EESrSTEIN'S NEW THEORY — INFELD 191 



would be the "field" aspects of the "gossip phenomenon." On the 

 other hand, we may be interested in the men who carry the gossip and 

 in the mechanics of their actions. These would be the "mechanical" 

 aspects of the gossip phenomenon. 



Yet, in describing the electromagnetic phenomena in the nineteenth 

 and early twentieth centuries, we did not use the field concepts alone. 

 Electrons — that is, negativel}' charged particles — produce an electro- 

 magnetic field while in motion. Thus, in Maxwell's theory, and later 

 in Lorentz's theory, we still find a mixture of the field and particle 

 aspects. Particles (electrons) move in an electromagnetic field and 

 influence the field by their motion. Yet it is the field aspect that is 

 the new and predominant feature of Maxwell's theory. 



The electromagnetic field is characterized at each point of space by 

 two arrows or vectors. One of these represents the electric field, the 

 other the magnetic field. But an arrow can be described by its three 

 projections upon three mutually perpendicular axes. (An arrow in 

 a plane is characterized by two projections, in a three-dimensional 

 space by three. ) Thus the field at each point ( remember that there are 

 two arrows) is represented by six numbers, three denoting the electric 

 and three denoting the magnetic field. These numbers change from 

 one point to another. These two arrows, and therefore the six num- 

 bers, change at the same point from one moment to another. Max- 

 well's equations tell us the laws of this change. Or: the electro- 

 magnetic field is characterized by six functions of space and time. 

 Maxwell's equations tell us how these functions change in space and 

 time. 



THE GRAVITATIONAL FIELD 



We can now characterize in general terms at least one aspect of 

 Einstein's second revolution: it did for gravitational phenomena 

 what Maxwell's theory did for electromagnetic phenomena. 



Newton's theory of gravitation follows the mechanical pattern. 

 Particles (moon, earth) are attracted by other particles (earth, sun). 

 In it there is no place for the concept of field, the scenery of which is 

 all of space. There is no place for a gravitational field spreading 

 through space in time. 



Einstein's theory of gravitation is not an improved version of New- 

 ton's theory ; it is an entirely new theory based upon new assumptions, 

 logically more satisfactory than those of Newton. Yet, the results, 

 which can be tested by observation, are very similar m the two theories. 

 There is a great area of agreement and a small area of disagreement. 

 The most famous new phenomenon, predicted by Einstein's theory 

 only, is that of light rays bending while passing near the edge of the 

 sun. Indeed, it was this phenomenon, discovered during a solar 



