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.