Gravitational Waves : Short Wave Science correspondent Nell Greenfieldboyce gives us the latest in gravitational waves and shares what scientists have learned (and heard) from these tiny ripples in spacetime.

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Gravitational Waves: Unlocking The Secrets Of The Universe

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Hello, SHORT WAVE voyagers. Today, we are chatting with NPR science correspondent, queen of the stars and planets, Nell Greenfieldboyce.

NELL GREENFIELDBOYCE, BYLINE: Oh, what a title. So listen. Emily, when you think of astronomers, what tool do you think of?

KWONG: Obviously, a pair of old-timey glasses. I'm kidding. I'm kidding. I think of a telescope.

GREENFIELDBOYCE: Absolutely. Of course, a telescope. I mean, that has been true ever since 1609, when Italian astronomer Galileo pointed a telescope up at the sky. And he saw the mountains and craters of the moon, the four largest moons of Jupiter. Ever since, astronomers have been building ever bigger and more fancy telescopes. But they all do the same thing, basically, which is capture light from distant objects.

CHASE KIMBALL: Throughout all of human history since we've been looking up at the stars, we've been gathering information just based off of the light that we see.

GREENFIELDBOYCE: So that's Chase Kimball, and he's a graduate student at Northwestern University. And he's just one worker in this big astronomy revolution going on right now that involves thousands of scientists. He says it's like people have always been watching the sky like it's a silent movie, and now they finally figured out how to switch on the sound.

KIMBALL: So we're getting, you know, brand-new information that we would've never gotten just from telescopes alone. We had previously just been, you know, watching the universe. And now we can listen to it.

KWONG: I'm super intrigued by this. So what does Chase mean by listen to it?

GREENFIELDBOYCE: All right. Let me give you just a little example.


KWONG: It sounds like a wind tunnel or water in a cave. What is this?

GREENFIELDBOYCE: That is two black holes colliding.

KWONG: Get out. What? No. How did anyone manage to record that?

GREENFIELDBOYCE: OK, so when big things collide out in space, they make waves in the very fabric of the universe. And these are not sound waves, but they're similar enough that the researchers can convert them into sound waves that we can hear. So, like, here's another one.


KWONG: (Laughter) That sent a weird feeling up my spine.

GREENFIELDBOYCE: That one is the so-called chirp of two neutron stars coming together. So neutron stars have more mass than our sun and are heavier, but they're only, like, the size of a city. So those two spiral towards each other and smash together. And the commotion made waves known as gravitational waves. And gravitational waves are ripples in the very fabric of space itself. Like, if you drop a pebble in a pond it makes waves, little ripples. This is the same idea.

KWONG: OK. Well, today, we take on gravitational waves - how scientists finally managed to catch them, what they're revealing about the universe so far and what astronomers could learn as they make their mind-boggling technology ever more sensitive. This is SHORT WAVE, the daily science podcast from NPR.


KWONG: OK, Nell, so I've been wanting to talk about gravitational waves with you for literally years. These are ripples in space-time. When I think of space-time, I think of Albert Einstein.

GREENFIELDBOYCE: OK, so first of all, literally years - like, you could just call me any time and talk about gravitational waves. Albert Einstein is a good place to start because he was the first one to come up with the idea of gravitational waves back in 1916. So, you know, this idea that space-time is a combination of space and time - it's the very fabric of the universe. Einstein's idea is that, you know, you can think of it a little bit like a trampoline. So if there's a disturbance somewhere, like someone jumping up and down on the trampoline, you'd get these waves rolling through, and space would be, you know, stretching and compressing just like the trampoline would.

KWONG: It's Einstein's trampoline, and we're all just bouncing on it, you know?

GREENFIELDBOYCE: You got to hand it to Einstein. He had some doubts about gravitational waves. He actually went back and forth on them. And other scientists thought OK, even if they exist, even if they're real, the effect of them would be so small, it would be impossible to ever detect.

KWONG: Yeah, I hear that. So how small are these ripples, actually?

GREENFIELDBOYCE: Really, really small. And they were eventually detected a century after Einsteins prediction on September 14, 2015. And those waves that they detected were generated by two black holes that merged to form a bigger, single black hole about a billion years ago. So they came together. The waves spread out. They traveled for more than a billion years. They finally reached Earth, stretching and distorting space. And they did that, you know, stretching and distorting by about one-one-thousandth the size of a proton. You know, and this moved mirrors inside a detector.

KWONG: That's itty-bitty. We're talking about space stretching by a fraction of the width of a subatomic particle.

GREENFIELDBOYCE: That's exactly right, yeah.

KWONG: But what detectors are so sensitive that they're even able to pick this up. You said they somehow use mirrors?

GREENFIELDBOYCE: Right - mirrors and lasers.


GREENFIELDBOYCE: So this is the sound of a shoe-scrubbing machine at the Laser Interferometer Gravitational-Wave Observatory. So that's the name of two identical massive detectors. They're funded by the National Science Foundation. They were conceived and built by MIT and CalTech. There's one in Washington state and one in Louisiana. They're identical. I went to the one in eastern Washington, which is kind of in this desert area. Mike Landry is in charge there. And so, you know, he had me clean my shoes in this scrubber. And then we put on booties. And then, you know, we went into this warehouse-sized clean room.

MIKE LANDRY: So - but we're just transitioning into the main experimental hall, the laser vacuum equipment area.

GREENFIELDBOYCE: You walk in and immediately see a huge metal tube that stretches off into the distance. And it is 2 1/2 miles long.

KWONG: Two and a half miles long? That's huge.

GREENFIELDBOYCE: Yeah. It sort of heads out of the building in a pipe. And there's another tube just like it the same length. They're joined together at a 90-degree angle. So if you were in an airplane flying above this place looking down, the whole thing looks like a giant capital letter L.

KWONG: OK. I'm picturing this, these two tubes. And what are they for?

GREENFIELDBOYCE: So the tubes are for the lasers, Emily Kwong, the lasers. So you get a laser. A laser is sent into this contraption. And the first thing that happens is that it hits a beam splitter. Half the laser beam goes down one arm of this L, and the other half goes down the other arm.


GREENFIELDBOYCE: And there's a mirror at each far end of the arm. So the light travels 2 1/2 miles down, bounces off the mirror, comes back. The light from both arms gets merged.


GREENFIELDBOYCE: And if a gravitational wave rolls through, stretching and compressing space and moving the mirrors...


GREENFIELDBOYCE: ...This setup can detect that. It can detect really subtle changes in the lengths of the arms and how far the light has traveled.

LANDRY: So we don't make an absolute measurement of the length of the arms. But if you send light down one arm and reflect it off the mirror at the end station and the same in the other arm and then combine that light, you have a real sensitive measure of the change in the relative length of the arms.

KWONG: Wow. That's amazing.

GREENFIELDBOYCE: It is really small, this change. It's like they're detecting the space between the sun and the earth lengthening by, like, the width of one hair.

KWONG: This is totally outrageous science. This is so cool. But how can they be so precise? Because it seems like almost anything could throw a measurement like that out of whack.

GREENFIELDBOYCE: It is a constant struggle. The control room there is filled with monitors to keep track of everything that might influence their equipment, like seismic activity.

KWONG: Oh, you mean like earthquakes?

GREENFIELDBOYCE: Yeah, and way more subtle ground movement. Amber Strunk works there at LIGO, and she says the ground is constantly shaking in ways that we humans cannot even perceive when we're just standing there.

AMBER STRUNK: So we use lots of suspension systems to reduce that ground motion. But then you have big ground motions like earthquakes. Anything over a 6.0 anywhere in the world will knock us out of alignment, and we can't take data for approximately an hour.

KWONG: That's a bummer.

GREENFIELDBOYCE: They told me the innards of the detector have to be about 10 billion times quieter than just everyday ambient ground motion.

KWONG: Wild. And what about other stuff that can move things, like the wind?

GREENFIELDBOYCE: The wind is a real problem out there.

STRUNK: And it rocks the buildings, and that rocks the mirrors. And so if the wind is high enough and we get a lot of wind out here, it can knock us out of lock. It can stop us from observing purely from the wind.

GREENFIELDBOYCE: And then there's just random problems you wouldn't expect. Like, one time they were trying to track down some weird noise in the data, and it turned out to be ravens pecking at some ice on pipes. Another time, they had an issue in the data, and it turned out to be the rumble of water being periodically released from dams on the nearby Columbia River. And then Landry told me, you know, they just have to think about the Earth's tides.

LANDRY: You know, every 12 hours, the solar and lunar tides stretch the crust of the earth between here and the end station by plus or minus 100 microns.

KWONG: And by micron, you mean one-millionth of a meter.

GREENFIELDBOYCE: Right. So that's, you know, a huge problem for them. They have to deal with that. Otherwise, that alone would swamp out any gravitational wave signal. And so, you know, it just goes on and on. When I visited, the detector was offline because they're doing some upgrades to make it even more sensitive. You know, they're going to basically try to cancel out noise from complicated quantum effects involving the laser and the vacuum.

KWONG: Got it. You know, I had heard there was a ton of skepticism we'd ever detect gravitational waves. But now I totally see why. Using mirrors to measure a pulse from a long time ago and all - it's amazing this ever happened.

GREENFIELDBOYCE: The National Science Foundation really persevered with this over decades. It was controversial. People said, you know, maybe spend all this money on a telescope, where you know it's going to work. But then, you know, when they made the first detection in 2015, it was such a big deal that it almost immediately won a Nobel Prize. And now there's gravitational wave detectors in Italy and Japan. There are plans underway for even more massive ones, with lasers going down pipes that are almost 25 miles long.

KWONG: So how many gravitational wave events have been detected since the first one in 2015?

GREENFIELDBOYCE: More than 50. So it's mostly been a bunch of black holes merging together. They've seen two cases of neutron stars smashing together. And at the end of June, researchers announced that they'd seen a couple of cases of black holes eating neutron stars, like, in one gulp.

KWONG: Wow. And I'm gathering that they can tell what's what by analyzing those signals, like the chirps we heard at the top.

GREENFIELDBOYCE: That's right. That's right. Landry says each time they improve the detectors, the pace of discovery just gets faster and faster. So they've gone from detecting one gravitational wave event a month to getting one every six days. And when they come back online next summer after these upgrades, he expects they'll be detecting about one event every day.

KWONG: Holy cow, Nell. OK. I mean, now that they're really figuring out the science, what can we learn about the universe by observing these events?

GREENFIELDBOYCE: Well, it's a way to study some of the most extreme powerful things happening in the universe that, previously, astronomers could never even hope to explore. Like, take black holes. They famously have such strong gravity that it sucks in everything, even light. And then, you know, weird stuff like neutron stars, these really odd ball stars - scientists think neutron stars might be slightly asymmetrical. Like, they might have little bumps on them. And, you know, as they spin around, that might make them put out a continuous stream of gravitational waves.

You know, or maybe you could have a black hole that's eating a neutron star and kind of rips it apart, you know, as it's eating it. And that might give off some light that people could see if they got a gravitational wave warning and we're able to point their regular telescopes at just the right spot. I mean, there's other possibilities, too. So, you know, maybe they'll detect waves from something like a supernova, you know, a massive star exploding. That would be, like, a big cymbal crash of gravitational waves.

KWONG: So they'll see more of the same, but they're also hoping to catch some new phenomena, it sounds like.

GREENFIELDBOYCE: Yeah. And the exciting thing is when you've got a new instrument, you know, a brand-new way of looking at things, you don't know what you might detect that you never even thought of because until now, you just weren't able to look at the universe in this way.

KWONG: Well, Nell, next summer, if they see something weird and new, please come back on the show and talk about it. Thank you for bringing us up to date on where we are with gravitational waves so far.

GREENFIELDBOYCE: Yeah. You don't have to, like, wait around for years, Emily. Just call me. I'll talk about gravitational waves any time.

KWONG: OK. Great. Great. Got you on speed dial.


KWONG: This episode was edited by Gisele Grayson, produced by Indi Khera, who also fact-checked, and Thomas Lu. The audio engineer for this episode was James Willets.


KWONG: I'm Emily Kwong. You're listening to SHORT WAVE, the daily science podcast from NPR.


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