Introduction

There is an ongoing series of symposia, at Tokyo, on ‘Foundations of Quantum Mechanics in the Light of New Technology’. Indeed new technology (electronics, computers, lasers, …) has made possible new demonstrations of quantum queerness. And it has made possible practical approximations to old gedankenexperiments. Over the last decade or so have appeared beautiful experiments on ‘particle’ interference and diffraction, with neutrons and electrons, on ‘delayed choice’, on the Ehrenburg–Siday–Aharonov–Bohm effect, and on the Einstein–Podolsky–Rosen–Bohm correlations. These last are of particular relevance for the particular themes of this paper. But those themes arise already in the context of technology which is neither new or advanced, as is illustrated by the following passage:

These notions, of cause and effect on the one hand, and of correlation on the other, and the problem of formulating them sharply in contemporary physical theory, will be the themes of my talk.