356 BELL SYSTEM TECHNICAL JOURNAL 



magnetic moment of the electron. Its empire would have been extended, 

 had the ratio of the magnetic moments of proton and electron been equal 

 to the reciprocal of the ratio of the masses of these two — actually the 

 former ratio is too great by a factor of 2.78. This contretemps has led 

 many to deny the title of "elementary" particle to the proton; while as for 

 the neutron, the fact that it lacks an apparent electric charge while never- 

 theless displaying a magnetic moment leaves it also open to suspicion. 



Few readers of these pages will be unaware that electrons are observed 

 proceeding out of nuclei: it may well be a source of wonderment that they 

 are denied a residence in these assemblages of protons and neutrons only. 

 This is of course another example of the mortality of the electron. Having 

 observed that it is subject to birth and to death, should we be deterred 

 from supposing that it is born as it quits the nucleus from which it comes? 

 This rhetorical question gives a false impression of the course of history. 

 There was indeed an era when electrons were believed to inhabit nuclei, 

 when nuclei were regarded as assemblies of protons and electrons only. 

 It ended in 1932; but the observation of the birth and the death of electrons 

 did not ensue for yet another year. What happened in 1932 was the dis- 

 covery of the free neutron. Only when this particle had been discovered 

 did a physicist (Heisenberg) think it worth while to begin to develop in 

 detail the theory that the components of nuclei are protons and neutrons 

 and no other particles but these. 



Now I bring this article to a close by fulfilling my promise to speak of 

 Einstein's relation between energy and mass, which on the one hand has 

 been rigorously tested in the realm of nuclear physics, and on the other 

 has extended that realm. 



The relation may be worded in several ways; I will employ the shortest: 

 energy has mass. 



Now imagine an assemblage of particles sticking together. "Sticking 

 together" is not the dignified phrase of a physicist; such a one would say, 

 more abstractly but more exactly, that energy must be given the particles 

 to take them apart. But energy has mass; therefore the mass of the as- 

 semblage must be augmented, when they are taken apart. Therefore the 

 mass of the interconnected assembly is less than the sum of the masses of 

 the particles when free. 



Now with a single stroke this principle does away with what otherwise 

 would have been a quite unsurmountable obstacle to the doctrine that all 

 nuclei are made up of protons and neutrons. For "proton" and "neutron" 

 are not merely the names of hv-pothetical particles whereof nuclei are 

 made up; they are also the names of the two lightest of nuclei. These two 

 lightest of the nuclei are so massive, that it could not possibly be said that 

 the other nuclei are made up of them, were it not for the detraction of mass 



