PREDICTIONS FROM THE LAW 147 



behaves in the simple way which we might at least hope 

 to find.* 



Predictions from the Law. I suppose that it is at first 

 rather staggering to find a law supposed to control the 

 movements of stars and planets turned into a law 

 finicking with the behaviour of measuring rods. But 

 there is no prediction made by the law of gravitation in 

 which the behaviour of measuring appliances does not 

 play an essential part. A typical prediction from the law 

 is that pn a certain date 384,400,000 metre rods laid 

 end to end would stretch from the earth to the moon. 

 We may use more circumlocutory language, but that is 

 what is meant. The fact that in testing the prediction 

 we shall trust to indirect evidence, not carrying out the 

 whole operation literally, is not relevant; the prophecy 

 is made in good faith and not with the intention of tak- 

 ing advantage of our remissness in checking it. 



We have condemned the law of gravitation as a put- 

 up job. You will want to know how after such a dis- 

 creditable exposure it can still claim to predict eclipses 

 and other events which come off. 



A famous philosopher has said — 



"The stars are not pulled this way and that by 

 mechanical forces; theirs is a free motion. They go on 

 their way, as the ancients said, like the blessed gods." f 



This sounds particularly foolish even for a philo- 

 sopher; but I believe that there is a sense in which it is 

 true. 



* On the other hand a quantum (see chapter ix) has a definite 

 periodicity associated with it, so that it must be able to measure itself 

 against a time-extension. Anyone who contemplates the mathematical 

 equations of the new quantum theory will see abundant evidence of the 

 battle with the intervening symbol V — *• 



t Hegel, Werke (1842 Ed.), Bd. 7, Abt. 1, p. 97. 



