Sean Carroll

Zierler:

Okay. This is David Zierler, Oral Historian for the American Institute of Physics. It is January 4th, 2021. I am so happy to be here with Dr. Sean M. Carroll. Sean, thank you so much for joining me today.

Carroll:

Oh, thank you so much for having me.

Zierler:

This is so exciting because you are one of the best interviewers out there, so it's a unique opportunity for me to interview one of those best interviewers. So, I'm really quite excited about this.

Carroll:

It's way easier to be on this side, answering questions rather than asking them. No preparation needed from me.

Zierler:

Alright, Sean. To get started, would you please tell me your current titles and institutional affiliations? I put an "s" on both of them. I might add, also, that besides your brick and mortar affiliations, you might also add your digital affiliations, which are absolutely institutional in quality and nature as well.

Carroll:

Thanks very much. I am a Research Professor of Physics at Caltech, where I have been since 2006. I'm also an external professor at the Santa Fe Institute, where I've just been for a couple of years. Literally, I've not visited there since I became an external professor because we have a pandemic that got in the way. Or, maybe I visited there, but just sort of unofficially. Online, I have my website, preposterousuniverse.com which collects my various writings and things like that, and I'm the host of a podcast called Mindscape where I talk to a bunch of people, physicists as well as other people. As long as it's about interesting ideas, I'm happy to talk about it.

Zierler:

Now, the academic titles. What does Research Professor entail to the larger audience out there that might not be aware of the different natures of titles within a university department?

Carroll:

It's actually a very rare title, so even within university departments, people might not understand it. Also, my individual trajectory is very crooked and unusual in its own right. So, there's path dependence and how I got there. But research professor is a faculty member. Someone at the status of a professor, but someone who's not on the teaching faculty. So, I don't have any obligations to teach students. To be perfectly honest, it's a teensy bit less prestigious than being on the teaching faculty. So, it's like less prestige, but I have this benefit that I get this benefit that I have all this time to myself. Part of that is why I spend so much time on things like podcasts and book writing. It's the time that I would spend, if I were a regular faculty member, on teaching, which is a huge amount of time. Otherwise, the obligations are the same. I have graduate students, I can teach courses when I want to, I apply for grants, I write papers. It's just like being a professor.

Zierler:

And you take external professor at the Santa Fe Institute to an extreme level having never actually visited. Is that a common title for professors at the Santa Fe Institute?

Carroll:

Well, I have visited, just not since I got the title. Yes, it is actually a very common title for Santa Fe affiliated people. The Santa Fe Institute is this unique place. It's a research institute in Santa Fe that is devoted to the study of complexity in all its forms. So, that's physics, but also biology, economics, society, computers, complex systems appear all over the place. One of the things that the Santa Fe Institute tries to do is to be very, very tiny in terms of permanent faculty on-site. There aren't that many people who, sort of, have as their primary job, professor at the Santa Fe Institute. There's a few, but it's a small number. There's a large number of people who are affiliated one way or the other. I think probably the most common is mine, which is the external professorship. You don't get paid for doing it. In fact, you basically lose money, because you have to go visit Santa Fe occasionally. You can mostly get reimbursed, but I'm terrible about getting reimbursed. But the idea is that given the interdisciplinary nature of the institute, they can benefit, and they do benefit from having not just people from different areas, but people from different areas with some sort of official connection to the institute. So, I will help out with organizing workshops, choosing who the postdocs are, things like that.

Zierler:

I wonder, Sean, given the way that the pandemic has upended so many assumptions about higher education, given how nimble Santa Fe is with regard to its core faculty and the number of people affiliated but who are not there, I wonder if you see, in some ways, the Santa Fe model as a future alternative to the entire higher education model in the United States.

Carroll:

In a sense, I hope not. I think that Santa Fe should be the exception rather than the rule. There should be more places like it, more than there are, but it's no replacement for universities. I will never think that there's any replacement for having a professor at the front of the room, and some students, and they're talking to each other in person, and they can interact, and you know, office hours, and whatever it is. I think that the vast majority of benefit that students get from their university education is from interacting with other students. So, maybe conditions down the line will force us into some terrible situation, but I would be very, very sad if that were the case. Having said that, you bring up one of my other pet crazy ideas, which is I would like there to be universities, at least some, again, maybe not the majority of them, but universities without departments. I think the departments -- the physics department, the English department, whatever -- they serve an obvious purpose in universities, but they also have obvious disadvantages. Someone like me, for example, who is very much a physicist, but also is interested in philosophy, and I would like to be more active even than I am at philosophy at the official level, writing papers and things like that. But I'm classified as a physicist. That's what I am. I see this over and over again where I'm on a committee to hire someone new, and the physicists want to hire a biophysicist, and all these people apply, and over and over again, the physicists say, "Is it physics?" I'm sure the same thing happens if you're an economic historian. Do you go to the economics department or the history department? Or a biochemist, right? So, I'm a big believer in the disciplines, but it would be at least fun to experiment with the idea of a university that just hired really good people. Let every student carve out a path of study. Let every faculty member carve out a disciplinary niche in whatever way they felt was best at the time.

Zierler:

I wonder if in some ways you're truly old fashioned in the way that what we would call scientists today, in the 17th and 18th century, they called natural philosophers.

Carroll:

Yeah, but you know, I need to sort of emphasize the most important thing, and then my little twist on it. The modern world, academically, broadly, but also science in particular, physics in particular, is very, very specialized. That's not by itself bad. You can make progress digging deeply into some specialized subfield. It makes perfect sense that most people are specialists within academia. But there's also, again, very obvious benefits to having some people who are not specialists, who are more generalists, who are more interdisciplinary. The problem is not that everyone is a specialist, the problem is that because universities are self-sustaining, the people who get hired are picked by the people who are already faculty members there. If everyone is a specialist, they hire more specialists, right? They promote the idea of being a specialist, and they just don't know what to do with the idea that you might not be a specialist. So, even though the specialists should always be the majority, we non-specialists need to make an effort to push back to be included more than we are.

Zierler:

Sean, before we begin developing the life narrative, your career and personal background trajectory, I want to ask a very presentist question. From the outside looking in, you're on record saying that your natural environment for working in theoretical physics is a pen and a pad, and your career as a podcaster, your comfort zone in the digital medium, from the outside looking in, I've been thinking, is there somebody who was better positioned than you to weather the past ten months of social distancing, right? So, I wonder, in what ways can you confirm that outside assumption, but also in reflecting on the past near year, what has been difficult that you might not have expected from all of this solitary work?

Carroll:

No, you're completely correct. Actually, your suspicion is on-point. In fact, my wife Jennifer Ouellette, who is a science writer and culture writer for the website Ars Technica, she works from home, too. So, between the two of us, and we got a couple of cats a couple years ago, the depredations that we've had to face due to the pandemic are much less onerous for us than they are for most people. Having said that, they're still really annoying. You do travel a lot as a scientist, and you give talks and things like that, go to conferences, interact with people. I'm very, very collaborative in the kind of science that I do, so that's hard, but also just getting out and seeing your friends and going to the movies has been hard. Now, I did, when the quarantine-pandemic lockdown started, I did think to myself that there are a bunch of people trying to be good citizens, thinking to themselves, what can I do for the world to make it a better place? A lot of people in science moved their research focus over to something pandemic or virus related. Now, I'm self-aware enough to know that I have nothing to add to the discourse on combatting the pandemic. That's not what I do for a living. But I do do educational things, pedagogical things. So, I took it upon myself to do this YouTube series called The Biggest Ideas in the Universe. It ended up being 48 videos, on average an hour long. Literally, two days before everything closed down, I went to the camera store and I bought a green screen, and some tripods, and whatever, and I went online and learned how to make YouTube videos. They were very bad at first. The production quality was very bad, and the green screen didn't work very well. So, you can see me on the one hand, as the videos go on, the image gets better and sharper, and the sound gets better. My hair gets worse, because there are no haircuts, so I had to cut my own hair. So, on the one hand, I got that done, and it was very popular. Hundreds of thousands of views for each of the videos. On the other hand, I feel like I kind of blew it in terms of, man, that was really an opportunity to get some work done -- to get my actual job done. Like you said, it's pencil and paper, and I could do it, and in fact, rather than having a career year in terms of getting publications done, it was a relatively slow year. I'm finally, finally catching up now to the work that I'm supposed to be doing, rather than choosing to do, to make the pandemic burden a little bit lighter on people.

Zierler:

I'm curious, in your relatively newer career as an interviewer -- for me, I'm a historian. I've been interviewing scientists for almost twenty years now, and in our world, in the world of oral history, we experienced something of an existential crisis last February and March, because for us it was so deeply engrained that doing oral history meant getting in a car, getting on a plane with your video/audio recording equipment, and going to do it in person. It was really a quite difficult transition to embrace and accept videoconferencing as an acceptable medium. I wonder, for you, that you might not have had that scholarly baggage, if it was easier for you to just sort of jump right in, and say Zoom is the way to do it. Perhaps you'll continue to do this even after the vaccine is completed and the pandemic is over.

Carroll:

You know, I'm still a little new at being a podcaster. Two and a half years I've been doing it, and just like with the videos, my style and my presentation has been improving, I hope, over time. Certainly, my sound quality has been improving. But I did learn something. I was absolutely of the strong feeling that you get a better interview when you're in person. I think that's true in terms of the content of the interview, because you can see someone, and you can interrupt them. You know when someone wants to ask a question. You can see their facial expressions, and things like that. There's no delay on the line. But honestly, for me, as the interviewer, number one, it's enormously more work to do an interview in person. Either I'm traveling and lugging around equipment, or I need to drive somewhere, or whatever. But also, even though, in principal, the sound quality should be better because I bring my own microphones, I don't have any control over the environment. Unlike oral histories, for the podcast, the audio quality, noise level, things like that, are hugely important. People are listening with headphones for an hour at a time, right? Often, you can get as good or better sound quality remotely. So, these days, obviously, all of my podcasts interviews have been remote, but I'm thinking most of them are just going to continue to be that way going forward. It's an expense for me because as an effort to get the sound quality good, I give every guest a free microphone. I don't want them to use their built in laptop microphone, so I send them a microphone. It costs me money, but it's a goodwill gesture to them, and they appreciate it. It's all worth it in the end.

Zierler:

Given how productive you've been over the past ten months, when we look to the future, what are the things that are most important to you that you want to return to, in terms of normality? Even if you can do remote interviews, even if it's been a boon to work by yourself, or work in solitude as a theoretical physicist, what are you missing in all of your endeavors that you want to get back to?

Carroll:

Well, I think it's no question, because I am in the early to middle stages of writing a trade book which will be the most interdisciplinary book I've ever written. The tentative title is The Physics of Democracy, where I will be mixing ideas from statistical physics, and complex systems, and things like that, with political theory and political practice, and social choice theory, and economics, and a whole bunch of things. I'm very, very close to phoning up my publisher and saying, "Can we delay it?" because a huge part of my plan was to hang out with people who think about these things all the time. It's one thing to do an hour long interview, and Santa Fe is going to play a big role here, because they're very interested in complex systems. This is an example of it. They don't frame it in exactly those terms, but when I email David Krakauer, president of SFI, and said, "I'm starting this book project. Do you have any pointers to work that's already been done?" And he says, "Yes, everything the Santa Fe Institute has ever done counts." Like, ugh. So, I think it can't be overemphasized the extent to which the hard detailed work of theoretical physics is done with pencil and paper, and equations, and pictures, little drawings and so forth, but the ideas come from hanging out with people. In fact, Jeffrey West, who is a former particle physicist who's now at the Santa Fe Institute, has studied this phenomenon quantitatively. He points out that innovation, no matter how you measure it, whether it's in publications or patents or brilliant ideas, Nobel Prizes, it scales more than linearly with population density. In other words, of course, as the population goes up, there's more ideas. But it goes up faster than the number of people go up, and it's because you're interacting with more people. You're being exposed to new ideas, and very often, you don't even know where those ideas come from. You can't remember the conversation that sparked them. So, again, I'm going to -- Zoom, etc., podcasts are great. You need to go and hang out with people, especially in the more interdisciplinary fields.

Zierler:

So, the salon as an enlightenment ideal is very much relevant to you.

Carroll:

Very, very much. Yes. That's right. In fact, on the flip side of that, the biggest motivation I had for starting my podcast was when I wrote a previous book called The Big Picture, which was also quite interdisciplinary, and I had to talk to philosophers, neuroscientists, origin of life researchers, computer scientists, people like that, I had a license to do that. I was a credentialed physicist, but I was also writing a book. So, I could call up Jack Szostak, Nobel Prize winning biologist who works on the origin of life, and I said, "I'm writing a book. Can I come talk to you for an hour in your lab?" And he's like, "Sure." When the book went away, I didn't have the license to do that anymore. All these cool people I couldn't talk to anymore. So, now that I have a podcast, I get to talk to more cool, very broad people than I ever did before.

Zierler:

I'm curious if your more recent interests in politics are directly a reflection of what we've seen in science and public policy with regard to the pandemic. In other words, you have for a long time been quite happy to throw your hat in the ring with regard to science and religion and things like that, but when the science itself gets this know-nothingness from all kinds of places in society, I wonder if that's had a particular intellectual impact on you.

Carroll:

I think to first approximation, no. It has not. Because the thing that has not changed about me, what I'm really fired up by, are the fundamental big ideas. Not the policy implementations of them, or even -- look, to be perfectly honest, since you're just going to burn these tapes when we're done, so I can just say whatever I want, I'm not even that fired up by outreach. I do a lot of outreach, but if you look closely at what I do, it's all trying to generate new ideas and make arguments. I mean, The Biggest Ideas in the Universe video series is the exception to this, because there I'm really talking about well-established things. I purposely stayed away from more speculative things. But in the books I write, in the podcasts I do, in the blog or whatever, I'm not just explaining things or even primarily explaining things. I'm trying to develop new ideas and understand them. So, my interest in the physics of democracy is really because democracies are complex systems, and I was struck by this strange imbalance between economics and politics. Like, econo-physics is a big field -- there are multiple textbooks, there are courses you can take -- whereas politico-physics doesn't exist. Why is that? Why don't people think that way? So, it's really the ideas that have always driven me, and frankly, the pandemic is an annoyance that it got in the way rather than nudging me in that direction.

Zierler:

But undoubtedly, Sean, a byproduct of all your outreach work is to demonstrate that scientists are people -- that there isn't necessarily an agenda, that mistakes are made, and that all of the stuff for which conspiracies are made of, your work goes a long way in demonstrating that there's nothing to those ideas.

Carroll:

Well, it's true. That's why I said, "To first approximation." To second approximation, I care a lot about the public image of science. I care a lot about the substance of the scientific ideas being accurately portrayed. And I've learned in sort of a negative way from a lot of counterexamples about how to badly sell the ideas that science has by just hectoring people and berating them and telling them they're irrational. That doesn't work. If you actually take a scientific attitude toward the promotion of science, you can study what kinds of things work, and what kinds of approaches are most effective. I'm not an expert in that, honestly. I've not really studied that literature carefully, but I've read some of it. Maybe you hinted at this a little bit in the way you asked the question, but I do think that the one obvious thing that someone can do is just be a good example. Rather than telling other people they're stupid, be friendly, be likable, be openminded. By the way, all these are hard. I don't always succeed. It's not just a platitude. It's much easier, especially online, to be snarky and condescending than it is to be openminded. Also, by the way, some people don't deserve open mindedness. Some people are just crackpots. Drawing the line, who is asking questions and willing to learn, and therefore worth talking to, versus who is just set in their ways and not worth reaching out to? These are all very, very hard questions. Much harder than fundamental physics, or complex systems.

Zierler:

Sean, let's take it all the way back to the beginning. I'd like to start first with your parents. tell me a little bit about them and where they're from.

Carroll:

Roughly speaking, I come from a long line of steel workers. My parents got divorced very early, when I was six. My father was the first person in his family to go to college, and he became a salesman. Certainly nothing academic in his background, but then he sort of left the picture, and my mom raised me. She never went to college. So, I went to a large public school. My mom was tickled. She loved the fact that I was good at science and wanted to do it. She never ever discouraged me from doing it, but she had no way of knowing what it meant to encourage me either -- what college to go to, what to study, or anything like that. Even the teachers at my high school, who were great in many ways, couldn't really help me with that. So, a lot of the reasons why my path has been sort of zig-zaggy and back and forth is because -- I guess, the two reasons are: number one, I didn't have great sources of advice, and number two, I wasn't very good at taking the advice when I got it.

Zierler:

Did you connect with your father later in life?

Carroll:

No, not really. It never really bothered me that much, honestly. I know that for many people, this is a big deal, but my attitude was my mom raised me, and I love her very much, and that's all I really need.

Zierler:

You didn't have really any other father figures in your life.

Carroll:

Not especially, no. My mom got remarried, so I had a stepfather, but that didn't go very well, as it often doesn't, and then they got re-divorced, and so forth. I don't know how it reflected in how I developed, but I learn from books more than from talking to people. I had some great teachers along the way, but I wouldn't say I was inspired to do science, or anything like that, by my teachers. My favorite teachers were English teachers, to be honest. But I loved science because I hung out at the public library and read a lot of books about blackholes and quarks and the Big Bang. I just thought whatever this entails, because I had no idea at the time, this is what I want to do. If I can earn a living doing this, that's what I want to do. I was ten years old. I don't recommend anyone listening that you choose your life's path when you're ten years old, because what do you know? But it's worked pretty well for me.

Zierler:

What neighborhood did you grow up in?

Carroll:

When I was very young, we were in Levittown, Pennsylvania. So, like I said, it was a long line of steel workers. There's a famous Levittown in Long Island, but there are other Levittowns, including one outside Philadelphia, which is where I grew up. Then, we moved to Yardley, not that far away -- suburban Philadelphia, roughly speaking -- because there's a big steel mill, Fairless Works. Or there was. I think it's gone by now. The world has changed a lot. My mom worked as a secretary for U.S. Steel. My grandfather was a salesman, etc. I think both grandfathers worked for U.S. Steel.

Zierler:

Were your family's sensibilities working class or more middle class, would you say?

Carroll:

I'm not quite sure I can tell the difference, but working class is probably more accurate. I guess, my family was conservative politically, so they weren't joining the union or anything like that. The unions were anathema. My stepfather had gone to college, and he was an occupational therapist, so he made a little bit more money. So, I think economically, during the time my mom had remarried, we were middle class.

Zierler:

Was the church part of your upbringing at all?

Carroll:

When I was very young, we went to church every Sunday. Then, when my grandmother, my mother's mother, passed away when I was about ten, we stopped going. It was clearly for her benefit that we were going. As far as I was concerned, the best part was we went to the International House of Pancakes after church every Sunday. But, you know, I do think that my religious experiences, such as they were, were always fairly mild. I was never repulsed by the church, nor attracted to it in any way.

Zierler:

Not to put you on the psychologist’s couch, but there were no experiences early in life that sparked an interest in you to take this stand as a scientist in your debates on religion.

Carroll:

No, quite the opposite. Here is a sort of embarrassing but true story, which, I guess, this is the venue to tell these things in. I went to church, like I said, and I was a believer, such as it was, when I was young. I just drifted away very, very gradually. I never was a strong atheist, or outspoken, or anything like that. Then, I went to college at Villanova University, in a different suburb of Philadelphia, which is a Catholic school. It was not a very strict Catholic school. You didn't have to be Catholic, but over 90% of the students were, I think. You took religion classes, and I took religion classes, and I actually enjoyed them immensely. But the thing that flicked the switch in my head was listening to music. In particular, there was a song by Emerson, Lake & Palmer called The Only Way, which was very avowedly atheist. It was my first exposure to the idea that you could not only be atheist but be happy with it. Be proud of it, rather than be sort of slightly embarrassed by it. You could actually admit it, and if people said, what are your religious beliefs? You'd say, "Oh, I'm an atheist." I think, like I said before, these are ideas that get put into your mind very gradually by many, many little things. Who knows what the different influences were, but that was the moment that crystalized it, when I finally got to say that I was an atheist. So, it wasn't until I went to Catholic university that I became an outspoken atheist.

Zierler:

Sean, if mathematical and scientific ability has a genetic component to it -- I'm not asserting one way or the other, but if it does, is there anyone in your family that you can look to say this is maybe where you get some of this from?

Carroll:

Not especially, no. I do think my parents were smart cookies, but again, not in any sense intellectual, or anything like that. Certainly, no one academic in my family. Let's just say that.

Zierler:

As a ten year old, was there any formative moment where -- it's a big world out there for a ten year old. How do you land on theoretical physics and cosmology and things like that in the library? What sparked that interest in you?

Carroll:

You know, I wish I knew. When I knew this interview was coming up, I thought about it, and people have asked me that a million times, and I honestly don't know. I got books -- I liked reading. That was always true. And probably, there was a first -- I mean, certainly, by logical considerations, there was a first science book that I got, a first physics book. But I don't remember what it was. For a lot of non-scientists, it's hard to tell the difference between particle physics and astronomy. I got a lot of books on astronomy. I got a lot of books about the planets, and space travel, and things like that, because grandparents and aunts and uncles knew that I like that stuff, right? I enjoyed that, but it wasn't my passion. It was really the blackholes and the quarks that really got me going. But I don't know what started it. People always ask, did science fiction have anything to do with it? I was a fan of science fiction, but not like a super fan. Also, I think that my science fiction fandom came after my original interest in physics, rather than before. So, I honestly just can't tell you what the spark was.

Zierler:

Did you have a strong curriculum in math and science in high school?

Carroll:

No, no. It was fine. Our senior year in high school, there was a calculus class. That was, I think, a very, very typical large public school system curriculum where there were different tracks. I was on the advanced track, and so forth. It wasn't even officially an AP class, so I had to take calculus again when I got to college.

Zierler:

With Villanova, it's clear enough it's close to home. The tuition was right. You got a full scholarship there, of course. But I wanted to come back to the question of class -- working class, middle class. Was something like a Princeton or a Harvard, was that even on your radar as an 18 year old? Was that something that you or a guidance counselor or your mom thought was worth even considering at that time?

Carroll:

Actually, this is completely unrelated but let me say something else before I forget, because it's in the general area of high school and classes and things like that. By far, the most intellectually formative experience of my high school years was being on the forensics team. Forensics, in the sense of speech and debate. I like the idea of debate. We had a wonderful teacher, Ed Kelly, who had coached national championship debate teams before. It was funny, because now I have given a lot of talks in my life. I've gotten good at it. This is what I do. But when I started out on the speech and debate team, they literally -- every single time I would give a talk, I would get the same comments. "The substance of what you're saying is really good, but you're so bad at delivering it. You're so boring and so stilted and so stiff." But I did overcome that, and I think that I would not necessarily have overcome it if I hadn't gone through it, like forced myself to being on that team and trying to get better at it. And of course, it just helps you in thinking and logic, right? Being on the debate team, trying to work through different attitudes, back and forth. Not to mention, socialization. Being with people who are like yourself and hanging out with them. So, this is again a theme that goes back and forth all the time in my career, which is that there's something I like, but something else completely unrelated was actually more stimulating and formative at the time. So, that was true in high school. As far as class is concerned, there's no question that I was extremely hampered by not being immersed in an environment where going to Harvard or Princeton was a possibility. We did briefly flirt with the idea that I could skip a grade when I was in high school, or that I could even go to a local private school. The Lawrenceville Academy in New Jersey we thought of, but number one, it cost money, and number two, no one in my family really understood whether it would be important or not, etc. That includes me. I'm not discounting me. There was no internet back then. It was hard to figure out what the options were. Certainly, I would have loved to go to Harvard, but I didn't even apply. We just knew we couldn't afford it. Roughly speaking, my mom and my stepfather told me, "We have zero money to pay for you to go to college." We were sort of in that donut hole where they made enough to not get substantial financial aid, but not enough to be able to pay for me to go to college. And things are much worse now, by the way, so enormously, again, I can't complain compared to what things are like now. But that narrowed down my options quite a bit. Then, Villanova was one of the few places that had merit scholarships. So, the ivy leagues had, at the time -- I don't really know now -- they had a big policy of only giving need based need. It would be completely blind to -- you don't get a scholarship just because you're smart. There were two sort of big national universities that I knew that were exceptions to that, which were University of Chicago, and Rice University. Rice offered me a full tuition scholarship, and Chicago offered me a partial scholarship. But Villanova offered me full tuition, and it was closer, so the cost of living would be less. So, Villanova was basically chosen for me purely on economic reasons. This is not anything really about me, but it's sort of a mention of sympathy to anyone out there who's in a similar situation. The benefits you get from being around people who have all this implicit knowledge are truly incalculable, which I know because I wasn't around them. Even as late as my junior or senior year as undergraduates in college, when everyone knew that I wanted to go to graduate school and be a professor, or whatever, no one had told me that graduate students in physics got their tuition paid for by stipends or research assistantships or whatever. So, I was still sort of judging where I could possibly go on the basis of what the tuition numbers were, even though, really, those are completely irrelevant. No one told me. Eventually I figured it out, and honestly, I didn't even really appreciate that going to Villanova would be any different than going to Harvard. I would have gone to Harvard if I could have at the time, but I didn't think it was a big difference. I think, now, as wonderful as Villanova was, and I can rhapsodize about what a great experience I had there, but it's nothing like going to a major, top notch university, again, just because of the other students who are around you. I had great professors at Villanova, but most of the students weren't that into the life of the mind. Some of them were, and I made some very good friends there, but it's the exception rather than the rule. When I went to graduate school at Harvard, of course, it was graduate school, but I could tell that the undergraduate environment was entirely different. Not necessarily because they were all bookish. Harvard is not the most bookish place in the world. It's the place where you go if you're the offspring of the Sultan of Brunei, or something like that. Powerful people from all over the place go there. But still, the intellectual life and atmosphere, it was just entirely different than at a place like Villanova, or like Pennsbury High School, where I went to high school.

Zierler:

Sean, I'm so glad you raised the formative experience of your forensics team, because this is an unanswerable question, but it is very useful thematically as we continue the narrative. That is, the extent to which your embrace of being a public intellectual, and talking with people throughout all kinds of disciplines, and getting on the debate stage, and presenting and doing all of these things, the nature versus nurture question there is, would that have been your path no matter what academic track you took? In other words, let's say you went to law school, and you would now have a podcast in an alternate [universe] or a multiverse, on innovation, or something like that. I remember -- who was I talking to? It was Mark Trodden who was telling me a story about you. I asked him, "In graduate school, the Sean Carroll that we know today, is that the same person?" And he said, "Absolutely. Absolutely the same person."

Carroll:

But he didn't know me in high school. That's a different me. It's funny that you mention law school. Law school was probably my second choice at the time. Knowing what I know now, I would have thought about philosophy, or even theoretical computer science or something like that, but at the time, law seemed like this wonderful combination of logic and human interest, which I thought was fascinating. The actual question you ask is a hard one because I'm not sure. I've only lived my life once, and who knows? I did always have an interest in -- I don't want to use the word outreach because that sort of has formal connotations, but in reaching out. In talking to people and sort of sharing what I learned. That's why I joined the debate and speech team. Talking in front of a group of people, teaching in some sense. I taught a couple of courses -- not courses, but like guest lectures when I was in high school. My teachers let me do, like, a guest lecture. Again, uniformly, I was horrible. Literally, my math teacher let me teach a little ten minute thing on how to -- sorry, not math teacher. I was taking Fortran. We learned Fortran, the programming language back then. There are numerical variables and character variables. I was very good at Fortran, and he asked me to do a little exposition to the class about character variables. I remember, on the one hand, I did it and I sat down thinking it was really bad and I didn't do very well. My teacher, who was a wonderful guy, thinks about it a second and goes, "Did you ever think about how really hard it is to teach people things?" which is probably not the nicest thing he could have said at the time, but completely accurate. Susan Cain wrote this wonderful book on introverts that really caught on and really clarified a lot of things for people. Absolutely, for me, I'm an introvert. I'm not someone who gains energy by interacting with other people. It is incredibly draining for me to do it. All of the ability I have to give talks, and anything like that, has come from working at it. It's not a sort of inborn, natural, effortless kind of thing. I can do it, and it is fun. I enjoy in the moment, and then I've got to go to sleep afterwards, or at least be left alone. The idea of going out to dinner with a bunch of people after giving a talk is -- I'll do it because I have to do it, but it's not something I really look forward to. But, yes, with all those caveats in mind, I think that as much as I love the ideas themselves, talking about the ideas, sharing them, getting feedback, learning from other people, these are all crucially important parts of the process to me. So, probably, yes, I would still have the podcast even if I'd gone to law school.

Zierler:

So, no imaginable scenario, like you said before, your career track has zigged and zagged in all kinds of unexpected ways, but there's probably no scenario where you would have pursued an academic career where you were doing really important, really good, really fundamental work, but work that was generally not known to 99.99% of the population out there.

Carroll:

I don't think so. It's hard for me to imagine that I would do that. I have enormous respect for the people who do that. Like, when people talk about the need for science outreach, and for education and things like that, I think that there is absolutely a responsibility to do outreach to get the message out, especially if the kind of work you do has no immediate economic or technological impact. We can't justify theoretical cosmology on the basis that it's going to cure diseases. It's only being done for the sake of discovery, so we need to share those discoveries with people. I think that responsibility is located in the field, not on individuals. I think there are some people who I don't want to have them out there talking to people, and they don't want to be out there talking to people, and that's fine. I love people who are just so passionate about their little specialty. That's all they want to do, and they get so deep into it that no one else can follow them, and they do their best to explain. It's just wonderful and I love it, but it's not me. I really do appreciate the interactiveness, the jumping back and forth. I have a short attention span. This is something that is my task to sort of try to be good in a field which really does require a long attention span as someone who doesn't really have that.

Zierler:

On that note, as a matter of bandwidth, do you ever feel a pull, or are you ever frustrated, given all of your activities and responsibilities, that you're not doing more in the academic specialty where you're most at home? Let's face it, quantum mechanics, gravitation, cosmology, these are fields that need a lot of help. There's still fundamental questions. Do you ever feel that maybe you should just put all of that aside and really focus hard on some of the big questions that are out there, or do you feel like you have the best of both worlds, that you can do that and all of the other things and neither suffer?

Carroll:

I think this is actually an excellent question, and I have gone back and forth on it. Since the answer is not clear, I decide to do what is the most fun. So, I could completely convince myself that, in fact -- and this is actually more true now than it maybe was twenty years ago for my own research -- that I benefit intellectually in my research from talking to a lot of different people and doing a lot of different kinds of things. That's less true if what you're doing is trying to derive a new model for dark matter or for inflation, but when what you're trying to do is more foundational work, trying to understand the emergence of spacetime, or the dynamics of complex systems, or things like that, then there are absolutely ways in which this broader focus has helped me. Now, can I promise you that the benefit is worth the cost, and I wouldn't actually be better off just sitting down and spending all of my time thinking about that one thing? No, I cannot in good conscience do that. Furthermore, anyone who has really done physics with any degree of success, knows that sometimes you're just so into it that you don't want to think about anything else. When you're falling asleep, when you're taking a shower, when you're feeding the cat, you're really thinking about physics. Then, I'm happy to admit, if someone says, "Oh, you have to do a podcast interview," it's like, ah, I don't want to do this now. I've got work and it's going well. But then there are other times when you're stuck, and you can't even imagine looking at the equations on your sheet of paper. Then, okay, I get to talk about ancient Roman history on the podcast today. Thank goodness.

Zierler:

Well, Sean, you can take solace in the fact that many of your colleagues who work in these same areas, they're world class, and you can be sure that they're working on these problems.

Carroll:

Yeah, again, I'm a big believer in diverse ecosystems. I'm a big believer that there's no right way to be a physicist. Not just that there are different approaches. Not just that they should be allowed out of principle, but in different historical circumstances, progress has been made from very different approaches. So, to say, well, here's the approach, and this is what we should do, that's the only mistake I think you can make. In fact, I would argue, as I sort of argued a little bit before, that as successful as the model of specialization and disciplinary attachment has been, and it should continue to be the dominant model, it should be 80%, not 95% of what we do.

Zierler:

Right. Let's get back to Villanova. I'm curious, is there a straight line between being a ten year old and making a beeline to the physics and astronomy department? Was that the game plan from day one for you?

Carroll:

I think so. The thing that I was not able to become clear on for a while was the difference between physics and astrophysics. I'm surprised you've gotten this far into the conversation without me mentioning, I have no degrees in physics. Both my undergraduate and graduate degrees are in astronomy, and both for weird, historical reasons. My stepfather's boss's husband was a professor in the astronomy department in Villanova. The astronomy department was just better than the physics department at that time. It was very small. There were literally two people in my graduating class in the astronomy department. Everyone got to do research from their first year in college. So, that's a wonderful environment where all of your friends are there, you know all the faculty, everyone hangs out, and you're doing research, which very few of the physics faculty were doing. I didn't think that it would matter whether I was an astronomy major or a physics major, to be honest. In some extent, it didn't. I took almost all the physics classes. There was one course I was supposed to take to also get a physics degree. I got a minor in physics, but if I had taken a course called Nuclear Physics Lab, then I would have gotten a physics bachelor’s degree also. This chair of the physics department begged me to take this course because he knew I was going to go to a good graduate school, and then he could count me as an alumnus, right? But I was like, no I don't want to take a nuclear physics lab. That's just not my thing. But the only graduate schools I applied to were in physics because by then I figured out that what I really wanted to do was physics. But I didn't get in -- well, I got in some places but not others. I did not get into Harvard, and I sweet talked my way into the astronomy department at Harvard. Again, I convinced myself that it wouldn't matter that much. Again, I was wrong. Again, because I underestimated this importance of just hanging out with likeminded people. I took all the courses, and I had one very good friend, Ted Pine, who was also in the astronomy department, and also interested in all the same things I was. So, he was an enormous help to me, but it's not like there were twenty other people who were doing the same kind of thing, and you hang out and have lunch and go to parties and talk about Feynman diagrams. That just didn't happen.

Zierler:

Sean, I wonder if you stumbled upon one of the great deals in the astronomy and physics divide. That is, as an astronomy student, you naturally had to take all kinds of physics classes, but physics majors didn't necessarily have to take all kinds of astronomy classes.

Carroll:

It's true, but I did have to take astronomy classes. I've forgotten almost all of it, so I'm not sure it was the best use of my time. The astronomy department at Harvard was a wonderful, magical place, which was absolutely top notch. Literally, my office mate, while I was in graduate school, won the Nobel Prize for discovering the accelerating universe -- not while he was in graduate school, but later. So, once again, I can't complain about the intellectual environment that that represented. But I still did -- I was not very good at -- sorry, let me back up yet again. By the time I got to graduate school, I finally caught on that taking classes for a grade was completely irrelevant. What mattered was learning the material. So, I audited way more classes, and in particular, math classes. I sat in on all these classes on group theory, and differential geometry, and topology, and things like that. Again, stuff that has not been that useful to me, but I just loved it so much, as well as philosophy and literature classes at Harvard. So, I did my best to take advantage of those circumstances. I guess, the final thing is that the teaching at that time in the physics department at Harvard, not the best in the world. There were some classes that were awesome, but there were some required classes that were just like pulling teeth to take. And guess what? I was an astronomy major, so I didn't have to take them. So, that was a benefit. I'm not sure if it was a very planned benefit, but I did benefit that way.

Zierler:

Sean, I wonder, maybe it's more of a generational question, but because so many cosmologists enter the field via particle physics, I wonder if you saw any advantages of coming in it through astronomy. Are there any advantages through a classical education in astronomy that have been advantageous for your career in cosmology?

Carroll:

I think that there -- I'm not sure there's a net advantage or disadvantage, but there were advantages. I think that's the right way to put it. The two advantages I can think of are, number one, at that time, it's a very specific time, late '80s, early '90s -- specific in the sense that both particle physics and astronomy were in a lull. These were not the exciting go-go days that you might -- well, we had some both before and after. So, it was difficult to know what to work on, and things like that. In particular, the physics department at Harvard had not been converted to the idea that cosmology was interesting. I remember having a talk with Howard Georgi, and he didn't believe either the solar neutrino problem, or Big Bang nucleosynthesis. I was like, okay, you don't have to believe the solar neutrino problem, but absolutely have to believe Big Bang nucleosynthesis. In retrospect, he should have believed both of them. But the astronomy department, again, there were not faculty members doing early universe cosmology at Harvard, in either physics or astronomy. Another bad planning on my part. But there were postdocs. The faculty members who were at Harvard, the theorists -- George Field, Bill Press, and others -- they were smart and broad enough to know that some of the best work was being done in this field, so they should hire postdocs working on that stuff. So, I got talk to a lot of wonderful people who are not faculty members at different places. Fred Adams, Katie Freese, Larry Widrow, Terry Walker, a bunch of people who were really very helpful to me in learning things.

Zierler:

I love historicizing the term "cosmology," and when it became something that was respectable to study. You get different answers from different people. Steve Weinberg tells me something very different from Michael Turner, who tells me something very different from Paul Steinhardt, who tells me something very different from Alan Guth. So, for you, in your career, when did cosmology become something where you can proudly say, "This is what I do. This is real physics. This is something that's respectable."

Carroll:

You know, I'm not sure I ever doubted it. I don't think that was a conversion experience that I needed to have.

Zierler:

Because it came before you, you mean.

Carroll:

Well, or I just didn't care. I guess, I was already used to not worrying too much. I think I probably took this too far, not worrying too much about what other people thought of my intellectual interests. As long as I thought it was interesting, that counted for me. Of course, Harvard astronomy, at the time, was the home of the CFA redshift survey -- Margaret Geller and John Huchra. On the observational side, it was the birth of large-scale galaxy surveys. Bob Kirshner and his supernova studies were also a big deal. So, the idea of doing observational cosmology was absolutely there, and just obvious at the time. As far as that was concerned, that ship had sailed. Everyone knew that was real. Now, there are a couple things to add to that. One is, it was completely unclear whether we would ever make any progress in observational cosmology. People had learned things, but it was very slow. I remember Margaret Geller, who did the CFA redshift survey, when the idea of the slow and digital sky survey came along and it was going to do a million galaxies instead of a few thousand, her response was, "Why would you do that? We've done a few thousand, what else are you going to learn from a few million?" We learned a lot is the answer, as it turns out. Margaret Geller is a brilliant person, so it's not a comment on her, but just how hard it is to extrapolate that. The other anecdote along those lines is with my officemate, Brian Schmidt, who would later win the Nobel Prize, there's this parameter in cosmology called omega, the total energy density of the universe compared to the critical density. Just like the Hubble constant, we had tried to measure this for decades, with maybe improvement, maybe not. It wasn't really clear. So, as the naive theorist, I said, "Well, it's okay, we'll get there eventually. We'll figure it out. We'll measure it." Brian, who was a working class observational astronomer said, "No we won't. You don't understand how many difficulties -- how many systematic errors, statistical errors, all these observational selection biases. It's just really, really hard." So, we made a bet. We made a bet not on what the value of omega would be, but on whether or not we would know the value of omega twenty years later. We bet a little bottle of port, because that's all we could afford as poor graduate students. Then, of course, Brian and his team helped measure the value of omega by discovering the accelerating universe. So, he won the Nobel Prize, but I won that little bottle of port. Okay? I just want to say. I don't want that left out of the historical record. I think the final thing to say, since I do get to be a little bit personal here, is even though I was doing cosmology and I was in an astronomy department, still in my mind, I was a theoretical physicist. It was really like quantum gravity, or particle physics, or field theory, that were most interesting to me. I wanted to do it all, so that included the early universe cosmology, but I didn't think of myself as being defined as a cosmologist, even at that time. It wasn't until my first year as a postdoc at MIT when I went to a summer school and -- again, meeting people, talking to them. It was a summer school in Italy. People are sitting around with little aperitifs, or whatever, late at night. Different people are asking different questions: what do you do? Someone asked some question, and I think it might have been about Big Bang nucleosynthesis. And I answered it. I explained it, and one of my fellow postdocs, afterwards, came up to me and said, "That was really impressive." It never occurred to me that it was impressive, and I realized that you do need to be something. You can't be everything, and maybe what I was a cosmologist. So, it wasn't until my first year as a postdoc that I would have classified myself in that way.

Zierler:

Sean, to go back to the question in high school about whether or not a Harvard or a Princeton was on your radar, I'm curious, as a junior or a senior at Villanova, given that economically, and even geographically, you were not so far away from where you were as a high schooler, what had changed where now a place like a Harvard would have seemed within reach?

Carroll:

Because they pay for your tuition. That's all it is. I would have gladly gone to some distant university. I never had, as a high priority, staying near Lower Bucks County, Pennsylvania. In fact, I did have this idea that experiencing new things and getting away was important. It just so happened, I could afford going to Villanova, and it was just easy and painless, so I did it. But when you go to graduate school, you don't need money in physics and astronomy. In fact, I got a National Science Foundation fellowship, so even places that might have said they don't have enough money to give me a research assistantship, they didn't need that, because NSF was paying my salary. In fact, the university or the department gets money from the NSF for bringing me on. So, then, I could just go wherever I wanted. Then, it was just purely about what was the best intellectual fit. Again, purely intellectual fit criteria, I chose badly because I didn't know any better. So, I went to an astronomy department because the physics department didn't let me in, and other physics departments that I applied to elsewhere would have been happy to have me, but I didn't go there. And who knows, it all worked out okay, but this sort of background, floating, invisible knowledge is really, really important, and was never there for me.

Zierler:

How did you develop your relationship with George Field?

Carroll:

Well, Harvard -- the astronomy department, which was part and parcel of the Harvard Smithsonian Center for Astrophysics -- so, the Smithsonian Astrophysical Observatory and the Harvard College Observatory joined together in the 1970s to form this big institution, which I still think might be the largest collection of astronomy PhDs, in the United States, anyway. So, there were all these PhD astronomers all over the place at Harvard in the astronomy department. So, they could be rich with handing out duties to their PhD astronomers to watch over students, which is a wonderful thing that a lot people at other departments didn't get. So, every person who came, [every] graduate student, was assigned an advisor, a faculty member, to just sort of guide them through their early years. By the way, I could tell you stories at Caltech how we didn't do that, and how it went disastrously wrong. It helped really impress upon me the need for departments to be proactive in taking care of their students. So, George was randomly assigned to me. I was a theorist. That was clear, and there weren't that many theorists at Harvard, honestly. I'm trying to remember -- when I got there, on the senior faculty, there was George, and there was Bill Press, and I'm honestly not sure there was anyone else -- I'm trying to think -- which is just ridiculous for the largest number -- there were a few research professor level people. George Rybicki was there, and a couple other people. They soon thereafter hired Ramesh Narayan, and eventually Avi Loeb, and people like that. But still, way under theorized, really, for the whole operation, if you consider it. So, it was a coin flip, and George was assigned to me, and invited me to his office and said, "What do you want to do?" And we started talking, and it was great. We hit it off immediately. I was unburdened by knowing how impressive he was. Let's put it that way. I had never heard of him before. When I told Ed Guinan, my undergraduate advisor, that I had George Field as an advisor, he said, "Oh, you got lucky." And now I know it.

Zierler:

Sean, another topic I love to historicize, where it was important and where it was trendy, is string theory. Where was string theory, and how much was it on your radar when you were thinking about graduate school and the kinds of things you might pursue for thesis research?

Carroll:

It was 100% on my radar, and we can give thanks to the New York Times magazine. There was a famous story in the New York Times magazine in the mid '80s. It might have been by K.C. Cole. I'm not sure, but it was a story about string theory, and the search for the theory of everything. Largely, Ed Witten was the star of the show, and that's why I wanted to go to Princeton. That was my first choice. So, becoming a string theorist was absolutely a live possibility in my mind. Even back then, there was part of me that said, okay, you only have so many eggs. Do you want to put them all in the same basket? Being a string theorist seemed to be a yes or no proposition. Either you bit the bullet and you did that, or you didn’t. I was a little bit reluctant to do that, but it did definitely seem like the most promising way to go. When I went to Harvard, there were almost zero string theorists there. There was Cumrun Vafa, who had been recently hired as a young assistant professor. Now, of course, he's a very famous guy. Big name, respectable name in the field, but at the time, being assistant professor at Harvard was just like being a red shirt on Star Trek, right? You're not going to get tenure. They chew you up and spit you out. So, you didn't even know, as a prospective grad student, whether he was someone you would want to pick as an advisor, because who knows how long he'd be there. But anyway, I never really seriously tried to change advisors from having George Field as my advisor. I could have tried to work with someone in the physics department like Cumrun, or Sidney Coleman would have been the two obvious choices. But instead, in my very typical way, I wrote a bunch of papers with a bunch of different people, including a lot of people at MIT. Alan Guth and Eddie Farhi, Bill Press and George Field at Harvard, and also other students at Harvard, rather than just picking one respectable physicist advisor and sticking with him. So, string theory was definitely an option, and I could easily have done it if circumstances had been different, but I never really regretted not doing it.

Zierler:

What was George Field's style like as a mentor? In other words, did he essentially hand you a problem to work on for your thesis research, or were you more collaborative, or was he basically allowing you to do whatever you wanted on your own?

Carroll:

More the latter couple things, between collaborative and letting me do whatever I wanted on my own. Like I said, I wrote many papers that George was not a coauthor on. We wrote a lot of papers together. Never did he hand me a problem and walk away. That's the opposite. When we were collaborating, it was me doing my best to keep up with George. He was a very senior guy. He'd already retired from being the director of the Center for Astrophysics, so you could have forgiven him for kicking back a little bit, but George's idea of a good time is to crank out 30 pages of handwritten equations on some theory that we're thinking about. So, that's what he would do.

Zierler:

What was he working on when you first met him?

Carroll:

He was in the midst of this, sort of, searching period himself. I think that's one of the reasons why we hit it off. He was trying to learn more about the early universe. Like I said, we had hired great postdocs there. Terry Walker was one of them, who's now a professor at Ohio State. George and Terry team-taught a course on early universe cosmology using the new book by Kolb and [Michael] Turner that had just come out, because Terry was Rocky Kolb's graduate student at Chicago. George didn't know the stuff. He had to learn it. He would learn it the night before and then teach it the next day. So, we were just learning a whole bunch of things and sort of fishing around. But he does have a very long-lasting interest in magnetic fields. Probably his most important work was on the interstellar and intergalactic medium. He knew all the molecular physics, and things like that, that I would never know. But the closest to his wheelhouse and mine were cosmological magnetic fields. So, we wrote a little bit about that, and he was always interested in that. The dynamo, the Biermann battery, the inverse cascade, magnetic helicity, plasma effects, all of these things that are kind of hard for my purely theoretical physicist heart to really wrap my mind around.

Zierler:

What was your thesis research on? Of all the things that you were working on, what topic did you settle on?

Carroll:

Well, as in many theoretical physics theses, I just stapled together all the papers I had written. The title was, if I'm remembering it correctly, Cosmological Consequences of Topological and Geometric Phenomena in Field Theories. So, like I said, I really love topology. I took courses with Raoul Bott at Harvard, who was one of the world's great topologists. Sidney Coleman, in the physics department, and done a lot of interesting work on topology and gauge theories. I was in Sidney's office all the time. He was a blessing, helping me out. So, I wrote some papers on -- I even wrote one math paper, calculating some homotropy groups of ocean spaces, because they were interesting for topological defect purposes. So, that was with other graduate students. Ted Pyne and I wrote a couple papers, one on the microwave background. One of these papers, we found an effect that was far too small to ever be observed, so we wrote about it. But of course, ten years later, they're observing it. We didn't know, so that paper got a lot of citations later on. Then, I wrote some papers with George, and also with Alan and Eddie at MIT. The whole thing was all stapled together, and that was my thesis.

Zierler:

Either then, or retrospectively, do you see any through lines that connected all of these different papers in terms of the broader questions you were most interested in?

Carroll:

No, not really. Other than being interesting at the time, theoretical physics questions. If I want to be self-critical, that was a mistake. I think it's fine to do different things, work in different areas, learn different things. I continued to do that when I got to MIT. I wrote about supergravity, and two-dimensional Euclidian gravity, and torsion, and a whole bunch of other different things. But no, they did not tie together in some grand theme, and I think that was a mistake. I think that it's important to do different things, but for a purpose. I think one thing I just didn't learn in graduate school, despite all the great advice and examples around me, was the importance of not just doing things because you can do them. When I first got to graduate school, I didn't have quantum field theory as an undergraduate, like a lot of kids do when they go to bigger universities for undergrad. So, I was behind already. When I wrote my first couple papers, just the idea that I could write a paper was amazing to me, and just happy to be there. Happy to be breathing the air. Seeing my name in the Physical Review just made me smile, and I kept finding interesting questions that I had the technological capability of answering, so I did that. It really wasn't, honestly, until my second postdoc in Santa Barbara, that I finally learned that it's just as important to do these things for reason, for a point. It's not just you can do them, so you get the publication, and that individual idea is interesting, but it has to build to something greater than the individual paper itself.

Zierler:

On the point of not having quantum field theory as an undergraduate, I wonder, among your cohort, if you felt that you stuck out, like a more working class kid who went to Villanova, and that was very much not the profile of your fellow graduate students.

Carroll:

I think I did not really feel that, honestly. Graduate school is a different thing. Graduate departments of physics or astronomy or whatever are actually much more similar to each other than undergraduate departments are, because they bring people from all these undergraduate departments. I really do think that in some sense, the amount that a human being is formed and shaped, as a human being, not as a scientist, is greater when they're an undergraduate than when they're a graduate. You go from high school, you're in a college, it's your first exposure to a whole bunch of new things, you get to pick and choose. Some have a big effect on you, some you can put aside. Then, you enter graduate school as more or less a fully formed person, and you learn to do science. So, my other graduate school colleagues, Brian had gone to the University of Arizona, Ian Dell'Antonio, who was another friend of mine, went to, I think, Haverford. We had people from England who had gone to Oxford, and we had people who had gone to Princeton and Harvard also. But honestly, no, I don't think that was ever a big thing. The much bigger thing was, Did you know quantum field theory? Like, that's a huge thing. That was always holding me back that I didn't know quantum field theory at the time.

Zierler:

And no one gave you advice along the lines of -- a thesis research project is really your academic calling card? It's the path to achieving tenure. Having all these interests is a wonderful thing, but it's not necessarily most efficacious for pursuing a traditional academic track. No one told you that, or they did, and you rebelled against it.

Carroll:

Honestly, maybe they did, but I did always have a slightly "I'll be fine" attitude. I won't say a know-it-all attitude, because I don't necessarily think I knew it all, but I did think that I knew what was best for myself. Again, I was wrong over and over again. This is not a good attitude to have, but I thought I would do fine. When it came time to choose postdocs, when I was a grad student, because, like I said, both particle physics and cosmology were in sort of fallowed times; there were no hot topics that you had to be an expert in to get a postdoc. So, I got really, really strong letters of recommendation. I'd written a bunch of interesting papers, so I was a hot property on the job market. I ended up going to MIT, which was just down the river, and working with people who I already knew, and I think that was a mistake. As much as I love those people, I should have gone somewhere else and really shocked my system a little bit. It would have been better for me.

Zierler:

So, you were already working with Alan Guth as a graduate student.

Carroll:

Yeah. What happened was there was a system whereby if you were a Harvard student you could take classes from MIT, get credit for them, no problem. So, many of my best classes when I was a graduate student I took at MIT. I learned general relativity from Nick Warner, which later grew into the book that I wrote. I took the early universe [class] from Alan. I took a particle physics class from Eddie Farhi. To his great credit, Eddie Farhi, taught me this particle physics class, and he just noticed that I was asking good questions, and asked me who I was. He didn't know me from the MIT physics department. I explained, and he said he had read this paper that he thought was interesting, by Richard Gott, on time machines, close time-like curves in gravity. Would I be interested in working on it with him? And I said, "Yeah, sure." We worked on it for a while, and we got stuck, and we needed to ask Alan for help. So, that's how I started working with Alan. Alan and Eddie, of course, had been collaborators for a long time before that.

Zierler:

Who was on your thesis committee? Who possibly could have represented all of these different papers that you had put together?

Carroll:

I had the best thesis committee ever. My thesis committee was George Field, Bill Press, who I wrote a long review article on the cosmological constant with. We never wrote any research papers together, but that was a very influential paper, and it was fun to work with Bill. Sidney Coleman, who I mentioned, whose office I was in all the time. It was certainly my closest contact with the Harvard physics department. And then, both Alan Guth and Eddie Farhi from MIT trundled up. There was a rule in the Harvard astronomy department, someone not from Harvard had to be on your committee. So, it made it easy, and I asked both Alan and Eddie. So, between the five of these people, enormous brainpower. Also, they were all really busy and tired. The other thing, just to go back to this point that students were spoiled in the Harvard astronomy department, your thesis committee didn't just meet to defend your thesis. They met every six months while you were a graduate student, after you had passed your second-year exam. Now, in reality, maybe once every six months meant once a year, but at least three times before my thesis defense, my committee had met. Bless their hearts for coming all the way to someone's office. Even from the physics department to the astronomy department was a 15-minute walk. So, they knew everything that I had done. They'd read my papers, they helped me with them, they were acknowledged in them, they were coauthors and everything. And I knew that. So, when it came time for my defense, I literally came in -- we were still using transparencies back in those days, overhead projector and transparencies. My thesis defense talk was two transparencies. One of them was a joke because one of them was a Xerox copy of my quantum field theory final exam that Sidney Coleman had graded and really given me a hard time. I got the dimensional analysis wrong, like the simplest thing in the world. So, I made the point that he should judge me not on my absolute amount of knowledge, but by how far I had come since the days he taught me quantum field theory. Then, the other transparency was literally like -- I had five or six papers in my thesis, and I picked out one figure from every paper, and I put them in one piece of paper, Xeroxed it, made a slide out of it, put it on the projector, and said, "Are there any questions?" That was my talk. Bill Press, bless his heart, asked questions. And Sidney Coleman, bless his, answered all the questions. And Bill was like, "No, it's his exam. He's supposed to answer the questions." And Sidney was like, "Why are we here? We knew he's going pass." So, I was done in 20 minutes. The whole thing was the shortest thesis defense ever.

Zierler:

I'm always amazed by physics and astronomy [thesis] defenses, because it seems like the committee never asks the kinds of questions like, what do you see as your broader contributions to the field? Because the ultimate trajectory from a thesis defense is a faculty appointment, right? So, it's not just that you have your specialty, but what niche are you going to fill in that faculty that hires you. Did you get any question like that? Were you thinking along those lines at all as a graduate student?

Carroll:

The person who most tried to give me advice was Bill Press, actually, the only one of those people I didn't write a paper with. Actually, I didn't write a paper with Sidney either. But Sidney, and Eddie, and Alan, and George, this is why I got along with them, because they were very pure in their love for doing science. As long as they were thinking about something, and writing some equations, and writing papers, and discovering new, cool things about the universe, they were happy. They all had succeeded to an enormous extent, because they're all really, really brilliant, and had made great contributions. Bill was the only one who was a little bit of a strategist in terms of academia. He was the one who set me up on interviews for postdocs and told me I need to get my hands dirty a little bit, and do this, and do that. I didn't listen to him as much as I should have.

Zierler:

He's a JASON as well, so he has lots of experience in policy and strategizing, and things like that.

Carroll:

Again, in my philosophy of pluralism, there should be both kinds. It's not a good or a bad kind. You really, really need scientists or scholars who care enough about academia to help organize it, and help it work, and start centers and institutes, and blaze new trails for departments. I do have feelings about different people who have been chosen as directors of institutes and department chairs. Some of them are leaders and visionaries, and some of them are kind of caretakers. Both are okay in their different slots, depending on the needs of the institution at the time, but I think that a lot of times the committees choosing the people don't take this into consideration as much as they should.

Zierler:

Sean, when you got to MIT, intellectually, or even administratively, was this just -- I mean, I'm hearing such a tale of exuberance as a graduate. There was so much good stuff to work on, you didn't say no to any of it, you put it all together. As a postdoc at MIT, was that just an opportunity to do another paper, and another paper, and another paper, or structurally, did you do work in a different way as a result of not being in a thesis-oriented graduate program?

Carroll:

It was mostly, almost exclusively, the former. Another paper, another paper, another paper. Again, I had great people at MIT. Dan Freedman, who was one of the inventors of supergravity, took me under his wing. When I got there, we wrote a couple of papers tighter. Really, really great guy. But mostly -- I started a tendency that has continued to this day where I mostly work with people who are either postdocs or students themselves. So, Wati Taylor, who's now an MIT professor, Miguel Ortiz, Mark Trodden. These were people who were at my level. That's almost all the people who I collaborated with when I was a postdoc at MIT. So, I still didn't quite learn that lesson, that you should be building to some greater thing. The one exception -- it took me a long time, because I'm very, very slow to catch on to things. There was one formative experience, which was a couple of times while I was there, I sat in on Ed Bertschinger's meetings. Ed is a cosmologist, and remember, this is the early to mid '90s. I was a postdoc at MIT from '93 to '96. We discovered the -- oh, that was the other cosmology story I wanted to tell. But we discovered in 1992, with the COBE satellite, the anisotropies in the cosmic microwave background, and suddenly, cosmology came to life, but only if you're working on the cosmic microwave background, which I was not. But still, it was a very, very exciting time. Parenthetically, a couple years later, they discovered duality, and field theory, and string theory, and that field came to life, and I wasn't working on that either, if you get the theme here. But the anecdote was, because you asked about becoming a cosmologist, one of the first time I felt like I was on the inside in physics at all, was again from Bill Press, I heard the rumor that COBE had discovered the anisotropies of the microwave background, and it was a secret. You're not supposed to tell anybody, but of course, everybody was telling everybody. And I got to tell Sidney Coleman, and a few of the other faculty members of the Harvard physics department. I got to reveal that we had discovered the anisotropies in the microwave background. So, they looked at me with new respect, then, because I had some insider knowledge because of that. Anyway, Ed had these group meetings where everyone was learning about how to calculate anisotropies in the microwave background. It's almost hard to remember how hard it was, because you had these giant computer codes that took a long time to run and would take hours to get one plot. People were very unclear about what you could learn from the microwave background and what you couldn't. Everyone knew it was going to be exciting, but it was all brand new and shiny, and Ed would have these group meetings. People like Chung-pei Ma and Uros Seljak were there, and Bhuvnesh Jain was there. All these people who are now faculty members at prestigious universities. They were all graduate students at the time. Ed would say, "Alright, you do this, you do that, you do that." It was clear that there was an army that was marching toward a goal, and they did it. They succeeded beyond anyone's wildest expectations. That group at MIT was one, and then Joe Silk had a similar group at Berkeley at the same time. People like Wayne Hu came out of that. Martin White. So, that was my first glimpse at purposive, long term strategizing within theoretical physics. I didn't really know that could be a thing, but I was very, very impressed by it. I was like, I can't do that, but it's very impressive, but okay. That was a glimpse of what could be possible.

Zierler:

Were you on the job market at this point, or you knew you wanted to pursue a second postdoc?

Carroll:

Well, that's interesting. When I applied for my first postdoc, like I said, I was a hot property. Actually, Joe Silk at Berkeley, when I turned down Berkeley, he said, "We're going to have an assistant professorship coming up soon. You should apply." And I didn't because I thought I wasn't ready yet. What happened was between the beginning of my first postdoc and the end of my first postdoc, in cosmology, all the good theorists were working on the cosmic microwave background, and in particle physics, all the good theorists were working on dualities in one form or another, or string theory, or whatever. And I wasn't working on either one of those. So, I was a hot property then, and I was nobody when I applied for my second postdoc. I did also apply, at the same time, for faculty jobs, and I got an offer from the University of Virginia. I decided to turn them down, mostly because I thought I could do better. I didn't really want to live there. I thought maybe I had not maxed out my potential as a job market candidate. It was a tough decision, but I made it. Also, I got on a bunch of other shortlists. I was on a shortlist at the University of Chicago, and Caltech, and a bunch of places. So, I thought, well, okay, I was on a bunch of shortlists. Now, next year, I'll get a job. And I didn't. So, I did eventually get a postdoc. I got two postdoc offers, one at Cambridge and one at Santa Barbara. I went to Santa Barbara, the ITP, as it was then known. Institute for Theoretical Physics. Now, the KITP. Honestly, I only got that because Jim Hartle was temporarily the director. I think that I would never get hired by the KITP now, because they're much more into the specialties now. Jim was very interdisciplinary in that sense, so he liked me.

Zierler:

Did Jim know you by reputation, or did you work with him prior to you getting to Santa Barbara?

Carroll:

By reputation only. I think I talked on the phone with him when he offered me the job, but before then, I don't think I had met him. We might have met at a cosmology conference. I'm not sure. We certainly never worked together. So, my three years at Santa Barbara, every single year, I thought I'll just get a faculty job this year, and my employability plummeted. I didn't even get on any shortlists the next year. I got on one and then got rejected the year after that because I was not doing what people were interested in.

Zierler:

It's remarkable how trendiness can infect science.

Carroll:

Well, the answer is yes, absolutely. You didn't ask a question, but yes, you are correct. It is remarkable. But I want to remove a little bit of the negative connotation from that. It's not just trendiness. Certain questions are actually kind of exciting, right? Like, if you just discovered the anisotropies in the cosmic microwave background, and you have a choice between two postdoc candidates, and one of them works on models of baryogenesis, which have been worked on for the last twenty years, with some improvement, but not noticeable improvement, and someone else works on brand new ways of calculating anisotropies in the microwave background, which seems more exciting to you? I think it's perfectly rational in that sense. Again, I think there should be more institutional support for broader things, not to just hop on the one bandwagon, but when science is exciting, it's very natural to go in that direction.

Zierler:

Perhaps, to get back to an earlier comment about some of the things that are problematic about academic faculty positions, as you say, yes, sometimes there is a positive benefit to trends, but on the other hand, when you're establishing yourself for an academic career, that's a career that if all goes well will last for many, many decades where trends come and go. So, what might seem very important in one year, five years down the line, ten years down the line, wherever you are on the tenure clock, that might not be very important then. So, how did you square that circle, or what kinds of advice did you get when you were on the wrong side of these trends about having that broader perspective that is necessary for a long-term academic career?

Carroll:

Well, I'm not sure that I ever did get advice. I think I figured it out myself eventually, or again, I got advice and then ignored it and eventually figured it out myself. We'll get into the point where I got lucky, and the universe started accelerating, and that saved my academic career. But I think, that it's often hard for professors to appreciate the difference between hiring a postdoc and hiring a faculty member. For hiring a postdoc, it does make perfect sense to me -- they're going to be there for a few years, they're going to be doing research. Let's pick people who are doing exciting research. Also, of course, it's a perfectly legitimate criterion to say, let's pick smart people who will do something interesting even if we don't know what it is. But it's less important for a postdoc hire. Whereas, for a faculty hire, it's completely the opposite. I have zero interest in whether someone is doing a hot topic thing for a faculty hire, exactly like you said. Hopefully, this person is going to be here for 30 or 40 years. We want to pick the most talented people who will find the most interesting things to work on whether or not that's what they're doing right now. I had another very formative experience when I was finally a junior faculty member. I was awarded a Packard fellowship which was this wonderful thing where you get like half a million dollars to spend over five years on whatever you want. Literally, "We're giving it to you because we think you're good. We don't care what you do with it." At the end of the five-year term, they ask all the Packard fellows to come to the meeting and give little talks on what they did. Again, and again, you'd hear people say, "Here's the thing I did as a graduate student, and that got me hired as a faculty member, but then I got my Packard fellowship, and I could finally do the thing that I really wanted to do, and now I'm going to win the Nobel Prize for doing that." I absolutely am convinced that one of the biggest problems with modern academic science, especially on the theoretical side, is making it hard for people to change their research direction. All the incentives are to do the same exact thing: getting money, getting resources at the university, getting collaborations, or whatever. We make it so hard, and I think that's exactly counterproductive.

Zierler:

And it doesn't work well from your approach of being exuberant and wanting to just pursue the fun stuff to work on.

Carroll:

It's also self-serving for me to say that, yes. I think people like me should have an easier time. That's really the lesson I want to get across here.

Zierler:

Sean, what work did you do at the ITP? Who did you work with? What were the most interesting topics at that time?

Carroll:

Again, I just worked with other postdocs. I worked a lot with Mark Trodden. That was sort of when Mark and I had our most -- actually, I think that was when Mark and I first started working together. Even though we overlapped at MIT, we didn't really work together that much. We started a really productive collaboration when I was a postdoc at ITP in Santa Barbara, even though he was, at the time -- I forget where he was located, but he was not nearby. Not only did I not collaborate with any of the faculty at Santa Barbara, but I also didn’t even collaborate with any of the postdocs in Santa Barbara. I just worked with my friends elsewhere on different things. And it was great. Mark and I continued collaborating when we both became faculty members, and we wrote some very influential papers while we were doing that. But it did finally dawn on me that I was still writing quirky things about topological defects, and magnetic fields, and different weird things about dark matter, or inflation, or whatever. So, I did finally catch on, like, okay, I need to write things that other people think are interesting, not just me. I looked around, and I'm like, nothing that I'm an expert in is something that the rest of the world thinks is interesting, really. What am I going to do? So, happily, I was a postdoc at Santa Barbara from '96 to '99, and it was in 1998 that we discovered the acceleration of the universe. The two groups, Saul Perlmutter's team, and Brian Schmidt’s and Adam Riess's team, discovered the accelerating universe. Should I explain what that is, or should we assume that people know what that means?

Zierler:

Please, please do.

Carroll:

Okay. So, like I said, we were for a long time in observational astronomy trying to understand how much stuff there is in the universe, how much matter there is. The obvious thing to do is to go out and count it. Like, here's the galaxy, weigh it, put it on a scale. It's not quite like that but watch how fast it's spinning and use Newton's laws to figure out how much mass there is. Do the same thing for a cluster of galaxies. Do the same thing for a large scale structure and how it evolves. Look at the dynamics of the universe and figure out how much matter there must be in there and compare that to what you would guess the amount of matter should be. What you would guess is the universe is expanding, and how fast it's expanding is related to that amount of density of the universe in a very particular way. There's one correct amount of density that makes the geometry of space be flat, like Euclid said back in the prehistory. If there's less matter than that, then space has a negative curvature. If it's more, then it has a positive curvature. People had known for a long time -- Alan Guth is one of the people who really emphasized this point -- that only being flat is sort of a fixed point. If you're positively curved, you become more and more positively curved, and eventually you re-collapse. If you're negatively curved, you become more and more negatively curved, and the universe empties out. So, the fact that we're anywhere near flat, which we are, right? Euclid's laws work pretty well. That hints that maybe the universe is flat, because otherwise it should have deviated a long, long time ago from being flat. So, then, you can go out and measure the mass density of the universe and compare that with what is called the critical density, what you need to make the universe flat. I will confess the error of my ways. When I was a grad student and a postdoc, I believed the theoretical naturalness argument that said clearly the universe is going to be flat. There's also the argument from inflationary cosmology, which Alan pioneered back in 1980-'81, which predicted that the universe would be flat. But even without that, it was still the most natural value to have. But the astronomers went out and measured the matter density of the universe, and they always found it was about .25 or .3 of what you needed. This didn't shut up the theorists. The theorists said, well, you just haven't looked hard enough. You're looking under the lamppost. It was true that as you looked at larger and larger scales in the universe, you saw more and more matter, not just on an absolute scale, but also relative to what you needed to see. So, there were these plots that people made of, as you look at larger and larger objects, the implied amount of matter density in the universe comes closer and closer to the critical density. But by the mid '90s, people had caught on to that and realized it didn't keep continuing. Neta Bahcall, in particular, made a plot that turned over. She's like, okay, this omega that you're measuring, the ratio of the matter density in the universe to the critical density, which you want to be one, here it is going up. As I look from a galaxy to a cluster to large-scale structure, it goes up, and it goes up to .3, and it kind of stays at .3, even as I look at larger and larger things. The theorists were just beginning to become a little uncomfortable by this, and one of the measures of that discomfort is that people like Andrei Linde and Neil Turok and others, wrote papers saying even inflation can predict an open universe, a negatively curved universe. They saw the writing on the wall. They saw that they were not getting to the critical density. But it's hard to do that measurement for reasons that Brian anticipated. It's a messy thing. Measure all the matter in the universe. That's a tough thing to do. The one way you could imagine doing it, before the microwave background came along, was you could measure the amount by which the expansion of the universe changes over time. So, it's one thing if you're Hubble in the 1920s, you can find the universe is expanding. Now, you want to say, well, how fast is it expanding now compared to what it used to be? And that gives you another handle on the total matter density. It doesn't need to be confined to a region. You're really looking out into the universe as a whole. So, if you can do it, it is a great thing. These two groups did it, and we could do a whole multi-hour thing on the politics of these two groups, and the whole thing. We won't go there, but the point is, I was friends with all of them. Brian was the leader of one group, and he was my old office mate, and Riess was in the office below ours. He wrote the paper where they actually announced the result. So, Perlmutter, who was the leader of the other group, he and I had talked in very early days, because he was the coauthor with Bill Press on this review article. So, I was on the ground floor in terms of what the observational people. I was also on the ground floor theoretically, because I had written this paper with Bill Press that had gotten attention. The paper was on what we called the cosmological constant, which is this idea that empty space itself can have energy and push the universe apart. An old idea from Einstein, and both Bill and I will happily tell you, when we were writing the paper, which was published in 1992, we were sure that the cosmological constant was zero. We just didn't know how you would measure it at the time. So, we talked about different possibilities. So, I'm doing a little bit out of chronological order, I guess, because the point is that Brian and Saul and Adam and all their friends discovered that the universe is not decelerating. All of them had the same idea, that the amount of matter in the universe acts as a break on the expansion rate of the universe. So, you can think of throwing a ball up into the air, and it goes up, but it goes up ever more slowly, because the Earth's gravitational pull is pulling it down. Likewise, the galaxies in the universe are expanding away from each other, but they should be, if matter is the dominant form of energy in the universe, slowing down, because they're all pulling on each other through the mutual gravitational force. So, literally, Brian's group named themselves the High Redshift Supernova Project: Measuring the Deceleration of the Universe. So, what they found, first Adam and Brian announced in February 1998, and then Saul's group a few months later, that the universe is accelerating. This is easily the most important, most surprising empirical discovery in fundamental physics in -- I want to say in my lifetime, but certainly since I've been doing science. Since I've been ten years old, how about that? It just came out of the blue. There were some hints, and I could even give you another autobiographical anecdote. In late 1997, again, by this time, the microwave background was in full gear in terms of both theorizing it and proposing new satellites and new telescopes to look at it. But the fruits of the labors had not come in yet. So, cosmologists were gearing up, 1997, late '90s, for all the new flood of data that would come in to measure parameters using the cosmic microwave background. So, late 1997, Phil Lubin, who was an astronomy professor at Santa Barbara, organized a workshop at KITP on measuring cosmological parameters with the cosmic microwave background. He asked me -- I was a soft target, obviously -- he asked me to give a talk at the meeting, and my assignment was measuring cosmological parameters with everything except for the cosmic microwave background. So, my job was to talk about everything else, a task for which I was woefully unsuited, as a particle physics theorist, but someone who was young and naive and willing to take on new tasks. So, it was to my benefit that I didn't know, really, what the state of the art was. I had an astronomy degree, and I'd hung out with cosmologists, so I knew the buzzwords and everything, but I hadn't read the latest papers. So, I read all the latest papers in many different areas, and I actually learned something. So, I gave a talk, and I said, "Look, something is wrong." This is December 1997. We theorists had this idea that the universe is simple, that omega equals one, matter dominates the universe -- it's what we called an Einstein-de Sitter in cosmology, that the density perturbations are scale-free and invariant, the dark matter is cold. All these different things were the favorite model for the cosmologists. I said, the thing that you learn by looking at all these different forms of data are that, that can't be right. No one had quite put that together in a definitive statement yet. There were hints of it. I was certainly not the first to get the hint that something had to be wrong. Maybe it was that the universe was open, that the omega matter was just .3. Maybe it was that there was some mixture of hot dark matter and cold dark matter, or maybe it was that there was a cosmological constant. These were all live possibilities. So, when Brian, Adam, Saul, and their friends announced in 1998 that there was a cosmological constant, everyone was like, oh, yeah, okay. That's it. Despite the fact that it was hugely surprising, we were all totally ready for it. I was in on the ground floor, because I had also worked on theoretical models of it. Greg Anderson and I had written a paper. He was another postdoc that was at MIT with me. We wrote a little particle physics model of dark matter that included what is now called dark energy interacting with each other, and so forth. So, I was not that far away from going to law school, because I was not getting any faculty offers, but suddenly, the most interesting thing in the universe was the thing that I was the world's expert in, through no great planning of my own. To my slight credit, I realized it, and I jumped on it, and I actually collaborated with Brian and his friends in the high-z supernova team on one of his early papers, on measuring what we now call w, the equation of state parameter. I wrote a couple papers by myself on quintessence, and dark energy, and suddenly I was a hot property on the faculty job market again.

Zierler:

And this was all happening during your Santa Barbara years.

Carroll:

All while I was in Santa Barbara. Literally, it was -- you have to remember, for three years in a row, I'd been applying for faculty jobs and getting the brush off, and now, I would go to the APS meeting, American Physical Society meeting, and when I'd get back to my hotel, there'd be a message on my phone answering machine offering me jobs. It was like suddenly I was really in the right place at the right time.

Zierler:

What were the faculty positions that were most compelling to you as you were considering them?

Carroll:

There were two that were especially good. There was one that was sort of interesting, counterfactual, is the one place that came really close to offering me a faculty job while I was at KITP before they found the acceleration of the universe, was Caltech. Caltech has this weird system where they don't really look for slots. It's rolling admissions in terms of faculty. Every year, they place an ad that says, "We are interested in candidates in theoretical physics, or theoretical astrophysics." So, you can apply, and they'll consider you at any time. They brought me down, and I gave a talk, but the talk I could give was just not that interesting compared to what was going on in other areas. I talked about topological defects, and it was good work, solid work, but they were honestly -- and this is the sort of weird thing -- they said, after I gave the talk and everything, "Look, everyone individually likes you, but no one is sure where you belong." That's the message I received many, many times. What they meant was, like, what department, or what subfield, or whatever. So, they said, "Here's what we'll do. We have this special high prestige, long-term post-doctoral position, almost a faculty member, but not quite. Apply for that, we'll hire you for that. You can come here, and it'll be a trial run to see if you fit in, and where you fit in the best." So, I said, "Okay, I'll apply for that. I'll go there and it'll be like a mini faculty member. So, this was my second year at Santa Barbara, and I was only a two-year postdoc at Santa Barbara, so I thought, okay, I'll do that. And the postdoc committee at Caltech rejected me. They did not hire me, because they were different people than were on the faculty hiring committee and they didn't talk to each other. So, I had to go to David Gross, who by then was the director of KITP, and said, "Could you give me another year at Santa Barbara, because I just got stranded here a little bit?" And he said, "Yes, sure." And then they discovered the acceleration of the universe, and I was fine. Once that happened, I got several different job offers. The two that were most interesting to me were the University of Chicago, where I eventually ended up going, and University of Washington in Seattle. Washington was just a delight. There were so many good people there, and they were really into the kind of quirky things that I really liked. Eric Adelberger and Chris Stubbs were there, who did these fifth force experiments. The astronomy department was great, the physics department was great. Ann Nelson and David Kaplan -- Ann Nelson has sadly passed away since then. And they had atomic physics, which I thought was interesting, and Seattle was beautiful. So, it was very tempting, but Chicago was much more like a long-term dream. Remember, I applied there to go to undergraduate school there. And I applied there to graduate school and to postdocs, and every single time, I got accepted. They wanted me, and every single time I turned them down. So, the idea that I could go there as a faculty member was very exciting to me.

Zierler:

To the extent, to go back to our conversation about filling a niche on the faculty, what was that niche that you would be filling? What would your academic identity, I guess, be on the faculty at the University of Chicago?

Carroll:

Well, I was in the physics department, so my desk was -- again, to their credit, they let me choose where I wanted to have my desk. The obvious choices were -- the theoretical cosmology effort was mostly split between Fermilab and the astronomy department at Chicago, less so in the physics department. The physics department had the particle theory group, and it also had the relativity group. I chose wrongly again. I do this over and over again. I clearly made the worst of the three choices in terms of the cosmology group, the relativity group, the particle theory group, because I thought in my naïveté that I should do the thing that was the most challenging and least natural to me, because then I would learn the most. I can do cosmology, and I'd already had these lecture notes on relativity. I knew relativity really well, but I still felt, years after school, that I was behind when it came to field theory, string theory, things like that. So, that's where I wanted my desk to be so I could hang out with those people. Not to give away the spoiler alert, but I eventually got denied tenure at Chicago, and I think that played a lot into the decision. I was less good of a fit there. If I had just gone to relativity, they probably would have just kept me. Who knows? Maybe not. It's at least possible. I thought and think -- I think it's true that they and I had a similar picture of who I would be namely bringing those groups together, serving as a bridge between all those groups. A few years after I got there, Bruce Winstein, who also has passed away, tragically, since then, but he founded what was at the time called the Center for Cosmological Physics and is now the Kavli Institute for Cosmological Physics at Chicago. So, he founded that. I was there. He invited a few of us. I will not reveal who was invited and who was not invited, but you would be surprised at who was invited and who was not invited, to sort of write this proposal to the NSF for a physics frontier center. As long as I was at Chicago, I was the group leader of the theory group in the cosmological physics center. So, that's what I was supposed to do, and I think that I did it pretty well.

Zierler:

Sean, just a second, the sun is setting here on the east coast. Let me just fix the lighting over here before I become a total silhouette.

Carroll:

I will get water while you're doing that. I'll be back.

Zierler:

Perfect. *pause* Ah, much better.

Carroll:

Good.

Zierler:

So, Sean, what were your initial impressions when you got to Chicago?

Carroll:

I mean, the good news was -- there's a million initial impressions. Again, I did badly at things that I now know are very obvious things to do. What you should do is, if you're a new faculty member in a department, within the first month of being there, you should have had coffee or lunch with every faculty member. Just get to know people. But it was kind of overwhelming. There are so many people at Chicago. The wonderful thing about it was that the boundaries were a little bit fuzzy. So, there was the physics department, and the astronomy department, and there was also what's called the Enrico Fermi Institute, which was a research institute, but it was like half of the physics department and half of the astronomy department was in it. There was the James Franck Institute, which was separate. So, there's just too many people to talk to, really. So, I kind of talked with my friends. It was -- I don't know. You'd need to ask a more specific question, because that's just an overwhelming number of simulations that happened when I got there.

Zierler:

I guess, one way of putting it is, you hear of such a thing as an East Coast physics and a West Coast physics. I can never decide if that's just a stand-in for Berkeley and Princeton, or it means something more general than that. But to the extent that you've had this exposure, Harvard and then MIT, and then you were at Santa Barbara, one question with Chicago, and sort of more generally as you're developing your experience in academic physics, when you got to Chicago, was there a particular approach to physics and astronomy that you did not get at either of the previous institutions?

Carroll:

There was, but it was kind of splintered because of this large number of people. Chicago, to its credit, these people are not as segregated at Chicago as they are at other places. At Caltech, as much as I love it, I'm on the fourth floor in the particle theory group, and I almost never visit the astronomers. They're across the street, so that seems infinitely far away. The idea of visiting the mathematicians is just implausible. I mean, I could do it. Again, I could generate the initiative to do that, but it's not natural, whereas in Chicago, it kind of did all blend into each other in a nice way. I really leaned into that. I started a new seminar series that brought people together in different ways. There's always some institutional resistance. My biggest contribution early on was to renovate the room we all had lunch in in the particle theory group. It was just a dump, and there was a lot of dumpiness. There's not a lot of aesthetic sensibility in the physics department at the University of Chicago. I remember, even before I got there, I got to pick out my office. It was like cinderblocks, etc., but at least it was spacious. They asked me to pick furniture and gave me a list of furniture. I looked at the list and I said, "Well, honestly, the one thing I would like is for my desk to be made out of wood rather than metal.” They had these cheap metal desks. The guy, whoever the person in charge of these things, says, "No, you don't get a wooden desk until you're a dean." Frank Merritt, who was the department chair at the time, he crossed his arms and said, "No, I think Sean's right. I think new faculty should get wooden desks. We should move into that era." So, I raised the user friendliness of it a little bit. Also, with the graduate students, it's not as bad as Caltech, but Chicago is also not as user friendly for the students as Harvard astronomy was. Let's put it that way. Sorry, I forgot the specific question I'm supposed to be answering here. Oh, kinds of physics.

Zierler:

Yeah, the kind of physics at Chicago.

Carroll:

Yeah, so actually, I should back up a little bit, because like I said, at Harvard, there were no string theorists. There was Cumrun Vafa, one person who was looked upon as a bit of an aberration. When I went to MIT, it was even worse. Well, sorry, also one string theorist: Barton Zwiebach was there. Also, assistant professor, right? Young people. In that era, it's kind of hard to remember. The first super string revolution had happened around 1984. So, by 1992 or 1993, it's been like, alright, what have you done for me lately? We were promised the mass of the electron would be calculated by now. It had gotten a little stuck. Then, through the dualities that Seiberg and Witten invented, and then the D-brane revolution that Joe Polchinski brought about, suddenly, the second super string revolution was there, right? So, it was explicable that neither Harvard nor MIT, when I was there, were deep into string theory. But when I was in Santa Barbara, I was at the epicenter. Santa Barbara was second maybe only to Princeton as a string theory center. Polchinski was there, David Gross arrived, Gary Horowitz, and Andy Strominger was still there at the time. So, it was really just a great place. It was a little bit of whiplash, because as a young postdoc, one of the things you're supposed to do is bring in seminar speakers. I suggested some speakers, and people looked at my list and were like, "These aren't string theorists at all. This is a weird list. We don't know what to do with this." And Chicago was somewhere in between. The particle theory group was very heavily stringy. Basically Jon Rosner, who's a very senior person, was the only theorist who was a particle physicist, which is just weird. The University of Chicago, which is right next to Fermilab, they have almost no particle physics. They've tried to correct that since then, but it was a little weird. They also had Bob Wald, who almost by himself was a relativity group. It had been founded by Chandrasekhar, so there was some momentum there going. Bob Geroch was there also, but he wasn't very active in research at the time. Then, of course, the cosmology group was extremely active, but it was clearly in the midst of a shift from early universe cosmology to late universe cosmology at the time. The emphasis -- they had hired John Carlstrom, who was a genius at building radio telescopes. The South Pole telescope is his baby. They hired Wayne Hu at the same time they hired me, as a theorist, to work on the microwave background. Soon afterward, they hired Andrey Kravtsov, who does these wonderful numerical simulations. So, the late universe was clearly where they were invested. So, despite the fact that I connected all the different groups, none of them were really centrally interested in what I did for a living.

Zierler:

To go back to the question of exuberance and naïveté and not really caring about what other people are thinking, to what extent did you have strong opinions one way or another about the culture of promoting from within at Chicago? In other words, like you said yourself before, at a place like Harvard or Stanford, if you come in as an assistant professor, you're coming in on the basis of you're not getting tenure except for some miraculous exception to the rule. Was that the case at Chicago, or was that not the case at Chicago?

Carroll:

No, and to be super-duper honest here, I can't possibly be objective, because I didn't get tenure at the University of Chicago. I do try my best to be objective.

Zierler:

But you're good at math. What are the odds? That's my question.

Carroll:

Well, most people got tenure. You were hired with the expectation that you would get tenure. Not a 100% expectation. Honestly, Caltech, despite being intellectually as good as Harvard or Princeton, if you get hired as an assistant professor, you almost certainly get tenure. The bottleneck is hiring you as an assistant professor. At Harvard, it's the opposite. Chicago is a little bit in between. I was on the faculty committees when we hired people, and you would hear, more than once, people say, "It's just an assistant professor. If they don't pan out, they just won't give him tenure." So, that was definitely an option. But I would guess at least three out of four, or four out of five people did get tenure, if not more.

Zierler:

Structurally, do you think, looking back, that you were fighting an uphill battle from the beginning, because as idealistic as it sounds to bring people together, intellectually, administratively, you're fighting a very strong tide. There's a lot of inertia. There's a lot of bureaucratic resistance to that very idea, even if the collaborations are going to produce great, great topics.

Carroll:

I think that is part of it. The specific way in which that manifests itself is that when you try to work, or dabble, if you want to put it that way, in different areas, and there are people at your institution who are experts in those specific areas, they're going to judge you in comparison with the best people in your field, in whatever area you just wrote in. And it's not just me. I mean, Angela Olinto, who is now, or was, the chair of the astronomy department at Chicago, she got tenure while I was there. I learned afterward it was not at all easy, and she did not sail through. One of the reasons why is she mostly does work in ultra-high energy cosmic rays, which is world class, but she wrote some paper about extra dimensions and how they could be related to ultra-high energy cosmic rays. So, the string theorists judged her like they would be judging Cumrun Vafa, or Ed Witten. So, it's not an easy hill to climb on. So, that's one of the things you walk into as a person who tries to be interdisciplinary. Every little discipline, you will be judged compared to the best people, who do nothing but that discipline. Maybe that's not fair. I hope that the whole talk about Chicago will not be about me not getting tenure, but I actually, after not getting tenure, I really thought about it a lot, and I asked for a meeting with the dean and the provost. They met with me, and it was a complete disaster, because they thought that what I was trying to do was to complain about not getting tenure and change their minds about it. I had no interest. Once I didn't get tenure, I didn't want to be there anymore. You couldn't pay me to stick around if they didn't want me there. What I wanted to do was to let them know how maybe they could improve the procedure going forward. It was a very casual procedure. I heard my friends at other institutions talk about their tenure file, getting all of these documents together in a proposal for what they're going to do. None of that at Chicago. At Chicago, you hand over your CV, and you suggest some names for them to ask for letters from. And that's the only thing you do. There's no other input that you have. As a result, the fact that I was interdisciplinary in various ways, not just within cosmology and relativity and particle physics, but I taught a class in the humanities. I went on expeditions with the dinosaur hunters as a public outreach thing. I did various things. I talked to the philosophers and classicists, and whatever, but I don't think anyone knew. They didn't even realize that I did these things, and they probably wouldn't care if they did. But it's absolutely true that the system is not constructed to cast people like that int he best possible light.

Zierler:

How do we square the circle with the fact that you were so amazingly positioned with the accelerating universe a very short while ago?

Carroll:

Well, you know, again, I was not there at the meeting when they rejected me, so I don't know what the reasons were. Various people on the faculty came to me after I was rejected, and tried to explain to me why, and they all gave me different stories. In retrospect, there's two big things. One is you do get a halfway evaluation. So, if you're assistant professor for six years, after three years, they look at you, and the faculty talks about you, and they give you some feedback. Honestly, the thought of me not getting tenure just didn't occur to me, really. It was so clear to me that I did everything they wanted me to do that I just didn't try to strategize. I didn't stress about that. So, the year before my midterm evaluation, I spent almost all my time doing two things. Number one, writing that textbook that I wrote on general relativity, space time and geometry. And number two, I did a lot of organizing of a big international conference, Cosmo '02, that I was the main organizer of. As a result, I think I wrote either zero or one papers that year. That was always temporary. I had that year that I was spending doing other things, and then I returned to doing other things. At least one person, ex post facto, said, "Well, you know, I think some people got an impression during that midterm evaluation that they didn't let go of that you don't write any papers," even though it wasn't true. I do remember, you're given some feedback after that midterm evaluation, and the director of the Enrico Fermi Institute said, "You've really got to not just write review papers, but high impact original research papers." So, again, I sort of brushed it off. I thought I knew what I was doing. I said, "Yeah, don't worry. I will." But then, the thing is, I did. I wrote papers that were hugely cited and very influential. Like, several of them. More than one. And at my post tenure rejection debrief, with the same director of the Enrico Fermi Institute, he said, "Yeah, you know, we really wanted you to write more papers that were highly impactful." And I said, "But I did do that." And he says, "Yeah, I saw that. I was really surprised." What could I do? I'm not sure of what I'm being asked for. I do think that people get things into their heads and just won't undo them. So, that was one big thing. I think that one year before my midterm, I blew it. And the other thing was honestly just the fact that I showed interest in things other than writing physics research papers. The biggest one was actually -- people worry that I was blogging, and things like that. They didn't know. They had no idea that I was doing that, but they knew --

Zierler:

But you were. You had already dipped your toe into this kind of work. You were starting to do that.

Carroll:

I started blogging in 2004, and I was rejected in 2005 from Chicago. But they did know that I wrote a textbook in general relativity, a graduate-level textbook. It just never occurred to me that that would be a strike against me, but apparently it was a huge strike against me. What you hear, the honest opinion you get is not from the people who voted against you on your own faculty, but before I got the news, there were people at other universities who were interested in hiring me away. I was still thought to be a desirable property. Of course, once you get rejected for tenure, those same people lose interest in you. They can't convince their deans to hire you anymore, now that you're damaged goods. But they told me, they said, "We talked to the people at Chicago, and they thought that you were just interested in writing textbooks and not doing research anymore." So, even if it's a graduate-level textbook filled with equations, that is not what they want to see. One of the people said to me afterwards, "We thought that you'd be more suited at a place with a more pedagogical focus than what I had." I could point to the papers I wrote with the many, many citations all I wanted to, but that impression was in their minds.

Zierler:

Is writing a graduate-level textbook in general relativity, might that have been perceived as a bit of a bold move for an assistant professor?

Carroll:

Oh, yeah, entirely. Again, while I was doing it, I had no idea that it would be anything other than my job, but afterward -- this is the thing. If I do get to just gripe, zero people at the University of Chicago gave me any indication that I was in trouble of not getting tenure.

Zierler:

So, you were blindsided, totally.

Carroll:

Completely blindsided. Like I said, it just didn't even occur to me. I thought it would be more likely that I'd be offered tenure early than to be rejected. Partly, that was because I knew I'd written papers that were highly cited, and I contributed to the life of the department, and I had the highest teaching evaluations. I did everything right. And part of it was because no one told me. Anyway, again, afterward, more than one person says, "Why did you write a textbook? Why did you do that?" Several of these people had written textbooks themselves, but they'd done it after they got tenure.

Zierler:

So, let's get off the tenure thing. Let's go back to the happier place of science. First, on the textbook, what was the gap in general relativity that you saw that necessitated a graduate-level textbook?

Carroll:

It's funny, that's a great question, because there are plenty of textbooks in general relativity on the market. Who hasn't written one, really? One of the best was by Bob Wald, maybe the best, honestly, on the market, and he was my colleague. Bob is a good friend of mine, and I love his textbook, but it's very different.

Zierler:

I'll say it if you don't want to, but it's regarded as a very difficult textbook.

Carroll:

It's difficult, yes. It's challenging. In my mind, there were some books -- like, Bernard Schutz wrote a book, which had this wonderful ambition, and Jim Hartle wrote a book on teaching general relativity to undergraduates. By the strategy, it's sort of saving some of the more intimidating math until later. Doing as much as you could without the intimidating math. Then, there were books like Bob Wald's, or Steven Weinberg's, or Misner Thorne and Wheeler, the famous phonebook, which were these wonderful reference books, because there's so much in them. So much knowledge, and helpful, but very intimidating if you're a student. So, I thought that graduate students just trying to learn general relativity -- didn't have a good book to go through. At least, I didn't when I was a graduate student. We used Wald, and it was tough. So, it's not quite true, but in some sense, my book is Wald for the common person. It's sort of the most important ideas there but expressed in a way which was hopefully a lot more approachable and user-friendly, and really with no ambition other than letting people learn the subject. Not any ambition to be comprehensive, or a resource for researchers, or anything like that, for people who wanted to learn it.

Zierler:

I wonder, Sean, if there's the germinating idea that would inform your interests in outreach, and in doing public science and things like that, it was that inclination that was bounded in an academic context, that you would take eventually into the world of YouTube, and hundreds of thousands of lay people out there, who are learning quantum gravity as a result of you.

Carroll:

Absolutely. I think that, again, good fortune on my part, not good planning, but the internet came along at the right time for me to reach broader audiences in a good way. In fact, that even helped with the textbook, because I certainly didn't enter the University of Chicago as a beginning faculty member in 1999, with any ambitions whatsoever of writing a textbook. That was not on my radar. But to go back a little bit, when I was at MIT -- no, let's go back even further. When I was at Harvard, Ted Pyne, who I already mentioned as a fellow graduate student, and still a good friend of mine, he and I sort of stuck together as the two theoretical physicists in the astronomy department. We both took general relativity at MIT from Nick Warner. Absolutely brilliant course. Nick is also a friend of mine, and he's a professor at USC now. And we just bubbled over in excitement about general relativity, and our friends in the astronomy department generally didn't take general relativity, which is weird in a sense. Harvard taught a course, but no one liked it. Everyone could tell which courses were good at Harvard, and which courses were good at MIT. So, Ted and I said, we will teach general relativity as a course. Bill Press did us a favor of nominally signing a piece of paper that said he would be the faculty member for this course. But he was very clear. He said, "As long as I have to do literally nothing. I don't have to go to the class, I don't have to listen to you, I'll sign the piece of paper." Good. So, we had like ten or twelve students in our class. We had problem sets that we graded. The whole bit. We made up lecture notes, and it was great. It was a huge success. Again, a weird thing you really shouldn't do as a second-year graduate student. It's not good time management, but we did it and we enjoyed it. Then, when I got to MIT, they knew that I had taught general relativity, so my last semester as a postdoc, after I had already applied for my next job, so I didn't need to fret about that, the MIT course was going to be taught by a professor who had gone on sabbatical and never returned. MIT was a weird place in various ways. So, that would happen. So, they actually asked me as a postdoc to teach the GR course. So, again, I foolishly said yes. And this time, first I had to do it all by myself, but because I was again foolishly ambitious, I typed up all the lecture notes, so equations and everything, before each lecture, Xeroxed them and handed them out. It was over 50 students in the class at that time. Wildly enthusiastic reception. Everyone loved it, I won a teaching award. That was great, a great experience. And then a couple years later, when I was at Santa Barbara, I was like, well, the internet exists. I have about 200 pages of typed up lecture notes. I'll just put them on the internet. So, I did, and they became very popular. Once you do that, people will knock on your door and say, "Please publish this as a textbook." So, I was sweet-talked into publishing it without any plans to do it.

Zierler:

Now, the high impact research papers that you knew you had written, but unfortunately, your senior colleagues did not, at the University of Chicago, what were you working on at this point? What were those topics that were occupying your attention?

Carroll:

Well, as usual, I bounced around doing a lot of things, but predictably, the things that I did that people cared about the most were in this -- what I was hired to do, especially the theory of the accelerating universe and dark energy. So, I wrote a paper, and most of my papers in that area that were good were with Mark Trodden, who at that time, I think, was a professor at Syracuse. So, we wrote one paper with my first graduate student at Chicago -- this is kind of a funny story that illustrates how physics gets done. I mentioned very briefly that I collaborated on a paper with the high redshift supernova team. So, they had already done their important papers showing the universe was accelerating, and then they want to do this other paper on, okay, if there is dark energy, as it was then labeled, which is a generalization of the idea of a cosmological constant. The cosmological constant would be energy density in an empty space that is absolutely strictly constant as an energy. Every cubic centimeter has the same amount of energy in it. Dark energy is a more general idea that it's some energy density in empty space that is almost constant, but maybe can go down a little bit. The obvious ideas, you have some scalar field which was dubbed quintessence, so slowly, slowly rolling, and has a potential energy that is almost constant. Well, how would you know? Well, you could measure the rate at which the universe was accelerating, and compare that at different eras, and you can parameterize it by what's now called the equation of state parameter w. So, w equaling minus one, for various reasons, means the density of the dark energy is absolutely constant. w of minus .9 or minus .8 means the density is slowly fading away. w of zero means it's like ordinary matter. So, the density goes down as the volume goes up, as space expands. So, dark energy is between minus one and zero, for this equation of state parameter. So, an obvious question arises. What about minus 1.1? Could the equation of state parameter be less than minus one? And the High-z supernova team, my friends, Bob Kirshner, and Brian, and Adam, and so forth, came to me, and were like, "You know, you're a theorist. What should we do? Should we let w be less than minus one?" And I thought about it, and I said, "Well, there are good reasons to not let w be less than minus one. There's good physics reasons. It would be bad. Here's a couple paragraphs saying that, in physics speak." So, they just cut and pasted those paragraphs into their paper and made me a coauthor. It was very funny, because in astronomy, who's first author matters. In physics, it doesn't matter, it's just alphabetical. But the High-z supernova team strategy was the whole thing would be alphabetical, except the most important author, the one who really did the work on the paper, would be first. So, it'd be a first author, and then alphabetical. Except, because my name begins with a C, if they had done that for the paper, I was a coauthor on, I would have been the second author. So, it would look like I was important, but clearly, I wasn't that important compared to the real observers. So, just for me, they made up a special system where first author, alphabetical, and then me at the end. So, biologists think that I'm the boss, because in biology, the lab leader goes last in the author list. So, that was just a funny, amusing anecdote. Anyway, even though we wrote that paper and I wrote my couple paragraphs, and the things I said were true, as. theoretical physicist, I kept thinking about it. I presented good reasons why w could not be less than minus one, but how good are they? And in the meantime, Robert Caldwell, Marc Kamionkowski, and others, came up with this idea of phantom energy, which had w less than minus one. What that means is, as the universe expands, the density of energy in every cubic centimeter is going up. So, it's not hard to imagine there are good physical reasons why you shouldn't allow that. Like, where's the energy coming from? But they imagined it, and they wrote down little models in which it was true. That leads to what's called the Big Rip. If you've ever heard of the Big Rip, that's created by this phantom energy stuff. So, Mark Trodden and I teamed up with a graduate student, my first graduate student at Chicago. Mark Hoffman was his name. We wrote a paper that did the particle physics and quantum field theory of this model, and said, "Is it really okay, or is this cheating? Is there something wrong about it?" What we said is, "Oh, yeah, it's catastrophically wrong. You would have negative energy particles appearing in empty space. You should not let w be less than minus one." So, we wrote a paper on that, and it became very popular and highly cited. Another follow up paper, which we cleverly titled, “Could you be tricked into thinking that w is less than minus one?” by modifying gravity, or whatever. And that got some attention also. Then, the other big one was, again, I think the constant lesson as I'm saying all these words out loud is how bad my judgment has been about guiding my own academic career. So, when I was at Chicago, I would often take on summer students, like from elsewhere or from Chicago, to do little research projects with. I was thinking of a research project -- here is the thought process. It's good to talk about physics, so I'll talk about physics a little bit. Here is my thought process. We have dark energy, it's pushing the universe apart, it's surprising. We don't know why it's the right amount, or whatever. We also have dark matter pulling the universe together, sort of the opposite of dark energy. But there was this interesting phenomenon point out by Milgrom, who invented this theory called MOND, that you might have heard of. He says that if you have a galaxy, roughly speaking, there's a radius inside of which you don't need dark matter to explain the dynamics of the galaxy, but outside of that radius, you do. For every galaxy, the radius is different, but what he noticed was, and this is still a more-or-less true fact that really does demand explanation, and it's a good puzzle. If you just plug in what is the acceleration due to gravity, from Newton's inverse square law? What is the acceleration due to gravity at that radius? It's the same for a whole bunch of different galaxies. The crossover point from where you don't need dark matter to where you do need dark matter is characterized not by a length scale, but by an acceleration scale. You can do a bit of dimensional analysis and multiply by the speed of light, or whatever, and you notice that that acceleration scale you need to explain the dark matter in Milgrom's theory is the same as the Hubble constant. The expansion rate of the universe, even though these two numbers are completely unrelated to each other. The Hubble constant is famously related to the dark energy, because it's the current value of the Hubble constant where dark energy is just taking over. This is what's known as the coincidence problem. Why is the matter density of the universe approximately similar to the dark energy density, .3 and .7, even though they change rapidly with respect to each other? What's so great about right now? So, my thought process was, both dark matter and dark energy are things we haven't touched. We've only noticed them through their gravitational impact. Everyone knows -- Milgrom said many years ago in the case of dark matter, but everyone knows in the case of dark energy -- that maybe you can modify gravity to get rid of the need for dark matter or dark energy. Hard to do in practice, but in principle, maybe you could do it. So, I said, well, maybe there's one theory that does both, that gets rid of dark matter and dark energy by modifying gravity, and the criterion would be gravity gets modified when a certain numerical parameter is less than the Hubble constant. The acceleration due to gravity, of the acceleration of the universe, or whatever. So, I said, well, how do you do that? So, this is when it was beneficial that I thought differently than the average cosmologist, because I was in a particle theory group, and I felt like a particle theorist. I wrote down Lagrangians and actions and models and so forth. So, I said, as a general relativist, so I knew how to characterize mathematically, what does it mean for -- what is the common thing between the universe reaching the certain Hubble constant and the acceleration due to gravity reaching a certain threshold? Well, you parameterize gravitational forces by the curvature of space time, right? And the simplest way to do that is what's called the curvature scalar. What's interesting -- you're finally getting the punchline of this long story. Sorry about that. What's interesting is something which is in complete violation of your expectation from everything you know about field theory, that in both the case of dark matter and dark energy, if you want to get rid of them in modified gravity, you're modifying them when the curvature of space time becomes small rather than when it becomes large. Everyone knows when fields become large and strengths become large, your theories are going to break down. The Planck scale, or whatever, is going to be new physics. No one expects that small curvatures of space time, anything interesting should happen at all. But maybe it could. So, I played around writing down theories, and I asked myself, what is the theory for gravity? It's literally that curvature scalar R, that is the thing you put into what we call the Lagrangian to get the equations of motion. It's the simplest thing you possibly could do. How could I modify R so that it acted normal when space time was curved, but when space time became approximately flat, it changed. It moved away. Well, I just did the dumbest thing. I said, well, what about R plus one over R? So, we'd already done R plus a constant. If you just have a constant, that's the cosmological constant. Everyone knows about that. Einstein did that, but nobody had done one over R. And it wasn't like that was necessarily motivated by anything. It was just -- could that explain away both the dark matter and the dark energy, by changing gravity when space time was approximately flat? So, I actually worked it out, and then I got the answers in my head, and I gave it to the summer student, and she worked it out and got the same answers. The answers are: you can make the universe accelerate with such a theory. You can explain the acceleration of the universe, but you can't explain the dark matter in such a theory. In fact, the short shield solution, the solution that you get in general relativity for spherically symmetric matter distribution, is exactly the same in this new theory as it was in general relativity. So far so good. Firing on all cylinders intellectually. Everything is going great. Came up with a good idea. But now, I had this goal of explaining away both dark matter and dark energy. I did not succeed in that goal.

Zierler:

We'd be having a very different conversation if you did.

Carroll:

Yes, well that's true. That would have been a very different conversation if I had. But it should have been a different conversation anyway, because I said, well, therefore it's not interesting. I didn't do what I wanted to do. And at least a year passed. I'm not sure how much time passed. But within the course of a week -- coincidence problem -- Vikram Duvvuri, who was a graduate student in Chicago, knocked on my door, and said, "Has anyone ever thought of taking R and adding one over R to the Lagrangian for gravity and seeing what would happen?" I said, "Well, yeah, I did. It doesn't really explain away dark matter, but maybe it could make the universe accelerate." And he goes, "Oh, yeah, okay." And then I got an email from Mark Trodden, and he said, "Has anyone ever thought about adding one over R to the Lagrangian for gravity?" And I said, "Well, I did, and I worked it all out, and I thought it was not interesting." But clearly it is interesting since everyone -- yeah. So, we wrote a paper. Mark and Vikram and I and Michael Turner, who was Vikram's advisor. The four of us wrote a paper. Huge excitement because of this paper.

Zierler:

Was this your first time collaborating with Michael Turner?

Carroll:

Yes, I think so. I think we only collaborated on two papers. That one and a follow up to that. So, yeah, we wrote a four-author paper on that. It became a big deal, and they generalized it from R plus one over R to f(R), any function of R. There's a whole industry out there now looking at f(R) gravity. And, you know, I could have written that paper myself. I had it. I had the results. We did some extra numerical simulations, and we said some things, and Vikram did some good things, and Mark did too, but I could have done it myself. Then, I would have had a single-author paper a year earlier that got a thousand citations, and so forth. And I didn't. So, taste matters. It's good to have good ideas but knowing what people will think is an interesting idea is also kind of important.

Zierler:

Sean, did you enjoy teaching undergraduates? I assume this was really a unique opportunity up until this point to really interact with undergraduate students.

Carroll:

I like teaching a lot. It's a lot of work if you do it right. Chicago was great because the teaching requirements were quite low compared to other places. It was on a quarter system: fall, winter, spring quarters. So, there's three quarters in an academic year. As a faculty member in a physics department, you only taught two of them. So, basically, there's like a built-in sabbatical. You get one quarter off from teaching every year. I taught both undergraduate and graduate students. I taught graduate particle physics, relativity. I started a new course in cosmology, which believe it or not, had never been taught before. Theoretical cosmology at the University of Chicago had never been taught before. I also started a new course, general relativity for undergraduates, which had not been taught before, and they loved it. I think that the secret to teaching general relativity to undergraduates is it's not that much different from teaching it to graduate students, except there are no graduate students in the audience. So, the undergraduates are just much more comfortable learning it. They're a little bit less intimidated. I actually think the different approaches like Jim Hartle has to teaching general relativity to undergraduates by delaying all the math are not as good as trying to just teach the math but go gently. Give them plenty of room to play with it and learn it, but I think the math is teachable to undergraduates. I taught what was called a big picture course. Bill Wimsatt, who is a philosopher at Chicago had this wonderful idea, because Chicago, in many ways, is the MIT of the humanities. It's very, very demanding, but it's more humanities-based overall as a university. It could be very interdisciplinary in some ways. It falls short of that goal in some other ways. But it gives lip service to the ideal of it. One of the things is that they have these first-year seminars, like many places do. First year seminars to sort of explore big ideas in different ways. But Bill's idea was, look, we give our undergraduates these first year seminars, interdisciplinary, big ideas, very exciting, and then we funnel them into their silos to be disciplinary. We teach them all these wonderful techniques and we never quite let them apply those techniques they learn to these big interdisciplinary ideas. So, he started this big problems -- I might have said big picture, but it's big problems curriculum -- where you would teach to seniors an interdisciplinary course in something or another. So, Shadi Bartsch, who is a classics professor at Chicago, she and I proposed to teach a course on the history of atheism. So, not whether atheism is true or false, but how it developed intellectually. It was a lot of fun because there weren't any good books. No one has written the history of atheism very, very well. Now, we did a terrible job teaching it because we just asked them to read far too much. It was like, if it's Tuesday, this must be Descartes, kind of thing. We did not give them nearly enough time to catch their breath and synthesize things. But it was a great experience for me, too, teaching a humanities course for the first time.

Zierler:

Sean, I want to push back a little on this idea that not getting tenure means that you're damaged goods on the academic job market. We can both quite easily put together a who's who of really top-flight physicists who did not get tenure at places like Harvard and Stanford, and then went on to do fundamental work at other excellent institutions, like University of Washington, or Penn, or all kinds of great universities. I want to ask, going to Caltech to become a senior research associate, did you self-consciously extricate yourself from the entire tenure world? Did you do that self-consciously? Were there tenure lined positions that were available to you, but you said, you know what, I'm blogging, I'm getting into outreach, I'm doing humanities courses. I'm going to bail from the whole enterprise. What was your thought process along those lines?

Carroll:

Well, by that point, I was much more self-conscious of what my choices meant. I was kind of forced into it by circumstances. So, I gave a lot of thought to that question. I really took the opportunity to think as broadly as possible. I forced myself to think about leaving academia entirely. Had I made a wrong choice by going into academia? No, no, I kind of like it here. I made that choice consciously. A lot of my choices throughout my career have not been conscious. I just did the next step that I was supposed to do.

Zierler:

That's the theme so far.

Carroll:

Yeah, so this is a chance to really think about it. So, then, the decision was, well -- so, to answer your question, yes -- well, sorry, I didn't quite technically get tenured offers, if I'm being very, very honest, but it was clear I was going to. If I had pursued certain opportunities, I could have gotten tenured.

Zierler:

Let me ask specifically, is your sense that you were more damaged goods because the culture at Chicago was one of promotion? In other words, an assistant professor not getting tenure at Stanford, that has nothing to do with him or her. That's just the system. Is your sense that really the situation at Chicago did make it that much more difficult for you?

Carroll:

I think so, but I think it's even an exaggeration to say that Harvard or Stanford don't give people tenure, therefore it's not that bad. I think it's bad in the following way. I had done a postdoc for six years, and assistant professor for six by the time I was rejected for tenure. You're old. People know who you are. You've been around the block a few times. You can't get a non-tenured job. You're just too old for that. Some places like Stanford literally have a rule. If you've been so many years past your PhD, or you're so old, either you're hired with tenure, or you're not hired on the faculty. So, you have to be hired as a senior person, as a person with tenure in a regular faculty position. The Caltech job is unique for various reasons, but that's always hard, and it should be hard. Hiring senior people, hiring people with tenure at a really good place is just going to be hard. You really have to make a case. You, as the physics department trying to convince the provost and the dean and the president that you should hire this person, that's an uphill battle, always. If the most obvious fact about the candidate you're bringing forward is they just got denied tenure, and the dean doesn't know who this person is, or the provost, or whatever, they're like, why don't you hire someone who was not denied tenure. I'm not making this up. This is literally the words that I was told. People shrugged their shoulders and said, "Yeah, you know, there's zero chance my dean would go for you now that you got denied tenure."

Zierler:

This is also the time when the Department of Energy is starting to fully embrace astrophysics, and to a lesser extent, cosmology, at the National Laboratories. I wonder if that was a quasi-alternative career that you may have considered at some point, particularly because you were so well-acquainted with what Saul Perlmutter was doing.

Carroll:

There are not a lot of jobs for people like me, who are really pure theorists at National Labs like that. There are theorists who are sort of very closely connected to the experiments. There's always exceptions to that. SLAC has done a wonderful job hiring string theorists, for example.

Zierler:

There's a strong theory group at Los Alamos, for example.

Carroll:

At Los Alamos, yes. That's right. But I think I didn't quite answer a previous question I really want to get to which is I did get offered tenured jobs, but I was still faced with a decision, what is it I want to maximize? What do I want to optimize for, now that I am being self-reflective about it? One option was to not just -- irrespective of what position I might have taken, to orient my research career toward being the most desirable job candidate I could be. Playing the game, writing the papers that got highly cited, being in the mainstream, and doing things that everyone agreed were interesting, which I did to a certain extent but not all the way when I was in Chicago. Or, I could say, "Screw it." No one goes into academia for fame and fortune. You go into it because you're passionate about the ideas, and so forth, and I'm interested in both the research side of academia and the broad picture side of academia. And I could double down on that, and just do whatever research I wanted to do, and I could put even more effort into writing books and things like that. And I did reflect on that option, and I decided on option B, that it was just not worth it to me to sacrifice five years of my life, even if I were doing good research, which hopefully I would do. But to shut off everything else I cared about was not worth it to me. So, the Caltech job with no teaching responsibilities or anything like that, where I'd be surrounded by absolutely top rate people -- because my physics research is always very highly collaborative, mostly with students, but also with faculty members. Being surrounded by the best people was really, really important to me. So, that combination of freedom to do what I want and being surrounded by the best people convinced me that a research professorship at Caltech was better than a tenure professorship somewhere else.

Zierler:

Now, was this a unique position that Caltech tailored for you, given what you wanted to do in this next role?

Carroll:

I lucked into it, once again. Planning, not my forte. Gordon Moore of Moore's law fame, who was, I think, a Caltech alumnus, a couple years before I was denied tenure, he had given Caltech the largest donation that anyone had ever given to an American institute of higher education. I think it was like $800 million. Honestly, I'm not sure Caltech quite knew what to do with it. Like, okay, this is a lot of money. What are we going to do? So, one of the things they did was within Caltech, they sent around a call for proposals, and they said for faculty members to give us good ideas for what to do with the money. Marc Kamionkowski proposed the Moore Center for Cosmology and Theoretical Physics. And they said, "Sure!" And gave him not a huge budget, but a few hundred thousand dollars a year. He used that to offer me a job, to pay my salary. So, without that money coming in randomly -- so, for people who are not academics out there, there are what are called soft money positions in academia, where you can be a researcher, but you're not a faculty member, and you're generally earning your own keep by applying for grants and taking your salary out of the grant money that you bring in. So, most research professors at Caltech are that. That's the job. The slot is usually used for people -- let's say you're a researcher who is really an expert at a certain microwave background satellite, but maybe faculty member is not what you want to do, or not what you're quite qualified to do, but you could be a research professor and be hired and paid for by the grant on that satellite. Theorists never get this job. So, the fact that it just happened to be there, and the timing worked out perfectly, and Mark knew me and wanted me there and gave me a good sales pitch made it a good sale.

Zierler:

I know the theme is that there's no grand plan, but did you intuit that this position would allow you the intellectual freedom to go way beyond your academic comfort home and to get more involved in outreach, do more in humanities, interact with all kinds of intellectuals that academic physicists never talk to. Did you understand that was something you'd be able to do, and that was one of the attractions for you?

Carroll:

Yeah, absolutely. I don't want to be snobbish but being at one of the world's great intellectual centers was important to me, because you want to bump into people in the hallways who really lift you to places you wouldn't otherwise have gone. That can happen anywhere, but it happens more frequently at a place like Caltech than someplace else.

Zierler:

And you mean not just in physics. You mean generally across the faculty.

Carroll:

Well, right, and not just Caltech, but Los Angeles. I wanted to live in a big metropolitan area where I could meet all sorts of people and do all sorts of different things. The specific thing I've been able to do in Los Angeles is consult on Hollywood movies and TV shows, but had I been in Boston, or New York, or San Francisco, I would have found something else to do. But, you know, I did come to Caltech with a very explicit plan of both diversifying my research and diversifying my non-research activities, and I thought Caltech would be a great place to do that.

Zierler:

Let's start with the research first. Now that you're sort of outside of the tenure clock, and even if you're really bad at impressing the right people, you were still generally aware that they were the right people to impress. Now that you're sort of on the outside of that, it's almost like you're back in graduate school, where you can just do the most fun things that come your way. So, was that your sense, that you had that opportunity to do graduate school all over again?

Carroll:

Yeah, it's what you dream about academia being like. I could have probably done the same thing had I had tenure, also. In fact, I'd go into details, but I think it would have been easier for me if I had tenure than if I'm a research professor. Research professors are hired -- they're given a lot of freedom to do things, but there's a reason you're hired. Theoretical cosmology was the reason I was hired. So, it's not quite a perfect fit in that sense. But that's okay. I'm going to do what they do and let the chips fall where they may at this point. So, I did start slowly and gradually to expand my research interests, especially because around 2004, so soon before I left Chicago, I wrote what to me was the best paper I wrote at Chicago. Not one of the ones that got highly cited. In fact, no one cited it at the time -- people are catching on now -- but it was on the arrow of time in cosmology and why entropy in the universe is smaller in the past than in the future. Something that very hard to get cosmologists even to care about, but the people who care about it are philosophers of physics, and people who do foundations of physics. This goes way back, when I was in Villanova was where I was introduced to philosophy, and discovered it, because they force you to take it. There were a lot of required courses, and I had to take three semesters of philosophy, like it or not. But I loved it. I ended up taking six semesters and getting a minor in philosophy. But interestingly, the kind of philosophy I liked was moral and political philosophy. I took some philosophy of science classes, but they were less interesting to me, because they were all about the process of science. Thomas Kuhn, Paul Feyerabend, how to scientists make decisions about theories, and so forth? It is interesting stuff, but it's not the most interesting stuff. What I discovered in the wake of this paper I wrote about the arrow of time is a whole community of people I really wasn't plugged into before, doing foundations of physics. So, they're philosophers mostly, some physicists. But they're really doing things that are physics. They're trying to understand not how science works but what the laws of nature are. It's just they're doing it in a way that doesn't get you a job in a physics department. It gets you a job in a philosophy department. I love that, and they love my paper. The cosmologists couldn't care, but the philosophers think this paper I wrote is really important. So, that gave me a particular direction to move in, and the other direction was complex systems that I came increasingly interested in. I became much less successful so far in actually publishing in that area, but I hope -- until the pandemic hit, I was hopeful my Santa Fe connection would help with that. All of which is to say, once I got to Caltech, I did start working in broadening myself, but it was slow, and it wasn't my job. I was hired to do something, and for better or for worse, I do take what I'm hired to do kind of seriously. So, most of my papers are written with graduate students. I have a lot of graduate students. I have group meetings with them, and we write papers together, and I take that very seriously. Having been through all of this that we just talked about, I know what it takes them to get a job. I'm not going to let them be in the position I was in with not being told what it takes to get a job. So, I want to not only write papers with them, but write papers that are considered respectable for the jobs they want to eventually get. So, for better or worse, this caused me to do a lot more conventional research than I might otherwise have done.

Zierler:

Yeah, and being at Caltech, you have access to some of the very best graduate students that are out there.

Carroll:

Oh, yeah, absolutely. Some of the papers we wrote were, again, very successful. I wrote a paper with Lottie Ackerman and Mark Wise on anisotropies. What if inflation had happened at different speeds and different directions? We made a new prediction for the microwave background, which was very interesting. I wrote a couple papers with Marc Kamionkowski and Adrienne Erickcek, who was a student, on a similar sounding problem: what if inflation happened faster in one side of the sky than on the other side of the sky? These are all things people instantly can latch onto because they're connected to data, the microwave background, and I always think that's important. I'm in favor of being connected to the data. So, we had some success there, but it did slow me down in the more way out there stuff I was interested in.

Zierler:

Sean, one of the more prosaic aspects of tenure is, of course, financial stability. I'm curious if you were thinking long-term about, this being a more soft money position, branching out into those other areas was a safety net, to some degree, to make sure that you would remain financially viable, no matter what happened with this particular position that you were in?

Carroll:

No, not really. I had never quite -- maybe even today, I have still not quite appreciated how important bringing in grant money is to academia. I've never cared. I've done it. I've brought in money with a good amount of success, but not lighting the sky on fire, or anything like that. I played a big role in the physics frontier center we got at Chicago. I got the Packard Fellowship. I'm on the DOE grant at both places, etc. Normal stuff, I would say, but getting money was always like, okay, I hope it'll happen. I'm not going to really worry about it. I'm crystal clear that this other stuff that I do hurts me in terms of being employable elsewhere. There's no real way I can convince myself that writing papers about the foundations of quantum mechanics, or the growth of complexity is going to make me a hot property on someone else's job market.

Zierler:

Sean, when you start to more fully embrace being a public intellectual, appearing on stage, talking about religion, getting more involved in politics, I'd like to ask, there's two assumptions at the basis of this question. You can challenge them if that seems right. As a public intellectual who has discussed, I mean, really, it's a library worth of things that you've talked about and [who you have] talked with, is your sense first that physics being the foundational science is the most appropriate place as an intellectual launching pad to talk about these broader topics? And then, even within physics, do you see cosmology as the foundational physics to talk about the rest of physics, and all the rest of science in society? In other words, if you were an experimental condensed matter physicist, is there any planet where it would be feasible that you would be talking about democracy and atheism and all the other things you've talked about? How do you understand all of these things?

Carroll:

I think, to some extent, yes. And I'm not sure how conscious that was on my own part, but there's definitely a feeling that I've had for a while, however long back it goes, that in some sense, learning about fundamental theoretical physics is the hardest thing to learn about. So, if I can do that, I can branch out afterwards. There's definitely a semi-permeable membrane, where if you go from doing theoretical physics to doing something else, you can do that. No one gets a PhD in biology and ends up doing particle physics. It's conceivable, but it's very, very rare. Whereas there are multiple stories of people with PhDs in physics doing wonderful work in biology. I don't think it has anything to do with what's more important, or fundamental, or exciting, or better science, but there is a certain kind of discipline that you learn in learning physics, and a certain bag of tricks and intellectual guiding stars that you pick up that are very, very helpful. So, I do think that my education as a physicist has been useful in my caring about other fields in a way that other choices would not have been. Having said that, the slight footnote is you open yourself up, if you are a physicist who talks about other things, to people saying, "Stick to physics." I get that all the time. I literally got it yesterday on the internet. Someone said it. There's a sense in which the humanities and social sciences are more interchangeable. Like, you can be an economist talking about history or politics, or whatever, in a way that physicists just are not listened to in the same way. And that's okay, in some sense, because what I care about more is the underlying ideas, and no one should listen to me talk about anything because I'm a physicist. It's not a matter of credentials, but hopefully being a physicist gives me insight into other areas that I can take seriously those areas in their own rights, learn about them, and move in those directions deliberatively.

Zierler:

Let me ask you that question specifically on the topic of religion. Is your sense that your academic scholarly vantage point of cosmology allows for some kind of a privileged or effective position within public debate because so much of the basis of religion is based on the assumption that there must be a God because a universe couldn't have created itself?

Carroll:

I'm not sure privileged is the word, but you do get a foot in the door. You do get a seat at the table, in a way, talking about religion that I wouldn't if I were talking about the economy, for example. Physics does give you that. Evolutionary biology also gives you that. There are a lot of biologists who have been fighting in the trenches against creationism for a long time. That's a recognized thing that's going on. But yeah, in fact, let me say a little bit extra. There is the Templeton Foundation, which has been giving out a lot of money. It's really the biggest, if not only source of money in a lot of areas I care about. Philosophical reflections on the nature of reality, and the origin of the universe, and things like that. It also has as one of its goals promoting a positive relationship between science and religion. I don't think the Templeton Foundation is evil. There are evil people out there. I don't think they're trying to do bad things. I just think they're wrong. I just disagree with where they're coming from, so I don't want to be supported by them, because I think that I would be lending my credibility to their efforts, which I don't agree with, and that becomes a little bit muddled. Almost none of my friends have this qualm. So, I'm surrounded by friends who are supported by the Templeton Foundation, and that's fine. I have no problems with that. But they often ask me to join their grant proposal to Templeton, or whatever, and I'm like, no, I don't want to do that. It's my personal choice. The point I try to make to them is the following -- and usually they're like, sure, I'm not religious. I'm an atheist. I don't agree with what they do. But they're going to give me money, and who cares? And my response to them is what we do, those of us who are interested in the deepest questions about the nature of reality, whether they're physicists, or philosophers, or whoever, like I said before, we're not going to cure cancer. We're not developing a better smart phone. There's no immediate technological, economic application to what we do. There are very few ways in which what we do directly affects people's lives, except we can tell them that God doesn't exist. That's a huge effect on people's lives. And I do think -- it's not 100% airtight, but I do think not that science disproves God, but that thinking like a scientist and carefully evaluating the nature of reality, given what we know about science, leads you to the conclusion that God doesn't exist. We have been very, very bad about letting people know that. So, I think, if anything, the obligation that we have is to give back a little bit to the rest of the world that supports us in our duties, in our endeavors, to learn about the universe, and if we can share some piece of knowledge that might changes their lives, let's do that. And the most direct way to do that is to say, "Look, you should be a naturalist. God doesn't exist, and that has enormous consequences for how we live our lives. Let's sit and think about this seriously." But very few people in my field jump on that bandwagon.

Zierler:

Sean, just as in earlier in life, your drift away from religion, as you say, was not dramatic. Was your pull into becoming a public intellectual, like Richard Dawkins, or Sam Harris, on that level, was your pull into being a public intellectual on the issue of science and atheism equally non-dramatic, or were you sort of pulled in more quickly than that?

Carroll:

I'm not exactly sure when it happened, but I can tell you a story. When I got to Chicago as a new faculty member, what sometimes happens is that if you're at a big name place like Chicago, people who are editors at publishing houses for trade books will literally walk down the halls and knock on doors and say, "Hey, do you want to write a book? Do you have any good plans for a book?" So, I was in my office and someone knocked on my door. Stephen Morrow is his name. He was an editor at the Free Press, and he introduced himself, and we chatted, and he said, "Do you want to write a book?" And I said, "Well, I thought about it." This is probably 2000. Maybe 1999, but I think 2000. I said, "I thought about it, but the world has enough cosmology books. What the world really needs is a book that says God does not exist. So, I would like to write that as a scientist. Like, here's how you should think about the nature of reality and whether or not God exists." And he was intrigued by that, and he went back to his editors. I think, they're businesspeople. They're not in the job of making me feel good. They made a hard-nosed business decision, and they said, "You know, no one knows who you are. No one cares what you think about the existence of God. No one would buy that book, so we're not going to do it." That's fine. I did not have it as a real priority, but if I did something, that's what I wanted to do. Then, of course, Richard Dawkins wrong The God Delusion and sold a bajillion copies. I almost wrote a book before Richard Dawkins did, but I didn't quite. But, you know, the contingencies of history. Stephen later moved from The Free Press to Dutton, which is part of Penguin, and he is now my editor. He's the one who edits all my books these days, so it worked out for us.

Zierler:

I'm curious how much of a new venture this was for you, thinking about intellectually serving in academic departments. Was your sense that religion was not discussed because it was private, or because being an atheist in scientific communities was so non-controversial that it wasn't even something worth discussing?

Carroll:

I think, both, actually. It is fairly non-controversial, within physics departments anyway, and I think other science departments, with very noticeable exceptions. One of my good friends is Don Page at the University of Alberta, who is a very top-flight theoretical cosmologist, and a born-again Evangelical Christian. He is not at all ashamed to tell you that and explains things sometimes in his talks about cosmology by reference to his idea about God's existence. Everyone sort of nods along and puts up with it and waits for the next equation to come on. I think it's more that people don't care. As much as, if you sat around at lunch with a bunch of random people at Caltech physics department, chances are none of them are deeply religions. Some of them might be. One of my best graduate students, Grant Remmen, is deeply religious. He's the best graduate student I've ever had. But the idea that there's any connection with what we do as professional scientists and these bigger questions about the nature of reality is just not one that modern physicists have. They just don't care. They're like, what is a theory? What can I write down? Where are the equations I can solve? Oh, there aren't any? Then why are you wasting my time? I think that's much more the reason why you don't hear these discussions that much.

Zierler:

But within the physical sciences, there are gradations in terms of one's willingness to consider metaphysics as something that exists, that there are things about the universe that are not -- it's not a matter of them being not observable now because we lack the theories or the tools to observe them, but because they exist outside the bounds of science. Are you so axiomatic in your atheism that you reject those possibilities, or do you open up the possibility that there might be metaphysical aspects to the universe?

Carroll:

So, two things. One is the word metaphysical in this sense is used in a different sense by the professional philosophical community. Metaphysics to a philosopher just means studying the fundamental nature of reality. You can be a physicalist and still do metaphysics for your living. So, I think what you're referring to is more the idea of being a non-physicalist. They come in different varieties. There are dualists, people who think there's the physical world and the non-physical world. There are substance dualists, who think there's literally other stuff out there, whether it's God or angels or spirits, or whatever. There are property dualists, who are closer to ordinary naturalist physicists. They're probably atheists but they think that matter itself is not enough to account for consciousness, or something like that. There's extra-mental stuff, pan-psychism, etc. Okay, with all that clarified, it’s funny that you should say that, because literally two days ago, I finished writing a paper on exactly this issue. Part of my finally, at last, successful attempt to be more serious on the philosophical side of things, I'm writing a bunch of invited papers for philosophy-edited volumes. So, I was invited to write one on levels of reality, whatever that means. It's taken as a given that every paper will have a different idea of what that means. That's how philosophy goes. That's okay. So, the paper that I wrote is called “The Quantum Field Theory on Which the Everyday World Supervenes.” Supervenience is this idea in philosophy that one level depends on another level in a certain way and supervenes on the lower level. If you change something at the higher level, you must change something at the lower level. It's never true that two different things at the higher level correspond to the same thing at the lower level. That's what supervenience means. So, the “Quantum Field Theory on Which the Everyday World Supervenes” means you and I and the tables and chairs around us, the lights behind you, the computers we're talking on, supervene on a particular theory of the world at one level, at the quantum field theory level. What I mean, of course, is the Standard Model of particle physics plus general relativity, what Frank Wilczek called the core theory. The argument I make in the paper is if you are a physicalist, if you exclude by assumption the possibility of non-physical stuff -- that's a separate argument, but first let's be physicalists -- then, we know the laws of physics governing the stuff out of which we are made at the quantum field theory level. We don't know the theory of everything. We don't understand dark matter and dark energy. We don't understand economics or politics. But if you want to say, okay, I'm made out of electrons and protons and neutrons, and they're interacting with photons and gluons, we know all that stuff. We will literally not discover, no matter how much more science we do, new particles in fields that are relevant to the physics underlying what's going on in your body, or this computer, or anything else. I think that's a true argument, and I think I can make that argument. Now, you might ask, who cares? Well, one ramification of that is technological. It used to be the case that there was a close relationship between discoveries in fundamental physics and advances in technology, whether it was mechanics, electromagnetism, or quantum mechanics. There haven't been any for decades, arguably since the pion was discovered in 1947, because fundamental physics has understood enough about the world that in order to create something that is not already understood, you need to build a $9 billion particle accelerator miles across. It does not lead -- and then you make something, and it disappears in a zeptosecond, 10^-21 seconds. It doesn't lead to new technology. So, that's one important implication. The other is this argument absolutely does not rule out the existence of non-physical stuff. Because, I said, you assume there's non-physical stuff, and then you derive this conclusion. But exactly because the Standard Model and general relativity are so successful, we have exactly the equation -- they're not just good ideas. There's an equation you can point to. If you want to tell me that is not enough to explain the behavior of human beings and their conscious perceptions, then the burden is on you -- not you, personally, David, but whoever is making this argument -- the burden is on them to tell me why that equation is wrong. To tell me exactly the way in which this extremely successful quantum field theory fails. No one does that. No one who wants to be in favor of pan-psychism or ghosts or whatever that tells me where exactly the equation needs to be modified. So, it's not a disproof of that point of view, but it's an illustration of exactly how hard it is, what an incredible burden it is. Whereas, if you're just a physicalist, you're just successful. You have the equation. You're still faced with this enormous challenge of understanding consciousness on the basis of this physical stuff, and I completely am sympathetic with the difficulty of that problem. But I think, as difficult as it is, it's an easier problem than adding new stuff that pushes around electors and protons and neutrons in some mysterious way.

Zierler:

Sean, as you just demonstrated, atheism is a complex proposition. So, I wonder, just in the way that atheists criticize religious people for confirmation bias, in this world that you reside in with your academic contemporaries and fellow philosophers and scientists, what confirmation biases have you seen in this world that you feel are holding back the broader endeavor of getting at the truth?

Carroll:

There are so many, and it's very easy for me to admit that I suffer from confirmation biases, but it's very hard for me to tell you which ones they are, because we all each individually think that we are perfectly well-calibrating ourselves against our biases, otherwise we would change them in some way. In my book, The Big Picture, I suggested this metaphor of what I called “planets of belief”. There's a different set of things than you believe, propositions about the world, and you want them to sort of cohere. There's a certain gravitational pull that different beliefs have that they fit together nicely. One thing that you want them to cohere with is reality, the evidence of the data, whatever it is. But there's an enormous influence put on your view of reality by all of these pre-existing propositions that you think are probably true. Even if you're not completely dogmatic -- even if you think they're likely true but you're not sure, you filter in what information you think is relevant and important, what you discount, both in terms of information, but also in terms of perspective theories. If someone says, "Oh, I saw a fuzzy spot in the sky. Maybe it was a UFO driven by aliens." I'm likely to discount that because of all various other prior beliefs whereas someone else might give it a lot of credence. So, you're asking for specific biases, and I'm not very good at giving you them, but I'm a huge believer that they're out there, and we should all be trying our best to open our eyes to what they could be.

Zierler:

The way that you describe your dissertation as a series of papers that were stapled together, I wonder the extent to which you could superimpose that characterization on the popular books that you've published over the past almost 20 years now.

Carroll:

You know, every one [of them] is different, like every child -- they all have their own stories and their own personalities. When I did move to Caltech circa 2006, and I did this conscious reflection on what I wanted to do for a living, writing popular books was one of the things that I wanted to do, and I had not done it to that point. So, here's another funny story. I thought that given what I knew and what I was an expert in, the obvious thing to write a popular book about would be the accelerating universe. People had mentioned the accelerating universe in popular books before, but I honestly didn't think they'd done a great job. Honestly, I still think the really good book about the accelerating universe has yet to be written. So, I wrote up a little proposal, and I sent it to Katinka Matson, who is an agent with the Brockman Group, and she said something which I think is true, now that I know the business a lot better, which was, "It's true maybe it's not the perfect book, but people have a vague idea that there has been the perfect book. People think they've heard too much about dark energy, and honestly, your proposal sounds a little workmanlike. It doesn't sound very inspired, so I think we'll pass." So, I thought, okay, and again, I wasn't completely devoted to this in any sense. I thought it would be fun to do, but I took that in stride. Then, a short time later, John Brockman, who is her husband and also in the agency, emails me out of the blue and says, "Hey, you should write a book." So, they had clearly not talked to each other. So, I said, "Yes, I proposed a book and your wife rejected it."

Zierler:

Awkward.

Carroll:

So, Katinka wrote back to me and said, "Well, John is right." She's very, very good. I like her a lot. She said, "John is right, and I was also right. You should write a book, and the book you proposed is not that interesting. What is it that you are really passionate about right now?" And she had put her finger on it quite accurately, because already, by then, by 2006, I had grown kind of tired of the whole dark energy thing. I had done that for a while, and I have a short attention span, and I moved on. But in 2004, I had written that Arrow of Time paper, and that's what really was fascinating to me. I really wanted to move that forward. So, I said that, and she goes, "Well, propose that as a book. We'll publish that, or we'll put that out there." That was what led to From Eternity to Here, which was my first published book. In many ways, it was a great book. In many ways, I could do better now if I rewrote it from scratch, but that always happens. The second book, the Higgs boson book, I didn't even want to write. I don't know how public knowledge this is. Remember, the Higgs boson -- From Eternity to Here came out in 2010. Late in 2011, CERN had a press conference saying, "We think we've gotten hints that we might discover the Higgs boson." Physicists knew, given the schedule of the Large Hadron Collider, and so forth, that it would probably be another year before they raised the significance to that to really declare a discovery. So, there was a little window to write a book about the Higgs boson. Steven Morrow, my editor who published From Eternity to Here, called me up and said, "The world needs a book on the Higgs boson. They need it written within six months so it can be published before the discovery is announced. The only person who both knows the physics well enough and writes fast enough to do that is you." Six months is a very short period of time.

Zierler:

He wasn't bothered by the fact that you are not a particle physicist.

Carroll:

I'm close enough. I'm enough of a particle physicist. I know the field theory. But it's not what I do research on. So, that's why I said I didn't want to write it. My response to him was, "No thanks." If I'm going to spend my time writing popular books, like I said before, I want my outreach to be advancing in intellectual argument. I want it to be proposing new ideas, not just explaining ideas out there. Writing a book about the Higgs boson, I didn't really have any ideas to spread, so I said, "There are other people who are really experts on the Higgs boson who could do this." So, his response was to basically make me an offer I couldn't refuse in terms of the financial reward that would be accompanying writing this book.

Zierler:

Sean, I'm sorry to interrupt, but in the way that you described the discovery of accelerating universe as unparalleled in terms of its significance, would you put the discovery of the Higgs at a lower tier? In other words, if you held it in the same regard as the accelerating universe, perhaps you would have had to need your arm to be twisted to write this book.

Carroll:

Yeah, there's no question the Higgs is not in the same tier as the accelerated universe. The Higgs, gravitational waves, anisotropies in the cosmic microwave background, these are all hugely important, Nobel-worthy discoveries, that did win the Nobel Prize, but also [were] ones we expected. No sensible person doubted they would happen. Whereas the accelerated universe was a surprise. There were people who absolutely had thought about it. I wrote a big review article about it. But most of us didn't think it was real. And that really -- the difference that when you're surprised like that, it causes a rethink. Honestly, here we're talking in the beginning of 2021. I still don't think we've taken it seriously, the implications of the cosmological constant for fundamental physics. So, I think it's a big difference. That's one of the things that I wanted to do. I thought that for the accelerated universe book, I could both do a good job of explaining the astronomy and the observations, but also highlight some of the theoretical implications, which no one has really done. So, anyway, with the Higgs, I don't think I could have done that, but he made me an offer I couldn't refuse. As it turned out, CERN surprised us by discovering the Higgs boson early. The discovery was announced in July. It was July 4th.

Zierler:

How rude of them.

Carroll:

Yeah. We were expecting it to be in November, and my book would have been out. But the good news was I got to be at CERN when they announced it. So, that appeared in my book as a vignette. And I did use the last half of the book as an excuse to explain some ideas in quantum field theory, and gauge theory, and symmetry, that don't usually get explained in popular books. So, I used it for my own purposes. But mostly, I hope it was a clear and easy to read book, and it was the first major book to appear soon after the discovery of the Higgs boson. As a result, it did pretty well sales-wise, and it won a big award. It won the Royal Society Prize for Best Science Book of the Year, which is a very prestigious thing. Actually, without expecting it, and honestly, between you and me, it won it not because I'm the best writer in the world, but because the Higgs boson is the most exciting particle in the world. They basically admitted that. They were like, how can you not give it to the Higgs boson book, right? Those poor biologists had no chance that year. But that gave me some cache when I wanted to write my next book. I had done what Stephen [Morrow] asked for the Higgs boson book, and it won a prize. So, basically, I could choose really what I wanted to write for the next book. So, that's when The Big Picture came along, which was sort of my slightly pretentious -- entirely pretentious, what am I saying? -- super pretentious exposition of how the world holds together in the broadest possible sense. A defense of philosophical naturalism, a brand of naturalism, like a poetic naturalism. So, basically, giving a sales pitch for the idea that even if we don't know the answers to questions like the origin of the universe, the origin of life, the nature of consciousness, the nature of right and wrong, whatever those answers are going to be, they're going to be found within the framework of naturalism. That's the case I tried to make. So, it was a very -- it was a big book. It was very long. I very intentionally said, "This is too much for anyone to read." So, I wrote very short chapters. There are a lot of chapters, but they're all very short. You can read any one of them on a subway ride. So, it didn't appear overwhelming, and it was a huge success. That was the first book I wrote that appeared on the New York Times best seller list. I don't know whether this is -- there's only data point there, but the Higgs boson was the book people thought they wanted, and they liked it. No one wanted The Big Picture, but it sold more copies. So, sometimes, you should do what you're passionate about, and it will pay off. Then, my final book, my most recent one, was Something Deeply Hidden. Again, going back to the research I was doing, in this case, on the foundations of quantum mechanics, and a sales pitch for the many-worlds interpretation of quantum mechanics, and the most recent research I've been doing on deriving how space time can emerge from quantum mechanics. So, I think that -- again, it got on the best seller list very briefly. It literally did the least it could possibly do to technically qualify as being on the best seller list, but it did. Believe me, the paperback had a sticker on the front saying New York Times best seller. But I think that book will have an impact ten and twenty years from now because a new generation of undergraduate physics students will come in having read that, and they will take the foundations of quantum mechanics seriously in a way that my generation did not. So, I'm very, very happy to have written that book.

Zierler:

Do you see the enterprise of writing popular books as essentially in the same category but a different medium as the other ways that you interact with the broader public, giving lectures, doing podcasts? Do you see this as all one big enterprise with different media, or are they essentially different activities with different goals in mind?

Carroll:

I think it's part of a continuum. They're not exactly the same activity, but they're part of the same landscape. They reach very different audiences, and they have very different impacts. To me, the book is still the most profound way for one person to say ideas that are communicated to another one. You have enough room to get it right. I laugh because I'm friends -- Jennifer, my wife, is a science journalist -- so we're friends with a lot of science journalists. Some of them also write books, but most of them focus on articles. A lot of them, even, who write books, they don't like it, because there's all this work I've got to do. I can't get a story out in a week, or whatever. I love it. I love writing books so much. You tell me, you get a hundred thousand words to explain things correctly, I'm never happier than that. But, okay, not everyone is going to read your book. You sell tens of thousands of books if you're lucky. Whereas, if I'm a consultant on [the movie] The Avengers, and I can just have like one or two lines of dialogue in there, the impact that those one or two lines of dialogue have is way, way smaller than the impact you have from reading a book, but the number of people it reaches is way, way larger. And you know, Twitter and social media and podcasts are somewhere in between that. I'm very pleasantly surprised that the podcast gets over a hundred thousand listeners ever episode, because we talk about pretty academic stuff. We get pretty heavily intellectual there sometimes, but it warms my heart that so many people care about that stuff. I'm a big believer that all those different media have a role to play.

Zierler:

Sean, given the vastly large audience that you reach, however we define those numbers, is there a particular demographic that gives you the most satisfaction in terms of being able to reach a particular kind of person, an age group, however you might define it, that gives you the greatest satisfaction that you're introducing real science into a life that might not ever think about these things?

Carroll:

It's a great question, because I do get emails from people who read one of my books, or whatever, and then go into physics. They decide to do physics for a living.

Zierler:

Oh, that's great.

Carroll:

Yeah, it absolutely is great. Or other things. I had this email from a woman who said, literally, when she was 12 years old, she was at some event, and she was there with her parents, and they happened to sit next to me at a table, and we talked about particle physics, and she wrote just after she got accepted to the PhD program at Oxford in particle physics, and she said it all started with that conversation. She could pinpoint it there. And, yeah, it's just incredibly touching that you've made an impact on someone's life. Having said all that, my goal is never to convert people into physicists. I think there are plenty of physicists. I mean, I'm glad that people want to physicists, but there's no physicist shortage out there. What I would much rather be able to do successfully, and who knows how successful it is, but I want physics to be part of the conversation that everyone has, not just physicists. I want people to -- and this is why I think that it's perfectly okay in popular writing to talk about speculative ideas, not just ideas that have been well established. I've said this before, but I want to live in the world where people work very hard 9 to 5 jobs, go to the pub for a drink, and talk about what their favorite dark matter particle candidate is, or what their favorite interpretation of quantum mechanics is. I want it to be okay to talk about these things amongst themselves when they're not professional physicists. That's when I have the most fun. The idea -- the emails or responses that make me the happiest are when someone says, you know, "I used to love physics, and I was turned off by it by like a bad course in high school, and you have reignited my passion for it." That's what really makes me feel successful.

Zierler:

Sean, I wonder if a through-line in terms of understanding your motivation, generally, to reach these broad audience, is a basis of optimism in the wisdom of lay people. In other words, you're decidedly not in the camp of somebody like a Harold Bloom, The Closing of the American Mind, where you are pessimistic that we as a society, in sum, are not getting dumber, that we are not becoming more closed-minded. You have an optimism that that's not true, and that what you're doing as a public intellectual is that you're nurturing and being a causative effect of those trend lines.

Carroll:

Yeah, I think that's right. And I think it's Allan Bloom who did The Closing of the American Mind. Harold Bloom is a literary critic and other things.

Zierler:

Oh, yes. I got my Blooms mixed up!

Carroll:

Oh, yeah. You get dangerous. I say this as someone who has another Sean Carroll, who is a famous biologist, and I get emails for him. You know, look, I don't want to say the wisdom of lay people, or even the intelligence of lay people, because there's a lot of lay people out there. I want to say the variety of people, and just in exactly the same way that academic institutions sort of narrow down to the single most successful strategy -- having strong departments and letting people specialize in them -- popular media tries to reach the largest possible audience. So, they keep things at a certain level. They have a certain way of doing things. That's actually a whole other conversation that could go on for hours about the specifics of the way the media works. I've appeared on a lot of television documentaries since moving to L.A. That's a whole sausage you don't want to see made, really, in terms of modern science documentaries. Some of them are excellent, but it's almost by accident that they appear to be excellent. So, I do think that in a country of 300-and-some million people, there's clearly a million people who will go pretty far with you in hard intellectual stuff. If I could get a million people buy my books, I'd be a really best-selling author. Like, crazily successful. You don't get that, but there's clearly way more audience in a world as large as ours for people who are willing to work a little bit. Not for everybody, and again, I'm a huge believer in the big ecosystem. I love the little books like Quantum Physics for Babies, or Philosophy for Dummies. Those are all very important things and I'm not going to write them myself.

The things I write -- even the video series I did, in fact, especially the video series I did, I made a somewhat conscious decision to target it in between popular level physics and textbook level physics. So, I realized right from the start, I would not be able to do it at all if I assume that the audience didn't understand anything about equations, if I was not allowed to use equations. So, in the second video, I taught them calculus. I taught them what an integral was, and what a derivative was. Not so they could do it. If you take a calculus class, you learned all these techniques, like the product rule, and what to do with polynomials. I didn't do any of that, but I taught them the concept. An integral is measuring the area under a curve, or the volume of something. A derivative is the slope of something. And then I could use that, and I did use it, quite profligately in all the other videos. And, you know, video sixteen got half a million views, and it was about gravity, but it was about gravity using tensors and differential geometry. Like, I did it. I do think that audience is there, and it's wildly under-served, and someday I will turn that video series into a book. There's this huge gap in between what we give the popular press, where I have to fight for three equations in my book, and a textbook, which is three equations every paragraph.

Zierler:

Sean, I'm curious if you think podcasting is a medium that's here to stay, or are we in a podcast bubble right now, and you're doing an amazing job riding it?

Carroll:

That's a very hard question. I don't know. Blogging was a big bubble that almost went away. People still do it. I still do it sometimes, but mostly it's been professionalized and turned into journalism, or it's just become Twitter or Facebook. Social media, Instagram. So, it's sort of bifurcated in that way. I don't know what's going to happen to the future of podcasting. It's still pretty young. Part of the reason I was able to get as many listeners as I do is because I was early enough -- two and a half years ago, all of the big podcasters were already there. Had it been five years ago, that would have been awesome, but now there's a lot of competition. So, it is popular, and one of the many nice things about it is that the listeners feel like they have a personal relationship with the host. This is a very interesting fact to learn that completely surprised me. Advertising on podcasts is really effective compared to TV or radio or webpages. Ads that you buy on a podcast really do get return. In part, it's because they're read by the host who the audience has developed a trusting relationship with. So, I suspect that they are here to stay. Audio, in one form or another, is here to stay. But I'd be very open minded about the actual format changing by a lot. I do long podcasts, between an hour and two hours for every episode. Some people say that's bad, and people don't want that. Some people love it. Go longer. Again, rather than trying to appeal to the largest number of people, and they like it. It will never be the largest. I'll never be Joe Rogan or Marc Maron, or whatever. But I get plenty of people listening, and that makes me very pleased.

Zierler:

I wonder, in what ways, given the fact that you have this tremendous time spending with all these really smart people talking about all these great ideas, in what ways do you bring those ideas back to your science, back to the Caltech, back to the pen and paper?

Carroll:

Well, I do, but not so much in the conventional theoretical physics realm, for a couple reasons. For one thing, I don't have that many theoretical physicists on the show. They're rare. They appear, but once every few months, but not every episode.

Zierler:

And that's by choice, because you don't want to talk to them with as much eagerness as you want to talk to other kinds of scientists or scholars.

Carroll:

It's not that I don't want to talk to them, but it's that I want the podcast to very clearly be broad ranging. Even if it were half theoretical physicists and half other things, that's a weird crazy balance. I want the podcast to be enjoyable to people who don't care about theoretical physics. And if one out of every ten episodes is about theoretical physics, that's fine. You can skip that one, but the audience is still there. And also, of course, when I'm on with a theoretical physicist, I'm trying to have a conversation at a level that people can access. It doesn't always work. Sometimes we get a little enthusiastic. Netta Engelhardt and I did a podcast on black hole information, and in the first half, I think we were very accessible, and then we just let our hair down in the second half.

Zierler:

Right. You nerded out entirely. I remember that.

Carroll:

It's funny when that happens. I did an episode with Kip Thorne, and I would ask him questions. He knew exactly what the point of this was, but he would say, "Why are you asking me that? You know the answer to that." And I'd have to say, "Yes, but maybe the audience does not know what a black hole is, so you need to explain it to us." However, because I am intentionally and dynamically moving into other areas, not just theoretical physics, I can totally use the podcast to educate myself. And I have been, and it's been incredibly helpful in various ways. Sometimes I get these little, tiny moments when I can even suggest something to the guest that is useful to them, which makes me tickled a little bit. So, this dream of having a truly interdisciplinary conversation at a high intellectual level, I think, we're getting better at it. We're creeping up on it. I wouldn't say we're there yet, but I do think it's possible, and it's a goal worth driving for. Let's put it that way.

Zierler:

Just to bring the conversation up to the present, are you ever concerned that you might need a moment to snap back into theoretical physics so that you don't get pulled out of gravity? [So that] you don't get too far away that you don't know how to get back in? Or are you comfortable with that idea, as so many other physicists who reinvent themselves over the course of a career are?

Carroll:

I think I'm pretty comfortable with that idea. I don't think I'm in danger of it right now, so who knows five or ten years from now? But, you know, my standard is what is it that excites me at the moment? So, if, five or ten years from now, the sort of things that excite me do not include cutting edge theoretical physics, then so be it. If they do, then I'd like to think I will jump back into it. Not even jump back into it but keep it up. I haven't given it up yet. I'm trying to finish a paper right now. I should be finishing this paper rather than talking to you, on quantum mechanics and energy conservation.

Zierler:

I'm terribly sorry! [laughs]

Carroll:

That's okay. But, I mean, I have no shortage of papers I want to write in theoretical physics. So many ideas I want to get on paper. But I'm unconstrained by caring about whether they're hot topics. So, it's incredibly liberating because I don't have to keep up with the billion other papers that people are writing in the hot topics. That's my secret weapon, that I can just write the papers I want to write. Now, look, if I'm being objective, maybe this dramatically decreases my chances of having a paper that makes a big impact, because I'm not writing papers that other people are already focused on. But the dream, the goal is that they will realize they should have been focused on it once I write the paper. The only way to do that is to try, so let's see what happens.

Zierler:

Are you particularly excited about an area of physics where you might yet make fundamental contributions, or are you, again, going back to graduate school, are you still exuberantly all over the place that maybe one of them will stick, or maybe one of them won't?

Carroll:

No, I think I'm much more purposive about choosing what to work on now than I was back then. I mean, infinitely more, let's put it that way. And I do think that within the specific field of theoretical physics, the thing that I think I understand that my colleagues don't is the importance of the foundations of quantum mechanics to understanding quantum gravity. I think that I read papers by very smart people, smarter than me, doing cutting edge work on quantum gravity, and so forth, and I still find that they're a little hamstrung by old fashioned, classical ideas. I think the reason why is because they haven't really been forced to sit down and think about quantum mechanics as quantum mechanics, all for its own sake. So, temporarily, this puts me in a position where I'm writing papers and answering questions that no one cares about, because I'm trying to build up a foundation for going from the fundamental quantumness of the universe to the classical world we see. Like, literally, right now, I'm interested in why we live in position space, not in momentum space. Why is there an imbalance in theoretical physics between position and momentum? And the answer is, to most people, there is. Move on with it. Get on with your life. That's not going to lead us to a theory of dark matter, or whatever. But I have a conviction that understanding the answer to those questions, or at least appreciating that they are questions, will play a role -- again, could very easily play a role, because who knows, but could very easily play a role in understanding what we jokingly call the theory of everything, the fundamental nature of all the forces and the nature of space time itself. So, there is definitely a sort of comparative advantage calculation that goes on here. There's very promising interesting work being done by string theorists and other people doing AdS/CFT and wormholes, and tensor networks, and things like that. There's a whole set of hot topics that are very, very interesting and respectable, and I'm in favor of them. But there's plenty of smart people working on that. There's nobody working on using insights from the foundation of quantum mechanics to help understand quantum gravity, or at least, very, very few people. So, that's, to me, a really good chance of making a really important contribution. It might fail, and I always try to say that very explicitly. It might be a good idea that is promising in the moment and doesn't pan out. We'll have to see. But we don't know yet, and it's absolutely worth trying.

Zierler:

Given the way that you rank the accelerating universe way above LIGO or the Higgs boson, because it was a surprise, what are the other surprises out there, that if they were discovered, might rank on that level of an accelerating universe? For example, integrating gravity into the Standard Model. Would that be on that level?

Carroll:

Well, that's not an experimental discovery. That's not data. You have to say, what can we see in our telescopes or laboratories that would be surprising? There's a bunch. Like I said, the reason we're stuck is because our theories are so good. So, just show that any of our theories are wrong. The thing that people are looking for, the experimental effort these days, and for very good reason, is aimed at things that we think are plausibly true. There's nothing like, back fifteen years ago, we all knew we were going to discover the Higgs boson and gravitational ways. We all knew that eventually we'd discover CMB anisotropies if you go back even farther than that. So, even though these were anticipated, they were also really good benchmarks, really good targets to shoot for. We're kind of out of that. Not only do we have a theory that fits all the data, but we also don’t even have a prediction for that theory that we haven't tested yet. Phew, this is a tough position to be in. We could discover what the dark matter is. That would be great. We could discover gravitational waves in the microwave background that might be traced back to inflation. We could discover that dark energy is not a cosmological constant, but some quintessence-like thing. There was, as you know, because you listened to my recent podcast, there's a hint of a possibility of a suggestion in the CMB data that there is what is called cosmological birefringence. The polarization of light from the CMB might be rotated just a little bit as it travels through space. This is not what you predict in conventional physics, but it's like my baby. The first paper I ever wrote and got published with George Field and Roman Jackiw predicted exactly this effect. Later on, I wrote another paper that sort of got me my faculty jobs that pointed out that dark energy could have exactly the same effect. So, I would become famous if they actually discovered that. My only chance to become famous is if they discovered cosmological birefringence. But look, all these examples are examples where there's a theoretical explanation ready to hand. It's not overturning all of physics. If you found something like a violation of Lorentz invariants, if you found something of the violation of the Schrödinger equation in quantum mechanics, or the fundamental predictions of entanglement, or anything like that. If you found that there was a fundamental time directed-ness in nature, that the arrow of time was not emergent out of entropy increasing but was really part of the fundamental laws of physics. In other words, the dynamics of physics were irreversible at the fundamental level. If you found that information was lost in some down-to-Earth process -- I'm writing a paper that says you could possibly find that energy is not conserved, but it's a prediction of a very good theory, so it's not a crazy departure. But those kind of big picture things, which there are little experiments here and there. You know, high risk, high gain kinds of things that are looking for these kinds of things. Those would really cause re-thinks in a deep way.

Zierler:

Sean, we've brought the narrative right up to the present, so much so that we know exactly what you should be working on right now. So, for the last part of our talk, I want to ask a few broadly retrospective questions about your career, and then a few looking forward. First, this conversation has been delightfully void of technology. We have not talked about supercomputers, or quantum computers. We haven't talked about 30-meter telescopes. We haven't talked about any of these things where technology is so important to physics. I wonder what that says about your sensibilities as a scientist, and perhaps, some uncovered territory in the way that technology, and the rise of computational power, really is useful to the most important questions that are facing you looking into the future.

Carroll:

Yeah, no, good. I would say that implicitly technology has been in the background. We talked about discovering the Higgs boson. We talked about discovering the cosmic microwave background anisotropies. The COBE satellite that was launched on a pretty shoestring budget at the time, and eventually found the CMB anisotropies, that was the second most complicated thing NASA had ever put in orbit after the Hubble space telescope. It was really an amazing technological achievement that they could do that. Not to mention, gravitational waves, and things like that. So, the technology is always there. I don't interact with it that strongly personally. That's absolutely true. In part, that is just because of my sort of fundamentalist, big picture, philosophical inclinations that I want to get past the details of the particular experiment to the fundamental underlying lessons that we learned from them. And, also, I think it's a reflection of the status of the field right now, that we're not being surprised by new experimental results every day. I think that if I were to say what the second biggest surprise in fundamental physics was, of my career, it's that the LHC hasn't found anything else other than the Higgs boson. It's sort of a negative result, but I think this is really profound. For similar reasons as the accelerating universe is the first most important thing, because even though we can explain them -- they're not in violation of our theories -- both results, the universe is accelerating, we haven't seen new particles from the LHC, both results are flying in the face of our expectations in some way. They seem unnatural to us. They don't quite seem in direct conflict with experiment. So, they're not very helpful hints, but they're hints about something that is wrong with our fundamental way of thinking about things. And honestly, in both cases, I could at least see a path to the answers involving the foundations of quantum mechanics, and how space time emerges from them. I can't quite see the full picture, otherwise I would, again, be famous. But I do think that there's room for optimism that a big re-think, from the ground up, based on taking quantum mechanics seriously and seeing where you go from there, could have important implications for both of these issues. So, in that sense, technology just hasn't had a lot to say because we haven't been making a lot of discoveries, so we don't need to worry about that. If I were really dealing with the nitty gritty of baryon acoustic oscillations or learning about the black hole mass spectrum from LIGO, then I would care a lot more about the individual technological implications, but my interests don't yet quite bump up against any new discoveries right now.

Zierler:

Sean, as a public intellectual with your primary identity being a scientist but with tremendous facility in the humanities and philosophy and thinking about politics, in the humanities -- there's a lot of understanding of schools of thought, of intellectual tradition, that is not nearly as prominent as it is in the sciences. With that in mind, given your incredibly unique intellectual and career trajectory, I know there's no grand plan. We've already established that.

Carroll:

Tell me about it.

Zierler:

But do you see yourself as part of an intellectual tradition in terms of the kinds of things you've done, and the way that you've conveyed them to various audiences?

Carroll:

Yes, but it's not a very big one. Maybe not even enough to qualify as a tradition. I would certainly say that there have been people throughout the history of thought that took seriously both -- three things. One, drive research forward. Learn new things about the world. Two, do so in a way which is not overly specialized, which brings together insights from different areas. Three, tell people about it. Spread the word. Don't just talk to your colleagues at the university but talk more widely. I think all three of those things are valid and important. Very, very important. More than just valid. I think there have been people for many, many years who have been excellent at all three of these things individually. There haven't been that many people who have been excellent at all three at once. Maybe I fall short of being excellent at them, but at least I'm enthusiastic about them. There are, of course, counterexamples, or examples, whichever way you want to put it. George Gamow, in theoretical physics, is a great example of someone who was very interdisciplinary and did work in biology as well as theoretical physics. He wrote wonderful popular books. Mr. Tompkins, and One Two Three Infinity was one of the books that I read when I was in high school. And I was amused to find that he had trouble getting a job, George Gamow. People didn't take him seriously. Bertrand Russell, on the philosophy side of things, did a wonderful job reaching to broad audiences and talking about a lot of things. I think I would put Carl Sagan up there. A lot of people focus on the fact that he was so good at reaching out to broad audiences, in an almost unprecedented way, that they forget that he was really a profound thinker as well. This is really what made Cosmos, for example, very, very special at the time. Like I aspire to do, he was actually doing. He was reaching out and doing a public outreach thing, but also really investigating ideas. He was doing intellectual work in the process of public outreach, which is really, really hard, and he was just a master at it as well as being an extremely accomplished planetary scientist, and working with NASA and so forth. So, all of those things. Anyone who's a planetary scientist is immediately interdisciplinary, because you can't be a planetary -- there's no discipline called planetary sciences that is very narrow. And, you know, in other ways, Einstein, Schrödinger, some of the most wonderful people in the history of physics, Boltsman, were broad and did write things for the public, and cared about philosophy, and things like that. There is a whole other discussion, another three-hour discussion, about how the attitude among physicists has changed from the first half of the 20th century to now, when physicists were much more broadly interested in philosophy and other issues. Part of that was a shift of the center of gravity from Europe to America. Part of it was the Manhattan Project and being caught up in technological development. Part of it was the weirdness of quantum mechanics, and the decision on the part of the field just to shut up and calculate more than to fret about the philosophical underpinnings. But there definitely has been a shift. So, yeah, I can definitely look to people throughout history who have tried to do these things. Maybe going back to Plato. I don't know if Plato counts, but he certainly was good at all these different things.

Zierler:

Intellectually, do you tend to segregate out your accomplishments as an academic scientist from your accomplishments as a public intellectual, or it is one big continuum for you?

Carroll:

For me, it's one big continuum, but not for anybody else. So, I recognize that. I pretend that they're separate. On my CV, I have one category for physics publications, another category for philosophy publications, and another category for popular publications. They are clearly different in some sense. Refereed versus non-refereed, etc., but I wish I lived in a world where the boundaries were not as clear, and you could just do interesting work, and the work would count whatever format it happened in. Michael Nielsen, who is a brilliant guy and a friend of mine, has been trying, not very successfully, but trying to push the idea of open science. Not just open science like we can read everybody's papers, but doing science in public. Like, a collaboration that is out there in the open, and isn't trying to hide their results until they publish it, but anyone can chip in. This turns out to work pretty well in mathematics. They actually have gotten some great results. I've seen almost nothing in physics like that, and I think I would be scared to do that. Talking about all of the things I don't understand in public intimidates me. But part of the utopia that we don't live in, that I would like to live in, would be people who are trying to make intellectual contributions [should] be judged on the contributions and less on the format in which they were presented.

Zierler:

Sean, in your career as a mentor to graduate students, as you noted before, to the extent that you use your own experiences as a cautionary tale, how do you square the circle of instilling that love of science and pursuing what's most interesting to you within the constraints of there's a game that graduate students have to play in order to achieve professional success? Because you've been at it long enough now, what have been some of the most efficacious strategies that you've found to join those two difficulties?

Carroll:

You know, students are very different. That's one of the things you have to learn slowly as an advisor, is that there's no recipe for being a successful graduate student. As the advisor, you can't force them into the mold you want them to be in. Some of them are very narrowly focused, and they're fine. What academia asks of them is exactly what they want to provide. You don't really need to do much for those. Others, I've had students who just loved teaching. And you'd think that's a good thing, but it's really not on the physics job market. If you spend your time as a grad student or postdoc teaching, that slows you down in doing research, which is what you get hired on, especially in the kind of theoretical physics that I do. And there are others who are interested in not necessarily public outreach, but public policy, or activism, or whatever. So, I try to judge what they're good at and tell them what I think the reality is. I wrote a blog post that has become somewhat infamous, called “How to Get Tenure at a Major Research University.” I was surprised when people, years later, told me everyone reads that, because the attitude that I took in that blog post was -- and it reflects things I tell my students -- I was intentionally harsh on the process of getting tenure. Not just because I didn't, but because I think the people you get advice from are the ones who got tenure. It worked for them, and they like it. The system has benefited them. So, and it's good to be positive about the great things about science and academia and so forth, but then you can be blindsided. You can be surprised. So, I intentionally tried to drive home the fact that universities, as I put it, hired on promise and fired on fear. They'll hire you as a new faculty member, not knowing exactly what you're going to do, but they're like, alright, let's see. This could be great. Hopefully it'll work out. But then when it comes to giving you tenure, they're making a decision not by what you've done for the last six years, but what you will do for the next 30 years. So, if you've given them any excuse to think that you will do things other than top-flight research by their lights, they're afraid to keep you on. To be perfectly fair, there are plenty of examples of people who have either gotten tenure, or just gotten older, and their research productivity has gone away. So, I think that when I was being considered for tenure, people saw that I was already writing books and doing public outreach, and in their minds, that meant that five years later, I wouldn't be writing any more papers. I'm the kind of person who would stop writing papers and do other things. I was a good teacher. All the warning signs, all the red flags were there. The idea that someone could be a good teacher, and do public outreach, and still be devoted and productive doing research is just not a category that they were open to. So, they weren't looking for the signs for that. And I think that I need to tell my students that that's the kind of attitude that the hiring committees and the tenure committees have. And I love it when they're interested in outreach or activism or whatever, but I say, "Look, if you want to do that as a professional physicist, you've got to prioritize getting a job as a professional physicist." And that's not bad or cynical. It's just, you know, you have certain goals in life. Let's do the thing that will help you reach those goals. You don't necessarily need to do all the goals this year. Maybe some goals come first, and some come after.

Zierler:

Sean, for my last question, looking forward, I want to reflect on your educational trajectory, and the very uncertain path from graduate school to postdoc, to postdoc to the University of Chicago. It seems that when you finally got to Caltech, it all clicked for you. You were at a world-class institution, you had access to the best minds, the cutting edge science, with all of the freedom to pursue all of your other ideas and interests. Is it the perfect situation? Is this where you want to be long-term, or is it possible that an entirely new opportunity could come along that could compel you that maybe this is what you should pursue next?

Carroll:

So, no, it is not a perfect situation, and no I'm not going to be there long-term. I'm definitely not going to be at Caltech, even two years from now. I'm on a contract. They go every five years, and I'm not going try to renew my contract. The reason is -- I love Caltech. I don't want to say anything against them. This particular job of being a research professor in theoretical physics has ceased to be a good fit for me. Part of it is what I alluded to earlier. The actual job requirements -- a big part of it, the part that I take most seriously, and care most about -- is advising graduate students. To do that, I have to do a certain kind of physics with them, and a certain kind of research in order to help them launch their careers. I just don't want to do that anymore. It's not what I want to do.

Zierler:

There's a moral issue there that if you're not interested in that, that's a disservice to the graduate students.

Carroll:

Absolutely, and I feel very bad about that, because they're like, "Why haven't you worked on our paper?" Because I know, if you're working with Mark Wise, my colleague, and you're a graduate student, it's just like me working with George Field. You feel like I've got to keep up because I don't do equations fast enough. Whereas, my graduate students, I do work, they do work, but I do other things as well. So, they have no trouble keeping up with me, and I do feel bad about that sometimes. And I want to write philosophy papers, and I want to do a whole bunch of other things. I might do that in an academic setting if the opportunity comes along, and I might just go freelance and do that. I have the financial ability to do that now, with the books and the podcast. I'm never going to stop writing papers in physics journals, philosophy journals, whatever. I've already stopped taking graduate students, because I knew this was the plan for a while. And it has changed my research focus, because the thing that I learned -- the idea that you should really write papers that you care about and also other people care about but combined with the idea that you should care about things that matter in some way other than just the rest of the field matters. The rest of the field needs to care. Some field needs to care. If literally no one else cares about what you're doing, then you should rethink. I do firmly believe that. I'm not someone who thinks there's a lone eccentric genius who's going to be idiosyncratic and overthrow the field. That's a romance, that's not a reality. But it needs to be mostly the thing that gets you up out of bed in the morning. You've got to find the intersection. This is the advice I tell my students. I say, "Look, there are things you are interested in. There are things the rest of the world is interested in. Look at the intersection of those and try to work in that area, and if you find that that intersection is empty, then rethink what you're doing in life." And I applied that to myself as well, but the only difference is the external people who I'm trying to overlap with are not necessarily my theoretical physics colleagues. Maybe it's them. I certainly have very down-to-Earth, standard theoretical physics papers I want to write. But maybe it's not, and I don't care. I can just do what I want. And that's what I'm going to do, one way or the other.

Zierler:

And it's owing to your sense of adventure that that's probably part of the exhilaration of this, not having a set plan and being open to possibilities.

Carroll:

Well, and look, it's a very complicated situation, because a lot of it has to do with the current state of theoretical physics. I can pinpoint the moment when I was writing a paper with a graduate student on a new model for dark matter that I had come up with the idea, and they worked it out. It was really hard, because we know so much about theoretical physics now, that as soon as you propose a new idea, it's already ruled out in a million different ways. So, coming up with a version of it that wasn't ruled out was really hard, and we worked incredibly hard on it. We wrote the paper, and it got published and everything, and it's never been cited. More importantly, the chances that that model correctly represents the real world are very small. It's a necessary thing but the current state of theoretical physicists is guessing. And a lot of it is like, What is beyond the model that we now know? That's not all of it. It's a very small part of theoretical physics. A lot of theoretical physics is working within what we know to predict the growth of structure, or whatever. But there's a certain kind of model-building, going beyond the Standard Model, that is a lot of guessing. And I've guessed. I've written down a lot of Lagrangians in my time to try to guess. And at some point, it sinks in, the chances of guessing right are very small. So, I want to do something else. I don't want to do that anymore, even if it does get my graduate students jobs. I want to go back and think about the foundations, and if that means that I appeal more to philosophers, or to people at [the] Santa Fe [Institute], then so be it. I'm very happy with that.

Zierler:

That's a great place to end, because we're leaving it on a cliffhanger. We'll see what comes next for you, and of course, we'll see what comes next in theoretical physics. Maybe it'll be a fundamental discovery that'll compel you to jump back in with two feet.

Carroll:

Absolutely. You know, there's a lot we don't understand. We're pushing it forward, hopefully in interesting ways, and predicting the future is really hard. I think I got this wrong once. There's a quote that is supposed to be by Niels Bohr, "Making predictions is hard, especially about the future." I think I misattributed it to Yogi Berra. Someone else misattributed it first, and I believed them. It sounded very believable. But apparently it was Niels Bohr who said it, and I should get that one right. So, he was right, and I'm learning this as I study and try to write papers on complexity. The space of possibilities is the biggest space that we human beings can contemplate. So, that's why it's exciting to see what happens.

Zierler:

Sean, thank you so much for spending this time with me. This has been an absolutely awesome four hours. I'm just thrilled we were able to do this. So, thank you so much.

Carroll:

David, my pleasure. Thank you for inviting me on. It's an honor.