popper-logic-scientific-discovery

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new appendices
moving freely between perfectly reflecting and smooth walls.’ 13 (We need
not go into the finer peculiarities of the arrangement.) ‘Now, in the
causal interpretation of the quantum theory’—that is, in Bohm’s
interpretation—‘. . . the particle is at rest’, Bohm writes; and he goes on
to say that, if we wish to observe the particle, we shall ‘trigger’ a process
which will make the particle move. 14 But this argument about observation,
whatever its merits, is no longer interesting. What is interesting is
that Bohm’s interpretation paralyses the freely moving particle: his
argument amounts to the assertion that it cannot move between these
two walls, as long as it is unobserved. For the assumption that it so moves
leads Bohm to the conclusion that it is at rest, until triggered off by an
observation. This paralysing effect is noted by Bohm, but simply not
discussed. Instead, he proceeds to the assertion that though the particle
does not move, our observations will show it to us moving (but this was
not the point at issue); and further, to the construction of an entirely
new imaginary experiment describing how our observation—the
radar signal or photon used to observe the velocity of the particle—
could trigger off the desired movement. But first, this again was not the
problem. And secondly, Bohm fails to explain how the triggering photon
could reveal to us the particle in its full, proper speed, rather than
in a state of acceleration towards its proper speed. For this seems to
demand that the particle (which may be as fast and as heavy as we
like) acquires and reveals its full speed during the exceedingly short
time of its interaction with the triggering photon. All these are ad hoc
assumptions which few of his opponents will accept.
But we may elaborate Einstein’s imaginary experiment by operating
with two particles (or billiard balls) of which the one moves to and fro
between the left wall and the centre of the box while the other moves
between the right wall and the centre; in the centre, the particles collide
elastically with one another. This example leads again to standing
waves, and thus to the disappearance of the velocity; and the Pauli-
Einstein criticism of the theory remains unchanged. But Bohm’s triggering
effect now becomes even more precarious. For let us assume we
observe the left particle by shooting at it a triggering photon from the
13 D. Bohm, in the same volume, p. 13; the italics are mine.
14 Op. cit., p. 14; see also the second footnote on that page.