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Interview of Joseph H. Taylor by David Zierler on May 22, 2020,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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In this interview, Joseph Taylor, the James S. McDonnell Distinguished University Professor of Physics, Emeritus, at Princeton University, recounts his upbringing in and around Philadelphia, and the centrality of Quakerism throughout his childhood. He describes his undergraduate experience at Haverford, where he developed his interest in physics and in experimental radio astronomy specifically. Taylor discusses his graduate work at Harvard, and why the mid-1960s was an exciting time for radio astronomy, and he describes his thesis research under the direction of Alan Maxwell on observing radio galaxies and quasars to create two-dimensional maps. Taylor describes the impact of the discovery of pulsars, just as he was completing graduate school, and he explains his decision to join the faculty at the University of Massachusetts to start the Five College Radio Astronomy Observatory. He describes the fundamental advances in pulsar research in the 1970s, and he recounts his early and soon to be significant interactions with Russell Hulse, and he describes the logistical challenges of setting up research at the Arecibo Observatory. Taylor describes the intellectual origins of discovering gravitational radiation, and he explains his decision to join the faculty at Princeton which centered around its strength in gravitational physics. He discusses the long period of time between his research and the Nobel Prize for which he was recognized, and he discusses the impact of the prize on his life and his research. Taylor discusses his tenure as Dean of Faculty at Princeton, and in the last part of the interview, he describes his current and recent interests in WMAP, and why he welcomes the strides his field has taken toward greater diversity.
Okay. This is David Zierler, Oral Historian for the American Institute of Physics. It is May 22nd, 2020. It is my great pleasure to be here with Professor Joseph Taylor. Joe, thank you so much for being with me today.
Happy to be with you.
To start, please tell me your title and institutional affiliation.
I am the James S. McDonnell Distinguished University Professor of Physics, Emeritus, at Princeton University.
Great, and a topic that you have written much about, and I'd love to hear it in your own voice. Tell me a little bit about your family background.
I was born in Philadelphia. When I was six my family moved from the Germantown section of Philadelphia to the New Jersey farm where my father had grown up, just across the Delaware river. I was the second child of a large family – an older brother, three sisters, and another brother. My parents actually raised eight children including two young cousins.
So, we're on a farm on the banks of the Delaware River, near a fairly small town and just across the river from a very large city. The farm was run by my grandfather, and it had been in the family since 1720. Both families, my mother's and father's, were Philadelphia Quakers, going back to Colonial times.
It's remarkable to think of that area as being so undeveloped as to be farmland.
And it still is a farm. A nephew lives there today and runs the place, although it's considerably smaller than it was in my day. In the 1940s and 50s the major crop was peaches, through most of the summer. But we also grew corn, tomatoes, and other vegetables and kept a few hundred laying hens. In those years the farm was still coming out of the Depression. The second war was over, so things were looking up, but farm life was still pretty tough in some ways. However, I never thought of it that way. It was a great place for kids to grow up.
Could you talk a little bit about how your parents shared with you their Quaker heritage and values?
Quakerism attracts people who like to think for themselves – who believe that one's “inner light,” or conscience, is as good any guide for living if you're careful to pay attention to it. To me Quakerism goes well with science because it is an experiential thing. You can gain guidance for life in the same kind of way that you gain from an experiment in physics. You see what works, and what is consistent with other truths that you already know. Maybe that's a bit of a stretch in some ways, but it means something to me.
In what ways did your Quaker upbringing influence your spiritual makeup, your possible belief in a creator, or not?
I don’t find describing “beliefs” a very useful approach to such topics. As a scientist, I want to understand how nature works. To do that, I and others perform experiments, formulate hypotheses, and test them in order to discover Nature’s truths. Similarly, I find that experience gained living in concert with others yields natural truths that are spiritual, in a sense – a philosophy of living to guide us in the way we interact with other people and with the world.
Would you say that's a fair summation of the overall Quaker worldview?
It's hardly correct to think of a single Quaker worldview. There are many variations. The most fundamental understanding is that there is something of God within each of us.
What were your parents' professions? What did your father do?
My father was a schoolteacher, then a school principal. First in Quaker schools, but then in public schools in southern New Jersey. My mother ran a home, and a family. Together they put eight children through school and through college.
Did you go to Quaker schools growing up?
Yes, through all but one of my school years. I then went off to Haverford College, another Quaker school.
At what point did you start to distinguish yourself in terms of your interest and ability in math and science?
I was generally a good student. In high school I liked my math courses, though we only got as far as trigonometry. I liked science courses too, although the physics I had in high school was rather backward. I didn't know anything of modern physics until going off to college.
As a child I probably did more physics on my own than in school. My older brother and I taught each other Morse Code and learned electronics on our own, from handbooks. We became avid radio amateurs. We spent many hours salvaging broken television sets, taking them apart, and using the parts to build amateur radio transmitters and receivers. I learned a lot of electronics and even the practical side of electromagnetic theory before going to college.
When you were thinking about what school to go to did you have physics in mind specifically for undergraduate study?
I did not have a well-formed plan. I was a soccer player, a basketball player; I was interested in sports, playing with radios, chasing girls. It was the family tradition that young men went to Haverford, and young women went to Bryn Mawr, or maybe Swarthmore. So, that's what I did.
When did you settle on physics at Haverford?
At first I thought I would be a math major, and I spent two years in a math sequence. But I did take physics as a freshman, and by the middle of sophomore year discovered that I was having more fun in the physics lab than when proving theorems.
Does that speak to your interest in doing things that are not so abstract, would you say?
Yes. I certainly appreciated the math I had taken, and I continued to do more. But in the end, all the way through graduate school, I learned more mathematics in physics courses than in math courses.
Who were some of the professors in the physics program at Haverford that you became close with?
My two favorites were Fay Ajzenberg-Selove and Tom Benham. Fay Selove, one of the pioneer women in physics, was a superb mentor. I wrote her a letter over the summer before my sophomore year, effectively saying “I’m not really sure I like what I'm doing. Should I be a physics major instead? I'll try to catch up to the others in my class.” She strongly encouraged me, saying "Go ahead, you'll catch right up."
And, as it turns out, she saved my letter. Years later, after my Nobel Prize had been announced, we were both present at a celebration at Haverford. Fay pulled out the letter and said, "I've been saving this letter for 30 years, to use against you at some time in the future."
What was her area of expertise?
Fay was a nuclear physicist. She had worked with Tommy Lauritsen at Caltech before coming to Haverford. Her husband Walter Selove was a physicist at University of Pennsylvania, and she later left Haverford to join him at Penn.
The other man I mentioned, Tom Benham, was also a unique individual in many ways. He was blind and lectured from Braille notes. Most physics professors, particularly in small courses like many at Haverford, like to lecture with sketches, diagrams, and graphs drawn on the blackboard. Tom was able to draw on the blackboard, but his lines weren’t straight and the graphs weren't very good. So we had to help him, but that was part of the learning process.
He was extraordinarily good at teaching concepts, conveying a feeling for the laws of physics. At the start of his full-year course on electricity and magnetism, he handed out a problem set for the entire first semester. The lectures would proceed independent of these problem sets, but it was understood that we must work on problems every week.
He scheduled a one-on-one session with each of us, every week, at which we came and reported on how we had solved each problem we had worked on that week. There was no point in handing in problem solutions because he couldn't read them. Sometimes we’d say, "Well, I didn't finish number 19, but I worked on it for a while, and this is how far I got … “ He would then provide a few hints about what to do next, or how you might think about it a different way, or where you had gone astray. It was a good way to learn physics, and I learned a lot from Tom Benham.
Right. How much lab work did you do as an undergraduate?
I think pretty much the standard labs. A junior-year lab had some of the classic experiments of physics: measuring the speed of light, the charge of an electron. I enjoyed those labs, but they were largely cookbook experiments. The apparatus was already there, and it was known to work. You didn't have to design anything.
In contrast, I very much enjoyed a senior thesis project where, on my own, I built a radio telescope that could detect five radio sources in the sky and measure their positions. That was a lot of fun, and it took great advantage of my practical experience with radio-frequency electronics and antennas, gained from my amateur radio hobby going back to middle school.
That preempts my next question which was: looking forward in terms of astrophysics, how well was your self-identity defined as a physicist by the time you graduated from Haverford? In other words, when you were thinking about graduate school, how well-defined were your ideas about experimental versus theoretical, the kinds of sub-fields you wanted to work on, how did that come to be developed as an undergraduate?
I started thinking about that near the end of my junior year, realizing that this has been a lot of fun, but it will be over soon, and what am I going to do next? I was about to start working on the senior thesis project of building an interferometric radio telescope. I was already thinking about doing a graduate degree in physics, and I decided that astrophysics might be a good choice. I was having fun with radio astronomy, already, and was doing a lot of background reading important for my thesis project.
Of course, this is a field that you've stayed in your entire career, so as an origin story, I'm curious. What was it about radio telescopes that captured your imagination? What were you looking to find out, both in terms of discovery, and in terms of building instrumentation?
I was not focused on a particular problem in astrophysics; I would have found nearly any problem interesting, especially problems outside the solar system. I was especially fascinated by extragalactic radio sources, because that's mostly what I could detect with my radio interferometer. Two of the five sources I detected were radio galaxies thought to be at huge distances, with radio noise-generating mechanisms that nobody understood at the time.
We now know that they were activated by supermassive black holes at their centers. Of course we didn't know this at the time, but the phenomena fascinated me. In some ways I was at least as much interested in the instrumentation as in the astronomical targets to be observed.
In terms of your senior thesis, what did you learn from that experiment about how to do physics properly, and what did you discover as a result of your research?
My senior thesis project was entirely self-directed. Tom Benham was my supervisor, and helped me to acquire some needed test equipment, coaxial cable, and the like, but otherwise I was pretty much on my own. At the time I was not really well trained in the “right” ways to do physics. I was figuring it out on my own. There's a lot to be said for such an approach, though it can involve considerable wasted time pursuing what turn out to be bad ideas. Things that you learn on your own tend to stay with you better than things you learn from a textbook, teacher, or colleague.
I know this is a theme that you've mentioned in some of your public writings. Part of it is make sure you're having fun along the way, too.
Yes, for sure! When you’re having fun doing something, you spend a lot of time at it – and you can become good at it.
Did you do any summer internships?
Yes, I spent the summer after my junior year at the Brookhaven National Laboratory on Long Island, in a particle physics group. They projects I worked on were mostly instrumental -- building or measuring things in the lab. For the first time I was working with real physicists, on real problems, and receiving good advice about how fundamental physics should be done. There were 20 or so summer students at the lab, each given something to work on, but there was plenty of “down time” for softball, tennis, the beaches, and evening bridge games.
In retrospect, many of the student assignments were close to the borderline of being make-work projects. I think we students understood we were watching people doing important physics, rather than contributing significantly to it ourselves.
Now, your undergraduate years mostly overlapped with the Kennedy administration. I'm curious if the beginning of the space age, and the lofty things that Kennedy said about space, had any impact on you in terms of the kinds of things you were developing.
Yes, that certainly did have an influence. There was an atmosphere in which we knew the sky was the limit. Any idea that you had was something that potentially might actually be possible to do. It was a time when we were not worried about the future job possibilities. I don't remember ever talking about that with other students. we just knew there were going to be jobs for us, and jobs probably in an area that we would have selected by choice.
Certainly, a golden age in the academic job market, for sure.
And the beginning of things like the National Science Foundation for funding fundamental science. Those were all wonderful tools to have at one's disposal.
In terms of thinking about graduate schools to apply to, did you get any advice about particular programs, or areas of interest? How did you go about choosing a graduate program?
I wasn't very original about that. I had a general idea that I wanted to stay in the Northeast. I ended up applying to Cornell and Harvard, and chose Harvard, in part, because two or three other Haverford seniors were going there in other fields. It seemed like, well, let's go off there together. I knew there were radio astronomy programs at Harvard and Cornell, and some people working in that area. They had instruments that would be accessible. This was a time when the National Radio Astronomy Observatory was being built in Green Bank, West Virginia, so big radio telescopes were on the way. It was a good time to start in a field like that.
When you got to Harvard, I assume most of your cohort had gone to bigger schools than you had. My question is: in what ways did going to a smaller school serve you well, and in what ways did you feel like perhaps you needed to catch up with students who perhaps had gone to places like MIT, or Caltech, or Berkeley, or places like that?
I never felt at a disadvantage. There were certainly students who had done more physics than I had done. But I was in the astronomy department at Harvard, and almost none of us had done astronomy before at any depth. We were all physics majors, basically.
So, astronomy was its own program at Harvard.
Yes. Its own program and its own department. Indeed it was the Harvard Observatory, something like a mile away from the physics department and the other sciences. Most of us took courses in physics and mathematics as well as courses at the observatory. I liked the back and forth, and the chance to move around within the university.
Did you have an idea of what professor or professors you wanted to work with before you got to Harvard, or you developed those relationships after you got there?
Entirely after I got there. In fact, I didn't know any names that were potential supervisors.
Who did you end up becoming close with?
I did my dissertation under Alan Maxwell, a solar physicist. It was an experience rather like my undergraduate thesis in being essentially self-motivated and self-driven. Alan worked in an entirely different field. He was an excellent mentor, pointing me in good directions and giving good starting advice. He then had the knack of getting out of the way and letting me get on with it. I liked that. It was a good way of helping me to become self-reliant, defining a problem on my own, and proceeding to address it.
Alan was good at opening doors for me. When I needed, for example, some time at the big facilities, like the National Radio Astronomy Observatory, he was happy to write introductory letters, putting me in touch with the scheduling committee, helping me to write proposals, and the like. But Alan never took part in the observations or told me how to do them. He just said, "Okay, you've got the time. Make the best of it."
General question: what were some of the big questions that were being researched in the astronomy program in those days? What were some of the big, exciting developments in the field, generally?
Quasars were first recognized as unique objects in 1963, my first year at Harvard. Nobody knew what they were. We didn't even know if they were galactic, or extragalactic. I spent the summer of 1963 at the Harvard observatory – not the administrative and teaching facilities in Cambridge, but the Observatory out in Harvard, Massachusetts. It was a very small summer program, half a dozen students. We had a radio telescope at our disposal there, a 60 ft. dish, with a good receiver for the 1420-megahertz hydrogen line.
We did a program that summer to measure the absorption line spectrum of the first known quasar, 3C273 – absorption lines created by the hydrogen in the spiral arms of our galaxy. We could establish that at least the quasar was farther away than these spiral arms, and therefore probably outside our galaxy. We had no clue what these objects were -- they were called “quasi-stellar”, and maybe they were some kind of funny star in the galaxy. Our observations pretty well showed they weren't. In fact, that summer project with Sam Goldstein really should have been published – it was important at the time. But we never sent it off anywhere, we just sort of put it in a drawer.
How did the discovery of quasars relate to broader questions about how the universe works?
Within a few years, smart people had a good notion that quasars had something to do with gravitational collapse – ideas that lead to an understanding of supermassive black holes in galactic nuclei. It took decades for these ideas to be fleshed out, but at the time we knew that by some mechanism these objects created big jets and a lot of radio emission. Most of the details and even the fundamental energetics were not yet understood, so it was a fascinating topic to study.
We were looking at the structure of the lobes of radio emission. My PhD thesis became a series of observations of these objects – radio galaxies and quasars -- by means of their occultations by the moon. I observed what happens when the moon, in its slow motion in front of the “fixed stars”, covers up the radio source. The emission disappears within a few seconds, and reappears about an hour later, when the moon has moved out of the way. By analyzing the disappearance and reappearance events carefully, one can obtain a one-dimensional map of the source. Typically it would be at a different angle on the moon's limb at disappearance and reappearance, so you get two one-dimensional maps – and you could start to put these together to make a full two-dimensional picture. This was the work of my thesis.
It was important because we did not then have reliable map-making radio telescopes. Any source that was smaller than the beam of a single-radio telescope was effectively a point-source: you couldn't tell what kind of unresolved structure might be present. Now we make pictures with interferometric arrays, but at the time those didn't exist. We didn't have the radio-frequency phase stability to make such an instrument work. So, mapping radio sources by lunar occultations was a uniquely powerful tool for a few years.
How much were you thinking about general relativity during this time?
Not much at all. I took the course in relativity at Harvard Observatory. It was obviously an important part of physics, but not particularly relevant to most of astronomy.
I'm curious, what theoretical concepts were relevant for your work?
Synchrotron emission was the model being explored. Relativistic motions of charged particles, but this is special relativity rather than general relativity. Relativistic motions of charged particles and magnetic fields generating synchrotron emission. That was the basic model for these radio sources. Even for some things closer at hand -- Alan Maxwell was working in solar radio astronomy, and the radio bursts that come from the active sun also generated synchrotron radiation.
Are pulsars in any way on the horizon in terms of what you're looking at and what you're thinking about during your Harvard years?
I turned in my thesis in January 1968. Pulsars were discovered in Cambridge in late 1967, but had been under wraps until published in Nature in early February 1968. So, at just the time I had turned in and defended my thesis, pulsars were discovered. I had won a post-doctoral fellowship to stay on at Harvard for another year or so, with pretty much a blank slate offer to work on anything attracting my attention. I thought pulsars sounded interesting, and got busy with them right away. Several of us at the Harvard Observatory were in a similar situation, with an ability to move quickly into a new project, so we got started. We were among the first anywhere in the world, other than those in Cambridge, to work on pulsars.
I'm curious if the discovery at Cambridge had happened a year or two earlier, how much would this have affected your dissertation, or did you tend to think of these as two separate topics?
They were separate topics. I certainly would have read about them and been fascinated by them, but I would not have changed the topic I was working on. I would have finished what I had been working on, I think.
I want to switch a little to the larger social scene. Obviously, in the late 1960s in Cambridge, a lot of significant political and cultural things are happening: anti-war protests, and civil rights, and things like that. I'm curious the extent to which you were involved in these issues and what your own personal politics were during this time.
I was not particularly political then. I had my opinions, but I was not demonstrating or going to Selma, Alabama, though I was very much in sympathy with the things that were happening there. I was deeply involved in finishing my PhD, and not worrying too much about affairs of the world at large. I was somewhat uncomfortable with having been deferred from the draft because I was in graduate school, while others in my cohort had their numbers coming up. As a Quaker, I would have requested and received status as a conscientious objector. I would then have been required to serve some alternative service, as indeed one of my brothers did a few years later.
When you say you felt uncomfortable to some degree, is that to say that you felt some notion of serving your country even if it wouldn't have been in an aggressive, hostile capacity?
Exactly. I was sensitive to the fact that others whose situations were similar to mine would not have had the opportunity to claim a fundamental right of not serving in an armed capacity.
To move back now to the research, another origin story question. When you first learned about pulsars, did you think this was something that would continue to capture your imagination for the rest of your career? Was there something special about this particular topic that you wanted to really dedicate yourself to?
I had no way of thinking ahead 50 years. I was thinking ahead by one year, or two years. I was thinking these objects are different from any kind of known celestial radio source. All the other sources we knew about were constant, or at least as constant as you could measure over timescales of a few years. Pulsar emission varied on timescales of milliseconds, so obviously, something was very different.
I was challenged by the instrumental issues involved with detecting and studying pulsars. I was fascinated by the brand-new tools we could bring to bear. We could actually imagine having small computers in the laboratory, or at the observatory, sampling the data rapidly and recording the information on magnetic media. So, suitable tools were just becoming possible at the same time that pulsars appeared as a target. That was a challenging and exciting prospect for an experimentalist.
When pulsars were discovered, what were the implications in terms of what this meant, and what immediate research questions did it beg?
For the first year we thought pulsars must be either white dwarfs or neutron stars. Those were the only known objects that seemed plausibly capable of generating such rapidly varying pulses. Within a year the white dwarf hypothesis was set aside and the notion of a strongly magnetized, spinning neutron star took over. Of course, the idea of a neutron star, something orders of magnitude more dense than a white dwarf, had been around since the 1930s, but it was only an idea. There was a vague notion that one of the early discovered X-ray sources might be a neutron star, accreting material from a nearby supergiant star. But that was not a known fact, yet. It seemed that pulsar radio emission was a completely unexpected back-door possibility for observing an object as small as a neutron star, even at interstellar distances.
How much was the challenge of understanding these things a technological challenge of available instrumentation, compared to how valuable it would be to answer the kinds of questions that you knew were being raised?
For me it was mostly a challenge involving instruments and techniques. There was no hope of making images of these objects. They are far too small and too distant even for radio-interferometric imaging. So, we focused on the time domain rather than the spatial domain. We planned to analyze the signals at millisecond or sub-millisecond timescales. This involved either very fast analog instrumentation, or possibly digitizing the data and working in that regime. At the time, those options were both viable, and we tried each of them.
We had fast analog chart recorders, we had fast analog magnetic tape systems, but we also were exploring the possibilities of recording the data digitally, and analyzing with computers. We did not have a computer at the telescope capable of such analysis in real time, but we could record data on digital magnetic tapes, ship them to the computer center, and proceed that way. After the Cambridge group had discovered the first four pulsars, we discovered a fifth one. That was the first use of a digital pulsar searching algorithm, one that I developed in 1968.
Parallel question to the limitations of instrumentation: given your interest in digitally recording this data: to what extent were you keyed into the limits of computational power, and the ability of computers to help you do the research that you wanted to do?
Computational power was growing rapidly at the time. It was archaic by today's standards: processors had speeds measured in kilohertz, not gigahertz. Many orders of magnitude slower than today's machines, but nevertheless they allowed us to do things we could not do any other way. So, we were developing algorithmic techniques at the same time that computer engineers were developing machinery that could use them effectively. These machines had started following Moore's law, even before Moore had thought of it.
Joe, at the end of your time at Harvard, after your post-doc, and you were thinking about your next move, I'm curious if joining an institution like NASA was something that you considered at some point.
The short answer is no. I could easily have imagined myself at a place like the National Radio Astronomy Observatory – a place where I knew the telescopes and the people. I could easily have imagined joining a staff like that. But even back to Haverford years, I pictured myself as a teacher, a college professor. So, that’s the direction I was looking toward.
I wasn't one to worry much about where a possible job might turn up. I waited until they fell into my lap, more or less. As I said before, this was a time when the sky was the limit; there were lots of possibilities. I just didn't worry about it much. During my post-doctoral year at Harvard, I received a few feelers. One of them came from Bill Irvine at the University of Massachusetts. It seemed an attractive prospect to me. Eventually the small group of us at Harvard who had been working on pulsars decided to move together, as a group, to the University of Massachusetts.
At the University of Massachusetts, was there a similar distinction? Did they have a separate astronomy program from the physics program?
No. At the time, it as a combined Department of Physics and Astronomy. It turns out, since I left there in 1981, the astronomy program has separated off, but it's still closely connected to the physics department.
The Five College Radio Astronomy Observatory, how well developed was this before you arrived?
It wasn't established at all. We started it. The Five College cooperation was itself a new program just getting started.
You mean the idea of a consortium of colleges.
Yes. Hampshire College, the fifth of the five colleges, was just opening its doors for the first time, in '68, or maybe it was '69. So, these things were happening together. It was a wonderful time for all of us who were part of that Five College astronomy program. The university was much larger than the four other colleges, but each college had an astronomer, or maybe two. This meant that the combined the group was an active and sizable astronomy department. This was good for all of us – and especially for the astronomers at the smaller colleges.
In terms of all of the new hires, the new faculty members, plus building this astronomy observatory, where were the main sources of funding for this coming from? These were obviously not inexpensive endeavors.
No, they were not inexpensive, although we developed a knack for building things on the cheap. I had great colleagues who were good at scrounging and acquiring government surplus and things like that. We applied for funding from the sources that you would imagine. The National Science Foundation was a major source from the beginning. We got a sizable grant for building the radio telescopes – the first instruments of the Five College Radio Astronomy Observatory – from the Cottrell Foundation. It's a private foundation that liked to fund small college enterprises. They were very generous to us at a time when we really needed it.
We got a lot of heavy equipment from the State of Massachusetts, mostly surplus equipment. It wasn't spendable money, but it was usable equipment of various kinds. The original pulsar telescope was built with telephone poles and fencing that we could buy form Sears-Roebuck – this made the reflecting surface – and some steel angle-stock, and things like that. An essential piece of equipment for putting everything together was a “bucket truck”, the kind of truck with a big auger drill on the back, used to drill the holes for the telephone poles. The bucket, on the end of an extensible arm, gave us access to the tops of poles where the steel girders could be attached. We acquired the bucket truck as surplus equipment from the State of Massachusetts.
In terms of building the observatory, it's such a tremendous opportunity to do it the way you want to. On the cheap, yes, but in terms of constructing the instrumentation to look at the things you want to look at. My question is, how much was it focused exclusively on pulsars, or did the observatory have a broader purview than that?
For about ten years the pulsar telescope was our main scientific instrument. Its greatest contributions were its use as a test bed for instruments which we would then take to even larger telescopes – the 300-foot dish at Green Bank, West Virginia, and the 1000-foot Arecibo dish in Puerto Rico. Much of the equipment essential to my work at Arecibo was developed and thoroughly tested at the Five College Observatory. We could test receivers, instruments we called “de-dispersers,” signal averagers, and even dedicated computers and software on our own telescope, doing some fundamental spade-work there, and then take the equipment to a much bigger telescope for the most important work.
From the first moment that pulsars are discovered through these early years at the observatory in the 1970s, how had your understanding of pulsars improved and expanded during this time?
In early years, much of our focus was on trying to understand the emission mechanism, and from that hope to gain an understanding of the pulsar itself, and its magnetosphere. We wanted to know where the radio noise originates, and what it can tell us about the immediate surroundings of a rapidly spinning, strongly magnetized neutron star. We carefully measured polarization to see what it could tell us about the emission environment.
Along the way, we developed ways of very accurately following the timing of the pulsar, the rotational phase of the spinning neutron star. We were curious to see how accurate they were, as clocks, and to see how accurately we could do the timing measurements. We discovered it was possible to use timing measurements over several years for doing high precision astrometry. We were able to measure the first proper motion of a neutron star, relative to the fixed stars. It turns out that most neutron stars acquire high velocities as a result of their formation in supernova explosions. This realization proved helpful to understanding how neutron stars fit into the overall scheme of stellar evolution.
Our attempts, and those of many others, to understand the radio emission mechanism of pulsars have never been very fruitful. It's too messy. It's messy physics, not clean physics.
What makes it messy? What do you mean by that?
My dear friend and colleague from Bangalore, Venkataraman Radhakrishnan, who everybody always called Rad, once described trying to understand the pulsar emission mechanism as being equivalent to standing in the parking lot outside a noisy factory trying to deduce how the factory machinery works by listening to its squeaks. The radio emission we get form a pulsar is something like one part in ten thousand of the rotational kinetic energy loss of the spinning neutron star. Almost all the energy is going off in some other form than what we observe. To use this tiny fraction of energy to tell us how the machinery works is a losing proposition. We're not going to figure much out that way.
When did you first meet Russell Hulse?
Russell was a first-year graduate student in the physics department at UMass when I was designing the system that we could take to Arecibo to discover pulsars. Russ was then interested in atomic physics and other areas of fundamental physics. We met at afternoon tea and other times, in the department, and one afternoon I told him about my plans and asked whether he might be interested in working on it. He thought about it for a short time and said yes. It was a shift of interest for him, but many of us do that in graduate school, doing something a little different from what we had imagined.
He joined the project in 1972. I had already developed a general plan for a computer-based pulsar-discovering instrument that I wanted to build and take to Arecibo. In addition to a small computer it involved a multi-channel receiver and some ancillary equipment. My plan was to put it all in a big crate, air freight it to Puerto Rico, and put it to work on the largest radio telescope in the world.
I'm curious when the transition in your mind happened from thinking about Russell as a student, and then ultimately as a collaborator and a peer.
We were collaborators throughout his time as a thesis student. We worked together to build an instrument and write the software for it. The software was a big task. I had roughed out most of it in advance, but had not written the detailed code. Mini-computers were new, and by our standards expensive. To save money we bought one that didn't have an operating system. It didn't have a compiler, an assembler, or a linker. It didn't have any of those things. We had to code every bit of every word the way we wanted it, and put those bits on a magnetic tape that could be read by a bootstrap loader.
It was hard work, and it was completely new to both of us – indeed, at the time it was new to almost anybody. Russ went to the factory where the computer was built, in Florida, and spent 10 days at a crash course learning how to program their machine. In the meantime, we were building the receivers in the lab at UMass. We got it all together, took it to Puerto Rico, and hooked it up with the Arecibo telescope. Russ stayed there for the best part of a year.
In 1973 the reflecting surface of the Arecibo telescope was being replaced with a new, more accurate surface. Telescope usability was therefore far less than 100%. The feed antennas could not be moved very often, so we devised a plan to set the feed at a particular spot and wait for Earth rotation to bring the desired part of the sky overhead. In this way we could get observing time in large quantities, so long as we could deal with requirements of the upgrading project. That turned out to be just what we needed, and it was good fun. I had to go back to UMass to teach courses and attend to other duties; I was back and forth fairly frequently, while Russ did the groundwork at the telescope when I was not there.
I'm curious, in terms of developing your collaboration with Russ, were there things that he was better at than you, and things that you were better than him, and you sort of drew on those strengths to build this research, or was there mostly overlap, and it was more about a division of labor?
There was a lot of overlap. Russ became very good at programming this computer. The algorithms were things that I had worked out earlier, but the details and specific implementation were very much his own work. He was resourceful and on the spot, doing all the things necessary to make such a project actually work. Of course, this was at a time before email, and even telephone phone conversations were expensive, so we didn't do that very often. We wrote letters back and forth.
That's amazing. Joe, can you talk a little bit about the culture of the observatory in Puerto Rico? What was it like there?
I've always loved my time at the Arecibo Observatory. It's a unique instrument. There's nothing like it anywhere in the world. The staff was around 150 people, during its heyday years.
Was it mostly Puerto Ricans there?
Yes. Probably 100 of the staff were locals, many with a lot of technical skills. A very good electronics lab. The scientists were mostly from stateside, with a scattering from other parts of the world. There was a fairly rapid turnover among scientific staff. People would come for a year or two, and this was good for the overall health of the institution. There was stability at the top, but a lot of refreshment with new ideas coming from outside.
And, of course, it was fundamentally a visitor facility – most of the instrument time was assigned to visitors from U.S. institutions, and others around the world. There was a lot of opportunity for give and take with scientists from other institutions than one’s own. The nature of the telescope – a fixed, spherical mirror, looking straight up -- means that you have to wait for Earth rotation to bring your target source into view, and you can follow it for only a couple of hours each day. To make effective use of all 24 hours, the telescope’s “back end” equipment must be sufficiently flexible that an experiment can be set up and started in a few minutes, run for three or four hours, and then taken down so the next experiment can be put up. Observers necessarily become adept at doing this quickly.
Our specially programmed computer was a new addition to the lab. It had to be given its own spot and its own connection points; our own receiver would be plugged into the tail end of the telescope’s normal chain of signal processing.
At what point do you start thinking about gravitational radiation, and is there a dramatic moment where you realize that you've confirmed its existence?
Hulse's project was basically to scan the portion of the Milky Way that goes overhead in Puerto Rico, looking for previously undetected pulsars. When we started only a few dozen pulsars were known. We estimated that we might double or even triple the number of known pulsars, and we hoped to sketch out how they are distributed along the galactic plane so we could infer how their evolution is related to that of other types of stars.
My proposal to the National Science Foundation, which produced the $30,000 that purchased our computer, mentioned the potential importance of finding even one pulsar in a binary system. We might then be able to measure its mass, an extremely important number because it would provide direct evidence on the “stiffness” of nuclear matter in bulk quantity. This was a sort of holy grail, but we weren't counting on its success: our bread and butter was to be just discovering pulsars.
The project started in 1972 and we were taking data by late 1973. By mid-1974 Russell had found several dozen new pulsars, and clearly there were more on the way. In July 1974 I received a letter from Russell saying he had a candidate pulsar with an unusually short period, about 59 milliseconds. Even more importantly, the period had the oddball characteristic of being not quite constant. He was following these changes, and he thought the pulsar might be especially interesting in some way.
Why was this an oddball characteristic?
Up to this time, all known pulsars had been found to be extraordinarily good clocks. Their periods changed by only small amounts in the sixth decimal place and beyond. These changes could be ascribed either to the Earth's motion (causing variable Doppler shifts) or to a tiny but measurable slowdown of the rotation rate. But this pulsar behaved differently. Its period varied in more like the third decimal place, and the changes were repetitive – although the repetition pattern wasn't quite obvious, at first. We could observe the pulsar for only two and a half hours each day. On subsequent days the period would go through nearly the same set of variations, but with a progressive lag of around 45 minutes per day.
It turned out that this pulsar is in orbital motion with orbital period just under eight hours. Of course, eight hours is one third of a day, so one day later, the pulsar is back at almost the same spot in its orbit, shifted by about 45 minutes. After two weeks we could establish that the pulsar is in a bound orbit of about seven hours and 45 minutes, and we could plot a reasonably accurate orbital velocity curve.
How much during these years are you thinking about the broader theoretical implications of this research?
Our principal interest was in pulsars as a phenomenon related to the endpoints of stellar evolution. We were not thinking about relativity at all, or even gravitation theory. For the orbital analysis we used Newtonian physics, which was initially adequate. We did a simple, freshman-physics-level calculation to show that if the pulsar is a neutron star of about 1.4 solar masses, its orbiting companion must have comparable mass. Maybe it's another neutron star, but in any case, it's something around one or one and a half solar masses. Two such masses in an eight-hour orbit means a mildly relativistic system.
Thus, within a few weeks of the discovery we knew that our binary pulsar was relativistically interesting. At that time, yes, we started to think about gravitational radiation, gravitational red shift, and precessing orbits. We realized that we could no longer rely on Newtonian physics to describe this orbit.
So, initially you're not thinking about proving Einstein correct. That's not really an item of concern for you.
Such ideas became interesting only after we realized that the binary pulsar system might be a nearly ideal experimental tool for probing relativistic gravity. We then became very much interested in them..
You mentioned that Russell had stayed on in Puerto Rico for longer stretches of time, but you had to shuttle back and forth because of your other duties in Massachusetts. I'm curious the extent to which the observatory in Massachusetts, and the observatory in Puerto Rico, and to some extent, the observatory in West Virginia, if you used all three observatories sort of toward one end, or you were maintaining different projects, or different research endeavors at each of these observatories.
Yes, I had active research projects at each of the three observatories. Work on the binary pulsar – the relativistic gravitation experiment – was entirely done at Arecibo. We needed the sensitivity of that big telescope. That's one of the reasons that we never had much competition in doing that experiment. Other radio telescopes around the world could detect the binary pulsar, but they couldn't do accurate timing anywhere near as well as we could with the sensitivity of the Arecibo dish.
Of course, at the time, we were also developing better instrumentation for doing that timing. Development of special instrumentation for timing the binary pulsar turned out to be very important also for timing millisecond pulsars, when they were discovered a few years later, in the early 1980s. So, we were in Arecibo and well prepared to do accurate measurements of those objects, as well.
Joe, in these intensive years of research, what were the conferences, or the journals, or the other means of communicating your research that were most useful to you?
The most important measurement of the binary pulsar system is that its orbital period is decreasing over time. This measurement was first reported at the Texas Symposium on Relativistic Astrophysics held at Munich in December 1978. I announced that we had measured an orbital period change consistent with the loss of energy by gravitational radiation, as predicted by general relativity. This work was published in Nature soon afterward.
I'm curious if the initial research on pulsars at Cambridge meant that Cambridge continued to be a player in pulsar discovery and research.
Pulsar work at Cambridge continued for a year or so, but decreased after that. The Cambridge instrument that discovered pulsars was built for a different purpose, measuring interplanetary simulations of quasars. It was a pioneering instrument with capabilities essential for making the initial discovery, but it was not a very good instrument for following up pulsar work. British contributions to pulsar astrophysics in the years following 1967-'68 were mostly done at Jodrell Bank, not at Cambridge.
When I asked you about the options you were considering after graduate work, you emphasized the importance of working within an academic environment. I'm curious if you could talk a little bit -- perhaps we'll return to this when we get to Princeton, but during your time in Massachusetts, what was so valuable to you about being in an academic environment, in terms of the way that you interacted with your colleagues, the way that you interacted with undergraduates and graduate students?
I enjoy teaching, and I wanted to continue doing that, but I was strongly focused on research as well. Research often proceeds most effectively with young people around – individuals not yet steeped in the traditions of the sub-field – because they ask questions and have new ideas that might not occur to those with more experience. My interactions with undergraduates and graduate students at UMass were important in those years, and have remained so in all my years at Princeton.
What have been some of your favorite courses to teach undergraduates?
By choice, I have mostly taught beginning and mid-level courses. At UMass I taught a range of astronomy courses. At Princeton I loved teaching the physics department’s large service courses, those for engineering and pre-med students. The interactions with students closest to my research work have mostly been one-to-one supervision of independent and dissertation work.
For the Astronomy 101 type courses where you knew that these students likely would never have this opportunity again, what are some of the most important things that you wanted to emphasize to those students about astronomy and physics?
I started teaching at a time when astronomy was blossoming. The early years of the space program, direct exploration of nearby planets, and so forth, were underway. We were also discovering things at the farthest reaches of the universe, with cosmological implications. Giving non-science undergraduate students a basic flavor of those exciting things was an interesting and worthwhile challenge. I enjoyed doing the demonstrations and the lectures, and I could see the students get a lot out of them. That effort is quite different from involving students more directly in research work. I enjoyed both sorts of interaction.
How did the opportunity at Princeton come about?
I was happy with my position at UMass and not particularly looking for a change. Research work was progressing well. I was starting to serve on various national advisory committees, the visiting committee of the NRAO, and places like that, so I was meeting colleagues from other universities fairly regularly.
After one such meeting David Wilkinson took me aside and asked if I might have any interest in moving to Princeton. This was an immediately attractive possibility – in part because of the distinction of the physics department at Princeton, but also because it would be more or less “going home” for me, returning to New Jersey. It did not take long for a formal offer to be made and accepted.
Was the Five College Observatory strong enough at this point, did it have its own legs, that when you left it would have a good trajectory going forward?
The Five College Observatory was thriving. Our pulsar telescope was not designed for long life; as I said earlier, it was built on the cheap. But by 1979 we were also building a highly capable millimeter-wave telescope designed for spectroscopy of interstellar molecules. That project was well staffed and became very productive.
When you moved to Princeton, was there an observatory opportunity for you there, either to build one or to join an existing observatory?
No. Major radio astronomy instruments are large enough and expensive enough that they are seldom owned and operated by single universities. Rather, they are owned by consortia or national facilities. My plan was to continue as a visiting observer at Arecibo and other radio telescopes at remote, radio-quiet locations. I did some work at the Molonglo and Parkes radio telescopes in Australia. I was quite happy to be a user of such facilities.
Your appointment was in the physics program at Princeton. Did Princeton have a separate astronomy department, or was it combined?
At Princeton we have separate departments of astronomy and physics, and you might well ask about why it was the physics department that attracted me.
That's the question.
The simple answer is that the Princeton Physics Department had an established effort known as the Gravity Group, which originally started around Bob Dicke and John Wheeler. Wheeler had turned his attention from nuclear physics to gravitational physics and relativity in the late '60s. Dicke was interested in a wide range of phenomena, including some astrophysical projects. Together with some younger colleagues, they formed a group interested in probing gravitation at its limits. The binary pulsar experiment fit very naturally into the gravity group.
What do you mean "at its limits"? What does that mean?
Newtonian gravity is well understood – no problems there in everyday circumstances – but pushing to the limits of strong fields, relativistic velocities, and gravity’s influence on the universe as a whole raises many interesting questions. Moreover, at that time these areas were mostly devoid of experimental data. Growing theoretical interest, for sure, but not yet any important experimental tests.
Bob Dicke was interested in the possibility that general relativity might not be the only way to describe relativistic gravitation. He did a widely followed experiment measuring the precise shape of the sun – one of the early experiments done in Princeton’s Gravity Group. Gravitational radiation was beginning to be more widely discussed, as well. An experimental tool like the binary pulsar, potentially a sensitive probe of the existence of gravitational radiation, was very attractive to the Princeton department.
When you joined the physics department at Princeton, what were your impressions of it overall, in terms of the big things that people were interested in?
Princeton’s Physics Department was widely known for important results in physics over the years, but the department is much smaller than those in many major research universities. Princeton physics has thrived because the department concentrated in a few areas and tried to do those things exceptionally well. We didn't try to cover everything.
Like a Berkeley, for example.
Yes, or MIT, or lots of other larger places. When I came to Princeton we had strong groups in particle physics, nuclear physics, and theory. Not much in condensed matter physics, especially not on the experimental side. We had the gravity group. We had some initial interest in what later blossomed as biophysics, but it was not yet a viable subfield. Partly because of the continual evolution of principal areas of interest, the department was a delightful and scientifically exciting place to work.
When you moved to Princeton, did you see this, in broad terms, as a continuation of your research, or were you looking for opportunities to diversify the things that you were working on?
I planned to continue what I had been working on, while on the lookout for related opportunities. We were trying hard to increase the number of known pulsars. I was involved in pulsar searches underway at Arecibo, at Green Bank, and in Australia. I was also continuing regular timing measurements of the binary pulsar.
About a year before the move to Princeton, Joel Weisberg joined me as a post-doc at UMass. I persuaded him to come with me to the Princeton department as an assistant professor. As things developed, Joel and I worked together on the binary pulsar timing experiment for more than 30 years. We developed better and better instrumentation for this work, taking advantage of the improved capabilities of small laboratory computers and ancillary digital equipment. We gradually increased the rate at which we sampled the binary pulsar data, thereby improving the timing accuracy. Most of this work was done after we moved to Princeton.
That gets me back to one of my previous questions. What about the telescopes themselves? How had advances in the telescopes also enhanced your research and what you were able to find?
The reflecting surface of the Arecibo telescope was being upgraded during the Hulse-Taylor pulsar survey. By the time I moved to Princeton its shiny new surface was available, with improved accuracy. Higher frequencies could now be used, and improved low-noise receivers were being built. Many advances in the electronic and digital regime: better receivers, lower noise temperatures, wider bandwidths, and improved data handling facilities. We contributed to some of those advances, but much of it was done by the observatory staff.
We participated in the planning of new instruments, often offering advice and requirements and so forth. The observatory was very good at carrying through with the construction of needed facilities. The observatory had a really good electronics department, and that was essential to the way in which the telescope could be an all-purpose instrument for a lot of different projects including our own.
Joe, given your focused area of research over so many decades, can you talk a little bit about how you ensured against diminishing returns, in terms of the value of the things that you had discovered? In other words, it's a really big deal when the pulsar is first discovered, right? But then how do you ensure that the value of the things that you continue to learn are also valuable throughout the decades?
The gravitational radiation detection experiment was one which inherently rewards patience. The longer you wait, the bigger the observable effect becomes. It's not just a linear increase, it's a quadratic increase. So, after two years, it's four times as good as after one year, and so forth.
Can you explain the science behind that? Why is that? Why is patience rewarded in these measurements?
To detect the effect of energy loss by gravitational radiation we try to measure a reduction in the time it takes the pulsar and its companion to complete one orbit. If the orbital frequency increases in a linear fashion, the phase of the orbit increases as a quadratic – that is, in proportion to time squared. A graph expressing the essence of the experiment Joel Weisberg and I did, over all those years, is a parabola that shows the quadratically increasing orbital phase. We put it on a tee-shirt. It had a parabola starting up here and going down at an increasing rate. (I'm waving hands at my computer, but I'm not very good at it!)
The longer we pursue the measurements, the more accurate will be the measurement of the quadratic term. If at the same time we're also improving the accuracy of the fundamental timing observations, we can do even better than improving as time squared. Measuring the accuracy of energy loss from the orbit to 20% or so, took us about four years. We announced at the Munich meeting in 1978 that that we were starting to see what appears to be orbital decay. But a 20-30% measurement is not all that interesting. We'd like to have a 1% measurement, and to see if it really agrees with the value relativity predicts. That took many more years.
By about 1990 the measurement accuracy was better than 1%. At that level, there are influencing effects having to do with gravitational accelerations of the system as it orbits around the galactic center, and similar quantities that have to be measured separately in order to proceed any further. Interestingly, we can turn the problem around and say, well, if we assume that relativity is correct in describing the orbital decay, what can we say about the astrophysics? That provides a way to measure the distance from the sun to galactic center. Such a measurement has been done even better now, by other techniques, but it was an interesting result along the way.
While we're talking about the issue of all of this research taking place over decades, I'm fascinated at how long the gestation period is of the research relative to when you're recognized for it with the Nobel Prize in 1993. The extent to which you understand or appreciate these things, why 1993? What was it about that year in particular that made it that this would be the time in which you and Russell would be recognized for your work?
I can't really answer for why 1993 was the right year, but I can easily understand that a lot of years were required. As I explained, this was a measurement which became increasingly interesting as its accuracy improved. 1% accuracy is a lot more convincing than 10%. Competing theories of gravitation were being widely discussed in those years. Until the mid-1980s it was not completely clear that the general relativity calculation we were comparing against our measurements was, in fact, the correct and rigorous result from general relativity.
The relativistic field equations are nonlinear, and the solutions are not simple. There was no possibility then of doing the whole thing numerically, with sufficient accuracy; and there was significant discussion, if not disagreement, about whether the calculations, approximated as they had been done, were giving the right result. There was a discussion about whether the Einstein paper from 1918 that contained the so-called “quadrupole formula”, which we were using to calculate the expected energy loss from the system, could be correctly applied to a self-gravitating pair of orbiting stars. Largely because of our experimental measurements, such theoretical issues finally came to a head. A rigorous theoretical treatment became an important goal, and was achieved in the early 1980s.
In terms of there being competing theories, and there being disagreements, what was your style as a scholar in terms of being a part of these debates? Did you engage with people who disagreed with you, or was your style more to just do the research and let the research speak for itself?
Our focus was entirely on the experimental aspect. Of course, I was interested in the theory, but I was not a significant contributor. I was very happy to let the theorists argue it out among themselves while we could stand aloof and say, "Well, guys, this is what nature is doing, according to our measurements. If we're going to have a theory that explains it, you're going to have to work it out."
I want to talk a little bit about the circumstances leading up to the big announcement in 1993. How does this work? Is there buzz? Are there inklings that this is developing, or is it really a surprise out of the blue where you hear stories about getting a call in the middle of the night from Sweden?
It was a surprise out of the blue for us. There had been hints over the years, but it was already 19 years after the discovery. The hints were of the sort that someone would say, "This is going to come to the attention of people in Stockholm someday." We didn't think about it that much. We were certainly not waiting for a phone call. When it came, it was a complete surprise.
When did it come, and who called you?
It came early in the morning, maybe 7 AM. Perhaps 2 PM. in Stockholm, so a pretty reasonable time. I was in bed asleep. My wife often got up a little before me; she came into the bedroom and said, "There's a reporter on the phone that says you've been awarded the Nobel Prize." It was not the Nobel Foundation calling: it was a reporter calling, from Stockholm. In fact, I never did receive a call from the Nobel Foundation, because the phone just kept ringing, one call after another, from different news organizations and then family members and friends. We finally unplugged the phone so that we could have some coffee and eat our breakfast.
Then, I went off to the physics department. By that time, it was already known about there, in part because the Nobel folks, having failed to reach me by telephone, had sent a fax to a machine in the corridor outside the department office. The corridor was accessible to anybody, and graduate students had already found the fax and were reading it.
That must have been one of the crazier days in your life, I imagine.
It was all of that. Obviously, a big honor, and a day of celebration. One celebration after another. A press conference later in the afternoon. Obviously, a lot of popping of champagne corks in the department that day.
Obviously, it's an unparalleled recognition, but I wonder if there were any other honors or recognitions that you have received that were equally valuable to you in terms of recognition, not just from a global body like the Nobel Prize, but more focused, like for example the Einstein Medal in 1991. I'm curious how you regard each of these recognitions.
One that was most important to me was the MacArthur Foundation Prize Fellowship in 1981.
Of which you were among the first, correct?
It was the first year of MacArthur Fellowship. But in those very early years, they did it twice a year; I was in the second group in that first year. It was also the year in which we moved to Princeton, so our kids were in new schools and I was a new faculty member in the Princeton department. I remember that the MacArthur Fellowship made me think "Okay, you're going to actually have to do something now. You're going to need to produce something."
In some sense, it was a bit of a burden at first. Obviously a very nice honor, and the money was welcome. To many MacArthur fellows, it provides an opportunity to do something completely different for a year. But I didn't want to do something different. I had just changed jobs; I had entered a new department; I was teaching physics courses instead of astronomy courses. I didn't want to stop doing any of those things. The MacArthur Fellowship didn't enable me to do something that I wouldn't have done otherwise, but it did enable me to pay off a mortgage.
Another very nice honor was the Wolf Prize, which came in 1989. Or was it 1990?
1992, okay. So, it was just a year before the Nobel Prize. I remember, at that time, somebody -- was it Sheldon Glashow? -- saying, when I saw him after the Wolf Prize, "Well, it's a good thing you got that one first, because you can't get that one after the other one." Something like that. So, the Nobel Prize came just a year later. Those were the most important recognitions.
The Einstein Medal was very nice. It has a uniquely important name attached to it, but as far as receiving a lot of newspaper attention it was a relatively small thing . It meant giving a lecture in Bern, and meeting physics colleagues there. That was enjoyable.
Joe, after the champagne, and the heady days, and after you got this recognition for the Nobel Prize, I'm curious if you ever gave thought in the weeks, and perhaps months after, that the Nobel Prize was not just a recognition for your specific area of research, but it was also an opportunity to serve as a platform for other things that were important to you, and perhaps now you had a recognition about you that might serve usefully in publicizing, or making known other issues that might be not entirely related to your research.
It certainly does give one an opportunity to do that sort of thing, and I've done some of it. But I haven’t often taken a leadership role. I get letters asking if I would be willing to co-sign a petition or document of some sort. I think about these carefully, and I join the effort sometimes, but sometimes not.
I have felt the Nobel Prize brings an important obligation to be available for public lectures and publicizing the importance of science. I did many such gigs for several years after the prize. Usually it would be a matter of accepting an invitation to give a lecture at a physics department somewhere, followed by "… and while you're here to talk to the department, how about giving a public lecture the evening before, or perhaps after?" So, it was often a visit to a campus, spending a couple of days, mixing with physicists, but also with the interested public. I enjoyed doing that – and it was, I felt, an important obligation to be available.
These things mostly stopped after 1997, when I was asked if I might be interested in serving as Dean of the Faculty at Princeton. I realized that such a university administration post would keep me from doing much public lecturing for a while. By then I was ready to turn the page and try working on something new.
What were your major goals and accomplishments as dean of faculty?
Most importantly it was the 3 Rs: recruiting, retaining, and retiring the faculty. Princeton has a very flat administrative structure. All department chairs report to the dean of the faculty, so I worked with 35 or 36 department chairs on their recruiting and retaining issues. Identifying new junior faculty prospects, considering our them for promotions, recruiting new senior faculty – all of those things. The overall quality of the Princeton faculty was my main responsibility and duty. I found it interesting and fun most of the time. It was an opportunity to get to know the rest of my university very well.
As I'm sure you can appreciate, most academics are rather narrowly focused on their own teaching and research. I had never paid much attention to what was going on in most other departments at Princeton. Even though I knew we had many pockets of excellence, and I admired things happening in other fields, I hadn't paid much attention to the details. When I was Dean, I paid lots of attention to those details.
How well were you able to continue with your research during this period?
Not very much. By that time, two junior colleagues, David Nice and Stephen Thorsett were carrying on much of the work that we had started together. Their work was continuing and productive, but I was mostly a peripheral part of it by then.
How many years were there after your ten years dean of faculty, and before your retirement in 2006?
I returned to the physics department in 2003, and taught for three more years.
When you were able to do research, in what ways did the Nobel Prize enhance your research in terms of opportunities that were available to you, and in what ways was it a burden because all kinds of people might have been interested in you for reasons that were sort of peripheral to the things that you were really interested about?
Both effects were present to some degree, as I suppose is common. There were advantages, but also burdens. Some prize-winners have surely been better than me at proceeding with further research, perhaps even in new areas. I didn't attempt to do that. One obvious advantage was that when I submitted a renewal request for a National Science Foundation grant, asking for a five-year budget rather than three years, they just said, "Okay, five years then." I'm sure this was mostly thanks to the folks in Stockholm. But overall, the prize was not an advantage to my additional research. Quite likely, I would have accomplished more by continuing as I had been working, without the additional distractions.
That's a tremendous irony.
Yeah, right. So, it gives you special admiration for individuals like John Bardeen, or Marie Curie, who were awarded twice.
Right, exactly. So, you retired in 2006, and in the past 14 years, what have been the things that you've been interested in with regard to astronomy and physics?
I continue to attend the research seminars of the department and try to keep up with what my younger colleagues are doing. I offer advice from time to time. But I haven't been calling the shots. Wisely, I think, my department decided not to continue allocating research positions to the narrowly focused work on pulsars that I had done since 1981. Instead, we chose to put our major gravity group effort into cosmology.
So, the group has been working on the Wilkinson Microwave Anisotropy Probe, WMAP, and other things that have evolved from that. Other ways of exploring the large-scale structure of the universe. That has been a long-time focus in our department, and a good way to concentrate our resources. Again, compared to some of the major research universities, we are rather small. We must make careful choices about fields of concentration. My pulsar work was a fascinating field for several decades, but in some ways it had become a mature sub-field. We were not aware of special opportunities that we would regret losing; we saw, instead, plenty of exciting opportunities in cosmology.
Well, Joe, now that we've brought it up to present day, essentially, I want to ask you for the last few questions during our talk, a few broadly retrospective questions about your career, and then a few forward-looking questions. The first is what do you see in the sum total of your research and your contributions as the most important contributions in the realm of theory, and then in the realm of experimentation?
My own contributions have been entirely on the experimental or observational side, but close collaborations with theorists have been essential. One of my favorite colleagues has been Thibault Damour, the French physicist who did much to help settle questions about the rigorously correct predictions of general relativity for the binary pulsar timing experiment. We worked together a number of times over years, both here and in France. Ours was a rich collaboration between a theorist who wouldn't know what to do in a laboratory, and an experimentalist who does not pretend to be able to do the calculations required for some of the theoretical work. We did things together that neither of us could do, alone.
On the experimental side my most important collaborator has been Joel Weisberg. We spent a year together at UMass and then about five years at Princeton, before he moved on to Carleton College. Joel had always pictured himself at a smaller college, and he thrived there. I was sorry at the time of the Nobel Prize that he was not recognized as a third awardee. Without question Russell Hulse's contributions were essential to the discovery of the binary pulsar. But the reason our discovery attracted attention resulting in a Nobel Prize was the pulsar timing experiment, which I did almost entirely with Weisberg.
Clearly, nobody asked you your opinion about whether Weisberg should be included in this.
Correct. And it should be noted that almost all Nobel Prizes have questions asked afterwards about whether so-and-so should have been included, or not included. It's hardly unusual. One thing that happens all too frequently is that somebody will say, "Hulse and Taylor shared the Nobel Prize for showing that gravitational radiation exists." The fact is, Hulse and Taylor share the Nobel Prize for discovering a binary pulsar; other work done afterward showed that gravitational radiation exists. It embarrasses Russell to have people say that he did these experiments that showed gravitational radiation because by that time he had moved on to other things. Weisberg and I were doing the binary pulsar timing measurements.
You mentioned before the phrase “messy physics”. Maybe this is messy prize recognition. Joe, I wonder if you can talk about either personally, or as a representative of your field, going all the way back to your days at Harvard, what are some days looking back to the 1960s that were truly not understood about how the universe works, and today are truly understood about how the universe works?
One of the biggest changes is that in the 1960s cosmology was a speculative backwater of astrophysics. There were ideas, but very little data. No accurate measurements of anything with cosmological significance. By cosmological, I mean the large-scale structure of the universe, and the evolution of how things must have changed over time. None of this was quantitative at the time. It was speculative. We imagined being able to do better measurements, but they were not yet possible. The situation today is completely different. The space program, and measurements made with new instruments and technologies from the ground, have provided data that could not have been foreseen 50 years ago. Cosmology today is a quantitative field with a lot of high precision measurements. We know that the Big Bang occurred 13.7 billion years ago, and a bunch of other numbers like that, with high confidence.
Another thing that has changed over these years are the participants in the field. When I was a graduate student at the Harvard Observatory we and our teachers were nearly all white males, most of us born in North America. I took one course from the woman astronomer, Cecilia Payne-Gaposchkin, on stellar spectroscopy. But she was an outlier. Today there are far more women, increasingly more people of color, and people from other parts of the world making major contributions to these fields in our universities. We have graduate students from all over the world in the Princeton department; that was not so true 50 years ago, or even 35 years ago. I had the personal good fortune of having four or five really good women graduate students over the years, and all of them are still active in the field. This has been an important change over these years. The demography and sociology of the field has changed for the better.
In terms of quantifying how these advances came about over these decades, there are advances in the technology, in the telescopes and the computers; there are eureka moments; there is the hard work, day in and day out of taking the data and understanding it. How do you rank these things in terms of the overall picture of how knowledge has advanced? What are the most important factors in considering these advances?
Good question. I'm not quite sure how to answer. I think having worked on a problem for a long time, several decades, refining the measurements for the same experiment gives one a chance to have seen it from a wide variety of different viewpoints. At first, we could begin to picture what might be possible, but were not sure how to do it. Over the years, we figured out ways to do it better, and then could see how, perhaps, even those techniques might be improved upon. Focusing for such a long time on a single goal gives one the opportunity to try a lot of approaches – some of which work, and some don't – and incrementally get closer and closer to the desired goal. This has been important.
It was helpful to me that our binary pulsar timing measurements could be a sort of “back burner” experiment, much of the time. We made measurements in sessions that lasted about two weeks, repeated every 12 to 18 months. We didn't have to do it continuously. We could go away and come back a year later and do it again. In the meantime, for the other 11 months of the year, we could think about other projects. We always had a number of projects in the works, with several graduate students working on different ones. In this way it was possible to keep a lot of balls in the air, only some of which were associated with the problem that earned a Nobel Prize.
In what ways do you see your research contributing to fundamental existential questions about the nature of the universe, where it came from, and how it started?
My work relates only indirectly to cosmological questions, but in an important way. We really do need to know what the correct theory of gravitation is. We have shown experimentally that a correct theory of gravitation must include radiative coupling and radiative energy losses. Gravitational radiation is an inherent characteristic of Einstein's theory; if we're going to use this theory to understand the overall structure of the universe, we've got to know whether all of its predictions are verifiable. That's why the binary pulsar experiment has been important.
To turn the question of advancing discovery on its head, going all the way back to your work as a graduate student, what are some things that stick out in your mind that are really not well understood, even to this day?
We still don't understand the detailed mechanism of pulsar radio emission – the problem we were so tied up with for the first five years or so of studying pulsars. We'd like to understand that better. We'd like to get up close to a neutron star, sometime, with suitable sensors and see what's going on at the magnetic polar caps and at the so-called “light cylinder”, where the co-rotation velocity is equal to the speed of light. I don't know whether those things will ever be possible. We certainly haven't done them yet. We'd still like to know why quantum mechanics works so well at the microscopic level, and yet we have no quantum theory of gravity.
Over the years it has been an extremely useful guiding principle to look for ways to unify the laws of physics. We haven't been able to do that yet with gravitation and quantum physics. Maybe string theory can lead us there, but we've been saying that now for thirty-some years. It would be nice to know if there's going to be an answer that unifies gravitation with the rest of physics.
Obviously, it's just an opinion, and you don't have any way of knowing, but do you think a grand unified theory is achievable?
I hope so. It has been a helpful guiding principle for several centuries. Maybe there's an endpoint to that kind of unification, but maybe not. I kind of like the idea that there is not. If only we could see what we're overlooking, we might find a way to make it all come together.
Perhaps this is a philosophical question as much as it’s a scientific question, but the fact that we have not yet achieved a grand unified theory, that's a reflection more on us, and not on nature, right? I mean, nature, of course, is a unified thing. The universe is unified, right? So, if we can't find one theory to put it all together, that's simply a reflection of our own limitations of understanding the universe, not what the universe is.
I would go along with most of that. We seem to have discovered that somehow nature is not connected with mathematics. Although mathematics seems to be wholly an invention by humans, it seems as though nature must know about mathematics also. Yet, we don't quite know why.
You seem to be implying in there that there's some disconnect between humans and nature, that a constructive humanity is somehow separate from nature.
Not humans as such, but human ideas and understanding. We still have little notion about what separates biological living matter from non-biological matter. It would be nice to know if that's describable by some mathematical law. We are certainly far from any such understanding.
Joe, now that we're in this realm of discussion, for my last question, and we've already been touching on it, personally, for you, what are you most excited about looking ahead?
I'll tell you what I'm spending my time doing these days, as a retired professor of physics. I do try to keep up with what my colleagues are up to, but my own time is spent teaching myself an entirely new field, communication theory, and applying it to my longtime hobby of amateur radio. I am having a lot of fun doing that.
You mean communication theory in a sociological context?
No, communication theory in an engineering context. How best to store information, transfer information from A to B, how the internet works, and radio communication at a distance, in a noisy environment. All of those kinds of things, with error control codes and ways of transferring information with extremely low probability of introducing errors. Why is that you can receive emails every day that have no transmission-induced errors in them? It's because the engineers have carefully worked out ways of sending redundant information in highly efficient ways.
I've been learning what these people have been up to. As a student, I always found that things I figured out for myself, I learned best. So, I have been approaching this communication theory in the same kind of way. I often work out something that seems interesting and possibly new, and then discover that communication engineers knew it 30 years ago. But still, it's nice to have come onto it myself.
Well, Joe, it's been an absolute pleasure speaking with you today. I want to thank you so much for spending the time with me.
My pleasure, David. It was good. Goodbye.