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Weih's data: what ad hoc explanations do local and non-local models give?

  1. Apr 16, 2012 #1
    Weih's data: what "ad hoc" explanations do local and non-local models give?

    From the thread on Nick Herbert's proof:

    harrylin:
    The more I become aware about the tricky details and the impressive experimental attempts to disprove "local realism", the more I am impressed by - to borrow some of Lugita's phrasing - the equally overwhelming survival of Einstein locality, but not definitive proof [if that were possible] due to various loopholes like detector efficiency and "noise".

    Luigita15:
    But the thing is that pretty much every one of the loopholes have been closed seperately, we just haven't managed to close all of them simultaneously in one experiment. So the local determinist is left with having to come up with an explanation of the form "In THIS kind of experiment, the predictions of QM only appear to be right because of THIS loophole, but in THAT kind of experiment, the predictions of QM only appear to be right because of THAT loophole."

    harrylin:
    [..] What we are concerned with is realistic measurements and different from what I think you suggest, I have not seen evidence of the necessity for more ad hoc "local" explanations of measurement results than for "non-local" explanations. [..]

    Lugita15 next brought up ion experiments while I was thinking of Weih's experiment and now Zonde brought that up, complete with a real data set. So now Weih's experiment is the topic of this thread but we should also start one on ion experiments - and hopefully also with real data!

    Thank you! :smile: - and the arrow above links to your post with the attachment (that will have to wait until tomorrow or the day after though)
     
  2. jcsd
  3. Apr 16, 2012 #2

    zonde

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    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    From "A Close Look at the EPR Data of Weihs et al":
    "I include the ranges Low 2, High 2, and High 3 for t B − t A near ±470 ns because I find them mysterious. They contribute too few coincidences to matter for determining whether a Bell inequality is violated."

    I have some comments to say about these mystery peaks but I will post them a bit later. Just the very salt of it - they are related to wrong handling of double detection in electronics.
     
  4. Apr 17, 2012 #3
  5. Apr 17, 2012 #4
    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    Another analysis of Weih's data:

    Is the Fair Sampling Assumption supported by EPR Experiments?
    http://arxiv.org/abs/quant-ph/0606122
     
  6. Apr 17, 2012 #5
    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    Many sources refer to the so-called 'bluescan' data set. Any idea if it is available for download?
     
  7. Apr 17, 2012 #6
    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    Yes. Send an e-mail to Gregor Weihs and he'll send you the link. That's how I got it.
     
  8. Apr 17, 2012 #7
    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    An interesting aspect of this paper I've not seen discussed here is this snippet:

     
  9. Apr 17, 2012 #8

    DrChinese

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    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    I think these analyses of the Innsbruck data are pretty cool. Peter Morgan has done some work in this area too, although he has not published anything at this point (that I have seen at least). He found some anomalies in the data as he was working with time windows.

    However, in all of these you have some issues with finding patterns post facto. As you know, it is not unusual to find variations after the fact if you cherry pick what you use. Most of the analyses have shown little to date (mostly supporting the Weihs et al conclusion). Even this reference comes to some rather unusual conclusions. They say they witness signalling from Alice to Bob. That is assuming I read this correctly: "Bob’s marginal probabilities clearly vary with Alice’s setting , in violation of the non signalling principle." That would be quite a find.
     
  10. Apr 17, 2012 #9
    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    I understood their conclusion differently that you. In their own words:

     
  11. Apr 17, 2012 #10

    DrChinese

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    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    Yes it was a little confusing to me too. Either there is violation of the no-
    signalling principle or not.

    I mean, either Bob is seeing something different on his own or he isn't. If he's not, and you have to compare data from both sides first, where is the violation of no-signalling?
     
  12. Apr 17, 2012 #11

    zonde

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    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    Bigelow in
    A Close Look at the EPR Data of Weihs et al
    p.9-12 under NO-SIGNALING VIOLATIONS is analyzing Adenier and Khrennikov's claims.

    "I demonstrate that Adenier & Khrennikov were mistaken; the coincidence counts
    identified in either window—I’ll illustrate for the wide window—can be modeled as a
    fair sample drawn from a set of sixteen coincidence counts that obey the no-signaling conditions."
    ...
    "In light of the above results it may occur to the reader to wonder how Adenier &
    Khrennikov could conclude that the fair sampling assumption “cannot be maintained to
    be a reasonable assumption as it leads to an apparent violation of the no-signaling
    principle.” In their normalization procedure, Adenier & Khrennikov assumed ..."
     
  13. Apr 20, 2012 #12
    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    OK, I haven't yet gotten around to looking at the data themselves, but I think that we could start with discussing precise and understandable issues that have been raised by others.

    For example, I'm interested in comparing the explanations that are given for the results with a time window of 200 ns, as in plotted in fig.2 of the paper by De Raedt et al - http://arxiv.org/abs/1112.2629 .

    Apparently they can explain it with a somewhat ad hoc local realistic model, and I would like to compare that with what QM predicts for that case. But regretfully, it appears that different explanations have been given, which also sounds ad-hoc to me. Does anyone know of a serious, numeric calculation or simulation attempt in defence of QM?
     
  14. Apr 20, 2012 #13

    DrChinese

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    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    You don't need to defend QM. Nothing is happening that is an issue. The question is how to define the universe of entangled pairs. QM only predicts that the entangled state statistics will come from polarization entangled pairs only. Pairs that are not entangled produce product state statistics. Since we know that the Weihs data worked as expected per QM when the window was 6ns (i.e. a Bell inequality was violated), all is good.

    About the 200ns coincidence window: considering that the original was 6ns, obviously the statistics change as we get further from that and expanding to 200 is a lot. What is an objective definition of an entangled pair? Who decides that? If we use a smaller window, we are progressively more certain we actually have such a pair. As long as there is no particular bias when that happens, what difference is there? So you would need to be asserting something like: "The true universe of entangled pairs includes pairs outside the 6ns window which, if they were included, would not violate a Bell Inequality." But you would also need to explain why ones within the 6 ns window DID! The obvious explanation being, those outside the window are no longer polarization entangled and you wouldn't expect them to produce entangled state statistics.
     
  15. Apr 20, 2012 #14
    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    We are looking for possible disproof of one of the models;that comment is putting things on their head. :tongue2:
    Thus you suggest that the different observation is mainly due to non-entangled photon pairs. I have also heard the incompatible explanation that the the different observation is mainly due to noise. Assuming that QM is a predictive theory and not ad hoc, then someone skilled in QM should be able to give numerical predictions of both observables if enough apparatus details are known.
     
  16. Apr 20, 2012 #15

    DrChinese

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    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    There is no "noise" other than photons. Filters keep out those that are not coming from the PDC crystal. What you don't know is what might cause decoherence of the entangled state. If that happens mid flight, it might be due to something that delays a photon. If so, it is not entangled when it arrives AND it is late. So no issue there.

    As I mention, we need an entangled source. Certainly, the 6ns window source is a pretty good entangled stream. We cannot make that statement once the window gets too large, however large that is.

    You will have to assert that the 200 ns source is ALSO entangled before we can use it for anything. How do you do that? You check for perfect correlations. Once you get a good set of entangled pairs, you can do your Bell test. (That is what Weihs et al did with the smaller window.)

    So it is meaningless to analyze the data this way (post facto) without FIRST having checked the source for being in an entangled state. Where is this accomplished? I haven't seen this. Unless Weihs agrees the larger window is suitable (obviously he didn't), you can't really criticize his conclusion.
     
  17. Apr 21, 2012 #16

    zonde

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    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    Can you list those different QM explanations? Maybe this list can be made shorter.

    And I am not sure that there can be QM prediction for 200 ns coincidence window because QM gives prediction for idealized setup where coincidence window is 0 ns. Reasoning how we go from idealized 0 ns case to 6 or 25 or 200 ns case and how much noise this adds is supposedly classical i.e. there are imperfections in measurement equipment like jitter, dark counts, delays in electronics, drift of clock, discarded events due to polarization setting change window and inefficient detection. Question about unpaired photons prior to detection should be within QM domain but I don't know how it can be handled.
     
  18. Apr 21, 2012 #17
    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    I did not even consider his conclusion, which was based on a different data analysis. But indeed, we need to consider the assertions that were made about the experimental set-up, and then look what can be put to the test with the existing data.
     
  19. Apr 21, 2012 #18
    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    One person asserted that it was noise. Another one asserted that it was non-entangled photons. I ignore if each of these explanations:
    - can produce the observed results
    - can be made non ad-hoc by means of post-diction, based on the used equipment.

    And I'm not sure if also detector inefficiency was proposed as possible "non-local" explanation.
    Are you sure that here you are not confusing a specific Bell test prediction with the capability of a presumably complete theory of physics to predict a statistical result?
    Exactly - what we expect of physical theories, is to enable us to predict the results of measurements under realistic predictions. For example, one doesn't get away with claiming that Newton's mechanics is perfectly correct, and then when measurement results deviate somewhat from the prediction for the "ideal case", make the ad hoc excuse that results are surely due to clock drift, jitter, thermal expansion and so on; these must be accounted for by means of plausible estimations.
     
  20. Apr 22, 2012 #19

    zonde

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    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    About noise. We have two different sources of noise - stray photons that do not come from our photon source and dark counts in detector.
    About dark counts Weihs et al wrote in their paper http://arxiv.org/abs/quant-ph/9810080:
    "The photons were detected by silicon avalanche pho-
    todiodes with dark count rates (noise) of a few hundred
    per second. This is very small compared to the 10.000 –
    15.000 signal counts per second per detector."

    There is nothing said about stray photons so I suppose that careful experimentalist can keep them out good enough so that they can be ignored.

    So I say that noise can be excluded from the list.

    I am fairly sure that I am not confusing anything.

    QM prediction is not concerned about technical difficulties of identifying which photons belong to the same pair.

    No, we expect from theories results for idealized setups.
    We approach reality perturbatively. So the theory provides mathematically simple baseline and then we add correction terms using some other theory or simply using empirical data and experience (of experts).

    So yes, we make excuses. But what we expect from valid test of the theory is that all excuses are made prior to experiment.
     
  21. Apr 22, 2012 #20
    Re: Weih's data: what "ad hoc" explanations do local and non-local models give?

    Thanks for the link to their paper and the clarification! :smile:
    That can't be right - but surely this is a simple misunderstanding about words. For as I stressed and you confirmed with your precisions here above, good experimenalists discuss the consequences of the deviations from the intended ideal and give quantitative predictions that account for these effects, as far as known. Not doing so would make experiments useless, as I also illustrated with my earlier example.
    I had the impression that the theory concerning essential points that we are interested in - issues like dark counts and non-entangled photons - happens to be in the field of QM. In particular such things as instrument noise can be predicted from QM.
    That's a fair but unrealistic expectation as many experiments showed: often some crucial detail was overlooked, and only discovered afterwards. A more realistic expectation is that no ad hoc explanations are provided, after-the-fact; what matters is explanations that are backed up by experiments or existing theory, such as the one by Weiss that you provided here above.
     
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