IRA FLATOW: This is Science Friday. I’m Ira Flatow. Imagine you’re an astronaut floating through the vastness of space with just endless solitude and quiet on all sides. Peaceful beauty, no? Or maybe just a little bit intimidating.

Now imagine, as this intrepid astronaut, you come across a black hole in your travels. And maybe you’ll lose your mind, and you decide to jump in. What would happen?

Well, here with me to talk about her new book, Black Hole Survival Guide, is Dr. Janna Levin, Professor of Physics and Astronomy at Barnard College of Columbia University in New York. Always good to talk with you, Janna.

JANNA LEVIN: Oh, Ira it’s so good, I would say, to be here, but we’re just sort of in the ether, aren’t we?

IRA FLATOW: We are, we are absolutely in the ether. Let’s get right to this idea of the book. This isn’t the first book you’ve written about black holes, and it’s not the first time you’ve been on Science Friday to talk about them, as we’re saying. What is it about black holes that keeps you coming back for more?

JANNA LEVIN: Yeah, it’s funny. Black holes are extraordinary in that they’re astrophysically real. So the first time Einstein was presented with the idea of a black hole, a friend writes him a letter from the Russian front during World War I, right after he publishes general relativity with this mathematical solution. But Einstein sensibly said, nature will protect us from their formation. So there they’re astounding because nature thought of a way to make them, which is incredible, by killing off a bunch of heavy stars.

But they are more than that. Black holes are almost fundamental gravitational objects. They’re almost like fundamental particles. There’s something foundational about them. There’s something theoretically impressive about them, that they’re this unbelievable terrain on which we think about things.

IRA FLATOW: And you open your book with the phrase that you repeat a lot, all throughout the book, and that is, black holes are nothing.

JANNA LEVIN: Yeah.

IRA FLATOW: I mean, this is different from what a lot of people think. And you go over that. A black hole is dark and bare and empty, you say, and that’s a big concept to start off with. I mean, we all think, and as I say, you mentioned this in the book, that we’re mistaught about what a black hole is.

JANNA LEVIN: Yeah, I mean, I had well versed friends in science who were like, well, it’s a really dense object. And so one of the sort of misconceptions I wanted to chuck away was this idea that black holes were things at all. They’re really not things.

A black hole is an empty spacetime. It may be formed by a very dense object, like the collapse of a star. But once that star creates the black hole, it’s gone. The star continues to fall.

And Sir Roger Penrose won the Nobel Prize in October for work he did in the ’60s proving this, at once the star creates a curved spacetime so strong that not even light can escape, the star itself is forced to continue to fall. And it’s gone.

IRA FLATOW: So where does it go?

JANNA LEVIN: Well, that remains a quandary. In the same paper where Penrose talked about the inevitability of the formation of the black hole, he talks about the singularity, which people get very hung up on, that the black hole might have a region in its center where the spacetime is so strongly curved that it creates an infinite curvature, and it’s a singularity.

But that’s not really the important part. We don’t know where it goes. We know that the star continues to fall, but that the most important part is the event horizon, this region it leaves behind. We’re protected from what goes on the inside by this event horizon.

So the mystery of what happens inside remains and is being debated. But to some extent, it’s not our business.

IRA FLATOW: There’s so much to talk about, and the event horizon is one of my favorite subjects with a black hole to talk about because I’m not quite sure what it looks like. For example, in the pictures we see of a black hole, the event horizon– and in movies where they try to make a black hole, the event horizon is a disk that comes out from the black hole. My question– is it a disk like a ring around Saturn? Or does that disk go around the whole black hole so that you see it on edge from wherever you are?

JANNA LEVIN: So this is very interesting because, really, the event horizon is a spherical point in space. It’s a place, not a thing. And it’s a shadow. It’s literally like asking about the shadow of a tree.

And so when you’re describing the rings of Saturn, like we saw in the movie Interstellar–

IRA FLATOW: Right, exactly.

JANNA LEVIN: –or what we saw with the Event Horizon Telescope, what you’re looking at is the luminous material around the black hole that’s casting the shadow. Imagine the shadow of a tree. You don’t have a shadow of a tree unless you have a source of light. And so you have to illuminate the tree to cast the shadow.

And so this disk that you’re describing is casting the shadow. It’s illuminating the area around it. And because the spacetime is so strongly curved that even the light from this disk, like the rings of Saturn, give you the illusion that they’re above and below the black hole– it is just a total illusion– that casts the shadow of the event horizon. The event horizon itself, if you were to fall across that event horizon, it would be as unspectacular as stepping into the shadow of a tree.

IRA FLATOW: Well, let’s talk about that. So that shadow does go around the whole black the entire black hole.

JANNA LEVIN: It does.

IRA FLATOW: It follows your eye around the entire black hole.

JANNA LEVIN: Yeah.

IRA FLATOW: Now, talk about your astronaut falling into the event horizon. Take us through what happens.

JANNA LEVIN: Well, black holes are so perilous from many angles, and falling inside seems to capture people’s imagination. It’s very interesting that if you fell inside a black hole about the mass of the Sun– the thing about black holes is they’re small for their heft. So you take something as heavy as a Sun, and it’s only 6 kilometers across that shadow. So you cross within that 6-kilometer shadow and you’re on the inside. You have a very short fraction of a second to live before you’re terrorized by the extreme curvature in the center.

So the event horizon itself is actually not that bad. You can just float right across. If it was dark against a dark background, you wouldn’t even know you were encountering a black hole. You could just kind of float happily, drift to the inside, where you would be shredded and pulverized and all the things that people describe, because the spacetime is so strongly curved that your feet are pulled away from your head, and your body and your ligaments are torn apart until you’re into your–

IRA FLATOW: I hate it when that happens.

JANNA LEVIN: And then your fundamental bits go the fate of the star that originally formed the black hole, which is to say, we don’t know exactly what that fate is.

IRA FLATOW: All right. Let’s go to a great question from a listener, Mary Lee in Pittsburgh, on her Science Friday VoxPop app.

MARY LEE: So when people describe black holes, they describe them as eating things or consuming things. But a black hole doesn’t chew anything or absorb its nutrients. So I’m just wondering what actually happens when it consumes something.

JANNA LEVIN: So I think the most dramatic example of this is when two black holes merge. And what you see when that happens is that the event horizon deforms around the object it’s absorbing. In this case, the two black holes, it so it’s strong the two event horizons just totally bubble and deform. And they create these sort of audible– I mean, very faint to our ears, that’s why we require a detector– but these audible ringing in spacetime until it settles down to being a quiet black hole.

So the really impressive thing about black holes, to your listener’s question, is that once it absorbs something, it deforms a little bit for a second. But then it becomes flawless and featureless again. It sheds those imperfections. And it grows a little bit, and then it quiets down and settles down.

That is unlike anything in the universe. Any other object in the universe, you can put a little mountain, a little flaw, a little deformity. You can change it, alter it, but not a black hole. A black hole is flawless.

IRA FLATOW: And you say in the book they are all identical.

JANNA LEVIN: Yeah, it’s really stunning. If you think of something like a fundamental particle, like an electron, there is no electron that’s a little bit heavier than any other electron. They’re exactly the same. There’s so much the same that we consider them to be technically identical. They’re interchangeable. There’s no history to them.

And once a black hole settles down after you’ve thrown something in, it is absolutely flawless and featureless and identical to every other black hole of that weight.

IRA FLATOW: And you say it’s perfect.

JANNA LEVIN: Yeah, they are literally flawless. You cannot have a mountain or a blemish on a black hole for long. It will shed it away.

IRA FLATOW: The process, I think, that was described, and I’ve read it in other places, that it sheds its hair. Is that the hair? That it’s settling down and then becomes smooth and bald, I guess.

JANNA LEVIN: I mean, this was– none of this was obvious. You think that when Schwarzschild first wrote that letter to Einstein in 1916, he wrote down this beautiful with a medical solution. But it was decades of people laboring over it to understand these things. The no hair theorems, which you’re referring to, exactly suggest that a black hole has to be featureless.

To put it in most simplistic terms, because it forbids the transmission of any information inside the event horizon, it has to be featureless. I mean, after all, if it had features, I could determine what was on the inside. But the event horizon tells me I can’t.

So yeah, in a sense, they really are perfect flawless objects.

IRA FLATOW: Interesting.

JANNA LEVIN: I want to say not objects, but places.

IRA FLATOW: Places, that’s better, because now we know there’s nothing inside a black hole. A good question from Dave on Twitter. What would happen if two black holes met? Would the one with a weaker gravitational pull get sucked into the other? Would weird singularity stuff get sucked out of the weaker one? People are thinking about this.

JANNA LEVIN: Yeah. It’s really interesting. When LIGO was so widely reported, when they recorded the sound of the spacetime ringing from the collision of two black holes, they didn’t really talk about this very much. But this is exactly what happened. A smaller black hole merged with a bigger black hole, particularly in the first event which was detected, which the merger happened over a billion years ago.

So yeah, what’s really happening is you have to nothings which are just deformations in spacetime, warping each other–

IRA FLATOW: You say that so nonchalantly. Two nothings that are just deformations in spacetime.

JANNA LEVIN: Yeah, like two shapes, basically.

IRA FLATOW: OK. OK, there you go. That’s good.

JANNA LEVIN: Two shapes in spacetime. And when they get close to each other, you notice that those shapes become unlike either one alone. And eventually what happens is they merge into a bigger black hole. But as with what happened with the gravitational wave experiment, which recorded the sound of the spacetime ringing, is that a significant amount of the energy comes out in the ringing of the spacetime, which is shedding away all of those imperfections that we’re talking about so that what results, even when two black holes of equal size merge, is a perfect flawless black hole. And all of the imperfections ring out in the spacetime and can be recorded by instruments like LIGO.

So we have the event of the merger of two black holes that was first seen in 2015, I guess, first detected, was the most energetic event we’ve detected since the Big Bang.

IRA FLATOW: That’s pretty big.

JANNA LEVIN: And none of it came out as light, none of it. Like, that’s wild. It all came out in the ringing of spacetime. And that’s just crazy.

IRA FLATOW: Speaking of perfect and flawless, I’m talking with Dr. Janna Levin, Physics and Astronomy Professor at Barnard College at Columbia University in New York. We’ll be right back after this break. I’m Ira Flatow. This is Science Friday from WNYC Studios.

This is Science Friday. I’m Ira Flatow. In case you’re just joining us, I’m talking with Dr. Janna Levin, author of Black Hole Survival Guide. This is a great little black book with a black hole on. It’s a cute little book.

JANNA LEVIN: It’s good.

IRA FLATOW: Everything you ever wanted to know about black holes, I think, is in this book. You know, I remember years ago, when I first started out in this business, I interviewed John Archibald Wheeler. Supposedly, he was the originator of the term “black hole”?

JANNA LEVIN: Yeah, so the story I’ve heard, apparently he was giving a lecture down Broadway and got exhausted– this is 1967– got exhausted saying the inevitable consequence of gravitational collapse, or I don’t know. He had to keep saying these really elaborate terms. And supposedly someone from the back row shouted, how about black hole?

And Wheeler, in his unbelievably witty way, just foisted the term on the physics community. He wrote shortly thereafter that, like the Cheshire cat fades from view, leaving only at smile– and butchering the quote– the black hole fades from view, leaving only its gravitational attraction. And in that phase, he foists black hole on us.

IRA FLATOW: That’s terrific. That’s a great story. Janna Levin, author of Black Hole Survival Guide, you remind me– I’m always looking for great physicists who can speak to the public, like you do. You’re a great communicator. And your book reminds me of one of my all-time favorite books about physics, Quantum Physics for Poets, written by the late great Leon Lederman, with Christopher T. Hill as co-author. Do you feel like you’re an evangelist for black holes?

JANNA LEVIN: You know, it’s really– I write about what I love. Maybe I should think more about your audience. But I don’t– my first port of call is the subject that I love.

IRA FLATOW: But you got interested in black holes very early in life, right? You write in your book how interested you were as a child.

JANNA LEVIN: Yeah. And I think one of the interesting things was that it just was normal to me to read about black holes. These were just normal– they had already been discovered, they had already been observed out in nature. And so to kind of reverse the process of rediscovering how remarkable they were and how remarkable their discovery was, was kind of moving because I took it for granted. Of course, there’s other galaxies out there.

I mean, when Einstein was writing his first papers, we did not know there were other galaxies out there. And we take all of this for granted. So I think there was this kind of reverse reinvention of it for me, rediscovering them.

IRA FLATOW: How much of the mathematics describing black holes, describing the singularity, understanding what’s inside that spot, how much of it is real? And how much of it is just mathematical–

JANNA LEVIN: This is profound because even the fact that that’s a question means that there is a crisis that we have to solve. What does it mean to say it’s real or not real? The star falls. It creates the black hole. What happens to it? We’re back to the opening question.

It’s not to be dismissed. It’s a major crisis in trying to understand what happens to what falls inside of a black hole. For a long time, I think there was this hope that, well, we’re protected by the event horizon. We don’t ever have to really know. It’s not our business know. It happens inside the black hole. But we’re beginning to realize that forcing ourselves to try to address that question might hold the clues to the theory of everything, like, really the theory that unifies matter and gravity together.

IRA FLATOW: Because that’s the big mystery, right? We haven’t found the gravitons or the quantum equivalents of gravity, right?

JANNA LEVIN: That’s right. We don’t know how to quantize gravity.

IRA FLATOW: That happens in a black hole?

JANNA LEVIN: Well, the black hole seems to be the terrain on which we have to figure it out. It’s really the only frontier that we know of which is both real in astrophysics and on paper, in math, that is giving us the clues to try to understand what that quantum theory of gravity is. And it’s so elusive that– it goes to Hawking’s observation that black holes evaporate through some very subtle quantum process, which is forcing us to try to ask, well, if it evaporates, we need to understand what happened to the stuff that fell inside.

We’re no longer protected. We’re no longer forever safely on the other side of that one-way window because if they evaporate, eventually the event horizon is yanked up and we’re forced to look inside and confront what happened to that star or that astronaut that fell in.

IRA FLATOW: Do we need new concepts in physics? Do we need new physics to explain this?

JANNA LEVIN: Yeah. And I think there was some question over whether or not I should go into the quantum aspects of the black hole in this book because it really is the hard stuff.

IRA FLATOW: We like the hard stuff on this show, so you can get into the grass a little bit more, in the weeds.

JANNA LEVIN: I love that. I love that stuff because when I was a kid, I thought scientists memorized equations and spat out facts. And that’s obviously terribly false in that and a terrible stereotype. And what I’ve realized is the most exciting part of science is when you don’t know the answer, and that’s where we are right now. We’re on the cusp. It’s almost within reach, but we don’t quite understand it.

So yes, the most interesting concepts coming out of theoretical physics right now are on the theoretical terrain of the black hole. So if something falls inside, is it connected by a quantum wormhole to the Hawking radiation on the outside? I mean, the ideas are wild, wild right now.

IRA FLATOW: You could go to another universe on the other side of a black hole, could be another universe.

JANNA LEVIN: The best way to survive a black hole is to hope that your quantum bits find life elsewhere, yes.

IRA FLATOW: So at least we know what we don’t know, right? We’ve narrowed down what we don’t know.

JANNA LEVIN: Yeah. That’s some things– black holes, in terms of just their spacetime description, are very knowable. We understand them beautifully. There aren’t paradoxes or mysteries. It’s the quantum aspects where we run into paradoxes and mysteries.

IRA FLATOW: All right, getting into the weeds a little bit more here. We have a question from Arde on Twitter, who says, something you mentioned, though, a bit ago, explain how black holes evaporate. Do they really disappear?

JANNA LEVIN: Wow. I mean, this is what earned Hawking his fame. And here was this sort of inimical character who is provoking people, literally sort of grinning well he caused people great duress. It’s very subtle process. The black hole doesn’t emit anything. It steals energy from the vacuum around it.

So a black hole is nothing, as we’ve described. There is this event horizon, which is just an empty region of space, where you’d have to travel faster than the speed of light to escape the black hole. There’s nothing there. But the black hole manages to steal from the quantum vacuum.

And this, if you want to get into the weeds, goes back to Schrodinger’s very profound observation that there is an uncertainty in where a particle is or what it’s doing. But that also means that you can’t say a particle is not there. There’s no such thing as nothing. You can no longer have absolute nothing because that would mean you had this infinite precision in knowing a particle was not there.

IRA FLATOW: Right, OK. We’re following.

JANNA LEVIN: If you know what I mean.

IRA FLATOW: Yeah. go ahead.

JANNA LEVIN: So because of that, there’s this sort of frothing possibility that the nothingness of spacetime has a frothing possibility of particles kind of being there sort of virtually. The black hole has this ability, that Hawking pointed out, to steal from the vacuum, steal from this potential.

So imagine there is a nothingness. There are two particles that are kind of– you can’t firmly say there are not there. They cancel each other’s qualities, so they’re perfectly match so that they match the nothingness of spacetime. You can’t have them both feet electrically charged, for instance. They have to be opposites to cancel everything.

The black hole has the ability to steal one of this virtual pair that’s bubbling out of the nothingness of the quantum vacuum and leaving exposed its partner. And its partner can’t go back to being nothing without its pair. And so the partner just escapes, and that’s what the Hawking radiation is. It’s an incredibly subtle process, but none of it originates from inside the black hole.

IRA FLATOW: So if you have enough of those events, then you’re black hole is leaking and evaporating.

JANNA LEVIN: That’s right. The black hole steals energy from the vacuum. It actually gets lighter in the process. It loses mass in the process, and this light escapes out into space time.

And in principle, we’ve never seen this because bigger black holes are cool. They don’t evaporate very quickly. We haven’t observed this yet. But tiny black holes would explode. It actually is inverse to what you might think.

And so what happens is the black hole, technically, over a very, very long timescale, will get smaller. And the event horizon will shrink. And yet, none of it originated from inside the black hole.

So here’s the crisis. What happened to the stuff that fell inside the black hole if it didn’t come out in the radiation, which would be logical– that would be fine. Like, it came out in the radiation. But you’re suddenly no longer protected from the horrors of the interior of the black hole if it evaporates.

IRA FLATOW: And yet, we don’t know what happened to it is what you’re saying.

JANNA LEVIN: But what’s beautiful about it is that it’s provoking the most profound conversations about quantum gravity.

IRA FLATOW: Well, we’re having one right here and now.

JANNA LEVIN: Yeah. So it’s giving us the sort of fundamental clues. It’s prodding us in the right direction, if you know what I mean. It’s giving us the signs along the way.

IRA FLATOW: That’s great. There are so many questions, so little time. Let me ask about the our own black hole that we have at the center of our Milky Way, correct? Is that a prerequisite for a galaxy now, to have a black hole in its center?

JANNA LEVIN: Well, I mean, nobody expected this. So even after black holes had been named, which took 50-some years, even after they had been observed, nobody expected black holes millions of times or billions of times the mass of the Sun. That was totally unanticipated. We call those supermassive black holes.

We knew that stars collapsing could form black holes tens of times the mass of the Sun, or maybe even hundreds, but millions or billions of times the mass of the Sun? That was unpredicted. And to answer your question, yeah, we think that basically every sort of normal galaxy has a supermassive black hole at its center. And that is hundreds of billions of galaxies in our observable universe. So there are hundreds of billions of supermassive black holes that we don’t know where they came from, how they formed.

But we do think that they had maybe a really important role in terms of sculpting the galaxies in which they live, forming regions which are hospitable for life. So they might be very in terms of our emergence.

IRA FLATOW: I don’t have a whole lot of time left because I love to talk about this. I’ll try to get this question in. It’s one of my favorite topics. Spooky action at a distance, Einstein made that up. What is it? And how could it play into black holes?

JANNA LEVIN: Oh, I wish I could say it in German. Sometimes it’s translated as “ghostly.” It may play a role in our quantum understanding of black holes. There are these very creative wonderful physicists, like Lenny Susskind, who think about Einstein’s thought experiment way back when, which was when he said that phrase, spooky action at a distance.

So what Einstein was talking about, he was talking about these pairs of particles that form which are so opposite that they cancel each other, essentially. I liken it to, like, a yellow droplet of paint in a blue droplet of paint combining to make green. If they came from green, they’d better be yellow and blue in order to make the green.

And so these pairs we call entangled pairs. There they have to cancel each other’s properties. And yet, there’s this sense in which they don’t necessarily– in that Schrodinger uncertainty principle way– don’t necessarily manifest in some concrete way.

And so right now, one of the biggest ideas about black holes is exactly about that, the idea that there are these wormholes, possibly, which are connecting a particle– so if a black hole stole a particle from the vacuum and released a particle that came out as Hawking radiation, maybe they’re connected through a wormhole. I know it sounds crazy.

IRA FLATOW: My hair is hurting.

JANNA LEVIN: But the idea is, maybe the particle on the inside and the particle on the outside are connected by an entangled wormhole in the spirit that Einstein– that he was thinking about back then. And so the stuff that fell in the black hole is getting out because it’s the same. It’s entangled or connected by a wormhole with the stuff came out.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios. In case you’re joining us now, we’re talking with Dr. Janna Levin, author of this terrific new book, Black Hole Survival Guide, everything– everything– you’ve ever wanted to know about black holes. But how do you prove that? Can you make any experiments? Science really needs evidence, right?

JANNA LEVIN: Well, I appreciate the question. I also believe math is evidence.

IRA FLATOW: Oh.

JANNA LEVIN: And if it makes predictions mathematically that are confirmed, then I think that’s encouraging. If not a smoking gun, it’s encouraging. And so a lot of these predictions are able, mathematically, to make predictions that correspond to sort of other calculations. They all fit together.

It’s like assembling a puzzle on your table and all the pieces fit together. It’s not exactly an observation. But on your table they ll fit together, which is encouraging.

IRA FLATOW: One last thing on our table– I want to shift gears a little bit and talk about astrophysics. Astrophysics didn’t win a Nobel Prize until the 1970s, and now this year the prize was awarded to three scientists for black hole-specific research. Are we seeing a Renaissance in black hole research? Should we expect some sort of breakthrough in any of these questions that you’re talking about?

JANNA LEVIN: I think it’s really interesting. So back when Hubble was observing galaxies for the first time in the ’20s, the first galaxies that we realized were outside of the Milky Way, he really lobbied hard for astrophysics to be considered for the Nobel Prize. And he did not succeed. Hubble did not get a Nobel Prize.

Astrophysics, like you said, was not considered for the Nobel Prize until the ’70s. And this century we’ve had at least two Nobel prizes for black holes alone, just black holes.

I would say that previous to the start of this century, black holes were fading in terms of their centrality for physics. And suddenly, they’re back again. And they’re back not just astrophysically, but also theoretically. So I feel like we’re kind of living in this century for black holes.

IRA FLATOW: That’s great. I wish we could talk more. This book is excellent, as all of Janna Levin’s books are. Black Hole Survival Guide, you need this when you’re going out into space. Take it with you. You can read it as your time is changing. We didn’t even get into the time, the aspect. That’s incredibly crazy stuff.

Janna, thank you for taking time to be with us. It’s a great book.

JANNA LEVIN: Thanks so much, Ira. So fun to talk to you.

IRA FLATOW: Dr. Janna Levin, Professor of Physics and Astronomy at Barnard College of Columbia University in New York, author of the new book, Black Hole Survival Guide. We’re going to take a break. And when we come back, we’re coming back down to Earth, talking with a scientist who takes the pulse of scallops, and a composer who makes music from the sounds of rivers.

I’m Ira Flatow. This is Science Friday from WNYC Studios.

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