Robert Cahn

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ORAL HISTORIES
Robert Cahn

Credit: Lawrence Berkeley National Laboratory

Interviewed by
David Zierler
Location
Video conference
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In footnotes or endnotes please cite AIP interviews like this:

Interview of Robert Cahn by David Zierler on July 21, 2020,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
www.aip.org/history-programs/niels-bohr-library/oral-histories/46762

For multiple citations, "AIP" is the preferred abbreviation for the location.

In this interview, David Zierler, Oral Historian for AIP, interviews Robert Cahn, Senior Scientist Emeritus at the Lawrence Berkeley Laboratory. Cahn recounts his childhood in the San Francisco area, and he describes his early interests in math and science, and he describes his undergraduate experience at Harvard, where he was influenced by Dan Kleppner and Ed Purcell. Cahn describes his summer internship at SLAC, and his travel experiences in Europe after graduating. He describes his decision to pursue graduate work at Berkeley and he explains the political tumult that had convulsed the campus in the late 1960s. Cahn discusses his work with Dave Jackson on Regge theory and his postdoctoral work at SLAC, which was focused on quark research. Cahn describes his work at the University of Washington, where he collaborated with Lowell Brown, and he explains his decision to join the physics faculty at University of Michigan, where he collaborated on several projects with Gordy Kane and where he became interested in parity violation in atoms. Cahn explains his decision to move to UC Davis, and he describes the opportunity at LBL that presented itself shortly thereafter. Cahn describes the way LBL has been integrated with the physics department at Berkeley, and he discusses his tenure as Director of the Physics division. At the end of the interview, Cahn describes LBL’s increasing involvement in cosmology, the fundamental discoveries that have been made over the course of his career, and he considers some of the philosophical or metaphysical issues that arise in investigating how the universe works.

Transcript

Zierler:

Okay, this is David Zierler, Oral Historian for the American Institute of Physics. It is July 21st, 2020. I am so happy to be here with Dr. Robert Cahn. Bob, thank you so much for joining me today.

Cahn:

It's a pleasure.

Zierler:

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

Cahn:

Well, I'm now a Senior Scientist Emeritus at the Lawrence Berkeley Laboratory.

Zierler:

When did you gain the Emeritus designation?

Cahn:

Technically, I retired in 2015. I was rehired part-time over the next couple of years. Since 2017, I'm truly retired. The Emeritus probably came in 2015, I guess.

Zierler:

So, truly retired means, absent the pandemic, you were no longer going into the lab after 2017?

Cahn:

No, that isn't what it means. I was going into the lab every day.

Zierler:

I figured as much. I just wanted to know.

Cahn:

What it means is that I wasn't getting any compensation whatsoever. That's what it means.

Zierler:

Right. As I've learned in this field, physicists never actually retire.

Cahn:

One of my colleagues at Berkeley, prior to the pandemic, was still coming into work every day, and I think he's 90 or 91. His thesis experiment was the discovery of the antiproton. Actually, I have a Zoom meeting with him on something tomorrow.

Zierler:

That's pretty good. Bob let's take it all the way back to the beginning. Let's start with your parents. Tell me a little bit about your mom and dad and where they're from.

Cahn:

My parents are from San Francisco, and I grew up in the San Francisco Peninsula. My father was a mechanical engineer and got his degree at Berkeley. My mother got her degree at Berkeley, my grandmother got her degree at Berkeley, my two grandfathers, one got a degree at Berkeley in pharmacology. The other got a law degree at the University of California's law school at the time, which was Hastings. So, we have a lot of Berkeley in our family.

Zierler:

A lot of Berkeley indeed, but not only that, but Berkeley going back multiple generations with a tradition of the women getting degrees as well.

Cahn:

Right, well, my grandmother who went to Berkeley, her mother went to college and went a Normal School. A Normal School was a school that trained people to teach. She graduated from San Jose Normal School in 1878, here in California. She was born in California, so we go back a ways.

Zierler:

Indeed, you do. Bob, where did you grow up?

Cahn:

I grew up on the peninsula, in Menlo Park.

Zierler:

Did you go to public schools throughout?

Cahn:

Yes.

Zierler:

At what point did you start to get interested in math and science, perhaps even before your formal education?

Cahn:

Probably around third or fourth grade.

Zierler:

What kinds of things were you interested in at that young age?

Cahn:

Primarily, math. My father was a mechanical engineer and was quite good in math. He taught me a lot. But also, science. Like many other people who became theoretical physicists, I read George Gamow's books. One, Two, Three... Infinity. That seems to be a common experience for many of us of my generation, and even ones a bit older. I still have it on my shelf here. It's a great classic.

Zierler:

Did your father involve you in his career when you were a kid? Did you get a sense of the kind of work that he did?

Cahn:

To a certain extent. He was a mechanical engineer who designed power plants. He taught me how motors and generators work, and maybe a little bit about electrical circuits. He had a good, strong training as a mechanical engineer at Berkeley. He certainly knew heat transfer, which was a big part of his job. They were building steam plants. He knew a lot about heat transfer. I still have some of his books on heat transfer. He would have been a good teacher. He taught me math until I was maybe 13 or so, and then I taught myself. There are some great books that you can learn from. Hogben's Mathematics for the Million is a great classic. I don't know if you know that one, but it's one of those things where you can teach yourself mathematics all the way though calculus just by reading an extremely entertaining book.

Zierler:

So, probably in math, you might have been even more advanced than some of your teachers in high school.

Cahn:

Yeah, but I had some sympathetic teachers. I remember when I was a sophomore, I had a very sympathetic teacher who gave me a book on advanced calculus, I guess. I did a very nice science project on non-Euclidian geometry with the support of my extremely good geometry teacher. I already read some stuff on non-Euclidian geometry, which was a lot of fun.

Zierler:

Bob, I assume you took physics in high school. Did that experience convince you to pursue physics as an undergraduate?

Cahn:

It was clearly something I could do pretty well. I guess, if I could put it somewhat ironically, I think what convinced me to be a physicist was taking an extremely hard math course my first year in college, which taught me I was not a mathematician. If you want to hear about that class, I can go into that. That was another experience.

Zierler:

Was that top line item that you realized that math at that level was going to be too abstract?

Cahn:

I wouldn't put it quite that way. I realized there were some people who were really, really good. I had two classmates who became tenured, one at Princeton and one at Yale. They were both tenured in math by the time they were 27.

Zierler:

Whole different league.

Cahn:

You learn some humility. They're both quite famous, actually.

Zierler:

Bob, what schools did you apply to when you were thinking about college?

Cahn:

The ones that it came down to in the end — I'm sure I must have applied to Berkeley and probably Stanford, but the two that it came down to were actually Harvard and Cornell, and that was because I had been in a summer program my junior year in high school that was connected in a way to Cornell through something called Telluride House. Those were the two places I looked at more seriously at the end.

Zierler:

Did you also see this as an opportunity to go to the east coast?

Cahn:

Yeah. I had never been east of Montana.

Zierler:

So, why, in the end, did Harvard win out over Cornell?

Cahn:

Probably not for any particularly good reason. Maybe because it was more famous. My parents had lived in Boston for a while during the Second World War. My mother had even attended Radcliffe for a year, finishing up her undergraduate education that was really at Berkeley. My father was working at MIT, and I don't know. I doubt that I knew enough to make a real decision until later.

Zierler:

At what point did you decide on physics as a major at Harvard?

Cahn:

I would say it was the second exam I took in physics my freshman year. After the first exam, I thought maybe I would be pre-law.

Zierler:

What was the exam in?

Cahn:

In mechanics. This was an honors physics course by an extremely fine professor, and quite famous, Dan Kleppner who was in fact, the first winner of this big prize that the APS set up recently. He was a superb teacher and was my advisor part of the time when I was an undergraduate. He teaches an extremely demanding course, and when I did pretty well in that course, I thought this makes more sense than trying to compete against my friends in math.

Zierler:

What were some other formative courses that you took as an undergraduate?

Cahn:

I took quantum mechanics from Ed Purcell, which was a phenomenally great course. Purcell was an extraordinary teacher. Dan Kleppner was very good, but Purcell was — I've been very fortunate. I've had course instruction by many of the really great teachers in physics, and certainly, Purcell ranks absolutely among them.

Zierler:

So, Purcell was the rare person who was as great of a teacher as he was as a physicist.

Cahn:

Absolutely. Later on, I got to actually T.A. for him. When I was an undergraduate, I guess I was T.A. in a physics lab. Purcell came to the physics lab. Here's this Nobel Prize winner who shows up in the beginning physics lab. He was a really impressive guy, and a truly great physicist. So, that was certainly one of the important courses that proved to me, yes, I want to be a physicist. I want to do what this guy does. I also had some great chemistry courses. I had some really fantastic chemistry teachers. I actually was a chemistry and physics undergrad at Harvard, not because I especially love chemistry, but it was a joint major where, if you played it right, you didn't have very many requirements. If you got into a special chemistry course, they gave you like 3.5 years of credit for 2 years of courses. In the end, you didn't really have to take very many courses, so you could take a lot of other things. I became a chemistry and physics major, and there were some outstanding teachers in that. We had Bill Lipscomb, who went on to win the Nobel Prize. We had E.J. Corey, in organic chemistry, who went on to win the Nobel Prize. And we had E. Bright Wilson, whose son won the Nobel Prize. E. Bright Wilson was another very great physical chemist who wrote a classic quantum mechanics textbook with Linus Pauling. Pauling and Wilson, extremely famous quantum mechanics text written in the '30s. So, I had some fantastic chemistry teachers. Ed Purcell, Dan Kleppner — these guys were extraordinary.

Zierler:

Bob, looking back, did you see any advantages in furthering your physics career by taking all of these chemistry classes?

Cahn:

No, I didn't. But I think that was a mistake on my part. That was a good thing to do. Maybe I saw something in it. We did thermodynamics, we did classical thermodynamics, and Wilson, especially, taught us a lot about atomic physics, molecular physics. Except for the organic chem, which I actually thought was a branch of magic. The inorganic parts of the chemistry program were extremely useful for physics, actually, even if I hadn't really appreciated it right away.

Zierler:

Did you have any exposure to lab work as an undergraduate, either during the year or in the summers?

Cahn:

Not as much as I should have. I had a lot of exposure — I actually have some stories. I had a lot of exposure in chem lab, partly because I was so slow. There are a lot of anecdotes. I was in this very special chemistry course. You have to understand that I'm the generation that's the response to Sputnik. The federal government was throwing money at science education, and one of the things they did was establish a special chemistry course at Harvard with about 20 kids in it. I went into the course, initially, hoping I would get into the course. At first, I wasn't accepted into the course, which didn't surprise me, because I didn't know any chemistry. Then, a couple weeks into the semester, I got a telegram — there used to be things called telegrams — I got a telegram from the professor - Bill Lipscomb -who was teaching it saying he had an opening and I should come see him. I went to this office and he asked me whether I wanted to join the course. I said I didn't really have a chemistry background and I didn't really know all this stuff about acids and bases and stuff, and he said, "It doesn't matter. We're going to skip all of that." So, I agreed to take this course. By the time I got into the course — this was an advanced course, but it was beginning chemistry — by the time I got into it he was doing the Schrödinger equation for hydrogen, and he had separated variables and was solving the differential equation. So, it was a pretty heavy dose for most freshmen. That was a fantastic course, and we had a lot of lab work, especially, there was an inorganic semester, two organic semesters, and then one more inorganic. We had a lot of labs in the organic chemistry. It was like eight hours a week of lab, for which you got no additional credit. That was just par to the course. We used to complain that students are taking English courses that met two or three hours a week, and we were taking three hours of credit and eight hours of lab for the same credit for this chemistry course. So, that was a lot of lab. In physics, the freshman lab was mostly memorable because my lab partner was Walt Hewlett, the son of Hewlett Sr. of Hewlett-Packard. We had a lot of fun together. The second semester was much more of a story. If you had done really well in the first semester, they told you were going to do a special lab second semester. This is going to go on for a while. I don't know if you want to hear all of this.

Zierler:

I want to hear every bit of it.

Cahn:

Okay. So, there were maybe fifteen of us who were put in this special lab, and the lab instructor was a guy named Gene Engels. Gene subsequently became a professor of physics at University of Pittsburg. We got there the first day for this special lab, and Gene says to us, "Your lab assignment for this term is to make up an experiment and do it." It was kind of — it's a very Harvard sort of thing. Maybe we can come back to that. Anyway, then he said, "Well, you know, I've been told by Professor Wilson," — that's Professor Richard Wilson, Dick Wilson, "that if you came up with an idea for something, he might be able to give you some time on the Harvard cyclotron." I should look it up. I don't remember if it was a 140 MeV proton machine, or 160 MeV proton machine. This was well past its time, to put it mildly. I had a friend in this class who was quite an amazing guy. He started as a math major and ended up as an English major. He could do absolutely anything. This guy was really a genius. The two of us said, "Let's make a proposal and do an experiment with cyclotron. That sounds like fun." We've taken one semester of mechanics. We know almost nothing, and we're just starting to take E and M. So, I went to the library and found a book that seemed to be about high energy physics. It was actually Rossi's book on cosmic ray physics: it is one of the great classics. I learned a little bit about what was going on, and we had already learned about what a differential cross section was in our freshman — we had actually learned about calculating the Rutherford cross section. My friend and I decided that if we were going to get time on this marvelous machine, we better propose something really serious, or people wouldn't give us time. We came up with this proposalto scatter protons off of polarized aluminum, would mean that we'd have to polarize the nuclei, but I'm sure we had no idea how you would actually do that. So, we went to the lab instructor — I guess, Gene was probably the assistant professor then, or something like that. We told him, "Here's our proposal. We'd like to scatter protons off of polarized aluminum." It took him about ten minutes to stop laughing, because, yes, it would be interesting, but it would be unbelievably hard. First of all, you'd have to go to very low temperatures, and you'd have to figure out how to polarize the aluminum. I didn't know whether it'd have the spin and can be polarized. Anyway, it was a ridiculous idea, and he laughed for about ten minutes, and then he said, "How about you do unpolarized?" And we said, "Oh, yeah, we just didn't think that would be interesting enough." He said, "Okay, I'll go to Wilson." So, he went to Wilson and told him what we wanted to do, and he said, "Okay, well can you get us a trial run?" He said, "Here's our equipment. This thing here is a Faraday cup. It collects the charge. And this thing that's attached to it is called an electrometer, and it'll measure the charge." The idea was that we were going to send this beam in, and some of it would scatter away, and we'd see how much beam got through and calculate the cross section by knowing the density of aluminum, and the thickness, and so on. Then, he said, "Unfortunately, the electrometer is broken, and we don't have any wiring diagram." So, we had this big piece of electronics here with a bunch of wires hanging out of it. My friend, George Teeter, who was my lab partner for this, somehow, he made this thing work. I think there were still some wires hanging out the back, but it worked. So, we ran some short thing which Wilson took as evidence that we could sort of do this. So, then he gave us 24 hours of running time. So, we were feeling pretty excited about this. This was the big time for us freshmen. We had 24 hours of running time on the Harvard cyclotron. We were hot stuff. So, we set up our experiment. I think we had a bunch of pieces of aluminum for target. Oh, first we had to recruit people for shifts. We decided we weren't going to run it for 24 hours. So, we recruited roommates to run shifts, and Gene Engels to run shifts, because we had 24 hours consecutively to run this thing. We had our targets on this cart that we were going to roll into the beam line when it was our turn. The fact that we maybe we weren’t at the frontiers of physics was suggested when they rolled out the previous target, which was a monkey. Wilson had gotten interested in medical physics. But we did run our experiment, and that was a pretty neat experience for a freshman. So, that concludes part of my lab experience, I guess.

Zierler:

I wonder if that started a pattern where some of your most productive learning was not in the classroom, but it was in a lab environment.

Cahn:

Probably not. Maybe the most exciting stuff was in that kind of an environment, but I don't know that learning took place in that environment. I then did some work with Gene Engels and his partner, Lou Hand. I didn't really have much time to do it, and I didn't do much at all, but then they offered me a chance to work with them during the summer of 1965, which was the summer of my junior year. We were going to be working at Brookhaven and at Harvard at the CEA, the Cambridge Electron Accelerator. I had been working with Lou, but I decided I didn't really want to work there, I wanted to work on west coast back at home. In part, because that's home, and in part because that's where my girlfriend lived. As it turned out, we got married that summer. In any event, they arranged for me to work instead at SLAC. So, I benefited in lots of ways. One was that I wasn't in the CEA when they had a major explosion that destroyed the places. Lou Hand was injured. They had a liquid hydrogen target that exploded, and big pieces of concrete from the roof fell down. I think one person was killed, and a lot of people were injured. I could have been there, but instead I was out on the west coast. I was working for Dick Taylor, who subsequently won the Nobel Prize for the elastic electron scattering. That was a fantastic experience, too. My idea of a physicist was really somebody like Dan Kleppner, or even more, Ed Purcell. Ed Purcell was just this fabulously brilliant, marvelous, elegant person. He was just incredible. I met Dick Taylor, and Dick Taylor was like a wild man from Edmonton, Alberta. He was profane, he was big, he was loud, he was in every way the exact opposite of Ed Purcell. That was also a great experience. This was before SLAC turned on, and I was told to design beam-position monitors. Of course, I didn't know anything about anything. I did get some experience then, and it was great fun, and I wrote a SLAC technical note about being position monitors. There was another Harvard student there with me, who you may know of as well. That's Henry Frisch. So, Henry and I were both working at SLAC that summer. That was very exciting, even if I didn't understand anything. I do remember being in a room with Pief Panofsky up at the blackboard designing End Station A.

Zierler:

What was it like to be in Pief’s presence during those heady days?

Cahn:

I mean, he was one of these people that was incredible, and you could tell that he was incredible. I got a chance to work with Pief, later on at a SLAC post doc. He was one of these amazing people who treated everybody equally, even if you were an insignificant post doc, or even more insignificant undergraduate. He still treated you like he treated everybody else. Pief is clearly one of the greats of particle physics, without any doubts. There are people who win the Nobel Prize like Dick Taylor, and then there are the people up on another level, like Pief Panofsky.

Zierler:

Did you have a sense, even at that relatively inexperienced age, that Pief had a master plan, that he had a whole idea of what SLAC was going to do in his head, before it even turned on?

Cahn:

The fact is that SLAC's idea of what they were going to do turned out to be completely wrong. That's why SLAC was so great. What they thought they were doing was rather mundane. They were measuring these form factors, the kind of stuff that Bob Hofstadter was doing. Even though Bj had started to develop some ideas in a very obscure way that would be more profound, I think people were still surprised with — I'd have to go back and look at the history very carefully to see the dates of all these things to get it exactly right. Bj, and also Richard Feynman —

Zierler:

That's Bj Bjorken, just so we're all clear.

Cahn:

Yes, yes. Bj Bjorken. Both came up with these ideas that the cross sections weren't going to fall off at high Q squared. Bj started from a rather formal approach, but Feynman started from a very down to Earth approach, where he said that inside the proton there are these hard little things called partons. We all knew that was a code for quarks, but in fact, we weren't completely right. It was a code for both quarks and gluons, but we didn't have any gluons in those days. Of course, the electrons weren't going to connect directly with the gluons, but they were part of the constituents. Then, when they did the experiment, what they found, of course, was that there was a scaling relationship which was easily understood in Feynman's language, and we were scattering off of fundamental, tiny charge objects. That was truly revolutionary. In Berkeley — well, I'm maybe getting ahead of the story here. That experiment's, I guess, 1966. I graduated from Harvard in '66, and had a year traveling thanks to a fellowship that I got from Harvard. So, my wife and I spent six months traveling around Europe.

Zierler:

Were you married at that point?

Cahn:

Yeah, we got married in '65, after my junior year. Fran had become a graduate student at Harvard in the ed school.

Zierler:

This was back when undergraduates got married.

Cahn:

Not many, but some of us. It's pretty rare these days. Our kids got married in their 30s. I was 20. So, my senior year, we were living together in married student housing in Cambridge. Then we had a year off, more than half of the year traveling through Europe on this fellowship for which Harvard gave me $3000 for the year, and we had another $1000 we could kick in. We spent $4000 traveling for six months in Europe, including the flights to and from the west coast.

Zierler:

This sounds like from a different planet.

Cahn:

It was. I like to tell people that our hotel room in Paris, including the breakfast for the two of us was $3.80. Now we can't get a cup of coffee for that.

Zierler:

Let alone in Paris, right? What was Harvard's expectation? Were they just looking to create well-rounded alumni?

Cahn:

Yeah, it was that sort of thing. They had money from benefactors. There were two of these kinds of fellowships, Sheldon and Shaw. The way I looked at it, the really great students win all the Marshall fellowships, and when they were gone, I was in mind to get one of these. So, I had friends who won the Marshalls, and then I got one of these, which was like a damn good second prize. So, I started one year later at Berkeley. I started in '67 at Berkeley.

Zierler:

Bob, between your undergraduate experience at Harvard and your time at SLAC, how well defined were you in terms of the kinds of physics that you wanted to pursue as a graduate student by the time you finished at Harvard?

Cahn:

It wasn't well-defined. In particular, when I started in Berkeley, I made a conscious choice that I wanted to start out, at least, doing solid state physics. The reason, I think, was pretty simple. I have an uncle, who was really my first cousin once removed. He was a very famous solid-state physicist, and he just turned 100. He's at Stanford and he's still doing his physics. His name is Ted Geballe.

Zierler:

Oh, wow! That's so great. I did know that you were related. I talked to him.

Cahn:

Okay, well, Ted was my mother's first cousin.

Zierler:

He is amazing. He is an amazing person.

Cahn:

He is absolutely amazing.

Zierler:

I got this very strong sense that he's still quite active.

Cahn:

Absolutely. I visit him, we talk physics, and he complains about referees. "These people weren't really understanding this quantity U. They're just wrong." He's always been absolutely amazing. He was an amazing guy. His brother, Ron, was also a physicist, and was a chairman of the physics department at the University of Washington for like 25 years. Yeah, Ted is quite amazing. Really, because Ted was a great solid-state physicist, I felt I really should learn about solid state physics. In a certain sense, I would say that I was sort of consciously staying away from particle physics, but I didn't try very hard. On the other hand, I did join the solid-state group at Berkeley that was headed by Charlie Kittel. He was always extremely kind to me. He gave me a summer job, probably the first year after I started graduate school. I helped make some figures and stuff for one of his fantastic books. Charlie was another one of the great teachers. I've had many of the greatest teachers in physics. Dan Kleppner I would count as one, absolutely Ed Purcell, Charlie Kittel, Dave Jackson, Gene Commins, less well-known but also fantastic, Eyvind Wichmann. These guys were all incredible. All the ones that I mentioned at Berkeley all won the Outstanding Teacher Award at Berkeley. These were amazing, amazing teachers. I just was really lucky to have all of them as teachers, and as people I got to know very well. I didn't get to know Purcell so well, but well enough so he wrote me a letter of recommendation when I graduated school, but the others became serious friends.

Zierler:

Before we get to your time at Berkeley, I want to ask about some of your developing political interests while you were an undergraduate. Now, 1966, when you graduated, was still relatively early in all of the movements that were to come later in the decade —

Cahn:

That's not quite true. It was very early in Cambridge, but it was not early in Berkeley.

Zierler:

Right, exactly. That's right.

Cahn:

I like to characterize the level of consciousness at Harvard by saying I remember seeing a march in support of the students at Berkeley, and they had a big banner with Berkeley misspelled. I think that was the level of consciousness at Harvard. I was not involved at all in politics at Harvard. I had been sort of political in high school. Valedictorian, I gave a speech that probably offended some people. I can remember going to a rally where we could heckle Richard Nixon, who was running for governor. But not at Harvard. Frankly, I was too busy working. I really worked hard as a student.

Zierler:

Right. That's exactly my question. I was curious if, since Harvard was certainly behind Berkeley, how engaged you were. Was the draft an item of consideration for you?

Cahn:

It was briefly, but it turned out that they passed a regulation saying that if you had been married before something like August 26, 1965, you were exempt, and I was married August 22nd. I know the exact date of when we were married, and where the cutoff was, I don't know, but it was like a week. I was okay, but my friends in Berkeley weren't, and there were people who went to Canada. When I got to Berkeley, I guess I was probably vaguely anti-war already. I had come back from our travels in February of '67, and by then things were really heating up. But I was pretty shocked when I got to Berkeley. If you went to a rally in Berkeley in 1967, some of them you could call anti-war rallies. I guess people call them anti-war rallies, but there were plenty of rallies where they didn't want peace now. They wanted an NLF victory. It wasn't that we were fighting the wrong war, it was we were on the wrong side. That was Berkeley in 1967. There were plenty of faculty who felt that way and were just about as radical as the students.

Zierler:

I'm curious, specifically, the physics department. Was there a lot of concern about the way that physics was being used by the Pentagon?

Cahn:

Is this an innocent question, or do you know about it?

Zierler:

It is an innocent question.

Cahn:

Okay, well, yes of course. In particular, one member of the factually with whom I became extremely close, Charlie Schwartz. Do you know about Charlie Schwartz?

Zierler:

I don't.

Cahn:

Oh, okay. Charlie was a theoretical physicist particularly in nuclear and atomic physics. Charlie's political awakening happened around the time that I tookquantum mechanics from him. This was about the time that we became aware of Andrei Sakharov's book on coexistence. I have a copy of that up here which I can bring out.

Zierler:

Oh, wow.

Cahn:

I'll show you the next page.

Zierler:

Oh, wow.

Cahn:

That's Sakharov's inscription.

Zierler:

Very cool.

Cahn:

We'll get to that eventually. So, Charlie became very political. At that time, that was my first year of graduate school, so this was '67-'68. I'm joining the solid-state group around that time. I worked with Marv Cohen, who you know about, I'm sure. I even wrote a band structure paper with Marvin. But then, my second year of graduate school, I took Dave Jackson's particle physics course and that was the end of my career in solid state physics. I was seduced.

Zierler:

So, Bob, just to be clear, it was never condensed matter in those days, it was always solid state.

Cahn:

It was called solid state. I think, typically, it was called that. I think Charlie Kittel’s advanced book is called something like Solid State Physics, maybe because they were mostly talking about the solid state as opposed to liquid state, which is also condensed. So, eventually, I asked whether Dave would take me on as a student. He had two real students ahead of me. One was Chris Quigg, and the other was Rick Field. I don't know whether you've talked to Rick.

Zierler:

I have not, no.

Cahn:

Do you know anything about Rick?

Zierler:

I don't.

Cahn:

Okay, I'll just tell you one thing to stimulate you. So, Rick and I were grad students together. Rick, I guess, was the same year I was as a grad student, and we were sharing an office. Rick says to me, "Hey, my sister gave me this really great birthday present. It's a color TV!" I was astonished. The idea that a graduate student had a color tv was just — we were really lucky we had a black and white tv. I said, "What is it your sister does?" He says, "Well, she's in these kids' TV shows." I said, "Like what?" And he said, "The Flying Nun." I said, "That's not a kids' show." So, Rick is Sally Field's brother.

Zierler:

Oh, okay.

Cahn:

Rick retired as a theoretical physicist at the University of Florida a couple of years ago. So, Chris, Rick, and I were the three Jackson students, basically, at Berkeley. Chris got out in '70, Rick in '71, and I got out in '72.

Zierler:

What was the transition to Jackson and away from solid state for you? How did that work out?

Cahn:

How did it happen, or what do you mean, how did it work out?

Zierler:

Yeah, how did it happen? How did you realize that was the area that you wanted to pursue?

Cahn:

It was just much more exciting to me. I was one of those people who wanted to understand the secrets of the universe. That's what I've been able to look at for the last 50 years.

Zierler:

What was Jackson working on at that point?

Cahn:

Dave was doing things mostly with Regge theory. I don't know whether that means anything to you.

Zierler:

It does.

Cahn:

That was a fad that lasted until just about that time. At the time I wrote my thesis, I didn't really work with Dave on the thesis. If I worked with anybody, it was with one of the post docs, MartyEinhorn. When I look back at it, it was an incredibly boring subject that we were — my thesis was unbelievably boring. If I showed it to somebody today, they'd say, "Why did you do that?" We didn't know any better. Especially, Berkeley — this perhaps is an important aspect of the history. The dominant figure in Berkeley theoretical physics was Geoff Chew. Geoff was a wonderful, wonderful person whom we all respected enormously. He was a deep thinker and he had his own view of how particle physics should work out. You can look at it as saying that it was completely wrong headed, or it was visionary. You could look at what Geoff was doing in either way. Geoff didn't believe in quarks. He didn't believe in field theory. He felt that field theory was doomed because we all knew that strong interactions were strong, and perturbation theory was never going to work. So, he tried to develop an approach to strong interactions. He didn't care about electromagnetism. That was all done. Strong interactions had to be based on a few basic principles. Take properly, those principles would uniquely determine the result. That's what he believed. Initially, those principles were very few, like analyticity, unitarity,Lorentz variance — a few things. In developing this, he ran into a young man who had come up with some extremely profound way of looking at analyticity issues of dispersion relations. That person was Stanley Mandelstam. Geoff recognized how extraordinary Stanley was and brought him to Berkeley. People made efforts to try to develop a theory that really just came out of these principles. Perhaps the best phrase that came out of all this work was "nuclear democracy," by which Geoff meant there were no fundamental particles. All particles were equivalently fundamental. It was a very profound way of looking at things. The great irony is that having rejected field theory, rejected quarks, his students, especially people like David Gross, went on to develop string theory. It carried through some of the same ideas that it wasn't all field theory, but there were basic things — I'm far from a string theorist, but string theory was just beginning at the time when I was a grad student. It wasn't called string theory. One had the Veneziano model. Have you heard of the Veneziano model?

Zierler:

Sure.

Cahn:

Good. So, Gabriele Veneziano wrote down this model which, let's put it this way, it almost satisfied all of the requirements that Geoff had laid down for what a theory had to do. He wrote down this equation — it's a gamma function times a gamma function divided by a gamma function. Gabriele said that's the answer, and people went crazy. Ultimately, that all developed into string theory. A lot of the basic work was done by post docs at Berkeley. At the time that I was a grad student, we had some incredible post docs, especially Michael Green from Green and Schwarz, and Miguel Virasoro, well known for the Virasoro algebra. These guys were fantastic. Then, Geoff wasn't doing this, but Stanley was. Stanley's students were also fantastic. Charles Thorn was, I think, the greatest student around in theory when I was at Berkeley. Also, Michio Kaku, who you probably know of. These guys were really fantastic. String theory was sort of growing at Berkeley, but there still wasn't any traditional field theory. Stanley was off doing string theory, Dave was doing Regge theory.

Zierler:

Bob, where are you in all of this? It sounds like with string theory you have a front row seat, but you're not directly involved yourself.

Cahn:

I was Dave's student. Dave isn't doing string theory, so what's Dave doing? Well, he was doing Regge theory. What I did in my thesis was related to Regge theory. It was something that Al Mueller had developed, which related to using Regge theory in a particular way, but it wasn't a real Regge analysis. As a Jackson student, you learn lots of different things. Of course, I continued to be a Jackson student for the next 50 years, almost.

Zierler:

What was Jackson's style as a mentor? Was he hands-on or hands-off, would you say? In other words, if you had a problem and you hit a wall, would you go to him and he would help you out, or you would work on it yourself?

Cahn:

No. Work on it yourself. The problem is if you went to him, he'd solve it right away and that would take away all the fun. My wife used to say to me, "How can you tell whether you're doing well? Dave never says. He never gives you any feedback or pats you on the back." I said, "Well, you can tell." He wasn't the kind of guy to constantly praise you. He had very high standards and you had to be able to understand him without him saying things. He expected a lot of people. I think that's not something that's known just to those of us who were his students. It's known to many thousands of students who have had to work their way through some part of his classical electrodynamics, a book that's famous and notorious. The problems in it are really exceptionally difficult, to put it mildly. He was a fabulous calculator with a very, very deep understanding of physics. We can get into that more if you want. Dave was certainly one of my closest friends for the rest of his lifetime.

Zierler:

Did you develop that friendship status during your time as a graduate student, or more afterwards?

Cahn:

More afterwards, but when I finished my thesis, I went backpacking with him, Marty Einhorn, and Dave's oldest daughter, with whom I'm still in communication, Nan Jackson. Nan just mailed me a collection of Dave's binders about this high of all the course notes from when he taught quantum mechanics, and a bunch of his incidental calculations over the last ten or fifteen years.

Zierler:

Oh, wow. Does he have a manuscript collection somewhere?

Cahn:

42 cartons of books in the University of California library.

Zierler:

Oh, wow.

Cahn:

So, if you're looking for something — I suspect I'm the only person who's ever consulted it. I went to it when I was writing his biographical memoir for the National Academy of Science. Do you know about the biographical memoirs?

Zierler:

I do, yeah.

Cahn:

Okay, so I wrote the one for Dave, and I'm in the process now with some of my colleagues of writing one for George Trilling.

Zierler:

Ah, right. Just for the record, he recently passed.

Cahn:

Yes, right. There's one incident I forgot to mention from my Harvard years which may connect with someone you know. I told you that I TA'd for Ed Purcell's class in the laboratory. There weren't many women taking physics at Harvard in those days, but there was a rather striking, tall blonde in that course. Her name is Kate Kirby, Executive Director of the APS. That was kind of a funny coincidence. I met her when she was a freshman and I was a junior, I guess. Something like that.

Zierler:

Bob, who was on your thesis committee?

Cahn:

My thesis committee was a chemist named Bob Harris, Gerson Goldhaber, Dave Jackson, and Mahiko Suzuki.

Zierler:

And Gerson you would go on to coauthor a book.

Cahn:

Right.

Zierler:

Why did you have a chemist on your committee?

Cahn:

You always had to have someone from outside your department, and I must have known Bob from political things. I still see him occasionally at concerts in Berkeley. There are many other stories about him. Of course, I vividly remember — this is the oral exam. I remember every moment of it. Bob asked a kind of obscure question, and I didn't really know a good answer to it. I said, "That's an interesting question." It had to do with Regge stuff. "That's an interesting question. What do you think about that, Geoff?" Geoff Chew was on the committee, too. I deflected the question and asked Geoff to think about it. So, I got away with that one. Mahiko asked me a bunch of questions about SU(3). I did all of them right. He tracked me on one because it turned out not really about SU(3). Everybody got a laugh when I got it wrong. Then, I thought I'd gotten through the whole exam and it wasn't so bad. The chair of the committee, of course, was my thesis advisor, Jackson. He turned around and said, "Does anybody have any more questions? This has been pretty short." And nobody had anything, and he said, "Well, I have some." He reaches into his jacket and pulls out a sheath of papers and starts grilling me. So, that went on for quite a while.

Zierler:

And that was a surprise to you. You didn't see this coming.

Cahn:

I should've because he was on my prelim exam. The prelim exam at Berkeley, when you start as a graduate student, I don't know if it works exactly this way now, but 50 years ago, you had to take a series of written exams that went on for like two or three days, and if you did okay on those, then you got to take an oral exam. My oral exam committee had Sherwood Parker, maybe Carson Jeffrey. I've forgotten them all, but one I do remember is this new guy who had just arrived at Berkeley, Dave Jackson. I just figured, well, he's new. He's not going to ask any questions. I can tell you every question that he asked. It's a long story, but that was my introduction to Dave Jackson. He had devastating questions where he wanted you to use basic physics. My Harvard education told me to write down a Lagrangian, which was a very bad idea. It was quite an excruciating experience, so I should have anticipated it.

Zierler:

You survived.

Cahn:

I survived, and I did better on the candidacy exam than I did on the prelim. That was pretty tough. So, you probably don't want to finish up Berkeley without going back to the politics.

Zierler:

Of course. That's exactly where I wanted to go. You said at Harvard you were working too hard to pay attention, but I imagine at Berkeley, you were working plenty hard, but you found the bandwidth to become more politically engaged.

Cahn:

Well, you couldn't avoid politics in Berkeley at that time. It's really hard to describe what it was like.

Zierler:

Were you marching? Were you actively taking part in protests?

Cahn:

Oh, yeah. Oh, yeah. There were a number of things. In '69, I think — I have a hard time getting each one of these events.

Zierler:

Are you thinking of the bombing of Cambodia?

Cahn:

That was later. That was what, '71 or '70?

Zierler:

The bombing of Cambodia was in '70, I believe.

Cahn:

Just to give you an idea of what Berkeley politics was like, that resulted in moving Berkeley to the right. Why did that happen? It happened because that was so outrageous that it mobilized a large fraction of the students. That moved the political center to the right of where it began. I mean, the right was still "get out of Vietnam," but the political activity in Berkeley was way to the left of that group. I got involved in a variety of things. One thing is I got involved through Charlie Schwartz — this is going to go on forever. There was a group called SESPA, Scientists and Engineers for Social and Political Action. I think it was started at MIT. A lot of this grew out of a Scientists' Day of Concern, which must have been in '69. Somehow, I was already involved enough, so I was going to be a speaker at an event in Pauley Ballroom, one of the main places on campus. I'm not going to get all the chronology right, but that even was unforgettable because before I got up to give my speech — there was some series of speakers — a bunch of Black Panthers marched in and took over. So, that was fairly memorable. I guess, I got involved in politics initially through Charlie Schwartz, and the group that was involved in — there was concern about what was going on at the Livermore Lab. Also, maybe even before that, there was a battle over free speech at the lab at LBL. Have you ever heard anything about that?

Zierler:

I have not, no.

Cahn:

Okay. So, what happened was that Charlie had wanted to use the auditorium at the lab as a place to have noontime talks on political subjects. The director of the lab, Ed McMillan, didn't want it. The reason was that Ed was afraid that it would lead to strife at the lab, just as there was strife on the campus. He didn't want that. In particular, he felt that a lot of people in the engineering area of the lab would not be sympathetic to the leftwing politics of grad students, and maybe even some of the faculty. So, he forbid these talks. Charlie tried to run these talks anyway. This ultimately led to Ed taking away Charlie's summertime employment at the lab. So, that was one part of it. One part of it was we can't have the lunchtime talks, one part is that Charlie is fired from his summertime job, and this led to enormous strife at the lab. You can go look it up in Science Magazine and New York Times.

Zierler:

Just to get a sense, at that juncture, how much of an outlier was Charlie among the faculty in terms of his activism?

Cahn:

It depends upon which particular thing you're talking about. When he asks students to sign a pledge that they wouldn't use their science for bad reasons, he was a big outlier. People were angry because loyalty oaths at Berkeley has a very special meaning.

Zierler:

But that's the right kind of loyalty oath, though, right?

Cahn:

That's what he would have said, but they said, "No loyalty oaths." When it came to anti-war stuff, probably Charlie wasn't much more radical than anybody else. I mean, he was more radical than some people. But this battle developed into an enormous battle within the laboratory. Some of it was aligned on political lines, and some of it was aligned on other lines. For example, there were some big personalities at the lab. Ed wasn't a really big personality. He was a great scientist, and a not very active director of the laboratory. But a really dynamic person, and somebody who I was told was called "the one genius in Berkeley," was Luis Alvarez. Luis was very right wing, but he hated Ed because he thought that he should have been the director instead of Ed. So, Luis thought our leftwing group really was great. Anything that was fighting Ed McMillan was okay by him. This reminds me of another story. When I'm a graduate student, one night I get a phone call from Luis Alvarez. I'm not in Luis's group. Why am I getting a phone call from Louie Alvarez? Luis says, "Are you willing to run for the executive council of the APS?" Of course, I had no idea what the executive council of the APS was. So, I said, "Well, okay." He explained a little bit, but I'm not sure I was a member of the APS, but I said, "Okay." They decided they needed a student on the executive committee. So, I said yes, and then I realized he had done this just to tweak at McMillan's nose. I was fighting with Ed, and Ed wanted to think of me as some crazy leftwing character, and then Luis gets me nominated to be on the executive committee of the APS. It was just his way of tweaking Ed's nose. So, that was just an example of how these things got mixed up between leftwing/rightwing politics on one hand, and battles within the lab. So, this battle over whether we could have lunchtime speakers on political topics went on for a number of years. The event that really made it big time occurred in the following way: the lab, which was really a very progressive place, and in the end, Ed was a good, liberal person whose politics probably weren't all that different from what mine is today. They had a program where they would have lunchtime speakers form the community, what today we would call diversity. So, they had had two speakers come in who were black radicals. They were really radicals. This was Berkeley in 1969. When I say radical, I mean, these guys were radical. They were running for city council. I said to one of my friends, "Hey, we've got them. He's allowed these political talks in the auditorium. We got them." He wouldn't let us have ours, so I phoned up the ACLU and said, "Hey, there's a situation. Will you take it?" And they said, "Yes." So, as a grad student together with another grad student, and a woman named Lonnie Hancock, who subsequently became the mayor of Berkeley later, another graduate student, not in physics, we sued the director of the laboratory in federal district court and won. So, this did not endear me to lab management very much. So, then, Ed's response to that was that, of course, he shut down all talks. Then we couldn't complain that we were discriminated against because there weren't any talks. Then there was a long battle that went on for years, and finally we won this thing. The first committee that was set up just said we shouldn't have these talks, and that didn't settle it and we continued to fight. Eventually, there was a vote in the faculty about this. The faculty voted on our side, that we should have these talks. Dave Jackson, after that faculty meeting, left a note on my desk listing all the various things I had done to outrage the lab. For example, I appeared at something that was called the War Crimes Tribunal. One of the events that was held by radical students and other was the War Crimes Tribunal. What was the tribunal for? It was for World War III. Since there wasn't going to be any tribunal after World War III, we better hold that tribunal now. This is all coming back to me. So, you come into whatever auditorium they'd rented for the War Crimes Tribunal, and somehow, I was supposed to be one of the speakers. You'd see around the room flags of the liberated countries of the world, like the Pathet Lao, and every revolutionary communist group you could think of, we had their flags around us. They're not exactly a neutral forum for our tribunal, which was supposed to be a neutral thing. So, I gave my speech, and I've forgotten what I said and probably don't have a copy of it. Afterwards, somebody gets up and says, "We've heard enough of this talk. Let's go march on Edward Teller's house." So, I got up and said, "Well, you know, this is a bit more serious than marching on Edward Teller's house." I tried to convince them that was a really stupid idea, and if they really cared about it, they should be organizing and not marching on Edward Teller's house. But they did march, and I guess they got stopped by police, or whatever. If you look in some of the books about Teller, you will see that stuff is misquoted. They conflated my talk with somebody afterwards, and basically said, "Ignore that guy Cahn. We're going to march on Teller's house." Ed McMillan was not happy about the idea that people were marching on Teller's house when they could march on McMillan's house. So, that was one of the many things I had been involved in that outraged Ed. Anyway, Dave's note, after this meeting with the faculty, which we won, it was something like Bob Cahn at the War Crimes Tribunal, and Bob Cahn at this, and Bob Cahn at that, nearly lost today, but there was a vote. So, I was so happy I pinned this up over my desk. The next day, I come back, and I see it's turned over. On the back, Dave has written, "What kind of conspirator are you anyway? Destroy all evidence." I still have this note. So, it was kind of like that when I was a graduate student and then — all these stories come back. At the end of my graduate career, at the time, whatever papers you generated at the lab, the preprints were physical objects in those days, and had to have a blue cover showing the LBL logo, and so on. Everybody's thesis got published as this preprint, and then mailed out all over the country and internationally. So, my thesis — I was pretty sure I wasn't going to get away with this — I wrote a preface saying anybody who has the technical background to read this thesis should make sure that our technological powers are used — I said something stronger about while I was writing this thesis, we were waging a war of aggression against the peoples of Southeast Asia. If you have the technical knowledge to read this, you should make sure technology doesn't get used there. I was pretty sure the lab was not going to print that preface. I kept waiting for this phone call saying, "No way are we printing this preface." And that phone call never tame. At the time, Dave was actually spending maybe a whole semester at Fermilab, which was just starting up in '72. So, finally, a big box of these preprints comes. It's my thesis! I open up, and the preface is gone. In the table of contents, the line for the preface is gone. So, I phoned up Dave Jackson. Dave was really angry. If they had said they weren't going to print it, okay. I couldn't argue with that. That was legitimate, I think, on their part to say there's no way they were going to print this. But to do it without asking — so, Dave phones Ed McMillan, and says, "You owe Bob Cahn a written apology." Ed McMillan is this Nobel Prize winner — fantastic, great physicist. All kinds of wonderful. And I'm this puny little graduate student. So, I get a phone call from McMillan. He says, "Hello Bob. This is Ed McMillan. Dave says I owe you a written apology. I don't think it has to be written. Do you?" Obviously, I wasn't going to disagree. He said, "I want you to know this was not a policy decision," by which he meant that he was not the one who made that decision. I believed that. I said, "Oh, well, in that case, you'll be willing to issue an erratum." He said, "I don't think we're going to go that far." So, like a week later, I get a big envelope. What's in the envelope are all the mailing labels that they used to send out the preprints. They said I can do whatever I want with the mailing labels. So, I made up a little sheet that had the preface that they removed on one side, and the other side explained what had happened. I mailed it out to all the places they sent the preprint, and that is how I left the lab in Berkeley at the end of my graduate student career. One of my friends, Willi Chinowsky — do you know who Willie is?

Zierler:

No.

Cahn:

Willie is now a 91-year-old high energy experimenter. He said to me, "They're never going to let you back." But eventually they did. Anyway, it was quite an interesting time to be in Berkeley. Another recollection from that time was during all this political activity, meeting with different groups of people who were much more radical than I was. I remember one day we were going from one meeting to another, and I'd forgotten what we were planning, but I'm in this car, and I said to myself, "This is not smart." The person driving the car was Eldridge Cleaver.

Zierler:

Wow. How did you find yourself in this car? What led to that?

Cahn:

I don't know. Meetings we were going to organizing some demonstration. I could figure out what it was but let me just say that at the meeting it was clear that among the people who were at the meeting, Cleaver was certainly the most conservative. This is something called 139 X. The campus, in order to mollify the students — this is again 1969. I don't know what year. I have to look this stuff up — they decided they would allow students to invent courses that would count as real courses, if they had real content. So, the radical students got together, and they formulated a course to be taught by Eldridge Cleaver. This drove the regents — University of California is overseen by something called University of California Regents, Ronald Reagan's people. He was the governor. And they went completely bananas. So, Cleaver taught this course on racism, or whatever. He was a good speaker, but it drove the regents absolutely crazy. It was probably in connection with that. It was called 139 X. "139 X, On Campus, For Credit, As Planned." That was his slogan for the time. There are many other stories, but that'll give you some sense of what it was like to be in Berkeley as a grad student from '67 to '72.

Zierler:

Did you find it difficult to maintain your focus on physics with all of these other activities?

Cahn:

I didn't feel that way. It was probably true. I probably should have devoted myself more to my physics, but I was making progress. I wrote a thesis. I got just the post doc position I wanted, so it seemed to have been okay.

Zierler:

So, you wanted to go to SLAC. That was the objective.

Cahn:

When I was finishing my thesis, Dave said, "Where would you like to go as a post doc?" I said, "I'd like to go to SLAC." As far as I know, I never wrote an application or anything.

Zierler:

Had you been following the developments there over the past two or three years?

Cahn:

More or less, but probably less. I mean, yeah, but not as well as I should have. I'll get to that in a minute. So, sometime later I get a phone call from Sid Drell, and Sid says, "We're going to offer you a post doc." I said, "I accept." He said, "Well, don't you want to think about it?" I said, "No." That's what I wanted to do. So, I remember I went down to SLAC to visit, I guess, shortly after that. This is the early spring, maybe even fall, but probably early spring '72, and I go down and have lunch with Sid and a bunch of people from the theory group. One of them is a guy named Pierre, whom you will not have heard of. Pierre was a nuclear physicist, did the three body problem, and had become an extremely radical politically. So, I knew of Pierre, and he knew of me. Sid is introducing me to everybody at the table and introducing me to Pierre. Pierre sticks out his hand with some kind of movement, handshake, or something, that I'm supposed to recognize. Sid was horrified. Sid was probably a moderate democrat and way to the right of all the students and graduate students and post docs. Sid must have wondered what he had gotten himself into. SLAC had been pretty active politically, especially Bob Jaffe. Do you know Bob Jaffe?

Zierler:

I interviewed him.

Cahn:

Okay. So, Bob was Sid's student the way I was Dave's student. We knew each other quite well. I haven't seen him often in the last years, but we were friends. He was a great guy. Great physicist, and he's done some really good things. He was quite politically active by Stanford standards, anyway. Not Berkeley standards, by any means.

Zierler:

Different world in Palo Alto.

Cahn:

Yeah. So, I moved out to SLAC actually before I finished my thesis and commuted back and forth when he sometimes worked down at Stanford. In those days, it was the "old boy" network. I don't know that any letter of recommendation or anything was ever written. Somehow, I told Dave that I wanted to go to SLAC, and Sid called me up. So, that was pretty nice. SLAC was the center of the universe. It became more the center of the universe after I left, but it was already a fantastic place.

Zierler:

In what projects did you get involved when you arrived at SLAC?

Cahn:

Well, I was a theorist, so there isn't a project. What happened was, I told you, in Berkeley, we learned electrodynamics, and we knew Feynman diagrams, but nobody talked about even elastic scattering. Absolutely nobody. There were some outside people who had talked about quarks. We had a visitor, Gordon Moorhouse, who talked about quarks and was really good. But Stanley didn't really talk about quarks at that time, I don't think. Dave didn't talk about them, and certainly Geoff thought they were anabomination. So, when I got to SLAC, it was all about quarks. One of my very good friends as a post doc, and somebody else you may know, Bill Colglazier —

Zierler:

No, I don't know that name.

Cahn:

Okay, Bill Colglazier was a post doc with me, and he went on to a very distinguished career in science policy. He was the executive director of the national research council, the NRC. He served with the state department. He's still active in D.C. You should meet him. His physics career didn't go on for very long, but he had an extremely distinguished career in public policy. Fantastic guy. He and I were very close friends, and his wife, Cathy. Anyway, he must have handed me Feynman's book, Photon Hadron Interactions. So, that's what I learned from, and quickly got involved in all the issues surrounding both deep inelastic scattering, and e+e- annihilation.

Zierler:

Did you meet Feynman when he would come to visit SLAC?

Cahn:

Okay, Feynman stories. Time for Feynman stories.

Zierler:

Sure.

Cahn:

I don't think I met him then. My first talk as a post doc I gave at Caltech. At a certain point in the talk, I said, "Here are three reasons for making this assumption." And Feynman says, "If you had one good reason, you wouldn't give us three." And I thought quickly and said, "You're right," and went on with the talk. Then, sometime later, I heard Feynman. They used to have annual meetings at Irvine in maybe December or January, and Feynman would come to them. There was one meeting where somebody's giving a talk and they say, "Here are five reasons for," and Feynman says, "If you had one good reason, you wouldn't give us five." So, I realized this was a set piece with Feynman, and I didn't feel so bad about it. So, I met Feynman, I guess, when I went to give this talk in Caltech. I met him maybe one or two times besides that. One time I was walking across the campus with Feynman, and I said to him, "Oh, I'm supposed to say hello to you from one of my mother's friends." I suddenly realized I only knew her married name, and she had known Feynman when they were both college age. She wasn't married. I said, "I don't know her maiden name. It's Irene." And he said, "Oh, Irene Levine." So, he remembered this friend of my mother's and I was impressed. There was this story which Irene claimed wasn't really true, but I think it may have been, that she was at a dinner party, and she was seated next to — I don't know whether it was Leonard Schiff or Felix Bloch, one of the people from the Stanford physics department. She learned that this person's a physicist, and she said, "Oh, I guess you don't know I used to date this fellow who was studying physics. I wonder how he's doing." Bloch says, "Oh, what was his name?" She says, "Dickie Feynman." Bloch says, "He's doing okay." At least I was able to confirm that it was true, that Dick Feynman certainly knew Irene. If I remember correctly, her sister was dating Lennie Bernstein, so it's a great story if it's true. Some of the work that I was doing with Bill Colglazier was relevant to some issues Feynman was interested in, and his trying to understand how the parton model works. I'd had the opportunity to talk to him about it, so that was really exciting. Feynman was Feynman, but I didn't know him well, certainly. The person who knew him well was Rick Field. Rick was on many papers called Feynman-Field. I told you about Rick's sister. What I didn't tell you was that Rick came to Berkeley as an undergraduate on an athletic scholarship and was a gymnastics champion. When he graduated from Berkeley, he was selected as the Athlete of the Year. Rick's family was a Hollywood family. Not just his sister being extraordinarily famous, but his stepfather was a stuntman. If you haven't read Sally Field’s autobiography, it's an outstanding book and you should read it. You'll learn about Rick in it, okay? If you go on Google, and you look up Sally Field and Higgs boson, you can find Sally Field trying to explain the Higgs boson to David Letterman. So, Rick started doing gymnastics in high school, and that was how he got this athletic scholarship to Berkeley. The story is that basically Rick had two choices in life. One was the direction he went, which was become a theoretical physicist who ended up working with Richard Feynman, and the other was to play Tarzan. He was this gymnast who could certainly do all these things, and he came from this Hollywood family. Instead of becoming Tarzan, Rick became a theoretical physicist. I like to tell that story. Rick is somebody who's also worth interviewing. He was very involved in LHC work, and even though he's a theorist, he's a member of the CMS collaboration. He would definitely have some interesting stories to tell, I think.

Zierler:

Obviously.

Cahn:

Extremely down to Earth guy. Absolutely. We met his sister back when we were graduate students. We had dinner one time with Rick and his sister. She wasn't as famous as she is now. She was already kind of famous. We've been to her house in Aspen once when she wasn't there.

Zierler:

Bob, was your time at SLAC a limited term appointment? Could you have stayed on longer if you had wanted to?

Cahn:

Well, I didn't stay on as long as I could have, but it was just a regular post doc. It probably was two year. I don't know whether it was two year or three year. I didn't stay the whole time because partway through it — I went in the spring of '72 even though my position there didn't really start until the fall. It turned out to be convenient to move. I started getting offers of slightly higher-level positions. I interviewed for a job at Harvard which I didn't get, and I think they really just interviewed me because they had some money left at the end of the year. The job went to Alvaro De Rújula. He participated in very famous papers there after the discovery of Psi. I was interviewed at Carnegie Mellon by Lincoln Wolfenstein, and he worked in Washington. I went to University of Washington with some slightly fancier title. The exact title I probably can't remember. It was less than a faculty position but more than a post doc.

Zierler:

I could answer that for you right now. Research assistant professor.

Cahn:

Okay, yeah, that sounds right. It was not a real faculty job, and in the end, I didn't stay because it wasn't a real faculty job, and I got offered a real faculty job at Michigan.

Zierler:

Bob, before we leave SLAC, really quick, in 1973, did you have a sense that the November Revolution the following year was coming, or did that happen too quickly?

Cahn:

I don't think anyone had that sense. There were people who just before came out. David Politzer had his idea of what charmonium would look like, and there was the search for charm paper by Mary K, and Ben Lee, and Jon Rosner. But nobody really understood how narrow it would be. No. The answer is no. Absolutely not.

Zierler:

What were the big questions that you recall when you were there?

Cahn:

I think trying to understand the parton model and trying to use that. This was while QCD was being developed. I wasn't really thinking about QCD, I'll admit. The gluon idea was not something I was working with, and I don't know whether it was anything that anyone was working with there at SLAC. That's a little bit later. Just barely later. You can say, "Okay, here's the date of the paper." But it takes a while before these things catch on. So, I think we were still mostly working in this — well, there were other people working in more formal ways. Maybe they were using gluons. People were using more field theory. Yitzhak Frishman from Weizmann was spending time at SLAC. There was a very close connection between SLAC and the Israeli physicists. I got to know many of them. We formed many of our close friendships that we have today when we were at SLAC. At 5:00, I'm going to be talking to Fred Gilman.

Zierler:

Oh, I talked to Fred Gilman for hours and hours and hours.

Cahn:

Okay. Well, we're close social friends with Fred and Barbara, and we're going to talk to them at 5:00. So, that was a really close friendship that we formed at that time. There were others. That's the closest of them. We've maintained this friendship, starting in 1972. That's pretty long.

Zierler:

Bob, were you writing papers during this time, or was this more just about soaking it all in at SLAC?

Cahn:

Oh, no. I was writing papers. Maybe not great papers. When I was at SLAC, there was some stuff I did with John Ellis. We were trying to understand why the e+e- cross section was rising, which wasn't understood. We understood that it could happen with more quarks. We were talking quarks, and there's a paper John and I wrote that's some APS proceedings where we were trying to figure this out. We talked about charm and things like that, but we didn't say you're going to find this fantastic spike at 3.091, or whatever it is these days. So, yeah, we were thinking about these problems of the quark model, and charm. Those things were in the air. Shelly was going around pushing charm. But people were still stunned. Psi was the great event of our lifetime in physics. Even more than the Higgs. It was the great event. I can describe exactly how I found out about it.

Zierler:

Please do. Where were you when it came out?

Cahn:

I was a post doc in Seattle. I had actually been working — I had a friend who passed away, Paul Mockett, experimenter. Our families are very close. There was some anomalous data in proton antiproton collisions at very low energy, a peak. One of the questions that I was thinking about was could that peak that we now call charmonium, could that be the bound state? It's at 1.9 GeV. I had a whole binder full of thoughts about this, and calculations. After learned about the Psi, I never opened that binder again. But the way I found out was I came in in the morning, November 12th, I guess, 1974. There was a note on my desk taken by our wonderful group secretary, Janey Spaeth. The note said, "Bob, call me or call SLAC." The note she had taken down was from Dave Jackson. So, I thought about it a minute and decided most likely this was coming out of SLAC. So, I phoned Sharon Jenson, the great theory secretary at SLAC, who was extraordinary, and everybody loved her. Anyway, I called Sharon and I said, "Dave said I should call SLAC." Sharon said, "I think you want to talk to Haim Harari." You know who Haim Harari is?

Zierler:

Sure.

Cahn:

You do? Okay. So, I talked with Haim and he told me what had happened. So, that's how I found out.

Zierler:

Did you regret a little bit that you weren't on the scene?

Cahn:

I don't think I felt that way. I felt I was in close contact with what was happening, and I still have Dave's contemporaneous notes from that weekend. Dave was told by the experimenters what was happening, and he went down to SLAC. He worked all through Saturday and Sunday calculating what was going on. This is another story. So, the width of the — I'm going to get a little bit technical here. The width of the resonance that SLAC saw was like 2 MeV. But the energy of each beam, the beams weren't monochromatic. They had a certain energy spread themselves. Basically, the width of the observe peak was through the energy spread. Dave realized that he had enough information to figure out what the real width of this was. This is a perfect example of knowing fundamentals. Resonances are covered by a Breit-Wigner shape. That's the way the world is. Knowing that and knowing the other facts that you have at this disposal, Dave figured out what the actual width was, which was not 2 MeV. It was a little less than 100 KeV, less that one twentieth of what SLAC was measuring. He could not convince the experimenters on the paper to state this fact. He was always very annoyed. I think he claimed the SLAC theorists wouldn't accept his argument. He was very annoyed that they couldn't understand a Breit-Wigner. Dave was very big on fundamentals. Anyway, that's a story that's persisted. I have all of Dave's notes in which he calculates all of this stuff. It's extraordinary. So, I was talking to Dave, and of course, this was before email, so we're all talking to everybody. The community went crazy. I think it was Marty Einhorn and Chris Quigg — Chris may have mentioned this. They wrote a paper in which they talked about a new particle. It wasn't charmonium. It was made of pandas and was called pandemonium. That's what it was like. Then, no sooner had we gotten used to this than SLAC discovered Psi-prime. We had a speaker come and give us a talk and see how — I'm not sure whether I knew her before she came. That was Vera. Do you know Vera?

Zierler:

Of course.

Cahn:

Okay. So, Vera came and gave a talk. I believe that the department offered her a job on the spot. That's my recollection.

Zierler:

That sounds quite plausible.

Cahn:

Vera is a very close friend over all these years. I got to know her at that time, and we must have gone down to SLAC in the summers then, because I got to know her better. We're still very close friends to this day. I'm not sure that I met her before that talk. Anyway, that pretty much told you that you had an atomic system. Some people tried to be skeptical, and Fred is one who tried to be skeptical, but when they saw the electromagnetic transitions, it showed you. You could calculate all this stuff from atomic physics stuff things, and that's some of what I did. Lowell Brown and I did some of that in Seattle. We haven't talked about Seattle physics yet. I arrived there in the fall of '73.

Zierler:

Bob, was your sense that the department was in building mode at that point? Was it pretty well established?

Cahn:

It wasn't in building mode because they didn't have positions. They had some very good people. They had a nice experimental group, and they had some extraordinarily smart theorists who — this is going to get delicate. Three students of Julian Schwinger: Marshall Baker, David Boulware, and Lowell Brown. They were all phenomenally smart people. I got to know Lowell the best of the three. We were very close, I think. In the end, I was disappointed that they didn't keep me. I think they made a mistake. I think Lowell and I worked very well together. Lowell was extraordinarily smart. Do you know Lowell?

Zierler:

I talked to him, too.

Cahn:

You talk to a lot of people.

Zierler:

I talk to a lot of people. I'll tell you one thing about Lowell: he's so understated relative to his contributions. It's remarkable. Like, everything is no big deal, but then you look at what he's accomplished.

Cahn:

Yeah. I think Lowell was frustrated.

Zierler:

Yes.

Cahn:

There are lots of different things to go into being a great physicist, and people like Steve Weinberg, who are great technically, and great imaginatively. And people like Shelly [Glashow] who are great imaginatively. There are people who are great technically, like Lowell. It takes lots of different talents. Lowell was exceptional if you gave him a problem. Nobody could beat Lowell. Same thing is true with David Boulware, who was fantastically smart as well. Incredibly strong, technically, these guys. Seattle was still kind of a backwater, despite that. Lowell surely should have gotten tenure at Yale. No question about it. He would have benefited, maybe, from being on the east coast.

Zierler:

Is part of the issue that Washington was a bit of a backwater?

Cahn:

Yeah, despite the fact that my uncle was the chair. It had good people, and it's gotten much better. But it wasn't building, because they didn't have positions. That was my frustration. I went there believing there would be a position that opened up. There was an assistant professor, Joe Weiss, who had been a Berkeley PhD a few years before me. I had hoped that there would be another position opening up, but it didn't happen. So, eventually,Gordy Kane came, and induced me to go to Michigan. Subsequently, Joe Weiss was killed in a tragic climbing accident.

Zierler:

Yeah, I heard that story.

Cahn:

Terrible. Steve Ellis, who came as a post doc while I was still there, eventually got a faculty position. They also hired Larry Yaffe.

Zierler:

What were some of your major research projects while you were at Washington?

Cahn:

Most of them were done with Lowell. I got back and looked, and maybe not all of them. But we had a graduate student who was exceptional. You may know him, Larry McLerran. Larry was a grad student, and Lowell, Larry, and I did some stupendous calculations together. If you interview Lowell, you're going to know all these stories.

Zierler:

No, not necessarily. I had to pull a lot out of him, actually.

Cahn:

At the time, there was an anomaly in muonic X-rays. I don't know whether you know muonic X-rays. Let me just briefly explain. Muons are much heavier than electrons, which means that the Bohr radius is much smaller by that ratio. So, it's like 200 times smaller. That meant that a negative muon that's captured before it decays falls into the nucleus and is way inside the electron cloud. It's an interesting way to probe what's going on in the nucleus. People are interested in measuring the X-rays that are emitted as the muon drops down from one energy level to the next inside the atom, really close to the nucleus. One of the reasons it's interesting is because for a heavy nucleus, now the electromagnetic interaction in the muon is more complex, because the field is stronger. QED tells you how to calculate these effects. One of our colleagues, a nuclear physicist, Larry Wilets, had calculated this, and Lowell looked at it and said, "This is garbage. This is wrong. We're going to do it right." Lowell was right, as he almost always is, that the calculation just didn't converge. It just didn't make any sense. Lowell realized we needed to be somewhere else in the complex plane to do this stuff. We wrote a series of three long papers and two letters that were really stupendous calculations. We calculated things to all orders in Z alpha. Z being the charge of the nucleus, alpha being alpha. So, usually you do perturbation theory. You do something to one order, and you do the next order. We calculated to one particular parameter to all orders. That meant that our calculations came out in terms of these spectacular special functions of Z alpha. This was a real tour de force. We all contributed in various ways. Of course, the shaggy dog story is that in the end the anomalies went away because it was experimental error. We never were able to explain the anomalies, and we calculated all these effects that might have been responsible. They weren't, and eventually it turned out that the experiments had some calibration problem. But we wrote these incredible papers with really extraordinary calculations. Some people have looked at them. Some Russians paid attention to it. But it was an enormous, enormous effort. Hard, hard analytical calculations. Lowell was just incredible. But so was Larry. We all did serious work. So, that was a lot of what we did, but then Lowell and I got involved in stuff related to Psi.

Zierler:

What remaining work was there needed on Psi?

Cahn:

Well, you had to understand things like the decays of the Psi-prime. Psi-prime goes to Psi-pi-pi,we addressed that and it continues to get attention still, mostly because sometimes it explains things and sometimes it doesn't. You apply it to other systems with a b instead of a c quark. Anyway, it didn't work out at Seattle, and when I got an offer from Michigan and Washington was not able to respond with a faculty position, I said goodbye. It was very hard.

Zierler:

Did they recruit you, or you were actively pursuing open positions?

Cahn:

Gordy Kane, of Michigan — you've probably talked to Gordy.

Zierler:

No, not yet. I'm going to have to get a list of names from you. You are a fountain of names here. This is great.

Cahn:

Gordy had been Dave's student at the University of Illinois before Dave came to Berkeley. There's no way for me to describe Gordy to you. You're going to have to hear form Gordy. I think Gordy maybe has retired now, finally. Gordy must be 80-something, or nearly 80. Gordy must have known of me from Dave Jackson. He was Dave's student and he knew about me, so he recruited me. It wasn't just Gordy, it was Marty Einhorn. My friend Marty, with whom I'd worked on my thesis, was at the University of Michigan. So, I always thought of it as Gordy recruiting me, but undoubtedly, Marty had a lot to do with it.

Zierler:

So, you were joining a pretty strong theory group at Michigan, was your sense.

Cahn:

Marty was strong. Gordy, I would say, got a lot more attention years later than he got at that time. Marty was very good, and there was Ed Yao, who was good, and Yukio Tomozawa, who was not as good. It was not a terrific crew. The Michigan experimental group was quite good, and quite diverse. That was a very strong experimental group. Of course, that was the place where Don Glaser invented the bubble chamber, and some of those people who worked with Don. Others of them came to Berkeley when Glaser came to Berkeley. This is a story I know very well now from all the work I'm doing on George Trilling's obituary. I know all the details on this. George worked with people like Jack Vander Velde, and Byron Roe, who were at Michigan. They were still at Michigan when I went there. Those were first-rate people. There were a lot of really good experiments at Michigan. My favorite among them passed away, tragically, two years ago. That was Rudy Thun. Rudy was a wonderful guy, about my age. They had some really fine people. Great crew. Michigan treated me extremely well. It's a great town, a great university. As a university, more of a big time than Washington, though I think Washington has come up since then. They're two of the great public universities, outside of the state of California. Maybe the two best. I did a number of things while I was in Ann Arbor.

Zierler:

Who were the people with whom you were collaborating during your time in Michigan?

Cahn:

I wrote a paper with Gordy. I'd have to go back and look. I don't think Gordy and I wrote more than one paper together, and I don't think I wrote anything with Marty. Maybe I was writing them by myself. I'd have to go back and look. I could do that, but I don't have my list in front of me. Good question. I'd have to go back and look at what I did then. I didn't collaborate close with anybody except Gordy on this one paper. It was really mostly me. It wasn't Gordy's calculation, and it wasn't all that important of a paper.

Zierler:

Did you take on graduate students?

Cahn:

Actually, there are some anecdotes here. I got very interested in parity violation in atoms. There were also some wonderful atomic physicists at Michigan, which was a long tradition at Michigan. There was an atomic physicist named Bill Williams. There were other people trying to do parity violation experiments in atoms. Norval Fortson in Seattle, and of course, Gene Commins in Berkeley. Bill Williams wanted to do parity violation in hydrogen because in hydrogen you can calculate everything. If you do parity violation in a heavier atom, calculations can't be precise because of the atomic physics. In hydrogen, you can calculate everything. Bill Williams was an expert. I think he had a student of Willis Lamb. He had collections of Lamb's paper, and I still have that. I think he gave me a copy of it. So, I did calculations for Bill on parity violation in atoms. There's a kind of interesting anecdote there. So, I had calculated in terms of whatever parameters you have for the weak interaction,including the neutral current interaction, and somehow Val Telegdi had gotten a copy of this. Val told Ernie Henley that my calculation was wrong. Ernie Henley was, at that time, the Chair of the Physics Department at the University of Washington. I guess, maybe, my uncle wasn't the chair when I was there. He must have been the dean. Anyway, I was really annoyed at Telegdi for going to Ernie Henley to say this wasn't right, because there was still a sore point between me and Seattle over the fact that they didn't give me a job, and that Val didn't come to me and complain. Not that I knew Val. Val was a very famous physicist, of course. Val's argument was that the quantity that I calculated as the parity violating quantity, he said this was the wrong answer because he was CP violating, and surely, we weren't looking at CP violation. It's true, we weren't looking at CP violation. But he was wrong because he didn't understand the behavior of various quantities, because the answer involved the lifetime factor of gamma. Gamma is a kind of funny thing in equations that one uses is. It enters into equations in a sort of phenomenological way. Therefore, its properties are not straightforward. Basically, Val was wrong. I knew I was right because I calculated it. If it had been CP violating, I would have gotten zero. Fortunately, I had someone I could make my case too. I was visiting Seattle at this time. We were in Michigan, but our hearts were still in Seattle. They are still in Seattle. Now, my daughter was a resident in Seattle, and my grandson's in Seattle. Anyway, we were visiting Seattle, and there was somebody there I could appeal to and discuss the calculation with, and that was Rudy Peierls. Do you know who Rudy Peierls is?

Zierler:

I don't, no.

Cahn:

Okay, well you need to know this. Rudy Peierls is one of the great physicists of the 20th century. Rudy Peierls was born in Germany, came toBritain, was a major figure in nuclear physics, and particle physics. Both in nuclear physics and solid-state physics — both. So, that's pretty rare, to put it mildly. He is a founder of both fields, and proved something called the optical theorem, which is one of the fundamental things you learn in quantum mechanics. He proved the optical theorem. Is there a time when people didn't know the optical theorem? He proved the optical theorem. He wasn't one of the people writing papers in 1925, but maybe by 1930. He's also famous for being the first person to calculate the critical mass for an atomic bomb. He's also famous because he brought to Los Alamos a young physicist, even younger than he, who had fled from Germany, a physicist named Klaus Fuchs.

Zierler:

I definitely know Klaus Fuchs.

Cahn:

Okay, so you need to learn about Rudy Peierls. He was one of the people who established quantum mechanics and how it works. Rudy agreed that I was right, so I felt very vindicated. Anyway, that's my Val anecdote. I never talked to Val about this. This was all between me and Ernie Henley and Rudy Peierls. The only other time I've interacted with Val was he came up to me one time when I was at CERN and complimented me on a paper I'd written with Fred Gillman. In fact, where we'd found some very clever way of getting an answer that was easy to understand. Anyway, that all has to do with my Seattle time, I guess. You asked what I was doing in Ann Arbor. One of the things was working with Bill Williams, who was tragically killed in a plane crash. He was flying, and it crashed into Lake Michigan. I think the rest survived. It was a very tragic story. He was a great atomic physicist. I'd have to go back and look and see what other papers I was writing at that time.

Zierler:

Did you take on graduate students in Ann Arbor?

Cahn:

No, I didn't.

Zierler:

Is that because that wasn't done for assistant professors?

Cahn:

Maybe. I didn't really think about it. I was too busy worrying about getting tenure myself. I was doing a lot of teaching.

Zierler:

What kind of courses did you teach?

Cahn:

I taught graduate quantum mechanics. Actually, there's another anecdote. I taught graduate quantum mechanics and being way too ambitious and naive assistant professor, I said, "I'm not going to teach out of some textbook. I have to write my own notes." I did use Feynman's third volume as a starting point, but I made up all my own problems. It was just an enormous amount of work; I think probably too much under the influence of Dave Jackson. I remember one problem I calculated on atomic transition, 3D to 1S. Some hideous calculation. It was really hard. I had Bethe-Salpeter, the famous book on one and two electron atoms. They had a calculation in there, and it's wrong. To make sure I got it right, I made Dave Jackson calculate it, too. I'm quite sure no student calculated it. So, I was spending too much time on that. Another thing I did, though, stimulated by one of the grad students there, was I decided to teach a course in group theory. There's a book on group theory by a guy named Wybourne, I think. It was really neat. It had all this fantastic stuff in it, something called Dynkin diagrams. Dynkin diagrams are a way of understanding Lie algebras, and not just simple Lie algebras like SU(2) and SU(3), but the so-called exceptional Lie algebras. So, I started reading Wybourne, and I got some other things. I got a book by Jacobson. This is a serious math book which meant it was really hard to understand. And then I went to the Michigan library and found the original papers of Mr. Dynkin. He was Russian, but eventually it ended up at Cornell. So, I still have my Xerox copies. This is from the '70s, so it's pretty primitive Xeroxing, too, of all of Dynkin's papers, a big binder full of papers. When I was reading Wybourne's book, it was written as sort of a text, and I was really surprised I couldn't do the problems. Eventually, I realized the problem was that Wybourne's book wasn't at a high enough level to understand to do the problems. What was going on? Well, it turned out Mr. Wybourne had plagiarized the book from Mr. Dynkin. When things got hard, he just didn't do that part. It was possible to prove that it had been plagiarized, first of all, because there were sections that were pretty clearly plagiarized, but there was a critical, obvious misprint in one of the theorems of Dynkin's papers. If you're really working through Dynkin, and you understand what's going on, you get to the part that says something that’s wrong, but it was some fundamental theorem and it was clear that there was a typo. Wybourne has printed it with the typo. That was the giveaway. The book was all plagiarized. I notified the publisher. I don't know if he ever did anything about it. Anyway, under the stimulus of this student to go learn all this stuff, I eventually wrote a book on Lie algebras, semi-simple Lie algebras, and Their Representations. I must have finished when I was in Berkeley. It was published by Benjamin-Cummings, and I gave them a photo ready copy, using some new thing called Tex. This was before there was LaTex or anything. This was pure Tex. I wrote the whole book in Tex in 1983. Anyway, that was one of the things that I undertook when I was in Michigan was learning group theory, and learning about Dynkin diagrams, and so on.

Zierler:

How did the opportunity at UC Davis come about?

Cahn:

It was really cold in Ann Arbor. We had like the two coldest winters they've ever had. We went 42 days without going above freezing. It was really cold.

Zierler:

That was too much for a California boy.

Cahn:

Oh, for a California girl.

Zierler:

Ah.

Cahn:

So, JackGunion at Davis had a position.

Zierler:

What were your tenure prospects at Michigan? Was that trajectory on track?

Cahn:

Once I had the offer from Davis, the dean at Michigan said, "If you stay, we'll give you tenure next year." So, that was not an issue. Michigan was always very good at how they treated me. So, Jack's at Davis. So, Jack invited me back to Davis. He had gotten money from DOE. It was supposed to be their way of helping produce more junior faculty positions. DOE was not really very happy with the idea that I was giving up a junior faculty position to go to Davis, but I guess, eventually, some of my friends prevented — DOE was talking about not letting me take the position once Jack offered it. But in the end, they relented. So, I visited Davis in February. If you visit Davis in February, you've left Ann Arbor, you're very vulnerable. There were all these orchards, and everything in blossom. So, I went to Davis with a funny position that I was supposed to be this position you mentioned before, Research Assistant Professor. That's what the position was supposed to be at Davis. But the committee that decides on these things decided that given my position in Michigan, they were going to make me Research Associate Professor. They were going to basically give me tenure in the research lab. The deal at Davis was eventually I would be on the research faculty but in fact, this committee sort of crossed out the physics department, who I think weren't very, happy that somebody outside of their department had granted me tenure, albeit in the research line. Anyway, I went to Davis and worked with Jack on a paper. It was actually not so bad except we sort of got it backwards. It would have been more interesting if we interchanged the initial state and final state. Shortly after I arrived at Davis, LBL opened up a position. So, a year later, I was back in Berkeley.

Zierler:

Why was the LBL offer so attractive that you would be at UC Davis for such a short amount of time?

Cahn:

Because it was Berkeley. Davis has become a much stronger, much stronger place in physics and altogether than it was then. It was truly a backwater, and Jack was doing his best to build it up. Am I allowed to just really gossip?

Zierler:

This is your record. As much as you want.

Cahn:

Jack had tried previously to recruit Helen Quinn, which showed extremely good judgment on his part, and I think he was not able to convince the faculty, or somebody there, to give him the position. Somehow it didn't happen even though Jack knew it was the right thing. He was certainly right. I think the point was that Jack was trying very hard to build up the department. It had a significant experimental group headed by Dick Lander, but it was still kind of a backwater, and Jack worked hard to build it up. The whole campus, of course, was also on the way up since that time. That was a long time ago. It was almost 50 years ago. Davis is a very different place now. Outstanding in cosmology, for example.

Zierler:

Yes. Bob, you emphasized, of course, it makes sense, it's Berkeley. That's why you go. But of course, you were going to LBL, not the department of physics. So, I'm curious if you saw in some way that you'd be leaving the professor's life to some degree.

Cahn:

Yup. That was a sacrifice. I think maybe the stronger comparison is with Michigan. Being on the faculty at Michigan — as I say, Davis was kind of a backwater, but if you make the comparison, faculty at Michigan is pretty high level. Is it worth trading it? Well, we wanted to be back in California, and I was going to be back with Dave Jackson, and there were other people there. Mike Chanowitz, who had been a post doc with me at SLAC.

Zierler:

Wait, Dave Jackson moved to UC Davis?

Cahn:

No, no, no. I'm talking now about Berkeley.

Zierler:

Oh, I see. Right.

Cahn:

Well, yes, it wasn't faculty, but it was a great place. At the time, the theory group was really a united theory group. The faculty and the LBL people absolutely worked together. The faculty had their offices at LBL, and they spent their time at LBL.

Zierler:

So, you felt a lot of integration with the physics department.

Cahn:

It was much better integrated on the theory side than it is today. Very much better.

Zierler:

And the title they offer, Divisional Fellow, what's the tenured equivalent of that?

Cahn:

You could call it either Assistant Professor or Associate Professor without Tenure. It was the line from which you became tenured. I looked into this. The Division Fellow position hadn't been around very long. One person who had gotten Division Fellow, and hadn't gone on to senior staff, was Jerry Nelson. It wasn't that he didn't get it. He invented the segmented telescope, and he went onto UC Santa Cruz — a truly gigantic achievement. He went on to bigger and better things, and that was an enormous step for astronomy. But the other division fellows had been Dave Nygren, Bill Carithers, Mike Chanowitz, and they hired Ian Hinchliffe together with me. We still hire people as Division Fellow with the intention that they're going to be tenured. It doesn't always happen, but it mostly always happens.

Zierler:

So, technically, there was a bit of a risk because you were leaving a tenured position for an untenured position.

Cahn:

Berkeley would not have taken me into this position unless they intended to give me tenure. It's just obvious. I was too senior for that. I'd been around a lot longer than Ian. I started there in '79, so about 7 years past PhD. That's pretty late.

Zierler:

What was the catalyst there? Who was central to bringing you on?

Cahn:

No one's going to tell me. I don't know. I can imagine, I would suppose Mike and Dave. I can guess. I never asked them, and I never knew, but it always seemed logical. But also, maybe the experimenters. They thought that I was somebody who would work with them, and I did.

Zierler:

And this came out of the blue. You weren't looking to leave UC Davis.

Cahn:

No. I thoroughly thought we would stay in Davis. It was the University of California. We weren't that far from the Bay Area, and our families. It was a nice place to bring up kids. Davis was fine. As I say, it's a much better place today.

Zierler:

What projects or research did you take on when you first got to LBL?

Cahn:

I need to look at a list of publications to remember what's what. I think I was quite active. The early days were really time when I was most active. '83, probably, was the most important paper I wrote, and that was with Sally Dawson. It's now called Vector Boson Fusion. That was interesting because it was an example of how you may think about some project that you don't carry out, and then many years later something comes to you. Well, here's the way to use it. So, when I was doing my thesis, one of the things that I thought about writing my thesis, along with something called two-photon physics. In an electron-positron machine — of course, electrons can emit photons, so it's possible for the electron and the positron that are coming at each other each to emit a photon. Either a real photon, or a virtual photon, a photon that isn't quite real but is pretty much like a real photon. Then, those two things can collide, and they can produce things. So, it's possible, effectively, to do photon-photon collision, which sounds crazy. You can't make a target out of photons, but you can do photon-photon collisions. That was something I had thought about doing. Stan Brodsky had written an important paper about this subject. I'd become involved in it, actually, probably before I wrote the paper with Sally. There's something called Yang's theorem, or maybe Yang and Landau's theorem that a spin-1 particle cannot decay into two photons. A spin-0 particle, like the pion, can decay into two photons. The spin-1 particle absolutely cannot decay into two photons. It's forbidden by gauge invariance together with Bose statistics. You use them in the theorem proved by both Landau and Yang. But if the photons aren't quite real, the theorem doesn't apply. A photon that's not quite real is not quite massless, is another way to put it. So, it's possible in experiments to make these not quite real photons, have them collide, and make a spin-1 particle. People at Berkeley have been measuring these things, and I wrote some papers on that. But somehow, because of thinking about these two-photon things, it occurred to me that in collisions, you start with really high-energy particles, quarks. They can emit virtual W bosons, or Z bosons, just the way electrons and positrons emit virtual photons. We knew that a Higgs boson if is existed, and is heavy, could decay into two Ws and two Zs. So, you could make it that way as well. So, I had this idea that we could find a new way to make the Higgs. I guess we already knew one other way to make the Higgs, which is the standard way, which is to collide two gluons. But I had this idea that maybe we could make Higgs bosons by colliding virtual Ws with virtual Zs, and there were two people I asked if they'd like to join me. One was Howard Georgi, who said no, and the other was Sally Dawson, who was a post doc. Have you talked to Sally?

Zierler:

I have not.

Cahn:

You should talk to Sally. Sally had a great, long, productive career as a particle theorist, and I would say is one of the pioneering women in particle physics. She wasn't the pioneer that Mary K was, but she's had an enormous career, and continues to do research, and have leadership roles. She's an exceptional person. She's an important person to talk to. So, Sally joined me in this, and we invented vector boson fusion. She did a paper on her own based on that, which was also important. So, that was one paper. I wrote a paper with Haim Harari that was great fun. If you're asking, most of the papers I've written have been not with famous people. I wrote a paper with Fred, that I mentioned. It was pretty nice, mostly because of its simplicity. A paper with Haim, and a paper with Shelly Glashow. That was about hypothetical, very heavy particles that might exist, and might not decay. It was a very bazaar sort of thing, but people haven't looked for these particles. So, obviously, those three papers were written with famous people, but most of the rest of the papers were written with people who were no more famous than I am. This one with Sally is an example. Then, the paper that followed on that — a few years later I wrote a paper — classic example of now not to write a paper — with Steve Ellis, Ron Kleiss, and James Sterling. It was based on the thing I did with Sally. What we did was to realize that these events where quarks coming at each other emitting W bosons, the quarks would recoil and come out at a large angle. You could detect those quarks as jets in high energy collisions. So, you could use that as a signature for those events, and then that would enable you to look at events that are very likely to have produced a Higgs boson. That has been used a lot. It's anoutgrowth of the paper with Sally. It started at CERN. When I say how not to do it — the basic idea is very simple, but of course, we spent a whole year calculating some part of it that wasn't very important. At the time, we thought it was important. We didn't really understand what was important and what wasn't important. So, we did an enormous calculation that would have been impossible without Ron Kleiss. Ron Kleiss was a great calculator, and so was James. We had fantastic people that Steve and I were working with, but it went on endlessly. The whole point could have been made in a 3-page paper that focused on the right thing, but we didn't know what the right thing was. I think that's not uncommon, that you don't really understand your own paper.

Zierler:

When did you eventually figure that out?

Cahn:

On this particular paper?

Zierler:

Yeah.

Cahn:

I'm not sure. In the subsequent years. That paper caught on, and other people jumped on that area and worked it very hard, which I probably should have done, where they showed how to use this tagging to find these events. It's a standard technique now. The problem is it becomes standard, and then ATLAS and CMS don't cite it anymore because they can't imagine that there was ever a time when it wasn't known. So, there's no citation. Most of the time they don't cite it because everybody knows that's how you do it. That's a problem with that paper, and sometimes with the paper with Sally. Students now don't know there's any history. They just know this is how you do it. That turned out to be an important idea. Another paper that I wrote around that time — all of these things have anecdotes. It's kind of interesting. Howard Georgi, who's a really great physicist, exceptional — he and David Politzer wrote a paper. This was back in the time when we were still trying to convince ourselves that gluons are real. So, they calculated this process in electron scattering from a proton, and they looked at the — there's a plane that's formed by the electron coming in and going out. So, those two lines give you a plane. Then, if you look down the direction of — I've forgotten now whether it's the electron or the virtual photon, but you look down this axis, the quark that comes out and has some azimuthal dependence around some axis. What they showed was that if there were gluons, there would be this particular azimuthal dependence. It was either cosine phi, or cosine 2 phi, and they argued that this was the signature that proved that there were gluons there. You remember, I've been influenced by Feynman's “Photon-Hadron Interactions”. Feynman had this attitude that you didn't need fancy calculations, though he was the greatest at doing fancy calculations. You could get everything from his dumb, naive parton model. So, I asked myself, maybe Feynman's right. Maybe I can get the same effect without the gluons, just using Feynman's parton model. So, I calculated this in the naive parton model, but I allowed the partons to have transverse momentum. So, the protons coming at you with all its partons, that is to say, quarks. They can have some transverse momentum. You calculate what happens. The electron emits a virtual photon, which kicks a parton quark that has transverse momentum. What you find is that there can be cosine phi and cosine 2 phi dependence for the outgoing quark. The size of that effect is proportional to the transverse momentum of the struck parton, and inversely proportional to Q, the mass of the virtual photon. So, what that tells you is that you can use this to measure the transverse momentum of the quarks inside a photon. So, what you find is that as you go to higher and higher Q squared, higher mass virtual photons, my effect goes away, and Howard's and David's remains. But at relatively low energies, mine dominates and measure the transverse momenta of the quarks. So, that can be a big deal in nuclear physics. There are nuclear physicists who redo our electron scattering experiments on nuclei and turned out to be very interested in all of this stuff. So, that paper caught on pretty well, even though, again, it was a pretty straightforward calculation. It was really my way of trying to show that Howard and David had overstated their case. You didn't need gluons. Of course, they could argue that the transverse momentum is from the recoil of the gluons inside the protons. But basically, I was showing that their argument wasn't so ironclad. Anyway, these things do get measured. It's been a big deal at Jefferson Lab. That was another thing in the '80s. Then, I got challenged by a very wonderful Russian physicist, Boris Ioffe. He didn't believe my calculation, so I had to write another paper convincing him, which I think I did. Those were all things that happened in the '80s, I guess. Probably my most active period.

Zierler:

Bob, how significant was it to be at LBL in terms of what was going on the experimentation side?

Cahn:

Well, I think it was stimulating.

Zierler:

Sure.

Cahn:

Where were we?

Zierler:

I was asking about what it was like to be at LBL in terms of experimentation.

Cahn:

There were experimenters coming, asking me things like how do we calculate this stuff for two-photon production of a spin-1 particle? How do we calculate this, that, or the other thing for charm particles? So, there was a constant interaction with people likeGerson Goldhaber and George Giddaland others. So, yeah, it was a great atmosphere in addition to having the other theorists around. Post docs, too. Post docs like Sally Dawson were great. We had some great — not just at that time, but a continues to be the case that Berkeley attracts really, really great post docs. It attracts great students. An example of a student who came through Berkeley, who was off scale, was Nima Arkani-Hamed. There are people who — Joe Polchinski was a student who had an office adjoining mine. Do you know about Joe Polchinski?

Zierler:

No.

Cahn:

One of the greatest string theorists ever. Fantastic guy who died at the age of like 60. It was just horrible, horrible, horrible. Tragedy. He was one of the sweetest guys. He was one of those graduate students, when he got his degree and left, it was a big loss to the department. Nima came back and bounced around. These were people who were great. Charles Thorne, Nima, Joe Polchinski, Steven Sharp, a number of others were graduate students — fantastic. We had fantastic post docs. I mentioned Lisa Randall. We've done very, very well at attracting both students, and especially post docs. We had a pretty good vehicle to do this, which is one of the reasons it's such a great place. People like to come to Berkeley.

Zierler:

Bob, how did your day-to-day change when you were named Director of the Physics Division in '91?

Cahn:

Completely.

Zierler:

Did you stop being a physicist, essentially?

Cahn:

A lot more than I anticipated. I had been involved with experiments before. I was involved with the design of an experiment when I was in Michigan, something that did not get selected, called HRS, a high-resolution spectrometer. Later, I was involved in something else, but before I was Division Director, I was very interested and involved in SDC, the Solenoidal Detector Collaboration. I became division director in '91. When did we start SDC? That may have been a little bit earlier. Yes, it was. So, I'd been involved a bit in planning for SDC, and I continued to be until the demise of the SSC. But suddenly I had completely different responsibilities. A lot of it had to do with our work on developing ASICs, Application Specific Integrated Circuits, for experiments, for CDF, for D-Zero and forBaBar. LBL was a real leader in the application using semiconductor detectors at colliders. The earliest work, I think, had been done with semiconductor detectors in high energy physics, but it was really led by our current LBL director, Mike Witherell. But for colliders, LBL played a big role in CDF. CDF work had a lot to do with its ability to be the first to detect the top quark. Carl Haber, and others, were instrumental in this work. Carl is a really important figure in detector development. Have you met him, or talked to him?

Zierler:

I've not, no.

Cahn:

He's great. He's also an amazing person. He's a MacArthur Fellow for having developed the means to play back recordings that you can't play, and you can't touch. We can talk about that sometime. Anyway, Carl was very involved in the development of silicon-strip detectors. LBL was a leader in silicon-strip detectors for both CDF and D-Zero, and then the development of pixel detectors. A lot of my energies as Division Director went into dealing with our integrated circuit engineers because they were constantly being wooed away by people in Silicon Valley offering them much bigger salaries. Some of them were also rather temperamental. So, that was a big part of my job, to try and keep all of these people happy and make sure that all of our projects moved ahead. The Division Director has lots of personnel issues to deal with, lots of bureaucratic work to deal with. It's really all-consuming, as our current Division Director could tell you.

Zierler:

Is part of it also continuing to attract these top-notch post docs that you mentioned?

Cahn:

The theory group mostly takes care of that. They're more worried about hiring people for positions that are going to become permanent. Post docs, by and large, won't become permanent. When I was finished being Division Director, I was interviewed by the committee that was the search committee for the new division director. They asked me, "What's the most important part of the job of a division director?" I said, "Personnel." Personnel is the key. You get the right people; you're going to do great. Hiring the best people is the whole story. Yes, there's also judgment as to which projects to pursue, and how to make cuts when you need to make cuts. Those are important, but there's nothing really that compares to the importance of hiring good people. That's the key. It's a challenge to maintain LBL and the campus high energy program at the level that it was for so many years. Having the bevatron and the bubble chamber made Berkeley really a center. How do you maintain that? They had all these Nobel Prize winners. Well, you get great people, like George Trilling, who are among the greatest physicists in the world. That's what you need. That's how you survive. I think we've been able to do that pretty well. We'll see how things turn out. When I look now at the people who are not quite my contemporaries, the 60-year-olds, yeah, we've got some really good people. Really fantastic people.

Zierler:

Bob, I wonder if you can comment more broadly, as you rose in the ranks at LBL, how it changed its overall focus, how it remained relevant, both on the theory and the experimental side.

Cahn:

When you say LBL, you mean the Physics Division, whereas there's another question, which is the Lab itself. The Lab itself, in my opinion, is thriving because of one particular lab director, and that's Andy Sessler. Andy was an accelerator physicist who became the director, I think, in '73. He's the one who saw that we should diversify what we're doing scientifically, and really set us on the path of the enormously successful laboratory that we have now. The budget now is $1 billion a year.

Zierler:

In what ways did he push the lab in new directions?

Cahn:

First of all, environmental science. Very early, Berkeley got into environmental science, in large part because of Art Rosenfeld. If you ask what scientists at LBL has had the most impact on the country, on the world? The easy answer is Art Rosenfeld. Art Rosenfeld's impact on environmental science is gigantic. He was joined by other people who came out of high energy physics, but Andy Sessler is the one who really, as the leader of LBL, was in the position to make that all happen. Now, of course, we probably do much more biology than we do physics, by far. The lab diversified, it got into the human genome project, and that was really important. At LBL, the physics division has not grown, and the high energy physics andnuclear physics have not grown, which is not surprising. We used to have the major accelerator, and now we don't. That makes a difference. But I think we've maintained a role of very significant leadership, and what I think of most important, experiments. There are two things: you need to choose the right experiments, but most of all, you need to have good people. I think, not just among the 60-year-olds, but among some of the younger people, we've got some really great people. You know we're going to do well. An example which we haven't gotten to is Saul Perlmutter.

Zierler:

Right. When did you first meet Saul and work with him substantively?

Cahn:

When I was Division Director. There was this supernova project. One person who doesn't get enough credit because he tends to rub people the wrong way, maybe, but deserves a lot of credit is Rich Muller. Rich Muller was a person who certainly helped GeorgeSmoot, and certainly helped Saul Perlmutter. If you helpedtwo people win the Nobel Prize, you deserve a fair amount of credit, I think. Rich and I were never great friends. Our politics were very different. Some people view Rich as a dilettante, but an effective dilettante. So, Saul — I knew about the supernova group when I started as Division Director, and they already were building a telescope. I think it was called the New Telescope. It started under my predecessor, under Pier Oddone, who's another person to talk to. An important person, as he's the director of Fermilab. Have you talked to Pier?

Zierler:

Oh, yes. I have.

Cahn:

Okay. So, even under Pier, there was already this supernova project that I guess maybe was headed by Rich Muller, and involved Gerson, who had joined, and Saul, and CarlPennypacker. So, I was aware of that, especially because I was close to Gerson. By this time, we had written our book. First edition. Gerson even came to me and said, "You should really put Saul in charge of this project." I felt I had to check with the other senior people, Rich and Carl. They said, "Yeah, put Saul in charge of this project." I had known Saul, but we weren't close prior to that. But then we became very closely involved as I sort of had to run political interference for him.

Zierler:

Why, what was needed?

Cahn:

Well, "Who are these upstarts in Berkeley that don't even do astronomy that think they're going to do supernovae? They don't know anything." I may have read all of this in Ursula's interview. I probably don't remember it as clearly as I did for her when I reviewed my notes, but we were real outsiders. There's no question about it. None of these people had any background. They weren't established people. Bob Kirshner, Alex Filippenko, these were people with real reputations. They knew all of the problems. Sometimes there's an advantage, of course, in not knowing the problems. I tried to convince Dan Kleppner to look for parity violation in hydrogen. He said to me, "No, no. I know too much. I know all the problems. I can't do this." And he was right. But Bob Kirshner thought you couldn't do cosmology with supernovae. initially, he thought. So, he was on an advisory committee that told BernardSadoulet to cut off Saul's funding. So, that's sort of where I had to intervene to protect Saul. It went beyond that, because I think Kirshner continually tried to deny opportunities. There isn't any question. He continually tried to deny opportunities to Saul, especially after he decided that Saul was right, and that this could be done. Kirshner was a smart enough and good enough astronomer to put together a team to compete. Kirshner is a very smart guy. He's a great scientist. He's a fabulous speaker on science. He's an extraordinarily talented guy, but it was pretty tough fighting him. Real tough fighting him. The chairman of the Astronomy Department at Harvard vs. a bunch of non-astronomers at LBL. Who's ever heard of astronomy at LBL? The fact that Jerry Nelson invented the segmented telescope, I might mention. So, yes, that was a continual battle. I think we're always quick to state that it was a great thing that Kirshner, in the end, built a competing team. It's really important to have competition.

Zierler:

It was good for the science, you're saying, to have that competition.

Cahn:

Absolutely. As I told people, either team would have been delighted to prove the other team wrong. Neither team would let the other team get away with anything, for sure.

Zierler:

Did you recognize this at the time, or this was only in retrospect that you realized this was a positive thing?

Cahn:

It probably took us a while to see it positively. It was a very tough fight. We were fighting a lot of powerful people. We were having a very hard time. People were stealing our ideas, and then getting telescope time for them. It was a very bitter fight.

Zierler:

And this was tense. This was not just a scientific disagreement. There was some bad blood here.

Cahn:

Absolutely. Ask Saul about the disagreements about who gets how much time to speak at the Nobel Prize ceremony. It didn't even stop when they won the Nobel Prize, for goodness sakes. They're interesting scientific controversies that continue, which are scientific controversies. What is H-naught, the Hubble constant? There's a discrepancy between two different ways of measuring it. Adam Reese is on one side, and a lot of people are on the other.

Zierler:

Bob, of course, there's a long gestation period between research that leads to the Nobel Prize. When was the buzz getting so loud that it seemed like this was really in the cards?

Cahn:

I can tell you a funny story about that. This is kind of bizarre. We have friends that we used to tailgate with at Cal football games, and he used to bug me and said I should win a Nobel Prize, because then you get a special parking space, and it would be easier to tailgate if I could have a Nobel Prize parking space. And I said, "No, listen. I'm not going to win the Nobel Prize, but Saul is going to win the Nobel Prize." So, one morning I'm driving to work, and I get a phone call. 2006, I get a call from Jay and he said, "What was the name of your friend who was going to win the Nobel Prize?" I said, "Saul." He said, "George?" And I said, "Oh, yes. George." So, George Smoot won it first, and rightly so. I have another anecdote about that. George made his discovery back when I was Division Director, and I knew zero about cosmology. I know a bit more now. But at the time, I didn't know much. George comes in one day, and I guess he showed me the figure that he had, and he was pretty excited, and I said, "George, is that going to be big?" George said, "Yes, this is going to be big." My point is that certainly it was clear that Saul should win the Nobel Prize. It was just a question of time. But, yeah, George won it first. So, LBL — people say, "They don't do astronomy," and stuff like that. Well, two of the biggest discoveries in cosmology were due to George and Saul.

Zierler:

Yeah. What did that do for LBL long term? Is it now recognized — the way you describe it, it's this brash, young upstart. It has no business being involved in these things. Did this cement its status, or was this more a brief period in time, and LBL went back to its wheelhouse?

Cahn:

No, no, no. It changed LBL I think more important, it changed DOE. That's the important thing. When I was a Division Director and Saul was doing this crazy stuff, I like to say that I believe that DOE's attitude was if you want to spend your money on that, that's okay by us, but don't expect to get any credit for it.

Zierler:

And their tune changed.

Cahn:

I was maybe spending $300,000 a year on Saul, which wasn't a lot. It's certainly not very much now, but even if you put in some escalation for cost of living, and so on, inflation, it still wouldn't be much. It'd probably be less than $1 million a year. But I think it changed DOE. In the end, we did get credit, and certainly, DOE embraced cosmology. So, that was a major achievement. It changed DOE. But yes, it established us. Winning the Nobel Prize is a really good thing, and it meant that we could then go on to do other things. Saul then had this idea of doing the supernova experiment from space, which would have big advantages. We'd done a lot of work developing this project, originally called SNAP. But in the end, trying to work with NASA became impossible, and that all failed. It may get resurrected through something they're still talking about. It didn't work out the way we wanted, but in the process, we became more aware of using this BAO technique, Baryon Acoustic Oscillation. We brought David Schlegel to the lab, and he became the center of the cosmology effort. BAO is fundamentally just a hell of a lot easier than supernova. We still have the supernova program, and there are still science objectives that we hope to carry out with it, but CMB and then supernovae were the two big steps, then BAO, and then it's going to be back to CMB, with something called CMB S4, which we're heavily involved in. So, yes, it changed LBL, but more important, it changed DOE.

Zierler:

That's a really significant comment about how DOE's approach to cosmology changed. What kind of line can you draw from that in terms of what's been done in cosmology because of DOE's embrace of it?

Cahn:

Well, if we go back to the beginning, instead of embrace, let's start off with tolerance. So, they tolerated us paying George. We probably paid George partly, and maybe he was paid through Space Sciences Lab. That was more tolerance, and with Saul, also, it was tolerance. The embracing, I think, came after that. DOE embraced SNAP without any question. They were very supportive. It's just that it became impossible to work with NASA. You'd have to ask Saul about the details of that. But out of that, grew this new effort. So, the lab now has CMB, it has supernovae led by Saul, and it has BAO led by David Schlegel. The cosmology program is equal in size to the high energy collider program, ATLAS. So, that's pretty significant. When we started, we were all high energy, and there was this tiny little piece of cosmology. Now, it's split equally between those two. So, that was a big change for us. But as I said, I think the more important thing was the change for DOE. We have to keep prodding them, but they've come along. It's taken them a while. They had a hard time dealing withtelescopes these folks, and the CMB — CMB, they didn't pay very much — the original experiment was all NASA. They, DOE, didn't have to pay very much. They were just paying some salary to George. But now they're talking about big money for CMB. And there's very big money for dark energy in the form of LSST. Our DESI experiment is small change, but LSST is big-time money.

Zierler:

To bring this narrative up to the present, it didn't just change LBL and DOE, you got much more involved in cosmology yourself.

Cahn:

Yes. I think the way that happened, really, was back when Jim Siegrist was still the division director, I think that the bookkeeping was kind of in disarray in cosmology. Jim asked me to take over, and I said I'm only going to do that if you let me run cosmology. But the way I got involved was through something else. The way I got involved in cosmology was actually due to Fred Gilman. We keep coming back to Fred. So, how was that? Well, there was a task force set up, and I think it was at the time when Fred was the chair ofHEPAP. It was ten people from astronomy and cosmology, and two people from high energy physics. Two were John Huth of Harvard, and me. It was obvious that Fred was the reason that I was chosen. There was no other reason. Well, there was a reason. I was connected with Saul, and Saul had, you know. So, there was a reason. By this time, Saul had discovered dark energy, and that's what this was all about, and I was the head of cosmology, or something. I was at least that in Berkeley. So, that was a fantastic experience. I had an 18-month tutorial from some of the best cosmologists in the world. It was unlike any other committee I've ever been on. People worked. They worked really hard. They devoted themselves to this project, and we wrote this really significant paper, the Dark Energy Task Force paper. The people on it were fantastic. There were people like Wayne Hu, Andy Albrecht, Gary Bernstein, Wendy Freedman — just the most wonderful people. Lloyd Knox. They were just incredible people. I decided that I was going to take it seriously. So, if they were going to calculate, I was going to work in order to calculate. These people helped me learn how to do the calculations, and I really learned a lot. That enabled me to get involved in cosmology. It was because of this committee work, really, more than Jim putting me in charge of cosmology. So, it was really the Dark Energy Task Force that got me involved.

Zierler:

To what extent did the task force really set the tone for the field going forward?

Cahn:

Absolutely. It was RockyKolb’s brilliance. I'll explain exactly how brilliant he was. Rocky said — have you talked to Rocky?

Zierler:

I did this morning, in fact.

Cahn:

If you want a guy who's entertaining, he's something.

Zierler:

He's amazing.

Cahn:

We ran into each other by chance at a little hotel in Spain last summer. That was crazy. At DETF Rocky says, "If we're going to sell this, we have to improve something by a factor of 10." Everything we measure, we improve things by a factor of 3. Rocky says, "Well, we need to multiply two things together." So, that's what we did. We took two uncertainties and multiplied them together, and we decreased each of them by a factor of three and got an order of magnitude. The Dark Energy Task Force figure of merit is some two-by-two determinant. So, effectively, you're multiplying together the two diagonal elements together. So, that was genius on Rocky's part. That was so clever. But then that would marry together with enormous work that was done by people like Andy, like Gary Bernstein, who's another fantastic person. These were just wonderful people. It was a real thrill to be on that committee. I learned so much, they were such nice people, and I do think it set out the program for dark energy. If there's any criticism that I would make of it, it's that we were too successful. The dark energy task force figure of merit became used by everybody, sort of unscrupulously, because you could redo it in different ways and get whatever answer you wanted. It became probably too powerful. But it set out directions that I think in retrospect were still solid statements. For example, about weak lensing, we said it's potentially the most powerful technique. I think it's probably the case that it's still potentially, and not proved. Potentially. I think we could foresee the BAO is going to have an easier time than supernova. So, I think all these things were laid out and done very well quantitatively. You have to read carefully what our assumptions are about systematics. But, to be honest, we would typically say, "Okay, here's what you can do optimistically, and here's pessimistically." I think that was the right way to do it. It was a great committee, with great leadership, great membership, and it was great to be a student on that committee, which is effectively what I was.

Zierler:

Bob, how bullish are you about the prospects of significant advances in dark energy?

Cahn:

It's tough. I think dark energy and particle physics are in very similar binds. The binds are too much success. We have theories, or models, that we've been unable to break. Moreover, we have a similar problem in that there are not good alternatives. The alternatives that you make up are pretty ad hoc. They're not convincing. So, let's take dark energy, is w equal to -1? That's the simple model. Well, there's no good alternative that say, no, it's -0.9. So, you get close to -1. What have I learned? Until you can prove it's not -1, you're in trouble, and without a good alternative, what kind of scale do you have for how good an experiment has to be? That's why the Dark Energy Task Force figured it was important and set a scale for an experiment. How can we judge how important some experiment is going to be? Well, what's its figure of merit? That's there because there isn't a good alternative. It isn't A or B. And in high energy, you sort of have a target in supersymmetry. We haven't found it. There's no good argument about what the scale is at which we're going to find something. So, I think that's very tough times in both fields, and also, they're not cheap. Cosmology is cheap compared to high energy, but LSST is a $1 billion experiment. That's not really cheap. Where could we get a breakthrough? We could get a breakthrough in any one of these things. I think for the money, the thing that's most attractive to me is dark matter because there, we know that there's a lot of dark matter. We don't know exactly where to look, but for sure, it's there. It's right around us. That's a good thing. And the experiments are not cheap anymore, but they're not on the level of building an accelerator. So, I find that maybe the most attractive direction right now. On the other hand, you asked about dark energy. But there's another issue in cosmology, which is inflation. That's where, I think, CMB fits in. So, my view, which maybe you would call pessimistic, is that — and you could call is parochial — that DESI is going to be the best dark energy experiment.

Zierler:

Oh, wow. Why?

Cahn:

Because I think LSST is going to be fantastically hard. Fantastically hard. Now, fortunately, they have some incredibly good people. So, maybe they'll figure out how to do this stuff. People like Gary and Rachel Mandelbaum. They have a lot of really great people, but it's going to be fantastically hard. It's so much harder than what we do. All we measure is redshifts. That's all we have to measure. The only thing. We don't care what the galaxy looks like, what shape it is. We measure red shifts.

Zierler:

Bob, you drew an interesting parallel between particle physics and dark energy. It's easy in particle physics to point to — we don't have the SSC; we don't have the ILC. So, what's the analogy there for dark energy, or perhaps even cosmology in general, in terms of long-term structural challenges?

Cahn:

One of the problems with cosmology is that there's only one universe. That's a big problem.

Zierler:

Not according to some string theorists, though.

Cahn:

Well, let them try to do an experiment in one of those other universes, then. You know, there's only so much you can do. We're not near the limit measuring all the galaxies we can measure, but we're going to make some dent in that. There's just a limit to how much you can — we're going to do a pretty good job on one third of the sky with DESI. So, there are real problems there. The problem with accelerators is different. It's cost and time. I have a hard time dealing with discussions of what accelerator is going to turn on in 2040. I'll be 95. There are younger people, and they can look at it. In high energy physics — maybe this is a strange way to put it — I think a major challenge for high energy physics will be morale.

Zierler:

Yeah.

Cahn:

It takes a lot of fortitude to say, "Yeah, let's work at this same energy for the next 20 years and see what we find." I think that's tough. One thing we didn't talk about was BaBar, where I actually was on an experiment. There, I remember that people felt that if we ever got to a point where it took two years, or certainly three years to double our data set, the experiment was over. Of course, it got terminated by DOE prematurely, but the point is that you're only willing to spend so much of your life doubling a data set. If you're only seeing your errors by square root of two, how many years of your life is that worth? I find that's a big problem for the high energy community. It's a very rough business. I would have been happier if the direction were somehow to get to high energy fast. 12 Tesla in the current tunnel. I worry that people won't have the fortitude.

Zierler:

So, what makes CMB and inflation different? What's untethered there that makes you so excited about this?

Cahn:

Those are experiments that can be done in a reasonable period of time, and they address real questions. There are predictions from inflation that we hope to measure, and we thought we had measured it. This r factor looking for the B modes. But there are things to measure, and it's a technique that's improving, and has proved its value. People know how to do it, and that's great. Mind you, DESI will do dark energy. Like I said, this is very parochial. And then we do CMB S4. What cosmology is after that, I don't know. The problem here has to do with reductive science. This Phil Anderson business, and how terrible reductionism is. But he had a point in the following sense, that one of the attractions of condensed matter science is there's no end to the questions. There's no end to materials, and there's no end in questions. There are ends to our questions.

Zierler:

Which is ironic, because the universe is like, you'd think there's no end.

Cahn:

But the problem is there's an end to our questions. We have great questions right now, but are there questions beyond those? We don't know that we can answer those questions. They're great philosophical questions. I don't know whether they're physical or metaphysical. How many constants are there that we could ever hope to calculate, and how many are incalculable, they're just givens? Einstein said, "God doesn't play dice with the universe." But I think current thinking is God plays dice with the universe in the sense that there's some universe, and there are a certain number of constants about which you can never know anything. They just are what they are, and if you're really lucky, you'll be able to calculate some of the other constants from them. So, I wrote a paper — actually one paper which I never mentioned, which you may or may not know. I wrote a paper called the 18 Parameters of the Standard Model in your Everyday Life. It had to be changed to 27 after we found out about neutrino masses, but the basic idea is there. Another way of looking at that problem is, well, how many could ever be calculated in terms of others? How many fundamental parameters are there? Well, the problem with supersymmetry is the number of parameters got to be like 140, which didn't look so elegant anymore. Even 18 or 27 sounds like a lot. Are they the roll of the dice? Is that the answer? It used to be very popular to try to figure out relations between quark masses and the CKM angles. Because they all come out of the same matrix. Somehow there must be a relationship. You'd like to think that there aren't that many parameters that God set at the time of inflation. But there is a problem with the reductionist approach. It means you run out of questions. You say, "This is an interesting question. How galaxies form? Okay, I'm not interested. She can go figure out how galaxies are formed. I don't care about how galaxies are formed." If you take that opinion, you run out of questions, or you narrow yourself down to questions you can't answer. I think that's a danger on the particle physics and cosmology side. Because it's so reductionist in the questions that it asks, you can run out of questions. It doesn't even really answer those questions, but if you say the only questions I care about are where's the CP violation that's responsible for the baryon symmetry, and what's dark matter, and what's dark energy, — if those are the only questions that you care about, you have a pretty narrow field, and you may not be able to answer those questions. So, that's a danger, I think. All the stuff that's near and dear to my heart.

Zierler:

You're saying that's a danger for the field, not a danger for the kinds of questions people are asking, because there might not be other questions to ask?

Cahn:

Yeah. I mean, maybe there are, and I don't want to get into metaphysical questions myself about multi universes and all that sort of stuff. I can't do an experiment in them. If you can get a real physical consequence in our universe, yes, I'm interested. But I do worry that we've been too successful, and what we desperately need is for these models to fail. We can't learn anything until they fail. Or until we make a new discovery, which is sort of like them failing. It's all worked too well. You can still say, well, what kind of answer is this if you told me there are 27 fundamental parameters? Is this an answer to anything? But I don't know what experiment to do to address that question. Once I've got those parameters, and I write down their values, the experimenter has done his or her job, and it's somebody else's job to figure out how to reduce the number of those parameters. At that point, the difference between physics and metaphysics starts to become a little bit blurry. So, I think our fields are in trouble because we narrow the questions. It's not our choice. That's the way the science worked. It's because we seem to have narrowed down to a few fundamental questions by answering so many questions. I mean, almost all the stuff we're talking about, everything we've been discussing was completely unknown when I wrote my thesis.

Zierler:

Right. That's unbelievable. That's amazing.

Cahn:

So, what's been achieved is really, absolutely incredible. But it's left us in a tough spot, I think.

Zierler:

Well, Bob, I think that's a great transition to my last question, and that is, using your powers of extrapolation, and all you've learned over the course of your career, and on the basis that physicists never retire, you obviously have thought deeply about where the field is headed next and this issue with diminishing returns on diminishing questions. What do you see personally as the most fruitful area and questions for you to work on yourself?

Cahn:

I think I could give an answer, but it isn't what I'm working on. It's what I should be working on.

Zierler:

All right, so what are you working on, and then really, what should you be working on?

Cahn:

I've been working with a young cosmologist, an astrophysicist, named Zack Slepian, who's at the University of Florida. He was at Berkeley, and that's why we were working together, but he's become Assistant Professor in Florida. We've been doing some work that grew out of work he had done earlier with Daniel Eisenstein on multi-galaxy correlations. Instead of just measuring the distance between two galaxies, you measure triangles, or quadrilaterals, and look for those kinds of correlations among the galaxies, which is to say the matter distribution in the universe. This has a very technical side to it, which has to do with using complicated combinations of spherical harmonics, a field that was really established by nuclear physicists in the '40s and '50s, and to a certain extent, adopted by particle physicists. We're going back to some of those techniques to develop means of analyzing data from experiments like these. How do you look for three particle correlations, four particle correlations? Particles here are galaxies. So, that's what we've been working on. A lot of it has been rather mathematical. Both Zack and I have a distinct weakness for mathematical, analytical things, which is dangerous. There's an anecdote I tell, which appears, I think, maybe in my biographical memoir for Dave Jackson, but when I was taking Dave's course in particle physics, and we were doing stuff with Regge poles, which is an extremely mathematical topic, especially using special functions and complex variables. That's something Dave was a master at. He has all the blackboards in this lecture room filled with these unbelievably complicated formulas, and special functions, and everything. It was so popular in the late 19th century, in England, and Dave stepped back from the blackboard, and looks up at it and says, "Born a century too late." So, he had this great weakness for Bessel functions, and such. I've been doing lots of stuff with Bessel functions with Zack. It's all too seductive and takes up too much of our time. We need to get done with it and go onto something more physics.

Zierler:

But it's fun, it sounds like.

Cahn:

It's fun, but we need to get done. There are other things we can do that are more physical. If a young person came to me and said, "What should I work on?" The answer is dark matter. I think that's a great problem for physical science. Whether it's in astrophysics, or cosmology, or particle physics, I think all of them have an obligation to figure out what 80% of the matter of the universe is that we haven't been able to find. I think that's easily the most pressing question because we know it's there. I mean, dark energy could be just this cosmological constant. And there you are. There's nothing more to say. The only ways we know particle physics isn't done is because we haven't found dark matter, and we know that dark matter is a particle, or something like a particle. It's particle-like. It's some kind of matter, maybe more wavelike. But it's not in our model, and it's physically out there. So, that, I think, is the great problem. That's what I think a lot of people should be working on, and a lot of people, both theorists and experimentalists are working on. I think it's a good choice. Of course, dark matter could be found with the LHC.

Zierler:

In the way you're saying it as a future historical artifact, right? It almost sounds tantalizing, that it's within reach.

Cahn:

Well, we don't know. It could be. That's the problem is we don't know. The Higgs boson, we didn't know what its mass would be. A lot of were expecting it to be much heavier. A lot of the work that we did focused on how to find a really heavy Higgs boson. One of the stories I didn't way was some of the work I did with Sally, when we were finding a way to make the Higgs, and the competing method was the gluon fusion, the gluon fusion goes through a top loop. To calculate that, you need to know the mass of the top. At the time, we didn't know it. So, when we compared our technique with that technique, they were sort of comparable. That's because we thought the mass at the top was 50 instead of like 170. It turns out, they come in as the square, so we're only a tenth as powerful as that technique. We didn't know the top mass when that was being done. Dark matter is what you need to care about right now, I think, because it's really there. The worst thing, of course, is that it could have only gravitational interactions, and then it’s just the astronomers and astrophysicists that have any chance of understanding it. I think if I had to wager money, I would guess there's a better chance of finding that than finding something at the LHC at the current energy.

Zierler:

Right. Well, we'll see. Historians will look back and –

Cahn:

I hope we'll see. I consider myself very lucky to have been alive when we found the Higgs boson. The first paper I wrote about the Higgs boson, I think, was in '79. It had been around, and people to talked about it for quite a while before that. So, I had to wait from '79 to 2012, but at least we found it. There are no guarantees on dark matter. Higgs boson, as I said, might have been a lot heavier. We thought about that. It turned out to be quite light. Light enough to be found at the LHC. We need to be lucky for something else now.

Zierler:

Well, Bob, on that note, it's very exciting. We’ll have to stay tuned, and we'll have to see where this all heads. I want to thank you for spending this time with me. It's been absolutely phenomenal talking with you, and I really appreciate your insights.

Cahn:

Yeah, well, it's nice taking to you. I hope you get a chance to talk to some of the people I mentioned who you haven't spoken to. Particularly, I think of Rick Field, and Sally Dawson as people that you would find really interesting.

Zierler:

Absolutely.