Daniel and Kelly’s Extraordinary Universe - What happens to event horizons when black holes collide?

Episode Date: June 30, 2022

Daniel and Jorge take you step by step through the process of black hole mergers . https://www.youtube.com/watch?v=Y1M-AbWIlVQ&t=47sSee omnystudio.com/listener for privacy information.

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I'm Dr. Scott Barry Kaufman, host of the Psychology Podcast. Here's a clip from an upcoming conversation about how to be a better you. When you think about emotion regulation, you're not going to choose an adaptive strategy which is more effortful to use unless you think there's a good outcome. Avoidance is easier. Ignoring is easier. denial is easier, complex problem solving, takes effort. Listen to the psychology podcast on the IHeartRadio app, Apple Podcasts, or wherever you get your podcasts. Hey, Daniel, I have a question about smashing things together.

Oh, well, you came to the right place. I know, you're a professional smasher, I guess. I'm actually wondering if it's the right way to study things. I don't know. I mean, what could go wrong? I mean, does it work for everything? Like, let's say you're trying out a new restaurant. Is smashing two plates together really the best way to test it? I mean, I would read that restaurant review, wouldn't you?

Or what about movies? Like, you would smash Blu-ray disc together? Maybe that's how they came up with awesome crossover events. Maybe that's the origin of the Marvel Multiverse. Somebody had a stack of DVDs on their coffee table and Eureka. Somebody smashed two comic books together. or a comic book with a Blu-ray. There you go. That's what happened.

Smash two Hollywood actors together. Hi, I'm Jorge. I'm a cartoonist and the co-author of Frequently Asked Questions about the universe. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I'm the other co-author. of frequently asked questions about the universe. What, what a coincidence? What are the chances that we would collide

on a podcast like this? What happens when you smash two co-authors together? Do you get one big author? You get a Voltron author maybe. Can I be like left foot? Jokes aside, you get a really fun book that neither of us could have written on our own, filled with amazing physics, insights, deep revelations

about the nature of the universe and hilarious cartoons. Yeah, tagos really. Amazing and frequent questions about the universe, like why we can't get to other stars, or is there an afterlife possible in this universe? Or why Daniel doesn't believe in time travel? Wait, you don't believe in time travel? Didn't you read the book, man? I'm going to have to go back in time and read it.

It's too late. You're out of time. I ran out of time. That's why. But anyways, welcome to our podcast, Daniel and Jorge, Explain the Universe, a production of Eyeheart Radio. in which we smash up the two most amazing things in the universe, your brain and the entire universe. We try to take everything that's out there, all the craziness, the insanity, the frothing quantum mess that is our reality, and squeeze all of it into your brain. Because we believe in you. We believe that your beautiful brain, even though it's a tiny part of the universe, can contain

within it a whole idea of the universe, that we can look out into the depths of space and actually understand what's going on out there. On the podcast, we talk about everything that's happening out there and explain all of it to you. That's right. We smash together scientific ideas and discoveries and collide them with bad puns and a lot of conversation here in order to pick up the pieces

and hopefully make sense of this amazing and wonderful cosmos that we live in. And you give me a hard time for it sometimes, but I really do think that smashing stuff together is really the best way to understand it. I mean, like, who hasn't tried a sample of their neighbor's plate at the table, right? And mixed it with their own dinner to create something new. Oh, wow. Do you ask for their permission for us, though, at least?

I mean, usually it's somebody in my family. So, yes, I'm reaching over to my wife's plate to try a French fry and dip it in whatever sauce is on my plate. And you never know. That could have been a culinary invention that rock the world. It just seems a little, you know, sort of a little destructive way of studying the universe. You know, I'm more of an engineering type. I like to take things apart, not smash it together. That's because you care about putting them back together.

I just want to know what's going on inside. Well, I mean, doesn't it seem a little destructive in a way? Like, you know, sort of like a little kid who smashes things out of anger. It is destructive, absolutely. But, you know, sometimes that's all you can do. Joking aside, if you have a toaster, yes, you can take it apart, carefully piece by piece and catalog what's inside it. And that probably is a better way to understand how a toaster works

than taking two toasters and making a toaster collider. But sometimes the forces that hold these things together are so strong that the only way to break it up to understand what's going on inside is to smash it up. And that's the case, for example, with protons. Have you actually looked? Maybe there's a screwdriver for protons. You just need to get the right one with the right, you know, shape. Yeah, the screwdriver for protons is another proton. I guess it's more like a hammer than a screwdriver.

But then what is that screwdriver made out of, Daniel? Exactly. That's the only tool you have. If everything in the universe is a proton, then basically you're just smashing protons together. Wait, doesn't everything eventually fall apart or break apart? Can you just wait for things to, you know, break open? I mean, I have grant deadlines and, you know, I got to get stuff done. I can't just wait until the heat death of the universe when everything collapses.

I see. It's a lack of patience, not a lack of better methods. You're encouraging procrastination to the heat death of the universe, right? That's really on brand for you. Yeah, there you go. And in the meantime, the grant could support you, right, while you wait. That's right. grant for waiting for 10 to the 14 years until the universe does its experiments for me.

We'll see how that goes. I'll cut you in if it gets funded. Yeah, yeah. As long as it's for 10 to the $14, I'm totally in. But smashing things together does seem to be the preferred way physicists like to explore things at the smallest levels because there is no screwdriver for opening things like protons or even quarks. There is no screwdriver. There are no tweezers. And it's something that we can actually do. We can manipulate protons. We can tune in their energy. We can smash them together to see what's going on inside. And the same thing is true for even bigger stuff. We can't take stars apart. We don't have the machinery to understand what's inside a planet.

So the best way to learn about it is to watch collisions of enormous astronomical objects to see what's going on inside. Yeah, I guess sometimes it's hard to bake things apart, like you said, right? Like it's hard to take a star apart. That would be pretty difficult. It's pretty hard to take a star apart. It's even hard to look inside a star. We had the Parker Solar Probe recently, which came super close to the sun and almost fried itself, but not quite. And he gives us a picture of what's going on on the surface and helps map a little bit of the insides. But you know, we have questions about what's going on deep, deep in the heart of our sun that we could only really answer by smashing it into another star. Yeah, I guess sometimes it's hard to look inside

the thing. So you kind of have to break them apart because they don't open up so easily. And to be honest, in engineering, we do sometimes smash things together or squish him until they break. Just trust test them. Now don't worry, I don't know how to build a star collider, so I'm not going to shoot Proxima Centauri at our star anytime soon. That's a grand proposal that will never be funded, but we don't have to build these colliders ourselves. We don't have to construct cosmic colliders to smash planets together because the universe is doing it for us. We just have to look out there into the skies and find the experiment already underway. Yeah, because it is a pretty big universe Even though it's huge and empty, it's pretty big and pretty full of stuff.

And so there's always something going on in the universe. And something that's going on is a big collision. We have seen comets slam into planets. We have seen binary stars collapse into each other. We've seen all sorts of crazy stuff smash into itself and learned an incredible amount in the process. Yeah, we've seen galaxy smash together, right? That's sort of how dark matter was confirmed. Yeah, we can see galaxies merging in the middle of this process of swirling around each other

and there's stars forming one new elliptical galaxy. And you're right, we've even seen galaxy clusters collide. The bullet cluster is two big groups of galaxies, enormous piles of galaxies smashing into each other. Dark matter coming out on either side, which tells us, as you said, dark matter is its own thing and not just some weird twist on gravity. Yeah, I guess smashing things together is a good way to explore things,

especially if they're sort of mysterious and kind of hard to know that they're there or what's going on inside of them, right? like smashing things with dark matter in them. So it helps you see the dark matter. Yeah, it helps you separate the dark matter from the rest of the stuff because different things smash differently, right? The gas and the dust in those galaxies smashed into each other, making huge explosions and bright flashes of light.

But the dark matter passed right through. So that tells you that dark matter really is different from normal kind of matter. So yeah, absolutely smashing stuff together. Great way to figure out what's going on. Great way to support physicists. We like to smash things as little kids. Yeah, but, you know, don't like smash your kids together if you're not sure what they're up to. There is a limit to this idea.

I see. Well, I think they usually smash themselves pretty good without your help or direction. All right, but in no way am I endorsing kids smashing on the podcast. I don't even know why you would bring it up. I guess kids are mysterious also. They're hard to understand. Yes, absolutely. Kids are hard to understand.

But there are better ways to understand what's going on inside your children than smashing them together. Right, right. I guess you could talk to them, I guess. You could just make references to them on the podcast and hope they listen. Yeah, maybe like 20 years from now when they're in therapy. They'll be like, what did my father think of me?

Oh, it's right here on this podcast. What? But anyways, there is something mysterious out there in the universe that we would like to know more about. We would like to know what's going on inside of them. But so far, they are one of the hardest things to look at and figure out. And a lot of people write to me and ask what happens when these two mysterious objects in the universe come together.

Is it just like other collisions or are some of the fundamental rules of the universe broken? So today on the podcast, we'll be tackling the question. What happens when black holes collide? This is a very sensationalist question, I feel. What happens when black holes collide? It's sort of like shark versus shark, like which shark eats the other one?

It's one black hole sucking in the world. the other one, they suck in each other? What does that even mean, man? Yeah, I know. It's like, can a hole fall into another hole? Like, you know? Holy moly, that's a complicated question. That is a whole lot of holes there in that theory. We need a holistic understanding of how this works. But this is sort of part of our, I guess, a recent theme we've had going on in the podcast. We can almost call it like smash month or smash week. Exactly. We got smashing on the brain over here at the podcast headquarters.

Hopefully it'll be a smashing success. But we have been smashing things together. We smashed photons together last time and we smashed. What else did we smash? We did a whole list in our episode's question about smashing stuff together. That was the theme, annihilation questions. And then we smash light together, which it turns out you can't smash together. And now we're smashing black holes.

What's next, Daniel? Smashing universes? Oh, wow, universe collisions. Actually, there is a theory about different bubbles in the multiverse and bumping into each other and leaving an imprint on the cosmic microwave background radiation. I just read about this theory, yeah. The Big Bounce. Yeah, there was a theory by Roger Penrose,

and he claimed to see evidence for it in the cosmic microwave background radiation, but nobody could confirm it. So it's definitely not something we've seen, but a pretty awesome idea. Yeah, also it's called The Big Bounce, not the Big Smash. So we can't talk about it. This episode, sponsored by Smash Burger. By Smash Mountain. But I remember the first time I,

heard about black holes being collided. And I thought, wow, that's incredible. Like two things that we definitely do not understand. And I thought to myself, I want to see what happens, what comes out, what's revealed in the shards of that collision. Like, show me the answer, universe. Yeah. What are the shards of the two sharks when they collide?

And it's sort of amazing, you know, that it happens out there in the universe and that we can see it. So to me, it feels like we are peeking under the rug of nature, really understanding something. something deep about the nature of space and time by looking for these extreme collisions when nature has to tell us how things work. Yeah, because black holes are pretty extreme in the universe, right? There are some of the most extreme conditions imaginable. Maybe they're breaking the laws of physics inside or at least the laws that we know.

And so you can't imagine amazing things are going to happen when you smash two of them together. Yeah, they're probably going to smash the laws of physics. And I guess maybe a more philosophical question is, is Daniel, can two holes actually collide? like what's actually hitting each other nothing's going to hit each other is there just two holes i guess you could think of them as like merging right if you and a friend are both digging holes in the ground you just keep digging then eventually just get one big hole right so those two holes can sort of merge so it's more of a black hole merger it is but actually the math doesn't quite work that way because the black hole that comes out is a little bit smaller than the sum

of the two black holes that went in which is pretty weird wait what well you just spoiled He said another hole comes out. I guess the two holes don't cancel each other. Yeah, they do an amazing dance of relativity to form something new that comes out. Well, then, as you said, this kind of thing happens all the time in the universe, and we get to observe it, right? We certainly do, and we learn a lot about the nature of space and time in the process. All right. Well, as usual, we were curious how many people out there had heard of black holes colliding

and what maybe they think happens when they do. So thank you to everybody who participates in these segments for our podcast. we hope you have a good time answering random questions without any chance to prepare. If you'd like to participate and hear your speculation on the podcast for everybody else to enjoy, please don't be shy. Write to us to Questions at Danielanhorpe.com. So think about it for a second. What do you think happens when two black holes collide?

Here's what people had to say. Yeah, I think we know this, right? So if two black holes collide, A, they make a bigger black hole, but B, I think some people have discovered that they make gravitational waves. but that seems like too simple an answer. So I read Black Hole Blues by Gianna Levin and my best guess is when black holes collide they circle around each other faster and faster and as they get closer to each other

they're circling almost at the speed of light at the very last split second and when they collide the force has enough energy to overpower the energy that an entire guy galaxy might put out. And that's why we can sense the gravitational waves, you know, galaxies away here on Earth with the new instruments we have. In general, I don't think there is a direct collision of a black hole, but rather they orbit each other closer and closer and closer with probably the stronger one feeding off of the weaker one. Ultimately, after all the fireworks are done, I would assume that the smaller one would be eventually absorbed into the larger one. And you have one substantially larger black hole.

When two black holes collide, I think they just merge into a larger one. We can observe gravitational waves happening while this occurs. But other than that, I think they just merge. When black holes collide, gravity waves make their way to our clever listening devices here on Earth. And I think probably there's a lot of energy released, and they become one black hole. When black holes collide, I mean, you have very, very heavy, heavily dense and very strong gravity coming from these things. So when they collide, it's got a kind of one winds out, which everyone is, A, more dense and B strongly, have stronger gravitational forces, and then it kind of absorbs. They make a lot of gravity waves, and then they make one big black hole.

All right. A lot of fun answers here. I like the one that said when they collide, they eat each other. Kind of like sharks. But who is eating who, man? That doesn't really answer the question. But if one shark starts eating the tail of the other shark and then the other sharks starts eating the tail of the other shark, what's going to happen?

It's like a yin-yang shark somehow. Yeah, a yin shark, yeah. Maybe the shark ends up eating itself. When you get a shark NATO? Because they're spinning around so fast. That's probably how that crossover event happened, right? A shark DVD and a tornado DVD, boom. That's right.

Yeah, one bad idea, smashed another bad idea. Yeah, why not, right? Who knows what happens when you collide the craziest things in the universe? So I love the creativity there. Thank you. Yeah, why not? Maybe that should be the title of the podcast. Why not?

Let's get smashing. Well, let's break it down for people. here, Daniel, maybe let's start with the basics. What is a black hole? A black hole, as you said, is a hole in space and time, but it's a really strange hole. You know, really what it is is a location where there's so much mass and energy in one spot that it's dense enough, that particles that are near it are doomed to fall in. You know, mass and energy tells space how to bend and then space tells particles and mass and other things how to move. So the more mass and energy you have somewhere, the more space curve, which is why, for example, satellites orbit the

earth instead of just flying away. You could think of all of gravity, in fact, as the invisible curvature of space rather than like a Newtonian tugging. And so black holes are where space is curved so much that there are particles that can never escape. So there's sort of holes in space and there are sort of holes caused by gravity, right? Like that's another way to do sort of think about it, right? It's like there's there's so much stuff and energy in them that it's just sucks everything in and it sucks them so much, they can never get out. Yeah, mostly you can think about gravity in two different ways. You can think about it like a force.

Something is pulling on you like the Earth is tugging on you. That's sort of Newton's idea of gravity. But black holes come out of general relativity, which encourages you to think about gravity in a very different way. It says that gravity isn't a force. It's just that space is curved, but you can't see that curvature. The only thing you can see is the effect of that curvature on the motion of objects. So you shine a flashlight, for example, through curved space, then it seems to you to bend.

But that bending is just because it's moving in a straight line through curved space you can't see. So when you apply that to black holes, you don't get like a really strong force of gravity. You get a place where space is bent so much that now it's just one directional. Things inside the event of horizon of a black hole always end up at the singularity, according to general relativity, because that's the only direction left in space. Right, yeah. But general relativity might be wrong too, right? Like there's this possibility that maybe gravity is a force and there are force gravity particles and everything, right? General relativity almost certainly wrong at some level, not in the sense that it's making mistakes about GPS or that we're getting the numbers wrong, but it can't really be the true description of nature, as you said, because, for example, it predicts singularities at the hearts of black hole. And you know, that's not as much a physical prediction like general relativity doesn't say there is a point of infinite density.

as much as it's a breakdown of the theory. It says, well, here's what I predict, and that seems sort of nonsense. So at this point, replace me with a better theory. So we don't know what's going on at the heart of black holes. It could be that the right picture of gravity is as a sort of quantum field, the way we have all the other forces. You can think about gravity is the exchange of gravitons.

So yeah, you're right. General relativity, almost certainly wrong. On the other hand, it predicts black holes and we see them. So it's right about a lot of stuff. Well, black holes are really strange. and there are a couple of really strange things about them. Like, first of all, light can't escape. So they just look like a giant hole in space.

But it also does interesting things, like slow down time. Yeah, there are two different kinds of time dilation in our universe from relativity. One is much more commonly talked about, which is velocity-based time dilation. If you see somebody moving fast through the universe relative to you, you see their clocks slowing down. That kind of time dilation is relative because if they look back at you, they see your clocks slowing down. So you two disagree about whose clock is slowing down, which is really weird and confusing. It makes you doubt like, you know, truth and the existence of reality. But the kind of time dilation that happens near a black hole is different.

The more space is curved where you are, the more your clock will slow down. And that's not a relative effect. It's absolute. So if your friend gets near a black hole where space is curved more, you will see their clock slowed down because they're in more curved space. They will see your clock going faster, right? It's the opposite of the relative time dilation that happens due to velocity. Here is absolute because everybody agrees. So if you are falling near a black hole,

your friend will see you slow down and you will see them sped up. Yeah, it's a pretty cool effect. And you will literally see them moving slow motion, right? And they will literally see the entire rest of the universe moving in fast forward. Yeah. And so if you see somebody falling toward the black hole, the closer they get, the more their time slows down. And so it actually takes an infinite amount of time for the last.

last thing to fall into a black hole. Like you toss a banana into a black hole. You don't actually see it enter the black hole past the event horizon until time equals infinity. Right. We've had I think whole podcast about this because it's sort of a little bit mind bending because you do sort of see black holes growing over time, right? And at some point they're going to overtake the banana, right? Yeah, that seems confusing because it suggests that black holes could never grow because nothing could actually fall into them. That's why I said it's only true for the last thing to fall into a black hole. Because as the banana falls towards the black hole, the event horizon actually grows before the banana crosses over. A black hole isn't like a pet where you need to put

something in it for it to get bigger. It doesn't have to like eat the banana. Besides, the event horizon reflects the total gravitational energy of the system. So as the banana falls into the black hole, the event horizon actually grows out to meet it. And then if you throw something else like an apple after the banana, that also pulls out the event horizon so it'll come and encompass the banana. So that's how we see black holes actually grow out there in the universe. There's a continuous stream of stuff falling into it and pulling out the event horizon. Right, right. You don't have to feed them, but it's nice to feed them, right?

You don't want a black hole to starve. I don't know. It depends where the black hole lives. If it lives in your basement, I don't think it's a good idea to feed it. I think if it lives in your basement, it's game over for your house. You know, there is some size of a black hole where it's radiating away energy and you could feed it at the same rate. So you can have a stable black hole that you keep as a pet. You can have a pet then.

You just contradict it yourself. Just don't pet it, I guess. Don't touch it. I wouldn't recommend it, but theoretically it's possible to keep a black hole stable. I see. And so something else that's interesting about a black hole is that there are only a few things we can know about them, right? I mean, there are black hole and stuff falls in and we never see it again. But there are a few things that you can tell about them.

Yeah, everything that falls into the event horizon is lost to us. And what happens to it, we cannot know. information about what's inside the event horizon can't escape. But that doesn't mean we can't measure things about the global black hole. Like a black hole has mass. It tugs on you even though you're outside the event horizon. And so you can use that to measure how much stuff is inside the event horizon. How much mass does this black hole have? So there are a few things you can measure from outside the event horizon. And that's the mass of the black hole. Also, the electric charge of the black hole because charge is conserved in the universe. You drop it on a lot of the world. You drop it

electron into a black hole that changes its charge and its electric field. The same thing for its spin. Black holes can spin. So there's this theorem called the no hair theorem that says those are the only three things you can know about a stable black hole. Mass, spin, and charge. Wait, why is it called the no hair theorem? How does hair fall in?

Get, fit into this? I think it's a joke that says that you can't know whether black holes are hairy. Like, you can't know what's going on inside the black hole. Does it have blonde hair? Does it have a mohawk? It's like, you know, just an example of something you can't know about a black hole. It could have been called the no tattoos theorem also. Yeah, that makes no sense to me.

But that's the official name. All right, well, that's a black hole. And so now the big question is, what happens if I take two black holes and I smushed them or smash them or merge them together? Apparently, a lot of things happen. So let's get into that. But first, let's take a quick break. Imagine that you're on an airplane and all of a sudden you hear this. Attention passengers.

The pilot is having an emergency and we need someone, anyone, to land this plane. Think you could do it? It turns out that nearly 50% of men think that they could land the plane with the help of air traffic control. And they're saying like, okay, pull this, do this, pull that, turn this. It's just, I can do my eyes close. I'm Manny. I'm Noah. This is Devon.

And on our new show, No Such Thing, we get to the bottom of questions like these. Join us as we talk to the leading expert on overconfidence. Those who lack expertise lack the expertise they need to recognize that they lack expertise. And then, as we try the whole thing out for real. Wait, what? Oh, that's the run right. I'm looking at this thing. Listen to No Such Thing on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts.

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I haven't audition in, like, over 25 years. Oh, wow. That's a real G-talk right there. Oh, yeah. We've got some of the biggest actors, musicians, content creators, and culture shifters sharing their real stories of failure and success.

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struggles, and all the issues affecting our Latin community. You feel like you get a little whitewash because you have to do the code switching? I won't say whitewash because at the end of the day, you know, I'm me. Yeah?

But the whole pretending and code, you know, it takes a toll on. Listen to the new season of Grasas Has Come Again as part of My Cultura Podcast Network on the IHartRadio app, Apple Podcasts, or wherever you get your podcast. Like we've recently been able to listen to black holes colliding and a lot of they had they happen more often than we thought. collisions were first observed in 2015, but it was a very, very long search for black holes. People started decades and decades before that, trying to invent systems that were sensitive enough to the radiation emitted from black hole collisions so that we could see it here on Earth.

This is something predicted by Albert Einstein, though he thought we could never actually observe it. He thought this is a cool effect. Too bad. It's too tiny for us to ever see. Oh, I see. So before we could listen to them with gravitational waves, people, People were trying to see them, but they never found any, right?

Yeah, people were trying to listen to them with gravitational waves for a long time. That was Einstein's prediction that they would create from gravitational waves, but that would be impossible for us to see these gravitational waves, to observe them. And you know, I'm in the company who agreed with Einstein for a long time. When I was thinking about grad school, I had a few different choices. And one was going to university that was deep into LIGO, that was developing the technology and trying to observe gravitational waves. And I remember thinking, that's cool, but they're never going to make that happen.

and so I'm going to go do particle physics instead. Yeah, yeah. I'm in the company of Einstein as well. I have crazy hair as well. But Einstein was wrong, and so was I, because they did see gravitational waves. They did see this crazy pattern of radiation emitted from the collisions of black holes.

Yeah, I think what I was asking is, like, could you see two black holes merging together? I mean, we can sort of see black holes out there in the universe, and we can definitely see their effect on the stars or the galaxy around them. Could you ever hope to detect a black hole collision without the gravitational waves detection? Like, could you ever see two black holes actually colliding? You can actually see them, but you're right.

You don't see the black holes themselves colliding. Black holes are surrounded by accretion disks, all sorts of matter, the sort of on deck for falling into the black holes. And so sometimes when two black holes collide, their accretion disks also collide and create light. They've seen this a couple of times where they've seen flashes of bright light at the same, time and in the same direction as they've observed gravitational waves. So they have seen in a couple of occasions bright flashes of light emitted from black hole collisions.

Right, but before 2015, like maybe you would see the bright flash of light, but you wouldn't be able to know if it was a black hole collision. Yeah, exactly. It just seemed like a flash of light. And there's lots of weird flashes of light in the universe. And you can't necessarily tell that one is from a black hole or from something else or just from two stars colliding or two blobs of gas collide. So really the unique signature, the thing that told us that black holes were colliding were these patterns of gravitational waves, which are like ripples in space and time itself.

And so far since 2015, we've seen a whole bunch or I've heard a whole bunch of these black hole collisions, like maybe like 10 a year or something, right? It's incredible. We have like 50 examples now. And to appreciate how amazing that is, realize that we didn't know how often black holes collided. When LIGO turned on, it was like a new kind of instrument and we're listening to something new in the universe or a new kind of eyeball. We're looking for things in the universe. It's all just an analogy because gravitational radiation is not something you can see or hear. We're just trying to translate it into sort of human experience. But we didn't know if this kind of thing happened once in a century, once in a millennium, or like 10 times a second. So when they turned this thing on, it could have been that they were waiting for years to hear the first one or that they came fast and hard.

And amazingly, we were lucky. And they're pretty common. And they saw gravitational wave in the first test run. Like they turned this thing on and they were just like doing calibration runs just to make sure everything was working. And boom, they saw a signal in the first calibration run. So they were like off to the races writing a paper on week two. Yeah, it's pretty amazing. Pretty cool.

And they happen pretty often maybe like once a month, once every month and a half. And do they happen here in our galaxy or are we listening to these collisions from like all over the universe? They happen all over the universe, and we can see these things really far away, like billions of light years. Now, the further you are away from these things, of course, the fainter they are. And so if they're closer, it's easier for us to see them. They're further away, then they need to be more dramatic, more powerful for us to observe them. But we've detected these collisions from black holes that are billions of light years away. But are they happening here in our Milky Way galaxy?

We see black hole collisions fairly commonly, but we can see them from really, really far away. and they don't happen actually that often in any individual galaxy. So we haven't actually seen what happened yet in the Milky Way. Remember the black holes are not that common. We have really big ones in the center of the galaxy. Then you have black holes created from stellar collapse. But to get two black holes to collide, you really need like two black holes in a binary system.

Because I guess 50 seems like a lot of black holes colliding, but it's a big universe, right? There are trillions of galaxies out there. So the fact that we're only seeing maybe one a year, it means that maybe they're not that common. Yeah, they're not that common sort of per galaxy. But they happen often enough for us to have a pretty nice data sample, which means that we can really study these things.

It's not just like we saw one and then we're wondering if that was typical or not. We have like dozens of these things. So we can start to ask statistical questions about what's likely and what's common. We can see which ones are weird, which ones are normal. It's really an awesome moment when you can start to do like population science on black hole collisions. Yeah, like statistical. surveys. All right, well, maybe step us through here. What happens? What's like step by step? What's going on when two black holes collide? Because, you know, I think one thing that a lot of people

might not know is that black holes can move, right? Like, it's weird to think of a hole moving, like a hole in the ground doesn't move, but black holes can move and they can sort of like fly through space and run into other black holes. Yeah, black holes have mass just like everything else. And so they have inertia and they can have momentum. Black hole can move in the same way you can move past a black hole. Remember that velocity is just relative in our universe. So if you're flying past a black hole from its point of view, then from your point of view, the black hole is flying past you, right? And so these things definitely can move. And a lot of people probably think about black hole collisions is like two black holes just flying through space and bumping into

each other, like two dogs in the park smashing into each other or something because they weren't looking where they were going. Instead, these two things have sort of been faded to collide since their birth. Remember that a lot of stars are born as binary systems. They were made near each other and gravitationally bound from the beginning, orbiting each other in a long dance. And that's how most black hole collisions happen. They start as a binary star system, then each one collapses into a black hole. Then you get black holes orbiting each other. So they've always been neighbors. It's not like they're just two strangers that smash into each other. And they're orbiting each other and they're slowly losing that energy, radiating away their orbital energy until eventually,

they collide. Well, yeah, because I guess most black holes come from stars. And so if you have a binary system and both stars turn into a black hole, then you have a binary black hole system, right? But isn't that sort of rare? I mean, it's a little rare for a star to turn into

a black hole. But now you need to have both of them in the binary star system turn into black hole. Exactly. And so those are the conditions you need. And we're still understanding, you know, black hole formation, but it depends on how much mass there was in each individual star. If it's massive enough, then eventually it will collapse into a black hole.

There's like no way to avoid it once it burns up its fuel. The thing I think is interesting is understanding why these things are inevitable. Like, why can't two black holes just orbit each other happily forever until the end of the universe? Why do they have to fall into each other? Right. Like in our solar system, you know, the planets are orbiting the sun pretty stably. Pretty stably, that's true.

We're not falling to the sun yet. Not today and not tomorrow. But, you know, these orbits aren't. not technically stable because every time you're in orbit around something, you're accelerating. And anything that's accelerating in our universe that's changing its velocity is generating gravitational waves. You know, what is a gravitational wave? It's just when your gravitational field changes.

If you have an object in space, it has a gravitational field. It's changing the shape, the curvature of space. If that object accelerates, then the curvature of space changes. But it doesn't change instantaneously. Just like if the sun disappeared, we wouldn't. notice for eight minutes or whatever. It would take time for that gravitational information to propagate. And so when something accelerates, it's changing the curvature of space. And that's what's happening when the Earth is going around the sun. It's accelerating. And so that radiates

away some energy in terms of the information about updating the curvature. Right, right. So we're slowly losing a little bit of energy in our orbit. And so eventually, I guess, in the very, very, very, very far future the earth is going to fall into the sun that's right although other things will happen before the earth falls into the sun due to radiating gravitational energy because the earth is not that massive but if you have two black holes that are really really massive they're going to radiate a lot more gravitational energy and so as they lose energy they fall into each other right they can't maintain their orbit if they don't have that energy so they're very orbit the thing that's accelerating them around each other is shaking the curvature of space around them creating these

gravitational waves and forcing them to get closer and closer and faster and faster. So it's more like a swirl in than a collision. Right. It's sort of like if you have a still lake or a still body of water and you take two fingers and you sort of rotate them or spin them around each other, they're going to be generating waves on the water. That's sort of how people see the two black holes kind of radiating out energy as waves. Exactly.

And it's a deep concept that's really applicable to lots of different physical phenomena. Right. Like how do you generate radio transmissions? You take electrons, which have an electric field, and you accelerate them up and down. You shake them. And that wiggles the electric field. That wiggle in the electric field is nothing more than a photon. It's passing of information and energy through that field. So you take a mass now. It has a curvature in space. You wiggle that mass. You accelerate it. That wiggling of space time is gravitational radiation. It carries away energy. Right. So they're not sort of, you know, aimed at each other. They're more like swirling together. But then at some point, they lose energy and they swirls faster and faster and closer and closer. And at some point, they start to touch, I guess, right? Or collide. Yeah, they start to touch. And to think about that, you have to think about what you mean by two black holes touching, right? Like, what is the edge of a black hole? What's the surface of it? Often we talk about it in terms of the event horizon. We talk about the event horizon as if it's like something physical, you know, like a search.

or a boundary or something, it's really just sort of like a location past which you can never escape the black hole. But it's not like there at the event horizon. There's no physical surface. It's just like past this point, you will never escape. Like the edge of a hole is not really a barrier. It's just where you fall in. Yeah, it's just where you fall in. The subtle point also is that you can't measure the event horizon. Technically to calculate where the event horizon is, you need to know like what happens to every particle that comes near it. Then you find sort of like the surface in which if a particle passed through it, nothing ever escaped. So you sort of need to know the fate of every particle to figure out exactly where the event

horizon is. Wait, wait, what? What do you mean? We can't tell where it is? Like, can we, didn't we just take a picture of a black hole recently? Doesn't that give us a pretty good idea by the way that the light bends around it, where the event horizon is? We did take a picture of black hole.

And that does give us clues about the size of the event horizon because actually what we're seeing there is the shadow of the black hole, which is larger than the event horizon because some light, for example, will pass in the air and get bent around it. So the shadow actually looks a little bit bigger than the event horizon. But check out our whole episode about the black hole image for details about that. But in principle, even that picture doesn't tell you exactly where the event horizon is. Like it could be that there's a particle that could pass a little bit closer to the black hole than we're seeing in that picture and then escape. You don't know for sure. Now, we can calculate it, right?

General relativity lets you calculate the size of the event horizon. So we have this short-styled radius. But it's not like something you can locally measure. You can't say I'm in or I'm out at any given moment in the universe. It's not like some device you could build that could tell you I'm inside a black hole or I'm outside of black hole. You can either calculate it from general relativity or you can shoot a bunch of particles at it and wait till the end of the universe and see which ones escaped and which ones didn't. So it's sort of a fuzzy boundary, I think, is what you're saying. Exactly. And we're going to have to keep that definition in mind as we think about what happens when the event horizons get close to each other.

All right. Yeah. So what happens? So I have one black hole and I have another black hole in each of my hands and I'm swirling them and I'm bringing them together and they're swirling and they're swirling and at some point where the event horizon would be or where we think it is or fuzzily where it should be, they start to overlap. They start to overlap. And when people write to me about this, something they're confused about is like a black hole, the event horizon is a sphere, right? it's like centered around the singularity. Now you have two black holes, both of which are spheres. What happens when they touch? Do you suddenly get a sphere at the center of the two? Does that mean the event horizon is like shrinking a little bit? What happens there?

And so the answer is you can't have like discontinuities with the event horizon is in one place and then one instant later. It's totally different. It's a smooth transformation from having two blobs to one blob. And a little bit surprisingly, that means that the event horizon is not always spherical in the transition between two black holes and one black hole that results it's a weird sort of peanut shape yeah i'm imagining i guess you know like if you take two ink plots like two blobs of ink and you sort of bring them together they're going to sort of like

touch maybe at the boundary and then sort of merge and but just a little bit first and then the blob sort of merges together the two blobs become a peanut shape and then they sort of blob blob blob together. Is that sort of what happens? That's sort of what happens. And to figure out exactly what the shape of the event horizon is, people do these numerical relativity calculations where basically they shoot a bunch of particles near these two masses and they figure out where the no-go zones are, where if a particle passed through it, it ends up in the singularity no matter what. They have to calculate the event horizon in this way to like figure out where the no-go zones are. And they do these incredible simulations and you can

find these images online if you want to look for the video. We'll put a link into the show notes. And what happens is you have like two blobs. And as they get closer to each other, there's like a filament that forms between them. Now the event horizon looks sort of like a dumbbells, like two big blobs with a very thin line between them. And then as they get closer and closer, that line grows and grows and grows and grows. Eventually you have like a peanut and then a tic-tac and finally a sphere. Wait, we skip past the peanut M&M. Or a whole bunch of other candies. All right. Well, let's get into what actually is happening with that event horizon.

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Now the black holes are swirling around each other, getting closer and closer and closer and just as they're about to touch, they actually sort of reach out and touch each other kind of in a way, right? Yeah, they become regions in space between the two black holes that are now effectively inside their event horizon, their combined event horizon, because if you're in that place, you will not escape. So you could have been outside the event horizon just before, but now this filament has formed where if you were right between them, you're no longer have any chance to escape the black hole. And to me, it's really fascinating this moment when the event horizon is no longer a sphere because it's an opportunity to learn something, to know something about the history of the black hole, what's going on inside, even from the outside. What do you mean? What can you learn or what could you hope to learn? Well, if you come along a black hole and it's a perfect sphere, you have no idea. what's inside. Is it bananas? Is it apples? Is it the result of a star collapsing or two stars collapsing? You have no idea about like the merger history of that black hole. But if you come

along at the moment when the black holes are still merging, then you know this must have come from two mergers. You know that there are two singularities inside that event horizon or you know something about the history of it more so than if you just come along to a sphere, right? So you know a little bit about what's going on inside the event horizon. I see. You're saying it tells you a little bit about what happens when you change a black hole, right? Because a non-changing black hole is sort of mysterious and impenetrable. But a black hole that's changing, maybe you can tell something about whether, you know,

all the stuff inside of it is concentrated in the middle or spread out evenly, things like that. Yeah, it's fascinating to me because black holes are like the most identical thing in the universe. Remember this, in theory, only three things to know about a black hole. There's only three numbers that totally determine it. And two black holes that have the same mass, spin, and charge are totally equivalent. There's no way you can do experiments to tell them apart. Except unless, and this is the crack in the no hair theorem, if you have a black hole with that mass and that's been in that charge, but it's still finishing its last merger, and you can know one little clue about that black hole.

You can know that it came from this merger. And that must mean that the innards of the black hole haven't quite settled down yet. they haven't like formed the singularity or the quantum fuzzball or whatever is going on inside because the event horizon has a different shape it's the same mass the same spin and the same charge but a different shape event horizon hasn't yet collapsed into a sphere oh i see you're saying we can learn about the resulting black holes but we don't really would know anything about the holes the two holes that went into it right like those would still be a mystery those would be a little bit of a mystery but you can know something about their relative masses for example

If two black holes that are equal in mass merge, then as they're merging, they look different than a black hole where one was like 99% of the final mass and the other one was 1% because it's more asymmetric. So you know something about what went into the black hole from the shape of the sort of peanut before it collapses into a sphere. I just think that's fascinating because it's a tiny little crack. And any crack to say like I know something about what's going on past the eventorizing, that's tantalizing because we know so little about what's inside. I see. You're saying like a regular black hole by itself, it's inscrutable. But if you see what two of them joining together, you're like, hey, I know a little bit about what happens in these extreme conditions. Yeah, you know a little bit about the history of this black hole. Whereas for another normal black hole, you know literally zilch, except for the mass, the spin, and the charge.

And here you have like a little bit more information. Plus, you get to describe these things in terms of a diagram that physicists call a pair of pants diagram, which is a lot of fun. Yeah, have you? I guess if you Google pants in black holes, you'll get a whole bunch of interesting images. I haven't tried it to me. Maybe we should have the adults in the audience check that first. The idea is that you have two patches of space time, which are sort of like the legs of the pants, and then they're merging and they form like the waist eventually.

So if you draw that out with like tubes of space time connecting each other, you start with two, you end with one. It's sort of like a pair of pants. Yeah. I think it's going to be hard to paint that picture. But I think the main point is that when the two black holes get together, they sort of reach out in the middle. They start merging.

That's sort of, I guess, where your in scene would be in the pants. And then they blob together, right? Is that sort of what happens? Like, at least that's what the simulation say, that they just blob together and become one big hole. Yeah, they blob together and become one big hole. And in the process, released an incredible amount of gravitational radiation. And we can learn something about that process from looking at the details of the

wiggles of that radiation. Right. And a lot of this energy comes from the angular momentum, right? Like maybe they're spinning around each other slowly when they're far apart. But then as the two black holes get together, they have to preserve the same angular momentum. By the time they get really, really close to each other, they're spinning at incredible speeds, right? Which makes for huge accelerations, which make for huge gravitational waves. And that's why probably every black hole out there is spinning. The simplest idea we have of a black hole, the short style black hole of a sphere is in the event that the mass is not spinning, it's just sitting there. But because things fall into a black hole and they will always swirl around before they fall in, because otherwise

they'd have to fall in directly to the center, like perfectly online with the singularity. Any deviation from that, they're going to fall in with a little bit of angular velocity. And so with momentum, and so they're going to end up spinning. So the final black hole has to be spinning every black hole out there in the universe is almost certainly spinning for that reason and two black holes spinning around each other you're absolutely right how a lot of angular momentum and they're going to generate a lot of gravitational radiation but the end result the what ends up happening at the end is they merge into one bigger black hole that's bigger than the two individual black holes but maybe like not as big as if you just added the mass of the

two black holes exactly they lose some of that mass right what happens when you lose energy as a black hole. You lose mass. Just like if a black hole is radiating hawking radiation, it's shooting out particles. It's losing mass. And so therefore, it's shrinking. Right. So black holes can evaporate. They can lose their mass through hawking radiation and get smaller and smaller. If they also lose energy by gravitational radiation, they are also getting smaller. And so there's so much energy released in these collisions that sometimes they can lose a lot of mass. Like as much mass is our sun. Right. And this energy goes out as gravitational waves, but also I imagine a whole bunch of light too, right? Like quasars have, you know, all that gas that was around each of them is, you know,

going through these extreme velocities and smashing against each other. A lot of that must go out as basically light as well. Most of the energy is radiated as gravitational radiation. It can be like 5% of the mass of the system is lost due to gravitational radiation. There can also, be light emitted, and this is sort of an open question. People have only seen a couple examples where they have seen flashes of light perfectly coincident with black hole collisions, and so something they're excited to study. This is an era of multi-messenger astronomy where you can see the same thing in electromagnetic radiation, light as you can in gravitational radiation, and so you can study it much more deeply. It's not something we've seen many examples

of, so it's not something that's very well understood yet. All right, so stay tuned as we get more samples of these collisions. But I guess one big question, it kind of goes back to what you were saying before, which is that time slows down near a black hole. So if I'm near a black hole, my time is frozen basically, or super slow motion. So how do these, but now if you get a black hole next to the other one, isn't one of them slowing time down for the other one? And wouldn't they just look to us like they're frozen in time?

Yeah, you might wonder, like, why do black holes ever collide? Don't they slow each other's time down so much that they basically just get frozen before they merge, right? Remember that a black hole is not a single point in space. So really what we're talking about is the merger of their event horizons. And so while the two singularities may orbit each other for a long time slowing down because of the time dilation, their event horizons can merge before the singularities come together. Right. But still, like the time should be slowing down almost to a slow.

stand still near the edge of each black hole. So as they start to merge, wouldn't things kind of freeze in time? So things sort of do freeze in time in the sense that they get slowed down. Like what are we seeing when we see black holes merge? We see a pattern of gravitational radiation that comes from the black hole. We see it speed up and go faster and faster and faster. Now when you look at that, you might wonder like, why isn't that slowed down? Why isn't it get like spread out and slowed down? Why don't we see the gravitational radiation get slower and slower. The answer is that we are. We are seeing the effects of that time dilation already. Like if there wasn't time dilation, then that gravitational radiation just for the collision

would be going insanely fast. So we are seeing the effects of time dilation already sort of built in when we see black holes collide. It would look different without the time dilation. Oh, I see. You're saying like things are so extreme. Things are moving so fast around these collisions and gravitational waves are being emitted so quickly and so intensely that even with almost freezing time, they still come out and they seem at a sort of a certain frequency for us. Exactly. That's built into those calculations. And when we do the numerical relativity to figure out like what's happening and where is the event

horizon, how much gravitational radiation is emitted, that's of course taken into account. And so we're seeing just what we expect, time dilated, slowed down collision, but still generating these gravitational waves. So it's not slowed down to zero. I see. So if you were like near one of these black holes as an observer, like it would be like insane, right? It would just like happen in a flash. Yes, exactly. It would happen much more quickly. If you were very close to the event horizons of these black holes as they were happening, so you had sort of the same clock as they would, you would see something very different. Just the same way, if you see somebody fall into a black hole from far away, you see their time getting slowed down. But they don't see that, right? They experience time normally.

they just fall in and end up hitting the singularity. They would look very, very different if you were in the neighborhood of the black hole. So that's pretty convenient, right? Like usually when you want to observe something colliding really fast, you have to use a high speed camera or you have to somehow slow down time or sample it's super fast so you get a good picture of what's going on. But this one has sort of like a built-in slow-mo setting. Exactly.

When the most interesting thing happens in the universe, it automatically goes slow-mo just like in special effects in the movies, right? Just like in Marvel movies where like the bad guy shoots at the good guy and it's like time slows down. So they can dodge it. Yeah, exactly. Just like in the matrix. They can make those crazy bins and dodge those bullets. All right.

Well, I guess then that's what happens when you collide two black holes. They sort of slowly reach out to each other. They start to merge. You get a peanut shape black hole, I guess. And then that eventually bloss into a bigger black hole. And some folks write and ask questions like, is it possible? for particles to escape the black hole during the merger because they've heard that black holes shrink a little bit. They radiate this energy away and so like maybe when the black holes are combining something can like sneak out the back, right, which is a fun idea. But unfortunately, no, black holes do not leak out any of this information when they merge. And the key thing to understand is that even though the mass of the two black holes is smaller than there's some, the volume of a black hole grows very quickly with its mass. So even though the final mass is not just the sum of the incoming mass,

the final volume can be like eight times the original black hole volume. So the event horizon is smaller than it would have been if it hadn't radiated gravitational radiation, but it's still bigger than either of the two black holes combined. But you're saying I think some things do sort of escape, right? Some information escapes, right? Like even if it's in a different form, in the form of gravitational waves,

I think you were saying earlier that, you know, you can learn a little bit of its history as stuff escapes, right? That's information, right? Yeah, the gravitational waves contain information about the mass of the black hole and its location and its velocity. It doesn't tell you anything about what's going on inside. But you're right, from the shape of the event horizon, you can tell a little bit about the history of this black hole. And you're right, that's also encoded in the gravitational waves. But if something was just like at the edge of the black hole as it was merging could somehow, you know, get lucky and somehow, you know, as these things are merging, it maybe pulls on the event horizon

in such a way that somehow it gives you a little bit of a window for like one particle to like shoot out. No, unfortunately not. That's what the event horizon means. The event horizon is not a physical surface. It's just like the location past which no information ever actually gets out. So you can't ask like, is it possible for something to get out? Well, that's like by definition. That's what the event horizon is. It's the point where nothing ever escapes. No information. leaks out. That's how we figure out where the event horizon is for these things at any given moment. We look into the future history of these black holes in our simulation and say, where's the point past which nothing ever escapes? Right, right. I guess I'm thinking

like, you know, like if you're accelerating the black hole really fast, isn't it possible for something to escape? You know, like if I accelerated the earth really fast, the things on one side would be squish against the earth, but maybe the things on the other side of the earth might fly off and get left behind. It's certainly true that if you did that to the Earth, you would cause incredible damage. And I like the way you're thinking about really destructive experiments just to learn about the nature of the universe. Kudos there.

You're really becoming a physicist. But if you did that to a black hole, you would, nothing would leak out of the event horizon. That black hole is enough curvature that even photons moving at the speed of light can't escape. And by accelerating the black hole, can't make anything travel faster than the speed of light. All right. Seems like it's plausible. Well, but I think the main question is what happens when two black holes collide and they sort of don't collide, right? They sort of smush together.

I guess it's the simple answer. They sort of grow to meet each other. And for a few moments there, you have a black hole that doesn't have a spherical event horizon, which is kind of an incredible revealing moment for the black hole. Yeah, you get a peanut, I guess, a peanut black hole. All right. Well, I think this really kind of points to some of the amazing things that can happen out there. in the universe, you know, the situations with that are not just extreme, but it's like you take two extreme situations and you smash them together. You get like extra extreme. The thing that

amazes me is that we can do these calculations. People perform these simulations using numerical relativity tools. They describe these incredible bonkers things that's happening. And then we can actually look out there in the universe and we see them. It really happens. And that makes you wonder, like, is this what's really going on out there? If I could fly out to the black hole and like watch it with my eyeballs. Is this what I would see? You know, and I hope that one day eventually we can travel the universe and we don't have to just see these black holes from billions of light years away. We could see these collisions close up. Yeah, just make sure you bring some peanuts to snack on while you watch. And smash your peanuts together with a chocolate bar and boom,

you invented chocolate M&M's. That's the extreme, Daniel. Extreme snacking with Daniel and Jorge. All right. Well, we hope you enjoyed that. And think about all the amazing things that are happening right now the universe whose echoes we're hearing right now that are washing through you right now and maybe telling you about what happened in that collision think about all the things that are happening in the universe and sending us information that we don't even know about that we're ignoring that future generations of scientists will discover and will use to learn incredible things about the universe yeah i mean there must have been hundreds or thousands or hundreds of thousands of black hole collisions whose crash whose mush sounds washed over human

but we never even mute. We see a tiny, tiny fraction of the universe and we understand even less of it. Thanks for joining us. See you next time. Thanks for listening. And remember that Daniel and Jorge Explain the Universe is a production of IHeartRadio.

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