FOUNDATIONS OF QUANTUM MECHANICS

82 CHAPTER IV. THE COPENHAGEN INTERPRETATION
(d) Heisenberg (1930) describes the path of an electron in a Wilson chamber as follows. Suppose
that the incoming electron can be described by a wave packet with fairly sharply defined position
and momentum. Upon free development this packet spreads out in the course of time so
that the position becomes less sharp. When the electron ionizes a molecule in the Wilson chamber
a macroscopic droplet is formed, which can be understood as a position measurement. As
a result the wave packet reduces to a packet which is rather sharply located, with a dimension
in the order of a molecule, which again spreads out until a next ionization takes place.
It can be shown that the successive spreading and contraction in position and momentum is,
according to the uncertainty principle, in agreement with the observation of a macroscopic
path. We cannot speak however of the path of an electron in an atom, not even approximately.
An observation of the position of the electron with an accuracy larger than the dimension of the
atom requires such a large recoil that the electron is generally pushed out of the atom entirely.
Therefore, of such an ‘orbit’ no more than one point is observable. Notice that observation plays
a vital role; the path in the Wilson chamber only comes into existence because we observe it.
(e) As a result of Heisenbergs discussion of the uncertainty principle the term measurement disturbance
was introduced in quantum mechanics. Initially the inclination existed to consider this
as a more or less classical physical process; the momentum of the electron is disturbed by the
collision with a photon. This is also indicated by Heisenberg’s use of the word ‘error’ for δq.
From the beginning, Bohr resisted this explanation of Heisenberg, and he put the emphasis on
the necessity to combine mutually excluding terms from a wave and particle picture in one description.
Especially because of EPR it later became clear that the ‘measurement disturbance’
cannot be an ordinary error.
IV. 2
BOHR AND COMPLEMENTARITY
The core of the Copenhagen interpretation lays, of course, in Bohr’s work. His articles are characterized
by an entirely own style. Remarkably, Bohr hardly uses the formalism of the theory, he
generally gives a qualitative argument instead. His difficult, and sometimes obscurely formulated,
long sentences are notorious, full of subordinate clauses and conditional definitions which do not
always clarify his intentions. A careful reconstruction and interpretation of Bohr’s point of view,
and its development in the course of time, has been given by E. Scheibe (1973, chapter 1), another
interpretation is the monograph of H.J. Folse (1985).
Centrally in Bohr’s consideration is the language we use to do physics. Bohr emphasizes that,
regardless of how abstract and refined the terms of modern physics may be, in essence they are only
an extension of everyday language, and they are nothing but means of communication we use to
communicate observational results to other people. Such an observational result, the outcome of a
measurement on a physical system in certain experimental circumstances, is therefore the basic element
of consideration. For this, Bohr uses the term phenomenon. Every phenomenon is the resultant
of a physical system S, a preparation apparatus P , a measuring apparatus M and their mutual interaction
in a concrete experimental situation.
The description of a phenomenon must always be made in unambiguous terms because of the
requirement of communicability. A statement like, for example, “the object is in a superposition