Blas Cabrera

Zierler:

OK, this is David Zierler, Oral Historian for the American Institute of Physics. It is February 9, 2021. I'm so happy to be here with Professor Blas Cabrera. Blas, it's great to see you. Thank you so much for joining me.

Cabrera:

Nice to meet with you.

Zierler:

So, to start, would you please tell me your title and institutional affiliation?

Cabrera:

Right, so I'm a Professor of Physics at Stanford University. In fact, it is a named professorship, the Stanley Wojcicki Chair Professor of Physics at Stanford University in the Physics Department.

Zierler:

When were you named to the Wojcicki chair?

Cabrera:

It was in 2012.

Zierler:

It must be nice. So many named chairs are from 50, 100 years ago, and Stan is still with us. And he can be a part of what's going on.

Cabrera:

That's right. It's been an honor to carry his name.

Zierler:

I want to ask a question before we develop your family history, and that is a very much in-the-moment question. In the pandemic, how has your research been affected in both ways? Perhaps with the mandate of physical isolation, you've been able to get more work done in terms of data analysis and things like that? And on the other hand, how has the lack of physical, interpersonal connection perhaps stymied the research that you were planning to do over this past year?

Cabrera:

I think I'll go ahead and let you know that I have, over the past several years, been fighting prostate cancer. And so, it's hard to disentangle the impact of that from the pandemic. But I can make some comments because certainly, we have, as of about halfway through 2020, been able to re-access my research laboratory. I had several post-docs and students that I work with that did continue to do research in the laboratory. And we discuss that research over weekly Zoom meetings. Sometimes more often than weekly. For me, the interactions with the research staff are very important. And I have noticed that Zoom does provide a mechanism for communication, but it does lack some of the interpersonal connections. And in particular the brainstorming kind of sessions don't work as well in Zoom as they do in person. The combination of the pandemic and my cancer forced me to significantly wind down my research efforts. A good bit of my current research, in the search for dark matter, is collaborative with the group of Richard Partridge at SLAC, which is a short distance away. And more and more, I've been working with them in an advisory role, and that's what I intend to continue doing in the next few years. I'm still maintaining my active faculty appointment on campus, including teaching. I'm teaching electricity and magnetism this quarter, which is always fun.

Zierler:

Well, Blas, let's take it all the way back to the beginning. And for you, it's such a special case. Perhaps you can tell me if your family situation is truly unique in the field. It's such a fun question. Of course, in particle physics, we have three generations of quarks and leptons. And for physicists, there are many multi-generations of physicists of two generations. In fact, two are your colleagues at Stanford. We have Steve Kivelson, whose mother, Margaret, is a prominent physicist, and we have Daniel Fisher, whose father Michael is a prominent physicist.

Cabrera:

And we also have David Gordon-Goldhaber.

Zierler:

Oh, that's right, yes! But for you, I'm wracking my brain, do you know of any other examples of three generations of physicists in one family?

Cabrera:

I do not, but I also have not carefully studied that question.

Zierler:

Well, I'm a historian of physics. I can look into this more closely. But minimally, it is quite unique, and perhaps singular. Let's go back all the way to the beginning. Where are your parents from?

Cabrera:

Both of my parents were born in Madrid. On my father's side, I have the same name as my grandfather, but it is not the same name in Spain. There his name was Blas Cabrera Felipe, and my name in Spain is Blas Cabrera Navarro.

Zierler:

And Navarro is what? What does that accord to?

Cabrera:

That's my mother's first last name. In fact, I've always thought the Spanish system was rather well designed in that women don't lose their names when they marry. In fact, officially, my mother's name continued to be Carmen Navarro Clavero, where the Clavero was her mother's first last name. And de Cabrera was added to the full name to recognize a marriage. Often, it was abbreviated where you have Navarro de Cabrera, and you leave out the second of the last names. They were both born in Madrid. They didn't know each other in Madrid. My father's family was mostly from the Canary Islands. In fact, at one point our relatives traced back Cabreras to about 1600, and on the island of Lanzarote, one of the islands of the Canary Islands, half of all of the last names are Cabrera. So that is a very long history there. Apparently each generation sent the eldest son to Madrid, typically to study law, and then they returned to the Canary Islands to head the family business. Most of the Canary Islands was involved in transport and shipping. For centuries, sailboats needed to head south to the Canary Islands to catch the prevailing easterlies to the new world. In fact, Christopher Columbus took off from the Canary Islands on his initial voyage to the new world. They were initially rediscovered around 1400, I think 1401. And then, initially there were trade routes along the African coast before the discovery of the Americas increased travel and commerce quite a bit. My grandfather went to Madrid to study law around 1900, but instead he got interested in science. I think the family then sent the second eldest son to Madrid to study law. My grandfather was very famous, as you are probably aware, including attending the Solvay Conferences of 1930 and 1933. That has always been a source of great pride for me. He passed away the year before I was born, so I didn't have direct contact with him. He had a relatively difficult time with the Spanish Civil War. Before the civil war broke out, he was president of the University of Madrid. Towards the end of the civil war in 1938, he initially went to France, and then when it was clear he was not going to be welcomed back to Spain, he immigrated to Mexico and spent the last four years of his life there. He arrived in 1941 and died in 1945.

Zierler:

And what were your parents' experiences during World War II?

Cabrera:

My father and mother both left Madrid towards the end of the Civil War. Roughly 250,000 Spaniards went to France initially. And then, about 50,000 went on to Mexico. Mexico was the most welcoming country in the Americas. So, towards the end of 1938, the end of the Spanish Civil War, both of my parents independently immigrated to Paris, France. My mother's side is a bit more complicated because my grandfather on my mother's side was an admiral in the Spanish Navy. He was stationed in the Spanish Embassy in London when the Civil War broke out. My mother was born in 1919, so in '38, she would have been 19. She had an older brother, my uncle Fernando, and a younger brother, Jose. There was a big argument in the family between my grandfather and grandmother about the youngest, who wanted to enlist in the Spanish Army and go to Spain to fight in the Civil War. My grandfather said he could not prevent him from doing that. So, he went to join the army and one month in he was killed in an accident with an anti-aircraft artillery shell. I think my grandfather never got over that loss and he died relatively young at about 54.

Zierler:

So, you didn't know your grandfather at all.

Cabrera:

I did not know either of my grandfathers. They both passed away before I was born. As for grandmothers, my mother's mother lived with us, so I knew her very well. Both of my parents independently ended up in Paris. They came to know each other through the Spanish community in Paris prior to World War II breaking out and they were married during World War II in 1942. Both lived and survived World War II in Paris. As I have related, my grandfather immigrated to Mexico in 1941. At that time that required going back through Spain because the only ships that were leaving in '41 were from Lisbon. Fortunately, my grandfather had friends in high places and he was allowed to make that journey through the corner of Spain into Portugal in '41. And then, he went on to Mexico City. My father is the youngest of three sons. The oldest for many generations was named Blas, so that's where my name comes from. My uncle Blas was actually in the cabinet of Negrin, who was the last President of the Republic, so he actually left Spain with the government in exile and went to Mexico with the government in exile. He then spent the remaining 42 years or so of his life in Mexico City. The three brothers did actually get together in 1978. There was a reunion in the Canary Islands honoring the 100 years since my grandfather’s birth after the death of Franco. My uncle refused to return to Spain while Franco was alive. And so, in '78, for the first time in 40 years, he met with my father and my other uncle Luis, who was the middle brother, in the Canary Islands. I had an opportunity to go to that conference and meet many of the family which I'd never met before. Initially, we went to Madrid and met a number of family members, and then there was a five-day affair on the island of Tenerife, the island of Lanzarote, and I think one of the other islands. So, it was quite an interesting trip, and I met a number of the dignitaries of the time. And I learned quite a bit about my grandfather from that as well from the talks that were given. Rockefeller had donated funds to build an institute for my grandfather's research in Madrid, and that building remains to this day. But it went into disuse after the Civil War. So, my parents ended up in Paris, they met through the Spanish community, married in '42, and I was born in '46 in Paris. My father, through connections of his father, worked at the International Bureau of Weights and Standards, which did maintain a presence even during the war. And that allowed continued research efforts, although they were attenuated. And my father, who was a solid-state physicist in crystal growth and oxide growth, and due to work he did at BIPM was invited by Neville Mott in Bristol to do research there. And at Bristol, he made the professional connections which ended up with him going in 1951 to join the faculty at the University of Virginia in the US.

Zierler:

Blas, before I turn the same question to you, do you have a sense if your father was inspired to become a physicist by your grandfather? Was it known whether your grandfather involved your father in his career, and your dad grew up knowing what it meant to be a physicist, which was his inspiration?

Cabrera:

Well, I don't remember talking in great detail to my father about that. But I do know that my father always said that he first wanted to be a historian. Always loved history and read many history books throughout his life. And in his university studies, he got interested in physics. The second oldest, Luis, who is the middle brother of my father's generation, started wanting to be a physicist and ended up as an architect in the Canary Islands. And by the way, depending on where they ended up, the eldest son, because of his connections with the government, ended up in exile in Mexico. The middle one continued in the Canary Islands as an architect. And then, my father ended up in Bristol, and then Paris, and then back to the US.

Zierler:

How long were you in Paris after your birth?

Cabrera:

We came to the US when I was 5. So, I remember the trip in a ship, the Liberte. And I definitely remember the Statue of Liberty going into New York. We ended up in Charlottesville, Virginia. And I think I’m said to have come back from the first day of playing with the kids saying, “It’s strange, but people here don’t know how to talk. They don’t understand.”

Zierler:

Did you pick up French at all? Or it was exclusively Spanish?

Cabrera:

I went to kindergarten in France, so I knew French. We always spoke Spanish at home, so I know Spanish from that. When we arrived in the US, the story that I learned from the family was that it took about three weeks from me to go from speaking French to speaking English, and that I had, during those three weeks, mixed sentences. I pretty much forgot French at that point. My parents used to use French as the secret language. But we always did speak Spanish at home, so I remained bilingual to some extent from that.

Zierler:

Blas, was your sense that, aside from professional opportunities that drew your family to the United States, was there any specific desire to avoid going back to Spain, either politically or because of emotional wounds? Was that something specific to be avoided?

Cabrera:

I think the science became very politicized in Spain, and so the quality of what was going on had diminished significantly. My father did return to Spain in 1970. This was while Franco was still alive. And in fact, he spent 15 years in the University of Virginia, and then he had an opportunity to go to the University of Mexico in Mexico City. So, he went there. But within a year or year and a half, he was invited to return to Spain. And so, going first to Mexico and then to Spain he clearly had the desire to help promote science in Spanish-speaking countries, including Spain. And there was a new minister of education under Franco in 1969, I believe, who decided that in order to improve the very conservative academic system in Spain, he would create two new universities modeled under the more US style of promotions and organization. And so, he created the Autonomous University of Madrid and the Autonomous University of Barcelona. My father was invited to form the Physics Department in the Autonomous University of Madrid. And so, he returned to Spain in 1970. Things went very well for about three years. But then, this progressive minister of education was deposed. The replacement was well-meaning, but the powerful Academic Council of the country forced the two new universities into their fold. All of the contracts were abolished, and roughly half of the department left Spain. I remember my father saying that he thought carefully about leaving, but he thought he could do more if he stayed. So, he did stay. It was a very difficult time initially, but with the death of Franco and formation of a more democratic society, things slowly improved, and connections with the rest of Europe improved as well.

Zierler:

How long were you in Charlottesville? What ages were you there?

Cabrera:

So, we went there in '51. And then I attended the University of Virginia for my college studies, and then I went to Stanford as a graduate student in physics.

Zierler:

Oh, so really, your whole childhood and undergraduate was in Virginia.

Cabrera:

That's right all of my schooling through college was in Virginia. I stayed in the US when I went to Stanford. At about that time, the rest of my family, my two younger sisters, mother, and father, went to Mexico and then to Spain. And so, the family has lived back in Spain since that time. Actually, both of my sisters are now in Madrid again, although one of them had been in Paris for much of her life.

Zierler:

What was the citizenship question like for your family?

Cabrera:

As soon as it was possible, which was after five years, my parents become US citizens. They had an incentive for doing that because my grandmother lived with us in Paris, and she was unable to immigrate to the US until they became citizens. So, after five years, they became citizens, and then my grandmother was allowed to come join us in Charlottesville. In 1962, my father took a sabbatical in Caracas, Venezuela. He took the family with him, my sisters and myself included. In preparation for that trip, the children were given derivative citizenship since my parents had become citizens. And that was needed to obtain our passports for traveling to Venezuela. In 1963, we also took a trip to Spain, which was my first recollection of Spain. I have been told that when I was 2 my mother returned to Spain for a brief trip to visit relatives when they were living in Paris after World War II. I was about 17, and all of Madrid had turrets on the intersections with machine guns. And the equipment was all German. So, I hadn’t realized that the Spanish Army had been supported heavily by Hitler. And then, there had been a falling out because Franco had refused to aid Hitler in his invasion of Britain. There’s a famous boxcar meeting on the border of Spain and France, where Hitler was apparently very unhappy, but Franco correctly judged that Hitler was not interested in opening another front. So, he, Franco, agreed to be neutral in World War II. Kept all of the German equipment that he had. And in '63, all of the billboards said 25 years of peace. And I remember my parents' indignation that, "Well, yes, but he caused the Civil War." It was only 25 years of peace, right?

Zierler:

Blas, I'm curious, for your identity growing up, given your family's experiences during the Civil War and then World War II, and how young you were coming to the United States, even though you spoke Spanish at home, did you feel like an American? Did you feel like an immigrant? Were these complex issues for you?

Cabrera:

Not particularly. In general, I did have more of a global view than the average person. Charlottesville was a more progressive university town. But when you went not very far outside of Charlottesville, it was definitely the South, both in terms of the accents and people's much more local sense of themselves. So, I think it helped to grow up in a university town. Turning now to the question of when I became interested in science, and physics in particular, I remember watching the gatherings in our home, which included many physicists. And I remember from a very young age, before I really understood what physics was about, that there was tremendous excitement in the discussions that people had about what they were doing. They talked about what they were doing with great glee and excitement. And even as a young child, I noticed when I went to friends' houses, who were, say, doctors or lawyers, they hardly ever talked about what they did. And that struck me from a young age. Really, I think the most important influence for me was a close family friend, John Mitchell, who was a chemical physicist. He was actually the reason my father had been invited to come to Charlottesville. And we spent hours and hours when I was young where I would ask questions about how things worked, and he would patiently answer all of the questions. And he also showed me how to solder circuits, and he brought chemistry sets and equipment to our house. So, I think that connection and the excitement having to do with the things that he showed and taught me were very influential in deciding at fairly young age, maybe 10 or 11, that physics seemed like a really neat thing to be striving for.

Zierler:

And did you have that sense that, with your grandfather and your father, this was really a family tradition by the time it got to you?

Cabrera:

Probably there was some of that as well, but I think more, it was the excitement of studying physics rather than the tradition. By the way, with my wife of nearly 50 years we have three sons of which we are very proud. One is an engineer at Google, in the second is a Sociology Professor at Laverne, and the third studied environmental biology in college and is politically active in progressive movements. So, I certainly do not feel badly about the tradition not materializing into the fourth generation. They did all study some physics, but physics has changed quite a bit over the last 100 years.

Zierler:

And you went to Charlottesville public schools throughout?

Cabrera:

I did attend first through third grade in public school, and then transferred to parochial school for grades four through eight. But let's step back. I went to third grade in public school, and I think my parents decided to put us in parochial school as tensions were beginning to increase over the integration issues. The year I was in the eighth grade, the public schools were closed, and there was much home schooling. The public schools reopened the following year and I went into grade nine in public high school. And so, I was in the first class in the high school that was integrated. I think there were eight Blacks. And I do distinctly remember the first day in biology class, there were these old-fashioned granite tables with two students each for dissecting and various things. And I came in and sat down next to one of the Black students. And I was surprised and taken aback after class when the teacher took me aside and said, "Is that going to be OK with you?" So, I didn't think that was a question that needed to be asked, but it definitely indicated the time.

Zierler:

Blas, this is to say though that your childhood was very much environmentally a segregated society.

Cabrera:

That's right. There were two high schools in Charlottesville. One was the white one, one was the Black one. And even after the integration, relatively small numbers of Blacks were in the white school.

Zierler:

And for you and your family, were you perceived as white? You have a Hispanic name. Obviously, your family's from Europe. But I'm curious how your family may have been perceived.

Cabrera:

Yeah, I'd say that I didn't really sense any negative aspects. I do remember giving someone my name, Blas Cabrera, over the phone and response repeating my name was Oscar Brown! So, our Spanish name was strange for most. We had friendly interactions with the neighbors. I do remember our home often attracted Spanish-speaking people from all over.

Zierler:

Which in those days was probably not that many.

Cabrera:

Right, but in addition to many academic visitors in the Physics Department, it included, for example, a worker from Puerto Rico who helped us with the gardening in the backyard. My mother did feel isolated in the US. And so, Spanish-speaking peoples from South America, from Puerto Rico, from Mexico tended to congregate at our house in get-togethers. And I found that very interesting.

Zierler:

I assume when it was time for you to start thinking about college, the tuition and the proximity was just too good to pass up.

Cabrera:

That was one reason, and then I applied to Cornell, Harvard but I was not admitted. And so, the University of Virginia was, I thought, a pretty good place to go. And definitely, it did seem to me that the very low economic strain on the family was something that I found a positive as well.

Zierler:

Was the plan for you to be a physics major from the beginning?

Cabrera:

Yes, I pretty much knew I wanted to be a physics major, and that was reinforced. One of the introductory classes I took, my father taught, which was an interesting dynamic because his accent in English was never great, and so I often had to interpret for some of the students. But I didn't get too much backlash from that. And, of course, the grading was all done by TAs. I do remember that distinctly. For me, the most influential member of the Physics Department was Bascom Deaver. He was an experimentalist studying superconductivity and I got really excited by that research. Bill Fairbank from Stanford, who had been Bascom's thesis advisor, they had together discovered flux quantization, which I always did think should have won the Nobel Prize. But that's one of those things that are always hit and miss. And having the four people with Doll and Nabauer from the German group as well was, part of the issue since the limit was typically three for the Nobel Prize. Bill Fairbank came to Charlottesville and gave several colloquia, and I was really excited by his many research areas, including detecting gravitational waves, measuring the motional drag of General Relativity in an Earth orbiting satellite, and many other exciting experiments. Do positrons fall up or down? Is gravity symmetric on matter versus antimatter? All these things which are fairly fundamental got me interested in going to Stanford and studying under Fairbank. So, after my undergraduate studies, I applied to Stanford and was admitted to graduate school at Stanford, and was able to join Bill Fairbank’s group. What struck me the most when I got there is that all these things that Bill had talked about, many which I thought were done deals, were things that were being worked on actively. So, there was still lots to do. He had about 25 graduate students when I arrived, and more than half had independent projects. There was a huge amount of diverse research going on. In the large group. There was one assistant professor, Bill Hamilton, who went on to LSU and set up the Louisiana LIGO system to detect gravity waves. I worked closely with him. Bill Fairbank was around at times, but he had so many things going on that the amount of time he spent with any one student was limited. But always high-energy excitement everywhere.

Zierler:

Blas, given the leg up that you had as a child of a physicist and how much exposure you had to the physics department, I wonder if you left the University of Virginia with your identity as a physicist fairly well-formed, at least insofar as you knew you would focus either in theory or experimentation.

Cabrera:

Yeah. I knew from a young age that experiment was what I wanted to do. My father straddled theory and experiment. He didn’t do experiments, but he often helped to analyzed experiments using theoretical techniques. So, he was a theorist but working with experimentalists. And I knew from an early age that I loved working with my hands. And I remember in high school repeating the Millikan Oil Drop Experiment, which was a fascinating thing to do. And so, I knew I wanted to be an experimentalist, and that's why Bascom Deaver, who was very much an experimentalist, and then Bill Fairbank, very much an experimentalist, were my role models. Also, the connections with John Mitchell when I was growing up had a lot of experimental aspects to them. How things work, and how you get things to work for you in the experimental world.

Zierler:

Blas, on the social side of things, in Charlottesville, by the time you graduated in 1968, had the 60s, so to speak, reached Charlottesville? Was the anti-war movement significant? Was the Civil Rights Movement a big deal there?

Cabrera:

Well, I remember the democratic conventions and the riots. For me, when I got to graduate school, I met a number of people, including Vietnam War veterans. And I do remember at one point thinking, "Is it really fair to be working in research when so many social problems are occurring in the country?" And in the end, I decided yes, it's a good thing for me to be doing.

Zierler:

Was the draft something that you had to contend with?

Cabrera:

I did, yes. In 1969 roughly half of the physics graduate students at Stanford had been able to obtain deferments from their draft boards. And that was true for me. Charlottesville was where my draft board was located. Before leaving for Stanford, I had been asked to register for the draft, to decide whether or not I was draft-able. They do a blood test. I remember the drill sergeant, I believe, saying, "If you go to college, you cannot fail this written test, so don't try." And then, "Pee in this bottle," and one of the potential draftees saying, "From here?" So, I remember that I was draft-able. The letter from the draft board prevented me from being drafted, and roughly half of the graduate students ended up not getting those letters, depending on the whims of their local draft boards. And so, most of them chose to enlist, which gave them more choice of what they were going to do than those who were drafted. But I did think that was rather unfair that the different boards behaved so differently. There was then a time, that I think was '72, when the rules were changed, and there was one year where everybody was eligible for being drafted independent of what your draft board had said in the past. And your birthday was assigned to a random number from 1 to 366. And mine just happened to be in the 300s, which made it relatively improbable that I get drafted, in fact I did not get drafted.

Zierler:

When you were thinking about graduate programs, was working with Fairbank the main goal? Did you bother applying elsewhere?

Cabrera:

I may have applied one or two other places, but really, working with Bill Fairbank on the exciting projects that I'd heard him talk about was my goal, yes.

Zierler:

Now, did you know Hamilton before you got to Stanford or at least know of him?

Cabrera:

I did not. I knew him after arriving to Stanford. In fact, the dynamics of Bill's group, I didn't know very much about.

Zierler:

Now, Fairbank and Hamilton had separate groups? Or they co-led one group?

Cabrera:

Pretty much, that was the assistant professor that was connected to Bill's group. It's, of course, an independent assistant professor position, which at that time, very improbable, maybe 10% of assistant professors were promoted to tenure. This was the time when, for example, Shelly Glashow was not promoted to tenure at Stanford. So not all decisions were wise. When I became assistant professor in 1981, I remember being told by a senior faculty in the Department that there was only a 10% chance for tenure. It didn't matter to me because the research was exciting. And generally, those that left Stanford without tenure, ended up in reasonably good places.

Zierler:

Another social question. For Palo Alto, when you got there, the rift between the physics faculty and the SLAC faculty was significant. Did you detect that at all from your vantage point?

Cabrera:

Absolutely. It was amazing to me, as a graduate student, how little contact we had with Panofsky, and Drell, and these famous physicists, Richter and so forth. Hofstadter did make connections with SLAC, particularly the crystal ball detector. Dave Ritson and Stan Wojcicki were faculty members who worked at SLAC and on campus. There were a few who were making connections, but there was definitely a high barrier. Felix Bloch in particular was pretty adamant about his unhappiness with the way SLAC had evolved.

Zierler:

What was Fairbank's group doing when you joined?

Cabrera:

As I said, they literally had 15 or 16 different projects. So, Francis Everitt was there with the relativity gyroscope experiment. In fact, I first began to work on ways to make ultra-low magnetic fields using superconductors, and the impetus for that was the Gyro Relativity Experiment needed to have very low magnetic fields. And so, I didn't directly work with Francis, but there was this connection. And then, after I got my degree, I was a post-doc on GP-B for five years and then became an assistant professor after that. So, each time, the chances of going elsewhere were fairly high. So, I looked at options but ended up deciding that that was an opportunity I didn't want to pass up.

Zierler:

Now, Fairbank and Hamilton were essentially coequal advisors for your thesis research? Or what was the arrangement exactly?

Cabrera:

I think Bill Fairbank was the thesis advisor. And I worked with Bill Hamilton fairly closely on a number of these things. He then went on to LSU, and Jim Opfer was the one who replaced him. I worked less closely with him. But each assistant professor would be involved with some of the experiments. About 1983 there was quite a conference in honor of Bill Fairbank’s 65 birthday called Near Zero that resulted in a book that Jane Fairbank edited over many years. Those proceedings contain a remarkable collection of all the experiments going on in his group. So, there was the production of low magnetic field that related to Gravity Probe B, and there was the question of whether antimatter falls up or down, which was being addressed. The gravity wave detectors, resonant bar detectors were being developed. There were biomedical experiments going on, so magneto-cardiology using SQUIDs was going on. There was the quark experiment, can you find fractional charge? And, of course, all of the interesting possible experiments that occurred in that period. There was a post-doc student, George Hess, who was setting up to measure Planck’s constant divided by the electron mass using a rotating superconducting ring experiment. And I basically went into his lab at Stanford, and he joined the faculty at the University of Virginia shortly thereafter. So, it was kind of a swap. But I became interested in that experiment. After completing my Ph.D., as a post-doc, I worked with GP-B, and then I came back to the h/m experiment when I became an assistant professor. And it was the first experiment that I worked on as a faculty member setting up my group. You rotate a superconducting ring, and the London moment, which is the readout of the GP-B experiment, you measure in units of the flux quantum in the ring. Experimentally you determine what rotation frequency equals to an integer number of flux quanta ends up giving you h/m, where m is the mass of the Cooper pair. We were able to do that sufficiently accurately to look at relativistic corrections to the Cooper pair mass.

Zierler:

As a graduate student, as you're developing your professional identity, this research hits on so many major subfields. There's astrophysics, there's solid state, there's particle physics. What would you have called yourself? Or how might you have envisioned a faculty position for you as you were developing your professional identity?

Cabrera:

That's a very good question because, for example, Bill, in the field of superconductivity, the flux quantization experiment dramatically demonstrates Cooper pairing because the flux quantum is hc/2e instead of hc/e. But I think the community that was studying superconductivity in detail pretty much left him out of the history over time. His presence didn't continue there sufficiently compared to Tinkham and many other famous faculty. So, that was an example where I noticed that Bill was being spread so thin was not necessarily a good thing. So, I also thought in an early time that working on a few experiments rather than many was the right direction. Making more progress on a few experiments.

Zierler:

To the extent that you were thinking big with regard to your thesis research, what were some of the broader questions in the field at that time, and how was your research responsive to those larger questions?

Cabrera:

In my PhD studies?

Zierler:

Yeah.

Cabrera:

The aspect of developing the low magnetic fields was directly connected with being able to do the gyro experiment. So that was the natural research to join as a post-doc and basically, take what I had learned for how to make low fields into the application that eventually became the lead balloon flying in the satellite. And then, if you look at my thesis, I think it's titled something like, "What you can do with low magnetic fields." How you get them, and what you can do with them. And several of the chapters are measurements searching for magnetically charged particles. In the low fields, basically we passed samples through a superconducting loop with sufficient sensitivity to easily see a Dirac magnetic charge with a single pass. And so, I remember looking around after having done those experiments and written several papers. Of course, the Alvarez Group had this enormous body of work where they had taken samples from the bottom of the oceans, and moon rocks, and so forth. And the sensitivity they had was quite a bit less than mine, 1,000 times less sensitive. But the thing about magnetic charges is that if you put them on a train and run them through a loop many times, the signal keeps getting bigger proportional to the number of times. So, I couldn't think of a reason why what I was doing would have an advantage until '79 when there was this wonderful talk given by Shelly Glashow at the Grand Unification Theory Conference. I didn't go to the conference, but I heard about this talk and read the proceedings. He suggested in that talk that dark matter may be made up of magnetically charged particles that come from the 't Hooft - Polyakov Monopoles that must exist if unification is the true theory of nature. Very powerful concepts. And 't Hooft and Polyakov, unlike Dirac, were able to calculate the mass on the particles. And they're just incredibly massive at 1016 times the mass of a proton, which is the mass of a paramecium instead of a normal elementary particle. So, the idea that, OK, maybe that makes up the dark matter, and just very few of these massive particles would be sufficient. And then, suddenly, it became clear to me that such a massive particle wouldn't ever be at rest in matter. So, all of the Alvarez experiments wouldn't have seen it because they would be pulled to the center of the Earth. The force of gravity's too big to keep it confined to material. So that the ability to have a detector which could detect a Dirac magnetic charge on a single pass meant that if we had these particles passing through the Earth and through us all the time, we could see them with these SQUID measurements. So that was the exciting part that got me started on the monopole experiment in 1979. And the famous St. Valentine's Day event occurred in '82.

Zierler:

Blas, to stay on your thesis research for a little bit, of course, during your dissertation years, there are major advances happening in the world of theory as well. I'm curious if any of those advances were particularly relevant for your research.

Cabrera:

Well, I was very interested in the unification directions. What was their significance relative to fractional charges? And then, also, the possibility of magnetic charges. Then, with 't Hooft-Polyakov, the very high likelihood of them existing. And '82 was very interesting because it was also the year that Guth, who was a post-doc at SLAC, developed inflation to solve the monopole problem directly. He said, "We don't want to lose this concept of unification which forces the existence of magnetic charges. We don't see them. How can we construct something that prevents us from seeing them?" And then, he had great success with inflation because it solves a number of other problems than just the monopole problem.

Zierler:

Did you meet Guth when he was at SLAC?

Cabrera:

I did not. No, it was only later that I learned that that was the case. And, of course, Andrei Linde came to Stanford later. And I think I did meet Andrei at a conference in Leipzig in 1982 before the Berlin Wall came down. So, he was talking about inflation at that point, too, from his point of view. And then, he and Renata were invited to come to Stanford.

Zierler:

Another subfield question for you. During this time, did you feel like your work was relevant for the burgeoning field of cosmology as well?

Cabrera:

Well, the Shelly Glashow talk about dark matter maybe being magnetically charged particles, that really set the rest of my career going because rather than the work that I had focused on of solid-state questions with the relativistic mass of the Cooper pair, I definitely turned my attention to, what is dark matter? And the concept that it might be magnetic charges was an opening that seemed like the right way to go. That's what I did. And then, as time went on, and it became less likely that magnetic charges formed dark matter. So, looking around at what other possible candidates there were lead to WIMP searches. What we always said about theory is, "Let's try to find theories which have staying power." If things keep changing around, then as an experimentalist, you're never going to catch up. But if you have something which seems to be fairly fundamental and stays around for a bit of time, that's a good thing. That's the impedance mismatch between theory and experiment that you always have to overcome. So, everything for the rest of my career was focused more on the dark matter question. I did finish up a set of measurements with the rotating ring experiments. And then we tackled the dark matter problem with silicon and germanium detectors, and then collaborating with the Berkeley Group. Around that time, we held quarterly informal conferences called BALTIC. This was the Bay Area Low Temperature Informal Conference, a loose-knit group of Berkeley, Stanford, Santa Clara, San Francisco State, and Livermore, they were all working on low temperature detectors, some for dark matter searches and others for high resolution x-ray spectroscopy. And there were exciting developments that occurred, and more and more sensitive detectors were developed. And our application was dark matter, but we did then collaborate with Berkeley who had similar interest in the dark matter question. And Livermore and other institutions were looking at x-ray spectrometers that could be made with these kinds of detectors. So again, it was the technology that spanned across different communities and different interests, which was the theme there.

Zierler:

I'll test your memory, who was on your thesis committee?

Cabrera:

I don't remember at this time. So, you probably looked it up, right? [Looking it up the title of my thesis was “The Use of Superconducting Shields for Generating Ultra-Low Magnetic Field Regions and Several Related Experiments,” and the committee included William Fairbank (Principal Advisor), CWF Everitt, Alexander Fetter, and Robin Giffard.]

Zierler:

Well, obviously Fairbank and Hamilton. Who would've been the third? Any recollection?

Cabrera:

Actually, Hamilton had left by then and was not on my reading committee. Often there was a theorist involved, too. The third was a theorist, Sandy Fetter and the other three experimentalists.

Zierler:

Was it a good experience, your oral defense?

Cabrera:

Yeah. I remember the back and forth with the questioning, I thought, went fairly well. And it's also one of these things where family members who could attend were there. I think one of my sisters was there. And so, that's always a good thing. I did talk about the measuring of the magnetic charges. And what you saw with samples, where any kind of dipole magnetic moment would give you an upward squiggle and then a downward squiggle, but it returns to the same line once it passes through, indicating no magnetic monopole charge. So, that was fun.

Zierler:

What kind of advice did you get? Or where was the most sensible or exciting place for you to go for your post-doctoral research?

Cabrera:

I was looking at GP-B because of what I thought was an interesting experiment to directly probe General Relativity in Earth orbit, and the application of the low magnetic fields, 10-8 Gauss levels that I had developed, that was a natural thing. I also was very excited about the possibility of going to IBM Yorktown Heights, and that was the second. They had research on superconducting computers and many other applications of low temperature physics. There was quite a bit of fundamental research going on there as well in superconductivity. So that's the other place that I thought was exciting. So, I didn’t think so much about going to faculty positions elsewhere. The national lab and the ability to really push the research area and the industrial lab in the case of IBM Yorktown Heights was one of the things that I was very interested in doing.

Zierler:

So, what was the ultimate decision?

Cabrera:

When the opportunity arose at Stanford, I did think through it. But the fact that I had quite a bit of equipment that had been developed and the opportunity to work for five or six years, even if I didn't get tenure. Tenure was assumed to be the less probable outcome at the time. And also, startup funds, I think I ended up having to push for $100,000 of startup funds at the time. But before me, I think there had been zero startup funds. First of all, the agencies tended to support incoming faculty starting the first year. They never do that now. Now, you have to essentially go through your startup funds before agencies end up supporting you. And that was interesting as well. I did have NSF funding for the superconducting experiments I was doing with the Cooper pair. And then, as the monopole came around, I got initially a fairly small amount of funding from the DOE. And then, that grew into my main support. And the NSF, over a period of time, wandered downward as I was doing more on the dark matter side.

Zierler:

Intellectually, did you see your post-doctoral research as more a continuation of your thesis research or an opportunity to take on new pursuits?

Cabrera:

I think a little of each. So, the technology was very directly connected to what I'd done before, but the general relativistic questions that were being asked, I found interesting as well. And so, that was the direction to go. With DOE, it was definitely high-energy physics and the theme of what makes up the dark matter? If the dark matter is a new particle, then definitely particle physics is interested in how it fits into the structure. We didn't have a good candidate in the particles we knew existed, so almost all the candidates were particles that may exist but hadn't been shown to exist yet.

Zierler:

Where was Shelly's talk that had this formative influence on you?

Cabrera:

I was not at the conference, and so I don't know exactly. I know there was one in North Carolina, but I believe that was the next one that occurred. And I did attend that one.

Zierler:

And you would've read this in Phys Rev Letters, that kind of thing?

Cabrera:

I think it was word of mouth, actually, between graduate students, and post-docs, and some SLAC connections, and so forth that talked about this. And then, I think I did go and read the proceedings. But typically, they don't come out for a while. And then, this was before you could go and download the slides off the web. But it definitely struck me. And before this talk, I had heard about 't Hooft and Polyakov developments, which were in '76. And that had struck me. And then, the idea of them being dark matter really came out of this talk of Shelly Glashow. So suddenly, there was a reason for attacking experimentally measurements which hadn't been there before.

Zierler:

I don't want to get you depressed, but I'm curious in those early years that the sense was the mystery of dark matter would be solved in the near term, that it wouldn't be this 40-plus-year ongoing endeavor.

Cabrera:

Yeah. What I tell people now is that I really no longer care about who it is that discovers dark matter, but I would like to know what it is before I pass away. With inflation theories, monopoles were somewhat of a long shot because you would've had to have had certain things be exactly right, like the reheating temperature, etc. On the other hand, for WIMPs, it was impressive how the density naturally came out of arguments about the weak scale annihilation. And that really is the reason so many experiments were performed across the world. And so, I thought that was a pretty good chance of success in terms of finding a candidate that made sense. So, it’s been a little disappointing. I think it was also in ’82 that I attended a Conference in Trieste. It was after the monopole because I think I gave the talk about that experiment at Trieste. And Dirac was at the conference. I do remember that vividly. I asked him two things. One was, did he remember my grandfather? Because he’s in the picture in the Solvay Conference. And Dirac very matter-of-factly said, “Yes, there was a Spaniard there.” And that was it, nothing else. And then, I asked him about magnetic monopoles and whether he still felt they were likely. He wrote beautiful papers back in ‘28 and ‘31 about how the existence of a magnetic charge forces the quantization of both electric and magnetic charges if quantum mechanics is to remain consistent. And he kind of looked up and said, “Well, it appears that nature hasn’t taken advantage of this beautiful theory.” So, he was late enough in his life where I think he was feeling the frustration in something which also looked fairly likely early on having not materialized. Of course, 't Hooft and Polyakov, in retrospect, it's not surprising that the mass of these particles should be much bigger because the energy associated with the quantization of magnetic charge is some 5,000 times greater than the energy associated with the quantization of electric charge because the fine-structure constant basically gives you the ratio between the two. That means, then, likely that anything that's a quantum of magnetic charge is going to be a massive energetic particle. Then, 't Hooft and Polyakov gave theoretical meat to that, and the natural energy being one over the fine-structure constant times the unification scale. So, if Grand Unification is the 1014 GeV, then the mass of the particle is 1016 GeV, which is only a factor of 1,000 from the Planck mass. So, this is a totally different regime. But looking at the universe if you start from infinite energy, all particles have to have formed at some point. And so, you have to have, through arguments of causality, some number of particles forming per unit volume. We don’t see any of these, we don’t see cosmic strings or the magnetic charges, domain walls, etc., which all have to exist if the unification theories are correct. And then, the beauty of inflation being able to dilute things, then we reheat up to the temperature that generates only the things we do see in a fairly natural way, allowing you to preserve the elegant unification theories, while explaining why it is you don’t see all the things they predict.

Zierler:

Who were your key collaborators during the GP-B years?

Cabrera:

That was mostly working in-house with collaborators in Aero-astro Department and the Physics Department. And I think there were also some in applied physics. So, it was all internal. There was also, of course, trips to Huntsville, Alabama. There were groups that were working with NASA. NASA had been supporting the program. So that was interesting to see how that worked in terms of the space program. I was somewhat turned off, I guess I would say, by the very large machinery that it took to do a space project. I remember looking also at Hofstadter's gamma ray observatory, how many years that took and how many people, etc. So, there were very exciting projects, but it was like an accelerator experiment in many ways. Those didn't directly attract me because I always did like the idea of having relatively smaller groups that made a more direct impact on a particular experiment. And, of course, as the dark matter experiments grew, they became, themselves, significantly larger endeavors. But I sort of had to be taken there a step at a time.

Zierler:

I'm curious how much of a background in General Relativity may have been useful for the GP-B work.

Cabrera:

Right. I had taken General Relativity in graduate school, I remember. And Schiff, of course, had worked the dragging of inertial frames calculations in detail. So, he had worked with the early GP-B development work. The original three faculty members were Schiff, the theory in physics, Fairbank, experiment in physics, and Bob Cannon, who was in the Aero-astro Department. They first discussed the experiment in 1959—I believe it holds the record for the longest-running single experiment.

Zierler:

I’m curious if theorists like Kip Thorne were interested in this work at all.

Cabrera:

So, there was politics involved, which I sort of at a distance knew about, and then as time went on, I heard more and more about. Should we be spending this large amount of money on a NASA project for a measurement that, if you analyze the post-Newtonian expansion of General Relativity, you should be getting to some of those same constants from other measurements that are being made, including the binary pulsar of Taylor? And so, there were significant issues in the Decadal reviews. I think that the experiment didn't do as well as you might have liked when being compared with others. Francis Everitt actually wrote quite a bit arguing about the importance of a direct measurement of frame dragging. He wrote a biography of Maxwell, by the way, which I always found interesting. And he told me, that when he went through Maxwell's manuscripts, it was remarkable that Maxwell would often take a paragraph and rewrite it with just a few changes in it for his publications, and that he had found in handwritten documents, a whole section about magnetic charges. So, Maxwell had understood that he could symmetrize the equations with the divergence of B giving you the magnetic charge and currents producing electric fields with no beginning and end, no curl. And that in this careful analysis of how it is you would put this additional component into electricity and magnetism, at the end, he concludes that the lack of experimental evidence of these particles means we set these terms to zero. And none of his publications actually mention that, but I found that to be interesting. And one of the things that I've always done in describing magnetic charges is that the symmetrization of Maxwell's equations is not a good reason for believing in magnetic charges. The reason is that you can make a chiral transformation where you take the existing electric and magnetic fields and combine them linearly, so you define a new electric field that is a linear combination of the existing electric and magnetic field and vice versa for the new magnetic field. And what that does is, it symmetrizes Maxwell's equations where the electric charge is everywhere proportional to the magnetic charge. And if you rotate this chiral rotation through 45 degrees, you end up with symmetric equations. And you haven't changed any of the physics. Everything's still the same. But the equations look symmetric. So, it turned out not until Dirac was there a really fundamental reason for believing that magnetic charges might exist. And that's this feature of quantizing both electric and magnetic. So that was very powerful since we know that electric charges are quantized, and we have no other theoretical reason for that quantization. So that struck me as being very important. And, of course, 't Hooft and Polyakov made even stronger statements in terms of the necessity for these charges of the Dirac size occurring with unification. By the way, it was a very interesting discussion with Bill Fairbank because, of course, if a free fractional charge existed with charge e/3, then the smallest magnetic charge has to be three times bigger than the Dirac charge. In fact, I was surprised, I remember discussions with Bill saying, "If that event we saw Valentine's Day really was a magnetic charge, that then puts into question the observation of fractional charges." And it surprised me that he said, "I believe your magnetic charge measurement more than I believe our fractional charge measurement." That struck me. That was interesting.

Zierler:

What were the circumstances of you joining the Stanford faculty? Were you on the job market at that point? Did they recruit you before you got to that stage?

Cabrera:

They didn't recruit. In fact, I think that this was the kind of follow-on to the assistant professors that had been fairly closely aligned with Bill Fairbank's group. And I forget whether he completed his term–I think he probably went on to another position prior to the tenure decision step. But that opening was coming up. And so, there was a posting, and I did decide that that was a worthwhile thing to apply for. And I'm trying to remember whether at that time I applied for other positions as well. I think again, as I was saying, the concept of industrial labs, like Yorktown Heights, was still in play in my mind of directions to pursue for my career. But the direct line of things I'd been doing would be very possible with the assistant professorship. And then, I did, of course, take off in different directions, particularly with the monopole experiment, which gave me a visibility which made it possible to obtain tenure versus very unlikely to obtain tenure.

Zierler:

One of the things that's so interesting about the search for dark matter is that there are so many fields within physics that are coming at this question from very different vantage points. So, I'm curious when sociologically–

Cabrera:

And that did strike me. So, the event occurred on Valentine's Day, and I think I saw it later that afternoon, it was a Sunday…

Zierler:

This is 1982.

Cabrera:

1982. And it did strike me that very rapidly after the event I started to get many inquiries from particle physicists that I think had been spread by word of mouth from graduate students on campus to graduate students at SLAC. And so, it quickly became clear that I had to decide what to do with this event. I made the decision to publish, being conservative in the claim being made, but also thinking that this was an interesting event. I remember when being asked, "What probability do you have of this being a magnetic charge?" I said, "Well, there are possible spurious ways in which you could imagine producing this event." But it looked pretty good to me. So, I gave it about a 40% chance of being real. So, I was pretty excited, but understood that the bar was super high for claiming a discovery.

Zierler:

Was anybody else doing exactly what you were doing? And I guess the question I’m really getting at was, what was the secret behind your detector that wasn’t achievable elsewhere?

Cabrera:

Not at that time. There had been the Alvarez experiments, which also used a superconducting ring and a little train that ran the sample through many times. And then, they would open a switch in the superconducting ring and measure how much current had been induced. And so, they were able to gain back a factor of 1,000 in sensitivity by running the train through 1,000 times before they opened the switch. Using a SQUID, I could easily sense about one percent or so of the Dirac charge on a single pass. So that was just a much more sensitive way to measure the current in the superconducting ring. And so, that struck me. And I hadn’t thought of a reason why that was important until the suggestion of super massive magnetic particles made it clear that you were not going to be able to find these particles at rest in matter.

Zierler:

I wonder if you can convey your emotions that night. What was going through your mind as this detection found us?

Cabrera:

It was exciting. And I remember a few weeks after the event Luis Alvarez came to visit my lab, and we spent remarkably little time, maybe half an hour or 45 minutes, during which I described the experiment and what we had seen. And he looked at me, and he said, “Just wait for the next one or two events. I don’t see anything wrong with this.” You may have recalled that Buford Price at Berkeley had interesting possible detections with his Lexan sheets, and that Alvarez has been rather brutal. By the way, just an interesting aside is that Buford Price was one of my father’s graduate students. My father always said that Buford and Bob Coleman, who was a solid-state physicist, were his two best graduate students coming from UVA. So, I found that to be an interesting connection.

Zierler:

You must get this all the time, but following Valentine's Day, 1982, was the going assumption that what you had detected would become–I don't know if commonplace is the right word, but it would not have been historically a one-off event?

Cabrera:

Right. And for example, I remember calling Luis Alvarez for advice after the science writer Walter Sullivan at New York Times had written an article about the experiment. And the question I asked Luis was, "Should I be cooperating with media interviews or just wait for the publication of peer-reviewed scientific papers?" And so, two things struck me about that conversation with Alvarez. One was, that was the night before he was giving a presentation at the National Academy of Sciences on the extinction of the dinosaurs, and it really struck me how nervous he was about that talk. And I said, "You've given thousands of talks. Why are you nervous?" He said he considered this to be one of the most important scientific contributions he had ever made, and he wanted to get it right. So, I found that to be striking that somebody that renowned and very confident in himself was nervous. So that was very interesting. And then, the other thing he said was, "You know that if another event or two happens in the detector, you'll be in Stockholm in the fall?" And I said, "Well, you can't think about that." He said, "No, you can't, but that's the reality." A year later, there was the famous telegram that came as a Valentine's Day card, that said, "Roses are red, violets are blue. The time has come for monopole two. It was signed the Harvard Gauge Group, and I later learned that it had been sent by Shelly Glashow.

Zierler:

To the extent that there were other groups now that leapt at the opportunity to build similar detectors…

Cabrera:

Absolutely. So as soon as this paper was written, there were probably a dozen groups that immediately started applying SQUIDs to this type of measurement. So, for example, IBM Yorktown Heights was one, University of Chicago with Henry Frisch got involved fairly early on. And at the conferences that occurred within the year, I'd say there were roughly a dozen efforts, of which maybe two or three extended over years competing with our experiments.

Zierler:

And was this more a spirit of cooperation or collaboration between you and these other groups?

Cabrera:

Mostly these were pretty straightforward experiments performed by small groups. We did think about building a very large detector, say 100 square meters. And that would be more like an accelerator experiment, which would then take a larger collaborative effort. But the ones that were being worked on for generation 1, 2, and 3, the third one actually used a section of the first superconducting accelerator at Stanford, which was also one of Bill Fairbank's research efforts, by the way. The niobium cavity superconducting accelerator was developed at Stanford, and they had spare long Dewars, which we turned into a monopole detector with eight faces around a cylindrical geometry. So, any kind of charge going through would trigger two and only two of the eight faces. So that was the largest one we did for generation 3. It took about ten years to complete those three generations. And in the end, we ended up with flux limits that were about 5,000 times more stringent than suggested by that original event. So it was hard to continue to believe that that original event was real, although the frustrating part was not being able to replicate a spurious mechanism for producing that event.

Zierler:

So, take me through an alternate history where the event that you recorded in February 1982, now that there are so many other groups involved, you're energized, you get a big grant about that. Take me through an alternative history where these events become commonplace. What are the bigger questions that are now known as a result of these commonplace events being detected?

Cabrera:

The things we talked about at the time–for example, Eric Cornell was an undergraduate at Stanford and worked in a summer program to develop a scanning system, so if a magnetic charge passed through a superconducting cylinder, it would leave a double vortex where it went into the cylinder and a double vortex where it went out. And then, imagine a small coil that we could scan to detect the presence of these vortices. And then, just continually map it, and as more and more particles went through, you would be able to tell which way they were going, and how many there had been. The ultimate idea would be that, if you can detect these particles, and if their density is anything reasonable, then could you imagine steering two of them into each other to annihilate and do particle physics experiments at an energy scale of 1016 GeV. So that was kind of the really pie-in-the-sky dreaming.

Zierler:

Which would tell us what about how the universe works?

Cabrera:

Well, these very, very energetic particles, their internal structure, how they relate to the unification theories, and what the decay products would be, would all be inputs for particle physics theory. So, instead of the 103 GeV now accessible with accelerators, in principle you would be able to explore energy scales that are 13 orders of magnitude larger. So, that is, all the particles between us and the 13 orders of magnitude, in principle, would be accessible through this annihilation of products.

Zierler:

I wonder if the process of recognizing long term that this event from February 1982 was unique, if you went through something like a 12-step process of grief, all the way from denial to acceptance, how did that work?

Cabrera:

I wrote papers mostly for conference proceedings describing how else you could've produced it in a spurious sort of way. But very quickly, the concept that if you have trap flux in the vicinity of the SQUID, that from a mechanical disturbance moves from one local minimum to another, you can generate a step smaller than a flux quantum in the SQUID, which is what you need. And it would just, by chance, have to be the right size to be the Dirac charge, which is the other part that was quite striking about this event. It was not only the presence of the step occurring in the data, but the size of the step was the Dirac charge to within 5%. Even though the event was striking, you could devise a spurious way to produce it. I do remember trying to do experiments with various mechanical disturbances to see what kind of disturbance it took to produce a similar event. We were able to produce such events, but none were clean in the sense that along with the step in the data there would be slow relaxation shifts that occurred. But when you cool systems down to liquid helium temperatures, you always hear pings and pops that occur. And you could imagine those occurring sufficiently locally that they don't generate the broader disturbance we were generating with the mechanical disturbances that we could produce externally. So, that was the way to understand it as a spurious event. Everything that we did in the studies made it hard to justify immediately identifying the spurious nature, but remember, whenever you have something which is so striking, to claim a discovery you have an extremely high bar. I typically have said in talks over the years, that the lack of any additional convincing candidates in the generation 2 and 3 experiments led us to conclude that the original event was more likely spurious than real. Both generation 2 and 3 experiments were coincident in the sense that multiple SQUIDs would register a real signal. However, neither of those experiments had single SQUID events similar to the original event. So, something close to one SQUID wouldn't be consistent with a magnetic charge passing through the apparatus. So that was then a coincident experiment that would've been very convincing to see any event in one of those experiments. We didn't see them. And so, in the end, the fact you can’t replicate the experiment in science means that you can’t accept it as being real. So, you have to say, "More probable that it was spurious than real." The largest experiments done were using scintillators with slow-moving particle electronics at roughly 100 square meters. And those set limits, which were beyond the so-called Parker Bound, which was one of the limitations in terms of how many magnetic charges there could be and not wipe out the magnetic fields that we know exist in our galaxy.

Zierler:

When did you start to turn your attention to WIMPs?

Cabrera:

So, in '83, '84, I was attending conferences in which the monopoles were being discussed. There were a number of theoretical talks about other things that should be seen if magnetic charges were present at densities anything like what was being suggested by the experiment. These included stimulated proton decay, for example. The theorists were beginning to take that more and more seriously. Also, that the natural density of magnetic charges, starting in a universe without inflation, was huge compared to the density of matter that we have in our universe. So, that's a problem. Why are there so few magnetic charges? Why do they make the density of the dark matter? There were no easy answers. When inflation was developed, one possible answer was that the reheating temperature was just enough to allow the monopoles to regenerate, but nothing more massive. Again, that would need to be very fine-tuned in order to obtain the density of dark matter that we see in our universe. So, these were the theoretical problems that began to develop, even within a year or two after the '82 event. And at the conferences, when dark matter candidates were being talked about, these were particle theory conferences and experimental particle physics conferences, axions were, of course, being discussed early on. The thing I never liked about axions is, I didn't see a natural reason why they should make up the dark matter. They could, and they should be looked at because they're an elegant construction for why the strong force obeys charge parity symmetry, but not the weak force. So that was the interesting alternative, for what else dark matter could be? Very quickly, I think this concept of the WIMPs, which were introduced in 1985 with the Goodman and Witten paper, really struck me. And so, there was a paper, which I wrote with Wilczek and Lawrence Krauss, in which we focused on neutrinos, but really, looking for dark matter was in our minds. That's what we were looking at. And you could also do interesting neutrino physics with those kinds of detectors.

Zierler:

I understand at this time, the rise of WIMPs as a serious contender for dark matter, it was primarily because of theoretical advances. There weren't new observations or experiments that allowed WIMPs to take center stage.

Cabrera:

Yes, but they did couple strongly to supersymmetry at the weak scale. Naturally in many theories the lightest supersymmetric particle was connected to the WIMP dark matter density. And, of course, the steppingstones to unification at higher energies as well is part of that. So those were the connections that there were around. But, of course, experimentally, there's a huge, wide swath of possible particle masses that can be the dark matter. But we do always point to the bullet cluster as being an example where the nature seems to have torn apart the dark matter from the baryonic gas, which makes up most of the baryonic mass in a galaxy. And the fact you could separate them means that having modified gravity seems less likely than actually a particle fluid for the dark matter. So that's one of the things that we always point to. But all that means is the particles interact weakly, and they still can be a whole bunch of different possibilities for what they are, including axions all the way to WIMPs, etc. We've gone through three generations of WIMP searches in our experiment, and there probably have been about 40 experiments worldwide. And the limits that have been set are impressive–the natural scale that we always talked about is that if the interaction of WIMPs annihilating each other through crossing arguments should be similar to the cross section for a WIMP scattering off a baryon and vice versa. And we are now maybe three or four orders of magnitude smaller than that natural scale. So, you can develop theories where the scattering off of baryons is three to four orders of magnitude weaker and more. But they all involve some form of fine tuning. And my theory colleagues tell me, “Well, that’s not a problem given how much fine tuning we already have to have with the cosmological constant, for example.” And so, a three or four-order fine tuning doesn't seem to bother them. But it does bother me as an experimentalist. It seems unlikely.

Zierler:

What was different in terms of your area of expertise in building detectors earlier in the 1980s as this would be applied to considering WIMPs as a candidate for dark matter?

Cabrera:

The first idea was this idea of crystals of semiconductors having incredibly small heat capacities at millikelvin temperatures. So, the idea that an interaction with a particle anywhere in the crystal, if you cool it down to millikelvin temperatures, you should be able to detect an elementary particle interaction, a single elementary particle interaction. And the idea of a tungsten transition where the superconducting transition temperature of tungsten is 15 millikelvin for the elemental tungsten. watching a small ring go normal from superconducting would provide a thermometer, for measuring the event. The real breakthrough in our group, occurred in 1994. By the way, the monopole experiments went on for ten years, so through the first few years of the 90s. Then around 1992 I was told by the DOE that, "You can either continue the monopole experiments, or you can do dark matter WIMP experiments. You can't do both, so you have to decide." And so, that was the point where I decided that the WIMPs did look more interesting because of two things. One was the very interesting particle detector technologies that we were developing, and then the second one was that I think there was more excitement in WIMPs than magnetic charges at that time. And then, in 1994 with one of my graduate students, Kent Irwin, who's now on the faculty at Stanford after being at NIST, Boulder for many years, we developed this novel method to read out a superconducting transition-edge sensor using a SQUID. The concept was very simple to describe; if you voltage bias a section of superconductor, the self-heating, which is Joule-heating, is V2/R. And so, that has the property that when heat flows into the system, the resistance goes up, and the Joule heating goes down. So, that's a self-stabilizing system where you set up a bias voltage, and automatically, the tungsten sits in the middle of its transition. And so, when either particles directly hit the tungsten as photons for example. We made measurements of the Crab Pulsar with one of these little tungsten slabs at McDonald Observatory, and that was a lot of fun. We also then fabricated these transition-edge sensors on large crystals and absorbed the phonons coming from the crystal, initially, into the aluminum fins. Then the quasi particles formed in the aluminum ended up in the tungsten transition-edge sensors. So, that's how all of our detectors have worked since the mid-1990s. And we've made larger and larger versions of those, in terms of how to detect dark matter. We were funded by DOE and NSF for SuperCDMS and told to specifically look for light WIMPs below a mass of 10 GeV, which the energy depositions from each event are small, and then LZ with the xenon detector was specifically asked to make the biggest detectors they could for WIMPs, above 10 GeV in mass. So those experiments continue to be active, where ours is being built in Sudbury, Canada in SNOLAB. And LZ is operating in the Homestake mine in the US.

Zierler:

What were the circumstances leading to CDMS becoming formalized as an international collaboration?

Cabrera:

It was a little at a time. So initially, there were these BALTIC, Bay Area Low Temperature Informal Conferences. And then, I think towards the end of the 80s, we had a loose agreement with Bernard at Berkeley and also with David Caldwell at UCSB. I'd say that the three of us, David Caldwell, Bernard, and myself formed the first collaboration. David had been doing experiments looking to see whether or not you could detect neutrinoless double beta decay in germanium. And at an underground site at the Oroville Dam, they had done experiments with a germanium detector that was well-shielded to look for this neutrinoless double beta decay. And then, Bernard had suggested to David, "Well, why don't you analyze your data near threshold to see what it says about dark matter interacting with your detector?" And so, that was the first dark matter experiment with existing technology, semiconductor diode detectors. And I joined the two of them to look at concepts of improving the detector technologies. And so, that was the formation of CDMS at the end of the 80s and early 90s. We then built the Stanford Underground Facility at a shallow site on the Stanford campus for doing the first phase of our experiments. And so, that was 17 meters of water equivalent overhead, which allowed the hadronic component of the cosmic rays to be shielded. And the first experiments we ran were in End Station III on the Stanford campus with detectors both from Berkeley and from Stanford. We had, actually, competing technologies, which did produce an interesting time when we had to down-select, decide what we were going to do moving forward. And so, there were aspects of technologies from both groups that we included in the final designs.

Zierler:

I assume by the time CDMS really got to a mature stage, the original tensions between the physics department and SLAC had essentially melted away.

Cabrera:

Yes. But I think it's because people died. When I was a graduate student, in 1974, Sid Drell gave a colloquium on campus, and that was the first time that I actually heard Sid Drell talk. And I graduated in '75, so I'd been there for five years at that point, or almost five years. And a couple of things really struck me. He was talking about his role in nuclear disarmament politics. He cofounded the Center for International Security and Arms Control at Stanford, which was very important at that time during the Cold War. I was immediately dumbfounded as he presented his talk at how much his presentation style mirrored that of Dirk Walecka, who was a nuclear physics theorist at Stanford, and who was actually Chair of the Physics Department around that time. I learned shortly thereafter that he had been a graduate student of Sid Drell. That's the first time that I noticed that your students not only learn physics from you but also how to make presentations including mannerisms. I then paid attention to my students and post-docs when they gave presentations, and I did notice that they pick up on some of my style and techniques. And so, even though there's no genetic connection, there definitely are generations of physicists through the thesis process, which learn not just the physics, but a number of other aspects, which I found to be quite interesting. I heard Drell give one other talk, which was titled The Correlation Between Music and Physics in physics theorists. He hosted a string quartet at his home, and he said that he had been fascinated by how similar the musical styles were to the theoretical styles of theorists. I found that to be interesting as well.

Zierler:

What were some of the technological, or engineering, or even administrative challenges of CDMS I? Why do it right at Stanford and essentially under Stanford? There's a lot of open land out there.

Cabrera:

Well, we were looking around in the Bay Area for a convenient place to get enough shielding for the hadronic component of the cosmic rays to be turned off, and then later we went much deeper at the Soudan mine to further reduce the cosmic ray muons. And recently, we've gone deeper again at SNOLAB in Canada. We went to the Sudan Mine in Northern Minnesota because there was a proton decay experiment ongoing, so there was a scientific presence. And also, it's a very interesting mine because it's not an active commercial mine, but it's still open because of historical mine tours that the state of Minnesota gives. So that was sort of a perfect situation for experiments. That's 800 meters underground, and SNOLAB is 2,070 meters underground.

Zierler:

I can't imagine for CDMS I that this was an easy sell for the president or the board of trustees to OK this directly under the campus.

Cabrera:

On campus at Stanford, there was a tradition for doing intermediate scale experiments including the first electron accelerator with Hansen. That was a 1-meter tube of copper. That developed into a 300-foot-long linear accelerator that was the workhorse for Bob Hofstadter's Nobel Prize, on probing the size of the nucleus. And then, Bill Fairbank got involved, and there was a tunnel dug underneath that 300-foot for developing superconducting accelerators with niobium cavities. And the Navy came and told them that they had year-end funds that needed to be expended, and so they built End Station III that was deep enough to connect to that tunnel. And so, that end station was a room that was some 60 feet underground. And it had beam dumps in the far wall. And so, that was an easy place to start digging for an underground chamber that would have 35 feet of dirt overhead. I was able to get the university to contribute; It cost a quarter-million to dig the tunnel, and that was fascinating as well because the people that we found to dig it are part of a very small community of tunnel diggers worldwide. We hired this small crew of three people. Mostly they dug chambers for wine cellars in Napa Valley. And they could dig a chamber and shotcrete the walls for less money than it cost to build a building on the surface and refrigerate it. So that was fascinating. But this guy, in an earlier career, he had worked on large projects like the Chunnel. And so, it was fascinating to hear all of his stories. Because digging tunnels ends up being one of the construction projects with the most unknowns. You don't know what you're going to run into. You try to do pre-drillings to find what kind of soils and so forth there are along the path you're digging, but you almost always run into something unexpected. When they dug the SLAC tunnel, by the way, they actually ran into dinosaur remains. So, there are many things, which are quite interesting, that you can run into. So, $250,000. The different parts of the university contributed half the cost. We ended up having the DOE contributing the other half distributed over some number of years for the use of facilities. We ran, in that facility, through about 2000, so basically eight years. And the CDMS I results were from that experiment. And then, we had begun working on Soudan in 2000, and we started running in 2003 or 2004, in the Soudan Mine and called the experiment CDMS II. Later, we ran a first round of SuperCDMS Soudan with bigger detectors in the Soudan mine facility. And now, we're completing the facility for even larger detectors called SuperCDMS SNOLAB, which can go down to very low thresholds, and therefore, detect small mass particles in Sudbury, Canada in SNOLAB.

Zierler:

What was the decision on the sequencing from CDMS I to II? Had I essentially run its course? Was it satisfactory in terms of what it found, and it was just time for the next project?

Cabrera:

So, we were limited by backgrounds at that point is the way to say it. We couldn't make additional improvements because we still had muons. The muons produced neutrons; the neutrons caused recoiling nuclei that are similar to what we were trying to detect. And so, going to where the muons were a smaller flux was the most important next thing to do. We kept running in Soudan mine until we were able to complete the most sensitive experimental limits which were background limited. One recurring problem is how do you have data for students to graduate on a reasonable time scale when you have significant gaps between experiments? And we tried to minimize those. It's still a problem because you never quite succeed in minimizing it.

Zierler:

Was CDMS I generally considered to be a success relative to its original goals?

Cabrera:

Yes. I mean, of course, the original goals were detecting dark matter, but we did set important limits.

Zierler:

Well, right. There's that.

Cabrera:

But the limits were significant. And I think there was a period when they led the field, but there were many competitors in deeper mines in Europe. And those competitors were fairly quickly getting to similar limits. And so, wanting to get Soudan up and going was significant for us. And that had a background sufficiently low that we could carve out new regions. And there, we did lead the field for a significant number of years as other experiments were catching up. The xenon experiments took a lot longer to catch up around 2010. And then, we bifurcated with our detectors being best for detecting the lowest mass WIMPs with the lowest threshold detectors, and xenon being able to go to larger and larger mass detectors in a natural way and search for more massive dark matter particles.

Zierler:

Were the same basic questions from CDMS I transferred to CDMS II? Or just by dint of doing the experiment, had some of the questions changed for Minnesota?

Cabrera:

That's a good question. I think the fundamental model we were after was WIMPs, and so that didn't change. But there were papers written about secondary measurements that we were making at the same time, for example, what can you say about electron recoils in the detectors? Which then relates to axion possibilities. And there's something called LIPs, which is lightly ionizing particles, which is the possibility of particles that have a smaller charge than the unit charge of the electron. Again, theoretically not very likely, but you can set limits on a very wide range of these lightly ionizing particles, which would show up as a line through a stack of detectors. So, there were a number of secondary measurements of that type. Typically, though, we've repeated them each generation, where you can do somewhat better, and that gives students a number of different projects to do in addition to the fundamental WIMP experiment. We also got very interested in the Effective Field Theory approaches, which are very powerful. These theories include all possible particle interactions categorized with about a dozen parameters. And then, looking to see, in your experiments, what limits you can set on each of those parameters. And so, that is a very powerful way to distinguish between the different possible theories of dark matter particles, how they might interact. And if you ever saw something, you could immediately turn those kinds of measurements into additional information about the particles. So that's the reason that those theoretical developments and bringing them into the experimental analysis was important.

Zierler:

What were some of the administrative and technological distinctions from CDMS II to SuperCDMS?

Cabrera:

Mostly going to larger crystals. We went from 1-centimeter thick, 3-inch diameter to 1-inch thick, 4-inch diameter. That increased the unit cell. And at the same time, having thresholds with the superconducting transition-edge sensors, which allow us to move into a new territory on very light particles. Those are the technological directions that we've taken. We also have developed at Stanford ultra-sensitive detectors. In a gram-scale silicon crystal detector, we have demonstrated the ability to detect a single electron hole pair thermally using the transition-edge sensors. We apply an electric field across the crystal, and when an electron hole pair forms, it accelerates and deposits heat proportional to the voltage across it. So, you add heat to the crystal equal to one charge times the applied voltage across the crystal. So, for about 100 volts across 4 millimeters, you can actually detect individual electron hole pairs. With a semiconductor detector at room temperature or liquid nitrogen temperature, the best sensitivity is about 100 electron hole pairs FWHM. So that was exciting, both for dark matter in this very light region where we can see whether or not particles are depositing an electron volt scale of energy, which is an electron hole pair, in silicon, for example. And we can set limits on those particles, even with the small crystal at the surface at Stanford. And we plan, then, to push that in the larger crystals at SNOLAB as a second phase add-on. So, you do the first experiment, and then we believe we can make large crystals, which can resolve individual electron hole pairs in a second generation of experiments in the SNOLAB facility.

Zierler:

What were some of the considerations in going from Minnesota to SNOLAB?

Cabrera:

Well, first of all, the large experiment that supported the Soudan facility was the neutrino oscillation experiment, where a neutrino beam at Fermilab was aimed at Soudan, had been completed. And they moved on to the larger, close-to-surface detector that's called DUNE. And so, that meant it would've cost us too much to keep running at the Soudan facility. Also, the muons at Soudan, which are roughly one per square meter per minute, became a limiting background in the experiments we were running. And SNOLAB, which is a factor of three deeper has 1,000 times less muon flux. So that gives you that much additional reach, as long as you can make the other backgrounds smaller, which you have to work hard to do. But that was the reason. Sudbury is actually a real town with a population of around 100,000, instead of Soudan, which is very tiny with a population of maybe 200. So, students and post-docs like it more. And the other interesting thing, by the way, is that Sudbury is 100 miles south of Soudan. Soudan is on that northern part above Lake Superior, and Sudbury is down where Lake Eerie comes down further into the US. So, I found that interesting that we're actually going south by 100 miles. And typically, the temperatures at Sudbury are 10 degrees or so warmer than at Soudan most of the year.

Zierler:

I'm curious if Art McDonald was at SNOLAB by the time you got there and was that part of the consideration.

Cabrera:

Yes, part of it. But remember, SNO, which was the Sudbury Neutrino Observatory experiment, was the first reason for setting up the lab. And then, they did choose to set up a multi-experiment facility, which they call SNOLAB. And our experiment was one of the natural ones to request to go there, and we were approved fairly early on. And so, the Canadians have felt that that's been important to support that facility. I think at Homestake in the US, that's somewhat trickier because they did get significant funding from South Dakota, which was important, to reopen the gold mine. But there's nowhere near the infrastructure you have at SNOLAB, in terms of a clean room facility that far underground. Also, SNOLAB is deeper than Homestake Mine—4,000 versus 6,000 feet. So, that made it attractive. Of course, we don't have access to Canada right at the moment, so that's one of the problems that develops from the pandemic. But it's not the only one. And hopefully, that will complete the installation over the next year.

Zierler:

Just as there was much anticipation moving sites from Stanford to Minnesota, what was some of the excitement and possibility about moving to SNOLAB?

Cabrera:

The construction is an entirely new facility with 100 times less background as the goal. And so, there was quite a lot of effort that went into how to do that. And we did it, actually, initially and wanted to build a larger volume for detectors in the future. And I think we did get scaled back in terms of financial considerations, where now, it is more a facility that will look for these light candidates versus the heavier candidates. And it's harder to get to larger masses. But that's OK. We've always said that if something convincing is detected, then it will be easy to argue for larger detectors in the future. And so, right now, the most important thing is to do careful experiments, and if something is there that you could convincingly demonstrate its existence…

Zierler:

And when did you become spokesperson, and what are some of the responsibilities in that role?

Cabrera:

I'm probably not going to have all the dates on hand, but I was co-spokesperson in the first decade for maybe three or four years. I then became spokesperson about 2012. And I stepped down from that role at the time that I became Project Director for the SuperCDMS SNOLAB project funded by DOE and NSF, and I did that for three years. I stepped down in 2017, and David McFarland is now in that role, who's a scientist at SLAC. And SLAC is the lead lab on the project that the DOE is doing. The NSF has contributed to it as well. The SuperCDMS Collaboration now includes 23 institutions worldwide including national labs [SLAC, Fermilab and PNNL] and universities [Caltech, Durham, NISER, Northwestern, Queen’s, Santa Clara U, SNOLAB, South Dakota School of Mines & Technology, SMU, Stanford, Texas A&M, TRIUMF, U British Columbia, UC Berkeley, U Colorado Denver, U Florida, U Minnesota, U Montreal, U South Dakota, and U Toronto].

Zierler:

Physically, how much time would you be spending on site, both in Minnesota and in Canada?

Cabrera:

In Minnesota, we actually had the requirement for about two visits per year. And in 2003, when we had some technical difficulties with getting the system cold and operating, I spent 75% of my time over about three or four months helping to solve the technical issues. And since then, I've participated a couple of weeks per year, which were the typical rotations in Soudan. We have not yet started SNOLAB because of the restrictions in travel, which have delayed the project probably by a good year. Right now, the project is going through a re-base line, as they call it at DOE, which is requesting sufficient funds to get us over the pandemic impacts. That's going to be helpful. And that's assuming the pandemic actually ends this calendar year. But given the variants, I think that's not a certainty. So, we have to see how it goes.

Zierler:

Given that the overall goal of the entire CDMS endeavor over the past more than 30 years now…

Cabrera:

CDMS officially started probably in the early 90s, maybe ‘92 or so.

Zierler:

So right around 30 years. Given that its central mission remains elusive, of course, the discovery of dark matter, what have been some of the most efficacious political or budgetary cases to make to funders like NSF to say, “Stick with us, we’re still onto something. We’re still learning important science. This is still important work to support”?

Cabrera:

That question is asked every review. For example, when SuperCDMS and LZ were chosen in a down select competition, which had maybe eight or so potential experiments. And so, the choice was that these two seemed to be the ones that were going to have the greatest reach for scientific discovery. And we made that case successfully. So, I think you continue to make those cases, for both projects, and then, also, each of the individual groups must request funding as well–for DOE support, groups propose to the comparative review that occurs typically every three years for each group. Other groups are funded by NSF, so they submit similar proposals. And the same question gets asked by those review committees. "Is this something that we should continue to be funding and doing? And what is the role of this group in the larger experiment? Is it a key role, is it important?" etc.

Zierler:

And is there a general sense over the decades that the fundamental question is getting closer? We're getting closer to understanding dark matter because of what CDMS has been able to accomplish?

Cabrera:

Well, what I always say is that science progresses as much by experiments which set limits as by experiments which make discoveries. Improved limits are carving out regions of possible phase space, in which you don't see something new. And I do remember the striking thing with Bill Fairbank, for example, he made his name in measurements of the Lambda point. So, this transition that occurs in superfluid helium. And his group was able to make measurements down to the nanokelvin level to show that you really have this very interesting divergence that occurs at the Lambda point. And there’s no reason a priori why you should’ve been able to do such a precise experiment. But they managed to do it. So, Bill had the remarkable luck, I would always call it, that the things he chose to do did end up having interesting results. I think he got carried away with how thin he stretched himself taking on too many different projects, which were all interesting, but you are not going to make progress in all of them at the same time. I think one of the things I've also found very satisfying is that the technology with the transition-edge sensors is now the state of the art in x-ray spectroscopy and looks like it will fly on satellites in the future for x-ray astrophysics. And also, it is the technology of choice in all of the CMB experiments at this point. So that's been rather pleasing to see that the technology development that we made for dark matter has had a number of applications in other fields of science, and also, technology for categorizing materials.

Zierler:

And what's the current status of the Cryostat facility in SNOLAB? Where's that now?

Cabrera:

So, the Cryostat is still at Fermilab. It's been tested several times and demonstrated that it gets to the base temperature, and also, the heat load that it can handle is within specifications. So those are both important. And we are ready to start assembling things in SNOLAB, but again, the inability to travel easily has meant that more work is going on getting things together at Fermilab and at SLAC before we actually have easy access to SNOLAB.

Zierler:

Let's switch over to some of your accomplishments on the administrative side of things. What years were you chair of the Department of Physics at Stanford?

Cabrera:

I was Chair of the Physics Department at Stanford for three years from '96 to '99. And there were three of us, Doug Osheroff, Steve Chu, and myself that agreed to three-year rotations. Actually, historically, Leonard Schiff was chair forever—several decades. And actually, Felix Bloch was the core of the Department, and Leonard Schiff as the administrator was the way it worked for years and years. After that, there were Walter Meyerhof, and Dirk Walecka, Stan Wojcicki and Sandy Fetter. We started a pattern of three-year rotations, which sometimes grew to five years. But the three-year rotations are ones where a faculty member with an experimental group can maintain their activity through the three-year term. Anything longer than that, it becomes difficult to maintain the group. We've always liked, at Stanford, the idea of having the chair be intimately aware of what it takes to do research. So that's been the reason, even though there are some downsides to rotating through, the upsides are pretty significant, including that you have people there who are aware of what it takes to do research, and, as time goes on, you have a collection of faculty who have been chairs before. And so, they understand what it takes, and they also help out at times. So that's been a good mix.

Zierler:

What do you see as some of your key achievements as chair?

Cabrera:

Well, while I was chair, we had a Nobel Prize each year. This is where the administrator gets to take credit, right? So actually, Doug Osheroff, Steve Chu, and Bob Laughlin were the three. And the year before I was chair, in addition, Marty Perl had won at SLAC. I do remember, actually, that after Marty won, I became chair the following year. And there are a number of requests that Marty made to the chair of the department, and he felt that in his stature as a Nobel laureate, he deserved extra attention. But, when we had additional Nobel laureates every year, that became less of an issue. So, that was good. But anyway, you can't really take credit for that. I'm trying to remember what kind of issues there were. There were always issues having to do with the qualifying exam, how you do that. There's recruitment, the diversity component, which was already an important component. Art Walker, who was a Black physicist, moved from applied physics to physics, and he was always a strong advocate for diversity. And so, I think the department, in those years, did better than it's done more recently, in terms of the graduate student populations, and with hiring. So, I think that was an important component for me. I do remember that 10% of the faculty took up 90% of your time. That's what administrators learn pretty quickly.

Zierler:

In what ways was your tenure as Director of the Hansen Experimental Physics Laboratory useful for some of the other collaborations that you were part of?

Cabrera:

I'd always very much appreciated HEPL, as we called it. It was originally the High Energy Physics Lab. And so, they cleverly changed the name to Hansen Experimental Physics Lab when it ceased to be an accelerator center. But at Stanford, it allowed what we called intermediate scale experiments to be done. So, for example, the ability to find a place to have the 17 meters or 35 feet of dirt overhead, that's unusual to be able to find at a university campus. Doing GP-B would not have been possible without the infrastructure of HEPL, both in terms of technical staff and locations for doing this intermediate scale. I was somewhat discouraged as time went on that the DOE and NSF funding levels have essentially remained the same, so inflation has diluted their effectiveness over time. And so, the ability to do the intermediate scale experiments is harder than it once was. Typically, they have an easier time generating a larger experiment, which has a large community that works to promote it versus the intermediate scale ones, where you typically have less push. And then, you still have the small-scale experiments, which support groups, and that continues. But again, I would say that I found it interesting that in 1990, I think, our DOE group was supported at about $400k per year, and that is still the same. And the last renewal that I had was about $400k per year and 30 years later. And so, inflation has made that 40% of what it was, something like that. But what I always tell students and post-docs, part of that really is also that, for good or for bad, before about '89, high-energy physics was supported under the nuclear security model, and that after the demise of the Soviet Union, that ceased to be the central theme. And so, the support for HEPL didn't disappear, but the impetus for keeping it up was reduced. And probably rightly so is what I would say, from a broad perspective. But it did mean that the communities have fought hard to keep accelerator centers. They've been fewer and fewer. Some of them have closed as time's gone on. But they have still succeeded in doing interesting science, with a fair amount of effort at the political level as well as the scientific level.

Zierler:

As you tell it, when you were a kid, you recognized intuitively that physics was where it was at because the faculty parties, they liked talking about their work. It's well-known that it's important for you to convey your love of science to students. And so, I'd like to ask some retrospective questions with regard to your career as an educator. First, what have been some of the most important and enjoyable classes that you've taught to undergraduates at Stanford?

Cabrera:

I'm teaching Engineering Physics right now, which I haven't actually done for 20 years, so that's interesting. But I'm doing it on Zoom, which is difficult. I think there's no doubt. I record lectures, and the students listen to them asynchronously because of the time zone differences. And then, we have office hours, where I interact with a small fraction of the students, maybe 20, 25 out of a class of 250. And those interactions are always enjoyable. We do have this chat thread where students ask questions, and I answer some of the ones that pertain to the material that I was presenting. And so, I think that's worked reasonably well. But I am ready for more in-person interactions with students. So which ones do I enjoy? I think all of the introductory ones. I did find it interesting when I taught pre-med. So, the pre-meds, interestingly, had more intellectual curiosity than the engineers, if that makes sense, which I found interesting. So, the discussions about sort of wider ranging connections to possible new theories, etc., that there was a lot of interest among the pre-meds. The engineers tend to be more, "How do you do this? What equations do I need to use? And that's enough. I don't know why this is from theory, I just need to know how it works." And I found that interesting. Of course, it's not across the board, but there is a component of that sense. And then, the 60 series, which is our advanced freshman physics, has always been a lot of fun to teach. Typically, most of those students are interested in physics, and they're definitely curiosity driven. It's a relatively small class, and so that's interesting. When I was a young Assistant Professor, Eric Cornell was one of my students in that 60 series, and I do remember when he worked in our group over the summer that I was very impressed with his capabilities, and I thought all Stanford undergraduates were going to be that capable. He's quite remarkable, as you know.

Zierler:

On the graduate side, who have been some of your most successful graduate students and post-docs? And in what ways does the work they've gone on to do make you optimistic about the long-term prospects of where the field is headed?

Cabrera:

That's a good question. Kent Irwin, of course, was key in this development of the transition-edge sensor. And he's now back at Stanford working on dark matter radio, which is looking for things like axions over a very broad range. And so, that's an exciting development to see. And I've enjoyed that. Adrian Lee is a grad student who is on the faculty at Berkeley and working on CMB experiments, a number of that type. There's Tali Figueroa, who's actually from Puerto Rico. He has joined the faculty of Northwestern. And he's very close to Fermilab, so he's definitely in the Super CDMS collaboration, working hard. Tarek Saab is at the University of Florida working with a collaboration. Who else comes to mind? I'm not going to get all of them. Martin Huber is actually at University of Colorado, working with the collaboration, and also, he's done other things in superconductivity, working collaboratively with condensed matter groups. So, that's been fun to watch. And Brian Dougherty, who was a very good post-doc, I think is at Los Alamos National Lab at this point doing many particle experiments. I know that one of my early graduate students went on to Varian to work on industrial research applications, and another went to University of Oregon and is on the faculty there. She was from South Africa originally, and it was very interesting to hear about all the politics in South Africa over the years. That was in the late '80s, so a very interesting time in South Africa as well. And I'm probably forgetting a bunch of people at this point. I think I've had 25 students over the years and maybe ten or so post-docs that I've worked with directly. And, of course, in collaboration, you work more broadly with students and post-docs from many other institutions. The week stints at Soudan have been very useful in making connections between students and post-docs that when you happen to be there at the same time, you actually learn quite a bit when networking occurs in person. And there are all of the places to eat, only about five or six of them, close to Soudan there. And so, you get to know members of the collaboration fairly well.

Zierler:

For the last part of our talk, I want to ask a few broadly retrospective questions about your career, and then we'll end with one looking forward. Going back to February 1982, if you allow for the possibility that what you detected was, in fact, correctly the signature for a magnetic monopole, and that for whatever reason, we just haven't seen it since, but that there's a possibility that this will happen in the future, what are your feelings about that?

Cabrera:

Well, the specific statement that I always make is that for a scientific discovery to be accepted, you need to repeat the result. And so, often, I end it saying that when I'm asked the question, "Do you think that was a real event or not?" in a sense, it doesn't matter what I think. If I can't reproduce it, then it's not part of scientific knowledge.

Zierler:

But to clarify, those experiments leading to that possible detection have stopped, and so how can we know if it's repeated or not?

Cabrera:

That's correct, but, for example, the limits that are set with the scintillators have pushed to more sensitive lower limits. But you're right, we're not running that experiment anymore. I've often been asked, "Why didn't you just keep that one running forever?" But we did run it quite a while. And I think the event occurred at 151 days into the data-taking. And that was five different cryogenic runs accumulated. And then, we ran it for a total of 383 days, without seeing anything else. So that was the original dataset. And then, we ran the second and third generations, which grew the size to an exposure 5,000 times larger than the original data set.

Zierler:

So basically, you've made peace with the fact that whatever it was, was not the detection of a magnetic monopole?

Cabrera:

That's right. I fact, the third-generation paper actually says that at this point, given how much further the limits have been pushed, we have to argue that the event in '82 was more likely spurious than not.

Zierler:

To go back to the earlier comment I made about how it's so fascinating in dark matter how so many different fields are coming…

Cabrera:

And I was going to comment on that, that the year after the ’82 event, when I probably gave about 25 talks, I was really struck that every other area of physics in one way or another had something to say. And it was a very special time in the sense I did get to meet many people in different research areas. And I remember giving a talk at one of the national labs, where there had been a recent paper with scintillators looking for slow moving particles, which had set limits significantly below what our event suggested. And so, I expected to be asked a question about that at the talk. But there was no question. But then, afterward, when I was being given a tour of the various facilities, the scientist who was lead author on the paper took me aside, and he said, "You know, when we turned on our experiment, we did see events, but we turned up the threshold until they disappeared." And that really struck me. So, there is always pressure to publish, but if you a priori are not going to be open to there being something there, then it really seems strange to me. And so, that struck me. There are various things that you learn along the way.

Zierler:

You provided such an elegant answer for all of the ways in which CDMS has advanced the science, even without discovering dark matter. But let's turn that question specifically on dark matter. It might be difficult, given that the endpoint remains a question mark. But what more do we know about dark matter, without understanding what dark matter is, as a result of CDMS?

Cabrera:

I'd say that, it's almost entirely in the sector of having weakened the case significantly for weakly interacting massive particles. That's the most important result that these experiments have provided. And as I said before, I'm uncomfortable with the fine tuning required to get the WIMP model to work down at the levels that we are probing now. So, I think that reduces the likelihood. And so, the theoretical structures that people are looking at now are significantly broader than weakly interacting massive particles models. These have the downside that there is not as elegant a reason why these particles should make up the dark matter. On the other hand, expanding the possibilities is what's happened from ruling out the most natural models. Since we don't see them, you have to look elsewhere. I've always said that what it is that makes up most of the matter of the universe is a pretty fundamental question. And that the dark matter is the structure which drove the formation of the galaxies since dark matter is about 85% of the matter. And so, the clumping of the dark matter is what formed the galaxies, which then fell to the center of the potentials to light up the stars, etc. So, the structure of the universe is determined by dark matter, and I'd really like to know what it is.

Zierler:

To use the well-trodden metaphor about looking for your keys only under the streetlamp, in the sense that one of the values, as you know, with CDMS is providing it a decent process of elimination, that's very good, but it only works under the streetlamp. How confident are you that that proverbial street is well-lit enough that we know that it's fruitful to eliminate these candidates?

Cabrera:

The agencies were actually pleased with having natural limits for the LZ and SuperCDMS experiments. There's something called the neutrino floor that, for the massive WIMPs above 10 GeV, once you get to where the atmospheric neutrinos dominate your signal, it's very hard to pull out dark matter signals from the neutrino signals. So that's the natural stopping point according to the agency. They love to have a natural stopping point. And in the lighter area for mass below 10 GeV, it turns out to be the solar neutrinos dominate. And so, again, there's a natural floor to how far you go with these dark matter searches, which is when the solar neutrinos become your dominant signal. You can always do modeling for the neutrino signal and see if there's anything else there, but quickly, that becomes a very difficult job to pull a signal out of a nearly identical background. That's the natural stopping point where the agencies look at a light at the end of the tunnel in terms of how much support they're going to need to provide over the long term. And remember, you can easily construct theories where the dark matter exists, and it's within the neutrino signal, which makes it hard to think of ways to look. You can also pretty easily construct, for example, gravitinos, the supersymmetric particle to the graviton, and we can't think of any experiment to detect those. So those could be the dark matter in principle, and we don't have an easy way to build an experiment. So it's possible that nature ended up being perverse and put dark matter into something that's very hard to see. But the science progresses by continuing to look in new places and see if there's something that hasn't been seen before.

Zierler:

On that note, last question, looking to the future. Let's say, best case scenario, it's SuperCDMS. The results are conclusive. You and your team have figured out what dark matter is. After we put away the champagne, and we know what dark matter is, what questions will have been answered, and what new questions will have been allowed to be raised that are not yet possible to ask?

Cabrera:

That's a pretty broad question, of course.

Zierler:

That's why I save it for the last one!

Cabrera:

Yeah. So that was the comment that I made about the Effective Field Theory, was in that direction, that as soon as you start seeing events that look convincing, then you start asking a bunch of questions about which of the parameters associated with all possible particle interactions are turning out to be the properties of that particle? So that's a particle physics direction, that you want to learn as much as you can, see where this particle fits into the structure of particle physics. And is it along the road to unification? Does this help or not help? Those are the kind of questions that, for me, are the most interesting. These things interact weakly, we already know that. So it's unlikely to be a new power source, in terms of propulsion or some other aspect of energy. This was the annihilation of supermassive magnetic charges to new very high-energy interactions was one of the things that arose from the monopoles. If these particles end up not being very massive compared to, say, 10 or 100 GeV, that's probably not an interesting direction. So, I think the fundamental understanding–what's the famous Maxwell quote about his equations? "What good are these equations?" And his response was, "What good is a newborn baby?" It's hard to tell where this is going to go. But if you understand the fundamentals, then it's a good thing moving forward.

Zierler:

Well, on that note, Blas, it's been an absolute pleasure spending this time with you. I'm so glad that we connected, and your insights and perspectives are really a historical treasure. So thank you so much. I really appreciate it.

Cabrera:

All right. Nice talking with you as well.