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EPR reasoning

  1. Jan 14, 2010 #1
    I'm slowly working my way through what I need to really understand the Bell papers. Everyone says that a good start is the EPR paper. I understand that Einstein may not have read the final version before publication and may not even have wanted his name appended (sort of convoluted logic and clumsy wording, and Einstein was actually really good at this stuff!!) There is a statement direct quote: . If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity. .Somehow, to me this doesn't seem to hold water. Maybe someone can clarify it. How can I predict the state of a system, much less without disturbing it? If I'm driving a car at 60 miles an hour, I can look at the speedometer and know I am going that speed, but even there, there is no prediction. The world doesn't seem to have dials and meters attached to it in any reasonable way, so, what the heck is meant by this statement?

    I know they use this definition later in the two particle wave function argument, but that seems to be using what they considered a result to really come up with the definition. Fairly circularly transparent, it seems to me. Am I missing something?
  2. jcsd
  3. Jan 14, 2010 #2


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    Yes, the issue that he is referring to is as follows: Say I have two pairs of socks of different colors. I pull out one member of a pair and give to Alice. I give the other sock to Bob, and tell both of them not to look at the color. Alice flies to Atlanta and then looks at the sock, and it is yellow. She immediately knows that Bob, who flew to Boston, has a yellow sock too. (Yellow is my favorite color, by the way.)

    So according to EPR, Alice was able to predict, with certainty, the results of an investigation into the color of the sock given to Bob. And she is able to do this without disturbing Bob or Boston in any way. Therefore, according to EPR, there is an element of reality to the color of Bob's sock and that is a completely objective statement. It is not observer dependent.

    BTW, I wouldn't worry too much about Einstein's objections to the EPR paper itself. He definitely believed the conclusion, as he restated it famously: "I think that a particle must have a separate reality independent of the measurements. That is: an electron has spin, location and so forth even when it is not being measured. I like to think that the moon is there even if I am not looking at it." That was an opinion that was held by many for years. Of course, now we know about Bell but Einstein didn't live that long.
  4. Jan 15, 2010 #3
    Thanks for your comments and explanation Dr. Chinese (interesting moniker ;-{) ) I probably should have included the rest of the paragraph after the excerpt that I quoted, vis:It seems to us that this criterion, while far from exhausting all possible ways of recognizing a physical reality, at least provides us with one such way, whenever the conditions set down in it occur. Regarded not as a necessary, but merely as a sufficient, condition of reality, this criterion is in agreement with classical as well as quantum-mechanical ideas of reality.. This still seems to be a pretty self-serving (regarding the argument in the second half of the paper) definition, and there may be others definitions that obviate the arguments in the first half.

    The paper seems to be in two quite distinct sections, with the first dealing with non-commuting operators and the HUP and the second with their gedanken conjecture. I keep harking back to classical conjugate variables which are Fourier transforms of each other. Classically, one can not say that, because I do not know (and may not even be able to determine) the value of some physical parameter that it does not exist and hence is not "real" in some sense.

    There seems to also be a bit of fancy footwork in the first half in making the momentum in the wave function a constant (hence surprize , surprize, it is measured as a delta function fixed value). No need for a reply, just thinking out loud (okay, in writing!!)
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