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Heisenberg's Uncertainty or hidden variable

  1. Mar 16, 2014 #1
    Intuitively it seems more plausible that there should be a hidden variable influencing the path of particles when they travel, than having a fundamental mechanism of nature be truly random, but what do I know. I just don't understand how you can ever completely rule out a hidden variable. I know that Bells theorem shows that because of quantum entanglement all determining variables cannot be local (ie a feature of the particle). I just read his paper and he shows an example of how entanglement can be used to influence the flight of a particle, thus proving that not all relevant variables can be purely local and disproving that part of Einsteins theory, but he does not (and doesn't claim to) completely rule out other hidden variables, local or not.

    So I guess my questions are these:
    In the abstract, how could you ever claim to disprove the existence of further unknown influencing variables, if your model cant predict a result with exactly 100% accuracy?
    Are there other indications that hidden variables do not exist?
    If not, why isn't the generally agreed upon theory not that hidden variables exist, given that it seems more intuitive and that is generally the default assumption about systems we cant (yet) model?

    Also with uncertainty you can now have a cat in a superposition of dead and alive, which again doesn't seem very intuitive. The thing that triggers the collapse of the superposition is supposedly observation, which begs the question if consciousness is required for this. I guess we can assume that a cat isn't conscious (I think it probably is to some extend, but that's irrelevant here), but if it was a human in the box, surely that person would experience either life or death? If wave functions really only collapse when being observed by consciousness, then the vast majority of the universe must be in a super position by now. Also all matter should be in superposition right after the big bang so the first life would have to arise from matter in superposition, but that shouldn't happen...
    I think I like the hidden variable solution a lot better.
     
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  3. Mar 16, 2014 #2

    atyy

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    Hidden variables may exist, and these are the de Broglie-Bohm theories and its variants.

    At present there is no consensus if these also adequately describe relativistic quantum mechanics. Many different hidden variable theories are possible, so while they provide a conceptual framework for quantum mechanics, picking any one of them as a more fundamental theory is not useful at the moment.

    The other major approach to solving the "measurement problem" of quantum mechanics is the many-worlds approach. I don't know if this approach really works, but David Wallace's book "The Emergent Multiverse: Quantum Theory according to the Everett Interpretation" is a very good and serious exploration of how such an approach may work.
     
  4. Mar 16, 2014 #3

    stevendaryl

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    It might be surprising that you could rule out a hidden-variables theory without knowing exactly what that theory might be, but Bell's Theorem does exactly that. There is an experiment, the EPR experiment (or variants of it) whose outcome is provably impossible for any hidden variable theory (with some caveats) in which causal influences travel at a speed equal to or slower than the speed of light.
     
  5. Mar 16, 2014 #4

    ZapperZ

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    Sorry, but intuition is a BAD measuring stick to challenge anything in QM. We all should learn about that by now, I would think.

    Zz.
     
  6. Mar 16, 2014 #5

    bhobba

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    Indeed.

    Well since no one claims that - what's your concern?

    I don't know where you got that from. There is no such general agreement, indeed DBB is a hidden variable theory many believe in, and virtually everyone, myself included, accepts as valid.

    Even Feynman's sums over histories approach everyone learns about is actually a hidden variable theory - but of a rather novel and non trivial type - that the particle takes all paths simultaneously is ascribing a variable - namely its path - that you really cant say it actually has until it's measured.

    Thanks
    Bill
     
    Last edited: Mar 16, 2014
  7. Mar 17, 2014 #6
    I agree but I don't think I implied that anywhere. What I am saying is that if you have two theories equally descriptive of nature (ie same math) and one is more intuitive, I would default to trust that one. That is not the same.

    I don't think this is true. Here is a direct quote from the conclusion of Bells's original paper:
    "In a theory in which parameters are added to quantum mechanics to determine the results of individual measurements, without changing the statistical predictions, there must be a mechanism whereby the setting of one measuring device can influence the reading of another instrument, however remote. Moreover the signal involved must propagate instantaneously, so that such a theory could not be Lorentz invarent."
    http://philoscience.unibe.ch/documents/TexteHS10/bell1964epr.pdf
    Einsten claimed that all variables influencing the path of the particle were local and there were hidden ones that influenced the flight of the particle. What Bell says in this paper is that not all variables can be local because he demonstrates how quantum entanglement can be used to influence the path of a particle (influencing spin and then using magnetism to change its path). He does NOT claim to prove that there are no hidden variables, local or otherwise! And I still don't see how this could ever be done, more than I can see how you can disprove god, lets say (he could after all be the hidden variable).

    Well stevendaryl a few posts above you seemed to claim that and you read and see it a lot in popular media (such as youtube video). Here is an example on Steven Hawkings homepage:
    "Hidden variable theories might seem to be the most obvious way to incorporate the Uncertainty Principle into physics. They form the basis of the mental picture of the universe, held by many scientists, and almost all philosophers of science. But these hidden variable theories are wrong. The British physicist, John Bell, who died recently, devised an experimental test that would distinguish hidden variable theories. When the experiment was carried out carefully, the results were inconsistent with hidden variables."
    http://www.hawking.org.uk/does-god-play-dice.html

    So I still don't really know if there are a big misconception out there or if I am missing something big.
     
  8. Mar 17, 2014 #7

    bhobba

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    The misconception is in you.

    'a hidden-variables theory' is not all hidden variable theories. We have theorems that rule out SOME hidden variable theories - which is exactly what Steven said. Probably and with caveats is the out here - there are plenty of theories that evade it.

    But that's not what you are saying - you are speaking about HV theories in general.

    I suggest you actually UNDERSTAND what is being said.

    You quoted John Bell saying:
    Well in that case you might like to explain to us exactly why its an issue if something isn't Lorentz invariant?

    There are many perfectly valid hidden variable theories such as DBB, Nelson Stochastics, Primary State Diffusion and even the path integral approach that evade Bells Theorem. DBB, for example, is not Lorentz invariant.

    Thanks
    Bill
     
    Last edited: Mar 17, 2014
  9. Mar 17, 2014 #8
    You are right, I misinterpreted stevendaryl comment (sorry stevendaryl).

    I am not quite sure what your point is here, if you are asking why Bell thought it was an issue if a hidden variable theory wasn't Lorentz invarent, he probably means to say that some variables propagate faster than light. As I understand it, his proof is saying that all hidden variable theories must include a Lorentz invarient factor. If your question is instead why I personally think it is a problem, my answer is I don't at all.

    Anyways I think I had my basic question answered, the hidden variable theory is still taken seriously even though in a lot of popular media it is portrayed as dead (See Steven Hawkings homepage or Minutephysics on youtube)
     
  10. Mar 17, 2014 #9

    bhobba

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    Most indeed it is.

    I think you are referring to the following from Hawkins site:
    'Einstein's view was what would now be called, a hidden variable theory. Hidden variable theories might seem to be the most obvious way to incorporate the Uncertainty Principle into physics. They form the basis of the mental picture of the universe, held by many scientists, and almost all philosophers of science. But these hidden variable theories are wrong. The British physicist, John Bell, who died recently, devised an experimental test that would distinguish hidden variable theories. When the experiment was carried out carefully, the results were inconsistent with hidden variables. Thus it seems that even God is bound by the Uncertainty Principle, and can not know both the position, and the speed, of a particle. So God does play dice with the universe. All the evidence points to him being an inveterate gambler, who throws the dice on every possible occasion.'

    Its misleading at best and strictly speaking downright wrong.

    What Bell showed is some hidden variable theories are wrong. But many exist that are consistent with the experiment he cited eg DBB.

    In Hawking's defence however, I must point out he would know this, but explaining subtleties to lay people is actually quite difficult and popular articles, even written by people of the highest calibre like Hawking, often veer from strict correctness.

    Thanks
    Bill
     
  11. Mar 17, 2014 #10

    stevendaryl

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    Yes, you're right. The conclusion of Bell's theorem is that the results can't be explained by hidden variables without FTL influences.
     
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