Parallel-Universe Portals

A solid material has been discovered that, when shaped into a ring and activated (by lowering its temperature below some critical point, let's say), it forms a parallel universe on the other side. This isn't a gateway into an existing alternate universe, but rather a duplicate of the host universe, identical in every way at the time of activation. However, over time (on the order of minutes), the two universes will desync due to slight quantum-scale differences that the portal produces on each side. (It’s the same type of parallel universe generation described in this previous Worldbuilding SE question, but I’m interested in a special case)

As long as a portal is active, it must be in the same position in both universes. If you move the portal from one end, it moves in the other. This means that a portal can never be used to cross distance or time. When I say that it is in the same position, I also mean that it faces the same direction. So if you walk into a portal, moving north, you will exit in another universe, still moving north. Walk south again? Back in your universe, moving south. Walk around the portal and walk north through it again? Back in your universe, moving north this time. (Moving through the portals isn't really relevant to this question, but I just wanted to give some examples so that what I'm describing is a bit clearer.)

Notably, portals duplicate existing portals. To illustrate this, imagine that you have an active portal linking your universe, universe 0, to another universe, 1.

enter image description here

After a few minutes, when the universes are desynced, your copy in universe 1 decides to open another portal (since it’s after the desync, you're able to choose not to open another portal in your universe). The end result is shown below, with 2 copies (joined by a blue portal) of your previous 2-universe system (joined by a red portal):

enter image description here

The Question

What happens if your copy doesn’t wait for a desync before opening the second portal? Suppose once the red portal opens, she waits only 1 second before opening the blue portal. Since you're both still synced, you'll open a portal at that time as well. So there are 8 universes, right? I don’t think so.

I think what happens is a “hall of mirrors effect.” Consider the two copies of you at the ends of this 8-universe chain. Why do they only have a red portal open? Shouldn’t they have also opened blue portals, since that’s what you did and they’re synced to you? By induction, the chain of universes should be infinite.

The question is: does the “hall of mirrors effect” produce a truly infinite chain of universes, or does something break the chain?

My current answer: the chain is infinite.

Reasoning: There’s some incredibly slight amount of desync that happens in that 1 second between the portals opening. If a limit exists, it probably depends on that. Let’s assume that 1 second of waiting causes a desync of 1 nanosecond (that is, that 1 second of waiting between portals cause you and your copy to behave almost the same, but with one of you on a 1-nanosecond lag). So the second blue portal opens 1 ns after the first.

I initially thought that this would make the chain finite, since each layer further from the starting layer would add time, until eventually the desync would be so large that the copies on the ends would be in a completely different state of mind and the hall of mirrors would be broken. But no, between 0 and 1 ns apart, the multiverse would resemble the second diagram, with 00 and 01 about to open their blue portals at the same time at t = 1 ns. Any role the tiny desync plays in the chain is gone by the first iteration.

Am I right? I’m not as sure about my answer as I’d like to be, and it feels a little wrong to have an actual infinity come up in this problem. After all, these portals are remarkably well-behaved in every other scenario I could think of. You can move them through each other and intersect them like chain links, all without making the multiverse implode or anything. I’m interested to see if this case has some subtlety I’m missing.

Answers could involve:

  • The desync between the portals
  • The time that light takes to travel between the portals
  • The absurdly small probability of a quantum fluctuation causing, say, the portal to stop working, on a human timescale

Answers should not involve:

  • Handwavium / built-in stopping mechanisms (i.e. the portals are designed not to work if another portal was opened less than a second earlier)
  • The portals causing the universe to collapse (for General Relativity reasons or otherwise)
  • Any assertion that the chain of universes doesn't exist at all (or at least think about it a while and draw some diagrams before posting. This is very easy to misunderstand. After a lot of thought, I'm confident that the chain starts, just not that it ends.)
  • $\begingroup$ Infinite is a concept. A number without limit or something so large its uncountable. Unfortunately, I believe the number of Portals your characters could created could be calculated. There are several limiting factors here. Portals can only occupy a certain space. Portals create parallel words which means you have an original world. Portals desync and we assume they stop duplicating past desync. The biggest question is, what happens when a portal closes. Does the parallel world completely disappear? or does it live on by itself, forever unlinked? I'll leave the maths to someone else... $\endgroup$
    – Shadowzee
    Commented Aug 4, 2019 at 23:50
  • $\begingroup$ Say I create a red portal, and then after the desync we agree my universe will be labeled A, and my double's universe on the other side of the red portal will be called B. Now I create a blue portal in my universe A, which at the moment of creation should lead to a universe C where my duplicate also has a red portal which leads to a universe which is at least just like B at that moment. But is it possible the red portal in universe C is actually permanently connected to universe B, instead of being connected to a universe that starts like B but desyncs? That could avoid infinite duplication. $\endgroup$
    – Hypnosifl
    Commented Aug 5, 2019 at 1:39
  • $\begingroup$ ...maybe that doesn't work though, because when my duplicate in B looks through his own red portal, he can't see both universe A and universe C if they have desynced, it can only be one or the other, thus the red portals in A and C can't both be connected to the same B eternally. So on second thought, I agree with you about the hall of mirrors effect. $\endgroup$
    – Hypnosifl
    Commented Aug 5, 2019 at 1:48
  • $\begingroup$ Its not quite what you are looking for, but you may be interested in free groups. They have a structure which might apply, if you wanted it to. $\endgroup$
    – Cort Ammon
    Commented Aug 5, 2019 at 4:41
  • $\begingroup$ In what way do then portals/metal interact with both universes at the same time? e.g. if you place the portal on a table that doesn't exist in both universes, will the portal float in the other? $\endgroup$
    – xpy
    Commented Aug 6, 2019 at 8:40

7 Answers 7


Let's pretend I have 2 doors. I am the first state(A)

enter image description here

I make a copy, then I have 1 door left in each of 2 universes. My second state (B) is linked by door #1

enter image description here

Then using my second door, I make a copy creating resulting in 2 universes with 1 available door, and 2 with none, My third state (C) is now connected to (A) by the second door

enter image description here

At about the same time, (B) is also opening his second door to make state (D).

enter image description here

(B) now makes both ends. Here is where the infinity thing happens. In the exact same quantum of time as they are made, they also copy themselves.

enter image description here


enter image description here


So, with this visualization we can see it is infinite because no matter how many times you "unfold it", the ends will always have a copy of the 2nd state (B) with an unopened door. For this to be an eventually terminating pattern, your outside copies would need to be 3rd, then 4th, then 5th generation copies, but you will never generate more than a second generation copy that still has a door to open.

EDIT: You can also see from the visualizations that the duplication process never causes an overlapping of portals as I originally hypothesized. This means that you will not get an infinite energy buildup; so, the mirror effect should not destroy the lab causing a forced termination from a run-away interaction between the dimensions.

  • $\begingroup$ Right. So the question is, is there a mechanism that ends it? You glossed over the desync times, saying "a nanosecond" and "within nanoseconds," but I think we need to answer the question of how the desync changes per layer. Does it add up to anything finite? It seems more likely that it decays to zero, preventing the chain from ever ending, but I'm not 100% convinced. $\endgroup$ Commented Aug 14, 2019 at 22:09
  • 2
    $\begingroup$ No, it is infinite because the copy who is making the copy is always a copy of the 1st universe, and not the 2nd, 3rd, 4th, 5th, etc. Think of it like this, if you use a copy machine to copy a copy, then copy that copy, etc. your image degrades from one to the next, but if you just leave the second copy on the scanner, you can just keep duplicating it indefinitely, and this is what you're doing here. It means there is no opportunity for things to drift apart. $\endgroup$
    – Nosajimiki
    Commented Aug 15, 2019 at 14:49
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    $\begingroup$ Ah, I see what you're trying to say now. But the issue is that quantum differences across portals don't just stay in the universes on each side. Rather, after an amount of time that's related to the distance between the portals, the quantum effects will propagate from one portal to the other. However, your argument has convinced me that if you open the portals so simultaneously that light doesn't have time to travel from one to the other, then it is truly infinite. No matter how fast the quantum information spread, the layers would accumulate faster than the differences could catch up. $\endgroup$ Commented Aug 16, 2019 at 16:30
  • $\begingroup$ The "doomsday lightbulb" you describe won't be the end result. The energy put off by a light source falls off as 1/r^2. If you have an infinite hallway of rooms, the total energy reaching you will be on the order of the sum of 1/n^2 as n goes to infinity, which is a finite value. You have a linearly increasing number of energy sources whose energy diminishes quadratically - you can open as many portals as you want, but there's an upper limit on how powerful the combined output of the lights will be when it reaches you. $\endgroup$ Commented Aug 16, 2019 at 17:26
  • $\begingroup$ It's not an infinite hallway, it is an infinite number of lightbulbs the exact same distance from you. Each copy makes a new portal in the exact same place. Not easy to visualize but an inevitable consequence unless the OP specifies that no two portals can occupy the same spot. If they can't exist in the same spot, you terminate at [B1][A0][A0][B1] because the B1s can't make portals where the A0s have already made one. $\endgroup$
    – Nosajimiki
    Commented Aug 16, 2019 at 18:32

Of course this is all theoretically suspect as it is impossible (according to the rules of quantum physics) to create an exact copy of an existing state. Thus it would be impossible for your mystery portal material to form an exact parallel universe.

But putting that argument aside, your portal appears to act like a mathematical operator with an inverse. To illustrate: if you construct a portal $A$ then going through it once takes you from your universe $x$ to a parallel (initially 'identical' but now divergent) universe $y$. Passing through it again does the inverse operation - taking you from universe $y$ back to universe $x$. That is fine as lots of operators are their own inverses (for example multiplication by -1).

So mathematically $y = Ax$ and $x = A^{-1}y$ . This is consistent as

$A^{-1} y = A^{-1}(Ax) = (A^{-1}A)x = Ix = x$

But now consider adding a second portal $B$. To simplify the argument imagine constructing both of them simultaneously in universe $x$, so now you have a choice of going through portal $A$ to universe $y$, or through portal $B$ to universe $z$. Note that as you constructed them simultaneously, both $y$ and $z$ will have 'copies' of $A$ and $B$.

Here is where it gets interesting. Going back and forth through portal $A$ just takes you back and forth between $x$ and $y$.

Similarly going back and forth through portal $B$ takes you back and forth between $x$ and $z$.

If you go through $A$, then through $B$, then back through $B$ and back through $A$, you will find yourself in $B^{-1}BA^{-1}Ax = (B^{-1}B)(A^{-1}A)x = IIx = x$, so you end up back home again.

But if instead you go through $A$, then through $B$ and then 'back' through $A$ and then 'back' through $B$ you will be in $B^{-1}A^{-1}BAx$ and if $A$ and $B$ are non-commutative operators then $x = B^{-1}BA^{-1}Ax$ $\ne$ $B^{-1}A^{-1}BAx = x'$

If you repeat the path again and again you will get from $x'$ to $x''$ to $x'''$ etc. so you will cascade through an infinite hall-of-mirrors of alternate universes.

If you want things to be that way then fine, but if you don't want this to be a problem, all you need to do is define that in your universe 'portal-operators' are commutative operators (via some mysterious property of the portal material itself), which would dismiss all recursive loops as then $x = B^{-1}BA^{-1}Ax$ $=$ $B^{-1}A^{-1}BAx = x$ by definition.

  • $\begingroup$ Interesting approach. I think that the commutative case that you discussed effectively means we form loops rather than chains (so 00 and 01 would be connected in the second diagram). However, I don't think this makes sense as soon as we start thinking about desyncing. It implies that when my copy in 10 makes a blue portal, another blue portal kind of magically appears in my universe, too, one that neither I nor my copy on the other side (01) made. Either that, or we're limited to 2 universes, and all portals just connect between them. Neither case is really what I'm describing here. $\endgroup$ Commented Aug 5, 2019 at 11:26
  • $\begingroup$ Regarding the first point, about it being possible on a quantum level: I think it only takes a slight adjustment to make the universes functionally identical without violating the uncertainty principle, to an acceptable level of hand waving. We could say that, within one Planck time of the portal being opened, the universes are maximally similar and it is not possible to prove or disprove that the universes are identical. After one Planck time, it's theoretically possible to prove them to be non identical; after a few seconds, it's practically possible; and after a few minutes, it's trivial. $\endgroup$
    – Robyn
    Commented Aug 5, 2019 at 11:30
  • $\begingroup$ @Robyn, the uncertainty principle isn't the issue, it's that parallel copies violate the no-cloning theorem. Look at the 4th bullet of the link to see a thought experiment where copying a state (in this case by creating parallel universes) could let you send messages faster than light. However, I think this could also be avoided as long as every state is entangled with its copy across the portal. As the universes desync, these entanglements slowly break. $\endgroup$ Commented Aug 5, 2019 at 13:34
  • $\begingroup$ @GiladM it's not my answer, but I may have a way to explain how desynch coul work. The basic is stating that if there is a maximum of 3 universes looping for ever, then if a person in universe z opens a portal toward x. There sinply must be a portal in x. Either the universes are not desynched quite enough for there to not be a portal in x. Or some other phenomenon has to create the portal. In the worst case, the person in z has to pump enough work to open a portal in x from the other side. (Which is possibly inpossibly hard, but theorically doable) $\endgroup$
    – 3C273
    Commented Oct 11, 2019 at 2:45
  • $\begingroup$ I'd think of it like an inductance or eddy currents. Where if you have a magnetic field going through metal, you have some current created. And the opposite is true where any current causes a magnetic field. You can counter the magnetic field or the current, but at the end of the day, you're juste fighting the symptoms. $\endgroup$
    – 3C273
    Commented Oct 11, 2019 at 2:53

Let's assume you don't open the two portals at exactly the same time. You always have 2 universes that have only 1 portal. Opening a portal in one of them gets rid of a 1-portal universe, but doubles the other one.

The rate of the doublings depends on the desync rate and the difference in portal-opening times. In your example, the second pair open a portal 1 second apart. That means your two clones have 1 second to desync. In that 1 second, how much do their opening times/probabilities change?

Then, consider the product of the probabilities of opening at each step. This infinite product is the probability of your infinite hall of mirrors.

  • $\begingroup$ Welcome to the site! I really like this answer; it gets to the heart of what I'm asking. I think you're right, that it all boils down to the function that converts the desync time of one layer to the desync time of the next, and the probability of a portal opening in some amount of time. I need to think for a bit about how those functions work, but I'm sure I'll be able to get a precise answer once I do. $\endgroup$ Commented Aug 17, 2019 at 9:02

I'll add to this letter number festival:

We have the starter universe where you have two buttons, one on your left and one on your right. The one on your right is A and the one on your left is B. This looks like this:

(1) 1[-]

One universe with no portals inside. You create portal A which links to another universe that is perfectly identical to the starter universe:

(2) 1[A]<>1[A]'

Note the apestrophe at the second universe "boundry" as upon creation the universes are absolutely identical! This changes almost immediately with quantum rippling effects through the universe but at the exact time of creation they will be identical! Now you immediately open portal B with one nanosecond before the second one opens:

(3) 1[B]'<>1[B+A]<>2[A]

The first paralel universe is now a different one, so its changed from 1[A]' to 2[A]. But thats not as interesting as 1[B]', which contains two exactly identical portals inside it... which means that even before 2[A] creates its second portal you have to write (3) like this:

(4) 1[B+A]'<>1[B+A]<>2[A]

But wait... since all A portals in universes 1[ ]' and 1[ ] are PERFECT COPIES of each other upon creation they can only lead to the exact same point, universe 2[ ]!

Tangent: if you do not accept that the identical portals will reach in the same universe than portal A inside 1[B+A]' would lead to universe 1[B+A]'', which would lead to 1[B+A]''' etc in an infinite loop that would happen even if you desynced before opening simply for having 2 portals simultaneously. Tangent over.

Then we move 1 nanosecond further and portal C opens while the other univers desyncs:

(5) 3[B+A]<>1[B+A]<>2[A+C]<>2[A+C]'

Again we see that upon then moment of creation 2[A+C]' will have the exact same portals at the exact same positions open, meaning that the A portal in it will lead back to the original unless you use the tangental interpretation which causes an infinite loop no matter if you open them synced or not.

Edit: testing if you have infinite universes.

Activate portal A.

Press the button for portal B, an automatic system fires a gun that will pass through portal B to portal A and activates portal B just before the bullet reaches it. This means that a bullet is in flight at perfectly the same trajectory and speed as the one in the other universe.

Both bullets pass through portal B and end up in each others universe, making it seem like nothing happened at all from the side-view. They then both pass through portal A simultaneously...

In an infinite portal universes the bullets simply hit whatever is behind portal A, a small target for example.

In a looped universe both bullets simultaneously try to exit portal A in the exact same position, speed and trajectory. A few things can happen:

The bullets coordinate systems function like a combined [3d+1]+[3d+1] coordinate system, the bullets simply collide with each other and fall to the ground.

The coordinate systems dont line up and each part of the bullet exiting will do so and occupy the exact same space as a perfect copy of itself. If string theory is right you might get lucky and the vibrations that make up the bullets information just double in size, transmuting the bullet to whatever its new vibration is.

Otherwise you now have not just fusion but actual particles down to the smallest gluon or whatever is smallest currently trying to force itself into the same position. This will likely mean it creates micro black holes that instantly evaporate as the bullet passes through (and the resulting explosion pushes back). Dont be there.

But perhaps the most interesting question... if you throw something through portal A in universe 2 it will have to exit in BOTH the other universes... What happens? Are they divided or copied?

Gag reel:

Schrödinger comes along and places a measuring device. Portal A is created, making both universes essentially a quantum-entangled entity. Portal B is opened and at the same time Schrödinger measures his own universe to be up or down.

Universe 3 flips upside down. In an infinite portals universe number 2 also flips.

In a looped universe, everything not yet desynced inuniverse 2 now has to be both up and down and cannot be up and down.

  • $\begingroup$ Upvoting this answer because you said "letter number festival" (in addition to being well-written) $\endgroup$
    – overlord
    Commented Oct 10, 2019 at 19:48
  • 1
    $\begingroup$ @overlord added some stuff. Hope you still like it. $\endgroup$
    – Demigan
    Commented Oct 10, 2019 at 20:56
  • 1
    $\begingroup$ In the months since I posted this, I've come to the conclusion that the chain is actually infinite - your aside about portals producing worlds labeled with primes of primes is actually what happens. The question has always been explicitly about a type of parallel universe that forms a chain, not one where A goes to B goes to B goes back to the same A. But I'm happy that this question tickled your brain like it did mine for a while. I hope to be able to write some sci-fi with this as the core mechanic someday. $\endgroup$ Commented Oct 12, 2019 at 14:17
  • $\begingroup$ Now that you've started your other questions about this subject I'm curious to your explanation as to why the loop is infinite. As far as I can tell before the desync happens the second portal in the new universe is identical to the one in the original (as per your own rules of perfect copy until quantum shenanigans), so both these portals have to connect to the existing one. Why wouldnt this be the case? $\endgroup$
    – Demigan
    Commented Feb 24, 2020 at 22:02

There isn't enough desync for one portal to open and not the other.... so you can still treat A and B as a single unit, so when AA and BB open, they are a mirror of A/B. Meaning AA and BB end up linking to each other leaving you with 4 universes, not 8 or infinity.

  • $\begingroup$ I don't think this works, see my last comment above. $\endgroup$
    – Hypnosifl
    Commented Aug 5, 2019 at 2:09

There's no definitive answer, following is a few possible explanations.

Trying to hold strictly to "syncing":

  1. Open a portal from universe A to universe B, portal is called A0<->B0 (bit to designate portal location)
  2. While synced, universes A and B each open a new portal. Since they are synced, these new portals are in the same location and go to the same universe, C. We now have A0<->B0, A1<->C1, B1<->C1. We have overlap at universe C's 1-location, with portals going to A1 and B1; we also have overlap at A0 and B0, since on C's 0-location must be identical to A's and B's 0-location.

You end up with 3 universes and the portals A0<->B0, A1<->C1, B1<->C1, A0<->C0, B0<->C0.

Without really holding to syncing (it generally breaks immediately):

  1. You create the first portal, ending up with universes A and B, with a portal A0<->B0 (again using bits to denote which portal location we're looking at)
  2. While synced you and your B-universe double create new portals. Presumably in the same place, since you're synced.
  3. Couple possibilities here:

    • This new pair of portals could either be another portal between universes A and B, so you have A0<->B0 and A1<->B1 (which means we de-synced immediately, because our 1-location portals go to different places; or each portal opens to both itself and the other universe's 1-location)
    • Or, this new pair of portals both go to new universes, so you've got A0<->B0, A1<->C1, and B1<->D1. (This would also appear to de-sync A and B immediately, since they have different portals in their 1-locations; or each portal could open to itself, and to both new universes (A1<->A1, A1<->C1, A1<->D1, B1<->B1, B1<->C1, B1<->D1))
  4. In the second case, universes A and B both have two portals, and we know where they both go. However, universes C and D each have a portal we haven't determined yet. And there again seems to be two options (there's more, but I'm ignoring the strict-syncing cases):

    • Portals C and D are literal copies of their parent-universe's portals: which means that in universe C, in the same place as universe A's A0<->B0 portal, universe C also has a portal to universe B. Same thing in universe D (based on universe B this time). So, we've got: A0<->B0, C0<->B0, A0<->D0, A1<->C1, B1<->D1. This is scary, because I don't know what happens when there are two portals to different universes in the exact same place (explosion? walking into a portal means you walk out of both other portals?).
    • The new portals go to new universes and we have A0<->B0, A1<->C1, B1<->D1, C0<->E0, D0<->F0. Now we have universes A, B, C, D with two portals each... but E and F with undetermined portals.
  5. Repeat 4 infinitely many times, instantaneously. This happens since these are simply copies of portals that already exist.

So, you end up with 2, 4, or infinitely many portals. There may be other solutions, trying to be strict with syncing is complicated.


For the chain to be possible, there would have to be a some delay in the creation of a parallel universe. In the case that the other universe is instantly created, it would be more like holding a mirror up to the existing reality. If there was a ‘pulse’ from the ring that created the other universe, the creation of a new portal would occur every time the pulse reached where an existing portal was. This chain would be infinite, or practically infinite. If portals act as a way to create a mirror copy of current reality, no chain would form.


Desync times would not have an effect in the case of a pulse as already existing portals are affected. Unless a ring was destroyed at any point in the chain during the formation(which would take an endless amount of time) the chain will be cut off at the mirror of that universe as each portal mirrors current connected reality.

I will post reference images later if you need clarification


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