Oscar Wallace Greenberg

Zierler:

Okay. This is David Zierler, Oral Historian for the American Institute of Physics. It is February 16th, 2021. I am so happy to be here with Professor Oscar Wallace Greenberg. Wally, it’s great to see you. Thank you so much for joining me. 

Greenberg:

Oh, it’s my pleasure. 

Zierler:

To start, would you please tell me your title and institutional affiliation?

Greenberg:

Professor Emeritus and Research Professor at the University of Maryland in College Park. I’m also Adjunct Professor at Rockefeller University. 

Zierler:

I see. And when did you go Emeritus from Maryland?

Greenberg:

2015. 

Zierler:

In what ways since have you remained active in the field? Are you teaching? Are you still reading papers? Or are you truly retired?

Greenberg:

I’m not teaching, I am reading papers, and I’m reviewing papers and discussing things with my friends and other members of the department. 

Zierler:

Wally, given your long-range career, what are the things that are most interesting to you today to look at? What papers are most compelling to you to read?

Greenberg:

I think, now, the most interesting thing is cosmology and our universe, actually. Particle physics is still interesting, too. And of course, quantum field theory. I’ve always been interested in quantum field theory. 

Zierler:

Wally, would you count yourself among so many of your colleagues in particle physics who later on pursued interests in cosmology and astrophysics?

Greenberg:

Yes, yes. 

Zierler:

In what ways have you used your background in particle physics to think about astrophysics and cosmology? 

Greenberg:

Well, there’s a big connection because your knowledge of particle physics provides information which is useful in cosmology and the discussion of the early universe. 

Zierler:

Wally, on the personal side, how have you been faring during the pandemic? In what ways have you been combatting the problems associated with physical and social isolation?

Greenberg:

Well, fortunately, I’m happily married, and that makes a big difference. Of course, we have Zoom, telephone, and so forth, so we can keep in contact. We are living in a retirement community called Riderwood, which has two or three thousand retired people, and there’s a big social community connected with Riderwood itself. So, even inside of this community, we have a lot of social contact, which is very important. And we get food delivered every day, so we’re very fortunate. We’re very sheltered, and very taken care of. I also have an older brother, six years older than I am, who in some ways was like a father when I was younger. So, I have this good contact with him, and I have three sons who I can speak with also, and two grandchildren. So, I’m socially connected. 

Zierler:

Wally, I know you’re fully vaccinated. Is the retirement community starting to think about social occasions and life getting back to normal at all?

Greenberg:

Just beginning. They really haven’t reopened things yet. 

Zierler:

Wally, let’s go all the way back to the beginning. I’d like to start first with your parents. Tell me a little bit about them and where they’re from. 

Greenberg:

Both of my parents are Jewish immigrants from Eastern Europe; my father from Poland and my mother from Russia. They both came here before the First World War. So, they came to the United States relatively early. I think their native language was Yiddish, which I never learned because it was a sort of secret language which they used to talk to themselves. My father also spoke Polish. I guess it was a kind of typical Jewish family from Eastern Europe, where education was very important. 

Zierler:

Where did your parents meet?

Greenberg:

I don’t really know where they met. They met in the United States, but I don’t know where. There was a Jewish community association called the Workman’s Circle. It was a social organization and also provided insurance and had a Jewish doctor who was usually starting out in practice. I remember it would cost seven cents to go across town. We never lived in a Jewish neighborhood. We always lived in a non-Jewish neighborhood. My father was a storekeeper. He had a luncheonette, and then later a shoe store. So, we always lived near the shoe store, or whatever the business was.

Zierler:

Was your sense that your parents’ families left Europe more because of antisemitism, or because of economic opportunity, or a combination of both?

Greenberg:

I think a combination of both. They came before the First World War, so they came early. They always spoke with a Jewish accent. 

Zierler:

Where did you grow up?

Greenberg:

I grew up first in East Orange, and then more time in Newark, New Jersey. As I said, I had an older brother, six years older than I, who became an academic administrator. He became Senior Vice President of Rutgers University, the state university in New Jersey. He was known for his integrity. He was a very organized and responsible and a careful university administrator—not an empire builder, but someone who serves the University. 

Zierler:

Wally, to what extent was your family Jewishly connected when you were growing up? Did you belong to a shul? Did you get bar mitzvahed? That kind of thing?

Greenberg:

Yes, I was bar mitzvahed. I was circumcised, and I belonged to a shul, yes. We had a large family, mostly on my mother’s side. My mother was one of eight children, so we had a lot of relatives on my mother’s side. My father had one brother, so we had some relatives on his side, too. There was a lot of family connection. In fact, many of the Jewish immigrants had businesses of one kind or another. We had our own internal family businesses that we went to for things, so if you needed a pair of shoes, people came to my father because he had a shoe store. If you needed haberdashery, you went to my uncle.

Zierler:

Wally, when did you start to get interested in science? Was it early on?

Greenberg:

Very early. I was interested in science from a very early age. I think I used to go out at night and make pictures of the stars all by myself. I remember, even in grade school, when I was in the second grade, they would bring me to the fifth grade, and have me read to the fifth grade students to show that a second-grade kid could read well, to sort of impel the fifth graders to read better. I remember that when I was eight years old in grade school, I made up a chart of the natural radioactive decay series, which I was asked to give to a sort of student assembly. So, I would go to the library. Also, mathematics. I loved mathematics. 

Zierler:

In high school, did you have a strong curriculum in math and science, would you say?

Greenberg:

No, no. I went to Barringer, which is the third oldest public high school in the United States, but it was nothing unusual. 

Zierler:

What kind of colleges did you apply to?

Greenberg:

Well, I went to Rutgers, the state university. I think I applied to Temple in Philadelphia. The point is that at Rutgers you could get a scholarship, so I never paid tuition. I went to Rutgers as an undergraduate, and then Princeton as a graduate student, and I never had to pay tuition for any of that. In graduate school I was supported. 

Zierler:

Did you not have the grades where you thought that a place like MIT or Princeton or Harvard was within reach?

Greenberg:

Well, I went to Princeton for graduate school.

Zierler:

But I mean for undergraduate, when you were a high school student. 

Greenberg:

Well, my grades were good, but the point is that Rutgers, as the state university of New Jersey, I could go for free for tuition. So, I didn’t seriously apply to other places for undergraduate school. It was a good school, and I was happy. 

Zierler:

Wally, when you go to Rutgers, was the plan for you to major in physics right away, or did that come later?

Greenberg:

Well, I majored initially in mathematics, and then switched to physics. I took a lot of mathematics also. 

Zierler:

What compelled you to make the switch?

Greenberg:

I think mathematics had a lot of sub-branches. Physics seemed like a more unified subject. I thought I’d do better in physics, but I took a lot of math courses also. I took graduate math courses while I was an undergraduate student. 

Zierler:

Who were some of the professors in the physics department that you may have gotten close with at Rutgers?

Greenberg:

I can’t remember their names. There were some, but—when I was an undergraduate, there was one other student, Al Robbins, who was a physics major. We were the only two physics majors. We used to do homework together in all night cafeterias on New Brunswick. 

Zierler:

Wally, did you know if you wanted to pursue theory or experimentation by the time you finished as an undergraduate? 

Greenberg:

Well, I knew I wanted to do theory. 

Zierler:

Why was that? Because of your mathematical interest, would you say?

Greenberg:

Yeah, I had no interest in laboratory work. I’m not sure I was suited for it. But I liked to do theory. I love theory. I did mathematics just for the fun of it.

Zierler:

And you would have graduated when, about 1952 or so?

Greenberg:

‘52 undergraduate, and ‘56 graduate, yes. 

Zierler:

And by the time you finished undergraduate, what were some of the most exciting developments in theoretical physics? What was going on at that point that you might have wanted to pursue for graduate school?

Greenberg:

Well, I think these particles were being discovered, the various hyperons, and various unstable particles which decay into protons and neutrons and electrons.

Zierler:

Did you get any advice about where to go for graduate school, or how did you go about making that decision?

Greenberg:

Well, Rutgers was a state university, and then from there, I became acquainted with the fact that Princeton was a good place. So, I went there. There was someone named Eugene Wigner, who eventually won the Nobel Prize in physics, and there was John Archibald Wheeler, and Arthur Wightman, who was my thesis advisor, who had come from Yale. And then Valentine Bargmann, a mathematical physicist. So, it was certainly a good place for graduate school.

Zierler:

What were your impressions when you first got to Princeton? Did it feel like a very different place than Rutgers?

Greenberg:

Yeah. Well, I graduated college, I guess, at 16. I got my Ph.D. at 20, so—

Zierler:

You were very young. 

Greenberg:

I was young, yeah. I remember, even in grade school, when I was in the second grade, they would bring me to the fifth grade, and have me read to the fifth grade students to show that a second grade kid could read well, to sort of impel the fifth graders to read better. 

Zierler:

Wally, how did you go about developing your thesis research? What was your dissertation on?

Greenberg:

It was called “The Asymptotic Condition in Quantum Field Theory.” It is the mathematical structure which enables you to calculate the scattering amplitude, the S-matrix, from a quantum field theory. So, my Ph.D. thesis was purely theoretical. The only numbers in the thesis were equation numbers and page numbers. There was no empirical information in the thesis at all. 

Zierler:

Who was your graduate advisor? 

Greenberg:

Well, the thesis advisor was Arthur Wightman. 

Zierler:

What was Wightman working on by the time you connected with him? What was his research?

Greenberg:

Well, he was interested in quantum field theory, and he was developing something which he called axiomatic quantum field theory, which turned out to be rather sterile. I think it was—physics is empirical science, and you don’t want a lot of axioms and things like that. You want to look at the world and describe the world, and not have axioms which constrain you in your understanding of what’s going on in the world. So, Wightman had a lot of influence. He was sort of the guru of quantum field theory, but I later got into the more empirical side of quantum field theory, more empirical side of particle physics. My best work was in that area rather than the axiomatic side. 

Zierler:

What was Wightman like as a person?

Greenberg:

Very hardworking, but he had his own way of doing things, and he wanted to impose his mode of thinking on other people, which I didn’t particularly like. It wasn’t easy to work with him. He was hard, and he made it hard for me to publish because his standards were unreasonably high and not necessarily realistic. My Ph.D. thesis never got published because of Wightman’s interference. It was only when I left graduate school that I started to publish more prolifically. 

Zierler:

Wally, to zoom out a bit from your dissertation, what were the conclusions of your research, and how did it relate to some of the broader questions in theoretical physics at that point?

Greenberg:

Well, I proved a number of theorems connected with—for example, the connection of internal and space-time symmetries, and things like that. I did a lot of that kind of work. I worked on something called the CPT theorem, which is a symmetry that a large category of theories obey. I think almost all of my publications are single publications. I didn’t collaborate with other people. I talked to people, but I didn’t collaborate with them. 

Zierler:

Wally, do you remember who else was on your thesis committee besides Wightman?

Greenberg:

Yeah, there was somebody named Bargmann, who was a German émigré who was very mathematically inclined. And then, there was [Alan] Shenstone, who was a spectroscopy experimentalist. So, he didn’t have much to do with it. It was Wightman, Bargmann, and maybe Wheeler, John Archibald Wheeler. 

Zierler:

Wally, after you defended, what opportunities were available to you? Were you looking for faculty positions? Did you want to do a post-doc? 

Greenberg:

I wanted to do a post-doc. My first post-doc was at Brandeis, where I worked with S. S. Schweber, where we worked on models of quantum fields. Then I worked at MIT with Francis Low, and we proved some theorems about high-energy limits of scattering amplitudes, and something called the Greenberg-Low high-energy bound. 

Zierler:

How did you initially get connected with Low?

Greenberg:

Well, I was looking for another post-doc position, and MIT had one. I got to MIT, and I sort of gravitated to work with Low. He was a very intelligent man. He was the sort of person, when you explained things to him, he absorbed it immediately. He picked up on things very quickly. He was also very athletically inclined, so I would play tennis with him. 

Zierler:

Did MIT feel like a much bigger place to you than Princeton?

Greenberg:

Oh, yeah. They cleaned the halls with machines that clean streets. They had these whirling sounds, and you have these street cleaning things coming down the halls at MIT—if you’re there at night, which I was often. 

Zierler:

Wally, how long did you stay at MIT?

Greenberg:

Just a year. 

Zierler:

Did you work exclusively with Low, or did you have other collaborators?

Greenberg:

Generally, I didn’t collaborate much. I talked to people, but most of my papers are single-author papers. 

Zierler:

So, what else were you working on at that time? What other projects did you have?

Greenberg:

I was working on high-energy limits and the general structure of quantum field theory. I had been influenced by Wightman to pursue general questions in quantum field theory. 

Zierler:

What was some of the early research related to quarks, and were you aware of what [Murray] Gell-Mann was doing at Caltech?

Greenberg:

Yes, yes. In fact, I realized some of the issues with quarks required some new information, so I introduced what’s now called color, which is a three-valued hidden charge carried by quarks. It is the color force that holds the quarks together in the proton and neutron. So, that turned out to be very important. 

Zierler:

Wally, let’s back up for a minute here, because before this time, some people were even questioning whether quarks existed. What were those debates like? What were the parameters of these discussions?

Greenberg:

Well, the point is, one of the things about quarks is, if you have two protons, if you separate them, the force decreases like a magnetic force that goes down. It’s 1/r² in the force. With quarks, you can’t separate. As you separate them, the force doesn’t decrease. There’s something called permanent confinement. The quarks are permanently confined, so that led to a lot of skepticism as to whether they were real or not. So, there were a lot of issues about how to decide whether quarks really existed as objects, even though they couldn’t be isolated. You couldn’t isolate a quark. 

Zierler:

Who besides Gell-Mann was a leading figure in these discussions?

Greenberg:

[Richard] Feynman was very important, and he was a much better communicator than Gell-Mann. There was someone named George Zweig who collaborated with Gell-Mann, but also went off independently. People did not know whether the quarks had fractional charge or integer charge. One of my colleagues at Maryland was pushing for integer charge, Jogesh Pati, but the fractional charges really worked. 

Zierler:

So that I understand the sequencing, your contribution with color charge rested on what Gell-Mann had discovered with quarks themselves. In other words, you could not have come up with the idea of color charge without Gell-Mann and Zweig’s work initially.

Greenberg:

Yes, that’s right. The color charge pertained to quarks. It was the color charge that held the quarks together in the proton and neutron. One of the issues is that the quarks were spin one-half, and therefore had to obey Fermi-Dirac statistics. The quark model was inconsistent with the Pauli exclusion principle unless you had another quantum number connected with quarks to make the statistics work out properly. Color provided that. Introducing color made the quark model consistent with the exclusion principle. So, that was very important. Of course, it was very controversial because a lot of people didn’t believe in quarks anyhow. There’s an anecdote: I did some of this work at the Institute for Advanced Study, and [Robert] Oppenheimer was the director for the Institute of Advanced Study, and also a physicist. I gave him a copy of my pre-print about having color for quarks. I met him a week later, and I said, “Have you had a chance to look at my paper?” He said, “Greenberg, it’s beautiful.” I was very happy. Oppenheimer was a decent physicist. And then he said, “But I don’t believe a word of it.” That was typical of Oppenheimer. He always had to make a clever comment. He was always someone who could summarize things in a very clever way. Not always completely accurately, but—

Zierler:

Wally, of course, Gell-Mann was proved correct, and Oppenheimer was proved incorrect. What was your understanding? How could there be so many eminent physicists who just denied the existence of quarks? What was that all about?

Greenberg:

Well, people grow up in a certain way, and they have certain beliefs. Even though physics is an empirical science, people stick to their beliefs. There was a kind of anti-quark feeling. People didn’t like the quark model.

Zierler:

What does that mean, an anti-quark feeling?

Greenberg:

The quark model was so simple. The math has simple numbers, like a third and two-thirds etc. and things worked out in a very simple way. People wanted more complicated theories. My thesis advisor, Wightman, wanted some overarching big theory. The idea that you could just do things with fractions like one-third and two-thirds and so forth, that was too simple for him. People had fixed ways ideas about things. The idea that something could be very simple—I always look for the simplest possibility. Inventing some new thing, like color, so things could be simple, other people want to make things complicated with what they had. I was willing to throw away what you had and just invent some other thing that made life simple. The point is, if you’re faced with a difficult problem, one way of handling it is to complicate the situation that you know about, introduce all kinds of bells and whistles to make the thing you know work. Another possibility is just to throw that stuff out and invent some new thing that’s simple, and that solves the problem. That was my attitude, to do the new thing, and not to become an expert in all the intricacies of what didn’t work. You had to introduce some new thing to make the quark model work. You had to have a new kind of quantum numbers connected with quarks that was invisible with protons and neutrons because it was sort of saturated. The quarks carry color, but when you put three quarks together to make a proton or neutron, it’s color neutral. So, the color is zero, so to speak. So, you had to invent a new thing that didn’t show up in the protons and neutrons to make the theory work. The point is, my attitude was not to complicate the current theory by adding new, extra possibilities, but just to throw it out and get some new thing that was simple. That’s what color provided.

Zierler:

Wally, after Zweig and Gell-Mann posted this about quarks, were the concept of quarks accepted quickly by the physics community, or was this a gradual process?

Greenberg:

No, it was too radical for people. They didn’t like it. It took about ten years for these ideas to be accepted. Very fine physicists, like Steve Weinberg, who is a Nobel Prize winner, did not like it at all, and just couldn’t accept it. 

Zierler:

Why did it take so long? A decade is a long time. What happened in the course of that decade? 

Greenberg:

Well, if you look at the excited states of baryons, what is called baryon spectroscopy, which a quark model could work out a simple picture of what the excited states of protons or neutrons, which are baryons, would be. I worked that out. It took some ten years for the experiments to show that the predictions I made about the excited states of protons and neutrons were correct. I predicted what the higher levels of the excited states of protons and neutrons would be, but it took some ten years for those experiments to be done to confirm what I proposed was correct. Then, after ten years, it was known, and it wasn’t new anymore. Also, I must say, I was not a good self-publicist. I tend to be rather self-effacing, and rather quiet. 

Zierler:

Wally, at the end of the ten years, observationally, or experimentally, what might have been proven about quarks that wasn’t available ten years earlier, or were these still theoretical concepts even ten years later?

Greenberg:

Well, we knew what the excited states of baryons were, what’s called baryon spectroscopy. That gave empirical information, and I predicted what those levels should be in great detail, and it was confirmed. It took some ten years for those experiments to be made. 

Zierler:

Where specifically? Where were these experiments that confirmed your prediction?

Greenberg:

I think mostly Brookhaven. 

Zierler:

Did you spend time at Brookhaven yourself?

Greenberg:

Summers, yeah. My visits to Brookhaven were not very important in terms of my own work. Of course, CERN—Brookhaven and CERN. 

Zierler:

Wally, I’d like to go back to the term color charge. As I’m sure you know, people like Dick Feynman did not like using the term color, because it didn’t accurately describe what was going on. Of course, we’re not talking about actual colors. So, can you talk about the decision to name it color, and why that may have been a good decision?

Greenberg:

Well, the idea was that if you combine three different colors of light, you can make white light. If you combine the three colors of quarks, then you can make things that are color neutral, the protons and neutrons. I didn’t give it the name color, actually. I’m not sure who did that. 

Zierler:

But to be clear, the word color is metaphorical. Quarks don’t actually have color in the sense that we think of colors. 

Greenberg:

That’s correct. It’s just a convenient label for the three-value charge that they carry. 

Zierler:

Wally, what do you see as your contribution with color charge? How do we understand quarks better now as a result of the concept or the theory of color charge?

Greenberg:

Well, I explained what we should expect when we excite protons and neutrons. So, the excited states, what’s called baryon spectroscopy, require the color charge to make it work. I worked out what the consequence of color charge would be, and over some period of ten years, my calculations were confirmed by experiment. You had to have high energy accelerators that could excite these excited states of protons and neutrons to verify my predictions. It took some time, about 10 years for my predictions to be confirmed.

Zierler:

Wally, in terms of where you were physically as you’re thinking about these things, were you mostly in College Park, or were you mostly at the Institute at Princeton?

Greenberg:

I was at MIT from 1959 to 1961, then at University of Maryland in 1961, and the Institute of Advanced Study in Princeton in 1964. Well, I was at MIT for two years as a Postdoctoral Fellow, where I worked on high-energy limits with Francis Low. And then I came back to Maryland. I was two years in the Air Force, from 1957 to 1959. I’m a Captain, U.S. Air Force, Retired. I had an ROTC commission from undergraduate school. Eventually, I was stationed at Air Force Cambridge Research Center in Bedford, Massachusetts, at Hanscomb Field. So, I was in the Air Force. 

Zierler:

And then you were an associate professor, a visitor at Rockefeller University in the mid-1960s. 

Greenberg:

That’s correct.

Zierler:

What was your interest in going there? What was happening at Rockefeller?

Greenberg:

Well, there was somebody named Abraham Pais, who was a good physicist, who was at Rockefeller. I never wrote a paper with him, but I had interactions with him. There’s somebody named Nick Khuri, who I had known from graduate school who was at Rockefeller also. 

Zierler:

Then, later in the decade, you spent time in the Weizmann Institute.

Greenberg:

Yeah, I spent two years at Weizmann, and Tel Aviv University.

Zierler:

What was your interest in that? Did you have any Zionistic interests?

Greenberg:

Not particularly. It was a good place. There was a lot of good physics going on there. I also met my first wife and got married in Israel. That marriage produced three children, but the marriage didn’t last. I got three sons from my first marriage. 

Zierler:

It’s interesting, because your first decade at the University of Maryland, you’re really not in College Park much at all. 

Greenberg:

That’s true. It was easy to get postdoctoral positions and fellowships and so forth. That’s right. 

Zierler:

When did you come back to College Park on a more permanent basis? Was it the early 1970s?

Greenberg:

I think so. I don’t have my CV in front of me, but that sounds about right. 

Zierler:

What were some of the projects that you took on at that point? There are so many exciting things that are happening in theoretical physics at this point. 

Greenberg:

I was very interested in high-energy limits. How big can the cross section get at high energy? So, I worked with Francis Low on high-energy limits. He was at MIT. I did know some theorems about the connections of space-time and internal symmetries. Space-time symmetry would be like rotation in the Lorentz group, and internal symmetries would be like isotopic spin, and things like that, how these symmetries are related and how they’re connected. I did some sort of mathematical things, too. I did some theoretical work on the possible relation of these kinds of symmetries to each other. 

Zierler:

How were advances in experiments useful to your research at this point? There are so many amazing things that are going on. 

Greenberg:

Of course, I had predicted what the excited states of protons and neutrons should be. Over some ten year period, my predictions were verified. So, that confirmed that color really did exist. Confirmation of color was [inaudible]. 

Zierler:

Wally, who were some of the prominent graduate students you’ve had as a professor at Maryland? 

Greenberg:

Well, my best student was Amit Raychaudhuri, who became a professor in India. He became Palit professor of physics and head of the Harish-Chandra Institute of Physics in India. He was my best student. 

Zierler:

Wally, what are some of your favorite classes to teach undergraduates? 

Greenberg:

Well, I taught history of physics-type things to undergraduates. That was fun. 

Zierler:

Is that an interest of yours, history of physics?

Greenberg:

Somewhat, yeah. I preferred to teach graduate courses like quantum field theory. I tended to do things in the more mathematical aspects of quantum theory. I probably taught everything in the curriculum at one time or another. At Maryland, the teaching load was very light. If you had a grant, and I did, then you only had to teach one course a semester. So, that was very light teaching. So, that’s what I did. 

Zierler:

Wally, were you ever interested or following the developments in string theory?

Greenberg:

Not particularly. I never did anything in string theory. String theory people had this idea that it was going to be the theory of everything, and I was always very skeptical about theories of everything. I took a more down-to-earth point of view. There was no smoking gun that showed that string theory was correct, and it became very mathematical. They started going into higher dimensional theories and so forth. It became a mathematical game after a while. 

Zierler:

Wally, let’s start with Einstein. When did you first meet Einstein? Was it at the Institute? 

Greenberg:

When I was a graduate student. The people at the Institute for Advanced Study were told by Oppenheimer not to bother Einstein. But I was a graduate student at Princeton University, so I didn’t know about that prohibition. You could call Einstein’s secretary and make an appointment and go see Einstein. So, I did that a number of times and had conversations with Einstein. I remember having a conversation with him at about lunchtime, and when we got up to go home for lunch, I had the impulse to help him on with his jacket, but I was too shy, so he had to put his jacket on by himself. But I walked him home from lunch, and he was very easy to talk to. 

Zierler:

What kinds of things did you talk to him about? Physics, or life in general?

Greenberg:

If Einstein explained something which I couldn’t quite understand, I couldn’t press him. I was too much in awe of him. But he had an assistant, Bruria Kaufman, and I could press her. I could get my questions answered by her. But, still, it gave me an entry into meetings with Einstein, which I found very, very exciting. At these meetings I also got to his home and had tea where his assistant would make tea for everybody. I actually introduced Einstein at his last lecture. We had it in Palmer Lab, in one of the classrooms, but we kept it quiet so that the whole town of Princeton wouldn’t come to hear Einstein talk. He gave a very general sort of talk. I remember he made some comment—if a person such as a mouse is such and such—he had this very picturesque way of speaking. 

Zierler:

When did you first meet Wheeler?

Greenberg:

Well, he was one of my graduate school professors. Wheeler was a very systematic guy. He would start at one end of the blackboard and write systematically, cover the board, and go back and erase it and start again. He was a very hardworking person. Once I asked him a question after class, and he said, “Well, I’ve been up all night. Can I answer that question tomorrow?” He would stay awake all night working. 

Zierler:

What about Feynman? When did you meet Dick Feynman?

Greenberg:

I met Feynman somewhat later. I met him probably at meetings. I remember meeting him once at a restaurant on Connecticut Avenue in Washington after he had given the last talk of the session. I was having lunch with Wheeler, who had been his thesis advisor. Feynman came in, and Wheeler was trying to get Feynman’s attention to tell him about some physics work. Feynman was so excited. He had just given a talk, and he was so excited he couldn’t sit down. He picked up a knife and was flicking breadcrumbs that were on the table, with his knife. Wheeler could never get his attention. Feynman was just too excited to settle down and listen to Wheeler. 

Zierler:

What did Feynman know of your work?

Greenberg:

Well, to make things work, you need color, so Feynman used it in his work. The thing is, Feynman tended to reinvent things for himself, so he didn’t pay attention to the literature too much. I think he didn’t initially give me the credit, but he was generous to give me credit once he understood the situation, once I explained it to him.

Zierler:

Did you make him understand that you were to be credited with this research?

Greenberg:

Yeah, I did, and he did give me credit.

Zierler:

In what way did he give you credit? In a paper, in a speech?

Greenberg:

In a paper, eventually. He cited my work. He had rediscovered some things, and then he realized that I had already done it. 

Pearl Katz (Greenberg’s Wife):

Can I interpose about this modesty in not being known? You can take this out, but it’s a perspective. He had a conference in 2015 called, “50 Years of Quarks and Color.” There were like 16 people, and at least three Nobel Prize winners; as many speakers as people there. So, he was walking across the camps at Maryland with the Dean, who was planning this. Wally told him that it wasn’t really known that he had done this work. So, the Dean stopped the first physicist that they came on as they were walking on campus, and this is a person that was at least 30 years in the department with Wally and spent at least five years carpooling with Wally. The Dean said, “Hey, Joe? Who did color in quarks?” Joe scratched his head, Wally is right in front of him, and Joe said, “Well, I think it’s someone in Indiana.” So, his not being recognized and known is very real, because he’s so genuinely modest. 

Greenberg:

Yeah, I wasn’t very good at tooting my own horn. 

Zierler:

Wally, what do you think explains that? Why were you so self-effacing, giving the enormity of your contributions?

Greenberg:

I think it had to do with my upbringing. My mother would say, “Don’t be too outstanding. The non-Jews will be jealous.” So, I was taught at an early age to be self-effacing, and I think it stuck. 

Zierler:

I wonder if Dick Feynman’s mom could have given him the same advice, though. 

Greenberg:

Well, we don’t know. 

Katz:

Well, it’s one of the wonderful things about his personality, but it certainly wasn’t very good for his career, for his recognition for what he did, because people just didn’t know that he did this. Gell-Mann also attempted at times, and [Harald] Fritzsch attempted at times, to take credit for this work. And there was a meeting in Moscow where a Russian physicist claimed to have done this earlier, the color work. But fortunately, this was published in Phys Rev Letters, so there was no questioning about it and the Russian physicist had cited it.

Zierler:

Wally, when did you meet Wigner? When did you start to know about Wigner?

Greenberg:

Well, Wigner was one of my professors in graduate school. He was very kind. I remember asking questions, and he brought me into his office, and gave me a really detailed explanation—among other things, we were at a conference, and we left very early in the morning. We were both very hungry and thirsty, so I had some wrapped pieces of chewing gum which I shared with Wigner so we could get some sugar into our systems. Wigner was obviously very intelligent, but a very approachable person. So, I learned a lot from Wigner. He was a master of group theory, among other things. He had a strong mathematical bent. 

Zierler:

Did you stay in touch with Wigner after graduate school?

Greenberg:

I did, yes. In fact, we wrote an article in Physics Today with both our names on it, which I actually wrote. Wigner wanted me to write the article, so I did. One of my important collaborations was someone named Albert Messiah. His nickname was Bacco. So, I spent some time in Paris working with him. We did some work on statistics of particles. Messiah had fought in the Second World War, and he had been in the Free French Army. He was Jewish, and he went to the Free France Army, and came to London and fought with the Free French Army of de Gaulle. 

Zierler:

Wally, given your interest in the history of physics, I’m curious about how you see the history of theoretical physics, specifically theoretical particle physics, over the course of your career. In other words, it’s so obvious in the 1950s, the 1960s, and the 1970s, the field was wide open to such fundamental discoveries. Do you think that tapered off at a certain point, or has that fundamental discovery kept pace, even up to the present?

Greenberg:

I think it tapered off. I think the quark model was a seminal moment in organizing particle physics. I think the emphasis now is on cosmology and the early universe, but the particle physics aspect has been very well developed. I think the frontier is now really cosmology and the early universe. 

Zierler:

What do you think explains the fact that theoretical particle physics has tapered off? Is it that we know everything that there is to know? Are we limited by our knowledge, or by our observations?

Greenberg:

Well, if you take atoms apart, you get nuclei. If you take nuclei apart, you get protons and neutrons. You take protons and neutrons apart, you get quarks. As far as we know, there’s nothing more elementary than quarks, so that finishes the subject. I tried to invent models in which quarks were composites, but it didn’t work because there’s no empirical evidence for quarks being composite. As far as we know, quarks are elementary. Maybe at some higher energy, which we can’t access, we will see quark structure. Quarks may have composite structure, but there’s no empirical evidence for the structure of quarks. Quarks seem to be fundamental.

One of the people that I had a lot of contact with was Yoichiro Nambu, who is just a wonderful person and first-rate physicist. I had a lot of interaction with him. I would visit him in Chicago, and he visited me in Maryland. Nambu invited my wife and me to his Nobel Prize ceremony—when he got the Nobel Prize. He didn’t go to Stockholm.

Zierler:

Wally, could you imagine an experiment operating at sufficiently high energy that would definitively determine whether or not quarks are truly elemental?

Greenberg:

Well, at the energies we have currently accessible, they seem to be fundamental. There’s no way of knowing whether at some higher energy they would become composite, we would see composite structure. There’s no evidence for composite structure. I don’t know what energy you would need. Maybe they’re not composite. Maybe they are elementary. If they are elementary, then no experiment could prove otherwise. 

Zierler:

I wonder if philosophically it’s possible that it goes in both directions. For example, if the universe is limitless, isn’t it possible, at least theoretically, that particles get smaller and smaller in a limitless fashion, or does that not sit well with you?

Greenberg:

Well, you need evidence. There’s no evidence for any internal structure of quarks. 

Zierler:

Would something like the SSC have helped resolve this question, do you think?

Greenberg:

There’s no way of knowing. Yes, if we had the higher energy of the SSC, then maybe we’d learn something new, but we don’t know whether that’s true or not. A number of these things are empirical, not philosophical questions. But without the data, there’s no way of answering. 

Zierler:

Wally, you mentioned you spent some time and did some work at CERN. When was that?

Greenberg:

I don’t have the years in front of me. I think it was probably in the summer of ‘64 before I was at the Institute.

Zierler:

Many people say that when the SSC collapsed, CERN was really the last best hope that there would be experiments at these energies that might answer some of these questions. I’m curious if you think CERN might be the place where some of these questions might be answered empirically. 

Greenberg:

Well, we need higher energies than CERN can produce. So, there’s no way of knowing. 

Zierler:

Wally, tell me about the event that marked your retirement in 2015. Who organized that?

Greenberg:

My wife, mainly, organized that. We had a lot of good people there. It was a very nice period. 

Katz:

In addition to the Nobel Prize recipients, Robbert Dijkgraaf, the head of the Institute came and gave a major talk. Dijkgraaf and Physics Today asked Wally to write an article about his talk, which he did. One of his talks was on the usefulness of useless knowledge, and how these ideas about baryons were things that were just circulating in his mind, that he was playing with, with no thought of any possible usefulness. I think you also did some work with Messiah on it, so when the whole question of quarks came up, these kinds of things had already been circulating in his mind. This didn’t come from nowhere, but it had to do with just pondering things that nobody thought of. I don’t know how much you shared with anybody except perhaps Messiah. So that when these paradoxes came up about the existence of quarks and such, these kinds of meanderings that he had thought about were relevant. That’s why he could posit this three-value charge. [ed. See O. W. Greenberg, “The Origin of Quark Color,” Physics Today 68 (1), 33–37 (2015): https://doi.org/10.1063/PT.3.2655]

Zierler:

Wally, tell me about this idea of useless knowledge. As a scientist who’s focused on basic research, discovery itself is useful because it’s discovery. What do you mean by the concept of useless knowledge?

Greenberg:

Well, the point is, worrying about how the world works is fun and intellectually satisfying, but what good is it? There’s an anecdote which I like to tell: Faraday showed a British Prime Minister some of his experiments on electricity and magnetism. The Prime Minister said, “Well, Mr. Faraday, that’s all well and good, but what good is it?” Faraday said, “I don’t know, but one day you will tax it.” So, the point is, these useless things can turn out—you don’t know what’s going to be useful and what’s not. For example, number theory goes back to the Greeks. It was very useless. But now, number theory is very important. It’s important in cryptography and commerce, and so forth. The number theory which was kind of a game has lots of practical applications. So, theoretical things are very important—they take years for them to be practical, of course, but they’re fundamental. 

Zierler:

Wally, just to illustrate the point, and we’re all going to be beneficiaries soon enough, the research that went into mRNA vaccines that hopefully will get us out of the COVID crisis, this came about from basic research. It did not come about because people were focused on COVID. 

Greenberg:

Right, right. Of course. 

Zierler:

So, Wally, on that basis, to come back to quarks and the concept of useless knowledge, in what ways has your research and the research of your colleagues actually contributed to things that are useful in everyday life?

Greenberg:

Well, I think, nuclear reactors, for example, are important. Some of the fundamental things that we’ve done have to do with nuclear reactions and nuclear power. 

Zierler:

That’s a great example. Well, Wally, for the last part of our discussion, I’d like to ask a few retrospective questions about your career, and then ones looking forward. I’m curious, first, if you feel lucky, just as a matter of timing, that your interests chronologically coincided with a very fruitful time in theoretical physics. How do you feel about that as you reflect back?

Greenberg:

That was very lucky, I think. People quote some of the great physicists of the 19th century, saying that physics is done and there’s nothing new to be done. But when I was growing up, lots of new things were being discovered. That was lucky. You could be in a period where nothing new is happening. I fortunately lived when things were happening, and there was a chance to make contributions. 

Zierler:

It’s so obvious with your research career, the questions of what were mysterious that are now understood, but I’ll refine that. Within your specific field, within quarks, what are things that we understand now, and what things remain unknown, even empirically? 

Greenberg:

Well, one thing that’s unknown: is there something more fundamental than quarks and electrons? We don’t know. Maybe, at higher energies, electrons turn out to be composite. There’s no evidence of that, though. The same thing with quarks. Are they the fundamental objects, or is it just because we don’t have enough energy to excite the higher states? That’s just not yet known. 

Zierler:

As you transferred later in your career into interests leading more into cosmology and astrophysics, in what ways did your training in particle physics inform these developments? And the inverse question is, in what ways has cosmology and astrophysics enhanced the field of theoretical particle physics?

Greenberg:

Well, there is some information about what the interaction would be at high energy. So, cosmology, the universe is kind of a laboratory that has high energies available that we don’t have on Earth. So, by studying the things we see in cosmology, we can see what possibilities there are. 

Zierler:

Wally, looking to the future, if you have advice that you give to younger people in the field, because there’s still fundamental work to do in theoretical particle physics, what would you tell them? What do you think are the most promising avenues for ongoing discovery in the field?

Greenberg:

I think I would tell them to go into biophysics rather than physics. I think. It’s going to be too expensive to get higher energies, so new discoveries in particle physics are probably unlikely. It’s more likely to find things in genetics and in biophysics. I think that’s the new frontier. 

Zierler:

Well, Wally, I’d like to thank you for spending this time with me. It’s been so wonderful hearing your perspective, and I’m so glad that we connected. Thank you so much. 

Greenberg:

Oh, it’s my pleasure. 

 

EDITOR’S NOTE: After the interview, Greenberg assembled the following list of his key contributions to be included as a technical addendum to the transcript:

1. Introduced the color degree of freedom in terms of parafermi statistics of order 3 for quarks in the same year, 1964, that quarks were introduced.

2. Introduced the symmetric quark model of baryons, which is that the quark wave functions of baryons are symmetric in the visible degrees of freedom. This followed from the recognition that quarks carry a hidden charge.

3. Introduced parastatiscs whose state that are bosons or fermions are in one-to-one correspondence with the states in the SU(3) color theory that are color singlets.

4. Proved Haag’s Theorem that the interaction picture does not exist in an interacting, relativistic, quantum field theory.

5. Proved a number of theorems connected with the connection of internal and spacetime symmetries, such as the resolution of the statistics paradox associated with the placement of the ground-state baryons in the 56 of the flavor-spin SU(6).

6. Demonstrated the Greenberg-Low high-energy bound on total cross-sections from the Wightman axioms.

7. Analyzed the phenomenology of possible small violations of Fermi and Bose statistics.

8. Introduced a new family of particle statistics that interpolates between Bose and Fermi statistics, called “infinite” statistics and “quon” statistics. This led to a quantitative bound on possible violations of the Pauli exclusion principle and of Bose statistics, and it stimulated high-precision test of statistics.

9. Showed that the spin-statistics theorem is distinct from the spin-locality theory.

10. Invented specific models, including generalized Free fields, Lie fields, and the Parton model.

11. Showed how Quantum Field theory leads to restrictions on high-energy limits of scattering amplitudes and cross-sections.

12. Showed that the saturation properties of fermionic composites made of parafermi quarks are in one-to-one correspondence with the color singlet states of quarks.

13. Developed, with Resnikoff, the “symmetric” quark model for baryon spectroscopy, and I calculated the excited states of baryons.

14. Developed the CPT theorem, which is that local and relativistically covariant quantum field theories must also obey CPT symmetry.

15. Showed that the LSZ asymptotic limit is a weak operator limit, and I derived it properties.

16. Introduced the rephasing-invariant analysis of CP violation.

17. Proved that violation of CPT also implies violation of Lorentz invariance.

18. Proved that variational principle specifically adapted to quantum field theory.

19. Proved spontaneous and dynamical symmetry breaking, superconductivity, and relativistic bound state, using the N quantum approach to quantum filed theory.

20. Proved that quantum field theory on noncommutative spacetime violates micro-causality.

21. Invented the quon statistics parameter which generalized the Wigner-Ehrenfest-Oppenheimer result that bound states of odd numbers of Fermions are Fermions, and all other cases are Bosons.

22. Derived bound states in exactly Galilean-invariant field theory.

23. Demonstrated the Froissart bound follows from the analyticity of the scattering amplitude in the relevant Lehmann ellipse.

24. Analyzed non-Bose, non-Fermi statistics from the phenomenological, quantum-mechanical, and quantum field theory points of view.