u8 GRAVITATION— THE LAW 



What has happened to the theory of the earth has 

 happened also to the theory of the world of space-time. 

 An observer at rest at the earth's centre represents what 

 is happening in a frame of space and time constructed 

 on the usual conventional principles which give what 

 is called a flat space-time. He can locate the events in 

 his neighbourhood without distorting their natural sim- 

 plicity. Objects at rest remain at rest; objects in uni- 

 form motion remain in uniform motion unless there is 

 some evident cause of disturbance such as hammering; 

 light travels in straight lines. He extends this flat frame 

 to the surface of the earth where he encounters the 

 phenomenon of falling apples. This new phenomenon has 

 to be accounted for by an intangible agency or demon 

 called gravitation which persuades the apples to deviate 

 from their proper uniform motion. But we can also start 

 with the frame of the falling apple or of the man in the 

 lift. In the lift-frame bodies at rest remain at rest; bodies 

 in uniform motion remain in uniform motion. But, as we 

 have seen, even at the corners of the lift this simplicity 

 begins to fail; and looking further afield, say to the 

 centre of the earth, it is necessary to postulate the acti- 

 vity of a demon urging unsupported bodies upwards 

 (relatively to the lift-frame). As we change from one 

 observer to another — from one flat space-time frame to 

 another — the scene of activity of the demon shifts. It 

 is never where our observer is, but always away yonder. 

 Is not the solution now apparent? The demon is sim- 

 ply the complication which arises when we try to fit a 

 curved world into a flat frame. In referring the world 

 to a flat frame of space-time we distort it so that the 

 phenomena do not appear in their original simplicity. 

 Admit a curvature of the world and the mysterious 

 agency disappears. Einstein has exorcised the demon. 



