Locality in Physics

When Michael Faraday challenged the Newtonian idea of “action at a distance” as a way of explaining electric and magnetic forces, he laid out the framework for today’s picture of the electromagnetic interaction as a field, propagating between the charges and currents which are the sources of that field, and acting on other charges and currents at various distances.

 

Faraday was not able to develop his idea into a set of equations describing the propagation of his fields; that fell to others of his contemporaries, above all James Clerk Maxwell. But his challenge to the Newtonian idea was extended by him to suggest that gravity should be propagated from place to place in a similar manner to electromagnetism.

 

At the end of the nineteenth century Faraday’s and Maxwell’s ideas had to be modified to accommodate the newly discovered electron, an apparently point charge of electricity having a mass, and therefore acted upon by a force resembling Newton’s gravitation, but with the important difference that the force depends on the velocity, as well as the position of the electron. This discovery meant that the field equations involved the state of the charged particles as well as the fields. The perfection of this Classical Electrodynamics was down to Lorentz, Poincaré, and ultimately the famous 1905 article of Einstein. Many changes to electrodynamics have occurred since then, principally because of the intervention of the quantum theory. Even within the classical regime, there have been important interventions, of which that of Wheeler and Feynman in 1945 stands out; they had a formulation in which the field variables were eliminated, so that everything was described in terms of the positions and velocities of the charged particles.

 

Einstein strongly criticized the changes brought about by quantum theory, because they seemed to imply that particles could disappear at one place and then reappear at a new place without passing through any intervening places. The classical theory of Wheeler and Feynman suffered from a related eccentricity; although the evolution of an electric system was now continuous, the motion of a particle depended on that of others at not only past moments, but also future moments of time! Their physics involved abandoning a basic feature of causality.

 

But by 1925 Einstein’s efforts in creative physics had transferred from electrodynamics to gravity, the giant step being taken in the theory of 1915, to which he gave the name General Relativity (GR) to distinguish it from the Special Relativity of 1905. Here he did not follow the ideas of Faraday, but instead introduced the formulation that “gravity is geometry”. His idea from then on was to absorb electrodynamics into the same system, giving a Unified Field Theory. On the face of it, one might be inclined to classify such a programme as the obverse of Wheeler-Feynman, which could be regarded as the first step towards a Unified Particle Theory. For myself I prefer to think of these two unification programmes as two aspects of the same programme. The logic of “gravity is geometry” is that the field of gravity is abandoned, and so Einstein’s unification, in order to succeed, would have to abolish the electromagnetic field.

I hope I have said enough here to indicate my view that Faraday’s idea that interactions between separated bodies are mediated by physical fields of force is as much opposed to a large part of Einsteinian, as to Newtonian physics. Strangely, in spite of remaining constant in his insistence that gravity is geometry, Einstein never acknowledged that his ideas ran counter to those of Faraday; indeed he continued to see himself as a disciple of Faraday. He elevated the field view of nature to the status of a philosophical statement in what he called the Principle of Local Action (PLA) in 1948, and he made this principle the central core of his criticism of Quantum Mechanics (QM). Although he never acknowledged that his own theory of gravity, that is GR, might suffer from the same lack of locality as QM, he showed some sign of doing so in particular areas of gravity theory, notably in his opposition to black holes and in his many changes of opinion about whether gravity waves exist. Though I do not presume to embed my own ideas about fields and locality in an overall philosophical world view, I have expressed some opinions of a philosophical and historical nature elsewhere on this page and also..