The Joy of x

Priya Natarajan on Black Holes and Mapping the Universe

In this episode, Priya Natarajan, professor of astronomy and physics at Yale University, speaks with Steven Strogatz about her lifelong fascinations, including black holes, mapping the universe and early personal computers.

In this episode, Priya Natarajan, professor of astronomy and physics at Yale University, speaks with Steven Strogatz about her lifelong fascinations, including black holes, mapping the universe and early personal computers. This episode was produced by Dana Bialek. Read more at Music and production is by Story Mechanics.

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Steven Strogatz: What would feel is easiest for you as a place to start? Should we talk about your Commodore computer, or you want to talk about black holes?

Priya Natarajan: Sure, you want to start by my childhood? Sure.

Strogatz [narration]: From Quanta Magazine, this is “The Joy of x”. I’m Steve Strogatz. In this episode, Priya Natarajan.


Strogatz: Is that an easy, comfortable place?

Natarajan: Yeah, I mean, that, that would be a smooth start.

Strogatz: Okay.

Strogatz [narration]: Priya’s a theoretical physicist who loves to use math to think about black holes. Her picture of the universe is one where black holes are not exotic oddities; they’re everywhere. They’re like orchestra conductors conducting the whole universe. Usually, when I get together with her, we don’t just talk about our work, though; we talk about our relationships and our stories, our lives. It’s easy for me to feel really close to her. Before this interview, we were talking on the phone, and she told me about an early model of a computer that she used when she was a teenager, a Commodore 64.

Strogatz: I, I see my, my friend Bert here, my engineer, got excited when I mentioned the Commodore 64. So, maybe some listeners won’t know what we’re referring to. What is — what’s the story, there?

Natarajan: So, the Commodore 64 was the sort of the first desktop computer, personal computer, that was marketed, and it’s, uh, it dates all of us to the late ’80s, mid- to late-’80s. And I think sort of my generation of young people — so, I was a young teenager then, and I got started programming, learning how to program. And so, it was sort of the first introduction to the digital age that really permeated, uh, society in a much broader way than could’ve been imagined. And it’s very hard now, when we all have laptops and we have phones and everything, personal, uh, items that we carry around, uh, to imagine what a breakthrough it was. What — it was exhilarating —

Strogatz: Huh.

Natarajan: — to have, like, a machine with that kind of capacity.

Strogatz: Yeah?

Natarajan: You know, I was sort of a science nerd, uh, interested in electronics and robotics and stuff at the time, so to me, it was such an incredible gift. It was a present from my dad — so I grew up in India, and my parents are professors, and they often went abroad for conferences and so on. And my father would always bring back, sort of, you know, exciting toys, books, things, and he brought back a Commodore 64 for me as a birthday present.

Strogatz: Wow. [LAUGHS] Is it the sort of computer — not something that you would assemble, it wasn’t that kind of — it was, it’s already out of the box.

Natarajan: No, it wasn’t that bad. I did that later, at MIT, but —

Strogatz: Yeah — did you? Uh-huh.

Natarajan: Yeah. But, um —

Strogatz: Huh —

Natarajan: — it had a graphics terminal, so you could actually see stuff —

Strogatz: Yeah?

Natarajan: — you could plot. And that, for me, was what was so thrilling, the fact that, not just that you could code, ‘cause I kind of learned to code, but that you could plot things and you could visualize them. And I was very visual person, I was always obsessed with maps and looking through atlases and… So, you know, the fact that you could visualize something, like, immediately after computing something was just astounding, to me, as a child. [LAUGHTER] I mean, I think — I, I felt that it was such a marvel, and I always wanted to do something more than just kind of play with it, do something useful, like a scientific calculation.

Strogatz: Yeah. Did you — so you say late-’80s, but just to help me remember, the, the —

Natarajan: Yeah.

Strogatz: — Macintosh was around in the mid-’80s, I think.

Natarajan: Yeah.

Strogatz: And, and, now, was —

Natarajan: I think this was around, roughly, the same time as a Mac.

Strogatz: Okay, but, but it had, uh, a little bit of a different relationship to the person using it, isn’t that true?

Natarajan: That’s correct, yeah.

Strogatz: So, so, in the sense that you could, you could program it, you could make it do things.

Natarajan: Yeah.

Strogatz: The Mac was more like, you know, you had to get software, and you — well, you didn’t get the software; it came with the Mac. But, uh —

Natarajan: Right.

Strogatz: — I’m trying to even remember: Did, did we actually purchase software somewhere and then have — maybe you did, you had a little floppy disk and you’d have to insert it, and then —

Natarajan: It had a floppy disk. It came with an initializing floppy disk, so, to get stuff going —

Strogatz: Yeah.

Natarajan: — but that was it. And you could store stuff on a floppy, so, ’cause I remember the first big code that I wrote, uh, which was, you know, making a star map for over Delhi, that was, like, the major accomplishment for me, at the time. And that I remember saving it on a floppy disk and —

Strogatz: Well, this is amazing, hold on. Uh, so, I have never tried to make a star map; I’m not even sure I know what a star map is. I, I was not the kind of kid that looked at the — well, is it — was it something that, like, would be posted in the newspaper, where the different stars are in the sky — That’s right — at a different — on a given day, where you should look, at what time?

Natarajan: Yeah, so I think in Delhi, at least, we had a custom, at that time, that they would publish a monthly star chart —

Strogatz: Yeah.

Natarajan: — a sky chart of the sky above Delhi. And, you know, they would mark out the constellations that you could see, and any planets, any naked-eye planets or planets that you could see with a small telescope, those, those would be marked out. And they published this, like, once a month or so. And I, you know, I was an amateur astronomer at the time, I had a telescope, and I was part of the amateur astronomers’ club at the Nehru Planetarium in Delhi, and so I was an avid sky watcher, so I used to follow those maps. And I showed up at the Nehru Planetarium, um, they had a new director, this wonderful woman, Dr. Raghavan. And I sort of showed up and I said, “Look, I’d like to do something. I have a Commodore 64 [LAUGHTER], and I can compute.” [LAUGHTER] And, so, I, I somehow thought that I had entered, like, you know, like, a new echelon of, uh, of science and I could do something useful.

Strogatz: [LAUGHS] Yes, well, yeah.

Natarajan: So, uh, yeah, I offered my services to her. [LAUGHS] So then she asked me what I liked, and I said, “Well, you know, I love maps, and I like the idea of mapping.” And, and she said, “What do you like about maps?” I said, “What I love about maps is that — “ you know, at that point, my experience with maps was from atlases, right, from atlases of both space —

Strogatz: Sure, yeah.

Natarajan: — and land. And I would say — I told her that I liked the fact that in maps, not only could you use maps to find places, but that maps also seem to reveal what is known and what is not known. Because maps often have a piece of —

Strogatz: Mm —

Natarajan: — land, for example, you know, um, denoted as terra incognita, so something you don’t know anything about.

Strogatz: Sure.

Natarajan: And to me, that … sort of that mystery —

Strogatz: Yeah.

Natarajan: — of knowing and not knowing, and both of them being sort of in the same place and showing you the frontier —

Strogatz: That’s awesome.

Natarajan: — showing you what the frontier is —

Strogatz: Yeah, yeah.

Natarajan: — like, what remains to be known, I mean, that was such an exciting thing for me.

Strogatz: It’s very, very exciting, especially where so much of the experience of school for someone at the age you were, at that time, is the teachers always telling you what is known.

Natarajan: Exactly.

Strogatz: And, and you don’t get a true picture of what science or any kind of —

Natarajan: Yeah.

Strogatz: — learning is really about. But, by the way, were there dragons —

Natarajan: Yes.

Strogatz: — drawn on some of the old maps? ’Cause what they always talk about —

Natarajan: “Here be dragons.”

Strogatz: — about, “Here be dragons,” and that.

Natarajan: Yes, yes.

Strogatz: You, you actually saw maps of that type?

Natarajan: Yes, I, I loved, also, these mythical maps — maps of mythical places as well as real. I mean, I didn’t really care as much about whether something was real or not real; that was not my preoccupation, at the time. My preoccupation was with —

Strogatz: Yeah.

Natarajan: — what the map was really trying to tell us, and what the story of that map was.

Strogatz: Wow. But, so, you, you tell, uh, the director, “I’m interested in maps,” and, you know, because of the edge between the known and the unknown. And then, she says, “Oh, I, I have an idea for something you could do.”

Natarajan: That’s right. Uh, and she said, “Oh, so, what about star maps?” I said, “Oh, yeah, I look at the star map at the dates published, uh, the monthly star map, I look at it, uh, for the one over Delhi. And, you know, I look at, you know, I look to see which constellations I can see, and so on.” And she said, “Hey, have you thought about making your own one of those maps?” And I said, “No. Ah, is that doable?” And she said, “Well, I can get you started. Uh, it’s a hard problem, but you have a Commodore 64, right?” [LAUGHTER] And I said, “Yes.” [LAUGHTER] And, um, so then she taught — she asked me if I knew, um, spherical geometry, did I know trigonometry, and I said, “Yeah, I know a little bit, and I can pick up what I need to know.” And so she —

Strogatz: [LAUGHS] So, let me interrupt — I’m sorry — I, I just want to ask you about that. Because I’ve heard, as a mathematician, I’ve heard about spherical geometry; it’s a word, a phrase you see. I don’t know that I’ve ever had it in any course. Um, I mean, I know what spherical coordinates are, I know how to locate a point on the —

Natarajan: Right, mm —

Strogatz: — surface of a sphere. But, but spherical geometry, where — I mean, that’s a pretty old-fashioned subject, right? I mean, that’s —

Natarajan: Right, that’s —

Strogatz: — they don’t teach it in school or college, anymore.

Natarajan: Right, but that’s where my Russian Center Physics and Mathematics for Everyone series came in handy.

Strogatz: Ah, oh, yeah?

Natarajan: I had read all about projections, various kinds of projections, and how you could project from, you know, the spherical sky onto a flat map, like, how that was done.

Strogatz: Ah —

Natarajan: And, um, so, yes, I had learned some of these very old-fashioned, um, sort of bits and pieces from, uh, that series. Go back and I spend, like, six-and-a-half weeks of my summer holidays cracking this problem. I crack it, and of course, then I produce the map for New Delhi, and then I show up at her door with the printout, right, and showing the map.

Strogatz: [LAUGHS]

Natarajan: And she looked at it and —

Strogatz: [LAUGHS]

Natarajan: And, you know, she told me later she did not believe that I had done it myself, first of all. So I showed her all my notebooks, and all my calculations, and how I did it, and I even had a printout of my program so that she could check it. And —

Strogatz: Wow. [LAUGHS]

Natarajan: — she still said that she was —

Strogatz: Yeah.

Natarajan: — she, she said that she, she was utterly impressed, but she thought, “This is too good,” right? Couldn’t have done it herself.” So then, she starts, starts, of course, she quizzes me, and, you know, and I explain how I did it, so it’s, she said, you know, later, when she’s recounting her sort of encounters with me —

Strogatz: [LAUGHS]

Natarajan: — when I was much older, she said, “And then I thought, ‘Hm, maybe this kid did do it,’” right? And then, she asked me a question, and she said that question settled it for her. And I don’t remember it, but, I mean, I remember answering all her questions, but I don’t remember a pivotal one. Her pivotal question — and it turns out it is so crazy-prescient, right? — she says, “Okay, this is all great, Priya, but what if you move to Boston —”

Strogatz: Yeah.

Natarajan: “— or Brisbane?”

Strogatz: Mm-hmm?

Natarajan: “So, what are you going to do about the night sky then? Who, who are you going to rely on for your star map?” And I said, “Aha, the way I’ve written the program, I — you can put the latitude —

Strogatz: Oh, boy, get outta here. [LAUGHS]

Natarajan: — “and longitude of any place on earth, and I can fire up the star map for you.” Yeah, and then she was, like, she said at that moment, she realized, “Okay, this is it, okay, I’m going to, I’m going to keep this kid close.” And I — she said that, you know, later on, she said that, at that point, she said, “Okay, uh, I really want to spend time with this kid.” [Laughs] So, and then, you know, she would give me little projects, so she was working on sunspots, so, she was doing, taking data, so I would plot things for her, or help her analyze some data, historical data. And essentially, I think what had happened is that she, you know, she introduced me to research, and I got completely seduced by this idea.

Strogatz: Yup, I hear it, that’s, that’s what this was —

Natarajan: Right.

Strogatz: — this was research, yeah.

Natarajan: And it was figuring things out. And, you know, it didn’t matter that somebody else was making a star map or that people knew how to do it; it didn’t have to be the first ever whatever, right? Just the thrill of doing it myself, that joy. And, you know, I still, yeah, and I still have that, I still have that, and that’s what I’m so grateful for. You know, so many years into the game of doing science, professionalizing, career, all of that, I still have that childlike thrill that you get when you figure something out and it works out.

Strogatz: [LAUGHS]

Natarajan: And, um, yeah, and I think that that has probably been one of the most important lessons, um, in my life —

Strogatz: Wow.

Natarajan: — that keeping that thrill alive.

Strogatz: And so, I, I love your emphasis on the point that the discovery doesn’t have to be a discovery for the world.

Natarajan: That’s right.

Strogatz: It was a discovery for you, it was new to you —

Natarajan: Yeah.

Strogatz: — and that was the fun. I mean, it wasn’t a competitive thing, like, “Oh, I solved something no one else has solved.”

Natarajan: No, yeah, not at all, it’s never been that, about that for me. It’s always been, uh, pushing my own cognitive limits, like, you know, “I couldn’t figure this out, like, two hours ago, two days ago, and now, I have figured it out.” And then I have this thing, this thirst, to master.

Strogatz: Uh, well, uh, just a very basic question, uh, is the — what is the night sky like in Delhi? Is it — I would’ve thought that there would be a lot of artificial light, but is that not true? It’s very dark?

Natarajan: It was much clearer when I was growing up in the ’80s than it is now. Now, it’s utterly polluted, I mean, there’s really no way you can really see the night sky —

Strogatz: Ah —

Natarajan: — or see the splendor of it. You might get a hint, now and then, of some constellation, but, you know, it’s really not that great. But when I was growing up, you just had to drive out — the city hadn’t expanded as much as it’s, it has expanded now, so you could just drive out, like, you know, 10 miles, 15 miles, and you would really, the — you could go away from the city lights. And it was marvelous, it was just really marvelous.

Strogatz: Mm-hmm. Well, let’s pick up about the, the broader theme of maps, because you said it was prescient for, um, the director, Raghavan, to have, you know, given you a mapping question.

Natarajan: Mm-hmm.

Strogatz: It’s become a — I don’t know, is it like the, the main central theme, would you say, of your career since then? Or, or one of them?

Natarajan: Well, I think that it informed — yeah, well, it, it sort of, it, you know, it’s the backdrop.

Strogatz: Okay, backdrop.

Natarajan: It’s the backdrop of everything that I do, and it’s the backdrop of my worldview.

Strogatz: Hm —

Natarajan: And it’s very fundamental to how I see my work, and also how I see the world, um, in terms of comprehending the world, understanding, making sense of the world, and, um, navigating the world, uh, both in science and life. Um, so, you know, she was prescient because, uh, somehow she had — was able to recognize — that you know, it also means that there’s a sort of a sense of exploration, right? Maps carry that connotation, and I definitely, you know, see myself as an explorer of some sort.

Strogatz: Mm-hmm. Mm-hmm?

Natarajan: And my younger brothers will immediately point out that, you know, I’m like this armchair — much as I wanted to be, like, you know, an explorer in the 16th Century on a ship, you know, finding terra incognita —

Strogatz: [LAUGHS] Yeah?

Natarajan: — my brothers who know me well said, “Well, I think you would not really relish a spot of scurvy or whatever.”

Strogatz: [LAUGHS] Yeah, no, I get that.

Natarajan: “You know, you’re so gentile that you would want, yeah, you would want the, you know, the glamorous comfortable version of the Age of Exploration voyagers,” right?

Strogatz: Yeah.

Natarajan: Which was not available then, [LAUGHS] so.

Strogatz: So, the term you were using was “armchair explorer.”

Natarajan: Exactly, yeah.

Strogatz: Well, ’cause I see, in some other interview with you that I read, um, that you mentioned that you — one thing you strive for, in a lot of your work, is a kind of depth of understanding that, that comes from an abstract point of view, like, uh, the mathematical —

Natarajan: Yeah.

Strogatz: — point of view getting to the essence of things. Um, that’s, that works well for armchair explorers of a certain type, yeah.

Natarajan: Absolutely. Uh, you know, and to give you an example of the, you know, when I talk about, like, mapping, right, sort of the — and you, and you must have this, right? — the, the, the sort of very peculiar ways in which one learns things. So, I learned the piano, and I was a very reluctant learner. Uh, you know, when I was very young, you know, I started learning and I didn’t really like it, and, you know, I felt — I, I really regret that now. But in order to get through my lessons and to learn them, you know, I mapped all the keys into numbers.

Strogatz: Oh —

Natarajan: And so, every piece was, you know, The Ode to Joy was 3-3-4-2-2-1, whatever, you know, I had that down. And I was … because I have a pretty good memory and that’s sort of how I learned. And, you know, I never realized that this also has to do with a sort of a facility for a certain kind of mapping, instinctually —

Strogatz: Yeah.

Natarajan: — wanting to map things. Map what you don’t know and what you’re learning to something you already know —

Strogatz: Hm —

Natarajan: — and to see if you can then push it, right?

Strogatz: Oh —

Natarajan: So —

Strogatz: Remarkable. So, does that work, as far as, like, when you play piano, do you sound like a player piano?

Natarajan: Well, I’m just, I’m just not a good enough player; I’m sort of mediocre.

Strogatz: [LAUGHS]

Natarajan: I would love to be better, but, um, you know, I’m, I’m hoping to now sincerely learn musical notation.

Strogatz: Mm, mm.

Natarajan: The problem with this mathematical transposition —

Strogatz: Yeah.

Natarajan: — is that I used that to circumvent learning musical notation, right?

Strogatz: Well, that’s what I was wondering, yeah.

Natarajan: [LAUGHS] Yeah.

Strogatz: It may be almost a crutch for you.

Natarajan: That’s right.

Strogatz: This theme of maps and, you know, the, the edge between the known and the unknown —

Natarajan: Yeah.

Strogatz: — is, is really brought together nicely, I think, in your work on mapping the universe and —

Natarajan: Yeah.

Strogatz: — mapping dark matter, and black holes as being objects at the edge —

Natarajan: Yeah.

Strogatz: — between what we know about physics and where physics breaks down. So, so maybe it’s time for us to try to get into you as the scientist you are now, and that you’ve been —

Natarajan: Yeah.

Strogatz: — for the, you know, like, say, since grad school. Um, when we talked, yesterday, you were super pumped up about talking about what you were calling “outflow” from black holes.

Natarajan: That’s right.

Strogatz: Which, is that the same thing as, I’ve read elsewhere, “burping”?

Natarajan: Yes. [LAUGHTER] Yes. There’ve been many —

Strogatz: So, tell me about — yeah.

Natarajan: Yeah. So, I, I think this was, um — You know, so I was in graduate school from 1995 onwards, at Cambridge, Cambridge England. And I was working, uh, with Martin Rees, and on trying to understand how to integrate the growth formation and evolution of black holes into the larger picture of the assembly of galaxies and structure in the universe. And around that time is really when we started, observationally, to get a hint that perhaps, you know, maybe every galaxy in the universe harbors a black hole in the center, and that the mass of this black hole has some sort of relationship, as in a correlation, with the mass of the stars in the innermost regions of a galaxy. So it was just starting to emerge that perhaps there was some connection between the assembly of galaxies, the formation of galaxies and structure in the universe, for which we had a very good theory, and, you know, black holes were sort of needed to now be incorporated into that scheme. So, for my thesis work, I did, uh, sort of the first real sort of synthetic modeling of putting these pieces together, the dark matter halos that are seeding and, uh, form the scaffolding for all structure formation. So, you know, galaxies form in the centers of dark matter halos, when gas falls in and then, you know, cools and forms stars, that’s how you assemble a galaxy. So, my work, at the time, was to develop a new framework and methodology that would somehow integrate the formation of the black hole into the scheme. And, you know, when I was doing that, um, I realized that, in order to successfully do that, you somehow had to couple some of the very small scales on which — were relevant to black holes, you know, like the event horizon, uh, the Schwarzschild radius, um, to the region which is very tiny compared to any galactic scale on which we see stars or stellar phenomena. And, and the black hole is so compact that it’s sitting right at the center of a galaxy.

Strogatz (narration): It’s going to be important to understand what Priya’s talking about when she says “event horizon.” It’s, uh, if you think of a black hole like a beachball, the event horizon would be like the rubber, the surface of the beachball. It’s the boundary that defines a region of space where, uh, anything inside that black hole cannot get out, not even light. You also heard Priya mention the Schwarzschild radius. That’s, essentially, the radius of this beachball. That’s what defines the event horizon.

Natarajan: But if the growth of the black hole somehow had to be connected to that of stars, as the correlation was suggesting, we didn’t know if there was causation or it was just, you know, a statistical fluke correlation, right? But if there was going to be causation, then there had to be a way in which the black hole could impact very large distances from the center of the galaxy. And up to then, all we had all been preoccupied with was how to feed black holes, how to grow them —

Strogatz: Mm-hmm.

Natarajan: — how do you get the gas into the center.

Strogatz: Yeah.

Natarajan: And we hadn’t though very much about what else black holes could do. So, you know, we were sort of, you know, we, we had realized, at that point, that black holes could either be feasting, they could be sitting in very gas-rich galaxies, in the centers of gas-rich galaxies, and feeding in a lot of matter very rapidly. And the quasar phenomenon was, that the matter that’s falling in, that’s being sort of heated up and glowing as it’s being swallowed up, lights up and you see a quasar.


Strogatz (narration): A quasar is one of the brightest objects in the universe; it’s thought to be caused by gas swirling into a super-supermassive black hole. When the gas falls in there, energy explodes out, and it is extremely bright; you can see it all the way across the universe.

Natarajan: What we didn’t know is how much of the matter that is actually right around the black hole finally makes it in. People suspected that they would actually dribble, like, black holes couldn’t feed fully, that there would be some material that would probably be ejected.

Strogatz: [LAUGHS] Hold, hold on, I — you’re, you’re giving me a whole — there’s so much good stuff coming at me, here. When I hear “feasting” and “dribbling,” I’ve gotta pick you up on that. Those are so vivid. The black hole — did you say that, “feasting”?

Natarajan: Yes, I did, yeah, feasting.

Strogatz: The black hole is feasting. So I’m picturing this gluttonous thing —

Natarajan: Yes.

Strogatz: — gorging itself on —

Natarajan: Right, gas.

Strogatz: — on gas?

Natarajan: Yes. [LAUGHTER] Right.

Strogatz: And then, it’s getting so, um, so worked up, occasionally, it kinda dribbles it out, like a little baby that you’re feeding, uh, the —

Natarajan: That’s right.

Strogatz: — baby food too fast, and it goes blurblurlbur?

Natarajan: Yeah.

Strogatz: [LAUGHS]

Natarajan: But it’s not as random in the sense that, you know, what’s really happening, it, it’s a very ordered physical process, right? So, this gas is being heated up, so, radiation is being released. This radiation has some pressure, so the dribble is not just going poof on the corner of your mouth, but it’s actually getting pushed out as an outflow.

Strogatz: Yeah?

Natarajan: So —

Strogatz: But, so, the first thought is, what, what happened to the idea that nothing can escape from a black hole? We’re not talking about that?

Natarajan: Oh, no, but this is, this is stuff — no, this is from way outside. This is well before he point —

Strogatz: We’re way outside the event horizon.

Natarajan: Yeah, absolutely.

Strogatz: So we’re just —

Natarajan: It’s, like, you know, 10,000 times the event horizon.

Strogatz: Okay.

Natarajan: Yeah. Or 100,000 times —

Strogatz: I see.

Natarajan: — outside the Schwarzschild radius. So, you know, this, you know, the, the, the gravity of the black hole has enormous pull —

Strogatz: Yeah.

Natarajan: — so it’s actually affecting the flows of gas from, like, way out.

Strogatz: Mm-hmm.

Natarajan: The only question is, how much of that gas would actually lose its spin or its angular moment, right, because it’s swirling? You’d need to swirl and lose all your angular momentum, to be captured by the black hole.

Strogatz: What time of — are we thinking about? Is this, like, uh, uh, uh, a bill —

Natarajan: Oh, any time in the universe.

Strogatz: Oh, so it’s —

Natarajan: So, a lot of galaxies have hydrogen, a lot of hydrogen sitting in the center, dust and gas.

Strogatz: So they’re just hydrogen out there in interstellar space, or we’re — ?

Natarajan: In the galaxy.

Strogatz: In the galaxy.

Natarajan: We’re already sitting inside a galaxy. So these are galaxies that have assembled, already, with their stars.

Strogatz: Okay, so, but — So, if the universe we’re saying, now, is 13.8 billion years old or something —

Natarajan: Right.

Strogatz: — you’re saying this could be at any time, we’re not just talking about the first billion years or anything?

Natarajan: Right, I mean, the, some of the details are slightly different in the sense that, you know, when the first stars light up and the first black holes form, we think that is roughly around the same time that these two phenomena occur. Because, you know, one way of making the first black holes is from the dead, you know, sort of the dead stars, the first episode of stars formation, the stars exhaust all their fuel, and they leave little black holes behind. So we think that could be one way to start off black holes, right, the black hole seeds. So, that could happen, we believe, when the universe was maybe, like, not even a billion years old, right?

Strogatz: Uh-huh.

Natarajan: And all the way to today. Except, in the very early universe was very gassy, there was a lot of gas, because nothing had cooled and formed stars, yet. And so, you’re progressively, over time, in the universe, converting a lot of gas into stars, stars and galaxies.

Strogatz: Mm-hmm.

Natarajan: And so, you’re locking up a lot of gas in stars. So, in early times —

Strogatz: So, when you use “gas” — sorry — when you, when you talk, keep talking about gas, I should think of things that haven’t condensed into bigger objects that we recognize as either gaseous planets or stars or —

Natarajan: Right, so, but you could —

Strogatz: Uh, you just mean, like, gas that’s kind of just out there as lots of molecules.

Natarajan: That’s right, so that’s early on.

Strogatz: Yeah.

Natarajan: And then, a lot of it gets locked up in stars, but there’s still a lot of gas amongst the stars sitting in the center of a galaxy.

Strogatz: Mm-hmm.

Natarajan: Because, you know, it’s the reservoir — there’s a reservoir of gas from which new stars keep forming and condensing out, right?

Strogatz: Uh-huh.

Natarajan: And there is a cosmic ambient distribution of gas that is trickling down to the centers of galaxies, like, a slow sort of trickle. So your gas is being replenished into galaxies over time.

Strogatz: Like, I remember my childhood astronomy books as showing stars and planets and empty space in-between them.

Natarajan: Mm-hmm.

Strogatz: And that’s not the picture we’re talking about now. I mean, yes, there’s a lot of empty space, but you’re telling me I need to be thinking about what I kept calling empty space —

Natarajan: The dense part.

Strogatz: — has a lot of gas in there that’s doing stuff.

Natarajan: That’s right.

Strogatz: That’s the reservoir —

Natarajan: There’s a lot of gas that we don’t see, right —

Strogatz: Yeah.

Natarajan: — which is not shown in those images, because it’s either too hot or it’s too cold —

Strogatz: Yeah, yeah.

Natarajan: — so it’s not glowing in the right temperature to be seen by us, visually.

Strogatz: Okay, nice. Right, so, back to, then, feeding, feasting and, and burping —

Natarajan: Right, so, yeah, and burping and all these sort of, uh, gastronomical analogies, which kind of work quite well.

Strogatz: [LAUGHS] They do.

Natarajan: So, um, the … so, a feasting black hole that is feeding, very actively — a lot of gas, uh… Because, you know, as the gas is falling in it, it gets, uh, heated up and it radiates. And that’s how, that’s how we see black holes. We see black holes indirectly, from the glowing gas that the dying gasps of this, you know, glowing gas that’s making its way in.

Strogatz: Uh-huh.

Natarajan: That’s how we see black holes.

Strogatz: Because what, that gas gets very hot and then starts radiating?

Natarajan: Heated to very high temperatures, yeah.

Strogatz: I see.

Natarajan: And starts glowing in the x-rays, typically.

Strogatz: Ah, so they’re firing out x-rays, which then ultimately make it to our x-ray detectors.

Natarajan: That’s right, that’s right. So, you know, the kinds of black holes I’m talking about, I should just clarify, are these super — they’re monsters. These are the outlier black holes that are the centers of galaxies. I mean, the universe is basically littered with black holes, big and small.

Strogatz: I think that’s something people don’t necessarily know, that’s a kind of a —

Natarajan: I know, I know.

Strogatz: I mean, I used to hear that — again, not being an expert — that black holes were these kinds of exotic phenomena. Now, from your work and your colleagues’ work, it’s, like, they’re at the center of every galaxy, they’re all over the place, they structure the universe.

Natarajan: Pretty much, yeah, that’s right. And so, at the center of the galaxies, you have these supermassive black holes. Anything above, above a million times the mass of the sun —

Strogatz: Uh-huh?

Natarajan: — is basically called a supermassive black hole. And then, we now have these even bigger monsters called —

Strogatz: Really?

Natarajan: — ultra-massive black holes, which are a billion times the mass of the sun —

Strogatz: A billion suns, wow.

Natarajan: — or bigger, they — yeah, a billion, isn’t that unbelievable?

Strogatz: It’s un — you can’t — well, first of all, nobody understands billion. [LAUGHS]

Natarajan: I know, it’s, like — that’s right. [LAUGHS] But then, you have all these little black holes, um, which are the end states of stars, that are littered everywhere. Because you have, you know, wherever there are stars, you have these littered black holes. And that’s what LIGO is detecting, you know, this detector that —

Strogatz: Yeah.

Natarajan: — detected gravitational waves from colliding black holes, they detected gravitational waves from these teeny-tiny 30 times the mass of the sun, 50 times the mass of the sun. You know, for me, they’re teeny-tiny, I mean, they’re still —

Strogatz: [LAUGHS] That’s a really fun point of view, for you they’re little, they’re the little specks, 30 times the mass of the sun. [LAUGHS]

Natarajan: That’s right.

Strogatz: That’s interesting.

Natarajan: Yeah, I, I know, right, my, uh, I often joke, yeah, I often joke with my friends, when… You know, so I love to cook, and — but, you know, I don’t — I mean, I — recipes are for guidance. I kind of really sort of figure out my own proportions. But if you ask me how much an ounce is, like, with my hand, I wouldn’t be able to tell you. But if you ask me how much a, a billion solar mass black hole —

Strogatz: That’s interesting.

Natarajan: — you know, I have enormous intuition about it. [LAUGHS] So it’s really crazy, right? You know, you know how it is, right? When you really get familiar, there’s a way in which you, your sense of scale gets recalibrated, because of the numbers that you deal with, right, that — it’s, uh, much more graspable, those kinds of numbers [LAUGHS], they’re much more graspable, uh, than even sort of real things, uh, that you encounter every day. So, in our galaxy, for example, LIGO is detecting all these little black holes that are colliding, right? These are all within our galaxy, so these are all little black holes that are sort of everywhere. And every galaxy has these —

Strogatz: Really? I didn’t get that —

Natarajan: — these —

Strogatz: Hold on, let me make you pause, there — sorry.

Natarajan: Yeah.

Strogatz: So, yeah, so, LIGO, which we all heard about —

Natarajan: Yeah.

Strogatz: — um, was it 2017 or something that we got the first —

Natarajan: Yeah, ’16, yeah.

Strogatz: 2016, okay, right —

Natarajan: No, ’17, ’17.

Strogatz: The Nobel Prize was 2017, right?

Natarajan: Yeah.

Strogatz: But they, um, you’re saying every time we keep hearing about this collision or that collision, it’s usually like a billion light years away —

Natarajan: No — that’s right.

Strogatz: — or something. And, but that’s the scale of our, just our galaxy?

Natarajan: Galaxy — yeah, it’s all within our galaxy.

[Editor’s note: The Milky Way galaxy is only about 100,000 light years across, which is the intended scale in their discussion.]

Strogatz: So that’s nothing.

Natarajan: That’s nothing, yeah, well, it’s all fairly nearby events.

Strogatz: Wow.

Natarajan: They are very nearby events.

Strogatz: Ah —

Natarajan: And, um, you know, and these supermassive black holes that are in the center of every galaxy, they also do collide. Because early on in the universe, you know, galaxies form by colliding with each other, as in —

Strogatz: Yeah.

Natarajan: — a bigger galaxy forms from the collision of two little ones, right? And if both of them have little black holes in their centers, they are going to merge and form a bigger black hole. And that process will generate gravitational waves, as well. Except that those gravitational waves have much lower frequency, and they are not detectable from the earth.

Strogatz: Oh, so you’re —

Natarajan: So, you know, LIGO, uh —

Strogatz: — telling me colliding galaxies, we —

Natarajan: And the colliding black holes in them.

Strogatz: — and the colliding black holes in, you know, their centers —

Natarajan: Supermassive black holes, yeah.

Strogatz: Yeah, that that, in principle, could give us other kinds of gravitational waves that would be too low a note for us to hear in our detectors.

Natarajan: That’s right. Uh, we are very sure that they do, and not that they may.

Strogatz: Uh-huh?

Natarajan: We’re very sure. We just haven’t heard them, yet, because we haven’t built the equipment, yet. You know, and what is really exciting about the collision of supermassive black holes is, because of where they are sitting, the center of galaxies, which are very gassy and very rich environments in terms of stars and, um, you know, packed with stars, dense, dense environments, you know, prior to these two black holes actually colliding, en route, we would start to see very interesting unique signatures that would emanate in electromagnetic spectrum.

Strogatz: Mm —

Natarajan: And when they collide, because there’s a lot of gas, right, you see a lot of imprints on the gas, so you’ll see radiation. So, for example, you know, one of the challenges with LIGO you might’ve noticed, right, is they find a gravitational wave event, and then people are scrambling to point their telescopes, all kinds of telescopes, to see where it happened.

Strogatz: Yeah.

Natarajan: Because there is no other accompanying —

Strogatz: Uh-huh.

Natarajan: — signal in the optical — not, you know, very rarely. It happens only when you have a neutron star and a black hole collision, right? So, um, in contrast —

Strogatz: So, to use a, just to use a very crude analogy for that, it would be sort of like if, in real-world, you heard something that, that caught your attention, and you have to whip your head around to look what, what made that noise, but there wouldn’t —

Natarajan: Yeah.

Strogatz: — be anything to see. And so, here you’d —

Natarajan: That’s right, there may not be —

Strogatz: Or there may not be, but — yeah.

Natarajan: — in the case if there is — yeah. For the pure black hole, black hole, little black hole, little black hole collision, there may not be anything at all, however —

Strogatz: But here you’re saying there would be something to see, because of the fireworks.

Natarajan: Oh, yeah, there’d be fireworks beforehand.

Strogatz: Okay. [LAUGHS]

Natarajan: Oh, yeah, there’d be fireworks beforehand, and I think that’s, that’s the kind of, uh, those are the kinds of calculations that people like me are doing.

Strogatz: I, I want to go back to this interesting —

Natarajan: Outflows, you want to go back to outflows, right? Yeah, yeah.

Strogatz: Well, I do, because there’s a very cool story, there, that I want to understand better.

Natarajan: That’s right.

Strogatz: You mentioned that if you have a big black hole —

Natarajan: Yeah.

Strogatz: — then the stars around it may tend to be big.

Natarajan: That’s right, the mass of the stars around the innermost regions — so there was a correlation, so, the bigger a black hole in a galaxy, the — its mass was correlated with the stars in the center.

Strogatz: Yeah, so that’s an interesting pattern.

Natarajan: And so, you found bigger black holes in bigger galaxies, so, by and large.

Strogatz: Uh-huh?

Natarajan: So, that was the hint of the correlation. So, for example, if we take the size of the earth, right?

Strogatz: Yeah.

Natarajan: So, the, if the black hole would be the size of a penny, the Schwarzschild radius would be the size of the, of a penny, at the center of the earth.

Strogatz: Okay?

Natarajan: And the stars whose masses, the mass of this black hole would be correlated with —

Strogatz: Yeah.

Natarajan: — would be the size of the earth. So, the question is, then, how does the, how does the circumference or the outer part of the, um, shell of the earth know about this little penny? Well, this little —

Strogatz: It seems unbelievable.

Natarajan: That’s right.

Strogatz: Right? It seems crazy, how could the little penny at the center of the earth have some influence to correlate its size with that of things on the surface?

Natarajan: Right, so the analogy, yeah, the analogy would be —

Strogatz: Yeah.

Natarajan: — tectonic plates and earthquakes, right? So that’s how they communicate.

Strogatz: Okay. Oh.

Natarajan: The, the center of the earth communicates with the surface —

Strogatz: Yes —

Natarajan: — via these sort of, you know, surface motions, motions through these sort of tectonic plates and deeper structures, right?

Strogatz: Uh-huh.

Natarajan: So, the idea was that, you know, black holes, therefore, all the gas that is, you know, heated up doesn’t make its way to the center, and maybe some of it gets pushed out. So what you have is an outflow of gas that reaches the equivalent of the penny pushing some gas out to the outskirts —

Strogatz: Huh —

Natarajan: — out to the surface, just like earthquakes —

Strogatz: Yeah.

Natarajan: — the same idea.

Strogatz: So the flowing gas in this, in this analogy —

Natarajan: Yeah.

Strogatz: — would be analogous to what?

Natarajan: In this case, it would be the flowing gas, yeah.

Strogatz: That would be analogous to the waves in the earth?

Natarajan: Right, right. So then, I had this crazy idea — I mean, people thought that there were, you know, there were smaller-scale outflows that, you know, stars were, you know, when stars form, there’s a lot of gas that’s pushed around, and there are winds from stars. So that was all kind of known at the time, but it was really not known if black holes, that energy that’s coming out of the radiation — uh, and in gas around the black hole, if that could be tapped mechanicallyto push gas out into a wind or an outflow. That was not known, at the time; there was no observational evidence. You know, it was theoretical, you could calculate it, and that’s what, um, I did with a collaborator of mine, who was a postdoc, at the time, at, uh, Cambridge. And, so, I realized that, you know, if a quasar could power this outflow, then, um, a couple of interesting observational tests could test such a picture. And one of them would be that this stuff would push out and it would push out a lot of gas in-between, because it would be very powerful outflow — outflow traveling at, you know, a fraction of the speed of light, which is still pretty fast. And it would sweep up a lot of the gas in-between that it encountered, right? So it was kind of really pushed through. And there would be a gas shell that… Literally, you would have a gas shell that was being moved out. And then, I calculated that, you know, what would the fate of that shell be, could that, would that shell break — it would, it would be gas, right? Could that shell form stars? Could that start to glow? And could we see these patterns of stars or little galaxies forming outside big galaxies, like, outside, you know, galaxies that harbored the quasar. So then I was thinking about, you know, could we see the gas? Would the gas itself glow? Would that shell glow before it broke up and formed stars? And then, I had this kind of real, uh, aha moment, when I was just doing the calculation and I realized that, in fact, this gas, if it hung around, and if it could hang around without forming stars quickly, then, it would glow itself. Because it would be cooling as it was getting pushed out, right? It would cool, cool, cool, and it would get very cool, and it would emit in the radio wavelengths.

Strogatz: Mm, mm.

Natarajan: Very, very long, low energy wavelengths.

Strogatz: Mm —

Natarajan: Like, sorta gigahertz sort of frequencies.

Strogatz: Mm-hmm?

Natarajan: And then I predicted, I did a calculation and I showed that if you had a supermassive black hole, or you had a super-duper-massive black hole — [LAUGHS] — you know, a billion times the mass of the sun, if you just tapped even one percent or five percent of the radiation energy in the radiation, to push mechanically, to give you this wind or outflow, then, you would have enough matter, enough gas pushed in that shell that it could glow and it could be detectable.

Strogatz: Wow, nice prediction.

Natarajan: So, I made that —

Strogatz: So this is really honest science, now.

Natarajan: Oh, yeah, totally.

Strogatz: This is big —

Natarajan: Totally.

Strogatz: You’re really sticking your neck out.

Natarajan: Totally sticking my neck out, but —

Strogatz: And you’re a grad student, right?

Natarajan: Ah, totally, yeah, grad student, yeah, hadn’t finished.

Strogatz: So, so you’re a grad student, you’re sticking your neck out, you’re saying there’s going be a — if this is true, it has a real prediction, this, these radio

Natarajan: Yeah, and then you should see it in the radio.

Strogatz: Uh-huh.

Natarajan: Of course, um, unluckily for me, at the time, there was no such instrument available —

Strogatz: Oh —

Natarajan: — operating at that frequency —

Strogatz: Yeah.

Natarajan: — at that time, 1999.

Strogatz: The frequency you’re talking about —

Natarajan: You know —

Strogatz: — the prediction of gigahertz detectors, gigahertz detectors?

Natarajan: Yeah, gig, yeah, gigahertz, yeah.

Strogatz: Oh, uh-huh?

Natarajan: And with the kind of sensitivity, so it’s a combination of sensitivity and operating in that frequency.

Strogatz: Okay, yeah.

Natarajan: Right? So, we didn’t have that, so, you know, that sweet spot wasn’t covered, yet. But, you know, fast-forward ten years, and there is talk of an instrument called ALMA, which is the large millimeter array in Chile, a radio array in Chile, that was going to be the most sensitive radio telescopes that we ever made, spanning a real range of frequencies, including this frequency where you could potentially see it. So, you know, I worked on this, I made this prediction, and I did a couple more theoretical papers on, you know, if that gas actually fragmented, and formed stars, and what would that look like, and so on and so forth, right? But then, I moved on. But you know, I kind of kept my eye out, sort of, you know, I’m not an observer, but I would sort of watch to see, “Okay, you know, is something turning on? Is this going to become feasible?” But then it turns out ALMA came on, and I noticed that, and I also noted that, you know, it would take a few years of ALMA running, for it to get to the sensitivity to detect this. You know, this would happen not in the most nearby black holes, because it turns out that the nearest supermassive black holes to us are not feasting. They’re actually fasting. [LAUGHS] Because they’ve evacuated pretty much, you know, they’re — they’re fed, they’ve reached their limit, they’re, you know, they’ve maxed out everything that’s there. They’re done, right?

Strogatz: [LAUGHS] Yeah, okay.

Natarajan: So they’re just sitting there. Yeah, like the, like the black hole in the center of the Milky Way, which is 4 million times the mass of the sun, is just sitting there, right? It’s doing nothing, except, occasionally, it’s going to catch anything that comes close, right?

Strogatz: Yeah.

Natarajan: It’s not feeding.

Strogatz: I see.

Natarajan: Uh, it’s feeding at a very, very negligible rate, right, you don’t see it as a quasar or anything, right? It doesn’t glow, at all.

Strogatz: Hm —.

Natarajan: And, um, you may see some piddly radio radiation from it —

Strogatz: Yeah.

Natarajan: — but nothing more. But, um, so, these black holes, these supermassive black holes that would be driving outflows would necessarily have to be ones that were feasting, and, therefore, they would be farther away from us. Which means you need an instrument that’s really sensitive enough to see more distant growing black holes. So, ALMA reached the sensitivity about two years ago.

Strogatz: Uh-huh, this is good.

Natarajan: And then —

Strogatz: You’re really building up the drama, here, I like this. [Laughter] And so it’s, so it’s —

Natarajan: I was not involved, you know, I was not kind of putting proposals in, because, you know, as I kind of alluded to sort of in my own nature, right, I mean, the prediction was out there, it was published, it was peer-reviewed.

Strogatz: Okay. So you say two years ago, ALMA is sensitive enough, and —

Natarajan: Was sensitive enough, and then, uh, less than a year ago, um, last December, I see a paper where they report the first detection of a quasar outflow in its radio signature. And that was — you know, it’s that same kind of thrill that I told you, the childlike — thrill [LAUGHS] you have when you sort of figured something out, but this was even better. [LAUGHS] Because, you know, this was a prediction, and it was, you know, taking a risk, a creative risk, and, uh, yeah, that was super exciting.

Strogatz: That is the ultimate. That’s what we live for.

Natarajan: That’s what we live for, right? And I just found out that, uh —

Strogatz: [LAUGHS]

Natarajan: So, I found out from this group that they sort of detected another one. You know, obviously, you know, just finding one isn’t enough.

Strogatz (narration): I love talking to Priya about the connections between art and science, and her emphasis on maps helps me connect the dots between what artists do and what scientists do.

Natarajan: I feel a very deep connection to art, in particular, sort of modern abstract art, uh, because I think there is something, at a very, very deep level, that is very similar about what we all do, like, how we think about the world and how artists see the world. And what they capture about the world and their imagination, and how they put it together and they create something, right?

Strogatz: Mm-hmm.

Natarajan: And I think we do something that’s very analogous: we look at nature, we look at the world, we make measurements. But then we also have, you know, the beauty of mathematics, the power of mathematics, to build a model. And then, we create something. There is a deep connection between the way in which artists map their inner world to the outer world, and the way we scientists do it. Again, it’s coming back to mapping, right?

Strogatz: Well, that’s what I’d like to understand, because I think you have some, some notion of maps that’s more generalized or, uh, you know… Like, most people, if they’re hearing “map,” they’re thinking a map, something you unroll on the table and it’s got drawings.

Natarajan: Mm-hmm.

Strogatz: But, but certainly, in science, we talk about mapping this problem onto that problem, you know —

Natarajan: That’s right.

Strogatz: I mean, we, we use the word “map,” and then there’s all these sayings about “the map is not the territory.”

Natarajan: That’s right.

Strogatz: So you’re probably thinking in terms of mental models versus reality —

Natarajan: That’s right, that’s right.

Strogatz: — and so on. So, can you —

Natarajan: Uh, no, I’m —

Strogatz: — expand a little for us —

Natarajan: Yeah, so —

Strogatz: — what you’re talking about here with, with maps, and what’s so appealing for you? I, I haven’t posed the question crisply. I’d just like to hear more of your thinking about maps.

Natarajan: So, I think that maps are used both, like, literally and metaphorically.

Strogatz: Yeah.

Natarajan: And literally, of course, they are sort of, you know, uh, things that you unfold and you, I mean, you can locate, right? It’s a way of spatial location and of, of, uh, of visualizing connections between things, right? That’s sort of broadly what sort of, you know, cartographic sort of maps are. But I think of maps as mental devices —

Strogatz: Yeah.

Natarajan: — because they give you the capacity to represent ideas, abstract things, concepts. And through a sort of a multilayered process, allows you to grow from the level of abstraction and conceptualization to something very, very real.

Strogatz: Yes.

Natarajan: And, uh, and, and the reality — and, as you said, in the kind of work that you do, too, right —  these sort of models, they start out being very abstract, and they are very sort of concentrated, right? They have this, sort of, you first start out with the model that has the essence of the problem that is simplified. And you hope that you have made the right approximations and, you know, neglected what you think are external extraneous things.

Strogatz: Mm —

Natarajan: And that you’ve got at the essence of it, right? And then you solve it, and then you try to understand it. And then you say, “Okay, now, let me go and reintroduce, like, one little bell and one little whistle, and then see how that changes,” right? So, I see, this is sort of what we do when we build models, and in sort of the scientific work, and sort of the, sort of the theoretical work that I do. And, you know, it’s informed by data, it’s very much informed by data. What I do is, of course, you know, these models are not, you know, sort of crazy out-there models. They are models that are constrained by what we know already, right? Uh, but the goal is, then, to push these forward and to make predictions for something that is not known, yet, not detected, yet, but is likely to be detected, right?

Strogatz: Mm-hmm.

Natarajan: So, I see this whole process is what I sort of, you know, also refer to as mapping.

Strogatz: Okay.

Natarajan: And as you said, you keep mapping, you keep moving the problem, um, and, you know, add complexity, you pare it down. I mean, this is the thing that I sort of really relish about the kind of work that we all do, is the fact that, you know, you have these very complex problems that you need to solve, with many interconnections, things feeding into each other, many things affecting each other. But then, there is a way that mathematics and physics allows you to pare things down.

Strogatz: Well, so, is this, uh, uh, when you — you keep emphasizing that aspect of, of your work or of the work that appeals to you. And, and is it a — is that why abstract art is the kind of art that appeals to you the most?

Natarajan: That’s right, absolutely, absolutely, you got me, absolutely.

Strogatz: So, because it’s about the essence and the parts that are irrelevant or not what the artists wants to focus on are just gone, it’s minimalism, we don’t —

Natarajan: That’s right.

Strogatz: — we don’t need to see that other stuff.

Natarajan: How you, how you extract the essence in art is much more freewheeling. For us, for us in maths and in physics, right, we have laws, right, I mean, which is just incredible. We have laws, we have axioms, we have theorems, and they sort of delimit a little bit, right, they circumscribe the kinds of extractions and paring down that we can do, like, what is permitted.

Strogatz: When I talk to scientists, I, I love to hear about the nitty-gritty. How do they actually work? What are their work habits? Helps me understand how they think. Do you sit behind a computer? Are you using pencil and paper? I’m, I suppose you’re doing both, but —

Natarajan: Yeah, I do both. So, I often start out with pencil and paper — I’m extremely visual, so I start out, often, sort of with, you know, like, a cartoony — so, I have sort of an intuition of how something might work physically. And, so, and I think that really does come out of, you know, like, a very sort of strong sense of being grounded in physics and foundations of physics and mathematics. And, so, I often come up with a diagram, like, a little diagram, that’s how I start —

Strogatz: Mm-hmm.

Natarajan: — when I’m thinking about a problem. So, you know, the problem that I’ve been working on for the last ten years, which I’m really thrilled is, like, moving in a very good direction and we have made some great predictions, is of a brand-new way to make the first black hole seeds. And, so —

Strogatz: The seeds, the seeds of the black holes, uh-huh.

Natarajan: Holes — uh, seeds of supermassive black holes.

Strogatz: Of supermassive ones, yeah.

Natarajan: Yeah, supermassive ones. So, and then, you know, as I said, the standard way to make them would be from the end states of the first stars that form, and the subsequent generations of stars that form. But it turns out those seeds are tiny, they’re not massive enough to explain the detected presence of supermassive black holes already in place as quasars when the universe was a fraction of its age. So you see these very bright quasars, very far away, and they’re glowing, and from their luminosities, you can figure out how massive the black hole is likely to be. And they’re already billions of times the mass of the sun when the universe was 15 percent of its current age.

Strogatz: Mm —

Natarajan: How is that possible? So —

Strogatz: That’s such an interesting mystery, that, you know, if you’re going to make something that’s a billion suns’ worth of mass —

Natarajan: Matter, yeah.

Strogatz: — but — or, matter, yeah — but you don’t have, you haven’t even made suns, yet, or you’re just barely making ’em —

Natarajan: That’s right.

Strogatz: — that’s a big profound mystery.

Natarajan: Yeah, so, how do you scoop up the mass, like, how do you scoop it up, how do you scoop up so much mass? So, so we have this idea that we — I developed with a postdoc of mine, uh, Giuseppe Lodato, um, you know, over about 15 years ago we started on this sort of, you know, which started out as a speculative, you know, just a germ of an idea, and then you develop it. And as I said, you know, we started out with a pared down idea, with basically a picture. So, the idea was that, you know, the way to resolve this is obvious, right? Which is, you need to somehow make a baby black hole to be much more massive from the get-go.

Strogatz: Yeah.

Natarajan: So you just need to make it, like, 1,000 times, 10,000, 100,000 times the mass of the sun, from the get-go. And how do you do that? You need certain sets of physical conditions, um, in the universe, so you need to, uh, you know, have — we know that in the universe, the early — before you form any stars, you have hydrogen gas, and that gas would settle down into some kind of disky structure. And if this — this disk normally fragments and forms – cools and forms stars, and that’s how you form the stars —

Strogatz: Uh-huh?

Natarajan: — that’s the standard lore. But if somehow you could keep this pregalactic disk, keep it intact —

Strogatz: I see.

Natarajan: — and introduce a particular kind of instability, you know, the kind of vortex instability that would have, you know, a bathtub, you pull out the plug in the bathtub and see the water sort of flowing down into that vortex?

Strogatz: Sure.

Natarajan: If, somehow, you’re able to trigger that kind of instability in the gas very early on, then you can siphon, scoop up a lot of gas down the center, to make a very massive black hole. So —

Strogatz: So, let me try a, let me try to give you a version of what I thought I just heard.

Natarajan: Mm —

Strogatz: Uh, if I had a big puddle of water —

Natarajan: Mm, yeah.

Strogatz: — which is sort be, going to be like your gas, I could splatter it into lots of little raindrops, little —

Natarajan: Yeah.

Strogatz: That would be like making stars.

Natarajan: Mm —

Strogatz: And that’s not going to work. Whereas, if I —

Natarajan: But that, that won’t help for the black hole, you’d have to wait.

Strogatz: It won’t help to make a supermassive black hole. The trick is to somehow spin up that puddle, like, as if it were all getting sucked down the drain —

Natarajan: Yeah.

Strogatz: — make a little tornado’s worth, so it all got focused, yeah.

Natarajan: Exactly, absolutely, you got it, right? So, I, so, for this particular problem that — I still have my notebook. So I still work with a fountain pen on notebooks, that’s what I do, and I have a notebook from Day One of when I started doing research. They’re lined up in my office. [LAUGHS] So, I can go back and show you that, you know, I have this drawing of the vortex. So, I got this idea because, for some reason, I — I don’t know if I was watching a tornado or I was sitting in my bathtub or something — [LAUGHS] — and then I thought, “Ah, that instability, if I could use that instability, somehow,” right? ’Cause you need to deliver matter very rapidly to a very compact place —

Strogatz: Yeah.

Natarajan: — in the center of a early galaxy. So that, so you start — it starts out with diagraming like that, and then sort of working out the physics of what, you know, what are the conditions that would give you that kind of vortex and that kind of flow. And then, and then sort of slowly realizing, okay, how long will it take to build up, you know, the kind of direct collapse black hole seed — that’s what they’re called, now — um, and so on, right? And then you realize, at that — there’s a, there’s a part of the problem that you reach where you basically now need a computer to look at the more general. So you can work out one specific scenario and say, “Okay, I can make it work in this instance, this circumstance, and thus the numbers work out.” And then, to sort of generalize it and to sort of put it in the larger context of what is known, you need a computer. So then I moved to modeling on the computer, either with an actual simulation or a solving of a set of mathematical equations, right? You know, the stages are quite clear in my head —

Strogatz: Mm-hmm?

Natarajan: — because I’ve been sort of, you know, marching along in this problem, over the last decade.

Strogatz: Wow.

Natarajan: And so, now, then, the question — the big question, of course — is are those conditions that, you know, under which this will happen, are they available —

Strogatz: Yeah.

Natarajan: — in the early universe, right?

Strogatz: Yeah, yeah.

Natarajan: So that’s the basic question, right, this is great, I figured out the temperature, the density, everything you need —

Strogatz: Really?

Natarajan:  — to generate this, but then are they available? And then, you need to go look at these cosmological simulations, these very large sort of, you know, simulations of, uh, the early universe, from the initial conditions to now —

Strogatz: Uh-huh?

Natarajan: — with dark matter and all the known physics. So then, we went and then we saw, ah, it turns out that, yes, in some rare places in the universe —

Strogatz: Yeah?

Natarajan: — these conditions are available. So this is a feasible process, right? So —

Strogatz: But is it too rare, would there be not enough supermassive black holes?

Natarajan: No, there — it turns out, that’s the thing, it was not — they don’t have to be too ubiquitous. Because these supermassive black holes that are billion times the mass of the sun —

Strogatz: Yeah.

Natarajan: — very early on, it’s a tiny handful of them.

Strogatz: Okay, so that’s working.

Natarajan: Most of them are garden variety.

Strogatz: That sounds like that’s working.

Natarajan: Totally working, yeah.

Strogatz: Whoa.

Natarajan: So, and then we worked out, over time, when we could do better simulations, more accurate simulations, um, of the early universe, we worked out that, actually, the statistics work out perfectly well.

Strogatz: Oh, boy. [LAUGHS]

Natarajan: Then the question is, what are all the physical conditions? So, one key physical condition, as I was telling you, we’ve got to prevent the formation of stars.

Strogatz: Yeah.

Natarajan: Right?

Strogatz: Right.

Natarajan: And so, and the way stars form is because, you know, gas cools, so you need molecular, um — so, uh, everything is atomic hydrogen, and then, you start forming molecules, and that means they’re going to cool the gas, the presence of molecules. So basically, what you need is you need some process that would prevent the formation of hydrogen molecules.

Strogatz: Mm-hmm.

Natarajan: And we know one way to do it, easy way, is radiation.

Strogatz: Hm —

Natarajan: So basically, this then tells you that you need a location in the universe where you’re forming stars already, so you have radiation. And then, nearby, there is a region where this kind of swirling vortex —

Strogatz: Mm-hmm.

Natarajan: — can form. And the radiation from the stars that are forming in the nearby dark matter halo early galaxy will suppress the cooling and allow this process to occur.

Strogatz: It’s, it’s such a Sherlock Holmes process you’re using, here.

Natarajan: I know, it is, right?

Strogatz: You’re assembling all these different clues, it would have to be, like, this, this, and this —

Natarajan: Right.

Strogatz: — and, uh.

Natarajan: And, yeah, and then, what you find is that (a) it’s viable, and not just that… Like, the basic physics has also told you the very specific, it’s helping you hone in on the specific sets of conditions —

Strogatz: Yeah.

Natarajan: — all the conditions that you need for this to be fulfilled. And then the question is, is this too restrictive, right? Again, the rarity, right?

Strogatz: Uh-huh, right, right.

Natarajan:  “Oh, my god, can this happen often enough?” It turns out it can.

Strogatz: Hm —

Natarajan: And so, so that hurdle — so, and, you know, and each of these things, this whole idea, um, you know, it took about ten years —

Strogatz: Wow.

Natarajan: — to get to this point. Because the simulations were not good enough, because the computers were not —

Strogatz: Yeah.

Natarajan: — fast enough.

Strogatz: Oh, boy. Yeah, yeah, interesting.

Natarajan: And so, everything had to kind of, you know, everything has been kind of converging, right, as time. And then, finally, a year ago, we reached the point where we could actually make a prediction of the spectrum of this object. And we were able to — this object, as it’s assembling —

Strogatz: Yeah.

Natarajan: — and it’s growing, what would it look like.

Strogatz: Uh-huh?

Natarajan: And it turns out that it would emit, again, in very long wavelengths, like, one to three to five microns, in the very early universe which is very dusty, and that’s where you would see the signature. And guess what, the James Webb Space Telescope has the exact window where it should be able to see it. So, I’ve put my neck out again. I’ve put my neck out —

Strogatz: It sounds like your neck is extremely stretched out.

Natarajan: [LAUGHS] Right?

Strogatz: I mean, first, uh, wait, so, micron wavelength, you’re saying? These are microwaves?

Natarajan: Yeah, one to, one to five, one to, yeah, one to five microns, one to ten microns is where this spectral signature is quite unique for this class of object.

Strogatz: But what, what would those, what’s — I just don’t remember my EM, uh, my electromagnetic spectrum — what would those waves be called, at that wavelength?

Natarajan: They’re just microwaves, right, so, so microns, yeah.

Strogatz: They’re microwaves, okay, yeah, I thought so, yes, so, so —

Natarajan: And so, these are infrared sort of mid, far, near infrared radiation.

Strogatz: But you’re saying it will have not just, uh, that it’s going to have a, a lot of power in that set of wavelengths, but it’ll —

Natarajan: Yeah, most of the power, so most of the power —

Strogatz: It’ll have a particular spectral pattern signature?

Natarajan: That’s right, a very spectral, clear spectral signature from the hydrogen.

Strogatz: Like a fingerprint, yeah.

Natarajan: A total fingerprint. A unique fingerprint. And unlike normal garden variety quasar, slightly later on in the universe, right, which is a slightly more assembled mature black hole —

Strogatz: Yeah.

Natarajan: — later on in the universe, where we talked about, earlier, much of the output would come in the x-ray, for these very early chaps, much — they, uh, the x-ray is suppressed and it all comes out in the infrared, because the universe is very dusty.

Strogatz: Did, did you just call them “chaps”?

Natarajan: Yes, I called them chaps, yes.

Strogatz: These “very early chaps”? [LAUGHTER]

Natarajan: [LAUGHS] I also anthropomorphize the objects that I’m studying.

Strogatz: I love it. Man, I gotta — I want to draw a picture of these things. We’ve got these chaps that are feasting, they’re belching —

Natarajan: [LAUGHS]

Strogatz: There’s like some kind of Shakespearean character. [LAUGHTER] Okay, sorry —

Natarajan: Perhaps, perhaps this anthropomorphizing is not something I quite wanted to reveal, but anyway. [LAUGHS]

Strogatz: We should, no, come on, this is how — the playful spirit of science is how the great scientists do their thing. You’re, you’re showing how it’s really done. This is, this is the thrill of doing science: you, you have a guess about how the universe is going to behave — and we’re not just making stuff up, I mean, this is a really —

Natarajan: That’s right.

Strogatz: — important process, if it exists, and it may not. But, I mean —

Natarajan: That’s right.

Strogatz: — right now, it’s, it’s like we’re waiting for the end of this movie, and we don’t know how this is going to turn out.

Natarajan: Yeah, right, yeah, we don’t know how it turns out, yeah.

Strogatz: And it might, it might be right, it might not be right, but it’s a beautiful idea —

Natarajan: Yeah.

Strogatz: — that came from this, um, you know, kind of like very down-to-earth thing, thinking about the, the bathtub vortex and —

Natarajan: [LAUGHS]

Strogatz: But then with a lot of detailed calculations —

Natarajan: Right, and time.

Strogatz: — ten years of tenacity — yeah.

Natarajan: No, no, but also, I mean, I have to tell you that, you know, when I started it, everyone — you know, it was always a peer-reviewed, published, everything, right? And people thought, “Mm, oh, another of Priya’s very, uh, original inventive but speculative ideas,” right? And, “Hm, it’s kind of speculative.” You know, “speculation” is also a loaded word, right, and there are people who, like, you know, uh, do crazy speculation. I don’t do crazy speculation —

Strogatz: No, no.

Natarajan: — it’s a very calculated speculation —

Strogatz: You’re very constrained, you’re paying attention to all the —

Natarajan: Uh, yeah, very, yeah, yeah. But, yes, I mean, I think I’m known as someone who, whose papers will have something, some kind of new kind of thing that pushes stuff.

Strogatz: Uh-huh.

Natarajan: You know, I’m not particularly religious, but I was born Hindu, but, and, but I have read the scriptures, I know Sanskrit, and I’ve read a lot of Indian philosophy, enough to understand the things that I like about it, and what I can take, and so on. And I think this, you know, the idea, uh, there’s a very strong theme, um, in Indian philosophical thought, about how you just perform the actions, and you don’t get invested in the result, right?

Strogatz: Uh-huh.

Natarajan: The outcome is not in your hands, and that’s not what you need to examine the motivations for why you are doing what you are doing. And it’s a very famous quote from the Bhagavad Gita. And, you know, you don’t realize it, and I probably would not have admitted to this about maybe even 15 years ago, but now that I look back and I think and I say, “You know what, that is sort of what has really guided me.” That’s sort of, um, that’s what has allowed me to develop the kinds of attitudes that I have, that have served me well, right, and that continue to serve me well. Um, try and not be obsessed with kind of outcomes and the sort of, even the sort of validation of theories and predictions and, you know, all of that, right? To not be trapped by that, to not be defined just by that. And that’s why, in a way, I talked a lot more about the process, uh, today, when talking to you —

Strogatz: Mm-hmm, yes.

Natarajan: — more than kind of listing to you all the discoveries and all of the major breakthroughsthat have come out of my work, blablabla, [laughs] right? And, and I think —

Strogatz: No, the process is what’s so interesting —

Natarajan: Yeah.

Strogatz: — and we can all relate to that. And of course, there’s so much luck in the outcomes and —

Natarajan: Oh, enormous, right?

Strogatz: — and as you say, it can be a trap. Yeah.

Natarajan: Yeah.

Strogatz: No, it’s, uh, um, and I think we can all relate to that, whatever our — whether in business —

Natarajan: Mm-hmm.

Strogatz: — or any creative field —

Natarajan: Mm —

Strogatz: — or anything: you have to think about your process.

Natarajan: Mm —

Strogatz: Um, do, and being, you know, being honest, being true to yourself and your art, love your art. And, yeah, you may get lucky and get noticed, or not, but at least you have —

Natarajan: Right.

Strogatz: — integrity. But also, I, I once had a mentor who said, “You know, if you really want something remarkable to have a chance of happening, you have to study something or work on something that grips you irrationally —”

Natarajan: Yes.

Strogatz: — “by the imagination.” That’s the way he put it to me.

Natarajan: That’s so beautiful.

Strogatz: Isn’t that a cool phrase? He says, uh — that was Art Winfree —

Natarajan: Yeah.

Strogatz: He said, “It has to grip you irrationally by the imagination.”

Natarajan: Yes.

Strogatz: And what I like about that, I mean, I guess you get it when you say it’s beautiful.

Strogatz: The irrational part is the key.

Natarajan: Yes, exactly.

Strogatz: That you can’t, you can’t even explain why you love the problem.

Natarajan: Right.

Strogatz: You just love it so much —

Natarajan: Right.

Strogatz: — that you think about it when you take a bath and, you know, when you’re driving or whatever, it’s just, you’re in love with it. And maybe something remarkable will happen. That’s what it takes.

Natarajan: Right, and it’s a certain kind of being consumed by it, right?

Strogatz: Yes. Well —

Natarajan: That, yeah, and it’s a suspension of, uh —

Strogatz: I mean, your ten-year voyage with this bathtub vortex that may have created the black holes, the supermassive ones —

Natarajan: [LAUGHS]

Strogatz: — and given structure to the universe, you know, I mean, clearly, you’re consumed at ten years on it.

Natarajan: Yeah.

Strogatz: And it’s not done, yet, and we don’t even know if it’s going to be right.

Natarajan: Oh, it’s not done, yet, yeah, yeah, yeah.


Next time on “The Joy of x,” Alex Kontorovich will tell us what it was like to work with the Michael Jordan of mathematics.


“The Joy of x” is a podcast project of Quanta Magazine. We’re produced by Story Mechanics.

Our producers are Dana Bialek and Camille Petersen. Our music is composed by Yuri Weber and Charles Michelet. Ellen Horne is our executive producer. From Quanta Magazine, our editorial advisors are Thomas Lin and John Rennie. Our sound engineers are Charles Michelet, and at the Cornell University Broadcast Studio, Glen Palmer and Bertrand Odom-Reed, though I know him as Bert. I’m Steve Strogatz. Thanks for listening.


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