universe each episode.
- The movie Donnie Darko which has gained a cult following, deals with an alternate universe.
- The Doctor Who serial Inferno features the doctor travelling to a parallel universe.
- The Amber series also features parallel universes, which are called "shadow".
- The acclaimed His Dark Materials trilogy by Philip Pullman features numerous parallel universes, a prominent example being a world where the human soul takes on a visible animal form known as a dæmon.
- The computer game, Half-Life features an attack of aliens from a parallel universe and dimension.
- The video game "Legend of Zelda: Majora's Mask" deals with the protagonist, Link, travelling to an alternate world to save the principal city from a moon destined to collide within 3 days.
- The game Myst has its background based on this theory. A people known as D'ni colonised Earth from another universe, and kept traveling to other universes (known as Ages) through Books. According to their cosmology, each universe is a leaf of the Terokh Jeruth the Tree of Possibilities.
In general, most of fantasy works feature an alternate Earth in a past prehistoric age that never existed (eg. Robert Howard's Hyborean Age or Tolkien's Middle-earth), or alternative middle-ages, alternative Victorian Era (Steampunk), alternative 20th century or wholly different realities and planets (cf. Moorcock's Multiverse), Pratchett's Discworld and many more).