Working with Bohr

Almost since I can remember, my ambition was to be a physicist. My parents had both studied physics and worked for a short time in the Cavendish Laboratory, and, although neither made a career in science, I was brought up knowing about physics. At both preparatory and secondary schools, however, my most inspiring teachers taught mathematics, and I left school with a maths scholarship to Cambridge and the ambition to work on quantum theory. That was in 1923, when Sommerfeld and others were still elaborating Niels Bohr's theory of orbits. But in 1926, when I took my final examination-which included some questions on quantum theory-the "new" quantum mechanics of Heisenberg and Schrodinger appeared and everything was changed. Bohr's orbits were dead. Most of physics and all of chemistry were now open to the theorist, once he or she could understand the new theory. But there was no one in Cambridge to teach it. Ralph Fowler, Rutherford's son-in-law, with an office in the Cavendish next to Rutherford's, was in charge of theoretical physics. He was the most helpful of men; but he was leaving for a sabbatical in America and, when I went to see him, told me to come back the following year. So I set out for Germany to learn some German and spent the next year reading the papers of Heisenberg and Schrödinger and also a paper by Max Born on the wave mechanics of collision processes. When I had understood these papers, I started work on collisions and was able to publish some papers. It was remarkable how quickly theoretical physics advanced at that time. In 1928, Paul Dirac in Cambridge startled the small world of the atomic physicists with the most brilliant piece of reasoning that I have seen- his theory of the electron's spin-and I began trying to apply it to collision problems. My first research had been on Rutherford's law for the scattering of alpha particles or electrons by atomic nuclei, and I wanted to see what changes the new theory would bring about. Some of my work must have impressed Fowler, for he recommended me to Bohr in Copenhagen and arranged a grant for working abroad. But it was rather a lonely time. I had few, if any, contemporaries to whom I could talk about what I was doing, and I do not remember any eager interest in quantum mechanics among Rutherford's research students. Patrick Blackett was always encouraging, but he was appalled by the complexity of the mathematics. Charles Darwin, professor at Edinburgh, was a help, for he had been trying to find a way of incorporating electron spin into quantum mechanics when Dirac forestalled him. Copenhagen, in contrast, was anything but lonely. We came to Bohr's institute in Blegdamsvej at about ten in the morning and left about six. We were in and out of each other's rooms all day, and so was Bohr. Nobody dreamed of keeping an idea to him-self; our joy in life was to tell it to other people, to get it criticized and if possible accepted. And if not, one could try again. Bohr himself, if he had a new idea, would come into the institute and tell it to the first person he could find; sometimes this was me. I later wrote of Bohr, "It is his enthusiasm, I think, that has created the school that has its centre in Copenhagen. When he begins to see something he talks for hours and hours, just bubbling with enthusiasm, and interrupting everyone till Mrs. Bohr comes and fetches him for dinner; often he won't leave then." I learned in Copenhagen what physics was all about, that it was a social activity and that a teacher should be with students, and I learned how beautiful physics could be. Most of my discussions with Bohr were about electron spin and how it affected collisions, particularly collisions between electrons. I remember that when I wrote a paper Bohr would read it, turn it inside out, walk round it and ponder it for weeks. Sometimes I felt impatient; I wanted to get it into press before someone had the same idea. But to follow Bohr's thought was a wonderful education. Letters to my parents at that time show how I appreciated it. After one of his parties, I wrote of "Bohr going round and talking shop to one person privately after another, fearfully eager looking, asking about one's own work, glowing when he talked of the big problem yet to be solved, the one to be discussed during Heisenberg's [upcoming] visit. That is the problem, perhaps, of the interconnection between relativity and the quantum theory, which contradict one another rather." In another letter I wrote, "I like the life here where half one's time is discussing. That is the great point of Copenhagen, I believe, though it's probably the same in Germany. Only Bohr knows everything that's being done and has a marvellous knack of finding the sense behind mathematics. Bohr is the kind of man who can tell one that one is talking nonsense without hurting-I don't think I can say more than that, do you? And he has his students alone in the evening to talk, and then walks home with them, telling how he discovered the theory of spectra. And then it's 1 a.m., per-haps." Bohr gave me the ambition to run a group of theorists myself, and I got the opportunity when I became professor of theoretical physics in the University of Bristol in 1933. The subject, as I had learned, could be a social activity, built on discussions and the exchange of ideas, and so the tea and coffee room would be the center of our activities. This was not so in Cambridge in those days. Also the discussions should be shared with the experimental workers; we were all physicists, not two different kinds of worker. And further, the head of the research group must have all the time in the world for his students and co-workers. I went back to Copenhagen the following year for a conference, a real gathering of the atomic physicists of the day, and after that I saw little of Bohr. At Bristol I worked on solid state. Once he asked me to explain what I was working on, but I could not arouse his interest in this less fundamental subject. Nonetheless, now, in my eighty-first year, I look back on the four months I spent in Copenhagen with Bohr when I was 23 years old as the experience that showed me most clearly what I wanted to do in life. One of the winners of the 1977 Nobel Prize in Physics, Sir Nevill held the Cavendish Chair at Cambridge from 1954 until his retirement in 1971. A Life in Science, his autobiography, was published this year by Taylor & Francis.

Working with Bohr

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