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Causality in Quantum Electrodynamics

Published online by Cambridge University Press:  01 July 2024

Henry Margenau
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Quantum mechanics, even in its early and simple phases, has often been regarded as a non-causal discipline. The argument supporting this view cites the uncertainty principle as prohibiting the ascertainment of complete knowledge concerning physical states upon which causal prediction could be based. Recent developments in atomic physics have added new and puzzling features to the problem of causality insofar as they operate, not only with intrinsically unmeasurable states, but also with time reversals which have been interpreted to mean that the effect can be prior to the cause. Feynman's theory of quantum electrodynamics is particularly rich in unorthodox suggestions which tantalise philosophers. The purpose of the present paper is to exhibit them, appraise their methodological function and see in what manner they violate the rules of causal description. This purpose, it seems, is best achieved by a sequential discussion of three questions: What does causality mean in physics? What is the new method of quantum electrodynamics? Is this new method compatible with the causal doctrine in some satisfactory form?

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Type
Research Article
Information
Diogenes , Volume 2 , Issue 6 , March 1954 , pp. 74 - 84
Copyright
Copyright © 1954 Fédération Internationale des Sociétés de Philosophie / International Federation of Philosophical Societies (FISP)

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1 Our survey of the meanings of causality is not quite complete. One deficiency lies in its failure to analyse further the laws which connect the states. They must in some sense be invariable, or time-free. This point has been discussed in my book, The Nature of Physical Reality (McGraw-Hill, 1950), where further reflections concerning the suitableness of probabilities to function as state variables will be found.

Also omitted has been a version of causality which, though extremely limited, has found its way into the technical literature under the label' causality conditions' (see, e.g., VanKampen, Phys. Rev., 89, 1072, 1953). It is nothing more than the requirement of relativity limiting the speed of a wave packet to the speed of light and says, in effect, that a cause at one point at time t cannot produce an effect at another point, a distance r from the first, at a time earlier than t+r/c.

2 Here and everywhere else in this paper, energies are understood to be frequencies, i.e., every energy is divided by Planck's constant.

3 The leisure for reflections that have led to this and other publications was afforded by the tenure of the Hill Foundation Visiting Professorship at Carleton College, for the award of which I am grateful.