MODERN PHYSICS JEANS 93 



Our balance at the bank always consists of an integral number of 

 pence, but it does not follow that it is a pile of bronze pennies. A 

 child may, however, picture it as so being, and ask his father what 

 determines which particular pennies go to pay the rent. The father 

 may answer " Mere chance " — a foolish answer, but no more foolish 

 than the question. Our question as to what determines which 

 photons get through is, I think, of a similar kind, and if Nature seems 

 to answer " Mere chance," she is merely answering us according to 

 our folly. A parable which replaces radiation by identifiable pho- 

 tons can find nothing but the finger of fate to separate the sheep 

 from the goats. But the finger of fate, like the photons themselves, 

 is mere pictorial detail. As soon as we abandon our picture of 

 radiation as a shower of photons, there is no chance but complete 

 determinismi in its flow. And the same is, I think, true when the 

 particle-photons are replaced by particle-electrons. 



We know that every electric current must transfer electricity 

 by complete electron-units, but this does not entitle us to replace 

 an electric current by a shower of identifiable electron-particles. 

 Indeed the general principles of quantum-mechanics, which are in 

 full agreement with observation, definitely forbids our doing so. 

 AVhen the red and white balls collide on a billiard table, red may go 

 to the right and white to the left. The collision of two electrons A 

 and B is governed by similar laws of energy and momentum, so that 

 we might expect to be able to say that A goes to the right, and B to 

 the left or vice versa. Actually we must say no such thing, because 

 we have no right to identify the two electrons which emerge from the 

 collision with the two that went in. Its as though A and B had tem- 

 porarily combined into a single drop of electric fluid, which had sub- 

 sequently broken up into two new electrons, C, D. We can only say 

 that after the collision C will go to the right, and D to the left. If we 

 are asked which way A will go, the true answ^er is that by then A will 

 no longer exist. The superficial answer is that it is a pure toss-up. 

 But the toss-up is not in nature, but in our own minds ; it is an even 

 chance whether we choose to identify C with A or with B. 



Thus the indeterminism of the particle-picture seems to reside 

 in our own minds rather than in nature. In any case this picture 

 is imperfect, since it fails to represent the facts of observation. The 

 wave-picture, which observation confirms in every known experi- 

 ment, exhibits a complete determinism. 



Again we may begin to feel that the new physics is little better 

 than the old — that it has merely replaced one determinism by 

 another. It has ; but there is all the difference in the world between 

 the two determinisms. For in the old physics the perceiving mind 

 was a spectator; in the new it is an actor. Nature no longer forms 



