Chapter VII 



GRAVITATION— THE EXPLANATION 



The Law of Curvature. Gravitation can be explained. 

 Einstein's theory is not primarily an explanation of 

 gravitation. When he tells us that the gravitational field 

 corresponds to a curvature of space and time he is giv- 

 ing us a picture. Through a picture we gain the insight 

 necessary to deduce the various observable consequences. 

 There remains, however, a further question whether 

 any reason can be given why the state of things pictured 

 should exist. It is this further inquiry which is meant 

 when we speak of "explaining" gravitation in any far- 

 reaching sense. 



At first sight the new picture does not leave very 

 much to explain. It shows us an undulating hum- 

 mocky world, whereas a gravitationless world would be 

 plane and uniform. But surely a level lawn stands more 

 in need of explanation than an undulating field, and a 

 gravitationless world would be more difficult to account 

 for than a world with gravitation. We are hardly called 

 upon to account for a phenomenon which could only 

 be absent if (in the building of the world) express pre- 

 cautions were taken to exclude it. If the curvature were 

 entirely arbitrary this would be the end of the explana- 

 tion; but there is a law of curvature — Einstein's law of 

 gravitation — and on this law our further inquiry must 

 be focussed. Explanation is needed for regularity, not 

 for diversity; and our curiosity is roused, not by the 

 diverse values of the ten subsidiary coefficients of curva- 

 ture which differentiate the world from a flat world, 

 but by the vanishing everywhere of the ten principal 

 coefficients. 



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