I QM: Interesting View - Get the Inside Scoop

[vanhees71]

? What is nice about this paper?
It is essentially devoid of physics, replacing it by a set of should's for the personal beliefs of individual agents, without telling how the agents come to a mutual, objective understanding of the physical world.

What is nice about this paper is the emphasis of locality, which is in accordance with locality in relativistic QFT. As I wrote today somewhere already in this thread, I also don't like the subjective interpretation of probabilities. The association with a (pure or mixed) state to preparation procedures is far better than just a subjective guess as all the highly accurate descriptions of real-world experiments with quanta demonstrates.

 

[A. Neumaier]

The association with a (pure or mixed) state to preparation procedures is far better than just a subjective guess as all the highly accurate descriptions of real-world experiments with quanta demonstrates.

Yes, but in your discussion of the postulates you also linked the mixed state to knowledge, which is a subjective entity. You also call the collapse subjective though it is objectively measurable for light passing a polarization filter.

qBism is at least consistent in this respect, while you mix subjectivity and objectivity to a syncretistic religion...

 

[facenian]

In relativistic QFT a very important defining ingredient is locality aka microcausality of local observables, i.e., the Hamiltonian density must commute with all local observables at spacelike separation of the arguments of the corresponding operators. That's the only solid definition of locality I know of, and it's sufficient to exclude spooky actions at a distance.

I agree that it is solid as a "definition". However, it is not clear to me that it excludes spooky action unless you again define the absence of spooky action by the commutativity of operators.
I guess It is hard to separate ideas from words.

 

[vanhees71]

Of course a mixed state also reflects some knowledge about the system as any probability distribution does. As I said, you need additional concepts to associate the "right" stat. op. to a situation of incomplete knowledge. One very succuessful use is the maximum entropy principle with the von Neumann-Shannon-Jaynes entropy. I know that you don't like this approach for whatever reason, but it works well in practice.

In which sense is collapse objectively measurable for light passing a polarization filter? For a single photon the photon gets absorbed or is let through as a whole and if the photon goes through its polarized in a direction given by the orientation of the polarization filter. There's no need for non-local interactions as claimed by a collapse but just the local interaction of the em. field with the filter.

 

[vanhees71]

I agree that it is solid as a "definition". However, it is not clear to me that it excludes spooky action unless you again define the absence of spooky action by the commutativity of operators.
I guess It is hard to separate ideas from words.

If the Hamilton density commutes with any local observable at space-like separated distances by construction there cannot be faster-than light interactions. That together with Poincare invariance of the S-matrix is why one makes this assumption to begin with.

 

[A. Neumaier]

In which sense is collapse objectively measurable for light passing a polarization filter? For a single photon the photon gets absorbed or is let through as a whole and if the photon goes through its polarized in a direction given by the orientation of the polarization filter. There's no need for non-local interactions as claimed by a collapse but just the local interaction of the em. field with the filter.

The collapse is not a nonlocal interaction; nobody claims that when they use this term.

The collapse is the fact that (due to the local interaction of the em. field with the filter) the ingoing photon can have any state but the outgoing photon (if there is one) is in the eigenstate of the projector to the polarization direction.

Thus, in the standard terminology, the state of the ingoing photon collapsed. This is the consensus of the physics community even though you don't use this terminology (and in fact misinterpret the concept in discussions).

 

[facenian]

Bell nonlocality is not a statement that the dynamics of microsystems is nonlocal. It is only a statement that if modeled by a classical hidden variable theory, quantum mechanics would be nonlocal.

I completely disagree with this view! Unfortunately, it is very widespread among working physicists. Shut up and calculate is a good strategy to make progress but should not be taken to the point of annihilating critical and logical thinking.
That statement is a mere tautology. It is not surprising that different theories give different predictions, therefore, predict different bounds for the result of a certain experiment, e.g., a Bell test experiment.

Indeed, Bell's assumption are completely independent of quantum mechanics!

Of course, they are!. It is about evaluating whether certain spooky predictions made by QM can find a local explanation by shifting to another theory. The relevant result is that we can't. Rejecting an explanation because it is purportedly classical means that QM's spooky predictions have no local explanation at all.
The only way out is to declare the absence of nonlocality by decree or reformulate our concept of locality more coherently. Fortunately, that can be done by shifting locality to no-signaling.

 

[A. Neumaier]

It is about evaluating whether certain spooky predictions made by QM can find a local explanation by shifting to another theory.

No. It is about evaluating whether certain predictions made by local quantum physics can also find a local explanation by shifting to a local deterministic (or classically probabilistic) theory.

 

[facenian]

No. It is about evaluating whether certain predictions made by local quantum physics can also find a local explanation by shifting to a local deterministic (or classically probabilistic) theory.

You said local quantum physics. Then I have to agree, you just have declared that QM is a local theory. Period.

 

[A. Neumaier]

You said local quantum physics. Then I have to agree, you just have declared that QM is a local theory. Period.

Local quantum physics in the sense of Haag's book is local in a very well-defined, meaningful sense. In spite of Bell's theorem (which looks for underlying hidden variables and proves that these hidden variables would have to be nonlocal) and the corresponding experiments (which agree with local quantum physics).

 

[facenian]

Local quantum physics in the sense of Haag's book is local in a very well-defined, meaningful sense. In spite of Bell's theorem (which looks for underlying hidden variables and proves that these hidden variables would have to be nonlocal) and the corresponding experiments (which agree with local quantum physics).

Once more, let me agree with you to a certain extent. The Bell theorem should have one unambiguous interpretation: local non-conspiratorial hidden variables cannot mimic QM or describe the real world.
The controversial part of the Bell theorem is its implications regarding QM's locality. The other issue that should not be controversial is that the last point is controversial.
I hope we can at least agree on that.

 

[PeterDonis]

The controversial part of the Bell theorem is its implications regarding QM's locality.

There are no such implications (although many discussions of Bell's theorem claim, implicitly or explicitly, that there are). All Bell's theorem tells us about QM is that QM cannot be a local hidden variable theory of the kind that Bell's theorem applies to. Bell's theorem does not show that a local hidden variable of that type is the only possible local theory; it simply doesn't address that general question either way.

 

[A. Neumaier]

The controversial part of the Bell theorem is its implications regarding QM's locality. The other issue that should not be controversial is that the last point is controversial.
I hope we can at least agree on that.

No, I am on the opposite side of you in this.

I find it obvious that Bell theorem says nothing regarding QM's locality, since the latter does not work with hidden variables. Once you drop these, Bell's theorem is vacuous.

Whatever controversy there is, it is because the concepts are used in a muddled way.

 

[facenian]

No, I am on the opposite side of you in this.

I find it obvious that Bell theorem says nothing regarding QM's locality, since the latter does not work with hidden variables. Once you drop these, Bell's theorem is vacuous.

Whatever controversy there is, it is because the concepts are used in a muddled way.

I have no doubts that for you is obvious. But saying that it is not controversial is denying the existence of others for whom that opposite is obvious or dismissing them as crackpots. I hate to rely on authority but I think that nobody would consider someone like Gerardus t' Hooft a crackpot. Well, he takes very seriously the nonlocality implied by the Bell theorem to the point that the resolves the nonlocality problem by recoursing to a violation of statistical independence(one of the two hypotheses necessary to derive the Bell inequality). Please do not tell me that he does not understand QFT.

One last logical point I would like to point out. The local character of a given natural phenomenon should be possible to evaluate in a theory-independent way. I don't find it cogent that the local character of a given phenomenon is dependent on the class of theory used to evaluate it. The spookiness of the phenomenon is present or absent independently of which theory describes it.
See for instance this paper https://arxiv.org/abs/2102.07524

 

[A. Neumaier]

Gerardus t' Hooft a crackpot. Well, he takes very seriously the nonlocality implied by the Bell theorem to the point that the resolves the nonlocality problem

Of course once someone looks for an alteration of quantum theory in terms of hidden variables, which is a perfectly meaningful enterprise, then Bell's theorem is relevant. But only then!

On the other hand, I always thought that not the nonlocality problem but the measurement problem motivated t' Hooft to look for hidden variable theories. Do you have clear evidence to your alternative view of his motivation?

The local character of a given natural phenomenon should be possible to evaluate in a theory-independent way.

No, because even to tell what 'local' means needs some theory! It is there where Bell and Haag differ.

this paper https://arxiv.org/abs/2102.07524

It just says that there are two different definitions of locality that are not in contradiction. This reinforces the fact that the notion of locality is not theory-independent.

 

[facenian]

It just says that there are two different definitions of locality that are not in contradiction.

I agree.

This reinforces the fact that the notion of locality is not theory-independent.

I disagree with this part. If you want to test the local character of a theory, the concept must be applicable to it.
In the example that concerns us here, QM does not pass the test for "local-causality" as defined by Bell and shown in the paper. However, QM passes the test for signal-locality.
I strongly opposed the vacuous dictum "QM is local because the Bell theorem is a classical result". That kind of loose talking has led to heated debates that lead nowhere like the exchange between Tim Maudling and Reinhard Werner.
The misunderstanding is worsened by popular and widespread incorrect derivations of the Bell inequalities like the ones based on counterfactual definiteness.

 

[A. Neumaier]

In the example that concerns us here, QM does not pass the test for "local-causality" as defined by Bell and shown in the paper. However, QM passes the test for signal-locality.

This reinforces the fact that the notion of locality is not theory-independent. One theory (hidden variables) says locality means Bell locality (what you call "local-causality" as defined by Bell), and one theory (of quantum field theory) says that locality means spacelike commutativity of fields (which implies what you call signal-locality). Both concepts are theory laden, just in different ways.

 

[AndreiB]

Of course once someone looks for an alteration of quantum theory in terms of hidden variables, which is a perfectly meaningful enterprise, then Bell's theorem is relevant. But only then!

This is true, but you should not forget that without hidden variables physics must be non-local as EPR proved.

Just look at the abstract of bell's paper:

On the Einstein Podolsky Rosen paradox
https://cds.cern.ch/record/111654/files/vol1p195-200_001.pdf

"THE paradox of Einstein, Podolsky and Rosen was advanced as an argument that quantum mechanics
could not be a complete theory but should be supplemented by additional variables. These additional vari
ables were to restore to the theory causality and locality "

OK, so you need hidden variables to restore locality. Bell investigated the only remaining local option, hidden variables. Any other theory is already shown to be non-local (if fundamental/complete).

 

[AndreiB]

one theory (of quantum field theory) says that locality means spacelike commutativity of fields (which implies what you call signal-locality).

If you assume QFT to be complete/fundamental it has to be non-local in the sense that one measurement has a causal influence on the other, space-like measurement. If QFT is seen as a statistical approximation it can be local, no problem.

 

[A. Neumaier]

If you assume QFT to be complete/fundamental it has to be non-local in the sense that one measurement has a causal influence on the other, space-like measurement.

No. Only that correlations are nonlocal. They are expressible in terms of 2-point functions which are manifestly nonlocal.

This is true, but you should not forget that without hidden variables physics must be non-local as EPR proved.

... based on the assumption that the detection events are particle properties. But mathematical models where the detector is treated by QM but light as external classical field predict a photoeffect with Poisson distributed events, just like the standard QM treatment for coherent light. Since the model has no particles, this proves that the detection events cannot be particle properties. They are artifacts of the binary yes-no nature of the detector.

Bell investigated the only remaining local option, hidden variables. Any other theory is already shown to be non-local (if fundamental/complete).

Bell assumed that the things measured are local variables and found a contradiction. But he didn't realize that the variables instead of the dynamics could be nonlocal. Classical mechanics is already full of nonlocal expressions which when measured do not behave like Bell's theorem claims - they don't satisfy his hypothesis.

I completed quantum mechanics without introducing additional variables. Reinterpreting what is already in the formalism was enough.

 

[AndreiB]

No. Only that correlations are nonlocal.

A correlation is a correlation. Two synchronized clocks are correlated. This has nothing to do with non-locality. What can be non-local is the mechanism by which those correlations are enforced. In the case of synchronized clocks the mechanism is local, past communication with a master clock.

... based on the assumption that the detection events are particle properties.

No such assumption is made by EPR. The argument assumes only locality and that the results are correctly predicted by QM. The physical embodyment of the hidden variables does not make any difference. It could be particles, fields, whatever.

The argument is really simple. If you can perfectly predict the outcome of a measurement, that measurement is predetermined. Whatever it is that determines it is called a hidden variable. It need not be a particle.

If the result were genuinely random you could only predict it with 50% accuracy on average. So, the assumption that EPR measurements are random has been falsified by our 100% prediction accuracy.

The only way to avoid hidden variables is to postulate that whatever measurement is first instantly determines the second, so a non-local mechanism.

Classical mechanics is already full of nonlocal expressions which when measured do not behave like Bell's theorem claims - they don't satisfy his hypothesis.

What do you have in mind?

I completed quantum mechanics without introducing additional variables. Reinterpreting what is already in the formalism was enough.

Can you please provide your local explanation of the EPR perfect anticorrelations in terms of your thermal interpretation?

 

[facenian]

This reinforces the fact that the notion of locality is not theory-independent. One theory (hidden variables) says locality means Bell locality (what you call "local-causality" as defined by Bell), and one theory (of quantum field theory) says that locality means spacelike commutativity of fields (which implies what you call signal-locality). Both concepts are theory laden, just in different ways.

Actually, no. Local causality(LC) and signal causality are concepts equally applicable to QM and hidden variables(HV).
If local causality made sense only with HV theories, it wouldn't make sense to say that QM violates it. This is a logical problem. You can't ask what's the taste of the color yellow (excluding synaesthesia). See discussion in section IV of the paper https://arxiv.org/abs/2102.07524, it is quite short and easy to read.
Pardon my insistence but the statement claiming that QM is local because the Bell inequality is a classical result is a vacuous meaningless tautology. It implies that poor John Bell was a moron.

 

[A. Neumaier]

A correlation is a correlation. Two synchronized clocks are correlated. This has nothing to do with non-locality.

Correlations at different points in space are nonlocal, by definition, because they depend on what happens at both points. No matter how you try to explain them.

Can you please provide your local explanation of the EPR perfect anticorrelations in terms of your thermal interpretation?

I discuss locality in Section 4.4-4.5 of Part II of my preprint series, and in Chapter 13.5-13.6 of my book.

 

[facenian]

No such assumption is made by EPR. The argument assumes only locality and that the results are correctly predicted by QM. The physical embodyment of the hidden variables does not make any difference. It could be particles, fields, whatever.

In fact, there is a problem here with regards to the EPR paper. EPR introduced a metaphysical concept, i.e., "elements of physical reality". This has produced much unnecessary confusion to this day. Einstein immediately reacted against this and explained (in a letter to Schrodinger) how to correctly argue against completeness. Bell referred to the EPR paper but never mentioned "elements of physical reality". Bell certainly was a clear thinker.

 

[A. Neumaier]

If local causality made sense only with HV theories, it wouldn't make sense to say that QM violates it. [...] See discussion in section IV of the paper https://arxiv.org/abs/2102.07524, it is quite short and easy to read.

I didn't find there a theory-independent definition of local causality. Without reference to a theory one can neither define cause and effect nor the meaning of local.

EPR introduced a metaphysical concept, i.e., "elements of physical reality".

This is not only a metaphysical concept but the basis of all our physics. If experimental facts are not elements of reality then all of physics is unreal.

 

[facenian]

I didn't find there a theory-independent definition of local causality. Without reference to a theory one can neither define cause and effect nor the meaning of local.

I guess the discussion could go forever. But it is in the nature of discussing ideas that are hard to express accurately in words. All that I mean is this: to ask if a certain theory like QM (or HV) possesses the property called local causality (LC), it should be possible to test the theory against it. i.e., the property should be applicable to the theory.
In other words, it does not make sense to ask whether QM has a property called "tasty" but it does make sense to ask whether it has a property called LC.

Since we are discussing two theories: QM and hidden variables(HV) and we want to know if either theory posses a property called LC, I pointed out that the concept should be applicable to both. In that sense, and only in that sense, the concept should be theory-independent.
The alluded paper(https://arxiv.org/abs/2102.07524) shows that it makes sense to ask whether QM is locally causal and that QM does not pass the test for local causality irrespective of the Bell inequality, so QM's failure to pass the test cannot be blamed on the violation of the Bell inequality as usually is.
The Bell inequality does not prove that QM is not locally causal, it only proves that we cannot fix that with non-conspiratorial common causes.
Of course, we can argue whether LC is the appropriate concept for "locality" but that is a different discussion.
I beg your pardon if I repeated things that you already know, but perhaps someone else will read this post.

 

[A. Neumaier]

All that I mean is this: to ask if a certain theory like QM (or HV) possesses the property called local causality (LC), it should be possible to test the theory against it. i.e., the property should be applicable to the theory.

I agree, but the 'should' is a requirement, not something accomplished. It requires that the property called local causality (LC) must be defined unambiguously and theory-independent. Where is this in the paper you cited?

The Bell inequality does not prove that QM is not locally causal, it only proves that we cannot fix that with non-conspiratorial common causes.

Yes, so it is not a statement about quantum mechanics but a statement about fixing the latter by non-conspiratorial common causes.

 

[facenian]

This is not only a metaphysical concept but the basis of all our physics. If experimental facts are not elements of reality then all of physics is unreal.

Allow me to disagree with an enphatic no! QM, our best theory, does not assume that. It is metaphysical because it assumes that physical properties actually exist before measurements. Experimental facts are about measurements, not what existed before the observation. Also, determinism does not imply pre-existence as is usually claimed.
Here again is a source of usual misinterpretation with the Bell inequality: that it assumes "elements of physical reality". The Bell inequality only uses determinism(derived or assumed). It does not assume pre-existence.

 

[PeterDonis]

Allow me to disagree with an enphatic no!

You're agreeing with him, not disagreeing with him. He's saying that experimental facts are elements of reality. So are you.

 

[A. Neumaier]

Allow me to disagree with an enfátic no! QM, our best theory, does not assume that. It is metaphysical because it assumes that physical properties actually exist before measurements. Experimental facts are about measurements, not what existed before the observation. Also, determinism does not imply pre-existence as is usually claimed.

Detectors are elements of our macroscopic reality in Einsteins sense. Measurements are elements of our macroscopic reality once measured. Without elements of reality, there is no objective physics.

The question is whether electrons have such elements of reality. Independent of Bell's experiments I don't think they have, except in semiclassical situations. But quantum fields have such elements of reality. This is explained in my book.

Here again is a source of usual misinterpretation with the Bell inequality: that it assumes "elements of physical reality". The Bell inequality only uses determinism(derived or assumed). It does not assume pre-existence.

I know that Bell does not assume elements of reality. But in his conceptual basis he assumes beables (in the form of hidden variables). These are his strong substitute for elements of reality.

 

[facenian]

You're agreeing with him, not disagreeing with him. He's saying that experimental facts are elements of reality. So are you.

Not in the EPR sense. This is difficult as I pointed out before. You perhaps are right he said "elements of reality" not "EPR elements of reality".

 

[vanhees71]

No. Only that correlations are nonlocal. They are expressible in terms of 2-point functions which are manifestly nonlocal.

What do you mean by "nonlocal" here? That the correlations are observable with space-like separated local measuremurements? That's of course true (e.g., for the polarization measurments on two photons in a Bell state). But also this doesn't contradict anything concerning the locality (microcausality) of QFT.

 

[A. Neumaier]

What do you mean by "nonlocal" here? That the correlations are observable with space-like separated local measuremurements? That's of course true (e.g., for the polarization measurments on two photons in a Bell state). But also this doesn't contradict anything concerning the locality (microcausality) of QFT.

Local = depending on one position.
Nonlocal = depending on two positions instead of one.
That correlations are nonlocal is a triviality and doesn't contradict anything concerning the locality (microcausality) of QFT.

 

[facenian]

Local = depending on one position.
Nonlocal = depending on two positions instead of one.
That correlations are nonlocal is a triviality and doesn't contradict anything concerning the locality (microcausality) of QFT.

Different positions in space are not sufficient for nonlocality. Position in time is also necessary. Events are nonlocal when space-like separated. Correlations can have local common causes or non-local common causes and finally do they have to be causally connected?
I think that by construction QFT is built to comply with necessary conditions of locality, not with sufficient conditions. Besides is built upon the principles of ordinary QM that is manifestly non-local in the sense of Bell's local causality (again this does not imply hidden variables or the Bell inequality)

 

[A. Neumaier]

Different positions in space are not sufficient for nonlocality. Position in time is also necessary. Events are nonlocal when space-like separated.

I was talking for simplicity about a fixed time.

Correlations can have local common causes or non-local common causes and finally do they have to be causally connected?

The definition of correlations is independent of this.

I think that by construction QFT is built to comply with necessary conditions of locality, not with sufficient conditions.

It is thought to be necessary and sufficient, though the mathematical analysis is currently too hard to prove it. But the causal framework of Haag is a sufficient condition.

Besides is built upon the principles of ordinary QM that is manifestly non-local

Ordinary QM of multiple particles is not relativistic, hence has no reason to be local in any sense. Newton's mechanics is also not local! Neither is the heat equation!

in the sense of Bell's local causality (again this does not imply hidden variables or the Bell inequality)

You still lack proof that your LC can be applied to QM. Where is the precise theory-independent definition of LC that contradicts QM?

 

[vanhees71]

In special relativity two events can be causally connected if and only if one event is in or on the light cone of the other event, because such and only such events have a frame-independent temporal order.

Space-like separated events cannot be causally connected, because they don't have a frame-independent temporal order.

Locality of a theory like classical or quantum field theory means that within this theory there cannot be any causal connection between space-like separated events. In other words there cannot be faster-than-light signal propagation.

I don't know what you mean by "ordinary QM". If you mean non-relativistic QM, then locality is not an issue anyway, because here by construction the temporal order of events is absolute anyway and there is thus no constraint on signal velocity at all. Indeed, in Newtonian physics instantaneous actions at a distance are the standard description, and Newton worried about this indeed in connection with his theory of the gravitational interaction. Today we know of course that Newton's worries were quite justified.

 

[facenian]

You still lack proof that your LC can be applied to QM. Where is the precise theory-independent definition of LC that contradicts QM?

Equations (7) and (8) in this paper https://arxiv.org/abs/2102.07524
As I said before, we can make QM local by shifting to another definition. The problem is that people usually don't do that and utter trivial tautologies devoid of any meaning like "QM is local because the Bell theorem is a classical result".

 

[A. Neumaier]

Equations (7) and (8) in this paper https://arxiv.org/abs/2102.07524

These equations assume the hidden variables $\lambda$, hence they do not apply to QM.

 

[facenian]

These equations assume the hidden variables $\lambda$, hence they do not apply to QM.

Please read from the initial paragraph above eq (7) "The fact of matter..."
There are no "hidden variables" when you take lambda to be the singlet state. It is just ordinary QM.
(Try to read it objectively without prejudices)

 

[A. Neumaier]

There are no "hidden variables" when you take lambda to be the singlet state. It is just ordinary QM.

It is not just ordinary QM, since you take the state to be the complete hidden variable that each particle carries. (8) only means that the state is not the complete common cause of the detection events. Nothing about locality is implied (or assumed) in such a simple argument!

Indeed, according to the statistical interpretation of QM, the state is not assigned to a single particle but only to an ensemble of identically prepared particles.

 

[facenian]

No - you take the state to be a hidden variable that each particle carries.

But according to the statistical interpretation of QM, the state is not assigned to a single particle but only to an ensemble of identically prepared particles.

Of course, I take the state to be the only hidden variable. That means you do not introduce anything foreign into the theory and you can apply the LC concept to the theory without changing or perturbing it.
There are no particles carrying anything or state assigned to anything. There are only two predictions made according to the laws of quantum physics that notoriously fail to pass the test. It is a laconic mathematical expression that ordinary QM does not pass.

 

[A. Neumaier]

You still lack proof that your LC can be applied to QM. Where is the precise theory-independent definition of LC that contradicts QM?

Equations (7) and (8) in this paper https://arxiv.org/abs/2102.07524

Why are these two equations a precise theory-independent definition of LC?

I see neither an encoding of local nor one of causality. (7) is just a denial of the independence of $a$ and $b$ given $\lambda$, which is obvious when you set $a=b$ - even classically.

 

[PeterDonis]

Thread closed for moderation.

 

[berkeman]

After a Mentor discussion and some edits, the thread is reopened.

 

[AndreiB]

In fact, there is a problem here with regards to the EPR paper. EPR introduced a metaphysical concept, i.e., "elements of physical reality". This has produced much unnecessary confusion to this day.

I don't see what is confusing about "elements of physical reality". It's a placeholder for some unknown physical property. It can be a field magnitude or a particle property or who knows what. The argument is clear and simple, unlike Bohr's response to it.

The mistake of EPR was to insist on the simultaneous existence of non-commuting properties. This is not necessary. You measure both particles on Z and you conclude that those measurements must be predetermined (hidden variables) or that one measurement determines the other (non-locality). Why bother with the X or Y spin at all? EPR's choice to introduce counterfactuals allowed Bohr to attack the argument.

 

[AndreiB]

Correlations at different points in space are nonlocal, by definition, because they depend on what happens at both points. No matter how you try to explain them.

OK, then a chess table is a non-local object according to your definition. Such "non-locality" is of no concern since it does not contradict relativity. The type of non-locality which is forced upon us by the EPR argument (if hidden variables are denied) does contradict relativity because you need to postulate a causal link between space-like measurements. Let's call this relevant type of nonlocality FTLC (faster then light causality).

I discuss locality in Section 4.4-4.5 of Part II of my preprint series, and in Chapter 13.5-13.6 of my book.

I red those chapters but I admit I do not understand what is your explanation for the perfect EPR anti correlations (I think it's better to only discuss the case of measurements performed on the same axis, say Z and not bother with rotating detectors).

You say:

"The thermal interpretation explicitly acknowledges that all quantum objects (systems and subsystems) have an uncertain, not sharply definable (and sometimes extremely extended) position, hence are intrinsically nonlocal.
Thus it violates the assumptions of Bell’s theorem and its variations."

I do not think this is true. A pair of rotating billiard balls or a long rod are extended objects, "intrinsically nonlocal" - according to your definition of locality - but you cannot violate Bell's inequalities with them. At no point does Bell assume that the entangled system must consist of a single point in space which seems the only object you would call "local".

"Attempting to literally interpret the two photons in a system with an entangled 2-photon state leads
to paradoxes related to seemingly acausal nonlocal correlations."

As pointed out earlier, the EPR argument makes no assumption regarding the nature of those photons, fields, or whatever they might be. It only looks at the properly recorded measurement results.

"Whatever Alice and Bob measure far away depends on the whole 2-photon system."

So, this extended 2-photon system is a hidden variable?

"Over long distances, the uncertainty intrinsic to the 2-photon system becomes huge"

And how is this supposed to help us? Being more uncertain does not seem to help achieving the perfect predictions possible in the EPR situation. And, again, why is this relevant?

"The object becomes vastly extended – so nonlocal that the assumptions in Bell’s argument are obviously violated."

No, not any large object can pass Bell's theorem.

"Alice’s and Bob’s position are causally unrelated."

Agreed, this is what relativity implies.

"But something else from Alice becomes known to Bob faster than light – conditional information."

This is another way of saying that EPR correlations are known to exist. We know that.

"In Bell-type experiments, the conditional information and the correlations become actual only when someone (like Charles) has access to the actual data resolving the condition"

True, but irrelevant.

"It is easily seen that extended causality is observed."

This is an assertion. It does not follow from anything you said before.

You conclude:

"This doesn”t explain everything about the observed correlations"

It doesn't explain anything. If your "extended object" determine the measurement results you have a hidden-variable theory and is consistent with relativity. If your "extended object" doesn't determine the results then it must behave like a perfectly rigid object which is impossible in relativity. Which is it?

 

[gentzen]

"It is easily seen that extended causality is observed."

This is an assertion. It does not follow from anything you said before.

Maybe it doesn't follow from what he said before, but it is seen easily nevertheless. The real issue is that "extended causality" is weaker than what QFT actually provides. And A. Neumaier both knows this, and also acknowledges this:

Eberhard & Ross [13] gives a proof of causality from relativistic quantum field theory, in the sense that no faster than light communication is possible.
...
This doesn”t explain everything about the observed correlations but casts some doubt on the validity of the stringent assumptions made in derivations of Bell-type inequalites.

So you complain: "It doesn't explain anything." Well, it only removes the paradox, but the analysis how it occurs still needs to be done. A. Neumaier acknowledges this by writting:

With the thermal interpretation, the measurement problem turns from a philosophical riddle into a scientific problem in the domain of quantum statistical mechanics, namely how the quantum dynamics correlates macroscopic readings from an instrument with properties of the state of a measured microscopic system. This problem will be discussed in Part III [39].

 

[AndreiB]

Maybe it doesn't follow from what he said before, but it is seen easily nevertheless. The real issue is that "extended causality" is weaker than what QFT actually provides.

He defines extended causality like that:

"Joint properties of an extended object depend only on the union of the closed past cones of their constituent parts, and can influence only the union of the closed future cones of their constituent parts."

OK, so how the "Joint properties" of the 2-photon relate to the observed measurement outcomes? If they strictly determine the outcomes we are in the case of a hidden variable theory. If not, the 2-photon behaves as perfectly rigid rod, violating relativity. Since Neumaier explicitly agreed that space-like events cannot cause each other he should say that what he proposes is a hidden variable approach. However, he claims:

"The thermal interpretation gives a natural, realistic meaning to the standard formalism of quantum mechanics and quantum field theory in a single world, without introducing additional hidden variables."

So you complain: "It doesn't explain anything." Well, it only removes the paradox, but the analysis how it occurs still needs to be done.

The measurement problem was not the subject of our discussion. The EPR correlations were the subject, and he does not explain them. This is why I said he explains nothing. He says that the 2-photon is a fuzzy/uncertain extended object, itself obeying extended causality. OK, great. So now what? What does this object has to do with the observed correlations? We will see in part III. I've just searched part III for EPR and got nothing.

 

[gentzen]

OK, so how the "Joint properties" of the 2-photon relate to the observed measurement outcomes? If they strictly determine the outcomes we are in the case of a hidden variable theory.

The "joint properties" of the 2-photon alone don't strictly determine the outcomes. The state of the world on the other hand does strictly determine the outcome. And since you we will never exactly know the state of the world, that state could be regarded as a sort of hidden variable. But in any case, it is not a hidden local variable theory, so Bell's theorem doesn't apply.

However, what is meant by "the state of the world"?

The thermal interpretation
...
• is description-dependent but observer-independent, hence free from subjective elements;
• is about both real systems and idealized systems, at every level of idealization;

So it is not "subjective", but still description-dependent. And it could be about an idealized system, instead of the real world. Since all our models (as long as they don't include quantum gravity) are idealized, "the state of the world" will basically always be about an idealized system. And since it is not "subjective", it is not what we know or want to assert about the state, but the mathematical state itself (which could potentially contain an infinite amount of information).

The measurement problem was not the subject of our discussion. The EPR correlations were the subject, and he does not explain them. This is why I said he explains nothing.

Since "the state of the world" in the thermal interpretation is basically the quantum mechanical state, the problem is less to explain the correlations, but to explain why we observe definite measurement outcomes. And that is the measurement problem.

 

[facenian]

I don't see what is confusing about "elements of physical reality". It's a placeholder for some unknown physical property. It can be a field magnitude or a particle property or who knows what. The argument is clear and simple, unlike Bohr's response to it.

It is not confusing as a well-defined concept, it is metaphysical and produces confusion. An example of this confusion is the belief that the Bell inequality requires such a hypothesis for its derivation. Another confusion is that determinism implies it. Determinism only means you can predict a result with certainty it does not have to assume its pre-existence, however, EPR assumes the pre-existence by definition. It is not "incorrect"; it is unnecessary and metaphysical because makes an assumption about the existence of what has not been actually measured.
Einstein with his keen insight immediately reacted to this since he did not write the paper.
Unfortunately, the concept is widespread and widely used except perhaps by experts in foundations.