As you've probably guessed, I have no idea how multiverses and parallel universes, or space-time travel, etc work. I don't know how Physics or Mathematics explains alternate universes. But, they are pretty much a driving plot point in my WIP.

Is it possible to accidentally travel from one universe to another, without intentionally trying to access the other universe?

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    $\begingroup$ This will take some hand waving the disconcerting facts away. If you are basing your ideas on string theory, the thing they do NOT tell you is that two universes would never be exactly the same. Very few alternative choices would ever be able to support anything resembling life. The laws of physics would be different in every one. Most would probably not have anything remotely like 'matter' as we know it. Using string theory to posit universes similar to our own faces unfathomable odds $\endgroup$ Oct 4, 2018 at 3:02
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    $\begingroup$ the reality-check tag is really not going to work here as the whole idea of traveling between alternative realities is just simply not something we have any theoretical (let alone experimental) basis for. $\endgroup$ Oct 4, 2018 at 4:05
  • $\begingroup$ Removed reality-check tag. Also removed the second question. $\endgroup$
    – elemtilas
    Oct 4, 2018 at 5:44
  • $\begingroup$ It's basically science fiction to think that there are multiple (parallel) universes (it appears like for some people the sheer amount of galaxies is simply not enough). So if it's impossible even if you'd put in all of Earth's efforts into that task for thousands of years, why would it be possible out of accident? The magnitude of hand waving necessary would conjure unimaginable storms and tornadoes upon the Earth which could threaten the survival of humanity. $\endgroup$
    – Battle
    Oct 4, 2018 at 9:00
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    $\begingroup$ Before you go into the details, you have to learn what the words you are using actually mean. I advice you to do research or ask questions about what this multiverse stuff is all about and then ask such advanced questions. There are still answers that might be good, but I wonder how much they help you in constructing a world. That foundation needs to be solid imo $\endgroup$
    – Raditz_35
    Oct 4, 2018 at 10:40

7 Answers 7


First off, the required disclaimer: There is zero scientific evidence to support parallel universes. Zip. Nada. In fact, if there was evidence, we'd typically choose to re-define the universe to include everything that we can interact with, so the idea of going to a parallel universe is almost silly when viewed that way. Everything we talk about when it comes to multiverses is pure speculation, without a shred of evidence.

Second, when you look at multiverses, it helps to look at what sort of parallel universes you might go to. Max Tegmark put forth the most popular classification of parallel universes:

  • Level 1: The same physics as our universe in every way. These universes just differ by the layout of the matter inside the universe, and are typically separated by far enough that they don't interact in any way.

  • Level 2: Same physical laws, but different constants. In these universes, $E$ still equals $mc^2$, but $c$ may be a very different number due to symmetry breaking events that change the nature of reality. Universes where these constants differ could be dramatically different than ours, so much so that few writers even think about talking about them. And it's going to get more alien from there

  • Level 3: Many Worlds. One interpretation of quantum mechanics is that there are many universes, such that every possible thing that could happen, happens. A level 3 multiverse covers that.

  • Level 4: The Ultimate Ensemble. Basically, if it has laws of physics that can be described in the language of abstract mathematics, it fits in level 4. This is such a wide classification that it has received criticism that it isn't even sufficiently defined.

So that gives you a sense of just how far and wide cosmologists have gone with this parallel universe thing. They have literally gone everywhere. There isn't an answer to your question which isn't a valid one.

In my opinion, the biggest question is whether the symmetries between these universes are continuous or discrete. If you have one universe where Wayne Gretzky scores a winning shot, and a universe where he misses, is there necessarily a universe where the pick glances off the goalpost? If so, you're building a world with continuous symmetry. Do you have two universes which are mirrors of eachother, and nothing else? Or one world where Abraham Lincoln is assassinated and one where he isn't, with no in-between case? Then you have a discrete symmetry. The two systems behave radically differently when you try to flesh them out into a consistent worldview, so its a good idea to get them straight ahead of time.

As an example, consider Heinlein's Future History multiverse. This was a world he wrote several books in and its defining trait is the "Time Corp," which is a group that does time travel in a multiverse setting. Heinlein, like many authors, had to keep it interesting. So the first symmetry between his multiverses is a common prefix. The time travelers can go back y years in the past, and find themselves in a universe where everything happened exactly the same as in their universe, but events after that point may be different. This is a continuous symmetry in that if you can travel back 2 hours in to the past, or 3 hours into the past, that implies you can also travel back to 2 hours, 43 minutes, 12 seconds, and some arbitrary fraction of a second.

By Heinlein's rules, the act of traveling back through time creates a fork in his timelines (they don't have to do that, it's simply what he chose). There'd be too many timelines to count if he went with this, so he simplified. He decided that some events were "cusp" events, where history would go one way or the other. For example, the landing on the moon was one of these events. Humans always managed to land on the moon, no matter what timeline you visited. However, the person who landed on the moon might be different. This is a discrete symmetry. Two timelines may have Neil Armstrong landing on the moon, and one my have Vladimir Komarov land on the moon first, but it's impossible to have a timeline where Armstrong half landed first and Komarov landed half first. As Heinlein chose it, many timelines played out almost the same, except for these events. So the timelines labeled "23 Neil Armstrong" all played out roughly the same, dominated by the fact that Armstrong landed on the moon first.

None of that was forced by physics, but it was forced by writing. He needed something to untangle the infinite variations that multiverses tend to permit.

Heinlein also took a stab at a Level 4 multiverse that contained the Future History series. He called this wider series the "World as Myth." His logic for this was "any world which has been imagined exists." Main characters were trained not to imagine particularly dark horrid universes, so they couldn't exist and try to reach out across the multiverse. This includes the literal devil and hell, though we find in the Book of Job (subtite: A Comedy of Errors), the devil isn't always who he appears to be. Personally, I found the books fun, but this World as Myth series was much, much harder to follow than the Future History series. It lost a lot of readers because that multiverse is just too wide. It's too hard to write about. The restrictions you place on yourself are essential for writing good multiverse fiction.

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    $\begingroup$ Would it be... well, not "possible", but at least "conceivable" that there exists another universe, completely identical to this one, with the only difference of starting "later"? People will make the same decisions, oceans would move the same way and so on, it's just that some it hasn't happened yet. This wouldn't be as useful for travelling (you'd change it), but to look into the past, or predict the future. $\endgroup$
    – ChatterOne
    Oct 4, 2018 at 7:56
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    $\begingroup$ The grumpy-pants view of the multiverse. An idea strongly resisted since Hugh Everett proposed it sixty or so years ago. While no empirical evidence exists for it, there is a growing body of theoretical work that works better if there was a multiverse. Funny how often mathematical constructs mirror physical reality. Pure speculation like antimatter and the neutrino. Fiction can use unfounded scientific concepts. Otherwise what's the role of imagination? Even so good exposition on the multiverse. $\endgroup$
    – a4android
    Oct 4, 2018 at 9:11
  • $\begingroup$ @ChatterOne That would be a Level 1 parallel universe. Of course, the interesting question would be how your travelers proved that said universe is a time-shifted copy of ours and not one that is almost a time-shifted copy. $\endgroup$
    – Cort Ammon
    Oct 4, 2018 at 14:34
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    $\begingroup$ @a4android True, We do tell ourselves a narrative where our universe is one of many. We also tell ourselves it'd be nice if complex numbers were real =) There is the age old question of whether the universe is defined using mathematics, or if we invented mathematics to make sense of the universe. Either way, I think coming to grips with your preferred symmetry between universes is key. Without some decent symmetry to work with, the odds of your character finding a world which is remotely interesting to write about is vanishingly small. $\endgroup$
    – Cort Ammon
    Oct 4, 2018 at 14:43
  • $\begingroup$ @a4android Re-reading my answer, you're right that I didn't give the possibilities justice. I pointed out that you have to identify the symmetries you use to make order of the infinite messiness of universes, but it kinda fell flat. I added some examples from Heinlein to try to make it less grumpy-pants. Though I still have to be a little grumpypants about the readability of his World as Myth series. $\endgroup$
    – Cort Ammon
    Oct 4, 2018 at 15:22

In terms of understanding the science and math, this is a good starting point for the layman.

(I'm expanding this answer on request, to make a more complete answer that addresses the different concepts in parallel universes.)

Let's start by pointing out that this essay is far from a complete resource on the topic of parallel universes and I'd encourage all who read it to supplement with their own research on the matter. I'll also point out that the very concept of whether or not a multiverse is possible or not is hotly contested between physicists across the world, even today. Finally, I'm going to point out that we don't even know all there is to know about this universe yet and it may never be possible to do so; scientists today often refer to the 'observable universe' as a concept that defines the radius of the universe (with Earth at the centre), the length of which is the distance that light can travel between the big bang and now. As such, we can never hope to know the 'complete' universe now, let alone the nature of a multiverse, but we do have a number of mathematical models and resultant theories we can explore.

Let's look at two of them; the many-worlds scenario and string theory.

The Many-Worlds Scenario is based on an idea from quantum physics that maybe really small sub-atomic particles don't work on a concept of waveform collapse, or put more simply, don't resolve into a final state from their 'superposition' before they're observed.

This concept of waveform collapse was explored by Erwin Schrodinger in his famous thought experiment about the cat that sits in the box, neither alive nor dead until 'observed', or interacted with. In the many-worlds model, the universe splits at the point that the cat is observed. In one, the cat is alive and in the other the cat is dead, leading to a virtually infinite number of universes that represent every possible 'state' of matter (and energy).

If we use this definition of the universe, then yes, we enter parallel universes all the time without even trying.

On the other hand, getting to the spirit of your question; can one accidentally move between these parallel universes so that one version of ourselves is in a universe 'intended' for a different version? No.

Again, starting with the many worlds scenario. What is not clear from my own research is where all these universes reside by comparison to each other, but let's assume for the moment that each of the many universes is in itself a fully formed 4 dimensional spacetime construct. Let's further assume that these universes are all finite in nature and exist side by side, with the closest one being the one that matches our existing situation most closely, the changes that differentiate them appearing only at the end of the spacetime construct.

To get to a universe in which you'd notice any substantial change between the two, you'd essentially have to instantly travel an (almost) virtually infinite number of universe lengths in an instant to get to the 'new' earth. This is prohibited by relativity of course, which means you need a wormhole.

The energy requirements alone for these phenomena would be prohibitive. They require negative energy to maintain, but that is likely going to require an equal amount of positive energy to establish, at the very least. To maintain a 1 cubic metre opening on one end of a wormhole, you need an energy mass of around the size of Jupiter. Passing through something that needs that much energy to maintain will be noticed, and very dangerous. It's probably not surviveable. As for accidental, well the probability of landing on a parallel earth in another 'dimension', with each dimension containing an observable universe the size of ours? Well, it's as close to mathematically impossible as makes no odds.

But, what about string theory?

Well, the parallel universes that arise from string theory math are even more exotic. Here we come across a concept called 'branes', short for membranes, of lower dimensionality that are tied to the 9-dimensionality of strings.

I won't go into explaining this in detail because it's not relevant to my answer, other than to say that this theory states that the complete mulitverse can be made up of many different branes of differing dimenionality, all comprised within a 9-dimensional construct of which quantum strings are a primary component. In this model, the parallel universes are far fewer, far more exotic by comparison to each other, and can only be traversed via extremely localised points at which the branes intersect.

Again, you're looking at wormholes, with all the energy baggage they carry. BUT, this time there are more problems. Your wormhole may not lead to a universe that has the same number of dimensions, may not have environments in them (and not be capable of supporting them either) that are conducive to human life, and their location in our universe would dictate that you can only have fixed wormholes, in fixed places, connecting to fixed alternate universes.

This is actually far better for your chances of the wormhole travel being accidental, but far worse for your chances of surviving the travel or ending up somewhere surviveable.

In short, regardless of whether your multiverse is the many-worlds or string theory model, if you were to (even accidentally) travel between universes;

1) You'd notice the transit
2) You probably wouldn't survive it
3) the probability of 'landing' in a safe, surviveable place on the other side alone would make the journey so implausible as to be impossible in every practical sense.

That said, yours would not be the first story to do it anyway. :)

  • $\begingroup$ Survive-able? Whatever happened to "survivable"? It is after all a word, whereas the hyphenated 'survive-able" is not. $\endgroup$
    – a4android
    Oct 4, 2018 at 8:50
  • $\begingroup$ Mmm! Your answer seems to be conflating two models of parallel universes. The many-worlds version of quantum mechanics & one version of general relativity's multiverse. MW allows for switching through parallel universes continuously. 10^1000 per second, IIRC. It is nigh on impossible to recognise one was in a parallel world. Assuming a hitherto unknown boost to MW switching a person might accidentally arrive in a parallel world sufficiently different they could notice the transition had happened. No need of wormholes too. Just suggesting. $\endgroup$
    – a4android
    Oct 4, 2018 at 8:58
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    $\begingroup$ @a4android tell that to Worldbuilding's spell check. We need an upgrade I fear. Ans as for conflating models; no. Just addressing all the various forms of the idea in a brief version. Happy to expand if necessary. $\endgroup$
    – Tim B II
    Oct 4, 2018 at 12:18
  • $\begingroup$ I'd be happy to see you expand your ideas. It seems to me that many-worlds quantum mechanics and general relativity's multiverse are very different versions of parallel universes. Travelling between universes would involve different mechanisms. Answering a question like this needs sufficiently detailed explanations to show how such transitions could, hypothetically, occur. Not easy, but I'm sure you're more than capable of being able to do it. $\endgroup$
    – a4android
    Oct 5, 2018 at 3:49
  • $\begingroup$ RE WB's spell checker: 'survive" and "able" are correct words. Putting them together is the problem. It has trouble coping with knowing which hyphenated words are genuine and those which are not. $\endgroup$
    – a4android
    Oct 5, 2018 at 3:51

Maths and Science isn't going to help you much with something that we ourselves have never experienced, encountered or observed. At best you have ideas and theories that might work and might not. Since you yourself don't have much experience in it, the best way to address it is with magic.

I'm going to address it the way I remember the did in the Golden Compass book series. You have a knife, a knife thats incredibly sharp. One side of the knife can cut through any material in the world like it was butter. The other side of the knife can cut through the fabric of the universe, allowing you to travel into parallel dimensions (or other worlds). The only issue is that you can only cut through the fabric of the universe when two worlds happen to align (in terms of the ground and environment. No sense cutting open a doorway into a pool of lava, or 100m in the air) and the fact that you need to close these portals ( by pinching the edges together ).

In your world, you have a similar device that someone else has used to travel through the many worlds in existence. Your main character simply falls through one of these open doorways which haven't been properly closed. This puts them in a new world with no idea how they go there and gives you a mystery character who can be a enigma or a driving force for plot developments.


Depending on how you want your multiverse to work, there could be many ways your protagonist could move between universes without meaning to.

If your protagonist is unknowingly some kind of super-being, then simply going about their daily lives may cause them to shift between universes. Consider an Amberite from Roger Zelazny's universe who has never walked the pattern, or had it's nature explained, but who nevertheless has an innate ability to traverse shadow without realising it. Every step they take makes a unconscious, infinitesimal changes to their universe, so it could take years before they start to notice inconsistencies between the world they are in now, and the world they remember being in previously.

Alternately, if your protagonist is a drug user, they might find themselves transported to a parallel universe when they use a particular drug in a certain state of mind. Again, they may have no concious control over this travel, as it is their unconcious mind making the changes.

Possibly your protagonist has access to technology or magic they don't understand. They have discovered by trial and error that when they type their wishes into a strange computer they found, or speak their desires into this glowing amulet, that those desires come true, but they have no idea that this works by transporting them into a parallel universe.

The possibilities are endless, you just need to find one which works for your story.


Let phrase it like this When it is possible to travel multiverses, then it can happen by accident.

In your world travelling multiverses is possible. So one might enter a vehicle for multiverse travel by accident, step through a gate accidentally, or being hit a a magic spell that transports to another universe by accident. Take your pick.

And, of course, you can intend to make a multiverse travel, but accidentally come out at the wrong place (some mistake in the preparations, some malfuncitioning part of the vehicle, some unexpected condition between the multiverses, whatever you want).


If you are interested in the fictional side of things: This has already been done. Two cases I know of:

Cascade Point (short story by Timothy Zahn) - Due to the specific manner in which FTL works, an unaccounted-for piece of equipment, not properly stored in the ship's shielded compartment during a 'jump', causes the ship to transition differently than expected. Several 'jumps' later, they arrive at their destination colony planet, only to find no evidence that it had ever been colonized. They manage to work out that they have accidentally slipped into an alternate universe. [then they have to work out how to get back!]

Mostly Harmless (Douglas Adams) - Beings who originate from one of the galaxy's 'plural' sectors are known to be liable to slip from one universe to another while travelling through hyperspace, and are explicitly warned against such travel by the star liners' brochures. Arthur Dent has already done this, which is why instead of arriving at Earth he arrives at a particularly dismal alternate Earth called 'NowWhat'. This is also how he got separated from his girlfriend Fenchurch (she slipped, he didn't).

  • $\begingroup$ @MarkBooth Thanks for editing the question. I can now delete my comment and this one too in the fullest of time, when you have had the chance to read it. $\endgroup$
    – a4android
    Oct 6, 2018 at 7:09

As others have said, we really know nothing about parallel universes, so you are free to invent anything you want here.

As for the accident part, I think the best way would be teleportation gone wrong.

In a magic setting you set up a spell to take you to another city. You arrive in a city that looks exactly like what you had in mind when casting the spell, but all the things you didn't think about are ... different.

In a sci-fi setting we have finally learned how to travel Faster Than Light. Hurray! We send an expedition to a remote exo-planet. Everything looks fine until they travel back home and arrive at a different Earth.

Either way, the effect can be distance related, so everything looks fine during testing. No noticeable effect, at least not noticeable before people start looking for these things.


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