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Interview of Berndt Muller by David Zierler on July 27, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with Berndt Müller, James B. Duke Professor of Physics at Duke University. The interview begins with Müller discussing his current work on quark-gluon plasma physics and the connections between nuclear physics and cosmology. Müller then recounts his family history in Germany during and after WWII, as well as his childhood in West Germany. He recalls his undergraduate studies at Goethe University Frankfurt, where it was the inspiring lectures that catalyzed his enthusiasm for physics. Müller explains the heavy ion research he was involved in at the time, as well as his master’s thesis on the Dirac equation. He recounts his first visit to Berkeley Lab in 1972 and his subsequent acceptance of a postdoc at University of Washington and a fellowship at Yale. Müller then returned to Frankfurt as an associate professor and explains how he got involved in quark-gluon plasma research. Müller talks about the creation of the RHIC and how that led him to pursue his next job in the US, landing at Duke. He discusses his involvement with the Institute of Nuclear Theory at the University of Washington, as well as his work at Brookhaven over the years. Müller recalls the pros and cons of the administrative side of academia, which he experienced as the Chair of the Faculty of Physics and then Dean of the Faculty of Natural Sciences at Duke. The interview concludes with Müller’s reflections on winning the Feshbach Prize and his predictions for the future of theoretical nuclear physics.
Okay. This is David Zierler, oral historian for the American Institute of Physics. It is July 27th, 2021. I am delighted to be here with Professor Berndt Müller. Berndt, it's great to see you. Thank you so much for joining me.
Well thanks for inviting me.
Berndt, to start, would you please tell me your title and institutional affiliation?
I am the James B. Duke Professor of Physics at Duke University in Durham, North Carolina.
When were you named to that chair, and in what ways is the benefactor possibly connected with your research or the things that you work on?
I was named to that chair in 1995, if I recall correctly. The Duke University was founded by James B. Duke with his tobacco fortune. He also donated a number of chairs across the university. I hold one of two James B. Duke in the physics department.
Berndt, just as a snapshot in time, what are you personally working on these days, and more broadly in the field, what are you following? What's interesting to you?
Well, my personal interests at this point, I'm still working on quark-gluon plasma physics. I mean there's a lot going on experimentally, but also there are quite a few theoretical developments, which I try to keep abreast of. And after my several years in the Brookhaven administration, there is quite a bit of catch up to do on that. But I'm in the process of writing one or two shorter papers over the summer. And then I'm particularly interested in the question of thermalization in quantum systems. That's connected to questions that have to do with how systems look thermal, although they may actually not be thermal in the traditional sense. There is a deep connection with the quantum coherence and quantum entanglement. It's a very active area and quite exciting.
Berndt, a very broad question. For all of the fields that you follow and are involved in, what is your state currently of the interplay between theory and experiment, or theory and observation? Where are the theorists out in front of the experimentalists, and where are the experimentalists out in front of the theorists?
I would say on the side of the quark-gluon plasma, there is no question that the experiment is ahead of the theorists, right? Because both the facilities at Brookhaven and the LHC are gathering tremendous amounts of data. And they have many more data that are interesting than theorists can actually make sense of. So there, I would say the experiment is clearly in the lead, although there are always theoretical speculations that are looking for experimental verification or falsification, but generally, I would say there, experiment is in the lead. And there is a lot of experimental data that theorists can dig their teeth into. On the other hand, the other area that I mentioned that has to do with thermalization of quantum systems and so forth, this is currently I believe a theoretically-led area. There are some very interesting experimental new developments, in the lab in particular, in connection with Bose condensates that can be probed in very elegant ways. But mostly this is a more theoretical and conceptual area that connects, of course, nuclear physics with gravitational physics like black holes, with quantum computing, and so forth. So in this case, I think theory is far ahead of the experiment.
I wonder if you could reflect broadly on where you see quark gluon plasma research in the way that it might connect nuclear physics with cosmology?
Generally, I think the connection between nuclear physics and cosmology was over-estimated in the beginning. I mean the whole area emerged out of the question, what is the matter that fills the universe in the beginning, when it was realized that the universe was originally hot. At the highest temperatures, what actually is the matter in the universe? And that was what originally drove the field. On the other hand at this point, we have come to understand that the implications of the details of what constitutes that matter in terms of cosmological evolution, is really not a dominant aspect. Much of what's happened at the highest temperatures is hidden by the black body radiation that, you know, is coming from the recombination of hydrogen atoms or just the neutralization of the cosmos at temperatures of a few degrees. Whereas we probe much, much higher temperatures now from the laboratory in heavy ion collisions.
However, there is a new connection that actually has developed in the last few years, and that is the very dense matter. Not so much hot matter, but dense matter, as it exists in neutron stars. This connection will be probed in the next decade or so, I think, through the observation of neutron star mergers. This is a very exciting, interesting field. This is not temperature-dominated, it is really density dominated. And lies probably outside the domain of conditions that nuclear physics, nuclear collisions can actually probe. Not far away from it, but somewhat outside the domain. Nonetheless, it's a very interesting new direction that is now being explored.
Berndt, I'd like to ask a question that perhaps has cultural or even generational components. With regard to your interest and your research in black holes, did you come of age scientifically in a place and at a time when black holes were real, that they were not just theoretical abstractions? Or were you part of that transition, where you saw that happening?
No, I think at the time when I was a graduate student, I began to think a little bit about black holes. Anyway, it was a purely theoretical construction. Nobody had identified one. As a matter of fact, there were discussions, "Do they, black holes, really exist, or do the laws of nature, of general relativity, break down in some form before a black hole can be formed?" These were questions in the early 1970s that were being pursued, and then of course the revolution of Hawking radiation and so forth happened in the mid-1970s and the understanding that black holes were most likely real and that they posed some truly fundamental theoretical questions that are still not fully understood. But the experimental evidence for black holes really emerged only over time, and by now I think nobody questions that they exist. A Nobel Prize was recently awarded for the discovery of a black hole in the center of our own galaxy, although it is a very indirect evidence. We now can observe black hole mergers by the gravitational waves they emit, and they fit general relativity very, very, very well. But all these are to some extent indirect probes of black holes, because I mean, it's obviously not possible to probe the interior of a black hole and come back safely.
Berndt, where do you see your work, again thinking very broadly, either as continuing to contribute to our understanding of the Standard Model as it currently exists, and possibly in building out new physics beyond the Standard Model?
I have not personally worked on extensions beyond the Standard Model. I think that's to some extent from a point of view and-- let me backtrack. The reason I like doing physics, in particular theoretical physics, is not only because it matches what I believe are my talents, that are a combination of mathematical skills and physical intuition, but also because it is connected to hard experimental evidence. At present we have absolutely no hard experimental evidence for any physics beyond the Standard Model. I mean, we know that the Standard Model doesn't explain everything. But it explains nearly everything, and none of the experiments that have been done at high energy colliders have brought clear evidence of physics beyond the Standard Model. Right? So in some sense, that area is very much filled with speculation that's very interesting and very tempting, but it is not the kind of physics I like to do.
A more contemporary question, tell me about your commitment to the creation of an electron-ion collider, what new science might come as a result, and why the Department of Energy more than other agencies might be the source in seeing this come to fruition?
Well, this is a combination of opportunity and scientific interest. You can think of the electron-ion collider as the ultimate electron microscope, which will probe the structure of atomic nuclei and in particular that of the hydrogen atom nucleus, the proton, with unprecedented precision. And we still don't really understand the internal structure of the proton and certainly we don't understand in any detail the quark-gluon structure of atomic nuclei that are more complex. The fact that we've just recently revised the value of the radius of the proton is an indication of how little we really know. So there is a big gap in understanding of how quarks and gluons build the matter that surrounds us. And the only practical way to explore this quantitatively and precisely is an electron ion collider. Now actually in principle, a muon ion collider could do that as well, but of course that's orders of magnitude more difficult to build. Because nobody has built a muon collider, although people have thought about it, and maybe once in the future that will actually be done, but the practical thing to do now is build an electron ion collider.
The opportunity aspect comes in because the US doesn't have to build such a machine from scratch. It has facilities that can be converted into an electron ion collider, and it chose the one at Brookhaven that already provides the needed ion beams. That's a roughly a $2 billion investment that's already there. Now it turns out that converting this into an electron-ion collider also will cost something on the order of $2 billion. But you can imagine that if you had to build this all from scratch, it would be far beyond what the Office of Nuclear Physics in the DOE or the scientific nuclear physics community in the US or even Europe could hope to build.
So in other words, there is an opportunity here for the US to do something and claim an area of research that's currently wide open and build on its existing infrastructure to do that. And I think there's great interest around the world to participate. To some extent it is not a new area in terms of energy or kinematics. We have data that probe in that domain already. However, it is a new area in precision. And as you know, there are always two frontiers in physics. One is the energy frontier and the other one is the precision frontier, and very often you'll find new phenomena by simply making much more precise measurements. So that is really what the electron-ion collider will do. It has been touted as the ultimate machine to discover and map out the structure of protons and nuclei, and I think that's a fair statement.
Berndt, finally, for this initial portion of our conversation, a question that we're all dealing with, that we still continue to deal with, and that is during the pandemic and the mandates of remote work, how has your science been affected one way or the other? What have been areas where you've been more productive, and what's been challenging in not being able to see your collaborators in person?
Over the past year and four or five months that we have been in this situation, science has progressed in my view as well as possibly could have been imagined. And probably better than most people would have imagined. The fact that we have these powerful teleconference tools now, Zoom being one of them, has allowed collaborations within institutions, across institutions, even on the international scale, to proceed almost uninhibited. And in some sense, I would say even there have been some benefits from people not having to go on travel, which takes a tremendous amount of time and effort. And thus one might make an argument, although I haven't seen this made quantitatively on the basis of data, one might make an argument that actually the scientific community of the past year-and-a-half has been more productive than it would otherwise have been.
Now there is the experimental aspect, and we've been fortunate in Brookhaven at the heavy ion collider, that we managed to figure out that after the initial few months of complete shutdown, we'd be able to restart the program and complete it and carry it out almost on time, and within the full range of what was planned thanks to the heroic effort of many people. I don't think that's maintainable for a decade. But it certainly has been maintainable for about a year and a half, and maybe will be maintainable for about another year or so, if necessary. But when it comes to building a facility like an electron-ion collider, which involves a lot of on-the-ground work, different trades coming in, many people coming in from the outside, assembly, and so forth, the current situation would, if it continues, lead to a significant delay. And so I would say at this point, I think science hasn't suffered. It may have even to some extent benefitted in terms of its productivity from new ways of interacting and avoiding time consuming travel. On the other hand, one has to recognize that getting to know in person other people, especially for young scientists, is absolutely critical. So, from the point of view of the next generation, if this continues, I think it would be really deleterious. So far, I think, it's been probably roughly neutral overall.
Well Berndt, let's go all the way back to the beginning. Let's go back to Germany and let's start first with your parents. Tell me about them.
Okay. Let me begin with my mother's side. My mother's family, and I'll go back a little bit further than may be commonly done. My mother's family originally was among refugees from Bohemia who had to flee religious persecution. They were musical instruments makers, and after the Thirty Years war, they had to flee. They settled in Saxony just across the border and brought their trade with them. In around 1860, my great-great grandfather founded a musical instrument factory there, which grew into one of the largest factories of its kind in the world. It was very well known. And so this is where my mother was born and grew up, and eventually I was born after the War. But as you know, in East Germany, which is where Saxony is located, the conditions both during the war and after the war became very problematic. Right after the war, my grandfather was considered by the Communists as an enemy of the people, as capitalist, was imprisoned, and died in one of the prisons run by the Soviet secret police.
My father, on the other hand, his father ran what's called a post station, in the 1920s and early 1930s, and at the beginning of the Depression, he went bankrupt. My father was being educated at a boarding school because they lived in a small town that didn't have a high school. He had to stop his education and begin learning a practical trade, so he became a salesman. And when Hitler came to power, he was drafted first into the Reich labor service, then into the German army. He was released just before the Second World War, but redrafted shortly afterwards and then spent the next four years fighting in, well, various parts of Europe. In particular, at the end, in Russia where he became a prisoner of war in 1944. He only came back, was released in 1949. So he had an incomplete high school education. My mother also had completed maybe six years of high school, but again at the time, young women were not necessarily considered to have to complete high school. She would have liked to, but her parents had other things in mind, and so they both didn't complete their high school education. But under other circumstances, I'm sure they would have.
I was born shortly after my father came back from the Soviet Union, and my parents married. Since my father was originally from West Germany, that's where he was released to. He moved to Frankfurt am Main, where he had relatives and began to build up an existence as a salesman. I stayed behind in Saxony, in the town where I was born, Markneukirchen, until 1953. Because my mother had joined my father in West Germany, I was basically brought up by my grandmother. She was tipped off in early 1953 that it was very likely that the border would be permanently closed. And so in May 1953, she took me and my aunt and fled to West Germany. Which was not that easy, because the way to do that was to go through Berlin. You traveled through the Soviet sector and took a tram, which many people did because there were, you know, commuters. People who lived in East Berlin, worked in West Berlin, and so they traveled back and forth every day. That was possible. But of course, two women with luggage and a young child were very conspicuous, and so they were apprehended and thrown into jail. Eventually my parents found out and contacted the American authorities. The U.S. High Commissioner in West Germany issued a piece of paper that authorized my transfer to West Germany.
Do you have any recollection of this?
No, but I have the documents. And my parents of course and my aunt told me the story many times. I was put on the plane and flown out to West Germany to be reunited with my parents, whom I hardly knew. And that's where I grew up. That's basically the history up to the point where I came to West Germany and grew up there.
Berndt, when you started to get older, would you ever attempt to ask your parents about their experiences during the war? Did they talk about that?
My father talked about it every day. I mean it was not something that was well known at the time or understood at the time, but he clearly had post-traumatic stress disorder. And from all the horrible experiences during the war, he was talking to us about them, and the prisoner of war time. He was talking about it to his family every day basically.
Did they ever talk or wonder about how Germany could have gotten to such a barbaric state with Nazism?
I think it is well understood that this... originated from a combination of the economic conditions at the time, the fact that Hitler brought order, and then a receptivity to propaganda. My father realized what was really going on, of course, when he was fighting at the eastern front in Russia. He saw the terrible, horrific things that were going on there. And he became very disillusioned with the situation. But as a common soldier and out in the field, there was very little he could do except avoid becoming a perpetrator of mass shootings and things like this himself. Which he did. The things that were going on were absolutely horrific. He told me often that when he came back on leave, home either to his family or my mother's family, he was shocked about the lack of understanding of what was really going on. The Nazi propaganda was very effective. And of course, anybody who realized what was going on and who tried to actively fight it risked his own life. On the other hand, he had quite positive words about the Russians, although he was five years in a Russian POW camp. Of course, many of the German soldiers there suffered from hunger and illness and so forth, and many died, but he always emphasized that the Russians themselves were completely poor and suffering from hunger. So relatively speaking, he felt that the German POWs were treated as well as was practical under the circumstances.
Berndt, was your sense from either of your parents that during the war, they were aware of what had happened to the Jews of Germany and of Europe? Were they aware of the Holocaust? Or that was kept secret from them?
Although the Nazis tried to keep the death camps secret, there were clearly visible signs that were impossible to ignore. Hitler himself had announced publicly that he wanted to exterminate the Jews. Everybody knew this unless they closed their eyes and ears. How much someone witnessed in person probably depended on where they lived. I wasn’t born, so it’s impossible for me to know for sure. What I do know is that my father, witnessing first-hand as a front-line soldier what happened at and near the war front, especially in the Ukraine, was well aware of mass shootings.
Berndt, when did you have the first sense that you were living in a divided Germany? Or that the Cold War was something that had obvious ramifications to your upbringing, your reality?
Well, I think I've been aware of as long as I can think back. I mean having fled from East Germany, this was something that was quite obvious, and so the fact that the country was divided and that the regime in East Germany was totalitarian and not a free society at all was quite obvious. So, it's not something that I became aware of but, as far back as I can think, it's always been present in mind. I've always been thankful for my grandmother for having taken me out of East Germany when it was still possible.
Berndt, tell me about your school, first as a young boy and then on to high school. What kinds of schools did you go to?
Going to elementary school was a great experience because I had a very engaged teacher who at the time led an experiment in which the classes were selected by aptitude, so he was leading a class of what was considered after the initial examination, the brighter students. Thus I was always surrounded by other bright young people, relatively speaking. I enjoyed school and I must say that as long back as I can think of, I enjoyed mathematics, which was really counting and calculating. I remember learning to count in Italian before I was ten years old.
Is this to say that long term mathematics was your entrée to physics?
Absolutely. Mathematics was the entrée into physics, and jumping forward, when I initially enrolled at the university, my major was supposed to be mathematics. But I double majored in physics and after a year, it became clear to me that I was more interested in physics because it was not just dealing with inventions of the mind, but actually with real things. In addition to dealing with relationships and concepts and constructs, it also involved the challenge of actually figuring out how nature worked. And that I found much, much more interesting than just, you know, proving theorems.
What kinds of opportunities did you have to pursue your interests in math and science, perhaps even outside of school, when you were growing up?
Interestingly, the teacher who is most vivid in my memory is my chemistry teacher. He was also one of these people who had been quite damaged by the war. In class, told us stories about his own horrific experiences that he had. And I'm sure in hindsight he probably suffered from similar problems as my father. He was happy to find me and a fellow student who were interested in, you know, pursuing chemistry experiments. He gave us basically free reign of the school lab. We could do whatever we want. The condition was that we would set up and prepare the experiments that he wanted to show during class. And I enjoyed that tremendously. It was not so much about mathematics, but it was really about exploring nature. In hindsight, I mean, this would not be possible today. We had access to any substance, how dangerous or poisonous it might be, at age 16. We could have done tremendous damage. (laughs) But it was fun, and we were smart, and we carefully read about the things we were doing. So nothing dangerous ever happened.
However, the experience taught me also that I was not good in experimental science. One occasion that's in my mind here is an experiment that's called Victor Meyer experiment. It's for the measurement of the molecular weight of volatile fluids. And the way it works is that it's a complicated glass apparatus which you have to heat, you have to evaporate the liquid, and then it goes over a series of tubes where you catch the vapor and measure the volume of air that it displaces. And then, by a relatively simple calculation knowing the molecular weight of air, you can deduce the molecular weight of that particular liquid. So we set this up for him, and because it's a tall apparatus, we put a Bunsen burner on the floor, and then we put the glass equipment up there, and eventually it came up onto a big work bench that was in the chemistry classroom. And because this was close to that wooden workbench, we put an asbestos plate in front of the wood to protect it from the flame. (No one was concerned about asbestos at the time.) Next time in class he performed the experiment. It takes about half an hour to actually do its work. And everything went smoothly. And then after class, we had to dismantle and store all the equipment. What we hadn't anticipated is that although the wood of that workbench would not go up in flames since it was protected by the asbestos plate, it would turn to charcoal under the heat. (laughs) And so in the place where that setup had stood, there was a huge black spot. Which stayed there for the rest of my time in high school. My teacher never said a word, but it was clear to me that I was not really good at thinking through all the consequences of an experiment. And there were other instances of that type.
This suggested to me early on that if I go into science, I should go into a theoretical science and not experimental science. Because I have a tendency of breaking things, but also I'm not really good at thinking through all the ramifications of an experiment. Which can be dangerous, or at least it can be, you know, suboptimal. So in school, I did a lot of reading in mathematics, advanced mathematics, on the one hand, but then also spent a lot of time in this chemistry lab and that was overall a lot of fun. But it was clear that I didn't want to become a chemist, and theoretical chemistry at the time was not what it is today, so that was not really an option. You really couldn't do many calculations in chemistry. That's only been possible now because of advanced computing. But at that time, that was not really an option.
Berndt, culturally, was it expected to stay home, to stay close to home, for college?
The answer is yes. Most students did this because there was not a particular hierarchy among German universities -- they were all state-funded -- and so it was fine to stay close to home. It was of course much less expensive if you didn't have to rent a place, you could live with your parents. And so that was the natural thing to do. And I think actually all those in high school that I knew at the time studied in Frankfurt. Very few went elsewhere. As a matter of fact, I tried-- because at age 18, I was eligible for the draft. And one of the conclusions I had drawn from the experience of my father, who had spent 13 years in uniform and as a prisoner of war, that under no circumstances I would join the military. Just as in the US, going to Canada was the way to escape from the draft during the Vietnam War, the logical way would be to study in Switzerland. And thus I applied to the ETH in Zürich. That was my preferred place. Unfortunately, they decided that they wouldn't accept me. A kind letter, but no, we're not interested in you. And so I stayed in Frankfurt and became a conscientious objector. I was accepted for that in a kind of trial. Thus I was not required to do military service, but in principle, I would have had to do civil service after completing my studies.
Berndt, as an undergraduate, what was most exciting to you? The lectures that were inspiring, the textbooks that you wanted to read more of? What were the things that spoke to you as you were developing your professional and academic identity?
No question it's the lectures. I found lectures most interesting, but on the other hand, there were not very many professors who gave really exciting lectures. There were two that stand out in my mind. One was a very senior mathematician. His name is Gottfried Köthe, who was working on differential equations, differential geometry and group theory. But he was almost at the point of retirement. And the other one was a young physics professor named Walter Greiner, who had just a few years before been appointed in theoretical nuclear physics. Maybe at the time he was the youngest professor in Germany, certainly in that field. He was able to give really exciting lectures and good in engaging the students. He would often pause during the lecture and turn around and say, "I'm stuck, who can help me?" And he also made it clear that not all the problems were solved. That there were many exciting problems you could work on if you're interested, and so his lectures were really inspiring. These were the main sources of inspiration. I also read textbooks of course, for example, Landau & Lipschitz I found very deep and concise and interesting, but they didn't generate interest. They were able to satisfy the interest in some topic, if you already had it. But to generate enthusiasm and so forth, I think there's nothing that replaces lectures, good lectures.
Was your transfer into physics from that mathematical interest perspective, would you say that was more gradual or was there a particular class or a professor that made you jump in with two feet rather quickly?
Well, I think that was Greiner's influence. I found that theoretically, physics was more exciting than mathematics because it dealt with the laws of nature rather than the axioms that people had invented. And also, it fundamentally had applications. Although mathematics also has applications. But they are a little bit further removed from the mathematics itself. Thus I decided after a year that I would switch over to a physics major. And it was clear from the beginning that I would not go into experimental physics.
And in the world of theory, what was exciting at that point, at least as it was conveyed by the faculty?
At that time in Germany, actually the region around Frankfurt, several universities had decided to build a new experimental facility for heavy ion research. Which is called Gesellschaft für Schwerionenforschung (GSI) which is now being upgraded into an even bigger facility. This was decided at just about the time when I became a student. It was an experimental facility that promised to explore regions of physics, of nuclear physics and atomic physics, that had been unexplored. And in particular the problem that fascinated me and the problem that my thesis advisor, Walter Greiner, suggested I should work on was to ask the question, what happens when an atom has a nucleus with a charge that exceeds one divided the fine structure constant, that is, 137? When you solve the Dirac equation for an electron around a point charge larger than 137, all hell breaks loose, and the solution fails to exist. Of course, there are no nuclei in nature with such a high charge, but the prospect existed that by fusing two nuclei it would be possible to actually extend the periodic table into that domain.
There had been a number of explorations, and there was a paper by Werner & Wheeler from the late 1950s which showed that if you take into account that the nucleus has an extended charge distribution, you can continue. And there had been a few calculations that said that you could do this until a nuclear charge of about 170. But if you actually had a beam of uranium nuclei, and collided it with another uranium nucleus, the total charge you get is 184. It thus seemed possible experimentally to actually probe into that domain once that new facility would be constructed.
And so that was what my thesis advisor identified as a problem that I was very interested in. He had a fellow student, Johann Rafelski, who actually had looked at how the Dirac equation could be modified in order to avoid this. The first modification that they came up with turned out to fail. It didn't avoid that problem. In other words, the problem existed even if you modified the Dirac equation. I wrote a Masters thesis then that showed the only way to avoid that problem coming up is if you violate gauge invariance, which was not something that anybody would consider seriously. And so, the problem was open. My advisor Greiner suggested that I should look at the bound states of a Dirac electron in the field of two nuclear charges, two nuclei that are approaching each other. Imagine a relatively slow collision between two uranium nuclei and ask how far does the Dirac equation work? At what distance between the nuclei does something strange happen?
And so that was my thesis. You could look at it from the point of view that this is the relativistic molecule problem involving the relativistic equation for an electron in the field of two charges. It's essentially an unstable molecule made up of two heavy nuclei. This is a difficult problem to solve, because it doesn't have analytical solutions, and it requires a lot of delicate numerical work. At that time, the computers that we had access to were very limited. In fact, the university didn't have any useful computer at the time when I began my graduate studies. The only way we could do calculations was on the weekends. The Deutsche Bank had, of course, a computing center. They didn't need it over the weekend, so on the weekend we could use the UNIVAC 9000 of the Deutsche Bank. That machine had 64k core memory. Not megabyte, kilobyte. And it was among the most capable machines at the time. And I needed to diagonalize large matrices to solve that problem. You could do just about a 100 x 100 matrix. That was just on the borderline of what was sufficient.
Really, in hindsight, it was insufficient for precise and reliable calculations. But that's what was available. This became clear fairly quickly, and my advisor said, "Well, why don't you, instead of solving this problem, first look at the question what happens if you had a nucleus with 184 protons, what would be the bound states?" It turns out that the binding energy of an electron around such a nucleus exceeds twice the electron rest mass. That means that the bound state is really degenerate with the state of an electron-positron pair. Instead of a true bound state, you have a bound state imbedded in a continuum of states. So, I set out to solve that problem. You can do this pretty much analytically with minimal numerical calculation, and so that was my first publication really in solving the problem of an electron in the Coulomb field of a nucleus that is, as we called it at the time, super-critical. That was the first real publication that I coauthored. It was accepted in Physical Review Letters, which was very nice. It solved the problem of what happens when you have two uranium nuclei that approach each other and eventually fuse into a compound nucleus. The molecular problem, however, still hadn't been solved, and so I continued to work on that in my Ph. D. thesis.
Berndt, to go back to that original question about the interplay of theory and experiment, what was going on if anything in the world of experiment that may have informed your research, that was driving it forward at this point?
There wasn't really anything happening yet in the world of experiment, but there was a prospect of it happening once GSI was completed. And that is what excited me, because you could really make a prediction. From the point of view of a theoretical physicist, there are two situations that are really interesting. One is when you're ahead of the experiment and you can make a prediction, which may be verified or falsified, or you have wonderful experimental data and you can find an explanation. I think most theorists would agree that making a prediction is more exciting. Because in a sense, you're ahead of the experiment. You are not following. If you do a calculation to explain the outcome of an experiment, either your calculation agrees with the data, in which case it's very nice. Or, if it doesn't agree with the data, it's useless. But if you make a prediction, then it is useful in any case because it motivates the experiment. Right? So the fact that there were no available data and it was an area in which you could make theoretical predictions was really the most exciting aspect of it. But knowing that the data would come within the decade was also important because if you make a prediction and it takes 50 years for it to be tested, that's not particularly rewarding unless you live very long.
Berndt, as a graduate student, how parochial was your research world? In other words, were you aware of what was happening outside of Germany? Were you following events in England, in France, in the United States, in Japan? Was there a more global purview you had, or that would only come later?
Absolutely. My thesis advisor had been a postdoc in the United States, and he was well aware of what was going on internationally. While I was still a graduate student I got a first-hand introduction to U.S. science, which was the next step in my evolution as a physicist. My thesis advisor used to spend the summer months at various places in the United States when the German university system had vacations. And so, in 1972, when I had just finished my Masters degree, he took me along to Lawrence Berkeley Lab, as it was called at the time. That was incredible experience because, first of all, if you can imagine a place that is very different from central Europe, it's the Bay Area in California. The Berkeley Lab had a number of Nobel Prize winners, really famous scientists. In particular, I remember meeting Glenn Seaborg there, who was leading the Nuclear Science Division and who with his staff had found a great number of new elements. And I met a number of other people, including Art Poskanzer, whom you've interviewed. Among theoreticians, Wlodek Swiatecky and quite a few others who were already doing things that the German community was dreaming to do.
This was a very exciting time. I met some graduate students from Berkeley also at the time. In particular, I remember Miklos Gyulassy, who was just a beginning graduate student and came up from campus once in a while. He also got interested in the same problem I was working on. Berkeley had much better computing facilities than we did. So I brought my computer program with me, which was all in punch cards. Unwisely, I had it shipped as it was a huge stack of cards, and the plane had luggage limitations. But thanks to the inefficiencies of the international mail system it arrived half burned, which made the punch cards useless. I had hoped to complete my thesis there, so I did other calculations, but what I enjoyed most was a tremendous broadening of my experience, much beyond what I could have had in Frankfurt. It turned out, and we didn't know this at the time, that there were similar questions being asked in the Soviet Union by Zel’dovich and Popov and others. But they published in Russian journals, and it took a while until these publications were translated. We had no direct contacts with them at the time, and so we only became aware of these publications, which solved essentially the same problems, after we had solved the problems ourselves. I had no connections to Japan at that time, but I was very well aware of what was going on in other parts of Western Europe and in the U.S.
Berndt, do you have a specific memory of when you thought you might leave Germany and pursue a professional career abroad?
Well, after I finished my Ph. D. I applied for postdoc positions in the US. And--
Specifically the US? You knew that's where you wanted to go?
Well, that was the place to go. I mean the U. S. was clearly seen as the most advanced place in nuclear and particle physics. So that's where you went. I got a postdoc position at the University of Washington in Seattle which, after having spent the summer in Berkeley, was considerably less exciting. It was a good university and still is a very good university, but it didn't have a national lab and there was a much smaller group of characters there. What made it very interesting for me is that this was in 1974-75, when the big revolution happened with quarks now becoming suddenly established, and quantum chromodynamics (QCD) having been invented by people like Frank Wilczek. QCD was becoming not just a theoretical construct, but actually useful in explaining experimental data. I learned a lot during that period, but from the kind of activity of research, it was much less exciting than what I had experienced before in Berkeley.
Was there a group at Yale that you slotted in with? Or you were working mostly on your own?
This was the University of Washington.
Oh, at Washington first, right.
Actually, there was supposed to be a group, a nuclear theory group, at the time under Ernest Henley and Larry Wilets. Unfortunately, Ernie at the time was either department chair or dean, so he was mostly unavailable. And Larry was going through a very problematic and all-consuming divorce. I saw him maybe half a dozen times in the whole year. So I was mostly on my own, which wasn't bad, because I had a lot to learn. However, before I actually went there, I spent a semester at Yale as a postdoctoral fellow. There was of course the lab led by Allan Bromley, Wright Nuclear Structure Lab. One of the professors there, Jack Greenberg, was very much interested in performing the experiments that would probe what happens in the super-strong Coulomb fields, when the facility at GSI would come online. He was performing, so to say, his preparatory experiments at the accelerator at Yale, looking for these kind of quasi-molecules that are formed out of slowly colliding nuclei. Not looking for electron-positron pair production, but searching for photon emission. I started to do calculations that could be compared with his data. One could make predictions, one could explain the data that were being measured, and so forth. That was the first time that I was able to make direct contact between data as they came out and the calculations that I was doing.
So that was quite exciting, but it was in a sense only the preparation for the experiments that were supposed to come later after the GSI facility was operational. What I enjoyed about Yale was that it had much more of an academic community than the University in Frankfurt. First of all, Yale is a residential campus. Most of the students at Frankfurt lived at home. You went to the lectures, but then afterwards you all went home. You had friends that you interacted with, but the university was not the place until you went to do graduate work. Whereas Yale had a lot more to offer in terms of a close-knit intellectual atmosphere. And it had a tremendous cultural atmosphere. It had a wonderful theater, wonderful music events and so forth. So, what I learned there was what a leading American research university can offer, not just on the research side, but also on the cultural side. And that I found very interesting.
Berndt, did you have opportunity to interact with Bromley?
Yes. I met him many times. He was very much interested in the research I was doing. And he was a very impressive character.
What was he working on at that point?
Well, he was managing the lab. And he also had already, I believe, become quite interested in science policy. He was similar to Glenn Seaborg, who at the time also was directing the whole nuclear enterprise at Berkeley. He was not directly doing any science, but he was very supportive of the science that people in his lab were doing. He was very interested in what I was doing, what Jack Greenberg was doing, which was very reassuring and motivating, and he was accessible. You could walk into his office and say, "Here is something really exciting," and he listened and was excited about it.
How long did you stay at Yale?
I stayed one semester, the spring term.
And then did you return to Washington, or you went back to Germany?
No, after I finished my Ph. D. degree, I first went for a semester to Yale, then I returned to Germany, and then I went to the University of Washington.
Okay, okay. The chronology is a little confusing. (laughs)
Yeah, I'm sorry. Seattle is a wonderful area to live in, but it was not at the time booming like today. Things were cheap, and there was beautiful nature all around and everything, but from the point of view of the research environment, compared to Yale, it was a step down.
And to foreshadow, was there any evidence that the INT was in the works at that early juncture?
No, it wasn't. The INT was founded in the late 1980s, so this was almost 15 years before the INT.
Yeah, yeah. Perhaps some of the absence that you're feeling informed the importance in the decision in creating the INT?
Well, yeah, I mean the driving force behind the INT was of course Ernest Henley. But as I said, during my stay he was deeply involved in the university administration, departmental and college management. And so he had no time for research. It would have been very different if he had been around doing science at the time.
What was your next move? What were you able to do next?
At that time my stay in Seattle came to a close, I had two offers. One was for an assistant professor at Yale, and the other one was for the equivalent of an associate professor at Frankfurt. I was quite tempted to accept the position at Yale, because I'd enjoyed very much being there. On the other hand, I was also quite aware of the fact that assistant professors at Yale were glorified postdocs. They almost never would be tenured. Assistant professor positions at Yale were a good stepping stone for the next position. But they were not a permanent position, so I had no expectation for a permanent position there. Which was different with the position at Frankfurt.
There was a second aspect, which was that my wife also studied physics. She hadn't completed her studies yet, and it was much easier for her to continue her studies and finish up at Frankfurt than at Yale. So I accepted the position at Frankfurt and returned there in 1976.
And the Frankfurt offer came with tenure, or it was clear that that would be achieved in the short term?
The Frankfurt offer came with tenure because, at that time, all the regular faculty positions in Germany were tenured.
Meaning there is no assistant professor slot in Germany?
There were positions that were clearly untenured, but they were not regular faculty positions.
Like a lecturer?
They were not really like lecturers in the British system, but more like, you know, the senior assistant to the professor. But all the faculty positions were tenured. That was changed later. Nowadays, you have a system similar to the one in the U. S. You have tenure track positions and so forth, but at that time any regular faculty position was tenured. And that was of course also attractive. What made it even more attractive is that, as I mentioned, the experimental facility near Frankfurt, GSI, was in construction. It was supposed to start operating very soon. That was where my research really was aimed at the time, and so it was a great place to be.
Berndt, what were the research implications of this choice? In other words, is it possible you would have pursued different projects had you accepted the offer at Yale? Or you were set in what you were going to do and it really didn't matter what your home institution was?
Well, I think I would probably have partly continued to pursue the research I was involved in, but I would probably have branched out into other directions. So I think my career probably would have developed quite differently if I had accepted the Yale offer. I don't know in which direction, but I'm pretty sure it would have developed differently. One of the downsides of the position in Frankfurt that I hadn't fully appreciated at the time-- it's both an advantage and a downside—is that I had access to many graduate students because the large number of students relative to the number of faculty positions. The available support funds for graduate students were an order of magnitude larger in Germany than they were in the U. S. An assistant professor at Yale would have had maybe one or two graduate students. In Frankfurt I had ten or 15. The advantage of that is, of course, that you can pursue a lot of different projects, in which you supervise them. But on the other hand, it leaves you very little time to do your own research. Because you become almost a professional advisor. That's a huge difference between the U. S. system and the German system at the time. There are advantages to both systems. But what I hadn't fully appreciated, and what I felt after a while, after ten years or so, I sensed that I became much more of a science manager than I wanted to be at that stage of my career.
And you're saying that's inevitable, the system is geared toward that?
The system is very much geared to that. It's now a little different, because the number of professors has increased very much in Germany. They now have a tenure track system. In the same institute in which you would have maybe three or four professors, now you have ten. So the ratio of students to faculty members has changed. Thus it is a little different now from it was at the time, but still the ratio of students to faculty in Germany is quite a bit larger than in the U. S. Which necessarily leads to a situation in which faculty members are much more occupied with supervising research than by personally doing it.
I had actually been aware of this because I had a very good friend, a senior professor at Stanford, Walter Meyerhof, who was an experimental nuclear physicist, but he was not of the Allan Bromley type. I mean, Allan Bromley ran the lab in a similar way as a German professor would. He had all the other people doing the work. He was managing, he was providing the funds, the organization, everything, and he was just like Seaborg, he was directing it. Walter Meyerhof was a typical American faculty member. He had maybe a postdoc, maybe two graduate students, and he would be at the lab doing experiments himself. He knew all the details. It was a very different way of doing science than I was accustomed to in Germany. This changes the way that a senior person does science, because if you do it yourself, you know all the details. Otherwise, you only check that everything is right. (laughs) But doing it yourself is very different from supervising. I was much more attracted by that way of doing science that was more the standard in the American system, than the way you're doing science that was the standard in the German system at the time.
Berndt, to the extent that you were able to devote the necessary time to the research, during those ten years what were the big projects? What were you focused on at that point?
The main project during that time was to work out all the details, the theoretical details, associated with the experiment trying to produce a super-critical Coulomb field in the collisions of nuclei and observing the process of spontaneous electron-positron pair creation. And one of the big issues with this was that in order for the pair production to happen, you had to have a vacant lowest bound state in the atom. But when you collide the atoms, because the nuclei are surrounded by many electrons, most naturally those bound states would be filled with electrons. And then the pair production wouldn't happen. So, you actually had to predict the probability with which that bound state that was super-critically bound was vacant. The collision provides a mechanism for that, because it's time-dependent. Thus you have a natural excitation mechanism that can create a vacancy.
One of the big questions was to figure out what that probability was. We found a way of doing the calculation-- it's approximately 1%-- and we also found ways of check the calculation from the data. This required a lot of detailed understanding of what happens in an atomic collision that's slow compared to the motion of the electrons, but highly relativistic from the point of view of the motion of the electrons. There were lots of other interesting phenomena. You could measure the duration of a collision by looking for interference patterns in the emitted radiation and in ionization ionization probabilities. There was a lot of atomic physics of this type that was pursued in close collaboration and competition between the theory group at Frankfurt and the experimentalists at GSI.
After a few years, I became interested in what happens when you collide nuclei at much higher energies, because there were proposals for experiments that had been developed originally going back to the time that I spent in Berkeley. But the experimental program that was being developed was developed at CERN, where you have high energy beams of protons. A number of experimentalists figured out that instead of accelerating a proton, you also could accelerate a heavy nucleus and collide it with another one at very high energies, orders of magnitude larger than what you could do at GSI. The idea was that in such collisions you would convert the kinetic energy of the nuclei into thermal energy, and so you could heat up nuclear matter to extraordinary temperatures.
It became clear towards the end of the 1970s that the state of matter that would be created was most likely a plasma in which quarks and gluons were free and unconfined. This state was called a quark-gluon plasma. The question arose, if this is what happens when you collide two nuclei at the highest energies that you can access in the lab, how would you know that it's a quark-gluon plasma? Again here, theory predated the experiment in the sense of actually making predictions before data had come out, but it was triggered by the promise of an experiment happening within a few years. That fascinated me, and over the years, beginning around 1980, I developed an additional research program that was looking at these very high energy collisions among atomic nuclei.
Was this a parochial existence for you, or did you make pains to stay involved with what was happening in the United States during these years?
I very much stayed in contact with what was happening in the United States.
Who were some of the key people that you were collaborating with, or who you were at least in contact with?
I wasn't collaborating with anyone in the US. I mean, this was competition at the time. People who were leading this field in the U. S. were Gordon Baym, Larry McLarren, and Joe Kapusta. T. D. Lee was also very much interested in this field and supported it. So there were a number of people in the United States who were interested in the same physics. Their interest really was triggered by the prospect of building the Relativistic Heavy Ion Collider (RHIC). That proposal was made to the U. S. nuclear physics community in 1983. It took another 17 years to come to fruition, whereas the experiments in CERN started in the mid-1980s. There were also other experiments at Brookhaven at the AGS beginning in the late 1980s.
Berndt, when did you first visit Brookhaven?
I'm trying to remember when that was. It must have been sometime in the mid- to late-1980s. I don't remember exactly the date.
And earlier, you had already spent time at CERN?
I spent much more often time at CERN. I mean that was a train ride away. I had colleagues who were there, especially my friend Johann Rafeski, and so I went on visits to CERN maybe once or twice a year. CERN was much closer than Brookhaven, and it was only around the late 1980s that it became clear that there was a reasonable chance for RHIC to be built. Before that, it was one of the projects that might very well just be an idea, but not something that really would be constructed. It was the 1989 nuclear physics Long Range Plan that ultimately said: yes, we really should go ahead and build RHIC. And at that time, I had become somewhat frustrated with the way the German academic enterprise worked, as I explained to you. And I had become interested in this quark-gluon plasma related physics, and so I began to think that maybe it would be a good idea to move to the U. S.
Berndt, let's develop that a little more. What was your initial decision or idea that got you more fully involved in quark-gluon plasma?
Well, the idea was that you needed to figure out how to show that a quark-gluon plasma is formed. There were quite a few people who proposed signatures for the quark-gluon plasma. You could look for photons, you could look for lepton pairs. You could look for all kinds of particles that are emitted. A particular area that I found most interesting was so-called strange quarks. Because strange quarks don't normally exist in nuclei. They have to be formed by reactions, and the prediction that my friend Johann Rafelski made in 1980 was that, if you would actually liberate gluons, the production of strange quarks would happen very abundantly. His prediction was that you would saturate the density of strange quarks. But that should only happen if gluons and quarks are set free so that you can produce them in copious ways.
He and I got together, and we made a number of predictions about, you know, the observable consequences, which at the time were looked at as quite speculative, and unlikely to be realized. Because strange particles were usually produced in rather suppressed abundances. And the prediction that they will be produced at chemical saturation was not at all accepted at the time. The experts didn’t think that this was likely to happen. On the other hand, that's a nice prediction to make, exactly because people would say: I don't think that will happen, right? In particular, there is a particular particle called the Omega baryon which contains three strange quarks. I think at the time it had only been seen maybe two dozen times in high energy experiments, because if one strange quark is unlikely to produce, to make three and get them into the same particle is an extremely rare event. We predicted that these would be abundantly produced. This was eventually verified in the experiments, but at the time when the prediction was made, people just didn't believe it.
It was around that time that I was invited by a university in Belgium, University of Liège, to give a set of lectures. And I thought of what I lecture on, and I said to myself, "Well, why not give a set of lectures about the quark-gluon plasma?" Nobody had done this before. I was interested in it, and I was intrigued by developing a set of lectures that introduced others to a new field. And so I gave that set of lectures and wrote it up. At the time, the way that you distributed a manuscript was that you sent a copy to the preprint library in Stanford, and then people would see in weekly updates that this preprint existed with the title, and they would then write to you asking for a copy of the preprint. And usually when you did this, you know, you would get maybe a dozen postcards asking for these preprints. I had sent in the lectures just before the Christmas vacations in 1983, and when I came back about two weeks later, there was a stack of 150 of these postcards waiting for me on my desk! And I said to myself, "Wow. People must be interested in that area." This made it clear to me that there was a lot of interest around the world. It showed that this was an area that captured people's interest and was worthwhile pursuing.
To connect what you said earlier about your realization that this line of research really required you to be in the United States, tell me about the range of considerations. Both in terms of the people to work with, the experiments to be more closely involved in, what was going through your mind at this point?
It was both of that. First of all, the theorists whom I respected for their early work in that area were mostly in the US, except Shuryak, who was in the Soviet Union, and secondly, the US had decided to build this fabulous machine (RHIC) that would be unique of its kind and very likely produce the data that would be necessary to verify theoretical predictions. And finally, as I mentioned already, I was attracted by the way science was done in the U. S., which was much more hands-on for senior scientists who would actually do calculations rather than just supervise calculations. And thus, in 1988-1989, I began looking for positions in the U. S. Eventually, I received two offers. One was for a senior scientist position at Brookhaven Lab, and the other one for a professorship at Duke.
The Brookhaven offer must have been compelling for you.
The Brookhaven offer was compelling, but there were some aspects that made me not so excited about it.
Who was there at Brookhaven that would have been exciting to work with?
Well, this is the point. The people I was interested in collaborating with were not at Brookhaven. Also, the Brookhaven offer came with an additional offer for an adjunct position in Yale, which is just on the other side of the Long Island Sound, to teach courses, maybe one course every other semester. I was told very clearly from the department chair in Brookhaven, "Well this is wonderful, but when you go there, from Brookhaven's point of view, you have to take vacation." And I thought that was just preposterous.
Welcome to the Department of Energy. (laughs)
Yes. Welcome to the Department of Energy. In the meantime, people have found ways around that and it's somewhat more relaxed than it was at that time. But at that time, this was just something that I found repelling. At the same time, the offer at Duke was quite attractive from a number of points of view. First of all, the nuclear theory group there had been led by Larry Biedenharn, who was about to retire. Larry was an eminent scientist, but he was also somebody who could not tolerate anybody beside him. But he was now close to retirement, and it was clear he would retire in about two or three years. Thus I had there the promise of actually building a nuclear theory group according to my own ideas over the next decade or so. In addition, it's a wonderful campus. When I talked to the Department of Energy, the program director for nuclear physics, Dave Hendry, indicated that he would be looking very positively on establishing a research group at Duke. There was a very successful experimental lab at the University's nuclear laboratory which had been in existence already for about 30 years. But they had failed to establish a serious theory effort relevant to their future research program. They were very much interested in doing that, and so after considering the two offers, I decided to accept the one from Duke.
Berndt, let's zoom out a little bit about the state of nuclear theory at that time, circa 1990. To the extent that all subfields in physics go through periods of waxing and waning. Where was nuclear theory in the United States? What were some of the big exciting issues? Was it attracting graduate students? Were there things that were really suggesting that the field had promises in front of it?
The field of nuclear physics in the United States had had great days in the 1950s, 1960s, and partly into the 1970s, but there hadn't been a major facility built for many, many years, and it was considered as backwards, especially by the high energy physics community. High energy physics was based on fundamental laws of nature, quantum electrodynamics, electroweak interactions, QCD, and nuclear physics was perceived as some kind of molecular physics with imprecise models and very little theoretical depth. All the people whom I mentioned before, who were interested in quark-gluon plasma, came from the high energy physics community. They were not nuclear physicists.
Nuclear physics at the time was overall at a low point. This was one of the motivations around that time to found the Institute of Nuclear Theory. Nuclear theory, in particular, had been declining. Many of the universities that had traditionally been strong in nuclear physics, including Princeton, MIT, Berkeley and so forth, were giving up on it. Among the two places that were emerging was Stony Brook, led by Gerry Brown, which was seen as the lead contender for the Institute of Nuclear Theory. Everybody expected it would go to Stony Brook. But there was also this group led by Ernest Henley in Seattle which competed and was ultimately successful. From this point of view, nuclear physics was ready to be rebuilt on the basis of the physics of quarks and gluons.
The first facility that was new was the Jefferson Lab. But when it was conceived and built, quark-gluon physics was really not yet firmly in the mindset of nuclear physicists. It was more an extension of more traditional electron scattering on nuclei and studying nuclear excitations. And only with the gradual increase in the energy of the electron beam did it morph into a lab that really looks at the quark structure of nuclei, but this had not originally been central at its foundation. So the Jefferson Lab was the first facility that really changed this. At the time that it began operating, RHIC was already under construction. These two facilities caused a complete change of emphasis: Trying to understand nuclear physics on the basis of quarks and gluons using quantum chromodynamics rather than on the basis of hand-waving models that had no direct connection to fundamental physics. It was the right time to begin this rejuvenation of nuclear physics, and particularly theoretical nuclear physics in the U. S.
And where is the INT in this for you?
The INT is a great place that has greatly helped making that transition. So as soon as I arrived at Duke in 1990, I participated in the third program that they ran. I spent a lot of time over the next decade at the Institute of Nuclear Theory. Under first Ernest Henley and then Wick Haxton’s leadership, it was a very exciting place, because it attracted a lot of bright people. In particular, it attracted many young people who were interested in rebuilding nuclear theory from a more fundamental point of view, many of whom came from particle physics.
Yeah. Yeah. Berndt, it's outside of your immediate area, but were you following the rise and the fall of the SSC at this point?
Very much so, because North Carolina was one of the states that competed for the SSC. A senior colleague of mine, Al Goshaw, was leading the North Carolina effort. It was pretty clear to me at the time that, compared to Texas, they didn't stand a chance because North Carolina didn't have either the depth of academic programs, nor the economic capabilities of Texas to support the effort. So it was for everybody, except the people directly involved, a doomed effort from the beginning. On the other hand, by competing you gain a lot in terms of attention and also focused the local community on a goal. It was helpful to do that, and so I was very much paying attention to what was going on.
Berndt, did you take graduate students right away when you got to Duke? Did that take some time?
No, actually, I was very lucky. There were two students that actually decided to join me that came from Frankfurt and transferred. And I found two very bright students at Duke. One of them came from an elite program from China. Called the CASPIA program, that T. D. Lee had created in the mid-1980s. 1990 was one of its last years, and I had a very bright student who immediately decided he wanted to work with me. Thus I had a good group of students right from the beginning.
And as always there's an indication where you're working with graduate students and you're thinking about developing their careers. What are the topics that you're focused on insofar as the next generation of students in nuclear theory are concerned?
I always had the principle that I would find an exciting thesis project for my students, but then once a student had finished, I would leave that project to them to further develop their career. Thus I immediately started out with having one student doing research together with the postdoc developing a detailed description of the nuclear collision process. We wanted to understand the process on the basis of quarks and gluons in the colliding nuclei. How is a quark-gluon plasma formed? How does it thermalize? To that end we constructed the first cascade model of interacting quarks and gluons, which could lead from the colliding nuclei to a quark-gluon plasma. Of course, today it's much more sophisticated than what we did at the time, but it was really the first comprehensive attempt of understanding how you go from two colliding nuclei to that hot state. That was one project.
The other one was triggered by a visiting scientist from Armenia, Sergei Matinyan. He first came for an extended visit but later joined our research group permanently. He talked to me about the discovery that he had just made together with his students. That they had looked at quantum chromodynamics as a dynamical theory, and they found that it's very different from electrodynamics. Photons don't interact among themselves. They interact with electrons, but not directly with each other. The way you express this mathematically is that electrodynamics is a linear theory. You can write down the general solution of any linear set of equations. The solution may be very complicated, but you can do it. There are explicit formulas for that. In the case of electrodynamics, they're called Lienhard-Wiechert potentials and have been derived on the basis of Maxwell’s equations and explored for decades. This allowed quantum electrodynamics to be constructed and worked out in great detail using the fact that you can solve Maxwell's equations analytically. The equations for the gluon field, the Yang-Mills equations, are nonlinear, and thus no rigorous solutions exist except for plane waves which don't include any interactions. Matinyan had discovered that when you looked at certain simple limits of these equations, they showed a property that in nonlinear dynamics is called chaos. In other words, that you could not analytically predict solutions because they are unstable against small perturbations.
And so I proposed to, again, a postdoc and that brilliant Chinese student I mentioned before, that they should try to explore these equations more generally. The way you do this is you put the equations on a grid or a lattice, and then you numerically solve them. And they showed that, indeed, it is a completely chaotic set of equations. In other words, there are no stable solutions. Any solutions that you find are inherently sensitive to very small perturbations. You could show how such a system, when it's highly excited, thermalizes. This was my second line of research, which led me to be interested in the physics of chaos. This was really the beginning of my attempt to understand how a system of quarks and gluons can thermalize very quickly.
Berndt, to go back to an earlier comment about computers from the beginning of your career, in 1990s, what was the relevance of the rise in computational power for your research at the time?
Oh, it was absolutely essential. I mentioned earlier that the university in Frankfurt didn't have a real computing center when I was a student, and we had to go to the banking center to do calculations. Later on, the university established a university-wide computing center and, together with the GSI lab, they had plenty of computing power. Whenever you try to make comparisons with experimental data for complex processes, the ability to detailed, large-scale computations is absolutely essential. When I came to Duke, I realized that the university didn't have that. One of the essential conditions for me coming there was that they would buy an adequate computer system, which they did. This was orders of magnitude more powerful of course than what I had as a graduate student, because it was a decade and a half later. As you know, computing power develops very rapidly over the years. This allowed us to do many calculations that even a few years before would have been impossible. But they were absolutely essential if you wanted to compare with data.
Tell me about the NSF support of your workshop on QCD vacuum structure. How did that come about?
I had a senior colleague at Brown University whose name is Herbert Fried. He is now in his 90s and long retired. Together with my former colleague from Frankfurt, Johann Rafelski, we thought it would be interesting to bring the people who have an interest in strong fields in quantum electrodynamics and in quantum chromodynamics together. So, we were looking for a way of supporting this, and we initially contacted NATO. They run conferences, especially in less-developed NATO countries. They actually supported one workshop in Corsica in, I believe it was 1990 or 1991. And then eventually, the NATO people said, "Once is enough." (laughs) "Find other sources." And so we went to the NSF and other sources, and got support to continue this series.
These were meetings that typically lasted about a week. We had to work this out on a shoestring basis, because we couldn't pay people to come. There had to be a motivating venue for them to come, and Fried knew the president of the American University in Paris very well. They had been fellow students, and he convinced him to make their facilities available at no cost during their vacations. We held that meting there several times. It was a very stimulating meeting, because you know, it’s theme was very broad, and the people who came were people who had some interesting idea, but not necessarily a mainstream idea. You would go there and you would sit through a series of talks for a week where very bright people would present, their out-of-the-mainstream latest ideas. I found this tremendously inspiring, and a number of projects that I later pursued actually came from ideas that occurred to me when I was listening to these talks. Usually conferences are much more narrowly thematic. If you want to go to a major funding agency and ask them for tens of thousands of dollars for a meeting, you have to give them a theme under which you want to hold the meeting. You have to tell them, "This is the plan, that's the outcome, and so forth." Ours was basically a shoestring meeting with very little funding, and so we were free to do what we wanted to do and to invite the people we thought would give interesting talks.
Berndt, given your interest forthcoming in extreme energy density, I wonder if you could paint a sort of broader picture? What is the meaning of that term and where does it come from as it relates to the things that you were working on prior to that point?
Well, extreme energy density relates to the quark-gluon plasma. You could say it's another word for it. But more broadly it is the question of how does matter behave under extreme conditions? What do the laws of nature tell us about situations in which the energy density is just extreme? This could be extreme in terms of a temperature that's very high. It could be a density that's very high. It could be coherent energy in terms of, for example, a field. It refers to situations where the energy density is large compared to the typical other scales in the system, such as the mass of the proton or the mass of the electron, or binding energies, and so forth. Then the processes that happen are not governed by the masses of particles, but they're governed by the strength of forces. And it can manifest itself in multiple different ways depending on the situation. I mean, is it primarily thermal? Is it primarily cold but dense? Is the energy concentrated in coherent fields? There are multiple ways in which the energy density can be manifested, but the common aspect is that the energy density is large compared with all other scales involved.
As your research at this point is taking on more relevance in astrophysics and cosmology, what new collaborations or new fields were you getting involved in, and specifically what might have been happening in the world of observation that was relevant for your research?
I think I need to be clear that over the last eight years or so, much of my time was spent at managing the nuclear and particle physics programs at Brookhaven. So, in many respects, I've been more of an observer than actually somebody who ventures into new territory and does research. Thus I can't really say that new developments on that side have triggered my own research. But what I find fascinating is clearly the physics that is now being explored by LIGO. The mergers of black holes, of black holes and neutron stars, of neutron stars with each other. I think there's a lot of very interesting physics that will come out of this. The other exciting development is our ability to probe closer and closer to the Big Bang in general cosmological observations. The whole question of how the universe developed at a time when its energy density was extraordinarily high is an interesting question. That's a question where we have very few direct probes, but we have an increasing number of observations that allow us to constrain models of what was happening there. These are clearly areas where I think there is a lot of development that's going to happen over the next 10-20 years because of much better measurements and much more interesting data, both from gravitational wave observations and the associated astrophysical observations. But also from precise measurements of what remnants of the structure of the early universe survive today.
In light of your--
My own interests have revolved around some of that, but I haven't decided yet what is the most interesting part of it that I want to pursue more actively.
Berndt, in light of your previous comments about some of your administrative concerns back in Frankfurt, I can only ask with a smile on my face about your tenure as chair of the faculty of physics, and then dean of the faculty of natural sciences at Duke. Tell me about some of those responsibilities, and if that was different in the way that you saw this balance at Frankfurt?
It was very different in the following sense. If you're part of a community, be it the department or the university or a scientific community in the nation, you have, if you have the ability and the opportunity, a responsibility to manage or help manage the affairs. But you shouldn't do this when you are 30 years old. That's the time when you should do your research.
And you probably shouldn't even do it when you're 40 years old. But when you're 50 or 60, it's a different situation, right? You have a lot of work that you already have done. You have hopefully realized many of your ideas. Some of which give you a prize. (laughs) But then it's time to give back to the community by helping to manage it and to safeguard its future. Right? And so, becoming department chair after I had been at Duke for roughly seven years was a natural thing to do, because it allowed me to help steer the department, not just my own research, but the department, into the right direction. What I discovered is the department chair is about the most difficult job that exists in academia, because fundamentally you have almost no power. Faculty members are very independent. You have a tremendous amount of responsibility, but no power to force people to do what you think should be done. In other words, it's all about convincing people.
The position as dean for the Natural Sciences-- this was quite different in that respect, because you actually have a very sizable budget, and although you can't tell people what they should do, because the faculty members are independent in their research, you have a lot of incentives for the faculty and the departments to go in certain directions. For example at the time, I was able to develop a nanoscience initiative within the College of Arts and Sciences. I was able to develop a diversity initiative where I convinced the university president, Nan Keohane, to make sizable startup funds available for senior women in the sciences that we could recruit, which was a game changer at the time. We were not able to recruit everyone we wanted, but we got some really outstanding senior women into the science faculty that we didn't have before. So these are the kinds of things you can do as a dean, which you cannot do as a department chair. I found that much more interesting and rewarding.
At that point, I had to decide whether I wanted to pursue an administrative career within the university system, and I ultimately decided that that was not what I wanted to do. At some level in the university administration, you spend a lot of time on fundraising. In my position as divisional dean, I could still have a research group and do some really exciting research at the time. Some of my most well-cited publications date from that time. This would not be possible if you became a senior dean or provost. And so I decided against that.
Brookhaven on the other hand, managing the nuclear and particle physics directorate there, which is about 750 people, was a different situation. First of all, I was older. I was about 60. And secondly, there was a real challenge because at the time, there was a threat that the RHIC would be closed because of lack of funding, because of a lack of a clear program for the future. I thought that I had an opportunity to really make a difference. Of course, it was clear that this would very severely reduce the amount of research I could do. But on the other hand, I think it had a significant impact on the community as a whole, which not only led to the continuation of the RHIC program, and the ability to complete its mission, but also ultimately to the decision to build an electron-ion collider. That's the kind of the thing that you should work on if you have the opportunity to do it, but not if you're 30 or 40 years old. Then you should do the science.
Berndt, what were the circumstances of you accepting this position at Brookhaven? Did you go on leave, did you resign your position and then come back? How did that work?
That was important for me. I effectively went on leave. The contract was that basically Brookhaven would pay Duke, and Duke would loan me to Brookhaven for a period of years. Thus I retained my tenured position. You know, when you take a position like this, you have hopes but you don't know how things turn out. I mean it could simply be that the whole facility will be closed depending on political decisions outside of your control. So convincing Duke that it would be a good idea to give me a multi-year leave was important, and the university graciously agreed to do that.
To go back to an earlier thing that you said about some misgivings you had when you had that choice about going to work for Brookhaven earlier, what had played out correctly as you had prognosticated, and what might have changed that convinced you to take this position, albeit in a very different capacity?
So eventually the position that I declined was taken maybe six years later by Larry McLerran, who came from the University of Minnesota. Larry did a fabulous job in building an outstanding nuclear theory group and developing a strong connection to the experimental program.
What were some of the technical challenges you came up against in this new role?
Coming from a university environment into a system of a national lab, that is, operating under DOE rules is quite a game changer. A positive side is that the amount of resources that you have at your disposal is much, much larger than you can hope to have at a university. It's a different scale. On the downside, the overall organization is a lot more constrained by operating in a way that it always has. In the case of Brookhaven, if you go back to the 1950s and 1960s, and there are still remnants of this, the physics department and the associated accelerators had operated almost like academic programs, but with a great deal of funding that they wouldn't have had at a university. But otherwise, the overall mode of operating, that was very academic. This is not something that is compatible with modern bureaucracy. And for better or for worse, DOE now has the expectations of having much more control over day-to-day affairs and over day-to-day decisions, and keeping that control. And so, one of the challenges is to work within the constraints of that system but to find ways of allowing science to flourish in spite of these constraints.
Ultimately, I think if you work at a university as a faculty member, you're only responsible to yourself about the research you're doing. If you're at a national lab, you have to justify why you're doing it and where you're spending the money. The burden is really on your ability to provide that justification, and you have to provide the justification in a way that the funding agency understands and finds exciting. So, the challenge comes down to developing and expressing a scientific strategy that justifies the funding, and then to execute meticulously and deliver on the promises. But if you can do that, then the system works extremely well. In that sense, it's very different from an academic system at a university. I think we've been able to do that over the past decade quite well at Brookhaven. But again, it requires a really detailed understanding of what works and what doesn't.
Berndt, during these years to what extent were you paying attention to what was happening at the LHC and where did you see areas of cooperation and where did you see areas of competition?
In the case of particle physics, the biggest activity of Brookhaven scientists is at the LHC. Brookhaven is a member of the ATLAS collaboration, and it is the largest outside contingent of scientists within the ATLAS collaboration. Brookhaven manages the entire U. S. participation in the ATLAS detector. Which is probably in the order of a thousand scientists. Thus in particle physics, the collaboration is very intense and Brookhaven acts as the conduit for the American participation in that particular aspect of the high energy physics program. Fermilab does the same for the CMS detector, so Fermilab and Brookhaven together are basically managing the U.S. participation in the LHC.
On the nuclear physics side, where there are programs at the RHIC and the LHC in quark-gluon plasma physics, there is real competition. The two facilities have different strengths. The LHC has much higher energy, but RHIC is much more versatile. And so there in terms of the competition view, the challenge is to identify the particular strength of the program that you can execute at RHIC. You have six months of beam time every year instead of one. You have a much larger range of nuclei that you have at your disposal. You can collide nuclei over a wide range of energies, almost a factor of 100. Whereas the LHC runs basically at one energy. So they are complementary programs and in a sense, that's ideal because there are many different questions that get addressed in that way. So, depending on the area, in particle physics it's really close collaboration, in nuclear physics, it's competition, but in both cases I think it's very favorable to the overall U.S. science situation.
What were some of the most exciting results coming out of RHIC that really stand out in your memory?
Well, the first and most exciting result, after establishing clearly that the quark-gluon plasma is formed, is the demonstration that this is the most perfect fluid that we have observed. In other words, the relative specific viscosity of the fluid is less than that of any other known fluid. Far below what typically the laboratory substances give you. And very close to the quantum limit. What this discovery triggered is not only the general science of fluids that are close to the quantum limit, but also many theoretical studies of how that comes about? What is the nature of this property? This engendered a really exciting understanding. If you want to express it in simple terms, imagine you take an atomic nucleus and begin heating it up. When you heat it up, eventually it breaks apart into protons and neutrons, so it becomes a gas of particles. And then when you heat it further up, there comes a point when these particles disintegrate into quarks and gluons, and it goes from a gas into a liquid. But it is maybe the only simple material in which a gas, with further heat, converts into a liquid. And vice versa.
What makes that happen? What is so special about this? Its impact on our understanding of the structure of phases, as temperature changes and as the interactions change, is something that triggered a lot of theoretical investigations. And renewed understanding of how fluids behave at the limits of quantum mechanics. The other discovery that RHIC has made is, because it is also a polarized proton collider, it has for the first time demonstrated clearly that gluons contribute to the spin of the proton. Originally, the idea was that the quarks carry the spin of the proton. They have spin one-half, and there were simple models that said, "Well, if you combine the three quarks, you get the spin of the proton." But then measurements showed that that's actually not true. The quarks only carry about a third of the spin of the proton, and it became a big question as, where's the rest? One hypothesis was that it resides in gluons, and RHIC demonstrated that indeed, gluons contribute a large fraction of the proton spin. Probably there's more to it since as the quarks move around in the proton, they also contribute to the proton spin through their orbital angular momentum. That's something that will be measured with precision at the electron-ion collider, but RHIC provided the first data that said, gluons contribute to the spin of the proton.
So these were the two major discoveries.
Were you involved in e-RHIC? Was that part of your responsibilities?
Yes, my responsibility was to develop the proposal for Brookhaven and, with the accelerator scientists, to develop the scheme in which RHIC would be converted into an electron-ion collider. One of the major challenges of the last three, four years was to develop a concrete proposal, develop the science plan, but ultimately develop a proposal that you could go to DOE with and say, "Here's how we will do this." A credible proposal. I mean, you can easily develop a proposal that looks very good on paper but then if people look at it, they say, "Can you prove that every step works?" If it's a $2 billion project, DOE wants to know that every step can work. They don't want to build something that ultimately will not meet the specifications. So the challenge was to develop a proposal that actually withstood all scrutiny.
And were the options that it would be Brookhaven or somewhere else? Or Brookhaven or nothing?
It was Brookhaven or the Jefferson lab. Brookhaven has the hadron facility, RHIC. It needs an electron accelerator and a storage ring. The Jefferson lab has the electron accelerator and needs a storage ring and an accelerator for protons and nuclei. I think the strength of the Brookhaven proposal is that it had a clear path to higher energies, and that it was somewhat less expensive because we have a larger part of the infrastructure already in place.
What was the overall budgetary environment as you were working through these considerations at the DOE? Was this a more generous time? Was this a difficult time?
It was initially a very difficult time. We were not sure from one year to the next whether RHIC would be able to continue operating. We were then lucky in basically starting 2015 that we had convinced DOE that RHIC was a priority and, in particular, that it was their best chance to build the next facility. Funding is never generous. But it was sufficient to carry out all the program that we had promised.
I'm curious if having a fellow nuclear theorist in Ernie Moniz as Department of Energy Secretary, if that was useful to you at all?
Well, the most important thing Ernie did was actually before my time. It was under his leadership as Under-Secretary of Energy that the Office of Science was established in the DOE. DOE had always supported fundamental science. But it was not a clearly independent coherent structure and organization within the DOE. You have to recognize that the Office of Science is a relatively small part of the overall DOE operation, a large fraction of which is the nuclear weapons program, and another one is of course applied energy research. Nuclear reactors and such things. By focusing all the scientific research programs into the Office of Science, they became much more prominent. If you want to go to Congress and get funding, then this is an invaluable advantage. The formation of the Office of Science under Ernie was absolutely critical in making that work.
And who was your main contact at DOE? Did you work with one person? was it you and a committee? How did that play out?
My area was supported by the Office of Nuclear Physics and the Office of High Energy Physics. And so there are two directors. In high energy physics it's Jim Seagrist, and in nuclear physics was Tim Hallman. Tim was a staff member at Brookhaven before going to DOE, so he knows the lab very well. Which can be an advantage and a disadvantage, because he has to be impartial, and he cannot create the impression that he's favoring Brookhaven in any particular way. But on the other hand, he understands the value of the science and so forth, and the interaction with these two leaders of the office, and of course with all the people who work with him, it was extremely positive.
Because there's a certain amount of salesmanship in this endeavor, in what ways did you communicate to DOE the importance of the science? The importance that, of all the things that they could do, this was really worth their support?
There are really two ways in which that happens. One occurs every seven or eight years is the long-range plan. The last long-range plan in high energy physics was in 2014, and that was just culminating at the time when I came to Brookhaven. The real challenge there from the point of view of the lab is to convince the DOE managers that the lab would follow the priorities of the long-range plan. And not continue doing what it had been doing for 30 years. In other words, that the lab was really identifying with the mission priorities of the Office of High Energy Physics. On the nuclear physics side, we managed to convince the Office of Nuclear Physics to make our proposed program a central component of the long-range plan in 2015, and to also convince the DOE that the next step would be an electron-ion collider. All we needed to do, basically, was work out a practical and affordable way of building it. So, there we shaped the long-range mission of the DOE in a very much more tangible way than on the high energy physics side.
What next is important is every year you have a budget meeting with the DOE. And during that meeting, you convince the DOE program managers that A) you have faithfully delivered what you promised, or more, but not less, and B) that you have a clear plan of carrying out the mission that was agreed upon in the long-range plan. I think we succeeded extremely well in doing both sides, so the funding that Brookhaven received in both areas grew substantially during the period I was the associate lab director. We grew our funding by about 30% over those eight years, and although it was the oldest area in the lab, it was the fasting-growing area in the lab at the time. Of course, many people contributed to this. You have to recognize that it's really all a team effort, but it requires a strong focus and also the ability to express the value of what you're doing and what you have achieved.
Berndt, what opportunities did you have in this role to serve as a mentor to junior scholars? Graduate students, postdocs, people just starting out.
I had one graduate student from Duke still, whom I mentored during that time and who is extremely bright. He finished about two and a half years ago, he's now at MIT and doing very well. In terms of the scientists at Brookhaven, I think I've been able to help them understand and to develop their own skills of how to be successful in an environment in which you want to do the best science, but you're also constrained by DOE rules. As I said, these rules can be quite cumbersome, but this is not because people are ill-meaning. It's because that's the way that the federal bureaucracy works today. There is a tremendous desire to control and to make sure that we stay on the bright side of Congress. You have to be able to express the value of the science that you're doing, and then you have to deliver or over-deliver on your promises, and that requires both great scientific creativity and ingenuity but on the other hand also a tremendous amount of responsibility and carry-through, and the ability to communicate well. I think I've been able to help quite a few of the younger staff members to develop their own capabilities in that respect, and it wouldn't have been possible without them.
In 2017, you presented a paper titled, "The Future of RHIC." I'm curious four-plus years out, what has aged well as you looked to the future in 2017?
Well, as I said, we've done everything we promised to do. And for example, one of the big programs was, because of the ability of RHIC to change the energy over a wide range, we promised to do a very high statistics beam energy scan where we changed the energy from 3 GeV to 200 GeV and looked at how the nuclear reaction changes over that range of energy. We set ourselves extremely high goals for that. And the lab actually over-delivered by a significant factor on those and carried it out on time. Which was not trivial, especially last year, when the facility had to shut down for three months simply because nobody could work. So I think the program laid out in that paper has held up very well. A large fraction has been executed, and there is more to come. They're building a new detector called sPHENIX that will be finished next year. It will form the core of RHIC’s science program until 2025 when the facility is planned to shut down.
Berndt, do you have a successor that has the same role, or was your role singular in what you did?
. I had a predecessor, Steve Vigdor, who came from Indiana University, and I have a successor who is actually also a professor at Duke, Haiyan Gao, the first time a woman to hold this position. She's an expert in the science of the electron-ion collider, so she is clearly well aligned with the future of the lab. What I find also good is that the model of having a university faculty member on leave managing this position continues. So I think that's altogether very nice.
Coming back to Duke, what opportunities did this present for new research projects?
Well, fundamentally... there is a wide-open space there. I've been back at Duke since the beginning of this year. I spent the first several months getting back into teaching, which was a challenge both because I hadn't done it for eight years, but also because it all had to be done online, rather than in person, and so that was a huge challenge. I am right now starting to think about what really is it that I want to do with my next years of research. I haven't made up my mind yet.
(laughs) There's a lot to work on.
There is no shortage of interesting projects. I mean as always, you have to choose wisely.
Berndt, tell me about winning the Feshbach Prize. What did that mean to you?
It was very exciting. For a number of reasons. First of all, I was a great admirer of Herman Feshbach himself. Both in terms of what he did in science -- some of my own research made use of developments that he had made in the 1970s -- but also as a person. He was very active in terms of opening up Western, especially American, science to the Soviet Union. He was active in nuclear disarmament. He was active on the side of bringing women into physics. So he was the kind of scientist you can really aspire to be. Years ago I had an opportunity to meet him myself. We had a wonderful dinner at my home where I found that he, as a person, was very likable, very funny, and very honest and open. That alone made it very rewarding.
The second thing is that at the time that the prize was established, I was chair of the Division of Nuclear Physics in the APS. I helped lead the campaign for funding of that prize. The idea of a nuclear theory prize actually came partly out of a committee that I had led that recommended that there should be a prize dedicated to nuclear, theoretical nuclear physics, which didn't exist. I was very glad the community followed up on that and decided to establish such a prize and provide enough funding for it through donations. And the last thing is I got a notice that I won the prize when I was in the hospital recovering from a major surgery I had just a few days before. So this was a very nice surprise.
A much-needed shot in the arm, so to speak.
Yes. (laughs) You couldn't have timed it better.
(laughs) In your address, given the high profile of this recognition, what did you want to emphasize about your research and about the larger significance of the recognition in the field?
What I tried to emphasize is two things. Number one, that whenever somebody wins the prize, there are many other people who have contributed to their research in very fundamental ways. It's not a one-man show or one-woman show. And in a sense, the prize also honors also those who have collaborated to make it possible. The second aspect that I wanted to demonstrate is that an area or particular duration of research often starts out with many people who doubt it. When not only I and my collaborators, but also others proposed ways of looking and identifying the quark-gluon plasma, there were many, many people, I would say by far the majority of nuclear physicists, who said, "Wonderful ideas, but this is so complicated that you will never be able to demonstrate that you've observed is what you think you observed." In other words, you will never have enough information and evidence to prove anything.
And a few people, me included but also others, were determined to make it work. We believed that you could, in spite of the complexity of a collision between two heavy nuclei, ultimately read out of the data with sufficient theoretical support that a new state had been discovered, and what its properties are. And how it had proceeded, and how you go from something that seems for many people impossible to something that's now well-established, is an interesting journey. That's what I wanted to show, as an inspiration to young people to pursue goals where many of the senior people say that probably doesn't work. But somehow that's how science progresses, that you pursue goals that may be very difficult to achieve and have many people who doubt they are achievable goals. But eventually, through a lot of hard work of many people, they become achievable. That's how science progresses.
Well Berndt, now that we've worked right up to the present, for our last part of our talk, I'd like to ask some broadly-retrospective questions and then we'll end looking to the future. So just right now in terms of a state of play in the field, going back to an earlier comment where when you got to Duke and you were saying how nuclear physics, theoretical nuclear physics, was in decline, where is it today, vis-à-vis RHIC and other developments, and more broadly, do you see what's happening at Brookhaven as a sign that the United States remains a leader in high energy physics, or is that really the last stand for high energy physics leadership in the United States?
I think there's no question in my mind that the United States is in a certain area of nuclear and high energy physics, the leader. That's very clearly so in nuclear physics. The country has many of the best facilities in the world, and with the electron-ion collider, has clearly set itself on a path to maintain that for the next two to three decades. On the true fundamental high energy physics side, it's also clear that the science is now global. And you cannot have one country be the leader. Because the resources, both in terms of funds but more importantly in terms of the number of people required to carry out the research, exceed what's available in any one country. So here clearly the CERN has established itself as the premiere global lab, but CERN would not work as well, and you might then make an argument that it might not work at all, without a very strong US participation. Because if you take the US community out of the LHC program, I'm not sure it's feasible. And so high energy physics, in a sense, is clearly beyond the national realm. It's a global enterprise, and it relies on functioning international collaboration, in which the US is the largest but not the dominant partner. On the nuclear physics side, the required resources not as large. And so there is still the ability for one country to be the leader and I think at this point, the US is clearly in a leadership position. And will be over the next one or two decades, at least.
Berndt, in all that you've worked on, looking back what gives you the most intellectual and even emotional satisfaction that you've been a part of?
I think it's just fundamentally the experience that as a theoretical physicist, you can develop an idea that is unprecedented, you can predict new phenomena, and if you're lucky, you actually find the experimental colleagues who make it reality. And if you're even more lucky, the data agree with your predictions. But even if they don't, you've learned something. Right? So, it's not that a prediction that's falsified doesn't further science. It can be deeply important. But I think what is most rewarding is when you're involved in both of these stages, namely both the prediction phase and then the phase in which you actually make sense of the data. And I was very lucky to have been able to be there.
In all of your research, what have been proven to be the most intractable theoretical problems to work on? No matter what you do, you always seem to hit a wall?
Well, I don't know how to answer that question, because one of the skills of a scientist is to figure out which problems are amenable to a solution, given the technologies of either mathematical, computational, or experimental capabilities at a given time. And to choose problems that are amenable to solutions. And not problems that are not ready for a solution or are, even if they can be solved, of no particular interest. If I would answer the question in a sense of which are the most intractable problems I have worked on, I would say that I would have chosen those problems unwisely. Let me give you a well-known example of a case where a problem proved to be intractable. After completing special and general relativity, Einstein spent the next 20, 30 years of his life trying to find a unified field theory. It proved to be an intractable problem. He couldn't find a solution. But the reason was simple: Most of elementary particle physics was unknown. In other words, the physics that would have to be unified wasn't even on a radar screen. And so pursuing a problem of that type, which is intractable for any number of reasons, is ultimately not a wise scientific choice. The challenge is to identify problems that can be addressed and resolved, either experimentally or theoretically, and to avoid problems that cannot be solved. In that sense, I would say there are many intractable problems in physics, as in other sciences. But these are not the ones that you should really waste your time on. The time to solve them hasn't come. There are plenty of interesting problems that are challenging that can be solved. I don't know whether that's a good answer, but...
It's a thoughtful answer. It's a considered answer, and I appreciate it. Berndt, do you consider yourself at all part of a particularly German intellectual tradition? Do those cultural differences mean much as you reflect on the way you've approached science?
As a matter of fact, no. When I was young, I was particularly enthralled by British philosophers and the British way of approaching science. People that I greatly admired, people like Bertrand Russel, and any of the English philosophers of the Enlightenment, Locke, Hume and so forth; on the scientific side, you know, Maxwell, Rutherford, and so forth... In other words, the British way of approaching science, which is a close combination of experimental physics and theoretical development, always attracted me more than the more obscure-- potentially obscure-- German approach to philosophy, that looks at the deepest mysteries. Einstein, we come back to that, is a prime example of someone where the German approach to science fortuitously led to two great discoveries, or multiple great discoveries. But you have to be very lucky for that approach to work. Usually, it is a much more empirical way of approaching science that has a closer interplay between experiment and theoretical, philosophical concepts than that which is very much ingrained in the German philosophical origin of science for which Kant is a prime example. So no, I was always more attracted by the British-American approach to the science than by the German approach to science. Which probably is what made me quite happy as a scientist in America.
(laughs) Right. Berndt, on that note, last question looking to the future. A topic we touched on earlier. Where do you see, with regard to your overall research agenda, where do you see a broader convergence in the various subfields that you're involved in, in the future, and where do you see ongoing relevance that nuclear physics has its own realm, theoretical particle physics has its own realm, cosmology and astrophysics, where they're separate and where do you see opportunities for additional convergence as more and more people are looking at similar and fundamental problems?
If there's one area where I would say there is a current connection that goes through many of these areas, it is that we're still trying to fully understand quantum mechanics. And that goes from understanding how complex quantum systems work in nuclear physics to how does quantum mechanics and gravity can be reconciled in particle physics-- where string theory is one model, but I mean we have no idea whether that's the right approach-- to quantum computing where really the question is, can you construct a system that works like an ideal quantum system, although it's embedded in a macroscopic world? And there's a fundamental difference between the macroscopic world and the quantum world because both are statistical, but they are statistical in a very different way, mainly that the quantum world is statistical on the basis of a quantum mechanical amplitude, and the other one in terms of the probability. There's a very fundamentally different way in which they operate. And we have some rough idea of how that transition happens, but we don't know enough to really know to what extent a quantum computer of macroscopic dimensions is possible. And where the system that is becoming of macroscopic dimensions and has many quantum processors. We can do a dozen quantum processors. We can do 50. Can you build a system that works in the same way based on 1000 or a million? We just don't know. Because we don't understand how quantum physics on a large scale in more than a simple model system works. So, I think that whole question of the interplay between quantum physics, complex systems, and the macroscopic world, is going through nuclear physics, particle physics... It certainly goes through cosmology and through quantum computing, and even condensed matter physics as a common connection. I think there will be lots of interesting discoveries in the next few decades as we try to really address that with modern technological developments.
There's much to be excited for, is what you're saying.
Oh absolutely. It's again something that will probably occupy the physics community for many, many years.
And attract the brightest minds.
Berndt, it's been a great pleasure spending this time with you. I'm so happy we were able to do this, and I'm so grateful for all of your insights and perspective that you will be able to share with the history of physics community. So thank you so much.
It was a great pleasure.