Charles Zemach - Session I

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ORAL HISTORIES
Charles Zemach

Photograph by J.D. Jackson, courtesy AIP Emilio Segrè Visual Archives, Jackson Collection

Interviewed by
David Zierler
Interview date
Location
Teleconference
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Interview of Charles Zemach by David Zierler on 2020 July 2,Niels Bohr Library & Archives, American Institute of Physics,College Park, MD USA,www.aip.org/history-programs/niels-bohr-library/oral-histories/45350-1

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In this interview, David Zierler, Oral Historian for AIP, interviews Charles Zemach, retired from the staff of the Hydrodynamics Group (T3), Theoretical Physics Division at Los Alamos. Zemach recounts his childhood in Manhattan as the son of Jewish immigrants and his experience at Stuyvesant High School. He describes his undergraduate work at Harvard and the influence he felt from Julian Schwinger and George Mackey, and he explains his decision to remain at Harvard for his Ph.D., which he earned under the direction of Roy Glauber. He describes some of the major questions in theoretical particle physics in the early 1950s and the excitement surrounding quantum electrodynamics, and he explains his research on neutron scattering, which grew out of Fermi’s work on simple delta-function interactions twenty years earlier. Zemach discusses his postdoctoral research at the University of Pennsylvania, and then at Berkeley, where he describes the relevance of his research on the bootstrap theory that Geoffrey Chew was developing. He describes the series of events leading to his work for the Arms Control and Disarmament Agency (ACDA) in Washington, which Sid Drell encouraged him to pursue because it would allow him to participate in some of the great challenges in nuclear arms control during the Nixon administration. He explains how the ACDA was set up to solidify Kissinger’s control of nuclear policy, and he describes his role in the SALT I and SALT II negotiations. Zemach discusses his subsequent work at Los Alamos, where Harold Agnew recruited him to become leader of the Theoretical Division and where he focused on fluid dynamics as it related to nuclear bomb design. At the end of the interview, Zemach discusses some of his activities in physics since his retirement in 1993, including his ongoing interest in fluid dynamics and his work on river rights in the Santa Fe area.

Transcript

Zierler:

OK. This is David Zierler, oral historian for the American Institute of Physics. It is July 2nd, 2020. It is my great pleasure to be here with Dr. Charles Zemach. Chuck, thank you so much for being with me today.

Zemach:

Pleasure to be here so far.

Zierler:

[laugh] All right, so to start, would you tell me your most recent title and institutional affiliation?

Zemach:

My most recent position and title was staff member in the hydrodynamics group, also called T3, in the theoretical physics division, also called T Division, of the Los Alamos National Laboratory.

Zierler:

OK. And now—and

Zemach:

I had a somewhat checkered career with a number of different [laugh]titles.

Zierler:

My first question is let’s talk about the man who connected us. When did you meet Shelly Glashow?

Zemach:

We were both graduate students in the physics department of Harvard University.

Zierler:

I see.

Zemach:

In my last year there, Shelly and I were roommates, so we were quite close. We got along very well.

Zierler:

Beautiful, beautiful. All right, Chuck, let’s start right at the beginning. Let’s start with your parents. Tell me a little bit about your parents and where they are from.

Zemach:

My mother was born in Russia, and my father was born in Poland. To be more specific, my mother was born in the city now called Tsaritsyn, which used to be called Stalingrad. And my father was born in Wolkowitz, a tiny eastern Polish town. But then he lived in Bialystok, and the two of them actually met in Moscow. This was in the period of World War I and the Russian revolution.

My mother was a student at Moscow University in 1917 during the outbreak of the revolution. And my father was some sort of a businessman. I don’t know quite what his business was. I’m not all that clear about the background. Anyway, they met in Moscow.

They were both Jewish, and my father was a leader in a sort of Hebrew nationalism. He founded a dramatic company, called “Habimah” which put on theater performances in Hebrew. This was in Moscow. The Habimah continues today in Israel. My mother joined that group; eventually some sort of magic happened, and they got married.

Zierler:

[laugh] Chuck, do you know if the performances were in Yiddish or were they in Russian?

Zemach:

No, they were in Hebrew.

Zierler:

In Hebrew?

Zemach:

Hebrew was not a common language at the time. Yiddish was mostly the common language of Jews in that part of the world. Introducing the language of the Bible into modern Jewish culture was a novel idea in that time and place.

Zierler:

Now, when did they come to America?

Zemach:

In 1926. What happened was that the company — with the permission of the Soviet government — was touring in Western Europe, and they got to Paris. In Paris, there was an episode. My father, according to my mother’s memory of it, wrote to a friend in Moscow, saying, “Well, how are things going with the revolution and everything?” And would his friend advise him to go back to Moscow?

The friend, writing from Moscow, said that “if you believe it is a good idea to go to hell to light a cigarette then by all means come back to Moscow.” Well, that’s a definitive piece of advice. They came to America instead, in 1926.

Zierler:

And, Chuck, when do you arrive on the scene? What year are you born?

Zemach:

I was born 1930 in Los Angeles. My father and mother, of course, arrived in New York, but then they went off to Los Angeles where they were involved in various kinds of theatrical efforts. My mother was actually in a number of American movies at the time.

Zierler:

Really?

Zemach:

Yes.

Zierler:

What was—did she have a stage name?

Zemach:

She was not a big star.

Zierler:

What did she go by?

Zemach:

Miriam Goldina. Her father’s name was Goldin, and in Russian, the feminine of that is Goldina. And her first name was Miriam. So that was her stage name. In 1933, we came to New York, and she used that name professionally.

Zierler:

Chuck, growing up were you—was your family Jewishly observant at all?

Zemach:

Not particularly. It was sort of tangentially observant. I did have opportunities for one reason or another to go to a synagogue at some time or other. But I don’t think that either of my parents was particularly observant.

Zierler:

Were you bar mitzvahed?

Zemach:

No.

Zierler:

Now, you grew up in New York?

Zemach:

Yes, from the age of 3 until I finished high school, and then I went off to college.

Zierler:

What neighborhood in New York did you grow up in?

Zemach:

Midtown New York, the western side. We were not far from Central Park. We lived in various rather humble brownstone housing, 89th Street and 90th Street and 92nd Street on the West Side of New York.

Zierler:

Did you go to public schools as a kid?

Zemach:

Yes, I went to public school.

Zierler:

Which PS?

Zemach:

PS 166.

Zierler:

PS 166. One school all the way through 12th grade?

Zemach:

No, that school finished off in the sixth grade. For seventh, eighth and ninth grade, there was a junior high school, which was actually a very new and modern school called Joan of Arc Junior High School on 93rd Street.

Zierler:

And where did you go for high school?

Zemach:

Stuyvesant High School. In those days, there were three science-oriented high schools in New York City. There was Stuyvesant, Bronx High School of Science, and the Brooklyn Polytechnical High School. Shelly Glashow went to Bronx Science.

Zierler:

Did you go to Stuyvesant because in junior high, you were particularly good at math and science?

Zemach:

Yes, I developed a great love for mathematics, and I think an aptitude for it. I only remember one course in physical science. There were a number of health courses: good nutrition, good health habits, how the eye and ear worked, etc.

Zierler:

What were your favorite classes at Stuyvesant?

Zemach:

All the math classes. Every semester, there was the math class. I went into Stuyvesant in my sophomore year because the freshman year was spent at the junior high school. And the first classes were in—well, there was math 4A which was plane geometry, then math 4B which was the second term of plane geometry. I really loved those.

And then there was always some other math course. There was intermediate algebra, and advanced algebra, and then trigonometry, and solid geometry. By my senior year, I’d gotten to know enough about mathematics so that Dr. Brodie—I remember him with great affection—Dr. Brodie, who was teaching a course in calculus, had me teach the course, with him sitting in the back of the room and occasionally offering advice.

Zierler:

So it was really math that was your first love in high school, not physics?

Zemach:

That’s correct. There was also physics, but that wasn’t all that interesting. Well, no, that’s unfair. It was interesting enough, but math was my main interest.

Zierler:

Chuck, what colleges did you apply to?

Zemach:

At the time I got to my senior year, there was a general fear among students that it would be very difficult to get into a college at all. So I applied to a lot to of them. Dr. Brodie did tell me that Harvard was the place to go, so I applied to Harvard. I also applied to Cornell and Princeton and a few others.

Zierler:

[laugh] And was your plan to major in math? That was the plan, to be a math major originally?

Zemach:

No, by that time, I was more or less confirmed to be a physicist.

Zierler:

That’s interesting. Even though that you really excelled and enjoyed math the most, the plan was for you to become a physicist. How did that happen?

Zemach:

I had the idea that math was not a useful subject—

Zierler:

[laugh]

Zemach:

—as a profession. I didn’t really understand the modern academic scene. I did not understand at that time that I might want to become a college professor. So what’s the point of math? Where will you go? I didn’t know. Physics seemed more useful. I was not well-informed in what all that situation was.

Zierler:

So where did you end up? Did you go to Harvard as an undergraduate?

Zemach:

Yes, and majored in physics. I knew enough math so that I never took any undergraduate math courses. I only took graduate math courses including in my freshman year.

I began with one elementary physics course, but then I had a certain pride about that. I decided it was beneath me to take undergraduate physics courses, so I only took graduate courses, with a couple of exceptions. I had to take thermodynamics, which was an undergraduate course.

I omitted optics, and that was a mistake. But optics was labeled an undergraduate course, and my pride prevented me from taking that, and that cost me in later life. Optics was a pretty important subject. I had to learn it on the side, and that was a difficulty.

But the important courses in physics—well, at that time, Julian Schwinger was the star of theoretical nuclear physics, and all physics students looked up to him. I either took his courses or audited his courses every semester as an undergraduate. He was teaching graduate courses, but I either took one or two of his graduate courses, and audited the others.

Zierler:

Were you able to develop a personal relationship with Schwinger?

Zemach:

No, certainly not as an undergraduate. Schwinger was, on the one hand, a brilliant man. On the other hand, he was shy and didn’t go out to other people much. And even as a graduate student, and later on in life when I did get to know him and did have some social interaction with him, it was marginal. He had a wife who much more outgoing, and she made him a little more human.

Zierler:

[laugh]

Zemach:

I don’t know if you’re interested in these kinds of anecdotes, but, for example, I mean, stop me if I’m wandering —

Zierler:

No, no, I love it, please.

Zemach:

There was a physics graduate student organization, which had a president and some vice presidents, and so forth. This organization every year would hold a picnic for the graduate students in physics and for the faculty. Every year, the president or someone from the student body would personally call up Julian Schwinger and invite him to the picnic. And he never came.

But in the year that I’m remembering, the students had a different idea. They called up Schwinger’s home at the time when they knew he was in class, and spoke to Christine Schwinger, inviting her to the picnic. So that time, they both came.

Zierler:

[laugh]

Zemach:

Not only did they both come [laugh] but Schwinger got into the baseball game, and it was amazing. He was a tremendous slugger. He must’ve been a very good baseball player in his youth. He was the star of the faculty-student baseball game.

Zierler:

I had not heard that one. That’s a good one.

Zemach:

Yes.

Zierler:

Chuck, who were some of the other professors that you got close with as an undergraduate?

Zemach:

Well, starting as an undergraduate, my first course in mathematics was with George Mackey. George Mackey is another one of the stars in my early career whom I revered. In later life, I and my wife and he and his wife got to be quite friendly, so I knew him a lot. He was teaching a course on functions of real variables.

That is a subject matter which is not too well-defined. But he made it a fascinating course, and he talked about Peano’s postulates and logical foundations of mathematics, and then metric spaces, and topological spaces, and topological groups, and measure theory—all sorts of things which just suited his fancy. But that was my introduction to Harvard intellectual life, and that turned out very well.

Zierler:

Now, Mackey was in the math program or in the physics program?

Zemach:

Oh, that’s math. George Mackey was a math professor, and that was in my freshman year. There were a lot of great physicists in the physics department, and I got to know them slightly or well. There was my undergraduate advisor, John Van Vleck, Ed Purcell, and Norman Ramsey, who each earned a Nobel Prize. And Wendell Furry, from whom we took electrodynamics, a fascinating teacher.

I I also took a course on advanced algebra from Oscar Zariski, which was quite an experience. My best non-technical class was a year of German literature, taught by Arthur Gardner, then a graduate student in the German department. It gave me a lifelong appreciation of German literature, especially poetry. That covers most of my undergraduate career.

In graduate school, I had the notion that I’d like to have Julian Schwinger as my PhD advisor, as was true of many other students. But Schwinger farmed out these applicants to various junior members of the staff, and my advisor was actually Roy Glauber, which was very profitable for me.

At the time, Glauber was a post-doctoral assistant to Schwinger. He wasn’t even an assistant professor. But he guided me through my thesis and so forth, and that was enriching. Another protégé of Schwinger, was Abraham Klein, and I also worked with him. My ultimate thesis was actually two problems. Glauber had given me a problem, and Klein had given me a problem, and I did both problems.

Zierler:

Chuck, I want to go back before we get to your thesis. First, by the time you had finished your undergraduate education, did you know that you wanted to focus on theory, or that only really became confirmed as a graduate student?

Zemach:

I think I always wanted to focus on theory. As to why, I don’t know.

Zierler:

Probably because of math. Math must’ve been a part of it, right?

Zemach:

Yes, that’s right. Probably it was also that when I was growing up, I was poor, and didn’t have many mechanical things. So I spent my time reading books from the library. I would get math books and occasionally physics books from the library. But that was the alternative to working with tools.

Zierler:

And when you were thinking about graduate programs, did you think about leaving Harvard? Were you encouraged to stay? How did that decision play out?

Zemach:

Well, I wasn’t sure. My undergraduate advisor was John Van Vleck, who was also the chairman of the department at that time. I asked his advice on what I should do for graduate school. What he said was, “Well, for the really good students,” and he included me in that, “we try to hold onto them. But it’s really better for most students to go away to some new institution, and get a new environment.”

So I pretty quickly decided I should go to either MIT or Harvard, and I was accepted by both. And then I was in the hallway of the physics building, and there were a couple of faculty members and they asked me where I was going for graduate school. I said I was unsure whether to go to Harvard or MIT, and could they give me any advice?

One of the faculty people was Harold Levine. He said that there was a difference in attitude in the Harvard people and in MIT — a difference in the schools. That Harvard was more theoretical and fundamentally based, whereas MIT was more practically oriented. Then Professor Furry, who was also in that little group, said, “MIT is a hellhole.” [laugh] So I decided on Harvard. Afterwards, I spent a year at MIT as a postdoc, and it’s not at all a hellhole.

Zierler:

[laugh]

Zemach:

It’s a great place—

Zierler:

[laugh]

Zemach:

—and with great faculty there.

Zierler:

Now, Chuck, I want to orient ourselves chronologically. You graduate Harvard as an undergraduate in what would it be? 1952?

Zemach:

I graduated in June of 1951. I got an award from Harvard as a consequence of graduating summa cum laude. The award was called a Shaw Traveling Fellowship, and allotted me $2,500 with certain rules. The rules were that I had to spend it traveling outside the United States, and I could not enroll in any university.

There were three other fellowships of that kind called Sheldon Traveling Fellowships, so there were four of them altogether. The Shaw was a little better because it paid $2,500 whereas the Sheldons only paid $2,400. The idea of the donors was that the student should get educated abroad by himself, and not at university.

I felt that was a wonderful idea. I spent a year in Europe, mostly in Western Europe, and in North Africa, and I got as far as Turkey, Greece, and Yugoslavia. At Harvard, I had done a pretty good job of learning German.

The routine in Harvard was that you were supposed to take two years of a foreign language. I had already taken Latin in high school, and I took the Latin qualifying test. It turned out that in Latin, I would only have to take one more semester. I don’t regret taking Latin. But instead of taking one more semester of Latin, I decided that two years of German would be more valuable for science.

So I took German 1, and then, skipping German 2, two semesters of German literature. We went through a whole set of classics in the history of German literature.

When I got to Europe, I was already pretty good at reading German. I was pretty helpless at speaking German, and my first attempts were a little clumsy. But one of my ambitions was to learn languages.

When traveling in German and Austria, I went to a movie and read a newspaper almost every day. After a while, I could understand movies pretty well. I would go to a German gasthaus in the evening where the old boys would gather around, drink beer. I was always an oddball because I was a kid, and obviously a foreigner. These old cronies would focus on me, wanting to know who I was and where I came from. I had a lot of practice in speaking German.

I could pass as German in some cases—not all. In addition, I spent time in France and in North Africa, where the second language was French. Professor Mackey told me that the way to learn French was the way he had done it his year at Nancy, which was to study Hugo’s French Simplified. I also carried Hugo’s Italian Simplified.

I bought a motorcycle, which was a little adventurous because at the time I graduated from college, I did not know how to drive a car. I had to learn how to ride a motorcycle and take a test in German for the driver’s license. That stuff was eventually overcome, but it caused me a little trouble. I rode around Germany and Austria on my motorcycle. I really loved western Austria, which is beautiful. And then, I traveled further through Italy and North Africa. Previously, I had been in Scandinavia and England, and I eventually got to Spain and then Turkey, Greece, and Yugoslavia. So I had quite a full menu there.

Zierler:

And, Chuck, I want to—just so I understand, this scholarship was purely to become worldly and well-traveled? This was not about learning physics abroad or anything like that?

Zemach:

It was actually opposed to learning physics. In the New England mentality and also in the Harvard mentality, there is something about being broadly educated. This was a device to have the recipients broadly educated, which I appreciated. And I continued traveling extensively thereafter whenever I got the opportunity.

Zierler:

Chuck, I’d like you to set the stage a little bit. As you enter graduate school at Harvard, and you’re—you know—you want to work with Schwinger, it’s 1951, 1952, what are some of the big questions that are being asked in theoretical physics in the early 1950s? What are the most important issues?

Zemach:

The previous decades had seen great advance in the understanding and application of quantum mechanics to states of molecules, atoms, and atomic nuclei (with many questions unanswered). When my undergraduate advisor Van Vleck was asked how he found problems for all his students, he said “Well, there are 92 elements.” But elementary particle dynamics was something new. The complete list of “elementary” particles at the time, both stable and unstable, included photons, electrons and positrons, protons and neutrons, neutrinos, and the mu, pi, and K mesons. Of the four types of forces, gravitation, weak interactions, electrodynamics, and strong interactions, gravitation at Harvard was pursued only by Bryce Dewitt, not a regular faculty member. Weak interactions, observed mainly through beta decay of nuclei, were a major interest, but not an active PhD topic, as I recall.

There had been some breakthrough experiments on hydrogen energy levels, and the magnetic moments of nucleons and electrons, and matching calculations which depended on quantum electrodynamics. Schwinger plus Cal Tech’s Richard Feynman were the leaders, by different methods, in solving the really difficult questions of quantum electrodynamics. Schwinger seemed to regard Feynman as an ignorable rival. A Schwinger student would never say “Feynman diagram” in Julian’s presence. But at Schwinger’s 60th birthday commemoration in Los Angeles, Feynman said that the two “had climbed the same mountain from opposite sides.”

The continuing construction of bigger and better accelerators in that era lead to knowledge of nucleon-nucleon interactions and other strong interactions with nuclei at increasingly higher energies. A lecture in my early graduate career on nucleon-nucleon scattering noted that for incident energies up to two or three million electron volts, only the S wave was relevant; such energies were defined “low”. The “high” energy range was upwards of three Mev. Before I finished my degree, energies below the threshold for the production of pi-mesons, that is, several hundred million electron volts, were “low”. “High” energy was beyond that. When I was still a student, the Chamberlain-Segrè group at Berkeley used the Bevatron (energies up to nine billion electron volts) to bombard protons on protons and neutrons, and produce antiprotons, again resizing the perspective on high, low. Today, one talks about trillions of electron volts.

The students who were guided by Schwinger, Glauber, and Klein in my day, a most congenial and mutually stimulating band of brothers, and one sister, grouped together in a complex of rooms in the basement of the old Harvard physics building, worked on quantum electrodynamics, or nuclear, or molecular physics. They all became academics: Sheldon Glashow (Berkeley, Harvard, Texas A&M, Boston U), Paul Martin (Harvard), Jeremy Bernstein (Stevens Institute of Technology and author of many articles in the New Yorker magazine), Roger Newton, {U of Indiana), Margaret Kivelson (UCLA), Stanley Deser (Brandeis), Al Peaslee (Los Alamos Laboratory), Robert Raphael (a period in France, then Catholic U of America), and myself.

Zierler:

So, Chuck, then I want to ask as your developing these relationships with Klein and Glauber, what are the kinds of questions that you’re asking where you feel like you can effectively contribute to all of these developments in theoretical physics that are going on around you?

Zemach:

I’m not sure I understand what your question is.

Zierler:

The question is in terms of you putting your dissertation together, right, what are the questions that are most interesting to you that you feel like you can make a contribution to theoretical physics at that time?

Zemach:

Well, I wanted to get the answers to my PhD problems. That was my immediate concern.

Zierler:

And what were those questions that Glauber and Klein posed to you?

Zemach:

In the 1930’s, Enrico Fermi showed that the scattering of low-energy neutrons, such as thermal neutrons, from atomic nuclei could be represented by a simple delta-function interaction. In early 1952, Glauber had incorporated this result into a quantum-mechanical operator formula which described neutron scattering, including elastic and inelastic channels, from a system of interacting atoms (neutron diffraction). This was relevant to the concerns of the developing nuclear reactor industry. Data on low-energy neutron scattering from methane gas had been measured at the Chalk River reactor in Canada. Glauber asked me to extract numbers from his rather opaque operator formula to match the Chalk River data and, more generally, to calculate neutron scattering from gases.

Klein posed a problem on hydrogen hyperfine structure, that is, on the small shifts in spectral lines due to the interaction of the proton and electron magnetic moments, arising from their spins. A “Fermi formula” gave the lowest-order contribution to this effect. The quantum electrodynamic corrections due to the electron’s behavior had also been calculated. These calculations assumed that the proton was a point charge with a point magnetic moment. In fact, the proton has a finite size, of perhaps 10-12 centimeters in extent, mainly due to its virtual interactions with the pi-meson field. So the proton electric charge has a certain distribution in space fe(r), and its magnetic moment has a distribution fm(r). My task was to find the effect on the hyperfine shifts of these distributions.

My thesis had a Part I on neutron scattering and a Part II on the hyperfine shift. The results of Part I appeared as Zemach & Glauber, "Dynamics of Neutron Scattering by Molecules and Neutron Diffraction by Gases", Phys. Rev. 101, 118, 129 (1956). My curves coincided with the Chalk River data exactly. A nuclear reactor official later congratulated me because the papers “solved the whole problem”. Later, a Berkeley engineer showed me a. Russian-language textbook on reactors in which one chapter was titled “Zemach-Glauber Theory”.

The results of Part II appeared as Zemach, "Proton Structure and the Hyperfine Shift in Hydrogen", Phys. Rev. 104, 1771 (1956). The paper introduced a new type of distribution, fem(r), which was the convolution of the electric and magnetic distributions fe(r) and fm(r). The proton-size correction depended only on the first statistical moment of this convolution distribution. Later work by others on hyperfine shifts in heavier nuclei also utilized this statistical moment, and it was called the Zemach moment. My daughter Dorothy, browsing through the internet, encountered this term; she said it sounded like the name of an exotic new perfume.

Zierler:

[laugh]

Zemach:

A couple of years later, there was a follow-up paper to Part I: Robert Mazo & Zemach, "Diffraction of Neutrons by Imperfect Gases", Phys. Rev. 109, 1564 (1958). This derived a virial expansion (series in powers of gas density) of the neutron cross-section which accounts for correlations among different gas molecules. As a by-product, a simplified rule for the Ursell-Mayer resolution of separable functions of gas-particle coordinates into cluster functions —needed for virial expansions — was obtained. We defined a generating function for the separable functions and a generating function for the cluster functions, and proved that the latter was the exponential of the former. Does that answer your question up to now?

Zierler:

Absolutely. Chuck, who was on your dissertation committee? Do you remember?

Zemach:

Roy Glauber because he was my thesis advisor. And Abe Klein. There was John Van Vleck, the chairman of the department, who was also my undergraduate advisor, so I wanted him on that. There must’ve been another physicist and somebody from the math department whom I don’t remember.

Zierler:

What year did you defend your dissertation, Chuck?

Zemach:

1955.

Zierler:

And what was your next move? Where did you go after that?

Zemach:

I had an NSF fellowship for a post-doctoral position which I took at MIT for a year. Abe Klein had become a professor at the University of Pennsylvania in Philadelphia. He offered me a job there as an instructor — a rank which I believe no longer exists anywhere —to begin September, 1956, and I took it.

Prior to that year, I had taken a trip out to California with a couple of friends, and visited the Stanford physics department, during the summer. That was something of a lark. It was just a way to spend the summer. However, I did fall in love with California, not only with the Bay Area but with the national parks and all the other parks and beaches and everything in California.

So although I then returned to Philadelphia for my first job, I was determined to go back to California. The following year, I was offered an assistant professorship at University of Pennsylvania, with assurance of an associate professorship to follow (that is, tenure}, but I turned that down in favor of a postdoc at Berkeley. This was the first of three occasions on which I abandoned a tenured job, or promised tenure, for an uncertain future. I always kept in mind the words of the featured speaker at my high school graduation, a Mr. Paul Schubert, who was a former graduate of my high school and had become a well-known naval authority. He said, “When you have a choice between security and opportunity, choose opportunity.”

Zierler:

So you started at Berkeley when? In 1956?

Zemach:

No, in Berkeley I was an assistant research physicist—that means a postdoc—beginning in 1957. In 1958, I was already an assistant professor of physics. In 1962, I was associate professor, and a full professor in 1965.

Zierler:

Chuck, what kind of new research did you take on when you joined the faculty at Berkeley?

Zemach:

Some of them were single shots at nuclear and particle physics: One was on pion scattering from nuclei (with Ken Watson). The nucleus was represented as a refractive medium and the index of refraction could be inferred from the scattering data. Another showed how a transformation of a possible Lagrangian for muons, electrons, and photons eliminates contact muon-electron interactions, consistent with experiments (with Nicola Cabibbo and Raoul Gatto). Several others were on properties of the Born series and analyticity for potential problems (with Abe Klein and with a math graduate student Farouk Odeh).

But then there were some profound and large questions.

One was the so-called bootstrap theory, and another was the task of analyzing angular correlations and other info from the stream of data emerging from the bubble chambers at Berkeley and elsewhere. The bootstrap theory was invented by Geoffrey Chew, or possibly by Chew plus his graduate student, Steven Frautschi. They published a paper which I thought — and I guess they agreed — was a fundamental conceptual advance in how elementary particles are put together.

A proton and an electron in a hydrogen atom are bound by the electromagnetic force, which is “weak”. Therefore, the mass of the hydrogen atom is only slightly less than the mass of the proton plus the mass of the electron. That was old history.

A proton or neutron interacting with a pi-meson is in the regime of strong forces, more than 1000 times the electromagnetic force. Such forces may possibly form a nucleon-pion bound state with binding energy comparable to the rest energies of the constituents.

There is a presumption that dynamical rules exist whereby, with input data about a physical system given, physical consequences can be deduced.The input data for an elementary particle system will consist of dimensionless quantities such as the numbers of particles of various spins, the ratios of their masses, interaction coupling constants, and whatever else is necessary to define the system. The output will include an enumeration of the bound states produced, their spins, the ratios of their masses, and other numbers of the same character as the input data. The bootstrap principle is that the output numbers must equal the input numbers.That is, all strongly-interacting “elementary” particles are composite.This principle produces a set of self-consistent equations, one solution of which, corresponding to the real universe, must exist. Although this means that “everything is related to everything else”, limited sets of self-consistent relations relating a few low-mass particles can be found. Thus, Chew and Frautschi, with a primitive dynamical method (The N/D method of Marshall Baker, Chew and Stanley Mandelstam), showed that rho mesons could be regarded as bound states of pi mesons and rho mesons, and their calculated rho-pi mass ratio and rho-pi-pi coupling constant agreed fairly well with experiment.

This was a remarkable idea, and I spent a fair amount of time with some colleagues testing it in other ways. Elementary particle physics efforts divide, roughly, into two areas: dynamics and symmetries. The study of bootstraps called for bigger and better dynamical methods. My two principal Berkeley colleagues of that period, Sheldon Glashow and Steven Weinberg, pursued symmetries.Well, that’s where the money was —

Zierler:

[laugh]

Zemach:

— and they both earned Nobel Prizes. Now the bootstrap idea was of Nobel stature, but not mine. I did have the notion that what I was working on would earn or was helping to earn a Nobel Prize for Geoffrey Chew and perhaps Steven Frautschi.

Then Fredrik Zachariasen (Cal Tech) and I did a bootstrap test which extended the Chew-Frautchi result: Zachariasen & Zemach, "Pion Resonances", Phys. Rev. 128, 849 (1962). The experimentalists hd turned up the omega meson, heavier than the rho, which decayed into three pi’s. We did a two-channel bootstrap of this system, with a two-channel N/D method, again in fair agreement with experiment, despite the crude dynamics and simplifying assumptions.

Zachariasen and I were close friends since our common year as postdocs at MIT. Zachariasen used to tell people he liked to collaborate with me because he could get his name first on the paper. We had another scheme for exploiting our common last initial. We would write a paper entitled “Zitterbewegung in high-Z-Nuclei” and list Dan Zwanziger and Bruno Zumino as co-authors. Nothing came of it.

The next step was more ambitious: Ernest Abers (then my graduate student and later chairman of the UCLA physics department) & Zemach, "Bootstraps and the Pion-Nucleon System", Phys. Rev. 131, 2305 (1963). This was a calculation of the pion-nucleon and pion-N* coupled channels. Here, N* refers to the 1238 MeV pion-nucleon resonance with 3/2 spin and 3/2 isospin.. Everyone took for granted that the N* resonance was due to attractive forces between pions and nucleons. We confirmed this and showed that the same forces led to a a bound state in the 1/2 spin, 1/2 isospin state, the same state as the nucleon. Therefore, it was the nucleon. There were no other lower-energy bound states or resonances in any spin-isospin combination of the pion-nucleon interaction. The bootstrap predicted the existence of states that did exist, the absence of states that didn’t exist, and our calculation gave qualitative confirmation of the nucleon/pion and N*/pion mass ratios, the nucleon-nucleon-pion coupling constant, and the N* width.

In those days, Berkeley was producing another heavy nucleon resonance or hyperon resonance almost every couple of weeks. Three higher-energy pion-nucleon resonances had been identified in certain higher angular-momentum channels while eleven channels lacked resonances. Dynamical calculation in these fourteen cases was beyond our ability, but whether the forces were attractive or repulsive could be roughly estimated. With two exceptions, attributed to the uncertainty in assessing forces in these cases, resonances existed in the attractive cases and were absent in the repulsive cases.

Two other papers followed in this sequence: Abers, Zachariasen, & Zemach, "Origin of Internal Symmetries", Phys. Rev, 132, (1963) and Zachariasen & Zemach, "Parity Conservation and Bootstraps", Pays. Rev. 138, B441 (1965). When the self-consistent bootstrap solution contains, as nature does, a multiplicity of particles of like spin, their masses and interaction strengths rvmay well be equal because the determining equations possess symmetry with regard to them. Parity conseation of the strong interactions, which appears to be true in nature, fits naturally into this program. This argument applies equally well to time reversal and charge conjugation. Simplified models are worked out in these papers, not to prove the validity of this explanation, but to persuade the reader that it is plausible, and in fact, attractive.

Also, R. H. Capps, showed in 1963 that, if mass equalities are assumed, the bootstrap principle implies full SU3 symmetry for interactions of pseudoscalar mesons and vector mesons. This could have been an example in our "Origin" paper, but Capps thought of it first.

We felt that the above work gave strong support to the bootstrap idea. But the crudeness of the N/D method and other simplifying assumptions pointed up the need for improved dynamics.

In 1951, Bethe and Salpeter, and also Schwinger, preceded by some preliminary work, set forth a relativistic-quantum-field-theory generalization of the non-relativistic Schrödinger equation for the interaction of two particles. The Bethe-Salpeter (BS) equation was applied successfully to obtain the relativistic shifts in hydrogen energy levels, because the weak electromagnetic force permitted perturbation expansions from familiar non-relativistic states. There had been no application to strong interactions. When relativity is incorporated into description of a quantum system, not only are kinematics changed, but also encountered are time-retardation of forces, anti-particles, and the production of additional particles from a two-particle state, linking a two-particle state to a multi-channel system.

In 1965, Charles Schwartz, whose office was next to mine at Berkeley, used a variational principle to calculate bound states of an approximate BS equation for two scalar particles interacting via a third scalar particle. This was followed by Schwartz & Zemach, "Theory and Calculation of Scattering with the Bethe-Salpeter Equation", Phys. Rev. 141, 1454 (1966), also via a variational principal. This approach produced data for the “intended” generalization of the non-relativistic problem, avoiding confrontation with the multi-channel character of the general problem.

At that point, one of my goals was to repeat the N-N*-pi calculation with the BS equation, to compare experiment with a better dynamical theory. The first step was to master the technology of the BS equation with scalar particles, a fourth-order partial differential equation, with the help of two graduate students. That took a while. All the differential-equation solvers offered by the Berkeley computer experts crashed early on, when programmed with our equation. The experts said the equation was “stiff”. We failed to improve any of these computer routines. The eventual solution, not dependent on the computer experts, was to expand the interaction and the wave function in series of powers of x, the four-dimensional radius variable, and determine their coefficients by iteration. When these functions vary something like e-x for x-values up to 10, and we aim at five-digit accuracy in the output, that takes a lot of terms in the series. But it worked.

This led to: Kershaw, Snodgrass, & Zemach, "Methods for the Bethe-Salpeter Equation I. Special Functions and Expansions in Spherical Harmonics", Phys. Rev. D 2, 2806 (1970) and Kershaw & Zemach, "Methods for the Bethe-Salpeter Equation II. Brackets and the N/D Method", Phys. Rev. D 2, 2819 (1970). The first of these prepared the formalism, including Green’s functions for the spherical-harmonic truncated solutions, and special functions which we called vector Bessel functions. As a by-product, a simplified technique for computing the nodes and weights for Gaussian quadrature was introduced. (A similar method was developed at Stanford at about the same time.) The second paper detailed the method for calculating bound-state energies and the two-particle eigenfunction component of the multi-channel state. The N/D method referred to had no connection with Chew’s approximation. For convenience, the interaction was multiplied by lamda, and for each total mass-energy from zero to threshold, the bound states were identified by eigenvalues of lambda. The actual data, including contour maps of the eigenfunctions, were published later in Zemach, "Observations on the Two-body Relativistic Wave Equation", Physica 96A, 350 (1979).

The next step was to look at the pion-nucleon-N* problem again. Because the nuclear spins coupled with orbital angular momentum in a relativistic context, the old Chew-Low separation of partial waves exploiting the Pauli spin matrix vector and the rotation group in three dimensions needed replacement by their analogs in four dimensions. This necessitated analysis utilizing the rotation group in four dimensions. I had worked out the details after the last-mentioned papers were completed.

But at that point, I had come to a parting of the ways with Berkeley—and with the University of California. I had decided to go to Washington, and join the Arms Control and Disarmament Agency (ACDA), an adjunct of the State Department, because the Strategic Arms Limitation Talks (SALT) were about to start up between the US and the USSR. And I never completed the bootstrap program.

Zierler:

Did the work in the bootstrap—Chuck—did the work in the bootstrap program continue on after you had left it?

Zemach:

No, as far as I know. The nature of frontier physics since then has completely changed. There’s quarks, gluons, new understanding of gauge theories and the relation of weak interactions and electromagnetism, the standard model, and the Higgs particle. I never did appreciate how electrons and photons might be tied to bootstraps; the heavy gauge bosons were unknown at the time (although suspected). I am still convinced that bootstraps make sense for the strongly interacting particles formerly regarded as elementary. The work of the 1960’s provides undeniable support. Recall one item: A model of pion-nucleon interactions good enough to predict the N* resonance in the 3/2 spin, 3/2 isospin state, at roughly the right mass, will also predict the nucleon as a bound state with 1/2 spin, 1/2 isospin, at roughly the right mass. Perhaps this can be passed off as an incidental implication of crossing symmetry, not invalidating a dynamical calculation starting from assumed interactions of quarks and gluons. See, for example, the lattice QCD calculation by Guralnick, Warnock, and Zemach cited later in this account. I did pursue certain other ideas while I was at Berkeley. I don’t know whether you want me to discuss…

Zierler:

Yes, please do.

Zemach:

OK. Now what else did I pay attention to at Berkeley? Well, this was—the Berkeley period was a splendid explosion of high-energy discovery—experimental discovery—largely following from the invention of the hydrogen bubble chamber. There was a question of how to derive experimental results by the spins and parities of the new particles and resonances which were formed. I did a comprehensive study of how to do that, which was quite different from the conventional approach. The conventional approach was to combine angular momenta by Clebsch-Gordan coefficients. My approach was to combine angular momenta through tensors which were functions of momentum vectors.