RECENT EVENTS IN RELATIVITY — ROTHMAN 



389 



the form of photons. Each photon has a certain energy which is 

 proportional to the frequency of the radiation. Associated with this 

 energy is a definite mass. When the photon rises from the surface 

 of the earth, it must do work against the gravitational field. It is 

 "pulled back" by the force of gravity. Therefore, the photon loses 

 kinetic energy. Since its velocity must remain a constant, this loss 

 of energy is observed as a decrease in the quantum energy — in other 

 words, a decrease in the frequency. 



Conversely, a photon falling toward the earth must acquire an 

 increased frequency. 



An alternative way of describing the same situation is offered by 

 the Principle of Equivalence. We imagine the source of radiation 

 and the observer to be located in a spaceship undergoing 1 g. of ac- 

 celeration (fig. 2). The source and observer are always the same 



o 



A ^ 



S 



Figure 2. — Light leaving the source A heads for the observer B. By the time the light 

 reaches the observer, the latter is at D, and is traveling faster than it was when the 

 light left the source. 



