332 COMSTOCK— ELECTRICITY AND MATTER. [April 22, 



us will admit that the concept of absolute motion need not be used 

 Z'ery often, at least, in the science of the future; if the foregoing- 

 views are true ones. The modern group of conceptions known as 

 the " Principle of Relativity " teaches that the idea of absolute 

 motion is entirely superfluous, and that the time honored concepts of 

 space and time, as independent of all motions, do not accurately fit 

 the real universe and should be modified. 



^Modified in what way? Real time is measured by real clocks 

 and real distance is measured by real rigid bodies, and we find an 

 unexpected discord between moving clocks and stationary clocks, 

 and between moving rods and stationary ones. We have, therefore, 

 no possible use for what might be called " universal time." We 

 might form a vague concept of a " cosmic second " pervading the 

 universe, but we could do nothing with it, and it would therefore 

 be entirely artificial. Whenever we wished to think about an actual 

 moving object and wished to measure some vibration frequency on 

 it, let us say, we would have to use some actual clock-beat or other 

 periodic phenomenon as a unit. So that the actual universe has us 

 hopelessly in its grasp, and our concepts of space and time to be 

 valuable must be in harmony with the habits of real things. 



The principle of relativity, therefore, makes changes in the 

 fundamental concepts of space and time for moving systems ; the 

 second in a moving system is longer, the meter, in the direction of 

 motion, shorter, than in stationary systems. The units then are in 

 harmony with real happenings in such systems, and this makes it 

 possible to introduce the last great synthesis of modern theory, the 

 deeper unity of physical law under the dominance of what we have 

 known as electromagnetic principles ; and this brings us one step 

 nearer to the last, ultimate generalization which is the unattainable 

 ideal of science. 



Mas.sachusetts Institute of Technology, 

 April 20, 1911. 



