Is the idea of "local realism" finally dead?

[deltasigma]

This experiment definitively closes the door on any loopholes regarding photons. Original journal article at link.

The new research, conducted at the Institute for Quantum Optics and Quantum Communication in Austria, closes the fair-sampling loophole by using improved photon sources (spontaneous parametric down-conversion in a Sagnac configuration) and ultra-sensitive detectors provided by the Single Photonics and Quantum Information project in PML’s Quantum Electronics and Photonics Division. That combination, the researchers write, was “crucial for achieving a sufficiently high collection efficiency,” resulting in a high-accuracy data set – requiring no assumptions or correction of count rates – that confirmed quantum entanglement to nearly 70 standard deviations.

[deltasigma]

This is a fairly significant achievement to say the least - as evinced in part by it’s publication in Nature - and given some of the threads I’ve seen here on quantum mechanics, I was hoping it would at least generate some comments. No?

[Chronos]

I’m sure there’s something there, or it wouldn’t be in Nature, but I’m not sure what this tells us that we didn’t already know.

[deltasigma]

@Chronos

I’m sure there’s something there, or it wouldn’t be in Nature, but I’m not sure what this tells us that we didn’t already know.

As the article explains:

Albert Einstein, among others, did not believe that was a complete description of nature, and argued that presumably entangled particles actually have inherent properties which are somehow hidden but which exist before measurement. This view has come to be known as “local realism.”

So what we now know with respect to photons would be that such hidden properties do not appertain. Yes?

[Half_Man_Half_Wit]

What the experiment did was to close the so-called ‘detection loophole’: if your detectors are insufficiently efficient, then you only measure a fraction of the photons; but the detected photons may not be representative of the complete ensemble, and thus, skew the statistics, making the observed violation of a Bell inequality less significant. You can show that you need at least a certain detector efficiency to close this loophole (for some special Bell inequality), and this is the first experiment to do that.

However, the models that are ‘local realistic’ and hide this fact via producing skewed detection statistics are extremely contrived, and I don’t think anybody has ever seriously proposed any as an alternative to the abandonment of local realism; to my mind (and most other physicists’), the issue was conclusively settled at least with Zeilinger’s experiment in the late 90s, if not already with the earlier Aspect experiments.

But all that notwithstanding, if you want to go really hardcore local realist, you can always appeal to the superdeterminism loophole: the measurement directions you chose were always predetermined in precisely such a way as to yield the observed violations. No experiment can close this loophole; but of course, it’s also not an attractive alternative.

[deltasigma]

It’s interesting that you mention Zeilinger since it was his group that performed the penultimate experiment in April:

The young academics in Anton Zeilinger’s group including Marissa Giustina, Alexandra Mech, Rupert Ursin, Sven Ramelow and Bernhard Wittmann, in an international collaboration with the National Institute of Standards and Technology/NIST (USA), the Physikalisch-Technische Bundesanstalt (Germany), and the Max-Planck-Institute of Quantum Optics (Germany), have now achieved an important step towards delivering definitive experimental evidence that quantum particles can indeed do things that classical physics does not allow them to do.

Also from that article:

Quantum theory describes entanglement as independent of any physical separation between the particles. That is, entanglement should also be observed when the two particles are sufficiently far apart from each other that, even in principle, no information can be exchanged between them (the speed of communication is fundamentally limited by the speed of light). Testing such predictions regarding the correlations between entangled quantum particles is, however, a major experimental challenge.

So in the absence of hidden variables, it appears to violate a basic principle of relativity.

Of course it would be easier for me to understand my error and your apparent disagreement with a major scientific paper if you explained things more simply. As Einstein once said.

If you can’t explain it simply, you don’t understand it well enough.

Of course given his feelings regarding quantum mechanics, that’s quoted mainly for the sentiment it imparts.

[Half_Man_Half_Wit]

@deltasigma

It’s interesting that you mention Zeilinger since it was his group that performed the penultimate experiment in April:

Yes, I know. (And the experiment was first reported on last year in December.)

So in the absence of hidden variables, it appears to violate a basic principle of relativity.

Well, that depends. Entanglement can’t be used to transmit information (quantum mechanics is an example of a class of ‘no signalling’-theories), and special relativity can be understood to place a speed limit on the transmission of information, thus yielding no conflict (and of course, it’s perfectly well possible to formulate relativistic quantum theories, e.g. quantum field theories). And hidden variables have no bearing on the issue: if there were hidden variables, then you’d still need to have them influence one another instantaneously, as is for instance the case in Bohmian mechanics; this is in fact much harder to square with special relativity.

Of course it would be easier for me to understand my error and your apparent disagreement with a major scientific paper if you explained things more simply.

I don’t disagree with anything in the paper, merely with the claim that this represents the final disproof of local realism physicists have been yearning for all of these years (which isn’t in the paper). It’s an important technical and experimental advance, and completely justified as a publication in Nature (though if you’ve got the name Zeilinger in the author’s list, you do have an easier time getting in), but it’s not more than that and there’s no need to tout it as such. And even beyond that, achieving a loophole-free Bell test is an important goal, both for the experimental challenges involved and to fulfill a desire of completeness. But at this point, it will not significantly change the distribution of people believing in quantum mechanics versus believing in local realism.

As for my explanation, I’m happy to expand on any point you find unclear, if you ask (rather than insinuate that I don’t know what I’m talking about).

[deltasigma]

I’m aware that entangled particles do not signal one another but you should be aware that this is not the issue. As for what I do or do not understand, that would only be relevant were this a private conversation. Was that your impression?

edit: Oh. The paper clearly does not claim to end the controversy and that was neither my claim. It DOES end the controversy regarding photons however. How you can dispute that is . . . ‘curious’ shall we say and one I think that warrants some explanation I think since it seems to be the claim made in the article - an explanation that can be appreciated by all preferably.

[Half_Man_Half_Wit]

@deltasigma

I’m aware that entangled particles do not signal one another but you should be aware that this is not the issue.

Regarding the compatibility with special relativity, it is indeed the issue. Regarding Einstein locality (a stronger condition not implied by SR alone), signalling is irrelevant, of course.

As for what I do or do not understand, that would only be relevant were this a private conversation. Was that your impression?

No, my impression was that you were asking a question you wanted answered, and did not understand a part of my answer. Hence, my offer to elaborate.

edit: Oh. The paper clearly does not claim to end the controversy and that was neither my claim. It DOES end the controversy regarding photons however. How you can dispute that is . . . ‘curious’ shall we say and one I think that warrants some explanation I think since it seems to be the claim made in the article - an explanation that can be appreciated by all preferably.

There just hasn’t been any meaningful controversy about this in years. Before the experiment, 99% of physicists believed in Bell inequality violation, and afterwards, 99% will continue to do so (the numbers probably carry an error of ~1%). Nobody (that I’m aware of, at any rate) has ever believed that Bell violations were due to violation of fair sampling, i.e. due to only a subset of photons that violate the inequality being detected, while the totality of photons is in agreement with it. But this was a theoretical possibility that is now closed, which I’ve never disputed.

[deltasigma]

So to quote once again from the article

Albert Einstein, among others, did not believe that was a complete description of nature, and argued that presumably entangled particles actually have inherent properties which are somehow hidden but which exist before measurement. This view has come to be known as “local realism.”

In the 1960s, Irish physicist John Bell formulated a definitive test for entanglement, showing that if local realism is true, there are mathematical limits on the correlations between different measurements performed on the particles. If those constraints are exceeded, then the entangled particles are obeying purely quantum-mechanical rules.

This appears to state that Bell’s inequalities are wholly inconsistent with local realism, ergo, whenever they have been shown to be inescapably true as in the case of photons, there is NO local realistic explanation. And it is your contention that this has always been the dominant belief in physics?

[Half_Man_Half_Wit]

@deltasigma

So to quote once again from the article This appears to state that Bell’s inequalities are wholly inconsistent with local realism, ergo, whenever they have been shown to be inescapably true as in the case of photons, there is NO local realistic explanation. And it is your contention that this has always been the dominant belief in physics?

Not always, but as I said, most physicists would probably consider the 98 experiment by Zeilinger’s group, if not the 81-82 experiments by Aspect et al., to have established this consensus.

[Frylock]

@deltasigma

Of course it would be easier for me to understand my error and your apparent disagreement with a major scientific paper if you explained things more simply.

deltasigma, the problem starts with what I just quoted. He did not ascribe an error to you, and he did not disagree with the paper.

[Asympotically_fat]

Yep, it’s important, but not Earth-shattering- it’s ruling out an implausible loophole in tests for local realism.

The experiment didn’t test for the possibility of other loopholes, so whilst the other experimental loopholes had been tested for previously, they may still be in operation in different combinations in different experimental set-ups and that only an experiment that simultaneously tests for all experimental loopholes can close them all.

Of course tests of Bell’s inequalities don’t rule out hidden variable theories, only local hidden variable theories under the assumptions that Bell made. For example Bohmian mechanics is a hidden variables theory whose predictions are indistinguishable from the predictions of quantum mechanics,