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Credit: Geoffrey Albrecht
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Interview of Andreas Albrecht by David Zierler on 2021 February 10,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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In this interview, Andreas Albrecht, Distinguished Professor of Physics and Director of the Center for Quantum Mathematics and Physics (QMAP) at the University of California, Davis, discusses his life and career. Albrecht describes the growth of the department since his arrival, his affiliation with QMAP, and the broader effort to integrate more mathematicians into the field of cosmology. He recounts his childhood in Ithaca as the son of two PhD scientists and family sabbatical visits to Santa Cruz and to the Soviet Union. Albrecht describes his budding interests in physics in high school, his undergraduate experience at Cornell and his early exposure to the ideas of Robert Dicke and Alan Guth. He discusses his graduate work at Penn and the circumstances that led him to become Paul Steinhardt’s mentee in cosmology. Albrecht conveys all of the excitement surrounding inflationary cosmology in the early-mid 1980s and the opportunity that led to his postdoctoral appointment with Steve Weinberg’s group at the University of Texas where he became interested in cosmic strings. He describes his subsequent postdoctoral appointment at Los Alamos where he worked with Wojciech Zurek and where his carpools with Geoffrey West proved to be a formative intellectual experience. Albrecht explains his decision to accept a staff position at Fermilab and the contemporary advances in cosmic strings scaling and why primordial nucleosynthesis was uniquely data-oriented relative to other fields in cosmology. He describes his subsequent faculty position at Imperial College in London and he emphasizes the productive and tight-knit cosmology community across the UK. Albrecht conveys the importance of the cosmic microwave background (CMB) experiments and how his ideas of equilibrium cosmology had changed over time and where the term “Boltzman Brains” originated. He describes how UC Davis was rapidly growing and how the opportunity to build a cosmology group was appealing to him. Albrecht explains the origins of his “arrow of time” concept and why this resonates with the broader public’s interests in the universe. He conveys the existential difficulty, and possible impossibility, of developing a credible theory of the beginning of the universe. Albrecht reflects on the spiritual dimensions of cosmological unknowability and the significance of the anthropic principle, and he discusses his efforts as department chair to enhance diversity in the field. At the end of the interview, Albrecht discusses his current work on decoherence and einselection, and he explains why avoiding prejudices in one’s scientific sensibilities is both singularly difficult and key to unlocking future discovery.
OK, this is David Zierler, Oral Historian for the American Institute of Physics. It is February 10, 2021. I'm so glad to be here with Professor Andreas J. Albrecht. Andreas, it's great to see you. Thank you for joining me.
Yeah, thank you for this interview. I'm looking forward to it.
So to start, would you please tell me your titles and institutional affiliations? And you'll notice I pluralized everything because I know you have more than one.
So I'm a Distinguished Professor of Physics at University of California, Davis in the Department of Physics and Astronomy, and I'm also Director of the Center for Quantum Mathematics and Physics, also at UC Davis. And this is a center that bridges physics and mathematics. So it's beyond just one department.
Now, do you have an affiliation with the UCD Cosmology group as well?
Yes. That's a much more informal designation, so that's just within the Department of Physics and Astronomy. We have different groups. And yes, I'm in that. Those things are seen in a little bit looser form, but I actually was recruited to UC Davis to create the Cosmology group. And that's been a lot of fun. It's a thing that has taken on a life of its own. It was an expansion of the department, which was then called the Department of Physics. And the idea that cosmology allowed us to expand in a particular exciting area. In recent hiring, the cosmology group has sort of spread its wings and said, "Well, we don't want to just be cosmology, we want to be astrophysics." And the hiring has been much more broad. It culminated in renaming the department Physics and Astronomy. I'm very excited to have all those people as colleagues.
Now, was QMAP part of your appointment in coming to UCD?
No, it didn't exist until a few years ago. One of the things that really did attract me to UC Davis was an environment where there was a lot of, I would say, adventurous ambition in terms of building new programs. And, of course, to start with, it was what attracted me to build the Cosmology group. That sort of became the template, which the department repeated again and again, and the campus repeated again and again. And it's interesting, when I was being recruited, somewhere in the whole process, someone showed me, "Oh, here's an idea that UC Davis has," and it was for a performing arts center. And someone had said, "Nowhere in Northern California is there a really awesome performing arts center. And our plan is to have one." And at that point, it was just a sketch on a poster and they were raising money.
The Performing Arts Center now exists (it is where I used to park my car [laugh]). Back then it was just talk, but to me, it was this really exciting vision. Why not try something really great? And why not think bigger than anyone expected us to think? So now, if you talk to artists, they debate which is better, our Mondavi Center, or the Disney Center in LA. Some think one, some think the other. But it's an awesome performing arts center. It's better than anything in San Francisco. And people come from all over to attend events there. That's the kind of spirit I love at UCD. So it's not just about physics or cosmology, but it's a spirit. And QMAP emerged from that same kind of spirit, where Provost Hexter, at the time, said, "We have all this hiring to do. Let's set aside a small percentage of the hiring to do something that no one would ever do in the normal scheme of things." So he ran a competition.
Individuals had to put together ideas that were different from what they would normally get from departments. People in the Physics Department and people in the Math Department had long been talking about how to bridge those areas. And I would say, already, there were more foundations for such a bridge than in a typical place. But it's quite a project to make a bridge like that. And somehow, it was hard to get things coordinated. Different people were ready to make more of an initiative at different times. And so, having this program from the provost really gelled a lot of that thinking. I was department chair at the time of the Physics Department. There were numerous proposals coming out of the department. And I saw my job not to prefer one or the other, but to nurture the success of as many as possible. At that point, I didn't see myself, certainly, as a particular leader for that proposal.
But they put together a really exellent proposal, and it was successful. It was awarded four faculty positions. Then, I was involved in the recruitment as a department chair supporting a search committee. And we hired really awesome people. When I was finished being department chair, people in what they ended up naming QMAP twisted my arm to become director. For me, five years as department chair was enough. I was ready to forget about administrative duties. But their vision was and remains so compelling that I thought, "Well, this is a way I can contribute." Fortunately it's much less work than the department chair.
Now, is QMAP administratively shared by the Department of Physics, Astronomy, and then the Department of Math? Or it's its own center, it's independent?
So it's complicated. My vision for what I could do as director is, I didn't want to spend my energy manipulating administrative structures. It's just not my thing. And so, as much as possible, people are faculty in one or the other department. And as much as possible, all the grant processing, promotion cases, all of that are handled in the departments. And the vision I have for my role as director is, how can QMAP add value? So I don't see myself as duplicating another department's structure. That may end up being a really great idea for QMAP eventually, but it'll be another director that does that. That's not my forte in terms of what I can contribute. Our big accomplishment so far is brand new space for all of us that's really beautifully, inventively created to nurture interactions and sense of community. It sort of opened over the summer, and we've not been able to use it yet due to the pandemic. I's a bit magical that it was completed despite COVID, despite endless other things. It's quite something that that came about at all. It’s very exciting.
Substantively, I can't help but think the desire to bring more mathematicians into cosmology research, it just reminds me of way back in the very early days of inflation, when Alan Guth decided to bring Harry Kesten on board. I wonder if intellectually, there's a heritage there where you recognize implicitly, mathematicians really do need to be involved in a substantive way if there's going to be advances in these fields.
Well, I think the relationship between physics and mathematics is just this amazing, rich, and complicated story. I think neither field can really manage without the other in some way. Certainly, physics can't manage without mathematics. And yet, certainly today, the fields have very different personalities. So, we're trying to bridge that. The relationship between the two fields is more than any one anecdote. It's the whole story of physics. You go back to Newton inventing calculus. Whatever you name it, the relationship is very profound. Stories have it that when mathematicians invented non-Abelian symmetries, they said, "Here's something finally that physicists will never use." And now, it's at the foundation of every theory of particle physics.
So it's just a remarkable relationship. I see QMAP as an experiment in terms of what we can do with that relationship. I think in terms of my priorities, in terms of helping build institutions and community in science, it's about supporting really awesome individuals. And that's much, much more valuable to me than designing an agenda, or a formula, or a detailed vision of, "This is exactly where we're headed." The thing I know for sure is that QMAP has some really awesome individuals in it. So in my view, that's the most fundamentally important piece. And then, we'll see where we all go with that.
Before we take it back to the beginning, I'd like to ask a very in-the-moment question. And that is, the going assumption for a theoretical physicist is that presumably, this time of the pandemic has been a very productive time, just because the mandates of physical isolation are such that you might have time to work on equations or papers that you might not have been able to. So to what extent is that true? Or alternatively, to what extent has the lack of physical, in-person interaction, like the beautiful space that now exists and is empty for QMAP, really stymied the science and research that you otherwise might've been looking forward to this past year?
Speaking very personally, from my own trajectory, it's been a real plus, actually. It's been welcome. I would say specifically for my science, it's been a plus, and also, in my larger sort of psychological space, it's been a plus. Because I think there's something about our culture that really embraces busyness in an unhealthy way and reveres busyness in an unhealthy way. Even if an individual feels empowered to find their own way through that, as I kind of do, it's still an influence. It's still part of the environment. And so, yeah, it's helped me create a lot of different kinds of space for my own work and for my own reflective side. But at the same time, it's really depressing not to be interacting with people. So I think both are true. I actually think that's one of the really remarkable things about science generally is the complicated relationship between sort of hiding away and doing your own thing, and then needing the interactions with other scientists.
At the very minimum, the interactions help us establish what’s good about what you've done. More generally, those interactions tend to improve one’s work either at the level of helping you see that you've gone down the wrong path, or perhaps helping you tune up the idea to make it stronger. So I think maybe the way to say it is that we're lucky, as scientists, to find a path through the current conditions that's beneficial. But we're still missing the other pieces. I've had numerous conversations where I've said to a colleague, "We're in the group where we always feel lucky to have a little bit more time on our own to think about things." And that's definitely true for me. But different colleagues have different styles.
I know some of our QMAP members have kept their seminars going every week straight through. It's all on Zoom of course. Personally I've found a different mix. One of the pluses is that, in my weekly research meetings, we can invite anyone we want from anywhere in the world. Certain things have been shaken up. And the theoretical physics community generally was already very strongly situated on Zoom before the pandemic. There was a lot going on. A lot of people were very comfortable and ready to use Zoom for their collaborations. Experimental physics, too. Physicists were very involved with these remote tools, just because physics and other sciences are really global. Another wonderful thing about science is it kind of transcends boundaries and political tensions.
You read these stories about the Coronavirus and how part of the story is the difficult relationships between China and the West, and particularly, the US. But in the meantime, the Chinese scientists had sequenced the thing, and the genetic sequence was all over the world before these clowns in charge could mess it up. And that's a particularly poignant story about science in the middle of something larger than just science. But that same kind of story repeats itself again, and again, and again.
Well, let's take it back all the way to the beginning. I'd like to start with your parents. Tell me a little bit about them and where they're from.
So my parents were both PhD scientists. My dad was a chemist on the faculty at Cornell. My mom was a biochemist. I believe she went by her maiden name, Genia Solomon, in her professional life. This could get very big, relationships with parents. There's a lot to say. But here's something I wish I had asked her more about. My mom pursued her career as a scientist through a post-doc at Harvard, and then completely let go of it to raise a family. I'm one of four kids, and she was devoted to raising us and paid no attention to her field. Then, when we were older, she was offered a lectureship at Cornell, where my dad was, to teach a particular biochemistry class. She spent the summer before that class basically catching up on everything that happened in biochemistry for the last 20 years, which must've been just awesome. It must've been just incredible. What an incredible journey. Because that certainly was a very, very active field, and still is, but to take that 20-year journey in the space of a few months must've been incredible.
Is your sense that it was a personal choice for your mom? Or she felt like the culture at the time was such that the expectation would be for her to give up her career?
So I think there's probably many answers to a question like that. At the time, she was very clear that her view was that this was her choice. And she expressed no bitterness or anything like that. She lost her mom to illness, she didn't die, but she became really disabled, when my mom was a teenager. And I never had this conversation with my mom, but looking back, I think she felt that loss in her life and wanted her role as a parent to be different. So to me, that's probably a really powerful part of the narrative. But not one I've verified with her. I think it was probably a passionate choice on her part, reflecting on all of that. On the other hand, she also related anecdotes about really negative attitudes from others about women in science.
Tell me about your father.
So my dad's dad started life as a German peasant and wound up as an anthropology professor in the US. And so, when we have all these conversations about first-generation college students, and the journey it takes, and the sort of stages people go generation to generation to get to that point, he did this huge leap in one lifetime. Certainly that helped my dad feel that an academic path was more accessible (and he became a renowned physical chemist). And by the way, my mom was actually first-generation. So she and her twin brother were the youngest in the family. Among their siblings, they were the first to go to college. So in my family the arrival in academia was pretty fresh on both sides. It made it seem a little bit more accessible for my dad and made it seem very accessible to me, to have parents who were in that role.
My dad had many great attributes. One of them was, and this, I would say, was definitely true of both my parents, really loved science for what it was without ego wrapped up in it. They cared about science because of how it felt to do science, not to prop up their status. And maybe a little bit to a fault. I would say coming away from that environment, I had to learn a little bit of the sharp elbows part of science, which I think I've come to value, actually. I think wrestling with ideas and different viewpoints is really good for science. But I feel like starting with that sort of modesty and purity of their interest in science was a really great place to start for me. So I'm really grateful for that. There's a whole other thread to that story.
Your parents gave you a very Central-European-sounding name. I'm curious, if you ever talked about that, why they wanted to do that.
So my dad was also Andreas Albrecht. And they insisted on a different middle name so that I wouldn't be a junior. I'm the eldest, and the story was my mom really wanted me to take my dad's first name. But between the two of them, they didn't want me to be a junior. They didn't want that kind of implication or something. So it's interesting, I never felt an expectation that I would be a junior copy of my dad. And I like the name.
You were saying there's another thread.
Well, there's another thread to my upbringing, which also influences my science. So my parents were extremely left wing. I would say to this day, they're probably the most left wing people I've ever met. And I felt that that piece of it was difficult for me. In the end, as I became an adult and found my way in life, I felt I couldn't embrace the extreme views that my parents had. They had this idea, this is their plan for how people should be. And it wasn't really working for me. I actually don't even know how to quite put this in words. I might use the word “rational”, but I don't even know quite how to use it for this topic.
The place I'm going is that when I finally got excited about physics, I really appreciated how grounding it is and that there's a reference point, that there's a right and wrong that you can trust through data. That you can have all the arguments you want, and at the end of the day, you can hope that data will come in and resolve the argument for good. And I've talked to colleagues who have shared similar stories, where sort of intense kind of politicized conversations of their childhood didn't work for them. And they found the sort of solidity of physics really welcoming, really inviting at that stage of their lives. That was certainly true for me. Of growing up in the 1960’s, political drama was everywhere (not just in my childhood home), and often to very good ends. Interestingly much as I still appreciate the grounding side of physics, for most of my career it is the opportunities for creativity in areas that have not yet become well-grounded which have excited me the most.
Another thread is when I was a junior high and high school student, especially high school, initially, my huge passion was the violin. My dream was to become a professional violinist.
And what derailed that was actually my high school physics class. One of the things I remember from that was not only that it was taught well, and that it was exciting, but it was in contrast to the previous science class I had, which was biology. That was taught in a way that was all about multiple choice tests, and if you thought too hard about the question, you'd get it wrong because it was just about reproducing the particular argument that they had laid out for you in the class. So, this idea that you could work out a physics problem, and once you worked it out, you could think all you wanted about it, and it wouldn't change the answer was, I remember, really appealing to me. And it took me a while to get used to it.
Were you born in Ithaca?
And you spent your whole childhood there?
No. So there were three different times away from Ithaca. One was in 1963-64, my dad was on a State Department-sponsored exchange program with the Soviet Union. And there are so many stories, I don't know where to start and stop with stories about that. And the program supported the family to go. So somehow, we were part of a State Department protocol. So someone stationed overseas, presumably there was some pay grade he was on, and that meant the family could go, and all of that. But as a little 6-year-old, all I knew was that all of a sudden, we were packing up and going to a city that was then called Leningrad. And I think one of the remarkable things is, how did my dad's politics play into that? Surely, whoever was deciding who went on this exchange program knew my dad's politics. He wasn't secretive about it. And what were they thinking?
And when you say left wing, like, so left wing that he was a communist sympathizer?
Absolutely. I remember going to picnics of lefties in my teens with him, and all the trendy lefties of the era were excited about Mao's cultural revolution. And he was busy defending the Soviet Union. So the others had ditched the Soviet Union as the ideal and were defending Mao, and he was defending Stalin. What a bunch! The cultural revolution and Stalin, the whole thing. So there must be lots of stories I don't know. What's his FBI file like? [laugh]
What memories do you have of the Soviet Union?
I'll get to that, but I just wanted to reflect, my best guess is that the decision to approve his exchange trip to Russia was probably an absolutely awesome reflection on what can be really great about the US. That they probably sat there with his file and everything and just said, "Fuck it, let's send him." [laugh] Or something. Or, "Maybe he'll learn a lesson." Or I don't know what. But they sent him. And I'm sure he was the most pro-Soviet person in his community in Russia. I was just 6, so it was mind-bending. It's mind-bending from an adult point of view to think back on it, but as a kid, it was a whole other thing. And I guess looking back, you think how adaptable kids are. So my parents sent me to what was actually called kindergarten. Age 6, the norm there was kindergarten, which meant I wasn't trying to do any particular academic subjects. It was a social thing. And before long, I felt part of it. I became fluent in Russian, and had interesting friendships and experiences. I have fond memories of that.
How long were you there?
And it probably helped that my parents were very excited. A year. A whole year. Yeah, I had my 7th birthday there.
And then you came back to Ithaca?
Yeah. And then, we went again ten years later for half a year to what was still Leningrad. It was in between those two trips that my dad spent a sabbatical at UC Santa Cruz. We lived in Santa Cruz for a year in must've been '70, '71. People think of Santa Cruz as the place the 60s never left. But in 1970, the 60s actually hadn't arrived yet in Santa Cruz. And it was interesting. My dad was being recruited to UC Santa Cruz. They wanted him to build a chemistry program there. UC Santa Cruz was this brand new campus at the time. And basically, the kids, me and my siblings, were pretty unhappy there for whatever reason. We didn't connect very well with our school environment. It's hard for people to believe this story, but as a kid, I was naturally identified with the somewhat older youth culture of the time. In the town of Santa Cruz, none of that was happening in 1970.
Whereas in Ithaca, you really felt connected with the whole scene. In Santa Cruz, it felt like a very conservative little retirement community with no connection to that. It was happening in the hills outside Santa Cruz and on campus. But that was far removed from what we experienced in the town. Looking back as an adult, UC Santa Cruz was the dream job for my dad. He loved the sort of no-grades ideals of the campus. Everyone thinks the UC Santa Cruz campus is gorgeous, but looking back, for my dad's particular aesthetic sense, it really was perfect for him, more than probably anyone else. And he turned it down because the family was unhappy. We went back to Ithaca. And I've always felt a little bit of a twinge that I've come out to California. I'm so thrilled to be in California. There's so much about being in California that makes me happy. And that I sort of found my way to that place, which my dad denied himself because of how I was feeling at the time about California.
When you got back to Ithaca, did you know that you wanted to pursue science for yourself? When did that start for you?
So I didn't have my high school physics class til junior year of high school. And so, all these trips had finished by then. And it took me completely by surprise. So the second trip to the Soviet Union, I had really stayed with my violin efforts, and had found a pretty special teacher there, and really stayed very, very committed to that. It was literally my high school physics class that changed everything for me. I had no idea I'd become a scientist. I actually remember being utterly miserable in some of my science classes before then.
Going to Cornell, the proximity and the tuition must've been irresistible.
Well, actually, someone was just asking me about that. The options that were compelling to me were Cornell, MIT, and Swarthmore. And it really felt important to me to get further away, to sort of become more independent. But I felt MIT was too focused on technical things, and I wanted a broader experience. And by the time I was deciding, in the end, I wanted something bigger than Swarthmore. I was ready for the intensity that comes from a larger place. Based on all that, Cornell in the end was my natural choice. And certainly, the tuition was helpful for my parents. But they tried to be very open for whatever choice I was ready to make, so I didn't feel a lot of pressure from that.
Was physics for you the plan from the beginning? Did you declare the major right away?
Oh that's a little bit of a story. It was totally my plan from the beginning, but this is something that people of different generations have trouble understanding. The culture of the time was that it was incredibly uncool to know what you wanted to do. [laugh] And it was especially so if what you wanted to do was a hard science. And at Cornell, then, you didn't have to declare your major til your junior year. There was never a doubt in my mind what I wanted to do, but I didn't declare that because I had succumbed to that pressure. And then, probably sophomore year, maybe even freshman, but at some point, I just said, "Screw this. I know what I want to do." And I went to the Physics Department, signed up for the major, and then I remember, I went out and bought some beer, which you could do at 18 back then, and brought it to my dining hall, and sort of chose the table with the people who were least into the idea that I was choosing hard science, and shared the beer around to celebrate and get in their face about it.
And I remember some of them were so uncomfortable, they almost didn't accept the beer. [laugh] It was sort of fun for me to realize that I wasn't going to play that game, that I was going to choose my path. And I think over the years, every now and then, I've encountered people a little bit older than me who tell me a story like, "Oh, yeah, I was a physics major," or, "I was planning to be a physics major, but it was the 60s, and there was so much pressure against that path." That science, especially physics, was associated with the military, with the atom bomb, and there was the Vietnam War. So all these associations were made. And they actually shared with me how they left physics because of that, and they really regretted it. So when I’ve had those conversations, I'm well-aware what they were feeling.
Would you say that Bob Richardson was the most formative intellectually on you during your Cornell years?
He was one of many. I would say just the whole environment was really special, having a whole array of people who were passionate about their research and what they did. Bob Richardson was just my assigned undergraduate advisor. And at some point, he said, "We have a position in our lab. Are you interested?" And I thought, "Oh, why not?" I'd been working my way through college doing dishes in the dining hall. And I think it was quite influential to be there, especially sensing all the passion of the grad students, and post-docs, and everyone, and feeling part of that. And research is just so different than taking classes. And I had a fun, small part, but it helped me feel I could do that.
What were some of the most exciting physics at the time? What was your sense among the faculty, the most exciting stuff that was going on? And professionally, as you looked forward, where did you see yourself slotting into that excitement?
So I think you could say I was naive. I was really sort of open. I loved physics, but I sort of had the feeling that anything you could do in physics would be fun. And I didn't have such a focus. That changed radically in grad school. I would say maybe more broadly, in college, as passionate as I was about physics, I was also interested in a lot of different things. I took seminars on poetry, studio art classes, and history classes, and really valued immersing myself in all of that. I would've forgiven my mentors for wondering, "Is this guy really serious?" just to put it bluntly. And when I became a graduate student, I had just such thoughts. I thought, "Oh my God, I better really get to work. If I'm going to make this happen, I better really get to work."
And I think for my temperament, I'm probably really lucky I went through that cycle because I think all the diverse things I did in college, they're really important to me, and I'm glad I had the chance to grow and exercise those sides of myself. And then, grad school is an absolutely awesome time to decide that you're ready to buckle down and get to work. And so, it was pretty good timing for me. One of the remarkable things in science is that there's so many different ways to be a scientist, and there's so many different personalities that get expressed different ways in their scientific presence. But I certainly know there's a lot of people who try to be very, very serious from the start. Some are very, very serious from the start, and they never look back. They just shoot up into wonderful careers in science. And then, others burn out with that. I think if I had tried to be totally serious from the start, I would've burned out. And I really feel lucky that I got serious when I did. Clearly, I was serious enough to do all right in college. So there's all kinds of interconnections in the story.
When you and I exchanged emails earlier, I mentioned the interview with Robert Dicke in this same archive. I'd recently been Googling around and sort of stumbled across that interview. Robert Dicke gave this very famous Messenger Lecture at Cornell, which Alan Guth was present at. I was a senior, then, at Cornell. So this is part of the story. And that lecture was vaguely on my radar, but I had some conflict and couldn't go. And I cared enough to ask someone who went, "How was it?" And they just said, "Oh, it was some weird thing about numbers. I don't know." And that was it. So, I was pretty disconnected. Maybe I could've pushed some things aside and made sure to go, but I wasn't quite that devoted. I remember Robert Wilson came through Cornell and gave a series of lectures, and one of them was in the Arts and Architecture colloquium, where he talked about designing the main building at Fermilab. And at the time, of the three different series he talked in, the Arts and Architecture one was the only one I was going to regularly. [laugh] So that was kind of a reflection of my path. I absolutely loved the physics I was learning, but I had this very diverse outlook.
So that does beg the question, to a certain degree, when it's time to think about graduate school and who you might work with as an advisor, how did you narrow down your broad range of interests?
So what happened was, I started grad school, and I actually felt pretty uninspired for a while. You're taking all these courses, and then I forgot how it all played out, but I think it was the spring quarter of my first year in grad school. I took a group theory class taught by Tony Zee and was really excited by it. I think the way he taught it, it was very simple and elegant. And I think a lot of the other things we were doing in the other courses were really messy. Just hard, messy stuff that you had to plunge in and learn. I think the conceptual elegance of the group theory class was really seductive to me. So then, I wanted to do theoretical physics. And so, it was the contrast with that, I think, that triggered my inspiration. It meant a lot to me.
So it wasn't just this rational thing, "Oh, let me see what's out there, and let me see what's good." It was much more about my passions. My passions being not excited to begin with, and then excited. But then, what happened was, I started working harder in all my classes. Once I was excited, I worked hard on everything.
And here is an interesting part of the story. This was in the middle of some huge recession. And the word on the street was that no theoretical physics students were getting jobs, and if you did a PhD in theoretical physics, certainly in particle physics, which was where I felt I was headed, you wouldn't even get a post-doc. The best students from Princeton weren't getting post-docs. This is all just gossip among the grad students, but there was certainly some truth to it. And so, meanwhile, Penn, where I was a grad student, had this really powerful condensed matter experiment program, which is the field where I had been working in the lab at Cornell. So it was a natural fit. And at the very minimum, anyone with a PhD from that program would get an offer from Bell Labs, which at the time, was this totally awesome place to be. Many people spoke of it as the best place to be for that kind of physics.
And I actually went through a huge amount of soul searching. Like, "What am I doing if just a year ago, I thought any kind of physics would be awesome? And here's a sort of reliable path to a career in physics available to me. Why would I choose one that was almost guaranteed to exclude me from a career in physics? Or at least a career in sort of basic research." And that was really hard. I lost a lot of sleep over that question. And then, I finally just decided, "I'm having so much fun, I'm so excited by this"–and this was before I even had an advisor. But I was so excited by those possibilities. And this is something I tell my students. It helped me to know that by studying physics, I was acquiring skills that would be useful somewhere. That I could get a job, I could make a living. And so, I sort of took that risk rather than thinking, "Oh, I really want to guarantee a path in basic research." I just said, "I'm going to have fun. I'm going to really just enjoy this while it lasts." And I really had no expectation that it would lead to the career that I've been so lucky to have.
Did you know about Paul Steinhardt before you got to Penn?
No, he wasn't there. So I asked Tony Zee if I could be his student, but he had just been recruited away to the University of Washington. So he wasn't an option. And then, next, I felt I was on this great track to work with Willy Fischler, who was at Penn at the time. He was Belgian, he had to go do his national service. He was an assistant professor. He'd been postponing his national service year after year, and he got a letter that said, "If you don't show up on this date, you'll be arrested next time you come to Belgium." So he, all of a sudden, he disappeared as well. And I really didn't know what was going to become of me. I'd had some oral exam that I didn't do very well on, and I thought they weren't going to even want me in the particle theory group at all. And then, this was before the internet, so rumors were much more precious and infrequent things, but somehow, the rumor got out among the grad students that this hotshot young theorist was hired by Penn. And I just made sure I was the first student at his door when he arrived.
I was desperate. And I was very excited by Paul's reputation. I made sure I was the first student at his door when he arrived. And he said, "Well, I'm not doing particle theory anymore. I'm doing cosmology." And my heart just sank. Because I was very excited about particle physics. I'd had a little bit of exposure to cosmology in an astrophysics class. And it wasn't connected with the exciting things that were just emerging. It was sort of old school and didn't seem that exciting. It seemed like a lot of big questions without any way to answer them. And so, I was actually really disappointed, but I was pretty desperate. And I also had the wisdom tucked away somewhere in my young and not terribly developed mind to say, "Maybe Paul knew better than I did what was exciting." And so, I said, "I'd love to do cosmology with you." And the first paper he assigned for me to read was Alan Guth's famous inflation paper. And he already, then, had ideas how to work around the problems that Alan had identified.
So that was actually my question there. Was he already in the mode of moving on from the original theory of inflation by the time you got to work with him?
Yes. He had very concrete ideas. So if you look at that paper, it's sort of all about something called the Coleman-Weinberg Limit. And he mentioned that our first meeting. He said, "Guth ran into this problem with inflation." And he's very clear in that paper, Guth is, about what's good about the idea, and what doesn't work. And Paul, from day one, was onto this new idea. The problem is this big bump in the potential you have to tunnel through to get out of inflation in Alan's picture. And Paul said, "Well, there's a certain limit of the theory where that bump basically goes away." And so, that was his agenda, to study that. So, really, all the pieces were in place to go down that path.
This is early Paul Steinhardt. This is not Princeton Paul Steinhardt yet. He's young. To what extent were you very quickly working with him, not just as a mentee, but as a collaborator or a peer, in a sense?
To this day, I have this feeling that there's some magic with your first grad student. It was so magical working with Paul. He's not that much older. And I think, for me, part of the transition was, as a student, being unsure of my future and being hungry to impress. Each exam, I'd want to do well, so there was this whole sort of structure I was climbing through and trying to do well. And suddenly, I felt all of that struggle was replaced with a struggle to understand. It was the pure joy of doing science, which is incredibly hard work, but really joyful as opposed to the sort of agony that one can feel as a student, wondering if you're doing the right thing or what have you.
It was just very different. I'm not sure I'm finding the right words, but it's very different from being a student and worrying about my future, just enjoying the science and enjoying the give and take of it. And Paul was really excited about that project. So, I really had access to him. I could just walk into his office, and he'd drop what he was doing to find out what result I had, or what problem I was stuck on. And we'd have phone calls over the weekend and all of that.
When you say drop everything, with Paul, at this time on any given day, was he working in condensed matter stuff as well?
You'd have to ask him that. Oh no, of course he was because he had another student, Dov Levine, who was doing condensed matter at the time. And I think in those particular moments, it was much harder for Dov because Paul was really excited about the cosmology. But as far as I recall the story, by the time we were finishing, Dov had some pretty exciting work as well, and I think, has done very well. So yes, in some form, Paul definitely had both condensed and cosmology matter going on.
How did this initial paper ultimately feed into what would become your thesis research?
Well, there was that paper, and a number of following papers. I'm very enamored of the view that the thesis itself is kind of an anachronism, and what you really produce are your papers for a physics degree, that it's really about the research, and the way we articulate research is through publications, articles. And I have a strict rule with my students that they have three weeks to make a computer file and combine their papers to make a thesis, and that's it. And after that it's back to writing papers. And Paul had more or less that view then. But he was a junior person, and actually, other people overrode that and wanted the thesis to be much more of its own writing. [laugh] So I remember feeling really disappointed about that because I had a lot of new projects going on, and didn't want to spend all that time perfecting the thesis.
Now, where does Andrei Linde come into the story?
So I forget the exact timing, but somewhere that fall. I think it must've been the fall or December. Michael Turner came through Penn and mentioned he had seen a pre-print from Andrei Linde. So we were really discouraged because we–I shouldn't speak for Paul, but what I recall of our concept of our relationship to the world of other theoretical physicists was, would anyone take our work seriously because we were taking the Coleman-Weinberg Limit of this potential? And we were concerned that would be seen as too exotic to be interesting, too much of a niche approach to be of interest. Not typical enough of the theory. And so, we were more concerned whether anyone would be interested rather than rushing things out quickly. And then, once we knew about that preprint, we all of a sudden realized, we better say what we had to say, and get something out, and stop worrying about all the many layers of things. To some extent, we still wonder about inflation. We had a lot on our minds about what to make of the whole theory, and we realized, we better say something. We better say what we knew already.
On that point, it's so early, and cosmology is still such an exotic field. Was there anything in the world of observation or experimentation that was useful that convinced you and your mentors that you were on the right track? Is it all equations? Is it all math at this point?
There's actually a lot to say about that. So there's different pieces. I'm just going to start with a memory. It doesn't quite fit in the sequence we're going, but it's a very powerful memory. So Paul was one of the first people to work out the perturbation spectrum from inflation. He was in one of the pioneering collaborations. There were several groups that brought those results to the Nuffield workshop, and I remember him coming back from that all excited. But he said, "We have this perturbation spectrum, and the astronomers can't tell us what they want." I remember him being really frustrated that here was a chance to compare something with data, and there was no data to speak of. There wasn't nearly enough.
That then connects back to my experience in the astrophysics class I'd taken earlier, where there was the sense that you could ask questions, but there wasn't enough data on these big scales to really feel you're going to do science.
A lot of the excitement around inflation was solving cosmological puzzles. The “tuning problems”. And one of the things we can get to is that my start in research was very, very focused on solving these tuning problems. The Alan Guth paper was the first serious experience reading a scientific paper I had. And the whole energy behind inflation was, at the time, solving these tuning problems. It wasn't even until a little bit later that the perturbation spectrum had come out, and then we didn't know what to make of it. And then, I would say, to this day, the question of solving the tuning problems and what that really means is quite messy. It's part of a wonderful struggle I continue to experience to this day in my research, what to make of that agenda. And we really should get to that in its own right in this conversation.
But much more concrete than the “tuning problems” of cosmology was the monopole problem. John Preskill had written this incredibly important paper pointing out that the idea of “Grand Unification” was deeply undermined by cosmological considerations. He showed that very generically Grand Unified Theories (“GUTs”) would produce so many “magnetic monopoles” in the big bang that they would be the dominant particle in the universe today. The fact that no one has ever seen a single one actual was a very potent experimental result, one that thanks to Preskill’s calculations had apparently destroyed the primary agenda of the particle theory community. So the particle theory community was incredibly interested in cosmic inflation, which offered a very concrete resolution to the monopole problem (by diluting the monopoles away by rapid inflationary expansion). I would say it was this aspect of cosmic inflation that initially captivated the community. And it was in some sense really a healthy mix of theory and experiment. The fact that you don’t need to build a $1B experiment to see that they universe is not made entirely of magnetic monopoles does not make it any less of an experimental result!
Still, to have a really healthy mix between theory and experiment you need more than just one observation, and in the early days the conversations about inflation were very theory-centric. Probably especially because of that disconnect from data, I really had the thought in the back of my mind that, "OK, this is a lot of fun. But I really want to find my way back to particle theory," which was sort of where I originally thought I was going to go. And that that would be a place where ideas and data could clash or resonate, but where real science could happen. And there was really no sign of that in cosmology. So I would say, cosmology of that era was really fueled by theorists' exuberance and not the sort of disciplined connection with data I had hoped for. That was the nature of the field.
I'll test your memory. Who was on your committee at Penn?
I have no idea. What I do remember was more of a talk, but I guess it was my thesis defense. And there was quite a crowd there, so it wasn't just my committee, and that's part of why I don't remember. So one of the things that I think we do much better now is demand students have experience giving presentations. In fact, I think today, students arrive in grad school with much more experience giving presentations. Students probably leave high school with more experience giving presentations than I had walking into my thesis defense. And I gave an absolutely terrible talk. It was just a really bad talk. Looking back, I can think, "Well, why should it be a good talk?" I had no idea how to give a talk. I just sort of squirreled away, and wrote transparencies, and got up and gave it. It was a bad experience. And I think all I remember was one person, who I barely knew, coming up to me afterwards and saying something very supportive, despite the disaster. So I actually have that very fond memory. And that was Gene Beier, who's an experimentalist. I have this clear memory of that.
During these early years, as you're finishing up your graduate school, you're thinking about post-doctoral opportunities, long term, was your sense that cosmology would be something that would make you employable in academic physics?
No, I was still stuck with this idea that theoretical physics wasn't employable at all. That was the deal I had embraced when I decided to go into theoretical physics. I would not expect an academic path. So it just sort of blew my mind that I got a post-doc. And I guess, to me, the huge amount of interest that inflation received early on was a bit mysterious because I was well-aware of the open questions, some of which remain open today. So, I really didn't expect it to last. One of the interesting features of that history is that, at the time, the main excitement in particle theory was the idea of Grand Unification. And most particle physicists were pretty well convinced they had the right theory of Grand Unification, which is unifying electroweak with strong forces. And that was the standard SU (5) model. And our paper, Paul's and mine, is about the standard SU (5) theory.
We didn't talk about “inflatons”, we talked about the Higgs field of the SU (5) model. We were working out the cosmology for that theory. And if you had asked me then what would be around decades later, or even a few years later, I would've bet on SU (5), not on inflation. That was just the culture. Inflation was just this sort of flash of excitement. It seemed very fluffy to me at the time. All this excitement emerged about an idea that was still so poorly developed. Whereas particle physics had this deep history, and electroweak unification was so successful. That was the thinking, that electroweak unification was so successful, that surely, Grand Unification was the next thing.
And specifically with testability, the concern then and the longstanding concern.
Yes. All these big accelerators were being built. Yeah. Exactly. That had the taste of real science.
What were the most compelling post-doctoral opportunities you had to choose from?
So I'm trying to recall if I had other ones. I went to the University of Texas with Steve Weinberg, and I don't recall the details. I just remember being really excited when I got the phone call. And by the way, getting a phone in those days–I didn't have a phone, but I got a message that Steve Weinberg was trying to reach me, and I had to call him back. I had to find some phone. Long distance calls were incredibly expensive. And I couldn't just pick up a phone and make a long distance call to Texas. So it was this whole rigamarole to get through. And then, I finally get connected with Weinberg. I'm standing in the middle of the Physics Department office on some phone, and then he says, "I'd like to offer you a job." And I was very excited. So that's the part I remember.
Was this specifically to join the gravity group at Texas?
Weinberg had his own group at Texas. It wasn't gravity. I think that's still true now, it's called the Theory Group.
Was he working on inflation at all during this time?
No. He was just interested in the topic. I think that's the best practice, hiring post-docs, is hiring people you think are going to be fun. I never wrote a paper with him. I got excited about cosmic strings during that time. And I found Weinberg very inspiring. We didn't do specific research together, but there was something important at that stage for me–there was still this mystique of the theoretical physicist, that you just sit there being brilliant and create effortless brilliant insights on cue. And getting to see Weinberg in action, there was no pretense there. He just worked hard until he understood things. He is incredibly bright, but not at all shy about the role of hard work. And that's the substance. I think this mystique is actually a really damaging thing for physics. It sort of applies to physics as a whole, and especially, theoretical physics. But that's not what it's really about. It's about the hard work and passion you bring to your efforts.
And so, it was incredibly valuable for me to see how Weinberg worked. Here's a funny story that sort of gives you a taste that era. By default, didn't have a computer account when I started my post-doc and didn't know if I was going to need one. When I realized I might use one, I went into Weinberg's office and said, "I think I'm going to start using a computer. Is there some way I can get an account?" And Weinberg looked up from whatever he was doing and said, "I have an account I never use. Just use mine." And so, some later day, I'm in the department at night, doing my work, and some other grad student comes bursting in. In those days, you'd use a computer at a terminal, and other people could see who was on the terminals.
So someone had determined that Steve Weinberg was in the department at the middle of the night in this computer room, on the computer, and came bursting in to see the spectacle of this Nobel Prize winner burning the midnight oil. And chances are, Weinberg was up late, working hard wherever he was. And the insight for me was, it was not a novelty. He is a passionate scientist, and the idea of him being up late, working hard on something wasn't a novelty to me. And that anecdote kind of illustrates these different perceptions people had that I think are really burdensome to the field. In physics, we really have a diversity problem. And I think this whiz kid image is part of the problem. This whole question of, "Do you belong?" And if you think you only belong if you're that whiz kid, it's actually wrong. It's wrong in so many ways. People play the whiz kid game, and then fall on their faces. And people who could be awesome scientists don't think they belong because they can't pull the whiz kid stunt.
What was your entree into cosmic strings? How did all of that get started?
I made a great friendship with Neil Turok when we were students together at a summer school, at Erice one summer in grad school. And we stayed in touch… we did a road trip together to visit the Aspen Center for Physics. At some point, I was having trouble settling in in Texas. And he said, "Why don't you just come out and visit me in Santa Barbara?" where he was a post-doc. I went there, and we got talking about all kinds of physics. He had worked on cosmic strings as part of his thesis. He told me of this work, and he said, "So to really understand this, you need to write a big computer program." And he said, "But that sounds really daunting." And very naively, I said, "No, I don't think that's going to be a problem. Let's just write it." And I had experience, I'd done a whole bunch of computation for my thesis. And I said, "No, we can do this." We started writing code for cosmic strings. I didn't know much about the topic til that project started. But that was my entree.
To what extent were you relying on your knowledge base in cosmology up to that point? And to what extent was your entree really new science altogether for you?
So you're sort of asking me to reconstruct my brain at that early time. [laugh] But what drew me energetically into that topic was a big picture cosmology consideration, which was that with the cosmic string picture, there was sort of the back-of-the-envelope argument why the cosmic strings could be the seeds for the formation of galaxies, and the way that argument worked was, to circle back to get the right size seeds, you had to have a certain mass per unit length of the string. And the mass per unit length you required was exactly what you'd expect from grand unified theories. So there was this coincidence of numbers that seemed utterly profound and exciting.
So it's really not just cosmology, but particle physics as well. Everyone talked about the connection between particle physics and cosmology, and these arguments that cosmic strings could be the source of structure was a really beautiful example of such a connection. Meanwhile, if you opened up the hood and looked at what you had to do with inflation models to get the numbers right for cosmic structure, it seemed to flow much less naturally. You had to sort of fiddle with things a lot more to get that to work. So there was something sort of magical-seeming about the cosmic string argument. That's what fueled my interest. So yeah, it was cosmology, and it was particle physics, too. And it was comparing it with inflation.
At the end of your time at Texas, were you on the job market? Did you feel like you wanted another post-doc first?
I totally forgot about that. I remember having a conversation with Steve Weinberg about that. And he said, "Well, someone in your position could be either looking at a faculty position or a post-doc." And that actually blew my mind because I was still in this mindset that there's no opportunities. And I didn't feel ready to shift to faculty. That seemed daunting. I wanted more time just for research. And I probably just indicated I was more interested in the post-doc. And more than one postdoc was certainly the norm. So I guess the thing to say is, I was completely unprepared for that conversation. I really hadn't thought it through. Probably by temperament, and then also by this perspective that I was just kind of winging it and living on borrowed time, in terms of the prospects for an academic job meant I just didn't invest much thought. I didn't have very many careerist thoughts. It was all about the passion I felt for my research. So I sort of felt like at some point, reality was going to hit and remove me from that world. But I wasn't going to help it along. I was going to enjoy every minute I could get of it. That was the feeling I had.
Was Los Alamos a logical next step? I know there was a theory group there, but how did that opportunity come together for you?
So Wojciech Zurek and I met at the Aspen Center for Physics the summer right after I got my PhD. And we really hit it off. He was doing a lot of astrophysics and cosmology then. That was his main effort. Well, no, that's not true. He was at Aspen for an astrophysics workshop, and one of the things is, I was trying to pull up some memories for our conversation today, and one of the fond memories I had was this conversation with Wojciech at Aspen where he said, "Oh, you're going to post-doc in Texas? Here are some tips about housing, and here are some people you should talk to." And somehow in that conversation, I was already getting interested in the arrow of time. Only understanding in my gut that it was something to do with cosmology, but not really understanding how intertwined it is. Just kind of following my nose. And I remember Wojciech saying, "Oh, yeah, that's interesting. Here are some papers you should read."
And I remember shortly after that workshop, I had arranged to pass through Austin for a few days to look for housing before actually moving there. And all I remember of those three days was sitting in the library, reading the papers that Wojciech had recommended. [laugh] So I did find housing, but that was not the most memorable part of those days (and that was actually reflected in the quality of the apartment I found as well). I actually don't remember which papers they are either. So Wojciech and I had this intellectual relationship that was already very broad. He certainly was interested inflation, interested in astrophysics, and he was in the astrophysics group at Los Alamos, which may not exist anymore. But we also had these extra layers of interest. Arrow of time. I don't think we talked much about quantum physics then. Maybe a little bit because inflation was predicting a perturbation spectrum via a quantum mechanical mechanism, and that was drawing in all kinds of attention. The community of physicists have a really weird relationship with quantum mechanics. And the fact that inflation was using such a quantum mechanical argument to come up with the perturbations was triggering that difficult relationship in all kinds of interesting ways.
And so, Wojciech and I might very well have had conversations about quantum physics already that summer. He was actually very assertive in trying to recruit me for the next post-doc. So he had me coming out to give a talk before I even was really thinking about my next post-doc. Which looking back is sort of a naive thing. Really, anyone on a two-year post-doc, which is what I had, should be thinking about their next post-doc from day one. [laugh] But that's not the way it was for me.
Did Los Alamos feel like an academic environment in your little circle? Did you feel the military component of it at all?
Oh, so there's lots of layers to that. At the time, there were really exciting people there. Geoffrey West, who's a fascinating intellectual figure, was a member of the particle theory group there. There was Stuart Raby. There were others. So Peter Carruthers had come out, and kind of in this group-building spirit that I ended up living in my own way later in life, he was invited by Los Alamos to just bring some really creative people together with no money worries. For a huge lab, having a bunch of creative theorists, you can just pick some money off the floor and fund a group without even worrying about it. So it was this really free and adventurous environment. But I actually really felt uncomfortable with the military side of it. It was a very confusing decision for me to make. I felt a wonderful intellectual connection to a whole range of people there, but I had concerns about the military piece.
How did your work evolve as a result of being in Los Alamos? What new ideas did you start to consider during that time?
Well, at the time my main effort was trying to pin down the cosmic strings, which had already started a little bit in Texas. But during my post-doc, and at subsequent visits to Los Alamos seeds were planted in my mind on the topics of emergence of classical from quantum and also the arrow of time. Conversations with Wojciech planted those seeds. And so, it was really this sort of combination of having this workhorse project of the cosmic strings and seeds being planted for fresh new ideas. It's hard for me to recall more details than that. For part of that period, I carpooled. I lived in Santa Fe and traveled up to the lab, and I carpooled with Geoffrey West. And he's just a wonderfully fascinating intellectual in so many dimensions. Well, not just intellectual, but a human being, including this very intense intellectual piece. And that was really stimulating. We talked about absolutely everything.
For me, part of the evolution of my career was, and probably still is, to be honest, the evolution from being young and seeing the world of physics as a solid, concrete edifice, which somehow I want to join and have a well-defined part in, sort of find a role and become the right person to fit that role. And I think what's missing from that point of view is that the world of physics is actually made out of all these different personalities, each of us doing physics, really, in our very own way, and somehow the fitting comes where we have to interact with our colleagues and have a shared perspective of some sort. But what is going on for each individual can be very, very different. And I think carpooling with Jeffrey West helped my evolution with that a lot. He's a very deep and reflective person. And quite a bit older than me, so he'd evolved more in that direction. Whether it was sort of commentary or anecdotes about different individuals, that was pretty important.
Were you actively on the job market? Did Fermilab recruit you? How did that play out?
I didn't see it coming at all. In fact, there was already talk at Los Alamos of promoting me there to stay in a scientist position. The relationship between physics research and fads is a pretty interesting topic. And I felt that the people in my circle at Los Alamos were really more independent of the fads than most, and I really valued that. I was drawing the balance between how I felt working at a military lab and appreciating the power of this lab that also created the opportunity to bring these really imaginative people together and sort of break the patterns of fad-following that were so evident other places. So I was able to see that as a more complicated thing than just simple labels. And I really cherished the intellectual environment there. It seemed very attractive to continue there. And then, out of the blue, Fermilab was very aggressive. A sign of cosmology coming into its own was Fermilab creating this cosmology group. And I was the one they wanted. And that was quite a transition for me, just in terms of my own idea of where I stood. I didn't expect that.
Again, I was just sort of in it for the fun. Oh, another thing that really influenced my perspective on Los Alamos was that Willy Fischler, who came up before as my potential mentor at Penn before joining the Belgian Army–by the way, that's a great story that you should really hear from Willy. As I recall it, he goes there from actually being assistant professor to bootcamp, and worrying about, "How am I going to handle tenure?" and all this stuff, and he's in the Army. And some officer felt sorry for him, and gave him a desk job, and let him do physics. Gave him a desk job and said, "But just do your physics." And he wrote some really important papers. So he was hot property. He and I moved from Penn to Texas at the same time. And he had been a post-doc at Los Alamos. And he, also, was very, very fond of the individuals there and the intellectual environment, and he sort of primed me to see that in a positive light, to be ready to appreciate that. And yeah, so the Fermilab thing was kind of out of the blue. It was a very different environment.
If I had wanted to move, I was really thinking I wanted to be a university professor. The Los Alamos thing, I knew how it was working for me, and it was pretty good. But the idea of packing up and moving, I thought, "Well, I would really want to be in a professor job." Because I did, even then, feel attracted to mentoring students and being part of the broader setting of a university. But Fermilab folk were sharp operators, and they went and grabbed me before I had any other options. They just offered me the job. "Here's your offer. Here's your deadline." So they were quite aggressive. And I ended up going there. It was certainly very exciting to be in a group deeply focused on the problems I was working on, and I enjoyed special friendships with Michael Turner and Rocky Kolb, who were the original members of that group. (In fact, Rocky and I met that first summer in Aspen as well.)
Who were your most important collaborators during this time? Who were you working with most frequently?
Well, most frequently, I was working with Neil Turok. We had a very intense collaboration going. And I would say in terms of actual collaboration, that was the main collaboration for me of that era. There were lots of people who were influential in my thinking, and ideas, and broader perspective on physics. So I really valued my community beyond that collaboration, I have very fond memories of all of that. And I think both Neil and I would agree that we got kind of stuck in that project. We were excited by a certain vision, which was you'd do the computer calculations and pin down the scaling solution, which would then be easy to extend across many decades of scale to talk about structure in the entire Universe. So we could put the Cosmic String Structure Model to rigorous scientific test.
But basically, that was too naive a picture. And the idea of cosmic strings scaling is just too simplistic. And to understand the evolution of a network of cosmic strings certainly exceeded the dynamic range of computers at the time, and I would say, even though some people feel they're making progress, from what I learned back then, I remain skeptical that even now, that's a computationally accessible problem. Because what we ended up learning was that many scales are important. And I should say, there's a piece of this that's very, I would say, unflattering to me and Neil. The computational shortcoming were really brought up by competitors. The competition was seeing these effects that were indicating a wider dynamic range for the problem. Neil and I were very junior at the time, but we were the senior people in the field of simulating cosmic strings. People looked to us for the deepest understanding of the topic. And we were really seduced by this scaling picture, and there was enough slop in the numerics that we could kind of dismiss these other competitors. But looking back, it's an example of sort of the privilege that comes with a certain status and the ability to carry on based on that status, even if you're wrong. But science won in the end. And most of those individuals wound up having great careers. So it worked out pretty well.
To come back to this idea of concerns about data and testability from inflation, was the broader environment at Fermilab, all of the amazing things that were going in there in the world of experimentation, where data, of course, is central, intellectually useful to you and the research you were pursuing?
There are probably several threads to pull on with that. [laugh] The one that's first coming to mind is that the climate in cosmology, still, at that point, was really the Wild West. It was this bursting forth of theoretical ideas. David Schramm was part of the group, and he was very influential. That was one of the really data-oriented things, was primordial nucleosynthesis and testing that against astronomical data. But I think overall, there was a sense in that group at the time that it was mainly about exciting ideas, and not about rigorous grounding in experiment or observation. Certainly, not like the field is today. And I think the sort of different cultures you sketched out, probably the main influence that had in my life was that there was resentment of the cosmology group.
So there was this large lab of many people, and there was sort of a resentment of that group. Which had been created by the director without any sort of due process. From what I heard, the director (Leon Lederman) just said, "Oh, cosmology, exciting. I'm going to make this group." [laugh] And so, when I came up for the equivalent of tenure at the lab, I failed to get it. And I think this clash of cultures that you talked about certainly had a role. So it was sort of a negative. So you were asking me a question in a very different way. But this is the answer that comes foremost in my mind. And I think there's lots of later stuff. Of course, that's quite a moment in someone's life for something like that to happen. And there are so many threads. I want to get back to the relationship with data because I have another story about that. So I would say, at Fermilab, I was mainly in this theorist bubble. And here's a story that I want to tell, and I can't remember where I was when I made this trip. Maybe it was when I was at Los Alamos, I forget. But I remember visiting Caltech, and giving a talk, and talking with a cosmic microwave background experimentalist.
And this is such an ancient memory, I don't remember who it was. It was probably Andy Lang. And he was saying, "We really want to know your predictions so we know what to look for in our experiments." I just remember having this reaction like, "What's he talking about? We have all these flaky theories." The theories seemed really fast and loose to me. They did not seem rigorous. And so, I sort of looked upon this experimentalist as the person that was going to keep us honest. The idea that he was looking up to us for guidance really seemed backwards to me, given my personal perspective on the qualities of these different kinds of work. I was very young then. Since then, I've come to realize that you can't raise money to do an experiment unless you have some idea what you're trying to accomplish with it.
And so, even though, at the time, these theories were very poorly developed, I now understand that this whole Wild West thing that I was kind of uncomfortable with, it's really thanks to that that experimentalists were able to raise money, and gather all this awesome data, and turn cosmology into a real science. So it's sort of a fun growing up story for me to have been through that, have these different perspectives, and come to see how it all has worked pretty well.
I wonder if Imperial College also came out of the blue for you. I can't imagine that that was really on your radar from Illinois.
So I was looking for my next job in the depths of another massive recession, and that was basically the only academic job in my field. And I was feeling pretty discouraged about my not getting tenure. There were all kinds of politics, and I know that the senior people in the cosmology group at Fermilab absolutely wanted me to stay. They were incredibly supportive, and they were so used to getting their way, I think, that they didn't try to say, "Well, you should be doing this and that to make sure you have the right case." But also, I was incredibly naive. Looking back, I didn't really pay any attention to the things I really should've been.
So my own feeling coming out of that was, after a period, of course, of pain, and bitterness, and all that, I thought, "God, what it takes to get tenure is a pretty wonderful thing. You've got to convince these really bright and interesting colleagues that you've accomplished something. What could be better than that?" In terms of getting something so important to one, what could be better than that? But I hadn't paid any attention to that stuff. So I was feeling down about it. And there were no jobs. It's not like I felt like I was the best or something. Lots of people had played the game better than I had. So I didn't know what to expect. And when a department does hiring, there's the whole question of even what field they want to hire in. So I didn't feel that the Imperial College job would come my way.
I was feeling very uncertain and insecure. In some sense, my record wasn't so beautiful from the point of view of a skeptical faculty member from another field, sitting down, opening up a folder, and looking at my record. I feel so indebted to Tom Kibble, who was the head of the Imperial College Theoretical Physics group then. I'd already encountered him because he was really the pioneer of cosmic strings, and we'd encountered each other on different occasions. And I would say, now, looking back and having a broader perspective at this point in my life, I realize, knowing Tom better now than I did then, and sort of getting a clearer picture, I think he appreciated more about me than I realized then. And Neil Turok was also on the Imperial College faculty. Neil and Tom, you can't think of more different people, personality-wise. But I know they were both really supportive of me for that job. And I'm really grateful. It saved my career at that point. And in my times at Imperial College, I really had so many positive interactions with Tom Kibble. He was a really influential person to me.
It goes deep in your family, and it's not the same country, but I wonder if being in Europe at some level felt more natural to you than it might otherwise for an American who had family going back generations in the United States.
So I tell people living in England taught me how American I really am. And I think a lot of American intellectuals sort of harbor a fondness for the old world. And there's probably something to that in the sense that I was more open to trying that than maybe other people would've been. Unfortunately, we arrived in England with three young kids, and it was a struggle for everyone to adapt. I think would've been easier in a country with a completely different language. Because we arrived, and we were just trying to make it work. When you have little kids, you can't just say, "Oh, let's go touring and look at the beautiful London things." First you have to provide the basics for your family. If the language had been different, I probably would've been more inclined to ask myself, "How do they do this here?" when I would go out to get something done (be it buy milk on a Sunday morning or buying a washing machine). Probably because the language was approximately the same, I felt I didn't need to ask myself those questions. But I should've. That’s what you need to do to find your way in a new country. So looking back I feel like I was something of the “ugly American”. But I was just focusing on trying to make things work the way I was used to and getting frustrated.
How might we translate the tenure rank from going to lecturer, to reader, to professor into American categories?
So already, as a lecturer, I was tenured. I arrived at Imperial with tenure.
This must've been a great relief to you. Not only the fact that it's the only job out there, but that you finally achieved tenure.
Yes. And the other thing I should say is, I talk about these cultural differences, but I always felt really welcomed and integrated into the department at Imperial. So I was sort of experiencing these cultural challenges elsewhere in life, but I just felt incredibly, warmly welcomed in the department. I should also mention the absolutely awesome intellectual community across the UK with interests in cosmology. This community was tightknit and fostered many dear personal friendships as well as great professional relationships. In so many ways, my work environment was absolutely thrilling. I have really fond memories of that. And it was actually really hard to leave years later when I moved to California. I have such fond feelings about that department. And in a sense, it was harder because I felt they had stuck up for me and created an opportunity. They saved my career. So it was hard for me to leave.
In the 1990s, cosmology, at least observationally, is starting to come into the forefront with all of the major and exciting observational advances, experiments. I wonder what some of those big projects were that were particularly relevant for what you were doing and what was important to you during those years.
Initially, it was absolutely the cosmic microwave experiments. There was this cumulative plot that would be made with all the CMB experiments on it, and I've totally forgotten which order which one appeared because there were all these balloon experiments and South Pole experiments. And rocket experiments. They'd send something up, and it would sort of sit high above most of the atmosphere for a little bit. All the data points had massive air bars. And it was pretty confusing. They all sort of made this big, hairy clump on the plot. It was pretty exciting that it was happening, but it was happening slowly. I say “slowly” because that is how it felt in the day-to-day life of a theorist eager to get answers. I suppose from a historical perspective it all happened in an instant.
I'll tell you the most exciting part of that era for me. So here I am, still wondering about cosmic strings. And basically, I had this flash of insight at some random point, and it was rooted in all kinds of different projects I had been doing.
But I had this flash of insight that the cosmic strings–you've surely seen plots of the cosmic microwave background temperature power spectrum that has these beautiful wiggles. So the insight I had was that cosmic strings could not produce those wiggles. And one of the fun parts of that was that the reason they couldn't produce the wiggles was exactly the same reason it was really difficult to calculate. So it was kind of this coming together of my experiences with that topic, where the struggle to calculate and the feeling I had failed to calculate what I'd hoped to calculate, the things I understood about cosmic strings from that actually came around and allowed me to make this really important point, which I did with some awesome collaborators, including Pedro Ferreira, my first PhD student, and Joao Magueijo, who was, I think, at Cambridge at the time. David Coulson was on that paper as well. But I remember feeling just this jolt of excitement that from that insight, we would know the answer.
So even though the data from the CMB at that time was a big, hairy mess, everyone knew there were instruments being built, satellites being planned, WMAP, and so on, and even before then, that were going trace out the microwave background power to wonderful detail, and we were going to know whether those wiggles were there or not. It was totally guaranteed that we would know it. And then, I had this insight that that would make all the difference, that this would actually discriminate between these two big ideas for where the structure came from. I really lost a lot of sleep from sheer excitement after having that realization. And then, I remember going to a conference in Cambridge, and Cambridge was only an hour or so away by train, so it was the local neighborhood. And when you go to a conference under those conditions, it's not a great way to go to a conference because you sort of go to part of it, and then you have some commitment back home. It's pretty complicated. So I remember there was going to be a talk about the latest data.
Lyman Page was going to give a talk with some new data on the CMB, and no one had seen it yet, so this was a big thing. But somehow, I had screwed up my commitments, so I actually had to go back to London and missed the talk. I remember, over lunch, sitting with Lyman Page, and he said, "Well, OK, I'm going to show you." And he carefully peeled open his folder of transparencies and let me have a very quick look. And the peak was really there. So that was pretty much the end for cosmic strings, at least as seeds for cosmic structure.
There's actually another great story about that, that I should tell you. Unless you're trying to track a particular path, I'm going to tell you this other story now.
It's related to the cosmic acceleration. After seeing Lyman’s data, we went into the mode of saying, "At what point do we quit on cosmic strings?" Because data was still not giving a complete picture of the peaks, it was just sort of shooting up toward the first peak. So the question was, "Is there anything in astrophysics that could make the power in a cosmic string model look like this sort of rising curve?" which was all we had. And we worked really hard to push all the parameters, and there was a lot of uncertainty about re-ionization in the universe, and that affects the microwave background. There were lots of pieces you could push. And one of the pieces we pushed was adding a cosmological constant (“a Lambda”). Which at that point was a tiny, little niche topic. So the famous results showing cosmic acceleration hadn't come out, and there was a tiny, little niche of a handful of theorists who pointed out that the data was somewhat supportive of a positive cosmological constant. And one thing that was on people’s minds is that cosmic inflation theory was having certain difficulties with the data that was available then, and the proponents of Lambda had sort of said, "Well, look, this will help save inflation." [laugh]
So we said, "OK, if we had a Lambda, and we make these sort of oddball assumptions about re-ionization," we could get the cosmic string curve to come up the way the CMB data was going. We submitted a paper, and there was this remarkable timing. Just as we're submitting it, the very, very preliminary supernova results come out. And now, I'm even forgetting the history. Which of the collaborations first reported results of the supernova projects in a conference saying Lambda was zero? So the very first report from one of those projects actually said, "We get Lambda of zero." And so, we get a referee report back saying, "This is an interesting paper. And we're supportive of it. But at least you should mention that you're using a positive value of Lambda, when it's actually just been reported to be zero." But in the meantime, as the referee report was on its way back to us, updated claims were made from these projects with supported a positive Lambda. So it was the best ever response to referee you could imagine. We were able to write back and say, "Actually, yeah, we will cite this data, and it's actually supportive of our assumption."
So we sort of pushed on that, and then finally, more data came in, and beautiful oscillating curves were coming out in the data. And then, I think, somewhere I wrote a paper saying, "That's it for cosmic strong models of cosmic structure." And to me, that's just really exciting. To this day, people say, "Well, what's really going on with inflation when you can dial parameters and get this spectrum or that spectrum, and it all seems very mushy?" And to me, I always refer back to that experience. The question of the origin of perturbations had this really profound fork in the road, and that's very, very, different physics that supports one picture or another. And we took the data, and nature made her choice. And here we are. So, to me, if we talk about my journey to try to do rigorous science and all the ups and downs, that was the gold standard. So that was very satisfying.
At what point did the accelerating university and dark energy really take center stage for your research agenda?
So there's a history for that. Part of it was, ruling out cosmic strings allowed space for that. And that was becoming very exciting. And I somehow stumbled into this idea (with my then-student Costas Skordis) about a particular model of quintessence, that's now known as the Albrecht-Skordis Model now. I just somehow stumbled into that, and that was fun. But, to me, I think there were two pieces to my journey with that. One was kind of a very early awareness of how exciting theoretically that result was, and a desire for that excitement to translate into better data. So I really wanted to rigorously study that exciting thing. And one of the things that was happening was, in a sort of observational astronomy world, there was a temptation to say, "Well, there's lots of parameters we can study. Why measure the acceleration? That's a really hard one. Why don't we measure an easier one? And it's just a number. It's just a cosmological constant. We can look it up in a textbook and see that a cosmological constant is a possible explanation, so what's the big deal?"
And so, from a particle physics point of view, it's a huge deal, and it's extremely disruptive to have a non-zero Lambda. I think, now, we can look back on all the different things theorists have tried to explain it or kind of explore it. The fact is, it's been hugely disruptive to the thinking of theoretical physicists. And I really embraced the challenge of trying to communicate that excitement and the importance of the subject to my friends in the observational astrophysics world to try to make sure that actual experiments were done. To make sure people did care about taking that data, rather than just sort of dismiss it. So I was very energized by that. And then, on a much, much more theoretical front, already in the 80s, there were discussions of the wave function of the universe and debates about, "Should it be the Hartle-Hawking wavefunction? Should it be the tunneling wave function that Vilenkin advocated?" And other people chimed in with different variations on that.
And to me, those debates always felt really empty because it's like, "Why should the universe be listening to what theorist A or theorist B is arguing is the reason why this should be the wave function?" It just seemed really disconnected. It just seemed like it was a political game rather than a science project at that level. And I thought, "If only I could envision the universe reaching equilibrium." What happens when a system reaches equilibrium is, it forgets where it has been before, and then that's the state of the universe. And so, if the universe could reach equilibrium, then you have this picture of the universe eternally sitting there and fluctuating. Then, cosmology would be a fluctuation out of equilibrium. In such a picture the probability of a given cosmology is given entirely by the laws of physics governing the equilibrium behavior. There is no room for theorists arguing about their favorite wavefunctions. So that was the realization I already had in the 80s. But there was absolutely nothing about the universe to suggest that it could reach equilibrium. So it was sort of a concept that was floating out there with no grounding in reality. But it was captivating to me, so it stayed in the back of my mind. And it was not long at all after the cosmic acceleration was discovered that I realized that if the acceleration is given by a truly constant fundamental cosmological constant, that that would provide us with a notion of equilibrium for the universe. And I forget when I learned what pieces of this because it was sort of an exploration that several people were undertaking. I know Tom Banks, Willy Fischler and Lenny Susskind were part of those conversations.
But the topic goes back further. If you have a positive cosmological constant, the universe becomes a de Sitter space at late times. And de Sitter space has a horizon that in certain respects is similar to that of a black hole. There's this old work from the 70s by Gibbons and Hawking showing that if you have a positive cosmological constant, and if you interpret the horizon entropy the same way you do with black holes, then the de Sitter space actually has the highest entropy of any state in that cosmology. And so, it's a natural notion of equilibrium. If you have a cosmological constant and you wait long enough, that's where you wind up, as a de Sitter space.
There's lots of other layers to the story. But what arrived for me as these different pieces came together was that here was a way to actually realize the notion of equilibrium cosmology that had long been on my mind. My interest in that topic has remained very strong and steady over the years despite the fact that there are deep intrinsic problems with the equilibrium idea (for which I coined the term “Boltzmann Brains”). By comparison, the quintessence thing feels a little bit more like a fling. Because there was this sort of way Costas Skordis and I found we could play with the scales of quintessence to make it seem a little bit more natural with a particular approach, and so that was fun. I guess my interest in cosmic acceleration caught fire for me in several different ways at once. It was nice to have it there for me when I ruled out cosmic strings. It was really nice to have all this exciting stuff waiting for me to jump into.
I should also add that very recently my interest in the Albrecht-Skordis model has been reignited by the “H_0 tensions” currently present in the cosmological data. These tensions suggest new ingredients may be needed beyond what is known as the standard model of cosmology. One of my current students, Arsalan Adil, and I are currently exploring if the Albrecht-Skordis model could be that missing ingredient. This work is especially fun because we have Lloyd Knox at UC Davis (our first hire years ago building out the cosmology program) who is probably the leading theorist in the world exploring the nature of these tensions. It also has been really fun to reconnect with Costas Skordis (now a senior professor at the University of Prague) on this topic.
And were you in direct touch with people like Saul Perlmutter or Adam Riess at this time?
Yes. Yes. So Saul's down at Berkeley, and we had numerous face-to-face meetings back and forth. We both remember those very fondly and kind of regret we're not doing it more now. But yeah, we were working very closely. We never wrote a paper together, but we came close. I think in the end, we talked a lot about how to model the supernovae, and there was an early paper I did with modeling the impact of future surveys, which I think Saul was almost on it, but then you've sort of got to decide at the end of the day who's on and who's off. And that's how it went. But we worked very closely. It was a lot of fun.
It's interesting because your first real professorship was in England. What were your responsibilities there with regard to undergraduate teaching, taking on graduate students, expectations of research?
They were considerable. I found the transition to teaching really challenging. I had learned to give good research seminars. Fortunately, not long after that terrible first talk at Penn, I learned to give good talks. But I found that the organization required for teaching undergraduates is quite different. And it was quite a journey for me to do that. One of the remarkable things that people tell me is still true is that there's sort of a culture in the UK that you should not require a textbook there for undergraduate classes. So, the students would regard the lectures as mainly this basic photocopying system, where I would write on the board, and they would write notes, and that would be their resource.
One of my favorite parts of giving a lecture is when I put my chalk down, and turn around, and just talk with the students about what that particular thing means to me, or how it's influencing the field, or whatever. That's where I really add value compared to just reading a textbook. And those were the moments where my classroom would come unraveled because no one knew what they were supposed to be writing. So I quickly learned to help the students out in that regard. In fact, I think because of those pressures, I was possibly the first person on the planet to put their lecture notes online. This was in 1992 or something. Someone told me about the World Wide Web. Someone came back from a conference and said, "Let me tell you what I just learned about, this World Wide Web. Look at this." And I immediately said, "Oh, that's going to help my students. Then I don't have to make my lecture a photocopying system. I can put all my notes online, and I can relieve my students of the pressure of getting their only reference from their lecture notes."
That was sort of a fun episode. And the department manager at Imperial actually tried to convince me that I should take my notes offline because the students were using up too much paper printing them out. And this is the point where Tom Kibble stepped in. Another moment in my life where Tom Kibble's wisdom prevailed over other forces, and I was able to keep the notes up. And the World Wide Web was so physics-centered at the time that for years afterwards, if you Googled my name, pages and pages would come up, mainly including various of my lecture notes. Even when I moved to California and all that, you'd Google my name, and there'd be these pages of lecture notes from Imperial College just sitting there.
On the personal side of things, did you think at a certain point that you'd make a life for yourself in England, that Imperial was going to be your long term employer, and you'd raise your family there?
That was very confusing to me. The way physics careers work, it kind of had to be the way we thought about it. But it was very hard for my family to transition to England. There were various factors that made it hard. And so, there was always this question of, "What would that really look like?" And I was keeping my eye out because of those pressures. I was keeping my eye out for opportunities to come to the US, even though I had such fond feelings for my situation there. And I have to say, looking back, there are dear friendships with colleagues and neighbors of that era that have sustained and grown despite time and distance. So in many ways we had a wonderful foundation for a good life there.
How did the opportunity at UC Davis come about?
I think at some workshop, maybe it was Michael Turner who said, "Do you know about this plan at UC Davis to build a cosmology program?" And at that point, when you're at a senior level, you really shouldn't go asking, they should come to you. If you're tempting enough, people should come to you. But I think it was Michael Turner who said, "You should just call them, and talk with them, and see about that." And he said he had been consulted, and he had recommended me. And I made a phone call and some initial connection, but they first tried other people. And then, at some point, they called me and said, "No, we're really serious. We want you to apply." It's funny because I was just recalling a particular moment of that.
So I applied and then I was really surprised that I didn't hear back. And I thought, "This is just weird because they said were really, really interested." Then I didn't hear back. And then, I thought, "OK, this doesn't feel right, but just in case there's been some miscommunication," I sent an email to the department chair, and it was a good thing because somehow, there had been a glitch, and they hadn't seen my application. And they were busy wondering why I hadn't applied. Just now, we were sorting through some old junk and found this ski pass, which is where I was, at a ski conference in France when I finally had this moment of, "OK, I'm going to send that email." And it was weird because at a conference, you're distracted, you're busy with other things. I said, "OK, I'm going to send this email to Davis, see what's going on." I just had this momentary lull where I could do that. And so, this ski pass just reminded me of that story.
Who was in the physics department that you might build up the program with?
Well, in the end, it was a joint effort with me and Bob Becker, who's an observational astronomer known for his work on Lyman-alpha forest, and quasars, and many other things. Also there was already a strong particle theory group at UCD that had interest in cosmology as well. But Bob and I were the co-chairs of the search committee, and we worked together closely on that. That was a really wonderful team. Once I was hired, there were three positions on offer, and we both agreed, and this is actually a bit anomalous in the world of academia, that what we'd do with those three positions is just put out very broad ads and hire the people we found most exciting. Not try to have a plan, and not try to say, "Well, Bob should get someone in his area, and I should get someone in my area," which is stuff that happens all the time in academia.
But I think getting to know Bob and knowing that we'd be working together and that we shared those sorts of views really helped me decide to come. And yeah, we agreed we wouldn't even have a prejudice theory versus experiment or observational cosmology, that we'd be very broad and see who we got. That worked well and we pulled together an eclectic group of really awesome individuals. And I remember, at early stages, various of our new hires came to me and said, "I really wish there were more people in my field." Typically, people came in from post-docs, where whatever they were doing, there was a larger environment, more senior people were doing it, and they were missing that. And I was so gratified that by the time things rolled around to do additional hiring, those same people were at the forefront of saying, "We've got to get people who are doing stuff really different from what we do." That's what makes an exciting group. So I really have good feelings about the people we hired and where they've taken things.
Was your sense that your hire was part of a broader mission? Within the UC system, there's so many powerhouses of physics: Santa Barbara, UCLA, of course, Berkeley. Was your sense that UC Davis was in growth mode in physics particularly? Or was there a larger initiative across the sciences for UC Davis at this time?
Well, I would say even beyond the sciences, UC Davis was just in growth mode. It was in growth mode, and it was very, very excited about using that growth to be better, which isn't always the way universities think. But UCD was really embracing the idea that this growth could raise the standing of UC Davis enormously, which it has, in many different fields. So scattered around the faculty are other people, who came like I did, from very high-powered places to Davis with the excitement and a sense of liberation of building something new. There's something kind of freeing and exciting about building something new. And that has been prevalent in many places across the campus.
As I sought advice about this move from colleagues outside of UC Davis, everyone said, "University of California has this very strong foundation as a strong university, is set up to be a strong university." And so, there was a sense that I would benefit from that, which we certainly have. Because it's a UC, there's some baseline of strength to build on. And also, in terms of hiring observers, access to the Keck Observatories was a huge thing.
The opportunity to build the cosmology program offers a really unique opportunity to engage your particular interests and broaden them out to bringing people in, doing seminars. In what ways did you connect your research interests at that time within the administrative opportunity that this presented at UC Davis?
I tended not to think of it quite that way. I tend to think of the raw material as sort of passionate, intellectual energy. And that definitely went into the group-building, in terms of trying to identify and attract really exciting individuals to hire. And then, in my work, it was more just choosing projects that I found exciting. And one of the things that was really influential in my decision to try to make things work at UC Davis was the internet. Which meant everyone was connected. So it didn't matter so much who was exactly there. So I sort of kept those things separate. I pursued the research interests that excited me most, and I hired the people who excited me most, really, based on the raw intellectual energy rather than a sort of piecing together of a jigsaw puzzle of interests. Of course emphasizing those values allows for all kinds of interplay and reinforcement, between the group-building and research sides, but none of that proceeded in a systematic way.
Were you still working on arrow of time stuff during this transition period?
It was taking shape, yes. So I still had this feeling that it was relevant, and I hadn't quite pushed it through very far. In fact, one of the things I had done to try to, I don't know, reconcile my limited economic situation and my intellectual interests in England was, I signed a contract to write a book about the arrow of time. And with the intuition that the arrow of time had more to do with cosmology than I understood at the moment. It had more to do with my actual research interest than I understood. It was supposed to be a popular level book. And the insights I would develop from writing that book would position me to understand more what I wanted to do with it in my research. The book was never actually written. I actually wrote a draft, and there was sort of a mix of situations where the publisher was sold, and sold again, and the new editor wasn't so excited, and I was running late. And meanwhile, I was in Davis and had fewer economic concerns. And so, the book was never written. If you Google it on Amazon.co.uk, it actually exists, there's a page for it. [laugh]
The arrow of time is one of those concepts in cosmology that the broader lay-public loves to learn about. So I wonder if that's one of the things that motivated you to conceptualize of this project as a popular book.
Absolutely. And it helped me get the contract. I thought it was a great mix of all these things. And interestingly, in the end, I captured a lot of what I was trying to do in this book in a TEDx talk. And to some degree, I feel like that accomplished it better. What I most wanted to do with the topic is not instruct people, but to get people curious so that their curiosity would take them in their own directions about the arrow of time. And in a sense, part of my struggle in putting the book together was, I hadn't found a way to do that well in book format. You need this many chapters, and you need to have the reader go from one to the next, so you have to have it all sorted out for them. And what I felt worked with my TEDx talk is, it provoked a lot of curiosity. The TEDx is ten minutes or so. One of my favorite kinds of public lectures to do is to show the TEDx talk, and then just take questions. The lecture is mostly Q&A, and it's wonderful.
How much were you thinking about Boltzmann's brains at this point?
In the 90s, probably not much. So Boltzmann's brain is a phrase I invented, but it's a concern people have had for 100 years. It's not a new question. It's basically, in equilibrium, a small fluctuation's more likely than a large one. And our universe is a whopping big fluctuation. So, "How come we're not part of a smaller fluctuation, but this one?" is sort of the question. And so, that must've been hovering on my mind to some degree. I think that was the kind of rigor I was hoping to get from the book-writing, just to think more carefully about all these things, which I hadn't so far. It was emerging. For me, I think the real driver was these ideas about de Sitter space that I mentioned and equilibrium, which sort of emerged in the very early 21st century.
But the arrow of time is an important central concept in pulling together different ideas about fine tuning in cosmology, which has been a theme of mine forever. So I really was attached to it from that point of view already for a long time. I think these things are really intertwined. In lots of ways, it's hard to even lay out for you. Because one of the things that's really coming home to me in recent years is how ill-prepared the world of physics is to converge on a clear idea of what we can say about the beginning of the universe. So I sort of feel my journey with that is that, the first paper I ever read was Alan Guth's paper, and it just laid out this agenda of, "Let's find dynamical mechanisms that explain the state of the universe." And then, that was the subtext of the ways we work with inflation for a number of years. But what soon captured my attention was how there's a whole question of how you start inflation. And there's the question of, "Does that require more tuning than just a plain big bang without inflation?" And that's actually a pretty complicated topic to wade into.
I found it quite distressing that so many of my colleagues didn't care about that question. What's sort of settled for me on that is that the community of physicists doesn't have an established protocol for deciding we have a good theory of the beginning of the universe. And there's lots of different ways of looking at it. By contrast, there's no doubt that we've discovered the Higgs particle. There's absolutely no doubt. There's not a serious scientist who says, "Oh, no, we didn't discover the Higgs particle. It’s just a glitch." It's very clear what it takes to discover a particle, and we've done it. But what it takes to say something useful about the beginning of the universe, there's lots of different ideas about that. And you're surely aware of all the passionate debates that have unfolded about the value of inflation, and I finally realized that it's all rooted in that. It's not this lockstep technical question of, "Do we have a theory?" It's not at all the same as, "Do we have a particle?"
It's like, "What does it even mean to have a good theory of the beginning of the universe?" And there's not a ton of common ground about that. Here is something I say very often. This is something you may have essentially already done at this point, once we're finished with this conversation. But I say someone should write a book about the different pioneers of cosmic inflation and the really, really different way each one of us thinks about what inflation means. [laugh] It's really different. People have really different ideas about what it even means.
To compare the discovery of the Higgs, if we look at these things as they develop in a narrative of history, we can discover the Higgs now in the present. It's here. Inflation requires time travel, to a certain degree. And so, I wonder if, as you've matured in the field, and the exuberance of Alan Guth's original paper is 40 years ago at this point, you've gotten more comfortable with the idea of the unknowable. That may have seemed quite knowable 40 years ago.
So I actually have two very different answers to that. And one of them, I'm really grateful you asked this question because it brings me back to another Tom Kibble story. One of the numerous wonderful conversations with Tom Kibble I've had in my life. It was some lunch at Imperial College, and I spouted off some version of what you just said, which is what many people are fond of saying, "Well, you can't reproduce the beginning of the universe, or you can't do repeated measurements, you can't build up the quality of your data," and all that. And Tom put his foot down in his very gracious but very firm Tom Kibble way and said, "No, that's absolutely wrong. There's data you can collect about the universe, there's data you can collect about particles.
There's always questions you can ask where the data you are able to collect isn't enough to answer, and there's other questions you can answer well." That's true with particles. Look at where we are with grand unification. There's particles people dream of that they never will never see in a lab. So Tom said, "It's absolutely the same thing. There's just an amount of data that you have, and there's things it's good for and things it's not good for. And there's always an opportunity for progress by collecting more data and being clever about ways you connect it with your theory." And he sort of offered at that level a complete parallel, which to this day, I find really compelling.
But having said all that, I also think there's something really profound about what quantum physics does to limit the kind of conversations we can have. I have this paper on the origin of probabilities. I think broadly speaking, one of the challenges in cosmology is that we have a lot of things that we do in a very intuitive way in the laboratory context.
But we then try to bring the same intuition to cosmology, it's possible to make an error doing that. That the parallel isn't as easy as you might try to make it. And one of those is the relationship between counting arguments and probabilities. There's this whole thing where people say eternal inflation predicts infinitely many of this and infinitely many of that. So if there's infinitely many, how can you tell which is more probable? And one of the pieces of that origin of probabilities paper says that's just the wrong argument. In many cases, counting is related to probabilities, and then more power to you for using that intuition. But it's actually not always connected. And so, I don't think we need to dig too deep into the specifics of that, but that's one example of where elevating the problem to a cosmological context requires that you think differently. And so, I guess it still fits with Tom Kibble's point, which is that there's some things you can test and some things you can't.
I still stand by that position, which I think is beautiful, and I was very rapidly convinced by Tom to that way of thinking. But the research I'm just writing up now is sort of pushing even harder along those lines. I have this whole piece of my research involving the emergence of classical from quantum. And that was stuff I wrote a bunch of things in the 90s about, and I've returned to that. There's a whole question of, how do you think about quantum physics in the absence of an external observer? The external observer is often a crutch we use to figure out how to work with quantum mechanical questions. But in cosmology, you don't have this external observer. In the 90s, there was a lot of interest in something called consistent histories, which is a way to try to impose some discipline on this question of, if someone hands you a wave function, how do you interpret it without talking about some external thing and measuring it?
And one of the features of that scheme is that there's not just one story that comes from that analysis. I can hand you a wave function, and you can come up with many different stories. And in some sense, there's many different stories, and there's the science probability story, and in that sense, it sort of has this standard kind of physics-y flavor to it. But there's also different sets of histories, and the theory gives you no way of preferring one over the other. So there can be this set of histories, and there's sort of a sense of a complete set. And among the set, you can assign different probabilities. But then, you could chuck that set out and choose a different set that gives a different set of stories. And at the time, I felt that was a real shortcoming of that framework, and that surely, there were ways do better than that. And I wrote some papers trying some other methodology, which seemed to tame that multiplicity to an interesting degree. But the problem I've been working on recently, which is really just the last few months–the reasons I told you to postpone our conversation til February were correct, but an added reason I didn't tell you is, I was hoping to get a little more clarity from this project. Because it seemed really important to me. And it's influencing, really, my very broad thinking.
That project, without getting into a lot of detail, has forced me to face up, first of all, to the power of the consistent histories' formalism, that I'm feeling like it's less of an optional thing and more of pretty much what we have to do to do a quantum theory of the universe. And then, also, it's how this ambiguity I was just talking about is really part of the story. So we can't just say, "These are the right histories." And I'm intrigued by how much that's impacting me, given that I've already been on a path of appreciating a lot of ambiguity that we're stuck with in quantum physics and cosmology. But somehow, this is bringing it to another level for me, so that's interesting. You started with this question of getting used to what's unknowable, and to some degree, that's really been happening to me over my career, that there's been important shifts in my thinking along the way.
And I think you characterize it very well that you launch into a career sort of embracing the culture of the moment in physics. And the culture of the moment was, "We can nail this. We can understand the initial conditions of the universe. And while I'm really excited about things I've done that feel like advances, I also feel that my journey over the decades has been humbling as well in terms of what we can expect. So it's sort of, I would say, really fun mix of both.
I'm not sure if it's personally relevant to you, but, of course, there are many religious traditions that would take great comfort in what you're saying on the basis that, "Well, of course, the universe comes from God, and God is unknowable, so there's no surprise there."
So you mentioned religion, and then I'm going to expand on that and say I count among my circle of friends and loved ones maybe people we might characterize as hippies, for lack of a better word. And in that sphere, there's no end of stuff like that. You tell someone you're a physicist and, oh my God, they get started about the relation to Eastern thought, and this, and that. My natural response to all of that is, "OK, that's fine." I sort of see all those things, the way people can take a lesson from physics and bring it into other spheres–I know a shaman who, when she starts her thing, she starts it by talking about what percent of the universe is unknown, and she gets all the numbers wrong. And I let go of that, and I say, "People are using physics here sort of as a resource for poetry”. And that's fine. That's what it means to them. But I also feel that what physics actually is to physicists is the exercise of finding, “what can you do by pursuing a much more rigorous, structured effort?”
And so, I try to talk to my friends about how separately I see physics. What is knowable and unknowable, I think, for a physicist is something of a mystery. We don't even know if quantum physics is absolutely right. So there's sort of this, "Within quantum physics, this is what's knowable and what's not knowable." But right now, I don't mean this in a negative way, but I feel like I've been backed into a corner with this consistent histories stuff where I didn't see myself going. And now, I'm sort of wondering, "Where does that leave me? Where are the things where I can say, 'Yes, this is something I can understand about the universe?'" So it's always this dynamical thing of, "Where can we go next?" Which is a very physics-y thing. However, with the benefit of age, and perspective, and all that, at the end of the day, physics is done by humans. And also, I would say, wherever you stand on deep beliefs and all of that, the fact of the matter is, religion as we know it is consumed and interpreted by humans. I'm not religious at all. I'm an atheist. It's an easy question to answer because I can just look inside, ask, "Do I believe in God?" and the answer is no.
But I think there's a lot of wisdom in religion. So I do respect how other people find a source of wisdom in religion and feel that in many cases is absolutely awesome. And so, somehow this lesson of God being unknowable may be a certain message about humbleness that I can relate to. So I gave you a little spiel to resist your comment about religion and God, but the fact of the matter is, I probably have another side, the human side of it all, where I probably can go there a little bit further than I might've suggested.
On the question of concentrating your efforts on the knowable, to go in the absolute other direction, I'm curious how you approach the problem of the multiverse. So Andrei Linde is convinced that the multiverse is a testable proposition, and then, of course, on the other end of the spectrum, for critics of string theory, particularly, the multiverse drives them crazier than anything else because of the fundamental un-testability of the multiverse. So I'm curious from your perspective, coming at it from the vantage point of probabilities, where do you see yourself slotted in on that broad debate?
“All over the place” is the easy answer. And I guess I don't see myself slotted in very well at all in that set of polarized views. I see myself reaching for opportunity wherever I can find it. And one thing I can say is that the notion of the multiverse isn't well enough developed to really know how to answer that. I think it's fair to say that the pieces Linde needs to believe about the multiverse to say the things he says, he finds very compelling. So he feels good about making such a statement because he feels the pieces that he has to assume make a lot of sense to him. And then, to people who are more skeptical, those things don't make so much sense. So again, we're sort of in this place where it's almost like a Rorschach test. It's a test of our personalities.
So it's sort of a perspective I have on how people navigate. Things that haven't been developed firmly and technically yet, but you think, "Surely, we'll get there," and then you build your intuition on that expectation, and things like the multiverse are grounded in a lot of different ideas about that. What can you assume is going to be worked out, and it's just a matter of time before people work them out? And what do you think is this gaping flaw, missing piece that no one knows how to complete? And I think that's sort of the character. So I've been historically very, very negative about the multiverse. My intuition has been very negative about the multiverse. And I felt that these measure problems that people encounter seem really profound and tricky. And we absolutely cannot finish this conversation without talking about the notion of infinity, which is really, deeply influential in my thinking and separates me from colleagues in certain ways. But I have to say that what I learned from the origin of probabilities paper is that maybe there's a way for this multiverse to work.
So maybe there's a way that the issues of the multiverse can be evaded, not in the way everyone's hoping they would be, but in the way of saying, "Nope. These questions can't be answered." And so, some set of questions can't be answered. And then, the question is, can you have a multiverse theory that despite the difficulty of answering leaves you with enough of a theory that it's compelling. So we could say, "OK, here's are all the things it can't answer. But it's still an awesome theory, and here's why we love it. Here's why the community has rallied around it, and this is the right way to think about the universe."
So I shared with you sort of a piece of this recent project that's on the unknowable end, but at the same time, this project has also opened up sort of interesting opportunities for doing cosmology. So it's a mix. And I have to say, it's still fresh in my mind how I end up feeling about that. It may end up being mainly a huge plus for certain ideas I'm very fond of, despite the sort of unknowable piece. So it's a really fun mix.
The question I wanted to ask before was, and perhaps it's as much a psychological question as a physics question, do you think that part of the interest in pursuing the idea of the multiverse stems out of a frustration around the unknowability around the Big Bang? In other words, if the multiverse is really, then doesn't it sort of deemphasize the primacy of the Big Bang as the be-all and end-all to understand where it all comes from?
It's a good question. So we've somehow managed to talk for hours about cosmology without mentioning the anthropic principle. And that's probably a great success. [laugh] There's so many threads to this. It's a lot of fun. So I would say, let's just try to follow our noses into this. I don't think there's any way to organize this conversation carefully. But one thing, which links back to an earlier part of the conversation, is that for many people, the multiverse is an abandonment of an agenda in physics, which was absolutely the agenda I recall from my early stages of my career, which is, "We will find The Theory. This is The Theory. This is the way to understand all the particles." Sort of a watertight picture of everything, all of physics. And so, people have been sort of pushed and pulled into this multiverse picture for a variety of reasons, but one of the problems people have is that it means letting go of that earlier agenda.
Actually, one thing I should say is that multiverse means many different things to different people. And there's the many worlds of quantum mechanics, that's one thing. But that, you can still have, even with a very strict set of laws of physics. Then there's the multiverse of the string theory landscape, and that's more provocative from the point of view of “laws and physics”. By the way, people don't even agree whether string theory really predicts this landscape. There's people who still think string theory's just going to deliver this one set of laws. So it's kind of up in the air where we're really headed. But I think a fascinating part of the story, and this really ties back to something I said earlier, which is that the notion of cosmic acceleration and the possibility of the cosmological constant, or whatever it is, dark energy, is so disruptive to theoretical physics that many people who absolutely believed in the old dogmas that I learned as a student have felt dragged with great difficulty into thinking about the multiverse because of what it means for Lambda.
And Steve Weinberg was one of the people who really pushed in that direction, really, based on trying to understand the cosmological constant. He's revered for many things, but one of the things he's revered for is actually having a theory that predicted there would be a cosmological constant before it was observed. So this is an absolutely awesome part of the history of physics. But when he came out with that theory, he was using anthropic thinking, using a sort of multiverse thinking. So part of anthropic thinking is, "This is what you need to have life," and then somehow, another part of that is that you need to somehow have some notion of how the laws of physics sample different possibilities. And all that can seem very radical against the context of the standard ways of thinking back in the day. And I remember running into Steve Weinberg at the conference shortly after that paper came out and saying, "Steve, I'm just so surprised, knowing what I know of you, to see you go down this path." And he gave this response that was very humble and honest. And he said, "You know that I've worked so hard to understand the cosmological constant in other ways. And it just hasn't worked. And so, I felt I should try this." And I think this is an example of how difficult that problem is, and also, ultimately, why I try to convince my astronomer friends that this is a really profound and exciting problem. This is the kind of revolutionary"–I still do, but especially in those early days, I'd go around saying, "This requires a revolution in our thinking about physics to understand it." And this is an example. We have all these people who never wanted to think about a multiverse, now thinking about a multiverse, thinking they have to think about a multiverse. And so, it's exciting. It's really at this cutting edge where we don't know where we're headed.
And this is a great place to raise the concept of infinity and your unique approach to this. I guess the question there is, do you need the multiverse to think rigorously about infinity?
So here's where I come from. I say, it's impossible to think rigorously about infinity. Infinity's a meaningless thing to a physicist. And then, that's not quite true. Let me elaborate a little bit. First of all, we will only ever have a finite amount of data. At any given time, we will only ever have a finite amount of data. And so, there always will be a finite theory that explains all the data. So then, there's the question of, what good is infinity? The notion of infinity can often make things simpler. That's the good infinity. And an example I like to give is, we think of space as a continuum. And so, between any two points, there's actually infinitely many points. That's what a continuum means. And if we didn't think space was smooth in that sense, was a continuum, then things like Lorentz symmetry wouldn't be defined. Lorentz symmetry assumes the continuum in space to do the Lorentz transformations on. And so, basic concepts that underpin our whole thinking about physics rely on that notion of infinity.
Now, technically, we don't know if space is a continuum. Technically, we could have little, tiny grid or something under there. And then, that would mean that the concept of a Lorentz transformation wouldn't be perfect. It would be flawed. But those flaws could be below the radar. And so, as a practical matter, rather than worrying about those flaws, and where they might be, and what they might do, we just assume a continuum and talk about Lorentz symmetries. And that's a great use of infinity, but I would say when infinity gets you into trouble, that's when you should start wondering what you even mean by it. And so, while the spacetime continuum is infinity doing good. Infinity can also do bad.
Picking up, now, a thread I touched on earlier, one of my objections to the multiverse that comes from eternal inflation, which is not necessarily as grand as the string theory landscape, is that it's presuming an infinity. So you presume you can have inflation go on forever, and there's a notion of a continuum behind that that lets you express that mathematically. And then, you get into all this trouble with the multiverse, and the measure problems, and all of that. But it seems like the starting point should be, why did you believe that infinity to begin with? It's not just making your life simpler. It's not helping you propose some elegant thing. It's actually creating this total disaster for you. And one of the pieces of this whole de Sitter space and equilibrium thing that I have been involved with over the years is the notion, and this comes from people like Willy Fischler, and Lenny Susskind, and Tom Banks, that when you think in a certain way about de Sitter space — as I mentioned, it's the maximum entropy you can have for a cosmological constant--the maximum entropy is finite.
And if you think about entropy as the logarithm of the number of states of a system, which is a standard formula, then it's saying that if the maximum entropy is finite, then surely, you could describe the universe with a theory with a finite Hilbert space, with a finite number of states. And that's, on one hand, a really radical thing. Because even a single harmonic oscillator has an infinite Hilbert space in physics. So we're not used to thinking of a world with a finite Hilbert space. And of course, the size of this Hilbert space is massive. It's an exponential of an exponential of this de Sitter entropy. So it's not a small Hilbert space. But to me, building physics in a finite Hilbert space is a really exciting idea because it inoculates us from this multiverse and the damage that comes from that belief in infinity. Perhaps the problems with the multiverse are nature’s way of telling us about its intrinsic finiteness. That's been sort of my party line about the multiverse and infinity, that actually, we're getting a message from nature that everything's really finite, and you can't have this extravagant multiverse, and all that. But then, this origin of probabilities project helped me see how it might be possible to tame the multiverse in a certain way, as I mentioned.
And so, that's caused me to feel a little less rigid. I originally felt that this de Sitter equilibrium picture really allowed me to draw the line and say, "No. This multiverse thing is nothing but trouble. And here's how nature's telling us nature has avoided this trouble. So we should shut up and listen. And here's how it's going to work." But having some window on how it could work differently through this probability work has sort of calmed that a little bit for me and made me think, "Oh, well now I should be curious." Because as I said, infinities are fine as long as they work out. As long as they help you, they're fine. So that's what I have to say about infinity.
I'd like to ask, back on planet Earth, one question I'm particularly interested in regarding your term as chair of the department from 2011 to 2016, I'm very interested in the concept of near and distant history as it relates to what science has been going through this past year with regard to diversity and inclusivity. Which, of course, now, is and very rightly so is on everybody's radar. So my question is, as chair, starting ten years ago, was UC Davis talking in those terms? Was there an emphasis on enhancing diversity and inclusivity? Specifically because the department and the university generally was in growth mode, perhaps the department did not have the emotional and historical baggage that a place like Caltech or Princeton would have.
I would say it's more complicated than that. And I think there's different forces at work. Certainly everyone was generally aware that that was a problem for our field. But when you're trying to come up in the world, and make your mark, and be taken seriously as a higher standing department than you had been, you want to show that you're hiring the best. And that can be complicated in terms of, are you just buying into the stereotypes? Or are you being open-minded? And I think we actually did very well. So the different hiring I've been involved with has expanded diversity, really, without ever making a thing of it. And one of the glaring things is how that's not really been working out with Black people in physics. But we have more women, we've expanded in terms of Latino faculty and different areas.
And at the administrative level, they were always scrutinizing our levels of diversity and promoting opportunities to increase it. So we knew when we saw an opportunity we could run with it. But these were always people we started out being excited about in their own right as scientists.
But here's something I can share that since I finished my job as chair, I sort of felt that became something of a different kind of issue for me because I ended up feeling like, "OK, I've had this experience, I've done well as chair, and I've helped, over many years, UC Davis become more prominent." And diversity still is a problem in physics. And I sort of felt like, "What can I do at this stage in my career with everything I've learned and the fact that I care so much about this? What can I do differently? How can I contribute more?" And I feel like at the heart of that lie conversations, which we actually touched on earlier in this interview, which is what it means to be a physicist. What does it mean to belong to the physics community?
And I feel it’s especially important to talk with young people about that. And we have students, I think, more energized than ever about diversity questions, and so there are more opportunities. As department chair, I had opportunities to talk with students about these things, various opportunities of different sorts. And you and I talked about this whiz kid thing, right? And that's still a disease. And speaking really frankly with students about that, or even being able to tell them that impostor syndrome works for department chairs, too. And they just kind of look in shock that that was imaginable. And it's the same thing I was sharing earlier about my own path, that I'd look up at this sort of pantheon of great senior figures and wonder what my place was in that. And, over time, I discovered it was the place I made for myself. There's sort of a place you have to fit into, but also the place you make for yourself. And the diversity of different people's natures brings them to different places.
And so, I think, clearly, we have so much need to improve, and I think different people are contributing different things. But, for me, having really frank conversations with students about this whole belonging thing, and then how many fallacious ideas swirl around it, and trying to break through that, trying to bring more diverse people in touch with the idea that they really have a path into physics–some of the scary things are just false. No, it's not about being a whiz kid. But another scary thing is you actually don't know. And that's just true. I try to tell people that by not knowing how it's going to turn out, you already belong. I don't even know. I'm writing a paper. I don't know how it's going to be received. I don't know. It may be just torn apart. There may be something I've missed that my colleagues just tear apart, and it's garbage the minute it touches the archive. And, of course, I'm saying this from a much more comfortable place than a student would be.
But that's still part of the game. It's people like Steven Weinberg staying up til midnight because they're just damn curious about something, even after they've won a Nobel Prize. This is what physics really is. And I feel that right now, the tool I have that I trust the most in helping our diversity problems is really engaging with students on these topics, and also learning from them. Understanding better how things look to them now, and from that learning what things can be addressed to improve the situation. UC Davis is getting involved in a bridge program run by American Physical Society to help improve diversity, to help draw more diverse students into physics. And I've volunteered for that specifically with the idea of contributing this kind of mentoring that we’ve just been talking about.
Just to bring the narrative up to the present, we talked about the pandemic year, but since you stepped down from chair to 2020, what have been some of the major projects you've been involved with more recently?
I should check which papers came out when, but one of the things that has been happening for me is quite an evolution into the quantum side of things, which was actually very fortuitous because there's actually a lot of money and grants in it these days. But it was something I actually started before I saw that coming. And it's sort of hearkening back to work I did in the 90s, but revisiting. So this project I was just talking about, about consistent histories and so on, has been quite a while coming to fruition, but I'm very excited about it. And it's sort of linking the arrow of time and quantum physics in very interesting ways. So this ties us to a number of things. I have this whole thing about finiteness, and one of the things that means is that if you do have inflation, it's only a finite amount.
So if you have inflation, it's not this eternal inflation multiverse thing. Something has to cut it off. And I've explored some ideas about that and wound up with the idea of, if there's only a finite amount of inflation, then there's some messy start to it that might actually be in the visible data. So rather than have this eternal thing where whatever started inflation's way out beyond our observable sphere–it turns out that the arguments we have based on the equilibrium cosmology, the finiteness of inflation is just enough. So you will just see the beginning of inflation as you look onto large scales in the university. And so, we ask, "Well, what if there's some messy start to inflation? What might we see?" And this goes under the heading of entanglement of the initial state of inflation, and my collaborators and I have written several which explore that. It's quite exciting because they're really potentially observable effects, which we haven't quiteI pinned down to tests yet.
But that's something I'm quite excited about. And then, I don't know how much to get into the details, but this project that I'm just writing up now links a lot of my favorite topics. So one of the key pieces of how classical emerges from quantum is something called decoherence and einselection, which is how quantum physics has its own mechanism for sort of getting rid of Schrodinger cat states, which are so annoying and so distractingly un-classical to us. And so, the special role of decoherence in all of this is called einselection. And einselection has only ever been understood in conditions which exhibit in arrow of time.
And so, I've asked the question, "Is einselection even possible if you don't have an arrow of time?" Imagining that one could create a link, say, "OK, well, to have classical physics, you need einselection, and to have einselection, you need an arrow of time. And to have an arrow of time, you need a low entropy start to the universe, which will tie things together in some interesting way." But what I've found is this is where this consistent history thing comes in. I have this toy model, and I put it in an equilibrium state, which doesn't have an arrow of time, and I ask, "Do I have einselection?"
And what happens is that when you just systematically pose that question very carefully, you wind up using consistent histories' formalism. And you find that within that formalism–remember, I said there's alternate narratives you can have about the same quantum state? And that allows you to extract, from an equilibrium state, narratives which do exhibit an arrow of time. And so, you can have einselection. So it's all a little bit different than I expected. And it's still kind of sinking in what I want to make of all that. But it's exciting. One of my favorite kinds of experiences as a scientist is when I start a project and get surprised by what I get. So I'm really enjoying this. That's one of the things.
Well, for the last part of our talk, I'd like to ask one broadly retrospective question to sort of tie it all together up to this point, and then we'll look to the future. So I guess the best way to go about this is, one of the things that's remarkable about your career and the narrative through-line is that really, since the beginning, your research has always brushed up against that boundary of the knowable and the unknowable. Inflation, multiverse, time, dark energy. I want to ask, intellectually, scientifically, do you tend to think of these different areas as interlocking insofar as advances to move toward the knowable in one of those fields will unlock the rest because physics is working toward that truly grand unified theory of everything? Or do you think that these things necessarily will remain separate to some degree? Insofar as we can approach understanding them.
So the remarkable thing about your question, which sounded like it was going to get all messy and involved, is I can simply say, to your first version of the question, yes. It's an easy answer for me. Yes. That is how I'm thinking, for better or worse. I'm always following my nose and trying different angles, and that may be much more of a locally determined step that I'm taking. But I'm very aware of all the interconnection of these things. And it is the sense that it really could all come together that keeps me so enthusiastic about any one of these small steps. But I feel we are a long way from actually piecing all this together. For example the toy model work about einselection I just mentioned. I am excited about what I am learning form it but it is a long way to go from my simple toy model to theories of the universe. I’m not sure where all that is headed. But I'm very happy to take small steps. I'm not always expecting that a grand synthesis is right around the corner. But the promise of it somewhere out there is always at least somewhere in my mind.
To the extent that there is one of these areas that you're most bullish on understanding first that will unlock the others, how might you rank them? Would dark energy come as the most tantalizingly close?
So I don't feel like we're that close. And my sort of visceral response to your question–I'm sort of doing some introspection based on your question, and I feel like what goes on inside of me with that is that I have a certain aversion to ranking these things out of concern that getting too comfortable with a ranking could create a missed opportunity to notice some new way of thinking. So I'm aware that all these pieces seem really important and that they have a chance to feed on one another. And I feel uncomfortable with trying to rank it for the reason I just said. One of the things I just find really intriguing about science, especially at this edge of what we understand, is that different people bring different prejudices about what they're willing to count on, and what they're willing to expect to require changes. And so, for example, I'm a total addict to quantum physics. I'm totally not one of these people who tries to think of an alternative to quantum physics. But that's just a prejudice, in the end. But it's one I'm very comfortable making. So I am comfortable making assumptions and building my work on certain assumptions I don't question very much, but apparently not in terms of the ranking you just asked me about.
Last question. Looking forward, especially because you have a graduate student who's waiting to talk to you, I want to ask, looking to the future for this graduate student or any of the others where you feel a certain responsibility to provide guidance that's going to make for a successful career, looking back at your own personal narrative and the enormous benefit, and as you called it, magic of that time when you connected with Paul Steinhardt, he was just on the cusp of coming up with a way of improving on inflation, and then off you go, what are the things now for your graduate students where, in good confidence, you can extrapolate into the field for their careers far down the line where you say, "These are the things to be excited about. Here are the areas where fundamental possibility and discovery may happen. And here's where I feel really good about fostering your interests along this path"? How do you feel about those things as you look to the future?
So I don't think that way at all. And it's actually been a struggle for me over the years to kind of come to terms with those responsibilities which you've outlined very well. And with the fact that in my own research, I've always followed my nose and never actually been careerist–that's not perfectly true. But I've been less so than I might've been. And I think for me, personally, the thing that always mattered most was that I was enjoying what I was doing.
But that's the point, it's not being careerist, it's encouraging the most exciting science. Because as your career demonstrates, when you pursue your interests, the opportunities make themselves apparent. That's what I mean.
Sometimes. I also feel incredibly lucky. To me, it looks like a lot of risk-taking that miraculously panned out, to be fair. And so, I try to have that conversation with my students very frankly. And there are different areas of physics where you can be much more practical-minded. Let me start with a slightly different comment, which is, when I started the job of supervising grad students, I felt like I was duty-bound to come up with the first project for students that was really well-defined to sort of get their feet wet and get started. And I actually have a lot of trouble doing that because either I don't know whether the project's any good, or if I do, I've sort of done it already. So there's this sort of split. I had this idea, "I should try to find this middle place." And I don't know that middle place very well in my experience as a researcher. And I started feeling terribly guilty about that.
And then, I started noticing after I had had a number of students, that every now and then, I'd sort of accidentally have a very formulaic project. But the students didn't do so well. And I realized that by exposing the student to the raw uncertainty of finding a project and how the fact of research is very un-formulaic, it actually was valuable to the student in many ways. They learned that they had to coexist with that aspect of science. And then, also, I think once things did finally get going on something, they were really glad to be there, and they threw themselves into it. They knew it wasn't to be taken for granted. So I've since made a certain kind of peace with that, and I usually tell my students. And there are professors who work very differently. And they have a very concrete idea that, "You do this, and you get a job." And it can work that way. But I just tell my students that's not me. And so, I have trouble predicting what would be the best thing to do for your future, but what I do is, I really take them seriously. I take them, and their efforts, and their research seriously. And I say, "All I can offer in terms of subjects is the stuff I happen to be excited about. And we don't actually know at any given moment if a particular thing is going to be a dud or not."
Now, I have the advantage now that I'm really well-established, and so I can write a letter for someone, and I can say, "Well, this project turned out to be a dud, but this is a really awesome student." It's never been quite that extreme, but people take my letters seriously. That's a nice thing. But I always feel like I need to be upfront with my students about the risks involved. And I share with them that for me, science is mainly about the adventure. And I guess the thing I do come back and tell my students–well, first of all, I think giving advice, people always tell what works for them. And I tell them, "You can go find different advice from different people and get any advice you want based on what works for them." But, for me, what worked was kind of liberating myself from the burden of having a plan. [laugh] Sometimes things didn't go too well because of that. But in the end, what worked was also because of that, feeling liberated from the need to have a plan and just following my curiosity and passion. And I sort of say, "That's what I can offer you. It doesn't work for everyone. It's not guaranteed to work for anyone. But that's what I can offer." That's the way I approach it.
Well, on that note, Andreas, it's been absolutely great spending this time with you. It's been an adventure in and of itself. I'm so glad we connected. And thank you very much for doing this. I really do appreciate it.
It's a lot of fun for me. I think you've navigated your part of this super well. That's made it a lot of fun for me. I just feel really lucky to have such fun experiences to share.