Undergrad QM objects do not have properties until measured?

[stevendaryl]

But in operational quantum mechanics, which Werner claims to be local, there is wave function collapse. If we take the wave function to be real, then operational quantum mechanics is manifestly nonlocal. The state space in operational quantum mechanics is not a simplex, and that doesn't seem to depend at all on whether one assumes the wave function to be real or not real.

I am puzzled by Werner's argument, although I can't say that I'm 100% certain that he's wrong. But even if Werner is right, his argument is murky enough that it's hyperbole to call someone a "crackpot" for not agreeing with him (as rubi called Maudlin).

It seems to me that in wave function collapse, you can either take the wave function to be something physical, in which case collapse is a nonlocal, physical process. Or you can take the wave function to just reflect our knowledge of the world, in which case the collapse is just updating our knowledge based on new information. The latter takes a "non-realistic" view of the wave function. (Or "non-physical"--I'm not sure what "realism" means). So I can sort-of see that whether collapse is local or not depends on whether you view the wave function realistically.

But the second choice, that the wave function isn't to be taken realistically/physically was exactly what Einstein assumed. He thought that both the probabilistic aspects of quantum and the nonlocal aspects were due to the fact that quantum mechanics was not a fundamental theory, but that the wave function was some kind of statistical summary of a microscopic reality that we don't have a theory for, yet. That was the whole point of the EPR argument, to show that there was some reality that was not being reflected in the wave function.

 

[rubi]

But even if Werner is right, his argument is murky enough that it's hyperbole to call someone a "crackpot" for not agreeing with him (as rubi called Maudlin).

I'm not calling him a crackpot because I don't agree with him. I'm calling him a crackpot because makes no effort to understand the criticism and rather prefers to respond polemically. If you read Werner's article, you will see that Werner considers Maudlin a crackpot as well. He's just polite enough not say it directly.

 

[atyy]

In standard operationalist quantum theory, the objects of interest are particles for instance (or fields in QFT) and their state is given by a wave function. If you claim that the wave function is real, this is a shift of perspective. The objects of interest aren't the particles anymore, but rather the wave function itself. It is a physical object rather than just a container of information about physical objects ("a state"), so it has a state itself, which contains the information about the physical object called "wave function". This state is an element of a simplex (the space of distributions on phase space, where by phase space I mean classical phase space as in Hamiltonian classical field theory).

Perhaps. But even if that is true, it doesn't show how operational quantum mechanics is local. Realism is a prerequisite for Maudlin's definition of locality, so at best one has to say that the notion of locality on operational quantum mechanics is empty. One can define another notion of locality, but then it wouldn't contradict Maudlin - it would just be a different definition.

Edit: Regarding state - would Werner's argument work if one were using an operational definition of state, eg. if we consider both classical and quantum states to be just containers of information? Thus for example, if we take the quantum state to be real, the "state" as defined in Eq 12 of https://arxiv.org/abs/quant-ph/0101012 would still not be a simplex, even though the theory is nonlocal.

 

[rubi]

But even if that is true, it doesn't show how operational quantum mechanics is local.

I agree with that. My point is however that we can't claim that it is non-local, which is what Maudlin does.

Realism is a prerequisite for Maudlin's definition of locality

I fully agree, but Maudlin denies that he needs to assume realism.

at best one has to say that the notion of locality on operational quantum mechanics is empty. One can define another notion of locality, but then it wouldn't contradict Maudlin - it would just be a different definition.

Well, there is one single notion of locality that applies to all theories that can be formulated on Lorentzian spacetimes (see my post #22 and wikipedia for additional information) and all other notions of locality must be derived from it. Bell's (and Maudlin's) criterion follows from the standard notion plus the assumption of classicality. It's a special case of the general principle.

 

[zonde]

I'm not calling him a crackpot because I don't agree with him. I'm calling him a crackpot because makes no effort to understand the criticism and rather prefers to respond polemically.

I don't understand the criticism either. Can you explain it?
Can you point out the flaw in this argument of Maudlin:
"if a theory predicts perfect correlations for the outcomes of distant experiments, then either the theory must treat these outcomes as deterministically produced from the prior states of the individual systems or the theory must violate EPR-locality. The argument is extremely simple and straightforward. The perfect correlations mean that one can come to make predictions with certainty about how system S1 will behave on the basis of observing how the other, distant, system S2 behaves. Either those observations of S2 disturbed the physical state of S1 or they did not. If they did, then that violates EPR-locality. If they did not, then S1 must have been physically determined in how it would behave all along. That’s the argument, from beginning to end. (That’s also the point of Bell’s discussion of Bertlmann’s socks.) So preserving EPR-locality in these circumstances requires adopting a deterministic theory. Where, in this argument, does any presupposition about the geometry of the state space play any role? Nowhere."

 

[rubi]

I don't understand the criticism either. Can you explain it?
Can you point out the flaw in this argument of Maudlin:
"if a theory predicts perfect correlations for the outcomes of distant experiments, then either the theory must treat these outcomes as deterministically produced from the prior states of the individual systems or the theory must violate EPR-locality. The argument is extremely simple and straightforward. The perfect correlations mean that one can come to make predictions with certainty about how system S1 will behave on the basis of observing how the other, distant, system S2 behaves. Either those observations of S2 disturbed the physical state of S1 or they did not. If they did, then that violates EPR-locality. If they did not, then S1 must have been physically determined in how it would behave all along. That’s the argument, from beginning to end. (That’s also the point of Bell’s discussion of Bertlmann’s socks.) So preserving EPR-locality in these circumstances requires adopting a deterministic theory. Where, in this argument, does any presupposition about the geometry of the state space play any role? Nowhere."

There is no flaw in this argument. The fact that Maudlin thinks that this part of the argument is what Werner considers to be faulty clearly shows that he didn't understand the criticism at all. Non-simplicial state spaces can also account for some degree of determinism. However, one cannot prove Bell's inequality from non-simplicial state spaces, so the simplex structure is a crucial extra assumption. Thus, a violation of Bell's inequality says nothing about theories modeled by non-simplicial state spaces.

 

[atyy]

Well, there is one single notion of locality that applies to all theories that can be formulated on Lorentzian spacetimes (see my post #22 and wikipedia for additional information) and all other notions of locality must be derived from it. Bell's (and Maudlin's) criterion follows from the standard notion plus the assumption of classicality. It's a special case of the general principle.

Sure, but if Maudlin's locality is what you are calling classical local causality in your language, and a state is operationally defined as in operational quantum mechanics, then Maudlin's argument would be that one form of Bell's theorem is:

Operational quantum mechanics cannot be embedded into a classical locally causal theory.

The statement is of the form "X cannot be embedded into Y". There is of course a requirement for classicality in defining Y but not in defining X. So if Maudlin is referring to what one can put for X, then it is correct that there is no requirement for the state space to be a simplex.

 

[rubi]

Sure, but if Maudlin's locality is what you are calling classical local causality in your language, and a state is operationally defined as in operational quantum mechanics, then Maudlin's argument would be:

Operational quantum mechanics cannot be embedded into a classically locally causal theory.

If that was the case, I'd be happy, since then we're back at "no local realistic theory can reproduce all predictions of quantum mechanics", which is the standard reading of Bell's theorem, which pretty much all physicist agree with. However, I doubt that this is what Maudlin believes. The abstract of his paper is:

"On the 50th anniversary of Bell's monumental 1964 paper, there is still widespread misunderstanding about exactly what Bell proved. This misunderstanding derives in turn from a failure to appreciate the earlier arguments of Einstein, Podolsky and Rosen. I retrace the history and logical structure of these arguments in order to clarify the proper conclusion, namely that any world that displays violations of Bell's inequality for experiments done far from one another must be non-local. Since the world we happen to live in displays such violations, actual physics is non-local."

Apparently, Maudlin believes that the violations of Bell's inequality imply that the world is non-local. In the paper, he makes it pretty clear in my opinion that he thinks that realism is not required as an additional assumption in Bell's theorem. Furthermore, in his reply to Werner, he leaves no doubt that he doesn't understand where the assumption of a simplicial state space is used. If Maudlin had just explained the standard reading of Bell's theorem, then the editors certainly wouldn't have asked Werner for a reply.

 

[atyy]

If that was the case, I'd be happy, since then we're back at "no local realistic theory can reproduce all predictions of quantum mechanics", which is the standard reading of Bell's theorem, which pretty much all physicist agree with. However, I doubt that this is what Maudlin believes. The abstract of his paper is:

"On the 50th anniversary of Bell's monumental 1964 paper, there is still widespread misunderstanding about exactly what Bell proved. This misunderstanding derives in turn from a failure to appreciate the earlier arguments of Einstein, Podolsky and Rosen. I retrace the history and logical structure of these arguments in order to clarify the proper conclusion, namely that any world that displays violations of Bell's inequality for experiments done far from one another must be non-local. Since the world we happen to live in displays such violations, actual physics is non-local."

Apparently, Maudlin believes that the violations of Bell's inequality imply that the world is non-local. In the paper, he makes it pretty clear in my opinion that he thinks that realism is not required as an additional assumption in Bell's theorem. Furthermore, in his reply to Werner, he leaves no doubt that he doesn't understand where the assumption of a simplicial state space is used. If Maudlin had just explained the standard reading of Bell's theorem, then the editors certainly wouldn't have asked Werner for a reply.

OK, but if we agree that Bell's theorem applies to operational QM, and operational QM has a state space that is not a simplex, then it is true that there is no requirement for classicality in the theories that Bell's theorem applies to.

 

[rubi]

OK, but if we agree that Bell's theorem applies to operational QM, and operational QM has a state space that is not a simplex, then it is true that there is no requirement for classicality in the theories that Bell's theorem applies to.

Bell's theorem still applies only to classical theories. It tells us what inequality a classical local theory must necessarily satisfy. It doesn't tell us anything about an inequality that a quantum theory (or any other theory with a non-simplicial state space) must satisfy, no matter whether it is local or not. The violation of the inequality thus only proves that those theories that predict its satisfaction must be excluded (i.e. all classical local theories).

 

[N88]

I don't understand the criticism either. Can you explain it?
Can you point out the flaw in this argument of Maudlin:
"if a theory predicts perfect correlations for the outcomes of distant experiments, then either the theory must treat these outcomes as deterministically produced from the prior states of the individual systems or the theory must violate EPR-locality. The argument is extremely simple and straightforward. The perfect correlations mean that one can come to make predictions with certainty about how system S1 will behave on the basis of observing how the other, distant, system S2 behaves. Either those observations of S2 disturbed the physical state of S1 or they did not. If they did, then that violates EPR-locality. If they did not, then S1 must have been physically determined in how it would behave all along. That’s the argument, from beginning to end. (That’s also the point of Bell’s discussion of Bertlmann’s socks.) So preserving EPR-locality in these circumstances requires adopting a deterministic theory. Where, in this argument, does any presupposition about the geometry of the state space play any role? Nowhere." (Emphasis added.)

I'm reluctant to intervene in helpful discussions on Bell's Theorem between Science Advisors and a Gold Member when another Science Advisor says:

Bell inequality violations have nothing at all to do with the measurement problem, hence should be off-topic in this thread. They address a completely different problem - that of local hidden variable theories.

But I see connections with my OP and zonde's question.

Isn't this the flaw in Maudlin's argument: "If they did not, then S1 must have been physically determined in how it would behave all along."

In the context of the OP question QM objects do not have properties until measured? I say that they do have SOME properties (spin s = 1/2, for example) before measurement. So, questioning Maudlin: S1 has properties that are correlated with those of its twin and these properties physically determine how it behaves all along; so, similar to human twins, there should be no mystery in the independent behaviour of widely-separated twins being correlated in Bell-tests?

 

[atyy]

Bell's theorem still applies only to classical theories. It tells us what inequality a classical local theory must necessarily satisfy. It doesn't tell us anything about an inequality that a quantum theory (or any other theory with a non-simplicial state space) must satisfy, no matter whether it is local or not. The violation of the inequality thus only proves that those theories that predict its satisfaction must be excluded (i.e. all classical local theories).

But isn't it the case that:

"Bell's theorem says that operational QM cannot be embedded into a classical locally causal theory" means that "Bell's theorem applies to operational QM"

?

 

[rubi]

But isn't it the case that:

"Bell's theorem says that operational QM cannot be embedded into a classical locally causal theory" means that "Bell's theorem applies to operational QM".

I'm not sure what you mean by "apply", but the direction of the argument is as follows:
1. Bell's theorem: Every classical local theory must satisfy Bell's inequality.
2. Quantum theory violates Bell's inequality.
3. Therefore, quantum theory can't be embedded into a classical local theory.
You want to run the argument backwards, but that doesn't work. Bell's theorem is only a theorem about classical local theories. But maybe I just don't understand what conclusion you want to draw.

 

[N88]

Bell's theorem still applies only to classical theories. It tells us what inequality a classical local theory must necessarily satisfy. It doesn't tell us anything about an inequality that a quantum theory (or any other theory with a non-simplicial state space) must satisfy, no matter whether it is local or not. The violation of the inequality thus only proves that those theories that predict its satisfaction must be excluded (i.e. all classical local theories).

But don't we have good reason to say that it is the classicality condition that provides the problem, not the locality condition?

In d'Espagnat's article http://www.scientificamerican.com/media/pdf/197911_0158.pdf at page 166 (and endorsed by Bell at page 147 in his 2004 book) we find: "These conclusions require a subtle but important extension of the meaning assigned to the notation A⁺ … … … ."

To me, that subtle but important extension seems to be exactly the classicality condition that Bell (1964) assigns to his λ? So, in the context of the OP, this Bell-endorsed "subtle but important extension" appears to be the false "classical" view that QM objects (which may have some properties in common, like spin s = 1/2) DO NOT have the properties measured in d'Espagnat's article and in typical Bell-tests prior to measurement? So reject the classicality of QM objects and retain locality?

 

[atyy]

I'm not sure what you mean by "apply", but the direction of the argument is as follows:
1. Bell's theorem: Every classical local theory must satisfy Bell's inequality.
2. Quantum theory violates Bell's inequality.
3. Therefore, quantum theory can't be embedded into a classical local theory.
You want to run the argument backwards, but that doesn't work. Bell's theorem is only a theorem about classical local theories. But maybe I just don't understand what conclusion you want to draw.

I agree with all three statements. Statement #3 is about quantum theory, so in that sense I would say that Bell's theorem does apply to quantum theory.

 

[N88]

I agree with all three statements. Statement #3 is about quantum theory, so in that sense I would say that Bell's theorem does apply to quantum theory.

It seems to me that Bell's theorem IS RELEVANT to quantum theory. So is this correct? Bell's theorem is relevant to the OP and QM because it shows that, prior to measurement and unlike typical classical objects, quantum objects do NOT have the properties measured in Bell-tests until they are measured.

 

[rubi]

But don't we have good reason to say that it is the classicality condition that provides the problem, not the locality condition?

In d'Espagnat's article http://www.scientificamerican.com/media/pdf/197911_0158.pdf at page 166 (and endorsed by Bell at page 147 in his 2004 book) we find: "These conclusions require a subtle but important extension of the meaning assigned to the notation A⁺ … … … ."

To me, that subtle but important extension seems to be exactly the classicality condition that Bell (1964) assigns to his λ? So, in the context of the OP, this Bell-endorsed "subtle but important extension" appears to be the false "classical" view that QM objects (which may have some properties in common, like spin s = 1/2) DO NOT have the properties measured in d'Espagnat's article and in typical Bell-tests prior to measurement? So reject the classicality of QM objects and retain locality?

The problem is either the classicality condition or the locality condition. I (and most physicists) would blame the classicality condition, since locality is probably the most successful heuristic we have in physics and dropping it would generate more problems than it solves, while dropping classicality seems to generate no intrinsic problems apart from being unintuitive. But of course everyone is free to choose their own conclusion, as long as they acknowledge that such a choice exists.

I agree with all three statements. Statement #3 is about quantum theory, so in that sense I would say that Bell's theorem does apply to quantum theory.

Well ok, you can put it that way, although I would prefer to say that Bell's theorem has consequences for QM rather than that it applies to QM, but that's just semantics. Those consequences are that QM cannot be both classical and local.

 

[atyy]

Well ok, you can put it that way, although I would prefer to say that Bell's theorem has consequences for QM rather than that it applies to QM, but that's just semantics. Those consequences are that QM cannot be both classical and local.

Yes, I believe it is just semantics between Werner and Maudlin. If in some sense one can say that Bell's theorem applies to QM, then it is true in some sense that Bell's theorem applies to theories whose state space is not a simplex (since the state space of QM is not a simplex).

 

[rubi]

Yes, I believe it is just semantics between Werner and Maudlin. If in some sense one can say that Bell's theorem applies to QM, then it is true in some sense that Bell's theorem applies to theories whose state space is not a simplex (since the state space of QM is not a simplex).

The problem with Maudlin isn't whether Bell's theorem applies to QM or not, but rather what the assumptions of Bell's theorem are. Maudlin claims that there is no assumption of classicality. The criticism is directed only towards this claim and this is more than just semantics. If there were no classicality assumption, then the violation of Bell's inequality would prove that QM is non-local. However, the classicality assumption is crucial and this is what Werner points out. Maudlin is objectively wrong when he claims that classicality is not an assumption.

 

[N88]

The problem is either the classicality condition or the locality condition. I (and most physicists) would blame the classicality condition, since locality is probably the most successful heuristic we have in physics and dropping it would generate more problems than it solves, while dropping classicality seems to generate no intrinsic problems apart from being unintuitive. But of course everyone is free to choose their own conclusion, as long as they acknowledge that such a choice exists. … …. Well ok, you can put it that way, although I would prefer to say that Bell's theorem has consequences for QM rather than that it applies to QM, but that's just semantics. Those consequences are that QM cannot be both classical and local.

Thanks, BUT: I see nothing counter-intuitive in expecting that sensitive objects (quantum objects) would be modified by measurements. So why would ANY physicist hold to the classical here? Why not, without question, reject the classical and retain the successful heuristic of locality? For, at the order of the quantum level, even classical objects are modified by measurements. Like the (now slightly dented) wall I just measured so that my partner could hang a picture "dead-center". (The external corner of the wall now dented by the measurement alone; even though, so far, only I have spotted it.)

 

[atyy]

The problem with Maudlin isn't whether Bell's theorem applies to QM or not, but rather what the assumptions of Bell's theorem are. Maudlin claims that there is no assumption of classicality. The criticism is directed only towards this claim and this is more than just semantics. If there were no classicality assumption, then the violation of Bell's inequality would prove that QM is non-local. However, the classicality assumption is crucial and this is what Werner points out. Maudlin is objectively wrong when he claims that classicality is not an assumption.

But it depends on what one is talking about when discussing whether the classicality assumption. The classicality assumption is needed in the definition of locality uses, but it is not needed in what Bell's theorem applies to (eg. QM), so if it is the latter that Maudlin is talking about, then he is correct.

 

[rubi]

Thanks, BUT: I see nothing counter-intuitive in expecting that sensitive objects (quantum objects) would be modified by measurements. So why would ANY physicist hold to the classical here? Why not, without question, reject the classical and retain the successful heuristic of locality? For, at the order of the quantum level, even classical objects are modified by measurements. Like the (now slightly dented) wall I just measured so that my partner could hang a picture "dead-center". (The external corner of the wall now dented by the measurement alone; even though, so far, only I have spotted it.)

Well, I and most people do reject classicality and keep locality. However, dropping classicality is worse than just saying "measurements modify the state", since that is also possible in a classical theory. Of course, having to drop even one of them is unfortunate, since both appear intuitive.

But it depends on what one is talking about when discussing whether the classicality assumption. The classicality assumption is needed in the definition of locality uses, but it is not needed in what Bell's theorem applies to (eg. QM), so if it is the latter that Maudlin is talking about, then he is correct.

Maudlin believes that Bell proved the following: "Every local theory, be it classical or not, satisfies Bell's inequality." This is definitely wrong and it spoils the rest of his argument. He wants to argue:
1. Every local theory, be it classical or not, satisfies Bell's inequality.
2. Quantum theory violates Bell's inequality.
3. Therefore quantum theory is not a local theory.
However, his statement of (1) is false and consequently, (3) is false as well, since it is based on a false premise. So the paper contains a severe mistake and Werner is right to criticize it.

 

[N88]

Well, I and most people do reject classicality and keep locality. However, dropping classicality is worse than just saying "measurements modify the state", since that is also possible in a classical theory. Of course, having to drop even one of them is unfortunate, since both appear intuitive.

I do not see why "classicality" of the type invoked by d'Espagnat and Bell appears intuitive. Locality, yes. Such classicality, no (for me). So could you expand on why you consider the dropping of such classicality is "unfortunate" and worse than just saying "measurements modify the state".

 

[atyy]

Maudlin believes that Bell proved the following: "Every local theory, be it classical or not, satisfies Bell's inequality." This is definitely wrong and it spoils the rest of his argument. He wants to argue:
1. Every local theory, be it classical or not, satisfies Bell's inequality.
2. Quantum theory violates Bell's inequality.
3. Therefore quantum theory is not a local theory.
However, his statement of (1) is false and consequently, (3) is false as well, since it is based on a false premise. So the paper contains a severe mistake and Werner is right to criticize it.

But Maudlin quite clearly qualifies his locality as "EPR-local", which is one of the usual synonyms for classical local causality.

 

[rubi]

I do not see why "classicality" of the type invoked by d'Espagnat and Bell appears intuitive. Locality, yes. Such classicality, no (for me). So could you expand on why you consider the dropping of such classicality is "unfortunate" and worse than just saying "measurements modify the state".

Well, for instance non-classicality implies that a particle can't have both a position and a momentum. How do you interpret this? Mathematically, it is not a problem, but I don't think it is intuitive.

But Maudlin quite clearly qualifies his locality as "EPR-local", which is one of the usual synonyms for classical local causality.

Maudlin believes that EPR-locality implies the conditions that are needed to prove Bell's theorem. However, EPR-locality does not imply classicality. It only implies a weak form of determinism, which can also be satisfied by theories that are formulated on non-simplicial state spaces. In order to prove Bell's theorem, you must make the additional assumption that the state space is a simplex. EPR-locality isn't enough.

Edit: If you claim that EPR-locality implies that the state space is a simplex, then I demand a proof for that.

 

[atyy]

Maudlin believes that EPR-locality implies the conditions that are needed to prove Bell's theorem. However, EPR-locality does not imply classicality. It only implies a weak form of determinism, which can also be satisfied by theories that are formulated on non-simplicial state spaces. In order to prove Bell's theorem, you must make the additional assumption that the state space is a simplex. EPR-locality isn't enough.

Edit: If you claim that EPR-locality implies that the state space is a simplex, then I demand a proof for that.

Hmmm, EPR locality is so vague that usually one just defines it to be classical local causality. At the heuristic level, there are two notions of locality (1) no superluminal transmission of information (2) classical local causality. Since EPR were not talking about (1), it is usually assumed that they were talking about (2). Operational QM is local in sense (1), but not (2). Is operational QM local in a sense that is neither (1) nor (2)?

 

[rubi]

Hmmm, EPR locality is so vague that usually one just defines it to be classical local causality.

Well Maudlin's argument is based on the idea that the assumptions of Bell's theorem are implied by the things he says earlier. He can't just define his earlier comments to prove the assumptions. Either they do, or they don't and if they don't, his argument is incomplete and he must admit that he needs an extra assumption.

At the heuristic level, there are two notions of locality (1) no superluminal transmission of information (2) classical local causality. Since EPR were not talking about (1), it is usually assumed that they were talking about (2).

(1) and (2) are not mutually exclusive, so you can't argue that EPR-locality must be either (1) or (2). I agree that it is a vague concept, but that doesn't free us from the obligation to make it formal if we want to use it in a mathematical argument like Bell's theorem.

Operational QM is local in sense (1), but not (2). Is operational QM local in a sense that is neither (1) nor (2)?

Every locality condition must imply (1), so QM is certainly local in the sense of (1). However, it can still satisfy a stronger locality condition in the sense of my post #22. The fact that Maudlins argument fails to imply a simplicial state space means that we still have the choice between rejecting classicality and rejecting locality. It is perfectly possible that there is no spooky action at a distance in QM.

 

[atyy]

Well Maudlin's argument is based on the idea that the assumptions of Bell's theorem are implied by the things he says earlier. He can't just define his earlier comments to prove the assumptions. Either they do, or they don't and if they don't, his argument is incomplete and he must admit that he needs an extra assumption.

Well, I won't defend Maudlin that far. I'm not a big fan of arguing from EPR. I prefer to just define classical local causality. I will say Maudlin is not a crackpot, and overall his message is not very far from what everyone agrees with: QM is local by no signalling, and not local by classical local causality.

Every locality condition must imply (1), so QM is certainly local in the sense of (1). However, it can still satisfy a stronger locality condition in the sense of my post #22. The fact that Maudlins argument fails to imply a simplicial state space means that we still have the choice between rejecting classicality and rejecting locality. It is perfectly possible that there is no spooky action at a distance in QM.

But is there really something between (1) and (2) that operational QM satisfies? There is a notion, but as far as I know, the notion is empty in operational QM. Looking at the other article by Zukowski and Brukner you linked to in post #20, they basically say locality should be defined as no superluminal signalling. Maudlin explicitly says QM is local if one defines it as no superluminal signalling.

Also, it seems (according to Maudlin) that Werner says that QM is local if we take the epistemic state to be the physical state. Isn't that problematic? How is that different from saying that the wave function is real, which as you agreed does make QM nonlocal?

 

[zonde]

Non-simplicial state spaces can also account for some degree of determinism.

I am trying to understand what Werner means by "assumption of simplex state space".
I found this:
"According to Maudlin, Bell makes no assumption of “realism” or (as I called it in my reply) of “classicality” (in short “C”), or a hidden-variable description."
And this:
"The first issue is the explanation of classicality “C”. I gave a technical definition, the simplex property, ... "

So as I understand "simplex state space" is basically the same as "hidden-variable description", right?

 

[zonde]

Isn't this the flaw in Maudlin's argument: "If they did not, then S1 must have been physically determined in how it would behave all along."

In the context of the OP question QM objects do not have properties until measured? I say that they do have SOME properties (spin s = 1/2, for example) before measurement. So, questioning Maudlin: S1 has properties that are correlated with those of its twin and these properties physically determine how it behaves all along; so, similar to human twins, there should be no mystery in the independent behaviour of widely-separated twins being correlated in Bell-tests?

Maudlin explains EPR dilemma in simple words:
you either say that entangled particles are like identical twins and therefore give perfectly correlated measurement outcomes under matching conditions
or
they secretly communicate instantaneously over unlimited distances.

And to be on the safe side we can state it more correctly by speaking about physical configuration in local neighborhood of detection events rather than particle properties alone.