popper-logic-scientific-discovery

appendix *xi 469
measure the momentum spectroscopically (either in a direct manner,
or by using the Doppler effect), and the spectroscope will be rigidly
fixed to the same frame as the first ‘instrument’. (The fact that the
spectroscope absorbs the particle B is irrelevant to the argument which
concerns the fate of A.) Thus an arrangement with a movable frame of
reference cannot be accepted as an essential part of the experiment.
Thirdly, Bohr does not explain here how to measure the momentum
of B with the help of his movable diaphragm. In a later paper of his, a
method of doing this is described; but this method seems to me again
impermissible. 7 For the method described by Bohr consists in measuring
(twice) the position of a ‘diaphragm with a slit . . . suspended by
weak springs from a solid yoke’; 8 and since the measurement of the
momentum with an arrangement of this kind depends on position
measurements, it does not support Bohr’s argument against Einstein,
Podolsky, and Rosen; nor does it succeed otherwise. For in this way we
cannot get the momentum ‘accurately before as well as after the passing’
of B: 9 the first of these measurements of momentum (since it
utilizes a position measurement) will interfere with the momentum of
the diaphragm; it thus will be retrospective only, and will not be of any
use for calculating the momentum of the diaphragm at the time
immediately before the interaction with B.
It does not seem, therefore, that Bohr in his reply adhered to the
principle of making only such idealizations or special assumptions
which favour his opponents (quite apart from the fact that it is far from
clear what he wanted to contest).
(5) This shows that there is a grave danger, in connection with
imaginary experiments of this kind, of carrying the analysis just as far
as it serves one’s purpose, and no further; a danger which can be
avoided only if the above principles are strictly adhered to.
There are three similar cases which I wish to refer to, because I find
them instructive.
(6) In order to meet a critical imaginary experiment of Einstein’s,
7 See Bohr, in Albert Einstein: Philosopher-Scientist, ed. by P. A. Schilpp, 1949; see especially the
diagram on p. 220.
8 Op. cit., p. 219.
9 Bohr, Physical Review 48, 1935, p. 699.