An Address delivered on May 5th, 1920, in the University of Leyden
An expanded form of an Address to the Prussian Academy of Sciencesin Berlin on January 27th, 1921.
How does it come about that alongside of the idea of ponderablematter, which is derived by abstraction from everyday life, thephysicists set the idea of the existence of another kind of matter,the ether? The explanation is probably to be sought in those phenomenawhich have given rise to the theory of action at a distance, andin the properties of light which have led to the undulatory theory.Let us devote a little while to the consideration of these twosubjects.
Outside of physics we know nothing of action at a distance. Whenwe try to connect cause and effect in the experiences which naturalobjects afford us, it seems at first as if there were no other mutualactions than those of immediate contact, e.g. the communication ofmotion by impact, push and pull, heating or inducing combustion bymeans of a flame, etc. It is true that even in everyday experienceweight, which is in a sense action at a distance, plays a veryimportant part. But since in daily experience the weight of bodiesmeets us as something constant, something not linked to any causewhich is variable in time or place, we do not in everyday lifespeculate as to the cause of gravity, and therefore do not becomeconscious of its character as action at a distance. It was Newton’stheory of gravitation that first assigned a cause for gravity byinterpreting it as action at a distance, proceeding from masses.Newton’s theory is probably the greatest stride ever made inthe effort towards the causal nexus of natural phenomena. And yetthis theory evoked a lively sense of discomfort among Newton’scontemporaries, because it seemed to be in conflict with theprinciple springing from the rest of experience, that there can bereciprocal action only through contact, and not through immediateaction at a distance. It is only with reluctance that man’s desirefor knowledge endures a dualism of this kind. How was unity tobe preserved in his comprehension of the forces of nature? Eitherby trying to look upon contact forces as being themselves distantforces which admittedly are observable only at a very smalldistance—and this was the road which Newton’s followers, who wereentirely under the spell of his doctrine, mostly preferred totake; or by assuming that the Newtonian action at a distance isonly apparently immediate action at a distance, but in truth isconveyed by a medium permeating space, whether by movements or byelastic deformation of this medium. Thus the endeavour toward aunified view of the nature of forces leads to the hypothesis of anether. This hypothesis, to be sure, did not at first bring with itany advance in the theory of gravitation or in physics generally,so that it became customary to treat Newton’s law of force as anaxiom not further reducible. But the ether hypothesis was boundalways to play some part in physical science, even if at first onlya latent part.
When in the first half of the nineteenth century the far-reachingsimilarity was revealed which subsists between the properties oflight and those of elastic waves in ponderable bodies, the etherhypothesis found fresh support. It appeared beyond question thatlight must be interpreted as a vibratory process in an elastic, inertmedium filling up universal space. It also seemed to be a necessaryconsequence of the fact that light is capable of polarisation thatthis medium, the ether, must b