26 SECTIONAL ADDRESSES 



rate to anyone who has made an elementary study of mechanics, I suppose 

 there is nothing more absolute than the law of causality. By this I mean 

 that the future is completely contained in the present. Passing over 

 obvious examples where this is true, like the path of a projectile or the 

 orbit of a planet, we may take an extreme case where we might expect 

 our faith in the principle would be most severely tried. Take the typical 

 case of chanciness, the tossing of a coin. We know that in a general 

 way there is an even chance of heads or tails, even though we sometimes 

 hear of gifted individuals with muscles so delicately adjusted that they 

 can control the event. But in the ordinary way the tossing of a coin 

 is complicated by being produced by a living organism, so let us simplify 

 the problem by designing a catapult of some kind to project it. Which 

 of us does not believe the coin would fall the same way every time if 

 such a mechanism could be made with really complete precision ? 

 When the machine fails to make it do so, we say it is because there may 

 have been a speck of dirt in the lubricant or something like that. In 

 other words, we do not feel that the fall of the coin is determined by 

 chance, but we regard the uncertainty we observe as due to our ignorance 

 of all the detailed causes. Ignorance is a confession of incompetence, 

 and so we regard the existence of chance as a blemish in our otherwise 

 admirable characters. This feeling goes very deep, since we are pre- 

 vented by it from having the complete control of our surroundings that 

 we somehow think should be our due. We start prejudiced against 

 probability and in favour of causality. So much for what we feel about 

 causality ; and about thirty years ago this feeling would have been 

 regarded as a piece of inescapable reasoning, with the same kind of 

 compelling power as a logical syllogism. We still have the feeling, but 

 now we know it is wrong, and what is more, we know that it is wrong 

 for a reason we never thought of. To understand this oversight we 

 must review the recent history of atomic theory. 



The history of the development of physics in the first quarter of the 

 twentieth century will rank as one of the greatest in the advancement 

 of knowledge, but it will also rank as one of the most curious in the 

 history of human thought. In 1901 Planck started the quantum theory. 

 Even this was curious. He was trying to find out the law of complete 

 radiation by the use of ordinary statistical methods, and observed that 

 he got his answer at what should have been the last stage but one of 

 his work. The last stage would have involved proceeding to a limit, 

 and he found that he got the experimental answer without doing so, 

 and an, absurd answer if he did. The work went rather deep into 

 statistical theory and there were many for long afterwards who were not 

 convinced of its compelling force, but it was the great merit of Planck 

 that he knew that he had got something involving a quite revolutionary 

 idea — the quantum. In succeeding years other phenomena were seen 

 to involve the same revolutionary idea : Einstein's theory of the photo- 

 electric effect, and of the ionisation produced by X-rays, his theory of 

 specific heats, later improved by Debye, and Bohr's theory of spectra. 

 All these things fitted in quite obviously with the quantum, but quite 

 as obviously they violently contradicted the physics of the nineteenth 

 century. What should a man think about a beam of light which accord- 



