John Green, Very Curious: God, I'm so old. Do you ever, like, see yourself in a zoom and just be absolutely shocked?

Dr. Katie Mack, Astrophysicist (chuckling before answering): Sometimes. But you can put on the - you can do the "touch up my appearance" thing on Zoom and then 

John: Can I do a baby filter that turns me into a nine-year-old? 

Dr. Mack: No, no, no, but it'll smooth you. So you'll be an inhumanly smooth person. That's always an option. 

John: 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. 

Dr. Mack: If you could see yourself from far enough away, you could see yourself as a baby, (chuckling)

John: Right. Yeah. 

Dr. Mack (chuckling before talking): You would have to have set up the mirror a long time ago, 

John: Yeah. 

Dr. Mack: but in principle. 

(theme music plays)

John: 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 going to 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 spacetime objects that are infinitely compelling, literally, and yet also, ultimately, unknowable. Here's our conversation.

(theme music plays)

John: So today we are going to pause from our cosmic timeline where we've been going through the first seconds and then millions of years of the universe, and we're going to take a quick gander at something I have genuinely, 

John Green: deeply no understanding of. Which is black holes. I mean, you are painting onto a black canvas right now, Dr. Mack. 

(Dr. Katie Mack chuckles.)

Dr. Mack: Okay. Alright. So black holes are, they're one of these topics that (like) everybody 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!

John: So there are lots of them? That's the first thing I didn't know. I don't think I knew that there are lots of them. 

Dr. Mack: 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.

John: A hundred million?

Dr. Mack: Yeah.

John: Wow. Okay. 

Dr. Mack: 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 know that there are a lot in our galaxy. We know that there's a super massive one in the center of our galaxy, and we know that there are supermassive black holes in the center of the 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 are and what they represent, they're 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

Dr. Katie Mack: 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're some of the brightest things in the universe. And

John Green: Oh.

Dr. Mack: 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 (supposed to) help us to map the cosmos.  

John: Now that is surprising

Dr. Mack: Yeah. 

John: from the perspective of somebody who's - who's interested in how these things get named. 

Dr. Mack: So. Okay. Let's just back up. So that's kind of the preview of why astronomers are excited about black holes, but let's go back up and (like) let's talk about what a black hole is. 

John: That would be super helpful for me because I don't even know whether to be anxious about all these black holes 

(Dr. Mack chuckles.)

John: until you tell me what they are. 

Dr. Mack: Okay. (chuckling while answering) It may not help when I tell you what they are. 

John: Sure. I don't expect it to help. I expect it to get worse, but I'm still excited. 

Dr. Mack: 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 important. Stars go through a lifecycle and the lifecycle 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 proto star. It forms a star. A star is technically born when (you know) nuclear fusion starts happening in the center,  it lights up, 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,

John: Hmm.

Dr. Mack: something like that?

John: Okay. 

Dr. Mack: 0.08 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 0.08 to about 0.4 solar masses. Those ones just (like) they burn hydrogen into helium in their core. They kind of burn slowly. 

Dr. Katie Mack: They don't have a whole lot of mass to (like) create a bunch of pressure in the core, so they kind of just glow for billions of years. So those red dwarf stars can just continue (to sort of) burning slowly for billions of years. Those are long lived stars 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. 

John Green: 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,

Dr. Mack: Yeah, yeah. 

John: Long, 

Dr. Mack: Long, peaceful.

John: not too much drama.

Dr. Mack: Yeah.

John: Peaceful.

Dr. Mack: Yeah, exactly.

John: I don't want to have a ton of nuclear explosions.

Dr. Mack: Right.

John: I want to have just a steady, steady diet of them, 

Dr. Mack: Yes. Yeah, just a nice kind of (you know) constant burning, but not

John: Right.

Dr. Mack: sort of inflamed.

John. Yes!

Dr. Mack: Yeah. Yeah, that sounds great. When you start to get a little bit bigger, that's when things start to get more dramatic. 

John: Mhm.

Dr. Mack: So our star, the sun is kind of an intermediate mass star. So this is a range from about 0.4 solar masses to about eight. These are the stars like our sun, where it's burning hydrgen into helium in the core, and it's going to keep doing that for billions of years, but at some point it's going to run out of hydrogen in the core to burn, and there's going to be a few processes that go on. IT's going to burn a little helium for a bit as it's getting toward the end of its hydrogen burning life. It's going to be expanding and turning red. 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 going to be it for us. 

John: Well, the earth part of us, 

Dr. Mack: The earth part of us. 

John: we'll be on so many planets by then, Dr. Mack?

Dr. Mack: Sure. I mean, we've got a billion yeas to figure it out, right? We'll sort something out. We'll find some other possibility. 

John Green: If we make it to the oceans boiling, I'm just going to say it. It'll be a dang miracle.    Dr. Katie

Mack: That's true. That's true. Yeah, that'll be a good run. 

John: I like our odds of making it through the next billion years at about zero. (Like) I would put it at about zero.  

Dr. Mack: (Yeah.) I don't blame you there. I think that would be impressive for sure. 

John: 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.  Dr.

Mack: That's true. 

John: So these are the stars that are about the size of our sun that include our sun. 

Dr. Mack: Yes. Yeah. So in the future, (you know) 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 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 fo these in the sky. They're called planetary nebula. They don't really have anything to do with planets, but it's in historical term anyway.

So there'll be this big colorful nebula created from the slough off outer layers of the star, and the core of 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 puff out the gas, then that gas can collapse on itself. Right now, the sun is kind of help 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 a kind of maximum density of

Dr. Katie Mack: regular matter where you can put a whole lot fo regular matter together and make it really, really dense. And we have materials that are super dense, really heavy metals can be very, very dense. But there's 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 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 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 volume of the earth. 

John Green: Oh. 

Dr. Mack: So it's condensing a whole lot of matter into a very small space 

John: Yeah. 

Dr. Mack: and creating this weird form of matter, this electron degenerate matter where you've got kind of the protons and neytrons and electrons kind of in this sort of strange space.

John: And just to do a quick though 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? 

Dr. Mack: You would definitely be crushed. You would be sort of squished onto the surface of a white dwarf. 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.

John: Okay.

Dr. Mack: In general, (you know) the more mass in the smaller amount of space, the worse the gravity is. Those words I did not put together in the right way, but 

John: But I know what you mean. You mean the more extreme the gravity is? Because there's still all the same mass in a 

Dr. Mack: in a small space. 

John: small space. Okay, I got it.

Dr. Katie Mack: Yeah so it's this compactness that's the issue. So a white dwarf is much more compact than a star. 

John Green: Okay. 

Dr. Mack: It's a way that you can pack in a whole lot of matter into a small space by kind of messing with how the electrons interact with each otehr so that they're not kind (of) held up in these levels the way that regular matter is. Now, there are some stars that are more massive than that, 

John: Hmm.

Dr. Mack: 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 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 carbon and nitrogen and oxygen, and it can create these kind of shells of burning of different elements as it goes through its lifecycle. So 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 our sun (you know) sort of blows off its outer layers, 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 -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 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,

Dr. Katie Mack: 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 of 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 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 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 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 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 a different process.

That's not an end of life supernova. That's something else that can happen, that 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 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. 

John Green: Our star is not going to do that.

John Green: When you say for a short period of time, sometimes that means (like) a picosecond, and sometimes that means (like) 2 million years.  

Dr. Katie Mack: It's days. It's like several days.

John: (Okay.) So you've to catch a supernova in a relatively short frame of time to be able to enjoy that beautiful explosion. 

Dr. Mack: (Yeah.) Yeah. That's right. 

John: (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.  

Dr. Mack: 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 there's a range. It's kind of weighted toward the low end, but there's quite a lot of stars that are more massive than our sun. Yeah.

John: (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. 

Dr. Mack: Yeah, yeah, exactly.

John: Okay. Alright. I mean, so far it seems fine because this seems like other galaxies' problems.  

Dr. Mack: Yeah. Although, I mean, we're a little overdue for one in our galaxy. 

John (grunts): Mmmm.

Dr. Mack: We haven't had one in a while, and 

John: Great. 

Dr. Mack: We're - we're kind of crossing our fingers because it would be, 

John: Whoa, whoa, whoa, whoa, whoa. Wait. Are we crossing our fingers in hopes that it does happen

(Dr. Mack: Yeah!)

John: or in hopes that it doesn't happen?

Dr. Mack: Yeah, because we would learn a lot. 

John: Oh, okay. 

(Dr. Mack chuckles.)

John: And we wouldn't die in the process. 

Dr. Mack: We wouldn't, no.

John: Great.

Dr. Mack: 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 dunno, some ridiculously long number of years. 

John: Great. 

(theme music)

John: So you know what that means. As a planet anyway, we're going to be here for a while. Long enough, 

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Policygenius, because there are no nearby stars about to go supernova. 

(theme music)

Dr. Katie Mack: So when that supernova happens, the remnant, 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 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 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 furtehr is called neutron degeneracy pressure. And basically what happens there is the whole star core part of the star becomes like a giant nucleus.

(John: Hmm.)

Dr. Katie Mack: 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 matter, so it's as dense as an atomic nucleus, but it's about, it's (like) the mass of the sun or a little bit more. So it's this supermassive nucleus basically. So 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 it's just about trying to (like) keep the particles existing that (it) holds it up. So it is this very, very extreme form of matter, and we don't understand all of the details of what that matter is. There's all sort of theories about what's going on inside. It might be a super fluid inside, (it) could be these weird vortices, could have a kind of strange structure that's like a sort of lasanga structure. There are all these models with (like), they call it nuclear pasta.

(Both Dr. Mack and John chuckle before Dr. Mack continues talking.)

Dr. Mack: 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. 

John: Wow.

Dr. Mack: It's like a couple of kilometers or 10 kilometers or something 

John: Wow,

Dr. Mack: that is extraordinarily dense matter, extraordinarily compact.   

John: wow,

Dr. Mack: And one cool thing about neutron stars is a lot of times when they're formed, when they're compressed that way, the magnetic fields of the star 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 radiation (like) positrons and gamma rays. And that jet of radiation can (like) 

Dr. Katie Mack: spin around because the magnetic pole and the rotation pole might not be quite lined up. So it's a little bit off kilter. And that means that, that jet of radiation can (like) sweep through the universe like a lighthouse, like a lighthouse beam. And if we are lined up with one correctly, then we see this pulsing beam of radiation coming to us.

Every time the neutron star spins, they can spin at period of milliseconds. So it depends on the stage of the neutron star that it's at, but it can be these millisecond pulses. These are called pulsars, these kinds of stars.

And we can use them kind of like clocks. They have a very regular pattern. (Because) They're just rotating. And every time they go through a rotation, they flash because of this beam of light.

And so you can use them kind of like lighthouses, like clocks, and you can use that to map out a lot of things about the galaxy by seeing these across the galaxy, these (you know) dozens of neutrons stars, these pulsars, and I'll talk later about how we can use pulsars to learn about supermassive black holes. (chuckles before continuing) There's a kind of cool connection by, (you know) we use 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. 

John Green: Wow. 

Dr. Mack: So that's a neutron star. So that's what happens when the initial star is somewhere around 20 or fewer solar masses.

John: Okay.

Dr. Mack: 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 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 extreme neutron degeneracy 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 compresses it more than that.

Dr. Katie Mack: And then there's, 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 runaway, just runaway collapse. 

John Green: You get this sort of cycle of collapse.

Dr. Mack: (Yeah.) So there's just nothing that can stop it. And so all that matter, that would've formed the core of that star just keeps going inward, keeps compressing and compressing and compressing. 

John: Like infinitely? 

Dr. Mack: Well, yeah, because there's nothing to stop it. (Right?) So this is the theory,

John: Okay. 

Dr. Mack: and this where things get fuzzy, because at some point 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) 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. 

John: (Mhm.) Sorry. I got really anxious there. 

Dr. Mack: (chuckling before answering) It's okay. There's a lot of weird stuff about black holes. 

John: So how come they don't suck in everything?

Dr. Mack: Well, because not everything is already falling toward the black hole. 

John: (Oh! All right.) So it's only things that are already falling toward the black hole. It doesn't (like) expand infinitely. 

Dr. Mack: (Yeah.) It can't reach out to distant things.

John: But if I got near it -

Dr. Mack: If you get near it, it's a problem. 

John: If I got in a spaceship and got close to it, that would be a big problem. 

Dr. Mack: Yeah. The way that gravity works, (like) let's say that you're in, (like) I dunno, a giant cloud of gas or something, and there's a whole lot density at the center, but you're kind of toward the outer edges. You're going to feel the gravity of everything within (like) closer to the center than you. (Like) You can draw a sphere and you're at the edge of that sphere and everything close to the center, 

Dr. Katie Mack: you're going to feel the gravity of that. If you get closer and closer to the middle, you're just going to feel the gravity of the stuff interior to you. And so if you can get really far away from that gas cloud, you're going to 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 this stuff interior to you. 

John Green: (Mhm.) Okay. Okay. 

Dr. Mack: 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 center of the earth. (Right?) 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 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. 

John: (Okay.) So if you made the earth half the size that it is now with the same density, 

Dr. Mack: Well, the same amount of stuff -

John: same overall amount of stuff, but in half the size, I would still walk around the earth feeling the way I feel now. 

Dr. Mack: 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. 

John: Right, right. (Okay.)

Dr. Mack: But if you actually compressed it all and then you were down there, right? If you compressed everything and they were down there, then you would feel more gravity because you're closer to the center of where all that gravity is.

John: Okay. Okay.

Dr. Mack: However you redistribute the matter, as long as it's till kind of interior to you in that sort of sphere, it's how far you are from, I mean, to first approximation, there are things that can change, but (like, you know,) how far away you are from the center.  So a neutron star, a white dwarf, (you know) 

Dr. Katie Mack: 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 they can be in the same sort of range of masses, but it would kill you a lot more. (To be-) The gravity would kill you a lot more to be on a white swarf or a neutron star than it would on something as massive as the sun, because you would be closer. All that matters much more compact. You'd be closer to the center of it. (takes a breath before continuing)

So it's the compactness of matter that really affects how much gravity you feel. 

John Green: And because these black holes are really, really, really compact. 

Dr. Mack: Then if you get close enough to them, you get realy 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 than 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. 

John: Got it. Okay. But if you get close enough, it will. 

Dr. Mack: If you get close enough, then things get really weird. 

(theme music plays)

John: 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 hige 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 

John Green: 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 get weirder from here. 

(theme music plays)

Dr. Katie Mack: 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 a big rubber sheet and you put a bowling ball in the center, 

John: Yes.

Dr. Mack: and then you can roll (you know) tennis balls or golf balls around. Then they make little orbits. This is 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 add another dimension, which makes it really hard to visualize, but essentially a massive object like the sun pulls the space in all around it. It's (kinda) tucking itself in all directions, (kinda) distorting space toward it in all directions, and that creates a kind of curvature of space so that the earth, instead of going 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 trying to go in a straight line, but the space is curved, and so it's following that curve around the sun.

John Green: Okay.

Dr. Katie Mack: If the sun were more massive or if you got closer to the sun, 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 go 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) has to be going more quickly. 

John: There's more that makes you want to fall in and so you (have) to go fast to not fall in. 

Dr. Mack: (Yeah.) You have to get more angular momentum, (like) more orbital inner momentum to not fall in.

John: Got it.

Dr. Mack: That curvature of the space also messes with things like the orbit of mercury (like) precesses. (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 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 shined 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 a similar way to how the light of (you know) a distance star is stretched out by the expansion of the universe, (you get the same - ) you get this red shifting of the light.

Dr. Katie Mack: So if you shined a flashlight from the surface of a white dwarf or neutron star, that light would be redshift(ed). So it would be redder

(John Green: Hmm.)

Dr. Mack: by the time it gets to the place it's going than when the light was emitted.

(John: Hmm.)

Dr. Mack: And there's also an effect on time. These are kind of connected, but it means 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 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. 

John: Wow.

Dr. Mack: That's called time dilation. Gravitational time dilation. And that's something that also happens in the presence of a strong gravitational field that can be observed on earth. 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 seconds will have gone by the end of the experiment on the bottom clock than on the top clock. 

John: Wow.

Dr. Mack: 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. 

John: So you can see a black hole in that sense. 

Dr. Mack: Well that's complicated.

John: Okay. 

Dr. Mack: You can see the light of the stuff that's around the black hole. 

John: 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. 

Dr. Mack: 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 

Dr. Katie Mack: 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. 

(John Green: Oh.)

Dr. Mack: So a black hole itself cannot produce light. There's one tiny caveat to that, that has to do with the end stage of a black hole in the distant, distant future, hawking radiation, and we can talk about that later. But just talking about 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. 

John: 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 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 a) like a town in size?

Dr. Mack: Okay, so the event horizon, the distsance from the singularity to the event horizon, that's called the Schwarzschild radius and the Schwarzschild radius for something as massive as the sun is three kilometers, 

(John: Oh.)

Dr. Mack: that's small, right? That's pretty little. 

John: But these are usually much bigger than the sun? (Right?) So we're talking about maybe like 60 kilometers? 

Dr. Mack: Yeah, so it's proportional to the mass. So something 10 times as massive as the sun, if that were a black hole,

Dr. Katie Mack:  it'd be 30 kilometers (but) in radius. 

John Green: Okay. 

Dr. Mack: So if you get closer than 30 kilometers to the (you know) singularity of a black hole 10 times as massive as the sun, then the only direction you're going to go is toward the singularity. 

John: Proper emergency. 

Dr. Mack: Now that's what the equations tell us. We can't know for sure what's going on beyond the event horizon because no information can escape. 

John: (Oh.) And we can never know if no information can escape then can we ever know?

Dr. Mack: I mean, it gets a little tricky. ('Cause) There's a lot of debate what 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 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, 

(John: Hmm.)

Dr. Mack: 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 otehr properties really, as far as we can tell. They can't have mountains. They can't have sort of stuff coming out of them. By the time a black hole forms, it's really just kind of a defect in space. (Like) It's just a pure spacetime object because the only observational thing that you have is the curvature of space around it. And (you know) maybe it could be spinning (right?), it could be distorting the space through spinning. It could even have a charge, although (you know) the ones in space that we know about, they don't seem to do that. They seem to neutralize in some way, but it can't be anythign but a sphere (you know) except 

Dr. Katie Mack: unless it's spinning in which case it can be kind of a sort of distorted sphere, but (like) one that's not spinning, say it is 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 event horizon through things 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. We don't really understand that.

There's a supermassive black hole in the center of our galaxy. It's about 4 million times as massive as the sun, and there are supermassive black holes that are billions of times as massive of the sun as in other galaxies. We got a picture of one of these, and what the picture looked like was 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 disc and the magnetic fields throwing things around. So there's a whole lot of light around this object, and what we saw was 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 -

John Green: Like right now? In every moment?

Dr. Mack: 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. 

John: So we're not really seeing the black hole. We're seeing where the light would've been if it weren't for the black hole. 

Dr. Mack: Yeah, that's one of the things that really shows us that it's a black hole because if it were any other kind of object, that light would've been able to get to us, there would've been light shining toward us, but because it's

Dr. Katie Mack: 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. 

John Green: Right. Okay. 

Dr. Mack: And so that's called the black hole shadow, 

(John: Oh.)

Dr. Mack: and that's one of 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 taking it into the event horizon where the only direction it can go is toward the singularity and we can't see it anymore. 

John: This would imply on some level that when we say matter cannot be created or destroyed, 

(Dr. Mack: Hmm.)

John: maybe not?

Dr. Mack: (You know) Yeah, that's a term that's gotten a lot of popularity. That matter cannot be created or destroyed. And I mean, you can change matter into energy and vice versa. That's something that you can do in lots of ways.

John: Well, information cannot be created or destroyed. 

Dr. Mack: But information, yeah, so it's possible 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 

John: Okay.

Dr. Mack: 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. You add to that singularity, the event horizon gets a little larger because the event horizon is just proportional to the mass of the thing. So put more energy in that kind of increases the mass. 

John: And that would happen because say a rogue planet happens across the path of a black hole goes past the event horizon and vrroomf.

Dr. Mack: 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. 

Dr. Katie Mack: 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 these (sort of) complicated quantum processes that can occur toward the edges of the black hole toward the event horizon. And so there's something that weird that can happen toward the edges of a black hole, and that can over time kind of slowly leach 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 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 five solar mass black hole, which is about the smallest black hole we know of. There might be some that are a little bit lower mass than that, but it's around there. For that (kind of) mass of black hole, the lifetime for this Hawking radiation thing is something like 10 to the power of 69 years. So

John Green: Oh,

Dr. Mack: it's a really long time.  JohnL I was thinking maybe on the scle of trillions, but no, we're talking on the scale of numbers that we don't have words for. 

Dr. Mack: Yeah, exactly. 

(theme music plays)

John: (Alright) 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.

John Green: And a black hole's 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 super massive black hole in the center of our galaxy? And I wasn't going to let the episode end without circling back to that. 

(theme music plays)

John: So I have two remaining questions, Katie. The first is that you've mentioned that there are these super massive black holes that can be billions of times the mass of our sun. 

Dr. Katie Mack: Yes. 

John: And that seems improbable to me 

Dr. Mack (chuckles before responding): Yeah.

John: because I cannot imagine that there is a star that could go supernova that would be billions of times the size of our sun. 

Dr. Mack: That's right. That's right.

John: So how do we get these black holes that are so massive that (like) the entire Milky Way galaxy is spinning around one?

Dr. Mack: This is an excellent question because it is something we still don't entirely understand. 

(John: Okay.)

Dr. Mack: What we know is that large galaxies all seem to have a super massive black hole in the center, and it's not really that everything is orbiting the syper massive 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. 

(John: Hmm.)

Dr. Mack: The mass of the black hole in the center of our galaxy is about 4 million times as massive as the sun. And so (you know) it's not super important to the whole  galaxy. 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

Dr. Katie Mack: galaxies seem to have a super massive black hole in the center, and we think that 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 a correlation.

So we think that they kind of grow up together. As matter is coming into the galaxy, matter is also going into the black hole. Snd 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 timescales,... it's kind of tough to get that to work out. 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 JWST 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 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 accreting 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 if you just throw a whole lot of matter at a black hole,

Dr. Katie Mack: some of it'll just fall in. But a lot of it will create a disk of matter called an accretion disk, 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 the splashy (like) white caps and you see stuff that's the leaves that are swirling around in the whirlpool. 

John Green: And some of that stuff gets spit off. It doesn't all fall in, some of it gets spit off in these jets of radiation. 

Dr. Mack: Yeah. Yeah. And so if you just try and throw more matter at a black hole that's secreting, then the intensity of the glow of that accretion disk 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 kinds of puffs out and it kind of blows itself away, right? Because just trying to put it all too much 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. (chuckling before continuing) There are a lot of people working on this problem of how do you make the black holes grow that fast?

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 headstart in growing really quickly. Other ideas are that (you know) if you let the matter fall in a particular way, it can kind of overwhelm the outward pressure of the radiation and it can kind of fall in anyway. So there are a couple of different ideas for this, but whatever happens (you know) somehow black holes can get really super massive, and pretty much all the large galaxies seem to have super massive black holes.

So we know that outs is about 4 millions times as massive as the sun. We call this black hole Sagittarius A*, and that's written Sagittarius and then a capital A and then an asterisk. It's one of the most annoying pieces of terminology in astrophysics,

Dr. Katie Mack: because every time you tell someone who's not an astronomer about it, you have to explain the name of five minutes. 

John Green: You are not going to be able to justify this one to me. 

Dr. Mack: No, (it's not-) it's just weird, historical reasons. 

John: Yeah. I mean, we call the sun "the sun", right? Like, we should call it something like that.

Dr. Mack: We should call it "the big sink".

John: The big sink!

Dr. Mack: Yeah.

John: The big sink! We did it!

Dr. Mack: Yeah.

John: It's over. "The big sink".

(theme music plays)

John: My second question, 

Dr. Mack: Mhm.

John: and this may be one of those things where it turns out tha y'all just, like, use 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 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?

Dr. Mack: Yes. Yeah. So 

John: Oh no. Is it possible that we are just like the what got sucked in by a black hole? 

Dr. Mack (chuckling before answering): No. No.

John: Oh, good. Great. 

Dr. Mack: We can measure 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 Big Bang singularity, maybe an infiniteky dense point

Dr. Katie Mack: of space. It's a place where spacetime becomes infinite in some way. It becomes sort of pinched together in some way. If you imagine spacetime as like a grid and you pinch a grid in a point, that's kind of what singularity means. 

John Green: So my big concern about the idea that the universe beginning in a  singularity, which I know it didn't necessarily, but

Dr. Mack: Hmm.

John: 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

(Dr. Mack chuckles.)

John: dense points of space.

Dr. Mack: 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. 

John: Right. But that seems less crazy to me if you're telling me that like infinite density is not unprecedented. 

Dr. Mack: Right. But the thing is, 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 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 something. 

John: I like that one. That's good. That's funny. 

Dr. Mack: Yeah. So 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?

John: Right. And you can't see...

Dr. Mack: You can't see past that. 

John: Like you literally can't because, 

Dr. Katie Mack: Like if you were just inside the event horizon and you shine a flashlight outward, that light will curve around and go in, (chuckling)

John Green: Right, right.

Dr. Mack: so it will not leave. 

John: Right.

Dr. Mack: 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 things get complicated and quantum, and we don't know. There are lots of ideas for what might be happeing inside a black hole. And so the idea of the singularity, that's what happens if you just follow the equation of general relativity to their conclusion, you reach this infinity. But in general, like, singularities, like infinities in equations and stuff are just a sign that something's gone wrong.

John: Oh.

Dr. Mack: Your model is broken in some way, and they're not generally not a good thing in our, sort of, physical models.   

John: 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 rid of the infinity. 

Dr. Mack: Yeah, exactly. Exactly.

John: Hmm.

Dr. Mack: 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, (chuckling before answering) you know, but we don't know because we can't observe beyond the event horizon.  

John: Wow. 

Dr. Mack (chuckling some more): Yeah. 

John: Wow.

Dr. Mack: So there's all sorts of wild things that happen. Like, so 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...     

John: Mhm.

Dr. Mack: By the tidal forces. It's called a tidal force when you have that gravity gradient like that?

John: Mhm.

Dr. Mack: And so there's a technical term for that process for getting stretched out by the black hole. And that technical term is 

Dr. Katie Mack: spaghettification.  

ohn Green (chuckling before answering): See, again, I feel like progress is being made. That's good.

Dr. Mack (close to laughter before answering): Yeah, yeah.

You're turned into spaghetti.

John: You get spaghettified.

Dr. Mack: I mean, you can also call it tidal disruption, but yeah, spaghettification is a term. 

John: Yeah. Spaghettification is perfect.

Dr. Mack: Yeah, yeah.

John: Even at my body size, the gravity would be so different between my head and my feed that I would get stretched.

Dr. Mack: Yes. Yeah.

John: Wow.

Dr. Mack: Everything would get stretched and disrupted. I mean, probably you'd be broken and destroyed.

John: I understand that. But like theoretically, 

Dr. Mack: Yeah. Yeah, and we can see things fall into supermassive black holes sometimes. These tidal disruption events where a star can be ripped apart by a black hole, and it created 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 disk around it. 

John: Great.

Dr. Mack: It doesn't have bright jets or anything like that, which is good. We don't want Sagittarius A* to be an active black hole. That would be bad. 

John: The big sink. We don't want the big sink. 

Dr. Mack: 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. (chuckling before continuing) So a little bit of stuff can happen, but with, you know, really distant black holes, like black holes in 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, that's when they're called quasars.

They have, 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 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 the expansion and so on. And so they're really useful bright objects in the universe and 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 star.

So it's kind of a similar thing, but 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

Dr. Katie Mack: black hole, and the other one is still kind of in its orbit. Sometimes they're eating, (chuckling before continuing) they're, like, pulling the material off of their neighbor star, and it creates an accretion disk 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-

John Green: Wow. 

Dr. Mack: -where we're pretty sure that it's a black hole eating its neighbor star. And that's what this bright x-ray source is. 

John: Thank God we don't have a neighbor star. I'm really glad we don't have a, we're not on one of those planets with two suns like Luke Skywalker.  

Dr. Mack: Yeah, yeah, that could be real unpleasant. Liek can you imagine 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."

(Both John and Dr. Mack laugh)

Dr. Mack: Anyway...

John: The universe is wild.

Dr. Mack: Yeah.

John: Wild, Katie.

Dr. Mack: Yeah, yeah.

John: This was thrilling for me.

Dr. Mack: Good. 

John: I feel like I was in an episode of Star Trek or something.

(Dr. Mack chuckles)

John: It was like, it had all the drama of a proper movie. I feel like, you know what I felt, 

Dr. Mack: Excellent.

John: I felt like I was in a Christopher Nolan movie.

Dr. Mack: Nice, nice. Good. Yeah. So the black hole in Interstellar,  the supermassive black hole in Interstellar, they call it Gargantuan. The visualization of that, it includes the black hole shadow. So the way that the light is distorted around the black hole in the imagery in that movie is pretty accurate. There are a couple of things they change to make it more cinematically engaging, but it's pretty accurate the way the light is lensed around the black hole because it's distorted. And there's kind of a fun, kind of a fun story where basically, like, Kip Thorne, he's a Nobel Peace Prize laureate who studies black holes, based at Caltech. So he was 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 code to figure out what this black hole would look like as a 

Dr. Katie Mack: way of - sure making the movie more realistic. But also they wrote some papers 

(Dr. Mack and John laugh)

Dr. Mack: based on this, and it's a great way to get, you know, supercomputing time. Convince Hollywood

John Green: Right. Yeah.

Dr. Mack: that it's a good idea. Then you don't have to, you know, apply for the supercomputing selection committee to use the supercomputers at the university. You use the Hollywood one. It's much better. 

John: That's amazing.

(Dr. Mack laughs)

John: That's amazing. So we need more science fiction films

Dr. Mack: Mhm.

John: about black holes. 

Dr. Mack: Yeah. Yeah.

John: Alright. I'll make the case the next time I'm talking to Christopher Nolan. 

Dr. Mack: Excellent. You know what you should do: the next time you write a novel, just put a black hole in there somewhere 

John: Yeah.

Dr. Mack: where there's a plot point about, you know, some property of it. And so when they make it into the film, they have to do that calculation. 

John: One thing you should probably know about the movie adaptations of my books is that they don't have that budget. 

Dr. Mack: Aww.

John: Unfortunately, the whole reason Hollywood likes my books is because there's no, like, explosions. 

Dr. Mack: Oh man,

(Dr. Mack and John chuckle on)

Dr. Mack: you got to work with me here, John. Come on.

John: No, no, no. They just want, they're like, "oh, people in a room conversing? Perfect!"

(Dr. Mack chuckling some more.)

John: So I will endeavor to write plot-ier fiction, but I think you might be talking to the wrong brother. I think we need to get Hank on this one. 

Dr. Mack: Okay. 

(theme music plays)

John: 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 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: 

John Green: dark matter. I'll see you then. 

(theme music plays)

John: This show is hosted by me, John Green, and Dr. Katie Mack. This episode is produced by Hannah West, edited by Linus Obenhaus, with music and mix by Joseph "Tuna" Metesh. Special thanks to the Perimeter Institute for Theoretical Physics. Our editorial directors are Dr. Darcy Shapiro and Meghan Modafferi. And our executive producers are Heather Di 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/crashcourse .

(theme music plays before a tap ends the video)