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Credit: Paul Langacker
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Interview of Paul Langacker by David Zierler on April 29, 2020,Niels Bohr Library & Archives, American Institute of Physics,College Park, MD USA,www.aip.org/history-programs/niels-bohr-library/oral-histories/44597
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In this interview, David Zierler, Oral Historian for AIP, interviews Paul Langacker, professor emeritus of physics and astronomy at the University of Pennsylvania. Langacker recounts his childhood in the Chicago area and his early interest in particle physics. He discusses his education at MIT and his graduate work at Berkeley and he describes the political situation there in 1968 , his work with Owen Chamberlain, and Mahiko Suzuki and the origins of his life-long interest in weak interactions. Langacker explains his work at Rockefeller University, which was building a program in particle physics, and the circumstances leading to his hire at Penn. He talks about his research at DESY and the tenure process, and explains what he worked to accomplish as chair of the department, and in particular, his interest in increasing the diversity of the faculty. Langacker discusses his more recent interest in connecting superstring theory to particle physics during his time at the Institute for Advanced Study. Toward the end of the interview, Langacker shares his views on string theory and its role in achieving a grand unified theory in physics.
This is David Zierler, oral historian for the American Institute of Physics. It is April 29th, 2020. It is my great pleasure to be here today with Paul Langacker. Thank you so much, Paul, for being with me today.
To start off, let’s have your current title and institutional affiliation.
I am an emeritus professor of physics and astronomy at the University of Pennsylvania.
Great. Let’s start right at the beginning. Tell me about your birthplace and your family background.
I was born in Evanston, Illinois, but my parents lived in Chicago. When I was five or six years old, they moved north of Chicago about 30 miles, near Waukegan, where I grew up. We had a lower middle class background. My older brother and I were the first ones in our family to go to college or anything like that. We went to a very good high school. They recognized my brother and my abilities and were very good to us. Although they didn't normally offer calculus, they paid a math teacher to teach a calculus course just for me and one other person. It was similar with advanced biology. I certainly appreciated that.
Were your parents from Chicago?
To be honest [laugh], I wish I had done more to learn about my parents’ background. They were both born in the South, but they both had some connections to Wisconsin. And I learned at some point that they were actually married in Dubuque, Iowa. I don’t know how that all worked. I should quiz my brother, who is a few years older. Maybe he knows more.
And what level of education did your parents attain?
High school for both parents. My father started out in some sort of clerical job. Then when they moved, he was actually working in a shipping department, but they eventually promoted him to be a bookkeeper. They gave him some kind of correspondence course, but I don’t think that actually led to a formal degree. So basically high school.
So given that both you and your brother were gifted in school, I wonder if there was some family basis for doing well that your parents had but were never able to articulate because of their opportunities.
I think that’s absolutely the case. My mother was quite intelligent, and I think my father was fairly intelligent, also. But not having the educational or cultural background, they weren’t able to really fully utilize it.
Did they value education, and did they encourage you and your brother to achieve degrees?
They certainly supported us and never gave us any sort of difficulty. On the other hand, they weren’t snobbish about it at all. They were very easygoing. I can’t really say they encouraged us per se. If one or both of us had not had that inclination, I don’t think they would have pushed it, but since we did they encouraged us. My brother went on and got a PhD in linguistics, and he’s a professor of linguistics, emeritus, at the University of California San Diego.
You went to public school through 12th grade?
Yes, that’s right.
When did you start exhibiting talent in math and science?
I guess it was around fifth grade. I think the teacher noticed it more than I did. At some point, my class took a standardized test. When the results came back, the teacher was asking the class, “Well, who do you think in the class came out on top?” And nobody could really guess. I didn't even guess. [laugh] But the teacher seemed to be not at all surprised that it was me. I was generally pretty good in elementary school. I went to a one-through-eight school. There was no kindergarten or anything like that. But I was a little lazy, or a little rebellious perhaps, towards the end of my elementary school. But once I got to high school, which was a four-year high school, I really excelled in all of the subjects. But I was most interested in math and science.
As a child, were you a tinkerer? Did you like to play with chemistry sets or anything like that?
Yes, I did. I had a chemistry set, and I played with it. Not systematically; I didn't work through all the things in the book. But I did some. So I did like to tinker a little bit. And I liked to read. I discovered Scientific American at some point and read the articles on biology and in physics. Somehow, it was early on, like in eighth grade or so, I just sort of realized I’d like to go into physics, especially elementary particle physics. It’s not that I had some inspiring person, or that there was any big event or anything. [laugh] I just sort of knew it.
What did you know about elementary particle physics as an eighth grader that inspired you to pursue this field?
Now that I think about it, it was probably a couple years later, in high school, that I zeroed in on particle physics. I didn't take a physics course until my senior year. I attended a science program in Chicago, probably in my senior year. I think it was held at the Adler Planetarium, although I'm not sure. They had a lecture series with two lecturers. One was René Dubos, a famous biologist, and the other was Willie Fowler, a famous nuclear and astrophysicist from Caltech who lectured on stellar nucleosynthesis. They each gave two lectures, and one other student and I took the train down to Chicago. I found Fowler’s lectures especially to be inspiring. So it was probably those lectures combined with Scientific American articles that convinced me. It wasn’t elementary school; it was towards the end of high school.
So what was it about those Fowler lectures that was so inspiring to you?
Oh, just the whole idea that a star is not just a big ball of fire. There are physical processes going on, and elements are being created. And he brought in a lot of particle physics as it was known at the time, because nuclear and particle physics combined with gravity drive everything in a star. Fowler’s lectures expanded my view of things. I don’t remember so much about the biology lectures, except that René Dubos had a terrible cold. [laugh]
[laugh] Did you have a good physics teacher in high school?
Not especially great. My science teachers were adequate but not terribly inspiring. I don’t want to knock them. And similarly, it was basically one math teacher, and he was pretty good. Oh, I should mention another thing. In the summer between my junior and senior years of high school, I went to a six-week mathematics program for high school students at Oregon State University and got my first introduction to calculus, statistics, group theory, and so forth. And computer programming, which I think helped inspire me also.
When you were thinking about colleges to apply to, were you specifically thinking about physics departments that you wanted to be a part of?
By then I was thinking about physics and mathematics. I wasn’t quite sure which direction I wanted to go. I didn't know that much about the possibilities, but I had a very good guidance counselor in high school, and she suggested MIT. So I applied there and got in.
That’s the only place you applied?
I applied to Michigan State University and was accepted there also, but those were the only two places.
That’s an interesting conversation to have with a guidance counselor. I mean, MIT—of course, what a wonderful place to be. But of all the schools to apply to, it seems like those are two interesting choices. Like why not Stanford or Caltech or Harvard or Princeton? And so on and so forth.
Any idea about that?
Remember, I was from a lower middle class background in the Midwest. Places like Harvard or Princeton were not really in my world view. Neither was MIT before she suggested it, but I think she felt that it would be a better fit. She was pretty confident that I would get into MIT [laugh] and I should probably just apply to one other school to be safe.
How long were you at MIT before you settled on physics?
It was [cough]. Excuse me, I have [cough]—
If you need to get water, go ahead. That’s fine.
Actually I have some water here. My cough is in my throat, not in my lungs, so I'm not especially panicked about the COVID-19 virus.
I had placed out of calculus, because I had had it in high school—[cough]—I hope I don’t lose my voice. Instead, in my first year at MIT I took a two-semester course in mathematical analysis, which was designed to teach prospective pure mathematicians about rigor. The first semester was difficult, but I managed all right. In fact, it was interesting—let me digress a little about that first course. The final exam was held at night starting about 6:00 or 7:00 p.m., and you could stay as long as you wanted. You could bring any reference you wanted as long as it didn't breathe. It was a true/false test consisting of 20 theorems. Was this theorem true or false? And it was graded plus one, minus one, or zero. Zero if you left it blank, minus one if you got it wrong. They weren’t known theorems or anything that was in any book, and there was absolutely no way to know whether one was true or false until you rigorously proved it or found a counterexample. I spent about eight or nine hours [laugh] on it. In the end, I left one blank, and I got 19 correct. [laugh] And so I got an A in the course.
I was very proud of that. If I had gotten an 18, I would have gotten a B. [laugh]
It was a tough course, but the second semester was much harder. It was taught by Isadore Singer, the famous mathematician, who was at MIT at that time. The whole semester was basically the most rigorous imaginable proof of Stokes’ theorem. Stokes’ theorem is sort of proved in an advanced calculus course, but not totally rigorously. Well, that just blew me away. It was so difficult that it convinced me that pure mathematics was not for me. I survived the course somehow, but it drove me out of mathematics. [laugh] I met Singer about ten or 15 years ago, at the Institute for Advanced Study when he was visiting, and I told him that story about how he had [laugh] driven me from mathematics into theoretical physics.
And he was amused by it. [laugh]
What was the issue? Was it too abstract for you?
Yeah, it was just so abstract. Frankly, it was just too difficult for me. It wasn’t just a matter of taste or what I enjoyed; I realized that I just didn't quite have what it would take to excel in that field. I could have conceivably majored in math from a more applied point of view, but somehow that was not an interest to me at the time. Although in my early years at Penn, I taught a graduate mathematical methods course, and I taught a lot of that in graduate quantum mechanics courses as well. So I came to enjoy applied math. But at the time, I never thought of majoring in it, and I sort of plunged into physics and never looked back. [laugh]
When did you formally declare the major in physics?
I have no real recollection of that, but it was probably the second or third year.
Who were some of the professors that you became close with, in the department?
There was Rai Weiss, who recently shared the Nobel Prize for his work on LIGO. He was a very young [laugh] assistant professor at the time. There was Anthony French, who was more specialized in education. MIT required a senior thesis, and my senior thesis advisor was Robert Hulsizer, an experimental particle physicist. I think those were probably the ones that I knew best. I also enjoyed other courses. I took a number of literature courses from Albert R. Gurney, the playwright, who taught humanities at MIT. I've always enjoyed non-science topics like literature and history.
In terms of course work, did you gravitate more towards the theoretical or the experimental kinds of courses?
Probably more towards the theoretical. In fact, MIT at the time was experimenting with not having an elementary physics lab, so I never took one. I did teach some elementary labs during my early years at Penn, and I enjoyed them. But the only lab course I took at MIT was an advanced lab in the junior year, and that was at a much higher level. But I think I realized in the course of that lab that I was probably going to electrocute myself at some point if I went into experiment.
Like I remember when we did the Millikan oil drop experiment. You were staring into the microscope, and you were manipulating controls with your hands, and it would have been fairly trivial to touch a hot electrical lead. I just realized it wasn’t for me. [laugh] And I’ll be honest; I never liked chemistry. Either in high school or in college, I never was any good in the chemistry labs. I never could get the experiments to work out right. The theoretical part of chemistry was OK, but that was not my thing.
Did you spend any summers as an undergraduate involved in physics work?
I spent my second summer and the summer after I graduated working as a computer programmer for a company near Cambridge that specialized in computer typesetting. That was instructive because at that time in the science courses, one only learned about IBM and Fortran and punch cards. The summer job introduced me to a completely different type of computing. The company had a DEC PDP-1 computer, and for breaks one could play the original “Spacewar!”, one of the first computer games, which had recently been developed by people at MIT. The summer after my junior year, I joined an international program that sent science students abroad to work in labs. I went to an applied physics lab in the Netherlands, in Delft, where they put me to work trying to set up and test a radioactive beta-gamma coincidence experiment. So, except for the junior year lab course, that was my first real lab experience. And then in my senior year, I did my senior thesis working with Hulsizer, writing computer programs to analyze data.
What was Hulsizer working on at that point?
It was measuring and interpreting the antiproton-proton elastic scattering cross-sections obtained from bubble chamber observations at Brookhaven.
And he gave you a project that was directly related to his work?
That is correct.
So what did you do?
I wrote the computer programs to apply the so-called optical model to the scattering data, and then fitting the data to determine the model parameters.
I'm curious, given your increasing fluency in computer programming—this is an opportune time to know these things—did you ever think about entering industry after MIT? Did you ever have any job offers in that field?
I never really considered a computer-oriented career. I did briefly consider working in an applied physics company. I attended an industry recruitment session during my senior year or maybe in the late junior year, and a company based in Washington, D.C. interviewed me and flew me down to Washington for further interviews. But I decided that I really wanted to go to graduate school instead, especially since I wanted to do particle physics.
What graduate programs did you apply to?
I applied to Berkeley, which is where I went. I probably applied elsewhere as well, but frankly I don’t remember. That was a long time ago! [laugh]
Was Berkeley where you really wanted to go?
Yes, it was.
And in terms of particle physics, was your sense that that was the place to be?
Yes, Berkeley was one of the most important centers for particle physics. I would have also liked to remain at MIT, but MIT discouraged their own undergraduates from staying. There were also excellent programs at other universities, such as Stanford, Caltech, Chicago, Harvard, and Princeton, but Berkeley was especially strong in both theoretical and experimental physics. Also, SLAC (at Stanford) had recently turned on, and many of the Berkeley experimenters were working there, including my initial advisor, Owen Chamberlain. Anyway, Berkeley especially appealed to me. I think I wanted to go to California, also.
So you get to Berkeley when? In the Fall of 1968?
This is a pretty wild time to be in Berkeley, entering a graduate program. Did you consider the fact that there was going to be campus unrest and that might impact you professionally and personally?
Well, that’s a major subject. [laugh] The short answer is no. Around my senior year, the anti-Vietnam War demonstrations were starting to heat up in many parts of the country. I was involved in that in a fairly minor way at MIT. I went to a couple of peace demonstrations. At that time, Berkeley was not really so difficult. It was only after I arrived that the real problems with the riots and disturbances happened. Now, I have to digress with an anecdote here. My current wife—I've been married twice—also got a PhD at Berkeley. She’s a little older than I am, and was there from 1963 to 1968, with her first husband. They loved it there. The Berkeley free speech movement began while they were there. It was mainly peaceful and positive at the time, so my wife has wonderful glowing memories about the free speech movement and Berkeley. My memories are just the opposite; I arrived just after they left (we didn't meet until decades later), just about the time that things were starting to get very ugly, not only at Berkeley but also at the University of Wisconsin and elsewhere. The Martin Luther King and Bobby Kennedy assassinations had occurred the previous spring, and the war was becoming increasingly horrible. The Kent State killings were a couple of years later. Anyway, most of the ugliness started about the time that I entered Berkeley. And it went on, not constantly, but part of each year, for the four years I was there. Sometimes the focus was the war. Sometimes it was the People’s Park or other issues. I quickly realized that at Berkeley and probably some other places, the valid causes that I believed in, were drowned out.
Which were what? What did you see as valid?
The Vietnam War, mainly. I was very much opposed to the war. Unfortunately, Berkeley and some other places were magnets for people who just wanted to riot for the sake of rioting, and they came in and turned everything very ugly. This was aggravated by overreactions on the part of the governor, Ronald Reagan, and others. So it was a hard time. I don’t know if you're familiar with the layout of Berkeley, but the Lawrence Berkeley Laboratory is on the hill over the campus. By my third year I was working up there. Each day, we would look out over Berkeley to see if there were clouds of tear gas somewhere, just so we’d know which route to take home. And my first wife, who was with me at the time, almost got caught between a cloud of tear gas and a line of National Guard soldiers with bayonets. She was not involved in any demonstration; she just happened to be at the wrong place at the wrong time. So it was a very ugly thing. Frankly, for the rest of my life I have had a very negative psychological view of Berkeley, even though I've been back there a number of times, and everything has been fine, and I know all the good things. And I've spent other times in California and enjoyed it. But those years sort of soured me on Berkeley and California forever.
In terms of the things that you felt personally connected to in terms of the protests with Vietnam, I wonder, being situated in the physics department, if you were concerned specifically with the role of the military in physics research. If that was an issue that you were aware of, and how you might have felt about that.
I remember people talking about that, but that was not something that especially bothered me. I was much more just concerned about the war itself.
OK, so the general idea of there being academic and DoD partnership on physics matters, that was not a concern for you?
It might have been a concern, but a minor one. Not something that I focused on.
In terms of working with Owen, did you go there with that intent? Was that one of the draws for you? Did you know you were going to work with him?
No. I believe I was just assigned to him. At Berkeley, when we first arrived we spent several days taking a very difficult written and oral prelim exam. And then I was assigned to him as my initial advisor, before I went into theory. I wanted to do theory all along, but at the beginning it’s better to work with an experimenter. So I worked with him, especially in the first summer. There was a shortage of office space, so I shared his office at the LBNL, or what’s now called LBNL. He was a very impressive person.
I'm always curious—coming from MIT to Berkeley, obviously your vantage point as an undergraduate to a graduate is very different, but I wonder if you could reflect at all about the ways that the departments operated, in terms of similarities and differences. Maybe in terms of the kinds of subfields that were emphasized at each program, the relative hierarchy of the theory versus the experimental, where funds and resources were sort of poured in more. Did you see any major differences between how the department at MIT approached physics and how Berkeley did?
To tell you the truth, no, I don’t. That’s more based on ignorance than on not seeing differences. I was just a student concerned about learning physics. That’s all I wanted to do. Learning physics and then dealing with all the sociological background and the disturbance on campus. Berkeley was wonderful in the sense that there was such a wide variety of courses and so forth that I could take. But as an undergraduate physics major at MIT you didn't have that much in way of electives. I do think that the physics curriculum at MIT in the late ‘60s was a little bit old fashioned, like courses on vacuum tubes, a lab on glass blowing, and stuff like that. [laugh]
Whereas I didn't have any of that at Berkeley. I thought they were both good places. I enjoyed the academic parts of both of them very much.
How much course work did you take at Berkeley? Were you mostly in the labs, or did you do a significant amount of course work?
I did a lot of course work, I think certainly for two years, and I probably took some courses in my third year as well. The first two years were basically all coursework. I never really worked in labs at Berkeley. Even when I was working with Owen Chamberlain, I was again writing computer programs, not physically working in the lab.
What were some of your favorite courses at Berkeley? Or favorite professors.
Well, Berkeley was an interesting place. That was the time of the S-matrix and bootstrap theory, which was one of the most popular areas in particle physics for a number of years before the standard model came in. The bootstrap was dominated by Geoffrey Chew. He was not my advisor, but I took courses from him. He was a good teacher. He continued to be a little bit active in physics until his death last year. I also took a summer reading course from Stanley Mandelstam that was very useful to me.
What’s a reading course? What does that mean?
It means it was like an independent study course. I think it was during my second summer. The way that they taught the particle physics courses at that time gave me the impression that things like dispersion relations and analyticity all just popped magically out of the mind of some S-matrix theorist. But in fact it was all motivated by field theory, especially from Feynman diagrams, even though the S-matrix program had in other ways abandoned field theory. Even though Mandelstam was an S-matrix person, he insisted that I study the second volume of Bjorken and Drell’s field theory book, which showed how it all came from Feynman diagrams. [laugh] That was very illuminating for me.
Was this your first exposure to the Feynman diagrams?
No, it was not. I took a course in field theory in my first year at Berkeley from Eyvind Wichmann, who was an axiomatic field theorist. Axiomatic field theory a la Wightman and others was another big thing at the time. It was an interesting course. Two of the three quarters were rigorous axiomatic field theory, and then in the third quarter, he did quantum electrodynamics and Feynman diagrams. It was a total switch. Anyway, I had learned some of that. The second year I was there, the course was taken over by Korkut Bardakci, and I was asked to be the TA, which mainly meant correcting homework papers. And he taught a very conventional Feynman diagram-type [laugh] field theory course.
What does it mean to teach conventionally?
Feynman diagrams, basically, which are an elegant perturbation theory formalism for calculating for weak coupling . Modern field theory is not all perturbation theory but rather includes all kinds of non-perturbative aspects. But at the time, to most people field was restricted to a Lagrangian with a weak coupling, allowing you to do calculations in perturbation theory, which is most conveniently done by Feynman diagrams. [laugh] Now, it’s much more sophisticated. Another important professor I had was Eugene Commins, who was an outstanding experimenter who did atomic physics. I took his course in weak interactions. Weak interactions can be calculated perturbatively, although at that time, before the standard model, there were divergences in higher order. After all of the exposure I had had to the strong interactions, S-matrix theory, and axiomatic field theory, the course in weak interactions opened up a new world to me. Commins had just written the first draft of the first edition of his famous textbook on weak interactions, which he used in the course. The course really blew my mind. Weak interactions became one of my major interests throughout my entire professional life.
What was it about weak interactions that was so compelling for you?
For one thing, the wide variety of processes that are described. Later, the standard model predicted even more processes, which were subsequently observed, and many of the processes could be predicted very precisely using perturbation theory to higher order. This allowed extremely precise tests of the theory. But I am getting ahead of myself; when I was in graduate school we didn't have the standard model yet, or at least it was not incorporated. Steve Weinberg had written his famous paper on SU(2)×U(1), which is the weak interaction part of the standard model, in 1967 but nobody paid much attention to it until after I had gotten my PhD. [laugh]
I'm wondering about that. Why is it, do you think, that no one paid—was he just that far beyond that it was just—people just couldn't grasp what he was saying?
No, I think he didn't pay that much attention to it, either. [laugh]
For one thing, it was written just as a theory of leptons, because it was too difficult to incorporate strongly interacting particles. And although the quark idea was around, the obvious extension to include the three quarks that were known at the time would have led to results that clearly contradicted experiment. Most of the ingredients in the standard model had precursors earlier—quarks, nonabelian gauge theories, SU(2)×U(1), charm, spontaneous symmetry breaking, and the Higgs mechanism. But spontaneous symmetry breaking and the Higgs mechanism had been mainly thought of as being relevant to the strong interactions. Weinberg’s big thing was realizing that it should be applied to the weak interactions instead. But it was a little early, and the time wasn’t quite ripe. And he didn't prove it was renormalizable. That was done by others a few years later. The SU(2)×U(1) model predicted weak neutral currents, but they were not observed until a few years later. Their properties, and those of the gauge bosons observed later, were the major early tests. Also, the discovery of the charm quark allowed an acceptable extension to quarks.
[laugh] Paul, given your exposure to all of these amazing intellectual pursuits and research projects, I wonder, how difficult was it for you to settle on a dissertation topic?
Well, most of what I just described happened when I was already writing my dissertation, or soon afterwards. My dissertation was motivated more by my thesis advisor, Mahiko Suzuki, who had just arrived at Berkeley in 1970.
How did you get connected? How did that happen?
I think I went to see him. I heard that he had come. He was not so interested in things like the S-matrix theory, but in current algebra. Current algebra sort of abstracts a lot of the relevant symmetry ideas from field theory without having to rely on a specific field theoretic model. I was looking for an advisor. Most of the people were working with Chew or Mandelstam. But by that point, I wasn’t so interested in the bootstrap-type stuff. So I thought I would go talk to him. I became his first student, or one of the first two. There was another who he took on at the same time.
Where had he come from, before he got to Berkeley?
He was educated at the University of Tokyo and then had come to the US, I think to Caltech, the Institute for Advanced Study, and Columbia. Anyway, I didn't really do a dissertation in the sense that you're probably thinking of. I wrote a series of research papers, some of which were related to current algebra, one was on the muon magnetic moment, and one was on weak interaction theory, trying to predict the possibility of weak interaction processes being relevant to scattering and not just weak decays. That was also something that got me interested in the weak interactions. So by the time I was ready to write my dissertation, I had written four or five papers, some in collaboration with Mahiko. There was one that was on Regge pole theory that I had thought of myself and did independently. And so my thesis was really just a collection of these individual topics, rather than one very large analysis. And I have to admit that none of the papers that I worked on as a graduate student were things I would look back on as having much lasting importance. I mean, they were OK papers [laugh] but none of them were earth-shattering.
And in terms of being an advisor, how hands-on was he in terms of helping you develop the project?
In most of them, he was very hands-on. We spent a lot of time together. Expect for the one on the Regge theory, he had suggested the papers. And so yes, I thought he was a very good advisor.
As you were putting the dissertation together, were you thinking about what its contributions to the field were, sort of beyond your own research, thinking about where it fit in, in a broader scope?
Probably not. I was a little too young and inexperienced for that. I just wanted to write down what I had done, and hoped that it would be important. But as I say, I cannot say now that anything was all that important.
Who was on your committee?
First, I should comment that at Berkeley at the time, one had to undergo at least two difficult oral exams. The first one was the oral part of the prelim that I already mentioned. Then when it looked like I was going to be drafted, I decided to take a master’s degree, and I had to take another oral exam for that. And that was interesting. Charles Kittel, the famous solid state physicist, and John Clarke, famous for SQUIDs, were on my committee. They and the two other members each asked me a very hard questions. After about an hour or 90 minutes, I finally finished the fourth question, and I was thinking, “Well, I hope I managed all right.” And then Kittel said, “Now, should we go around again?”
I thought he was joking, but he wasn’t. [laugh]
So they put me through another four. I passed the exam, [laugh] but it was hard.
How much were these questions focused on your own research, and how much—
Oh, that was the master’s oral that I took sometime in my first or second year. So that was general physics. Now, the thesis defense, who was on it? Certainly Suzuki, Geoffrey Chew, Dave Jackson, the author of the famous electrodynamics textbook, and a mathematician named Stephen Diliberto who I had taken a math course from. I don’t remember anybody else, so certainly those four. They also asked me very hard questions, but in particle physics . They were not directly on my thesis, but were on related topics inspired by it. Just to blow my own horn, I think I did very well. And Geoffrey Chew—was it Chew or Jackson? I think it was Chew—told me that he was going to recommend me for postdoc positions above his own students. [laugh]
That was very gratifying.
Do any of the questions stick out in your memory?
One of them was to sketch the proof of why electrons and protons have the same electric charge, even though one of them has strong interactions and the other doesn't. That’s a symmetry argument, and fortunately I knew enough about it to sketch the argument. They also asked me to describe some research project I was thinking about but had not put into the thesis. At the time I had an idea about how to improve the efficiency of dispersion relation analyses —and again, this was my idea, not Suzuki’s. A dispersion relation is basically an application of the Cauchy theorem involving a line integral in a complex plane, in which you have experimental data for part of the region, and then you try to determine something about the other parameter regions. I had some ideas that I described. I think they liked that, even though it didn't really work out in the end and I never wrote a paper on it. Dave Jackson asked another question about electron/positron annihilation. One of the projects with Suzuki was an extremely clever way of putting a bound on the strong interaction contributions to the then-unmeasured anomalous magnetic moment of the muon. When I say extremely clever, it was entirely Mahiko’s idea, not mine. So I had talked about that, and there were going to be a series of new electron-positron colliders coming online in the next couple of years, such as SPEAR at SLAC, that would be relevant to the subject. And so Jackson asked me to talk about all of the new colliders [laugh] in that field. I sort of got through that one, but I had not really [laugh] thought that much about the details of the different accelerators and experiments.
I'm curious, with these questions—were the professors, were they more looking for the right answer, or were they more looking for your fluency in the relevant research, and being able to understand the research and being able to convey it, whether or not there’s a definite right or wrong answer at the end of that?
The latter, definitely. They wanted to see how well I could think on my feet and handle myself. They would usually start out—especially in the prelim and master’s orals—with something so difficult that I wouldn't know how to begin, and then they would give me some hints. They wanted to see how I would run with those hints.
Now, you defend, and you're thinking—do you have the Rockefeller University opportunity—is that set up before you defend, or that comes together later?
The time scale for applying for postdocs at that time was later than it is now, so it was some time in the spring semester of my last year. I probably had had the thesis defense already or at about the same time.
Was Rockefeller where you wanted to be, or was that—how did that come together?
I applied to many places. Two of the earliest offers I received were from Rockefeller University and from the theory group at Brookhaven National Lab. Rockefeller is mainly known for biology and medicine, but at the time they were trying to expand into other areas, and they had a small particle theory group with several excellent people. Brookhaven offered to fly me out to give a seminar. While I was there, they graciously allowed me to also visit Rockefeller and the Institute for Advanced Study. I liked Rockefeller and accepted their offer. I later received nine or ten other offers, but I had already accepted Rockefeller.
And you said that they were building the program at this point.
Yeah, they had four people in particle theory. And they were building an experimental program, headed by Rod Cool. They had other groups in statistical mechanics and mathematics. The theory group was led by Abraham Pais, who wrote a famous biography of Einstein. I worked with Heinz Pagels, who initiated chiral perturbation theory, which is still a big subject now, 50 years later. [laugh] He later became director of the New York Academy of Science and wrote several popular science books. Unfortunately, he died in a mountain climbing accident in 1988.
As a program that was in development, that was building, did you get the sense that there was a different institution that they were modeling it after, that they were thinking about emulating? Like the Institute, or something like that?
Well, they may have, but it probably was closer to the Institute in that it was not a PhD program. It was a research program with postdocs. And just like the Institute, they could have graduate students from other local universities. There were a number of graduate students and postdocs at Rockefeller while I was there, for two years.
Interdisciplinary collaboration, was that emphasized at Rockefeller? Were you able to talk to people from different fields?
No, it was not emphasized. I think they were really just trying to do what they could to build these fields. And then they later sort of backed off. There may still be some efforts in those areas, but not much.
Were your goals to continue to revise and refine your work on your dissertation, or were you looking to take on new projects at Rockefeller?
At first I was thinking about refining my dissertation projects. But then Heinz Pagels asked whether I would be interested in working with him on chiral perturbation theory. I was somewhat familiar with the subject, because I had heard about it when I was a graduate student, and that was one of the things that attracted me to Rockefeller. And so I said yes. I worked almost exclusively on that for the two years that I was there. One exception was a theoretical data analysis on total cross section measurements, which I did with a number of people. That was motivated by a new precise experiment that Rod Cool and others had made at the recently opened National Accelerator Lab (Fermilab). I did lots of stuff like that later in my life, but that was the first one.
How much were you publishing and attending conferences and doing these kinds of interactive things?
I went to my first conference, in Irvine, California, when I was still a graduate student in my final year. That was my first introduction to Weinberg’s theory, the electroweak standard model. He wasn’t there, but someone else, I think it was Francis Low, gave a talk about it. He was talking about the Higgs mechanism and everything. Those ideas were so far from what most people were thinking about that people were laughing, as if it were a joke. While a graduate student, I went to Caltech once to give a talk. That was probably about it. But once I got to Rockefeller, I went to several conferences, including a big APS meeting in Chicago. I summarized the conference for the group when I came back. I think I also visited some universities to give seminars. I never kept track of things like seminars, so I don’t remember all that well, but I was starting to do those things. I did it much more later.
When did the opportunity at Penn first come along? How did that happen?
I told you that I had had around ten offers for postdocs when I first graduated. It was very different after Rockefeller. At first, I didn't have any decent offers at all.
Does that say something about sort of the larger availability of faculty positions in physics at that time? Had departments stopped really hiring like crazy like they had previously?
Yes. when I got my PhD in ’72, that was the time that the job market really crashed. It was a combination of two things. First of all, there was a general crash in all fields of academia—humanities, all the sciences, et cetera. Places were just saturated. Baby boomers were getting their PhDs, and the university job markets were saturated. In physics, it was worse than other fields, because there had been the big push after Sputnik to increase physics faculties and physics research, and that got saturated at about the same time. So it was really a bad time. So I don’t think any of my contemporaries in physics had any hope of getting a faculty position initially. We were just worried about getting postdocs.
You're talking about after Rockefeller, or after Berkeley?
After Berkeley. But it went on for some time. I think there were around a dozen of us who got our PhDs in particle theory at Berkeley the same year, and only a couple of us survived in the field in the US. There were a couple who were a year or so earlier, like Chris Quigg and Rick Field, and one Turkish physicist survived in Turkey. But it was a hard time. Even after Rockefeller, I was only looking for postdocs. And Heinz told me that part of the problem was that so many places had offered me a position two years earlier that they probably didn't [laugh] want to come back again. So I was actually getting pretty frustrated.
Does that mean, Paul, that there weren’t even jobs to apply to, after Rockefeller, in 1974?
Well, there were lots of postdocs you could apply to.
No, I'm talking about assistant professor lines.
There may have been a few at research-oriented institutions, but I don’t think I applied to any, and other people in my same situation were also focusing on postdocs.
Just because you would not be competitive at that stage?
There were a lot of people a little bit older who had had postdocs, and I think whatever few number of jobs that were being filled at the assistant professor level were going to the older people. Anyway, I didn't get any offers for a while, and then when I was at that conference in Chicago, I got a phone call one morning in my hotel room from Gino Segrè at Penn, offering me a postdoc, or at least offering for me to come visit and give a seminar. I forget which. And so I did go visit there later, and I accepted the postdoc. That’s how I started at Penn. Somewhere in the middle of my first year, they decided to promote me to assistant professor.
Ah, OK. So did you have a sense that it might turn into that, or that was a pleasant surprise when you were already there?
It was more of a pleasant surprise. [laugh]
And a big relief, I'm sure, too.
When you first got to Penn, was your sense that this was a program that was really looking to establish itself and compete at the top level vis a vis other departments?
It was a good program. They had a number of strong people. It was not at the same level as Berkeley or MIT, but it was a pretty good program.
When did you start taking on graduate students?
Let’s see. [pause] I started as an assistant professor in 1975. I think I tentatively took on one or two students within the first couple years, but eventually they left or went into some other area of physics. At that point, one day, the chairman of the department walked into my office and told me I was not going to get tenure. So I didn't want to [laugh] take any students at that point.
That obviously turned out to be incorrect.
[laugh] Well, they later changed their mind, but Penn was never as supportive as I would have liked of my more phenomenological type of theory, and that was part of the reason. But anyway, they were pretty good about allowing me to take a leave of absence for a year, which I spent at SLAC, Berkeley, the Institute for Advanced Study, and the University of Wisconsin, Madison. At the end of that time, they reconsidered. Frankly, I think they were a little embarrassed that they hadn’t promoted me in the first place, because a number of senior important theorists from other institutions told me that they were kind of outraged and had let Penn know it. [laugh] But anyway, I don’t know really what was going on there.
So what was the reversal of fortune where your fate was in fact not sealed, with the tenure issue?
I think they just realized that they had made a mistake, but they didn't want to immediately just change the decision. So what they did was to say, “OK, we're going to consider hiring somebody at the tenured associate professor level,” and they brought in several candidates, including myself, and then they gave it to me. [laugh] I should say that I always have a somewhat difficult problem with time frames, and what was happening when, but I think they tried to make up for it a little bit by promoting me to full professor somewhat sooner than normal, within a couple of years. Anyway, so coming back to my graduate students. I did not have that many graduate students in my life. I think there were five official students, as well as three who I unofficially co-advised. Only four of those ended up in tenured faculty positions. I was more successful with postdocs.
Why do you think that was?
Part of it was that to survive at all in particle theory, you had to be very strong. Most of the very promising prospective students went to just a few top institutions, and Penn was not quite at that level, at least not in particle theory. Also, most of the strongest students were more interested in formal areas like string theory, while I was more interested in connecting theory and experiment. Anyway, my first student who ultimately completed his PhD with me must have started around 1985. I did a sabbatical at DESY in Hamburg in ’87 - ’88, and he went with me there.
What was going on at DESY? What project did you get involved in?
Well, by that time, I had developed my own program in weak interactions. There were a number of types of projects, including model-building and studies of the implications of various possible extensions of the standard model, i.e., of new physics. But mainly in the global analysis of experimental data. There were lots of experiments, especially in neutrino scattering initially. Typically each experimental group would analyze their own data and extract some parameter or parameters from it, often with differing theoretical assumptions and not treating systematics the same way. And so one of the things that I had done most, and the first major work was actually done before I was [laugh] rejected for tenure at Penn, was global analyses. My collaborators and I would take all of the experiments, reanalyze them in a consistent theoretical framework, making the same assumptions about radiative corrections, about QCD corrections, how to parameterize the results, and trying to properly correlate common systematic errors between the experiments. This was done both within the context of the standard model, and in the more general context of likely extensions, such as involving a larger gauge group or additional particles, or of completely different models. I had started this early on and spent some of my time on it for my entire professional career. So I spent a fair amount of my time at DESY just continuing that work. I had a very difficult time getting my computer programs to run. They had an idiosyncratic homemade computer operating system at DESY, and I spent weeks and weeks [laugh] getting my programs to run.
Was that simply a nice place to do your work, or was there a natural fit in terms of your project and what DESY offered?
It was a nice fit. Roberto Peccei was then the head of the theory group at DESY. I had visited there the previous summer, and we had written a paper together with Tsutomo Yanagida. It’s not so much that I worked with a lot of people; I worked with my student who was there, and with a postdoc from Canada, David London. David and I wrote several papers that I think were very influential on possible extensions of the standard model. So it was a good environment. Also my wife was originally from Germany, and so that was another aspect. Later in life, they tried a couple times to recruit me to go to Germany. After the unification, DESY acquired a lab in Zeuthen in the former Eastern Germany. They tried to recruit me to go and head the theory group there. And later there were overtures from the Max Planck Institute in Munich. But by that time, my wife was Americanized [laugh] and we decided not to do it.
Did you keep up relations with DESY after your visiting professorship there had ended?
Not so much. Peccei left at about the same time that I did, and went to UCLA. David London went back to Canada. My student Sankagiri Umasankar got his PhD and went back to India. There were two young graduate students of Peccei’s from Argentina who got their degrees and left. They are both well-known now in the field—Carlos Wagner and Marcela Carena. So a lot of the connection was lost, although I did visit there a couple times.
And then later on at Penn, you got a named chair—the William Smith Term Professor.
That’s right. That was something they gave me when I was thinking of going to Zeuthen. I should say I was terrible negotiator [laugh]. I should have negotiated a higher salary or something, but I didn't, but they did offer me the Term Chair.
Were you the inaugural holder of that chair, or there were people before you?
I don’t think it was inaugural. And “term” means it was not a permanent chair. It was only for a couple of years. But still, it was better than nothing. [laugh]
Do you know who William Smith was? Was he alive?
He was the first provost of Penn.
And then in 1996, you become chair of the department.
Was that more of a situation where it was your turn, and you weren’t looking forward to it, or this is something that you embraced?
Well, it was something I embraced as a duty. It was not a matter of somebody’s turn. I was talking earlier about the lack of jobs around ’72. Well, by ’95, there was still a gap in people of my age level in most physics department in the country. And so at about the time when I was about the right age to become a department chair, there were very few other people in the department anywhere close in age. There were a lot of people younger, and a lot of people much older. Also, I felt at that time that we really needed to do something to reinvigorate the department. Many of the senior people were still very effective in teaching, research, or administration, but frankly there was some dead wood. [laugh] We also had previously had a separate astronomy department that was on the point of demise. There was a lot of interest in building an astrophysics program in the physics department, and these facts had led to a merging of the two departments.
It was clear that we really needed to improve our teaching, because another factor of the Sputnik and postwar eras was that some of the older people had a disdain for teaching. They thought it wasn’t important. But by this time, if you couldn't do a good job with undergraduate teaching, your department was not going to be well supported by the university. A young person was not going to get a faculty job or promotion unless he or she was a good teacher, although research was still more important. I knew that there had to be a shake-up in the department, and that we had to do something to improve our undergraduate teaching for both physics majors and for service courses, and we needed to reinvigorate our graduate program. I think most people in the department realized that I was the only suitable person who was likely to be able to shake things up in this way, so I agreed to be a candidate. It’s interesting; as an academic at a university, I had absolutely no experience or training or background in administration, and suddenly I was in this administrative position. I could talk to the dean and try to convince her or him that physics was important. And I could talk with faculty and to candidates for faculty positions. I think I had some degree of success in dealing with those things. But I also had to deal with routine housekeeping matters, such as dealing with staff personnel issues, building maintenance and renovations, and condolences for deaths.
In terms of your discussion with your dean, what was your sense of how valued the physics department was within the overall school?
It was respected for its research efforts, but not so much for its teaching. We didn't have very many undergraduate majors, and there were occasional threats that the engineering school would try to take over the service courses in physics for engineers. So there was some sense that even though we had quite a bit of research money from the DOE and NSF, we weren’t quite pulling our weight. But I always felt that the two regular deans and one acting dean, and also the associate deans, while I was chair were on my side. They wanted to do whatever they could to help, even if the resources were limited. Resources basically means faculty slots. I had hard negotiations—sometimes very hard—and I often didn't get anything like what I wanted, but they were never adversarial.
So if you had to issue yourself a report card in terms of what you accomplished and didn't accomplish as chair, what would be your grades, and in what areas would you be grading yourself?
I had three major goals, and I think I was pretty successful in all of them. The first goal was to rebuild the faculty, including encouraging retirements of some of the people who needed to retire.
Those are always very sensitive conversations, right?
Oh, yes! Very. And to try to get new faculty slots for young people in areas we were supporting. I think I was successful there. By the end of my five and a half years I had overseen either the appointment or promotion of around 20 faculty members in a faculty of about 35. So that was a major thing. Very time-consuming, also. A faculty appointment or promotion is an enormous job at a major university. Second, I mentioned before that we merged with astronomy to become a Department of Physics and Astronomy. The last two people from the old department retired soon thereafter. So building a new astrophysics group of around seven faculty was a major priority for me. I succeeded in the sense that we managed to hire a number of excellent people. Unfortunately, some of them were raided by other institutions [laugh] very quickly. Which I thought was not very nice, since we were trying to build a new group and the raids tended to destabilize our efforts. Nevertheless, the effort ultimately succeeded even though there was a turnover of people, so most of the people there now actually came later.
When you have that sensitive conversation with a professor who should really become an emeritus professor, what’s the winning strategy in terms of how you don’t make an enemy for life, and how you keep that person sort of engaged and happy with the department? What’s that winning strategy?
I don’t know. [laugh]
My worst case, I had to actually threaten to try to have him removed for incompetent teaching. He didn't like it, but he finally gave in. And there were a couple of others where I was probably not as diplomatic as I should have been. A related problem was that I had found it necessary to move the emeritus faculty from their large, cushy offices to smaller ones—they had many of the nicest offices, even the people who would only come in every three weeks or so. And I made a lot of enemies that way [laugh], even of people who I normally would have thought were very nice. One of my colleagues from nuclear physics, Abraham Klein, now deceased, apologized to me for the behavior of the other emeritus faculty.
Anyway, there is no good strategy for asking someone to retire. There is an old saying—"What’s the difference between a tenured faculty member and a terrorist? You can negotiate with a terrorist.”
As long as a tenured faculty shows up for class, is not drunk, and doesn’t harass the students, it’s hard to do very much. Anyway, the third thing I did was similar to astrophysics in that it mainly bore fruit later. I thought we were woefully underrepresented in women. There was one senior woman who had been there a long time, Fay Ajzenberg-Selove, a nuclear physicist who had had her problems before I came. [laugh] And then there was one woman, Mirjam Cveti?, who I brought in before I became chair. There was what I think was completely unjustified hassel about her promotion, but in the end the department did the right thing. Thankfully, because she has been extremely successful since then. Anyway, there were only the two women in the department. I thought we needed a lot more and pushed hard for that.
What about diversity in terms of African Americans and other underrepresented groups?
We had one African American, Larry Gladney, and that was it. Frankly, it’s hard enough with women, because there are lots of good women out there. The demographics are much worse for African Americans and other minorities—there just are not that many candidates in physics. I supported Larry a lot, and he later became department chair, not my successor but the next one. However, I really made no progress with minorities. I only managed to bring in one woman while I was chair, and she was soon lured away to another university. But I had pushed the issue very much, and the department hired several women soon thereafter. I don’t know how many women they have now, but eight or nine would be my guess. So I count that as a success. Just in terms of the academic side, the department now has thriving efforts in particle physics, astrophysics, and condensed matter--both hard and soft. We had previously had a nuclear physics program with an in-house accelerator, but that was closed while I was chair. It was not my doing that closed it; that decision came from higher-up, but I agreed with it. I think I was fairly successful in terms of building the faculty. There were other things as well. For example, I established a summer physics program for high school students. Most of the work for that was done by our staff people who ran the undergraduate labs.
During your time as chair, I imagine inevitably your own research sort of took a back seat. Were you able to keep up at all, or you put everything on hold?
I was able to keep up, but I like to say that there are two time constants for a job like this. One time constant is how long it takes to really accomplish major changes--that is five years, in my opinion. The other is for being able to maintain your research program. That is about three years. [laugh] So many people have found that they really could not get back into research. I was most active with postdocs and collaborators for the first few years, but it became more and more difficult later. In my case, I managed to come back into research and be successful. But it’s hard, because being chair of a big department is a huge job.
Had your research focus changed at all, or were you still maintaining the projects that you had prior to becoming chair?
There was some change. I continued working on global analyses, weak interactions, and additional gauge interactions, which I mentioned earlier, as well as neutrino physics and grand unification. This was usually with current or former students and postdocs, including Jens Erler, Naoya Hata, Nir Polonsky, and Ming-xing Luo, and (mainly later) with Vernon Barger of the University of Wisconsin. I also became increasingly interested in trying to connect superstring theory to particle physics. I was never a real string theorist--it’s very abstract and very mathematical, and just not my thing, or my talent. Most phenomenologically oriented theorists tend to snub string theory, and most string theorists tend to snub experiment and ordinary particle theory. I wanted to connect the two, just like I had always tried to connect theory and precision weak interaction experiments. So I wrote a lot of papers on possible implications of string theory, especially on the types of new physics that often occurred in specific string constructions.
All of this work was done in collaboration with a real string theorist, Mirjam Cveti?, who I mentioned earlier, and some of her students and postdocs, including Lisa Everett, Jing Wang, Tao Liu, and (later) Jim Halverson. Incidentally, Mirjam, Lisa, Jing, and I once wrote a four-author paper on which I was the only male author—somewhat unusual in particle theory. Mirjam and I had done some string physics earlier but got into it much more while I was department chair. And that program continued through the rest of my career as well. I also became more interested in supersymmetry as a result. The string theory work was one of my motivations later for going to the Institute for Advanced Study. That’s probably the top place in the world for string theory, but there the theorists—people like Ed Witten, Juan Maldacena, and Nati Seiberg—do not disdain the experiment and phenomenology. They support it very strongly.
So is there a place where this disdain is located? Is there the polar opposite of the Institute, where they sort of serve the other side of the issue?
A lot of places and a lot of individuals. I'm not going to single one out.
So when it came time for you to become emeritus yourself, how did that decision-making process happen?
Well, there were a number of factors. Remember, I was not given tenure initially, and there was never all that much support for what I did at Penn, even amongst the particle experimenters. There were some string theorists at Penn who had this disdain, and they influenced the experimenters and people in other parts of subfields. That doesn't apply to everybody, but to some. I was never very good at self-promotion or at pushing for my own thing. So like a lot of people, I was much more respected in the rest of the world than I was at my own institution. In addition, Penn had, and probably still has, an attractive early retirement incentive--they allow people who have been there for a long time to retire at age 60 and to continue receiving their salary for two years. And so I jumped on that when I turned 60, which was in 1976, I guess, or—when would that have been?—1986? No, it was 2006. [laugh] I'm finding as I get older, I can’t do arithmetic in my head anymore. [laugh]
[laugh] That’s OK!
Anyway. So I just jumped at it, to take the early retirement. And I had had various sabbaticals at the Institute for Advanced Study. That’s located about an hour’s drive from where I live, so it was convenient for me to go there. Usually, on a sabbatical, I would take one of their studio apartments, and I would come home one or two days a week. I really enjoyed it. And I had good relations with the string theorists, with Steve Adler, and with John Bahcall, who headed the astrophysics group, which included a big neutrino physics program. Neutrino physics was another of my major interests early on, and I had done a lot of work in that area. So it just seemed natural that I would go to the IAS and continue my research. Unfortunately, I had arranged that a couple years in advance, and in the interim, John suddenly died. It wasn’t really sudden, but almost nobody knew that he was sick. So the neutrino program at the Institute quickly died. But still, it was attractive for the other reasons. I enjoyed interacting with all the people. They always had some phenomenological postdocs.
They had a good young assistant professor, Lian-Tao Wang, at Princeton University, and I interacted with him and his postdocs as well. And later Nima Arkani-Hamed joined the IAS faculty. I had an extremely productive time at the Institute. While there I wrote many research papers and review articles, and two editions of a graduate textbook The Standard Model and Beyond. I stayed at the IAS from 2006 until I fully retired last fall [of 2019]. I had been phasing out for several years, but I finally realized that it was time to stop. I was slowing down and finding it harder to keep up with the young people. Also, I was very lucky in that my professional career coincided almost exactly in time with the development and testing of the standard model, so I thought it was a good time to stop.
The cartoon version of the Institute is that there’s tea time, and you can see all the major geniuses in the world, and they're all interacting from their various disciplines, and wonderful conversations are happening, and brilliant research is coming out as a result. Was your experience—did it, in any way—was it reflected in that kind of general idea of what happens at the Institute?
Yeah, to some extent. Not so much at the tea time, at least not in the latter years. It was more like that when I did my first sabbatical there in 1981. But yes, it is a lot like that. [laugh] It’s really a wonderful place. I have nothing but good memories. And while I was there, they needed somebody at Princeton University to teach the graduate course in particle theory, so I did that for three years, and had a long-term appointment even after that, as a senior scientist or something. But I was mainly at the Institute. I wrote my textbook and a second edition there, and a semi-popular book for Princeton University Press.
What was that popular book on?
It was on particle physics. It was called Can the Interactions be Unified? It was their title, not mine. It was meant to be at the level of a physics colloquium or an undergraduate physics major—easily understandable to a condensed matter physicist, for example. But it was not a book for the general public.
In terms of those interactions at the Institute, did any of those interactions materially advance your own research, or was it just sort of like a wonderful place to be, and a good place to get your mind active and to be thinking great thoughts and apply them?
The interactions that materially advanced my research were mainly the ones with the postdocs, the assistant professor from the university, and my outside collaborators. Not so much with the senior people per se at the Institute. Much more, it was just such a nice environment.
Paul, at this point, I think we've basically gotten up to the narrative points. So I want to ask you, in the last portion of our discussion, some sort of broad-based questions about assessing your overall career. The first one is—I'm fascinated by your emphasis on connecting the experimental and the theoretical worlds in physics. My first question on that is, where does that interest come from, in terms of you personally and what you're interested in? And what do you see as the benefits of your endeavor? What’s the sort of feedback that your work in this area has been effective and better research has happened as a result?
The way it started was mainly that in the early to mid ‘70s, when I was first at Penn, they had discovered the weak neutral current, a prediction of the Weinberg-Salam model. But there were alternative models as well. And there were some early experimental results that were suggesting that some of the alternative models might be needed. So I started, mainly in collaboration with Gino Segrè, on making some models that would have different properties from the original one. But then I got interested in how could one experimentally tell the difference? By then, there were starting to be a lot of experiments, mainly at CERN but also at Fermilab and elsewhere on neutrino scattering and the weak neutral current. As I mentioned before, I got involved with trying to globally analyze them. I emphasized earlier that if you have several similar experiments, you should analyze them the same way, and not with different assumptions. And when you have different reactions as well, you need a framework that relates them. You can beat them against each other and see whether the standard model is consistent, or whether there’s a totally different theory that works better, or whether there’s a perturbation on the standard model that works better. So I realized that this type of global analysis would contain much more information than the individual experiments. Now, I don’t want to say that nobody had ever thought of this before. Other people had done similar things. But it’s just, frankly, that my various collaborators and I did it better [laugh] in the long run than anybody else. It’s interesting that for a number of years, some of the experimenters, especially at CERN—I spent a lot of time in Europe in those days—really resented it, because they would take the view, “Oh, we did the experiment. We should get all the glory. And you're just some American theorist who takes our results and combines them with somebody’s else’s results, and then goes and gives talks at conferences all the time.” So I had a somewhat hard time.
But after a few years, the attitude started changing, and I noticed that more and more of the experimenters were anxious to talk to me, because they realized that their results were having much more importance than they ever would have if it had just been the one experiment and one measurement. So perhaps a culmination of this was some years later when the electron-positron Z-pole colliders LEP at CERN and SLC at SLAC turned on. They increased the precision of the tests of the electroweak standard model enormously, as well as other things such as QCD studies. There were four major experiments at LEP. They formed the LEP Electroweak Working group, in which representatives of the four experiments did basically the same sort of joint analysis of their data that I was talking about, of trying to have uniform theoretical assumptions and treatment of common systematic errors, et cetera. They eventually incorporated the SLC data as well. I could go a step further and combine their joint results with other types of experiments. I don’t know in detail how these joint working groups came about, but I like to think that my efforts and those of my collaborators over all those years to pioneer that sort of analysis might have influenced them. Nowadays, collaborative analyses of theorists and experimenters from different experiments are the norm. In terms of recognition, I mention that at the end of the successful running of LEP, they had a symposium at CERN celebrating their achievements in all of their areas, including the electroweak and the strong interactions, heavy quarks, etc. And they invited me to attend and give the summary talk on the electroweak interactions, rather than the experimenters or any of the CERN theorists. That was very gratifying.
I’ll give another example, from neutrino physics. You know, Ray Davis had his famous chlorine experiment to detect solar neutrinos, but he only observed about one third of the expected rate. There were later experiments by other groups using other targets. None of them seemed to agree with the standard solar model, and that left the question of whether there was something wrong with John Bahcall’s calculations of the solar production rate, or was it due to the properties of the neutrinos, such as oscillations of one type into another if they have a small mass. So I got interested, again, in combining the experiments, because the different types were sensitive to different parts of the solar energy spectrum. Combining them in a global analysis was a crude way measuring the spectrum. It was clear from the analysis that I did with Sid Bludman, Naoya Hata, and Dallas Kennedy that the culprit was almost certainly the properties of the neutrinos. That was because the middle of the spectrum was suppressed much more than the higher-energy neutrinos, and there was no way for that to occur by any plausible nonstandard astrophysical scenario that anybody had proposed. But it worked perfectly well with neutrino oscillations, or more precisely with the closely related MSW effect, which involves both neutrino mass and interactions in the sun. So that was one of my early results on neutrinos. It convinced me, and I think a lot of other people, that neutrinos have mass and were responsible for the solar neutrino deficit. This was confirmed definitively by later experiments.
I’ll give you an anecdote that was one of my, I think, major unheralded contributions to physics. [laugh] Lincoln Wolfenstein’s original paper on what became known as the MSW effect had a sign mistake. This was critical, because for a given mass ordering the MSW effect is more important for neutrinos than anti-neutrinos, or the other way around, depending on the sign. And Mikheyev and Smirnov just copied the sign without thinking about it critically. Well, I had done some work some years earlier on a related topic, which I won’t go into, but in which I realized that there had been a sign mistake. When I saw Wolfenstein’s paper, I realized, “Ah! It’s the same sign!” And so I carefully went through and checked, and found that his sign was wrong. By then, people were starting to pick up on the effect, and Hans Bethe wrote a paper in which he wrote something like, “The muon neutrino has to be lighter than the electron neutrino.” Well, that was based on the wrong sign [laugh] and it’s really the other way around. So I decided, probably stupidly, that rather than embarrass people, I would just write privately to all the people who would be really interested in this. And it was a long struggle to convince everyone, because Ray Davis was at Penn by then. He would come back after a trip and say, “Well, such and such famous person disagrees with you.” Then that person would send me their notes, and I would find a trivial sign mistake in it. [laugh] So in the end, the sign was corrected, but I didn't get much credit for it [laugh]. Wolfenstein was a wonderful person generally, but he was not very generous about this. He was embarrassed by the sign mistake, so he never gave me much credit.
Did you ever have personal interaction with Wolfenstein, on that or other topics?
Yes. I had many interactions. I never worked with him, but he would visit the theory group at the Berkeley lab when I was a graduate student there, and so I got to know him then, as well as many times later. We were quite friendly, but I was disappointed about his reaction to the sign. Bethe revised his paper, and he acknowledged me, but in one of his lectures that I heard he made it sound like I had just corrected a typographical error.
But you saw it as more fundamental than that, obviously.
Oh, it was. I had basically bet my reputation on the sign, at least to the neutrino community. Literally hundreds of subsequent papers describing or trying to explain the neutrino masses would have been wrong if it had not been corrected. Unfortunately, the early calculations were done in the language of the index of refraction for neutrino scattering, which is a lousy way to do the calculation, as opposed to the simpler method of working directly with the field equations. I couldn’t find a very good fundamental derivation of the index of refraction in ordinary optics, even in the standard texts, especially of whether it is greater than or less than one. That’s because everybody knows that it’s greater than one in optics, and so [laugh]—nobody worries very much about deriving it. It is even trickier for neutrinos because of charge conjugation noninvariance, and I had to derive the result carefully from scratch. Anyway, I remember Mike Turner telling me afterwards that I should have waited a few months and then written a paper, correcting it. Then, he said, I would have become famous. [laugh] John Bahcall had a lengthy footnote in his book on neutrino astrophysics in which he correctly told the story. That was surprising because I didn't really know him when he wrote it. I got to know John very well later, but I never asked him how he got his information. [laugh] So anyway, that was one of my unheralded contributions to physics. There have been a couple of others, but one’s enough to talk about.
That’s a great story about an unheralded contribution. What do you see as your most heralded contribution, or your most significant contribution, and in what field?
The precision global analyses of the electroweak physics, which ultimately showed that the standard electroweak model is correct up to possible perturbations, and led to successful predictions for the ranges of the top quark and Higgs boson masses. Even after LEP came on and much later, I continued this work. The LEP and SLC people combined the results of their experiments, but then I would go on and combine their results with all the other types of experiments. I did this for 40 years or so. [laugh] And for decades, I wrote the electroweak article for the Particle Data Group, with Jens Erler joining me for something like the last 20 years. I’d say on the whole, that was my biggest contribution.
Another contribution was that I did one of the detailed analyses with one of my students, Ming-Xing Luo, on the running of the three gauge coupling constants, showing how the supersymmetric extension of the standard model is more consistent with the gauge coupling unification expected in grand unified theories than is the model without supersymmetry. That was a complicated analysis. It was done by a couple of other groups simultaneously, and superseded an earlier pre-LEP study with Bill Marciano and others. I think the paper with Ming-xing has been cited well over a thousand times. A later study incorporating some new input data and new theoretical elements that I did with another student, Nir Polonsky, was also important.
I also did a lot of work on the implications of superstring constructions with Mirjam Cveti? and others. It’s very difficult to get just the standard model or the supersymmetric standard model out of concrete superstring constructions. Almost all of the constructions predicted certain types new physics, which were things that I call top-down or remnant physics, things that don’t necessarily solve a standard model problem or have a bottom-up motivation, but rather are just there. The most common examples are more complicated gauge sectors with heavy neutral gauge bosons. Z primes, they’re called. And more complicated Higgs sectors and more complicated fermion sectors, not just a fourth generation, but fermions that have a completely different character. Initially, most people didn’t want to think about such things, because they didn’t solve standard model problems, and in fact many string theorists would not even consider such constructions. Nowadays, that’s the sort of thing that many people are looking for. [laugh] The Z primes were a special interest of mine. I wrote papers on all aspects of Z primes, not just the string implications, over more than 30-years.
I finally mention two non-research efforts that had some influence. One was my lectures at advanced summer schools, review articles, and books. The other is a letter that I wrote with Pierre Ramond and Mary K. Gaillard when I was co-organizer of a workshop at the Santa Barbara Institute for Theoretical Physics in 1995. At that time, many of the world’s theorists were becoming more and more interested in supersymmetry as perhaps the most plausible extension of the standard model, and there was much experimental effort at most of the labs to search for it. The one exception was Fermilab, which had the highest energy accelerator, but apparently no effort in that direction. We wrote a letter to the heads of Fermilab and the experimental collaborations urging the importance of supersymmetry searches, and obtained signatures from many very prominent theorists. The Fermilab people never acknowledged our letter, but within a year they were carrying out many analyses of their data to look for it. In the end they did not discover supersymmetry—and even now we do not know whether it is there at observable energies—but that is not the point. It was a promising possibility and needed to be looked for.
On that note, I know since you've thought so much about fundamental theories both from a theoretical and an experimental perspective, I want to know if you can explain how you see the standard model in relation to some future grand unified theory. Is it a way station? Is it separate? Will it be supplanted if that unified theory is ever achieved? How do you see these things working out?
Well, I don’t have a crystal ball, but I think that it will be just a way station, and that there is something more fundamental, likely a string theory. I think that’s by far the best game in town. People talk about trying to unify relativity and quantum mechanics; well, string theory does that, at least at the perturbative level. [laugh] The difficulty is just that there are so many different solutions to string theory, the landscape of vacua.
So then if I can ask an obvious question—why is string theory not simply the theory of something? What’s the difference?
Well, it would probably be the theory of everything if it’s correct.
[laugh] OK. So the theory of everything of course has to be correct for it to be the theory of everything. [laugh]
Yes. I think there’s unlikely to be a smoking gun, but I think it’s a good chance that that we may observe some types of the remnants that are very common in the string constructions. There are other things that you rarely get and which would go far towards falsifying string theory if they were observed. Like high-dimensional representations of the symmetry group, which are very hard to obtain in string theory, whereas fundamentals, adjoints, and bifundamentals are ubiquitous. I pushed that idea for a long time, and gave lots of talks on what I called string-favored and string-disfavored types of new physics, things that were likely to be found in string constructions, and things that were not. Now, there’s a modern related set of ideas called the swampland conjectures, which are much more sophisticated than anything I ever did. [laugh] I'm proud of the work that I did on remnant physics, especially the Z primes, but I never wrote a seminal paper or anything. I think the ideas were ahead of their time, but are very fashionable nowadays.
Paul, the idea that probably there’s no smoking gun—that inevitably raises—whether it’s a philosophical question or it’s a spiritual question. Is that to say that objectively there is no smoking gun, or the human mind simply isn’t built to perceive the deeper reality that would reveal what that smoking gun is?
Some of each, I think. But I have not given up on the possibility of more evidence for or against string theory. It may be just a matter of luck. [laugh] Consider classical electrodynamics. Maxwell’s equations are simple, but you can think of unlimited numbers of configurations of electromagnetic fields and charges. So there are a huge numbers of solutions. And the same with string theory. There are an enormous or infinite number of string vacua, characterized in part by the geometry of the extra dimensions. Physics depends on which vacuum we're living in. Many of them will have these remnants, and there are also possible cosmological consequences. There might be a multiverse, in which different regions of space have different physics. And if that’s the case, then it’s conceivable that very precise cosmological observations might give a clue about that. Also, most cosmologists think that some form of inflation is probably correct, but even there, there’s not a really definitive smoking gun.
But at some point observations and theoretical progress on inflation might give insight into string theory. I also expect mathematical progress in our understanding of string theory. I guess my biggest hope would be that there will be more understanding of the mathematics to try to zero in on the most likely string vacua, combined with experimental progress and seeing some of these remnants. Also, many but not all string vacua involve supersymmetry at low enough mass scales to be observable in the laboratory. The new supersymmetric particles have so far not been seen, but they could still show up at somewhat higher energies. It depends on luck, to some extent, on how nature is. But an analogy I’d like to give is this—I have sort of a layman’s interest in the evolutionary origins of homo sapiens. Thirty years ago or so all we really had was the fossil evidence. How many people then imagined the progress that was made possible by DNA testing.? Maybe something like that will happen in physics. It might not be next week. It might not be next year. It might be in 100 years or 500 years. But I think the scientific possibilities for progress are excellent. This includes prospects for further experiments in particle physics and cosmology, including new techniques such as Wakefield plasma acceleration, and for theoretical progress in physics and mathematics. Whether they pan out in the political and economic climate is another issue which I won’t comment on.
Maybe a more concrete answer just in terms of what we know presently—I wonder if you can talk about, surveying from the beginning of your career to now, if you can talk about what things are really understood now that at the beginning of your career really were not.
At the beginning of my career in particle physics, we understood quantum electrodynamics, but had no satisfactory mathematical theories of the weak or strong interactions. We had a lot of factual knowledge about the weak interactions and the strong, interactions, and many ingredients, such as quarks and symmetry groups, but no real theories. Now, we have a mathematically consistent standard model that really is consistent with just about all phenomena observed down to the size of about a thousandth of the size of the atomic nucleus. We don’t understand dark matter or dark energy, or why there is an excess of matter over antimatter, but everything else, we do. I should admit that the standard model has a lot of arbitrary parameters that we don’t understand fundamentally, like the masses of the quarks and leptons, but they are measured and that is not an inconsistency. There could be new physics relevant to small distance scales and higher energies. There’s the oft-cited problem of reconciling general relativity and quantum mechanics, but as I already described we have a perfectly good superstring theory that does just that, at least at the perturbative level. And it’s finite, not just renormalizable.
What does that mean that it’s finite and not just renormalizable?
When you start calculating Feynman diagrams, you run into divergent integrals. It turns out that in a subset of field theories, the so-called renormalizable ones, these only affect things like the electric charge and the mass of the electron. So you start out with a parameter in the equations of motion that you call the bare electric charge, and one called the bare mass. But then you calculate corrections to these from QED, and you add the corrections to the bare ones to get the physically observed ones. Well, those corrections are infinite. [laugh] There is a simple classical analog. If you take a classical ball of electric charge, of radius r, with a bare mass, and calculate the energy stored in its electric field, and then shrink the radius to zero, you'll get an infinite correction to the mass of the system. However, if you express everything in terms of the total mass and assume it is finite, then everything is fine. In essence, the infinity in the field energy cancels an infinite bare charge, leaving a finite result. Similarly, in a renormalizable field theory if you rewrite everything in terms of the physically observed quantities to begin with, the infinities disappear. But still there is still a feeling that one has cheated somehow. The infinities are associated with the point-like or zero radius nature of the particles. However, that all disappears in string theory because nothing is point-like. Everything is smoothed out and fuzzy.
Paul, I think for my last question, I want to ask you something that’s sort of more forward-oriented or future-looking, and that is, either in what you can see in your own lifetime and where you feel your field is headed, what are the things that you're most excited about? Either from advances in experimental work or in theoretical work. What do you think can be achieved?
That’s a hard question, because the straightforward advances in particle physics experiment are very big and expensive. There is certainly a possibility of pushing the experiments up another order of magnitude in energy, and there are proposals for this, and there is a very good chance that one would observe new physics. So far no supersymmetric particles have been observed, but they could well show up at higher energy. One might also observe some of the remnants I've been talking about, or something totally different. We may also develop new accelerator technologies that are smaller and cheaper. There are also high precision experiments. There are hints of some possible deviations from the standard model, but none are yet conclusive. The magnetic moment of the muon seems to be a little bit off from what’s expected. That could be real or it could be an experimental problem. That will probably be settled fairly soon. But as I was saying earlier, I think there’s room for a lot of progress in our theoretical understanding in string theory. But the more immediate prospects are in astrophysics and cosmology. There are new instruments that are revolutionizing the study of dark matter and dark energy. I think we're likely to get a real insight and progress there. Gravitational wave experiments are mainly relevant to astrophysics, but could also give clues about particle physics, e.g., through indications of phase transitions in the early universe. And maybe we'll pin down whether inflation is real, and how it connects to particle physics and to string theory. So I'm especially looking for progress in those areas, at least on shorter time scales.
Paul, it has been a great pleasure speaking with you today. I really appreciate the time we've spent together.
Oh, you're very welcome.