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Speaker 1: Few words are as heavily loaded as the word singularity.

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Speaker 1: To AI futurists, it means the moment that ais can

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Speaker 1: teach themselves and might accelerate beyond human control. To physicists,

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Speaker 1: it means a divergence, a divide by zero, a point

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Speaker 1: of nonsense, a failure, a breakdown. To the broader public,

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Speaker 1: it represents a mystery, an unseen, unknown point at the

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Speaker 1: heart of black holes, something fascinating and mind bending, something

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Speaker 1: we are a little bit afraid of, but also entrance

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Speaker 1: by a bizarre extremum that might reveal something deep about

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Speaker 1: the nature of the universe. To you, our listeners, it

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Speaker 1: means a whole episode dedicated to demystifying, detoxifying, deconstructing the

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Speaker 1: several subclasses of singularities. So welcome to Daniel and Kelly's

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Speaker 1: Extraordinary Singular Universe.

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Speaker 2: Hello. I'm Kelly Wienersmith. I studied parasites and space, and

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Speaker 2: I'm excited to talk about singularities today.

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Speaker 1: Hi, I'm Daniel Whitson. I'm a particle physicist, but I'm

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Speaker 1: actually not a singularity. There are other Daniel Whitson's. How many,

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Speaker 1: I'm not sure I've only ever found one he's an

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Speaker 1: artist in the UK and he's quite good, if I

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Speaker 1: have to say so myself.

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Speaker 2: Oh fantastic. It's something in the name. Maybe. I don't

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Speaker 2: think there's any other Kelly Wienersmith, which is why we

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Speaker 2: picked this name. Have I told you the story about

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Speaker 2: how I ended up with this name?

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Speaker 1: No, tell us the story, all right.

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Speaker 2: So I was working on my master's degree and I

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Speaker 2: published my first paper on smallmouth bass, and there's a

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Speaker 2: lot of papers on smallmouth bass, and so I looked

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Speaker 2: to see how hard was it going to be for

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Speaker 2: people to find my first paper by looking up papers

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Speaker 2: by Kay Smith, and there were literally like over one

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Speaker 2: hundred thousand, and if you did k L Smith, there

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Speaker 2: was like, you know, eighty thousand or there were still

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Speaker 2: so many that it was going to be really hard

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Speaker 2: to find me, Okay. And then I fell in love

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Speaker 2: with Zach, and I was like, all right, I don't

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Speaker 2: really want the last name Wiener. But with the order.

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Speaker 1: You're telling this story makes it sound like you picked

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Speaker 1: Zach because he had an unusual last name. No.

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Speaker 2: No, So you know, I dated some guys with some

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Speaker 2: pretty great last names.

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Speaker 1: Wow.

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Speaker 2: And then there was Zach, who I fell in love with.

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Speaker 1: And despite the last name I see.

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Speaker 2: So I looked up how many papers there were by K.

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Speaker 2: Wiener and K. L Weener and they were literally still thousands.

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Speaker 1: Wow.

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Speaker 2: And I was like, I don't really know if I

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Speaker 2: want to take this name if it only helps me

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Speaker 2: by like one order of magnitude. And so we smoothed

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Speaker 2: our names together and we our little family are the

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Speaker 2: only Wiener Smith's I found. There's a Wiener hyphen Smith

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Speaker 2: who's an obstetrician, which is amazing because she's helping people

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Speaker 2: Smith Wieners, which I love and.

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Speaker 1: Uh, professionally accurate last names. I love it.

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Speaker 2: That's right. But now Google scholar Kelly Wiener Smith, you

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Speaker 2: only get me. All right, we've gotten off track. I

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Speaker 2: am a singularity.

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Speaker 1: You are the one and only. Congratulations.

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Speaker 2: Oh thanks. I don't think there's a lot of vying

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Speaker 2: for this position.

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Speaker 1: And Kelly, if you were a point particle, then there

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Speaker 1: would be an infinite density if Kelly Wiener Smith's at you.

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Speaker 2: There are days where it feels like that's happening as

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Speaker 2: we cruise onto the holiday season, and that is.

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Speaker 1: Exactly what we're talking about here today. You're listening to

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Speaker 1: this episode in January, after surviving the holiday season, potentially

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Speaker 1: gaining mass, potentially losing mass, however you do it. But

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Speaker 1: today we're talking about that very massive question of gravitational singularity.

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Speaker 2: Amazing, and whenever we have a massive topic to handle,

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Speaker 2: we always go to the extraordinaries to see what they

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Speaker 2: think we do.

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Speaker 1: This question actually was inspired by an extraordinary he wrote

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Speaker 1: to me. Julie Bud wants to learn more about singularities.

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Speaker 1: Here's her question.

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Speaker 3: Hi, Daniel and Kelly, I'd love it if you could

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Speaker 3: do an episode about all things singularities. I have this misconception,

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Speaker 3: which I think maybe a common one, of a singularity

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Speaker 3: just being a tiny point in space where density becomes infinite.

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Speaker 3: But I recently learned it's actually kind of just a

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Speaker 3: term for where our understanding of physics breaks down and

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Speaker 3: we don't really know what happens. I also learned that

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Speaker 3: there are different types of singularities inside black holes, like

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Speaker 3: infalling and outflying singularities, and a BKL singularity, which I

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Speaker 3: can almost wrap my head around but also not really.

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Speaker 3: So I'd love if you could do a deep dive

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Speaker 3: into what singularities are and the different types of them.

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Speaker 2: Thanks, fantastic question from Julie. Frankly, I don't know the

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Speaker 2: answer here, so I think I'm going to learn a

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Speaker 2: lot today.

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Speaker 1: And I love this kind of question because one of

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Speaker 1: the things I love doing on the podcast is going

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Speaker 1: deeper than your typical pop side treatment, breaking through to

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Speaker 1: the next layer of knowledge, and really sharing with people

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Speaker 1: a lot of the subtlety and the nuance and how

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Speaker 1: people on the forefront of knowledge are actually thinking about

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Speaker 1: this stuff.

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Speaker 2: Yeah, and before we get to how the scientists at

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Speaker 2: the forefront of knowledge you're thinking about this stuff, let's

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Speaker 2: see how the people on the street are thinking about

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Speaker 2: this kind of stuff.

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Speaker 1: So I went out there and asked folks, what is

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Speaker 1: the best kind of gravitational singularity. Here's what people had

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Speaker 1: to say.

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Speaker 4: Gravitational singularities weren't real, so maybe the best one is

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Speaker 4: one you can study in your particle collider kind it

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Speaker 4: would make me lose ten pounds. I like classical gravitational

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Speaker 4: singularity is the best because I don't like the idea

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Speaker 4: of space being broken up into pieces.

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Speaker 1: As far as I can tell, it's being a grandparent,

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Speaker 1: the kind that's close enough to provide insight into the

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Speaker 1: universe and far enough away that I don't have to

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Speaker 1: worry wry about her breaking my stuff.

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Speaker 4: I would say a black hole a naked singularity, because

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Speaker 4: you could observe it and learn from it.

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Speaker 1: I have always had a fond spot for neutron stars.

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Speaker 1: Black holes are the best kind of gravitational singularity, the

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Speaker 1: singularity where it all started with the Big Bang, The

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Speaker 1: one that lets you view itself from afar, doesn't spaghetify

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Speaker 1: you and tells you all it's secrets. The one that

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Speaker 1: doesn't have any side effects.

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Speaker 4: All the singularities, all the singularities, all the singularities. Some

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Speaker 4: cosmic grads up being the telescope. It looks like there's

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Speaker 4: a ring on it. That's the accretion disking. Gravity is

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Speaker 4: lensing it. We'd like to study quantum gravity inside of it,

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Speaker 4: but if we visited, we just s begetify in it

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Speaker 4: will uh oh.

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Speaker 1: The kind that stays the heck away from me.

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Speaker 3: Is a black hole or neutron star.

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Speaker 2: Well, I think from now on the rule is that

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Speaker 2: everybody has to sing their answer to the tune of

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Speaker 2: a Beyonce song.

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Speaker 1: Wow, dude, really raise the bar for everybody, didn't he?

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Speaker 2: Thank you? Zach?

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Speaker 1: All right, So let's be singular in our focus and

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Speaker 1: talk about what singularities are. And I want to start

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Speaker 1: by talking about singularities sort of mathematically and theoretically, because

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Speaker 1: like many things in science, this word is overused.

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Speaker 2: I think these days I hear about it most and

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Speaker 2: as it relates to tech. But we are not at

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Speaker 2: all talking about tech singularities today.

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Speaker 1: We are not talking about that moment when ais can

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Speaker 1: teach themselves and accelerate beyond human control, and whether that's

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Speaker 1: already happened or not. We're talking about singularities from a

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Speaker 1: mathematical and physical point of view, and unfortunately, there's not

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Speaker 1: a whole lot of agreement on what singularities mean. There's

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Speaker 1: sort of two different kinds of singularities, though there are

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Speaker 1: ways to unify them in your mind. The first one

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Speaker 1: is essentially what Julie mentioned, which is where there's a

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Speaker 1: breakdown in your theory because you predict something unphysical and infinite.

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Speaker 1: Like if I flip a coin and I want to

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Speaker 1: calculate the probability for the coin to come up heads,

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Speaker 1: and my answer gives me infinity, I'm like, well, that

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Speaker 1: can't be right. The probability has to be less than

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Speaker 1: one so infinity is wrong. Something went wrong in my

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Speaker 1: theory I divided by zero, or I just made a

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Speaker 1: dumb error, or the theory is broken. Right, that's an

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Speaker 1: unphysical prediction, something which can't happen. It's outside the bounds.

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Speaker 1: And that happens a lot when math confronts physics. Right, mathematics,

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Speaker 1: let's you do lots of things which the physical world

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Speaker 1: doesn't let you do. A simple example of that is

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Speaker 1: like when you throw a ball, it follows a parabola.

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Speaker 1: That parabla moves forward, but also it could move backwards

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Speaker 1: in time, and the same solution allows the ball to

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Speaker 1: like go deep into the earth behind you or whatever. Obviously,

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Speaker 1: that's not going to happen. There's a boundary there that

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Speaker 1: you haven't included in your calculations. But there's lots of

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Speaker 1: times when you're solving problems in math and there's an

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Speaker 1: unphysical solution that you don't consider. And so one idea

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Speaker 1: of a singularity is when your math predicts infinity but

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Speaker 1: physics rejects that.

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Speaker 2: I guess I hadn't thought of singularity as another word

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Speaker 2: for were we're probably wrong about this, but is that

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Speaker 2: the right way to think about it?

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Speaker 1: It's an indication that something is probably wrong with your theory.

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Speaker 1: Not all the time. It could be that the infinities

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Speaker 1: are real. We don't really know. But if your physics

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Speaker 1: is telling you that the infinities are impossible and the

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Speaker 1: mathematics is predicting infinity, then there is something wrong with

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Speaker 1: your theory. That's the idea, okay, and it doesn't have

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Speaker 1: to be gravitational. You can take, for example, an electron

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Speaker 1: and think about the electric field of the electron. How

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Speaker 1: does that electric field vary with distance from the electron? Well,

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Speaker 1: it goes like one over distance square, like most things

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Speaker 1: in physics. So you twice as far away, the electric

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Speaker 1: field drops by a factor of four. Well, what happens

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Speaker 1: if you get twice as close. The electric field goes

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Speaker 1: up by a factor of four. What if you get

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Speaker 1: twice as close again, up by another factor of four.

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Speaker 1: If the electron is a point particle, you can get

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Speaker 1: infinitely close to it. What is the electric field at

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Speaker 1: the electron itself? Well, you're dividing by zero or it's

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Speaker 1: approaching infinity, And so the electric field in classical electrodynamics

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Speaker 1: is predicted to go to infinity at the electron itself.

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Speaker 1: There's a divergence there, a singularity and infinity. We don't

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Speaker 1: believe the electric field really is infinity there. This is

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Speaker 1: a breakdown of the theory because the electron is not

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Speaker 1: really a point particle. There's a limit to how close

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Speaker 1: you can get to it. So that's an example of like,

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Speaker 1: you didn't build all the physics into your theory, and

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Speaker 1: then you took the theory who literally and extrapolated it

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Speaker 1: beyond where you thought it was relevant, and so you

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Speaker 1: got a singularity. It's a warning sign from the mathematics

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Speaker 1: to say, oh there, buddy, back up and think about

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Speaker 1: what you're doing.

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Speaker 2: Interesting, So when you get a singularity like that, how

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Speaker 2: much of the theory do you throw away or do

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Speaker 2: you just say this theory is good up until this

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Speaker 2: point where we don't understand something.

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Speaker 1: Yeah, exactly, you can just put a boundary and say, well,

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Speaker 1: this theory works up to here, beyond that it's not applicable.

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Speaker 1: And that's the case where basically every theory of physics

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Speaker 1: we have right, every theory has boundaries because they start

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Speaker 1: from some assumptions, and those assumptions are only true in

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Speaker 1: limited cases. Even like the standard model of part of

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Speaker 1: physics are best. Quantum theory that predicts things to ten

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Speaker 1: decimal places. We know it's not valid everywhere because in

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Speaker 1: order to do any calculations we have to ignore gravity.

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Speaker 1: And a lot of times you can ignore gravity. When

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Speaker 1: two particles interact to the Large Hadron Collider, Gravity's are relevant,

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Speaker 1: but it's there, and if those particles get really really massive,

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Speaker 1: it's no longer irrelevant. So when things get really really massive,

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Speaker 1: we're really really dense, then you can no longer ignore gravity,

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Speaker 1: and you can't just calculate blindly assuming gravity doesn't exist.

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Speaker 1: So they're always boundaries to your theory. And when principle,

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Speaker 1: you'd love to find a new theory that doesn't have

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Speaker 1: those boundaries, or works beyond the boundary or something. But

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Speaker 1: there's lots of different possible approaches.

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Speaker 2: And I seem to remember infinities coming up in our

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Speaker 2: understanding of gravity when we were talking with Jonathan Oppenheim

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Speaker 2: and Thomas van reet Am. I remembering that correctly.

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Speaker 1: Yeah, exactly. String theory is an extension of these theories

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Speaker 1: that avoids some of the infinities that appear in the

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Speaker 1: standard model and in gravity. So it's an example of

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Speaker 1: a quantum gravity theory that avoids some of those infinities

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Speaker 1: by replacing points with lines. Essentially, instead of having dots,

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Speaker 1: you have strings. And of course it's still very controversial,

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Speaker 1: but that's an example of how you might extend a

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Speaker 1: theory to be more broadly applicable. Or you could just say, look,

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Speaker 1: this theory works here, and we have a different theory

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Speaker 1: over there. That's what we do. For like water, we

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Speaker 1: have a theory for fluids, and we have a different

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Speaker 1: theory for crystals and different theory for vapor And we

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Speaker 1: don't say we want a general theory of water, you know,

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Speaker 1: we're like, look, Navria Stokes works here, and crystal theory

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Speaker 1: works there, and we're fine. And so there's lots of

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Speaker 1: different approaches, and so a singularity more generally is where

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Speaker 1: things go to infinity. And in the case of gravity,

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Speaker 1: for example, you have gravitational curvature in the heart of

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Speaker 1: the black hole. We'll talk about that more in a minute.

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Speaker 1: That goes to infinity. And that's a question is does

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Speaker 1: that really happen or is it a sign that the

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Speaker 1: theory is broken somehow? Or in the early universe, the

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Speaker 1: Big Bang was a singularity, and in the future if

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Speaker 1: there's a big crunch that could be a.

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Speaker 2: Singularity, don't I don't want there to be a big crunch, Daniel, what.

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Speaker 1: You want doesn't really matter, unfortunately, Kelly.

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Speaker 2: Yeah, that's a little harsh. But also because we're extrapolating

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Speaker 2: an infinity, I'm going to decide that physicists are probably

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Speaker 2: just wrong about that.

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Speaker 1: Okay, great, I hope that works for you. That's one

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Speaker 1: concept of a singularity, where things get infinite as sign

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Speaker 1: that probably your theory is breaking down. There are other

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Speaker 1: concepts of a singularity. One that I think is really

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Speaker 1: fascinating are paths that are not extendable. So imagine you

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Speaker 1: are moving through space. You can go here, you can

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Speaker 1: go there. There's always a place you can go right,

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Speaker 1: you can walk around, you can go downstairs, you can

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Speaker 1: go upstairs, you can take off from the Earth, you

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Speaker 1: can go to Mars, whatever. There's always somewhere to go.

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Speaker 1: So your path is infinitely extendable. But what if there

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Speaker 1: was a place where a path couldn't continue, where like

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Speaker 1: it just ran into a dead end, like a boundary

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Speaker 1: or an endpoint. A great example of this is the

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Speaker 1: heart of a black hole. Inside a black hole, space

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Speaker 1: is curved and there's only one direction of space. It

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Speaker 1: just goes towards the center. And so your path will

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Speaker 1: continue towards the heart of the black hole, and that's it.

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Speaker 1: It just ends there. You'll be there forever at the

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Speaker 1: heart of the black hole. So that path is not extendable.

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Speaker 1: And so that's another way to think about a singularity.

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Speaker 1: It doesn't have to be in conflict, but it starts

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Speaker 1: from a different definition and so sort of leads you

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Speaker 1: in a different direction conceptionally.

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Speaker 2: So like, how generalizable is that use of the word singularity?

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Speaker 2: Like I get lost when I'm driving all the time.

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Speaker 2: So when I go down a wrong road and I

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Speaker 2: hit a dead end, can I turn around to my

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Speaker 2: passengers and be like singularity or is that? Is that

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Speaker 2: the incorrect usage of this word.

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Speaker 1: Yeah, you can't blame math for the reason you get lost, Kelly. Sorry, Unfortunately,

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Speaker 1: you know math is not that useful. It needs to

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Speaker 1: really be a non extendable path, you know, partner space

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Speaker 1: where you just cannot continue.

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Speaker 2: All right, So you hinted that we're going to get

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Speaker 2: into black holes again, which I think is maybe one

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Speaker 2: of the top topics that the extraordinaries ask us about.

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Speaker 2: Nobody ever gets tired of hearing about black holes. So

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Speaker 2: tell me about a black hole singularity. No, wait, first,

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Speaker 2: remind me what a black hole is.

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Speaker 1: Yeah, So to understand what a black hole singularity is,

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Speaker 1: we have to remind ourselves how black holes work. And

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Speaker 1: remember that black holes have very powerful gravity, so powerful

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Speaker 1: that not even light can escape. They have an event

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Speaker 1: horizon beyond which nothing can return. But you need to

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Speaker 1: unshackle yourself from you intuitive sense of like Newtonian gravity,

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Speaker 1: where gravity is pulling on stuff, where black holes are

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Speaker 1: like huge vacuum cleaners sucking on things because force. It's

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Speaker 1: not the right way to think about gravity, because gravity

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Speaker 1: is not a force and also doesn't work for black holes.

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Speaker 1: Like black holes, we know can bend the path of

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Speaker 1: light and can trap light, but light has no mass.

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Speaker 1: And then Newtonian gravity the gravitational forces between two objects

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Speaker 1: with mass, and so photons shouldn't feel black holes in

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Speaker 1: Newtonian gravity, but they do in the universe, so we

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Speaker 1: know Newtonian gravity is wrong. Actually had a list are

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Speaker 1: writing last week to correct me and say, actually, photons

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Speaker 1: do have mass. We know that because they feel gravity

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Speaker 1: because they get sucked into black holes. And it was like,

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Speaker 1: interesting argument. Let's unwind this to see where you went wrong.

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Speaker 2: Anyway, And I remember that email. They titled it a

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Speaker 2: massive correction. Yeah, that was great.

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Speaker 1: They definitely had a lot of gravitas in their email.

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Speaker 1: I welcome it. Please if you think I made a mistake,

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Speaker 1: right to me, I'm pretty sure I'm right on this

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Speaker 1: one that photons do not have mass. And the right

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Speaker 1: way to think about black holes is not in terms

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Speaker 1: of a force, but in terms of the curvature of space. Right, So,

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Speaker 1: mass bends space, which means it changes the relationships between

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Speaker 1: objects their distances. This kind of gravitational curvature is very

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Speaker 1: tricky to think about. I think about it in terms

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Speaker 1: of relative distances between pairs of objects. And if you

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Speaker 1: like throw a bunch of ping pong balls into space,

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Speaker 1: and then you measure all the distances between those ping

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Speaker 1: pong balls, and you're like, this one is for me

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Speaker 1: to from that one, this one is six meters from

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Speaker 1: that one. Then you have all these like pair wise distances.

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Speaker 1: You have all your ping pong balls, and you know

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Speaker 1: the distances between all of them. Right now, the way

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Speaker 1: to think about curvature is to think about could you

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Speaker 1: actually like put meter sticks between all of those ping

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Speaker 1: pong balls and have it all line up. If you can,

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Speaker 1: then space is flat. You've measured those spaces and you

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Speaker 1: have those meter sticks, and everything sort of cooks together.

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Speaker 1: But if you can't, if you can't do that with

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Speaker 1: like straight meter sticks, then it's because the relationships between

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Speaker 1: the points have become distorted. Because space is curved. You

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Speaker 1: can't see the curvature of space directly, but you can

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Speaker 1: measure it by like putting stuff in space and measuring

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Speaker 1: the distances between them and see, like do they click together?

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Speaker 1: Can I describe that without gravitational curvature?

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Speaker 2: Okay, to be honest, I always have trouble with this

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Speaker 2: idea of space being curved. But would it be similar

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Speaker 2: to say, like, all right, so an object isn't going

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Speaker 2: into a black hole because a black hole is pulling

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Speaker 2: it in. It's going into a black because space is

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Speaker 2: curved in such a way that it's directing it into

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Speaker 2: the black hole. And that that's a little bit different.

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Speaker 1: Yes, So now we're talking about the consequences of the curvature.

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Speaker 1: So if you have the idea of a curvature in

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Speaker 1: your head, now what happens to stuff?

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Speaker 3: Now?

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Speaker 1: The space is curved. Is that the concept of like

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Speaker 1: moving in a straight line is different. Your intuition about

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Speaker 1: a straight line assumes flat space. You're like, I'm going

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Speaker 1: to go from here to there. I'm just going to

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Speaker 1: go from here to there straight line, a shine of flashlight.

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Speaker 1: It's going to go from here to there in a

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Speaker 1: straight line. But what is a straight line in curved

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Speaker 1: space is following the curvature of space itself. And so

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Speaker 1: if you just have an object in free fall, it's

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Speaker 1: moving under the influence only of the curvature of space.

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Speaker 1: It's going to follow the curvature of space. And so

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Speaker 1: if space is flat, it's just going to drift the

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Speaker 1: way you would normally into it. But if it isn't,

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Speaker 1: then it's going to bend. It's going to follow that

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Speaker 1: invisible curvature of space. And so the Earth goes around

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Speaker 1: the Sun because it's following that curvature, and things fall

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Speaker 1: into the black hole. Yes, Kelly, because they're following that

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Speaker 1: curvature of space.

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Speaker 2: And it does gravity make that curvature or these two

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Speaker 2: different things.

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Speaker 1: So gravity sort of is that curvature. Right. Gravity is

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Speaker 1: not a force, it's just the effect of that curvature.

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Speaker 1: We can't see the curvature, but we see the curvature's

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Speaker 1: effect on stuff, and we call that gravity. And we

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Speaker 1: know it's not a force because like when you jump

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Speaker 1: off a building, you are feeling no force, no acceleration.

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Speaker 1: You're moving towards the center of the Earth. We say,

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Speaker 1: under the influence of gravity. But if you like held

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Speaker 1: up a scale underneath you, you would feel no acceleration. There's

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Speaker 1: no force happening to you there. It's not like a

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Speaker 1: chemical rocket. If you fire a chemical rocket, then you

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Speaker 1: definitely feel that acceleration. Right, If you like hit the

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Speaker 1: brakes in your car, that's acceleration you feel. Those Those

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Speaker 1: are real forces. Gravity is not a real force because

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Speaker 1: it doesn't create those feelings of acceleration. You're just naturally

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Speaker 1: following the curvature of space, which means something amazing. If

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Speaker 1: you're falling into a black hole, you don't feel any gravity,

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Speaker 1: but you're dying, especially if you let Kelly do the driving.

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Speaker 2: Oh all right, all right, you know what, I need

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Speaker 2: some time to recover from that insult. So let's take

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Speaker 2: a break and when we get back we'll talk more

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Speaker 2: about this concept of black hole singularities. All right, I've

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Speaker 2: recovered from the wicked burn Daniel gave me regarding my

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Speaker 2: driving skills.

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Speaker 1: You're the one who said you're driv into singularities.

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Speaker 2: I said, well, mate, what happened to?

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Speaker 3: Yeah?

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Speaker 2: Maybe I did? All right, you got me? All right,

411
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Speaker 2: tell me about black hole singularities. Let's stop talking about

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Speaker 2: my driving record.

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Speaker 1: All right. So we have this concept of spatial curvature

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Speaker 1: and it explains the effect we call gravity. And so

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Speaker 1: what happens near massive objects is things move towards those

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Speaker 1: massive objects because that's where the curvature of space is.

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Speaker 1: And a black hole is an example of that curvature,

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Speaker 1: growing so powerful that life becomes trapped inside that curvature

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Speaker 1: because the direction of space is just one d inside

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Speaker 1: that black hole. Beyond the event horizon, space is curved

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Speaker 1: so much it only points towards the core, and at

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Speaker 1: the core, according to the calculations of general relativity, that

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Speaker 1: curvature becomes infinite. Right, So the curvature of space itself

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Speaker 1: becomes infinite. So like, what does that mean? Well, if

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Speaker 1: you have two points in the opposite sides of a

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Speaker 1: black hole, what is the distance between them? If it's

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Speaker 1: going to pass through the singularity. That distance is infinite.

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Speaker 2: So is this a case where the equations are breaking

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Speaker 2: down or is this like actually what we expect will happen.

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Speaker 1: Yeah, that's a great question. My suspicion is that the

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Speaker 1: equations are breaking down because what we've done here is

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Speaker 1: extrapolate general relativity beyond the point of its relevance. We're

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Speaker 1: doing a calculation in general relativity and we're ignoring quantum mechanics.

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Speaker 1: And most of the time you can do that. When

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Speaker 1: you're calculating how two black holes are going to fall

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00:21:54,240 --> 00:21:56,520
Speaker 1: into each other, or how Jupiter moves around the Sun,

437
00:21:56,760 --> 00:21:59,159
Speaker 1: you can ignore quantum mechanics. It doesn't really matter the

438
00:21:59,200 --> 00:22:01,640
Speaker 1: same way you can or gravity when you're doing quantum

439
00:22:01,640 --> 00:22:05,280
Speaker 1: mechanical calculations on individual particles. But when you're talking about

440
00:22:05,359 --> 00:22:08,440
Speaker 1: super tiny, dense objects like the heart of a black hole,

441
00:22:08,600 --> 00:22:12,320
Speaker 1: quantum mechanics matters. Again. Quantum mechanics says that that prediction

442
00:22:12,440 --> 00:22:14,360
Speaker 1: is wrong. It can't happen. You can't have that much

443
00:22:14,440 --> 00:22:17,480
Speaker 1: mass isolated with that much energy in that small space.

444
00:22:17,480 --> 00:22:20,440
Speaker 1: It violates the Heisenberg and certainty principle. It definitely would

445
00:22:20,520 --> 00:22:22,719
Speaker 1: not happen. So what you've done here is take in

446
00:22:22,760 --> 00:22:25,760
Speaker 1: general relativity, ignore quantum mechanics, but done it in a

447
00:22:25,800 --> 00:22:28,840
Speaker 1: region where quant mechanics can't be ignored. So it's in

448
00:22:28,840 --> 00:22:31,360
Speaker 1: that sense kind of a nonsense prediction. It's also sort

449
00:22:31,359 --> 00:22:34,160
Speaker 1: of a useful diagnostic to say, like, well, is general

450
00:22:34,200 --> 00:22:37,600
Speaker 1: relativity valid here? I think this is the universe telling

451
00:22:37,680 --> 00:22:41,200
Speaker 1: us no. Quantum mechanics says it can't happen. General relativity

452
00:22:41,200 --> 00:22:43,560
Speaker 1: says it does happen. We don't have a great theory

453
00:22:43,560 --> 00:22:46,600
Speaker 1: of quantum gravity to make these calculations, and so it's

454
00:22:46,640 --> 00:22:47,600
Speaker 1: sort of a question mark.

455
00:22:47,880 --> 00:22:51,639
Speaker 2: Is this level of nuance conveyed in like pop culture

456
00:22:51,680 --> 00:22:52,879
Speaker 2: treatments of black holes?

457
00:22:53,119 --> 00:22:53,639
Speaker 1: Or is it?

458
00:22:54,000 --> 00:22:57,000
Speaker 2: How is this usually conveyed in pop science?

459
00:22:57,320 --> 00:22:59,600
Speaker 1: Most of the popular science treatments just say that there's

460
00:22:59,600 --> 00:23:01,320
Speaker 1: a single already at the hearts of the black holes,

461
00:23:01,440 --> 00:23:04,199
Speaker 1: but scientists don't understand it. I think more often we

462
00:23:04,200 --> 00:23:07,159
Speaker 1: should underline that this is a prediction of a classical theory,

463
00:23:07,200 --> 00:23:10,119
Speaker 1: and we already know that classical theory is limited and

464
00:23:10,160 --> 00:23:12,560
Speaker 1: that we shouldn't trust its predictions. So doesn't mean you

465
00:23:12,560 --> 00:23:14,920
Speaker 1: shouldn't make those predictions and use them again, like as

466
00:23:14,960 --> 00:23:19,399
Speaker 1: a diagnostic. But I think some of that nuance is missing, Yeah.

467
00:23:19,160 --> 00:23:22,440
Speaker 2: Okay, and what do experiments tell us about whether or

468
00:23:22,520 --> 00:23:25,040
Speaker 2: not this is where things are breaking down or if

469
00:23:25,040 --> 00:23:26,440
Speaker 2: we're really describing the universe.

470
00:23:26,800 --> 00:23:30,199
Speaker 1: Experiments have been really influential in our understanding of black holes. Originally,

471
00:23:30,520 --> 00:23:32,760
Speaker 1: this whole idea, people thought, well, it's ridiculous, there's no

472
00:23:32,840 --> 00:23:35,200
Speaker 1: way the universe is going to let a black hole form,

473
00:23:35,480 --> 00:23:37,720
Speaker 1: not to mention, is there a singularity? And it's hard,

474
00:23:37,800 --> 00:23:40,560
Speaker 1: but do black holes exist at all? For a long time,

475
00:23:40,600 --> 00:23:42,439
Speaker 1: they were just like a curiosity of the math, and

476
00:23:42,440 --> 00:23:44,480
Speaker 1: people are like, well, this is definitely a sign that

477
00:23:44,560 --> 00:23:46,840
Speaker 1: something is wrong and the universe would never let this happen.

478
00:23:47,080 --> 00:23:49,040
Speaker 1: But then we found things that look a lot like

479
00:23:49,119 --> 00:23:51,800
Speaker 1: black holes that are very very dense and very very

480
00:23:51,840 --> 00:23:56,080
Speaker 1: massive and have incredibly powerful gravity on the things near them.

481
00:23:56,440 --> 00:23:58,520
Speaker 1: We see them in the hearts of galaxies, We see

482
00:23:58,520 --> 00:24:01,640
Speaker 1: them where stars have colapsed. Do we know that these

483
00:24:01,640 --> 00:24:04,040
Speaker 1: are actually black holes that have an event horizon and

484
00:24:04,080 --> 00:24:05,600
Speaker 1: potentially singularity within them?

485
00:24:06,080 --> 00:24:06,159
Speaker 4: Not.

486
00:24:06,440 --> 00:24:09,920
Speaker 1: Technically, we have only indirect evidence that these things are

487
00:24:09,920 --> 00:24:12,360
Speaker 1: black holes, in that there's nothing else in our physics

488
00:24:12,480 --> 00:24:15,440
Speaker 1: that is so massive and so dense and so compact

489
00:24:15,520 --> 00:24:18,000
Speaker 1: all at the same time. They can't be neutron stars,

490
00:24:18,000 --> 00:24:20,240
Speaker 1: they can't be white dwarfs, they can't be anything else

491
00:24:20,280 --> 00:24:22,639
Speaker 1: in our physics. So the only thing left in the

492
00:24:22,680 --> 00:24:25,200
Speaker 1: box is a black hole. But you know, there are

493
00:24:25,200 --> 00:24:27,800
Speaker 1: other ideas out there of things that they could be.

494
00:24:28,080 --> 00:24:31,760
Speaker 1: String theory predicts things like fuzzballs. There are other quantum

495
00:24:31,840 --> 00:24:35,280
Speaker 1: versions of these predictions. So we have only indirect evidence

496
00:24:35,280 --> 00:24:38,399
Speaker 1: that black holes actually exist, even though they're treated as

497
00:24:38,480 --> 00:24:41,200
Speaker 1: like definitely existing and having been observed in the sort

498
00:24:41,200 --> 00:24:42,560
Speaker 1: of broad popular literature.

499
00:24:42,720 --> 00:24:45,400
Speaker 2: Yeah, and I really enjoyed our recent conversation about Bechdel

500
00:24:45,480 --> 00:24:48,520
Speaker 2: stars and black holes are confusing, but we're we're learning

501
00:24:48,520 --> 00:24:49,240
Speaker 2: more all the time.

502
00:24:49,400 --> 00:24:51,679
Speaker 1: Yeah, and that's not a criticism of astronomers, like it's

503
00:24:51,720 --> 00:24:54,480
Speaker 1: amazing what they've found. And you know, seeing black holes

504
00:24:54,560 --> 00:24:56,680
Speaker 1: is hard. And even the event horizon telescope, which just

505
00:24:56,720 --> 00:24:59,480
Speaker 1: image the accretion disk around the black holes, tells us

506
00:24:59,520 --> 00:25:03,880
Speaker 1: a lot, but isn't direct observation of the event horizon itself. Right.

507
00:25:04,160 --> 00:25:09,119
Speaker 2: Okay, so we've talked about black hole singularities, right, are

508
00:25:09,200 --> 00:25:12,399
Speaker 2: there other kinds of gravitational singularities to talk about?

509
00:25:12,800 --> 00:25:14,920
Speaker 1: Well, all the singularities we're going to talk about which

510
00:25:14,960 --> 00:25:18,800
Speaker 1: makes me think of Beyonce. The singularity are at the

511
00:25:18,840 --> 00:25:22,399
Speaker 1: hearts of various kinds of black holes, okay, and so

512
00:25:22,720 --> 00:25:26,679
Speaker 1: less catalog those the sort of vanilla singularity, and the

513
00:25:26,760 --> 00:25:29,360
Speaker 1: classic one is at the heart of a black hole

514
00:25:29,400 --> 00:25:32,600
Speaker 1: that's very simple. It's spherical, it has a lot of mass,

515
00:25:32,800 --> 00:25:35,480
Speaker 1: it has no spin or no charge. Remember that the

516
00:25:35,600 --> 00:25:38,080
Speaker 1: kinds of things a black hole can have, according to

517
00:25:38,119 --> 00:25:41,240
Speaker 1: general relativity, are just three. There's only three things you

518
00:25:41,240 --> 00:25:44,280
Speaker 1: can know about a black hole. It's mass, it's spin,

519
00:25:44,680 --> 00:25:47,040
Speaker 1: and it's charge. You can't know anything about how those

520
00:25:47,080 --> 00:25:49,920
Speaker 1: things are arranged within the event horizon, and you can't

521
00:25:49,960 --> 00:25:53,840
Speaker 1: measure any other physical properties. So those three things completely

522
00:25:53,880 --> 00:25:58,040
Speaker 1: determine the black hole, again according to classical general relativity.

523
00:25:58,400 --> 00:26:00,720
Speaker 1: So if you have the simplest kind, which use a

524
00:26:00,760 --> 00:26:03,679
Speaker 1: short style black hole, just a really massive stuff with

525
00:26:03,720 --> 00:26:06,239
Speaker 1: no spin and no electric charge, then you get an

526
00:26:06,240 --> 00:26:09,240
Speaker 1: event horizon that's spherically symmetric, and at its heart is

527
00:26:09,320 --> 00:26:13,560
Speaker 1: that point that's singularity, the location of infinite density or

528
00:26:13,560 --> 00:26:17,399
Speaker 1: infinite curvature, and that's the sort of the nilla classic singularity.

529
00:26:17,440 --> 00:26:19,600
Speaker 1: I think most people think about when they think about

530
00:26:19,600 --> 00:26:21,040
Speaker 1: black holes and singularities.

531
00:26:21,320 --> 00:26:23,720
Speaker 2: Okay, sorry, I'm still absorbing. Okay, So these are all

532
00:26:23,800 --> 00:26:26,560
Speaker 2: examples of stuff at the middle of a black hole

533
00:26:26,720 --> 00:26:28,760
Speaker 2: and just differences in the way that we think about them.

534
00:26:29,000 --> 00:26:32,360
Speaker 1: Yeah, exactly. And these characteristics of the black hole determine

535
00:26:32,359 --> 00:26:34,399
Speaker 1: what's happening and ats the core. So if you just

536
00:26:34,440 --> 00:26:36,880
Speaker 1: think about it, a lot of stuff, a star collapsing

537
00:26:36,880 --> 00:26:39,360
Speaker 1: into a very dense object, that's a short style black hole.

538
00:26:39,400 --> 00:26:42,280
Speaker 1: And that was the first solution to Einstein's equations ever

539
00:26:42,320 --> 00:26:45,760
Speaker 1: found to describe a black hole. No spin, no charge,

540
00:26:46,320 --> 00:26:46,959
Speaker 1: just mass.

541
00:26:47,160 --> 00:26:47,520
Speaker 3: All right.

542
00:26:47,560 --> 00:26:49,439
Speaker 2: I think it would be pretty awesome to have a

543
00:26:49,440 --> 00:26:52,360
Speaker 2: type of black hole named after yourself, but that's probably

544
00:26:52,400 --> 00:26:54,199
Speaker 2: not going to happen for me. But all right, so

545
00:26:54,240 --> 00:26:57,440
Speaker 2: we've got swar style, if I could say it, we've

546
00:26:57,440 --> 00:26:59,320
Speaker 2: got the first kind of black hole we talked about

547
00:27:00,200 --> 00:27:02,560
Speaker 2: what what? Okay, so we're going to be looking at

548
00:27:03,000 --> 00:27:06,960
Speaker 2: variations on mass, spin, in charge. Our next kind has

549
00:27:07,080 --> 00:27:08,000
Speaker 2: what spin?

550
00:27:08,280 --> 00:27:11,920
Speaker 1: Right, It's very unlikely that something collapses into a black

551
00:27:11,920 --> 00:27:15,480
Speaker 1: hole and isn't spinning. Why because everything in the universe

552
00:27:15,520 --> 00:27:17,439
Speaker 1: is spinning. All the stars out there are spinning, all

553
00:27:17,480 --> 00:27:20,159
Speaker 1: the galaxies out there are spinning. Everything is spinning, and

554
00:27:20,160 --> 00:27:21,960
Speaker 1: The reason everything is spinning is the reason we have

555
00:27:21,960 --> 00:27:25,480
Speaker 1: any structure at all. You started with vast clouds of hydrogen,

556
00:27:25,840 --> 00:27:27,960
Speaker 1: but then you had little regions of over densities and

557
00:27:28,000 --> 00:27:31,120
Speaker 1: underd densities. With gravity took hold and gathered stuff together

558
00:27:31,160 --> 00:27:33,919
Speaker 1: and collapsed it and overall in the universe we think

559
00:27:33,920 --> 00:27:35,880
Speaker 1: there's probably no spin, but if you take a random

560
00:27:35,960 --> 00:27:38,520
Speaker 1: region of the universe, there's going to be, on average

561
00:27:38,520 --> 00:27:41,800
Speaker 1: a little bit of spin. It's like taking a million

562
00:27:42,000 --> 00:27:44,560
Speaker 1: coins and flipping them and saying, do you get exactly

563
00:27:44,600 --> 00:27:47,000
Speaker 1: fifty percent heads? Now you get a little bit extra

564
00:27:47,040 --> 00:27:49,320
Speaker 1: tail sometimes or a little bit extra heads another time.

565
00:27:49,880 --> 00:27:52,760
Speaker 1: To randomly chop the universe up into chunks, each one's

566
00:27:52,760 --> 00:27:54,320
Speaker 1: going to have a little bit of spin. They all

567
00:27:54,320 --> 00:27:56,320
Speaker 1: add up to zero, but each one's going to have

568
00:27:56,520 --> 00:27:58,520
Speaker 1: a little bit of positive a little bit of negative spin.

569
00:27:58,560 --> 00:28:01,760
Speaker 1: It's not impossible to have zero, but most likely it'll

570
00:28:01,760 --> 00:28:04,520
Speaker 1: have non zero spin. And then that collapses into stars

571
00:28:04,760 --> 00:28:07,760
Speaker 1: and gets exaggerated because as things collapse, they spin faster,

572
00:28:08,200 --> 00:28:11,399
Speaker 1: like a figure skater pulling in her arms. So if

573
00:28:11,440 --> 00:28:14,520
Speaker 1: you have a star, it's most likely spinning when it collapses.

574
00:28:14,800 --> 00:28:17,560
Speaker 1: Where does that spin go, Well, it can't just go away.

575
00:28:17,960 --> 00:28:20,840
Speaker 1: Spin is conserved in the universe, and so a spinning

576
00:28:20,880 --> 00:28:24,200
Speaker 1: star has to turn into a spinning black hole. So

577
00:28:24,560 --> 00:28:25,920
Speaker 1: I think that most of the black holes in the

578
00:28:26,000 --> 00:28:29,640
Speaker 1: universe are not schwartzy black holes. They're curve black holes.

579
00:28:30,040 --> 00:28:31,160
Speaker 1: They have mass and.

580
00:28:31,359 --> 00:28:34,199
Speaker 2: Spin, and so they're called curve black holes.

581
00:28:34,359 --> 00:28:38,040
Speaker 1: curR kerr from the New Zealand physicists who came up

582
00:28:38,080 --> 00:28:38,360
Speaker 1: with them.

583
00:28:38,680 --> 00:28:42,280
Speaker 2: Ah, all right, curve black holes. Okay, so you wrote ringularity.

584
00:28:42,320 --> 00:28:43,640
Speaker 2: What does ringularity mean?

585
00:28:44,040 --> 00:28:46,120
Speaker 1: Yeah? So what is at the heart of a spinning

586
00:28:46,160 --> 00:28:50,160
Speaker 1: black hole? Well, it can't be a singularity. Why not

587
00:28:50,600 --> 00:28:55,520
Speaker 1: because a singularity a point can't have spin. Oh right,

588
00:28:55,560 --> 00:28:58,280
Speaker 1: If you have something which literally has no extent, then

589
00:28:58,520 --> 00:29:02,040
Speaker 1: it can't spin. There's nothing to spin. And so what's

590
00:29:02,080 --> 00:29:03,680
Speaker 1: at the heart? How do you make something which is

591
00:29:03,720 --> 00:29:07,160
Speaker 1: infinitely dense and still can spin. The prediction is at

592
00:29:07,200 --> 00:29:09,840
Speaker 1: the heart of a spinning black hole is not a

593
00:29:09,920 --> 00:29:12,560
Speaker 1: singularity but a ring gularity.

594
00:29:13,120 --> 00:29:14,560
Speaker 2: Love it? Did you cop with that term?

595
00:29:14,680 --> 00:29:16,080
Speaker 1: I did not come up with that term, but I

596
00:29:16,160 --> 00:29:18,120
Speaker 1: love it so much I'm gonna say that at every opportunity,

597
00:29:18,400 --> 00:29:20,840
Speaker 1: imagine a ring like literally like the kind of ring

598
00:29:20,880 --> 00:29:23,800
Speaker 1: around your finger and it's spinning, so it's a circle

599
00:29:23,880 --> 00:29:27,480
Speaker 1: of infinite density, and it carries that angular momentum and

600
00:29:27,520 --> 00:29:30,560
Speaker 1: it can't collapse into a point like singularity because it

601
00:29:30,560 --> 00:29:32,520
Speaker 1: can't get rid of its angular momentum.

602
00:29:32,640 --> 00:29:36,600
Speaker 2: Okay, so we've talked about how you almost certainly have spin,

603
00:29:37,600 --> 00:29:39,440
Speaker 2: and it feels to me like you'd almost certainly have

604
00:29:39,640 --> 00:29:41,920
Speaker 2: charge because what is the probability that all the charges

605
00:29:41,960 --> 00:29:46,120
Speaker 2: cancel out? And so we determined that a schwarz Child

606
00:29:46,200 --> 00:29:49,520
Speaker 2: black hole is probably less likely than a cur black hole.

607
00:29:51,520 --> 00:29:54,479
Speaker 2: Is a curb black hole also less likely than whatever

608
00:29:54,560 --> 00:29:58,880
Speaker 2: black hole we're gonna probably talk about that also has charge.

609
00:29:59,440 --> 00:30:01,920
Speaker 1: Yes, I think that most black holes in the universe

610
00:30:01,960 --> 00:30:05,920
Speaker 1: probably have mass and spin and charge. And the effect

611
00:30:05,960 --> 00:30:08,280
Speaker 1: of having charge is very similar to the effect of

612
00:30:08,320 --> 00:30:12,520
Speaker 1: having spin. It changes where the event horizon is. So

613
00:30:12,640 --> 00:30:15,360
Speaker 1: number one, if you have spin, you have a ringularity

614
00:30:15,400 --> 00:30:17,440
Speaker 1: instead of a singularity. If you add charge, then you

615
00:30:17,480 --> 00:30:20,959
Speaker 1: get a charged ringularity. It doesn't change the structure of

616
00:30:21,000 --> 00:30:25,160
Speaker 1: the singularity, because even a point could carry charge. Having

617
00:30:25,240 --> 00:30:28,120
Speaker 1: charge doesn't determine the structure of the singularity.

618
00:30:28,280 --> 00:30:30,560
Speaker 2: Sometimes you all do get naming right, good job.

619
00:30:33,280 --> 00:30:35,560
Speaker 1: But the spin and the charge do have a really

620
00:30:35,600 --> 00:30:38,520
Speaker 1: interesting effect on the black hole. The spin forces it

621
00:30:38,520 --> 00:30:42,320
Speaker 1: into aringularity instead of singularity, but it also changes where

622
00:30:42,360 --> 00:30:46,479
Speaker 1: the event horizon is. So as you speed up a

623
00:30:46,480 --> 00:30:49,200
Speaker 1: black hole, you spin it faster and faster, the event

624
00:30:49,240 --> 00:30:53,200
Speaker 1: horizon actually shrinks. Or as you add electric charge to

625
00:30:53,240 --> 00:30:56,560
Speaker 1: the black hole, the event horizon actually shrinks, which creates

626
00:30:56,600 --> 00:31:00,160
Speaker 1: a maximum spin or a maximum charge to a black hole.

627
00:31:00,560 --> 00:31:03,160
Speaker 1: Because you can spin the black hole so fast, the

628
00:31:03,200 --> 00:31:06,160
Speaker 1: event horizon shrinks to zero, and then boom, you get

629
00:31:06,200 --> 00:31:09,400
Speaker 1: what's called a naked singularity, which you talk about a

630
00:31:09,440 --> 00:31:12,240
Speaker 1: little bit more in a minute, a singularity that's not

631
00:31:12,520 --> 00:31:15,320
Speaker 1: behind an event horizon, a singularity you could actually like

632
00:31:15,520 --> 00:31:18,040
Speaker 1: stare at, though it would be rude. Please don't ogle

633
00:31:18,120 --> 00:31:19,360
Speaker 1: our naked singularity.

634
00:31:20,240 --> 00:31:23,080
Speaker 2: Wait, could you escape from a naked singularity if there's

635
00:31:23,080 --> 00:31:26,400
Speaker 2: no event horizon or my misunderstanding event horizons.

636
00:31:26,120 --> 00:31:29,840
Speaker 1: No, you're exactly correctly understanding the consequences of not having

637
00:31:29,880 --> 00:31:31,640
Speaker 1: an event horizon. Absolutely.

638
00:31:31,800 --> 00:31:32,480
Speaker 2: Whow cool.

639
00:31:32,680 --> 00:31:35,120
Speaker 1: The other fascinating thing about spinning black holes is to

640
00:31:35,160 --> 00:31:38,200
Speaker 1: have much more complex space time than people typically think about.

641
00:31:38,640 --> 00:31:41,200
Speaker 1: You think about the space time near a short sized singularity,

642
00:31:41,200 --> 00:31:43,800
Speaker 1: there's an event horizon, things fall in. That's the end

643
00:31:43,800 --> 00:31:46,600
Speaker 1: of the story. But a spinning black hole has much

644
00:31:46,600 --> 00:31:49,400
Speaker 1: more complex space time. It doesn't just have an event horizon.

645
00:31:49,880 --> 00:31:53,160
Speaker 1: It has another horizon within the black hole called a

646
00:31:53,240 --> 00:31:56,080
Speaker 1: Cauchhi horizon that we'll talk about later. And then outside

647
00:31:56,120 --> 00:31:59,800
Speaker 1: the event horizon there's a region called the ergosphere where

648
00:32:00,360 --> 00:32:04,160
Speaker 1: itself is spun. Because the black hole is spinning, there's

649
00:32:04,160 --> 00:32:07,720
Speaker 1: an effect in general relativity called frame dragging. So Einstein's

650
00:32:07,720 --> 00:32:11,040
Speaker 1: general relativity is not just like Newton's gravity, but you're

651
00:32:11,080 --> 00:32:15,040
Speaker 1: replacing the force with curvature. It predicts different stuff. So,

652
00:32:15,080 --> 00:32:19,440
Speaker 1: for example, a spinning object in Einstein's theory has different

653
00:32:19,480 --> 00:32:22,880
Speaker 1: gravity than a non spinning object. Whereas like Newton says

654
00:32:22,880 --> 00:32:25,240
Speaker 1: it doesn't matter, like if the Earth is spinning or

655
00:32:25,320 --> 00:32:29,600
Speaker 1: not spinning, Newton predicts exactly the same force. Einstein says, no, no,

656
00:32:29,680 --> 00:32:33,200
Speaker 1: that spin matters. It contributes to the gravitational energy, and

657
00:32:33,240 --> 00:32:35,600
Speaker 1: not in a simple way. You have an object near

658
00:32:35,680 --> 00:32:38,840
Speaker 1: spinning black hole, it will pick up a spin right,

659
00:32:38,920 --> 00:32:41,720
Speaker 1: So the curvature doesn't just pull things in, it can

660
00:32:41,760 --> 00:32:45,040
Speaker 1: also spin things. We have these satellites around the Earth

661
00:32:45,080 --> 00:32:48,720
Speaker 1: called gravity pro b which measured these very small effects

662
00:32:48,960 --> 00:32:52,480
Speaker 1: with incredible detail using these quartz balls that are the

663
00:32:52,520 --> 00:32:56,520
Speaker 1: most spherical thing humanity has ever constructed. They have this

664
00:32:56,560 --> 00:32:59,440
Speaker 1: incredible process where they create these things in Germany, they

665
00:32:59,480 --> 00:33:02,440
Speaker 1: send them to our Gentina for this careful polishing and

666
00:33:02,480 --> 00:33:06,760
Speaker 1: they're like incredibly spherical and they're spinning in gyroscopes in

667
00:33:06,800 --> 00:33:09,000
Speaker 1: space to measure the effect of the Earth's spin, the

668
00:33:09,000 --> 00:33:12,280
Speaker 1: gravitational effect of the Earth spin on these walls. Really

669
00:33:12,320 --> 00:33:13,560
Speaker 1: an incredible experiment.

670
00:33:13,680 --> 00:33:16,080
Speaker 2: I love that multiple nations were needed to make that happen.

671
00:33:16,160 --> 00:33:16,840
Speaker 2: That's wonderful.

672
00:33:19,360 --> 00:33:22,160
Speaker 1: I'm imagining some like old lady in Argentina with like

673
00:33:22,160 --> 00:33:24,440
Speaker 1: a really smooth cloth and she's like rubbing it and

674
00:33:24,520 --> 00:33:25,560
Speaker 1: rubbing it, rubbing.

675
00:33:25,280 --> 00:33:27,440
Speaker 2: It, and she's the best smoother in the world. So

676
00:33:27,480 --> 00:33:29,680
Speaker 2: it had to go to Argentina exactly.

677
00:33:30,200 --> 00:33:32,760
Speaker 1: You know, there are really bespoke processes, like also for

678
00:33:32,880 --> 00:33:36,480
Speaker 1: developing the massive lenses needed for telescopes. There's like a

679
00:33:36,520 --> 00:33:38,800
Speaker 1: few people who like know how to do this and

680
00:33:38,840 --> 00:33:41,960
Speaker 1: cook it just right, and like it's still an art anyway.

681
00:33:42,680 --> 00:33:45,160
Speaker 1: Outside the event horizon is the ergosphere and you can

682
00:33:45,200 --> 00:33:47,960
Speaker 1: do cool stuff like drop stuff into the ergosphere and

683
00:33:48,000 --> 00:33:49,760
Speaker 1: you'll pick up a spin and then come back out,

684
00:33:50,120 --> 00:33:52,800
Speaker 1: and so you can extract energy from the black hole

685
00:33:52,840 --> 00:33:53,240
Speaker 1: this way.

686
00:33:53,560 --> 00:33:55,960
Speaker 2: Oh oh man, So if we were closer to a

687
00:33:56,000 --> 00:33:57,920
Speaker 2: black hole, we could have like free energy.

688
00:33:58,200 --> 00:34:00,960
Speaker 1: Yeah, this is called the Penrose process, like slowing down

689
00:34:01,000 --> 00:34:03,200
Speaker 1: the spin of a black hole by stealing its spin.

690
00:34:03,840 --> 00:34:09,000
Speaker 2: Absolutely, something about that feels unsafe.

691
00:34:09,600 --> 00:34:11,360
Speaker 1: Again, don't let Kelly drive.

692
00:34:11,320 --> 00:34:15,160
Speaker 2: Nope, nope, nope, or extracts the energy from a black hole.

693
00:34:15,280 --> 00:34:17,720
Speaker 1: But much more interesting than what's outside the black holes,

694
00:34:17,719 --> 00:34:20,319
Speaker 1: of course, is what's inside the black hole. Let's talk

695
00:34:20,320 --> 00:34:22,920
Speaker 1: about a naked singularity. To me, this is like one

696
00:34:22,920 --> 00:34:26,160
Speaker 1: of the most tantilizing ideas in physics because seeing the

697
00:34:26,200 --> 00:34:29,200
Speaker 1: singularity would tell us so much about the universe. Right

698
00:34:29,239 --> 00:34:31,960
Speaker 1: we talked earlier, like gravity tells us it should be

699
00:34:31,960 --> 00:34:35,160
Speaker 1: a point or a ring. Quantum mechanics says, no, that's impossible.

700
00:34:35,600 --> 00:34:38,560
Speaker 1: But doing those calculations is hard. People have tried it.

701
00:34:38,600 --> 00:34:41,319
Speaker 1: They predict x, they predict why. What's really there? If

702
00:34:41,360 --> 00:34:43,319
Speaker 1: we could just see it, If the universe could just

703
00:34:43,360 --> 00:34:46,800
Speaker 1: tell us what's going on, that would be such valuable

704
00:34:46,800 --> 00:34:49,440
Speaker 1: insight and guide us in forming a deeper theory in

705
00:34:49,480 --> 00:34:52,400
Speaker 1: physics and understanding the whole nature of reality and the universe,

706
00:34:52,840 --> 00:34:56,080
Speaker 1: so seeing the singularity would be incredibly powerful.

707
00:34:56,280 --> 00:34:57,719
Speaker 2: And when we come back, we're going to talk about

708
00:34:57,719 --> 00:34:59,600
Speaker 2: how to take the clothes off a black hole so

709
00:34:59,600 --> 00:35:22,080
Speaker 2: that you can and see inside. All right, Daniel, you

710
00:35:22,160 --> 00:35:24,600
Speaker 2: and I are having a creepy afternoon talking about how

711
00:35:24,640 --> 00:35:26,839
Speaker 2: to de robe a black hole. Let's talk some more

712
00:35:26,840 --> 00:35:29,240
Speaker 2: about naked singularities. How can you see inside?

713
00:35:29,440 --> 00:35:32,080
Speaker 1: Do you think it's inappropriate to like undress nature and

714
00:35:32,120 --> 00:35:33,359
Speaker 1: try to understand how it works?

715
00:35:33,440 --> 00:35:35,520
Speaker 2: I mean, I think animals are walking around naked all

716
00:35:35,560 --> 00:35:37,800
Speaker 2: the time. We're very comfortable with it. It's the physicists

717
00:35:37,840 --> 00:35:38,759
Speaker 2: that are uncomfortable.

718
00:35:40,400 --> 00:35:44,200
Speaker 1: All right, that's fair, that's fair. So how do you

719
00:35:44,280 --> 00:35:46,719
Speaker 1: take a part in event horizon? We always hear you

720
00:35:46,719 --> 00:35:48,960
Speaker 1: can't see inside an event horizon. That's true for a

721
00:35:48,960 --> 00:35:51,920
Speaker 1: schwartziled black hole, But for a black hole that's spinning,

722
00:35:52,280 --> 00:35:55,040
Speaker 1: where the event horizon is depends also on that spin,

723
00:35:55,160 --> 00:35:58,920
Speaker 1: and also on the electric charge. Remember that Einstein's gravity

724
00:35:58,960 --> 00:36:02,480
Speaker 1: is more complicated than Newton's gravity. Newton's gravity just basically

725
00:36:02,560 --> 00:36:05,719
Speaker 1: has charges and where they are, but Einstein has a

726
00:36:05,920 --> 00:36:09,520
Speaker 1: stress energy tensor, and different things contribute in different ways.

727
00:36:09,920 --> 00:36:12,520
Speaker 1: Remember the episode where we talked about like why potatoes

728
00:36:12,800 --> 00:36:14,959
Speaker 1: moving near the speed of light don't turn into black

729
00:36:14,960 --> 00:36:16,840
Speaker 1: holes because they have a lot of energy. Well, that

730
00:36:16,960 --> 00:36:19,880
Speaker 1: kinetic energy doesn't contribute the same way that the mass

731
00:36:20,000 --> 00:36:22,439
Speaker 1: energy does, which is why potatoes don't turn into black

732
00:36:22,440 --> 00:36:24,920
Speaker 1: holes at high speed. And so in the same way

733
00:36:25,040 --> 00:36:27,680
Speaker 1: like the spin of the black hole or it's electric charge,

734
00:36:27,760 --> 00:36:30,640
Speaker 1: they do contribute to the gravitational energy of the object,

735
00:36:30,840 --> 00:36:32,680
Speaker 1: but not in the same way. It's just like adding

736
00:36:32,760 --> 00:36:35,200
Speaker 1: mass to the black hole. So if you add spin

737
00:36:35,280 --> 00:36:38,120
Speaker 1: to the black hole, you do change the gravity. But remember,

738
00:36:38,360 --> 00:36:41,799
Speaker 1: like an example of gravity pro b it's complicated. And

739
00:36:41,880 --> 00:36:44,319
Speaker 1: so what happens as you spin up the black hole

740
00:36:44,400 --> 00:36:46,360
Speaker 1: is the event horizon actually shrinks.

741
00:36:46,560 --> 00:36:49,520
Speaker 2: And is it harder to see because it's shrinking. No,

742
00:36:49,800 --> 00:36:52,080
Speaker 2: we decided that that makes it so you can see it.

743
00:36:52,080 --> 00:36:54,440
Speaker 1: It doesn't become harder to seeze just now it's spinning.

744
00:36:54,440 --> 00:36:55,960
Speaker 1: And some of these black holes in the universe we

745
00:36:56,000 --> 00:36:59,200
Speaker 1: think are spinning super duper fast, so their event horizons

746
00:36:59,239 --> 00:37:02,400
Speaker 1: are smaller than they would be otherwise. But if you

747
00:37:02,400 --> 00:37:05,640
Speaker 1: could spin it beyond some threshold, then in principle you

748
00:37:05,680 --> 00:37:08,719
Speaker 1: would undress the black hole. Its event horizon would be

749
00:37:08,719 --> 00:37:10,319
Speaker 1: at radius zero, and.

750
00:37:10,280 --> 00:37:11,719
Speaker 2: So if you wanted to try to spin it up,

751
00:37:11,719 --> 00:37:13,680
Speaker 2: so you could do that, could you throw a bunch

752
00:37:13,719 --> 00:37:16,640
Speaker 2: of extra mass at it that was spinning the right

753
00:37:16,680 --> 00:37:17,920
Speaker 2: way to make that happen.

754
00:37:18,160 --> 00:37:22,239
Speaker 1: Yeah, So this is an area of much debate and confusion.

755
00:37:23,360 --> 00:37:27,440
Speaker 1: General relativity in principle allows naked singularities, but it doesn't

756
00:37:27,440 --> 00:37:29,920
Speaker 1: tell you how to make one right. And this is

757
00:37:29,960 --> 00:37:31,719
Speaker 1: the problem with general relativity. A lot of times. It's

758
00:37:31,719 --> 00:37:34,440
Speaker 1: like general relativity allows for wormholes, like if one existed

759
00:37:34,440 --> 00:37:36,960
Speaker 1: in the universe, it wouldn't be breaking the rules, but

760
00:37:37,040 --> 00:37:38,919
Speaker 1: it doesn't tell you how to go from we don't

761
00:37:38,960 --> 00:37:41,120
Speaker 1: have a wormhole? Do we have a wormhole? It's like

762
00:37:41,160 --> 00:37:44,560
Speaker 1: saying slew flays are possible, Okay, but what's the recipe.

763
00:37:44,640 --> 00:37:46,840
Speaker 1: I don't know how to make that arrange the particles

764
00:37:46,840 --> 00:37:50,200
Speaker 1: into a southflay, Like it's important distinction. And so people

765
00:37:50,200 --> 00:37:53,080
Speaker 1: have tried to go from like non spinning black holes

766
00:37:53,120 --> 00:37:56,000
Speaker 1: and think about adding electric charge or think about adding spin,

767
00:37:56,080 --> 00:37:58,800
Speaker 1: like keep dropping stuff into the black hole and angle

768
00:37:58,840 --> 00:38:01,239
Speaker 1: it away from the core. That when you're adding the master,

769
00:38:01,239 --> 00:38:04,600
Speaker 1: you're also adding spin, right, spinning up the black hole

770
00:38:04,640 --> 00:38:07,920
Speaker 1: getting faster and faster and so people have wondered about, like,

771
00:38:08,200 --> 00:38:10,600
Speaker 1: what happens when you reach the maximum spin? Is it

772
00:38:10,760 --> 00:38:13,880
Speaker 1: possible in our universe to overspin a black hole and

773
00:38:13,960 --> 00:38:18,080
Speaker 1: reveal the singularity inside of it? Roger Penrose has his

774
00:38:18,239 --> 00:38:22,680
Speaker 1: cosmic censorship hypothesis. He just predicts like, no, it can't happen.

775
00:38:22,760 --> 00:38:24,719
Speaker 1: There's something in the universe that's going to prevent them

776
00:38:24,719 --> 00:38:28,080
Speaker 1: from existing. But remember people also thought that about black

777
00:38:28,120 --> 00:38:30,200
Speaker 1: holes in general, and then they were like, oh my gosh,

778
00:38:30,200 --> 00:38:33,800
Speaker 1: these things are real. What In nineteen ninety one, Stephen

779
00:38:33,840 --> 00:38:37,479
Speaker 1: Hawking bet John Preskill and Kip Thorn that these things

780
00:38:37,520 --> 00:38:41,160
Speaker 1: are impossible, and this bet today still unsettled.

781
00:38:41,480 --> 00:38:44,560
Speaker 2: Ah, I guess Hawking wouldn't pay up either way.

782
00:38:44,760 --> 00:38:48,359
Speaker 1: Yeah, exactly. And so some people have done calculations try

783
00:38:48,360 --> 00:38:50,000
Speaker 1: to figure out like, well, what happened if you tried

784
00:38:50,040 --> 00:38:52,440
Speaker 1: to overspin a black hole? Like literally, what would prevent you?

785
00:38:52,840 --> 00:38:55,160
Speaker 1: And there's some calculations out there that suggest that it's

786
00:38:55,200 --> 00:38:58,720
Speaker 1: impossible that as you drop an object in, it loses

787
00:38:58,800 --> 00:39:02,400
Speaker 1: angular momentum due to a back reaction. The acceleration of

788
00:39:02,440 --> 00:39:06,200
Speaker 1: the object into the black hole causes gravitational emission, which

789
00:39:06,360 --> 00:39:09,800
Speaker 1: effectively loses that angular momentum, so it's going to radiate

790
00:39:09,840 --> 00:39:12,480
Speaker 1: away some angular momentum, just the same way that like

791
00:39:12,680 --> 00:39:15,759
Speaker 1: if you accelerate an electron, the way that happens is

792
00:39:15,800 --> 00:39:18,719
Speaker 1: by emitting photons. Like for an electron to turn left,

793
00:39:18,760 --> 00:39:21,120
Speaker 1: it has to emit a photon right. And so the

794
00:39:21,200 --> 00:39:23,760
Speaker 1: idea is if you drop stuff into a black hole

795
00:39:23,920 --> 00:39:26,600
Speaker 1: in a way that would increase its spin too close

796
00:39:26,640 --> 00:39:28,840
Speaker 1: to the maximum, it's going to radiate away some of

797
00:39:28,880 --> 00:39:30,840
Speaker 1: that spin as gravitational waves.

798
00:39:31,160 --> 00:39:35,040
Speaker 2: So if we can't make it spin faster, is there

799
00:39:35,120 --> 00:39:38,200
Speaker 2: anything else that could make it spin faster or is

800
00:39:38,239 --> 00:39:40,400
Speaker 2: this just something that can never happen because nothing's going

801
00:39:40,440 --> 00:39:41,439
Speaker 2: to spin up a black hole.

802
00:39:41,640 --> 00:39:44,200
Speaker 1: There's a lot of disagreements, and these calculations are hard,

803
00:39:44,400 --> 00:39:47,600
Speaker 1: so nobody knows is the answer, if this is possible

804
00:39:47,680 --> 00:39:49,560
Speaker 1: or not. I would love to have a black hole

805
00:39:49,560 --> 00:39:51,040
Speaker 1: and shoot a bunch of stuff into it and see

806
00:39:51,040 --> 00:39:55,920
Speaker 1: what happens. We don't know, of course, Okay, we just

807
00:39:56,080 --> 00:39:59,440
Speaker 1: don't know. So maybe naked singularities are possible in the universe,

808
00:39:59,520 --> 00:40:02,320
Speaker 1: or maybe these voculations are correct and you just can't overspin,

809
00:40:02,520 --> 00:40:05,759
Speaker 1: or very similar argument for overcharging a black hole, that

810
00:40:05,760 --> 00:40:08,560
Speaker 1: there's a maximum charge to a black hole. We just

811
00:40:08,600 --> 00:40:12,120
Speaker 1: don't know. But that's a really fascinating kind of singularity

812
00:40:12,120 --> 00:40:13,520
Speaker 1: and naked singularity.

813
00:40:13,840 --> 00:40:16,080
Speaker 2: Okay. And so if I can take a step back

814
00:40:16,120 --> 00:40:18,840
Speaker 2: for a second and do a big overview. Basically, so

815
00:40:18,880 --> 00:40:21,280
Speaker 2: we're talking about what's happening at the center of black holes,

816
00:40:21,280 --> 00:40:25,319
Speaker 2: and the answers are either something completely different than everything

817
00:40:25,320 --> 00:40:27,680
Speaker 2: we've talked about because we get infinities, which means maybe

818
00:40:27,680 --> 00:40:28,160
Speaker 2: we're wrong.

819
00:40:28,400 --> 00:40:30,080
Speaker 1: Most likely outcome yes okay, Or.

820
00:40:30,360 --> 00:40:33,640
Speaker 2: You can have a naked black hole, a schwarz Child

821
00:40:33,680 --> 00:40:36,680
Speaker 2: black hole, and a cur black hole if those equations

822
00:40:36,719 --> 00:40:40,320
Speaker 2: are correct, and it turns out we are describing something real, okay.

823
00:40:40,360 --> 00:40:41,919
Speaker 1: And if you have a black hole that has spin

824
00:40:42,040 --> 00:40:45,319
Speaker 1: and charge is called Akur Newman black hole. And there's

825
00:40:45,320 --> 00:40:48,640
Speaker 1: another variation where there's a black hole that's not rotating

826
00:40:48,640 --> 00:40:51,600
Speaker 1: but it has charge, and that's called a reister Nerdstrum

827
00:40:51,640 --> 00:40:54,040
Speaker 1: black hole. So you have all the varieties there in

828
00:40:54,080 --> 00:40:56,719
Speaker 1: general relativity. But those are all the classical predictions all

829
00:40:56,719 --> 00:40:58,000
Speaker 1: ignore quantum mechanics.

830
00:40:58,160 --> 00:41:00,680
Speaker 2: So we've talked about black holes that have charge, spin,

831
00:41:00,880 --> 00:41:04,919
Speaker 2: and mass. Have we looked at every permutation of those

832
00:41:04,960 --> 00:41:06,360
Speaker 2: three traits?

833
00:41:06,680 --> 00:41:09,759
Speaker 1: Now, yes, we talked about all those variations, but there's

834
00:41:09,760 --> 00:41:13,239
Speaker 1: still other kinds of singularities that we can explore. Like

835
00:41:13,400 --> 00:41:16,640
Speaker 1: even in those scenarios, we're assuming that the black hole

836
00:41:16,800 --> 00:41:20,600
Speaker 1: is symmetric, that as the mass falls in, it's like

837
00:41:20,640 --> 00:41:23,560
Speaker 1: a sphere, because the star is mostly a sphere, right,

838
00:41:24,080 --> 00:41:27,480
Speaker 1: But in the universe, nothing is ever perfectly a sphere.

839
00:41:27,840 --> 00:41:31,680
Speaker 1: So there's another kind of singularity called a BKL singularity

840
00:41:31,760 --> 00:41:34,359
Speaker 1: after three physicists whose name start with BK and L.

841
00:41:34,880 --> 00:41:36,759
Speaker 1: That describes the chaos you might get at the core

842
00:41:36,800 --> 00:41:39,480
Speaker 1: of a black hole if it's not actually symmetric.

843
00:41:40,000 --> 00:41:42,080
Speaker 2: Okay, yeah, I guess to be honest, I'm having a

844
00:41:42,080 --> 00:41:44,719
Speaker 2: little trouble imagined. So when things spin, I always think

845
00:41:44,719 --> 00:41:47,719
Speaker 2: of them as becoming more spherical. I'm having trouble imagining

846
00:41:47,760 --> 00:41:51,080
Speaker 2: an asymmetrical black hole. But that is the limit of

847
00:41:51,080 --> 00:41:52,880
Speaker 2: my imagination. So okay, how would.

848
00:41:52,719 --> 00:41:55,200
Speaker 1: Well look at the Sun? Is the Sun perfectly spherical?

849
00:41:55,320 --> 00:41:57,279
Speaker 1: It has features on the surface, which means that you

850
00:41:57,320 --> 00:41:59,960
Speaker 1: can tell one location on the Sun from another location,

851
00:42:00,000 --> 00:42:03,279
Speaker 1: and it's not perfectly spherical. And you might think, okay,

852
00:42:03,280 --> 00:42:06,080
Speaker 1: those are small things, but the problem with gravity is

853
00:42:06,120 --> 00:42:10,920
Speaker 1: it's very non linear, so small deviations create larger deviations.

854
00:42:11,000 --> 00:42:12,760
Speaker 1: Like the whole reason we have structure in the universe

855
00:42:12,800 --> 00:42:15,440
Speaker 1: is because gravity is nonlinear and it started from very

856
00:42:15,480 --> 00:42:20,120
Speaker 1: small deviations from perfectly smooth to make like stars and galaxies,

857
00:42:20,640 --> 00:42:24,239
Speaker 1: and gravity is very powerful. Another difference between new Tony

858
00:42:24,280 --> 00:42:27,400
Speaker 1: and Einsteini and gravity is that in Einstein gravity, the

859
00:42:27,440 --> 00:42:32,320
Speaker 1: gravitational energy itself has gravity. And so think about what happens.

860
00:42:32,320 --> 00:42:35,440
Speaker 1: For example, as things are collapsing, you get title forces,

861
00:42:35,440 --> 00:42:38,200
Speaker 1: which we've talked about a lot, right, things get stretched

862
00:42:38,200 --> 00:42:41,799
Speaker 1: out and squeezed as you approach the singularity. But if

863
00:42:41,800 --> 00:42:46,360
Speaker 1: you follow those calculations through, those title forces contribute also

864
00:42:46,520 --> 00:42:49,719
Speaker 1: to the gravity, and so actually the title forces end

865
00:42:49,800 --> 00:42:53,120
Speaker 1: up oscillating. So you get something which was compressed like

866
00:42:53,160 --> 00:42:55,800
Speaker 1: in the long way, like a football, Then those title

867
00:42:55,840 --> 00:42:58,200
Speaker 1: forces give a feedback effect and now the thing gets

868
00:42:58,200 --> 00:43:01,360
Speaker 1: squished and it gets compressed in another direction. So the

869
00:43:01,400 --> 00:43:03,920
Speaker 1: point is if you're not perfectly smooth, you're going to

870
00:43:03,960 --> 00:43:08,520
Speaker 1: create all sorts of crazy chaotic fluctuations. Any tiny deviation

871
00:43:08,600 --> 00:43:12,359
Speaker 1: from perfectly smooth symmetry is going to create chaos. So

872
00:43:12,400 --> 00:43:16,160
Speaker 1: this collapse is not symmetric but very chaotic. So that's

873
00:43:16,160 --> 00:43:20,120
Speaker 1: the idea of a BKL singularity. It's just like chaotic gravity,

874
00:43:20,480 --> 00:43:22,400
Speaker 1: and we're not used to thinking about gravity as like

875
00:43:22,480 --> 00:43:26,160
Speaker 1: having fluctuations and randomness. This is still deterministic, but it's

876
00:43:26,400 --> 00:43:29,239
Speaker 1: so hard to calculate because it's so sensitive to the

877
00:43:29,280 --> 00:43:32,200
Speaker 1: initial conditions that we call it chaos. It's not random,

878
00:43:32,280 --> 00:43:35,880
Speaker 1: it's chaotic. So that's what a BKL singularity is in

879
00:43:35,920 --> 00:43:36,640
Speaker 1: broad terms.

880
00:43:36,920 --> 00:43:40,560
Speaker 2: So you have made me switch from thinking how could

881
00:43:40,560 --> 00:43:44,200
Speaker 2: it not be a perfect sphere? To how could it

882
00:43:44,280 --> 00:43:47,840
Speaker 2: possibly be a perfect sphere? So that and that's probably

883
00:43:47,880 --> 00:43:49,240
Speaker 2: the right way to think about it, right.

884
00:43:49,120 --> 00:43:51,319
Speaker 1: Yeah, it's just much much harder. And so you know,

885
00:43:51,320 --> 00:43:53,440
Speaker 1: in physics, what we do is we say the universe

886
00:43:53,520 --> 00:43:55,880
Speaker 1: is too complex to model completely, so let's take a

887
00:43:55,880 --> 00:43:58,280
Speaker 1: simplified approach and get rid of some of the details

888
00:43:58,320 --> 00:44:00,680
Speaker 1: we hope are irrelevant. We can get sort of the

889
00:44:00,719 --> 00:44:03,239
Speaker 1: big picture view, which is why you start with like

890
00:44:03,520 --> 00:44:05,839
Speaker 1: spherical objects that are not spinning and have no charge,

891
00:44:05,880 --> 00:44:07,640
Speaker 1: and you do that calculation and then you're like, Okay,

892
00:44:07,680 --> 00:44:09,760
Speaker 1: I think we figured that out. Let's add this feature,

893
00:44:09,840 --> 00:44:12,080
Speaker 1: let's add that feature. And so we're trying to move

894
00:44:12,200 --> 00:44:15,920
Speaker 1: more gradually towards a more realistic description of the universe.

895
00:44:16,360 --> 00:44:18,839
Speaker 2: Okay, so if it's chaotic, then I imagine it's also

896
00:44:19,160 --> 00:44:22,640
Speaker 2: like inconsistent, Like if you look at it, you know,

897
00:44:23,000 --> 00:44:25,320
Speaker 2: year to year, it probably has a slightly different shape.

898
00:44:25,360 --> 00:44:26,719
Speaker 2: Wouldn't that be fair to say?

899
00:44:26,920 --> 00:44:29,160
Speaker 1: Yeah, but we're talking about what's happening inside the black hole,

900
00:44:29,280 --> 00:44:31,560
Speaker 1: so we can't see it. It probably also means that

901
00:44:31,600 --> 00:44:34,399
Speaker 1: the event horizon has these kind of ripples too, right,

902
00:44:34,440 --> 00:44:38,000
Speaker 1: because the event horizon is determined by the curvature. But

903
00:44:38,080 --> 00:44:41,240
Speaker 1: these effects might be subtle. And again, we've never directly

904
00:44:41,280 --> 00:44:43,839
Speaker 1: measured the extent of the event horizon, but that could

905
00:44:43,880 --> 00:44:47,200
Speaker 1: be super cool. See a fluctuating, oscillating event horizon.

906
00:44:47,320 --> 00:44:47,640
Speaker 2: Yeah.

907
00:44:47,760 --> 00:44:49,960
Speaker 1: Another kind of singularity people talk about a lot is

908
00:44:50,000 --> 00:44:53,520
Speaker 1: the Big Bang singularity, and this is another kind of singularity,

909
00:44:53,560 --> 00:44:56,120
Speaker 1: but it is quite different from a black hole singularity.

910
00:44:56,160 --> 00:44:58,480
Speaker 1: People really like to make these connections, be like, is

911
00:44:58,480 --> 00:45:01,120
Speaker 1: our universe inside a black hole? Because black hole is

912
00:45:01,120 --> 00:45:03,319
Speaker 1: a singularity and the Big Bang was a singularity. Dot

913
00:45:03,360 --> 00:45:07,399
Speaker 1: dot dot, we're inside a black hole. Awesome, pop science points, right,

914
00:45:07,800 --> 00:45:10,280
Speaker 1: But the singularity that might have existed at the beginning

915
00:45:10,280 --> 00:45:13,440
Speaker 1: of the universe is a very different kind of singularity

916
00:45:13,480 --> 00:45:14,960
Speaker 1: from the one that's at the heart of a black hole.

917
00:45:15,040 --> 00:45:17,799
Speaker 1: It's a singularity not in space, right, a black hole

918
00:45:17,800 --> 00:45:20,640
Speaker 1: singularity and principles should last forever and is at one

919
00:45:20,680 --> 00:45:24,719
Speaker 1: location in space. Right. The Big Bang singularity says the

920
00:45:24,840 --> 00:45:28,799
Speaker 1: universe is old and cold. That's where we live now.

921
00:45:28,960 --> 00:45:31,080
Speaker 1: But if you wind the clock back, things get hotter

922
00:45:31,280 --> 00:45:34,839
Speaker 1: and denser, right, And so the universe was denser, and

923
00:45:34,880 --> 00:45:36,960
Speaker 1: as you go back in time, things get denser and

924
00:45:37,000 --> 00:45:40,360
Speaker 1: denser and denser, and eventually you get a singularity in

925
00:45:40,480 --> 00:45:45,000
Speaker 1: density at one moment in time, but everywhere in space.

926
00:45:45,920 --> 00:45:49,040
Speaker 1: So black hole singularity one point in space, everywhere in time.

927
00:45:49,560 --> 00:45:53,280
Speaker 1: Big Bang singularity everywhere in space, one moment in time.

928
00:45:53,560 --> 00:45:57,200
Speaker 1: So technically a singularity in the same sense of singularities

929
00:45:57,280 --> 00:46:00,360
Speaker 1: is having infinities, but physically very very different from a

930
00:46:00,360 --> 00:46:01,520
Speaker 1: black hole singularity.

931
00:46:02,120 --> 00:46:03,200
Speaker 2: Was it spinning.

932
00:46:05,320 --> 00:46:08,440
Speaker 1: We don't know. That would be the whole universe spinning, right,

933
00:46:08,480 --> 00:46:11,160
Speaker 1: And we think probably the whole universe on average has

934
00:46:11,200 --> 00:46:14,719
Speaker 1: spinned zero. But that's a whole other fascinating topic. And

935
00:46:14,800 --> 00:46:17,520
Speaker 1: also remember that this prediction that the universe goes to

936
00:46:17,560 --> 00:46:21,200
Speaker 1: a singularity and density that also ignores quantum mechanics. What

937
00:46:21,360 --> 00:46:23,280
Speaker 1: really happens is you go back to a certain moment

938
00:46:23,320 --> 00:46:26,160
Speaker 1: thirteen point eight billion years ago, and beyond that we

939
00:46:26,200 --> 00:46:29,400
Speaker 1: can't predict with just gravity. We need to incorporate quantum mechanics,

940
00:46:29,440 --> 00:46:31,840
Speaker 1: and we don't know how to do that. So modern

941
00:46:31,920 --> 00:46:34,399
Speaker 1: theories of the Big Bang do not include a singularity.

942
00:46:34,600 --> 00:46:36,879
Speaker 1: They have a question mark before this moment when things

943
00:46:36,920 --> 00:46:39,640
Speaker 1: are so dense we can no longer calculate. But there

944
00:46:39,719 --> 00:46:42,680
Speaker 1: is a concept of a singularity, which again ignores quantum

945
00:46:42,719 --> 00:46:45,080
Speaker 1: mechanics if you want to go that far. But it's

946
00:46:45,120 --> 00:46:48,360
Speaker 1: different from a black hole singularity. Okay, this moment beyond

947
00:46:48,360 --> 00:46:50,400
Speaker 1: which you can no longer calculate without knowing how to

948
00:46:50,400 --> 00:46:53,759
Speaker 1: do quantum gravity sometimes called the plank temperature, like when

949
00:46:53,800 --> 00:46:56,680
Speaker 1: things get so hot we can no longer ignore gravity.

950
00:46:56,920 --> 00:47:00,239
Speaker 1: Or my favorite description of it is absolute hot. You've

951
00:47:00,239 --> 00:47:02,919
Speaker 1: heard absolute zero, like the universe can't get blue zero.

952
00:47:03,280 --> 00:47:06,280
Speaker 1: This is absolute hot. It's like the hottest possible temperature

953
00:47:06,680 --> 00:47:09,239
Speaker 1: beyond which, like temperature, doesn't make sense anymore, like we

954
00:47:09,280 --> 00:47:11,279
Speaker 1: don't know it can't get hotter. We just we don't

955
00:47:11,280 --> 00:47:14,040
Speaker 1: know anything about what happens beyond the plank scale.

956
00:47:14,120 --> 00:47:16,400
Speaker 2: Do we know what temperature is absolute hot?

957
00:47:16,600 --> 00:47:18,920
Speaker 1: Yeah, it's the plank temperature. It's a very big number.

958
00:47:19,160 --> 00:47:22,879
Speaker 2: Oh okay, but it's not like five thousand. It's no,

959
00:47:23,000 --> 00:47:24,840
Speaker 2: it's just some very big number, got it?

960
00:47:24,880 --> 00:47:28,040
Speaker 1: Okay, all right, So let's talk about the last two

961
00:47:28,120 --> 00:47:31,400
Speaker 1: kinds of singularities that Julie mentioned. These are in flying

962
00:47:31,640 --> 00:47:36,279
Speaker 1: and outgoing singularities. These are not standard terms for singularities.

963
00:47:36,280 --> 00:47:39,520
Speaker 1: You don't find them often in the literature in general relativity.

964
00:47:39,760 --> 00:47:43,000
Speaker 1: These are phrases made up by Kip Thorn and explained

965
00:47:43,080 --> 00:47:47,080
Speaker 1: in his book The Physics of Interstellar Kip Thrn definitely

966
00:47:47,080 --> 00:47:49,279
Speaker 1: knows a lot of general relativity, a lot more than

967
00:47:49,280 --> 00:47:51,720
Speaker 1: I do, due to the total expert in a badass

968
00:47:51,719 --> 00:47:54,799
Speaker 1: and a Nobel Prize winner for general relativity. So no

969
00:47:54,920 --> 00:47:57,880
Speaker 1: criticism implied in any of these comments. It's just like

970
00:47:58,080 --> 00:48:00,360
Speaker 1: he's very poetic with these words, and not all of

971
00:48:00,400 --> 00:48:03,120
Speaker 1: those words have caught on everywhere. So if you google

972
00:48:03,120 --> 00:48:05,080
Speaker 1: this stuff, you don't find like textbooks on it. But

973
00:48:05,120 --> 00:48:07,600
Speaker 1: the dude knows what he's talking about. When he refers

974
00:48:07,640 --> 00:48:11,200
Speaker 1: to an inflying singularity, he's talking about the complex space

975
00:48:11,239 --> 00:48:15,840
Speaker 1: time structure inside a spinning or charge black hole. So Remember,

976
00:48:15,880 --> 00:48:18,200
Speaker 1: this black hole is not just a sphere of mass

977
00:48:18,239 --> 00:48:20,920
Speaker 1: that's gotten compressed into a point. It's spinning, and so

978
00:48:20,960 --> 00:48:23,319
Speaker 1: the space time is much more complicated on the outside

979
00:48:23,520 --> 00:48:26,799
Speaker 1: and on the inside. So there's the event horizon, but

980
00:48:26,800 --> 00:48:30,120
Speaker 1: if you go past the event horizon, there's another horizon.

981
00:48:30,200 --> 00:48:34,040
Speaker 1: It's called the Kaushi horizon. So again these space times

982
00:48:34,040 --> 00:48:36,839
Speaker 1: have more structure than just an event horizon and a singularity.

983
00:48:37,080 --> 00:48:40,520
Speaker 1: There's this Kaushi horizon. Outside the Kaoshi horizon, you can

984
00:48:40,560 --> 00:48:43,960
Speaker 1: actually survive like you can live. People have theories that

985
00:48:43,960 --> 00:48:46,480
Speaker 1: you could have like stable orbits within the black hole,

986
00:48:47,000 --> 00:48:49,680
Speaker 1: and maybe even life could evolve, which could be amazing.

987
00:48:50,200 --> 00:48:53,920
Speaker 1: Within the Kaushi horizon, things get crazy. The Kaoshi horizon

988
00:48:54,000 --> 00:48:58,640
Speaker 1: essentially is the point where classical determinism breaks down, where

989
00:48:58,640 --> 00:49:02,000
Speaker 1: there's not enough information to predict what's going to happen

990
00:49:02,040 --> 00:49:05,400
Speaker 1: in the future. Remember that the equations of general relativity

991
00:49:05,400 --> 00:49:09,440
Speaker 1: are nonlinear, which means they blow up. Sometimes small deviations

992
00:49:09,440 --> 00:49:12,279
Speaker 1: in your initial conditions can lead to very large differences

993
00:49:12,520 --> 00:49:16,560
Speaker 1: in your predictions. Right You're going from infinitesimal changes in

994
00:49:16,600 --> 00:49:20,919
Speaker 1: your conditions to huge predictions in the future. That's also

995
00:49:21,000 --> 00:49:23,960
Speaker 1: a kind of infinity, and so this is what Kip

996
00:49:24,000 --> 00:49:26,400
Speaker 1: Thorne means by a singularity.

997
00:49:25,680 --> 00:49:28,600
Speaker 2: Here, Okay, And so just to confirm it, it sounds

998
00:49:28,640 --> 00:49:30,239
Speaker 2: like what you're saying is this is the kind of

999
00:49:30,320 --> 00:49:35,160
Speaker 2: singularity where we don't understand it at all as physicists.

1000
00:49:35,160 --> 00:49:37,040
Speaker 2: And so when you say there's not enough information to

1001
00:49:37,080 --> 00:49:40,520
Speaker 2: predict the future, that's not like people living in the

1002
00:49:40,520 --> 00:49:44,279
Speaker 2: Cauchy horizon can't predict the future. It's us. We can't

1003
00:49:44,320 --> 00:49:46,080
Speaker 2: predict what happens in that situation.

1004
00:49:46,480 --> 00:49:50,600
Speaker 1: We can't predict it with our limited understanding of relativity

1005
00:49:50,600 --> 00:49:53,680
Speaker 1: and general relativity and our current ability to do calculations.

1006
00:49:54,000 --> 00:49:56,840
Speaker 1: It doesn't mean that it's impossible. Okay, got it. And

1007
00:49:56,880 --> 00:49:58,840
Speaker 1: so in that sense, is this a physical thing? Is

1008
00:49:58,880 --> 00:50:01,319
Speaker 1: to reflect the limit in our theory. There are some

1009
00:50:01,440 --> 00:50:05,160
Speaker 1: interesting physical effects here, like in falling radiation or matter.

1010
00:50:05,280 --> 00:50:08,440
Speaker 1: Blue shifts very dramatically as it approaches the Kushi horizon,

1011
00:50:08,920 --> 00:50:12,040
Speaker 1: and so the energy density diverges, like you get these

1012
00:50:12,080 --> 00:50:15,920
Speaker 1: predictions of essentially infinite energy density which don't seem to

1013
00:50:15,960 --> 00:50:17,920
Speaker 1: make sense. And so this is what he means by

1014
00:50:17,960 --> 00:50:21,520
Speaker 1: the inflying singularity. There's this other threshold after you come

1015
00:50:21,560 --> 00:50:25,240
Speaker 1: into the black hole, as you fall in towards the ringularity,

1016
00:50:25,600 --> 00:50:29,880
Speaker 1: you experience this inflying singularity of the Kushi horizon.

1017
00:50:30,160 --> 00:50:32,960
Speaker 2: Okay, but what if you were trying to get out,

1018
00:50:33,680 --> 00:50:36,000
Speaker 2: because it sounds like you don't really want to be

1019
00:50:36,040 --> 00:50:38,120
Speaker 2: at the middle of a black hole. So what is

1020
00:50:38,160 --> 00:50:39,520
Speaker 2: an outgoing singularity?

1021
00:50:39,680 --> 00:50:42,120
Speaker 1: And outgoing singularity is related to the topic we talked

1022
00:50:42,120 --> 00:50:46,080
Speaker 1: about earlier, the BKL singularities. You have a collapse of

1023
00:50:46,160 --> 00:50:49,200
Speaker 1: black hole that's not symmetric, right, or maybe you have

1024
00:50:49,280 --> 00:50:51,880
Speaker 1: like two black holes that collide, and so that's very

1025
00:50:51,960 --> 00:50:54,920
Speaker 1: much not symmetric. You have an axis along which they collide.

1026
00:50:55,200 --> 00:50:57,600
Speaker 1: So again, the nonlinearity of the Einstein equations lead to

1027
00:50:57,640 --> 00:51:00,360
Speaker 1: chaotic effects, and you can get things like proping gation

1028
00:51:00,480 --> 00:51:04,640
Speaker 1: of a curvature shock wave that's moving out from the core.

1029
00:51:05,040 --> 00:51:08,319
Speaker 1: So you can get this outgoing singularity. What we mean there, again,

1030
00:51:08,480 --> 00:51:12,000
Speaker 1: is a singularity in the curvature, so like huge gredients,

1031
00:51:12,400 --> 00:51:17,080
Speaker 1: very nonlinear reactions, not a dot of matter that's infinitely dense,

1032
00:51:17,320 --> 00:51:20,360
Speaker 1: but a deviation in the curvature that's moving in space

1033
00:51:20,400 --> 00:51:20,800
Speaker 1: and time.

1034
00:51:21,200 --> 00:51:23,680
Speaker 2: Okay, and this is maybe not the most important question

1035
00:51:23,840 --> 00:51:25,560
Speaker 2: that I could be asking right now, but you said

1036
00:51:25,560 --> 00:51:27,919
Speaker 2: this was in a book called the physics of Interstellar.

1037
00:51:28,280 --> 00:51:30,520
Speaker 2: Are these concepts part of that movie which I have

1038
00:51:30,560 --> 00:51:31,480
Speaker 2: to admit I've never seen.

1039
00:51:31,640 --> 00:51:35,160
Speaker 1: You have not seen in Interstellar? Wow? Oh wow? And

1040
00:51:35,239 --> 00:51:37,080
Speaker 1: married to a nerd never seen that movie.

1041
00:51:37,200 --> 00:51:39,040
Speaker 2: He's never seen it? I know.

1042
00:51:39,480 --> 00:51:41,920
Speaker 1: Oh my gosh. Is that on purpose? You guys like

1043
00:51:42,360 --> 00:51:43,360
Speaker 1: boycotting that movie?

1044
00:51:43,440 --> 00:51:45,719
Speaker 2: No, just never no interest, just never got around to it.

1045
00:51:45,800 --> 00:51:46,560
Speaker 2: Maybe I need to.

1046
00:51:47,120 --> 00:51:49,640
Speaker 1: Well, there's lots of fascinating and wonderful stuff about that movie.

1047
00:51:49,640 --> 00:51:52,680
Speaker 1: It's also stuff I can't stand about that movie. So anyway,

1048
00:51:53,800 --> 00:51:56,240
Speaker 1: a very popular movie, and a lot of real physics

1049
00:51:56,320 --> 00:51:59,239
Speaker 1: went into it. Not all the plot elements are based

1050
00:51:59,280 --> 00:52:00,880
Speaker 1: in sound physics, but a lot of it is. And

1051
00:52:00,920 --> 00:52:03,680
Speaker 1: so Kip wrote this book, The Physics of Interstellar for

1052
00:52:03,680 --> 00:52:06,600
Speaker 1: people who are interested diving more deeply into these topics.

1053
00:52:06,880 --> 00:52:08,680
Speaker 1: So some of the stuff in the book the Physics

1054
00:52:08,719 --> 00:52:11,319
Speaker 1: of Interstellar, is not covered in the movie. So we

1055
00:52:11,400 --> 00:52:14,479
Speaker 1: don't have in flying or outgoing singularities in the movie

1056
00:52:14,560 --> 00:52:15,800
Speaker 1: as far as I can recall.

1057
00:52:15,920 --> 00:52:17,680
Speaker 2: Okay, got it all right.

1058
00:52:17,560 --> 00:52:20,480
Speaker 1: So that's our tour of singularities. We think that the

1059
00:52:20,480 --> 00:52:23,040
Speaker 1: appearance of these singularities in our predictions is a sign

1060
00:52:23,080 --> 00:52:26,520
Speaker 1: that those predictions are probably nonsense, but we don't really know,

1061
00:52:26,920 --> 00:52:30,319
Speaker 1: because sometimes the universe acts sort of nonsensical in a

1062
00:52:30,360 --> 00:52:32,560
Speaker 1: way that seems crazy at us, but later we come

1063
00:52:32,600 --> 00:52:35,719
Speaker 1: to learn actually makes more sense than anything we could

1064
00:52:35,719 --> 00:52:39,120
Speaker 1: have imagined, and so we always should be ready for surprises.

1065
00:52:39,600 --> 00:52:42,600
Speaker 1: And as usual, there's a lot more interesting, rich physics

1066
00:52:42,600 --> 00:52:45,160
Speaker 1: here that goes well beyond what you typically see in

1067
00:52:45,239 --> 00:52:46,280
Speaker 1: popsie coverage.

1068
00:52:46,320 --> 00:52:48,680
Speaker 2: And thank you so much to Julie for inspiring this

1069
00:52:48,880 --> 00:52:51,640
Speaker 2: rousing conversation about the many kinds of singularities.

1070
00:52:52,040 --> 00:52:54,480
Speaker 1: Let's check in with Julie to see if we scratched

1071
00:52:54,520 --> 00:52:55,520
Speaker 1: her singular.

1072
00:52:55,160 --> 00:52:59,680
Speaker 3: Ititch consider my singular itch scratched, or maybe my reingular

1073
00:53:00,719 --> 00:53:03,880
Speaker 3: I did not know regularities were a thing. I like

1074
00:53:03,920 --> 00:53:07,319
Speaker 3: how you described singularities as a warning sign from the

1075
00:53:07,360 --> 00:53:10,319
Speaker 3: math and an indication that something is wrong, like the

1076
00:53:10,360 --> 00:53:13,080
Speaker 3: math and the physics contradict each other. But also we

1077
00:53:13,120 --> 00:53:15,640
Speaker 3: can't say for sure, so if you physicists could figure

1078
00:53:15,640 --> 00:53:18,080
Speaker 3: out quantum gravity for the rest of us, that'd be great.

1079
00:53:18,520 --> 00:53:21,319
Speaker 3: He also inspired new questions to ponder, like if there

1080
00:53:21,320 --> 00:53:24,439
Speaker 3: are fuzzballs instead of black holes, or if life could

1081
00:53:24,480 --> 00:53:29,000
Speaker 3: evolve inside a spinning or charge black hole. Wild and yep.

1082
00:53:29,120 --> 00:53:32,120
Speaker 3: I absolutely did read the Signs of Interstellar by Kip Thorne,

1083
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Speaker 3: which is where I got those terms, so it was

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00:53:33,960 --> 00:53:35,960
Speaker 3: helpful to have that terminology cleared up.

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Speaker 4: Thanks again, all.

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Speaker 1: Right, thank you very much everybody for tuning into this

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Speaker 1: survey of several singularities. We hope this helps stretch your

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Speaker 1: mind to the limits of what humans can conceive of.

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Speaker 2: Until next time, Daniel and Kelly's Extraordinary Universe is produced

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Speaker 1: about this Extraordinary Universe.

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Speaker 2: for future shows. If you contact us, we will get

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Speaker 1: We really mean it. We answer every message. Email us

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