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This was prompted by a previous question I had here, but I decided it was a good stand-alone question.

This is a general science fiction question and not specific to any particular world.

Is there a difference between a parallel universe and an alternate universe? It seems the two words are used interchangeably, but according my last question, they might be considered two different things. If they are different, what makes them different? And if they are the same, do you have any proof or anything that might back up your claim?

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  • I personally would use parallel for geometric differences and alternate for event differences. But there's no way to make everyone so disciplined. The show Sliders for example features an infinite number of universes that exist independently. The definition gets muddled because some of those universes are so similar they have nearly identical timelines. And timelines tend to noticeably change from time travel. So alternate just means different. And star trek has parallel universes AND alternate timelines. There's no universal answer and is very dependent on the specific fiction's rules
    – lucasbachmann
    Mar 16, 2022 at 23:42

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They're not the same thing. Taken from the definition of parallel universe:

A parallel universe or alternative reality is a hypothetical self-contained separate reality coexisting with one's own

And from the definition of alternate universe:

An alternative universe (also known as alternate universe or alternate reality), commonly abbreviated as AU, is a type or form of in which canonical facts of setting or characterization in the universe being explored or written about are deliberately changed.

So looking at the definitions, we see some differences. A parallel universe would be a completely separate universe, possibly containing similar characters or facts, but definitively a separate entity. An alternative universe would likely take place in the same universe, but with altered facts (i.e. "what-if" scenarios).

Using Star Wars as an example, as that's what I'm most familiar with, E.T. would be (mostly) a parallel universe to Star Wars. An alternate universe would be something appearing under the Star Wars: Infinities label (deliberately non-canon stories) or fan fiction not approved by Lucas Enterprises.

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    The definitions seem to indicate that they are the same: "or alternative reality" ... "or alternate reality"...
    – NominSim
    Jun 29, 2012 at 15:17
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    No, those are what they are sometimes known as. That is why there is confusion (the whole reason the question came up in the first place) - the two terms are often used interchangeably. But if you actually read the answer, you notice the definitions are completely different.
    – The Fallen
    Jun 29, 2012 at 16:32
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    To put it succinctly, an "alternate universe" is just a literary artifact, where things about a single universe just happen to differ between different works, and a "parallel universe" is a physical fact within the fictional setting.
    – Spencer
    Mar 16, 2022 at 19:03
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Writers aren't very strict when it comes to these two particular terms. Most of the time, the two terms are interchangeable, unless they've been defined to be different within that continuity.

For example, in Star Trek, the word "parallel" is taken to mean "alongside", not that the two universes developed in parallel/mostly in parallel. They have timeline splits for universes that were identical up until the divergence point, an antimatter universe that appears to be otherwise completely identical to ours, and at least one "parallel universe" that's on a difference plane of existence.

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    That said, I'm of the opinion that "alternate universe" should be an umbrella term for "parallel universe" and other, non-parallel universes, like the Sphere Builders, who have nothing in common with our universe.
    – Izkata
    Jun 28, 2012 at 23:01
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Parallel universe is a term describing a (theoretical) physical concept of having a similar yet somewhat different universe separate from the "given" one; this concept may be used as one of the elements in SFF setting.

Alternate universe is a term describing a literary (or other media) technique of pretending that an existing universe is somehow different.

Bear in mind that neither has an "official" definition in SFF and as such, it is quite possible that different works/authors can use either one with different connotations or even meanings.

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The terms are essentially interchangeable; some people may make distinctions, but there is no consensus.

The people behind the Historical Dictionary of Science Fiction have studied the usage of these terms (alternate reality, alternate world, parallel universe, etc.) more seriously than anybody else; they have considered most every combination of adjective and noun you can think of, and they have dredged a large body of literature for evidence. Here are their definitions (follow the links to see many historical examples of the use of each term):

alternate reality = alternate world

alternate universe = alternate world

alternate world one of many possible universes, which may have different physical laws or a different history than our own

alternative reality = alternate world

alternative universe = alternate world

alternative world = alternate world

parallel universe a universe conceived of as existing alongside our own, having many similarities to it but usually differing from it in some significant way (as having a different history or different physical laws)

parallel world a world that exists alongside or in addition to the known world; = parallel universe

If these three definitions are not exactly the same, the differences are obscure and insignificant. Jeff Prucher's Hugo-winning book Brave New Words: The Oxford Dictionary of Science Fiction is largely based on the material at the Science Fiction Citations site, precursor to the Historical Dictionary of Science Fiction. The definitions in Prucher's book differ slightly in wording but amount to the same thing:

alternative world n. one of many possible space-time continua, having a different history or physical laws than our own space-time continuum

parallel universe n. a universe or space-time continuum that exists alongside ours, and which may possess different physical laws or a different history. Also used fig. Compare PARALLEL WORLD.

parallel world n. a PARALLEL UNIVERSE.

Of the terms discussed here, only parallel universe and parallel world have entries in the online OED at this time:

parallel universe n. (a) (orig. and chiefly Science Fiction) a universe conceived of as existing alongside or in addition to our own, having many similarities to it but usually differing from it in some significant way; also fig.; (b) (in the many-worlds interpretation of quantum mechanics) a universe regarded as actually existing alongside our own, albeit with no possibility of detection (see many-worlds n. at MANY adj., pron., n., and adv. Compounds 2).

parallel world n. fictional or hypothesized world held to exist alongside or in addition to the known world; = parallel universe n.

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When I think of parallel I see this: Parallel Lines

When I think of alternate I see something like this:

alternate

What I'm heading at is that I think if a universe A is parallel to universe B then they were both created at the same time. And if universe C is alternate to universe D then either one of them was created first and the other one is a "spin off".

========

Parallel universes will have same tendencies and/or directions as well as have comparable parts, some analogous aspects and/or recognized similarities.

I can only assume that alternate universes will change back and forth between some conditions and/or states, actions. I don't think that alternate universes would have same direction (I might be wrong about that so don't quote me).

This picture

tthe image is split horizontally, with boundless blue water, gently rippling, below and a blue sky with light clouds above. A line of mirrored spheres emerge from and progressively rise above the water. Each sphere reflects the sky, the water, and the previous sphere in the line.

should shed some light on this topic.

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Think of an alternate universe as a line that as it progresses through time more and more branches "generate" from the "original" line. So each bifurcation represents a different outcome of any situation; for example, this morning you had the option to stay home or go to work, but you chose to go to work, an alternate universe would branch off from the decision of not going to work. A PERFECT example of this would be the "Butterfly Effect" movies, somehow the protagonists can travel between different lines of the same but ALTERNATE universe... furthermore, in that alternate universe there is not another version of the same person: this is another way how parallel and alternate universes are different; if you travel to a parallel universe you will find a slightly different version of yourself, a perfect example would be the series "Fringe". In Fringe when a protagonist travels to the parallel universe then he/she finds another version of the self (however in the series they sometimes refer to the parallel universe as alternate universe which is incorrect). A HUGE DIFFERENCE is that parallel universes (as the name suggests) NEVER touch each other, and if they do it would theoretically destroy one or both of the universes (as in Fringe); Alternate Universes generate from ONE SAME universe so at some point in time they were the same.

EDITED:

One more thing, Fringe got it wrong about Deja Vu's... Deja Vu's are nothing but the impression of recalling something not a window to an alternate universe. So what causes Deja Vu's? The best explanation I've read states that our brain uses a chemical compound when we are trying to recall something in order to help remember faster and better; when we have a Deja Vu it simply means that that chemical has been produced mistakenly when we were not trying to remember anything.

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Sorry that I'm very late in this game, but here's my two cents about all this. Parallel universe and alternate universe can be distinguished, as seen above. But they could also be one and the same, at least in the realm of science fact. (Sorry if I'm posting this on a science fiction group.)

I'm having in mind the many-worlds interpretation and other multiverse interpretations in the field of quantum physics. In a nutshell, such hypotheses posit that all alternate or "what-if" histories, both that have already occurred and will occur in the future, take place in real life, just that they're in parallel universes or alternate universes. This way, "what-if" histories don't have to be restricted to the realm of a fictional alternate space in this universe - in other words, not restricted to the conditional, as in to the "could have been", "would be", etc. And multiverse theories could serve to reduce, if not eliminate, the suspension of disbelief in terms of alternate histories. It's possible that there's an infinite number of such universes in the cosmos.

Having said that, alternate histories or alternate universes could still be considered fictional, though often based on plausibility, in the sense that one can only approximate, based on real-life examples from our own universe, what such parallel/alternate worlds are like.

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  • Add references (links, pictures if you can) and try to switch from a personal standpoint to a third person perspective. Your answer is valid, but this site is big on people acting as repeaters, rather than originators, of ideas. Still, here's your first +1: keep it going, make the changes, and have a good time.
    – user40790
    Aug 10, 2016 at 16:50
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The way I see it is this. I'm going to be using the time travel logic presented in Marvel's "Loki" and "What If...?" as reference.

Loki is where we get our first proper introduction to the multiverse in the MCU. In Loki the TVA monitors "Variant Timelines." Now the term variant can mean either parallel or alternative depending on context.

Let's talk about one single universe, it's singular timeline. In Endgame the time travel logic is this. When something changes it doesn't change the timeline but creates a "branching" timeline, like a branch in a tree. The branch is an ALTERNATE variant timeline, in which they coexist as a part of the same universe, a branch of the same tree.

Then there are PARALLEL variant timelines, or an entirely different, separate timeline with its own alternate timelines or "branches," a separate tree.

Alternate universe in this context means a branch in a tree, and parallel means an entirely different tree.

The Multiverse in the MCU is like a forest, each tree (parallel universe/timeline) has branches (alternate universe/branches/"branching timeline")

The terms are used interchangeably since many don't really see the distinctions, which is understandable. I thought this analogy would help to better understand the distinction.

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  • Is there any evidence that MCU writers (or the writers of any other notable sci-fi) make the same distinction between parallel and alternate universes as you do? Or is this just your own personal headcanon? Headcanon answers aren't considered good answers around here. A good answer should try to establish what the creators intended.
    – LogicDictates
    Mar 16, 2022 at 18:02
  • That's presented in the MCU, not headcanon. They presented alternate versions of Loki in the Loki show, as well as PARALLEL versions like Sylvie. The main character is an ALTERNATE version of the main Loki from 2012 in the Avengers. Even in the MCU wiki, under the Multiverse page it says that Kang chose many different parallel universes for his "Sacred Timeline" The tree analogy was the best explanation I could think of for clarification. It's kind of like the "cliffnotes" explanation
    – Dylan
    Mar 16, 2022 at 18:08
  • I used the Loki show as a reference to the question since I believe that show presents the two concepts better than almost any other show or movie involving time travel/timelines.
    – Dylan
    Mar 16, 2022 at 18:12
  • Two points. Firstly, the question states that this is "a general science fiction question and not specific to any particular world", so a good answer should attempt to establish consistent definitions that apply to multiple franchises, not just one. And in order to demonstrate they apply to multiple franchises, you'd need to show evidence relating to multiple franchises, which you've not done here. Alternately, you could use evidence to demonstrate that the terms aren't used consistently from one franchise to another, and that'd be an equally valid answer, but you've not done that either.
    – LogicDictates
    Mar 16, 2022 at 18:54
  • Secondly, you haven't convincingly established that your definitions are correct even in relation to the MCU alone. The only source you've cited is a fan-written wiki, which isn't official, and doesn't strictly say that Kang chose parallel rather than alternate universes to form the Sacred Timeline, anyway. It just says that he chose "a collection of universes", without clarifying which of the two categories those universes fell into.
    – LogicDictates
    Mar 16, 2022 at 18:55
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There would be many parallel universes, possibly a huge number, possibly very similar to ours or very different. But essential is that there are multiple ones. Travel between them may be possible. Time travel can be explained as going to a universe which is exactly like hours was 50 years ago or like hours might be in 50 years time.

If we talk about an alternate universe, there is/can be only one, and our universe cannot exist. It is essentially our universe, except it is more or less subtly different.

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Spencer made a comment observing,

To put it succinctly, an "alternate universe" is just a literary artifact, where things about a single universe just happen to differ between different works, and a "parallel universe" is a physical fact within the fictional setting.

This can be understood as both terms belonging to different domains of discourse. "Alternate universe" is a literary or storytelling term, and "parallel universe" is a concept in physics. In fact, you could say that the term "alternate universe" is a term that belongs to the writers of a story and their perspective while "parallel universe" belongs to the characters and their perspective.

Some examples in SFF would qualify as both. For example, the "Mirror Universe" in Star Trek is both a parallel universe in the sense that in-universe characters from both sides treat the existence of the other as a physical reality, as well as an alternate universe because it allows the writers to explore a different side of the characters (e.g. Evil Kirk).

Star Wars Legends would be an example of an alternate universe that is not a parallel universe. Legends is not a parallel universe that characters from the main Star Wars universe can travel to though a wormhole or other plot device, it is simply a label for stories that do not fit into the main canon.

For the third case, the various planes that exist in specific editions of Dungeons and Dragons can be considered parallel universes but not alternate universes. Characters can travel between planes but these planes do not represent alternate forms of the same reality. For example, the Elemental Plane of Fire is not just an alternate version of the Prime Material Plane where everything is on fire, it's a distinct place with its own story.

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To make it simple as possible, A Alternate Universe would simply be a universe where President Kennedy never gets killed or in 2016 the Warriors beat the Cavs. Another example would in this universe you decided not to go to work, in another Alternate Universe you decide instead to go to work.

A Parallel Universe would be instead Of humans becoming the dominant species, it would be the Neanderthals. Robert J. Sawyer book the Neanderthal Parallax is a good example of that. Another example would be Star Wars and Star Trek.

What about other Dimensions or better yet planes.

This is obviously more in fantasy but it also does occur in Science Fiction as well. Other Dimensions or planes, are essentially about beings living in a different plane of existence or realm. Different planes of existence is essentially different facets, frequency or Spectrum. This not the best example but let’s use Infrared light. Your eyes can not detect it but it does exist, certain sounds your ears cannot detect but the sounds indeed exist. So another plane of existence in Sci-fi or fantasy really is beings that vibrate at a different spectrum or frequency.

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  • Hi, welcome to SF&F. Your example of The Neanderthal Parallax ignores the possibility that it is also an alternate universe, with a single branching event, just from a point far in the past. As such, I'm not sure you're making a meaningful distinction. While examples aren't bad, it would be much more useful to find usages of the terms in contexts that make the difference clear.
    – DavidW
    Oct 29, 2020 at 2:54
  • A parallel universe is it own universe and does not depend on another Universe. A Alternate Universe depends on another universe. A alternate universe is created by an individual choice or in sci-fi a time traveler decision to change the course of history. A Parallel Universe is created by chance in the course of evolution. There can be multiple parallel universe and they will all be independent from each other. There could be multiple alternate universes but it’s all dependent on one universe because it depends on a individual decision making. A Parallel Universe is created by Evolution.
    – Dave
    Oct 31, 2020 at 17:36

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