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Entanglement and Bell's theorem. Is the nonlocality real? 
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#1
Dec1212, 07:37 PM

P: 29

The results of measurements of phase entangled particles together with Bell's theorem provide pretty convincing evidence that the Universe contains nonlocal interactions.
Yet I'm lead to wonder. Let's imagine the usual idealised experimental scenario, where there is an emitter of particles in a twin state and two measuring devices on opposite sides of the system performing measurements in a spacelike separated way. The measurements on one side of the system are not interesting in themselves. They are just random. They only become interesting when they are compared with the measurements from the other side, with a correlation being observed. We know that when performed appropriately, this will show that the measurement results are correlated in a way that, by Bell's theorem, cannot be explained by any local interaction  the measurements appear to be nonlocally linked. Now step back. Consider that the above is performed in complete isolation, except that the results of the final step  comparison of the measurements, is transmitted to an outside observer. For the comparison to be made, the results have to be transferred from where they are made to a common place. Since the experiment, except for the last step, is performed in isolation, the outside observer can regard the entire experimental situation as a superposition of quantum states with no decoherence except at the last step. The "measurements" are nothing but further entanglements between the twin state particles and the measuring apparatus. The last step involves an interaction between particles that represent the results of the earlier "measurements", with the states of particles for the two measurements for a given twin state pair of particles being already entangled. Those particles now further interact locally to produce the transmitted result of the comparison to the outside observer. So the outside observer can (in principle, at least) calculate the evolution of the system, and the final transmitted results (in a probablistic sense), without needing to assume any nonlocal interactions. In particular, the outside observer cannot use Bell's theorem to prove that the system is nonlocal, because nothing in the system has a definite value that Bell's theorem requires. From this perspective, it looks as if the appearance of nonlocality in the system results from a false assumption by observers embedded in the system that they are somehow independent of it, and that their measurement results are definite values before they are compared. A possible objection is that we can posit yet another observer outside the enlarged system that consists of the first system and the first outside observer, and do that again and again, making this look like some kind of infinite regression. However the first outside observer sits at the first place where a definite result can be obtained immediately without needing the enlarged system to evolve further. Sylvia. 


#2
Dec1312, 12:23 AM

P: 79

The bottom line is that it cannot be definitively said, from Bell tests, that nature is nonlocal. Not because of experimental loopholes, but because the Bell formulation of local hidden variable models might not be general. I've omitted the rest of your post because I think it's irrelevant. Even if all experimental loopholes in Bell tests are eventually closed, then what does that mean? It means that Belltype formulations of quantum entanglement are not viable. And whether or not nature is nonlocal remains an open question. Why Belltype formulations of quantum entanglement preparations are nonviable has been the subject of numerous publications. Here's another thing to consider. Suppose that it's found that absolutely no local model of quantum entanglement can be formulated. Does that mean that quantum entanglement is nonlocal? No, it doesn't. This is because the correspondence of theory to reality is, and will always be, essentially unknown. 


#3
Dec1312, 02:00 AM

P: 29




#4
Dec1312, 04:48 PM

P: 79

Entanglement and Bell's theorem. Is the nonlocality real?
The correlations are such that empirical laws from classical optics are applicable to the QM treatment of entanglement. This simple fact would seem to indicate that whatever is going on in quantum entanglement has nothing to do with nonlocality. Bell's theorem (experimental violations of Bell inequalities) can't show that nature is nonlocal. For those who want to believe that nature is nonlocal, then that's a metaphysical assumption for which there's absolutely no evidence. 


#5
Dec1412, 12:57 PM

Sci Advisor
PF Gold
P: 5,388

Your alternative is to postulate macroscopic entanglement, which you would need to keep the entanglement alive. Because usually the results are encoded to computers and then compared. In your view: when the results are brought together, the entanglement ends. 


#6
Dec1412, 06:26 PM

P: 29

I'm pondering whether one can come up with a version of the EPR experiment where the entanglements are sufficiently clearly specified that the system appears nonlocal from the inside (an admitedly vague term in this context), but is manifestly local from the outside (ditto). I confess that I haven't made much progress there. Sylvia. 


#7
Dec1912, 09:07 PM

Sci Advisor
P: 8,470

I think this is the essence of the argument made by advocates of the manyworlds interpretation in explaining why there doesn't need to be any violation of locality in their interpretationbasically if each measurement just puts each experimenter into a superposition of different measurement results, then there is no need for nature to decide which measurement result "here" is paired with which measurement result "there" until there's been time for a signal from each experimenter to reach someone at the midpoint. See for example Vindication of Quantum Locality by David Deutsch and The EPR paradox, Bell's inequality, and the question of locality by Guy Blaylock.



#8
Dec2012, 08:11 PM

PF Gold
P: 692

An interesting paper on this topic that came out today:
http://lanl.arxiv.org/pdf/1212.5214.pdf 


#9
Dec2012, 08:59 PM

P: 29

The background to my interest in this issue is the experiments that continue to explore the lower bound on the speed of some hypothetical influences that implement the apparent nonlocality. I think the experimenters themselves do understand that the results they're getting only have meaning if the influences are real, but this caveat tends to get lost when the results are reported in the popular science media. I was wondering whether these experiments are even worthwhile in the absence of any concrete evidence of the existence of the influences, and while other models of reality remain viable. Of course, one can say that doing experiments that test the limits of theory is necessarily part of what science is about. And that's true, but there are not infinite resources available for doing experiments, so it's inevitable that they get prioritised. Just yesterday I came across this recent paper http://arxiv.org/pdf/1210.7308.pdf in which the authors claim to have proved that if phase entanglement involves influences that travel at a superluminal but finite speed, then it's inevitable that actual FTL communication be possible. That is, it's not possible for such influences to be forever hidden. Note that their proof does not purport to apply to influences that are instantaneous (necessarily in some privileged reference frame), so some versions of the pilot wave theory would remain possible without allowing FTL communication. Sylvia 


#10
Dec2012, 11:38 PM

P: 915

Question: If phase/quantum entanglement is considered random (a basic premise in QM) then even if "influences" are assumed to travel FTL, I cannot see how, the author can show that, FLT communication/information would, even theoretically, be possible 


#11
Dec2112, 01:32 AM

P: 29

So if entanglement is actually mediated by FTL finite speed influences, then QM is capable of being falsified, with the required experiment simply not having been done yet. But the second question is whether it would actually then be possible to exploit the influences. In the paper they show a simple case where an exploit is possible if entanglement is only mediated by such influences, but then show that the exploit is trivially avoided by adding a shared hidden variable. That leaves the question of whether an exploit could still be achieved despite any mechanism based on both FTL finite speed influences and hidden variables. They appear to be showing (I haven't checked the math  I'd have to learn some more of it first) that based on QM, it is possible to construct an exploit. Though as I write this, it occurs to me that their proof appears to be based on a theory, QM, that is, ex hypothesi, in any case wrong. The ramifications of that are currently beyond me. Sylvia. 


#12
Dec2212, 08:31 PM

P: 53

I posted yesterday a short paper in which I show a deterministic model of quantum spin interactions that achieves the same correlations as QT with only luminal signaling in EPR/CHSH experiments. The paper includes the source code for the simulation. Although not indicated in the paper, in the talk I gave I also showed an experiment than can confirm or rule out the hypothetical mechanism that is the basis for the model. I am working on getting endorsed for arxiv, but in the meantime here it is:
https://docs.google.com/open?id=0BxB...XR2R3ExTlEyNm8 


#13
Dec2212, 10:36 PM

P: 29

Second thoughts, scrub that, a model is permitted to do that, provided it achieves it locally. 


#14
Dec2212, 10:39 PM

P: 1,583




#15
Dec2312, 01:18 AM

P: 53

Thanks for looking at it, Sylvia! That code is there to identify the alpha2 and beta2 angle set upon coincident detections for which correlation and anticorrelation are swapped in the tally. An experiment by Rowe et al has a really clear articulation of CHSH in the paper. I'm at a bar right now or I'd find a proper reference.



#16
Dec2312, 02:28 AM

P: 29

Ok, I am bothered by the fact that a lower difference in the angles of Bob and Alice leads to a greater probability of simultaneous detection together with the code that inverts the meaning of correlation for angle differences less than PI/4. This latter seems exactly equivalent to inverting one result, say Alice's, immediately after detection, but only when the angles differ by less than PI/4. That would be manifestly nonlocal.



#17
Dec2312, 03:00 AM

P: 53

It is an exploit of a class of coincidence loopholes. It is based on the relative size of the phenomenon to the size of the coincidence window. The counting is correct. If you add a linked list for the sequence of past states you can expand the window. You will also have to reduce the step of the walk of Emmitt. This is the process of finding the constants in a new theory. It's hard...help needed!



#18
Dec2312, 03:34 AM

P: 53

Note also the other event may never happen. We're not necessarily talking particles travelling and not being counted. Rather, the events actually happen at separate times or perhaps the other never happens.



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