Speaker 1: Hey, Daniel, would you say you understand general relativity?
Speaker 2: I mean i'd say I understand it better.
Speaker 1: Every week, so that means you don't understand it.
Speaker 2: I think it still like makes my head spin.
Speaker 1: To be honest, what's the trickiest part of it?
Speaker 2: I think the hardest part to get your mind around
Speaker 2: is that spinning stuff has different gravity than stuff that
Speaker 2: doesn't spin.
Speaker 1: Yeah, okay, there's no way to spend that. That is
Speaker 1: just confusing.
Speaker 2: Sometimes I just want to like spin up extra brains
Speaker 2: to help me figure out how space itself can spin.
Speaker 1: That is quite a yarn you are spinning.
Speaker 2: Maybe I'm spinning a web of physics or a web
Speaker 2: of lies.
Speaker 1: I'll take that for a spin.
Speaker 2: Hi, I'm Daniel. I'm a particle physicist and a professor
Speaker 2: at uc Irdine, And those are all the spin related
Speaker 2: jokes I could think of.
Speaker 1: Hi, I'm Katie, and I cannot put a new spin
Speaker 1: on those jokes. I am not a particle physicist, but
Speaker 1: I enjoy physics in the universe and I host an
Speaker 1: animal themed podcast.
Speaker 2: All right, I got one more spin related joke for
Speaker 2: you. You ready, wait?
Speaker 1: Wait, okay, now I'm ready.
Speaker 2: Why did scientists stop watching the Earth spin.
Speaker 1: I don't know, Daniel why they.
Speaker 2: Got bored and called it a day.
Speaker 1: Oh that's my soul leaving my body.
Speaker 2: That was a good one, and Welcome to the podcast
Speaker 2: Daniel and Jorge explain the Universe, in which we try
Speaker 2: to avoid your brain spinning inside your cranium as we
Speaker 2: take a tour of how the universe works, all of
Speaker 2: its weird and wonderful ways of operating, from the tiniest
Speaker 2: quantum particles to the most massive of super massive black
Speaker 2: holes spinning at the centers of galaxies. Motion and rotation
Speaker 2: and spinning and translation, and all of these things are
Speaker 2: essential to the way the universe works in some sense.
Speaker 2: Understanding how things move and what they are is what
Speaker 2: physics is all about. That's what we try to do
Speaker 2: on this podcast, break down why things move, how they spin,
Speaker 2: and how that affects everything around them. My friend and
Speaker 2: co host Orge can be with us today, but we're
Speaker 2: very happy to have Katie here with us to take
Speaker 2: physics for a spin.
Speaker 1: You know, I used to do ballet when I was
Speaker 1: a kid, and one of the things you have to
Speaker 1: learn to do in ballet is to spin without getting
Speaker 1: dizzy and falling over and so you kind of try
Speaker 1: to keep your head looking at a point of reference
Speaker 1: and sort of twist your head around so that you
Speaker 1: don't get dizzy. Are you saying that when I'm a ballerina,
Speaker 1: I have more gravity than regular non ballerinas.
Speaker 2: I'm saying when you do that spin, you drag the
Speaker 2: fabric of space time along with you, like sticking a
Speaker 2: fork in spaghetti and twisting it.
Speaker 1: That's really cool. Now, I wonder my teacher was so
Speaker 1: strict she didn't want us tearing holes in the fabric
Speaker 1: of the universe with our off tempo spins.
Speaker 2: She was really just looking out for your safety, it
Speaker 2: sounds like. And as scientists have tried to understand the
Speaker 2: nature of gravity, why things fall down, why things orbit
Speaker 2: each other in the sky, we have learned so much
Speaker 2: about the very nature of the universe around us. From
Speaker 2: Aristotle just telling us things fall down because they like
Speaker 2: sort of going down, to Newton telling us that gravity
Speaker 2: is a force between objects that have mass, to Einstein
Speaker 2: telling us that actually gravity is the bending of space,
Speaker 2: invisibly curving in front of us and affecting the path
Speaker 2: of everything that moves through it every time. It requires
Speaker 2: a huge conceptual change in the way the universe works
Speaker 2: around us, often with surprising results and strange phenomena that
Speaker 2: really test our ability to un understand the nature of
Speaker 2: the universe.
Speaker 1: Do we now understand what gravity is like? Has that
Speaker 1: answer been solved for good?
Speaker 2: We're going to solve it today on the podcast Katie
Speaker 2: that's right.
Speaker 1: Oh, let me get out of notepad.
Speaker 2: After this podcast, buy your ticket to Stockholm. No, gravity
Speaker 2: is definitely not understood. In fact, we know that our
Speaker 2: current understanding of gravity is limited, that there are parts
Speaker 2: of the universe it just fails to describe, and parts
Speaker 2: of the history of the universe that just don't make
Speaker 2: sense if you take literally Einstein's theory of general relativity.
Speaker 2: And one of those places is at the heart of
Speaker 2: black holes. Black holes are famous because they're weird and
Speaker 2: they gobble stuff up, and yet they actually exist in
Speaker 2: the universe. But they hold within them mysteries and secrets
Speaker 2: that might tell us how gravity actually works at the
Speaker 2: quantum mechanical level. The thing that we don't know how
Speaker 2: to do is merge Einstein's picture of gravity with the
Speaker 2: way quantum mechanics tells us that reality operates at the
Speaker 2: smallest level. Because we can't go so inside black holes.
Speaker 2: We have to do other things to them. And one
Speaker 2: thing we can do to black holes is to spin
Speaker 2: them and see what happens when they stick their fork
Speaker 2: into the spaghetti of space time.
Speaker 1: Okay, when you say we can spin black holes, do
Speaker 1: you mean we can just sort of twirl them around
Speaker 1: like a giant top or something, or a huge destructive
Speaker 1: bay blade. Are these things just out there naturally spinning
Speaker 1: or are we shooting ping pong balls at them to
Speaker 1: try to get them to spin both?
Speaker 2: Actually, all the black holes we know about are too
Speaker 2: far away for us to experiment on in the laboratory.
Speaker 2: There aren't any very close by. But we do think
Speaker 2: that all of the black holes that exist in the
Speaker 2: universe do spin. Basically, everything out there is spinning. Our
Speaker 2: planet is spinning, the Sun is spinning, Our planet spins
Speaker 2: around the Sun. The Sun spins around the center of
Speaker 2: the galaxy. The galaxy orbits the center of the galactic cluster. Basically,
Speaker 2: everything out there is spinning, and because angular momentum in
Speaker 2: the universe is conserved, we think that everything that went
Speaker 2: into the black hole was probably spinning. So the black
Speaker 2: holes themselves are almost certainly spinning, which means every black
Speaker 2: hole out there is probably a spinning object. And the
Speaker 2: fascinating thing about spinning a black hole is that it
Speaker 2: changes its gravity. But there's a catch. Black holes can
Speaker 2: only spin so fast before they might tear themselves open.
Speaker 1: I mean that's also true in ballet. So the Earth
Speaker 1: is spinning? Is that why the Earth has gravity? And
Speaker 1: it keeps us on it?
Speaker 2: The Earth would have gravity even if it wasn't spinning.
Speaker 2: But the Earth's gravity is actually subtly different because it
Speaker 2: does spin. It doesn't just pull on things, It also
Speaker 2: twists things in orbit around it. We'll dig into all
Speaker 2: that when we talk about frame dragging. But because spinning
Speaker 2: of an object changes its gravity, it can also affect
Speaker 2: the size of a black hole and puts a limit
Speaker 2: on how fast a black hole is allowed to spin
Speaker 2: before it cracks open and breaks the rules of physics.
Speaker 1: Wait, so can a black hole actually crack open? Or
Speaker 1: is the just some kind of theoretical limit.
Speaker 2: The rules of physics actually say that there's a maximum
Speaker 2: speed that black holes are allowed to spin before they
Speaker 2: crack open. Nobody knows what would happen if you broke
Speaker 2: that rule. Would it actually crack open the black hole?
Speaker 2: Would you go to physics jail Kai's old ballet teacher
Speaker 2: come and scold you. Let's find out.
Speaker 1: Oh, that's the scariest option.
Speaker 2: And so today on the podcast, we'll be answering the
Speaker 2: question how fast can a black hole spin?
Speaker 1: I'm gonna say fifty miles per hour? I get it.
Speaker 2: Don't tell me that's the fastest speed you can imagine
Speaker 2: in your mind, Kitty, you live in Italy. The drivers
Speaker 2: there drive faster than that all the time, just when
Speaker 2: they're going to the corner store.
Speaker 1: I guess, but it's in kilometers per hour, and I
Speaker 1: haven't figured that out yet.
Speaker 2: How long you've been there already?
Speaker 1: You know, a couple kilometers? I'm not really sure.
Speaker 2: Right, We won't quiz you on you, but I did
Speaker 2: go out there and ask our listeners what they thought
Speaker 2: about how fast a black hole could spin. We're very
Speaker 2: grateful to the listeners who participate in this segment of
Speaker 2: the podcast, giving us a sense for what people know
Speaker 2: and what they're confused about. If you would be willing
Speaker 2: to add your voice to this group of volunteers, please
Speaker 2: don't be shy. Write to me to questions at danielandthork
Speaker 2: dot com. So think about it for a moment before
Speaker 2: you hear these answers. Do you know how fast a
Speaker 2: black hole can spin? Here's what people had to say.
Speaker 1: Faster than sonic the hedgehog.
Speaker 3: I would guess up to the speed of light. I
Speaker 3: think nothing can go faster than that through space, so
Speaker 3: that would be a limit. But I don't see any
Speaker 3: reason why it couldn't get up to that speed.
Speaker 2: I actually learned that black hole spain the other day.
Speaker 4: I have no idea how fast they go. I believe
Speaker 4: that the rate of rotation of a black hole has
Speaker 4: to do with its size, so the smaller ones will
Speaker 4: rotate faster. How fast can they go? I have no idea.
Speaker 5: Well, I would assume that they spin very fast, because
Speaker 5: neutron stars such as pulsars spin I believe many thousand
Speaker 5: times per minute, and black holes are perhaps even smaller
Speaker 5: and denser, which the contraction of their size would make
Speaker 5: them spin even faster, just like a figure skater holding
Speaker 5: their arms in would make them spin faster.
Speaker 6: My instinct is that the only limit ON have asked
Speaker 6: a black hole could spin is the speed of light. However,
Speaker 6: I imagine there might be some kind of loophole that
Speaker 6: may allow it to be faster than that.
Speaker 7: Very intrigued, Well, black holes, you mean the thing that
Speaker 7: makes the black hole or the stuff going into the
Speaker 7: black hole, Because the stuff going into the black hole
Speaker 7: can spin really fast, but this thing that is the
Speaker 7: black hole is the singularity, and can a singularity even
Speaker 7: spin I don't even know.
Speaker 2: I don't even understand what a singularity is. Mass that's
Speaker 2: been infinitely mashed up.
Speaker 8: Black holes would not spin so fast that any given
Speaker 8: part would have such a high angular momentum to fly
Speaker 8: off the black hole. But being such massive bodies, they
Speaker 8: can spin really fast.
Speaker 1: I do enjoy the Sonic the Hedgehog answer that it
Speaker 1: could be even faster than Sonic the Hedgehog, who probably
Speaker 1: does spin faster than fifty miles per hour, Like, would
Speaker 1: you say that a black hole is faster than mister
Speaker 1: Sonic the Hedgehog.
Speaker 2: You know, I've been asked a lot of physics questions
Speaker 2: in my career, a lot of random questions from the public.
Speaker 2: I've never been asked to compare a black hole to
Speaker 2: a fictional video game character, and I just don't know
Speaker 2: how to answer that question. Like the Sonic the Hedgehog
Speaker 2: even follow the laws of physics. Are the two things
Speaker 2: comparable at all? Or is it like comparing miles per
Speaker 2: hour to kilometer per second?
Speaker 1: Wise answer given how many Sonic the Hedgehog fans are
Speaker 1: out there ready for blood. But yeah, I mean, it
Speaker 1: seems like some people think it's limited by the speed
Speaker 1: of light, and so some people think that it's got
Speaker 1: to be faster than something like a bigger skater, or
Speaker 1: smaller things rotate faster, or a lot of really interesting
Speaker 1: theories from people.
Speaker 2: Yeah, and people are definitely right that the surface of
Speaker 2: a black hole shouldn't be able to move faster than
Speaker 2: the speed of light. But it turns out there's another
Speaker 2: limitation to black hole speeds that if black holes spin
Speaker 2: too fast, they might crack open and reveal what's inside
Speaker 2: of them. So it's a fascinating question to think about,
Speaker 2: like what is a black hole, how do they spin,
Speaker 2: how fast do they spin? What is the maximum speed?
Speaker 2: And what would happen if you overspun a black hole.
Speaker 1: It's like if you spun pizza dough too fast and
Speaker 1: it tore itself apart. But I did think black holes
Speaker 1: weren't so much a physical object like a pizza. I
Speaker 1: thought it was kind of this just sort of sinkhole
Speaker 1: in the universe of like of incredibly incredib dovidibly dense
Speaker 1: matter that has an incredibly strong pull of gravity. I
Speaker 1: could use a refresher on what exactly a black hole
Speaker 1: is made out of.
Speaker 2: Black Holes are both physical objects and that we know
Speaker 2: there's something inside of them. There's some mass in there,
Speaker 2: but there's also a sort of conceptual layer to them,
Speaker 2: which is the event horizon. Then we'll talk about in
Speaker 2: a minute, But in the beginning, it just starts with
Speaker 2: a blob of stuff. You know, stuff has gravity, but
Speaker 2: gravity is sort of surprisingly weak, like it's the weakest
Speaker 2: fundamental force that's out there. Gravity is so much weaker
Speaker 2: than like electromagnetism or the weak nuclear force or the
Speaker 2: strong force. And you can discover this yourself. Every time
Speaker 2: you put like a fridge magnet up on the fridge,
Speaker 2: that tiny little magnet is holding it up. It's beating
Speaker 2: the gravity of the entire Earth. Right, This enormous planet
Speaker 2: with all of its rocks and magma inside of it
Speaker 2: is pulling on that thing, defeated by a tiny fridge magnet.
Speaker 2: That tells you how a weak gravity really is, and
Speaker 2: yet if you accumulate enough mass, gravity can become pretty powerful.
Speaker 2: I mean it's holding you to the surface. The Sun's
Speaker 2: gravity is holding the entire Earth in orbit, right, the
Speaker 2: gravity of the galaxy is holding it together. It really
Speaker 2: is pretty impressive on a cosmic scale. So if you
Speaker 2: build enough mass, you can get very powerful gravity. But
Speaker 2: if you take one more step, that's when you get
Speaker 2: to the crazy town. You take a lot of mask
Speaker 2: and you also squeeze it down to a really small
Speaker 2: space so you can get really close to the mass.
Speaker 2: That's when gravity really gets bunkers.
Speaker 1: So you have like say, the Sun could fit in
Speaker 1: a handbag, but it still retains the gravity of the
Speaker 1: entire Sun. And then you fit a bunch of those
Speaker 1: little handbag sized suns into a small area, then you
Speaker 1: have just an enormous amount of gravity.
Speaker 2: That's right. You could turn the Sun into a black hole. First,
Speaker 2: imagine you're standing on the surface of the Sun.
Speaker 1: Oh hot, too hot.
Speaker 2: I assume you're wearing appropriate foot whear, you know, not
Speaker 2: like ballerina.
Speaker 1: Shoes flop flops for the Sun.
Speaker 2: Now, the gravity on the surface of the Sun is
Speaker 2: already going to be very powerful. Because the Sun is very,
Speaker 2: very massive, but it's not a black hole, right, it's
Speaker 2: emitting light. Now, take the Sun and shrane get down
Speaker 2: to an object like a few kilometers across. If you're
Speaker 2: still at the same distance from the center, you're at
Speaker 2: where the surface of the Sun used to be, then
Speaker 2: you're gonna feel the same thing. Nothing has changed for you.
Speaker 2: You're still going to feel the gravity of the Sun
Speaker 2: in the same way because all you've done is change
Speaker 2: the internal configuration of the stuff inside the Sun. So
Speaker 2: nothing will change for you in terms of the gravity
Speaker 2: you feel if you're still at the same distance. But
Speaker 2: shrinking the Sun down to a region like the size
Speaker 2: of Los Angeles means you can actually get much closer
Speaker 2: to it. Right, instead of having to stand of the surface,
Speaker 2: you can now just be a few kilometers from the
Speaker 2: center of the Sun with all of that mass concentrated there.
Speaker 2: So that's why the gravity gets much much stronger. It's
Speaker 2: not just about having a lot of mass. It's about
Speaker 2: having a lot of mass in a very small space.
Speaker 2: And if you did that, if you shrunk the Sun
Speaker 2: down to the size of Us Angels, it would form
Speaker 2: a black hole.
Speaker 1: And that black hole is called traffic in Los Angeles. Okay,
Speaker 1: So it's really about how tightly packed this matter is
Speaker 1: such that it has a really strong effect of gravity
Speaker 1: and a small amount of area, and so that is
Speaker 1: what gives a black hole such powerful sort of I
Speaker 1: guess drainage, universal drainage, Yeah, exactly.
Speaker 2: In Newtonian gravity, the force between two objects depends on
Speaker 2: the distance between them, like one over the distance squared.
Speaker 2: So as the distance gets smaller, the force gets more powerful.
Speaker 2: And because it's one over distance squared, the force gets
Speaker 2: much much more powerful. If you're twice as close, it's
Speaker 2: four times as powerful. If you're a thousand times as close,
Speaker 2: it's a million times more powerful. So this force gets
Speaker 2: extraordinarily powerful if you can shrink the distances between the objects.
Speaker 2: That's what compacting the sun does for you. But then
Speaker 2: you go over this incredible threat. It's not just like
Speaker 2: the force gets stronger and stronger and stronger. It does
Speaker 2: something sort of incredible, which is to create this event horizon,
Speaker 2: a region of space where anything that enters can never.
Speaker 1: Escape, even with a really really strong rope.
Speaker 2: Even with a really really strong rope, even light that
Speaker 2: enters the event horizon is trapped.
Speaker 1: What does that mean for light to be trapped?
Speaker 2: This is where Newtonian gravity no longer works to describe
Speaker 2: what's going on. You're imagining light moves at the speed
Speaker 2: of light, why can't it escape the black hole? And also,
Speaker 2: light has no mass. In the Newtonian picture, gravity is
Speaker 2: just a force between two objects that have mass. Photons
Speaker 2: have no mass. How can the black hole pull on
Speaker 2: them at all? So the Newtonian picture really totally breaks
Speaker 2: down there. Instead, the way you need to think about
Speaker 2: it is in Einstein's view of the universe, where gravity
Speaker 2: is not a force, it's a curving of space time.
Speaker 2: Space itself is not always just like an evenly spaced grid.
Speaker 2: It has wiggles and curves in it. And if you
Speaker 2: shoot like two laser beams through space, if it's totally flat,
Speaker 2: then those laser beams will fly forever parallel, never touching.
Speaker 2: But if space is curved, those laser beams will bend,
Speaker 2: sometimes they may even cross, or they can diverge away
Speaker 2: from each other based on the curvature of space. And
Speaker 2: that curvature is not something we can see directly right.
Speaker 2: We can't look at a piece of space and say, oh,
Speaker 2: I see the curvature. Instead, we just see the effect
Speaker 2: of that curvature on the motion of stuff. And so
Speaker 2: Einstein tells us that when you have stuff in space, mass, energy,
Speaker 2: anything like that, it changes the shape of space. So
Speaker 2: back to the question of the photons. What's happening when
Speaker 2: you have a huge amount of matter in a really
Speaker 2: tiny space is that you've curved space so much that
Speaker 2: photons moving through that cannot escape. Photons are affected by
Speaker 2: mass because mass changes the shape of space and photons
Speaker 2: move through that space.
Speaker 1: So the gravity of the black hole causes space to
Speaker 1: sort of warp into this funnel shape through which photons
Speaker 1: have no choice but to travel, you know, through this
Speaker 1: funnel right exactly.
Speaker 2: Effectively, the shape of space is changed, is so distorted
Speaker 2: inside the black hole that every direction forward is towards
Speaker 2: the center of the black hole. When we talk about
Speaker 2: space being curved, we really mean like it's changing the
Speaker 2: organization of space, the way points are connected and the
Speaker 2: distances between them, and so inside the black hole is
Speaker 2: really just one direction of space, which is towards the.
Speaker 1: Center that's unnerving.
Speaker 2: And so this region we call this the event horizon.
Speaker 2: It's the region around the black hole where nothing can
Speaker 2: escape anything that enters that will never escape the event horizon,
Speaker 2: even at time equals infinity at the end of the universe.
Speaker 2: It's actually what we mean by the event horizon. The
Speaker 2: technical definition is that region of space which after infinite time,
Speaker 2: particles that enter will never leave. And the size of
Speaker 2: that event horizon depends on the parameters of the black hole,
Speaker 2: Like the bigger the mass inside the black hole, the
Speaker 2: more space is curved, and the larger the event horizon.
Speaker 2: It's actually a very simple calculation. It's called the short
Speaker 2: siled radius that tells you the radius of the black hole,
Speaker 2: which just depends on the mass, the gravitational constant G,
Speaker 2: and the speed of light.
Speaker 1: All Right, so we've got this sort of space funnel
Speaker 1: that happens that you cannot escape because you're inside of
Speaker 1: warped space that only goes in one direction, and that
Speaker 1: is into I guess, the tummy of the black hole.
Speaker 1: So say you could go inside the space funnel without
Speaker 1: dying horribly and sort of turning into spaghetti, what would
Speaker 1: you find there? Just like a really dense clump of matter, Like,
Speaker 1: what is in there?
Speaker 2: Boy? Do I wish I knew the answer that question?
Speaker 2: Because I would be headed to Sockholm to pick up
Speaker 2: my Nobel Prize. It's really one of the deepest questions
Speaker 2: in physics because we don't know what would be there.
Speaker 2: We have a prediction from general relativity, the theory that
Speaker 2: tells us how space bends and how matter moves through
Speaker 2: that in space, and that theory tells us that at
Speaker 2: the heart of a black hole is a singularity because
Speaker 2: things that fall in a black hole have to proceed
Speaker 2: towards the center. Eventually everything does, and the power at
Speaker 2: the center is so incredible that it's like a runaway effect.
Speaker 2: It just keeps compacting, compacting, and compacting, so that everything
Speaker 2: that falls into a black hole ends up at this
Speaker 2: one point, a point of infinite density, infinite curvature of space.
Speaker 2: That's the prediction from general relativity. But most physicists don't
Speaker 2: view that as a real prediction. They view that as
Speaker 2: an indication that the theory is failing. Anytime you get
Speaker 2: nonsense predictions from your theory, you're like, well, maybe the
Speaker 2: theory is not being applied correctly. I look at my kids,
Speaker 2: for example, and I say, oh, in the last fifteen years,
Speaker 2: they've grown from basically zero to two meters high. Does
Speaker 2: that mean in the next fifteen years they're going to
Speaker 2: grow to four meters high and then six meters high.
Speaker 1: Only time will tell I.
Speaker 2: Would say that's a nonsense prediction. It tells me that
Speaker 2: I've extrapolated beyond the region where the theory makes any sense.
Speaker 2: You know, a simple linear extrapolation doesn't make sense there.
Speaker 2: I need a new idea for how humans live after
Speaker 2: age fifteen. And that's what's happening in general relativity. We
Speaker 2: don't think there really is a point of infinite density.
Speaker 2: We think something else is going on, and we need
Speaker 2: a new theory to describe it, because that infinite density
Speaker 2: also violates fundamental principles of quantum mechanics tells us we
Speaker 2: have an incredible amount of energy concentrated in a single
Speaker 2: point without any sort of quantum fuzz, and so quantum
Speaker 2: mechanics tells us you need some sort of quantum fuzz
Speaker 2: when you have that much energy. So otherwise you know
Speaker 2: something's location and it's momentum very very precisely, and that's
Speaker 2: just not allowed by the Heisenberg uncertainty principle. So what's
Speaker 2: inside a black hole would tell us how space actually works.
Speaker 2: What is gravity in the quantum sense, Is it a
Speaker 2: quantum theory, is actually a force or is there some
Speaker 2: sort of space foam that bubbles up from fundamental quantum mechanics.
Speaker 2: We just don't know. But if we could look inside
Speaker 2: a black hole, we would get an enormous clue about
Speaker 2: how space actually works, what gravity really is, and how
Speaker 2: to unify general relativity and quantum mechanics.
Speaker 1: That's really interesting because like I hear about these things, like,
Speaker 1: you know, inside a black hole, you have sort of
Speaker 1: this infinite amount of I guess suckage. But I guess
Speaker 1: in a way that you're right, Like, it doesn't really
Speaker 1: make sense for there to be a single, infinitely dense point,
Speaker 1: especially with what you were describing with sort of the
Speaker 1: quantum fuzz the uncertainty we know exists in quantum mechanics.
Speaker 1: But I guess, like, are there any other theories? Are
Speaker 1: there any other leads that we have as to what
Speaker 1: is going on there? Like, how do we even go
Speaker 1: about figuring out what could be happening?
Speaker 2: Yeah, great question. It's really hard because we can see
Speaker 2: inside a black hole. We do have various other theories.
Speaker 2: We talked about a few of them on the podcast
Speaker 2: ideas that black holes are actually slowly collapsing stars, or
Speaker 2: maybe they're fuzzballs made out of strings, or maybe there's
Speaker 2: something else entirely. We do have a bunch of theories,
Speaker 2: but testing them is difficult because all the predictions they
Speaker 2: make a hidden behind this cosmic veil, which makes it
Speaker 2: pretty hard to know which one is right. But there
Speaker 2: are other features to a black hole which present a
Speaker 2: really fascinating opportunity because, as we talked about earlier, gravity
Speaker 2: is not just based on the amount of mass something has.
Speaker 2: It's also based on its spin and its energy in
Speaker 2: terms of electromagnetism. So these other features of a black hole,
Speaker 2: the spin and the charge, might present opportunities for cracking
Speaker 2: them open and seeing what's inside.
Speaker 1: Okay, well, I am really excited to find out what
Speaker 1: is inside these ballerina black holes. But first let's take
Speaker 1: a quick break. All right, and we are back and
Speaker 1: we were talking about the universe's deadliest ballerina, a spinning
Speaker 1: black hole, and what might be inside of their So
Speaker 1: we have just gotten to the part where we are
Speaker 1: talking about how black holes spin. So I'm having a
Speaker 1: hard time picturing what it means for a black hole
Speaker 1: to spin, Daniel, can you let me know, is this
Speaker 1: like a black hole doing a pure What is the
Speaker 1: spin of the black hole?
Speaker 2: Yeah, it's a good question, but what is spinning exactly?
Speaker 2: But we know that black holes are made of stuff, right,
Speaker 2: there's stuff that has fallen in, there's mass, there's energy
Speaker 2: in there. And when stuff falls into a black hole,
Speaker 2: it has angular momentum. Like if you drop a ping
Speaker 2: pong ball straight in towards the center of a black hole,
Speaker 2: then you're not making it spin at all. But if
Speaker 2: you drop a ping pong ball and it hits the
Speaker 2: black hole sort of far from the center, then you're
Speaker 2: like giving it a push. The way somebody on the
Speaker 2: Merry Go Round is spinning around and you give them
Speaker 2: a tug near the edge of the Merry Go Round.
Speaker 2: You're applying a torque to the Merry Go Round. Same
Speaker 2: way you shoot a bunch of ping pong balls at
Speaker 2: a black hole, but you do it off center. You're
Speaker 2: going to add some spin to the black hole. So
Speaker 2: as long as things fall into a black hole and
Speaker 2: don't head directly for the center, if they spin around first,
Speaker 2: then as they fall in, they're going to keep that spin.
Speaker 2: Because in our universe, angular momentum is conserved can't just
Speaker 2: go away, which means because the black hole is made
Speaker 2: of stuff that was spinning around it, eventually a black
Speaker 2: hole itself is spinning. What's actually spinning? I mean, if
Speaker 2: you imagine like a singularity at the heart of the
Speaker 2: black hole, it's just a point. Points themselves cannot spin.
Speaker 2: So people imagine that what's going on inside a black
Speaker 2: hole is not a point spinning but a ring. Instead
Speaker 2: of having a point of infinite space, you have like
Speaker 2: a ring of singularities, and that ring is spinning. Again,
Speaker 2: this is the general relativity prediction, which we think is
Speaker 2: probably wrong, but that's the more complex picture of what's
Speaker 2: happening inside a spinning black hole.
Speaker 1: So you have multiple singularities that form sort of a
Speaker 1: ring inside potentially mm hmmm.
Speaker 2: Because a ring can spin, it has some spatial extent,
Speaker 2: it has some spatial extent at least in one dimension,
Speaker 2: and that ring itself can spin to preserve the angular momentum.
Speaker 2: Because again, angular menum can't just go away in our universe,
Speaker 2: we think it's conserved.
Speaker 1: Okay, so how do we even know that black holes
Speaker 1: can spin?
Speaker 2: Yeah? Great question. According to the theory, they can spin
Speaker 2: in general relativity, and also, as we talked about later
Speaker 2: in the podcast, we've seen a bunch of black holes
Speaker 2: and we've seen the effect of their spinning on the
Speaker 2: stuff around them. And this is a really fascinating feature
Speaker 2: of general relativity, that the spin of an object does
Speaker 2: change its gravity. I mean, again, in the Newtonian picture,
Speaker 2: which I think is pretty intuitive for most people, gravity
Speaker 2: is just a force between two objects that half mass,
Speaker 2: like the Earth is pulling on you. If you're a
Speaker 2: satellite in space orbiting the Earth, you think that your
Speaker 2: orbit is affected only by the mass of the Earth.
Speaker 2: Why would it change the gravity if the Earth is spinning, right,
Speaker 2: the formula is just like gmm over r square. There's
Speaker 2: no opening in there to describe this spinning of the object.
Speaker 2: And Newton would say it doesn't matter if the Earth spins,
Speaker 2: it's like, got the same mass configuration everywhere. As long
Speaker 2: as it's a perfect sphere, it should have the same gravity.
Speaker 2: But in Einstein's picture of the universe, that spin does
Speaker 2: change the gravity of the object.
Speaker 1: And so we're on the Earth and so we are
Speaker 1: experiencing a different gravity from the Earth because it is spinning.
Speaker 1: Then if it was staying still, and so things around
Speaker 1: a black hole would also be experiencing a different effect
Speaker 1: from a spinning black hole versus a stationary black hole exactly.
Speaker 2: And the crucial thing to understand is that in general relativity,
Speaker 2: it's not just that mass bend space, but the things
Speaker 2: that bend space come together in a complicated dance. It's
Speaker 2: described not just by like a number, as it is
Speaker 2: for Newton, like a single number the mass, but this
Speaker 2: stress energy tensor tensor is just a fancy way of saying,
Speaker 2: like a matrix, a bunch of stuff all organized. So
Speaker 2: the mass definitely affects things, energy affects things, can energy
Speaker 2: affects things. Also, Electromagnetic energy and spin all come together
Speaker 2: in this complicated dance. But it's not just like you
Speaker 2: add up all the energy and you get a number
Speaker 2: and that's how much the curvature of space is. It
Speaker 2: is more complicated, and spin affects the curvature of space
Speaker 2: in a really weird way in that it spins space.
Speaker 2: They call it frame dragging because it really is like
Speaker 2: spinning space like a fluid. So what happens to an
Speaker 2: object around something that's spinning is that you're not just
Speaker 2: pulled towards it by its gravity. You're also spun. So
Speaker 2: if you have a satellite orbiting the Earth or a
Speaker 2: black hole and that Earth or black hole is spinning,
Speaker 2: it's going to apply a little torque, a little twist
Speaker 2: to the thing orbiting. We've actually measured this. We have
Speaker 2: a satellite orbiting the Earth called gravity Probe B. It
Speaker 2: has very very precise gyroscopes to measure its direction and
Speaker 2: its spin. We have a whole podcast episode about this,
Speaker 2: and they've detected this effect that the spinning of the
Speaker 2: Earth applies like a little torque to this satellite. It's
Speaker 2: very very subtle. In the case of the Earth, it
Speaker 2: requires extra ordinary precision even to detect it, but it's there.
Speaker 2: We've confirmed it.
Speaker 1: So if you drop like a marble down a funnel,
Speaker 1: like straight in the center down a stationary funnel, it
Speaker 1: just goes straight down. If you drop it sort of
Speaker 1: on the side of a stationary funnel, it rolls down
Speaker 1: the side and then into the center. But if the
Speaker 1: funnel like is spinning, like if you start like sort
Speaker 1: of moving the funnel around and then put the marble
Speaker 1: on the side of the funnel, the marble itself will
Speaker 1: start to like sort of circle around as it goes
Speaker 1: down into the center of the funnel. Is that kind
Speaker 1: of what's happening.
Speaker 2: That's sort of what's happening. There's some tricky bits there
Speaker 2: about the funnel pushing on the marble, but in effect, yeah,
Speaker 2: the spinning of the black hole or the spinning of
Speaker 2: the object applies a spin to stuff near it. So
Speaker 2: in the case of a spinning black hole, you don't
Speaker 2: just have the event horizon, you also have this region
Speaker 2: past the event horizon, called the ergosphere, where the black
Speaker 2: hole is applying a certain amount of twist to stuff.
Speaker 2: We talk once about how you can ext diract energy
Speaker 2: from a black hole basically by dropping rocks near it.
Speaker 2: Those rocks get spun as they go through the ergosphere,
Speaker 2: extracting some energy from the black hole basically for free.
Speaker 2: So I think Freeman Dyson or maybe Roger Penrose came
Speaker 2: up with this plant to use black holes as a
Speaker 2: power source, which.
Speaker 1: Is pretty awesome, sounds really easy. All we need to
Speaker 1: do is get really close to a black hole.
Speaker 2: Exactly. So these spinning black holes have this weird effect
Speaker 2: on space time, and it also affects their event horizon,
Speaker 2: like how close you can get to a black hole
Speaker 2: and still escape depends not just on the mass of
Speaker 2: the black hole, but also on its spin and its
Speaker 2: electric charge. Because these things enter into the stress energy
Speaker 2: tensor that general relativity uses to do this calculation. It
Speaker 2: can change the size of the event horizon if you
Speaker 2: spin your black hole or if you give it electric charge.
Speaker 1: So are we kind of getting into the limitations of
Speaker 1: how fast a black hole can spin, because if it
Speaker 1: is affecting the event horizon, and presumably there has to
Speaker 1: be some characteristics to the event horizon to keep that
Speaker 1: black hole stable. Otherwise, like if you change that event
Speaker 1: horizon too much, what could happen to the black hole?
Speaker 2: Exactly right, And the crucial thing to understand is that
Speaker 2: spinning a black hole will shrink its event horizon. This
Speaker 2: is counterintuitive because you feel like, well, you're adding energy
Speaker 2: to the black hole. Isn't that just putting more fuel
Speaker 2: on the fire. Remember that spin and things like electric
Speaker 2: charge have a complicated way that they contribute to the
Speaker 2: curvature of space. It's not really very intuitive, but adding
Speaker 2: spin or electric charge to your black hole will shrink
Speaker 2: the event horizon, will make it smaller. It's possible to
Speaker 2: get closer to one of these objects if it's spinning
Speaker 2: or has electric charge than otherwise. And as you were saying,
Speaker 2: there's a breakdown point. If you spin the black hole enough,
Speaker 2: the event horizon shrinks down to zero, which means there
Speaker 2: is no event horizon, which means you've like cracked open
Speaker 2: the black hole and revealed the brand mystery that's inside
Speaker 2: of it.
Speaker 1: I bet it's nugat. I bet it's filled with nugat.
Speaker 2: I really think you should have a snack before we
Speaker 2: do these episodes, Katie.
Speaker 1: I'm sorry if you make space sound so delicious, Well
Speaker 1: that's I mean. But presumably we don't know if this
Speaker 1: has ever happened or if it could ever happen.
Speaker 2: We don't know if this has ever happened or if
Speaker 2: it actually could ever happen. Again, it's sort of a
Speaker 2: prediction of general relativity, And we don't think that general
Speaker 2: relativity is the law of the actual universe. It's just
Speaker 2: the law of the universe we describe in our head,
Speaker 2: which so far works perfectly to describe everything we see.
Speaker 2: We think that it probably fails the inside of a
Speaker 2: black hole, but we've never seen that, So this is
Speaker 2: a prediction of general relativity that if you overspin a
Speaker 2: black hole, you will reveal a naked singularity, you will
Speaker 2: crack it open, you'll shrink the event horizon down to zero,
Speaker 2: which is really fascinating because it seems like, on one hand,
Speaker 2: an opportunity to learn this secret of the universe, like,
Speaker 2: let's go out and overspin a black hole. I want
Speaker 2: to see what's inside of it. Right. On the other hand,
Speaker 2: it's like, well, is that really allowed? Is there something
Speaker 2: in the universe that would prevent that from happening?
Speaker 1: So do we have any calculations as to like how
Speaker 1: fast a black hole can spin before it breaks itself apart?
Speaker 2: Yeah, we do actually have the calculation from general relativity,
Speaker 2: and it tells us that if there's more energy in
Speaker 2: the spin than there is in the mass, then the
Speaker 2: radius of the event horizon becomes a negative, which basically
Speaker 2: means there is no event horizon. So that means the
Speaker 2: bigger the black hole, the faster it can spin. So
Speaker 2: it's not like there's a maximum spin rang for all
Speaker 2: black holes, but there's like a ratio if you have
Speaker 2: more energy in your spin than you do in your
Speaker 2: mass than the event horizon shrinks. It's actually the same
Speaker 2: thing for electric charge. We did a podcast episode about
Speaker 2: that recently. If you overcharge the black hole, meaning adding
Speaker 2: more energy to it in electromagnetism by adding like electrons
Speaker 2: for and there's a certain ratio of mass to charge
Speaker 2: you cannot exceed because it will shrink the event horizon
Speaker 2: down to zero. So now there's the interesting question of like, well,
Speaker 2: what happens if you do it? Or is there a
Speaker 2: physical way to actually make this happen?
Speaker 1: I mean, I think it sounds like you got to
Speaker 1: put a sign up at the teacups ride that you
Speaker 1: must be this big of a black hole to be
Speaker 1: able to spin this bath.
Speaker 2: Let's just regulate the problem away. Huh. How very progressive
Speaker 2: of you, Katie. But there's sort of two interesting questions
Speaker 2: here to tease apart. One is like does the universe
Speaker 2: say this is actually impossible? And the other is could
Speaker 2: you make it happen? Right? Some things are not against
Speaker 2: the rules, but you have no way of making them happen,
Speaker 2: no way of like going from our universe to creating
Speaker 2: some configuration that you're curious about, Like, not everything is
Speaker 2: possible to build. You have to assemble things, right. You
Speaker 2: need a recipe for saying I'm gonna build this house,
Speaker 2: I'm gonna put one brick and then the next brick,
Speaker 2: and then the next brick, a way to step by
Speaker 2: step put it together. And so while we don't think
Speaker 2: that it's against the rules of general relativity to crack
Speaker 2: open a black hole this way by overspinning it, we
Speaker 2: don't think that's actually a recipe that would let you
Speaker 2: do it. There's no way to, like, step by step
Speaker 2: make this.
Speaker 1: Happen, because if you got close enough to a black
Speaker 1: hole to build something, it seems like the black hole
Speaker 1: would just suck all your tools away.
Speaker 2: That's one example of a practical reason why you might
Speaker 2: not be able to do this. But people have thought
Speaker 2: about other thought experiments, like say you have a black
Speaker 2: hole and you want to try overspinning it. Take the
Speaker 2: suggestion you made earlier, which is like, well, let's just
Speaker 2: shoot ping pong balls at it, but not right at
Speaker 2: the center. Right. Every time you shoot a ping pong
Speaker 2: ball at a spinning black hole, you're giving it a
Speaker 2: little push. So can't you just keep doing that forever,
Speaker 2: eventually exceeding this ratio and cracking open the black hole. Well,
Speaker 2: it turns out that's not actually possible. What happens when
Speaker 2: you shoot something into a spinning black hole is that
Speaker 2: the black hole causes it to accelerate, which gives off
Speaker 2: some gravitational radiation, because every acceleration requires some kind of
Speaker 2: radiation to conserve momentum. You can't just like zoom in
Speaker 2: one direction without pushing off in the other direction. And
Speaker 2: this gravitational radiation, it turns out, will decrease the angular
Speaker 2: momentum of the thing you're throwing in. So basically, a
Speaker 2: black hole that's right on the edge of being extremal
Speaker 2: that's almost overspun will basically despin stuff that you throw
Speaker 2: into it, so that it's impossible to overspin the black hole.
Speaker 1: So it's self limiting in a way.
Speaker 2: Yeah, exactly, it's sort of self limiting. This is called
Speaker 2: the back reaction. Like the black hole and the object
Speaker 2: have effects on each other before they fall in, and
Speaker 2: so they're pulling on each other, tugging on each other,
Speaker 2: radiating away some gravitational energy. And it's very complicated calculation,
Speaker 2: but people have gone through this and try to imagine
Speaker 2: all possible ways you can throw things into a black hole,
Speaker 2: and none of them can actually push it over the edge.
Speaker 2: That's not saying this configuration is impossible. It's not saying
Speaker 2: overspun black holes are not possible in the universe. It's
Speaker 2: saying we don't know of a recipe for building one,
Speaker 2: because as soon you get close to overspun black holes
Speaker 2: become very very good at rejecting any more angular momentum.
Speaker 1: I mean, it's hard to feel like this black hole
Speaker 1: is not intentionally creating rules so that it doesn't rip apart.
Speaker 1: But I guess that's a very homo sapient mindset that
Speaker 1: there's some intentionality here.
Speaker 2: No, I think you're totally onto something really fascinating, which
Speaker 2: is like the secrets of the universe are hidden behind
Speaker 2: this event horizon. Then as soon as we're clever enough
Speaker 2: to think about, ooh wait, here's a loophole, maybe we
Speaker 2: can break it open, it turns out there's like another
Speaker 2: reason why that can't work. It does feel almost like
Speaker 2: a conspiracy theory, like we have all the answers in
Speaker 2: this little box, but there's no way to open the box.
Speaker 1: We'd find the off switch to the universe in there,
Speaker 1: and they don't want us to find it.
Speaker 2: This is very similar to a topic we talked about
Speaker 2: recently about overcharging the black hole. In a similar way,
Speaker 2: if a black hole has a lot of electric charge,
Speaker 2: then it repels stuff you try to throw in it
Speaker 2: that has more electric charge charge, so it prevents you
Speaker 2: from overcharging the black hole. This is the way black
Speaker 2: holes prevent you from over spinning themselves with this back
Speaker 2: reaction on stuff you throw in. And then people try
Speaker 2: to be even more clever, like, well, what about internal
Speaker 2: quantum spin? You know what if you like take something
Speaker 2: that has internal quantum spin, like electrons, and you drop
Speaker 2: them straight into the black hole without angular momentum, wouldn't
Speaker 2: that add to the overall spin of the black hole?
Speaker 2: Great idea. So people went off to do the calculations
Speaker 2: and it turns out there are spin spin interactions like
Speaker 2: the spinning of the black hole and the quantum spin
Speaker 2: of this object will induce an interaction between the two things,
Speaker 2: which again will lower the angular momentum of the falling
Speaker 2: in object, avoiding going over the threshold. So the summary
Speaker 2: is that we know of no way to overspin a
Speaker 2: black hole. We currently think it might be impossible again
Speaker 2: not impossible for it to exist as an overspun object.
Speaker 2: To break this limit, we just don't know how to
Speaker 2: make it happen.
Speaker 1: Man, that really does seem like the black hole is
Speaker 1: just trying to frustrate particle physicists like you.
Speaker 2: It doesn't take a lot. You're just going to have
Speaker 2: the secrets of the universe and hide them from us.
Speaker 1: So I'm also curious to find out because we've talked
Speaker 1: about how black holes have a maximum speed limit that
Speaker 1: the black hole Highway patrol manages to keep in check
Speaker 1: through these radiation, these spin spin interactions. But I'm also
Speaker 1: curious if there is a minimum speed limit for a
Speaker 1: black hole, if a black hole could get away with
Speaker 1: not spinning. But maybe first we should take a quick
Speaker 1: break to make sure that we don't get ticketed by
Speaker 1: the black hole police. So we are back and we
Speaker 1: are keeping this podcast under the speed limit. But as
Speaker 1: we know, with speed limits, it's possible to go too
Speaker 1: slow on a highway, people get mad. So is this
Speaker 1: the same thing with black holes? Do they hack to
Speaker 1: spin or can they just laze around and stay still.
Speaker 2: It's a great question because most of the description of
Speaker 2: black holes you hear about out there are these very simple,
Speaker 2: short style black holes, just stuff in space with massive
Speaker 2: gravity and not even talking about the spin, and that's
Speaker 2: the sort of like generic description of a black hole.
Speaker 2: But if you go out there and look for stuff
Speaker 2: in space, there's basically nothing that isn't spinning. Right, Everything
Speaker 2: in space is spinning. There was inherent angular momentum too stuff,
Speaker 2: and that can't go away, and so stuff keeps spinning.
Speaker 2: So it really makes no sense to imagine a huge,
Speaker 2: massive object with all of this stuff thrown into it,
Speaker 2: somehow perfectly balanced between positive and negative angle momentum so
Speaker 2: that it doesn't spin. It's like flipping a billion coins
Speaker 2: and getting exactly one half of them heads and exactly
Speaker 2: one half of them tails. That's what you would need.
Speaker 2: So it's not impossible for a black hole to have
Speaker 2: no spin. It's just extraordinarily improbable. It's like an enormous
Speaker 2: pencil balanced on its very tip forever.
Speaker 1: I guess that's a little bit scary. The idea of
Speaker 1: a black hole that's perfectly imbalanced, somehow defying probability and
Speaker 1: just still kind of sucking things in it.
Speaker 2: Is pretty weird. And black holes are weird and powerful objects.
Speaker 2: And all the ones that we've seen out there in
Speaker 2: space are spinning. We can tell they're spinning because the
Speaker 2: stuff around them is spinning. Like the black hole picture
Speaker 2: that we've seen, what we're actually looking at when we
Speaker 2: look at that picture is not the black hole itself.
Speaker 2: There's like a blank spot in the middle where the
Speaker 2: black holes like not giving off any light. What we're
Speaker 2: looking at is like the Crispy Kream donut of stuff
Speaker 2: around the black hole that's falling in, and that stuff
Speaker 2: is glowing. It's so hot because of the tidal forces,
Speaker 2: the frictional gravity of the black hole, rubbing and squeezing
Speaker 2: all that stuff as it orbits, the swishing around the
Speaker 2: black hole before it actually gets flushed down this cosmic
Speaker 2: toilet bowl. And every black hole that we've ever seen
Speaker 2: has this very strong spin to the stuff around it,
Speaker 2: which means that it is also spinning.
Speaker 1: I mean, you did scold me about needing a snack
Speaker 1: before the show, and then you describe the stuff around
Speaker 1: a black hole as Krispy Kreme donut getting mixed messages here,
Speaker 1: and then you described it as like a toilet bowl,
Speaker 1: so getting really mixed messages here. So we have measured
Speaker 1: things around a black hole. How do we do that?
Speaker 2: Yeah, that's a great question. One thing we can do
Speaker 2: is look at the orbits of stars around the black
Speaker 2: hole like in other galaxies. We can't resolve individual stars,
Speaker 2: but we can tell how fast stars are spinning. We
Speaker 2: can look at like the velocity of streams of stars.
Speaker 2: We can't make out the individual ones, especially near the
Speaker 2: centers of those other galaxies, but we can tell how
Speaker 2: fast they're moving because of the red shift, and we
Speaker 2: can tell the density, and so we can infer the
Speaker 2: presence of these super massive black holes in other galaxies.
Speaker 2: And sometimes we can directly see the effect of spin
Speaker 2: on the stuff around it, Like if the black hole
Speaker 2: is spinning, it changes like how close stuff can get
Speaker 2: to the black hole, and it also changes how that
Speaker 2: stuff is spun. There was one case where we saw
Speaker 2: two black holes orbiting each other. So like a really
Speaker 2: big black hole called OJ two nine seven. It has
Speaker 2: like almost nineteen billion solar masses, one of the biggest
Speaker 2: black holes out there that we know about. It's orbited
Speaker 2: by another black hole that has just like one hundred
Speaker 2: and fifty million solar masses. I mean, already a gigantic object,
Speaker 2: but tiny in comparison to the big Mama black hole
Speaker 2: it's orbiting, and the orbit of this smaller black hole
Speaker 2: is processing. Imagine like an elliptical orbit, not a perfect circle,
Speaker 2: but the smaller black hole is moving around in an ellipse,
Speaker 2: and ellips is like an elongated circle. Well, the direction
Speaker 2: of the elongation is changing as the little black hole
Speaker 2: orbits the big black hole. This is like the spin
Speaker 2: of the orbit of the little black hole around the
Speaker 2: big black hole, and the procession of that orbit tells
Speaker 2: us something about the spin of the big black hole
Speaker 2: because its spin is contributing to that procession.
Speaker 1: That's crazy. I mean first of all that a big
Speaker 1: black hole can suck up a littler black hole, but
Speaker 1: also just that you can see that change in movement
Speaker 1: that would only be possible given sort of the spin
Speaker 1: of the bigger black hole. Can we actually measure how
Speaker 1: fast the big Mama black hole is spinning given the
Speaker 1: procession of the smaller black hole.
Speaker 2: Yeah, we can, Actually, we can make a calculation, and
Speaker 2: the surface of that black hole is moving at almost
Speaker 2: sixty thousand kilometers per second, which is like twenty percent
Speaker 2: of the speed of light. Sort of amazing. Now, this
Speaker 2: thing is so big though, it's radius is so huge.
Speaker 2: It's like three hundred and sixty au it's the distance
Speaker 2: between the Earth and the Sun. It's so large. Even
Speaker 2: though it's moving to twenty percent of the speed of light,
Speaker 2: it still takes five million seconds to complete one spin.
Speaker 2: So it's such a big bear that it still takes
Speaker 2: a huge amount of time to spin once.
Speaker 1: Five million seconds is how many years?
Speaker 2: Five million seconds is like sixteen percent of a year.
Speaker 1: Okay. I can't think in seconds, so that's like two months.
Speaker 2: So this thing is spinning at twenty percent of the
Speaker 2: speed of light, still takes two months to complete one spin.
Speaker 1: Okay. Because my perception of seconds are different. Like when
Speaker 1: I'm in this podcast, they go by very fast because
Speaker 1: I'm really interested. If I'm waiting for a bus to come,
Speaker 1: the seconds are very long.
Speaker 2: But there's another black hole that's spinning even faster. This
Speaker 2: one is really small. It has only two million solar masses.
Speaker 2: It's NNGC thirteen sixty five. But the cool thing about
Speaker 2: this black hole is it doesn't have like a baby
Speaker 2: black hole orbiting it, but it's close enough that we
Speaker 2: can study the accretion disc in great detail.
Speaker 1: Is the accretion disc the stuff that is caught up
Speaker 1: circling the black hole.
Speaker 2: It's the stuff that's on deck about to go into
Speaker 2: the black hole. It's like falling in, but because it
Speaker 2: has angular momentum, it just doesn't fall straight in the
Speaker 2: same way like the Earth doesn't just fall into the
Speaker 2: Sun because of the Sun's gravity it's in orbit. This
Speaker 2: stuff also has angular momentum, which slows it down, which
Speaker 2: prevents it from falling immediately in. It has to like
Speaker 2: radiate away some of its energy in order to fall in.
Speaker 2: So inside the accretion disk, this stuff is like bumping
Speaker 2: and grinding and glowing, giving off some of its energy
Speaker 2: just before it falls into the black hole. And so
Speaker 2: by studying what's happening inside that accretion disc, it's like
Speaker 2: a probe for what the gravity of the black hole
Speaker 2: is doing to it. We can understand the tidle force
Speaker 2: is the strength of the gravity and also the effect
Speaker 2: of the spin, because the spin of the black hole
Speaker 2: changes like how close the accretion disc can get to
Speaker 2: the event horizon.
Speaker 1: So, if it's spinning faster, can the accretion disk get
Speaker 1: closer to the event.
Speaker 2: Yes, exactly. If it's spinning the same way as the
Speaker 2: accretion disc, then the accretion disc can get very very
Speaker 2: close to the black hole without actually falling in. Whereas
Speaker 2: if the accretion disc is spinning one way the black
Speaker 2: hole is spinning the other way, then you end up
Speaker 2: with this like weird gap between the black hole and
Speaker 2: the accretion disc where stuff in that gap can't exist
Speaker 2: because it would just fall immediately into the black hole.
Speaker 2: But if they're spinning together, then the spin of the
Speaker 2: black hole like accentuates the spin of the accretion disc
Speaker 2: and it helps it spin faster. It can actually get
Speaker 2: closer in before falling into the black hole. So by
Speaker 2: looking at like the size of the gap between the
Speaker 2: event horizon and the start of the accretion disk, they
Speaker 2: can measure the rate of spin of the black hole itself.
Speaker 2: And this guy is spinning at eighty five percent of
Speaker 2: the maximum rate allowed for this mass black hole.
Speaker 1: Man, he's got like the universe police kind of following him.
Speaker 1: They haven't put their lights on yet, but they're keeping
Speaker 1: an eye on them.
Speaker 2: Exactly, And so this is fascinating, Like, Wow, this guy's
Speaker 2: close to an overspun black hole. Be really interesting to
Speaker 2: watch as that stuff falls in, to see like, is
Speaker 2: it's beating up? Does it gradually approach ninety nine point
Speaker 2: nine nine nine percent? Or are we wrong about how
Speaker 2: black holes can't be overspun and maybe this one will
Speaker 2: actually get cracked open, maybe that stuff will fall in
Speaker 2: and erase the event horizon.
Speaker 1: We better keep an eye on it also, because the
Speaker 1: faster it spins, the closer you can get to the
Speaker 1: event horizon without falling in, Daniel, is this the kind
Speaker 1: of black hole that you would want to travel to
Speaker 1: and try to sort of drop some sonar into the
Speaker 1: middle of the black hole?
Speaker 2: I mean, I don't even travel the conferences anymore. There's
Speaker 2: no way that I'm going to visit a black hole.
Speaker 2: Absolutely not. I am too old for that. But I'm
Speaker 2: very happy to send up you know, co hosts, guest hosts,
Speaker 2: all these kind of people who would happily volunteer.
Speaker 1: It's an interesting pool of people. You'd be okay sending
Speaker 1: to a black hole as long as you bring.
Speaker 2: Your travel mike and tell us what you're learning as
Speaker 2: you fall in.
Speaker 1: Yeah. I do have a good wind screen on it,
Speaker 1: so it should still come through clear.
Speaker 2: But you know, I think the less to take away
Speaker 2: from this is that space is so much weirder than
Speaker 2: we thought it was. Even the idea of space being
Speaker 2: bent into curves by mass is weirder than Newton thought of.
Speaker 2: But there's so much more to it than just that
Speaker 2: space is not just bent into curves. It's also spun.
Speaker 2: It's flowing like a fluid around very massive objects, dragged
Speaker 2: along with them like a fork twirling that spaghetti. And
Speaker 2: that changes the way that black holes operate. The event
Speaker 2: horizon of a black hole is not just a function
Speaker 2: of its mass. It's also a function of its spin
Speaker 2: and its electric charge.
Speaker 1: I feel like this is a great personality test of
Speaker 1: who wants to see the black hole over spin and
Speaker 1: break apart and maybe break the universe? And who would
Speaker 1: like the black hole to not break apart and keep
Speaker 1: things safe?
Speaker 2: Oh my god, who would be in that second category?
Speaker 2: Who would not want the black hole to wreak the
Speaker 2: rules the universe? Who we've been talking about here, not me?
Speaker 2: Remember that seeing the universe break the rules, it's not
Speaker 2: a failure, it's not a problem, it's an opportunity. Those
Speaker 2: are the moments when we learn what the real rules
Speaker 2: of the universe are. The universe is out there, we
Speaker 2: think following some set of rules, and we have in
Speaker 2: our heads some internal description of that, a model of
Speaker 2: how we think the universe works, and when those two
Speaker 2: things clash, that's when we get to update our list.
Speaker 2: Were like, oh, turns out we were wrong about what's
Speaker 2: going on over here. Let's fix things. But we need
Speaker 2: those clues sometimes, we need the universe to show us
Speaker 2: what's wrong with our understanding of how it works by
Speaker 2: contradicting our predictions. And so if we see the universe
Speaker 2: breaking the rules that we thought, it's not a failure.
Speaker 2: It's a huge leap forward. It's an open door to
Speaker 2: a deeper understanding of how the universe actually works.
Speaker 1: Well, kids, you heard Daniel. You gotta break rules to
Speaker 1: learn exactly.
Speaker 2: Unfortunately, it seems like black holes are fortresses of knowledge.
Speaker 2: Not only do they have these event horizons that keep
Speaker 2: us from seeing what happens at extraordinarily extreme circumstances, but
Speaker 2: also there very wildly at protecting those event horizons. You
Speaker 2: thought you could overcharge a black hole, it repels your idea.
Speaker 2: You thought you could overspin a black hole. It's got
Speaker 2: its own spin doctors to keep you from doing it. So,
Speaker 2: for now, at least, we don't know how to crack
Speaker 2: open a black hole.
Speaker 1: We need a rogue black hole, a maverick.
Speaker 2: We need to put a black hole in witness protection.
Speaker 1: What would that even look like?
Speaker 2: Put a black hole inside another black hole? I don't know,
Speaker 2: we'll ask the SBI. Well, thanks very much everybody for
Speaker 2: going on this journey inside the mysteries of the universe,
Speaker 2: wondering what's going on inside a black hole and how
Speaker 2: we could ever know and whether we ever will. And
Speaker 2: thanks very much Katie for taking this crazy physics tour
Speaker 2: and breaking all the rules with us.
Speaker 1: Thank you for not shooting me into the center of
Speaker 1: a black.
Speaker 2: Hole, not yet, at least stay tuned. Thanks everyone, tune
Speaker 2: in next time. Thanks for listening, and remember that Daniel
Speaker 2: and Jorge Explain the Universe is a production of iHeartRadio.
Speaker 2: For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts,
Speaker 2: or wherever you listen to your favorite shows.