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Spooky action at a distance

  1. Jun 24, 2009 #1
    Coming at this problem from the angle of philosophy/psychology and an unhealthy relationship with the Journal of Consciousness Studies, I'm interested to know how you Physicists interpret the process of collapse, or rather the concept of entanglement. I've read so much new age rubbish all over the place (although I wouldn't call Penrose or Evin Harris Walker new-ager's). Does the violation of Bell's Inequalities demonstrate that quantum "spooky action at a distance" is not merely correlation, but some real physical process? Is this only the case if you try to interpret QM in local realistic terms?

    I suppose what I really want to know is how does photon A "connect with" photon B, such that a measurement on A instantaneously acts on B? Is there no fact of the matter at the moment, or is it all down to your particular flavour of philosophical interpretation?
     
  2. jcsd
  3. Jun 24, 2009 #2
    PlayUK, Welcome to PF...

    Murray Gell-Mann gives an analogy I like a lot when interpreting "spooky action at a distance":

    Professor X has a peculiar habit, he puts on a BLUE sock and a RED sock every day instead of wearing identical pairs like normal people. The foot he chooses to put on these socks, however, is random. Therefore one day he could put on a blue sock on his right foot,but the other day he could do just the opposite. You, as the observant student, cannot know which color will end up in which foot before seeing one of his feet (and no complicated theory will help you predict that because it's really random), but once you see one of his socks, you immediately know the color of the sock you didn't see. There's no mechanism, no spooky action at a distance, when you see the the blue sock, you KNOW where the red sock is.

    This is Gell-Mann's interpretation (I think it's originally attributed to someone else but I can't remember it now) and could be found in his book "The Quark and the Jaguar" . So if anybody is going to attack this with their own view on the subject, MGM is the man to talk to.

    But I have a feeling his interpretation would be far more convincing than any other that I'll ever see in this forum.
     
    Last edited: Jun 24, 2009
  4. Jun 24, 2009 #3
    If that's all that "action at a distance" is then I don't see what all the fuss is about.
     
  5. Jun 24, 2009 #4
    There's no fuss among the top physicists . It's perfectly clear. The fuss is more among the public.
     
  6. Jun 24, 2009 #5
    sokrates,

    I agree with your analysis completely. Thanks for the nice example with professor's socks.

    I think the reason why there are still endless discussions of the "spooky action at a distance" is that people (intuitively) try to understand quantum mechanics in the language of 19th century "physical mechanisms". I guess their idea is that the wave function is some kind of material "fluid", that superposition of states is a real thing, and that wavefunction's collapse is an objective physical process. People tend to think that (while not observed) professor's socks *really* exist in a superposition state. So, when the left sock has been observed, there should be some kind of superluminal physical agent/messenger which rushes to the right sock and tells it: "Hey, don't you know that your fellow left sock has been observed and collapsed to red color? You must immediately collapse to blue color. Otherwise you will be found in violation of the laws of quantum mechanics and punished".

    The true lesson of quantum mechanics is that it doesn't make sense to think about "physical mechanisms" of events that are not directly observed. In any case, quantum mechanical formalism (superpositions, wave functions, etc) is just a calculational tool, not a physical model.
     
    Last edited: Jun 24, 2009
  7. Jun 24, 2009 #6

    DrChinese

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    Bertlmann’s socks and the nature of reality, Bell, 1981. But this is a simple analogy and does not actually explain Bell test results at all.
     
  8. Jun 24, 2009 #7
    What kind of explanation you want?

    We have a theory (quantum mechanics) which predicts experimental results (correlations between different polarizations of separated photons) with great accuracy. Is there anything else needed?
     
  9. Jun 24, 2009 #8

    DrChinese

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    I'm good with that. I simply pointed out that socks are not a good analogy. As you say, physical mechanisms aren't easily paired with QM.
     
  10. Jun 25, 2009 #9
    So true. Maybe we should change the way we think about QM rather than complaining about its unpredictability all the time.
     
  11. Jun 25, 2009 #10
    That's it. Thanks Dr. Chinese!
     
  12. Jun 25, 2009 #11

    Fredrik

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    This interpretation was completely crushed by Bell. Theories like this one predict correlations in the results of measurements called Bell inequalities. (See e.g. 215, 216). QM predicts that Bell inequalities will be violated. Experiments have confirmed that they are.
     
  13. Jun 25, 2009 #12
    Yes, I liked the socks analogy, but it doesn't square with what I've read, i.e. if this were the case, Bell's Inequalities would hold, as DrChinese et. al. have pointed out. So given Bell's result, I would be right in thinking that this is not merely correlation?
     
  14. Jun 25, 2009 #13

    alxm

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    Essentially. It's an interpretation thing, which tends to get debated endlessly here, despite the fact that most working physicists don't really concern themselves with interpretations much at all. It's one of those things that's usually brought up in popular-scientific accounts and introductory textbooks, on how weird and confusing QM is. But once you know about it, most of us just accept the 'weirdness' and get on with it, since it's not relevant to putting QM to practical use. (personally I tend to just think "Who says Nature is obliged to not be 'weird?')

    Anyway, so yes, the flaw of the 'socks' argument is that the color is a hidden variable. At all points in time the blue sock was blue and the red sock was red, and the probabilities arose from our lack of knowledge of these variables. Well, the Copenhagen interpretation says there aren't any such hidden variables. They're genuinely indeterminate. Which then raises the question of how, once the blue sock has been observed to be blue, the red sock 'knows' it must be red. And the Bell-test experiments showed that there are no such (local) hidden variables. And there may not be nonlocal ones either, IIRC correctly some more recent results DrChinese pointed out (I'll defer to him on these interpretational matters).

    Addressing your original ponderings; I'm more qualified to answer there, being a chemical physicist I supposedly know something about the area where quantum mechanical phenomena become chemical and biochemical phenomena. And my 'professional opinion' about 'quantum consciousness' and its ilk, is that it's basically a load of nonsense. (not least the new-agey stuff which is a horrible pseudoscientific garbage)

    (I wrote a somewhat long and detailed criticism here, but opted to delete it as not to bog down the thread with a giant wall of text that may not be of interest. But in short: Bad idea to think it's quantum until 'conventional' explanations fail, chemistry is intrinsically quantum-mechanical, hence they're actually suggesting a hitherto-unknown chemical phenomenon - unlikely since it's generally believed that all biochemistry works just the same as any other chemistry - only the molecules tend to be bigger)
     
  15. Jun 25, 2009 #14
    I have seen this argument used several times on this forum. Does somebody have a reference for this?

    The fact that most working physicist will probably work on another subject and just have to accept this 'weirdness' of nature for their dayly work doesn't necessairily mean they are happy with the current answers.
     
  16. Jun 25, 2009 #15

    HallsofIvy

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    My understanding is that it is not just a matter of observing that electron (not photon) A has positive spin and immediately "knowing" that electron B (which may now be a great distance away from initially paired electron A) has negative spin, as in seeing that one of the professor's socks is red and immediately knowing that the other is blue. The theory says that electron A does NOT HAVE a well defined spin until you observe it. And when you do observe it that causes electron B to immediately have opposite spin to whatever you observe A to have.
     
  17. Jun 25, 2009 #16
    I agree with ajw1 - this is only true in the sense that, for example, most working physicists don't really concern themselves with the physics of nuclear reactors. We all have different fields of study, and we assume that someone somewhere knows how nuclear reactors work.

    In fact there is a very large community of physicists who do concern themselves with interpretations. They have conferences and stuff.

    To which they don't invite alxm, clearly..
     
    Last edited: Jun 25, 2009
  18. Jun 25, 2009 #17
    Well I don't want to turn this thread into a metaphysical debate on the nature of Consciousness. I am happy to admit that I don't know the answer here, but would side against the functionalists based on intuition alone (a very bad guide, I know).
     
  19. Jun 25, 2009 #18
    Jaynes explains this in:

    Jaynes, E. T., 1989, `Clearing up Mysteries - The Original Goal, ' in Maximum-Entropy and Bayesian Methods, J. Skilling (ed.), Kluwer, Dordrecht, p. 1

    Bernouli's Urn Revisited
    Define the propositions:
    I : "Our urn contains N balls, identical in every respect except that M of them are red, the remaining N-M white. We have no information about the location of particular balls in the urn. They are drawn out blindfolded without replacement."
    R_i : "Red on the i'th draw, i = 1; 2; ..."

    Successive draws from the urn are a microcosm of the EPR experiment. For the first draw, given only the prior information I , we have

    P(R_1|I ) = M/N (16)

    Now if we know that red was found on the first draw, then that changes the contents of the urn for the second:
    P(R_2|R_1,I ) = (M-1)/(N-1) (17)​
    and this conditional probability expresses the causal influence of the first draw on the second, in just the way that Bell assumed.But suppose we are told only that red was drawn on the second draw; what is now our probability for red on the first draw? Whatever happens on the second draw cannot exert any physical influence on the condition of the urn at the first draw; so presumably one who believes with Bell that a conditional probability expresses a physical causal influence, would say that P(R_1|R_2,I) = P(R_1|I).
    But this is patently wrong; probability theory requires that
    P(R_1|R_2,I) = P(R_2|R_1,I) (18)​
    This is particularly obvious in the case M = 1; for if we know that the one red ball was taken in the second draw, then it is certain that it could not have been taken in the first.
    In (18) the probability on the right expresses a physical causation, that on the left only an inference. Nevertheless, the probabilities are necessarily equal because, although a later draw cannot physically affect conditions at an earlier one, information about the result of the second draw has precisely the same effect on our state of knowledge about what could have been taken in the first draw, as if their order were reversed.
    Eq. (18) is only a special case of a much more general result. The probability of drawing any sequence of red and white balls (the hypergeometric distribution) depends only on the number of red and white balls, not on the order in which they appear; i.e., it is an exchangeable distribution. From this it follows by a simple calculation that for all i and j ,
    P(R_i|I ) = P(R_j|I) = M/N (19)​
    That is, just as in QM, merely knowing that other draws have been made does not change our prediction for any specified draw, although it changes the hypothesis space in which the prediction is made; before there is a change in the actual prediction it is necessary to know also the results of other draws. But the joint probability is by the product rule,
    P(R_i,R_j|I) = P(R_i,|R_j,I )P(R_j|I) = P(R_j|R_i,I)P(R_i|I) (20)​
    and so we have for all i and j ,
    P(R_i|R_j,I ) = P (R_j|R_i,I) (21)​
    and again a conditional probability which expresses only an inference is necessarily equal to one that expresses a physical causation. This would be true not only for the hypergeometric distribution, but for any exchangeable distribution. We see from this how far Karl Popper would have got with his "propensity" theory of probability, had he tried to apply it to a few simple problems.
    It might be thought that this phenomenon is a peculiarity of probability theory. On the contrary, it remains true even in pure deductive logic; for if A implies B, then not-B implies not-A. But if we tried to interpret "A implies B" as meaning "A is the physical cause of B", we could hardly accept that "not-B is the physical cause of not-A". Because of this lack of contraposition, we cannot in general interpret logical implication as physical causation, any more than we can conditional probability. Elementary facts like this are well understood in economics (Simon & Rescher, 1966; Zellner, 1984); it is high time that they were recognized in theoretical physics.
     
  20. Jun 25, 2009 #19
    In deriving his inequalities, Bell has several hidden assumptions. Two of them are as follows:
    (1) That a conditional probability P(X|Y) expresses a causal influence exerted by Y on X.
    (2) Not all local hidden variable theories are included in his equations.

    Assumption (1) has been addressed in the my previous post, quoted from Jaynes.
    Several other authors have addressed assumption (2) including Jaynes himself in the quoted article above:

    See for example:

    Exclusion of time in the theorem of Bell
    K. Hess et al 2002 Europhys. Lett. 57 775-781

    Breakdown of Bell's theorem for certain objective local parameter spaces.
    Hess and Philipp, PNAS February 17, 2004 vol. 101 no. 7 1799-1805

    Therefore, it has been shown that the spooky action was due to the mistaken assumption that a
    conditional probability must signify a physical influence, and it has also been shown that Bell arguments do not consider all possible local hidden variable theories. Bell's inequalities are only limitations on what can be predicted by Bell-type theories.

    Therefore, some of the conclusions of the Aspect-type experiments are premature. At most, such experiments show that Bell-type theories are untenable.
     
  21. Jun 25, 2009 #20

    DrChinese

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    Please, mn4j, this is off topic and belongs in a separate thread. I would be happy to discuss Jaynes' concepts in that context, but this thread is not a "Why Bell is wrong" thread. It is not fair to make every thread mentioning Bell yet another opportunity to confuse newbies with your Local Realist ideas.​
     
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