A NEW PICTURE OF GRAVITATION 115 



absolute, of which we must try to devise an appropriate 

 picture. For reasons which I shall presently explain we 

 find that it can be pictured as a curvature of space and 

 time. 



A New Picture of Gravitation. The Newtonian picture 

 of gravitation is a tug applied to the body whose path 

 is disturbed. I want to explain why this picture must 

 be superseded. I must refer again to the famous incident 

 in which Newton and the apple-tree were concerned. 

 The classical conception of gravitation is based on New- 

 ton's account of what happened; but it is time to hear 

 what the apple had to say. The apple with the usual 

 egotism of an observer deemed itself to be at rest; 

 looking down it saw the various terrestrial objects includ- 

 ing Newton rushing upwards with accelerated velocity 

 to meet it. Does it invent a mysterious agency or tug 

 to account for their conduct? No; it points out that 

 the cause of their acceleration is quite evident. Newton 

 is being hammered by the molecules of the ground 

 underneath him. This hammering is absolute — no ques- 

 tion of frames of reference. With a powerful enough 

 magnifying appliance anyone can see the molecules at 

 work and count their blows. According to Newton's 

 own law of motion this must give him an acceleration, 

 which is precisely what the apple has observed. New- 

 ton had to postulate a mysterious invisible force pulling 

 the apple down; the apple can point to an evident cause 

 propelling Newton up. 



The case for the apple's view is so overwhelming that 

 I must modify the situation a little in order to give 

 Newton a fair chance; because I believe the apple is 

 making too much of a merely accidental advantage. I 

 will place Newton at the centre of the earth where 



