NEWTON’S LIGHT RAY
If someone asks about the image of a light ray to Newton, it is most likely to receive an answer like this: a light ray is a straight line, being the trajectory of a particle of light. Everybody knows that the image that Newton had of the light is that of a stream of particles moving with a constant velocity, and it seems only natural to infer that an ideal light ray in his opinion is only the trajectory of a single light particle. This image is in direct contrast with that given one century after Newton by Fresnel, in which the light ray is just a geometrical line, the locus of the centers of ellipsis of polarization or, in older terms, the locus of origins of the light vector. Both these images, although in sharp contrast with each other, have in common the idea that along the light ray acts no force towards the source of light: from a classical dynamical point of view the light propagation is a free motion. In order to explain the light one relies heavily upon mathematics of the continuum mechanics or electromagnetism.
Therefore both these, by now classical, images of light are in contradiction with the Mach’s principle. According to this the classical free motion is an expression of the overall interaction of a material body with the whole matter in the Universe, rather than being an expression of no interaction, as the first principle of classical dynamics seems to claim. Indeed there is no way to isolate a body in order to check how it moves in those conditions: the first principle of classical dynamics is not a falsifiable proposition. This contradiction will remain as such as long as we cannot judge the uniform motion in terms of forces, exactly as we judge, say the Kepler motion. If the light, for instance, can be judged this way, we have a model, if not of free motion, of a motion with constant speed anyway.
It is a pleasant surprise to see that Newton’s theory of the forces “invented” by him fits this bill, pointing towards an image of a light ray in full accordance with the electromagnetic theory of light as in the attached figure. At any moment the electric vector is acted on by central forces directed towards the source of light – they can be repelling as well as attractive forces – while rotated by the magnetic vector. This image of a light ray and, implicitly, of the constant speed motion is based upon dismissal of two common misconceptions. The first one of these is that the central forces have magnitude depending exclusively on the distance, and the second one is that the magnetic vector is a… vector, while in this instance is a skew-symmetric tensor.