ON THE PRINCIPLE OF ACTION BY CONTACT

Abstract

THE idea of contact-action is an essential part of the theories of classical physics, at least when they are formulated in terms of the field concept as required by relativity. But can or need the principle be reconciled with quantum mechanics ? The exhaustive description of quantum phenomena reveals socalled causal anomalies.' The double-slit diffraction experiment performed with electrons is a well-known illustration. If the wavelanguage is chosen to describe the events on the screen, we must say that the extended wave front is suddenly contracted into points, i.e. the scintillations. Or we may describe what happens at the slits in terms of the particle-language. Then we cannot avoid saying that an electron passing through one slit is influenced by the presence of the second slit since the diffraction pattern (i.e. the distribution of scintillations on the screen) is seen to differ whether one or two slits are used. In other words, we seem to require an instantaneous action at a distance if an exhaustive description of quantum phenomena is given, and this constitutes the causal anomaly. This has long been known and it is now understood that these anomalies are of a specious sort. If we reject the exhaustive in favour of a restrictive description no reference to causal anomalies need be made. Such a restrictive description speaks only about what is actually measured in a given situation and does not mention how action is transmitted or whatever is not observed under the circumstances. This kind of description can always be carried out since the uncertainty relations in fact rule out the interpolation to unobserved events, e.g. the happenings at the slit when the screen is observed. And we cannot describe the events in terms of two