On 23-03-2022 02:11, Bruce Kellett wrote:

On Wed, Mar 23, 2022 at 10:26 AM smitra <smi...@zonnet.nl> wrote:Let's consider this whole non-locality issue right from the start.Probably a good idea. The discussion has become rather confused. We should sort out exactly where we agree and where we disagree.The violation of Bell's inequalities proves that QM cannot have an underlying local deterministic theory, i.e. one in which the measurement outcomes arise deterministically as a result of local interactions. In Aspect's experiments, the polarizer angles being set at space-like separations, rules out local hidden variable theories where the photons are still influenced by both polarizers via local interactions.Yes.What the violation of Bell's inequalities does not prove, is that QM is non-localWell, these results certainly show that some work needs to be done if you are to recover a completely local theory.

`The unitary time evolution in QM is local. There is only an issue with`

`collapse, as the mechanism for this is usually left unspecified. But`

`apart form the issues with collapse QM is a local theory. The violations`

`of Bell's inequalities have no bearing on QM itself, only on hidden`

`variable theories that seek to explain QM in terms of classical`

`deterministic concepts. Such theories cannot be local.`

What is true is that there is a problem with collapse interpretations in precisely the sort of entangled states used in Bell-type experiments,I think there are problems in non-collapse interpretations as well. It is a mistake to put all the problems down to collapse models -- the collapse in most models can be eliminated, and that does not make these theories local.

`The problems can only be with collapse as without collapse you have a`

`manifestly local theory. If you think that (unitary) QM is non-local`

`then you need to prove that, because all the known theorems apply only`

`to hidden variable theories, not to QM itself.`

because the measurement outcome at one location provides one with information about the other measurement outcome.That is not true in general. If the polarizers are known to be parallel, then Alice's result, whatever it may be, tells her that Bob's result will be the opposite. But this is not the case in general. If the polarizers are not parallel (and not known in advance to be parallel), then all Alice can infer from her result is that Bob could get either up or down. She cannot know which, and she cannot, without knowing the relative polarizer angle, assign probabilities to the two possible results. By hypothesis, in this situation, she does not know the relative polarizer angle, so she has no information about which result Bob will get.

`Yes, I agree, the relative angle must be known and then for every spin`

`measurement for Alice she'll get some amount if information about Bob's`

`result, ranging from 0 to 1 bit per bit of her results.`

If the polarizers are parallel or antiparallel then the measurement result at one site provides one with perfect information about the measurement outcome at the other side. While doing the experiment with parallel or anti-parallel polarizers does not prove that QM has no underlying local deterministic theory, that's also unnecessary as this is already an established fact.I thought that was what was in dispute here.

`The violation of Bell's inequalities proves that QM does not have an`

`underlying local deterministic theory. So, we are then allowed to assume`

`this and then consider another experiment where the polarizers are`

`parallel. That experiment then does not have to reproduce this known`

`fact about there not being a local deterministic theory underlying QM.`

We can then use that fact and say that because we already know that Bob's photon does not decide in a deterministic way whether or not to move through the polarizer based on the local physical state, that Alice having the information on whether or not Bob's photon will move through Bob's polarizer, demonstrates a nonlocal feature of QM.But Alice does not have any such information after her measurement.

`She does if both polarizers were agreed to be chosen parallel to each`

`other, and if there is a real collapse. In that case her measurements`

`predicts what Bob will find and vice versa. So, Bob then knows that the`

`violation of Bell's inequality means that before he measures his spin`

`that the information about what he is about to find does not exist`

`locally in the physical state of the system, but that it does exist at`

`Alice's location die to her state collapsing after she performs her`

`measurement. That's the non-locality in collapse interpretations that`

`does not exist in the MWI.`

But, of course, this is only true in collapse interpretations where in Bob's sector there exists a unique result for Alice. This is not the case in the MWI, so here there is no issue with non-locality at all.That does not follow, because, rather than Alice not having a unique result in Bob's sector, it is the case that Alice has split into two branches, in each of which she has a definite result. So when she meets Bob, he also splits into a copy for each of Alice's sectors. So, for Bob, the Alice he meets will have a definite result. There is no ambiguity coming from the many worlds situation here. Whenever Bob looks to Alice, he knows he will see the definite result that she obtained.

`The version of Alice that Bob will mat is undetermined in advance. Bob`

`splits due to local interactions with Alice's sector.`

One cannot avoid the consequences of Bell's theorem by this manoeuvre. Even if one can claim that the fact that Alice splits into two copies, one for each result she could get, means that she does not (from Bob's perspective) get a definite result voids the applicability of Bell's theorem, this still does not provide a local explanation for the violation of the Bell inequalities. The violation of these inequalities is an established experimental fact, and an explanation of this fact is required. If it is claimed that many worlds can provide a local explanation, then it is up to those who make this claim to provide this local explanation. This you have failed to do. Merely claiming that Bell's theorem does not apply in many worlds theories (non-collapse theories) is not an explanation of anything. The correlations are still there, and still in need of explanation.

`Bell's theorem doesn't even apply to QM itself, let alone the MWI.`

`Bell's theorem applies to local hidden variable theories. And the`

`nonlocality issue with collapse is also a different thing than what`

`Bell's theorem is about.`

`Unitary QM is a manifestly local theory, so there is nothing to explain.`

`If you believe that (unitary) QM is nonlocal, you ought to prove this.`

One can then say that there is still an issue with locality in Bell-type experiments due to the correlations depending on the relative angle. The relative angle is only available non-locally.Only in Aspect-type experiments where the polarizer angles are set at spacelike separations. This was done to rule out the possibility of some local influence on the hidden variables that might be possible if the detector angles were known at the time of the creation of the entangled pair, as in the original Freedman-Clauser experiments. This was always a somewhat extreme possibility, and no attempt has ever been made to provide a hidden variable theory (local or not) that could achieve this. So the fact that the polarizers are set non-locally is not really an issue, apart from ruling out some somewhat bizarre possibilities.However this sort of non-locality is put in by hand, and what you put in must come out in the final results.This is just silly. This non-locality is not "put in by hand". It is either inherent in the experiments on non-separable entangled pairs or it is not -- nothing is put in by hand.

`Of course, the entangled state is a nonlocal object that is used in the`

`experiment, but this is also created using local interactions.`

Take e.g. this trivial experiment. Two copies are made of a file containing a random bitstring and given to Alice and Bob. Alice and Bob move away from each other, and at a spacelike separations they both apply a random cyclic permutation to the bitstrings and save the results in their file, overwriting the old data by te new data. They then meet each other and compute the correlations of their bitstrings. Obviously, the correlation will be (almost) zero unless they have chosen the same cyclic permutation. And this is also information that is only available non-locally.No, it is not available only non-locally. They might have agreed before the experiment on a cyclic permutation that both could use. In your toy example, such a possibility is not as bizarre as the thought that something similar could enable a local explanation of the Freedman-Clauser results.So, the mere fact that a correlation depends on nonlocal information that was put into the experiment, does not demonstrate that there is an essential non-local feature in the laws of physics. What you out into an experiment that affects the results must come out.Your simple example is not really appropriate, and it does not demonstrate that the non-locality was put in by hand.

`It demonstrates that the mere fact that a correlation depends on`

`nonlocal information in an essential way, does not prove that there is`

`anything nonlocal about the processes that explains the correlation.`

In the Bell-type experiments this is a useful method to demonstrate to rule out a local dependence on both polarizer settings. The mere fact that the correlation then depends on a non-locally chosen relative angle is not the argument here.No, the real argument here is the provision of an actual account of how the correlations are formed in a many worlds setting if one is not to rely on the inherent non-locality available in the so-called 'collapse' models. Your claim is that many worlds can provide this local account. So far you have not provided any such account.

`The account exists in the form of the unitary time evolution of the`

`state describing the entire system. This is local. It is your assertion`

`that unitary QM is nonlocal.`

Merely claiming that many worlds theories violate the assumption that Bell made that experiments have definite results does not amount to an account of anything.

`Bell's theorems do not apply to QM, only hidden variable model. What`

`matters is that QM violates Bell's inequalities, which then proves that`

`QM does not have an underlying local deterministic model. QM itself is`

`local except possibly for the collapse part depending on how you model`

`that, or if collapse happens at all.`

It can readily be argued that, while Bell probably thought that experiments gave unique results, that assumption did not invalidate the main conclusions of his argument. After all, Alice and Bob can only meet in one world, and in that world in which they do meet, each had definite results for their measurements. The fact that there are several copies of both Alice and Bob, does not alter the fact that only one copy of one can meet only one copy of the other. The correlations have to be explained for each such pairing of Alices and Bobs.

`Those correlations then imply that there is no local hidden variable`

`theory possible that can explain the individual measurement outcomes.`

Saibal

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