Daniel and Kelly’s Extraordinary Universe - How fast can a black hole spin?

Episode Date: August 8, 2023

Daniel and Katie get their heads spinning as they spiral into the center of spinning black holesSee omnystudio.com/listener for privacy information.

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there's a good outcome. Avoidance is easier. Ignoring is easier. Denials easier. Complex problem solving takes effort listen to the psychology podcast on the iHeart radio app apple podcasts or wherever you get your podcasts so that means you don't understand it. Hey Daniel would you say you understand general relativity? I mean I'd say I understand it better every week so that means you don't understand it I think it still makes my head spin, to be honest. What's the trickiest part of it? I think the hardest part to get your mind around is that spinning stuff has different gravity than stuff that doesn't spin.

Yeah, okay, there's no way to spin that. That is just confusing. Sometimes I just want to, like, spin up extra brains to help me figure out how space itself can spin. That is quite a yarn. You are spinning. Maybe I'm spinning a web of physics or a web of lies. I'll take that for a spin. Hi, I'm Daniel. I'm a particle physicist and a professor at UC Irvine, and those are all the spin-related jokes I could think of.

Hi, I'm Katie, and I cannot put a new spin on those jokes. I am not a particle physicist, but I enjoy physics in the universe, and I host an animal-themed podcast. All right. I got one more spin-related joke for you. You ready? Wait. Okay, now I'm ready. Why did scientists stop watching the Earth spin?

I don't know, Daniel. Why? They got bored and called it a day. That's my soul leaving my body. That was a good one. And welcome to the podcast. Daniel and Jorge explain the universe. in which we try to avoid your brain spinning inside your cranium as we take a tour of how the

universe works. All of its weird and wonderful ways of operating from the tiniest quantum particles to the most massive of supermassive black holes spinning at the centers of galaxies. Motion and rotation and spinning and translation and all of these things are essential to the way the universe works. In some sense, understanding how things move and what they are is what physics is all about. That's what we try to do on this podcast, break down, why things move, how they spin, and how that affects everything around them. My friend and co-host Jorge can be with us today, but we're very happy to have Katie here with us to take physics for a spin. You know, I used to do ballet when I was a kid, and one of the things you have to learn to do in ballet is to spin without

getting dizzy and falling over. And so you would kind of try to keep your head looking at a point of reference and sort of twist your head around so that you don't get dizzy. Are you saying that when I'm a ballerina, I have more gravity than regular non-ballerinas? I'm saying when you do that spin, you drag the fabric of space time along with you, like sticking a fork in spaghetti and twisting it. That's really cool. No wonder my teacher was so strict. She didn't want us tearing holes in the fabric of the universe with our off-tempo spins.

She was really just looking out for your safety, it sounds like. And as scientists have tried to understand the nature of gravity, why things fall down, why things orbit each other in the sky, we have learned so much about the very nature of the universe around us, from Aristotle just telling us, things fall down because they like sort of going down, to Newton telling us that gravity is a force between objects that have mass. to Einstein telling us that actually gravity is the bending of space, invisibly curving in front of us and affecting the path of everything that moves through it. Every time it requires a huge conceptual change in the way the universe works around us, often with surprising results and strange phenomena that really test our ability to understand the nature of the universe. Do we now understand what gravity is? Like, has that answer been solved for good?

we're going to solve it today on the podcast candy that's right oh boy let me get out a notepad after this podcast buy your ticket to stockholm no gravity is definitely not understood in fact we know that our current understanding of gravity is limited that there are parts of the universe it just fails to describe and parts of the history of the universe that just don't make sense if you take literally Einstein's theory of general relativity and one of those places is at the heart of of black holes. Black holes are famous because they're weird and they gobble stuff up and yet they actually exist in the universe. But they hold within them mysteries and secrets. It might tell us how gravity actually works at the quantum mechanical level. The thing that we don't know how to do

is merge Einstein's picture of gravity with the way quantum mechanics tells us that reality operates at the smallest level. And because we can't go inside black holes, we have to do other things to them. And one thing we can do to black holes is to spin them and see what happens. when they stick their fork into the spaghetti of space time. Okay, when you say we can spin black holes, do you mean we can just sort of twirl them around like a giant top or something or a huge destructive bay blade? Are these things just out there naturally spinning

or are we shooting ping pong balls at them to try to get them to spin? Both, actually. All of the black holes we know about are too far away for us to experiment on in the laboratory. There aren't any very close by. But we do think that all the black holes that exist in the universe do spin. Basically, everything out there is spinning. Our planet is spinning. The sun is spinning.

Our planet spins around the sun. The sun spins around the center of the galaxy. The galaxy orbits the center of the galactic cluster. Basically, everything out there is spinning. And because angular momentum in the universe is concerned, we think that everything that went into the black hole was probably spinning. So the black holes themselves are almost certainly spinning, which means every black hole out there is probably.

probably a spinning object. And the fascinating thing about spinning a black hole is that it changes its gravity. But there's a catch. Black holes can only spin so fast before they might tear themselves open. Hmm. I mean, that's also true in ballet. So the Earth is spinning. Is that why the Earth has gravity and it keeps us on it?

The Earth would have gravity even if it wasn't spinning. But the Earth's gravity is actually subtly different because it does spin. It doesn't just pull on things. it also twists things in orbit around it. We'll dig into all of that when we talk about frame dragging. But because spinning of an object changes its gravity, it can also affect the size of a black hole and puts a limit on how fast a black hole is allowed to spin

before it cracks open and breaks the rules of physics. Wait, so can a black hole actually crack open, or is this just some kind of theoretical limit? The rules of physics actually say that there's a maximum speed, that black holes are allowed to spin before they crack open. Nobody knows what would happen if you broke that rule. Would it actually crack open the black hole? Would you go to physics jail?

You kiddie's old ballet teacher come and scold you? Let's find out. Oh, that's the scariest option. And so today on the podcast, we'll be answering the question. How fast can a black hole spin? I'm going to say 50 miles per hour. Do I get it? Don't tell me that's the fastest speed you can imagine in your mind, Katie.

You live in Italy. The drivers there drive fast than that all the time just when they go into the corner store. I guess, but it's in kilometers per hour and I haven't figured that out yet. How long have you been there already? You know, a couple kilometers. I'm not really sure. Right. We won't quiz you on units.

But I did go out there and ask our listeners what they thought about how fast a black hole could spin. We're very grateful to the listeners who participate in this segment of the podcast, giving us a sense for what people know and what they're confused about. If you would be willing to add your voice to this group of volunteers, please don't be shy. Write to me to questions at Danielandhorpe.com. So think about it for a moment before you hear these answers. Do you know how fast a black hole can spin? Here's what people had to say. Faster than Sonic the Hedgehog. I would guess up to the speed of light, I think nothing can go faster than that through space. So that would be a limit, but I don't see any reason why it couldn't get up to that speed.

I actually learned that black hole spin the other day. I have no idea how fast they go. I believe that the rate of rotation of a black hole has to do with its size. So the smaller ones will rotate faster. How fast can they go? I have no idea. Well, I would assume that they spin very fast because neutron stars such as pulsars spin, I believe, many thousand times per minute, and black holes are perhaps even smaller and denser,

which the contraction of their size would make them spin even faster, just like a figure skater holding their arms in would make them spin faster. My instinct is that the only limit on how fast a black hole could spin. is the speed of light. However, I imagine there might be some kind of loophole that may allow it to be faster than that, very intrigued. Well, black holes, you mean the thing that makes the black hole or the stuff going into the black hole?

Because the stuff going into the black hole can spin really fast. But the thing that is the black hole is the singularity. And can a singularity even spin? I don't even know. I don't even understand what a singularity is. mass that's been infinitely mashed up. Black holes could not spin so fast that any given part would have such a high angular momentum to fly off the black hole. But being such massive bodies,

they can spin really fast. I do enjoy the Sonic the Hedgehog answer that it could be even faster than Sonic the Hedgehog, who probably does spin faster than 50 miles per hour. Like, would you say that a black hole is faster than Mr. Sonic the Hedgehog? You know, I've been asked a lot of physics questions in my career, a lot of random questions from the public. I've never been asked to compare a black hole to a fictional video game character. And I just don't have to answer that question.

Like, does Sonic the Hedgehog even follow the laws of physics? Are the two things comparable at all? Or is it like comparing miles per hour to kilometer per second? That's a wise answer given how many Sonic the hedgehog fans are out there ready for blood. But yeah, I mean, it seems like some people think it's limited by the speed of light. And so some people think that it's got to be faster than something like a figure skater or smaller things rotate faster. Or a lot of really interesting theories from people. Yeah, and people are definitely right that the surface of a black hole shouldn't be able to move faster than the speed of light.

But it turns out there's another limitation to black hole speeds that if black holes spin too fast, they might crack open and reveal what's inside of them. So it's fascinating question to think about like, what is a black hole? How do they spin? How fast do they spin? What is it the maximum speed and what would happen if you overspun a black hole? It's like if you spun pizza dough too fast and it tore itself apart. But I did think black holes weren't so much of physical object like a pizza.

I thought it was kind of this just sort of sinkhole in the universe of like of incredible. incredibly, incredibly dense matter that has an incredibly strong pull of gravity. I could use a refresher on what exactly a black hole is made out of. Black holes are both physical objects and that we know there's something inside of them. There's some mass in there. But there's also a sort of conceptual layer to them, which is the event horizon. Then we'll talk about in a minute. But in the beginning, it just starts with a blob of stuff.

You know, stuff has gravity. But gravity is sort of surprisingly weak. like it's the weakest fundamental force that's out there. Gravity is so much weaker than like electromagnetism or the weak nuclear force or the strong force. And you can discover this yourself every time you put like a fridge magnet up on the fridge, that tiny little magnet is holding it up. It's beating the gravity of the entire Earth, right? This enormous planet with all of its rocks and magma inside of it is pulling on that thing defeated by a tiny fridge magnet. That tells you how a weak gravity really is. And yet, if you accumulate enough mass, gravity can become pretty powerful. I mean,

it's holding you to the surface. The sun's gravity is holding the entire Earth in orbit, right? The gravity of the galaxy is holding it together. It really is pretty impressive on a cosmic scale. So if you build enough mass, you can get very powerful gravity. But if you take one more step, that's when you get to the crazy town. You take a lot of mass and you also squeeze it down to a really small space. So you can get really close to the mass. That's when gravity really gets bonkers. So you have like, say the sun could fit in a handbag, but it still retains the gravity of the entire sun.

And then you fit a bunch of those little handbag-sized sun into a small area. Then you have just an enormous amount of gravity. That's right. You could turn the sun into a black hole. First, imagine you're standing on the surface of the sun. sun. Ow, hot, too hot. I assume you're wearing appropriate footwear, you know, not like ballerina shoes. Footflops for the sun. Now the gravity on the surface of the sun is already going to be very

powerful because the sun is very, very massive, but it's not a black hole, right? It's a meeting light. Now take the sun and shrink it down to an object like a few kilometers across. If you're still at the same distance from the center, you're at where the surface of the sun used to be, then you're going to feel the same thing. Nothing has changed for you. You're still going to feel the gravity of the sun in the same way because all you've done is change the internal configuration of the stuff inside the sun. So nothing will change for you in terms of the gravity you feel if you're still at the same distance. But shrinking the sun down to a region like the size of Los Angeles means you can actually get much closer to it, right? Instead of having to stand at the surface,

you can now just be a few kilometers from the center of the sun with all of that mass concentrated there. So that's why the gravity gets much, much stronger. It's not just about having a lot of of mass. It's about having a lot of mass in a very small space. And if you did that, if you shrunk the sun down to the size of Los Angeles, it would form a black hole. And that black hole is called traffic in Los Angeles. Okay, so it's really about how tightly packed this matter is such that it has a really strong effect of gravity in a small amount of area. And so that is what gives a black hole such powerful sort of, I guess, drainage, universal drainage. Yeah, exactly.

In Newtonian gravity, the force between two objects depends on the distance between them, like one over the distance squared. So as the distance gets smaller, the force gets more powerful. And because it's one over distance squared, the force gets much, much more powerful. If you're twice as close, it's four times as powerful. If you're a thousand times as close, it's a million times more powerful. So this force gets extraordinarily powerful if you can shrink the distances between the objects. That's what compacting the sun does for you.

But then you go over this incredible threshold. It's not just like the force gets stronger and stronger and stronger. It does something sort of incredible, which is to create this event horizon, a region of space where anything that enters can never escape. Even with a really, really strong rope? Even with a really, really strong rope, even light that enters the event horizon is trapped. What does that mean for light to be trapped? This is where Newtonian gravity no longer works to describe what's going on. You're imagining light moves at the speed of light.

Why can't it escape the black hole? And also, light has no mass. In the Newtonian picture, gravity is just a force between two objects that have mass. Photons have no mass. How can the black hole pull on them at all? So the Newtonian picture really totally breaks down there. Instead, the way you need to think about it is in Einstein's view of the universe, where gravity is not a force.

It's a curving of space time. Space itself is not always just like an evenly spaced grid. It has wiggles and curves in it. And if you shoot like two laser beams through space, if it's totally flat, then those laser beams will fly forever parallel, never touching. But if space is curved, those laser beams will bend. Sometimes they may even cross or they can diverge away from each other based on the curvature of space.

And that curvature is not something we can see directly, right? We can't look at a piece of space and say, oh, I see the curvature. Instead, we just see the effect of that curvature on the motion of stuff. And so Einstein tells us that when you have stuff in space, mass, energy, anything like that, it changes the shape of space. So back to the question of the photons, what's happening when you have a huge amount of matter in a really tiny space is that you've curved space so much that photons moving through that cannot escape. Photons are affected by mass because mass changes the shape of space and photons move through that space. So the gravity of the black hole causes

space to sort of warp into this funnel shape through which photons have no choice, but to travel, you know, through this funnel, right? Exactly. Effectively, the shape of space is changed. It's so distorted inside the black hole that every direction forward is towards the center of the black hole. When we talk about space being curved, we really mean like it's changing the organization of space. The way points are connected and the distances between them. And so inside the black hole is really just one direction of space, which is towards the center. That's unnerving.

And so this region, we call this the event horizon. It's the region around the black hole where nothing can escape. Anything that enters that will never escape the event horizon, even at time equals infinity at the end of the universe. It's actually what we mean by the event horizon. The technical definition is that region of. space, which after infinite time, particles that enter will never leave. And the size of that event horizon depends on the parameters of the black hole. Like the bigger the mass inside the black

hole, the more space is current and the larger the event horizon. It's actually a very simple calculation. It's called the short-siled radius that tells you the radius of the black hole, which just depends on the mass, the gravitational constant G, and the speed of light. All right. So we've got this sort of space funnel that happens that you cannot escape because you're inside of warped space that only goes in one direction. And that is into, I guess, the tummy of the black hole. So say you could go inside the space funnel without dying horribly and sort of turning into spaghetti. What would you find there? Just like a really dense, clump of matter? Like, what is in there? Boy, do I wish I knew the answer to that question because I would be headed to Stockholm to pick up my Nobel Prize. It's really one of the deepest questions in physics because we don't know what would be there. We have a prediction from general relativity,

the theory that tells us how space bends and how matter moves through that bend space. And that theory tells us that at the heart of a black hole is a singularity. Because things that fall in a black hole have to proceed towards the center, eventually everything does. And the power at the same thing. center is so incredible that it's like a runaway effect. It just keeps compacting, compacting, compacting so that everything that falls into the black hole ends up at this one point, a point of infinite density, infinite curvature of space. That's the prediction from general relativity. But most physicists don't view that as a real prediction. They view that as an

indication that the theory is failing. Anytime you get nonsense predictions from your theory, you're like, well, maybe the theory is not being applied correctly. I look at my kids, for example, and I say, oh, in the last 15 years, they've grown from basically zero to two meters high. Does that mean in the next 15 years, they're going to grow to four meters high and then six meters high? Only time we'll tell. I would say that's a nonsense prediction. It tells me that I've extrapolated beyond the region where the theory makes any sense. You know, a simple linear extrapolation doesn't make sense there.

I need a new idea for how humans live after age 15. And that's what's happening in general relativity. We don't think there really is a point of infinite density. we think something else is going on and we need a new theory to describe it because that infinite density also violates fundamental principles of quantum mechanics tells us we have an incredible amount of energy

concentrated in a single point without any sort of quantum fuzz. And so quantum mechanics tells us you need some sort of quantum fuzz when you have that much energy so otherwise you know something's location and its momentum very, very precisely and that's just not allowed

by the Heisenberg uncertainty principle. So what's inside a black hole would tell us how space actually works. What is gravity in the quantum sense? Is it a quantum theory? Is it actually a force? Or is there some sort of space foam that bubbles up from fundamental quantum mechanics? We just don't know.

But if we could look inside a black hole, we would get an enormous clue about how space actually works, what gravity really is and how to unify general relativity and quantum mechanics. That's really interesting because like I hear about these things like, you know, inside a black hole you have sort of this infinite amount of. I guess, suckage. But I guess in a way that you're right, like it doesn't really make sense for there to be a single infinitely dense point, especially with what you were describing with sort of the quantum fuzz, the uncertainty we know exists in quantum mechanics. But I guess like are there any other theories? Are there any other leads that we have as to what is going on there? Like how do we even go about figuring out what could be happening?

Yeah, great question. It's really hard because we can't see inside a black hole. We do have various other theories. We talked about a few of them on the podcast, ideas that black holes are actually slowly collapsing stars, or maybe they're fuzz balls made out of strings, or maybe there's something else entirely. We do have a bunch of theories, but testing them is difficult because all the predictions they make are hidden behind this cosmic veil, which makes it pretty hard to know which one is right. But there are other features to a black hole which present a really fascinating opportunity because as we talked about earlier, gravity is not just based on the amount of mass something has. It's also based on its spin and its energy in terms of electromagnetism. So these other features of a black hole, the spin and the charge, might present opportunities for cracking them open and seeing what's inside. Okay. Well, I am really excited to find out what is inside these ballerina black holes. But first, let's take a quick break. I'm Dr. Joy Harden-Bradford, and in session 421 of Therapy for Black Girls, I sit down with Dr. Ophia and Billy Shaka to explore how our hair connects to our identity, mental health, and the ways we heal.

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radio app, Apple Podcast. or wherever you get your podcasts. All right, and we are back, and we are talking about the universe's deadliest ballerina, a spinning black hole, and what might be inside of there. So we have just gotten to the part where we are talking about how black holes spin. So I am having a hard time picturing what it means for a black hole to spin. Daniel, can you let me know?

Is this like a black hole doing a pirouet? What is the spin of the black hole? Yeah, it's a good question. What is spinning exactly? But we know the black holes are made of stuff, right? There's stuff that has fallen in, there's mass, there's energy in there. And when stuff falls into a black hole, it has angular momentum. Like if you drop a ping pong ball straight in towards a center of a black hole, then you're not making it spin at all.

But if you drop the ping pong ball and it hits the black hole sort of far from the center, then you're like giving it a push. The way somebody on the merry-go-round is spinning around and you give them a tug near the edge of the merry-go-round, you're applying a torque to the merry-go-round. Same way you shoot a bunch of ping-pong balls at a black hole, but you do it off center, you're going to add some spin to the black hole. So as long as things fall into a black hole and don't head directly for the center, if they spin around first, then as they fall in, they're going to keep that spin. Because in our universe, angular momentum is conserved. It can't just go away, which means because the black hole is made of stuff that was spinning around it, eventually a black hole itself is spinning. What's actually spinning? I mean, if you imagine like a singularity at the heart of the black hole, it's just a point.

Points themselves cannot spin. So people imagine that what's going on inside a black hole is not a point spinning, but a ring. Instead of having a point of infinite space, you have like a ring of singularities and that ring is spinning. Again, this is the general relativity prediction, which if we think is probably wrong, but that's the more complex picture of what's happening inside a spinning black hole. So you have multiple singularities that form sort of a ring inside potentially? Mm-hmm. Because a ring can spin.

It has some spatial extent. It has some spatial extent, at least in one dimension. And that ring itself can spin to preserve the angular momentum. Because again, angular momentum can't just go away in our universe. We think it's conserved. Okay. So how do we even know that black holes can spin? Yeah, great question.

According to the theory, they can spin in general relativity. And also, as we talk about later in the podcast, we've seen a bunch of black holes and we've seen the effect of their spinning on the stuff around them. And this is a really fascinating feature of general relativity, that the spin of an object does change its gravity. I mean, again, in the Newtonian picture, which I think is pretty intuitive for most people, gravity is just a force between two objects that have mass. Like the Earth is pulling on you. If you're a satellite in space orbiting the Earth, you think that your orbit is affected only by the mass of the Earth. Why would it change the gravity if the Earth is spinning, right?

the formula is just like gmm over r squared there's no opening in there to describe the spinning of the object and newton would say it doesn't matter the earth spins it's like got the same mass configuration everywhere as long as it's a perfect sphere you should have the same gravity but in einstein's picture of the universe that spin does change the gravity of the object and so we're on the earth and so we are experiencing a different gravity from the earth because it is spinning than if it was staying still. And so things around a black hole would also be experiencing a different effect from a spinning black hole versus a stationary black hole.

Exactly. And the crucial thing to understand is that in general relativity, it's not just that mass bend space, but the things that bend space come together in a complicated dance, it's described not just by like a number as it is for Newton, like a single number of the mass, but this stretch. Press energy tensor. Tensor is just a fancy way of saying like a matrix, a bunch of stuff all organized. So the mass definitely affects things, energy affects things, kinetic energy affects things.

Also electromagnetic energy and spin all come together in this complicated dance. But it's not just like you add up all the energy and you get a number and that's how much the curvature of space is. It is more complicated. And spin affects the curvature of space in a really weird way in that it spins space. They call it frame dragging because it really is like. spinning space like a fluid. So what happens to an object around something that's spinning is that you're not just pulled towards it by its gravity. You're also spun. So if you have a satellite orbiting the

earth or a black hole and that earth or black hole is spinning, it's going to apply a little torque, a little twist to the thing orbiting. We've actually measured this. We have a satellite orbiting the earth called gravity probe B as very, very precise gyroscopes to measure its direction and its spin. We have a whole podcast episode about this, and they've detected this effect that the spinning of the earth applies like a little torque to this satellite. It's very, very subtle in the case of the earth. It requires extraordinary precision even to detect it, but it's there. We've confirmed it. So if you drop like a marble down a funnel, like straight in the center down a stationary funnel, it just goes straight down.

If you drop it sort of on the side of a stationary funnel, it rolls down the side and then into the center. But if the funnel like is spinning, like if you start like sort of moving the funnel around and then put the marble on the side of the funnel, the marble itself will start to like sort of circle around as it goes down into the center of the funnel. Is that kind of what's happening? That's sort of what's happening. There's some tricky bits there about the funnel pushing on the marble. But in effect, yeah, the spinning of the black hole or the spinning of the object applies a spin to stuff near it. So in the case of a spinning black hole, you don't just have the event horizon. You also have this region past the event horizon called the ergosphere, where the black hole is applying a certain amount of twist to stuff.

We talked once about how you can extract energy from a black hole, basically by dropping rocks near it. Those rocks get spun as they go through the ergosphere, extracting some energy from the black hole, basically for free. So I think Freeman Dyson or maybe Roger Penrose came up with his plan to use black holes as a power source, which is pretty awesome. Sounds really easy. All we need to do is get really close to a black hole. Exactly. So these spinning black holes have this weird effect on space time and it also affects their event horizon. Like how close you can get to a black hole and still escape depends not just on the mass of the black hole, but also on its spin and its electric charge. Because these things enter into the stress energy tensor that general relativity uses to do this calculation, it can change the size of the use. event horizon if you spin your black hole or if you give it electric charge. So are we kind of getting

into the limitations of how fast a black hole can spin? Because if it is affecting the event horizon, presumably there has to be some characteristics to the event horizon to keep that black hole stable. Otherwise, like if you change that event horizon too much, what could happen to the black hole? You exactly right. And the crucial thing to understand is that, spinning a black hole will shrink its event horizon. This is counterintuitive because you feel like, well, you're adding energy to the black hole. Isn't that just putting more fuel on the fire? Remember that spin and things like electric charge have a complicated way that they contribute to the curvature of space.

It's not really very intuitive. But adding spin or electric charge to your black hole will shrink the event horizon. We'll make it smaller. It's possible to get closer to one of these objects if it's spinning or has electric charge than otherwise. And as you were saying, there's a breakdown point. If you spin the black hole enough, the event horizon shrinks down to zero, which means there is no event horizon, which means you've like cracked open the black hole and revealed the grand mystery that's inside of it. I bet it's nougat. I bet it's filled with nougat.

I really think you should have a snack before we do these episodes, Katie. I'm sorry if you make space sound so delicious. Well, that's, I mean, but presumably we don't know if this has ever happened or if it could ever happen. We don't know if this has ever happened or if it actually could ever happen. Again, it's sort of a prediction of general relativity. And we don't think that general relativity is the law of the actual universe. It's just the law of the universe we describe in our head, which so far works perfectly to describe everything we see. We think that it probably fails the inside of a black hole, but we've never seen that.

So this is a prediction of general relativity that if you overspin a black hole, you will reveal a naked singularity. You'll crack it open. You'll shrink the event horizon down to zero, which is really fascinating because it seems like on one hand, an opportunity to learn this secret of the universe like, let's go out and overspin a black hole. I want to see what's inside of it, right? On the other hand, it's like, well, is that really allowed? Is there something in the universe that would prevent that from happening? So do we have any calculations as to like how fast a black hole can spin before it breaks itself apart? Yeah, we do actually have the calculation from general relativity and it tells us that if there's more energy in the spin than there is in the mass, then the radius of the vent horizon becomes a negative, which basically means there is no event horizon.

So that means the bigger the black hole, the faster it can spin. So it's not like there's a maximum spin rate for all black holes, but there's like a ratio. If you have more energy in your spin than you do in your mass, then the event horizon shrinks. It's actually the same thing for electric charge. We did a podcast episode about that recently. If you overcharge the black hole, meaning adding more energy to it in electromagnetism by adding like electrons, for example, then there's a certain ratio of mass to charge. You cannot exceed because it will shrink the event horizon down to zero.

So now there's the interesting question of like, well, what happens if you do it? Or is there a physical way to actually make this happen? I mean, I think it sounds like you got to put a sign up at the teacups ride that you must be this big of a black hole to be able to spend this fast. Let's just regulate the problem away, huh? How very progressive of you, Katie. But there's sort of two interesting questions here to tease apart. One is like, does the universe say this is actually impossible?

And the other is, could you make it happen? Right. Some things are not against the rules, but you have no way of making them happen. No way of like going from our universe to creating some configuration that you're curious about. Like not everything is possible to build. You have to assemble things. Right. You need a recipe for saying, I'm going to build this house. I'm going to put one brick and then the next brick and then the next brick. You need a way to step by step put it together. And so while we don't think that it's against the rules of general relativity to crack open a black hole this way by overspinning it, we don't think this is actually a recipe. be that would let you do it. There's no way to like step by step make this happen. Because if you got close enough to a black hole to build something, it seems like the black hole would just suck all your tools away. That's one example of a practical reason why you might not be able to do this. But people have thought about other thought experiments. Like say you have a black hole and you want to try over spinning it. Take the suggestion you made earlier, which is like, well, let's just shoot ping pong balls at it, but not right at the center. Every time you

shoot a ping pong ball at a spinning black hole, you're giving it a little push. So can't you just keep doing that forever, eventually exceeding this ratio and cracking open the black hole? Well, it turns out that's not actually possible. What happens when you shoot something into a spinning black hole is that the black hole causes it to accelerate, which gives off some gravitational radiation. Because every acceleration requires some kind of radiation to conserve momentum. You can't just like zoom in one direction without pushing off in the other direction. And this gravitational radiation, it turns out, will decrease the angular momentum of the things.

you're throwing in. So basically a black hole that's right in the edge of being extremal that's almost overspun will basically de-spin stuff that you throw into it so that it's impossible to overspin the black hole. So it's self-limiting in a way? Yeah, exactly. It's sort of self-limiting. This is called the back reaction. Like the black hole and the object have effects on each other before they fall in. And so they're pulling on each other, tugging on each other, radiating away some gravitational energy. And it's a very complicated calculation, but people have gone through this

and try to imagine all possible ways you can throw things into a black hole and none of them can actually push it over the edge. That's not saying this configuration is impossible. It's not saying overspun black holes are not possible in the universe. It's saying we don't know of a recipe for building one because as soon as you get close to overspun,

black holes become very, very good at rejecting any more angular momentum. I mean, it's hard to feel like this black hole is not intentionally creating rules so that it doesn't rip apart. But I guess that's a very homo sapient mindset that there's some intentionality here. No, I think you're totally onto something really fascinating, which is like the secrets of the universe are hidden behind this event horizon. Then as soon as we're clever enough to think about, ooh, wait, here's a loophole. Maybe we can break it open. It turns out there's like another reason why they can't work.

It does feel almost like a conspiracy theory. Like, we have all the answers in this little box, but there's no way to open the box. We'd find the off switch to the universe in there, and they don't want us to find it. This is very similar to a topic we talked about recently about overcharging the black hole. In a similar way, if a black hole has a lot of electric charge, then it repels stuff you try to throw in it that has more electric charge. So it prevents you from overcharging the black hole. This is the way black holes prevent you from overspinning themselves with this back reaction on stuff you throw in. And then people try to be even more clever, like, well, what about internal quantum spin?

You know, what if you like take something that has internal quantum spin like electrons and you drop them straight into the black hole without angular momentum? Wouldn't that add to the overall spin of the black hole? Great idea. So people went off to do the calculations. And it turns out there are spin-spin interactions like the spinning of the black hole and the quantum spin of this. object will induce an interaction between the two things, which again will lower the angular momentum of the falling in object, avoiding going over the threshold. So the summary is that we know of no way to overspin a black hole. We currently think it might be impossible.

Again, not impossible for it to exist as an overspun object. To break this limit, we just don't know how to make it happen. Man, that really does seem like the black hole is just trying to frustrate. particle physicists like you. It doesn't take a lot. You're just going to have the secrets of the universe and hide them from us. So I'm also curious to find out because we've talked about how black holes have a maximum speed limit that the black hole highway patrol manages to keep in check through these radiation,

these spin, spin interactions. but I'm also curious if there is a minimum speed limit for a black hole, if a black hole could get away with not spinning. But maybe first we should take a quick break to make sure that we don't get ticketed by the black hole police. I'm Dr. Joy Harden Bradford. And in session 421 of Therapy for Black Girls, I sit down with Dr. Othia and Billy Shaka

to explore how our hair connects to our identity, mental health, and the ways we heal. Because I think hair is a complex language system, right? In terms of it can tell how old you are, your marital status, where you're from, you're a spiritual belief. But I think with social media, there's like a hyper fixation and observation of our hair, right? That this is sometimes the first thing someone sees when we make a post or a reel is how our hair is styled. You talk about the important role hairstyles play in our community. the pressure to always look put together and how breaking up with perfection can actually free us.

Plus, if you're someone who gets anxious about flying, don't miss session 418 with Dr. Angela Neil Barnett, where we dive into managing flight anxiety. Listen to therapy for black girls on the iHeartRadio app, Apple Podcasts, or wherever you get your podcast. I'm Dr. Scott Barry Kaufman, host of the Psychology Podcast. Here's a clip from an upcoming conversation about exploring human, potential. I was going to schools to try to teach kids these skills and I get eye rolling from teachers or I get students who would be like, it's easier to punch someone in the face. When you think about emotion regulation, like you're not going to choose an adaptive strategy which is more

effortful to use unless you think there's a good outcome as a result of it if it's going to be beneficial to you. Because it's easy to say like go you go blank yourself, right? It's easy. It's easy to just drink the extra beer. It's easy to ignore, to suppress, seeing your colleagues, who's bothering you and just, like, walk the other way. Avoidance is easier. Ignoring is easier. Denial is easier. Drinking is easier.

Yelling, screaming is easy. Complex problem solving. Meditating. You know, takes effort. Listen to the psychology podcast on the Iheart radio app, Apple Podcasts, or wherever you get your podcasts. Have you ever wished for a change but weren't sure how to make it? Maybe you felt stuck in a job, a place, or even a relationship.

I'm Emily Tish Sussex. and on she pivots, I dive into the inspiring pivots of women who have taken big leaps in their lives and careers. I'm Gretchen Whitmer, Jody Sweeten, Monica Patton, Elaine Welteroff. I'm Jessica Voss. And that's when I was like, I got to go. I don't know how, but that kicked off the pivot of how to make the transition. Learn how to get comfortable pivoting because your life is going to be full of them. Every episode gets real about the why behind these changes and gives you the inspiration and maybe the push to make your next pivot.

Listen to these women and more on She Pivot, now on the IHeartRadio app, Apple Podcasts, or wherever you get your podcasts. I always have to be so good, no one could ignore me. Carve my path with data and drive. But some people only see who I am on paper. The paper ceiling. The limitations from degree screens to stereotypes that are holding back over 70 million stars. Workers skilled through alternative routes rather than a bachelor's degree. It's time for skills to speak.

speak for themselves. Find resources for breaking through barriers at tetherpapersealing.org, brought to you by Opportunity at Work and the Ad Council. So we are back and we are keeping this podcast under the speed limit. But as we know as speed limits, it's possible to go too slow on a highway. People get mad. So is this the same thing with black holes? Do they have to spin or can they just blaze around and stay still?

It's a great question because most of the description of black holes you hear about out there are these very simple short-style black holes, just stuff in space with massive gravity and not even talking about the spin. And that's a sort of like generic description of a black hole. But if you go out there and look for stuff in space, there's basically nothing that isn't spinning, right? everything in space is spinning. There was inherent angular momentum to stuff and that can't go away and so stuff keeps spinning. So it really makes no sense to imagine a huge massive object with all

of this stuff thrown into it somehow perfectly balanced between positive and negative angle of momentum so that it doesn't spin. It's like flipping a billion coins and getting exactly one half of them heads and exactly one half of them tails. That's what you would need. So it's not impossible for a black hole to have no spin. It's just extraordinarily improbable. It's like an enormous pencil bounced on its very tip forever. I guess that's a little bit scary. The idea of a black hole that's perfectly in balance somehow defying probability and just still kind of sucking things in. It is pretty weird. Black holes are weird and powerful objects and all the ones that we've seen out there in space are spinning. We can tell they're spinning because the stuff

around them is spinning. Like the black hole picture that we've seen, what we're actually looking at when we look at that picture is not the black hole itself. There's like a blank spot in the middle where the black hole's like not giving off any light. What we're looking at is like the crispy cream donut of stuff around the black hole that's falling in. And that stuff is glowing. It's so hot because of the tidal forces the frictional gravity of the black hole rubbing and squeezing all of that stuff as it orbits. The swishing around the black hole before it actually gets flushed down this cosmic toilet bowl and every black hole that we've ever seen has this very strong spin to the stuff around it which means that it is also spinning i mean you did scold me

about needing a snack before the show and then you described the stuff around a black hole as crispy cream donut getting mixed messages here and then you described it as like a toilet bowl so getting really mixed messages here so we have measured things around a black hole how do we do that Yeah, that's a great question. One thing we can do is look at the orbits of stars around the black hole. Like in other galaxies, we can't resolve individual stars, but we can tell how fast stars are spinning. We can look at like the velocity of streams of stars. We can't make out the individual ones, especially near the centers of those other galaxies.

But we can tell how fast they're moving because of the red shift and we can tell the density. And so we can infer the presence of these supermassive black holes in other galaxies. And sometimes we can directly see the effect of spin on the stuff around it. Like if the black hole is spinning, it changes like how close stuff can get to the black hole and it also changes how that stuff is spun. There was one case where we saw two black holes orbiting each other. So like a really big black hole. It's called OJ 297.

It has like almost 19 billion solar masses. One of the biggest black holes out there that we know about. It's orbited by another black hole that has just like 150 million solar masses. I mean, already a gigantic object, but tiny in comparison to the big mama black hole is orbiting. And the orbit of this smaller black hole is processing. Imagine like an elliptical orbit, not a perfect circle, but the smaller black hole is moving around in an ellipse. An ellipse is like an elongated circle. Well, the direction of the elongation is changing as the little black hole orbits the big black hole.

This is like the spin of the orbit of the little black hole. around the big black hole. And the procession of that orbit tells us something about the spin of the big black hole because its spin is contributing to that procession. That's crazy.

I mean, first of all, that a big black hole can suck up a littler black hole, but also just that you can see that change in movement that would only be possible given sort of the spin of the bigger black hole.

Can we actually measure how fast the, big mama black hole is spinning given the procession of the smaller black hole? Yeah, we can actually. We can make a calculation and the surface of that black hole is moving at almost 60,000 kilometers per second, which is like 20% of the speed of light. Sort of amazing. Now, this thing is so big though. Its radius is so huge. It's like 360 AU. It's the distance between the earth and the sun. It's so large that even though it's moving at 20% of the speed of light, it still takes 5 million seconds to complete one spin.

So it's such a big bear that it still takes a huge amount of time to spin once. 5 million seconds is how many years? 5 million seconds is like 16% of a year. Okay. I can't think in seconds. So that's like two months. So this thing is spinning at 20% of the speed of light still takes two months to complete. one spin. Okay, because my perception of seconds are different. Like when I'm in this podcast,

they go by very fast because I'm really interested. If I'm waiting for a bus to come, the seconds are very long. But there's another black hole that's spinning even faster. This one is really small. It has only two million solar masses. It's NGC 1365. But the cool thing about this black hole is it doesn't have like a baby black hole orbiting it, but it's close enough that we can study the accretion disc in great detail. Is it the accretion disc the stuff that is caught up circling the black hole? It's a stuff that's on deck about to go into the black hole. It's like falling in, but because it has angular momentum, it just doesn't fall straight in.

The same way like the Earth doesn't just fall into the sun because of the sun's gravity. It's in orbit. This stuff also has angular momentum, which slows it down, which prevents it from falling immediately in. It has to, like, radiate away some of its energy in order to fall in. So inside the accretion disc, This stuff is like bumping and grinding and glowing and giving off some of its energy just before it falls into the black hole. And so by studying what's happening inside that accretion disk, it's like a probe for what the gravity of the black hole is doing to it.

We can understand the tidal force is the strength of the gravity and also the effect of the spin because the spin of the black hole changes like how close the accretion disc can get to the event horizon. So if it's spinning faster, can the accretion disk get closer to the event horizon? Yes, exactly. If it's spinning the same way as the accretion disk, then the accretion disc can get very, very close to the black hole without actually falling in. Whereas if the accretion disk is spinning one way and the black hole is spinning the other way, then you end up with this like weird gap between the black hole and the accretion disc where stuff in that gap can't exist because it would just fall immediately into the black hole. But if they're spinning together, then the spin of the black hole like accential.

the spin of the accretion disc and it helps it spin faster and can actually get closer in before falling into the black hole. So by looking at like the size of the gap between the event horizon and the start of the accretion disk, they can measure the rate of spin of the black hole itself. And this guy is spinning at 85% of the maximum rate allowed for this mass black hole. Man, he's got like the universe police kind of following him. They haven't put their lights on yet, but they're keeping an eye on them. Exactly. And so this is fascinating. Like, wow, this guy is close to an overspun black hole. It would be really interesting to watch as that stuff falls in to see, like, is it speeding up?

Does it gradually approach 99.999%? Or are we wrong about how black holes can't be overspun? And maybe this one will actually get cracked open. Maybe that stuff will fall in and erase the event horizon. We better keep an eye on it. Also, because the faster it spins, the closer you can get to the event horizon. and without falling in. Daniel, is this the kind of black hole that you would want to travel to to and try to sort of drop some sonar into the middle of the black hole? I mean, I don't even travel conferences anymore. There's no way that I'm going to visit a black hole. Absolutely not. I am too old for that, but I'm very happy to send up, you know, co-hosts, guest hosts, all these kind of people who would happily volunteer.

It's an interesting pool of people you'd be okay sending to a black hole. As long as you bring your travel mic and tell us. what you're learning as you fall in. Yeah, I do have a good windscreen on it, so it should still come through clear. But, you know, I think the lesson to take away from this is that space is so much weirder than we thought it was. Even the idea of space being bent into curves by mass is weirder than Newton thought of, but there's so much more to it than just that.

Space is not just bent into curves. It's also spun. It's flowing like a fluid around very massive objects, dragged along with them. them like a fork twirling that spaghetti and that changes the way that black holes operate the event horizon of a black hole is not just a function of its mass it's also a function of its spin and its electric charge i feel like this is a great personality test of who wants to see the black hole overspin and break apart and maybe break the universe and who would like the black hole to not break apart and keep things safe oh my god who would be in that sense

second category. Who would not want the black hole to break the rules of the universe? Who are we even talking about here? Not me. Remember that seeing the universe break the rules, it's not a failure. It's not a problem. It's an opportunity. Those are the moments when we learn what the real rules of the universe are. The universe is out there we think following some set of rules and we have in our heads some internal description of that a model of how we think the universe works. And when those two things clash, that's when we get to update our list. We're like, oh, turns out we were wrong about what's going on over here. Let's fix things.

But we need those clues sometimes. We need the universe to show us what's wrong with our understanding of how it works by contradicting our predictions. And so if we see the universe breaking the rules that we thought, it's not a failure. It's a huge leap forward. It's an open door to a deeper understanding of how the universe actually works. Well, kids, you heard Daniel. You got to break rules to learn.

Exactly. Unfortunately, it seems like black holes are fortresses of knowledge. Not only do they have these event horizons that keep us from seeing what happens at extraordinarily extreme circumstances, but also they're very wildly at protecting those event horizons. You thought you could overcharge a black hole? It repels your idea. You thought you could overspin a black hole? It's got its own spin doctors to keep you from doing it.

So for now, at least, we don't know how to crack open a black hole. rogue black hole, a maverick. We need to put a black hole in witness protection. What did that even look like? Put a black hole inside another black hole? I don't know. We'll ask the FBI. Well, thanks very much, everybody, for going on this journey inside the mysteries of the universe, wondering what's going on inside a black hole and how we could ever know and whether we ever

will. And thanks very much, Katie, for taking this crazy physics tour and breaking all the rules with us. Thank you for not shooting me into the center of a black hole. Not yet at least. Stay tuned. Thanks, everyone. Tune in next time. Thanks for listening.

And remember that Daniel and Jorge Explain the Universe is a production of IHeart Radio. For more podcasts from IHeart Radio, visit the IHeart Radio app, Apple Podcasts, or wherever you listen to your favorite shows. I'm Dr. Joy Hardin-Bradford, host of the Therapy for Black Girls podcast. I know how overwhelming it can feel if flying makes you anxious. In session 418 of the Therapy for Black Girls podcast, Dr. Angela Neal-Barnett and I discuss flight anxiety. What is not normal is to allow it to prevent you from doing the things. that you want to do, the things that you were meant to do. Listen to therapy for black girls on the IHeart Radio app, Apple Podcasts, or wherever

you get your podcast. Hi, it's Honey German, and I'm back with season two of my podcast. Grazias, come again. We got you when it comes to the latest in music and entertainment with interviews with some of your favorite Latin artists and celebrities. You didn't have to audition? No, I didn't audition. I haven't auditioned in like over 25 years.

Oh, wow. That's a real G-talk right there. Oh, yeah. We'll talk about all that's viral and trending With a little bit of cheesement And a whole lot of laughs And of course, the great bevras you've come to expect Listen to the new season of Degas Come Again

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