John Wheeler - Session IV

GOING TO COPENHAGEN

Absorption of radiation by matter, by atoms of this, that, and the other atomic number, was a topic that I talked about more than once with Breit and some of the people around Breit while I was at New York University in the year 1933- 34. I remember somehow getting onto some of the subtleties of that problem with Harvey Hall. I think maybe then I spoke to him about how it would be great to be able to talk about that with Neils Bohr. It was either in that connection or some similar connection that Harvey said to me, "One has to be a mature physicist before one can get much out of being with Bohr," as if to imply that I wasn't yet in that category.

I can't recall ever formally discussing with Bohr before I went to Copenhagen the idea of going to work with him there. I suppose I must have written him because I would think there had to be some statement of willingness to accept me before the National Research Council would validate my fellowship for overseas. One of the things about validating a fellowship for overseas was something special: It was assumed that the Fellow would not be married. That was part of the deal. I'm not sure if I had the OK from them to go to Denmark whether they might not also have given, if I had put it up to them, the OK to get married on the fellowship.

At any rate, I went. I can well believe that Breit had to write some letter to Bohr about me. I assume that would have been easy because I suspect he had to write a letter about me for the fellowship renewal. I don't know that there was anybody specific I asked about the idea of going to work with Bohr. I got the impression when I mentioned this—what intrigued me—that everyone thought it would be great. It would be a little like going to work for Michelangelo if you were a sculptor. It would be the thing to do.

Reasons for choosing Bohr? I feel so grateful to that book of Henrick Anton Lorentz on problems of modern physics, his 1927 lectures at Caltech, so grateful because it put the quantum and relativity in central place. I did not expect to learn anything about relativity from Bohr. I did not think of that as a negative. The quantum was exciting enough, and issues of modern physics were exciting enough.

WORKING WITH BOHR

It took some little doing in talking with Bohr to get to the point that the issue at hand was fully developed, before he was charged up. Until that point was reached, he would utter words like, "That's beautiful" or ". . . interesting[?]."But then, after he got going, if I didn't respond, the whole discussion would die down. He depended so much on dialogue in making his advances in physics. I recall once, stopped for a traffic light on my bike, this car came up. There I could see Bohr and his son talking back and forth. That was the way he liked to operate—dialogue, back and forth, discussion.

There were some people in the world of physics who tended to talk in an authoritative way, not waiting for the give and take of discussion. There were two occasions I know of where that led to a negative reaction. In one case, the physicist, who had come to work in Copenhagen, got steered by Bohr to go to Hamburg to work with other people interested in magnetic resonance. These cases were people who did not fit in.

I do not know how much was the product of discussion between Niels Bohr and his wife Margrethe. I have a feeling that she knew a lot more about what was going on than she ever let on in any conversation. I remember her saying to me when I remarked about something or other, "Yes, Niels has so many ideas, and so many of them were then published by other people." I never heard him say a single word about such things.

The Thomas precession, that was worked out by L. H. Thomas at Copenhagen: The effect had been a great puzzle. I don't know, but I can well believe that Bohr asked the key question there that opened up the subject. It had to do with relativity. I would have to go back to look at L. H. Thomas's paper to see if he speaks about any influence of Bohr on that point.

PHYSICS IN COPENHAGEN - COSMIC RAYS AND PENETRATION OF PARTICLES THROUGH MATTER

His [Bohr's] work with Rutherford's group at Manchester on the penetration of charged particles through matter [made it possible] to deduce the energy of an alpha particle from its penetrating power—5 cm of air, or what not. That was a central topic in those days, when magnetic spectrometers weren't available to make a direct measurement of energy. To use every approach that one could think of was natural. This was the time in which Bohr dreamed up the method of the equivalent field. That is to say, the charged particle when traveling through matter travels typically some distance away [from nuclei - transcript here interrupted by a phone call].

At any rate, Bohr realized that these what one might call distant collisions were the vital ones in transferring energy out of the alpha particle to the matter through which the alpha particle was penetrating. He conceived the idea of analyzing the effective field of the traveling particle—its electric field—into equivalent radiation in the sense of a spectrum of photons. Then one could use the known absorbing power of matter for radiation as a way to calculate the energy taken up by the air or the material that the alpha particle is penetrating.

This method of equivalent radiation he was applying to new problems when I arrived in Copenhagen. Already there at the Institute was someone whose name I don't remember now [later recalled to be E. J. Williams]. He and Bohr were concerned with this issue of the penetrating power of the cosmic rays. Bruno Rossi and Rossi's collaborators had observed that the cosmic rays could go through a 10-cm slab of lead. How could a particle have such a high penetrating power? An alpha particle from a radioactive substance going through 5 cm of air, the equivalent of five-thousandths of a cm or less of lead, an unbelievably small distance, [is stopped].

I never heard mentioned in connection with that work the London International Conference on Physics of October 1934 [said 1933 on tape, but 1934 meant] although this penetrating power of cosmic rays was there a central topic, in many ways the most vital issue of the whole conference. At that conference, Bethe had reported on his calculations of what the existing electron theory of Dirac would say about the penetrating power of an electron through air—the electron going through air, passing an atomic nucleus, experiencing a sudden deflection, and in that way being caused to radiate, and the energetic photons it radiated going on to create new particles in the air below, so that those new particles then went on and repeated the history of the incoming particle, on a lesser energy scale. So here there was a multiplication process giving rise to an electron shower.

Who had been the first to study electron showers? That I don't know. The cosmic rays, I came gradually to realize, made a romantic subject, all the way from Hess m. g a balloon flight and finding radiation increasing in strength as it rose. And Marcel Shein at Chicago with his measurements, and Arthur Compton and the great debate between Compton and Millikan—whether these cosmic rays were particles or photons, Millikan arguing for photons and Compton arguing for particles. So it became so important for Compton to go to places on the earth with very different magnetic field so that one could see from the ability of these particles—this radiation—to get through the magnetic fields surrounding the earth that they must be charged particles rather than photons. This latitude effect, so called, was a romantic feature of the subject. One might say it was also romantic that the antagonism somehow was focused at Chicago, because that's where Marcel Shein was working, that's where Compton was, and that's where Millikan had been before he went to California Institute of Technology.

Millikan was such a forceful character, dynamic. I recall his having a little black notebook, and as he went to a Physical Society meeting, it was explained to me—I can't recall whether it was by Millikan himself or by somebody who knew him—that he was making notes on promising people. He wanted to make Caltech a place of leadership. He had missed out on Einstein, because he thought he had Einstein safely lassoed to go to Caltech, only Abraham Flexner got in and won out in the bidding.

Then other romantic feature of the cosmic ray subject was the Italians, Bruno Rossi and others who worked with him or independently, taking their equipment up in the Alps to find the effect of altitude. Still another fascinating feature was the fact that I learned only later, that the cosmic ray intensity at Denver is such that people there get a dose of radiation which is more than many people at sea level were prepared ever to accept.

THE 1934 LONDON CONFERENCE (AND HUNGARIANS)

When I got to Copenhagen, I had about two weeks to go around and get an apartment. Then I took the boat train [to England]. That's when I first found myself surrounded by Hungarians. In all my life, I seem to have been directed where to go and what to do by a triangle[?] of Hungarians. On board that train was Hevesy, who told me about his interest in heavy water and an experiment that he'd done, drinking heavy water to see if it had any effect on him—something that got a big play in the newspapers in Copenhagen. And then there was Edward Teller and his bride Mici—they'd just been married.

MORE PHYSICS IN COPENHAGEN

E. J. Williams, when I arrived in Copenhagen after the London conference—my second arrival in Copenhagen, but my real arrival—was working with Bohr, applying this method of equivalent radiation field to analyze the very processes that Bethe had talked about in showers, because there was a great cloud hanging over the work of Bethe in the words of Oppenheimer a few months earlier that you couldn't believe electron theory above an energy of 137mc2, 60 or 70 million electron volts. Why that figure? I would have to go back to see if Oppenheimer ever wrote down anything on this, but if you are going to question electron theory and say it's got to break down somewhere, that was the only number you could pull easily out of the hat as a place where it would break down—137mc2.

NEW PARTICLES

K: Was there any thought of a new particle in 1934?

The advent of the neutron in 1932 had made a big impression on everybody, and you've probably seen the letter of Pauli where he proposed the neutrino (what came later to be called the neutrino), the letter that begins with the words, "Dear Radioactive Ladies and Gentlemen."

The cosmic rays moved us all into a realm of energies and therefore, by translation, into a realm of particle masses much greater than physics had ever dealt with before, to the best of my knowledge. It would have been impossible, I think, for Anderson to have confidently claimed the existence of a new particle if it had not been that these arguments about penetrating power were so firmly established. If one had doubted one's knowledge about energy loss, it would have been conceivable to try to interpret Carl Anderson's findings in a cloud chamber at Caltech as proof that particles of radioactive substances penetrate, but the positron [...] It would be a fascinating thing to work out the scenario of the might-have-beens if it were not clearly established that high-energy particles lose energy by radiation and shower production.

K: Was there any reverse logic? That is, the acceptance of those penetrating-power calculations supported Anderson's experimental finding and the proposal of a meson. But was the reverse true, that when you were there the penetrating-power calculations made people suggest that perhaps there must be a heavier particle, prior to the experimental evidence?

I can't recall the idea that there must be another particle there coming forward at Copenhagen. I think it really came from Caltech, but it was certainly taken up at Copenhagen, because that's where the "moral authority" really lay through this method of equivalent radiation field. A wonderful thing about the method of the equivalent radiation field was that the argument depended (a) on relativity, which nobody could fight against, and (b) on the response of matter to the lower frequencies in the equivalent radiation field. So one had got himself clean away from the problems and the doubts that had been expressed about high energies into a realm of certainty: (a) relativity and (b) response of matter to low-frequency radiation. Without those arguments, the Caltech evidence would have lost its punch.

But were people looking for more particles? I've just been reading about Yukawa and his idea of two kinds of mesons. This brings us to a period after Copenhagen, but where the Copenhagen considerations were central. There was the work of Powell at Bristol using photographic emulsions to record cosmic rays, and the perfectly marvelous discovery of two kinds of mesons instead of the one that had been talked about before.

THEORIES OF NUCLEAR FORCE

I must say that the concept of meson theory of nuclear forces seemed to me like an evangelical religion with a theme that I found it hard to believe. But those who took it up had a faith which was beyond understanding. I just read a tribute to Marshak, following his death a year ago. That's where the meson theory of nuclear forces found an enthusiastic disciple. I'm afraid somewhere I wrote a statement about this that was making a little fun of it, the meson theory of nuclear forces. It's probably somewhere in that business about Men and Moments.

I'm trying to recall another current of ideas and how it made out at Copenhagen at this time. I'm referring to Heisenberg's theory that the force that binds one nucleon to another arises out of exchange of neutrinos between the particles—a neutrino theory of nuclear forces, which had some of the flavor of the pi meson theory of nuclear forces, but more difficulty. It was about this time that Wigner came forward with the idea of the neutron and the proton as two states of one particle. just as there can be a spin for a proton that is up or down, so here this characteristic could be one that gave one a proton or one that gave one a neutron.

K: I had previously attributed that idea to Heisenberg. But it was actually Wigner's idea, the idea of a two-state nucleon?

Well, that I would like to know. I have to find out. I can well believe that Marshak in his book on the subject has gone into that.

HEISENBERG

One paper that I learned about at Copenhagen that I have never read is a paper Heisenberg wrote when he despaired about the negative energy states of the election and how one would ever fit them into physics. An argument either somehow that they don't exist or that they never come into play. But then after the production of pairs was observed, he had to give that up. So I never heard him or anybody else push his ideas. Heisenberg came to Copenhagen about Easter time of 1935. I can recall a break in the several-day-long conference of the kind that Bohr was accustomed to have once a year at Copenhagen. A break in the conference when we were all walking outdoors, and in the garden in front of the Institute building, I found myself walking beside Heisenberg and asking him questions. But I cannot for the life of me remember what [questions].

THE COMPOUND NUCLEUS OTHER NUCLEAR MODELS

I speak of Easter time conference, It must have been a little before Easter time, because it was at Easter time that Moller went to Rome and learned about the experiments of Fermi, and came back full of excitement about them. But it was also occasion for much puzzlement, both in Rome and Copenhagen. That's when I remember Bohr getting up and interrupting Moller and walking up and down in back of the lecture table before the blackboard with his head down, pondering over what Moller had said, saying over and over, "Now it comes. Now it comes. Now it comes." Suddenly the idea did come out, of the compound nucleus.

It was shortly after that that Bohr was going to London—or at any rate to England—giving a lecture. If I understand right, he took along a model where you could see a marble roll into a little tray full of marbles, the particles trading energy and some particle coming out totally different from the one that went in. The compound-nucleus model of nuclear reactions. I thought that at that time he had also mentioned the liquid drop, but Peierls thinks not. He thinks it was not till fall that Bohr got onto the liquid drop as a special instance of a compound nucleus.

There was such a nice old gentleman who came to Copenhagen some time during the year—what was his name? Was it Rubinowicz [1, who had worked in spectroscopy and who, in an earlier day, had been briefly at Copenhagen? He kindly invited me to visit his house when I was in Warsaw after the war for a physics conference.

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 alyze 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]