Why Are There Giant Concrete Tunnels in the Desert?

This was going to be one of the next huge discoveries.

And then it happened.

And we're going to go where it happened.

[ELECTRICAL WHIRRING] [MUSIC PLAYING] WOMAN: 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 would 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 going to discover gravitational waves.

It could be in the next 10 years, it could be the next 50.

We don't know if this detector is going to 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 eight 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 is 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 minus 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.

WOMAN: Those liquid nitrogen tubes that go inside, that carry the liquid nitrogen inside.

DIANNA: Yeah.

And they were getting ice around those tubes.

And the ravens were coming out and pecking at that.

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?

AMBER STRUNK: This is one of the sections of the concrete structure, yeah, the beam tube cover.

The concrete tunnels house two 4-kilometer long tubes made of metal that are actually under vacuum, which is surprising, because the metal is pretty thin.

AMBER: This is the section of the beam tube itself.

DIANNA: Gotcha.

So that is holding the vacuum.

Whoa.

DIANNA: A powerful laser is shot out and split it 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 bypassing 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.

So we're trying to do everything possible to keep any particulates out.

Which is hairnet and shoe covers.

We're going into the experiment hall.

From what I remember, because I've been here before, this is the laser.

That enclosure over there is the laser.

Absolutely.

DIANNA: 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 they come back and merge here as well.

So down this way, we've come to where we've got-- The photo diodes.

This is where the gravitational wave signal is detected-- detected, yeah.

DIANNA: That was the photo diode that detected the actual first gravitational wave.

And I got to hold it, kind of.

I got close to it.

Wow.

They told me not to touch it.

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 because 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 4 kilometers, 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 2,000 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 4-kilometer-long arms.

But they're actually effectively 1,200 kilometers.

And the light bounces back and forth, on average, about 300 times.

So that makes the arm length not just 4 kilometers but effectively 1,200 kilometers long.

That's crazy.

And length matters here.

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 going to 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.

MICHAEL: Two black holes, take the first event.

You can tell from the waves on the ring down that this thing was 62 solar masses.

So 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 as light.

It's all coming out in gravitational waves.

That makes it this super exotic object.

Many people asking, can you just to set off a nuclear bomb and measure the gravitational waves there?

It's 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 give me my laptop?

Thank you.

So it says three solar masses, 5 times 10 to the 47 joules versus 2 times 10 to the 17 joules.

The number of nuclear bombs in this collision, about 2 times 10 to the 30.

That's 2 with 30 zeros.

OK, that's not even a number I can fathom.

Thank you.

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.

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.

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 that 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, in general relativity, it's effectively the Young's modulus of space-time, which in materials, Young's modulus tells you how compressible something is.

You could say, oh, here's 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 20 times, 20 orders of magnitude, stiffer than steel.

The numbers we're dealing with here are just unbelievable.

That's why we need black holes and neutron stars, stars exploding, the birth of the universe.

These are the kind of things that are actually going to 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 1/1,000 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 small.

I'm just lost.

You talked about breaking our brains, like the melting of our brains just thinking about scale alone.

I think it helps us understand our place in the universe 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, because 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.

Phew, LIGO is so freaking cool.

And the people at LIGO were so amazing.

I feel incredibly lucky to have been able to go back there, because I actually went once before for the Stellar episode with PBS, you may have seen.

It 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 going to make an entire video with just that interview so you can look out for that next week.

It's going to 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 physics-ing.

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