FORCES AND ATOMS 341 



smear of chalk to symbolize it on the blackboard. Let us say that space 

 is empty and leave the blackboard black between the dots, without deeming 

 ourselves deprived of the right of saying that the particles exert and suffer 

 forces on and from one another. 



What is there to be said about the forces? A very great deal! for most of 

 theoretical physics is made up of beliefs or ideas about the forces, augmented 

 by the mathematical operations— very hard and very long-winded, in far 

 too many cases — required for making the ideas really useful. So great a 

 programme is indicated by that sentence, that I am wasting words in adding 

 that it will not be fulfilled in one or two lectures, nor in the whole of the 

 course. Only the most general of statements can be made in what follows. 

 Of these I lay down at once the first, which is negative and self-evident: 



The forces cannot be purely repulsive. For if they were, all of the particles 

 would rush off into the uttermost depths of space, and we should have no 

 model at all for a universe which, with all its faults, does manage at least to 

 stick together. 



Therefore there must be attractive forces, and these by and large must 

 overpower the repulsive ones, if any such there be. 



But need there be any repulsive forces at all? (Let the sophisticated 

 reader now forget for a little that there are electrical forces which are repul- 

 sive, so that he may enquire with an open mind as to whether such could 

 be avoided.) At first, it may not seem so; and one may invoke the great 

 authority of Newton, who is often thought to have contented himself with 

 assigning to all bodies the power of attracting one another with the force 

 of gravity. He did not so content himself, and we shall learn this shortly. 

 For the moment, let it be remembered that forces of attraction unopposed 

 would tend to draw all of the particles of the universe into a single compact 

 clump. If the volume of each particle were infinitely small, so also would be 

 that of the ultimate clump; if the volume of each particle were irreducible 

 below a certain minimum— but we shall ere long find what that idea can 

 involve us in! Briefly, there must be something to oppose the attractive 

 forces. To call this something by the name of "force", or even to call it 

 by any single name, would be to limit it unduly. So to the second general 

 statement I give the form: 



There must be attractive forces, but there must also be antagonists to them. 



If someone wanted a particular problem of the theory of physics identified 

 to him as the profoundest, the problem of these antagonists might well be 

 selected as such. 



There is indeed one famed and spectacular case, which makes one antago- 

 nist clear. It is the case of the heavenly bodies: the planets revolving 

 around the sun, the satellites around the planets. Why does not the moon 

 fall onto the earth and the earth fall into the sun? Newton's laws of motion 



