RELATIVITY RUSSELL. 211 



This side of the subject, although deeply interesting to the mathe- 

 matician, and also to the philosopher, is not a matter of practical 

 concern, principally for the reason that it does not deal with the 

 facts of nature themselves, but entirely with the mathematical lan- 

 guage which we employ in describing them. 



FUNDAMENTALS OF EINSTEIN'S THEORY SUMMARIZED. 



The fundamental physical facts concerning nature which have 

 developed in connection with the theory of relativity may be briefly 

 and somewhat crudely stated in this fashion : 



1. Our methods of measuring space and time are tied up with our 

 assumption as to whether and in what direction we are moving in a 

 manner which, if we assume our motion to be very rapid, greatly 

 modifies the results of these measurements, but which, for motions 

 that are not more rapid than those of the planets or at most of the 

 stars, produce no difference in these measurements which could be 

 detected except by the most delicate and refined methods of observa- 

 tion, and usually not even a difference great enough to be so detected. 



2. The new conceptions are, therefore, of very little or no import- 

 ance to the practical man, but are of very great interest to the phi- 

 losopher, since they indicate that the old traditional conceptions of 

 space and time are not the only conceptions of this sort which the 

 human mind is capable of forming, and, what is more, that when 

 the comparison is made very precise these newer and apparently 

 bizarre conceptions of space and time fit the facts of nature more 

 closely than the simple common sense ones. 



3. It has more recently been shown that the previous assumption 

 that gravitation and the motion of material bodies on the one hand, 

 and electricity, magnetism, and light on the other, formed two sepa- 

 rate sides of nature, not connected with one another, is incorrect. 

 These two great complexes of natural phenomena and forces are 

 actually parts of one still greater whole, although the connection be- 

 tween them is of such a character that it produces measurable results 

 in only a very few cases. 



The theory of relativity does not supersede the older scientific con- 

 ceptions or destroy them, but leaves them as very close and very use- 

 ful approximations to the facts of nature. As is usually the case 

 with great scientific advances, it leaves us with a view of nature 

 which is more complex and harder to understand and to work with 

 than our previous conceptions, but which at the same time reduces 

 what previously appeared to be disconnected things to manifesta- 

 tions of a single underlying unity of principle. 



