Do Black Holes Have Hair?

00:00:08
Speaker 1: Hey or he have you started to go gray yet?

00:00:11
Speaker 2: I'm not sure I ask you to answer that question. You know, I want to preserve my air of mystery. How about you.

00:00:17
Speaker 1: I found a silver hair too in my beard, but so far I'm still all brown up top. But you know, I'm getting a little.

00:00:24
Speaker 2: Impatient, getting impatient. What do you mean you want to go gray?

00:00:28
Speaker 1: I wouldn't mind that gravitas that comes from having a little bit of gray.

00:00:32
Speaker 2: You mean you don't have enough gravity right now?

00:00:36
Speaker 1: My gravity has actually been increasing.

00:00:38
Speaker 2: There you go. It sounds like your body's doing it for you. But are you trying to counteract the you know, whole physicist look with the shorts and the sandals.

00:00:46
Speaker 1: Yeah, exactly, I'm trying to look a little bit more grown up.

00:00:48
Speaker 2: I'm not sure gray hair is going to help you there.

00:00:50
Speaker 1: Maybe I should just shave my head.

00:00:52
Speaker 2: There you go. That's one way a little older, more distinguished. And after you shave it, you could paint it gray.

00:00:59
Speaker 1: I'm not sure that's how gravitas is accumulated.

00:01:02
Speaker 2: Sounds like physicists don't know how gravity or gravitas works. Both big mysteries. Hi, I am Horehammick cartoonists and the author of Oliver's Great Big Universe.

00:01:26
Speaker 1: Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and I never really gone after gravitas.

00:01:34
Speaker 2: What do you mean you just inherently have it? Can you say you're a physicist?

00:01:38
Speaker 1: People go ooh, No, exactly the opposite, trying to break down those barriers, you know. That's why I wear socks and sandals every day. Don't try to create any distance between myself and my students.

00:01:49
Speaker 2: Or apparently your feet and the exterior world.

00:01:53
Speaker 1: And everyone knows that's another reason to live in southern California, right, Socks optional.

00:01:59
Speaker 2: I guess that's what physics is. It's all about, you know, breaking barriers, connecting with the universe. Anyways, Welcome to our podcast, Daniel and Jorge Explain the Universe, a production of our Heart Radio.

00:02:09
Speaker 1: In which we try to break down the barriers between ourselves and nature, between you and a complete understanding of everything we do and don't know about the universe. We think the universe is the greatest mystery and we are here to crack it.

00:02:23
Speaker 2: That's right. The universe is full of barriers of things we can't see things we can't understand and things that we may never understand, including barriers about fashion and dress codes at universities.

00:02:35
Speaker 1: You know, people say, focus on what you're good at, and so that's why I just don't pay attention to fashion.

00:02:40
Speaker 2: So you're good at wearing sandals, it sucks.

00:02:42
Speaker 1: I'm good at ignoring fashion. I just do my own thing.

00:02:45
Speaker 2: Well, I am totally with you. I think the most comfortable thing in the world to wear is socks and sandals. If anyone does not try that out there, I highly recommend it, or or socks with flip flops even better.

00:02:57
Speaker 1: I'm so glad this is an audio only medium.

00:03:00
Speaker 2: We don't have smell of vision, thankfully, though.

00:03:04
Speaker 1: I'm terrified at the mental images you are creating in the minds of our listeners.

00:03:08
Speaker 2: Actually have socks that have the little notch for your flip flop strap.

00:03:14
Speaker 1: You're like one step away from those horrendous toe shoes.

00:03:17
Speaker 2: Oh no, no, no, that's where I draw the line.

00:03:19
Speaker 1: I see one notch is fine. Four notches two too many.

00:03:25
Speaker 2: I don't need each of my individual toes wrapped in its own little pocket.

00:03:29
Speaker 1: Again with the troubling mental images.

00:03:31
Speaker 2: Here, well, but toes are a big part of the universe theories of everything, and so physicists have been on the lookout and on the search for one SU's theory that can explain everything in the universe, including all the mysterious bits of it.

00:03:43
Speaker 1: And some of the most mysterious bits of the universe are those pieces that we cannot see, things hidden behind the event horizon of black holes. What's going on in there? Are there singularities or is there something else? How does gravity work for quantum particles? All the answers are waiting for us behind these barriers.

00:04:03
Speaker 2: Yeah, black holes seem to be the sort of the epitome of mystery in the universe. It's almost like a little pocket that takes you out of the universe.

00:04:10
Speaker 1: Right, Yeah, it's sort of like they are their own little pocket universes because they are cut off from us. And maybe the most tantalizing thing is that we can only know a few things about what's gone past that barrier.

00:04:21
Speaker 2: Yeah, they're almost like cosmic sensors, you know, they're like hiding information, blacking it out for everyone to uh see or a Nazi.

00:04:29
Speaker 1: And physicists hate when they lose information. When some knowledge is hidden from them, and so we spend a lot of time wondering about what we can know about the things that have fallen into a black hole. Is it possible to show up to a black hole and know what has been tossed into it?

00:04:46
Speaker 2: Yeah, we have a lot of questions about black holes, but maybe not as important as the one we're asking today. So today on the podcast, we'll be asking the question do black holes have hair?

00:05:03
Speaker 1: And do black holes wear socks with their sandals?

00:05:06
Speaker 2: I think the real question is black holes have black hairs or gray hairs? Or does it depend on the age of the black hole or it's gravitas exactly?

00:05:15
Speaker 1: Maybe the really big super black holes are like the silver backs of the universe.

00:05:19
Speaker 2: I guess. Don't all black holes by definition have gravitas? I mean you kind of have to take them seriously, right, I guess so if gravity is the bending of space time, then gravitas is like the bending of the social structure. Like, are there lightweight black holes out there or you know, frivolous black holes? Probably not right.

00:05:36
Speaker 1: They probably aren't like teenage black holes that the parent black holes thing should be taking their life more seriously?

00:05:42
Speaker 2: Where did that come from?

00:05:44
Speaker 1: Frivolous. You're like, what are these black holes doing with their lives anyway? Or maybe it's just because I have teenagers in high school.

00:05:51
Speaker 2: I think you might be bringing us the home issues here on the Physics podcast.

00:05:57
Speaker 1: Well, in the end, we are trying to understand the universe through the lens of our own minds, so it's impossible to separate personal issues from physics issues.

00:06:05
Speaker 2: You could be a family physicist. You know there's family physicians. Yeah, there are a lot of dynamics in the household.

00:06:11
Speaker 1: Yeah exactly. Maybe black hole therapy can solve some problems.

00:06:15
Speaker 2: Yeah, just throw the whole family to black hole and they have to get along.

00:06:21
Speaker 1: It might get kind of hairy.

00:06:23
Speaker 2: But Harry, apparently black holes can be, or might be or could be. And so that's the question we're asking here today, which is kind of a weird question. I mean, who thinks of black holes having hair?

00:06:32
Speaker 1: Physicists? Physicists use hair as a metaphor for all sorts of crazy stuff.

00:06:37
Speaker 2: Well, it's a fun question, and so as usual we were wondering calming people out there, I thought about this, Harry question.

00:06:43
Speaker 1: So thanks very much to everybody who answers these questions. For this really fun segment of the podcast, and we are always looking for more volunteers and we want to hear from you, So join the group right to me to questions at Danielandjorge dot com.

00:06:56
Speaker 2: So think about it for a second. Do you think black holes have hair? Here's what people had to say.

00:07:01
Speaker 3: I suppose you'd have to ask yourself what exactly you're talking about when you mean hair. My assumption is that you would be referring to a biological process, and since black holes are not a biological entity, I would struggle to believe that they create hair in it of themselves.

00:07:15
Speaker 4: Well, I don't know about actual hair, but I do know that when things fall into a black hole, they get spaghettified. So maybe black holes are surrounded by a whole bunch of spaghettified ragg doll hair.

00:07:26
Speaker 5: I'm assuming by hair you're asking about something that I have never heard of, and not asking about like hair on your head.

00:07:35
Speaker 1: But since that's the only.

00:07:36
Speaker 5: Definition that I know, I'm going to say, no, black holes do not have hair on them.

00:07:40
Speaker 4: The only thing I can really equate hairs to would be like cosmic strings of some sort. I really don't know. I don't know.

00:07:50
Speaker 1: Well, if black holes have hair, it's yet another thing that tests more hair than I do.

00:07:55
Speaker 2: All Right, some great answers here, pretty creative talking about like is a cosmic a hair?

00:08:02
Speaker 1: That's right? Cosmic strings? Are these cracks in space time we talked about once, which do seem kind of hairy, but are completely separate from what we mean when we say black hole hair.

00:08:11
Speaker 2: I also wonder if them you can take this question literally, like, if the Earth falls into a black hole, it would have a lot of hair in it literally, right, or would it? I don't know.

00:08:20
Speaker 1: Yeah, well that sort of goes to the heart of the question. Really, are black holes what they eat? Or does it not matter what you put into a black hole? Can you not tell the history of a black hole in any way?

00:08:31
Speaker 2: I see these are internal hairs, hairs you don't really want to see.

00:08:37
Speaker 1: Maybe somehow we ended up talking about ingrown hairs on the podcast.

00:08:42
Speaker 2: Yes, do black holes have ingrown hairs?

00:08:46
Speaker 1: Next week, the comparison between black holes and black heads?

00:08:50
Speaker 2: Yeah, oh boy, those are very popular on the TikTok.

00:08:55
Speaker 1: And we're back to teenage issues.

00:08:57
Speaker 2: Here you go, black hole popping right here on the podcast, it's the sound of a black hole popping. But anyways, let's get down to it, Daniel, and let's maybe recap for people what exactly is a black hole and what do we know about them?

00:09:09
Speaker 1: So we don't really know what a black hole is. We have a theoretical concept from general relativity that tells us that if you have enough matter or enough energy density in a small region of space, that that space will be so curved that no information can escape. No photons, no particles, no gravitational waves, nothing from beyond this event horizon can ever propagate out and tell you anything about what's hidden behind that curtain. That's the sort of general relativity idea of a black hole. And we've seen some things out there in the universe that really closely resemble a black hole. There's something at the center of our Milky Way, there's a bunch of collapsed stars, there's something at the center of other galaxies that really resembles a black hole. But you know, because you can't see inside the event horizon, we're not exactly sure that what we've seen out there in the universe aligns with Einstein's idea of what a black hole would look like.

00:10:01
Speaker 2: Yeah, it's basically like an actual hole in space. Right, Like, once you have enough mass into one spot and it's tense enough, that region of space becomes a hole. Right like anything you throw in there, it's going to stay in there.

00:10:13
Speaker 1: It's a hole in that sense. But a hole sort of gives you an idea of like a discontinuity, like there's a gap or something. But remember that space is smooth, it's continuous. So what we're talking about is space being bent, space being curved, you know the way that like the Earth doesn't move in what looks like a straight line to us because it's following the curvature of space. When you put mass inside space, it bends it. It changes the relative distances between things, which changes how things flow. And so near a black hole, space is curved and curved very very intensely, and as you get closer, that curvature gets stronger and stronger. But it is continuous, right, there's not like a sharp cut off. But there's a point on this smooth curvature past which nothing can escape, and that's the radius we call the event horizon. And so you could say that's sort of like a threshold. It's like a hole if you fall into you'll never escape.

00:11:00
Speaker 2: Yeah, I think you're saying that there's no cank in space, but there is sort of a point, that sort of a discontinuity where light can no longer escape.

00:11:07
Speaker 1: Yeah, there's a discontinuity sort of in your fates. Like if you have photons on one side of it and photons on the other side of this radius, the ones past the radius can escape and fly through the universe, and the ones inside the radius will never escape. So there's a sort of discontinuity in the outcomes of particles. But remember the event horizon. It's not a physical barrier. It's just this difference in the outcomes of particles.

00:11:28
Speaker 2: It's sort of like a real hole, right, Like there's no barrier, you just follow. At some point you're on the hole, and some point you're not on the hole. Yeah, that's right, all right. So then what do we know about black holes, if anything at all.

00:11:40
Speaker 1: So the frustrating thing is that we have this theoretical concept of what might be inside a black hole. In Einstein's pictures, famelessly, this infinitely dense dot, a singularity where gravity has had this runaway effect compressing and compressing and compressing forever with nothing to resist it and creating this infinitely dense dot at the heart of a black hole. But of course we can't see inside a black hole. All we can do is observe them from the outside. No information escapes from the inside of a black hole, according to Einstein's theory. But we can know a few things about the black hole, like we're on the outside of it. But we can still measure some things about the black hole without going inside. For example, we can know the mass of the black hole. We can measure the black hole's impact on space time even past the event horizon, So we can know something about the black hole without even seeing past the event horizon.

00:12:31
Speaker 2: You mean, like we can know how much gravity it exerts onto the things around it.

00:12:36
Speaker 1: Exactly. The curvature of space time continues past the event horizon, right, there's no kink, Like we said, it's smooth, so past the event horizon, it's still exerting an influence on space time, and we can use that measurement of the curvature to tell how much mass there is inside the event horizon, in the same way that like you can measure the mass of the Earth just by seeing its gravitational effect on the Moon or on a satellite. Right, all that gravity adds up and has the same effect on the point. Doesn't tell you anything about like the configuration of the Earth or whatever. And you could replace the Earth with a point particle to have the same gravitational effect. But from a distance you can measure the overall gravitational force. And we can do that for a black hole obviously.

00:13:18
Speaker 2: And the idea is that like, the more gravity of black hole exerts and of the things around it, the more massive it is.

00:13:24
Speaker 1: Exactly, we can do the calculation to say how much space time gets bent by black holes of a certain mass, and so we can back that up and say, well, we measure this amount of curvature, and so therefore black hole must have this mass.

00:13:36
Speaker 2: I wonder if like mass is the actual right term for it, right, Like, we don't even know if what's inside of a black hole is mass or just a whole bunch of pure energy.

00:13:45
Speaker 1: Right, that's exactly right. We use mass as a way to sort of measure the energy that's inside the black hole, because remember that space time is really curved by energy density, not necessarily just by mass, like a proton bends space time, even though it's just made of a bunch of quarks which are very very low mass. But there's a lot of energy stored inside the proton energy in the bonds of those quarks, which contributes to its mass and contributes to the curvature of space time. So on one hand you could say, well, it's really just the stored energy of the black hole. On the other hand, that's kind of what mass is. Mass is the internal stored energy of an object, and so on one hand, yeah, it's energy. On the other hand, that's what we call mass.

00:14:24
Speaker 2: Wait, so it's like it's inertial mass or gravitational mass. Does that mean a black hole can have kinetic energy as well well?

00:14:31
Speaker 1: In general relativity, inertial and gravitational masses are the same thing. So yes, this is its inertial mass. This is like how hard it is to get it moving and how hard it is to slow it down, et cetera, et cetera. And yes, black holes can have kinetic energy. They can move right, and you can move past a black hole, and velocity is all relative. So if you're flying past a black hole at half the speed of light, you see it moving towards you at half the speed of light. So yes, black holes can definitely move, they can have kinetic energy.

00:15:00
Speaker 2: All right. So then how would you measure the mass of a black hole. You would sort of like put a scale near it or throw a pebble to see how it swings around it. How would you do it?

00:15:09
Speaker 1: Unfortunately, we're not near enough any black holes to do any experiments, but fortunately in astrophysics we can watch these experiments happen. So the best way to measure the mass with black hole is to see the motion of stuff near it, like the mass of the black hole the center the Milky Way. We measure by looking at stars that whiz past it and seeing the gravitational force on those objects and knowing how much mass has to be to create that gravitational force to make the stars bend the way they do. And for distant black holes and other galaxies, we can see like the sort of swirl of stars around the center of the galaxy and use that to measure the gravitational force from the black hole.

00:15:45
Speaker 2: Yeah, that's what I meant, you know, a pebble star.

00:15:48
Speaker 1: Yeah, basically, yes, same thing, the pebble.

00:15:50
Speaker 2: Method, all right, So then what else can we know about a black hole?

00:15:54
Speaker 1: Another thing we can know about a black hole is it's electric charge. If you put electrons into a black hole, Well, charge is concerned in the universe. So if an electron falls into the black hole, the black hole now has an overall negative charge. At another electron, get another negative charge. So you can know the overall charge of a black hole.

00:16:13
Speaker 2: Well, you mean like a black hole can have a voltage.

00:16:15
Speaker 1: Yeah, a black hole can have an overall charge and it can have electric fields. Right in the same way that black hole has mass, and that makes for effectively a gravitational field or bending of space time past the event horizon. The black hole can have a charge and that creates an electric field which can also be pasted the event horizon.

00:16:34
Speaker 2: Interesting, but I guess if I wonder if a black hole has a negative charge, can you use it as a battery? Probably not, right, Like you can't get electrons to flow out of it, can you?

00:16:44
Speaker 1: You can use a charged black hole the way you can use any other kind of charged particle. Right, You can use it to create electric fields. You can use it to repel stuff or attract stuff. But I think batteries involved like the flow of electrons through materials and I don't know that's chemistry.

00:17:03
Speaker 2: That's too hairy for you.

00:17:04
Speaker 1: Yeah, But essentially, you can think of a black hole as just like an enormous particle, right the way like an electron sort of has a charge attached to it, but you don't really think about like where is the charge. It's just like a property of the electron. You could think the black hole as just sort of like enormous particle. You don't have any details about, like where is the electric charge. It's just sort of like assigned to the entire event horizon, the same way you assign the mass to the entire event horizon. You don't really know what's going on inside, how it's that mass arranged? Is it still in bananas? Is it squished into something else? Is it nuclear pasta? You don't know anything about the internals. You just assigned it to the exterior, and then you can treat it like anything else that has a charge.

00:17:42
Speaker 2: I see, sort of like maybe the Earth. You could do that with the Earth too, right, Like, I'm sure the Earth has an overall.

00:17:46
Speaker 1: Charge exactly, any extended object with an overall charge from a distance, you can treat like a point particle with a charge, the math is exactly the same. And the case of the Earth, of course, we can know it, and it's not hidden beyond an advent horizon, So we could learn about this stuff in other ways. In the case of a black hole, you can't. And there's something sort of similar there about a black hole and a particle, the way that like two electrons are identical. You know, you can't tell the difference between this electron and that electron. They have all the same properties. That's what defines them. Two black holes with the same mass and the same charge, and it'll talk about it in the same spin, are really two identical objects?

00:18:22
Speaker 2: All right? Well, talk about spin? What a spin for a black hole?

00:18:25
Speaker 1: So the same way that you can toss stuff into a black hole that has charge, you can also toss stuff into a black hole that makes it spin. You throw something exactly towards the center of the black hole, it'll make it grow and make it more massive. But if you throw something into the event horizon but a little bit off center, then it's sort of like you're giving it a push. The way if you're like holding a bicycle wheel on an axle and you hit the rim, it'll start to spin. Or if you push on a merry go round, it'll start to spin. If you throw particles into the edge of the black hole, you can get it to spin because angular momentum, like charge, doesn't disappear in our universe. So that black hole then has to accumulate that angular momentum, which means it has to spin.

00:19:02
Speaker 2: But I guess the angular momentum in the universe is not like a fundamental thing. Is It's not like energy or regular momentum. Is it like? It's really just like the difference between two points in your body and how they're moving relative to each other, isn't.

00:19:15
Speaker 1: It Now, angular momentum is just as fundamental as linear momentum, and maybe even more fundamental than energy number. Energy not actually conserved in our universe because conservation of energy requires space to be static, and our universe space is expanding, so energy is not conserved. But linear momentum and angular momentum are both conserved in our universe. It comes from deep symmetries. Linear momentum comes from the fact that space is the same everywhere, that like translation from here to there, shouldn't change your experiment, and angular momentum comes from the fact that there's no preferred direction in space, that every direction is equivalent, and so angular momentum has to be conserved and that makes angular momentum something very fundamental to the universe.

00:19:55
Speaker 2: All right, Well, so then a black hole can have angular momentum, and that is that what it's been exactly.

00:20:00
Speaker 1: The spin of the black hole is how it stores angular momentum. And there's all sorts of fascinating consequences there in general relativity, because like a singularity can't spin. So the idea is that inside a spinning black hole is maybe not a singularity but a ringularity, like a whole circle of singularities, and those singularities are all spinning together, which is how they store the angular momentum. But nobody really knows what's actually going on inside.

00:20:27
Speaker 2: All right, So the things we can know about a black hole are its mass, it's spin, and it's charge. Now the big question is can we know how hairy it is? So let's dig into that question. But first let's take out a quick break. All right, we're talking about the hairs of a black hole. Can a whole half hairs.

00:20:58
Speaker 1: That's exactly the question. And when physicists say hair, they don't really mean hair. They're just taking a word that exists that has another meaning and giving it a meaning. In physics, which is, you know, has a great tradition of being very confusing what.

00:21:12
Speaker 2: You mean, like in this case it actually is like a cosmic string or a particle or something, or is it just a metaphor, it's.

00:21:19
Speaker 1: A metaphor for anything else. Essentially, general relativity tells us that we can know three things about a black hole. Its mass, it's been its charge, and nothing else. Nothing about its history, no interesting little details, no texture, no hair.

00:21:34
Speaker 2: Is that because of the event horizon. Basically, like if you wrap the Earth around an impenetrable, you know, black shield, you could also not tell anything about the Earth, whether it's t had hairs or not.

00:21:46
Speaker 1: I think that's true, but maybe only because an impenetrable black shield would basically have to be an event horizon of a black hole. There's no other way to make something truly impenetrable. If you just build something really solid and black, it would like read it it's some temperature, and that would tell you about what's going on inside of it. The only way to really be impenetrabled to give up no information would be to make a black hole.

00:22:08
Speaker 2: I guess that's what I mean. It's like, the reason we can't know anything else about a black hole is because it has an event horizon which keeps all the information inside. It's not something like that information is destroyed, right.

00:22:20
Speaker 1: We don't know what's going on exactly. And that's the deepest question, is like is the information destroyed? Is it actually contained within black hole somehow or is it actually destroyed? Like is the internal state of a black hole the same, no matter what you've put into it. Make a black hole out of bananas and you make another one out of apples. Do you really get exactly the identical black hole out of those two things if you put in the same amount of mass? Or is there some history there some way to tell a banana from an apple black hole?

00:22:51
Speaker 2: I guess I'm trying to, you know, relate it to the earth, and like you could make an earth out of bananas, and if you put it behind your impenetrable shield, you could also not tell the Earth what's made out of bananas or apples.

00:23:03
Speaker 1: That's exactly right, but it conflicts with sort of other things. We've seen in the universe and our quantum intuition, because quantum mechanics tells us that information is not destroyed, that the present is uniquely determined by the past, and that therefore from the present you can always derive the past, that like the past, has left a permanent imprint on the present. Quantum mechanics tells us that we can run the laws of physics forwards or backwards in time, so we could use the present to predict the future in the same way we could use the present to reveal the past. And this is in conflict with that. This says, no, no, no, there's no information here. Once you create a black hole, you can't tell what its history is at all. There's no texture to grab onto, no little details if you zoom in, there are no hairs there to reveal the history of the black hole.

00:23:47
Speaker 2: I wonder if maybe the real question is not whether a black hole has hair. The question seems to be more like, if I toss a hair into a black hole, does the hair get destroyed forever? Or is it just going to stay inside the black hole and it's going to be there, but we can't tell if it's there.

00:24:03
Speaker 1: I think those are both really interesting questions, but I think there are separate questions. In general relativity, we can't know anything that passes the event horizon. But maybe general relativity black holes are not the black holes we see in our universe. Maybe there are little ripples and hairs on the event horizon that we could use to learn about what has fallen in. And then the second question is like what happens to stuff that falls in does form some new state of matter? Is that information still there? Because we think that black holes eventually evaporate, that they radiate away all of their energy, they lose their mass, and if that information has been destroyed within them, then that information is lost forever, which would be very confusing from the point of view of quantum mechanics.

00:24:41
Speaker 2: Well to the second question, I know we've talked about in the podcast before, how it is possible to go into a black hole and survive. Right, Like, for a big enough black hole, the event horizon happens. You don't have to get spaghetified or shredded apart to go into technically the event horizon of a super massive black hole, right, That's right.

00:24:58
Speaker 1: And if for really large black hole holes, if they do have significant charge and or spin. There are even stable orbits within the event horizon of the black hole.

00:25:08
Speaker 2: So you could like fall into a super massive black hole and stay in there but still be hole, still be there.

00:25:14
Speaker 1: And still be inside. According to general relativity, yes that is possible. Again, we don't really know right what is going on inside there, so we don't really know the fate of any of this stuff until you know, you jump into a black hole and figure it out.

00:25:26
Speaker 2: Well, not me, Maybe it should be the physinessist. I mean, I think you need the cartoonist outside to draw what happens and then figure out if it's a good idea. But I guess what I mean is you can throw a hair into a black hole and it can survive, Like you can have a hair inside of a black hole. It's she said, if somebody else comes by, they won't be able to know whether you threw a hair in there or not.

00:25:46
Speaker 1: Exactly that's true. According to general relativity. You could build a black hole out of hairs and nobody else could tell the difference between that and a black hole made out of toenails or something else.

00:25:56
Speaker 2: But you would know there was a hair in there.

00:25:57
Speaker 1: You would know because you saw it falling in.

00:25:59
Speaker 2: You mean, yeah, or because I tossed it into the hole.

00:26:02
Speaker 1: That's true, you would know the history, but there'd be no way to measure it from the objects itself. There'll be no record on the outside that would tell you. Again, according to general relativity.

00:26:11
Speaker 2: Okay, so then what does that mean? Does that mean? Quantum mechanics says something differently.

00:26:16
Speaker 1: Quantum mechanics definitely says something different, And quantum mechanics tells us that this whole picture of a black hole according to general relativity is very very likely wrong. Remember that general relativity is not built on the foundation of quantum mechanics. It makes very different assumptions about how the universe works. It assumes that space and time are continuous and smooth, that you can have like infinitely small distances and infinitely small masses. Quantum mechanics gives us a very different picture. It says everything is discreet, it's quantized, it's chunked up into pieces, and there's a limited amount of information we can know about the universe. General relativity says that you can have tiny little objects moving in smooth paths, so you can perfectly know how they move. Quantum mechanics says that's not possible, and they come directly into inflict at the heart of a black hole, where general relativity says you have this point of infinite density and quantum mechanics says, no, that's not possible. And so what we need is some sort of merging of the two, a theory of quantum gravity that tells us what happens when quantum objects feel very strong gravity.

00:27:16
Speaker 2: I guess maybe the question is what does quantum mechanics say about the black hole hair question?

00:27:22
Speaker 1: So quantum mechanics as we have it now doesn't know how to deal with gravity for particles. Right, particles are very different from things like sand, or rocks, or base balls or even pieces of hair. Those things are basically classical objects, so we know where they are and we can talk about them as if they always have a specified location. But quantum particles are different. They have like probabilities of being here and probabilities of being there, So we don't know how to do gravity for particles. Like if a particle has a probability to be here and there, does it have half the gravity here and half the gravity there, or is there a gravity probability we don't know. What we need is a theory of gravity for particles, and nobody has one, and so until you have that theory, you can't actually know what quantum mechanics even says about what's going on inside a black hole, or whether there are ripples in texture on the surface of the black hole that you can use to figure out what's inside.

00:28:13
Speaker 2: I see, I think this is what I'm trying to get to, is like, what do you mean like quantum mechanics says there are ripples on the surface that might be able to tell you if a black hole is made out of hairs or bananas.

00:28:23
Speaker 1: Unfortunately, we have no perfect theory of quantum gravity, but people are doing some calculations to try to figure out is it possible for a quantum black hole to have hair? Wait?

00:28:31
Speaker 2: Wait, wait, what's a quantum black hole?

00:28:33
Speaker 1: So quantum black hole would just be a black hole as described by a theory of quantum gravity, instead of a classical black hole as described by general relativity. Right, Einstein's classical black holes are a singularity with an event horizon around them, with no hair whatsoever, a perfectly smooth surface, and you can't tell anything about what's inside or what was used to build it. Quantum black hole is a description of a different theoretical object, one that follows rules of quantum gravity.

00:28:58
Speaker 2: All right, let's dig into that. Is a quantum black hole, hole that's both black and white at the same time.

00:29:03
Speaker 1: No, no, but that sounds awesome. I wish that were true. Carlo Rovelli has this fun theory that black holes might be collapsing stars that are slowed down by gravitational time dilation, and eventually they turn around and become white holes. And we had a fun conversation with him and some of his colleagues about exactly that how a black hole can be in a superposition of a state maybe being a black hole, maybe being a white hole. So basically, yeah, your theory is one of the contenders.

00:29:31
Speaker 2: There you go. So, then how is a quantum black hole different than a regular relativity black hole.

00:29:37
Speaker 1: So there's a few different answers to that, because there are a few different theories of quantum gravity. Right, we have no perfect theory. We don't even have a theory that's complete and it works on paper, not to mention a theory which has been tested against what's actually happening out there in the universe. So people are working in different directions in quantum gravity, and some of them are working specifically on this problem and have ideas for the consequences of their particular quantum gravity on the hairiness of a black hole. So in some theories of quantum gravity, black holes have different kinds of hairs.

00:30:07
Speaker 2: Like long hair, curly hairs, luster's hairs. What did to say about what black holes could be like?

00:30:13
Speaker 1: Yeah, so let's go through some of the options. The first real progress in quantum gravity was Stephen Hawking. He said, actually, let's figure out whether black holes radiate a weigh any information. And he has his famous theory of Hawking radiation that says that black holes generate particles. You know, if you're near the surface of a black hole, it'll actually be shooting off particles at you. This is Hawking radiation. The thing about Hawking radiation from black holes is that it's supposed to contain zero information that just depends on the mass of the black hole and nothing else about the internal configuration, about whether it's apples or bananas.

00:30:47
Speaker 2: And that's because it's like generating particles out of the vacuum, right out of nothingness, out of pure energy that's inside the black hole. Like, it doesn't depend on what kind of energy, whether it has hairs or been in it's inside. It just kind of creates stuff. As I think, I understand that it just creates stuff and then radiates it out.

00:31:07
Speaker 1: That's right, that it creates stuff based just on the energy the mass of the black hole. Actually it's formulated fascinatingly in terms of like black hole thermodynamics. So you can think about the temperature of the black hole. But you have to be very careful about applying Hawking's arguments to like a microscopic particle picture of what's happening about creating these particles, because Hawking doesn't have again, the full theory of quantum gravity. He did this sort of semi classical calculation where he thought about quantum fields and near event horizons, and he figured out that these quantum fields have to radied. He doesn't have a microscopic picture. And you'll often hear this story about Hawking radiation, about particles and antip particles created near the event horizon. One falls in and one escapes, et cetera, et cetera. That's all hand wavy storytelling. That's not accurate it doesn't actually hold together in terms of what's happening with Hawking radiation.

00:31:54
Speaker 2: Wait, it doesn't hold together, or we don't know if it's true.

00:31:57
Speaker 1: If you start from that microscopic picture and try to build up a prediction of Hawking radiation, it doesn't work.

00:32:02
Speaker 2: What do you mean, Like, the radiation we measure from a black hole doesn't match what you would predict from that.

00:32:07
Speaker 1: Well, number one, we've never measured the radiation from a black hole. This is still just theoretical.

00:32:11
Speaker 2: So then what do you mean.

00:32:12
Speaker 1: As soon as you make yourself this microscopic picture and then you have these quantum particles, you have all sorts of questions you can't answer, like about fuzziness and probability, and whether the particles can fluctuate past the event horizon or not. And so the quantum mechanical picture isn't really complete about what's happening to these little particles. Nobody can start from those particles and calculate up and predict Hawking radiation. That's not what's happening. Hawking has started from sort of a bigger, broader picture, just from understanding like the energy flow the quantum fields near the event horizon, and predicted that the emit radiation. But again there's no microscopic picture that really holds together. But you want to know more details about how that works. We have a whole episode about Hawking radiation where we can review that in detail.

00:32:51
Speaker 2: But I think the main picture you're saying is that we do know that black holes radiate Hawking radiation. We just don't know like the specific details of what's happening at the border of a black hole to make that happen.

00:33:01
Speaker 1: That's right, Hawking didn't come up with a complete theory of quantum gravity, so he doesn't have a microscopic picture, and he predicts this Hawking radiation, which again should contain no information. So a Hawking black hole basically has no hair, even though it's kind of a quantum black hole. But there are other theories of quantum gravity people who've been working on, and some of these do predict black hole hair.

00:33:21
Speaker 2: All right, let's get into the hairy details of these theories and whether or not black holes have hair or quantum hairs or no hairs at all, and what it could mean about our understanding of the laws of physics. But first, let's take another quick break, all right, we are braiding our knowledge of the universe here talking about black hole hairs and whether they have it or not. And so according to Einstein, who is famous for his hair, by the way, his gray hair, even as black holes don't have hair, which seems kind of mean given how much hair he had. But it seems like quantum mechanics at first said they didn't have hairs, but now there were some theories about quantum gravity that say, maybe they do have hairs.

00:34:10
Speaker 1: That's right, And all these calculations are approximate. You know, nobody has a full theory of quantum gravity that they can start from and predict these things from first principles. Everybody is doing sort of approximations. They're saying, well, we don't have the full theory, but maybe it looks a little bit like this, And if it looked like this, then what would be the answer to this question? But a lot of is approximate and handwavy and inconsistent with other things. But you know, this is how progress is made. We don't always just have a flash of insight with the whole answer that we can work from. We put things together and try to patch them together, and eventually maybe it comes together into a bigger picture of everything, all.

00:34:44
Speaker 2: Right, Well, so then how do these fuzzy quantum gravity theories say that black holes have hair?

00:34:50
Speaker 1: So there's a paper about ten years ago that try to study the configuration of gravitons far away from a black hole, so outside the event horizon. Really interesting question to think about gravity as a quantum force because if gravity isn't the curvature spaced time, if quantum gravity lies in the direction of like figuring out how to express gravity in the same kind of language that we express other forces like the weak force and electricity, magnetism and the strong force as forces mediated by particles the way they like the photon mediates electromagnetism. Then gravity would have a particle, the graviton that you use to mediate gravity. And so in this picture this is like graviton based quantum gravity. People thought about, like, what's happening with all the gravitons in the vicinity of a black hole?

00:35:39
Speaker 2: Like what they get sucked in too? Right?

00:35:41
Speaker 1: Yeah? Would they get sucked in? Is there information in the flow of the gravitons. People are often writing in and asking me if the mass of the black hole is contained within the event horizon and gravity is quantum mechanical, wouldn't it need to like shoot out gravitons in order to mediate gravity, and wouldn't that break the area of the black hole? So you can see how it's tricky to think about gravity as a quantum force. If there really is an event horizon there.

00:36:08
Speaker 2: Whoa Okay, So let me see if I got this right, and a quantum theory that has gravitons in it, which we don't know if they exist or not, like the gravity of a black hole, you would feel it because it's shooting gravitons at me or am I shooting gravitons at the black hole? Couldn't both be the case?

00:36:25
Speaker 1: Yeah, both would be the case. And one question is whether those gravitons contain any information about what's inside the black hole or not. On one hand, you can imagine the event horizon itself just generating a gravitational field and shooting off gravitons from the event horizon. No information about what's going on inside, only information about the total mass. Right, Like if you could change the configuration inside, if you build it out of bananas instead of apples, you get the same exact distribution of gravitons in the outside, or is there information in the graviton ripples that tells you about what's happening inside the event horizon. So there's this paperback ten years ago that deduced some small changes in the graviton field far away from the event horizon based on the mass that was inside. Essentially like a little bit of a history imprinted on the space around the black hole that could tell you what had fallen in. You could like show up to a black hole a million years later and say, oh, there was a banana and then an apple, and then three granola bars thrown into this black hole.

00:37:26
Speaker 2: What do you mean, Like, how would the banana versus the apple affect the graviton that the black hole shoots out, So.

00:37:32
Speaker 1: It affects the gravitational field outside. The gravitons themselves don't pass the event horizon, but the event horizon itself has some shape to it, because remember, in Einstein's general relativity, a black hole is a perfectly spherical object, right like, as you zoom into it, you never see any bumps, any ripples, any change. It's perfectly spherical. But quantum mechanics says that can't be the case, right, that there's always a discritizations like a pixelization inherent to the universe, which means the event horizon has to have some ripples to it's some texture to it, and those tiny little deviations can change the ripples of the gravitons on the outside, and they're caused by the history of stuff that you've thrown into the black hole. So you make a very slightly different black hole if you put in bananas than if you put in apples. That information is not lost, it's somehow contained on the event horizon of the black hole.

00:38:23
Speaker 2: Like if a black hole has a banana at its center or an apple at its center, you're saying that it might cause a different texture of the event horizon at the border of it.

00:38:33
Speaker 1: Exactly. Yeah, And you could detect this if you could measure gravitons in the vicinity of the black hole.

00:38:39
Speaker 2: How would the banana or the apple affect the texture all the way at the edge of the black hole? Like if I put a banana or an apple on the center of the Earth, would it I guess it would affect the surface of the Earth in some microscopic way.

00:38:50
Speaker 1: You have to think about this quantum mechanically right, we're talking about the quantum states of all these objects, which contain the history of everything that's happened to them. Remember, in a quantum world, the present is uniquely determined by the past, which means that every possible past has a different present, which means you can invert it. You can say, well, because we're in this present, we can tell what the past was. So all the tiny ripples, the details of the quantum states of everything around you could be used in principle to like rewind time and tell you about what's happened in the past. For folks who watch that show Devs, that's the whole basic principle. And so the idea is that you throw a banana or an apple into the black hole, it creates a slightly different black hole in a way that imprints that information. I don't know the details. I can't tell you, like a banana black hole looks like this, or an apple black hole looks like that. But in quantum mechanics, there's a lot of potential information on the surface because the surface is not totally smooth, it can't be whereas a gr black hole has to be perfectly smooth with no information.

00:39:51
Speaker 2: Well, I think you're saying maybe it's not specific to quantum black holes. Like in a relativity black hole, you could also throw a banana and everything inside of the black hole could remember the banana. You just couldn't tell from the outside because it's perfectly smooth. According to Einstein, you're saying that in a quantum black hole, the black hole is not perfectly smooth. You could maybe read from its surface what you threw it.

00:40:12
Speaker 1: Yes, exactly. That was one theory in a paper about ten years ago that effectively the gravitons could tell you about the texture of the surface, which would in turn tell you about what's going on inside. The problem is that gravitons may be impossible to ever detect. Remember, gravitons are not gravitational waves. Gravitational waves are huge waves in the gravitational field that we can detect. Gravitons would be like drops in the ocean compared to waves in the ocean. So if they exist, they're super duper tiny and it may be impossible to ever see.

00:40:43
Speaker 2: What does it have to be gravitons? I wonder if there is a texture to a quantum black hole, wouldn't you also maybe detect other particles coming from the Hawking radiation off of it, and maybe could you could tell its texture from that.

00:40:56
Speaker 1: You could, And there was a theory a couple of years ago that suggested, maybe you remember we talked about on the podcast, that there might be wormholes that connect the interior and exterior of the black hole, and that Hawking radiation could in fact give you information about what's going on inside. That it's not the way Hawking described it, totally information free, but maybe actually has information about what's going on inside, because these wormholes are like bridges that are connecting quantum entangled particles, some of which on the inside and some of which on the outside of the black hole. So there are versions of quantum gravity in which Hawking radiation does have information in it.

00:41:32
Speaker 2: All right, So then what are some of the other theories about quantum black holes?

00:41:35
Speaker 1: So all these would be very very hard to detect because you're talking about detecting Hawking radiation and differences in it which we've never even seen, or detecting gravitons, so we don't even know exist, and if they do, would be very hard to see. Recently, there was an idea to look for hair on extremal black holes. We've talked about this on the podcast recently. Black holes have maximum spin or maximum charge that they can maintain, beyond which a vent horizon disappear and things go crazy. But when black holes are near this extremal state, things that fall into them might leave instabilities on the event horizon in some theories of quantum gravity, and if so, it would generate ripples in their gravitational waves, not in their gravitons, and gravitational waves are things we can detect that we might be able to see sometime in the future.

00:42:22
Speaker 2: What do you mean instabilities on the event horizon? What does that mean?

00:42:25
Speaker 1: Well, think about what happens when something falls into a black hole. It's going to fall into a specific spot. Right you toss a banana into a black hole, You're tossing it in one side of this sphere, not on the other side, and so immediately what's going to happen is that the event horizon is going to grow out to meet you. So the event horizon is not going to be spherical for a moment. Then it slurps in this banana, it falls into the singularity, and the black hole rings back down to a perfect sphere that's in general relativity at least.

00:42:53
Speaker 2: Wait, what do you mean if it's not perfectly spherical? What shape is it? Like a blong? Does it go oblong for a second?

00:43:00
Speaker 1: Yeah, it goes oblong for a second. And if you remember, we talked about merging black holes. When black holes merge, their event horizons grow together, and in some moments they look like a dumbbell or they look like something else. And I commented on that podcast. So that's exciting because it tells you something about the history of the black holes in a way that you can't otherwise know. In gr but there's no hair theorem applies to the stable state of the black hole. Like you put a banana in a black hole, you expect it it's event horizon and have a little bit of a funky shape as it sort of settles back down to a new stable object. But in this theory of quantum gravity, it says that those instabilities persist that you create like waves on the surface of the black hole which ripple basically forever, they don't ever.

00:43:40
Speaker 2: Go away, sort of like a gravitational wave.

00:43:44
Speaker 1: Almost yes, exactly, and these would generate gravitational waves, which, according to this paper, we might be able to detect, probably not with our current observatories like LEGO, but with future gravitational wave observatories like LISA or some of the other projects, we might be able to actually see these gravitational waves from these hairy black holes.

00:44:04
Speaker 2: So wait, you're saying, like, even if I drop a hair or a banana into a black hole maybe a billion years ago, and it happens to be one of these extreme black holes even today, it might be generating gravitational waves that tell me what was dropped in a billion years ago.

00:44:19
Speaker 1: That's the theory, Yes, exactly. So if you're planning to commit crimes and drop your evidence into a black hole, this might be a loophole.

00:44:27
Speaker 2: Wouldn't all that information just be radiated away or it would stay there for a billion years.

00:44:32
Speaker 1: It would fade with time for sure. So if you want to catch a killer using clues dropped into a black hole, your best bet is to use your gravitational wave observatory immediately afterwards, because it would fade away. Even these ripples would eventually fade away. As you say, energy is being lost, right, it's being radiated away, but in principle it still persists. Forever. Technically, it just gets fainter and fainter.

00:44:52
Speaker 2: I see, like the ripple stays on the surface of the black.

00:44:55
Speaker 1: Hole somehow, mm hmm, exactly.

00:44:57
Speaker 2: All right, Well, what does this all mean about our understanding black holes? Like, there's still a big mystery. It seems we don't really even understand what black holes are. We've seen these things out there in space. We didn't even know if they actually are black holes, if they are gr black holes, if there are quantum black holes.

00:45:13
Speaker 1: And this is really at the heart of the question, what's going on out there? I suspect that what's really happening in the universe is something different from any of these theories. You know, it's going to be a big surprise. And the exciting thing is if black holes do have hair, we might be able to measure it. We might be able to get these subtle signatures from the ripples on the surface of the event horizon to tell us what's going on inside and maybe give us some clues about what kind of universe we live in.

00:45:40
Speaker 2: Would that be the holy grail of black holes or the hairy banana.

00:45:44
Speaker 1: Exactly the way Einstein's hair is the holy grail for all physicists.

00:45:49
Speaker 2: Is it think I think like Brian Cox, is it popularized and you kind of physicists here?

00:45:55
Speaker 1: Yeah?

00:45:56
Speaker 2: The whole like emo kind of hipster aging pop star. Yeah, yeah, there you go.

00:46:01
Speaker 1: I don't think I'm ever going to be an aging pop star. But you know, there are all these really deep questions about the nature of the universe we live in. Is information destroyed or not? It might be the black holes are like cosmic toilet, it's just flushing away this information to be destroyed. Or it could be that they're preserving that information. They're radiating it back out into the universe somehow, and then information is not destroyed in our universe. It's a pretty deep question about the nature of reality.

00:46:28
Speaker 2: Yeah, and the nature of information itself, Like could you ever destroy information? Or does the universe always remember everything? Is there always a paper trail?

00:46:38
Speaker 1: Yes, exactly. If you committed a crime a billion years ago, justice might still be coming.

00:46:43
Speaker 2: All right, Well, more deep questions about the universe and a big reminder that there's still a lot for us to discover and to understand and to learn about and potentially to comb over as well. We hope you enjoyed that. Thanks for joining us see you next time.

00:47:06
Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. Or more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows.