What happens to event horizons when black holes collide?

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Speaker 1: 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 restaurants. 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. I mean, 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 to Hollywood actors together. Hi. I'm or Hamry, cartoonists and the co author of Frequently Asked Questions about the Universe. Hi, I'm Daniel. I'm a particle physicist and a professor at U C 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 can? Do 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 insides, deep revelations about the nature of the universe, and hilarious cartoons. Yeah. It tackles 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? Terrible? Didn't you read the book? Man, I'm gonna 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 My Heart 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 with 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 smashed 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 makes 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 sauces on my plate, and you never know that could have been a culinary invention that rocked the world. It just seems a little, you know, sort of a destructive way of studying the universe. You know, I'm more of an engineering type. I'd like to take things apart, not smashing 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, it's sort of like a little kid which 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 and 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. But have you actually look. Maybe there's a screwdriver for protons. You need to get the right one with the right you know shape. Yeah, the screw driver for protons is another proton. I guess it's more like a hammer than a screw driver. But then what is that screw driver 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 grand deadlines, and you know, I gotta get stuff done. I can't just wait till the heat death of the universe when everything collapses. I see, it's a lack of patients, not a lack of or methods. You're encouraging procrastination to the heat death of the universe. Right, that's really on brand for there you go, And in the meantime, the grant could support you. Right, that's right. I'm going to write a grant for waiting for ten to the fourteen years until the universe. Does it experiments for me? We'll see how that goes. I'll cut you and if it gets funded. Yeah, as long as it's for ten to the fourteen dollars, I'm totally in no. But the 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 courts. There is no screwdriver, there are no tweezers, and it's something that we can actually do. We can manipulate protons, we can tune 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 take to 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 it gives us a picture of what's going on in 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 of the things, so you kind of have to break them apart because they don't open up so easily. And to be honest, an engineer and we do sometimes match things together us 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 Proximus 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. And 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 some things that going on is a big coalition. We have seen comets slam in the 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 smashed 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 their 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, the 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 are 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 a great way to figure out what's going on, great way to support physicists. Would 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, seeing 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 you know. 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 then 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 twenty 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 him, But so far, they are one of the hardest things to look at and figure out. 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? There's a very sensationalist question, I feel, what happens when black holes collide. It's sort of like shark versus shark, which shark eats the other one? You know, one black hole sucking in the other one, these sucking each other. What does that even mean? Man? Yeah, I know, it's like kind of 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 will be a smashing success. But we have been smashing things together. We smashed photons together last time, and we smashed what else do we smash? We did a whole listener episodes question about smashing stuff together. That was the theme annihilation questions. And then we smash light together, which turns out you can't smash together. And now we're smashing black holes. What's next, Danuel 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 yet. 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 a smash Burger. 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 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 peaking under the rug of nature, really understanding something deep about the nature of space and time by looking for these extreme collisions when nature has to tell us how things were. Because black holes are pretty extreme in the universe, right, there's some of the most extreme conditions imaginable. Maybe they're breaking the laws of physics inside, or at least the laws that we didn't know, and so you can't imagine amazing things are gonna happen when you smash two of them together. Yeah, they're probably gonna smash the laws of physics. And I guess maybe a more philosophical question is, is Daniel, kid, two holes actually collide? Like, what, what's actually hitting each other? Nothing's gonna 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 map doesn't quite work that way because the black hole that comes out is a little bit smaller than the some of the two black holes that went in, which is pretty weird. Wait what, well, you just spoiled it, 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 years, 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 like to participate and hear your speculation on the podcast for everybody else to enjoy, please don't be shy. Right to us two questions at Daniel and Jorge dot 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, hey, they make a bigger black hole? Would be I think some people have discovered that they make gravitational way. Put that seems like two simple and once there. So I read black Hole Blues by Jane Levin, and my best guess is black holes Clyde, they circle around each other faster and faster, and as they get um closer to each other, they're circling almost at the speed of light. At the very last split second, and when they Clyde. The force has enough energy to overpower the energy that an entire galaxy might put out, and that's why we can sense the gravitational waves galaxies away UM here on Earth with the new instruments we have. In general, I don't think there's 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 merged 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 UM I think probably is 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 come from these things. So when they collide, it's got a kind of one window which everyone is a more dense and be strongly have stronger gravitational forces, and then it's kind of absorbs. They make a lot of gravity waves and then they make one big 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. But if one shark starts eating the tail of the other shark, and then the other shark starts eating tail the other shark, what's going to happen? It's like a yin yang shark somehow, Yeah, ying shark. Yeah, maybe the shark ends up eating itself when you get a sharknado 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 hell 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. Black hole, as you said, is a hole in space and time, but it is 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's it's dense enough that particles that are near it are doomed to fall in. You know, mass and energy hells space how to bend, and then space tells and 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 or with 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's sort of holes caused by gravity, right, Like that's another way to do sort of think about at it, right, And it's it's like there's there's so much stuff and energy in them that it just sucks everything in and and it sucks them so much you can never get up Yeah. Mostly you can think about gravity in two different ways. You can think about like a force something is pulling on you, and like the Earth is tugging on you. That sort of Newton's idea of gravity. But black holes come out of general relativity, which encouras you to think about gravity in a very different way, since the 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 at a place where space has ben 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 to right, Like, there's this possibility that maybe gravity is a forcedome. There are forests, 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 is, and 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, I like can't escape, so they just look like a giant whole and 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, And that kind of time dilation is relative because if they look back at you, they see your clocks slowing down. So you too, disagree about whose clock is slowing down, which is really weird and confusing makes you doubt like you know, truth and the existence of reality. But the kind of time dilation that happens to 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, is absolute. So if your friend gets near a black hole where space is curved more, you will see their clocks slow 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 absolutely 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. Yet it's pretty cool effect. And like 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 towards 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 thing to fall into a black hole. Like you toss the 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 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 that they're going to overtake the banana. Right. Yeah, that seems confusing because it suggests the 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, doesn't have to like eat the banana the sides. 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 will come and encompass the banana. So that's how we see black holes actually grow out there in the universe is a continuous stream of stuff falling into it and pulling out the event horizon. Right, But you don't have to feed them, but it's 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 up in your basement, it's game over for you for here 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 could have a stable black hole that you keep pet You can't have a pet then you just contradicted yourself. Just don't pet it. I guess you 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, they're a black hole. Stuff falls in and we never see it again, but there are a few things that you can tell about them. Everything that falls into the event horizon is lost to us and what happens to it, and 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 an electron into a black hole that changes its charge and its electric yield. The same thing for its spin. Black holes can spin. So there's this theorem called the no hair theorem that says those the only three things you can know about a stable black hole, mass, spin, and charge. Wait, what is it called the no hair theorem? How does hair falling 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 that have blonde hairs, have a mohawk. It's like, you know, just an example of something you can't know about a black hole. M 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 smashed them or merged them together. Apparently a lot of things happen. So let's get into that. But first let's take a quick break. Alright, we're talking about smashing black holes together, and Daniel, this happens all the time, right, Like we've recently been able to listen to black holes colliding, and a lot of they had. They happened more often than we thought in black hole collisions were first observed in two thousand and fifteen, 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, that 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 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 it 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 gonna go do particle because 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, you know, detect the black hole collision without the gravitational waves tection, Like, could you ever see two black holes actually colliding? You can't 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 that sort of on deck for falling into the black holes, and so sometimes when two black holes collide, there are creation discs also collide and create light. They've seen as 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 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 colliding. So really, the unique signature and the thing that told us that black holes were colliding where these patterns of gravitational waves, which are like ripples in space and time itself. And so far since we've seen a whole bunch of I've heard a but a whole bunch of these black hole collisions, like maybe like ten a year or something. Right, it's incredible we have like fifty examples now and to appreciate how amazing that is, realized that we didn't know how often black holes collided. When Lego 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 here. We're just trying to translated 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 ten 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 a 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 and week two. 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 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 they're closer, is easier for us to see them there, further away than the head needs 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 milk away 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 one happen yet in the Milky Way. Remember, the black holes are not that common. We have really big ones in the center of the galaxy, the black holes created from stellar collapse. But they get two black holes to collide. You really need like two black holes in a binary system, because I guess it fifty 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 you know, maybe one 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 you know 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 fair to think of a whole moving like a hole in the ground doesn't move, but black holes can move when they can sort of like fly through space and and run into other black holes. You know, black holes have bass just like everything else, and so they have inertia and they can have momentum. Black hole can move in the same way that you can move past a black hole. Remember that velocity is just relative in our universe. So if you're flying past the 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 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 obitting each other and they're slowly losing that energy, radiating away their orbital energy until eventually they collide. WHOA, 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 first start to turn into a black hole. But now yet you need to have both of them in the binary star system turned into black holes exactly. And so those are the conditions you need. And we're still understanding your black hole formation. But it depends on how much mass there was in each individual star. If it's a massive enough, then eventually it will collapse into a black hole. There's like no way to avoid it when it burns up its fuel. And 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, are the planets are orbiting the Sun pretty stably, pretty stable, e that's true, We're not falling to the Sun yet, not today and not tomorrow. But you know, these orbits are not technically stable because every time you're in orbit around something, you're accelerating, and anything that's accelerating in our universe, it'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. Is 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 gonna 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 gonna 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 now 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, all 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 swirl faster and faster and closer and closer, and at some point they start to touch, I guess right, or 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 talked about the event horizon as if it's like something physical, you know, like a surface 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's anything 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 do? 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 uson because actually what we're seeing there is the shadow of the black hole, which is larger than the event horizon because you know, some light, for example, will pass near it 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 that 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 a black hole. You can either calculate from general relativity where you can shoot a bunch of particles at it and wait till the end of the universe and see which one's 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 gonna have to keep that definitely 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 a one black hole and I have another black hole and meat in my hands, and I'm swirling them and I'm bringing them together, and they're swirling and 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, and they start to overlap. And when people write to me about this, something they're confused about is like a black hole's event horizon is a sphere, right, It's like centered around the singularity. Now you have two black holes, both of rich or spheres. What happens when they touched? You suddenly get a sphere at the center of the two? You know, 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 happing two blobs to one blob, and a little bit surprisingly that means that the event horizon is not always spherical. 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 if you take two ink plots, like two blobs of ink and you sort of bring them together, they're gonna sort of like touch maybe at the boundary and then sort of merge and but just a little bit first, and then the blobs sort of merges together, the two blobs become a peanut shape, and then they sort of blob block 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 at 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 film mint that forms between them. Now, the event horizon looks sort of like a dumbbells, like two big blocks with a very thin line between them. Then as they get closer and closer, that line 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 and m or of a whole bunch of other candies. All right, well let's get into what actually is happening with that event horizon and where do all the gravitational waves come from. But first let's take another quick break. All right, we're talking about smashing two black holes together. And the scenario we're picturing is two black holes that came from a set of binary stars and each start became a black hole. 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. They're 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 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 what can you learn? 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 too. 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 spear, 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 the black holes 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 the 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 that charge, but it's still finishing its last merger, 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 inners of the black hole haven't quite settled down yet. They haven't like formed the singularity or the quantum fuzz of all 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 wouldn't know anything about the holes. The two holes that went into it right like they would, 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 of the final mass and the other one was one 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 event horizon. That's tantalizing because I mean, it was 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 an other normal black hole, you know literally zilch except for the mass, 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 the physicists called a pair of pants diagram, which is a lot of fun. Yeah. I guess if you google pants and black holes, you'll get a whole bunch of interesting images. I haven't tried it to make we should maybe should have. The adults in the audience checked that first. The ideas that you have two patches of space time, which is 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 two 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 very 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 sort of I guess whether you're in scene would be in the pants. And then they blocked together, right, Is that sort of what happens? Like? Is that what this? At least that's what the simulations say, that they just blocked together and become one big hole. Yeah, they blocked 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 when as the two black holes get together, they have to preserve the same angular momentum as other. By the time they get really really close each other, they're spinning at incredible speeds, right, which makes for huge accelerations, which make for a huge gravitational way. And that's why probably every black hole out there is spinning the simplest idea we have of a black hole, the short stile black hole of a sphere is in the event that the mass is not spinning, is just sitting there. But because things fall into a black hole, and they will always swirl around before they fall in, because otherwise they have to fall in directly to the center of like perfectly online with the singularity. Any deviation from that, they're gonna fall in with a little bit of angular velocity and so with momentum, and so they're gonna 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 hole 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, what ends up happening at the end is the emerge 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 map ass, 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 if 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 ways. But also I imagine a whole bunch of like light to write 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 five of the mass of the system is lost due to gravitational radiation. 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 when you can see the same thing you know, 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, well, one big question. It kind of goes back to what you were saying before, which is that you know, time slows down near a black hole. So if you're if I'm near a black hole, my time is frozen basically or it's super slow motion. So how do these But but now, if you get a black hole nextly 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 rozen 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 stand still near the edge of each black hole, so as they start to merge, when things kind of freezing time. So things sort of do freezing 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 its 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 gets slower and slower. The answer is that we are. We are seeing the effects of that time down lation 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 build 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 ways 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 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 are like near one of these black holes as an observer, like it would be like insane, right, It wouild just like happen in a flash. Yes, exactly, would happened 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 experienced time normally. They just fall in and end up hitting the singularity if you look very very different, if you were in the neighborhood of the black hole. Mm. 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 sampled super fast that you get a good picture of what's going up. 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, 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 bends and dodge those bullets. All right, Well, I guess then that's what happens when you collide to black holes. They sort of slowly reach out to each other. They start to merge, you get a peanut shaped black hole, I guess, and then that eventually blogs 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 the 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 emerge. 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're saying earlier that you know, you can learn a little bit of the 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. 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 pools 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, is 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 pass 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 squished 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 they're you're the becoming a physicist. But if you did that to black hole, you with 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 one. All right, it 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 smushed 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, 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 described 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 it 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't 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. Eminem's that's too 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 in the universe. Who's you know echoes we're hearing right now that are washing through 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 just cover and we'll 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. Who's crash, whose mush sounds washed over humanity, but we never even knew. We see a tiny, tiny fraction of the universe, and we understand even lesson. Thanks for joining us, see you next time. Thanks for listening, and remember that Daniel and Jorge explain the Universe is a production of I Heart Radio. Or more podcast from my heart Radio, visit the I heart Radio app, Apple Podcasts, or wherever you listen to your favorite shows.