NUCLEUS OF THE ATOM — ANDERSON 241 



One of the chief mathematical problems of the day was to incorpo- 

 rate the theory of relativity into quantum mechanics. In this con- 

 nection the endeavors of P. A. M. Dirac resulted in considerable suc- 

 cess when he wrote down his famous equation, now known as the 

 " Dirac electron-equation." 



This equation proved remarkably successful in solving a variety 

 of problems which had hitherto baffled the theorists; but it contained 

 one very striking feature which was a source of considerable annoy- 

 ance. It required that electrons should under certain conditions 

 have a negative energy and negative mass; they should have less 

 than zero energy and weigh less than nothing. Dirac considered 

 each point in space, including empty space or a perfect vacuum, to 

 be " filled " with an infinity of such negative energy electrons. He 

 also made the assumption that these negative mass electrons were 

 unobservable and that it was a property of free space that they 

 should be there. Dirac stated in 1930 that if one of these electrons 

 should be removed, the " hole " in space that remained would mani- 

 fest itself as an electron of positive electrical charge and of posi- 

 tive mass and energy. 



The logic is perfect, for taking away less than nothing from space 

 is equivalent to putting something there. 



Because no positive electrons had ever been observed and because 

 of a natural repugnance toward the idea that an infinity of electrons 

 of negative mass should occupy each point in space, practically all 

 theoretical physicists considered this feature of Dirac's equation an 

 unfortunate weakness. Because of the success of his equation, how- 

 ever, they continued to use it. 



But the discovery of the positron seemed to provide just the par- 

 ticle to correspond to one of these " holes " in space. The corre- 

 spondence is indeed very close as is shown by the fact that in agree- 

 ment with the Dirac theory, the fastest negative electrons had 

 energies 1,000,000 electron volts greater than the fastest positive 

 electrons. 



This observation provides evidence for the correctness of the view 

 put forward by Blackett and Occhialini, who on the basis of the 

 Dirac theory suggested that the positive and negative electrons might 

 be created in pairs out of the incident radiation. Fortunately for 

 the physicists of today, the theory of relativity as developed by Ein- 

 stein shows that there exists a very close relationship between mass 

 and energy — so close in fact that mass and energy may be considered 

 as two aspects of the same entity. According to this view, if a pair 

 of electrons is created out of the gamma radiation, then an amount 

 of energy would be used up in the act of creation depending upon 

 the masses of the particles formed. If one take the known mass of 

 the negative electron, and assume the same value for the mass of the 



