Note Note 17 — § 4 THE DIFFICULTIES IN THE PRINCIPLES OF CLASSICAL MECHANICS

(p. 40). The new points of view as to the nature of inertia are based upon the study of the electromagnetic phenomena of radiation. The special theory of relativity, by stating the theorem of the inertia of energy, organically grafted these views on to the existing structure of theoretical physics. The dynamics of cavity-radiation, i.e. the dynamics of a space enclosed by walls without mass, and filled with electromagnetic radiation, taught us that a system of this kind opposes a resistance to every change of its motion, just like a heavy body in motion. The study of electrons (free electric charges) in a state of free motion, e.g. in a cathode-tube, taught us likewise that these exceedingly small particles behave like inert bodies; that their inertia is not, however, conditioned by the matter to which they might happen to be attached, but rather by the electromagnetic effects of the field to which the moving electron is subject. This gave rise to the conception of the apparent (electromagnetic) mass of an electron. The special theory of relativity finally led to the conclusion that to all energy must be accorded the property of inertia.