FOUNDATIONS OF QUANTUM MECHANICS

14 CHAPTER I. CONCEPTUAL PROBLEMS
given wave function ψ must be replaced by a mixture of eigenstates. Such eigenstates of position and
momentum are simply not available in quantum mechanics. The possibility to assign values to P 2
and Q 2 attacks the complementarity idea in the heart.
EPR anticipated the objection that only that which has been measured is real (EPR 1935, p. 780),
Indeed, 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. This makes the reality of P and Q depend upon the process of
measurement carried out on the first system, which does not disturb the second system in
any way. No reasonable definition of reality could be expected to permit this.
They conclude their article with the next paragraph,
While we have thus shown that the wave function does not provide a complete description
of physical reality, we left open the question of whether or not such a description exists.
We believe, however, that such a theory is possible.
The problem whether a complete theory is possible or not, is called the hidden variable problem. The
so - called ‘hidden variable theories’ are attempts to solve this problem. We will come back to this in
chapter V.
Bohr’s (1935a) response to the argument of EPR aims at the question to what extent the condition
for an element of ‘physical reality’, as worded by EPR, is fulfilled in their example. The next quotation
is from Bohr (1935b, p. 700),
From our point of view we now see that the wording of the aforementioned criterion of
physical reality proposed by Einstein, Podolsky and Rosen contains an ambiguity as regards
the meaning of 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. (Emphasis added.)
It is not easy to completely comprehend what Bohr says here. Evidently, he abandons the original
idea that the measurement disturbance creates the measurement results, or, at least, that such a creation
can be understood as a physical process. It is replaced by the idea that applicability of physical concepts
depends on the context of measurement. Performing a measurement on one of the particles
is considered as determinative for the applicability of concepts to the other particle. Bohr says that
the measurement disturbance is not a mechanical disturbance; apparently LOC(EPR) continues to
apply for him if we, using the term ‘influence’, refer to a mechanical interaction, but not if we mean