An objection which has often been raised by non-mathematical students is the following: They argue that since energy moving with the velocity of light must possess an infinite mass, we should be crushed under the tremendous impact of light rays falling on our bodies. In an elementary book of this sort we cannot go into mathematical details, but we may say that the argument is fallacious. It is only ponderable matter constituted by bound energy which would become infinite in mass when moving with the velocity of light. Light rays represent another type of energy, known as free or loose energy, and calculation shows that the momentum produced by this form of energy moving with the velocity of light is not infinite, but exceedingly minute. Of course the theory of relativity is not one of pure mathematics. Einstein was not solely interested in juggling with equations and laws to make them fit into his scheme. The final verdict of the correctness of a theory of mathematical physics must always rest with the experimenter, and it remained to be seen whether the Einsteinian equations of mechanics corresponded more accurately to facts than did the classical ones. We may state that the most precise experiments have proved the correctness of the Einsteinian laws of mechanics and that Bucherer’s experiment proving the increase in mass of an electron in rapid motion is a case in point.[51] [Pg 160] It should be clear by now that very important differences distinguish the theory of Einstein from that of Lorentz. Lorentz also had deduced from his theory that the mass of an electron should increase and grow infinite when its speed neared that of light; but the speed in question was the speed of the electron through the stagnant ether; whereas in Einstein’s theory it is merely the speed with respect to the observer. According to Lorentz, the increase in mass of the moving electron was due to its deformation or FitzGerald contraction. The contraction modified the lay of the electromagnetic field round the electron; and it was from this modification that the increase in mass observed by Bucherer was assumed to arise. In Einstein’s theory, however, the increase in mass is absolutely general and need not be ascribed to the electromagnetic field of the electron in motion. An ordinary unelectrified lump of matter like a grain of sand would have increased in mass in exactly the same proportion; and no knowledge of the microscopic constitution of matter is necessary in order to predict these effects, which result directly from the space and time transformations themselves.