What can possibly replace the singularity? What are Planck stars and Gravastars? What’s it like to fall into a rotating black hole? I discuss these questions and more in today’s Ask a Spaceman
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So what really happens at the center of a black hole? Well, the short answer is we don't know. And if you just wanna stop there, I won't blame you. Just just end the episode, move on to the next because that's exactly the direction this show is going. We don't know.
But if you're interested in the long answer, well, listen on. We use general relativity to understand black holes. In fact, that's how we predicted the very existence of black holes. And that's where we get the language of the event horizon and the inescapable gravity. And at the center of the black hole, when viewed in general relativity, which is, like I said, our only way to view black holes right now, at the center is something called the singularity.
A singularity is a point of infinite density. It's the place where everything that formed the black hole, that ever fell into the black hole, everything associated with the black hole has collapsed down into an infinitely tiny point, like a literal geometric point. I want you to understand, in general relativity, the singularity of a black hole takes up no space. It has no spatial extent. It has no volume.
It has no length or width or even depth. It is a literal geometric point. That's the singularity in general relativity. Matter crammed down to an infinite density. This singularity appears in general relativity in the description of black holes, and so far everything we've been able to test about black holes has agreed with what's predicted from general relativity.
And so if we want to take this to its ultimate conclusion, singularities are at the center. But singularities aren't real. Like, you can't cram matter down to an infinitely tiny point and make it have no spatial extent. The the singularity, the presence of the singularity in the mathematics of general relativity when describing black holes tells us that we're doing something wrong. It tells us that there is a breakdown of physics.
It's telling us that general relativity isn't good enough. It's telling us that we need something new. Specifically, we need a theory of quantum gravity. We need to understand strong gravity at small scales where the force of gravity meets the realm of quantum mechanics. We do not have such a theory.
We wish we did, but we don't, and so we don't know what's happening at the center of black hole. All we know right now is that it's a singularity. And according to general relativity, singularities only appear inside black holes. In fact, the singularity, like, you can think of the black hole is really the singularity. The event horizon, the production of an event horizon is a consequence of the presence of a singularity.
It's just an invisible line in space where, space itself is flowing towards the singularity faster than speed of light. Like but the black hole really is the singularity in general relativity. We think that it's possible that all singularities are are cloaked or hidden behind event horizons. If if a singularity outside event horizon is called a naked singularity, and those are bad. They're bad because singularities are places where physics breaks down.
We have no physical description of the environment near a singularity. We simply don't. And so all of our understanding of causality breaks down, of cause and effect, and action and reaction. It's just all the stuff that makes physics physics breaks down. All abilities to predict future events from known physics breaks down.
So, you know, like I said, physics breaks down near a singularity. Let's say let's say singularities actually existed, that there is a place where all laws of physics breaks down. You want those to be behind event horizons, then it's no biggie because if it's behind an event horizon, it doesn't affect the rest of the universe. And anything that falls through the event horizon is totally cut off from the universe anyway, so it just doesn't matter if, like, if you really want there to be a place in our universe where all laws of physics breakdown, where all causality breaks down, you want it behind the event horizon because then you don't have to care. It's just stuffing it in the closet and then never opening the door again.
It just doesn't affect your daily life. But if there were a naked singularity, a singularity without an event horizon, that would mean that there's a place in the universe that is accessible where time, space, and causality and all of reality breaks down, which would be bad. Like, how could it be that there's a place that we can reach in our cosmos that you can travel to where all sense of time and space and causality and all of real just everything breaks down. That would be bad. So some physicists have proposed something called the cosmic censorship hypothesis.
If singularities really do exist, if you really do want a place where all causality is broken, then they are always behind an event horizon. It appears that this is the case with general relativity that anytime it produces a singularity, it is behind an event horizon. Although you can make a lot of assumptions and tweak the math and sometimes maybe produce a naked singularity, but it doesn't, like, look like it's physically possible in the universe, which is good, or the singularity doesn't exist. And there's no need for a cosmic censorship hypothesis because we need to replace the singularity with something else. What is that something else?
We don't know. Like we said, whatever is happening near and at the singularity, it is at a meeting point between gravity and quantum mechanics. It requires new physics, requires a new physical theory of nature and we don't have that physical theory of nature. And but, hey, this is why we're so interested in singularities because understanding the singularity would reveal a more fundamental picture of reality. So it's it's a little bit of interest in the topic in the community.
Here's an option. We can replace potentially the singularity with something else. Maybe it's something called a Planck star. Planck star. So this comes about from an idea called loop quantum gravity, and I haven't done an episode on loop quantum gravity yet.
I probably should. A lot of you have asked. I did a few episodes on string theory. String theory is one attempt to unify quantum mechanics with our understanding of gravity and give us a theory of quantum gravity. String theory ain't doing so well, but there are alternatives out there.
One of them is loop quantum gravity. Now unlike string theory, string theory claims to be a theory of everything where it gives us a theory of all the things. Loop quantum gravity says no. No. No.
No. Not not for me. I'm just gonna give you a quantum theory of gravity, and that's it. And loop quantum gravity, which I'll dig into once I do an episode on it, says that space and time itself are quantized, that space and time itself come in discrete little chunks. And and from our perspective, way up here in the macroscopic world, space and time appears well, I like a wave my arm around.
It looks smooth and continuous. I feel the the the march of time as some continuous flowing river. But in loop quantum gravity, space and time are actually divided up into tiny little chunks, But really, really small, somewhere around the Planck scale, which is, like, 10 to the minus 35 meters, which is all really small. I don't even have an analogy to describe how small it is. Like, it's oh, it's the Planck scale is to an atom or, like, an atomic nucleus as an atomic nucleus is to, like, the universe or something.
It's it's it's tiny. But that's where space and time are quantized. And so you can't have anything smaller than that. You can't have spatial scale smaller than the fundamental spatial unit of space, and you can't have time scales shorter than the fundamental unit of time. You just can't.
It's not physically allowed because our universe is discrete and chunky, and you can't have a chunk smaller than the smallest possible chunk. You're simply not allowed. So in the view of the Planck star, we don't have singularities because singularities are infinitely tiny, and that is definitely smaller than the Planck scale, smaller than the smallest possible thing that's allowed. So instead, everything that has ever fallen into a black hole, gas, dust, cats, your worst enemy, gets scrunched down to not to infinity, but to something very, very, very small. One of the benefits of the existence of a Planck star is that this can potentially help avoid that black hole information paradox, which we did in episode on way back when.
And you'll recall, like, the information paradox is information goes into a black hole. And if it's just the information gets locked away forever, who cares? But then Hawking discovered Hawking radiation, good thing it was called Hawking radiation because he discovered it, shows that informations eventually evaporate, and that radiation in its simplest understanding carries no information with it. So if all the information goes into a black hole and then it emits radiation that has no information and then it disappears, what happens to the information? Hence, paradox in a lot of theoretical interest.
One way to avoid that is that, oh, oh, instead of a singularity, it gets scrunched down into a Planck star, and then the black hole evaporates through its normal Hawking radiation carrying no information with it. Instead of just blinking out of existence, it leaves behind the little Planck star nuggets, this tiny tiny object that has all the material that ever flowed into the black hole compressed in this tiny little volume, and presumably there's enough volume there to contain all the information of everything that fell in. That's nice. Another interesting thing about Planck stars is that they this fundamental quantum nature of reality, this quantum nature of space and time provides a repulsive force. Just like degeneracy pressure.
Like, the fact that electrons can only exist in certain states, if you try to squeeze electrons down to smaller and smaller states, they will resist you. There will be a pressure, and this is what holds up white dwarves and neutron stars. The fundamental chunkiness of space time itself can provide a sort of degeneracy pressure, like the ultimate kind of degeneracy pressure. Like, something's preventing ultimate gravitational collapse, and this is it. But this actually forces a rebound where material falls in, scrunches down, feels this repulsive force that eventually flings it back outwards.
So eventually, black holes explode and not in the nice calm quiet way of Hawking radiation like a big boom, but this takes a long time because of the extreme gravity in a black hole. There's a lot of time dilation going on. So even though this process might take only a second from the point of view of the material inside the black hole itself, from the outside world, it could take tens of billions or hundreds of billions or even trillions of years to actually play out. So Planck stars actually predict that black holes will blow up, which is kinda cool. And it potentially solves the information paradox problem, which is kinda cool, and it totally gets rid of the singularity, which is kinda cool.
Problem, we don't know if this is correct. We don't know if Planck stars are right are correct. We don't know. It does all this requires space and time to be quantized. We don't know if space and time are can be quantized.
Loop quantum gravity hasn't come up with any definitive predictions. It isn't a fully fleshed out physical theory of nature. It's in a different kind of mathematical muck, but still a mathematical muck same as string theory. Like, string theory is just tangled up in mathematical knots, pun intended. Loop quantum gravity is all tangled up in mathematical knots as well.
It we can't have a physical theory. It can't make predictions. It can't explain how the universe works. We don't know if loop quantum gravity is correct. We don't know if that if we had a correct version of loop quantum gravity and space time really was chunky, that that would lead to the existence of Planck stars?
We honestly don't know. We don't know. So it's a cool idea, but a very untested hypothetical idea. So here's another option that's been proposed, something called gravastars. Gravastars.
I'm not making up these names, folks. Gravastars, it's like a black hole, but not. When you think of a plain black hole, it's it's you've had the singularity at the center with infinite density, and then you have some empty space. Then you have the event horizon, that boundary, the invisible boundary, and then you have more empty space on outside of it. You know, that's the simplest possible description of a black hole.
The gravistar idea is that instead of being empty space on the inside, it's filled with dark energy. Now dark energy, if you'll remember, is the and if you know, I'm about to repeat it. Dark energy is the name we give to the accelerated expansion of the universe. Right? Our universe is getting bigger and bigger faster and faster every day.
We call this dark energy. One possible explanation for this accelerated expansion is that there's something in the vacuum of space time itself, which is repulsive, has an anti gravity property. Yes. Gravity is allowed to be repulsive under very special conditions, and we'll see another special condition later in this episode. But one of those conditions is called dark energy.
So maybe black holes aren't filled with a singularity to center and then just a bunch of empty space. Maybe they're filled with dark energy, and so all the material that falls into a black hole just gets stuck on the event horizon because there's all this anti gravity on the inside that is supporting it. Like, maybe that's what's preventing ultimate gravitational collapse is some sort of vacuum energy, some sort of dark energy itself. There's no event horizon When it comes to gravastars, it's just you just get stuck. Like, you're trying to fall in that you just get blasted on the surface.
So but that means light can pass through. Light can pass through. There's no event horizon. Light can pass through regions of dark energy just fine. But other than that, it looks and acts totally like a black hole.
And bonus, there's no singularity because all the matter is spread out on the surface, not crunched down in the middle. Instead, you just have a ball of dark energy. Double bonus, gravastars might create a new universe because some material can potentially collapse inwards, like when it's forming, pass through the center of the gravistar and explode in a new dimension, and then the dark energy inside the gravistar can pass through as dark energy as the other side and potentially explain the dark energy that we find in your our universe. Like, maybe we're just on the inside of a giant of gravistar in another universe. Let me just pause to catch my breath here and to caution against ideas that seem to solve every problem in physics.
I always get suspicious. Like, we don't understand singularities. Okay. I've come up with a solution for singularities, and by the way, I've also explained the nature of dark energy. I've also explained the origins of the universe.
I've also explained, you know, why that dinner didn't go so well last night. Like, anything that that starts to explain so many different things, I always get naturally suspicious because that's just that isn't how it works, and it doesn't even take into account the existence of Patreon. You can go to patreon.com/pmsudder to learn how you can keep the show going. Doesn't even talk about that. Like, Gravastar is missing a major component of the way our universe works, and that's through Patreon and my never ending gratitude.
And this idea of the Gravastar, like, it sounds cool. It's super sci fi. It's very mathematical. These are like it's like the barest sketch of an idea, and it doesn't even come close to confronting reality. Like, hey.
Okay. Gravastars are really objects that are filled with dark energy. That sounds cool. How do you make one from the, star that's collapsing? Like, we know how to make black holes in our universe.
You just take a star, yank out its power source, and watch it collapse, and it forms a black hole. How does that get filled with dark energy? Intentional silence here because there is no answer. Like, how do you actually physically create a gravistar? Nobody knows.
Nobody knows. And, also, we've watched black holes collide or listen to the gravitational waves emitted when black holes collide, and most gravistar models are simply ruled out because they do talk about the nature of the event horizon. They do make a prediction about what the event horizon looks like and acts like and feels like, and, like, every almost every gravistar model has been ruled out. And I say most because the sufficiently motivated theorist can always cook something up that barely agrees with the latest observations. But as time goes on, those models become ever more complicated and untenable.
So, yes, there are still surviving Gravastar models out there that have passed observational test, but they're kind of complicated. And, also, we haven't answered the riddle of how to actually create a gravistar in using the physics of our universe. So kind of bummer there. The last option I'm gonna explore is to look at a more realistic black hole. You know, the picture that we have of black holes of a singularity, a point of infinite density surrounded by empty space, then there's this shell, the invisible line called the event horizon.
That's that's all well and good, and that's fine. That's the simplest possible form of a black hole. That's a black hole that isn't electrically charged and that isn't spinning. But what if we add electrical charge or rotation to a black hole? Like, does this even happen?
Well, well, charged black holes probably aren't really a big concern because if a black hole were to say be net negative charge, it would very, very quickly attract positive charges, which would fall in and neutralize it. And the new the universe itself is neutral on balance anyway. So we don't think charged black holes are really gonna be a real thing that we need to worry about in the universe. But rotating black holes, that's another thing. Stars rotate, and when big stars die, they turn into black holes.
The black holes are gonna be rotating. Matter can fall in, and it's spinning around. Like, yeah. Yeah. Black holes spin.
They rotate. That's a real thing. Does this change anything in our story? Does this eliminate the singularity? Does it make it more complicated?
Do we get an out once we start looking at more physically realistic black holes? Well, we can model what happens to a black hole once it's spinning. It's actually a very complicated solution. A Roy Kerr was the first one to figure it out back in the sixties, I believe. Like, if you're going to encounter a black hole in our universe, and I recommend you don't, but if you were, it would not be a static uncharged fixed nonrotating object.
It would be an uncharged object, but it would be rotating very, very quickly, so you'll have to deal with what a rotating black hole will do to you and what you'll experience. On the outside, there's all sorts of cool stuff. There's a region called the ergosphere where space itself is being dragged along with the black hole. Like, if you set a, like, a a coffee table on a rug and you start spinning the heavy coffee table, the rug itself will get pulled. That's pretty cool.
The event horizon itself gets stretched out. That's pretty cool. You fall in, though, and then you're gonna die. You'll pass through the event horizon, but the singularity that's at the center isn't a point of infinite density. Instead, because of the extreme rotation, it gets stretched into a ring.
And there's another force to contend with inside of a black hole. Inside of a static black hole, it's just gravity. It's just sucking. It's just pulling and pulling and pulling and pulling and pulling until you hit this singularity and you die if you manage to survive that long. You get crunched in that infinitely tiny point, which we know is wrong, but that's the story provided by GR.
The story provided by general relativity for rotating black holes is that there is another force, and that's the centrifugal force. You see a black hole is rotate. Like, you're let's just talk about the gravity. The gravity that you experience from a black hole, you can think of as emanating from the sing singularity. So because that's where all the stuff is.
It all scrunched down to that tiny little point. That's like the power source. That's the heart. But in a rotating black hole, there's still all that stuff way at the center, still at the singularity. But because of the extreme centrifugal force, it's bent into a circle.
And that centrifugal force is large because all that material, all the gas and dust, all those solar masses have crunched down to the singularity. And through conservation of momentum, which is gonna hold even inside of a black hole, it's going to spin faster and faster and faster. And so you have an indescribable amount of rotation at the center of a black hole, a rotating black hole. Indescribable. So I won't even try describing it.
But something strange here happens here. Because the centrifugal force is ludicrously extreme, it creates antigravity. This is one of the weirdest things of general relativity. Like, Newtonian gravity is all pulling all the time. That's the only thing that gravity can do.
In general relativity, however, gravity can become repulsive under certain special conditions. One of those conditions is dark energy, and that's the accelerated expansion of the universe. We see it. Another condition is when you have a lot of centrifugal force. Strong enough centrifugal force can create antigravity.
Now we don't feel this in our normal everyday lives because the antigravity produced by, say, like, a merry-go-round, like, a merry-go-round has centrifugal force that produces an anti gravity, but it's so insanely weak compared to the normal gravity of all that mass that you don't even notice it. It's barely a fraction there. It's like technically even it exists, but come on. No one can even measure it. It's impossible to even measure.
But inside of a rotating black hole, the centrifugal force is strong enough that its antigravity becomes repulsive. I'm gonna leave that aside for a second. It's still possible for you to pass through the ring singularity of a rotating black hole. And if you follow the mathematics of general relativity to the letter and number, that ring singularity is the entrance to a wormhole. When you pass through the ring singularity without dying because you're not touching the singularity itself, you enter a wormhole.
You travel to a distant part of the universe, and then you get spat out of the polar opposite of a black hole, which is a white hole. Black holes don't let anything out. Their opposite white holes don't let anything in. If you fall into a black hole, space itself is flowing inwards towards the singularity faster than the speed of light. The interior of a white hole space is flowing outwards faster than the speed of light, so you have no choice but to be ejected, and you will be deposited in some random part of the universe.
That's what it says. That's what the math of GR says that if you have a the center of a rotating black hole is the entrance to a wormhole that is connected to a white hole in some other part of the universe. Taking aside the hole, how do you actually construct this from a real collapsing star? Just looking at the plane mathematics itself, this doesn't happen. And it doesn't happen because of that centrifugal force and the antigravity that it produces.
There's a meeting point inside the event horizon of the black hole, of a rotating black hole. When you first cross the event horizon, you're pulled by the gravity of the singularity, so you sink down and down and down and down and down. But then you start to feel that resistance, that antigravity from the centrifugal force pushing back at you, and then there's a meeting point where they're in balance. That meeting point is called the inner event horizon. Now you will pass through the inner event horizon because you yourself have mass and you're gravitationally attracted to the ring singularity and so you can overwhelm that repulsion, but light can't.
A beam of light that passes into a rotating black hole will cross the event horizon, travel down to the singularity, keep going, but then meet that resistance of the antigravity. And then at the boundary, at that inner event horizon, it will get reflected back, but then hang out there. It'll get stuck, and it will continue to gain energy. It will constantly again, this is just following the math. It will constantly wiggle back and forth between the inward pull of gravity and the outward pull of anti gravity.
You'll get stuck there ramping up in energy to infinite energy, infinitely blue shifted. So if you were to pass into a rotating black hole before you touch the ring singularity and enter that magical wormhole, you will have to pass through the inner event horizon which will contain the entire past history of the universe sped up to infinity and energized to infinity. It'll be an infinitely hot, infinitely bright wall of energy. This is probably going to kill you, but it also tells you that we're doing something wrong. Because the whole underlying assumption of a rotating black hole.
Like like, the the what you write down and say, okay. I'm going to describe the the gravity associated with this arrangement of material. And that assumption is that all the matter and energy of a black hole rotating black hole is concentrated in the singularity. That's how you get the solution. That's how Roy Kerr got his solutions.
Okay. If I put all the matter and matter and energy in the center and then set it rotating, what do I get? But the inner horizon that results from this is also a place of extreme matter and energy, another place of infinite energy. So where's the infinite energy in a rotating black hole? Is it in the singularity, or is it in the inner horizon?
You can't have both. It's a contradiction. It shows that the solution is breaking down. The solution leads itself to a contradiction. We are going to assume in order to construct how a rotating black hole behaves, we are going to assume that all the matter and energy all the energy is concentrated in the center in the singularity.
And then as you work out the results, you find another place with infinite energy, and that's bad. That breaks your very assumptions of how you got the math in the first place. We know that we're able to describe rotating black holes very, very well on the outside because it matches up with everything we see, all of our observations, including gravitational waves, and I should do an episode devoted entirely to gravitational waves and all the black hole merger discoveries. Feel free to ask. But this is telling us that our solution is breaking down, that we haven't avoided the problem of the singularity.
There's no wormhole. There's no white hole. We haven't been able to shove that issue to the side. Instead, it's once again telling us that we don't understand the center of black holes. Every time we try to come up with a physical description of the centers of black holes, the physics breaks down.
Non rotating, rotating, doesn't matter. Gravastars don't look promising. Planck stars, who the heck knows? What's really at the center of a black hole? We don't know.
I told you. Thank you so much for listening, and thank you to Andy p on email, Britney on YouTube, Jeff j on email, Robert s on YouTube, Vladimir b on YouTube, Jack s on email, at Grubelard on Twitter, James l on email, and Peter e on email for the questions that led to today's episodes episode. I only did one. And thank you so much to my top Patreon contributors. That's patreon.com/pmsutter.
The top contributors this month were Matthew k, Justin z, Justin g, Kevin o, Duncan m, Corey d, Barbara k, Nudardu, Chris c, Robert m, Nate h, n or f, Chris l, Cameron l, Nalia, Aaron s, Kirk t, Tom b, Scott m, Billy t, and Rob h is your contributions plus everyone else's that keep this show going. This is my job, folks. This is my job and I love it. Please keep those questions coming. Feel free to go to iTunes to drop a review.
I really appreciate it. Tell your friends I'm also on Spotify. And, hey, if you haven't bought How to Die in Space, we I do talk about how to die in a rotating black hole. That's How to Die in Space. You can find it on booksellers nationwide, also Audible and Amazon.
You know the deal. Keep those questions coming to ask a space man at g mail dot com or hit me up on social media. I'm Paul Matt Sutter, and I'll see you next time for more complete knowledge of time and space.