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Where is the flaw with predetermined entanglement state?

  1. May 1, 2015 #1

    LsT

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    Not to repeat what is already said, my thoughts are more or less the 4 points mentioned here:

    http://physics.stackexchange.com/qu...ement-an-illusion-based-on-a-wrong-assumption

    The upvoted answer to what gpgemini's said is that this assumption is essentially what local hidden variables theory propose, and by that its ruled out because experiments demonstrate Bell's inequality violation.

    But to my understanding, a predetermined state is a different thing than local hidden variable theory:

    Within a local hidden variable theory, entangled particles somehow supposed to have infinite (?) embedded information with them about what will the outcome be in any possible measurement, such as the outcome will emulate what is interpreted as puzzling FTL instantaneous action in other QM interpretations. Referring to this, Bell showed that there are limits to the probability distribution of all possible outcomes if the information the entangled particles carry, is predetermined the moment the entangled pair is created and not changed until measurement. Experiments repeatedly violate local hidden variables probability limits, ruling them out (assuming no loopholes)

    But with a predetermined entanglement, particles do not carry ANY information, just exists a definite "opposing" state between them (such as a specific polarization angle difference or opposing spin direction in x axis). Bells inequality clearly does not apply here, because you cannot calculate probability distribution limits of NO information. And to my understanding (wrong?) any entanglement experiment using polarization direction of photons, because of Malus law CAN violate the inequality, without ruling out, this predetermined state hypothesis.
    ...
    I am for sure missing something fundamental, but what?
     
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  3. May 1, 2015 #2

    DrChinese

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    You are saying there is predetermined entanglement and yet it is local. How does that make sense? If there is predetermined entanglement between 2 particles, and it is an actual state (not just a correlation), then it cannot be local. Do you see why?

    And if it IS local and predetermined, and is a correlation, then it cannot agree with the predictions of QM (a la Bell's Theorem).

    Please keep in mind that measurement of 2 entangled particles always show 100% predictable results when measured at the same angles. If it is local, this implies predetermination. And the particles MUST be carrying that information, since they cannot be in contact with each other.
     
  4. May 1, 2015 #3

    LsT

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    Hello DrChinese and thank you for your answer.

    (Here I think that I should make clear that I am trying to do nothing more than to understand some subjects using my own thought experiments, I am completely aware of my ignorance, and grateful to anyone helps me to reduce it :) )

    Hmm..I guess I have used wrong terminology here and I apologise for this. What I am talking about is an actual state and local of course. Not non-local. Not hidden information. I am hypothesizing an entangled particle pair eg photons that each has a definite very specific polarization angle determined at their inception/creation. The polarization angle of each photon can be "random" but their difference is allways constant. Obviously that is not properly called predetermined (maybe just determined is better?) but that is what I am talking about.
    If by correlation we exclude that it is an actual state, no I am not talking about this. I understand that Bell's Theorem applies here. But as I stated I think that Bell's inequality does not apply to an actual state because of the lack of (hidden) information. If I am wrong here please enlighten me.
    Please elaborate this. I fail to see why a (local) definite state as I hypothetized above will fail to produce the same results along the lines of QM. (please keep this talk about photons/polarization angle if you get into details).
     
  5. May 1, 2015 #4

    DrChinese

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    By the way, welcome to PhysicsForums, LsT!

    This is the conclusion of Bell's Theorem, which shows that QM and a theory as you describe cannot be reconciled. You are saying the state is local and predetermined. Therefore the choice of measurement for Bob's photon cannot influence the outcome of a measurement by Alice on her photon, correct? Your premise is certainly reasonable - Einstein essentially believed as much. But he didn't have the benefit of Bell and Aspect et al (the team who first tested Bell Inequalities).

    Please check out one of my Bell Theorem web pages. The easiest for math is:

    http://drchinese.com/David/Bell_Theorem_Easy_Math.htm

    Or read the original Bell:

    http://www.drchinese.com/David/EPR_Bell_Aspect.htm
     
  6. May 1, 2015 #5

    zonde

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    I like this informal proof of special case for entangled photons https://www.physicsforums.com/showthread.php?p=2817138#post2817138
    If you think this proof does not apply to your model it should be easy to point out the place where is the difference.
     
  7. May 2, 2015 #6

    LsT

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    @DrChinese
    I like your website! And your explanation of the Bell's Theorem is really nice. So as I started reading your article I stumbled upon this phrase:
    AFAIK hidden variables as an idea came after the EPR paper. If possible please elaborate how this proof is fomulated (just a simple description, give me some points). And I will repeat again that my point of view is that local hidden variables are not the same as a determined state in the way I am describing it in this thread.

    @zonde
    That is really easy to follow even for me! I'll try to point out where I see the difference(s):
    Ok
    In my "model/thought experiment", step Three is INDENTICAL to step Two, because no single particle carries any information regarding the the outcome of a single detector, CONTRARY to what hidden variables theory postulates. The "information" eg the existing state is defined only between them, and so only in relation to the angle DIFFERENCE of the detectors. So what we have done until now is to repeat two times a 30 degree difference. And, irrespective of my assumptions, I think there is an error in this statement: the mutual misalignment (lets say that mutual misalignment = AnlgleA - AngleB) is not -30 degrees if we turn the B detector in the opposite direction. The mutual misalignment is 30 degrees, exactly the same with step 2.
    Nothing can be expected. We tried only a 30 degree difference, wich gives 25% errors. The 60 degree difference results are unknown and depended (in the case of photons/pollarization angle) only in the real/defined pollarization angle between the photons, the angle between detectors and Malus' law.
    Maybe local hidden variables are not including all cases of local realism?
    As I have pointed, the above is correct only for a local hidden variable theory. In my "model" the outcome is defined/makes sense only in relation to the angle difference of the detectors. So yes, detector A output does not depend on the output of detector B, because there is no defined output of A at its own and vice versa. (because the output of A or B at its own is random (1/2), as is the output of a single polarization filter against a single photon). But the Output = OutputA - OutputB is of course related to both.
    ...
     
  8. May 2, 2015 #7

    atyy

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    In the Copenhagen, the quantum state is not real. But there is also nothing wrong with treating the quantum state as if it is real for all practical purposes (reality is just a tool to calculate the probability of measurement outcomes). Here I will therefore treat the state as real.

    In quantum mechanics, one can see that the EPR and Bell tests clearly violate locality because after Alice measures her particle, the wave function will collapse. The wave function collapse is nonlocal, as it occurs on a slice of simultaneity. So the quantum mechanical explanation of the result is clearly nonlocal, for all practical purposes.

    The question arises whether there is another formalism without collapse, without quantum mechanics that can provide a local explanation of the correlations predicted by quantum mechanics. Bell's theorem tells us the answer is no - there is no local hidden variables explanation of the correlations predicted by quantum mechanics - with some loopholes such as retrocausation, superdeterminism, or allowing an experiment to produce more than one outcome.
     
  9. May 2, 2015 #8

    LsT

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    atyy, thank you for you answer

    Aside from the more general information you provided, and is more than welcomed by me, what I understand from your answer is that my "hypothesis" falls clearly under the local hidden variables category (or not?). Maybe you can be more specific, as to WHY this is the case? I am not asking for some simplified easy understandable answer btw (it would be better for me of course), just a specific answer.
     
    Last edited: May 2, 2015
  10. May 2, 2015 #9

    atyy

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    If I understood you correctly, my thinking was that your example is a nonlocal hidden variable hypothesis. Here the hidden variable is the entangled state that is initially created. The simplest way to see that this is nonlocal is to have the Alice measure before Bob in the Bell test. Then Alice will collapse the wave function, which is manifestly nonlocal.
     
  11. May 3, 2015 #10

    zonde

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    The above is correct for any local model. With variables we sort of describe the potential certainty of some interaction.
    Your model is clearly classified as non-local. Because angle difference of distant detectors is non-local variable.
     
  12. May 3, 2015 #11

    LsT

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    @zonde
    I don't see how the angle difference is non-local. Irrespective of their distance, let's assume that the 2 detectors are physically connected (part of a single stiff object) such as when I change the objects angle, B and A detectors change angle (their difference remains constant). With the right construction, B can easily "follow" A a lot faster than if B was receiving information from A with the speed of light.

    And if the angle difference of the detectors is indeed non-local (in a way I don't understand), how can Bell's Theorem also use it to in/validate locality?
     
  13. May 3, 2015 #12

    stevendaryl

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    The angle difference is nonlocal because the two experimenters (I always call them Alice and Bob) can choose their angles independently at a moment's notice.

    As to the second question, that's the point of Bell's argument. It's easy to explain the result using nonlocal, instantaneous effects. But it is not possible to explain the result using local effects.
     
  14. May 3, 2015 #13

    LsT

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    So, in a Bell experiment with the two detectors connected in a way that makes their angle difference local, my hypothesis is to be considered?
     
  15. May 3, 2015 #14

    stevendaryl

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    Well, it would be weird if there were one mechanism for the case where the angle difference is fixed ahead of time, and a different mechanism for the case where they are not fixed ahead of time.
     
  16. May 3, 2015 #15

    ZapperZ

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    I'll try to approach this in the simplest, possible manner. What you are describing here is the case for what is commonly known as "realism". This means that you are saying that in the EPR-type experiment, for example, the two entangled pair actually already have a definite state before they are measured. So when one is finally measured, then it is automatic what the other one will be in. Is this correct?

    The problem here is that such a concept of classical realism is not compatible with what we have observed in experiments. An example is this one. Tony Leggett has also formulated a case where it is almost implausible to maintain the idea of classical realism.

    So in this case, the experiments just simply do not match the idea of an already predetermined state.

    Zz.
     
  17. May 3, 2015 #16

    LsT

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    @stevendaryl
    Yes, I understand what you mean, but no one can argue for the case that the angle difference can be local, which is not the case for non-local. For example is there a proof saying that you can know the angle difference of the detectors at all times without violating non-locality? To my hypothesis, if you have to know the angle difference at all times, is the equivalent of a making a local connection between the detectors, however fast you move them. Maybe I can say that by using a clock as reference, you make the system equivalent of a local.

    @ZapperZ
    Quoting from S. Groeblacher et al.

    But that is saying nothing about local realism except it has been disproved by Bell's experiments. I here make a hypothesis about local realism and the possibility that it is out of the scope of Bell's theorem.

    In fact, that is my bet also. But I am not aware of such an experiment.
     
  18. May 3, 2015 #17

    ZapperZ

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    Then you are contradicting the concept of quantum superposition, which has been shown to be valid in numerous observation, especially in Chemistry. You will have to start from there.

    Zz.
     
  19. May 3, 2015 #18

    atyy

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    There is no such proof, and in fact this is one of the well-known loopholes to Bell's theorem - superdeterminism. The other famous loophole is retrocausation. Bell's theorem does not forbid a superdeterministic, or a retrocasual theory from being local and also explaining the correlations predicted by quantum mechanics.

    http://arxiv.org/abs/quant-ph/0301059
    "Now “free choice” is a notion belonging to philosophy and I would prefer not to argue about physics by invoking a physicist’s apparently free choice. It is a fact that one can create in a laboratory something which looks very like randomness. One can run totally automated Bell-type experiments in which measurement settings are determined by results of a chain of separate physical systems (quantum optics, mechanical coin tossing, computer pseudo-random number generators). The point is that if we could carry out a perfect and succesful Bell-type experiment, then if local realism is true an exquisite coordination persists throughout this complex of physical systems delivering precisely the right measurement settings at the two locations to violate Bell’s inequalities, while hidden from us in all other ways."

    http://arxiv.org/abs/1303.2849
    "A second problem is that we may never be sure that the choices of measurements are really “random” or “free”. For instance, in the experiments (Scheidl et al., 2010; Weihs et al., 1998) the measurement choices are decided by processes that are genuinely random according to standard quantum theory. But this need not be the case according to some deeper theory. Some people have argued that a better experiment for closing the locality loophole would be to arrange the choice of measurement setting to be determined directly by humans or by photons arriving from distant galaxies from opposite directions, in which case any local explanation would involve a conspiracy on the intergalactic scale (Vaidman, 2001).

    The point of this discussion is that an experiment “closing” the locality loophole should be designed in such a way that any theory salvaging locality by exploiting weaknesses of the above type should be sufficiently conspiratorial and contrived that it reasonably not worth considering it."
     
    Last edited: May 3, 2015
  20. May 3, 2015 #19

    LsT

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    Maybe I am not getting somethin right (again), I mean, when conducting a Bell's experiment you have to know the angle difference at all times right? So you can corelate it with coincidences. Why is this not the equivalent of locality, if all you care about is this angle difference? What difference does it make if each in relation to you appears to rotate completely random, with superfast speeds. You make the equivalent of a physical connection just with software and clocks.

    Yes, maybe that's what I am doing :)

    Please point to me why you came to that conclusion.

    Can you please also give me a good place to start reading about that (actual) experiments?
     
    Last edited: May 3, 2015
  21. May 3, 2015 #20

    atyy

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    Bell's theorem assumes that the measurement settings on each side are independent of each other, independent of the hidden variable, and independent of the measurement outcomes. In order to satisfy this assumption, we make the measurement settings as random as we can. So from our point of view, a good effective description of the measurement settings is that they are random. If in fact we failed in our attempt to make the measurement settings random, then the assumptions underlying Bell's theorem do not hold, and the theorem does not apply.
     
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