So our first panel participant is a Professor
of Physics at Princeton University. Uh, in
addition to his studies of gravitational collapse,
black hole mergers and cosmic singularities,
he designs algorithms to efficiently solve
Einstein's equations on large computer clusters
The next participant is an assistant professor
in the Gravitational Wave Physics and Astronomy
Center of California State University, Fullerton.
She has developed a widely used model for
dense matter inside neutron stars. She's the
senior lead on the Extreme Matter group within
the LIGO Scientific Collaboration Please welcome
Jocelyn Read.
Also joining us. He is the lead astrophysicist
in the LIGO Scientific Collaboration. She
directs the Center for Interdisciplinary Exploration
and Research in Astrophysics at Northwestern
University where she is the Distinguished
Professor of Physics and Astronomy. Please
welcome Vicky Kalogera.
And our final participant this morning is
a professor of physics at Syracuse University
whose research involves using gravitational-waves
observations to understand the nature of the
universe. Please welcome Duncan Brown.
You know, suppose that some, somebody from
some alien civilization, a very advanced civilization
comes to earth and asks us how good are you,
what have you achieved? Then, one very good
way to answer them would be we have detected
gravitational waves from merging black holes
and neutron stars. And the reason that this
would summarize in an extraordinarily good
way our achievement is that number one, on
the theoretical side, it would tell these
people that we have reached a situation where
we understand already that space time is not
just the backdrop of cosmic events, as was
once thought, but is actually an active participant.
And spacetime can stretch. It can curve and
warp. Uh, and that gravity is not, we also
understood that gravity is not some mysterious
force acting across distances in space, but
rather it is an expression of the curvature
of spacetime.
So the sun, you know, as if it's standing
on a trampoline or something, it's warping
space and then the planets find the shortest
paths in that curved space. That's what gravity
is all about. So this is on the theoretical
side. Then on the experimental side, the fact
that we have detected these gravitational
waves, we will tell this alien civilization
that we have reached such a level of technology
that we are able to detect displacements that
are thousands of times smaller than the size
of the proton, which is really astounding
when you think of this. So with this one sentence,
we have detected gravitational waves from
merging black holes, we would tell these civilizations
where we have reached.
So let me start with Frans. And we could say,
I will make this statement and you will tell
me if I'm wrong, that, by the LIGO discovery
of gravitational waves from merging black
holes, uh, this actually was the very first
direct discovery of an Einstein black hole.
Am I right?
Yes, yes, I agree. And it's actually the,
the, the history of black holes is fascinating.
As probably most people here know, Einstein
came up with, with the Theory of Relativity
finally after many years in 1915 and within
a few months Karl Schwarzschild discovered
the first black solution, the theoretical
solution that describes what we know as black
holes today. And no one took that seriously.
It was such an absurd solution that predicted
so many things that people couldn't understand.
Even Einstein thought it was just a mathematical
oddity. Um, and then in the sixties and seventies
though, when perhaps what several people,
including Kip Thorne called the golden age
of black hole physics, theoreticians finally
sort of got an understanding what the solution
represents. And yes, there are a lot of bizarre
things about it, but we sort of understood
it that it's actually perhaps not quite as
strange as what people thought.
And astronomers also discovered various objects
in the universe, which was very mystifying
if they weren't black holes.
Or good candidates.
Good candidates, but then a sort of a strange
thing happened if you think that the doubt
that was there in the scientific community
about black holes and then from the sixties
and seventies when these objects are to be
discovered, it almost went completely the
opposite. That black holes became this thing
which almost had to be there. It was almost,
for what else could they be out there? But
if you think about it, until this LIGO discovery,
what evidence, what scientific evidence that
we actually have, that there really are black
holes that are described by Einstein's theory.
And by that you mean really this extreme warping
of space time.
The extreme warping of space time. So it's
in the sense that. So what the evidence that
there was before was, it was, I'd say there
was incontrovertible evidence that there were
very dense, compact, massive objects out there,
but that there was no direct evidence that
they were black holes of general relativity.
In other words, these manifestations of space
and time were space and time that were so
warped that event horizons formed.
But then with LIGO's discovery, we saw the
gravitational waves coming from this warping
of space time around two merging black holes.
To me, that was the most astonishing thing
about seeing their data for the first time,
is that it was the first direct evidence for
black holes as described by the theory of
relativity.
Right? Just to continue on the trampoline
example, I mean black holes, they warped the
thing so extremely that they essentially punch
a hole through the drum. So this was the evidence
for that. Jocelyn, neutron stars are extraordinarily
dense object. In fact, one cubic inch of neutron
star matter has a mass of maybe a billion
tons or thereabouts. Now you take such two
neutron stars and collide them. So it's a
little bit like a, an accelerator experiment
only on a cosmic scale. Tell us a little bit
about that.
Yeah. So, uh, so neutron stars are um the
remnants left over after massive stars, not
quite massive enough to collapse to a black
hole uh, reached the end of their lives and
they compact down into some. The core of the
star is more than the mass of the sun, about
one and a half times the mass of our sun,
but compacts down into a region about the
size of Manhattan. So it's this phenomenally
dense object. And one of the interesting things
about these dense objects is that we don't
exactly know what happens at their cores.
So the matter in neutron stars is denser than,
than any physical matter you might be familiar
with. So dense that we don't anymore have
nuclei with electrons kind of fuzzing around
them, but the nuclei have been compressed
so close together that a neutron star is almost
like an atomic nucleus the size of a, of a
city.
Let me just stop you for one second. So basically
if you take a normal atom, the size of the
atom is about 100,000 times bigger than the
size of the nucleus, but in this case you
have compressed everything so much that the
nuclei touch each other. There is no more
that space of 100,000 times. Please go on.
Yeah, so, so we have this, this bizarre form
of matter and it's possible when we get to
these densities we're still describing standard
model physics, but we don't understand how
to translate that into what happens. And it
might not be that there are protons and neutrons
like nuclei anymore, but there are just direct
quark matter interacting with itself in ways
that we can't create and reproduce in any
terrestrial laboratory. So these cosmic accelerators
smash neutron stars together, they take some
of the densest matter and the gravitational
waves trace out the dynamics of this collision.
And then they collide and they throw out the
source of an array of, of electromagnetic
counterpart observations, uh, in this really
dramatic, this dramatic kind of event that
we now, for the first time observed. So we
see the traces of the densest stable objects
in the universe, in this panonopoly of observation
Quarks just for those who don't know are the
fundamental particles that make up protons.
For example, there are three quarks that make
up a proton and so on, and in this case, you
really reach those densities where the quarks
seem to interact with each other and so on.
Vicky, so we're talking about collisions of
black holes, of neutron stars in this, but
how do these systems form at all? How do we
get to the situation which, you know, then
we have something to collide.
Well, um, theoretical astrophysicists have
ideas about how these systems form. But we
don't have the perfect answer to your question.
So, um, over the years we, uh, we have developed
some hypothesis for how this can happen in
nature. What we know for sure is that it happens
in nature. We know that these binary systems,
two black holes in orbit, one around the other,
just like the earth is going around the sun.
But now imagine two black holes or two neutron
stars, like as dense as Jocelyn was describing
in orbit one around the other exist in nature,
and the way they might form, um, is, uh, there
are two classes of potential formation mechanisms.
One is you take two stars that, that form,
they are born in pairs. They are born one
going around the other, unlike our sun, which
is a single star as we are.
About 50 or so of all stars are in such pairs.
Are in binaries. Exactly. So we know from
observations with regular telescopes that
most of the stars out there actually are born
in pairs. So they are born in pairs and they
start burning the nuclear fuel. They're producing
a light, uh, they're gonna go through the
nuclear revolution. Eventually their cores
are going to collapse and depending on their
mass, they're extremely massive, um 20, 30,
50 times the mass of the sun. Then they're
gonna form black holes. If they're lucky enough,
then at the time of the formation, the two
black holes would stay in orbit and the system
will not disrupt. And then you're going to
get a binary or you might get a binary with
two neutron stars and then eventually they're
gonna come together in this spiral dance.
They're going one around the other, emitting
gravitational waves which now we have detected,
and then they have no other option but to
collide together. Uh, so this can happen with
black holes or neutron stars. There's a whole
other class of formation channels that broadly
speaking can be described in the following
way.
You can be in a extremely dense stellarly
junk, not the kind of region we live in, thankfully.
Otherwise we won't have a quiet planet like
we have it. But there are parts of the galaxy
where stars are extremely packed together
in a small volume. So you have a collection
of stars, we call them clusters of stars.
And the stars may be individual stars and
they're going to go through their nuclear
revolution. And eventually they're gonna form
the death remnants. Neutron stars or black
holes. Those, especially the black holes,
are going to be, end up being the heaviest
objects in that cluster. And as uh, imagine,
um, I guess in a glass of, I don't know, I'm
thinking a glass of milk with heavy, I don't
know.
The heavy stuff sinks to the bottom.
I'm thinking of coffee, coffee beans covered
with chocolate. So, uh, so they're going to
sink in the bottom because they are the heaviest
things. Then as they sink in the bottom, they're
also moving at high velocity and they're interacting
together and they can actually, even though
the black holes were formed separately, they
can actually combine together into pairs and
then eventually with the emission of gravitational
waves they are going to end up colliding.
So you have pairs that were born as stars
together and they stayed together all their
lives and others that went through the dance
floor. They had many, many partners along
the time, the time of their lifetime. And
eventually they collided with some partner.
Let me just add one thing to both of their
comments and that is, I mean, they both describes
stellar evolution, in stellar evolution and
stars basically spend their lives trying to
fight gravity. If you don't have any opposing
force, everything because of gravity will
collapse to the center. So stars spend their
life trying to fight gravity, and they do
that by doing nuclear reactions that produce
a lot of energy and they, which builds a lot
of pressure and that holds them against gravity.
But as they go through their nuclear fuels,
at some point they ran out of fuels and then
gravity finally has the upper hand. And that
leads it eventually to these collapse, which
forms, neutron stars or black holes.
Duncan, I say to that experimentally. You,
you are at the closest from these groups to
the experimental side. What does LIGO do,
and how does it do it?
Sure. So, so LIGO is a, it's basically a very,
very precise ruler for measuring, for measuring
length. And to give you an idea of how precise,
think about the, you know, you have these
two black holes or neutron stars going around
each other, many, many, many billions of light
years away, producing these ripples in space
time. And the physical effect of these ripples
in space time is to stretch and squeeze space
itself. So gravitational waves, you know,
weak gravitational waves are probably passing
through this room right now, stretching and
squeezing everything in this room, but that's
so tiny that we don't notice. You know, fortunately
for us, there's no nearby binary's producing
gravitational waves that you would feel and
otherwise we don't live in a quiet planet.
So the, the, the stretching and squeezing.
So you gave an analogy that that over a couple
of miles you're measuring a distance that
is a change in distance that let, that is
less than the size of a proton. Or another
way of saying that if you think about the
distance between the sun and the nearest star
to the sun, which is about four light years
or so, you're measuring that distance to the
width of a human hair. So that's how precise
the measurement has to be to detect these
incredibly weak gravitational waves. And so
LIGO itself is both simple and complicated.
It's simple in the sense that it's an experiment
that you can build as an undergraduate in
the lab. You take a laser beam, you shine
it through a mirror that's partially silvered,
and so half of the laser beam goes this way
and half of the laser beam goes the other
way.
You put another mirror at the end, another
mirror this end in this L shape, you bounce
the light back so the light comes back towards
the what we call the beam splitter. This,
this mirror that split the beam, and then
the light leaks out towards a a, a, a light
detector here, a photodetector right here,
and if you set this up so the lengths of those
two arms are perfectly balanced, then you
can set it up. So if in LIGO we think classically
we think of light as a wave and you set this
up so the the peaks and troughs of the waves
line up and so you get what we call constructive
interference coming back towards the laser
and destructive interference going towards
the photo detector.
Constructive interference is when the light
from the two things amplifies each other and
destructive is when it kills each other.
Right, and so all the light comes back towards
the laser when the thing is perfectly balanced,
as the gravitational wave passes through,
it stretches one arm and squeezes the other
arm, and so it upsets that perfect length
balance between these two arms. Now in reality,
these arms are two and a half miles long with
40 kilogram mirrors at the end of each arm,
so as a gravitational wave goes past, it changes
this delicate balance of the two arm lengths
and some light just leaks out towards this
photo detector and that light. The light that's
leaking out towards the photo detector encodes
the length change which encodes the ripples
in space time, which are the gravitational
wave signal
So they hit these things with miles long arms
and they measure the thing changing by the
length of a proton. This is the experiment.
This is why I said if you tell somebody we
have achieved this, you know, there is nothing
more dramatic that you can say that we have
achieved from a technological perspective.
It is my, it is actually the most precise
measurement we've ever made. Humans in any,
in any field of science. Technology.
Yes, and I think coming back to your question
to Frans, you know, have we directly detected
black holes? You know, we, I think everyone
believes that there was firm evidence for
black holes, for observing the light given
off as material falls into black holes or
the light given off by stars orbiting black
holes. But with LIGO, you actually, those
gravitational waves, those ripples in space
came from the black holes themselves. So this
machine reached out and touched the event
horizon of those black holes with a machine
that we built here on earth.
Right? So Frans, we've now talked a little
bit about how these things emit gravitational
waves and what not and so on. And these are
these perturbation in spacetime. Tell us a
little bit about that, and also tell us a
little bit about how do you simulate what
happens in these things.
Right? Right. So I guess as people have talked
about and Duncan, so these gravitational waves
or ripples in space and time. So Einstein's
theory of general relativity is not a theory
about a force of gravity. It's a theory about
space and time and it says that space, space
and time or space time can be described by
a geometric structure, distances by things
separated by a certain amount of space or
distances in time. We are moving in time as
we speak now, um, and what I...
Too fast unfortunately.
And so, and what Einstein's theory says. So,
you know, like what Mario described in the
beginning. I mean if you think something like
the earth or the sun is producing this sort
of bend sheet, which sort of represents the
curvature of spacetime. But when, uh, when
objects accelerate, when they move. And one
example of acceleration is two black holes
or two stars orbiting each other, well the
earth going around the sun, would actually
be producing gravitational wave right now,
but that kind of acceleration produces these
little ripples in the geometry of space and
time. They propagate out at the speed of light
and as Duncan has mentioned, the exact details
of how, of the shapes of these ripples encodes,
what produced them, so you can imagine there's
just some two things going around like that.
That produces a very nice sinusoidal pattern
in the wave.
Let. Let me just stop you for one second.
One of the things about Einstein's theory
is that gravity doesn't act instantaneously.
It propagates at the speed of light, so these
disturbances propagate at the speed of light.
That is a very important feature of Einstein's
general relativity. Please go on.
We so so that that perhaps brings to one problem
with something like LIGO in the sense that
as with Duncan described, it's not a telescope,
it's not focusing gravitational waves onto
an image, so we can sort of see what things
look like in gravitational waves. Perhaps
a better analog is a seismometer. There's
something that produces an earthquake in certain
space and time. These ripples propagate and
LIGO measures these little fluctuations, so
now say, okay, we've seen some ripple. What
does that mean? What does it represent? How
can we say that it's two black holes that
collided. It was two neutron stars collided,
and the way that we do that is we try to solve
the Einstein equations using various theoretical
tools, pencil and paper methods, computer
simulations, and try to predict what Einstein's
theory says for each one of these systems
and vary the parameters of the system. And
then when then for the events at LIGO saw,
we take these waveforms, these little ripples,
and we try to match them up to our various
predictions and when we get good matches we
can say that the parameters acquainted the
simulations. That is how we interpret the
ripples.
Maybe we can have the the video? So here we
have these two objects and they spiral around
each other and you can see that they are getting
closer and closer. And they emit these ripples
which are these gravitational waves. And the
frequency of the waves becomes higher and
higher the closer they get together. And eventually
boom, they merge together. And in the case
of neutron stars, they produce this thing
which we happen to call a kilonova simply
because it's about a thousand times brighter
than what we call a normal nova. And basically
that's what we've seen. So please go on.
The problem with these simulations. The Einstein
equations are very, very complicated and that's
perfect for computers. They don't care about
complication except it just takes a lot of
processing power, a lot of computers. So for
example, to do an actual simulation, like
something like that, which lasts for perhaps
a few dozen orbits, the merger calculates
how the gravitational waves propagate. It
might take anywhere from say 100,000 to a
few million CPU hours. So your typical, like
the power in your cell phone these days, cellphones
are.
So don't try this at home.
So if you have like a million cell phones
all working together for an hour. They'd be
able to produce one waveform.
Right. Can you just address this? I mean there
is this thing that is known as the Chirp.
Yeah. So right, so let me explain. So I said
I get to the structure that signal tells us
sort of what's going on, um, and for, for,
for binary black holes, that's called this
chirp waveform. And so what's happening is,
so now we've got these two black holes of
in orbit. So when they're pretty far apart,
um, it's a, it's a very nice, essentially
circular orbit. And I said, you know, we get
through the sinusoidal wave and you can actually
also, it's nice for these black hole mergers
because the frequency that they're emitting
at it is in the audio range. You can almost,
you can say, LIGO is listening to the sounds
of the universe, so say two black holes, they
were orbiting at a certain frequency. So it's
at, when they're far apart, it's essentially
a monotonic frequency. So just a very pure
tone. Um, but this, this orbital motion produces
gravitational waves and that drains energy
from the system. And so that's why two black
holes that are in orbit, they're not going
to stay in orbit because they lose the energy
to gravitational waves. And the way that that
affects the system is that they start, they
start getting more, more bound, they start
moving, they're spiraling closer to each other.
So when they're closer it to each other, um,
they, they have to go faster in some sense,
you know, now there's a stronger, if you will,
gravitational force, so they have to start
moving faster. So the sine wave that they're
emitting increases in frequency, so with the
period decreases and it also increases in
amplitude because they're going faster, but
now they're going faster so they emit more
gravitational wave energy.
So the sound is stronger but the pitch is
higher, right?
So, so, so it's sort of a runaway process.
And so they start spiraling around faster
and faster and faster. And eventually they
collide and it's at, it's, if you think of
like what, what happens to this monotonic,
well almost monotonic wave that's increasing
in amplitude and frequency. And it's as this
woop sound like, that's what's called the
Chirp. And then finally when they collide
together there's that final burst of the Chirp.
And then very, very quickly, in fact, astonishingly
quickly, the black hole, the, the two black
holes, they merge into a single larger black
hole and it sells down to a stationary black
hole that's completely quiet. It doesn't emit
gravitational waves and they've had final
stages called the ring down phase. And you
can almost think of it's like these two black
holes. They're busy chirping because of the
motion. They smashed together. It's like having
taken a bell and like giving it a big sort
of. You hit it with a hammer. Now bells are
very efficient at ringing. They ring, they
make a good sound for a long time. Black holes
are terrible bells. It's like throwing two
pieces of party together. They almost stopped
ringing immediately, which is actually an
astonishing prediction. Like how can objects
stop ringing that quickly? Be it black holes
or like that.
Thank you. Now Jocelyn, I want to ask you
something but I'll just make a small introduction
to my question so that it will become clear.
So for example, on earth we have tides and
the tides are because of the moon. Basically
what happens is that the point that is closest
to the moon feels a little bit of a stronger
gravity than the center of the Earth, and
so the sea goes a little bit higher. Similar
thing happens at the farthest from the moon
point because there, it's the center of the
earth that moves a little bit farther. So
still the, the sea is higher, now believe
it or not, these two neutron stars, even though
they only are, they have a mass, a little
bit larger, larger than the sun, but they
are just six miles, you know, in radius. They
still, when they get very close, they can
raise tights like this. So tell us a little
bit about this and how can you tell whether
there are tides or not?
Yeah. So, so as, as Frans said, so the, the
gravitational wave is draining energy from
the system and that's ah, and then so that's
changing how the orbit happens. And then slowly
as the stars get closer and closer together,
um, different features of the stars come into
play in the dynamics of how they're orbiting.
So when they're far apart, the only thing
that really matters is their mass, uh, depending
on the mass of the star, they'll, they'll
orbit at a particular frequency and the frequency
changes in a particular way. As they get closer,
the influences of their spins and how different
the masses starts to come into play. And then
as they get very close to each other, the
stars of tides become more and more significant.
And what happens is, in addition to energy
being drained away by the gravitational waves,
some of that energy goes into deforming the
star.
So you can think of a star at rest has a certain
energy and to deform the star to raise the
tides, that changes the energy. It pulls the
star out of equilibrium and that, that takes
energy to do. So, the forming the tides is
another drain on energy. And so it causes
the inspiral to accelerate. Um, and then of
course, you know, there, there's also a factor
that depending on the size of the star, they
eventually crash into each other. And if they're
very compact, they can do a few extra orbits,
if they are large, they'll, they'll interfere
with each other more quickly. So the Chirp
at the very end of the pattern of the ripples
will either get accelerated by tides and then
cut off by the stars smashing together. Or
if the stars are very compact, the tides are
weaker, the, it's harder to disrupt a very
compact star than a larger, fluffier star.
So that more compact stars will continue their,
their black hole like orbit, and then eventually
merge in a more simplified way. So that's
encoded in the pattern.
Basically, how, how much you can deform the
star by these tides depends on well we called
these, the equation of state. Basically, it
depends on how hard it is to deform it, how,
you know, how does it respond to trying to
change, you know, by applying a force to it
out, trying to change that. So basically,
isn't it true that what happens is that these
tides at some level tell us this equation
of state, I mean, how the pressure depends
on the density of the star and so on.
Right. So, so we're, we're trying to find
out what are the properties of the mysterious
matter in the core of the stars. And the key
property that we're interested in is at a
given density, how much pressure does that
matter provide? So this, uh, this is matters'
last stand against the crush of gravity, so
it has to be providing a lot of pressure at
a very high density, but if that pressure
is low, the collapse happens a little further
and you have a compact star. If the pressure
is high, you have a larger star. And the larger
the star is the larger the impact of the tides,
the outer material of the star is farther
away, and it's easier to be disturbed by the
tidal force of the other star. So a larger
star deforms more, pulls more energy from
the system.
Vicky, we mentioned this a little bit, but
I want to get a little bit into the more nitty
gritty. Black holes are, you should understand
that black holes are actually very simple
objects. They are characterized by two numbers.
Well three in principle, but one is the mass.
The second is the spin, how fast they rotate,
and the third principle is their electric
charge, but in astrophysical objects, there
is no real electric charge. So it's two numbers
basically. But from these things we can try
to determine the mass and the spin and also
the orientation of the spin, namely is it
rotating around these axes or that axis, and
so on. Well and also things like the distance
and things like- Walk us a little bit through
how do you determine all these parameters
from the observations?
Yes. Um, so it goes back to what, uh, I think
Frans mentioned it first, the chirp. Okay.
So, uh, all this information about the mass
and the spin and the distance of the source
of these, uh, uh, dense spiral dances and
eventual, uh, collisions and the tides that
are all encoded in that chirp we're observing
with the gravitational wave detectors. What
we measured directly is basically the squeezing
and stretching and squeezing and stretching
of the space time here on earth, which is
the, uh, is the propagation of the wave that
started from the source, when the wave is
reaching us it's disturbing our spacetime
here. And what we measure is the amplitude
of the wave. We measure the frequency of the
wave and we also measure the duration of the
wave. And during that duration of the signal,
the frequency is not steady, but it actually
changes.
So we can measure what we call a frequency
derivative, so the frequency of the wave is
not a simple sinusoid as you were saying,
but the peaks of the, um, of the sinusoid
that are coming closer and closer together.
The rate of change.
The frequency is changing. And we can measure
that rate with which the frequency is changing.
So we have amplitude, frequency and frequency
derivative, uh, so that rate of change of
the frequency. So these three are the fundamental
measurements we make, uh, and, and we can
use these three fundamental properties with
a little bit extra information that is hard
to get into right now. Uh, but these are the
three main things that allow us to decode
masses, spins and the distance because the
amplitude of the wave is telling us something
about how far away is the source and also
how massive the object are. And the change
of the amplitude with time and the change
of the frequency with time is also telling
us something about the spins. Sometimes, not
always. Sometimes we go straight in the spins,
but sometimes we get very weak constraints
on the spins.
Maybe we can have an image just so in the
case of the neutron stars in particular, we
also observed this in ultraviolet light, optical
light and so on. And we have an image of what
was actually observed with a swift experiment.
So look in that box and you will see that
thing which suddenly, well you see the period
disappearing ah, it's in a galaxy that's,
well has a telephone number of 4993 and so
on, but um, so just realize that everything
they told you, these are things that happen
sometimes billions of light years away. So
this is the type of thing we see and it is
from that type of information that you get
all these details that you see here. And that
leads me to you Duncan because in this case
of the neutron star merger are, there was
somewhat of a mishap in that in one of, you
see they have this alert system where when
something happened because they want to alert
other telescopes and this and that, when this
neutron star merger happened, it, the alert
worked in one of the locations but not in
the other. Tell us a little bit of what happened
then.
That's right. So, so to set the context is,
as Frans said, gravitational wave detectors
are like listening to the universe. You're
listening to the sounds of space time, the
ripples and gravitational wave detectors don't
just look in one direction. They look in all
different directions. So just like my ears
can hear sounds from all over this room. If
someone over there made a noise, then I could
say, okay, my, my brain can localize that.
The sound is coming from over there and I
can turn, I can point my eyes and I can look
over there and I can see what's going on.
And that's what we wanted to do with, with
gravitational wave detectors and electromagnetic
telescopes. We wanted to say, let's listen
out for gravitational wave signal coming from
somewhere on the sky, somewhere out there
in the universe. And then when we hear this
signaling gravitational ways to then say,
point telescopes over there and look over
there and see if you can see light in all
its different forms coming from these two
objects colliding.
So we had a system that was set up that would
alert us to these, these, uh, these gravitational
wave detections. And in order to figure out
where sounds are coming from, I have two ears
and a very complicated signal processing unit
inside my head. So I can, I can detect and
I can localize sources. With gravitational
wave detectors, we have three detectives.
There's the two US LIGO detectors, one in
Washington state, one in Livingston. And then
there's a French Italian detector just outside
Pisa in Italy. So these three detectives make
a network and the idea is they triangulate
on the sky, where were the sources? And so
you can say gravitation wave over there, go
look. And so they all have to work together.
All three detectors have to work together
in order to figure out where the source is,
like my two ears have to work together.
Unfortunately what happened was these gravitational
wave detectors are incredibly sensitive instruments,
meaning as we said, you're measuring changes
in length that are smaller than a proton.
And the real world is a noisy, messy, dirty
place. Things glitch, things ping. There is
all these complicated control loops that keep
the detector stable. And what happened on
the morning of August 17th when we, uh, when
we detected the binary neutron star is the
LIGO Hanford detector, one of the detectors
as computer software that scans continuously
for these patterns, these chirp patterns of
gravitational waves looking for these signals
in the detector data and the software that
was looking at the Hanford data in Washington
state said, I think I've seen something really
interesting. You should take a look at this.
And it sends out alerts to people's cell phones
and emails and people start to get text messages
saying, hey, hey, come and take a look at
this. And we looked and I used to tell my
students, you know, when we finally see a
binary neutron star signal, you know, won't
be that obvious. We'll be pulling out the
noise. We'll look. And it was just obvious.
It was just a beautiful thing. I'm like, okay,
well that's clearly. There you go.
I'm happy to be wrong. That look, you can
see a chirp. There it is. Um, but it was,
the computers only found that in the Hanford
detector, they didn't find it in Virgo and
they didn't find it in Livingston, which is
key to knowing where on the sky that it is.
And so this is where the humans come in. This
is why people get PhDs and spent many years.
I tried to understand what's going on in these
detectors and figuring out what's going on,
so the humans came along and said, okay, we
have to take over, the computers have failed
us. What's going on? And and we realized very
quickly a team of people who've dedicated,
large fractions of their lives to understanding
how these detectors work, realize that there
was a glitch, what you call a glitch in the
Livingston detectors, like having a scratch
on a record or a or a CD.
You're listening to the music and you suddenly
hear a click and the computers are trained
to ignore clips and pings and things that
don't look like gravitational waves. So they
said, oh, we're ignoring that. Nevermind.
No signal there. Nothing to see here. So the
humans actually went in and said, no, no,
no. Look, we can see a signal in Livingston,
we can see a signal in Hanford. There's just
this giant glitch in Livingston at the same
time that the humans can figure out what the
computers couldn't. So what we did is we leapt
into action, understood where the glitch came
from. This is kind of a frenzied, maybe four
or five hours on that morning trying to figure
out where, the where, you know, what had gone
wrong in the instrument, how to remove this
and excise it from the detector data, reanalyze
the data together with Virgo and Hanford,
and Livingston, bring the detector from, the
data from all three detectors together, so
then finally triangulate the source and say
it's there and send an email out to the observing
community to say, here's the patch on the
sky where we think the signal is, go point
the telescopes. And fortunately we managed
to do that quickly enough that we didn't really
lose that much. I think if we'd move very,
very quickly, telescopes in South Africa,
it would have been overhead in South Africa.
We could have caught some very important information
from the early parts of the, uh, the signal.
So. So next time we're going to do this better,
but we got pretty much everything you could
have.
Vicky, you wanted to say something.
Yeah, I wanted to say that the, that we were,
as you said, we were a little lucky because
we only took four or five hours, but these
early four or five hours. Uh, the source was
in the southern sky and those are four or
five hours was daylight in the, in the south.
So even if we had the early localization,
it was still daylight even in southern South
Africa. So we were still delayed by a little
bit, but we didn't miss the whole day. We
missed only maybe a few hours at the end of
the day. Yeah. So it could have been worse.
Can you imagine. Here is an event that was
perhaps the most exciting scientific event
in the almost history of humankind. And you
had the glitch at one of the sites of the
experiments. I mean, this is how science works.
But humans saved the day.
Yes, yes, yes, yes.
I want to thank these wonderful panels. So
please join us in a round of applause.
