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Credit: Cliff Will
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Interview of Cliff Will by David Zierler on June 2, 2020,Niels Bohr Library & Archives, American Institute of Physics,College Park, MD USA,www.aip.org/history-programs/niels-bohr-library/oral-histories/XXXX
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In this interview, David Zierler, Oral Historian for AIP, interviews Cliff Will, Distinguished Professor of Physics at the University of Florida. He recounts his childhood in Ontario, Canada, and explains his decision to enroll at McMaster University, which was both nearby and offered an excellent physics program. He describes his studies with Bertram Brockhouse and how he developed his skills and interests in theory. Will explains his early impressions of Caltech, and how different California felt in the late 1960s. He describes his graduate research in general relativity under the direction of Kip Thorne, and he explains the significance of his calculation of the n-body equations of motion, which was the first post-Newtonian approximation of general relativity. Will explains the import of recent experimental advances in general relativity and how this advanced theoretical work. He describes his postdoctoral research at the Fermi Institute and his attraction at the concept of working with Chandrasekhar. He explains his decision to join the faculty at Stanford, and the state of the field in general relativity and gravitational radiation in the early 1970s. Will describes the circumstances leading to his work at Washington University and the research he did at the McDonnell Center for Space Sciences. He discusses his service work for the National Research Council and his advisory position on the Stanford-NASA space mission called Gravity Probe-B. Will describes his interest in conveying scientific concepts to the broader public, and the excitement he felt in joining the LIGO collaboration. He discusses his recent research interests at the University of Florida and his ongoing collaborations in France. At the end of the interview, Will reflects on what has been confirmed and improved in the field of general relativity since the time of Einstein.
This is David Zierler, oral historian for the American Institute of Physics. It is June 2, 2020. It’s my great pleasure to be here with Professor Clifford Will. Cliff, thanks so much for being with me today.
Thank you. It’s a pleasure being here.
So to start, please tell me your title and institutional affiliation.
I’m Distinguished Professor of Physics at the University of Florida. I am also McDonnell Professor of Space Sciences, Emeritus, at Washington University in St. Louis, which is where I was for 31 years before I moved to Florida. I’m also a Chercheur Associé at the Institute for Astrophysics in Paris where I spend substantial amounts of time each year.
Okay. Now let’s go right back to the beginning. Tell me about your family background and your early childhood in Canada.
I was born in 1946, and in fact I’m a ground zero Baby Boomer. I was born literally nine months after my father was demobilized by the Canadian Army in early 1946.
[Chuckles] Strategic timing.
Indeed. These days being called a Boomer is not necessarily such a good thing, of course. My parents were working class. They had high school educations, but not beyond. They both worked in my hometown of Hamilton, Ontario for the Proctor and Gamble company, a big soap manufacturer in those days. But they were intelligent people. They grew up in the Depression, so they didn't have the opportunities to go beyond high school; they had to go out and work to earn money. At the end of his career at P & G, my father functioned as a professional engineer with the company at a very high level, even though he had no formal educational training in the field. He learned on the job. My parents both insisted that my younger brother and I study hard and go to college and so on, and so education was a very important thing in our family. My brother had training as a chemist and had a long career in the specialty gas business in Canada.
So after graduating from high school, I went to McMaster University, my hometown college, which in those days had a very strong physics department. They had Canada’s first research nuclear reactor. One of my professors in 1964 was Bertram Brockhouse who later won the 1994 Nobel Prize in physics (with Clifford Shull) for his work using slow neutrons to study crystal structure in materials, both at McMaster and at the Chalk River nuclear reactor near Ottawa.
Cliff, just to back up to high school, I’m curious, when you were thinking about schools to apply to, if you were thinking about physics programs specifically.
Yes. By the eleventh grade I wanted to be a physicist. That was settled.
How did you develop such an early interest in physics?
Actually, in the ninth grade, I wanted to be an architect, because I liked drafting. In the tenth grade I wanted to be a geneticist. This was the early days of DNA and things.
I wanted to work for the World Health Organization, believe it or not. (Ironically WHO has become a political football during the current coronavirus pandemic). But by the eleventh grade I’d done a little reading of popular books on physics and astronomy, and so that pretty much made up my mind that that’s what I wanted to do. So I took all the physics, chemistry and math courses I could in high school, and then just registered as a physics major at the registration desk on day one at McMaster. No waiting to see.
Did you have a good physics teacher in high school?
I did. Glendale High School was very small at the time. It was built as the expanding population of Hamilton moved into the suburbs. My class was the first to attend and graduate from the school. So there was basically one science teacher and he taught both physics and chemistry.
He was a very inspirational teacher. One year in either physics or chemistry class (I don’t remember), I crossed a line with him. A friend and I were rather cutups in the class. We were both the best students in the class, and he couldn't decide whether to put us at the back of the room so we wouldn't disturb the students with our whispering and commentary, or put us in the front of the room so he could keep his eye on us. But it was clear we were his top students.
So one day he was teaching about the Bohr atom with the nucleus at the center and the electrons going around in planetary-style orbits, and we were snickering and whispering because, of course, it’s wave functions and quantum mechanics. So he finally became fed up and said, “Okay, Cliff. You're such a smart ass.” He didn't say exactly that, but it’s what he implied. “I want you to come back next week and give a lecture to the class about quantum mechanics and the quantum mechanical model of the atom.” Now another teacher would have just sent us to the principal’s office to be smacked down, but he said, “Okay, why don't you tell the class what’s the right picture?” [Laughing] So I did. But even though we annoyed him, he saw that we needed to be nurtured, and he did what he could to motivate us because we were a bit above what he was teaching. So I always admired that about him, and in fact I kept in touch with him over the years afterward.
Now your parents, you said your parents were working class. When you told them that you wanted to study physics as a career, did that register with them? Did they ever think that this was a concern; you wouldn't be able to support yourself?
I don't think so. I think that, while I wasn’t a super genius, I always had top grades, and it was clear that I would do well in whatever I did. I think from their point of view, physics was sort of like engineering, and that was a good career. It’s actually something that drives me crazy these days when I hear about high school counselors encountering a student who is interested in physics and they say, “Okay, why don't you become an engineer?” because they don’t realize that Physics can be an actual career.
But I think in those days, my parents somehow thought it was like engineering and so that would be okay.
Yeah. So McMaster was not only a top physics program; it was in your backyard.
That’s right, it was home. I just drove to school. I lived at home, at least until my senior year. By that time, I realized that I’d be going to graduate school, and it would be unlikely I would stay in Canada because there just were many more opportunities in the US in the late 1960s. I wanted to do theoretical physics, but I didn't know what kind. And I was going to apply to Princeton, Caltech and Cornell, the usual fancy places, with a few backups in Canada. So it was clear I was going to leave, and so I figured I’d better spend a year living away from home figuring out how to live independently.
I’m curious, Cliff. In your burgeoning interest in theoretical physics, was that sort of negatively defined? In other words, were you not that good in the labs, or were you not much of a tinkerer? Or you just had a stronger inclination towards theory?
I had a stronger inclination. I wasn’t a natural experimentalist. You know, I never built radios, or had a telescope. I had the usual chemistry set that most kids had, but I wasn’t fascinated by that aspect of it. Generally speaking, I hated the lab courses that I had to take at McMaster. The best part of the lab courses in my view was developing a complicated model or theory to explain my crappy data.
That appealed to me, trying to figure out why I was getting such bad results, and then analyzing that part. The actual measurements interested me less. In fact, I was accepted, among other places, to Cornell for graduate school, but I turned them down because they required all first-year grad students to take a lab course.
[Laughs] No-go for you.
Of course, it’s ironic that I’ve spent my entire career—and we’ll get into that later—thinking about experimental tests of GR …
…and talking to the experimentalists who make all of these things work!
I guess Caltech didn't have that first-year requirement.
No. So I got into Caltech. I actually think I got in on their second round because the acceptance letter came late. I’d actually already accepted another university, but this was within the period when you could change your mind without being frowned on, and so I turned down the other place and accepted Caltech.
So Cliff, I’m curious. Before we get to the academic side, you arrive at Caltech in the late 1960s. It’s quite an interesting time to be in California. What was the scene like there when you arrived?
Right. So in fact, when I told my classmates at McMaster that I was going to the US for graduate school, they were horrified. In 1967 there had been “race riots” in big US cities; in the summer of 1968, we’d had the Martin Luther King and Bobby Kennedy assassinations. Canadians by nature tend to be somewhat anti-American or at best have a love-hate relationship with the US. My classmates were unhappy that I would even consider going to the US. I mean how could you go to a place like the US with the Vietnam War going on.
Sure. People were leaving the US to go to Canada, not the other way around. [Chuckles]
Exactly, and people were worried that I’d get killed. The US was viewed so negatively then that my fellow students were just appalled. Now on the other hand, Caltech was not exactly a hotbed of social protest. I mean I remember during the Cambodia bombings we all put on black armbands, but it was not exactly Berkeley in terms of protests and unrest. Students had too many homework sets to do to have time for protesting. [Laughs] But it was a very interesting time to be in the US and California. I arrived in August of 1968, and the next three years we experienced Vietnam and Nixon, and all the stuff going on was very fascinating. Not to mention the warm weather and the beach!
Now did you connect with Kip Thorne immediately or did that develop later on?
It turned out that at that time there was a fairly substantial band of Canadian grad students in physics at Caltech. I think the view was that Canadian students—especially from Ontario because we had five years of high school, and the universities were good—were better prepared than many American students. So in those days, Caltech, among other places, admitted a lot of Canadian students, especially in the physical sciences.
So I knew a bunch of Canadian students already before I went down there, and so when I arrived, they kind of took me under their wing and helped me get acclimatized. But I was pretty clueless. I mean I didn't even realize that as a teaching assistant I was assigned an office. It was at the end of my first year and one of the secretaries said, “We noticed you haven't used your office,” and I said, “What? I had an office?” [Laughter]
Also, Kip had just joined the faculty at Caltech in 1967, and the graduate brochure that they had sent me did not have him listed. I had no idea that he existed. So after about a month and a half into the semester, I was taking my classes and studying and so on. My Canadian friends said, “What are you interested in doing for your PhD?” and I said, “Well, I know I want to do theory, but I’m not sure what kind.” Of course, in those days many students went to Caltech wanting to work with Richard Feynman or Murray Gell-Mann, not realizing that they took very few students. So the choices were a bit limited and they said, “Well, why don't you check out this new faculty member. He’s kind of weird. He has this flaming red hair. He looks kind of odd and he has a weird name, “Kip”. Why don't you go talk to him?”
So I went to talk to him and he said, “Well, if you're interested in general relativity research at all, you're out of luck because I’m teaching the GR course as we speak, and it’s only taught every other year. So you’d have to wait till your third year to take it, and that would be hopeless.” So I quickly dropped an astronomy elective that I had signed up for and signed up for his course. I had to make up a month and a half worth of homework sets and lectures. I mean, that semester I dropped 20 pounds. I really had to work hard to catch up. But evidently it worked out all right because in the spring semester of the course I wrote a term paper, and Kip was sufficiently impressed with it that he asked if I was interested in sitting in on his group meetings.
What was the paper that impressed him so?
It was a calculation that in many ways formed the foundation of my entire career. I calculated, in the first post-Newtonian approximation of general relativity, the so-called N-body equations of motion, in which you have N particles treated as point masses. This had been done by de Sitter in 1916 and by Einstein, Infeld and Hoffmann in 1938. I used an approach based on Chandrasekhar’s development of post-Newtonian hydrodynamics. I treated each body as a fluid ball and then integrated over each body to find an equation for its center of mass. At the end of the day, of course I got the same equations as de Sitter and EIH, but I carefully went through the hydrodynamical details, which is quite different from what they had done. Today it’s almost a trivial calculation, but I had to learn it and do it, and Kip thought it was pretty good., and
Cliff, I’m curious if at McMaster you had already developed an interest in general relativity?
I had no idea about GR. There was nobody on the faculty who was what you would call a relativist, nobody who would be called an astrophysicist or an astronomer. Since then McMaster has developed a very strong astrophysics program, but in those days there was nobody. The emphasis was on solid state physics and nuclear physics because of the strong tradition of Brockhouse and a physical chemist named Harry Thode who had really developed those branches of physics as a strong focus in the department. I mean I knew about Einstein, but I didn't know general relativity. I had no idea about black holes. We’d heard vague things about them, but there was no one there capable of teaching anything related to GR.
Now looking back, was that emblematic of academic physics as a whole? Was general relativity in the early, mid-1960s—had it sort of fallen out of favor relative to the other disciplines?
Pretty much. This is a story that I’ve written about in my popular books and other articles, and many historians of physics have written about it. After the big publicity over the measurements of the bending of light in 1919, from the late 1920s on, research on GR as a field of physics pretty much died away so that by the late 1950s and early 1960s, it was considered a backwater of physics.
A backwater because it was already so well established that there were diminishing returns?
No. In fact, in some ways the opposite. The experimental evidence for GR was still very weak, and no one felt that it applied to anything interesting. Even in the one place where it might be of some importance, cosmology, it seemed to fail. In the 1950s, the measured value of the Hubble constant implied that the universe was younger than the Earth. So it just was not a robust field. The field was also very small; there was not a community of people working on it who could interact and help build it up. In fact, Kip, who had been an undergraduate at Caltech, tells the story that when he graduated and was about to head for Princeton to go to graduate school, a very famous Caltech astronomer told him that under no circumstances should he study general relativity because it would never have anything to do with normal physics or astronomy. Luckily for me and for many others, he fell under the spell of John Wheeler and did his thesis in general relativity.
Right, right. So was it a bold move on your part to sort of not take that advice to heart, or was this something that just sort of captured your imagination and you didn't care about the implications on your career?
I think in those days, I was not focused on “my career”. I think that students in general were not as career oriented as they seem to be today. I mean I just sort of loved studying these things, and I loved being at Caltech. I had no idea that general relativity was still considered a rather marginal field. What got me at the end of the day was Kip. He was a charismatic scientist, a wonderful teacher and an inspiring mentor. As I said, I finally started taking his course. He was teaching out of draft chapters of what would become “Misner, Thorne, and Wheeler” the famous GR textbook. He would bring in chapters in preprint form and pass them out for us to read for the week’s material, and these were the chapters that would be going into the book. So the atmosphere was that of being at the beginning of some amazing thing. I didn't quite know what it was going to be, but it was like being on the ground floor of something.
Yeah, yeah. Was your sense that Kip was on that ground floor? Was he part of that?
He was. By the spring of 1969, I was sitting in on Kip’s group meetings, just absorbing what was going on (I didn’t really understand very much). Most of Kip’s students were working on various astrophysical implications of general relativity. Because Kip was part of what’s called Kellogg Lab at Caltech, which is where the astrophysicists like Willy Fowler lived, he was very much in that mode of thinking about astrophysical problems as opposed to the more mathematical side of the theory.
One of Kip’s students, Richard Price, was working on oscillations of the Schwarzschild solution and the gravitational waves they would emit. The name “black hole” had just been attached to this solution by Wheeler a few years earlier. The name had not been totally popularized or accepted. One of his students, Jim Ipser, was working on whether or not the redshifts of quasars could be explained by quasars simply being very dense clusters of stars whereby the redshift was a gravitational redshift from the strong gravity of the cluster, not from the expansion of the universe. Ipser showed that if you had a star cluster of that density, it would be unstable and would simply collapse into a supermassive black hole. So that could not explain the redshifts of quasars. So everything in Kip’s group was linked somehow to astronomical observations.
But that spring of 1969, Joe Weber announced the first detection of gravitational waves. This was a big crisis because, if he was correct, the strength of the waves he was detecting was orders of magnitude larger than anyone could ever imagine.
What were the immediate theoretical implications of this?
Well, they evolved very quickly, and certainly in Kip’s group. One possibility was that instead of looking at oscillations of the Schwarzschild black hole, which produced gravitational waves too weak to explain Weber’s events, maybe if you looked at the rotating black hole, the Kerr solution that had only been discovered five years previously, maybe weird things could happen. Maybe the rotation of the black hole, which induces a twisting of spacetime around the hole, could cause relativistic beaming of the radiation. This problem was attacked by Kip’s students Bill Press and Saul Teukolsky. But another possibility that Kip realized, and this had an impact on me, was that maybe general relativity is simply wrong. You know, maybe we’re using the wrong theory.
Is that a stand-in for saying that maybe Einstein himself is wrong? I mean, what’s the overlap there?
Well, already in the early 1960s, an alternative theory of gravity called the Brans-Dicke theory had been invented, and at the time it was a viable competitor to general relativity. There was not strong experimental evidence constraining Brans-Dicke theory, so it was a viable possibility. On the other hand, it didn't do any better at explaining Weber’s results. But still, the experimental evidence in favor of GR was sufficiently weak that maybe it isn't the right theory. So in my memory, I picture Kip looking around the room. I might have been hiding behind another graduate student. He pointed at me and said, “Cliff, I want you to find out everything that we know currently about the experimental tests of general relativity because maybe it’s wrong and we have to consider that possibility. It may be important to shore up the experimental foundations of it if we’re to really understand what’s going on with these gravitational waves.” It didn’t exactly happen that way, but it pretty well summarizes what was probably a discussion over several days between him and me about what I should work on.
What’s your game plan? How do you go about assessing the full field of experimental tests?
Well, I read a lot of articles. But also at that time, the nearby Jet Propulsion Laboratory (which is overseen by Caltech jointly with NASA), had just launched the Mariner 6 and 7 missions to Mars, and were planning future missions. One of the aspects of Mariner 6 and 7, kind of a side experiment, was to measure the Shapiro time delay, a prediction of general relativity discovered theoretically by Shapiro in 1964. This the delay of a radar tracking signal as it passes by the Sun when the planet is on the far side of the solar system.
This is the Irwin Shapiro delay.
The first test of the effect had been done by Shapiro and his team by bouncing radar signals off Venus and Mercury, and the JPL people wanted to do better with Mariner 6 and 7. They were also looking at future missions like Mariner 9 and Viking. So Kip told me to go up to JPL, talk to the people up there who were planning these missions, and find out what accuracy they thought they could achieve and so on. This got me thinking about how advanced technology and space missions could be used to test GR.
In my survey of the literature about various other tests, I discovered the remarkable papers by Kenneth Nordtvedt at Montana State University. In 1967, Ken did a calculation of the N-body equations of motion, using not general relativity but the Brans-Dicke theory. He had discovered that, in this theory, when you have bodies that are massive and are held together by their own gravity (such as planets or stars), then that gravitational energy could fall in an external field with a different acceleration than normal matter. In particular that would make a difference in the way the Earth and the Moon fall toward the Sun. With lunar laser ranging about to start with the planned placement of a laser retroreflector on the moon by Apollo 15 astronaut Neil Armstrong in the summer of 1969, he proposed that it would be possible to test this effect. Today we call it the Nordtvedt effect. The effect is zero in general relativity. GR says all bodies fall with the same acceleration, but lunar laser ranging could test GR against alternative theories such as the Brans-Dicke theory, which predicts a small difference.
Along the way, he developed a way to take this post-Newtonian approximation of general relativity and Brans-Dicke theory and to stick arbitrary parameters in front of the various terms, generalizing things so that you could do these calculations in a wide class of gravity theories. So one of the things I did that first summer was to study his papers in detail.
Ken had developed a point-mass approach to this parameterized framework for describing theories, and I realized you could generalize his approach to perfect fluids as I had done in my term paper for Kip. So you treat each body, such as the Earth or Moon, as a realistic ball of fluid with internal pressure and energy density rather than as a cloud of point masses held together only by their mutual gravity. Within a year I had discovered that in some theories of gravity, if you're moving relative to the average rest frame of the universe, there were observable effects that could be tested. And I discovered other properties of general theories of gravity by analyzing this framework. This ultimately evolved into what’s today called the Parametrized post-Newtonian (PPN) framework, which is the workhorse tool for studying experimental tests of GR.
I actually must give some of the credit for the success of the PPN framework to Sandor Kovacs, a fellow student in Kip’s group, later to become a noted cardiologist at the Washington University Medical School in St. Louis. In 1971, right after my graduation from Caltech, Ken Nordtvedt organized a summer workshop on experimental tests of GR at Montana State University in Bozeman. During that summer, Sandor suggested that Ken and I unify our two different approaches into one. Ken used point masses, I used fluids. My version had more PPN parameters than his. We even used different conventions for subscripts (Greek vs Latin) on various quantities. Largely to put and end to Sandor’s incessant nagging, Ken and I made the necessary compromises and wrote two large papers that summer, one establishing the “canonical” PPN framework, and another describing many new experimental tests that could place bounds on the PPN parameters. Apart from a few minor tweaks that I made later in my review articles and books, that framework is still used today.
Yeah. Cliff, I wonder if you could explain the term post-Newtonian, right? It’s like everything is post-Newtonian.
So what does that mean specifically in the context in which you're using it?
You imagine systems where things move slowly relative to the speed of light and where the gravitational potential is small compared to c2. In this approximation, you can expand general relativity, roughly speaking, in powers of v/c and in powers of the gravitational potential divided by c2. At the lowest order of the approximation, you recover Newtonian gravity. The first corrections to the equations are then called post-Newtonian, the next are post-post-Newtonian or 2PN and so on. This approximation works as long as you're not too close to black holes or neutron stars, or as long as the bodies don’t move too fast, but in the solar system and in many other contexts, it’s a good approximation.
I see. Okay.
Of course, these days we go out to 2PN, 2.5PN, 3PN, and beyond, especially motivated by gravitational wave detection. In my group and other groups around the world, we’ve been carrying these approximations up to enormously high post-Newtonian orders.
Is there any theoretical limitation to the PN, how high it can go?
I don't know. It depends on the method used. It becomes, of course, unbelievably complicated. Most of this work could never be done without algebraic software like Maple or Mathematica, because it just becomes too complicated. But for all the purposes for which we want to apply general relativity, it’s been calculated to an order that works unreasonably well. So for example, for the first detection of gravitational waves, the analysis of those signals was done with a combination of post-Newtonian theory carried to suitably high orders and numerical relativity (direct solutions of Einstein’s equations using massive numerical computation), which you need to do for the very final stage of merger of the two bodies. Those two approaches work so well and particularly so in an overlap region that you can really stitch them together to get a complete picture of the whole gravitational wave signal of a coalescing binary from very early on where post-Newtonian theory works fine, all the way through to the final oscillations of a final black hole. People have developed analytic formulas that are very accurate fits to these combinations of techniques. But in the places where they overlap, they agree unbelievably well. It’s really been very satisfying to be able to work in this approximation to really see how well it works.
Yeah. So Cliff, let’s get back to the narrative at Caltech.
How do you go about putting your dissertation topic together? Does Kip essentially hand you a research question, or you were doing this mostly on your own?
I was able to do it mostly on my own, because basically it was low-hanging fruit. I mean it was a problem that was ripe for exploitation: first generalizing Ken’s framework to a broader one, looking at some of the new consequences of it that people hadn't realized before. If you focus on general relativity, you have no idea that, because the solar system is moving relative to the cosmic background radiation, there are effects that you could measure and test. They’re all zero in general relativity (GR exhibits a kind of Lorentz invariance for gravity), so you would never even know the effects were potentially there if you focused only on that theory. So the failure to detect an effect can be just as important as detecting an effect. Ken Nordtvedt was fond of stating that “zero is just as good as any other number”.
But if you broaden your attention to other theories, you see there are things that could be tested. Many tests were carried out over the years, leading to very tight limits on most of these PPN parameters, all consistent with general relativity.
And I’m curious if when you say it was low-hanging fruit and only a few years earlier GR was a backwater, did you see your work as part of restoring GR to its rightful place? Or were you still ahead of the curve in that regard?
I don't know if I really thought about it at that time. I mean, when you're a graduate student, you're really working on calculations, making sure they're right and so on. Within a few years, especially after the discovery of the binary pulsar in 1974, it had become clear that this whole process from sort of the mid-’60s onward constituted what I have called the “renaissance” of general relativity.
I was just a player in it and carried along by the tide, but I didn't have such lofty thoughts as a graduate student. [Chuckles] I was just trying to get my work done. But in terms of thesis, I mean Kip pretty much left me alone. For example, I found these so-called “preferred frame” effects, the fact that moving through the universe could have observable consequences in some theories. Kip didn't believe it at first because he was so wedded to Lorentz invariance as a big part of gravity somehow. It took a while for me to convince him that it actually was correct if you look at various alternative theories of gravity.
So as it turned out, my PhD thesis just consisted of two reprints and two preprints of papers stapled together with a four-page introduction. That was a model that was allowed at Caltech in those days. I didn't actually have to write a formal thesis. I just put together the papers that I had written and I was done. I don't know if I still do, but at that time I think I held the world’s record for PhD speed at Caltech. It was about two and a half years. I entered in 1968 with only a Bachelor’s degree, and I did my defense in the spring of 1971. And I beat my late friend David Schramm, a student of Willy Fowler and Gerry Wasserburg, and a founder of “astro-particle” physics, by about two months.
What was the secret to your speed, would you say?
Well, again, this problem, as I said, was low-hanging fruit. I wrote one paper after another just finding these new things because few people apart from Nordtvedt had thought about it before. This resurgence of doing experiments and building a theoretical background for doing them was something that was just new and it was ripe.
And in terms of—you know, not particularly having lofty thoughts about the general place of GR in the field at large—were you thinking about the impact of your research and the way that it would have larger ramifications on GR itself?
I mean I certainly felt that it was important to be able to carry out these new and different tests of GR. But even though I thought I hated experiment, I certainly appreciated the fact that you have to have experimental confirmation if you're to believe the theory, and the work that I was doing, Ken and others after me, we were finding new ways to add to this kind of catalogue of tests, and also new understanding of what they mean.
So for example, two students who followed me in Kip’s group, David Lee and Alan Lightman, extended this notion. David later became quite successful in the finance world and is now Chairman of the Board of Trustees at Caltech, and Alan became a well-known novelist and science writer. They focused on a classic test of the foundation of general relativity, the so-called Eötvös experiment, which verified the equality of acceleration of different bodies, of different materials. Well, David and Alan developed a framework that really helped you to understand the implications of those measurements for various classes of theories, including general relativity and including other theories that today we call non-metric theories of gravity.
So this really appealed to me, doing theory that helped you to really understand what experiment was telling you in a way that’s deeper than pure phenomenology. You know, you can say that bodies fall with the same acceleration to some precision and stop there, but here one could now say, “This now tells us that this class of theories is dead in the water. This class of theories survives,” and so on.
Who was on your thesis committee?
I don't remember everybody, apart from Kip. I know Tom Tombrello was on it. I can tell a story about Tom Tombrello. But I actually forget who else was on it.
Let me hear the story about Tom.
So Tom was a well-known nuclear physicist at Caltech. It was clear I was going to sail through my defense, but I was still nervous like every graduate student. During my presentation, I was talking about my work on the PPN framework and everything was going smoothly, but I made the big mistake of mentioning neutron stars. So Tom said, “Oh, that’s interesting, Cliff. Tell me about neutron stars.” So I said, “Oh, they’re stars maybe one and a half times the mass of the Sun, very compact, 10-15 km in radius, the basis for pulsars, and they’re made of neutrons.” Tom said, “Well, neutrons are unstable with a half-life of 1,000 seconds. How come the neutrons survive?” and I literally stood there for maybe 20 minutes. In reality it was probably a minute and a half, you know. Why don't they all decay? I had no idea. And finally, after what seemed like forever, Kip said, “Well, let’s move on.” [Laughter] I tell this story to students as a lesson in how teaching helps you to learn. It’s only much later when you actually teach a course in something like statistical mechanics that you really understand what’s going on, right? The density in the neutron star is so high that the Fermi level is higher than the mass of the electron, so the neutrons can't decay because there are no quantum states for the electron to go into because of the high density.
So to be clear, Tom was asking an answerable question; you just didn't know it at the time.
I just didn't know the answer! [Laughing] But something like 15 years later, I met Tom at a conference on teaching modern physics, and I told him this story, that he had really almost destroyed me, and Tom said, “Oh no, I would never have done that. You were one of our best students.” I said, “You did it, man!” [Laughter]
You did! [Laughing] So what were you thinking after you defended? What were your options post-thesis?
Well, there were two things. One, Kip was desperate to keep me for another year, not because of me but because in the meantime I had married the woman who was his secretary, and he didn't want her to leave because she was fantastic. His group was growing rapidly and my wife was crucial to keeping his empire going.
So he arranged for me to be an instructor for a year. Actually, I co-taught a quantum mechanics course with him. And in fact, I claim to have taught cosmologist Michael Turner, who was then a Caltech undergrad, everything he knows about quantum mechanics.
Michael and I are good friends. So I stayed on for another year as an instructor, but then finally I had to leave. It took Kip many years to forgive me for taking Leslie away from him, but I finally had to leave.
Did you enjoy teaching, your early experience teaching?
Yes. I enjoyed it. It’s very nerve-racking when you're new to it, but over time I was considered a very good teacher. I taught both at Stanford and Washington U the full menu of courses, from physics for non-scientists to graduate courses, and I always got good ratings. I really enjoyed it and I was good at it. So I think that early experience helped. Sure.
Was your experience that teaching undergraduates at Caltech—were they some of the strongest students that you taught?
They were very good, and I mean in a way, just being barely a PhD myself, it was pretty intimidating. But Kip was such a great teacher. I tried to follow his model.
So Kip was known to be an excellent teacher also.
Already he was known as one of the best teachers in the department at Caltech, so he really inspired me. He emphasized teaching and explaining things clearly not only to students but also to the public and to non-scientists. This something I’ve taken very seriously in my career. So he was very, very inspirational.
So what was your next move? How did the opportunity at the Fermi Institute come about?
My experience is really an example of how times have changed. If I had known any differently, maybe I would have done things differently, but I sort of had this sense that everything would turn out all right. I had two offers to consider. One was an offer from UC Santa Barbara working with Jim Hartle; this was complicated because Jim was being recruited by the University of Illinois at the time. So if he moved there they were going to hire me at Illinois. But then he finally decided to stay at Santa Barbara, and arranged a job there for me. So it would be a tenure-track assistant professorship at Santa Barbara versus a post-doc at Chicago, and I took the post-doc.
Today people would say, “What?! Are you nuts?”
Did you know Jim? Did you know Jim and his work at this time, or this was new to you?
He was a contemporary of Kip, and had been a student of Murray Gell-Mann at Caltech. He was working with Kip a lot on a series of papers on rotating relativistic stars, so he came to campus a lot. So I knew him fairly well, and I had visited both Illinois and Santa Barbara as part of the recruitment and I liked him. On the other hand, at that time, strangely enough, Santa Barbara was a bit of a backwater.
The Institute for Theoretical Physics (now the Kavli ITP) hadn't come yet. The only person there who was noteworthy in our field was Jim, basically. He had gone to Santa Barbara, and his presence then helped to nucleate something that got a lot bigger, leading ultimately to the ITP.
So clearly, despite the job security, there was something really pulling you to the Fermi Institute. What was it?
Well, part of it was Chandrasekhar.
I mean Jim Hartle was a great guy, but Chandra was much more famous. Another was Jim Ipser, a former student of Kip’s. He and I and our wives are very good friends, so it was a chance to go there and spend a couple years with them. And it was the Fermi Institute. I mean, it was astronomers and physicists. It was a very famous place. Santa Barbara wasn’t so prominent, and so somehow I didn't think about job security. Maybe it’s a sign of me and my confidence, misplaced or not. Maybe it’s a sign of the times, although it was only in retrospect that I realized that in fields like particle physics, the 1970s were terrible for jobs. Later in that decade, various things like the discovery of the J/psi particle changed the scene for particle physics. But our field of gravitational physics, we sensed, was on the rise. It was a hot field in some ways, so somehow we felt that we’d come out all right in the end.
Was the Fermi Institute really known to be like one of the places to be for GR?
Yes. In addition to Chandrasekhar and Ipser, the famous mathematical relativist Bob Geroch was there. At that time, Abhay Ashtekar, John Friedman and Steve Detweiler were graduate students, so there were several future famous people there. There was also the Laboratory for Space Research (LASR) where Chandra was housed, so there was a lot of astrophysics stuff going on. And the physics department had lots of famous people in many fields. So it was a prominent place to be, and Chicago, it just was appealing to go to Chicago and live there for a while.
Right, right. What was the setup for you? Was the expectation that you would go there and be handed projects from senior people, or were you going to go there and work on things that you wanted to work on?
It was pretty open-ended. Because it was the Fermi Fellowship, you were not tied to anyone’s specific funding or projects. In many ways, I think that Chandra was rather disappointed in me as a post-doc because I didn't particularly jump into collaborating with him on his projects. I did a few things related to stuff that interested him, but I didn't become a full collaborator, and I think he was a bit disappointed. I had many projects of my own, including follow-ups to my thesis, and projects related to gravitation redshift experiments, extending the work that David Lee and Alan Lightman had done. I was doing a bunch of things that were just so hot I felt I had to keep working on them. So although Chandra and I didn't actually write any papers together, I did write a few papers related to differentially rotating stars and rotating black holes, which was what he was working on at the time.
Did you stay in close touch with Kip during your time in Chicago?
Nothing huge. I mean, we stayed in touch, but…
But you weren't specifically collaborating with him, I’m asking.
No. We actually only wrote two papers together the whole time I was at Caltech, one a little commentary paper on the status of experimental tests of GR, based on the reading that I had done that first year, and the first paper of the series that I wrote on the PPN framework. But all the rest I did on my own. I think Kip sort of felt that he had birthed me and I was…
On your own, yeah.
…good to go.
I’m curious. What was the institutional relationship between Fermi and the Department of Physics at Chicago?
I don't know. I didn't really think too much about it. Again, as a post-doc we don't…
Right. But I’m just curious in terms of the workaday. It was a very separate world from the Department of Physics.
Well yes, because Chandra and his group were situated in LASR (the Laboratory for Astrophysics and Space Research) which was part of the Enrico Fermi Institute. It was a fairly newish building separated by maybe a block, block and a half from the physics department building. So I rarely went over there. Maybe I went there for colloquia, and that’s about the only time I would go into that building. So I really didn't encounter too many of the people like Val Fitch and others of the famous particle physics and condensed matter types who lived there.
Right, right. So what was--
Again, we were just busy doing our own thing.
Right, right. So what would you say, looking back, was your most significant work at the Fermi Institute?
What did I do there? [Chuckles] I have to actually look at my CV or something to see. So inspired by David Lee and Alan Lightman’s work on using the Eötvös experiment to test a class of non-metric theories of gravity, I came up with an idea to do what are called “null” gravitational redshift experiments. So instead of measuring the shift in frequency of a spectral line between the surface of the Sun and the Earth or between a clock in orbit and an identical clock on the ground, you take two different clocks, made using different atoms, or based on different fundamental physical processes. It’s sort of like the Eötvös experiment, comparing the fall of different materials, but here you put these different clocks side by side in a changing gravitational field, say, due to the Sun. As the Earth rotates and moves in its elliptical orbit in the Sun’s field, the laboratory moves in and out of the Sun’s field. If certain aspects of the principle of equivalence were violated, then these clocks would tick at different rates relative to each other. So you just compare their rates and see if there’s any offset or modulation as a function of distance from the Sun.
This turned into an actual experiment carried out in 1978 at Stanford, one of the few experimental papers I’ve actually been a coauthor on, although they didn’t allow me to set foot in the lab. The first clock was a hydrogen maser clock that was built by Bob Vessot, who had done the famous Gravity Probe A experiment in 1976, putting a hydrogen maser clock on a rocket and measuring its rate relative to an identical clock on the ground. He had built a clock that was destined to be installed at the Deep Space Network for satellite tracking in California. So we arranged for him to swing through Stanford where they were building microwave cavities that were cooled to liquid helium temperatures. So the cavities were superconducting. These were all part of a prototype superconducting linear accelerator they were building, but they also could serve as very stable atomic clocks. But the physics of the two kinds of clocks is sufficiently different that if the equivalence principle is violated, then as a function of the solar field, they should drift relative to each other in an oscillatory manner once per day because of the Earth’s rotation, superimposed on a linear drift, caused by the fact that in April 1978, when we did the experiment the Earth is midway between perihelion and aphelion in its orbit. So we actually did that experiment over a two-week period at Stanford and published a very nice paper that was later picked as one of the century’s thousand seminal papers by the Physical Review when they celebrated the 100th anniversary of that journal. So we were very happy about that.
Was your time at Fermi very productive in terms of writing papers and presenting at conferences?
In the summer of 1972, between Caltech and Chicago, I lectured at a summer school in Varenna, Italy on experimental gravity. After that we went immediately over to Les Houches in France where I was a student in the famous black hole summer school that had Stephen Hawking, Kip, Jim Bardeen, Brandon Carter, Igor Novikov and others as lecturers. At that school, Brandon, Stephen, and Jim wrote the famous paper on the “four laws of black hole mechanics” that became the conceptual foundation of black hole thermodynamics. So that was a fantastic summer.
Were you just there as a spectator mostly or did you--
At Les Houches I was a student. In Varenna I was a lecturer at the summer school, so I hung out with Bob Dicke, Bob Vessot and Francis Everett and a lot of my experimentalist friends. But I had to write up those lectures for publication. Those lecture notes later turned into my first book, Theory and Experiment in Gravitational Physics. I also wrote an article for Scientific American. So I was very busy with writing projects during my stay at the Fermi Institute.
Yeah. Now when it was time to enter the job market, you were talking a little bit about what deep trouble particle physics was in during these years. What was your sense in terms of the transition of GR from the backwater? Where was it as a symbol of the job market in the mid-1970s?
I think many of us felt that the field was growing, though not by leaps and bounds. I mean, my contemporaries at that time were Saul Teukolsky, Bill Press, and Bernard Schutz (Caltech graduates) and Bob Wald (Princeton). All these people were of similar age coming out, but of course we’d all come from top places and so we somehow had a sense that we’d make out all right. It’s interesting to note that all of us were later elected to the National Academy of Sciences. So an alternative view might be that in this period around 1968, the emerging field of GR attracted a rather unique group of people. This is something for historians of science to ponder.
And I’m curious in terms of that expansion. Were the kinds of opportunities that you were looking at—were they looking to add as opposed to replace a retiring GR person for most of these faculties?
I don’t think it was as dramatic an expansion as you might have thought, given the growth of the field. I think it was only modest growth because departments then were still dominated by particle physicists and condensed matter physicists who might give a token nod to GR and assign a single faculty slot, but still regarded themselves as the bread and butter of physics. I think that a more dramatic expansion has happened since LIGO’s success, particularly in smaller universities. You know, being able to get a member of the LIGO collaboration on board and then build up a small but active group at a smaller place. That’s been important.
So in those days I wouldn't say it was a huge expansion, but I think it was enough that for us coming out of our first post-docs, we felt that it was still a pretty positive outlook.
Yeah, yeah. So how did Stanford come together?
Well, the main person who recruited me to Stanford was Bob Wagoner. I had applied to Cornell, but they hired Saul Teukolsky. Bob Wald went to Chicago, Bill Press went to Harvard. I had an offer from Jet Propulsion Laboratory working on GR because, as I mentioned, they had a large group involved in using satellite data to test GR, for example using the Shapiro delay. They even had a group of theorists there who were working on some mathematical aspects of GR. So I had that job offer. But ultimately I picked Stanford because it was a full tenure-track faculty position. We were also happy to be returning to California. My wife was born in San Francisco, but grew up in Burbank, so she was a California girl. But the main attraction was Wagoner. He had been a post-doc at Caltech (just before my time there) working with Willy Fowler and Fred Hoyle on seminal calculations related to Big Bang nucleosynthesis. I knew he was a great guy, and so I decided to go there.
What was Bob working on during the years when he was recruiting you?
He was working a lot on aspects of post-Newtonian theory and on gravitational waves. I think he had pretty much moved on from Big Bang nucleosynthesis, and was working more on relativistic astrophysics. He was certainly interested in GR, but had his feet firmly in the astrophysics camp because that was his training and background. I think he liked me because I was an expert in GR, but I also wanted to work on things that had observational implications. I wasn’t off doing formal mathematical things or quantum gravity or stuff like that.
Right. So you were specifically interested in having more interplay with experimentalists. That was one of the attractions.
In addition to Bob Wagoner, there was a large group headed by Bill Fairbank building gravitational wave detectors to try to prove or disprove Weber’s claims of detection. Francis Everitt was leading the early development of the Gravity Probe B gyroscope experiment to test GR, so there was certainly some experimental gravity activity there and that was interesting.
Was there anything that was going on at SLAC that was interesting to you at the time?
No. Of course, they were just about to discover the J/psi particle, so they were fully in particle physics mode. Obviously, SLAC has broadened into astrophysics since then, but at that time it was pure particle physics.
Right, right, right.
I can tell you two stories about Bob Wagoner. Bob is one of the most enthusiastic people I know. The first thing he said to me before we even got there was “When you come, you must buy a house. If you don't agree to buy a house, I will rescind the job offer. This place is going to be taking off soon and you have to buy a house. Don't rent.”
Smart guy! [Laughs]
Of course, we couldn't afford to live in Palo Alto on an assistant professor’s salary.
No one could afford that, although Bob lived on Frenchman’s Hill, which is the “low-cost” housing development on the Stanford campus. But only full professors or law or medical school professors could afford those houses. So we moved south and ended up in Sunnyvale where the declining price of houses met our budget. [Chuckles] Even then we had to borrow from parents. We lucked into a 4% assumable VA loan on the house. I mean it was a complete stretch. I had a newborn daughter and a 10-year-old daughter and a wife. We were totally broke.
We lived not far from Sunnyvale-Saratoga Road. Soon after we moved there in late summer 1974, my brother-in-law called me from Los Angeles and said, “Cliff, there’s a company that two guys have just started not far from you. It’s up there in Cupertino, two guys working in their garage, and the company’s named after a fruit—orange, pomegranate…no, apple. Something like that, and they’re looking for investors.” [Laughter]
You weren't interested.
I had to take a pass.
Oh, boy. [Laughing]
The second story about Bob happened within a month of my arrival. It was in mid September. I mean I had just put my books on the bookshelves in my office. Bob runs into my office waving an IAU telegram. Do you know what they are?
I know what telegrams are certainly, yeah.
Well, in those days, new discoveries in astronomy were announced by sending telegrams from the International Astronomical Union to all the various observatories and universities that were on their subscription list. Bob ran into my office and said, “They’ve just discovered a binary pulsar. Whatever you're working on, drop it. We’ve got to work on this.” This of course was the binary pulsar discovered by Joseph Taylor and his graduate student Russell Hulse.
Yeah, yeah. That one you took. You took that advice.
Now how much did you know about binary pulsars up until this point, or pulsars?
I knew what every student knew at some level just from reading, including my nearly fatal bit of knowledge that they were made of neutrons!
But you immediately recognized.
So we knew what they were, but of course in those days, there were very strong arguments that proved that a binary pulsar could not exist.
A supernova explosion of one star within a binary to produce a pulsar would simply disrupt the binary system. It could not survive as a binary.
So you shared in Bob’s enthusiasm that this was a very big deal.
Yeah, we really jumped on it. We didn't collaborate, but we each wrote separate papers basically over a weekend. Bob wrote a very influential paper on the gravitational wave damping of the orbit, pointing out that, given the current accuracy of the timing of the radio pulses and the size of the effect, it would take 10-15 years to detect it, but it would be detectable. Of course, Joe Taylor beat that prediction, measured the effect in just four years, but that was because they were motivated to. [Chuckles] But anyway, Bob’s paper really influenced the thinking about what you could do with it. I wrote a separate paper on the implications of measuring the advance of the pericenter of the orbit because that was stuff I’d been thinking about a lot.
Did you see binary pulsars as a departure from GR, or was it just sort of a natural transition for you?
We thought of it as a new testing ground—you know, the first place to test GR outside the solar system.
Yeah. So what were some of the immediate research questions for you that this discovery begged?
Many, many people wrote papers. There were maybe 100 papers written in the course of a few months about looking at all the various relativistic effects you could measure and their implications. So it was clear this would be a real place to think about GR. So I started working on the gravitational-wave damping predictions of a wide range of alternative gravity theories.
But in 1975, a year after the discovery, Jürgen Ehlers, Arnold Rosenblum, Peter Havas and Josh Goldberg wrote a paper arguing that the standard formula in GR that people like Bob had used, called the “quadrupole formula” for calculating the gravitational radiation induced damping of an orbit such as that of the binary pulsar, might not be correct. They argued that the theoretical foundations for that formula were flimsy at best. In fact, Rosenblum was doing calculations that suggested that the formula might in fact be wrong numerically, that the numerical coefficient should be something else. Ehlers et al. argued that the theory that was used to justify this formula had holes big enough you could drive trucks through them. This paper annoyed many of us, because it’s like your mother telling you, “Eat your spinach.” You know deep down that spinach is good for you, but you don't want your mother telling you to eat your spinach.
They were very correct in many of their assertions. Gravitational radiation from realistic systems is not something you can calculate exactly in general relativity. You must use some form of approximations and simplifying assumptions. Their paper argued that the standard approaches made uncontrolled approximations; that using point masses to treat the particles was fundamentally wrong in GR; that we don’t know whether the post-Newtonian approximation converges, and so on. But despite our annoyance, the paper was very influential in the sense that it motivated many people, me included, to start thinking hard about the foundations of gravitational-wave theory.
Mm-hmm [yes], mm-hmm [yes]. Now when you say many of the assertions they were correct in, that certainly makes me wonder what were they not correct in?
It’s not so much being incorrect, but that some of their critiques were more important than others. I mean, some were things that you could say, sure, but when you really think about it, that’s under control. Others were more serious. And of course in Rosenblum’s case, he never completed the calculation that he claimed gave a different result. Sadly, he died prematurely before he could finish it, although it was pretty clear to most of us that his approach didn't have much hope of panning out.
So the next question I wanted to ask was in these early months, really—not even years—as all of this was happening, what were some of the big questions in gravitational wave theory that became immediately apparent to you?
Right. So one of the big questions was whether or not using point masses was justified. The reason is that since gravity is nonlinear, point masses are problematical. Point charges work fine in electrodynamics, but if you take a massive body and shrink it, its gravitational binding energy gets bigger and bigger. Gravity is nonlinear, so that growing energy generates its own gravity, so how do you control that? How do you shrink something to a delta function without having its self-energy of gravity become infinite? And we all know that when you let a distribution of mass shrink, it ultimately becomes a black hole, not a point mass, but these are harder to deal with mathematically.
How do you systematically calculate the gravitational waves taking into account the fact that it’s a retarded field. You have to have boundary conditions at infinity so that you don't have incoming gravitational radiation. That turned out to be a relatively simple or straightforward thing to resolve. But an issue that has never been resolved is whether this post-Newtonian approximation converges. It probably doesn't, and so how do you estimate the errors? You go out to some order, and how do you know that you're even close to the right answer?
Ehlers et al. argued that in most textbook examples, we derived the damping or shrinkage of the orbit by saying we calculate the energy flux to infinity, assume that energy is conserved, and so the orbital energy must decrease by the same amount, and that’s what makes the orbit shrink at the appropriate rate. Well, that concept of “energy balance” was an unproved assumption, according to Ehlers et al.
So what we had to do was to calculate the radiation damping effect from first principles locally without relying upon the flux radiated to infinity. You calculate the forces within the system itself that damp its motion. These calculations are tricky. They had been done at various crude levels before Ehlers, et al. (sometimes leading to contradictory results) but many of us started to take the problem seriously and to work it through and grind out the details. Some of the early work I did on this was in collaboration with a mathematical relativist named Martin Walker.
So years later, in 1999, I was at a conference in Kyoto, and Jürgen Ehlers was there. We were both lecturing, and I was talking about the progress we had made on all these calculations of gravitational radiation. Of course, by then we had experimental evidence that the formula was right simply because it agreed with Taylor’s data from the binary pulsar. Somebody asked the question, “Well, would Jürgen agree that things are in much better shape than they were when they wrote that paper?” So I asked him, “Jürgen, would you agree?” and Jürgen said, that he wouldn't completely agree, but he was willing to admit that it was much better. [Laughter]
Cliff, I’m curious. To what extent did you rely on collaboration with experimentalists for this line of work? Was this a totally theoretical enterprise for you or was there experimentation that was required for this?
There was no real experimental requirement. Everything I did was purely theoretical. Many of the things I did, the ideas that I put out about what you could do with an experiment were then later done by experimentalists, and many of them would invite me to their labs to show me their apparatus and what they were going to do. But they did the experiments. They didn't need me to help them make the measurements, so in that sense there was no sort of serious collaboration, except for that one example back at Stanford. So in some ways I was independent, but I also did interact with a lot of experimentalists, so I got to know a lot of famous experimentalists like Bill Phillips, Eric Adelberger and Dave Wineland. There was a period during the cold trapped atom era where they were measuring whether or not the energy levels of atoms were independent of the orientation of the atom relative to directions in space. This is something that I and my Stanford PhD student Mark Haugan had worked out in detail theoretically. Again, what would be the implications and what limits could you place on certain parameters of these non-metric type theories by doing these sorts of energy isotropy tests? So with these trapped atoms, they could measure spectral lines that were extremely narrow because the atoms are so cold. You get exquisite precision in measuring transition frequencies, and so they could put an atom in a magnetic field and let the Earth rotate, and as the magnetic field rotates relative to our velocity direction through the universe, they could see if there’s a modulation. They could put limits on any variation of the frequencies at levels of parts in 1022! We could interpret that as a verification of Lorentz invariance, for example. So these were among the ideas that I was working on in the 1980s. I did pure theory, but I interacted with experimentalists, and often they would invite me to their places so that I could see the pretty equipment. SAC here?
Right. Now on the mentor/teacher side, you did have graduate students at Stanford?
I only had Mark Haugan who went through the full PhD. One of the things about Stanford during that period is that they would hire assistant professors, would evaluate them periodically, and then after six years would evaluate them for tenure with the working assumption that they would never get tenure. When they wanted to hire tenured faculty, they always looked outside.
Yeah. Were you aware of this when Bob recruited you or were you sort of naïve about it?
Everyone was aware of it. The department chair told us about it pretty much up front. When we met with other junior faculty, they all told us that “They say it’s a fair process, but you’ll never get it.” Students are also aware of the system, and so if you want to work with a junior professor, unless he’s part of a group where there are many professors you could then latch onto if the person has to leave, you tend to be wary.
So Haugan, now a professor at Purdue, came to me in my second year or so and graduated before seven years were up. I also collaborated briefly with Michael Turner, but he was really Bob Wagoner’s student. We worked on a very nice gravitational-wave project during that time. Just before I left Stanford I wrote the first paper on perturbations in inflationary cosmology with Joshua Frieman, who is now a famous cosmologist at the University of Chicago. He was an undergraduate at Stanford at the time. It was just a few months after Alan Guth had proposed the inflationary scenario for cosmology, and so we looked at the perturbations of that inflationary cosmology. It turns out there are much better ways to do it than our simple-minded approach, but it was a fun paper and Josh ultimately had a very successful career in cosmology.
But at the end of the day, like many before me, I didn't get tenure.
So you did not sort of try to stay ahead of the curve. You went up for tenure review.
Yes. The review process was during the fall of the sixth year. We weren't quite ready to fully go on the job market and to move at that point, so I figured I’d take a chance, though I did apply for a few jobs that looked interesting. If the decision was negative I’d have the seventh year to find a job. The rules for the review at that time were very explicit. They would solicit letters of recommendation from various people in the field, and to get tenure, you had to be ranked by these people as being in the top three in the world among a list of your contemporaries. Well, the list they sent around for me had Bill Press, Bob Wald, Saul Teukolsky, John Friedman, Jacob Bekenstein, all the people you would expect, but it also had Martin Rees, Kip Thorne, and Stephen Hawking.
[Laughs] Who are not peers!
Not peers. Kip wrote back to them complaining about this, saying they’re a whole generation older, and that Rees is in a completely different field from me.
Meaning that it’s designed to not give you tenure.
It’s designed to get a certain outcome.
And I have to say that when I didn't get tenure, the morale among the assistant professors at that time went through the floor because they figured--
Right, the idea being like if you didn't get tenure, there’s no help for anybody.
If I couldn't get it, there was really no hope for them.
Just to play devil’s advocate, Cliff, is there any wisdom in this system? Is there anything that it’s good for?
I don't know. I mean I used to think that such a system couldn’t possibly work to a department’s benefit. They hired all these young, enthusiastic people. We had all the committee duties. Many older faculty did nothing, right? My first year I was put in charge of the colloquium. So we worked our butts off. We taught like crazy. We served on committees. We did everything to run the department, right? And then they chucked us out. So you would think, what a failed model.
The model continued for several more years where they always just hired tenured people from the outside. Now the trouble is that during that following seven or eight years they hired from the outside as tenured professors Douglas Osheroff, Steven Chu, and Robert Laughlin, three future Nobel Laureates. So my theory did not past the experimental test! But I think a lot of the existing system was a holdover from the attitudes of Felix Bloch and Robert Hofstadter, famous Stanford physicists from an earlier era, who had a long-lasting influence on the department. I think that over time it has changed to some degree. So for example, Peter Michelson and Blas Cabrera, who were both Stanford graduate students while I was there, were both ultimately hired as assistant professors and then promoted to tenure in the normal manner. So I think they evolved into a more reasonable system from what they had at that time.
Mm-hmm [yes], mm-hmm [yes]. So in the end, Washington University was more than happy to take you on, I’m sure.
And here I want to pause for a moment and note that I’ve been really blessed in my career by encounters with four remarkable people. The first, of course was Kip, who got me going in GR but who taught me about good teaching and the importance of reaching out to the general public. The second was Chandra, who taught me about doing meticulously detailed calculations that can have important physics implications, and also taught me an appreciation of physics history. The third was Bob Wagoner, who taught me about enthusiasm: he said that if you don't love what you're doing, you're in the wrong business. The fourth person, Robert M. Walker, brought me to Washington University in St. Louis, but he was not at all in my field. He pioneered the nuclear track etching technique with Buford Price. He then moved into lunar and planetary science, and was a pioneer of analyzing moon rocks, dust from the moon and dust collected on spacecraft. He conjectured that grains from distant stars could be found in meteorites and could be used to study distant astrophysical processes, and then verified it in the lab. He was in charge of what’s called the McDonnell Center for the Space Sciences at Washington U. In 1979-80, my penultimate year at Stanford (during the tenure evaluation), they had a job search. Bob’s viewpoint was you should hire the best people available.
So they had a search in theoretical astrophysics, very broadly defined, and at the tenured associate professor level, and they got an application from this general relativist, me. They had no one on the faculty in general relativity, but Bob took the application seriously. It turned out that I was their second choice, but their first choice (in another branch of astrophysics) turned them down. In the summer of 1980, they then offered me the position, and they had funding to hire a second person to build up a group in general relativity and theoretical astrophysics. But this is a field totally distinct from Bob Walker’s own direct field of interest.
He was the director of the Mac Center, and the faculty slots were funded by Center endowment money. But he had the vision to convince the department to hire somebody who was doing exciting science, even though it was in a different field from his own or from other faculty in the Center. Over my years at Washington U, I found that chatting with him about science was always an exhilarating experience. You walked away with just such a great feeling about his vision for science, how to do science, the important questions in science, whatever kind of science it was.
So I give him a lot of credit, too.
On the real estate side, I’m sure you were delighted by the housing opportunities in St. Louis. [Laughs]
Oh, man. We bought a house twice as big for half the price!
Of course, meanwhile in California our house had tripled in value because of the whole Silicon Valley thing.
So Bob was right. [Laughs] I thank my lucky stars for Bob Wagoner!
It wasn’t great for your tenure at Stanford, but at least it was good for your pocketbook.
But those were seven great years. I had no regrets. I mean, the tenure process was a stressful time, though perhaps not as stressful as it could have been. You knew it was a long shot, so there were no devastating surprises.
Yeah, yeah. So Cliff, can you talk a little bit about the McDonnell Center for Space Sciences? How well developed was it when you arrived?
When I arrived in 1981, it was about ten years old. Using an initial gift from the James S. McDonnell Foundation, Washington U had hired Walker from General Electric Research, where he had begun his career in the industrial sector, to build up an effort in space science and astrophysics. They already had a very substantial cosmic ray group there, and so it was a natural thing to do. The McDonnell Foundation then gave additional funding to actually endow the center, with money for faculty slots and post-docs, money to give for projects that are a little risky, and so on. It was jointly part of the Physics and Earth and Planetary Science Departments. So some of the people hired were in Earth and Planetary; some were in physics.
Did you see this as a natural fit in terms of your own research? Was this going to be a particularly good place for you?
I didn't expect to have much scientific interaction with the people already there. Mostly Center people were looking at meteorites and cosmic rays and lunar samples from the Apollo program. The attraction was not so much “fit”, since there was nobody in GR there, but rather the opportunity to build a group in astrophysics and GR with Mac Center support. We hired another person soon after I arrived, a theoretical astrophysicist named Jonathan Katz, who at the time was very well-known in x-ray astronomy, worked on gamma ray bursts, and later moved into a variety of different fields outside astrophysics. They also had a very good particle theory group, led by Carl Bender. So as a matter of fact, in terms of research colleagues in GR, I was alone, but somehow it didn't bother me. I’m not one of these super-collaborative people anyway, and so I felt very comfortable there.
And you're sort of far along in your career now where you can--
That’s right, and I was officially established. I had graduate students. I had some funding from the Center. I also was able to effectively transfer my chunk of Bob Wagoner’s NSF grant over to Washington U, so I was funded from the start. I also got NASA grants from time to time, so I had funding for post-docs and various visitors and so on. So I certainly did not lack for junior people to have around me.
Now when you arrived, was your sense that the Physics Department was in growth mode? Was it looking to build its stature and become sort of more nationally ranked, and was your hire there—did you see that as part of that process?
I don't think so. At the time, the Physics Department was relatively stable. They were well known in some areas. Certainly in space physics, they were very well known through the Mac Center. They had a pretty decent particle theory group, as I mentioned. They were very strong in things like NMR, condensed matter physics, low temperature physics, and biomedical physics. So it was certainly a well-established, small department. Washington U was kind of a funny place in that it was very dominated by the Medical School, generally considered one of the top five in the country. We always felt that the Med School was the tail that wagged the dog; that anything biomedical was favored. Anything that wasn’t was kind of ancillary, and things like the Physics Department were there to teach freshman physics to the premed students.
I think it’s an extreme statement, but there’s some element of truth to it. So I don't think Physics at Washington U was in a particular growth mode. The department wanted to stay strong, and they hired some very good people to replace retirements, but overall we did not grow substantially and I think it still hasn’t grown. I was department chair for ten years, and despite my efforts, we didn't grow substantially. The Administration liked where we were and they did not envision making physics a “jewel in the crown” the way some universities do.
They tended to think of physics as this fundamental field, and it is good to have a Physics department. But for them, the jewel in the crown was the biomedical side.
Right, right. Did you take the move as an opportunity to branch out into new theoretical endeavors, or were you looking to continue essentially what you had been working on at Stanford?
No, I did a bunch of new things, but in large part, I sort of fell into them or had an idea that panned out. So for example, I spent a fair amount of time working on oscillations of black holes called “normal modes”. Today they are also called the “ringdown” modes of the black hole. One of my former graduate student colleagues from Caltech, Bernard Schutz, who by then was a professor at the University of Cardiff -- later he would become a founding director of the Max Planck Institute for Gravitational Physics in Germany -- came to spend a sabbatical with me. We started thinking about these black hole normal modes and realized we could calculate the frequencies and decay times of these normal modes rather accurately using standard WKB methods from standard quantum mechanics. So he and I, and later one of my PhD students Sai Iyer and I wrote what turned out to be very influential papers. Black hole normal modes are present in the gravitational-wave detections made by LIGO and will be important for the LISA space-based detector. That was a new direction of research that lasted several years.
What other major projects were you working on during those years?
Well, let’s see. In the ’80s, what did I do in the ’80s? Man.
It’s 40 years ago already!
That’s right, and those brain cells are dying. It’s a lost decade, the ’80s. [Laughter]
I mean one thing in 1986, of course, you publish a major book.
Yes. But during say the first 10 years at Washington U, in addition to black hole normal modes, I worked mainly on further aspects of clearing up the “quadrupole formula” controversy inspired by Ehlers et al. I worked on the interpretation of tests of GR, especially using high-precision clocks, and on gravitational waves in the Brans-Dicke theory of gravity. I also chaired a committee for the National Research Council where we studied whether or not the Air Force program that was running the early incarnation of the Global Positioning System (GPS) was taking the timing corrections due to Relativity properly into account. It turns out that the GR corrections are so large that they must be accounted for, otherwise the system would fail to achieve its stated navigation accuracy. Our conclusion was that they were doing it just right.
Later on at Washington U, I spent 13 years, from 1997 to 2010, chairing a NASA scientific oversight committee for the famous Stanford-NASA space mission called Gravity Probe-B, which put superconducting gyroscopes in orbit around the Earth to test general relativity. My committee learned all the nitty-gritty details of the project, from instrumentation to mission operations to data analysis, and advised them on how to achieve the best and most credible scientific result. And this is the guy who once imagined never thinking about experiments again!
Did you ever work directly with Russell Hulse and Joe Taylor?
No. I mean I knew Joe pretty well over the years, but we didn't collaborate. I wasn’t involved at all in aspects of the data analysis. My colleague Thibault Damour was much more closely involved with Joe in terms of working out how to do the best job of looking for GR effects in the data. My main contact with Russell Hulse was when I was writing my popular book Was Einstein Right? I wanted to tell the story of how they detected the binary pulsar. It was Russell who actually made the detection; he was there at the Arecibo radio telescope.
So I called him up and said, “I’d like to learn the story,” and he said, “Well, give me a few days and I’ll call you back.” After his PhD in radio astronomy, he had moved into plasma physic and was then at the Princeton Plasma Physics Laboratory. So he went up to his attic and got his old notebooks. He said, “Cliff, I haven't looked at these notes in ten years”. So he sent me photocopied pages of the notebook showing the day by day results and how he had to continually scratch out the pulsar’s pulse period and write in a new one because it kept changing like crazy, until he realized that the cause of the large variations was orbital motion and not some manic-depressive pulsar. So we had very nice chats about his reliving what he had done to make the discovery. That formed a very nice part of my book. I was able to tell the whole story in detail. The capstone to this particular story is that, when Joe and Russell received the Nobel Prize in 1993, the Nobel Foundation invited me and my wife to attend the ceremonies and festivities in Stockholm, all expenses paid!
What motivated you to write a book for a more broad audience about general relativity? It’s a unique thing. Not many physicists do that, so I’m curious what your motivations and your thought process were about that particular topic.
There were several things. One was Kip. I was inspired by his attitude that you have to tell science to the general public. A lot of scientists look down on popularizing science, but he didn't, and wrote things for Scientific American and other magazines. He was just starting to write his book Black Holes and Time Warps: Einstein’s Outrageous Legacy. It took me nine months to write mine; it took him four years to write his, but he’s a much busier guy. [Laughter]
Also, although I was a physics major in undergraduate school, I really only spent half my time doing physics. The other half of my time I was a student journalist. I worked for the campus newspaper, and I learned from that experience a style of writing that I think has served me very well in writing for the public. You have to have a good first sentence, a strong lead. Use short sentences. It is a style of writing designed to draw the reader in and to get the reader hooked to your story so that the reader will read at least partway and be involved.
So around 1985 I realized that I knew a lot about this subject, but more importantly I knew the people. I’ve heard a lot of stories. I have anecdotes. I’ve spent time with Ken Nordtvedt, Bob Vessot, Irwin Shapiro and Joe Taylor. So not only could I tell the science; I could tell some of the personal side of it. So I just sat down and “spilled my guts” as they say, and as I said, it took nine months.
Fundamentally, what was it that you wanted the broader public to understand about general relativity?
I think what motivates a lot of us who write for the public is to help them to understand the way that science works. There is a huge misconception about science, and there are notions that science is part of a conspiracy of elite people who press their views on the masses. You know, people don't fully understand the self-correcting nature of science. The lack of understanding of the scientific process is very deep.
And of course this is a matter that’s in far too sharp of a focus in our current crisis.
Exactly. Also, I just thought I’d spent 15 years having a huge amount of fun doing this, and I thought it might be fun for other people to read about it. That’s all. So I put myself into the book at various levels because I was there, and I think it made it a little more personal for readers and kept them reading along, although there are parts of it that are a bit of a slog. Naturally, you have to describe the science in a way that’s not so simple or superficial that it really loses the scientific content. You know, like the rubber sheet analogy for curved spacetime; we know it’s an analogy and it’s not right, but it’s useful. I tried to describe the science as accurately as I could but still keep it interesting and lively for the reader. I felt I could do it, and so it worked out.
Did you enjoy the experience?
Doing like the book tour thing and interacting with the public?
Yes. It was great fun, and as I said, I’ve enjoyed given public lectures. Particularly being based in St. Louis and the Midwest at the time, I was traveling around and giving lots of public talks in various towns and cities where having a well-known scientist come and give a public talk could be a big deal. There were places where I filled auditoriums. I enjoyed doing it, and I felt it was an important part of my obligation to give these kinds of lectures. So I did it a lot, especially while at Washington U. In fact in 2005 during the “World Year of Physics” celebrating the centenary of Einstein’s famous 1905 papers, the Canadian Association of Physicists sent me on a 4-week, 21 city speaking tour of Canada from one coast to the other, giving my “Was Einstein Right?” talk.
I mean that’s interesting to go back to the influence of Kip because what you feel as an obligation, it’s largely a unique impetus among many people you might consider peers.
I’m curious. When you became chairman, right, there are different stories about… You know, it’s like it’s your turn and you can't avoid it and you have to do it. Was that your sense, or did you embrace the opportunity?
Well, first of all, there was no question of “my turn”. I was only the fifth chairman of that department since Arthur Holly Compton was chair from 1920 to 1923.
[Chuckles] So these are really reigns.
It was while Compton was Chair at Washington U that he did his famous scattering experiments in 1922. Of course, Chicago stole him away in time for him to win the Nobel Prize for it. So he was chair in those early days, and there were four between him and me, Arthur Hughes, George Pake, Eugene Condon and Richard Norberg. Hughes served for 30 years, while Norberg served for 29 years.
So it’s not a rotating job as it is in many universities. [Chuckles]
I see. I see.
And of course, I never imagined that I would take on such a job. About five years after I got to Washington U, I was recruited to apply to be the founding director of the Canadian Institute for Theoretical Astrophysics (CITA) in Toronto. But I turned it down, saying “No way I’m ever going to be an administrator.
It would take you away from the work.
Yes. But within five years I was Department Chair.
When Dick Norberg announced he was going to step down, the line started forming outside my office, one after another. “You're the only person in the department we trust.” You know, we all take on committee duties, right, as a faculty member.
I made the mistake of doing those jobs efficiently, on time, without pissing anyone off, and with some level of wisdom, you know? [Laughs]
[Laughs] Look how you're rewarded for it!
Exactly! You know, you’ve got to be incompetent and irascible at the same time! So I decided to do it, and because I’m a pretty compulsively organized person, it did not seriously impact my research. A big factor was that at that time my group had a sequence of extremely good graduate students and post-docs.
For example, when I was elected to the National Academy of Sciences in 2007, the citation included two things: first, my development of the PPN formalism used for experimental tests, and second, the development of these high post-Newtonian gravitational wave calculations that later played a role in the LIGO detections. Well, the second part of the citation, all that work was done while I was department chair. Admittedly, I had a lot of students working on the projects, but it’s theoretical work, so I was checking and doing calculations alongside them in parallel. So I was able to balance the two, the Chair stuff, as well as research and teaching. And I served for two five-year terms, so it was fine.
When did you get involved in LIGO?
I was never directly involved with LIGO, in that I was never part of the collaboration. I studiously avoid signing up for such things because you had to obligate yourself to specific tasks and timelines (to say nothing of zillions of telecons). My style of research is to do whatever I want to do when I want to do it. But I got seriously involved in LIGO-related research without actually joining LIGO.
In the late 1980s, when LIGO was in the initial stages of development, Kip and people within LIGO were already discussing the idea that an important target source for detection would be the inspiral and merger of two neutron stars. In 1989, my graduate student Craig Lincoln and I published a paper describing a lot of the details of the gravitational-wave signal that would be emitted in such a process. Apparently, upon seeing our paper, Kip called his group together and said, “Cliff and Craig Lincoln have written this paper on neutron star inspiral. We’ve got to get on top of this because they may get ahead of us.” Their response to our paper was a 1993 paper entitled “The Last Three Minutes,” a play on Steven Weinberg’s book about cosmology, The First Three Minutes.
This was the last three minutes, pointing to the final phase of the inspiral and merger of two neutron stars. They made the case that the data analysis envisioned for LIGO was going to be so precise that you would need an equally precise prediction for the inspiral gravitational waves so that you know that what you're measuring is the actual wave form, and more importantly, so that you can measure the masses and spins of the bodies from the detailed shape of the wave form. An implication of this was that we would need calculations beyond the leading order in the post-Newtonian approximation, which is what Lincoln and I had done. Soon after the Last Three Minutes paper, around 1994, Kip organized a workshop at Caltech to try to motivate theorists to attack this problem of very accurate wave calculations. In fact, he was very worried about whether they would have theoretical predictions for the wave signals in time for the detections. I mean, he was panicked.
So a bunch of us went to the workshop, and it was there that my student Alan Wiseman and I, and the group in Paris headed by Thibault Damour and Luc Blanchet, along with their Indian colleague Bala Iyer, agreed that we would try to get the second post-Newtonian (2PN) corrections to the gravitational wave formulae. Bob Wagoner and I had actually done the first post-Newtonian (1PN) corrections back in 1976. We were going to try to do the 2PN calculations because obviously LIGO was going to need this stuff eventually. Of course, today we’re up to 4.5PN and beyond, but at that time such calculations were considered a tour de force. We agreed that each group would do the calculations separately. There would be no communication between us until we had a final answer for an agreed upon formula. We’d compare that final formula only when we were done; before that we wouldn't interact at all.
So at the end of the day, Alan Wiseman and I were about three or four months behind them. They had their result, but they didn't say anything about it. We finally got our result. This was the early days of the nascent internet, so we wrote out the formula and faxed it to Paris. And then of course, because of the time zone difference, we had to wait till the next day to get an answer back. The formula for the energy flux contains a series of terms with coefficients that are horrible rational numbers consisting of, say, a seven-digit number divided by a six-digit number. When we compared with the Paris group, the coefficient of every single term agreed, exactly the same rational number in front of each of the terms that we were comparing, so that was extremely exciting.
So already in the early ’90s I was involved in the calculations that were going to be so important for LIGO. It was clear that I was part of this whole effort, though I did not formally sign up.
Cliff, you mentioned that you had some really excellent graduate students during your time in St. Louis. Who are some of the standouts, the really…the collaborations with graduate students or post-docs that were really tremendously productive for you also?
So a couple of graduate students from that time were key. One was Alan Wiseman, who is now at the University of Wisconsin, Milwaukee. He joined the LIGO collaboration and became a leader of the computer infrastructure team for data analysis in LIGO.
Another was Larry Kidder. He is now a senior research scientist at Cornell and is a key member of the numerical relativity group there led by Saul Teukolsky and others.
A very key post-doc at the time was Eric Poisson. We wrote a number of papers on post-Newtonian theory, in particular on the effect of the new 2PN gravitational wave formulae on the accuracy with which LIGO could measure things like masses and spins of the merging neutron stars. He had been a post-doc at Caltech during the “Last Three Minutes” work and then came to me. Sadly for me, the University of Guelph in Canada offered him a faculty position, and he left after only one year. I gave him my blessing to go because it was a great opportunity (he is Canadian), and of course it was right next door to what later became the Perimeter Institute in Waterloo. He’s had a great career, winning the Hertzberg Medal from the Canadian Association of Physicists, for example. We’ve recently cowritten a big textbook on post-Newtonian theory and gravitational waves. He’s one I really remember.
I’m curious about the titles of the books. You know, Was Einstein right? and Is Einstein Still Right? I’m curious because is that the kind of title that the publisher likes because everybody knows Einstein, or do you think it’s really useful in terms of focusing on the individual and not the larger issues for which he was a major person, but of course not the only one.
No. It’s much more crass. Einstein sells.
[Chuckles] Right, right.
Was Einstein Right? was my working title from the very start. Nicolás Yunes and I had many go-arounds about the title of the current book, but we again agreed that it’s got to have Einstein in the title. So our book is called Is Einstein Still Right? The book comes out actually in three weeks in the UK and then in September in the US.
So I wonder if you can talk a little bit about as an updated edition or a sequel what happened in the past decades that compels the question.
I had been vaguely thinking about an updated version of Was Einstein Right? for some time. But other projects intervened. Eric Poisson and I wrote our textbook, an exposition of everything you wanted to know about post-Newtonian theory. We finished that in 2014, then I had to do a major overhaul and update of my old 1981 technical monograph Theory and Experiment in Gravitational Physics. I completed that in 2018.
As for Was Einstein Right? I had actually contacted the publisher prior to 2005, the World Year of Physics, when Einstein was the big theme of the celebrations. I figured this would be perfect timing, but they weren't very interested. They didn't see the potential sales. These days, you know, if you can't sell 100,000 books, most trade publishers aren’t interested.
So I set the idea aside and ended up working on the other two books. After they were done, I started thinking once more about a WER? update. Then, out of the blue I got an email from Nico Yunes. He had been an undergraduate student at Washington University when I was there, and we had written a paper together on testing theories of gravity using gravitational waves, and I’d kept in close touch. By this time he was a professor of physics at Montana State University, Ken Nordtvedt’s old stomping grounds. In the email he said that he was thinking about writing a popular book and wondered what I thought. He didn't want to step on my toes if I was thinking about the same thing, and so I thought, “Well, why don't we just do it together?” He’s young and energetic and knows a lot of the players currently active and involved in these things. I sort of have the background of the old book, plus perhaps a deeper sense of the history of the subject by virtue mainly of my advanced age. We would import and buff some of the old stuff from WER?; other stuff we would drop because it’s not so relevant anymore. And of course we’d focus a lot of attention on the gravitational wave discoveries and also things like the supermassive black hole in our Galactic Center, and the recent images from the Event Horizon Telescope. So it was a good collaboration because I had the writing background. His second language is English (he was born and raised in Argentina), so he sometimes had less familiarity with American idioms, but that’s fine. I think our talents were very complementary.
So in terms of work in the past few decades, again, why does the question need to be re-asked?
Largely because there are hints in nature that we might at some point need to go beyond Einstein. We don't currently have a quantum theory of gravity, and the very nature of Einstein’s theory makes the standard approaches to such a quantum theory problematic. Sadly, we may never know unless we can find some experimental way to decide between, say, string theory and loop quantum gravity or some of the other models. Nevertheless, there’s something about general relativity that in a way predicts its own demise, namely, the existence of singularities.
Is your sense that Einstein would have appreciated that himself, that it would have predicted its own demise as you see it? Or it was only because of later advances that that’s even a legitimate question?
I don't think Einstein understood much about the nature of singularities that the theory predicted. That understanding really didn't come until the 1960s.
What allowed for that? I mean why in the 1960s? What had happened in the interim period to allow for that understanding of singularities?
Mainly trying to understand the nature of the interiors of black holes and also Big Bang cosmology because by then revisions of the Hubble constant had made Big Bang cosmology look respectable once again.
People like Roger Penrose and Stephen Hawking proved rigorous theorems about the inevitability and the nature of singularities in general relativity. That really put into sharp focus that these are predictions of the theory. Certain classes of them are inside black holes and so we probably don't worry about them, because the event horizon of the black hole prevents them from doing any mischief that might escape. The other one is the Big Bang singularity, and it is a one-off, and so what? But it was still unsettling that an otherwise well-behaved theory would predict unavoidable infinities within itself. Maybe quantum mechanics would cure these singularities and give us a regular, finite picture of the world everywhere in spacetime. So that was part of the motivation for pursuing quantum gravity. Another motivation was that the other interactions of physics are quantized, so why not gravity?
A more recent motivation for considering gravity beyond Einstein, of course, came from the discovery of the acceleration of the universe in 1998. Pure general relativity requires the universe to decelerate its expansion. There were basically three ways out of this conundrum. One was just the good old Cosmological Constant introduced by Einstein himself around 1917 to allow static universes, then withdrawn when the expansion of the universe was discovered. That’s my favorite option. My view is that there are several constants of nature. This cosmological constant is one we didn't know was relevant before and we’re now in a position to measure it. So we can now list in a table G, e, ħ, and c, and now we have Λ. The fact that its measured value drives particle physicists crazy doesn't bother me one bit.
A second option is dark energy. This would be produced by some new cosmic field that has to be built in a specific way to induce the universal acceleration. It probably varies with time. It could be an extension of the standard model of particle physics, or it could be something very new. The third option is a direct modification of GR, of gravity itself, that only matters on these cosmic scales, but that preserves all the small scale and astrophysical things that pure GR predicts and that have been confirmed. This has given rise to a rather substantial industry of constructing “modified gravity” theories. Dark matter has played a slight role in this too because, instead of dark matter, you could try to modify gravity on galactic and cluster scales so that you don't need dark matter to explain things like the rotational structure of galaxies. So since the late ’90s, there has been growing work on these modified gravity theories that could then account for the acceleration of the universe in a natural way without invoking dark energy or a simple cosmological constant.
Mm-hmm [yes]. Cliff, I’m curious. As far as metaphors go, and to the extent that we can really use language at all to describe these things, do you like the term Big Bang? Does that do a relatively good job at explaining the event at the beginning?
Well, it certainly doesn't from the point of the view of the public because to them, a big bang means something very localized that expands into a void like ….
A firecracker. A firecracker.
Yes, and of course it absolutely wasn’t like that. The universe even at the Big Bang was probably spatially infinite, but still infinitely dense. Everything started to expand, but there’s nothing outside. There’s no boundary. It’s just that everything everywhere began to expand. So in that way the term evokes the wrong images in terms of the general public.
Now you say it evokes the wrong images. Does it also evoke the wrong historicity—in other words, the narrative of the Big Bang that looks like it works well within like the first two lines of the Old Testament, right? The idea that it implies that there was nothing and then there was something. Is that also problematic?
Sure, because people have always asked, “What came before it?”. Again, from a technical point of view, that’s simply a non-question.
Why is it a non-question from a technical point of view?
Because according to classical GR, it’s a singularity and you can't trace time back through the singularity into an earlier epoch. Time literally begins at that point, and so there is no “before”.
But you're saying that within the framework of classical GR. Does that imply that there might be a different framework for which those same questions might be more applicable?
Well, not in classical GR because of the theorems of Hawking and Penrose that there are no other possibilities. In other words, if you go backwards in time and the matter gets more and more dense, there’s no choice but to hit this singularity. This has been shown to be true even if the universe is wildly anisotropic so that it expands differently in different directions. You get some very complicated behavior in these models, but at the end of the day, it’s still an initial singularity and time cannot be traced into the past beyond it. On the other hand, if you turn on quantum mechanics, all bets are off. One of the key assumptions of those Hawking-Penrose theorems is called the positive energy assumption. Quantum mechanics can violate that assumption, so it opens up lots of possibilities, such as quantum tunneling from the past into the current universe.
Where do you see your work fitting in with this pursuit for a larger, a grand unified theory?
You know, I don't see my work fitting in because my focus is on experiments that are performable realistically. It’s going to be very hard to test grander theories experimentally, such as any kind of quantization of gravity. Even for these modified gravity theories that may play a role at the largest scales of cosmology, the observations aren’t very precise, and the interpretations are highly model-dependent. The measurements may rule out some extreme models or theories, but I predict there will be lots of other theories that will be able to fit all of the data. So what do you do to distinguish among them?
So I like to view my work in a less grandiose perspective, in that it will help us to understand to what extent Einstein’s theory is correct on the scales of the solar system, astrophysical systems, and galaxies, where we can do good enough experiments and observations to really test theories and distinguish among them. Beyond that, my viewpoint is that if you have a proposal for quantum gravity or for modified gravity at cosmological scales, you just have to guarantee that it agrees with all the experiments that have been done, as laid out in my books and review articles, you know?
[Laughs] That’s a tall order!
If you can't do that, I wash my hands. [Laughter] I take no responsibility, as our president says.
So Cliff, back on planet Earth, I’m curious about… Well, one question I want to ask before we get to your tenure at the University of Florida is I’m just curious. Do you see your election to the National Academy—is that just sort of a nice thing to be recognized at that level, or did you feel that that had any sort of specific advantages for you in terms of opening doors or anything like that?
No, it didn't have particular payoffs in that sense. Typically it happens late enough in life for most people – I was 61 when I was elected -- that if you're that prominent, it’s not going to help your career directly.
You know, the Chancellor and Dean of Washington U sponsored a nice reception for me. I think I may have gotten a raise, but these are not big deals. And there are a lot of people who don’t take these things terribly seriously. Still, I thought it was amazing. It’s gratifying to have your work recognized, and especially by people not in your specific area of expertise. I’m now sort of active in the Academy, and I see how the election process plays out. You have to be nominated by an existing member of the Academy, then voted on by members spanning all sub-fields of physics, then by people spanning all the physical sciences (chemistry, math, etc), and finally by members across the entire Academy. So how does a person ever get elected?
I like to think that one of the reasons I got elected was because my work involved the experimental consequences of GR. So I spent a lot of time interacting with experimentalists in many different branches of physics, such as atomic physics, nuclear physics, particle physics and astrophysics. Academy members like Bill Phillips, Eric Adelberger, Joe Taylor, and Irwin Shapiro already knew me pretty well through my research that directly connected to their work. It wasn’t just this narrow group of GR people in the Academy who recognized my name.
Physics is a big field and it’s hard to know everybody. And all the nominations are glowing and talk about the wonderful the work the nominee has done, but I think I might have had a bit of an edge because I was actually known to a much broader swath of physicists than your average theoretical general relativity person.
But still, it was just tremendous and a complete surprise. I know what the Academy is, of course. I’ve served on many Academy panels and studies, so I know how it functions. I had attended an annual meeting of the Academy as a guest speaker in a symposium on Einstein. Other than that, being elected was not on my radar at all. I remember that one day, Michael Turner sent me an email asking me if I was going to be available the following day (a Tuesday) for a phone call, say, around 9:30. I figured, Oh, it’s Michael Turner and he’s surely calling me up to serve on a committee of some sort because Michael’s a mover and a shaker and he’s always involved in this kind of stuff. He wants me to serve on something. [Laughs]
So the next morning he called me and he said, “You know, I’m here at the Annual Meeting and I’m happy to report that you’ve been elected to the National Academy. Your election brings the average age of the Academy from deceased to 80.” [Laughter] I was 61 at the time, which is relatively young. In recent years the Academy has been really pushing to elect younger people. It’s also making a big effort to broaden the membership in terms of the usual forms of diversity—gender, race/ethnicity and age. But it is also pushing geographic diversity, you know, getting away from so many members being at Princeton, Berkeley, Caltech, Harvard, and so on, and trying to identify people who are doing Academy-level work in Kansas and other places. In that sense, I was diverse because I was from Missouri, although there were plenty of Washington U medical school people already in the National Academy.
To fast forward to 2019 with your recognition with the Einstein award, right, I’m curious in the ways in which that kind of recognition is different than the recognition bestowed by the Academy.
Well, it’s a different sort of organization. It’s a foundation in Bern, Switzerland that runs, among other things, the Einstein Museum in Bern that’s housed in one of the apartments where he lived while working in the Patent Office. It tries to preserve his legacy and his works, so it’s not a huge organization. But the medal was a very cool thing because it’s fairly new. The first one was given to Stephen Hawking 40 years ago, and the list of past winners contains all the famous people in the field. So it’s just very nice to be recognized for having made a contribution to the field.
Yeah. Now when you moved to Florida in 2012, you retained an affiliation with Washington University.
It’s a pretty loose affiliation. I’m the James S. McDonnell Professor, Emeritus, which is a purely honorific title I’m allowed to keep. Other than that, I don’t have any direct involvement with the department there.
My move to the University of Florida came about because of a confluence of factors. In the late 1980s, we hired a young faculty member named Wai-Mo Suen who worked in numerical general relativity, and together he and I built up a successful GR group that was known around the world as WUGRAV. But by about 2010 or so, Wai-Mo was thinking seriously about moving back to Hong Kong where he was from, and building a research group there, and also getting involved in the commercial side of the scientific supercomputer business. So I tried to convince the department and the university that they should immediately replace him and maybe even hire someone else because pretty soon—this was 2010—they’re going to detect gravitational waves. I felt that WUGRAV had a strong presence in this whole game, and Washington U ought to stay active in it and be ready when the discoveries came.
Well, one of the things I loved and hated about Washington U is that it has a St. Louis mind-set. St. Louisans are very conservative and very cautious. Washington U rarely does anything bold. When Harvard, Yale and other places were investing their endowments during the early 2000s in risky, high-gain financial instruments, Washington U played it very safe. When the financial crash came in 2007, Washington U hardly noticed. They did not cancel a single faculty search. They did not delay any building construction. Almost nothing changed because they invested very cautiously and conservatively. When their endowment went over a couple of billion dollars back around 2000, several of us Science Department chairs went to the powers that be and said, “Your endowment has grown to this huge amount. You should be spending some of it, and by the way, here’s a list of excellent ideas from the Science Departments which you could spend it on.” [Laughter]
They knew we were scientists so they explained how they have these humongous viscosity factors built into their models for how they sell and buy and trade and spend. So they told us that they were not going to go on a buying spree. So when the crash occurred, and nothing happened at Washington U we said, “Thank you very much!” [Laughs]
[Laughing] Right! Right.
But they did not feel that they wanted to make a big investment in my field at that time, despite the impending departure of my colleague. A particular Dean at the time was unsympathetic to making a bold move to really keep the department in a game that was about to explode. Meanwhile--
So this was very much… Your response to this was you were looking for a new position? This is connected?
At the same time, my wife and I had thought that when and if I ever separated from Washington U—of course I would never stop doing physics—that we might move to Florida. One of my daughters lives in the St. Petersburg area, and if we made such a move, I would figure out something about hooking up with some university.
So when it started to emerge that Washington U wasn’t going to respond to my proposal to keep the WUGRAV group going, I contacted colleagues at the University of Florida in Gainesville. It happens that they had a huge effort in GR. They were part of the LIGO collaboration, for one thing. (The data-analysis algorithm that made the first detection of gravitational waves in 2015 was actually designed by the UF group.)
There were several theorists there, Jim Ipser, Bernard Whiting, Richard Woodard and Steven Detweiler, whom I’ve known for years (sadly, Detweiler died unexpectedly a few years ago). My good friend Jim Ipser from Caltech and from my postdoc time at Chicago, had recently retired, but we felt it would nice to reconnect with our friends of almost 50 years standing. There was also a group very active in the LISA project for a space gravitational-wave detector. I mean if you want to do GR, UF was a very good place to go.
So I sent them an email saying I’m wondering if they might be interested in maybe having me come to Florida and have some kind of an affiliation at UF; and a little bit of remuneration might be nice, although it wasn’t critical. I mean I had started investing in TIAA-CREF back when I was a poor Assistant Professor at Stanford. The first year I could sign up, around 1977, I put in the max that I possibly could, and kept doing that at Stanford and Washington U, and so we were not too concerned financially, but a small salary would be nice.
Well, in what you might call the inverse of Washington U’s caution, within about six weeks we had negotiated a deal whereby I would be a non-tenured research professor, with a salary equal to 1/3 of my final-year Washington U salary. No teaching duties. No committee duties. I live in a condo on the Gulf beach near St. Petersburg. I go to the campus in Gainesville, (about 2 1/2 hours by car) two days a week. I’ve been going to Paris regularly for extended visits since 2003, and I wanted to continue that for five months every year. So I did the math and figured that would be about a one-third commitment to the department, and within six weeks we had a deal. I even got some start-up funding!
Life is good!
Life is good. And I soon got emails from other colleagues that said, “Cliff, how can I get a deal like that?” [Laughing]
Yeah, right! Right!
So it has really worked out well. I have continued to be funded by the NSF, and I have two PhD students.
How did that come about, your affiliation with France and the CNRS?
So for my very first sabbatical from Washington U in 1996, I went to Paris. I actually took a year off from being Department Chair. They had a colleague serve as Chair for one year while I did that; on my return I served a second five-year term. We did six months there and then six months at Hebrew University in Jerusalem. I went there mainly to collaborate with Thibault Damour and other GR people at the Observatory of Paris in Meudon, just outside the city. We’d all been working on these post-Newtonian calculations, and so it would be a great place to be scientifically, and of course it was Paris.
Seven years later, I was then planning another sabbatical. We decided not to go to Jerusalem because unlike 1996 when peace was breaking out all over in Israel, the security situation in 2003 was much worse. They had had the Second Intifada in 2000. In 1996, we lived in a flat on Smolenskin Street three doors down from the Prime Minister’s residence, and I walked past it every day on my way to the bus to Hebrew University. Today the street is blocked, and if you even look at the residence too closely, people with guns start waving at you menacingly. Why go if you can't go out and explore and do what you want to do without constant worry?
So we decided to go to Paris for the whole 15 months, two summers and the academic year. By that time, half the group at Meudon, including my friend and colleague Luc Blanchet, had moved to the Institut d’Astrophysique in the heart of Paris, which was even more convenient because of the short subway trip. Damour had moved to an institute even farther outside Paris. So it was a great GR group there and a huge institute—astronomers and astrophysicists doing every kind of astronomy you can think of.
That year, 2003-04 we decided we loved Paris so much we wanted to go at every possible opportunity, so we started going for summers. I occasionally organized my teaching duties so that I would double up for the Spring semester and be off the Fall semester, so we’d stay on for all or part of that Fall semester. The deans weren’t happy; they want your butt in the chair on campus, but I didn't worry about it. Then for the next sabbatical 2010, we did the same thing for the full year.
So when this Florida move came up, going to France annually for five months was part of the deal. So we’ve been very happy. My wife asked me after we had been in Florida about a year, “Do you ever spend time thinking about whether we made the right decision?” I said, “By time, do you mean anything longer than a nanosecond?” [Laughing]
Yeah, right! Exactly, exactly. Well, Cliff, now that… I think we’re right up to the present day in terms of the narrative. I just have one last question for you. I’ll make it both a broadly retrospective question and one that sort of has implications into the future, and that is in terms of the fundamental questions that you’ve been involved with for so many decades, right, what’s the question that might stand out in your mind that remains most compelling to you and that has ongoing interest in terms of things that might not be fully understood, certainly 40 or 50 years ago, maybe not today, but at some point in the future we might actually get there? What really stands out in your mind in terms of those kinds of questions that you’ve devoted your career to?
Well, in terms of a very big question, I would phrase it this way: is Einstein going to be right, is general relativity going to be correct, all the way down to the Planck scale? Most of us agree that at that scale, corresponding to a time 10-43 seconds after the Big Bang, something’s got to give in terms of a quantum description of gravity.
I’m sorry. What happens at the Planck scale? What does that mean?
Well, that’s when you really have to have a quantum theory of gravity. However, with things like dark energy, dark matter, and the acceleration of the universe, people are thinking that quantum gravity effects might emerge at different scales, not the miniscule Planck scale, but perhaps at the scale of the GeV phase transition in the early universe, or even at the Hubble scale, different scales where some weird things might happen and general relativity might have to be modified. So to me, the question is will standard, non-quantum general relativity actually be right all the way down to the natural Planck quantum scale? By “standard GR” I mean GR including the cosmological constant for the acceleration of the universe. You would use standard GR for all galactic and astrophysical scales. You’d use it all the way down to just after the Big Bang. It will be correct at all those scales. All you need is the cosmological constant, but to me that’s like GR, too. I mean that’s the teeniest modification you’ll make.
It doesn't violate GR; it extends it.
It doesn't violate GR. It really only affects stuff at the largest scales.
But it’s not a fundamental violation of the foundations of GR, and of course Einstein felt he could add it because it didn't violate any of his deeply cherished principles. He only added it because he thought the universe had to be made static, and that’s what he needed to do it. So just to be more precise, it would not surprise me if GR plus the cosmological constant turned out to be correct all the way down to the Planck scale.
Right. Right. So you are a true believer.
I’m a true believer. Now of course, you have to think about how and when you might answer that question empirically. So I’d like to tell the story of baseball star Yogi Berra, whom I actually met once. That was in 1992, when a local St. Louis restaurant call Blueberry Hill had its annual ceremony of installing Hollywood-type stars in the sidewalks along the street near the restaurant, honoring famous St. Louisans. That year, I represented the Physics Department when they inducted Arthur Holly Compton for his contributions to Washington U; two other inductees that year were Yogi Berra and baseball announcer Joe Garagiola, who both grew up in the Italian neighborhood of St. Louis. So I got to meet and have lunch with Yogi Berra! When I was a kid growing up in Canada, I loved the New York Yankees and especially Yogi Berra, Whitey Ford and Mickey Mantle, all those great Yankees.
But here’s where Yogi, whose mind had its own unique logic, has something to say about proving quantum gravity. When Yogi was long retired from baseball, his wife Carmen came to him and asked, “Yogi, you were born in St. Louis, you played baseball in New York, and we now live in New Jersey. When you die, where do you want me to bury you?” and Yogi said, “Surprise me.”
So it wouldn’t surprise me if general relativity with the cosmological constant was true down all the way to the Planck scale.
Right. Right, right. But you're willing to be surprised.
I’m willing to be surprised. But my day-to-day focus is still on testing GR at more modest scales. Today we’re thinking about testing general relativity at scales of things like black holes and neutron stars. I’ve been interested recently in testing aspects of black holes using observations of the stars orbiting the Galactic Center supermassive black hole, and have been collaborating with the group at UCLA that is one of the leading observational groups studying these stars. Testing GR near neutron stars and black holes using the information contained in gravitational waves is another big interest of mine. I still follow solar system and binary pulsar tests, but I mean my daily work is on these kinds of topics. But these are still fairly local scales, not cosmological scales or tiny quantum scales. I’ll leave those to other people.
Well, Cliff, it’s been so fun talking with you today. I’ve been on quite the adventure with you. I really want to thank you for spending the time with me.
Yeah. It’s been fun reminiscing.
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