Local realism ruled out? (was: Photon entanglement and...)

akhmeteli

As far as I know, it is impossible to prove violations in dBB without using some assumptions beyond unitary evolution, otherwise such a proof could be transferred to standard quantum theory. If you disagree, could you please give a reference to such a proof?


I gave my reasons to think that fundamental relativity has not been ruled out - absence of loophole-free demonstrations of violations and absence of contradiction-free proof of violations in quantum theory.

Ilja

My point was not about Newton's thinking, but about the equations. The link http://plato.stanford.edu/entries/ne...osophy/#ActDis has been already postet here and shows that Newton was aware that there is an action at a distance in the equations, and has considered the lack of mediation as a problem.

dBB does not give any answer, and does not even try to give one. So the situation is quite similar to Newtonian gravity, where the formulas do not tell us anything about an explanation for gravity.

And, similarly, I think this is an interesting open problem and can be a hint for developing some subquantum theories. A theory which, for example, restricts the maximum speed of this spooky action should violate quantum theory.

This would be a second hint for subquantum theory, the first being that QM fails for very small values of ψ. That's because around ψ(q)=0 the dBB velocity becomes infinite, even if only in a quite harmless way (increasingly fast rotation around the 0).

bohm2

Not sure if Gisin's experiment was posted in this thread but his group suggested that the speed of this non-local connection must be is at least 10,000 times the speed of light:
Testing spooky action at a distance
http://arxiv.org/pdf/0808.3316v1.pdf

harrylin

Nice find! I'll read it.
Note that your Arxiv link is a version of a reviewed publication:
http://www.nature.com/nature/journal...ture07121.html

PS. a quick question: I quickly looked over it but I could not immediately "get" the idea behind it.
What is in a nutshell their method for determining the minimal speed of "spooky action at a distance? They mention two-photon interference, which sounds somewhat like MMX (even more like KTX). Where is "Bell" in all that?

akhmeteli

Let me just note that the article in question does not claim simultaneous elimination of both the detection and the locality loopholes (probably, the detection loophole still exists, as is customary for all experiments with photons), so, strictly speaking, their experiment does not even demonstrate violations of the Bell inequalities.

Ilja

The idea is that if there is another, greater limiting speed, say 100c, then there has to be a corresponding superlight cone and there will be space-like separated event pairs for this superlight cone too. And for such event pairs the Bell inequalities should hold. So one has to test the violation of Bell inequalities for large enough sets of event pairs so that there will be no place for the 100 c superlight cone.

The next idea was that there is a reasonable hypothesis for the place of the superlight cone - one can guess that the time of the rest frame for the background radiation will be time-like in the superlight cone too. So there is no need to rule out all those skew superlight cones, and all one needs is to care about the much smaller set of superlight cones compatible with the background radiation rest frame. So, one only has to look for event pairs which have approximately equal time in the CMBR frame.

harrylin

Ok, thanks - it's starting to dawn on me now.
I have the impression that in that last paper they tried to be so general as not to need the CMBR hypothesis.

Anyway, I now find my hunch that such an interferometer experiment is unlikely to yield anything "spooky" supported by an old discussion on this forum (keyword I searched with was "Franson"):

http://www.physicsforums.com/showthread.php?t=229393

However, there is for me not enough explanation there ...

bohm2

This has been posted here before but was just published in Nature Physics. It argues that barring loopholes, if the non-local effects observed in Bell-type experiments propagate at any finite speed, then non-locality could be exploited for superluminal communication:
Looking Beyond Space and Time to Cope With Quantum Theory
http://www.sciencedaily.com/releases...1028142217.htm

Quantum non-locality based on finite-speed causal influences leads to superluminal signalling
http://www.nature.com/nphys/journal/...nphys2460.html

Full article posted in arxiv:
http://arxiv.org/pdf/1110.3795v1.pdf

bohm2

I know this isn't likely to sway any opinions but for completion and future reference this is a follow-up piece by Gisin to the J. D. Bancal et al. Nature Physics paper linked above that was just posted on arxiv:
Quantum correlations in Newtonian space and time: arbitrarily fast communication or nonlocality
http://lanl.arxiv.org/pdf/1210.7308.pdf

bohm2

Lecture from lead author JD Bancal from Perimeter Institute:
http://pirsa.org/displayFlash.php?id=11110145

harrylin

That reminds me of De Raedt et al who more than agreed with that.
As a reminder:
- http://www.physicsforums.com/showthread.php?t=499002

akhmeteli

Three years have passed. As far as I know, all the loopholes have not been closed in one single experiment. For example, reporting some further progress in a recent article http://arxiv.org/abs/1212.0533 , Zeilinger e.a. still admit that “The realization of an experiment that is free of all three assumptions – a so-called loophole-free Bell test – remains an important outstanding goal for the physics community”.

I am writing this without any Schadenfreude. I do appreciate that a loophole-free experiment can be performed any moment now. It looks like the race to conduct the first experiment of this kind is really fierce. E.g., the following quote is interesting (SCIENCE, VOL 331, P. 1380 (2011)): “Zukowski thinks the race to close all the loopholes simultaneously will soon be over. “Conservatively, it could take another 5 years to complete, but it could also be done tomorrow,” he says. “We’re at the stage where everyone is scared to read their competitors’ papers, in case they find they have been beaten. The only real question is: Who will win?””

I also had this impression of a fierce race listening to talks on quantum foundations experiments at several conferences last year. On the other hand, some experimentalists admitted (typically, not in their official talks:-) ) that they encounter some formidable challenges.

So I am just trying to say that these three years since the start of this thread have demonstrated again that it is extremely difficult to demonstrate violations of the genuine Bell inequalities. Will they be demonstrated by the fiftieth anniversary of the Bell’s article next year? Or ever? My prediction is “no”. But I may be mistaken.

Peter Morgan

I guess you're just bashing your response out, so this is not intended to be a substantive criticism, but it's not a correlation that's higher, it's a sum of absolute values of a sum and a difference between four different correlations $|A-B|+|C+D|$ (at least, that's what it is in the CHSH formulation, in no case is the difficulty for local realism that a correlation coefficient simpliciter is too high).

akhmeteli

Dear Gordon Watson,

I tried to explain in my post 753 in this thread why I cannot agree with you.

akhmeteli

So Santos, Marshall, nightlight are not rational quantum physicists, in your book. They are in mine. Let me add that I gave those "reasons" and properly published them (although I just repeated other people's arguments, as far as those "reasons" are concerned).

Another thing: "the foundations of quantum mechanics ... remain hotly debated in the scientific community, and no consensus on essential questions has been reached." (Schlosshauer, Kofler, Zeilinger, http://arxiv.org/abs/1301.1069 ). So disagreement is a "new normal" for quantum foundations.

nanosiborg

Agree. 1 and 2 are facts, and 3 seems to me to be a much more reasonable position than that taken by the loophole people.

akhmeteli

You are satisfied, I'm not.

Not unless you ignore the loopholes.

Not unless you use as assumptions mutually contradictory postulates of standard quantum theory, such as unitary evolution and the projection postulate. However, if you use mutually contradictory assumptions, you can get any conclusion, however absurd.

This is an opinion, not a fact.