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Under Everett's interpretation I understand the key notion is that every quantum possibility is physically realized in its own universe, such that any one universe only contains any one physical realization stemming from the "same" parent measurement. I've also understood that it's popular to say that, under this interpretation, the physical world(s) branch out - creating a tree - based on every such possibility.

But, physically speaking, if we take this branching approach naively at face value - are we supposed to understand that from any given state where you have X possible outcomes, what is now 1 physically real world, turns into X physically real worlds (one for each possibility)?

I see this perpetuated all over the place - that X and Y possibilities exist in their own, physically distinct universes, and that this removes some problems in Copenhagen or simplifies some of them. But it's also very confusing, because if that is the case, how do we explain this cloning-the-universe-up-until-this-point into however many possibilities there were? Where does that energy come from? Not to mention - how is it possible to copy all the information of an entire universe in an instant (without violating a bunch of physical laws in the existing universe)?

So - is it really as simple as this answer says; "it is just mathematics"? So that when the many-worlds interpretation says that every possibility is physically realized, you'd think of it in the sense that is to some degree analogical to how you'd think of the electron physically realizing all its possible paths in the double-slit experiment?

Or does the interpretation claim that there are actually physically real and distinct worlds - universes - for each possible outcome? As in, there actually exists a space-time (even if it's inaccessible to us) that is identical to its twin space-time(s) with the only exception being the outcome of that particular measurement? If that is the case, how do proponents of the MWI resolve the problems I posed earlier about information-copying?

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  • $\begingroup$ The interpretation indeed says that there are those "paralell realities", and yes, they do physically exist. There is no violations of any information conservation principle, since this interpretation was put forth precisely to recover the conservation of information. In a few words, there is no copying of information : when the cat is half-dead and half-alive, there is information on both states. One will be "realised" in one universe, the other in the other one. As for energy conservation, for each observer energy is conserved, so there is no contradiction of conservation laws. $\endgroup$
    – Frotaur
    Commented Feb 24, 2020 at 14:26
  • $\begingroup$ To suggest that there is a new universe for every action of every electron is ludicrous. $\endgroup$
    – R.W. Bird
    Commented Feb 24, 2020 at 18:28
  • $\begingroup$ @Frotaur: But if you have 1 universe, and you come to an impasse where you have to resolve a quantum possibility - let's say the superposition of spin up and spin down - how do you get to 2 universes if there is no copying? I understand your sentiment that "all the information" is contained in the original - but where & how does the second universe come from, if it didn't already exist? And surely, it doesn't exist prior to the resolution of the possibility, because I've also understood that MWI is not a multi-verse interpretation? $\endgroup$
    – Vegard
    Commented Feb 25, 2020 at 9:27
  • $\begingroup$ @R.W.Bird, and what interpretations of quantum mechanics isn't "ludicrous", in your point of view ? For Vegard, in Everett's point of view, both the spin up and the spin down "possibility" is realised. When you become entangled with the electron by measuring its spin, the state of the system is described by $|\uparrow>\otimes |you see up>+|\downarrow>\otimes |you see down>$. There are effectively "two version of you", one that sees spin up, and the other spin down. Those are the two "separate" universes of Everett. $\endgroup$
    – Frotaur
    Commented Feb 25, 2020 at 11:23
  • $\begingroup$ @Frotaur: But this goes precisely to my question - in your opinion, are these "separate universes" actual physically separate universes, or are they as the other answerer below claims overlapping probabilities superimposed on the same universe? I understand that in Everett, when the wavefunction is split, mathematically it looks the same for any given branch as if that branch is the only world that exists - so you might say, each branch is functionally its own world - but is that really the same as saying that there physically, actually exists separate universes for all those branches? $\endgroup$
    – Vegard
    Commented Feb 25, 2020 at 12:01

2 Answers 2

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Everett's "worlds" are as you described: "analogical to how you'd think of the electron physically realizing all its possible paths in the double-slit experiment". The worlds are not physically distinct and separate. They are coexisting, superimposed probability distributions.

What distinguishes one such world from another is mutual exclusivity within the global probability distribution that encompasses all of the "worlds": "In this set of worlds, the electron was found to have taken the left slit; in that set of worlds, it was found to have taken the right slit; while in all the other worlds it was found to have passed through both slits." This principle extrapolates to Schroedinger's cat, and to all the other consequences of Everett's interpretation.

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  • $\begingroup$ Thanks for the answer, this does make more sense. In the interest of checking my own confirmation bias, are you familiar with any serious argument that would hold a different position? $\endgroup$
    – Vegard
    Commented Feb 25, 2020 at 10:35
  • $\begingroup$ There are lots of people who seriously doubt Everett's interpretation, but among those who accept the interpretation and have a good grasp of physics, I don't know of any serious disagreement $\endgroup$
    – S. McGrew
    Commented Feb 25, 2020 at 12:56
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Yes . .. ..In a quantum computer where collapse does NOT take place on a superposition , we are --as a matter of fact-- experiencing branchification of world/universe into event-making sub-branches where up-or-down symbolization no longer is restricted to one quantum . There must be "quantA" involved to make it possible for interference taking place in , say , a multiple-term's equation so that the intended equation might be solvable with its so many terms only thru a QUANTUM computer and NOT by an ordinary one .So sharp a division about what appears to be a reasonably well-defined question cannot maintain coming together of two ( or more ) 3-D worlds until and unless there are an infinite number of universes, each with its own parameters, its own structure. . . . ... yeah : Physically/Physicalistically . .. .. It is NOT the observer’s consciousness that “forks” or “splits,” because only those subspaces of Hilbert space describing definite macroscopic measurement results satisfy psychological conditions for supporting a unified consciousness. The ONLY other option would be to totally deny any interference and any resulting inference .

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  • $\begingroup$ How does the world branch, in practice? If there exists let's say 1 world to start out with, what's the mechanism for how it branches when the quantum event takes place? Since information only exists in one place - in that 1 world - how does it get to the 2nd world, and at what speed? How is the 2nd world created, and with which energy? $\endgroup$
    – Vegard
    Commented Mar 16 at 5:52
  • $\begingroup$ When it branches , this plainly means branches , not "metamorphoses" from one universe onto another . The mechanism is , again , like all other modern physics' mechanisms , is no other than a patch of Mathesis capable of phenomenologically describing the so-called "reality" . Info does NOT stay merely in one place , as you evidently believe . What makes it all tick are analytical non-partial differential equations that have to deal with incrementally calibrating data-flux phase spaces . h $\endgroup$ Commented Mar 16 at 17:35
  • $\begingroup$ Information travels at the speed of light, so whenever it's in more than one place at the same time, it assuredly had to travel from point A to point B in some manner of speaking. If the position is that MWI branching creates actual physical worlds, then how does information not have to travel? It existed in world 1, and then it exists in both world 1 and world 2 - how? For that reason, it seems obvious to me that MWI cannot possibly describe simultaneous physical worlds but rather discrete possible configurations of the same world - within the phase space you mentioned. $\endgroup$
    – Vegard
    Commented Mar 21 at 8:55
  • $\begingroup$ I think NOW you have arrived at what I was trying to convey : $\endgroup$ Commented Mar 21 at 16:18

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