202 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1921. 



The light waves came back over the two paths in exactly the same 

 interval of time. 



They tried it again and again at different times of the year when 

 the earth was moving in different directions around the sun, so that 

 even though the earth might have been at rest in space on some one 

 of these days it certainly was not at rest on all of them. But they 

 always met the same negative result. 



II. EINSTEIN'S ASSUMPTION THAT ONLY RELATIVE MOTION IS 



POSSIBLE OF STUDY. 



Other optical experiments of a more intricate nature and even 

 greater delicacy were attempted with the same object of detecting the 

 motion of the earth through the ether and they all failed. 



After it became clear that the trouble was not in the apparatus or 

 the experiment, it was evidently necessary to account for the absence 

 of the predicted effect. 



After various minor hypotheses had been tried, Einstein started in 

 with the bold assumption that these experiments had unveiled a new 

 law of nature, viz, that the universe was so constructed that it was 

 not possible by any physical experiment, optical or otherwise, to 

 detect the existence of absolute, uniform, straight-ahead motion, or 

 indeed to determine whether the observer's frame of reference was at 

 rest or in such uniform translational motion. 



If this is true, it follows that it is only the relative motions 

 of material bodies in the universe which we can study at all. 



Hence the name of the " Principle of relativity." 



A second principle, following naturally from the experiments which 

 led to the first, is that the velocity of light in empty space will always 

 come out the same, whether measured by an observer moving, with 

 his apparatus, in one direction at one rate or by one similarly moving 

 in another direction and at a different rate. 



NOVEL CONSEQUENCES OF EINSTEIN'S HYPOTHESIS. 



This principle sounds harmless enough, but the consequences which 

 follow from it are so different from our old preconceived opinions 

 that they often appear to us grotesque to a degree. 



Take one of the simplest ones. Let us go back to the observer with 

 a ring of mirrors surrounding him, from all of which the reflections 

 of his flash of light reach him at the same instant. If he thinks that 

 he is at rest in space he will say that these mirrors are distributed 

 around a perfect circle with his own position as center. 



Now suppose he chooses a different frame of reference, in uniform 

 motion compared with his original one. That is, suppost that he 

 thinks that he and the mirrors together are moving uniformly in 

 some particular direction and at a high velocity. 



