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Loopholes in optical Bell tests

  1. Sep 19, 2005 #1
    In reading about Bell tests I found this link:http://www.abc.net.au/science/features/quantum/, which has discussion of numerous loopholes in both theory and experimental design of Bell tests, which when combined, appear to suggest that "local realist" explanations of all Bell test violations should exist. That is, no single Bell test does not have at least one of the following seven loopholes, and since it only takes one loophole (1 of 7) to invalidate test results as suggesting, "local realism" does not exist, in fact, there are no Bell tests that disprove "local realism". Second part of thread, is anyone aware of yet additional "loopholes" in the published literature ?

    Bell Test Loopholes (no single Bell test does not contain at least one loophole)
    1. Fair sampling loophole
    2. Subtraction of accidentials loophole
    3. Failure of rotational invariance loophole
    4. Synchronization loophole
    5. Enhancement loophole
    6. Double detection loophole
    7. Locality-light cone interpretation loophole
  2. jcsd
  3. Sep 19, 2005 #2


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    You appear to have gone one step too far. These so-called loopholes do NOT "invalidate" the results. That's like saying measurement of zero resistivity invalidates the existence of superconductors simply because superconductivity has other properties that resistivity measurement could not measure. On the contrary, each of the experiments, no matter which loopholes they have, clearly gives a strong indication that "local realism" isn't as convincing as QM description. Think about it. No matter which loophole an experiment has, none of them ever indicate a result that is not consistent with QM's predictions.

    Furthermore, you seem to have ignored a more stringent test of local realism based on multipartite system, and the more stringent extension of Bell theorem in the CHSH theory. Again, collect ALL of the bell-type experiments that cover a huge amount of experimental variation and find those that claim an inconsistent result with QM. You make everyday decisions based on orders of magnitude LESS certainty than this.

  4. Sep 19, 2005 #3


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    This is illogical. You can run any test and see that the presence or absence of any of the above "loopholes" makes no difference to the outcomes. All outcomes support QM and reject Local Reality. It is like asking: "Do you get the same results on Monday as you do on Tuesday?" Or "Do you get the same results in the Northern Hemisphere as in the Southern Hemisphere?"

    Is it a loophole that every permutation of every experiment in science is not performed each and every time? No, of course not. The object is to determine what variables make a difference to the outcome. How is it that none of the 7 listed individually make any difference but several collectively do?

    It is not generally accepted that these are loopholes anyway. The actual tests are very convincing and leave little room for doubt. I would spend more time reading the actual tests, and then you will see how ridiculous these claims really are.
  5. Sep 19, 2005 #4
    Not wishing to be argumentative, just passing on what I read. Thus, the CHSH-theory brings forth yet two more loopholes, as read at this link:http://www.math.uu.nl/people/gill/Preprints/poster.pdf [Broken]
    8. Detection Loophole
    9. Coincidence Loophole
    and here is the conclusion of these authors: "Closing each separate loophole in different experiments is just not good enough. Maybe it is not a coincidence, that no one could close all loopholes in one experiment, despite 20 years progress".
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  6. Sep 20, 2005 #5
    OK, here is one manuscript where Bell tests were studied and they concluded that at least one loophole is very convincing indeed--I do not find their logic ridiculous, perhaps open to debate, but not ridiculous:

    I seem to have hit some sort of QM nerve with this thread--why so defensive of Bell tests ? This is an unscientific position--one must always show how results of experiments may be incorrect based on faulty experimental design and or false conclusions based on holding false premises (so-called loopholes). That fact that a number of physicists claim these loopholes are valid is for me a red flag--Bell tests are not all that convincing afterall that local realism does not exist.

    It seems to me that this "loophole" problem is very valid--QM makes an extraordinary claim--that local realism at both the macroscopic and microscopic level does not exist--thus it requires extraordinary experimental evidence from "at least one experiment" that is free of any and all "loopholes". As far as I know, no such experiment exists, and at least one author has suggested that it is impossible for such an experiment to ever be conducted. It is clear from the numerous discussions about loopholes such as above internet link that QM does not prove that reality is non-local--it is only a hypothesis.

    But I shall stop hitting nerve cells and let others that have detailed knowledge of Bell loopholes continue the dialog on this thread.
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  7. Sep 20, 2005 #6


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    If you are asking: The opinions of those authors is not really mainstream science. Try a quote from something more authoritative. If you bother to actually check the experiments, instead of someone else's biased views, you will see that the evidence is very convincing. Try this, for example:

    http://arxiv.org/PS_cache/quant-ph/pdf/9810/9810080.pdf [Broken]

    This is peer-reviewed science. It is considered definitive by scientists, although I doubt that any Bell experiment can be constructed to satisify some people.

    If you are telling: Some people think the world is flat - they see loopholes in the theory that the earth is round. Meanwhile, the rest of us fly around the globe.

    Ultimately, you are free to believe whatever you want.
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  8. Sep 20, 2005 #7


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    You are quoting Caroline Thompson, an obnoxious crackpot who was banned from this board. I can tell you from personal experience that there is something seriously wrong with her. And besides denying the validity of Bell tests, she also denies the existence of photons. If you also question the existence of photons, please let us know now and save the rest of us the time we might otherwise spend discussing the subject with you.

    As to the nerve you are hitting: this subject has been debated ad nauseum here. PhysicsForums is focused on mainstream and cutting edge science; those who deny accepted/published scientific experiments are neither. If you have something useful to add to the discussion, that is great. If you are planning to parrot the words of Caroline, you likely won't last long here.

    As I said previously, it is instructive to read the actual experiments and understand Bell before you try to tear it down. This area has been studied at great depth by some pretty good scientists.

    Now to the science itself:

    I will tell you that there are in fact 2 separate issues: a) Bell's Theorem and b) tests of Bell's Inequality. Bell's Theorem says that QM and Local Reality are incompatible in some respects. Tests of Bell's Inequality uniformly show that Local Reality is violated. Separate tests of QM uniformly support it. So the problem is a lot bigger for the local realist than it might first appear. You must explain not only why LR is violated (that's the loopholes you mention); but you must also explain why the experimental error always takes you to the QM predicted value! That's a tall order, and one that not a single LR supporter has ever endeavored to address. On the other hand, if there is no significance to the purported loopholes, then the QM position is fine as is.

    LR predicted value + significant experimental error = observed value

    The above is what a local realist asserts, but the problem is you have to come to grips with the following coincidence which cannot be denied:

    QM predicted value = observed value

    So I assert the simplest explanation of all: There is no experimental error. So you see, the local realist has a tremendous burden. If you do not follow this logic, then look at the LR predictions at various angles and you will quickly see how far off you are from observed results.
  9. Sep 20, 2005 #8


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    Oh, now this is getting somewhere.

    You are quoting someone who (i) cannot derive the wave equation from the Maxwell equation even though she's claiming that photons are waves and (ii) have been known to say that maxwell equations are nothing more than "just math". So this is the person you pay more attention to?

    Here's something you might want to ponder. NO EXPERIMENT and no experimentalists have made any conclusion that the results of their experiment are inconsistent with QM's prediction. The ONLY people still clinging on to such possibilities are either a very small and obscure number of theorists, or crackpots like your hero. None of these people have actually gone out and DO the experiment themselves! Why? Because they can't, or do not understand the nature of detectors and thus, have zero understanding on how such measurements are done.

    This important fact is totally ignored when someone comes here and claim these things. The amazingly large number of experiments that covers a large variation of techniques, but giving CONSISTENT conclusions is astounding! The logic you used would also cause you to claim that superconductivity experiments are "invalid" because they didn't measure everything at once. Would you like to go out on the limb and claim that too?

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  10. Sep 20, 2005 #9
    That's a lot of loopholes. But, experimenters are quite careful
    about their experimental designs and adjustments to the raw data.
    Bell tests are getting better. Instrumentation and detection is getting
    better. I'm betting that when a test is done that everybody can agree
    is loophole free, then that test will show a violation of the inequality
    that was applied to it.

    The only questions remaining to be answered will then have to do with
    how the results are interpreted. What do experimental violations of the
    inequalities mean? Apparently most physicists don't take
    the idea that nature is nonlocal too seriously. Keep in mind that
    "quantum nonlocality" doesn't mean superluminal propagation
    of anything. I would also guess that most physicists think that
    there is a real world out there full of phenomena that exist whether
    we happen to be looking at them or not -- and that these phenomena
    have certain definite properties at any instant, and function according
    to certain rules

    So, what exactly does it mean to say that experimental violations
    of Bell inequalities contradict local realism, or that such violations
    disallow local realistic models? I'm not sure.

    Here's Wikipedia's definition of local realism:
    In physics, the principle of locality is that distant objects cannot have direct influence on one another: an object is influenced directly only by its immediate surroundings. This was stated as follows by Albert Einstein in his article "Quantum Mechanics and Reality" ("Quanten-Mechanik und Wirklichkeit", Dialectica 2:320-324, 1948):

    "The following idea characterises the relative independence of objects far apart in space (A and B): external influence on A has no direct influence on B; this is known as the Principle of Local Action, which is used consistently only in field theory. If this axiom were to be completely abolished, the idea of the existence of quasienclosed systems, and thereby the postulation of laws which can be checked empirically in the accepted sense, would become impossible."

    Local realism is the combination of the principle of locality with the assumption that all objects must objectively have their properties already before these properties are observed. Einstein liked to say that the Moon is "out there" even when no one is observing it.

    According to this definition, locality means that the settings (or results or whatever) at A don't affect the results at B, and vice versa. There's nothing in the experiments themselves to indicate that A and B are affecting each other. Saying that A and B are affecting each other is a matter of interpretation. My current approach is to assume that the experimental results are more or less correct, and that nature is local (ie., that the speed of light is a limit) -- so there's something wrong with interpretations
    that conclude that violations of the inequalities means that 'something' is propagating superluminally between A and B.

    Realism here is defined as "the assumption that all objects must objectively have their properties already before these properties are observed." Well, nobody really knows what the term "photon", as defined in quantum theory, corresponds to in nature.
    In optical Bell tests, the photons that are detected are transmitted by
    the filters. It isn't known exactly what is actually happening 'in nature' at the level of the interaction between the emitted light and the filter. It isn't known exactly what is happening between emission and detection, or during the emission process. There's no qualitative apprehension of entities or events or processes at the 'quantum level' of nature. There are models which relate experimental preparations and results of course, but these don't give a clear qualitative picture of the 'quantum world'.

    Einstein wanted a more realistic theory. Everybody would like to have
    a more realistic theory -- ie., an accurate 'picture' of what's happening at
    the level of the quantum. But there isn't one.

    So, what does it mean to say that a photon has certain properties
    independent of detection? Not much so far -- all attempts, afaik,
    to model Bell tests in a 'realistic' way have been contradicted.
    Of course that doesn't mean that the light incident on the filters
    doesn't have any physical properties. It wouldn't make any
    sense to say that. But the 'realistic' models are not up to speed yet.

    The optical Bell tests that use atomic cascades (see, for example, Aspect et al. 1982) indicate that in order to get entanglement (ie., violation of an inequality), coincidental detections have to be causally linked to the same oscillator (the same atom). Quantum theory says that two photons emitted in opposite directions by the same atom will have identical polarization. This doesn't refer to any property of the emitted light prior to detection. Nevertheless, the tacit assumption is that light emitted in opposite directions by the same oscillator at the same instant has some common physical characteristics (which would correspond to Schroedinger's
    meaning when he coined the term, entanglement) -- which is the thing that Bell's theorem, as interpreted by some, is supposed to disallow.

    The whole thing is an intriguing mystery, a significant part of which is evaluating the physical meaning of Bell's theorem. My current understanding is that, while experimental violations of inequalities can be used as a fairly trustworthy indicator of entanglement, the Bell tests aren't necessarily telling us that nature is nonlocal or that a more realistic theory is, in principle, impossible.

    Anyway, instead of worrying about the loopholes I would rather marvel at the promise of actually being able to do stuff with quantum entanglement.

    It's not that experimental loopholes aren't an important consideration -- it's that statements about nature being nonlocal or realistic theories being impossible because of Bell's theorem aren't an important consideration.

    Inequalities based on a Bell-type model of the prototypical optical Bell test will always be violated. What A and B are measuring individually is not the same as what A and be are measuring jointly. In the joint context (the Bell test context), the settings and results at one end aren't separable from (aren't independent of) the settings and results at the other. They're being considered *together*. Inequalities based on a model that separates them will always be violated.

    Keep in mind that the only physical connection that A and B have is that they're jointly measuring something that came from the same oscillator at the same instant. The coincidence correlations are produced by the joint settings of the filters.

    So, there's only two possibilities. Either the filters are jointly analyzing the same thing, or the two arms of the setup are 'communicating' or 'influencing each other' via some unknown and undetectable, superluminally propagating 'disturbance' that has no upper limit. My guess is that most working physicists are inclined to believe the former rather than the latter.
  11. Sep 20, 2005 #10


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    It is true that there has not yet been a single experiment where raw data as such have demonstrated a violation of a Bell inequality (at least in 2-photon experiments, I'm less aware of others). In order to be able to do so with 2-photon systems, the individual photodetectors should have a quantum efficiency of the order of 86% and neglegible noise counts, and this is technologically not realized yet (I'm less well up in my reading concerning more than 2 photons). So experimental corrections are necessary.
    As such, it has to be said, that the entire family of local realist theories has indeed not been definitely excluded BY RAW DATA. What has been done, however, each time, is that using the standard knowledge people have of 1) the experimental devices (such as quantum efficiency) and 2) of simple quantum mechanics, they found results IN AGREEMENT with these predictions. Now, each time, you could also think up A MODEL MADE UP ON PURPOSE that would also wiggle in to explain the data, but these models have huge difficulties predicting OTHER things that are correct, and moreover have very peculiar views on how well-known apparatus (such as photodetectors) operate. Usually it is erroneously claimed that "nobody knows exactly how these detectors/polarizers... work".
    For instance, one model is the classical field approach with some added vacuum noise. No electrons, no photons, just classical fields. I'm talking here about Stochastic Electrodynamics (SED). However, SED has difficulties explaining things like the helium atom.
    The theory that fits ALL data well is quantum theory. This does not exclude of course that something else might not exist, but you have a hard time doing so. The idea is that if quantum theory had a serious problem, one would see DEVIATIONS from the predictions, using the usual properties of the apparatus.

    It is a bit as if you are, in court, confronted with:
    - witnesses have seen you 5 minutes before the crime near the house,
    - you have a great financial motive to have killed the person
    - your car got a parking ticket 20 meters from the place of the crime 10 minutes before the crime
    - there were blood stains of the victim found in your car,
    - a week before the crime, you said, when drunk, on a party that you were going to cut that bastard's throat
    - a knife fitting the victim's wounds was found in your dustbin.

    Each individual element can have another explanation:
    - you were just having a walk there
    - you are not really interested in the money
    - you parked your car there just to have a walk
    - you forgot to close the car with the key, and the true murderer tried to steal your car, hence the blood stains
    - one should not take you seriously when you are drunk
    - you simply happened to have a similar knife which you happened to throw away by coincidence that day.

    Ok, this is indeed possible. But what will the jury decide, do you think ?

    Local realist explanations sound a bit like this explanation. True, sometimes an innocent gets convicted...
  12. Sep 20, 2005 #11
    Thank you Sherlock...your detailed explanation and the conclusions you reach above "make logical sense" to me, since I hold a realistic view of existence. Now, as you state, what Bell tests "test" has nothing to do with local realism, but has much to say about "entanglement", which is a very important topic. So, now I can fly around the world happy that the world really could care less that I (or any other human) exist.
  13. Sep 21, 2005 #12


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    Just because that makes you sleep better at night doesn't mean that is what it does. There are OTHER experiments that make quantum entanglement even more pronounced than Bell-type tests. The experiemnt that shows 2 entangled photons beat the diffraction limit is MORE convincing. If all we need are tests of QM entanglement, we already have that without doing the Bell tests.

    Yet, the Bell and EPR-type experiments are still being done AND refined. Why? Because each one of them points to a CONSISTENT evidence of the failure of LR. It isn't JUST an experiment on entanglement, because if it is, the "loopholes" would not matter a single bit!

    Like I have said, YOU make decisions based on LESS convincing evidence than what we have seen about QM entanglement. Yet, these don't seem to bother you one bit.

  14. Sep 21, 2005 #13

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    I don't understand why some people are so determined to prove that Quantum Mechanics is "not the complete picture". The accusation has often been made that scientists are not being scientific in their "blind" or "dogmatic" adherence to the tenants of QM over local realism, but it seems to me that just the opposite is true. The irrational and persistent attempt to get around the results of QM by proposing all kinds of hidden variable theories is, it seems to me, the very definition of personal bias in science.

    Do these more complicated hidden variable theories make any predictions that QM does not? It seems to me that these theories are tailored to give exactly the predictions of QM, and that they exist only so some researchers can put their minds at ease on a number of non-scientific philosophical points. When an experiment produces a result which QM fails to describe, then I will listen to competing theories. Until then I'll stick with the simple scientific theory of QM and justly call everything else philosophy.
  15. Sep 21, 2005 #14


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    And to provide evidence of what you have just said, the very fact that there CONTINUE to be more and more experimental tests of this phenomenon is clear evidence that physicists simply won't take things for granted on anything. If we all have already decided that this is a done deal, why all these more refined tests? That makes no sense.

    On the other hand, these people who want to uphold Local Realism, NONE of them are doing ANY kinds of experiments! All they do is piggy-back on top of existing experiments (all of which claim failure of LR and supports QM's picture) and take the easy way out by pointing out the "loopholes". At best (and that itself is rather dubious), all they can say that these experiments cannot prove definitively that LR is dead. They certainly can't show anything that the results are inconsistent with QM's description. However, the OP of this thread took this an illogical step further by claiming that these experiments "invalidates" QM. Oy vey!

  16. Sep 21, 2005 #15


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    As usual, well said. And just to drive the point home: in all of science, it is normal and common to refine tests to get even more and more accurate results and limit further the possibility of any deviations from accepted theory. Examples: tests of the value of c; tests of general relativity; tests of special relativity; tests of the equivalence principle; tests of the mass of the electron; etc. etc. and this is just in physics.

    So while I assert that the results of Bell tests are conclusive, in the same breath I want to see them performed with greater efficiency and accuracy. You never know what you might find!
  17. Sep 21, 2005 #16
  18. Sep 21, 2005 #17
    OK, a very nice paper dealing with mesons---then in a few seconds looking on the internet I find this recently posted (2005):http://arxiv.org/PS_cache/quant-ph/pdf/0501/0501039.pdf [Broken], and yet another loophole is identified, now with attempt to use mesons for Bell tests.

    I did not start this thread to start an argument of facts--not possible because I know little about Bell tests. My point of the thread is that I find it very interesting that every time someone puts forth a Bell Test that claims to prove that local realism does not exist, yet another paper follows that shows the claim to be suspect--not conclusive--to have an interpretation loophole or one of at least nine other loopholes. I know of no loophole arguments against theory of gravity, not a single paper in > 100 years published on loopholes of cell theory in biology, second law of thermodynamics--no loopholes. Yet, each and every one of so-called Bell tests have papers published that identify loopholes. Is there not a pattern here that at least causes a scientist to raise an eyebrow ?--that is all I am doing at this point--eyebrow raising.

    Perhaps those that hold local realism to be valid in physics have no need to waste limited and expensive lab time attempting to prove the obvious, that reality exists independent of human observation. Perhaps it is non-sense to even attempt to test such a proposition if in fact it is an axiomatic given, because in philosophy, and mathematics, from which physics derives, an axiom cannot be falsified by logic or reason or experiment--it just must be accepted (I will call this Bell's laspe). If in fact local realism is such an axiom concept, then it must be predicted that no single test can ever be devised to falsify local realism that will not always be found to have some loophole. And this is exactly the state of affairs since Bell over the past 40+ years, as predicted if local realism is an axiomatic concept, not a single Bell test that does not contain at least one loophole (e.g., 100% experimental verification by all Bell's Tests in favor of the hypothesis that local realism is an axiomatic concept of both physics and philosophy).

    How about if we ask an experimental local realist physicist to chime in on the thread as to why they do not spend time testing Bell Inequalities--does anyone know of such a person ?
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  19. Sep 21, 2005 #18


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    This is where you get your "facts" wrong. Look at ALL the experimental paper on such test. Now count how many peer-reviewed papers that appear to counter those conclusions!

    There is no such thing as "yet another paper follows". There is, however something like "yet another INTERNET WEBPAGE" production that make such claims. Do these people manage to even write a rebuttal to those papers (you do know that one CAN write a rebuttal to such things if one believes that those papers are wrong, don't you?) in the respective journals? Nope!

    You really should seriously consider the credibility of your sources that you somehow are putting a lot of weight on.

    Good luck in finding one. I have my own opinion on why such a person does not exist.

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  20. Sep 21, 2005 #19

    Physics Monkey

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    If I may, I would draw everyone's attention to the second sentence of the introduction to the most recent "loophole" paper (http://arxiv.org/PS_cache/quant-ph/pdf/0501/0501039.pdf [Broken]).

    The author states, "Even if now SQM is confirmed by a huge amount of data and represents one of the pillars or modern physics, the fundamental quest about a possible realistic theory reproducing its results remains open."

    What are the key phrases here? First, the author admits a huge amount of data supports QM. Second, the author admits that QM is justly regarded as a pillar of modern physics. Third, the author states that the goal of a local realistic theory is to reproduce the results of QM.

    I think the author's description of this search as a "fundamental quest" is particularly apt. For local realists, the fundamental desire is to describe the world in terms of local realistic theories. This is their overriding goal and it constitutes a severe personal bias.

    We can debate all the day long the question of local realism, and local realists can continue to show that they can construct excessively complicated theories that reproduce QM. However, until the day an LR researcher can propose some experiment that will distinguish LR from QM, the question remains in the realm of metaphysics.

    At least that's how I see it.
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  21. Sep 21, 2005 #20
    I agree with you 100%--it is to peer reviewed papers on loopholes that we must put greatest weight of evidence. Here is my initial count for tonight--I note that they range from 1970 to 2004, very much in line with the time period of Bell tests that attempt to prove that local realism does not exist, some of these papers are reviews that cover many different Bell tests, not just one.

    1. P. M. Pearle, Phys. Rev. D 2, 1418 (1970)
    2. J. F. Clauser and M. A. Horne, Phys. Rev. D, 10, 526 (1974)
    3. T. W. Marshall, E. Santos and F. Selleri, Local realism has not been refuted by atomic-cascade experiments, Phys. Lett. A, 98, 5-9 (1983)
    4. T. W. Marshall and E. Santos, Stochastic optics - a reaffirmation of the wave nature of light, Foundations of Physics, 18, 185-201 (1988).
    5. T. W. Marshall and E. Santos, Stochastic optics: a local realist analysis of optical tests of the Bell inequalities, Phys. Rev. A, 39, 6271-83 (1989).
    6. C. H. Thompson, Subtraction of ?accidentals? and the validity of Bell tests, Galilean Electrodynamics 14 (3), 43-50 (2003)
    7. C. H. Thompson, The Chaotic Ball: An Intuitive Analogy for EPR Experiments, Found. Phys. Lett. 9, 357 (1996)
    8. A. Cabello, E. Santos. Phys. Lett A. 214, 316 (1996)
    9. E. Santos, Phys Rev A, 69, 022305 (2004)
    10. O. A. Barut. The fallacy of arguments against local realism in quantum phenomenon. in Waves and particles in light and matter. A van der merwe et al. eds. Plenum (1993).


    Finally, although I know you will object, I include what I believe to be a recent balanced and fair defense of the local realist position about Bell tests by professor E. Santos. I personally would put weight on this internet post because Santos has published in recognized peer reviewed journals on this issue:

    http://arxiv.org/PS_cache/quant-ph/pdf/0410/0410193.pdf [Broken]
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