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 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"

The idea that multiple universes can exist has been used together with the anthropic principle to explain why our universe seems fine_tuned for intelligent life. Life as we know it could not exist, or would be overwhelmingly unlikely to exist, if any of the major physical constants were changed by even small amounts. However, if a very large or infinite number of universes exist, each with a different set of arbitrary physical constants, then at least some will have the right conditions for intelligent life to develop.

Parallel universes in philosophy

Concepts related to the idea of parallel universes in physics exist in philosophy and theology, usually referred to as "possible worlds". See for example Gödel's ontological proof of God's existence.

Parallel universes in fiction

Many science fiction and fantasy stories feature parallel universes. Distinguishing these sub_genres from alternate history stories is problematic, but one might confine parallel-universe stories to those in which at least two universes come into contact. An early example is H. G. Wells's Men Like Gods (1923). There are far too many such stories to list, but other fictional instances of parallel universes include:

• Murray Leinster's story "Sidewise in Time" (1933), showing different parts of the Earth somehow occupied by different parallel universes, was influential in science fiction.
• C. S. Lewis's Chronicles of Narnia (first volume published in 1950) featured three universes and a "Wood between the Worlds" from which one could enter any of a huge number of universes.
• The Star Trek episode Mirror, Mirror shows four principals translated into the "Mirror Universe" in which the same characters are present, albeit evil.
• Isaac Asimov's novel The Gods Themselves depicts scientists in our universe who find a way to "import" small amounts of matter from a universe having different physical laws, with unforeseen consequences.
• Robert Heinlein's novel The Number of the Beast is focused around a 'time machine' that also proves to be able to travel sideways and other directions in time, allowing for crossing into other realities, even ones previously considered fictional by the protagonists.
• DC Comics and Marvels respective bodies of work each reside in multiple interconnecting simultanious realities.
• 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 "shadows" of the one true universe.
• 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.
• Stephen King's Dark Tower series deals heavily with alternate worlds and doors that allow travel between these worlds. In turn, many of King's novels are somehow related to the universe depicted in the Dark Tower books.
• The Japanese animated films Laputa Castle in the Sky and Kiki's Delivery Service are set on alternative Earths.
• The cult television series, Buffy the Vampire Slayer, has featured a number of parallel universes, bridging the gap with magic.
• The Caste Of The Metabarons comics by Alejandro Jodorowsky features two wars between our universe and parallel ones.

In general, most fantasy works feature an alternate Earth in a past prehistoric age that never existed. (e.g., Robert E. Howard's Hyborean Age), alternative middle-ages, an alternative Victorian Era (Steampunk), an alternative 20th century or wholly different realities and planets (cf. Moorcock's Multiverse, Pratchett's Discworld and many more).

The Garden of Forking Paths, a short story by Jorge Luis Borges, is curious in that it presents a parallel universe theory of time that is consistent with the many-worlds interpretation (of which Borges knew nothing at the time), but it does so as an academic fashion, without actually contacting these other universes.

External links

• Max Tegmark's Parallel Universes paper (http://www.hep.upenn.edu/~max/multiverse.pdf) (pdf)
• Many Worlds & Parallel Universes (http://timetravelportal.com/viewtopic.php?t=288)