Daniel and Jorge Explain the UniverseDaniel and Katie get their heads spinning as they spiral into the center of spinning black holes
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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 like 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, that's there's no way to spend 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're ready, Wait?
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.
Oh 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 super massive 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 Orge 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. Now, I 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 Katie That's right.
Oh boy'll like 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 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 that 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. 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 pingpong balls at them to try to get them to spin both?
Actually, all 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 of 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 conserved, 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 a spinning object. And the fascinating thing about spinning a black hole is that it changes it's gravity. But there's a catch. Black holes can only spin so fast before they might tear themselves open.
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 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 it actually crack open the black hole. Would you go to physics jail, Kay's old balley 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 gonna say fifty miles per hour. I get it.
Don't tell me that's the fastest speed you can imagine in your mind, Katting, you live in Italy. The drivers there drive faster than that all the time, just when they're going to the corner store.
I guess, but it's in kilometers per hour, and I haven't figured that out yet.
How long you've 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 Danielandthork dot 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 spain 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 this 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. It's being really fust I.
Do enjoy the Sonic the Hedgehog answer that it could be even faster than Sonic the Hedgehog, who probably does spen faster than fifty miles per hour? Like would you say that a black hole is faster than mister 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 through a fictional video game character, and I just don't know how to answer that question, like the Sonic the Hedgehog even follow the laws of physics, either 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 bigger skater or smaller things rotate fast, 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 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 a physical object like a pizza. I thought it was kind of this just sort of sinkhole in the universe of like an 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 that 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 mask 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 bunkers.
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 suns 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.
Oh 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 emitting light the Sun and shrane get 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 gonna 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 of 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 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 and 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 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 curve 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 through this funnel.
Right exactly, Effectively, the shape of space is changed, is so distorted inside the black hole that every direction forward is towards the center of the black 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 curved, 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 Sockholm 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 center is SoC credible that it's like a runaway effect. It just keeps compacting, compacting, and compacting, so that everything that falls into a 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 fifteen years they've grown from basically zero to two meters high. Does that mean in the next fifteen years they're going to grow to four meters high and then six meters high.
Only time will 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 fifteen. 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 it's 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 fuzzballs 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, 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 ballery in a black holes. But first let's take a quick break.
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All right, and we are back and we're talking about the universe's deadliest ballerina, a spinning black hole, and what might be inside of theirs. So we have just gotten to the part where we are talking about how black holes spin. So I'm 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 pure What is the spin of the black hole?
Yeah, that's a good question. What is spinning exactly? But we know that 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 the center of a black hole, then you're not making it spin at all. If you drop a 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 gonna 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 gonna keep that spin. Because in our universe, angular momentum is conserved 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 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 hmmm.
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, angularmentum 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 talked 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 half mass 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 square. There's no opening in there to describe the spinning of the object. And Newton would say it doesn't matter if the Earth spins, It's like got the same mass configuration everywhere. As long as it's a perfect sphere, it 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. Then 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 the mass, But this stress 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 saddle light orbiting the Earth called gravity Probe B has 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 talk 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 this plant 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 you uses to do this calculation. It can change the size of the 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?
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 space. It's not really very intuitive, but adding spin or electric charge to your black hole will shrink the event horizon, will make it smaller. It's possible to get closer to one of these objects if it's spin 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 nugat. I bet it's filled with nugat.
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 ever seen that. So this is a prediction of general relativity that if you overspin a black hole, you will reveal a naked singularity. You will 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 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, than the radius of the event 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 than 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 spin this fat.
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 gonna build this house. I'm gonna 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 it's actually a recipe 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 overspinning it. Take the suggestion you made it earlier, which is like, well, let's just shoot ping pong balls at it, but not right at the center. Right, every time you shoot a ping pong ball at a spinning black hole or 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 thing you're throwing in. So basically, a black hole that's right on the edge of being extremal, that's almost overspun will basically despin 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 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 that 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 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 over spinning 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 ideas, 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.
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So we are and we are keeping this podcast under the speed limit. But as we know, with 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 hack to spin or can they just laze 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 the 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 too 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 momentum so that it doesn't spin. 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 balanced on its very tip forever.
I guess that's a little bit scary. The idea of a black hole that's perfectly imbalanced, somehow defying probability and just still kind of sucking things in it.
Is pretty weird. And 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 holes like not giving off any light. What we're looking at is like the Crispy Kream 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 bla 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 describe the stuff around a black hole as Krispy Kreme 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 called OJ two nine seven. It has like almost nineteen 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 one hundred and fifty million solar masses. I mean, already a gigantic object, but tiny in comparison to the big Mama black hole. It's 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, and ellips 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 sixty thousand kilometers per second, which is like twenty percent of the speed of light. Sort of amazing. Now, this thing is so big though it's radius is so huge. It's like three hundred and sixty AU. It's the distance between the Earth and the Sun. It's so large that even though it's moving to twenty percent of the speed of light, it still takes five 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.
Five million seconds is how many years?
Five million seconds is like sixteen percent of a year.
Okay. I can't think in seconds, so that's like two months.
So this thing is spinning at twenty percent 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 NNGC thirteen sixty five. But the cool thing about this black hole is it doesn't have like a baby black hole orbiting it, but it's enough that we can study the accretion disk in great detail.
Is the accretion disc the stuff that is caught up circling the black hole.
It's the 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 disk, this stuff is like bumping and grinding and glowing, giving off some of its energy just before it falls into the black hole. And so by studying what's happening inside that accretion disc, it's like a probe for what the gravity of the black hole is doing to it. We can understand the tidle 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 disc, then the accretion disc can get very very close to the black hole without actually falling in. Whereas if the accretion disc is spinning one way 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 accentuates the spin of the accretion disc and it helps it spin faster. It 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 eighty five percent 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's close to an over fun black Hole'd be really interesting to watch as that stuff falls in, to see, like, is it's beating up? Does it gradually approach ninety nine point nine nine nine percent, 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 without falling in, Daniel, is this the kind of black hole that you would want to travel to and try to sort of drop some sonar into the middle of the black hole?
I mean, I don't even travel the 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 mike and tell us what you're learning as you fall in.
Yeah. I do have a good wind screen 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 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 over spin 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 second category? Who would not want the black hole to wreak the rules the universe? Who I've been 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. Were 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 gotta break rules to learn exactly.
Unfortunately, it seems like black holes are fortress 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.
We need a rogue black hole, a maverick.
We need to put a black hole in witness protection.
What would that even look like?
Put a black hole inside another black hole? I don't know. We'll ask the sbuy 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 iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact. But the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions house US Dairy tackling greenhouse gases. Many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit us dairy dot COM's Last Sustainability to learn more.
There are children, friends, and families walking, riding on paths and roads every day. Remember they're real people with loved ones who need them to get home safely. Protect our cyclists and pedestrians because they're people too. Go safely California from the California Office of Traffic Safety and Caltrans.
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