SCIENCE, COMMON SENSE, AND DECENCY 5 



physics, which have for nearly twenty-five years seemed irreconcilable, 

 just so must we recognize two types of natural phenomena. First, those in 

 which the behavior of the system can be determined from the average 

 behavior of its component parts and second, those in which a single dis- 

 continuous event (which may depend upon a single quantum change) 

 becomes magnified in its effect so that the behavior of the whole aggregate 

 does depend upon something that started from a small beginning. The first 

 class of phenomena I want to call convergent phenomena, because all the 

 fluctuating details of the individual atoms average out giving a result that 

 converges to a definite state. The second class we may call divergent 

 phenomena, where from a small beginning increasingly large effects are 

 produced. In general then we may say that classical physics applies satis- 

 factorily to convergent phenomena and that they conform well to the 

 older ideas of cause and effect. The divergent phenomena on the other 

 hand can best be understood on the basis of the quantum theory of modern 

 physics. 



Let me give some illustrations of divergent phenomena. The wonderful 

 cloud chamber experiments of C. T. R. Wilson show that a single high- 

 speed electron or an alpha particle from an atom of radium, in passing 

 through a gas, leaves a trail of ions. By having moisture in the gas and 

 by causing the gas to expand just after these ions are produced, drops of 

 water are made to condense on the ions. By a strong illumination it thus 

 becomes possible to see or photograph this track of ions as a white line on 

 a dark background. The time at which an alpha particle will be emitted 

 from a radium atom is inherently unpredictable. It would be totally con- 

 trary to the teachings of modern physics to suppose that our inability to 

 predict such an event is merely due to our ignorance of the condi- 

 tions surrounding the particular atom. The uncertainty principle requires 

 that even if these conditions were absolutely fixed the time of emission 

 and the direction of emission of the alpha particle are subject to the laws 

 of chance and thus for a single particle are unpredictable. 



The occurrences in the Wilson cloud chamber following the disintegra- 

 tion of a single radium atom are typical divergent phenomena. The single 

 quantum event led to the production of countless thousands of water drop- 

 lets and these made the track of the alpha particle visible and led to its 

 reproduction in a photograph. This track may show some unusual feature 

 of particular interest to the scientist ; for example, it may have a kink 

 which indicates that the alpha particle collided with the nucleus in one of 

 the molecules of gas. The photograph may therefore be published — may 

 start discussions among scientists tliat involve thousands of man hours — 

 may delay one of them so that he misses a train on which he might other- 

 wise have suffered a fatal accident. Examples of this kind, any number of 



