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Interview of Katherine Freese by David Zierler on April 23, 2021,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
For multiple citations, "AIP" is the preferred abbreviation for the location.
Interview with Katherine Freese, Director of the Weinberg Institute for Theoretical Physics, the Jeff and Gail Kodosky Endowed Chair in Physics at UT Austin, and the Director of the Texas Center for Cosmology and Astroparticle Physics (TCCAP). Freese begins the interview with an overview of terminology, such as cosmology, astrophysics, and astroparticle physics and the delineation between these fields. Then she describes her childhood in Bethesda, Maryland where both her parents were scientists. Freese recalls beginning college at age 16, starting at MIT and then transferring to Princeton. She recounts taking time off after her undergraduate studies, before deciding to pursue graduate studies. Freese began grad school at Columbia but switched to the University of Chicago to work on neutrino physics with David Schramm. She discusses her first post-doc at Harvard, working on WIMPs and dark matter, and then her second post-doc at Santa Barbara with Frank Wilczek. Freese then recalls returning to MIT as a professor where she worked with Alan Guth and Josh Frieman on cosmic inflation. She talks about her transition to the University of Michigan and the exciting developments in cosmology at the time, as well as her introduction to dark energy. Freese describes her more recent involvement with NASA’s SPIDER experiment, as well as the honor of being named to the National Academy of Sciences. Freese discusses the amazing opportunity of being the Director at the Nordic Institute for Theoretical Physics and ends the interview with her hopes for the future of cosmology, namely her hope for finding dark matter.
Note from K. Freese:
Since the time of the interview, my title has changed. Since fall 2022, I am the Director of the Weinberg Institute for Theoretical Physics, the Jeff and Gail Kodosky Endowed Chair in Physics, and the Director of the Texas Center for Cosmology and Astroparticle Physics (TCCAP).
Okay. This is David Zierler, oral historian for the American Institute of Physics. It is April 23, 2021. I am delighted to be here with Professor Katherine Freese. Katie, it is great to see you. Thank you so much for joining me today.
Happy to be here!
Katie, to start, would you please tell me your current title and institutional affiliation?
I am at the University of Texas in Austin. I’m the Jeff and Gail Kodosky Endowed Chair in Physics. I also simultaneously spend time in Stockholm as a Professor of physics there.
What is your affiliation at Stockholm?
Well, I’ll give you the history.
I went originally in 2014 as Director of Nordita, the Nordic Institute for Theoretical Physics. One of the premier theoretical physics institutes in the world. Basically, around the same time I received from the Swedish government an absolutely enormous grant (equivalent of 15 Million dollars) over ten years to do theoretical physics. The way that works is that you spend half of every year as a professor in Stockholm and then the other half at your home institution. This idea was to bring in foreign researchers in a substantial way, to expand connections to the world community.
So as a consequence, I’ve built up a group there. In 2021 I had eight post-docs, which is insane. I mean, that’s an institute. That’s more than Fermilab astrophysics has or SLAC, you know? [Laughs] So it has been very generous funding, and I’ve had a lot of fun running a group there. We’re going out of historical order here, but you asked me what I’m doing there. It’s a slightly longer story to explain.
Katie, I’m going to guess that in the pandemic the boundaries that would prevent you from not being able to be in a particular place, they’ve worked against your calendar, that you’ve been probably more busy than ever as a result of Zoom and being remotely in touch with your colleagues and collaborators.
That’s an interesting question. I think one of the… Well, first of all, as you know Zoom is very difficult—teaching by Zoom, eliciting… You’re psychologically trying to elicit responses from people that you’re just not going to be able to get. The body language is very hard to determine when you’ve got this sea of faces in front of you, and actually, I only get about ten faces in front of me and the rest just put up their names because they don’t want you to see them. And you don’t even know if they’re really there! So, I find that psychologically very draining. Everybody does, what they call Zoom fatigue.
I think that’s probably the biggest factor, but there’s also the fact that normally when you give a colloquium somewhere, and I do a lot of that… Let’s see. In this last semester, I gave a colloquium at the Harvard Center for Astrophysics. I gave a colloquium at Tufts, at Wash U St. Louis where I also gave a public lecture, and you know, public lectures by Zoom just aren’t the same. But anyway, normally you would go to the place, which means you would find a substitute to teach for you and you wouldn’t be expected to show up at seminars at your home institution because people would know you’re traveling.
But now, somehow you’re supposed to be doing three things at the same time because in principle you could log in. “Why haven’t you logged into us?” because they know you can. Then you have to explain, “Well, because I’m doing this other thing.” So I think that’s… But on the other hand, it’s a lot less exhausting. I’m not crossing the Atlantic every other day. I’m not getting on the airplanes every other day, so you know, pros and cons. So I don’t quite know how to answer your question, at least not briefly. [Laughs]
Katie, a nomenclature question, from the beginning of your career in physics to now and the way that these names may or may not have changed. So for you, we have cosmology, we have astrophysics, we have particle astrophysics and we have astroparticle physics. Where are the differences and the boundaries between these sub-disciplines in physics for you?
So I would have to say… All right, you asked me historically how things have evolved. My PhD advisor was David Schramm at the University of Chicago, and he was one of the founders of the field of astroparticle physics --- the smallest particles in the universe explain the largest structures, connections between particle physics and astrophysics in the large scales. While I’m on the subject of Dave Schramm, shall I tell you a little bit about him?
He was an extremely inspiring person. He was also a large person. [Laughs] We called him Schrammbo! He was an incredible athlete; he nearly went to the Olympics in wrestling. I think he was an Olympic coach at Caltech or something like that; I’m not sure, but he was a great wrestler. Every year we would go to the Aspen Summer for Physics, which was a wonderful thing that was… Because one of the founders of the city of Aspen is actually a physicist, there’s now the Aspen Center for Physics and we are very fortunate that every year there are winter meetings that are for both theorists and experimentalists. In the summer you go for several weeks at a time to think and network and so on.
He had a house in Aspen and we would go skiing along with the conferences in the winter. His legs were so big that where there was a mogul beforehand, after he went through it, it was just gone. [Laughter] Sadly, he died at age 52 flying to Aspen in his private plane, Big Bang Aviation. There’s a lot of that, I guess, private planes crashing, particularly in Aspen. It was tragic. I don’t know, but anyway I just had to mention him.
So because of Dave Schramm, one of the founders of particle astrophysics, I actually switched fields. I was originally a student at Fermilab outside of Chicago working on an experiment looking for a neutrino mass. Didn’t find it then; it was found decades later, but anyway. Then I decided I’d like to get into the city of Chicago, so I signed up for his class… You know, I don’t know if I should tell you all the humorous stories.
Absolutely, but we can come back to it in chronological narrative, if you want also.
Yeah, I’m going way out of order.
I should answer your question about the delineation between fields. Okay, I’ll do that. I think particle astrophysics still means the same thing that it did. Cosmology is… I don’t know about the evolution of the name, but it’s a broader topic in that it also encompasses observers who don’t work on particles at all. Some of the things that would be closest to what I do—you have the cosmic microwave background experimentalists and they’re usually in physics departments rather than astronomy departments. This I don’t know why this historically is true, but you also have people looking at galaxies, clusters, large-scale structures. Cosmology has become… What has changed is that cosmology is a bigger and bigger part of not just physics but also astronomy, so cosmologists are taking over astronomy. I don’t know if I want to keep that, but… [Laughs]
You’re not the first person who’s said it, though.
You’re not the first person who has said it.
Yeah, cosmology is becoming… Well, it’s just been so successful! The data that have poured in over the last couple of decades is just mind-boggling. I’m not sure the terminology has changed, but the focus in physics and astronomy departments—everybody has to be doing cosmology now. It is not… You know, in the 1950s it may have been thought to be science fiction, but not anymore. It’s a major, major part of physics, a major part of astronomy.
Katie, let’s go all the way back to the beginning. Tell me about your parents and where they’re from.
Well, see, here’s where I wish I could show you the pictures. I have pictures in my talk that I gave to the National Academy of Sciences. Okay. On my father’s side, I come from a family of artists and architects. My grandfather was a Bauhaus architect in Germany and my grandmother was a painter. When my father as a child was tinkering around in the basement with chemistry sets, his mother said to him, “You’ll never make a living in science. Why don’t you go into the family business, the arts?” Well, he defied that advice. He was actually a PhD student with Heisenberg in Gottingen, and then a post-doc with Enrico Fermi in Chicago. After that he went to Caltech where he switched to biology and became one of the founders of the field of molecular biology. He founded the department of molecular biology at the University of Wisconsin. My mother was from a town on the Swiss-German border called Konstanz, and she actually got her PhD in biology at age 22.
Yeah, and then went to Caltech as a postdoc. People say I’m a pioneer for women in science but I am nothing compared to her! At Caltech she met my dad. Then both of them moved to the National Institutes of Health in 1962, which is why I grew up in Bethesda, Maryland. So they were very big influences on me. The idea that a woman could be a scientist was not exactly foreign to me. In fact, unlike many of my peers at that time, I was raised, first of all, you will earn your own living, but also you can do whatever you want. You’re really smart. I was told, “Do it, do it, do it!” I’m very grateful for that.
For our local crowd, the AIP is in College Park, so we know Bethesda. What neighborhood did you grow up in?
Near Walt Whitman High School.
Did your parents involve you in their careers? In other words, even as a young girl, did you know what it meant to be a scientist?
At the dinner table… They tried not to talk about it, but of course they did, so I became familiar with the language, the thinking… Oh, you know what? Can I add about… I wanted to add this about my dad.
The contributions that he made were really seminal in the foundations of how evolution works—mutations, point mutations. You know the bases A, C, G, T? One of them switches out for another in a point mutation. Having been trained as a physicist, he called them transitions and transversions, so really, really super-important stuff. Groundbreaking work. Mutations are the way evolution works, and his discoveries were fundamental. I did want to add that about him.
They did talk about stuff at the dining room table and then as kids we’d go visit their labs and play with pipettes and petri dishes, so the idea of… I don’t know. It was visible. I think for a lot of people in the world, they have no idea what a scientist is, what a scientist does, and so that was information I had.
Besides their influence, Katie, when did you become interested in science yourself?
Well, I was always really good at math, how satisfying it is to solve a problem and I really liked that. Then when I was 15, I went to Exeter summer school where I took a physics class, and I have to admit at first I thought, “Oh my god! This is too hard. I’m going to fail!” Then at some point I realized, “No, actually I’m doing just fine.” We also did some… That’s when I was first exposed to relativity and I just thought was so fascinating, the paradoxes, and I thought, “I have to understand this,” because you just get an inkling of it in these introductory classes. So I wanted to learn more about that.
Then I went to college when I was 16. I went to MIT, but I was underprepared because back in Bethesda I went to Holton-Arms School, an all-girls school which at the time offered no physics, no calculus, nothing. So going to MIT at 16 was a tall order because you had the Bronx High School of Science kids who were, you know, years ahead of me. It was really tough that freshman year, but I don’t know. We could have a five-hour interview where I tell you all the ups and downs of my career, but that was a tough one. I went in there thinking, “I’m brilliant. I’m at the top of the world.” I scored well on every possible entrance exam. By the end of that year, it was a really demoralizing experience. I thought I was stupid, so it’s kind of sad to do that to a 16-year-old. [Laughs]
What did you do next?
I transferred from MIT to Princeton and was still pretty lost sophomore year, which was also tough for me because the subject matter was still the same mechanics and electromagnetism that you were supposed to know from high school physics that I never had. But then junior year, all of a sudden, it was something completely different. It was quantum mechanics and statistical mechanics and thermodynamics. It was new to everybody, not just to me. Plus I decided to actually buckle down and really study. To my amazement, when the first midterm grades came back… They show you on the board the histogram. The average was like a 45 and somebody had an 85 and we all thought, “Oh, we hate that person!” Guess who that person was? It was me!
And I was like, “Oh my god! Maybe I’m not so stupid!” Yeah.
Katie, did you transfer to Princeton with the intention of studying physics?
Yes. I did.
Why would you think if you were underprepared at MIT that Princeton would be better for you?
Well, that’s not why I transferred. I transferred because MIT was a miserable environment. [Laughs] It was really horrible. Don’t forget I was one of few women…
Back-- Actually, the female to male ratio at MIT was 1:7, and then at Princeton was 1:3, which was much better. As far as I know I was the second female physics major at Princeton ever because women hadn’t gone to Princeton before, you know? And Princeton was better for me because I had friends from high school who went there so I was less lonely.
Were there any faculty members, graduate students, post-docs who were women?
You mean at MIT or Princeton or both?
No, at Princeton.
I’m trying to remember. In my class among the majors, I have a picture again here I can show you, but there were 40 physics majors and two of us were women. So that was in class, okay? On the faculty there was one woman, Pam Surko, who was an assistant professor, and she was a bit of a mentor, actually. Other than that, I’m trying to remember if there were any female faculty. No, there were zero female faculty.
I know this because I know the first ones that came along after me, you know? So there weren’t any. She might have been the only one. But she didn’t stay there. Went to Bell Labs.
Now you were saying with Fermilab, your early interests were in experimentation.
What kind of lab work at Princeton was formative for you?
Nothing. A couple things at Princeton that I really liked. We did junior papers and senior thesis, so you actually worked with a professor (two different professors as a junior and then one as a senior). But I wrote papers. I researched topics and wrote up what I learned, so I didn’t do any actual experimentation.
Who were the professors you worked with on those projects?
Yeah, this is going back in memory. Oh, the junior papers were with Val Fitch, who won the Nobel Prize for CP violation, and with Frank Calaprice. Then the senior thesis was with Hartmut Sadrozinsky. He is an experimentalist and so I did some of the math behind how the… You know, I don’t remember the details, but I wrote up stuff. I didn’t do any actual lab work.
Besides just the numbers at Princeton, were you ever discouraged, just from the vibe of the department, that you weren’t welcomed?
The reason I left MIT is because of the atmosphere there. It was very much… In fact, in all of physics, when people talk about the macho environment, I think what they’re referring to is actually what you’re affected by more than anything are your peers. So the students, all students have this impression that you have to be Einstein to survive as a physicist. So they all come in—not all, but many come in insecure as hell, and the way that young men handle that is bravado. “This is easy.” “It’s trivial. It’s easy. I can do this.” Dadadada. Well, it’s not easy for anybody, but for a young woman, you don’t understand that it’s just a show they’re putting on and you have the impression that, “Uh-oh. If it’s easy for them, I don’t belong here.” So that was an issue. At MIT that was really pervasive.
I think MIT students-- You know, there are 1,000 in a class and they’re all valedictorians from high school and they all come in realizing, “I’m just average here.” In fact, MIT is aware of this problem, which is why they switched freshman year to pass/fail because they used to have a lot of suicides. Every single one of those 1,000 students is really, really smart and should be told that fact every day. Is it a problem with the institution or is it… I think it’s… What studies have shown, for example, in astronomy once you have 20% women in the community, the whole culture changes. It’s a more normal community, and so you can’t blame anybody for this, but it’s an uphill battle to get from zero to 20%.
And this bravado issue was less pronounced at Princeton?
Well, Princeton, you don’t only have scientists and engineers. You have all kinds of different fields where women have always felt more accepted.
Moving forward to when I was Professor at Univ. of Michigan: when you’re on your way, going to the office, your elbows are already out. You’re thinking, I’m entering a battlefield. That’s in many physics departments. By the way, I have to say UT Austin is the opposite of that. Do you want me to tell you about what it’s like there?
I would love to hear it.
So that’s one of the reasons that I’m so happy there, is… So for example, the theory group is all on the ninth floor of the building, and it is a warm and fuzzy environment. People are so nice and so supportive, even though we’re all working in different areas. I mean, I’m in cosmology and then other people are in string theory and others in particle phenomenology. Austin is the only place I know where all of us talk to each other, attend seminars together, are super friendly. It’s a variety of topics we work on, but everybody goes for lunch together, walks for 20 minutes to the faculty club, chatting on the way, and this whole thing is organized by Steve Weinberg, one of the founders of the Standard Model of particle physics, one of the most brilliant people in the world. He sets a phenomenal tone because he’s a real… I don’t know what word to use. He’s a gentleman. He’s…
I can share with you an amazing story that Andreas Albrecht told me when he was a post-doc with Steve Weinberg, and that was Weinberg worked as hard as everybody else. What that demonstrated implicitly was that if somebody like Steve Weinberg has to work hard, that means that people might start to think of themselves not as outsiders who see these people as unattainable, but as insiders and they belong in the field as well. He set that tone.
I understand. Mm-hmm [yes]. Steve is very inclusive, invites everybody to lunch. He has some funding for that and pays for everybody’s lunches. It’s a wonderful experience, those lunches. Then visitors come and we talk about science. Steve leads the conversation and it’s always about something interesting. We talk about history. We talk about all kinds of stuff. The human side of physics and the people is there, so this weekly lunch is a big deal. And the tone—if the tone from the senior person is so positive, that trickles down. So the environment is absolutely wonderful, exceptional, and as far as I can tell, I’m sure it wasn’t always true, but the department as a whole has a level of civility that is also probably influenced by Texas because Texans are very… Well, it’s more than civil. They’re friendly, warm and fuzzy. The word polite—you can be polite and very cold, so that’s the wrong word. I’m not finding the right words, but anyway.
Well, you get to live in Austin on top of everything else, and that’s pretty good.
Yes. I love Austin. It’s a really fun town.
By the end of Princeton, what did you want to do next? When did you realize you wanted to pursue physics at the graduate level?
Jesus. My history is… I mean, there’s also the fact that junior year at Princeton was really, really hard. It was… I’m telling you how I did well, but the price that I paid was that you had to work around the clock, and you still didn’t get it done. You had to guess what was going to be on the exam because you couldn’t possibly learn everything they threw at you. In fact, nobody finished. We had a lab course. Nobody finished it. We all took incompletes and had to finish it over the summer, so we were kind of a famous class, for about a decade after. “They screwed up.” I mean the physics department screwed up. That’s not acceptable, you know?
So actually, the work was so intense that I cried every night. I could barely survive it. I could barely do it. It was really too much. By the time I graduated, I had been accepted to 14 of the 16 grad schools I applied to. I didn’t think I’d get into any, so I applied to everything I could think of. I got into almost all of them and started at Stanford and three days later called up my girlfriend and said, “Come on. Let’s go travel around the world.” So I quit. I dropped out. I needed a break. I needed to do something. I was 20 years old and I needed a break.
We went to Tokyo where we taught English to earn some money. She also dropped out of grad school in art history—not dropped out, but you know, hiatus. Then I ended up also working in a bar serving drinks and so on and off did that for a couple of years, but my life is a little complicated. In the middle I also went back to Georgetown grad school in biology for a semester. [Laughs] And at the same time took one physics class. I couldn’t help it. I had to do the physics, so I went…
In the middle of those two years… Well, towards the end of those two years, in Tokyo I got appendicitis and it had started to burst, so I was in the hospital. They didn’t give me enough painkillers and I don’t know why, maybe because I’m bigger than Japanese people, so they miscalculated. I don’t know. The pain was so intense. Am I going on and on about these details too much or did you want to hear?
This is amazing. Please, please.
All right. The pain was so… I was just… The only thing I was aware of was a hole in my belly for a good part of a week, okay, and you fade in and out of consciousness. Let’s face it. So then at the end of that week, all of a sudden I look over and realize I’m in a double room with another person. I hadn’t even known I’d been sharing a room for that whole week because all of my consciousness was concentrated on that hole in my belly. So finally when I’m coming to, you know, there’s another person over there.
So as I’m coming to, I get bored instantaneously because that’s who I am. I get bored quickly and I need something new every day. That’s, by the way, why I had to become a theorist, because I have to do something new all the time. I don’t have the attention span to work on the same experiment for that long a period of time. I like to have many things that I’m doing.
Back to the hospital in Tokyo… Believe it or not, I had with me a physics book called Spacetime Physics by Taylor and Wheeler, and it is at the level of a good high school student. It doesn’t even have calculus, but it has all these really cool paradoxes. I poured over that book in the hospital. Once again, it was special relativity that grabbed me. I thought, “Oh my god! This is so cool!” and I decided to reactivate one of the grad schools I’d gotten into. So in August I contacted Columbia University and they let me in for September.
Why, of those 14, Columbia? Did you want to be in New York?
Yes. I didn’t know that I was a city girl until I spent time in Tokyo and I had a great time. So I wanted to be in New York. Yep.
With the intention of doing a PhD there?
Yep. However, after you finish… God, you know, this career has been full of tough things! There was the freshman year at MIT that really nailed me, there was the junior year, and then there was again the qualifying exam at Columbia University. They offered it on the week of January 3rd, okay? So the first year I thought, “To hell with you. I’m having New Year’s,” you know? So I didn’t study at all and of course I failed. I spent a lot of my first semester at Studio 54. Most famous nightclub of our lifetimes. No regrets there. But failing the qualifier the first time made the second year really, really awful because they’re going to kick you out if you fail again, you know? I was terrified that whole year, but it’s okay. I passed. [Laughs]
And again, the whole intention at this point is, before David Schramm, it’s all experimental physics for you. That’s your trajectory.
Yes. Don’t ask me why. I just thought particles are really cool. Something inside you just gets turned on for whatever reason, and I thought the idea that the fundamental building blocks of the universe—how much more interesting can you get than that?
That’s right. That’s right.
So I wanted to go after that.
So why Chicago? How did that happen?
As a Columbia University grad student, I went to Fermilab, but as I just told you, I realized I’m a city girl and there I am with buffalo on a farm an hour outside of the city and I’m thinking, “I’ve got to get into the city!” So then I started taking this course from Dave Schramm, and I always tell people that…
Okay, so let me tell you what dark current is. My job at Fermilab was to unplug these phototubes. So you twist them, you unplug them, and you see is there still a signal even when they’re not plugged in? That’s called a dark current, and that would be bad. Then that means it’s a bad phototube, right? So you do that over and over again, a thousand times for a thousand phototubes and your hands are bleeding. I always say I switched from dark current to dark matter and dark energy [laughs] because I just got so inspired by this course by Dave Schramm. Oh yeah, more stories. You know, I’m going on and on too much. You can cut this stuff if it’s too much. So there I was--
Not at all. What was the original point of contact with Dave where you realized you were going to be a theorist?
Well, I wanted to take an acting class in Chicago just to get into the city, but those classes had already started. By some miracle, University of Chicago is one of the few universities that starts in October, which is the timeframe we’re talking about, so I looked. What can I take at the University of Chicago, and I saw cosmology. It was that simple. I just decided to take this class. I didn’t know anything about him, so I hit the jackpot! I guess what I’d say about me is that I kept trying, looking, looking, looking, until I found… Many people just take the first thing that comes along. I’ve left that-- I haven’t even told you all the other things I tried, so I’m an unusual person. I look, look, look, look, look and when I hit the jackpot I knew it. This is for me. And I’m not talking about fame and fortune; I’m just saying what makes me happy is what I’m doing.
What was Schramm working on when you connected with him first?
Well, I can’t answer that generally, but I can tell you the connection that it had with me. The answer is neutrino physics because I was doing this experiment at Fermilab on neutrinos, and he asked me… Well, I’m sorry, but I have to tell you this story. I was auditing this class, so it didn’t matter what grade I got, right? Then all of a sudden before the midterm I saw the damn test sitting on his secretary’s desk. I could have looked at it and I thought, “No, I’m not going to do that. I’m an auditor anyhow,” and besides I wouldn’t have anyway.
So then again, same story. There was an average grade on the midterm which was really bad and I was at the top of the class. Same story again and he noticed and then he said to me, “Would you like to do a project on neutrino physics with me?” I came back to him and I said, “How about I switch and I become your student?” and he went for it. So I transferred to the University of Chicago. I’d already been accepted there before anyhow, and they had to… They couldn’t find the paperwork, but eventually they reactivated the acceptance.
How did you develop your thesis topic with Dave?
I didn’t write a thesis. Theses are a waste of time. What matters is publications. So at University of Chicago, if you have a single-author paper, you don’t have to write a thesis. But I can tell you the topics that I worked on. You know, cosmology is a very powerful probe of neutrinos, and I’m still working on this… I dropped that subject for a long time, but I’m back on it now because how are you going to go after a neutrino mass? Well, there are oscillation experiments, but there’s also cosmology. It affects… Neutrinos play a major role in the growth of structure and they affect the cosmic microwave… Well, that’s the main thing; they affect the growth of the structure and so you can see evidence for neutrino mass in the structure. That was the first paper I wrote. But then I wrote a bunch of…
I also have to mention another mentor at Chicago and that was Michael Turner. He was an assistant professor and so I wrote papers, actually with both Dave and Michael, on magnetic monopoles. Well, you know how there are electric charges. There are point charges. An electron has a charge. The question is whether or not there’s a magnetic analog, and that would be called a magnetic monopole. So refrigerator magnets are dipoles; you have a positive and a negative. Can you have just a positive magnetic charge all by itself?
At the time, there were experimental hints. We called it the Valentine’s Day event because there were some hints in experiments on that day that there was a monopole, which turns out it was a fluke, and no, we have not discovered them. But anyway, monopoles have… This is again this particle astrophysics connection. Monopoles would affect what happens inside stars, so they would actually eat up stars. So you look at the densest stars, which are white dwarfs or neutron stars, and if they would eat up these things, then they wouldn’t be around anymore, so you can go look for them that way. So we did that and we ended up placing such incredibly tight bounds on the number of monopoles, we didn’t find them, of course. That’s one of the things that I did. That was a focus for what I did in grad school.
Was there a thesis defense even though there wasn’t a big emphasis on the thesis itself?
Yes. I remember talking on… Yeah, it was put together very quickly. I was offered a post-doc at Harvard Center for Astrophysics without having completed, it turns out, some of the requirements for my PhD. Dave Schramm had to talk them out of it after I’d already gone to Harvard. I didn’t even know this. I never took… I guess it was called solid state physics at the time, and I didn’t know I was supposed to have done that. I kind of regret I didn’t, but anyway.
So the thesis defense was, “Okay, we’re doing it next week,” and then I got sick. Then I remember being on the phone with Gene Parker—you know the Parker Solar Probe? That’s the Gene. He’s a wonderful brilliant guy. Discovered the solar wind. Wrote a fabulous book on astrophysical magnetic fields. What a witty… What a wit. I sat in on one of his classes. He was very witty and the students did not understand him on the whole, but I got along with him. He was really fun. Anyway, he said, “Just show up.” This is the guy that everybody’s afraid of and I thought, “Oh, good.” So I just showed up, and I don’t know. I knew my stuff. I answered the questions. That was it.
Actually, I remember Bob Wald asked me a question and I thought, “That’s an interesting one. Somebody should do that,” and still to this day nobody has done it. I don’t remember what it was.
Was it a gravity question?
It had to do with buoyancy of magnetic fields in stars, actually, but I can’t tell you more than that. No, it was not.
What was your research for your post-doc at Harvard? What were you working on at that point?
So that’s where something good happened to me. Well, good things have happened to me all along, but I went to… In Israel, there are these winter schools in Jerusalem, so I went to that one and there was another attendee, a guy named Andrzej Drukier, and he knew where to go for New Year’s, which is not the same day in Israel as it is in the US. So on December 31st we went to this place called the Cinematheque.
I started talking to him and he had done with Leo Stodolsky some work on how you could detect neutrinos. Neutrinos are weakly interacting particles, and the coherent scattering of neutrinos… They thought about how you’d build a detector to look for that. It was something that was actually measured for the first time a few years ago, by Juan Collar, really creative guy.
But then there are other weakly interacting particles, namely WIMPs (weakly interacting massive particles). These are proposed solutions to the dark matter problem which I have spent a lot of time thinking about since then. Using the same technology they thought of, you could also look for WIMPs. So Goodman and Witten wrote a paper suggesting that, and then Andrzej …
I was a post-doc at the Harvard Center for Astrophysics. I brought Andrzej back with me to Harvard where David Spergel was a grad student. We were both hired by Bill Press, who is now a computer scientist at UT Austin. So Andrzej, David, and I wrote the second paper ever on the idea of Direct Detection. In the experiment you’ve got a nucleus sitting here. These weakly interacting particles traveling around in the galaxy come into the detector and scatter elastically off of the nuclei in the detector. A small amount of energy is deposited in the nucleus, which you can try to measure with very sensitive detectors.
The galaxy would be full of WIMP dark matter particles. In fact, there would be a billion of these going through your body every second, but they don’t have electromagnetic interactions. They don’t have strong interactions. There are four fundamental forces, so the question is, other than gravity, what if they also have weak interactions? That’s the basic idea.
So given that they have a weak interaction, which is why they would behave similarly to neutrinos, you can calculate… So I did the scattering rates. I did the Feynman diagrams for the scattering that you would have off of your detector to then hit the detector, and then David added the astrophysics. “We have this whole sea of dark matter around us in the galaxy,” which we called the halo of the galaxy, “and so what would be the distribution? How many WIMPs of what velocity would be coming through your detector?” You put all these pieces together and then we came up with numbers. Experimentalists saw these predictions, and so that started this whole field of direct detection. I guess you could say we pioneered the ideas that led to the field of Dark Matter Direction Detection. Now 30 years later, the experiments are worldwide. It’s even… So all over…
So in Europe there are a bunch of experiments, in the United States, in Canada, in Asia, even at the South Pole. You have to go into underground laboratories. You have to be a mile below ground, so that means either in abandoned mines or underneath mountains because otherwise you’re swamped by other particles and cosmic rays flying around in the galaxy. So there’s been a search to look for these types of interactions, and the sensitivity of these experiments is insane! The latest, for example, is xenon detectors. You’ve got a ton of liquid xenon. It’s a giant vat that’s waiting to be hit by WIMPs. Really, really big detectors.
So of all these detectors, there is only one that has hints of a signal based on work by me and Andrzej and David. Our work is called annual modulation. The fact that the… Okay, the Earth moves around the sun, and as a consequence, the event rate should go up and down—maximum every June, minimum every December. That is what DAMA has, this experiment made of sodium iodide crystals outside of Rome and underneath the Apennine Mountains. They do see exactly the signal that we predicted, so there’s no question after 13 years of data or whatever. They have an annual modulation, but the question is, is it WIMPs or is it something else?
This is a very weird situation. They won’t share their data, which is not the normal… It’s not usual, so that’s kind of weird. And then other experiments don’t see anything. Yeah, well, but the other experiments are made of different material, so then you have to really nail down the theory to be able to compare them and we don’t know how to do that.
Katie, was there a sense of optimism in those early days that dark matter was something that would be understood in the short term?
Yes!! After Harvard Center for Astrophysics, I was a post-doc in Santa Barbara at the Institute for Theoretical Physics. (There was no K yet; it wasn’t Kavli yet.) I gave a colloquium upon arriving there telling everybody, “Oh, this will be discovered in ten years.” I gave a colloquium at Berkeley: “This will be discovered in ten years.” Wrong. So that’s really frustrating.
What was the source of the optimism? Why did you think, and your colleagues, that dark matter would be understood in the short term?
Well, WIMPs made so much sense. First of all, there are only four fundamental forces. Dark matter doesn’t have electromagnetic interactions; otherwise it would give off light. It most likely doesn’t have strong interactions or we would know. Other than gravity, the fourth remaining force is weak the weak force. If you postulate that dark matter does have weak interactions, then an amazing coincidence happens; some call it “the WIMP miracle.” We know how to make the calculations of how many particles of all types there are in the early universe, and how many are left today. These particles are their own antimatter. If you postulate that the annihilation happens through weak interactions, you end up with the right abundance of them leftover today from the hot early universe. That’s a big deal. Number two, there are outstanding problems in theoretical physics and the proposed resolutions to those problems automatically predict new particles that could be dark matter. One of those big problems would be resolved if you had supersymmetry, which doubles the numbers of particles that we have. There’s the Standard Model, but then there would be the supersymmetric sector and the lightest one of those would also be WIMPs. So you had a lot of theoretical motivation to believe that WIMPs made sense. So a lot of--
This is, of course, Katie, right at the time when there was optimism that the SSC was actually going to be built and perhaps supersymmetry would be found then.
Yeah, that too. Mm-hmm [yes]. And now the LHC has not found supersymmetry.
So if WIMPs made so much sense and here we are later, what’s the problem? What happened?
Well, you can be very clever, theoretically self-consistent, and nature just doesn’t do what you want it to. You’re just so… The fun part about being a theorist is you get to be very creative, inventive, make stuff up, and so I’ve done that a lot in my career. Then you have to be really lucky that what you just made up actually fits nature.
Tell me about your time in Santa Barbara. Who were some of the key people you worked with at the ITP?
Well, Frank Wilczek was there, and we started a number of projects… Well, we did similar projects and talked a lot, but we never actually wrote papers together there. But one of the great things about it was the other post-docs. I learned a tremendous amount from them. These are not people I’ve worked with, but in my office on one side of me was Bill Bialek who is a biophysicist, on the other side Andy Strominger who is a string theorist. I don’t know. It was just a very…
Oh, you know what else I did? I went to seminars that were meant for grad students. Professors introduced themselves; you know, “Here’s what I do.” So I would go to that and then Jim Langer gave some talk and I realized, “Oh, wow! Is this going to apply to cosmology?” He’s a condensed matter physicist, but I ended up writing papers with him. So I’ve always kept my eyes open. Two plus two is four: there’s a two over here in one field, another two in the other field, and then you put them together; you can come up with something new. That’s what I’ve always tried to look for. My dad told me that was a way to find new ideas, so I kept that in the back of my head always.
Katie, with all of the optimism surrounding WIMPs, what was the chronology that it was WIMPs and not massive compact halo objects for dark matter?
Yeah. Back when I was in grad school, we did not know what the geometry of the universe is. We did not know what the total content of mass and energy in the universe is. So it was still possible that it was entirely made of Standard Model particles, although from Big Bang nucleus synthesis—and my advisor Dave Schramm worked a lot on this—it was clear, if you want to match the element abundances from three minutes after the Big Bang, that you couldn’t…restricted the numbers of ordinary particles, but it still could have been the whole universe. But if inflation was right, then ordinary matter would only be at most 10% of the total, so there was already a battle going on between those who said, “Look, ordinary matter is enough. Dark matter could be made of ordinary matter in the form of MACHOs (massive compact halo objects): these would be faint stars, or planetary type objects, or stellar remnants like white dwarfs, things like that,” I mean, because if you go to lower mass, there are more and more stars. So there could have been a lot of really faint ones or substellar objects planetary size, things we didn’t see.
But then there were the other people who said, “No, no, no. Inflation is telling us what the total mass-energy content is and so you must go to exotica like axions or WIMPs or whatever.” So it’s kind of fun. The battle was the MACHOs vs. the WIMPs. I worked on both sides of this debate. We looked at a combination of theory and data and showed that the MACHOs made of faint stars are only 3% of the total of our galaxy and the white dwarfs may be at 10%, so you can’t explain all the dark matter in terms of MACHOs.
I can’t help but notice that in bemoaning the bravado in the field, you decisively ruled out MACHO for dark matter. That’s pretty fun.
Yeah. I don’t know. You remember the movie with Madonna? I used to go around saying, “MACHOs are dead. Desperately looking for WIMPs.” Then CERN’s Alvaro de Rujula in one of my talks came up to me and said, “I’m a wimp!” [Laughter]
Did you go on the job market after the ITP or did you want to do another post-doc and that’s how Berkeley came together?
I went on the job market while at ITP and… How did this happen? So at Berkeley I got a Presidential Fellow of the University of California, which was meant for diverse candidates, and actually, the year after me Lisa Randall did the same thing. These fellowships increasing diversity… I don’t know. I think they’re good. People worry if they can be detrimental to careers. It didn’t hurt either one of us. I was only at Berkeley for three months because then I got the MIT position. I got married in August, got pregnant in September, and my ex-husband was looking for… He worked on star formation, so he was looking for a post-doc, and he had decided he was going to get one at the Harvard Center for Astrophysics, okay, hubris, I guess, but he did get one. So I applied. I just wrote to… People hadn’t even advertised. I wrote to every school in Boston and I got an MIT professorship. [Laughs]
How did you feel coming back to MIT at a very different stage in your career?
Oh god, it was miserable again. Ohhh…
They hadn’t changed.
No… It was awful. I showed up. The Department Head was Bob Birgeneau, and the first thing they do is have a meeting with the assistant professors. Instead of saying, “You’re some of the most wonderful, brilliant people on earth,” which is true—every single person there is fabulous—they say, “You know only one in three of you will get tenure.” Then you pretty quickly realize that of those one in three, only one in three was not an MIT PhD student, so your odds are actually one in nine because they keep in-house. Not true anymore. Places change, you know? So I was like, “What a wonderful beginning that really makes you all happy.” Isn’t that horrible, that kind of behavior? Jesus.
Katie, at this point, would you say professionally, given that this is now your first faculty job, are you working in cosmology? Is that sort of the niche that you’re filling in the department?
Yeah. Oh, yeah. Yeah, yeah, yeah.
Who else at MIT in the department at that point would have called themselves a cosmologist?
Alan Guth, three doors down from me. Inventor of inflationary cosmology. Wonderful, wonderful, wonderful human being and friend and brilliant. That was really fun. We had a great time. Like I said--
Were you following all of the excitement in inflation during the 1980s?
Of course! I used to go in to him with a crazy idea like one after the next and then he would kill them, you know? But then eventually I came on to natural inflation with Josh Frieman and Angela Olinto and that was a really good idea. There was, at the time, a fine-tuning problem in inflation. You have to have a potential that’s very, very flat, and we figured out how to do that.
Well, I remember Josh Frieman was giving a talk. I don’t remember why this came up in his talk, but axions, the QCD axion, that thing has two mass scales that give you… You can explain a flat potential if the height and the width come from two different scales. It all comes from a symmetry that kept the potential flat, the shift symmetry. So I went to Josh and said, “Well, why don’t we use this for inflation?” and so we did.
So the idea that you could use some kind of shift symmetry or axionic type of particle, I thought it was a great idea at the time, and it is really… It has sort of taken off. I mean, it is really… People call it axion inflation now. They don’t even necessarily remember… Well, they do. So natural inflation is the idea of using an axion, but the specific version of it at the time that we had was a cosine potential and that is being tested against data from cosmic microwave background data. But the big picture of using axions as the particle responsible for inflation is more general and lots of models use that idea. These aren’t the original axion, they are at higher energy scales, could be from string theory.
Now was Frank Wilczek talking about axions during the ITP days?
The two inventors of axions independently were Steve Weinberg and Frank Wilczek, and Frank Wilczek is now… You know he’s also in Stockholm just as I am. He also took one of these professorships. He is building an axion group and is really going after axions. But at the time, at ITP, I didn’t talk to him about them that much at the time, no. But I know that right now it is a focus for him again.
Did you take on graduate students at MIT?
I did. My first grad student ever was a Barnard undergrad, Janna Levin, and she is now a Barnard/Columbia professor.
It was wonderful, and she has quite a public persona as well doing a lot of outreach, writing books, and working on black holes. So she’s had a really great career.
After three years, what were your prospects for tenure? What was the writing on the wall?
Okay, so the timeframe for tenure at MIT is seven years, but there were two factors that made me leave. One of them was my husband. He did get that job at the Center for Astrophysics as a post-doc but he needed a permanent job. The two of us had to get jobs together, so we applied together to get faculty positions and got them both at Michigan.
There’s also the fact that I had a child while I was at MIT, and you know there was zero maternity leave. So it was, again, a situation of crying every night. Am I crying-every-night kind of person? No! I mean, these were really, really, really tough circumstances! How the hell did I survive this? I don’t know! So, we had no money. They didn’t give us enough money to pay the babysitter… Worse that than at first, we couldn’t even find a babysitter. Eventually we did and my husband’s entire postdoc salary went to her. And no maternity leave. Out of three things you can handle two of them: having a newborn, teaching, and research. You can handle two out of three, so I begged them to give me a semester off teaching. Instead, they made sure I didn’t get time off teaching. In fact, they actually changed the rules of the Sloan Fellowship. Another assistant professor Ed Bertschinger had gotten a semester off teaching using the Sloan Foundation money so he could go to Santa Barbara for a workshop. I got a Sloan Fellowship a year later and I also wanted to use it for a year off teaching. Instead, I was sitting there pregnant in the faculty meeting while they decided, “Faculty are no longer allowed to get a semester off with Sloan fellowships” so they wouldn’t let me do it. Did they use me as the test case because I was pregnant? Who knows.
Was there any other women faculty at MIT at that point?
Yes! Herman Feshbach during his time as Department Head had hired about five female faculty, and they were senior to me. Yes. MIT was looking good for that reason, yes. So anyway, those two things… Oh, and I remember the Head of the Physics Dept at my time, Bob Birgeneau, telling me, “Oh, children are much more difficult when they’re 20 than when they’re newborn.” It makes me furious to remember that line. Probably true if you have a wife at home. Here I was struggling to the point of crying every night and I had to listen to that man. He told me, “Maternity leave is a benefit MIT cannot afford.” Am I being too negative in this interview? I think this stuff is important for people to know about.
It is important….
Katie, how did the opportunity at Michigan come up? Were you actively looking?
Did they recruit you?
Now where is the two-body problem in this?
Well…yeah. So my ex-husband—his name is Fred Adams—we had to get jobs together. So we applied together to places and we actually ended up with two offers, ironically Michigan and Wisconsin [laughs], which are both in the Midwest, college towns. It’s kind of funny. One of the reasons that we chose Michigan was because Wisconsin had no maternity leave [laughs], so I thought, “I’m not doing this again!” But then we ended up getting divorced instead of having more children anyway, partly I think to a large extent because the situation with the first child was such hell. Thank you MIT. So the idea of having another one was more than my ex could take. I should have been more sympathetic than I was, I guess, but anyway. [Laughs]
What were your impressions of the Michigan department of physics when you first arrived?
Much better than MIT. Oh god. You know, this whole interview is full of negativity. I don’t know what… I’m usually a pretty positive person, but these are things I had to face, right?
Absolutely. Well, but the positive thing is look what you’ve accomplished despite the negative aspects. That’s the point.
Yeah, kind of amazing, huh? Yeah, I don’t know how.
But you didn’t answer my question yet. What were your impressions when you first arrived?
Well, first of all you’re allowed to be a parent. People would bring their kids to school. It’s a very different thing, so that was a plus. I was the first woman hired on to the faculty, and what I didn’t know… [Sighs] See, I told you I’m getting negative already. I didn’t know this at the time, but apparently a lot of the faculty didn’t want to have female… And then the chairman made it happen anyhow. The wonderful Homer Neal was Physics Chairman at the time.
And you came on a tenure line or it was a tenured offer?
At that point I was tenured. I was 34 years old and I had tenure, so yeah.
How did tenure affect (or not) your research? In other words, if you could be more adventurous; you didn’t have to worry about those kinds of things.
Yes, that’s for sure, although if you look at what I did before tenure, like going into this dark matter field, that was really novel. Doing the natural inflation, that was really novel. Those are some of the best ideas I ever had. So I did it anyhow even beforehand. What I’ve always--
Now with Wisconsin, was IceCube up and running at that point? Was that a factor in your decision making?
No, it wasn’t. Wisconsin actually does a lot of…does more astroparticle physics than most places, but I underestimated that. Yeah. Anyway, no, it wasn’t.
Did you take graduate students with you from MIT?
Actually, Janna Levin, my first graduate student, still stayed at MIT. It was just another year, you know, so it didn’t make sense to move, and then she became a post-doc at CITA in Toronto. So no, I did not. And right now-- I just moved to Texas from Michigan, and I still have a student at Michigan, actually, so the same thing. People don’t want to move when there’s just not so much time left.
You know, you uproot your life for a year or two is just not worth it.
Same question as from MIT. Who at Michigan would have called themselves a cosmologist at this point?
Actually, the three of us were hired—me, my ex-husband Fred Adams, and also Gus Evrard—roughly the same time (Gus a year ahead of us) because they wanted to go into astrophysics. Actually, Gus also does cosmology. He does simulations of large-scale structure, so that’s something that the department decided they wanted to get into. So we were the first.
On the experimental side, Michigan historically has wonderful high energy experiment, and a number of those people moved into cosmology. At the time, Greg Tarle was already getting into… I think he was already then searching for…studying cosmic rays, including potentially hints of dark matter annihilation. He was switching, so a lot of people moved from high energy more into cosmology or astrophysics, and since then there’s been a lot more of that at Michigan. So they were interested. They saw, “This is a hot new field. Let’s do it. Let’s go into it,” which I think was a really smart thing to do because it’s true. It’s really a… You know, it’s been an incredibly growing field as we talked about at the beginning.
Katie, a broad question in the 1990s. What were some of the most exciting advances observationally in cosmology? What was going on that was most relevant for your interests?
Okay, the cosmic microwave background, no question about it. The COBE satellite… It’s the leftover light from the Big Bang, and the initial question was… It serves as a proof the Big Bang is right, first of all, because if you had a hot early time with all the particles bumping into each other, then you would have a blackbody spectrum for the photons, and so the COBE satellite found actually the most accurate, the most perfect blackbody known to humans within ten minutes of data. So it really nailed that. The other thing that it’s done…well, more around the year 2000, got the geometry of the universe which tells you the total mass and energy content, the parameters of the universe. So the cosmic microwave background has been hugely influential. It also tested inflation models such as the natural inflation model. That’s one of the big ones.
Then the advances in studies of large-scale structure are other probes—all these things. It’s like a jigsaw puzzle. All the pieces have to fit together. Supernovae were, again, really more at the turn of the millennium, the discovery that it looks like there’s a dark energy there, but it really took a combination. When the supernovae first came out, I didn’t believe it. I thought, “This can’t be right. We must not understand these objects that well.” But then when you have supernovae, cosmic microwave background, clusters, baryonic acoustic oscillations, everybody agreeing that you’ve got 25% dark matter, 70% dark energy—well, that certainly has a major impact on all of us, especially me because I’m trying to figure out what is the dark matter? What is… The dark energy I’ve backed off on trying to figure that out for a while. I tried it first, but that’s really hard.
When did you first get involved with dark energy?
Around the year 2000. I was working with a post-doc at Michigan, Dan Chung, who is now a faculty member at Wisconsin. Those were interesting times because results from string theory were being studied in the context of cosmology. String theory requires extra dimensions --- ten spatial dimensions— extra dimensions in addition to the three normal dimensions that we’re used to. The implications of that (the extra dimensions) for cosmology, that was a hot topic. I mean, it was kind of a new hot topic, and so one of the things we realized is that if you have… Let’s say you have extra dimensions and our normal three-dimensional universe is a membrane sitting inside these extra dimensions. Like the surface of a drum. Then there could be a second 3D membrane, away from ours, and the stuff in between is called the “bulk”. So, we could be living on this three-dimensional surface, but the bulk could change the equations governing the expansion of our 3D universe. If that were true, you could have a theory of dark energy that wouldn’t require any vacuum energy. You can think of it as these other…the extra dimensions pulling on our three-dimensional universe, changing the equations. The changes to the equations would only become important in the recent epoch of the universe and that could cause the acceleration of the universe today. You wouldn’t need anything other than atomic and dark matter to get the accelerated universe even without any vacuum because of the modified equations.
We called this idea Cardassian expansion because, honestly, Lisa Randall was giving lectures where she talked about the warp factor. I didn’t realize it was a term used in studies of general relativity; I had only ever seen it called the conformal factor. I thought she took the name from Star Trek, so I thought, “Okay…” I’ll call my new theory “Cardassian,” also from Star Trek. Then I came up with a good reason for the name afterwards, which is that Cardassians look really weird, but they’re actually made of the same stuff we are, and their goal is accelerated expansion of their evil empire. So the dark energy is actually nothing but matter and radiation. It just looks weird, but it’s just ordinary stuff that causes acceleration.
Were you in touch with Michael Turner at all at this point on dark energy?
Mmm…not really. [Laughs] He coined the name dark energy. We’re good friends, but we did not work together on this, no.
Did you see that more-- To go back to the nomenclature, did you divide up your research interests in terms of what was astrophysics and what was cosmology, or were those distinctions really never important for you?
Oh, I never… No, not important. It’s more along the lines of something flashes and you have an idea that comes into your head. The nice thing… One thing I like about cosmology is it draws from. Let me back up. Cosmology draws from so many areas. It draws from particle physics. It draws from… I mean, I’ve worked on stars, so I really have to know how stars work. It draws from astronomy, the large-scale structure, so it draws… In the examples I’ve been telling you about, you mix up all these different fields. I don’t pay any attention to what names they are; it just is a problem that you want to go after and then you have to learn something new. I mean, I really had to learn about stellar structure, you know.
And on that point, when did you start working on stars?
Well, there was the graduate school work about monopoles in white dwarfs, but then completely differently, we came up with the idea of dark stars, which is that dark matter could power stars. Oh, man. My stories are going to go on too long.
No, not at all!
So I was a Miller Professor at UC Berkeley, but they were renovating the offices, so I ended up spending a lot of time in Santa Cruz. There was a former Michigan student, Doug Spolyar, and we were trying to look for, okay, some interesting effects of dark matter at different points in the universe. At some point I got so frustrated I took the entire pile of papers that we had and threw them in the garbage.
Then we started looking at the first stars, 200 million years after the Big Bang. The very first stars that form would be in a very dark-matter-rich environment, and so people had not really paid enough attention to that. Clouds of hydrogen and helium from the Big Bang start to collapse at the centers of protogalaxies, called minihaloes, en route to making stars. We asked, “What would all that dark matter inside the collapsing clouds do?” If the dark matter is self-annihilating, such as WIMPs, the annihilation products could dump heat into the cloud. We had to put together a lot of pieces to make it work.
Then we started talking with Paolo Gondolo and we realized, “Oh my god. Dark matter can actually power stars.” He knew how to calculate that the collapsing hydrogen would drag in even more dark matter. We realized that you can have a star made entirely of hydrogen and helium almost, 0.1% made of dark matter, and the dark matter can be the power source for the star. They’re very weird-looking things. At first, when we did this we just thought, “Okay, we’re changing the history of the first stars.” We didn’t know it was a star; we just knew we were stopping the collapse of the clouds to make stars. But then another insight a year later, we realized, “Holy shit. These are actual stars.” So hydrostatic equilibrium, thermal equilibrium, the four equations of stellar structure are all satisfied —these things are real stars and in fact, they’re really bright! And in fact, the James Webb Space Telescope might find them because they start out one solar mass in size; they can grow up to a million solar masses, a billion solar luminosities. Let’s find them! Oh no, I’m getting a little excited here.
Yeah! You mentioned COBE. What was valuable to you for WMAP?
The parameters of the universe that I was telling you about. Well, the geometry of the universe, the fact that the geometry has no curvature; it’s a flat geometry—that came a little bit before WMAP with TOCO and BOOMERANG. But WMAP then had very accurate data and it nailed… I mean, at this point with WMAP and Planck there are seven peaks in the data. The parameters of the universe have to go into a computer code to fit the data, and so the amount that you learned is ridiculous. You learn the total content in terms of normal matter being 5%. That comes out of the second peak. The dark matter and dark matter content comes out of a combination of the other peaks and constraints on inflation, etc. etc. So all of that was really beginning to be nailed down by WMAP and more accurately now with Planck. This is hugely influential stuff.
Katie, you had visiting positions at the Planck Institute and at CERN. In what ways was this really valuable for your research?
So you’re talking about the Max Planck in Munich, I guess…
I went there over many years repeatedly, actually. Well, so Leo Stodolsky was a director there. I think he’s emeritus now, but he’s one of the guys that I mentioned with Andrzej Drukier there, a very creative guy to talk to. I met Paolo Gondolo. I’ve done a lot of work with him, and so again, it was our lunches. We would just brainstorm. So I started working with Paolo and I’ve done so over the years. That was a lot of fun.
CERN—I didn’t go there very long, you know. I was there for three weeks at two different times. Did I start working with people there? Yes, I did, actually—some extra dimension stuff that I did with people there, yes.
Katie, a general question. You’ve secured funding from both the NSF and DOE for your work. What are some of the similarities and differences in these funding agencies?
I was NSF-funded at MIT and then at first at Michigan. Then I switched over I guess five years later to DOE. DOE tends to have…did have umbrella grants and so you would join… Instead of applying as an individual, you would apply as a member of a group. Now you apply as an individual at both institutions. But at the time, Michigan was like 60% of the faculty were all under the same DOE umbrella or something, I don’t know, so I joined that umbrella. That’s the logical thing to do, you know? So I am still DOE-funded and just applied again.
Katie, tell me about some of your work on D-branes.
Well, this is what I was telling you about, changing Einstein’s equations, so that was the main thing. That’s one thing we did. Another project we did that’s really interesting—people call it shortcut metrics now, but we wrote the first papers on it. If our universe is this three-dimensional surface, then in principle… So light, for example, or gravitons or whatever… Let’s do gravitons. Anyway. Something could leave our three-dimensional surface, go out into the extra dimensions, and come back at a later time, and depending… Think of the brane we live on as a curved surface; so then if you leave the brane and go in a straight line in the extra dimensions that runs into the curved surface again later on, it’s actually a shorter distance. So you could have information that looks like it’s going faster than the speed of light from the point of view of our brane, but it’s not. It’s just going out into the extra dimensions and coming back. That might be a way to explain… The large-scale smoothness of our universe doesn’t make sense because it violates causality. You need somehow something to go faster than the speed of light, and so inflation is one solution, but this might have been another. This would be another idea, to send information off into the extra dimensions. That was the other thing we did.
Tell me about your work on primordial black holes when you were thinking about the cosmological monopole problem.
Hmm. You know, I’m not going to answer that one. I’m going to tell you about something else. One thing that we’ve done recently is… So primordial black holes are dark matter candidates. They could form at phase transitions in the early universe. People want… There’s a lot of interest in them lately because LIGO found black holes, and it’s possible some of those could be primordial black holes. So we thought about, okay, what about the next gravity wave detector LISA? If you had a bunch of primordial black holes around the big supermassive black hole in the center of the galaxy, then you could have a signal that LISA could see. That’s something we proposed, and I think that could be pretty interesting.
When you first started thinking about what would become dark stars, is there any hope that they’ll be observed?
Yes! That’s what I’m saying. They start out relatively small, the mass of the sun, but they’re weird… They’re 10 AU in size, with a radius ten times the distance between the Earth and the sun. Also their temperatures are very cool. The surface is not hot. There are no ionizing photons coming off, so there’s nothing to prevent them from accreting mass. They can keep growing and growing. As they get more massive, they get brighter. They can end up a million times as massive as the sun and a billion times as bright. But we’re talking about the first stars, so we’re talking a long time ago, which means that you have to have a telescope that is capable of probing pretty far back and that is the James Webb Space Telescope. That will launch in the next few years. John Mather and Jon Gardner have been encouraging us to make predictions for what you would see and so we’ve done that, what these things would look like, because the normal first stars are really tiny…are small. They’re 100 solar masses and harder to see unless you see a whole galaxy of them. But if you have a big and bright one like we are predicting, then you could see an individual one, so that’s a really exciting prospect.
Katie, a really broad question on inflation, which has had such an interesting history over the past 40 years. When in your career did it seem that inflation was most accepted?
Oh, I mean it made a big hit right away. As I said, people were saying, “Okay, so the geometry of the universe has to be flat because that’s what inflation would give us. Therefore, we have to have exotic dark matter.” So it immediately transformed the field. But then what’s happened since then is that the predictions of inflation are coming true. One of the big things that inflation does is it gives you seeds for galaxy formation, and we really don’t have an alternate mechanism for that. At the end of inflation you have small patches of the universe that are more dense than others and they accumulate more mass and eventually make galaxies. Each individual inflation model makes different predictions for these fluctuations. Also, primordial gravitational waves are predicted by inflation models. Those two things, the density fluctuations and primordial gravitational waves, are different for every model and are being tested by Cosmic Microwave Background measurements. That’s why I joined two CMB experiments: the SPIDER balloon experiment at the South Pole and the Simons Observatory in Chile.
So big picture, first of all, inflation predicted a flat universe and that came out correct. It predicted a number of other things that any inflation model would have and they all came out correct. Now it is also differentiating between models, and a lot of them actually have been ruled out. So you’re narrowing down the individual model, for Christ’s sake, but I think the fact that inflation has been observationally…the predictions have been confirmed one after the next—that’s a big deal. So is this some kind of effective model that we’ve got at the moment that will prove to be more complicated in detail? That’s certainly possible, but there really seems to be something right about it, so that’s very exciting.
Is this to suggest that you don’t necessarily see string gas cosmology as a viable alternative to inflation?
Well, you know, I don’t know much about string gas cosmology, so I’m not going to … I think inflation is great. One of the alternatives to inflation for making structures was cosmic strings. At certain types of phase transitions, you would get these topological objects that looked like strings and they would move around and cause matter to move around and give rise to galaxies. That’s ruled out. And the cosmic microwave background has a Doppler peak at 1 degree. That’s not consistent with cosmic strings, so they’re gone. So string gas cosmology, what it predicts I don’t really know, but is it really an alternative to inflation? I don’t think so. Do people say that it is?
Some present it as a viable alternative to inflation.
Okay. Well, I guess it’s not… Yeah, so…
It’s simply not on your radar. That’s fine.
More broadly from there, from cosmic strings, Katie, in what ways has string theory generally been useful for your research?
I’m sorry. You blanked out. What was that?
In what ways has string theory more generally been useful for your research?
I think I told you some of this brane cosmology. That’s part of string theory, and I’ve told you about two of the ideas that we had there, so let me just stick with that.
Okay. Now IceCube was not yet up and running when you were considering Wisconsin.
By the time it was, how did you get involved in that?
So when I was a post-doc at Harvard Center for Astrophysics, in addition to what I’ve told you about before, I realized another thing, which is dark matter particles, as they pass through the Earth, like WIMPs, one in ten of them get captured by the Earth, sink to the core, annihilate, and you would get neutrino signals coming out. That paper was simultaneous with one by Krauss and Wilczek. We both came up with the idea at the same time. Anyway, if that’s true, then that’s a way to look for dark matter. So IceCube and its previous and future variants—that’s one of the things they look for, but that… Yeah. So anyway.
I’m also friends with Francis Halzen. He’s a really great guy.
He helped make IceCube happen, along with many others. Wisconsin is like… I think what they’ve done is just incredible. IceCube is incredible! Have you read the popular-level book Telescope in the Ice by Mark Bowen? The story of getting that experiment to work is something else. One of the hardest things apparently is they had to drill bore holes into the ice by shooting hot water down there. That’s where the kilometer long string of phototube detectors had to go into the ice. Oh my god. The whole thing… It’s an extreme environment. Operating at the South Pole is a big deal!
Katie, when were you most interested in the LHC possibly finding supersymmetry?
Huh. Well, I mean before it started. [Laughs] It looked like a great way to go after supersymmetry and WIMPs, right? I remember talking to Nobel Laureate David Gross and I asked him what his hopes were for the LHC (other than, of course, the Higgs) and he said, “SUSY, SUSY, SUSY!” [Laughs] So everybody was really hepped up on trying to find supersymmetry, which has not happened. On the other hand, we also knew before they even started that you might just need to go to higher energies to find it. We knew that from the beginning, you know?
That leads me back to my comment about the SSC and possibly finding it there.
Yeah, that would have been… Yeah, that’s a shame, a damn shame.
Katie, when did you start following LIGO?
I mean, I knew about it when I was in grad school, so…
But I mean, it is an incredible engineering feat, right? It’s just… And political feat that they got the money to keep doing this is phenomenal! So what is… Yeah, the idea that… You know, moving matter. You wave your hand; you’re making gravitational waves. Good luck finding those! But merging black holes is a better target. The fact that they were able to do this is a beautiful thing.
One of the things that I’ve worked on since then with a grad student at Texas, Josh Ziegler, is they were not supposed to find black holes of—I don’t remember the numbers now; I’ll get it for you later—about 100 solar masses. There aren’t supposed to be any black holes. It’s called the mass gap, the reason being that the precursor stars were supposed to completely explode and leave no black holes. So what Josh Ziegler and I did, since I am familiar with stellar codes now, we put in… Well, Josh did the work. He put an additional heat source into these precursor stars to see if it would prevent them from blowing up, and yes, he got that to work. So we’re able to produce the missing black holes that weren’t supposed to be there in standard stellar evolution. That’s something that I think is interesting.
Katie, I wonder if you might compare the significance of the discovery of the Higgs in 2012 and the detection of the gravitational wave a few years later.
Yeah, I don’t know how to do that. They’re both… Okay, so gravitational waves, we know they have to be there. It was a question of can you do the experiment to see them, and the answer is yes. It was finally done. Once you can do it, you can explore this whole new… It’s a whole new window into the universe, so it opens up many doors. We did know it had to be there. The Higgs, it was a predicted part of the Standard Model, but maybe it wasn't going to be there and then that was going to leave a theoretical conundrum with people fiendishly trying to explain alternatives. So the Higgs was not as certain that it would be there, but now that it is, it just is! Whereas gravitational waves, we knew they had to be there, but now that they’ve been discovered, it opens new observational avenues.
Now by definition, this answer is going to be purely speculative, but where do you see the Muon g-2 anomaly experiment at Fermilab headed? What might be the significance of this, if you had to guess?
I don’t know. The question is… Okay, so experimentally it does seem… The measurements seem to be not what the theory predicted, and that seems to be consistent between Brookhaven, Fermilab. So the measurements are agreeing with each other. However, the theory is not yet agreeing. There’s the standard theory, but then there are these lattice calculations that kind of bridge the standard theory and the new experimental results. That’s a really tough one because the lattice calculations, that’s really… They used every supercomputer in the world to do it, and so how are you going to check this? I don’t know where that’s going, but it’s fascinating, the idea that we really want new physics, you know? But can I tell you another anomaly that is in cosmology that is more directly related to my work?
The Hubble tension. The Hubble constant, the expansion rater of the universe, there are two different measurements that are in disagreement. You can extract it from the cosmic microwave background and you get the number 67 km/s/Mpc. You can also look at late-time data like the supernovae and you get a different number, 74. It’s supposed to be the same object, but you are testing at different times in the universe, so what does that mean? There’s an idea called early dark energy that was originally proposed by Marc Kamionkowski. If you add in a vacuum energy just before the microwave background is created, at the level of 10% of the total of the universe, then that would cause the value of the Hubble constant extrapolated out of the cosmic microwave background… It would lead them to agree, you know? So I got excited. I saw his talk and I thought, “Oh my god! It looks like a phase transition!”
In the same way that the original inflation model… Oh, I don’t want to go into a physics lecture, but anyway, we have a model called chain early dark energy now. If I had my way, the idea is you have a series of phase transitions, so imagine bumps… It’s like a tilted cosine. This is a vacuum energy that could give you inflation early on. Then it would become subdominant. Then it would reappear at the 10% level to give you the dark energy and then it would be gone. Then it would reappear to give you today’s dark energy. That’s my fantasy, to make that work. At the moment, we have a model that gives you early dark energy and today’s dark energy with phase transitions. Yeah, so I’m having fun.
What has some of your more recent research in the cosmic microwave background been?
As I said, my group has joined two CMB experiments. We joined SPIDER, which is a balloon experiment at the South Pole and also the Simons Observatory in Chile, with the goal of going after inflation and neutrino mass. That’s my goal, anyhow. My people in Stockholm are involved with both of these experiments. During my time at Michigan, we were involved there also. [Note: post 2022, we hired Nick Galitzky, a fabulous experimentalist on SO, to UT.]
Katie, tell me about the idea that became your fantastic book, The Cosmic Cocktail. How did that all come together?
I first started trying to write a book ten years earlier, but it was really dry, so I kind of gave up on it. What I was writing wasn’t fun. Then I was simultaneously… I was teaching a course for… It’s called a freshman seminar in cosmology. It was a program that the president of the University of Michigan created for small classes of 25 freshman. They’re not going to be physics majors; they just want to learn something interesting for their lives. So in teaching that course, I learned how to tell people about cosmology, how to talk about the subject matter along with the personalities, the people who do the research, and some of my own stories. I discovered if you combine stories with the science, then people will take in a lot more. They enjoy themselves. They stay awake through the whole class and they get excited and they remember. They learned a lot! In fact, every year that I taught it, one or two of them would switch majors and become physics majors.
It’s an opportunity. It was really great, and so I realized this is how you reach out to non-scientific people. Well, some of them apparently are/were scientific, but to reach a broader audience, I learned how to do it.
Speaking of a broader audience, were you surprised how much it caught on and how many languages it’s been translated into?
You mean the book? Yeah, that’s really fun, isn’t it? Yeah.
You might be having this same effect all over the world.
The building I lived in in Stockholm, I don’t have that apartment anymore. I had the same place for the first seven years that I was there. The landlord told me that he was in Hong Kong and all of a sudden he saw me on TV talking about the book! [Laughter] That was fun. Another time, somebody at the gym recognized me, too…in Sweden! [Laughs] That was fun.
Katie, more recent history. What were the broader considerations in moving from Michigan to Texas?
Well, first of all, I love Austin! When you’ve been in the same place for 25 years, you’ve got to go do something different. I’m a city person, so I’m moving back to a city. It’s an exciting place. I love it. I also love the atmosphere, as I’ve told you, in the theory group and in the department more generally. They’re really good to me. Michigan was not so good to me. It took me a long time to figure out what was going on, but I was, for raises, ranked at the bottom of the department even when I was Nordita director, okay? So I was always… Then I had to hire a lawyer to get my raise up. So I told them, “I’ll stay if you guarantee me an average raise for the department for the rest of my life,” because I just couldn’t get one! [Laughs] So then of course, the minute I left I became a member of the National Academy of Sciences. They screwed up, I’d say. [Laughs] But I’m not sure if I want this in print or not, but like I said, I was the first woman ever hired on the Michigan physics faculty.
In what ways, if at all, has the department improved in that regard over the 25 years you were there?
There was a wonderful chair, Myron Campbell, and a wonderful dean, Terry McDonald. Individuals can play such a major role, and between… I don’t know whose idea this was. I think it was the dean’s idea, actually, instead of searching for individual fields, to have broad searches for faculty members, and the dean said, “Give me a list of ten.” Women always make it into the list of ten. If you have to pick one, it will never be that one. Statistics bear up what I’m saying. So out of that list of ten, the dean would say, “Okay, I’m going to give you these three,” and there would be a woman in there. So suddenly all these women got hired, and as I’ve said, once you get a certain level of female faculty in the department, the whole place changes. I think that is a major factor. The current chair is wonderful. I mean, it’s a really… Places change. It could be that wherever I was 20 years ago I would have had the same experience. I think the whole world is changing.
In what ways has the past year-- We’re just right after the conviction of the murderer of George Floyd, and this obviously has prompted a moment of self-reckoning in STEM more generally. In what ways do you think the physics community will respond positively to this emphasis on diversity and inclusivity over the long term, that this really is a structural shift and not just a moment in history that the community might move beyond?
It is unbelievable, the change I’ve seen in the last year or two. Diversity has become a real focus, so all of a sudden people are really paying attention to potential junior faculty who are women and minorities and really making an effort to evaluate, is this somebody that we want? Really special attention paid to this issue. It’s really a sea change, and I think that’s everywhere and I think it’s here to stay. So it’s really encouraging. It’s wonderful.
It will be good for the science as well.
Well, sure! The more broad perspectives that you have, the better.
What did it feel like when you were named to the Academy?
Yippee! [Laughs] You know, I’m not as good on…you know, when you write this up. I’m much better, I think, if you do the visual and my personality.
Oh, it will come through. It will come through. I’m not concerned about that.
I was thrilled. What can I say? I was really honored and thrilled.
I’m asking specifically because after a career where being validated by your peers at every career stage was challenging for you, in some ways--
Okay, wait a minute. That’s not true. Validated… I always, when I went… In my field, I was always validated. I hope that comes through. Every time I’d go to conferences, I’d give a talk, people would get excited. They would like what I did.
Allow me to clarify. I don’t mean--
My work was validated all along.
Let me clarify. I don’t mean scientifically. I mean administratively, like in the way that you didn’t get the raises that you should have at Michigan, for example. That’s the point. So my question is in what ways is being a member of the Academy…does it fill a gap?
You know, I’m going to remove the bit about the raises anyway, so I don’t think this… Yeah. I’m afraid. Sorry.
I’m really, really happy to be a member. It’s a huge thrill and…yeah. I don’t know. I don’t know what else to say.
Is it just an honor or is it also an opportunity to advance your science?
You know, I just, two days ago, gave my ten-minute welcome speech, so we shall see. I can’t answer that. But I’m obviously going to meet some really interesting people, and whenever you network and meet interesting people, new ideas come up. I also would hope that the recognition… I don’t know. Yeah, I don’t know yet, you know?
Katie, since we’re right up to the present, for the last part of our talk, I want to ask a few broadly retrospective questions. We’ll end looking to the future. So first, of all of your collaborations, of all of your research endeavors that you’ve been a part of, what’s been the most fun? Is there something that sticks out in your mind as the most fun project to work on?
I’d say that that would have to do with the people that I’ve worked with where there are more ideas going around, and so I’ve had a couple of people that I’ve loved working with. I guess what I really love is when you have a flash of inspiration, you see something and you’re like, “Let’s try that!” I’ve had that happen a couple of times and I’ve tried to tell you about the times it has happened—not all of them, but some of them. So I’m not going to pick out one, no. Having an idea is just… That’s such a thrill, thinking of something, and like I said, nine out of ten times it doesn’t go anywhere, but it’s just a thrill when it does. I think I’ve told you the different periods in my life where that happened, and so--
Conversely, those nine out of ten times when you hit a wall, what’s been most frustrating? What’s something that you thought you were initially optimistic about that remains so difficult for a breakthrough?
Well, obviously the dark matter problem. I really thought that we’d have this figured out by now, so that’s really… That’s a disappointment.
And this gnaws at you--
That’s a big disappointment!!
This gnaws at you more than dark energy?
Oh, yeah. Dark energy, I don’t think we have a clue. We don’t know what the hell is going on. Dark matter, we know it exists. We know there is something that has mass that’s pulling on stuff. It’s really… I don’t think there’s any way around it, and so it is a problem that we can attack, that we can solve. Dark energy is too hard. I mean, will I have the brilliant insight to solve this tomorrow? I don’t think so. I don’t know what it’s going to take.
Is it a theoretical breakthrough or is it an observational breakthrough for dark matter, do you think?
Oh, for dark matter? Yeah, I think an experiment has to find it. Yeah. Experimental.
Katie, one of the values of these oral histories is that an outsider just looking at your CV might think that it all came easy to you. How else could you have possibly produced this scholarly body of work? So I want to ask in the way that things have not been easy for you, have you put that to productive use? In other words, do you take that negativity and turn it around into your scholarship, or is your productivity, are your accomplishments despite those difficulties?
Here’s one I turned around. I wanted to become the director of the Michigan Center for Theoretical Physics. I deserved to be. It had been around for ten years. I had been one of the people writing the bylaws, running it at an associate director level. But then I was passed over. So then when Sweden asked me, “Would you like to apply to be director of Nordita in Stockholm?” Nordita is one of the premier institutes for theoretical physics in the entire world. I thought… I had been willing to sacrifice, to do anything to run that Michigan thing, okay? But when I didn’t get it anyhow, I thought, “Okay. Maybe I should apply to Stockholm.” So I wouldn’t have done that if they’d… If they’d done the right thing in Michigan, I wouldn’t have had to go be a big shot. [Laughs]
It really helped my career to go to Sweden, and my god, this group that I’ve been telling you about, I mean these post-docs and students—I’ve never… I mean, it’s been phenomenal. One of the students is now a post-doc at Stanford. The other student is now the Kavli-Newton Fellow at Cambridge. The third student is a CMB guy and he’s at Princeton for three years followed by two at the CCA in New York. So I mean, these people that I have been able to work with and mentor have been just incredible, and none of that would have happened if I’d stayed in Michigan. So it really…
I was kind of forced to think bigger, and that was really good for me. I guess that’s what I would say was a plus of the whole thing, that… I think a lot of people just stay where they are and they keep doing their projects and that’s great. I had to travel all along to work with post-docs elsewhere. I had a sabbatical at Columbia University and I was in the same room, their giant room on the ninth floor with Will Kinney and Richard Easther. Later on, Justin Khoury was in that room. I can’t remember who else, but I got to know… Really, these are the best people, you know? Will and I have written lots of papers together on inflation. One of the more fun collaborators I’ve had is Will Kinney. So I would leave Michigan because I wasn’t happy there and then that would take me to other places where I would start other collaborations. That was really good for me.
Was the Swedish approach to gender relations refreshing for you?
Absolutely. Yes. Yeah, absolutely.
It’s not difficult to be a woman in that environment.
Yeah. I mean it’s… Oh god. These are such big questions. Let me tell you… As I said, I had no maternity leave. Places in the US now all do, but nobody has what Sweden has!
Both parents get…I don’t know if it’s between six months and a year purely maternity and paternity leave. One of my post-docs over there just went on paternity leave with his six-month-old daughter, six months of no work fully paid. Wow!
That’s pretty good!
Wow! I could have used that, you know, the six months. So that’s wonderful that they have that. On the other hand, they don’t succeed… The way they do the faculty hiring, they never hire any women into the physics department, so that system is flawed. It’s a very weird system. You don’t even get letters of recommendation. You just get two people that write up what they think of the candidates. So that’s a downside, but on the whole it is a very gender-friendly nation. Yes.
Katie, your publication list really… You look at it… Within cosmology and astrophysics, the question is what haven’t you worked on, right?
So my question is, is there some overall explanation for how you pick any given project? Is there something that ties it all together?
Yeah, I guess… When I give colloquia, I always start by saying, “Over the turn of the…” Well, let me back up. A hundred years ago we didn’t know anything and then along comes Albert Einstein and general relativity. Look where we’ve come in a hundred years! We’ve got the Big Bang. We’ve got, since the turn of the millennium, the shape of the universe, the age of the universe. We’ve learned so much, but there are two remaining big questions: what is the universe made of? How did it begin? So I guess those would be the two…for me, the focus of what I’d like to know. So how did it begin? Well, that is an endless question, but I do work on inflation. I’ve tried to figure that out. I’ve also had a model called the phantom bounce with Will Kinney where we had a bouncing universe. In another paper I’ve got to mention, Will Kinney and I also worked on the fate of life in the universe. We’ve worked on both the beginning and end of the universe. What does that even mean? And what is the universe made of? I mean, the fact that we don’t know 95% of the content, that’s… What a fun project to work on, you know?
And when you focus your question on where did it all begin, are you looking at t = 0 or t = epsilon?
Well, what does t = 0 mean? [Laughs] You know, I do what I can… These inflation products that I do or -- The bounce idea, you would be going on into the infinite past and infinite future potentially, so I’d like to do what I can, or if you believe… I don’t work on string theory, but they’re certainly trying to understand quantum gravity and what are the consequences of that for cosmology, so as much as we can as far as going back.
From all that you’ve learned, would you guess that we are in a multiverse or not?
I am not a fan of the multiverse. I think that’s philosophy more than science. I do understand that string theory has 10500 vacua. By the way, in chain inflation, we use a field that tunnels from one minimum to the next and we are using a few hundred or a thousand of the vacua, so I don’t think that’s a problem. [Laughs] But the idea of the multiverse that there could be all these different types of universes out there and that we happen to live in the one we do because otherwise we couldn’t live at all (anthropic principle), I’m not a fan of that. I think we have to understand the fundamental constants, why they take the values they do. We have to understand the cosmological constant, why it is what it is. I’d like a dynamical solution to all the problems that we have. I don’t want to resort to either averaging over all the other universes or just saying, “Oh, we couldn’t live anywhere else anyhow.” I find that very unsatisfying. Maybe there are other universes beyond ours that we can’t contact, that we never will be able to contact. Well, in that case I’m not interested. I’d like to work on what we can probe, and if someday in a couple hundred years people take wormholes to other universes, okay, they can worry about it. [Laughter]
Katie, for my last question, given the way you answered mine about the diversity and the breadth of your research agenda so far, you answered it by referring to the fundamental questions.
So with that in mind, as you look to the future of your career, however long you want to remain active, what are you most optimistic about in terms of achieving--
20 more years.
20 more years?
Okay, great! So in 20 years, what are you most optimistic about in terms of achieving concrete answers to these most existential fundamental questions in physics?
Could we please find the dark matter??? And could some variant of natural inflation please be correct? And could we please find dark stars? [Chuckles]
Given all the excitement--
The ideas that I had decades ago are testable and those tests are coming up, and so that’s something that I really look forward to.
And in 20 years we’ll have a lot more data than we have now.
Oh, yes. Absolutely. Certainly on finding dark stars, that data is going to be… There could be some data on that soon. James Webb Space Telescope. And inflation, that data… That’s why I joined these CMB experiments is we’re going somewhere with that. Dark matter, I don’t know. That’s really… That’s tough. Apparently it’s a harder problem than any of us bargained for. It’s a 90-year-old problem at this point! I want to say something interesting. I always like to tell people about this. One thing I learned in Sweden is that Knut Lundmark, a Swedish guy, was onto dark matter in 1930 already.
He noticed the same thing, that things are moving too rapidly around the centers of clusters and so on and that dark matter could be a resolution to that. He was so unpopular that they kicked him out of Stockholm and sent him off to Lund, which ironically is a great place to do particle physics now, but at the time it was a backwater.
[Laughs] Who knows what will happen in 90 years?
Yeah. 90 years and we still don’t know what it is. That’s just… That’s weird. So then there’s always… Dark matter I’m really convinced is there, but dark energy—you know, you start to wonder. Is this like epicycles? You know what I’m talking about? That was the theory of Ptolemy that lasted a thousand years. They thought the Earth is at the center of what we now call the solar system, but, well, the data weren’t matching very well. So if you want to have other things going around us, then they had to have things in circular orbits around us and then other things in circular orbits around them and dadadada. The whole thing sort of fell apart; it didn’t work very well. Then there was a breakthrough by Copernicus, which is that if you put the sun at the center, then the math becomes much simpler. You can work in whatever reference frame you want. You can work in the Earth’s reference frame, but it’s pretty stupid because we’re all moving around the sun.
So it is possible that there’s some shift in world view that we’ve got coming to explain dark energy, which is what I was looking for. Get rid of dark energy; just have matter and electromagnetic radiation and change the equations. That’s what I was looking for, but since then people have done a lot more sophisticated work on that. But most of those models are ruled out because of LIGO, actually, because gravitons seem to move at more or less the speed of light. So that ruled out most of these models that modify Einstein’s equations. So do we need some radical new thinking? Maybe, yeah. We need an Einstein! [Chuckles]
Well, we’ll check back in 20 years. We’ll see what we’ve learned since.
Katie, thank you so much for doing this, for clearing your precious schedule to spend this time with me. I’m so appreciative that we were able to do this.