In Part I, I claimed that Bohr simply took for granted the conclusion that EPR argue for, namely, that quantum mechanics does not yield a complete description of physical reality, and that where they part ways is the unargued-for suggestion that a more complete theory is possible.
If this is right, why isn't it clear to every reader of Bohr's reply to EPR?
The reason, I think, is that it wasn't clear in Bohr's own mind. When I read Bohr, it often seems to me that he is conflating two distinct questions. One is the question of whether there could be a theory whose state-descriptions go beyond what is allowed by quantum mechanics. The other is whether we might ever be in a position to know more about the state of a quantum system than is allowed by the uncertainty relations.
They aren't the same question. Think of classical mechanics. In classical mechanics, a complete state-description is a specification of precise values of all the system's dynamical variables. For systems composed of many molecules, it would be hopeless to even come close to knowing the precise state of the system, and so we resort to probability distributions over the set of precise states. It is irrelevant, for the way the theory is used, whether these limitations on knowledge of the precise state are pragmatic limitations, or limitations in principle.
Einstein's view was that quantum wave functions had a status similar to the probability distributions used in classical statistical mechanics; they represented incomplete knowledge of a precise state that would occur in some other theory (not necessarily classical). If anyone, prior to Einstein's death, presented a good argument for why one shouldn't think of quantum states in this way, I haven't seen it. No such argument is found in Bohr's reply to EPR.
Though I don't understand Bohr's response to Einstein, I do understand what Einstein attributes to Bohr as a reply, in his Replies to Critics in the Schilpp volume. There are two ways to reject the conclusion of an argument. One is to find a flaw in the reasoning; the other is to accept the reasoning that leads from premises to conclusion, and to reject one or more of the premises. Bohr's reply to EPR reads as if he thinks he's found a flaw in the reasoning; he says he's detected an ambiguity in the EPR reality criterion, an ambiguity fatal to the argument. But in the Replies, Einstein has Bohr reject a premise of the argument.
Of the "orthodox" quantum theoreticians whose positions I know, Niels Bohr's seems to me to come nearest to doing justice to the problem. Translated into my own way of putting it, he argues as follows:
If the partial systems A and B form a total system which is described by its ψ-function ψ/(AB), there is no reason why any mutually independent existence (state of reality) should be ascribed to the partial systems A and B viewed separately, not even if the partial systems are spatially separated from each other at the particular time under consideration.
So: I understand that as a potential reply to EPR, though I also think that it would be incumbent on someone who replied that way to answer Einstein's challenge, at the end of the Dialectica article, to point to some phenomenon that suggests that we should reject the premise.
As it appears to me, there can be no doubt that the physicists who hold the quantum mechanical manner of description to be, in principle, definitive, will react to these considerations as follows: They will drop requirement II of the independent existence of the physical realities which are present in different portions of space; they can rightly appeal to the fact that the quantum-theory nowhere makes explicit use of this requirement.
I grant this, but note: if I consider the physical phenomena with which I am acquainted, and especially those which are so successfully comprehended by means of quantum-mechanics, then, nevertheless, I nowhere find a fact which makes it appear to me probable that one has to give up requirement II. (Einstein 1948, translation in Howard 1985)I don't think that Bohr, or anyone else, answered that challenge in Einstein's lifetime.
Einstein, Albert (1948). Quanten-mechanik und wirklichkeit. Dialectica 2, 320–324
Einstein, Albert (1949). Remarks concerning the essays brought together in this co-operative volume, in P.A. Schilpp, ed., Albert Einstein: Philosopher-Scientist (Chicago: Open Court Press), 665–688.
Howard, Don (1985). “Einstein on Locality and Separability.” Studies in History and Philosophy of Science 16, 171–201.