Andrea Ghez
Big Brains podcst

Unlocking the secrets of black holes, with Andrea Ghez (Ep. 81)

Nobel-winning scientist examines the monster at the center of our galaxy—and how we got here

Andrea Ghez
Big Brains podcst

Show Notes

If you know anything about black holes, it may come as a surprise to learn that there’s actually one lurking at the center of our galaxy. It was uncovered by UCLA astrophysicist Andrea Ghez, and in 2020 she won a Nobel Prize for this discovery. But how do you go about finding something that emits no light? How do you see the unseeable?

In this episode, Ghez explains how she proved this supermassive black hole was hiding in the Milky Way and answers all our pressing questions like, including: Are we being sucked into this monster? And could researching it prove Einstein’s theory of relativity is actually wrong?

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(Episode published November 18, 2021)



Paul Rand: There’s a monster lurking at the center of our galaxy.

Tape: It’s a monster, all right.

Tape: A rip in the very fabric of space and time.

Paul Rand: It’s been there your whole life, and maybe as long as the life of our galaxy itself.

Andrea Ghez: Well, it’s a million to a billion times the mass of the Sun, so you might call it monstrous in its mass.

Paul Rand: That is Andrea Ghez. She’s a professor of physics and astronomy at UCLA, and a graduate of the University of Chicago Laboratory Schools. And, last year, she won the Nobel Prize in physics for proving that, at the center of the Milky Way, is a black hole.

Tape: This year’s prize is about the darkest secrets of the universe.

Paul Rand: And not just any black hole, but a supermassive one.

Andrea Ghez: Today, we think that most, if not all, galaxies harbors black holes at their centers, and that this black hole plays a really essential part in both the formation of the galaxy, our own is the Milky Way, and the evolution of galaxies, which are really the fundamental building blocks of our universe.

Paul Rand: If you’ve seen or read any sci-fi, your first reaction is probably to fear that we’re being sucked into this black hole.

Andrea Ghez: So black holes have this very bad reputation of being cosmic vacuum cleaners. That is an unfortunate, and incorrect, conceptualization. For the most part, most things that are nearby it will happily orbit the black hole the same way that the planets orbit the Sun. That you don’t get the pulling, or dragging in, of objects.

Paul Rand: The idea that there’s something at the center of our galaxy that you not only can’t see, but that we don’t understand, is terrifying. But it bothers scientists on a fundamental level. Their job is to find things out, and not only do they not know what happens inside black holes, but they think if they can find the answer, it may hold the keys to understanding all of space and time.

Andrea Ghez: Black holes are such interesting objects because they’re some of the most simple objects that we have in our universe, and yet they’re the most complex because we don’t have the physics to describe them.

Paul Rand: From the University of Chicago podcast network, this is Big Brains: a podcast about the pioneering research, and pivotal breakthroughs that are reshaping our world. On this episode, the monster black hole that lurks at the center of our galaxy, and the quest to prove it exists.

Paul Rand: I’m your host, Paul Rand.

Paul Rand: Andrea Ghez may have won the Nobel Prize in 2020, but she’s been thinking about space ever since she was four years old.

Andrea Ghez: Oh, gosh. The scale of the universe was really what caught my fantasy. Or, sorry, that’s what caught my fancy. And my fantasy. I think I was so ... I think troubled probably was the first place.

Paul Rand: Oh, interesting.

Andrea Ghez: And it was that early recognition that you’re insignificant on the scale of the universe, both in terms of space ...

Paul Rand: Which is great when you’re four, isn’t it?

Andrea Ghez: Yeah. A little terrifying. I mean, I really distinctly remember this. It kept me up at night. And all these things that, before you have the mathematical tools, our every day experience don’t prepare us for thinking about, well what does it mean to talk about the beginning and end of time, or the edge of space? I mean, that’s how you think about your world at age four.

Paul Rand: Right. Right. Of course.

Andrea Ghez: So, I think those were the things that stuck with me, and stuck with me for a really long time.

Paul Rand: And there’s almost nothing more terrifying out in space than black holes, so naturally that’s where Ghez was drawn. But not all black holes are created equal. Some stand out more than others.

Andrea Ghez: There are two forms of black holes that we think exist astrophysical, and they’re distinguished based on mass, and we also think that they’re distinguished based on how they were formed.

Paul Rand: The first is probably what you’re thinking of: a star, kind of like our Sun, but bigger, that collapsed at the end of its life.

Andrea Ghez: So, theory predicts that stars that start their lives off with a lot of mass, much more massive than the Sun, they should end their life as stellar mass black holes. And they are roughly ten times the mass of the Sun, on order of ten. Okay. So we predicted them theoretically, and then we found them observationally. That was about, oh gosh, half a century ago.

Paul Rand: Then there’s the other kind, and they made those look like small potatoes.

Andrea Ghez: The supermassive black holes, by their name you know that they’re much more massive, so they’re roughly a million to a billion times the mass of the Sun.

Paul Rand: And supermassive black holes are really Ghez’s specialty. She was awarded the Nobel Prize in 2020 for proving that one is at the center of the Milky Way.

Andrea Ghez: So observations of the centers of galaxies revealed phenomena that weren’t easily explained by other things that we recognized. And that lead people, those observations, lead people to suggest that they could exist. And that’s what’s such a big deal about this result, is that we’ve demonstrated that this other type of black hole, the ones that are a million to a billion and that reside at the center of galaxies, exist.

Paul Rand: But wait a minute. Black holes are called black holes because we actually can’t see them.

Tape: Getting towards blackness.

Paul Rand: They emit no light. So how do you prove something exists if you can’t see it?

Tape: It’s all blackness.

Paul Rand: To see the unseeable, Ghez has an A star near the black hole.

Andrea Ghez: So SO-2 is my favorite star in the universe.

Paul Rand: Okay. That’s quite a designation. Why is it your favorite?

Andrea Ghez: It’s my favorite because it’s the one that has the most capacity to tell us what lurks at the center of the galaxy.

Paul Rand: What Ghez did was take very good measurements of the things you can see, like the stars nearby, and she could see they were orbiting something invisible, but very big.

Andrea Ghez: How long does it take these stars to orbit whatever’s at the center? So SO-2, my favorite, goes around the center of the galaxy every 16 years. So once we could figure out both the size of the orbit, like how far away it gets, the shape of the orbit, and the time scale of the orbit, we can figure out the mass that’s driving it’s motion. So we, at that point, could say it is four million times the mass of the Sun inside a region that corresponds to the scale of our solar system, and that is the proof of a black hole.

Paul Rand: So what came first in the realm, the galaxy or the black hole?

Andrea Ghez: Well, that’s a great question. In fact, it’s kind of like the chicken or the egg question.

Paul Rand: Uh huh.

Andrea Ghez: And for many years, people didn’t know, and, in fact, in the early years of this research, that’s how it was phrased. Which came first, the galaxy or the black? And, in fact, as a community we had explanations that could support one or the other, and today we, I think, shifted to understand that that’s even the wrong question. That the mass of the central part of the galaxy seems to be always correlated with the mass of the black hole, and the scale ... the size, the scales are so different, that suggests that whatever formed one had to form the other synergistically, and that there’s some feedback mechanism that keeps that relationship fixed over the lifetime of the galaxy. So it means that there’s something really important in terms of the controlling feature, or that relationship between the black hole and the galaxy. So they formed together.

Andrea Ghez: But, in fact, because we think that there’s such an intimate relationship between the supermassive black holes and their host galaxies, the supermassive black holes are found at the very center of these galaxies, we think that they formed from the process that gave rise to the galaxy in the first place.

Andrea Ghez: So if you think about the universe, which started off as an incredibly simple place: it was big, empty, kind of boring.

Paul Rand: Little dark.

Andrea Ghez: A little dark. Very dark. But there were fluctuations of densities, of very simple gas, that collapsed. So it was a collapsed process that led to the formation of galaxies. So in some sense it’s very similar to the formation of stars, but on a much bigger scale.

Paul Rand: So then is the idea as you describe it that every planet, every star, eventually becomes a black hole?

Andrea Ghez: Oh no. Actually, we have to be super careful about this.

Paul Rand: Okay.

Andrea Ghez: So it’s only the most massive stars in which we get ... you can describe it as a balancing of forces ...

Paul Rand: Mm-hmm (affirmative).

Andrea Ghez: ... or ... and here I want to be careful. I’ll describe it as a tug-of-war for conceptualization. So you have gravity on one side, and at the other side you have other forms of forces. Black holes form when gravity wins over anything that can be put on the other side.

Paul Rand: Okay.

Andrea Ghez: So the Sun, there’s not enough gravitational force associated with that mass for the Sun to overcome ... effectively, it’s actually the fact that the electrons don’t like each other, or don’t like to share the same space. So that creates a force that counteracts gravity, so you have to get to much more massive objects to overcome that force and other forces that can get in the way of that collapse. So we think that stars in our own galaxy that start their lives off with more mass than 30 times the mass of the Sun will end up as stellarmass black holes. In other words, you’ve created that environment in which gravity wins.

Paul Rand: You’ve also talked about describing this monster, and I think I remember this, where you talked about it as having an unusually large meal, maybe playing this analogy out a little bit more. So why the nightmares are there are beginning to come up a little bit more. Does that still hold as you think about this description? What did that mean as you said it?

Andrea Ghez: So it’s interesting in terms of talking about things where we’re really at the edge of, or the frontier of, our knowledge. How do you create these analogies that work?

Paul Rand: Yes.

Andrea Ghez: So if we go back to thinking about what observations made us think about the existence of supermassive black holes, one of those observations was the emission of a tremendous amount of light at the center of galaxies that was unlike anything emitted by stars or gas. And today we think that that’s from matter falling onto the black hole. So the way we think about those galaxies that we first looked at to say that there were supermassive black holes are called active galactic nuclei. So in other words, they’re galaxies where their nuclei, or centers, are active. That’s a lot of mission. And we say, well that’s probably lots of matter falling onto the black hole, or falling through the event horizon, the last point that light can escape, onto the black hole. So that’s where I like to make that analogy, that these are black holes or galaxies that are having a Thanksgiving feast, but they’re dining on a lot of matter that’s just outside the event horizon. In other words, available, close enough to the event horizon to be pulled in. So the giant dining feast analogy works really well.

Andrea Ghez: And it’s useful to make that analogy when you’re talking about our galaxy, because our galaxy is very quiet in comparison to these active galactic nuclei. There is a source that looks like a very wimpy version of active galactic nuclei in terms of its characteristics, but it’s wimpy. So you can talk about ... it’s like our galaxy, if you think that that’s the same phenomena, is having a snack.

Paul Rand: And do they ever end, black holes in general, ending? Do they stop?

Andrea Ghez: Well, that’s also an interesting question that forces one to think about the ultimate evolution of the universe.

Paul Rand: Okay.

Andrea Ghez: So our universe has been around for roughly 14 billion years. That seems like a long time scale to us.

Paul Rand: It does.

Andrea Ghez: But the universe is going to go on for much longer times scales, if you just ... these things ... all the objects in our universe live for a long time, but black holes live for a really long time. There’s a wonderful book by Fred Adams called The Dark Ages, which just runs, basically, a simulation of the universe out to its absurd endpoints, and what you have is a very dark place, and hence the name, and a lot of black holes just hanging around.

Paul Rand: Okay. I think I’ve been in a bar like that before.

Paul Rand: After the break, Einstein, a black hole, and quantum mechanics walk into a bar. We’ll talk about what happens next.

Paul Rand: Hello Big Brains listeners. The University of Chicago podcast network is excited to announce the launch of a new show. It’s called Entitled, and it’s about human rights. Co hosted by lawyers and Uchicago law school professors Claudia Flores and Tom Ginsburg, Entitled explores the stories around why rights matter, and what’s the matter with rights.

Paul Rand: The best thing about black holes is that their possibilities are literally infinite. A black hole is like a playground for physicists. Even how things circle around this black hole can tell Ghez secrets about the universe.

Andrea Ghez: So they’re quite simple, and yet we don’t have the physics to describe them, and so they represent the frontier of our knowledge, and in particular our ability to make our description of the physical world that’s small, which is the world of quantum mechanics, work together with the description of the world that describes how gravity works. So that’s the realm of general relativity, what Einstein is so famous for. So today, we don’t know how to make those two fields meet, and while black holes are a prediction, or an expected outcome of general relativity, we can’t describe the details because you have to understand how to make that description of quantum mechanics work with the description of general relativity.

Andrea Ghez: So, in a sense, they’re really exciting, because they say if you want to understand physics, from a pure physics perspective, push our understanding of physics forward, these are objects that are incredibly exciting because they represent where that mixing happens.

Paul Rand: What Ghez wants to test using black holes isn’t just some obscure theories, but the fundamental natures of our universe.

Andrea Ghez: Gravity is one of the four fundamental forces of our universe, but in fact, it’s the least tested of the four fundamental forces. So we’ve been able to get into this new era of asking, well how does gravity work near a supermassive black hole? Does gravity work in the way, in this region, outside the event horizon, in the way that Einstein predicted? And so we were able to do the first set of those observations, or measurements, in 2018.

Paul Rand: But there’s been some discussions about this idea of co-mingling of space and time, and can you explain, when that phrase is used, what that refers to?

Andrea Ghez: Absolutely. I mean, it’s really one of the most key elements of Einstein’s revolution on the description of how does gravity work. So Newton had a great description that we used for many, many, many years, and in that description there’s no connection between space and time. So what Einstein is effectively saying is that space and time, how we think about space and time, are no longer independent when you get into a very strong gravitational field. So in the case of the black hole at the center of our galaxy, out at large distances, you can actually use Newton’s laws of gravity to describe some of these orbits. But for the closest star, SO-2 in particular, as it goes through closest approach, we can now actually see and verify how Einstein has modified that theory of gravity to work in strong gravitational fields, which is that co-mingling.

Andrea Ghez: And, in fact, if you go into science museums, you often see the gravity well, that funnel where they encourage you to roll down a quarter or something that goes into the black hole, and what that visualization is supposed to help us with is it’s pretending that the surface at the top is space. So it’s collapsed one dimension of space, so that’s just two dimensional space, and it’s using the third dimension to describe time. So there’s this deformation as you go towards the center.

Paul Rand: And let’s play out a little bit more as you’re digging into Einstein and, at least as you’re talking about the theory of relativity, you’ve said that it appears that “Einstein’s right, at least for now.” And I wonder if you can explain that a little bit more, and does that mean you think it’s going to be proven wrong?

Andrea Ghez: So when I said before that Einstein is right, at least for now, it means that our ability to see these effects increases with time. So in other words, as you continue to make these measurements, your understanding of how gravity works improves. So, the other joke I like to make: it’s like a bottle of fine wine. It just keeps improving with time.

Andrea Ghez: So yes, it’s passed the test today, but the test is going to become more stringent as we go forward in time, and there are future tests that are emerging. So, in fact, as we make our way, or as SO-2, as we see it, is making it’s way towards its furthest approach, other tests, which are in some sense more fundamental, how the object itself moves through space and time rather than photons moving through space and time, are emerging. So we’re just now making that last push to really see how this star and others are moving. And once again, it’s a super exciting phase of the experiment, because we’re starting to see signals that we couldn’t see before, and you’re at that stage in the experiment where you have to ask is what you’re seeing physics, or is what you’re seeing the inaccuracies in how you’ve analyzed the data? In other words, as your ability to make the measurements improve, you have to be very careful about what we call systematic errors creeping in.

Andrea Ghez: So how do you ... the visual I have in my head when I think about what we’re doing today, it’s like going around the cars and kicking the tires, like is what we do robust enough to claim that what we see if true physics versus some mistake?

Paul Rand: Andrea Ghez is one of only four women to ever win a Nobel Prize in physics.

Andrea Ghez: In all honesty, let’s look at who was the first: Marie Curie. She set the bar so incredibly high. She not only won one Nobel Prize, she won two, and her daughter won a Nobel Prize. So I remember when I was young reading biographies of Marie Curie, so she was already an important figure in terms of my understanding that it was possible.

Paul Rand: When she’s not talking about black holes, Ghez advocates for more women entering the sciences, and she hopes her Nobel Prize may inspire even more women.

Andrea Ghez: What I can say is that, from first-hand knowledge, I think that having strong and visible role models is super important. So if I think about my own evolution, my very first science teacher was at the University of Chicago in a chemistry class, and what I didn’t understand at the time was how fortunate I was to have, already, a model of a woman who was a scientist. I didn’t question it in high school, why should I? It was just there.

Andrea Ghez: She actually did something that was really important. At the time I remember applying to colleges, and I had decided that I really wanted to go to MIT, and somebody said, well, MIT doesn’t accept girls, and I remember going and talking to this science teacher, and she was great. She just said, “well, what’s the worst thing that can happen? They say no.” So, one, it was clearly the place I was going to look for the advice. Clearly there was already, early, this ... I mean, you’re starting to feel that discouragement. So even though I had the good fortunate of being in a family, an environment, in a school environment where there was tremendous support, somehow culturally, there’s still that skepticism. If you just look at all our images of scientists, they tend to be men.

Andrea Ghez: And, in fact, I think that’s why Star Trek was so important for my generation, is that there were women on the space trap. So I really guess I’ve really come to appreciate how important it is to have women role models, so at MIT and then at Cal Tech as a grad student, there were very few women scientists, and I can remember there were moments where I just ... actually, the way I used to think about it is I’m in the wrong playground. You sit in these classes, especially in physics, there would be 100 people, and there’d be four women, and you just knew something ... it’s like, do you have the fortitude, passion? I mean, for me, it was always, yeah, this is what I like. Why shouldn’t I be able to do it? But I think it’s what also makes me so interested in teaching at the undergrad level. To teach the earliest classes, where you can help change the notion, just from a ... it’s almost that unconscious bias perspective. How do you think about a scientist? What do they look like?

Andrea Ghez: So receiving the Nobel Prize, and having this highlighted as the fourth time, it just makes me realize that that’s a really important visibility piece, because, again, it changes the notion.

Paul Rand: Of course, diversity is important in every field, but I’m wondering, as you think about your field, and in terms of the disparity that we’re talking about, what disadvantages it actually presents that greater intermingling could actually be beneficial to science itself.

Andrea Ghez: It’s interesting to think about. I mean, the science itself doesn’t care what gender you are. Actually, it doesn’t even care about your existence. We’re talking about scales that are so much larger than we are, it’s a very humbling field.

Andrea Ghez: I guess the things that have come to mind as I get older is there’s something very liberating, actually, about being ... in terms of suggesting new ideas and being, maybe, challenging the status quo in terms of how people think. There’s some social costs for doing that. So if you’re in the majority, there’s more social cost for suggesting new ideas. If you’re already on the outside, I think there’s less cost, and it maybe gives you an opportunity to not only think differently, but also speak differently.

Andrea Ghez: What I really think, and I feel strongly about this, is that diversity of thought is really important, and, in fact, it’s why competition is so important. So, another aspect of this prize is that both I and my competitor were awarded the prize, and it’s also given a wonderful opportunity to talk about the benefit, not only of competition, but also the idea that two different points of view is really valuable. I mean, I think that we’ve not only kept each other on our toes, but we often think quite different about the work.

Andrea Ghez: You learn. I mean, you learn ... I mean, while these groups are independent, they’re not ... the fact that we’re all part of a community, which means that we’re constantly presenting our results at meetings or publishing our result means that you are learning from each other. So there’s been this interesting ratcheting up of understanding. And, in fact, I think the work has gotten so much further for the existence of these two groups. So I think that really highlights how much further you can get when you have different modes of thought, whether or not that comes from your background, your training, your just overall experiences. I think science will always benefit from multiplicity of viewpoints.

Paul Rand: One final question that I’m wondering is, we’re coming out of a period where the climate reports are more precarious by the day. You’re out in California where the wildfires and other ramifications of climate change are really starting to ravish the earth in a lot of different ways. How in the world that you operate in, do you look at what is happening into our world, and either gain some comfort out of that, gain a different perspective on that, or it doesn’t impact you in your thinking in any way?

Andrea Ghez: Oh I think it’s hard to be human and not have this impact your thinking. We as humans are having incredible impact on our environment and our ability to survive. I, personally, think about this from the point of view of how important it is for people to understand what scientists have to say about the status of our climate, the status of our global health, and this is where I think astronomy plays a really important role because I like to think of astronomy as the gateway science. It is a science that so many people have a fundamental curiosity about. It inspires us to think about this amazing, fascinating universe that we live in. It’s hard not to be engaged by those notions from a very early age.

Andrea Ghez: So I think it’s made me much more passionate about teaching than I ever have been before. I got into this field from a research perspective, because I was really curious and engaged in driving the frontier of knowledge forward, but the older I get, the more engaged I get with the other part of new knowledge in universities, which is the training of that next generation, and I think astronomy has a great role there.

Paul Rand: If you’re getting a lot out of the important research shared on Big Brains, there’s another University of Chicago podcast network show you should check out. It’s called Not Another Politics Podcast. Not Another Politics Podcast provides a fresh perspective on the biggest political stories, not through opinions and anecdotes, but through rigorous scholarship, massive data sets, and a deep knowledge of theory. If you want to understand the political science behind the political headlines, then listen to Not Another Politics Podcast, part of the University of Chicago podcast network.

Matt Hodapp: Big Brains is a production of the Uchicago podcast network. If you like what you heard, please give us a review and a rating. The show is hosted by Paul M Rand, and produced by me, Matt Hodapp, with assistance from Alyssa Eads. Thanks for listening.

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