Friday 16 October 2015

Bohr's reply to EPR (Part III)



Did Bohr find a flaw in the EPR argument?

He says he did.  In a brief note (Bohr 1935a) published soon after the publication of the EPR paper, in his fuller reply to EPR (1935b), and in his later (1949) recap of his discussions with Einstein about QM, Bohr claims to have discovered an ambiguity in the EPR reality criterion.

The EPR reality criterion reads:

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 (EPR 1935, p. 777).

Here’s what Bohr says about the alleged ambiguity:

the wording of the above-mentioned criterion of physical reality proposed by Einstein, Podolsky and Rosen contains an ambiguity as regards the expression “without in any way disturbing a system.” Of course there is in a case like that just considered no question of a mechanical disturbance of the system under investigation during the last critical stage of the measuring procedure. But even at this stage there is essentially the question of an influence on the very conditions which define the possible types of predictions regarding the future behavior of the system.  Since these conditions constitute an inherent element of the description of any phenomenon to which the term “physical reality” can be properly attached, we see that the argumentation of the mentioned authors does not justify their conclusion that quantum-mechanical description is essentially incomplete (Bohr 1935b, p. 700; 1949, p. 324).

EPR consider two systems that interact for a while, and then, after a certain time, no longer interact.  On the basis of this absence of interaction, they conclude that what is done to the first system produces no change in the state of the second system.  According to Bohr, this is ambiguous between two readings.


  1. They could mean that an experiment performed on the first system produces no mechanical disturbance of the other.
  2. On the other hand, they could mean that an experiment performed on the first system has no effect on the types of predictions that can be made about the other.

I submit that there is no ambiguity, because (2) is not a possible reading of what EPR meant.  Of course the choice of experiment done on one particle has an effect on the sorts of predictions that can be made about the other; that’s the whole point of the argument!  And EPR do note that, since the choices are mutually exclusive, to make a choice means losing the opportunity to make the other sort of prediction about the other particle:

one would not arrive at our conclusion if one insisted that two or more physical quantities can be regarded as simultaneous elements of reality only when they can be simultaneously measured or predicted. On this point of' view, since either one or the other, but not both simultaneously, of the quantities P and Q can be predicted, they are not simultaneously real (p. 780).

Bohr says that that the conditions that define the possible types of prediction that can be made about a system constitute an inherent element of the description of any phenomenon to which the term “physical reality” can be properly attached.  This sounds to me as if he saying that being able to predict the outcome of a position “measurement” is a condition for applying the concept of position, and being able to predict the outcome a momentum “measurement” is a condition for applying the concept of momentum. But, if that’s what he’s saying, he’s simply taking the out that EPR offer at the end of their paper.  They note that this way out has the consequence that the reality of properties of one system depends on what is done to the other.  If that is, indeed, what Bohr is saying, then one wishes he had said so straightforwardly!


References

Bohr, Niels (1935a).  Quantum Mechanics and Physical Reality. Nature 136, 65.

——— (1935b). Can Quantum-Mechanical Description of Physical Reality Be Considered Complete? Physical Review 48, 696-702.

——— (1949).  Discussions with Einstein on Epistemological Problems in Atomic Physics, in P.A. Schilpp, ed., Albert Einstein: Philosopher-Scientist (Chicago: Open Court Press), 199–241.

Einstein, Albert, Boris Podolsky, and Nathan Rosen (1935), Can Quantum-Mechanical Description of Physical Reality Be Considered Complete? Physical Review 47, 777-780.





           


8 comments:

  1. No, that was not Bohr's meaning wrt to context - the possible types of predictions - being an inherent element of the description. All he meant was what kind of measurement was being performed. In the spin case, it would be the orientation of the Stern-Gerlach apparatus and the analyzers. For a fuller account of all of this, look at this paper of mine:

    https://www.academia.edu/1294403/What_makes_a_classical_concept_classical_Toward_a_reconstruction_of_Niels_Bohrs_philosophy_of_physics

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  2. From Howard Wiseman, via facebook:

    One could indeed wonder why Bohr didn't just give a coherent defence of operationalism. I suspect that he didn't want to do that because he saw complementarity as more than that --- he saw it as offering insights into all manner of things outside quantum physics.

    But I don't quite agree on your analysis of the two meanings you offer to reflect Bohr's "ambiguity". You say "the choice of experiment done on one particle has an effect on the sorts of predictions that can be made about the other; that’s the whole point of the [EPR] argument!" I don't think that's true. The ability to make different predictions by different measurements is the whole point of EPR. The inability to make other predictions (by making further measurements), having made a certain type of measurement, is what Bohr meant by non-mechanical disturbance.

    Interestingly (to me at least), Bohr's non-mechanical disturbance at a distance does not require entanglement to manifest:
    http://arxiv.org/abs/1208.4964

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    1. Ok, "whole point" is overstatement.

      But, while it's true that Bohr emphasized the exclusivity of the measurements more than EPR do, EPR do make the point that one has to make a choice, and that one choice excludes the other. If the experiments weren't exclusive, then there'd be no need for the argument; one could ascertain both position and momentum at the same time, and it would be evident that QM description is incomplete.

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    2. Serendipity: this just showed up on my fb feed. Video from New Scientist about discord as the mark of quantumness.

      https://www.youtube.com/watch?v=CDorBWsRd6w&feature=youtu.be&utm_source=NSNS&utm_medium=SOC&utm_campaign=hoot&cmpid=SOC%257CNSNS%257C2015-GLOBAL-hoot

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  3. Sure. And I agree that (2) is not a plausible reading of what EPR meant. So by "ambiguity" Bohr seems to mean "You have not really specified what you mean by disturbance, so this is my opportunity to attack your argument at a technical level by interposing my own definition".

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    1. Do they really not specify? They say that after a time, the systems no longer interact. We know how to model interactions in QM: via interaction terms in the Hamiltonian. I think it's clear that what they mean is absence of interaction in this ordinary sense.

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  4. Oh yes, they are clear that certain conditions (no longer interacting, in the ordinary physical understanding) imply no disturbance. And that is in the spirit of the rest of their paper --- they only need a sufficient condition, so they only give a sufficient condition. That's why I agree that (2) is not what they could mean by no-disturbance. They don't actually define what "disturbance" is, but Bohr is surely knowingly rejecting their notion.

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