Daniel and Jorge Explain the UniverseWhat convinced people that black holes are real?
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Hey Jorge, do you still have that pet black hole in your backyard?
Well, you know, my physics lawyer tells me as you neither confirm nor deny.
That that's good advice. So in that case, I have a question for your physics lawyer.
Uh oh, he charges a lot better be good.
All right, here's my question. How do you actually know that it's a black hole?
Well, it's either a black hole or something else that eats a lot of bananas.
All right, So you either have a black hole or King Kong living in your backyard.
Yeah, either way, I definitely need more bananas. I am Orhan, a cartoonist and the creator of PhD comics.
Hi, I'm Daniel. I'm a particle physicist, and I think the universe is kind of bananas.
Welcome to our podcast Daniel and Jorge Explain the Universe, a production of iHeartRadio.
In which we take you on a tour of everything that's crazy and amazing about our universe, which turns out to be most of it. We talk to you about the things that we do know, and about the things that we don't know, the things that scientists are trying to figure out, and the things that you are wondering about this incredible, glittering cosmos we find ourselves in.
Yeah, all the amazing stuff out there that's mysterious and interesting and intriguing and potentially mind blowing. And sometimes we also like to talk about how we discover things, because you know, I think that how we discover things sometimes is almost as important as knowing the thing itself, because that's how we know it's there and tells us a little bit about how science works.
Yeah, as an experimentalist, I always want to know, like, how do we know that's true? You're telling me this thing exists in the universe. How do we really know? What is the evidence that convinced people? Because there was a moment in science when we didn't think it existed, and then all of a sudden things shifted and everybody was convinced. What were the pieces of evidence that came together in people's minds that made them believe something new and wacky and unbelievable was real?
Daniel, is there the equivalent of like a behind the scenes clip for physics? You know, like when you publish a paper, do you also publish the commentary or the behind the scenes or bloopers?
You know? Sometimes people accidentally leave little comments in their paper. I mean not visible in the final product, but they're sort of hidden inside the text that creates the paper, and you can scan through and you can see people authors arguing about what they should put in the paper, like this next line is hot garbage, We should cut it out?
Why?
Stuff like that?
Yeah, no kidding.
It's the equivalent of like leaving little comments in your Microsoft word document. We usually write our stuff in La tech, and you can put comments inside the source, and sometimes people forget to pull that stuff out, and so you can get some pretty hilarious insights into how a paper was.
Wow, does anyone ever say we totally made this up? But don't tell anybody no?
And it's not usually that dramatic. It's not like, you know, the last dance that documentary about Michael Jordan. We don't have the You know that much of drama in physics usually if you're writing papers with people, then you mostly agree. Most of the drama is between papers, like you know, this paper says the other paper was wrong, and those folks over there in that university don't know what they're talking about. That's where most of the conflict really is, see gossip. That's right. Hey, the stakes are hi. You know, we are trying to figure out the nature of the universe itself here, man, it's not just basketball game.
You can't get caddy enough when it comes to the universe. So to be on the program, we'll be talking about the discovery of something that I guess most people have heard of this. You know, everyone who's interested in science and space probably has heard of these, but I've bet not a lot of people know how they were discovered.
That's right. It's something that's extra weird and fascinating in the universe, and it took science a long time to come to grips with the idea that they could actually be out there, that they could really be a real weird thing in our universe. And this is something that happens a lot sort of in physics, that we have an idea for something weird and strange, we think, well, that's just like some mathematical artifact that's not actually real, it doesn't really happen out there. And then we discover, Wow, the universe actually is that weird. Quantum mechanics is real, electrons really are determined by weird probabilities. And so we're sort of coming to grips with We're waking up to realize that the universe is stranger than we ever imagine.
Yeah, and this thing is extra strange and extra weird. It's one of the maybe true like pockets of mystery in the universe that we may never even discover what's inside of them. So to the on the podcast, we'll be asking the question, how do we discover black holes? So, Daniel, I assume we didn't just fall into one by accident.
We're inside one right now. Welcome to the podcast Inside the black Hole.
That noty we'll ever hear.
We are literally screaming into the void.
There's an idea that maybe we are inside of a black hole. Isn't that a possibility?
Yeah, that's a possibility. It's theoretically possible that our entire universe is inside another black hole. We had the Loop quantum gravity theorist come on. We asked her what's inside a black hole? And she actually said maybe an entire universe, And so that's fun to think about. But you know the problem with black holes is that we can't see inside, so we don't know what's inside them, and anybody who does survive the journey into a black hole can't then shout to us about what they find. So, as you said, they may be eternal pockets of mysting.
Yeah, so we might be live inside of a black hole right now along with everybody else.
That's right, along with all those socks you've lost that probably went into a black hole.
Somewhere that's an extra black hole, that's like a laundry hole. Yeah, it's a big question, like how do we know that black holes really exist? What's the evidence? And more kind of maybe interesting is how do we come to think of them? And how did people become convinced that they exist even before we had ever seen one?
And so, as usual, I pulled our listeners and I asked them if they knew about the history of the discovery of the black hole. What was the crucial piece of evidence that moved it from the category of like crazy bonkers theoretical idea to crazy bonker's real actual facts.
Think about it for a second. How do you think we discovered black holes? Here's what people have to say.
A star found that was turning red and blue, but it didn't but there was no recognizable binary pair or another star for it. So that's how they knew that it must be a binary pair with a black hole.
I would say that this is true, somebody observing the space and has seen some sort of gravitational lensing happening, that the black hole has gone in between a observed star and a observer.
I believe that they were initially discovered by in more of a theoretical sense, by not being able to explain the gravity that was missing due to the rotation universe and what was holding it together. They were officially discovered by the gravitational lensing I believe.
I think black holes were predicted by Einstein, but how they were discovered was by seeing lensing in stars.
An idea I rather believe is that when two black holes collide or a black hole is like born. So when a star collapses into a singularity that these occurrences leads like this leads to an emission of race maybe like X rays or gramma rayce I'm not sure, and these rays were like measured on Earth.
I'm not sure, but I would guess some gravitational lens effect from what I can remember, Einstein were the one that figured out that black holes were a thing with just equations and stuff, and then it took years and years and years until they finally found the actual evidence of it in real life.
The only way I can think the black holes were discovered were maybe because of their gravitational effect on the surroundings.
I assume that it was probably a mathematical possibility for the existence of such a body which had such strong gravitational pull that even light cannot escape, and maybe later on they finally found it.
All right, pretty interesting answers.
Yeah, people do have an idea that black holes were first thought of and then later discovered, which is pretty cool. But the consensus tends to be here, some gravitational lensing that you could like see a black hole passing in front of a star and distorting it, and that's true in theory that if that happened. You might be able to see it, but that's definitely not how black holes were discovered.
So not through gravitational lens, but maybe through some other gravitational means.
Yeah, and this actually has a lot of parallels to other big mysteries of the universe, like dark matter. We've talked about dark matter on this podcast a lot. It's something we know is there but only have sort of indirect hints of its existence, and all those hints are gravitational. And in a very similar way, black holes are very strong, very powerful, very important to the universe, but also very hard to see directly because they're mostly gravitational objects and gravity is very weak.
Yeah, I guess it's hard. How do you see something that's in space, especially.
It's the perfect camouflage, well done, black holes.
It's hiding, But what is it hiding from?
It's hiding from us.
I guess all right, Well step us through here, Daniel. Because black holes are sort of interesting in that they were thought of theoretically first, or there was some discovered theoretically first, probably a long time, almost one hundred years before we actually ever saw one.
Yeah, and you might be wondering, like, what does it mean to discover something theoretically right, after all, for things to exist that have to be in our universe, and so experiments of the old things that can really discover something right, Well, you can actually make discoveries theoretically. You can say, here are the laws of the universe as we think we understand them, what are the consequences of them? What are some predictions we can make from these laws that would maybe be surprising. And so if you like look in corners of the space and say, oh, if these laws do this and these laws do that, is there something that we hadn't anticipated that these laws can do. And that's precisely what happened with black holes. Came to some new understanding of the way gravity worked, and then started to look at the consequences that what does that mean? What possible weird stuff can gravity do? And people almost literally stumbled over this weird, bizarre prediction for what gravity could do.
Yeah, I guess you can theoretically discover things theoretically, is kind of what you're saying.
Yeah, And that's exactly what was done, for example, with the Higgs boson. The Higgs boson was an idea which first came about theoretically. People looked at the pattern of the particles and they thought, you know, this would make more sense if there was this other thing that existed, and then we found it. Very much in contrast to dark matter. Dark matter is something that was seen experimentally. It was like a puzzle in the universe. We didn't understand what we were seeing until later we came up with an idea to explain it. But black holes were found theoretically, and it really, as you said, goes back to the genesis of general relativity. Back in nineteen fifteen when Einstein published his final paper on the field equations for general relativity and.
Then he dropped the mic. He's like estein out.
He sort of did. And the thing I understand about his field equations is that they are nasty and complicated, Like he discovered sort of how space and time talk to matter, you know, And what he discovered is that space isn't just like an empty backdrop, but it's something that's dynamical and that it responds to matter. So matter tells space how to curve, how to bend, how to shape, and then space tells matter how to move. So it's like a complex system, a thing with a lot of feedback, and that makes it very difficult to know, like, well, what is the solution what actually happen in various circumstances. And for a long time, the only thing people could ever figure out in terms of the Einstein field equations were super simplified universe like the universe filled with homogeneous dust, or a totally empty universe like nobody's ever solved the Einstein field equations for our actual universe.
I guess it. You can sort of explore with equations, right, Like if you find that the things around you obey a certain law or equations like F equals and a you can then kind of tweaked the numbers and the parameters to kind of explore more extreme conditions than what do you have around you. Right, you could add like what happens if the mass goes to zero, or what happens if the force goes to zero, and the equations would tell you.
That's right, and that's exactly what happened. So Einstein published these equations in nineteen fifteen, and then he sent them to his friend and colleague, Short Child, and short Chiled looked at these and he played around with them, and he actually found a solution. Just a few months later, he found what's one of the first exact solutions to the field equations, like a configuration of matter and the definition of space that satisfied those equations that could be real in the universe. So he found this solution, and he found some things about it that were kind of weird.
Well, I get to step us through a little bit. What did you mean by a solution to the equations? Like the equations kind of related space and matter, and then you have to find a solution for them, and the solutions what do the solutions tell you?
The solutions tell you how space curves. So if you have a configuration of matter, if you say I'm going to put a big blob of stuff right here in the middle of the universe, then the solutions tell you how space curves all the way through that universe. And so that's a solution.
Meaning how the space bends, or like how things move around.
It, how space bends, and then how space bends determines how things move, right, Like we know that having the Sun and the center of our Solar system bends the space in its vicinity, so that therefore the Earth moves in an orbit around the Sun rather than just flying off in what otherwise looks like a straight line. So you start with the mass, you say, I'm going to put this mass into the universe. That tells you the shape of space, and then that lets you determine the equations of motion how something would actually move through that space. So he was the first one to figure out, if you put a really massive object in the universe that's spherically symmetric, what is the shape of space around it? And what he found was really weird. He found that if you put in a really heavy mass, enough mass that there's this sort of edge to it, that there's this point where the curvature of space sort of becomes infinite, right like space is curved like it is around the Sun. But if the mask gets large enough, then you have this threshold, this point which we now call the short Child radius, where the curvature of space gets a singularity, where like the field equations have this infinity in them. And it's not something that he understood at the time the way that we understand it now. He didn't say, oh, this is the event horizon of a black hole. He was like, all right, I found a solution. But it has some weirdness at a certain distance from this audition.
Oh, I see, he just seems sort of like turned the knob on the mass and then he found that the equations suddenly kind of got walked.
Yeah, they got wonky, and people were like, huh, that's weird, and you know that happens a lot in theoretical physics. You're like, I found a solution to this set of equations. It makes sense of it here. I don't really understand what's going on in that part, but let's just put that aside for now. And people studied it for you know, ten years, twenty years, forty years before they really had an understanding for what that meant. Initially, they only look at it as well, this curvature space gets really strong here, so time slows down in the vicinity of a lot of curvature, and so it might be something like a frozen star. They thought, if time slows down as you approach this heavy, heavy mass, then you'll see time slow down as things approach this thing, and it's sort of like time stops when you get to that point. So they didn't call it a black hole back then. They called it a frozen star.
Wow, that's almost a little better.
Yeah. Well, and I think that they thought that if you saw one of these things in nature, it wouldn't be a black emptiness. It would be like a star, but just like frozen in time. You know, like if a star grew so massive that it passed this threshold, it would just like freeze in whatever, like crazy flaming moment it happened to be in right.
But it would stop emitting photons, in which case it might be black.
Yeah. Well that's not something that they understood until much much later.
All right, So then it was kind of wonky. And didn't they think that maybe the equation was wrong, like like, this is a weird result and predict something that seems that would make time stop. Maybe our equations are not meant to work in these extremes.
Yes, definitely. For a long time people thought, well, this is an interesting sort of mathematical curiosity, but they thought it couldn't be real. They thought instead that it only happened under a certain very special, perfectly symmetric conditions that you could achieve sort of on the page, but would never actually happen in reality, that in reality something else would interfere, would muck it up so you wouldn't get this weird behavior. So for a long time it was like, hey, look at this cute, little weird mathematical effect. Of course that's not real, like that would never really exist. The universe is not that insane.
But they had to give it some second thoughts later, and then later there were big discoveries about it. So let's get into those. But first let's take a quick break.
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All right, Daniel, so what do I do about the pet black hole in my backyard? Should I call the physics vet?
Or I feel like this is a legal trap. No matter what advice I give you, I me either complicit in your death or the death of your neighbors somehow.
Well, may it's just a theoretical black hole that I have in my theoretical yard. All right, So we're talking about the discovery of black holes. And so they were sort of this weird mathematical kind of oddity in the equations, and maybe the equations or wrong, or maybe these things were real but people didn't know. So what kind of pushed people to think that maybe they were real?
There was a period in the late nineteen fifties when there was a lot of theoretical activity because people finally understood theoretically what this might mean. There's a guy named David Finkelstein, and he was actually working on something totally different. He was trying to understand quantum gravity, was trying to bring together quantum mechanics and gravity, and he cooked up a really weird system that had some gravitational kink in it. And it was a weird system, but it gave him an idea. He was like, you know what, in my weird system, I had this concept of an event horizon where information can go in one direction but not the other. And he was looking at his weird theory and he thought, you know what, this might be what we're talking about in Schwartzile's theory. And he went back and he looked at these frozen stars and the Schwartziled radius and the singularity in the gravitational equations, and he said, you know what, that's what's happening here. He like theoretically reinterpreted this and said this is what an event horizon is.
Oh, he came out of a different direction.
Yeah, he just had like a moment of insight, like how to look at these equations. But still people thought, all right, well, that's cool. This makes this weird mathematical curiosity more interesting, Like how weird would that be if information it could only flow in one direction across a threshold in space? Right, But then they started seeing weird stuff out there in the universe that slowly built up the evidence that these things could be real. And the first thing was the discovery not of black holes, but actually of pulse songs.
These are like special stars, right.
Yeah, these are special stars. They are neutron stars. These are stars that are at the end of their life. And this is the leftover cored and it's so dense that all the protons and electrons have merged together to form neutrons. It's a very dense kind of object, but again, it was a theoretical object. People thought, well, potentially, if you had enough stuff together, it could collapse into a neutron star, but nobody really believed it existed until they were discovered. In nineteen sixty seven. A special version of them, pulsas, which rotate and have a very strong beam coming at the top of them, were discovered and people thought, oh, my gosh, maybe these very massive, gravitationally collapsed objects really do exist in the universe, and people started to take these mathematical solutions a little bit more seriously.
I see, pulsars are not sort of related to black holes, but they are sort of extreme and crazy and maybe also kind of like a weird equation oddity. And so when they finally they're like, hey, maybe maybe that applies to black holes too.
Yeah, maybe we should start taking these gravitational oddities that are theoretical. We should start taking them more seriously, because you know, if neutron stars are real, maybe black holes are also.
It's like, hey, he got a noble price, I want one too. Let's dig into this.
And it's a fascinating story because there's a lot of different threads. At the same time. You have this theory thread where people are finally starting to understand like what these equations mean. Then you have this thread from the neutron stars where people discovered pulsars, and then totally separately, people were launching rockets into the atmosphere to try to study X rays. And in White Sand's missile base in New Mexico, a couple of folks outfitted a rocket with this X ray detector because they wanted to see, like what does the sky look like in the X.
Ray Because the atmosphere stops a lot of X rays from coming down to Earth. So if you want to see X rays, you're going to have to go out into space.
That's right, and it's a great opportunity to see something new. It's just to look at the universe in a new way. Like we've looked at the universe using our eyeballs, and we build telescopes that are more powerful to look in visible light, but we also like to look at the universe in invisible light. You know, infrared or radio waves or X rays, and X rays are extra powerful because they come from gas that's at millions and millions of degrees. There are things out there that emit only in the X rays, and you can only see their X rays. They don't emit visible light. So people thought, well, let's take a look at the universe using a new set of eyeballs. So they flew these rockets up to the top of the atmosphere and outfit of them with X ray detectors. And this is not like you know in orbit. It just like goes up. It's sort of suborbital.
Yeah, it takes takes a selfie, then drop down.
It takes a university sort of scans the sky for like eight degrees and then it comes back to I like that. And they saw some really weird stuff. They found eight new very bright sources of X rays that nobody had ever seen.
Before, like spikes, and they could tell where was coming from.
Yeah, they could tell where it was coming from. They were like points in space. It was like, you know, you develop your picture and you see these bright dots in the sky, and this one, sickness X one, which turns out to historically be the most important, was the brightest one. And if you look up in the sky, there's nothing there, Like you don't see anything in the optical. Your eyes don't tell you there's anything, but there's an incredible source of X rays coming at you from this dot in the sky.
Interesting, and that's weird. So's it's not emitting visible light, but it's emitting X rays.
Yes, And that was really weird. So people thought, well, what's there. So then these same folks that are like, well, let's follow up on this, and they built a satellite with NASA and they put it up in space to orbit, and this gave them more data and more precision, and they were able to figure out exactly where it was coming from. This is now nineteen seventy, and they learned something else really fascinating about this source was that it was variable, Like it wasn't just emitting X rays. It would emit a bunch of X rays and then not very much, and then a bunch of X rays and then not very much. It was highly variable, and it was variable on a really short timescale. It's not like it would take a year to change. It could go like on and off in less than a second.
What and was it consistent or was it sort of random?
It was sort of random and sporadic. But the fact that it would go on and off in like less than a second, gave them a really really valuable clue about how big it is, because something that turns on and off in less than a second can't actually be that large. Why not because of the speed of light. Like if this is all caused by a single event, you have some event which is causing this thing to flare up, then there's a certain amount of time that the information has to travel across an object. So that limits how big that object can be if it's going to sort of operate coherent.
Like if it's too big, then you would see it fade in and out kind of.
If it's too big, then they would have like lots of different pockets, right, they have a little pocket over here that's doing something, a little pocket over there that's doing something. But for an object to act like as one, like one coherent source turning on and off, means it has to be pretty small because all that stuff has to sort of be in communication within.
Light speed, in sync. Right, Yeah, it.
Has to be in sync exactly, just like the boy band. It has to move in one direction, if you know what I'm talking about.
Yeah, no, totally. I'm a big fan of black hole street boys.
Also they were the best anyway, So they knew that this thing was very powerful, and then it had to be smaller than the Sun, like the time variability of it. Given the clue that this thing was like smaller than about ten percent of the Sun. So then that was really interesting because now you know you have something there very bright in the X ray and very very small.
I see, and it could be one of these crazy like neutron stars or did they think it was a new kind of star.
That was the next thing. It's like, well, maybe it's a neutron star. And so to figure out whether or not it was a neutron star, they had to figure out how heavy is it? Because neutron stars have a maximum mass, like you can't get a neutron star more than like three or four times the mass of the Sun. If they get that big, they should collapse to a black hole. So the next thing was to figure out like, well, how heavy is this? And the good news is that this X ray source had a star nearby. There was another a really big star as blue supergiant that was near it, that was orbiting around, and that one was bright and that when you can see in the visible and so this object, whatever it was that was making the X rays, was orbiting around this blue supergiant star. Really they're orbiting each other. It's like a binary system. And you knew that the X rays were not coming from that other star like that can't make X rays, they're not hot enough. You could tell, like their X rays coming from this separate thing that's orbiting the star. And based on how fast the star and this new mystery object were orbiting each other, you could figure out the mass of that mystery interesting and what did they find? How massive was it? It was really pretty big. It was like fifteen times the mass of our Sun. And this new object was orbiting this super giant star like every five days. Like this is not you know, a year long orbit or something. It's been pretty close to each other. Yeah, this is like cosmically very violent. And so you knew it was very massive but not very large, and you knew that it was really dense, and you knew that it was dark. And in the end, all of the evidence for the observation of black holes basically comes down to an argument like that, like you have a huge amount of mass in a small amount of space, and it's not radiating, so therefore it must be a black hole.
Oh, the thing itself had to be a black hole, because what Neutron stars can't be that heavy.
Neutron stars cannot be that heavy. If they get any heavier, they should collapse gravitationally to being a black hole.
Theoretically though, Like, but at the time, they didn't know if black holes were real, So couldn't they just have assumed that it was a super duper dense neutron star.
Yeah, well, you could rule out neutron stars because neutron stars actually do make visible light. I mean, they have a surface, and when stuff falls under the surface of a neutron star, you can see it radiates. Right, so neutron stars can be visualized. But you're right, you could say, well, how do we know it's actually a black hole? How do we know it's not something else? If it's just really massive and really small, how do you know it is a black hole, not some like weird preon star or a quark star or some other new kind of non black hole matter.
Right, But the theories back then predict that black holes emitted X.
Rays, So the X rays don't actually come from the black hole itself. It comes from the gas that's swirling around the black hole, the accretion disk, And so what we're seeing are not X rays from the black hole, but from the gas that's about to go in the black hole. The black hole was slurping out gas from this super giant blue star that was near it, and there was this like stream of gas and that it would swirl around the black hole, and as it was swirling around the black hole, it gets rubbed against each other, a lot of friction there, and that's when the gas is then emitting in these millions of degree situations, is emitting these X rays.
But did they know that back then, did they know that about the accretion disks, that that was all part of their model of black hole, that was all.
Part of the model of black holes. Yeah, but you know, it's still it's a little bit indirect, like how do you really know that it's a black hole? And even to this day, like our evidence is limited to basically that kind of argument. It's like, there's nothing else that we can think of that could describe this, nothing else that could be this dense and this massive and radiate in these certain ways and in no other ways. Black hole is sort of the only candidate we have to me that's not like total slam dunk evidence. You know, it's about as good as I think we can get. I'm not criticizing, but there's still there's a level of indirection there. It's like, yeah, I don't really solved the murder until you've seen the body.
Right, right, but you could still maybe find the person guilty. Yeah, the stellar object guilty.
And you know, there was a lot of debate and discussion in the community like is this thing real? And you know, like in the early seventies, I think most people were convinced in the astrophysics community that this was a black hole. But there was one holdout, very notable holdout.
Oh who was it?
Well, Stephen Hawking. Stephen Hawking. It was nineteen seventy four and he had just come up with his theory of like black hole thermodynamics and Hawking radiation, and he really moved the whole like theoretical field of black holeology I suppose forward. But he wasn't sure that it was a black hole, and he made sort of a famous bet with Kip Thorn. Hawking bet Kip Thorn that it wasn't a black hole.
Really what made him think it wasn't? What was he skeptical about.
I'm not sure he was actually skeptical. Later, when he finally can heated it, he said that he was just hedging his best. But this way, either it was a black hole, which was awesome for him, or he won a bet against kip Thorn, which was also awesome for him. So this way he got something.
Either way, he's playing all the angles.
Yeah, sort of like Pascal's wager with black holes.
Maybe he's superstitious. He's like, if I bet against myself, maybe he'll come through and then I'll get a noble price.
Yeah. And so that's the early seventies, and we have this evidence for a source, you know, that's very intense mass and a small space has the right radiation profile. And then this one last thread, which is quasars. Quasars are these very very bright source of radiation from very deep in the universe, and for a long time nobody really understood. They seemed to be coming from really far away, yet they were still really bright, which meant that whatever was making them was extraordinarily bright. For a long time, nobody really believed.
They didn't believe that they were black holes that was actually like a coded black hole.
No nobody even even thought it was black holes. For a while, people just didn't even really believe that the data was right. They thought, you know, how could something be this bright and so far away, because then it had to be ridiculously bright at its source. But people eventually believed these really are super bright sources and then finally came to understand them as super massive black holes at the centers of galaxies. And so this thread of quoasars was sort of helped along by the discovery of black holes as a real thing. People are like, oh, well, black holes are real. Maybe we can use that to explain quasars.
Also, mm, because we've seen quasars, we just didn't know what could be making that much energy.
Yeah, and this could explain it. A really dense, compact gravitational mass capable of squeezing the gas in its environment enough to generate this incredible radiation. And now we know that we have one. For example, at the center of our galaxy, center of the Milky Way, is a huge black hole four million times the mass of the Sun. It's called Searius a star. It's funny, it's called a Star there because the guy who named it was so excited and he thought Star made something exciting.
What really, Yeah, he didn't think putting an asterisk would make people somehow suspicious of it.
I know in the sports world an asterisk means like, well maybe not right, But in chemistry, star means excited state. For him, it's like an exclamation.
Mark, exclamation yeah, like a smiley face at the end, I discovered Sagittarius, a smiley face emoji, star emoji, yeah, telescope star night.
So you have all these threats coming together, this theoretical understanding of it as an event horizon beyond which no information can pass, and then the discovery of these X ray sources which had no corresponding optical signature, and then come together with this line of thinking about quasars. What are these weird emitters at the centers of these galaxies?
Plus trying to prove Stephen Hawking wrong. I mean, that's that's some motivation right there. All right, but all this is still a sort of circumstantial evidence, and so let's get into how we actually see them today. But first, let's take another quick break.
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Hi.
I'm David Eagleman from the podcast Inner Cosmos, which recently hit the number one science podcast in America. I mean neuroscientists at Stanford, and I've spent my career exploring the three pound universe in our heads. We're looking at a whole new series of episodes this season to understand why and how our lives look the way they do.
Why does your memory drift so much?
Why is it so hard to keep a secret, When should you not trust your intuition? Why do brains so easily fall for magic tricks? And why do they love conspiracy theories? I'm hitting these questions and hundreds more because the more we know about what's running under the hood, the better we can steer our lives. Join me weekly to explore the relationship between your brain and your life by digging into unexpected questions. Listen to Inner Cosmos with David Eagleman on the iHeartRadio app, Apple Podcasts or wherever you get your podcasts.
All right, Daniel, So we're I guess we're in the seventies and the eighties, and we have all this evidence for black holes, and there's a lot of stuff that we're seeing that could be or would be explained by black holes. And also everyone wants Stephen Hawking to be wrong. So what sort of sealed the deal for black holes? Like, was it us seeing them or ten taking a picture last year for the first time, or were we pretty convinced before.
Well, it's been a sort of slow accumulation of evidence. And the best way to convince yourself that something weird is real is to see it in lots of different ways, because that prevents you from having made one particular mistake or misunderstanding one kind of data, or screwing up your lenses or something. And so the first piece of evidence was you know, this sickness X one, this X ray source from a very compact object. But now we have several ways, also still indirect, to point to these black holes.
All right, so what are some of these ways? Did we lose something out there in space?
Or Number one is that we started seeing a lot of quasars and by now we've seen like one hundred thousand galactic quasars, and so each of these, yeah, each of these are probably a super massive black hole the center of a galaxy, and so we see the radiation from them. You can see the accretion disk around some of them if they're close enough, really, and so this is pretty strong evidence that those black holes exist.
You can actually see the grease in this or you can just sort of see something really bright.
Well, you can see something really bright coming from the center of the galaxy. And we'll talk in a minute about the direct imaging of a black hole that came up much later. But this is pretty strong evidence for sort of black holes to exist in the universe. But you know, there's two kinds of black holes. There's these super massive black holes with the center of galaxies that are very powerful and crazy, and then there's the kind of black hole that I think most people think about, like a star at the end of its life that collapses and turns into a black hole.
Those are like the ped black holes.
Yeah, that's right. Those are the kinds you can pick up at the local mall. And we think that there are a lot of those, but we've only actually ever seen a few because they're much harder just spot. So while we've seen like one hundred thousand Galacti quasars, we only have a few dozen stellar mass black holes that we've actually observed.
But don't we see stars turning into black holes a lot? Like you know, these supernova? Don't we see those pretty often?
We see supernova I mean not that often, but not every supernova turns into a black hole. Some of them turn into neutron stars or something else. And so it has to be particular size to turn into a black hole. And also you see a supernova, the black hole doesn't necessarily form immediately and isn't visible immediately. It's surrounded by this huge cloud of stuff still for a while, and so it's going to take a long time for the accretion disc to sort of gather together and make the black hole visible own.
I hadn't realized that we've seen many more super massive black holes than the smaller black holes.
Yeah, these stellar mass black holes are harder to spot.
All right, So that's one growing body of evidence. What else is there?
Another indirect way to see a black hole is to look at the stars around them, Like if you see nothing in the sky, but then you see things swirling around it as if there was a very strong amount of gravity there. That you can sort of the same argument. You can say, well, there has to be a big blob of stuff there that's providing the gravity. And we see this, for example again at the center of our own galaxy. We can watch the path of stars as they near the center of the galaxy, and we can tell that they are bent in a curve as if there was an incredibly strong gravitational source there. We can't see the black hole directly, of course, but we can see the motion of stars around it.
Right, And you can find movies of this online probably Right. It's like a picture of basically a black screen, but you see stars kind of moving and going really fast around nothingness.
Yeah, there's a group at UCLA that's been watching the center of our galaxy for like two decades now and plotting the motion of these stars and there's one star in particular, it's called S two, which passes really close to the center of the galaxy and then whips around who has sort of a shorter period. And because they've been watching for two decades, you can see like complete orbits of some of these stars, and then you can do the calculation and you can tell how much mass there is, and because the stars passed so close to the center, you can get a sense for how small it has to be. Right, the star's trajectory limits the size of this thing, and so at some point you're like, well, there's a huge amount of gravity in a small amount of space. Boom black hole, right.
And so wow, we can actually point a telescope at the center of our galaxy and get a picture like that.
Yeah, you can. You can see these stars. It's difficult because there's so much gas and dust and so you have to sort of see through that and look in the near infrared light and also use like fancy adaptive optics. But you can actually see that.
So we do have kind of a pet black hole in our backyard.
Yeah. Yeah, it's tens of thousands of light years away. So if your backyard is that big, then yes, you have a black.
Hole inside, or maybe we're the pets of the black hole in their yard.
And then recently we have a whole different kind of evidence for black holes, and that's the gravitational waves that come when they collide with each other.
Right, that's what Lego discovered recently, right, a couple of years ago.
That's right. If black holes get close enough to each other, then they slurp each other in. But usually they have some like angular momentum around each other, so they can't just like approach head on. It's like a near miss. And then they swing around and they come back around and they swirl a little bit, just like stuff going down the toilet bowl. It swirls a little bit until eventually it finally collapses into a single black hole. And in those last moments when they're swirling around each other really really fast, then the gravity is changing really really quickly, and so the gravitational field goes like up and down and up and down, up and down as the black hole swirls around. And that's what we call a gravitational wave.
Right.
It's kind of pulling and pulling and pushing really quickly and making waves in the fabric of space time.
That's right, because if you think about gravity not as like a gravitational field, but instead curving space, which is what general relativity tells us, then what happens is you're seeing these ripples in space time. And that's what we saw here on Earth using Lego, and we have a whole episode about that. But the point is that there's a signature there in this shaking of space. It starts and then it goes fast and faster and faster and faster, boom, until the black holes collide. And that looks a certain way, and you expect it to look a certain way if you see two black holes, and it looks different if you see like a black hole eating a neutron star. And so this is really pretty good evidence that those black holes are real. They're outt right.
So that's pretty recent. And then even more recently, we actually took a picture of a black hole.
Yeah, we did this direct image from the event Horizon telescope. It looked a black hole the center of a nearby galaxy M eighty seven, which has a really super duper black hole at its center. And they try to like focus in and they try to separate the part of the black hole that's the very center of the actual event horizon from the gas that's around.
It, and we got a picture, like there's an image you can look up online and see it which sort of proves all these theories.
Yeah, you have a picture. It looks sort of like, you know, a fuzzy donut or something. It's not too spectacular unless you like really understand the context of it, which is what you're seeing is the gas swirling around on the black hole. And then at the very center you see the shadow. You see nothing, right, there's nothing there.
I mean you see the hole. You see the hole and it's black.
Yeah, you see the hole and it's black. And you know what's different about that blackness, And just like the random blackness of a patch of space, it's really the stuff around it. It's this incredibly hot gas that's swirling around and it looks exactly the way you would expect. And it's impossible to get that configuration of high speed gas emitting X rays without a huge gravitational mass. So we know there's a gravitational mass right in the center of that picture where the black part is, but there's no light being emitted. So again it's a direct picture of what a black hole would look like. Is it actually a black hole or something else that doesn't emit light? You know, then you get into semantics about what is and is not a black hole. It's something very dense, very compact, very strong gravitationally that does not emit any visible light.
Well, I'm going to make a bed with you, Daniel, just like Stephen Hawking made a bed. I'll bid you that it is actually a giant fuzzy door.
Okay, And how you can approve that? You can take a trip out there and take a bite.
Well, we may never set a lot of baden.
When that image came out, I saw people online taking pictures of Krispy Kremes and saying, hey, look, my picture of Krispy Kreme looks just like this crazy discovery. How come I'm not getting any press by the Science News. But it's fun to actually look at that picture and think about, like what am I seeing? And you know, if you look at the very center of it, you're looking at the event horizon itself. Right, There's no photons come to your eyeballs from the very center because any photon that could would have had to come out of the black hole, and so that's impossible.
Right, So it is like a black hole, and it does and in subtle ways too. I heard that it really confirms a lot of our theories about what's happening around a black hole. Like one side of it is brighter than the other side of the doughnut because the light is going faster when one side and not the other. So it really does sort of confirm and look like what we predicted. Like in the movies Interstellar, they sort of simulated black holes and they made up pictures of it, and the real one sort of looks like that.
Yeah, and it's amazing. And remember that what you're seeing is not what's there. You're looking at an image. Just like when you look through distorted glass outside the trees look all wibbly and wobbly and whatever. That's not what's actually happening. That's the image you're seeing. So that's what's happening here. And the reason you're seeing an image and not just you know what's there is that space is being bent. Right the environment around a black hole. Space is curved, and so light doesn't travel in straight lines. And so they predicted this image, as you're saying, they predicted if you have a black hole with an accretion disk around it, what would you see? What would it look like? And they predicted all these distortions by retracing all those photons, and as you said, what we see is what they expected. And so that's pretty good confirmation that they understand the physics of what's happening there.
So do you feel like that picture was kind of the nail in the coffin that finally said, yes, now we can rest easy and know for sure that black holes are real? Or were people pretty convinced before we saw the picture.
People were pretty convinced that black holes were real before we saw the picture. The picture is like an even more stringent test in a new, fascinating and frankly visually appealing way of the black hole theory. So I think, you know, since the mid nineteen seventies, black holes have been generally accepted as real, but they just get cooler and cooler as we learn more and more about them.
Yeah, I guess it's you know, it's really amazing to think, not just the kind of the long path that we've taken here, like seeing it in the equations, coming up with solutions, finding circumstantial evidence, but just to think that, you know, these crazy ideas are real. You know that space really does kind of form these pockets where nothing can come out, and that they can exist, and that you can actually kind of go out there and touching them, be around them.
Yeah, and it makes you wonder about the sort of primacy of math thematics, because you know, all these ideas came from just following the mathematics. We were expressed our physical laws in terms of math, and we followed the consequences and we got this weird result and then it turns out to be real. It makes you wonder like, is math just something in our heads or is it like fundamental to the universe itself, because it seems like the universe is following these mathematical rules regardless of their absurdity.
Are you thinking math is better than physics?
Now?
I know that all of my colleagues in the math department find it to be more fundamental than physics.
But we all know love is the true fundamental power in the universe.
According to that according to the original boy band the.
Utles, I was just about to say the same thing.
And remember a lot of the listeners suggested that we could see black holes by seeing their lensing, Like if a black hole passed in front of another star, you would see it distorting. And that's true theoretically. We just haven't observed that yet, and so that's a possibility. It's something we might get to see in the future, and that would be a very nice additional piece of evidence. But there haven't been any micro lensing events observing.
Okay, but we've seen it in other ways for sure.
Yeah, we've seen it in lots of ways. Yeah, But that's just something to look forward to. That's something you the listener out there, might be the first person to ever.
Accomplish, right, So keep looking up at the stars and don't look away. All right. Well, we hope you enjoyed that. Thanks for joining us.
Thanks for tuning in, and thanks for sending us your questions and sharing your curiosity with us.
Even if we do sometimes go down a black hole. See you 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 Apple 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. How is 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.
We're just days away from our twenty twenty four iHeartRadio Music Festival, presented by Capital On.
The biggest headliners in live music will be taking over to Mobile Arena, Las.
Vegas lost some special surprises and moments you are not going to want to miss.
Stream only on Hulu.
The iHeartRadio Music Festival and listen on iHeartRadio the most anticipated live music events up the.
Year This Friday and Saturday, starting at ten thirty pm Eastern, seven thirty Pacific.
Guess what Will?
What's that Mango?
I've been trying to write a promo for our podcast, Part Time Genius, But even though We've done over two hundred and fifty episodes.
We don't really talk about murders or cults. I mean, we did just cover the Illuminati of cheese, so I feel like that makes us pretty edgy. We also solve mysteries like how Chinese is your Chinese food?
And how do.
Dollar stores make money? And then of course can you game a dog show? So what you're saying is everyone should be listening. Listen to part Time Genius on the iHeartRadio app or wherever you get your podcasts
