(February 3, 1929)
presented to the general public, in terms that the average person could understand (or so he thought).
The History of Field Theory
("Olds and News of Field Theory")
By Albert Einstein
While physics wandered exclusively in the paths prepared by Newton, the following conception of physical reality prevailed: Matter is real, and matter undergoes only those changes which we conceive as movements in space. Motion, space and also time are real forms. Every attempt to deny the physical reality of space collapses in face of the law of inertia. For if acceleration is to be taken as real, then that space must also be real within which bodies are conceived as accelerated.
Newton saw this with perfect clarity and consequently he called space "absolute". In his theoretical system, there was a third constituent of independent reality; the motive force acting between material particles, such forces being considered to depend only on the position of the particles. These forces between particles were regarded as unconditionally associated with the particles themselves and as distributed spatially according to an unchanging law.
The physicists of the nineteenth century considered that there existed two kinds of such matter, namely, ponderable matter and electricity. The particles of ponderable matter were supposed to act on each other by gravitational forces under Newton's law, the particles of electrical matter by Coulomb forces also inversely proportional to the square of the distance. No definite views prevailed regarding the nature of the forces acting between ponderable and electrical particles.
The Old Theory of Space
Mere empty space was not admitted as a carrier for physical changes and processes. It was only, one might say, the stage on which the drama of material happenings was played. Consequently Newton dealt with the fact that light is propagated in empty space by making the hypothesis that light also consists of material particles interacting with ponderable matter through special forces. To this extend Newton's view of nature involved a third type of material particle, though this certainly had to have very different properties from the particles of the other forms of matter. Light particles had, in fact, to be capable of being formed and of disappearing. Moreover, even in the eighteenth century it was already clear from experience that light traveled in empty space with a definite velocity, a fact which obviously fitted badly into Newton's theoretical system, for why on earth should the light particles not be able to move through space with any arbitrary velocity?
It need not, therefore, surprise us that this theoretical system, built up by Newton with his powerful and logical intellect, should have been overthrown precisely by a theory of light. This was brought about by the Huygens-Young-Fresnel wave theory of light which the facts of interference and diffraction forced on stubbornly resisting physicists. The great range of phenomena, which could be calculated and predicted to the finest detail by using this theory, delighted physicists and filled many fat and learned books. No wonder then that the learned men failed to notice the crack which this theory made in the statue of their eternal goddess. For, in fact, this theory upset the view that everything real can be conceived as the motion of particles in space. Light waves, were, after all, nothing more than undulatory states of empty space, and space thus gave up its passive role as a mere stage for physical events. The other hypothesis patched up the crack and made it invisible.
The ether was invented, penetrating everything, filling the whole of space, and was admitted as a new kind of matter. Thus it was overlooked that by this procedure space itself had been brought to life. It is clear that this had really happened, since the ether was considered to be a sort of matter which could nowhere be removed. It was thus to some degree identical with space itself; that is, something necessarily given with space. Light was thus viewed as a dynamical process undergone, as it were by space itself. In this way the field theory was born as an illegitimate child of Newtonian physics, though it was cleverly passed off a first as legitimate.
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 favored 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 repels a second body, this was for him brought about not by a direct action from the first body on 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 versa. The "field" thus provided a conceptual apparatus which rendered unnecessary the idea of action at a distance. Faraday also had the bold idea that under 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 so simple to us nowadays and which finally build the bridge between the theory of electro-magnetism and the theory of light. It appered that light consists of rapidly oscillating electro magnetic fields.
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 themselves to the idea that the physical states of space itself were the final physical reality, especially after Lorentz 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 distributed.
Join Us
