348 BELL SYSTEM TECHNICAL JOURNAL 



netic forces, we are to treat their motions by the method of quantum 

 mechanics, and not by the method of classic mechanics. 



I will not pretend that this is a slight innovation, nor try to represent it as 

 anything less than a great and difficult revolution in some of our most 

 cherished habits of thought. Concepts formerly sharp, even those of posi- 

 tion and motion themselves, become hazy; there are pitfalls and labyrinths; 

 the mathematical technique is novel and hard. Yet in the picture of the 

 universe as now presented, there are particles possessed of charge and mass; 

 there are electromagnetic forces between the particles; there is motion of 

 the particles; there is radiation, which it is just barely permissible to dis- 

 regard in an outline like this one, and which I am disregarding; and outside 

 of the realm of "nuclear" phenomena, there is nothing else. The stability 

 of the world, that is to say, of the picture, is assured by attractions and 

 repulsions electrical in nature, and by motion, with radiation playing an 

 essential part. 



The hydrogen atom appears before the eye of the mind as a system of a 

 nucleus and an electron : two particles of known, equal and opposite charges, 

 of known unequal masses, attracting one another by electrostatic force. 

 The force draws them together, but there is kinetic energy and there is 

 motion, and so they stay apart. It takes a definite amount of energy to 

 separate them, and the theory derives its actual value very exactly from a 

 basic principle. Any other atom appears before the mind as a system of a 

 nucleus and two or more electrons. The nucleus bears a positive charge, 

 the electrons are negative; the nucleus attracts the electrons, but they repel 

 one another; there is motion; between the attraction and the motion and the 

 repulsion, there is stability. A molecule is a system of two or more nuclei 

 positively charged and two or more electrons negatively charged, and the 

 same three qualities hold the balance. A tangible piece of metal is an 

 enormous multitude of nuclei and electrons, these latter enjoying a very 

 wide variety of motions, some moving almost as freely as though the metal 

 were a vacuum: again the balance is held, the metal tending neither to shrink 

 nor to explode. 



All this is a programme for the explanation of Nature; and it is a pro- 

 gramme which has been largely fulfilled — wherefore this lecture and a 

 portion of the course. Not everything has been explained, nor ever will be. 

 Quite apart from the phenomena called nuclear, there are countless things 

 and happenings on earth which are so complicated, that they may well obey 

 our fundamental laws without ever giving us the chance to prove it. If 

 we should apply our assumptions to them and start to work out the conse- 

 quences, it would take a geological era to finish the job. Perhaps all phe- 

 nomena of life are of this type. The most that can be asked for is, that the 

 theory should deal capably with all the phenomena for which it cannot 



