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confusedashell
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I just wonder to the doubters after so many experiments proving nonlocality, what type of "Proof" would be "proof" enough for the people out there who still think a local interpretation could ever describe reality?
confusedashell said:I just wonder to the doubters after so many experiments proving nonlocality, what type of "Proof" would be "proof" enough for the people out there who still think a local interpretation could ever describe reality?
confusedashell said:I thought Bell in essence proved that if any theory are to fit the emperical evidences of Quantum Theory it has to be nonlocal?
confusedashell said:So either you must believe quantum theory is wrong, or accept nonlocality?
akhmeteli said:As far as I understand, it is believed that Bell proved that a local theory cannot be compatible with predictions of quantum theory.
Predictions, but not existing empirical evidence. In other words, experimental results incompatible with local theories have not been obtained yet.
jtbell said:Make that "a local and realistic theory." Here "realistic" means that variables such as position or spin orientation have definite (although possibly "hidden") values at all times, even before a measurement of those variables.
jtbell said:As far as I know, the experimental evidence so far is consistent with the predictions of QM and therefore inconsistent with the predictions of any local realistic theory (per Bell's Theorem).
The entire point of the Aspect and other EPR-Bell type experiments is that they confirm the predictions of QM in regions where the Bell inequalities are broken.akhmeteli said:I am afraid this is a non sequitur. Experimental evidence consistent with the predictions of QM is not "inconsistent with the predictions of any local realistic theory (per Bell's Theorem)" as long as it does not break the Bell inequalities. And as far as I know, no existing experimental evidence breaks those inequalities.
You misunderstood something.akhmeteli said:Experimental evidence consistent with the predictions of QM is not "inconsistent with the predictions of any local realistic theory (per Bell's Theorem)" as long as it does not break the Bell inequalities. And as far as I know, no existing experimental evidence breaks those inequalities.
Doc Al said:The entire point of the Aspect and other EPR-Bell type experiments is that they confirm the predictions of QM in regions where the Bell inequalities are broken.
Demystifier said:You misunderstood something.
akhmeteli said:I agree, that was the entire point of those experiments. The problem is, strictly speaking, they failed to make their point. Abner Shimony (and it seems he knows what he is talking about and he is no fan of local realistic theories of QM) wrote the following in his article on the Bell theorem in the Stanford Encyclopedia of Philosophy (http://plato.stanford.edu/entries/bell-theorem/#3 ):
"The incompatibility of Local Realistic Theories with Quantum Mechanics permits adjudication by experiments, some of which are described here. Most of the dozens of experiments performed so far have favored Quantum Mechanics, but not decisively because of the “detection loophole” or the “communication loophole.” The latter has been nearly decisively blocked by a recent experiment and there is a good prospect for blocking the former."
Therefore, as I said, until recently (the article is marked "Copyright 2004"), there have been no experiments demonstrating (without additional, more or less arbitrary assumptions, such as the "fair sampling" assumption) a violation of the Bell inequalities. Again, maybe something radically new has happened since then that I am not aware of. In such case I would appreciate if somebody more knowledgeable than I could enlighten me.
ZapperZ said:I posted this just 2 days ago:
https://www.physicsforums.com/showpost.php?p=1698898&postcount=67
Zz.
akhmeteli said:Thank you very much for the reference.
For the benefit of others: as far as I could understand, the experiment in the cited paper (I guess there is a version in arxiv as well) demonstrates the Bell inequalities with the detection loophole closed. The locality loophole is still there big way though: to close it, they need to increase the spatial separation from 1 m to 15 km. Thus, it looks like the general conclusion still stands: there has been no experimental proof of nonlocality so far (44 years since the Bell's article), and I don't hold my breath waiting for such proof. And not just because of experimental difficulties or because local realism is any relative of mine, but because, for reasons outlined in my post #6 in this thread, such proof would mean that unitary evolution of quantum theory is not universal. And this may be too big for me to swallow.
peter0302 said:Well, there've been other experiments in which the distances were sufficient to close the locality loophole. I know you want both loopholes closed in one experiment but I think you can syntehsize the two results and deduce what would happen...
peter0302 said:Show me information transferred faster than the speed of light and then I'll believe in non-locality. Until then, what you call the "loopholes" look to me like symtoms of the fundamental problem, i.e., that locality is real and that realism is wrong.
ZapperZ said:At some point, you need to step back and look at the body of evidence here.
There's a whole slew of experiments that have closed the locality loophole. All of them involved photons.
There's another slew of experiment that have closed the detection loophole. All of them involved some form of particles, from protons, neutrons, muons, etc.
Now, you are telling me that, somehow, each one of the still "open" loophole conspires to make themselves be THE factor that can still account for the apparent violation of Bell inequality. Don't you just find that to be rather a very unlikely coincidence?
ZapperZ said:There's another glaring aspect of this. All, and I mean 100%, of the experiments on Bell inequality all make the same claim of the violation. Now, one would think that someone who isn't convinced of this, or who is claiming that such-and-such a loophole is responsible for the apparent violation, would at least be able to conduct his/her own experiment, show the data, and argue conclusively that others who have performed the experiment have erroneously analyzed the data, and that the <insert favorite loophole here> loophole is there, in the data. Now, can you find me such an experimental report? I have found none. Don't you find this rather curious? Why is that?
ZapperZ said:I will tell you why. In the detection loophole, for instance, even without a 100% efficiency in photon detection, people who do such experiments have to first of all learn about the behavior of their detectors. Everyone who depends on any form of photodetectors have to do this, including high energy experimentalists. We need to know how these instruments behave, what they can do, and more importantly, what they can't do. We need to know when the data we have is reliable, and when we are over-reaching.
So when experiments involving entangled photons are performed, even without a 100% efficiency, we have an excellent idea of the performance of the detectors to say with reasonable confidence of what the actual data are. To me, it is why you have never, ever seen such experiments that contradict the conclusion of violation of local realism so far. It is because once you learn and understand the behavior of such detectors, you'd never pay attention to the weak "detection loophole" argument. The argument against the validity of a data set can only be made by experts not only in the physics, but also in the detection scheme. When Talayerkhan claimed to detect fusion in his bubble fusion experiment, his detractors were not someone who have no clue on the experimental method he was doing. In fact, many of them were world-renowned experts in neutron detections, and they pointed out exactly where the device he's using and the method he adopted can easily produced faulty results. This then threw a lot doubt in the data and subsequently the conclusion (I haven't yet mentioned the fact that others who tried to reproduce the experiment did not get the same result). I have never seen that done with any of the Bell experiments done so far. Considering that there have been plenty of such experiment, and with 100% agreement on the conclusion, I find the lack of contradicting experimental results to be a very obvious shortcoming of those who claim otherwise.
So to me, the continued stubbornness in proclaiming that local realism is still valid because this loophole is still open, or that loophole is still open, has nothing to do with not having convincing experiments. That's like saying Evolution isn't true simply because there are still "gaps" in our knowledge, or that QM isn't right because it still can't be reconciled with GR. It rings hollow because of what they all CAN do already, whereas the alternative have done nothing. All the experiments have produced ONE very convincing argument in favor of violation of non-local realism per the Bell theorem. That is what all these papers have argued and concluded. The paper that I had recently cited simply tried to start hammering down the last nail in the coffin - by being the first to attempt at closing BOTH locality and detection loophole simultaneously. The alternative, being local realism via the non-violation of Bell inequality, have ... er ... zero experimental evidence!
Zz.
Why do you say this? The projection postulate is part of (some interpretations of) quantum mechanics; I don't see where it's used in deriving Bell's inequalities.akhmeteli said:You see, you cannot prove the Bell inequalities without the projection postulate (please advise if I am wrong).
One can, in principle, never prove non-locality with entanglement experiments. Oneconfusedashell said:I just wonder to the doubters after so many experiments proving nonlocality
That is a good question. If it is very unlikely that there is a true loophole, i.e., if it is very unlikely that actually QM is wrong and nature is local, then why new experimental confirmations of the things that we "already know" are regularly published in journals such as PRL and Nature? I really don't understand it. Does anybody?akhmeteli said:Otherwise why all these attempts to close the loophole? Why publish the results of such attempts in PRL?
Demystifier said:Just for the record, I do think that QM is correct and that nature is nonlocal.
akhmeteli said:I don't know anything about any conspiracy. There is just no experimental evidence of violation of the Bell inequalities (the assumptions of the inequalities require sufficient spatial separation). What am I supposed to do?
I readily admit that I don't know much about experiments testing the Bell inequalities. But my understanding is there is a consensus among knowledgeable people (including you), no matter what they think of the interpretation of QM, on what has been demonstrated experimentally and what has not (actually, you are not saying that I misrepresented the experimental situation. If you believe I did, please advise). So I don't see any need to perform any special "local realism inspired" experiments. I hope the bulk of the existing experiments were conducted by knowledgeable people, and I don't care whether they were local realists, positivists, solipsists, whatever. There is no disagreement on what experimental results were obtained. But everybody is free to draw their own conclusions, as long as such conclusions are consistent with the established experimental data.
Look, you know a lot about photodetectors, I know next to nothing about them. Does this mean that I am not in a position to have my own opinion on the violations of the Bell inequalities? I don't know if people who first raised the issue of the detection loophole knew a lot about photodetectors.
Are you telling me I must accept the fair sampling assumption just because I don't know anything about photodetectors? Am I supposed to believe in god just because I have not read the bible or the quran? Then in which god am I supposed to believe - the god of the bible or the god of quran? I just know that the fair sampling assumption is not generally recognized, so I am free to accept or to reject it. I choose to reject it, and I am left with no experimentally observed violations of the Bell inequalities.
OK, forget about local realism for a moment. In my posts I was trying to explain why I don't admit (as long as it is possible without a contradiction with experimental data)that there is any experimental evidence of violations and why I don't expect any such evidence to appear. You might have noticed that my reasoning had little to do with local realism. Furthermore, I actually swore by quantum theory. You see, you cannot prove the Bell inequalities without the projection postulate (please advise if I am wrong). The projection postulate introduces irreversibility, whereas unitary evolution of the quantum theory allows no such thing. You cannot have it both ways - unitary evolution and the projection postulate. You have to choose. I choose the unitary evolution, which is quantum theory, pure and simple. You may say: but we have to accept both mechanics (classical or quantum), where there is no irreversibility, and thermodynamics with its irreversibility. Yes, but we understand that the irreversibility of thermodynamics is just a very good approximation, as the underlying mechanics does not allow any rigorous irreversibility. You need some "manual" interference, however subtle, to obtain irreversibility. And it seems that the analogy between mechanics vs. thermodynamics, on the one hand, and unitary evolution vs. projection postulate, on the other hand, may be deep enough, as arXiv:quant-ph/0702135 (Phys. Rev. A 64, 032108 (2001), Europhys. Lett. 61, 452 (2003), Physica E 29, 261 (2005)) demonstrates, using a rigorously solved model, how the projection postulate evolves as a result of thermodynamic irreversibility.
They are not realists, i.e., they do not believe that there is objective reality existing even without measurements.Hans de Vries said:What about all the people who believe that both QM and SR are correct?
Demystifier said:They are not realists, i.e., they do not believe that there is objective reality existing even without measurements.
Maybe you are right that most people think that way, but is their view logically consistent? Do they understand the Bell theorem? I don't think so.Hans de Vries said:I completely disagree here. Most of the people who believe that both QM and SR are
correct do not even consider a world without objective reality.
Hans de Vries said:Most of the people who believe that both QM and SR are
correct do not even consider a world without objective reality.
Demystifier said:Do they understand the Bell theorem?
First, even if you are right, this is not what most people think (we were talking about the majority, right?).Hans de Vries said:In post #22 I reasoned that non-locality can not be proved by the failure of Bell's specific type of hidden variable theory.
Demystifier said:First, even if you are right, this is not what most people think (we were talking about the majority, right?).
Second, I disagree with you. I think that the Bell result disproves ALL local hidden variable theories.
Third, there are even more direct proofs that QM and local hidden variables are incompatible. The Hardy proof is the simplest one.
Is that supposed to be in contradiction with something that I said?Hans de Vries said:A majority of Quantum Field Theorists does not believe in causal relations outside the
lightcone and the textbooks teach accordingly, Peskin and Schroeder, Weinberg...