Interview with Gerard ’t Hooft Interviewer: Shigeki Sugimoto Physics Had Been Primary than learning to read and Interest since Very Early write. In Europe, in general, Age when you’re 6 years old Sugimoto: Thank you very you learn how to read and much for letting me have write at least the basic stuff. this opportunity to talk to Before that, I could not read you today. There are a lot of or write but I could add and things that I wanted to ask subtract, and I knew that I you. was interested in that. ’t Hooft: Yes. Sugimoto: In mathematics? Sugimoto: First, I would like ’t Hooft: In mathematics and to ask you when and how you physics. Physics was running got interested in science. in my family to some extent. ’t Hooft: I think that I was My uncle was a theoretical somewhat exceptional. physicist. He was well known When I was still very young, in his eld. My grand uncle maybe even at nursery was Frits Zernike, who also school, I really knew that I had a Nobel Prize in physics. was going to be interested I was very much inspired by in the world of nature. I that and from an early age had much more difculty in on I knew I was interested in understanding humans than physics. nature. I also found adding Sugimoto: I see. When did and subtracting much easier you decide to be a physicist? ’t Hooft: Well, I don’t know when I really decided to be a Gerard ’t Hooft is Distinguished physical scientist but physics University Professor at Utrecht University (since July 2011). He shared had been always my primary the 1999 Nobel Prize in Physics interest. Maybe 9, 10, or 11 with Martinus J. G. Veltman“ for elucidating the quantum structure years old, I knew that I was of electroweak interactions.” He going to be a physicist. has also received many other distinguished awards including the Sugimoto: Then, you Dannie Heineman Prize in 1979, became a Ph.D. student of Wolf Prize in Physics in 1981, Lorentz Medal in 1986, Franklin Medal in Veltman and soon after 1995, Oskar Klein Medal in 1999, that you started to work and Lomonosov Gold Medal in 2010. He received his Ph.D. from Utrecht on the renormalizability of University in 1972. In 1977, he Yang-Mills theories. Is that became a full professor at Utrecht University. right? 14 Kavli IPMU News No. 30 June 2015 Started to Work on the to replace it with something Renormalizability of better, something where you Yang-Mills Theories don’t have to renormalize ’t Hooft: Yes. Veltman was away innities. There were working on the problem of many electroweak theories. how to renormalize Yang- One approach was called the Mills theories and he had scattering matrix approach, developed some very good, another was called current sound techniques which algebra; there were all sorts fascinated me. But he said, of algebraic ideas about how “This is very difcult” and to understand elementary “It may be better for you to particles, but eld theory was work on something else.” not at all popular in those But my reaction was“, Well, I days. like the problem that you are Sugimoto: I see. What was working on very much. I want the reason that Veltman and to understand more of that.” you believed in gauge theory? From the beginning I said“ I ’t Hooft: Veltman was very understand your difculty, so pragmatic. He understood I want to see what I can do there was a basic problem about it.” in understanding the weak Sugimoto: I heard that many interactions. He learned people were skeptical about about the experimental gauge theory at that time. observations. Experimentally, ’t Hooft: At that time, yes. a lot was already known It is a bit difcult to say about the weak interactions how the history developed and about other properties of because now people are very particles and forces. Also, their much tempted to say that symmetry structure was very there was such a thing as well understood since Gell- the electroweak theory and Mann’s group theory became the only question was how a well-known topic. It was to renormalize it. But that is understood how important Interview not how they looked at their problems at that time. I mean Shigeki Sugimoto is Professor at the majority of physicists did the Yukawa Institute for Theoretical not want to think in terms Physics, Kyoto University. He is also Visiting Senior Scientist at the Kavli of eld theory. They wanted IPMU. 15 group theory, Lie groups in to do it. completed the proof of Found the Way to Cancel particular, is for physics. That Sugimoto: I see. renormalizability? All the Anomalies was clear but how to get the ’t Hooft: He had all the ’t Hooft: I rst thought, ’t Hooft: I realized,“ Well, exact description of these ingredients but there was “I will need to convince now I understand exactly particles was a big mystery. still no answer about how people that this is the way how to do it and I have Now, Veltman even did not to renormalize this theory to do things,” because there to ll in some details,” care pretty much about properly. At that time this was was a sentiment against but those were secondary eld theory although he thought as just one possible renormalization. So I realized details. The most important liked the general formalism, approach to physics and it that people were going to detail was the anomalies. It but he just repeated what was not expected that this criticize whatever I had done wasn’t obvious that if you everybody else said that eld was going to be the way to and Veltman had the same renormalize this diagram theory is probably not going understand all the forces in response as well“. Maybe you using this counter term, to be the answer. But in the nature. have something interesting and that diagram using that meantime there was nothing Sugimoto: Were there many here, but people will ask this counter term, if you combine better to do. He thought people trying to prove the and that. Do you have your the whole thing it will still that eld theory might not renormalizability at that answers ready?” I realized be unitary; and indeed a be the answer. But it was time? that people were going to ask counterexample was known. the thing I understood that I ’t Hooft: No, there were not quite a lot of questions which There were examples of could do in principle, except so many people who were I could not answer. This is a theories where this would we did not understand the studying renormalizability. very mathematical problem. break down; that was the details. We basically did not There was Abdus Salam, Mathematicians are very case when you have chiral understand how to handle there was Steven Weinberg, accurate and I was somewhat symmetry; left and right the renormalization effect of but they were asking more sloppy in the way of phrasing particles are different. particles with the exception of generic questions like“ How things. He said“, This is where Sugimoto: The chiral the photon. The photon was in the world can these things you have to be m ore precise. fermion. understood; actually the best- hang together?”“ How do we Otherwise nobody will believe ’t Hooft: The chiral fermion understood particle. describe these forces?”“ How you.” has anomalies in it. Those Sugimoto: Right. can we understand what the Sugimoto: Were you excited anomalies would be ’t Hooft: That was a vector next particle will be that is about this? disastrous. Now not every particle, so why was it so going to be discovered?” and ’t Hooft: Yes I was very theory has such anomalies, at difcult to renormalize other so on. But, renormalization excited because this was least not that we knew. But vector particles? The problem was not very fashionable. certainly the moment when still there was this danger; was in the masses of these Sugimoto: Did you think I realized the importance of maybe there are more such particles. Veltman understood you would succeed when you the Higgs mechanism. I didn’t anomalies. While formulating that the weak interactions started tackling this problem? really call it that because I the rules to renormalize the are caused by vector particles ’t Hooft: Well, as long as I didn’t know the papers by theory, we have to prove that with mass, and he tried to hadn't been able to answer Higgs and Englert very well. everything hangs together understand how to make the main questions I didn’t I had heard that there were without any anomalies a unitary renormalizable know whether I would people thinking along these because if they were there, theory for such particles. He succeed or not, but I was lines. So I accepted that I was we could understand that had discovered many of the very ambitious and I knew not the rst to write down renormalization would problems that were there. He for sure I wanted to get the these theories, but I did feel destroy unitarity, which had his own approaches to best answers I could nd. So I I was the rst to understand would imply that it would the problems using gauge thought“ If this problem can how exactly the Higgs not really work. invariance and he was be solved at all I will try to mechanism was solving the Sugimoto: Right.