TEE THEORY OF RELATIVITY 443 



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 satisfac- 

 tory without a brief survey of the history of the development of the sub- 

 ject. As has been stated, for many years the ether theory of light has 

 found general acceptance, and up to about twenty-five years ago prac- 

 tically 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 under- 

 taken 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 hypoth- 

 esis has had a dignified following. The Michelson and Morley experi- 

 ment, together with certain others undertaken for the same purpose, 

 remained for a number of years as an unexplained fact — a contradic- 

 tion to ascertained well-established and orderly physical theory. Then 

 there appeared in 1905, in the Annalen der Physih, a modest article 

 by A. Einstein, of Bern, Switzerland, entitled, " Concerning the Electro- 

 dynamics of Moving Bodies/' In this article Einstein, in a very unas- 

 suming 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 as- 

 sumptions. 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 mov- 

 ing 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 



