Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Status of deBroglie-Bohm pilot wave theory

  1. Jan 8, 2009 #1
    It seems clear from Bell's 1986 article "Six possible worlds of quantum mechanics" (reproduced in J.S. Bell, Speakable and Unspeakable in Quantum Mechanics, 2nd ed., 2004) that he considered the pilot wave theory a radical, but also valid and simple potential solution to the wave-particle duality problem. (My interest in Bohm's theory was piqued when his text was used in my grad school QM course).

    I have two questions about the pilot wave theory:

    1. Isn't this a form of hidden variable theory? If so, why is the theory not incompatible with experiments demonstrating violation of Bell's inequality, and thus that a QM theory cannot contain a hidden variable component? Is it somehow the "guidance" of particles provided by the pilot wave that makes the pilot wave theory compatible with entanglement?

    2. Has there been further theoretical work on the pilot wave theory? Has any theoretical (or experimental) work since Bell's 1986 article cited above either ruled out or made less likely the pilot wave interpretation of quantum mechanics?
     
  2. jcsd
  3. Jan 9, 2009 #2

    Demystifier

    User Avatar
    Science Advisor

    Yes, of course.

    Experimental violation of Bell's inequalities rules out only LOCAL hidden variables. Nonlocal hidden variables are still allowed. DBB pilot wave theory is a nonlocal hidden variable theory.

    Yes, because of nonlocality.

    Yes, a lot.

    No. (Perhaps somebody will disagree with that, in which case I can elaborate.)
     
  4. Jan 9, 2009 #3

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

    Demystifier is well versed on dBB, and his knowledge is deep. There have in fact been a number of efforts made to probe dBB experimentally, including "A first experimental test of de Broglie-Bohm theory against standard quantum mechanics (2002)". Demystifier and other proponents of dBB do not accept these results as meaningful, and I am not in a position to debate the matter either way. Similarly, you may be interested in S. Groeblacher et al., "An experimental test of non-local realism, Nature v.446, p.871 (2007)". If you are interested, Demystifier can explain the objections to these works.

    In my personal opinion: Most non-local alternative theories have the following 2 issues: a) they claim 100% identical predictions to standard QM (SQM), while claiming to be more or less deterministic at their core; and b) they don't actually provide any better explanation (than SQM) of how entangled particles communicate their measurements so they can follow the cos^2(theta) regime. Demystifier would likely not agree with b).

    Currently, there is no single fully fleshed out version of dBB, and this is a field of active research by a number of dedicated scientists.
     
    Last edited: Jan 9, 2009
  5. Jan 9, 2009 #4
    Interesting. I was told Bohm could show that his theory was indistinguishable from classical quantum mechanics. Is that false?

    The problem was supposed to be that it neither does well with SRT nor QFT nor density matrices.
     
  6. Jan 10, 2009 #5

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

    I think the answer is both yes and no. It is indistinguishable in the general sense that all of the predictions of QM are to be matched. However, there is no SINGLE dBB-type theory agreed upon by all. And in the ideal, there will be some predictive difference that appears in the dBB version that is NOT present in SQM. I.e. it is inevitable that the dBB theory will go further (than SQM) in its explanation of the underlying mechanics. It must really, that is its primary "advantage" and therefore it will end up opening itself up to some potential difference eventually.

    So what is currently happening is that as more specific versions of dBB are being put forth, tests are being conceived and executed. Again, without debating the merits, dBB has not passed those tests.

    One of the problems, as I see it: In a Bell test, Alice and Bob are observed at different times, and in fact their polarizers can be changed in mid-flight both before and after the photon passes by. The wave state is to be determined by effects from all particles everywhere (since it is non-local). Yet the effect of everything in the universe cancels out except for the settings of Alice and Bob, which aren't even both set at a single point in time (imagine one photon travels 5km and the other travels 20km to their detectors. And yet it is ONLY Alice and Bob's polarizer settings, at different points in time, which contribute to the cos^2 equation. If we are looking at a non-local instantaneous connection, this doesn't seem to make sense (to me anyway). Now I know perfectly well that the dBB theory is to be indistinguishable from SQM in its predictions, but I just don't see how this scenario pans out. What I am hoping for is a mechanical description of this type of entanglement. Obviously the SQM explanation is simply the formalism, and no mechanism is provided. But if dBB is to move into the mainstream, I think it needs to address this head on. Otherwise, it ends up being nothing more that a curiosity to the great majority of scientists.
     
  7. Jan 12, 2009 #6

    Demystifier

    User Avatar
    Science Advisor

    As DrChinese said, there are a few different versions of the dBB interpretation. Some of them are compatible with SRT and QFT better than the others. (If you want, I can give you further references.) Concerning the density matrices, they are all compatible with it, provided that one accepts that the whole world allways has a well defined wave function (pure state), while density matrices only describe our incomplete knowledge about subsystems.

    I would like to mention only one very fresh reference:
    http://xxx.lanl.gov/abs/0811.1905 (yesterday accepted for publication in Int. J. Quantum Inf.)
    because, in my opinion, it is the most explicit demonstration so far of the compatibility between (one version of) dBB and SRT.
     
    Last edited: Jan 12, 2009
  8. Jan 12, 2009 #7

    Demystifier

    User Avatar
    Science Advisor

    Actually, I agree with you that dBB does not provide a better explanation of HOW entangled particles communicate. Nevertheless, it does provide a less vague description of that communication, in the sense that it provides a clear picture of what these particles (the communication between which should be explained) are.
     
  9. Jan 10, 2010 #8
    I think there is a lot to be said for not blindly accepting the SQM, but with that being said I question the fundamental motive that drives people to run from the implication of that model. It is less elegant than some others because, to be blunt, it is less of a concocted theory to explain existing evidence, and more of a work in progress that has had people scratching their heads for nearly a century. I wouldn't place my bets firmly in either camp, but without anything but thought experiments of dubious value, I find the SQM more explanatory if not so comforting.
     
  10. Jan 10, 2010 #9
    While some might attribute the motivation of those who look to dBB pilot wave theory to an inability to come to grips with the wacky nature of quantum mechanics, I believe that for many, this motivation is something more profound.

    The SQM is essentially an end of the road. It says, there are no more answers here, this is all that we have and will ever have. Nature behaves thus and we cannot know more than this. Most physicists believe this: that nature is statistical and we cannot know more than what lies deeper because of the Heisenberg Uncertainty Principle; that there is no deterministic basis for the behavior we see in Quantum Mechanics.

    For some who pursue dBB, there lies a tenacious need to discover new physics and new routes for explanations to the great question which has driven science from the very beginning: Why?

    It may be that there is no answer beyond what we have in SQM. Nevertheless, the appeal of dBB pilot wave theory for some—and I count myself among these—is that it offers a potential path for further discovery of new physics, new understanding, and new possibilities. It represents an area of largely uncharted new territory.

    We've tried thinking of the world largely in SQM terms for more than 60 years, we haven't been able to answer the big questions yet. Why not try something new as long as we stick to the facts and experimental data?

    It may be that the seemingly small difference of allowing for the possibility for separation of the wave from the matter is enough to germinate some new ideas that, in turn, open some breakthrough on the big questions of the day: unifying QM and gravity, dark matter, or dark energy, etc.

    If there is something more deterministic underlying the behavior we clearly see and that is modeled very well with SQM, wouldn't it be good to know what it is? If it exists, we will never find it if we don't explore all the routes that at least admit of its possible existence. You won't generally find, or even look for, what you don't think is possible.
     
  11. Jan 10, 2010 #10
    There has been a great deal of interest in deBB theory over the last ten years, and a great many papers published, despite the desperate (and often ignorant) resistance put up by traditionalists. The reasons are essentially as inflector says (I couldn't have put it better myself!). They're even teaching it at mainstream universities now. If you're interested in learning more about it I usually recommend the Cambridge University course:

    http://www.tcm.phy.cam.ac.uk/~mdt26/pilot_waves.html

    I recently saw the author of this give a popular public lecture which is very interesting and a good introduction at a much lower level than the graduate course:

    The return of pilot waves, or, why Bohr, Heisenberg, Pauli, Born, Schroedinger, Oppenheimer, Feynman, Wheeler, von Neumann and Einstein were all wrong about quantum mechanics.
     
  12. Jan 10, 2010 #11
    Two points... first, wanting something doesn't make it true. Wanting the universe to be purposeful or elegant or anthropic doesn't go anywhere towards making that a reality. The second is the QM is not the end of the line anymore than GR was the end of the line. Most respected physicists, when pressed (and not during grant season) will probably tell you that beyind a unification of GR and the SQM would be a new physics, and maybe one requiring new mathematics to explain it.

    Yes, the concept of Beckenstein Bounds, or the Planck Scale seems superficially to limit any possiblity of a deeper understanding of nature than is offered by QM. This is untrue for the reasons stated however, although once again the question of determinism would be up for grabs. Personally, I don't find a deterministic view of nature comforting or intuitive in the least. An emergence of space-time from chaos that we can't comprehend seems entirely possible given that we're just slightly evolved chimps. The limits to what reality is may not be reflected by our ability to describe it, sad as that is to consider.

    It may be then, that no theory has the neatness or elegance we want because we can't get that precise. The very nature of our existene could limit us... and so on. The only reason I get philosophical here, is because again, by no means does QM represent an end to any roads.... and even if it did none of us will live long enough to see that confirmed (if true). So, take heart in the inevitability of death at least. ;)
     
  13. Jan 10, 2010 #12

    DrChinese

    User Avatar
    Science Advisor
    Gold Member

    That might be a little melodramatic, the end of the road part. By my count, there were over 1000 papers published on entanglement alone last year. I would estimate that no more than 1% of those advocated the Bohmian perspective or otherwise were derived based on pilot wave theory. So presumably, the remaining 99% were either based on SQM (or possibly some other interpretation). That doesn't sound like the end of a road to me.

    We have been hearing about the advantages to a new approach for many years now (not just in this area but with string theory and other avenues too) and yet it would take only a clear success to convince. My point being that SQM is giving us something right now (99%) and the new approach has yet to produce anything convincing. I think this is pretty much indisputable. I am, of course, open to any new advance which the Bohmian approach is able to provide us; and would welcome a non-local interpretation if that proves to be better in the end.
     
  14. Jan 10, 2010 #13
    I think the name of that lecture is too short. To be fair, "Hawking, Thorne, Susskind, etc... etc..." would be less cute and flattering, but more accurate. Then again, maybe just, "Why everyone except the people in this room are wrong" would work? Either way, teaching something is hardly a concession to the reality of a theory, any more than NOT being allowed to teach something makes it valid. For instance, I believe they teach religion at Cambridge and most other universities in the world. That doesn't really make religion anything more than popular enough to form a class. Sorry.

    I am a little disturbed by the number of arguments that center on liking one theory over another for reasons that seem to have less to do with the science and more to do with what is most palatable. I'm not sure why a work in progress (QM) needs an interpretation that makes a series of assumptions for the sake of remaining at least semi-classical.
     
  15. Jan 10, 2010 #14
    You may think he's being cute, but what he says is in fact not controversial. With the exception of Einstein (and possibly Schroedinger), all of the people on the list, at one time or another, said that it was impossible to reformulate quantum mechanics as a hidden variables theory i.e. - in the case of deBB - to convert QM from a statistical theory of observation into a dynamical theory of particle trajectories. Von Neumann even proved mathematically that it was impossible.

    Irrespective of which, if any, interpretation of QM has anything to do with 'reality', we all now know that this is incorrect and that it is possible to formulate QM in such a way. Therefore the people in the title were wrong on this point (though of course right on many others). That's quite interesting and certainly something that the well-rounded student of QM should be aware of.

    I forget why he said Einstein was wrong. It was something about not taking advantage of the existence of deBB in his propaganda battle against Bohr.

    Twenty years ago there were no classes on this anywhere in the world, and probably less than 10 people did research in the field. Now there are quite a few classes on the subject, and hundreds of researchers. So it is readily apparent that people are finding the theory increasingly interesting. It is a perfectly reasonable question to ask why this is so.

    Your argument about teaching religion is irrelevant. In science there are criteria for what subjects are taught. String theory, orthodox QM, and de Broglie-Bohm theory are taught because there appears to be a point to doing so. Phlogiston theory is not.

    As has been discussed here before, de Broglie-Bohm theory makes concrete predictions that in extreme circumstances are different to those of orthodox QM, and so it can in principle be tested. That's science for you.

    And there are certainly reasons for formulating a theory in a more palatable manner. When you're teaching students, why confuse them with a needlessly obscure interpretation which denounces explanation on principle and pretend that this has to be the last word? Why not at least make students aware that if you make the single change that electrons exist when no-one is looking at them, then one can logically deduce that QM is just the statistical mechanics of particles with trajectories made non-classical by the influence of a wave. Irrespective of whether it's true, it's the only clear explanation that currently exists, and again this is highly interesting.
     
  16. Jan 10, 2010 #15
    Funnily enough, those percentages reflect almost exactly the proportion of students that are actually taught about the de Broglie-Bohm theory.
     
  17. Jan 10, 2010 #16
    I don't think he's being cute, I think he's being incorrect. My point however, was meant to be cute, but I'm guessing it fell short of the mark. As for my point about religion being taught, unless we're talking about two completely different worlds, this is one in which science is not immune to the pressures of a vocal minority in academia.

    As for deBB making predictions, that's lovely, but as yet untested and untestable. Arguing that a minority are informed makes sense only when a theory is relatively young and unvetted. Until and unless an experiment can be formulated, performed, and repeated in which deBB is distinguished from QM, it's still just an ad hoc theory. That's science too.

    As for the notion that 1% of students (which students, where?) are taught? That is one statement for which sources are needed before I buy it, sorry.

    As for believing that a Local Hidden Variable Theory can be valid, I'm firmly in the camp that says no, no it can't AND that's precisely the reason why dBB advocates changed the argumnets over time to be nonlocal and therefore sidestep the issue. You are doing a bit of sidestepping yourself, going back to an "end of the road" argument when that is simply NOT the implications of either theoretical framework.

    Oh... and give Phlogiston time, I'm sure it'll make a comeback! lol... Seriously however, String Theory(ies) are lovely and interesting, but they're contributing to other areas of math and science. Hell, Knot Theory alone makes String Theory interesting enough to teach. I'm not clear that dBB makes any similar contributions except to let some people cling to a deterministic view. If, to go into an unrelated discipline, Phrenology had any predictive merit it might still be taught. After all, fMRI's are fueling something like the next step in that way of thinking, albeit less... wrong. Until and unless dBB yields results of value to anyone save its proponants, I don't really care that some academicians are willing to have it taught.

    As for Einstein vs. Bohr, that is a matter of history and record like Edison vs. Tesla. Was it true? Oh yes. Does the fact that we have this in detail as part of an open historical record mean that argument would be more compelling if the THEORY had more inherent traction? Also yes. Being persecuted can be the result of advocating compelling "heretical" knowledge, but that alone isn't an argument for or against a theory.
     
  18. Jan 10, 2010 #17
    Doesn't matter what you think. It is possible to formulate QM as a hidden variables theory. The guys that were cited in the Cambridge lecture said it wasn't. Therefore they were wrong, even if you personally don't like the theory in question.
    Indeed it is. Isn't that the point Lee Smolin and Peter Woit were making in their anti-string theory books?
    Well, progress is being made. See e.g. the following papers - full references to these and other similar papers on the Cambridge web site Further Reading section..

    - Inflationary cosmology as a probe of primordial quantum mechanics A. Valentini (2008).
    - De Broglie-Bohm prediction of quantum violations for cosmological super-Hubble modes, A. Valentini (2008).
    - Astrophysical and cosmological tests of quantum theory, A. Valentini (2007).
    Of course you're right - I simply made up the number (as indeed did Dr. Chinese). That said, there are probably several tens of thousands of physics students in the world, and I don't think it's unreasonable to suppose that a couple of hundred of them have hidden variables mentioned in their QM classes, since we know that at least some of them do.
    It has been demonstrated experimentally essentially beyond reasonable doubt that local hidden-variables theories are not valid - this is hardly a controversial viewpoint.

    So what do the experiments actually show? Many people conclude that they show the universe is indeed non-local, in the sense that certain kinds of influences can propagate instantaneously. If this is so, then the non-local aspect of deBB theory is actually a point in its favor. The alternatives to believing the universe is non-local are (simplifying somewhat) things are not real, there are an infinite number of parallel universes, or influences can travel backwards in time. Which of these four bizarre things do you believe? Many people consider the idea that influences can travel very fast to be the least unreasonable.

    Also, DeBB advocates did not change their arguments over time, as you state.
    And we don't care that you don't care.. :smile: So just let us get on with it, and let's see what we can do. Isn't science fun?
     
  19. Jan 10, 2010 #18
    You seem tense, or maybe a bit high-strung? (badum cha, intention double pun). I'm unimpressed. I also recognize an intractable issue when I confront one, and my interest in seeing you argue for the general lack of regard of a theory as one of its selling points has moved from amusing to absurd. We're not going to settle this today... actually WE'LL never settle any of this... but I want to make somethings clear...

    String Theory... I like it. It's nifty. It's probably a complete concoction of ad hoc theories just like deBB. I'm not arguing for the perfection of QM and sattelite theories, rather I'm arguing (much as Dr. Chinese is) that the Standard Model has been a source of progress in the field and deBB has not. Ok... deBB has progressed on the heels of SQM, but again: only in an ad hoc fashion. Until it is something more than a theory that is long on EXTREMELY vast promises (a return to determinism, wow) and short on anything even approaching results. Discussions of theoretical results don't equal results. Hell, at least String Theorists and Brane Cosmologists can hope that the LHC produces something for them. deBB is so diffuse right now, especially the pilot wave theory, that experimental verification seems as likely as observing photon decay. :) See, I can exaggerate too!

    EDIT: "Things are not real" is a very sloppy way of putting it. If you're referring to the holographic principle, that hardly argues for unreality. The fact that MW or wavefunction collapse (as per Copenhagen), doesn't mean that something equally strange isn't "real". The idea that a species with a biology that is practically DEDICATED to filtering input from our senses would have an intuitive grasp of what is "real" (a metaphysical term if you ask me) is absurd. To simplify Brane Cosmology, virtual pair annihilation, a holographic principle, or many worlds interpretation as "unreal" is grotesque. They are conjectures to explain mounting theoretical advances and (trailing) evidence that what you seem to consider a "real" world just isn't what we expect. I for one find that acceptable, if not entirely comprehensible.

    Oh... and the not caring bit... well, you'd pretty much have to build a thick skin after a while in your particular line of research wouldn't you? That, or cultivate fast reflexes to dodge the overripe fruits and custard pies. :D
     
    Last edited: Jan 10, 2010
  20. Jan 10, 2010 #19
    Not me, mate. Very relaxed. Sitting here gazing out the window with a glass of Montenegro.
    It's not intractable, it just requires hard work. Maybe you don't want to do it, but that's OK.
    You're right. It is absurd. Which is why I never said any such thing.
    True enough. Which soccer team do you support, then?
    Look, I know you don't care, and you're not going to read any of the references I gave you, but it was invented before SQM (1923-27) so one could honestly say that SQM is just deBB theory with the absurd notion added that electrons disappear if you're not looking at them. You couldn't make it up.
    Well, so you say, but I like it..

    If you want to talk about something, why don't you answer my question from my previous post. Which of the four bizarre things do you believe? (none of them, right?).

    EDIT: OK - now you have (for people reading this, at the end of the previous post).
     
  21. Jan 10, 2010 #20
    See what happens when you edit your posts while I'm replying to them. Whose turn is it now then?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Status of deBroglie-Bohm pilot wave theory
Loading...