i 9 6 THE QUANTUM THEORY 



This is the famous Correspondence Principle enun- 

 ciated by Bohr. It was at first a conjecture based on 

 rather slight hints; but as our knowledge of quantum 

 laws has grown, it has been found that when we apply 

 them to states of very high quantum number they con- 

 verge to the classical laws, and predict just what the 

 classical laws would predict. 



For an example, take a hydrogen atom with its elec- 

 tron in a circular orbit of very high quantum number, 

 that is to say far away from the proton. On Monday, 

 Wednesday and Friday it is governed by classical laws. 

 These say that it must emit a feeble radiation continu- 

 ously, of strength determined by the acceleration it is 

 undergoing and of period agreeing with its own period 

 of revolution. Owing to the gradual loss of energy it 

 will spiral down towards the proton. On Tuesday, 

 Thursday and Saturday it is governed by quantum laws 

 and jumps from one orbit to another. There is a 

 quantum law that I have not mentioned which prescribes 

 that (for circular orbits only) the jump must always be 

 to the circular orbit next lower, so that the electron 

 comes steadily down the series of steps without skipping 

 any. Another law prescribes the average time between 

 each jump and therefore the average time between the 

 successive emissions of light. The small lumps of 

 energy cast away at each step form light-waves of period 

 determined by the h rule. 



"Preposterous! You cannot seriously mean that the 

 electron does different things on different days of the 

 week!" 



But did I say that it does different things? I used 

 different words to describe its doings. I run down the 

 stairs on Tuesday and slide down the banisters on 

 Wednesday; but if the staircase consists of innumerable 



