244 SCIENCE AND THE HUMAN MIND 



velocity of light in free space appears the same to all 

 observers, regardless of the motion of the observer 

 or of the source of light. 



These two statements taken together form what 

 is called the Principle of Relativity. They are in 

 accordance with all facts known at present, and, 

 although they lead to unexpected and startling results, 

 none of those results have been yet shown to be 

 inconsistent with each other or with the results of 

 observation and experiment. 



If we imagine two bodies to be moving past each 

 other in free space on parallel paths, as two railway 

 trains may pass each other on opposite lines, a simple 

 geometrical construction will convince us that, which- 

 ever body we assume to be at rest and whichever in 

 motion, on the principle of relativity, the unit of time 

 on the moving body is to the unit on the stationary 

 body as unity is to Ji v 2 /c 2 , where v is the velocity 

 of the body, and c the constant velocity of light. 

 Thus, to an observer on either body, the time scale of 

 the other body seems different from his own. Similarly, 

 there is a corresponding change in the unit of length, 

 a change equal to that suggested by Lorentz and 

 Fitzgerald. 



When we pass to dynamics, we find once more that 

 the principle of relativity requires that the mass of a 

 moving body should vary, and, in terms of the mass of 

 a stationary body, should be i/ ^/i v 2 jc 2 . If the body 

 moved with the velocity of light, v would be equal 

 to c, and the mass would become infinite. Here 

 again, the principle leads to the same result as the 

 ordinary electromagnetic theory, and is in equal 

 accord with the results of experiment. 



