Timeline for Why do people still talk about bohmian mechanics/hidden variables
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| Apr 13, 2017 at 12:39 | history | edited | CommunityBot |
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| Mar 23, 2011 at 20:12 | comment | added | iii | BTW, regarding your comment about contextuality. Surely all models of quantum mechanics are contextual. And despite what you said Bohmian mechanics is contextual too! Here gives a good introductory explanation for Bohmain mechanics. pirsa.org/C11001 | |
| Mar 23, 2011 at 19:17 | comment | added | iii | Dear Luboš, Thanks for suggesting me the paper. I read the paper by Zeilinger et. al. which seems the only real scientific one. On page 3 you see "It is clear that other classes of non-local theories, possibly even fully compliant with all quantum mechanical predictions, might exist that do not have this property when reproducing entangled states. Such theories may, for example, include additional communication [23] or dimensions [24]. A specic case deserving comment is Bohm's theory" | |
| Mar 23, 2011 at 10:06 | comment | added | Luboš Motl | Dear @Raskolnikov, the term "realism" has a very specific technical meaning in this debate about interpretations of QM, and rejecting "realism" surely doesn't mean to deny the reality of anything. It only means to refute the assumption that all the information about quantum systems that is ultimately revealed by observations already exists prior to the measurements. In reality - which is governed by quantum mechanics - the information is only decided randomly at the moment of the measurement. The reality of the measured outcomes is the only thing science needs, so science is doing just fine. | |
| Mar 23, 2011 at 10:02 | comment | added | Luboš Motl | Dear @Sina Salek, yes, even contextual hidden-variable theories have been excluded. Just read at least one of the sources I am giving you - e.g. the Zeilinger et al. paper. I can give you many others. Most of the arguments that exclude hidden variables have really been known from the 1920s. ... Concerning the word "contextual" itself, in some sense, all the quantities in proper quantum mechanics are "contextual". None of them is "primitive", which is, in the Bohmian philosophy, the antonym. However, no QM variables are "hidden" and none are "classical" in QM. | |
| Mar 23, 2011 at 9:59 | comment | added | Luboš Motl | Dear @Raskolnikov, John Bell got to his research with the desire to defend realism but the actual research ended exactly with the opposite conclusion than what he wanted. Bell's theorem itself only falsifies "local realism" but e.g. the paper I mention - and be sure that Zeilinger is a top guy in this discipline - refutes non-local realism as well. There's no doubt today that "realism", and not "locality", is the wrong assumption, and what Bell was dreaming about can't change anything about it. Bell was just wrong, OK? There's no doubt about it today. | |
| Mar 18, 2011 at 19:50 | comment | added | Marek | @Raskolnikov, good, I am looking forward to clearing this up. If I completely misunderstood Bell's theorem I'd rather know sooner than later. Still, I am pretty sure I am right, haha :) | |
| Mar 18, 2011 at 19:35 | comment | added | Raskolnikov | @Marek: So it seems we have a fundamental disagreement. It's time I make my answer to the question. I'll try to address several points that have been brought up. | |
| Mar 18, 2011 at 18:27 | comment | added | Marek | @Raskolnikov: incorrect. Bell's theorem states that a theory that has hidden variables and is local must satisfy Bell's inequality. We know from experiment that this inequality is violated 100% in every system. This means that the original assumption that the theory can be both local and contain hidden variables is invalid. But it surely doesn't rule out locality if we are willing to let go of hidden variables, i.e. more complete description of nature, i.e. realism. Agreed? | |
| Mar 18, 2011 at 17:32 | comment | added | Raskolnikov | @Marek: OK, but you do agree with me that Bell's theorem does not rule out hidden variables? What it rules out is locality. Because if one assumes locality, this inevitably implies the need of hidden variables and this then leads to a contradiction (violation of Bell inequality). But if one doesn't assume locality, there is no problem. So, it's not a choice between hidden variables or locality. It's more radical than that, it says there is no locality. | |
| Mar 18, 2011 at 16:29 | comment | added | Marek | @Raskolnikov, perhaps I am using the wrong definition of realism, but for me it's equivalent to hidden parameters in this context. That is, assuming that we could know more about nature than what basic QM tells us (which is that there are incompatible observables and so our knowledge about nature, even in principle, must be strictly smaller than in the classical theory). This kind of realism is what Bell's theorem rules out, if one assumes locality. But I wouldn't be surprised if your definition of realism differed; after all, there are dozens of them out there (e.g. on wikipedia)... | |
| Mar 18, 2011 at 16:25 | comment | added | Raskolnikov | @Marek: But discarding realism is tantamount to discarding science altogether. And I don't see how quantum mechanics forces us to that conclusion. What would it mean to "discard realism"? Everything goes? We live in a magical fairyland where everything is possible? Or does it mean that we just are not capable of describing how the world works? That we just should give up on science or what? What does it mean to abandon realism? If you mean "naïve realism", I'd agree with that, but that is trivial and we didnt' need quantum mechanics to know that. | |
| Mar 18, 2011 at 16:16 | comment | added | Matt Reece | @Raskolnikov: I have read many (but not all) of Bell's articles in that book. While his inequality is important, I disagree strongly with his interpretations of it and of quantum mechanics in general. He seems to view everything through the lens of wanting hidden variables to work (and of a strong affection for Bohmian mechanics). At times he even wrote that Bell's inequalities imply an incompatibility of QM with relativity! This shows that he failed to really understand the lessons of quantum field theory (despite making important contributions to it himself). QFT is very much about locality. | |
| Mar 18, 2011 at 16:12 | comment | added | Marek | @Raskolnikov: what the hell are you talking about? Bell theorem doesn't discard anything. It just tells you that locality and hidden parameters are incompatible. Whether you choose to discard realism and retain locality (which is most sensible and what most people do) or discard locality and retain realism is totally up to you. | |
| Mar 18, 2011 at 12:28 | comment | added | Raskolnikov | @Matt: Actually, the fact that Zeilinger is a co-author is precisely what rised my suspicion even more. The guy doesn't understand shit about Bell's results. He might be a first rate experimentator, he's a second rate theorist. But I didn't want to say that at first, because that's more of an ad hominem. As for your remark: NO, definitely no. Bell's theorem discards locality. It doesn't say anything about realism. I don't know where you're getting that from. I suggest you read "Speakable and unspeakable in quantum mechanics" by Bell. | |
| Mar 18, 2011 at 2:08 | comment | added | Matt Reece | @Raskolnikov, what Bell's theorem discards is "local realism." This doesn't mean giving up locality, if you are willing to give up realism. Of course, this is what quantum mechanics (especially quantum field theory) tell you that you should do. (Also: given that Zeilinger is among the authors of the paper Lubos linked, it seems a bit rash to accuse the authors of not understanding quantum mechanics on the basis of their abstract alone....) | |
| Mar 17, 2011 at 18:31 | comment | added | iii | You still haven't answered his question on why people still talk about non-local hidden variables such Bohmian model! So are you saying even contextual hidden variable theories are impossible? And based on what? | |
| Mar 17, 2011 at 18:31 | comment | added | Raskolnikov | Unfortunately, just reading the abstract of the paper you mention, the authors already fail at understanding Bell's theorem. Bell's theorem does not discard realism. It only discards locality. Bell had to reiterate that point many times over during his lifetime. Most misunderstood physicist ever. | |
| Mar 17, 2011 at 18:22 | history | answered | Luboš Motl | CC BY-SA 2.5 |