Forces and Atoms: The World of the Physicist* 



By KARL K. DARROW 



ONE of the signs whereby a physicist may be known is a fondness for 

 putting dots upon blackboards. This is not an irrational habit, but a 

 S3anbolic practice. It is a symbol of his manner of regarding the world as 

 a multitude incredibly enormous of particles incredibly small. The dots 

 stand for the particles, and the bare regions of the blackboard for the empty 

 spaces between them. The habit has not indeed been universal. Many a 

 thinker has preferred to consider the world as a continuum, a solid or jelly 

 or fluid; and we shall see that this alternative has always been very near in 

 the background, even when the "atomists" were at their most triumphant. 

 Let me however defer this other idea, and derive as much as possible from 

 the notion of particles in a void. 



But when the dots are set down on the otherwise clean board with regions 

 of black emptiness between, the story is far from completed. It is, in fact, 

 only begun, for the major part is yet to be written : the account of the forces 

 among the particles. Though these last be separated from each other by 

 spaces apparently empty, yet they are not unconscious of each other, for 

 each of them is subject to a force — the resultant of many forces, due to aU 

 rest. 



One might attach an arrow to each dot, to signify the strength and the 

 direction of the force which acts upon it. One might draw wandering 

 curves all over the board, to intimate at every point the direction and 

 strength of the force which a particle would feel, were it to be at that point. 

 This is accepted practice, but it would be worth the doing only if our assump- 

 tions and our ambitions were much more specific than for the present they 

 are. Perhaps at least the blackboard should be smeared with a uniform 

 coating of chalk, to signify that a particle in space is not left entirely to 

 itself, but feels the influence of the others. Among our not-so-distant 

 ancestors there seems to have been a psychological need for a gesture of the 

 sort; they talked about space as though it were filled with a "medium" or 

 "aether", because it seemed wrong to them to say that space is empty if 

 the particles which wander in it are subject to forces. Our generation has 

 nearly lost the need, whether of an aether to occupy actual space or of a 



* Opening lecture of a course on "Nuclear Physics and Theory of Solids" delivered 

 in the Spring semester of 1941, during the author's tenure of the William Allan Neilson 

 chair at Smith College. 



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