William G. Unruh

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ORAL HISTORIES
William Unruh

Credit: Texas A&M Dept. of Physics and Astronomy

Interviewed by
David Zierler
Interview date
Location
Video conference
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Interview of William G. Unruh by David Zierler on May 20, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
www.aip.org/history-programs/niels-bohr-library/oral-histories/47038

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Interview with William "Bill" Unruh, Professor of Physics and Astronomy at the University of British Columbia, and Hagler Fellow at the Institute for Quantum Science and Engineering at Texas A&M. He credits his mentor John Wheeler for the steady progress of interest and work in general relativity over the decades, and he reflects broadly on the original debates among the relativists and the founders of quantum mechanics. Unruh explains the inability to merge these foundations of physics as the source of his attempts to understand the black hole evaporation as found by Hawking. He recounts his upbringing in Manitoba as part of a Mennonite community and his early interests in Euclidean geometry, and he describes his undergraduate education at the University of Manitoba. Unruh explains his decision to pursue a PhD with Wheeler at Princeton on topology and general relativity, and scattering cross sections of black holes to scalar fields. He describes his postgraduate appointment at Birkbeck College where he worked with Roger Penrose and he narrates the origins of his collaboration with Stephen Fulling and Paul Davies. Unruh discusses his time at Berkeley and then at McMaster and he historicizes the point at which observations made black holes more "real," and he explains his first involvement with decoherence. He explains his involvement with LIGO from its origins and its quantum mechanical nature, and he narrates his reaction of amazement when gravitational waves were detected. Unruh describes the impact of his work in quantum mechanics on computation, and he explains some of the advances that have made observation more relevant to his recent research. At the end of the interview, Unruh describes his efforts to launch a Gravity Archive at UBC, he expresses his frustration with people who insist we do not know quantum mechanics, and he quotes Wheeler, quoting his favorite Grook to convey that he is having fun and wants to learn as much as he can, while he can.

Transcript

Zierler:

OK, this is David Zierler, Oral Historian for the American Institute of Physics. It is May 20, 2021. I am delighted to be here with Professor William George Unruh. Bill, it's great to see you. Thank you for joining me today.

Unruh:

Thank you for asking me.

Zierler:

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

Unruh:

Well, I'm Professor of Physics and Astronomy at the University of British Columbia. I've also have a research position for five years at the Institute for Quantum Science and Engineering at Texas A&M where I've been a Hagler Fellow there for the last three years.

Zierler:

Are you still active at the Perimeter Institute?

Unruh:

I am. The last year, obviously, and this year, I haven't been able to visit. But I'm still a Distinguished Research Chair. And I've been spending about a month a year there.

Zierler:

How did the connection at A&M come together, and how do you incorporate that affiliation into your overall research?

Unruh:

It basically came through Marlan Scully, who's a quantum optics researcher there. I've known him for a long time, since the late 1970s, early 1980s. And he's the Director of the IQSE, and he got me Hagler Fellowship, and then invited me over, and arranged this whole thing. He's gotten very interested, especially, in acceleration radiation, i.e., the fact that a detector which is accelerated in the vacuum, where you would think there's nothing, sees that vacuum as populated by a thermal bath of particles. So, what one means by particle depends on your state of motion. And he's gotten very interested in it. A lot of the techniques from quantum optics have been, and can be, applied fairly straightforwardly to this motion of the detector. We've been looking at some of the features of that. He's had some insights on the problem and I've been trying to teach him my understanding of acceleration radiation, and he has been teaching me Quantum Optics. So that's been a fun and fruitful interaction.

Zierler:

A question we're all dealing with right now, how has your research fared in the past year-plus in the pandemic, working remotely, not seeing your colleagues in person, not going to conferences? In what ways has that afforded you more bandwidth to work on problems, and in what ways has it just been difficult because of the isolation?

Unruh:

I don't know how much more bandwidth it has afforded me. It probably should've. But unfortunately, the whole isolation and all of COVID has come at a psychological cost as well, so that the amount of mental energy one's able to put into it has not been as much as one could've hoped or anticipated. The other thing is, a lot of things happen in the corridors of conferences, and visits, and so forth. One of the major projects that I'm engaged in right now came about because Joerg Schmidtmeyer, a Bose-Einstein condensate experimentalist from Vienna, and I were chatting in the hallway at a little meeting at the University of Nottingham that Silke Weinfurtner organized there. And he asked me a question, and I said, "Yeah, I think we could do that." And that's morphed into a major experimental and theoretical project that we're carrying out to try and see whether or not one can see analogues of this acceleration radiation in a Bose-Einstein condensate.

So that's been one of my major pushes during the past year. At present, that's primarily theoretical. We meet about once a week on Zoom. But that's still not enough. You want to walk down the corridor, and start chatting, and say, "OK, I've had this idea. What does it sound like?" And that's just very hard to do, because even with Zoom, you've got to make an agreement, send an email, decide to meet at some time or another, instead of just being able, in a two-minute period, to walk down the corridor and talk to people.

Zierler:

You're saying that science needs spontaneity.

Unruh:

Science definitely needs spontaneity. Science needs that sort of casual interaction between people. And so much happens because of that. Even formal talks might trigger ideas in some people, but they tend not to produce the kinds of interactions one needs. They happen much more one-to-one, over coffee, or wandering down the streets, as you're looking at the views where you happen to be. And that casual interaction is vastly underrated, I think, by many people, as to how important that is for the way in which science works.

Zierler:

Well, before we go back and develop your personal narrative, I'd like to ask some broad thematic questions that survey the whole of your research career, spanning all the way back to the 70s, but beyond, as it relates to what you learned from your mentors. So first, as I've come to appreciate, general relativity, as a sub-field, has gone through ups and downs over the decades, where it was very popular, and where it was considered a backwater. In very broad brushstrokes, since the time when you were a graduate student and became interested in GR, what have been the ups and downs?

Unruh:

Well, I think it's been mainly ups. Basically, since that time, it's been a steady progression, where it has become more and more popular, and more and more at the center of science. We look at the Nobel Prizes over the last five years, and the majority of them have gone to exactly the field of general relativity, cosmology, black holes with Penrose and so forth, cosmic microwave background radiation, etc. All of these things. And the astonishing thing is that, almost all of those areas were really developed since Einstein died. And they were areas in which even Einstein had immense problems. He was arguing, even almost at the end of his life, that black holes did not exist. He, of course, hadn't called them black holes, because Wheeler only popularized that name in the late 1960s. Einstein called it the Schwarzschild singularity and came to the conclusion that they were unphysical and could not occur in nature. His arguments were very bad. But that was his feeling, even till the end of his life. And almost all of the growth in understanding them and detecting them came after his death.

And it's also hard for me to see what the general attitude toward general relativity was because John Wheeler, who was my PhD supervisor, was one of the people who clearly led the charge in changing interest in general relativity. He had been a particle physicist, developing S-matrix, and R-matrix techniques for example, and had been heavily involved in the Manhattan Project. And then, in his late 40s, early 50s, he started to become interested in general relativity and spent most of the rest of his life doing studying general relativity, popularizing it, and coming up with weird insights. He was the kind of guy who everybody, listening to a colloquium of his, would say, "This guy's just gone completely off the deep end." But 20 years later, everybody would be working on exactly the kinds of questions that he was asking in that colloquium. An example was his insistence, already in the late 50’s, on the importance of topology in General relativity and physics. He also loved inventing catchy, memorable names for various effects. An example was his phrase “black holes have no hair”, which tended to cause sniggers (as reported for example by Jeremy Bernstein about when he first heard the phrase in the late 60’s) rather than insight. But they were remembered.

So that's the milieu I was in. We were a bunch of graduate students in Relativity and we all felt the situation we were in was normal. Only afterwards as we went into the rest of the world did we realize it was far from normal. When I was at Princeton around 1970, we had seven or eight graduate students in Relativity at Princeton, all working with Wheeler, or with one of his post-docs, or with one of the people that he brought to Princeton. He had Remo Ruffini as a post-doc, he had Karel Kuchar and Jimmy York as assistant professors at the time. So it was, I think, the largest relativity group in the world at that time. Of course, we all thought this was just normal.

I've talked to Bob Wald about this, who was also a graduate student there at the time, and he had the same feeling. It was only afterwards that we realized that, no, this was not normal. So from my point of view, not only was there intense theoretical interest, but people were also interested in experimental approaches to on the field. Joe Weber at Maryland, was, at that time, working on gravity wave detection and thinking that he had actually detected them, which unfortunately wasn't true. His work however was the catalyst which made people realize that perhaps measurement of Gravitational Waves was not as impossible as it seemed and led to their detection 60 years later.

So there was, really, a lot of intense excitement about the field at that time, and it just kept growing. Cosmic microwave background radiation was discovered. Again, there was another group at Princeton --Bob Dicke, Jim Peebles, David Wilkinson, and so forth, -- of people working on the experimental work on CMB, early cosmology, etc. Unfortunately, there was little interaction between, at least, the students in the two groups. And the controversy over solar oscillations -- whether or not the sun has a quadrupole moment, which would've upset the calculations of the procession of Mercury, was also going on at Princeton at that time. So I arrived at Princeton into the thick of it. What was going on in the rest of the world didn't really impact me until very much later, by which time many people were starting to get interested in the field. That interest just been increasing constantly since that time. The beginnings of laser interferometry for gravity wave detection were occurring just at that time as well, especially through Ray Weiss at MIT. Kip Thorne had just been a student of Wheeler's. He'd left a couple years before I arrived there. Charlie Misner at Maryland had been a student of Wheeler's 20 years before that.

So it was a hive of really intense interest in General relativity that I was involved in at the time. I think part of the general neglect of the field that has been seen as occurring in the 30s and 40s was, in part, because of the War. Other areas of physics attracted the attention of most physicists. To a certain extent, it was much easier to find things to do in those other areas. Experiments were ongoing, particle accelerators were being built, brand new effects were popping up in condensed matter. So they were areas where a lot more could be found just by wandering casually through the field. General relativity was harder. The mathematics was daunting, and the concepts were foreign . Instead of an open field, everything seemed to be hidden inside haystacks, which you had to burrow through to find interesting problems, or had to have insights, as Wheeler had, that there's something to be found by burrowing in some particular haystack. I was extremely lucky to have entered the field just at that time.

Zierler:

As we approach the centennial anniversary of the 1927 Solvay Conference, what aspects of the foundational disagreement between Bohr and Einstein are resolved, and which are not resolved?

Unruh:

Well, I can't remember exactly what happened at that conference. I do remember the Bohr-Einstein debate about foundations of quantum mechanics from the 1930 Solvay conference, where Einstein presented his argument against ?E, ?T to Bohr, which was: “You weighed a little box to figure out how much energy is in the box. A clock mechanism in the box then briefly opens the box. It is important that the clock is in the box since an external clock opening and shutting it could easily feed energy into, or extract energy from the box. While the box is open, a photon is emitted. Because of the clock, you know exactly when the photon was emitted. You then weigh the box again and thus know exactly how much energy was emitted via the photon. Thus, you know both the energy of the photon, and exactly when it was emitted, violating Heisenberg’s Energy-Time uncertainly relation.” After an uncomfortable night of thought, Bohr came up with his brilliant psychological argument that in the weighing of the box, the clock, which determines when the shutter was opened, would, by Einstein’s own theory of General relativity, be red-shifted by an unknown amount in that process of weighing the box, the more so the more accurately you weighed the box. Bohr argued that the resultant uncertainty in the time of emission (due to the that unknown red-shift) and the uncertainty in the energy would obey Heisenberg’s relation. Bohr thus used Einstein's own theory against him.

The year after I wrote my general exams as a graduate student, the Princeton department had put that question on that year’s general exams. I thought that was unfair. It took Bohr all night to try and figure out an answer to that puzzle, so, if you had never seen the answer before, it would have been hopeless for you to answer in a half-hour on the exam. Furthermore, the argument just seemed to be fallacious because it seemed to be saying that in order for quantum mechanics to be consistent, general relativity had to be true. And that just seemed far, far too much output from the little bit of input. About 8 years later, in the late 1970s, when I first came to the University of British Columbia, Geoff Opat was visiting UBC on sabbatical from the University of Melbourne. We started talking about the puzzle and realized (and published in the American Journal of Physics), that Bohr’s argument actually did not depend on general relativity. Any theory in which energy had weight would give the same answer. Since Einstein’s posing of the problem used the fact that energy had weight, it was solely the assumptions of the problem which implied Bohr’s solution. The use of General relativity in his counter argument was, of course, a brilliant psychological tactic on Bohr's part.

This is an example that a number of the questions raised at the Solvay conferences have been answered, sometimes much later than they should've been. At the same time they've been important to me, because they raise some really interesting questions and deepened my understanding of Quantum Mechanics. There's another one of these early puzzles, which occurred is a little bit later, which was the Bohr-Rosenfeld uncertainty relations for the electromagnetic field. In trying to understand how the uncertainties came about during the process of measurement, they found that they had to introduce a weird compensatory device in their measurements of the field in order to show that they could satisfy the Heisenberg relations.

In the mid-1980s, I was teaching a graduate course on Measurement theory here -- a course which was usually about experimental techniques for making measurements. I hijacked it into a course on Quantum Measurement theory. While going through the Bohr-Rosenfeld argument, I realized that that compensatory mechanism was unnecessary. They were simply looking at the measurement apparatus in the wrong way. Using the Heisenberg representation it was obvious that if they measured the change in a certain combination of the momentum and position of the apparatus, the Heisenberg relations fell out almost trivially. Their compensatory mechanism was completely unneeded.

This strengthened my belief that, at least for many foundational questions, the Heisenberg representation is far better than the Schrödinger representation, despite the fact that most of the physics community loves the Schrödinger representation far more than the Heisenberg. For many problems, the Schrödinger equation is much easier to solve. But while that is true, I think, conceptually, the Heisenberg representation is much clearer on some questions than is the Schrödinger representation.

For example, the Schrödinger representation too easily leads to the idea that quantum mechanics is just a field theory like electromagnetism, and that the wave function has a reality just as the electromagnetic field does, leading to many, largely pointless, arguments in the field of the foundations of quantum mechanics. The insight into the importance of the Heisenberg representation also turned out to be important to my career in that I applied the same techniques in my research into the quantum behavior of the LIGO type laser interferometers to measure gravity waves.

So yeah, many of those early arguments have certainly come back and certainly influenced people trying to answer them, trying to understand what's going on in them. But I think by this time, we have a pretty good handle on most of the disputes.

Zierler:

This is a question that's as much a nomenclature and sociology of science question as it is a science question. But in your recollection, when did cosmology become a respectable sub-field in physics, something that you could pursue a PhD in?

Unruh:

Well, as I said, I was extremely lucky in being in one of the places where that had been true since the 1960s. Both John Wheeler and Bob Dicke, at Princeton, had certainly felt all along, certainly since the 1950s, that cosmology was a valid area of research, and that, especially with Bob Dicke’s leadership, one could actually do experiments, make observations which could answer questions within that field. He built up a whole experimental program to do exactly that -- to measure some of these cosmological parameters, etc. So from my point of view, it had been obvious that cosmology was a respectable area ever since I came into the field. I know there are all the famous stories about Rutherford saying, "If anybody comes into my lab and begins to speak about the universe, I tell them to leave." And to some extent, I think that was the attitude of at least part of the physics community. But I was, I guess, cocooned from that by being in one of the places where people had been getting PhDs for 20 years already by thinking about the universe, about cosmology.

Jim Peebles, for example, who comes from my home city, Winnipeg, got his PhD with Bob Dicke about 15 years before I got mine with Wheeler, doing cosmology, thinking about the early universe, thinking about the Hot Big Bang and whether or not that could actually be experimentally detected. He realized and calculated that there should be this relic cosmic microwave background radiation, calculated what that temperature should be, and that that temperature should be measurable. Dicke proceeded to set up an experiment to measure it. They were however scooped by a couple of people who had no idea what they were seeing, namely Penzias and Wilson, who had to talk to Dicke and his group to figure what they had detected. Jim Peebles tells a story that he was in a lab with Dicke, Wilkinson, and so forth, and the phone rang, and Bob Dicke answered, saying, "Hello," and then standing there listening. Finally he put his hand over the receiver and said, "Boys, we've been scooped." Anyway, these stories are just to show that the interest in cosmology had been there all along at Princeton.

So I was insulated from the anti-cosmology attitude, an attitude which may have existed in other physics departments, where condensed matter or particle physics were the fashion. The number of experiments in gravity or cosmology that one could carry out in the 1950s to the 1970s was extremely small. On the other hand, they were also extremely small because people hadn't been looking for what experiments one could do. But in cosmology, there were only a few people who felt that measurements were possible. I remember from the first Texas symposium I attended in 1970 Sandage and de Vaucouleurs giving talks on the Hubble constant, one of them getting about 50km/sec per Megaparsec, and the other getting about 100. And I think it was de Vaucouleurs who emphasized that one should not simply take the average of the two of them because there were good reasons why his result was better than the other one.

Of course, we know now that the average would have been a pretty good estimate. Both of them claimed an accuracy of plus or minus 10%. It was the average that was within plus or minus 10% of what we now believe to be the right answer. So they were interesting times.

Black holes were something that everybody at Princeton knew, loved, worked on, and understood, and believed strongly that they existed. In the 1970 -- and other early Texas Conferences on Relativistic Astrophysics -- most of the astrophysicists thought black holes were just one of these figments of the theorists' imaginations, which had absolutely no relevance to what they were doing. On the other hand, that was also just at the time at which observations were beginning to be made of systems that seemed to have massive dark stars in them. Binary systems with large (a few solar mass) companions but no light coming from them. And the only thing anybody could think of was that those companions must be black holes. And quasars were being discovered, whose immense energy output seemed only model-able by matter falling into black holes and converting the gravitational energy into light emission due to the friction of fluid flow near the black hole. So this was just at that time when the number of experiments of relevance for General relativity were starting to expand.

I think that part of the reason I went into the field I did was for theological reasons–in my youth, I was interested in both theology and general relativity. I remember taking Fock's textbook on general relativity out of a public library in Winnipeg when I was about 15 or so, to more or less read the words. The equations, I didn't understand at all. But I sill remember it because in the introduction to the book, Fock professed thanks to Lenin and his dialectic materialism for allowing him to get these insights into what general relativity really means.

And clearly, thinking back on it, that was a political decision on his part because Einstein was looked down on in the Soviet Union, being Jewish, being a theorist, both of which were things that they didn't regard very highly, and so he sort of had to put that at the beginning of his book. One of my first interactions with a Russian physicist was with Leonid Grishchuck, who worked in cosmology, at a meeting, I think, in Boston. We got together in my hotel room one evening just to chat about physics. He had read my paper on acceleration radiation and said to me, "When I first looked at your paper, I thought it was a very idealist paper. But then, when I read it more carefully, I realized that it really was a realist paper."

And I could understand what he was getting at. On the other hand, for anybody to think about physics with those kinds of categories in mind just was so strange to me. That was what I found so unusual and I guess one of the things I found unusual in Fock's book. On the other hand, John Wheeler had taught me that thinking philosophically about physics was a perfectly good way of doing physics, of coming up with ideas, questions, and so forth. So it was interesting.

Sorry, I've gotten myself so off track from your question, I don't remember what you asked. [laugh]

Zierler:

It was about cosmology as a respectable discipline.

Unruh:

Yeah, so for me, it was a respectable discipline all the way along. As was thinking about foundations of quantum mechanics. Again, that was one thing that John Wheeler was constantly doing, and I guess one of the reasons why everybody else in the world thought he was nuts. I remember, even in the 1990s, Gell-Mann gave a lecture. He had gotten interested in the foundations of quantum mechanics, and he made this apology saying that most people thought that thinking about foundational interests in quantum mechanics is only there for old professors who were no longer able to think about physics anymore. He was trying to convince us that this was a respectable thing to do. And for me, I'm just sort of looking at him thinking, "Why are you trying to defend this interest? There's absolutely no need to defend it. " So yeah, I was lucky.

Zierler:

A more recent question, what are your feelings on the current and future prospects that string theory will provide a plausible path to understanding quantum gravity?

Unruh:

There are some really interesting questions that they're attacking. However, I've never been able to follow the arguments. I've tried to learn string theory at times, I read one of the standard textbooks, got to page ten, and couldn’t figure out why they're doing what they're doing. And so, I've always had trouble. Clearly, there's been a huge amount of work, there've been a lot of mathematical insights that it's generated. It's links to physics, and its tie-in to experiments and so forth has always been so remote that it's been hard for me to get a handle on exactly why they've been doing some of the things they've been doing.

So as far as its promise for the future, I don't know. Is it something that I'm going to work on? The answer is no. I think it would be even harder for me to figure out what's going on there, the foundations of the string theory, now than it was 30 or 40 years ago for me. Unfortunately, it's been a field that has tended to live on hype too much. Outrageous statements have been made, promises have taken the place of arguments, never mind demonstrations, or what passes for proof for physicists. And I'm very willing to accept that a physics proof is nowhere near a mathematical proof. On the other hand, one almost always has some physical system that one can use as a crutch to lead one through the places where one gets confused by the mathematics. The physical system gives one a glimmering of what you're going for. As a result, one can avoid most of the swamp holes that could envelope you if you were just randomly walking over the landscape. So I didn't know. I think there is quite probably a kernel of truth or insight in string theory, which will prove to be lasting. But what it is, I'm not sure at all.

Zierler:

What are alternative pathways in your mind to reconciling general relativity and quantum mechanics?

Unruh:

Obviously, this has been something that's bothered me my whole career. It's a question I still don't have an answer for.

Zierler:

Are there experimental limitations for which theory is stuck until we operate at higher energies, for example?

Unruh:

Well, part of the problem with experiments is, and part of the strength of experiments, is to winnow out competing theories. You've got a whole bunch of theories, which make different predictions. You go and look at the world, and you find that this theory’s prediction is supported, and that one’s isn't. And as a result, you can rule out certain ideas in favor of others. Unfortunately, in the area of quantum gravity, one is in the opposite situation, that one has no theories. Now, as we know from the history, especially in quantum mechanics, having no theories and being led to something via experiments was the way in which quantum mechanics developed. And unfortunately, in this case, all of the experiments that one could hope to think of doing are so remote, so difficult that one just simply can't get a handle on them.

People have gotten very excited in recent years that perhaps this experiment or that experiment might give us some insight to quantum gravity. I think almost all of them have been, shall I say, too hopeful as to whether or not they could actually see something and as to whether or not they would actually say very much about quantum gravity. So, one is still thrown back on theoretical speculations. My attitude has always been to try to ground those speculations in things that we're fairly sure of, which is one of the reasons why I've been working in quantum field theory and relativistic or curved-space-time background, because both of those areas, general relativity as a theory of space-time, and quantum field theory, are on pretty solid foundations. And so, one has a lot more faith that things that one does there will actually have lasting value.

And it's one of the reasons why I've tried for so long to understand what's going on in black hole evaporation that Hawking found. Because I think there is a kernel of insight there that one has to get a hold of. It's one of the reasons why I've always been a little bit suspicious of the black hole entropy kinds of arguments. For me, black hole entropy is really strongly founded on the fact that the radiation emitted is thermal, not on sort of any fundamental aspect of statistical mechanics and so forth. And understanding the thermodynamics, I think, has to come from temperature rather than entropy. That's still a very minority position in the field, I think, but has been part of what's driven what I've been doing. So, trying to understand black hole evaporation has been one of the key themes of research that I've been carrying on throughout my whole career.

Zierler:

Well, let's take it all the way back to the beginning. Let's go back to Manitoba and even before. Tell me about your parents and where they're from.

Unruh:

My mother was born in Southern Manitoba, in what later years became the chicken shack on her father’s farm. Basically, it was a small, two-room house. The middle wall separating the two rooms of the house was a wood-furnace. Two brick lined narrow walls separated the two rooms, and you would just build the fire between them to heat the shack. In winter night you would build the fire which you would hope would keep things warm till morning (it could be -40 0 outside). A few years later, her father built a big, more regular farmhouse on that land. So that was very much a farming community. But my grandfather, and certainly all of his kids, were really interested in education, and thinking, and arguing with each other, discussions, etc. He was also one of the driving forces behind building a residential high school (Mennonite Collegiate Institute) in the 500 population town to service the Mennonite community in all of southern Manitoba. (Both my mother and father went there.)

My father was born in the Ukraine, just north of the Crimea and west of what is now the city of Zaporizhzhia. One of the things that physics gave me was the ability to actually visit the area on one of my conference trips to Moscow.

One of the great things about physics is how wonderful the people in it are. This is especially true in General relativity. In some of the other fields, I hear stories of back-sniping and people being nasty to each other. And I've rarely seen that in the field of general relativity. Anyway, Slava Mukhanov, a recent PhD graduate of Vitali Ginzburg, arranged that I take a trip down to Crimea to see if I could find my father’s birth town. Mukhanov had met a computer entrepreneur, a Mr. Levi from Zaporizhzhya, when selling him a computer that Mukhanov had bought on one of his trips to the West and had persuaded him to help me in my quest. Levi then, took a couple of days of his life to shepherd Leonid Grishchuk and me around to try to find this town.

When my father lived there it was called Waldheim and now Vladovka. I had a little map that had been published in 1900 of the Mennonite region there. I expected that I could get a map current map of the area, but all they had was a 5x5cm map of the whole of the Ukraine. Using my 1900 map, we finally found the town, and even the nearby village where they had caught a coal train to Riga and the beginning of their trip to Canada. When my 12-year-old father left Waldheim, it had two streets (about 2km long) separated by about a kilometer and a stream. The farms would be plots of land, which would be maybe, let's say, 50 or 100 meters wide versus half a kilometer long down to the river. And that would be the farm extending back from the houses along the streets. When I visited, it was still the same layout and size as it had been in 1900. It hadn't changed in the 100 years (except having had to be rebuilt after the devastation of the WW2).

And my father's uncle, a theologian, who had gone to Germany -- I come from a Mennonite community, and this was a Mennonite town in Russia -- wrote to my grandfather in about 1924 and said, "I've heard that the railway company in Canada is loaning money to attract settlers for the prairies." The government of Canada had built the CPR railway to unify the country, especially against the fear that the US was still anxious to annex land in Canada. But then the CPR had to make a go of it by hauling freight. They needed people and industries that could produce goods that needed transportation. On the prairies that was farmers and grain. And so, they put up money to attract settlers, who would then settle Manitoba, Saskatchewan, and so forth, farm it, and produce the grain, which could then be transported by the CPR. The breadbaskets of Europe, in the chaos of the early 20th century, were areas they had their sights on attracting people from. My grandfather was a minister in the Mennonite Brethren church in Waldheim, and was tired of being called in to the Communist offices to be grilled for hours on the sermon he had given on Sunday. He decided to grab this opportunity to emigrate. At that time the Communists were happy to allow these undesirables to leave.

One of the stories told about him was that he had to go to a nearby town, Tokmak, in order to visit a Communist office and get permission. The first time he went, the official looked at the application and said, "Well, look, this son of yours (my uncle) is 21. He's of draftable age. We can't let him go." And my grandfather was very depressed, but the official said, "Come back in a week's time." Coming my grandfather said to my uncle, "If you don't go, nobody's going." And my uncle says he told his mother, "You all go. If I disappear at some point, I will find you." He was planning on going down through Turkey and to make his own way over to Canada if need be. My grandfather went back the in a week’s time, and the official said, "You're, what, 45 years old? With eight kids. You and your family are going to go over to this new country, you don't know the language, you don't know anything or anyone there. Why in the world would you want, at 45 years, to do that?" And my grandfather reportedly answered, "Because I want to live another 45 years." The official burst out laughing, stamped the papers, and said, "Go." So that's how my father ended up in Canada, in a small town, Steinbach, in Southern Manitoba where my grandfather became the minister in a new Mennonite Brethren church in the town.

Zierler:

Do you have a sense of how quickly he picked up English?

Unruh:

My father was 12 years old at the time. So he picked it up pretty fast. My grandfather was a minister of the church there and was atrociously paid, if paid at all. So the whole family basically had to go out to work. The women in the family, there were four, became maids in Winnipeg, which was the nearest big city. The men did various and sundry things, mainly working on farms around the area. So, I think they had to learn English pretty quickly. Again, education was very important, and my father was the only one of his class who finished Grade 12 (boarding at the MCI). They still spoke German in the Steinbach.

The Mennonites had moved from northern Germany to the Ukraine under Catherine the Great to farm the rich lands in the Ukraine and had retained German, both Low as the everyday language, and High as the “Sunday-church” language. Steinbach was dominantly Mennonite. German was still the standard language. But all of them picked up English, especially my father, being as young as he was. So they were fluent in it. However even after my parents married during the war, the home language was still German. When I was born, my first language until I was about 4 or 5 years old was German. Before I started school, they decided I really should know some English, so after that the home language became English (my younger sisters learned very little German at home.) At the same time my father was a teacher in the public school system in the area south of Winnipeg before the war, and in Winnipeg after the war, and so worked in English.

Zierler:

Have you retained it at all?

Unruh:

Yeah, I've retained some of it. I can make sure that I'm not being sold, as a low German expression has it. I was in Germany for a while a few years ago, going on a tour through the Zollverein coal mine monument in Essen, and this tour was all in German. I could follow it and even ask questions. So enough of it remains. The grammatical structure is fairly well there. The vocabulary still tends more to be a 5-year-old's vocabulary rather than a 20-year-old's vocabulary.

Zierler:

What are some of the rhythms of Mennonite communal life, in terms of the church, customs, holidays? What sticks out in your mind?

Unruh:

Well, I think the importance of the church. You had the church service every Sunday. The church services, until I was 15 or even 20, when I basically left home, were still all in German. So this kept my German up well beyond age 5. When I was 5, my parents decided that I should speak English, so they started speaking English in the home for me. So I don't remember ever having had any problem in school or anything. And then, there were a few other things with the Mennonites. Not supposed to drink, no dancing, there were disputes at the time when I was young about whether or not one should even have a radio or, certainly, television, both of these letting the world inside one's home, which was a very dangerous thing to do. But many of those restrictions gradually disappeared under the pressures of city life as I grew up. They disappeared, not by discussion, but because people more and more just ignored them. Another of the things that set the Mennonites apart was both the philosophy of adult baptism (from whom the Baptists in the US got that attribute in the 18th century) and non-resistance, meaning a refusal to serve in any armed services, or to swear allegiance to any king or government. I think in me that has clung on as inability to fear authority, and to believe that my own considered opinion is worth defending against any authority.

Zierler:

Did Mennonites have their own schools? Or did you go to public school?

Unruh:

I went to public school from grades 1 to 9. In grades 10 and 11, there was a Mennonite high school (MBCI) that I went to. And in grade 12, around the time of Sputnik, the United States set up funds in order to try and improve science education. My father had become a high-school science teacher. That was part of how I learned physics. When I was in grade 4 or 5, I would read through the grade 10 or 11 textbooks that he was teaching from. I would basically look at the pictures and read the words rather than the equations, which I didn't follow all that well.

Anyway, he heard about this program in the United States which paid for high school teachers to take a year at a university to upgrade their knowledge of science. He got a sabbatical to go to Rhode Island for a year, where he just took undergraduate physics courses at Brown University to update himself on physics. It was one of the times when I realized that being a student is incredibly hard work because I don't think he had ever worked that hard in his life. Often the common feeling is that being a university student is a relaxing time, when you can skive off and not do much for four years, before you get out into the real world and really do something.

And it's just simply not true. It really is hard work. I saw how difficulty it was on my father during that year. He was a bright man, but had not been a student for 15 years and found it really hard. Anyway, I went to just a public high school there. They were teaching the new PSSC physics program in that high school (in the fairly rich town of Barrington, R.I.). This new high-school program was a result of the post-Sputnik attempt to increase the knowledge of science in the high schools. It was a part of reform of the science educational system the US introduced at the time.

As an aside, one of the advantages I had in my Manitoba education, was that we actually also got taught Euclidian geometry in Math class -- for example, the whole business of taking a triangle and proving that the bisectors of the angles all meet in one point, just by doing geometrical logic starting with some primitive axioms. This has vanished from the curriculum almost everywhere, and I think that's been an incredible shame. Even in the 70s, it started to vanish, but the 80s and 90s with under the “New Math”, and they got rid of this 2,000-year-old stuff. It is in my opinion a real shame because it was, I think, the one place in my education where I really got both the importance of and techniques of thinking logically. One starts off with a certain set of premises, and just using those, one can (well, Euclid could) by small logical steps, build those premises up into discovering things that one would never have expected to have been true. That was, I think, one of the key things that I ever took away from my high school education, learning Euclidian geometry. It had the biggest impact on my mental processes. And I'm really sorry that that's disappeared from the curriculum.

Zierler:

As you mentioned earlier, you found this amazing book in the public library as a 15-year-old. But what were your interests or inclinations that led you to have this interest earlier?

Unruh:

I'd had that kind of interest even from when I was very young. I remember my parents bought the family a set of the Books of Knowledge when I was young, and the Encyclopedia Britannica when I was about 15, which was a pretty expensive purchase. I would just enjoy sitting there and reading through those things. And I remember, even when I think I was in grade 2, I and one of my friends would make up question sheets for each other, where we would sort of write down 15 or 20 questions for each other, (found by reading things like the Book of Knowledge) and we'd give them to the other one to solve and we’d mark them. And I just enjoyed doing it, both in making up and in answering the questions. I enjoyed solving these kinds of puzzles. Also, my father, being a science teacher, had had these science books around. And I would leaf through them, and as I said, read the words rather than necessarily the mathematics. So I think that was part of it. And I was good at it, so I continued doing it.

To some extent, even when I went to university, I had no idea what I wanted out of it or why I wanted to go there. But physics attracted me, both because it was studying fundamental issues about how the world worked and because I was good at it. I had been good at it in high school and so forth, so, "Let's try it." And I kept doing it well, so I continued. I had no real understanding or reasoning about where it was going to take me.

Zierler:

Now, was the decision to go to the University of Manitoba automatic in terms of finances and geography? Was leaving farther from home as an undergraduate in the realm of possibility at all?

Unruh:

Basically, the answer was no. The attitude in Canada was very different from that in the US. I saw, when I was in the States for my grade 12, that all of my fellow students were applying to various and sundry universities far away from home. But the attitude in Canada was that there were good provincial universities, and they were just where you went. You could stay at home, and you wouldn't have to find money in order to do your own housing and so forth. Most of the Canadian universities have tended to be, because of that, very poor in terms of student housing. Even University of British Columbia, I think, has on-campus housing for less than 20% of the students, so students are forced to take basement apartments in the surrounding areas, if their homes are far away, or, if they live within Vancouver, they will tend to live at home. That's just the standard ethos in Canada even now.

In the United States, graduation from high school is a real graduation, where you expect never to see your fellow students again, except occasionally when you come home or to a reunion. In Canada, you're still living in the same place, you have the same friends, though you of course make new ones. So the answer was no. It was really only when going to graduate school that moving away was a thought.

Zierler:

Was majoring in physics always the plan for college?

Unruh:

Well, first year, you go in there, and you don't know what you want to do. I was pretty definite that it would be science. Physics, I probably felt was most attractive of the various areas. But I was pretty open. Again, I did well in most of the science courses. But I guess I liked physics best in terms of the kinds of puzzles that one had to solve. Chemistry, I did well in as well, but it seemed to be more sort of memorization of weird facts about the world rather than trying to understand it. And I was attracted to physics because there, you really could have a few things that you started off with (eg, Newton’s laws), and from those, develop one's ideas. In a sense, it was closer to Euclidian geometry than it was chemistry or biology. Those other fields seemed worse for me, in that you just had to learn these random facts, which seemed to have no particular links between them. So I think that's why I stayed in physics.

Zierler:

Who were some of the professors you had as an undergraduate who were mentors to you?

Unruh:

It's hard to say as an undergraduate. It was a very good, demanding curriculum. We had a new head of department who came in, and this was in, I think, third year or maybe second year. I wanted to take some extra math courses. And he just said, "No, you can't do that." And then, he made the mistake of saying, "I will let you take extra math courses only if your average last year was over 95%." And my average happened to have been 96%. So I tended to spend more time in, I think, the math courses. I also remember some of the math courses were atrocious, even though the content was fascinating. I remember in one of my math courses in fourth year, the lecturer came in on the first lecture. He had assigned this textbook. This was on differentiable manifolds. It was a very dense, mathematical textbook, where if you skipped over one sentence, you couldn't understand the next paragraph at all. He walked in, and he had these handwritten notes that he proceeded to copy onto the blackboard, which we very rapidly realized were exactly the contents of the textbook. He was a new teacher, and I guess had no idea how to teach.

So he had copied the textbook onto his notes and then copied those onto the blackboard, and expected us to then copy those into our notebooks. And I just said to myself, "I can read the notes as well from the textbook as I can from the blackboard." So instead, one of my friends had this lecturer in a Shakespeare course in a seminar type format, who was a very intellectually charismatic. He absolutely loved Shakespeare and loved explaining what was going on in the plays. Instead of attending the math class, I just sat in on those seminars, which was far more rewarding than watching somebody copy a textbook onto the blackboard. So even at that time, I wanted a lot of different stuff in my life. It wasn't just that physics was the center. There were so many things that one could learn and look at that attracted me within my undergraduate education.

Zierler:

To what extent as an undergraduate did you appreciate the binary in physics between the world of theory and the world of experimentation?

Unruh:

The problem with undergraduate education almost everywhere is that it's extremely strongly theory-based. And labs are something that, even now, most physics departments struggle with figuring out exactly what they want the students to learn out of the labs. They feel very strongly that labs are important, and I think that they're very important. But they're very unclear, still, as to what it is that they want the students to take out of them and how they want to teach them. This is slowly changing. In the last ten to twenty years at UBC a real effort has been made by at least some teachers to change, to give the labs a purpose. Sometimes when a new head of the labs comes in there can be some back-slipping. I know one of the labs I was involved in--as part of my teaching duties, you have tutorials or labs, and I usually like working in the labs--was going in a direction I thought was good, and then another person took them over, and it just slipped back into the sort of recipe style of labs, rather than teaching the students how to think with things, rather than just with abstract concepts.

In my fourth year at the University of Manitoba, one of our labs was, basically, atrocious, because everything became following some rules without explanation. I remember one of the things that the lecturer wanted to do was to give us a feeling for statistics. To do that, he fed us this paper copy of a massive bunch of data coming out of a gamma ray counter -- the number of counts per second for about a half hour of data. We were supposed to plot the number of counts per second onto a bar graph, so that we could feel the Poisson distribution growing under our hands. A couple of my friends in the class got the idea of doing this via computer instead (which in 1967 were very new things). The professor got incredibly annoyed with them because he wanted them to do this tedious work by hand. So my experiences with the labs were minimal.

But at the same time, I was always strongly attracted to it. Having grown up with my relatives being farmers, I was used to tinkering. And physics really was an experimental science. So when I was at Princeton, one of my summer jobs -- in the first year, I needed to get some jobs in order to help pay for tuition at Princeton -- I worked in Donald Hamilton's atomic physics lab, where I actually designed a little experiment. So in my education in graduate school rather than undergraduate, I actually got my “hands dirty” in the lab and appreciate the importance of laboratory work. Just as with Euclid one could start out with a pile of seemingly random material and create something which would give you an insight into how the world worked. It was another way of solving puzzles. And I think I have appreciated that ever since.

So in the last 15 years, for example, I've been heavily involved in this analogue gravity work, where it turned out, as I showed in about 1980, that there are, condensed matter systems, which mimic things like black hole radiation, the thermal radiation coming from black holes, and that one could actually do experiments on them. And I got a couple of pos-docs coming to me, first Ralf Schützhold from Germany, and then Silke Weinfurtner, who had gotten her PhD in New Zealand with Matt Visser, was also really interested in experiments. Ralf and I had been thinking about the possibility of doing some of these analog experiments and had even interested Greg Lawrence, a civil engineering professor at UBC in the possibility of doing experiments. But Ralf left before we had a chance to set anything up. A few years later when Silke came as my post doc, she said, "Look, we have to do this," and had the forcefulness to actually do it-- to gather up a team including Greg and his former student. And I was in the machine shop, using the milling machine to design an obstacle for the flume to create a white-hole/black-hole analog to see stimulated Hawking radiation.

Zierler:

As an undergraduate, what kind of education, if at all, did you have in GR?

Unruh:

I'm trying to remember if I had a course in GR at all. I think I must've. The book we used was Adler, Bazin, and Schiffer. Which was an OK introductory book. In a sense, I didn't learn my general relativity from courses. I took a course in Princeton from John Wheeler, which, again, the textbook he used was by Anderson, and I didn't find it great. I found that he made mistakes in places. And when you start finding mistakes, you start distrusting the textbook. So the learning hasn't been from textbooks. The learning has been much more from osmosis, and reading books on my own, and from discussions with people, than it has been from formal courses.

Zierler:

What kind of advice did you get, if any, about graduate schools to apply to, advisors it would be good to work with? In other words, Princeton's a long way from Manitoba. How did you figure out where you wanted to go?

Unruh:

Well, there were some of these big names that I applied to, places like Stanford and Harvard. Princeton, I think, it was Erich Vogt at the University of British Columbia, who had encouraged me to apply to Princeton. (He had been one of Wigner’s students 15 years earlier). Erich had been a neighbor of my father’s family in Steinbach, and had been taught by my mother in grade 2 for two months, at which point she shoved him up a year. She promoted him because he was ahead of everybody in the class. He came to visit Manitoba to give a talk, and I talked to him about graduate schools. He suggested Princeton as a possible place to apply to.

And then, I applied to some of the universities in Canada, like University of Toronto and McGill. And I think Princeton sort of offered me the better deal. Erich encouraged me to go to Princeton, said it was a good place. I had gotten a Woodrow Wilson Fellowship at that point. At that time the Woodrow Wilson Foundation (since renamed) had these one-year graduate fellowships they would give after an interview (in Minneapolis – my first airplane trip). They've changed the way in which they work now. Then, people like myself in physics could apply. So I finally decided to go to Princeton. But I didn't have any strong feelings about why I went there. It was just a good school to go to.

Zierler:

Did you have any inklings, going to a United States campus in the late 1960s, about all of the campus protests? Had you seen that at all in Manitoba?

Unruh:

No, not really. I went in '67, so it was more or less while I was there that they all broke open. Kent State occurred while I was at Princeton. And the Berkeley protests were in the early 1970s. So when I went there, the protests were just getting started. There's an interesting story about John Wheeler and the protests. I was doing my first post-doc in 71-72, and for my first post-doc, I had gotten a National Research Council (of Canada) Fellowship, which I could take out of the country. And I remember being at the Texas Symposium in Austin that year, and Roger Penrose was there, giving one of his talks on black holes. And I approached him and said, "Look, would it be possible to come and do my post-doc with you, assuming I get money to support me for it?" And so, I went there, and then Wheeler had put me up for a Miller Fellowship at Berkeley, and when I was awarded the Miller, I had to try and decide what to do because it overlapped with my fellowship in England.

I talked to John Wheeler about it, and Wheeler, of course, has this reputation of being an ultra-conservative, right-wing kind of guy. And he said, "Berkeley is an extremely interesting place. There's a lot of stuff going on there." And he did not mean in physics. It was the liveliness and foment of the protests that seemed to attract his attention. This was the time of the occupation of that little plot of land just off the university (People’s Park?). So it was a very fraught time, if you will. And yet, that was where he was suggesting I should go. He was a very complex individual. You couldn't simply sort of slot him as a standard ultra right-wing guy. In physics as well, he was willing to go way out in his ideas, publicly, and be willing to be made fun, of because I'm sure he realized that most people thought what he was doing was insane.

Zierler:

Did you connect with Wheeler right away, or was that developed over time?

Unruh:

No, I remember even that first year I was at Princeton, there, I knew of him. The first time I saw him, he was in the main physics office, talking on the phone, possibly to his wife, about curtains for their house. And I was just sort of flabbergasted. Here's this famous physicist, and he's talking about curtains. [laugh] The realization that even famous physicists are human beings.

Zierler:

How quickly did you settle on him being your advisor?

Unruh:

It was after I passed Generals. The first two years at Princeton I spent getting ready for the Generals examinations. And so, I spent all of my time sort of learning physics, going to courses, although in Princeton, there were no course requirements at that time. One of the courses I took was taking, Stat Mech-- a year-long course. And I took the course for the whole year. But at the end of the year, I found out that I had dropped it at Christmas. I had never known that I had dropped it at Christmas. Wigner had dropped me from the course. He had apparently decided that I wasn't serious or annoying or something. I didn't push it because courses didn't matter at all. Your future depended only on the Generals. When I passed those, Wheeler was the first person I approached about being my advisor, and he said, "Sure."

Zierler:

And what was Wheeler working on at that point?

Unruh:

Issues in quantum mechanics, the self-regarding universe. Quantum mechanics as a game of 20 questions. So, issues in the foundations of quantum mechanics were at least part of his research. He was also interested in black holes, pushing black holes (which name he had promulgated), the initial value conjecture. Is the initial value problem valid? What of the problem of time in quantum gravity? The thin slice conjecture, that instead of giving the conjugate momentum and the spatial metric on one time slice, take two slices and just give the metric on those. Is that a valid way of proceeding? Etc. I didn't get involved in that, but I heard about it.

It was hard working with him because he was never there. During the last two years of my PhD, he tended to be gone a lot. In part, he was writing the textbook, Gravitation, with Charlie Misner, and Kip Thorne, his former students. They spent a lot of time getting together to work on that. He was consulting in the government. So it was really hard to actually get to see him.

I remember once, towards the end of my fourth year, I wanted to talk to him about something in my thesis. And I heard that he was coming onto campus for a brief visit. I was waited around the entrance to Jadwin Hall (the Physics building at Princeton) and happened to be upstairs at the elevator. Elevator door opened, there he is, and there are ten people around him. And he had this sort of choppy walk, very fast. He would go walking down the hallway with all these people around him, and I just looked at that and said, "Forget it."

On the other hand, I had some really good interactions with him, but they tended to be brief. He was going to California to work with Kip on their book. And he said, "Look, do you want to come along?" I said, "Sure." And he didn't pay for any of this. But I had enough money, I bought an airline ticket and flew off to Caltech. And we spent the first night I was there, talking for about four or five hours. Next day, I think he had a seminar there and then he, Kip and Charlie went off to Baja to work on the book over the long weekend. When they came back, he said, "Oh, I have to give a colloquium at Berkeley. Do you want to come along?"

So we flew to Berkeley and talked on the plane a bit. But it was one of those California flights, where the plane is trying to sample all altitudes at the same time. One of those quantum flights. [laugh] So both of us were a bit distracted, trying to keep the airplane in the air. But that trip was great because I would follow him around, and he would talk to various people. And he almost always had this ability to always concentrate on what the other person was interested in. He never had trouble talking to people because he was able to get them talking about what they were most interested in. And seeing that was a great technique to learn. Then, he gave his colloquium, and I think, just at the end of the colloquium, he heard that his father had died. So I think I saw him for a total of five hours, just the two of us together, on that trip two or three week trip. And that's the sort of interaction we had. Or, back at Princeton, he would have meetings where he'd have his whole group together, and people would explain what they were doing, and he would talk about something that interested him.

Zierler:

What was the process for you developing your thesis research? And how closely did you work with Wheeler on that?

Unruh:

There were two questions that I tried to answer in my thesis. Wheeler had always -- ever since his early student, Misner -- been interested in topology and general relativity. And he had mentioned that, I think, in one of these group meetings. A few weeks later, I was supposed to come and talk to him (one of those irregular meetings with one’s supervisor). The meeting was in a week's time, and I was desperately trying to think of something to say because I knew I had done piss-all. I was trying to think of something that I could say to indicate that I wasn't a complete dolt. I came up with this question, whether there some topological charges that one could make up out of just the metric. I had absolutely no idea how to do that. So I went and presented this to him. He suggested some people that I might want to phone and talk to.

But that was not the end of the meeting. The next thing he said was, "Oh, by the way, there's this conference in Switzerland next summer that I'm going to. And this topic would fit in perfectly with that conference. Why don't you send them this telegram signed by me asking them to invite you to the conference as well?" And he handed the text of the telegram to me. I knew I had absolutely nothing to say at that point, and that raised a question. Was I going to be able to have something to say in eight months' time? I had no idea whether I was going to have anything to say, but Switzerland was quite an attraction. It took me about a day of wandering around the campus before I finally found the courage to send off the telegram.

And they did invite me. It was very unusual for them, they said later on, because it was sort of a workshop kind of meeting. And they hadn't really thought about having students there. But I guess because it was Wheeler who asked, they did invite me. So that meant that I then had eight months in which to work on this. And I phoned some people (like Ted Newmann) who gave me a few hints as to possible ways I might want to try to look for such charges. Fortunately, I actually managed to have something to say by the time I got there.

Zierler:

What did those people suggest?

Unruh:

Well, I think Ted mentioned the Newman-Penrose formalism, which I looked at a little bit and used later on when I was a post-doc in Britain extensively. They suggested that that might be one way of attacking the problem. I'm trying to remember who else I talked to. Maybe Misner suggested that I look for closed but not exact forms, about which I knew zero about and had to quickly learn about. That was the technique that I used in trying to attack the problem.

Zierler:

What was going on in the world of experiment or observation that may have been relevant, if at all, to your thesis research?

Unruh:

Basically, nothing. So the second half of my thesis was that-- looking for gravitational topological charges. The first half, also, was inspired by him in the sense that, while I was in Caltech visiting him (but he was no longer there), he sent me a paper that he and Remo Ruffini had been writing on scattering cross sections of black holes to scalar fields. And I looked at it, and I said, "I can do that better." And so, the first half of my thesis was me doing it better.

Zierler:

And what exactly was the game plan? How would you go about doing it better?

Unruh:

Well, I developed this technique, which I discovered later was the sort of matched asymptotic expansion for solving the equation. You have this potential barrier around a black hole for waves in the vicinity of the black hole. If you're near the black hole, very near the horizon, or very far away from the horizon, the equations are easy to solve -- just plane waves of Bessel functions. You can completely neglect this potential barrier. Under the potential barrier, you can solve the equations if you neglect the frequency dependence of this wave. The trick is that you can match the three together in a region where all three are reasonable approximations. And so, I developed that technique which would allow me to approximately solve the equations and get a much more accurate answer than they had done in the paper.

Zierler:

How mathematical would you say your thesis was?

Unruh:

Parts of the second part were very mathematical. Learning about co-homologies and forms, etc. This was purely mathematics, and it was purely a matter of taking derivatives of the metric and seeing how you could create one of these forms. And a last part of it, I had to use some fairly advanced theorems in order to do them. At that Switzerland conference, Bob Geroch asked a question, which made it seem as though almost everything I had done was trivial. When I thought about it later on, I realized it wasn't. But his comments gave me another approach to the problem which I added after the meeting. I, then, had to learn about a bunch of cohomology tricks that I knew nothing about beforehand. The first part, the scattering cross section, was more a matter of calculation. The matched asymptotic expansion was more a matter of doing calculations, solving differential equations, than it was, really, any kind of forefront mathematics. That was stuff that you could've done in the 19th century. Although matched asymptotic expansions turned out to a relatively recent discovery by others.

Zierler:

Besides Wheeler, who else was on your thesis committee?

Unruh:

Second reader was Karel Kuchar. Writing the thesis was interesting because Wheeler, all of a sudden, said to me, "Bill, I'm going away in the middle of May. And I'd really like to have the thesis finished by that time." And this was something like the beginning of April. So I had to very rapidly start writing the thesis. I would turn up at my friend’s door at 8AM, just as he was heading off to the university, plop myself on his floor, and hand the pages as I was writing them to his wife, who typed them. Another friend of mine (Vince Ruddy, one of Peeble’s rare students) would write in the equations onto the typed pages. It's not a well-written thesis. I get embarrassed every time I open it, and so, I almost never open it.

The way Princeton Physics worked was that a second reader had to read the thesis and OK it a few weeks before the defense. I delivered the thesis to him at 9 o'clock one evening and said, "Well, actually to meet Wheeler's deadline, you have to hand in your OK by noon tomorrow." And he did it, which was going way, way, way beyond the bounds of anything I could expect.

Zierler:

That's asking a professor to pull an all-nighter.

Unruh:

Yeah. I would never do that for a student nowadays. [laugh] And then, they had a larger committee. Sam Trieman was on it. Wheeler, Kuchar, Wightman I think were on the committee who put me through the oral exam and granted me my thesis.

Zierler:

What kind of interactions did you have with your fellow Manitoban Jim Peebles during these years?

Unruh:

Very little. He was one of the gods up there. He was 15 or 20 years older than I was. So he was on one of the three oral exams that was part of the Generals, and asked me a question. This was about the experiment that I had done in Hamilton’s lab. As part of the Generals, Princeton physics demanded that all theorists had to do an experiment-- either improving one of the undergraduate lab experiments, or working with some experimental group. I couldn't answer Peeble’s question in the General’s exam orals, which embarrassed me. Two hours later I finally figured it out.

One of the weird things at Princeton at that time was that you had these two relativity groups. You had Wheeler's group, and you had Dicke's group. And the amount of interaction between the two was extremely small. There weren't any joint seminars, people from one wouldn't drop in and talk to the other. Vince Ruddy was a good friend, but I never talked to him much about his thesis. There was no animosity between the groups or between Wheeler and Dicke, just little interaction. Thinking about it now, that was an incredible shame.

One of the experimental efforts we heard about was Weber’s gravitational wave detector experiments. Weber was invited to Princeton to gave a seminar on his preliminary results in about 1970, which, of course, were very controversial. But Wheeler was somebody who appreciated experiments a lot, who was constantly coming up with Gedanken experiments and trying to think about the world in the sense of, "What does this mean in terms of physics?", not in terms of what it means in terms of mathematics. I would say that he was never that strong in terms of mathematics.

I remember I was at one of the Texas conferences either in the late 70s or early 80s, and I was talking, I think, to Kip Thorne, who said that Feynman had told him that one of the great things about working with Wheeler was that you always felt that you were a better mathematician than he was. (It is good for a graduate student’s ego to feel that at least in some are you come up to the mark.) But Wheeler could ask the most far-out questions, which seemed crazy, but one realized that they struck at the heart of key issues. He seemed to have this insight into what was important in physics. For him, the questions were more important than the answers.

Zierler:

Meaning intuition was really important.

Unruh:

Beyond what almost anybody else had. But in terms of actually implementing it, I would say he wasn't any great shakes. And so, it was much more the sort of motivational stuff that was far, far more important for him. And you also had to have the right frame of mind to appreciate his questions. If you didn't have, as most physicists didn’t have, the right frame of mind, then you would just think, "This guy's gone completely crazy."

Zierler:

As you were wrapping up at Princeton, what post-doc opportunities were available to you?

Unruh:

I didn't even apply to that many. The main thing I did was apply for a scholarship from Canada, the NRC post-doctoral fellowship. I had already talked to Penrose about the possibility of my working with him if I got the fellowship. At that time, he was still at Birkbeck College and certainly wasn't famous, as he later became. But the work he had done, I had always been very impressed with. So when the scholarship came through, it was just natural to go there. He was in England. Part of the attraction of working with him was going to another country.

Going to England, I was thrown on my own. In all my prior work, as an undergraduate at home, and as a graduate student, I lived in the dorms. Basically someone else took care of me -- where to live, where to eat. In England, suddenly, I would be completely on my own, which was scary and attractive at the same time. It was an adventure, and I looked forward to it. And I was incredibly fortunate that the fellowship, in British terms, was rich. I think I made more money from that fellowship than I think Roger made as a professor at Birkbeck. I saw advertisements on the buses for bus drivers, where the weekly rental cost of my apartment was more than those bus drivers were being paid a week.

Zierler:

What was the group that you joined? What was the research culture there?

Unruh:

Birkbeck College was founded 150 years ago or so by George Birkbeck as an adult education college. Its goal was to provide education for the working man—ie, people who had to work during the day. Thus, all of the classes were in the evening. It became part of the University of London system in 1830. Penrose worked in the maths department where was clearly the star. Unfortunately, the rest of the the college, including Physics, was about a kilometer away from Mathematics, so I never got to know it at all. David Bohm was in the physics department there, but I saw him maybe once. So the group was basically Penrose, and he had another post-doc, who was a fellow Canadian, and a couple of students. This gave me a lot of time to think about physics. It also gave me time to do things in the other areas of my life, which was also important.

I got involved in a folk club in the Crypt of St. Martin-in-the-Fields (on Trafalgar square) through an early girlfriend. One night Hugh Maddox, one of the vicars there, said, "We need more actors. We're putting on this sort of folk passion play called Alive." (This was the time of Jesus Christ Superstar and Godspell). The play had been created about a year earlier through improv. I said, "That sounds like fun," so I joined with them. This was also an incredibly important to me because I saw parts of England that I would absolutely never have seen otherwise. Hugh had friends all over England from his days in seminary at Oxford, and he would get us invitations to places like Marlborough Boys' School to put on the play, Winchester College, Eaton, the Ashford Remand Home, which was where young offenders were thrown (it apparently had a horrible reputation, although we did not see that).

We also went to Wormwood Scrubs, which is Britain's maximum security prison on the outskirts of London, to put on the play there. Our play was “in the round”, so we would be running through the audience, and the guards were just incredibly nervous. You have this audience full of murderers, rapists, etc., and here we are, running through them. I think guards had nightmares of us being grabbed, and being held hostage, and so forth. (They did restrict attendance to I think their model prisoners, but still...). It turned out to be one of the most rewarding experiences of the whole play because the audience was so involved. Toward the end we had the trial of Jesus, and and the audience would shout -- "Give him life. Give him life." "He's guilty," being shouted out by the audience. Experiences like that was something that I just would never, ever have had otherwise.

Another advantage of working with Roger at Birkbeck was that he had much more time then. He basically had this one course that he had to teach in the evenings so he was available during the day.

About five years later, he was invited to Oxford and became professor there. And when I saw him there, like Wheeler had been at Princeton, he was constantly busy. At Birkbeck, I'd often have a question for him, I'd knock on his door, and I would stumble out three hours later, as we had covered humongous tracts of physics and mathematics in this discussion. The question I actually wanted to ask might've taken the first 20 minutes. But the whole conversation just kept going. And he had the time to do that. So in a sense, I didn't need any “rest of the group”. I had enough stuff to do on my own. And these kinds of interactions with Roger were such that they could keep me going.

Zierler:

And what kinds of things would you engage Roger on?

Unruh:

I don't even remember the discussions anymore. Black holes, etc. One of the things I got interested in was the problem of radiation reaction in electrodynamics. And there was this integral that he had discovered a few years earlier, and he suggested that I look at that, as it might be helpful. Part of the problem with it was that it was all done in this Newman-Penrose formalism (which I had learned some of during my thesis). It is a great formalism, with a formidable notation. There was an awful lot you have to learn before you even understand what in the world is going on there. And so, I spent my time reading their papers and learning that formalism in order to write that paper. So that was one of the examples I worked on. Another one, he and a student were working on radiation scattering from black holes. Because that had been part of my thesis, he got the student to talk to me, and we worked on that a little bit. I just gave him some advice on it. And they eventually published a paper together on that. So it was a very rich experience, both from the point of view of physics and from the point of view of my life.

Zierler:

Now, to foreshadow just a few years, when did you become aware of the work that Stephen Fulling and Paul Davies were doing?

Unruh:

Stephen Fulling was a fellow graduate student in Princeton, and he had sent me a copy of his thesis where he had raised the question of whether the quantization of fields in flat space-time was unique. That thesis and answering that question turned out to be very important in my career. Paul Davies, I had met at King’s College just down the road from Birkbeck. But the main interaction came after Hawking had written his paper on thermal emission by black holes. When I was a Miller Fellow at Berkeley, I was supposed to be working with Ray Sachs, but unfortunately, Ray was on sabbatical. So, I spent almost all my time working on my own or talking to some of the fellow post docs in physics and maths. But basically, I worked on my own.

But I still had a girlfriend in England, Pat (Truman then, Unruh now), and I wanted to see her. She was (and is) a musician who I met because she did much of the music for the play Alive I joined. So every conference that I could imagine, I would find some way of getting invited. Even if it was only tangentially related to the work I did, I would fly over there and see her (and attend the conference of course). In December of '73, Abraham Taub in the math at Berkeley department got a preprint. He gave it to his post-doc, Vince Moncrief, and Vince showed it to me. It was the draft of Hawking’s first paper on the thermal emission by black holes.

And so, I tried to understand it, found a mistake in it, was really having trouble figuring it out and was sort of dubious about the result. When a black hole forms, everyone expected it to radiate quantum particles briefly, because or the rapidly time changing metric. But Hawking discovered that it kept radiating constantly, without pause, with a thermal spectrum. It was a mystery how it could do so.

I had been doing some work on fields around rotating black holes and horizons , and had just written a paper on it. Because of that, I had been invited to a conference in England at the Rutherford lab, which was the first conference where Stephen gave a talk about his work on black hole thermal emission. One of the other people invited to give a talk at that conference was Paul Davies. I think I had met him while I was a post-doc with Roger as well. Paul and Joe Taylor had worked on black hole thermal emission and were quite critical about it in their talk.

I had seen one of the organizers, Denis Sciama, a few days before the conference, and had given him a copy of my paper on fields around a Kerr black hole, and he organized that I be given 5 minutes to talk on that work. I felt Stephen’s work was more important that what I had done, but I was still confused. Using work I had done (but not yet written up) developing on Fulling’s work on alternative quantizations in flat space-times, I tried in 5 minutes to present the ideas, and ask which types of particles Hawking’s work depended on. But, due to my own confusion, those five minutes were basically a jumble. It was not a coherent talk at all. It gave Jane Hawking, Stephen’s wife, the idea that I was siding with Paul Davies and John Taylor. And I remember at lunch, after that session, Jane just lit into me, defending Stephen.

(The mistake I found in Stephen’s paper had been dealt with in the first sentences of Stephen’s talk when he said “Some of you might have found this mistake, but that has been corrected”. It made no difference to the result. Davies’ and Taylor’s objections were also not really relevant although some of their concerns highlighted characteristics of the Hawking process which are still puzzling.) But that conference also started Paul and me talking, and then at various conferences and during my visits to London, Paul, and Stephen Fulling, and I started working together. We wrote our first paper together about the regularized energy momentum tensor in 1+1 dimensional black hole evaporation. Davies and Fulling had started working on the the calculation, and then I got involved in the paper as well. Later, when I was at UBC, I had Paul over here with his family on a sabbatical for a year, and we wrote some papers together as well.

Zierler:

And what were the broader questions in the field for which this effect was responsive? How did you engage with people more broadly in the field? What were the bigger questions?

Unruh:

Well, the thing that was most important for me was, you have these particles being emitted by the black hole. And Steve Fulling, who had been a fellow graduate student of mine at Princeton who had entered the graduate school a year after me, had sent me a copy of his thesis where he raised the question: “In a quantum field theory, what is a particle?” Leonard Parker had written papers on particles and particle creation in cosmology in the late 60’s already, and had inspired some of Fulling’s work. Parker’s was, I felt, a very mathematical approach to what a particle is, which just didn't work as far as a black hole because it relied heavily of a definition of particle that was spatially global, while particles for me were something that was local in space. In addition, Jim Hartle had written for a paper for Wheeler's 60th birthday Festscrift in which he argued that electrons on either side of a black hole horizon could not influence each other via the weak interaction because of the lack of quantum correlation in the naive quantization of the neutrino field in a black hole space-time. This just seemed physically wrong to me, although perfectly acceptable mathematically. These all raised the question of "What in the world was meant by particles in quantum field theory, and where are these particles coming from in black hole thermal emission?"

That question, and the confusion I had over Hawking’s result led me to the idea of building a model (Gedanken) particle detector. If you want to know what a particle is, use a particle detector. So I applied that, first in flat space time where I calculated the effect of accelerating the detector, and then near a black hole. I knew even before Hawking had done his work that there was this alternative notion of particles. And that was sort of what I had rambled on about at that Rutherford meeting. And I was still sort of grappling my way into view of particles were. So Stephen, I knew from previous meetings, and Paul, I got to know mainly after that.

After that, we started to work together on Hawking radiation and and other issues of quantum fields in gravitational situations, trying to understand, for example, where Hawking’s particles came from. Part of my drive toward defining particles by particle detector was also a very Wheeler-esque kind of question, "What's the physics here?" Hawking's (and Parker’s) analysis was very mathematical, and it wasn't clear entirely what in the world their calculations meant physically. And I wanted to know what the physics was. Where are these particles? How are they being created? And that's been a question that took, on one side, 30 years to answer, and on another side, one we're still grappling with it.

Zierler:

Specifically on that point, I wonder if you can address this question of whether the Unruh effect has been observed and what that even means.

Unruh:

The Unruh effect is the name that some give to the fact that if you accelerate a particle detector in the state of the quantum field (called the vacuum, which you would think had no particles present) the detector would respond as though it were immersed in thermal bath with temperature proportional to the acceleration. So, there are a few ways one could imagine observing it. The first suggestion that it had been observed was by Bell and Leinaas back in the 80s, who argued that one can regard the spin of an electron in a magnetic field as a sort of thermometer. So if the electron is in a magnetic field, then one of the spin directions is going to have a higher energy than the other. And if you are in a thermal bath, then that will mean that you get excitations, and you won't be able to keep all of the electrons in one of the energy levels. In my paper, what I had looked at was the effect of uniform linear acceleration. Unfortunately that is essentially impossible to see experimentally. The accelerations were too large, and the detector would leave even the largest lab well before the experiment was finished unless this lab were larger than the size of the solar system.

So forget it, that's not going to be observed. But other people had worried, also, about the circular acceleration, and Bell and Leinaas argued that that case looks approximately thermal as well. They argued that the spin depolarization, (start off with the electrons all in the lowest energy state of the spins, and if you wait awhile, you will find some of them in the higher energy state. This will look as though that electron has absorbed radiation.) JD Jackson, for example, had looked at in the early 70s for electron storage rings using straightforward but very messy Quanum Electrodynamics calculations. Bell and Leinaas argued that this could be explained much more simply as the circular acceleration made the electromagnetic vacuum look like it was full of approximately thermal photons, which were absorbed by the electrons. Jackson did not believe this.

Jackson and I were both invited to a conference in Monterey, organized by Pisin Chen, in part to examine this topic. Jackson gave a skeptical analysis of the Bell-Leinaas’s paper, pointing out non-thermal features of the response of such a spinning electron detector. His analysis worried me, and caused me to go home and do the daunting calculation myself.

A few months later, I sent the results to Jackson by email. He looked at it, and he said, "Yeah, it's interesting. I've got to think about it some more. But here, in your equation number whatever, you have this fraction of," and I'm just making this up because I don't remember the actual numbers, "2,946 over 1,310. Are you sure that shouldn't be 1,432 divided by 837?" I went back to my notes, redid the calculation, and sure enough, he was right. And it just blew my mind that he was able to take this horrible calculation that had taken me months to do, and in a few days, he'd been able to find this error in the calculation.

But anyway, I think I finally convinced him that the effect could be looked at as Bell and Leinaas did. The features that had made him doubt that could be explained because of interference between two separate thermometers that that electron in the storage ring represented. One of them is a spin thermometer, measured by the spin depolarization of the electron. The other thermometer is the vertical oscillation of the electrons in their path around the storage ring. Vertical oscillation of the electron induces spin flips through the Thomas precession, and the vertical oscillations are caused by the same vacuum state fluctuations of the electromagnetic field that cause the direct spin flips. Since they come from the same source, there is interference between the two sources of spin flipping. That interference explained the weird features he had pointed out and had led him to say, "There's no way this could be thermal." But Jackson was an incredible physicist, an incredible calculator. He's a Canadian as well. And he got denied tenure at McGill, which was how he ended up at Berkeley. There's no way he should've been denied it. [laugh]

Zierler:

How long were you at Berkeley?

Unruh:

A year and a half. I had these two post-docs, which overlapped, so I took a year and a half at Birkbeck and a year and a half at Berkeley. And then, I started applying for post-docs because there was nothing magical coming up. There were no grants I could apply for. And so, I think I applied to about 300 different places.

Zierler:

And to go back to the nomenclature question, for the job market, what jobs were relevant to you at that point? Was it astrophysics, cosmology, general theory? How were you defining yourself professionally at that point?

Unruh:

For post-docs, it was more general relativity, etc. Because that's where I was looking for the post-docs. I also applied to some faculty positions, one at Williams College, which they actually offered me, but I finally decided not to go there because they kept emphasizing that this was just a two-year position, and that there was zero chance of my getting tenure there. And then, through a back door, at McMaster Applied Math department where they had heard from Physics that I was looking for a job. And they offered me a faculty job, and part of the reason I was happy to accept was that it was back in Canada. But it was actually in applied math, so the title was general mathematical physics. At Williams College, it had been undergraduate physics, basically. So it was just a general physics teaching kind of job. When I turned down the Williams job, I was maybe a bit of an idiot because I had no other job offer at the time. [laugh]

Zierler:

What did you take on after that?

Unruh:

It was while I was at McMaster that I wrote that compendium paper Notes on Black Hole Evaporation. It started with a review of Hawking’s result and presented the work on the alternative quantizations of fields in a black hole space-time, and tying them to Hawking’s calculations. In the second section I introduced the definition of particles in terms of detectors and calculated the response of an accelerated detector in flat space-time, and near a black hole. The third section was a generalization of paper by Beverly Berger, Dattakumar Chitre, Vince Moncrief (a fellow post doc with me in Berkeley), and Yavos Nutku in which I analysed the theoretical structure of a “min-superspace” quantization of scalar field coupled to purely spherically symmetric gravity, where everything could be treated exactly. So this paper, “Notes on Black Hole Evaporation”, was sort of a grab bag of various things that I'd worked on in that last while. They all had something to do with quantum mechanics and black holes, but should've been broken up into three or four papers. I don't like writing papers, so the fewer I write, the better, in some sense. Which is a mistake, but it's just my personality.

Zierler:

I'm not sure where to orient this in the chronology, but the question of whether black holes were real or not. Do you have a specific recollection of when the field achieved sort of a critical mass in agreeing that they were real?

Unruh:

It depends on whom you mean by the field. Being at Princeton, they were real from the day I got there. Everybody there was working on them. John Wheeler had written up the calculations that Kruscal had done on the structure of the black hole horizons already in 1960, putting Kruscal’s name alone on the paper. You've heard that story, I presume. Wheeler had also popularized the name “Black Hole”. And so, they were as real as anything else in physics at least at Princeton. And at that time, people started seeing the x-ray emissions from Cygnus X-1, which could not be explained by any kind of star besides a supermassive star, but there was nothing there that you could see as a source for the emissions.

And then, quasars were being discovered, and the reasonable explanation was the conversion of the gravitational energy of the in-falling matter onto a black hole into heat and x-rays of quasars as black holes. The astrophysics field, I think, was much more resistant to the idea of black holes. And it took at least another ten years before they started to become at least a little bit comfortable with black holes. In the early 70s, there were just small groups of Astrophysicists that thought they were of any importance, other than perhaps mathematical importance. Which, of course, is part of what gives me pause about string theory, because it's sort of in a situation where it's of mathematical importance, but not really of much physical importance yet.

Zierler:

And to go back to that question of the interplay between theory and experimentation and observation, at what point did black holes become more real not simply as a theoretical proposition?

Unruh:

Well, I think the evidence was accumulating around that time that there was something going on in stars like Cygnus X-1 that we really can't explain in terms of conventional physics but can fairly straightforwardly in terms of black holes. So that would've been about the beginning of 1970. It was at that time at the Texas Symposium on Relativistic Astrophysics where Penrose gave his talk on black holes and white holes – the title introduced the term white holes. Roger got up, and he says, "Well, my title is “Black Holes, White Holes”. I have no idea what white holes are, but the best I can come up with," and he turned the transparency upside down that he had just used to describe what black holes were with time going vertically, "is the time reverse of a black hole."

White holes were introduced by the organizers of the Texas Symposium as joke (which gave a bit of the flavor as to what the attitude toward black holes was). But the relativistic astrophysics community was starting to say, "Maybe there's something here to at least think about. Maybe it's not simply a theorist's imagination, but maybe it has something to do with the real world." So I would place it at around that time-- the late 70’s. As I said, it's hard for me to gauge because I came out of a milieu in which the existence of black holes was just completely self-evident. So there was nothing surprising about their existence, nothing that had to be overturned.

The existence of the cosmological constant or dark energy were much harder for me because I'd come out of a milieu where that idea was just silly. Wheeler followed Einstein that the introduction of the cosmological constant was an error, that simplicity was too important to be abandoned easily. And I know, even in the late 1980s, when Peebles started saying "Maybe we have to take the cosmological constant seriously," I would put out my hand and say, "Jim, how can you be such a traitor?" (Yes, that was supposed to be a joke).

Zierler:

How long were you at McMaster for?

Unruh:

Two years.

Zierler:

Was that shorter than you were planning?

Unruh:

I didn't really have a plan. I just went there. It was a job. I had a job to do. I was doing research. An opportunity for going to British Columbia came up, and then Erich Vogt, who I'd mentioned before, had written me a letter saying, "You might think of applying for this." I talked to Wheeler at that time and asked his advice about the possibility of moving, little realizing that it was just at the time that he was making up his mind to move to Texas from Princeton. And so, he was quite encouraging about moving instead of staying. The problem with McMaster, I was that I was in the applied math department, which was a little bastard child. It had been formed as a department by physicists and engineers who were very unhappy with the way mathematicians were teaching mathematics to physicists and engineers, and wanted some practical mathematics taught. All this epsilon, delta kind of stuff that mathematicians liked seemed to them to be pretty useless.

They wanted the students to at least learn some things like differential equations, statistics, linear algebra, spatial vectors, etc., that might be useful to them in their physics careers. And so, they persuaded the university to for form a separate department about five or six years before I got there. There were only about seven faculty members, and they dumped in people who did not fit elsewhere. A couple statisticians, a couple of mathematical physicists, of which I was the third, a couple of computer scientists because they had no computer science department in the university at that time. And about two years after I left, the department disappeared. Applied maths teaching went back to mathematics. A computer science department and a statistics department were set up. So McMaster was sort of a funny place anyways. I think the difficult status of the department also helped encourage my move to British Columbia.

Zierler:

When did you get involved with decoherence?

Unruh:

I think right from the beginning. I didn't quite call it that, but having to worry about what happens in, for example, black hole evaporation, "Why does the emission outside look thermal?" "Because of this partner mode that's inside the black hole." So there's this entanglement between inside and outside, and if you trace out over one, you get a density matrix over the other one. And that, then, started being called decoherence. So it had been there as an interest of mine ever since who knows when. I met Wojciech Zurek in 1980 when he got his PhD in Texas in Prigogine’s group. He switched and became a post-doc of Wheeler's just at the time that I had a half-year sabbatical at U Texas. And so, we'd been doing a lot of talking together, and he was, of course, very interested in the foundational questions of Quantum Mechanics. Wheeler himself was concentrating almost completely on The Quantum, rather than gravity. Wojciech and I wrote a paper in the late 80s on decoherence when he had moved to Los Alamos. So concerns about the foundations of quantum mechanics and of decoherence were there almost since the beginning of my physics career.

Even in graduate school. I might not have called it decoherence, but it was an interest of mine. Because of all of the things with quantum mechanics and gravity, it was something that was important as far as gravity was concerned. And so, if I was going to try and understand quantum mechanics and gravity, I would have to understand that and learn techniques that, I guess, ultimately came from the quantum optics community. Many are techniques which weren't and still aren't as well-known in the generic physics community as in quantum optics. Which is, I guess, the way in which I got my connection with Marlan Scully. So, it had been an interest ever since who knows when.

Zierler:

Tell me about the Cosmology and Gravity Program, for which you were the director at the Canadian Institute for Advanced Research?

Unruh:

That started because I wanted to get Jim Peebles to UBC. Jim had spent a year in BC on sabbatical, especially in Victoria, and I felt that there was a possibility that he might be persuaded to move. Unfortunately that attempt never went anywhere. At the same time, Sidney van den Bergh, an observational astrophysicist at the DAO (Dominion Astrophysical Observatory in Victoria), was involved in a new group being formed in Toronto, who were looking for subjects to support where they could make a real difference. It was called the Canadian Institute for Advanced Research (then CIAR, now CIfAR-- pronounced SeeFar). They had no idea what they should support, except that they had decided on a first program in Artificial Intelligence and Robotics. Sidney thought, "We should have a program involving astrophysics." He had gotten in touch with a couple of the astronomers in physics at UBC about getting together to brainstorm, and one of them had mentioned to me offhand in the corridor that they were going over the next day to Victoria. And I said, "Oh," thinking of Peebles, "that sounds really interesting. I'm going to come along."

So I more or less inserted myself on that and got to know about this Institute. At the same time, the UBC Board of Governors had decided it would be good to get to know what was going in the university they were governing, and set up meetings where they would invite researchers to come talk to them informally. One of them, Alan Crawford, who owned a high-tech company in Vancouver, was also involved with CIAR from the financial/donations side, and he got me in touch with Fraser Mustard, who was the head of the CIAR. And so, I thought, "OK, why don't I get together a proposal on Cosmology and Gravity to see if they might be interested".

Using some seed money they gave me, I invited a number of people (including Peebles of course, but also Schiff) to give a little sort of Saturday seminar in Toronto to the people on the board of the CIAR. They had their first program on artificial intelligence and robots, and perhaps Cosmology and Gravity might interest them. After about a year, in which we gathered some of the top young people to a real physics workshop, the CIAR finally decided to make it their second program. And since I had been the push behind it, I was also made the first director of the program. I do know that Fraser was always somewhat suspicious that he was being sold a bill of unknown goods. He was a medical researcher (one of the key people in establishing the proof that taking baby aspirin decreases the probability of heart attacks and strokes). I think it was the interplay of an intensely theoretically driven approach feeding into highly improbable experiments that he just found hard to grasp. But he was willing to take a gamble.

And then, very fortunately, I got to know Dick Bond, who had been one of the people I'd invited to this preliminary thing as to what kinds of areas we could look in. And he's always been a real macher (a Yiddish/Low German word literally meaning “maker”-- someone who pushes the envelopes and tries to get things done). And so, he was pushing me very strongly to do things, expand things, and the program grew. And astonishingly enough, it's still going. At one point CIAR ran into funding trouble -- the donations were simply not keeping up with expenses. They were thinking of closing programs. Ours being one of the earliest programs, we had had about five years of support by that time. "They've had their chance, their kick at the can." was one attitude in the board. But it was the other programs that CIAR had started by then and their directors who said to CIAR, "No, you can't close Cosmology and Gravity. They set the standard of excellence of the CIAR. You can't get rid of them." I knew nothing about this until afterwards. That was very satisfying.

And about 35 years later, the program is still going. It has been renamed (Gravity & the Extreme Universe) and Vicki Kaspi (a neutron star observer) at McGill is now the director. After I stepped down as director after 10 years, Scott Tremaine (who moved from Toronto to Princeton at that time) was the director. And then, Dick Bond took over for 15 years, and now Vicki Kaspi. And it has been surfing the growing wave of interest in the field -- three of the most recent Nobel Prizes have gone to the field. And the program always, even though it started as a purely theoretical effort always had had a strong link with experiment. That was always a really important part of that program.

Zierler:

Were you paying attention to the earliest iterations of LIGO? Or did you pay more attention later on?

Unruh:

No, I was there, in a sense, right from the beginning, even before LIGO. What really interested me there was both the gravitational wave detection, but it was also the quantum mechanics that I was interested in. It rapidly became clear that the quantum nature of detectors (these were 1 tonne bars of aluminum, or lasers with kilo to megawatt power in them – about as far from quantum as one could imagine.) And so, when in the mid to late 70’s Kip Thorne at CalTech and Braginsky in Moscow became interested–Braginsky realized that such detectors would be limited by quantum noise. He wrote the first paper pointing out the problem and suggesting a solution. I looked at that paper and realized that his technique didn't work. This got me involved in the interplay between quantum mechanics and gravity wave detection at that time.

By 1980, the group at Caltech were also becoming very interested in laser interferometer detectors, and the quantum limits to these. I began thinking how some of the issues that I'd been looking at for the bar detectors--Braginsky's analysis was on the bar detectors--could be applied also to the laser gravity wave detectors, asking how the quantum limit applied there, and how one could evade those limits. I realized that the key was that signal itself, the gravity wave, was highly classical. Quantum gravity effects were completely negligible. Thus, the problem was looking at a classical system where the quantum mechanics of the detector was the problem. That gave one ways of reducing that quantum noise in the detectors.

I realized that the quantum radiation pressure on the mirror meant that the mirrors acted in some sense as amplifiers, and the quantum noise was a form of squeezed states. In addition, the other noise source, the shot noise of the ultimate photon detectors at the output to the interferometer was also a form of squeezed state, and that these added coherently so that by feeding into the interferometer an appropriate squeezed state, one could cancel these effects and therefore, cancel out the quantum noise that was introduced by both the radiation pressure noise and the shot noise. Just this year, 40 years later, the LIGO team has demonstrated experimentally that this works. In a sense, my career has been one of the examples that counter the the short-termism that seems to infect so much of science these days. “Impact factors” do not measure what happens 40 years in the future, where the research today becomes a crucial part of solving problems that become important 40 years from now. Two years (the “impact factor” timescale) is far too short a time to see what will ultimately be important to science.

My work on analogues, which I did in 1980, is also only now coming to fruition 40 years later. Similarly, this work on quantum noise in interferometric detectors and so forth is really coming to fruition now, 40 years later. In two years, you can measure fashions. You can decide whether skirts should be short or long in a two-year time span, but it's a very bad measure in physics.

My son, Daniel, is a Classics scholar and he tells the story of Solon and Croesus, the richest man in the world at the time. Coesus surrounded by his gold and slaves, asked Solon who was the happiest man in the world. Solon answered and it was not Croesus. “What about me?” Solon answered that it was really only at the end of life that one could determine whether or not a man’s life had been happy. Years later Croesus was defeated by the Persians. He was captured and tied to a stake to burned alive. He cried out “O, Solon, you true seer! O Solon, Solon!”, which was heard by Cyrus the Great, King of the Persians, who asked why Croesus said that. Cyrus was so impressed with the story that he spared Croesus’ life.

True value, like true happiness, cannot be determined in two years.

Zierler:

I wonder if you might explain a little more some of the quantum mechanical perspectives that are of interest with LIGO.

Unruh:

Well, LIGO is an amazing piece of apparatus. You have these great, big, 40-kilogram mirrors. One of the nice things about gravity waves is that the mass of the mirror doesn't matter. The mirror just follows a geodesic, the same path if 40kg or 40ng. So that doesn't change the coupling to the gravitational field. It moves as much as it does. If you're trying to detect a force, for example, then the lighter the object is, obviously the more impact the force has on the motion of the object. Gravity, on the other hand, has the same impact on the motion, no matter whether it's very light or very heavy.

However, outside forces (atoms hitting the mirror, stray electric fields, or fluctuations in the laser force on the mirror) have a smaller impact on the motion, the larger the mass. So you have these very heavy mirrors, which helps you in reducing the amount of noise that the quantum fluctuations in the laser beam produce in the motion of the mirrors. On the other hand, the larger the output laser intensity, the larger are the fluctuations in the laser intensity, but the smaller the fractional fluctuations in the output that you use to determine the difference in the lengths of the two arms of the interferometer. If your purpose were to measure the position of the mirror itself, then there in an optimum intensity for any given mirror mass, where those two kinds of quantum noise are just equal. This is called the standard quantum noise. It is large enough that it hides almost all of the expected signals.

You could try and make the mirrors heavier, but that runs into practical difficulties in supporting the mirrors. You could make the laser beam stronger, but that soon gets to the point where it starts frying your mirrors, which is not a good idea either. However, the two noise sources turn out to come from the same place, quantum vacuum fluctuations coming in, not with the laser beam, but coming into the output of the interferometer, even when no light (the vacuum) comes into that port. Even nothing is noisy when mixed with the laser beam. But by making that input noisier (technically it is called squeezing the vacuum) you can actually decrease the quantum noise coming out from the interferometer. This extra noise that you feed in makes knowing the position of the mirror harder, but cancels out if you want to know what the gravity wave amplitude is.

I showed theoretically that this could be done back in about 1980. It was questions like this, how can one get rid of the quantum noise in measuring the gravity wave, that fascinated me. This work also got me into the community of people who were working on and building LIGO.

I didn't get involved in this huge, astonishing engineering tour-de-force that they carried out in actually building LIGO. It always amazed me. I remember going to Caltech and having Ron Driver (who Kip persuaded Caltech to steal from Glasgow) showing me around their little model 40-meter laser interferometer and their tests of noise isolation for that test instrument. The best thing that they had found at the time for isolating the mirrors/lasers from seismic noise were these little foam-rubber toy cars which they squeezed between steel plates. Those poor little foam cars sitting all around provided the damping for seismic isolation. It’s like going into a high-tech lab and finding everyone playing with rubber duckies. That morphed into scientific and engineering wonder we have today, 40 years later. So yeah, I was involved from very early on, but primarily trying to understand its quantum mechanics and seeing how one could manipulate quantum mechanics in order to make this kind of interferometer detector possible.

Zierler:

When the announcement of the detection was made a century after Einstein, emotionally, scientifically, what were your reactions? What did you feel at that moment?

Unruh:

I thought, A) it was expected, and B) I was just absolutely astonished that it was possible. Because I knew how weak these signals were. It was just so incredible that they could actually see them. The fact that this gravity wave coming by moved these mirrors by just one part in 10 to the 22. Well, it's impossible to even think of one part in 10 to the 22. That's a number that's so small that you can't even wrap your brain around it. And this detector was finally was able to see it. The fact that those gravity waves were there was of zero doubt to me at any time.

So from that point of view, it was, "Yeah, OK, I've known about this for 50 years of my life." [laugh] But the fact that it was actually able to be detected was astonishing. There is such a difference between that kind of faith in one’s theory, and actually “seeing” it. But the biggest surprise was that not only did you see the gravity waves, they also clearly came from huge black holes. You saw an effect that could only have come from black holes larger than anyone had thought possible. Thus, one not only saw the gravitational waves, one also got absolutely incontrovertible evidence that black holes exist.

They weren't as large as one of these black holes at the center of the universe, but all you can see is the orbits the orbits of the stars around them. Those stars are millions of times the black hole size away from the black hole. "There must be something in there. The only thing we can think of that is so big but does not shine is a black hole." Well, here, there's just absolutely no question. There's no other way in which you can explain what's going on is that these waves were created by two very large black holes orbiting each other, 30-100 times a second. The gravity waves are coming from a distance of only a few times the size of the black hole away from it. Some people still try to come up with other explanations (and it is always good to test one’s assumptions).

But then people also come up with alternative explanations for the evidence that the earth is spherical. The observations of gravitational waves kill two totally disparate birds with one stone–gravitational radiation exits, and black holes exist. But the third thing that really hit everyone with any interest in LIGO was the astonishment that you had these humongous black holes, 30, 40, 60-solar mass black holes out there. Everyone had expected 1, 2, maybe a 5-solar mass black hole. Nobody expected these sizes. In preparing for the detections, numerical relativists created gravity wave templates, the predictions of the form of the gravitational waves, from various sizes of black holes. They needed these because they expected the wave strength to below the noise at any time, and that they would have to look for signals by matching the noisy signal with the expected waveform. If you look at a catalog of these templates, you see a large number clustering down around one to five solar mass. As you look for larger masses the catalog gets sparse, which a few examples up to about 100 solar masses. (“Might as well throw a few in there just in case, but don’t spend much time calculating anything about them because that would be a waste of time -- such large ones simply will not exist”).

They basically said, "Well, we put them in there just in case." But nobody believed at all that that part of the template space would be of any relevance whatsoever to the first detection. And there on the first detection the sizes were almost at the top right-hand corner of the mass-mass plot they had made templates for. Fortunately, they didn't really need templates. That first detection was so big that you could see it by eye above the noise. Search was not necessary. The signal was bigger than the noise for each cycle of these black holes around each other. And this occurred just one week after they turned the improved detector on.

Zierler:

Just for context, with all of this discussion of black holes, we haven't even talked about the image from the Event Horizon Telescope.

Unruh:

Right. That was amazing. However, it didn't hit me as much.

Zierler:

Why not?

Unruh:

I think partly because I'd lived with it so long. In about 1978, I made a movie. I was giving a large public talk at UBC on black holes. And so, with Maurice Pryce at UBC and and Leigh Palmer at Simon Fraser University, we made this film where I took about the 300 brightest stars in the sky and showed what the sky would look like if you were in orbit around that black hole. And you see this black hole swim through all of the displaced images of the stars, the stars swirling around the black hole, etc. So the idea that black holes could focus light was something that I had lived with and seen (of course in simulations) for 50 years. When I first saw those movies that we produced, I was just blown away. But that was 50 years ago.

So I'm afraid I was a bit more blasé. The ability to use the earth as a humongous single telescope to see micro-arc-second structure in the center of the galaxies there was definitely astonishing. So, for me, it was more their ability to do it, than what they saw that was astonishing. Part of the problem with those experiments is that, as you're working on your experiments, you've got to keep persuading people to fund you. So you've got to keep making talks and showing these images of, "If only we had this thing, this is what we'd see." So I'd seen a number of pictures of what they would see “when it all worked” before I saw the the real thing, and those images looked almost identical to the real thing. I’m sort of going, "Been there, seen that." That is of course incredibly unfair to the the people who created that amazing tour-de-force, but you asked for my reaction.

Zierler:

It's well-appreciated that your research in quantum mechanics has had important effects on our understanding of computation. Have you been involved specifically in those advances, or is it more other people looking at your research and applying it to computation?

Unruh:

I got involved early on, through Wojciech Zurek, who got interested very early on. He invited me to a conference in Santa Fe, where Peter Shor gave one of his early talks, and Charlie Bennett was there, who I'd known for 15 years already through my interest in the foundations of quantum mechanics. And Ralf Landauer was there who was so important in understanding the physical basis for quantum computing. I had also heard Feynman’s lecture on quantum computing a few years earlier. But that Santa Fe workshop was what got me interested. Having worked with Wojciech on decoherence, I was very concerned about its effect on quantum computing. So I wrote a paper on the effect of decoherence on any such computation, and argued that it would destroy the coherence needed to create a quantum computer on an incredibly short time scale. Not that others were not also worried, but my paper was the first. That made me pretty doubtful that quantum computation was going to work. It also made me learn what was happening in the field, and I gave a series of lectures at UBC explaining things like the Shor factoring algorithm, Grover’s search algorithm, reversible classical and quantum computation, Landauer’s erasure theorems.

On the basis of that paper, I was invited to give a talk at an NSA sponsored workshop on quantum computing, which was great, I got to see the big black cube which is the NSA headquarters (from a distance only). And at the boundary of the restricted area, they've got this little museum so that the hoi polloi could go in and see something about early cryptography. It also got me interested in cryptography, especially algorithms like RSA. I do computer kinds of stuff as a sort of hobby. I administer a small group of computers around here as well. And learning about cryptography from that point of view and the Shor point of view was also fun. But it wasn't so much really doing research in it, except for that one paper, and kibitzing. One of my former post-docs, Raymond Laflamme, became very heavily involved when he was at Los Alamos in quantum computing.

Anyway, I gave a talk at that conference on my concerns about the effects of decoherence on the possibility of creating a quantum computer. About a year later, my concerns about the effects of decoherence were essentially alleviated , when, independently, Peter Shor, and Andrew Steane showed that one could carry out error correction even on quantum systems. This was surprising to me because classical error corrections tends to rely heavily on erasure -- using dissipation and entropy to get rid of errors. And erasure and dissipation themselves produce decoherence and destroy quantum computation. They however showed that it could be done completely reversibly, and while maintaining the quantum coherence that is absolutely crucial to quantum computing.

Zierler:

To get back to your interactions with Marlan Scully, what have been some of the advances in instrumentation or technology more generally that made you more involved in the field of quantum optics?

Unruh:

Well, the involvement with him was far more theoretical. He recently got interested in things like acceleration radiation and Hawking radiation. He's been interested in that for many years and would keep bugging me about it. He came up with some interesting ideas about what happens in black holes, a pseudo-equivalence principle. In my case, what I'd looked at in my work on detectors, both in the acceleration and also the black hole case, was that if you're a constant distance away from the horizon, and you have, either in the acceleration case with the Minkowski vacuum, or in the black hole case, the sort of vacuum that you might expect when matter collapsed to a black hole, that you'd see thermal radiation.

He and a group mainly at Texas A&M (including my old friend Steve Fulling) had been showing that if you had, instead, what's called the Boulware vacuum (in the black hole case) or the Rindler vacuum (in the flat space-time case), (they are the states in which one sees no particles if one is held a fixed distance away from the horizon) that if you drop something in (like a charged particle, or a particle with say magnetic spin) , then from the point of view of the outside observer, that object seems to be accelerating. It is not accelerating in and of itself, but relative to you it is. Then, due to the interaction with the Boulware or Rindler vaccum (which is what you would consider the vacuum state) that falling object will emit radiation (not detect radiation, but emit radiation) which looks to you like it has a thermal spectrum. It is a rather weird sort of equivalence principle.

He persuaded Texas A&M to make me a Hagler Fellow which brings me there to work with them on a yearly basis. It has been a fruitful collaboration. I am learning many more of the techniques of quantum optics, and they are learning my point of view about how the Hawking radiation is created, and why an accelerated detector becomes excited as though immersed in a thermal bath.

I have also been involved in experiments, primarily with Silke Weinfurtner, who was a post-doc of mine, and is now in England at Nottingham. She is the head of a UK Research and Innovation program, qSimFP (Quantum Simulators for Fundamental Physics) a large consortium of physicists, both theorists and experimentalists, working on issues of quantum mechanics in cosmology and gravity in an analog context. And one of the proposals is to carry out an experiment to measure an analogue for acceleration radiation in a Bose-Einstein condensate. However, instead of dragging a detector through the condensate in its ground state, you use a moving laser beam as a transformer, which transforms quantum density fluctuations in the Bose-Einstein condensate into fluctuations in the light, which are far easier to measure. Accelerating the spot where the laser beam intersects the BEC in a circle give one the ability to measure the Bell-Leinaas quasi-thermal effect. So it's being able to actually see an analogue of the acceleration radiation ideas.

Zierler:

And best case scenario, what does the experimental verification exactly look like with this?

Unruh:

Basically, you have this Bose-Einstein condensate in a thin sheet like a flat dinner plate. You run a laser beam through that with the intersection being in a single spot, and you have that spot run in circles around the BEC. Looking at the fluctuations induced in that laser beam downstream, and they should have a thermal-like spectrum. And looking for that thermal spectrum is what we hope to accomplish. It turns out to be much more difficult than the black hole ones. My 1980 paper, where I suggested one could do experiments where one could see Hawking radiation in similar analog systems. Experiments are actually being carried out to see this analog Hawking effect. However the acceleration radiation case is experimentally more difficult, and includes many of the ideas, especially regarding the quantum limits, from LIGO. Instead of a spatial interferometer where the two arms of the interferometer go in different directions in space, in this proposal the two arms travel on different trajectories in frequency space, but the analysis is almost identical to that of LIGO.

There have been a number of stimulated emission experiments. One of the first was with Silke and me, and with a civil engineer, Greg Lawrence. In that case we looked at the surface waves in water, with the flow arranged so that analogs of both the white and black hole horizons to those surface waves were created. We stimulated the Hawking radiation by launching waves toward the horizon (using the white hole horizon was easiest) and measuring the scattered radiation. We found that this stimulated effect produced a thermal spectrum, just as predicted by Hawking for black holes) and by me for analog systems. Recently, Jeff Steinhauer in Haifa has been carrying out experiments in a BEC and seeing the quantum entanglement of the radiation being given off on the two sides of a horizon. The radiation is also consistent with a thermal spectrum. So analog Hawking effect experiments are already being done. Acceleration radiation experiments still isn't there, and so that's one of the other things that I've been working on. But to understand and design the experiment one really has to understand quantum optics, and how one detects fluctuations in laser beams, and what an interferometer really is.

It is fascinating that many of my interests over the past 40 or 50 years really seem to be coming together in my current research.

Zierler:

Now that we've worked right up to the present, for the last part of our talk, I want to ask some broadly retrospective questions, and then we'll end looking to the future. So first, I can't help but contrast your responses, where in graduate school, your research was totally aloof from experimentation and observation. It was a different world. And here you are, 40-plus years later, and you're deep into the world of experimentation. So I wonder if you might sort of reflect on the larger implications or what we might learn from this, in terms of the development of the field over the course of your career.

Unruh:

Part of the problem, of course, in general relativity was that there were almost no experiments that one could think of doing. From my own trajectory, I think at least part of a big influence was John Wheeler, who always wanted to think about things physically. He was willing to dive into sort of very, very esoteric mathematics and so forth, but always wanted to understand it in a much more intuitive fashion, partly because that was what he was so good at. And so, I think I was inculcated with that whole philosophy very strongly. And I'd also been interested in doing things, getting my hands dirty if you will, ever since I was who knows how high. Building transistor radio kits. Fixing my bicycle. While I was at Princeton, I built an amplifier kit for my hi-fi system. So that kind of getting my hands dirty was certainly not foreign to me. I remember in the 1980s, sort of proudly saying to someone, "Nobody is ever going to get their hands dirty in my line of work," and promptly proceeded to do so.

So for me, I think it's important to carry both of them at the same time. And it's a shame, though I don't know how to solve it, as I mentioned earlier, this business of teaching. How in the world one does one teach undergraduates the importance, and the excitement of both. Part of the problem for us professors, is that teaching theory is far easier. You have 300, 400 years of people working on theory to fall back on. When you become stuck for something to say in class, "Well, I can now go ahead, and prove such-and-such," and spend the next 45 minutes filling up the class time by going through the proof because that requires almost no thought on my part. How one does that in an experimental situation is far more difficult. So how one brings experiment into one's undergraduate education much more than one does now, I'm still struggling with. And I think the whole profession is still struggling with that.

So I think that's one of the things that was important. The field of gravitation physics itself, as you say, it was primarily a theoretical field. The only way in which one could get ahead was to do theory because the experiments didn't exist. And one of the things that has surprised me is, that at Harvard and MIT, general relativity was considered somewhere way down there. They didn't teach it.

Zierler:

This gets me to my very first question about the ups and downs of GR, and how you were cocooned from that at Princeton.

Unruh:

Princeton was incredibly strong in theory. And similarly, Syracuse around Peter Bergmann was very strong with theory. The Boston area tended to be weaker. And yet, some of the most innovative experiments came from Boston. Pound and Rebka measuring the link between time and gravity using the newly discovered Mossbauer effect. Robert Vessot using a H-maser to measure the same to an accuracy of 10-4 by launching the maser to an altitude of 10000 miles. Irwin Shapiro discovering and measuring the time delay effects of GR for light passing near the sun. Ray Weiss, the key figure in the development of the laser interferometer gravity wave detector. And all of them having very little theoretical support from their colleagues. So why were some of the most innovative experimental ideas coming out of a place where the most innovative theory didn’t exist? I don't understand. And it's something that needs looking into; trying to disentangle the history of what was going on in general relativity and the relation between theory and experiment. This brings me to another project I am participating in here at UBC, namely the setting up a gravity archive and a gravity initiative.

There is one of those people that only England can produce. When Michael Wright was a graduate student, he decided that his purpose in life, was to keep track of what was going on, first in some areas of Mathematics and the philosophy of Mathematics and then in General relativity. He would go to every meeting he could and would record it, audio at first, and then in the 1990s, he brought in video to record the conferences that he could go to. So, he has thousands of hours of recordings of what was going on in the field as it was happening from 1970s on. He concentrated on England in the early stages, but afterwards, traveled around the world, mortgaging his house in order to be able to do this. It is an incredible treasure-trove of history. The plan is to set it up as a cataloged digital archive that anyone can access. Not just a transcript, but also archiving the actual video and audio, so that somebody can go back to those original recordings and see, "OK, at 17 minutes and 37 seconds into this recording, when Roger Penrose first talked about twistors, what was it he said, and how did his ideas develop." Our hope is that anyone will be able to use this, and elucidate what happened to the field intellectually, and how did it change.

A lot of these tapes are on old audio tape. I don't know if you've ever played with audio tape, but as it gets old the binding between the carrier of the information, the iron oxide, and its binding to the plastic tape gets weaker and weaker. I learned that myself. I had an old tape recorder, and I recorded some shows. 20 years later, I went back to listen to them, and the erase head and the recording head would get all crudded up with rust that flaked off the tapes. So these recordings have to be transcribed into digital format, which we hope will last a little bit longer, and being cataloged so one knows what is there.

And there are other treasures out there as well. John Wheeler was someone who, whenever you talked to him, he had these huge notebooks, about 14x10, hardcover notebooks. And as you would talk to him, and he would be recording the gist of the conversation into the notebooks. If he came across an interesting idea he could put it into the book, gluing preprints and his annotations into the notebook.

There are something like 60 of these notebooks, each of them about 200 pages long. Well, they're sitting in the Philosophical Society of Philadelphia. Yes, you can see them by going there and getting permission, or if you happen to be a member of the society, you can just go there on spec and try and find in this 50 feet of shelf space containing everything from the receipt for his curtains in his house to these notebooks. You can't do anything with that. It needs to be online and properly cataloged. I got interested in some of the history of black holes, and in particular, the understanding of the horizon of black holes. From Charlie Misner I learned that Martin Kruskal and Wheeler would get together occasionally for lunch. And Wheeler was still involved in the bomb development and reactor design, and Kruscal in the creation of plasma fusion energy reactors. Both were top secret, and they thus could not talk about their work.

So they would talk about peripheral issues. As I understand it, one day, Kruskal says, "I just did this little calculation a few nights ago," and explained it to Wheeler a bit, and Wheeler scribbled it into his notebook, as he did with everything. But the importance didn't hit him at all until about a year later, when Charlie Misner mentioned that he and David Finkelstein (Georgia Tech U) had been doing some work on horizons of black holes, and Wheeler remembered this conversation with Kruscal. Kruskal was on sabbatical in Paris, so Wheeler went back into his notebook, repeated the calculation, wrote up the paper, sent it in to be published with only Kruskal's name as author to PhysRev. The first that Kruskal knew about the paper was when he got the Galley-proofs with his own name as only author.

He immediately suspected where this had come from. But what is in those notebooks? I would love to see it. Well, I'm not going to fly to Philadelphia and spend my time there to do it. And I have no idea where it is in the notebook so I cannot ask them to make a copy for me, since I have no idea where to tell them to look for the appropriate entry. They could hire somebody to go through all the notebooks looking for this. Well, come on. That would be more than me flying there.

Zierler:

Sounds like we need a major digitization project.

Unruh:

Right. It's stuff like that that really needs to be preserved, so that people can go back in history and answer some of these historical questions, which can be as fascinating as the physics itself. One item about the history of my field that I only learned about extremely recently was part of the history of Einstein-Rosen bridge. That paper turns out to be historically interesting from two points of view. One is, they talk about this connection between two infinities via the throat of a wormhole. Well, it turns out that Flamm had done that in the year 1916, within a year of Schwarzschild’s publishing his solution. Flamm had shown that if you take a constant time slice in the Schwarzschild metric, that you get this throat which goes through to the other side. Spatially the metric is exactly that of a double trumpet horn. Flamm didn't make a big deal of it, and the paper is not very well written, but it's really all there. And I'm damn sure Einstein saw that. But there's no reference to Flamm in the paper that Einstein and Rosen wrote 20 years later. The second interesting thing in that paper is that this metric which we now usually call the Rindler metric is already in that Einstein-Rosen paper as a footnote. They write down the metric of for an accelerated observer, if you will, and say, "It is known that...". By whom? Unfortunately in physics, if somebody gets an insight and even publishes it, that doesn't mean that just because they got the insight, it's remembered.

And sometimes, it has to be reinvented three or four times before it finally sticks. And being able to tease that kind of stuff out, I think, is fascinating. My interest in the history of the field has probably been primarily triggered because of Lee Gohlike in Minnesota. His primary occupation is renovating old 1920s and 1930s Mercedes cars. And he has and has had access to the original drawings, so he can reproduce lost or damaged parts to exactly the original factory specification and drawing of these cars. This means he can reproduce even the exact metallurgy that was used in order to build the various parts. But besides old cars, he's really, really interested, also, in the history and philosophy of physics. So every year, he holds a meeting (The Seven Pines Symposium), which I started getting invited to via Bob Wald, on the history and philosophy of science at a inn that Lee owns in eastern Minnesota.

And so, you have these historians and philosophers of physics, who really know their physics. One often has an image of philosophers, "Yeah, they go off and philosophize, but they don't know what they're talking about in general." Well, these are not those philosophers. These guys really know the history and philosophy, and the physics a hell of a lot better than many physicists do. And so, being involved in that group year after year is fascinating.

Philip Stamp at UBC is the key person pushing a gravity archive project, and looking for people who would have the money to support it. I think it's great for the history and philosophy Charles W. of the field and being able to see what was really going on at the time, rather than having some old man like me remembering what happened, which is always iffy.

Zierler:

I couldn't disagree more strongly.

Unruh:

Everybody has a great ability to edit their own history.

Zierler:

As Socrates tells us, the more you know about something, the more you know you don't know. I wonder how you might apply that dictum to quantum mechanics.

Unruh:

It's two-sided. I get really, really tired of people who tell us that we don't really understand quantum mechanics. Because it's been an astonishing theory. It was developed to try and explain this very limited little thing, the spectrum of atoms. It got generalized so rapidly to new situations and effects, like spins, for example, which were completely different kinds of animals than anything that they had looked at. It got applied to quantum field theory which is a very different kind of theory that quantum mechanics. It got applied to non-linear Charles W. quantum field theories, Yang-Mills theories, etc. And in each one of those cases, it took a lot of work, but in many ways, you look back, it was pretty straightforward.

And to say you do not understand something, when you can go ahead and use it appropriately and correctly in all kinds of brand new situations, can generalize to absolutely new situations with great facility seems to me to be a gross misuse of the term “not understand”. It is your understanding of the word understand that seems to me to be the problem. If you can do all of that, then what in the world does “not understand” mean? I think it's not that we don't understand it, but that we have this prejudice as to how one should describe the physical world and how one should understand it, how one should intuit the physical world. As Einstein said, one of the purposes of doing physics is not just to be able to do calculations and make predictions, but to have a gut feeling about what's going on. And I think it's development of that gut feeling that people have had such trouble with and is what they mean when they say, "I don't understand it."

And I think the developments over my career, if you will, of people being willing to look at simple situations and really understand those situations and develop an intuition for them has sort of broken through that problem of, "I don't really understand it." Sometimes, I think people just start off looking at it from the wrong point of view. As I mentioned, I like the Heisenberg representation far, far better than Schrödinger, in part because it makes clearer distinction between dynamics and, if you will, knowledge, which in Schrödinger, gets all tangled together and completely confusing people. For example, this whole business of “reduction of the wave packet”, and this notion that many physicists seem to carry around with them that the wave function is this thing that sits out there in the universe and actually has physical presence out in the universe, is a problem that primarily arises out of the Schrödinger representation. In the Heisenberg representation, it's absolutely clear that you have things out there, which are described by these operators, in a very strange way, but we know how to handle that.

But because it's a probabilistic theory, things can happen without prior cause, as you would expect from a probabilistic theory. So once you have that extra knowledge, something has actually happened without prior cause, how do you incorporate that new knowledge that into your view of the future world, or even the past world? Cosmology is all about making observations now and talking about the past, not making observations now and talking about the future. And quantum mechanics handles that without any problem if you think about it in just the right way. Yakir Aharonov has been trying to teach us that for 40 years, and everybody has been totally confused by him.

So that development of a good quantum intuition is, I think, the thing that we're most seriously lacking. And finding sort of simple situations that you can examine, understand well enough that you can incorporate it as part of your intuition is where we're still lacking. I would compare it a little bit to the philosophical position that happened with Newton, where he developed this theory, especially of gravity, action at a distance, and so forth, which it took people 100 years or so to really become comfortable with. This idea that you didn't have to have vast vortices between the sun and Jupiter, as Descartes had, that the sun, in its turning, did not produced these fluid vortices in the ether, which then caused Jupiter to follow its path around the sun, was hard to accept.

And people would say, "But you've just stuck in this extra bit of magic. How in the world can the sun do anything to Jupiter just like that? I surely need an explanation for why the sun does that to Jupiter." And Newton says, "No. You just need that there is a radial force, falling off as 1/r2, that the sun exerts on Jupiter" And over 300 years, we've gotten extremely used to that. We no longer think it's miraculous or weird. And then, of course, general relativity comes along and says, "No, no, no, no. There's not something mysterious. What happens is, the sun causes time to change in its vicinity, and that change in time, together with Jupiter traveling along a straight line, that is what causes Jupiter to go around the sun." [laugh] So in a sense, both of my fields have had these problems with intuition. How does one develop an intuition? Because it just doesn't fit in with the mechanistic intuition that people developed over the 300 years from Newton ‘til the present.

Zierler:

And of course, this thinking is very much a legacy of Wheeler and his approach.

Unruh:

Yeah.

Zierler:

Last question. Looking to the future, what do you want to accomplish? What is next for you?

Unruh:

I don't know. I'm having too much fun just playing around, understanding smaller problems. It would be nice to get some deeper insight into some of the huge questions, and I hope that some of the small problems that I'm huddling around with might help produce some answers to those bigger questions. But I don't have any dreams that what I do now is going to directly answer them. All one can hope is that what I do will act as some goal to some way of inspiring somebody who's younger than I am to see a way out of the morass. So it's sort of like, I guess, my grandfather moving to Canada. Despite all of the problems, "I'm doing it because I want to live another 45 years." I'm doing it because I want to understand a few more of the problems like the ones I've understood in the past. John Wheeler had a favorite quote, a Grook by Piet Hein:

 

I’d like to know

what this whole show

is all about

before it’s out.

 

It's still too much fun for me playing with the world.

Zierler:

I love it. Bill, it's been an absolute pleasure spending this time with you. I'm so glad we were able to do this. So, thank you so much.

Unruh:

Oh, you're welcome.

 

What is now proved was once only imagined-- Wm. Blake, Marriage of Heaven and Hell