John Wheeler - Session V

The compound-nucleus model took time to make its way in the world. Nobody helped Bohr translate it from general thought into concrete realization more than Fritz Kalckar. Kalckar belonged to a Danish intellectual family. He died early from heart failure, but his brother Jørgen Kalckar is at the Institute for Theoretical Physics in Copenhagen today, and has been editing the part of Bohr's paper[s] concerned with the meaning of quantum theory. I have in the office somewhere—I have to go over it—a typewritten manuscript of that material that's to come out, to be, I think, the last missing volume of the collected papers of Niels Bohr. Fritz Kalckar capitalized on an idea that [...] had come up at that Easter-time seminar, where Bohr himself came out with the compound-nucleus model. I had once thought that there was the place where he had come out also with the liquid-drop model of the nucleus, but Rudolph Peierls, in his notes to the collected papers of Bohr on nuclear physics, argues that it was really the fall of 1935 and not April of 1935. But it would be a mistake to forget George Gamow, who in earlier years—I can't give chapter and verse—had talked about a liquid-drop model for the nucleus. That thought had fallen in abeyance in the meantime. Perhaps this is one more of those illustrations of how a thought can sneak in insidiously, or unawares, to a line of reasoning from sources that one doesn't recall.

That was 1935, and by the fall of 1935, I was already teaching physics at the University of North Carolina at Chapel Hill.

I stopped in to see my first graduate student, Katherine Way, when I was in North Carolina a month ago. She's in a retirement community much like the one where I live. She is, however, 91 years old. She broke her hip about five months ago and has been in care, in the infirmary part of the retirement community, for some time. Her thesis was on the magnetic moment associated with the liquid-drop model of the nucleus. It had turned out that if a liquid drop is rotating too fast, it falls apart. It will undergo what today we would call fission. This instability set a limit to the kind of angular momentum and the kind of magnetic moment that a nucleus could have. Having once come upon that instability, it would have been a natural thing to investigate other forms and causes for instability, which would have led us to an analysis of fission before fission, but we did not think of that.

I recall that Eugene Wigner was acutely uncomfortable about any such continuum picture. He was much more impressed by the pattern of spins that one saw among the atomic nuclei, a pattern that led Maria Mayer and [Hans] Jensen to put forward their ideas about shell structure in the nucleus, about an ordering of states somewhat analogous to the ordering of states that one sees in an atom. It was natural for Maria Mayer to take up that line of work, because she had worked in earlier days on the shell-model picture of atoms, going as far as could be gone easily in the shell model of atoms, based on the Fermi-Thomas atom model. It sounds perhaps like a contradiction in terms to use a continuum treatment like the statistical atom model at the same time one is dealing with a shell model. But it's not actually a contradiction because the key feature of the statistical atom model is the field of force produced jointly by the atomic nucleus and the electrons that are going around. That field if force is not overly sensitive to details of shell structure. But once one has such a field of force in hand, then one can go ahead and analyze the possible states of motion of an electron in that field of force, and come out with conclusions about the order of occupancy, filling these energy levels—therefore the shell structure of atoms. So Maria Mayer had from that source a splendid background for taking up the shell model of nuclear structure, the subject for which she was awarded a Nobel Prize. The decisive experimental input were the spins and angular momenta of the atomic nuclei. That was a great achievement of experimental nuclear physics to have so much data on that topic at hand at the time she was exploring this world of ideas.

So Wigner, my colleague here at Princeton, was much more impressed by reasoning of that kind than by any kind of liquid-drop picture. But I'm getting ahead of the game, because I was not a colleague of Wigner until the fall of 1938, when I came to Princeton, and it was about that time that he ate some oysters from a source which wasn't so good. He got hepatitis and was laid up in the infirmary of the University, which is the building next to the physics building. So it was easy for me to visit him there after the discovery of fission and talk about the business of going over the fission barrier. Wigner had worked with Michael Polanyi on related issues in molecular physics—the probability of a molecular transformation depending on a point in configuration space going over a potential summit. Today that line of reasoning is very much to the fore, especially because one has sources of light that will explore stages in the passage over the molecular potential-energy barrier at a time scale of femtoseconds.

K: Before we leave Copenhagen, could you speak a little bit about the way you worked with Bohr, or used Bohr. You and Bohr did not co-author any papers at that time (JAW: That's right), yet he must have been influential on your work.

RESONATING GROUP STRUCTURE IN COPENHAGEN

In Copenhagen, I think I had given too much of my time to following up an idea I had had before I went to Copenhagen—the concept of a nucleus which was described by a mixture of different possible states, just as one can describe a molecule by a wave function, or probability amplitude, which is a combination partly of binding between ions and partly of binding between neutral atoms.

Just as one speaks of a molecule as resonating between these two possible states of binding, so I was using a similar description for the nucleus, with the nucleus resonating between a state where it consisted of individual neutrons and protons and a state where it consisted in large measure of alpha particles. I somehow found myself talking about this with Ed Condon when I visited Princeton in company with Breit at the time Einstein was giving his first lecture in Princeton. Condon said, "Why don't you call it resonating group structure?" So I owe that name to him. So here I was, employing the concept of resonating group structure to analyze the collision and interaction between two helium nuclei—two alpha particles. Experiments on the scattering of one alpha particle by one helium nucleus had been reported and discussed at the London meeting, and that seemed a timely reason to take up that topic and come out with a conclusion about it. But here was I, doing calculations with a slide rule at a time when the calculations were really quite imposing, and I never came through with a believable conclusion from the standpoint of resonating group structure.

GAMMA RAY SCATTERING BY LEAD

The other project that I remember particularly taking a lot of my time in Copenhagen was with Milton Plesset. He too was a National Research Council Fellow, and we wanted to see if we could understand one of the great puzzles that had turned up in the London meeting, that is to say, the powerful back scattering of gamma rays by lead. No known elementary process would give such strong back scattering. We were concerned with a totally different picture, that is, that in the lead there takes place a mini-shower analogous to the showers that one already knew take place in the upper atmosphere. Just as in the upper atmosphere, so in the lead, our picture was: The photon comes in in the field of force of the nucleus—in this case, a lead nucleus rather than a nitrogen nucleus—producing a pair of positive and negative electrons. Those two particles go forward but they are scattered because the scattering power of a lead nucleus is enormously greater than the scattering power of a nitrogen nucleus, so they find themselves going in directions far different from the forward direction, which one sees so vividly in the normal cosmic rays in the atmosphere. So the idea was that the gamma rays that came backward were produced by these positive and negative electrons going backward—electrons radiating in the field of force of nuclei by which they passed and positrons radiating in the field of force of the nucleus, giving gamma rays going generally in the direction they were traveling. It was important in this subject—and I am afraid it was I more than Plesset that was involved in this side of the subject. Another side of the subject that he and I got involved in was what could you truly expect in the way of honest scattering by the nucleus itself. We decided that the key factor there was how much could we truly expect in the way of absorption of gamma rays by the nucleus itself, because from absorption and from the Kronig-Kramers relations between scattering and absorption, we could expect to figure the scattering of the nucleus.

We produced a paper, but Bohr, with his desire always to have a comprehensive account of whatever topic came up, seemed to think we needed to do more on the topic. We never did bring that to conclusion; we never did publish that paper. I don't know where it sits now. I remember seeing Plesset a couple of years ago. He remembered a little about it, but he didn't show a lot of enthusiasm to resurrect the topic and go on with it. I've still got it on the books, so when some student comes by who might be interested in such a topic,...

K: What was the reaction of you and Plesset to being sort of put down by Bohr? Did you feel a little bit crushed by that, or did you feel that it was perfectly reasonable for him to do that?

I can't recall our ever really discussing how we felt. I can't recall. (K: You just felt that he was the Pope? JAW: Yes [laughs].) It would be wonderful to discover any notes about what we did and felt at that time. I am afraid I didn't keep a lot of notes from those days. In my paper about Feynman, I think it was, in Physics Today, I think I reproduced a little of the notes I had made on this general topic at that time.

K: At any rate, you don't have a recollection of being sort of psychologically scarred by that experience?

No, no. And I don't recall any great white hope that Bohr imparted to us that if we did so-and-so, that would bring this thing to a triumphant conclusion. I need a good analogy to that. I suppose it would be as if an art expert came in and looked at a painter's work and said, "Well, that's interesting," and went out. [Laughter]