Einstein's later thoughts on EPR

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In re-discussing the EPR scenario in his autobiographical notes (1946), Einstein wrote:
One can escape from this conclusion [that statistical quantum theory is incomplete] only by either assuming that the measurement of S1 (telepathically) changes the real situation of S2 or by denying independent real situations as such to things which are spatially separated from each other. Both alternatives appear to me equally unacceptable.
The first option (i.e. "telepathically changes the real situation") is just Einstein's unacceptability of non-locality as it goes against relativity but what does Einstein exactly mean by his unacceptability of the second option: "denying independent real situations as such to things which are spatially separated from each other"? Wiseman in one paper has suggested that what Einstein means by the second option is "extreme subjectivism", denying that there are matters of fact about distant observers:
Statistical QM as a theory with a single observer A allows ψ to be interpreted as A’s knowledge of the quantum world. The fact that this changes instantaneously now poses no mystery, because it is merely a change in A’s knowledge. But precisely because it is only a change in A’s knowledge, this change in ψ cannot affect the result B obtains, even when (as with the EPR state) the results that A and B obtain may be perfectly correlated. Statistical QM as a theory of A’s knowledge evades this problem, however, simply by denying that B has an independent existence. All that exists (if the theory is complete) is A’s knowledge of B.
Maudlin also discusses this second of Einstein's options but appears more non-committal to it, especially given that he feels it leaves Einstein's argument for the incompleteness of QM untouched:
In this passage Einstein offers two possible ways to reject the conclusion of his argument: accept telepathy or reject the claim that systems spatially separated from one another even have ‘independent real situations’. Unfortunately, Einstein never discusses this second option in detail. My guess is that Einstein couldn’t even really imagine what this second option could be like. But more than that, it is not even clear to me exactly how this move is supposed to interfere with the argument. If we use the EPR criterion of an element of reality, then we get the existence of an element of reality in S2 that was not represented in the quantum mechanical description of S2, so the quantum-mechanical description is not complete.
Is it accurate to say, that if we accept this second option of non-realism in Bell's scenarios, does it mean a more radical non-realism as argued by Wiseman:
On the other hand, if one assumes that relativity is fundamental, then Bell experiments have proven that there is no real world. To be more precise, under this assumption Bell-experiments have proven that the real world exists only within one’s own past light-cone. It cannot be stressed enough that this does not mean merely that microscopic particles or fields cannot have determinite properties. It means macroscopic objects, other conscious observers even, are not real at the present time.
From Einstein’s Theorem to Bell’s Theorem: A History of Quantum Nonlocality
http://arxiv.org/pdf/quant-ph/0509061.pdf

What Bell did
http://iopscience.iop.org/1751-8121/47/42/424010/pdf/1751-8121_47_42_424010.pdf
 

atyy

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Yes. In the second option there is no violation of the Bell inequalities at spacelike separation, hence reality need not be nonlocal. It is not comntroversial that this form of nonrealism can save locality. Quantum Bayesianism and RQM are examples of such approaches. My favourite one is Nikolic's Solipsistic Hidden Variables :)
 
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It is not comntroversial that this form of nonrealism can save locality. Quantum Bayesianism and RQM are examples of such approaches. My favourite one is Nikolic's Solipsistic Hidden Variables :)
How can it save locality? Doesn't the very notion of “locality” already presuppose "realism"? This as an argument I've come across a few times. For instance, Norsen writes:
...the choice between rejecting Locality and rejecting Metaphysical Realism is not a choice in the ordinary sense – in particular, one cannot “save Locality” by rejecting Metaphysical Realism. And this is because the very idea of “Locality” already presupposes Metaphysical Realism, a point that is undeniable once we remember what we are using the term “Locality” to mean: the requirement that all causal influences between spatially separated physical objects propagate sub-luminally. The point here is this: to reject Metaphysical Realism is precisely to hold that there is no external physical world. And once one rejects the existence of a physical world, there simply is no further issue about whether or not causal influences in it propagate exclusively slower than the speed of light (as required by Locality). Or put it this way: “Locality” is the requirement that relativity’s description of the fundamental structure of space-time is correct. But relativity theory is thoroughly “realist” in the sense of Metaphysical Realism. If there is no physical world external to my consciousness, then, in particular, there is no space-time whose structure might correspond to the relativistic description... – and so that description’s status would be the same as, for example, that of claims about the viscosity of phlogiston or theories about the causes of cancer in unicorns: false in the strongest possible sense. And so the idea of giving up Metaphysical Realism as an alternative to giving up Locality (relativity’s account of space-time structure) is simply nonsense.
http://arxiv.org/pdf/quant-ph/0607057.pdf
 

DrChinese

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The Einstein quote can be read several different ways. Here is what Einstein said in the text preceding bohm2's quote:

"There is to be a system that at the time t of our observation consists of two component systems S1 and S2, which at this time are spatially separated and (in the sense of the classical physics) interact with each other but slightly. The total system is to be described completely in terms of quantum mechanics by a known ψ-function, say ψ12. All quantum theoreticians now agree upon the following. If I make a complete measurement of S1, I obtain from the results of the measurement and from ψ12 an entirely definite ψ-function ψ2 of the system S2. The character of ψ2 then depends upon what kind of measurement I perform on S1. Now it appears to me that one may speak of the real state of the partial system S2. To begin with, before performing the measurement on S1, we know even less of this real state than we know of a system described by the ψ-function. But on one assumption we should, in my opinion, insist without qualification: the real state of the system S2 is independent of any manipulation of the system S1, which is spatially separated from the former. According to the type of measurement I perform on S1, I get, however, a very different ψ2 for the second partial system (φ2, φ21, . . . ). Now, however, the real state of S2 must be independent of what happens to S1. For the same real state of S2 it is possible therefore to find (depending on one's choice of the measurement performed on S1) different types of ψ-function. One can escape from this conclusion [that statistical quantum theory is incomplete] only by either assuming that the measurement of S1 (telepathically) changes the real situation of S2 or by denying independent real situations as such to things which are spatially separated from each other. Both alternatives appear to me equally unacceptable."


It is a mistake to attempt to recast the EPR assumptions to drop realism as an assumption. Realism (=2 or more simultaneous elements of reality) is explicit in EPR, and without it there is no EPR argument. In the quote above, Einstein again assumes that there are 2 or more simultaneous elements of reality (according to which of several different choices of measurement are executed).
 

morrobay

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Yes. In the second option there is no violation of the Bell inequalities at spacelike separation, hence reality need not be nonlocal. It is not comntroversial that this form of nonrealism can save locality. Quantum Bayesianism and RQM are examples of such approaches. My favourite one is Nikolic's Solipsistic Hidden Variables :)
I read the first option as non locality and second option as non realism. Both of which could account for inequality violations.
 

atyy

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How can it save locality? Doesn't the very notion of “locality” already presuppose "realism"? This as an argument I've come across a few times. For instance, Norsen writes:

http://arxiv.org/pdf/quant-ph/0607057.pdf
Norsen also writes "Perceptual Realism is needed to arrive at the claim that Bell’s inequalities are, in fact, violated." Norsen correctly claims that if the bell inequalities are violated at spacelike separation, then there is no local explanation for the correlations - nonrealism cannot save locality, because a "nonreal explanation" is just playing with words. However, in the second alternative mentioned in the OP, there is no violation of the Bell inequalities at spacelike separation, so there are no nonlocal correlations to be explained, only one's receipt of the report of nonlocal correlations.

Norsen gives a more detailed explanation in the context of his interpretation of MWI, but it can equally apply to the second alternative of the OP [bolding mine]: "One could indeed follow MWI in retaining Locality by rejecting Perceptual Realism, but this is not a move one makes as a response to the experimental data; rather, it is a move one makes to justify rejecting the data as systematically failing to reflect the true state of the world."

So Norsen is not claiming that all forms of nonrealism cannot save nonlocality.
 
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Realism (=2 or more simultaneous elements of reality) is explicit in EPR, and without it there is no EPR argument. In the quote above, Einstein again assumes that there are 2 or more simultaneous elements of reality (according to which of several different choices of measurement are executed).
Einstein’s Boxes thought experiment before EPR made no such assumption and reaches a similar conclusion; that is, non-locality or incompleteness. Moreover, even in the EPR scenario, is it clear that one actually needs that particular "realism" assumption of 2 or more simultaneous elements of reality to reach Einstein's conclusions? Norsen writes:
But because the Boxes thought experiment in no way relies on a choice between two complementary quantities to measure,it seems immune from Bohr’s criticism that there is a kind of “semantic disturbance” effected by this choice. There is no question with the Boxes experiment about which quantity will be measured. We are going to open B1 and look for the particle there, period. So there appears to be no room for “an influence on the very conditions which define the possible types of predictions regarding the future behavior of the system.” Given the assumption of locality/separability (with which Bohr appears to agree given his denial of a “mechanical disturbance”), the Einstein’s Boxes thought experiment establishes the conclusion of incompleteness (that is, it establishes the dilemma between locality and completeness) in a straightforward way that seems immune to Bohr’s criticism of EPR....
Moreover,
The fact that one can, in the EPR situation, predict with certainty just the position of particle 2 (without disturbing it in any way) is sufficient, according to the reasoning set up by EPR, to conclude that, if local, quantum theory is incomplete. For that alone means that the position of particle 2 is an element of reality, and one that has no counterpart in the original, pre-measurement entangled wave function. Maudlin refers to the attempt to (in addition) beat the uncertainty principle by also establishing the real existence of particle 2’s momentum as “an unnecessary bit of grandstanding” which plunged “the previously simple EPR argument into the muddy waters of [modal logic and counterfactual definiteness].”
Einstein’s Boxes
http://arxiv.org/pdf/quant-ph/0404016v2.pdf
 
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morrobay

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http://arxiv.org/pdf/quant-ph/0509061.pdf On page 8 of this paper from the OP:
One might have expected the arguments by EPR and Schrodinger to have led to renewed interest in theories that sought to complete
QM since such theories offered the prospect of solving both the non-locality problem and the quantum measurement problem.
And on page 9 : Einstein suggested that one of the following is false
1. The completeness of statistical QM ( minimal interpretation ) 2. locality. 3. Independent reality of distant things. (realism).
So why not more emphasis on option one and or option three for Bell inequality violations explanation ?
 

atyy

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http://arxiv.org/pdf/quant-ph/0509061.pdf On page 8 of this paper from the OP:
One might have expected the arguments by EPR and Schrodinger to have led to renewed interest in theories that sought to complete
QM since such theories offered the prospect of solving both the non-locality problem and the quantum measurement problem.
And on page 9 : Einstein suggested that one of the following is false
1. The completeness of statistical QM ( minimal interpretation ) 2. locality. 3. Independent reality of distant things. (realism).
So why not more emphasis on option one and or option three for Bell inequality violations explanation ?
Einstein's argument that QM may be incomplete relied on locality, ie. Einstein was arguing for local hidden variables to complete QM. However, Bell's theorem rules out local hidden variables that reproduce QM, so Einstein's argument for the incompleteness of QM has been done away with.

Option 3 is still a possibility, and has not been ignored, eg. Quantum Bayesianism, Relational QM, and (best of all IMHO) Solipsistic Hidden Variables http://arxiv.org/abs/1112.2034.

Option 1 is also a possibility, but the hidden variables cannot be local. The argument for 1 is usually not based on Einstein's argument, which was too weak, but on the measurement problem: http://arxiv.org/abs/0712.0149.
 
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Norsen also writes "Perceptual Realism is needed to arrive at the claim that Bell’s inequalities are, in fact, violated." Norsen correctly claims that if the bell inequalities are violated at spacelike separation, then there is no local explanation for the correlations - nonrealism cannot save locality, because a "nonreal explanation" is just playing with words. However, in the second alternative mentioned in the OP, there is no violation of the Bell inequalities at spacelike separation, so there are no nonlocal correlations to be explained, only one's receipt of the report of nonlocal correlations...So Norsen is not claiming that all forms of nonrealism cannot save nonlocality.
In a recent article Norsen discusses this second alternative of Einstein in more detail:
Strictly speaking this inference requires the additional assumption “(which ... Einstein makes explicitly) that systems have real factual situations”. Recall that “denying independent real situations as such to things which are spatially separated from each other” was one of the two things that Einstein jointly described as “entirely unacceptable”. In my opinion, and probably that of Einstein, one must clearly accept that spatially-separated systems have their own “real situations” before one can even meaningfully ask whether locality is respected...

The additional required assumption here would thus seem to be a logical precondition for discussing locality, rather than something one might coherently deny instead of locality. Note that this point is closely related to the important point that Bell would later express by insisting that the notion of locality must be formulated “in terms of local beables”.
Are there really two different Bell’s theorems?
http://www.ijqf.org/wps/wp-content/uploads/2015/03/IJQF2015v1n2p2.pdf
 

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