Is MWI Considered Local in Quantum Mechanics?

[DrChinese]

a. None of this means the MWI does not have to account for the experimental results. Of course it does, just as any QM interpretation does. It just doesn't do it by appealing to "mutual influences" or "remote changes".

b. It does it by, first, saying that the wave function is all there is; second, saying that there is no collapse, so all of the possibilities contained in the wave function actually exist (meaning that measurements don't have single results--all possible results happen); and third, saying that the wave function is what enforces the correlations such as are observed in Bell inequality violations, entanglement swapping, etc.

a. Again: once someone explains how nonlocal effects are made to happen using purely local mechanisms, I can determine this for myself. But without that, I can't agree with you.

b. All possible results occur, that's exactly the problem! QM precisely predicts a) some results NEVER happen; and b) some results occur far more or far less than I would expect from MWI. Neither of these are ever explained in a detail example.

I have requested a detail explanation of a specific referenced experiment above, and I believe there is a reader who follows MWI closely enough to walk me through the logic of that example. Specifically:

Photons 1 and 4 share no common past and are distant. A equally remote observer can choose to entangle them or not. How is it that if all outcomes are possible, we always see perfect correlations when those photons are measured at the same angle settings? And in fact Photon 1 can be measured BEFORE the remote observer chooses to entangle it with Photon 4, and even before the measurement setting for Photon 4 is selected? (Keeping in mind here that the same mechanism must also produce Bell inequalities when the angle setting are different.)

And of course I am really asking how this is accomplished through some hypothetical local mechanism.

 

[Morbert]

i) But there are experiments in which photons become entangled that have never existed in a common past light cone. See my #25.

ii) Not sure how this statement even is supposed to make sense. OK, if there are nonlocal extended regions with information that is not present in subregions, how do remote photons 1 & 4 get entangled?

The light cones of the two 2-photon systems span the BSM. To demonstrate action at a distance to a MWI proponent, the BSM that establishes the branches with entanglement would have to be spacelike separated from both 2-photon systems, which are "type-ii" nonlocal systems. Only then would you have an eventual quantum state determined by the state outside its past light cone.

 

[DrChinese]

a. I'm not sure that would be possible as you state it here--the wave function would play a role similar to a "global variable", since it contains entangled degrees of freedom which can be widely separated in 3-dimensional space. (But, as has already been noted, that is true of any interpretation since they all use the same underlying math, which contains the wave function playing that same role.)

b. The first thing one would have to do to describe any experiment using the MWI would be to get rid of anything that says or implies that measurements have single results or that wave functions collapse. For example: It's not just the photon: it's everything.

c.If you ask what makes it so that everything matches up correctly in each branch, that's simple: those are the only possibilities that exist in the wave function. There is no branch of the wave function in which both photons 1 and 4 are measured along the same direction but their polarizations aren't the same. That is what it means to say that photons 1 and 4 are entangled in the parallel polarization state.

d. Whether all this counts as "local" is a different question. No "nonlocal influence" has to go between the photons during the measurements to enforce the correct correlations between the measurement results, because that is already enforced by the wave function itself, as above.

a. Global variables sound confusingly nonlocal to me. And the issue we are discussing is whether MWI is local, not the nonlocal elements of other interpretations. We already (mostly) agree that standard QM has nonlocal elements. The nonlocal extent of an entangled system is something even @vanhees71 agrees to.

b. I'm accepting this point as being a tenet of MWI. And this is actually its most appealing point, in my opinion. It would solve a lot of conceptual problems.

c. Whoa, that's completely impossible! Photons 1 and 4 aren't entangled yet! That doesn't occur until our distant observer chooses to perform the swap or not. And that can be done AFTER Photon 1 is already measured, and there has been a splitting into a V> branch and an H> branch. Photon 4 has no connection to Photon 1 whatsoever, any more than it has a connection with any other photon anywhere. There is nothing at this point that gives an indication that they will be entangled in the future.

d. This is my point. It cannot be local because distant events have yet to occur that will change Photon 4's relationship with Photon 1 from Product State to Entangled State. That occurs in the future, and MWI is supposed to strictly reject anything which does not follow Einsteinian causality. (And please, don't ask me to define that as I think everyone understands that term the same way.)

 

[mattt]

It doesn't matter what experiment we are talking about.

The mathematical derivation of the correct results, pertinent to the given experiment, using the mathematics of standard non-relativistic quantum mechanics, is exactly the same (or mathematically equivalent) in all the interpretations of non relativistic Quantum Mechanics.

The only difference is how they translate it into English words.

 

[DrChinese]

a. The light cones of the two 2-photon systems span the BSM.

b. To demonstrate action at a distance to a MWI proponent, the BSM that establishes the branches with entanglement would have to be spacelike separated from both 2-photon systems, which are "type-ii" nonlocal systems. Only then would you have an eventual quantum state determined by the state outside its past light cone.

a. Photons 2 and 3 meet in this particular experiment, true enough. But that is absolutely not a requirement of QM. They do not need to ever meet or exist in a common light cone either. And in fact quantum repeaters rely on that attribute to work. I can supply the references if you are not sure on this point.

b. I don't understand this angle of the argument. Photons 2 and 3 are supposed to be localized, although I concede that Photons 1 & 2 (likewise 3 & 4) can be considered to have overlapping wave functions in spacetime. When the decision is made to entangle Photons 1 & 4, however, there is no time for any change of change to propagate from that location to either Photon 1 or 4 - which you must concede had no prior connection or correlation at all*. So there must be some kind of action at a distance to explain the change in statistics for Photons 1 & 4 from Product State to Entangled State.*From the reference: "A successful entanglement swapping procedure will result in photons 1 and 4 being entangled, although they never interacted with each other. ... We confirm successful entanglement swapping by testing the entanglement of the previously uncorrelated photons 1 and 4."

 

[DrChinese]

It doesn't matter what experiment we are talking about.

The mathematical derivation of the correct results, pertinent to the given experiment, using the mathematics of standard non-relativistic quantum mechanics, is exactly the same (or mathematically equivalent) in all the interpretations of non relativistic Quantum Mechanics.

The only difference is how they translate it into English words.

Well, I don't think that's actually the case. From SEP:

"The causes of our experience are interactions, and in nature there are only local interactions in three spatial dimensions. These interactions can be expressed as couplings to some macroscopic variables of the object described by quantum waves well localized in 3 D -space, which are in a product with the relative variables state of the object (like entangled electrons in atoms)..."

We all know that Bell prohibits local realistic interpretations from agreeing with the predictions of QM. So I am flat out saying that MWI must be nonlocal, precisely because its most important tenet is the realism of the wavefunction.

 

[Morbert]

a. Photons 2 and 3 meet in this particular experiment, true enough. But that is absolutely not a requirement of QM. They do not need to ever meet or exist in a common light cone either. And in fact quantum repeaters rely on that attribute to work. I can supply the references if you are not sure on this point.

Yes, a reference would be useful.

 

[PeterDonis]

All possible results occur, that's exactly the problem!

If you don't accept the MWI, yes, of course it's a problem. But MWI proponents don't see it as a problem, they see it as a solution: the solution to the "mystery" of "collapse of the wave function". If collapse never actually happens--if the actual dynamics is always unitary--things get a lot simpler, according to MWI proponents.

QM precisely predicts a) some results NEVER happen

In cases where this is true--because those results have zero amplitude--the MWI predicts that they don't happen as well--that there is no branch of the wave function that contains them.

and b) some results occur far more or far less than I would expect from MWI

I don't know what you mean here. MWI assigns exactly the amplitudes to each result that appear in the wave function. Those are the same amplitudes that are used to compute the probabilities of the results.

There is (at least for critics of the MWI) a serious issue with how to make sense of probabilities in the MWI, since using the concept of probability appears (at least to critics) to require that each individual measurement only has one result. MWI proponents sometimes agree that this is an issue that needs to be addressed, but they also appear to believe it has been addressed in the literature they have published.

Of course if you are an MWI skeptic, you won't agree with its proponents on claims like the above. But that's not going to be resolved here.

 

[PeterDonis]

Photons 1 and 4 share no common past and are distant. A equally remote observer can choose to entangle them or not. How is it that if all outcomes are possible, we always see perfect correlations when those photons are measured at the same angle settings?

When you say "all outcomes are possible", what do you mean? If you mean all outcomes contained in the wave function are possible, then the MWI agrees with that--and it says they all happen, because they all have branches in the wave function. And, as I have already said, the wave function is what enforces the correlations.

If you mean there are outcomes that are possible that aren't contained in the wave function, what are you basing that on? We are doing QM here, and in QM, the possible outcomes are the ones that are contained in the wave function.

If you are wondering how the MWI explains what happens when the settings are changed--when the distant observer makes the choice to either entangle or not entangle the photons--the MWI answer is that the wave function contains both possibilities for that as well (at least, assuming that some kind of quantum indeterminacy is involved in making the choice). The wave function in the MWI contains everything: the experimenters, the equipment they use, and the processes they use to choose measurement settings. The "settings chosen to entangle the photons" branch of the wave function only includes the possible results that show the required correlations; the "settings chosen not to entangle the photons" branch of the wave function only includes the possible results that don't show those correlations.

 

[PeterDonis]

c. Whoa, that's completely impossible! Photons 1 and 4 aren't entangled yet! That doesn't occur until our distant observer chooses to perform the swap or not.

As I noted in post #44 just now, the distant observer and the possible choices they can make are included in the wave function.

that can be done AFTER Photon 1 is already measured, and there has been a splitting into a V> branch and an H> branch

Since all of the measurements involved commute--and that is a fact of the basic math of QM, independent of any interpretation--the order in which the branching occurs in the MWI doesn't affect the results. It doesn't matter if the photon 1 measurement branching occurs before or after the choice of swap or no swap branching.

To put this another way, branching occurs in Hilbert space, not 3-dimensional space (or 4-dimensional spacetime if we are doing QFT). The events that are connected to branching are localized in 3-dimensional space (or 4-dimensional spacetime if we are doing QFT), but the branching itself is not. (As I have already remarked, I think this is a valid reason not to consider the MWI to be local. But it doesn't change the fact that this is how the MWI explains things.)

It cannot be local because distant events have yet to occur that will change Photon 4's relationship with Photon 1 from Product State to Entangled State. That occurs in the future

See my remarks above.

MWI is supposed to strictly reject anything which does not follow Einsteinian causality.

Do you have a reference where an MWI proponent makes this claim? As far as I know, MWI is an interpretation of QM, not relativity, and says nothing whatever about any particular concept of causality. It only claims to explain the predictions of QM.

 

[DrChinese]

Do you have a reference where an MWI proponent makes this claim? As far as I know, MWI is an interpretation of QM, not relativity, and says nothing whatever about any particular concept of causality. It only claims to explain the predictions of QM.

The fact that distant measurements commute is either a) simply a restatement of the idea that QM does not respect the usual Einsteinian locality/causality (i.e. quantum nonlocality is evident), or b) there are locality constraints at play (what you imply). For our discussion though, it is meaningless. QM predictions for entangled statistics are based on a future distant context. There are no experiments that demonstrate otherwise. The statements "A causes B" and "B causes A" cannot be distinguished in these as it would in a classical model. Arguing otherwise is circular reasoning.

As to the reference request: I already provided that in post #41: The causes of our experience are interactions, and in nature there are only local interactions in three spatial dimensions. These interactions can be expressed as couplings to some macroscopic variables of the object described by quantum waves well localized in 3 D -space, which are in a product with the relative variables state of the object (like entangled electrons in atoms)..." That is an absolute statement of the direction of causality being one way only.

And again I point out that a) I am the only person here providing references; and b) this thread is about someone who is a believer in MWI trying to explain how MWI operates without nonlocality. I would appreciate someone explaining how Alice observes Photon 1, splitting things into an H> branch and a V> branch, and then point out: where does the branching occur that places distant Photon 4's H> result into the same branch as Photon 1's H> result (likewise pairing the V> side) in each and every instance - without any element of Photon 1 or Photon 2 ever being near to Photon 4 (and Photon 3 never being close to Photon 2 while it is also close to Photon 4).

I say MWI is nonlocal, and experiments as referenced prove it. The Nobel committee commented about this and other related works recently: "[Anton Zeilinger's] research group has demonstrated a phenomenon called quantum teleportation, which makes it possible to move a quantum state from one particle to one at a distance." That would be action at a distance, which is what I keep saying is generally accepted science. Is the Nobel committee wrong? Does anyone seriously think they have never heard of MWI? Or are they just using loose lingo?

Apparently every MWI adherent claims these experiments are easily explained, yet I cannot find a comprehensible explanation anywhere. It's all hand waving, with no reasonable attempts to explain Bell/swapping/GHZ/etc. (I have read works by Deutsch and Vaidman and found them sorely lacking). I have provided a straightforward example from an important paper, and would appreciate someone taking some time to discuss that example without quibbling over every word.

@PeterDonis, I will gladly continue to discuss with you as long as you are willing to put in the time, but so far you have refused to make an actual statement related to the experiment I cited. Why split hairs over words when my example is right there to discuss? If you think MWI does not specify it follows Einsteinian causality, then say so unambiguously - and perhaps YOU could provide a reference for that. It is clear to me it claims to follow traditional notions of locality and causality, and goes so far to claim it is local realistic. No need for me to cite or prove that, because if someone comes along and says it doesn't, then my questions are answered.

 

[PeterDonis]

The fact that distant measurements commute is either a) simply a restatement of the idea that QM does not respect the usual Einsteinian locality/causality (i.e. quantum nonlocality is evident), or b) there are locality constraints at play (what you imply).

I'm not sure I understand. If the measurements are spacelike separated, we would expect them to commute, even on classical grounds.

The mysterious part, from a classical point of view, is how such measurements can commute even when they are timelike or null separated. But "distant" is not a word I would use to describe the measurements that situation. I would describe it as a situation where, classically, we would expect causal ordering, but quantum mechanics doesn't give us that. The measurements in question could just as well take place at the same spatial location; QM doesn't require that they be spatially distant.

QM predictions for entangled statistics are based on a future distant context.

They are based on an overall context that can include measurements that are in the causal future of other measurements, but still, as noted above, commute (where classically we would expect them not to).

As to the reference request: I already provided that in post #41: The causes of our experience are interactions, and in nature there are only local interactions in three spatial dimensions. These interactions can be expressed as couplings to some macroscopic variables of the object described by quantum waves well localized in 3 D -space, which are in a product with the relative variables state of the object (like entangled electrons in atoms)..." That is an absolute statement of the direction of causality being one way only.

I'm afraid I don't agree. It is a statement of how "interactions" work in QM. In itself it is not a statement of any particular notion of causality. It certainly is not a claim that the MWI requires "Einsteinian causality".

"Interactions" is not the same thing as "entanglement". In more technical language, "interactions" refers to terms in the Hamiltonian, not properties of the quantum state. Interaction terms in the Hamiltonian are as the quote describes them. But if they are acting on entangled degrees of freedom, in QM they can do things that make no sense classically, like end up producing correlations that violate the Bell inequalities between particles that have never interacted, and indeed don't even have to have ever existed at the same time.

this thread is about someone who is a believer in MWI trying to explain how MWI operates without nonlocality.

If you think this refers to me, I don't see where you are getting it from. I have never claimed that MWI operates without nonlocality. If we define "nonlocality" as "Bell inequality violations", then obviously MWI must accept nonlocality, just as any QM interpretation must. (I have not even claimed to be a "believer in MWI".)

What I am trying to do in this thread is to make it clear what the MWI actually says and doesn't say. I am not trying to argue that the MWI is local or that it is not. But being clear about what the MWI says and doesn't say seems to me to be an essential prerequisite to even trying to evaluate whether or not the MWI is local.

I would appreciate someone explaining how Alice observes Photon 1, splitting things into an H> branch and a V> branch, and then point out: where does the branching occur that places distant Photon 4's H> result into the same branch as Photon 1's H> result (likewise pairing the V> side) in each and every instance - without any element of Photon 1 or Photon 2 ever being near to Photon 4 (and Photon 3 never being close to Photon 2 while it is also close to Photon 4).

I have already answered this question. And I have repeatedly referred you to my answer when you have repeated this question in previous posts. I don't know why you keep ignoring what I have already said.

so far you have refused to make an actual statement related to the experiment I cited

I disagree. I have made one, and I have repeatedly referred you to it. In the interest of facilitating discussion, I'll repeat the gist of it once more: the wave function enforces the correlations. The wave function already contains all the correlations you describe. That is how the MWI explains them.

If you want me to go into more detail in actually writing down how the wave function does that, that's fine, I can take a stab at it. But it would be nice if you would at least acknowledge that I have given the above answer, multiple times now.

 

[DrChinese]

And just in case anyone thinks I am mischaracterizing something about MWI, this is from Vaidman in Plato:

a) The MWI is a deterministic theory for a physical Universe and it explains why a world appears to be indeterministic for human observers. ... The quantum state of the Universe at one time specifies the quantum state at all times.

b) The MWI does not have action at a distance.

c) The most celebrated example of nonlocality of quantum mechanics given by Bell’s theorem in the context of the Einstein-Podolsky-Rosen argument cannot get off the ground in the framework of the MWI because it requires a single outcome of a quantum experiment.

---------------

My comments:

a) Deterministic here means there is only forward in time causality. I would also say this is a claim of realism

b) No action at a distance here means it is local and there is no nonlocality. This is refuted by any swapping or teleportation experiment, see my post above with this from the Nobel committee: ""[Anton Zeilinger's] research group has demonstrated a phenomenon called quantum teleportation, which makes it possible to move a quantum state from one particle to one at a distance.""

c) Bell Theorem requiring "a single outcome from a quantum experiment" is not even true, but regardless: If this isn't hand-waving, I don't know what hand-waving is. It is typical of local realism proponents to invent "tacit" or "hidden" assumptions in Bell and then deny them - and end discussion there (as Vaidman has).

Bell's theorem requires only that an observer in any branch see a single outcome from a quantum experiment. There is no requirement that there are no other branches containing other observers. )Bell was certainly familiar with Everett's thesis and certainly never thought his theorem didn't apply to MWI. If you are interested, you can read about some of Bell's shifting ideas about things like Everett, Bohm and GRW here.)

 

[PeterDonis]

Deterministic here means there is only forward in time causality.

No, it doesn't. Deterministic laws can be evolved backward in time just as easily as forward. The quote you give even says so: it says the quantum state at one time specifies the quantum state at all times, not just all future times. If we could know the full quantum state of the entire universe now, according to the MWI, we would know it for all times, including all the way back to the Big Bang.

No action at a distance here means it is local and there is no nonlocality.

I would want to see a lot more detail about what Vaidman actually means by "action at a distance" before accepting any such claim (and I'll read through the article you reference again to see if I can find such detail). If, for example, you think Vaidman is claiming that the MWI does not predict Bell inequality violations, then I would expect Vaidman himself to object, since the MWI is an interpretation of QM and QM itself, independent of any interpretation, predicts Bell inequality violations.

Bell Theorem requiring "a single outcome from a quantum experiment" is not even true

I'll need to read through the article again to evaluate this as well.

I note, btw, that Zeh, in the paper you reference, appears both to be an MWI proponent (since he says he believes the Schrodinger equation is always valid) and to have no trouble accepting that the MWI is nonlocal. I also note that in the paper, he mentions David Deutsch's version of the MWI and contrasts it with his own. I would be interested in any other comments he has on the writings of the other MWI proponents you have referred to.

 

[PeterDonis]

No action at a distance here means it is local and there is no nonlocality.

I would want to see a lot more detail about what Vaidman actually means by "action at a distance" before accepting any such claim

And it didn't take me long to find, since it's in the same paragraph of the article as the quote that you, @DrChinese, gave in post #48. Here are the last three sentences of that paragraph, which you failed to quote:

Although the MWI removes the most bothersome aspect of nonlocality, action at a distance, the other aspect of quantum nonlocality, the nonseparability of remote objects manifested in entanglement, is still there. A“world” is a nonlocal concept. This explains why we observe nonlocal correlations in a particular world.

So based on this, I definitely do not agree with the claim quoted at the top of this post.

 

[DrChinese]

a. I'm not sure I understand. If the measurements are spacelike separated, we would expect them to commute, even on classical grounds.

b. If you think this refers to me, I don't see where you are getting it from. I have never claimed that MWI operates without nonlocality. If we define "nonlocality" as "Bell inequality violations", then obviously MWI must accept nonlocality, just as any QM interpretation must. (I have not even claimed to be a "believer in MWI".)

c. What I am trying to do in this thread is to make it clear what the MWI actually says and doesn't say. I am not trying to argue that the MWI is local or that it is not. But being clear about what the MWI says and doesn't say seems to me to be an essential prerequisite to even trying to evaluate whether or not the MWI is local.

d. I have already answered this question. And I have repeatedly referred you to my answer when you have repeated this question in previous posts. I don't know why you keep ignoring what I have already said. ... I disagree. I have made one, and I have repeatedly referred you to it. In the interest of facilitating discussion, I'll repeat the gist of it once more: the wave function enforces the correlations. The wave function already contains all the correlations you describe. That is how the MWI explains them.

a. If there is action at a distance, then measurements might commute or they might not. In QM, there is no time ordering to its predictions. Therefore if there is AAD in QM, then they WILL commute.

On the other hand, if there is no AAD, then classically all such measurement also commute. So saying they commute only tell you that there is no AAD of the type which follow a direction from the past to the future.

b. No, I have never had the impression you are a particularly a proponent of MWI nor a proponent of the idea that MWI is local. I think you do an excellent job of discussing the wide variety of interpretations here. Often, you act as sort of a "devil's advocate". Sometimes that approach is useful, but not always.

c. Well, certainly there are a lot of claims made about MWI. I have attempted to distill what I believe is common to most descriptions, and I have read a few. They all pretty much claim that the wave function evolves locally. So how do nonlocal effects appear? If I have misrepresented some material point, how about YOU reference something rather than challenge me word by word? Fair is fair. If you don't have time to provide such reference, then skip the point rather than quibbling with me. I have made the question quite clear.

d. "The wave function enforces the correlations"? That's an answer? That isn't even a summary of an answer. The question from above is:

I would appreciate someone explaining how Alice observes Photon 1, splitting things into an H> branch and a V> branch, and then point out: where does the branching occur that places distant Photon 4's H> result into the same branch as Photon 1's H> result (likewise pairing the V> side) in each and every instance - without any element of Photon 1 or Photon 2 ever being near to Photon 4 (and Photon 3 never being close to Photon 2 while it is also close to Photon 4).

I'd like someone to walk us through the splitting and evolution of the systems point by point where they can be discussed. I think we start off in agreement, there is splitting when Photon 1 is measured (H> branch and V> branch). At that time, Photons 2 and 3 are distant to the Photon 1 measurement, but heading towards each other - soon to be swapped (or not) by the experimenter. Photon 4 is distant to Photons 2 and 3 (Photon 1 has ceased to exist). The polarization of Photon 4 will be measured last (not that ordering actually matters, it's just easier to discuss).

When and where do the H> and V> branches next split? And what are the consequences to the remaining Photons on the outcome of the branch they are in due to the splitting related to the measurement of Photon 1?

 

[DrChinese]

And it didn't take me long to find, since it's in the same paragraph of the article as the quote that you, @DrChinese, gave in post #48. Here are the last three sentences of that paragraph, which you failed to quote:

Although the MWI removes the most bothersome aspect of nonlocality, action at a distance, the other aspect of quantum nonlocality, the nonseparability of remote objects manifested in entanglement, is still there. A“world” is a nonlocal concept. This explains why we observe nonlocal correlations in a particular world.

So based on this, I definitely do not agree with the claim quoted at the top of this post.

Ah, I can't go there. This is exactly the hand-waving I spoke of. Yes, I had seen that and intentionally did not include it because it makes no sense in this context.

It still does NOT explain how the wavefunctions would work in a swapping setup as I have established. So what if there is nonseparability in remote objects? Photons 1 and 2 are the remote object! Photon 4 has nothing to do with anything at this point!!

 

[DrChinese]

a. No, it doesn't. Deterministic laws can be evolved backward in time just as easily as forward. The quote you give even says so: it says the quantum state at one time specifies the quantum state at all times, not just all future times. If we could know the full quantum state of the entire universe now, according to the MWI, we would know it for all times, including all the way back to the Big Bang.

b. If, for example, you think Vaidman is claiming that the MWI does not predict Bell inequality violations, then I would expect Vaidman himself to object, since the MWI is an interpretation of QM and QM itself, independent of any interpretation, predicts Bell inequality violations.

c. I note, btw, that Zeh, in the paper you reference, appears both to be an MWI proponent (since he says he believes the Schrodinger equation is always valid) and to have no trouble accepting that the MWI is nonlocal. I also note that in the paper, he mentions David Deutsch's version of the MWI and contrasts it with his own. I would be interested in any other comments he has on the writings of the other MWI proponents you have referred to.

a. Deterministic laws can (theoretically) be calculated forward and backward, true. But that's a far cry from 2 way causal laws. In retrocausal type interpretations, there are causal elements in both the future and the past. There aren't, as far as I know, any MWI proponents claiming causality works backwards to what we human perceive as the direction of time. So this is just another quibble that is not furthering the discussion.

b. Vaidman of course thinks Bell does not apply.

c. Agreed. I assume that Bell didn't think of Everett as local either, but I can't find detail on that specific point. Not that his opinion would decide the debate anyway, I just thought it is interesting to see his thoughts. Of course, GHZ only appeared right around the untimely death of Bell and no experiments had been done on that yet. And quantum teleportation much the same. Those would have certainly influenced Bell in some way.

 

[PeterDonis]

"The wave function enforces the correlations"? That's an answer? That isn't even a summary of an answer.

I disagree that it's not even a summary of an answer. And I have posted more than just that once sentence about it. But at least now you're acknowledging it and we can move on from there.

I'd like someone to walk us through the splitting and evolution of the systems point by point where they can be discussed.

Sure, I said I would take a stab at it and I will.

Let's first describe the overall setup to be sure we have it right. We have four photons. In the initially prepared state, photons 1 and 2 are entangled, and photons 3 and 4 are entangled. Both entanglements are maximal so by monogamy of entanglement there can't be any other entanglements involved.

In the middle of the experiment, so to speak, photons 2 and 3 are brought together and an experimental choice is made of whether or not to induce an entanglement swap. If a swap is induced, then after the swap (where "after" does not refer to time ordering, since as already noted it is possible to run this experiment where, for example, the photon 1 and 4 measurements are in the past light cone of the photon 2 and 3 swap/no swap decision--"after" only refers to logical ordering in terms of the analysis we are doing), photons 2 and 3 are entangled, and photons 1 and 4 are entangled. Again, both entanglements are maximal.

At the end of the experiment (which might, as noted, be in the past light cone of the "middle" described above, but it is logically the end for purposes of analysis), photons 1 and 4 are measured. To keep it simple, we will assume they are both measured along the same polarization axis, so if they are entangled, the measurements will always agree. If they are not entangled, there is an equal chance for the measurements to agree or disagree.

Now we can talk about how the MWI describes what happens to the wave function in the above. The starting wave function is, schematically (and ignoring normalization, which I will do throughout):

$$
\ket{\Psi}_0 = \ket{\psi}_{12} \ket{\psi}_{34} \ket{\text{ready}}
$$

where the lower case $\psi$ kets on the RHS are subscripted with the photons that are entangled in them, and the "ready" ket describes the state of the swap/no swap decision apparatus.

When photons 2 and 3 come together, either a swap happens or it doesn't. So after that decision is made (and in the MWI, the dynamics of that decision would be encoded in the Hamiltonian and would affect the wave function), the wave function becomes

$$
\ket{\Psi}_1 = \ket{\psi}_{14} \ket{\psi}_{23} \ket{\text{swap}} + \ket{\psi}_{12} \ket{\psi}_{34} \ket{\text{no swap}}
$$

In MWI-speak, we have had a split into two worlds: in the first, the swap happened and the entanglements are changed; in the second, the swap didn't happen and the entanglements are not changed.

When the photon 1 and 4 measurements are done, we have the final state, which will be:

$$
\begin{matrix}
\ket{\Psi}_2 = \left( \ket{\text{1 and 4 up}} + \ket{\text{1 and 4 down}} \right) \ket{\psi}_{23} \ket{\text{swap}} \\
+ \left( \ket{\text{1 and 4 up}} + \ket{\text{1 up and 4 down}} + \ket{\text{1 down and 4 up}} + \ket{\text{1 and 4 down}} \right) \ket{\psi}_2 \ket{\psi}_3 \ket{\text{no swap}}
\end{matrix}
$$

Here we have two more splits of worlds, in MWI-speak: on the "swap" side, we have a split into two worlds, in one of which photons 1 and 4 are both up and in the other they are both down; on the "no swap" side, we have a split into four worlds, corresponding to the four possible combinations of photon 1 and 4 results (since if there is no swap they are uncorrelated). (Note that I have assumed that no measurements are made on photons 2 and 3, so they just stay however they were after the swap/no swap decision is made and executed.)

Of course none of this says that the MWI is "local". The wave function itself, I would say (and Zeh says in the paper you referenced from him), is a nonlocal object, because it includes degrees of freedom that are spatially separated. But it's a perfectly good explanation of the correlations: as I said, they are enforced by the wave function, and more specifically by the possibilities that appear in the wave function. "Perfect correlation" between photons 1 and 4 if there is a swap just means the only possibilities that appear in the wave function if there is a swap are ones in which the measurement results on photons 1 and 4 agree.

 

[PeterDonis]

This is exactly the hand-waving I spoke of.

It seems pretty clear to me: Vaidman is acknowledging that the wave function is a nonlocal object, just as Zeh says in his paper. In other words, he is saying that he agrees that the MWI is nonlocal. He agrees that the MWI predicts things like Bell inequality violations and entanglement swapping. He just doesn't think the MWI attributes these things to any "action at a distance".

 

[mattt]

And just in case anyone thinks I am mischaracterizing something about MWI, this is from Vaidman in Plato:

a) The MWI is a deterministic theory for a physical Universe and it explains why a world appears to be indeterministic for human observers. ... The quantum state of the Universe at one time specifies the quantum state at all times.

b) The MWI does not have action at a distance.

c) The most celebrated example of nonlocality of quantum mechanics given by Bell’s theorem in the context of the Einstein-Podolsky-Rosen argument cannot get off the ground in the framework of the MWI because it requires a single outcome of a quantum experiment.

I don't see anything controversial in those three points (and the rest of the article).

If you understand what he is trying to say, I find it a quite straightforward translation of the mathematics of quantum mechanics into the English words of the Huge Everett interpretation.

 

[martinbn]

@DrChinese This is a bit of a side questions, but it was said a few time, so I'd like to understand what the claim is. You said that quantum teleportation proves non-locality, but which form of non-locality? If you mean violations of Bell's inequalities, then no one disagrees. If you mean something else, what exactly? And why do you think the references you gave support that kind of nonlocality?

 

[PeterDonis]

the Huge Everett interpretation

Huge?

 

[Morbert]

@DrChinese I'm still interested in a reference re/post #42. I think it's useful to emphasise the timelike relation between the BSM and the preparation of both 2-photon systems that, according to MWI, gives rise to the resultant decoherent branches.

 

[DrChinese]

a. Let's first describe the overall setup to be sure we have it right.

b. We have four photons. In the initially prepared state, photons 1 and 2 are entangled, and photons 3 and 4 are entangled. Both entanglements are maximal so by monogamy of entanglement there can't be any other entanglements involved.

c. In the middle of the experiment, so to speak, photons 2 and 3 are brought together and an experimental choice is made of whether or not to induce an entanglement swap. If a swap is induced, then after the swap (where "after" does not refer to time ordering, since as already noted it is possible to run this experiment where, for example, the photon 1 and 4 measurements are in the past light cone of the photon 2 and 3 swap/no swap decision--"after" only refers to logical ordering in terms of the analysis we are doing), photons 2 and 3 are entangled, and photons 1 and 4 are entangled. Again, both entanglements are maximal.

d. At the end of the experiment (which might, as noted, be in the past light cone of the "middle" described above, but it is logically the end for purposes of analysis), photons 1 and 4 are measured. To keep it simple, we will assume they are both measured along the same polarization axis, so if they are entangled, the measurements will always agree. If they are not entangled, there is an equal chance for the measurements to agree or disagree.

e. Now we can talk about how the MWI describes what happens to the wave function in the above. The starting wave function is, schematically (and ignoring normalization, which I will do throughout):

$$
\ket{\Psi}_0 = \ket{\psi}_{12} \ket{\psi}_{34} \ket{\text{ready}}
$$

where the lower case $\psi$ kets on the RHS are subscripted with the photons that are entangled in them, and the "ready" ket describes the state of the swap/no swap decision apparatus.

f. When photons 2 and 3 come together, either a swap happens or it doesn't. So after that decision is made (and in the MWI, the dynamics of that decision would be encoded in the Hamiltonian and would affect the wave function), the wave function becomes

$$
\ket{\Psi}_1 = \ket{\psi}_{14} \ket{\psi}_{23} \ket{\text{swap}} + \ket{\psi}_{12} \ket{\psi}_{34} \ket{\text{no swap}}
$$g. Of course none of this says that the MWI is "local". The wave function itself, I would say (and Zeh says in the paper you referenced from him), is a nonlocal object, because it includes degrees of freedom that are spatially separated. But it's a perfectly good explanation of the correlations: as I said, they are enforced by the wave function, and more specifically by the possibilities that appear in the wave function. "Perfect correlation" between photons 1 and 4 if there is a swap just means the only possibilities that appear in the wave function if there is a swap are ones in which the measurement results on photons 1 and 4 agree.

a. Thanks as always for your time.

b. Agreed. For convenience of discussion, I am also specifying that the initial entangled states for each pair is HH+VV. That doesn't change anything of import.

c. Only difference here is I want to specify that the measurement of Photon 1 occurs prior to the swap, and the 2/3 swap prior to the Measurement of Photon 4. I recognize fully that there is no predicted difference to the outcomes, but this sequence more clearly highlights the conceptual problems of MWI. This is perfectly feasible if the experiments are performed in the same inertial reference frame.

Further: I assumed you understood that my use of the word "distant/distance" (which matches the Nobel committee's terminology) means a distance too great in spacetime to be traversed at c. Thus no signal can go from the measurement of Photon 1 to the location of the swap of Photons 2/3. Similarly, no light signal can go from the swap (BSM) on Photons 2/3 to the location of the measurement of Photon 4. And Photon 4 is likewise too far from Photon 1's measurement for any light signal to get their either.

The only events in a common light cone is the initial entangled creation of Photons 1 & 2 (and likewise with the creation of Photons 3 & 4). So I accept that under MWI, the spreading of the "joint" wave function of Photons 1 & 2 (likewise 3 & 4) ultimately means that the swap brings together the wave functions of Photons 2 & 3. That despite the fact that no signal can go from there (location of the swap) to the location of Photon 4's measurement.

Please say if these specifications are not acceptable for our discussion, these variations have all been physically realized in various swapping experiments already.

d. Agreed that the Photon 4 measurement is to be done on same basis as Photon 1, so we desire the H> branch of Photon 1 to always contain the H> branch on Photon 4. And likewise we desire the V> branch of Photon 1 to always contain the V> branch on Photon 4. Note that the choice of H/V basis is arbitrary (I am sure no issue with that). And further that it is possible (but not important here, at least not at this time), that the choice of H/V basis could be selected midflight in a spacetime location isolated from the creation of the initially entangled pairs. Further, that basis choice need not be transmitted to the swapping mechanism (which is distant).

e. Agreed, we have a Product State of 2 entangled systems which have had no interaction of any kind. In fact, the only overlap they will ever have is if the swap is executed.

f. We are not agreed on this yet, because this is what we seek to prove. The MWI proponent (and I will not dispute) would say that immediately before the swap (which is after measurement of Photon 1), we have this:

f.i)
$$
\ket{\Psi}_1 = \ket{HH}_{12} \ket{\psi}_{34} + \ket{VV}_{12} \ket{\psi}_{34} {\text{ swap ready}}
$$

We want this to evolve somehow to what you say:

f.ii)
$$
\ket{\Psi}_1 = \ket{\psi}_{14} \ket{\psi}_{23} \ket{\text{swap}} + \ket{\psi}_{12} \ket{\psi}_{34} \ket{\text{no swap}}
$$

But this is not possible. QM predicts no swap can occur for either branch of the right hand side. It is an absolute requirement of swapping that Photons 2 & 3 are indistinguishable. They aren't any longer! Photon 2 is either in the H> branch or it is in the V> branch. In either branch (which by MWI tenet cannot interfere), Photons 2 & 3 can be distinguished on the basis of polarization. And this fact can be demonstrated by experiment: place an H> filter in the path of Photon 2. No swap will occur.

This point is not mentioned in the reference, as they assume everyone knows this. So I am providing the following reference:

Experimental loophole-free violation of a Bell inequality using entangled electron spins separated by 1.3 km
https://arxiv.org/pdf/1508.05949.pdf
"If the photons are indistinguishable in all degrees of freedom, the observation of ... [photons 2 & 3] in different output ports projects ... [Photons 1 & 4] into the maximally entangled state ..."
Note: photons are used for the swap in this particular experiment, and the final entangled pair is actually electrons. I modified the quote (in brackets []) to match our labeling.

Entanglement Between Photons that have Never Coexisted
https://arxiv.org/pdf/1209.4191.pdf
"One can also choose to introduce distinguishability between the two projected photons. In this case, ... the first and last photons do not become quantum entangled but classically correlated."

Further, it wouldn't matter if the order of events changed. In no version of branching will there be indistinguishable photons present for a swap unless (I guess) you specify the swap occurs before the measurement of Photons 1 & 4. But we already know from experiment that makes no difference at all, because explicit delayed-choice swaps (swap occurring after Photons 1 & 4 were measured) have been documented. See for example:

Experimental Nonlocality Proof of Quantum Teleportation and Entanglement Swapping
https://arxiv.org/abs/quant-ph/0201134
"Such a delayed-choice experiment was performed by including two 10 m optical fiber
delays for both outputs of the BSA. In this case photons [2] and [3] hit the detectors delayed
by about 50 ns. As shown in Fig. 3, the observed fidelity of the entanglement of photon [1] and
photon [4] matches the fidelity in the non-delayed case within experimental errors. Therefore,
this result indicate that the time ordering of the detection events has no influence on the
results..."

I am adding some specific MWI quotes in my next post as something of an extension to this one.

 

[DrChinese]

Quotes from the Vaidman article in SEP (my comments in brackets):

a) ...the ontology of the universe is a quantum state, which evolves according to the Schrödinger equation or its relativistic generalization... [which of course the Schrödinger equation evolves respecting c]. ... The Schrödinger equation itself does not explain why we experience definite results in quantum measurements.
b) The wave function of all particles in the Universe corresponding to any particular world will be a product of the states of the sets of particles corresponding to all objects in the world...
c) The difficulty with the concept of probability in a deterministic theory, such as the MWI, is that the only possible meaning for probability is an ignorance probability... [there is no actual uncertainty, since every outcome occurs there is realism at all times].

From Everett per SEP:
a) "All elements of a superposition must be regarded as simultaneously existing." [they are in different branches, of course]
b) [the following paraphrased by Barrett:] There is a sense in which A nevertheless gets a perfectly determinate measurement record...
c) "Let one regard an observer as a subsystem of the composite system: observer + object-system. It is then an inescapable consequence that after the interaction has taken place there will not, generally, exist a single observer state. There will, however, be a superposition of the composite system states, each element of which contains a definite observer state and a definite relative object-system state. Furthermore, as we shall see, each of these relative object system states will be, approximately, the eigenstates of the observation corresponding to the value obtained by the observer which is described by the same element of the superposition. Thus, each element of the resulting superposition describes an observer who perceived a definite and generally different result, and to whom it appears that the object-system state has been transformed into the corresponding eigenstate."
d) "... reality as a whole is rigorously deterministic. This reality, which is described jointly by the dynamical variables and the state vector, is not the reality we customarily think of, but is a reality composed of many worlds. By virtue of the temporal development of the dynamical variables the state vector decomposes naturally into orthogonal vectors, reflecting a continual splitting of the universe into a multitude of mutually unobservable but equally real worlds, in each of which every good measurement has yielded a definite result... [emphasis added].

------------------------------------

Of course, Everett's work appeared pre-Bell so he had no clue of what was to come. Clearly, MWI imagines a worlds in which a measurement on Photon 1 leads to 2 different branches: one with the definite H> result for all time to follow, and another with the definite V> result for all time to follow. These branches can never lead to entanglement of Photon 1 with anything - unless of course we have action at a distance. Again, see post#60 for the explicit proof.

 

[DrChinese]

@DrChinese I'm still interested in a reference re/post #42. I think it's useful to emphasise the timelike relation between the BSM and the preparation of both 2-photon systems that, according to MWI, gives rise to the resultant decoherent branches.

Sure. You can have as many BSMs between Photons 1 & 4 as you like with quantum repeaters. Each intermediate (except the ones associated with photons 2 & 3) can be placed before or after measurement (or creation) of either Photons 1 or 4. The BSMs associated with photons 2 & 3 can only be placed after their creation of course. But can be placed before or after the measurement of photons 1 & 4.

So the point is that you can select to perform an experiment in just about in order you like. QM doesn't care. ordering is never a factor. But because MWI evolves deterministically to the future, that leads to important differences.

Multistage Entanglement Swapping
https://arxiv.org/abs/0808.2972
Abstract: We report an experimental demonstration of entanglement swapping over two quantum stages. By successful realizations of two cascaded photonic entanglement swapping processes, entanglement is generated and distributed between two photons, that originate from independent sources and do not share any common past. In the experiment we use three pairs of polarization entangled photons and conduct two Bell-state measurements (BSMs) one between the first and second pair, and one between the second and third pair. This results in projecting the remaining two outgoing photons from pair 1 and 3 into an entangled state, as characterized by an entanglement witness. The experiment represents an important step towards a full quantum repeater where multiple entanglement swapping is a key ingredient.

See Fig. 1, and note that there is no theoretical limit to the number of stages (BSMs) in repeaters of this type. In this experiment, Photons 1 & 6 are entangled even though they never share a common light cone with each other, and their partners 2 & 5 also need never share a common light cone.

 

[PeterDonis]

Only difference here is I want to specify that the measurement of Photon 1 occurs prior to the swap, and the 2/3 swap prior to the Measurement of Photon 4.

That's fine. I already included cautions about time ordering vs. logical ordering in my post. As you say, the results are the same regardless of the time ordering, so any time ordering you want to specify is fine. It doesn't change anything I posted.

no signal can go from the measurement of Photon 1 to the location of the swap of Photons 2/3. Similarly, no light signal can go from the swap (BSM) on Photons 2/3 to the location of the measurement of Photon 4. And Photon 4 is likewise too far from Photon 1's measurement for any light signal to get their either.

Ok, so all of the measurements are spacelike separated. Again, the predictions are the same regardless, so if you want to specify this, that's fine. It doesn't change anything I posted.

The MWI proponent (and I will not dispute) would say that immediately before the swap (which is after measurement of Photon 1), we have this:

f.i)
$$
\ket{\Psi}_1 = \ket{HH}_{12} \ket{\psi}_{34} + \ket{VV}_{12} \ket{\psi}_{34} {\text{ swap ready}}
$$

That's the same state I wrote down, you're just expanding out what I wrote as $\ket{\psi}_{12}$.

We want this to evolve somehow to what you say:

f.ii)
$$
\ket{\Psi}_1 = \ket{\psi}_{14} \ket{\psi}_{23} \ket{\text{swap}} + \ket{\psi}_{12} \ket{\psi}_{34} \ket{\text{no swap}}
$$

Yes.

But this is not possible. QM predicts no swap can occur for either branch of the right hand side.

If this claim of yours is correct, it means QM cannot make correct predictions for entanglement swapping. Nothing I have written down is specific to the MWI; it is just basic QM, unitary evolution from $\ket{\Psi}_0$ to $\ket{\Psi}_1$ using the Schrodinger Equation with an appropriate Hamiltonian. If that does not produce $\ket{\Psi}_1$, then QM cannot make correct predictions, because $\ket{\Psi}_1$ is the correct state to describe the possible results of the swap mechanism. MWI interprets that ket to describe two branches that both "exist", instead of just describing possible results, but that's interpretation, not math. The math is the same on any interpretation.

So at this point I don't understand what your argument is. Are you arguing that we have to change how QM makes predictions in order to correctly predict the results of entanglement swapping?

It is an absolute requirement of swapping that Photons 2 & 3 are indistinguishable. They aren't any longer!

They aren't after a swap takes place, if a swap does take place. But the swap only requires that they are indistinguishable before the swap takes place. In other words, we have a Hamiltonian that describes the swap process; the input state that enables that process has to have photons 2 & 3 indistinguishable; but the output state after the Hamiltonian is applied does not have photons 2 & 3 indistinguishable (because once a swap has taken place, they aren't).

Photon 2 is either in the H> branch or it is in the V> branch.

If you are referring to $\ket{\Psi}_0$, the state before the swap Hamiltonian has been applied, there are no such branches. There is only one branch of the wave function at that point, according to the MWI. That's why I wrote it as a single term. Splitting what I wrote as $\ket{\psi}_{12}$ into two terms doesn't change the fact that there's only one branch; it just obfuscates it. Branching occurs in the MWI when decoherence occurs, and no decoherence has occurred at the point where $\ket{\Psi}_0$ is the overall wave function.

In either branch (which by MWI tenet cannot interfere), Photons 2 & 3 can be distinguished on the basis of polarization. And this fact can be demonstrated by experiment: place an H> filter in the path of Photon 2. No swap will occur.

You've changed the experiment. If there is an H filter in Photon 2's path, the Hamiltonian changes, and therefore so does the resulting state. We're not analyzing that different experiment. We're analyzing the experiment where it is possible for a swap to take place. In that experiment, there are no interactions except the swap operation. Other than photons 2 & 3 being "inside" the swap mechanism, where if they are indistinguishable a swap can occur, the photons travel freely and there are no unitary operations done on them besides free propagation. That is the experiment we are analyzing.

 

[DrChinese]

@DrChinese This is a bit of a side questions, but it was said a few time, so I'd like to understand what the claim is. You said that quantum teleportation proves non-locality, but which form of non-locality? If you mean violations of Bell's inequalities, then no one disagrees. If you mean something else, what exactly? And why do you think the references you gave support that kind of nonlocality?

The specific form of nonlocality - action at a distance (AAD) - that I claim is the generally accepted science is the remote change of quantum state by an experimenter's choice. That experimenter can choose to teleport/change the quantum state of a remote system (as measured by c). Note that entanglement swapping uses the same essential protocol as quantum teleportation. See any of the swapping experiments I have referenced such as these by teams associated with Zeilinger:

https://arxiv.org/pdf/quant-ph/0201134.pdf (2002-2008)
https://arxiv.org/pdf/0809.3991.pdf (2008)

As mentioned, the 2022 Nobel committee summarized these works and others as follows: [Anton Zeilinger's] research group has demonstrated a phenomenon called quantum teleportation, which makes it possible to move a quantum state from one particle to one at a distance."

If you want to claim it was not mainstream physics before 2022, then go ahead. Not sure how much clearer one needs to be here, this stuff has been around for 20 years. The GHZ experiments prove exactly the same thing as swapping experiments do, but the complexity of the math and the implementation is much more complex. With GHZ, you can remotely change a quantum state as well by the experimenter's choice of basis. If the experimenter makes a choice here that measurably changes a state there, how is that not action at a distance?

I strictly follow mainstream physics based on the thousands of papers I have scanned (unless I occasionally note otherwise). You probably have noted that I keep a lot of bookmarks on the best. So what I am saying is well accepted by the experimental community. All I can say is that there are a lot of people that have entrenched positions of denial that might do well to read a new more experiments and take a fresh view.

 

[DrChinese]

a. That's fine. I already included cautions about time ordering vs. logical ordering in my post. As you say, the results are the same regardless of the time ordering, so any time ordering you want to specify is fine. It doesn't change anything I posted.

Ok, so all of the measurements are spacelike separated. Again, the predictions are the same regardless, so if you want to specify this, that's fine. It doesn't change anything I posted.

b. If this claim of yours is correct, it means QM cannot make correct predictions for entanglement swapping. Nothing I have written down is specific to the MWI; it is just basic QM ... Are you arguing that we have to change how QM makes predictions in order to correctly predict the results of entanglement swapping?c. They aren't after a swap takes place, if a swap does take place. But the swap only requires that they are indistinguishable before the swap takes place. In other words, we have a Hamiltonian that describes the swap process; the input state that enables that process has to have photons 2 & 3 indistinguishable; but the output state after the Hamiltonian is applied does not have photons 2 & 3 indistinguishable (because once a swap has taken place, they aren't).d. If you are referring to $\ket{\Psi}_0$, the state before the swap Hamiltonian has been applied, there are no such branches. There is only one branch of the wave function at that point, according to the MWI. That's why I wrote it as a single term. Splitting what I wrote as $\ket{\psi}_{12}$ into two terms doesn't change the fact that there's only one branch; it just obfuscates it. Branching occurs in the MWI when decoherence occurs, and no decoherence has occurred at the point where $\ket{\Psi}_0$ is the overall wave function.

e. You've changed the experiment. If there is an H filter in Photon 2's path, the Hamiltonian changes, and therefore so does the resulting state. We're not analyzing that different experiment. We're analyzing the experiment where it is possible for a swap to take place. In that experiment, there are no interactions except the swap operation. Other than photons 2 & 3 being "inside" the swap mechanism, where if they are indistinguishable a swap can occur, the photons travel freely and there are no unitary operations done on them besides free propagation.

a. In QM, no change. In MWI, big change. And why is that?

QM is contextual. A future context - and ONLY a future context - dictates the quantum prediction. More importantly, the entire future context must be considered in QM. But that same statement cannot be made in MWI, because MWI evolves deterministically. Time and ordering are absolutely relevant, despite any MWI proponent's argument to the contrary. As each stage in the MWI version occurs, there is a branching (which is loosely akin to "collapse" in QM). But in indeterministic QM, there is only 1 collapse for the entire context. In deterministic MWI, there are many branching events for a swap - at least 3. Those lead to branches that do NOT match the QM result for all branches.

b. You have skipped the branching of MWI, your description is simply the QM version. No, of course I am not questioning that. I am citing mainstream physics here.

c. Again, you need to acknowledge that after Photon 1 is measured, there is an H> world and a V> world under MWI. This is absolutely and unquestionably a tenet of every version of MWI. If you can show me otherwise, please show me. On the other hand, I (the only one providing quotes and references) showed you otherwise in my post #61. In MWI, each observer sees a definite outcome.

d. If Photon 1 is H>, then by rule Photon 2 is H>. That is true in MWI. Nothing to see here. Of course, the swap polarization test can be performed at any angle, and if that angle is different than the Photon 1 test, we are now in counterfactual territory - so no claim is really possible for QM.

e. For MWI, it's no change if we add an H> filter when Photon 2 is already H>. After all, 100% of those will pass without any change. Of course we would lose the intensity from the V> stream.

 

[martinbn]

The specific form of nonlocality - action at a distance (AAD) - that I claim is the generally accepted science is the remote change of quantum state by an experimenter's choice. That experimenter can choose to teleport/change the quantum state of a remote system (as measured by c). Note that entanglement swapping uses the same essential protocol as quantum teleportation. See any of the swapping experiments I have referenced such as these by teams associated with Zeilinger:

https://arxiv.org/pdf/quant-ph/0201134.pdf (2002-2008)
https://arxiv.org/pdf/0809.3991.pdf (2008)

As mentioned, the 2022 Nobel committee summarized these works and others as follows: [Anton Zeilinger's] research group has demonstrated a phenomenon called quantum teleportation, which makes it possible to move a quantum state from one particle to one at a distance."

If you want to claim it was not mainstream physics before 2022, then go ahead. Not sure how much clearer one needs to be here, this stuff has been around for 20 years. The GHZ experiments prove exactly the same thing as swapping experiments do, but the complexity of the math and the implementation is much more complex. With GHZ, you can remotely change a quantum state as well by the experimenter's choice of basis. If the experimenter makes a choice here that measurably changes a state there, how is that not action at a distance?

I strictly follow mainstream physics based on the thousands of papers I have scanned (unless I occasionally note otherwise). You probably have noted that I keep a lot of bookmarks on the best. So what I am saying is well accepted by the experimental community. All I can say is that there are a lot of people that have entrenched positions of denial that might do well to read a new more experiments and take a fresh view.

Two comments. First, action at a distance is ypur wording. Do you have a reference with that exact wording? Second, you are confused about how teleportation works. There is no remote change of state!

 

[PeterDonis]

In QM, no change. In MWI, big change.

No, in MWI, no change. MWI makes the same predictions as QM. Just as any QM interpretation does.

If you are going to continue to make false claims about what the MWI says, there is no point in further discussion. I am quite willing to give more details about how the MWI explains the QM predictions. But I am not willing to entertain false claims that the MWI does not make the same predictions that QM does.

QM is contextual.

There is a sense of "contextual" in which this is true, but I'm not sure you are using the word in that sense. Again, you seem to be arguing based on "contextual" that the MWI does not make the same predictions that QM does. That is not a valid basis for discussion. See above.

I am citing mainstream physics here.

Not as far as your claims about the MWI's predictions are concerned. See above.

in indeterministic QM, there is only 1 collapse for the entire context

Please cite a reference for this very surprising claim. My understanding of "collapse" for interpretations that use that concept (I'm not clear about exactly what interpretation you are referring to as "indeterministic QM", but apparently it uses the collapse concept) is that it occurs when decoherence occurs. Decoherence occurs in at least 3 places in the experiment under discussion: when the swap/no swap decision is made, and when photon 1 and photon 4 are measured. So "collapse" would occur in all those places as well, on any "collapse" interpretation.

You have skipped the branching of MWI

I have done no such thing. I have given the correct MWI criterion for branching: it happens when decoherence occurs. That is the same criterion for when "collapse" happens in interpretations that use that concept, including whatever it is you mean by "indeterministic QM".

after Photon 1 is measured, there is an H> world and a V> world under MWI

That is at the end of the experiment--the state I labeled $\ket{\Psi}_2$. In that state, yes, there are separate branches for the two photon 1 measurement results. That state is the result of applying the appropriate photon 1 (and photon 4, since $\ket{\Psi}_2$ also takes into account that measurement) unitary measurement operators to the state $\ket{\Psi}_1$.

If Photon 1 is H>, then by rule Photon 2 is H>.

In state $\ket{\Psi}_0$, yes, that would be the case. But no measurement is carried out on that state.

For MWI, it's no change if we add an H> filter when Photon 2 is already H>.

False. MWI makes the same predictions as QM. See above.

 

[PeterDonis]

In QM, no change. In MWI, big change.

For MWI, it's no change if we add an H> filter when Photon 2 is already H>.

As I said in my previous post, both of these claims are false; the MWI makes the same predictions as basic QM does.

However, you also say:

MWI evolves deterministically.

But if you are going to object to the MWI on these grounds, you need to make the correct objection. That requires doing at least two things:

(1) You need to specify the scenario so that time ordering makes a difference. But you have specified that, for this discussion, you want all three of the decoherence events--the photon 1 and 4 measurements, and the swap/no swap decision--to be spacelike separated. That means their time ordering can't make a difference. If you want to ask how the MWI, with "deterministic evolution", explains entanglement swapping, you need to, for example, put the photon 1 and 4 measurements in the past light cone of the swap/no swap decision, so that, at least on its face, "deterministic evolution" would require that the photon 1 and 4 measurements can't possibly be made on a state that has decohered due to the swap/no swap decision.

(2) You need to ask how the basic math of QM--independent of any interpretation--accounts for entanglement swapping under the conditions I just described. And the answer to that is that it just handwaves it: it says, without any supporting argument, that we have to apply the photon 1 and 4 measurement operators to the state $\ket{\Psi}_1$, not the state $\ket{\Psi}_0$, even if the photon 1 and 4 measurements occur in the past light cone of the swap/no swap decision. (Your claim that "QM is contextual" explains this is not basic QM independent of any interpretation; it's a particular interpretation.) And then you need to explain why QM interpretations can't just handwave that the same way. [Edit: I was too pessimistic with this item; the basic math can in fact explain the results without any handwaving. See post #79 and follow-ups to it below.]

 

[PeterDonis]

MWI evolves deterministically.

Btw, another comment on this can be made as well: the "deterministic evolution", the Schrodinger Equation, is in Hilbert space, not ordinary spacetime. Relating evolution in Hilbert space to evolution in ordinary spacetime is by no means straightforward (in fact one might say that that relationship is at the core of the issues with QM interpretations). So one can't just assume that "deterministic evolution" requires everything to go "forward in time" in ordinary spacetime.

 

[PeterDonis]

I (the only one providing quotes and references)

You have not given any references that support your claims that the MWI makes different predictions from standard QM.

It would be nice if someone had a reference from an MWI proponent that gives an MWI viewpoint on the entanglement swapping experiments you have given references for. Unfortunately, so far despite a fair bit of searching I have not been able to find one. So I am doing the best I can myself to give what I believe to be the MWI viewpoint.