206 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



without saying where it happened and also with respect to what 

 frame of reference you define both when and where. 



All these spectacular changes, however, reach perceptible amounts 

 only for objects which are moving with at least a moderate fraction 

 of the velocity of light ; and the actual motion of the planets is so 

 much slower than this that no perceptible differences will be intro- 

 duced by our choosing frames of reference which are attached to the 

 earth, the sun, the planets, or the stars. 



III. RECENT ASTRONOMICAL EXPERIMENTS CONFIRM EINSTEIN'S 



HYPOTHESIS. 



Not content with these remarkable results, Einstein proceeded a 

 few years ago to generalize his theory further, in imagining another 

 tvpe of question which did not come within even the wide view of the 

 older relativity theory. 



To make this idea clear let us imagine two observers, each with his 

 measuring instruments, means of subsistence, et cetera, in a large and 

 perfectly impervious box, which forms his "closed system." 



The first observer, with his box and its contents, alone in space, 

 very remote from all gravitating bodies and entirely at rest. 



The second observer, with his box and its contents, is, it may be 

 imagined, near the earth or the sun or some star and falling freely 

 under the influence of its gravitation. 



To be more precise, we imagine him in what is called a " uniform 

 gravitational field," where the gravitational force is exerted on all 

 objects in exactly the same direction and is not converging toward 

 the center of the attracting body, where it is always of exactly the 

 same amount, and there is nothing to interfere with an indefinitely 



long fall. 



This second box and its contents, including the observer, will then 

 fall under the gravitational force, that is, get up an ever increasing 

 speed, but at exactly the same rate, so that there will be no tendency 

 for their relative positions to be altered. 



According to Newton's principles, this will make not the slightest 

 difference in motions of the physical objects comprising the system or 

 their attractions on one another, so that no dynamical experiment can 

 distinguish between the condition of the freely falling observer in 

 the second box and the observer at rest in the first. 



But once more the question arises, what could be done by an 

 optical experiment? 



According to the beliefs which have been held from the time of 

 Maxwell, who first developed the electro-magnetic theory of light, 

 until the present, it has generally been believed that gravitation, 

 however powerful, has no effect whatever upon light, and that light 

 would therefore travel in a straight line through a field of gravita- 

 tional attraction exactly as it would through empty space. 



