EINSTEIN S RESPONSE TO THE NEW DYNAMICS 455refers to our ordinary perception.' Bohr's deep concern with the role of language
in the appropriate interpretation of quantum mechanics never ceased. In 1948 he
put it as follows:Phrases often found in the physical literature, as 'disturbance of phenomena by
observation' or 'creation of physical attributes of objects by measurements,' rep-
resent a use of words like 'phenomena' and 'observation' as well as 'attribute'
and 'measurement' which is hardly compatible with common usage and prac-
tical definition and, therefore, is apt to cause confusion. As a more appropriate
way of expression, one may strongly advocate limitation of the use of the word
phenomenon to refer exclusively to observations obtained under specified cir-
cumstances, including an account of the whole experiment. [B9]This usage of phenomenon, if not generally accepted, is the one to which nearly
all physicists now subscribe.
In contrast to the view that the concept of phenomenon irrevocably includes the
specifics of the experimental conditions of observation, Einstein held that one
should seek for a deeper-lying theoretical framework which permits the descrip-
tion of phenomena independently of these conditions. That is what he meant by
the term objective reality. After 1933 it was his almost solitary position that quan-
tum mechanics is logically consistent but that it is an incomplete manifestation of
an underlying theory in which an objectively real description is possible.
In an article written in 1935 with Boris Podolsky and Nathan Rosen [E27],
Einstein gave reasons for his position by discussing an example, simple as always.
This paper 'created a stir among physicists and has played a large role in philo-
sophical discussion' [BIO].* It contains the following definition. 'If without in any
way disturbing a system we can predict with certainty (i.e., with a probability
equal to unity) the value of a physical quantity, then there exists an element of
physical reality corresponding to this physical quantity.' The authors then con-
sider the following problem. Two particles with respective momentum and posi-
tion variables (p\,q) and (p 2 ,q 2 ) are in a state with definite total momentum
P=p\ + pi and definite relative distance q= c/, — q 2. This, of course, is possible
since P and q commute. The particles are allowed to interact. Observations are
made on particle 1 long after the interaction has taken place. Measure p\ and one
knows p 2 without having disturbed particle 2. Therefore (in their language), p 2
is an element of reality. Next, measure qt and one knows q 2 again without having
disturbed particle 2. Therefore q 2 is also an element of reality, so that both p 2 and
q 2 are elements of reality. But quantum mechanics tells us that p 2 and q 2 cannot
simultaneously be elements of reality because of the noncommutativity of the
"This stir reached the press. On May 4, 1935, The New York Times carried an article under the
heading 'Einstein attacks quantum theory,' which also includes an interview with another physicist.
Its May 7 issue contains a statement by Einstein in which he deprecated this release, which did not
have his authorization.