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Interview of Roger Blandford by David Zierler on April 29, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with Roger Blandford, the Luke Blossom Professor at the School of Humanities and Sciences at Stanford University and Professor of Physics at SLAC. He discusses his current work developing alternate understandings of the Event Horizon Telescope image, on fast radio bursts, and on the notion that handedness has astrophysical origins. Blandford describes the history of cosmology as a respectable discipline within physics, and he credits the rise of VLBI in the 1960s and 1970s for demonstrating the evidence of black holes. He recounts his childhood in England, his early interests in science, and his education at Cambridge, where his thesis research on accretion discs and radio sources was supervised by Martin Rees. Blandford discusses his postdoctoral work on astrophysical particle acceleration and plasma and QED processes in pulsars and a formative visit to the Institute for Advanced Study and to Berkeley. He describes his initial impressions of Caltech where he joined the faculty and where he worked closely with Roman Znajek, and he explains the distinctions between radio jets and relativistic jets. Blandford explains his reasons for moving to Stanford to set up the Kavli Institute and he describes his involvement with the Astronomy and Astrophysics Decadal Survey. At the end of the interview, Blandford contends that the most exciting developments in the field have been on exoplanet research, why the possibilities in astrobiology give him cause for optimism, and why the concept that astronomical discovery arrives as “logically unscripted” resonates with him.
Okay. This is David Zierler, Oral Historian for the American Institute of Physics. It is April 29th, 2021. I am delighted to be here with Professor Roger David Blandford. Roger, it's great to see you. Thank you for joining me today.
Roger, to start, would you tell me your titles and institutional affiliations? You'll note that I pluralize everything because I know you have more than one.
Well, I'm the Luke Blossom Professor at the School of Humanities and Sciences at Stanford University, and I'm also Professor of Physics at SLAC. I have a business card on the bottom of my laptop with the full title on it.
Perfect cheat sheet for you. Roger, who was or is Luke Blossom?
Luke Blossom endowed two chairs at Stanford University. I had the privilege of meeting him, but he passed away soon afterward, and I attended his memorial. Luke had broad and eclectic interests and was genuinely interested in science. I was given some of his library. Despite a serious physical disability, he built a radio telescope to look at the sun from his backyard. He also painted, and I have one of his paintings on my wall. I wish I'd had the opportunity to get to know him better. He was clearly a remarkable man with lots of spirit.
While we're on the topic of benefactors, perhaps you could tell me about the chair for which you were named at Caltech, Richard Chace Tolman.
Tolman was a scientist, with broad interests, including chemistry and relativity, which I share. Tolman died before I was born, but I do have copies of a couple of the books he wrote, which were remarkable. The previous incumbent was Richard Feynman, an impossibly hard act to follow.
Roger, a question we're all dealing with right now, how has your science been impacted during the pandemic? In other words, as a theoretician, have you found more time and bandwidth to work on longstanding problems, or alternatively, has the mandate to be physically isolated from your colleagues really slowed things down for you?
Right now, the answer is I have had more time for old problems. However, even theorists are social animals, and so we do need to sniff our colleagues occasionally and I miss that. I have been impacted less than somebody who needs to interact with large groups of people or work in a lab. Obviously, for them, it has been crippling. More generally, the challenges faced by, and the sacrifices asked of so many others have been far greater and I am grateful for what they have done and humbled by their service, in many cases carrying significant personal risk.
The business of teaching has been seriously impacted. I taught three courses over Zoom. They were all different, but my candid assessment is that, overall, it takes twice as much time and effort to be half as effective. All the cliches about it mattering to be in the room with students and colleagues are true. If I ever had any doubts, I don't anymore. Despite this, I think we can learn from the Zoom experience.
Just a snapshot in time, what are you working on currently? What's most compelling to you?
You mean, in terms of research?
Too many topics. Let me give you three or four answers. The first is a heterodox view of the world-famous image from the Event Horizon Telescope of the ring in M87.
Well, first, Roger, let me hear the orthodoxy, before we get to the heterodoxy. What's orthodox in this image?
Well, the orthodox view is that we’re looking at a six or so billion solar mass black hole, orbited by very hot gas. Basically, the power that we observe comes from the gravitational energy of the gas flowing onto the black hole. That was a view that I and others helped develop forty years ago and it could still be right. The heterodox view, which I'm writing a paper on, is that the Event Horizon Telescope folks are looking at a magnetosphere -- a magnetically-dominated region – the current sheets and the high energy processes going on there. Gas has to be there, but it is cold and on a larger scale than their image. In this case, the source is powered by the spin of the black hole, just like a pulsar. The spin power is so great that it creates the famous jets in M87, and also pushes away the gas.
I think it is important to explore alternatives at this stage. One way I like to frame the question is to ask if the M87 ring is caused by nature or nurture. I am suggesting that it's the rotational nature of the black hole that is ultimately responsible for what we observe. I don’t believe that all black holes behave this way; the mighty quasars, and so on, are mostly nurtured.
This is the topic that comes to mind first, as you think about all of the topics you're working on.
I'll give you a second research area. I’m afraid it’s another trendy topic. (In my defense, I've been thinking about these matters for a long time, but recent observations have been highly stimulating.) This is the puzzle of Fast Radio Bursts. They are bursts of low frequency radio emission, intrinsically a few milliseconds in duration, that were first seen over a decade ago. The observations have improved dramatically over the last few years. My original hunch is now, I think, probably true. Most of them seem to be magnetars. These are neutron stars with unusually strong magnetic fields, about a million billion times that on Earth and it is the consequences of magnetic flares on their surface that we observe. It's like a solar flare but under extreme conditions.
Again, I have a heterodox view of how they emit. It's a work in progress but might also be relevant to the gentler radiation from pulsars, which is still very impressive. Basically, I see the powerful radio bursts as being expressions of continuum electrodynamics, not relativistic plasma physics. To give an analogy, an oceanographer is fully aware that water is composed of molecules formed by oxygen and hydrogen atoms and so on but would never describe a tsunami in these terms. Instead, she uses fluid dynamics.
Roger, two questions that I'd like to ask that I think –
I can give you the third thing, if you want.
The third topic is quite different and involves work led by my colleague Noémie Globus. I'm very much the junior collaborator on this one. This is also a bit “out there.” The idea is that the handedness that one associates with you and me, in terms of DNA and amino acids and all the rest of it -- the fancy word for this is chirality -- is causal and mediated by cosmic rays. It is not a matter of chance. So, at the time and place where life first began, possibly on Earth, possibly elsewhere, the cosmic rays create muons and electrons with magnetic moments preferentially anti-aligned with their momenta. This causes a small chiral bias in the rate at which primitive bio-molecules, starting to replicate, are ionized. This tiny bias can be amplified by evolution over, perhaps, a billion generations to give us homochirality. Ultimately this is due to cosmic ray protons being positively charged and, so, this connects to analogous processes that have been postulated to occur in the early universe.
So, that's top three. We can keep going if you want.
Well one more, also associated with cosmic rays. This is concerned with the origin of the very highest energy particles. They have energies about that of a well-hit baseball and there are relatively few places where these can have been accelerated. One candidate is the strong shock fronts caused by gas falling supersonically onto clusters of galaxies. Shock fronts are known to be efficient particle accelerators, but the mechanisms involved are only partly understood. My colleagues and I take the view that most of the entire cosmic ray spectrum from MeV to ZeV energy derives from a hierarchy of shock fronts, associated with stars, galaxies and clusters, with the output from one scale being the input for a larger scale.
I won't bore you anymore! I'm doing more orthodox things as well. I haven't completely lost it!
Roger, I'd like to ask two broad questions that I think will punctuate our discussion as we develop your career trajectory. The first is one of nomenclature. The terms cosmology, astrophysics, and astronomy, I'd like you to reflect on how those terms have changed over the course of your career, and what the implications of those changing boundaries or fields may have been.
Yes. There's obviously been a lot written on this going back to Aristotle and earlier. Cosmology, as we understand it today, has evolved from a matter of almost metaphysical speculation, and some observations, but very few –
For example, when you were a graduate student, would any of the faculty at Cambridge have identified themselves as a cosmologist?
Oh, yes. It was a strongly cosmological habitat. The doyen, I suppose, of cosmology was Fred Hoyle. He was the Director of our Institute, but rarely around. As a child, I listened to him on the radio, and I read his books, and I was a great fan. I remain a great fan. I think he did a lot for astronomy and astrophysics, and also for cosmology. He was a firm believer in the so-called steady state theory. His nemesis, Martin Ryle, was the leading radio astronomer in Cambridge and espoused an evolutionary view that the universe wasn't the same in the past. As we now know, Martin Ryle turned out to be right. The debate in the '60s was intense and highly personal. The evidence that Ryle originally produced was flawed. But it got sorted out by the time I became a graduate student in the '70s. Most younger people at Cambridge, excluding those who out of age or loyalty still supported steady state cosmology, accepted the observational evidence like everyone else. The microwave background, the evolution of quasars, the development of large-scale structure, and so on, strongly favored what Hoyle had pejoratively called the “Big Bang.” It was just a huge preponderance of evidence that one theory was, in essence, correct, and the other was, in essence, wrong. That happens in science, and it wasn't as though it was a bad theory, or you had good reason for always doubting it. Discoveries happened and the matter was settled by observation, as it usually is. Some echoes of steady state cosmology endure in modern discussions of the context for inflation.
This was a huge change. Cosmology has continued on its glorious path of measurement giving us a simple description of the universe which works remarkably well. Today, everyone talks about dark energy and dark matter as new discoveries. The very first book I ever read on physical cosmology was in the technical library in Birmingham, which is where I grew up. It was by Hermann Bondi written in 1952. He was originally one of the steady state people, but he had a very broad view, and he just went through what he saw as many different possible cosmologies. One of them was, essentially, the universe we inhabit today, flat Lambda-CDM. And he worked out the math for it. The scale factor goes as the two thirds power of the hyperbolic sine of the cosmic time, and so on. All of that is in there. When I went to classes as a research student, the cosmological constant and the idea that there could be more matter than we see were options and, a bit later, the observational evidence from galaxies and clusters for dark matter became more of the discussion. Likewise, the cosmological constant was introduced and rejected by Einstein. Lemaître took it seriously for good reasons. Eddington in the 30s, said it had to be there for other interesting reasons. Somewhat later, particle physicists got into cosmology, big time. For many of them, the cosmological constant, if you think about it in a reductionist way, didn't make any sense because it was a terribly small number compared to what you might naively expect, and the terribly small number you'd normally use in those instances is zero. So, many of them dismissed it. And then, when the evidence for it became observationally irresistible, for obviously psychological reasons, it became a big discovery, However, it never really went away theoretically since 1917, when Einstein first proposed it. It is a truly great observational discovery that raises important and interesting questions that derives from the explosion of reliable measurements, leading to the Standard Model.
For me, the observational centerpiece is the fluctuations of the microwave background. That is really what defines the Standard Model as well as anything. All the other things that we know about are basically consistent with this. There's nothing, in my view, out there that's really inconsistent with the Standard Model. There is the Hubble tension and so on, but I'm not sitting on the edge of my seat waiting for that to change cosmology. It's a really interesting matter what's going on with some of these observations, but if I had to put a modest amount of money on it, I would bet that basically the Standard Model is not going away. Just like in particle physics. There were tensions that suggested that the Standard Model is all wrong, etc. And then three-sigma goes down to two-sigma, and one-sigma, and so on, and that's where we are. And within their limits, I see both Standard Models as a job very well done (by others!).
Cosmology has seen a huge change from metaphysical discussion to something that's firmly based in empirical science. I think that's remarkable, and I'm very admiring of this. Where is the field going now? There are much harder questions beyond the Standard Model. The basic idea of inflation is doing very well right now. And then there are physics issues like baryogenesis and the influence of massive neutrinos where there are choices to be made. Answering these questions is going to be very challenging and I believe we will have to be patient. I suspect that, in the nearer term, there will be much more progress in completing the narrative history of the universe from cosmic dawn onward.
Which would include cosmic eschatology, all the way to the end?
Actually, I'm going to say not. I'm really talking paleontology, or archaeology, if you like. And I think it's a very different style of science from the physics questions which do, indeed include the anticipating the fate of the universe. Here, I’m thinking about sorting out the sequence of events from small growing over-densities of matter to the huge diversity of galaxies, stars and planets we observe with our telescopes today. Understanding their births, marriages and deaths. Piecing together the story, using all the Burgess Shales out there, to find the order in which things happened, including understanding how the big black holes got started, and what was their environmental impact. I suspect that roughly half of the job is done. I'm just not sure which half. We need to see this more as a movie script than as a matter of precise simulation, quantitative calculation and measurement. It is technique, and it is important, but telling it as a story is where the science is actually going to happen. To me, galaxies are more like people than elementary particles and describing their evolution is just as scientific as writing down a number or a likelihood. This is where I may differ than many of my colleagues.
A question that would require painting with very broad brushstrokes –
Broad brushstrokes, but with some confidence that this history actually happened. Another aspect of this is that I see astronomy and astrophysics as mostly driven by observation and measurement. Theory and simulation are an essential part of the process, but telescopes are judge and jury. It's been thrilling to be a witness and occasionally a litigant in these proceedings. So much has been learned.
I sometimes partition our field into God, sex, war – cosmology, stars and planets, extreme astrophysics - the three topics that sell newspapers and get you “above the fold on the front page of the New York Times.” We have discussed cosmology. Progress on extrasolar planets has been spectacular. Again, we've gone from dodgy observations and much speculation to serious and more-detailed discoveries of a diversity of planets found by four different techniques. It's amazing. I used to go to talks where there was nothing that you could bite into. Now there's too much, and it's going to get better and better. High energy, or extreme, astrophysics now involves the whole 140 octave spectrum of electromagnetic radiation plus cosmic rays, gravitational waves, and neutrinos. So far. This is almost all accessible, albeit with varying sensitivity. Even third-rate scientists can make discoveries under these circumstances, given these tools, and astronomers are first rate scientists. It’s a nice problem to have!
Roger, if you would survey the field of black hole research from your time at Cambridge all the way up to now, when during that period of time would you say observation has really led theory, and when has theory led observation?
Oh, gosh. That's a very interesting question. I think there's “Theory” and there's theory. Remember, black holes were regarded with extreme suspicion by very capable relativists, let alone anybody else for a long while.
Right. That's a good point.
The theory of classical black holes as initiated, unwittingly, by Schwarzschild and, wittingly, by Lemaître and Oppenheimer is a triumph. It was picked up by the heroes of the 1960s, Kerr, Wheeler, Zel’dovich, Novikov, Penrose, Hawking, Carter, Thorne and so many more who showed that, for astrophysics purposes, (ignoring important theoretical generalizations), the Kerr metric describes, essentially, everything we're likely to observe. Just two numbers. Simpler than electrons!
Einstein himself clearly didn't want them, and he wrote a rather confused paper in 1939 trying to protest. Eddington also. I think it was fear of the singularity. (There really ought to be a Greek coinage for this psychological malady). Roger Penrose, famously, made this into a formal complaint. Under conditions that astronomers believe can be satisfied, classical physics must fail in a finite proper time. However, these relativists also conjectured that Nature would save us from our embarrassment by drawing a cosmic curtain on all the unspeakable dissipation and quantum depravity that is rumored to take place. Nowadays, almost everyone accepts the principle of cosmic censorship, although the cognoscenti will add a few, legalistic caveats. There is no escape from behind the event horizon enveloping a classical black hole. Hollywood notwithstanding.
We have learned that black holes not only exist, but are prevalent, as a common endpoint of massive star evolution. We observe remarkable behaviors in X-ray binaries which I like to call “quasars for the impatient.” We almost certainly hear their birth cries as gamma ray bursts, of both major types – core collapse supernovae and neutron star mergers. The wonderfully frequent LIGO-VIRGO black hole mergers provide another manifestation of black hole formation. I think theorists led the charge on this, but observers now say, "OK, we buy into this, but please explain whence, how and whither?" This is a work in progress but there have been indisputable advances in developing different answers to these questions. For example, the success of general relativity in fitting gravitational radiation waveforms is impressive. However, there are many parts of these interpretations where we do not have commonly accepted explanations and this is exciting.
We also observe a million to ten billion solar mass “massive” black holes in the nuclei of most galaxies. As I have indicated I believe that there are many interesting possibilities still in play as to how they behave. Another puzzle, which disturbs many of my colleagues, is how can some of them have grown so fast so early in the universe. Personally, I am not too surprised by this.
Why is this not surprising to you?
Because I think they can indulge a voracious appetite when small and well-fed. They do this by trapping the escaping radiation and dragging it across the horizon. Alternatively, they can lose energy by radiating neutrinos.
Returning to your question, I think much of the early evidence for massive black holes came from radio astronomers who developed VLBI in the 60s and 70s. Their relativistic jets kept getting smaller, leaving non-black hole explanations of active galactic nuclei by the roadside. Somewhat analogous to showing that some compact stellar objects were too massive to be neutron stars or white dwarfs and therefore they must be black holes.
Personally, I was an early believer in black hole prime movers, but I suspect I viewed the evidence selectively. But now, with the Event Horizon Telescope, the matter is surely settled. Allow me to say that I am unstinting in my admiration for getting all of these idiosyncratic telescopes, astronomers and funding agencies to work together to produce coherent images even though my interpretation is contrary to what they advertise. Although some of my colleagues say this is just what they expected and they are not surprised, I share the public awe that the black hole is where it is supposed to be, has about the right size and seems to behave like Albert Einstein’s equations say it should. It isn’t some crazy pentagram off to the side! It’s the second order stuff that we are quibbling about.
And just currently, where the field is now, what would you say in terms of who is leading who?
I actually think it's led now by observation. I still think there are opportunities for completely fresh theoretical ideas. But I'm not seeing them coming at the rate that I saw in the early '70s, when I was a graduate student. It was an exciting time, especially for theory. For example, the properties of the Kerr metric were expressed in the language of thermodynamics. Originally, this was a cute analogy but then it turned out that tiny black holes really did have a temperature as Stephen Hawking showed. (I think I had the office next to his in the Institute of Astronomy at the time and I recall talking to him about this. To me, who was quite ignorant of the problems that it raised, and which ultimately got fixed, it was such a lovely idea. I could not believe it was wrong). Hawking radiation is big theory but, at the time, it also belonged in astronomy as people started looking for it. I even worked on a couple of approaches. Of course, nothing was found but the classical limit of the theory is central to understanding what we do observe.
And as you're saying, this is cyclical. Right now, if so much excitement is in observation, then the question is going to be analyzing all of that data, and then the theorists are going to have a lot of work to do.
Absolutely. Let's follow particle physicists and distinguish a pure theorist from a phenomenologist. Today phenomenologists are well integrated into the world of observers. Often, they're the same people. They adopt general relativity uncritically and put most of their physics effort into magnetohydrodynamics, plasma physics, radiative transfer and so on. Not really fundamental but very complicated. Like the weather. Right now, I think the really big breakthroughs and surprises are coming from this joint community.
By contrast, the pure theorists have indulged their prurient curiosity to speculate about what happens behind the event horizon. They have extensively debated the fate of information in Hawking radiation by semi-classical black holes and the behavior of wormholes on the quantum gravity scale. None of this has impacted observational astronomy beyond unsurprising upper limits. So far. Nonetheless, all of this neo-scholastic disputation – an extensive enterprise – has been very fruitful in forcing theoretical physicists to use their febrile imaginations to confront the most serious foundational questions in physics and here, there has been real progress. It is now widely accepted that information is not lost, firewalls, as a practical matter, do not form and so on. While, a single, usable and tested theory of quantum gravity still seems as far in the future as ever, the major, formal advances in string theory and other approaches are having an impact well beyond the questions that birthed them. In an anti-metaphorical sense, black holes have been givers, not takers.
So, if, as you say, this is cyclical then now is an opportune time for theorists to try to jump ahead of contemporary observing and to predict some completely new manifestation of black holes to be sought. Maybe we have been watching them explode after all; maybe they are enshrouded by light, axionic dark matter; maybe the LIGO black holes formed in the very early universe and produce cosmological fingerprints. Personally, I remain highly skeptical about classical, traversable wormholes, but I would love to be proven wrong!
Well, Roger, let's go all the way back to the beginning. Let's go to England. Let's start first with your parents. Tell me a bit about them and where they're from.
Well, that's hardly a cosmological perspective! My father was a fresh meat wholesaler, like his father and grandfather who worked in the Birmingham meat market.
Where did he serve during the war?
My father was in the RAF from 1939 to 1946 when he married my mother who served in the WRENS in Liverpool. My father was a navigator, dropping bombs on Germany. He was shot down over Belgium and managed to escape by parachute. He was hidden in private homes for nearly six months before it was judged safe to send him on to Switzerland. He kept in touch with the families who sheltered him. They were ordinary people, but extraordinarily brave. He passed away in 2013, on Armistice Day. He knew that the following year I would lead a Solvay conference on astrophysics and cosmology in Brussels and that I would address the King. He asked me to say “Thank You,” which I was glad to do.
Wow, wow. Now, were your family in Birmingham endangered from German air raids?
Oh, yes. My mother grew up in a suburb of Birmingham called Erdington. Birmingham is an industrial city, so it was a target. My grandmother was in a truck taking food and blankets around after the raids. There was conspicuous bomb damage to the houses on our street. Door frames skewed by large angles. The Second World War was a backdrop to my upbringing.
Now, am I hearing a Birmingham accent, would you say?
Probably not. But I can still speak “Brummie” if you want. I wasn't actually born in Birmingham. After the RAF, my father returned to the meat trade, and he was placed in charge of a depot rationing meat in Grantham, a town in Lincolnshire on the main road to Scotland. I was actually born four or five miles away from the birthplace of Isaac Newton and the apple tree. I boast about this to my students, and they're thoroughly unimpressed. Grantham was also the home of Margaret Thatcher. My parents knew the family, because, as my father was the source of the occasional side of beef which would come into town. He quickly ascended the social ladder into the realm of Alderman Roberts, who ran a grocery and was Margaret Thatcher's father.
Would you say that you had a more working class or middle-class upbringing?
Middle class, I suppose. These insidious distinctions were important when I was young, but they got shaken up in the sixties. My parents went to grammar school but left to start work when they were about fifteen. We returned to Erdington when I was five to a street next to the one where my mother had grown up. I went to the same primary school with the same headmaster.
What kind of schools did you go to growing up?
I went to the local primary school, and the teaching was excellent. I was surrounded by other pupils who seemed to understand things I did not. I saw my first science experiments performed – completely extra-curricular – and I was entranced. At the time I did not appreciate this. As kids, there's all sorts of other stuff going on and no one seemed unduly eager to learn, but in retrospect, I feel so fortunate to have been in this environment. Another year, another school could have been quite different. At 10, I passed a competitive exam to attend what was probably the best grammar school in Birmingham. The teaching was, again, quite traditional and first rate. As adolescents in the sixties we were, collectively, starting to question authority but never once did we have cause to doubt that our teachers knew their subjects inside out.
Roger, was Sputnik a big event that sticks out in your memory?
That was 1957. I was 8 years old then, but I do remember it. It did register.
And the Space Race, the early Kennedy Space Race years, was this something that captivated you?
Although I was interested in astronomy, I was less conscious of the Space Race. I do have clear memories of the Cuban missile crisis which was very frightening to us and the shock of learning, at a school dance, that Kennedy had been shot. He was very much admired. I was in grammar school from 1960 to the start of 1967, so there was a lot going on. We weren't consciously manning the barricades, but we knew that big social changes were happening. The old fusty and conservative governments were collapsing and being replaced by ones that were much more people-centered. There was also a lot of cultural change. Music was a big deal. Birmingham surely lagged and was more muted than London, France, Germany and the US but Rhodesia, South Africa and even Vietnam were changing attitudes, generationally.
Would you say that you had a strong curriculum in math and science in high school?
Absolutely. Again, it was quite accelerated, streamed and specialized. They started teaching us some calculus – slopes and areas - in my second year. From the age of fourteen, three quarters of my instruction was in math, physics, and chemistry. It was excellent but focused and relatively applied. (When I left school, we had been taught many tricks for doing integrals and solving differential equations, but I never saw any formal analysis until I went to university). And I don't think they ever tried to teach us much in an extracurricular way. The teachers were quite keen on keeping us focused on university entrance exams and not distracted by telling us about special relativity, say, as it was not on the syllabus. The one exception to this was after the exams were over, we had to write essays on topics that interested us. I decided to teach myself group theory and I duly wrote an essay of which I was very proud. One of the mathematics teachers went through it, red pencil to hand, explaining, firmly but clearly, the many places where I had simply not understood what I was writing about. He was teaching me a very important lesson which I am still trying to learn!
I left school early to work as a math (and sports!) teacher in a boarding school in the north of Scotland. It was in a castle. A long way from Erdington! I was actually teaching students older than I was, and part of my job was not to let them know this! I was also teaching “New Maths” including transformational geometry and matrices which was not part of what I had been taught. The whole experience was fabulous.
Given your interest in math and physics and astronomy, how did you settle on one particular focus at Cambridge?
I started as a chemistry student, but I found myself getting a bit frustrated. We learned quantum mechanics in our first term in a rather prescriptive fashion. This was followed by thermodynamics. Likewise. I found the physicists’ approach, which came later, more satisfying. Then, in my second year I got a vacation job pushing out rowboats in a local park. It rained the whole time, and we had no customers. So, I read Feynman’s lectures. We had been warned not to do this until we had completed a first pass through the topics covered in traditional lectures which had happened. Of course, there was a lot that passed over my head, but I found them to be wonderful and became a zealous convert to theoretical physics.
At Cambridge, were the departments of physics and astronomy integrated? Were they separate?
No. Astronomy was a research activity. There was really no direct teaching of it to undergraduates although astronomers taught regular physics courses. As a child in my primary school, I was fascinated by astronomy and avidly read books in the children’s section of the local public library. However, at university, I had really lost interest and was not paying much attention to the great discoveries that were happening. (I did go to hear talks in the university society including one, most memorably, by Tony Hewish on the pulsars which Jocelyn Bell and he had just discovered).
Who were some of the professors at Cambridge that you became close with?
I would say none of them because it wasn't like that. There was a system there called supervision, which meant occasionally you'd get a lecturer, never a professor, but usually it would be a graduate student who would teach you once a week for an hour. (Later, I gave these supervisions to bright undergraduates who could ask me any question on any topic that they could not understand. It was a great educational experience. For me). You got to know your supervisors, but the professors were distant figures. Like, being at a concert. You don't actually meet the performer. Some of them I got to know later. I thought they were tremendous. The same story as in my primary and grammar school. I feel privileged to have had the opportunity to listen to people who knew whereof they spoke. Let me give three examples.
We were taught quantum mechanics by Neville Mott. Some of my fellow students got upset because they said he wasn't teaching us modern quantum mechanics. (This was the sixties). I think that was the time when he was completing the research that won a Nobel Prize. Then, there was a lecturer called Jeffrey Goldstone who may not have conformed to the highest standards of modern pedagogy. I still have his notes where I can find ways of thinking about differential equations that I have never seen anywhere else. My first class in quantum field theory was taught by Phil Anderson, much to the amusement of my particle physics colleagues. It was all about lasers! It was unconventional, but this was somebody who was a scientist at a different level. That was clear.
In retrospect, it was what was said, the content and the organization, that mattered most. It was not performance art. No tricks. No instant gratification. I think that, especially in physics, you have to be patient and give ideas time to settle. For me the long-term value came from stuff you'll never see anywhere else, rather than the next chapter of the standard textbook. (Dirac stopped lecturing before I would have taken his class, but I went once just to listen to him. He did use a textbook! Subsequently, I came to see it as the greatest physics book since the Principia). Of course, I didn’t appreciate this at the time and I do understand that you have to work to keep students engaged. Especially today.
Did you give thought to going elsewhere for graduate school, or you were at Cambridge and that was the best place to be?
At that time it was more common for research students to stay at their undergraduate university. The custom at Cambridge was, that if you got a first-class undergraduate degree, then you could get a studentship. And if you didn't, you'd go somewhere else. I did interviews at other schools and I would have been very happy at any of them. However, I did have to decide what research field I wished to join. Initially, it was particle physics but I had friends who were graduate students who said "I wish I hadn’t done this. Nothing's happening. People are just writing papers." And every time I talked to an astronomer, they said, "I haven't got time to talk because something new has happened." It was a totally different environment. I quickly recovered my childhood interest in astronomy and decided this was what I wanted to do. But this was 1970, just before the theoretical ideas developed in the 60s were so productively deployed, and J/?, the November Revolution, happened. The early 1970s was a golden age in particle physics. I watched this vicariously from the sidelines but had no regrets about my choice which was the correct one for the wrong reason.
So, then, how, ultimately, did Martin Rees become your advisor?
I had a choice between going into the radio astronomy group, under Martin Ryle, or into a new theory institute, IOTA, under Fred Hoyle. The former was the safe choice, but I was advised to try to meet Martin Rees. He seemed very young, uncommonly well-dressed and he sat on a desk talking fast about a host of astronomy problems for about 20 minutes. It wasn't a personality thing. The parts I could follow were so clear and interesting. Deep questions explained in simple terms. I decided there and then, this is what I wanted to do but I had a lot to learn. I enrolled in a famous summer course at the Royal Greenwich Observatory in (another) castle in Herstmonceux. It was idyllic and it was here that I met my wife, Liz. We were given lectures on astronomy, projects and got to help with the observing through the frequent sea mist and develop photographic plates.
What was Rees working on at that point?
Everything. Cosmology, radio sources, the coming impact of X-ray astronomy, the intergalactic medium, the microwave background star formation as well as some more metaphysical matters. He kept abreast of observations and seemed to understand papers without reading them.
It wasn't just Martin. The rest of us were in that environment, too. After I started at the Institute, we would meet every morning for coffee, and you would talk to someone different about something new. Very different from today’s research world which is more feudal and focused. It was hard to actually accomplish anything, because you were overstimulated. And you weren't really working for anyone except yourself. But there were plenty of people on hand to help but basically, you had to do it.
Given all there was to work on, how did you narrow that down for your thesis research?
I started off working on molecular spectroscopy because I thought my chemistry would help. Martin suggested accretion discs and radio sources where much more was happening, and I changed to the second of these because I thought I had a better grounding in electromagnetism than fluid mechanics. It turned out that both lines of inquiry soon converged.
What was Rees's style like as an advisor? Was he hands-on? Was he really involved with your research?
Yes and no. The tradition was that you were expected to think for yourself. Having said that, Martin was always available and very quick in understanding difficulties and suggesting simple and economical ways to get around them. Short discussions were revelatory. His insight, intuition and understanding about what's likely to be significant and correct was, and remains, the best I’ve ever encountered. Although he was trained as a mathematician, he always resisted formalism if he could see a quicker approach. In my third and final year as a graduate student, he left for Sussex. I saw him once for an afternoon where he set me back on track although we exchanged postcards. Donald Lynden-Bell took over as my advisor and I learned a lot from him, too, in a very different way.
Who was in your thesis committee besides Martin?
In Britain at that time, there was a totally different system from that in the United States. The examiners were distinct from the lecturers while other people marked the exams on which everything depended. The same with the PhD exams. Your supervisor did not examine you. You were on your own with your thesis. So I went down to Sussex where Leon Mestel, whom I got to know much better later, and Roger Tayler, examined me. I wrote the thesis in a great hurry because my wife and I were going on holiday, and then had to wait four months to be examined!
What postdoc opportunities did you have available at that point?
I did not anticipate being able to stay in astronomy. It was a time of economic challenge. I was genuinely interested in plasma physics. So I expected to apply for a job at Culham as controlled thermonuclear fusion was imminent! And then I got offered a fellowship in Cambridge at St. Johns College.
So, you decided to stay in Cambridge.
Yes. St. Johns was fabulous. Again, I was extremely lucky. Again, these were sort of formative times in my life. One of the things I appreciated a lot was the opportunity to sit down and break bread with people with very different interests, who were scholars. You could be talking with anybody about anything, which for me is as important a feature of academic life, as writing your paper or book. (Once I got to sit next to Dirac, whom I idolized. It was not much of a conversation. He was kind, but taciturn and I kept thinking he must have been my age in 1927…) I sometimes miss this in the United States where people are more stove-piped.
So, what was your research for your postdoc? What were you working on?
I worked on astrophysical particle acceleration and, with Ted Scharlemann on plasma and QED processes in pulsars. I took a year off to come to the United States. John Bahcall invited me to spend a year at the Institute for Advanced Study, and I took three months out of that to go to Berkeley. So, we drove across the country and then returned. Two memorable adventures.
Who were you working with at the Institute?
Quite a lot of people, actually. It was a very exciting time. I went with no plan, but the big thing at that time was the discovery of the Hulse-Taylor binary pulsar. I dropped everything to work on this. It was not what I'd expected.
Did you interact with Joe [Taylor] while you were at Princeton?
Oh, yes. I went to his home in Amherst to discuss how to analyze the pulse arrival times and Joe was very encouraging. I was also working with Saul Teukolsky on this and thinking about the more astrophysics aspects with Larry Smarr. Bill Press, Brian Flannery, Ed van den Heuvel and Jonathan Katz were also very interested and I got to know them as well.
What about Stu Shapiro? Was he there at that point?
No, he wasn't. I met him later. Alan Lightman and Doug Eardley as well. These were all very impressive people and I was in awe of them. I thought every postdoc must be this good! Once again, I was surrounded by people who just knew things of which I was ignorant. It wasn't as though I thought I had to compete. It was just wonderful to be in this environment. They thought about things in different ways, and I enjoyed this. Paul Schechter was another colleague. His was more of an observer. I remember him telling me about two pieces of research he'd done as a student. They are still center stage in cosmology, 45 years later.
What was so exciting at Berkeley that it pulled you away from the Institute?
I got to know Jon Arons, and Claire Max in Princeton. They're still my friends, and they said, "We're taking jobs in Berkeley. Come on over." So, I did. Chris McKee was somebody I actually met there. I actually worked more with Chris than I did with Jon and Claire when I went to Berkeley. Nonetheless, we all got together, and we all talked, and I learned a lot from them. I worked with Chris on trying to figure out what a relativistic supernova explosion might look like. That turned out to be useful for gamma ray bursts, and so on.
Now, at this point, were you having such a positive experience in the States that you thought, perhaps I'll make a career for myself here?
I don't think I had a career plan beyond returning to Cambridge, where I had another year on my fellowship. Much to my surprise I was offered a faculty job at Caltech and, after much trepidation, I accepted it, deferring it for a year because my wife and I both felt obligated to return. It was a big step for us. We loved California but we did not want to cut ourselves off from home.
How did the opportunity at Caltech come about? What was the point of contact there?
Peter Goldreich, Wal Sargent and Jim Gunn whom I met in Cambridge which they visited over the summer.
What were your impressions of Caltech initially, when you first arrived?
Well, it was too frenetic to have any impressions of anything. We were in a completely new environment. The sun shone! We had a four-week-old son. (I kept reminding myself that Albert Einstein's first child was born in 1904!). We owned a used car, which I bought because it was called Galaxie, that seemed bigger than the home we left! We were actually house sitting in the Shoemaker’s large home with a beautiful garden before we got ourselves sorted out. There was not time to contemplate our circumstances or wonder what we were doing, it was exhilarating and exhausting. Most of all it was exciting, scientifically and socially.
And what were you working on when you got to Caltech? What was interesting in the field?
Just before I left Cambridge, I had worked with Roman Znajek on a paper on extracting energy from spinning black holes using magnetic fields in order to power relativistic jets. I naturally gravitated towards the radio observers, Marshall Cohen and, later, Tony Readhead, who became a very good friend. I talked to them about astrophysics while they were busy dealing with quite technical things, where I was no help whatsoever.
Did you take on graduate students right away?
Yes, I did. My first graduate student was Arieh Königl who was great. (As were all the students I had). We worked together on trying to explain the radio jets and he wrote some nice papers on relativistic fluid mechanics and gamma rays. Quite a lot of my time was spent thinking about jets, and so on, and now in my old age, I seem to have come back to that sort of thing.
Now, radio jets and relativistic jets, these are separate topics?
Arieh and I worked in the ones in active galactic nuclei, which are both, although we now know there are jets all over the place. We also know that the jets from active galaxies can be much more powerful gamma ray sources. But at that time, it was the “superluminal” radio jets associated with quasars that people were most interested in.
And I wonder, to foreshadow a bit, how this work was important for, eventually, the Event Horizon Telescope.
Well, the Event Horizon Telescope folks don’t think they see the jet in M87. It's always been a question how the jets, etc. are powered. It's like asking how does the sun shine? We know there's something in the middle of the sun and it keeps us warm but figuring out that this was due to nuclear reactions not gravitational energy took a while. Today we understand the nuclear physics, neutrinos have been detected and the oscillation modes measured. We have a very good model.
Reverse engineering quasars is a similar puzzle. One of the first papers I wrote with Martin proposed that pulsars were responsible, and we now know that this is not right. Instead, jets come from black holes and the accretion discs that orbit them. But then the question is are the jets powered by gravitational energy from the accretion disc, or the spin energy of the black hole - like nuclear power but even more efficient - as Roman Znajek and I suggested and my Caltech colleagues Sterl Phinney and Kip Thorne further developed. A long sequence of steadily improving VLBI observations (where Marshall Cohen and Tony Readhead were pioneers) and culminating in the EHT observations is pointing strongly to black hole rotational energy supplying the power. However, the disks are essential for anchoring the magnetic field which collimates the jets. Not everyone agrees with this, but I would say most astronomers now do so. In this case, the jets should be pretty much invisible close to the black hole. It’s only the confining material that is luminous and as we discussed at the start, the observers think this is gas and I believe it is magnetic field. We shall see.
As with any foundation collaboration, I always like to ask how the merging of personalities and expertise was relevant. With that in mind, what did you bring to the table, and what did Roman bring to the table?
Well, Roman was a student and I was an older postdoc, but it was an equal collaboration. He had more of a mathematics background while I was more of a physicist. We had both published papers pointing in the same direction and so it was natural to work together. By the time we were done, I was fussing over the mathematics and he was elaborating the physics. (Roman also went on to develop some far-reaching ideas about the nature of event horizons). I have to say, it was one of the happiest and most exciting collaborations I've ever had. We learned from each other. We're still in touch, despite his going off into other interests including politics.
To go back to an earlier comment you made about even well-regarded general relativists early on didn't even accept that black holes existed. When was there a critical mass in the community that it was accepted that black holes existed? When did that happen, roughly?
Well, the fact that you emphasized, even among the GR crowd, that's significant in and of itself, because if the GR crowd questions this, who cares what other physicists beyond the specialty might have to say?
Well Kip Thorne wrote a very nice book about all of this. Roy Kerr announced his solution at the same Texas Symposium in 1963 where Maarten Schmidt introduced quasars. The astronomers and relativists were not communicating very well but some attendees saw a possible connection. I think that among practicing relativists, the impact of what Roy Kerr had wrought sank in by 1967. Among astronomers, Lynden-Bell’s paper in 1969, followed by Bardeen’s generalization to spinning black holes were pretty influential. These were followed in a couple of years by the evidence for black holes in X-ray binaries. By this time most astronomers were no longer frightened by black holes. In the Soviet Union, papers by Zel’dovich and collaborators laid out many of the key ideas earlier, but they were not widely known in the west.
What years were you most intensively in contact with Steven Hawking?
We never worked together though I did have a few great discussions with him from the early 1970s to the late 90s. I co-authored a chapter for two volumes he put together to commemorate Newton and Einstein. Stephen’s contributions to relativity were extraordinary. His bravery and inspiration were even greater. The movie portrayal by Eddie Redmayne was remarkable.
What were some of the earliest conversations that made it obvious that black holes would be useful for quantum information studies?
I am not sure as I have never worked in this business. In addition to Stepehen Hawking I would call out Jacob Bekenstein and my Stanford colleague Lenny Susskind for setting out the issues and contributing to the resolution. John Preskill and Kip Thorne at Caltech also took this very seriously. It has spawned a large field of theoretical physics. Culturally, it was a dialectical engagement between those who, like me, had been brought up to see relativity as essentially geometrical and those who saw it as essentially quantum mechanical, like everything else. The former group, and by extension astronomers, were relieved to find that the large black holes that are observed behaved just as they had assumed; the latter group were comforted by understanding how sacred principles like unitarity were maintained. Everyone should be happy!
Roger, in the 1990s and early 2000s, what was going on in observation, particularly with supernovae, that was interesting. to you?
I never really got into the supernova business. I was interested in it, for sure, but more vicariously, as a spectator. I saw people who made bombs for a living trying to make core-collapse supernovae as a hobby. They weren't terribly successful. (It made one worry about the bombs). It's clearly a hard and subtle problem, and probably has a rather complicated solution. An even bigger surprise is that the Type 1a explosions are such good standard candles despite our not even agreeing what it is that is exploding. Cosmologists, meanwhile, see lemons, and make lemonade.
Roger, what were the considerations for moving to Stanford from Caltech?
I was excited about the prospect of starting up a new institute called the Kavli Institute.
Was this right at the beginning, when Kavli was funding institutes all over the world?
Yes. We were the first new institute. It was the vision of my colleagues here at Stanford who got this going. Fred Kavli put serious support into what was then the Institute for Theoretical Physics in Santa Barbara, and then became the Kavli Institute for Theoretical Physics. This has been a big success. I co-ran an early program there and have always enjoyed visiting. That was really the first Kavli Institute. The people at Stanford, Steve Chu, Jonathan Dorfan and Doug Osheroff had the vision to start a new institute and to bring astrophysics with a physics focus, to Stanford. Hence the name: Kavli Institute for Particle Astrophysics and Cosmology or KIPAC, for short. Fred liked it, I don't know to what extent he was thinking about this, then, but it wasn’t long before more institutes were started. So, I was excited by the prospect of building it up. I was very happy at Caltech. It's a sort of Mecca for astronomy, and I'm still working with people down there, but the challenge was enticing. From 2003 to 2013 the center of my academic life became growing KIPAC. Since 2013, I just teach, do research and follow other interests.
I'm curious why you took on the joint appointment with SLAC.
Well, that was the way it was set up - as a collaborative enterprise between campus and SLAC. SLAC built a great accelerator which is now being used as a light source in a very productive way. And it can build other things, too. People who work in particle physics are pretty good when it comes to taking on projects. They don't do things that they have no clue about, but they've got a large experience base. Just as I had hoped, SLAC personnel have worked successfully on many astronomy and cosmology projects as well.
So, at Stanford, your home department was never physics?
No, I'm a physics professor.
But the appointments are SLAC, you're directing the Kavli Institute, and you're professor in the School of Humanities and Sciences.
Oh, that's just the name of the chair. That means they can give the chairs to anyone in the school. It probably ought to resonate with what Luke Blossom was interested in, and he was interested in physics, for sure.
I see. In what ways has the appointment at SLAC been useful for you? Particularly, I should say –
I think mostly for building large projects, like the LSST camera, which is too large for a university professor to do it out of his or her back pocket. It should be based in a national center, lab or observatory.
I was curious, specifically, about SLAC, because you come at a time when SLAC's getting much more involved in astrophysics.
Well, yes. One of the conceits when I came was there's no astronomy and astrophysics at Stanford. But this was quite untrue. There was a lot going on in practice, and there was more that was incipient. Steve Kahn and I both knew that we were not starting from scratch. For me, one of the big attractions was the Fermi Gamma-ray Space Telescope led by my co-Blossom professor, Peter Michelson, which is an international collaboration, much of it centered at SLAC both instrumentally and scientifically. Not only did it work superbly, it made discoveries in areas we had not anticipated. Many of them. This was an exciting time. And it is still working and delivering results.
Tell me how you got involved with the Astronomy and Astrophysics Decadal Survey.
I certainly didn't volunteer! I was on a program selection committee, I had had involvement with three previous surveys and had worked with the three main agencies, NASA, NSF and DOE. So, I did have some relevant background and few illusions about how big the job would be. After Ralph Cicerone called me to discuss my leading it. My first instinct was to say no but further conversation with colleagues persuaded me to accept. I am glad that I did. I certainly got to work closely with some wonderful people and I am proud of what we delivered.
I wonder if the timing was such that you saw opportunity in the incoming Obama Administration where there may have been problems with the George W. Bush Administration, in terms of funding and support for astrophysics and astronomy.
Well, this started before the 2008 election, but I did see opportunity whatever the outcome. The agencies did not advertise much free energy and as time went by, they became more pessimistic, but I hoped that a compelling science case would carry us forward. I also felt strongly that the future had to be seen in an international, not a national context and that this would help.
Given the importance of public support -- ultimately, these are tax dollars that are going to be supporting these projects. To what extent were you sensitive to the kinds of things that might be exciting for the public?
I think all astronomers are very sensitive to the responsibility for and opportunity in public outreach. In general, we are actually very eager to share what we are learning, and, in this, we have probably been ahead of most scientific disciplines. We've got a huge advantage because of the power of the image. If you're a chemical engineer, you can transform the world with a new plastic, say, but it's hard to make that sound exciting. Seeing the image of the Event Horizon Telescope, or the LIGO waveform, or one of the cosmology movies, is visceral. They catch the attention of otherwise busy or distracted people. New graduate students expect to be spending time doing outreach, just like when I was a research student, I expected to be spending six hours a week teaching. It’s part of the job.
There’s another aspect to this which came up in the survey, and has featured more prominently since then, and this is inclusion – making the world of American science look like all of the United States. As well as a desire for equity, there is the simple observation that a diverse group of scientists with different backgrounds and experience is more likely to have fresh approaches. I do think astronomers have been ahead of the curve on this and really hammered away at it.
Yes, I've heard that.
This has been a benefit when it comes to trying to explain what's going on to people of different ages and backgrounds. The communication is far more effective when the communicator looks like they may indeed have come from a background similar to your own. For example, that's why Fred Hoyle was so effective in the 50s and 60s. He had a pronounced regional accent and didn't sound at all like a professor, but was extremely clear, and very compelling. I think astronomy has been an unusually good recruiter for science and technology as a whole, especially for those from under-represented backgrounds.
I'm curious about the DSc that you got at Chicago. Was this an honorary degree?
Yes, that was an honorary degree. It was very kind of them. It was a nice event and I got to catch up with some old friends there.
I wonder how you might compare the honor of your membership to the Royal Society versus the Royal Astronomical Society.
Well, I'm proud to be included in the ranks of both of them. I actually think that all of these scientific societies have an opportunity and a responsibility to impact public life, and science. Both the Royal Society and the Royal Astronomical Society take this very seriously. I won't presume your politics, but in the US, during the last administration, this sort of communication was a challenge, shall we say.
That's a very diplomatic way of putting it.
Covid brought the need into the open. Going forward, there's no shortage of global challenges and existential crises. The large interdisciplinary societies, like the Royal Society and the National Academy, have been taking them on. It is important that clear and consistent messages are delivered in all countries. There are no easy answers, but well-considered and prescriptive options are invaluable.
Professional Societies like the Royal Astronomical Society and the American Astronomical Society take on less consequential matters where they have special expertise. One that is very important right now is the deployment of communications satellites in low-earth orbit. There may be 100,000 of them before too long. They will compromise optical, infrared and, especially, radio astronomy. Making bandwidth available to people in underdeveloped countries is laudable but there is a need for more mitigation and regulation, and astronomers need to be more involved.
Roger, in what ways has serving on the Solvay Conference Advisory Board been valuable?
It's been fun. I've had exposure to other fields including biophysics and I’m looking forward to one on quantum information. There’s no way I'm going to work on that, but I'm interested. I was supposed to spend time in Belgium but I have had to give five lectures from my study. The Solvay conferences are important in the history of physics – physicists often show the photos from meetings involving Einstein, Curie et al (occasionally inserting their own images!) - but the series had declined in impact. David Gross and Marc Henneaux have done a great job reinvigorating them. I'm a great fan of these international societies because, in an increasingly xenophobic world, I think it is more incumbent upon us than ever to make public demonstrations of collegiality among scientists.
Just to bring the conversation up to the present, we've been talking about advances in observation. Just over the last ten years, what are some of the things you're personally most excited about for observation that's either already in the works, or it's being planned?
Oh, gosh. We haven't got time.
You might delineate between land-based telescopes and space-based telescopes.
I think one can do that, but I tend to take a more holistic view in that modern astronomy is more topic- or source-centered, and this leads to multi-wavelength and multi-messenger programs. This is most obvious in high energy astrophysics but when you think about how the standard cosmological model was assembled - all the measurements and upper limits –cosmology is not so different.
You ask about advances over the past decade, it’s a tough competition but exoplanets may be the winner. That is what my younger colleagues in astronomy seem to have concluded. I've had a long interest in this but not so much research involvement.
Is there an astrobiology interest there, specifically?
For me, yes. When you asked me what I was working on, I mentioned the possible role of cosmic rays in establishing biological homochirality. I still think it's an interesting idea, and has grown along three different directions involving physics, astronomy and biology. Experiment, observation and calculation. Another topic I thought about was panspermia. This is the idea, going back to Anaxagoras in Ancient Greece that life was seeded on earth from outer space. Fred Hoyle, whom I mentioned, was a proponent. Forty years ago, I would have dismissed this, but I take it much more seriously now because we know that exoplanets are common and they can be in dynamically chaotic environments. They must be ejecting detritus into the interstellar medium all the time. Some of those might carry dormant life. (I once tried to call a paper on this “The Guide to the Hitchhikers’ Galaxy”). The other development is that we have found so many different types of “extremophile” here on Earth. It is less of a stretch to imagine surviving a billion years sufficiently deep below the surface of a former asteroid. It would be very cold but shielded from cosmic rays.
Roger, now that we've worked right up to the present, I'll ask an impossibly broad, sweeping question that brings it all together for you, that has retrospective, and also forward-looking ramifications. It's clear you have a very well-developed sense of what we don't know, as we peer out into the universe. You've referenced Ancient Greece, you've referenced Aristotle. So, let's bring Socrates into the mix as well, who tells, of course, that the more we know about something, the more we know we don't know. So, looking over the course of your career, with what aspect of your research does that dictum most resonate? In other words, what are things we thought we knew, but then we learned something more, and that opened up a whole world of appreciation of what we don't know? And then looking to the future, where are you most optimistic about finding clarity on those topics?
We should all try to emulate Socrates. But only up to a point! I think my answer must be cosmology, although I have been less directly engaged in this research. Forty years ago, cosmology seemed to be in stasis. As I have tried to explain, the intellectual framework was largely in place. We simply did not know which out of many possible universes we inhabited. Now, we do. In retrospect, the biggest surprises were that the Standard Model was so simple, and that the universe was so cooperative in not erecting impenetrable foregrounds. (None of this takes away from the broad technical, accomplishment of observational cosmologists culminating in the Planck satellite). So, now we have the stage. There is so much more.
Physicists are intrigued by the set and the lighting. Wearing that hat, I see identifying the nature of dark matter as one of the greatest, answerable questions in physical science. Somewhat less accessible, but no less important, is deconstructing the scenery, the baryons and the photons, the neutrinos and the fluctuations. Understanding how they were assembled in the early universe, going back to inflation. As I explained, other physicists see dark energy as the biggest puzzle.
Wearing an astronomer’s hat, the play’s the thing. We do not understand the environmental science of galaxy formation including their stars and planets, their nuclei and their circum-galactic media. In short, there is so much we don’t know now after the success of the Standard Model.
And we'll get there, you think.
I am an optimist – you have to be in this business. I believe that much of this is knowable. However, I do think we need a better-optimized mix of approaches. When you know what you're doing, you want to do a mighty, LHC-style experiment that finds something that you know ought to be there.
That would be the Higgs, of course, you're referring to.
Yes. There, the big discovery would have been finding absolutely nothing whatsoever. That would have been fantastic but finding the Higgs is like verifying that there’s a black hole in M87, a feat that we should all be immensely proud of. It required an enormous and well-managed enterprise to do it. You had every piece of the jigsaw puzzle fit together. Except one which had dropped on the floor. And they found it.
But, to use dark matter, as an example, this is different. There are so many choices. It’s an open search and I think it makes sense that we diversify it and not be surprised if the next big discovery comes from an unexpected source not through one mighty experiment that may end up setting an upper limit on one possibility.
Perhaps even a theorist, you mean.
Well, a theorist could still have a good new idea. Yes. But we're not short of theoretical possibilities right now. My guess is still that it is a particle or, perhaps a new, essentially classical, field and not a modification of gravity, because I just don't see how nature can be so perverse to make things like the microwave background fluctuations work as well as they do with an alternative theory of gravity. I’m not sure that even God could pull that off.
Well, God may well have a sense of humor like that. Who knows?
I hope so. But I still suspect that the answer will be something we’ve already thought about, revealed in an unexpected place.
In the exoplanet business, there are great problems, like the dynamics, the chemistry and the planetary science in these exotic environments. But they keep coming back to life. Learning if we're alone and if not, getting to know the neighbors. My hunch here is that the universe is full of life with a diversity that will astound us. It will be fun for everyone to find out.
And technologically, this is feasible, to make this observation.
Technologically, it's absolutely feasible to make even better observations and better searches.
To clarify, though, when you're talking about technological feasibility, you're talking about bio-signatures, not being able to look any more closely than that.
There are so many approaches. Sample returns, for sure. We are awaiting the results from Hayabusa 2 and more missions are coming. Spectroscopy of exoplanet atmospheres should be transformed by JWST. Breakthrough Listen may deliver a signal. We can even look on earth. Suppose we found an isolated, extremophile colony in a deep-sea vent with the opposite chirality! We should cast our net wide. (I think it was Freeman Dyson who suggested trawling the interplanetary medium for “freeze-dried fish’’ – meteorite splashes from extraterrestrial oceans laden with novel lifeforms)!
There’s a common theme here. I think that most scientific discovery in astronomy has been, in Peter Medawar’s felicitous phrase, “logically unscripted.” Astronomers seeking one thing and finding something totally different. A signal that won't go away. You might say this is because astronomers don't know what they're doing! But I believe that, as a community, astronomers are self-selected to discover new phenomena which rarely enter through the front door but, instead, knock quietly, but insistently, at a side window. Sometimes, these hints are missed, but our record is pretty good for following them up. I think that new telescopes and techniques will keep these discoveries coming for the foreseeable future and there will be plenty more to enjoy.
Well, Roger, it's been an absolute pleasure spending this time with you. Thank you so much for doing this.
The pleasure has been mine.