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William A. Fowler’s Interview

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William A. “Willie” Fowler was a Nobel Prize-winning physicist at Caltech, who knew J. Robert Oppenheimer from before the war. In this interview, he talks about how Oppenheimer and his “school” of students and post-docs would travel each year from the University of California, Berkeley to Caltech, where Oppenheimer had an appointment on the faculty. He describes how Oppenheimer’s theoretical knowledge and perspective supplemented the experimental research being conducted at Caltech, including Fowler’s own. He also talks about the lives and careers of other physicists who interacted with Oppenheimer in Pasadena, including Charles Lauritsen, Richard Tolman, and Robert Millikan. The interview concludes with a discussion of Fowler’s friendship with Frank and Jackie Oppenheimer.

Date of Interview:
March 29, 1983
Location of the Interview:


Martin Sherwin: This is Martin Sherwin, I’m on my way to interview William Fowler at Caltech in Pasadena. Today is March 29, 1983.

William A. Fowler: Yes, I came to Caltech just fifty years ago in 1933 as a graduate student, and I have been here ever since. I am now the oldest graduate student at Caltech. I got to know Robert immediately, because he, in those days, had a joint appointment at Caltech and at [the University of California,] Berkeley. He would come down to Caltech when the Berkeley season ended. Berkeley started early in those days, in August, and ended in April, I guess. That’s about the time that the third quarter here at Caltech. So Robert and all of his students and post-docs would come down.

I took courses from him. Then in addition, my work in this laboratory, the Kellogg Radiation Lab, which had been built just two years before I came by Charlie Lauritsen, who was my professor. We were engaged in nuclear physics, and that was Robert’s great interest in those days. I saw quite a bit of him, because he was really interested in and involved in what I was doing as a graduate student.

Sherwin: What did you do your dissertation on?

Fowler: It was called “Radioactive Elements of Low Atomic Number.” In those days, if you had an accelerator, you went in one morning and bombarded some target and produced a radioactivity and measured the spectrum of the electrons or positrons that it emitted, measured the half-life. It was the initial studies of what we now call the weak interaction.

Let me say first that although he was not officially the supervisor of the theoretical aspects of my work, in actual practice, he was. Charlie Lauritsen was an extremely good experimentalist, a great experimentalist. But he left the theory of what was done largely up to Robert Oppenheimer and to a certain extent to Richard Tolman. The three of them were very, very good friends.

I eventually became, I think, a close friend of Robert’s. Although I was always younger, and I think I amused him. We had such entirely different characteristics, both in background and in behavior.

Sherwin: How would you describe those differences?

Fowler: I would say the simplest thing was, Robert was much more serious about things than I was. He also thought much more deeply about the significance of physics, both as a science and as a sociopolitical force.

My whole life, I haven’t taken things all that seriously. I enjoy life. So there was that difference. I was a Middle-Westerner from a small community in Ohio. Robert had a rich cultural background, which I did not have. So we were quite different. 

Sherwin: But you got along?

Fowler: With Robert, I never got into any deep arguments with him, in the way that many people did, as you must realize. I have always considered myself a liberal, always voted the Democratic Party ever since Franklin Delano Roosevelt was the first man I got to vote for. Robert and I, I would say, had a very friendly relation. I have always felt that he was somewhat amused by me, by anyone who would be called “Willie,” such as I am, as you must know. But I didn’t mind that, and I suppose I really revered Robert.

Contrary to what many people will probably tell you, I found him—at least in private conversations with me, in particular when we were discussing the implications of what I had done in my thesis, and then as I became a graduate student and an assistant professor and so forth—he was always very patient with me, very patient. Would go to extreme lengths to try to make sure that I understood what he was driving at.

Now, it is true, when there were other people involved, Robert could be—I guess sardonic is the best word. But not always. With Charlie Lauritsen and Richard Tolman, he had the really most remarkable relationship, I feel. Of course, they were both older than he, whereas I was younger, you see. But I could just tell that, in the same sense that I revered him, he revered Lauritsen for being such a fine experimentalist and such a really great man, and Richard Tolman for being a wonderful theorist and a very great man. I have frequently thought to myself that my relation with Robert was somewhat different than what I surmised the relationship of other people to be, although one never really knows.

But you asked me what was my thesis, and that’s a good indication. My thesis was to build a cloud chamber. Here’s some pictures that were taken with it later, much later, on the lower left there. The cloud chamber had a magnetic field around it. We bombarded a target that was inserted in a thimble in the roof of the cloud chamber, a thin walled thimble so that the electrons or positrons that were produced during the bombardment, then you would shut off the bombardment and watch the positrons or the electrons emerge from the target.

The cloud chamber had a magnetic field perpendicular to it, so the electrons and positrons were curved one way or the other so you could tell which was which. Furthermore, from the known magnetic field, which you had to measure, and from the radius of curvature of the particles, you could figure out their energy. You could count how many fell within a certain energy. If we got the spectrum, which is a continuous one, it was a great puzzle until [Wolfgang] Pauli came up with the idea much later of the neutrino. No, I shouldn’t say much later, just slightly earlier [in 1931].

We knew about the neutrino and we understood why the spectrum should be continuous, because the electron or the positron had to share the energy with the neutrinos. But the end point that we could measure, the highest energy, was the full energy available in the transition. It was the case when the electron or positron got it all, and the neutrino got none. That told us the difference in energy between the parent nucleus and the daughter nucleus. The nuclei that we produced were called “mirror nuclei.”

Sherwin: Mirror?

Fowler: Mirror, M-I-R-R-O-R. We can produce nitrogen-13, for example, which has seven protons and six neutrons and is radioactive. Decays to the stable isotope, the heavy isotope of carbon, carbon-13, which has six protons and seven neutrons. The fact that one is seven/six and the other is six/seven, we came to use the term mirror, mirror nuclei.

Sherwin: I see.

Fowler: Well, we studied carbon-11, which goes to boron-11; nitrogen-13, which goes to carbon-13; oxygen-15, which goes nitrogen-15; and fluorine-17, which goes to oxygen-17. I found that the maximum energy, namely the difference in energy between the mirror pairs, increased.

It was Robert who pointed out to me, “Willie, that’s quite natural. That difference is just the Coulomb energy difference. The parent has more protons than the daughter, and so its Coulomb energy is greater. What you’re showing is that the nuclear forces between a pair of protons is the same as the nuclear forces between a pair of neutrons.” This is called the symmetry of the nuclear forces.

With this method, you got nothing about the force between a proton and neutron. But since there are the same number of pairs of that on both sides, that just cancels out. He pointed out to me that my results meant that the nuclear forces were charge-symmetric. People had suspected that theoretically, but it was, I think it is fair to say, the first experimental proof that this very important facet of nuclear forces was the case.

And that continued. I got my degree in ’36, and became a post-doc until ’39. Then the war started, we went into defense work. Then when we came back after the war, Robert was here. He did not go back to Berkeley. He came to Caltech for a short time after the war, but then he was invited to become the head of the Institute for Advanced Study in Princeton, and much to my despair, he left. But we all realized that it was really what he wanted to do. There was no doubt about what he was going to do.

There was always this very close association between what we were doing here in the laboratory and Robert more or less serving as our theoretical mentor. It meant that what we did always turned out to have a significance that I do not think either Charlie or I would ever have come up with. In that sense, I have always felt a great debt to Robert.

You see, nuclear physics was new. Well, you could say in a sense we started in ’30. There had been nuclear radioactivity for a long time, all the alpha and beta and gamma rays that [Ernest] Rutherford and everybody—

Sherwin: ’32 would be—

Fowler: But ’32, you see, was when [Carl] Anderson discovered the positron, [Harold] Urey discovered deuterium [misspoke: 1931], [James] Chadwick discovered the neutron. And perhaps even more important from a practical standpoint, [John] Cockcroft and [Ernest] Walton showed that with a low energy accelerator, you could disintegrate the light nuclei.

Just before I came—that was discovered in ’32—Lauritsen had been doing X-ray research in this building, and in what was then the old High Potential Laboratory. It’s now the Sloan Laboratory, right next door. His X-ray tubes were powered by alternating-current transformers, so the electrons were accelerated on the appropriate part of the cycle. In order to get positive ions, like protons, deuterons, or alpha particles, all he had to do was put in an ion source instead of a filament that gave off electrons, and these positive ions were accelerated on the other part of the AC cycle.

By the time I got here, Lauritsen and Dick Crane, who was one of his graduate students at that time, and the one who worked most closely with Charlie. He later on became professor of physics at [the University of] Michigan. You really ought to talk to Dick Crane.

Sherwin: I’m going to be in Michigan next week.

Fowler: Right. You should see H. Richard Crane, because he can also tell you things about Robert. I do not think that Dick was ever as close to Robert as I got to be, but it is awfully hard to know. One is so self-centered [laughter] that it is really hard to know how much interaction there was between Dick and Robert. You can find out from him.

Another example was, in the midst of our work, Charlie and I were studying gamma rays during prompt reactions. They are called “capture reactions.” The incident particle, like a proton, is captured by a target like fluorine. The thing radiates radiation. Also sometimes the alpha particles are given off. That leaves oxygen-16 nuclei, and the oxygen-16 nuclei can be left in excited states. This is a long story.

But what we found was that in addition to the gamma radiation, which came when these excited states of the oxygen cascaded down to the stable ground state, we found that electron/positron pairs were emitted directly from the target. When the gamma rays would hit, say, a metal foil that we put in the middle of the cloud chamber, they would produce electron/positron pairs, which you would see as a kind of a V-shaped thing. But the gamma ray would not make any track coming in. Then we saw these darn things coming right out of the target.

It was Oppenheimer who explained that to us. It turns out that the first excited state of oxygen has the same spin and parity: zero spin, plus parity as the ground state. And photon emission—gamma ray emission—is strictly forbidden.

Sherwin: Now, was he able to explain this because he was a theorist? Did Tolman look at it also?

Fowler: Well, Robert knew all of the rules that governed nuclear transitions, and applied them much more closely than Tolman ever did. I am sure that Tolman knew the rules, because they are essentially the same as in atomic physics. Except in atomic physics, in those days at least, practically all the radiation was electric dipole. In the nuclear regime, where the energies are higher, you get magnetic dipole and electric quadrupole, the whole gamut. But electric monopole, that can’t occur.

Robert immediately realized that what we were doing was producing a state that could not radiate photons, and it turned out to have six million volts of energy. So it had more energy than necessary to create the rest mass of an electron and positron, which adds up to one million volts. And so it produced pairs. If you are below a million volts, what you get is two-photon emission, but it is very rare—very slow, I should say. Once the electron/positron emission becomes possible, it just swamps the two-photon emission. 

Oppie knew that. He had Julian Schwinger as a post-doc, I think, at that time. So Oppenheimer and Schwinger wrote a theoretical paper, which went along with our experimental one. It was a constant interaction before the war. Of course, after he went to the Institute, then it petered off. We brought one of his students here, Bob Christy, who is Professor Robert Christy, whom you must talk to, because Bob Christy was much closer to Robert than I, I am sure. So we brought Christy here, and then Christy played the role that Oppenheimer had, and did a wonderful job. He is also an extremely smart guy.

Sherwin: Christy was his student where, at Berkeley in the ‘30s?

Fowler: Yeah, and he may have been a post-doc.

Sherwin: After the war?

Fowler: One never knew which of the ones that came down were Robert’s students or whether they were his post-docs. That was one of the most amazing things. When he would come down in the spring, he would bring his school.

Robert was the first one in the United States to have a school of theoretical physics. That is the best word for the fact that he had around him a lot of young people. Nobody else did that sort of thing. It was just unheard of in the United States. It was common in Europe. But Robert started the first school of theoretical physics. There were people like Bob Serber, Willis Lamb—

Sherwin: Phil Morrison.

Fowler: Yes, Phil Morrison, Leonard Schiff, Julian Schwinger—I could go on—George Volkoff, Hartland Snyder. They would all come down and to this desert. Caltech was a desert in theoretical nuclear physics, except when Robert and his gang were there. It made a terrific difference. Tolman was only interested because Charlie was doing nuclear physics and Robert was explaining it.  Of course, Robert also got onto neutron stars, and Tolman did a great deal of work on neutron stars. In fact, if you look at the famous paper by Oppenheimer and Volkoff, which first showed—

Sherwin: Was this the black hole paper?

Fowler: No, it’s the neutron star paper.

Sherwin: Neutron star paper.

Fowler: Neutron star paper, which showed that the maximum mass of a neutron star, if you did not put in nuclear potentials, was seven tenths of a solar mass. Well, that’s right. Now we know that the attractive nuclear potentials, or the repulsive nuclear potentials, can increase that maximum mass to somewhere between two and three solar masses. But Volkoff and Oppenheimer did the first correct calculation, all the general relativistic details done correctly.

Tolman worked very hard on the problem. He tried to use standard polytrophic structure and got quite sensible answers. But the elegance and beauty of what Oppenheimer and Volkoff did just convinced everybody. Then Snyder and Oppie did the collapse to the black hole. That was Hartland Snyder; that was his second paper.

Tolman was fascinated by that, but he also was interested in the nuclear physics, just because Charlie was doing it and Robert was explaining it. I was also pretty close to Richard, insofar as a younger man can be with an older man. I used to go over to his [house]—he lived right over here. He lived where the Bachers—

Sherwin: Yeah, I’m going—

Fowler: That home was the Tolman home, and I used to go. There is a beautiful study in the back. I used to go and talk to Richard in his study. He was an incredibly wise fellow. I remember he was working on his book. I took his course before his book was actually published, and I was fascinated by his course. It was on relativity. I would go ask him questions, and we were friendly enough that he would let me. That may have been when I was a graduate student, too.

But anyhow, I had kind of special privileges with Richard and Robert, because I was working for Charlie and they were such good friends with Charlie Lauritsen.

Sherwin: Were you involved enough to help me to describe that friendship and that relationship, and how it evolved, what it included, the kinds of things they did together? Were they, in addition to being intellectually involved with each other—Robert was single, Tolman was married, and Lauritsen was married.

Fowler: Married and had a son, who was also very close to Robert, but they are both dead now. I would say that socially, Robert was much, much closer to Richard. Lauritsen was not much of one for social affairs, with one exception. He had a large group of graduate students. There were five or six when I was a graduate student. We had a seminar every Friday night.

The first year I took it, we used [Radiations from Radioactive Substances by] Rutherford, Chadwick, and [Charles] Ellis, a big thick book on natural radioactivity which had just come out [in 1930]. Each graduate student had a chapter, one week after another, and we would all get up and describe what was in the chapter and then people would ask questions.

But then invariably, after the seminar we would go to Charlie’s home. [His wife] Sigrid Lauritsen would serve beer and something to eat. Charlie just loved that. He usually drank hard liquor. But he loved that. His social life was with his students. I think that Charlie and Robert were close mostly on the intellectual level.

Sherwin: How much older was Charlie than Robert?

Fowler: Oh, my goodness. I would think Charlie was about ten years older, at least ten years older, than Robert [actually twelve years older]. Charlie was fairly old when he came here as a graduate student.

Sherwin: And he stayed also?

Fowler: Yes. But he had had quite a career as a radio engineer before he came here. I think it was the Atwater Kent Company, or maybe Collins Radio [actually Kennedy Radio Company]. [Robert] Millikan came to St. Louis, where they were located at the time, and gave a talk on cosmic rays, I guess. Charlie said, “That’s for me.”

So he packed up Sigrid and baby Tommy and came out here. He got into graduate school, although he did not meet any of the requirements. But Millikan and [Earnest] Watson realized that he was a very capable and dedicated man. He got his degree with Millikan on cold emission, and then, as I said, he went into X-rays. Then eventually, by the time I got here, he was into nuclear physics.

Sherwin: Now, I have heard—let’s put it this way—that there was less enthusiasm on Millikan’s part for Oppenheimer than on the part of other people at Caltech. That’s even put euphemistically.

Fowler: Well, you have to realize that Millikan was a deeply religious man. He was very, very Republican and conservative. He built Caltech by getting money from wealthy, conservative people. Robert’s radical views and his style just did not match Millikan’s.

I can believe that Millikan might not have been very attracted by Robert personally. But he went out of his way to make it possible for Oppenheimer to have this full professorship at Caltech on a part-time. That broke all the rules, and it was Millikan who had to do it.

Sherwin: You mean after the war?

Fowler: No, no, before the war, this arrangement. That he could be an associate professor or even an assistant professor without being fulltime. That’s against the rules.

Sherwin: Yes. I see.

Fowler: And it was Millikan who did that. Now, it was probably after a great deal of persuasion by Lauritsen and Tolman, who realized that Caltech needed a man like Oppenheimer.

In those days, nuclear physics was new, it was all the rage. The other theorists at Caltech—the other theorists at Caltech in the physics department—just could not have cared less. It was extremely important, if Caltech was to do any significant experimental work in this area, that it have a theorist. So Millikan supported Robert, and I would guess paid him rather more than he paid most of the faculty, as you have probably heard. So, no, it is perfectly true that Millikan didn’t cozy up to—well, that is a terrible term.

Sherwin: Now, somebody said to me that [Millikan]—and I could not pin down the dates—but in fact one quotation is, gave Robert “insulting raises,” and things like that. But that must have been later on.

Fowler: But look, Millikan did that to everybody. He did that to everybody. Some people he would not pay if they had private funds of their own. He felt that working at Caltech was the greatest thing in the world, and that you should not have to be paid.

The greatest thing was when [Lee] DuBridge came here and we all got a raise from $5,000 to something like $10,000 a year. Millikan, that was dictated by the fact that he ran this place on a shoestring, really.

Sherwin: It wasn’t until after the war that things really—

Fowler: Oh, the war changed everything, of course. But boy, in the Depression, he got all the faculty to take a 10% cut, just before I got here, I remember. I had friends, Graham Lange in the English Department, he just hated Millikan, because Millikan paid them such small salaries in the humanities. He says, “You’re at a technical school and you’re to teach scientists and engineers, and you don’t deserve the same salary as the guys in physics and chemistry get, or biology.” But he had to do it in order to make the place work.

I remember Millikan once said to me something derogatory about Robert’s personal habits: the fact that his fingers were so brown from smoking, you know, and the fact that Robert put pepper on everything. Millikan just abominated that. I don’t know whether people told you that—

Sherwin: Did he ever say anything about Robert being Jewish?

Fowler: Never to me. I would not think that that would be a fair criticism of Millikan. It is true he came from a deeply religious family, but I never saw on his part in the whole time any prejudice against Jews, frankly. But, you see, he brought [Paul Sophus] Epstein here.

Sherwin: No, it’s that I just—

Fowler: I would say that anyone who tells you that Millikan had prejudice against Jews is just dead wrong. Now, you have to realize that another person that I revered was Millikan. We called him “The Chief.” We knew if we had an ounce of brains in our head that he was the man who got the money for us, that he was the man who made it possible for us to get paid for having fun. I remember I complained sometime or other to Charlie and he says, “Oh, stop complaining, Fowler. What if you had to work for a living?”

That was just the way we looked at it, really, and Millikan made it all possible. Without him, this place would have been nothing, in my book. It is true [George] Hale played a great role and [Arthur] Noyes played a great role.

Sherwin: But Millikan was the center.

Fowler: Millikan was the man who made it all possible. Tou know, he tried awfully hard to get Einstein here. Oh, my goodness, in ’31, ’32—I forget the exact dates—just before Einstein, I guess, left Germany and came to the Institute for Advanced Study, Millikan tried very hard to get him to come here. But I think the most he wanted to pay Einstein was $5,000 a year. Einstein was going to get $10,000. This was before the war, mind you, when other people were getting $3,000 as full professors. Graham Lange was a full professor in 1938 and he got $3,000 a year.

Sherwin: The story I have heard with Einstein is that when they asked him what kind of salary, he asked for something extremely modest. They just did not pay attention to it, and they doubled it or tripled it. It still was not anything spectacular.

Fowler: Well, that’s not the way I have heard it. It is probably true that Einstein’s main reason for going to the Institute for Advanced Study rather than to Caltech was that he did not particularly want to teach formal classes, and he would have had to have done that here. But my understanding was that there were considerable negotiations in which Einstein felt that what Millikan was offering him was not really fair, even though Einstein himself was not asking for a great deal. But Millikan again just didn’t have the money, probably.

Sherwin: Now, how about the politics. Back in the early ’30s, I suppose there wasn’t very much activity, but certainly by ’36 and the Spanish Civil War? Now, Frank Oppenheimer was here by ’36, wasn’t he?

Fowler: Yes.

Sherwin: Did you know him, too?

Fowler: Yes, I knew Frank. Frank was a student in this lab, too, so during the time I was a post-doc, I forget, Frank must have come about ’36, ’35? Yeah, well, I got my degree in ’36, and he went right to work in this lab. He worked right in the room below here. I got my degree and I was a post-doc and so I got to know Frank and his wife, Jackie, very well. In fact, Jackie and I wrote a paper together, because I had a lot of data on those cloud chamber pictures I took that could be dug out.

Sherwin: She was a social worker, wasn’t she?

Fowler: Well, I guess she was, but after she married Frank, she wanted to do something. I let her measure these tracks, as we called them—

Sherwin: Oh, I see.

Fowler: —in the cloud chamber. Then she and I wrote a paper. Her name was Jaquenette, Jackie, and so there was W. A. Fowler and J. Oppenheimer. Many people always thought I wrote a paper with Robert, which I did not, it was Jackie. So I got to know Jackie and Robert and Frank. Frank had a Cadillac touring car. I used to borrow it from him.

But, I had always considered myself a liberal, although my father was very Republican, but the Depression made a Democrat out of me. 

I realized that both Jackie and Frank were considerably more liberal than I, and that they had associations that I just did not have time for. But it never entered my mind that they might have members of the Communist Party. That just never occurred to me.

Sherwin: Which they were.

Fowler: Which, it turns out, that they were. On the other hand, I am pretty sure that Robert never was. But Frank and Jackie were far more outspoken, at least to me, about their views than Robert. Robert always seemed to be, to me, rather ambivalent. He had such a strong sense of the cultural background of things. My feeling is he never went as far in his points of view as Frank and Jackie did.

Not that I criticize Frank and Jackie whatsoever. That was their business. I take a dim view of what happened to Frank, although he certainly turned the tables on people. This Exploratorium that he has built single-handedly up near San Francisco is a great achievement. Just nothing has pleased me so much as [that]. Because I was devastated when Frank was fired from [the University of] Minnesota. And then he went—

Sherwin: Colorado.

Fowler: —to [the University of] Colorado, and eventually he had to leave there.

Sherwin: I think he left there. The story they told me was that he said to Jackie, “You know, I don’t want to be buried here.”

Fowler: Yes, well, then for a while he had a ranch in Colorado.

Sherwin: Yeah, and then he got into the high school system, and then Ed Condon got him into the University [of Colorado].

Fowler: Yeah, well, you know more of the details. I went to see him in Pueblo, [Colorado] it was down near Pueblo someplace. There he was raising a herd of cattle, and Jackie was working hard. It was just fabulous.

Copyright 2005 Martin J. Sherwin. This transcript may not be quoted, reproduced, or redistributed in whole or in part by any means except with the written permission of Martin J. Sherwin. Rights granted to Atomic Heritage Foundation.