Image courtesy Paul Guetter Photography
Image courtesy Paul Guetter Photography

Director of the Simons Center for Geometry and Physics

Interview by Maria Shtilmark

Let us begin with Stony Brook where you obtained your PhD in 1981. How did your thesis and your advisor influence your future in theoretical physics?

There was quite an influence! I came to Stony Brook at the end of August 1978, passed the qualifying exams in January, and started working at the C.N. Yang Institute for Theoretical Physics (then called the ITP). My idea was to work in supergravity. As an undergraduate, I had read a nice article on supergravity by Dan Freedman and Peter van Nieuwenhuizen. At the time there was a student exchange agreement between my university and Stony Brook, which had just started as Spain was coming out of the Franco period. So first I went to the army, and once I finished military service I was fortunate to be accepted here, and to start working after the qualifiers with Dan Freedman. That was a very nice experience, he was a wonderful advisor, and when he moved to MIT in 1980 I went with him. I finished my thesis at MIT, as I was working on projects that he suggested. The only issue was whether I should have defended my thesis at MIT or Stony Brook. At MIT they said that if I wanted to defend the thesis I would have to pass the qualifiers again. We thought this was nonsense and my defense took place here, March 12, 1981.

In fact, in December 1980 I was already granted a fellowship at the Harvard Society of Fellows, so fortunately, I had a postdoc position to go to, even if I didn’t pass the thesis. It is a society of scholars, and to become a Junior Fellow you were supposed to be good at what you do, but also be able to interact with people in other areas of academic activity, and a PhD thesis wasn’t even required. But I passed the thesis, and obviously Dan Freedman’s letters to the Harvard Society of Fellows was important for my access to this privileged position. Dan Freedman has always been a great support in my whole career. I am in great debt to him.

It is fascinating to learn about your service in the army! What was your rank?

Second lieutenant. First, I was in a boot camp for three months, and then at A Military Academy, close to Madrid. I went to the Artillery Academy, ending up as a second lieutenant, and then six months of practice. And I was doing it while I was studying for my BA in Physics. At the time there were so many strikes at the university that I could only take few real courses. Either students, or professors were always on strike. So, I worked at home and decided I would complete my military duties (as at the time it was mandatory) while also working on my university degree. I had two six-month breaks, in the fourth and the fifth year of studies. Fortunately, I finished the army at the same time as I finished my BA in physics. At the time physicists and mathematicians were sent either to naval or artillery academies. At least, they trusted we could do some ballistics.

I noticed that a lot of your interviews are in Spanish, and you are greatly involved with science in Madrid. How important is it for you to come back to Spain and support science?

Ever since my PhD I’ve been visiting Spain regularly. I even took part in the construction of “The Institute of Theoretical Physics” at the Autonoma University in Madrid. I did my best for this to happen, but of course the locals had the biggest responsibility and credit. I went there frequently to lecture and teach graduate courses. When I was at CERN I directed some theses from students coming from Spain. I felt it was my duty towards the young people of my country to help them get an opportunity. I’ve had quite a number of Spanish students, and many are professors in Spain, or elsewhere. At CERN we can’t grant PhDs, but we can share students with member states, and here at the Center we can probably share students with the faculty in the Mathematics or Physics Departments. This way we are not officially PhD directors, but morally we are co-directors.

Since CERN has been brought up − During your time at CERN what was the most exciting discovery there, in your opinion, the one that meant the most to you and was special for your work?

In a sense, there are positive and negative discoveries. I think the best discovery of all has been the fact that now we all talk about the standard model of particle physics. Of course, many laboratories, e.g., SLAC, Fermilab, Brookhaven, CERN, etc., have contributed to that. But LEP (Large Electron-Positron Collider) set the standard model on its feet. It allowed for precision measurements, essentially for all aspects, except for the Higgs. The interesting thing is that because it is a machine based on leptons, electrons and positrons, you could actually predict the mass of the top quark, and interestingly enough, that information was passed to Fermilab which eventually made the discovery of the top quark around 1994-95. This information was important because it is a very difficult measurement and there is a large background, so for Fermilab that was a very useful piece of information. And finally, although LEP was made, among other things, to try and discover the Higgs particle, the scalar particle of the standard model, we had to wait until 2012 to have a machine discover it: LHC. So, in a sense I think that the last nearly 20 years of CERN have been to vindicate and to close the standard model. Then, of course, pushing the boundaries for dark matter, pushing the boundaries for supersymmentry, but I think the important legacy for these 20 years has really been to leave the standard model standing on good ground. And to see that happening was very exciting.

Could we talk about your book, “An Invitation to Quantum Field Theory”1? The goal is very well explained — “We have selected representative topics containing some of the more innovative, challenging concepts, presenting them without too many proofs or technical details;” “the book tries to motivate the reader to study QFT, not to provide a thorough presentation;” and you also “focus on some conceptual subtleties,” and you mention realization of symmetries in particle physics. Are you happy with the results in general, and of this particular focus on symmetry?

There are topics in QFT that somehow pass from one book to the other without any critical revision. Then some archetypes, or mantras, are created, and the reader is not asked to think thoroughly about some of those concepts. Unfortunately, this has led to some misconceptions propagating in time. Some of the deeper aspects of QFT concepts, discrete symmetries, e.g. breaking of parity, matter-antimatter asymmetry, CP violation, and CPT, one of the holy grails of the subject, are not explained in enough depth, the approach to them being often more of an engineering approach. We believe it is possible to explain to the reader that the CPT symmetry follows from three simple things, which are properties of our current knowledge: special relativity, quantum mechanics, locality. These three things are enough to prove the CPT and to study its consequences in particle physics. And this is one of the reasons we came up with the book — we don’t have to use the engineering approach with the symmetries. You can understand them at a deeper level from fundamental principles in physics. The interplay between symmetries and conservation laws, the properties of broken symmetries are often presented in ways that we did not find satisfying. We were trying to explain not the standard recipes, but what are the underlying concepts behind them.

Please, tell us a few words about how your article on Gravitational Anomalies, written with Ed Witten, came about, as it made a big impact on string theory.

That was when I was a Junior Fellow at Harvard. You could spend one year wherever you wanted, and Edward was kind enough to invite me to spend a year at Princeton. Edward had just come from a conference, I think it was in Texas, where he had been speaking with Sir Michael Atiyah precisely on this type of issue, so he drew me into this work. Clearly, at the time I was fairly ignorant on anomalies, so it was a great privilege to be working with Edward and to complete this work with him. We showed that the type IIB supergravity in ten dimensions was anomaly free through a rather remarkable cancellation of different contributions. Somehow we did not study other possible cancellations in more detail, and we missed what is called the Green-Schwarz anomaly cancellation mechanism. A real pity. We also worked on different things, like the descent equations and more mathematical formal structures of anomalies, and we missed, or at least I did, the possibility of exploring other anomaly cancellations, the ones that eventually led to the formulation of the heterotic string. Of course, working with Edward Witten is always an awe-provoking experience.

Let us return to Stony Brook — what are your memories from your student years here?

At the time graduate students were living in a place called Stage 12, not what you would call a ghetto, but something different from a normal American campus. It was a very international community. Many people made successful careers, like Ashoke Sen, who came in the same year as I (1978), Sunil Mukhi, Rohini Godbole, Rabindranath Akhoury, Ergin Sezgin… it was serendipitous to have all these people in the same place at the same time, and we really enjoyed being together. It felt like an extended family away from home. We shared life together, not only scientific interests.

At the time I was married, and my first son was born, so as soon as my family joined me I had to leave campus, because children were not allowed on campus housing. With the immense salary we were getting as teaching assistants, we had to share a house with an Italian mathematician and an American physicist. We used to call the place where we lived “Selden Horror.” The house was in really bad shape, and we could pay a good fraction of the rent by remaking the house, so we spent one year doing that, from the plumbing to the roof, new floors, etc. We really re-built the house, and eventually the owner sold it for a very good price. There was also a lot of conflict between different clans, or families in the area, but fortunately, we got along with them well.

For Christmas we would go back to Spain to see the family (of course the family would send us tickets as we couldn’t afford them). Before my trip, I went to one of the clan leaders, who was our neighbor, and I asked him if he could keep our keys and keep an eye on the house while we were away. He was so surprised that someone would put so much trust in him that he was our protector for the rest of our stay. People were very friendly and nobody disturbed us…

How does it feel to be back?

It feels like coming home. Many professors that I worked with as a teaching assistant are still active, and I have plenty of fun memories. I was here at the beginning of my scientific life, then I was away for 30 years, and now, at the dusk of my life, I came back. It feels like opening a new cycle, it feels nice. It is a homecoming in a sense. The place has changed, but not that much. And I’ve also known the people who are at the Center from outside the Center many years ago. Now I also need to get acquainted with the University administration. Something that one usually does not do as a graduate student.

Could you share your vision of the Simons Center’s place on the international map of the similar institutions?

I think that the Simons Center is unique in many respects. Clearly, one aspect is service to the community (programs, workshops), and that works extremely well. It is very well-financed and very well-organized. The staff of the Center are great. They are a small team of people who are very efficient and very motivated. This service component has been very successful, and will continue to thrive in the future. I believe we need to strengthen the research that is produced at the Center itself. We want to have it not only as a great place for meetings, workshops, and discussions, but also as one of the top centers for high-quality research. So far we’ve had great people hired, hence the initial conditions are excellent. But, given that we are a small group, it is important to share common goals with the Math Department and the Physics Department, especially the YITP. I think it is important to see how we can all collaborate, like a symbiosis. How we can use the Center’s resources and facilities to help in the common plan. There are various ways to go about it, and I have a number of ideas on how to proceed, but we will see. Last May I presented some of these ideas at the Board of Trustees, and their reaction was quite positive. So, I think it’s possible to implement them and take the Simons Center to the next level. I think we’ve reached the cruising altitude, but now we have to determine the height.

I love the metaphors you use — is literature important to you, the written word?

Very important. Literature has always been my passion. You can’t work, or at least I can’t, only on physics, because you become less efficient. It is always good to engage in creative or artistic activity that allow you to relax and return to your research problems with renewed energy. And for me these have always been literature and music.People who do research are very obsessive, almost neurotically so. And you need some strong attraction to remove your attention from what you’ve been working on for weeks, months, maybe years. For me literature has such power of attraction, along with music. Some of my favorites are Russians, like Dostoyevsky, Tolstoy, Lermontov, Pushkin, and Bulgakov and Andreyev, among the more modern. From the Spanish I would name the classics, and more modern ones, such as Borges, Cort.zar, Garc.a M.rquez, Vargas Llosa. My all-time French favorite is Balzac. I also enjoy reading essays, biographies, and history books.

Do you prefer an e-copy or a paper copy?

The good thing about e-copy is that I can travel with it. But I like to touch the book. It’s a good feeling to have it in your hands and to be able to browse it. So I buy both, hard copy and e-copy. The physical feeling of satisfaction of touching a book can’t be obtained by electronic media yet, but of course it allows you to carry thousands of books around in a very small device. Or when you can’t sleep, you can read without disturbing your partner.

I also know that you play piano?

I do my best. I started playing music, or shall I say learning music when I was 40. I didn’t know how to read music before, so I do my best, but age does not help…

This is very unusual and shows very broad talent.

Perhaps more obsession than talent. When I am at home we always have classical music playing. There are pieces that you know essentially by heart, note by note, and to play them in the background while you work is very soothing, it helps me concentrate. When I am little distracted I can latch on to the music and I know how it unfolds, it allows me to focus. But after listening to music so much I developed this yearning to play it and I said “before I am 40 I have to start,” as after 40 your brain begins to lose plasticity at a higher rate. I started to learn music theory, then took playing lessons, and eventually a wonderful professional pianist in the Geneva Conservatory decided to take me as a student. She found it amusing that this old guy had so much interest. I was one of the few non -professional students she had. I took a 90-minute class weekly, and practiced at home for an hour every day. My aim was to play Beethoven sonatas, and I am getting there. Some are already accessible, and this gives me immense pleasure. I have tried pieces of varying difficulty from mostly classical authors. I haven’t gone into jazz, I am absorbed by music up to the beginning of the 20th century.

I have to look for a teacher now, and of course I would love to have access to a piano. Being able to play music is very important for me, as I will be alone a lot. Cinzia [DaVia] is going to be at the Physics Department 50% of the time, starting January 2017. Until then she will be going back and forth from the UK. She will be based in Manchester where she is a professor of Physics. So, playing music will keep me company.

What upcoming programs at the Center are you especially looking forward to?

I am interested in the program on Entanglement. I’ll have to see what happens with Mathematics of Gauge Fields, since most organizers are mathematicians… I fear the worst (laughs). They may not be discussing the aspects of gauge theory that I am more interested in, their quantum properties. I am looking forward to a number of them, like the one on Turbulent and Laminar Flows, certainly looking forward to that. The programs are great, as for the workshops, I will attend most of them, time permitting.

You have already become an organizer of one — on gravitational waves.

I am organizing the Universe through Gravitational Waves with an expert on numerical relativity, Vitor Cardoso, and one of our local experts on gravity and cosmology, Marilena Loverde. I think it will be good, not just for the mathematics, but all aspects on how to detect them, what can we learn, and there are things about the sources of gravitational waves that are going to surprise us for decades. In fact, in different ranges gravitational waves will open windows to the universe that are not accessible otherwise, e.g. the Big Bang. We could even nearly see how the Universe begun. String Theory and Scattering Amplitudes are organized by two of my former graduate students at CERN, Katrin and Melanie Becker, so it will be fun to collaborate with them again. I will be involved, at least initially, in both of them, and then depending on the time available, I will see how involved I can be. I think it is important to go to the opening talk. And I want to make sure that every time there is a workshop, there is also a colloquium, accessible to Physics and Math Departments, which should highlight the reasons why this workshop is taking place, so that not only 30 specialists, but the Stony Brook community at large can appreciate it.

What is your main acknowledgement of your predecessor, John Morgan and the work that he did?

John started the Center, under his mandate very good people were hired, and some of them are still here. All are accomplished and world famous. So, to get it started and to bring it to this level, is already a great achievement.

September 6, 2016