6
$\begingroup$

Recent fluid-mechanical experiments by the groups of Couder in Paris and Bush at MIT, mimic a surprisingly wide range of quantum effects. The essential ingredient of these fluid-mechanical systems is a background or pilot-wave that guides the droplets.

Now, surprisingly, a simple analysis of a Bell-type experiment shows that, in the presence of a background field, one of the premises of the Bell inequality, namely measurement independence (MI), is violated. See the paper "No-Go Theorems Face Background-Based Theories for Quantum Mechanics" (available on arxiv). Therefore such classical droplet experiments could violate a Bell inequality. More importantly, if this analysis is correct, background-based hidden-variable theories are admissible, even if they are local (in the sense of ‘involving only (sub)luminal interactions’) and even if they are compatible with free will.

My question: to me the analysis seems fully sound, but maybe there is still an unphysical hypothesis that slipped in ?

$\endgroup$
  • 2
    $\begingroup$ Your link is broken. One can mimic quantum effects in a classical computer, be it analog or digital, this one just happens to be analog. Absolutely nothing stops you from simulating superluminal effects, by the way, so I am not sure where you are going ontologically. A simulation can always do "physics as usual", "physics 2.0" and, if you want it can also do "no physics at all". $\endgroup$ – CuriousOne Apr 20 '16 at 23:44
  • 2
    $\begingroup$ Could you update the link, please, and preferably give a citation so that link-rot is not a problem? I would really like to look at the paper! Thanks! $\endgroup$ – CuriousOne Apr 21 '16 at 1:58
  • $\begingroup$ @CuriousOne. Could be: arxiv.org/abs/1406.0901 $\endgroup$ – Stéphane Rollandin Apr 21 '16 at 9:20
  • $\begingroup$ @CuriousOne. It looks like you can win $1000 (from the author himself) if you find a flaw in the math of this paper. See minkowskiinstitute.org/Vervoort-r1.html ... $\endgroup$ – Stéphane Rollandin Apr 21 '16 at 10:12
  • $\begingroup$ @StéphaneRollandin: He will certainly not lose that money to me. At least the paper is somewhat thought provoking. I am already happy with that. $\endgroup$ – CuriousOne Apr 21 '16 at 15:35
4
$\begingroup$

For the sake of argument, I will assume that all of the calculations of the authors are correct- I don't see any obvious reason that they cannot be.

A few notes:

  1. As the authors note, the measurements independence criterion of the Bell Inequality is a well-known assumption. So pointing it out, by itself, is not an interesting contribution. What one could hope is that analysis of these droplet experiments leads to a plausible model for how this assumption could be violated.

  2. The authors show that suitable background correllations could in principle lead to a Bell violation in a droplet experiment, but they do not specify what observable would actually exhibit these correllations. It is presumably the case that such an observable would have to be 'fine-tuned,' in the sense that you would have to work to figure out how to make a measurement that is suitably affected by the background. There is a good reason they do not propose a specific way of doing this- they do not know one, and it may well be that any suitable observable would be too complex a measurement to be practical.

  3. In general, their model predicts significant deviations from quantum mechanics. As they note, they predict a Bell violation that depends on how fast one chooses the measurements, and as I mentioned it should depend on the observable chosen as well. Of course, one could imagine that we have picked just the wrong frequency range and observables in all of our Bell experiments to see this disagreement.

  4. In a recent claim of a loophole free Bell test, the random choice of measurement comes from both physical processes that are believed to be random, and also from streams of bits that are derived from things like files of various movies and television shows. So a model of Bell's inequality that violated background independence in this case would have to plausibly explain how all these things could be correlated, or how there could be some exploitable glitch in how these random bits are actually implemented as measurement settings. Needless to say, I have not seen such a model yet.

$\endgroup$
  • $\begingroup$ On your notes: 1,2. The topic of the article is not really to argue that a Bell inequality can be violated in droplet experiments (this is just a corollary), but to show that background-based hidden-variable theories for QM are admissible. 4. Thanks for the ref. Another recent ‘loophole-free’ experiment is done by the Zeilinger group. However, if a background field exists (vacuum fluctuations, the ether, or a fancy dark field), then one cannot close the freedom-of-choice loophole: see details in arxiv.org/abs/1602.01859. If correct, the article stands ! $\endgroup$ – LouisV Apr 26 '16 at 23:03
  • 1
    $\begingroup$ One does not necessarily need such a fancy background field- one point I have heard made (by one of the authors of the paper I linked to) is that all the electronics in the experiment are of course hooked up to the same power grid, and one could in principle imagine some weird electrical surge that is just right to trick us. My personal feeling is that until I see a plausible physical model for such 'conspiracies,' I do not find them very interesting. But, I certainly freely admit that they exist as a logical possibility. $\endgroup$ – Rococo Apr 27 '16 at 2:52
  • 2
    $\begingroup$ To stress @Rococo argument : To explain the experiment reported in arXiv:1511.03186, such a background field should direct the bits of a MPEG4(?) file encoding the Back to the future movie. Which means that such a fully deterministic theory implies specific correlations between the mind of Robert Zemeckis in 1985 and the polarization measurement of some photons in 2016 : something probably impossible to disprove from physics, but still quite implausible. $\endgroup$ – Frédéric Grosshans May 10 '16 at 13:05
  • $\begingroup$ oil drop experiments are not claiming to violate bells's inequality, as they have not yet been able to get to an analogue of entanglement. They argue thought that a non local fluid (a a fully correlated one could result in entanglement, but that is still speculative. $\endgroup$ – user83548 May 22 '16 at 21:06
0
$\begingroup$

To Rococo: Your notes do not address the question and the model that is referred to in the article. To be sure we are talking about the same thing, the question is whether the article http://arxiv.org/abs/1406.0901 (published in Found. Phys. 2016) is correct in its main claim, namely that local hidden-variable models that include a background field are possible (and plausible) after all. This is important, since it goes against Bell’s theorem (as it is usually understood). Concerning your notes: 1. You say “What one could hope is that analysis of these droplet experiments leads to a plausible model for how this assumption [measurement independence] could be violated.” My answer: this is precisely what is what is proposed in the article, in a detailed way. If a background is present (e.g. one that has similar properties as the fluid’s pilot wave in the droplet experiments), the article shows that measurement independence and the Bell inequality can be violated. The arguments are pretty straightforward, so if you find a math error or an unphysical assumption, I would be much interested ! 2. You say: “The authors show that suitable background correllations could in principle lead to a Bell violation in a droplet experiment, but they do not specify what observable would actually exhibit these correllations”. The topic of the article is not to argue that a Bell inequality can be violated in droplet experiments. The article concerns the admissibility of background-based hidden-variable theories for quantum mechanics. Only as a corollary, in a few lines, the article suggests that a Bell inequality can possibly be violated in a Bell-type experiment on droplets, because such systems contain a background (the surface or pilot wave on the fluid film). 3. You say: “one could in principle imagine some weird electrical surge that is just right to trick us. My personal feeling is that until I see a plausible physical model for such 'conspiracies,' I do not find them very interesting.” The goal of the article is to show that there is nothing weird to be assumed: just a background field (the physical vacuum, a zero-point field, a dark field,…). Note that that is all what quantum field theory is about. There is no conspiracy at all going on ! Really, your “personal feeling” is the standard opinion; but it is precisely what the article tries to counter, by a straightforward physical model. If you wish to help to assess the validity of the model, you would have to dig into it, and assess the validity of the physical assumptions and of the math.

$\endgroup$
  • 1
    $\begingroup$ (1/2) "the question is whether the article arxiv.org/abs/1406.0901 (published in Found. Phys. 2016) is correct in its main claim, namely that local hidden-variable models that include a background field are possible (and plausible) after all." I think we have no disagree about the first part- yes, they are possible, and your paper gives a general example of how this could come about. I don't dwell on this only because I think it is uncontroversial. $\endgroup$ – Rococo Apr 27 '16 at 23:42
  • 2
    $\begingroup$ (2/2) The point of our disagreement is the "(and plausible)" part. This is by definition subjective, so all I can offer is an opinion. But as detailed in my answer, my feeling is that while the work in papers such as yours is an important first step, a plausible model should specify what this background field is and show what physical couplings to matter it must have to coordinate seemingly random measurement choices in a way that fools every Bell test ever done. Of course, one must also show that it does not cause any disagreement with experiment. $\endgroup$ – Rococo Apr 27 '16 at 23:53
  • $\begingroup$ I think this is the last thing I have to say about this matter, but I certainly welcome your comments, especially if you think I am misunderstanding the situation. $\endgroup$ – Rococo Apr 27 '16 at 23:58
  • $\begingroup$ (1/2) I am happy you agree with the conclusion of the article, but it IS highly controversial that local hidden-variable theories can violate a Bell-inequality. The usual interpretation of Bell's theorem says that this is impossible. $\endgroup$ – LouisV Apr 29 '16 at 17:03
  • $\begingroup$ (2/2) I agree that wat is plausible is subjective. But I think one should not confuse 1) to debunk Bell's theorem, and 2) to construct a full-blown sub-quantum theory. 2) is for a future Nobel prize winner, but 1), if succesful, seems important to me too because it may indirectly motivate 2). When Bell started, very few people found his work important. But it led to important results in QM and Q Inf. Science... So abstract results can lead to concrete results. $\endgroup$ – LouisV Apr 29 '16 at 17:15

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.