DAVID GREENE, HOST:
This being summertime, our colleagues at NPR's science desk are going with the flow, with a series on waves.
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GREENE: Nice. Today, it is gravitational waves. Now, Einstein predicted that these waves must roll across the entire universe. But as NPR's Geoff Brumfiel reports, it took 100 years to actually find one.
GEOFF BRUMFIEL, BYLINE: In February, the announcement finally came, and it made headlines around the world.
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DAVID REITZE: Ladies and gentlemen, we have detected gravitational waves. We did it.
BRUMFIEL: The waves came from two black holes, which collided long ago. The ripples fanned out across the fabric of space and time, distorting the very dimensions we live in. Now, fact is most of us didn't notice. So how did scientists detect them? To find out, I went to see where they did it. It's just off Interstate 12, in Livingston Parish, La. To get there, you head through town, past the Gold and Guns pawn shop and up a country road.
There's a rusted, old pickup truck and another rusted, old pickup truck, even more rusted than the first one.
Turn onto an empty lane lined with pine trees, and keep going. Oh, I think I see some buildings up ahead.
This is the Laser Interferometer Gravitational-Wave Observatory That's kind of a mouthful, so scientists just call it LIGO. Physicist Joe Giaime is in charge here. He says measuring waves in space-time might sound complicated, but the basic concept is pretty simple.
JOE GIAIME: The thing we're measuring is length. Everybody kind of knows what length is.
BRUMFIEL: Because gravitational waves warp space, they literally change how long things are. And LIGO is basically the world's most complicated tape measurer. We walk up a little hill overlooking the machine. A drab, concrete pipe stretches off towards the flat Louisiana horizon. Giaime explains that this is one of LIGO's two arms.
GIAIME: And the other one is - juts off from the main building at a - at a right angle from the one we're on.
BRUMFIEL: So it's a giant L?
GIAIME: It's a giant L. That's right.
BRUMFIEL: When a gravitational wave passes by, one arm of the L gets a little shorter. The other one gets a little longer. The machine measures the difference. That's all there is to it, at least in theory. In practice, it's a lot tougher. By the time gravitational waves get to Earth, they stretch and shrink dimensions by less than a thousandth of the width of a subatomic particle. And on Earth, there are lots of bigger waves that can drown them out, like seismic waves from earthquakes. A big quake anywhere on the planet can set this whole machine shaking.
GIAIME: And so we just stop and wait it out.
BRUMFIEL: The machine is so sensitive, it can feel the vibrations from passing trucks, falling trees, even storms in faraway oceans. Overcoming all this background noise is the real trick to LIGO. And to see how they do that, we get into Giaime's car and set off on a road alongside the concrete tube. As we drive, he explains that inside the tube is a laser. The beam travels two-and-a-half miles to the end of the tunnel, where there's a mirror. Then, the laser bounces off the mirror and goes back.
GIAIME: So it's pretty boring what's going on here in the middle.
BRUMFIEL: The light's just going back and forth and back and forth, basically?
GIAIME: That's right.
BRUMFIEL: The laser light constantly monitors the length of the tube. We reach the end of the arm. There's a little building with a mirror inside. We head in. We have to wear hairnets and booties over our shoes. That's because these mirrors are perfectly adjusted and carefully controlled.
GIAIME: Contamination can ruin all those things.
BRUMFIEL: So basically don't touch anything.
GIAIME: Well, you won't get anywhere near it. Don't worry about that (laughter).
BRUMFIEL: We go through a second door. In this room, there's a big, steel tank. The mirror is inside.
GIAIME: The mirror weighs about 100 pounds, I guess.
BRUMFIEL: It hangs in a vacuum, isolated from the noisy world, just waiting for a gravitational wave to stretch the tube. The search started over a decade ago, but for a long time, LIGO didn't see anything.
GIAIME: Up until last year, you know, we would give tours here to little kids, who, at the end of the tour, would look us in the eye and say, you know, so what have you seen? What have you measured? And the answer is nothing - nothing yet.
BRUMFIEL: And Giaime began to get nervous. He had moments of doubt.
GIAIME: I can certainly say, personally, that I was wondering whether maybe there was some misunderstanding about - about what was out there in the universe.
BRUMFIEL: But he and the rest of the team kept at it, upgrading the lasers, tweaking the mirrors. And finally, on September 14 at 5:51 a.m., a wave passed through the detector. The whole machine vibrated, like a giant tuning fork listening to space and time.
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BRUMFIEL: That's it. That weird, little chirp is the space-time distortion changing the length of LIGO's tunnels. Moments later, an identical detector in Washington state picked it up, too.
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BRUMFIEL: That's all that's left of the massive wave created when the two black holes collided billions of years ago. It's small now, but at the moment of the merger, the power released was greater than all the stars combined. It truly was the biggest wave in the universe. Geoff Brumfiel, NPR News.
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