DEVELOPMENT OF NEW THEORY 207 



the appearance of the paper. That does not give long 

 for development; nevertheless the theory has already 

 gone through three distinct phases associated with the 

 names of Born and Jordan, Dirac, Schrodinger. My 

 chief anxiety at the moment is lest another phase of 

 reinterpretation should be reached before the lecture 

 can be delivered. In an ordinary way we should describe 

 the three phases as three distinct theories. The pioneer 

 work of Heisenberg governs the whole, but the three 

 theories show wide differences of thought. The first 

 entered on 'the new road in a rather matter-of-fact 

 way; the second was highly transcendental, almost 

 mystical; the third seemed at first to contain a reac- 

 tion towards classical ideas, but that was probably a 

 false impression. You will realise the anarchy of 

 this branch of physics when three successive pre- 

 tenders seize the throne in twelve months; but you 

 will not realise the steady progress made in that time 

 unless you turn to the mathematics of the subject. 

 As regards philosophical ideas the three theories are 

 poles apart; as regards mathematical content they are 

 one and the same. Unfortunately the mathematical 

 content is just what I am forbidden to treat of in these 

 lectures. 



I am, however, going to transgress to the extent of 

 writing down one mathematical formula for you to con- 

 template; I shall not be so unreasonable as to expect 

 you to understand it. All authorities seem to be agreed 

 that at, or nearly at, the root of everything in the phy- 

 sical world lies the mystic formula 



qp—pq = ih/2Tz 



We do not yet understand that; probably if we could 

 understand it we should not think it so fundamental. 



