>>Janna Levin (Astronomer, Barnard College):
It’s so nice to be here.
I love this venue.
It’s such a romantic venue.
We’re literally under the planetarium.
I love it.
Yeah, I - this building has a special place
in my heart.
It’s really nice to see everyone come out
for science.
Woo, science.
Yeah, we believe in science.
Science is great.
So, I wanted to talk tonight about this recent
gravitational wave discovery.
On February 11 of this year,
an otherwise obscure experiment made an announcement that sort of made the entire world
kind of freeze and pause for a second.
How many people heard around February
11 about the gravitational wave detection?
Okay, excellent.
It was an amazing moment.
I don’t even think that the experimentalists
expected the world to pay that much attention.
After about an hour, how many people felt
like they understood
what the gravitational wave detection was
about?
Okay, yeah.
[Laughs] Oh, yeah, this one guy.
This one guy did. [laughs]
Yeah, it’s very, very difficult to understand.
And so I want to spend tonight picking apart
what that discovery was,
why it was so monumental and why it was so
moving.
Even though very few people had heard about
LIGO before that day, it cost 50 years of
people’s lives.
Some of the original LIGO experimentalists
started this in the late ‘60s
when people didn’t even know if black holes
were real,
when people didn’t even know if gravitational
waves were real.
Einstein was arguing about the existence of
gravitational waves for decades.
He kept changing his mind.
He famously wrote a paper saying gravitational
waves do not exist,
and then between acceptance and actual putting
it to press,
he snuck in a completely different paper that
said that they did.
So, he was sort of all over the place.
And it’s an incredibly difficult subject.
So, imagine starting this experiment in the
late ‘60s, and now it’s 2016.
It’s the centenary of the year, or the day
that Einstein—or the year that Einstein
first proposed
the existence of gravitational waves, and
they make this detection.
So, I really want to spend some time talking
about what it all really means.
So, here it is.
We’re about to listen.
And this is already something you should feel
is strange.
We’re going to listen to the discovery.
In astronomy, we look at discoveries.
This time, we’re going to listen to the
discovery.
This is—it’s going to play twice.
It’s going to play the sound of the gravitational
wave recorded, and then it’s going to
increase it in pitch because the human ear
does not do well with the lower notes.
So, you’re going to hear it better the second
two times.
Okay, here we go.
[audio plays]
Want to hear it again?
[audio plays]
Okay, so in the second time, all that’s
happened is it’s been falsely increased
in pitch,
and you could hear it again.
Now, you should already be wondering
why this giant machine, which spans four kilometers
of which there are two on two different coasts
in the U.S.,
made a recording at all.
Why didn’t it take a picture like a telescope?
And so that’s what we’re going to spend
some time picking apart.
And why can we hear it?
It literally lands in the human auditory range,
so we’ll talk about that as well.
So, what was the gravitational wave discovery
about?
So, let’s first start with what we do know.
This is actually from the digital atlas here
at the American Museum of Natural History.
It’s a compilation of known observations
of the universe.
Essentially, every object that has ever been
observed is placed in this atlas.
And then you can drive around the atlas if
you want, kind of like Google Maps, only bigger.
And here is a map.
This is not a cartoon or a simulation.
It’s a map showing what our own galaxy looks
like.
This is the Milky Way galaxy.
It’s a collection of 100 billion stars.
We have never gotten this far outside of the
Milky Way galaxy to take a look at it.
This is some hundreds of thousands of light
years from our current location.
We have never been able to get there.
Where we live is inside the Milky Way.
So, here we are.
Let’s go and figure out where we are in
the location of the entire universe
before we figure out how we observe it.
We’re coming inside into the spiral arms.
New Yorkers are always very offended to find
out
that they actually live in the galactic suburbs.
I know that’s terribly disappointing.
[Laughter]
But it’s better than the center of the galaxy
because there’s a supermassive black hole there.
And we are slowing spiraling into that supermassive
black hole,
but it’s 26,000 light years away.
It will take a very long time.
There’s our sun.
It’s quite an average sort of yellow star.
We have gotten not even this far outside of
our solar system.
Well, here we have.
We have, we have.
Voyager, which was launched in the ‘70s
just broke out of the sun’s magnetic influence
and is basically interstellar after decades.
And that’s the farthest we’ve flung anything
in the universe.
Here we are on the third rock from the sun.
It’s a very nice place, you may have heard.
Good real estate.
[Laughs]
And you just saw the great view we had of
the galaxy.
Okay, here we are.
Most of us are bound to this rock.
Very few of us have left this rock.
Most of our satellites orbit this rock.
Very few things make it away from the earth
or outside of the solar system.
So, everything we know about the universe
comes to us from light.
And that’s already stunning.
It comes to us as we receive light sent to
us across the universe.
And because it takes light traveling to us
some time,
the further away we see something, the further
in the past we’re looking.
So, we have a map not only of a universe 90-some
billion light years across,
but nearly 14 billion years old.
And we get it all by taking pictures.
Since Galileo first pointed a telescope at
the sky, we’ve gotten this kind of silent movie.
And what we’re doing differently now is
trying to detect something about the cosmos
in complete darkness; in the absence of light.
And to do that, we really use gravity.
This is another, again, digital atlas.
It’s not a cartoon.
Every object you see in this digital atlas
is an entire galaxy.
So, there are as many galaxies in the observable
universe as there are stars in the Milky Way.
There are hundreds of billions of galaxies,
each one with hundreds or more billions of stars.
And although this is a lot of stuff that we’ve
seen, most of the universe is actually dark.
Ninety-five percent of the universe is not
luminous at all.
It will never send us light.
We will never see it with a telescope.
The cosmos is largely darkness.
We’re lucky we can do astronomy now because
the future is getting even darker.
So, here we are—and I didn’t mean that
as a political statement or anything.
[laughter]
Don’t read too much into it people.
So, here we are.
What do we do with a universe that is largely
dark when we cannot take pictures of the sky?
We want to use pure gravity.
And that’s where Einstein comes in.
So, Einstein proposes his most radical, most
important idea.
It’s finally written down beautifully in
1915.
Did I say 1916?
I lied.
Nineteen-fifteen.
It’s all right, you can look it up.
Einstein once said he didn’t know his phone
number
because why memorize something you could look
up.
So, that’s kind of how I feel about some
of these things.
So, in 1915 Einstein writes down his most
radical idea: the general theory of relativity.
And in this idea, he expresses his entire
theory of spacetime.
He says—and we’re going to say it in one
sentence very simply—
that mass and energy, like the sun, curves
space and time around it.
So, things fall under natural curves in space.
If you think about, if I was floating in empty
space
and I were to throw my clicker, what path
do you imagine it would take?
I mean, totally empty space; no earth, no
nothing.
What path do you imagine it would take?
Straight line.
Because what else is it going to do?
Straight line.
If I throw the clicker in this room, what
path is it going to take?
You again.
It’s going to go down.
It is literally tracing for you a curve in
the shape of spacetime.
It’s a stunning idea.
It’s an absolutely stunning idea.
Mathematically it’s very hard, but the intuition
is obvious once you realize that
it’s my hand that’s in the way of gravity
right now.
And once I let go, it’s going to trace for
me the natural curves in spacetime.
And so when the International Space Station
is orbiting the earth,
it is falling freely along a circular curve
in spacetime.
It’s absolutely falling.
It’s just thrown so fast that it always
clears the horizon.
It never crashes into the surface of the earth.
But it’s actually the astronauts in the
space station are falling.
That’s what they’re doing all the time.
So, there’s Einstein’s theory in a nutshell.
Not bad.
Now, you don’t have to go to graduate school
for five years.
Here’s the most extreme example of Einstein’s
theory.
It’s the black hole.
Here’s my portrait of a black hole.
[laughs]
So, in the same year that Einstein publishes
his great theory, Karl Schwarzschild,
who’s an infantry soldier during the war
on the Russian front,
is between calculating cannon fire trajectories.
Starts reading the proceedings of the Prussian
Academy of Sciences, as you do,
and solves for the curved spacetime around
a mass, imagining—
pretending really, just as a fantasy—that
all the mass is crushed to a point.
It’s not a physical or real solution.
It’s just an idea.
Imagine that it wasn’t the earth, but everything
was crushed to a point.
He writes down this solution.
It’s very obscure.
Einstein’s very impressed with it.
He helps him get it published, but Einstein
thinks these things will never form.
This is not reality.
Nature will protect us from such strange things.
The strangeness we now know as the black hole,
it didn’t earn its name until the ‘60s.
Nineteen-sixty-seven, I believe it earned
the name from the famous relativist John Wheeler.
So, here is a black hole.
This is not a cartoon.
It’s a mathematical model
by a physicist Andrew Hamilton who does these
stunning visualizations of black holes.
In this computer simulation, a black hole
about the size of our sun has formed.
And if you took the entire sun and made it
a black hole, it would be about six kilometers
across.
It would comfortably fit in Manhattan.
Six kilometers across the entire mass of the
sun.
And what do we mean by six kilometers across?
We mean that there’s a shadow cast because
the space is so strongly curved
that even light takes a path that always points
inward.
There are no paths that point outward.
No curves in spacetime.
Saying it another way, you would have to travel
faster than the speed of light
to escape from that shadow.
If you were to go up to the black hole, there’s
nothing there.
We have this myth that black holes are dense
crushes of matter.
Black holes are empty spacetime.
They’re like places more than they are things.
And here we’re seeing a little model of
the earth, which is self-illuminated because
the sun’s gone.
So, the little model of the earth is self-illuminated,
and notice it’s looking very warped as it
passes behind the black hole sun.
And that’s just an effect of the light following
bent paths as well.
So, all of this is about the bending of light.
We’re almost at gravitational waves.
So, here Einstein begins to think immediately,
look, if the sun and the earth
and these strange things, which don’t have
a name yet we now call black holes,
can curve the space and time around them,
surely when they move they curves have to
follow them.
And if the curves have to follow them, nothing
can travel faster than the speed of light,
so those curves must follow them at the speed
of light.
And what you create is a wave literally in
the shape of spacetime,
and the curves you fall along as the objects
move.
So, in this cartoon due to LIGO—courtesy
of LIGO—you see how the curves in spacetime
follow the object.
And if I have two objects moving, they create
a wave in spacetime.
Those are the gravitational waves that LIGO
detected.
They’re literally curves in the shape of
space.
If you were floating freely near the two black
holes as they collided,
you would kind of bob on the wave like something
floating on the ocean.
And it would be darkness, completely darkness,
but you would bob on the wave as it passed.
Now, we’ll talk about what was exactly detected
in a minute.
So, here’s how LIGO works, this crazy idea—imagine
it’s the late ‘60s.
Ray Weiss is a young professor at MIT.
He’s working on other projects,
but he has this mad idea to measure these
waves in the shape of spacetime
that people don’t even know are real.
People don’t even think black holes are
real.
Black holes are crucial for the conversation
because only the most cataclysmic events
can ring spacetime loud enough for anybody
to have any kind of hope of detecting it.
So, you need something like the intensity
of black holes,
which as they orbit each other are like mallets
on a drum.
And they will literally ring spacetime, emanating
these waves outward
while we wait to receive them here on earth.
Now, Ray’s idea was kind of like in this
cartoon.
Imagine spacetime is changing shape where
you are.
If you hang mirrors at the end points of this
L-shaped instrument,
and you bounce light along the L-shaped instrument,
the light will come back at different times
because it will have traveled different lengths
as the mirrors bob on the wave.
And in that way, you could record basically
the floating mirrors and the bobbing wave.
In some ways, LIGO is like the body of an
electric guitar recording the shape of the
guitar string.
If you pluck an electric guitar, it technically
doesn’t make a sound.
You attach it to the body of the electric
guitar, and it records the ringing shape
and plays it back through an amplifier.
This is how LIGO works.
It’s like a giant musical instrument.
As the mirrors bob on the wave,
LIGO records the shape of the bobbing and
it literally plays it back through a conventional
speaker system.
Bear in mind that these are billion-dollar
machines that they took 50 years to build,
and now a team of nearly 1,000 people work
on them.
That they span four kilometers.
When the light is going down the arms, each
one of these arms is four kilometers long.
And there’s two of these machines: one on
the coast of Louisiana
and one in Hanford, Washington.
So, when the experimentalists are in the control
room, they are literally listening to the
detector.
Let’s take a look at the real LIGO.
This is the real LIGO.
So, here are the long arms of the instrument.
This must be Hanford.
You can tell Louisiana because there are swamps
along the sides.
I swear, swamps.
There’s Louisiana, and there are alligators
in the swamp and mysteriously bass.
No one knows how the bass got there.
So, you see these four-kilometer-long arms.
We’re about to go into a cartoon
where we’re inside the tunnel.
There’s a laser that’s shot down this
tunnel,
and the light has to travel the four kilometers.
It’s a powerful laser.
This is a vacuum inside here.
That vacuum represents the largest holes in
the earth’s atmosphere.
There’s less stuff in the LIGO arms than
there are in regions of intergalactic space.
And these are the beer chambers, as they’re
sometimes fondly known.
There’s not actually beer in them, but there are the instruments, the mirrors, the suspension systems.
And this is what the instrument sounds like
in the control room.
[audio plays]
It just sounds like noise.
It sounds like noise unless a signal hits.
So, there is one instrument on the Gulf Coast,
one on the West Coast.
And if something happens in the universe—some
cataclysm event that rings spacetime—
these waves will travel and they’ll travel
all the way to the earth.
And they’ll strike the instruments, the
mirrors will bob, they’ll be recorded.
That’s the aspiration.
Now, in the year 2000 the first LIGO instrument
was built and listened for 15 years nearly
and heard nothing.
So, this was an incredibly successful technological
achievement in the year 2000.
And the skies were silent.
And they needed to build an advanced machine,
and then things like this happened.
The Hanford machine is on the same site as
the plutonium separation facilities
that were used for the original atom bombs.
And so there’s security patrols that drive
around,
apparently with a poor knowledge of geography,
[laughter] in the middle of the night no lights
or anything
and crashed into one of the LIGO arms.
Luckily, he didn’t puncture the vacuum.
If you ask the experimentalists what would
have happened,
they said we’d all go home.
It’s over.
The vacuum was drawn in 1998 and has never
been brought up to atmosphere since.
The vacuum is punctured, the experiment would
have been over.
That would have been it.
He broke his arm, but fortunately not the
LIGO arm.
[Laughs]
And it might have been like opening a vent
in space.
I don’t know how deadly it would have been
had he broken the vacuum for him.
It’s like opening a door on the space shuttle,
but the vacuum’s on the inside, not the
outside.
There was also an incident in Louisiana were
hunters started to shoot up the Louisiana machine.
They said by accident, but people didn’t
really buy that story.
One of the main experimentalists on the Louisiana
site said the Europeans just think we’re
so American.
Now, all we need is a hamburger incident or
something like that.
But we survived all of this.
The year 2015 comes.
They’ve taken out of the beer chambers,
which you can separately isolate from the vacuum,
and installed new components.
They basically replaced everything but the
nothing, everything but the vacuum.
And by 2015, the machine was operational.
That year in August, Ray Weiss, who’s now
in his 80s who’s onsite all the time,
said to me if we don’t detect black holes,
this thing is a failure.
If you asked anybody else on the ground when
or if we would ever detect black holes,
they said not until 2018, 2020.
Just don’t even think about it.
It’s not coming anytime soon.
Ray was pushing, pushing for the centenary.
He’s like I want it, goddammit.
Oh, Ray loves to swear, by the way.
He loves to swear.
I want it, goddammit.
I want the centenary of Einstein’s paper,
and pushing for it.
But even he started to think, oh, it’ll
never happen.
So, here it is, it’s September 13.
Ray’s onsite looking for radio interference.
And a lot of the experimentalists are interrupting
the machine.
They feel they’re not ready yet.
So, they decide to do tests.
They’re running these aggressive tests on
the machine,
postponing their science run.
And these tests really disturbed the machine.
They really basically ruin any potential for
detection.
But they just didn’t feel they were ready
yet.
It comes early Monday morning September 14
and in Louisiana these—
a lot of young graduate students and post-docs
working on this instrument at 4:00 in the
morning.
They get tired.
They decide to go home.
They put down their tools.
They’re done for the day.
Same thing happens in Washington state.
They were going to run all night, but they
just get fed up.
They leave the machines locked, though, in
observing mode.
Within the span of an hour, this signal, which
had been travelling for 1.3 billion years—
I know, it’s crazy—strikes Louisiana,
rings the machine there.
It’s beautifully recorded.
Middle of the night, nobody’s awake.
Nobody hears it in the control room because
it’s too fast.
It’s like seven or five milliseconds later.
It cruises across the continent and rings
the machine in Washington
and is beautifully captured there.
Eight-thirty a.m., Ray wakes up and looks
at the logs, as he always does,
and thinks what the hell is going on here.
What’s this?
And here’s what they detected that morning.
So, this is a computer simulation of two black
holes, each one about 30 times the mass of
the sun.
One was a little bit bigger than that.
So, they were big.
We were surprised how big they were.
One was maybe 29, one was maybe 35 times the
mass of the sun.
This is drastically slowed down, so that we
can watch it.
But we caught the final few orbits where these
two black holes,
which might have been orbiting for a billion
years for all we know,
are in their final few orbits.
These orbits took one-fifth of a second.
That’s what was caught by the instrument;
one-fifth of a second.
In that final one-fifth of a second, the ringing
of spacetime was finally loud enough for the
instrument to record.
They merged.
They formed one bigger black hole.
It’s about 62 times the mass of the sun.
It very quickly sheds off its imperfections
and goes quiet.
I think we have to see that again because
it’s so beautiful.
No telescope could see this event.
It happened in utter darkness.
And, again, it’s drastically slowed down.
This means that the original LIGO instrument
very well could have had these very gravitational
waves
passing over them when the black holes were
further apart, going slower,
but it was too quiet to detect.
It was only the final fifth of a second that
was loud enough because
what LIGO measured was a deviation in the
mirrors swinging over four kilometers
of less than 1/10,000 of the width of a proton.
And it was not until they could do that that
they could even catch that final instant.
So, there it is, that stunning result.
So, now what you should be listening to—
I’m going to play you two different things.
What you should be listening to when you’re
listening to the gravitational waves,
you should be thinking, oh, it’s like ringing
drum.
And the LIGO instrument’s like a musical
instrument that’s recorded the shape of
the ringing drum
and that’s why we’re listening to it,
just like you listen to a guitar in some ways.
And also you should be listening for the sweep
up.
The reason why it sweeps up is because the
black holes go faster and faster as they get
closer together.
And so the ringing sweeps up in pitch.
And we’re going to listen to two different
detections.
Because on December 22 there was a second
detection of two black holes colliding on
Boxing Day.
Two smaller black holes.
And so we’re going to listen to the Boxing
Day detection and, again, the original detection
from September 14.
And then we’re going to hear them again
scooped up in frequency—I mean, in pitch.
And the reason we do that is because the human
ear’s not so good at the low notes.
So, here we go.
The low notes are particularly bad on a computer.
[audio plays]
So is it going again?
Oh here we go, increased pitch.
[audio plays]
Who thought black holes would sound like that? [laughs]
It’s not what you expect.
But it’ll be your ring tone any day now.
So, when you think about how remarkable that
was, let’s think about that.
These black holes came from somewhere in the
southern sky.
Right now, they formed a big black hole that’s
dark and quiet.
We cannot see it with a telescope.
We cannot find it with a telescope and we
can’t hear it anymore.
And it’s out there moving away from us with
the expansion of the universe.
And when we start to ask ourselves how many
are out there, we’re already—
once LIGO’s operational all the time, we’re
going to start recording black hole collisions
monthly,
and now many neutron stars, other dead stars.
Maybe stellar explosions.
Maybe other kinds of collisions.
But out here, this was the Hubble deep field.
Again, every one of those objects is a real
galaxy.
And this is a patch of sky about a tenth of
the moon.
And this is how many galaxies we see.
In every one of these galaxies there might
be a billion black holes.
There’s a supermassive black hole, millions
or billions times the mass of the sun,
in probably every one of those galaxies.
And so we’re looking for those future—not
just when we’re going to be recording sounds
of space,
but maybe something we haven’t even thought
of yet.
I mean, that’s what we’re all really after
in truth.
Scientists love nothing more than the unexpected.
Being confirmed right after $1 billion is
kind of a drag.
[Laughs]
But with the first LIGO detection we already
made new discoveries.
Those black holes were huge.
We have never detected two black holes.
We’ve never detected black holes that were
completely dark.
We’ve never detected gravitational waves.
So, it was like a clean sweep.
But what we’re looking forward to ultimately
in the future in the life of the universe,
when you think about where we’re going—as
we will fall into black holes eventually.
It’s going to take a very, very, very long
time,
but eventually everything that can will fall
into black holes.
Eventually, all of those black holes will
evaporate, as Stephen Hawking told us they must
they’ll evaporate into radiation.
And the universe will expand and it will go
both dark and quiet,
which is why a friend of mine says we have
to do astronomy now.
[Laughs]
But I think what we have to look forward to
is really a remarkable future
and really what we’re all hoping for—LIGO
just came online, I think, this week maybe;
I’ve got to check in, to do more detections—is
what we’ve always gotten
when we've done astronomy,
which is something totally unexpected.
Thank you.
