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Bell Tests

  1. Dec 9, 2011 #1
    So I've been reading a lot about Bell tests. I have a question that I can't seem to find an answer to. The tests indicate that a photon decides to be horizontally or vertically polarized at the moment of detection. What are they basing this on? To make such a counter-intuitive assumption seems like it would require a lot of data, but having read z dozen papers on Bell tests by Aspect and others, I don't see it. It seems more likely that the photon has a polarization to begin with but it changes due to the detection method, thus causing an artificial correlation. If this were true, it would mean reality does exist but we erase it when we try to detect it.
     
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  3. Dec 9, 2011 #2

    Fredrik

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    It follows from the fact that the result of the long series of measurements doesn't satisfy the relevant Bell inequality. Every theory that says that the result of each polarization measurement was already decided before the measurement, predicts that the result of a long series of measurements will satisfy that Bell inequality.

    Experiments agree with QM, even when QM says that Bell inequalities will be violated.
     
    Last edited: Dec 9, 2011
  4. Dec 9, 2011 #3
    But isn't it possible that the variables were decided beforehand and changed by the measurement? The observer would THINK that the measurement decided the variable and that nothing had previously been decided, but the results should be identical if the variables weren't undecided, but were changed by the measurement.
     
  5. Dec 9, 2011 #4

    Fredrik

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    What do you mean by "changed by the measurement"? If you just mean that the state after the measurement (if the measurement doesn't destroy the particle) is different from the state before the measurement, that's not relevant here. Either the result (what the measuring device will display when the measurement is over) is determined before either of the particles begins to interact significantly with a measuring device, or it's not. If it is, then the results of a long sequence of measurements will satisfy Bell inequalities.
     
  6. Dec 9, 2011 #5

    f95toli

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    Maybe the OP is refering to a form of superdeterminism?
    If so, then no, there is no test that can rule that out.
    Although this form of "paranoid" determinism (which basically implies that nature is actively trying to "trick" us) is quite far fetched in my view; even compared to say MW.
     
  7. Dec 9, 2011 #6
    The CI version of what is happening in a Bell test would say that a photon has no polarization before it is measured, at which point its wave function collapses, which is due to the photon interacting with the detector.

    I'm suggesting that the photon has a polarization prior to being measured, but measuring it alters its polarization.

    There's of course no way to prove this with our current technology. This is more of an ontological question that could perhaps be measured through weak measurement. Kocsis et al. recently used weak measurement to work out trajectories for single photons in a double-slit experiment, and they exactly followed the predictions of Bohmian mechanics.

    QM suggests that photons do not have trajectories until they are measured. I'm suggesting that new research is pointing toward a non-local realism that does exist but we haven't previously been able to measure.
     
  8. Dec 9, 2011 #7

    Fredrik

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    I still don't see if you mean that the result of each measurement is a certainty (unknown, but still determined by this "polarization prior to being measured") before the measurement begins, or that it's not.
     
  9. Dec 9, 2011 #8

    DrChinese

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    Do you have a reference? I couldn't find anything in the archives.

    I have to say that your comment about research pointing to non-local realism is highly subject. My own reading is definitely towards the other direction.

    Also, you seem to be pretty familiar with the background on Bell tests and yet you use some language which confuses me. When you say an entangled photon has a definite polarization, i.e. is realistic, do you mean to say that is independent of the act of observation? Does it have definite values for any possible measurement angle?
     
  10. Dec 9, 2011 #9
  11. Dec 9, 2011 #10
    Interesting proposal.
     
  12. Dec 9, 2011 #11

    Cthugha

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    I do not think the results point towards any interpretation. As they measured average trajectories, orthodox QM also predicts the same "trajectories" but interprets them differently.

    Let me directly quote the paper: "Single-particle trajectories measured in this fashion reproduce those predicted by the Bohm–de Broglie interpretation of quantum mechanics (8), although the reconstruction is in no way dependent on a choice of interpretation."
     
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