190 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



laws tliat govern the suns and the nebulae are different from those 

 that govern the electrons in the atom, that no unifying principle 

 embracing both is, or ever will be, possible. Yet on this very problem, 

 Einstein has thought incessantly, finding solutions and rejecting them 

 because they did not satisfy his high standards of logical simplicity 

 and beauty. "VVliile discussing a theory, Einstein would often re- 

 mark to me : "Could God have created the world this way?" A good 

 physical theory, Einstein feels, must reflect the beauty and the glory 

 of the universe. 



In 1949, Einstein was 70 years old. In this year, he believes, he 

 found the solution for which he strove for 30 years. The results of 

 his last two decisive steps appeared, one in the Canadian Journal of 

 Mathematics (published for the Canadian Mathematical Congress 

 by the University of Toronto Press), the other in a tliird edition of 

 "The Meaning of Relativity" (published by the Princeton University 

 Press). 



Did Einstein solve the great problem of finding one law for the 

 large- and small-scale phenomena? It may be a long time before 

 mathematical analysis and observation pronounce their verdict — 

 before we find the treasures hidden in Einstein's new equations. No 

 one yet knows whether the third Einstein revolution in science has 

 been accomplished. 



THE ELECTROMAGNETIC FIELD 



To understand, even in general terms, the problem on which Ein- 

 fetein has worked for 30 years, we must go back to the nineteenth 

 century, to the times of James Clerk Maxwell, who was the first to 

 create a successful field theory. 



From the broadcasting antenna to my radio receiver, the radio 

 wave — that is, the electromagnetic wave — spreads with the velocity 

 of light. From the atoms in a neon tube to my eye, optical rays — 

 that is, an electromagnetic wave — spread with the velocity of light. 

 Radio and optical waves are governed by the same laws, expressed 

 by Maxwell's equations. They tell us how the electromagnetic wave, 

 or, as we say nowadays, the electromagnetic field, changes in space 

 and time and what its physical properties are. Maxwell's theory is 

 a field theory because it considers changes in time and in our three- 

 dimensional space. It is very different from a mechanical theory 

 that deals with such problems as the motion of the moon aroimd the 

 earth. In a mechanical theory, the particles and their motion are 

 important; in a field theory, the important factors are changes of a 

 field in space and time. 



If we analyze a piece of gossip we may be interested in the speed 

 with which this gossip spreads and how far it penetrates. These 



