Is an experiment planned to discern determinism and randomness in QM

[Jazzdude]

Since relativistic local QFT is local ...

QFT is just as local as ordinary QT with a Hamiltonian generating only local interactions. The non-locality of quantum theories is facilitated by the non-local construction of the state space that allows remotely entangled states. QFT has spatially separated Bell-states, so it's non-local in that sense. This is in part what makes non-locality in quantum theory so difficult to grasp: It's not caused by non-local interactions.

 

[gentzen]

Bell's theorem does not exclude all deterministic theories. Bell's theorem requires that the hidden variables should be independent of detectors' settings. Local deterministic models that violate that assumption have been proposed, like:

Quantum mechanics from classical statistics
C. Wetterich
Annals Phys. 325 (2010) 852
DOI: 10.1016/j.aop.2009.12.006

Have you read this? Is it useful? Is it even intended as a local deterministic model? I know Wetterich is a big name, but there are certain small words in the abstract that hint at a certain arbitrariness: "Quantum mechanics can emerge from classical statistics. ... Their expectation values define a density matrix if they obey a "purity constraint". ... No concepts beyond classical statistics are needed for quantum physics - the differences are only apparent and result from the particularities of those classical statistical systems which admit a quantum mechanical description. ..."

And then we read:

"We conclude this section by showing that despite these peculiarities, we cannot use spin-correlation measurements to transmit any useful information between two macroscopically separated points."

So, Sakurai does not provide any explanation for the EPR perfect anti-corelations. He does not say if information has been transferred from A to B or not, only that "useful" information cannot be transferred.

Needless to say this is a non-explanation. Relativity does not distinguish between useful and non-useful information. It says that if A and B are space-like A cannot cause B. If A causes B you have a conflict with relativity regardless of how useful that communication may appear to you.

The point is that the non-random part which can be influenced, namely the measurement settings, does not allow to transmit any information from A to B. The random outcomes of the measurements are not independent, but they don't contain any "useful" information, because they are random. Gisin even inverted that argument, namely that the measurement results must be truly random, because otherwise they would allow to transfer "useful" information.

 

[AndreiB]

Have you read this? Is it useful? Is it even intended as a local deterministic model? I know Wetterich is a big name, but there are certain small words in the abstract that hint at a certain arbitrariness: "Quantum mechanics can emerge from classical statistics. ... Their expectation values define a density matrix if they obey a "purity constraint". ... No concepts beyond classical statistics are needed for quantum physics - the differences are only apparent and result from the particularities of those classical statistical systems which admit a quantum mechanical description. ..."

You are right, it's not the best paper to help my point, although it says some important things about Bell's theorem. I think a better example would be this paper, also from Wetterich:

Quantum fermions from classical bits:

https://arxiv.org/pdf/2106.15517.pdf

The model is a probabilistic cellular automaton. The evolution of the automaton is deterministic, but the initial state is given in terms of probabilities. As far as I understand one could view such a probabilistic automaton as an ensemble of automatons, each with a sharp value in each cell. The model is both local and realistic.

The point is that the non-random part which can be influenced, namely the measurement settings, does not allow to transmit any information from A to B. The random outcomes of the measurements are not independent, but they don't contain any "useful" information, because they are random. Gisin even inverted that argument, namely that the measurement results must be truly random, because otherwise they would allow to transfer "useful" information.

I agree with what you are saying, but I don't agree that labeling the information as "non-useful" makes its instantaneous transfer compatible with relativity, so it's a red-herring.

If a judge asks a suspect if he spoke with the victim and the suspect answers that he didn't speak with the victim in the last week, would the judge be satisfied with that answer? Probably not, since the question was not about the "last week". Similarly, Sakurai should have said that either "no information was transferred" or "information has been transferred". He avoids giving a clear answer by inventing an adjective, "useful", no one asked him about. This makes me suspect that he actually believes that some non-useful information was in fact sent from A to B but he is not willing to admit it.

 

[AndreiB]

1. Certainly it does. The EPR-Bohm thought experiment - which tended to support the EPR position - was performed after Bell and was instead used to contradict the EPR position. The EPR paper itself was not bolstered by the advent of EPR-B.

Unless the experiment refuted QM's prediction (it didn't) it couldn't have changed anything about the argument. All its premises (locality+existence of correlations) remain valid. The reason why Bell's theorem is seen as a refutation of EPR is a logical fallacy. Bell did not assume such a position and his conclusion, clearly presented in his paper, is not that the world is non-deterministic, but that the world is non-local. He also embraced a non-local deterministic theory, de-Broglie - Bohm interpretation which further confirms my understanding.

2. There are plenty of local non-deterministic interpretations still on the table. For example, time symmetric/retrocausal/ascausal interpretations. In these, the future observables of a system cannot be explained solely by prior states.

Those are not local. Relativity does not allow future to past transfer.

3. Contextuality contradicts predetermination of measurement results.

While Bohmian Mechanics (a viable interpretation) also implies predetermination of measurement results, it is considered to be contextual as well.

Can you spot a contradiction between the above statements?

4. Obviously, you cannot predict the outcome of a measurement of an entangled property of a system - it's random.

I didn't make such a claim.

Note that once you make the initial measurement on that system (say Alice's), a later similar measurement (say Bob's) would be redundant

Yes, but why is it redundant? If Bob's measurement is not predetermined and so it's a genuine "act of creation" from Bob's part, it should not be redundant. But since it is we need to conclude that Bob didn't "create" anything new, he just discovered what was already there.

EPR said that any such redundant measurement implied that ALL possible such measurements would similarly be predictable. And in a way, that is accurate - for the special case of redundant measurements. However, EPR did not account for measurements that were NOT redundant - i.e. those in which the measurements were, to some degree, orthogonal (non-commuting). Most/many entangled spin pair measurements fit that profile.

I agree with this part. The EPR argument works for the measured properties. If the X-spin was measured we can conclude that the X-spins were predetemined. We cannot say anything about the Y-spins.

The QM correlation prediction for entangled spin pair measurements is explicitly contextual, dictated only by a function of the angle between (theta). There are no other input parameters. Bell showed that this contextual formula was incompatible with the basic EPR model (which they claimed must be assumed to be non-contextual). It should be obvious that a contextual formula would be incompatible with a non-contextual model, but this took decades to be better understood.

I agree.

In summary:

a. EPR showed that the special case of redundant measurements (and locality assumed), there were no obvious contradictions between QM and a non-contextual model in which outcomes must be predetermined prior to measurement. They SPECULATED (not proved) this precluded contextual models from being viable. In their view: a measurement "here" should NOT affect the outcome of a measurement at spacelike separated "there".

b. Bell showed that in the general case of measurements on partially non-commuting observables (and locality assumed), there were obvious contradictions between QM and any non-contextual model (i.e. in which outcomes are independent of choice of measurement bases). In Bell's view: a measurement choice "here" COULD affect the statistical outcome of a measurement at spacelike separated "there".

EPR's was a special case, and is not in contradiction with Bell's general case. I would conclude per b., that no non-contextual model is viable.

I agree with that conclusion. Only contextual models are allowed. our disagreement only comes from your assumption that contextual models need to be non-deterministic.

 

[Demystifier]

This point of view seems to me pretty complete, at least FAPP

I absolutely agree that it's complete FAPP. Bell himself (who coined the FAPP acronym) also often emphasized that. But the whole point of quantum foundations is to say something beyond FAPP. It's impossible to discuss quantum foundations and discard all aspects which are beyond FAPP, at the same time. A very dishonest thing to do is to accept only those beyond FAPP aspects which fit your own philosophical prejudices and discard all the others by claiming that you only care about FAPP.

 

[vanhees71]

QFT is just as local as ordinary QT with a Hamiltonian generating only local interactions. The non-locality of quantum theories is facilitated by the non-local construction of the state space that allows remotely entangled states. QFT has spatially separated Bell-states, so it's non-local in that sense. This is in part what makes non-locality in quantum theory so difficult to grasp: It's not caused by non-local interactions.

Non-relativistic QT is not local, as is all Newtonian physics, where action at a distance is the way to describe interactions (e.g., Newton's theory of the gravitational interaction). In non-relativistic QFT the microcausality condition doesn't hold.

 

[Lord Jestocost]

It's not wrong, it's incomplete. Why? Because you don't specify what do you mean by state. Do you mean the state of an ensemble, or the state of an individual system? If you mean the ensemble, then it's incomplete because the individual system can be prepared too, on which you say nothing. If you mean the single system, then it's incomplete because you need something beyond the state in the Hilbert space (because you adopt the Ballentine interpretation according to which state in the Hilbert space only describes the ensemble, not the individual system).

Indeed, one has to specify exactly what one means by "system state". In their paper “A Proposed Solution of the Measurement Problem in Quantum Mechanics by a Hidden Variable Theory“ (Rev. Mod. Phys. 38, 453, 1966), Bohm and Bub show it in an exemplary fashion:

“Now, in one of the most widely accepted interpretations of the quantum theory, i.e., that of the Copenhagen school,^1,2 the physical state of a system is assumed to be completely specified by its wave function which, however, defines only the probabilities of results that can be obtained in a statistical ensemble of similar measurements.”

 

[Jazzdude]

Non-relativistic QT is not local, as is all Newtonian physics, where action at a distance is the way to describe interactions (e.g., Newton's theory of the gravitational interaction). In non-relativistic QFT the microcausality condition doesn't hold.

I'm afraid you're missing the point here. I explicitly stated that you have to use a Hamiltonian generating only local interactions. The point is that the locality of the theory, in the same sense you call QFT local, depends on the interactions, whereas the locality relevant for the Bell inequality emerges from the construction of the state space.

 

[AndreiB]

Ok, then tell me what's wrong with Weinberg's statement of Bell's model: ...

Where exactly is Weinberg's local explanation of the EPR correlations? I can't see it here. Can you please post that as well? The fact that his version of hidden variables does not violate Bell's inequality does not imply that without hidden variables he can explain the EPR perfect correlations. Just like in Sakurai's case, that wonderful local and non-deterministic explanation of EPR is mysteriously absent.

 

[vanhees71]

I'm afraid you're missing the point here. I explicitly stated that you have to use a Hamiltonian generating only local interactions. The point is that the locality of the theory, in the same sense you call QFT local, depends on the interactions, whereas the locality relevant for the Bell inequality emerges from the construction of the state space.

QFT is local in the sense that the Hamilton density is a local observable, and the commutator between that Hamilton density with any local observable vanishes for space-like separated arguments. That's not the case in non-relativistic quantum theory, where a typical interaction Hamiltonian is
$$\hat{H}_{\text{int}}=\frac{1}{2} \int_{\mathbb{R}^3} \mathrm d^3 x_1 \int_{\mathbb{R}^3} \mathrm{d}^3 x_2 \hat{\psi}^{\dagger}(\vec{x}_2) \hat{\psi}^{\dagger}(\vec{x}_1) \frac{q^2}{4 \pi |\vec{x}_1-\vec{x}_2|} \psi(\vec{x}_1) \psi(\vec{x}_2),$$
describing the Coulomb interaction between charged particles. $\hat{\psi}$ is the annihilation operator of the (non-relativistic) Schrödinger field. That Hamiltonian is definitely not the integral over a local operator, but of the "action-at-a-distance" type.

 

[vanhees71]

Where exactly is Weinberg's local explanation of the EPR correlations? I can't see it here. Can you please post that as well? The fact that his version of hidden variables does not violate Bell's inequality does not imply that without hidden variables he can explain the EPR perfect correlations. Just like in Sakurai's case, that wonderful local and non-deterministic explanation of EPR is mysteriously absent.

Of course Weinberg doesn't explain the EPR correlations with a local deterministic HV theory. The point is to show that it cannot be explained with such a model to begin with.

 

[Jazzdude]

QFT is local in the sense that the Hamilton density is a local observable, and the commutator between that Hamilton density with any local observable vanishes for space-like separated arguments. That's not the case in non-relativistic quantum theory, where a typical interaction Hamiltonian is
$$\hat{H}_{\text{int}}=\frac{1}{2} \int_{\mathbb{R}^3} \mathrm d^3 x_1 \int_{\mathbb{R}^3} \mathrm{d}^3 x_2 \hat{\psi}^{\dagger}(\vec{x}_2) \hat{\psi}^{\dagger}(\vec{x}_1) \frac{q^2}{4 \pi |\vec{x}_1-\vec{x}_2|} \psi(\vec{x}_1) \psi(\vec{x}_2),$$
describing the Coulomb interaction between charged particles. $\hat{\psi}$ is the annihilation operator of the (non-relativistic) Schrödinger field. That Hamiltonian is definitely not the integral over a local operator, but of the "action-at-a-distance" type.

Again, you're totally missing the point. I've never said anything about Coulomb fields or Newtonian gravity. I'm arguing that you can formulate a local Hamiltonian and still have the non-locality that is relevant for the Bell inequalities. Bell's argument is also entirely independent of such details. It only uses the fact that the state space allows for remotely entangled states, and those states are also present in QFT. So your local QFT is in the same sense non-local as ordinary QT.

 

[vanhees71]

Again, I use locality only in the sense Bell also used it: The impossibility of causal effects between space-like separated events, i.e., microcausality. The long-ranged correlations described by entanglement have nothing to do with locality or non-locality. Rather it's what Einstein calls inseparability. It's clear that local QFT is fully compatible with and describes the observed inseparability of far-distant entangled parts of quantum systems. I don't accept to call a local QFT nonlocal, because that's a contradiction in itself, and this hast to be avoided in scientific discussions.

 

[DrChinese]

DrChinese said:

In summary:

a. EPR showed that the special case of redundant measurements (and locality assumed), there were no obvious contradictions between QM and a non-contextual model in which outcomes must be predetermined prior to measurement. They SPECULATED (not proved) this precluded contextual models from being viable. In their view: a measurement "here" should NOT affect the outcome of a measurement at spacelike separated "there".

b. Bell showed that in the general case of measurements on partially non-commuting observables (and locality assumed), there were obvious contradictions between QM and any non-contextual model (i.e. in which outcomes are independent of choice of measurement bases). In Bell's view: a measurement choice "here" COULD affect the statistical outcome of a measurement at spacelike separated "there".

EPR's was a special case, and is not in contradiction with Bell's general case. I would conclude per b., that no non-contextual model is viable.

I agree with that conclusion. Only contextual models are allowed. our disagreement only comes from your assumption that contextual models need to be non-deterministic.

Contextual models don't need to be non-deterministic. For example: Bohmian Mechanics is both deterministic and contextual. However, contextual models *imply* non-determinism, as the quantum expectation is solely based on the context. But I agree, it is not a strict requirement of contextuality. Note that EPR *implies* the existence of local hidden variables, but we now know that is not a strict requirement either. So I can see why my words in earlier posts might be interpreted differently than I intended.

DrChinese said:
2. There are plenty of local non-deterministic interpretations still on the table. For example, time symmetric/retrocausal/ascausal interpretations. In these, the future observables of a system cannot be explained solely by prior states.

You replied: "Those are not local. Relativity does not allow future to past transfer." This statement is not correct on several levels. First, there are time symmetric/retrocausal/ascausal interpretations, and all of them strictly respect c. That is in fact their hallmark: they reject hidden variables and classical causality while holding to locality. There are no FTL effects of any kind.

Second, there is nothing about special relativity that would work differently if the arrow of time were reversed. That we see an arrow of time does not preclude the existence of effects from our future impacting our present, alongside effects from our past impacting our present. I'm not saying there is any specific evidence of this, just saying that these are viable interpretations that cannot be rejected out of hand (other than for reasons of personal preference).

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

While this is not the place for a roundup of such interpretations: retrocausal interpretations, for example, feature backward in time "offer" waves that do not exceed c. Acausal interpretations, such as Relational BlockWorld (RBW), feature interactions between quantum objects that similarly respect c. In such interpretations, effects that appear FTL are explained by handshaking between the future and the past. A context spans both space and time coordinates, and there is no distinct causal direction.

 

[DrChinese]

Jazzdude said: "I'm arguing that you can formulate a local Hamiltonian and still have the non-locality that is relevant for the Bell inequalities. Bell's argument is also entirely independent of such details. It only uses the fact that the state space allows for remotely entangled states, and those states are also present in QFT. So your local QFT is in the same sense non-local as ordinary QT."

The long-ranged correlations described by entanglement have nothing to do with locality or non-locality. Rather it's what Einstein calls inseparability. It's clear that local QFT is fully compatible with and describes the observed inseparability of far-distant entangled parts of quantum systems.

Just a reminder: those "remote" and "far-distant" systems can span not only space, but time itself. It is possible to fully entangle particles that have never interacted, and in fact never even co-existed. See for example the following experiment:

https://arxiv.org/abs/1209.4191

 

[gentzen]

The long-ranged correlations described by entanglement have nothing to do with locality or non-locality. Rather it's what Einstein calls inseparability.

I just tried how it sounds for me if I talk of "inseparable randomness" instead of "nonlocal randomness". The immediate effect was that I got aware that the inseparable randomness does not only concern two (potentially) spacelike separated events, but can also affect more than two events.

This first somewhat reduced my satisfaction with the explanation that the inseparable effects in quantum mechanics concerns randomness and not signaling or causality. But then I remembered the fact that the possible entanglement is maximal between two systems and goes down significantly if more than two systems are entangled. This was somewhat reassuring for me, the only problem is that I never studied the details of entanglement between more than two systems. Well, I guess now I have to learn a bit more about that, to get back to my previous levels of satisfaction with my understanding of the non-intuitive aspects of quantum mechanics.

Regarding the words, "nonlocal randomness" goes down smoothly and therefore is nicely soothing for me. The words "inseparable randomness" feel less smooth and therefore trigger more thoughts. And "inseparable" invokes a more active picture, one of a spatially extended system, while "nonlocal" just invokes the picture of a randomness generator outside of space and time that simultaneously distributes its randomness to two different places. Both pictures are wrong in their own way, not sure which of the two pictures is more dangerous or misleading.

 

[gentzen]

You are right, it's not the best paper to help my point, although it says some important things about Bell's theorem. I think a better example would be this paper, also from Wetterich:

I agree with what you are saying, but I don't agree that labeling the information as "non-useful" makes its instantaneous transfer compatible with relativity, so it's a red-herring.

Thanks for your answers. I appreciate that you don't have a problem to agree with some of my points. I see that many other commenters replied to you and expect some sort of reaction, so I will not further bother you for the moment.

 

[vanhees71]

Jazzdude said: "I'm arguing that you can formulate a local Hamiltonian and still have the non-locality that is relevant for the Bell inequalities. Bell's argument is also entirely independent of such details. It only uses the fact that the state space allows for remotely entangled states, and those states are also present in QFT. So your local QFT is in the same sense non-local as ordinary QT."
Just a reminder: those "remote" and "far-distant" systems can span not only space, but time itself. It is possible to fully entangle particles that have never interacted, and in fact never even co-existed. See for example the following experiment:

https://arxiv.org/abs/1209.4191

Yes, entanglement swapping is a clearly demonstrated fact. Note, however, that again you need the entangled photon pairs to begin with, and these are also locally produced by parametric down conversion. It's also nothing contradicting standard (mircocausal) QED.

 

[vanhees71]

I just tried how it sounds for me if I talk of "inseparable randomness" instead of "nonlocal randomness". The immediate effect was that I got aware that the inseparable randomness does not only concern two (potentially) spacelike separated events, but can also affect more than two events.

This first somewhat reduced my satisfaction with the explanation that the inseparable effects in quantum mechanics concerns randomness and not signaling or causality. But then I remembered the fact that the possible entanglement is maximal between two systems and goes down significantly if more than two systems are entangled. This was somewhat reassuring for me, the only problem is that I never studied the details of entanglement between more than two systems. Well, I guess now I have to learn a bit more about that, to get back to my previous levels of satisfaction with my understanding of the non-intuitive aspects of quantum mechanics.

Regarding the words, "nonlocal randomness" goes down smoothly and therefore is nicely soothing for me. The words "inseparable randomness" feel less smooth and therefore trigger more thoughts. And "inseparable" invokes a more active picture, one of a spatially extended system, while "nonlocal" just invokes the picture of a randomness generator outside of space and time that simultaneously distributes its randomness to two different places. Both pictures are wrong in their own way, not sure which of the two pictures is more dangerous or misleading.

The advantage of the word "inseparable" is that it doesn't interfere with the various meanings of "locality/nonlocality". Einstein was not only an ingenious physicist but also a master of scientific (German) prose! This becomes the more evident, if you compare the EPR paper (where the argument is "swamped in erudition") with Einstein's single-autor paper of 1948 (A. Einstein, Dialectica 2, 320 (1948)).

Nonlocal randomness is not too surprising. What's so surprising about entanglement is the stronger-than-classical fardistant correlations described by it.

Concerning the entanglement of more than 3 particles/photons a nice example is the famous GHZ state ;-)):

https://en.wikipedia.org/wiki/Greenberger–Horne–Zeilinger_state

 

[DrChinese]

Yes, entanglement swapping is a clearly demonstrated fact. Note, however, that again you need the entangled photon pairs to begin with, and these are also locally produced by parametric down conversion. It's also nothing contradicting standard (mircocausal) QED.

Yes, with entanglement swapping:

You don't need to start with the entangled photon pair that is time separated (let's call this the "subject pair"). And in fact the subject pair can be entangled *after* the fact of their respective detections. The pair that is sent to the Bell State Analyzer (BSA), which casts the subject pair into an entangled state, need not be local to either member of the subject pair at the time of the cast. The subject pair is NOT entangled via PDC, they are entangled by the vehicle of the BSA (entanglement swapping).

So basically, there is no way to draw a reasonable context diagram showing forward in time local causality, since the subject pair can be entangled after they measured. Time ordering is of absolutely no consequence, in terms of experimental results. Of course, when you look in terms of an overall context, each component respects c even though the entangled subject pair is far separated in spacetime, with no direct interaction at any point (they couldn't have, since they never coexisted).

The questions here: How does the subject pair become entangled? Further, how can they become entangled *after* they are measured? I don't think any forward-in-time interpretation does a particularly good job explaining this. Certainly, the QFT regime of spatially-extended systems falls short. You need to have temporal extent too. In other words: you can't require systems to obey the usual causal order, any more than you can require systems to obey the usual locality limitations (you call a PDC entangled system that doesn't obey the usual locality limitation "inseparable").

 

[Jazzdude]

I don't accept to call a local QFT nonlocal, because that's a contradiction in itself, and this hast to be avoided in scientific discussions.

It doesn't matter if you accept it or not. The notion of locality in local QFT is not strong enough to allow the deduction of indeterminism from the violation of Bell's inequality. You would also need to eliminate the non-local nature of the state space in addition to asserting the locality of the interactions that generate the state evolution.

 

[Demystifier]

Contextual models don't need to be non-deterministic. For example: Bohmian Mechanics is both deterministic and contextual. However, contextual models *imply* non-determinism, as the quantum expectation is solely based on the context.

I have no idea what are you trying to say. For example, how does Bohmian mechanics imply non-determinism?

 

[AndreiB]

Of course Weinberg doesn't explain the EPR correlations with a local deterministic HV theory. The point is to show that it cannot be explained with such a model to begin with.

Does he explain them with a local and non-deterministic theory?

At this point you would probably reply that QFT is local and non-deterministic. however, i think such a position is not justified.

The formalism of QFT is compatible with both local and non-local interpretations, I think. In other words you cannot deduce from QFT that A measurement did not cause B, am I right? Also, you cannot deduce from QFT that there are no hidden variables. Sure, QFT does not postulate those variables, but one could see QFT as a statistical (incomplete) theory of a deeper layer of hidden variables.

So, by saying that:

1. QFT is local, and
2. QFT does not involve hidden variables

is misleading, because it might be the case (and I think EPR proved that beyond any reasonable doubt) that 1 and 2 cannot be both true at the same time.

This is why (if you, for some reason reject EPR's conclusion) I am asking for an explicitly local explanation of EPR perfect correlations by an explicitly indeterministic theory. I think this is impossible. How could you make two independent coin flips to always agree? They either are not independent (and since they are space-like you have non-locality) or they are predetermined (hidden variables).

 

[Demystifier]

This is why (if you, for some reason reject EPR's conclusion) I am asking for an explicitly local explanation of EPR perfect correlations by an explicitly indeterministic theory. I think this is impossible. How could you make two independent coin flips to always agree? They either are not independent (and since they are space-like you have non-locality) or they are predetermined (hidden variables).

Arguments of this type require thinking in terms of real objects which are not measured/measurable. Standard/orthodox/Copenhagen school of thought rejects any arguments based on such objects, except when used in favor of standard/orthodox/Copenhagen school of thought.

 

[AndreiB]

Arguments of this type require thinking in terms of real objects which are not measured/measurable. Standard/orthodox/Copenhagen school of thought rejects any arguments based on such objects, except when used in favor of standard/orthodox/Copenhagen school of thought.

The EPR argument (in the form presented by me here, taking into account only the properties that were actually measured) does not make any assumption regarding the "reality" of the objects or even the reality of the observations. In other words, the EPR argument is effective even against a hard solipsist. The argument proves that his mind cannot consistently reject hidden variables and accept relativity. As both QM and relativity are statements about his mind, the minimal assumption that his mind is logically consistent would require him to accept hidden variables.

A Qbist for example would be confronted by two types of "agent experiences". One type, coming from relativity would tell him that space-like "experiences" cannot cause/influence each other. Another type, from QM, would (if he rejects hidden variables, as any proper QBist would) tell him that space-like "experiences" can cause/influence each other. This contradiction is a proper falsification of QBism.

The trick QBists or other non-realists employ to avoid EPR is to assume that only QM is about "experiences" or "knowledge", whyle relativity is about the "real" world. The conflict is presumably avoided because QM and relativity are about different things. But this is inconsistent. There is nothing that distinguishes "quantum" experiments from the "relativistic" ones. So, by rejecting realism, the QBist achieves nothing. He just moves the contradiction from the objective external world to his own mind.

 

[Demystifier]

accept hidden variables

If one accepts hiden variables, then one accepts objects which are not measured/measurable.

 

[AndreiB]

If one accepts hiden variables, then one accepts objects which are not measured/measurable.

The hidden variables are certainly measurable. It is what you actually measure. Of course, one can reject them, but then one must accept the inevitable logical conclusion that space-like events cause each other.

If you reject the EPR's conclusion then you are forced by logic to reject at least one of its premises. This applies to QBists as well.

 

[vanhees71]

This is a typical "delayed-choice" situation. You can do the necessary coincidence measurements and just store them in corresponding protocols, one for each local measurement also noting the necessary "time stamps". Then you can share the information in the protocols to choose certain subensembles each of which corresponds to one of the four entangled states of the "new" pairs. That this really are the corresponding entangled states is certain, because you had two entangled (but not mutually entangled) pairs in the beginning. There is no retrocausation nor faster-than-light communication necessary to explain this.

 

[vanhees71]

It doesn't matter if you accept it or not. The notion of locality in local QFT is not strong enough to allow the deduction of indeterminism from the violation of Bell's inequality. You would also need to eliminate the non-local nature of the state space in addition to asserting the locality of the interactions that generate the state evolution.

I don't know in which sense you mean the state space is local or non-local.

 

[AndreiB]

This is a typical "delayed-choice" situation. You can do the necessary coincidence measurements and just store them in corresponding protocols, one for each local measurement also noting the necessary "time stamps". Then you can share the information in the protocols to choose certain subensembles each of which corresponds to one of the four entangled states of the "new" pairs. That this really are the corresponding entangled states is certain, because you had two entangled (but not mutually entangled) pairs in the beginning. There is no retrocausation nor faster-than-light communication necessary to explain this.

EPR is based on only 2 premises:

1. locality (space-like events do not cause each other)
2. The predicted perfect correlations exist.

As far as I can tell, nothing you said here, about delayed choices, has anything to do with these two premises. So, the conclusion of the argument (hidden variables exist) necessary follows.