Parallel universes in fiction


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 three 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. The term "parallel universe" is more general, without 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.

Overview

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.
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: travelling along latitude is akin to time travel moving through past, present and future, while travelling 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 - are alternate realities.
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 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 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 setting the books on a parallel universe instead of "our" reality is to allow for magic on the Disc. The Chronicles of Narnia also utilises this to a lesser extent - the idea of parallel universes are 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.

Science fiction

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. The idea of a parallel world was first introduced in comic books with the publication of The Flash #123, "Flash of Two Worlds".
In written science fiction, "new dimension" 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.
's Flatland is set in a world of two dimensions.
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, Lineland, and Spaceland 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. Wells also used the concept of parallel universes as a consequence of time as the fourth dimension in stories like The Wonderful Visit and Men Like Gods, an idea proposed by the astronomer Simon Newcomb, who talked about both time and parallel universes; "Add a fourth dimension to space, and there is room for an indefinite number of universes, all alongside of each other, as there is for an indefinite number of sheets of paper when we pile them upon each other."
There are many examples where authors have explicitly created additional spatial dimensions for their characters to travel in, to reach parallel universes. In Doctor Who, the Doctor accidentally enters a parallel universe while attempting to repair the TARDIS console in "Inferno". 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 similar to the many-worlds interpretation of quantum physics, although according to the novel they were more 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.

Hyperspace

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:
  1. It is possible to enter and exit from this hyperspace with reasonable ease.
  2. There is reason to enter the hyperspace and exit rather than conventional travel.
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.
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.

Time travel and alternative history

Technically, alternative histories as a result of time travel are not parallel universes: while multiple parallel universes can co-exist simultaneously, only one history or alternative history can exist at any one moment, as alternative history usually involves, in essence, overriding the original timeline with a new one. As a result, travel between alternative histories is not possible without reverting the timeline back to the original.
There are exceptions to the above, and an alternate history doesn't necessarily overwrite the old one. There are no rules written in stone regarding this. Modern ideas of time travel pose the idea of branching timelines, such as in Avengers: Endgame. Technically, if a timeline isn't explicitly stated to have been erased, it's still there.
Parallel universes as a result of time travel can serve simply as the backdrop, or it may be a central plot point. The Guns of the South by Harry Turtledove, where the Confederate Army is given thousands of AK-47 rifles and ends up winning the American Civil War, is a good example of the former, while Fritz Leiber's novel The Big Time where a war between two alternative futures manipulating history to create a timeline that results in or realizes their own world is a good example of the latter.
Subscribing to the many-worlds interpretation of quantum physics, alternative histories in fiction can arise as a natural phenomena of the universe. In these works, the idea is that each choice every person makes, each leading to a different result, both occur, so when a person decides between jam or butter on his toast, two universes are created: one where that person chose jam, and another where that person chose butter. The concept of "sidewise" time travel, a term taken from Murray Leinster's "Sidewise in Time", is used to allow characters to pass through many different alternative histories, all descendant from some common branch point.
Often, worlds that are more 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, novels such as Frederik Pohl's The Coming of the Quantum Cats and Neal Stephenson's Anathem explore human-scale readings of the "many worlds" interpretation, postulating that historical events or human consciousness spawns or allows "travel" among alternative universes.
Universe 'types' frequently explored in sidewise and alternative history works include worlds whose Nazis won the Second World War, as in The Man in the High Castle by Philip K. Dick, SS-GB by Len Deighton, and Fatherland by Robert Harris, and worlds whose Roman Empire never fell, as in Roma Eterna by Robert Silverberg. Romanitas by Sophia McDougall, and Warlords of Utopia by Lance Parkin.

Counter-Earth

The concept of Counter-Earth might seem similar to a parallel universe, but is actually a distinct idea. A counter-earth is a planet that shares Earth's orbit but is on opposition, therefore, cannot be seen from Earth. There would be no necessity that such a planet would be like Earth in any way, although typically in fiction it is practically identical to Earth. Since Counter-Earth is not only within our universe but within our own Solar System, reaching it can be accomplished with ordinary space travel.
Gerry and Sylvia Anderson used this concept in their 1969 movie Doppelgänger, in which a Counter-Earth is detected by astronomers and a manned mission launched by a US-European space consortium to explore it.
Though not impossible, the idea is rather implausible - since none of the other planets in the Solar System has such a "twin" sharing its orbit.
H.G. Wells placed the world of his 1903 Modern Utopia within our own universe - but much further away: "Out beyond Sirius, far in the deeps of space, beyond the flight of a cannon-ball flying for a billion years, beyond the range of unaided vision, blazes the star that is our Utopia’s sun. To those who know where to look, with a good opera-glass aiding good eyes, it and three fellows that seem in a cluster with it—though they are incredible billions of miles nearer—make just the faintest speck of light. About it go planets, even as our planets, but weaving a different fate, and in its place among them is Utopia, with its sister mate, the Moon. It is a planet like our planet, the same continents, the same islands, the same oceans and seas, another Fuji-Yama is beautiful there dominating another Yokohama—and another Matterhorn overlooks the icy disorder of another Theodule. It is so like our planet that a terrestrial botanist might find his every species there, even to the meanest pondweed or the remotest Alpine blossom.... Only when he had gathered that last and turned about to find his inn again, perhaps he would not find his inn!" However, Wells did not explain how such a precise duplication of our world could occur, nor how could a person be suddenly transported in twinkling of an eye from our world to the precise equivalent spot in that other world.

Convergent evolution

is a biological concept whereby unrelated species acquire similar traits because they adapted to a similar environment and/or played similar roles in their ecosystems. In fiction, the concept is extended whereby similar planets will result in races with similar cultures and/or histories.
Again, this is not a true parallel universe since such planets exist within the same universe as our own, but the stories are similar in some respects. frequently explored such worlds:
A similar concept in biology is gene flow. In this case, a planet may start out differently from Earth, but due to the influence of Earth's culture, the planet comes to resemble Earth in some way. Star Trek also frequently used this theory as well: in "Patterns of Force |Patterns of Force", a planet is discovered to be very similar to Nazi Germany due to the influence of a historian that came to reside there, who believed that the Nazi fascism itself was not evil and under benevolent leadership could be "good government"; while in "A Piece of the Action |A Piece of the Action", the Enterprise crew visits a planet that, 100 years after a book Chicago Mobs of the Twenties that had been left behind by previous Earth craft, their society resembles mob ruled cities of the Prohibition era United States.

Simulated reality

Simulated realities are digital constructs featured in science fiction such as The Matrix.

The Parallel Universe Theory

In 1954, a Princeton University doctoral applicant named Hugh Everett III concocted a final thought; that there exist parallel universes precisely equivalent to our Universe.
These universes are entirely connected with our own; in fact, they branch far away from our own, and our Universe diverges from others.
Inside these parallel universes, our wars have had unexpected results in comparison to those we as a whole know. Species that are wiped out in our Universe have developed and adjusted in others. In different universes, we people may get wiped out.
Everett’s give Many-Worlds Theory, as it came to be known, was his endeavor to respond to some unanswered inquiries raised by the consequences of examinations acted in the developing field of quantum material science.
Ideas of parallel universes or measurements that look like our own have shown up in works of imagination and utilized as clarifications for mysticism.

Fantasy

Stranger in a strange land

It is common in fantasy for authors to find ways to bring a protagonist from "our" world to the fantasy world. Before the mid-20th century, this was most often done by hiding fantastic worlds within unknown, distant locations on Earth; 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. 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.
However, increasing geographical knowledge meant that such locations had to be farther and farther off. Perhaps influenced by ideas from science fiction, many works chose a setting that takes place in another, separate reality. As it is now not possible to reach these worlds via conventional travel, a common trope is a portal or artifact that connects our world and the fantasy world together, 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 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 whether the dream-world is as real as the waking world. Science fiction often employs this theme in the ideas of cyberspace and virtual reality.

Between the worlds

As mentioned above, in many stories the parallel universe mold is simply transport a character from the real world into the fantasy world where the bulk of the action takes place. Whatever method is used ceases to be important for the most of the story until the ending until the protagonists return to our world.
However, in a few cases the interaction between the worlds is an important element, so that the focus is not on simply the fantasy world, but on ours as well. Sometimes the intent is to let them mingle and see what would happen, such as introducing a computer programmer into a high fantasy world as seen in Rick Cook's Wizardry series, while other times an attempt to keep them from mingling becomes a major plot point, such as in Aaron Allston's Doc Sidhe our "grim world" is paralleled by a "fair world" where the elves live and history echoes ours, where a major portion of the plot deals with preventing a change in interactions between the worlds.

Fantasy multiverses

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. The most epic and far-ranging fantasy "multiverse" is that of Michael Moorcock, who actually named the concept in a 1963 science fiction novel The Sundered Worlds. Like many authors after him, Moorcock was inspired by the many worlds interpretation of quantum mechanics, saying:

Fictional universe as alternative universe

There are many examples of the meta-fictional idea of having the author's created universe rise to the same level of "reality" as the universe we're familiar with. The theme is present in works as diverse as H.G. Wells' Men Like Gods, Myers' Silverlock, and Heinlein's Number of the Beast. Fletcher Pratt and L. Sprague de Camp took the protagonist of the Harold Shea series through the worlds of Norse myth, Edmund Spenser's The 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 they meet Stephen King and alter events in the real Stephen King's world outside of the books. An early instance of this was in works by Gardner Fox for DC Comics in the 1960s, in which characters from the Golden Age would cross over into the main DC Comics universe. One comic book did provide an explanation for a fictional universe existing as a parallel universe. The parallel world does "exist" and it resonates into the "real world". Some people in the "real world" pick up on this resonance, gaining information about the parallel world which they then use to write stories.
Robert Heinlein, in The Number of the Beast, quantizes the many parallel fictional universes - in terms of fictons. A number of fictional universes are accessible along one of the three axes of time which Dr. Jacob Burroughs' "time twister" can access. Each quantum level change - a ficton - along this time axis corresponds to a different universe from one of several bodies of fiction known to all four travellers in the inter-universal, time travelling vehicle Gay Deceiver. Heinlein also "breaks the fourth wall" by having "both Heinleins" visit an inter-universal science-fiction and fantasy convention in the book's last chapter. The convention was convened on Heinlein character Lazarus Long's estate on the planet "Tertius" to attract the evil "Black Hats" who pursued the main characters of The Number of the Beast through space and time in order to destroy Dr. Burroughs and his invention. Heinlein continues this literary conceit in The Cat Who Walks Through Walls and To Sail Beyond the Sunset, using characters from throughout his science-fictional career, hauled forth from their own "fictons" to unite in the war against the "Black Hats".
Heinlein also wrote a stand-alone novel, ', whose two protagonists fall from alternative universe into alternative universe, and after a number of such adventures die and enter a stereotypically Fundamentalist Christian Heaven. Their harrowing adventures through the universes are then revealed to have been "destruction testing" of their souls by Loki, sanctioned by the Creator person of the Christian God. The Devil appears as the most sympathetic of the gods in the story, who expresses contempt for the other gods' cavalier treatment of the story's main characters.
Thus, ' rings in the theological dimension of parallel universes, that their existence can be used by God. It manages also to have a fictional multiverse angle in that references are made to Heinlein's early SF/fantasy short story "They", a solipistic tale in which reality is constantly being transmogrified behind the scenes to throw the central character off his guard and keep him from seeing reality as it is, which was set in the same Heinlein fictional universe as The Moon is a Harsh Mistress.

Elfland

, or Faerie, the otherworldly home not only of elves and fairies but goblins, trolls, and other folkloric creatures, has an ambiguous appearance in folklore.
On one hand, the land often appears to be contiguous with '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 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. Terry Pratchett's Discworld series proposes that the world of the Elves is a "parasite" universe, that drifts between and latches onto others such as Discworld and our own world. In the young teenage book Mist by Kathryn James, the Elven world lies through a patch of mist in the woods. It was constructed when the Elven were thrown out of our world. Travel to and fro is possible by those in the know, but can have lethal consequences.

Isekai

, is a subgenre of Japanese fantasy light novels, manga, anime, and video games revolving around a normal person being transported to or trapped in a parallel universe. Often, this universe already exists in the protagonist's world as a fictional universe, but it may also be unbeknownst to them.

Films

The most famous treatment of the alternative universe concept in film could be considered 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. At times, alternative universes have been featured in small scale independent productions such as Kevin Brownlow and Andrew Mollo's It Happened Here, featuring an alternative United Kingdom which had undergone Operation Sea Lion in 1940 and had been defeated and occupied by Nazi Germany. It focused on moral questions related to the professional ethics of Pauline, a nurse forced into Nazi collaboration.
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 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 essence of a being described as Satan, trapped in a glass canister and found in an abandoned church in Los Angeles, is actually an alien being that is the 'son' of something even more evil and powerful, trapped in another universe. The protagonists accidentally free the creature, who then attempts to release his "father" by reaching in through a mirror.
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 "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.
More recent films that have explicitly explored parallel universes are: the 2000 film The Family Man, the 2001 cult movie Donnie Darko, which deals with what it terms a "tangent universe" that erupts from our own universe; Super Mario Bros. has the eponymous heroes cross over into a parallel universe ruled by humanoids who evolved from dinosaurs; The One 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 take their energy; and , the main character runs away from a totalitarian nightmare, and he enters into a cyber-afterlife alternative reality. The current Star Trek films are set in an alternative universe created by the first film's villain traveling back in time, thus allowing the franchise to be rebooted without affecting the continuity of any other Star Trek film or show. The 2011 science-fiction thriller Source Code employs the concepts of quantum reality and parallel universes. The characters in The Cloverfield Paradox, the third installment of the franchise, accidentally create a ripple in the time-space continuum and travel into an alternative universe, where the monster and the events in the first film transpired. This concept has been also been passively depicted in the view of a romantic couple in the Indian Tamil Film Irandam Ulagam.

Television

The idea of parallel universes have received treatment in a number of television series, usually as a single story or episode in a more general science fiction or fantasy storyline.
Sometimes a television series will use parallel universes as an ongoing 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 two encountered parallel realities 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 character's presence.
Once Upon a Time often talks about alternative realms or universes in which all different forms of magic, and non-magic may occur, depending on the realm. According to the Mad Hatter, they "touch each other in a long line of lands, each just as real as the last". He referred to our world's tendency to deny such things as arrogant.
In the season 1 finale of The Flash, the Reverse-Flash opens a singularity that connects his world to a parallel universe called Earth-2. In the second season, The Flash starts facing villains from that earth who also have doppelgangers on Prime Earth sent by Zoom. The array of Earth-2 villains consists of Atom Smasher, Sand Demon, King Shark, and Dr. Light; all are sent by Zoom to kill The Flash with the assurance of being taken back home. However, they are not the only ones who arrive from the singularity; this also includes the Earth-2 Flash after a close death and loss of speed from a confrontation with Zoom. When the Earth-2 Flash introduces himself to Team Flash, Barry distrusts him at first and places him in the metahuman pipeline at S.T.A.R. Labs. When The Flash starts having a hard time facing off against Sand Demon, he frees Jay so that he could help him as well as train him in his speed. With a new trick taught by Jay, Barry defeats Sand Demon. Later on, the Earth-2 counterpart of the Reverse-Flash, Harrison Wells, arrives in Prime Earth as well. He steals a weapon from Mercury Labs and saves Barry from the Earth-2 King Shark. When Jay confronts and sees Wells again, the argument gets heated between them before Barry intercedes.
The "Alf Stewart Rape Dungeon" series, created by artist Mr Doodleburger, uses footage from the Australian TV drama show Home and Away, but through the use of clever overlaid audio tracks, casts one of the main characters of the show, long running character Alf Stewart as a vicious violent character in a parallel version of Home and Away. see main article Alf Stewart Rape Dungeon Series

Television series involving parallel universes

There have been a few series where parallel universes were central to the series itself.

Comic books

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. and Multiverse DC's Michael Moorcock's Multiverse collected 12 issues in 1999 with an introduction by Moorcock which offered a sophisticated description of his rationale.
DC Comics inaugurated its multiverse in the early 1960s, with the reintroduction of Golden Age superheroes the Justice Society of America now located on Earth-Two, and devised a "mirror universe" scenario of inverted morality and supervillain domination of Earth-Three shortly afterwards, several years before devised its own darker alternative universe. There was a lull before DC inaugurated additional alternative universes in the seventies, such as Earth-X, where there was an Axis victory in World War II, Earth-S, home to the Fawcett Comics superheroes of the forties and fifties, such as Captain Marvel, and Earth-Prime, where superheroes only existed in fictional forms.
Therefore, 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. DC's version of "What If..." is the Elseworlds imprint.
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 and an Earth devoted to the Charlton Comics heroes of DC. Countdown and Countdown Presents: The Search for Ray Palmer and the 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 1981's Days of Future Past, 1995's 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 franchise 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.
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.
The graphic novel Watchmen is set in an alternative history, in 1985 where superheroes exist, the Vietnam War was won by the United States, and Richard Nixon is in his fifth term as President of the United States. The Soviet Union and the United States are still locked in an escalating "Cold War" as in our own world, but as the Soviet Union invades Afghanistan in this world and threatens Pakistan, nuclear war may be imminent.
In 1973, Tammy published The Clock and Cluny Jones, where a mysterious grandfather clock hurls bully Cluny Jones into a harsh alternative reality where she becomes the bullied. This story was reprinted in Misty annual 1985 as Grandfather's Clock.
In 1978, Misty published The Sentinels. The Sentinels were two crumbling apartment blocks that connected the mainstream world with an alternative reality where Hitler conquered Britain in 1940.
In 1981, Jinty published Worlds Apart. Six girls experience alternative worlds ruled by greed, sports-mania, vanity, crime, intellectualism, and fear. These are in fact their dream worlds becoming real after they are knocked out by a mysterious gas from a chemical tanker that crashed into their school. In 1977 Jinty also published Land of No Tears where a lame girl travels to a future world where people with things wrong with them are cruelly treated, and emotions are banned.
The parallel universe concept has also appeared prominently in the Sonic the Hedgehog comic series from Archie Comics. The first and most oft-recurring case of this is another "mirror universe" where Sonic and his various allies are evil or anti-heroic while the counterpart of the evil Dr. Robotnik is good. Another recurring universe featured in the series is a perpendicular dimension that runs through all others, known as the No Zone. The inhabitants of this universe monitor travel between the others, often stepping in with their Zone Cop police force to punish those who travel without authorization between worlds.
In more recent years, the comic has adapted the alternative dimension from the video games Sonic Rush and Sonic Rush Adventure, home to Sonic's ally Blaze the Cat. The continuities seen in various other Sonic franchises also exist in the comic, most notably those based on the cartoon series Sonic Underground and Sonic X. For some years, a number of other universes were also featured that parodied various popular franchises, such as Sailor Moon, Godzilla, and various titles from Marvel Comics. Archie has also used this concept as the basis for crossovers between Sonic and other titles that they publish, including Sabrina the Teenage Witch and Mega Man.
The various Transformers comics also feature the parallel universe concept, and feature the various continuities from different branches of the franchise as parallel worlds that occasionally make contact with each other. Quite notably, the annual Botcon fan convention introduced a comic storyline that featured Cliffjumper, an Autobot from the original Transformers series, entering an alternative universe where his fellow Autobots are evil and the Decepticons are good. This universe is known as the "Shattered Glass" universe, and continued on in comics and text based stories after its initial release.

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