August 20, 2020 • 46 mins
What convinced people that black holes are real?
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Speaker 1 (00:08):
Hey, Jorge, do you still have that pet black hole
in your backyard? Well, you know, my physics lawyer tells
me as you'd neither confirm nor deny that that's good advice.
So in that case, I have a question for your
physics lawyer. He charges a lot better be good. All right,
is my question. How do you actually know that it's
(00:30):
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. Either way, I definitely need more bananas.
(00:56):
I am or handmade cartoonist and the creator of PhD comic. 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 explained the university production of I Heart Radio,
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
(01:17):
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,
(01:39):
I think that how we discover things sometimes it's 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
(02:01):
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 behind the scenes
(02:23):
or bloopers? You know, sometimes people accidentally leave little comments
in their paper. I mean they're 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. Stuff like that. Yeah, no kidding.
(02:47):
It's the equivalent of like leaving little comments in your
Microsoft word document. We usually write our stuff in l 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 the paper
was Does anyone ever say we totally made this up?
But don't tell anybody no? And it's not usually a
(03:08):
dramatic It's not like you know, the Last Dance that
documentary about Michael Jordan's We don't have you know, that
much 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
(03:28):
with you. That's right, Hey, the stakes are high. You know,
we are trying to figure out the nature of the
universe itself. Here, man, it's not just basketball. You can't
get catty 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 been not
(03:50):
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
(04:12):
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 imagined. Yeah, and
(04:33):
this thing is extra strange and extra weird. It's one
of the maybe a true like pockets of mystery in
the universe that we may never even discover what's inside
of him. So to be on the podcast, we'll be
asking the question how do we discover black holes? So, Daniel,
(04:54):
I assume we didn't just fall into one by accident.
We're inside one right now. Welcome to the podcast Why
inside the black Hole that notity will 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 possibility? Yeah,
that's a possibility. It's theoretically possible that our entire universe
(05:15):
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 them, so we don't know what's inside them. And
anybody who does survive the journey into a black hole
(05:36):
can't then shout to us about what they find. So,
as you said, they may be eternal pockets of missing.
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
(05:58):
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 do people become
convinced that they exist even before we had ever seen
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
(06:20):
crazy bonkers theoretical idea to crazy bonkers real actual facts.
Let's think about it for a second. How do you
think we discovered black holes? Here's what people had to say.
There was 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
(06:40):
that it must be a binary pair with a black hole.
I would say that this is true. Somebody observing the
space and and has seen some sort of gravitational lensing
happening that black hole has gone in between a observed
star and a observer. I believe that they were initially
(07:01):
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 was holding it together. They
were officially discovered by the gravitational lensing, I believe. I
think black holes were predicted by Einstein um, but how
(07:26):
they were discovered was by seeing lensing in stars. An
idea in which I rather believe is that when two
black holes, Clyde or black hole is like born, so
when a star collapse isn't a singularity that these occurrences
leads like this leads to an emission of race, maybe
(07:49):
like X rays or gramma rays, I'm not sure. And
these race were like measured on Earth. I'm not sure,
but I would guess some gravitational lens effect from what
I can remember. Instantly, we're the one that figured out
the black holes were a thing. Was just equations and stuff,
and then it took years and years and years until
until I finally found actual evidence of it in real life.
(08:11):
The only way I contain the black holes were discovered
were maybe because of their gravitational effect on the surroundings,
and 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
(08:31):
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
(08:53):
to see it, but that's definitely not how black holes
were discovered, so not through gravitational lensing. But maybe there's
some other gravitation 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
(09:13):
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. I
guess it's hard. How do you see something that's dark
in space? Especially, it's the perfect camouflage, well done, black holes.
(09:37):
It's hiding what but what is it hiding from? It's
hiding from us. I guess, alright, well, step us through here, Daniel,
Because black holes are sort of interesting in that they
were thought of theoretically first, or there was sort of
discovered theoretically first, probably a long time, almost hundred years
before we actually ever saw one. Yeah, and you might
be wondering, like, what does it mean to discover something theoretically? Right?
(10:00):
After all, for things to exist, they have to be
in our universe, and so experimentally in things that can
really discover something right, Well, you can actually make discoveries theoretically.
You can say, here 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
(10:21):
look in the 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.
We 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
(10:45):
for what gravity could do. Yeah, I guess you can
theoretically discover things theoretically. It's 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,
(11:06):
dark matter is something that was seeing experimental. 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
(11:28):
relative and then he dropped the mic. He's like, I'm out.
He sort of did. And the thing to 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
(11:51):
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 would actually happened in various circumstances,
and for a long time, the only thing people could
ever figure out in terms of the Einstein field equations
(12:12):
were super simplified universe like the universe field with homogeneous
dust or totally empty universe like. Nobody's ever solved the
Einstein field equations for our actual universe. I guess you
can sort of explore with the equations, right, like if
you find that the things around you obey as certain
law or equations like f equals and make and then
kind of tweaked the numbers and the parameters to kind
(12:34):
of explore more extreme conditions than what you have around you. Right,
you could add like what happens if the mask coast
to zero, or what happens if the force coast 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 Stild, and short Stile looked at these and he
(12:56):
played around with him, 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 guess to
step us through a little bit, what did what did
(13:17):
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
(13:39):
way through that universe. And so that's a solution meaning
how 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
in 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 lying off
(14:00):
in what otherwise looks like a straight line. So you
start with the mass. You said, I'm gonna 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
(14:20):
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 space sort of becomes infinite, right, like
space is curved like it is around the Sun. But
if the mass gets large enough, then you have this threshold,
(14:42):
this point which we now call the short styles radius,
where the curvature of space gets a singularity, or 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 salut, but it has some
weirdness at a certain distance from this audition, I see.
(15:03):
He just sort of like turned the knob on the
mass and then he found that the equations suddenly kind
of got wonk. 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 over 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
(15:27):
they really had an understanding for what that meant. Initially,
they only looked at it as well, the curvature of
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,
(15:47):
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.
(16:09):
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 wonkey, And didn't they think that maybe
the equation was wrong, like like, this is a weird
(16:32):
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
(16:52):
achieve sort of on the page, but would never actually
happen in reality. Then 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
(17:14):
some second thoughts later, and then later there were big
discoveries about him. So let's get into those. But first
let's take a quick break, all right, Daniel, So what
(17:34):
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 trapping no matter what advice I
give you, I mean, the 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. Alright,
So we're talking about the discovery of black holes, and
(17:54):
so they were sort of this weird mathematical kind of
oddity in the equations, and maybe the equations were 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
(18:15):
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
(18:37):
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 short stiles theory.
And he went back and he looked at these frozen
stars and the short siled radius and the singularity in
the gravitation equations and he said, you know what, that's
what's happening here. He like theoretically reinterpreted this and said
(18:58):
this is what an event horizon is. He came at
it from 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
(19:19):
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 pulsings. These are like
special stars, right, yeah, these are special stars. There are
neutron stars. These are stars that are at the end
of their life, and this is the leftover chord, and
(19:39):
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, collapse into a neutron star,
but nobody really believed that existed until they were discovered
in nineteen sixty seven a special version and of them pulsars,
(20:01):
which rotate and have a very strong beam coming at
the top of them. We're discovering 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. Pulsars are not sort
of related to black holes, but they are sort of
extreme and crazy and maybe also kind of like a
(20:24):
weird equation oddity, and so when they find it, they're like, hey,
maybe need 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 Nobel price, I want
one to Let's dig into this and it's a fascinating
(20:46):
store because there's a lot of different threads. At the
same time. You have this theory thread where people are
finally starting to understand 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
Sands Missile based in New Mexico, a couple of folks
(21:07):
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 can. I 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
(21:27):
at the universe using our eyeballs, and we build telescopes
that are more powerful to look invisible 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
(21:49):
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 picture. It takes
selfie and drop takes the university sort of scans the
(22:13):
sky for like eight degrees and then it comes back.
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 spiked, and they
could tell where it 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
(22:35):
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 it's it's not
(22:56):
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
(23:17):
something else really fascinating about this source was that it
was variable, Like it wasn't just emitting X rays. It
would admit a bunch of X rays and then not
very much, and then a bunch of BAST rays and
then not very much. It was highly variable, and it
was variable on a really short time scale. 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?
(23:39):
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
(24:01):
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
(24:21):
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, totally. I'm a
(24:43):
big fan of black Hole Street Boys. 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 p the Sun,
So then that was really interesting because now you know
you have something They're very bright in the X ray
(25:06):
and very very small, I see. And it couldn't 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
(25:26):
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 that one was bright
and that when you can see in the visible and
(25:47):
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 because stars 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.
(26:08):
And based on how fast the star and this new
mystery object we're orbiting each other, you could figure out
the mass of that mystery Interesting 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 pretty close to each other. Yeah,
(26:32):
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
(26:52):
it's not radiating, so therefore it must be a black hole.
But the thing itself had to be a black hole,
because what nutrons rs 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 black holes were real,
so couldn't they just have assumed that it was a
(27:14):
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,
(27:37):
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 predicted 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
(27:59):
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 there would swirl around
the black hole, and as it was swirling around, the
black hole gets rubbed against each other, a lot of
friction there, and that's when the gas is then emitting
(28:21):
in these millions of degree situations, is emitting these X rays.
But did they know that back then? Did they know
that about the equation discs, that that was all part
of their model of black holes, 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? Even to
this day, Like our evidence is limited to basically that
(28:41):
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 holes
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,
(29:03):
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,
but you can still maybe find a person guilty, 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,
(29:23):
I think most people were convinced in the astrophysics community
that this was a black hole. But there was one holdout,
very notable holdout. Who was it well, Stephen Hawking. Stephen
Hawking in the 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 hoology I suppose forward, But he
(29:47):
wasn't sure that it was a black hole, and he
made sort of a famous bet with Kip Thorne. Hawking
bet Kip Thorne 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, lay he when
he finally conceded it, he said that he was just
hedging his best. But this way either it was a
black hole, which is awesome for him, or he wont
(30:08):
to bet against Kip Thorne, which was also awesome for him,
And so this way he got something. 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 Nobel price. Yeah.
And so that's the early seventies, and we have this
evidence for a source, you know, that's very intense mass
(30:30):
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
(30:51):
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
riduculously bright at its source. But people eventually believed these
(31:14):
really are super bright sources, and then finally came to
understand them as supermassive black holes at the centers of galaxies.
And so this thread of quasars was sort of helped
along by the discovery of black holes as a real thing.
People like, oh, well, black holes are real, Maybe we
can use that to explain quasars. Also, because we seem quasars,
(31:36):
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
(31:57):
four million times the mass of the Sun. It's called
Sagittarius 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 of putting an asterix would make people somehow
suspicious of it. I know in the sports world an
asterisk means like, well maybe not right, but in chemistry,
(32:20):
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 phase, emoji, star emoji, telescope star night.
So you have all these threads coming together, this theoretical
understanding of it as an event horizon beyond which no
(32:42):
information can pass, and then the discovery 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 hawkingrong. I mean, that's that's some
motivation right there. Alright, but all of this is still
sort of circumstantial evidence, And so let's get into how
(33:05):
we actually see them today. But first let's take another
quick break, all right, Daniel, So we're we're I guess
we're in the seventies and the eighties, and we have
(33:26):
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. Um, so what sort of sealed
the deal for black holes? Like, was it us seeing
them or tend taking a picture last year for the
first time, or did were we pretty convinced before? Well,
(33:46):
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
(34:07):
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 what are some of these ways?
Can 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 a hundred thousand galactic quaisars.
And so each of these, yeah, each of these are
(34:29):
probably a supermassive 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
accretion disk, or you can just see something really bright,
but you can see something really bright coming from the
center of the galaxy. And we're talking in a minute
(34:50):
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 linds the black holes. There's these
supermassive black holes at 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
(35:11):
and turns into a black hole. Those are like the
pet 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
to spot. So while we've seen like a hundred thousand
galactic quasars, we only have a few dozen stellar mass
black holes that we've actually observed. But don't we see
(35:31):
stars training 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
a neutron stars or something else. And so it has
to be particular sized 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
(35:55):
this huge cloud of stuff still for a while, and
so it's gonna take a long time for accretion disk
to sort of gathered together and make the black hole visible.
I hadn't realized that we have seen many more super
massive black holes and 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?
(36:17):
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, then you can make 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
(36:39):
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 take a picture basically a black screen,
(37:01):
but you see stars kind of moving and going really
fast around nothingness. Yeah, there's a group at U c
l A 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.
(37:24):
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 star
has passed so close to the center, you can get
a sense for how small it has to be right there.
The stars trajectory limits the size of this thing, and
so at some point you're like, well, there's a huge
amount of gravity and a small amount of space boom
(37:44):
black 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 inner
(38:05):
back yard. Yeah. Yeah, it's a 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. And then recently
we have a total 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,
(38:27):
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
(38:48):
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. So the gravitational field goes
up and down and up and down, up and down
as the black hole swirls around. And that's what we
call a gravitational way. Right. It's kind of pulling and
pulling and pushing really quickly and making waves in the
(39:09):
fabric of spacetime. 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.
(39:29):
It starts, then it goes faster 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, that they're
out right, and so that's pretty recent. And then even
more recently, we actually took a picture of a black hole. Yeah,
(39:52):
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
(40:13):
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 the black hole. And then at the very
center you see the shadow. You see nothing, right, there's
(40:36):
nothing there. I mean, you see the hole, you see
the whole, and it's black. Yeah, you see the whole
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
(40:56):
of high speed gas emitting X rays without huge gravitational mass.
So we know there's a gravitational mass right in the
center of that picture where the black part is right,
but there's no light being admitted. 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 admit light? You know, then you get into semantics
(41:18):
about what is and is not a black hole. It's
something very dense, very compact, very strong gravitationally that does
not admit any visible light. Well, I'm gonna make a
bed with you, Daniel, just like Stephen Hawkins made a bit.
We'll bid you that it is actually a giant, fuzzy doughnut. Okay,
And how are you going to prove that you're gonna
take a trip out there and take a bite. Well,
(41:39):
we may never set a lot of When that image
came out, I saw people online taking pictures of Crispy
Kreams 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 to think about,
like what am I seeing? And you know, if you
look at the very sent tour of it, you're looking
(42:01):
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 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,
(42:22):
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 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 it's
the real one sort of looks like that. Yeah, and
it's amazing. And remember that what you're seeing is not
(42:45):
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 enough, just you
know what's there is that space is being bent, right,
the environment around a black hole. Space is curved and
(43:05):
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 acretion disc 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
(43:25):
physics of what's happening there. So do you feel like
the picture was kind of the nail in the coffin
that people 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 the 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
(43:48):
way of the black hole theory. So I think, you know,
since the mid nineteventies, black holes have been generally accepted
as real, but they just get cooler and cooler as
we learn more and more about them. Kind of 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,
(44:09):
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
(44:30):
of mathematics, because you know, all these ideas came from
just following the mathematics. We 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 absurtainty. Are you
(44:54):
thinking math is better than physics, Doyle, I know that
all of my colleagues in the math department find to
be more fundamental than physics. But we all know love
is the true fundamental power in the universe, according to
all according to the original boy band the Beatles, I
was just about to say the same thing. And remember
(45:16):
a lot of the listeners suggested that we could see
black holes by seeing their lensing, Like if a black
hole past 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 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 observed yet. Okay, but we've
(45:38):
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 come 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
(45:58):
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 I
Heart Radio. For more podcast from my Heart Radio, visit
(46:20):
the I Heart Radio Apple Apple Podcasts, or wherever you
listen to your favorite shows.
Hey, Jorge, do you still have that pet black hole
in your backyard? Well, you know, my physics lawyer tells
me as you'd neither confirm nor deny that that's good advice.
So in that case, I have a question for your
physics lawyer. He charges a lot better be good. All right,
is my question. How do you actually know that it's
(00:30):
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. Either way, I definitely need more bananas.
(00:56):
I am or handmade cartoonist and the creator of PhD comic. 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 explained the university production of I Heart Radio,
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
(01:17):
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,
(01:39):
I think that how we discover things sometimes it's 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
(02:01):
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 behind the scenes
(02:23):
or bloopers? You know, sometimes people accidentally leave little comments
in their paper. I mean they're 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. Stuff like that. Yeah, no kidding.
(02:47):
It's the equivalent of like leaving little comments in your
Microsoft word document. We usually write our stuff in l 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 the paper
was Does anyone ever say we totally made this up?
But don't tell anybody no? And it's not usually a
(03:08):
dramatic It's not like you know, the Last Dance that
documentary about Michael Jordan's We don't have you know, that
much 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
(03:28):
with you. That's right, Hey, the stakes are high. You know,
we are trying to figure out the nature of the
universe itself. Here, man, it's not just basketball. You can't
get catty 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 been not
(03:50):
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
(04:12):
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 imagined. Yeah, and
(04:33):
this thing is extra strange and extra weird. It's one
of the maybe a true like pockets of mystery in
the universe that we may never even discover what's inside
of him. So to be on the podcast, we'll be
asking the question how do we discover black holes? So, Daniel,
(04:54):
I assume we didn't just fall into one by accident.
We're inside one right now. Welcome to the podcast Why
inside the black Hole that notity will 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 possibility? Yeah,
that's a possibility. It's theoretically possible that our entire universe
(05:15):
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 them, so we don't know what's inside them. And
anybody who does survive the journey into a black hole
(05:36):
can't then shout to us about what they find. So,
as you said, they may be eternal pockets of missing.
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
(05:58):
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 do people become
convinced that they exist even before we had ever seen
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
(06:20):
crazy bonkers theoretical idea to crazy bonkers real actual facts.
Let's think about it for a second. How do you
think we discovered black holes? Here's what people had to say.
There was 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
(06:40):
that it must be a binary pair with a black hole.
I would say that this is true. Somebody observing the
space and and has seen some sort of gravitational lensing
happening that black hole has gone in between a observed
star and a observer. I believe that they were initially
(07:01):
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 was holding it together. They
were officially discovered by the gravitational lensing, I believe. I
think black holes were predicted by Einstein um, but how
(07:26):
they were discovered was by seeing lensing in stars. An
idea in which I rather believe is that when two
black holes, Clyde or black hole is like born, so
when a star collapse isn't a singularity that these occurrences
leads like this leads to an emission of race, maybe
(07:49):
like X rays or gramma rays, I'm not sure. And
these race were like measured on Earth. I'm not sure,
but I would guess some gravitational lens effect from what
I can remember. Instantly, we're the one that figured out
the black holes were a thing. Was just equations and stuff,
and then it took years and years and years until
until I finally found actual evidence of it in real life.
(08:11):
The only way I contain the black holes were discovered
were maybe because of their gravitational effect on the surroundings,
and 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
(08:31):
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
(08:53):
to see it, but that's definitely not how black holes
were discovered, so not through gravitational lensing. But maybe there's
some other gravitation 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
(09:13):
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. I
guess it's hard. How do you see something that's dark
in space? Especially, it's the perfect camouflage, well done, black holes.
(09:37):
It's hiding what but what is it hiding from? It's
hiding from us. I guess, alright, well, step us through here, Daniel,
Because black holes are sort of interesting in that they
were thought of theoretically first, or there was sort of
discovered theoretically first, probably a long time, almost hundred years
before we actually ever saw one. Yeah, and you might
be wondering, like, what does it mean to discover something theoretically? Right?
(10:00):
After all, for things to exist, they have to be
in our universe, and so experimentally in things that can
really discover something right, Well, you can actually make discoveries theoretically.
You can say, here 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
(10:21):
look in the 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.
We 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
(10:45):
for what gravity could do. Yeah, I guess you can
theoretically discover things theoretically. It's 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,
(11:06):
dark matter is something that was seeing experimental. 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
(11:28):
relative and then he dropped the mic. He's like, I'm out.
He sort of did. And the thing to 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
(11:51):
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 would actually happened in various circumstances,
and for a long time, the only thing people could
ever figure out in terms of the Einstein field equations
(12:12):
were super simplified universe like the universe field with homogeneous
dust or totally empty universe like. Nobody's ever solved the
Einstein field equations for our actual universe. I guess you
can sort of explore with the equations, right, like if
you find that the things around you obey as certain
law or equations like f equals and make and then
kind of tweaked the numbers and the parameters to kind
(12:34):
of explore more extreme conditions than what you have around you. Right,
you could add like what happens if the mask coast
to zero, or what happens if the force coast 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 Stild, and short Stile looked at these and he
(12:56):
played around with him, 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 guess to
step us through a little bit, what did what did
(13:17):
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
(13:39):
way through that universe. And so that's a solution meaning
how 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
in 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 lying off
(14:00):
in what otherwise looks like a straight line. So you
start with the mass. You said, I'm gonna 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
(14:20):
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 space sort of becomes infinite, right, like
space is curved like it is around the Sun. But
if the mass gets large enough, then you have this threshold,
(14:42):
this point which we now call the short styles radius,
where the curvature of space gets a singularity, or 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 salut, but it has some
weirdness at a certain distance from this audition, I see.
(15:03):
He just sort of like turned the knob on the
mass and then he found that the equations suddenly kind
of got wonk. 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 over 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
(15:27):
they really had an understanding for what that meant. Initially,
they only looked at it as well, the curvature of
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,
(15:47):
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.
(16:09):
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 wonkey, And didn't they think that maybe
the equation was wrong, like like, this is a weird
(16:32):
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
(16:52):
achieve sort of on the page, but would never actually
happen in reality. Then 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
(17:14):
some second thoughts later, and then later there were big
discoveries about him. So let's get into those. But first
let's take a quick break, all right, Daniel, So what
(17:34):
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 trapping no matter what advice I
give you, I mean, the 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. Alright,
So we're talking about the discovery of black holes, and
(17:54):
so they were sort of this weird mathematical kind of
oddity in the equations, and maybe the equations were 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
(18:15):
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
(18:37):
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 short stiles theory.
And he went back and he looked at these frozen
stars and the short siled radius and the singularity in
the gravitation equations and he said, you know what, that's
what's happening here. He like theoretically reinterpreted this and said
(18:58):
this is what an event horizon is. He came at
it from 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
(19:19):
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 pulsings. These are like
special stars, right, yeah, these are special stars. There are
neutron stars. These are stars that are at the end
of their life, and this is the leftover chord, and
(19:39):
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, collapse into a neutron star,
but nobody really believed that existed until they were discovered
in nineteen sixty seven a special version and of them pulsars,
(20:01):
which rotate and have a very strong beam coming at
the top of them. We're discovering 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. Pulsars are not sort
of related to black holes, but they are sort of
extreme and crazy and maybe also kind of like a
(20:24):
weird equation oddity, and so when they find it, they're like, hey,
maybe need 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 Nobel price, I want
one to Let's dig into this and it's a fascinating
(20:46):
store because there's a lot of different threads. At the
same time. You have this theory thread where people are
finally starting to understand 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
Sands Missile based in New Mexico, a couple of folks
(21:07):
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 can. I 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
(21:27):
at the universe using our eyeballs, and we build telescopes
that are more powerful to look invisible 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
(21:49):
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 picture. It takes
selfie and drop takes the university sort of scans the
(22:13):
sky for like eight degrees and then it comes back.
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 spiked, and they
could tell where it 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
(22:35):
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 it's it's not
(22:56):
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
(23:17):
something else really fascinating about this source was that it
was variable, Like it wasn't just emitting X rays. It
would admit a bunch of X rays and then not
very much, and then a bunch of BAST rays and
then not very much. It was highly variable, and it
was variable on a really short time scale. 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?
(23:39):
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
(24:01):
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
(24:21):
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, totally. I'm a
(24:43):
big fan of black Hole Street Boys. 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 p the Sun,
So then that was really interesting because now you know
you have something They're very bright in the X ray
(25:06):
and very very small, I see. And it couldn't 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
(25:26):
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 that one was bright
and that when you can see in the visible and
(25:47):
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 because stars 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.
(26:08):
And based on how fast the star and this new
mystery object we're orbiting each other, you could figure out
the mass of that mystery Interesting 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 pretty close to each other. Yeah,
(26:32):
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
(26:52):
it's not radiating, so therefore it must be a black hole.
But the thing itself had to be a black hole,
because what nutrons rs 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 black holes were real,
so couldn't they just have assumed that it was a
(27:14):
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,
(27:37):
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 predicted 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
(27:59):
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 there would swirl around
the black hole, and as it was swirling around, the
black hole gets rubbed against each other, a lot of
friction there, and that's when the gas is then emitting
(28:21):
in these millions of degree situations, is emitting these X rays.
But did they know that back then? Did they know
that about the equation discs, that that was all part
of their model of black holes, 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? Even to
this day, Like our evidence is limited to basically that
(28:41):
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 holes
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,
(29:03):
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,
but you can still maybe find a person guilty, 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,
(29:23):
I think most people were convinced in the astrophysics community
that this was a black hole. But there was one holdout,
very notable holdout. Who was it well, Stephen Hawking. Stephen
Hawking in the 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 hoology I suppose forward, But he
(29:47):
wasn't sure that it was a black hole, and he
made sort of a famous bet with Kip Thorne. Hawking
bet Kip Thorne 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, lay he when
he finally conceded it, he said that he was just
hedging his best. But this way either it was a
black hole, which is awesome for him, or he wont
(30:08):
to bet against Kip Thorne, which was also awesome for him,
And so this way he got something. 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 Nobel price. Yeah.
And so that's the early seventies, and we have this
evidence for a source, you know, that's very intense mass
(30:30):
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
(30:51):
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
riduculously bright at its source. But people eventually believed these
(31:14):
really are super bright sources, and then finally came to
understand them as supermassive black holes at the centers of galaxies.
And so this thread of quasars was sort of helped
along by the discovery of black holes as a real thing.
People like, oh, well, black holes are real, Maybe we
can use that to explain quasars. Also, because we seem quasars,
(31:36):
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
(31:57):
four million times the mass of the Sun. It's called
Sagittarius 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 of putting an asterix would make people somehow
suspicious of it. I know in the sports world an
asterisk means like, well maybe not right, but in chemistry,
(32:20):
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 phase, emoji, star emoji, telescope star night.
So you have all these threads coming together, this theoretical
understanding of it as an event horizon beyond which no
(32:42):
information can pass, and then the discovery 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 hawkingrong. I mean, that's that's some
motivation right there. Alright, but all of this is still
sort of circumstantial evidence, And so let's get into how
(33:05):
we actually see them today. But first let's take another
quick break, all right, Daniel, So we're we're I guess
we're in the seventies and the eighties, and we have
(33:26):
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. Um, so what sort of sealed
the deal for black holes? Like, was it us seeing
them or tend taking a picture last year for the
first time, or did were we pretty convinced before? Well,
(33:46):
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
(34:07):
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 what are some of these ways?
Can 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 a hundred thousand galactic quaisars.
And so each of these, yeah, each of these are
(34:29):
probably a supermassive 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
accretion disk, or you can just see something really bright,
but you can see something really bright coming from the
center of the galaxy. And we're talking in a minute
(34:50):
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 linds the black holes. There's these
supermassive black holes at 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
(35:11):
and turns into a black hole. Those are like the
pet 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
to spot. So while we've seen like a hundred thousand
galactic quasars, we only have a few dozen stellar mass
black holes that we've actually observed. But don't we see
(35:31):
stars training 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
a neutron stars or something else. And so it has
to be particular sized 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
(35:55):
this huge cloud of stuff still for a while, and
so it's gonna take a long time for accretion disk
to sort of gathered together and make the black hole visible.
I hadn't realized that we have seen many more super
massive black holes and 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?
(36:17):
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, then you can make 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
(36:39):
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 take a picture basically a black screen,
(37:01):
but you see stars kind of moving and going really
fast around nothingness. Yeah, there's a group at U c
l A 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.
(37:24):
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 star
has passed so close to the center, you can get
a sense for how small it has to be right there.
The stars trajectory limits the size of this thing, and
so at some point you're like, well, there's a huge
amount of gravity and a small amount of space boom
(37:44):
black 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 inner
(38:05):
back yard. Yeah. Yeah, it's a 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. And then recently
we have a total 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,
(38:27):
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
(38:48):
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. So the gravitational field goes
up and down and up and down, up and down
as the black hole swirls around. And that's what we
call a gravitational way. Right. It's kind of pulling and
pulling and pushing really quickly and making waves in the
(39:09):
fabric of spacetime. 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.
(39:29):
It starts, then it goes faster 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, that they're
out right, and so that's pretty recent. And then even
more recently, we actually took a picture of a black hole. Yeah,
(39:52):
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
(40:13):
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 the black hole. And then at the very
center you see the shadow. You see nothing, right, there's
(40:36):
nothing there. I mean, you see the hole, you see
the whole, and it's black. Yeah, you see the whole
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
(40:56):
of high speed gas emitting X rays without huge gravitational mass.
So we know there's a gravitational mass right in the
center of that picture where the black part is right,
but there's no light being admitted. 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 admit light? You know, then you get into semantics
(41:18):
about what is and is not a black hole. It's
something very dense, very compact, very strong gravitationally that does
not admit any visible light. Well, I'm gonna make a
bed with you, Daniel, just like Stephen Hawkins made a bit.
We'll bid you that it is actually a giant, fuzzy doughnut. Okay,
And how are you going to prove that you're gonna
take a trip out there and take a bite. Well,
(41:39):
we may never set a lot of When that image
came out, I saw people online taking pictures of Crispy
Kreams 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 to think about,
like what am I seeing? And you know, if you
look at the very sent tour of it, you're looking
(42:01):
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 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,
(42:22):
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 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 it's
the real one sort of looks like that. Yeah, and
it's amazing. And remember that what you're seeing is not
(42:45):
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 enough, just you
know what's there is that space is being bent, right,
the environment around a black hole. Space is curved and
(43:05):
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 acretion disc 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
(43:25):
physics of what's happening there. So do you feel like
the picture was kind of the nail in the coffin
that people 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 the 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
(43:48):
way of the black hole theory. So I think, you know,
since the mid nineteventies, black holes have been generally accepted
as real, but they just get cooler and cooler as
we learn more and more about them. Kind of 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,
(44:09):
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
(44:30):
of mathematics, because you know, all these ideas came from
just following the mathematics. We 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 absurtainty. Are you
(44:54):
thinking math is better than physics, Doyle, I know that
all of my colleagues in the math department find to
be more fundamental than physics. But we all know love
is the true fundamental power in the universe, according to
all according to the original boy band the Beatles, I
was just about to say the same thing. And remember
(45:16):
a lot of the listeners suggested that we could see
black holes by seeing their lensing, Like if a black
hole past 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 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 observed yet. Okay, but we've
(45:38):
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 come 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
(45:58):
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 I
Heart Radio. For more podcast from my Heart Radio, visit
(46:20):
the I Heart Radio Apple Apple Podcasts, or wherever you
listen to your favorite shows.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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