Birth and Rebirth From Stellar Dust to Supermassive Blackholes
Transcriber: Judy YS
Reviewer: Amanda Zhu
OK, thank you very much, everyone,
for being here with us.
And thank you very much for the MCs
for such a wonderful introduction.
So our journey today starts here, OK?
This is the Nobel Prize of 2020
that was awarded
for the discovery and for understanding
of the black holes.
So what are black holes?
So half of the Nobel Prize was given
to the understanding of black holes.
And gravity,
you can understand that
as curvature of spacetime.
Here, you can see the Earth,
curving spacetime around it.
And we have another mass -
let’s say the moon -
it naturally follows the curvature
that the Earth produces.
And that’s what we call gravity.
But now imagine we take a lot of mass,
and we condense it into a single point.
What happens then?
It pretty much punctures spacetime.
And that’s what we understand
to be a black hole.
Now, the other half of the equation,
the other half of the Nobel Prize
was given for the actual discovery,
for the black hole
at the center of our galaxy.
So here we can see actually
how this discovery was done.
Here you can see, over time,
we’re tracing stars
right at the center of our Milky Way.
And we can see all the objects
moving around something,
something very massive.
But there’s no light
coming from that rare area.
And if we think about it,
we calculate how much mass
should have been there,
then we arrive at this astonishing number
of million times heavier than our own sun.
Yet it doesn’t emit light.
And that’s our first evidence
for the existence of a black hole.
Now, recently, you may have seen
this picture in the news.
This was the first picture
taken of a black hole.
Now, it may sound strange, right?
This is a black hole.
How can you take a picture
of a black hole.
Well, we didn’t, in fact, actually
take a picture of the black hole,
but instead, we actually took a picture
of the shadow of a black hole.
So here, you can see an animation
which shows this concept.
You can see the light rays going around
and starting to bend
from behind a black hole.
And all the light that then, you know,
is bended towards our way,
we can see it,
but of course, at the actual black hole,
it’s black.
We can’t see anything.
And so that’s how we end up
arriving with an image like this,
where you can see the ring
and something black in the middle.
So black holes are something that we have
fair confidence in its existence.
And, you know, these are mystical things
that kind of captured our imagination
ever since Einstein theoretically
came up with the concept
that a black hole must exist.
But how are they formed?
How can we form a black hole
that is more than a million times
heavier than our own sun?
And what I want to do today
is actually guide you through the process
and show you some
of our current understanding
in terms of simulations, right,
just our understanding of what happens.
But I’m going to do
something extra as well.
I’m going to also give you an audio tour
of how this happens,
and I’ll explain how this works.
OK, now let’s start at the very beginning,
where we have dense dust
everywhere in the universe.
And, you know,
through the process of gravity,
it attracts more and it attracts more,
and you get a bigger
and bigger cluster of dust.
And at some point,
this creates so much pressure
and adds so much more mass to it
that the star turns on
and starts emitting light.
And that’s how we create a star.
And the star then continues
to burn up its fuel, right,
its hydrogen, its helium and so on,
until at some point
it burns up all its fuel,
it can no longer sustain itself,
and it goes what we call supernova.
It explodes.
It sheds away its shell.
And then leaves behind something
that is only a remnant,
only a tiny fraction of its mass.
Now, it can either, depending on the size,
leave behind what we know
to be a neutron star,
which is just a big object,
about 10 kilometers,
but which has the mass
of an entire sun condensed into it,
which is almost like a single atom.
Or if the star’s even heavier,
it could even collapse
into what we know a black hole.
And the result of this process
gives us a black hole
that’s about several times
the mass of the sun.
But again, you know, we’re still not there
at the millions of times of the sun.
Now, this process of a supernova
has actually been observed,
and this is actually an animation
where you can kind of zoom in,
you can pan in,
on one of the brightest events
that happened in our night sky,
which was a supernova in 1987.
And what we can see nowadays,
if we point a telescope to it,
is it’s remnant.
We call it a nebula, right?
So its mass that it’s shed
in this big explosion,
is what we can actually observe now.
And this actually makes
for the beautiful imagery.
But still, we want to go
to the millions of solar masses.
How do we get there?
Now, to understand this process better,
we actually need to turn
to another Nobel Prize,
which was given in 2017.
And these three gentlemen got it
for the discovery of gravitational waves.
And let me explain to you
what gravitational waves are.
So gravitational waves
are distortions in spacetime
that travel at the speed of light.
And a distortion of spacetime
is actually just the changing of length.
It’s a compression of spacetime.
And you can see a vastly
exaggerated image here,
where you can see the Earth
being completely distorted
by a gravitational wave.
Now, these actual distortions
are very very small,
and they actually need to be picked up
by kilometer-size instruments
that we’ve built.
And this is an example
in the US called LIGO,
where there’s a three-kilometer,
sorry, four-kilometer machine
that we need to pick up
these tiny ripples in spacetime.
And the way this happens
is that there’s actually
a laser being shot -
There’s a laser being shot at a mirror,
and it bounces back,
and then the whole system,
you know, is locked in.
But when there’s a gravitational
wave compressing space,
you can see a flickering of this laser,
of the laser output.
And that gives us the image
of a black hole.
But better yet,
I should actually call it sounds.
These ripples actually caused
the disturbance in the laser,
which we can turn into sound.
And that’s something
that I wanted to do today with you.
So I don’t only want to show you
the process of what happens
when we form black holes,
but I also want to give you
the audio sense
of what it sounds like
for black holes to be born.
So this is the first example.
I talked about a supernova
producing neutron stars and black holes.
Now, first what I’m going to show you
is a simulation of
how an explosion actually occurs
And you can kind of see this rumbling,
this kind of, you know, mess going on,
and then at some point, you know,
it starts to wiggle
and then it will free itself.
Now, let me play the sound
the gravitational wave
associated with this.
(Low rumbling)
You can hear a vague rumbling.
It’s kind of gradual;
it’s almost like noise.
Now even in this process
we can actually look at the formation
of what happens inside,
in that process in the star.
And so this is a different example
where, in this case,
I’m showing the creation
of a compact object-based transition.
And you can kind of see it.
Suddenly, there’s this change, big change,
and there’s a shock wave being sent out,
and then the rest of the star explodes.
And again, let me show you
what it sounds like
in gravitational waves.
(Low rumbling)
(Pitch increases suddenly)
So you could hear the same rumble
that started in the beginning,
but suddenly this big “pop”
and then it fade off.
And this is, for example,
a way that we can figure out
what was produced in this explosion.
Was it a neutron star?
Or was it a black hole?
Now, upon creation, we can still continue
to listen to the sound.
So first, what I’m going to show you
is how does a neutron star sound?
Neutron stars have
their own characteristic sounds.
And so first, again,
I’m going to show you
the oscillation of a neutron star.
And you can kind of see
this neutron star rotating around,
and you can see the mass
being thrown around.
Now, again, the sound of a neutron star.
(Low rumbling with discernible beats)
OK, you can hear
this continuous kind of pitch,
but then it slowly fades over time.
And a neutron star is stable,
so you can hear this continuous sound.
Now, what about a black hole?
Black holes are very very stiff.
They are the most extreme objects
that we have in the universe.
How does that sound?
(A short blip)
So what you hear is a very faint blip.
It’s just a blip - blip! - and it’s gone.
It’s pretty much, you know, a single blip.
And then because
the black hole is so stiff,
all the sound,
all the gravitational waves
immediately damped.
OK, now to the central question again,
we know how to create black holes that are
several times the mass of the sun.
And we’ve seen black holes
that are a million times
heavier than the sun.
But how do we get
from one place to the other?
And that’s actually
what we call, you know,
a process where we want to see
if we can find the so-called
intermediate-mass black holes,
the black holes that are in between
several times the mass of the sun
and millions times heavier than the sun.
Now, we don’t necessarily know
how this process happens,
but we did get clues.
We did get clues.
Now, the first clue
comes from the actual discovery,
the first discovery
of gravitational waves.
And what we saw there was
what we call a collision of black holes.
Here, so you can indeed see
two black holes orbiting each other,
and this process emits gravitational waves
so that the objects get closer and closer,
radiates ever more gravitational waves
until at some point,
it collides and forms a single black hole.
Beautiful process.
So, this is actually an image
that you would have seen
if you were able to get close enough
and see the background stars behind it.
Now, black holes are too far away,
and especially too far away
for us to see a process like this happen.
But it doesn’t mean
that we can’t understand this process.
And in fact, what I want to now show you
was the first sound recorded
from exactly this process -
the collision of two black holes.
Ok. Take a listen.
(Low rumbling)
(High-pitch rumbling
with intervening chirps)
(Low rumbling)
(High-pitch rumbling
with intervening chirps)
So what we just heard was, first time,
you heard it in normal speed.
And you know, if you’ve got good ears
you would hear a thin blip.
And the second time,
we kind of increased the frequency,
so you can actually hear it
and you can hear this beautiful chirp,
the amplitude and the frequency -
loudness and the pitch - going up
as the two objects get closer
and collide to form a single black hole.
So now we have a process
where we take two lighter black holes
and form a heavier black hole.
This is a first way to making
ever more heavy black holes.
Now, to highlight the process
of how we understand
how massive the black holes were.
I’m going to play you now the same sound
but from two different collisions,
one heavier and one lighter.
And I want to show you how that sounds.
(Low rumbling glides to a higher pitch
and stops abruptly)
(Similar audio)
(Similar audio)
(Similar audio)
So, especially if you’re musically trained
and you’re able to hear
the difference in pitches
and also how the pitch evolved,
how it ramps up,
that’s how we scientists can understand
how heavy were the black holes
and what end product did they make
after their collision.
Now, that by itself is great.
We now have a firm grasp
of how we can make
out of lighter black holes
somewhat heavier black holes.
And our way towards the intermediate-mass
black hole actually came in 2019,
when we saw a merger.
Again, this is something
that I just showed you before -
there is just two black holes
orbiting each other, colliding,
forming a heavier black hole.
Now, that by itself
isn’t very special anymore in 2019,
but what was special
is the mass that this collision had.
And the mass, indeed, here,
was that that fell into
this intermediate-mass black hole region.
Now, to give you the sound.
I want to play to you the sound.
(A short, low sound)
Now, what you heard, again,
is just a faint blip.
It’s a very short blip.
I would always call it a bark
rather than a chirp.
And just because of that shortness
and that burst of energy,
that burst of sound,
we can infer that this black hole
was actually something
that was in this so-called
intermediate-mass black hole region.
Here we can see all the black holes
that LIGO has found over the years,
and you can see, highlighted,
the one that I just talked about,
the one that pushed us over this region
where we’re no longer talking
about stellar mass black holes.
But now we’re talking
about intermediate-mass blackholes
So indeed, we’re actually going towards
finding yet ever heavier black holes
and on our way to discover
how we can go from light black holes
that come from the process of supernovae
all the way to seeing black holes
that are of the size or the mass
that we found in the center of our galaxy.
So indeed, we now are, you know,
with these modern tools,
with these modern scientific tools,
we’re now getting
towards mapping out the process
of the birth and rebirth of black holes
that have such a fundamental importance
in our whole evolution
and everything we see around us.
Thank you.