Chapter VI 

 GRAVITATION— THE LAW 



You sometimes speak of gravity as essential and inherent to matter. Pray 

 do not ascribe that notion to me; for the cause of gravity is what I do not 

 pretend to know, and therefore would take more time to consider of 

 it. . . . 



Gravity must be caused by some agent acting constantly according 

 to certain laws; but whether this agent be material or immaterial I have 

 left to the consideration of my readers. 



Newton, Letters to Bentley. 



The Man in the Lift. About 19 15 Einstein made a 

 further development of his theory of relativity extending 

 it to non-uniform motion. The easiest way to approach 

 this subject is by considering the Man in the Lift. 



Suppose that this room is a lift. The support breaks 

 and down we go with ever-increasing velocity, falling 

 freely. 



Let us pass the time by performing physical experi- 

 ments. The lift is our laboratory and we shall start at 

 the beginning and try to discover all the laws of Nature 

 — that is to say, Nature as interpreted by the Man in 

 the Lift. To a considerable extent this will be a repeti- 

 tion of the history of scientific discovery already made 

 in the laboratories on terra firma. But there is one 

 notable difference. 



I perform the experiment of dropping an apple held 

 in the hand. The apple cannot fall any more than it 

 was doing already. You remember that our lift and all 

 things contained in it are falling freely. Consequently 

 the apple remains poised 'by my hand. There is one 

 incident in the history of science which will not repeat 

 itself to the men in the lift, viz. Newton and the apple 

 tree. The magnificent conception that the agent which 



in 



