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Interview of Gene Beier by David Zierler on 2020 April 27,Niels Bohr Library & Archives, American Institute of Physics,College Park, MD USA,www.aip.org/history-programs/niels-bohr-library/oral-histories/44446
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In this interview, David Zierler, Oral Historian for AIP, interviews Gene Beier, professor of physics at the University of Pennsylvania. Beier recounts his childhood in Illinois and his undergraduate experience at Stanford University, where he became close with Sid Drell, who encouraged him to pursue his graduate degree at the University of Illinois. Beier describes his work at the University of Illinois with his advisor Louis Koester, and his research at Brookhaven and Argonne Labs where he was involved in the search for a lepton heavier than the muon, but with a smaller mass than the K meson mass. Beier explains his decision to join the faculty of the University of Pennsylvania and he describes collaborations with some of his graduate students, his work at Fermilab, and the impact of quantum chromodynamics on his research. In the last portion of the interview, Beier explains the history of neutrino flux and his longstanding research on the atmospheric neutrino effect.
Okay. This is David Zierler, oral historian for the American Institute of Physics. It is April 27th, 2020. It is my great pleasure to be here with Professor Gene Beier. Gene, thank you so much for being with me today.
Okay, so to start, can you tell me your title and your institutional affiliation?
I'm professor of physics at the University of Pennsylvania.
Okay. And now let's start right at the beginning. Tell us about your childhood, your birthplace, and your family.
Well, I was born in Harvey, Illinois, which is a small town just south of Chicago. My parents were from central Illinois farm country. My father was, his family was part of the German farmers who moved to Illinois in the 1840s. And my mother's family had been around the United States since the early 1700s She grew up partly on the farm and partly in the town there.
Did your father, was he a farmer for a living?
He (laughs) started out being a farmer, but he saved up some money, and he was afraid that his father would take his money and invest it badly, so he decided to go to college. (both laugh)
What did he go to school for?
Well, there was a Teachers College, Illinois State Normal University, nearby, and he went to become a teacher and eventually he was a science teacher.
Yes. And he ended up being a physics teacher, but then he went into administration in his later life.
Did you ever have the pleasure of taking a class with him? Were you in the same school where he taught?
Yes, I did. There was always a joke that if I acted up in class, he would send me down to the class administrator. He was the class administrator. (both laugh) Who would send me home to my father. (both laugh)
So you grew up with science in the household, so to speak?
Yes. I would go too, when he would go in on the weekends to set up the laboratories for the following week when I was young. I would go in with him and sort of play around with some of the stuff. But they were just toys to me.
Right, right, right. And when did you start showing an aptitude in science yourself?
I don't know. Math was particularly easy for me. My interest in science, I remember one incident when I was about eight or ten years old, I can't tell you exactly when, where my father told me about molecules, atoms, atomic nuclei, neutrons, and protons. And that stuck with me. It was something really interesting, but it never occurred to me that that was something I would be involved in later. At that time, I wanted to be a right fielder for the Chicago Cubs. (both laugh)
Did you like the fact that you can visualize things that were so small? Was that what was attractive to you?
No, I think the thing that was attractive was that there was a structure, a rational structure, to matter at the small scales.
And obviously, you must have done very well in high school for you to have ended up at Stanford?
Yes. I was valedictorian. My parents were both teachers in central Illinois during the Depression. When they got married, my mother had to quit her job because you couldn't, it wasn't respectable to have two jobs in one family. And a few years later, my family moved to Harvey, where I was born. So... What was the question again?
Well, the question was that you did well in high school. You must have done well in high school.
Yes. So, the town that they moved to was a real mix of cultures. It was an industrial town, and it had everybody from the professionals, the doctors, and lawyers, to all of the factory workers. It was a real cross-section of America.
Was it commutable to Chicago?
Oh yes, there was a commuter train. It was about 20 miles south of Chicago, and so there was a lot of cultural inspiration. We used to go to the Museum of Science and Industry and the Field Museum and various places like that.
And why Stanford?
Well, my father had a fixation on me being an engineer, I think. And he wanted me to go to MIT. I had absolutely no interest in MIT. I knew people from prior years from our school who had gone there, and what they described wasn't for me.
What did they describe?
Well, the studies they were doing just didn't appeal to me. Also , we had had a number of family vacations in the western part of the United States, and I wanted to go west. And my father said, "If you don't go to MIT, you go to the University of Illinois." So the two things came together. We took a trip down to the University of Illinois, and we met a physics faculty member named Jim Smith, whose father had been a biology professor at Stanford. And Jim had gone to Harvard and MIT. And he said that Stanford was a perfectly good place to go if you didn't want to go to MIT. And I also had an uncle who was a businessman on the west coast, and he said, "There are two good schools, CalTech and Stanford, and if he doesn't want to go to MIT, he doesn't want to go to CalTech..." So Stanford was the result.
Berkeley was never in the mix?
Well, I had applied to Berkeley on my own, and my father was unhappy that I was planning to go to a state school. (laughs)
Yes, he said, "Go to Illinois."
Oh I see, if you're going to go to a state school, you may as well go to Illinois.
Right, right. So did you major in physics from the beginning at Stanford?
Well, almost. When it came to fill out what you wanted to major in, I had no idea. And my father said, "Put down engineering." So I did, and I got there and within... I didn't enroll in any engineering courses. I enrolled in the sort of standard undergrad, which was history and English and math, things like that. There was probably a science course in there too. Physics , I think. But the people I knew where I lived in the dormitory who were engineers were doing stuff that was really unappealing to me. So about six weeks after I got there, I switched my major to physics. Not because of strong motivation in physics, but because it got me out of engineering.
What was it about engineering? Too much math?
No, no, math was what I wanted. No, they were doing things basically from textbooks. The knowledge was given to them.
Oh, I see.
They weren't developing the knowledge, they were applying it.
And that didn't appeal.
So what about math itself as a major?
No, I'm experimental.
No, but I'm saying about math. Was math too theoretical for your tastes?
No, no, no, not at all. I came within one analysis course of having a degree in math, too, and in fact, my undergraduate experience in physics was that it wasn't really all that interesting because the mathematics I was learning was obviously applicable to physics, but it wasn't being used in the physics courses. And in fact, after the first year of calculus, almost all the math courses I had had physics graduate students in them. And they were going back to learn the stuff that I was learning as a sophomore and junior.
So looking back, what did that tell you about the way that the physics department was structured and the kinds of things that were emphasized and the kinds of things that were not emphasized?
Well, they taught all the right courses, but they didn't teach them as well as, I think, I've taught them during my teaching career. Things were not made intuitive and there was all this mathematical structure that was obvious in the physics, but it was never being used in the physics.
What would be a good example to illustrate the point?
Well, in say junior-level electrodynamics. We learned about vector calculus but it was just something that was there, it wasn't made intuitive. It just didn't seem to be a major part of the course.
Was it assumed that vector calculus was something that you would've already known?
No, no. not at all. When I have taught that same course, which I've done quite a few times, I find the students often would say that they had learned all of that in the math department, but it never made sense until I gave them the intuitive physical approach to specific problems. And then it seemed to fit together for them. It didn't fit together for me as an undergraduate, and so that helped me, in my teaching.
Were there any professors that you became close with at Stanford?
I think the closest was Sid Drell. Do you know the name, or?
Sure, sure. There's rarely an interview that goes by without a Sid Drell story. It's either Sid Drell or Feynman or both. Those are the constants.
Yes, so I lived in a dormitory across from Sid's home. And one of the--
Oh that's because-- So Stanford professors lived on campus, right?
Yes, and so I first met him, our dormitory had a program where I would guess a dozen or more faculty from different departments could just drop in and have lunch anytime. So I first met him having lunches with him, and then since we got to know each other, we would often find ourselves walking home at the end of the day together. And we would talk, you know, just casual things. But when it came time to think about graduate school, at that time I didn't know whether I wanted to go into applied math or physics. He and a few other professors had gathered the senior physics majors one evening and gone through, oh, 20 or 30 possible graduate schools and rated them A, B, C, D according to specific topics. And after four years on the west coast, I had decided I didn't want to stay on the west coast, I wanted to go somewhere else. And I went through that process with him, and then one day walking home at the end of the day, I was having a conversation about it, and he said, "You should go to the University of Illinois." Which sort of closes the circle.
But I did not know at that time that he was a graduate of the University of Illinois.
And had you already decided at that point that you wanted to continue on with experimental? Theoretical was never going to be something to pursue?
No, no. Not at all. At Illinois, my advisor was Louis Koester. And in experimental physics, Albert Wattenberg had a big influence. But a lot of my coursework was either taught or motivated by Dave Jackson, I don't know, you know him?
Okay. So he had just finished his classical electrodynamics book my first year at Illinois, and he taught three semesters of quantum mechanics, which was--
How long had Jackson been at Illinois by the time you got there?
Mmm... I don't think I could tell you that without looking it up somewhere.
But he'd been there for a while?
He'd been there for a while, and the year I left was the year he left to go to Berkeley.
So, it turned out that the quantum mechanics course was, I think, at 10 o'clock in the morning on Monday, Wednesday, Friday, and classical electrodynamics came after that, and I soon found that I didn't really care for the classical electrodynamics course. I could do it all on my own. Especially since Jackson's book had just come out. So after quantum mechanics, I would go off to the coffee room, and Jackson would go off to the coffee room, and we would sit there and chat and talk about things, and if I did have an electrodynamics question, I would ask him over coffee instead of going to class. (both laugh) So, those three people I think had the largest influence on me.
And how much, what was the split between coursework and lab work?
As a required course, there's no required lab course. But I went to work in an experimental lab, after my first year of classes. And ended up staying there, but to go on. In my second year, I took a course in particle physics from Jackson. And we've got along pretty well. Gordy Kane, who was a senior student at the time, wanted me to come on and take up the things that he hadn't completed in his thesis. And--
What was his thesis on?
I can't remember. (laughs) But that's a long time ago. And at that time, I was taking field theory from a man named Nishijima. He convinced me-- Well, maybe, he didn't convince me, but I realized that he was working at a much higher level than I was ever likely to work, and I didn't think I could be a successful theorist at that level. I thought Jackson, who was more phenomenology-oriented, I could handle that kind of work. But I also enjoy working in the lab. And so I ended up working in the lab, from then on.
How did you put your dissertation topic together?
(laughs) Al Wattenberg basically handed it to me. I had been thinking about doing an experiment at the recently-built Zero Gradient Synchrotron at Argonne Lab , and to do that, we were going to study charged K decays. It was in the heyday of CP violation in the K meson system And I built a differential Cherenkov counter that was fairly sophisticated to tag charged kaons in a beam. It was built on a design that Ted Kycia at Brookhaven had built for Brookhaven. But it was adapted to the ZGS characteristics.
Now, had you visited Argonne, or you were just aware of the work that was going on there?
I had visited it, but they didn't have a beam where I could test this. So we went off to Brookhaven and tested the device I built there. It was mostly me and a technician and occasionally, some other graduate students. And Al Wattenberg told me to do some things, put up a stack of lead bricks at the end of the device and make some measurements, and the motivation was to search for a lepton heavier than the muon, but with a smaller mass than the K meson mass, so the K meson would decay into it. And then to see if there was any signature of that from particles that could penetrate the lead bricks.
Which would tell you what?
Which, in retrospect, probably couldn't've told me anything. (both laugh) The setup was rather crude , But I was able to set limits on the existence of heavy lepton in that mass range, and that was useful information, but not critical to the way the world works. But it stimulated my thinking, because it was looking for a particle that, at that time, was only... was not predicted by anything, it was only a possibility. It's like the muon was not predicted. So why not look again?
Right, right. Can you explain the process of building this device? Are you starting out with the theoretical concept and then you have to engineer something to fit those specifications?
No, I was copying, as I said, a device that had been already built at Brookhaven and adapting it to the Argonne beam characteristics. So I basically had an engineer working with me, but we had the drawings from the Brookhaven device, so we knew pretty much which direction to go.
And where was your funding coming from for this?
Department of Energy. I don't think it was called the Department of Energy then. (laughs) I think it was called the AEC .
Right, right. Not til '77... The forerunner.
Now, were you at the natio-- did you visit any national laboratories during your summers as a graduate student?
Well, Brookhaven and Argonne were the places I was working all the time, so.
And in the summer? Yes?
No, well, during the year, too.
Oh, during the year, too? Uh-huh.
Yeah, I mean, once I completed classes, I was working on research and thesis full time.
I'll test your memory. Do you remember the title of your dissertation?
I don't trust this to be exactly right.
But I think it was, "A search for heavy leptons using a differential Cherenkov counter."
Okay. And who was on your committee?
Oh, my advisor, Lou Koester. Jim Smith. Roy Shult who was a theorist, Ulie Kruse , who was an experimentalist. And a mathematician, I had a minor in math. I can't remember his name.
Did you ever have a sense of how and why the physics department at Illinois was as highly-regarded as it was? In other words, I mean if you look at other major physics departments, they're sort of in step with the university that they're attached to in terms of the overall ranking. But the physics department at Illinois, which I've heard described in many contexts and in many eras, as really one of the best in the country, if not the world, right?
I mean, Illinois is a good school, but you don't hear of the University of Illinois generally referred to as one of the best universities in the world, like you would like, you know, a Harvard or a Stanford or something like that. So when you were there, did you ever get a sense of what it was about the physics department in particular that made it so spectacular?
Everybody I talked to said that it was due to a man named Wheeler Loomis, who was department head at the end of World War II. And he had brought in a number of really outstanding faculty, and then a number of really outstanding graduate students came. So I think that was it. The physics building at Illinois is named after him.
Yes. But this is before your time, you never crossed paths with him?
I never crossed paths with him. My first formal interaction when I got to Illinois was with the department head then. And the only thing I remember about that was he asked me to sign a loyalty oath. (laughs)
Loyal to who?
The United States.
It was left over from the McCarthy Era.
And I said, "Why do I need to do this? (laughs) I'm loyal, what's the big deal?"
And he says, "The state requires it and we can fight it and we will win, but there will be a cost." And we had a conversation, and he eventually had me sign the loyalty oath, which it was a matter of principle, it wasn't a matter that I was disloyal.
Yes. I'm curious, when you, after you defended, was the draft an issue for you?
Oh yes. I imagined that I would get a draft letter any day.
And how old were you in 1966?
But people were being drafted, and even young faculty members were being drafted. So I think, in fact, that goes back to some possible favoritism, and that was that because my father had been a school teacher and an administrator, he was well-known in the city where I was registered, Harvey, where I was registered for the draft. And I heard at one point that they would always accept my educational deferments because I was my father's son. So maybe there was some favoritism that had been used. I don't know of that first-hand.
So what were your options after you defended? What were you thinking about doing?
Well, I never really thought much about a career, except playing right field for the Chicago Cubs. (laughs)
Oh still? (laughs) You still were interested in that. (both laugh)
But I had done some work after my thesis using this device I built, and we had some nice results, and Al Wattenberg arranged for me to give some talks and one was at the University of Chicago, and one was at the University of Pennsylvania. I had no idea they were job talks, I thought people were just really excited about what I was doing. (laughs) But both of them came back and offered me assistant professorships. So then I had a really hard time choosing.
So no post doc? You didn't end up doing a post doc?
Well, there was nine months between when I finished my thesis and the start of the next academic year, so I was kept at Illinois as a post doc. But that was mostly to get me in sync with the academic year.
And why did you choose Penn?
Eventually, it was because the Princeton Pennsylvania accelerator was handy, and although Chicago had a synchro-cyclotron, I thought that there was more opportunity to do what I wanted to do. Penn also offered me a lot of freedom. I didn't come attached as an assistant to a professor. As some situations have.
So I eventually ended up finding a faculty member who had the same interests as I did. And then we went on to work together. But they had said that they would give me some funding if I wanted to collaborate with somebody at another university. So that freedom to do what I want made a lot of difference. If I went to Chicago, it was intended that I work with a certain faculty member for two years and at the end of two years, then you're pretty much set in what you're going to do. You're going to continue that. So I liked the freedom that Penn had..
So you started--
Penn was not very highly ranked then. I think it's come up in the world since.
Yes. So this would've been 1967, when you started at Penn?
And who were some of the senior faculty? Some of the more well-known faculty at that point?
Al Mann was the one that I ended up collaborating with a number of times. Walter Selove was building a big device to measure bubble chamber pictures, and he actually discovered a particle, which he named after his wife. It was the f-zero after his wife Fay. The others, Sherman Frankel became a good friend, but he did a lot of work, actually, with Sid Drell, who he went to graduate school with at Illinois, on issues of nuclear security. There was a man named Jules Halpern who I did not interact with a lot. And then there were a couple of somewhat younger people who, one of whom went into administration, one of whom I collaborated with on an experiment further down the line. So that was--
When did you start taking on graduate students?
Well, the first thing I did was an experiment where the students were actually Al Mann’s students. But I worked with them on the day-to-day basis. So the next experiment that I actually did on my own after that, or with another assistant professor, was the first one where I had my own graduate students. But there was some intervening history there, which might be interesting.
Al and I were thinking about Fermilab coming on the air, and doing neutrino physics there. And Al decided to recruit a couple of people. Carlo Rubbia and David Cline. . And that would be three groups, Rubbia was at Harvard, to take advantage of Fermilab at the time. And so that would be three groups, would be a substantial collaboration for taking on an experiment. And I had, in the sort of conceptual design, I had promoted and Penn actually built, wide gap spark chambers for tracking devices because I had used them in a previous experiment in Brookhaven with Al. And I also introduced the idea of calorimeters for measuring energy deposit. And Dave Cline brought on the magnet for measuring the signs of muons that came out the back end. But somewhere along the line, Al told me that Carlo thought that I wanted to play a larger role in the experiment, than he was going to let me do. And I said, "Okay, if that's the case, I won't do it." And if you look historically, there was an Experiment 1 at Fermilab, and it has my name on it. And there's an Experiment 1a, which was actually done, that does not have my name on it. Or proposals, rather. And it turned out that I had been talking about doing some hadronic physics with an assistant professor. We had an experiment that we proposed to Fermilab, but it wasn't going to be accepted as a first round experiment, so we actually did it at Brookhaven. It was one that could scale from Brookhaven energies to Fermilab energies. So we did it at Brookhaven and then went on to Fermilab and did it. And that was actually quite beneficial, because here I was pretty young. I was on my own for the most part.
And did you see your work in research as pursuing one central theme, or did you feel like there were largely unconnected research projects that you were working on at any given one time?
I think at that time, I was looking for the most interesting problems.
And what was interesting to you? How did you define "interesting"?
Well, the strong interactions were a mess at that time. There was no good theory of them. People had been trying to develop S-matrix theory, it was called, which was... You could not use perturbation theory to calculate the strong interactions with the tools that existed at the time.
Because the couplings were so large, you couldn't do a perturbation expansion. Perturbation expansion means that the first-- largest terms have the biggest effect, so the corrections for that are small, and so you can do it systematically in different orders of corrections. You couldn't do that with large coupling, because all of the effects were large. So we did that experiment, and it worked. It gave us some very nice results that got... Somebody somewhere did a nice review in Physics Reports of it. But by the time we finished, quantum chromodynamics had come along, and that was a completely different take on the strong interactions. And so it made what we had done, although we'd done it correctly, it wasn't as interesting as it might have been.
What was so different about it?
Different? Well, quantum chromodynamics?
Yes, what was-- it was a different way of looking at it?
Oh, yes, it was entirely different. It led you to understand that the strong interactions were working at relatively large distances, and when you got to smaller distances, the interactions became weak and became calculable. There was also a complete theory, so you didn't have to guess at things. It introduced, it was based on the idea of quarks and gluons interacting with each other. But when we started out on the experiment we were doing, quarks probably existed. . And nobody had heard of gluons.
When you say "probably," what was the over-under on that?
The experiment at SLAC that showed evidence for structure below the level of the proton had been done, and was consistent with quarks, but there was no overall theory to go with that, to interpret it yet.
When did experiment 734 begin?
After I finished that series of strong interaction experiments. . One of my colleagues at Penn, Brig Williams, along with Rubbia and Al Mann, had done an experiment on neutrino-proton elastic scattering at Brookhaven. And it was... It saw the signal, but it was very crude, so to speak. So we decided, I can't remember who (laughs) among us came up with the notion, but we decided to do it again at Brookhaven with a more sophisticated detector. And we developed that in collaboration with some people at Brookhaven. DOE, which did exist by that time, told us we needed more collaborators, so Bob Lanou and his group at Brown University, (Bob was a good friend anyways), joined in. And we were moving along towards doing a really good neutrino-proton elastic scattering experiment when Ronnie Rau who was the associate director for high energy physics at Brookhaven, called up and said, "We've got an opportunity that you can't refuse." And that was that a Japanese group wanted to get involved in an experiment like that. And that turned out to be an outgrowth of Brig Williams spending an extended period at KEK, the accelerator in Japan, to advise them about how to do a neutrino program. And his advice is, he told me, was that you shouldn't do it at KEK, you won't get enough neutrinos. You should do it at one of the big accelerators in the US.
Why couldn't you get enough neutrinos there?
The energy was too low. So Yori Nagashima and his group from Osaka joined us, and they brought enough money that we could make the detector much larger, so we could do neutrino-electron scattering as well as neutrino-proton scattering. And the cross-section, that is the probability of getting an interaction in neutrino-electron scattering, is a little more than 1000 times smaller than for neutrino-proton scattering. So that became the real motivation, and the thing we really wanted to do.
And how well-developed was the standard model at this point? Were you self-consciously aware of it and thinking about how to contribute to it? Or that only comes after the fact?
Oh, it wasn't called the standard model. We were measuring the coupling constants for all the things that later fit together to become the standard model.
So there was no notion of a standard model at this point?
Well, there was a notion that the physics was being unified, but one didn't know exactly what the couplings were, so there wasn't really a well-parameterized model. It was only when all of the parameters of the model were measured and people saw how they fit together that it became the standard model. This is in the 80s. And I think the people at the end of the 80s were talking about a standard model, but it was when the LEP experiments at CERN finished at the end of the 90s that people really felt that they had nailed down everything in the standard model.
And how did you see this research contributing to the creation of the standard model?
Well, like I said, we were measuring the couplings of neutrinos to electrons and neutrinos to protons, or...
And what else, I mean, if there were other things that also contributed to the standard model, where does this research fit in vis-à-vis those other projects?
Well, there were issues of what the weak interaction couplings of electrons were. They were measured in a polarized electron scattering experiment at SLAC. Also there were experiments in atomic physics to measure electron couplings, and so that was... And the ones at LEP were all measuring electron couplings. So there was a world of electron couplings and a world of neutrino couplings. They were being measured everywhere you could measure, but our experiment, I think, was one of the more successful ones.
And a definitional question, what is "standard" about the standard model? What does that mean?
I mean in the sense that like, you know, the unified theory. It's like it's self-explanatory, right? You're putting all the theories together for, you know, a unified theory. What is "standard" in the standard model?
No, the unified theories are not. There are many ways to make a unified theory. But the standard model is a particular choice of parameters. It's not a complete theory by any means. It has 25 or more parameters in it. So until you can predict what those parameters are, it's just a model. And it's "standard" in the sense that every experiment that is done agrees with the predictions of the stan-- Or, I don't know if they're predictions or, you know, what you would expect from a standard model. Cosmology is the same way in more recent years, in that it has developed a standard model too. This is more or less similar now that the cosmologists know a lot of the parameters of their field.
Who came up with the idea of using the sun as a source of neutrinos for this research?
Well, there's two options there. John Bahcall and Ray Davis. They got together back in the late 60s, I believe. And John thought he could calculate the neutrino flux from the sun from his nuclear physics and stellar evolution. And Ray thought from his nuclear chemistry, that he could measure the interactions of neutrinos from the sun. And so that became a famous duo and as it happens, the number that John predicted was much larger than what Ray measured. And the reactions to that were: a) you can't predict that very well, so the theory's wrong. b) was that that chemistry, if all you're doing is counting some atoms, how do you know you got them all? So the experiment's wrong. Nobody knew.
How would you get them all? What would that even look like to be sure that you got them all?
Well, Ray had all sorts of controls in the experiment. Not in the first round, but as he developed it, he would inject isotopes of argon that were different from the ones produced by the solar neutrino interactions in a known quantity. And he could recover themAnd quantitatively see that he got them all. So yes there were controls.
And how did this research lead directly into your research?
So Al Mann was at a conference somewhere in the US, I think in Utah, where—he was there, I wasn't there. He was interacting with Masatoshi Koshiba, who had built the Kamiokande nucleon decay detector in Japan. Al and I had been talking about ways of doing nucleon decay experiments in the US. The Koshiba group had found four events in it over in a year of running that looked like nucleon decay, but they didn't have enough resolution to see if they were background or nucleon decay. And so Al offered that we could help in an upgrade. The Japanese would put a veto around the detector, that is to tell what was coming in. So cosmic ray muons couldn't be the source, you could tell them. And we would build some electronics. The Japanese, interestingly enough, had very crude electronics. It was American electronics... This was the heyday of Japanese commercial electronics. So our group at Penn had developed the capability of building electronics. It still has it. So we built electronics that would add timing information to the photo tubes. The Japanese had only been measuring the charge. And the photo tubes were inside a very large water tank And part of the motivation was to do the nucleon decay but it was also, on our end, to see if we could lower the threshold enough to be able to measure electrons that were scattered by solar neutrinos. Now the Japanese also had this on their shopping list. They showed me when I first got there, a brochure written in Japanese, but as I learned a little bit of reading katakana, not much, but enough, I could tell that, yes, they had put that they wanted to study solar neutrinos in this. So we were in sync and proceeding with that, as well as with the nucleon decay. That was 1984, when we first started.
And why were you interested in the atmospheric neutrino effect? What were the bigger research questions surrounding this project? What were you looking to find out?
Well, the atmospheric neutrinos were entirely different situation. We weren't motivated by that at all. That's something that just happened. We were really looking for the solar neutrinos, and we eventually found them.
And if you could just explain the basic science, what's the difference between the solar neutrinos and the atmospheric neutrino effects?
The atmospheric neutrinos are relatively high-energy. One or two GeV. . And they're made by cosmic ray interactions in the Earth's atmosphere. And solar neutrinos are made by nuclear reactions in the center of the sun. And those are low-energy, the end point for the spectrum is 15 MeV. . Which is very much lower. So actually, I'll sidetrack this a little bit. As we were really beginning to get sensitive to the solar neutrinos, a supernova went off in 1987 in the Large Magellanic Cloud. . And we managed to see the neutrino interactions from that event.
How did that play out? Who first noticed the supernova?
Well, it was first seen by astronomers in the Southern hemisphere. I think it was a Canadian graduate student, actually, who first saw it. And so the word went out that the supernova had gone off. I got a fax. I was in Japan, in Tokyo, at the time. And I got a fax from Sid Bludman at Penn, who was a particle physicist turned astrophysicist, telling us about this. But it came before I got into work, and by the time-- I was living in Tokyo about an hour's commute from the university, and I always waited until the rush hour in Tokyo was over, so I would come out alive on the other end. (both laugh) And by the time I got in, the Japanese had seen the fax. They had called their astronomers, and everything was known, so they called off to the mine, which was in western Japan where the Kamiokande detector was, and it turns out that the graduate student who was on shift had done his weekly chore of putting all of the data tapes on a bus to send them to Tokyo. So the data was on the bus for a few days.
(laughs) Oh no.
Until it got back to us. And when it came in, it came in late in the day, and two of the graduate students, one named Nakahata , I think he may be the director of the Kamiokande Observatory right now, and another one, Hirata , just started making a very long plot, that they plotted on a line printer the number of triggers in the detector as a function of time. And this went up page after page of line printer output. I think they ended up with a couple or three feet pile. But they just stood by the line printer and they watched the events go by, and bang, there was one that was way off-- Not off-scale, but way above all of the backgrounds.
Can you just help--
And that turned out to be--
Can you help visualize the event? When you say a supernova "went off," what does that mean exactly?
Okay, there are many types of supernova, , but in this type, a heavy star has burned up all of its nuclear fuel, and the radiation pressure from the burning of the nuclear fuel then dies out, so the star collapses gravitationally. And there's nothing to stop it until it gets down to the neutron star, or possibly a black hole. But when it gets to be very, very small, then the primary way that it can lose energy is by emitting neutrinos, and so there's a huge burst of neutrinos. What you see in the sky is, I don't know, 1% of the energy, and the rest is in neutrinos.
And is this, how rare, historically, is this kind of occurrence? I mean, when this happened, is this like a once in a lifetime opportunity?
Well, this was the first one that had been visible in something like 350 years. So (laughs
And visible-- And that, irrespective of telescope technology?
Yeah, visible to the naked eye. And anything in our galaxy that became a supernova would be visible to the naked eye.
And what's the window of opportunity? I mean, this thing goes off, and how much time do you have to make the measurements that you need to make?
Well, we're taking data continuously. And so this just appears as an approximately ten seconds burst of events during the time that we're taking data. So.
And what exactly is the-- I mean, you're doing these experiments before the supernova even goes off. So what impact does this fact that the supernova goes off, what impact does that have on your research, if you were perfectly content to, you know, you didn't know it was going to happen, and you were perfectly content to continue on with your research had it not happened. So what impact did this event have on your research?
Well, it was huge. People had predicted this neutrino emission for some number of years. (clears throat) Excuse me. But there hadn't been any supernovae, of course, to test it. And in fact, this was the first data to appear that confirmed that people understood what was going on inside stars was as they had been calculating. So that was a big deal. In fact, I was exhilarated by it. I felt that if that was the only thing I did that was interesting in my life, that that would be enough. (laughs) But another way of looking at it was that it was a test beam that showed that the detector that we had could detect low-energy neutrinos.
And that those neutrinos from the sun are very rare. They don't come in a burst. They came one every few days, when we first started out.
Right. Right. And I wonder if you can connect, I mean, just for the broader audience that's going to be so interested in this. How does this research, and what you were able to confirm, how does that work into sort of broader concepts of physics and how we understand how the universe works?
Well, I'm not sure what level to answer that question. It's one of these things that you put together science from many, many different fields. It requires nuclear physics, particle hysics, astrophysics, atomic physics. You put it all together and it actually fits together and gives you a picture. To me, it's really amazing that we are able to do that, to understand our place in the world and how the world works well enough to make these models and then to be able to test them.
And what do you mean, "our place in the world"? What does that mean?
As human beings, as animals. That we are able to understand our surroundings. And that applies on the very small level, as in atoms. It applies on all of the levels understanding medical physics, understanding nature. Understanding the cosmos. That we understand this. People didn't understand these things 500 years ago.
Right, right. Can you talk about your more recent research with the Sudbury Neutrino Observatory?
Sure. in the neutrino experiment we did at Brookhaven, we looked for neutrino oscillations and one of the people who was interested was Herb Chen , who was at the University of California at Irvine at the time. A few years later, I took a sabbatical and I went to CERN for half a year, and then I worked in Herb Chen’s lab, actually trying to figure out how to use liquid noble gases to do neutrino-less double beta decay. But when I finished that sabbatical, I realized I couldn't carry on research on the west coastand went my own way. And Herb came up with the idea of using heavy water as a target for solar neutrinos. And he was fully aware of what we were doing in the Kamiokande experiment in Japan. So we would talk about things so that he could understand how to build the detector using heavy water. And heavy water would have the advantage that the solar neutrinos could interact with almost free neutrons. Heavy water has the heavy isotope of hydrogen, deuterium, which has a proton and a neutron. Ordinary water wouldn't work because the solar neutrinos are low energy, and they don't interact with protons. But they do interact with neutrons. So that's the key thinking there. And so I've been interacting with Herb. In one of our conversations it somehow came up that I might eventually become a collaborator on this. This was still mostly an idea at the time, although he had been recruiting people in Canada, which had a reserve of heavy water. And unfortunately, Herb came down with leukemia and passed away. But, the people in Canada carried on, and at one point, they apparently were in contact-- I guess, the Irvine people were still involved, but they didn't have a leader. Fred Reines from Irvine called me up, and said that the Canadians were looking for somebody like me to join into their experiment. And so I got in touch with the Canadians, or they got in touch with me. I can't remember. And I went to several of their collaboration meetings and somewhere in the end of 1987, I signed up to be part of the collaboration that became the Sudbury Neutrino Observatory collaboration. And that experiment had a real appeal to me, because it was a no-lose experiment. The idea was that this deficit that Davis had seen relative to what John Bahcall had predicted was confirmed by our work in Kamioka. So that was the first time that was confirmed. And it was done with the directional real-time detector. So we knew that the neutrinos were coming from the sun. It was because they pointed back to the sun.
And when you found that the neutrinos undergo "flavor transformation," what does that mean?
Oh just a minute, you're getting ahead of the story here. Because the thing is that with the Sudbury Neutrino Observatory, it had the capability of distinguishing whether the total number of neutrinos was the number that was predicted by John Bahcall or whether something else was going on. So when we measured the, it's called the charge current reaction, the neutrinos scattering off neutrons, the result could have been just consistent with what we had seen in Kamioka, and what Davis had seen, but the other possibility was that the neutrinos were as John Bahcall had predicted for the sun and were changing to other "flavors" and that there were neutrinos that would be missed by other detectors, but those would be seen in a neutral current interaction in the Sudbury Neutrino Observatory. And so that meant that we might see everything that Bahcall had predicted, and that some of the neutrinos had transformed into other flavors, namely the muon and tau type.
And the other possibility was that we wouldn't see that. In which case, there would be a real mystery that perhaps the neutrinos were transforming into sterile neutrinos, neutrinos that didn't interact.
And what would have been so mysterious about that?
Nobody would-- Well, people had suggested that possibility, but nobody could fit them into anything at all. No, for the standard model for neutrino oscillations to exist, you have to go beyond the standard model and give neutrinos mass. But the standard model doesn't accommodate sterile neutrinos at all. That would be a big step beyond the standard model.
And where is the current research on this?
Well, neutrino physics as a whole has come together, so that now, with a number of experiments, we've measured almost all of the parameters of the oscillations of neutrinos, but there are two missing points. And those are being worked on right now. One is that the neutrino masses, the mass of the electron-neutrino, muon-neutrino, and tau-neutrino can come in two possible hierarchies. It turns out that that's all that's left after you do the solar neutrinos and make that measurement. Whether two neutrinos are heavy and one is light, or one is heavy and two are light. And so that's called the mass hierarchy. And that can be measured in experiments that are ongoing now. And the other one is CP violation, which is whether neutrinos and anti-neutrinos behave the same way, or behave differently. And CP violation was discovered back in 1964, it's been seen in many quark systems, but just this past month, there was the first indication that it's been seen in neutrinos from a follow-on to the Kamiokande experiment in Japan. And so most of the modern neutrino physics is aimed either at measuring the mass hierarchy or CP violation, or exploring whether there are sterile neutrinos. There are hints of sterile neutrinos from a number of experiments, but nothing that's convincing.
Well, that brings us up to present. So, I wanna ask a few sort of retrospective questions. First, is your career as a teacher and as a mentor. First, on the teaching side, I wonder if you could talk a little bit about, what are some of your favorite courses to teach? And to whom? Do you like to teach classes to, you know, like a Physics 101, where the students are not necessarily physics majors, but they're interested in some general concepts? Do you like teaching upper-level undergraduate courses? Do you like teaching more graduate students? I'm just curious, both the kinds of students you most enjoy teaching and what courses you enjoy the most to teach?
Well, I enjoy the introductory courses a lot, but the ones that I really like are the undergraduate majors. And I think the reason is that the introductory courses have a lot of people taking the course who are there because they have to be there. It's a part of a requirement. And for the undergraduate majors, they're all there because they want to be there. And they're really interested. And it's also showing them at a more sophisticated mathematical level why the things that they learn in the introductory courses actually work. With the graduate students, you're working at a yet a more sophisticated mathematical level, but they've all seen the physics before. So I find that less interesting.
With the undergraduates, you know, in the introductory class, the ones that don't necessarily wanna be there, you have a unique opportunity with this captive audience. If there are things that you hope that they take away, even if they never think about physics again, what do you try to convey at that level to those kinds of students about physics?
Well, I, in a very general sense, I think it's that nature is describable in rather simple ways. I once asked a friend who was a doctor what his takeaway was from his pre-med physics, and he said that he never really believed until he took physics that the world could be described by numbers.
Wow. (both laugh)
And that had a big effect on me. That was sort of the attitude that I took towards teaching the introductory courses.
Uh-huh. Uh-huh. And what kinds of advice do you give undergraduates who are interested in pursuing a career in physics? In terms of graduate school, whether to enter into industry, what are the kinds of things that you emphasize that are important?
In the syllabus, the syllabus is rather fixed, to some extent it appears in the grading, which may be a function of their interest rather than their ability, and that is the A students are ones who are likely to be going to really first-class graduate schools, the B students will be successful in physics if they choose to. And maybe won't get into first-class graduate schools, but they'll get into respectable schools. And the C students probably shouldn't be going to graduate school. Now, having said that, I would say that about half of the undergraduate majors that I was teaching were actual dual majors. And what they were dual in would range over all sorts of things, from Russian literature to finance to microbiology.
I wonder if you have any comments, given Wharton and the rise of the, what are they called? The physics geniuses who go to teach, who go to make a lot of money on Wall Street? I wonder if you have any comments about the application of physics and finance?
Well, I don't particularly, but a number of our former students have gone that route. and become quite successful. I remember one of my colleagues, who was a theorist, had a graduate student who came back a few years later and was making twice what his mentor was making.
Sure, sure. And on the question of mentor, I wonder if you could talk a little bit about some of your most successful graduate students and the kinds of collaborations you pursued with them and what kinds of careers that they went on to achieve?
I think that interesting physics attracts the best graduate students, and the best graduate students I've had have been the ones with the Sudbury Neutrino Observatory. And a number of them have gone on to faculty positions, or research positions. There's one who's actually a faculty member at the University of Illinois. Another one at Cornell. There's a young woman who's at Heidelberg in Germany. So those are really good students.
A sweeping question, you know, looking back in your career, and I think it lends itself particularly well, given the kinds of experiments and research that you've been involved in. If you sort of trace from the beginning of your career to now, can you explain in your field and to you personally, what was really not understood when you started off in the career in physics, and what is understood today? Partly as a result of your work and partly as a result of your involvement in your specific field?
Well, when I started out, there wasn't a lot of order in any of the field. People who worked with bubble chambers and other techniques, were discovering a zoo of particles. I remember that when I took Dave Jackson's elementary particle course, after the first month or so, he would come in each Monday with Physical Review Letters and discuss a new particle that had been observed. And that happened almost weekly.
So you really felt like you were operating on the frontier, in a way?
Yes, we had no idea which way this was going. The parity violation had been seen shortly before I started graduate school. CP violation was seen, but there was no context for it. We had the Fermi theory of weak interactions, which was very nice, but couldn't possibly be the ultimate theory.
And so it was in the late 60s when Weinberg and Salam came together with electroweak theory and then later on Glashow added quarks into the theory. And so that's getting into 70s now. And as I said, the quantum chromodynamics, the theory of the strong interactions didn't come along until the middle 70s. But then there was this, I think, exciting time when everybody was making measurements that sort of put what we call the standard model together. I actually had a graduate student in roughly 1990 who commented that I had lived through the golden age of particle physics.
And I think he hit it on the head there.
Had that occur-- I mean, I was thinking that also. Had occurred to you, sort of, in thinking about how well-positioned you were in terms of the area you were working on and all that remained to be discovered when you were working on it?
Not until the student mentioned it to me. (both laugh) Yes so.
So to flip that question on its head, what remains unknown or mysterious to this day that may or may not be surprising to you, given all of the advances over the course of your career?
It's hard to say where particle physics is going or can go, given the limitations of doing experiments in the field.
And what are those limitations? What are you referring to?
You have to build gigantic machines to get the high energies. There isn't a very strong motivation right now in the sense that we know that something has to be observable when we spend all that money.
And is there, do you really see a direct correlation between discovery and cost and more energy? Are those all things, I mean is it certain, if you were... In other words, if you were testifying before a budget committee in Congress, and they wanted you to really, you know, explain why it's worth pouring all of this money into this new-- Would you be able to say with conviction that, you know, "We will discover new things if you give us the funds to do it"?
I am not sure that I could do that as easily in particle physics as a cosmologist could. Cosmologists started their golden age just when my graduate student made that comment to me, and there is a lot more to be seen in cosmology than... So there are huge experiments, or facilities. The James Webb telescope, for example, which I guess is delayed in its launch now until 2022. And I hope it works, since we're putting $15 billion of instrumentation in a place where we can't service it.
Have you ever experienced a dramatic sort of eureka moment, like a burst of insight that came quickly? Or do you see most of your work as incremental, and it's only in retrospect that you see where the discoveries really were?
The only thing that came in a burst was that supernova.
Literally. (both laugh)
Well, it was literally. Yes. Some of my colleagues in other sciences say that they need more instant gratification than (both laugh) we do, because our projects take a long time.
But when we finally do get the answers, like when we got the answer, the discovery results in the Sudbury Neutrino Observatory, that experiment, that was quite exhilarating. It was also the same week that my son graduated from college. (laughs) And so it was complicated because I was hustling there, and I was on the phone with news reporters everywhere.
Looking back, are there any experiments that you wished you had been a part of that you weren't? Or were there any experiments that you were a part of that you would have done differently given the opportunity?
No, I think I chose wisely. There were opportunities, for example, when we were doing a lot of work at Brookhaven. Around 1980, they were building a collider called ISABELLE , and some of us were working very hard on designing a detector for that. Brig Williams and I, when the ISABELLE project was canceled, went off to Fermilab and were interested in joining CDF, the mislabeled collider detector facility. It was still-- it doesn't detect colliders. And I then went off to my sabbatical and I ended up getting very much interested in neutrino-less double beta decay, and never went back to the collider world. The thing that we're working on currently is neutrino-less double beta decay, but that's, what is it? 30-some years later.
Yes. Well, Gene, I think that gets me to my last question, which is that forward-looking question. What, both personally and in your field, looking forward, what motivates you? What excites you? What discoveries are out there that are within grasp that you think are, you know, might point the way to whatever the next golden age in your field might be?
I don't know whether it will point the way, but I think neutrino-less double beta decay is one of the most interesting things, because if it is seen, then we know that lepton number is not conserved. And the neutrinos have at least a component that is different from the one that was presumed many years ago. And that would imply some mechanism for how the neutrinos get their mass, which would involve physics that we don't understand some very large energy scale. Now whether we could actually ever get to that energy scale is doubtful, I think. But it would imply that there's more out there, at least.
And when you say there's physics involved that we don't understand, what-- Obviously, if we understood it, you'd be able to answer the question precisely, but what is it that we don't understand?
Well, we don't understand--
What are we lacking right now?
We don't understand the mechanism that gives neutrinos their mass. We have models, but they all involve physics at some greater energy scale.
Yes. And what's--
And that's what we don't understand.
What's standing between the current state of play and getting to that level of understanding? Is it technology? Is it the next genius? What is it?
No, it's technology again. The experiments have become so sensitive that they have to become very, very large and very, very background-free. And that is a real technological challenge.
Uh-huh. But achievable? The budget is the bigger question than whether the technology is achievable?
I think that we will learn how to do the technology. We may not have it all yet. It's something that has to be done in stages.
But you learn at each stage.
Well, Gene, it's been an absolute delight speaking with you. I really appreciate our time together.
Great, I've enjoyed this too.