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Interview of Stanley Brodsky by David Zierler on April 22, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with Stanley Brodsky, Professor Emeritus at SLAC. Brodsky surveys his current projects after his retirement last year following 54 years of service to SLAC; they include new initiatives on hadron physics and his interest in the muon G-2 experiment at Fermilab. He recounts his upbringing in St. Paul, his early interests in electrical engineering, and his decision to stay close to home and attend the University of Minnesota for his undergraduate education. He explains his decision to remain at Minnesota for his thesis research, where he worked under the supervision of Donald Yennie on computing atomic levels from first principles in quantum electrodynamics. Brodsky describes his postdoctoral appointment at Columbia, where he worked with Sam Ting at DESY computing the QED radiative corrections for Bethe-Heitler pair production. He recalls his original contact with Sid Drell and his decision to come to SLAC to join the theory group in support of the many experimental programs in train, and he recounts the November Revolution and Sam Ting’s visits to SLAC. Brodsky describes some of the key differences in East Coast and West Coast physics in the 1970s, and he discusses his collaboration with Peter Lepage at the beginning of QCD’s development. He highlights the importance of thinking beyond conventional wisdom and he references his work on intrinsic heavy quarks to illustrate the point. Brodksy discusses his research on the Higgs VEV and the long range value of the Brodsky-Lepage-Mackenzie procedure, and he reflects on the many surprises in QCD color confinement that he has encountered. He explains the value of supersymmetry in his research and he considers why it has not been seen yet and why Maldacena’s work on AdS/CFT has been revolutionary. Brodsky describes SLAC’s increasing involvement in astrophysics and how he has managed his research agenda by working on many different projects at the same time. At the end of the interview, Brodsky emphasizes the significance of Bjorken scaling, he historicizes the first work in physics that explored beyond the Standard Model, and he reflects on the importance that luck has played in his career, simply by finding himself, at so many junctures, in being at the right place at the right time.
Okay. This is David Zierler, Oral Historian for the American Institute of Physics. It is April 22nd, 2021. I'm delighted to be here with Dr. Stanley Brodsky. Stan, it's great to see you. Thank you for joining me today.
Thank you for inviting me.
Stan, to start, would you please tell me your title and institutional affiliation?
Yes. I am now Professor Emeritus, at SLAC, The Stanford Linear Accelerator Center at Stanford University.
When did you go Emeritus?
At the beginning of 2020 — January 1st, 2020. Thus, I have worked for 54 years at SLAC and Stanford. It does seem amazing.
And then the pandemic hit.
That's right. Yes, that's true. In fact, there was supposed to be a ‘Stan-Fest’ for me at SLAC; an international physics conference dedicated to me and my students and collaborators; but it has not happened because of the Coronavirus. However, there was a ‘Stan-Fest’ at CERN last year — one day at a major international physics conference in Geneva which was held in my honor. A number of my colleagues, collaborators, and former students were able to come and participate. That was great.
Stan, in what ways do you want to remain active in physics? Retired, but you're still interested in things. What is most interesting to you to follow in retirement?
Yes, even though I've retired and now 81, I have still been initiating new work, publishing new physics papers, collaborating with colleagues all over the world, as well as giving invited talks. Somehow, I have been able to publish 27 papers since the beginning of 2020. It is a great privilege to work in science, and I would not want to give this up. Even at 81, I feel I am still able to contribute new ideas to physics.
What kinds of work are you doing currently? What's most interesting to you?
My collaborators and I have recently initiated a new field in hadron physics, called “light-front holography.” This work has mainly been done with my collaborators, Guy de Téramond at Costa Rica and Hans Günter Dosch at Heidelberg. Light-Front Holography provides a new analytic, comprehensive way to explain fundamental dynamical and spectroscopic properties of hadron physics, consistent with color confinement and other remarkable features of quantum chromodynamics. It reproduces the complete spectrum of the observed hadrons, the mesons and nucleons. It also implements the algebra of supersymmetry in a novel way.
Our approach does not involve the usual prediction of supersymmetry: new fermions and bosons. There are no squarks or gluons. Nevertheless, in our holographic approach, we show that are remarkable supersymmetric relations between the existing mesons and baryons in hadron physics — both in their dynamics and their spectroscopies. Many successful predictions have come out of this new approach. This has become a very exciting field, and thus it has been very hard for me to stop working on new applications and developments. Light-front holography is probably the most exciting thing I am working on. But I continue to work on several other areas of quantum chromodynamics.
For example, an important problem in theoretical physics is how to eliminate the renormalization scale ambiguities which have plagued the predictions of QCD and other quantum field theories. I am continuing to work on a new approach called the “Principle of Maximum Conformality” and its applications. For years theorists have thought that setting the value of the renormalization scale is an unsolvable, inherent ambiguity of perturbative QCD predictions; however, the PMC method, which I developed with my colleagues, rigorously eliminates such ambiguities, thus allowing precise theoretical predictions. The PMC predictions are also independent of the choice of the renormalization scheme, another fundamental requirement of quantum field theory. I also try to follow the current work that's going on in many areas of particle physics. I worked very early on the muon G-2 problem, so I'm closely following this field.
Yes, Stan, I wanted to ask about that specifically. Just a snap judgment from you, do you think this is new physics, or no?
Well, if I had to bet money, I think that the conventional predictions from the Standard Model will be confirmed. Sid Drell and I wrote a pioneering paper on this problem — showing how the muon G-2, tests the underlying short distance physics of muons. I worked with Tom Kinoshita on the first three-loop high-order calculation in QED; specifically, what is called the light-by-light contribution to the muon anomalous moment. At the time it was an incredibly challenging computation — it actually used up a large portion of the SLAC computer resources for months. Tom Kinoshita has continued to be the world leader for such calculations — now he's computing the QED corrections at five loops. In my own work, I have recently developed a powerful new method using light-front Hamiltonian perturbation theory, based on Dirac's Front Form, to calculate these high-order contributions in a new, more physical, way.
Stan, let's take it all the way back to the beginning. Let's start first with your parents. Tell me a little bit about them and where they're from.
Yes, I grew up in St. Paul, Minnesota where my parents, Sidney and Esther Brodsky, also grew up. My father was multi-talented — he could play the piano by ear, dive from a three-story diving platform, and was even on the football team at the University of Minnesota. One of my earliest memories was when we lived on Hague Avenue in St. Paul. I remember that when I was barely 3 years old, when my father fell asleep, and I walked by myself across the street and over to a major thoroughfare and streetcar line, Selby Avenue. My goal was to find my mother and go with her on the streetcar to downtown St. Paul. Fortunately, our neighbors rescued me.
We then moved to 1506 Grand Avenue in St. Paul, to an eight-plex of one-bedroom apartments. My brother and I shared the single bedroom, and my parents slept on a pull-down Murphy bed in the living room. My father worked hard at two full-time jobs, just to support us. In contrast, Grand Avenue, was only one block away from Summit Avenue, a gorgeous historic street of wealthy estates. We lived very modestly but it was great, actually, and I had good friends in that apartment building. I also benefitted from reading all of my father’s college textbooks that he had left in a drawer, from books on poetry to a medical texts.
Was this a Jewish neighborhood mostly, in St. Paul?
St. Paul had a strong Jewish community in the 1940’s: Orthodox, Reform, and Conservative congregations. We were Conservative Jews, and I had my Bar Mitzvah at Temple Beth Jacob in St. Paul. But, no, it was not really a Jewish neighborhood. There was some antisemitism, but I think that this mostly occurred in Minneapolis. It was a mixed neighborhood, but we did have a few relatives living a few blocks away.
I walked to school about a half-mile beginning when I was just four -- well, you might be amused by this story: My mother lied about my age. She listed my birthday as November 9th, 1939, when I was really born January 9th, 1940, just to get me into kindergarten at Ramsey School a year early. And for nine years I kept up the lie and gave the wrong birthdate to my teachers. I walked to school from our apartment house, down Grand Avenue with its traffic and street cars, walked across Snelling Avenue which was a major truck highway, Minnesota # 51, and then I walked across Macalester College — all of this while I'm four years old. Nowadays, they would arrest a parent for doing that. But at that time in 1944, there was no problem at all.
Stan, do you have any early memories of the United States being involved in World War II?
My parents adored Franklin Roosevelt, and we understood there was Nazi persecution of the Jews in Germany, but they really kept it away from me. I think we were more concerned about the polio epidemic and learning about kids in iron lungs. My parents were very concerned. One of my closest friends on Grand Avenue, Jeremy Waldman, actually had polio. That was very frightening. But I have great memories seeing my first television for the first time at a neighbor's house. All of the neighborhood kids were invited to see the earliest television shows to see Kukla, Fran, and Ollie, and of course, Howdy Doody. Eventually, our family got our own television, a 12.5-inch RCA Victor set, which continually went out of vertical and horizontal hold.
Do you have a sense that with better economic opportunities, your father would have had a more professional career?
Yes, for sure. He worked at Swift & Company on the assembly line in the morning, and then he worked in the evening and night at the Railway Express, in downtown St. Paul, shipping goods by freight trains. It was amazing how hard he worked. When I learned to drive at 15, I would drive my father all the way to the Swift & Co. plant in South St. Paul before 7 AM each morning. He really did work two full-time jobs.. When I went to the University of Minnesota it was only $112 a quarter and I paid for it all myself. I worked at 17 as a carry-out boy at the Cut Price Supermarket in St. Paul. I was also a cashier, a truck driver, and a stock boy, which was how I put myself through school for the first couple years. I never needed any funds from my parents.
Stan, when did you first get interested in science?
Well, I always knew I wanted to be an electrical engineer. I had a very close friend, Robert “Bubsy” Wood, who also lived also in the apartment building on Grand Avenue. We had fun doing all sorts of mathematical and science tricks together as kids. I remember that we invented what we called the “double-dabble” method for computing binary numbers, how to add them and multiply them. I recently looked at Wikipedia and found an article on the “double-method”, the same method that we invented before 1950! I am amazed by that. I had wonderful teachers at St. Paul Central High School. My senior year mathematics teacher stands out, Stan Thorson. He gave me read his own calculus books to read because they didn't teach calculus at that time in high school. So, that was wonderful. I really felt I loved mathematics, and the physics classes, and chemistry. I was very fortunate: St. Paul Central was an outstanding public high school that really helped put me on my way, preparing me for the University of Minnesota and my science career.
Stan, did you give any thought to going farther away for school? Was that within the realm of possibility for you?
That was not even conceivable, because of our financial situation. But it was a great opportunity that you could just go to Minneapolis, only about five miles away, and there would be the classic University of Minnesota campus. It was a dream.
Did you live at home? Did you live on campus?
I lived at home until I was in graduate school — until I got married. After Grand Avenue, we were able to move to a home on Juliet Avenue in the to a nicer home on Hartford Avenue in the Highland Park area of St. Paul. So, we sort of made our way up the hill.
So, what was it that turned you on to physics in college?
My father had a friend, Louie, in West St. Paul, who gave me the opportunity in high school to become a technician in his radio repair shop. Louie’s repair shop was actually in his garage, in the back of his house. Louie taught me how to fix radios, how you diagnose what's wrong. I was fascinated by this. That was the time in the 1950’s when electronics was just tubes, condensers, and resisters. Louies said, "Stan, you're going to make your own radio." So, I picked out a schematic from a book — a five-tube superheterodyne radio, and I assembled the whole radio myself. I bought all the parts, assembled it, and it worked perfectly and even had good sound. I still have that radio, but unfortunately, it fell off the shelf during the 1989 earthquake. I keep thinking I should try to fix it, but it is not simple to buy the ancient tubes.
Stan, was it a professor or a course, really, that focused your interests?
I think Stan Thorson who taught advanced mathematics at Central High School in St. Paul was probably the one who inspired me the most. He told me that I have to really do science. I appreciated all of the math and physics classes at St. Paul Central — the teachers always had great senses of humor. But I really had not conceived of becoming a real scientist. I just thought I'd be an engineer, assemble and create electronic things. But when I entered the University of Minnesota in 1957, I enrolled in the Institute of Technology, and received all A’s in my first year. My second year, as a sophomore, I got my first B in the required drafting course, which was devastating to me. That's because I had two close friends, Jim Loken, and Stan Ecklund, (who eventually both came to SLAC). They got all As, of course, so, they always made fun of me. I got that first (and my only) B in drafting — mechanical drawing — since I was a terrible artist.
So, I went to the University of Minnesota mathematics department, and I said, "I will transfer to become a mathematician if I don't have to take another quarter of that awful class." And they said, "Since you've matriculated through the Institute of Technology, you must take another quarter of mechanical drawing.” I then went over to the physics department that very day, walked into the physics office, and they said, "We don't care if you take drafting. Do you want to be a physics major?" I became a physics major and began to love physics. That was a wonderful break. They also had an advanced course for physics majors which I took. I remember one of the physics teachers, Edward Ney. He had travelled to Africa to observe a solar eclipse, but his jeep toppled over, and he had broken his clavicle. When he came back, I remember, Professor Ney said to our class, "I didn't want to stop the science expedition, just because of my accident.” So he continued to live his whole life with a broken clavicle. That was impressive. You do not forget when your teacher tells you “Science is more important than your own body.”
Stan, did you have the sense that the American response to Sputnik specifically created opportunities for you that might not have otherwise been there?
I'm sure you're right — that was one of the big incentives to United States going full blast into science. I don't know if I really made the connection that you're saying, but we certainly were impressed by the Russians' advances in space and then our NASA developments.
When it was time to consider graduate school, what were your options? Did you think beyond Minnesota?
Yes. I did receive a very good undergraduate education, and I had the opportunity to go to Caltech and other schools. But the physics department at the University of Minnesota, in its wisdom, arranged a National Science Foundation Cooperative Fellowship for me, which allowed me to be fully funded for three years in the U of M graduate school. Since I was just getting married, it just seemed like this is the natural thing to do. Also, I had a wonderful advisor in theoretical physics, Professor Donald Yennie, at that time who helped me decide to stay at the U of M.
Actually, I tried to become an experimentalist during my first year of graduate school, but I absolutely failed at that. As an undergraduate, I had received valuable training in experimental science. I worked at the electrical engineering department in their vacuum tube laboratory, and as a technician, I had learned glass blowing, welding, and other skills. I then worked as a student technician in the Van de Graaff laboratory at Minnesota, which gave me another incentive to become a nuclear experimentalist. I started out by trying to create a thin foil nuclear target coated with lithium for a scattering experiment using the Van De Graaff proton beam. I designed some sophisticated glassware, put the the metal foil and lithium inside the glasswork, and pushed the button of an RF machine to heat up and deposit a lithium compound. I could see everything was working perfectly for gaseous lithium to coat the metal and create the nuclear target that I had needed for my experiment. However, I could not get the glassware to open —the glassware valves had frozen tight. I had somehow invented a new glass glue, which prevents you from ever opening up glassware. I went to my advisor, Professor Morris Blair, and I said, "This is awful. I could never have anticipated this problem, and I had wasted $300 worth of equipment.” He said, "No problem at all." Well, the next day, I went to Don Yennie and became a theorist. I actually switched to theory that day.
Now, did you know Yennie as an undergraduate, or you only got to develop that relationship as a graduate student?
It was mainly as a graduate student, but certainly, I knew him earlier. He was a star at Minnesota, of course.
What was his research when you connected with him? What was he doing at that point?
Donald Yennie was recognized as a world leader in precision atomic physics calculations, computing the Lamb shift at very high orders in quantum electrodynamics, as well as other high-order calculations. I recall when he would bring me to his home in St. Louis Park, Minnesota, and we worked together for hours.
He was the most amazing algebraist that I've ever seen - he could just go from one complex line of algebra to the next line without flaws. I was absolutely amazed by this. He taught me great theory tools and very much inspired me, so that I had a wonderful experience with my thesis research. In my thesis I calculated the radiative corrections of the hyperfine splitting of atomic levels in hydrogen atoms to high order — a calculation that had never been done before. These accomplishments were all because of Yennie’s great training. I was able to complete graduate school within the three-year period of my NSF fellowship. I was also motivated to finish my Ph.D. within three years because Don Yennie had received an offer to join the faculty at Cornell University. He gave me the option to go to Cornell, or just complete my graduate school studies right then. So, I took that opportunity and within about two and a half years, I had my PhD, which was great — even after I had wasted six months trying to be a nuclear physics experimentalist.
Stan, what was the process for you developing your thesis topic?
Yes, I was very much guided by Don Yennie, and also another graduate student, Glen Erickson, who later became a professor at Davis. At that point in time, the topics of my thesis were at the leading edge of studying how well atomic levels could be computed to high precision from first principles in quantum electrodynamics. The Lamb shift and the hyperfine spin of hydrogen were he highest precision measurements being done. I became fascinated with this topic. The theoretical analysis was very challenging — we didn’t have the computer tools of today, so everything had to be done analytically, by hand. In many ways, that's great training for a theorist. I worry sometimes about present students, if they only use Mathematica, algebraic computer programs such as Mathematica, and numerical simulations; are they really learning all the intricacies of mathematics? So working in precision atomic physics was fantastic training. May analysis and thesis work are still quoted.
What were some of the conclusions of your thesis?
Well, basically, that you can carry out a calculation of bound state spectroscopy to high order, not just using perturbation theory but in a non-perturbative framework. One is dealing with a complex nonperturbative bound state problem, with high order perturbation corrections. My thesis was an extension of early calculations of the Lamb shift by Bethe and others, but the analysis that was required used completely new tools to analyze bound state phenomena.
Besides Yennie, who else was on your thesis committee?
There were at least two other professors on my thesis examining committee and my PhD oral, including Professor Stephen Gasiorowicz. He wrote the leading textbook at that time on particle physics. I had a great graduate physics course from him, as well. There also was a mathematician from the university faculty on my committee. My thesis oral went well.
What post-doc opportunities were available to you?
Somehow, without even applying, I received several great post-doctoral offers, including an offer from Stanford University from Professor Leonard Schiff, himself. I even talked to him on the phone. He offered me a post-doc position at Stanford, which was my dream. If you're from Minnesota, your goal all of your life is to live in California! However, my advisor, Don Yennie, said, “Stan, if you take that job from Stanford now, they're going to throw you out in two years, and you'll never see California again! You should first go to the East Coast — and in two years, you will be able to go to California.” Fortunately, I also had excellent post-doc offers from both Princeton and Columbia Universities.
Who would you have worked with at Princeton?
I am not sure. The Princeton offer was, of course, very hard to turn down, but the post-doc salary was a serious factor — it was very little at that time — it was only $7,600 for a year, and Columbia had offered $14,400. I had just gotten married, so it was not conceivable to take the Princeton position, so I accepted the offer from T.D. Lee at Columbia University; fortunately, that turned out to be great.
Did you know you were going to work with T.D. Lee before you accepted the offer?
Although I had great conversations and counseling from T.D, I ended up working with Professor Sam Ting more than anybody else at Columbia. As you know, Sam Ting went on to become a Nobel Prize winner. Sam knew of my experience calculating radiative corrections, and he invited me to go to the DESY laboratory in Hamburg with him and his experimental group. The DESY laboratory had just started, and Ting had set up an experiment to measure a fundamental process called Bethe-Heitler electron-positron pair production. In this experiment a photon (from the DESY accelerator) creates an electron-positron pair in the Coulomb field of a nucleus. At that time, a remarkable result on the Bethe-Heitler process had been reported by Harvard Professor Frank Pipkin. Pipkin had led an experiment at CEA, the Cambridge Electron Accelerator, which reported a result for the production cross section in strong disagreement with the predictions of quantum electrodynamics. Well, Sam Ting guessed out that Pipkin’s experiment could be misidentifying the production of pion pairs, not electron-positron pairs.
To check this hypothesis, Ting designed a new double-arm magnetic spectrometer for DESY which had huge rejection against pion pairs: 10^6 rejection for each element of the spectrometer. Since there were two spectrometers on each side, his design had 10^12 rejection on each side and thus 10^24 total rejection against pion pairs versus electron-positron pairs. And sure enough, Ting was right. QED was verified by his measurement at DESY. What I did was to compute the QED radiative corrections for Bethe-Heitler pair production, which at that time had never been done, and my analysis was used in the publication by Ting’s group. Fortunately, I had the skills to carry this intensive calculation out during the short two-month time I had while I was visiting DESY.
Now, where did you stand? Where did you first meet Sam? Was it at Columbia?
That must be correct, because he invited me and made all of the arrangements for me to come with him and his group to DESY. We became lifelong friends. Ting was very impressive. He really drove the DESY experiment. I met many of his colleagues there. One of his young colleagues at Columbia was Charles Jordan — We are still close friends. Pipkin was a Harvard professor, and Ting had shown his experimental result was totally wrong because of the lack of rejection of pion pairs. I don't think Pipkin ever published again. I believe that Ting learned from this experience to be doubly careful not to publish wrong experimental results. Ting became famous for his high standards and for having two independent experimental groups check every result. When Sam’s group found the J particle later at Brookhaven, I believe that he did not announce it for more than a year until he could make intensive cross checks. He did not want to ever make a mistake comparable to Pipkin’s.
What was it like to work with Ting? I've heard that he could be pretty intense at times.
Yes, I saw his intensity, interacting with members of his experimental group. But since I was a theorist, we had a fine collaborative relationship. He was of course very happy that I could do the radiative corrections which were needed for the final analysis of his experiment. Same and I also published a highly cited theory paper on what is called time-like processes,
What were your impressions of DESY?
DESY had just started up, following in SLAC’s pioneering path. In many ways, SLAC was the preeminent physics laboratory at that time. Panofsky had created this wonderful laboratory a Stanford, and DESY was following a similar program developing a program utilizing both electron and photon beams.
How much time did you actually spend at Columbia for your post-doc?
Yes, I had a two-year position. Except for that time at DESY, I was at Columbia continuously. In addition to the outstanding physics faculty at Columbia, I met Norman Christ, a very impressive graduate student of T.D. Lee at the time and Will Happer from Princeton. Other Columbia post-docs at the time included Carl Carlson, Marvin Weinstein, and Moshe Kugler -- we had a close theory community there. I remember experiencing the famous Manhattan power blackout. We picked up flashlights from the physics department and brought them to the Columbia Women’s hospital. Since there was still no power, I walked with Moshe Kugler up to the 13th floor of his apartment.
What was T.D. Lee like?
T. D. Lee was very certain about everything. I was fortunate that every week he would give me a private physics lecture. His public talks were amazingly perfect, every word. I did not work with him directly omens a project, but he was always encouraging and a good counselor. I was lucky to be able to work directly with Ting and his experimental group. Otherwise, I would have probably just continued to do calculations in precision atomic physics. I started to branch out to particle physics because of my theoretical analysis of the radiative corrections to high energy processes and my resulting contribution to Ting’s pair production experiment. I had not even started thinking about job offers for the next two years, when I unexpectedly received an invitation from Sid Drell to join his new theory group at SLAC. That was really a fantastic opportunity for me. Don Yennie was right: I did get to go to California after all — and I never left!
What was the original point of contact with Drell?
That is a good question. I received a letter for invitation from Sid, but I don't think I talked to him at all until we met at SLAC in the fall of 1966. Drell had founded the SLAC theory group, and he was central in setting it up. The laboratory was also starting its pioneering experimental program in 1968. I had come to SLAC at just the right time — it was really incredibly fortunate for me. The two-mile accelerator had just been built. A famous story: Panofsky has asked the California Highway Department to build the freeway bridge over the two-mile accelerator well before the Interstate 280 freeway was built. The bridge stood alone for several years before the freeway was actually completed. Pief had made this request because he did not want the construction of the freeway to interrupt the experimental program at SLAC. And he succeeded!
That's amazing. What were your impressions of SLAC when you first arrived?
It was a dream come true. First, of course, there was California. And then there were the absolutely dedicated and talented people at SLAC. Sid Drell had done a great job hiring people and setting up a theory group. They were all fantastic. In fact, virtually everyone in the initial groups of postdocs eventually became permanent faculty worldwide. The senior faculty in the theory group were amazing. BJ (James Bjorken) was great; we collaborated on a paper, which is still highly referenced. Sam Berman had a strong influence on me, but he left particle physics to lead a group at Berkeley on energy conservation. The location of the theory group offices was not far from the accelerator complex, which was then being set up for SLAC’s pioneering experimental program. We could just walk over and see all the construction. We could go into the End Station A and see the three gigantic spectrometers.
The famous experiment which discovered Bjorken Scaling in inelastic electron-proton scattering had just began to run when I came to SLAC. The SLAC theorists became immediately became tied into the laboratory program. I had great theory colleagues -- including Jerry Sullivan, Don Levy, Tom Appelquist, and Henry Abarbanel. I recall that on my very first day at SLAC: Jerry Sullivan drove Bill Weisberger and myself in a government car to hear a seminar at Berkeley. Bill Weisberger sat in the front seat talking to Jerry about all the exciting things he was going to be doing his next year — it was his last days at SLAC, and he would be leaving to take up a new position at Stony Brook. I was absolutely overwhelmed hearing Bill talking about so many innovative physics ideas. But since it was just my first week at SLAC, I had no idea what he was talking about!
Stan, what were some of the big issues of the theory group at SLAC when you joined? What was going on then?
We all wanted to support the experimental program, not just deep inelastic electron proton scattering. Plans had started for the SPEAR storage ring for studying a new field of electron-positron annihilation. I started thinking about “two-photon physics”, where the electron and positron did not annihilate, but instead each radiated a high energy a photon which then could collide and create new particles. I realized that you could actually do a whole range of novel physics using two-photon collisions. I developed this field with Kinoshita and Terazawa, and since then, it has become a major field of particle physics.
Just being at SLAC, one could see the potential for studying many different fundamental physics reactions. We could walk into the experimental areas which housed new facilities such as Joe Ballam’s bubble chamber experiment and Robert Mozley’s Streamer Chamber. They could study all sorts of hadronic physics with the SLAC electron and photon beams. I remember we could even walk through the operating hadron beam since it had such very low intensity. I got to meet all of the SLAC experimentalists who were doing pioneering measurements. The SLAC program in 1966 really was the seed that led to the development of what is now called the Standard Model: The discovery of Bjorken scaling gave the first proof of the quark structure matter; this was followed by the discovery of charm, and then the tau lepton at SPEAR. All of this was discovered at SLAC. It was amazing: one astonishing discovery after another.
Who were some of the key visiting physicists that you recall in the early days?
I remember a memorable visit by Sam Ting. He was a member of the SLAC program advisory committee at SLAC, and he arrived in Palo Alto on a Sunday evening, the night before the PAC meeting. As I mentioned, Sam and I were very good friends. He called me from his motel and said, "Stan, guess what?”, and then he told me about an amazing discovery by his group at the Brookhaven laboratory. They had created a new heavy particle which he called the J. (The letter J actually is the symbol for Ting in Chinese.) I was very excited by this news, and I made the connection with an incredible new discovery at SLAC that I had just heard about that same Sunday. I said, "Sam, I have to tell you something that has just happened at SLAC." I had been told that day about the discovery of the Psi particle at SLAC, and I realized it could well be the same particle that Ting had described to me.
Stan, do you remember who told you? Who told you about the Psi?
An experimental colleague and close friend, Gary Feldman, had called me earlier that Sunday, and he told me about the discovery of the Psi. Gary said, "Stan, you cannot say a word, but this is what we've discovered at SPEAR.” I was of course amazed by this, and then Sam Ting calls me and tells me about the J. I said, "Sam, don't go away. I'll have Gary Feldman call you within an hour. He can tell you about something exciting that has also happened at SLAC, which I am sure that you will be very interested in." But Sam replied, "No, there is no need for Gary to call.” I guess he had already had a hint of SLAC’s discovery. In fact, I think that Ting had been sitting on his discovery of the J for perhaps a year, but he was very cautious to announce its discovery because of the concerns that I mentioned before — he did not want to report a false signal.
So immediately after my call with Sam, I called Gary Feldman, and he called his colleague, Roy Schwitters. They also realized what was happening and also understood this amazing coincidence in physics history. A joint colloquium at SLAC was then arranged for the next day, Monday, for both Sam Ting and Roy Schwitters. Sam announced his group’s discovery of the J at BNL, and Roy announced the discovery of the Psi at SPEAR. It was clear from both talks that the J/psi was a vector meson, a bound state of a heavy quarks — which we now know is a charm/anticharm quark pair. So I was part of this amazing moment when the two groups came together to make a joint discovery: the J/psi particle and the new fourth quark flavor, charm.
Stan, when Gary first shared this news to you, how did you know immediately what a game changer this was? What was so significant right in that moment?
It was clear that the SLAC group at SPEAR had discovered a new degree of freedom of nature — a vector-meson bound-state of a heavy-flavor quark pair at SPEAR at a threshold in energy just enough to produce a new type of particle. This new quark flavor is now called charm. I was immediately conscious of this when Sam called — I realized his J particle and the SLAC psi particle were the same hadron, a heavy quark-antiquark bound state. Sam’s experiment produced the J using real photons, whereas the SPEAR storage ring produced the Psi from electron-positron annihilation to the charm-anticharm pair. I had realized it was the same vector meson and had understood it was the same physics. I did tell Sam, "I'm going to have Gary Feldman call you back." But he said, "Don't bother. It's okay." This does suggest to me that somehow, he already had a hint about SLAC’s discovery. There have been rumors that he did know, but I do not know this for sure.
Stan, you're uniquely well-positioned to consider what might explain this amazing instance of multiple independent discovery. How is it that these two totally separate groups came to the same finding at basically the same time?
Well, as I said before, Sam was evidently sitting and re-verifying his discovery for a year. After his famous pair production experiment at DESY as an assistant professor at Columbia, he got hired away by MIT. He then created the experiment at Brookhaven, setting up two high precision magnetic spectrometers similar to his DESY experiment. He evidently had the J for quite a while, but he did not want its discovery announced prematurely — to be certain there was no mistake, no false signal, in the experiment. He had seen what had happened to Pipkin.
Obviously, he was more terrified by the Pipkin experience than he was by getting scooped.
I think that is precisely right. You might talk to Sam Ting and see if my understanding of his psychology is right. He really did not want to make a mistake; he had become well-known for doing double-blind experiments, where two independent groups do the analysis. He was aware how can lose scientific reputation by making a fundamental mistake.
Stan, broadly speaking, how did the November Revolution change SLAC?
For one thing, it certainly confirmed Burt Richter’s scientific vision. Richter had proposed the SPEAR storage ring as a new experimental facility at SLAC, but the facility was unaccountably not approved by the SLAC PAC as a new facility— so SPEAR was built as an “experiment,”, not a full-scale research facility. But now one looks back at SPEAR as an amazing scientific advance. Of course, not long afterwards, came the amazing discovery at SPEAR by Martin Perl and Gary Feldman and their collaborators of a new lepton, the “tau”. So Richter’s pioneering storage ring had opened up to the physics world a new generation of particles: the charm quarks and the tau lepton. This set of particles is now referred to as the third generation.
This revolution in science was a great accomplishment of the SLAC experimentalists. These developments and the discovery of Bjorken scaling, which confirmed the underlying quark structure of matter, also justified the wisdom and foresight of Wolfgang Panofsky. Pief had shown great leadership to build the two-mile accelerator at Stanford. I understand that Panofsky and Drell had to fight against the leaders of Stanford physics department, Leonard Schiff and Felix Bloch, in order to create a new and separate academic department of the university for SLAC. In the 1960’s the conventional wisdom of the high energy physics community was that only hadron beams were a worthwhile scientific tool, not an electron beam. Pief in fact convinced Eisenhower to provide the starting $113 million that was needed to build the two-mile accelerator — without the approval of the high energy physics community.
At that time physics in the United States was dominated by East Coast physicists whose focus was on using hadron beams to study physics. When I now look back at that time, I realize how incredibly fortunate I was to be at SLAC, just at the time when these great discoveries were happening. By the way, there was a wonderful public exhibit at SLAC which recorded that the digging of the two-mile tunnel for SLAC was stopped when the skeleton of an ancient five-million-year-old mammal, called the Paleoparadoxia, was discovered. Pief’s wife, Adele Panofsky, became an anthropologist, and she constructed an amazing skeleton of this mammal from the bones recovered at the tunnel site. I also want to mention another dedicated person at SLAC, Louise Addis, who developed the SLAC library. Every Friday, the SLAC theorists would go into the library to see the physics preprints collected that week. Louise then helped develop the computer program, SPIRES, which gave worldwide web access to papers of the whole physics community. This development was a tremendous advance for communication in physics — immediate access for virtually every paper. SPIRES started with SLAC at the initiative of Louise Addis.
Stan, when did you first become aware of what Gross and Wilczek, and then separately, Politzer, were doing with asymptotic freedom?
I do not remember the exact time, but their work of course made a huge impact in the SLAC theory group. Gross, Wilzcek, and Politzer deserve great credit for proving that the underlying interactions of QCD has the feature of asymptotic freedom — that the strength of the fundamental coupling of QCD weakens with increased momentum transfer. Asymptotic freedom was also the physics required for Bjorken scaling. This crucial feature of QCD was especially important to me because it allowed my student, Peter Lepage, and myself to develop in 1979 the theory of hard “exclusive” processes—reactions where each particle in a scattering process experiences high momentum transfer. Asymptotic Freedom allowed us to calculate hard exclusive processes directly from the fundamental principles of QCD. We developed new factorization theorems, where the physical scattering amplitude can be written as a product of “hadron distribution amplitudes” for each incident and outgoing hadron, convoluted with an underlying quark and gluon scattering amplitude. We also derived the ERBL “evolution equation” which rigorously predicts the change of the shape of the hadron distribution amplitude as the momentum transfer increases.
Our factorization theorem allows one to calculate virtually any hard-scattering exclusive scattering process involving hadrons, photons and leptons, including wide-angle scattering two-body scattering, Earlier, in 1973, when Glennys Farrar visited me at SLAC from Cal Tech, she and I developed “counting rules" for exclusive process, which predict from first principles the nominal power-law fall-off of any hard scattering reaction with momentum transfer. Our counting rules were developed well before the discovery of asymptotic freedom; they have now all been verified by experiment. Glennys and I understood the key features of QCD underlying the calculation of hard processes, but it wasn't until my work with Lepage that one could understand how asymptotic freedom makes the power-law predictions and factorization properties for exclusive processes rigorous.
When you were named an associate professor at SLAC, did that change your day-to-day at all?
Actually, several years earlier, in 1968, I had been offered a “permanent staff position” at SLAC. A memorable moment still flashes in my mind: Professor Pierre Noyes, who was the head of the theory group at that time, said "Stan, come into my office, and shut the door." I thought; “I must be in trouble.” Then Pierre said, "Stan, we want to offer you a permanent position here at the laboratory — permanent staff." I immediately said, "I'll take it." Noyes replied, "Don't you want to think about it?" I said, "I have!” So, that was an incredible event for me — right at that moment, I realized that I would have this permanent position at SLAC to do research. That changed my life.
And that was the best for you. Nowhere else could have even competed.
Right. You know, when I look back, SLAC really was the preeminent theory group at that time. Everyone there was contributing to the advances of our field. Thanks to Drell, Bjorken, and the theory group faculty, we had a fantastically productive, collegiate community. And we also had the resources of Stanford University. So, it couldn't be better. A few years later, I was offered a faculty position as Associate Professor at SLAC, which is a tenured Stanford university position.
Does that give you opportunity to teach in the physics department if you wanted?
I did occasionally teach courses on campus, but this was not a dominant activity. One of the remarkable advantages of a faculty position at SLAC is that you could concentrate on research, as well as counsel graduate students; teaching was basically optional. Each time I was offered an opportunity to teach a special purpose course, I of course did that. But, I was lucky to be able to devote almost full-time to my research. Maybe that's why I have somehow been able to publish more than 700 papers so far. I also had the freedom and opportunity to accept visiting faculty position at institutions such as Cambridge University, the Institute for Advanced Study, Jefferson Laboratory, and a von Humboldt fellowship in Heidelberg.
So, there is tremendous advantage to not have obligatory teaching. However, the SLAC theory group also had Stanford physics graduate students who were anxious to do research with us. Stanford graduate students can do their Ph.D. under the direction of SLAC faculty members, both experimental, and theory. I have been able to counsel and collaborate with wonderful Stanford students. Virtually all of my PhD students eventually received faculty positions. The one student that stands out is Peter Lepage, who became a professor at Cornell, also the Dean of Science. Peter and I collaborated on many pioneering projects. I was very fortunate to have these wonderful graduate students working for me and to be part of a laboratory within an outstanding academic university.
What were some of the big developments in theory going into the 1980s?
I think that the most exciting developments were in hadron physics, where rigorous predictions could be derived from the fundamental gauge theory, quantum chromodynamics, QCD.
When were you working most with Lepage?
Mostly in 1979 and 1980, at the beginning of the development of QCD. There was one event which I will never forget. Soon after Lepage and I had developed our theory of hard exclusive processes, I gave an invited talk at Caltech at the first physics meeting devoted to QCD. The conference was a major international physics meeting which was arranged by Professor Murray Gell-Mann. Gell-Mann also gave the summary talk of the meeting. The many exciting new developments in hadron physics made it a truly memorable conference. Just after I gave my talk, Professor Richard Feynman came up to me, and said: “Brodsky, I have to talk with you.” He then said: “Meet me at my restaurant.” Feynman had his favorite restaurant in Pasadena. I said, “May I bring Peter Lepage? He is here, too.” “Certainly.”
So, we are sitting at a booth in his restaurant, with Feynman across from me, and Peter sitting on my left. Feynman leaned over the table and said, “Brodsky, what you said today was wrong!” I thought to myself: “My life is over.” I asked “What is the problem?” Feynman replied: “Every hadron has an infinite number of gluons in each of its Fock states.” In fact, a central point of our analysis was that the valence Fock state of a hadron wavefunction in QCD only contains quarks - no gluons. So, I said to Feynman, “You know, when a pion decays, its quark and antiquark in its valence Fock state annihilate to produce a muon and a neutrino, there are no gluons remaining in the final state.” Feynman replied: “I know that — It's nothing but PCAC. It doesn't mean a thing.” So, then, I replied, “When the rho meson decays to an electron-positron pair, its quark and antiquark annihilate to a virtual photon which makes the lepton pair— there are no gluons leftover. The decays occur from its quark-antiquark valence Fock state without any gluons.” Feynman looked at me, and said, “You're right.” I said, “Can I explain anything more?” “No, it's not necessary.”
The next day, Gell-Mann gave his summary talk. Gell-Mann said: “Brodsky has reported on his work with Lepage at our meeting yesterday. So last night, I walked down the hall and I knocked on Dick's door. Through the door, I said, 'Is it right?' And through the door, Dick replied: ‘It's right.’ And then Gell-Mann said: ‘Well, I guess it must be right.’” And that is how he summarized my talk. That was it. It was astonishing, actually. There are very few times in your life where you can actually say you told Feynman that he was wrong and he agreed! We were always friends after that.
You could tell him he's wrong, as long as you're right.
For sure. But, my god, it was a little bit scary to talk to Feynman like that and correct him on such a fundamental issue! So, certainly, my paper with Lepage was a major development in physics. It now has over 3,500 citations. The theoretical methods that we developed, including the rules for light-front perturbation theory, that we developed, are used throughout the QCD field. Our approach is built into the QCD technology and based on a basic property, the conformal invariance underlying quantum field theory. The counting rules for exclusive processes that I had developed were also essential for our work. We learned later that Glennys had also worked with her Cal Tech student, Darrel Jackson, on the calculation of the pion form factor from QCD. I understand that Feynman gave her trouble probably because of this same issue involving the structure of the pion’s Fock state. So, Glennys also deserves much credit, and there was also independent work by theorists in Russia as well.
Stan, tell me about your collaboration with H.C. Pauli.
Chris Pauli came to SLAC as a visiting professor from Heidelberg University. I learned later that he had originally promised to come to Stony Brook to work with Professor Gerald Brown. Jerry Brown later told me, "Stan, I will never forgive you for stealing Chris Pauli away!” Chris Pauli was experienced in using matrix methods to solve Hamiltonian theory, and together we developed a new computer-based method to solve hadronic bound state problems in QCD, called DLCQ: discretized light-cone quantization. (Actually light-front quantization, would have been a better terminology, since the light-front is adapted from Dirac's front-form.) DLCQ provides a rigorous, first-principle method on how to solve quantum field theories on a computer using light-front methods. The results are frame independent: Lorentz and Poincare invariant. DLCQ provides a discretization of spacetime which is independent of the Lorentz frame, allowing quantum field theories to be solved on a computer.
In our subsequent collaboration with my Stanford student, Kent Hornbostel, we showed how to use DLCQ to solve for the spectrum of eigenstates for virtually any quantum field theory in one-space and one-time dimensions. Our method is now also often used to also solve the quantum field theories which underly string theory. In the case of QCD 1+1, one can obtain the complete spectroscopy of meson and baryons. However, the size of the matrix representation becomes very large for QCD in physical 3+1 space time. Recently, James Vary, his group at Iowa State University and I have developed an extended method of DLCQ called “basis light-front quantization” together with James Vary and I and his group at Iowa State university which already has shown great promise for QCD(3+1). My work with Lepage on exclusive hadron processes and my work with Pauli and Hornbostel on DLCQ has shown the power of light-front methods. Over the years I have applied light-front quantization to many physics projects, so I’m sort of identified with it, in people’s minds.
Light-front theory provides a frame-independent way to analyze quantum field theories, particularly QCD, including the light-front wave functions, which underly the calculation of hadronic observables. Light-front theory also provides an interface between perturbative and non-perturbative physics. My work with Lepage was crucial in bringing Chris Pauli to SLAC, because he knew of our work and its potential for understanding nuclear as well as hadronic spectroscopy and dynamics. I think, these efforts have provided a milestone for solving problems in QCD. Our light-front methods also provide an alternative to lattice gauge theory. It is a useful tool but it which is frame independent, and operates in Euclidian space, not in physical Minkowski space.
And now in recent years, I have developed with my colleagues, Guy de Teramond and Hans Guenter Dosch, an exciting new approach to hadron physics called "light-front holography”. Light-Front Holography is based on the fact that string theory in five dimensions of Anti-de Sitter space is dual to QCD in 3+1 physical space-time, but at fixed light-front time. Dirac would have loved this field since the crucial ingredient is his front form. Because of this melding of fields, we are obtaining more insight into hadron dynamics and spectroscopy than ever before. Chris Pauli was a wonderful, talented theorist, but he was very unhappy that he was required to retire at the University of Heidelberg because of its mandatory retirement rules. We are fortunate that here in the United States, we do not have fixed retirement dates. I continued on the Stanford faculty until I became Emeritus Professor at age 70 in January 2020; my conscience told me that I should not hold up the appointment of a SLAC faculty position for another person. However, I have continued to work in many new areas on many new projects since then.
Stan, tell me about your work developing precision tests for QED, Quantum Electrodynamics.
Yes, well that goes back to my experience with Donald Yennie and how to compute atomic energy levels which involve both perturbative and non-perturbative dynamics. That training and experience was very important for me, and it became a background of analogous challenges in QCD. In one of my interdisciplinary papers, I showed with Patrick Huet, that if one takes the analytic number of colors N_C to zero in non-Abelian QCD, you obtain QED, the Abelian theory. One can take any QCD result and verify its validity in QED by taking the analytic limit, N_C goes to zero, keeping a parameter called C_F fixed. This limit thus provides an important constraint on QCD analyses. People often assume that the Abelian limit is N_C =1, a single color. In fact it is actually zero colors. Related to this, Sid Drell and I wrote a paper on the muon G-2 which provided an important analysis tool which is sensitive to substructure of leptons. My background in QED has been crucial in designing suc tests and revealing new phenomena in non-Abelian QCD. Another of my papers, which was influenced by my experience in QED, had impact in high energy physics.
There is an interesting phenomenon involving the target spin-dependence of particle production reactions called the “Sivers effect". Dennis Sivers is a friend of mine, and he had proposed studying single spin phenomena in QCD which could be tested using polarized hadron beams. I somehow didn't know of this work, and I also did not know at that time that John Collins had published a paper showing that the Sivers effect was zero because it apparently violated time reversal symmetry. Not knowing of their work, Ivan Schmidt, Dae Sung Hwang and I developed the theory of the Sivers effect. We showed that the spin-dependent amplitudes underlying single-spin-dependent hadron production have a complex phase consistent with time reversal. To his great credit, John Collins wrote a paper, soon after our analysis was published, stating that he agrees with Brodsky, Hwang, and Schmidt, and that his own paper is wrong! One rarely sees a theorist publish a paper admitting that they had made a mistake.
As we had shown, single-spin-dependent hadron production does not violate time reversal because of the complex phase of the production amplitudes. I knew this because of my experience with analogous single-spin effects in QED. I could analyze complex a hadron phenomenon in QCD by relying on its analog in atomic physics, and because of the great early training that I had received from Don Yennie. Our paper is highly cited and our predictions have been validated by experiment. I think the reason I understood how to carry out such analyses was because of the great early training that I had received from Don Yennie. I could analyze complex a hadron phenomenon in QCD by relying on its analog in QED, atomic physics.
Stan, tell me about your work with intrinsic heavy quarks when you were working on novel QCD properties.
That's another good example of going beyond conventional wisdom. When you look at the proton and its quantum fluctuations, you usually think about it as simply a bound state of three quarks. However, quantum fluctuations also lead to higher Fock states, such as three quarks and gluon, and the gluon can in turn make another quark-antiquark pair, which could be a charm pair for example. This phenomenon is described by what is called DGLAP evolution. This conventional perturbative QCD process, is actually only one way that heavy quarks heavy quarks are created. If one calculates self-energy contributions to the proton eigenstate contributions which are the analog of light-by-light scattering in QED, and gluon-by-gluon- scattering in QCD, then heavy quark pairs will be created within the proton by amplitudes where multiple gluons attach to the heavy quarks — processes which are not described by DGLAP evolution. This novel phenomenon is called “intrinsic heavy quarks”.
A key feature of intrinsic charm quarks is that they are is produced with a high momentum fraction of the proton’s momentum when a high energy proton interacts in a target. Some physicists are so used to the conventional DGLAP picture where a single gluon creates a quark pair, that they find it hard to accept intrinsic heavy quark production originating from multiple gluon subprocesses within hadrons. But virtually every experiment now shows that the intrinsic charm Fock state of the proton, such as the five-quark state (with three valence quarks plus a charm-anticharm quark pair) has approximately 1% probability. There is also an unexpected asymmetry produced between the charm and anti-charm quarks, which is another novel feature of intrinsic heavy quark production. There are now hundreds of theory and experimental papers on intrinsic heavy quarks, and it has even been verified by lattice gauge theory.
But I think that if you would take a survey of theorists right now, some would say, "No, this cannot be possible — heavy quarks can only made by a single gluon splitting as described by DGLAP evolution." This example shows that one must think carefully about the physics and not just accept conventional wisdom. In QED, the probability of a heavy lepton produced by light-by-light diagrams falls as 1/M^4, mass to the inverse fourth power. However, because of the non-Abelian coupling of gluons, the fall-off is only 1/M^2 in QCD. In fact , if one take the limit as N_C goes to zero, the 1/M^2 terms go away, and only the 1/M^4 Abelian terms survive. In many ways, my training in atomic physics became a guide to the essential physics.
For example, one can you ask the question, does positronium have intrinsic muons in its wave function due to QED quantum fluctuations? The answer is yes, coming from the light-by-light scattering self-energy diagrams o0f positronium, giving the intrinsic muon contributions. There are also vacuum polarization diagrams, which lead to the analog of the conventional DGLAP muon pairs. This is another example how the training I received from Don Yennie in atomic physics has always guided me — if a phenomena is true in atomic physics, it's has to also have an analog in QCD, just more enriched.
Stan, I wonder if you can explain how you were applying QCD to Compton scattering.
It is well known how Compton scattering works in atomic physics. A photon scatters scatter on the bound state electrons. Similarly, photons can scatter on the bound state quarks in a hadron. One of my papers which I am proud of illuminated what is called the “J=0 Fixed Pole” contribution to Compton scattering in atomic and hadronic physics. This is a special effect arising from the product of two currents, where the initial and the final photons interact at the same point in space and time on a bound electron in an atom of a bound quark in a hadron. If there was a spin-zero scalar electrons in the atom or a scalar quark (“squark”) in the hadron, this effect would be due to the well-known “seagull” diagram where two photons interact at the same point. The seagull amplitude has a very special feature, that it's actually energy independent and had a real phase.
But even in the world of spin 1/2 quarks and leptons, one also finds the same local two-photon seagull amplitude. This is a consequence of light-front Hamiltonian theory, where the seagull contact term appears, even though it is not explicit in a Feynman graph computation. When one uses light-front quantization, the seagull contribution appears from the exchange of a fermion acting instantaneously between two currents in light-front time. In the case of deep elastic scattering lepton-hadron scattering, the cross section comes from the imaginary part of the forward amplitude. However, when one analyzes the forward Compton amplitude, one also obtains the fixed-pole seagull contribution from the real part.
Theorists such as Ken Wilson developed the “operator product expansion” to prove momentum conservation, momentum sum rules, etc. However, I have recently shown with my colleagues Ivan Schmidt and Valery Lyubovitsky, that if one analyses the class of events called “diffractive deep inelastic scattering,” where the target proton or nucleus stays intact, the operator product expansion cannot be applied, and thus the traditional momentum and other sum rules are inapplicable. But again, I use the guide of atomic physics, and look at the atomic analog and see whether or not the hadronic analysis is also correct in atomic physics. I was clearly fortunate to have training in both fields.
Stan, to go back to an earlier comment, you emphasized when you first got to SLAC that the theory group was really focused on supporting the experimental work. So, with that in mind, I'm curious what the connection may have been when you were looking from QCD to high energy photon-photon collisions if there was anything going on experimentally at SLAC that was relevant.
Good question. I realized that from studying events at SPEAR, and later from other electron-positron storage rings, that one can observe processes derived from photon-photon collisions, reactions distinct from the events where the electron and positron annihilate. The two-photon events, are fact, dominant at high energy, because the two-photon production cross section is logarithmically increasing, whereas the annihilation cross section decreases inversely with energy squared. I helped to develop this field when I was on a sabbatical at Cornell University in 1971, working out the relevant formulas with Tom Kinoshita and Hidezumi Terazawa. Our publications are among my most highly cited papers. We realized that if an electron and positron collide, there are processes where the electron and positron each create a photon which then collide. The two photons can then annihilate and make a new pair of leptons or quarks. Although such reactions are higher order, the resulting cross section increases with the logarithm of the collider energy squared.
Thus, unlike electron-positron annihilation, the production rate from piton-photon collisions does not fall at all. I was always conscious from my training in QED. that two-photon physics was important because it of its remarkable energy dependence. I also worked with Kinoshita and Terazawa on the concept of the “photon structure function”, which in QCD reflects the underlying quark structure of a photon, a quantity which was also developed by Ed Witten. We also computed other two-photon processes, such as quark jet production. I also wrote a series of papers with Peter Lepage showing from first principle how to compute the rate for two photons to produce hadron pairs. Such processes are now an integral part of collider physics studies, Two-photon physics also can be studied in electron-election collisions, without a positron beam, using two rings, where each electron emits a photon.
Stan, tell me about your collaboration with Robert Shrock, Craig Roberts, and Peter Tandy. Where did you meet them and how did it all get started?
I worked with Robert Shrock first. He became interested in a provocative statement I made which is perhaps still controversial. It had become conventional to think about the vacuum in quantum field theory, as having an infinite number of quantum fluctuations. That is in fact true if one analyzes the vacuum state in “instant form”; i.e., at a single instant of time, but it is not true in front form. When you analyze the vacuum structure in Dirac's front form, one is looking at the state at a fixed light-front time, like a flash photograph, you do not have the huge quantum fluctuations that people normally ascribe to the vacuum state. I think it was at one of my visits to Stony Brook, when Bob got very interested in this.
Theorists have written papers claiming that vacuum fluctuations lead to a cosmological constant of the universe which is 60 orders of magnitude larger than observed; however, this assumes that one can make observations of the vacuum at a single instant of time. This violates causality, since it requires information to be correlated beyond distances limited by the finite speed of light. Furthermore, as Dirac emphasized, the instant form is frame-independent; it depends on the Lorentz frame of the observer. If you use the front form, which Dirac pioneered, then the vacuum of a quantum field theory is much simpler. There is then no cosmological constant problem. The only light front vacuum structure is what's called the zero mode, such as the Higgs constant vacuum expectation value. Bob and I also we invented the concept of “in-hadron condensates” vacuum-like effects localized with a hadron. This concept has been developed further with Craig Roberts and Peter Tandy, who are experts in Bethe-Salpeter methods, which, like the front-form, are frame independent. The main principle that phenomena cannot depend on the observer’s frame choice.
When did you meet Prem Srivastava?
I was fortunate that Prem chose to spend his sabbatical and work SLAC with me starting in 1999.
Your most significant work would have been with the Higgs VEV.
Yes, we wrote a series of papers on the light front quantization of quantum field theories including QCD in the Feynman gauge, culminating with a comprehensive paper on the Standard Model and the Higgs theory using light front quantization.
You were working on the Higgs VEV, the zero-mode scalar background field.
Exactly. The interpretation of the Higgs VEV is natural in the front-form, and it avoids the conflict with the huge cosmological constant predicted by conventional theory. This description of the Higgs theory and the Standard Model provides a consistent mathematical way to introduce quark and lepton masses based on each fermion’s coupling to the Higgs vacuum expectation (VEV), which is takes the form of a constant zero mode in the LF theory. The front-form, is frame independent and thus only allows a zero mode VEV for scalar particles. In effect it is a background field which exists throughout the universe of this Higgs field. Philip Mannheim, who has also recently adopted a similar viewpoint. The Higgs mode may seem unnatural physically, but I think it is now the only way to understand how to have the universal quark and lepton masses. However, it is possible that someday we will find another deeper description of Higgs physics.
Tell me about Mackenzie in the Brodsky-Lepage-Mackenzie procedure.
The BLM procedure sounds like the political slogan, but it is provides a consistent and rigorous to eliminate a persistent ambiguity when applying renormalization to perturbative predictions in quantum field theories. Peter Lepage and I collaborated with Paul Mackenzie when we were sabbatical visitors at the Institute for Advance Study at Princeton in 1982-1983. Paul was a visitor at IAS from Fermilab at that time, and he helped develop consistency tests of the BLM procedure.
The seeds of the BLM methods go back to QED, where there is no renormalization scale ambiguity. In QED, vacuum polarization graphs immediately determine the scale of the running coupling in each order of perturbation theory. This is called the Gell-Mann-Low procedure. One simply shifts the renormalization scale to absorb and thus incorporate all of the vacuum polarization contributions into the QED running coupling. In BLM, one applies the same method to QCD. In this case, the vacuum polarization and other non-Abelian contributions which control the running of the QCD coupling are called the “beta” terms. Again one shifts the argument of the QCD running coupling to absorb the beta terms and that sets the renormalization scale. In our papers, we applied BLM scale setting to the first nontrivial order.
There have been countless successful applications of BLM in the literature. More recently, Matin Mojaza, Xing-Gang Wu and I extended the BLM method to all orders, We named our extended scale setting method as “principle of maximum conformality” (PMC), since the coefficients of the resulting series match the series of the corresponding conformal scale-free theory with zero beta function. This allows one to systematically set the renormalization scale at every order of perturbation theory, and totally eliminate the scale ambiguity for virtually any QCD application. Moreover, the PMC result are independent of the theorist’s choice of renormalization scheme, satisfying another requirement of renormalization group invariance.
Another remarkable feature of the PMC is that the “renormalon” divergence problem is eliminated: there is no n-factorial growth of the perturbative coefficients. There have been many highly successful applications of BLM/PMC which have greatly increased the precision of perturbative QCD predictions and tests. I believe that this is a truly fundamental advance for hadron physics. However, some theorists who would read this discussion will insist: "No, no. I know that you cannot fix the scale." But they should recall that QED does not have any renormalization ambiguity. Not only is there no ambiguity of the scale choice in QED, but the physical results do not depend on the choice of the the renormalization scheme. I find it hard to understand this mindset of some theorists, but apparently they were trained at an early stage to simply use the MS-bar renormalization scheme and then just guess the renormalization scale. They use the MS-bar scheme just based on the intuition that it must be an optimized choice for physics.
However, the MS-bar scheme is just a human convention. I am reminded of story which I think Bill Bardeen told me himself. Bill was working at his desk at home, developing dimensional regularization as a method to regulate divergences in perturbative QCD. He realized that if you subtract the logarithm of 4pi, the Euler constant, and an additional 5/3, then the result of the dimensional regularization procedure coincides with the standard Gell-Man-Low scheme used universally for QED predictions. Bill’s wife, Marge, then calls him to dinner, but when he goes back to his desk, he forgets to subtract the 5/3! This accidental step actually defines what is called the MS-bar renormalization scheme.
So, theorists use MS-bar as the standard renormalization scheme for dimensional regularization, but it is clearly an arbitrary convention. In fact, you can choose any subtraction to define the scheme. Obviously, the actual physics answer cannot depend on this convention — it was a total accident that Bill dropped the -5/3. If he had kept this term, then we would at least have the same renormalization scheme for QED, QCD, and the electroweak theory — but now, if one want to have a unified procedure using MS-bar, you must use a different scale choice for QED and QCD. Clearly physics results cannot depend on a human convention. In a related advance, I developed with Mojaza and Wu a new set of applications of the BLM/PMC method called “commensurate scale relations”.
A commensurate scale relation rigorously relates any perturbatively-calculable physical observable to any other physical observables without any dependence on a choice of scheme or scale ambiguity at correct relative scales. I do not think it is generally appreciated how fundamental these relations are. Rod Crewther, who I met during one of my trips to Melbourne, is famous for the “Crewther relation”. The Crewther relation is an unexpected relation connecting the cross section for spin-dependent deep elastic lepton-hadron scattering to the annihilation cross section to hadrons. Crewther derived his relation specifically for conformal theory, where the coupling does not run.
However, Matin, Xing-Gang and I defined the “Generalized Crewther Relation,” which gives the relation between the two physical observables for physicalQCD with nonzero QCD beta terms. One thus obtains an exact first-principle scale-fixed relation between the two observables. In fact, the PMC allows any two observables which are perturbatively calculable in QCD to be related to each other, without any scale or scheme ambiguities. Thus, there is no dependence on a human convention. If we would meet physicists from another planet, they would not know about MS-bar — and of course they would not have known about Bill Bardeen's interaction with his wife! Clearly, one must predict and test relations between physical, measurable observables. This will provide checks of theory at a fundamental level without arbitrary conventions.
Hopefully, the theoretical community will eventually understand this — but it is interesting to see the human psychology if you have been trained with the misimpression that QCD predictions are always scheme dependent and that the renormalization scale cannot be fixed. I’ve had endless discussions with theorists on this; they eventually agree with the correctness of the first-principle BLM/PMC procedure, but they then still go back to the old convention of choosing an arbitrary scheme and guessing the scale. We can greatly increase the precision of every test of the Standard Model if people would consistently apply the PMC. As I have explained before, the reason for the name “principle of maximum conformality” is that after one sets the renormalization scale to eliminate the beta terms, the coefficients of the resulting perturbative series are identical to the corresponding conformal theory with a constant coupling. Thus, the PMC also allows one to recover the underlying conformal basis of QCD.
Stan, to branch out a bit from SLAC, I'm curious if there was major experimental work elsewhere that was relevant to you. Perhaps the Tevatron or what was happening at the LHC. I'm thinking about the application of PMC methods.
My colleagues and I have written many papers doing just that, making PMC predictions for collider physics at the Tevatron and the LHC. I have continued to collaborate with a talented group of theorists led by Professor Xing-Gang Wu from Chongqing University in China. Xing-Gang originally spent a year at SLAC with me. His colleagues and I, together with another innovative theorist, Leonardo di Giustino from Insubria University, have now published more than 35 papers giving PMC scale-fixed predictions for collider physics. So, at least in China, theorists have no doubt that one can make scale-fixed, renormalization scale- and scheme-independent predictions.
Tell me about your work with Guy de Téramond.
Guy is my most frequent collaborator — we have actually published 100 papers together on a large range of physics topics in hadron physics. Guy is totally brilliant and amazing, someone who virtually never makes a mistake. It is wonderful to have collaborators like Guy, who have sound judgment and are almost infallible doing calculations.
Well, it was with him and also Hans Günter Dosch that you worked with, and this was the work on the confinement potential.
Right. Guy and I worked with our colleague, Hans Günter Dosch, in a exciting new area of hadron physics called “light-front holography". Guenter is also an amazingly talented theorist. Light-front holography provides novel insights into hadron spectroscopy, hadron dynamics, and color confinement, physics which underlies virtually all aspects of hadronic phenomenon. Our number-one passion has been the development of light-front holography and applications in every possible direction.
I will take credit for an initial observation that helped to start this field. I once studied a famous paper by Juan Maldacena on physics in five-dimensional anti-deSitter space, a geometrical construction which is often used as a basis for string theory because of its underlying conformal features. This topic at first seems complicated and apparently unphysical, but I noticed that the equations describing a bound state in five-dimensional anti-deSitter space seem to have the same form as the equations that I knew arise when describing the bound states of hadrons in light-front physics. I also noticed that the AdS_5. formula for the form factors of bound states in five dimensions, which had been derived by Polchinski and Strassler, had the same form as the Drell-Yan-West formula for form factors of hadrons in physical 3+1 space-time, but at fixed light-front time.
It was just a matter of identifying and matching the relevant variables. I realized that one could match the variable z in the fifth dimension of anti-deSitter space to a light-front radial variable called zeta in the physical world of three space dimensions at fixed light-front time. The fifth spatial dimension z in anti-deSitter AdS_5 space appears to be only mathematical, but it matches an ordinary kinematic variable zeta, in the physical world of 3+1 QCD.
This initial observation began the field of “light-front holography”. Another advantage of knowing this correspondence, is that given what is called the “dilaton” modification of AdS_5 theory in the fifth-dimension z of AdS_5 theory, one obtains a confining potential in physical space-time. The dilaton has the analytic form of an exponential of a constant times z squared. The constant then becomes the mass scale underlying the color confinement of quarks and gluons in QCD. This observation started us off, since the “holographic” matching of z in AdS_5 space to zeta in 3+1 physical space time gives a light-front Schrodinger equation for quark-antiquark bound states. One thus implements color confinement, and remarkably, the eigenvalues of the resulting Schrodinger equation matches the observed spectroscopy of mesons. We found this startling correspondence compelling. The Polchinski-Strassler formula for hadronic form factors also matches the Drell-Yan-West formula for form factors evaluated from the overlap of the initial and final state LF wave functions, if one just matches the same variables z and zeta. This holographic correspondence is in fact valid for gravitational, electromagnetic, and weak form factors.
Stan, this is a rather broad question, but I wonder what some of the biggest surprises in QCD with regard to color confinement and hadron mass scales have been for you.
Okay, I will focus on one aspect. Theorists who work in the historic field called Regge theory have noticed that the square of the masses of all hadrons follow a simple pattern called Regge trajectories. If you plot the square of the mass of each known meson as a function of the internal angular magnetic momentum L between its quark and antiquark, one finds that the masses follow a linear trajectory. That is the square of the meson masses is proportional to L times a universal constant called kappa squared. The quark-antiquark confinement potential between a quark and antiquark must have a simple universal form in order to give this simple linearity.
Furthermore, if one plots the square of the meson mass M squared versus n, the number of nodes in the bound state wave functions — the number of times the bound state wavefunction crosses zero, one finds the identical slope. Thus, the Regge trajectories have the identical behavior in n and L. And even more amazing, if one repeats this procedure for the baryons, the protons, even baryons with strange and charmed quarks, one finds the same universal Regge trajectories in n and L with the same slope. In fact, as we have shown, this duality is the prediction of light-front holography derived using what is called “superconformal algebra”. But how could the spectroscopy of three-quark bound states, like a proton, have anything in common with the spectrum of the two-body quark-antiquark mesons? The essential physics principle is that the three quarks first arrange themselves as a quark and a diquark. For example, in the case of the proton, two of its three valence quarks, a u and a d, will cluster together as a spin-0 [ud] diquark. The diquark has the same color charge as an antiquark. The color interaction between the [ud] diquark and the remaining u quark is then exactly the same as a quark and an antiquark in a meson. The observed excitations of the proton are then the orbital excitations in the orbital angular momentum between its quark and diquark.
Remarkably, the slopes of meson and baryon Regge trajectories are predicted to match, not only in L but also in n. But then there is another miracle: light-front holography predicts that if one identifies L in a baryon with L+1 of the corresponding meson — the orbital angular momentum L between the quark and the diquark, and the baryon’s orbital angular momentum L between the quark and the antiquark plus 1, then the observed masses of the meson and baryon excitations coincide. It is in fact astonishing, that one observes this correspondence in the hadron spectroscopy of the real world. One is observing is a form of supersymmetry in hadron physics, because one is relating mesons on one hand, which are bosons, to baryons which are fermions. In fact, super-conformal algebra predict that hadrons are arranged as a 4-plet of mesons, baryons, and “tetraquarks” which are themselves composites of diquarks and anti-diquarks. This 4-plet mass degeneracy appears valid for the spectroscopy of the entire universe of hadrons, even including the heavy charm and bottom quarks. There are no new parameters in light-front holography, except for the quark masses.
Moreover, the underlying light front wavefunctions of mesons, baryons and tetraquarks have supersymmetric features. Our analysis utilizes the mathematical structure of super-conformal quantum mechanics pioneered by Haag and others. The pion is exceptional in light-front holography, since it is a quark-antiquark bound state with zero relative orbital angular momentum and thus has no baryon partner. Since L=0, light-front holography predicts that the pion mass vanishes for zero quark mass. In fact, the pion mass squared is predicted to be linearly proportional to the masses of its quark and antiquark, in agreement with the famous Gell Mann-Oakes-Renner relation. However in contrast to GMORT, there are no quark vacuum condensates. Moreover, since the pion does not have a baryon superpartner, one understands why there are no massless baryons. It is remarkable that you can actually derive all of the properties from light-front holography. I know that I should write a book on light-front physics, including the fascinating and profound features of light-front holography and its implications for non-Abelian gauge theory, QCD.
You mentioned before, supersymmetric relations between the masses of mesons and baryons. More broadly, how has supersymmetry been relevant for your work, and what might you share about the theoretical assumptions around the fact that we haven't yet seen supersymmetry?
A great question. The only time I've ever used supersymmetry has been in the applications of superconformal algebra to QCD. As you know, I have focused my studies on existing physical phenomena. It could well be that there are very heavy supersymmetric squarks and sleptons, and that supersymmetry will eventually be incorporated into the underlying theory of nature. The hope of many theorists is that since supersymmetry is such a beautiful principle, it must be a fundamental feature of nature. However, we haven't found any supersymmetric particles, despite intensive experimental searches.
However, as I have discussed, one does observes supersymmetry in hadron physics between mesons and baryons and even tetraquarks. I like to joke about this: it is very hard to solve three-body problem analytically, such as a bound state of three quarks. But if two of the three quarks first cluster as a diquark which then binds to the third quark, then we only have to solve a sequence of two-body problems, making our theoretical physics life simple. As I mentioned, super-conformal algebra predicts tetraquarks, diquark plus anti-diquark bound states, with the same mass as their mesonic quark-antiquark, and baryonic quark-diquark partners. One does not need to solve a four-body problem. The tetraquarks that are predicted by super-conformal algebra are in fact observed in the spectrum of hadrons as Marina Nielsen and I have shown. You can also analyze the role of diquarks in the higher Fock states of the hadrons.
For example, protons are not just bound states of three quarks in quantum field theory. It has higher Fock states, quantum fluctuations such as three quarks plus a gluon, and three quarks plus a quark and antiquark. But again, in light-front holography, these Fock states can organize themselves as a sequence of two-body states. The diquark clusters appear because the diquark is an attractive configuration in QCD. The diquarks thus become essential foundation stones underlining the supersymmetry between mesons, baryons and tetraquarks and their Fock state excitations.
Regarding your work in renormalization theory, I'm curious if you ever got to know Ken Wilson.
Oh, yes, I met Ken when I was a visitor at Cornell University. Although we did not collaborate directly, we had much overlap when we both were visitors at the Snowmass physics meetings of the DPF division of the APS. Ken led a theory group at Snowmass focusing on light-front physics, One of Ken’s colleagues, Stan Glazek, and I wrote a paper on a remarkable feature of the quark-antiquark confinement potential of light-front holography. We showed that if one takes the heavy mathematical limit of the light-front holographic potential, where all the quarks are very massive, then one not only recover spherical symmetry and nonrelativistic quantum mechanics, but one also finds that the color-confining potential predicted by light-front holography coincides with the standard linear potential of heavy quark theory, the linear potential established primarily by Peter Lepage and his collaborators. Thanks to Ken, I have Stan Glazek as a great collaborator.
You mentioned Maldacena before. I wonder, more broadly, if you can explain what was so revolutionary about AdS/CFT for your work?
Maldacena showed that five-dimensional Anti-deSitter space provides a geometrical representation of conformal symmetry, an essential scale-invariant property of quantum field theories. The soft-wall dilation modification of AdS_5 has the mathematical property that a mass scale can appear in the Hamiltonian and the resulting equations of motion, while retaining the conformal symmetry of the action. This physical principle was first emphasized by Fubini and his collaborators; it provides a natural way to introduce the mass scale underlying color confinement for QCD and the spectroscopy of hadrons. The numerical value of the scale, in GeV units for example, is then actually irrelevant, since only the ratio of hadron masses needs to be predicted. I always emphasize the importance of introducing a mass scale without affecting the underlying conformal invariance of the action, since this procedure, dictates the form of the dilaton of AdS space, which in turn, using light-front holography, fixes the form of the confinement potential in physical space-time. This appears to be a fundamental feature of hadron physics and an essential guide to nature. I thus believe that what we're doing in light-front holography has fundamental importance. It is not just mathematics, but relevant to the real world.
I'm curious, in your work combining hadron physics and photon science, if you ever had opportunity to work with Artie Bienenstock.
We know each other, of course, but we have not worked directly together.
Different research areas.
Right. Artie Bienenstock was a key pioneer developing the photon science program at SLAC. Although the SPEAR electron-positron storage ring was originally designed to study particle physics, the copious and well-controlled photon radiation from the stored electrons in the SPEAR ring has great value for physics and has led to the remarkable field of photon science. One can also credit this development to the close interactions of the SLAC and Stanford campus faculties.
Stan, as you well know, one of the big physics stories in the late 20th century, early 21st century, is the utility of particle theory to cosmology. I wonder how you see your research contributing in any way to that trend.
My most relevant work in cosmology are my publications with Robert Shrock on the features and properties of the vacuum of quantum field theories which are illuminated by using light-front quantization. We have shown that, contrary to the usual conception of the vacuum based on instant form (quantization at ordinary fixed time t), that quark and gluon condensates do not, in fact, contribute to the vacuum nor the cosmological constant. In light-front theory, which unlike instant form, is frame independent, the QCD condensates only have spatial support within hadrons. The light-front condensates thus contribute to the hadron masses, but not to the energy of the QCD vacuum.
Have you seen -- again, a very broad question -- from your perspective, how SLAC has changed its overall research mission from when you started into things like astrophysics and cosmology?
The SLAC laboratory has made truly significant changes over the years. When he first created SLAC, Pief was famous for saying that “every ten years SLAC has to change its direction.” Remarkably, it really has done that, incorporating many scientific fields outside of particle physics, such as photon science. We have also been very lucky to have Stanford University and its extraordinary resources. For example, the university provided its land for the SLAC two-mile accelerator, along Sand Hill Road all the way out to Portola Valley. It's really wonderful how to have this resource, this amazing open land right in the middle of the San Francisco Peninsula. There are only a few communities until you get to the ocean. Helen Quinn, an outstanding SLAC theorist, has been an environmental leader in preserving the land on the mid-peninsula from development. We have also been fortunate that Stanford University created SLAC as an academic department of the university, so that the leaders and faculty of SLAC are also Stanford professors and can teach on campus.
Another amazing advantage is that Stanford devotes its land for faculty housing. Many of the members of the Stanford and SLAC faculty live right on the campus. I’ll mention one memorable story: When Burt Richter won the Nobel Prize for the discovery of the Psi, hundreds of us from SLAC went to celebrate at his home on the Stanford campus. We were out on the Richters’ porch, and the Stanford band came marching down the street below us, banging their drums. And as they marched along, they're singing: “And if you win another one, you can march with us!” — If you win another Nobel Prize!
It was wonderful. Has anybody told you this story?
I have not heard that one.
Other memories of the close SLAC physics community come to mind. Pief and Sid created wonderful congeniality at SLAC. In the early years of SLAC, the Panofskys regularly invited the SLAC physicists to their home in Los Altos Hills so that we could actually have experimental seminars at their home. Also, Sid and Harriet Drell would invite everyone to their campus home for a SLAC picnic every year after the annual baseball game between the theorists and the experimentalists. (The experimentalists almost always won!) That was when I first met the Drell’s daughter, Persis, when she was 10 years old — she is now Provost of Stanford University.
As soon as I became permanent at SLAC, we contracted to have a new home built in a new area Frenchman’s Hill of the campus that the university was opening for faculty housing. (I proudly installed the intercom system throughout the house myself.) The great environment that faculty people have at Stanford is wonderful. We had an ideal community for families, including excellent schools for our children. One memory: Jane and George Collier, who were both anthropology professors, lived across the street from us on Cathcart Way. Their two children, Lucy and David, and our two boys, David and Stephen, took the parts and read the Shakespeare plays together. The four kids also loved computers, but at one point, they somehow broke into the Stanford computer system via a phone modem. They were caught, but instead of yelling at them, the four children were offered jobs in the computer center. You can tell it was pretty interesting to live in that wonderful area.
Stan, you've been a visiting professor and lecturer all over the world, so many places. Do any really stick out in your mind as being either really productive scientifically, or enjoyable or meaningful personally? What sticks out in your memory?
I have had great sabbaticals and visits with the faculties at Cornell, the Institute for Advanced Study at Princeton, Helsinki, and Heidelberg. Judy and I also had some bizarre adventures, such as getting arrested on our first day at Heidelberg for going the wrong way on a tram.
I'd like to ask a work-style question. It's not so much a scientific question, but an efficiency question. Your record of scholarship is kind of mind bending, all that you've done; how productive you've been. Is your style to work on multiple projects at the same time?
Absolutely, yes. I think that John Ellis, a leading theorist at CERN and now at Kings College in London, holds the championship on the number of physics publications —well over 1,300 compared to my 720. (We are also collaborators on several papers.) But yes, I do work on multiple projects all of the time, even now when I am retired. The ability to work with colleagues all over the world is one of the great advantages of the interconnected computer world. We collaborate, both offline, and online. I try to keep productive and have four physics projects ongoing right now. I think it's a good idea not to have to concentrate on just one physics project, so that if you are stuck, you can come return later and resolve the problems. It is very good to have feedback from collaborators and not work in isolation.
What have you learned over the years about getting unstuck from a theoretical problem?
Yes, I rarely end up being permanently stuck. I think it is best to approach physics problems from as many directions as possible, and explore analogs — particularly, analogs in atomic physics in my case. I don't remember many times where I've actually approached a problem, and then not being able to resolve it.
Given all of your collaborators, in terms of personality, in terms of sensibility, is there something that you generally bring to the table, no matter who it is that you're working with, or is it always a unique blend of personalities?
I try to bring a sense of humor to our interactions. The people I work with are very talented. I am often absolutely amazed what they can do, analytically. It is great to have such outstanding people working with you. It obviously makes our physics projects productive — a multiplier effect.
Being stuck in the pandemic has made me think a lot about the value of working side-by-side; real in-person interaction. It's obvious for experimentalists why that's so important, but I wonder if you can convey, even for theorists, why it's important to be together in a room.
Yes, we certainly miss do miss being at a blackboard, and just writing down an equation, with a colleague saying, "Yes, but you can also do this and then do that." One advantage of online Zoom meetings is that you can see the slides of the talks very well on your own computer. However, no-one wants to ask a foolish question; this is one of the negatives of seminars and conferences on Zoom.
Over the course of your career, of course, sometimes the experiments are leading the theory, and sometimes the theory is leading the experiment. How would you chart that roughly over the decades? When has which led the other?
Well, certainly, the discovery of Bjorken scaling was one of the crucial results that really just changed the physics world. It happened right here at SLAC, the discovery of the quark structure of matter, confirming the underlying point-like structure of matter and the underlying quark composition of hadrons. This really was a profound development for me. The discovery of bound states of heavy quarks such as the J/Psi, was also crucial. It showed that underlying composite structure of hadrons was analogous to the bound states of atomic states. Before the discovery at SLAC of Bjorken scaling, theorists had regarded hadrons as complex objects, without any concept of their fundamental composition. The discovery of the W and the Z confirmed the expectations of the quantum theory of the weak interactions underlying the Fermi interactions. The discovery of Bjorken scaling also led to the development of quantum chromodynamics, QCD. QCD, together with QED and the electroweak theory, then provide the underpinning of a unified quantum theory for the electromagnetic, weak and strong interaction, the crucial elements unified in the Standard Model.
In my case, I concentrated on aspects of QCD which predict novel hadronic phenomena, such as “color transparency”, a topic which was developed by Al Mueller and myself. Al and I showed that a hadron can actually traverse a nucleus target without absorption if it scatters in the target at high momentum transfer. This is due to the fact that the three constituent quarks of the proton interact as small color-singlet configuration. The discovery of QCD also led to other unexpected phenomena in hadron physics, such as “intrinsic heavy quarks” which carry a high momentum fraction of the momentum of hadrons; e.g., in the 5-particle Fock states of baryons.
This prediction, which was made by Paul Hoyer and myself, has important implications for collider phenomenology. QCD also predicts novel nuclear phenomena, such as the shadowing and anti-shadowing of nuclear structure functions. This complex phenomenon is connected to diffractive processes in deep inelastic lepton-nucleus reactions and subtle quantum mechanical interference of the nuclear amplitudes. I often reflect on how the complex physics of quantum field theory actually occurs in nature. Lattice gauge theorists use advanced supercomputers for many hours in order to calculate the hadronic contribution to the anomalous magnetic moment of a muon — its G-2, which is measured to high precision.
Of course, a muon does not have the resources of a gigantic computer to compute its properties. It somehow computes the correct universal result. There is an important three-loop contribution to the muon G-2 in quantum electrodynamics from light-by-light scattering, a multi-dimensional integration contribution which was first computed by Kinoshita and myself. There are also analogous high order QCD contributions to the muon G-2 involving virtual quarks and gluons. What is the mechanism which allows such incredibly complicated phenomenon to be done by a single muon? Perhaps there is an underlying principle, such as the minimization of the action, that can explain how complex physical phenomena is actually evaluated by nature.
Stan, as you mentioned, when you first got to SLAC, the Standard Model didn't even exist at that point. When did you feel like the Standard Model was complete, and when was the emphasis starting to shift on physics beyond the Standard Model?
Yes, all good questions. Over the years one could see the ingredients of the Standard Model being confirmed — the tau lepton, the charm quarks, the bottom and top quarks, the Z and the W, at both the LHC and the Tevatron. It is now hard to propose a viable alternative to the Standard Model. The remarkable empirical successes of the present theory does make one skeptical that new phenomena beyond the Standard Model will soon appear. I worked on a well-known paper with Sid Drell showing how high precision measurements of the muon G-2 can expose its structure at very short distances. Perhaps some variation from conventional Standard Model physics will appear when the Higgs sector is further explored. I think that a signal for a composite Higgs boson could be observed by measuring Higgs-Higgs interactions at a high energy collider. I also hope we can find a better theoretical description of the Higgs particle and the Higgs vacuum expectation value. I find it hard to imagine that the universe is filled with a background zero-mode Higgs field.
It's a purely speculative question, but it's relevant given your comments right now, if the SSC had been built, what would particle physics look like today as a result?
I can only speculate. If we had built the SSC, we hopefully would have discovered something unexpected; e.g., surprising features of the Higgs by colliding Higgses with each other together, Higgses with Zs and Ws and so on. Perhaps the SSC would also have discovered a new fourth flavor, beyond the top quark. The mass range of the known quarks, let alone the neutrinos, is truly astonishing — from the MeV mass of the lightest quarks to the TeV mass of the top quark. Maybe the SSC could have discovered even heavier fundamental particles.
Stan, I wonder if you've ever reflected on the caricature of the theoretical physicist just being a brilliant person who sits and thinks brilliant thoughts all day long, and there isn't the appreciation that hard work is really central to the whole endeavor. I come back to what you were saying about your father and how hard he worked, and perhaps how you may have inherited some of those traits and applied them to your field of scholarship.
That's a good point. My father could just sit down at a piano and play beautiful music, without any training at all: he could “play by ear”. He obviously had talents which he was never able to exploit. He worked hard his whole life and was dedicated to his family. But you asked about the caricatures of theorists. People imagine that people like Einstein and Dirac had profound thoughts all the time. Theorists do have talents in mathematics which they use to analyze complex topics. But really, most of the time, theorists are normal human beings.
Is there any "eureka moment" that stands out in your memory, either something that you understood for yourself, or something that you understood that really pushed the needle forward in the field?
The counting rules for the power-law fall of cross sections and form factors that I derived with Glennys Farrar may be an example. The fact that the counting rules work so well immediately suggests an underlying scale-free conformal theory for hadron physics. The work I did with Peter Lepage was certainly very gratifying; it gave a rigorous fundamental mathematical basis for exclusive scattering amplitudes. Lepage and I also defined the concept of distribution amplitudes and developed what is known as their ERBL (Efremov, Radyushkin, Brodsky, Lepage) evolution. These are truly beautiful mathematical results.
More recently, I have begun to appreciate how light-front holography provides the actual light-front wavefunctions and distribution amplitudes which underly hadron structure and dynamics, The fact that the pion mass squared is predicted to be linear of the quark masses is also a major prediction. How this property emerges from the strong interaction theory is a profound question, but we now know that comes directly out of light-front holography in a remarkable way. The light front holographic derivation contradicts the usual assumption that quark condensates which fill the vacuum are required.
Conversely, going in the opposite direction from so-called "eureka moments," have there been any intractable problems in theoretical physics? Things that have gnawed at you over the years, where no matter what you do, you always seem to hit a wall?
I think it is better to work on problems that you can make progress in!
How do you know early on how to identify the successful problems to work on?
I still use experiment for a guide. If one observes a new pattern emerging from phenomenology, then one has a powerful hint that one can explain it from new features of underlying theory. Any time one sees a recursive rule or uniform pattern of nature, we hopefully will find an underlying explanation which will lead to new fundamental theory.
Stan, you've been richly honored over the course of your career. I'd like to ask you specifically what it felt like to win the Pomeranchuk Prize in 2015.
Yes, I felt very, very honored. It was great to share the Pomeranchuk Prize with Victor Fadin, a great Russian theoretical physicist and also a collaborator. Receiving this award was a wonderful experience. One unusual thing: somehow, I was not allowed to enter the building of ITEP, the Institute of Physics in Moscow which hosts the Pomeranchuk Prize although I had been inside the ITEP building in previous years. The building and grounds resemble the Institute for Advanced Study at Princeton. I remember a previous visit to ITEP, when Boris Ioffe, Lev Lipatov, and my other Russian colleagues showed me the office room they had preserved in honor of the great theorist, Lev Landau. His picture was there, and they thought that somehow Landau and I look alike, which was a great compliment.
But this time, in 2015, my third visit to ITEP, they did not allow Americans inside the ITEP building. So, instead Lev Lipatov arranged a wonderful ceremony for us in downtown Moscow at the National Academy of Science. Unfortunately, many of my colleagues and friends in Russia, did not know they had changed the location of the ceremony, which was somewhat of a disappointment. But it was a tremendous honor to receive this honor from Boris Yaffe and Lev Lipatov. I feel bad for my Soviet colleagues now, because their restricted academic freedom, which keeps them from collaborations outside Russia. You do not hear of many new developments from our Russian colleagues now. I am also worried about new restrictions that could happen as well with my collaborators in China.
Cold War redux, in sense.
Yes, it's sad.
Well, Stan, maybe we should take a cue from Alexa and stretch. So, on that point, let me ask for my last question, if you look at your career from the beginning, it would be easy to say how lucky you were to be present at the creation of so much foundational work in theoretical particle physics. However, as I'm listening to you talk about your continuing activity in the field, it would suggest that there's still lots of interesting work to be done. So, I wonder, on that point, if you might push back a little against the idea that all of the really important work in particle theory has always been done, and how that might be inspirational for the next generation of theorists coming up, who are just beginning their careers now.
Yes, I have do have thoughts on this. First of all, I agree that I was totally lucky and fortunate being at the right place at the right time. I greatly appreciate how fortunate my contemporaries and I were to be part of a field which was developing so rapidly in so many fundamental ways. It does look hard to see anything comparable developing in our field now, so it might be better for young theorists to concentrate on fields ready for making comparable advances. I still think that there can be tremendous opportunities in QCD, such as describing nuclear dynamics at a fundamental level, but this is not the same as going into a new field and developing it from the ground floor up.
And yet, there are still things that are interesting to you now.
Oh, yes, absolutely. We still do not understand the fundamental mechanisms underlying color confinement, and this problem has continued to be a prime focus right now. I want to understand the analytic structure of confinement and find new basic principles that may have been missed before.
That's right. Stan, it's been a great pleasure spending this time with you. I'm so happy we were able to do this, and that you were able to share your insight and all of your scientific knowledge with me. So, thank you so much.
I also greatly appreciate your interest and patience. Thank you.