ii 4 GRAVITATION— THE LAW 



meters, and all measuring apparatus are relieved of 

 hammering and their indications can be received without 

 misgiving. The space- and time-frame of the falling lift 

 is the frame natural to observers who are unsupported; 

 and the laws of Nature determined in these favourable 

 circumstances should at least have not inferior status to 

 those established by reference to other frames. 



I perform another experiment. This time I take two 

 apples and drop them at opposite ends of the lift. What 

 will happen? Nothing much at first; the apples remain 

 poised where they were let go. But let us step outside 

 the lift for a moment to watch the experiment. The two 

 apples are pulled by gravity towards the centre of the 

 earth. As they approach the centre their paths con- 

 verge and they will meet at the centre. Now step back 

 into the lift again. To a first approximation the apples 

 remain poised above the floor of the lift; but presently 

 we notice that they are drifting towards one another, 

 and they will meet at the moment when (according to 

 an outside observer) the lift is passing through the 

 centre of the earth. Even though apples (in the lift) 

 do not tend to fall to the floor there is still a mystery 

 about their behaviour; and the Newton of the lift may 

 yet find that the agent which guides the stars in their 

 courses is to be identified with the agent which plays 

 these tricks with apples nearer home. 



It comes to this. There are both relative and absolute 

 features about gravitation. The feature that impresses 

 us most is relative — relative to a frame that has no 

 special importance apart from the fact that it is the one 

 commonly used by us. This feature disappears alto- 

 gether in the frame of the man in the lift and we ought 

 to disregard it in any attempt to form an absolute pic- 

 ture of gravitation. But there always remains something 



