How Do Black Holes Die? | Ars Technica Video | CNE

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Astrophysicist Paul Sutter returns to once again take us to the Edge Of Knowledge, this time to ponder the paradoxes of black holes. If nothing is supposed to escape a black hole, how can they emit miniscule amounts of radiation? What's the fate of matter sucked into a black hole once the black hole itself dies? Dr Sutter is joined by fellow astrophysicist Dr. Moiya McTier to discuss what we know, what we don’t, and how we’re trying to find out more.Find more with Dr. Paul Sutter here:Website: https://pmsutter.com/YouTube: https://youtube.com/paulmsutterTwitter: https://twitter.com/PaulMattSutterFacebook: https://facebook.com/PaulMattSutterInstagram: https://instagram.com/PaulMattSutterTiktok: https://tiktok.com/@paulmattsutterFind more with Dr. Moiya McTiertwitter.com/GoAstroMo twitter.com/ExolorePod

• Edge Of Knowledge

Released on 3/29/2022

Transcript

00:00 I like to think of black holes 00:01 as the ultimate roach motel in the universe. 00:04 Once you go in, you don't go out. 00:06 [shudders] 00:08 But beginning with the work of Stephen Hawking 00:11 we realize that, don't worry, dude, 00:14 you're in this episode a lot. 00:15 We realize that black holes emit radiation. 00:17 They actually glow just a little bit 00:19 and they eventually dissolve, completely disappearing. 00:22 And this opens up a nasty paradox 00:24 called the Black Hole Information Paradox. 00:27 What happens to all the information that flows 00:30 into a black hole after it disappears? 00:32 The full answer relies on a complete description 00:36 of quantum gravity, which we don't have. 00:39 [contemplative music] 00:50 Let's take a step back. 00:52 In Einstein's Theory of Relativity, 00:55 black holes are extraordinarily simple objects. 00:58 You just need three, only three numbers, 01:00 to completely describe any black hole. 01:03 You need to know its mass, its electric charge, 01:06 and its spin, and that's it. 01:09 And then the black holes themselves 01:11 are extraordinarily simple. 01:13 There's a singularity at the center, which is a point 01:15 of infinite density and also the subject of another episode, 01:18 and then it's surrounded by an event horizon 01:21 and that is the boundary, the entrance to the roach motel. 01:25 But that event horizon is even more boring 01:28 than you think it might be. 01:30 Take a look at this waterfall. 01:32 Where is the point of no return? 01:34 It's not the waterfall itself, it's somewhere up river. 01:38 There's a place on the river where the flow is too strong 01:42 for you to be able fight against the current. 01:44 That is the event horizon. That is the point of no return. 01:49 And this shows us how boring event horizons really are. 01:53 They don't look any different 01:55 from any other part of the river. 01:56 You only discover them when you try to escape. 02:00 But Stephen Hawking found that event horizons 02:03 are anything but boring. 02:04 There's a weird interaction happening there 02:07 between the event horizon itself 02:09 and the quantum fields that surround it. 02:11 And this interaction gives rise to the emission 02:14 of particles, particles that have a temperature. 02:17 Black holes glow and that's really messed up. 02:22 So let's go to the chalkboard to check this out. 02:25 Stephen Hawking's result was extraordinary 02:29 and, honestly, counterintuitive. 02:31 Who knew that that black holes, the ultimate bottomless pits 02:36 of destruction, would glow or would have a temperature? 02:40 And this temperature is something that we can calculate? 02:44 Hawking's formula for the temperature 02:46 of a black hole is simply 02:49 temperature is equal to the reduced Plank's constant, 02:54 which is Plank's constant divided by two pi, 02:58 times the speed of light cubed, divided by eight pi, 03:04 Newton's gravitational constant, 03:06 times the Boltzmann constant, 03:08 times the mass of the black hole. 03:11 This is telling us that the temperature 03:13 of a black hole is incredibly small 03:16 because the mass of a black hole, 03:18 even the smallest black holes have a mass 03:21 of a few times that of the sun, 03:23 makes this a very, very big number, 03:25 which makes temperature a very, very small number. 03:28 So this is incredibly, incredibly small. 03:31 We're talking a typical black hole emitting, you know, 03:34 like one or two photons a year. 03:37 But it's not zero. 03:39 Now black holes emit radiation through this crazy, 03:43 complex quantum mechanical process. 03:45 Any kind of radiation has a spectrum, has a structure. 03:50 But to describe the radiation a minute by a black hole, 03:53 I need some sort of inspiration, 03:56 a light bulb moment, if you will. 03:59 Thank you. All right. 04:01 This is an old school incandescent light bulb 04:04 and the kind of radiation, the spectrum of radiation 04:07 emitted by this light bulb 04:08 is something we call black body radiation. 04:12 Now, light bulbs aren't the only thing 04:14 in the universe to have black body spectra. 04:17 Human bodies emit radiation with a similar spectrum. 04:20 The sun emits radiation with the same spectrum. 04:24 And so do black holes. 04:27 Let's say we have a very cold object. 04:30 Very cold objects have a black body spectrum 04:33 of a flux versus wavelength that looks something like this. 04:40 And very hot objects have a similar kind of spectrum, 04:44 but it's shaped more like this. 04:46 It's peak-ier and then it goes down like this. 04:50 Any object in the universe that emits 04:53 as a black body radiator will have something looking 04:58 like these curves, including black holes. 05:02 That's one of the most remarkable things 05:04 about Hawking's result, is how simple the radiation emitted 05:08 by black holes really is. 05:10 It's this crazy complex quantum mechanical process 05:15 that leads to a simple equation for the temperature 05:19 and then a well-known spectrum 05:21 that we've known about for over a century. 05:24 But that simplicity makes black holes even more confusing 05:29 because this is the exact place where the paradox comes in, 05:34 because the black body radiation emitted 05:38 by a black hole doesn't carry any information. 05:41 It's very boring, actually. 05:44 So let's say you make one black hole made out of, 05:48 I don't know, a potato. 05:50 All right. 05:51 And you make another black hole made out of a plant. 05:54 Oh, yeah. So we've got two black holes. 05:57 One is made outta potatoes and one is made outta plants. 06:00 A lot of potatoes and a lot of plants, but you get the idea. 06:03 There's a lot of information in this plant 06:06 and in this potato and it's locked up in the black hole. 06:09 But then the black hole starts radiating 06:11 and they have the exact same mass, 06:13 they have the exact same temperature. 06:16 There's exact same black body radiation, 06:19 there's no information carried away. 06:22 I don't know which one is the potato black hole 06:24 and which one is the plant black hole. 06:26 But as they radiate, they lose mass, 06:29 and eventually they disappear. 06:31 So where did the information go? What happened to that? 06:34 This is the paradox. 06:36 So let's to an expert to dig 06:39 into this mystery a little bit more. 06:40 [science explodes] 06:43 I am Dr. Moya McTier. 06:46 I am an astrophysicist, a folklorist, 06:48 and a science communicator. 06:50 Well, let's talk about space, then. 06:51 We're talking about black holes. 06:53 The universe makes stars 06:54 and some stars turn into black holes. 06:56 How does that happen? 06:58 How do you go from this giant ball 06:59 of radiation and light and warmth to a black hole? 07:03 Black holes can form in a few different ways. 07:05 Astronomers are still trying to figure out 07:07 how the very first or early big black holes formed. 07:11 We call these primordial black holes. 07:12 But the most common way for black holes to form 07:16 is through the life cycle of stars. 07:19 So you have a star that's massive enough, 07:22 let's say it has to be about 40 times 07:24 more massive than our sun. 07:26 By the time it runs out of all of the hydrogen in its core, 07:29 something called hydrostatic equilibrium breaks. 07:33 So the way that stars work is that 07:35 they're constantly imbalanced between gravity crushing in 07:39 and the pressure that you get 07:40 when you have nuclear fusion pushing out. 07:43 And when you stop fusing hydrogen in your core, 07:46 you lose the outward pressure. 07:48 So gravity forces the star to collapse in on itself 07:52 and then there's this rebounding effect. 07:54 That creates a supernova explosion. 07:56 So in the explosion, a lot of the material 07:58 from the star gets flung out from the system. 08:01 But what's left, if it's dense enough, 08:03 or if it's massive enough, is a very dense black hole. 08:07 These are small black holes, 08:08 they're called stellar mass black holes, 08:09 only a few times more massive than our sun. 08:12 But if you get enough of them together, 08:14 then they can grow into a bigger black hole. 08:17 How many black holes are we talking about in a galaxy? 08:20 One, two, maybe 10? 08:22 We have one supermassive black hole 08:25 in the Milky Way galaxy, but there are tens 08:28 of thousands of these smaller stellar mass black holes 08:32 in the Milky Way alone. 08:33 And there are hundreds of billions of galaxies, 08:35 so there are lots of black holes out there in universe. 08:38 You mentioned this supermassive black hole 08:39 in the center of the Milky Way. 08:41 How do you get something to be supermassive? 08:44 I think in a lot of space contexts, 08:47 astronomers are still trying to figure out 08:49 how these really massive black holes form, 08:51 but we are pretty sure that 08:54 at least regular supermassive black holes, 08:57 not these hyper massive ones, 09:00 that they form through galactic collisions. 09:02 Because a lot of galaxies, most galaxies, 09:05 have black holes at their centers 09:07 and that's just another effect of gravity. 09:09 This really heavy thing is going to sink 09:11 towards the center of the galaxy. 09:14 And when these galaxies collide, 09:16 so do their black holes after many millions of years 09:19 of spinning in towards each other. 09:21 When you get all of these black holes colliding, 09:23 then their masses add up, 09:24 so you get a supermassive one in the center. 09:26 These black holes, they just sit there, 09:28 minding their own business 09:29 and they don't do anything else, right? 09:31 [laughs] Oh, I wish. 09:33 Well, it actually is a common misconception 09:35 that black holes suck things into them. 09:37 And they do have very strong gravitational pulls, 09:40 but they aren't like vacuums, 09:43 actually suctioning stuff in. 09:45 It's just a gravity well. 09:47 It is a pit for stuff to fall into. 09:49 If we look at the anatomy of a black hole, 09:51 there's the black hole in the center, 09:53 and the edge of the black hole is called the event horizon. 09:57 After the event horizon, 09:59 if the black hole is massive enough and active enough, 10:01 it has something called an accretion disk. 10:03 So this is the disk of material around the black hole 10:07 that it's eating. 10:08 It's material that has been gravitationally attracted 10:12 to the black hole and is actively in the process 10:14 of spinning into to this gravity pit. 10:17 Sometimes there's enough friction and heat 10:20 in that accretion disk that it lights up, so we can see it. 10:23 That's how we study a lot of black holes. 10:26 Or we study their gravitational effect on things around it, 10:29 like the stars that are orbiting it. 10:32 One of the things we're exploring in this episode 10:34 is Hawking Radiation. Ooh! 10:37 Can you just talk 10:38 about Hawking radiation and how annoying it is? 10:40 [laughs] Yes I can. 10:42 So we say that black holes are so dense 10:45 that nothing can escape them unless they're traveling faster 10:48 than the speed of light. 10:49 But that creates this paradox 10:52 because black holes have energy 10:55 and they can dissipate that energy. 10:57 But if something can't escape a black hole, 11:00 how can it possibly dissipate this energy? 11:02 Stephen Hawking came up with this hypothesis 11:05 for radiation that gets trapped, basically, 11:08 on opposite sides of the event horizon. 11:11 Some get sucked in, some doesn't. 11:13 The stuff that doesn't get sucked in 11:14 can be radiated away as energy, as this Hawking radiation. 11:19 Yeah, I've been thinking a lot 11:21 about the long term end of the universe. 11:23 And one of the potential ways that the universe could end 11:27 is in the big freeze. 11:29 This is the scenario where the universe keeps expanding, 11:33 forever, long enough that all 11:35 of the gas gets made into stars 11:38 and then all of those stars die and they cool off. 11:40 And eventually, many, many trillions of years from now, 11:45 the only thing left in the universe is black holes 11:48 after all of the stars have cooled down 11:50 to dark chunks of rock. 11:52 So some scientists have tried to figure out 11:55 what happens after that? 11:57 How do the black holes lose energy? 11:59 Because the only way for the universe 12:01 to really be dead is for there to be no energy in it. 12:04 So one way that black holes can lose their energy 12:08 is through this Hawking radiation. 12:09 And it would take many, many billions of years 12:13 for a single black hole to lose all of its energy 12:15 through Hawking radiation, but it is theoretically possible. 12:20 So the ultimate fate of the universe rests 12:24 in understanding Hawking radiation? 12:27 The ultimate fate of the universe 12:28 in this one potential scenario. 12:30 Thank you, Moya. 12:31 Yeah, thank you. 12:32 [hyper speed whirs] 12:34 To achieve his result, Hawking combined 12:36 our understanding of gravity, which is general relativity, 12:39 with our understanding of the very small, 12:41 which is quantum mechanics. 12:43 But he only did that in an approximate sense. 12:45 He assumed that the gravity was relatively weak 12:48 at the event horizon that it didn't couple strongly 12:50 to the quantum fields that were there. 12:52 It could be that in a full theory of quantum gravity, 12:55 something much more complex is happening 12:57 at the event horizon, something we call a firewall. 13:00 Now studio regulations prevented me 13:02 from having an actual wall of fire 13:05 to show you what would happen, 13:06 so, instead, I'm just gonna smash some eggs. 13:09 The key idea here is that in physics, 13:12 information is preserved and everything 13:15 can be traced back to its roots. 13:17 If I smash this egg, 13:19 [egg squishes] 13:21 all the information is preserved. 13:23 If I wanted to, I could retrace all the steps 13:26 of all the molecules and reconstruct the egg. 13:29 It would be extraordinarily difficult, 13:31 but it wouldn't be impossible. 13:33 So what happens in firewall theory 13:34 is that if this egg were to hit the event horizon, 13:37 it would get obliterated and all its information 13:40 would be spread across the event horizon. 13:42 And then that information would get tangled 13:45 up with the Hawking radiation leaking out into space. 13:48 That way when the black hole disappears, 13:50 all the information is still there in the universe 13:53 and the paradox is resolved. 13:55 A little bit messily, but still resolved. 13:58 You know what, this is actually, oddly, satisfying. 14:01 Anyway, we should move on. 14:02 Is there someone here to clean it up or, 14:06 it's me, it's me, okay. 14:09 [stars explode] As intriguing 14:11 as firewall theory is, it does have its shortcomings, 14:15 most importantly, we don't actually know how it works, 14:19 how the information actually gets encoded 14:21 on the event horizon and how it gets tangled up 14:24 in the Hawking radiation. 14:25 So it doesn't actually solve the paradox, 14:27 it just moves it somewhere else. 14:29 But in our studies of black holes, 14:31 we found something very intriguing. 14:33 And that has to do with holograms. 14:35 Yeah, yeah, Holograms, but not 14:36 the holograms you're thinking of, 14:38 holographic theory where multidimensional information 14:41 is stored on a lower dimensional 14:43 of the substrate, if you will. 14:45 We found that when information flows into a black hole, 14:49 its surface area increases in proportion 14:52 to the amount of information, not its volume. 14:55 So this is telling us that, somehow, 14:57 the information that goes into a black hole 14:59 is getting stored or encoded on the surface area, 15:03 not within the bulk of the black hole itself. 15:06 What is this telling us? We're not exactly sure. 15:08 It could mean that a full theory of quantum gravity 15:12 really only lives in two dimensions, 15:14 not three or something else. 15:16 I mean, honestly, black holes are just plain confusing. 15:20 Thanks, Stephen. 15:22 [contemplative music ends] 15:26 [piano tinkles] Share on Facebook Tweet Embed Send Feedback