RELATIVITY RUSSELL. 199 



farther off than the naked eye stars, revealed extremely rapid mo- 

 tions. 



If we try now to hang a frame of reference, so to speak, to the 

 average of these nebulae, it begins to look as if our solar system 

 was moving, compared with this, at a speed of something like 400 

 miles per second, which motion of course the system of stars visible 

 to the naked eye must substantially share. 



But now, which of all these systems is really moving? 



Are the stars at rest and the nebulae moving, or are the nebulae 

 at rest and the stars moving, or are they both moving past each 

 other in different directions, and is there anything at rest? Can 

 we really find anything anywhere in the material universe upon 

 which we can really set the feet of our imagination and say " J'y 

 suis, j'y reste" with the conviction that we are at last upon the firm 

 rock of the absolutely motionless? 



It is from a search for an answer to this question that the theory 

 of relativity grew. 



The first great contribution was made by Newton. An immediate 

 consequence of his fundamental principles of physical science is 

 that if we have a number of objects moving together in space, which 

 we may call a system, acting upon one another in any fashion, how- 

 ever complicated, but free from outside influence, then the relative 

 motions of the bodies in that system will not depend at all upon the 

 rate at which the system as a whole is moving through space, or the 

 direction of its motion, but only upon the mutual interaction of 

 its parts. 



Simple uniform motion in a straight line (what we technically 

 call a " motion of translation ") does not influence the things that 

 happen in the system at all, even to the minutest degree. Therefore 

 an observer within the system cannot hope to detect it unless he has 

 something outside to observe. It is on account of this great dynamic 

 principle that we are unconscious of the motion of the earth about 

 the sun. 



In our proposed search, then, for "absolute motion" we must 

 use some other means, and our most efficient tools are likely to be the 

 waves of light. We know that light spreads out from any hot body 

 into space in all directions and at the great speed of 186,000 miles 

 a second. 



TAKING THE ETHER AS A BASIS IN THE SEARCH FOR ABSOLUTE MOTION. 



Despite this enormous velocity, something real actually travels 

 outward, because it carries with it energy, which is, to the modern 

 physicist, one of the most fundamental of all realities. 



