- This was gonna be, you know,
one of the next huge discoveries,
and then it happened,
and we're gonna go to where it happened.
(upbeat music)
There's this place in the Washington desert,
where giant concrete tunnels
run for miles out into the sand.
For 20 years, they protected a strange technology
that will become humanity's first glimpse
into an unseen universe.
Hey, I'm Dianna and you're watching Physics Girl.
Last month, we traveled to Hanford Washington
to see the site where gravitational waves
were first detected.
Because gravitational waves are one of my obsessions.
Gravitational waves are invisible inaudible waves
in space itself and time, rippling through the universe,
through earth, through us all the time.
It turns out space-time can stretch like a slinky
and can carry waves across the universe.
My professors told me, "we don't know
when we're gonna discover gravitational waves.
It could be the next 10 years,
it could be the next 50.
We don't know if this detector is gonna work."
And then when the announcement was made in 2015,
which was only like four years after I graduated,
I was like, "What?, No"
- That's so cool.
- Gravity is cool.
It's also weird.
It doesn't affect space instantaneously.
If the sun suddenly disappeared,
poof, the earth would continue on orbiting
as if the sun were still there,
for 8 minutes and 20 more seconds
until that information about
the change in the gravitational field
would get to us, and then earth would fly off into space.
But at that same instant light would reach us too,
and we would see that the sun has gone.
But you know what that means?
The speed of gravity and the speed of light are the same.
That's one of the many things that we've confirmed
down to the order of 10 to the -15,
from the detection of gravitational waves.
And those giant concrete tunnels,
those are the exoskeleton of two giant arms
of the first gravitational wave detector.
LIGO.
Gravitational wave detectors have to be huge
because they need to detect waves
coming from billions of light years away.
They're so sensitive that trucks nearby affect the signal.
When we were there, they told us
to accelerate and decelerate slowly,
so as not to disturb the detector.
And also the AC unit is not even connected to the building.
They had to put it in a separate enclosure on springs,
so that wouldn't mess with the signal.
And then there's this.
- [Amber] Those liquid nitrogen tanks have tubes
that go inside that carry the liquid nitrogen inside.
And they were getting ice around those tubes
and the ravens were coming out and pecking at that.
- Oh my God.
- And that little bit of vibration,
coupled back into the interferometer.
- But that insane sensitivity allowed us
to detect an entirely new type of phenomenon.
I should tell you how the detector works.
So this is like a cross-section
of what the cement tunnel would look like.
- This is one of the sections of the concrete structure.
Yeah. The beam tube cover.
- The concrete tunnels house two, 4 Km long tubes
made of metal that are actually under vacuum,
which is surprising because the metal is pretty thin.
- This is the section of the beam tube itself.
- Gotcha.
So that is holding the vacuum. Wow.
A powerful laser is shot out and split into two beams,
and each is sent off into the X and Y arms of the detector.
That's what they call them.
Then, depending on the distance those laser beams travel,
when they meet back up again, the waves add
and they interfere and make patterns;
like waves that overlap in water making patterns.
If space is stretched or squeezed
in one of the directions by passing gravitational waves,
the laser beam in that arm will travel a shorter
or longer distance, affecting how the two waves,
the two beams add up when they're recombined.
And we can tell space has been stretched
by a gravitational wave.
- Now, we're trying to do everything possible
to keep any particulates out.
- We've just hairnet and shoe covers.
(happy music)
We're going into the experiment home.
From what I remember, because I've been here before,
this is the laser that like,
that enclosure over there is the laser.
That infrared laser beam
comes out of that big building thing over there
and through here, and then
this is where it splits into the two beams.
- And then when they come back and they emerge here as well.
So down this way,
we've come to where we got
- The photodiods.
- This is where the gravitational wave signal is detected.
Detected, yeah.
- That was the photodiod that detected
the actual first gravitational wave.
And I got to hold it, kind of, like I got close to it.
Wow.
They tell me not to touch it.
Wow.
Now the next thing to consider is
where to put one of these detectors.
You can't just put it anywhere.
They have to be far away from earthquakes or people.
'Cause they move too much.
Some people can't stop moving.
You can't even have the laser going through the air,
because there's too much noise,
and just the gas in the atmosphere.
So the tubes of the detector arms
are completely evacuated of everything.
Plus there are other crazy considerations.
The tunnels are so long that they have to correct
for the curvature of earth.
- Over four kilometers at the earth,
drops about a meter and a half.
- The detector is so sensitive,
it's affected by quantum fluctuations,
which is essentially randomness
caused by quantum mechanics in the mirrors.
Some of the problems that arise from noise
are solved by having an identical detector,
nearly 2000 miles away doing the exact same thing.
There's another LIGO in Livingston, Louisiana.
And when a gravitational wave passes through both detectors,
the signals line up perfectly.
And one more thing,
both detectors have four kilometer long arms,
but they're actually effectively 1200 kilometers.
- And the light bounces back and forth
on average about 300 times.
So that makes the arm length, not just four kilometers,
but effectively 1200 kilometers long,
and length matters here.
You need, the longer the arm,
the larger the overall effect
that the gravitational wave has on the arms.
- You know a clever way to get an even bigger detector?
Put it in space. This is a real plan.
I was like, "how are they gonna
launch vacuum tubes long enough up into space?
Isn't that hard?"
And then they politely reminded me
that space is a vacuum.
Excellent point space scientists.
Now, what kinds of things do you see with these detectors?
Oh, just super violent events
like neutron stars smashing into each other.
The first detection was two giant,
enormous black holes colliding
1.3 billion light years away.
- Two black holes take the first event.
You can tell from the waves on that,
on the ring down that this thing was 62 solar masses.
So three in that process,
three times the mass of the sun
were converted into gravitational wave energy.
At the time, it was the most powerful astronomical event
known to human beings because of that conversion.
Three times the mass of the sun in a quarter second,
that outshines all the stars in the known universe
for that quarter second, none of it's coming out in light.
It's all coming out in gravitational waves.
So that makes it this super exotic object. Right?
Many people asking, you to set off a nuclear bomb
and measure the gravitational waves there,
still a paltry amount compared to what you need
to actually see them.
- You heard him right.
A nuclear bomb puts out an insignificant meager
amount of energy compared to what is measurable
with gravitational waves.
I wonder how many nuclear bombs this actually is.
Can you hand me my laptop? Thank you.
So, it's three solar masses,
five times 10 to the 47th Joules
versus 2 times 10 to the 10th, 10 to the 17 Joules.
The number of nuclear bombs in this collision,
about 2 times 10 to the 30th.
That's 2 with 30 zeros.
(Dianna laughs)
Okay.
That's not even a number I can fathom. Thank you.
(Dianna Laughs)
- The only reason they're so small when they get to us,
is they've been traveling through the universe
for a billion years. When they first happened,
they were very concentrated here, right?
And as they start radiating outward,
they don't attenuate as they go through matter.
But as it goes out, you're now covering a bigger volume.
The whole wave together
would have the same amount of energy.
- Right.
- But the amplitude has to go down
because it's been spread out so much more.
So by the time it comes to us a billion years after
the collision, they're exceedingly small.
One of the reasons of the waves are so small
is that space is really, really stiff.
It resists being stretched by a lot.
It's the stiffest strongest stuff thing.
I'm not really sure what word to use to describe space,
but it's the stiffest medium that we know of.
- If you look at, what is the in general activity?
It's effectively the Young's modulus of space time.
- Yeah.
- Which in materials, Young modulus tells you
like how compressible something is.
You could say, "oh, here's the link
Young's modulus of space-time.
And let's compare that to Steel."
The Young's modulus is kind of a funny number
to use as a reference for the stiffness of space,
because it's usually a number that describes
how stiff materials are. So for example,
if you compared space to steel,
space is 10 to the 20 times
20 orders of magnitude stiffer than steel.
The numbers we're dealing with here are just unbelievable.
- That's why we need, you know,
black holes and neutron stars,
stars exploding the birth of the universe.
These are the kinds of things
that are actually gonna create vibrations in space,
that will be large enough in amplitude for us to see.
- That first collision event
was 1.3 billion light years away,
which means that it took 1.3 billion years to get to us.
That's why, when it finally got here,
it only changed the length of one of the arms
of the detector by 1/1000th the width of a proton.
If you can wrap your head around how small that is,
then you're saying to yourself right now,
"no, no, that's not possible. That's too small."
And if you can't wrap your head around how small that is,
congratulations, you are human.
- It's so mall. I'm just lost.
- You talked about breaking our brains, right?
Like the melting of our brains,
just thinking about scale alone.
I think it helps us understand
our place in the university too
and then helping people see
that it's this beautiful science
that explains so much.
- Yeah.
- I couldn't ask for a better job.
- I relate so hard. I relate to all of this.
- Since the first collision,
LIGO has detected 30 more
collisions of black holes, neutron stars,
violent events, powerful enough
to create gravitational waves that we could detect.
And we've learned such incredible things
from these collisions.
'Cause this is a new type of astronomy.
We're able to learn that when two black holes collide,
which are usually very spherical objects,
they have a ring-down phase
where they turn into this lumpy weird thing
that rings down like a bell.
And what we see at the end of the signal is that ring down.
We can learn something about a phenomenon
that we would never be able to see
with our current technology with a telescope.
Ooh, LIGO is so freaking cool.
And the people at LIGO were so amazing.
I feel incredibly lucky
to have been able to gone back there.
'Cause I actually went once before
for the stellar episode with PBS,
you may have seen that, went out a couple of months ago.
But this time I brought a bunch of your questions
from Twitter and asked them to Dr. Michael Landry,
who is the head of LIGO Hanford.
And I was so fascinated by his answers
that we're gonna make an entire video
with just that interview.
So you can look out for that next week.
It's gonna feed all of your curiosity,
but mostly my curiosity on gravitational waves,
and LIGO, and space and more.
Check back in for that next week.
But in the meantime,
thank you so much for watching and happy physicsing.
(happy curious music)