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EPR paradox

  1. Apr 4, 2013 #1
    how can we know the state of particle which is far? Is the quantum mechanical result right?
     
  2. jcsd
  3. Apr 4, 2013 #2
    The quantum mechanical result has been experimentally verified, if that's what you mean.
     
  4. Apr 4, 2013 #3
    In fact you can only make the statistical deduction/constatation that the spin are opposite in an eprb experiment, i.e. When both subsystem are measured.
    If you measure only one subsystem you can't predict a measurement thereafter on the second subsystem.
    In a real experiment both effect occur and the contribution due to randomness has to be substracted from the covariance so that it is not exact but reaches 90 percent. See for example the covariance curve at the end of the article 'Violation of Bells inequalities by photons more than 10km apart' by Gisin's group in geneva.
     
    Last edited: Apr 4, 2013
  5. Apr 16, 2013 #4
    In the EPR paradox, using a singlet state, the quantum result is calculated easily after an undergrad course in quantum. Using the singlet state, the correlation between two spins as they encounter filters oriented along vector a and b is <sigma(a)sigma(b)> = -a.b

    Now this -a.b is experimentally confirmed, so that means to a lot of people (not me) that the singlet state stretches out between the two particles. This, of course, is non-locality, firmly established in physics but not believe by me.

    Your question is: what is the state of these separated particles? Some people believe that each EPR particle carries "half a quantum state" (That is half the entangled singlet--poppycock) I do not believe that.

    If you assume spin has two axes of quantization, rather than one, then each spin carries a complete quantum state and their product accounts for the EPR correlation. This state is a superposition of the two axes of quantization and has eigenvalues of +/-root(2) rather than +/-1. If you measure that spin, you get the usual point particle spin because one orthogonal axis decohers.
     
    Last edited by a moderator: Apr 17, 2013
  6. Apr 16, 2013 #5

    DrChinese

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    This is not correct, and is easily refuted by the Bell logic. You should not be posting stuff such as the above. Please check forum rules regarding personal theories which are in disagreement with established science.
     
  7. Apr 17, 2013 #6
    As a scientist we question, especially things that do not make sense. What I said is completely backed up by objective reasoning.

    I have simulated the EPR correlations, one coincidence at a time and these agree completely with experiment and quantum mechanics. The model is both local and real. It is also not a classical treatment.

    What you say is that Bell's theorem easily refutes local realism, but does it? First Bell's theorem follows from believing Einstein Locality is incorrect in order to get the violation. But it is his spin assumption that is wrong, not his locality assumption.

    I find that if an isolated spin has two orthogonal axes of quantization then local realism resolves the EPR paradox.

    So my question is quite simple to you. If you believe in Bell's theorem, then tell me how two separated EPR pairs remain entangled over space like separations. If you can do this without using nonsense words like "quantum weirdness", then you will be able to do what no one else has done.

    Non-locality makes no sense. It must, therefore, be wrong.
     
    Last edited by a moderator: Apr 17, 2013
  8. Apr 17, 2013 #7

    DrChinese

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    That it makes sense or not misses the point of science, which is to describe as best possible.

    Either there is non-locality, or there is non-realism (or both). Bell's Theorem shows us such, and I can only assume you are not familiar with it. We have no obligation to review your model for mistakes, but by the forum rules you have an obligation NOT to post links such as above to unpublished material. So please cease, the next time you post similar to the above I will report you.
     
  9. Apr 17, 2013 #8
    I am surprised by your hostility. I am not asking you to review my model for mistakes, just have an open mind to new ideas. I can assure you that I am fully versed in Bell's work and, like quite a few others, believe that Bell's theorem is incorrect. After all Bell showed that von Neumann was incorrect, so why cannot I claim that Bell is incorrect, especially when I have objective reasons for it?

    If Bell's theorem is correct, then you have to explain in a scientific way how non-locality works. I think that is an obligation scientist have. If you believe in something, then you should be able to explain it to your mother. Since you cannot, and no one can, then there must be something wrong or something we have missed.

    So if you wish to report me, I think that the moderators will conclude that it is healthy to question, and I have not only questioned but proven that local realism works. I am submitting the paper to Physical Review A within the next week.

    Sorry if I got your back up.
     
  10. Apr 17, 2013 #9
    Welcome to PF Bryan! Much confusion will be cleared if you read our guidelines here
    https://www.physicsforums.com/showthread.php?t=414380

    Please let us know the result of your paper submission. Good luck!
     
  11. Apr 19, 2013 #10
    Bells theorem proves a case of strong emergence in physics which is not very held there. It is when the whole is more than the sum of its parts. See wikipedia.
    Douglas hofstadter also speaks about reductionism and holism in his book Goedel, Escher Bach.

    For a physicist wholeness is often viewed as arising because of a lack of knowledge but maybe some global aspects are not reducible and it is then the goal of science to study those global aspects too.
     
  12. Apr 20, 2013 #11
    whiteboard1 - It's a little different in different setups, there are several EPR/Bell-type setups. A common setup is an entangled pair of spin-half particles in a singlet state. In that case one knows that the sum of the two particles spin is zero. So if one measures the spin of one of the particles to be say -1/2 one instantly knows the spin of the other one is +1/2 (along the same axis of measurement).
    And, yes, all experiments so far agree with the predictions of quantum mechanics. They also violate the Bell limits, meaning that the experimental results can never be reproduced with any model that is both local and objective. (Ignoring here some "loophole-arguments" which basically says that we do something wrong in each and every of those experiments...)


    No, you have not. I have no idea what you misunderstood or do wrong in you simulations. But this is precisely what Bell showed is mathematically impossible. It has nothing to do with quantum mechanics, nor common sense. Just mathematics. He showed that any local and objective ("real") model has limits on correlations between separated measurements. If the model you simulated was both local and real, it would obey those limits. In certain setups however, quantum mechanics predicts results that violate those limits. Which is not that surprising since QM looks explicitly non-local (or non-objective depending on which interpretation you prefer). What is surprising and really intriguing is that experimental results violates those limits too. They happen to agree with the QM predictions as well (but thats not the important point!).

    This is not correct and shows that you did not understand Bell's theorem. Bell makes two assumptions when deriving his limits; locality and some form of objectivity. But the whole point of the theorem is that when results do violate those limit, those results can never be reproduced by any local and objective theory.

    That the speed of light should be constant for all observers make no sense to my intuition. Must it therefore also be wrong? No - science is all about keeping an open mind and accepting what repeatable experiments shows us.
     
  13. Apr 21, 2013 #12
    This is in theory true, but in experiments they don't get a 100% prediction for that case. See the violet curve at the end of : http://prl.aps.org/abstract/PRL/v81/i17/p3563_1 or http://arxiv.org/abs/quant-ph/9806043

    This is due to uncorrelated photons that are counted in the experimental result, some kind of noise.
     
  14. May 9, 2013 #13
    in fact there is a quantum way to calculate that 'noise' which correspond to single event and you get a prediction with 87% accuracy whereas experiment give 85 percent.

    But i cannot put in this forum since it is not mainstream accepted physics.
     
  15. Jun 4, 2013 #14
    What Bell has shown is something different, namely that von Neumann has made, in his proof, assumptions which have been unreasonable. In particular, there was the example of de Broglie-Bohm theory, which is a completely reasonable hidden variable theory. And this theory violates an assumption used by von Neumann.

    So the theorem proven by von Neumann remains valid. It is not wrong, but irrelevant, because the assumptions made by von Neuman have been unreasonably strong.

    Similarly, there is no chance that Bell's theorem is incorrect. There is only a very weak chance that there appears a reasonable theory which violates one of the assumptions made by Bell, but appear, nonetheless, "local" (that means Einstein-causal) and "realistic" (with some modified but reasonable definition of realism).

    A claim "Bell's theorem is wrong" is as nonsensical as "the theorem of Pythagoras is wrong".

    Wrong. This would be an interesting question, but there is no such obligation.
     
  16. Oct 13, 2013 #15
    Btw if we look at the experimental result concerning EPRB, I could find 3 experiments measuring CHSH, the values are :

    Aspect : 2.69, error 0.05
    Gisin : 2.38 error .09
    Wineland : 2.25 error .05

    we see that all 3 a greater than 2, hence implying a kind of non-locality according to Bell. However considering the experimental errors there is no overlap of the results possible. Does this mean that the error is in fact bigger than measured, or that we cannot compare in that way different experiments ?
     
  17. Oct 13, 2013 #16

    DrChinese

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    Good question. As these experiments have very different efficiencies, they are not directly comparable in the sense you suggest. What they are designed to do is test the local realistic prediction of a max of 2. I believe they all also give a value for of the QM expectation in some fashion. I believe 2.82 is about the max ideal, and less than ideal is always lower.

    It is worthy to mention that although the local realistic max is 2, that is not the prediction for all separable product state models. Some would be even lower. There really is no local realistic prediction per se because there are no viable local realistic models left on the table.
     
  18. Oct 13, 2013 #17
    If those figures are correct and they are indeed measuring the same quantity, it is very likely that one or more of those measurements involved some significant, unaccounted for, systematic error.

    The Gisin and Wineland results do cross within one standard deviation. It is the Aspect result that looks like the outlier, but in copying each other's techniques, the other 2 could easily have copied each other's systematic errors. Alternatively they could have independent systematic errors.

    Alternatively, as Dr. Chinese suggests, they could be measuring slightly different quantities, all of which cross at the Bell threshold of 2.
     
    Last edited: Oct 13, 2013
  19. Oct 13, 2013 #18

    DrChinese

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    OK, take a deep breath here. These are top experimental teams (Wineland won the Nobel last year, for example). There is no reason to speculate like this. Please stick to the science.
     
  20. Oct 13, 2013 #19
    Sure, I'm not suggesting that they aren't experts. I'm just looking at those errors and thinking 5 standard deviations is a lot. Wouldn't you agree?

    Systematic errors do end up in experiments. Do you remember the faster than light neutrino experimets recently?
     
  21. Oct 13, 2013 #20

    DrChinese

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    You mean, the neutrino experiment that could not be replicated?

    As I said, the EPR experiments are not identical for a lot of reasons. There are hundreds of ways to test local realism, and the boundary of 2 is somewhat artificial. By convention, most experiments are designed to make it that 2 is the boundary. On the other hand, any pair analyzed which is not entangled at the time (due to inefficiency) gives a result closer to 2. So there are a lot of variables at play.

    And therefore the results of Wineland's experiment will not match the result of Zeilinger, and yet both rule out local realism.
     
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