RELATIVITY OF ACCELERATION 129 



Einstein's law, are mass (or its equivalent energy) 

 momentum and stress (pressure or tension). In this case 

 the wrinkle might correspond to stress. That is reason- 

 able enough. If left alone the earth must pursue its 

 proper curved orbit; but if some kind of stress or pres- 

 sure were inserted between the sun and earth, it might 

 well take another course. In fact if we were to observe 

 one of the planets rushing off in a straight track, New- 

 tonians and Einsteinians alike would infer that there 

 existed a stress causing this behaviour. It is true that 

 causation has apparently been turned topsy-turvy; ac- 

 cording to our theory the stress seems to be caused by 

 the planet taking the wrong track, whereas we usually 

 suppose that the planet takes the wrong track because it 

 is acted on by the stress. But that is a harmless accident 

 common enough in primary physics. The discrimination 

 between cause and effect depends on time's arrow and 

 can only be settled by reference to entropy. We need 

 not pay much attention to suggestions of causation aris- 

 ing in discussions of primary laws which, as likely as 

 not, are contemplating the world upside down. 



Although we are here only at the beginning of Ein- 

 stein's general theory I must not proceed further into 

 this very technical subject. The rest of this chapter will 

 be devoted to elucidation of more elementary points. 



Relativity of Acceleration. The argument in this chapter 

 rests on the relativity of acceleration. The apple had an 

 acceleration of 32 feet per second per second relative to 

 the ordinary observer, but zero acceleration relative to 

 the man in the lift. We ascribe to it one acceleration or 

 the other according to the frame we happen to be using, 

 but neither is to be singled out and labelled "true" 

 or absolute acceleration. That led us to reject the 



