In 1916, this is the year after Einstein wrote
his paper on the general theory of relativity,
Einstein continued to think about the theoretical
ideas and he wrote a paper which we have here,
where he was thinking about the possibility
that if space and time can warp and curve,
then it might be the case that space and time
can also ripple, right?
The image to have in mind is think of a trampoline,
right, a trampoline, you put something heavy
in the middle.
It has a nice curvature to it, but if you
have kids that are jumping all around on the
trampoline, the shape doesn't stay nice and
static.
It ripples.
It vibrates.
So he was wondering whether it might be the
case that space itself might be able to undergo
these kinds of ripples, these kinds of vibrations,
and this would be known as a gravitational
wave.
Now interestingly, he wrote the first paper
in 1916 and in 1918, he corrected an error
in the 1916 paper and he continued to struggle
with whether or not he actually believed that
gravitational waves were a prediction of the
general theory of relativity and he worked
on this by himself.
He worked on it with collaborators, Nathan
Rosen being one of them, and a couple of years
after this paper where they expressed some
confidence that gravitational waves were real,
Rosen writes another paper where he basically
says that he thinks they're all just a mathematical
artifact.
And I think many historians think that Einstein
himself kind of had that view that there really
weren't these ripples in the fabric of space
and yet, by the 1960s when the mathematical
methods had been refined, where we could really
look at Einstein's equations and extract from
them the actual physical predictions with
certainty, it became clear that gravitational
waves were a prediction of Einstein's theory,
which would mean that if you had say two objects
like two neutron stars rotating around, they
would so disturb the environment that they'd
send out this train of gravitational waves
and that would mean in principle, you could
detect these because downstream, if you're
in the wake of one of these gravitational
waves, you will experience this kind of an
effect: a stretching and squeezing, a stretching
and squeezing.
Now, I should say this animation is not to
scale.
When you actually do the calculation, you
find that for a typical astrophysical phenomenon,
you'd find that the stretching and squeezing
would be less than an atomic diameter.
So the question is how could you ever measure
something so fine, and yet as we will discuss
now exactly that kind of a measurement has
been achieved and so to talk about that, let's
turn to our next guest to take a closer look
at gravitational wave as the lead astrophysicist
in the LIGO scientific collaboration.
She is a distinguished Professor of Physics
and Astronomy at Northwestern.
Please join me in welcoming Vicky Kalogera.
So thank you for being here.
Thank you.
And we'd like to get some insight into how
gravitational waves have been detected.
And LIGO is the facility.
So what does LIGO stand for-
LIGO is a double acronym actually.
It stands for a laser interferometer gravitational
wave observatory, and laser itself is an acronym,
uh, so it's a double acronym.
And what's that acronym?
Laser is such ingrained in my head, I don't
remember the full acronym-
And I think it's light amplification by stimulated
emission of radiation, but I'm not sure.
Okay.
So here's our detector, here's our gravitational
wave telescope.
It's a new type of telescope, uh, it doesn't
look like your traditional telescope, does
it?
So it has this corner station where you can
see there and uh, and then it has these tubes,
and it's like tubes of a regular telescope,
except these are on the ground and there are
two of them.
And from the corner station, we shoot down
lasers in a right angle.
And they travel down these four kilometer
tubes, vacuum tubes, and they reach the end.
They bounce off mirrors and they come back
and we study the light, the laser light that
comes together.
And through the interference pattern of the
laser light, we can tell whether the murals
at the end of those arms are being shaken
in these squeeze and stretch, squeeze and
stretch motion, that gravitational waves that
are supposed to affect, uh, space and time,
but space is what we can think about easier.
And you can actually measure shaking by atomic
distances with that kind of a device.
In fact, it's even smaller than that.
It's, uh, smaller than one, uh, thousandth
of the nucleus on the scale of the four kilometers.
It's the most accurate measurement we have
ever achieved, humans have ever achieved anywhere
in any field of science or, and engineering.
Now, how do you know that the shaking is a
gravitational wave and like not someone just
kicking the equipment-
There's a lot of shaking going on everywhere
and that was the reason why we didn't just
build one of these detectors.
We built two detectors, one in Louisiana state
and one in Washington, really far away from
one another, because if the shaking is happening
and it's affecting the one detector and it's
coincidence, I'm sorry, and it is earth based,
then it's very hard to reproduce the exact
kind of shaking, the exact kind of squeezing
and stretching and have it happen in two different
locations, independent locations so far away,
so having coincidence as we call it, at the
same time with exactly the same squeezing
and stretching pattern, it was extremely important,
so we needed to be able to claim such an unprecedented
claim that we detected gravitational waves.
It was really important to have observations
of the exact same signal at the same time
in two different independent locations.
And this, and this first happened, this was
the first achieved in 20-
2015, September 14.
Well, the world didn't know on September 14,
2015.
Some of us did.
Some of us did.
Um, and uh, that was a life changing day for
the hundreds of scientists and engineers who
are members of the LIGO scientific collaboration.
The world found out on February 11th, 2016
when we made the first announcement.
And, and so what was found?
So what was found is, uh, that basically two
black holes, one in orbit around the other,
uh, were disturbing spacetime, uh, not very
close to us, not at the center of our galaxy,
but actually over a billion light years away,
uh, at some other galaxy.
And the two black holes were coming together
because of the emission of gravitational waves.
They were disturbing spacetime around them,
generating these ripples that you talked about
earlier.
And these reports were traveling for over
a billion years at the speed of light.
And on September 14, they came, approached
the earth from the south, they hit our Louisiana
detector first and seven, about 7 milliseconds
later, they hit our Washington state detector
second.
And that's what you expected because that's
how long it would take light to travel.
Exactly.
So there has to be a finite delay because
of course, gravitational waves just like light
doesn't travel instantaneously, it takes time.
So the two black holes as they were coming
together in their orbit, losing energy because
of gravitational wave emission, at the end,
they merged, like in this other movie as well,
they merged and I'll explain the sound in
a minute because it's not self explanatory,
I should say.
Not really.
Not everybody gets it.
Well, I, I did a version of this on, on the
Stephen Colbert show.
And his interpretation is God Bugs Bunny.
That that's how he-
That's his interpretation.
I should say Stephen Colbert is a Northwestern
undergraduate.
I've met Stephen and we have talked about
the gravitational wave discovery.
So, uh, so the two black holes are coming
together and eventually they have nowhere
to go.
They're coming and they're basically physically
touching, except there is no actual surface,
there is no hard surface that is that imaginary
surface where light can't escape.
And the two black holes merge into a single
bigger black hole, they form a single black
hole and then that single bigger black hole
settles and the disturbance stops and the
gravitational wave signal stops.
So it's a finite, transient signal.
There is this whole turmoil in spacetime and
by the time you formed a single signal black
hole, the whole thing ends.
Now we talk about the actual signature as
shaking these devices in Washington and Louisiana
by less than an atomic diameter.
But what was that signal like when the black
holes actually collided, way out there, a
billion light years away?
Um, I don't have that particular number for
you, but it does scale as one over the distance.
So if we take 1.3 for that first detection,
if we take 1.3 billion light years away, it
was that many times bigger when it was generated.
So I'm calculating.
Yes.
So that would be about 50 times the energy
output of every star in the observable universe
if I did my calculation-
By energy.
Yes.
Yes.
You are quick.
I'm very quick.
Yeah.
So, so that's, so, so the basic lesson though
is it's a huge-
It's a huge explosion in gravitational wave
energy for a very small amount of time.
So the signal was 0.2 seconds, uh, and for
that-
0.2 seconds when it-
That lasted within, our, the frequency range
that LIGO, the LIGO detectors are sensitive
too.
So you should think you heard about electromagnetic
waves and they come in different frequencies.
Optical is the optical frequency range, infrared,
radio waves, etcetera.
These are all different frequencies of the
same type of wave, electromagnetic waves.
Gravitational waves have frequencies themselves,
so LIGO can't see every single gravitational
wave out there, can only see frequencies between
about 20 Hertz to about now, maybe hundreds
of Hertz, let's say 700 of Hertz at best.
Um, so in that frequency range, this first
binary black hole collision lasted about 0.2
seconds.
In that short amount of time, the collision
and energy generated, uh, in gravitational
waves, outshined by a factor of 50, all the
light generated by all the stars in the whole
visible universe, not visible by eye, but
in the whole universe that we know of and
we could ever detect.
And it just dilutes as it travels.
It does.
Yes.
So, so, you know, I've been asked about- Because
this is such an amazing discovery that you
and your team, I mean it's just, it's fantastic,
but how-.
Can we look at data, because then I want to
ask you a question.
Can you bring up?
Can you raise the volume?
I can't really hear it.
So that's a tiny signature that you were talking
about.
That's the tiny signature.
So you can just hold it up there if you would.
There's a few things to talk about.
Well, my general question is how can that,
that little tiny window of data give you so
much insight into what the source was?
Yeah.
The reason is that this scribble that we can
see on the screen, uh, and the banana that
you can see on the screen carries a lot of
information.
Okay.
Uh, and I'll, I'll, uh, I'll take a few, uh,
you know, maybe a minute or two to explain
it.
Yeah.
First, let me start by explaining the significance
of the sound.
Okay.
I don't want anybody in this room to have
any misunderstanding.
Gravitational waves are not sound waves.
Okay, so don't go away telling anybody that
gravitational waves are sound waves.
However, we can take the frequencies, remember
I said something about 20 hertz to a few hundred
hertz?
If you know something about music and the
frequency of sound that our ear is sensitive
to, that's about the range of the frequencies
of sound that our ear is sensitive to.
So we can take the gravitational wave frequencies
and pretend it's a sound, it's not a sound,
but pretend it's a sound and convert it into
sound and say, what would the gravitational
wave sound like if it were a sound?
And this is what it sounds like.
It sounds like a bird's chirp, which is not
quite what you heard, but that's how it came
out my mouth.
But can I ask you one question on that.
One question on that.
So, so it's laudable to be clear on that,
but just to also be- If our eardrums were
able to vibrate via the gravitational wave
influence, if, then that is what we would
hear.
Kind of, if- There is one distinction, if
I may, that-
It's not a transverse wave?
Exactly.
So sound waves are transverse waves, so the
oscillation is along the direction of propagation.
Gravitational waves, and electromagnetic waves
oscillate perpendicular to the direction of
propagation.
And that kind of has, it's important for how
it affects our ears.
Physiology.
Yeah, exactly.
If we go back to the image up there, uh, the
oscillation that we measure how the mirrors
at the end of that L shape telescope are being
displaced, are basically recorded by the scribble
we can see at the bottom of each banana.
The two graphs are what we measured at the
two different detectors, a handful in Washington,
Livingston in Louisiana.
So we had the signal was detected independently
in the two detectors at the same time with
only a slight time delay, as I said.
And what you see in the signal, the real data
is the scribble, uh, so we see that the oscillation
of the middles increased as time progressed.
The duration, the axis under that scribble
is not shown, but the duration is those 0.2
seconds.
The amplitude went up as you can see, and
then it went down and petered off.
That's when the two black holes came together,
formed the stable black hole, and then there
were no more gravitational waves.
Alright, so that's one thing.
The amplitude went up.
The sound you hear becomes louder before it
dies off.
The la- the second thing is that the peaks
of each oscillation come closer and closer
together.
That means the frequency of the oscillation
is increasing, and that the pitch of the sound
is becoming higher and higher.
Um, so that's the chirp.
Um, so that's what we call a gravitational
wave chirp.
The banana, you see, is another way of representing
the exact same phenomenon.
The banana gets brighter, that's the amplitude
of the stretching, become stronger and stronger.
And the frequency, goes up as a function of
time, and that's how you get that curvature
and the closer to the two black holes are
coming together, the faster the frequency
goes up until it dies off because you formed
a single black hole.
All right.
Now, now I'm going to finish by coming back
to your initial question.
What we measure is the amplitude of the wave,
the frequency of the wave, and the fact that
that frequency of the gravitational wave is
changing as a function of time.
So we're measuring a frequency derivative.
These three pieces of information are encoding
the masses of the two black holes and how
far away the system was.
So that's how by studying that progression,
that scribble, as I call it, and comparing
it to templates we have from Einstein's theory-
So tell us about those.
What are the templates?
If we take, if we take Einstein's theory of
general relativity, and uh, ask ourselves
if two black holes that are at some distance
and we follow spacetime and we solve, with
general relativity equations, the change in
spacetime as the two black holes are moving,
as the egg beater is messing up spacetime
around them, we can calculate what is the
amplitude, the frequency, and the frequency
of volution with time of the gravitational
waves being produced.
Calculate with computers.
So supercomputers in fact, because this calculation,
this simulation is very, very hard to do and
you need supercomputers to do it.
And some of these simulations may run even
for months at a time.
Um, so, but, but we have, we, they're all,
we, this is actually not my own personal work,
but uh, relativitists have been able to do
these simulations for about a little over
10 years now.
So we can create templates of these kinds
of signals, and when we correlate them with
our data from the detector, we can then find
the best fitting template that then tells
us that that particular signal came from a
pair of black holes that is that far away
and the masses in this particular case was
about 20 or 30 solar masses, uh, plus or minus.
Of course, there's always errors in every
measurement we make.
So that was 2015, 2016.
So there have been discoveries since.
Can you give us a sense of what's been going
on the last couple of years?
Yeah.
So since that first one that shook our world,
and honestly, it shook humanity, if I may
say, because we, on that day of the announcement,
we were- Media people at all the universities
did the count, not ourselves, but on that
one day we were on front covers, more than
900 newspaper front covers across the whole
world.
Now we have announced another five or so more
collisions of black holes that we have detected
in our data.
And uh, another, a third gravitational wave
detector in Italy has joined a operation.
So now there's more confirmation of more of
these events, independent confirmation, and
now we're discovering a population of binary
black holes in the universe.
Now you've also gone beyond black holes.
Exactly.
And that has made, again, that's sort of the
second most significant detection, which is
two neutron stars.
Quickly tell us what a neutron star is?
Yes.
And we're gonna we're gonna.
I'm gonna tell you what a neutron star is,
and have you all in New York, look at the
Chicago skyline since Northwestern is in Chicago.
So, um, so this is actually, this is to scale
unlike his movie.
Uh, so this is the Chicago skyline.
Um, so a big city, you know, about 10 miles
across and the shadow you see hanging above
the Chicago skyline is the edge of a neutron
star to scale.
Now in neutron studies, the death remnant
of a star that maybe 10 times the mass of
the sun, uh, will end up forming when it runs
out of nuclear fuel.
So it will form, it will be about the death
remnant will be about one and a half times
the mass of the sun, and it will be about
as big as a big city downtown.
Okay.
That's roughly the scale.
You can imagine how big the circle is.
Um, thankfully no neutron star is hanging
above Chicago as we speak or above New York
City.
Uh, but that gives you some sense.
So on our second biggest discovery, uh, which,
um, reached us on August 17, 2017.
So we're approaching the one year anniversary
soon-
That was right around the eclipse.
And I was, uh, that was, uh, another memorable
day of course for many of us.
Um, so it was just a few days before the eclipse.
Now it used to be that the big event of that
month was going to be the eclipse.
And if this had not happened, I would remember
the date of the eclipse.
But right now I forgot the date of the eclipse
because what I remember is August 17th.
Um, so two neutron stars came together in
a similar fashion.
The two neutron stars came together and we
got another banana, except, uh, we, this time
we had three detectors.
So you see again, Hanford, Livingston, and
we had an Italian detector operating as well,
and we saw the two bananas in the two detectors.
The scale, you see the signal duration is
much longer, it's not longer, just a fraction
of a second.
What you see on the screen is about 30 seconds.
But actually in our data, this is what you
see visually here- But in our data we extracted
the signal lasted 140 seconds, so a couple
of minutes, so really long signal which tells
us that the masses that came together in this
death spiral were actually much smaller, so
about one and a half solar mas-
As opposed to 30 or so.
As opposed to 30.
So the lower the mass, the longer the signal,
the longer this death spiral lasts.
Now the third detector doesn't show a banana
partly because the third detector is not as
sensitive, and partly because it was at the
various spatial location on the sky that the
third detector didn't have good visibility,
let's say.
Um, and then, and then spectacular things
happened after the collision of the two neutron
stars, unlike two black holes which come together
in peace, form a single black hole and nothing
else happens after that, two neutron stars
actually gave us a whole set of fireworks
in electromagnetic waves.
So you could actually not only see them in
gravitational waves, you can see them.
Yes, you can see them in light.
In real light.
Uh, and that started a whole other type of
astronomy, multi-messenger astronomy.
Multi-messenger, meaning two types of waves
came out of the same source, gravitational
waves and electromagnetic waves.
And what, what have people learned from the
neutron star collision?
There's a lot of talk about new ways of thinking
about nuclear astrophysics.
Yes.
So we learned a couple of different things.
So first of all, in the electric- learned
things from these multi-messenger character
of the source.
So one thing is that the first thing we saw
in electromagnetic waves was a gamma ray signal.
This is the highest frequency electromagnetic
waves we can detect.
And we knew gamma ray, a short burst of gamma
rays existed.
We had detected them for many years, since
the late sixties, uh, but- and we had hypothesized
that maybe merges of neutron stars were responsible
for them, but we had no proof.
The proof came from the gravitational waves
because only in the gravitational waves we
can measure masses.
So the measurement that chirp the banana tells
us that it was two stars that collided.
So we've associated with- For the first time,
we had proved that two neutron stars collide
can give you a gamma ray burst.
So that multi-messenger combination proved
the origin of short gamma ray burst are due
to neutron star collisions.
That was one big discovery.
Yup.
The second was that the two neutron stars
came together and as the, uh, uh, neutrons
collided, they actually formed the heaviest
elements we know on earth.
So elements like gold and platinum, a lot
heavier than iron, we of course know they
exist.
We have them on our planet.
We have them in on our rings and earrings
or whatever.
I lost my wedding rings.
I used to have one of those.
Does your wife know?
She does.
It's a bit of a sore point, but let's move
on.
Okay.
Yeah, um, so, so we know they exist, but,
but actually astrophysically or physically,
we didn't know for sure, how they are formed.
They are not formed in the centers of stars,
that we knew.
And through nuclear reaction.
So a hypothesis was maybe they're formed in
neutron star collisions.
With this one event from August 17th, we got
the experimental proof through the electromagnetic
waves and gravitational waves telling us two
neutron stars collided.
We have now proved that gold was formed at
this one event, and therefore we solved that
mystery as well.
That's fantastic.
Congratulations.
Well, I take credit, not just myself, but
there is a collaboration of hundreds of scientists.
Absolutely.
So we're reaching the end of our segment,
but there's one other question or maybe one
and a half questions.
So have you received any data that doesn't
fit the templates or more generally have a-
How can you imagine testing Einstein's general
relativity in this extreme environment using
gravitational waves?
We always try to test Einstein.
Yep.
Okay?
Uh, we pushed that frontier.
He made- I mean his theory must breakdown
at some point because it involves the singularity
at the center of the black hole and we don't
like singularities.
That's where the math just breaks down.
Exactly.
That's where the division by zero doesn't
make sense.
So somewhere general relativity has to break
down.
With gravitational wave observations, we are
observing black holes moving at half the speed
of light, 60 percent of the speed of light.
This is the strongest regime of gravity we
have ever probed with anything.
Nothing else has probed that the regime.
So so far we have not seen anything in the
gravitational wave data that disagrees with
general relativity, but we keep pushing that
frontier, and maybe one day we'll see something.
So it's always at the back of our minds.
And.
And would you say that sort of would be the
culmination of this enterprise to, to push
the understanding of gravity to the next place?
The physicist in me might say that.
So that's always something we try to keep
testing that theory.
The astrophysicist in me wants to know about
these black holes and neutron stars.
How do they form?
What's their masses?
How can nature form these pairs of black holes
and neutron stars in such high numbers that
we see so many of them, so this is what we're
after on the astrophysics side.
Well, good luck with all those fantastically
interesting projects, and everybody please
join me in thanking Vicky Kalogera.
Thank you.
Thank you.
