EINSTEIN'S NEW THEORY — INFELD 195 



born because of his dissatisfaction with General Relativity Theory, 

 a weak point of which was the artificial mixture of geometric and 

 physical concepts. But another weak point is perhaps still more 

 important. Both the electromagnetic and the gravitational theories 

 are dualistic theories. In both of these theories, we have sources of 

 the field (charges, particles) and the field itself. Thus we see in both 

 theories a mixture of two concepts: matter and field. It would be 

 philosophically much more satisfactory if we were able to build a 

 unitary theoiy based on only one of these concepts. The triumphs 

 of field theory were too great to allow us to abandon the field concept. 

 Hence, Einstein's aim was to build a pure field theory. In such a 

 theory we would have only field concepts and equations of the field. 



But we could argue : How can we be satisfied with field equations 

 alone? We know that matter is as real as the stone on which we 

 stumble. The supporter of the unitary field view would say that the 

 existence of what is known as matter should be deduced from the field 

 equations alone. "Wliat is regarded as matter is situated in regions in 

 which the field is especially strong. Motion of matter means that the 

 regions in which the field is especially strong change with time. 

 Hence, a resting electron has to be represented in a unitary electro- 

 magnetic theory by a small region, inside which the field is very strong, 

 and outside which it dies out quickly. Such a region, with a strong 

 but finite field, represents concentrated energy — that is, matter. 



A good field theory describes and interprets matter in terms of strong 

 fields. Hence, from the point of view of logical simplicity, great prog- 

 ress would be achieved if both Maxwell's theory and General Relativ- 

 ity Theory were to change into a pure field theory. Such a theory 

 would deal only with the concepts of the electromagnetic field, charac- 

 terized by 6 functions, and of the gravitational field, characterized 

 by 10 functions. But the laws of these fields would have to be changed. 

 Unlike Maxwell's theory and General Relativity Theory, such new 

 theories would have to admit solutions representing matter. The old 

 theories failed to do that. 



But even if we were to succeed in formulating a pure field theory, 

 such a theory would still be tainted with another fault. We saw, in 

 the old theories, that the gravitational field was a geometrical field too, 

 but the electromagnetic field was a purely physical field. This divi- 

 sion is again artificial, and, according to Einstein, a satisfactory theory 

 ought to have the following features : 



1. It ought to be a pure field theory. 



2. In it, electromagnetic and gravitational fields ought to be treated 

 on the same footing — that is, both should characterize the geometry 

 of our universe. 



