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

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Speaker 2: I'm not sure I ask you to answer that question.

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Speaker 2: You know, I want to preserve my air of mystery.

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Speaker 2: How about you.

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Speaker 1: I found a silver hair too in my beard, but

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Speaker 1: so far I'm still all brown up top. But you know,

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Speaker 1: I'm getting a little.

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Speaker 2: Impatient, getting impatient. What do you mean you want to

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Speaker 2: go gray?

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Speaker 1: I wouldn't mind that gravitas that comes from having a

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Speaker 1: little bit of gray.

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

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Speaker 1: My gravity has actually been increasing.

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Speaker 2: There you go. It sounds like your body's doing it

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Speaker 2: for you. But are you trying to counteract the you know,

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Speaker 2: whole physicist look with the shorts and the sandals.

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Speaker 1: Yeah, exactly, I'm trying to look a little bit more

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Speaker 1: grown up.

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

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Speaker 1: Maybe I should just shave my head.

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Speaker 2: There you go. That's one way a little older, more distinguished.

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Speaker 2: And after you shave it, you could paint it gray.

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

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Speaker 2: Sounds like physicists don't know how gravity or gravitas works.

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Speaker 2: Both big mysteries. Hi, I am Horehammick cartoonists and the

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Speaker 2: author of Oliver's Great Big Universe.

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Speaker 1: Hi, I'm Daniel. I'm a particle physicist and a professor

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Speaker 1: at UC Irvine, and I never really gone after gravitas.

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Speaker 2: What do you mean you just inherently have it? Can

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Speaker 2: you say you're a physicist?

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Speaker 1: People go ooh, No, exactly the opposite, trying to break

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Speaker 1: down those barriers, you know. That's why I wear socks

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Speaker 1: and sandals every day. Don't try to create any distance

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Speaker 1: between myself and my students.

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

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Speaker 1: And everyone knows that's another reason to live in southern California, right,

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Speaker 1: Socks optional.

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Speaker 2: I guess that's what physics is. It's all about, you know,

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Speaker 2: breaking barriers, connecting with the universe. Anyways, Welcome to our podcast,

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Speaker 2: Daniel and Jorge Explain the Universe, a production of our

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Speaker 2: Heart Radio.

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Speaker 1: In which we try to break down the barriers between

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Speaker 1: ourselves and nature, between you and a complete understanding of

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Speaker 1: everything we do and don't know about the universe. We

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Speaker 1: think the universe is the greatest mystery and we are

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Speaker 1: here to crack it.

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Speaker 2: That's right. The universe is full of barriers of things

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Speaker 2: we can't see things we can't understand and things that

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Speaker 2: we may never understand, including barriers about fashion and dress

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Speaker 2: codes at universities.

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Speaker 1: You know, people say, focus on what you're good at,

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Speaker 1: and so that's why I just don't pay attention to fashion.

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

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

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Speaker 2: Well, I am totally with you. I think the most

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Speaker 2: comfortable thing in the world to wear is socks and sandals.

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Speaker 2: If anyone does not try that out there, I highly

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Speaker 2: recommend it, or or socks with flip flops even better.

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

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

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Speaker 1: I'm terrified at the mental images you are creating in

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Speaker 1: the minds of our listeners.

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Speaker 2: Actually have socks that have the little notch for your

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Speaker 2: flip flop strap.

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

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

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

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Speaker 2: I don't need each of my individual toes wrapped in

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Speaker 2: its own little pocket.

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Speaker 1: Again with the troubling mental images.

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Speaker 2: Here, well, but toes are a big part of the

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Speaker 2: universe theories of everything, and so physicists have been on

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Speaker 2: the lookout and on the search for one SU's theory

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Speaker 2: that can explain everything in the universe, including all the

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Speaker 2: mysterious bits of it.

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Speaker 1: And some of the most mysterious bits of the universe

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Speaker 1: are those pieces that we cannot see, things hidden behind

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Speaker 1: the event horizon of black holes. What's going on in there?

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Speaker 1: Are there singularities or is there something else? How does

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Speaker 1: gravity work for quantum particles? All the answers are waiting

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Speaker 1: for us behind these barriers.

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Speaker 2: Yeah, black holes seem to be the sort of the

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Speaker 2: epitome of mystery in the universe. It's almost like a

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Speaker 2: little pocket that takes you out of the universe.

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Speaker 1: Right, Yeah, it's sort of like they are their own

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Speaker 1: little pocket universes because they are cut off from us.

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Speaker 1: And maybe the most tantalizing thing is that we can

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Speaker 1: only know a few things about what's gone past that barrier.

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Speaker 2: Yeah, they're almost like cosmic sensors, you know, they're like

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Speaker 2: hiding information, blacking it out for everyone to uh see

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Speaker 2: or a Nazi.

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Speaker 1: And physicists hate when they lose information. When some knowledge

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Speaker 1: is hidden from them, and so we spend a lot

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Speaker 1: of time wondering about what we can know about the

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Speaker 1: things that have fallen into a black hole. Is it

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Speaker 1: possible to show up to a black hole and know

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Speaker 1: what has been tossed into it?

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Speaker 2: Yeah, we have a lot of questions about black holes,

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Speaker 2: but maybe not as important as the one we're asking today.

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Speaker 2: So today on the podcast, we'll be asking the question

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Speaker 2: do black holes have hair?

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

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Speaker 2: I think the real question is black holes have black

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Speaker 2: hairs or gray hairs? Or does it depend on the

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Speaker 2: age of the black hole or it's gravitas exactly?

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Speaker 1: Maybe the really big super black holes are like the

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Speaker 1: silver backs of the universe.

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Speaker 2: I guess. Don't all black holes by definition have gravitas?

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Speaker 2: I mean you kind of have to take them seriously, right,

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Speaker 2: I guess so if gravity is the bending of space time,

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Speaker 2: then gravitas is like the bending of the social structure. Like,

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Speaker 2: are there lightweight black holes out there or you know,

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Speaker 2: frivolous black holes? Probably not right.

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Speaker 1: They probably aren't like teenage black holes that the parent

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Speaker 1: black holes thing should be taking their life more seriously?

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Speaker 2: Where did that come from?

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Speaker 1: Frivolous. You're like, what are these black holes doing with

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Speaker 1: their lives anyway? Or maybe it's just because I have

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Speaker 1: teenagers in high school.

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Speaker 2: I think you might be bringing us the home issues

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Speaker 2: here on the Physics podcast.

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Speaker 1: Well, in the end, we are trying to understand the

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Speaker 1: universe through the lens of our own minds, so it's

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Speaker 1: impossible to separate personal issues from physics issues.

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Speaker 2: You could be a family physicist. You know there's family physicians. Yeah,

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Speaker 2: there are a lot of dynamics in the household.

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

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Speaker 2: Yeah, just throw the whole family to black hole and

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Speaker 2: they have to get along.

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Speaker 1: It might get kind of hairy.

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Speaker 2: But Harry, apparently black holes can be, or might be

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Speaker 2: or could be. And so that's the question we're asking

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Speaker 2: here today, which is kind of a weird question. I mean,

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Speaker 2: who thinks of black holes having hair?

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Speaker 1: Physicists? Physicists use hair as a metaphor for all sorts

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Speaker 1: of crazy stuff.

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Speaker 2: Well, it's a fun question, and so as usual we

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Speaker 2: were wondering calming people out there, I thought about this,

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Speaker 2: Harry question.

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Speaker 1: So thanks very much to everybody who answers these questions.

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Speaker 1: For this really fun segment of the podcast, and we

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Speaker 1: are always looking for more volunteers and we want to

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Speaker 1: hear from you, So join the group right to me

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Speaker 1: to questions at Danielandjorge dot com.

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Speaker 2: So think about it for a second. Do you think

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Speaker 2: black holes have hair? Here's what people had to say.

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Speaker 3: I suppose you'd have to ask yourself what exactly you're

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Speaker 3: talking about when you mean hair. My assumption is that

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Speaker 3: you would be referring to a biological process, and since

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Speaker 3: black holes are not a biological entity, I would struggle

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Speaker 3: to believe that they create hair in it of themselves.

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Speaker 4: Well, I don't know about actual hair, but I do

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Speaker 4: know that when things fall into a black hole, they

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Speaker 4: get spaghettified. So maybe black holes are surrounded by a

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Speaker 4: whole bunch of spaghettified ragg doll hair.

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Speaker 5: I'm assuming by hair you're asking about something that I

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Speaker 5: have never heard of, and not asking about like hair

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Speaker 5: on your head.

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Speaker 1: But since that's the only.

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Speaker 5: Definition that I know, I'm going to say, no, black

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Speaker 5: holes do not have hair on them.

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Speaker 4: The only thing I can really equate hairs to would

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Speaker 4: be like cosmic strings of some sort. I really don't know.

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Speaker 4: I don't know.

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Speaker 1: Well, if black holes have hair, it's yet another thing

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Speaker 1: that tests more hair than I do.

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Speaker 2: All Right, some great answers here, pretty creative talking about

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Speaker 2: like is a cosmic a hair?

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Speaker 1: That's right? Cosmic strings? Are these cracks in space time

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Speaker 1: we talked about once, which do seem kind of hairy,

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Speaker 1: but are completely separate from what we mean when we

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Speaker 1: say black hole hair.

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Speaker 2: I also wonder if them you can take this question literally, like,

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Speaker 2: if the Earth falls into a black hole, it would

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Speaker 2: have a lot of hair in it literally, right, or

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Speaker 2: would it? I don't know.

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Speaker 1: Yeah, well that sort of goes to the heart of

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Speaker 1: the question. Really, are black holes what they eat? Or

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Speaker 1: does it not matter what you put into a black hole?

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Speaker 1: Can you not tell the history of a black hole

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Speaker 1: in any way?

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Speaker 2: I see these are internal hairs, hairs you don't really

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Speaker 2: want to see.

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Speaker 1: Maybe somehow we ended up talking about ingrown hairs on

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Speaker 1: the podcast.

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

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

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

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

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Speaker 2: Here you go, black hole popping right here on the podcast,

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Speaker 2: it's the sound of a black hole popping. But anyways,

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Speaker 2: let's get down to it, Daniel, and let's maybe recap

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Speaker 2: for people what exactly is a black hole and what

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Speaker 2: do we know about them?

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Speaker 1: So we don't really know what a black hole is.

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Speaker 1: We have a theoretical concept from general relativity that tells

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Speaker 1: us that if you have enough matter or enough energy

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Speaker 1: density in a small region of space, that that space

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Speaker 1: will be so curved that no information can escape. No photons,

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Speaker 1: no particles, no gravitational waves, nothing from beyond this event

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Speaker 1: horizon can ever propagate out and tell you anything about

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Speaker 1: what's hidden behind that curtain. That's the sort of general

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Speaker 1: relativity idea of a black hole. And we've seen some

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Speaker 1: things out there in the universe that really closely resemble

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Speaker 1: a black hole. There's something at the center of our

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Speaker 1: Milky Way, there's a bunch of collapsed stars, there's something

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Speaker 1: at the center of other galaxies that really resembles a

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Speaker 1: black hole. But you know, because you can't see inside

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Speaker 1: the event horizon, we're not exactly sure that what we've

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Speaker 1: seen out there in the universe aligns with Einstein's idea

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Speaker 1: of what a black hole would look like.

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Speaker 2: Yeah, it's basically like an actual hole in space. Right, Like,

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Speaker 2: once you have enough mass into one spot and it's

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Speaker 2: tense enough, that region of space becomes a hole. Right

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Speaker 2: like anything you throw in there, it's going to stay

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Speaker 2: in there.

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Speaker 1: It's a hole in that sense. But a hole sort

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Speaker 1: of gives you an idea of like a discontinuity, like

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Speaker 1: there's a gap or something. But remember that space is smooth,

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Speaker 1: it's continuous. So what we're talking about is space being bent,

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Speaker 1: space being curved, you know the way that like the

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Speaker 1: Earth doesn't move in what looks like a straight line

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Speaker 1: to us because it's following the curvature of space. When

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Speaker 1: you put mass inside space, it bends it. It changes

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Speaker 1: the relative distances between things, which changes how things flow.

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Speaker 1: And so near a black hole, space is curved and

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Speaker 1: curved very very intensely, and as you get closer, that

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Speaker 1: curvature gets stronger and stronger. But it is continuous, right,

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Speaker 1: there's not like a sharp cut off. But there's a

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Speaker 1: point on this smooth curvature past which nothing can escape,

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Speaker 1: and that's the radius we call the event horizon. And

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Speaker 1: so you could say that's sort of like a threshold.

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Speaker 1: It's like a hole if you fall into you'll never escape.

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Speaker 2: Yeah, I think you're saying that there's no cank in space,

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Speaker 2: but there is sort of a point, that sort of

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Speaker 2: a discontinuity where light can no longer escape.

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Speaker 1: Yeah, there's a discontinuity sort of in your fates. Like

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Speaker 1: if you have photons on one side of it and

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Speaker 1: photons on the other side of this radius, the ones

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Speaker 1: past the radius can escape and fly through the universe,

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Speaker 1: and the ones inside the radius will never escape. So

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Speaker 1: there's a sort of discontinuity in the outcomes of particles.

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Speaker 1: But remember the event horizon. It's not a physical barrier.

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Speaker 1: It's just this difference in the outcomes of particles.

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Speaker 2: It's sort of like a real hole, right, Like there's

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Speaker 2: no barrier, you just follow. At some point you're on

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Speaker 2: the hole, and some point you're not on the hole. Yeah,

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Speaker 2: that's right, all right. So then what do we know

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Speaker 2: about black holes, if anything at all.

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Speaker 1: So the frustrating thing is that we have this theoretical

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Speaker 1: concept of what might be inside a black hole. In

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Speaker 1: Einstein's pictures, famelessly, this infinitely dense dot, a singularity where

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Speaker 1: gravity has had this runaway effect compressing and compressing and

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Speaker 1: compressing forever with nothing to resist it and creating this

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Speaker 1: infinitely dense dot at the heart of a black hole.

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Speaker 1: But of course we can't see inside a black hole.

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Speaker 1: All we can do is observe them from the outside.

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Speaker 1: No information escapes from the inside of a black hole,

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Speaker 1: according to Einstein's theory. But we can know a few

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Speaker 1: things about the black hole, like we're on the outside

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Speaker 1: of it. But we can still measure some things about

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Speaker 1: the black hole without going inside. For example, we can

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Speaker 1: know the mass of the black hole. We can measure

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Speaker 1: the black hole's impact on space time even past the

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Speaker 1: event horizon, So we can know something about the black

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Speaker 1: hole without even seeing past the event horizon.

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Speaker 2: You mean, like we can know how much gravity it

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Speaker 2: exerts onto the things around it.

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Speaker 1: Exactly. The curvature of space time continues past the event horizon, right,

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Speaker 1: there's no kink, Like we said, it's smooth, so past

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Speaker 1: the event horizon, it's still exerting an influence on space time,

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Speaker 1: and we can use that measurement of the curvature to

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Speaker 1: tell how much mass there is inside the event horizon,

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Speaker 1: in the same way that like you can measure the

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Speaker 1: mass of the Earth just by seeing its gravitational effect

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Speaker 1: on the Moon or on a satellite. Right, all that

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Speaker 1: gravity adds up and has the same effect on the point.

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Speaker 1: Doesn't tell you anything about like the configuration of the

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Speaker 1: Earth or whatever. And you could replace the Earth with

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Speaker 1: a point particle to have the same gravitational effect. But

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Speaker 1: from a distance you can measure the overall gravitational force.

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Speaker 1: And we can do that for a black hole obviously.

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Speaker 2: And the idea is that like, the more gravity of

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Speaker 2: black hole exerts and of the things around it, the

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Speaker 2: more massive it is.

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Speaker 1: Exactly, we can do the calculation to say how much

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Speaker 1: space time gets bent by black holes of a certain mass,

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Speaker 1: and so we can back that up and say, well,

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Speaker 1: we measure this amount of curvature, and so therefore black

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Speaker 1: hole must have this mass.

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Speaker 2: I wonder if like mass is the actual right term

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Speaker 2: for it, right, Like, we don't even know if what's

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Speaker 2: inside of a black hole is mass or just a

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Speaker 2: whole bunch of pure energy.

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Speaker 1: Right, that's exactly right. We use mass as a way

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Speaker 1: to sort of measure the energy that's inside the black hole,

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Speaker 1: because remember that space time is really curved by energy density,

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Speaker 1: not necessarily just by mass, like a proton bends space time,

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Speaker 1: even though it's just made of a bunch of quarks

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Speaker 1: which are very very low mass. But there's a lot

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Speaker 1: of energy stored inside the proton energy in the bonds

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Speaker 1: of those quarks, which contributes to its mass and contributes

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Speaker 1: to the curvature of space time. So on one hand

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Speaker 1: you could say, well, it's really just the stored energy

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Speaker 1: of the black hole. On the other hand, that's kind

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Speaker 1: of what mass is. Mass is the internal stored energy

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Speaker 1: of an object, and so on one hand, yeah, it's energy.

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Speaker 1: On the other hand, that's what we call mass.

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Speaker 2: Wait, so it's like it's inertial mass or gravitational mass.

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Speaker 2: Does that mean a black hole can have kinetic energy

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Speaker 2: as well well?

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Speaker 1: In general relativity, inertial and gravitational masses are the same thing.

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Speaker 1: So yes, this is its inertial mass. This is like

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Speaker 1: how hard it is to get it moving and how

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Speaker 1: hard it is to slow it down, et cetera, et cetera.

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Speaker 1: And yes, black holes can have kinetic energy. They can

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Speaker 1: move right, and you can move past a black hole,

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Speaker 1: and velocity is all relative. So if you're flying past

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Speaker 1: a black hole at half the speed of light, you

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Speaker 1: see it moving towards you at half the speed of light.

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Speaker 1: So yes, black holes can definitely move, they can have

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Speaker 1: kinetic energy.

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Speaker 2: All right. So then how would you measure the mass

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Speaker 2: of a black hole. You would sort of like put

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Speaker 2: a scale near it or throw a pebble to see

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Speaker 2: how it swings around it. How would you do it?

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Speaker 1: Unfortunately, we're not near enough any black holes to do

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Speaker 1: any experiments, but fortunately in astrophysics we can watch these

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Speaker 1: experiments happen. So the best way to measure the mass

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Speaker 1: with black hole is to see the motion of stuff

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Speaker 1: near it, like the mass of the black hole the

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Speaker 1: center the Milky Way. We measure by looking at stars

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Speaker 1: that whiz past it and seeing the gravitational force on

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Speaker 1: those objects and knowing how much mass has to be

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Speaker 1: to create that gravitational force to make the stars bend

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Speaker 1: the way they do. And for distant black holes and

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Speaker 1: other galaxies, we can see like the sort of swirl

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Speaker 1: of stars around the center of the galaxy and use

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Speaker 1: that to measure the gravitational force from the black hole.

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

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

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Speaker 2: Method, all right, So then what else can we know

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Speaker 2: about a black hole?

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Speaker 1: Another thing we can know about a black hole is

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Speaker 1: it's electric charge. If you put electrons into a black hole, Well,

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Speaker 1: charge is concerned in the universe. So if an electron

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Speaker 1: falls into the black hole, the black hole now has

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Speaker 1: an overall negative charge. At another electron, get another negative charge.

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Speaker 1: So you can know the overall charge of a black hole.

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

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Speaker 1: Yeah, a black hole can have an overall charge and

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Speaker 1: it can have electric fields. Right in the same way

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Speaker 1: that black hole has mass, and that makes for effectively

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Speaker 1: a gravitational field or bending of space time past the

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Speaker 1: event horizon. The black hole can have a charge and

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Speaker 1: that creates an electric field which can also be pasted

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Speaker 1: the event horizon.

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Speaker 2: Interesting, but I guess if I wonder if a black

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Speaker 2: hole has a negative charge, can you use it as

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Speaker 2: a battery? Probably not, right, Like you can't get electrons

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Speaker 2: to flow out of it, can you?

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Speaker 1: You can use a charged black hole the way you

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Speaker 1: can use any other kind of charged particle. Right, You

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Speaker 1: can use it to create electric fields. You can use

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Speaker 1: it to repel stuff or attract stuff. But I think

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Speaker 1: batteries involved like the flow of electrons through materials and

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Speaker 1: I don't know that's chemistry.

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Speaker 2: That's too hairy for you.

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Speaker 1: Yeah, But essentially, you can think of a black hole

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Speaker 1: as just like an enormous particle, right the way like

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Speaker 1: an electron sort of has a charge attached to it,

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Speaker 1: but you don't really think about like where is the charge.

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Speaker 1: It's just like a property of the electron. You could

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Speaker 1: think the black hole as just sort of like enormous particle.

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Speaker 1: You don't have any details about, like where is the

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Speaker 1: electric charge. It's just sort of like assigned to the

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Speaker 1: entire event horizon, the same way you assign the mass

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Speaker 1: to the entire event horizon. You don't really know what's

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Speaker 1: going on inside, how it's that mass arranged? Is it

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Speaker 1: still in bananas? Is it squished into something else? Is

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Speaker 1: it nuclear pasta? You don't know anything about the internals.

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Speaker 1: You just assigned it to the exterior, and then you

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Speaker 1: can treat it like anything else that has a charge.

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Speaker 2: I see, sort of like maybe the Earth. You could

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Speaker 2: do that with the Earth too, right, Like, I'm sure

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Speaker 2: the Earth has an overall.

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Speaker 1: Charge exactly, any extended object with an overall charge from

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Speaker 1: a distance, you can treat like a point particle with

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Speaker 1: a charge, the math is exactly the same. And the

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Speaker 1: case of the Earth, of course, we can know it,

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Speaker 1: and it's not hidden beyond an advent horizon, So we

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Speaker 1: could learn about this stuff in other ways. In the

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Speaker 1: case of a black hole, you can't. And there's something

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Speaker 1: sort of similar there about a black hole and a particle,

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Speaker 1: the way that like two electrons are identical. You know,

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Speaker 1: you can't tell the difference between this electron and that electron.

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Speaker 1: They have all the same properties. That's what defines them.

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Speaker 1: Two black holes with the same mass and the same charge,

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Speaker 1: and it'll talk about it in the same spin, are

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Speaker 1: really two identical objects?

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Speaker 2: All right? Well, talk about spin? What a spin for

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Speaker 2: a black hole?

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Speaker 1: So the same way that you can toss stuff into

404
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Speaker 1: a black hole that has charge, you can also toss

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Speaker 1: stuff into a black hole that makes it spin. You

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Speaker 1: throw something exactly towards the center of the black hole,

407
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Speaker 1: it'll make it grow and make it more massive. But

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Speaker 1: if you throw something into the event horizon but a

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Speaker 1: little bit off center, then it's sort of like you're

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Speaker 1: giving it a push. The way if you're like holding

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Speaker 1: a bicycle wheel on an axle and you hit the rim,

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Speaker 1: it'll start to spin. Or if you push on a

413
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Speaker 1: merry go round, it'll start to spin. If you throw

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Speaker 1: particles into the edge of the black hole, you can

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Speaker 1: get it to spin because angular momentum, like charge, doesn't

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Speaker 1: disappear in our universe. So that black hole then has

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Speaker 1: to accumulate that angular momentum, which means it has to spin.

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Speaker 2: But I guess the angular momentum in the universe is

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Speaker 2: not like a fundamental thing. Is It's not like energy

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Speaker 2: or regular momentum. Is it like? It's really just like

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Speaker 2: the difference between two points in your body and how

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Speaker 2: they're moving relative to each other, isn't.

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Speaker 1: It Now, angular momentum is just as fundamental as linear momentum,

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Speaker 1: and maybe even more fundamental than energy number. Energy not

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Speaker 1: actually conserved in our universe because conservation of energy requires

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Speaker 1: space to be static, and our universe space is expanding,

427
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Speaker 1: so energy is not conserved. But linear momentum and angular

428
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Speaker 1: momentum are both conserved in our universe. It comes from

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Speaker 1: deep symmetries. Linear momentum comes from the fact that space

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Speaker 1: is the same everywhere, that like translation from here to there,

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Speaker 1: shouldn't change your experiment, and angular momentum comes from the

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Speaker 1: fact that there's no preferred direction in space, that every

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Speaker 1: direction is equivalent, and so angular momentum has to be

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Speaker 1: conserved and that makes angular momentum something very fundamental to

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Speaker 1: the universe.

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Speaker 2: All right, Well, so then a black hole can have

437
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Speaker 2: angular momentum, and that is that what it's been exactly.

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Speaker 1: The spin of the black hole is how it stores

439
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Speaker 1: angular momentum. And there's all sorts of fascinating consequences there

440
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Speaker 1: in general relativity, because like a singularity can't spin. So

441
00:20:10,000 --> 00:20:12,720
Speaker 1: the idea is that inside a spinning black hole is

442
00:20:12,760 --> 00:20:17,080
Speaker 1: maybe not a singularity but a ringularity, like a whole

443
00:20:17,160 --> 00:20:21,240
Speaker 1: circle of singularities, and those singularities are all spinning together,

444
00:20:21,280 --> 00:20:24,399
Speaker 1: which is how they store the angular momentum. But nobody

445
00:20:24,440 --> 00:20:26,919
Speaker 1: really knows what's actually going on inside.

446
00:20:27,000 --> 00:20:28,639
Speaker 2: All right, So the things we can know about a

447
00:20:28,720 --> 00:20:31,520
Speaker 2: black hole are its mass, it's spin, and it's charge.

448
00:20:32,040 --> 00:20:35,119
Speaker 2: Now the big question is can we know how hairy

449
00:20:35,200 --> 00:20:38,160
Speaker 2: it is? So let's dig into that question. But first

450
00:20:38,200 --> 00:20:53,040
Speaker 2: let's take out a quick break. All right, we're talking

451
00:20:53,119 --> 00:20:56,040
Speaker 2: about the hairs of a black hole. Can a whole

452
00:20:56,119 --> 00:20:57,679
Speaker 2: half hairs.

453
00:20:58,720 --> 00:21:01,959
Speaker 1: That's exactly the question. And when physicists say hair, they

454
00:21:01,960 --> 00:21:04,960
Speaker 1: don't really mean hair. They're just taking a word that

455
00:21:05,040 --> 00:21:07,919
Speaker 1: exists that has another meaning and giving it a meaning.

456
00:21:08,000 --> 00:21:10,719
Speaker 1: In physics, which is, you know, has a great tradition

457
00:21:10,760 --> 00:21:12,080
Speaker 1: of being very confusing what.

458
00:21:12,119 --> 00:21:14,560
Speaker 2: You mean, like in this case it actually is like

459
00:21:15,040 --> 00:21:18,080
Speaker 2: a cosmic string or a particle or something, or is

460
00:21:18,119 --> 00:21:19,520
Speaker 2: it just a metaphor, it's.

461
00:21:19,400 --> 00:21:22,760
Speaker 1: A metaphor for anything else. Essentially, general relativity tells us

462
00:21:22,960 --> 00:21:25,520
Speaker 1: that we can know three things about a black hole.

463
00:21:25,680 --> 00:21:29,760
Speaker 1: Its mass, it's been its charge, and nothing else. Nothing

464
00:21:29,760 --> 00:21:34,520
Speaker 1: about its history, no interesting little details, no texture, no hair.

465
00:21:34,880 --> 00:21:38,840
Speaker 2: Is that because of the event horizon. Basically, like if

466
00:21:38,840 --> 00:21:42,920
Speaker 2: you wrap the Earth around an impenetrable, you know, black shield,

467
00:21:43,160 --> 00:21:45,760
Speaker 2: you could also not tell anything about the Earth, whether

468
00:21:45,800 --> 00:21:46,639
Speaker 2: it's t had hairs or not.

469
00:21:46,800 --> 00:21:50,000
Speaker 1: I think that's true, but maybe only because an impenetrable

470
00:21:50,040 --> 00:21:52,720
Speaker 1: black shield would basically have to be an event horizon

471
00:21:52,800 --> 00:21:55,119
Speaker 1: of a black hole. There's no other way to make

472
00:21:55,160 --> 00:21:58,760
Speaker 1: something truly impenetrable. If you just build something really solid

473
00:21:58,840 --> 00:22:01,159
Speaker 1: and black, it would like read it it's some temperature,

474
00:22:01,720 --> 00:22:03,840
Speaker 1: and that would tell you about what's going on inside

475
00:22:03,840 --> 00:22:06,080
Speaker 1: of it. The only way to really be impenetrabled to

476
00:22:06,080 --> 00:22:08,640
Speaker 1: give up no information would be to make a black hole.

477
00:22:08,920 --> 00:22:10,600
Speaker 2: I guess that's what I mean. It's like, the reason

478
00:22:10,640 --> 00:22:12,800
Speaker 2: we can't know anything else about a black hole is

479
00:22:12,840 --> 00:22:15,760
Speaker 2: because it has an event horizon which keeps all the

480
00:22:15,800 --> 00:22:20,320
Speaker 2: information inside. It's not something like that information is destroyed, right.

481
00:22:20,280 --> 00:22:22,560
Speaker 1: We don't know what's going on exactly. And that's the

482
00:22:22,600 --> 00:22:25,800
Speaker 1: deepest question, is like is the information destroyed? Is it

483
00:22:25,840 --> 00:22:30,800
Speaker 1: actually contained within black hole somehow or is it actually destroyed?

484
00:22:30,840 --> 00:22:33,680
Speaker 1: Like is the internal state of a black hole the same,

485
00:22:33,760 --> 00:22:35,880
Speaker 1: no matter what you've put into it. Make a black

486
00:22:35,920 --> 00:22:37,920
Speaker 1: hole out of bananas and you make another one out

487
00:22:37,960 --> 00:22:41,600
Speaker 1: of apples. Do you really get exactly the identical black

488
00:22:41,640 --> 00:22:43,439
Speaker 1: hole out of those two things if you put in

489
00:22:43,480 --> 00:22:47,199
Speaker 1: the same amount of mass? Or is there some history

490
00:22:47,240 --> 00:22:49,919
Speaker 1: there some way to tell a banana from an apple

491
00:22:50,000 --> 00:22:50,600
Speaker 1: black hole?

492
00:22:51,000 --> 00:22:53,280
Speaker 2: I guess I'm trying to, you know, relate it to

493
00:22:53,280 --> 00:22:55,399
Speaker 2: the earth, and like you could make an earth out

494
00:22:55,400 --> 00:22:58,240
Speaker 2: of bananas, and if you put it behind your impenetrable shield,

495
00:22:58,280 --> 00:23:00,359
Speaker 2: you could also not tell the Earth what's made out

496
00:23:00,400 --> 00:23:02,800
Speaker 2: of bananas or apples.

497
00:23:03,000 --> 00:23:05,920
Speaker 1: That's exactly right, but it conflicts with sort of other things.

498
00:23:06,000 --> 00:23:08,840
Speaker 1: We've seen in the universe and our quantum intuition, because

499
00:23:08,920 --> 00:23:12,040
Speaker 1: quantum mechanics tells us that information is not destroyed, that

500
00:23:12,119 --> 00:23:15,959
Speaker 1: the present is uniquely determined by the past, and that

501
00:23:16,000 --> 00:23:19,080
Speaker 1: therefore from the present you can always derive the past,

502
00:23:19,320 --> 00:23:22,040
Speaker 1: that like the past, has left a permanent imprint on

503
00:23:22,119 --> 00:23:24,280
Speaker 1: the present. Quantum mechanics tells us that we can run

504
00:23:24,320 --> 00:23:27,320
Speaker 1: the laws of physics forwards or backwards in time, so

505
00:23:27,359 --> 00:23:29,399
Speaker 1: we could use the present to predict the future in

506
00:23:29,400 --> 00:23:31,399
Speaker 1: the same way we could use the present to reveal

507
00:23:31,440 --> 00:23:34,760
Speaker 1: the past. And this is in conflict with that. This says, no, no, no,

508
00:23:34,920 --> 00:23:37,600
Speaker 1: there's no information here. Once you create a black hole,

509
00:23:37,880 --> 00:23:40,160
Speaker 1: you can't tell what its history is at all. There's

510
00:23:40,200 --> 00:23:43,320
Speaker 1: no texture to grab onto, no little details if you

511
00:23:43,400 --> 00:23:46,439
Speaker 1: zoom in, there are no hairs there to reveal the

512
00:23:46,480 --> 00:23:47,560
Speaker 1: history of the black hole.

513
00:23:47,720 --> 00:23:49,760
Speaker 2: I wonder if maybe the real question is not whether

514
00:23:50,119 --> 00:23:52,359
Speaker 2: a black hole has hair. The question seems to be

515
00:23:52,400 --> 00:23:54,520
Speaker 2: more like, if I toss a hair into a black hole,

516
00:23:54,600 --> 00:23:58,000
Speaker 2: does the hair get destroyed forever? Or is it just

517
00:23:58,680 --> 00:24:00,720
Speaker 2: going to stay inside the black hole and it's going

518
00:24:00,800 --> 00:24:02,960
Speaker 2: to be there, but we can't tell if it's there.

519
00:24:03,280 --> 00:24:05,359
Speaker 1: I think those are both really interesting questions, but I

520
00:24:05,359 --> 00:24:08,760
Speaker 1: think there are separate questions. In general relativity, we can't

521
00:24:08,800 --> 00:24:11,840
Speaker 1: know anything that passes the event horizon. But maybe general

522
00:24:11,840 --> 00:24:14,480
Speaker 1: relativity black holes are not the black holes we see

523
00:24:14,520 --> 00:24:17,400
Speaker 1: in our universe. Maybe there are little ripples and hairs

524
00:24:17,440 --> 00:24:19,560
Speaker 1: on the event horizon that we could use to learn

525
00:24:19,600 --> 00:24:22,200
Speaker 1: about what has fallen in. And then the second question

526
00:24:22,320 --> 00:24:24,280
Speaker 1: is like what happens to stuff that falls in does

527
00:24:24,400 --> 00:24:27,280
Speaker 1: form some new state of matter? Is that information still there?

528
00:24:27,480 --> 00:24:30,560
Speaker 1: Because we think that black holes eventually evaporate, that they

529
00:24:30,720 --> 00:24:33,919
Speaker 1: radiate away all of their energy, they lose their mass,

530
00:24:33,960 --> 00:24:36,760
Speaker 1: and if that information has been destroyed within them, then

531
00:24:36,760 --> 00:24:39,720
Speaker 1: that information is lost forever, which would be very confusing

532
00:24:39,720 --> 00:24:41,400
Speaker 1: from the point of view of quantum mechanics.

533
00:24:41,600 --> 00:24:43,520
Speaker 2: Well to the second question, I know we've talked about

534
00:24:43,560 --> 00:24:46,000
Speaker 2: in the podcast before, how it is possible to go

535
00:24:46,240 --> 00:24:49,040
Speaker 2: into a black hole and survive. Right, Like, for a

536
00:24:49,080 --> 00:24:52,240
Speaker 2: big enough black hole, the event horizon happens. You don't

537
00:24:52,240 --> 00:24:55,200
Speaker 2: have to get spaghetified or shredded apart to go into

538
00:24:55,359 --> 00:24:58,399
Speaker 2: technically the event horizon of a super massive black hole, right,

539
00:24:58,480 --> 00:24:59,000
Speaker 2: That's right.

540
00:24:58,840 --> 00:25:01,120
Speaker 1: And if for really large black hole holes, if they

541
00:25:01,160 --> 00:25:04,560
Speaker 1: do have significant charge and or spin. There are even

542
00:25:04,680 --> 00:25:08,240
Speaker 1: stable orbits within the event horizon of the black hole.

543
00:25:08,400 --> 00:25:10,560
Speaker 2: So you could like fall into a super massive black

544
00:25:10,600 --> 00:25:14,200
Speaker 2: hole and stay in there but still be hole, still

545
00:25:14,200 --> 00:25:14,800
Speaker 2: be there.

546
00:25:14,640 --> 00:25:17,639
Speaker 1: And still be inside. According to general relativity, yes that

547
00:25:17,840 --> 00:25:20,639
Speaker 1: is possible. Again, we don't really know right what is

548
00:25:20,680 --> 00:25:22,919
Speaker 1: going on inside there, so we don't really know the

549
00:25:22,960 --> 00:25:24,800
Speaker 1: fate of any of this stuff until you know, you

550
00:25:24,920 --> 00:25:26,560
Speaker 1: jump into a black hole and figure it out.

551
00:25:26,720 --> 00:25:29,560
Speaker 2: Well, not me, Maybe it should be the physinessist. I mean,

552
00:25:29,560 --> 00:25:32,199
Speaker 2: I think you need the cartoonist outside to draw what

553
00:25:32,280 --> 00:25:35,480
Speaker 2: happens and then figure out if it's a good idea.

554
00:25:35,520 --> 00:25:37,240
Speaker 2: But I guess what I mean is you can throw

555
00:25:37,240 --> 00:25:40,159
Speaker 2: a hair into a black hole and it can survive,

556
00:25:40,280 --> 00:25:41,960
Speaker 2: Like you can have a hair inside of a black hole.

557
00:25:42,000 --> 00:25:44,240
Speaker 2: It's she said, if somebody else comes by, they won't

558
00:25:44,280 --> 00:25:46,280
Speaker 2: be able to know whether you threw a hair in

559
00:25:46,320 --> 00:25:46,679
Speaker 2: there or not.

560
00:25:46,760 --> 00:25:49,840
Speaker 1: Exactly that's true. According to general relativity. You could build

561
00:25:49,840 --> 00:25:52,360
Speaker 1: a black hole out of hairs and nobody else could

562
00:25:52,359 --> 00:25:54,320
Speaker 1: tell the difference between that and a black hole made

563
00:25:54,320 --> 00:25:55,880
Speaker 1: out of toenails or something else.

564
00:25:56,080 --> 00:25:57,600
Speaker 2: But you would know there was a hair in there.

565
00:25:57,720 --> 00:25:59,520
Speaker 1: You would know because you saw it falling in.

566
00:25:59,640 --> 00:26:02,680
Speaker 2: You mean, yeah, or because I tossed it into the hole.

567
00:26:02,760 --> 00:26:04,600
Speaker 1: That's true, you would know the history, but there'd be

568
00:26:04,600 --> 00:26:06,880
Speaker 1: no way to measure it from the objects itself. There'll

569
00:26:06,880 --> 00:26:09,880
Speaker 1: be no record on the outside that would tell you. Again,

570
00:26:09,920 --> 00:26:12,320
Speaker 1: according to general relativity.

571
00:26:11,800 --> 00:26:14,600
Speaker 2: Okay, so then what does that mean? Does that mean?

572
00:26:14,680 --> 00:26:16,760
Speaker 2: Quantum mechanics says something differently.

573
00:26:16,520 --> 00:26:19,400
Speaker 1: Quantum mechanics definitely says something different, And quantum mechanics tells

574
00:26:19,480 --> 00:26:22,400
Speaker 1: us that this whole picture of a black hole according

575
00:26:22,400 --> 00:26:25,919
Speaker 1: to general relativity is very very likely wrong. Remember that

576
00:26:26,000 --> 00:26:29,160
Speaker 1: general relativity is not built on the foundation of quantum mechanics.

577
00:26:29,160 --> 00:26:31,959
Speaker 1: It makes very different assumptions about how the universe works.

578
00:26:32,320 --> 00:26:35,960
Speaker 1: It assumes that space and time are continuous and smooth,

579
00:26:36,400 --> 00:26:39,680
Speaker 1: that you can have like infinitely small distances and infinitely

580
00:26:39,720 --> 00:26:42,840
Speaker 1: small masses. Quantum mechanics gives us a very different picture.

581
00:26:42,840 --> 00:26:46,119
Speaker 1: It says everything is discreet, it's quantized, it's chunked up

582
00:26:46,119 --> 00:26:49,080
Speaker 1: into pieces, and there's a limited amount of information we

583
00:26:49,119 --> 00:26:52,280
Speaker 1: can know about the universe. General relativity says that you

584
00:26:52,320 --> 00:26:55,159
Speaker 1: can have tiny little objects moving in smooth paths, so

585
00:26:55,200 --> 00:26:57,760
Speaker 1: you can perfectly know how they move. Quantum mechanics says

586
00:26:57,800 --> 00:27:00,600
Speaker 1: that's not possible, and they come directly into inflict at

587
00:27:00,640 --> 00:27:03,240
Speaker 1: the heart of a black hole, where general relativity says

588
00:27:03,280 --> 00:27:06,439
Speaker 1: you have this point of infinite density and quantum mechanics says, no,

589
00:27:06,640 --> 00:27:09,040
Speaker 1: that's not possible. And so what we need is some

590
00:27:09,080 --> 00:27:11,680
Speaker 1: sort of merging of the two, a theory of quantum

591
00:27:11,720 --> 00:27:15,600
Speaker 1: gravity that tells us what happens when quantum objects feel

592
00:27:15,720 --> 00:27:16,840
Speaker 1: very strong gravity.

593
00:27:16,920 --> 00:27:19,400
Speaker 2: I guess maybe the question is what does quantum mechanics

594
00:27:19,480 --> 00:27:21,880
Speaker 2: say about the black hole hair question?

595
00:27:22,200 --> 00:27:24,560
Speaker 1: So quantum mechanics as we have it now doesn't know

596
00:27:24,600 --> 00:27:28,280
Speaker 1: how to deal with gravity for particles. Right, particles are

597
00:27:28,320 --> 00:27:31,040
Speaker 1: very different from things like sand, or rocks, or base

598
00:27:31,119 --> 00:27:34,080
Speaker 1: balls or even pieces of hair. Those things are basically

599
00:27:34,119 --> 00:27:36,480
Speaker 1: classical objects, so we know where they are and we

600
00:27:36,480 --> 00:27:38,159
Speaker 1: can talk about them as if they always have a

601
00:27:38,200 --> 00:27:41,199
Speaker 1: specified location. But quantum particles are different. They have like

602
00:27:41,320 --> 00:27:44,479
Speaker 1: probabilities of being here and probabilities of being there, So

603
00:27:44,520 --> 00:27:47,600
Speaker 1: we don't know how to do gravity for particles. Like

604
00:27:47,760 --> 00:27:50,200
Speaker 1: if a particle has a probability to be here and there,

605
00:27:50,320 --> 00:27:52,399
Speaker 1: does it have half the gravity here and half the

606
00:27:52,400 --> 00:27:56,240
Speaker 1: gravity there, or is there a gravity probability we don't know.

607
00:27:56,320 --> 00:27:59,760
Speaker 1: What we need is a theory of gravity for particles,

608
00:27:59,800 --> 00:28:02,760
Speaker 1: and nobody has one, and so until you have that theory,

609
00:28:02,800 --> 00:28:06,639
Speaker 1: you can't actually know what quantum mechanics even says about

610
00:28:06,640 --> 00:28:08,879
Speaker 1: what's going on inside a black hole, or whether there

611
00:28:08,920 --> 00:28:11,040
Speaker 1: are ripples in texture on the surface of the black

612
00:28:11,040 --> 00:28:13,680
Speaker 1: hole that you can use to figure out what's inside.

613
00:28:13,880 --> 00:28:15,879
Speaker 2: I see, I think this is what I'm trying to

614
00:28:15,880 --> 00:28:18,000
Speaker 2: get to, is like, what do you mean like quantum

615
00:28:18,000 --> 00:28:20,400
Speaker 2: mechanics says there are ripples on the surface that might

616
00:28:20,400 --> 00:28:21,800
Speaker 2: be able to tell you if a black hole is

617
00:28:21,840 --> 00:28:23,200
Speaker 2: made out of hairs or bananas.

618
00:28:23,320 --> 00:28:25,720
Speaker 1: Unfortunately, we have no perfect theory of quantum gravity, but

619
00:28:25,760 --> 00:28:28,280
Speaker 1: people are doing some calculations to try to figure out

620
00:28:28,480 --> 00:28:31,840
Speaker 1: is it possible for a quantum black hole to have hair? Wait?

621
00:28:31,880 --> 00:28:33,520
Speaker 2: Wait, wait, what's a quantum black hole?

622
00:28:33,640 --> 00:28:35,879
Speaker 1: So quantum black hole would just be a black hole

623
00:28:35,920 --> 00:28:39,040
Speaker 1: as described by a theory of quantum gravity, instead of

624
00:28:39,080 --> 00:28:42,280
Speaker 1: a classical black hole as described by general relativity. Right,

625
00:28:42,320 --> 00:28:45,640
Speaker 1: Einstein's classical black holes are a singularity with an event

626
00:28:45,640 --> 00:28:48,960
Speaker 1: horizon around them, with no hair whatsoever, a perfectly smooth surface,

627
00:28:48,960 --> 00:28:51,360
Speaker 1: and you can't tell anything about what's inside or what

628
00:28:51,520 --> 00:28:53,440
Speaker 1: was used to build it. Quantum black hole is a

629
00:28:53,480 --> 00:28:57,280
Speaker 1: description of a different theoretical object, one that follows rules

630
00:28:57,280 --> 00:28:58,280
Speaker 1: of quantum gravity.

631
00:28:58,440 --> 00:29:01,080
Speaker 2: All right, let's dig into that. Is a quantum black hole,

632
00:29:01,200 --> 00:29:03,320
Speaker 2: hole that's both black and white at the same time.

633
00:29:03,800 --> 00:29:08,440
Speaker 1: No, no, but that sounds awesome. I wish that were true.

634
00:29:08,800 --> 00:29:11,520
Speaker 1: Carlo Rovelli has this fun theory that black holes might

635
00:29:11,600 --> 00:29:16,480
Speaker 1: be collapsing stars that are slowed down by gravitational time dilation,

636
00:29:16,920 --> 00:29:19,680
Speaker 1: and eventually they turn around and become white holes. And

637
00:29:19,720 --> 00:29:22,000
Speaker 1: we had a fun conversation with him and some of

638
00:29:22,000 --> 00:29:24,600
Speaker 1: his colleagues about exactly that how a black hole can

639
00:29:24,640 --> 00:29:26,680
Speaker 1: be in a superposition of a state maybe being a

640
00:29:26,680 --> 00:29:29,400
Speaker 1: black hole, maybe being a white hole. So basically, yeah,

641
00:29:29,600 --> 00:29:31,120
Speaker 1: your theory is one of the contenders.

642
00:29:31,320 --> 00:29:33,600
Speaker 2: There you go. So, then how is a quantum black

643
00:29:33,600 --> 00:29:36,960
Speaker 2: hole different than a regular relativity black hole.

644
00:29:37,000 --> 00:29:38,959
Speaker 1: So there's a few different answers to that, because there

645
00:29:39,000 --> 00:29:41,840
Speaker 1: are a few different theories of quantum gravity. Right, we

646
00:29:41,880 --> 00:29:44,240
Speaker 1: have no perfect theory. We don't even have a theory

647
00:29:44,280 --> 00:29:46,760
Speaker 1: that's complete and it works on paper, not to mention

648
00:29:46,920 --> 00:29:49,440
Speaker 1: a theory which has been tested against what's actually happening

649
00:29:49,440 --> 00:29:51,959
Speaker 1: out there in the universe. So people are working in

650
00:29:52,000 --> 00:29:55,040
Speaker 1: different directions in quantum gravity, and some of them are

651
00:29:55,080 --> 00:29:58,200
Speaker 1: working specifically on this problem and have ideas for the

652
00:29:58,200 --> 00:30:02,560
Speaker 1: consequences of their particular quantum gravity on the hairiness of

653
00:30:02,560 --> 00:30:05,280
Speaker 1: a black hole. So in some theories of quantum gravity,

654
00:30:05,400 --> 00:30:07,840
Speaker 1: black holes have different kinds of hairs.

655
00:30:07,760 --> 00:30:11,360
Speaker 2: Like long hair, curly hairs, luster's hairs. What did to

656
00:30:11,400 --> 00:30:13,160
Speaker 2: say about what black holes could be like?

657
00:30:13,280 --> 00:30:14,960
Speaker 1: Yeah, so let's go through some of the options. The

658
00:30:14,960 --> 00:30:19,000
Speaker 1: first real progress in quantum gravity was Stephen Hawking. He said, actually,

659
00:30:19,080 --> 00:30:23,440
Speaker 1: let's figure out whether black holes radiate a weigh any information.

660
00:30:23,880 --> 00:30:26,320
Speaker 1: And he has his famous theory of Hawking radiation that

661
00:30:26,440 --> 00:30:30,040
Speaker 1: says that black holes generate particles. You know, if you're

662
00:30:30,040 --> 00:30:32,480
Speaker 1: near the surface of a black hole, it'll actually be

663
00:30:32,520 --> 00:30:35,480
Speaker 1: shooting off particles at you. This is Hawking radiation. The

664
00:30:35,480 --> 00:30:37,680
Speaker 1: thing about Hawking radiation from black holes is that it's

665
00:30:37,720 --> 00:30:42,200
Speaker 1: supposed to contain zero information that just depends on the

666
00:30:42,240 --> 00:30:44,880
Speaker 1: mass of the black hole and nothing else about the

667
00:30:44,920 --> 00:30:47,800
Speaker 1: internal configuration, about whether it's apples or bananas.

668
00:30:47,880 --> 00:30:51,520
Speaker 2: And that's because it's like generating particles out of the vacuum,

669
00:30:51,560 --> 00:30:54,600
Speaker 2: right out of nothingness, out of pure energy that's inside

670
00:30:54,600 --> 00:30:58,120
Speaker 2: the black hole. Like, it doesn't depend on what kind

671
00:30:58,120 --> 00:31:01,040
Speaker 2: of energy, whether it has hairs or been in it's inside.

672
00:31:01,560 --> 00:31:04,640
Speaker 2: It just kind of creates stuff. As I think, I

673
00:31:04,720 --> 00:31:07,120
Speaker 2: understand that it just creates stuff and then radiates it out.

674
00:31:07,200 --> 00:31:09,400
Speaker 1: That's right, that it creates stuff based just on the

675
00:31:09,520 --> 00:31:12,640
Speaker 1: energy the mass of the black hole. Actually it's formulated

676
00:31:12,680 --> 00:31:15,560
Speaker 1: fascinatingly in terms of like black hole thermodynamics. So you

677
00:31:15,560 --> 00:31:18,040
Speaker 1: can think about the temperature of the black hole. But

678
00:31:18,120 --> 00:31:21,040
Speaker 1: you have to be very careful about applying Hawking's arguments

679
00:31:21,080 --> 00:31:24,960
Speaker 1: to like a microscopic particle picture of what's happening about

680
00:31:24,960 --> 00:31:27,880
Speaker 1: creating these particles, because Hawking doesn't have again, the full

681
00:31:27,920 --> 00:31:30,360
Speaker 1: theory of quantum gravity. He did this sort of semi

682
00:31:30,400 --> 00:31:33,360
Speaker 1: classical calculation where he thought about quantum fields and near

683
00:31:33,400 --> 00:31:35,920
Speaker 1: event horizons, and he figured out that these quantum fields

684
00:31:35,920 --> 00:31:39,280
Speaker 1: have to radied. He doesn't have a microscopic picture. And

685
00:31:39,320 --> 00:31:42,720
Speaker 1: you'll often hear this story about Hawking radiation, about particles

686
00:31:42,760 --> 00:31:45,880
Speaker 1: and antip particles created near the event horizon. One falls

687
00:31:45,920 --> 00:31:48,240
Speaker 1: in and one escapes, et cetera, et cetera. That's all

688
00:31:48,280 --> 00:31:52,120
Speaker 1: hand wavy storytelling. That's not accurate it doesn't actually hold

689
00:31:52,160 --> 00:31:54,640
Speaker 1: together in terms of what's happening with Hawking radiation.

690
00:31:54,840 --> 00:31:56,840
Speaker 2: Wait, it doesn't hold together, or we don't know if

691
00:31:56,840 --> 00:31:57,400
Speaker 2: it's true.

692
00:31:57,480 --> 00:31:59,720
Speaker 1: If you start from that microscopic picture and try to

693
00:31:59,720 --> 00:32:02,640
Speaker 1: build up a prediction of Hawking radiation, it doesn't work.

694
00:32:02,720 --> 00:32:04,760
Speaker 2: What do you mean, Like, the radiation we measure from

695
00:32:04,760 --> 00:32:07,160
Speaker 2: a black hole doesn't match what you would predict from that.

696
00:32:07,240 --> 00:32:09,440
Speaker 1: Well, number one, we've never measured the radiation from a

697
00:32:09,480 --> 00:32:11,520
Speaker 1: black hole. This is still just theoretical.

698
00:32:11,760 --> 00:32:12,480
Speaker 2: So then what do you mean.

699
00:32:12,520 --> 00:32:14,800
Speaker 1: As soon as you make yourself this microscopic picture and

700
00:32:14,800 --> 00:32:16,880
Speaker 1: then you have these quantum particles, you have all sorts

701
00:32:16,920 --> 00:32:20,120
Speaker 1: of questions you can't answer, like about fuzziness and probability,

702
00:32:20,600 --> 00:32:23,560
Speaker 1: and whether the particles can fluctuate past the event horizon

703
00:32:23,680 --> 00:32:26,600
Speaker 1: or not. And so the quantum mechanical picture isn't really

704
00:32:26,600 --> 00:32:29,280
Speaker 1: complete about what's happening to these little particles. Nobody can

705
00:32:29,280 --> 00:32:32,280
Speaker 1: start from those particles and calculate up and predict Hawking radiation.

706
00:32:32,400 --> 00:32:34,440
Speaker 1: That's not what's happening. Hawking has started from sort of

707
00:32:34,440 --> 00:32:37,600
Speaker 1: a bigger, broader picture, just from understanding like the energy

708
00:32:37,640 --> 00:32:41,000
Speaker 1: flow the quantum fields near the event horizon, and predicted

709
00:32:41,000 --> 00:32:44,280
Speaker 1: that the emit radiation. But again there's no microscopic picture

710
00:32:44,280 --> 00:32:46,400
Speaker 1: that really holds together. But you want to know more

711
00:32:46,440 --> 00:32:48,400
Speaker 1: details about how that works. We have a whole episode

712
00:32:48,400 --> 00:32:51,240
Speaker 1: about Hawking radiation where we can review that in detail.

713
00:32:51,400 --> 00:32:53,360
Speaker 2: But I think the main picture you're saying is that

714
00:32:53,400 --> 00:32:56,400
Speaker 2: we do know that black holes radiate Hawking radiation. We

715
00:32:56,560 --> 00:32:59,200
Speaker 2: just don't know like the specific details of what's happening

716
00:32:59,200 --> 00:33:01,520
Speaker 2: at the border of a black hole to make that happen.

717
00:33:01,600 --> 00:33:04,360
Speaker 1: That's right, Hawking didn't come up with a complete theory

718
00:33:04,360 --> 00:33:07,160
Speaker 1: of quantum gravity, so he doesn't have a microscopic picture,

719
00:33:07,240 --> 00:33:09,960
Speaker 1: and he predicts this Hawking radiation, which again should contain

720
00:33:10,080 --> 00:33:13,840
Speaker 1: no information. So a Hawking black hole basically has no hair,

721
00:33:14,160 --> 00:33:16,360
Speaker 1: even though it's kind of a quantum black hole. But

722
00:33:16,400 --> 00:33:18,680
Speaker 1: there are other theories of quantum gravity people who've been

723
00:33:18,680 --> 00:33:21,560
Speaker 1: working on, and some of these do predict black hole hair.

724
00:33:21,600 --> 00:33:23,720
Speaker 2: All right, let's get into the hairy details of these

725
00:33:23,760 --> 00:33:26,600
Speaker 2: theories and whether or not black holes have hair or

726
00:33:26,680 --> 00:33:29,040
Speaker 2: quantum hairs or no hairs at all, and what it

727
00:33:29,080 --> 00:33:31,800
Speaker 2: could mean about our understanding of the laws of physics.

728
00:33:32,360 --> 00:33:47,520
Speaker 2: But first, let's take another quick break, all right, we

729
00:33:47,600 --> 00:33:50,760
Speaker 2: are braiding our knowledge of the universe here talking about

730
00:33:50,800 --> 00:33:53,400
Speaker 2: black hole hairs and whether they have it or not.

731
00:33:54,040 --> 00:33:56,680
Speaker 2: And so according to Einstein, who is famous for his hair,

732
00:33:56,760 --> 00:33:59,080
Speaker 2: by the way, his gray hair, even as black holes

733
00:33:59,120 --> 00:34:01,280
Speaker 2: don't have hair, which seems kind of mean given how

734
00:34:01,360 --> 00:34:04,040
Speaker 2: much hair he had. But it seems like quantum mechanics

735
00:34:04,120 --> 00:34:06,600
Speaker 2: at first said they didn't have hairs, but now there

736
00:34:06,600 --> 00:34:09,359
Speaker 2: were some theories about quantum gravity that say, maybe they

737
00:34:09,360 --> 00:34:10,040
Speaker 2: do have hairs.

738
00:34:10,120 --> 00:34:12,960
Speaker 1: That's right, And all these calculations are approximate. You know,

739
00:34:13,040 --> 00:34:15,520
Speaker 1: nobody has a full theory of quantum gravity that they

740
00:34:15,560 --> 00:34:18,360
Speaker 1: can start from and predict these things from first principles.

741
00:34:18,440 --> 00:34:21,799
Speaker 1: Everybody is doing sort of approximations. They're saying, well, we

742
00:34:21,800 --> 00:34:23,800
Speaker 1: don't have the full theory, but maybe it looks a

743
00:34:23,880 --> 00:34:25,799
Speaker 1: little bit like this, And if it looked like this,

744
00:34:25,920 --> 00:34:28,120
Speaker 1: then what would be the answer to this question? But

745
00:34:28,160 --> 00:34:30,920
Speaker 1: a lot of is approximate and handwavy and inconsistent with

746
00:34:31,000 --> 00:34:34,040
Speaker 1: other things. But you know, this is how progress is made.

747
00:34:34,160 --> 00:34:36,239
Speaker 1: We don't always just have a flash of insight with

748
00:34:36,320 --> 00:34:38,680
Speaker 1: the whole answer that we can work from. We put

749
00:34:38,680 --> 00:34:41,200
Speaker 1: things together and try to patch them together, and eventually

750
00:34:41,200 --> 00:34:44,719
Speaker 1: maybe it comes together into a bigger picture of everything, all.

751
00:34:44,640 --> 00:34:47,840
Speaker 2: Right, Well, so then how do these fuzzy quantum gravity

752
00:34:47,880 --> 00:34:49,960
Speaker 2: theories say that black holes have hair?

753
00:34:50,040 --> 00:34:52,719
Speaker 1: So there's a paper about ten years ago that try

754
00:34:52,800 --> 00:34:56,680
Speaker 1: to study the configuration of gravitons far away from a

755
00:34:56,719 --> 00:35:01,200
Speaker 1: black hole, so outside the event horizon. Really interesting question

756
00:35:01,280 --> 00:35:04,960
Speaker 1: to think about gravity as a quantum force because if

757
00:35:05,000 --> 00:35:08,319
Speaker 1: gravity isn't the curvature spaced time, if quantum gravity lies

758
00:35:08,360 --> 00:35:11,360
Speaker 1: in the direction of like figuring out how to express

759
00:35:11,440 --> 00:35:14,480
Speaker 1: gravity in the same kind of language that we express

760
00:35:14,520 --> 00:35:17,920
Speaker 1: other forces like the weak force and electricity, magnetism and

761
00:35:17,960 --> 00:35:21,600
Speaker 1: the strong force as forces mediated by particles the way

762
00:35:21,520 --> 00:35:25,560
Speaker 1: they like the photon mediates electromagnetism. Then gravity would have

763
00:35:25,600 --> 00:35:29,840
Speaker 1: a particle, the graviton that you use to mediate gravity.

764
00:35:30,040 --> 00:35:32,720
Speaker 1: And so in this picture this is like graviton based

765
00:35:32,840 --> 00:35:36,200
Speaker 1: quantum gravity. People thought about, like, what's happening with all

766
00:35:36,239 --> 00:35:39,040
Speaker 1: the gravitons in the vicinity of a black hole?

767
00:35:39,719 --> 00:35:41,640
Speaker 2: Like what they get sucked in too? Right?

768
00:35:41,719 --> 00:35:44,600
Speaker 1: Yeah? Would they get sucked in? Is there information in

769
00:35:44,640 --> 00:35:47,400
Speaker 1: the flow of the gravitons. People are often writing in

770
00:35:47,440 --> 00:35:49,959
Speaker 1: and asking me if the mass of the black hole

771
00:35:50,000 --> 00:35:54,320
Speaker 1: is contained within the event horizon and gravity is quantum mechanical,

772
00:35:54,480 --> 00:35:57,759
Speaker 1: wouldn't it need to like shoot out gravitons in order

773
00:35:57,760 --> 00:36:00,640
Speaker 1: to mediate gravity, and wouldn't that break the area of

774
00:36:00,680 --> 00:36:03,480
Speaker 1: the black hole? So you can see how it's tricky

775
00:36:03,520 --> 00:36:06,680
Speaker 1: to think about gravity as a quantum force. If there

776
00:36:06,719 --> 00:36:08,480
Speaker 1: really is an event horizon there.

777
00:36:08,760 --> 00:36:11,160
Speaker 2: Whoa Okay, So let me see if I got this right,

778
00:36:11,280 --> 00:36:14,479
Speaker 2: and a quantum theory that has gravitons in it, which

779
00:36:14,480 --> 00:36:16,680
Speaker 2: we don't know if they exist or not, like the

780
00:36:16,719 --> 00:36:19,200
Speaker 2: gravity of a black hole, you would feel it because

781
00:36:19,200 --> 00:36:23,040
Speaker 2: it's shooting gravitons at me or am I shooting gravitons

782
00:36:23,080 --> 00:36:24,839
Speaker 2: at the black hole? Couldn't both be the case?

783
00:36:25,040 --> 00:36:27,960
Speaker 1: Yeah, both would be the case. And one question is

784
00:36:27,960 --> 00:36:32,040
Speaker 1: whether those gravitons contain any information about what's inside the

785
00:36:32,040 --> 00:36:34,719
Speaker 1: black hole or not. On one hand, you can imagine

786
00:36:34,719 --> 00:36:39,200
Speaker 1: the event horizon itself just generating a gravitational field and

787
00:36:39,239 --> 00:36:42,520
Speaker 1: shooting off gravitons from the event horizon. No information about

788
00:36:42,520 --> 00:36:46,279
Speaker 1: what's going on inside, only information about the total mass. Right,

789
00:36:46,440 --> 00:36:48,840
Speaker 1: Like if you could change the configuration inside, if you

790
00:36:48,840 --> 00:36:51,279
Speaker 1: build it out of bananas instead of apples, you get

791
00:36:51,320 --> 00:36:54,719
Speaker 1: the same exact distribution of gravitons in the outside, or

792
00:36:54,840 --> 00:36:58,040
Speaker 1: is there information in the graviton ripples that tells you

793
00:36:58,440 --> 00:37:01,480
Speaker 1: about what's happening inside the event horizon. So there's this

794
00:37:01,520 --> 00:37:05,520
Speaker 1: paperback ten years ago that deduced some small changes in

795
00:37:05,560 --> 00:37:09,160
Speaker 1: the graviton field far away from the event horizon based

796
00:37:09,200 --> 00:37:12,439
Speaker 1: on the mass that was inside. Essentially like a little

797
00:37:12,440 --> 00:37:15,360
Speaker 1: bit of a history imprinted on the space around the

798
00:37:15,400 --> 00:37:18,120
Speaker 1: black hole that could tell you what had fallen in.

799
00:37:18,280 --> 00:37:20,160
Speaker 1: You could like show up to a black hole a

800
00:37:20,160 --> 00:37:22,600
Speaker 1: million years later and say, oh, there was a banana

801
00:37:22,760 --> 00:37:25,320
Speaker 1: and then an apple, and then three granola bars thrown

802
00:37:25,360 --> 00:37:26,239
Speaker 1: into this black hole.

803
00:37:26,840 --> 00:37:28,840
Speaker 2: What do you mean, Like, how would the banana versus

804
00:37:28,840 --> 00:37:32,319
Speaker 2: the apple affect the graviton that the black hole shoots out, So.

805
00:37:32,320 --> 00:37:36,200
Speaker 1: It affects the gravitational field outside. The gravitons themselves don't

806
00:37:36,280 --> 00:37:39,640
Speaker 1: pass the event horizon, but the event horizon itself has

807
00:37:39,719 --> 00:37:43,319
Speaker 1: some shape to it, because remember, in Einstein's general relativity,

808
00:37:43,440 --> 00:37:46,880
Speaker 1: a black hole is a perfectly spherical object, right like,

809
00:37:47,200 --> 00:37:49,960
Speaker 1: as you zoom into it, you never see any bumps,

810
00:37:50,000 --> 00:37:53,920
Speaker 1: any ripples, any change. It's perfectly spherical. But quantum mechanics

811
00:37:53,920 --> 00:37:56,160
Speaker 1: says that can't be the case, right, that there's always

812
00:37:56,160 --> 00:37:59,719
Speaker 1: a discritizations like a pixelization inherent to the universe, which

813
00:37:59,760 --> 00:38:02,319
Speaker 1: means the event horizon has to have some ripples to

814
00:38:02,360 --> 00:38:05,600
Speaker 1: it's some texture to it, and those tiny little deviations

815
00:38:05,960 --> 00:38:09,000
Speaker 1: can change the ripples of the gravitons on the outside,

816
00:38:09,320 --> 00:38:12,200
Speaker 1: and they're caused by the history of stuff that you've

817
00:38:12,239 --> 00:38:14,439
Speaker 1: thrown into the black hole. So you make a very

818
00:38:14,480 --> 00:38:17,319
Speaker 1: slightly different black hole if you put in bananas than

819
00:38:17,360 --> 00:38:19,880
Speaker 1: if you put in apples. That information is not lost,

820
00:38:19,920 --> 00:38:23,120
Speaker 1: it's somehow contained on the event horizon of the black hole.

821
00:38:23,320 --> 00:38:25,279
Speaker 2: Like if a black hole has a banana at its

822
00:38:25,280 --> 00:38:28,040
Speaker 2: center or an apple at its center, you're saying that

823
00:38:28,200 --> 00:38:32,280
Speaker 2: it might cause a different texture of the event horizon

824
00:38:32,400 --> 00:38:33,160
Speaker 2: at the border of it.

825
00:38:33,320 --> 00:38:35,799
Speaker 1: Exactly. Yeah, And you could detect this if you could

826
00:38:35,840 --> 00:38:39,040
Speaker 1: measure gravitons in the vicinity of the black hole.

827
00:38:39,120 --> 00:38:41,600
Speaker 2: How would the banana or the apple affect the texture

828
00:38:41,840 --> 00:38:43,479
Speaker 2: all the way at the edge of the black hole?

829
00:38:43,560 --> 00:38:44,880
Speaker 2: Like if I put a banana or an apple on

830
00:38:44,920 --> 00:38:46,600
Speaker 2: the center of the Earth, would it I guess it

831
00:38:46,640 --> 00:38:50,160
Speaker 2: would affect the surface of the Earth in some microscopic way.

832
00:38:50,239 --> 00:38:52,359
Speaker 1: You have to think about this quantum mechanically right, we're

833
00:38:52,360 --> 00:38:55,360
Speaker 1: talking about the quantum states of all these objects, which

834
00:38:55,400 --> 00:38:58,720
Speaker 1: contain the history of everything that's happened to them. Remember,

835
00:38:58,760 --> 00:39:02,799
Speaker 1: in a quantum world, the present is uniquely determined by

836
00:39:02,840 --> 00:39:06,279
Speaker 1: the past, which means that every possible past has a

837
00:39:06,360 --> 00:39:09,200
Speaker 1: different present, which means you can invert it. You can say, well,

838
00:39:09,239 --> 00:39:11,880
Speaker 1: because we're in this present, we can tell what the

839
00:39:11,920 --> 00:39:15,320
Speaker 1: past was. So all the tiny ripples, the details of

840
00:39:15,400 --> 00:39:18,200
Speaker 1: the quantum states of everything around you could be used

841
00:39:18,200 --> 00:39:20,680
Speaker 1: in principle to like rewind time and tell you about

842
00:39:20,680 --> 00:39:23,160
Speaker 1: what's happened in the past. For folks who watch that

843
00:39:23,239 --> 00:39:26,520
Speaker 1: show Devs, that's the whole basic principle. And so the

844
00:39:26,640 --> 00:39:29,000
Speaker 1: idea is that you throw a banana or an apple

845
00:39:29,040 --> 00:39:31,960
Speaker 1: into the black hole, it creates a slightly different black

846
00:39:31,960 --> 00:39:34,960
Speaker 1: hole in a way that imprints that information. I don't

847
00:39:34,960 --> 00:39:36,720
Speaker 1: know the details. I can't tell you, like a banana

848
00:39:36,760 --> 00:39:38,919
Speaker 1: black hole looks like this, or an apple black hole

849
00:39:39,160 --> 00:39:41,600
Speaker 1: looks like that. But in quantum mechanics, there's a lot

850
00:39:41,640 --> 00:39:44,839
Speaker 1: of potential information on the surface because the surface is

851
00:39:44,880 --> 00:39:48,080
Speaker 1: not totally smooth, it can't be whereas a gr black

852
00:39:48,080 --> 00:39:51,000
Speaker 1: hole has to be perfectly smooth with no information.

853
00:39:51,400 --> 00:39:54,400
Speaker 2: Well, I think you're saying maybe it's not specific to

854
00:39:54,560 --> 00:39:57,759
Speaker 2: quantum black holes. Like in a relativity black hole, you

855
00:39:57,760 --> 00:40:00,440
Speaker 2: could also throw a banana and everything inside of the

856
00:40:00,440 --> 00:40:02,799
Speaker 2: black hole could remember the banana. You just couldn't tell

857
00:40:02,840 --> 00:40:05,760
Speaker 2: from the outside because it's perfectly smooth. According to Einstein,

858
00:40:05,920 --> 00:40:08,040
Speaker 2: you're saying that in a quantum black hole, the black

859
00:40:08,040 --> 00:40:10,440
Speaker 2: hole is not perfectly smooth. You could maybe read from

860
00:40:10,440 --> 00:40:11,880
Speaker 2: its surface what you threw it.

861
00:40:12,040 --> 00:40:14,359
Speaker 1: Yes, exactly. That was one theory in a paper about

862
00:40:14,400 --> 00:40:17,920
Speaker 1: ten years ago that effectively the gravitons could tell you

863
00:40:17,920 --> 00:40:19,799
Speaker 1: about the texture of the surface, which would in turn

864
00:40:19,840 --> 00:40:22,880
Speaker 1: tell you about what's going on inside. The problem is

865
00:40:22,920 --> 00:40:27,399
Speaker 1: that gravitons may be impossible to ever detect. Remember, gravitons

866
00:40:27,440 --> 00:40:31,680
Speaker 1: are not gravitational waves. Gravitational waves are huge waves in

867
00:40:31,719 --> 00:40:35,200
Speaker 1: the gravitational field that we can detect. Gravitons would be

868
00:40:35,320 --> 00:40:38,640
Speaker 1: like drops in the ocean compared to waves in the ocean.

869
00:40:38,719 --> 00:40:41,160
Speaker 1: So if they exist, they're super duper tiny and it

870
00:40:41,280 --> 00:40:43,000
Speaker 1: may be impossible to ever see.

871
00:40:43,120 --> 00:40:45,960
Speaker 2: What does it have to be gravitons? I wonder if

872
00:40:46,040 --> 00:40:48,920
Speaker 2: there is a texture to a quantum black hole, wouldn't

873
00:40:49,040 --> 00:40:52,560
Speaker 2: you also maybe detect other particles coming from the Hawking

874
00:40:52,640 --> 00:40:55,000
Speaker 2: radiation off of it, and maybe could you could tell

875
00:40:55,080 --> 00:40:56,320
Speaker 2: its texture from that.

876
00:40:56,239 --> 00:40:57,640
Speaker 1: You could, And there was a theory a couple of

877
00:40:57,760 --> 00:41:00,960
Speaker 1: years ago that suggested, maybe you remember we talked about

878
00:41:01,000 --> 00:41:04,560
Speaker 1: on the podcast, that there might be wormholes that connect

879
00:41:04,600 --> 00:41:07,719
Speaker 1: the interior and exterior of the black hole, and that

880
00:41:07,760 --> 00:41:11,279
Speaker 1: Hawking radiation could in fact give you information about what's

881
00:41:11,320 --> 00:41:14,520
Speaker 1: going on inside. That it's not the way Hawking described it,

882
00:41:14,719 --> 00:41:18,719
Speaker 1: totally information free, but maybe actually has information about what's

883
00:41:18,760 --> 00:41:21,840
Speaker 1: going on inside, because these wormholes are like bridges that

884
00:41:21,920 --> 00:41:25,080
Speaker 1: are connecting quantum entangled particles, some of which on the

885
00:41:25,120 --> 00:41:27,000
Speaker 1: inside and some of which on the outside of the

886
00:41:27,000 --> 00:41:29,680
Speaker 1: black hole. So there are versions of quantum gravity in

887
00:41:29,680 --> 00:41:32,719
Speaker 1: which Hawking radiation does have information in it.

888
00:41:32,760 --> 00:41:34,359
Speaker 2: All right, So then what are some of the other

889
00:41:34,400 --> 00:41:35,840
Speaker 2: theories about quantum black holes?

890
00:41:35,920 --> 00:41:37,800
Speaker 1: So all these would be very very hard to detect

891
00:41:37,840 --> 00:41:40,799
Speaker 1: because you're talking about detecting Hawking radiation and differences in

892
00:41:40,840 --> 00:41:43,759
Speaker 1: it which we've never even seen, or detecting gravitons, so

893
00:41:43,840 --> 00:41:45,719
Speaker 1: we don't even know exist, and if they do, would

894
00:41:45,719 --> 00:41:48,000
Speaker 1: be very hard to see. Recently, there was an idea

895
00:41:48,040 --> 00:41:51,759
Speaker 1: to look for hair on extremal black holes. We've talked

896
00:41:51,760 --> 00:41:54,799
Speaker 1: about this on the podcast recently. Black holes have maximum

897
00:41:54,880 --> 00:41:58,759
Speaker 1: spin or maximum charge that they can maintain, beyond which

898
00:41:58,760 --> 00:42:01,359
Speaker 1: a vent horizon disappear and things go crazy. But when

899
00:42:01,400 --> 00:42:05,040
Speaker 1: black holes are near this extremal state, things that fall

900
00:42:05,080 --> 00:42:08,279
Speaker 1: into them might leave instabilities on the event horizon in

901
00:42:08,320 --> 00:42:11,719
Speaker 1: some theories of quantum gravity, and if so, it would

902
00:42:11,760 --> 00:42:16,120
Speaker 1: generate ripples in their gravitational waves, not in their gravitons,

903
00:42:16,200 --> 00:42:19,600
Speaker 1: and gravitational waves are things we can detect that we

904
00:42:19,719 --> 00:42:21,840
Speaker 1: might be able to see sometime in the future.

905
00:42:22,880 --> 00:42:25,400
Speaker 2: What do you mean instabilities on the event horizon? What

906
00:42:25,440 --> 00:42:25,879
Speaker 2: does that mean?

907
00:42:25,960 --> 00:42:27,880
Speaker 1: Well, think about what happens when something falls into a

908
00:42:27,920 --> 00:42:30,560
Speaker 1: black hole. It's going to fall into a specific spot.

909
00:42:30,760 --> 00:42:33,239
Speaker 1: Right you toss a banana into a black hole, You're

910
00:42:33,239 --> 00:42:35,680
Speaker 1: tossing it in one side of this sphere, not on

911
00:42:35,719 --> 00:42:38,960
Speaker 1: the other side, and so immediately what's going to happen

912
00:42:39,360 --> 00:42:41,560
Speaker 1: is that the event horizon is going to grow out

913
00:42:41,560 --> 00:42:43,359
Speaker 1: to meet you. So the event horizon is not going

914
00:42:43,400 --> 00:42:45,880
Speaker 1: to be spherical for a moment. Then it slurps in

915
00:42:45,960 --> 00:42:48,880
Speaker 1: this banana, it falls into the singularity, and the black

916
00:42:48,880 --> 00:42:51,880
Speaker 1: hole rings back down to a perfect sphere that's in

917
00:42:51,920 --> 00:42:53,160
Speaker 1: general relativity at least.

918
00:42:53,239 --> 00:42:55,800
Speaker 2: Wait, what do you mean if it's not perfectly spherical?

919
00:42:55,880 --> 00:42:58,840
Speaker 2: What shape is it? Like a blong? Does it go

920
00:42:58,920 --> 00:42:59,920
Speaker 2: oblong for a second?

921
00:43:00,040 --> 00:43:02,640
Speaker 1: Yeah, it goes oblong for a second. And if you remember,

922
00:43:02,640 --> 00:43:06,480
Speaker 1: we talked about merging black holes. When black holes merge,

923
00:43:06,480 --> 00:43:09,319
Speaker 1: their event horizons grow together, and in some moments they

924
00:43:09,320 --> 00:43:11,720
Speaker 1: look like a dumbbell or they look like something else.

925
00:43:12,160 --> 00:43:14,879
Speaker 1: And I commented on that podcast. So that's exciting because

926
00:43:14,880 --> 00:43:17,040
Speaker 1: it tells you something about the history of the black

927
00:43:17,040 --> 00:43:19,040
Speaker 1: holes in a way that you can't otherwise know. In

928
00:43:19,120 --> 00:43:22,080
Speaker 1: gr but there's no hair theorem applies to the stable

929
00:43:22,080 --> 00:43:23,760
Speaker 1: state of the black hole. Like you put a banana

930
00:43:23,760 --> 00:43:26,040
Speaker 1: in a black hole, you expect it it's event horizon

931
00:43:26,040 --> 00:43:27,879
Speaker 1: and have a little bit of a funky shape as

932
00:43:27,920 --> 00:43:31,120
Speaker 1: it sort of settles back down to a new stable object.

933
00:43:31,320 --> 00:43:33,440
Speaker 1: But in this theory of quantum gravity, it says that

934
00:43:33,480 --> 00:43:37,000
Speaker 1: those instabilities persist that you create like waves on the

935
00:43:37,000 --> 00:43:40,120
Speaker 1: surface of the black hole which ripple basically forever, they

936
00:43:40,160 --> 00:43:40,760
Speaker 1: don't ever.

937
00:43:40,640 --> 00:43:44,040
Speaker 2: Go away, sort of like a gravitational wave.

938
00:43:44,200 --> 00:43:48,399
Speaker 1: Almost yes, exactly, and these would generate gravitational waves, which,

939
00:43:48,440 --> 00:43:51,640
Speaker 1: according to this paper, we might be able to detect,

940
00:43:52,040 --> 00:43:55,160
Speaker 1: probably not with our current observatories like LEGO, but with

941
00:43:55,280 --> 00:43:58,480
Speaker 1: future gravitational wave observatories like LISA or some of the

942
00:43:58,520 --> 00:44:01,440
Speaker 1: other projects, we might be able to actually see these

943
00:44:01,480 --> 00:44:04,680
Speaker 1: gravitational waves from these hairy black holes.

944
00:44:04,920 --> 00:44:06,759
Speaker 2: So wait, you're saying, like, even if I drop a

945
00:44:06,840 --> 00:44:09,920
Speaker 2: hair or a banana into a black hole maybe a

946
00:44:09,960 --> 00:44:12,239
Speaker 2: billion years ago, and it happens to be one of

947
00:44:12,239 --> 00:44:15,680
Speaker 2: these extreme black holes even today, it might be generating

948
00:44:15,719 --> 00:44:18,719
Speaker 2: gravitational waves that tell me what was dropped in a

949
00:44:18,760 --> 00:44:19,600
Speaker 2: billion years ago.

950
00:44:19,880 --> 00:44:22,840
Speaker 1: That's the theory, Yes, exactly. So if you're planning to

951
00:44:22,840 --> 00:44:25,680
Speaker 1: commit crimes and drop your evidence into a black hole,

952
00:44:26,320 --> 00:44:27,440
Speaker 1: this might be a loophole.

953
00:44:27,520 --> 00:44:30,279
Speaker 2: Wouldn't all that information just be radiated away or it

954
00:44:30,320 --> 00:44:32,480
Speaker 2: would stay there for a billion years.

955
00:44:32,520 --> 00:44:34,880
Speaker 1: It would fade with time for sure. So if you

956
00:44:34,960 --> 00:44:37,200
Speaker 1: want to catch a killer using clues dropped into a

957
00:44:37,200 --> 00:44:39,880
Speaker 1: black hole, your best bet is to use your gravitational

958
00:44:39,960 --> 00:44:43,040
Speaker 1: wave observatory immediately afterwards, because it would fade away. Even

959
00:44:43,080 --> 00:44:45,720
Speaker 1: these ripples would eventually fade away. As you say, energy

960
00:44:45,760 --> 00:44:48,400
Speaker 1: is being lost, right, it's being radiated away, but in

961
00:44:48,480 --> 00:44:51,760
Speaker 1: principle it still persists. Forever. Technically, it just gets fainter

962
00:44:51,840 --> 00:44:52,240
Speaker 1: and fainter.

963
00:44:52,680 --> 00:44:55,160
Speaker 2: I see, like the ripple stays on the surface of

964
00:44:55,200 --> 00:44:55,760
Speaker 2: the black.

965
00:44:55,520 --> 00:44:57,400
Speaker 1: Hole somehow, mm hmm, exactly.

966
00:44:57,560 --> 00:44:59,400
Speaker 2: All right, Well, what does this all mean about our

967
00:44:59,480 --> 00:45:02,239
Speaker 2: understanding black holes? Like, there's still a big mystery. It

968
00:45:02,239 --> 00:45:05,200
Speaker 2: seems we don't really even understand what black holes are.

969
00:45:05,360 --> 00:45:07,600
Speaker 2: We've seen these things out there in space. We didn't

970
00:45:07,640 --> 00:45:09,680
Speaker 2: even know if they actually are black holes, if they

971
00:45:09,719 --> 00:45:12,800
Speaker 2: are gr black holes, if there are quantum black holes.

972
00:45:13,320 --> 00:45:15,520
Speaker 1: And this is really at the heart of the question,

973
00:45:15,760 --> 00:45:19,160
Speaker 1: what's going on out there? I suspect that what's really

974
00:45:19,160 --> 00:45:21,319
Speaker 1: happening in the universe is something different from any of

975
00:45:21,320 --> 00:45:23,920
Speaker 1: these theories. You know, it's going to be a big surprise.

976
00:45:24,200 --> 00:45:27,120
Speaker 1: And the exciting thing is if black holes do have hair,

977
00:45:27,200 --> 00:45:29,600
Speaker 1: we might be able to measure it. We might be

978
00:45:29,600 --> 00:45:32,880
Speaker 1: able to get these subtle signatures from the ripples on

979
00:45:32,920 --> 00:45:35,520
Speaker 1: the surface of the event horizon to tell us what's

980
00:45:35,560 --> 00:45:38,279
Speaker 1: going on inside and maybe give us some clues about

981
00:45:38,360 --> 00:45:40,040
Speaker 1: what kind of universe we live in.

982
00:45:40,400 --> 00:45:43,680
Speaker 2: Would that be the holy grail of black holes or

983
00:45:43,719 --> 00:45:45,400
Speaker 2: the hairy banana.

984
00:45:44,840 --> 00:45:47,480
Speaker 1: Exactly the way Einstein's hair is the holy grail for

985
00:45:47,520 --> 00:45:48,319
Speaker 1: all physicists.

986
00:45:49,600 --> 00:45:52,719
Speaker 2: Is it think I think like Brian Cox, is it

987
00:45:52,800 --> 00:45:54,920
Speaker 2: popularized and you kind of physicists here?

988
00:45:55,719 --> 00:45:55,959
Speaker 1: Yeah?

989
00:45:56,280 --> 00:46:00,560
Speaker 2: The whole like emo kind of hipster aging pop star. Yeah, yeah,

990
00:46:00,560 --> 00:46:00,960
Speaker 2: there you go.

991
00:46:01,800 --> 00:46:03,240
Speaker 1: I don't think I'm ever going to be an aging

992
00:46:03,239 --> 00:46:07,399
Speaker 1: pop star. But you know, there are all these really

993
00:46:07,480 --> 00:46:09,680
Speaker 1: deep questions about the nature of the universe we live in.

994
00:46:09,880 --> 00:46:12,759
Speaker 1: Is information destroyed or not? It might be the black

995
00:46:12,800 --> 00:46:15,560
Speaker 1: holes are like cosmic toilet, it's just flushing away this

996
00:46:15,680 --> 00:46:18,560
Speaker 1: information to be destroyed. Or it could be that they're

997
00:46:18,600 --> 00:46:21,560
Speaker 1: preserving that information. They're radiating it back out into the

998
00:46:21,640 --> 00:46:25,680
Speaker 1: universe somehow, and then information is not destroyed in our universe.

999
00:46:25,760 --> 00:46:28,920
Speaker 1: It's a pretty deep question about the nature of reality.

1000
00:46:28,719 --> 00:46:31,440
Speaker 2: Yeah, and the nature of information itself, Like could you

1001
00:46:31,560 --> 00:46:35,720
Speaker 2: ever destroy information? Or does the universe always remember everything?

1002
00:46:37,040 --> 00:46:38,360
Speaker 2: Is there always a paper trail?

1003
00:46:38,520 --> 00:46:41,080
Speaker 1: Yes, exactly. If you committed a crime a billion years ago,

1004
00:46:41,480 --> 00:46:42,720
Speaker 1: justice might still be coming.

1005
00:46:43,000 --> 00:46:46,040
Speaker 2: All right, Well, more deep questions about the universe and

1006
00:46:46,280 --> 00:46:49,319
Speaker 2: a big reminder that there's still a lot for us

1007
00:46:49,360 --> 00:46:52,280
Speaker 2: to discover and to understand and to learn about and

1008
00:46:52,360 --> 00:46:55,600
Speaker 2: potentially to comb over as well. We hope you enjoyed that.

1009
00:46:56,000 --> 00:46:58,479
Speaker 2: Thanks for joining us see you next time.

1010
00:47:06,360 --> 00:47:09,160
Speaker 1: Thanks for listening, and remember that Daniel and Jorge Explain

1011
00:47:09,239 --> 00:47:13,239
Speaker 1: the Universe is a production of iHeartRadio. Or more podcasts

1012
00:47:13,239 --> 00:47:17,880
Speaker 1: from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever

1013
00:47:17,960 --> 00:47:19,680
Speaker 1: you listen to your favorite shows.