Marshall Baker

David Zierler:

This is David Zierler, oral historian for the American Institute of Physics. It is August 21st, 2020. I am so happy to be here with Professor Marshall Baker. Marshall, thank you so much for joining me today.

Marshall Baker:

Well, thank you for inviting me.

Zierler:

OK. So, to start, would you please tell me your title and institutional affiliation.

Baker:

I’m now professor emeritus at the University of Washington in Seattle, Washington.

Zierler:

And when did you go emeritus, what year?

Baker:

In 2002.

Zierler:

2002.

Baker:

When I was 70 years old.

Zierler:

OK. All right. Marshall let’s go all the way back to the beginning, the origin story of your life, and, of course, let’s start with your parents. Tell me a little bit about your mom and dad and where they are from.

Baker:

My father was born in Lithuania, came to the United States when he was about 3 years old, and we lived most of our life in Salem, Massachusetts. And my mother was born actually in Boston, but her father was also from Russia.

Zierler:

And what were the circumstances of your parents’ families leaving Eastern Europe?

Baker:

I don’t know too much about that. I’ve asked my mother and father about that and I just have no good picture of what their life was like when they were there. It must be interesting but it’s still a mystery, and it will probably always remain a mystery.

Zierler:

Where did your parents meet?

Baker:

I’m not even sure about that, how they met. I don’t know that, either.

Zierler:

But you were born in Boston?

Baker:

I was born in Boston and we moved to Salem when I was about 3 years old because my father’s father had a business in Salem.

Zierler:

What was his business?

Baker:

Oh, it was a clothing business, a very modest clothing business.

Zierler:

[laugh]

Baker:

And it remained modest; never changed from that.

Zierler:

Did your father stay in that business for his career?

Baker:

Well, that was one of the stories of my father’s life which wasn’t so good. He actually went to law school but when he graduated it was near the Depression and so he didn’t actually practice law but stayed in the business until the business sort of collapsed when he was about 60 years old, at which point—since he was a very, very social person, so he knew everyone in the city by then—and he then began to practice law at the age of 60. And then his life started, as far as his professional life, and even his health got better.

Zierler:

Really!?

Baker:

So that was a pretty good story.

Zierler:

How long was he able to practice after he started at 60?

Baker:

Until he died at 78.

Zierler:

Oh, wow. Good for him. And what about your mom, did your mom work outside the home?

Baker:

No. She just stayed at home and took care of everything, took care of us, and later in her life her father moved in and she took care of her father, too, who lived with us.

Zierler:

So, you spent your whole childhood in Salem?

Baker:

My whole childhood. All my schooling was in Salem.

Zierler:

And was it a suburban upbringing? Was it more urban?

Baker:

Not really Urban. Salem was a small town, about 40,000 people. It was far enough from Boston, 20 miles from Boston, that I didn’t think of it as a suburb of Boston.

Zierler:

Were there religious observances in your family growing up?

Baker:

Yes. I had a step uncle, a great uncle, I guess, who was an Orthodox Rabbi who spoke no English, actually, who lived across the street. And my mother kept a kosher house, and they went to all the observances, but they weren’t really deeply religious, I would say.

Zierler:

Were you bar mitzvahed?

Baker:

I was bar mitzvahed, yes. That was fun. What I remember very well though is that it took place basically in what I used to call the Front Street temple. There were two synagogues in Salem. There was the old one which was essentially upstairs, had a room upstairs of a Greek men’s club and it had people with long beards moving up and down. And there was a conservative, another more-fancy synagogue, too. So, I sort of thought all that was fun.

Zierler:

Did you stay involved with any Jewish observances after your bar mitzvah later in life?

Baker:

Well, right now, actually, my daughter, who also isn’t religious, was very concerned to maintain at least some—

Zierler:

Traditions.

Baker:

—and so I think we have a seder at her house to remind us of that and a few other things. But it doesn’t play too big a role.

Zierler:

Marshall, did you go to public schools throughout your childhood?

Baker:

Yes.

Zierler:

And were they good schools? Did you feel like you had a good education?

Baker:

Oh, I was satisfied with my education. I went to one school, which was right at essentially the end of our street, for eight years, the Saltonstall School. What I remember most about it, when I was in the sixth grade, I had an incredible English grammar teacher, Ms. Paine, who instilled me with the love of English grammar. You must imagine, how could anyone ever be in love with grammar? [laugh]

Zierler:

[laugh]

Baker:

But it had a profound effect because I think it got me interested the first time in really learning the languages, traveling a lot. I didn’t think of it that way, but...And in high school, I had a wonderful mathematics teacher, my freshman algebra teacher, who I remember vividly, and he played a very strong role in my life.

Zierler:

Marshall, when did you remember that you first started to get interested in science and how the natural world works?

Baker:

Well, I’m never very good with my hands and so it usually was more abstractly that I got very interested in science. After my class with Mr. Fillian, my math teacher, I then also had a very good chemistry teacher. So the science that I really knew when I graduated high school, I had any feeling for, was natural science which was actually chemistry. And that reflected the fact I had a wonderful chemistry teacher, and that’s it, too. He was number two teacher after the math teacher. And so the teachers have always influenced me.

Zierler:

So, when you were thinking about college, you were not thinking specifically about pursuing a degree in physics from the beginning?

Baker:

That I don’t really remember. What I do know, I think probably I was at that time, but I did start off actually majoring in mathematics and I switched to physics in my junior year because I got very interested in real things. And I realized about that time—It was at that time I realized I was aware of the great, exciting things which were happening in quantum electrodynamics. And there was a very famous professor, Julian Schwinger—

Zierler:

Right.

Baker:

—who everyone had heard about. You didn’t have to even be interested in science. And so he sort of played a big role. And I actually even managed to take his first-year graduate class in quantum mechanics when I was a senior, and so I had this experience already then. So, I would say that that was a big factor in my, not only going into physics, but studying the subject the way I did.

Zierler:

Now, what other schools did you apply to besides Harvard for undergraduate?

Baker:

I think the only other one was MIT.

Zierler:

Uh-huh. So obviously you had very strong grades in high school to consider MIT and Harvard for acceptance?

Baker:

Well, yes, I had strong grades in high school. However, the situation was quite different than it is now. The percentage of people accepted at these schools was probably 10 times the percentage that are now accepted, so life was different.

Zierler:

Did you get into MIT also?

Baker:

Yes, yes, yes.

Zierler:

And so why did you choose Harvard, you wanted a broader education?

Baker:

I wanted a broader education, that’s right.

Zierler:

Did you take advantage of that, Marshall? Did you take advantage of being able to take humanities and classics and those kinds of courses?

Baker:

Yes, somewhat. Not as much as I should’ve, but I remember an extremely good class in the study of the Supreme Court. I remember another class, The Democratic Institutions and Its Critics, which had Locke and Hobbes, so I did take advantage of that. But I could’ve taken even more advantage in retrospect.

Zierler:

Was Harvard, in those days—did it feel like a very old-fashioned place? Did it feel formal? Were you separated from women? Were you able to interact with professors on a friendly level? What were your impressions of it as an undergraduate?

Baker:

Well, I thought it was OK, but one of the main things that I remember about being an undergraduate—I had some teachers who were very good; I think most of the teachers were good—but the thing I really remember is my relations with other students. And I had a very interesting freshman roommate whom I was friendly with in a sense the rest of my life. And I actually helped him with chemistry and all that. But it turned out that later I didn’t room with him, and I roomed with somebody else. And this personal disappointment played such a big role that it almost blinded me and made all the other academic things sort of background fluctuations in the context of my personal relations. But, as far as your question about formality, that didn’t affect me. I thought it was very formal maybe, but the people who were fancy and formal I didn’t even know who they were—

Zierler:

[laugh]

Baker:

[laugh]—so I couldn’t really be affected by them.

Zierler:

Marshall, looking back, the fact that you were a mathematics major until your junior year, in what ways was that advantageous and useful for your entrée into physics?

Baker:

Well, it was very useful because I was confident about anything mathematical and I had, of course, been taking physics classes at the same time, so I was not behind in any way. But it gave me confidence, I would say.

Zierler:

Now, you mentioned Schwinger as a reason for you to switch over to physics. Did you grow close with Schwinger? Were you able to develop a relationship with him?

Baker:

I don’t think I ever talked with him when I was an undergraduate.

Zierler:

[laugh]

Baker:

He was very, very shy. Yeah. He’s a very, very shy person and is sort of a mythical cult figure who would come in and give these magnificent, magnificent lectures which would last an hour and a half, beautiful, over at the board, and then just leave. No one ever asked a question. I don’t remember any of that ever happening. So, it was a different kind of relation. Maybe it was a relation just of a distant admirer, but since I eventually became his student, of course, I talked to him in graduate school and he was always very shy, but no problem in talking with him. He didn’t have any desire to make you feel inferior to him.

Zierler:

So, he was just introverted, it’s not that he was unapproachable?

Baker:

Introverted, yes. That’s exactly the word to use. He was introverted.

Zierler:

What other faculty did you become close with as an undergraduate?

Baker:

I knew some of the faculty people, but I don’t remember being close to any other one. If I had more of a long-time chance, I might pick one out, but I don’t know. I think eventually I got to know Professor Glauber.

Zierler:

Right.

Baker:

Yeah. But that was when I was a graduate student.

Zierler:

What classes as an undergraduate were particularly formative for you in terms of you developing your own identity as a physicist and the kinds of physics that you would want to work on in graduate school?

Baker:

Well, I enjoyed very much, the first half of my freshman physics course with Professor Purcell, who had a Nobel Prize in nuclear magnetic resonance. And he was a very good teacher. And then I enjoyed also a class with Robert Karplus in electricity and magnetism, and another class by Norman Ramsey an introduction to quantum mechanics, which I took in my junior year before I took Schwinger’s quantum mechanics class in my senior year.

Zierler:

Over the course of your undergraduate degree, what did you realize you had the most talents in?

Baker:

Well, my talent was, if I said anything, trying to put things together, different things, trying to connect them up. That’s sort of the way I’ve always worked and my whole life has been like that, just taking pieces and finding maybe they fit together. And I probably overdid it, sometimes trying to make pieces fit together which really wanted to blow apart. But that’s what I think my—if I have a talent, that’s what I would call it.

Zierler:

When it was time to start thinking about graduate school, did you consider leaving Harvard and pursing a Ph.D. elsewhere?

Baker:

Yes. In fact, I thought it was a good idea. I had never essentially been out of the East, so I thought it was a good idea to do something different. And I also got very interested in this quantum field theory from Schwinger and so I then went to the opposite coast, to Caltech where there was Feynman, who was the second light in the revolution in quantum electrodynamics. And so I went one year to Caltech. It turned out I took classes with Feynman, I actually talked with him. He was very extroverted, even about his personal life. However, I wasn’t happy there. I didn’t like it as much for personal reasons, I think, and I had decided to leave. In the middle of the year I decided I’m going to come back to Harvard but, I’ll finish this year in Caltech. However, I then met one of the graduate students, Frederik Zachariasen, who I then became very friendly with, and then I stayed that whole summer at Caltech enjoying myself before I came back to Harvard, having met both Feynman and Fred Zachariasen, both of whom played important roles later.

Zierler:

What were your impressions of meeting both Feynman and Zachariasen?

Baker:

Well, Zachariasen was just full of life and friendliness and a little bit eccentric in his unspoken—not ridicule—he was very good at making jokes in a nice way about other people. He was so funny. I couldn’t imagine. And Feynman was sort of—he was a wonderful teacher. He had his own way of doing it, but, in particular, for some reason, I had a couple of days where, I don’t know how it occurred, but I spent almost a whole day with Feynman where Feynman was telling me his life and his sadness with the death of his first wife. And he just chose me to open up to. So, I had a different opinion of Feynman than you hear about in the books, that he was just a big joker. There was the famous book he wrote, Surely You’re Joking, Mr. Feynman! I don’t think that represented—that represented one part of Feynman, but he had more, and it was a different side and that’s the side I saw.

Zierler:

Why ultimately did you choose to pursue your degree at Harvard?

Baker:

Oh, I had sort of made my decision because of my first half of year at Caltech, when I was unhappy. That was before I met Fred, before I really had much interaction with Feynman. And then I almost changed it, but I did come back to Harvard, and, of course, I was very glad that I did that because things worked out very, very fortunately. That’s another thing. I had a lot of lucky breaks. That’s another feature.

Zierler:

What would you consider the lucky break in this regard?

Baker:

The lucky break in this regard is that, when I finally got my Ph.D., I had worked both with Schwinger and Feynman.

Zierler:

Yeah.

Baker:

And everyone else in Harvard when I was a graduate student, they, of course, were doing the things the way Schwinger did—but I had first learned everything from Feynman, so this combination was just a big lucky break.

Zierler:

Yeah, yeah. Probably unique, also, right? Who else ever was able to do this? You either learned with Schwinger or with Feynman, not both.

Baker:

Yes, I know. That’s why I say it was lucky. It just happened.

Zierler:

Marshall, it’s such a great opportunity. I want to ask broadly, not technically, what did you learn about the different ways of doing physics from Feynman and Schwinger? Because they obviously shared many talents and characteristics, but, of course, they were very different people, including in their approach to physics. So, I’m wondering what your takeaway was, having this very special opportunity to work with both of them?

Baker:

Well, Schwinger wanted to make the physics very beautiful from a very formal point of view. He would take the electromagnetic field, which is a classical field, and he formulated very, very clearly the quantum theory of fields, and it was just beautiful, and his lectures were beautiful. And so I took that general belief that the quantum field theory was something worth pursuing as a lifetime project, which it actually turned out to be. On the other hand, quantum field theory has lots of ways of looking at it. And, of course, Feynman, like everybody, I learned very quickly about Feynman diagrams, basically, which are actually the solution of quantum field theory, the perturbative solution of quantum theory before I learned of quantum field theory. So, I had a very good feeling for how to do calculations, looking at the force between two electrons in the quantum field of electricity and magnetism as just being two electrons exchanging a photon or two photons to three photons: they can be virtual photons. Each exchange can be represented in terms of Feynman graphs. So, I had another way to think about it, and I used both.

Zierler:

Besides their differing approaches, was your sense that they were asking fundamentally different questions or were they asking similar questions during your time with them?

Baker:

Similar questions, not fundamentally different questions.

Zierler:

And what were those questions?

Baker:

The questions had to do with quantum electrodynamics, the quantum theory, which had been developed actually in 1930s or so, and it allowed electrons and positrons to emit and absorb photons. And, as a result, if you tried to do something like calculate an energy level of an atom like the hydrogen atom, the electron that’s in the hydrogen atom, aside from interacting with the proton itself would interact with the radiation field and would be emitting and absorbing photons—and that would change the energy of the electron. However, if you tried to calculate it according to the perturbative rules, you would find you couldn’t calculate, you’d get infinity, and this lasted until Schwinger and Feynman actually showed how to do this by realizing that most of the infinite energy that occurred is due to the interaction of an electron and an atom that was already present for a free electron and could be explained by just identifying this infinite change with just a change in the mass. Therefore, realizing that all you ever do is measure the physical mass, you can avoid that problem and get results, physical results; for example: the result of the shift of the energy levels of the hydrogen atom, the so-called Lamb shift, and also the change of the interaction of an electron in a magnetic field, the so-called anomalous magnetic moment, which were observed just before I came to college. This was 1948. And Schwinger and Feynman both—I know what Schwinger did. Schwinger was able to calculate both of these and get a finite result.

Zierler:

And on that point, Marshall, I wanted to ask: During your graduate school time and your exposure to what Schwinger and Feynman were working on, can you reflect back on the impact of experimentation on theory? What advances were being made in the world of experimentation that helped move these fundamental theoretical discoveries forward?

Baker:

Well, there was some—I really didn’t know so much about that. I read a little bit about the experiment that Lamb and Retherford did on the atom, but it had a lot to do with—advances made during the war on microwave technology issues in the vicinity of 3cm wavelength and various other things. But I just don’t know the answer to that question.

Zierler:

How did you go about putting your dissertation topic together?

Baker:

Well, that was an interesting story, too. When I came in, there were a group of 12 students who wanted to work with Schwinger. And the way Julian Schwinger would work, he would go out to lunch with his group, including his postdoc Ken Johnson, who had been a first year graduate student when I took Schwinger’s quantum mechanics class along with him. And then, people who wanted to work with him would line up at his office and just wait in the line and he might suggest a problem. It might be “just work on this thing,” it might be a one-sentence thing. In my case he suggested a problem which he had actually started to work on but really had lost interest in it because it wasn’t relativistic field theory, it was nonrelativistic. It didn’t involve those questions. And it was such a good problem that anyone could have solved it—so it didn’t take much imagination. And after five meetings with him over a period of the year, I had a thesis. And it was a thesis on the interactions of mesons and nucleons at low energy, not the subject I have devoted my life to, but I worked on it somewhat after I got my degree.

Zierler:

Marshall, I’m sure you were narrowly focused at the time but reflecting back, in what ways did your dissertation help answer some of those bigger questions that were being posed in the world of theoretical physics at the time?

Baker:

I think it helped answer some of the smaller questions.

Zierler:

[laugh]

Baker:

Because people did use that to try to understand how to apply quantum field theory to meson nucleon interactions, and with some colleagues we worked on using this method for a while. It got into quantum mechanics books, it was used. But I wouldn’t say it was fundamental. It was a technique that was very good to solve quantum mechanics problems. And what I did in my thesis was show that it also could be used in problems like meson nucleon scattering where there are actually particles being produced all the time. So, I don’t think it did anything for fundamental physics.

Zierler:

Who was on your committee?

Baker:

Oh, who was on my committee? I should know that. I have a feeling—I know the mathematician on the committee was, I believe, Stakgold, Roy Glauber and Julian Schwinger were there. Since Roy Glauber was on my committee, I got to know Roy Glauber very well. And, in fact, it turned out that the way I got my first job at Stanford was the fact that—(I learned this later), which I’d never even applied for—that Roy Glauber had talked to Sid Drell at some meeting. It wasn’t Julian Schwinger; he’s too close. He knew me and I happened to be, luckily at that year—again, that was a tremendous bit of luck—with this group of 14 students in B24 down in the basement of the Jefferson Laboratory at Harvard. There were only two, for various reasons, who got—I told you I got mine fairly quickly—that got their degree that year. The other was Charlie Sommerfield. Charlie Sommerfield had an NSF fellowship so he could choose any—he didn’t need money from anyplace. So, I was the only one of those 14 students who was available for a job. And so Roy Glauber had no one else. So, I was very natural, just by the exclusion principle, and so that was another tremendous lucky break. And I got this job. I got this job, and now there was something else I found out later. Someone else got a job at Stanford that year. It turned out to be my old friend, Fred Zachariasen.

Zierler:

Ah.

Baker:

And Fred Zachariasen had worked with Sid Drell, so Fred knew Sid Drell very well. And Sid was the person who hired us at Stanford. And so I’m sure I also got a good word from Fred. I never discussed Fred, but between Fred Zachariasen and Roy Glauber speaking to Sid Drell, it just happened. And I got a call from Sid offering me this job.

Zierler:

You must’ve been very excited about that.

Baker:

Of course. I was amazingly excited, so that’s another accident.

Zierler:

When you got to Stanford, were you looking to continue on with your research and build and revise your dissertation, or were you looking to pursue new projects?

Baker:

Both. However, there’s another story. This job, the position I had was a postdoc. At that time, there was the Stanford accelerator, linear accelerator, and so they had people—in fact someone who had been there who I was essentially replacing was Geoff Ravenhall. He had been sort of their house theorist. He would do the calculations that the experimentalists needed to do. And so, when I was hired, I was told that I would have to spend—half of my job would be to take over what Ravenhall did, work with the experiments doing all those things, help them out, and the other half I could pursue whatever I wanted. Well, another lucky break. It turned out that this—not a lucky break—the experimentalists can very often analyze this stuff very well. And by the time they had worked with Geoff Ravenhall for two years, they could do everything themselves. So, when I came there, they decided they didn’t need me to do that, so I had 100% opportunity to do what I wanted to do.

Zierler:

And so what did you do with that opportunity?

Baker:

What did I do with that opportunity? The first problems I did, I did two papers with Fred right away, one having to do with calculating meson-meson interactions using this method, the determinental method that I had developed for my thesis. And another one working on a problem that Fred was an expert on, the interaction of electrons with nuclei which is due to an exchange of a photon so that’s determined by the so-called form factor, which is the probability amplitude for a photon to be absorbed by a nucleon. And so we wrote some papers on that, too.

Zierler:

Marshall, were you following all of the exciting developments with what was going to become SLAC?

Baker:

No. When you said following all the exciting developments, that reminded me of something I forgot about graduate school. When I was a graduate student, the exciting development was the discovery of the violation of parity in weak interactions by Lee and Yang. Now, it turned out that T.D. Lee came to give a series of lectures on parity violation. Could you excuse me a minute? There’s a little extra noise in the background.

Zierler:

That’s OK.

Baker:

I’ll be right back in 20 seconds.

Zierler:

Sure.

Baker:

OK. Sorry. Here I am again. So, anyway, I had quite a bit of extra time. I finished my work a little bit early and so Charlie Sommerfield and I decided we would go and write up a set of notes, so I learned very well this subject. It wasn’t a big thing, but I was knowledgeable about it at a sort of elementary level. Well, that turned out to be kind of important because when I was at Stanford, another lucky thing—Walter Myerhof was at Stanford. You asked me about the exciting things about accelerators, but this is a little different. He was in the nuclear physics laboratory, and they were very interested in weak interactions of nuclei. And so he asked me whether I—this was in my second year at Stanford, the middle of my second year—whether I would give a series of lectures in weak interactions to this group. And I managed to do it very well because I had been prepared. And, of course, what the whole point of that was they were testing me. This was a trial to be promoted to assistant professor. And so that was a nice thing. And then, later, what I continued to work on, and I worked a little bit on the spontaneous breaking of elementary particle symmetries which forms the—that is to say you can have theories which have a certain symmetry where the solutions don’t have the same symmetry. This leads to zero mass particles, et cetera. And I worked with Shelly Glashow on that.

Zierler:

When did you meet Shelly?

Baker:

He was a graduate student at Harvard. He was one of the people in that group of 14, but he came one year after.

Zierler:

One year after. OK.

Baker:

He was there. He graduated one year after, so I knew him very well, of course. In fact, when I was married, he made a party for us.

Zierler:

Oh! [laugh]

Baker:

He then came to Stanford, too. That’s right. After I came, he was there, so Shelly Glashow was there. And then I’ve known him very well since.

Zierler:

Did you keep in touch at all with Feynman or Schwinger during your Stanford years?

Baker:

No. Well, the last time I saw Schwinger was when I was on my way to Stanford and he was giving some lectures at Banff and we had some incident where I finally found him. I was asking him about my thesis, didn’t he want to have his name on my thesis. Because, you see, it was really all his ideas. I wrote a very long paper for the Annals of Physics, about a 50-page paper. Well, I made some contributions, but the underlying thing was Schwinger, so I went to look for him on this trip to Stanford. And, of course, I realized he wasn’t—but I couldn’t find him. Finally, I found him near a lake called Moraine Lake, and as soon as we came out, there was a bear between us, a big bear. And so that was my contact, so we had not a discussion about physics but a discussion about what do you do if you see a bear. And I even have pictures of Schwinger and that bear. Even now I keep them. So that was my—and, of course, at various other times there was a—the other time I remember seeing him in—was it 1980, I believe—what year was it? He became a professor at UCLA and there was a 60th birthday celebration; he was born in 1918, so it was 60 years after that. And we talked a little bit then. He was in a good mood, but very little conversation. Maybe there was one other time. That’s it. Feynman I never talked to again. I had this very intimate conversation, but I never essentially saw him again. Oh, yes. Once I came back to give a talk at Caltech—my friend Fred was at Caltech, so he invited me when he went to Caltech from Stanford. I was still at Stanford. And I remember Feynman stamping out of the room. He hadn’t remembered me at all, of course. And I must say it was a horrible talk, [laugh] but it was a little devastating. He was justified in stamping out of the room. That’s my last contact with Feynman.

Zierler:

Marshall, in what ways did your life change when you joined the faculty after being a postdoc?

Baker:

Oh, it changed tremendously because, as a result, my last three years—so I had three years to teach and I taught, and that teaching that I did those last three years were fundamental to everything I did afterwards. First of all, I taught electricity and magnetism, graduate electricity and magnetism for two years, and so I got there and know Jackson inside and outside, or whatever the book was we were reading—I think that was it—which I would later use. And then the last year I was there I taught the quantum field theory class. And I taught the quantum theory class, I was very excited about that because it was a math class that I incorporated both the methods of Schwinger and Feynman. So, I worked very, very hard, I think, that last year when I was teaching that class. I did nothing else, and it was so much fun. And it was so much fun that it was a whole year of class ending in June, but, foolishly or not, I told the students I was continuing my lectures because I wasn’t finished with what I wanted to say. [laugh]

Zierler:

[laugh]

Baker:

And, of course, it was optional, I wasn’t going to give any grade. But actually there was a finite number of students asking questions, and it was during this work that I realized what I was going to—I knew what problem I wanted to work on. I wanted to work on the problem that asked these questions, were these infinities that you got in quantum electrodynamics in the interaction of electrons and photons fundamental? Were they really there or were they somehow only there because the calculation used some approximation method? I thought there might be a way which might be able to attack that question. And that’s why I started that at that time.

Zierler:

How realistic was it to pursue solving quantum electrodynamics?

Baker:

Well, I would say that—I should’ve asked myself the question. I never asked myself that question because I was just interested in doing—and I just felt like working on it. It was just what I felt like doing. And probably if you had asked me that question then, I would’ve said it’s completely unrealistic—

Zierler:

[laugh]

Baker:

—but I’m going to do it anyway. [laugh]

Zierler:

How did that research turn out? How far did you get?

Baker:

Well, I worked on it for the next 15 years. It turned out that when I got this job—I didn’t tell you that. Let’s see. What’s the order of events? That’s right. I got a job at University of Washington, was my next job. I actually had a chance to—Stanford offered to give me a second assistant professorship but there were problems in relations between the accelerator people and the department people. And Sid Drell said, “Take the Washington job.” I took the Washington job with the understanding that I would be able to teach the same course the next year. And I started to work, and I started to develop this work. Meanwhile, I invited my friend, Fred Zachariasen, who at that time (1963) was at MIT, to give a seminar at the University of Washington. During his visit to Seattle I mentioned to Fred that I was beginning a study of the short distance perturbative divergences of QED. He immediately replied "You know that Ken Johnson, who is now a professor at MIT, is doing the same thing." Just two days later, after Fred's return to MIT, I received a call from Ken suggesting that we collaborate, and then we both started to work.on this problem, and we collaborated basically until his death in 2000. And then we wrote a large number of papers on that. We actually concluded the problem—should I say one sentence about what the problem was?

Zierler:

Absolutely.

Baker:

Well, let’s talk about the charge. The problem was that both the mass and the charge that you tried to calculate due to the interaction of an electron with radiation were incalculable. And what people did was say, well, let’s not bother about the calculation, let’s just try to forget it and ignore but then just use the observed, physical charges of the masses when you try to calculate energy levels; when you try to relate two experiments we’ll forget that problem. Well, the problem was that if you have—let’s look at the problem with the charge. The problem with the charge was that you have, say, electron, and in the radiation field around it there are electron-positron pairs.

Zierler:

Mm-hmm.

Baker:

And because of screening, the positron pairs get brought closer to the electron that has the opposite charge, and the other electrons move further away. As a result, the effective charge near the electron is less than the original charge. And it turns out that no matter how—that’s what’s called screening, and the screening is stronger and stronger and stronger. In fact, you could show that you essentially get infinite screening. So, what happens is, no matter what charge you start with, no matter what bare charge you started with, because the screening turns out to be infinitely strong, you end up with no charge. And so, if you look at it in some fundamental way, if you really demand the existence of a point charge, you find the end of electrodynamics. You get no theory whatsoever, and this was actually argued by the Russian physicist Lev Landau and he made this a little bit more convincing, except he wasn’t quite—what happened, what happened? Let’s see. What do I do? Oh, good. So that was the problem we tried to solve. We found actually that—and that result was that we actually found there was a particular value of the bare charge for which all these infinite screening terms which made the physical charge equal to zero, therefore the theory meaningless, actually could all cancel. And there was a possibility that the theory could be finite if the bare charge had a certain value. But we didn’t have an exact way to calculate whether there was such a charge.

Zierler:

Why was it difficult to make that calculation? What was missing?

Baker:

Because it involved calculating—the only way we could calculate it—we were looking for some root of an equation, which was some function of the bare charge. We were only able to calculate it was just complicated to—we calculated it in the perturbation theory. We couldn’t think of any way to do it except in perturbation theory. Finally, in 1978, we wrote a paper using a different method in which we did not get a conclusive result, so we still didn’t know the answer. But I must admit that after that I didn’t think it was likely that the equation determining that charge had any solution. But I learned an awful lot about quantum electrodynamics in the process of carrying out this work with Ken.

Zierler:

Marshall, I want to ask about the transition to the University of Washington. Of course, you must’ve known when you accepted an assistant professorship at Stanford that there was not a culture of promoting from within, right?

Baker:

Yes.

Zierler:

You must’ve known that your prospects, no matter how impressive your research was, of gaining tenure at Stanford were low from the beginning?

Baker:

Yes, yes, absolutely so. I was just thinking on the short term, enough to be lucky for a couple of years. And maybe if you can just hang on long enough, just stick at what you were interested in doing, it’ll work out.

Zierler:

And so, by 1962, were you sort of given notice that tenure was not going to be working out or did Washington recruit you before you got to that point?

Baker:

I guess I wasn’t clear before. What happened was some point in that—it was 1961, I guess, I left. In 1961 I was offered an extra three years. I was offered a second assistant professorship—it’s sort of an unusual situation—halfway at Stanford, so I could’ve stayed three more years. But then I was told that—even with that situation, I didn’t feel it was likely, and, as I mentioned, there was the problem of a conflict in the physics department between the regular department and the linear accelerator people, and Sid Drell said, “Don’t stay here because it’s too dangerous.” I left for two reasons. So that was the story. As far as I remember, again, I didn’t apply to Washington. I just don’t know how—I don’t remember applying, actually. I just got a call offering me a—things were different. I mean, it’s hard to believe. I wasn’t particularly—I had luck of working with Schwinger and Feynman, but I don’t remember. Maybe I applied. I just don’t remember applying. I must’ve applied, I guess, a few places.

Zierler:

Did you know anyone on the faculty before you arrived?

Baker:

I knew of people. I knew of Ernie Henley who had been a collaborator with Sid Drell, but I would say not really. I had no really personal contact with—no, the answer is.

Zierler:

And, Marshall, what were your impressions when you arrived in Seattle? Where was the physics department strong and where did you see opportunity for growth in the physics department?

Baker:

Well, the physics department was very strong in nuclear physics. And they obviously needed to grow in elementary particle physics.

Zierler:

So, your hire was part of that need to grow?

Baker:

That was the situation, yes, yes. I think that was it. And I think they also later wanted to grow also in condensed matter physics, too. It was an exciting time. I came in 1961. That’s right. That was the time of the Cuban Missile Crisis, I believe.

Zierler:

Yeah.

Baker:

And a lot of things were going on. There was the World’s Fair in Seattle, and my wife was expecting a baby, who came about six weeks early. So, a lot of things were happening. Can you hear me? Sometimes I drop my voice.

Zierler:

No, no, no. I hear you loud and clear. You’re fine. Oh, yeah.

Baker:

No problem with it, OK.

Zierler:

No problem.

Baker:

But tell me to speak up sometimes.

Zierler:

I will, I will, I will.

Baker:

OK.

Zierler:

When you got to the department, did you feel like you were part of a building up of the department? Were you part of that wave of hires that was looking self-consciously to make the physics department a top-flight program in the country?

Baker:

Yes. I felt that way, but, furthermore, I felt like I was immediately part of the department. It was an amazing relationship. Almost every single faculty member in the department had us over for dinner, more than the chairman, many times. It’s completely different than what happens these days. So, I felt just like I was inside. I knew Ernie Henley and, of course, I knew he had done work on this and quantum electrodynamics, too, so I knew him. He was older then. So, I right away felt a part of everything.

Zierler:

And you came in with tenure, right? You were at the associate level from the beginning?

Baker:

Yes. But I was called, actually—since I only had published seven papers or so at that time, and they varied, all these topics that I did mention. And so I was called an acting associate professor. But after three months I was given tenure the first year. But, strictly speaking, I had to wait three more months.

Zierler:

But you knew that was coming; it wasn’t actually a gamble?

Baker:

I wasn’t worried about that.

Zierler:

Marshall, did you have graduate students at Stanford? Did you bring anyone with you?

Baker:

I had, yes. I think I had one graduate student at Stanford who came with me, but he didn’t switch to University of Washington, he just came to Seattle and we had discussions and things. He, I think, got his degree at Stanford. But what was happening right away—I must tell you why I wasn’t thinking about that so much—I told you when I came to Stanford that I got this call from Ken Johnson and so I already had this project with Ken Johnson. No, wait a second. It was when I came to Washington. The first year I started this project with Ken Johnson. But, furthermore, when I came to University of Washington, I had told them I had an Alfred P. Sloan Foundation fellowship, and so that meant—I told them that I was going to ask for a leave of absence for a year. My second year would be on leave. And during that period, I went to Buenos Aires, Rome, and Sendai, Japan. So, I was away that whole first year. It was a very eventful year also. So, I didn’t have any graduate students at that point because I was away. One graduate student who was a graduate student at Stanford was offered a postdoc with me at Washington, so he came and worked with me at Washington. This graduate student, Ray Willey from Stanford, contributed a lot to our work with Ken Johnson. At that time I also worked with another postdoc—Ivan Muzinich, and with him I worked on these strong interactions of pions and nucleons.

Zierler:

Right, right. And so, Marshall, can you talk a little more broadly—the early 1960s, mid 1960s was such a phenomenally exciting and fundamental time in theoretical particle physics. Can you convey some of the bigger questions that were being asked, some of the exciting developments and where you saw your expertise and role in these developments?

Baker:

OK. A lot of stuff was happening say at Berkeley.

Zierler:

Yeah.

Baker:

OK. I came then to Stanford in the fall of ‘57, I guess. It turned out that my friend, Charlie Sommerfield—he comes back again—had taken a job at Berkeley. And we would travel, I would travel to Berkeley and he would come to Stanford all the time. So, I quickly got to know all the people at Berkeley very soon. And, at that time, there was the counter field theory revolution, the idea being what you do about the strong interactions. There were all these new particles—hyperons and mesons, strange particles, et cetera, a large number of—so many particles. Then a large group of people thought—which particle is more fundamental than the other ones? We can take two of the particles as fundamental and assume the third particle is a composite of both of these. So, in fact, what’s called S-matrix theory was developed at Berkeley, which is not inconsistent, I would say, with field theory. But, instead of asking the question, what are the fundamental constituents of everything, let us figure a way to use all the general properties of physics that we know to constrain all scattering amplitudes, all masses. And so the idea is you assume certain masses and forces, you calculate everything and see whether, after you do the calculation, you get them out as a conclusion. That’s called the bootstrap. And so people worked on it, so I learned all about that, too, and it was kind of very interesting. And then, connected with that, was Stanley Mandelstam. I don’t know of you’ve heard of Stanley Mandelstam—

Zierler:

Sure. Of course.

Baker:

—the Mandelstam representation. And I was with Charlie Sommerfield. Stanley Mandelstam was a very close friend, and so I was always talking about the Mandelstam representation and all this. While parallel—that’s even a little bit before—at the same time working with Ken Johnson. So, I was very influenced by that. OK. Since you asked that question, let me continue about Mandelstam.

Zierler:

I want to ask specifically, Marshall, on bootstrap. Were you working on this also or you were just interested in what other people were doing?

Baker:

My colleague, Fred, had been there. Did I work with—I don’t remember. I didn’t publish anything with bootstrap. No, I’m sure I didn’t. But maybe I talked and worked on a few things with Fred, but I did not work on bootstrap except only later in a way when I worked on Regge trajectories a little bit later with nonlinear trajectories. So, yes, but the answer is I did, but in a very indirect way, and it would be a diversion, I think, to our conversation. But let me quickly, since I brought the subject up, say, in about 1970—this was a later time—the duality had been discovered that was the Veneziano formula for scattering amplitudes. And what we did was with Darrell Coon, who was a postdoc, is to redo the usual work in Regge theory which used linear trajectories. We wrote a number of papers with nonlinear trajectories. A linear Regge trajectory means that you have a class of particles for which there’s a linear relation between the angular momentum J and mass squared. We worked with a theory in which there was a more general relation. But that never really went anywhere. It was very interesting mathematics, but it never had any physical implication. Maybe that was a time we worked very hard on that, and I was very interested, but no important physics came out of that.

Zierler:

And so I interjected there, Marshall. The other big things that you wanted to talk about?

Baker:

OK. I knew all about bootstraps, I knew all about Regge—should I say a word about Regge trajectories?

Zierler:

Absolutely.

Baker:

OK. We said in S-matrix theory—two incoming particles can exchange a particle, like two incoming electrons exchange a photon, and the two electrons scatter. If a single particle of high spin is exchanged—the resulting cross section becomes infinite. The idea of Regge trajectories was that if you had a sum of an infinite number of particles being exchanged, that would actually give a finite result. And that was the idea; instead of trying to describe particle interactions by individual particle exchange, you explain them by Regge exchange. Regge exchange is the exchange of an infinite number of particles. There is a whole theory of this. This theory was particularly developed by a Russian physicist, Gribov, in Leningrad. Now, I have to say where this connection is. So, remember I told you I was going to go abroad? In 1963-64, I go to South America, Rome, and eventually to Japan. When I was in Japan Sendai, all of the sudden, I got a call from the University of Washington, would I like to attend the meeting of what used to be called the Rochester Conference. You remember what the Rochester Conference is?

Zierler:

I’ve heard of it, yeah.

Baker:

It’s the high energy—they don’t call it the Rochester Conference, but it’s the high energy physics conference that they have, changes from place to place every year, but then it was still called the Rochester Conference. Anyway, I got a call from the Rochester Conference, when I was in Sendai, would I be the representative of the University of Washington. The Rochester Conference which was going to be in Dubna in the Soviet Union. Well, I was very excited about—I knew this is, of course, where Gribov—Gribov was going to be there. I did go, and this was transforming. I went to Dubna and there I met Prof. Ter Martirosyan of the Institute of Theoretical and Experimental Physics (ITEP) in Moscow, who became a lifelong friend. He encouraged me to spend time at ITEP. Later I did go to Moscow in 1966, 1970, and 1974. In 1974 we wrote a big review article on Regge theory.

Zierler:

Marshall, why was it so important for you to develop these contacts in Russia? What were they doing there that was so important for you to be a collaborator?

Baker:

They were doing important work on Regge theory (Gribov’s reggeon calculus). Also, as I mentioned earlier, Landau had concluded that quantum electrodynamics had zero charge. The people at ITEP knew all about this. When I gave a talk at ITEP on our work with Ken Johnson on that subject, I remember everyone said, “no, the charge is zero.” Landau has shown that. And so we had a lot of lively discussions. There were also lively discussions and other topics, too, which I had worked on.

Zierler:

What did you see as some of the big promises in theoretical breakthroughs with this research? What were you aiming for?

Baker:

Well, I was aiming to understand as much as I could and put as much of the things I could together. I wrote a paper with a graduate student I had at University of Washington, Larry McLerran. You might’ve heard of him. And we actually made a closer connection between field theory and Regge theory, and so that was part of my program, to try to put these two pieces together. If we just look at the long-term point of view, at the same time—it was in 1964 or 1965 that Gell-Mann came out with the quark model saying that protons are made up of three quarks. And then people would say quarks are fundamental, too. The mesons are made up of a quark and an antiquark. And there are up quarks with charge 2/3 and down quarks with charge minus 1/3. So, the proton, having charge one, is two up quarks with charge 2/3, which gives charge 4/3, one down quark with a charge minus 1/3, and that’s the proton. Gell-Mann, in putting this together, could fit a lot of data just on quantum numbers. He actually never believed—there’s always a controversy about this that these were really particles rather than mathematical objects. As far as I know, there wasn’t really that much on the quark model for quite a while.

But the important thing—and here’s where my long-term interest comes in again—there were two things that happened sort of simultaneously. One we realized that the various problems with the quark model could be eliminated—not eliminated—could be removed, could be compensated for by other effects if there was not just one up quark and one down quarks, but there are three up quarks, three down quarks. They each had some kind of a charge, given the name “color.” (Color may not be a good name, but it’s used.) There was a red up quark, a green up quark, and a blue—whatever it is. Now, once you did that, you had all these extra objects and the idea was that the real theory of the strong interactions—now we’re going back to the field theory point of view of the strong interactions—would be due to these colored quarks. Say the up red quark would emit a particle (very much like electrodynamics) which would be something like a photon except that that photon, that colored photon, would carry that red color charge. And that red color charge, 'that gluon, that colored photo, emitted by a colored quark, would itself carry color charge. But once the gluon carries color charge, the theory completely changes; a quark can emit many gluons, which, since they are charged, start interacting among themselves. And, lo and behold, what happens when they interact with each other? There’s a new interaction. These gluons then go back to the quark and could screen the quark, but they don’t screen the quark, they anti-screen the quark.

So, what happens is these colored photons have an anti-screening effect. They kill the screening effect that you had in electrodynamics. You end up with a charge which does not go to zero, which is perfectly fine. You get a very nice interacting theory at short distance and high energies, and these theories were then—the quantum field theory of these called QCD, was done by a group at Princeton; Gross and Wilczek and Politzer at Caltech, in 1973. They were able to show that this occurred from a consistent theory. It had already been known that the strong interactions at high momentum are weak (Bjorken scaling). Then everyone immediately had the idea that sind this consistent theory QCD, is very good at short distances, well, there must be something wrong with it at large distances. It must get stronger, this interaction. But, of course, this behavior is what everyone wants, and that’s quark confinement. You’re waving your hand with a big, big, big wave, and therefore not only will QCD explain high-energy momentum transfer events it will also eventually explain quantum field theory. Anyway, well, it explained confinement. So that occurred more or less when I was just finishing this Regge work. And, of course, I was also interested in the field theory. I came back—

Zierler:

And, Marshall, roughly what years are we talking about now?

Baker:

Now we’re talking about—the years we’re talking about are the following: S-matrix theory, Regge theory was 1960-1975. Then the quark model, it was first introduced in 1965, but without any real theory of the interactions. That’s right. And 1973 is when QCD—Gross, Wilczek, and Politzer, that was 1973. And 1973 was when I was just finishing up that work on Regge poles that I had mentioned, and I had come back to the University of Washington and had given one talk on this Regge theory. And I asked if I could teach the field theory class, and that was in 1976.

Zierler:

Now, Marshall, you know, in the early 1970s, the other thing that I want to ask you about, where is string theory in all of this for you? Are you paying attention to what’s going on with the two models in 1970 and 1972?

Baker:

Well, yeah. Yeah, I’m paying a little bit—I paid most attention—not really until about 1985, but at layman’s level. I was paying attention. It was clear that QCD had to lead to an effective string. Once you had confinement, the simplest model for confinement was, what do you think of the quark and antiquark coupled by—a slack string. You pull it apart, the string becomes taut, in this way, you get the potential. Of course, then the string theory was then based on, well, maybe the string is fundamental and so you try to do quantum mechanics of strings. And that eventually got related to gravity, but I was not paying too much—attention at that time. So, I got more interested in QCD and then I wrote a paper after I taught this class with a postdoc, C.K. Lee, on the general properties of this Yang-Mills—they call it the Yang-Mills theory, the Yang-Mills-like theories. And then I decided to try to do what Ken Johnson and I did in electricity and try to work on that and try to make a theory—try to calculate the interactions of quarks by gluons using similar methods to what we did with electricity except it’s much, much more complicated. But I started that work and then I was giving a talk on this in 1979. And it turned out my friend, Fred Zachariasen, was there again. He told me he was giving a talk on the same subject with a slightly different approach, so we again started to work on that, and we worked on that subject for a number of years. I’m maybe wandering too much. Did I answer your—what was your question? Did I answer it?

Zierler:

Well, the question was how closely you were paying attention to string theory in the early 1970s.

Baker:

I was paying more attention—I had a sabbatical at Paris in 1985. 1985 is the first time. By that time, there was a whole theory of string theory explaining confinement, the string was going to explain everything, gravity and all that. And there was a very beautiful course by Gervais at the Ecole Polytechnique in 1985, and there was a very good set of notes. And I spent a lot of time and I looked at those—keep looking at those for a long time. But, again, it was not for someone who is doing research but someone who is trying to make an effort to keep up with it. And I listened to the lectures of people who gave courses, even at the University of Washington. But I always was—I just wanted to know what was going on.

Zierler:

[laugh] Marshall, I asked the question about Feynman and Schwinger, and I want to pose that same question to you as you were involved with all of this fundamental work in the 1970s and 1980s. In what ways were the advancements in experimentation relevant for your theoretical work? Were you paying attention to what was going on at places like Fermilab or Brookhaven or SLAC? Was that relevant to the things that you were thinking about?

Baker:

The work at SLAC was very relevant. So, I studied quite hard. I knew Bjorken very well and we would discuss the analysis of their group, but that was, again, somewhat relevant and I wanted to know how things worked because this was trying to explain how quarks become weakly interacting in high-momentum transfer, et cetera. And the other thing, however, the other experiments that I’ve tried to keep up with, the ones at Brookhaven weren’t until later. Later at Brookhaven they actually saw the first time the quark gluon plasma. And that was very valuable. There was a deconfinement transition at a certain temperature of 200 MeV; that was at a later time. I think these experiments on the quark gluon plasma at Brookhaven were the ones I probably kept closely attached to. But I was very bad in understanding the geometry in all these experiments. So I had many notes and every time I left those notes for a couple of days, as if I hadn’t read but I knew where they were so I could go back and find them, so it didn’t really bother me. So that was the only time, with the quark gluon plasma.

Zierler:

Marshall, to move into the mid-1980s, what are you working on at this point in your career?

Baker:

In the mid-1980s,—With Fred Zachariasen and Jim Ball of the University of Utah, we had looked at QCD and showed in roughly 1982 that you could find a nonperturbative solution for the so-called gluon propagator, which was effectively like an electric propagator with a dielectric constant which vanished at charge distances. So, from this, we were able then to construct—but we couldn’t really go further. This was an approximate solution of QCD. We then realized that we could then get an effective theory of QCD if we introduced a few additional interactions. So then, at that point, we developed an effective theory of QCD which included—what you had to do is you had gauge fields, but you also had additional scalar fields. And so we were working on this—in 1985 we were, calculating things and—wrote a paper on the heavy quark potential in this effective field theory, and we continued to work on this. We worked on the deconfinement transition. We worked on that really for another 10 years, from 1984 to 1994 where, in that case, we used it to compare with the energy levels of heavy mesons made up of heavy quarks. We used that to try for hadron phenomenology. And that’s what I was really working on I would say in the mid ‘80s through to the mid ‘90s.

Zierler:

Marshall, I want to ask—it’s so interesting—when you talk about working on a particular project for 15 years, for 10 years, can you convey—how do I say this? [laugh] Why does it take so long? What happens over the course of a decade or more where you feel like there are still things to explore, there are still open questions out there? How do you know in terms of feedback mechanisms that you’re making progress and that you should keep at it, and how do you know, like, at year eight, you know what, we’re still not getting anywhere; time to back away from this? Can you talk a little bit about that?

Baker:

Well, we have to stop pretty soon, I see. Yes. How do I know? I don’t know.

Zierler:

Wait, why do we have to stop?

Baker:

I don’t know. Maybe you know, you decide. I thought we—

Zierler:

Marshall, we’re going to go as long as we need to. There’s no endpoint here.

Baker:

OK. Fine.

Zierler:

In fact, if you need to take a break, if you want to take a break at any time, just let me know.

Baker:

So, share your question again so—I’ll try to answer your question, not another question.

Zierler:

The question is: Your earlier research, you said, “we were working on this for 15 years,” right?

Baker:

Yeah.

Zierler:

And then you’re talking about your research in the mid-1980s, you’re working on this until 1994. So my question is: How do you know when you’re working on a problem for such a long period of time, what are the feedback mechanisms that you can rely on that tell you either you’re onto something and you should keep pushing it forward, and when do you say, you know, we’ve been at this for seven, eight, nine years; maybe we should back off and work on something else? How do you know when to pursue and how do you know when to stop?

Baker:

OK. Each one gives a little feedback. For example, in the work with electrodynamic systems, the work with Ken Johnson, I got very good feedback from other people, including some S-matrix people. They invited me to give a talk at the West Coast meeting of High Energy Physics at Berkeley in the first session, where Mandelstam was presiding. I gave the first talk, on quantum electrodynamics. Geoffrey Chew gave the same talk on S-matrix theory and the bootstrap. And the reason I was there was that I was supposed to represent maybe an attempted revival of field theory. And so little things like that, and then I got positive responses. So that was a positive aspect. (On the other hand, during the years, there are periods I didn’t know what to do with myself. You know, I just didn’t know what to do, so you find something else you want to learn. So, I learned fluid mechanics. I studied something else, and, oh, just learned some other new topics.) There have always been my collaborators, even when there was no paper—All this work as I mentioned to you, always had collaborators. And so I had encouragement from other people.

Furthermore—that’s a good question—in 1995, when I was still working on QCD—I’m trying to give you some examples—I met at a conference in Japan an Italian physicist named Nora Brambilla. She was a student of Prosperi’s, and, for some reason, she fell in love with this work we were doing with Fred and Jim, and she started to say we should collaborate with them using their approach and our approach. Of course, that’s just what I like to do, put two different things together, and so we did. How does this relate to your question? It relates to the question—it turns out that after 2003 or ‘04, I almost always worked alone, so it was really a big problem. I wrote a few papers. I’m invited to many conferences and things, but most of my work was always alone. Nora would be always giving me—every once in a while, I would get a reference, a link to a paper that I should be interested in related to what I’m doing.

And so I had that source. But I was just interested in the problem. And there are a lot of conferences, and there are certain conferences that have certain people always come for that conference. I went actually to a number of conferences, but there was one I used to always go to which there was a group of people who were always very interested in what we did, and so there was always a core group that was interested. And when I couldn’t think of anything to do, I told you the answer, I just thought of other things to learn and do.

Zierler:

And so, when you get to the end of it, when do you know that you’re satisfied that you’ve come onto something and you’re satisfied that you can move onto other research? In terms of theoretical discovery, what’s that endpoint where you say we’ve been at this X number of years, here’s what we’ve learned; I’m satisfied with the result and now it’s time to move onto the next project?

Baker:

I don’t really just—I must subconsciously ask myself that question, but I somehow keep—I hope that it won’t be a different project but it will be something else which will make use of everything I know.

Zierler:

Yeah.

Baker:

But to put it more relevantly, sometimes I cancel out everything that’s wrong in everything I did before and change one thing which would make it more interesting so I can continue. That’s sort of how I think. And that’s really sort of how it worked. As I look at everything, just reviewing what I already said to you, I was interested in field theory. Field theory is a very general subject. It includes the interactions of particles, quantum fields, QCD, electricity and magnetism, the standard model. The general principles are those of electrodynamics. There’s a few extra refinements in it but it isn’t anything basically different. The ideas are the same ideas, which were developed by Schwinger and Feynman in 1948. They are still used now, but there are new problems and questions. There’s weak electromagnetic theory, there’s whatever else, the standard model, but it’s all connected. It’s not the case that you complete one project, you stop, and you start a new project.

Zierler:

Right. I guess I’m asking a broader question about basically how theoretical physics works, because, of course, in the experimental world, those feedback mechanisms are quite obvious, right?

Baker:

Yeah.

Zierler:

You either see the particle or you don’t, or you detect the black hole, or you don’t, right? Things like that. But in theory, you don’t really have those clear markers where you say, we’ve really made progress on this or we haven’t; you just sort of keep on pushing the thread further and further.

Baker:

A hard question. What you said is just right. So, I just don’t know exactly—where are the markers? Let me think. So, you say there should be markers, you should know when you go from one project—

Zierler:

Well, let me ask like this: So, for example, the work that Politzer and Wilczek and Gross were recognized for with asymptotic freedom, right?

Baker:

Right, yeah.

Zierler:

The Nobel only came because experimentally their theory was proven time and time again, right? And so that would be a great example of a marker in the world of theory where they had all of the assurance that they needed that their theory was proved correct in the natural world, right?

Baker:

Yeah.

Zierler:

And so I’m curious, with your theoretical work, if experimental advances ever gave you that kind of satisfaction that the things that you were working on had borne out?

Baker:

All the things that I worked on—I also did some work on Regge scattering, et cetera, when I was at University of Washington related to experiment, the hadron deuteron scattering. I did work which some of my papers made certain predictions, but they were always small, it wasn’t so dramatic. It wasn’t earth shaking. We also tried to explain the lattice simulations of all the heavy quark potentials. This is, again, in 1995 period. And we were able to explain lattice data quite well, but I wouldn’t call it a breakthrough. It basically came down to a physically based parameterization with very few parameters, a lot of low energy levels, a lot of lattice data, but nothing fundamental. But basically the idea was to continue—the first half of my life was trying to understand quantum electrodynamics at short distances. The second half of my life has been trying to understand quantum chromodynamics at large distances. And they were connected by a quantum field theory once we added color to it. And nobody still understands.

So what I would say is the reason I’m continuing to work on that is that there’s still—everyone has a different model of understanding quark confinement, whatever it is, and I had one result, one prediction on which I wrote a paper in 2008. Fred, Gribov, Ter Martrosyan—everyone had died that I’d ever worked with, basically, and I wrote a paper on this deconfinement/confinement transition in our model. Now, the problem with our effective theory model, I said it introduced extra fields, so it threw something in by hand, and that’s sort of ugly. I mean, you try to explain something, you put something else in, you’re putting in as much as you’re explaining, you’re not explaining anything. Well, it turns out that we were able to get some predictions for this. It was one of the few predictions which was unambiguous without these fields, this stuff. But then I was limited. To do anything more—I was not an expert. In fact, I’m an ignoramus on computing and things. So, working alone I realized that I couldn’t do anything more on it. But I’m still very interested in this basic problem of confinement and things like this. It turned out that roughly about 2010 or 2011 there was an upsurge in lattice gauge theory, interest in confinement and strong interactions. There were a number of groups who were then starting to do lattice simulations—they would put in heavy quarks, various things they were trying, and then try to compute the field distributions, the color fields around them to try to get pictures of that. There was a Portuguese group (it was from Nora Brambilla. I probably got this reference), saying, you should talk to these people. And so this was this Portuguese group with Pedro Bicudo, who I happened to know because, in 2002, there had been a conference in Lisbon and we had discussed all about that. And they became very, very enthusiastic. They gave me all the data. I tried to say what, from our theory, what we should like to do. And then, all of the sudden at the same time, there was a second group from Bari, Italy, Italian. And they also were very interested in talking to me, and so we had extended conversations.

Now, the Portuguese group simulated a quark and antiquark, but they did not want to calculate the fields, they wanted to calculate the square of the fields, essentially the energy distribution. In that, you don’t get quite a picture of what the direction of the fields are, so you get more of the energy but less of a picture because it just computes the fields squared. The Italian group was actually computing the fields. Well, I started particularly to have a series of conversations with them over the past seven years. So, for the last seven years or six years, although I’ve been working alone, I haven’t really been working alone because I’ve always had these conversations. And then, in particular, I had some ideas about how to use our analysis, which still had in it—we had these extra fields, these extra scalar fields, which really weren’t part of anything. In the dual superconductivity model of quark confinement there are magnetic Higgs field so that the magnetic vortices of superconductivity are replaced by dual electric vortices of dual superconductivity which then give rise to a linear potential between quarks. We had a particular theory of that type; but I was never happy with that, but I wrote a paper—

Zierler:

Why not, Marshall? What would’ve made you happy with that? Where could you have gone that would’ve given you the satisfaction you were looking for?

Baker:

I was desperate to find something. I’d been working on all this other stuff, so I had a whole history of string theory. For example, one of the things I didn’t mention that we did get on the way is that my last student in 1999-2002, Ron Steinke, he was able to show how we’d get an effective string theory from our effective field theory. An effective string theory, not a fundamental one. Once you have an effective string theory you get confinement and you can calculate a lot of things. You can compute Regge trajectories. So, it came back. This was all done. And I was very happy about this. This was in 2002. Although the theory still had these scalar fields—we could now predict many of things and I could really understand a little bit more about Regge trajectories than I ever had before. A Regge trajectory you could understand in terms of rotating quarks of high angular momentum. So that was satisfactory. So that was a good job. But after that, I had no other person until these lattice people came in. Then it turned out—here’s another lucky break—it turned out I was in Italy in January of 2018. No one had invited me from Italy, none of the Italian group, even though they were interested in what we did. They even referred to it in some of their papers. However, another lucky break I had. My wife, who is a cloud physicist, has a colleague who’s a professor in Zurich and he was having his 60th birthday and some celebration, and she was invited to go to that. I mentioned that to the Bari people, you know, I can come to Italy for that time. And they were very nice—they invited me down to give a talk, and I gave a talk on this stuff. And I met a very nice group of people. Oh, there’s one thing before that. The previous year, at the Institute of Nuclear Theory in Seattle, one of the Bari people, a young woman who was a graduate student in Bari at that time, Francesca Cuteri came, and we talked with each other.

And so I had already a personal introduction with her, a very dynamic person. She came to dinner and we had a lot of discussion. So, she already knew me. That was one connecting person. This happened in probably 2017, the year before I went. So, anyway, I went there, and I gave a talk and I talked a bit about things. And we made some program of—I remember being on the blackboard in Bari, all of us discussing it, and some questions we could try to answer and simulations they’re supposed to do to check some of my ideas. Now, one of my ideas, you remember, was that I was so desperate to try to find predictions, so I had written a paper in 2016 which had some definite predictions about how large these extra fields had to be. It was called New Constraint on Effective Field Theories of Long-Distance QCD. Well, discussing this and going through this, I realized that that—when did I realize it? That that paper, all the general stuff in it was very good, but the extra constraint was complete hogwash.

Zierler:

[laugh]

Baker:

It demanded an essential role for the scalar fields.

Zierler:

And why is that hogwash? What’s the problem?

Baker:

The problem was with that constraint the effective theory looks like dual superconductivity on the border between a type 1 and a type 2 superconductor. In order for that to happen, you have to have a balance between the forces produced by the Higgs scalar fields and the pure gauge forces. That doesn’t happen; they can test that out. So, I realized there was a problem. So, when I gave the talk in Bari, I didn’t particularly—the paper was all there, but I was smart enough not to emphasize the constraint in my talk because they were supposed to find out.

And so I gave the talk and I said, basically, what you have to do is, if there’s going to be a confining force, there’s going to be long-distance linear potential, constant force, but there’s also going to be some residual Coulomb type effect. And you had to somehow separate them out to see what you’re doing. OK. That’s what I talked about at Bari, and we’re talking about January 2018. I wrote to them, but they only wrote back about the honorarium. I wasn’t interested, ‘cause I wanted to know about the physics. So, I stopped waiting around—so I stopped hoping, not waiting. I kept waiting but stopped hoping. There was nothing I could do so I started writing, why haven’t you written me? Nothing could be more irritating than that. But, lo and behold, in June of 2018—remember, I had left Bari in January of 2018, I gave this talk, had two days of discussion, knew all the people including Francesca Cuteri—I get a letter from them saying “We have used your idea: your idea seems to be a very good way for separation between the linear force and the Coulomb correction. And, in fact, we have a new colleague from the Ukraine, Volodya, who is going to give a talk on this at the lattice conference. But the deadline—(I came in on a Friday and I’m not in my office every single day—I happened to be there, and got this mail.) The deadline is tomorrow, so you have to tell us immediately whether you want to be on this paper.” And they said, well, if you’re not in this paper we’ll put you in a follow up paper. But, of course, for someone like me who had had no collaborators for 17 years, to all of the sudden asked to be put on a conference proceeding, I said, surely, yes. And it was written; I helped.

And so that started a collaboration which had been going on continuously and is still going on now. And we actually then wrote two papers and I think, using their data, they produced the light in the tunnel that I was blocked from. I would’ve already stopped. I’d worked so hard after this stuff. But then I realized that their data can be used to give a physical picture, I think, of actually how confinement works, and this physical picture is very simple, that—do you want me to mention that?

Zierler:

I am having a great time, Marshall. You keep going. This is great.

Baker:

OK. See, what we have is—when you calculate—I mentioned, say, these Portuguese calculations. There are color fields and they calculate squares of them. There are many kinds of electric fields. The electric meaning can mean ordinary electric; they have this color-charged electric, where electric means electric regarding the color charge. What the Bari people have calculated—and there’s another group I think in Pisa who works in it—developed a method of calculating that is quite simple. You have a quantity, an electric field which has exactly the same space-time properties as the electric field in electrodynamics. They can also calculate a magnetic field that way. There are just three components. It is a invariant under rotation in color space: i.e. it is gauge invariant. And what that is is a very simple idea to understand. No physical quantity could depend on color. The Bari group has found a way to measure on the lattice the field produced in the color—the color direction of the quark is varying. So, you take a statistical sample. So, if you put in a quantum theory, a quantum theory means you don’t have a definite known color. But if you actually fix the color, just look at one of the fluctuations, fix the color, and ask what’s the force emitted, not the total color force, but what’s the color force that’s due only to gluons of the same color. Furthermore, they have been able to separate the contributions of the fields giving rise to the linear and the coulomb potentials between a quark and antiquark. But they’ve been able to develop a technique to actually measure the field to produce exactly [phone ringing]—

Zierler:

Let’s wait for the phone to stop ringing.

Baker:

I’m sorry. No. I wasn’t going to pick it up. I knew it was going to stop.

Zierler:

OK.

Baker:

It’s so bad it would destroy this interview. We were talking about getting rid of it. So sorry.

Zierler:

I’m sorry. Continue.

Baker:

Well, what the Bari group measures is the field in the region of space in the same direction as the color of the quark and antiquark that emitted and absorbed it. So, essentially, they’re measuring directly a field which doesn’t depend on a particular color. So, what they have done is essentially, abstracted from all the stuff in this flux tube something that looks like an electromagnetic field. It has all the symmetry properties, I don’t know, of Maxwell theory. Now, that isn’t everything. That’s what they’ve measured. The Portuguese group measured different things. The field measured by the Bari group doesn’t contain all the energy because there are eight gluons, and this is just what represents one of them. The hypothesis I’m making is the following: I have to go beyond what we did before.

Zierler:

Right.

Baker:

I have to get rid of stuff. So, really, I like this—but please tell me if I’m not being clear with this one little point.

Zierler:

OK. Please.

Baker:

So, we know the field they calculate doesn’t compute all the energy but our assumption is that these other color fields, whatever the energy they contribute to, they do not contribute to the part of the energy which is producing the confining force. They might contribute to the energy due to Coulomb corrections or something else. So, we assume then that what the people in Bari measure really is the essence of confinement. That I call the Maxwell picture of confinement. That’s getting back to Maxwellism. Now, what this says is, since you have just a field, if you look in elementary electricity and magnetism, you see an electron and a positron and two charged particles, and then you see field lines. All the field line is a line connecting the electron and positron. The field, at every point, is just parallel to the line. It’s a way of just giving a picture of the electrostatic field between two charged particles. Going continuously, while if you have two positively charged particles, the field lines repel each other and they go out. And if you look at it, you can understand forces in terms of field lines. Going back to quarks, from what they calculate, they get a picture of the fields between a quark and antiquark. If you look at their field lines, they look exactly like the fields between charged particles except they don’t spread out as much. And the fact they don’t spread out is what makes the force actually a confining force. The picture you get is that of a confining force. The distribution of those confining field lines is symmetric around the quark- antiquark axis. The Bari group can actually calculate magnetic currents from the simulated field. We call this the Maxwell picture of confinement. We have written two papers, and we have elaborated on this idea. The Maxwell picture is that the confining force is produced by a single gauge invariant, Maxwell-like color field, and that the other fields, the fluctuating fields that are not measured, these other ones, only contribute to Coulomb corrections to the linear potential, these. In the second paper we computed the force between the quark and antiquark using the Maxwell picture a range of values of the quark-antiquark separations. And within, again, these errors, the calculated force was consistent with the Maxwell picture. This provides a picture of how this flux tube develops between quarks and antiquarks. But more has to be done. And if it stops here, that’s OK for me. If it stops here it stops here.

I’ve been in contact with them and I have discussed other calculations that they can do to predict whether this really works when you look at it more closely. They’ve gotten a big grant to do simulations with so much more accuracy. They might actually be able to test this. I hope very much that we’ll be able to pursue this further and find out whether this picture really is realized. But it’s a picture which is an explanation, I would say, of a part of confinement, a partial explanation which involves, instead of extra fields to try to fit the data, it uses the lattice simulations to determine the field and then determine the currents that confine the field, and use the data to determine what these currents are that circulate around the flux tube and prevent it from spreading out and yielding then a linear potential. OK. Well, that’s what we’re working on now. They were very interested. I hope they still are.

Zierler:

And how optimistic are you, Marshall, that this is really going to yield some significant results?

Baker:

Well, everyone is always hopeful. What I would say—I would be afraid—let’s put it the other way—I’m very happy that at least we wrote two papers. So even though it contain too much material so they are hard to read, it’s all written down and published. I very happy. I think that it requires even more work and a shorter paper concentrating just on the main ideas now that we really understand it. Whether or not is another paper is written with this idea, I’ll be very happy because at least I know about it and our group of people does. And I like the idea; it’s closer to understanding this question than I ever had dreamed to realize. But, to answer your question, I don’t know if it’s going to make an impact. It’s just too many things out there. It’s too many things in the paper. You asked how optimistic; I don’t know. The question is will it have an impact? It hasn’t happened yet. But it’s there now. It’s there, it’s written down. So, even though my memory—I’ll probably forget what I said to you in a few minutes—I can always look at the paper and I always do if I forget something. And, furthermore, with the extra computing power, they can do more calculations. If that really gets realized—there’s no reason why it shouldn’t, it’s for everyone’s benefit—then I’d say there’s a better chance that it will have an impact.

Zierler:

[laugh] Marshall, best case scenario, if all of this plays out just as you would hope, what would be the advancement in discovery? If everything plays out just as you want it to, what new physics will we know as a result?

Baker:

We would know what part of the field between a quark and antiquark produces the confining force. And it would then give you this very simple picture, you would see the field lines look more like electricity and magnetism, a little bit more compressed. From the confining field we can calculate the confining currents the magnetic currents which are the confining components, giving a physical picture of the confining flux to be. It’s not the complete picture, it doesn’t have everything, but it’s a picture which doesn’t depend upon—introducing a hundred extra parameters, or even one extra parameter, no extra parameters. And that still doesn’t mean anyone’s interested, but that’s what I think we would learn. We go a step toward a tighter picture of confinement without arbitrary assumptions and parameterizations. No one’s ever calculated a string tension or confining magnetic currents before from fields.

Zierler:

Marshall, you contribute to the truism I’ve learned, which is that physicists never retire.

Baker:

Well, I, you know—

Zierler:

You love it. You love it.

Baker:

Well, yeah, I sort of think this is a good time—sometimes there’s periods I can’t think—for example, there was a lot of time when I didn’t know what I was going to do. From the period of about 2003 to—seven years I’ve went through all the courses in biochemistry and things like that. So, I spent a lot of time learning other things when I was discouraged. But it’s because of this COVID-19 that actually I’ve been able to, since I haven’t been in my office since probably the middle of March because of all this, I’ve actually been able to do much more work.

Zierler:

Yeah. Right. I’ve learned that, too. The pandemic has been rough on experimentalists and pretty good to theorists.

Baker:

Very good. It’s very good. It’s been very, very good. So tell me about yourself a little bit.

Zierler:

No, no, no. This is your interview. I want to ask you, now that we have worked up to the present, and even you’ve given some ideas about where you hope this research heads to the future, I want to ask some broadly retrospective questions about your overall career. So, the first one is: In surveying all of the collaborations and projects you’ve been on, are there any that stand out in your mind that you’re either most proud of or you feel have contributed most fundamentally to advancing theoretical physics over the decades? Anything that particularly stands out in your memory?

Baker:

Well, I thought this work with Ken Johnson, although it ended up without a definitive result, we learned a lot about electrodynamics. A lot of people were interested in it while we were doing it, but it then led to the fact that it really couldn’t work out. The natural thing then, afterward—we had to add something else, and that led from that QED to QCD because I was—suppose you say, we if you can’t find a solution to your equation, you have to go someplace else. You have to do something else. And then Gross and Wilczek did it by adding gluons. So, I always thought that our work at the end was sort of some clue to QCD. But, as a long life thing, it’s—on the other hand, I notice now, with the Bari people, electricity has come back again in that—I’m really begging your question because I just can’t tear the physics from the people. I have Ken Johnson, I have Fred Zachariasen, I have Nora Brambilla, and now I have Francesca Cuteri and all the people of Bari. I associate each thing with them, so when I think of—and the Russian people, Karen Ter Martirosyan and Gribov with the Regge work, or Mandelstam and people I knew there. And so when I think about it, I don’t really think of the—I just think how I was working with all these people. It’s the people that made all the difference. And I just want to make always sure that I remember what their connection was. And all the work sort of was connected. I always had the point of view that everything sort of was connected. But the thing that probably got the first original excitement when I was the youngest was this work with Ken Johnson. But I think now what I’m excited about is—what these lattice people are doing.

Zierler:

It’s an important lesson you raise, as well, that even in the world of theory, even if a definitive result remains elusive, there’s tremendous opportunity to learn new physics along the way.

Baker:

Oh, of course. I always learn. When I got stuck, then I would go do something else for a while. But there’s always—it can be elusive but if it’s fun you—the rule is, be patient a little bit. It’s a meaningless statement. I was very lucky, I was very patient, but I was in a different time. It was easier to get a job. I didn’t have to worry. I always was offered jobs. I was lucky. Everything came. So, if I would say be like me, it would be a disaster for anyone now.

Zierler:

[laugh]

Baker:

Don’t be like me. In an ideal world, I think that’s a nice way to be. Do what you like to do, that’s sort of the mythical picture that people have. But that’s not life anymore. But then, you’re not asking me the question, I’m not supposed to—hopefully we’ll now have a new life for all the young people when all this terrible stuff with everything ends.

Zierler:

Well, we hope so. Well, Marshall, on that note, it’s been an absolute pleasure speaking with you today. I’m so glad we were able to do this.

Baker:

Yes, yes. Yeah. I was so nervous about this.

Zierler:

No, no, no.

Baker:

I must say one thing, a last one thing: In preparation for this I had already been, as you know, in contact with another historian.

Zierler:

Right.

Baker:

I had gone over very carefully all my history of all these things, which I had forgotten. So now I have organized everything I’ve done for my own sake.

Zierler:

Yeah.

Baker:

And it’s been a wonderful contribution you made to my knowledge of myself.

Zierler:

I’m so happy to hear that. And I hope that this interview—I want you to know that this interview—it’s so special to hear your perspectives and insights over your long career in physics. And it’s really quite important to include these stories into our historical record. So that’s my thank you to you. And if this has been helpful for sort of organizing your own thoughts and perhaps writing a bit about your experiences yourself, I’d be thrilled to hear that, as well.

Baker:

OK, certainly. And thank you very much.

[End]