Spotlight

Driven by the Forces: A Conversation with Prof. Sunil Mukhi

Sunil MukhiA particle physicist by training, Sunil Mukhi works on quantum field theory and string theory, areas in physics that have tried to identify and explain the physical forces of nature. Mukhi began his research career as a PhD student at Stony Brook University where he studied supergravity and supersymmetry. After a postdoctoral stint at the International Centre for Theoretical Physics, Italy, he returned to India and joined the Theoretical Physics group at the Tata Institute of Fundamental Research, Mumbai in 1993. He has moved to IISER Pune in 2012 where he continues to explore string theory and its applications. Apart from numerous publications pertaining to his research, Mukhi has authored a book on Mathematical Methods in Physics along with Prof. Mukunda and is working on a book on Particle Physics that draws upon experimentally established facts about elementary particles. Mukhi is a Fellow of the Indian National Science Academy (INSA) and Indian Academy of Sciences, Bangalore. He has earlier received the Bhatnagar Prize and the JC Bose National Fellowship. In October 2014 Mukhi has been named a Fellow of TWAS, The World Academy of Sciences. In this interview, Prof. Mukhi talks with us about his career and the two vital forces of his life: research and music.

SK: Your research work is in the broad area of understanding the fundamental forces of nature. We would like to get you talking on a fundamental question of another kind: why physics? What led you to choose the discipline and the specific areas that you have pursued over the years?

SM: I was always interested in science. In school I was interested equally in chemistry, physics and mathematics, and honestly much less in biology. Somehow living organisms were a little more challenging for me; I found physical objects and chemical reactions interesting and math very interesting. In college, it was a toss-up which of these to do and frankly, I could have gone into any of these fields. Many of my good friends who were doing physics felt that it addresses ultimate questions about what is nature and how nature works. Of course, biology and chemistry deal with nature but physics deals with the underlying laws in some sense and that was nice. Math was even more abstract and for that reason precisely I did not feel like going into it but I always had a great love for mathematics.

It is really in the canteen of St. Xavier’s College (Mumbai) that I got my inspiration to do science: poring over the text books, trying to do problems sitting there with cups of tea and dosas and evaluating integrals. I remember that period very fondly.

SK: Could you tell us about experiences during PhD and postdoctoral training: what did you start working on and how you were led to string theory?

SM: From Xavier’s I went to Stony Brook, U.S.A. to do PhD, at that time you could join graduate school right after a BSc. I was barely 20 and it was hard to cope with initially but managed to keep my head over water. Once I was there, my inspiration to pursue research in Physics became stronger talking to some of the outstanding faculty and other people over there. Shortly after I had joined Daniel Freedman’s group, he moved to MIT and couldn’t take me along. This was probably the single and only unfortunate incident in my career. It could have gone badly, but I later joined another good group, that of George Sterman. I had to switch my research area because of this but was able to eventually get back to my primary area of interest.

During my PhD I was working on supergravity, an approximation to string theory at low energies, which is a forerunner of string theory although at that time I did not know anything about string theory. During this time I learnt techniques which would be very useful later when string theory became a more popular subject, about three years after I completed my PhD.

There is in fact an interesting story on how I began as a graduate student. My first advisor asked three students-one of them was me-who wanted to work with him, to compete by checking a Russian paper to see if the calculations were correct. This paper had invented the concept of supersymmetry, which was a symmetry relating bosons and fermions.  The other two students did not seem to get very far, but I found that the paper was almost correct with some crucial errors. Based on this work, my first paper described a correction to this error and generalized the work in a way that connected supersymmetry to supergravity. Many years later when I looked at my paper, I found to my horror that the Russian scientists were not cited in my paper. I remembered this had to do with the prevailing atmosphere of hostility between the U.S. and Russia at that time and that my advisor probably was not keen on quoting the Russians. I now feel that the earlier paper should have been quoted irrespective of the political situation, although at that time, I was too inexperienced to have insisted on it.

I did a postdoc in International Centre of Theoretical Physics (ICTP) in Trieste, Italy founded by Abdus Salam, a Nobel Laureate who himself had worked extensively on supergravity. I did have some discussions with him considering our overlapping interests, but I started working on a slightly different topic exploring other theoretical aspects of supersymmetry. From there I got a job as a postdoctoral fellow at TIFR, Mumbai. By that time, I was very keen to come home to India to my family. Within two years, I was made a faculty member at TIFR.

SK: What did you start working on at TIFR?

SM: Very soon after I joined the institute, it became clear that string theory was suddenly exploding as a very popular field because of some developments that took place during that year, 1984: a dramatic paper from scientists in the U.S. and the U.K. and also a lot of advertisement for the string theory by Edward Witten, who had done extensive work on string theory and is considered one of the most brilliant physicists in the world. There was a sense that string theory provided a new way to understand two fundamental problems: how to understand gravity as a quantum theory which was previously understood only as a classical theory and how to unify different forces in nature. For me these questions were very attractive.

The second paper of my PhD suddenly became useful as a tool to study string theory although it was not written with that motivation. It was a solid 50 page paper that I took more than a year to write, I was thrilled that this work (Annals of Physics 1981) had suddenly become central in string theory. With close to 500 citations, this is my so far most cited paper. My work at TIFR, done mostly alone and with a few PhD students, visited topics from my postdoc and PhD days along with some new ones.

There was a lot of excitement about string theory, there were a lot of discussions and seminars at TIFR along with a couple of other colleagues working in similar area. The first meeting on string theory in India was held in 1986 in IIT Kanpur.

In 1984, the key realization was that string theory could also account for violation of parity, a bizarre phenomenon in our world. Left/right symmetry is known to be violated in nature in the weak interactions (discovered in late 1950’s to 1960’s by Yang and Lee for which they got a Nobel Prize). It was thought previously that gravitational attempts to understand all forces/interactions would not give a parity violating theory; but in 1984 the breakthrough showed that it is possible to get parity violation through string theory and that was very striking. That was the final nail in the coffin which convinced people that string theory was worth working on.

SK: Being an important route to reconcile the areas of general relativity and quantum mechanics, string theory has caught the imagination and excitement of several scientists. However, there are a sizeable number of skeptics who are unsure if the theory is testable and scientific. What is your take on such criticism?

SM: Many were indifferent to the area initially; the real skeptics came because the subject grew to such an extent that it started to gain popularity over other subjects of physics. And this made people not working in this area rather furious. I can understand now, especially on hindsight, because after all string theory was neither an experimental science nor a theoretical science that explained an experimental observation. It was rather an overarching framework which potentially could explain experiments; it could explain broad facts about our universe but did not so far produce any hard numbers despite a lot of work. There was a backlash when people felt that it was all empty talk. I feel that it is not correct for people to assume this; the theory is very much scientific but it has remained in the sphere of a potentially useful subject rather than actually resulting in a detailed understanding of the real world. The expression “real world” is commonly used in string theory as the theory addresses many possible worlds. Some physicists will say that there is no point studying any world other than the real world. But there is value to doing so as one can make hypotheses about the real world based on understanding of a simpler ideal world. You can also learn new mathematics by doing this. So it’s a combination of many things that string theory has achieved but not real hard numbers that can be compared with experiment.

SK: Could you describe to us the spread of the kind of research questions you have been addressing over the years?

SM: For the first 10 years after I began to work as an independent investigator, I was focusing on questions some of which came from my PhD, later I also drifted into the study of string theory in very low dimensional space, a beautiful 2-dimensional toy world in which string theory could propagate and where the calculations become more tractable. In 1995 my work took a major new direction when a concept of duality entered string theory. Although the concept was around for several years, it was largely brought to the forefront through the work of Ashoke Sen, a friend and colleague since my days at Stony Brook. He showed dramatic results in 1994 about electric-magnetic duality. This became a very exciting subject around 1994-1995.

Working with a new PhD student (now a faculty member at McGill University, Canada), I stumbled on a new duality relation which was not known in string theory and we published that in December of 1995. It was a great moment for both of us as a day after we put it up on the internet, we got an email from Witten saying that he likes our paper and that he was working on the same topic and we got there first! And then, being the gentleman he is, he said that he would like to put out his version of the ideas and sent a draft of his paper for our comments.

We continued to work on duality in string theory and each one of the papers that came out of our work in the following years has been highly cited. In the late 90s-early 2000, I was working on non-commutative geometry, an apparently outlandish idea although realized in magnetic systems in nature.

Within string theory, every five years or so, I have changed my subject completely. For example, I’m currently working on the theoretical nature of black holes, a very new area for me. I like to acquire new expertise. Often when things are going well and I have a run of good number of papers, I move on to a newer area. It is somewhat scary as one cannot be assured of success in each new area but it is exciting at the same time.

SK: Inspiring teachers could influence and determine one’s choice of a career. Did you come by such teachers?

SM: I did know a few good teachers, the one who inspired me the most was in my 9th or 10th class in school. He taught modern mathematics and in those days the syllabus was undergoing revision and several of us got to choose mathematics with two components, elementary and advanced. It so turned out that there was no syllabus and no board exam for advanced mathematics. We had a gifted teacher who taught us, concepts that we found strange and new and we had one year of fun. It was almost like free play, some of our friends were in fact playing outside, about ten of us were inside the class but the mental play was as much fun. And we really learnt a lot. He was a truly inspiring teacher. A few teachers in Stony Brook were inspiring too; in particular, C.N. Yang, a Nobel Laureate who discovered parity violation and proposed the Yang-Mills theory, which is the basis of all particle physics today. He had taught us a course on group theory which was very inspiring.

SK: What is your approach on mentoring, how do you find managing a research group?

SM: Managing a research group comes with its set of challenges. I learnt through my mistakes, in fact I’m still learning through them. My students often influence what I work on because I ask them to read and think and discuss ideas. I generally do not hand out problems to study, always try to generate the problem with the student. That often doesn’t work if the student is not willing to experiment. In terms of graduate students, I have been successful with relatively few, but those few have done well because they got the training which helped them to think on their own. I feel happy about this.

I personally grew up becoming independent early on, partly because of this incident of my advisor leaving Stony Brook in the middle of my PhD and me having had to change hands. Many people continue to get guidance from their advisor long after their PhD, they never completely break free. While this is a convenient arrangement, I feel happy to have been thrown out of the orbit of an advisor, it made me more independent earlier. It’s an uphill struggle though.

SK: You write a blog, you are a strong believer of music: you have a wide range of interests outside of physics and research. Would you please talk about some of these interests?

SM: Most scientists I know are single minded and focused on their work. For me, I need to switch myself off every now and then and music is a way to do that. I really, really love music. All my life I have nurtured an interest in Indian classical music but I also have a parallel interest in rock and jazz music. I have been learning to play the guitar and more recently drums as well. I have heard a lot of music over the years and even though I may be unfamiliar with playing an instrument, I know how they ought to sound, so the prospect of learning to play an instrument is very exciting for me.

My biggest hero in music is Pandit Kumar Gandharva about whom I have written on my blog. A new book is soon to be launched about him in which I’ve written a chapter having been invited by his family to write. It is thrilling to be writing about Kumar Gandharva alongside real music experts who have contributed to the book.

Prof. Mukhi’s home page is a comprehensive resource on string theory for physicists and non-physicists alike. His blog, tantu-jaal, features his interests and ideas outside research including music and literature.

– In conversation with Shanti Kalipatnapu

(Featured Image Photo Courtesy: Mimamsa Team, IISER Pune)

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