456 THE QUANTUM THEORYmomentum and position operators of a given particle. Therefore quantum
mechanics is incomplete.
The authors stress that they 'would not arrive at our conclusion if one insisted
that two... physical quantities can be regarded as simultaneous elements of real-
ity only when they can be simultaneously measured or predicted' (their italics).
Then follows a remark that is the key to Einstein's philosophy and which I have
italicized in part:
This [simultaneous predictability] makes the reality of p 2 and q 2 depend upon
the process of measurement carried out on the first system which does not dis-
turb the second system in any way. No reasonable definition of reality could be
expected to permit this.
The only part of this article that will ultimately survive, I believe, is this last
phrase, which so poignantly summarizes Einstein's views on quantum mechanics
in his later years. The content of this paper has been referred to on occasion as
the Einstein-Podolsky-Rosen paradox. It should be stressed that this paper con-
tains neither a paradox nor any flaw of logic. It simply concludes that objective
reality is incompatible with the assumption that quantum mechanics is complete.
This conclusion has not affected subsequent developments in physics, and it is
doubtful that it ever will.
'It is only the mutual exclusion of any two experimental procedures, permitting
the unambiguous definition of complementary physical quantities which provides
room for new physical laws,' Bohr wrote in his rebuttal [Bl 1]. He did not believe
that the Einstein-Podolsky-Rosen paper called for any change in the interpre-
tation of quantum mechanics. Most physicists (myself included) agree with this
opinion.
This concludes an account of Einstein's position. He returned to his criterion
for objective reality in a number of later papers [E28, E29, E30, E31], in which
he repeated the EPR argument on several occasions. These papers add nothing
substantially new. In one of them [E30], he discussed the question of whether the
quantum-mechanical notion of phenomenon should also apply to bodies of every-
day size. The answer is, of course, in the affirmative.
Bohr was, of course, not the only one to express opposition to objective reality;
nor was Einstein the only one critical of the complementarity interpretation.* I
have chosen to confine myself to the exchanges between Einstein and Bohr because
I believe that Einstein's views come out most clearly in juxtaposing them with
Bohr's. Moreover, I am well acquainted with their thoughts on these issues
because of discussions with each of them. Bohr was in Princeton when he put the
*In 1950 Einstein mentioned Schroedinger and von Laue as the only ones who shared his views
[E32]. There were many others who at that time (and later) had doubts about the complementarity
interpretation, but their views and Einstein's did not necessarily coincide or overlap (see [E33]).
Note also that the term hidden variable does not occur in any of Einstein's papers or letters, as far
as I know.