popper-logic-scientific-discovery

light’. And this assertion, we have shown, is simply false: it is refuted by
almost all typical quantum mechanical experiments.
But what then did Jordan wish to assert? Perhaps that there was no
experiment which would bring forth all the wave properties and all the
particle properties of light? Clearly this cannot have been his intention
since even an experiment is impossible which would bring forth, at the
same time, all the wave properties—even if we drop the demand that it
should bring forth any of the particle properties. (And the same holds
the other way round.)
What is so disturbing in this argumentation of Jordan’s is its arbitrariness.
From what has been said it is obvious that there must be
some wave properties and some particle properties which no experiment
can combine. This fact is first generalized by Jordan, and formulated
as a principle (whose formulation by Jordan, at any rate, we have
refuted). And then it is illustrated by an imaginary experiment which
Jordan shows to be impossible. Yet as we have seen, that part of the
experiment which everybody admits to be possible actually refutes the
principle, at least in Jordan’s formulation.
But let us look a little more closely at the other half of the imaginary
experiment—the one introduced by the words ‘on the other hand’. If
we make arrangements to determine the slit through which the particle
has passed, then, it is said, we destroy the fringes. Good. But do we
destroy the wave properties? Take the simplest arrangement: we close
one of the slits. If we do so, there still remain many signs of the wave
character of light. (Even with one single slit we obtain a wave-like
density distribution.) But it is now admitted, by our opponents, that
the particle properties exhibit themselves very fully, since we can now
trace the path of the particle.
(12) From a rational point of view, all these arguments are
inadmissible. I do not doubt that there is an interesting intuitive idea
behind Bohr’s principle of complementarity. But neither he nor any
other member of his school has been able to explain it, even to
those critics who, like Einstein, tried hard for years to understand
it. 19
19 Cf. Albert Einstein: Philosopher-Scientist, ed. by P. A. Schilpp, 1949, p. 674.
appendix *xi 479