popper-logic-scientific-discovery

appendix *xi 467
mechanical work without transferring heat from a higher temperature
to a lower temperature—must agree that they are concessions. Idealizations,
clearly, become impermissible for the purpose of critical
argumentation whenever this rule is violated.
(3) This rule may be applied, for example, to the discussion
initiated by the imaginary experiment of Einstein, Podolsky, and
Rosen. (Their argument is briefly re-stated by Einstein in a letter here
reproduced in appendix *xii; and this discussion is further commented
upon in my Postscript, section *109.) Einstein, Podolsky, and Rosen
attempt, in their critical argument, to make use of idealizations acceptable
to Bohr; and in his reply, Bohr does not challenge the legitimacy of
their idealizations. They introduce (cf. section *109 and appendix *xii)
two particles, A and B, which interact in such a way that by measuring
the position (or momentum) of B, the theory allows us to calculate the
position (or momentum) of A which has meanwhile moved far away
and cannot be any longer disturbed by the measurement of B. Thus A’s
momentum (or position) cannot become blurred—or ‘smeared’, to
use a term of Schrödinger’s—as Heisenberg would have it. 5 Bohr, in his
reply, operates with the idea that measurement of a position can be
achieved only by ‘some instrument rigidly fixed to the support which defines the
space frame of reference’ while measurement of the momentum would be
done by a movable ‘diaphragm’ whose ‘momentum . . . is measured
before as well as after the passing of the particle’. 6 Bohr operates with
the argument that in choosing one of these two systems of reference
‘we . . . cut ourselves off from any . . . possibility’ of using the other, in
connection with the same physical system under investigation. He suggests,
if I understand him properly, that though A is not interfered
5 Heisenberg thought, of course, of the smearing of one particle only, the one which is
being measured. Einstein, Podolsky, and Rosen show that it must be extended to another
particle—one with which the measured particle had interacted at some time, perhaps
years ago. But if so, how can we avoid having everything ‘smeared’—the whole world—
by one single observation? The answer is, presumably, that owing to the ‘reduction of the
wave packet’, the observation does destroy the old picture of the system, and at the same
time creates a new one. Thus the interference is not with the world, but merely with our
way of representing it. This situation is illustrated, as will be seen, by Bohr’s reply which
follows in the text.
6 Bohr, Physical Review 48, 1935, pp. 696–702. The quotations are from pp. 700 and 699.
(The italics are mine.) See also the note on p. 232, above.