Katie Mack

Zierler:

Okay. This is David Zierler, oral historian for the American Institute of Physics. It is October 5th, 2020. I am so happy to be here with Professor Katherine J. Mack. Katie, thanks so much for joining me today.

Mack:

Thanks for having me.

Zierler:

Okay, so to start, would you please tell me your title and institutional affiliation?

Mack:

Yeah. I'm an assistant professor of physics at North Carolina State University, and I'm also a member of the Leadership in Public Science Cluster, which is an initiative to connect the public with science in many different ways.

Zierler:

I can't help but point out also that in the field, both your twitter account and your website are institutions as well to some degree. Can you talk a little bit about your website, AstroKatie?

Mack:

Yeah, so my website, astrokatie.com, is just a sort of collection of where you can find my stuff online, and I put a bunch of my writings and videos and things like that on there. I should probably update it more often. (both laugh) It's useful because I have an institutional website, at NC State, that talks a little bit about my research, but because I do so much public stuff, it's very useful to also have a separate place that really brings together all of the science communication that I've done in various ways.

Zierler:

Katie, what are some of the challenges in conveying complex scientific concepts over Twitter?

Mack:

Twitter is a very unique space. The challenges in Twitter, there are a couple of things. One is obviously the brevity, right? For each tweet, you have 280 characters, and that's not very much. But you do also have the option of embedding photos and videos. You have the option of putting it in links. So it's not as limiting. The space is not as limiting as you might think, and you can also do threaded tweets. But the main thing is that you have to be able to be very, very concise and clear, because if you're not, then people will right away know or get the wrong idea—and that can be a big mess—or be confused and then reply, "What does this mean, what does that mean?" And so there's a strong incentive to be as clear as possible, as concise as possible, and that's just not the way that academics are usually trained to write.

Zierler:

Right. (both laugh)

Mack:

So it's definitely a very different medium from that perspective. And it's also a very, very interactive one, and that's the best thing about it, which is that you have immediate engagement. You know, if you say something and people understand it, then you have immediate feedback of people saying, "Wow, that's really interesting." Or, "Oh, I never thought that before, this is great." Or they have follow-up questions, and you can tell people are really digging into the topic. And if you say something and they don't understand it, then you immediately get all the questions, and so you find out what it is that people have misconceptions about, and you find out how to better-express that idea. Which is very, very helpful. Or you know, sometimes you get people being very angry about something, and so you have to, you know, you learn what sets people off in various ways. So that immediate interaction is very, very helpful if you want to learn how to talk to people who are not scientists in an effective way. In a way that's clear and concise and understandable and hits the right emotional tone. That's something that you also get feedback on very quickly. So, I think it's great. And it's also great as a sort of community forum, where it's not just about broadcasting, you know. You also listen to what people say, and you hear what people are interested in and what people care about and are excited about and you learn different perspectives and you are able to connect with people you might not otherwise see or interact with, and get a lot of different viewpoints. And that's especially important if you want to reach out to a broad audience with diverse backgrounds and so on. If you follow a lot of people who have different backgrounds from yours, then you learn a lot about those communities and what matters to them.

Zierler:

Sort of a broad sociology of academia question. Given how important Twitter and social media is in terms of developing an academic persona, and all of the effort that's put into it, do you think at some point Twitter will be something that will be considered among an overall tenure decision in terms of academic output? Or do you see that as, you know, it's totally an extracurricular activity and it's something that you do outside of your duties as a professor?

Mack:

Well, I think that there's certainly a growing understanding that it's a form of science communication, or it can be used as a form of science communication. So my hope is that it's already being considered in the way that writing books or op-eds or whatever could be considered, which is that it's a form of public engagement that is important for scientists to do sometimes. In my case, the position I have at NC State explicitly includes public engagement as part of my agreement. And so it is part of my tenure case. It's in my dossier. I haven't applied for tenure yet, but I've just gone through the reappointment process, and my Twitter presence was a big part of that. So this is not entirely a theoretical question. But I think that it's a form of communication like any other medium, and I think that academia is starting to recognize the importance of different kinds of media reaching different audiences in different ways. And so I do hope that people who find ways to connect outside of academia in whatever way, will have that effort recognized.

Zierler:

One of the big narrative lines of 2020 of course is how important it is for scientists to communicate to the public. And although your field of expertise is far away from the coronavirus, I wonder if you see your work in those sort of broader terms of the importance of being able to communicate clearly, how scientists work, how science works, and how important it is for there to be that level engagement with the broader public?

Mack:

Yeah, I do think that's super important. And that's a big part of what I do online. So one of the things that I do on Twitter is I talk about the latest spacecraft or result or whatever's going on in my academic field. But I also talk about just things that I care about as a person. I talk about academia and what it's like to be an academic, what my life is like, how we interact with each other, how we communicate, how we come to conclusions about the world. And part of the reason I do that is that it's part of a broader strategy to help people understand how scientists work, who we are, what we think about, what we care about. The fact that we are people with concerns outside of our academic discipline. The purpose of all that is really to help people have more trust in scientists, or at least just understand our motivations and to understand the motivations of the scientific process and scientific institutions. Because I think that when it comes to situations in which people don't trust scientists, a lot of that is because of a lack of understanding of why we do what we do and how we do what we do. And so even though I'm not working in a field that has any public health or national security importance, whatever, I can be a part of the process of demystifying the institution of science for the general public, in a way that can help to, you know, allow people to have appropriate context for what scientists are doing.

And you know, I don't think that... I don't want to pitch it as, "Everybody should just believe scientists no matter what and just do whatever the scientist says." I also try to be open about the ways in which science has been used to harm people and oppress. That's a big part of our history as scientists, and we need to be aware of that and open about that, especially because a lot of the communities we might be trying to talk to are certainly aware of that, and that factors into how they think about science as an institution and scientists as people. And so acknowledging all of those aspects of science and how it's been used and who we are is really important. And it helps to have the kind of dialogue we need to have with the public.

Zierler:

And the idea of, you know, pushing against the idea that society should just believe the scientists. Embedded in there is the assumption that scientists always believe in each other and they never disagree.

Mack:

Yeah.

Zierler:

And of course a lot of your work demonstrates that there are fundamental disagreements, you know, among scientists about how the universe works, and that's certainly part of that story as well.

Mack:

Sure, yeah, yeah. Absolutely. Yeah and also I think that, I mean, coming back to the scientific disagreement as a topic, I think that it's also really important to make it clear how that process works, that it's not just everybody believes A, and then someone comes along and says B, and then everybody believes B. It's, you know, the fact that science is a very incremental thing, and we don't throw away everything Isaac Newton wrote as soon as we have Einstein's relativity. These things interact with each other and are refinements or tweaks, or sometimes we find we were actually totally wrong about something, but usually not. And so the kinds of disagreements we have are not like, "Oh, everything was wrong and we have to start all over from scratch." It really is a matter of sort of building context and altering the sort of edges of our understanding. And I think that's an important thing, because a lot of times when people don't "believe" science, what's happening is that there's something about some institution or result that doesn't align with the way they think about the world or with their interests. And they say, "Well, you know, science doesn't know everything. They used to believe the world is flat; maybe all this is wrong too." And I think that kind of misunderstanding about how we refine our theories, how we resolve disagreements, can be very misleading to people and allow people to think, "Oh, well maybe absolutely everything that the scientists are saying is totally wrong." It's not. (laughs) So you know, having that be clear I think is a very important thing.

Zierler:

Well, Katie, let's take it all the way back to the beginning. Let's start first with your parents. Tell me a little bit about them and where they're from.

Mack:

So my mom is a... So she's had a very varied career in a lot of different fields and all of them connected to science. So she started out in psychology and then got a degree in nursing and became a PhD oncology nurse for a number of years. And then got into... Started working for a drug company and got into regulatory affairs, so she became a lawyer. And--

Zierler:

Wow.

Mack:

And so did some, practiced law for a while on the side. And...

Zierler:

Where is she from? Where did she grow up?

Mack:

She grew up all over, because her father was in the military. In the Navy, and so she kind of bounced around the world. So she was in Sweden for a while, she was somewhere in the U.S. on the East Coast for a while, she's kind of just been everywhere. They used to move every three years, I think. But yeah and then after she got her law degree, she became a tenure-track professor teaching health sciences and health law and stuff like that. And ethics and medical ethics and so on. And she's just retired from being a professor, but she's still teaching classes and now is considering going into journalism, so (both laugh) she's, you know, has a big range of interests. And she's really one of the people who got me involved in science, because -- Well, she was always interested in science fiction when I was growing up, and so I sort of got my love of science fiction through her, I think. And then she's always been interested in physics and astronomy as well. And used to take me to public talks about time travel and stuff like that. And black holes or whatever. And took me to the observatory for meteor showers and things like that. So she's definitely been a big influence and a role model for me, in terms of science and also just in terms of academia. Like, I knew a lot about how academia worked before I started college, because she was studying for her PhD while I was in high school. So I got a good first-hand view of how that whole system works.

Zierler:

And where did you grow up?

Mack:

I grew up in Los Angeles-- Well, Long Beach, California. So the LA area.

Zierler:

College, I mean public school throughout, up until college?

Mack:

Yeah, but I went to magnet schools. So when I was in fourth and fifth grade, I was at a performing arts school. And then when I was in high school, I was at a math and science school. So they were public, you know, I didn't pay for school, but they were places you had to apply and they had a different population of students. So they were, yeah, they were kind of different kinds of schools, but all through the sort of Long Beach Unified School District kind of system. And then I went to college at Caltech.

Zierler:

Katie, were you interested in science before you figured out you were good at it academically?

Mack:

Yeah, I mean I've always been interested in science. I think I grew up being the kind of kid who always wanted to figure things out, and take things apart, and I was always taking apart the remote control, or toy cars, or whatever, trying to figure out how it all fit together. And I had a little toy microscope for a while when I was a kid and I was trying to see what everything was made of, and then I would play with little electric motors and magnets and whatever I could get my hands on, you know? So I was always really into that, and when I started reading about physics, reading about things like black holes and spacetime and the Big Bang, that really, really caught my interest. So I remember reading A Brief History of Time and being really fascinated by the idea of all the sort of weird, mind-bendy stuff. And that's when I decided I wanted to get into cosmology. I think I was ten years old or something when I made that connection.

Zierler:

So when you were thinking about colleges, you were thinking specifically physics programs? Even before you started?

Mack:

Oh yeah. I was thinking specifically Caltech, honestly, because my grandfather on my mom's side had been to Caltech. So he was in the Navy and he did meteorology. He was a meteorologist in the Navy and so he did his meteorology study at Caltech during World War II, I think. And so he had all these stories about all the nerds at Caltech and (laughs) and the geniuses and these weird things that people did, and so I knew about Caltech and I had gone to public lectures at Caltech. Stephen Hawking used to visit there a lot, and so I would go see him speak there. And so to me, Caltech was this, you know, this amazing sort of nirvana of science and research and I wanted to go there.

Zierler:

Given how well-developed your ideas were, specifically with cosmology, how did you go about setting up your course of study in terms of, there's astrophysics, there's cosmology, there's astronomy. How did you sort of maximize your coursework toward your goals?

Mack:

Well, I mean I was a physics major at Caltech, and the track is pretty sort of straight-forward, for the physics major there. I knew I wanted to get a degree in physics. Caltech has an astronomy degree as well, but I wanted physics because I thought it would be more versatile, because I'd also done some work in particle experiment at the time, and I knew I liked particle physics as well as cosmology, and so I wanted to make sure that I was kind of best-placed to do anything in that sort of area. And so mostly it was just a matter of trying desperately to pass all my classes, right? Caltech is really hard, actually. (laughs) So I took a couple of astronomy electives as a physics major, but mostly I was just going through the ordinary physics track and just trying to do well with it.

Zierler:

Now, given your interests as a tinkerer, growing up, I wonder if that influenced your ideas about whether to focus on experimentation or theory?

Mack:

Well, a lot of my earlier experience was with experiment. I was very fortunate to have the opportunity to work at a neutrino detector while I was still in high school. So my high school, this little math and science magnet school, was on the campus of Cal State Dominguez Hills, and at that university there was a professor who was involved with the Super-Kamiokande neutrino detector. Professor Ken Genezer. And he had some funding to send undergrads to the detector, and I was-- I kind of sort of talked my way into his lab as a high school summer research assistant. And he decided that he didn't have any undergrads who were interested in going to work at the detector, and I was close enough, so he was able to send me to work at the detector for a couple of weeks in the summer while I was still a high school student. And so I got a lot of experience with my involvement in Super-K and also the long baseline experiment K2K, which I worked on a couple summers later with, you know, laboratory science on a big particle physics experiment.

And then later on, when I started at Caltech, I did a summer research project in a nuclear physics lab with lasers and all of that. And I enjoyed that but I think what really drew me into theory instead of experiment was that I also, I liked the idea of being able to just sort of speculate wildly and do whatever—you know, work on different subjects that drew my interest, rather than being sort of concentrated on one experiment over a long term. Because with experimental science, you have to kind of invest in a project for a long period, and there's a lot of work to just develop that experiment over the course of a long period of time, where you don't have results yet and you're kind of just refining things, and then eventually there will hopefully be a big payoff, but in the meantime it's a lot of very important precision work to get everything put together. And I kind of liked the idea of flights of fancy that you could do with theory, where you can really sort of be very creative and have very, very broad interests, and get, you know, sort of... I like the way that within theory, you can take very disparate, different topics and synthesize new ideas from having a very, very broad overview of a topic.

And I like that kind of thinking. I like that kind of like creative synthesis kind of thinking. So I think that's what made theory more appealing to me, the way that you can have your sort of creative epiphanies that involve, oh, you know, “I heard a talk about radio sources and radio telescopes the other day, and now I'm thinking about cosmic strings and actually that there's this connection that you can make and you can actually get a good interesting result by putting these things together in a way that hasn't been done before.” And that kind of making-connections thinking is really fun for me.

Zierler:

Did you think of yourself as a cosmologist, or as somebody who wanted to go into cosmology specifically by the time you were wrapping up at Caltech?

Mack:

Yeah, I think so. I had done a summer research project in theoretical astrophysics at that point, and I enjoyed that. But yeah, I think I wanted to go into cosmology, and when I was thinking about grad schools, I applied to grad schools in physics and astronomy, and one of the reasons I wanted to go to an astrophysics program specifically rather than a sort of general physics program was because I wanted to make sure that I got all of the necessary astrophysics knowledge that you get when you do a program specifically in astrophysics. And then I would kind of... I wanted to make sure that I didn't miss out on understanding how stars work, or what the interstellar medium is made of, because I knew that the theoretical physics was going to be something I would pursue either way, but there were, I knew there were things about astrophysics that I needed to know, that I didn't know how they might be useful, and would be a lot harder to learn separately, so I wanted to make sure I had a really good astrophysics grounding in my program, and then if I needed to brush up on general relativity or something, I would be able to do that from a textbook more easily, I think.

Zierler:

What were some of the big trends, or most exciting questions, in cosmology during your time as an undergraduate?

Mack:

Well, gravitational waves were just starting to be exciting when I was in undergrad. LIGO was being built.

Zierler:

Did you have interactions with people like Kip Thorne or Barry Barish at that time?

Mack:

Yeah, yeah. I interacted with Kip a little bit. And Sterl Phinney, who was one of the people who envisioned the LISA gravitational wave observatory, the space-based one that hasn't yet been built but hopefully will be. So he talked a lot about that when I was in undergrad there. He was my undergrad advisor, so gravitational waves were just a really big deal, and a lot of the LIGO technological development was happening on campus. And so I would sit in on some of those talks and I very briefly sort of did a little bit of work for the local LIGO test bed on campus. So that was definitely one of the big things that was exciting at the time. And CMB experiments were getting really interesting then as well, so WMAP was launched, I guess somewhere around the time I graduated, but the BOOMERANG experiment was going on while I was at Caltech, and led by some people there. And yeah, there was a lot of really interesting stuff about gamma ray bursts happening at the time as well. That was still fairly new and exciting. And one of the big GRB people was also at Caltech, so yeah, it was a very exciting time. And I was also able to be involved with some of the early thinking about 21cm astronomy, which has become a really important thing for looking at the epic of reionization and the first stars and galaxies, and so people were very interested in the prospects for that at the time as well.

Zierler:

Katie, earlier on, Caltech had a less-than-ideal track record, especially in physics in terms of the experiences of women. I'm wondering during your time there, if you experienced, you know, anything ranging from overt to microaggressions, or anything that would suggest that it was not the most welcoming place for women? Or if that history had sort of improved and was essentially gone by the time you were there?

Mack:

You know, it was interesting. Growing up, most of my hobbies were the kinds of things that were pretty male-dominated. So I mean for example when I was 13 years old, I was the only girl in the flag football league. I was kind of just really used to being around men all the time, like that was... So the fact that when I was at Caltech, the male: female ratio was about 2:1, was not the most extreme environment I've been in by any means. And even in my physics classes, the fact that very few of us were women just didn't feel like a big deal to me because I was used to it. And that doesn't mean that it wasn't a problem. But it was also something that I just didn't feel I had a lot of awareness of, or didn't pay all that much attention to in some sense. It was definitely, the ratio, people would talk a lot about the ratio. They would call it The Ratio, you know. The Male: Female Ratio. It was a big part of undergraduate life that people would complain about that, because this was a small campus. There were something like 900 undergraduates. And so everybody knew everybody, and the fact that there were way more men than women was a strain on how people interacted in various ways. And you know, women would complain about being hounded by groups of men-- Just, not in usually a sort of explicitly harass-y kind of way, but just like there were always men around. And women would complain about that. And so it was certainly something that was a strain on the social fabric of Caltech. But it was also just something I just didn't really think that much about, honestly, at the time. And part of that was that I didn't have any really big problems. I wasn't harassed in any way that I noticed. And given that it wasn't directly massively affecting me in a way that I could see, and I was just so busy working on my classes and trying to stay afloat, it just wasn't something that was really so much a part of my conscious experience.

Zierler:

Yeah, well that's great to hear. (laughs) That really is great to hear.

Mack:

I mean, it's not so much the… it would have been good if I'd had some awareness, because certainly other people did have problems, and I just didn't think about it. I didn't pay attention to it. It's not great to say, like, "Oh, I got lucky and therefore it's fine." I mean, a lot of women had really big problems, and I just didn't know. You know? I wasn't paying attention.

Zierler:

So you're careful to distinguish between your experiences and the broader reality?

Mack:

Yeah. Yeah, yeah. And there's nothing... I don't think there's anything good about being oblivious to other peoples' suffering and challenges and so on, right? I think that, again, I think I was lucky to not have experienced any really bad things in that regard, but that doesn't mean that it's good that I wasn't aware of it.

Zierler:

Did you think about staying at Caltech for your PhD? Were there any specific pieces of advice you got about whether that was advisable or not?

Mack:

Everybody advised you have to go somewhere else for grad school.

Zierler:

Yeah, yeah.

Mack:

So it didn't even occur to me. I didn't apply to Caltech for grad school. I knew I would end up somewhere else, so I applied to a bunch of different places, and Princeton seemed to be the place that had sort of the best cosmology in terms of the people who were working in that field and they had a really good program as far as I could tell, so I ended up at Princeton. I actually got in on the waiting list. I wasn't the first selection there, but you know, I took it.

Zierler:

Did you know specifically that you wanted to work with Paul Steinhardt?

Mack:

No, I didn't know. I knew of several prominent people. I mean, Princeton had just a huge list of amazing researchers in astrophysics and cosmology and theoretical physics, and I didn't know I'd work with Paul, but I was very happy to end up in his group in the end. I worked with a few different people. I worked with Jerry Ostriker when I first got there. Then I worked a little bit with Bohdan Paczynski, although we didn't end up writing up anything from that. And I talked a lot with David Spergel. He was sort of an informal mentor of mine. But I ended up doing my thesis with Paul, and I was very happy that he was willing to have me in his research group, and I just learned a lot from being part of his group.

Zierler:

Yeah. What was Paul working on? I know that with Paul, that's a dangerous question because of all the things that he could be working on at any given time. Not even in cosmology, for example.

Mack:

Yeah, yeah.

Zierler:

What was he working on in cosmology when you got to know him?

Mack:

He was working on some ideas around inflation. He was developing his ekpyrotic cosmology at the time when I first got to know him, so he and Neil Turok were working on putting together the first sort of formulation of ekpyrotic cosmology. And he was thinking about axions, which is what I ended up working on with him. So we worked on different ideas about axions and their role in early universe physics.

Zierler:

What was the requirement in terms of coursework at Princeton? How much did you need to sort of be in the courses, as opposed to, you know, do your own independent study?

Mack:

So at Princeton, the first two years are largely coursework, and then you rotate around doing a few different research projects in the first two years. But the coursework was things like the interstellar medium, stellar structure, cosmology, galaxies, I think those were the main... Maybe those were the main things. Yeah, I have a list somewhere. And so you take these sort of core courses, and then there's a general exam, which is an oral exam at the end of two years, that covers the topics from all of those courses, and sets you up for candidacy for doing your PhD. So yeah, you kind of have to learn all of astrophysics in the first couple of years and have a really good grounding in those things. And then I also took another cosmology course that Paul Steinhardt was teaching, and then there were courses in the physics department that I could have taken as well, but I ended up doing a slightly different thing, which is that I spent a year at Cambridge University sitting in on the Part III Maths course to get all the theoretical physics there.

Zierler:

What was your motivation in doing that?

Mack:

There were a few different things. One of them was, I knew I liked Cambridge a lot. I'd been there. I spent a semester there as an undergrad in an exchange program, and then I spent a couple months there between undergrad and grad school as a visiting summer researcher. I did a little summer research project there with Jerry Ostriker, who was visiting there, and also George Efstathiou. And so I'd already spent time at Cambridge, and I knew I liked that place a lot, and it seemed like a really efficient way to build up my theoretical physics background, because it's a year-long, very intense course, right? And I wasn't officially enrolled in the course. I was just an auditor. But I was able to sit in on all the classes. So I took quantum field theory and a course on symmetries in physics, black holes, what else did I take? Supersymmetry. Of course particle stuff. Yeah just a big range of theoretical physics courses, and it was all sort of packed into one year. So I was able to get a good sort of lightning-overview of all of the tools of theoretical physics. That was a really good opportunity. And then also while I was there, I did some research with people there. And that was a useful bit of experience too.

Zierler:

And what an extraordinary level of exposure. I mean, between Caltech and then Princeton and Cambridge, this is quite an amazing level of exposure you have to, you know, physics working at the highest levels that is offered anywhere, essentially. That must have been something that you appreciated even at the time.

Mack:

Yeah, yeah. It was great to be connected to so many of the leaders in the field and to be in those incredibly intense environments. I found that very stimulating. Very sort of intellectually stimulating and motivating to be around so many incredibly productive and intense people. And to be in a place where everybody's just, you know, really, really working hard and really trying to be at the very top of the field. And then of course for my first postdoc, I went back to Cambridge and got to spend more time there, which was great.

Zierler:

So I'll ask this with the perspective of your postdoc in mind, but it's more of a cultural question than anything else. Did you see any different ways that physics was done institutionally or departmentally, you know, like the Caltech approach or the Princeton approach or the Cambridge approach? Did any of those differences sort of make an impact on you?

Mack:

Yeah, it was really interesting to see the differences in these three, all extremely... institutions. One of the biggest differences I noticed was the approach to work-life balance of these different places. So at Princeton, there was a certain subgroup of Princeton people who considered it to be a badge of honor to be sleeping in your office. And just never go home. And that was unheard of at Cambridge, you know? People would not be there on the weekends. They would not be there late at night. Like occasionally, you'd run into someone. But it was, people would go home, you know? And they would have hobbies. And it was encouraged that people should have hobbies. People would ask each other about their hobbies. And that to me felt like a revelation, because I was just not used to that. I was not used to the idea that it was good to have life outside of work. And, you know, it's not like people at Cambridge are less productive, right? (laughs) I think that it's a different approach. It's a different way of working and I like the idea that you can have some kind of balance, and that's a good thing. There was also, one thing I noticed a lot with the students, was that at Cambridge, there's an explicit focus on transferrable skills, where the students are expected to learn transferrable skills, meaning: skills that are useful outside of academia. And so there are workshops and things for certain kinds of programming or whatever. Like things that you won't necessarily use within an academic career. That's something that wasn't really discussed at Princeton or Caltech. At those places, it's like, well of course you're going to be an academic. And I think that that attitude is changing everywhere. I think that it's becoming more accepted in general that not every graduate student in physics or astronomy is going to become a professor in physics or astronomy. And we should, you know, pay attention to that in some ways. But I think that was an awareness that was already there in Cambridge before I saw it in other places.

And so that was an interesting thing. So in general, it just felt like, I don't know, people had a sort of healthier approach to the intersection between academia and life in Cambridge. But at the same time, Cambridge was way more hierarchical. If you don't have a high enough rank, you're not allowed to walk on the grass. Like, there's just these weird things. You know, they have the high table at the formal dinners. There was a definite... There was a hierarchy that exists in the UK system that is not so prominent in the US system. I mean, but still everybody still went by first names. So it's not... I don't know.

Zierler:

It's not Victorian.

Mack:

Yeah. Yeah, exactly.

Zierler:

Katie, on the question of skills, at what point, probably in graduate school I would assume, did you understand that you had this special talent for sort of being a public persona in science? That you could-- You know, it would be expected that if you're going to pursue an academic life, of course you're able to do the research and you're able to write the papers and collaborate on these big projects. But the unique skillset of, you know, being able to do these awesome presentations and engage with the community. The larger, broader public in social media and things like that. At what point did you understand that you had a talent in that area? Above and beyond what a professor is expected to do?

Mack:

Yeah. You know, it just started with writing. So I've always enjoyed writing. And all kinds of writing. When I was in high school, I used to write horrible, angsty poetry. (both laugh) For fun. And I always loved writing long emails and letters and things like that. And in college, I took a creative writing course and, you know, just did really well in with that kind of writing. And so I've always been somebody who loves written expression, you know? Using language in interesting ways, and so in grad school, I got some opportunities to do science writing through... I took a course on science journalism with Michael Lemonick, who's at Scientific American now as an editor. I took a course on that with him and had some practice writing about science in that context. I also wrote some stuff for Sky & Telescope, and American Scientist as a freelancer, and that was through, well, Sky & Telescope reached out to Princeton astrophysics and was like, "Hey guys. Somebody write something for us." They just wanted, they were trying to recruit people to do some freelance stuff for them through the department. So I had a little bit of experience doing writing about science. And even as an undergrad, I'd entered an essay into a science writing contest with Griffith Observatory and got an honorable mention. So I'd already had some experience just writing clearly and writing about science as something that I was trying to do that I liked to do. And I don't know, I think at some point I just realized that that was a skill that I had, that that was a talent that I had.

Zierler:

And how well did that flow into the art of public presentation? You know, just being able to strap on a microphone and really engage a big... I mean, we don't do this anymore. It seems like ancient history. But back in the days when we would get on stage and talk to a big community of people who are interested in-- Did you see that skill naturally flowing from your abilities as a writer? Or is that an entirely different art form?

Mack:

I think it's a similar thing. I think being able to make a clear argument and to explain things clearly, that was all part of the same expression. But not having stage fright, not being afraid to stand up in front of people and talk about science, I feel very fortunate that I was in that position. I mean, I went to a performing arts school in fourth and fifth grade, so I'd already had some experience just being on stage. I'd played a bunch of instruments, so I'd done performance like that. I had to be in some plays and things in school. And then in high school and in college, a lot of courses involved giving presentations to the class. And in grad school, we had a lot of practice giving talks as part of the curriculum. So I'd had experience just standing in front of people. And at some point I just realized that it didn't scare me as much as it scared other people. And maybe it was because I had a lot of practice with it. But public speaking has just not ever really been a big fear of mine. And so between the fact that I wasn't scared to get up on stage, and the fact that I had this practice with communicating science and trying to explain things clearly, I think that just kind of all flowed together. And turns out I like it. (laughs) I really enjoy standing up in front of people and talking about science. And I enjoy performance in general. When I was in Cambridge, I joined a samba band. And I played a little drum (laughs) in these performances around town. That kind of thing was fun for me. So I think that that made it a lot easier to do that kind of work. To do presentations and to be in the spotlight in some way. And then, you know, really most of those opportunities have come through the fact that I'm known on Twitter and known as a writer. And so having the ability to write about science clearly was, sort of kicked off all of that. So I don't know, it kind of all comes together as being able to write, not being afraid to stand up on stage. Kind of just naturally sort of put me in that position of being the person to be a public face in some way. It's still weird to think about, though. (both laugh) It still is strange.

Zierler:

Katie, cutting back to Princeton and as sort of an entree to a question about Paul's style as a graduate advisor. When you were going about the process of developing ideas for a dissertation, how closely were your research interests aligned with what Paul was doing? In other words, did he essentially give you a problem to work on that was related to his research, or did you come up with your ideas on your own and you bounced them off of Paul and that's how it developed?

Mack:

So I had already done a few things that were kind of my own ideas. So that year that I spent in Cambridge, I worked on a project about cosmic strings that I developed with people who were not at Princeton. So I'd already been kind of thinking about a few ideas that were outside of what people at Princeton were doing. When I started working with Paul, it was really that he was interested in axions and he had this idea about axions and I just went with that. So I did--

Zierler:

What was that idea? Do you remember what was so interesting about axions at the time?

Mack:

Yeah, yeah. Well, he was interested in the question of axions at the string scale. So there was this... Well, so the standard QCD axion was starting to get sort of narrowed into a small box of parameter space and it wasn't clear if that was going to be totally ruled out or not. But it turned out that a lot of the string theory people were talking about the so-called string-scale axions, which are in a totally different part of the parameter space, but require sort of special initial conditions to not be ruled out by cosmology immediately. And so the question that Paul had was, if those special initial conditions were really consistent with what we know about inflation, and how the kind of fine-tuning that you need to get those special initial conditions connects to what we know about inflation how that sort of-- How does that fit in to the big picture of inflation and string theory and axions, and what are the intersections between all those ideas? And so the project we worked on was thinking about how much do you really need to tweak all these ideas to get the string scale axion to work in cosmology? And is it even worth it? Like is it so fine-tuned that you may as well scrap it and go back to the problem that motivated the axions to begin with, the strong CP problem? So he sort of formulated this question about how these things fit together, and then I did the calculations to figure out what the scale of tuning was, and how to kind of compare that to what you have without the axion. So...

Zierler:

Katie, this makes me wonder if you spent any time at the Institute? If these were sort of questions that were, you know, there were seminars at the Institute that might have been useful to you.

Mack:

Yeah, I went to the Institute from time to time. And I had spoken before with a few of the people there. I talked to Peter Goldreich a bit about projects, although I never ended up working with him on anything, really. And we corresponded a bit with Ed Witten about axions while we were working on that project. So I went to the Institute from time to time. I found the Institute super intimidating. (laughs) So I didn't--

Zierler:

Getting up on the stage of thousands of people, no problem, but the Institute was a little much for you?

Mack:

Oh yeah, yeah. Yeah, because there were all these incredibly intense geniuses there. I don't know. (laughs)

Zierler:

Although you are, I mean, your graduate advisor is Paul Steinhardt, who's, you know, doesn't take a back seat to anybody.

Mack:

Yeah, yeah, no, I know, of course, of course. I think the thing is that the vibe at the Institute was a bit different. So, you know, so they'd have these lunch meetings where you'd go and they basically like interrogate you about your research and start with the assumption that your work wasn't interesting, and then go from there. (both laugh) And I never went to those lunches because I was afraid to go to those lunches, but there was a very strong sense of this sort of "trial by fire," if you go to the Institute. And I think a lot of that was driven by John Bahcall's style. So he passed away while I was at Princeton, but he had sort of set the tone of this super intensity and this slightly antagonistic attitude toward other people's science.

Zierler:

And this is a gender-neutral kind of observation? This would apply to everybody?

Mack:

Oh yeah, yeah, yeah. Yeah. I don't know if sexism was a factor or not. But it was… everybody was afraid of the Institute. One of the postdocs at Princeton called it the Snake Pit.

Zierler:

Yeah. Yeah.

Mack:

So that was my impression of the Institute (laughs), and it was sort of built up by the fact that it's this very stately looking kind of, everything's shiny and they have those fancy tea and cookies and just, all these rooms that look like a smoking room at a gentleman's club in London, like that kind of, you know... It was just a very, very intimidating place.

Zierler:

Yeah. I thought of the Institute because I was curious where, you know, after David Gross leaves Princeton, perhaps the Institute is more the place to be for string theory than the department.

Mack:

Yeah, I think most of the string theory was over at the Institute, but I wasn't doing string theory. I was kind of doing stuff with, you know, connected to string theory and with implications for string theory potentially, but I was never into the heavy theory. I was much more [focused on] phenomenology.

Zierler:

Yeah.

Mack:

So it wasn't, yeah, I don't know. It's funny, because I spent a week at the Institute last year, or I guess, yeah, a little over a year ago. And it was lovely. I was there as a guest of Nima Arkani-Hamed. And I'd go to the lunches and I'd go to the talks and everybody was really nice and welcoming. And it was great. (laughs) But I think yeah, for some reason, I had been totally scared of it when I was a grad student.

Zierler:

How did you finish off putting your dissertation topic together? How did it all come together for you?

Mack:

Well, you know, I knew I wanted to work with Paul basically, and this project sounded like a really interesting one, and so we talked about it and we put together a plan for what I would work on, and then kind of after the fact, we kind of figured out how all of my previous research fits into kind of a similar area, and then put the dissertation together as through sort of kind of arguing that those were all parts of the same theme. But it was just like any research project, really. Just trying to come up with an interesting result, and I would meet with Paul a couple times a week and talk about where things had progressed and just kind of grinded away at it.

Zierler:

Who was on your committee besides Paul?

Mack:

I had a huge committee for complicated reasons. I don't know. But so Paul was on the committee, David Spergel, I think... So we had somebody from U Penn. I'm blanking on his name right now. So we had an external person. I should know who that is, and I can see his face in my mind-- Trodden, Mark Trodden.

Zierler:

Oh, Mark Trodden, sure. Yeah.

Mack:

Yeah. So he was on the committee. And a couple other people at Princeton were part of the committee just because they have big committees there. So Jeremy Goodman. Works on stars. I don't know. It's all a big haze now. It was a very sleep-deprived time of my life. (both laugh) But yeah, I had a large committee partially because I had kind of got into my thesis a little late, and I had been working with Jerry Ostriker before that, and so there were people sort of part of my committee from earlier. But anyway, it was a large group of people and I remember very clearly because when I had my thesis defense, there were five of them and they all had notes that they wanted to ask about on the thesis. They had all read it and (laughs) it was a grueling defense.

Zierler:

But that's a good sign. They were really interested in what you had done.

Mack:

Yeah. Yeah, no, it was great. But it was funny because I was not at all prepared for it, because I'd only seen thesis presentations in the physics department, not the astronomy department, and the physics department ones are very, very short. And so I thought like, "Oh, 20 minutes, I'll give my talk and then they'll ask a couple questions, and then that'll be it." And it went for like two hours.

Zierler:

Oh wow.

Mack:

(laughs) I was so exhausted.

Zierler:

They were probably like, they wanted to learn some stuff probably. (laughs)

Mack:

I mean, you know, it went well. They called me doctor at the end, so it's all fine.

Zierler:

Nice. Nice, that's always a good sign.

Mack:

Yeah.

Zierler:

Katie, to zoom out for a second, you know, maybe in retrospect if, as a graduate student you're so focused on your own research, but in cosmology at the time, what were some of the bigger questions? And as you were sort of developing your identity as a cosmologist, as a physicist, how did you see your dissertation being responsive to those bigger questions?

Mack:

So some of the big questions of the time, I mean one of them was about whether inflation happened.

Zierler:

Yeah.

Mack:

Right? And my thesis problem--

Zierler:

And we're talking, this is still like the Alan Guth inflation?

Mack:

Yeah.

Zierler:

We're talking about, yeah.

Mack:

Yeah, yeah. And my thesis project was partially looking at whether inflation is compatible with string theory and axions as, you know, can you fit all these things together? And one of the sort of conclusions of my project at the time, and I'm not sure if this would hold now, but the idea was you can have axions, inflation, or string theory, but you can only have two of those three because if you put all three together, they're not consistent with each other. You get too much of a fine-tuning problem.

Zierler:

What would be some of the theoretical underpinnings of that assertion? Where you can't have all three together?

Mack:

Well, the idea is that if the axion is a string scale axion, then it causes problems in the context of inflation. So you get too many isocurvature modes, or too much dark matter. So you have to tune the inflationary scale way, way down in order for that to work, and then you get this big fine-tuning problem. So if you don't have axions, then you don't have a problem. If you don't have inflation, that's fine. If you don't have string theory, then you can have the QCD axion and you're fine. But putting all those three together, you end up with a big fine-tuning problem. So that was, you know, I mean Paul Steinhardt has been a vocal critic of inflation for a number of years, and so that was kind of part of his thinking about inflation, was this idea that it doesn't work well with axions in this way. And so his preference for which two of those things to keep is keep string theory and axions, but lose inflation, right? (both laugh) Whereas other people might take a different approach on that.

Zierler:

And he's in the minority, right, in that regard?

Mack:

Yeah.

Zierler:

I've heard that the rough ratio, it's like 80/20, or something like that. 80% of the field is, they accept inflation, and 20% don't.

Mack:

I think it might have been more extreme at the time actually, because WMAP had made its beautiful results. So WMAP had confirmed everything to such high precision that it really looked like inflation fit to that picture perfectly, but we hadn't seen tensor modes. We still haven't tensor modes. And so I think that the fact that we still haven't seen tensor modes has allowed more people to question inflation than they had at the time. Because at the time, you wouldn't have expected it. So at the time it was like, "Well, everything fits with inflation, and now pretty soon we'll see tensor modes and then it'll be totally proven." And so that was kind of the thinking at the time, and Paul was unusually skeptical of inflation among his peers. And I think that he's less-unusually skeptical now. And also because there are more sort of alternative ideas, or there's more of an openness to alternative ideas now than there was at the time.

Zierler:

Yeah.

Mack:

So that question of, you know, the question of inflation, and how does it work? Did it even happen? That was a huge part of everybody's-- that was a big topic of discussion at the time, when I was in grad school. And then also, you know, dark energy was still fairly new. I mean I started grad school in 2004, and dark energy was only discovered in 1998, right, so it was still fairly new, so there were a lot of surveys trying to measure w and figure out some kind of... to figure out anything at all, really, about dark energy. So those were big topics, and then all of the things that WMAP was measuring for the first time, or you know, measuring precisely for the first time. About curvature and the ratios of the components and all of that. That was a huge topic. And then reionization. In astrophysics, figuring out when reionization happened was a really big deal. And that was something that was fairly new, because we had just been detecting redshift 6 quasars and finding that the universe was largely neutral at redshift 6, and then WMAP came along and said, "Oh actually, reionization happened at redshift 12," and trying to reconcile these different kinds of measurements of reionization was a big topic. Did it happen all at once? Was it an extended process? How do we bring together what the quasars are saying with what the cosmic microwave background results are saying? So that was a big topic in astrophysics at the time.

Zierler:

I want to ask a question, sort of out of narrative context, but while we're still in Princeton world, given how intensively you were thinking about the early universe, did you have even an idea at that point that you would develop a book project sort of on the other extreme end of history on how it all ends? Was that seed germinated even as a graduate student?

Mack:

No. You know, I wasn't thinking about the end of the time. I was thinking a lot about the beginning. And it's interesting how much thinking about the beginning drove me to thinking about the end, eventually.

Zierler:

Absolutely, right. You're the ultimate historian, by the way. (both laugh)

Mack:

Yeah. Yeah. But no, I hadn't thought about it at all. I mean, I'd thought about bouncing cosmologies, because that was Paul's thing, but no, it didn't occur to me. And I wasn't thinking about writing books either, you know, that was way... That was not on the horizon at the time.

Zierler:

Do you see now that any serious research into how the universe ends, requires as a prerequisite sort of, understanding how it begins? Is there that coupling that's really necessary there?

Mack:

I do think that that's necessary. I do think that it's all part of the same question, you know? I think we... the ending is going to be determined largely by the circumstances, the broader context in which the universe exists, and that broader context is set up by how it began. Or if we understood how it began, we would know how it fits into a larger space, or a timeline that extends before the universe in some way. Like, it would be clearer if we understood the beginning. And I think that, really, when we're asking about the end of the universe, a lot of what we're asking about is just the fundamental structure of reality. And we can't observe the end of the universe, but we can observe the beginning to a large degree. And so if we want to learn about the fundamental structure of reality, the beginning is the best bet we have. And we get that through cosmology, we get that through studying a lot of the same things we studied to try to learn about the future evolution. But that's the big question, really.

Zierler:

Did you know right from the beginning when you were at Cambridge that you wanted to go back for a postdoc? Was that sort of always your ambition after defending your dissertation?

Mack:

I mean I just loved being at Cambridge, so it was certainly a dream. You know, it was something that I definitely thought about and I looked into what kinds of things I could apply for, and I had... I met with people and talked about the fellowship programs and stuff. So it was certainly something that I was working toward, yeah. I wanted to be there.

Zierler:

And physically, where were you there at Cambridge? Was it at the Kavli Institute?

Mack:

It was at the Kavli, yeah. Yeah. So when I was there during grad school as a visitor, I had two offices. I had an office at DAMTP, in the theoretical cosmology group, and also an office at the Institute of Astronomy. But that was before the Kavli was built. And then when I went back as a postdoc, I was just at the Kavli. And I would sort of go back and forth.

Zierler:

And what was institutionally the relationship between Kavli?

Mack:

You mean between Kavli and the Institute, or?

Zierler:

Yeah.

Mack:

So it's connected to the Institute of Astronomy. Physically connected. It's on the same space. And now they have a whole other building, they have more connections with the physics department, and I think probably stronger connections with DAMTP as well, but at the time, they were sort of bringing together the three departments that do cosmology, and trying to put some people in the same place. So they had a few people from the other departments inside the building at the Institute of Astronomy. And I think the connections are stronger now.

Zierler:

Katie, in terms of your own research interests, when you got to Cambridge, was this specifically an opportunity for you to start delving into new areas in cosmology?

Mack:

Yeah, potentially. Yeah, I mean I wanted to work with George Efstathiou and Antony Lewis and some of the people there who were doing a lot of interesting early universe stuff, and stuff connected at the CMB. And I wanted to work more with people at DAMTP as well. Unfortunately, when I arrived as a postdoc there, the timing wasn't great, because it was right when Planck was starting up, and so everybody was behind these closed-door meetings all the time, because it was a very secretive collaboration, and so they would draw the curtains when they had these meetings, and so a bunch of people got swallowed up by that, and then a few people left for other institutions. So Hiranya Peiris left, and Antony Lewis left and it became a lot harder to work with the people who I wanted to work with while I was there, so it wasn't as productive a time as I'd hoped it would be. Personally because it was my first postdoc and I was still trying to figure out how to be an independent researcher and all that, but also because all the people I wanted to work with were busy or gone in a way that, like, I just didn't really know how to deal with that. And because I was on a fellowship, I wasn't anybody's responsibility, you know? So I didn't have the kind of guidance that I probably could have benefitted from.

Zierler:

Yeah.

Mack:

I mean, you know, I thought I was super independent, and maybe to some degree I was, but I also felt a little bit out to sea. Because of just, you know, where everybody was, and all that. So I ended up doing some of my work with Stuart Wyithe at University of Melbourne, just remotely, and I went and visited there during the time I was at Cambridge. And then that kind of set it up for my next postdoc, which was at University of Melbourne.

Zierler:

How did you connect with Wyithe initially?

Mack:

We met at a conference, and I was familiar with his work anyway, but then we chatted at a conference, and he said, "Oh, you should come to Melbourne sometime, we can talk about this stuff." And I was like, "Yeah, I want to go to Australia. That sounds awesome."

Zierler:

And what was he working on? What was his research at the time?

Mack:

So he's interested in 21 cm astronomy and reionization. And that was something that I had worked on before anyway, and it was such a big hot topic. I was keen to get more involved in that.

Zierler:

And what were some of the big questions surrounding this field at the time?

Mack:

Just like what we could really learn from it, you know. It was very new, and so the project that we worked on at that time was about the 21 cm forest, so using the 21 cm line to kind of study neutral gas along the line of sight. You know, in a frequency-resolved kind of way. So that was the project that we were looking at. But there was...Yeah, there were just a bunch of questions about, like, what are we going to learn about reionization? What are we going to learn about the first stars? How precisely can we measure this, and what are the best ways to do that? And what are the star formation histories? How are quasars involved versus stars? You know, what are the parameters? Like the whole thing was kind of an open question, and Stuart Wyithe was producing simulations of what we were going to see with these telescopes and so that was something that I was interested in. You know, just, really: what can we learn? Can we learn anything about exotic physics or some new, interesting ideas from studying this whole new phase of the universe that we'd never dealt with before? 

Zierler:

Okay. This is David Zierler, oral historian for the American Institute of Physics. It is October 12th, 2020. I am so happy to be back with Professor Katherine J. Mack. Katie, thanks so much for joining me again.

Mack:

Thanks for doing this.

Zierler:

Okay, so we left off last time, the 21 inch? Or centimeter-- Centimeter.

Mack:

Centimeter.

Zierler:

21 cm line in Australia. How long were you in Australia for?

Mack:

I was a postdoc there for five years. I had spent a few months, I think I'd done a three month trip or something? While I was still at Cambridge, but then when I went for my postdoc, I was there for five years.

Zierler:

Five years is on the long side for a theoretical postdoc. Did you extend a shorter initial offer?

Mack:

Yeah. So I was there for a three year fellowship. The Discovery Early Career Researcher Award. And then I stayed on for two years funded by a couple of centers of excellence in Australia. So there was the Center for All-Sky Astrophysics and there's the Center of Excellence for Particle Physics at the Terascale. And so jointly, those two. the astro center and the particle center, funded my final two years at University of Melbourne.

Zierler:

Katie, I want to ask sort of a broad cultural question. Coming from the United States, but also coming from quite elite institutions that are also known for their intensity, specifically in physics, right? Even though Caltech and Princeton are very different, they probably share that similarity of being places where physics is done on an intensive matter. I wonder what your impressions were in Australia, you know, culturally in terms of how physics was done there.

Mack:

Yeah. Yeah, it was interesting. It was just definitely the general vibe in Australia is more casual in a lot of different ways. You see that in academia, that it's much more-- People are just less formal in the way they talk, the way they interact, and all that. And the physics community is a lot smaller. And connected across the country in a way that isn't really done in the US, even though the US is a similar sort of physical size. It's rare for there to be such strong connections between people at institutions stretched across the whole country. But because the Australian community is quite small, there were these centers of excellence that connected the whole country, and I was part of two of those. And so I felt much more integrated with the entire scene than I had in either the US or the UK. I think that it's, you know, it's a matter of being a small population and somewhat sort of geographically disconnected from some of the other bigger research centers. There was more of a sense of people needing to be working together, collaborating across institutions, and so I think that colored the research environment substantially. There was also a lot more connection with Asia just because of proximity, I guess. And sort of historical connections, and so there were a few conferences I went to and things that were the Asia-Pacific region.

Zierler:

How international was your research community? In other words, was it parochial enough that people assumed you were Australian until they heard your American accent?

Mack:

I mean people knew who I was, so there wasn't that assumption in the research community. In public science it was quite interesting, because people did assume I was Australian. Because I was based in Australia. But the research community was very international. There were very few Australians among the groups I worked with, really. I mean generally speaking, the senior people were mostly Australian, and the graduate students were maybe half Australian, and then everybody in the middle was mostly international.

Zierler:

And sort of an overall question. Was there any unfinished work or overarching answers you still wanted to pursue from your dissertation during your time as a postdoc, or was this really new opportunities, new projects to work on?

Mack:

Well, when I was at Cambridge, I worked on a few projects that were sort of carried over from my old dissertation work. So two of my publications during that time were basically thesis work that was sort of finished up at Cambridge. And then when I was in Australia, I worked on some 21 cm astronomy stuff that was connected to work that I started as an undergrad, really. So there were certainly common threads that I was pursuing throughout my postdoctoral work. But I guess the main thing is that I was moving more toward dark matter being a focus and developing more of the 21 cm galaxy formation connection.

Zierler:

And what is the connection between dark matter and the 21 cm line? What's, how do those two go together?

Mack:

Yeah, well, so the 21 cm line is a way of probing the physical conditions during the time of Cosmic Dawn and reionization and potentially back to the Dark Ages, the Cosmic Dark Ages, which is before the first galaxies. Although that's observationally much, much harder to get to. And so one of the themes that I've been working on.

[pause]

Zierler:

Okay, so dark matter 21 cm line.

Mack:

Right, right. So basically, the thing that I've been thinking about a lot lately is how different versions of dark matter could affect the evolution of the first stars and galaxies, the intergalactic medium during the time that the first stars and galaxies are forming. So doing that--

Zierler:

And so when you say, "different versions of dark matter," currently or over time, dark matter has changed?

Mack:

So what I mean is different models of how dark matter interacts with regular matter.

Zierler:

Okay.

Mack:

So I've looked into that before, so I had a paper during my first postdoc that was about how primordial black holes evaporating would affect the 21 cm signal that we'd look for in terms of early galaxy formation and so on. Just because you have an exotic energy source dumping energy into the intergalactic medium, and therefore potentially changing this 21 cm signal. And then more recently I've been looking at if dark matter is annihilating, how that affects the 21 cm signal, how it affects the first stars and galaxies, how it affects that intergalactic gas. And so I've been thinking a lot about annihilating dark matter specifically, but in principle any kind of dark matter that has some interesting particle physics interaction with gas would have potentially a stronger effect during that period than now, and also the sort of astrophysics going on during that period is in some sense much simpler, because all it is is gas collapsing to form stars and galaxies. And so it could be a really interesting signal, and this is kind of a new window on that time period. So I've thought a lot about kind of what kinds of exotic physics—by which I mean just anything other than non-interacting cold dark matter plus baryons—how any kind of exotic physics could affect the 21 cm line. And you know, part of that is motivated by the fact that there's a huge amount of observational effort going into this, and we're going to see a lot of really interesting new data soon, looking at that signal. And so we should figure out what kinds of cool, interesting things we might find in it.

Zierler:

Did you ever think about pursuing a longer-term opportunity in Australia?

Mack:

Yeah, I did apply for one faculty job there and I applied for some additional funding and such, but the number of-- Because it's a small community, the number of permanent jobs is kind of small in Australia, and it is really far away from the rest of the physics and astronomy community. Just sort of physically.

Zierler:

Katie, did you start to do your popular science writing when you were in Australia?

Mack:

I started, really, when I was in grad school.

Zierler:

Yeah.

Mack:

But then I got much more invested in it in Australia. So I started writing a column for Cosmos Magazine, which is one of the biggest popular science magazines there.

Zierler:

I was asking because perhaps science communication, science writing, was a way for you to be sort of more connected given the fact that Australia is so far away from everything else.

Mack:

Yeah, I think that did make a difference. I think that I was more known outside of Australia because I was becoming just more known for the public stuff. But I also did try to keep connected with the rest of physics and astronomy community by attending conferences a lot. So I usually spent maybe two or three months out of the year not in Australia when I was living there, just traveling around giving talks, going to conferences, that kind of thing.

Zierler:

I want to ask sort of a broad question that, I could ask this at any point, but I think now is a good time. Particularly because I want to develop this narrative of when you started to think about the end of the universe. So cosmology, we usually think of-- I'll take a very liberal arts perspective on this, but if you'll permit me, right?

Mack:

Sure.

Zierler:

Early universe is the past, so you're like the historians, right?

Mack:

Mmhmm.

Zierler:

Then people who study the universe as it currently exists might be journalists, it's the present. And in the future, right, you're talking about in the future time when the universe ends, you would be like a prophet or something like that, right? (Mack: (laughs) Okay, sure, sure.) So when did you start to think, or is that a reasonable way to understand how some of the concepts that you've been wrestling with might exist in time, right? In other words, do you think of cosmology specifically as studying the past, the origins of the universe, the past 13.8 billion years ago, and something that happens in whatever it's going to be x number of billion years into the future is something where it's a prediction that has not happened yet, and so by definition it has to be at some level a prophecy?

Mack:

The way I think about it, more than that, is that the early-- I kind of think of end-of-the-universe cosmology as a branch of early universe physics. Because I think that it's really all about figuring out how the entire timeline of our universe fits into the fundamental physics of the cosmos and the governing principles. So, you know, our context within the larger theoretical framework of where we come from in the cosmos, how we fit into any kind of bigger picture that there might be, that really directly tells us about both the beginning and the end. So if we understood inflation, and if inflation even happened, then we would understand what sets up the initial conditions of the cosmos and what the ingredients of the universe are. And that pretty straightforwardly determines the ending, right, because whatever's in the cosmos had to come out of the very beginning, so if we really knew it was happening then, we would know what's happening in the end. Similarly, I mean basically the only ways that we can be destroyed as a cosmos are we can have dark energy do us in one way or another (and there are a couple of different ways that can happen), but if we understand the beginning of the universe, we'll understand where dark energy comes from. We'll understand the sort of over-arching model that includes dark energy.

Zierler:

Katie, is that to suggest that at some point-- I'm sorry. Is that to suggest at some point you started to focus more on dark energy coming from dark matter?

Mack:

No, no, not really. I still consider dark energy to be a little outside my purview. But dark matter is one of the… trying to understand dark matter is one of the ways we try to understand the early universe and the fundamental physics governing the cosmos. And dark energy is part of that story. So if we understood dark energy, if we understood the early universe, we would probably understand dark energy better and therefore we'd have, you know, we'd know a direction to go with those kinds of models. Then there are also models where the universe ends via vacuum decay, which is directly-- that comes through the Higgs field and what the Higgs field is doing, and that's early universe physics all the way, right? The Higgs field was instrumental to setting up the conditions for the current universe. If we understood how the Higgs field worked, we'd know if it's going to be decaying in the future, or transitioning. And that's an end of the universe scenario. And then the other one, really, that comes up is some kind of cyclic cosmology, and cyclic cosmologies are pretty much always brought up as a way to explain the initial conditions of the cosmos, and therefore the end of a previous universe is the beginning of ours, and the end of ours is the beginning of the next one, if there is a cycle. Otherwise there are some models where the Big Bang is a different kind of transition point, but it's really—the end is connected to the beginning.

Zierler:

Roger Penrose is talking a lot about this right now.

Mack:

Right, Roger Penrose's idea or Paul Steinhardt's stuff about cyclic cosmologies, or even some of this newer stuff from Latham Boyle, Neil Turok and their colleagues at Perimeter Institute where you have this, it's called a CPT-symmetric universe, where there's a kind of anti-universe/universe pair, and the anti-universe is the previous part… but anyway, they come out of a transition point at the Big Bang, and so the future of our universe is determined by what happened in the previous universe, basically.

Zierler:

And just to make sure we get our terms correct here, "multiverse" would be concurrent universes, and what you're talking about are sequential universes.

Mack:

Yeah. Yeah, yeah. But also, you know, multiverses come into the question as well, because some multiverse ideas involve many different universe possibilities with different values of dark energy, different cosmic evolutions, but those are also the kind of thing where you get that information by learning about the early universe. You know, if there's eternal inflation, for example, or if you understand string theory, then maybe that'll tell you something about that and that's something you'd learn about by studying the early universe. So, yeah, in my mind, in terms of the categorization, cosmic eschatology is a branch of early universe physics, and because I'd always been interested in early universe physics, and I worked on that during grad school and to a lesser extent during my postdoctoral work, it was natural to start thinking about endings and then it was really when I started getting interested in vacuum decay that I realized that this could be the subject of a book and a branch of my own research.

Zierler:

I'll do the editorializing here, though, Katie. When you say it was natural, it was natural for you, right? The number of physicists who think about the early universe, that's a much bigger community than people who think about the end. In fact, how big is it? How many of your colleagues are working on the things that you've been working on recently?

Mack:

Well, the vacuum decay crowd is kind of small. I guess maybe there's like two dozen people who are primarily interested in issues around vacuum decay. But anybody working on the Higgs field, the Higgs potential, and certain issues in kind of string theory would be touching on that. So in some sense, it's a small community who's really writing papers with the term "vacuum decay" in them, but it's a big deal in particle theory right now for sure. Less of a big deal in cosmology, per se, but particle theory is very concerned with this because the Higgs field is such a big topic. So I don't know, the number of people working on that is fairly small, I guess, but part of a larger community working on very connected topics. And then in terms of stuff like the future based on dark energy models, that's a real small group. And I don't really do that myself. I've never written a paper on dark energy per se. Although I'm interested in the big rip and phantom dark energy, those kinds of models, and right now I have a student working on some questions around that. But that's maybe a dozen people interested in that side. And then the cyclic stuff, again, is a small group of people, but also very specialized, so that's maybe another couple dozen who are really working on cyclic universes.

Zierler:

And I wonder if you can talk about the broader theoretical basis for the very idea that the universe has an end to it. For example, we haven't talked yet about the expanding universe, the accelerating universe, and one possible take-away from Saul Perlmutter's research is that, maybe it just goes on infinitely, right? Where's the end point in the expanding and accelerating universe?

Mack:

Right, so the end point you get in a kind of standard lambda-CDM concordance cosmology universe is a heat death. And that's the kind of ending where all structure in the universe is over, and you reach a maximum entropy state. So in some sense, the universe continues existing forever. It's just that nothing happens in it, and also time no longer means what you think it means. (laughs) Because once you reach maximum entropy, there's no arrow of time. Based on defining the arrow of time based on the second law of thermodynamics, which says that entropy has to increase going forward. So when time loses meaning and all structure is destroyed, and the universe is empty aside from a trace amount of radiation coming from the cosmic horizon, at that point, I say: it's over.

Zierler:

You're calling it? (laughs)

Mack:

You know, yeah. Yeah, I think at that point, for all intents and purposes, that universe is done. Something might come out of it, some future universe in some way. There would be like the Penrose cyclic cosmology, but I call that an ending even though in principle space and time still exist. Whereas there are other endings where they could really be destroyed. Like a vacuum decay scenario, you're destroying at least our kind of spacetime, or a big rip scenario, or a big crunch. Those can be more sort of permanently destructive, I guess. Whereas a heat death is just, the universe continues forever, but uselessly, I guess.

Zierler:

And how closely connected is that? To go back to this idea that cosmic eschatology is a branch of early universe studies. This scenario that you're describing, how close is that in your mind to whatever the universe looked like before the Big Bang? Is it essentially the same, or is it different?

Mack:

Ooh, it depends. So yeah, it depends. I mean you can have, according to general relativity, you can have a universe that starts with some kind of singularity, and ends with a heat death, and the two are not really connected. Except in the idea that no new physics comes in to change anything. We kind of need something like inflation to happen in between, and that's a kind of new physics, but it doesn't necessarily change the future evolution of the cosmos. So really by saying that you have a heat death, that that conclusion comes out of early universe physics is saying that early universe physics confirms the lambda-CDM concordance model and confirms that nothing else is going to happen. So that's when you would arrive at just the standard heat death scenario, by saying that you're sticking with the default model, basically. And that's something that we could confirm or rule out based on discoveries in early universe physics moving forward. Also if we somehow confirmed that dark energy is a cosmological constant. Which is something that you can do by degrees by studying dark energy itself, but you can't really confirm in any solid way without really understanding the big picture, fundamental theory on which our whole universe is based. So if we can really convince ourselves that dark energy is a cosmological constant, then the heat death is just where we get from that. So it's a little bit tricky, because it may not be possible to ever really know for sure that dark energy is a cosmological constant, but we can potentially, just by degrees, be more and more confident of a heat death ending.

Zierler:

Is that to say, if we can't ever confirm whether or not dark energy is a cosmological constant, is that tantamount to saying that we'll never understand what dark energy is?

Mack:

I mean potentially. The problem is that there aren't that many things you can measure, and the usual thing that we use to quantify how sure we are of dark energy is by measuring the equation of state parameter, w. And w equals exactly -1 is a cosmological constant. Anything less than that is a different kind of dark energy. Less than that or more than that. Anything not exactly equal to -1. And it's very possible that we will continue to measure w with shrinking error bars centered on -1 forever. And you can never get zero error bars. So if that's the main tool we use to determine the nature of dark energy, we're never going to be 100% sure. However, if we discover some fundamental governing principle of the universe, where dark energy is a component of it—let's say that we understand something about eternal inflation that sets this cosmological constant parameter through some deterministic means that makes sense—then we can say, okay, we know we have a cosmological constant. This is the value.

Zierler:

Where is the integration of gravity into the standard model? Where does that fit into all of this?

Mack:

Well, yeah, so that's another issue. So the standard lambda-CDM model includes general relativity being 100% correct, basically. We know that that doesn't work very well with quantum mechanics, so in principle, we should find some alternative version of gravity, but it may or may not affect the large scales, the evolution of the cosmos in any way that's detectable. Or that changes the picture of dark energy. So it may be that the thing that changes is our understanding of quantum mechanics, and we just keep general relativity more or less intact. We don't know that yet. But lambda-CDM is basically built with general relativity equals gravity, and that's that whole story.

Zierler:

What are some of the theoretical propositions or modeling or extrapolation that you would use, the way that we could go back to the Big Bang and calculate 13.8 billion years, or you know, whatever it is? How do you do that fast-forwarding into the future? How are we calling the end of the universe in a time scale?

Mack:

Right well so, that's mostly extrapolation via the Friedmann equations, which govern the expansion of the universe, the evolution of the components of the universe. Those go forward as well as backward, and so if we trust those equations, if we trust that kind of cosmic evolution, then you very straightforwardly get to a universe that's just governed by entirely dark energy. And that's kind of, it's happening already, right? Because matter and radiation are becoming less and less important over time, and we can see that trend and we can see that evolution continuing perfectly well into the future via these equations. And then if we have something like vacuum decay, then we have timescales associated with that transition, and we can just predict sort of probabilistic expectations for when that might happen, but we can basically know what that evolution would look like based on understanding the Higgs field and what the true vacuum state would look like if that transition did occur. But then when you get to things like these cyclic models, then the future evolution is based on the parameters of the theory, and when the transition to the new universe occurs based on how that theory is built up.

So for example in the ekpyrotic model, you have a scalar field that's governing the sort of potential of the universe, and that has some-- the potential is an element of the theory. And so once you write down the potential, you figure out the early universe evolution, and it just gives you the late universe evolution as well. So there are, generally speaking with these models—if you take the cosmic evolution equations that we use to describe the early universe, those continue, and then if you have something weird jumping in like vacuum decay or cyclic models, that's built into how we understand that those things are possible at all.

Zierler:

To orient the conversation sort of back to planet Earth and the narrative, at what point did these ideas sort of congeal for you into, this is a book. I have to write a book about all of this stuff. Was it in Australia?

Mack:

Yeah. So when I was in Australia as a postdoc there, that was when I was I was first receiving solicitations from publishers and agents asking if I was planning to write a book. Because of my science writing and my profile on Twitter and so on. So I started to get emails from agents, publishers. Because I guess they saw that I was writing for Cosmos Magazine, and they saw that I had a big Twitter following and stuff, and so they started writing me asking if I was going to write a book. And so I started talking to a few of them and thinking about what I would write a book about, and at first I was thinking about something about dark matter, but then I realized that when I was giving talks about cosmology in general, people were really responding to the topic of the end of the universe, and specifically vacuum decay. Because I had gone to a conference, while in Australia, where somebody was talking about vacuum decay and the latest results on it, and I had not really been familiar with the topic before that, and I started reading about it and I was just fascinated. And I wrote an article for Cosmos Magazine about it, and gave a few talks where I discussed it, and I thought-- And then I made the connection that it could make a really interesting book to talk not just about vacuum decay, but lots of different ways the universe might end, because they're all kind of things that I've been fascinated by before. When I was in grad school, I gave a presentation about the Big Rip, because I thought that was such a cool, weird idea, and that was a sort of side interest of mine. And then working with Paul Steinhardt, of course, I'd learned a lot about cyclic models.

And so I realized that in my past experience, although I hadn't really done research in these topics, I'd been following them and I'd found them really fascinating. And I thought this would be something that the general public might not have heard much about, and would be a fun way to talk about physics and astronomy in general, because talking about the end of the universe ties in so many different ideas from cosmology. So then I started talking to one of the agents about that particular topic as an idea, and then I got the job at NC State right around the same time. And I realized that that would be a perfect opportunity to actually write the book, because I had a job in which public engagement was specifically written into the job description.

Zierler:

But what you couldn't have known is that publishing in 2020, when it feels like the world is ending for so many reasons, would be the perfect time to write this book. (laughs)

Mack:

Yeah, although you know when I first pitched the book to publishers, it was early 2017, and it also felt like the end of the world in that time.

Zierler:

Yeah, that's right. It's been a long decay, actually. (laughs)

Mack:

Yeah, yeah. It has, it has, so I think I actually worried when I was getting the book ready for publication, and the pandemic was messing with the publication date and canceling the book tour and all that, I worried that, like, we overshot. Like the world is too depressing for people to read about the end of the universe. But it turns out that people have found it cathartic, so...

Zierler:

Yeah. I mean, I can share with you my own reaction. It's like, well first of all, our problems are rather small in the grand scheme of things. You know?

Mack:

Yeah, yeah, yeah. There's that, and I think also cosmology in general is a nice escape from any kind of immediate worries, and so it makes sense to—even if it's something about ultimate future destruction, it is at least temporally very distant from us, and also, you know, it's a big picture thing that takes you out of your… We're all sitting in little boxes right now, right? And being able to think about these big ideas and this larger context is a really nice escape.

Zierler:

Now, how did the opportunity at NC State come about? Did you have this from Australia, or you came back to the States and were on the job market from home?

Mack:

No, I applied from Australia. So I actually, I was recruited for the position. A colleague of mine who was already part of the Leadership in Public Science cluster had just been hired by NC State to be part of that public science group, contacted me--

Zierler:

Beyond physics, you mean? The public science beyond physics?

Mack:

Yeah, yeah. So the Public Science Cluster includes at the moment five faculty members. I'm the only one in the physical sciences. So there are a couple people working in sort of ecology related areas, one person in informal science education, and one in science communication. And so I'm sort of, you know, in the physics side of things, but nobody else in the cluster is. So yeah, so he contacted me and said, "Hey, there's this group that we're starting up, I've just joined it. I think it's going to be a great opportunity and you should apply for this job."

Zierler:

This is a very unique endeavor, to have this sort of (Mack: Yeah.) cluster. Do you know what the intellectual origins of this were?

Mack:

So NC State has a number of clusters that are funded through sort of provost/chancellor investment. I don't know exactly where the money comes from, but they're not funded through the department. Specifically, they're funded through a university effort. So there's a cluster called the Living Embryo, which is sort of biophysics and biology. There's a cluster on something to do with narrative storytelling that goes through a few different departments. There are clusters on energy, like future energy investment that go through a few different departments like ecology and engineering. So the idea is that the university has identified a number of topics, problems that they want to invest energy in, they want to invest research in, and so they define the topic and then hire people who are going to be interdisciplinary who can work together across departments to work on that particular problem. And so public science, broadly defined as any kind of science that connects with the general public (and there's several different ways of defining that), that was one of the investment areas. So they put together this cluster, they included people across the university, people in the public education section, people in the museum. So there's a natural history museum that people have ties with the university through. They got some people who were connected with that to be involved, and they designed the idea of the public science cluster, and then hired five of us to be faculty members who are, you know, our salary comes through the provost's office, and then we're placed within our departments.

Zierler:

So given that you had this pre-existing interest in science communication, that aspect of the job seems like it was perfect for you. But I wonder if you could talk about some of the challenges or opportunities that you see being, you know, not at a Harvard or a Stanford or something like that, where you, you know, it's a very different environment. And I wonder if you could just talk about what challenges might come with that, and what opportunities do you also see?

Mack:

Yeah, so I mean one of the things that I've noticed throughout my career, is that the name of the institution that's attached to you can open doors, right? So when I was an undergrad at Caltech, if I wanted to work with somebody at another institution, another place, the fact that I was a Caltech student gave me some automatic sort of benefit of the doubt. And then when I was at Princeton and at Cambridge, the same thing, right? People would automatically take you a little bit more seriously, take your calls, so to speak. Because of the institution you're attached to. In Australia it was a slightly different situation, because everybody was already connected to everybody, and so it was a different kind of scene there. But yeah, when I was thinking about NC State, I knew that just the name NC State was not going to be an automatic benefit of the doubt situation. But I also knew that no other institutions, none of these sort of big name, Ivy League type of institutions, were putting investment into this kind of innovative public science thing. I think that the Ivy League-type places, what they tend to do in terms of public engagement strategies, is they have their people who are sort of research superstars, who get tenure and then do a lot of public engagement, you know? You see that at places like Harvard and Caltech and you know...

Zierler:

You flipped the model, is what you've done.

Mack:

It's a different, I mean yeah. It's like, an institution that is not sort of put on that list of top-tier research institutions has different incentives in terms of raising their own profile, connecting with the public, and specifically NC State, they make a lot out of the fact that they're a land grant institution, which from their perspective comes with an obligation to do more publicly-engaged science, or research, or somehow connect with the community in a stronger way than a place that has a private endowment might feel. So I think that there are a couple things. One is that sense of connection to the community, obligation to the community that might not be there for private institutions. And then also the incentives for connecting to the public are going to be different, because you might not have the superstar Nobel laureate who's starting a multibillion dollar center at your institution, but you can find ways to increase visibility and connect with the community differently, through different kinds of investments.

Zierler:

What about graduate students? I mean, given your stature in the field, do people want to work with you? I mean, to flip that question about the name recognition of the school attached to your name, what about the name recognition of your name irrespective of the school? In terms of graduate students and who you're able to work with.

Mack:

Right so I have two graduate students working with me right now, and one postdoctoral researcher. And one of the graduate students, he was at NC State before I got there, and he came to me through looking around at who's working when he was looking for a thesis position. The other graduate student got in touch with me while I was still in Australia, because he had read about my work, and he wanted to work on things I was interested in. And so he and I were in touch before I even got to NC State. And then he applied to NC State because he wanted to work with me. And so that was a case where my visibility, my public profile, directly led to getting a student who is interested in the same things doing very good work. And then for my postdoctoral fellow, I got a number of really great applicants for that position because I had a network and because I had some visibility, and so I was able to advertise maybe more broadly than others might have been able to, and then I think I got some really good applicants because they specifically wanted to work with me. So I got a number of very good applicants. I got two extremely excellent applicants and in terms of people I really wanted to work with, and then one of them ended up coming to work with me. So that was something that I think when a postdoc is looking for a position, sometimes they look for an institution for name recognition, but sometimes they just look for who they want to work with and what they're working on, because the number of professors who are hiring postdocs is fairly small, and so if there's a particular thing you want to work on, the person you want to work with may or may not be at a big name place.

Zierler:

Right. Right. Well, Katie, just to bring the narrative up to the present, the book is out, right? The universe has ended, we've got that. What now? What are you working on now? What's the most exciting stuff currently and what you're looking on into the future?

Mack:

Well, I mean I was asked this the other day, and my answer was, well I've got two research proposals due before the end of the year, so I'm working on those.

Zierler:

What are they? What are the proposals?

Mack:

So they're two NSF proposals. Both of them kind of refining proposals that were sent in last year that got good reviews but that didn't get funded. So one of them is about dark matter annihilation in the cosmic dawn, and the other one is about early universe, sort of the exotic early universe models of dark matter. So dark matter with a thermal history that doesn't match the sort of standard assumption and the astrophysical consequences of that. So these were just two dark matter-related things. I'm also planning to put in a proposal for a vacuum decay related project, but that's still more in development. So what I'm doing in terms of that is I'm applying for funding to follow up on my dark matter research and get back into some of the research that had to be sadly neglected during the book push. So more dark matter research and also I have just some ongoing research projects that I want to develop more. One of them that's looking at dark matter and extra dimensions and primordial black holes, so tying in a few of the topics I've worked on before. And one of them with vacuum decay and some interesting possibilities with vacuum decay. And then I have a couple of projects that are kind of-- have been in development for a number of years that I'm just kind of trying to finish off, and those are on slightly different topics, but more connected with galaxies than dark matter.

Zierler:

Well, for the last part of our talk, I want to ask sort of two broad questions that have a forward-looking tint to them. And the first one is, I mean it's an impossible question to answer, but that's the kind of business that you're in, so I'm going to ask anyway. Are you surprised that dark matter and dark energy is where it is? That given all of the advances in so many other aspects in physics, and I mean I'm thinking like Michael Turner 1998. Is that a long time ago for dark energy, or is it not a long time ago, right? Given just sort of a broader perspective on all of the advances in physics, is it surprising to you or not how far there is to go, both for dark matter and dark physics in terms of really understanding both of them.

Mack:

I think that, I guess there are a couple of areas, and I would include dark matter and dark energy in these, where we were real optimistic for a while there that the simplest answer we could think of at the time was just going to work out. So we thought that would happen with supersymmetry. And the Large Hadron Collider was going to find supersymmetry, and then we'd be on our way to string theory and everything would be great. That hasn't happened. And then dark matter, we thought, okay, it's going to be the supersymmetric neutralino, thermally produced, annihilates in high-density regions. It'll be produced in the Large Hadron Collider, we'll find it in our direct detection experiments right away. That didn't happen.

Zierler:

But just to interject on that point there, are you specifying the Large Hadron Collider because if the physics was there, LHC would have found it? Or do you tend to believe that maybe we're hampered or hamstrung by not operating in higher energies? That LHC cannot offer.

Mack:

If it were the simplest model, we would have seen it at the LHC already. I think that's a reasonable statement. So we thought-- if supersymmetry were where we thought it was going to be, and dark matter were where we thought it was going to be, we would have seen both of them already in the LHC. So the LHC limits right now are uncomfortable for both of the expected outcomes within the theory there.

Zierler:

That's sort of a tough thing to swallow, but that's where we are.

Mack:

Yeah, yeah, exactly. So we were real optimistic that those two things would be sorted out by LHC and these detectors, and that has not happened. As for dark energy, that's a trickier one. Because the kind of default model, the cosmological constant, was never going to be easy to really, really pin down. I think that we might have been a little optimistic that we would figure out this whole 120 order of magnitude discrepancy and we have not. So the fact that the magnitude of the cosmological constant appears to be very difficult to reconcile with quantum field theory. I think that maybe we thought we would figure that out, and we have not gotten anywhere near figuring that out yet. So I think it's a matter of, am I surprised that we haven't sorted these things out? I don't know. I think that there was never any particularly good reason to think that we would have, other than our first kind of stab at it suggested that we should see it. But in terms of what we actually know about dark matter, for instance—and I'll focus on dark matter because that's where my expertise lies—what we know about dark matter is that it's present in the cosmos and has no detectable interaction with light. And therefore is this kind of collisionless stuff. And there's nothing in that that suggests that it has to be detectable in any kind of experiment. Because there's nothing in that that suggests, oh it definitely has weak interactions at a particular scale. We don't know, right? And there's no observational hint of that happening. It might be that dark matter does have a weak scale interaction, or an interaction through the weak force, but we don't have any a priori reason to believe that other than theoretical bias. And the fact that it would be a really convenient theory. (both laugh) Because all we see is it not interacting, you know. We see that it gravitates but otherwise does not interact in any detectable way. And so how surprised should we be? I don't know, right? Maybe we got so excited about our first stab at the problem that we got too invested in that idea. I mean, the same with axions, right? Axions could have been dark matter. It could still be that axions are the dark matter. But the experiments we've done so far haven't found them, and whether or not those experiments should have found them is kind of unclear, so we have not managed to kind of close the window on axions, and I don't think we should be super surprised about that either. So I don't know, I think there could have been a universe in which we got lucky or in which we were more immediately right. But I don't think we should be too surprised or dismayed that we don't live in that universe.

Zierler:

Well, looking ahead for my last question, relatedly. Where do you see technology in crossing the rubicon with these things? We're on the cusp with quantum computing, for example. The next generation of space-based telescopes are going to be extraordinarily exciting. Now, obviously, you're coming on the theoretical side of things, but how excited are you? Technology is always great for discovery, but how excited or optimistic are you that without these next-generation advances in both computing and observation, you know, the kinds of questions that remain so mysterious at that point might actually become better-understood?

Mack:

You know, it's interesting. I think this is one of the questions where working on the book really changed my perspective on it because one of the chapters in the book is about what we're going to see in the future, how we're going to answer all of these questions, and so I had the opportunity, thanks largely to the book grant I got from the Sloan Foundation, to go around and talk to a lot of my colleagues about that question and just contact everybody I knew who I thought had an interesting opinion on this. And go and interview them. And one of the common threads of those discussions was we don't know where it's going to go next, we don't know what technology is going to be most important. But we're going to get a lot of data. And historically speaking, every time you get a lot of data, you find useful things in it. And so that applies to these galaxy surveys, you know, the Vera Rubin Observatory and the LSST survey, and what that's going to find... the Nancy Grace Roman Observatory (WFIRST), and also James Webb and all these new CMB experiments that are coming up to really dig out every shred of data from the CMB that you can get. And that also applies to the LHC, you know, high luminosity LHC and the future colliders, either a future circular collider or an international linear collider, or however that goes. And then the whole new window of gravitational wave astronomy, and what we're going to get from LIGO, VIRGO, KAGRA, and the pulsar timing arrays and LISA and, you know, just the huge sort of flourishing of that field. We're going to get a ton of new data in a totally different direction than we had before. So everybody I talked to is just really optimistic that, you know, we don't know how it's going to go, but we do know that we're going to see so much more of the universe in such a new way, that probably it'll help (laughs) you know.

Zierler:

And maybe it will even reformulate the questions that we've been able to ask so far, (Mack: Sure, sure.) which have not yielded... I mean, you know, poorly-conceived questions will yield poorly-conceived answers.

Mack:

Yeah, and also in terms of dark matter in particular, one of the things that also came up in my discussions with people was the diversity of approaches to dark matter, and the diversity of dark matter models that are now being investigated. You know, since our first naive guess didn't work out as quickly as we hoped, now people are looking at just a huge range of different kinds of dark matter models. Sterile neutrinos, and axion-like particles, and fuzzy dark matter, and new ideas about primordial black holes, and just-- there's this huge blossoming of diverse dark matter models that each have different observational signatures. There are new kinds of observatories or detectors being built, new kinds of ways to analyze data from colliders and other things. So I think, really, what gives me hope is not any particular technology, but just the fact that we're branching out and we're just going to get a ton of new information. And we're not focusing on just one thing. We're not putting a huge investment into one piece of the question or one direction and hoping that works out. We're kind of just going everywhere. And I think that that's a very good approach.

Zierler:

This is why, you know, the subtext of your book, and why that concern about it being too depressing for the times that we live in, of course the answer there is that we don't have to really be so upset about what happens billions of billions of years into the future. Right now, the bigger story is is that this is just an extraordinarily exciting time. For all of these things.

Mack:

Yeah, absolutely. Absolutely. Yeah, yeah, definitely. And when I give talks about the end of the universe, I usually end with a bit about vacuum decay and then in the process of trying to reassure everybody that we shouldn't panic about it, I put up a slide of the pie chart of what the universe is made of, right? So you've got dark matter is about 26-ish%, or something, and dark energy is 70-something%, and then there's this like 5% slice that's regular matter. And I say, "Look, the entire standard model of particle physics is in that little 5% slice."

Zierler:

Right.

Mack:

We have these big mysteries. We have dark matter, we have dark energy, we have no idea what those are. And so we have the opportunity to figure that stuff out, and maybe that'll totally change our picture of the future of the universe, maybe that'll give us something completely new and different to think about, but that's the opportunity, and that's where the investment is, and that's going to be really exciting one way or another. That we do have these huge mysteries, and that we're tackling them in bunch of different ways, and I think it is an exciting time. That we both know so much and also so little, and we have sort of a good balance there between having a very good understanding of what the universe… how much of everything is out there, and a good tracing of the history and several interesting extrapolations in the future, and yet no fundamental understanding of what it all is. And I think that we're going to make a lot of advancements on the second part of that in the coming years.

Zierler:

Well, on that note, Katie, it's been an absolute blast to talk with you. Thank you so much for doing this.

Mack:

Thank you.

Zierler:

It's just quite exciting to be able to get your perspective in long form, not just at 140 characters at a time, and there's going to be, I don't have to tell you, there's going to be so many people who are going to be so excited to hear your perspective on this platform. So thank you so much for doing this, I really appreciate it.

Mack:

Thanks so much. It's been a really fun thing to go through all these topics, so thank you.

Zierler:

Fantastic.