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 constitute 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 of relationship, with our own universe. A universe where the very laws of nature are different – for example, one in which there are no relativistic limitations and the speed of light can be exceeded – would in general count as a parallel universe but not an alternative reality. The correct quantum mechanical definition of parallel universes is "universes that are separated from each other by a single quantum event."
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 a 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, 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 of quantum mechanics, postulating that historical events or human consciousness spawns or allows "travel" among alternate universes.
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, SS-GB by Len Deighton 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. Time travel and alternate history
Time travel is the concept of moving between different points in time in a manner analogous to moving between different points in space, either sending objects (or in some cases just information) backwards in time to some moment before the present, or sending objects forward from the present to the future without the need to experience the intervening period (at least not at the normal rate).
Although time travel has been a common plot device in fiction since the 19th century, and one-way travel into the future is arguably possible given the phenomenon of time dilation based on velocity in the theory of special relativity (exemplified by the twin paradox), as well as gravitational time dilation in the theory of general relativity, it is currently unknown whether the laws of physics would allow backwards time travel.
Any technological device, whether fictional or hypothetical, that is used to achieve time travel is commonly known as a time machine.
Some interpretations of time travel also suggest that an attempt to travel backwards in time might take one to a parallel universe whose history would begin to diverge from the traveler's original history after the moment the traveler arrived in the past.
Alternate history or alternative history is a genre of fiction consisting of stories that are set in worlds in which history has diverged from the actual history of the world. It can be variously seen as a sub-genre of literary fiction, science fiction, and historical fiction; different alternate history works may use tropes from any or all of these genres. It is sometimes abbreviated AH. Another occasionally-used term for the genre is "allohistory" (literally "other history").
Since the 1950s, this type of fiction has to a large extent merged with science fictional tropes involving cross-time travel between alternate histories or psychic awareness of the existence of "our" universe by the people in another; or ordinary voyaging uptime (into the past) or downtime (into the future) that results in history splitting into two or more time-lines. Cross-time, time-splitting and alternate history themes have become so closely interwoven that it is impossible to discuss them fully apart from one another. "Alternate History" looks at "what if" scenarios from some of history's most pivotal turning points and presents a completely different version, sometimes based on science and fact, but often based on conjecture. The exploration of how the world would look today if various changes occurred and what these alternate worlds would be like forms the basis of this vast subject matter.
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.
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.
A well known and often 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. A previous episode for the Trek series first hinted at the potential of differing reality planes (and their occupants)-titled "The Alternative Factor". A mad scientist from "our" universe, named Lazarus B., hunts down the sane Lazarus A.; resident of an antimatter-comprised continuum. His counterpart, in a state of paranoia, claims the double threatens his and the very cosmos' existence. With help from Captain Kirk, A traps B along with him in a "anti"-universe, for eternity, thus bringing balance to both matter oriented realms.
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.
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 goatee, like the "mirror" version of Mr. Spock.
Buffy the Vampire Slayer experienced a Parallel universe where she was a mental patient in Normal Again and not really "The Slayer" at all. In the end, she has to choose between a universe where her mother and father are together and alive (mother) or one with her friends and sister in it where she has to fight for her life daily.
The animated series, Futurama, had an episode where the characters travel 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.
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 democratic 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. According to the Doctor in "Doomsday", a new parallel universe is created by every decision made.
The OC had an episode where two main characters fell into a coma, and into an alternate/parallel universe.
Friends had an episode in which the characters wonder how different their lives would be with different choices.
Parallel universes/alternate futures are also featured in Heroes, and the idea of a parallel universe and the concept of deja vu was a major plot line of the first season finale of Fringe, guest-starring Leonard Nimoy of Star Trek.
In the 2010 season of Lost, the result of characters traveling back in time to prevent the crash of Ocean Flight 815 apparently creates a parallel reality in which the Flight never crashed, rather than resetting time itself in the characters' original timeline. The show continued to show two "sets" of the characters following different destinies, until it was revealed in the series finale that there was really only one reality created by the characters themselves to assist themselves in leaving behind the physical world and passing on to an afterlife after their respective deaths.
The anime Turn A Gundam attempted to combine all the parallel Gundam universes (other incarnations of the series, with similar themes but differing stories and characters, that had played out at different times since the debut of the concept in the 1970s) of the metaseries in to one single reality.
The anime Neon Genesis Evangelion features a parallel world in one of the final episodes. This parallel world is a sharp contrast to the harsh, dark "reality" of the show and presents a world where all the characters enjoy a much happier life. This parallel world would become the basis for the new Evangelion manga series Angelic Days.
In the animated Disney series Darkwing Duck, the title character's archenemy, Negaduck, comes from a parallel dimension called the Negaverse (not to be confused with the similarly named dimension in the Sailor Moon series).
In the Family Guy episode, "Road to the Multiverse", Brian and Stewie get a look at life in other universes that are at the same time and place as Quahog, but under different conditions.
In the Star Trek: The Next Generation episode "Parallels", Lt. Worf traveled to several parallel universes when his shuttlecraft went through a time space fissure.
As an ongoing subplot
Sometimes a television series will use parallel universes as an ongoing subplot. Star Trek: Deep Space Nine and Star Trek: Enterprise 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 Lois & Clark: The New Adventures of Superman.
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. The latter could also be seen as the point of the alternative reality portrayed in the movie It's a Wonderful Life.
Parallel universe-based series
There have been a 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; Sliders, where the characters travel across a series of "alternative" Earths, trying to get back to their home universe; and Charlie Jade, in which the titular character is accidentally thrown into our universe and is looking for a way back to his own.
In 1986, Disney produced a pilot episode for an animated children's show about interdimensional travel called Fluppy Dogs.
In the TV series Fringe the plot involves two parallel universes fighting with science until only one remains.