for bodies in relative motion, or we must define it in such a way as will free it of this ambiguity, and this is exactly what the relativity mechanics attempts to do.
Any discussion of the theory of relativity would be hardly satisfactory without a brief survey of the history of the development of the subject. As has been stated, for many years the ether theory of light has found general acceptance, and up to about twenty-five years ago practically all of the known phenomena of light, electricity and magnetism were explained on the basis of this theory. This hypothetical ether was stationary, surrounded and permeated all objects, did not, however, offer any resistance to the motion of ponderable matter. There came then, in 1887, into this fairly satisfactory state of affairs, the famous Michelson and Morley experiment. This experiment was directly undertaken to discover, if possible, the so-called ether drift.
In this experiment, the apparatus was the most perfect that the skill of man could devise, and the operator was perhaps one of the most skilful observers in the world, but in spite of all this no result was obtained. Physicists were then driven to seek some theory which would explain this experiment, but with varying success. It was proposed that the ether was carried along with the earth, but a host of experiments show this untenable. It was suggested that the velocity of light depends on the velocity of the source of light, but here again there were too many experiments to the contrary. Michelson himself offered no theory, though he suggested that the negative result could be accounted for by supposing that the apparatus underwent a shortening in the direction of the velocity and due to the velocity, just enough to compensate for the difference in path. This idea was later, in 1892, developed by Lorentz, a Dutch physicist, and under the name of the Lorentz-shortening hypothesis has had a dignified following. The Michelson and Morley experiment, together with certain others undertaken for the same purpose, remained for a number of years as an unexplained fact—a contradiction to ascertained well-established and orderly physical theory. Then there appeared in 1905, in the Annalen der Physik, a modest article by A. Einstein, of Bern, Switzerland, entitled, "Concerning the Electrodynamics of Moving Bodies," In this article Einstein, in a very unassuming way, and yet in all confidence, boldly attacked the problem and showed that the astonishing results concerning space and time which we have just considered, all follow very naturally from very simple assumptions. Naturally a large part of his paper was devoted to the mathematical side—to the deduction of the equations of transformation which express mathematically the relation between two systems moving relative to each other. It may safely be said that this article laid the foundation of the relativity theory.
Einstein's article created no great stir at the time, but within a
