The telegram reached Bohr soon after he received from Dirac a copy of his textbook, The Principles of Quantum Mechanics. Even if the author’s name were not on the cover, his identity would have been obvious to Bohr from a quick flick through: the unadorned presentation, the logical construction of the subject from first principles and the complete absence of historical perspective, philosophical niceties and illustrative calculations. This was the vision of a mathematically minded physicist, not an engineer. Dirac’s peers marvelled at its elegance and at the deceptively plain language, which somehow seemed to reveal new insights on each reading, like a great poem. Many of the students – especially the less able ones – were bemused, dissatisfied and sometimes even dispirited.31 The book had been written with no regard for his readers’ intellectual shortcomings, without the slightest sign of emotion, with not a single leavening metaphor or simile. For Dirac, the quantum world was not like anything else people experience, so it would have been misleading to include comparisons with everyday behaviour. He scarcely mentioned empirical observations except at the beginning, where he described an experiment that demonstrates the failure of classical theory to account for matter on the atomic scale and, hence, motivates the need for quantum mechanics. In its 357 pages, The Principles of Quantum Mechanics featured neither a single diagram, nor an index, nor a list of references, nor suggestions for further reading. This was, above all, a personal view of quantum mechanics, which is why Dirac – usually someone who abjured personal pronouns – always referred to it as ‘my book’. Physicists immediately hailed it a classic. Nature published a rhapsodic review by an anonymous reviewer who – to judge by the eloquence and sharp turn of phrase – may well have been Eddington. The author made it clear that this was no ordinary account of quantum mechanics:
[Dirac] bids us throw aside preconceived ideas regarding the nature of phenomena and admit the existence of a substratum of which it is impossible to form a picture. We may describe this as the application of ‘pure thought’ to physics, and it is this which makes Dirac’s method more profound than that of other writers.32 The book eclipsed all the other texts on quantum mechanics written at about the same time – one by Born, another by Jordan – and became the canonical text on the subject in the 1930s. Pauli warmly praised it as a triumph and, although he worried that its abstractions rendered the theory too distant from experiment, described the book as ‘an indispensable standard work’.33 Einstein was another admirer, writing that the book was ‘the most logically perfect presentation of quantum theory’.34 The Principles of Quantum Mechanics later became Einstein’s constant companion: he often took it on vacation for leisure reading and, when he came across a difficult quantum problem, would mutter to himself, ‘Where’s my Dirac?’35 But some of Dirac’s undergraduate students were not pleased to find that the book was largely a transcript of his lectures: why, these students wondered, was it worth bothering to go and listen to him? Yet others found the course uniquely compelling.36 He would enter the lecture theatre punctually and in full academic garb, wearing the traditional uniform of gown and mortarboard. Otherwise, there was nothing else theatrical about him. He would clear his throat, wait for silence, then begin. For most of the lecture, he would stand still and erect, enunciating each word, addressing what one of his students described as his ‘personal unseen world’.37 At the blackboard, he was an artist, writing calmly and clearly, beginning at the top left-hand corner, then methodically working downwards, writing every letter and symbol so that someone at the back of the room could see it clearly. The audience was usually quiescent. If a student asked a question, he would dispatch it with the economy of a great batsman and then move on, as if nothing had disturbed his flow. After precisely fifty-five minutes he would draw his presentation to a close and then, unceremoniously, gather his papers together and walk out.