IPMU Interview with David J. Gross Interviewer: Hirosi Ooguri

Elementary particle physics interested in science in general, was a goldmine in the 1960’s in particular in such an esoteric but theorists were powerless subject as elementary particle Ooguri: You have established physics. When did you decide the current paradigm of you want to be a physicist? elementary particle physics Gross: Long before I knew by discovering asymptotic what it really meant. I decided freedom, for which you were I wanted to be a theoretical awarded the Nobel Prize, physicist roughly at the age of and you have also made 13 or 14. great contributions to more Ooguri: That is pretty early. Not prospective areas of particle many people at that age know physics such as string theory. such a subject even exists. In addition to your scientišc Gross: I did not really know achievements, over the past what it meant to be a 10 years or so as the Director theoretical physicist, but I was of KITP, you have transformed inspired mostly by reading this place into the center of popular science books, such as theoretical physics in the world. the ones by George Gamov. So, we have many things to What excited me was that you learn from you, especially as could šgure out how the real we try to establish this new world works and solve the institute in Japan. It is an honor puzzles of the universe just to talk to you today. using your mind. That seemed I would like to start out by so exciting that I decided to asking when you became become a theorist and try to calculate the properties of the David J. Gross is Frederick W. Gluck world. I was very lucky since so Professor of Theoretical Physics many people are unsure exactly and Director of Kavli Institute for Theoretical Physics (KITP) at the what they want to do until University of California, Santa later in life. Barbara. He was awarded the 2004 Nobel Prize in Physics with David Ooguri: Then, you went to Politzer and Frank Wilczek for“ the Berkeley as a graduate student, discovery of asymptotic freedom in the theory of the strong interaction.” which at that time was“ the Among many other distinguished place” in particle physics. awards, he received the Dirac Medal in 1988 and France’s highest Gross: There were some great scientišc honor, the Grande Médaille theorists in Berkeley at that D'Or from the French Academy of Sciences in 2004. time, and it was certainly the

16 IPMU News No. 8 January 2010 IPMU Interview center of the experimental fashioned way of teaching. We particle physics. At that time, teach our students the same with David J. Gross elementary particle physics was way a master artist will teach a true goldmine, a host of new an apprentice by bringing Interviewer: Hirosi Ooguri particles were being him into his workshop and discovered every month and having him participate in his it was not hard to discover creation of works of art. Not new particles and new all students are able to engage phenomena. It was a very in research immediately, but exciting time experimentally, the best students from a and experimentalists were place like Princeton or other the masters of the šeld. The great research universities are theorists were pretty powerless. certainly able to start doing Ooguri: But your enthusiasm science from an early stage. towards theoretical physics was Ooguri: When you moved from not diminished. Berkeley to the East Gross: No, because there were Coast, you also so many problems. It was clear changed the that almost everything was not direction of understood, and the little your research. understanding one had seemed Gross: Berkeley ad hoc and paradoxical. It was was dominated by my exciting that constantly new advisor, Geoffrey Chew, things were discovered that who had this idea of the changed the way people looked bootstrap – a theory without at elementary particle physics. a theory. This was a very Ooguri: After graduating from anti-“šeld theory” approach, Berkeley, you went to Harvard which said that šelds cannot and then to Princeton, where be measured, are unphysical, you had great success with and one should not construct graduate students also. a dynamical theory in terms Gross: Well, it is easy to have of unobservable šelds. Rather great success with graduate one should only postulate the students when you are at a general principles that constrain place which has many great the S-matrix, which was students. Frank Wilczek (Nobel observable. The hypothesis was Prize winner) was my šrst that there was only one unique graduate student, and I think S-Matrix consistent with these Ed Witten (Fields medalist) must general principles. I got quite have been my third or fourth. I tired of this approach even Interview thought that was the norm. before I left Berkeley because One of the interesting things you could not do very much about science and mathematics with it. Moving to the East is that we still have a very old- coast was good because there šeld theory was still tolerated. Hirosi Ooguri is a principal investigator of IPMU. He is also Ooguri: But, even in the East, Fred Kavli Professor of Theoretical šeld theory was not yet the Physics at the California Institute of Technology. mainstream. there was no quantum šeld Gross: In 1968 when string Planned to prove field theory I was ready when the useless, but discovered a theory that would describe theory was born, that was superstring revolution theory that works asymptotic scaling. a period where I was just happened in 1984 Gross: It was certainly not Ooguri: So, you set out to beginning to think about Gross: Well, to some extent, the mainstream, largely prove that šeld theory was strong interactions and short but not totally. Remember because of its impotence. It useless and then instead you distances and deep inelastic that during those years there is essential for physicists to discovered a theory that works. scattering. At that time I was was a lot of development be able to calculate, to probe Gross: There were really three convinced that what one of supersymmetric theory, the limits of their theories parts of the program. The šrst really needed for to describe which came out of string and to make predictions that was to show that you needed the strong interaction was theory originally. Everyone could falsify or conšrm their asymptotic freedom to get the something totally revolutionary. in the šeld was interested in ideas. Field theory at that time observed scaling. The next thing String theory was directly along supersymmetric theories by that was quite insufšcient for the was to prove that there were that line of thought. time. At Princeton, Ed Witten strong interactions, since only no asymptotically free šeld I got involved in string theory and I always had a continued perturbative techniques theories, which with Coleman quite early, but I also realized interest in string theory. John – Feynman diagrams – were I did, with the exception of that this was not going to Schwartz used to visit Princeton available for calculation. non-Abelian gauge theories. explain hadrons. I was focused once or twice a year because Ooguri: Then you discovered The last part of the program, on trying to understand what his mother lived there, and he asymptotic freedom and which I did with Frank Wilczek, was going on in short distances would always come and tell us changed the people’s was to look at non-Abelian inside the proton. One of the what was happening or give a perception about the gauge theories, which much nicest features of string theory seminar. usefulness of quantum šeld to my surprise turned out to was the soft interactions, but So I was ready when theory as the language of be asymptotically free. It was those gave rise to very strong the superstring revolution elementary particle physics. almost one, two, three, QCD. falloff at large momentum, happened a year later, when Gross: The phenomenon of There was no choice. If you which was very different the Green-Schwarz anomaly asymptotic freedom was the wanted to explain the scaling, than what was seen in the cancellation was discovered. answer to the search for a you had to have a non-Abelian experiment – exponential as Suddenly, a lot of interest was theory that could explain why gauge theory. opposed to power falloff. So, generated. the strong interactions seemed Ooguri: After that, most of the string theory was not a good Ooguri: That led to your to behave as if they were free community moved to quantum place to try to understand the construction of the heterotic at short distances. It led to QCD, šeld theory. simple scaling behavior at short string theory. the theory of the strong force. Gross: Well, because you could distances, and I stopped doing Gross: Well, it was the But more generally, having a calculate and furthermore, string theory at that point. But unexpected answer to an theory that was totally well even better, the calculations I continued to follow it even obvious question – how to behaved and under control in worked. And then there were through its darkest days. It was realize E8 x E8. And the answer the ultraviolet gave enormous some spectacular experimental always fascinating. was not so hard, once one calculation ability and resolved conšrmations over the years. In 1983, I went off for a realizes that one could treat a lot of the lingering doubts But for me the major problem sabbatical to Paris and decided right and left moving waves about quantum šeld due to its was not so much continuing that this was a good time to on the string differently. It was ultraviolet singularities. with calculating tests of get back and learn more about a very exciting time because People’s attitude totally asymptotic freedom or QCD, string theory. suddenly all of these beautiful changed, including my attitude, but rather understanding Ooguri: Did you smell anything ideas fed together and you because I had been convinced conšnement, which turned out when you decided in 1983 that could have a reasonable unišed that šeld theory was not to be a lot harder. you want to get back and study gauge group and chiral matter. going to provide the answer Ooguri: Fast-forwarding to string theory? That sounds Ooguri: I remember I was a for a theory of the strong the mid 80’s, you went back like such a good timing, just graduate student when this interactions. In fact, my original to string theory again but in a a year before the superstring happened. One of the things research plan was to prove that different context. revolution. that impressed me was the fact

18 IPMU News No. 8 January 2010 that the families of elementary physics goal as well as the can arise. For example, we are right questions and the right particles emerge very naturally newer cosmology goal. planning a focus week where atmosphere because in the from the geometric structure of Ooguri: Some of the future statisticians and astrophysicists end scientists are focused on a Calabi-Yau manifold. experiments in cosmology will come together to try the question, on the science. Gross: Most of the previous and astrophysics may to develop new statistical It is necessary to establish an explanations for the number of have direct relevance to methods. That can point atmosphere in which people families, or for the hierarchies the unišcation goal. For toward a new interdisciplinary are willing to ask stupid of Yukawa couplings, or for the example, measurements of way for mathematicians and questions, explain obvious chirality of fermionic matter, the polarization of CMB or experimenters to collaborate. things, take the time and all of these were ad hoc or gravitational waves from the Gross: I think one of the most approach science as an exciting based on rather uninteresting inœation era could teach us important roles that an institute adventure together. symmetries. For the explanation about Planck scale physics. like the KITP or IPMU can play Ooguri: These are great lessons to emerge from geometry was is to bring in together scientists for us at IPMU. incredibly beautiful. Right problem and right from different communities to Gross: I think you have started people needed for Ooguri: Since then, string successful interdisciplinary collaborate with each other off very well. During the theory not only made progress collaborations on common problems. That summer, I met some of your toward the unišcation of Gross: We have so few handles is one thing that universities young postdocs, and I heard elementary particles, but its on Planck scale physics that any do not do very well. They very good things about how connections with many other conceivable way of learning create departments which are your institute is going. That is areas of physics have also about what happened at closed and largely ignore other the best review you can get. It emerged. For example, right very short times or length departments. Some of that is an excellent effort. now at KITP, you are having scales must be followed up. is for good reasons, because Ooguri: It is time for my last the workshop connecting The dreams of astrophysicists people have to concentrate question. Do you have any string theory to condensed and cosmologists and and focus in order to push message to our friends in Japan? matter physics. Condensed of experimental particle science forward. But institutes Gross: I am delighted with matter physicists attending the physicists are really heroic. like ours have the ability and this new institute and workshop seem very excited What I am impressed is just the responsibility to try to everything I have seen so far. about this development. the unbelievable difšculty of encourage interdisciplinary I am impressed with the fact Gross: String theory has turned doing these experiments or efforts. that you have brought all out to be intimately related to observations. We need such Ooguri: KITP, especially under these groups together, that quantum šeld theory, which is heroic efforts since it really is your leadership, became a range from mathematics to the language not just of particle essential for our šeld to get role model of interdisciplinary experimental cosmology. This physics but also of quantum some clues, not just from the collaborations. What do you is a remarkable collection many body theory, which is of beauty and power of our think you did particularly right? of entities and remarkable great interest to condensed mathematical descriptions that Gross: There are a few collection of people involved. matter theorists. String theory we are perfecting, but also important points. One is to I think it is really a very is rich and big and contains a from nature itself. Your identify the right problem. You wise move of the Japanese lot within it. institute includes all of these cannot force people to get Government to support this One of the great attempts – experimental as interested in other people’s high quality science at the developments in the last well as theoretical. problems. They have to be level of support that is really Interview few years has been the Ooguri: Yes, it is inspiring to interested in the problem necessary to make something realization of that old dream meet and talk to people who themselves. happen. The enthusiasm that of understanding the strong are working on experimental You also need the right you all have translates into a interactions in terms of string projects such as detection of people. You need the lot of work and effort. I have a theory. So, the circle closes. dark matter in the universe. willingness to explain to other feeling it will pay off, and it will But still my ultimate goal is It gives us a focus on what scientists in ways that they contribute greatly to science. the unišcation of the forces, theorists should be doing. can understand. All of that So, you are off to a great start. namely the ultimate particle Sometime, a new connection is possible if you have the

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