PRACTICAL APPLICATIONS 59 



tending to bring it gradually to rest. The effect might 

 be of great importance in the study of stellar motions; 

 it would mean that on the average old stars must have 

 lower speeds than young stars — a conclusion which, as 

 it happens, is contrary to observation. 



But according to the theory of relativity "coming to 

 rest" has no meaning. A decrease of velocity relative 

 to one frame is an increase relative to another frame. 

 There is no absolute velocity and no absolute rest for 

 the star to come to. The suggestion may therefore be 

 at once dismissed as fallacious. 



2. The B particles shot out by radioactive substances 

 are electrons travelling at speeds not much below 

 the speed of light. Experiment shows that the mass 

 of one of these high-speed electrons is considerably 

 greater than the mass of an electron at rest. The theory 

 of relativity predicts this increase and provides the 

 formula for the dependence of mass on velocity. The 

 increase arises solely from the fact that mass is a relative 

 quantity depending by definition on the relative quan- 

 tities length and time. 



Let us look at a 3 particle from its own point of view. 

 It is an ordinary electron in no wise different from any 

 other. But it is travelling with unusually high speed? 

 "No", says the electron, "That is your point of view. 

 I contemplate with amazement your extraordinary 

 speed of 100,000 miles a second with which you are 

 shooting past me. I wonder what it feels like to move 

 so quickly. However, it is no business of mine." So 

 the p particle, smugly thinking itself at rest, pays no 

 attention to our goings on, and arranges itself with the 

 usual mass, radius and charge. It has just the standard 

 mass of an electron, 9.10" 28 grams. But mass and 

 radius are relative quantities, and in this case the frame 



