The Joy of Insight: Passions of a Physicist
In the fall of 1950, when I was a senior at Harvard, I decided that I wanted to study the quantum theory. This was only moderately unreasonable. I was a mathematics major and had taken a solid course in freshman physics given by the theoretical physicist Wendell Furry. I had also read some popular books and had a great many philosophical talks about the quantum theory with my teacher Philipp Frank. While I knew some higher mathematics, when it came to physics I knew next to nothing. Nonetheless, I decided to enroll in the first-year graduate course in quantum theory taught by the then reigning genius in theoretical physics at Harvard, Julian Schwinger.
To say that Schwinger’s lectures were both brilliant and impenetrable would be an understatement. They were very brilliant and impenetrable. Schwinger was, it turned out, trying out an entirely new formulation of the theory on us—the old one would have been hard enough—and since he lectured from memory questions were discouraged. (Years later Robert Oppenheimer once said to me of Schwinger that when most people discuss a problem they show you how to solve it, but when Schwinger discusses a problem he shows you that only he can solve it.) After a few weeks I was lost. I was commiserating with a friend, a physical chemist, who was also taking Schwinger’s course. He said, what you should do is come with me to MIT and audit “Viki.” Viki, I learned, was Victor Weisskopf, who was Schwinger’s analogue—I am tempted to say antiparticle—at MIT.
My visits to Viki’s class in quantum mechanics at MIT were, in every way, a culture shock. The class and the classroom were both huge—at least a hundred students. Weisskopf was also huge, at least he was tall compared to the diminutive Schwinger. I do not think he wore a jacket, or if he did, it must have been rumpled. Schwinger was what we used to call a spiffy dresser. Weisskopf’s first remark on entering the classroom, was “Boys [there were no women in the class], I just had a wonderful night!” There were raucous catcalls of “Yeah Viki!” along with assorted outbursts of applause. When things had quieted down Weisskopf said, “No, no it’s not what you think. Last night, for the first time, I really understood the Born approximation.” This was a reference to an important approximation method in quantum mechanics that had been invented in the late 1920s by the German physicist Max Born, with whom Weisskopf studied in Göttingen. Weisskopf then proceeded to derive the principal formulas of the Born approximation, using notes that looked as if they had been written on the back of an envelope. Along the way, he got nearly every factor of two and pi wrong. At each of these mistakes there would be a general outcry from the class; at the end of the process, a correct formula emerged, along with the sense, perhaps illusory, that we were participating in a scientific discovery rather than an intellectual entertainment. Weisskopf also had wonderful insights into what each term in the formula meant for understanding physics. We were, in short, in the hands of a master teacher.
After I got my degree in physics in 1955 I had the chance to see Weisskopf in a somewhat different role—as a sounding board for scientific ideas. By this time I was a postdoctoral fellow working in the Harvard Cyclotron Laboratory. Only a few theoretical physicists then could be found on the staffs of Harvard and MIT. We used to meet once a week for lunch and during these lunches Schwinger would try out his new ideas on Weisskopf. The styles of the two men could not have been more different. Schwinger always built extraordinary mathematical castles in the air and Weisskopf had a wonderful way of trying to anchor them to the ground. One of the things I learned from those sessions was not to be afraid to say, “I don’t understand,” even in the most exalted company, something that stood me in very good stead when I left Cambridge for the Institute for Advanced Study at Princeton.
I also got from these discussions with Weisskopf a sense that physics was a culture with a history full of colorful, larger-than-life characters. Weisskopf, who had been born in 1908, only three years after Einstein created the theory of relativity and invented the quantum, had worked with most of the major figures in twentieth century physics, including Niels Bohr, Werner Heisenberg, Wolfgang Pauli, and Oppenheimer. He was, for me, a precious link to these people. For all these reasons, when I heard a few years ago that he was writing an autobiography I was very pleased, thinking I would soon be able to read in detail about many of the subjects I had heard him talk about in passing over the years. The autobiography, The Joy of Insight, is, as I expected, charming, and it is sometimes revealing, but it falls short of what I had hoped it would be. A clue to why this is so may be in a brief statement that Weisskopf makes about himself. He writes, “I am aware that I am not a good psychologist. Often in my life, I have overlooked the negative qualities of a person and seen only the positive side.” This quality no doubt makes for a more agreeable life, but it tends to make an autobiography less than satisfying.
Weisskopf was born in Vienna to a well-to-do and highly assimilated Jewish family. A great uncle had changed his name from Cohn to Colbert. The household included a cook, a chambermaid, and a nurse for the children, and the family was highly musical. Weisskopf learned to play the piano and has had a lifelong passion for classical music. Indeed, at some point he had to choose between going to a conservatory to study to be a conductor and continuing his studies in physics. The Christmas and summer holidays were spent at the Weisskopf country house in the mountains in Alt Ausee. Weisskopf was an excellent student, athletic enough to enjoy skiing, and with enough of a social conscience to become a socialist. He recalls writing songs and sketches for a political cabaret.
Weisskopf entered the University of Vienna in 1926 just at the time the new quantum mechanics was being formulated. After a two-year course, he was advised by his professor, Hans Thirring, to leave Austria since the universities there could no longer teach him anything useful. He chose Göttingen. As he writes, he might have gotten a better education, especially in mathematics, if he had gone to Munich to study with Arnold Sommerfeld, whose pupils included Pauli, Heisenberg, and, later, Hans Bethe. In particular, Weisskopf thinks he would have become a better mathematician. People often think that all theoretical physicists are mathematicians. Some, like Freeman Dyson, are, but most are not. Of course theoretical physicists use mathematics, but a physicist like Weisskopf, who is not a mathematician, uses as little as possible. Theoretical physics is a science of approximation. What counts is the ability to see what is “big” and what is “small”—which effects are important and which can be neglected. Many mathematicians who try to do physics don’t have a feeling for this at all. They think of physics as an abstract logical structure, which it isn’t. Weisskopf’s special ability lies in his physical intuition, which enables him to pick out the features of a problem that are important; mathematics are secondary.
Weisskopf had intended to study with Born. But Born was, under the best of circumstances, aloof, and to make matters worse, he suffered a stroke, from which he eventually recovered. Weisskopf found himself on his own; but he soon hit on a problem involving the radiation emitted when an electron in an atom makes a quantum jump from one excited state to another. In his book, Weisskopf does not hesitate to describe his scientific work, and he does so in a way that an interested layperson should be able to follow without much difficulty. In the problem he invented for himself he needed more mathematical skills than he had.
It was at this point that Eugene Wigner came to the rescue. If there ever was a mathematical physicist it was Wigner, although he had been trained as a chemical engineer. The two young men, Weisskopf was twenty-two and Wigner a bit older, published their paper in 1930, and it established Weisskopf as a physicist of promise. Fortunately Weisskopf’s family was willing to pay for another year of study with Heisenberg in Leipzig, where he went after taking his degree in Göttingen in 1931. But after only six months he received an invitation from Erwin Schrödinger, in Berlin, to come to Berlin to be a paid assistant.
Thus, within a year, he had a chance to work with the two men who did the most to invent the quantum theory. This was followed by a Rockefeller grant which he used to go to Copenhagen to study with Niels Bohr. In 1933 he was invited to come to Zurich to be Wolfgang Pauli’s assistant. No better education in physics could be imagined and Weisskopf had the intelligence and self-confidence to make the most of it. Pauli, with whom Weisskopf did important work on the quantum theory of fields, was known for his almost childlike honesty as well as his brilliance. Of a young physicist he once said, “So young and already so unknown.” Oppenheimer said that he was almost identical with his caricature, and he was. A reader of Weisskopf’s chapter on Pauli will come away with a sense of his genius and integrity. It was for me a little sad to find that Weisskopf refers to Pauli, in the present tense, as the conscience of physics. He died in 1958 and my own students, for example, know nothing about him apart from references to him in textbooks.
In 1932, while still in Copenhagen, Weisskopf married his Danish wife, Ellen, whom he met while working with Bohr. She died in the summer of 1989, and the book is dedicated to her. (There was a theorem at Bohr’s institute that no young man could stay in Copenhagen for more than two years without marrying a Danish girl.) She went with him to Zurich and, in 1936, to the Soviet Union, where Weisskopf thought he might accept a permanent job. One year there was enough. The Stalin terror was much in evidence and it affected many of the scientists he knew. In 1937 he had an offer from the University of Rochester where he remained until he went to Los Alamos in 1943. It was Weisskopf’s good fortune that he was able to bring his mother, brother, and sister to the US from Austria. Indeed, one is struck again and again by the combination of good luck and common sense that enabled him to stay clear of the political disasters that surrounded him. No member of his or his wife’s immediate family was killed by the Nazis. But for Weisskopf, like so many of the others at Los Alamos, the effects of the Nazi aggression changed his life.