Ep. 5: Black Holes

God, I'm so old. Do you ever like see yourself in a zoom and just be absolutely shocked? Sometimes, but you can you can put on the you can do the the touch up my appearance thing on Zoom. Can I do like a baby filter that turns me into like a nine-year-old? No, no, but it'll it'll smooth you So you'll be like an inhumanly smooth person. That's that's always an option. It just goes in one direction Time Well, or maybe it doesn't actually come to think of it I don't know if I've learned anything in the last few weeks It's that I have no idea if time goes in one direction if you could see yourself from far enough away You can see yourself as a baby Right. Yeah, you would have to have set up the mirror a long time ago Yeah, but you know in principle Welcome back to the universe so far Katie and I have been talking about the astonishing amount of information that we have been able to learn about Our universe but in this episode we're gonna focus on a mystery of our cosmos black holes on one hand black holes are a valuable tool their distinct properties or lack thereof Help us to map the universe But there are also things about them that by their nature we are unable to learn about So join me as I attempt to wrap my head around these space-time objects that are infinitely compelling Literally and yet also ultimately unknowable. Here's our conversation So today we are gonna pause from our cosmic timeline or we've been going through the first seconds and then millions of years of the universe and We're gonna take a quick gander at Something I have genuinely deeply no understanding of which is black holes I mean you are you are painting onto a blank canvas right now dr. Mack. Okay All right, so so black holes are they're one of these topics that like Everybody is is just constantly fascinated by black holes because they are genuinely one of the weirdest Objects that we know exists in the universe and they're everywhere. So there are lots of them I that's the first thing I didn't know. I don't think I knew that there are lots of them There are lots of them. We know there are a lot of black holes in our galaxy There are somewhere around 50 that we can identify and point at and say we know that there is a black hole right there But the estimate is that there's probably closer to like a hundred million in our galaxy a hundred million. Yeah Wow Okay, the way it works some stars become black holes when they die It's a process that happens often enough that black holes just kind of are everywhere in the universe And I'll talk about how they happen, but we we know that there are a lot in our galaxy We know that there are there's a supermassive one in the center of our galaxy And we know that there are supermassive black holes in the centers of other galaxies pretty much every large galaxy appears to have a Supermassive black hole in the center. So they really are completely ubiquitous objects in the universe they exist all over the place and That's a fascinating thing because when you get into the Details of how they work and what they are and what they represent They are just an incredibly bizarre kind of thing to be in the universe they really are one of the places where our intuitive understanding of physics and Even our detailed understanding of physics gets very very challenged. There are things about black holes We still don't understand especially in the interiors There are aspects of black holes that we may never have information about Because of the way that they sort of limit our ability to understand them. They are truly awesome and to an astrophysicist They're incredibly useful which Sounds strange, but that's partially because in some ways They are some of the brightest things in the universe and and we'll talk more about that when we talk about the supermassive black holes because supermassive black holes tend to Be some of the brightest things that we can see in the cosmos and there's some of the things that help us to map the cosmos Now that is surprising from the perspective of somebody who's interested in how these things get named So, okay, let's let's just back up So that's kind of the preview of why astronomers are excited about black holes But let's let's go back up and like let's talk about what a black hole is that would be super helpful for me because I don't even know Whether to be anxious about all these black holes until you tell me what they are, okay? It may not help when I tell you what they are sure I don't expect it to help I expect it to get worse, but I'm still excited. Okay. Okay, great So a black hole is something that happens when a really massive star dies So I'll tell you about how they form first because that's kind of that's kind of important stars go through a life cycle and The life cycle the star goes through depends on how much mass it starts with the first thing that happens is a bunch of gas gets together In a protostar it forms a star a star is technically born when you know nuclear fusion starts happening in the center It lights up right that's that's the birth of the star and stars can start out at a lot of different kinds of masses the lowest mass stars start at About sort of 8% the mass of the Sun something like that point point zero eight times the solar mass That's the kind of lowest mass star that can still be burning hydrogen into helium in its core And those really low mass stars about point zero eight to about point four solar masses Those ones just like they burn hydrogen into helium in their core. They kind of burn slowly They don't have a whole lot of mass to like create a bunch of pressure in the core So they they kind of just slow for for billions of years So those those red dwarf stars can just continue sort of burning slowly for for billions of years that there those are long live Stars that that don't burn very hot. They don't do anything spectacular when they die They just kind of burn out eventually but over a very very very long time now before you go any further I just want to say that my great ambition as a person is To have a life that is as much like one of those small stars as possible. Yeah, yeah long long Much drama. Yeah peaceful. Yeah, exactly. I don't want to have a ton of nuclear explosions. Right. I want to have it just a steady Steady diet of them. Yes. Yeah, just like a nice a nice kind of you know Constant burning but not right not sort of inflamed. Yes. Yeah, yeah, that that sounds great When you start to get a little bit bigger, that's when things start to get more dramatic So our star the Sun is kind of an intermediate mass star So this is a range from about point four solar masses to about eight These are the stars like our Sun where it's burning hydrogen into helium in the core And it's gonna keep doing that for billions of years but at some point it's gonna run out of hydrogen in the core to burn and there's gonna be a Few processes to go on is gonna burn a little helium for a bit as it's getting toward the end of its hydrogen burning life It's gonna be expanding and turning red. So So eventually our Sun will turn into a red giant star So over the course of about a billion years, it'll get brighter and brighter And it'll become bright enough to burn off the oceans of the earth. So that's gonna be it for for us Well the earth part of us the earth part of us will be on so many planets by then Dr. Mack sure, I mean we've got a billion years to figure it out, right? Like we'll sort something out. We'll we'll find some other Possibility if we make it to the oceans boiling. I'm just gonna say it'll be a dang miracle. That's true. That's true Yeah, that'll be a good run. I like our odds of making it through the next billion years at about zero Like that's I would put it at about zero. Yeah, I don't I don't blame you there I think I think that would be that would be impressive for sure I think it'd be great if we're in the first quarter of human history I don't think we're in the first 1% but I've been wrong before and one thing about me is I will not be around to find out that's true Okay, so these are the stars that are about the size of our sun and that include our sun Yes, yeah, so in the future, you know And in a few billion years the Sun will turn into a red giant star and at that point It'll go through changes in the core It'll it'll sort of blow off the outer layers of the star and that'll be really cool because that'll create this like big nebula We see a lot of these in the sky. They're called planetary nebulae They don't really have anything to do with planets, but it's an historical term Anyway, so there will be this big colorful nebula created from the sloughed off outer layers of the star And the core of the star will collapse into an object. We call a white dwarf So a white dwarf is the super dense core of a star And what happens is that when you stop having nuclear reactions to kind of you know puff out the gas Then that gas can collapse on itself right now You know the sun is kind of held up against its own gravity by The pressure from these nuclear reactions in the center So there's this balance between the outward push of those nuclear reactions and the inward Push of the gravity of just all of the stuff trying to fall together and collapse together under its own gravity And so when you get to the point where you stop being able to have those nuclear reactions holding everything up It can collapse and it can get really really dense So there's there's a kind of maximum density of regular matter Where you can put a whole lot of regular matter together and make it really really dense And you know we have materials that are that are super dense, you know really heavy metals can be very very dense But there's a a point where even That can't hold up the matter anymore where the electromagnetic forces that hold up atoms and molecules Can't hold up that matter anymore and it gets condensed even more That creates what's called a white dwarf where it's it's called degenerate matter Electron degenerate matter where you kind of push everything together in this way that's sort of denser than ordinary matter It's called degenerate because it has to do with how the electrons are kind of In each other's energy levels in this weird way. It's it's a different kind of matter. That's just super super dense So a white dwarf can be About the mass of the sun in about the volume of the earth Oh So it's condensing a whole lot of matter into a very small space Yeah And creating this weird form of matter this electron degenerate matter where you've got kind of these the protons and neutrons and electrons kind of in this this sort of strange space And just to do a quick thought experiment if I were to visit a white dwarf I assume it would be a big problem for me because the gravity would be really intense even though it would be an earth Sized object potentially it would not be an earth-like experience for me as a visitor You would definitely be be crushed You would be sort of squished onto the surface of of a white dwarf It's it would not be a pleasant thing to be because it would just be this super super dense object that you're very very close to Okay In general, you know The more mass in the smaller amount of space the worse the gravity is I mean those words I did not put together in the right way, but I know what you mean like you mean like the more extreme the gravity is because There's still all the same mass In a small space in a small space. Okay. I got it. Yeah, so it's it's this compactness That's that's the issue. So so a white dwarf is much more compact than a star Okay It's a way that you know, you can you can pack in a whole lot of matter Into a small space by kind of messing with how the electrons interact with each other So that they're not kind of held up in these sort of levels the way that that regular Matter is now there are some stars that are more massive than that Um that have a different cycle that they go through so a higher mass star more than about eight times the mass of the sun So when it goes through its life cycle It starts out doing the same thing. It sort of burns hydrogen in the core But when it finishes up all the hydrogen it can start to burn higher elements heavier elements too So instead of just like giving up and collapsing when it finishes its hydrogen It has there's enough matter pushing everything in there's enough pressure and temperature in the core that it can burn heavier elements So it can go through like carbon and nitrogen and oxygen And it can create these kind of shells of burning of different elements as it goes through its its life cycle So these these really massive stars They can burn heavier elements and this is part of how we get a lot of these heavier elements in the universe is through burning inside really massive stars Also through the death of stars like our sun when when our sun, you know, sort of blows off its outer layers It it creates some heavier elements in that whole process too But higher mass stars are doing this kind of interior to themselves But when a higher mass star gets to a certain point It's burning through heavier, heavier elements when it gets to iron it can't burn iron into a heavier element There's this thing that happens in nuclear physics That's I guess it's a little complicated to explain if you put together Light elements into heavier ones that creates energy Up to a point So all the elements lower than iron if you put the lighter elements together to make heavier elements that creates energy But on the heavier side on the higher side if you try to put together Things heavier than iron it would take energy to do that And so on the higher side if you split the nucleus apart, that's what creates energy This is why you can create a bomb out of either splitting Uranium or plutonium or whatever splitting the big heavy elements or by using hydrogen and fusing it into helium So those are two different kinds of big bombs that you can do because on the low end Fusion putting elements together creates energy on the high end fission pulling elements apart creates energy So what that means is that when you get to iron when you're burning up all these heavy elements and you get to iron You can't fuse beyond iron. You can't just kind of push things together and create energy beyond iron So if you when you get to iron you're not creating any more energy When you're trying to push those elements together anymore You're not creating new pressure to to hold up the star And so at that point the star can't hold itself up anymore and that's when it starts to collapse So for these these really massive stars they start to collapse once they get to the iron burning stage and The way that collapse happens It's a really massive thing the collapse is more violent. It implodes and then it explodes Spectacularly there's like a bounce off the core and this is creates a supernova So it's only these high mass stars that can do that supernova as the end of their life There's another kind of supernova that can happen that involves white dwarfs sort of Gathering more mass from neighbor stars and that can create a supernova too, but that's that's like a different process That's not an end of life supernova. That's something else that can happen. You can destroy white dwarf stars But in terms of the end stage of a star It's only the high mass ones that die by blowing up as a supernova and a supernova is just it's an explosion of the star that creates A really bright, you know explosion. It outshines its own galaxy for a short time It can be seen billions of light years away. It's a spectacular thing Now our star is not going to do that our star is not going to do that When you say for a short period of time sometimes that means like a picosecond and sometimes that means like two million years It's days. It's like like several days. Okay, so you've got to catch a supernova In a relatively short frame of time to be able to enjoy that beautiful Explosion. Yeah. Yeah, that's right. Okay. And so just to make sure I've got it Stars that are larger than eight times the size of our sun, which is a fairly large number of stars Yeah, I mean most stars are low mass most stars are lower mass than our sun But some stars are heavier. So there's a range, you know, and they can be much more massive than our sun So so there's a range that's kind of weighted toward the low end But there's quite a lot of stars that are more massive than our sun. Yeah, so and when those stars Die They kind of run out of the elements that they can burn they get to iron They can't burn it and then there's this massive Implosion followed by a massive explosion that becomes the brightest thing in that galaxy for a few days. Yeah. Yeah, exactly Okay. All right. I mean so far it seems fine because this seems like other galaxies problems Yeah Although I mean we're a little overdue for one in our galaxy We haven't had one in a while and great. We're we're we're kind of we're kind of crossing our fingers that because it would be I mean, whoa, whoa, whoa, wait. Are we are we crossing our fingers in hopes that it does happen? Or yeah, it doesn't happen. Yeah, because because we would learn a lot Oh, okay, then we wouldn't die in the process. We wouldn't know great So so to give you some comfort there are no stars within the like lethal range of us that we think could go supernova Anytime in the next like I don't know some Ridiculously long number of years great So you know what that means as a planet anyway, we're gonna be here for a while Long enough frankly for all of you listening to this podcast To need to worry about life insurance. That's right. I didn't even try to bury the lead on this one You need life insurance and our sponsor policy genius is here to help you make it easy with policy genius You can find insurance policies that start at just 292 per year for one million dollars of coverage Some options are 100 online and let you avoid unnecessary medical exams Policy genius has no incentive to recommend one insurer over another so you can trust their guidance and they have thousands of five star reviews on Google and trust pilot from customers who found the best fit for their needs So get peace of mind by finding the right life insurance with policy genius head to policy genius comm slash crash course Or click the link in the description to get your free life insurance quotes and see how much you could save That's policy genius comm slash crash course Policy genius because there are no nearby stars about to go supernova So when that supernova happens the remnant the the core of that star That did the implosion It can go a couple of different ways So it can either become a neutron star or a black hole And i'm going to say a little bit a little bit about neutron stars first because Neutron stars are these really amazing objects So if the star is less than Something like 20 times the mass of the sun but more than eight then when that supernova happens and the core collapses It's too massive to be held up by even electron degeneracy pressure even this this white dwarf thing It's too massive to be a white dwarf It compresses even further and the thing that holds it up when it compresses even further is called neutron degeneracy pressure And basically what happens there is the whole star the whole like core Part of the star becomes like a giant nucleus So it smashes all of the protons and neutrons together It turns most of the protons into neutrons. You have this giant Kind of nucleus of of matter. So it's as dense as as an atomic nucleus But it's about it's like a the mass of the sun or a little bit more So it's this super massive nucleus basically So it's it's held up basically by the fact that you can't have two particles in the same place at the same time Just sort of quantum mechanically There's this weird thing that that it's it's just about trying to like Trying to keep the the particle existing that that holds it up So it's this it's this very very extreme form of matter and we don't understand all of the the details of what that matter is There's all sorts of theories about what's going on inside. It might be like a super fluid inside could have these like weird vortices Um could have a kind of strange structure that's like a sort of lasagna like structure They're all these uh models with like they call it nuclear pasta Like what the what the configuration of stuff inside the neutron star is it's a really weird kind of stuff But it's so dense that you can take the mass of the sun and if you turn that into a neutron star It's now the size of a city. Wow It's like a couple of kilometers or like like several like 10 kilometers or something. Wow That is extraordinarily dense matter extraordinarily compact wow and one cool thing about neutron stars is a lot of times when they're formed When they're compressed that way the the magnetic fields of the star are are kind of compressed and twisted around And the star is born like spinning really rapidly So you can end up with this like strong magnetic field in the neutron star and it's spinning like a little magnet And it can create these jets of radiation from its poles from its magnetic poles that like throw out uh radiation like positrons and and gamma rays and that that jet of radiation can like spin around because the the magnetic pole and the rotation pole might not be quite lined up So so it's like a little bit off kilter And that means that that that jet of radiation can like sweep through the universe like a like a lighthouse uh like a lighthouse beam And if it's if we are lined up with one correctly then we see these this like pulsing Beam of radiation coming to us every time the the neutron star spins They can spin at like periods of like milliseconds So it depends on on the stage of the neutron star that it's at but it can be these millisecond pulses This these are called pulsars these kinds of stars and we can use them kind of like clocks because they have a very Regular pattern because they're just rotating and every time they go through a rotation They they flash because of this uh this beam of light And so you can use them kind of like lighthouses like like clocks and you can use that to kind of map out a lot of things about The galaxy by seeing these across the galaxy these you know dozens of neutron stars these pulsars And i'll talk later about how we can use pulsars to learn about super massive black holes Because it's it's there's a kind of cool connection by you know We we use these these really super extreme objects as sort of markers throughout the universe because we can see them really well And they have these very distinct properties. Wow So that's a neutron star. So that's what happens when when the initial star is somewhere around 20 or or fewer solar masses If it's bigger than that if the star starts out more massive than that And I don't know exactly where all the boundaries are on these things But if it's if it's a little bit more massive than that then when that collapse happens at the supernova stage You can get a situation where there's so much matter That even that extreme Neutron's generacy pressure cannot hold it up. So even by turning the whole thing into a nucleus It's still there's too much gravitational force It it compresses it more than that and then there's just there's no force known to nature that can hold it up There's nothing at all that we know of that could provide an outward push that can counter that much gravity So you get this like runaway just runaway collapse You get this sort of cycle of collapse Yeah, so there's just there's just nothing that can stop it and so all that matter That would have formed like the core of that star Just keeps going inward keeps compressing and compressing and compressing like infinitely Well, yeah, because we there's no there's nothing to stop it, right? So this is the theory and and this is where things get fuzzy because at some point we we cannot learn more about that kind of matter about that process because Based on the theory If there's nothing to stop that collapse It'll just it'll come to a point in the center the center of that gravity And it'll just become an infinitely dense point. We call that a singularity Sorry, I got really anxious there It's okay. There's a lot of weird stuff about black holes. So How how come they don't Suck in everything well because not everything is already falling toward the black hole Oh, okay. So it's only things that are already falling toward the black hole. It doesn't like Expand infinitely. Yeah, it can't like reach out to distant things But if you get if I got near it if you get near it is a problem If I got in a spaceship and got close to it, that would be a big problem Yeah, the way that gravity works like let's say that you're in like I don't know like a giant cloud of gas or something and it's it's there's there's a whole Ud density at the center, but you're kind of toward the out outer edges You're gonna feel the gravity of everything within like closer to the center than you Like you can draw like a sphere and you're at the edge of that sphere And everything close to the center, you're gonna feel the gravity of that If you get closer and closer to the middle, you're just gonna feel the gravity of the stuff Interior to you And so if you get really far away from that gas cloud You're gonna feel as much gravity as if all of that whole glass cloud was compressed to a single point in the center The amount of gravity that you'll feel is the same because you're still just feeling all of the gravity of the stuff interior to you Okay, okay So what matters is how close you are to The center of like where the matter is concentrated So like on the earth, we're not that close to the the center of the earth, right? We're we're feeling the gravity in a sense of all of the matter interior to us. So like the whole planet We're feeling it the same as though all of that mass were concentrated at the center The amount of gravity we feel from it is the same So if if we could compress all that matter to a smaller space, we could get closer to the center But as it is if we got closer to the center of the earth, there would be less matter between us and the center of the earth So we wouldn't feel all of the matter of the earth You know Interior to us. Okay. So if you made the earth half the size that it is now But the same density well the same amount of stuff same overall amount of stuff But in half the size I would still walk around the earth feeling the way I feel now Let me let me put it this way If you could keep the outer layer of the earth But compress all of the rest of it into a smaller space you would feel the same Right, right, right. Okay, but if you if you actually compressed it all and then you were close You were down there, right? If you compressed everything and they were down there then it would feel you would feel more gravity because you're closer to the center Of where all that gravity is. Okay. Okay However, you redistribute the matter as long as it's still kind of interior to you in that sort of sphere It's it's how far you are from I mean to first approximation you can there are things that can change but like, you know How far away you are from the center? So like a neutron star a white dwarf, you know, the sun They can all be around the same mass like around a solar mass There's some variation neutron stars have to be a little bit more massive But you know, they can be in the sort of in the same sort of range of masses But it would it would kill you a lot more to be To the gravity would kill you a lot more to be on a on a white dwarf or a neutron star than it would On something as massive as the sun because you would be closer all that matter is much more compact You'd be closer to the center of it So so how it's the compactness of matter that really affects how how much gravity you feel and because these black holes are Really really really compact then if you get close enough to them you get really screwed up But if you're far away Like if you have a black hole that has the same mass as the sun if you're far enough away from it You don't feel any different gravitationally Then if it were just a regular star So the same reason the sun is not sucking up all the matter around it The black hole won't either got it. Okay, but if you get close enough It will if you get close enough then things get really weird Okay, so to recap when a high mass star reaches the end of its life It sort of blows off its outer layer creating a nebula basically a huge cloud of gas and dust And eventually the core of the star collapses and then explodes This is a supernova an explosion so bright that for a few days at least it can be seen billions of light years away After the star goes supernova it could either become a neutron star Or a black hole when a high mass star below a certain mass collapses It compresses into an extraordinarily dense star and essentially becomes a giant nucleus known as a neutron star And when a high mass star above a certain mass collapses It compresses even further past the point where any force we know of could hold it up And it continues to compress presumably infinitely But the details of this part are beyond what we are currently able to learn And that spacetime object is known as a black hole And it only gets weirder from here So not only does the gravity get strong in a way that becomes really super lethal, but also it distorts space In a complicated way and that's connected Our understanding of gravity from Einstein from general relativity is that gravity Is the result of the distortion of space So I don't know if you've seen these kind of demonstrations where you take like You take like a big rubber sheet and you put a bowling ball in the center Yes, and then you can roll, you know tennis balls or golf balls around then they make little orbits This is like a sort of two-dimensional representation of what gravity does to three-dimensional space That analogy is pretty good. So the analogy that you know, you put something really heavy in the center of this sheet It makes a big dent. You put something less heavy. It makes a little dent And because of the way that the gravity distorts the space that causes the space to be bent so that objects don't follow straight lines They go around the more massive object That's more or less how we think that orbits work in the universe But you have to you have to kind of add another dimension which makes it really hard to visualize But essentially a massive object like this the sun kind of pulls the space in all around it It's kind of tucking itself in in all directions kind of distorting space toward it in all directions And that creates a kind of curvature of space so that the earth instead of going, you know, just in a straight line It's following that curve of the space around the sun and that's why it makes an orbit because it's it's trying to go in a straight line But the space is curved and so it's following that curve around the sun If the sun were More massive or if you got closer to the sun that that curvature is stronger So for example mercury has to go around the sun a lot faster to not fall in For the same reason that you know in those demonstrations Like if you're trying to get the golf ball to go around the bowling ball toward the center, you have to push it really fast Or else it'll fall in whereas on the outside. It can go really slowly because the space is more curved toward the center and So it kind of you know, it has to be going more quickly There's more that makes you want to fall in and so you have to go fast to not fall in Yeah, you have to get more angular momentum like more orbital inner momentum to not fall in Got it that curvature of the space also messes with things like the orbit of mercury like processes It's not it doesn't trace the same shape all the time. It kind of makes these weird little sort of loopy Shapes in its orbit because I mean it's an ellipse, but it's an ellipse where The longer side kind of shifts around as it's going around It distorts the orbit of mercury because the space is so distorted That it changes the way that that planet moves around Now with something like a neutron star or a black hole The space is even more distorted around that object and you can start to get These other effects that get even weirder. So for example near a neutron star or even a white dwarf There's so much gravity there there's so much curvature of space that it changes the way that light moves around In that area. So if you if you shine a flashlight from The surface of a white dwarf or a neutron star Then because the space is so distorted it's stretching out the light in this in a similar way to how the light of You know a distant star is stretched out by the expansion of the universe You get the same kind of you get this red shifting of the light So if you shine a flashlight from the surface of a white dwarf or a neutron star that light would be red shifted So it would be redder by the time it gets to the places going than when the light was emitted And there's also an effect on time These are kind of connected But it means that that time moves more slowly In a gravitational well than outside it. It kind of stretches time as well. So if you were You know at the surface of a neutron star or white dwarf you would experience time more slowly Than someone on the outside. So you'd look out and look like it would look like everything outside is moving a lot more quickly And people looking down at you would think that you were moving really slowly. Wow, that's called time dilation Gravitational time dilation and that's something that that also happens in the presence of a strong gravitational field And that can be observed on earth like you can take two clocks You can put one at the bottom of a tower and one at the top of the tower The one at the bottom of the tower will You know fewer fewer seconds will have gone by by the end of the experiment on the bottom clock than on the top clock Wow So these are all effects that get even more extreme with black holes So if you get close to a black hole the red shifting of light gets really extreme The time dilation gets really extreme The curvature of space is so extreme that light gets bent around black holes very strongly So for example, there's been this effort to take images of black holes So there've been a couple of these images of black holes that have been produced by the event horizon telescope And in those you can see this extreme distortion of the light from going around the black hole So you can see a black hole in that sense. Well, that's complicated Okay, you can see the light of the stuff that's around the black hole. Oh, right Okay, so you can't see the black hole itself, but you can see the light that's getting bent around the black hole Yeah, the reason you can't see the black hole itself is because the kind of definitional property of a black hole Is that it has an event horizon and an event horizon is a kind of region around the black hole where Anything that gets closer than the event horizon cannot ever escape and that includes light So a black hole itself cannot produce light There's one tiny caveat to that has to do with the end stage of a black hole and the distant future Hawking radiation and we can talk about that later But just just talking about sort of astrophysical black holes regular black holes right now Whatever goes into the black hole cannot come out and light cannot be emitted by the black hole because Once you get to the event horizon There's only one direction you can go and that's toward the singularity So essentially what's happening here is the space gets so curved that it's impossible for anything to Leave to go in a direction away from the singularity anymore. So at a certain point The space is curved such that all paths point toward the singularity And so if you get past that event horizon if you're closer To the black hole than the event horizon even if you're Light you're you're going in and so you go into this very very dense point Very very dense area. Can you help me understand how big these are? It was helpful for me when you were like the white dwarf is like the earth and A neutron star is like a city Is a black hole like uh Like a town in size So okay, so the the event horizon the distance to the from the singularity to the event horizon That's called the source child radius and the source child radius for something as massive as the sun is three kilometers Oh, that's small Right, that's pretty little But but these are these are usually much bigger than the sun, right? So we're talking about maybe like 60 kilometers Yeah, so it's it's proportional to the mass So something 10 times as massive as the sun if that were a black hole be 30 kilometers in radius Okay, so if you got closer than 30 kilometers To the you know singularity of a black hole 10 times the mass of the sun Then the only direction you're going to go is toward the singularity proper emergency Now that's what the equations tell us we we can't know for sure what's going on beyond the event horizon because no information can escape Oh, and we can never know if no information can escape then can we ever know? I mean It gets it gets a little tricky because there's a lot of debate about what really happens to information that goes into a black hole so this is a debate that's been going on for decades called the black hole information paradox and the paradox part is that there's some principles of physics that say that information cannot be destroyed and then there's black holes and And black holes seem to suggest that if you throw a dictionary into the black hole that dictionary is destroyed that information is destroyed But then there's other arguments to say well somehow that information should be encoded in some property of the black hole that could be Read in some way, but as far as we know black holes don't really have properties like they have a mass They can have a spin They can have an electric field. They can have a charge But they can't have any other properties really as far as we can tell they can't have like mountains They can't have sort of stuff coming out of them by the time a black hole forms It's really just like a kind of a defect in space like it's just a pure space time object because the only thing that you have is the curvature of space around it And you know, maybe it's it could be spinning right it could be Uh distorting the space through through spinning it could even have a charge although You know the the ones in in space that we know about they don't seem to to do that They seem to neutralize in some way, but it can't be it can't be anything but a sphere you know except unless it's spinning in which case it can be kind of a Uh sort of distorted sphere, but like one that's not spinning say it's just defined by There's a singularity somewhere in the middle But all we can do is we can observe that around this event horizon things fall in and don't come back Like the light gets distorted inward at that place And so we can observe some properties of the the event horizon through things like Like this event horizon telescope what it was looking at was it was looking at a supermassive black hole So something billions of times as massive as the sun and I'll talk about how those are formed later because we don't really understand that Um, there there's a supermassive black hole in the center of our galaxy. It's about four million times as massive as the sun And there are supermassive black holes that are billions of times as massive as the sun as in other galaxies We got a picture of one of these and what the picture looked like Was like there that black hole has an accretion disk around it. There's matter falling into it that lights up There's jets of radiation coming out from the matter and the accretion disk and the magnetic fields throwing things around So there's a whole lot of light around this object And what we saw was like a ring of light and a dark hole in the center And the reason we saw the dark hole in the center is because even though there's light all around it Some of that light is being twisted into the black hole like right now Like in every in every moment in every moment And so there are certain directions that if you look at it from that direction All you see is darkness because every bit of the light that should have come at you from that direction Instead got redirected into the black hole so We're not really seeing the black hole. We're seeing Where the light Would have been if it weren't for the black hole Yeah, that's one of the things that really shows us that it's black hole because if it were any other kind of object That light would have been able to get to us. There would have been light shining toward us But because it's a black hole it pulled in the light that could have gone from that direction We can work out the geometry of where all the light rays are going and there's going to be some direction some vantage point From which you look at it where in that direction It'll always be dark because all of the light that could have come to you from that direction goes into the black hole instead Right, okay, and so that's that's called the black hole shadow And that's one of the the ways that we can like observe the event horizon of a black hole Is through the way that it just swallows Light and kind of removes it from our universe by by taking it into the event horizon where the only direction it can go Is toward the singularity and we can't see it anymore. This would imply on some level that When we say like matter cannot be created or destroyed Maybe not You know, yeah, that's that's a term that's gotten a lot of popularity that matter cannot be created or destroyed And I I mean you can change matter into energy and vice versa That's something that that you can do in lots of ways. Well information cannot be created or destroyed but information. Yeah, so so It's possible that that black holes really do destroy information Although there are some theories that if you wait long enough a black hole will somehow release that information in some Unreadable but technically existing kind of way Okay, based on our kind of understanding of how black holes grow If you put matter or energy into a black hole, it just gets more massive Like you just you add to that singularity the the event horizon gets a little larger because the event horizon is just Proportional to the mass of the thing So, you know put more energy in that kind of increases the mass And that would happen because say a rogue planet Happens across the path of a black hole goes past the event horizon and Roof. Yeah, anything that gets too close will go in and so a black hole should just grow over time The event horizon should just get bigger over time But there's a theory that quantum effects that happen near the event horizon Can kind of pull energy out of the black hole Just by just sort of these complicated quantum processes that can occur Toward the edges of the black hole toward the event horizon And so there's something the weird that can happen toward the edges of a black hole and and that can over time Kind of slowly leech energy out of the black hole This is called hawking radiation because Stephen Hawking was one of the people who came up with this idea And so if you have an isolated black hole like let's say you just have a black hole that you that there's nothing falling into it It's just pure vacuum around it if you wait long enough then that black hole will shrink It'll radiate a little bit of energy toward the edges as it gets smaller as it gets You know lower mass it gets brighter and it radiates more and more energy more quickly And so toward the end when it gets tiny It may kind of explode toward the end and like destroy itself toward the end But for astrophysical black holes for black holes of the masses of things that we see in the universe This process would take I think I've worked out for a five solar mass black hole Which is about the smallest black hole we know of there may be some that are a little bit lower mass than that But it's around there for that kind that mass of black hole the lifetime for this hawking radiation thing Is something like 10 to the power of 69 years So it's a really long time I was thinking maybe on the scale of trillions, but no we're talking on the scale of numbers that we don't have words for Yeah, exactly All right, so to summarize there's a lot we still don't know about black holes, but there's also a lot we do know Once formed black holes distort space time and light through gravity Which is why when we look at a black hole we don't see the object itself But the light warping around it and a black holes event horizon essentially refers to the point of no return The distance you can be from the black hole Before being like irrevocably consumed by it In simpler terms once something passes the event horizon Nothing can escape it including light and information Which raises the question does everything that gets pulled in get destroyed? Well, we don't know for sure at least not yet Now katie also offhandedly mentioned that there is a supermassive black hole in the center of our galaxy And I wasn't going to let the episode end without circling back to that So I have two remaining questions katie the first is that you've Mentioned that there are these supermassive black holes that can be billions of times the mass of our sun Yes, and that seems improbable to me because yeah, I cannot imagine that there is a star That could go supernova that would be billions of times the size of our sun. That's right. That's right. So so how do we get these? Black holes that are so massive that like the entire milky way galaxy is spinning around one This is an it's an excellent question because it is something we still don't entirely understand Okay, what we know is that large galaxies all seem to have A supermassive black hole in the center and it's not really that everything is orbiting the supermassive black hole per se It's more that The supermassive black hole is in the center because it kind of falls to the center of the galaxy or it grows up in the center of the galaxy The mass of the black hole in the center of our galaxy is about four million times as massive as the sun And so, you know, it's not super important to the whole galaxy. It's not a it's not a large fraction of the mass of the whole galaxy It's a small fraction of the mass of the whole galaxy But it's at the center because that's where the most massive thing would naturally be So we think that most massive galaxies seem to have A supermassive black hole in the center and we think that the the supermassive black holes kind of grow up with the galaxy There's a strong correlation between the mass of the galaxy and the mass of the black hole So really massive galaxies tend to have more massive black holes Really low mass galaxies tend to have low mass black holes Depending on how you sort of measure the mass of the galaxy, but there's there's a correlation So we think that they they kind of grow up together like as matters Coming into the galaxy matters also going into the black hole and they kind of It seems like what happens is that when you get a whole lot of matter together to create galaxies Black holes form through, you know, the end stages of stellar evolution or something They kind of coalesce in the center and pull in matter and grow through the accretion of matter over time Now when you work out the details and the time scales It's kind of tough to get that to work out We don't we still don't know exactly how they grow as quickly as they do because Even really really early galaxies seem to have Really massive black holes So some of those galaxies we talked about before that jdlbsc is seeing that are really really early galaxies They seem to have supermassive black holes that are pretty bright in the sense that there's a lot of matter falling into them And so we see that brightness of of the matter that's falling in that's heating up as it's falling in There are very very distant quasars a quasar is a supermassive black hole That's are creating a lot of matter and that matter is lighting up and creating jets of radiation And those are really really bright because there's a whole lot of matter falling into a really massive thing And it gets heated up as it's swirling around those quasars can be really really bright and really really distant and Those seem to have grown up very very quickly in the early universe And we don't know how to get that much matter into that small space that quickly So if you try and work it out like it seems like Like if you throw if you just throw a whole lot of matter at a black hole Like some of it will just fall in but a lot of it will kind of create a disc of of matter called an accretion disc Kind of like a whirlpool like if you see a whirlpool in a lake or something You might not see the water so much but you see like the splashy Like white caps and you see stuff that's that's like the leaves that are swirling around in the whirlpool And some of that stuff gets spit off right it doesn't all fall in some of it gets spit off in these jets of radiation Yeah, yeah, and so if you just try and throw more matter at a black hole It's accreting then the intensity of the glow of that accretion disc lights up You know brightens and there's more pressure from all the other matter that's trying to fall in And so it kind of puffs out that seems like it should slow down The accretion of matter it seems like if you put in too much at once Then it should it kind of puffs out and it kind of blows itself away Right because you're just trying to put it all too much You know in the same place at the same time and we don't know exactly how to resolve that at the detailed level like It seems like these things At least in the very early universe grow very very efficiently in a way that seems like they shouldn't grow quite that fast There are a lot of people working on this problem of how do you make the black holes grow that fast? You know one idea is that the first stars were really really massive And so they left these really massive remnants remnant black holes And then so they got a head start in growing really quickly other ideas are that you know If you if you let the matter fall in in a particular way it can kind of overwhelm the outward pressure Of the radiation that and that can kind of fall in any way So there are a couple of different ideas for this But whatever happens, you know somehow black holes can get really super massive and and pretty much all the large galaxies seem to have Supermassive black holes. So we know that ours is about four million times as massive as the sun We call this black hole Sagittarius a star then that's written like Sagittarius and then a capital a and then an asterisk It's it's a it's one of the most annoying pieces of terminology in astrophysics because Every time you tell someone who's not an astronomer about it. You have to explain the name for like five minutes You're not going to be able to justify this one to me. No, it doesn't it's not it's just Weird historical reasons. Yeah, I mean we call the sun the sun right like We should we should call it something like that. She's just called the big sink The big sink the big sink we did it. It's over the big sink I My second question And this may be one of those things where it turns out that y'all just use like the same word for things Regardless of whether they're the same again not to be critical. I only learned what a black hole was about 20 minutes ago But you you keep referring to the singularity That the center of a black hole is an extraordinarily dense Singularity single point And then you said at the very beginning that it's possible that our Universe that the big bang began In a singularity Are those related terms? Yes Yeah, so no, is it possible that we are just like the What got sucked in by a black hole? No, no good great We can measure the the curvature of space and we can see that light can move in lots of different directions And so we're not in the interior of a black hole. So the term singularity means in this case a singularity is an infinitely dense point in space Or in the case of the the big bang singularity may be an infinitely dense point of space It's a place where space time becomes Infinite in some way it becomes sort of pinched together in some way if you imagine space time as like a grid And you pinch a grid in in a point That's that's kind of what what singularity means So my big concern about the idea the the universe beginning in a singularity, which I know it didn't necessarily But my big concern about that has always been well, it seems very unlikely to me That there is an infinitely dense point of space But what you're telling me is that there are actually lots of infinitely dense points of space Well, the idea of the big bang singularity is that all of space was In that point. So like when the big bang happened It created all of space by that point expanding Right But that seems less crazy to me if you're telling me that like infinite density is not unprecedented Right, right. And but the thing is like we can't observe any of that, right? We can't observe a big bang singularity if it happened or not. We don't know and we can't observe it um because Well, partially because we think that this cosmic inflation happened and that kind of obscures the view in some way And then partially because like if it really was infinitely dense like you just can't get information out of something like that And with black holes, there's a theorem that says that every singularity has to be shrouded by this event horizon It's called the cosmic censorship Cosmic censorship, uh, something I like that one. That's good. That's funny. Yeah, so we you can't have a naked singularity Is what that is called and that's because if you get too close to any singularity then the curvature has to be so Strong that light can't escape anymore and that is by definition an event horizon, right? Right and you can't see You can't see past that like you literally can't because like if you if you were just inside the event horizon And you shine a flashlight outward that light will curve around and go in So it will not leave Right, but we don't know for sure So there are lots of theories about what happens inside the black hole that you know, maybe there's some kind of like Stringy ball fuzz thing that has to do with string theory and like things get Complicated and quantum and like we just we don't know there are there are lots of ideas for what might be happening Inside a black hole and so the the idea of the singularity That's what happens if you just follow the equations of general relativity to their conclusion you reach this infinity but in general like singularities Like infinities in in equations and stuff are just a sign that something's gone wrong Like you've just you've oh your model is broken in some way and they're generally not a good thing In in our sort of physical models one of the reasons black holes would be so interesting then is that it's a place where At least so far you can't get rid of the infinity. Yeah, exactly exactly. So this might be a place where there really is There really is just a singularity there and it just kind of breaks space and time, you know But you know, we don't know because we can't observe beyond the event horizon Wow Yeah Wow, so there's all sorts of wild things that happen like You know, for example, the space is so curved that if you were to fall toward a black hole Let's say you're falling in feet first Right at some point the space is so curved that The gravity on your feet is way way Stronger than the gravity on your head. And so your feet get pulled in really fast And so you kind of get stretched out By the tidal forces it's called a tidal force when you have that gravity gradient like that And so there's a technical term for that process for getting stretched out by the black hole and that technical term is spaghettification See again, I feel like progress is being made. That's good Yeah Yeah, you're turned into spaghetti you get spaghettified. I mean you could also call it tidal disruption, but yeah spaghettification is a term Yes spaghettification is perfect. Yeah. Yeah, even at my body size The gravity would be so different between my head and my feet that I would get stretched. Yes. Yeah Everything would get stretched and disrupted. I mean, you'd probably just you'd be broken and destroyed. I understand that but like theoretically Yeah, and we can see things fall into super massive black holes sometimes These tidal disruption events where a star can be ripped apart by a black hole and it creates a big x-ray explosion Which is kind of cool to see and like our own black hole our own black hole isn't eating very much It doesn't have like a big accretion disc around it. It's not a bright jets or anything like that Which is good. We don't want we don't want Sagittarius a star to be Super like of an active black hole. That would be a sink. We don't want the big sink Yeah, yeah, but every once in a while like a gas cloud gets really close to it and gets kind of pulled in and the astronomers get Really excited that we get to see a little bit of gas fall into the black hole Like a little bit of stuff can happen But with you know with really distant black holes like black holes and centers of other galaxies when they have that strong accretion There's a whole lot of stuff falling in and they have these jets of radiation those that's when they're called quasars They have the that they're really really bright and we can see those across the universe in very very distant galaxies And because they're so bright we can use them as like Signposts we can use them as like markers to measure things in the universe and measure like the distribution of matter On really large scales in the universe and measure other things about like the expansion and so on and so they're they're really useful Um bright objects in the universe and in our in our own galaxy We can see a lot of black holes too, but the ones we see here mainly we see them when they're eating their neighbor stars So it's a kind of similar thing but like a black hole can be in a binary system with another star So maybe because they're born together and one of them explodes and becomes a black hole And the other one is still kind of in its orbit. Sometimes they're eating They're like pulling the the material off of their neighbor star And it creates an accretion disc around the black hole and that can light up in x-rays and those are called x-ray binaries And we see somewhere around 50 of those in our galaxy Wow Where we're pretty sure that that it's a black hole eating its neighbor star and that's what this right x-ray source is Thank god, we don't have a neighbor star. I'm really glad we don't have this We're not on one of those planets with two suns like uh, luke skywalker Yeah, yeah, that would be that could be real unpleasant Like can you imagine like, you know watching the other sun and like it's getting close to going supernova and you're like, don't do it Don't do it Anyway The universe is wild Wild katie. Yeah, yeah This was thrilling for me. Good. I felt like I was In an episode of star trek or something It was like it had all the drama of a proper movie I felt like you know what I felt I felt like I was in a christopher nolin movie Nice nice good. Yeah, so the the black hole in interest teller Um, the supermassive black hole in interest teller they call it gargantuan The visualization of that it includes the black hole shadow So the the way that the light is distorted around the black hole in the in the imagery in that movie Is pretty accurate. There are a couple things they change to make it more cinematically engaging But it's pretty accurate the way the the light is like lensed around the black hole because it's distorted And it's there's kind of a fun Kind of a fun story where basically like kip thorn who's um, He's a noble laureate who studies black holes basic haltech. So he was he was like the science advisor for the film I think he sort of came up with the idea of the film but anyway, he was working with the filmmakers And he kind of convinced them to run the the code to figure out what this black hole would look like As as a way of you know, kind of sure making the movie more realistic, but it also they wrote some papers I'm based on based on this and it's a great way to get you know, super computing time Confins hollywood Then you don't have to apply for like the super computing, you know a selection committee To get to use the supercomputers at the university use the hollywood one. It's what's better That's amazing. That's amazing. So we need more science fiction films About black holes. Yeah. Yeah. All right I'll make the case the next time i'm talking to christopher nolin Excellent, you know what you should do next time you write a novel just put a black hole in there somewhere Where like there's a plot point about you know some property of it And so when they make it into the film They have to they have to do that calculation One thing you should probably know about the movie adaptations of my books is that they are they don't have that budget Unfortunately, the whole reason hollywood likes my books is because there's no like explosions. Oh man You got to work with me here, john. Come on. No, no, no, they just want they're like, oh people in a room conversing perfect So I will uh, I will endeavor to write plottier fiction, but I think you might be talking to the wrong brother I think we need to get Hank on this one. Okay Considering all the well known aspects of our universe that I don't understand It's a real thrill to talk about something that bumps up against the edge of what proper experts don't know Also, I was mildly thankful that I didn't live on tatooine before this conversation and now like I'm Very thankful But I do have some regrets. I should be incorporating more black hole subplots into my novels next episode Katie tries to walk me through something I somehow know even less about than black holes Dark matter I'll see you then This show is hosted by me john green and dr. Katie mac This episode was produced by hannah west edited by linus open house with music and mix by joseph tunna meddish special thanks to the perimeter institute for theoretical physics Our editorial directors are dr. Darcy Shapiro and megan motiferry and our executive producers are heather d diego and seth radley This show is a production of complexly If you want to help us keep crash course free for everyone forever You can join our community on patreon at patreon.com slash crash course