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Interview of Michael Creutz by David Zierler on June 3, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with Michael Creutz, Senior Physicist Emeritus at Brookhaven National Laboratory. Creutz surveys where lattice gauge theory is “stuck” and where there are promises for breakthroughs in the field. He recounts his birthplace in Los Alamos, where his father was a physicist, and his upbringing in Pittsburgh and then San Diego. Creutz describes his undergraduate education at Caltech and his graduate research at Stanford, where Sid Drell supervised his work on deep inelastic scattering. He explains his decision to take a postdoctoral position at the University of Maryland, and he discusses becoming involved in lattice gauge theory following his exposure to Ken Wilson’s work on renormalization. Creutz describes Brookhaven’s focus on proton scattering when he joined the Lab, and he explains his work during the discovery of the J/psi. He explains his motivation for writing a textbook on lattices, and the value of ever-more powerful computers for lattice gauge research. Creutz explains his “controversial” approach to staggered fermions, and his work on topology in lattice theory. At the end of the interview, Creutz discusses his current interests in chiral symmetry, he reflects on the burst of intellectual activity at the dawn of lattice gauge theory, and he explains why parity violation in neutrinos continues to confound theorists.
OK. This is David Zierler, oral historian for the American Institute of Physics. It is June 3rd, 2021. I’m delighted to be here with Dr. Michael Creutz. Mike, it’s great to see you. Thank you for joining me today.
Well, thank you for inviting me.
Mike, to start would you please tell me your title and institutional affiliation?
OK. I am a senior physicist emeritus at Brookhaven National Laboratory.
Now what does the title in a National Laboratory setting, senior physicist, connote?
Well, physicists can be tenured. Senior physicist is when you’ve been around a long time and they want to make you a little more special. [laugh]
When did you go emeritus?
January of ’14.
Now, as I well know, physicists never actually retire.
How have you remained connected with Brookhaven since you went emeritus?
We have a weekly group meeting which we do by Zoom nowadays. And I go to that. I haven’t been publishing very much. I’ve done a few things since then. And given a few seminars. But it’s slowed down and I’m doing a lot of traveling on my own.
And just for a snapshot in time circa June 2021. What are you personally working on? And more broadly in the field, what’s interesting to you right now?
Well, because I’ve been doing lattice gauge for many, many years, I’m going to be lecturing at the Institute of Nuclear Physics. They’re going to have a summer school on the lattice. So, right now that’s what I’m working on. Preparing some lectures for that. I’ll be doing four lectures there. Other than that, my main interest in recent years has been why do chiral symmetry and the lattice conflict with each other in many ways? And I think it’s teaching us a lot very deep. Some of it’s turned out to be rather controversial, but I’m still playing with that.
I’ve been having a lot of fun these past few weeks asking a purely speculative question. And that is feelings about the G-2 muon anomaly experiment that’s causing so much excitement at Fermilab right now. What’s your sense and given your long tenure at Brookhaven and the original experiment there, what might that tell us about where things are headed with the Standard Model?
It’s tricky. I mean, it’s not very many standard deviations. So, it could just be a fluke. I was very critical of one recent calculation. They’re trying to reduce the error a lot and I think they’ve overdone it. But what it means, we don’t know. I mean, there are things we don’t know out there, so whether this is connected to any of the other things like dark matter or whatever, we don’t know. [laugh]
More broadly, Mike, in what ways right now is lattice gauge theory stuck? And in what ways is it really on the cusp of bigger and better things?
Well, course, computers keep getting better and better and better, so people can calculate things more and more accurately. And I think it’s a little discouraging that there hasn’t been any great breakthroughs of any kind. It’s just gradual progress. But it’s progressing. And we still have our annual meeting; well, the annual lattice conference has had some problems because of COVID. But hopefully that will start again next year. I believe it’s to be held in Bonn.
What is a breakthrough in lattice gauge theory? What would it look like and how would you know when you see it?
What would be a breakthrough? Well, one breakthrough I’d love to see is a much better way to treat the quarks. The fermions. What we’re doing right now, it kinda works. But it’s very, very sluggish and if we didn’t have any fermions it would be piece of cake. So, that’s of course, something that I’ve been saying for probably 25 years. That hasn’t changed. The other issue which has bothered me for a long time is that in the Standard Model there’s parity violation in the weak interactions. And the neutrinos are one-handed. And we do not know how to do that on the lattice. There are various ways being explored, but none have really been proven to be right. So, that’s probably the biggest unsolved problem there.
Lattice gauge theory, of course, is a community as well as it is a subfield in physics.
Where do you see the community of lattice gauge theory right now both in institutional terms, in terms of where the most exciting work is taking place, and in interpersonal terms in terms of your own collaborators?
Well, I tend to work by myself most of the time except you know, occasionally somebody will come work with me. I think it’s pretty stable in the size of the community. I mean the annual meeting has always been running 300-400 people. It’s distributed amongst some very strong groups. I mean, Brookhaven has its group. There’s JLab group and the Fermilab group. And then in Europe, BMW, and around DESY and Mainz there are very strong groups. These days with computer communication you can work on it anywhere.
Mike, a very much a road not travelled question. Next year is going to be your 50th anniversary at Brookhaven. I wonder if you’ve reflected—
[laugh] I guess that’s true, because I came in ’72, so.
That’s right. That’s right. [laugh] I wonder if you can reflect on what being in that environment has done for your work as opposed to if you had taken a more traditional route in an academic environment at a university as a professor?
I often wonder that, actually. I’d like to teach more although there’s a lot of work involved in teaching. But I have over the years done a lot of lectures at schools and things and lectured a lot of places. The lab has been very good about letting me do what I want. They’ve never really directed me to work on any particular thing. So, it’s a good environment for that. They leave people alone as long as you’re doing something. [laugh]
What about more broadly, the intellectual environment of being close to cutting edge experiments? Is that useful for your research at all?
It’s certainly very nice. And how useful, [that] can depend. My work has always been a little bit away from that. But to have the stimulating environment is important. I mean, I got my PhD at SLAC which was a very exciting place. And then, of course, we had the J/psi at the lab and all that. So, actually that period in the 70s, from an experimental point-of-view, it was absolutely spectacular for me. It really revitalized the field. It’s been going much slower since then. Nowadays there are a lot of people searching for weird, exotic things and never finding them. Well, that’s a little tricky. The G-2 experiment is, of course, very interesting because if it really is true, then there’s something missing in our understanding.
What experiments over the course of your career really have moved the ball forward in terms of the theory?
I think it was the discovery of the J/psi. I mean, that really convinced us that quarks were real.
It really changed the field tremendously. And it was very important to the development of QCD. I mean, before that there were speculations about it. I guess QCD technically started about a year or so before that. But that was really the biggest discovery of my career.
1974 is a long time ago. That’s a long time to wait for another experiment of that magnitude.
Well, of course, the other thing is the discovery of the three neutrinos and the fact that they have masses now.
Do you feel like we’re closer to pushing beyond the Standard Model at this point more than any other in your career?
I’m pretty conservative. I like the Standard Model. [laugh] So, you know, I think there’s a lot of speculation out there which is not very motivated. I never liked supersymmetry. It just didn’t seem natural to me. I mean, fermions and bosons are different. [laugh] And there’s absolutely no experimental evidence for it. I’ve never been happy with string theory either because ordinary renormalizable quantum field theory seems adequate to describe everything we have. So, you know, it’s fun to listen to these people from time to time, but I’ve never really quite wanted to switch to string theory. [laugh]
Mike, is that to say that you like string theory so much that it might not need to be moved beyond?
I think it’s going to fade away. I mean, there are some really marvelous things about 2-dimensional field theory, which is what string theory is kind of based on. But really you need to prove a lot there. But I think it’s going fade away. [laugh]
Not just be improved upon, you mean?
It’s got to make some predictions. There are very smart people working on it and I am content that they’re doing it. But a lot of it’s really mathematical.
Well, Mike, let’s go 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.
Oh. Yeah. My dad was at Los Alamos. I was the first non-Indian born in Los Alamos. [laugh]
And so, of course, I don’t remember those years. Although I’ve visited Los Alamos since and it’s a beautiful place. And so, my dad’s been a physicist all along. And so, that was kinda natural for me to just go into physics.
What was your father’s work at Los Alamos, if you can talk about it?
Well, he likes to tell me he blew up a lot of uranium. [laugh] He did what was called the Creutz test, actually; they made a dummy implosion, but not with enriched uranium. And there’s interesting stories in that. It looks like when they first did the test they thought the real one wouldn’t work. But then Bethe redid the calculations and said it would work and it did work. Of course, cause my dad had many connections there and over the years I have met many of the most famous people. Wigner and Bethe and Teller and all.
Where was your father from originally?
He grew up in Wisconsin.
And what was his education? What was his research in physics on?
He was doing proton scattering. He was an experimentalist. Apparently, he was Breit’s student which is interesting because I’ve heard other people that say what an obnoxious guy Breit was, but my dad would never say anything against him. And then he went to Princeton for a little while. You’ve probably heard of Feynman’s inverse sprinkler.
Well, my dad did that with him.
And my dad liked to remark that after the carboy blew up Feynman just disappeared and left him to clean it up.
And he learned that about theorists. Theorists are very good about disappearing if something goes wrong. [laugh]
[laugh] Do you know the story of how he got to Los Alamos? Was he recruited?
I believe he was recruited. I think he was, I’m not sure what his position at Princeton was. It was probably some kind of postdoc equivalent these days. And they recruited a lot of people there. He was mostly involved with the metallurgy of uranium. Apparently, very little was known about uranium at the time. They didn’t even know the melting point very accurately.
And what about your mom? Where was your mom from?
She was also from Wisconsin. She was born in Superior, Wisconsin. And they met, I believe, at University of Wisconsin where they were both students. She was a math major. She never took on a real career. I think she kind of regretted that. Although I think they did use all the women in Los Alamos for some kinds of calculations, but I was too young for that.
And how long were you in Los Alamos for?
Oh, I was less than a year old when we left. My dad went to Carnegie Tech then and became chair of the physics department at Carnegie Tech.
So, after the war was he no longer needed? Or did he want to move back to an academic setting? Or how did that work out?
Basically, I don’t really know. I think that everybody pretty much went back to their universities after the war. You know, Bethe went back to Cornell and all that.
And you spent the rest of your childhood in Pittsburgh?
No. We lived in Pittsburgh until I was about 12, I think. And then my dad got involved with General Atomic. That was a new big research place in San Diego. And he became the assistant director in charge of research there. That’s where we met a lot of very famous people because he would bring them all out there to try to get the place going. Bohr actually was in our yard in California.
Yeah. My dad was very impressed by him and I was just a little kid, so.
I didn’t think anything of it.
Now, did your dad involve you in his work at all growing up? Did you have an idea of what it meant to be a physicist?
Well, I think he always talked about physics. I mean he worked very hard. When I was, I must’ve been eight or ten or something, he gave me a 6-volt battery and a coil of wire and showed me how to do various things like make a buzzer and make an electromagnet and turn water into hydrogen and oxygen. All that kind of stuff which you can all do with just a coil of wire and a 6-volt battery. So, that really impressed me as a kid. [laugh]
Did you have a strong curriculum in math and science in high school?
Well, it was decent enough. I did go through the regular public high school. And I had some good teachers and some not so good teachers. But it wasn’t until I went to Caltech that the science really became strong.
Yeah. Now, between family finances and geographic decisions and grades what kind of schools did you want to apply to for college?
Well, I wanted to apply to ones with very strong science. So, I applied to Caltech, Harvey Mudd, Wisconsin, I believe. The one funny thing is I had a cousin who was at Stanford and my mother wanted to know I would get into Stanford. So, she had me apply there and I was rejected. [laugh]
She just wanted to show I was as good as my cousin. [laugh]
What were your impressions of Caltech when you first arrived?
It’s a fantastic place. It was a big change for me to go off to being in such an environment. Of course, the fact that it was all-male at the time kind of warps you. In high school I was one of the smartest kids. Although the smartest kid I later married. At Caltech I never expected to really stand out, my grade point was 3.2 or something when I got done with it.
Did Pasadena feel like a small town at that point? Or did it feel like a really, part of Los Angeles?
It was part of LA. It was also quite smoggy at certain times. When it rained you could see the mountains which was nice. We would on a clear night go out to Mount Wilson and look out over the city which was quite a pretty thing to do. But it was a big change for me.
Now did you declare the major in physics right away or that came later on?
I probably kind of figured I’d be a physicist all along.
And I always kind of liked geology a lot too, but physics was the natural way to go.
And who were some of the professors that you learned a lot from or grew close to as an undergraduate?
Well, Feynman, of course, was incredible. [laugh] He was spectacular. But we had pretty good teachers the first couple years. I was the year behind the famous, Feynman course. So, he was not giving it to us. Leighton did my freshman year; Cohen did the sophomore year. And then Feynman did the quantum mechanics quarter for us. But his presence was always felt. He was spectacular.
Now to the extent as an undergraduate you recognize the binary in physics between theory and experiment, did you gravitate particularly to one or the other?
I think I always wanted to be a theorist. I know when I went to Stanford, Drell encouraged me to look into being an experimentalist, but I never really took it very seriously.
Were there any lab work opportunities that were interesting or important to you as an undergraduate?
Well, during the summers I worked at General Atomic. They were developing cells which were a potential way to produce power in space. They were going to use thermionic devices, which never really panned out. So, I did work in a lab with that. That was very nice.
What were some of the most exciting things in theory, at least as your professors communicated them, when you were an undergraduate at Caltech?
I don’t know. I think we never really were brought up to the latest stuff. I mean, of course, this was the time of S-matrix theory and all that. It wasn’t really until Stanford with the deep inelastic scattering that everything really took off. I guess what I like to say is as an undergraduate you learn everything and then as a graduate student you learn you know nothing.
[laugh] Now, what kind of advice did you get about graduate programs? Where to apply, even who to work with?
I don’t remember why I picked Stanford.
Well, you didn’t get in the first time, so you had to prove that. Your mom was happy.
That’s right. I got in the second time. [laugh] Where did I apply? I probably applied to Wisconsin cause that’s where my dad had been. And my brother was going there. I don’t really remember why I picked Stanford. I mean, it was a good school. It was close by, sort of.
Now was SLAC part of the equation? Did you know even in those early years that it would be an exciting place to be?
No, I didn’t. I didn’t initially realize SLAC was there. When I was in my first year as a grad student I went around, talked to various physicists. And then they said I should go out to SLAC and talk to people there. And Drell just inspired me. So, he seemed to be a very good person to work with.
How did you go about developing that advisor relationship with Drell?
Well, it was a little bit loose because he was at the time a presidential advisor. And so, he spent a lot of time in Washington. But it was the environment at SLAC. The continuous really good seminars all the time. There were six of us graduate students who had desks all in this same room. It was the seminar room with desks around the walls, and that’s where we all were. And so, we couldn’t avoid the seminars. And of course, at this time Feynman spent a lot of time up there because he got really excited about the deep inelastic. And so we all got to interact with him more there.
What was the intellectual process of developing what would become your thesis research?
Drell suggested a couple of things. I think my thesis research is basically forgotten. It’s irrelevant in a way these days. It was on deep inelastic scattering with different ways of treating it. And there was a part of it which was on e+/e- producing a photon and then hadrons. This way one could handle even charge conjugation. But you know, it’s out there, but it never really blew up into anything.
At what point did you start to spend more and more time at SLAC?
Once I was a student, that’s where I would spend the time. I guess, well, the first year was all courses, so that was on campus. And then the second year I kind moved out there, got a desk and spent most of the time there.
Did you have a sense of some of the tensions between the SLAC faculty and physics faculty?
It was there. Yeah. I never really got involved with it. I mean, I guess Hofstadter was not very happy with what was going on at SLAC. But I was not really involved in any of that.
Who were some of the other senior people at SLAC that you interacted with?
Well, there’s BJ and Drell.
Bjorken, you mean?
Yeah, Bjorken. Everybody calls him BJ. [laugh]
Let’s see, Fred Gilman was there. Tom Appelquist was there. Both of which have had very successful careers. Then Marty Einhorn who’s at Santa Barbara now. Who were some of the other people? Oh, Joel Primack who’s at Santa Cruz. Also Probir Roy at TATA.
Mike, was your world at SLAC a very theoretical world? Were you paying attention to some of the experiments?
Well, everybody was talking about the experiments because they were receiving so much attention at the time.
Which was what? What was happening during your time?
Oh, that was the deep inelastic scattering. We were discovering that there was something point-like inside the proton. The connection with quarks and the spectrum came later.
Did you have interaction with Feynman when he would come up and find all of this so fascinating?
Oh, yes. I mean Feynman was very interactive in his personality. He loved to tell stories. And so, we would always crowd around him when he would tell these stories.
Science and non-science stories, I take it?
Oh, yeah. Oh, yeah. [laugh] He was a great storyteller. You can tell it from his books. His books are just the stories being repeated.
Right, right. [laugh] Besides Drell, who else was on your thesis committee?
Let’s see. Frank Von Hippel, who has since gone into public policy, a condensed matter theorist and a mathematician. [My thesis is in my office, but because of Covid I’m not allowed to go there to check.]
How mathematical was your thesis research?
Not very. In fact, I get upset with so many theoretical papers these days where I can’t even read the words. This was the West Coast, which was very different than the East Coast at the time. We were much more practically oriented I thought. So, you know, there’s a little bit of math in there, but nothing deep.
What opportunities were available to you in terms of postdocs or faculty positions after you defended?
Well, the one interesting thing was that I never applied for a postdoc because Drell said, “Don’t apply.” He was in the old boys’ network. And he called around and I had offers from basically, I don’t remember. I think I may have had a second offer, but I chose Maryland. So, I went to Maryland as a postdoc. So, that was interesting. Those were the days of the old boys’ network. [laugh]
Just a phone call.
Yeah, exactly. And by the next time when I left Maryland, it was the time of the pool. You know, you would apply to this pool and applications would go all over the place. Then I had an offer from Cornell and Brookhaven. Brookhaven offered me the assistant physicist level, which was one above the postdoc, so that’s why I went there.
Now, what group did you work with at Maryland? Or who did you work with at Maryland?
Well, Wally Greenberg was sort of the leader then. And Joe Sucher. Mohapatra, the neutrino guy, was there at the same time. I worked a lot with Siddhartha Sen. He was another postdoc. He went off to Trinity College for many years. He’s retired and back in India now. But he and I wrote several papers together.
Do you remember what Wally Greenberg was working on at that point?
That must’ve been around the time of color and all that, I would guess.
Those para-statistics may have been a little earlier. But yeah, he deserves a lot of credit for that because it really is color.
Now, did you look at the postdoc at Maryland as an opportunity to move onto new projects? Or did you want to expand and improve upon your thesis research?
I think I never quite know what I’m going to work on next. That was always my attitude. I just look at what’s interesting and dabble to understand how particle physics works. I’ve never been really stuck in one thing. Well, the lattice, of course, took over my life, but that was later.
When did you first become aware of what Ken Wilson was doing at Cornell?
Well, let’s see. I spent a lot of time trying to understand what confinement was all about. And so, I wrote some papers on how the gluon field spreads and all that. But then I went to the Erice school in ’75. There Ken Wilson gave his lectures on the lattice. And I’d been thinking about confinement and I suddenly realized that is the way to go.
Why? What spoke to you at that moment?
Because it was non-perturbative. That it got away from the old diagrams and everything else which weren’t telling us anything. And so, I wrote a little paper on putting the bag model on the lattice which was kind of fun. Then we had a condensed matter guy, Bob Swendsen, who gave a talk on Monte Carlo renormalization group and I had never known anything about Monte Carlo before. And I said, “This looks like fun.” So, I started playing with the lattice model with a Z2 gauge field on a little computer, on a calculator, basically. And it started to give interesting numbers. And so, it just took off. I mean, we did a bunch of stuff with discrete groups which are pretty easy. You can do them on your phone these days. And then at some point I said, “Well, I have to try SU(2),” just to have the program. And I had it and it ran very slowly. Quickly I gained many orders of magnitude with a little work on it. And then it just kinda took off. So, we started doing all kinds of different models.
What was going on at Brookhaven by the time you joined?
I think a lot of it was proton scattering. Those were the days of finding all of the resonances and also Regge theory. They were talking about building this ISABELLE machine which was going to be, 200+200 GeV or something like that. This got canceled and got revived years later as RHIC. This tunnel was in some stage of being made at the time when they canceled ISABELLE.
Now, how did the November Revolution at SLAC, how did that resonate all the way in Brookhaven for you?
We found it too.
In fact, I think we found it a little before. There are rumors that maybe some of Sam Ting’s results leaked to SLAC and they looked right away, but I don’t know if that will ever be known. [laugh]
Did you have much interaction with Sam Ting?
Not personally, no. Well, there was one time he stayed in our house when we were away. But not directly. But, of course, the Revolution affected everybody at the time. I remember Ling Lie Wang (now Chau) and I were very quickly figuring out the width of the J/psi and all that from initial the data they had. It was fun.
What opportunities did this present for new research in both quantum mechanics and quantum fluctuations?
Well, to me, I think I’ve understood quantum mechanics ever since the Feynman lectures. So. [laugh] I don’t think it changed our understanding of that. The main thing it changed was really convincing us that this charm quark was there. That there was another quark. And that quark was bound in non-relativistically. And the Cornell people did their game with the J/psi spectrum which all worked well. I don’t think it changed our understanding of quantum mechanics any. It was just quantum mechanics. [laugh]
What happened next in lattice gauge theory?
Ken had been talking about maybe you could do this on a computer, but the common attitude was, “Well, if Ken thinks it’s going to be hard, nobody was trying to do it.” So instead I started playing with this much simpler model. I said, “Well, we’re not going to be able to do the real model. But we can probably do a Z_2 model with a gauge interaction.” And it worked! So, we gradually built up to bigger systems. SU(2) was the first non-abelian group. And there were hints that confinement was really there. The force between sources was not falling with distance. And that was the one paper of mine which has the most citations.
When did you feel like you were contributing most to the Standard Model as it was being developed?
I guess I was just getting used to the idea that it was quarks and non-Abelian gauge fields, ideas that were floating around. Also, asymptotic freedom, of course, is very important to the picture because that’s how you can take continuum limit. I kind of remember at the Erice school when ‘t Hooft was there and I mentioned I wasn’t sure confinement was really going to happen. But then when I saw it in SU(2) and it was definitely happening.
How did you see it? What was SU(2) telling you?
Oh. First of all it was a non-Abelian group. So, the gluons are charged with respect to each other. And then Ken had described the Wilson loop. And so, that was the natural thing to measure. So, we measured Wilson loops. One of the earliest surprises was that the Z2 gauge theory, which is a generalization of the Ising model, had a very strong first order transition which was very easy to see. And I don’t think that was anticipated. People would kind of speculate it would probably be more like the Ising model second order transition. But it was extremely strong even on a little 34 site lattice. You could see it immediately which was very nice.
And then when do SU(5) and then SU(6) enter the picture?
Oh. Once I had the program, I could put in any old group I wanted. Something interesting that happens when you get up to bigger groups. SU(4), SU(5) seems to have a transition. And it’s not a deconfining transition; actually, I should go back and think about what it really means in modern language. But it has a disordered phase at very strong coupling. Another interesting thing appeared when we were doing the Z_Ns. Then there were two transitions if N got big enough. What is fun is once you’ve got one of these programs written you just change the group. That’s all. It’s easy. At the time the programs were all very, very simple. Nowadays these big groups get their big, fancy packages which are absurd. [laugh]
In what ways were Monte Carlo simulations really relevant for this?
Oh, the path integral is a statistical thing. You’re summing over all paths, weighted by the gauge fields. So, you want to generate a bunch of configurations of the system weighted by the action. And then it turns out a small fraction of them dominate. The joke I like to use in my talks on this is to think of a glass of beer as a statistical system. You can put those molecules all over the place. But you don’t need to know all those configurations. All you need is about six glasses of beer and then you know its important properties. [laugh]
[laugh] That’s great.
Yeah, you generate a few very typical configurations and then you can measure lots of things about the system.
Mike, what were your motivations in writing the textbook Quarks, Gluons, and Lattices?
I think somebody from Cambridge may have asked me. Because the field was very quickly developing.
Was there not a standard text at that point?
No. Not at all. No the lattice was new, there was nothing. The field had developed so fast. So, it is really the first textbook on lattice gauge theory. And it did very, very well.
Did you ever update it? Has it gone through multiple editions?
Well, there were a couple minor changes. First, second edition and all that. But nothing major. So, I just recently wrote another book with the World Scientific. This hasn’t received much attention yet; it needs more advertising! [laugh]
Who was the audience for the textbook, the original textbook? Was it undergraduates? Graduates?
Undergraduates don’t really have the training for that. And the field was growing very, very quickly. So, it was for graduate students and more senior physicists.
Mike, were you involved in all or were you paying attention to the earliest planning for the SSC?
Well, it was kind of going on. I was actually on HEPAP for a couple years there when a lot of that was going on. So, I have sitting in my office which I can’t go into, an early map of the potential sites around the country. So, it was being discussed. I didn’t have major role in it.
Now, you said earlier you’ve never liked supersymmetry. Is that to say that you’ve never wondered, had the SSC been built, would we have seen the supersymmetry? Because as far as you’re concerned maybe even if it was built we wouldn’t see it anyway cause there’s nothing to look for?
[laugh] I guess that’s my attitude. Yeah. [laugh]
I wonder if you can develop that a little more? Because as you know, the field is full of people who passionately feel like supersymmetry must be out there. Why do you say it’s not?
Well, it’s a mathematical game. One silly reason is that when we do the simulations on the lattice we treat fermion so differently, and it’s particularly difficult to put supersymmetry on the lattice. People have played with it a lot though. All this stuff we have discovered at CERN we would’ve discovered a lot earlier with the SSC. We did hold a lattice meeting in Dallas just after they cancelled it. It was a bit strange.
What were some of your early appreciation of the value of supercomputers to lattice gauge theory and to quantum field theory for that matter?
Well, a supercomputer just means the fastest computer at the time. And that, of course, evolves. I did a lot, my original work, on the CDC 7600 which these days your phone probably beats. But I the fact that we’ve got much, much bigger computers means we can simulate things closer to the real physical parameters and all that. But it’s incremental.
As lattice gauge theory was maturing in the 1980s and the 1990s, in what ways was it contributing more broadly to theoretical physics?
Well, we certainly convinced the world that confinement was there. That’s probably the main accomplishment of the time. And indeed, we are starting to get useful predictions. It’s only in recent years that we’ve been able to get the quarks light enough. The lattice also convinced people there was a transition to a quark gluon plasma even if it’s not a true transition. But there is a very big change in behavior at very high temperature. So those are some of the big things.
And what was the state of play with chiral symmetry at this point?
It’s always been a puzzle because the Wilson approach to doing fermions very violently breaks chiral symmetry. And so, I guess it must’ve been in the late 90s is when I started saying, “Well, this is a problem to work on more.” So, I started thinking more about chiral symmetry. And some of that’s gotten me in trouble. By exploring chiral symmetry, I came to understand a lot better that there is this parameter called theta which is in QCD, but it cannot be seen in perturbation theory. Perturbation theory doesn’t know about it. And I think you get the best understanding what theta is all about from looking at chiral symmetry. Theta is an angle which occurs in the chiral Lagrangian. And it raises a lot of interesting issues. What do you mean by the mass of a quark? And that’s what’s gotten me in a lot of trouble.
When you say a lot of trouble who are—you don’t have to name names, but I guess what fields are critical of this line of thinking?
OK. The most controversial thing I’ve been saying—well, first of all, I’ve been saying that the whole staggered fermion approach is incorrect, but that’s a more specific thing. There’s been a lot of discussion of whether a massless up-quark could solve the strong CP problem. And I’m saying that you can’t solve it unless you can define a quark mass. And I’m saying there’s an ambiguity in defining quark masses when they’re not degenerate. If they’re degenerate, then you’ve got chiral symmetry. If the up and down quarks are degenerate, and massless, then the pions will be massless. So, that’s a very physical thing. But if they’re not degenerate there’s a fuzziness. And that’s what I’ve gotten in a lot of trouble over.
So, what exactly is the orthodoxy that you’re challenging here?
People say that a vanishing up-quark mass would solve the strong CP problem. I’m saying it’s got nothing to do with it. [laugh]
Mm-hmm. And experimentally, how might this be resolved at some point?
Well, first of all, experimentally the up-quark mass is sufficiently far from zero that it’s really only an academic point. The up-quark is about half the mass of the down quark, phenomenologically. But intellectually a lot of interesting things happen. For instance, you can have a quark mass be negative and not have the physics get kind of weird. And it’s tied to theta because if you have three quarks and they’re all negative in mass that corresponds to theta being pi. And I think everybody agrees that then there’s a spontaneous breakdown of CP and all that. But in perturbation theory the sign of the quark is irrelevant. You can change variables and it goes away.
And where is the antiquark in all of this?
You’ve got to have quarks and antiquarks. I mean, that’s just fundamental and goes back to Dirac, right? That’s another talk I like to give to high school kids sometimes. Why do we have to have antimatter? And if you don’t have antimatter you can send signals faster than the speed of light which is a pretty cool thing.
What have been some of the most significant values of Yang-Mills to lattice gauge theory?
Well, Yang-Mills is the basic thing; gauge field theory is the Yang-Mills theory. In ’54 they said, “Well, what would happen if you have a symmetry other than the U(1) of electromagnetism?” And they put in an isospin, because isospin was just discovered at the time. And so, they said, “Well, what if your gauge fields have isospin?” And that is the whole framework of non-Abelian gauge theories, the Yang-Mills theory.
And when did you start thinking about topology in lattice gauge theory?
That is deeply involved with this question of quark masses, etc. It’s this question of what does theta really mean? What is this parameter? And it’s tied to the topology. And there are issues there because in the path interval you can prove that the typical gauge configuration is not a differentiable thing. It involves non-differentiable paths. That’s even true for just a particle in a potential. The typical path is non-differentiable. And that makes topology a bit tricky. It’s clearly connected to these issues. There’s a close connection between topology and zeroes of the Dirac equation and the theta parameter. And so, this is why I started to think about chiral symmetry, which was about ’96 or so.
Mike, an overall question for your research agenda. Were your interests in computational physics always routed in lattice gauge theory? Or there was a separate line of inquiry there?
I think I really never did much computing before. You know, I took a little course on algol in graduate school to learn what computing was all about. But after Swendsen described to me the Monte Carlo renormalization group, I said, “Oh. I can play this game too.” At the time we had a little desk calculator which was pretty powerful. A Hewlett-Packard thing. And so, I started programming that. And it was kind of fun. I never really considered myself a computational scientist as such. But then it worked for this one thing, so we kept going.
I’m curious about your affiliation with the C.N. Yang Institute at Stony Brook. When did that start and what were your motivations for joining?
Well, first of all we’ve always had joint seminars with them. Forever and ever and ever. So, we had a connection with them. At one point they said would I like to be an adjunct? And I said, “Sure.” Not that I asked to do it or anything. [laugh] I think it was probably Sterman who asked me.
And was that your first opportunity to teach since you’d been at Brookhaven?
Oh, I only taught once there. I only taught the one course. A lattice gauge course. Most of my teaching related things has been at various summer schools and stuff. I first went to Erice as a student and I went back a few years later as a lecturer. And one thing was clear, lecturers do get treated a lot better than students. [laugh]
But everybody gets treated well at Erice. If you have never been there it’s a spectacular place.
It’s on top of a mountain and views out over the sea. I’ve lectured at other summer schools, i.e., Banff and Karpacz. And I’m doing it this summer school at the end of the month. Of course, that’ll be on Zoom. It’s much more fun to go in person.
Yeah. Soon enough.
But when the lattice really took off, I was doing a lot of speaking. A lot of traveling.
What’s been some of the more recent work in strong CP violation?
There’s not much really new. I’ve been tossing out my crazy ideas. There’s still no evidence for it being there in the strong interaction, which is the strong CP puzzle. Because we know the other interactions do violate CP and so if there’s any kind of unification, why is it not in the strong interactions? And that’s not really understood. But very little new on that.
What about pion masses? What’ve been some of the more recent research in pion masses?
The pion mass is just supposed to be a consequence of the up and down quarks not being quite massless, and there’s no explanation in that. If you’re doing a lattice simulation, you just put in the quark masses and adjust them until you get the pion right. And there’s nothing fundamental about the number. It would be nice if there was, but we don’t know of any real reason for it to be what it is. And of course, the big puzzle is why are the quark masses so different? You know, the top quark is thousands of times heavier than the up and down quarks. And then why are the neutrinos so much lighter? I don’t know. [laugh]
Brookhaven has been good about allowing you to travel in the summer and be involved beyond New York?
Yes, until a couple years ago. [laugh] The lab decided about two years ago that they would no longer pay any travel for emeritus people.
So, the first couple years as emeritus I did a couple of conferences—I never charged them very much. So, now I have to go on my own if I’m going to go. And I will be going to meetings. But I think that was a very bad decision because it really discourages people from going emeritus.
But I think that was a very bad decision because when I initially retired they said, “Well, we’ll give you a few thousand for travel.” So, I went to a few meetings. Although there is another nice thing about being emeritus is I have more freedom. I went to a meeting in Russia at a time when the DOE said, “We’re not doing any travel to Russia.”
And so, I said, “Well, I’m not really an employee. I can go on my own.” And I went on my own. At the time Laurence Littenberg ask, “Don’t put Brookhaven’s logo on your slides.” [laugh]
But it was a very good meeting too because I gave what I thought was a very nice talk which was on this controversial subject. Stirred up some people.
And Mike, just to bring our conversation up to the present. More recently, what has been some of your work in numerical simulations in quantum field theory?
I’m really not doing numerical work anymore. I’ve been worried more about the issues with chiral symmetry. What’s going on there.
Is that just more interesting to you intellectually?
I think so. Yeah. I mean, I never was really a computer person as such, although I do play with my Raspberry Pi and all that. [laugh] But it’s not the computational side. It’s the physics side of things I’m more interested in.
And in terms of chiral symmetry, what does that teach us more broadly about the notion of handedness in particle physics?
Well, the fact that we don’t understand it for neutrinos.
That we don’t know how to put it on the lattice I think is a very serious question. It’s trying to tell us something about nature. Now, t’ Hooft did show that because of this chiral symmetry breaking, baryons have to decay. Although it’s an extremely small rate that he calculated. But it’s there. So, you’ve gotta incorporate that in your theory somehow. And we don’t know how to do that.
What do you think it’s going to take? It’s more a theoretical breakthrough or an experimental breakthrough?
I think theoretical. Because we don’t understand the theory. Things like proton decay, they are so rare that it’s going to be very hard to see—well, if they ever discover it experimentally that, of course, will be a revolution.
Well, Mike, now that we’ve worked right up to the present, I’d like to ask for the last part of our talk some broadly retrospective questions about your career. So, the first is the role of computers. Obviously, the power of computers has grown exponentially over the course of your career. Have you seen that progress as mostly steady or have there been revolutions in computational power insofar as that’s been relevant to what you’ve been able to do at any given point?
Well, nowadays simulations are actually being done with physical values of the parameters. I mean when we started off we were doing SU(2), not SU(3). When people started putting quarks in they were always much heavier than the physical quarks because when the quarks get lighter the calculation gets slower and slower and slower. But I don’t think there was any specific point where there was a dramatic change. So, it’s still a lot of the same questions. But now we’re doing it at the physical point.
I’ll ask a similar question about charting change over time. But this one is about emotions, about optimism. When from the beginning of your involvement with lattice gauge theory all the way up to now was there most optimism that breakthroughs and really helping to push physics to the next level was most apparent? Was that also steady? Or did those breakthroughs happen in fits and starts?
I think the only real breakthrough was when we got started. People started to say, “Maybe we can do this.” And before that nobody had thought about doing numeric simulations. Beyond that, it’s just been gradual and we’re getting down to where we can really do some real predictions.
So, as we’re refining what is it that we’re waiting for as the calculations become more precise? What’s the pot of gold at the end of the lattice gauge theory rainbow, I guess is what I’m asking?
I think this is sort of why I’m not doing the simulations right now. I don’t see it. We now pretty much believe that the calculations will reproduce the physical hydronic spectrum. There’s no evidence that there’s a problem there. And so, yeah. That’s why I got interested in this other problem, the chiral symmetry which was never really quite there. And then the question of neutrinos is always there in the back of my mind. How should we treat them? But I’ve been saying that for a long time. [laugh]
[laugh] From all of your advisory work on things like HEPAP has the field been in your sense well-supported according to its needs over the course of your career?
I think so. I mean, you know, there was this SCIDAC project, which is now called the USQDC lattice collaboration, which has been getting lots of good resources. And I think it’s been adequate. I mean, I’m not particularly greedy. It’s nice to keep it, it’s still coming. I think one of the points is that we tend to be the community which can use the new computers the fastest. We are always ready for them. And so, and then somebody builds a new, faster computer. We’re there and we’ll use it. And so, they’re happy to have us come in. I think it’s been adequate.
Mike, as you well know, one of the big narrative stories in physics over the past 50 years is the migration of particle physicists into astrophysics and cosmology. Do you see any of your work being relevant in those fields?
I don’t know. I think the other migration, of course, has been nuclear physics coming in to take over for a lot of it. So, originally it was a particle thing, the lattice, and then nuclear physicists are kind of taking over it. Astrophysics, I don’t know. I actually wrote a paper on black holes recently. But that was mostly for fun. [laugh]
In other words, does lattice gauge theory possibly have anything to tell us about dark matter or dark energy or cosmic inflation?
I have no idea. I mean one can probably do some simulations of inflation and stuff. And I think probably people have been doing that. But since you don’t quite know the rules…that’s one of the nice things about QCD. It’s a very precise theory. We have a precise Lagrangian. We know what we’re doing even if the people can do it wrongly. So, I don’t know. That’s the future. Dark matter, of course, is there. And so, we’d like to know what it is. Dark energy makes no sense at all. [laugh] But it also seems to be there.
Now, this influx of nuclear physicists into the field. What do you think accounts for that and in what ways have they pushed the field forward in ways that would not have happened otherwise?
Well, they’ve been mainly interested in the quark gluon plasma which is motivated a lot by the RHIC accelerator and then also stuff being done at CERN. But that community tends to think differently than I do in some ways. [laugh] So, it’s a strange melding of the communities. But the lattice is showing that there is some kind of quark gluon plasma which is a good thing.
Mike, in all of your research what stands out in your memory as being most intellectually satisfying? A lightbulb moment? Or really putting something together that wasn’t understood before?
That has to be after Wilson’s lectures. I said, “This is the way to do it!” [laugh]
That was very, definitely one. A much more minor stage, I think in about, I think it was around ’96 I started looking at what chiral symmetry would mean in terms of the pion spectrum. And I think the insight there was very nice.
Conversely, what problems have always gnawed away at you? No matter what you’ve done, you always seem to hit a wall. What stands out in your memory in that regard?
Parity violation in neutrinos. [laugh]
And I think always in the back of my mind is the question of why is it so much harder to put fermions into a computer simulation than to put bosons. Bosons are trivial. What is it that makes that so much harder? I don’t know.
And as you say, this is going to require a theoretical breakthrough.
I think so. Of course, predicting breakthroughs is not totally easy. [laugh]
Well, it’s a very nice segue to my last question, Mike, looking to the future. First, if you were advising a graduate student who was interested in lattice gauge theory, but also had concerns about the viability of their research for a long term career, would you suggest that this is a field that they should go into? And what might that tell us either way about the future of your research?
I think right now I would probably want somebody to tend more toward condensed matter physics. They’re doing some incredible things there. It’s not unrelated. You know, a lot of the topological insulators are very related to some of the ways we’ve been treating fermions on the lattice. I think high energy physics has kind of slowed down and there’ve been a lot of really tremendous results in condensed matter physics. So, I would maybe push people in that direction more.
And that’s to say even if an ILC was built? Or there are exciting additions at CERN that make things possible that aren’t true today? Or is what you’re saying not necessarily coupled to those potential advances in accelerators?
I guess I don’t see what they’re going to discover that we don’t know now. We do know there are exciting things going on in condensed matter physics and we don’t know what’s going to happen there.
So, it’s hard. I think too much of particle theory these days emphasizes beyond the Standard Model of physics. This is very speculative. People invent some crazy new particle and then see what it’s consequences are. But there’s usually not much motivation for it.
And as an addendum to that, for you personally, however long you want to be active in the field. What do you hope to accomplish?
I don’t know. I’m slowing down a lot. I mean, I’m 76 years old! [laugh] I’d like to keep going to our group seminars and seeing what people are working on. And I’d like to convince people that there’s a problem with defining the masses of quarks. [laugh] I did write in a little paper recently on black holes on how our concept of time remains a little muddled when you start talking about general relativity. So, I’d like to think about that some more.
And like it’s not at all obvious to me that there’s an information paradox. But I don’t know. It’s more of a hobby.
[laugh] Well, Mike, it’s been great fun spending this time with you. I’m so glad we were able to do this. Thank you so much.
OK. Well, it’s been fun.