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Courtesy: Bernard Sadoulet
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Interview of Bernard Sadoulet by David Zierler on November 19, 2020,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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In this interview, Bernard Sadoulet, Professor of the Graduate School at the University of California, Berkeley, discusses his time working in France as well as the study of dark matter. He discusses getting his Master’s degree in theoretical physics at the University of Paris in Orsay, and how this background was beneficial for his interest in experimental physics. Sadoulet speaks about his time working at CERN as part of the official committee managing the collaboration between Europe and the Soviet Union. He also details his work on the UA1 experiment while at CERN. He describes his role in the “Wise Men Committee,” and there task of producing a report about civil nuclear programs in France. Sadoulet discusses his time as a postdoc at Berkeley and his discovery of the Chi states. He speaks about his growing interest in dark matter in the 1980s and the interest he had in the possibility of building detectors to search for dark matter particles in the halo of our galaxy. He describes his collaborations with Blas Cabrera Navarro at the Center for Particle and Astrophysics. Lastly, he reflects upon how to meaningfully involve the public in science.
Okay. This is David Zierler, Oral Historian for the American Institute of Physics. It is November 19, 2020. I am delighted to be here with Professor Bernard Sadoulet. Bernard, it's so nice to see you. Thank you for joining me today.
Thank you for working on the oral history of physicists.
To start, Bernard, would you please tell me your most current title and institutional affiliation?
I am Professor of the Graduate School at the University of California in Berkeley, in the Department of Physics. This title means that I am retired but still strongly involved in research, and service activities on the campus.
When did you move to Hawaii?
I retired about eighteen months ago, on the first of July in 2019, but I had moved a few months earlier and was commuting to Berkeley for my teaching for the last semester.
Was the move to Hawaii purely a personal decision as a nice place to live, or there were other reasons to be in Hawaii?
A personal decision. This was a long-time dream of my wife, and with my impending retirement, it became possible. It's difficult for experimental work, of course, but a lot of our activities are taking place in teleconference meetings anyway-- we were spending a lot our time in Zoom meetings even before the pandemic. We have a collaboration from some fifteen institutions all over the United States and Canada.
Bernard, I'm sure you'll agree. I've heard many physicists say that they choose to retire so that they could actually get some work done.
Yes, but my wife and I have here a fairly large property which also needs a lot of work, which also absorbs a lot of my time,
What kinds of things are you involved with, or have you been involved with since retirement?
Of course, our research on dark matter, our Cryogenic Dark Matter Search (CDMS), I'm in charge of the NSF aspects of the construction of a new experimental set up to be deployed at SNOLAB in Ontario, Canada. I'm still the PI of the NSF part, which is a third of the total cost of the experiment. Of course, we have been impacted by COVID-19 and we also encountered technical problems. So, the program management is still taking a fair amount of my time. I'm also involved in the research itself, in the science. Our main focus at Berkeley has been trying to dramatically increase the energy sensitivity of our dark matter detectors to explore the region of very low mass dark matter particles. Although this R&D is mostly directed by Matt Pyle, an assistant professor at Berkeley who took over my group, I'm closely working with him and, of course, with our students and post docs. In the last two years, I have also been trying to wind down all the service responsibilities that I had at Berkeley, while attempting to transfer all the initiatives I has launched so they do not die with my retirement. I was, for example, the faculty lead for “Science at Cal,” which is basically a coordinating unit for outreach of the science and engineering departments at Berkeley. We are trying to organize activities with the public at large, with schools and so on. We had to find a new Faculty lead, and a new Executive Director of the program, we had to move the program into a different administrative unit, find a firmer funding and so on. But that's still in process. These transition tasks are still taking some of my time.
Bernard, let's go all the way back to the beginning with your family background. Let me hear a little bit about your parents and where they're from.
Okay, I grew up in France. I'm French. I'm also an American citizen. My parents were educated. My father was an engineer in mostly hydraulics. I'm convinced that that he would have been a physicist, if this has not been for World War II at the time when he became active. I certainly benefited from this engineering outlook that he had on nature and the physical phenomena around us. So, since I was three or four years old, we had a lot of discussions about how things work. He was also a very good handyman. When I was a kid I participated in his projects and am still fairly good with my hands. So, that's one aspect of my education. My mother had a bachelor’s in philosophy, and this was also quite important in my upbringing and my general outlook on life. She could not practice philosophy because she had six children, and of course, in the early fifties in France with all the restrictions after World War II, it was a fairly difficult life. But we kept speaking about philosophy, in particular about how we know and how we can think about stuff around us. I basically drunk Kantian philosophy at the same time I was drinking milk. So, this philosophy and humanities background was also quite important in my growing up. I had the chance to take advantage of the equivalent of double major which existed in French high schools at that time, where the better students could choose a rigorous education both in science and in humanities. So, until I was sixteen, I had a mixed training with a lot of French literature, Latin, Greek, and in the last year of high school in philosophy. At the same time I received a fairly good training in mathematics, physics and chemistry (modern biology was still emerging and not very present in our curriculum). So, when I was sixteen, I had to choose what I wanted to do: philosophy or science. I was clearly leaning on the science side, and I chose to go to a “preparatory school” for École Polytechnique, which is one of the two prestigious university level schools in France (what we call “Grandes Ecoles”). I had the chance to enter the school after two years, although it typically took three years. Ecole Polytechnique has been established in 1794 during the French Revolution to provide France with high level civil servants with a strong scientific and engineering. It offered a two-years program with an interesting mix of discussions of economics, political science and global societal issues on one side and a deep introduction to advanced mathematics and modern sciences on the other. After these two years which gave me both a deep commitment to public service and to science, I had to choose between a two-year “Applications School” or a Masters in a scientific field. I clearly opted for the Masters. My natural inclination was to choose a master’s in physics, but a number of senior advisors suggested that I look seriously at biology, because molecular biology was taking off and there was a lot of talk in France about the progress being made. There are French biologists actually got in 1965 the Nobel Prize in Medicine and in Physiology for their work on enzymes, and I happen to speak with one of them, François Jacob, after one of his seminars at Ecole Polytechnique, and a few minutes with him had a strong influence on my outlook of science. What he taught me, through this short interaction, is that you have to be both a scientist and a “poet,” by which he meant that you have to learn how things work but also keep enquiring about the meaning of the universe you discover. In any case, I finally decided that biology was too complex for me. I wanted to do something fundamental. Molecular biology is clearly fundamental, but evolution has built very intricate mechanisms that were just started to be uncovered, and the complexity look daunting. I took the easy route, so to speak. It was probably a book by Louis Leprince-Ringuet describing his experience in physics, and my father’s influence in my formative years, which tilted the balance towards particle physics. Leprince-Ringuet directed a laboratory attached to École Polytechnique and it was natural for me to join his lab while doing my master’s at the University of Paris in Orsay. Given my background, I knew that I wanted to do experimental physics, but there was no experimental physics master’s at that time, and my master’s was in theoretical Physics.
Bernard, did you want to do experimental physics in part because of your father's background in engineering?
Absolutely, absolutely. And I like to do things with my hands. So, this gave me, actually, a very interesting training, which is basically double training in theoretical and experimental physics. Experimental physics is more coming from my father, in some sense, and also what I did early in my career, but the strong mathematical training of Ecole Polytechnique and the theoretical physics master’s gave me a formal training in mathematical physics. This gave me a different perspective from that of most of my experimental colleagues on physics. I was able to redo theoretical calculations, which were rather technical. This allowed me a perspective which I would characterize in the following way: many experimental physicists are just trying to take models in the literature and trying to apply them to their data, whether the models are fundamental or just feeble attempts at trying something, which, in most cases, is artificial and doesn't work. So, one thing that this training gave me, which was important, is the ability to understand what is generic and what is not generic in the models. I used to say that I'm not interested in testing a model that one of my theoretical friends thought early in the morning, wrote down by the end of the afternoon, and totally forgot the following week. Experimental physics is a much longer timescale, and we have to put our effort on something which is fairly generic and likely to be important. I will come back to that a little later, but this helped me all along my career.
Bernard, it sounds like you were set on experimentation, but your background in theory has been useful to you for your entire career.
Yes, and I hope I have been able to transmit this approach to my students. I encourage them to do a little bit of the same thing and get involved in some theoretical work at the same time as experimental.
Bernard, I'd like to ask, as a student, how parochial, or not, was your worldview with regard to physics? In other words, was your understanding of physics contained within France, or were you aware what was going on in the rest of Europe, in the United States, in Japan?
I was certainly aware of what was going on in the United States, because basically all my teachers in France -- we are speaking of the mid-sixties -- were partially educated in the U.S. France did not have a good physics research support system before the war. There was not yet the CNRS, the equivalent of the NSF in the US. This only was established after World War II. So, many of my research mentors got some kind of post doc stint in the U.S., at least one year, and in several cases, several years. So, this was a normal thing. We were quite aware of what was happening in the U.S. The European cosmic ray physics community, the training ground of most of my mentors, was fairly well connected and extremely competitive: we knew quite well what was happening in the U.K., and in Germany for instance. Moreover, when I began to be involved in research in 1965, the focus of most of these groups was shifting to the new accelerators that were being built, NIMROD at the Appleton Rutherford Laboratory in the UK and the PS at CERN in Geneva in addition of course to Berkeley and Brookhaven in the US. CERN was put together in 1958 as an effort by Europe to create a center of research which would allow it to be a major player in the new field of particle accelerators.
Bernard, when you say Europe, to specify, did you understand that to mean specifically Western Europe, or would Eastern Europe and the Soviet Union be part of that shared effort?
No, at that time there were essentially no contacts with Eastern Europe.
So, CERN was very much a product of the Cold War, in a sense.
Yeah, it was in the middle of the Cold War, so there was absolutely no contact. Physicists of my age even didn't know that some of our colleagues -- I'm thinking about Bruno Pontecorvo -- were in the Soviet Union at that time. We did not know that. We knew that Fermi and similar people were in the States. We knew that scientists had fled Italy one way or the other, but the young people like me were not aware of those who “defected” to the Soviet Union. This might have been a bit of a taboo, in the older generation. I think the first time I heard of Bruno Pontecorvo, and neutrino oscillation experiments, and so on, was in the early seventies. I think theorists probably knew a little more. We knew about the Course of Theoretical Physics of Landau and Lifshitz, which become available in English in the 1960s and 1970s, and I am not sure when the Soviet Physics Reviews began to be systematically translated but there was very little contact before the early 1970s. At least this is when as a young physicist I began to be aware of the work in the Soviet Union. Okay, let me correct a little bit what I said. I think there were beginning to be contacts in the late sixties, because we knew about Serpukhov, the big accelerator that they were building and put in operation in 1967. And they had contacted CERN to start collaborations. It was enough for my wife and me to make a feeble effort in early 1970s to learn Russian in case I will be sent by CERN to the Soviet Union to work a common experiment. However, my career took a different path and my contacts with our Soviet colleagues took a different form. I went for the first time to Soviet Union in 1974. I was at that time a postdoc in Berkeley and was asked to join an American delegation to a conference in Dubna, the equivalent of CERN for the Eastern Block, in part to report to the US Department of Energy at what stage the Soviets were in terms of electronics and instrumentation. Clearly, we were all impressed by what they showed us, including one senior electronics engineer who was actually “scared” by the level of sophistication of their designs and their ability to copy our integrated circuits. I am sure this was reported up the chain of command. Then, when I came back to CERN in '76 as permanent staff member, I was assigned, between 1977 and 1984, to the official committee managing the collaboration between Europe and the Soviet Union. This would be a whole story in itself, as this was the time of the Human Rights issues (and half of the Soviet part of the committee was KGB!). Hopefully we were useful in maintaining a dialog with the Soviet authorities, at a time when the American Physical Society severed all the contacts with the Soviets, and maybe we helped protect some of our colleagues who were at the limit of dissidence. Coming back to your original question about the intellectual landscape, unfortunately, my training was fairly specialized. It was particle and nuclear physics, and there was very little of what was called at that time condensed matter physics, or solid-state physics. There was no biophysics, of course.
Bernard, was Orsay the best place to do the kind of research that you were interested in?
Orsay, yes. That's where actually the master’s took place, the lectures and so on. The Sorbonne, which is the main university of Paris, was barely teaching modern physics. A more multidisciplinary institution was the Collège de France, a prestigious, typically French institution established by Francis I in 1530. If you appointed as a professor there, you are basically paid to give one series of lectures a year and make progress on your own research. Leprince-Ringuet, for whom I initially worked, was actually professor at Collège de France. We would go and listen to these lectures. Some of us joked that sometimes we had to prepare part of his lectures. But unfortunately, we would not go to the lectures in other fields, which would have widened our horizon. There were probably much fewer avenues of multidisciplinary interactions, at least for a young physicist like me. This is not true, probably, for some of the theorists that taught our Masters. I became later on aware of their activity in condensed matter, for instance.
Bernard, what did you work on for your thesis research, your dissertation research?
I got this master’s degree in Paris (Orsay), and then I decided to go to CERN. So, I was actually a young student at CERN, and I worked on bubble chamber data. My PhD thesis was a generalized analysis of multiple sets of data around the idea of Regge poles, which were dominating the phenomenological discussions at that time. However, nobody knows anymore what Regge poles are. When I arrived in particle physics in '65, there was nothing significant happening. Nothing happened until about '72 or '73. The last discovery was the Omega minus, and people were starting to speak about quarks, partons, symmetries and so on. The theoretical community was, I think, looking for the next big ideas. On the experimental side, we were getting a little worried, unsure of how to make progress. Of course, we were very busy with our experiments, but we made no obvious breakthrough. So, when I defended my thesis in 1971, it was kind of broad analysis of what we knew about 2-body collisions leading to 2-body final states with transfer of charge, strangeness etc. After I finished my thesis, I still had this fellowship at CERN and decided to switch from bubble chamber techniques to electronics techniques. I was not very happy with the bubble chamber work, it was mostly analysis work of film data, and that's not what I wanted to do. BEBC, the Big European Bubble Chamber] which was coming online at that time, were basically mostly the work of engineers. They were too delicate and expensive to meaningfully involve young, irresponsible physicists like me. So, I decided to go to electronics techniques. A general theme of my work in the following decade and a half was basically trying to develop the next electronic instrumentation, which gave detailed information similar to what bubble chambers provide but with all the advantages of speed that the emerging electronics techniques were bringing. So, basically, I invested quite a bit in multiwire proportional chambers, a promising technique where we could detect ionization in gas from charged particles and measure a trajectory of these particles, e.g., in a magnetic field. We obtained the information in real time, and it did not suffer from the long recovery time of Geiger counters and spark chambers. I was actually a good friend of George Charpak, who got the Nobel prize for the invention of this technique. Although I never worked in his group, I was in very close contact with him and his group. So, just after my thesis was finally done in 1971, I was involved in what was called the Omega Spectrometer, a big magnet that we filled with proportional channels. This was a little unconventional in the division where I was in at CERN. Charles Peyrou, the director of the division, was a bubble chamber pioneer. Charles and I were actually fairly close, he was one of my influential mentors. Although he asked of course, why I was switching, he understood my reasons and gave me the authorization to do it. Some people considered me a little bit like a traitor, which is kind of silly. So, anyway, that's what's I spent the next two years working on, and Omega turned out to be a fairly successful instrument, providing interesting physics results, unfortunately after I had to leave CERN. This was 1973, I was arriving close to the end of my fellowship at CERN, and Charles Peyrou, told me: "Look, I would like to give you a permanent contract, but we cannot do that because you are not thirty yet. So, disappear, as far as you can, for example, to the U.S.", and he provided me with excellent recommendations. So, that's the time when I came to Berkeley, in summer 1973, as a post doc at LBL.
Bernard, I want to stop right there, before we get too far away from France. I'd like to ask you, as we talk about some of your service in the political realm later in your career, in the late 1960s when you were a graduate student in Paris, of course, there was a lot of social and political unrest among students and other populations.
I was not in Paris in 1968. I was living in Geneva.
Oh, you were in Geneva. So, you were away from that.
I moved to Geneva in '66.
Full time. You were not going back and forth.
I was not going back and forth, and my wife and I felt a little isolated in Geneva because there were all these things taking place in Paris in 1968. My family and family-in-law were living in Lyon, so we'd go back and forth a little bit between Geneva and Lyon. Maybe once a month we'd go. Generally speaking, we were supportive of the demands of the students. On the other hand, my father-in-law was actually a high-level official in Lyon, and this gave us the perception of what was going on the governmental side. But I never went to a demonstration. The closest that we did with my wife, who was pregnant at the time, was when we happened to be close to a demonstration in Lyon, we asked ourselves whether to join and concluded that no, we shouldn't do that. This would not have been safe for the baby to be born. Anyway, the distance gave us perspective, but at the same time, we found a little bit cut off. There was also in 1967 the Six-Day War between Israel, Egypt, Syria and Jordan which struck me quite a bit as I was in the reserve of the French Army and could imagine how difficult it had been for some young Israeli officers in a situation similar to mine to be called and have to serve for a cause they did not necessarily fully endorse.
Bernard, what were your politics with regard to nuclear weapons? How did you feel about nuclear weapons?
I was generally against nuclear weapons, although this was not at the top of my agenda at that time, and there was some French pride about developing our own nuclear force. The French tested their prototypes first in Algeria till the independence in 1966 and then in French Polynesia and it became increasingly scandalous that the French went on testing till 1996. I had some contacts, of course, with colleagues of mine who were working in a nuclear energy center in Saclay. They typically were involved not only in nuclear physics but also in civil nuclear power and nuclear weapons. I don't remember a lot about the discussions, but I knew that personally, I did not want to work on weapons. That was very clear. I was on the other hand more open to civil nuclear power. Moreover, as a French Navy reservist, in case of conflict, I would have been probably assigned to the submarine nuclear program. I would have tried to limit my involvement to nuclear propulsion, but I was never recalled. But the question of the relation between Science and military research became important for me in '72, when we became aware at CERN of the whole JASON controversy in the U.S. with a number of physicists working on issues like Vietnam and the bombing of Cambodia. And I was sufficiently interested that I was one of the organizers of discussions between the scientific staff at CERN and American physicists involved in JASON members around the general theme of the responsibility of the scientists. I remember in particular a panel discussion that we organized with a number of panelists including Sid Drell (that I did not know well at the time; twenty years later, I worked on a number of US committees with him), and Murray Gell-Mann, who was already very prestigious. I vividly remember their intervention. Sid basically took a very courageous stand, saying, "Look, you have to choose between working in the system or outside the system. Both ways of doing things are important. I chose to work within the system where I can have some influence.” Basically, he came totally unscathed out of the debate. His stand was very well received and as you know, Sid Drell played a very important role in disarmament discussions with the Soviet Union. Unfortunately, Murray Gell-Mann took a more ambiguous position. “I did not want to do it, but I was forced to participate because of my seniority, and so on.” This didn't go well with the audience. So, this was one panel discussion. We also organized at least one other discussion about the applications of science, where I remember George Charpak speaking about the application of nuclear instrumentation to medicine, in particular “his” multiwire proportional chambers. There was also some discussion about cancer treatment with accelerators, which was starting at that time. But the remark I remember most was by Val Telegdi, a professor at the University of Chicago, something like: “All of this is very good, but CERN will have little impact if it stays so specialized. The lack of multidisciplinary will kill its intellectual life.”
Bernard, did you come to Berkeley with the understanding that the postdoc might possibly turn into a proper faculty appointment?
No, I came back to CERN for eight years in between. I started my postdoc position at LBL at Berkeley in August 1973 to get “as far as possible from CERN” as advised by Charles Peyrou. On the day of my thirtieth birthday, I received a phone call from Charles saying, "Okay, now that you are thirty, would you consider coming back to CERN, and becoming a member of the senior scientific staff?" I said, "Yes, but I am currently doing pretty interesting stuff that I would like to finish: Can you give me two years to come back?" Which he did. When I had arrived in Berkeley, I joined the group of Louis Alvarez, a Nobel Prize winner for his work with Bubble Chambers. There was another post doc, Alan Litke, arriving at the same time, and we were discussing what to do. We were given essentially complete freedom of what experiments we could join. There was this fascinating result from the Mark I experiment at SPEAR, an e+e- collider at SLAC: the e+ e- annihilation rate they measured was very much larger than expected. This was totally unexpected. So, we decided to go and join that group. We were put in charge of constructing a cylindrical proportional chamber to place around the beam pipe to improve the particle reconstruction and vertex identification capabilities of Mark 1. It turns out that a lot of the theoretical speculations about the high annihilation rate, such as electrons having a hadronic component, were misguided. It is not what was happening. This was due to the production of charmed particles as evidenced by a more careful energy sweep which showed a clear step in the rate at a particular energy on which two spectacular resonances are superimposed, the Psi and the Psi-prime. It turns out that was supposed to be on shift at our experiment at SLAC, a particular Sunday, November 10, 1974, but decided not to go as I had to give what I thought was an important presentation at the SLAC Program Advisory Committee the following day; there were enough people on shift and it was more important for me to work on my talk instead. Then, that night I received a phone call from one of my colleagues saying, "You know what? We made a major discovery in your absence." And of course, it was very spectacular: the Mark I spark chambers were very noisy electronically, and every time they would trigger, there was a little "tack" in the intercom in the control room. If the energy of the machine was off the Psi resonance, it would be going "tack" and then "tack" and then "tack”, once every few seconds. When we were on resonance, it was "tack tack tack tack tack", several times a second. So, it was indeed very spectacular. I had been part of the discussions the previous Friday where we decided what to do for the weekend. We were supposed to go to high energy, but the cavities were not working, so we decided to go back and try to understand the dispersion of results that we had in that energy region. Anyway, I missed the Psi, and I am a member of Leon Letterman’s “prestigious club of people who missed the Psi", as he told me jokingly. I’ll come back in a minute to Sam Ting who concurrently discovered the resonance at CERN. In any case, in addition to the cylindrical proportional chamber which significantly increased the Mark I reconstruction accuracy, my main contribution on the analysis side to the experiment was the discovery of what now are called the Chi states. They are excitations of the two-charm pair. I was quite involved in showing that these states existed and establish their spectrum, which looks a little bit like excitation of atoms. I gave a talk at the APS meeting presenting the spectrum, which was quite complex. One of my colleagues told me afterwards, "I came to your talk, but I thought I was in the wrong session because it was clearly atomic physics." Anyway, clearly, we were sitting on the gold mine, and, as I explained to Peyrou, I did not want to go back to CERN immediately, so I stayed in Berkeley until May 1976.
Bernard, what were some of the major research questions that you were interested in at this point. And the experimentations that you were involved in, how were they responsive to those questions?
Okay, as I said at the beginning of my career until about '72, the field was a little bit looking for questions. I was only vaguely aware of the effort at SLAC by Bjorken and company of trying to formulate the deep elastic results obtained at SLAC at that time, in terms of “partons,” small hard scatterers inside the nucleon which were later understood to be quarks and gluons. Then, in '72, there was the beginning of the neutral currents in weak interactions, with what we jokingly called the 3-phase alternating neutral currents depending on whom was speaking, Carlo Rubbia, David Cline or Alfred Mann. They were collaborators in the same experiment but were initially reporting results contracting each other: one day, it was a clear proof of neutral currents and at the next conference a clear proof of absence of neutral currents. It was indeed tempting, given the oversized personality involved to make fun of these 3-phase alternating neutral currents. But the story stabilized and there were indeed neutral currents (which, 10 years later, we showed to be due to the Z particle). Then in 1974 SPEAR with the Psi gave a clear indication of the charm quark. As I said before the initial results were initially puzzling but between 1974 and 1976 the pieces came down and fit nicely together. We did not know, or at least I did not know, that Sam Ting had found the same particle as the Psi, that he called the J (and now we tend to call it the J/Psi). There has been a lot of debate about who got the result first, but this is another story, not very important anyway, as Burt Richter, our boss, and Sam Ting shared the Nobel prize.
Bernard, did you cross paths with Sam Ting at all at this point?
I did not know Sam at all at the time of the J/Psi, but when I was at CERN, I was in contact with him fairly regularly during the construction of the pbar-p collider, Sam wanted to actually build an experiment in the same interaction region, so we were in constant contact with him. There was a strange relationship between Carlo Rubbia and Sam Ting. Sam had a very different style from Carlo's, but he was also a fairly intense guy. And Carlo did not want to say directly no to Sam but managed to fully occupy the available space which meant that our cohabitation was impossible. On a different note, I remember a breakfast at the CERN cafeteria a few days after Burt Rucher’s and Sam Ting’s Nobel Prize was announced. I was with Martin Breidenbach, a key member of the Mark I team, whose technical dexterity was largely responsible for the beautiful results we had. Martin was visiting for some reason, and when Sam asked whether he could join us at our table, we of course invited him to sit down with us and spoke about the Nobel Prize. We asked him whether he would bring a number of collaborators to Stockholm. He said, "No, I would like instead to bring my family from Asia." So, it was an interesting conversation. Anyway, where were we? To answer your question about precisely —what were the questions which were motivating me— it was the most fundamental questions of the moment which we could attack with the new emerging instrumentation. One constant in my career has been to try to apply the best instrumentation available to explore a new field. That's what we did with e+ e-. When I came back to CERN, I was hesitant about working with Carlo Rubbia and started to look for other opportunities. I knew Carlo of course from his reputation, but also a little bit personally from my previous stay at CERN. For me, he was really smart and focused on some of the important questions of the moment. But I was more worried about his spotty publishing habits, his authoritarian management style, and the way he was treating his younger collaborators. But this was the time when he and Cline came up with the pbar-p idea. This seemed actually the only game in town: this would allow finally to look for the W and Z, the Weak Interactions Intermediate Vector Bosons that we were convinced had to exist around one hundred times the mass of the proton. But we needed the energy to produced them. Antiproton-Proton (pbar-p) head-on collisions was one way to obtain the required energy, but we needed to obtain enough of antiprotons. LEP was far in the future. The short version of the story that in spite of my reservations, six months later, I was working closely with Rubbia, but with an agreement, supported by the CERN management, that a small team of physicists and engineers would be directly in charge of the management, while Rubbia will retain the general leadership of the experiment. We put together a group of maybe five people, all very professional and disciplined counterbalancing some of the worst habits of Carlo’s but fully benefiting from his insights and his leadership. And we did not depend on Rubbia for our own career and had the full support of the CERN management. Carlo somewhat reluctantly accepted this arrangement, but this actually worked fairly well for at least eight years with a number of interesting results and a Nobel Prize for Carlo. So, anyway, six months later, I was working with Rubbia, and we focused on trying to demonstrate the accelerator physics that was necessary to construct a pbar-p collider.
Bernard, you mean the need to operate at higher energies?
No, we had to “cool” them down. The problem -- we can produce antiprotons. That's not a major problem. But we cannot inject them directly into the small hole of an accelerator in sufficient numbers. So, we have to “cool” them down and basically make a nice narrow and intense beam that we can inject into our accelerators. So, we had this kind of initial cooling experiment, ICE, which was an attempt to demonstrate that. It was actually a very interesting team where both particle physicists, Carlo Rubbia, and me, and accelerator physicists, for instance Simon van der Meer, and Guido Petrucci worked very closely together to basically demonstrate that this would work. I was in charge of the machine itself. It was a small accelerator where we did the testing, which was interesting because it was a reversal of roles; we could have imagined that Simon or Guido would be responsible, but they were occupied with the cooling challenge. So, I went to see my good friend Emilio Picasso. We were looking for a magnet ring to put our accelerator in. Emilio had a suitable no longer in operation: He had just used it for the famous g-2 to experiment where his team measured a fundamental magnetic property (the g-2) of the muon which will place fundamental limits on particle physics beyond the Standard Model. Emilio had these beautiful magnets which were machined in place for maximum precision, and I wanted not only to use them but also cut the lovely, machined pole pieces to transform this machine into an alternating gradient storage ring. I remember Emilio’s face when I asked, "Can I cut the magnets and machine the pole faces?" But Emilio, of course, after a brief hesitation, said yes. Building ICE was really quite an interesting endeavor because of the mix of culture. To give you an example, when finally, the machine was ready, we were injecting, trying to get particles -- not antiprotons yet, but particles from the proton synchrotron at CERN into the machine, and something was wrong. We could not see the particles coming through. I was in charge of the shift, actually, and I said, "Look, I know what to do. Give me give a few minutes." I went to my lab to get a scintillator with its phototube readout and brought it to the control room. I knew, because I had tested detectors close-by that every time you were injecting protons from the PS, there would be a big burst of particles that we could see in any kind of detectors. So, I was there with my scintillator in the middle of the control room, with lots of people watching, because that's the tradition at CERN when we are trying to start a new machine. They said, "What are you doing?" I said, "I'm looking for the particles -- the protons that you are losing are somewhere, including around here.” We were 200 nanoseconds off, or something like that -- I don't remember. So, the accelerator physicists were saying, "Can you really see the particles when we are trying to send to your machine?" "Yes, of course I can." So, they get scared, actually. So, what is the radiation level? That was the engineering aspect. Are we irradiating ourselves more than we should? We were not. And then we corrected the timing and successfully injected the protons in our storage ring. The point that I am making with this example is that because of the difference of cultures, the particle physicists were bringing different ways of doing things than the accelerator physicists. I think this was a very interesting time. So, we were able to demonstrate that we indeed could cool the antiprotons, and then succeeded to convince the upper management of CERN to allow us to do to make the modifications to the main CERN accelerator, the SPS, to transform it into a pbar-p collider. That's where Rubbia was actually very important, not only because he has good taste in physics, there is no doubt about that, but he can be extremely convincing. We had two directors at that time, John Adams for the accelerators, and Leon Van Hove for the science. We had to persuade them that this was the right thing to do. So, we are speaking now of probably '79, or something like that.
Bernard, at this point, as you're maturing in your career, are you operating at all in an administrative capacity, or is this all strictly scientific work for you?
So, the initial cooling experiment, I had very little administrative responsibility. We were purely science. But as soon as we began to put together an experiment which became the UA1 experiment, I was actually very involved in its management and in budgets. CERN was trying to protect a little bit its physicists from that. Management overload was one of the reasons actually I have gone back to Berkeley in '84 and switched fields to cosmology was that I thought I was not doing enough science. I was doing mostly budget and organization. I used to say at that time if I wanted to do big organization, I would not have joined IBM or General Motors. But I am a physicist. I want to do some science. So, that's the motivation which led me to attempt this switch to cosmology. I wanted more science, smaller teams, and at the same time something which was multi-disciplinary. Although I was part of the small team which discovered the data, the W and the Z, so I cannot complain about not doing any science with UA1. We did important science, and that was a very exciting time. I forget the exact date, but probably proposed to modification of the accelerator and the experiment around '78 or '79 and began to build it and roll these things out at the end of '83 and '84. But the analysis of the W and Z data in which I was deeply involved only represented three or four months out of five years. Our Physics goals were quite clear. We knew that something had to be there; people have suspected for a long time that there was a W Intermediate Vector Boson to explain the weak interactions, but nobody has been able to see it. Using pbar-p collision scheme, we certainly had access to it, as we will be well in the energy range which was indicated by a number of models. Because of the neutral currents, we thought there would be also a Z, the neutral companion of the W. The design of the experiment that we needed to achieve these goals was also pretty clear: The easiest channel to detect the W was when it decays into an electron with a high transverse energy and a neutrino which is not seen but also carries away a large amount of transverse energy. An event with a large energy electron and a corresponding large missing energy would be a clear candidate. We needed therefore a detector that measures as completely as possible what emerges from the collision. Rubbia, our collaborators and I rapidly agreed on this principle and started to design what we called a “hermetic” geometry trying to cover as much solid angle as possible, in a very similar way to what our SLAC-LBL team did with Mark 1, at SPEAR. Although, we used a dipole magnet with a magnetic field perpendicular to the beam instead of the solenoid with a field parallel to the beam as we had used for Mark I. This was the result a fairly hit debate between Carlo Rubbia and me: I was a defender of the solenoid which was more symmetrical, and Carlo was the defender of the dipole which was better at small angle with respect to the beam. Of course, Carlo won. I think we paid some price with that decision which made the analysis more delicate, but the most important goal was to get a detector running rapidly and, in the grand scheme of things, the magnetic field geometry was a detail. We convinced the CERN advisory panels and management that we had a good design, and this became the UA1 experiment that we built in 1980-1983. Anyway, putting together UA1 was an interesting experience. I was in charge of the central detector, these huge drift chambers 2.3 meter in diameter and 6 meters long which were giving us a bubble chamber like picture of the interactions. Carlo and I actually worked very well together on this project. Basically, he was focusing on the strategic issues and he left me and the team of physicists and engineers we have brought together, decide the details. We knew what we were looking for. And we were able to mobilize the CERN resources with the highest priority.
Bernard, on that point, I want to ask just to zoom out a little bit. What were some of the broader theoretical propositions that made you look for the W and Z bosons? What was built to create the UA1 experiment?
So, there was what we now call the Standard Model. It was starting to be very well established. This model of Weinberg-Salam-Glashow, which was basically unifying the electromagnetic and weak interactions, was partially vindicated by the presence of the neutral currents. So, the missing piece were really the W and the Z. Indirectly, there were some ideas about the mass, which was thought to be roughly 100 times the mass of the proton.
Bernard, was your understanding that the discovery of the W and the Z would complement the standard model, or enhance it?
It was a missing piece. So, we knew that the standard model was not complete. In our goal of trying to unify the electromagnetic and weak interactions, the W and Z were the missing piece. Since our discovery of the W and Zs, '83, '84, the particle physics field has focused on different problems. The missing top quark was identified at Fermilab in the mid-1990s. Another missing piece was the Higgs, responsible for giving mass to the hadronic particles and this was another spectacular discovery of CERN, culminating in 2012 with a major announcement of very strong evidence by both Atlas and CMS, the two major experiments at LHC. The focus then shifted to “Physics Beyond the Standard Model”: As you probably know, the Standard Model is unstable. Something has to happen at high energy, or the whole framework basically collapses. This is where the ideas of supersymmetry were coming from. Trying to stabilize the Standard Model. This may be very much related to dark matter that we will discuss a bit later on. But so far, we have not seen anything of the type at LHC. Coming back to UA1, we of course tried very hard to identify signs of physics beyond the Standard Model. We had a number of intriguing hints, but they all disappeared, often after refinements of our background models to take into account the improved understanding of the standard model phenomenology. We simply did not have enough energy for this new physics. In any case, working on the UA1 experiment was actually a very nice time for all the members of our team. Let me say, it was an intense time. And unfortunately, after the obvious goals of the W and Z were reached, it was more difficult to keep a strong focus and vibrant enthusiasm, a number of people (including me) switched to other projects, and things began to disintegrate a little bit and data taking was completed in 1990.
Bernard, on that note, did you know early on how successful UA1 was going to be? Was it immediately apparent how fundamental the discoveries were?
Oh, we were hoping it would be.
I’m asking, how long did it take for that confirmation?
I forget exactly when we started to look seriously for the W: it was probably in late 1982. At the beginning, it was, of course, a new experiment. It was a mess, and we had to debug it patiently, and so on. The accelerator physicists had also to debug the collider. As soon as both the experiment and the collider were working reasonably well, we were convinced that we could see something if it was there. At that time, we observed a number of strange events which could, superficially, have been the W and Z or something even more exotic. This was very exciting, but I had to remind my colleagues that we did not know well our detector yet and had to calibrate its response. When this was done there were much simpler and mundane explanations. Then the collider performance improved rapidly and suddenly we began to see very spectacular events with a very high energy electron and a lot of missing transverse energy, exactly what we expected from a W decaying into electron and neutrino. In a few weeks we convinced that there were no other possible explanations and published very rapidly. A few months later we had also a handful of Z candidates, with very spectacular pair of very high energy electrons. So, difficult to miss! But, as expected, the production rate was smaller, so we had to wait a little longer to get a decent number of them. Many people had been telling us that we won't be able to see anything because there would be so much junk together with the W and the Z. I remember that one argument that I kept making at that time with all my theorist friends, "Yes there will be junk. There will be emission of gluons, and jets all over the place. But we will see this junk. It's not something which will prevents us to see the Ws and Zs. Our detector is powerful enough to give us a complete and detailed picture of the events.” A less powerful detector where we did not see the junk would have produced much less convincing observations! So, anyway, that is the story. During the Initial Cooling Experiment and the design of UA1, I was actually quite involved in another very interesting non-professional project. From the US where I was between 1973 and 1976, I was following closely the ascension of the French Socialist Party. So, when I came back to Europe in '76, I spoke with a friend, Jacques Attali, who was quite involved in the Socialist Party and asked him "Is there anything that I can do to help?" Jacques was actually a close adviser of François Mitterrand, who was the head of the socialist party (Mitterrand will become in 1981 the President of the Republic of France). The socialist party was in the opposition. It was the second major party in France at that time. Jacques came back to me after a few weeks after consulting Mitterrand and said, "If you could put together a committee of experts on the question of civil nuclear power, that would be very useful." In hindsight, I know now what Mitterrand was thinking. He knew that his alliance with the communist party was fragile, and that he wanted to bring the ecologists on board to still assure a majority in case he had to break with the communists. So, I was kind of the perfect guy, so to speak, to put something like that together: I was scientist with some good knowledge of the nuclear technology; I was not communist; and as a young socialist, I was some ecologist sensitivity. So, I accepted the challenge and with the help of Jacques and other senior members of the party, we put together a committee to review civil nuclear power-- I insisted at that time that it shouldn't be just nuclear physicists. It should be something broader. So, we had a theologian on the committee. We had a sociologist. We had nuclear energy pioneers and high-level nuclear engineers. We had energy economists. They were all prestigious and well-known people, and I was acting as the secretary and organizer of this group. And because the Socialist Party was likely to win the next election in 1981, we had broad access to the upper levels of the French civil servants. The French press called us the Wise Men Committee. Our mandate was to produce a public report for the socialist party about what to do with the civil nuclear program. France was on track to get -- I forget the number -- thirty nuclear power stations, with a design very similar to the American Westinghouse pressurized water reactors. This program already represented some seventy percent of its electricity supply. Major, major push. This was never debated at parliament. It was purely a plan defined by high level civil servants, that the French often caricature as “technocrats.” No political input, no involvement of the population. That was one of the problems. The second problem was that the Atomic Energy Commission and EDF, Electricity de France, the national electricity company, were pushing very strongly a very ambitious breeder reactor program, which is very difficult technically, and problematic in terms of safety; the coolant is liquid sodium, which is very flammable. Contrary to conventional fission reactors with water moderator, such a breeder reactor is not intrinsically stable. If you lose the coolant, you can really a runaway situation with a small nuclear explosion. So, anyway, the mix of technical challenges and societal aspects was very interesting. I had to travel quite a bit to Paris. But we rapidly converged on recommendations: we suggested a moratorium in the construction of reactors, a re-evaluation of the breeder program and nuclear fuel reprocessing and a public debate of our energy policy in the parliament. The main message was that the problem was not with the nuclear energy per se. The problem was with the way it was deployed, a purely technocratic and authoritarian process without buy-in from the people. The standard joke in France was that if there were local opposition, the government was just sending the riot police instead of engaging a dialogue with the local population. This was not very far from reality unfortunately and rather typical of the French way of dealing with opposition to “progress”. In this convergence process, we had to deal with a few leaks before the report was finalized. Politicians like to make spectacular announcements, even if they are premature and Mitterrand was rather typical in that respect: he started to speak about the conclusions that we were developing to try to attract the support of the ecologists. Anyway, I had a few meetings with him but mostly with close colleagues of his. It was an interesting experience. Then, after when report was finished, I was surprisingly put in charge of the energy commission of the Socialist Party to refine the propositions. The Socialist Party indeed won the Presidential and Legislative elections in '81 and Mitterrand became President. We were naively expecting that the new Socialist government will implement our propositions. This was unfortunately not going to be the case! Too many of the high-level civil servants in charge, for instance the head of the Atomic Energy Commission, dug in their heels. We had interviewed them for our reports. So, they knew what we were speaking of. They explained to the prime minister that this is a much more complex problem than the Committee of Wise Men envisioned. France was for instance bound by international agreements on nuclear fuel reprocessing that we are to fulfill, the breeder program involved international partners and so on. So, basically, the technocrats just tried to resist any meaningful change of policy. Probably the main error that we made is that we didn't brief in detail the incoming Prime Minister, who didn't know very much about the whole thing. So, basically, not very much happened. There was a debate at the Parliament. That's the only thing we got. I was very disgusted after having spent five years of my life on this project. But politicians are living in a different world. Lesson learned!
Bernard, did you see this work as tangential to your research, or did you look at these as two parts of the same coin?
Well, technically, it was tangential, because it's not even nuclear physics or particle physics that we were working on. I learned what I had to about pressurized water reactors, the new breeder, reprocessing and so on, but that was not aligned my own research. The other thing, the theme of the report was more about the need of debating for the public at large these big technical decisions. Try to involve the citizen in the decisions, to not just impose the decisions made in secret, even if the deciders are technically very competent. The main person who shaped the report was Alain Touraine, who is a sociologist. So, the report was mostly a sociology report, or political report, with a lot of the technical underpinning as necessary. It was about the need for debate, and choice between big technical options. So, I saw that as an integral component of my mission as a scientist, to try to bring information. We were speaking of Sidney Drell earlier. I certainly don't want to compare myself with him. We had very different political opinions, and he was much more prestigious, but that was also what Sid was saying. This was the reason he got involved in nuclear disarmament agreements with the Soviet Union. This was just an incredibly important question and where he had an essential influence. As a scientist, I also thought it my duty to get involved in these issues of important technical choices and citizen involvement.
Bernard, how did you get involved as a dark matter research? Did you see this as a natural progression from UA1, or a new field entirely for you?
Okay, so we are speaking of '84. I was looking for a refocus of my career, as I said, with something with a bit more science every day, and but also for something which was really fundamental, and multidisciplinary. There were some discussions at CERN at that time about cosmology, which was clearly taking off at that time. There was actually a conference at CERN in 1983, the 1st ESO/CERN Symposium on the Large Structure of the Universe, Cosmology and Fundamental Physics, this involved big names of the field. This was partially related to the possibility for CERN to take over ESO, the European Southern Observatory which was being established, or at least have a strong technical and management partnership with it. Finally, the decision of the governments was to have a different organization, but CERN housed ESO while they were building new headquarters in Garching, Germany. A strong collaboration between the two organizations was very attractive for several young physicists like me. It was clear to us that CERN should get active on the astronomy side, the cosmology side, because of the strong potential scientific overlap.
Bernard, wait, at that early time in 1983, were people talking about weakly interacting massive particles?
No, not yet. The people were speaking about dark matter, clearly. This had been a problem starting in the 1930s. Vera Rubin, and Sandy Faber, and people like that refined the observational evidence in the mid-seventies. Then, there were a number of theorists such as, Peebles, Davis in the US and Zeldovich in the Soviet Union, which was then really part of the debate. So, for instance, there was this question of whether the dark matter in our galaxy could be made of neutrinos. Zeldovich on one side, and Davis and collaborators and Peebles on the US side showed that it doesn't work. Massive light neutrinos alone cannot form the large-scale structure that we observe today. This was also the beginning of microwave background studies. Actually, I should say the second generation of microwave background studies within particular Paul Richards at Berkeley, and David Wilkinson and his group at Princeton. So, it was clearly something which was going on, which was interesting, and the link between particle physics and cosmology was getting more obvious. In the early seventies, with Hoyle, Fowler and Wagoner could explain the primordial abundance of Helium and Deuterium with the Big Bang assumption and what we know in nuclear physics, establishing the link between nuclear physics and cosmology. We were now speaking about dark matter, and the formation of large-scale structure, to get information about physics at very high energy. Vice versa, some particle physics ideas such as inflation could help explaining central puzzles of cosmology. Coming back to the ESO-CERN symposium, it was the sign of there was a lot of intellectual fermentation at the border between particle physics and cosmology. In May 1984, a similar conference, the Inner Space and Outer Space workshop was organized at Fermilab, and it was clear that many young people like me, theorists and experimentalists alike, were fascinated by this emerging new field and wanted to contribute. Frankly, after the high-pressure work of building UA1 and analyzing the data which leaded to the discovery of the W and Z, I needed some new, at least to take a sabbatical. When I began to explore the idea of sabbatical away from CERN, the management of CERN initial reaction was, "No, no. You can't do that. We need you to manage the UA1 experiment", It was finally Jack Steinberger who convinced the CERN management, "Look, this guy is in danger of burning out. He needs to take some time off." In summer ’83 we started to seriously explore possibilities for a sabbatical starting in Summer ‘84: an attractive one was for me to go to the Goddard Space Flight Center, and I was offered a 1-year position there. My wife had been promised a position at the World Bank — she's an economist. This was a nice way to solve our “two-body problem” as we could live with our three kids in Washington DC. It turns out that at the last minute, my wife’s job at the World Bank collapsed. So, the only thing we could do in a very short amount of time was to go back to Berkeley where we had connections and a number of friends who could find a position for both of us. So, we came back to Berkeley in July ’84. The main purpose of this sabbatical for me, was to try to understand whether I could switch to cosmology. I was “earning my keep” so to speak (CERN was paying ½ of my salary) by doing some electronics for Charlie Townes, who was building a novel Infrared Red stellar interferometer. Then, after nine months or something like that, it was time to think about what next: going back to CERN or trying to extend my sabbatical. I was clearly interested in cosmology, and wanted to switch, but it would take some more time to get more familiar with the field. The other question of whether CERN would be interested in starting a new experimental effort in Cosmology. My contact with the CERN upper management was not encouraging. Herwig Schopper was the director of CERN at that time. He said, "Look, I'm not optimistic. CERN is governed by a treaty focused on nuclear and particle physics and there is probably not a way to divert resources to cosmology" And Rubbia, who was Director General Elect basically implied that he would prefer if I were to find a place somewhere else. So, I did not get a lot of support from the CERN management. I spoke a bit with my Berkeley colleagues, and George Trilling, who passed away just a few months ago, said, "Okay, so maybe we should try to get you to stay at Berkeley." Trilling started to drum up support to offer me a professor position in the Berkeley Physics department, with the help of Charlie Townes, Gerson Goldhaber and people who knew me from my postdoctoral stint at Berkeley. I was eventually offered in May 2015 a tenured faculty position that I accepted with the condition that in two years’ time I would make the decision whether to stay or to go back to CERN, which had granted me a leave of absence.
So, you didn't go to Berkeley committing to make a life for yourself in The States.
Not at that time. My wife and I had been there for three years before, we knew that we would adapt well. There were some transition problems with our kids, in particular with our fifteen-year daughter cutting school, because of the lack of attention she got at Berkeley High. But actually, our decision to stay in Berkeley helped solve these small issues. Initially, I was more thinking about doing gamma ray astrophysics, which is closer, in some sense, to particle physics. Same kind of instrumentation, and so on.
So, at this point, you start to really think of yourself more as an astrophysicist, and not a particle physicist. That's part of the transition.
That's part of the transition, but still with a strong link to particle physics -- -- working at this emerging intellectual border between particle physics and astrophysics and bringing some of my instrumentation background to astrophysics. So, at the beginning I was exploring what I could do on the question of understanding the X-ray and gamma ray background, which could offer a window into the early universe. We know now that is probably related to the emission from massive black holes at the center of large galaxies, but we did not know at that time. The question of origin of this background deeply interested me. And I started to develop new gamma ray detectors based on high pressure Xenon. This was funded by NASA and represented a significant fraction of my research till the late nineties when I realized that with the growing technical challenges of our approach that the technique was less promising than I initially hoped. So, even though we still could support it through NASA, I decided to terminate this aspect of my research, at a convenient time when my students and postdoc were moving to other endeavors. I had also grown much more much more interested in the dark matter question and cosmology in general. For me it started with the seminal paper of Goodman and Witten in ‘85. There had also been a very insightful article by Ben Lee and Steven Weinberg, in1977 but I got aware of it in ‘85. So, there was this sudden realization that maybe we could detect dark matter with particle physics methods. There was an intense time where I began to explore the possibility of building detectors to search for dark matter particles in the halo of our galaxy. I made a number of basic calculations about rates, background and sensitivity, continually comparing my conclusions with other colleagues. This looked very promising on the experimental side. `I spoke also with a number of theorists, including in particular Joel Primack and his student Kim Griest at Santa Cruz. We realized that the argument that Lee and Weinberg were making linking the density of the dark matter particles today with their interaction cross section was actually a very powerful and generic argument: If we assume that dark matter is made of particles produced in the early universe, and there was a thermal equilibrium at that time, their density today is inversely proportional to the interaction rate at the time it dropped out of equilibrium; if you input the density of particles today, you get in rough terms an interaction rate which is typical of weak interactions as if the physics on the W and Z had something to do with the dark matter. Particles interacting at the weak interaction scale could naturally form the dark matter today. Michael Turner introduced the term of Weakly Interacting Massive Particles (WIMP) for this class of dark matter candidates. This was a very powerful idea! This is often called the “WIMP miracle” but I prefer, as Turner does, to call it a powerful hint. So, how to detect these putative WIMPs? The Lee &Weinberg argument gives an order of magnitude of the interaction rates of these particles in a detector which, as Goodman and Witten remarked, makes them detectable, especially if we take into account coherence effects. If the halo of the galaxy is made of dark matter particles, they will go through our terrestrial detectors, and they will scatter and deposit energy As I was saying in ’86 at many conferences and seminars, it could be as high as a few events per kg and per day The basic issue is that because the WIMPs have the velocity typical of the halo of 300km/s, a thousandth of the velocity of light, very small by particle physics standard , they deposit very little energy. So, we need very sensitive detectors. Moreover. the rate is very small, and we need detectors with very low dark count rate. Therefore, they should have very low radioactive background, be carefully shielded and located deep underground. I was asked to summarize intriguing possibilities at the December 1986 Texas meeting in Chicago, a major Astrophysics conference, I was, of course, not the only person fascinated by this possibility, and soon after the Goodman and Winten paper, a number of colleagues in the US, Germany and France started to look at methods to detect WIMPs. A seminal workshop was organized in 1987 at the Rindberg Castle, a conference center of the Max Planck Society close to Munich. This symposium gathered a few of us who were interested in the dark matter detection and we started to compare notes and exchange ideas. This workshop was the first of what became the regular Low Temperature Detectors workshops, Why low temperature detectors? Basically because of their potential excellent energy sensitivity and maybe a misconception that Blas Cabrera and I had. I knew Blas Cabrera a little bit beforehand, but we started to collaborate informally around that time. The misconception that we had, together with most of our colleagues at that time, is that after a WIMP interaction on a nucleus it will recoil in the detector medium, without producing any ionization or scintillation. We only have energy in the form of heat, or acoustic vibrations that are called phonons. We thought we could not use any of the standard methods of particle physics which rely on ionization or scintillation. So, we were basically focusing on this idea of measuring heat or more generally phonon energy. The simplest idea which I was pushing was that in a crystal at low temperature, an interaction will raise the temperature, and we can measure that rise, especially at very low temperature where heat capacity decreases dramatically. By the way, that's also the method that microwave background people are using mostly these days. Blas introduced the more general idea of athermal phonons which could be detected through the breaking of Cooper pairs in a superconductor. So, if we expected no ionization in WIMP scattering on nuclei, the kind of technologies that I knew well, proportion counters, and scintillators, and things like that did not seem adequate. I had to switch to methods of detecting energy at very low temperature, something that I had no experience in. Why low temperature? Because in order to be very sensitive, you have to get rid of the thermal fluctuations as much as possible and they decrease rapidly with temperature. Blas on the other hand was a low temperature physicist and knew a lot about superconductors. I had just been appointed to Berkeley and chose to set up a low temperature lab so my group could be involved in these promising technologies, but they are very hard. I chose to focus on calorimetric methods where we measure the rise in temperature of a crystal with a high-performance thermometer. This was based in large part on what Paul Richards and company were doing to measure the microwave background and I had access to Eugene Haller, who gave us Neutron Transmutation Doped Thermistors. similar to what Paul was using. We tried to adapt the technology to large crystals and lower temperature, and I learnt from Paul and Eugene a lot of solid-state physics. In any case, we made rapid progress. During this period, roughly ‘85 to '90. I was invited to give many talks on the idea of detection of dark matter. And I could not only advertise the fascinating idea of detecting dark matter but also show some progress with novel types of detectors. One time, I think around 1987, I gave a Colloquium in my department, and after my talk, Marvin Cohen who is a prestigious condensed matter physicist, came to me and remarked, "I enjoyed your talk, but there is a small detail which I think is incorrect. It is not true that a slow recoiling nucleus in a condensed matter medium does not produce any ionization. It does produce ionization, but at a very low level. You should read the articles of Lindhard in the mid-sixties". So, then we began to realize that maybe we can measure the ionization at the same time as we measure the energy position. This could give us an additional handle to identify the dark matter interactions on nuclei which would recoil with a really small amount of ionization energy. Our main background in this kind of experiments is radioactivity which gives rise mostly to electron recoils, and for those events, the amount of ionization is between five to ten times larger than a nuclear recoil of the same energy. So, I spoke with Eugene Haller and other condensed matter physicists. The conclusion of these conversations was that it could work in principle, but we will never be able to extract the ionization from this germanium crystal that we are using. That won't work. So, I shelved this idea in my mind. In the late of '89, a student of mine was very smart but somewhat lazy, decided to save himself some work by wiring a prototype that we were working in a fairly unconventional way. I was furious and said, "Why did you do that?" Anyway, we ran the device, which was already in our fridge and unexpectedly we began to see very short pulses in addition to the slow thermal pulses that we were accustomed to see. Thinking a little bit about this, it was obvious that we were measuring ionization contrary to what people in the field had told me. So, we began to develop this idea of measuring ionization at the same time as the total energy. This trick was to give what became CDMS an unique sensitivity advantage from the early '90s to roughly 2010, when the xenon technology began to be competitive. The xenon technology uses, basically, a similar idea. They measure ionization and scintillation, which gives them a handle of separating nuclear recoils from electron recoils.
Bernard, when did you first connect with Blas Cabrera Navarro?
Probably '85, '86, something like that. Blas is an amazing physicist, with outstanding creativity, broad knowledge, and excellent laboratory skills. This is what we needed to start this new field of dark matter. Initially it was mostly exchanging ideas and interacting strongly at conferences. I was doing my thing and he was doing his. Then, in '88, Berkeley responded to a call from the National Science Foundation for proposals for a science and technology center. This was for the first generation of such centers. Surprisingly, our cosmology center was approved, and speaking of somebody who did not want to be involved in administration, I was chosen as the PI of the award and the director for the Center for Particle and Astrophysics for its duration from 1989 to 2001. Anyway, at the time of the proposal, I had involved Blas as one of the major contributors to our center research. So, we were really starting to work together very closely in '89, or even '88 when we were writing the proposal. We had agreed on a strategy where each of us was working on complementary aspects: low temperature calorimetry and ionization on my side and athermal phonon detection on his. We became very close, both technically and in terms of friendship. And we still are after some thirty-five years of collaboration! To summarize, the switch for me to cosmology in general occurred in '84, '85, and the specialization in dark matter in '85 with the first meeting at the Rindberg Castle in May '87. So, I had the joy of seeing the field going from five people to probably 2000 people, now, in the world, working directly on attempts to detect dark matter.
Bernard, can you talk a little bit about why the cryogenic approach is so promising for fundamental understanding of dark matter?
From 30,000 feet, you could say the energy depositions are very small. It depends on the mass of the particles, but because of their low velocity, we are speaking of kilo-electron-volts to electron-volts or even below, much smaller than the typical millions of electron-volts that usual particle physics detectors rely on. Moreover, you're looking for very rare events. So, you want to protect yourself against anything, everything which could disturb your measurement, and therefore going to low temperature to get rid of thermal vibrations in the crystal fluctuations. This naturally led to the use of quantum devices such as SQUIDs. In the long run, I think that this field, in particular for the search of very light dark matter particles, will evolve more and more to quantum limited instrumentation. In any case, the detection problem is that you lack sensitivity to detect the event but also redundancy to discriminate against backgrounds because you are close to your energy threshold. If comparing, for instance, liquid xenon and the low temperature detectors, it takes about 10 eV for a single incident electron to extract an electron ion pair and some 20 eV to emit a scintillation photon. If we go to solid state detector working at liquid Nitrogen, the quanta of excitation are ten times smaller, of the order of the electron-volt. And if you go to measure the vibrations inside the crystal with an athermal photon sensor at low temperature. you are now of quanta of the order of a milli electron volt. So, the energy sensitivity requirement for low mass WIMPs pushes you toward these low temperature techniques. Of course, we couple the sensors to the best amplifiers in the world, for instance SQUIDs and more generally quantum limited amplifiers. This is one of the frontiers in the field. The next challenge is to combine very sensitive methods and reasonably large target mass. This is relatively simple for liquid xenon and argon detector as the detector itself is a big vat of liquid. Their energy sensitivity and their discrimination are sufficient for dark matter particles above 5 keV/c2, and they benefit from self-shielding from radioactivity coming from outside. These detectors are therefore very well adapted to the mass region favored by the WIMP hint, hundred times the mass of the proton as I described before. Going to ton scale on the other hand is very difficult for our low temperature technology. So, CDMS decided in 2014 for its new SuperCDMS project, to shift its emphasis from large WIMP masses that we leave to the noble liquid experiments to low WIMP mass., where energy sensitivity and detailed information on events is a premium. In some sense we came back to our sources Initially, we were speaking of very low thresholds. But the WIMP hypothesis pushed us to higher WIMP mass (the natural scale of the supersymmetric particles, one of the main candidates was between a few and a thousand GeV/c2. We therefore did not push hard on the energy sensitivity and emphasized the target mass and dominated the field while Xenon was being developed. But when the noble liquids people showed us that they could explore this WIMP Hint region much more easily, we refocused on what our technology does best, the low mass WIMP region. So, the field has bifurcated: the noble liquid technology, exploring the High Mass WIMP region, and the low temperature detectors and small devices exploring the low mass region. We should clearly do both. Note that this compelling argument which lasted about 30 years, that the dark matter density points to the weak interaction scale, with supersymmetry as the main candidate, what we called the WIMP hint before, may be beginning to break down.: The Large Hadron Collider seems to indicate that at least the simple models of supersymmetry are not correct. Given this situation, while the noble liquids explore with high sensitivity the WIMP hint region, we had to open a new frontier, the low mass dark matter particle: region masses. It is nearly completely unexplored both theoretically (we lost our “lamp post” of the weak interaction scale) and experimentally. We have no idea what is there, and that's where our emphasis for SuperCDMS has been. Our new set up at SNOLAB can accommodate 10-kilogram scale targets. Wille we are building this new set up, our Berkeley group and others are also exploring methods to explore at lower mass than SuperCDMS, preparing for possible upgrades with even more sensitive and discriminating detectors.
Bernard, to be clear, this is a universal search for dark matter -- all kinds of dark matter, not just a particular kind.
It's very generic, but it's not universal. First, we have to assume that this is a particle, but it has to be a particle which somehow interacts with ordinary matter. There is no guarantee. You can imagine that there is a shadow universe, and that's where the dark matter is, and it doesn't interact except gravitationally. It's somewhat unlikely at least in our own naive particle physics expectation: Any particle is expected to interact in the initial soup of other particles at the really high temperature existing after the Big Bang and it would tend to retain part of its interaction strength even now, but that is not impossible that their interactions be negligible. Take for instance cosmological neutrinos which would require enormous detectors. In addition, there are particles such as the axion which have a somewhat different way of interacting. They mostly interact with photons which can be provided by the electromagnetic field in a tunable cavity. Another candidate is a dark photon that could convert itself into a real photon. Specially designed electromagnetic field sensors such as ADMX for axions or Dark Radio for dark photon are the preferred way to go and are currently being implemented. But our technologies of solid-state detectors can be useful if they have good enough energy sensitivity.
Bernard, can you talk a little bit about how advances in the technology behind detectors have made you more and more confident that fundamental discovery in dark matter is increasingly likely?
Clearly, the sensitivity of our detectors and the mass of the detectors used for dark matter searches is increasing very rapidly. Some of my colleagues are keen to show the equivalent of the Moore’s Law for the speed of integrated circuits and computers: If we plot the target mass versus time, we see a fast-exponential growth. But I like to argue that such a plot has to be at least two-dimensional. It is not only the mass which counts, but also the energy sensitivity. You should probably add additional dimensions such are the background levels and the discrimination power. So, mapping experiments capabilities a little more complex that just target mass but we are making fast progress with our detectors on all these fronts. Roughly speaking our field seems to split into two tracks: One is to arrive at a firm conclusion on the canonical WIMPs. As I said before, we do not see anything in our dark matter experiments around a mass of 100 times the mass of the proton and at a rate within a few orders of magnitude of the generic rate predicted by applying Lee and Weinberg argument. Moreover, experiments at the Large Hadron Collider have not seen any sign of Supersymmetry. Therefore, if there is any truth in that general idea, it is more complex that we thought. The liquid xenon and argon experiments are attempting to improve dramatically the sensitivity to detect conventional WIMPs of mass say between 10 and 1000 times the mass of the proton. They do that by increasing their target mass and decreasing their backgrounds. Eventually they will be limited by solar neutrinos. If they see something, we might have to build a more complex form of Supersymmetry. If they don’t see anything at the level of the solar neutrinos, the general WIMP approach is probably quite dead. This is the important task for xenon and argon experiments. A second direction of the field is to explore the low and very low mass region for dark matter particles, which may have very different physical origin. We can look for very low energy deposition from interactions with electrons, nuclei, or even the crystalline structure. This requires sophisticated detectors such as SuperCDMS and the following generation of ultra-low energy threshold detectors and we are rapidly progressing on the energy sensitivity. One nice thing is that at low dark matter mass, you do not need a large target mass. This is because, the mass density of these particles should sum up to the known halo density. The lighter they are, the higher their number will be, and for a given interaction rate, the lower will be the target mass needed. The field is nearly totally open with very few experimental limits and small detectors can bring huge improvements. However, there is little guidance from the theory on what to expect in terms of interaction probability and this is a definite weakness of this exploration. In absence of a general argument, you can dial at will the interaction probabilities between these particles and ordinary matter, you can do whatever you like. The only constraint is to prevent these particles to be produced at accelerators—otherwise we would know about them. In the articles I've been reading, and I should read a few more, but I don't have lots of time, unfortunately, there is nothing very convincing saying: “Okay, you should look at this region, and you should see something, otherwise this approach is wrong”. We were speaking before about this question which was already very present in the mid '70s: Is there physics beyond the standard mode?”. Clearly, dark matter is physics beyond the standard model. But the WIMP hint seems to be in trouble. We have to confirm that at high mass and we are exploring the lower mass region but have very little idea of what we could find. By the way, dark energy is also physics beyond the standard model and we do not know its origin either.
Bernard, on that point, I want to ask, to what extent do you see this research being relevant to those broader ongoing questions in physics, such as the search for dark energy, or searching for ways of integrating gravity into the standard model?
Okay, dark energy, in my mind, has clearly to be related to the quantization of gravity and vacuum energy. It is clearly a sign that we do not understand gravity one way or the other. The quantum vacuum energy is expected to behave very similarly in an expanding universe to the dark energy we observe. But, as you know, in the standard model of particle physics, if you compute the energy of the quantum vacuum, you get astronomical numbers, 10 to the hundred twentieth power greater than the dark energy we observe in the universe. So, it's another failure of the standard model. The universe wouldn't have existed, basically. That's another famous argument of Steven Weinberg. If the dark energy were much bigger than the one that we have, then the universe wouldn't have existed in the form it has. So, in that sense, trying to understand the nature of dark energy is strongly related to understanding the quantum aspects of gravity. This may also require fixing the standard model. By the way, supersymmetry was not fixing the standard model for that. It ended up being wrong by 10^60 instead of 10^120. So, it's progress, but that's not a very good prediction either. Anyway, for me, and for many people involved the various National Academy reports that have been written over the last twenty-five years, these questions of dark matter and dark energy are among the central issues in physical sciences. It is related to complementing the standard model and to incorporating gravity in a way which makes sense. So, to answer your first question, am I optimistic that we will find something? Yes and no. Yes, because new instrumentation always leads to surprises -- I should not say always but often leads to surprises. And for about thirty years, we had a very good guidance from the WIMP hint that we spoke about. So, I was bold enough to write a small opinion article, for Science in 2007, which was titled “Are We at the Brink of a Discovery?” The LHC was coming online. Our technologies for dark matter, we're making rapid progress. The Fermi telescope was on the horizon, and so on. I claimed that the next five years would be decisive! Of course, I was wrong: we have not found anything! However, I still advise young physicists to get involved. In my own experience, seeing what has happened, for instance, in particle physics in the period of 1965-1972, is you can be at an impasse for many long years; Suddenly something happens, and we are unstuck. Everything comes back together, and we make rapid advances till we are stuck again because of new questions we cannot readily answer. So, cosmology might be in a similar situation. Modern precision cosmology has been making tremendous progress, of course, in particular the very precise measurements of the microwave background, the extensive surveys of the large scale of the universe and the discovery of massive black holes and the gravitational waves that they produce when they merge, and so on. But this impressive progress generated in turn lots of questions that we cannot yet answer. We don't know, for instance, why dark matter and dark energy and ordinary matter have similar densities, within a factor of a few from each other. It doesn't make a lot of sense and our current description appears as convenient but unjustified parametrization. Of course, it could be a coincidence. I am somewhat reluctant to go in the direction of anthropic ideas, for instance their “landscape” implementation in string theories. If we don't understand something, it's not necessarily a good idea just to say that's because it just happened in that way. It was just random. You can try to justify such an approach by invoking a selection process —only such universes could have observers like us—., but it's a difficult and maybe misled way of doing science. As you know, there is a general difference of opinion among the cosmologists and particle physicists. Most of the people from my generation and a little older have difficulties with the anthropic landscape. An exception was Steven Weinberg, who unfortunately passed away recently. I asked Steve one day: “If Steve Weinberg doesn't understand something, does this mean that it has to be random?” He laughed.
That's great. Bernard, on the topic of difficult science, I want to ask, you're optimistic, and hopefully rightly so about discovering dark matter. What will it look like when that day comes? In other words, when with LIGO the detection happened, there was a theoretical basis going back 100 years, all the way to Einstein, to know what a gravitational wave is and what it would look like to find it. What are the theoretical foundations that you're confident exist that will allow us to be certain that dark matter has been correctly identified and now understood?
Okay, so, let's think about LIGO. This was, of course, a marvelous discovery. We began to see the beginning of a new form of astronomy. Moreover, these merging objects that we see provide a beautiful strong gravity environment that we will be able to study in detail. So, we learned both something about astronomy and cosmology, and, maybe more importantly, we are learning something about gravity. So far, it seems that the relatively simple model of general relativity works. I would not be surprised if there was something which we discover later on with added sensitivity, which would partially oblige us to tune it, but I have no idea. So, in terms of the dark matter if there is a discovery of unknown particles in our detectors, there are a number of things that we can do to fully test if they constitute the dark matter in universe. Several experiments should be seeing it, it should be different technologies with different biases and backgrounds. Then there are directionality and modulation arguments that can be used to link those particles to the halo of our galaxy. This is what we eventually will need to be sure they are indeed the galactic dark matter. The measurement of the direction of the incoming dark matter particles requires very difficult technology, but the dark matter should come mostly from one direction, because of our movement inside the galaxy. So, these are the internal tests that we can make with our direct detection of dark matter. There are external tests involving the LHC and the gamma ray telescopes and neutrino telescopes, which might bring additional confirmation. Vice versa the LHC or the gamma ray telescopes may give us hints that experiments attempting directly detect dark matter may use. When we are sure that we have discovered dark matter we will try to decipher what are its properties. So, depending on the mass, and the interaction cross sections, there are a number of experiments which can be done. As a third stage, we can transition to dark matter observatories, trying to understand the properties of dark matter velocities and directions inside the halo of our galaxy. For instance, the merging history of our galaxy, which has been absorbing and is still absorbing smaller galaxies, is uncertain in its details: if we can observe spikes in the velocity and direction, we could possibly provide information about this merging history. These are examples, but obviously it's been kind of different sets of experiments, going from general consistency, to performing complementary observations.
So, in sum, it's fair to say you're confident you'll know it when you see it and therefore, you're pretty confident that you haven't seen it already and missed it.
Yeah. So, to give you an example of the debates, it turns out that a number of experiments at low mass have something at low masses, low energy deposition. I am ninety-nine percent sure that is background. But what if, and that's always the question we should ask, what if it was the dark matter?
Our work as instrumentalists or experimentalists is to try to get rid of these backgrounds. I always think of the railway crossing in Europe. There are signs saying, "Beware, one train can hide another one." So, be careful, you might see one train, but there could be one behind it that you don't see. It's actually similar to the job of scientists in this kind of field: get rid of the background that you can see, and then, most likely, events which were buried in the first background will appear; our task is then to convince ourselves that whether they are coming from a new background that we have unraveled or from the real thing. We have already made a few iterations in the process and built several generations of experiments increasing able to suppress or identify the backgrounds that we have identified. But we are not done yet!
Bernard, you mentioned telescopes. I'm curious, there are so many exciting observational projects that are happening right now. Which of those are most compelling and relevant to the questions you're asking?
The microwave background observations are not made by usual optical telescopes but with a kind of specialized telescopes, but clearly these very high accuracy measurements provide a window on the physics in the early universe at a time where we fully understand the physics; we can therefore very reliable information on what happened in the universe in the very early universe (around 300,000 years after the Big Bang, some 13.7 billion years ago). I believe that the detailed understanding of the formation of structure is quite important. In spite of a number of increasingly broad surveys, we have only partial understanding, in part because the physics involved is much more complex. We do not know for instance the role the black holes in the formation of the galaxies (large galaxies have a very massive black hole in their center) and so on. So, the new generation of survey telescopes, in particular the Vera Rubin telescope (LSST) will bring new information, but it's certainly one type of measurements which is quite important. The James Webb Space Telescope will allow us to peek very far towards the formation of first galaxies and very likely will give us surprising results. The forthcoming EUCLID and Nancy Grace Roman (WFIRST) telescopes will improve our quantitative information on dark energy and provide consistency checks between three different measurement approaches. We already know the major thing: the ratio of the pressure to the dark energy density is very close to -1, which really looks to me like vacuum energy. So, yes, we should try to measure more accurately and any statistically significant inconsistency between methods may bring surprises, but my guess is that it will be -1. So, we are looking for another idea to bring new information on dark energy, but I have not heard of any which is compelling. We mentioned black holes already. The study of black holes, either through direct observations in X-rays, or radio, and combining that with LIGO would be quite important. For me, it's a combination of trying to understand the role of these objects in the formation of the structure that you have today, and the formation of the universe, but at the same time trying to understand the physics at strong gravity. As usual, there is always a problem of separating the astrophysics (what is often described by physicists as “gastrophysics”) from the fundamental physics. In any case, there is a lot of action, and I strongly invite young people to join the field. The key is bringing new information from progress in instrumentation. I am convinced myself that we will see revolutions brought about by an instrumentation based on quantum technologies. Starting in this field, it's certainly one thing that a young person should think about: where can advanced quantum technologies have an impact in cosmology.
Bernard, we began our talk with a discussion about your current research interests. I want to ask, for the last part of our discussion, two sort of broad-based future oriented questions. The first is on the policy side. You came up in your career, of course, during an era of public support in science, against the backdrop of the Cold War. Are you optimistic that western society, for the next fifty years, will continue to generously support this science? Is that something that you're confident about? If not, what can be done to ensure this kind of support continues in the way that it needs to for these discoveries to happen?
Okay, this is indeed an important aspect which we can talk about: How to meaningfully involve the public in science, and share our questions, results, enthusiasm and awe in an intelligent and effective way, so that the citizens and taxpayers understand that their money is well spent on things which are relevant to them. Frankly, our track record has not been very good. For the Superconducting SuperCollider (SSC), being built and then cancelled in the early ‘90s, there were many possible causes for its demise. But I'm not sure that the particle physicists did a great job in explaining in simple ways the goals of this huge project. I was not really involved at that time. I was already working on dark matter. However, I was in some other committees of particle physics. There was a certain arrogance displayed, or at least a lack of understanding how to communicate these things, the importance of the Higgs and so on, to the public at large. This is not the only reason, by the way. Another probable reason is the failure of our fields to attract bright young minds to science in a way which is also representative of our society. This was particularly dire in the early 1990s. We've seen with the Black Lives Matter movement an increasing recognition among my colleagues that we have not done enough to support the participation of African Americans in our field. Although it should be easier, after very rapid progress since the ‘70s we have not even been very successful in increasing the number of women physicists beyond a plateau of 20% or so. This is a question I've always been very interested and very engaged in, since I came to Berkeley. It was clearly a major problem to increase the number of women and minorities among faculty at Berkeley. This is a difficult problem. I am ashamed that there are still only very few women leaders in dark matter. Another problem is the pervading anti-science attitude. It might be related in part to the perceived opposition between science and religion. I hope that we will finally win the battle around this kind of silly battle on Evolution, which is only present in The United States, by the way. I have regularly taught a freshman seminar at Berkeley on the Big Bang, where I was trying to introduce young students to the frontier of our science. I always had one or two students —often not with a science intended major— in the audience, who joined because they had questions about their religion. They compare their religion with modern cosmology and are worried about what appears to them as contradictions. My answer was that you have to understand what science says and what your religion says. My basic message was that science is trying to establish facts. Religion is about meaning. There is no fundamental conflict: you have to understand that religion does not speak about facts and that, by the way, science does not speak about meaning. That's where the conflicts arise when there is arrogance or intolerance on one side or the other. So, I hope that we will get beyond that. But the anti-science atmosphere in this country goes beyond the perceived conflict between science and religion. The previous Obama administration has made some progress in trying to inform policies with what science and engineering are telling us on possible options.
Almost, not quite. [Unintelligible]
There is something that we can learn from COVID-19, in that we had better listen to scientists. A major example where science is providing important input to societal choices, is climate change. For example, it's getting clearer and clearer, in the mind of many people, that national security is not just a question of weapons. It's a question of the impact of these destabilizing issues. It's not only cyber warfare, which is a real problem, but climate change will lead to migration of people, pandemics, all these kinds of issues which have to be understood. Society has to make some choices. Walking back to what I was describing, in this report for the socialist party, in the late '70s, we argued that there are fundamental choices which our society have to make and in France, for the civil nuclear power, there has been very little public discussion, just decisions made by technocrats about what is best for France. Every citizen has to be involved, and science and engineering has to provide technical input to those discussions. We, scientist have understood how to better manage this relationship between the scientific input and the public discussions of options. It is essential. I think there is a need. We should both educate the public and educate the scientists to understand these interactions better, strip the scientists from their arrogance and help the often poorly informed public understand the technical aspects in simple terms. The goal to raise the discussion to the fundamental political issues behind the technical choices. We really need also to dissipate the fundamental misunderstanding of what science is about. The American Physical Society and the American Institute of Physics are important actors in this effort. I always wanted to teach a course about science and society, but our teaching load in our department did not allow it. However, I was able to introduce such themes in the Berkeley Connect course that with the help of younger colleagues I launched and taught for several years. The basic idea was to help all Physics undergraduates think beyond their formal academic course. Trying to understand the relationship between what they study at the moment with the key questions in science in general, the connection to science; Trying to understand what they learn at the moment with the skills they will need in their career, and what they do at the moment in the university community and they will have to do in society. This is a very informal course that is still going on. I give you two examples. One is the chance having Ray Weiss visit for a general talk on the campus, just after the first discovery of LIGO. We had lunch with Ray Weiss and sixty undergraduate students. A marvelous experience. Of course, Ray was very good. Another example is a small panel discussion about physics and weapons. I was somewhat careful in organizing that because this was a taboo subject in the department. I think this was around 2018. We had a historian of science, we had people involved in the mitigation of the Fukushima disaster, and people involved in weapons; I had agreed with my colleague, Charlie Schwartz, who tends to dominate and polarize such debates, for him to be in the audience and ask only one question. It's actually very interesting dialogue, including Charlie’s intervention and I wish we had longer. I wish our department could organize more of these types of discussions. The fundamental question is what is the role of a physicist in society. As I said before, nuclear weapons are not the only issue. Even in terms of national security, as I said before, they are only one of the issues, and the more urgent nuclear weapon aspects are less technical than political, with the importance of disarmament and non-proliferation. There are lots of other challenges that we have to face, and society needs scientists to explore potential courses of actions and explain the technical underpinning behind the choices we need to make.
Bernard, for my last question, I want to ask, because you've been involved in understanding dark matter for over three decades, and because you're an optimist that fundamental discovery and understanding will happen sooner than later. My question by definition is in some ways speculative and will ask you to use your imagination. When that day comes, and the fundamental puzzle of dark matter is solved, what questions that we don't even know to ask about the universe today will we be able to ask as a result of this discovery?
I actually already partially answered when I said that it might provide us with a more detailed history of the universe. Our search experiments with morph into the dark matter observatories which will join other precision cosmology observations and allow us to understand better the phases of energy release in the universe history, the formation of our galaxy and the large-scale structure. Direct dark matter observations will be another probe at our disposal, and likely speed up the progress in cosmology. To give an example that I have already touched on, one of the current puzzles is why are we forming massive black holes in the very early in the universe? We don't know. This might very well be related to dark matter. For instance, a good friend of mine, Katie Freese, had this idea that maybe instead of ordinary stars, the early universe was forming huge dark matter stars. This would have profound consequences on our understanding of the structure formation history and maybe this could explain the large number of those massive black holes which puzzle us. And we could test this hypothesis with a number of observations. I don't think the idea at the moment is favored, but it's still a possibility. The other big puzzle for me beyond dark matter is of course, the dark energy. Are there observations we can make which would bring qualitatively new information on its nature. Can we learn anything about it with the strong gravity measurements that gravitational waves observatories are starting to provide? I do not know, of course, and cannot predict the future. But it is likely that increasing the variety of probes we use in cosmology, part of the trend we call “multi-messenger astronomy, will lead to a profound modification of our paradigms, what Thomas Kuhn was calling a scientific revolution.
Bernard, I want to think you so much for spending this time with me. It was a delight learning all about your recollections and insights in the field. This will be a really wonderful addition to our collection for the Niels Bohr library. So, thank you so much.
Yes, okay. I look forward to reading (and likely correcting) the transcript.
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