popper-logic-scientific-discovery

seen) by Heisenberg’s imaginary experiment which is the intuitive
source of it all.
There seems to me a fairly obvious connection between Bohr’s
‘principle of complementarity’ and this metaphysical view of an
unknowable reality—a view that suggests the ‘renunciation’ (to use a
favourite term of Bohr’s) of our aspirations to knowledge, and the
restriction of our physical studies to appearances and their interrelations.
But I will not discuss this obvious connection here. Instead, I will
confine myself to the discussion of certain arguments in favour of
complementarity which have been based upon further imaginary
experiments.
(11) In connection with this ‘principle of complementarity’ (discussed
more fully in my Postscript; cf. also my paper ‘Three Views Concerning
Human Knowledge’, now in my Conjectures and Refutations, 1963,
chapter 3) Bohr has analysed a large number of subtle imaginary
experiments in a similarly apologetic vein. Since Bohr’s formulations
of the principle of complementarity are vague and difficult to discuss, I
shall have recourse to a well known and in many respects excellent
book, Anschauliche Quantentheorie, by P. Jordan (and a book in which,
incidentally, my Logik der Forschung was briefly discussed). 15
Jordan gives a formulation of (part of) the contents of the principle
of complementarity that brings it into the closest relation to the problem
of the dualism between particles and waves. He puts it as follows. ‘Any one
experiment which would bring forth, at the same time, both the wave
properties and the particle properties of light would not only contradict
the classical theories (we have got used to contradictions of this
kind), but would, over and above this, be absurd in a logical and
mathematical sense.’ 16
Jordan illustrates this principle by the famous two-slit experiment.
(See my old appendix v.) ‘Assume that there is a source of light from
which monochromatic light falls upon a black screen with two [parallel]
slits which are close to each other. Now assume, on the one hand, that
the slits and their distance are sufficiently small (as compared with
the wave length of the light) to obtain interference fringes on a
15 Jordan, Anschauliche Quantentheorie, 1936, p. 282.
16 Op. cit., p. 115.
appendix *xi 477