Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [104]
Dirac had a thorough grasp of Einstein's theory of relativity, which had generated a firestorm of publicity around the world in 1919 while he was still an engineering student, but he knew very little about Bohr's decade-old quantum atom. Until his arrival in Cambridge, Dirac always considered atoms 'as very hypothetical things', hardly worth bothering about.65 He soon changed his mind and set about making up for lost time.
The quiet, secluded life of a budding Cambridge theoretical physicist was tailor-made for the shy and introverted Dirac. Research students were largely left to work alone in either their college rooms or in the library. While others might have struggled with a lack of human contact day after day, Dirac was perfectly happy to be left alone in his room to think. Even on a Sunday as he relaxed by walking in the Cambridgeshire countryside, Dirac preferred to do it alone.
Like Bohr, whom he met for the first time in June 1925, Dirac chose his words, written or spoken, very carefully. If he gave a lecture and was asked to explain a point that had not been understood, Dirac would often repeat word for word what he had said before. Bohr had gone to Cambridge to lecture on the problems of quantum theory and Dirac had been impressed by the man, but not by his arguments. 'What I wanted was statements which could be expressed in terms of equations,' he said later, 'and Bohr's work very seldom provided such statements.'66 Heisenberg, on the other hand, arrived from Göttingen to give a lecture having spent months doing just the sort of physics that Dirac would have found stimulating. But he did not hear about it from Heisenberg, who chose not to mention it as he spoke about atomic spectroscopy.
It was Ralph Fowler who alerted Dirac to Heisenberg's work by giving him a proof copy of the German's soon-to-be-published paper. Heisenberg had been Fowler's house-guest during his brief visit and had discussed his latest ideas with his host, who asked for a copy of the paper. When it arrived, Fowler had little time to study it thoroughly and so passed it on to Dirac, asking him for his opinion. When he first read it in early September, he found it difficult to follow and failed to appreciate what a breakthrough it represented. Then, as one week turned into two, Dirac suddenly realised that the fact that A×B did not equal B×A lay at the very heart of Heisenberg's new approach and 'provided the key to the whole mystery'.67
Dirac developed a mathematical theory that also led him to the formula pq–qp=(ih/2)I by distinguishing between what he called q-numbers and c-numbers, between those quantities that do not commute (AB does not equal BA) and those that do (AB=BA). Dirac showed that quantum mechanics differs from classical mechanics in that the variables, q and p, representing the position and momentum of a particle, do not commute with one another but obey the formula that he had found independently of Born, Jordan and Heisenberg. In May 1926, he received his PhD with the first-ever thesis on the subject of 'quantum mechanics'. By then physicists were beginning to breathe a little easier after being confronted by matrix mechanics, which was difficult to use and impossible to visualise, even though it generated the right answers.
'The Heisenberg-Born concepts leave us all breathless, and have made a deep impression on all theoretically orientated people', Einstein wrote in March 1926. 'Instead of dull resignation, there is now a singular tension in us sluggish people.'68 They were roused out of their stupor by an Austrian physicist who found time while conducting an affair to produce an entirely different version of quantum mechanics that avoided what Einstein called Heisenberg's 'veritable calculation by magic'.69