Parallel universe or alternative reality is a self-contained separate reality coexisting 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 comprise physical 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 thereof) with our own universe. A universe where the very laws of nature are different (for example, it has no relativistic limitations and the speed of light can be exceeded) would in general count as a parallel universe but not an alternative reality.
Fantasy has long borrowed the idea of "another world" from myth, legend and religion. Heaven, Hell, Olympus, Valhalla are all “alternative universes” different from the familiar material realm. 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 Arabic literature, "The Adventures of Bulukiya", a tale in the One Thousand and One Nights (Arabian Nights), describes the protagonist Bulukiya learning of alternative worlds/universes that are similar but different to 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. In such stories, the nature of this other reality is often left mysterious, known only by its effect on our own world.
Often the alternative worlds theme in science fiction is framed by postulating that every historical event spawns a new universe for every possible outcome, resulting in a number of alternate histories. This literary interpretation is sometimes rooted in the many-worlds interpretation of quantum mechanics formulated by the physicist Hugh Everett in 1957, an alternative to the Copenhagen interpretation originally formulated by Niels Bohr and Werner Heisenberg around 1927. (See: multiverse.) This kind of alternative universe is often the backdrop of stories involving time travel and is often used to rationalize the logical paradoxes that arise when an author allows characters to travel backward in time. (See: grandfather paradox.)
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 SF examples is John Wyndham's Random Quest about a man who, on awaking after a laboratory accident, finds himself in a parallel universe where World War II never happened with consequences for his professional and personal life, giving him information he can use on return to his own universe.
While this is a common treatment in SF, 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; 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 time flow 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. 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 tend to occupy a much broader middle ground.
While technically incorrect, and looked down upon by hard science-fiction fans and authors, the idea of another “dimension” has become synonymous with the term “parallel universe”. The usage is particularly common in movies, television and comic books and much less so in modern prose science fiction.
In written science fiction, “new dimensions” more commonly — and more accurately — refer to additional coordinate axes, beyond the three spatial axes with which we are familiar. By proposing travel along these extra axes, which are not normally perceptible, the traveler can reach worlds that are otherwise unreachable and invisible.
In 1884, Edwin A. Abbott wrote the seminal novel exploring this concept called Flatland: A Romance of Many Dimensions. It describes a world of two dimensions inhabited by living squares, triangles, and circles, called Flatland, as well as Pointland (0 dimensions), Lineland (1 dimension), and Spaceland (three dimensions) and finally posits the possibilities of even greater dimensions. Isaac Asimov, in his foreword to the Signet Classics 1984 edition, described Flatland as "The best introduction one can find into the manner of perceiving dimensions."
In 1895, The Time Machine by H. G. Wells used time as an additional “dimension” in this sense, taking the four-dimensional model of classical physics and interpreting time as a space like dimension in which humans could travel with the right equipment.
There are many examples where authors have explicitly created additional spatial dimensions for their characters to travel in, to reach parallel universes. Douglas Adams, in the last book of the Hitchhiker's Guide to the Galaxy series, Mostly Harmless, uses the idea of probability as an extra axis in addition to the classical four dimensions of space and time. Though, according to the novel, they're not really parallel universes at all but only a model to capture the continuity of space, time and probability. Robert A. Heinlein, in The Number of the Beast, postulated a six-dimensional universe. In addition to the three spatial dimensions, he invoked symmetry to add two new temporal dimensions, so there would be two sets of three. Like the fourth dimension of H. G. Wells’ "Time Traveller", these extra dimensions can be traveled by persons using the right equipment.
See main article: Hyperspace (science fiction).
Perhaps the most common use of the concept of a parallel universe in science fiction is the concept of hyperspace. Used in science fiction, the concept of “hyperspace” often refers to a parallel universe that can be used as a faster-than-light shortcut for interstellar travel. Rationales for this form of hyperspace vary from work to work, but the two common elements are:
Sometimes "hyperspace" is used to refer to the concept of additional coordinate axes. In this model, the universe is thought to be "crumpled" in some higher spatial dimension and that traveling in this higher spatial dimension, a ship can move vast distances in the common spatial dimensions. An analogy is to crumple a newspaper into a ball and stick a needle straight through, the needle will make widely spaced holes in the two-dimensional surface of the paper. While this idea invokes a "new dimension", it is not an example of a parallel universe. It is a more scientifically plausible use of hyperspace. (See wormhole.)
While use of hyperspace is common, it is mostly used as a plot device and thus of secondary importance. While a parallel universe may be invoked by the concept, the nature of the universe is not often explored. So, while stories involving hyperspace might be the most common use of the parallel universe concept in fiction, it is not the most common source of fiction about parallel universes.
The most common use of parallel universes in science fiction, when the concept is central to the story, is as a backdrop and/or consequence of time travel. A seminal example of this idea is in Fritz Leiber’s novel, The Big Time where there’s a war across time between two alternate futures each side manipulating history to create a timeline that results into their own world. Time-travelers in fiction often accidentally or deliberately create alternate histories, such as in The Guns of the South by Harry Turtledove where the Confederate Army is given the technology to produce AK-47 rifles and ends up winning the American Civil War. (However, Ward Moore reversed this staple of alternate history fiction in his Bring the Jubilee (1953), where an alternative world where the Confederate States of America won the Battle of Gettysburg and the American Civil War is destroyed after an historian and time traveller from the defeated United States of that world travels back to the scene of the battle and inadvertently changes the result so that the North wins that battle.) The alternate history novel 1632 by Eric Flint explicitly states, albeit briefly in a prologue, that the time travelers in the novel (an entire town from West Virginia) have created a new and separate universe when they're transported into the midst of the Thirty Years War in 17th century Germany. (This sort of thing is known as an ISOT among alternate history fans, after S.M. Stirling's Island in the Sea of Time: an ISOT is when territory or a large group of people is transported back in time to another historical period or place.
The concept of "sidewise" time travel, a term taken from Murray Leinster's "Sidewise in Time," is often used to allow characters to pass through many different alternate histories, all descendant from some common branch point. Often worlds that are similar to each other are considered closer to each other in terms of this sidewise travel. For example, a universe where World War II ended differently would be “closer” to us than one where Imperial China colonized the New World in the 15th century. H. Beam Piper used this concept, naming it "paratime" and writing a series of stories involving the Paratime Police who regulated travel between these alternative realities as well as the technology to do so. Keith Laumer used the same concept of "sideways" time travel in his 1962 novel Worlds of the Imperium. More recently, Frederik Pohl used the idea in his novel The Coming of the Quantum Cats which is explicitly based on upon a human-scale reading of the "many worlds" interpretation of quantum mechanics, postulating that every historical event spawns a new universe for every possible outcome. Frequent 'types' of universe explored in sidewise and alternative history works include worlds in which the Nazis won the Second World War, such as in The Man in the High Castle by Philip K. Dick and Fatherland by Robert Harris, and worlds in which the Roman Empire never fell, such as in Roma Eterna by Robert Silverberg and Romanitas by Sophia McDougall. In his novel Warlords of Utopia, part of the loosely linked Faction Paradox series, Lance Parkin explored a multiverse in which every universe in which Rome never fell goes to war with every universe in which the Nazis won WWII. The series was created by Lawrence Miles, whose earlier work Dead Romance featured the concept of an artificially created universe existing within another -specifically, within a bottle - and explored the consequences of inhabitants of the 'real' universe entering the Universe-in-a-Bottle.
In the His Dark Materials trilogy, the universe the protagonist starts in is a Victorian counterpart to ours, although it takes place at the same time. It also appears that the Protestant Reformation never happened.
Fantasy authors often want to bring characters from the author's (and the reader's) reality into their created world. Before the mid-20th century, this was most often done by hiding fantastic worlds within hidden parts of the author's own universe. Peasants who seldom if ever traveled far from their villages could not conclusively say that it was impossible that an ogre or other fantastical beings could live an hour away, but increasing geographical knowledge meant that such locations had to be farther and farther off. Characters in the author's world could board a ship and find themselves on a fantastic island, as Jonathan Swift does in Gulliver's Travels or in the 1949 novel Silverlock by John Myers Myers, or be sucked up into a tornado and land in Oz. These "lost world" stories can be seen as geographic equivalents of a "parallel universe", as the worlds portrayed are separate from our own, and hidden to everyone except those who take the difficult journey there. The geographic "lost world" can blur into a more explicit "parallel universe" when the fantasy realm overlaps a section of the "real" world, but is much larger inside than out, as in Robert Holdstock's novel Mythago Wood.
After the mid-20th century, perhaps influenced by ideas from science fiction, perhaps because exploration had made many places on the map too clear to write "Here there be dragons", many fantasy worlds became completely separate from the author's world. A common trope is a portal or artifact that connects worlds together, prototypical examples being the wardrobe in C. S. Lewis' The Lion, the Witch, and the Wardrobe, or the sigil in James Branch Cabell's The Cream of the Jest. In Hayao Miyazaki's Spirited Away, Chihiro Ogino and her parents walk through a long tunnel into the spirit world. The main difference between this type of story and the "lost world" above, is that the fantasy realm can only be reached by certain people, or at certain times, or after following certain rituals, or with the proper artifact.
In some cases, physical travel is not even possible, and the character in our reality travels in a dream or some other altered state of consciousness. Examples include the Dream Cycle stories by H. P. Lovecraft or the Thomas Covenant stories of Stephen R. Donaldson. Often, stories of this type have as a major theme the nature of reality itself, questioning if the dream-world can have the same "reality" as the waking world. Science fiction often employs this theme (usually without the dream-world being "another" universe) in the ideas of cyberspace and virtual reality.
Most stories in this mold simply transport a character from the real world into the fantasy world where the bulk of the action takes place. Whatever gate is used (such as the tollbooth in The Phantom Tollbooth by Norton Juster, or the mirror in Lewis Carroll's Through the Looking Glass) is left behind for the duration of the story, until the end, and then only if the protagonists will return.
However, in a few cases the interaction between the worlds is an important element, so that the focus is not on one world or the other, but on both, and their interaction. After Rick Cook introduced a computer programmer into a high fantasy world, his wizardry series steadily acquired more interactions between this world and ours. In Aaron Allston's Doc Sidhe our "grim world" is paralleled by a "fair world" where the elves live and history echoes ours. A major portion of the plot deals with preventing a change in interactions between the worlds. Margaret Ball, in No Earthly Sunne, depicts the interaction of our world with Faerie, and the efforts of the Queen of Faerie to deal with the slow drifting apart of Earth and Faerie. Poul Anderson depicts Hell as a parallel universe in Operation Chaos, and the need to transfer equivalent amounts of mass between the worlds explains why a changeling is left for a kidnapped child. Interactions between magical and scientific universes, and the protagonists' attempts to restore and maintain the balance between them, are major plot points in Piers Anthony's Apprentice Adept series; he depicts two worlds, the "SF" planet Proton and the fantasy-based Phaze, such that every person born in either world has a physical duplicate on the other world. Only when one duplicate has died can the other cross between the worlds. Several of his Xanth novels also revolve around interactions between the magical realm of Xanth and "Mundania".
Multiple worlds, rather than a pair, increase the importance of the relationships. In The Lion, the Witch, and the Wardrobe, there are only our world and Narnia, but in other of C. S. Lewis's works, there are hints of other worlds, and in The Magician's Nephew, the Wood between the Worlds shows many possibilities, and the plot is governed by transportation between worlds, and the effort to right problems stemming from them. In His Dark Materials, the two protagonist Lyra and Will find themselves lost amongst many worlds, and travel them looking for the other. In Andre Norton's Witch World, begun with a man from Earth being transported to this world, gates frequently lead to other worlds - or come from them. While an abundance of illusions, disguises, and magic that repels attention make certain parts of Witch World look like parallel worlds, some are clearly parallel in that time runs differently in them, and such gates pose a repeated problem in Witch World. In the radio sitcom Undone, the main character, Edna Turner, prevents people from a parallel version of London called "Undone" from moving to London and making the city too weird. There are other parallel versions of London, and one of the main plots in the series is the attempt by The Prince to unite all versions of London together.
Linking rooms of various types (not all actual rooms) can hook together any number of worlds. The characters may chose only one, but the choice is all important in determining the worlds.
The idea of a multiverse is as fertile a subject for fantasy as it is for science fiction, allowing for epic settings and godlike protagonists. Among the most epic and far-ranging fantasy "multiverses" is that of Michael Moorcock. Like many authors after him, Moorcock was inspired by the many worlds interpretation of quantum mechanics, saying:
It was an idea in the air, as most of these are, and I would have come across a reference to it in New Scientist (one of my best friends was then editor) ... [or] physicist friends would have been talking about it. ... Sometimes what happens is that you are imagining these things in the context of fiction while the physicists and mathematicians are imagining them in terms of science. I suspect it is the romantic imagination working, as it often does, perfectly efficiently in both the arts and the sciences.Unlike many science-fiction interpretations, Moorcock's Eternal Champion stories go far beyond alternate history to include mythic and sword and sorcery settings as well as some worlds more similar to our own. However, the Eternal Champion himself is incarnate in all of them.
Roger Zelazny used a mythic cosmology in his Chronicles of Amber series. His protagonist is a member of the royal family of Amber, whose members represent a godlike pantheon ruling over a prototypical universe that represents Order. All other universes are increasingly distorted "shadows" of it, ending finally at the other extreme, Chaos, which is the complete negation of the prototype. Travel between these "shadow" universes is only possible by beings descended from the blood of this pantheon. Those "of the blood" can walk through Shadow, imagining any possible reality and then walk to it, making their environment more similar to their desire as they go. It is argued between the characters whether these "shadows" even exist before they're imagined by a member of the royal family of Amber, or if the "shadows'" existence can be seen as an act of godlike creation.
In the World of Tiers novels by Philip José Farmer, the idea of godlike protagonists is even more explicit. The background of the stories is a multiverse where godlike beings have created a number of pocket universes that represent their own desires. Our own world is part of this series, but interestingly our own universe is revealed to be much smaller than it appears, ending at the edge of the solar system.
There are multiverses also in Warcraft universe, in The Chronicles of Narnia, Terry Pratchett's Discworld series, and in Diana Wynne Jones's Chrestomanci, Howl's Moving Castle and Deep Secret books and in her standalone book A Sudden Wild Magic.
See main article: Fictional universe. There are many examples of the meta-fictional idea of having the author's created universe (or any author's universe) rise to the same level of "reality" as the universe we're familiar with. The theme is present in works as diverse as Myers' Silverlock and Heinlein’s Number of the Beast. Fletcher Pratt and L. Sprague de Camp took the character Harold Shea in the Incompleat Enchanter series through the worlds of Norse myth, Edmund Spenser's Faerie Queene, Ludovico Ariosto's Orlando Furioso, and the Kalevala - without ever quite settling whether writers created these parallel worlds by writing these works, or received impressions from the worlds and wrote them down. In an interlude set in "Xanadu", a character claims that the universe is dangerous because the poem went unfinished, but whether this was his misapprehension or not is not established.
Some fictional approaches definitively establish the independence of the parallel world, sometimes by having the world differ from the book's account; other approaches have works of fiction create and affect the parallel world: L. Sprague de Camp's Solomon's Stone, taking place on an astral plane, is populated by the daydreams of mundane people, and in Rebecca Lickiss's Eccentric Circles, an elf is grateful to Tolkien for transforming elves from dainty little creatures. These stories often place the author, or authors in general, in the same position as Zelazny's characters in Amber. Questioning, in a literal fashion, if writing is an act of creating a new world, or an act of discovery of a pre-existing world.
Occasionally, this approach becomes self-referential, treating the literary universe of the work itself as explicitly parallel to the universe where the work was created. Stephen King's seven-volume Dark Tower series hinges upon the existence of multiple parallel worlds, many of which are King's own literary creations. Ultimately the characters become aware that they are only "real" in King's literary universe, and even travel to a world - twice - in which (again, within the novel) they meet Stephen King and alter events in the real Stephen King's world outside of the books.
See main article: Álfheim.
On one hand, the land often appears to be continuous to ordinary land. Thomas the Rhymer might, on being taken by the Queen of Faerie, be taken on a road like one leading to Heaven or Hell.
This is not exclusive to English or French folklore. In Norse mythology, Elfland (Alfheim) was also the name of what today is the Swedish province of Bohuslän. In the sagas, it said that the people of this petty kingdom were more beautiful than other people, as they were related to the elves, showing that not only the territory was associated with elves, but also the race of its people.
While sometimes folklore seems to show fairy intrusion into human lands - "Tam Lin" does not show any otherworldly aspects about the land in which the confrontation takes place - at other times the otherworldly aspects are clear. Most frequently, time can flow differently for those trapped by the fairy dance than in the lands they come from; although, in an additional complication, it may only be an appearance, as many returning from Faerie, such as Oisín, have found that time "catches up" with them as soon as they have contact with ordinary lands.
Fantasy writers have taken up the ambiguity. Some writers depict the land of the elves as a full-blown parallel universe, with portals the only entry - as in Josepha Sherman's Prince of the Sidhe series or Esther Friesner's Elf Defense - and others have depicted it as the next land over, possibly difficult to reach for magical reasons - Hope Mirrlees's Lud-in-the-Mist, or Lord Dunsany's The King of Elfland's Daughter. In some cases, the boundary between Elfland and more ordinary lands is not fixed. Not only the inhabitants but Faerie itself can pour into more mundane regions.
The most widely known and imitated example is the original Star Trek episode entitled Mirror, Mirror. The episode introduced an alternative version of the Star Trek universe where the main characters were barbaric and cruel to the point of being evil. When the parallel universe concept is parodied, the allusion is often to this Star Trek episode. Another example is "Spookyfish", an episode of South Park, in which the "evil" universe double of Cartman (who is pleasant and agreeable, unlike the home universe's obnoxious Cartman) sports a beard, like the "mirror" version of Mr. Spock. Another animated series, Futurama, had an episode where the cast travels between "Universe A" and "Universe 1" via boxes containing each universe, and one of the major jokes is an extended argument between the two sets of characters over which set were the "evil" ones.
One of the earliest television plots to feature parallel time was a 1970 storyline on soap opera Dark Shadows. Vampire Barnabas Collins found a room in Collinwood which served as a portal to parallel time, and he entered the room in order to escape from his current problems. A year later, the show again traveled to parallel time, the setting this time being 1841.
Multiple episodes of Red Dwarf use the concept. In "Parallel Universe" the crew meet alternate versions of themselves: the analogues of Lister, Rimmer and Holly are female, while the Cat's alternate is a dog. "Dimension Jump" introduces a heroic alternate Rimmer, a version of whom reappears in "Stoke Me a Clipper". The next episode, "Ouroboros", makes contact with a timeline in which Kochanski, rather than Lister, was the sole survivor of the original disaster; this alternate Kochanski then joins the crew for the remaining episodes.
In "Rise of the Cybermen", an episode of Doctor Who, Rose Tyler, The Doctor and Mickey Smith accidentally travel to a parallel universe where Britain is a republic and Rose's father is alive; Rose and her mother are later stuck there permanently. In the earlier episode, "Inferno", the Doctor accidentally travels to a parallel universe where Britain is a fascist republic.
Sometimes a television series will use parallel universes as an on-going subplot. and elaborated on the premise of the original series' "Mirror" universe and developed multi-episode story arcs based on the premise. Other examples are the science fiction series Stargate SG-1, the fantasy/horror series Buffy the Vampire Slayer, Supernatural and the romance/fantasy . Following the precedent set by Star Trek these story arcs show alternative universes that have turned out "worse" than the "original" universe: in Stargate SG-1 the first encountered parallel reality featured Earth being overwhelmed by an unstoppable Goa'uld onslaught; in Buffy, two episodes concern a timeline in which Buffy came to Sunnydale too late to stop the vampires from taking control; Lois & Clark repeatedly visits an alternative universe where Clark Kent's adoptive parents, Jonathan and Martha Kent, died when he was ten years of age, and Lois Lane is also apparently dead. Clark eventually becomes Superman, with help from the 'original' Lois Lane, but he is immediately revealed as Clark Kent and so has no life of his own.
In addition to following Star Trek's lead, showing the "evil" variants of the main storyline gives the writers an opportunity to show what is at stake by portraying the worst that could happen and the consequences if the protagonists fail or the importance of a characters presence. The latter could also be seen as the point of the alternative reality portrayed in the movie It's a Wonderful Life (see below).
There have been relatively few series where parallel universes were central to the series itself. Two examples are the short-lived 1980s series Otherworld which transported a family from our world to an alternative Earth; and Sliders, where the characters travel across a series of "alternative" Earths, trying to get back to their home universe. In 1986, Disney produced a pilot episode for an animated children's show about interdimensional travel called Fluppy Dogs.
The most famous treatment of the alternative universe concept in film could be considered the The Wizard of Oz, which portrays a parallel world, famously separating the magical realm of the Land of Oz from the mundane world by filming it in Technicolor while filming the scenes set in Kansas in sepia.
A later example is the Frank Capra movie, It's a Wonderful Life where the main character George Bailey is shown by a guardian angel the city of Pottersville, which was George Bailey's hometown of Bedford Falls as it would have been if he had never existed. Another notable depiction of a parallel universe in movies is the second film in the Back to the Future trilogy (1985, 1989) by Robert Zemeckis, starring Michael J. Fox and Christopher Lloyd, showing an accidentally created alternative present and future. Like It's a Wonderful Life, The Big Time, and many other time travel stories using this conceit, it is clear that these alternative presents/futures are mutually exclusive with the protagonists' own - so, strictly speaking, the universes aren't parallel in that they cannot co-exist, rather they oscillate between one or the other.
Another common use of the theme is as a prison for villains or demons. The idea is used in the first two Superman movies starring Christopher Reeve where Kryptonian villains were sentenced to the Phantom Zone from where they eventually escaped. An almost exactly parallel use of the idea is presented in the campy cult film The Adventures of Buckaroo Banzai Across the 8th Dimension, where the "8th dimension" is essentially a "phantom zone" used to imprison the villainous Red Lectroids. Uses in horror films include the 1986 film From Beyond (based on the H. P. Lovecraft story of the same name) where a scientific experiment induces the experimenters to perceive aliens from a parallel universe, with bad results. The 1987 John Carpenter film Prince of Darkness is based on the premise that the Christian Satan is actually an alien being that is the son of something even more evil and powerful, trapped in another universe. The protagonists accidentally free "Satan", who then attempts to release his "father."
Some films present parallel realities that are actually different contrasting versions of the narrative itself. Commonly this motif is presented as different points of view revolving around a central (but sometimes unknowable) "truth", the seminal example being Akira Kurosawa's Rashomon. Conversely, often in film noir and crime dramas, the alternative narrative is a fiction created by a central character, intentionally - as in The Usual Suspects - or unintentionally - as in Angel Heart. Less often, the alternative narratives are given equal weight in the story, making them truly alternative universes, such as in the German film Run Lola Run, the short-lived British West End musical Our House and the British film Sliding Doors.
Recent films that have more explicitly explored parallel universes are: the 2001 cult movie Donnie Darko, which deals with what it terms a "tangent universe" that erupts from our own universe; Super Mario Bros. (1993) has the eponymous heroes cross over into a parallel universe ruled by humanoids who evolved from dinosaurs; The One (2001) starring Jet Li, in which there is a complex system of realities in which Jet Li's character is a police officer in one universe and a serial killer in another, who travels to other universes to destroy versions of himself, so that he can become 'the one'; and (2004), the main character runs away from a totalitarian nightmare, and he enters into a cyber-afterlife alternative reality.
Parallel universes in modern comics have become particularly rich and complex, in large part due to the continual problem of continuity faced by the major two publishers, Marvel Comics and DC Comics. The two publishers have used the multiverse concept to fix problems arising from integrating characters from other publishers into their own canon, and from having major serial protagonists having continuous histories lasting, as in the case of Superman, over 70 years. Additionally, both publishers have used new alternative universes to re-imagine their own characters. (See Multiverse (DC Comics) and Multiverse (Marvel Comics))
Because of this, comic books in general are one of the few entertainment mediums where the concept of parallel universes are a major and ongoing theme. DC in particular periodically revisits the idea in major crossover storylines, such as Crisis on Infinite Earths and Infinite Crisis, where Marvel has a series called What If... that's devoted to exploring alternative realities, which sometimes impact the "main" universe's continuity.
Recently DC Comics series 52 heralded the return of the Multiverse. 52 was a mega-crossover event tied to Infinite Crisis which was the sequel to the 1980s Crisis on Infinite Earths. The aim was to yet again address many of the problems and confusions brought on by the Multiverse in the DCU. Now 52 Earths exist and including some Elseworld tales such as Kingdom Come, DC's imprint Wildstorm Comics and an Earth devoted to the Charlton Comics heroes of DC. Countdown and Countdown Presents: The Search for Ray Palmer and the upcoming Tales of the Multiverse stories expand upon this new Multiverse.
Marvel has also had many large crossover events which depicted an alternative universe, many springing from events in the X-Men books, such as Days of Future Past, the seminal Age Of Apocalypse, and 2006's House Of M. In addition the Squadron Supreme is a DC inspired Marvel Universe that has been used several times, often crossing over into the mainstream Universe in the Avengers comic. Exiles is an offshoot of the X-Men franchsie that allows characters to hop from one alternative reality to another, leaving the original, main Marvel Universe intact. The Marvel UK line has long had multiverse stories including the Jaspers' Warp storyline of Captain Britain's first series (it was here that the designation Earth-616 was first applied to the mainstream Marvel Universe).
Marvel Comics, as of 2000, launched their most popular parallel universe, the Ultimate Universe. It is a smaller subline to the mainstream titles and features Ultimate Spider-Man, Ultimate X-Men, Ultimate Fantastic Four and the Ultimates (their "Avengers"). The line in many ways both inspired and was inspired by aspects of the new movie franchises in addition to creating younger versions of the modern heroes.