View Poll Results: What do observed violation of Bell's inequality tell us about nature?  
Nature is nonlocal  11  32.35%  
Antirealism (quantum measurement results do not preexist)  15  44.12%  
Other: Superdeterminism, backward causation, many worlds, etc.  8  23.53%  
Voters: 34. You may not vote on this poll 
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What do violations of Bell's inequalities tell us about nature? 
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#109
Feb1713, 06:47 AM

P: 733




#110
Feb1713, 07:42 PM

PF Gold
P: 674

http://lanl.arxiv.org/pdf/0912.1475.pdf http://www.fqxi.org/community/forum/topic/994?search=1 But since only entities localized in spacetime can ever be observed, it's not clear if "progress" can be made on this issue which kind of hilites Einstein's concerns; nevertheless, I found these 2 questions/problems discussed in the paper below very interesting and would support what you are suggesting: http://arxiv.org/pdf/1206.6290.pdf 


#111
Feb1813, 01:56 PM

P: 733

Before this thread goes quietly into the night, I would just like to point out one last time that  despite the fact that "antirealism" won the poll by a large margin  not a single person has been willing to answer my challenge. Here it is one last time in case anybody missed it...
Bell's inequality, as everybody knows, is a constraint on the correlations that can be exhibited between the outcomes of spin measurements on pairs of entangled particles, as the alignments of the measuring devices are changed. In principle, to be empirically viable, a theory needs to be able to make the correct predictions for the statistics that will be observed for *all possible* alignments. But for the sake of discussion, let us focus here on a very small and simple subset  namely, just the case where both Alice and Bob measure the spins of their particles along the zdirection. Clearly, to be empirically viable, i.e., to be able to make the right predictions for *all possible* measurements, a theory will have to at least make the right predictions for this particular case. As it turns out, experiment tells us that, in this case, there is a perfect (anti) correlation of outcomes: whenever Alice's particle goes up, Bob's goes down, and vice versa. So here is the challenge. People who answered "antirealism" in the poll evidently believe that there exists a theory that is (a) local and (b) nonrealist and which is empirically viable. As noted, this theory must surely be able to explain what is empirically observed in the special case of parallel measurements, if it is really empirically viable. So... what theory is this? Explain how the perfectly anticorrelated outcomes (in just this case where Alice and Bob both measure along the zdirection) can be accounted for in a local but nonrealistic model. Or, if you can't do that, please have the dignity to retract your vote. Thank you very much. 


#112
Feb1813, 03:42 PM

P: 724

ttn, you make it sound like this is the first time that a classical explanation for a quantum phenomenon appears inadequate and incoherent. Of course, this is not the case  classical intuition is the number one barrier, you could raise the same newtonian objections towards the uncertainty principle for instance and the people voting antirealism are merely acknowledging the reality of observations. Quite a number of experiements have been performed that prove that quantum particles do not have fixed properties at all times, as you would expect classically. I do not understand why a quantum physicist would ever go on a rampage about something as undefensible as realism in quantum physics unless he wanted to turn known physics upsidedown. Do you?



#113
Feb1813, 06:05 PM

P: 79

The only way to make explicit, to codify, the assumption of locality in a model of quantum entanglement is via the formal expression of statistical independence. Bell inequalities are based on the correlational boundaries imposed by this formal constraint, which means that any and all 'explicitly local' theories of quantum entanglement can't possibly violate a Bell inequality. Bell tests are designed to produce statistical dependence (via entirely local means), and a model explicitly based on statistical independence would not be expected to reproduce all the results of experiments based on statistical dependence. All of this is fine for Bell's main purpose, which was to see if local (hidden variable, but as we've seen HVs are superfluous) theories of quantum entanglement can be compatible with QM. Or, in other words, if QM could be interpreted locally  and he proved that it can't be. However, many people want to extend the applicability of Bell's theorem to say that it means that nature is nonlocal. Which means that statistical dependence of the sort designed into Bell tests is impossible in a local universe. But that doesn't seem reasonable to me, so I wondered where it came from. Those who believe that Bell's theorem proves that nature is nonlocal have assumed that (via codifying locality as statistical independence) in a local universe, we should expect the angular dependence (the correlation observed experimentally) to be bounded such that it can never reproduce the Malus' Law angular dependence that's observed experimentally. Prior to the adoption of statistical independence as being formally synonymous with the assumption of locality, the Malus' Law angular dependence is what would have been expected from Bell tests. Following the adoption of statistical independence as being formally synonymous with the assumption of locality, and applying this in models of experiments designed to produce statistical dependence via local means, it was expected that the angular dependence produced in Bell tests would not only not be Malus' Law but would in some cases even be linear  an expectation that runs contradictory to known empirical optics laws. In considering this, it seemed to me then that the oddity wasn't the angular dependencies produced in Bell tests, but the fact that Bell inequalities are based on angular dependency expectations that have no foundation in physics. In fact, their sole foundation is the application of models based on statistical independence to experiments based on statistical dependence. So, there seems to me to be a basic problem with extending the meaning of Bell's theorem to encompass nature. What Bell's theorem does, and the only thing it does (as far as I can tell), is definitively rule out local theories of quantum entanglement (a nonetheless monumental result). And here I'll restate my position regarding bohm2's poll. Violations of Bell inequalities tell us nothing about nature. 


#114
Feb1813, 06:09 PM

P: 733

Also, part of your comments above suggest that you misunderstood the challenge. I never said that people voting (b) should provide a "classical" (also local and nonrealist) explanation of the perfect correlations. The explanation can be "quantum" (whatever that means exactly) or whatever flavor you like. It just has to be local. Surely the reasoning here is clear? If somebody thinks we get to choose whether to reject realism or locality in the face of Bell inequality violations, and opts for rejecting realism, surely they believe that the empirical data can be explained locally. I'm just saying: put up or shut up. Show me a local nonrealist way to explain the perfect correlations or retract your false vote. Simple. 


#115
Feb1813, 07:19 PM

P: 733

http://arxiv.org/abs/0707.0401 


#116
Feb1813, 08:02 PM

PF Gold
P: 674

http://arxiv.org/ftp/arxiv/papers/0811/0811.2862.pdf 


#117
Feb1813, 09:19 PM

P: 79

It seems we're at an impasse on this, so, for the moment, we can just agree to disagree. Of course I do agree with your opposition to the "2." ('antirealism) votes and your clarification of the issue and (non)relevancy of 'realism'. I admire your contributions to your field. I'm willing to conjecture, even bet on, that nothing that applied physics can actually use (ie., no physical faster than light anything) will ever come from the assumption of nonlocality in nature per se. The most parsimonious 'explanation' for this will remain simply that there's no 'nonlocality' in nature. 


#118
Feb1913, 03:11 AM

P: 724

Bu we already know that noncontextual chairs and table do not exist according to quantum mechanics, so realism as is usually(naively) defined is broken at the level of atoms and electrons. Given that, who needs additional magic like nonlocality at all costs and what does explain better? He has no qm explanation for the reality of chairs and tables that matches both the postulates of qm and our experience of them, so adding nonlocality brings nothing substantial. Though it seems obvious that if realism fails, so does locality and nonlocality is implied by the consistence of the classical world and in the end both will be found to be incorrect and incompatible with qm. 


#119
Feb1913, 08:39 AM

P: 733

But then nanosiborg comes along and says: not so fast. There is an inconsistency between the assumption that everybody held (namely that the earth was flat) and the "experimental design" (meaning that the experiment actually shows that the earth is round). This inconsistency (which I guess is just the fact that there is a conflict between what many people *expected* and what the experiment actually *showed*) means that actually we cannot conclude from the experiment that the earth is round. The most we can say is that theories according to which the earth is flat are no longer viable. But this tells us nothing about nature. Tell me how what you're saying isn't just parallel to that (I think, manifestly absurd) response to the hypothetical scenario. Anyway, read the papers I mentioned. It's clear (to me at least) that you are clinging to loopholes that don't in fact exist, because you don't yet fully appreciate what Bell did. You need to study his work carefully before you take a strong position on whether he screwed up or not. 


#120
Feb1913, 08:48 AM

P: 128

ttn: here is the paper by Wallace where he describes some of the ideas regarding ontology: http://arxiv.org/abs/1111.2189



#121
Feb1913, 08:51 AM

P: 733

Don't agree? Then please please please address the challenge I keep posting: tell me how to explain even just the one simple subset of the data (namely, that there are perfect correlations when Alice and Bob measure along the same axis) (You I think mean to be pointing out that we already know that "realism" is false. Presumably you are thinking of the KochenSpecker and other similar "nohiddenvariable" theorems. I agree. Realism in that sense is already known to be false. But as bohm2 has explained, this is just a red herring here. To say that Bell's theorem does not prove nonlocality because we already know that realism is false, is like saying that the earth doesn't go around the sun because we already know the earth is round. It is just a total nonsequitur. It is possible to know more than one thing, so discovering X does not in any way preclude or automatically refute a purported later proof of Y.) 


#122
Feb1913, 08:53 AM

P: 733




#123
Feb1913, 09:11 AM

P: 3,181

http://physicsforums.com/showthread.php?p=4236787 Thus, apparently Bell implied with "realism" that it "is meaningful to assign a property to a system (e.g. the position of an electron) independently of whether the measurement of such property is carried out." IMHO that fundamentally disagrees with QM while it is not required for the concept of physical reality, as an electron could be extended as a wave without having a precise, single position. That it should have such a position is an unrealistic definition of "realism" as it only corresponds to a specific subset of models of reality. 


#124
Feb1913, 09:41 AM

P: 733

I don't understand your sentence starting "Thus, apparently Bell implied..." Who are you quoting there? I'm pretty sure that isn't a statement of Bell's! Indeed, I don't recall Bell ever talking about "realism". The whole idea that "realism" is somehow relevant to Bell's theorem is an invention of the people who haven't actually studied/understood Bell. I think I agree with your last couple of sentences, but again isn't the point just that "realism" is used to mean a number of rather different things, so people should be careful to define exactly what they mean whenever they use the term? For example, if the goal is to have what is sometimes described as a "realist interpretation of QM"  that is, some kind of good oldfashioned style physics theory that makes postulates about what kind of stuff exists and how it acts (rather than, e.g., operationaliststyle postulates about laboratory procedures)  then, no doubt, it would be ridiculous to demand from the outset that electrons must have definite sharp positions at all times. Maybe it will turn out that electrons are like little fuzzy clouds, or like groups of ripples on a pond. Such models would be perfectly "realist" in this sense and certainly shouldn't be ruled out a priori at the outset. I think that was your point, and I totally agree. But I think people who voted for "antirealism" in the poll did *not* mean that *this* sort of "realism" is refuted by Bell's theorem. They meant instead the idea that there should exist deterministic noncontextual hidden variables for all (?) "observables" recognized by QM. They are correct that this other sort of "realism" is indeed false, but they are wrong to think that this is the lesson of Bell's theorem. We already knew this realism was wrong, from von Neumann, KochenSpecker, etc. Bell taught something new, something that has nothing to do with realism. Anybody who disagrees should explain how to account for the perfect correlations in a local but nonrealist way. (Everybody knows you can explain the perfect correlations in a local way *with* this deterministic noncontextual HV sense of "realism". But then everybody also knows that you can't explain the *rest* of the QM predictions with that kind of theory. The question is whether the QM predictions can be accounted for by a theory that is local but nonrealist. The challenge is to show that such a model can even account for the perfect correlations subset of the QM predictions. No takers so far, unfortunately.) 


#125
Feb1913, 10:09 AM

P: 141

A useful criterion to classify "locality" that covers both QM and classical probability theories is this: A theory is local if an event in one region of spacetime can't influence the experimental outcomes of an experiment in a spacelike separeted region. In that sense, QM predictions can be explained by completely local quantum theories. Of course nonrelativistic QM doesn't count, but relativistic theories like Wightman QFT's can explain the predictions. Locality is even an axiom there. If you worry about the nonlocal correlations of QM, let's make a simple gedankenexperiment: You have a green ball and a red ball and put them in two identical boxes. You send these boxes to two different people. These people know that you started with a green and a red ball. So the probability to get green/red is 1/2. When person 1 opens his box, he will get a definite result. Let's say he gets red. Then he knows immediately that person 2 has the green ball in his box, even if that box hasn't been opened yet. This is definitely a nonlocal correlation, but nobody would consider this as an action at a distance. Up to now, this isn't quantum mechanics yet. But let's do the same experiment with qubits instead of bits. Instead of green and red balls, we put particles with spin into these boxes. We create 2 particled with orthogonal spin states, put them in the boxes and repeat the same experiment. Of course we get nonlocal correlations again, because we separated two particles that were created with correlation locally. So all the weirdness concerning "nonlocality" is gone and what remains is the standard QM weirdness about the existence of superpositions of states. 


#126
Feb1913, 10:28 AM

P: 3,181




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