156 GRAVITATION— THE EXPLANATION 



Maxwell that these waves were not strangers but were 

 already familiar in our experience under the name of 

 light. The method of identification is the same. It is 

 calculated that electromagnetic waves will have just 

 those properties which light is observed to have; so too 

 it is calculated that the ten coefficients of curvature have 

 just those properties which energy, momentum and stress 

 are observed to have. We refer here to physical pro- 

 perties only. No physical theory is expected to explain 

 why there is a particular kind of image in our minds 

 associated with light, nor why a conception of substance 

 has arisen in our minds in connection with those parts 

 of the world containing mass. 



This leads to a considerable simplification, because 

 identity replaces causation. On the Newtonian theory 

 no explanation of gravitation would be considered com- 

 plete unless it described the mechanism by which a piece 

 of matter gets a grip on the surrounding medium and 

 makes it the carrier of the gravitational influence radi- 

 ating from the matter. Nothing corresponding to this 

 is required in the present theory. We do not ask how 

 mass gets a grip on space-time and causes the curvature 

 which our theory postulates. That would be as super- 

 fluous as to ask how light gets a grip on the electro- 

 magnetic medium so as to cause it to oscillate. The 

 light is the oscillation; the mass is the curvature. There 

 is no causal effect to be attributed to mass; still less is 

 there any to be attributed to matter. The conception 

 of matter, which we associate with these regions of un- 

 usual contortion, is a monument erected by the mind to 

 mark the scene of conflict. When you visit the site of a 

 battle, do you ever ask how the monument that com- 

 memorates it can have caused so much carnage? 



The philosophic outcome of this identification will 



