5 7 2 D O C . 3 8 7 T H E N E W F I E L D T H E O R Y illegitimate child of Newtonian physics, though it was cleverly passed off at first as legiti- mate.To become fully conscious of this change in outlook was a task for a highly original mind whose insight could go straight to essentials, a mind that never got stuck in formulas. Faraday was this favoured spirit. His instinct revolted at the idea of forces acting directly at a distance which seemed contrary to every elementary observation. If one electrified body attracts or re- pels a second body, this was for him brought about not by a direct action from the first body to the second, but through an intermediary action. The first body brings the space immediately around it into a certain condition which spreads itself into more distant parts of space, according to a certain spatio-temporal law of propagation. This condition of space was called “the electric field.” The second body experiences a force because it lies in the field of the first, and vice ver- sa. The “field” thus provides a conceptual apparatus which rendered unnecessary the idea of ac- tion at a distance. Faraday also had the bold idea that in appropriate circumstances fields might detach themselves from the bodies producing them and speed away through space as free fields this was his interpretation of light. Maxwell then discovered the wonderful group of formulae which seems simple to us nowadays and which finally built the bridge between the theory of electro-magnetism and the theory of light. It appeared that light consists of rapidly oscillating electro-magnetic fields. THE REVOLUTION IN PHYSICS. After Hertz, in the ‘80s of the last century, had confirmed the existence of the electro-magnetic waves and displayed their identity with light by means of his wonderful experiments, the great intellectual revolution in physics gradually became complete. People slowly accustomed them- selves to the idea that the physical states of space itself were the final physical reality, especially after Lorents had shown in his penetrating theoretical researches that even inside ponderable bodies the electro-magnetic fields are not to be regarded as states of the matter, but essentially as states of the empty space in which the material atoms are to be considered as loosely distrib- uted. At the turn of the century physicists began to be dissatisfied with the dualism of a theory admit- ting two kinds of fundamental physical reality: on the one hand the field and on the other hand the material particles. It is only natural that attempts were made to represent the material parti- cles as structures in the field, that is, as places where the fields were exceptionally concentrated. Any such representation of particles on the basis of the field theory would Quantum theory, where the theory of the continuum (field theory) and the essentially discon- tinuous interpretation of the elementary structures and processes are fighting for supremacy.We shall not here discuss questions concerning molecular theory, but shall describe the improve- ments made in the field theory during this century. These all arise from the theory of relativity, which has in the last six months entered its third stage of development. Let us briefly examine the chief points of view belonging to these three stages and their relation to field theory. The first stage, the special theory of relativity, owes its origin principally to Maxwell’s theory of the electro-magnetic field. From this, combined with the empirical fact that there does not exist any physically distinguishable state of motion which may be called “absolute rest,” arose a new theory of space and time. It is well known that this theory discarded the absolute character of the conception of the simultaneity of two spatially separated events. Well known is also the courage of despair with which some philosophers still defend themselves in a profusion of proud but empty words against this simple theory. On the other hand, the services rendered by the special theory of relativity to its parent, Maxwell’s field equations. But the special theory of relativity showed that this causal correlation corresponds to an essential identity of the two types of field. In fact, the same condition of space, which in one coordinate system appears as a pure magnetic field, appears simultaneously in another coordinate system in relative motion as an electric field, and vice versa. Relationship of this kind displaying an identity between dif- ferent conceptions, which therefore reduce the number of independent hypotheses and concepts of field theory and heighten its logical self-containedness are a characteristic feature