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Oral History Transcript — Dr. Robert Serber

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Interview with Dr. Robert Serber
By Charles Weiner and Gloria Lubkin
At Columbia University
February 10, 1967

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Robert Serber; February 10, 1967

ABSTRACT: Engineering physics at Lehigh, 1926–1930; graduate work in physics at
Wisconsin, 1930-34; Ann Arbor summer school, 1934; reputation and major interests of theoretical group at Berkeley, mid-1930s; nuclear force studies; migrations of Berkeley theorists to Caltech; major discoveries during the 1930s, communication through journals; interaction between Berkeley theorists and experimentalists during the '30s; influence of cosmic' ray and astrophysics research on nuclear physics; beta decay; betatrons and synchotrons, pre- and postwar; significance of fission; contributions of war research to nuclear theory and techniques; end of war planning for high energy accelerators; mission to Hiroshima and Nagasaki, 1945; accelerator improvements, straight sections, and phase stability, mid-1940's; effect of higher energy experiments on nuclear structure theory, postwar to early 1950s; development of the optical model after 1949; the stripping reaction; motivations for shifting into particle research in early 1950s; reactions to the revived shell model; collective model; leading centers and scientists, and major discoveries, 1945–50; development of scattering theory and manybody theory.

Transcript

Weiner:

I'd like to start off by asking you, Professor Serber, about your first interest in physics. You took a Bachelor of Science, and completed it in 1930, and it's not at all clear from the information I have whether it was in Physics.

Serber:

Yes. I went to Lehigh, which was an engineering school, and started out as a general engineering student, and Lehigh had just started a program called Engineering Physics. I guess at the end of the freshman year you specified what branch of engineering you wanted to go into, and I went into the Engineering Physics section; that concentrated very much on physics. Partly because it was a brand new discipline and there weren't any real rules, you could do pretty much what you wanted. So I took mostly physics and mathematics courses at college. Of course it was the height of the depression then and jobs were awfully scarce, but I managed to get a teaching assistantship at Wisconsin.

Weiner:

In the Physics Department?

Serber:

In the Physics Department.

Weiner:

In your physics work at Lehigh, were you aware of the new quantum mechanics? Did this get introduced into the teaching at all?

Serber:

Not to any particular extent. I don't remember anything about it at Lehigh, as a matter of fact we had just standard courses: electromagnetic theory and light and sound. There was still a rather old fashioned training at that time.

Weiner:

Apparently you did well enough, at least to get an assistantship at Wisconsin in Physics. This wasn't the routine procedure, was it, considering that it was the depression period?

Serber:

Well, I must have had some good letters of recommendation, but that was just on the basis of the course work at Lehigh not on anything to do with modern physics.

Weiner:

Was there anyone in particular there who was really the strong man in the Department, or who had any special influence?

Serber:

Bidwell was the Chairman of the Department and I took some courses from him. I didn't have any particular personal contact with him beyond that. I don't think there was anybody really outstanding there.

Weiner:

When you applied for the assistantship at Wisconsin and received what was the source of the funds, do you know? Was it from university funds, or some grant?

Serber:

No, it was purely a teaching assistantship, and I earned $800 a year from the University.

Weiner:

They gave you $800?

Serber:

Yes.

Weiner:

And what teaching did you have to do?

Serber:

It was almost all teaching laboratory sections. As a matter of fact I remember that the first class ever taught, there were two sections of the same laboratory at the same time, and the instructor for the other half was Lee Haworth—I think it was the first course he ever taught too!

Weiner:

Did you work under any specific man in your early days there?

Serber:

Yes, I worked with Van Vleck the whole time at Wisconsin. He was in England the first semester I was there.

Weiner:

I know he was at a Solvay Congress that year.

Serber:

I think he was away on leave, I forget whether it was sabbatical or a leave of absence. He was away the first semester, and I guess he came back in February, and as soon as he came back I started to work with him. Things seemed to be a little more flexible in those days. Nowadays a student has to stay around and take all the courses, normally. As a matter of fact Van had just finished writing his book on electric and magnetic susceptibility. It was in galley proofs, and he asked me to read the galleys for him and I had a good time doing that for a while, and meanwhile I was learning some quantum mechanics.

It was a funny situation there because jobs were so scarce and nobody wanted to finish getting a Ph.D. So there wasn't a change in population at all—the same people would just stay, and Van taught quantum mechanics. Then the next year he taught advanced quantum mechanics. The following year he taught advanced quantum mechanics II—with always the same gang hanging on.

Weiner:

Were you aware of him before you came? In other words, why did you choose Wisconsin?

Serber:

The only reason was that they offered me a job. I don't think I had ever heard of him. I was quite ignorant of what went on in the physics world. I think it's still true that a student very rarely has any rational reason for the choice that he makes of where to go.

Weiner:

Sometimes the prestige of the institution attracts them.

Serber:

Frequently, the prestige being what it was like 30 years ago.

Weiner:

In that case did you have any special feeling of what place was important in physics in this country?

Serber:

Probably not, or I would have known the names of the biggest universities, and I would have known whether they were particularly good in physics.

Lubkin:

Were you very much aware of nuclear physics at that time?

Serber:

No, not at all. I knew a little about radioactivity and what Rutherford had done. I had no idea what else might be going on.

Weiner:

When did you become interested in it, and how did you first become aware of what was going on?

Serber:

Well, the work was all on atomic physics at Wisconsin, atomic and molecular physics. And it wasn't until I went to Berkeley as a National Research Fellow in 1934 that I learned anything about the nuclear physics at all.

Weiner:

In 1932 when the neutron was discovered, was there any reaction that you recall at Wisconsin? Was there any discussion of the implications of this?

Serber:

Well, there was interest. There were seminars and people talked about it but nobody at Wisconsin was following it up in any way, and there was no work of that kind going on there.

Weiner:

Who would present it at the seminars? Do you recall anyone in particular who took this as a special area of interest? I understand what you meant, that no one really pursued it as a specialty.

Serber:

No, I don't remember. There was nobody at all in any field connected with nuclear physics. Somebody would have talked about it, , if something interesting happened.

Weiner:

You mentioned this group that stayed on during the depression because it was as good a place as any to be, and the advanced work in quantum mechanics that this group was able to pursue. You mentioned one person in the group. Any others that you recall?

Serber:

There was [Ragnar] Rollefson, who was later chairman of the Department at Wisconsin for a long time. And Whitelaw in astrophysics. Amelia Frank who married Wigner—Wigner's first wife— and Manny Piore, whom I shared an office with for four years. And Ray Herb. He was a little younger; I guess he was still an undergraduate until the end. And I guess Kerst was an undergraduate.

Weiner:

Did you know him?

Serber:

No, I didn't know him at all. I did know him later at Illinois. And Haworth.

Weiner:

What about the faculty? Was Warren Weaver there at that time?

Serber:

Warren Weaver was there at that time. I took electro-magnetic theory from him. It was the year that he published that book with Max Mason. We were the guinea pigs they tried the book out on.

Weiner:

We have lecture notes taken by Ralph Winch at Wisconsin at some of Warren Weaver's lectures, and in the archives we also have some of Max Mason's papers.

Serber:

And Rudolph Langer in the Math Department. He was very good to the physicists; he gave a whole series of courses on classical mathematics that physicists use.

Weiner:

What was your dissertation topic? Was it in Professor Van Vleck's areas of interest?

Serber:

Yes, it was on assorted problems in atomic and molecular physics-the Faraday effect, the binding energies of hydrocarbon molecules-that kind of thing, and work on rotation bands and molecular spectra.

Weiner:

Was there much laboratory work in research—not teaching— done at Wisconsin in this period?

Serber:

Yes, there was quite a bit. A lot was going on for instance in the photo effect and there was also work done on the equation of the state of helium. There were still some people around that were doing the classical kinetic theory problems and statistical mechanics problems. Of course, Professor Van Vleck had a very active school in quantum mechanics, but it wasn't about nuclear physics.

Weiner:

After you got your degree, you went to Berkeley under a National Research Council fellowship. Why did you select Berkeley?

Serber:

As a matter of fact, originally I elected to go out to Princeton to work with Wigner, but on the way East between Madison and Princeton we stopped to attend the summer school at Ann Arbor, and Oppenheimer was there at the summer school. I became fascinated by Oppenheimer, and never had met Wigner—Wigner was in Europe at the time, and I think he wasn't going to be back for another six months.

Weiner:

He used to split his time, half a year abroad and half a year there.

Serber:

Anyway he was in Europe, I hadn't met him at that time. So I went to Princeton. The National Research Council said they needed a release from Wigner. Wigner wasn't there and, of course, nobody was there in the summer time. Ed Condon was in New Hope, so we went out to his place in New Hope and got our release from Condon, who didn't care one way or another.

Weiner:

Hadn't you met Wigner at Wisconsin while he was there?

Serber:

No, he came later.

Weiner:

He swapped with Breit for one year.

Serber:

That was before Breit. Van was there. I guess Van left about 1935, a year later, and then Breit came and Wigner. I am not quite sure about the Breit-Wigner part as that started the year after I left.

Weiner:

We talked to Wigner and it's all in there, but I don't recall. Had you gone to any of the summer school sessions prior to this one in the summer of 1934?

Serber:

No, this was the first one.

Weiner:

Were you invited, or did you know that was just the place to go?

Serber:

That was the thing to do—it was the one big summer activity. We had been pretty isolated at Wisconsin, and didn't know what was going on in the rest of the world. This was the first time I went out and met people.

Lubkin:

Was this your first contact with nuclear physics?

Serber:

I can't remember how much nuclear physics was involved, but I remember particularly that Oppenheimer was lecturing on the Dirac theory.

Weiner:

What form did it take? It would be interesting, since there are no real records of the summer school except people's recollections, to know your impressions of it, on the way they operated, and how the ideas were kicked around, who said what sorts of things and how a student—in your case a student isolated just prior to this—was drawn into the scene.

Serber:

It was very lively. I can't remember who was there. I was back a few times later so the trouble is I've forgotten who was involved with what when. I remember particularly that Oppenheimer was lecturing. think R.H. Fowler was there. He had lectured for a while at Madison, as a matter of fact. I am not sure whether he was at Michigan too. There was a lot of discussion about the lectures going on and there was a good deal of social life, and mixing of the students and the lecturers, and going out to the lakes to swim. We were invited to parties at the various professors' houses— I remember a party at Dennison's for instance. It was very exciting, at least to me. It was all new fields I was being introduced to then—field theory and things I didn't know anything about.

Weiner:

Were there many graduate students? What was the approximate ratio of faculty to graduate students? Of course by that time you had your Ph.D., didn't you?

Serber:

Yes. There were probably three lecturers at the time, maybe, half a dozen people lecturing all summer, and I would guess maybe 30 or so would be graduate students and young Ph.D.s, plus staff from Michigan.

Lubkin:

Did you have to pay to attend?

Serber:

No.

Weiner:

How were you maintained where you stayed? Was there any charge for that?

Serber:

We just rented an apartment in town. Nobody was supporting us then, and I had a National Research Fellowship which didn't begin until after the summer.

Weiner:

You said "we." Were you married?

Serber:

Yes, I was married the last year in Madison. People weren't used to being supported. We lived on $800 teaching assistantship, and $2000 from the National Research Council; it seemed like a lot.

Weiner:

You took an interest in Oppenheimer after hearing him. Did you have personal discussions with him at that meeting too?

Serber:

Well, don't remember specifically. I remember at a party at Dennison's for instance, Charlotte and I were with Oppenheimer. I remember there was some social contact with Oppenheimer, but I don't suppose it was an awful lot. I was still a student.

Weiner:

How did it come about then? Was there specific discussion at Ann Arbor clinching the decision to go to Berkeley?

Serber:

I asked Oppenheimer whether I could come out. As a matter of fact, as it turned out, that was the fashionable thing to do. There weren't many National Research Fellowships then, there were maybe 5 or 7, of which half were in theoretical physics. I am not sure whether all of them went to Oppenheimer that year, but it was pretty close.

Lubkin:

Who were the others who went?

Serber:

Ed Uehing, and Fred Brown—who disappeared from physics—that makes three.

Weiner:

From Wisconsin, or in general?

Serber:

Just in general. I kind of doubt that there were more than 4 in the whole country. So almost everybody ended up at Berkeley.

Weiner:

You mean 4 National Research Council Fellows?

Serber:

Oh, I've forgotten exactly, but the total number was something like 7. Apparently the word had gotten around somehow, and Oppenheimer had the most lively school in theoretical physics in the country then. He was just getting started but it was pretty well established by the time I got there.

Weiner:

Was there any knowledge of Lawrence's work at Berkeley? Did this play any role?

Serber:

In my going there? No.

Weiner:

Were you aware of his early cyclotron work?

Serber:

I had heard of the cyclotron, but that was about all. I probably knew in general the kind of thing they were doing.

Lubkin:

When did you first start thinking about nuclear physics problems?

Serber:

Well, at Berkeley the main interest when I first got there was in electron theory. The Dirac equation was fairly new, and the positron guess hadn't really been discovered yet. Carl Anderson was working on it.

Weiner:

That was 1932? The mesotron came in 1936-37.

Serber:

Yes, then I was wrong about the time. Oppenheimer had been working with Carl Anderson and the positron had been discovered, but the consequences hadn't been worked out. Oppenheimer and Carlson—it must have been 1933—had published a paper, and Furry was there too. Furry's theorem which is the charge conjugation, had been coming out, and Uehing was working on vacuum polarization, part of the Lamb shift. Uehing was trying to explain the spectroscopic evidence of the Lamb shift.

Lubkin:

Even then?

Serber:

Oh yes, the spectroscopists said 0.03 wave numbers and they were right. So it was known then. It was one of these things where there was an argument between them, whether we observed it or not. Some people said that the amount of error was just as big as the observation. And then Uehing started trying to explain and did so entirely on the basis of vacuum polarization. He calculated that right, and was only a small part. But we were interested then principally in quantum electrodynamics, and in the divergences, trying to reckon suggestions for various limiting processes, trying to get rid of the divergences.

Lubkin:

I note that you had a paper on "A Note on Positron Theory" in 1936.

Serber:

That probably was on self energy. One of the reasons we didn't do too well was that we were much more ambitious than people were afterwards, and we were trying to explain the mass of the electron, not just to show that we could renormalize it away. We were really seriously trying to make a self-consistent and complete theory in which presumably a correct mass would be understood. That was the point of view; the idea of renormalizing wasn't around then. In some way it was an attempt to learn to make the infinite integrals finite.

Weiner:

Did these questions serve as the focus of the colloquia held jointly with Stanford and Berkeley?

Serber:

Yes. Bloch was at Stanford, and Nordsieck was a student at Berkeley. The Bloch-Nordsieck treatment of radiation was done then, and a little later the Shower theory by Oppenheimer and Snyder mostly. But the way nuclear physics got in of course was strictly through connection with Lawrence's laboratory.

Weiner:

How did that become effective? Here is a group working on quantum electrodynamics, and on the same campus is Lawrence building up some other capability.

Serber:

That's where Oppenheimer was most successful, in producing a school of theoretical physics. He was interested in virtually everything. Stromgren's reference book came out during that time, 1934 to 1938, his book on astrophysics, the stellar model, and he did a lot of work on stellar models of neutron stars.

Lubkin:

Really, on neutron stars then?

Serber:

Oh yes, I was working on a theory of neutron stars, trying to put in nuclear forces, and see what difference it made.

Lubkin:

Your own first paper was on nuclear forces? You had one in '36 on proton-proton forces and the mass defect curves.

Serber:

Yes, and I've forgotten what the mass defect curves means now! I know what proton-proton forces were about. Let me get back a minute. Our friends of course were of equal age. They were working with Lawrence: McMillan, Alvarez, Reg Richardson, Lyman, Milt White. They would bring up the things they were working on and ask us questions. Then of course there was Oppenheimer and Lawrence. It wasn't a separate community; we were all living together. Naturally, when the questions came up we would think about them and try to answer them. Actually the first work on proton-proton scattering was started by Milt White and I guess he didn't get around to finishing it before he left. I think it was Bob Wilson's thesis problem.

Lubkin:

These were experiments?

Serber:

Yes. I did the calculations for Milt White's experiments and put them in terms of what kind of potential it took to give the scattering he observed. I think he was the first one to establish that the general order of magnitude of proton-proton forces was the same as the neutronproton.

Lubkin:

About when was that?

Serber:

1936, I would guess.

Lubkin:

Had there been a theoretical belief by then that they were the same?

Serber:

No, none at all, there wasn't any basis for it at all then. As a matter of fact it wasn't until there were some better experiments which really established a quantitative relationship that we had any idea that they were the same. I am not quite sure of the exact time of this. It was Merle Tuve, I guess, who first did experiments, to show they were almost equal.

Weiner:

I don't remember the timing of

Serber:

It was shortly thereafter.

Lubkin:

When did the terms like "charge independence" and "charge symmetry" first start to be appropriate to use?

Serber:

It wasn't until after the proton-proton scattering experiment gave the result a little better than Wilson that the forces were approximately equal, and that was Tuve. Then I guess Ray Herb worked with his linear accelerator on that problem for a long time. But I think Wigner probably pointed out the symmetry principle and pointed out that there could be an isotopic spin conservation law. I think a paper by Oppenheimer, Kalckar and I first applied it to nuclear physics, in a light element reaction, interpreted the low rate or lack of certain reactions as due to isotopic spin conservation.

Lubkin:

And about when was that?

Serber:

Probably 1937.

Lubkin:

Were you in on that paper?

Serber:

Yes.

Lubkin:

I was not sure if you meant that only Oppenheimer and Kalckar had done it.

Weiner:

The one on "Note on Resonances, in Transmutations of Light Nuclei," - Kalckar, Oppenheimer and Serber in 1937.

Serber:

That's probably it.

Lubkin:

This was then almost immediately after the experimental observation?

Serber:

Yes.

Lubkin:

This was right after Bohr and Kalckar's paper on the compound nucleus?

Serber:

That's right. Bohr came and paid a visit and brought Kalckar with him, and Kalckar stayed on in Berkeley for a semester.

Lubkin:

When did you first hear about the compound nucleus?

Serber:

Berkeley was physically isolated but letters passed back and forth, and I probably heard just about the time the ideas were developing. We did a little work on it and got completely different answers. I forget, but I think we had been hearing about it before Bohr; I think it was Weisskopf who probably paid us a visit and straightened us out. The idea that phases were random was what we had left out. [Note from Robert Serber: It later turned out that random phases had nothing to do with it.] We used coherent phases in trying to work it out, which just produced different results. But to go back a little earlier you could also say that Oppenheimer was also a professor at Caltech. At Berkeley the school started in August, and the first semester was over before Christmas, and then, the second semester ended in April. Then we would go down to Caltech, spend a couple of months there, and of course Lauritsen and his group were doing nuclear physics at Caltech.

Weiner:

This is Charles?

Serber:

Charles Lauritsen, and Willie Fowler, and Delsasso and Tom Bonner.

Weiner:

And the Oppenheimer entourage would go down the Coast.

Serber:

Yes, and we got more intensive—we got pushed in the direction of nuclear physics while in Caltech, more than at Berkeley.

Weiner:

And why is that? There were facilities in each place for nuclear physics experimental work, so how do you account for the difference?

Serber:

Well, there was more concentrated effort in this one little laboratory—everybody was very close, and very close socially in the Lauritsen bunch, and they adopted us. They took us in, and we lived in that atmosphere completely while we were down there. Even before the isotopic spin symmetry, the mirror nuclei symmetry was discovered by Lauritsen and Fowler and Delsasso in 1935 or 1936. But people forgot. It was rediscovered years later and Wigner's name got attached to but Lauritsen and Fowler and Delsasso had pointed it out and explained it in some detail. In fact, I remember doing calculations on Coulomb energy correlations.

Lubkin:

I see. Did they have enough energy levels at that time to verify that the positions were about right?

Serber:

Well, they had most of the ground state energies, but we'd have to look it up in the paper. They had a fair-sized series, all the first half of the sequence.

Weiner:

This migration back and forth went on each of the years that you were at Berkeley?

Serber:

Yes.

Weiner:

What about the others at Berkeley who were working around Lawrence—McMillan and Alvarez—did they ever visit Caltech? Was there much contact between these groups, or was it just the theoretical group that moved down?

Serber:

It was the theoretical group that moved down. Ed McMillan came down there occasionally. He was a native of Pasadena, and also a friend of Lauristen. But by and large, the experimental people didn't exchange. Of course in Berkeley you had occasion to know what was going on there in nuclear physics.

Lubkin:

Do you feel that the experiments with the cyclotron contributed anything to the knowledge of nuclear physics at that early stage before 1936?

Serber:

Quite a few things were discovered. Of course it wouldn't have made an awful lot of difference. I can't think of anything much that was done at Berkeley that wasn't done shortly thereafter at other places. Berkeley wasn't unique in that sense. Of course the one thing they could do, they could make an awful lot of radio-isotopes, things like that, in the cyclotron; they were way ahead on that. But, for in stance, they didn't discover artificial radioactivity first, and while they did do the first p-p scattering, it was shortly thereafter done more accurately. But Berkeley was finding out how to do a lot of things then, and was beginning to get the general picture of artificial radioactivity.

Meanwhile there were a lot of nice things they were doing, such as the effect of pair production on the absorption of gamma rays; was part of nuclear physics. It had to do with the absorption of nuclear gamma rays. You know there is a minimum; the absorption of gamma rays is falling off as the energy goes out. Then of course the pair production starts going up again at higher energies. This is something that had never been observed and it led to a lot of confusion in identifying the gamma rays of light elements. People were using the downcoming curve, and they weren't allowing for the fact that there might be two answers.

It might be on the high side, and the curve could come up to the pair production. I think Ed McMillan was the first one to point that out and establish what these gamma rays were and that the curve really came up again. The trouble with the interpretation was this double valuedness of the absorption curve. A lot of things like that were going on, but it was hard. It was laying the groundwork for a lot of other unique work that Berkeley was able to do later. It's awfully hard to think of a lot of things which Berkeley did which nobody else was doing, or couldn't do at that time.

Lubkin:

Would you say that at that time one could just as well use a natural source of bombarding particles?

Serber:

Oh, the cyclotron had a lot of advantages, but it takes time to learn how to use these tools. We were talking about before 1936 and I would say that that was mostly the learning period. You could find out a lot better from Segrè about what actually was done. There may be some things I just don't remember that were quite important. But my general impression is that it was sort of a learning period on how to use the cyclotron and have it all organized—to run an accelerator means a completely different kind of a laboratory than physics laboratories previous to that.

Lawrence invented high energy physics. Then he had to invent a laboratory, an organization, and a method of financing it , and how you decide who runs the machine and when, and so sorth. All this was going on and he was developing the techniques of a modern physics laboratory. A lot that was valuable was happening, but I don't know how much of it was unique in nuclear physics.

Weiner:

Where else would you look for relevant results in that period?

Serber:

Of course a lot was going on at the various Van de Graaffs and other kinds of accelerators, which were on a small scale, and also had more precise energy, not a blunderbuss affair like the cyclotron. So for precise energy level studies, for precise measurements, they were better; they had advantages. And so Lauritsen's lab at Caltech and Ray Herb's at Wisconsin, and Tuve's in Washington.

Weiner:

How about Europeans?

Serber:

Well, of course the Cavendish lab was doing a lot; Goldhaber was still there, and Cockcroft.

Weiner:

Did you look to them for results, or were you getting the type of results that you were most interested in and that were most useful to you from these groups you just mentioned in the United States?

Serber:

Well, let me see. I think that Cavendish and also the French lab, Frederic Joliot Curie, were still doing quite a bit at that time.

Lubkin:

What about the Fermi experiments, were you at all interested?

Serber:

Yes. Then of course the slow neutron business came along.

Lubkin:

That's a little later around '35.

Serber:

Yes, within this period. And that of course was quite a big influence—the Italian men. They had a very good school in nuclear physics in Rome.

Weiner:

How did you learn of all this work at the time? You mentioned correspondence. I'd like you to comment on the relative role of private correspondence in this period, and whether you were involved in it or whether you learned of this work through Oppenheimer.

Serber:

It was through Oppenheimer. But I would say a good deal of news came in journals, a good deal more than now. It was before the days of preprints. In theory, if it sounded really exciting, Oppenheimer would be likely to get a letter from somebody about it from Europe, from some friend of his, or from someone in the East. But by and large the news came through the journals.

Weiner:

Which journals were the ones that you regarded most highly for this purpose at that time?

Serber:

At that time it would be the [Proceedings of the] Royal Society, the Comptes Rendus, and the Physical Review, and then of course the Nuovo Cimento.

Weiner:

Who could understand that journal; who read that?

Serber:

Let's see, when did Segrè arrive? In 1936. But then Seaborg was there working on the transuranics. The chemists were doing a lot of nuclear physics.

Lubkin:

When did that start?

Serber:

They started before 1938.

Lubkin:

There were attempts to make transuranic elements?

Serber:

Yes, I remember Seaborg talking about it

Weiner:

Did you have much contact with Gilbert Lewis at Berkeley?

Serber:

I don't remember much. I remember giving a seminar on molecules when I first arrived at Berkeley, on quantum mechanical interpretation of bonding energies in hydrocarbon, and Gilbert Lewis told me that the chemists had known all that for 100 years [laughter]. And then I remember Gilbert Lewis working on these neutron experiments for a while, and on paraffin lenses.

Lubkin:

Can you identify the key experiments that were made in the early 30s, say until 1936? You mentioned the slow neutrons of Fermi, and the p-p scattering.

Serber:

Well, the discovery of the photodisintegration of the deuteron.

Lubkin:

Who did that?

Serber:

Goldhaber and Chadwick. Of course artificial disintegration in the first place—transmutation. One of the main ideas was the isotopic spin symmetry. Originally it was the shell model that was popular before the liquid drop model. The discovery of the neutron resonances was very important, and the drop model was partly a regression. It was one of these things that was half true and half false and led everybody off on the wrong track a good deal, too.

Lubkin:

You are saying that before the Bohr-Kalckar paper that the shell model was already...

Serber:

Oh sure, everybody did calculations by shell model. They didn't have the spin orbit force, but all calculations were done with shell model technique.

Lubkin:

Was there a recognition that a compound nucleus explanation was necessary to explain...

Serber:

No, not until the slow neutron work showed the sharp resonance. And then Bohr was so persuasive with his liquid drop that he could persuade everybody completely over to that side. It was years before they got back to the shell model aspects again.

Lubkin:

Did you yourself feel that Bohr's picture was correct at the time?

Serber:

Yes.

Lubkin:

Would you say anybody was a holdout during the 30s?

Serber:

Not that I know of. At least if they were holdouts, they didn't make any noise about it. I don't remember anybody. It wasn't until after the war that they really began to revert to a modified form of the original idea.

Weiner:

What role do you think the various models may have played for the experimenters themselves? Do you think that the dominance of a particular model influenced the kind of work they did?

Serber:

Experimental people are very often influenced by what the theorists are talking about, and very often this isn't a help to the experiments.

Weiner:

How much of this was there at Berkeley? This leads back to something that you started to develop but didn't complete; that is, that the group of young people of your age working with Lawrence discussed common problems with the group of young people working with Oppenheimer. We got as far as that, I think, in the discussion, but it wasn't clear then how the work that you were doing, on models and the nuclear forces, was related in these discussions with the experimental people.

Serber:

Take the proton-proton scattering as an example: Wilson was doing that experiment with a cloud chamber, I guess, and he was observing nuclear scattering. [I believe it was White I should be talking about here, not Wilson.]

Lubkin:

He was using cyclotron-energy protons?

Serber:

Yes. He was observing the scattering, and the question came up, "What does this mean in terms of nuclear potential?" I did that calculation for him with a square well, and calculated how much scattering that would give us as a function of the depth and this would determine the depth of the square well to give as much scattering as he observed. There was this kind of talk. Wilson would tell me what he was getting and I would figure out what kind of a potential was needed and tell him the answer, and then he'd come back the next day, of course, and tell me that it was a little different, and so forth.

Weiner:

Did this influence your own ideas, this interaction with his experimental work? Did it lead you to any changes in your own views?

Serber:

Well, in that particular case there was no basis really to have any views. There was complete lack of evidence. Heisenberg wrote his paper about nuclear forces. [Interruption]

Lubkin:

You were talking about the p-p scattering experiments. At that time were you thinking about how long a range the nuclear force would have to have?

Serber:

Yes, that was one of the problems. A model had mostly been made just to suit the neutron-proton forces. That's what Heisenberg did in the first place, presumably just because of it being the simplest thing to do, there not being any real necessity for anything else. In fact it was rather embarrassing to have like particle forces. That didn't help the saturation properties; it made it look more complicated. It was purely a phenomenological question, no theoretical basis to say anything at all. Even after Yukawa suggested the meson, [it seemed possible the like particle forces were smaller.] If there were only a charged meson and you used perturbation theory you'd only get neutron-proton forces. Nobody knew that there was a neutral meson too, which could be exchanged by two protons. In fact I [never] heard an argument which led me to suspect that there were large [like] particle forces.

Lubkin:

Then would you say that your paper with Kalckar and Oppenheimer was the first recognition that there were proton-proton forces?

Serber:

Oh no. The proton-proton were definitely established by the scattering experiments. And I think it was Wigner who pointed out that there could be selection rules in it as a consequence. I think ours was the first application as to specific reactions in nuclear physics of that sort of thing. This could be an example of Wigner's selection rule.

Lubkin:

You wrote quite a few papers on nuclear forces in the vicinity of 1936-38. Were you influenced by the Yukawa paper, or had you already started work of your own?

Serber:

The Yukawa paper was one of the big turning points. That was an original idea, and as far as i know nobody else had thought of it. Of course we did a lot of work in that direction—variation on the Yukawa theory.

Lubkin:

You had been thinking about the problem before, however.

Serber:

Yes, but from a phenomenological point of view, interpreting experiments and then taking the forces that we got out, putting them back, trying to make nuclear models.

Lubkin:

What progress would you say occurred in the theory of nuclear forces in the latter [part] of the 30s then?

Serber:

The main progress was just in finding out what they were, by direct scattering experiments. I think there was practically no progress by any indirect arguments. As a result what was found out was very limited because only low energy particles were available. So essentially all that was found out was the S range, that is its lowest angular momentum state, what the forces were in that state. So there was no experiment or evidence beyond that. You see, a suggestion like Wigner's, really was quite an extrapolation. It wasn't proven at all by the experiments about the isotopic spin symmetry. There wasn't anything you could say about whether the forces were equal in the P state. The P state was quite different. It was a pure guess that what was observed for the S state was true for forces in general.

Lubkin:

But was that idea well accepted then, without any experimental basis?

Serber:

It wasn't well accepted in the sense that nobody paid much attention to it. It didn't have much impact at the time. Wigner wrote papers on nuclear models using the symmetry which had its influence much later. It wasn't really until they got into high energy physics, 100 MeV physics, that experiments could really tell more about the forces. There wasn't anything at all known about the shape, except the fact that they were short-range of some kind. People didn't realize at that time that they were floundering around trying to prove something about the shape. It was shown later by the effective range approximation. The information just wasn't there. It was only after you could begin to do 100 MeV scattering that they began to find out anything in detail about the forces. And then at that point, or very shortly thereafter, people could make mesons, and that brought in the question of isotopic spin symmetry which leaped to the fore at that point.

Really up until then [with] what was known about nuclear forces, the evidence was rather against isotopic spin symmetry ... In fact, it still is, if you try to interpret the experiments purely on the basis of experiments on nuclear forces, you will never come to the conclusion of isotopic spin symmetry. Nowadays the question is: What kind of a potential must there be subject to the requirement of invariant isotopic spin. That's because a much better place for testing the isotopic spin is with the π-meson.

Lubkin:

Would you say that in the 30s the work on nuclear forces developed almost independently of nuclear models?

Serber:

Yes. Nuclear models originally suggested things like exchange forces. But beyond this, they didn't have any other ... They put on certain requirements to explain saturation of nuclear forces, and everybody thought that they could deduce something about the nuclear forces. It turned out to be completely false. The saturation requirement really was just misleading. The saturation came from the fact that there was a repulsive core, not from all these other requirements. But in the beginning people were naive and thought that an interaction could be simple, such as when you have the Coulomb interaction. They didn't like anything but a monotonic potential. The idea of an attractive region and a repulsive region - that seemed too complicated.

Weiner:

I just want to comment that the way you are relating, and interpreting and criticizing the developments is very useful for us. Some of this is very new information because it's not just factually establishing what idea came along when, but more how it was received and what effect it had at that time. It's exactly what we want.

Lubkin:

Of what importance were cosmic ray studies to nuclear physics in this period?

Serber:

We did a lot of work on cosmic rays at Berkeley, principally because of Oppenheimer's close connections with Carl Anderson and also to Bob Brode at Berkeley. Just looking back you see the connections which weren't so obvious then. Aside from showing there were some kind of mesons, I am not so sure there was much interaction with nuclear physics in those days, except it was clear that high energy protons had a large interaction with the nuclei in the air, but no details were known then, except that the general order of magnitude seemed reasonable. think the interaction with astrophysics was a lot more important when Bethe did the carbon cycle and was talking about the hydrogen cycle and working out the nuclear reactions for the sun—the stellar energies.

Lubkin:

Were you working on that too?

Serber:

Yes. We were misled on the carbon cycle by some odd experiments at Caltech at the time.

Lubkin:

Was that after the Bethe paper on the carbon cycle?

Serber:

Well, we were misled before. Bethe was smarter and didn't believe in the experiments.

Weiner:

Which experiments were these?

Serber:

Oh, let me see. I think one that was wrong was probably the last stage—nitrogen 15, recapturing and going back to carbon 12 and an alpha. My recollection is that there were some results from Lauritsen's group. This might have been very temporary, it might have been just this particular day when we asked him. The results indicated that instead of going to carbon 12 and an alpha you get oxygen 16 plus a gamma, so you wouldn't get the cycle. I think we were misled on that point.

Lubkin:

You say that this was before Bethe had worked out the carbon cycle, and by that time the data was better?

Serber:

No, I think Bethe was smarter; he hadn't heard about one of the experiments. [laughter] It could work either way.

Weiner:

It pays to be isolated in the East.

Lubkin:

Was your paper on stability of stellar cores a follow up on the carbon cycle?

Serber:

No, that was [?] and Chandrasekhar, who, working on the theory of white dwarfs, had shown that the stability limit was not much bigger than 1.2 times the mass of the sun. If it gets much bigger it would collapse. I've forgotten now who suggested neutrons, who talked about neutron stars first, but you could make the same calculations for neutron stars. Then the question came up of what difference nuclear forces would make in there. We knew awfully little about nuclear forces, but I did assume something or other and found out it made some difference, but it wasn't an awful lot.

Weiner:

Now, you mentioned the role of nuclear physics in connection with astrophysics. What was the interaction? In other words it was an application of nuclear physics to understand what was going on in the stars. But how did this in turn influence the development of nuclear physics?

Serber:

Well, at Caltech it did quite a bit. Willie Fowler got interested in measuring the nuclear reaction rates they needed for the astrophysical problem, and measuring very low energies. A good part of their experimental program was based on this interest of Fowler's.

Weiner:

And so this was a closer influence than the cosmic ray work, at the time?

Serber:

Yes. The cosmic ray work had more influence on quantum electrodynamics than it did on nuclear physics, in things like shower theory, convincing people to what extent electrodynamics was good at high energies.

Weiner:

Were you in touch with Bethe during this period when you were working on stars?

Serber:

I don't remember in particular. I mean, I was in touch with Bethe occasionally, but I don't remember anything in connection with this particular star question.

Weiner:

I was really asking as part of a general question. During the period from 1934 to 1938 did you visit the East or visit other universities or attend conferences that brought you into contact with wider groups?

Serber:

Not very much. There really wasn't any money available for traveling. We didn't have a contract we could charge it to; when we came East we paid for it ourselves. I occasionally came East to see the family in the summertime, not to go to meetings.

Weiner:

What meetings did you go to during this period?

Serber:

West Coast meetings of the Physical Society. There was one cosmic ray meeting in Chicago. Oppenheimer and I went out from the ranch to that meeting one summer.

Weiner:

Did you spend any summers at the summer school in Ann Arbor during this period?

Serber:

Not during this period.

Weiner:

And you mentioned seeing Bethe occasionally. Where would this be? On a visit that he would take?

Serber:

I remember I saw him at Oppenheimer's ranch in New Mexico once, and that's the only recollection I have of seeing Bethe during that particular period.

Lubkin:

What about Wigner? When did you first meet him?

Serber:

At Ann Arbor, probably in 1938 or 1939. [it wasn't 1938.]

Weiner:

You were already back East by that time, or heading back East?

Serber:

Yes. Either we were on our way East or thinking about it. [?]

Weiner:

Did the Oppenheimer ranch serve as an informal gathering place

Serber:

No, it wasn't an informal gathering place. As a matter of fact not very many people visited there. It was rather hard to get to and didn't have very many facilities. I remember Bethe being there, and Wigner, and Elsasser who visited there when he first arrived in this country, Ed McMillan, and of course Frank [Oppenheimer]. I can't think of anybody else.

Weiner:

Weisskopf mentioned stopping off there and meeting the "Oppenheimer boys," as he says, and that was one of his first contacts with a group of American physicists. He was very impressed.

Serber:

I remember that they were talking about how the hydrogen cycle was started.

Weiner:

This is when he came earlier and slept on the porch or something like that?

Serber:

Oh, everybody slept on the porch.

Lubkin:

I gather it didn't rain much.

Serber:

Well, the porch had a roof and you could always pull up close to the wall if it rained.

Lubkin:

What about the Fermi theory of beta decay? Did this have much effect on nuclear physics?

Serber:

Well, of course, the whole field of radioactivity had quite an effect, and a lot of work went on for a long time trying to establish what kind of interaction it was, whether it was scalar, vector and so forth. It took a long time before it got straightened out. Lubkin? You were working on the beta decay theory in the 30s?

Serber:

Yes, I had not done an awful lot.

Lubkin:

In 1939 you published a paper: "Beta-Decay and Mesotron Lifetime."

Serber:

That was sort of an early version, I suppose. I've forgotten exactly what it was, but it was some sort of early version of the Goldberger-Treiman relation. Of course we didn't know what the right kinds of interaction were and all that—V minus A. So I don't remember exactly what the relations were, but we were working towards the universal Fermi interaction, although I guess that wasn't really till after the war that it was in recognizable form. I don't remember. After all Yukawa had something about it right at the beginning, but I've forgotten what did that was different from what he did.

Lubkin:

Did Yukawa discuss the beta decay as well in his original work?

Serber:

Yes. He wrote in the original paper on the possibility that beta decay might happen through the meson.

Lubkin:

But the development of the theory had very little effect on the course of nuclear physics research in that period. Is that right?

Serber:

No, as a matter of fact there was quite a bit of work on beta decay in nuclear physics on spectra of various beta decays and what the electron spectrum looked like—trying to see whether it fitted in the right shape, and to get the lifetimes and see if you get the right f-t values. There was a great deal in nuclear physics connected with the Fermi beta decay period.

Lubkin:

What kind of response was there when this work was first published?

Serber:

There wasn't much at Madison; I was still in Madison then. hoped to get somebody interested enough to react to it in a professional way at Madison. It wasn't anybody's field of interest. At Berkeley we did the same things that everybody else did. We discovered for ourselves how many kinds of interactions there were, and what different shapes of beta spectrum you could get, and rates of forbidden reactions.

Lubkin:

While you were still at Berkeley, Bethe and Rose had published a paper on the limitation for cyclotron energy. This was presumably 1937. What kind of a response did Lawrence have for example?

Serber:

Lawrence laughed because the Berkeley cyclotron had already exceeded their limit.

Weiner:

Oh, really? By the time the paper was published?

Serber:

Well, it hadn't really exceeded it. It was right about on the edge of what they had been doing, and Lawrence really didn't believe in it at all. However, they were quite right in principle that there was a limit, but I think they underestimated it. But the fact that there was a limit was true, and that really wasn't understood until Ed McMillan did the synchro-cyclotron. Bob Wilson had done something on I forget exactly what. I think Bob Wilson probably understood it better than Bethe did.

Lubkin:

You mean the limitation of the energy?

Serber:

Yes. I've forgotten the whole history, but I know Bob had a paper, and he probably knew more about machines than Bethe did. I would guess that he probably had a more realistic answer.

Lubkin:

Do you think it affected the accelerator designs in particular?

Serber:

No.

Weiner:

Did it provoke anybody into doing some analysis and paying some attention to the ...

Serber:

I am not sure whether Wilson's paper was before or after Bethe's paper, and whether Bethe's paper provoked Wilson or not; I don't remember.

Lubkin:

Wilson's paper was, you say, a better calculation, a more realistic one?

Serber:

I don't remember whether it was better, but I wouldn't be surprised if it was better because Wilson certainly knew a lot more about machines than Bethe. But I am not sure. Wilson's may have been the first attempt at a calculation and Bethe's may have been more refined.

Weiner:

While we are on the subject of papers by Bethe, what was the response to the 3-part article—or the 3 articles—in Reviews of Modern Physics, that he had written along with Bacher and Livingston?

Serber:

It was very useful to have all the material collected so nicely, but at least as far as we were concerned, I don't think it was any news. It was a textbook.

Weiner:

As a textbook, as a compilation of existing knowledge, was effective at Berkeley? Was it used?

Serber:

Oh yes. Oh yes. When did that come out?

Weiner:

One in 1936 and the rest in '37.

Serber:

Yes, the students particularly used it a lot for quite a few years. It was used by people who were learning the subject a great deal and it was a very convenient reference.

Lubkin:

Then in 1938 you went to the University of Illinois. What decided you to move on?

Serber:

I was a National Research Fellow for two years and then Oppenheimer's research associate. And there wasn't any opening at Berkeley, at least I wasn't offered one there. Finally when Wheeler Loomis was filling up the University of Illinois, he offered me an assistant professorship. I guess it was Rabi who persuaded me that it was the right thing to do—to strike out, as it were.

Weiner:

What contact had you had with Rabi prior to that?

Serber:

I had met him in Berkeley a few times. He came out occasionally but I don't think I ever met him in the East.

Weiner:

What expectations did you have when you came to Illinois in 1938? In other words, did you regard this as an up-and-coming center of nuclear physics research, or what view did you have of it as an institution?

Serber:

Well, I knew there were a lot of good people who were just coming there that same year. Maurice Goldhaber came and Haworth, Manley and Lyman.

Weiner:

These were people you had had contact with before?

Serber:

Yes, Ed Jordan, the spectroscopist, and Don Kerst.

Weiner:

Kerst came from Wisconsin?

Serber:

Yes. It was quite an impressive list of people for a department to get all at once.

Weiner:

Where did all this support come from?

Serber:

Jobs were awfully scarce in those days. Somehow or other Loomis managed to persuade the University to support physics. It didn't take an awful lot—an assistant professor got $3,000 then. I think he got enough money for half a dozen assistant professors. There may have been two or three other jobs in the whole country.

Weiner:

Where did research funds come from?

Serber:

He must have persuaded the University. Well, it was something special of course, I guess both Wisconsin and Illinois had research foundations, which had some lucrative patents—vitamin D added to milk, enriched milk and irradiated—things that would make a lot of money.

Lubkin:

Were these originally state foundations?

Serber:

They were connected with the University, I believe. The patent rights were assigned to the University. think it was true both at Illinois and Wisconsin, that some money was available from patents.

Weiner:

This was probably part of the original land-grant structure, land-grant colleges with the emphasis on agricultural research. It s something that we can pursue. It's a very interesting thing.

Serber:

Well, don't know much about it, except that I think it probably had an influence. Somehow or other, when everybody else was broke, they managed to get money to start out things, and they were able to attract some very good people. And also Loomis was a very unusual man, and he ran the most successful Department in the country for years. Nobody ever left, essentially. They didn't have the nerve to tell Loomis that they were going elsewhere.

Lubkin:

Did he start out to establish a nuclear physics department?

Serber:

Not specifically, just a good physics department.

Weiner:

Just prior to this period you had begun to come in contact with many European physicists who were newly arrived here. I'd like to know your reaction. Was there a difference in style? Did they bring a different approach?

Serber:

Oh no, not in physics at all. The American physicists had all been trained with them. Oppenheimer got his degree in Göttingen, and Van Vleck had been abroad—I forget where he got his degree. I mean, all the Americans had been through the same schools, so there was no difference at all in style. There may have been differences in the social graces, but still Oppenheimer remained Oppenheimer. Oppenheimer was simply one generation removed, but brought up in the same tradition.

Lubkin:

How did you get involved in the work on the betatron? How did that come about?

Serber:

When I arrived there Kerst was already quite interested in and he was starting to do orbit calculations. The idea wasn't brand new, other people had had it, but nobody had succeeded in making it work. The whole trick in being successful was to really very carefully analyze what happened, so that you could figure out a way of injecting the electron. He was starting to try to integrate these things numerically and I came along and did the analyses for him with a more powerful mathematical method. He undoubtedly could have convinced himself that it would work by the hard way. But he showed me the problem, and I worked on it with him and we solved it pretty well together.

Weiner:

Since you were both new arrivals, how long after you got there did you start collaborating on this?

Serber:

Oh, a month or two, I don't remember; it was quite soon after.

Lubkin:

What effect did the development of the betatron have on nuclear physics?

Serber:

The betatron itself didn't have so much effect because, first of all, it was made in small sizes and by the time the first big one got going it was interrupted by the war. By the end of the war the betatron was improved. That is, the synchrotron was a direct lineal descendant of the improved betatron. So it had a big effect, but in its improved version, after Ed McMillan and Veksler had gotten their hands on it and fancied it up a little.

Lubkin:

In what sense did the betatron contribute to the synchrotron?

Serber:

The synchrotron simply is a betatron using an electric field to accelerate instead of a magnetic core. The rest is identical. As a matter of fact if the war hadn't come along, Kerst was already talking about putting in an electric field. He had that idea in 1940-41, and he probably would have worked it out if the war hadn't come along. We didn't have the idea about synchrotron oscillations and stability, that hadn't been thought out, but the idea of using an electric field to accelerate was. As a matter of fact, he did not get to the point of understanding stability. It was just that one step that McMillan and Veksler did. They worked at stability in electric fields. The rest of the orbit problem is identical to the betatron.

Lubkin:

Then would you say that the very large betatrons that were finally built were more useful in non-nuclear physics research?

Serber:

Yes, by and large. The synchrotron also—the small one—had a fairly limited usefulness. The main reason was that, of course, the synchro-cyclotron was developed simultaneously, and the big Berkeley cyclotrons were a lot more useful for nuclear physics and high energy physics than the synchrotron, just because they were using protons rather than electrons and gamma rays as targets. And the electrons and gamma rays were quite interesting but not at such low energies. It was only when you got up to high enough energies that you could study meson production and things like that. The Berkeley machine was able to do that, and they did a lot of nice work on it. After they started building a bigger electron synchrotron—the one at Caltech was around a BeV—then they shut down the lower energy. But there were a few years when the electron synchrotrons of 300 MeV range were doing important work in the production of mesons. For most nuclear physics the 184-inch cyclotron at Berkeley was a pattern. If that synchrotron hadn't been at Berkeley it might have been a very different story, but they were right next to each other, in very direct competition, and then they did some very nice work for a few years. When bigger synchrotrons got built, there wasn't much point in continuing with the one at Berkeley. And then Kerst ran a big betatron at Urbana for a while. And they had one at Chicago. They did quite a bit of work for a few years after the war. There were various ones around.

Lubkin:

Was it mostly in photo-nuclear experiments?

Serber:

Yes. It probably was that my own interest shifted away from nuclear physics to high energy by this time.

Weiner:

By the end of the war?

Serber:

After the war. I don't remember what they were doing at the lower energy level; I am probably giving a distorted picture.

Lubkin:

Can you identify the key experiments that were made in the late 30s and the early 40s, until the war?

Weiner:

From 1935-36 on.

Serber:

Yes, let me see. The discovery of the mesotron by Anderson and Street could certainly be one.

Lubkin:

Do you feel that that was important to nuclear physics?

Serber:

Oh yes, as far as understanding nuclear forces—the experiments on n-p and p-p scattering.

Lubkin:

Who was continuing those after the original work?

Serber:

Ray Herb did most of the very good work. The slow neutron work guess comes from that period of discovery. The beta decay work established the energy spectrum and showed that you got the Fermi spectrum. This certainly deserves to be on the list. Also the experiments establishing nuclear levels in light elements, for instance, and the mirror nuclei, the isotopic triplets, that kind of thing.

Lubkin:

When did it become clear that you could make triplets as well as mirror nuclei?

Serber:

I don't remember exactly, but my guess would be that it was toward the end of that period.

Lubkin:

Was this work that many groups were doing—the identification of the energy levels?

Serber:

Yes, various groups that had Van de Graaffs—Lauritsen and Herb, and maybe two people in England. We're still talking about nuclear physics, and that was the question, I assume. And then of course the work on transuranics, and finding fission a little bit later. And that sort of ended that period.

Lubkin:

Do you feel that the discovery of fission was important for nuclear physics as well as for the bomb?

Serber:

Well, obviously pile reactors were a great help in nuclear physics, both as a source of radioisotopes and as a source of slow neutrons.

Weiner:

In that sense it's a tool, then? But what about in the conceptual understanding?

Serber:

Not much; it was trivial. It was the kind of thing that when you heard about it everybody was kicking: Why didn't I think about it? I knew enough. I was in Urbana, and got a letter from Oppenheimer who said he'd just gotten a letter from somebody the day before. And he mentioned the existence of fission, and he mentioned the possibility of making a weapon; I think those were the two things he mentioned. That evening I gave a talk at the Journal Club, and by the time I got there in the evening, I had looked up the formula in the hydrodynamic section of the Handbuch der Physik for the frequencies of the oscillating drop, and worked out the general idea of the whole thing. There was nothing to it.

Lubkin:

Could you predict ...

Serber:

To try out the general idea, you can predict where the energy comes in. I think I predicted an atomic number of around a hundred. It's just something you can do in 15 minutes.

Lubkin:

You mean you could predict that the fission should be for Z a hundred rather than ...

Serber:

Well, yes, you had to look it up. It was just a matter of balancing Coulomb energy versus surface tension energy, and determining the frequency of oscillation of the drop. So when the frequency went to zero, or the zero point amplitude was equal to the radius of the nuclei. There was nothing to it.

Lubkin:

Was this before Meitner got to work on it, that you were aware of the Hahn and Strassmann result? Which work was Oppenheimer informed of?

Serber:

I am not quite clear about the history of it. Didn't the news come through Meitner in the first place?

Weiner:

Hahn, Strassmann, Meitner and Frisch; then Bohr came to this country.

Serber:

I never understood what Meitner was supposed to have done about it. It seems to me this was one of these romantic interludes. What did Meitner do?

Weiner:

It's not clear to me. Frisch is coming over and there will be a session at the April meeting of the Physical Society; Frisch is one of the speakers. It's on the history of fission.

Serber:

Meitner carried the news as far as I know. I know that Meitner and Frisch did a little more to establish that what Hahn and Strassman did was properly interpreted. The same thing happened on the experimental side. When the news came then Dunning, for instance, set up an experiment and in one afternoon he saw a fission fragment. The only reason why Dunning claims he did anything faster than anybody else is the time it took for the news to get across the country. Also at Berkeley, the first afternoon they heard about it they did it. These aren't very great feats of accomplishment; these are trivial things. After somebody has discovered them, anyone can do what's perfectly obvious in an afternoon; nobody can build a reputation on that.

Lubkin:

Do you feel that the paper of Wheeler and Bohr brought anything additional to the theory of fission?

Serber:

Oh yes, that went into a lot more detail. They could say a lot more than just a rough order of magnitude. For practical application, the wealth of detail becomes important. But as far as advancing knowledge of nuclear physics, in any deeper sense it doesn't contribute anything.

Lubkin:

All the material was already there in the compound nucleus idea, is that it?

Serber:

Yes, it's interesting as a consequence of an already known idea. It doesn't really add anything new.

Weiner:

What happened in that period from 1939 to 1942 when everyone was [became] involved full-time in the project in one way or another? What about the on-going research? Did the discovery of fission and its implications already begin to divert nuclear physicists from what they were doing, or did that work keep on?

Serber:

Well, of course the war started in '39 and people were diverted in the first place to the radar and the radiation lab at M.I.T. An awful lot of people left for that. Then by '41—it probably started a little earlier—a good fraction of everybody that was left had been absorbed by the fission project.

Weiner:

You went to Chicago on a project in 1942. What research was going on at Illinois from '39 to '42? Betatron?

Serber:

The betatron work. The Goldhabers were there, and I guess as recent German refugees they had more inconvenience and trouble in getting cleared for these projects. Goldhaber had lots of good ideas. A lot of the ideas were supposed to be a great secret, but Goldhaber worked at them in great detail; he knew all about them. Haworth had left for the Radiation Lab. He did neutron work. am not sure whether Manly was still at Illinois. He was doing neutron work, but he rapidly got absorbed by the Manhattan project. I never was actually in Chicago. I was in Berkeley that year—1942-3—working with Oppenheimer in Berkeley on setting up the Los Alamos business and working on the bomb.

Weiner:

But you were officially at the laboratory in Chicago?

Serber:

Yes. I guess you could call Berkeley a branch, as an organization, of the metallurgical laboratory.

Weiner:

What about the time prior to your going out in '42? What were you doing in your own work at Illinois?

Serber:

I was working with Dancoff on field theory, on meson strong coupling theory.

Lubkin:

Were these the papers that were published in '42-'43 on nuclear forces?

Serber:

Yes. Phil Morrison came there. I left shortly after Phil came. I think it was mostly meson theory that I was working on then, and betatron problems.

Weiner:

Then you went to Berkeley for the first stage of the Los Alamos thing. What was done in the Manhattan Project that related ultimately to the theory of nuclear structure and nuclear physics in general? In other words, what was the effect of the war on the field? Did it accelerate or retard it?

Serber:

When you look specifically at specific questions and certain parts like the diffusion theory, you discover it was worked out very thoroughly, and a lot of specific properties of nuclei capture, neutron cross sections, people worked on intensively. There was lots of money backing it up, so a lot of information was gathered. The major effect of course was that new techniques were developed. For the first time a lot of money and a lot of people were concentrating on it and this made for very rapid technical developments; plus the developments in radar, microwaves, and electronics generally. The reputation of the projects [after the war] made research money easy to get. After the war, of course, there was a tremendous increase in the possibility of having more research funds and techniques available.

Weiner:

Would you characterize this as a tuning-up period?

Serber:

Mostly; a lot of detail was filled in, and not very much was learned in principle.

Lubkin:

Would you say that nuclear reaction theory advanced at all?

Serber:

Nuclear reaction theory advanced somewhat.

Lubkin:

How important is the nuclear reaction theory in general to nuclear physics?

Serber:

It's an important part of it. When did Wigner write a whole series of paper on nuclear reaction theory?

Lubkin:

You mean after the Breit-Wigner formula?

Serber:

Yes. I've forgotten the exact time, among other things. Once the main outline of a theory like that is laid down, the applications are of practical importance, but in principle you understand the main things involved, and so it's not an important part of nuclear physics in that sense; you are not learning anything new from

Weiner:

What was the social effect of bringing together such a large group of physicists—younger men with senior men, Americans with Europeans-in the tight environment of Los Alamos under the circumstances of a crash program?

Serber:

Of course that always was a very closely knit community. Everybody knew everybody else quite well. I had known most of them quite well before. It was quite interesting having everyone together at once. Afterward the habits changed, I suppose largely because money for travel was available, and there were a lot more specialized meetings, not just the Physical Society meetings, and a lot more getting together and traveling around. It was a continuation. You saw the same people who had been in Los Alamos. I think that what made it possible was that the government paid for it.

Weiner:

Through different government branches such as A.E.C. or the Defense Department? Was there much discussion towards the end of the Los Alamos period about the future, about what was on the agenda for physics?

Serber:

Yes. The future development of high energy physics and high energy nuclear physics was certainly discussed. Ed McMillan in the last few months started to invent the synchrotron. He started to think about what he would do after the war and what Berkeley should do after the war.

Lubkin:

Was he aware of the r-f techniques that were coming out of the Radiation Lab? Did he benefit at all from the techniques that had been developed?

Serber:

You mean as far as the synchrotron discovery? No. I mean, they undoubtedly were of benefit to him when they went to build it, but as far as getting the ideas that went into it, I don't think there was much connection.

Lubkin:

Was he at Los Alamos all that time?

Serber:

Yes. I believe he was probably at the Radiation Lab for a while before Los Alamos; I'm not completely sure.

Lubkin:

Were there any other fundamental advances that occurred while you were still at Los Alamos?

Serber:

Not in physics. People were interested in getting back to the universities and starting up. They had all kinds of ideas about what they could so.

Weiner:

What sorts of ideas?

Serber:

Well, just starting up research at the universities again. They knew a lot of new techniques and new ways of doing things and they wanted to go ahead and apply them.

Weiner:

Was the chief interest in applying the new techniques [—] with the new confidence in knowing that you could get support [—] to old problems that were left hanging? You mentioned some of them—the Lamb shift, for example, came out just at the end of the war.

Serber:

Well, that was, of course, an example of new techniques being available for an old problem. At least for the part of nuclear physics I was interested in, I guess you could say it was on the nuclear force problem. It was clearly understood by that time that there were limits as to what you could find out with the energies of protons and neutrons that were available previously. The thing to do was to build high energy machines to get high energy protons and neutrons. You were attacking old problems still. The fact is that the proton-proton scattering problem had been with us all the time, but just kept going up in energy.

Lubkin:

It seems to have been one of your favorite problems.

Serber:

Yes, I had been working on it for a long time. Every time the energy got up another few billion volts I'd come back to it.

Weiner:

After the war, was there a scrambling or reshuffling of people in different positions, while there was a good deal of recruiting going on at Los Alamos? How did you come to elect to return to Berkeley?

Serber:

Ed McMillan and Luis Alvarez asked me to come to Berkeley. The main trouble with Illinois was Urbana, or rather Champaign, a small town in the Midwest and the life there. I was much happier near San Francisco. Of course we also had a lot of friends in Berkeley, and it was clear that Berkeley was going to be the center of high energy physics for quite a number of years.

Weiner:

Why did you feel that it was going to be the center?

Serber:

The 184-inch cyclotron they started to build before the war was sort of a brute force way of getting high energy and of getting around Bethe's limitations just by accelerating particles fast. But then as soon as Ed had the synchro-cyclotron idea it was clear that you could convert this machine and get it up to 400 MeV. It seemed like a cinch. They already had the magnet built. Nobody else had Berkeley's facilities.

Weiner:

Who else went from Los Alamos to Berkeley as new recruits?

Serber:

Let me see. I don't remember that any other theorist went; I don't remember any of the experimental people. I don't think any major figures went. Undoubtedly a lot of the younger people did.

Lubkin:

Before we go on to the postwar period, can you tell us what you did right after the explosion of the bomb, when you presumably went to Japan, judging by the title of your article. What kind of responsibilities did you have?

Serber:

Bill Penney and I went to Nagasaki and Hiroshima to make a first quick analysis of the damage. Besides the two of us, there was also a medical team from Los Alamos that went and worked at a lot of things in hospitals, and to see what residual radio-activity was left. Penney and I were mainly interested in the actual bomb damage. We did things like trying to pick up samples of oil cans and seeing how many miles away they were flattened. We'd collect samples of concrete and we would analyze the strength. We found shadows on the walls, and from the length of the shadow you could sight back and see how high the bomb was when it went off. You had to use your ingenuity to reconstruct what happened and measure the radiation damage and much of the physical effects of the bomb.

Weiner:

How long did you spend there?

Serber:

About two months.

Weiner:

What was your reaction to the scene of destruction?

Serber:

It was pretty rough. It was quite remarkable how humans organize for self-protection; it's really remarkable how in a very short time you can adjust to almost any situation. Once you get into a situation of complete destruction and damage, in about two days you get used to it and just go about your business and you ignore the human aspect of it pretty much.

Weiner:

You were in both cities. Did you live in the city or in some military camp?

Serber:

In both cases we were there before the cities were occupied by the Americans.

Lubkin:

It must have been pretty dangerous.

Serber:

Well, it was quite remarkable. The Japanese seemed to be a very well-disciplined people. The Emperor said they should cooperate with the Americans, and Penney and I wandered around all over the city two or three weeks after. Everybody let us; we wandered around, nobody ever threatened us. People seemed quite friendly. You couldn't imagine it in reverse in this country.

Weiner:

Were you in civilian clothes?

Serber:

No.

Lubkin:

Had you been involved in assessing the damage from the test bombs as well? Is that how you happened to go?

Serber:

No. I saw the test explosions. But they thought it was a good idea to have some kind of theorists in the area to see the ins and outs of the business in case some hitches would develop. It ended up that the kind of thing I did was reassure the pilots of the planes that they would be far enough away when the bomb went off, what it would feel like when the shock waves hit them, if they hit them. No hitches developed, so we didn't really have anything very serious to do.

Lubkin:

You didn't go along on the mission? Did Alvarez?

Serber:

No. Alvarez went on the first one and I was on the second-that's a long story.

Lubkin:

Did you say he was or you were?

Serber:

I was, but I got put off for not having a parachute.

Weiner:

Why couldn't you get one at the last minute?

Serber:

The plane was going down to the end of the runway waiting to take off; they were revving up the engine; the pilot called for a parachute check and there was one missing. Since I was the only outsider, I was the one that was put off. They had forgotten in the meantime that the only reason that plane was going was so that I could take pictures.

Weiner:

Well, let's get back to Berkeley. You had a paper that was published in 1946. It's not at all clear where and when this was written. This was called: "Orbits of Particles in the Racetrack."

Serber:

That was written at Berkeley. I went back to Berkeley, and a large part of the work I did in the beginning was connected with the machines they were building. There was an electron synchrotron—McMillan was doing that—and Lawrence, of course, was building the big cyclotron and Alvarez was building his linear accelerator.

Lubkin:

That was an electron linear accelerator?

Serber:

No, it was a proton linear accelerator, 30 MeV; Panofsky was working with Alvarez on it. I think Dick Crane was also planning to build a synchrotron at Michigan, and he came up with the design of straight sections, not just a ring. I worked out the equations for the straight sections and pointed out the disadvantages, and what you had to pay for the extra convenience. Of course, I was very stupid. At that point I had the alternating gradient machine— I had it all right there, all the mathematics was identical—if I'd just thought of generalizing that problem a little bit, you would have had the alternating gradient machine right there.

Lubkin:

Why is that so? Why does this resemble the problem?

Serber:

It was identical with the problem. You see, you had alternating gradients already; you had field and then zero, then field and then zero. I just didn't think of putting field and reverse field, field and reverse field. If I'd done that, it would have been the alternating gradient. It was completely trivial to have tried it for the other case.

Lubkin:

Did this straight section device get built?

Serber:

Yes, it was built in Michigan.

Lubkin:

And that was the first accelerator with straight sections?

Serber:

Yes.

Lubkin:

Did he introduce that for convenience in access?

Serber:

Yes. It was a great experimental convenience to get the beams out, otherwise you always had to try to do things inside a magnet.

Lubkin:

How quickly was McMillan's idea picked up? It was in '44 that he got the idea of phase stability. Was it generally known or did the war prevent the idea from spreading?

Serber:

It was practically a few months before the end of the war when Ed began to think of what he would do and what Berkeley would do when the war ended. Of course Ed was one of the major lights at the Radiation Lab. Ed just talked to Lawrence and things were under way.

Lubkin:

But elsewhere, aside from Berkeley, did other institutions pick it up? What about Michigan or other universities? One of the other universities actually beat them to the punch building the first synchronous machine.

Serber:

That was a gag to prove that the principle would work. Somebody had a little cyclotron and I guess they tapered off the magnetic field. They didn't have a real relativistic effect. They imitated a relativistic effect by deliberately reshaping the field and then showed they could compensate for it by using the stability. It demonstrated the principle, but it had nothing to do with building, with actually getting the machine built.

Lubkin:

What kind of contribution did you make to the design of the 184 inch? You were a coauthor according to the original paper?

Serber:

Well, in a big project like that there is the design group. There are two parts of it — the engineering design and the theoretical design, orbit calculations and things like that. The theoretical group was under my direction and it did all the orbit calculations and the theoretical design.

Lubkin:

Have you ever gotten your hands dirty?

Serber:

Not really.

Weiner:

Did your undergraduate training in engineering physics have any influence on your ability to work so closely with machine builders?

Serber:

I doubt if engineering physics training had any influence. Probably the fact that I chose to go to an engineering school in the first place might show I had that bent to begin with.

Weiner:

Is this unusual in recent physics, for the theoretician to be this closely involved in the very early and continuing stages of construction?

Serber:

No, it's changed now pretty much. There are theoreticians involved but they've sort of split apart. They're 100% machine-oriented like Ernie Courant at Brookhaven. They've split into a separate profession. In fact it's very hard to persuade theorists to learn about designing machines.

Weiner:

Were there any other theorists who worked along the same lines, working as closely with machine builders as you were in that period?

Serber:

At Berkeley I had people working for me, like Lloyd Smith, and Dave Judd, also now at Brookhaven. I don't know exactly when Ernie Courant got into it. It must have been pretty early. I don't know who else was working at Brookhaven [Hartland Snyder] on it. I remember that a number of people were interested, and in England too; I've forgotten who they were now. I remember that papers were being written, but it was a rather limited number of people. As say, it rapidly split off to be a separate profession.

Weiner:

Was that because of refinement in the techniques themselves?

Serber:

No, it was sort of snobbery, a subtle social scale—what you did in theoretical physics—high energy and particle physics was on top. And then going down the scale, working on machines probably was pretty far down.

Lubkin:

Would you say that that was true before the war?

Serber:

There were isolated cases of people ... Bethe wrote a paper, Bob Wilson wrote a paper and L. H. Thomas invented the Thomas cyclotron. And then there were assorted people inventing betatrons and things like that. It was sort of scattered. Of course it was the main interest of some people, but I guess people like that suffered even then from being machine builders.

Lubkin:

In other words, they were looked down upon?

Serber:

Well, yes, there was a social scale in physics.

Lubkin:

Were theorists still top dog then also?

Serber:

Yes.

Lubkin:

What effect on nuclear physics did the new high energy accelerators have then once they were built?

Serber:

It was sort of interesting how it changed the whole picture very rapidly. Quite aside from obviously being able to measure proton-proton scattering, they found out more about the forces. One big thing was that you could find out a lot more about nuclear forces by direct experiments in high energy neutron and proton scattering. As far as the nuclei, there were different kinds of reactions that were possible, completely different mechanisms. There was also a peculiar kind of kick-back to the whole idea of low energy reactions again. The liquid drop idea had taken over completely. Everybody had forgotten everything else. And this broke down in two directions: one from the shell model of Maria Mayer, and in another way one started to think about the problem in a different way—in terms of [high] energy reaction[s].

While they were building the machines in Berkeley, and all the physicists there were involved in building accelerators, they felt they were getting out of touch with what was happening in physics, and so I was asked to give a series of lectures about twice a week, for perhaps two years, on topics in nuclear physics and high energy physics. Of course in working out those lectures it dawned on me that the liquid drop model wouldn't work anymore. Mainly what would happen was that the cross sections would [be] expect[ed] to fall off with energy, as the energy got higher, the cross sections would get quite small. And then when the cross section got small a particle wouldn't just go in and get stuck. In fact if the cross section was small enough the mean free path would get bigger than the radius of a nucleon, and would go right through and come out the other side.

So there was a completely new set of ideas about what nuclei would be like in high [energy] models. For instance we developed the optical model, and before the optical model we developed a model just of high energy reactions. We made cascades of particles, spallation reactions and all that kind of thing. And then the optical model was taken down to lower energies, the point being that the exclusion principle (at 100 MeV) reduced the cross section quite appreciably, and some of the collision processes [where] you could have a scattering of a proton by a free proton, would be cut put and the proton wouldn't be [scattered] some of the [phase] space it would have to go into was already occupied by other nuclei. The exclusion principle reduced the cross section.

We got this idea by thinking about high energy. It worked the opposite way, as the energy got lower, the exclusion principle got more and more important. Nuclei got more transparent rather than less, and it led you back to the shell model idea again. There were two things: the cross section going down and this exclusion principle coming in. As the energy went down, first it got more absorbing, down to a certain point. Then the exclusion principle took over and it began to get more transparent again. It led to applying the optical models to low energy neutron scattering, which Weisskopf and Feshbach did.

As a matter of fact the cloudy crystal ball model was actually a name invented by Karl Darrow. It was at a Physical Society meeting. Was lt New York? I've forgotten where it was. When the optical model was applied to 100 MeV neutrons—I gave an invited paper on that. Karl Darrow was in the audience, and he got up at the New York meeting and said he would like to call this a cloudy crystal ball model. And Viki was there and he made some comment about it. Then Viki adopted the name, I guess, when he applied it to low energy. [Viki was chairman.]

Lubkin:

But he seems to use it synonymously with optical model. Would you say that the original usage was for low energies only?

Serber:

Darrow called the optical model in general cloudy crystal ball. But I think it first got into the literature when Viki used it.

Lubkin:

You were thinking about being led to the optical model before the shell model ideas were introduced. Is that right?

Serber:

That was because I was thinking about what would happen with high energy neutrons hitting a nucleus. Berkeley was starting the experiments on 100 MeV neutrons. As a matter of fact McMillan, Sewell and Mayer did the experiments with the 100 million volt neutrons with various nuclei and we explained them with the optical model.

Lubkin:

Did you come up with the model before McMillan and the others had their results done?

Serber:

Yes. The model existed; nothing had been published about It wasn't till they did the experiments that we determined and checked the constants in the model against the experiments, and published the results together with the experimental paper.

Lubkin:

I see. So was the model basically developed in 1948 or not until 1949? The paper is dated 1949.

Serber:

It probably went back to '47. They didn't publish that stuff.

Lubkin:

Were other people aware of your ideas?

Serber:

The Berkeley people were. Those two volumes of lectures were published, entitled Serber Says, and the third volume was the one that had to do with this part—high energy nuclear physics. Murray Lampert was the secretary, and I remember he did write up the third volume, and I was supposed to look at it. He had it in stencil form. I didn't get around to looking at it before Murray left Berkeley. So the third one never did get published. Serber Says was published by the Berkeley Radiation Lab.

Weiner:

Would it be one of the University of California publications?

Serber:

I presume so. I wonder if I still have a copy.

Weiner:

Did you save your manuscripts and notes that you were using in that period in the working out of these ideas?

Serber:

I might be able to find some of them if look around.

Weiner:

It would be helpful to see the evolution of your thoughts. You indicated that they changed over time, that it was a gradual realization.

Serber:

These papers circulated all over the country. I remember Murray Gell-Mann told me he studied them at M.I.T. when he was a student.

Weiner:

Do you have any idea how many copies were made?

Serber:

I don't know, but it must have been a few hundred.

Lubkin:

After you published the optical model paper, how did the idea get developed further? There were certainly subsequent papers by Bohm and Ford and Adair and Feshback, Porter, and Weisskopf. How did the original idea get modified in these papers?

Serber:

For the original calculation we took a very simple model, just a sphere with a uniform charge, density, and a sharp edge, and people then improved it by using more complicated but more realistic models of nuclei with rounded edges. They tried different shapes. I think it was mostly in that direction. If you take fancy enough shapes then you have to use computers and if you are at lower energy sometimes there are different techniques for calculating the result. think the idea wasn't changed much, but the same idea was applied to much more general kinds of models. Then of course there was a much bigger change when trying to apply the same idea to elementary particles; later on, one got in to the BeV range.

Lubkin:

Who did that?

Serber:

I think the first publication I know was by Rarita and myself. After that a lot of people did work on

Lubkin:

Would you say that the subsequent development of the optical model after 1955 was in the direction of high energies?

Serber:

Not completely. I mean, there were a lot of new developments in that direction using a lot of high energy field theory calculations, particle physics calculations.

Lubkin:

Was the theory that you published in '49 able to predict the resonances that Barschall found in '50?

Serber:

It was able to, and when it did it was in the hands of Weisskopf and Feshbach. I mean, you had to know what constants to put in.

Lubkin:

In other words, you needed some experimental basis.

Serber:

Yes, sure. They interpreted the data with the model. Then they predicted, as in a sense they were able to guess in advance, what the optical parameters would be.

Lubkin:

I see. Did you follow that development with much interest or had you lost interest by that time in nuclear physics?

Serber:

I wasn't paying too much attention to nuclear physics.

Weiner:

This might be a good time to ask about this change that did take place where many people did lose interest in nuclear physics and went into high energy. How would you account for this from a personal viewpoint?

Serber:

We were talking about the optical model and about its influence on nuclear physics—as far as nuclear reactions go, the high energy model was the direct interaction. And of course that also had quite a bit of influence on nuclear physics when they began to do it at higher energy—not very high energy, but say at intermediate energy, when the direct interactions played a part. And also the stripping theory; the stripping mechanisms were invented then for the Berkeley 184-inch cyclotron. Actually for a very practical reason, the source of a high energy neutron beam.

Lubkin:

You mean the idea of the stripping reaction was invented?

Serber:

Yes, sure enough. The stripping reaction. Stripping is a Los Alamos word.

Lubkin:

Who came up with this idea?

Serber:

I wrote a paper on stripping.

Lubkin:

I know that you did.

Serber:

Well, stripping was a Los Alamos word. And Ed McMillan was in charge of a program where the idea was to shoot a bullet through a hole and see if you could take out a layer or a cylinder, or something on the outside, and that was called "stripping." And the stripping mechanism was used to get high energy neutron beams in the Berkeley cyclotron. This was one place where we certainly fell down. Carl Helmholz in Berkeley began to see the wiggles when he looked at the high energy—I've forgotten exactly what the reaction was—but the Butler kind of stripping that goes to one particular state, and you see these wiggles. Carl Helmholtz observed that in Berkeley in high energy experiments looking at the fastest particle, and I had a graduate student working on it trying to find an explanation. Then Butler's theory came out, scooped us on it; we didn't have the right idea at all. But then of course the stripping reaction again came down from high to low energy, and we'd all been trying the other way.

Lubkin:

You needed the idea to explain the high energy phenomena, but then it turned out that it applied at lower energies too.

Serber:

Of course it extended it a good deal by taking the stripping with the caught particle going into one particular state. What I mean is, it added to it. It was interesting. Everybody was frozen so long by the liquid drop that they had to get away from it and then come back again to approach more realistic models for the intermediate energies and then for quite low energies.

Lubkin:

Coming back to the question of when you think you became a highenergy physicist ...

Serber:

It was really mostly when energies got high enough to begin to produce pi-mesons—that's the critical point.

Lubkin:

You mean that you could use an accelerator to make pi-mesons?

Serber:

Yes. Then the interest shifted to experiments with pi-mesons and nucleons rather than nuclear physics. People of course tried experiments with mesons and nuclei. It rapidly became apparent that it was much harder to disentangle what was going on than if you did it with just protons in the first place. Well, the best we could do usually was to use protons and deuterons. It was much simpler to understand and get unambiguous answers if the nuclear physics aspect was just dropped out of it. Of course in the meantime they were beginning to discover other kinds of particles, so everybody took off in that direction, trying to understand the families of particles.

Lubkin:

How do you think this selection process went on? Who became high energy, and who remained nuclear?

Weiner:

Do you mean by who, what types of physicists or their names?

Lubkin:

Either their names or the kinds of people?

Serber:

The bright young generation almost all went into particle physics, not into nuclear physics. Nuclear physics was second-rate physics and particle physics was the intellectual excitement. That started as soon as the machines got high enough energies to produce all these particles. Well, not only particles, I should include field theory in general, not just particle physics literally. I mean, we shouldn't forget the advances in electrodynamics after the war, too. In more detail, I think the bright young people switched to electrodynamics rather than nuclear physics. That was the excitement immediately after the war. And then they went into field theory and high energy physics. That was the way it really went.

Lubkin:

It was, then, a natural effect for the young kids just getting their doctorates, to become interested in field theory and high energy.

Serber:

Yes, those were the things that were most exciting.

Lubkin:

But what about the people who had already been working in nuclear physics?

Serber:

Bethe and Peierls talked at the New York meeting on nuclear models last week.

Lubkin:

Do you think that you were unusual to have made the transition?

Serber:

Well, not really, I was always associated with the experiments in the laboratory more than most theoretical physicists[,] in Berkeley. And when I left [t]here I went out to Brookhaven, and was closely connected with them there.

Weiner:

Actually you described your work in Berkeley as a graduate student along the lines of Oppenheimer's interests. There was interest in field theory then, and the straight nuclear physics interlude was rather a short one, if you take out the war period. I don't see you changing paths; I see a consistent trend.

Lubkin:

The work on nuclear physics was going on from 1935 until after 1950.

Weiner:

But the other work was also going on.

Serber:

It's perfectly true that Oppenheimer's group was never restricted to nuclear physics. Nuclear physics was one of the things that group did. It did field theory, astrophysics, cosmic ray physics. It worked in a number of fields, and I suppose also in that sense it wasn't such a big switch for me, since it was never particularly immersed in nuclear physics. But it was more that I always happened to be where the biggest machines were.

Weiner:

Let us consider how much time you can spare us and then we'll determine which questions are most important.

Serber:

Another 15 minutes?

Lubkin:

Then perhaps we should return to the shell model; we didn't talk about it too much. How did you feel when the first evidence for the magic numbers was introduced. Was it a surprise?

Serber:

It seemed awfully odd. I wasn't directly concerned with it. There were some very curious things about it. When it got back to the shell model again I was sort of surprised; everything seemed to be as it had been 20 years before. It was this big loop and back again. Then you wonder how fashions managed to change, how the two things could ever be compatible. You could have a shell model and a statistical model at the same time. It was more a psychological problem, how you could concentrate on one aspect to the complete exclusion of the other for such a long time. It was interesting to find out how they reconciled the shell model with the statistical model. But I never really worked on it at all myself.

Lubkin:

Are you referring to the work in the middle 50s?

Serber:

Yes. Wigner did quite a bit on it. I mean, the residual signs of a shell model even in the region where there are so many levels of statistical [nature].

Lubkin:

During the early postwar period, where would you say that the centers of nuclear physics seemed to be?

Serber:

Of course Berkeley was one of the centers. They had this 60-inch cyclotron running. I guess there was still a good deal of nuclear physics. Segrè was there and Seaborg; they were doing a lot of nuclear physics. And of course Chicago was a center, and Brookhaven, and Columbia—Rainwater invented the collective model. Rainwater and Wu did an awful lot of nuclear physics.

Lubkin:

When did the Rainwater work on the collective model first appear?

Serber:

It must have been in the late 40s.

Lubkin:

But it was before Bohr and Mottelson?

Serber:

Yes, no question of that. They got the idea from Rainwater.

Lubkin:

Did Rainwater's ideas take into account the rotational levels?

Serber:

He pointed out that the nucleus became elongated, and as soon as it wasn't spherical anymore, you got rotational levels. I don't remember whether he actually discussed the rotational levels. I know we talked about it, but whether he actually worked it out, I don't remember.

Lubkin:

There was no experimental evidence for the rotational levels until '53 or so?

Serber:

Yes, I think that's correct; there wasn't any at the time. It was mainly the quadrupole moments that interested Rainwater.

Lubkin:

Just the quadrupole moments of the light particles?

Serber:

Of the ground state of the nucleus.

Lubkin:

You mentioned only centers in the United States after the war. Do you feel that the European centers were in the background then?

Serber:

What happened is that they declined rather rapidly.

Weiner:

After Rutherford's death?

Serber:

When did Rutherford die?

Weiner:

1937, before Goldhaber left.

Serber:

Certainly after the war it did decline. At least the stories that I heard from people who were there in the 40s were that they were so old-fashioned, they were still using electroscopes, gold-leaf electroscopes. Some good people went along on inertia for a while, but you can't keep up old-fashioned techniques for very long and survive. But after the war it went down quite a bit, for a while.

Lubkin:

Do you feel that by the end of the 30s that accelerators were the technique of choice for all experiments in nuclear physics? Had the transition been made before the war?

Serber:

I wouldn't say all nuclear experiments, but it was for the most part. There was a lot of neutron work, slow neutron work if you could get a good radioactive source. I am not talking much about the European places because I didn't know them as well, and I didn't know exactly what was going on.

Lubkin:

In this early postwar period, who were the leading theorists?

Serber:

What was happening was that the most exciting things were in electrodynamics. There was Schwinger, Feynman, and Dyson ...

Lubkin:

And among the experimenters?

Serber:

There were a lot of good people at Berkeley, as you know, should also mention Cesar Lattes who discovered the mu mesotron in the Berkeley machine. There was Jack Steinberger; he was at Berkeley and then after, he came here. The younger people were good experimental people—Owen Chamberlain, Piccioni, who came early after the war. And a little later there were Leon Lederman, and Panofsky, of course. And Herb York started out well. Then Hofstadter and Bob Wilson.

Lubkin:

Now, in that first rush of experiments with the new equipment, what were the key experiments that were made, say from 1946 to 1950?

Serber:

As far as nuclear physics there was a whole bunch of things done by Seaborg, in the way of radio chemistry, spallation reactions, and all that sort of thing. Then as far as nuclear forces are concerned there were experiments on p-p scattering after the initial ones, mainly by Segrè and Chamberlain. Then other people got in the trend and discovered various pi-meson [reactions], production of pi-zero, first by [Moyer and York] and by Steinberger.

Lubkin:

Would you say that's already getting into high energy physics, when you talk about pi's?

Serber:

Yes, mainly because in nuclear physics things get thoroughly mixed up, talk about beta decay and weak interactions. Of course [in] nuclear physics {that] was in a thorough snarl. All the evidence in decay of and decay of mu, and decay of the tau meson, it had all been thrown together before the whole story was straightened out and the parity [isolation was found]. The idea came from high energy.

Lubkin:

Would you say that the experiment was a nuclear experiment?

Serber:

Yes. It was all founded on a nuclear experiment. But the whole straightening out of the beta decay theory depended a lot on [the tau meson.]

Lubkin:

What happened, after the new results of the higher energy[,] to the theory of nuclear forces?

Serber:

The immediate thing was that people found out what the scattering was, and the most important thing that emerged from that was the existence of a repulsive core. The saturation question was settled, as not due to having a proper kind of exchange forces there, but as due to the existence of a repulsive core. That was probably the main thing.

Lubkin:

And when was that recognized?

Serber:

It was mainly done by Jastrow. He came to Berkeley in '48 or '49. He rapidly pointed out the evidence for a repulsive core in the scattering.

Lubkin:

How did the theory of the many-body problem interact with scattering theory?

Serber:

I think there was a strong interaction. In trying to understand the strong interaction in general, people developed studying the scattering theory a good deal more than they had before, and started studying a lot more the problems that came up when particles strongly interacted. Previously the techniques had been developed mainly for electromagnetic interactions which are weak. I guess the techniques that were developed about scattering theory just led to enough extra sophistication, so that Brueckner could go ahead and see how to make much better approximations for nuclear matter problems. This came from a concentration of theoretical physicists on hard scattering problems, which they couldn't solve, so they tried to understand better the formulas of the scattering and learned a great deal more about S-matrix theory and formal treatment of scattering problems.

Lubkin:

Do you think that it's been very important to nuclear physics as opposed to high energy physics?

Serber:

What does "that" refer to?

Lubkin:

Well, I know of course that the scattering amplitudes and so on are very important in high energy physics calculations, but you don't usually hear much about scattering amplitudes at low energy.

Serber:

It does come in in the Brueckner type calculation. It's the same technique of calculating.

Lubkin:

Do you feel the many-body theory has been very fruitful for nuclear physics?

Serber:

Not yet, but I think it's in the middle still. So far they've got reasonable results for infinite nuclei, and now the calculation for finite nuclei—that's what Bethe was talking about last week. Bethe was quite optimistic that he was getting near to some interesting looking models. Peierls got up immediately afterwards and said, ["]Now you have to be careful.["] But I think as far as nuclear physics goes it will end up being a real contribution, and it will be very satisfying to see exactly how it goes along—predictions about density distribution of nuclei and things like that. But it hasn't gotten to that point yet, though it shows some promises.

Weiner:

I have a few final questions. One is on the general state of nuclear physics today. Would you characterize it as being at the end of an historical period of its development, or on a plateau, or in the midst of transition?

Serber:

I don't follow it very closely, but I have the impression—aside from the general questions about nuclear matter—that most of the experimenters now are pursuing acute technical fine points and things that are interesting, but it's not too clear to me what they are learning from them. I mean, they may be learning something about nuclear physics, but not from the point of view of science in the sense of learning the laws of nature instead of just describing a lot of phenomena. not at all clear to me what's happening in nuclear physics right now. They are doing a lot of solid state physics which often leads to very interesting technology in its practical applications, but it's sort of splitting off from physics and getting off on its own.

Weiner:

What could reverse this trend, or change it? Or do you see it as a logical development?

Serber:

It's logical. It's often true that frequently you don't exhaust a subject. If somebody knows how to put the right questions to it and knows techniques for doing good enough measurements, he can frequently find out fundamental things by very accurate measurements on complicated processes. And this undoubtedly will still be true in nuclear physics. But by and large it's not clear whether this is splitting off as a separate sort of technology rather than science.

Weiner:

I have two personal, and final questions: one is the reason for your move from Berkeley to Columbia in 1951, and then the second is a reflective question which I'll ask later.

Serber:

A good deal had to do with the loyalty oath problem. I was willing to sign it myself, but when they began to fire my colleagues— people like Professor Meck [Wick] (?)—that was unpleasant, so eventually I went to Columbia.

Weiner:

This was a difficult decision for a lot of people in that period. But what was the attraction at Columbia? There could have been other institutions that you could have chosen.

Serber:

Well, my wife's family was living in New York, and she liked the big city. One of the disadvantages of Berkeley was that it was hard for her to get a job there that she'd be interested in. Also, Columbia is one of the best universities, plus the fact that I knew Rabi [, and he] had something to do with it. Brookhaven was then obviously going to be an important place, so it made it perfectly possible to move without losing too much.

Weiner:

You had some misgivings about losing the close ties with the machines at Berkeley?

Serber:

Yes, and also with a lot of old friends at the Radiation Lab.

Weiner:

Now the final question. In thinking back over the work that you've done, which work has given you the most personal satisfaction?

Serber:

I think probably the answer to that is that it was the general work on high energy nuclear reaction. The reason probably is that nobody else was thinking about it, and Berkeley was the only place where there was any prospect of having any high energy particles to work with. Most other theorists were thinking about electrodynamics, so I had the field to myself. Nobody else was concerned with it at the time, it was a nice fresh virgin field. I enjoyed that part of it.

Weiner:

And do you feel that it was your most significant piece of work, aside from your personal satisfaction, in terms of either the short range impact on physics, or long range impact? Or would you select something different on that score?

Serber:

Oh, I don't know, maybe the work on the machines really was the most effective, as far as the effect it had on physics.

Weiner:

It is quite possible to have differences in personal satisfaction and long range impact. It's a very hard question to answer.

Serber:

It may very well be that working on all these accelerators did have a long-range impact on physics.

Weiner:

Some day we'll know. I think we've exceeded the time limit quite a bit. Thank you very much.