320 BELL SYSTEM TECHNICAL JOURNAL 



thereof from each other. When I say that we "assign" it, I do not 

 for a moment mean that we are doing something arbitrary or un- 

 testable. We know that these elementary particles are charged, and 

 indeed we have measured their charges. Atoms do normally appear 

 to us uncharged, but we know that that is because the elementary 

 particles of which they in their turn are made up are some of them 

 positive, some of them negative, and the balance normally happening 

 to be perfect. Some particles, the neutrons and the corpuscles of 

 light, seem permanently chargeless; but perhaps some day we shall 

 find it expedient to regard them as groups of smaller particles having 

 charges which balance one another. Apart from these two cases, we 

 may say with assurance that whenever we penetrate as far into the 

 fine structure of substance as we are able to go, we find the elementary 

 particles invested with mass and with charge. 



And now I arrive at the subject of this talk, the third property of 

 the elementary particles: the property which is called "angular 

 momentum," or "spin" for short. Now of course I am speaking as 

 to an audience of physicists, for if this were an audience of laymen it 

 would certainly be frightened by such a term as "angular momentum." 

 This is a misfortune, and perhaps a defect of general education; for 

 angular momentum is about as important as mass or charge, not 

 only on the scale of the elementary particles but also on the scale of 

 the visible world. Think what it would mean if there were no such 

 thing as the conservation of angular momentum! the earth might 

 cease from turning, it might cease from providing us with the regular 

 alternation of day and night, and with our standard of the flow of 

 time; it might even cease from traversing its regular orbit, and fly 

 off into space or fall into the sun. Well, I do not wish to scare you 

 with any such dire imaginings — I only want to remark on the fact 

 that the human race has been acquainted for a very long time indeed 

 with angular momentum as something which is unvarying, imper- 

 turbable, incessant; for of all the unvarying and imperturbable and 

 incessant things in the world, the rotation of the earth is the most 

 obvious and the most striking. So striking it is, that you might 

 reasonably expect that all the philosophers and all the physicists of 

 the past would have conferred the property of spin on all the atoms 

 which they have invented. Well, they did not; the notions of the 

 spinning atom, the spinning electron, the spinning nucleus are among 

 the newest in physics. I think that some of the reasons for the delay 

 will be evident later on in this talk, but it remains partly mysterious, 

 at least to me. Looking back on the situation with the well-known 

 advantages of hindsight, I do feel a good deal of surprise that the 



