# Einstein's later thoughts on EPR

1. Jan 4, 2015

### bohm2

In re-discussing the EPR scenario in his autobiographical notes (1946), Einstein wrote:
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:
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:
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:
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

2. Jan 4, 2015

### atyy

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 :)

3. Jan 4, 2015

### bohm2

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

4. Jan 4, 2015

### DrChinese

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).

5. Jan 4, 2015

### morrobay

I read the first option as non locality and second option as non realism. Both of which could account for inequality violations.

6. Jan 4, 2015

### atyy

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.

Last edited: Jan 4, 2015
7. Jan 4, 2015

### bohm2

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:
Moreover,
Einstein’s Boxes
http://arxiv.org/pdf/quant-ph/0404016v2.pdf

Last edited: Jan 5, 2015
8. Jan 6, 2015

### morrobay

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 ?

9. Jan 6, 2015

### atyy

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.

Last edited: Jan 6, 2015
10. Mar 21, 2015

### bohm2

In a recent article Norsen discusses this second alternative of Einstein in more detail:
Are there really two different Bell’s theorems?