What Does a Black Hole Collision Sound Like?

For centuries, the primary way that astronomers studied outer space was through sight. But just ten years ago, scientists successfully established a way to ‘listen’ to our cosmos – detecting gravitational waves created by huge cosmic events that took place billions of light years away. NPR science correspondent Nell Greenfieldboyce https://www.npr.org/people/4494969/nell-greenfieldboyce explains how scientists detect those gravitational waves, what kind of cosmic events we’re detecting now, and what they could tell us about our universe. Interested in more stories about the cosmos? Email us your question at [email protected] mailto:[email protected]. Listen to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave http://plus.npr.org/shortwave. See pcm.adswizz.com https://pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences. NPR Privacy Policy https://www.npr.org/about-npr/179878450/privacy-policy

Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.

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• 0:12Regina Barber?

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• 0:36Speaker 02

  You're listening to Short Wave from NPR.

• 0:40Regina Barber?

  Hey, short wavers. It's Regina Barber, and this week marks a very special anniversary in the world of physics. It's the 10th anniversary of a chirp, the chirp heard around the world.

• 0:53Regina Barber?

  Did you hear it? I'll play it for you again.

• 0:58Regina Barber?

  That is a gravitational wave. Physicists have converted it to sound waves so our ears can hear it. It may sound cute and maybe like a sonogram, but don't be fooled. Gravitational waves are produced by some of the most extreme violent events in our universe, like colliding black holes or two neutron stars smashing into each other. And the existence of these waves was first predicted by Albert Einstein over a century ago.

• 1:23Speaker 03

  Yeah, but he thought no one would ever detect them.

• 1:26Regina Barber?

  That's NPR science correspondent Nell Greenfield. Boys, hey, Nell.

• 1:29Speaker 03

  Hey, there.

• 1:29Regina Barber?

  So I know you've been covering gravitational waves for a long time now. I remember actually, right after LIGO was built, LIGO stands for Laser Interferometer Gravitational Wave Observatory. That's what's been finding these gravitational waves. It was built in 1999, and like, people from the observatory would come to my university because we were kind of nearby, and they'd give talks. And they were so excited. They said that they would be detecting gravitational waves very soon. But it wasn't soon. It took over a decade.

• 1:56Speaker 03

  Yeah, I mean, they were at this for a long time.

• 1:59Regina Barber?

  So long.

• 1:59Speaker 03

  It was amazing, actually, that the National Science Foundation, which funded this, kept it going.

• 2:03Regina Barber?

  Yeah.

• 2:04Speaker 03

  Eventually, you know, they detected them on September 14th, 2015. It was a huge, huge deal. And pretty soon after, some of the key people won the Nobel Prize. It's hard to believe it's been 10 years.

• 2:16Regina Barber?

  Yeah, no, I remember the excitement when it happened. And I like to say that a gravitational wave is like a ripple in reality itself. But maybe that's not very helpful. So what do you, what do you think is the best way of explaining like what a gravitational wave really is?

• 2:30Speaker 03

  Huh, a ripple in reality? I mean, that's kind of what it is. I mean, reality in terms of like space and time, which I think people sort of experience as these sort of solid, you know, sort of things, like you knock on a table, it's solid. Or like the earth is solid.

• 2:44Regina Barber?

  The moon is solid.

• 2:45Speaker 03

  But, you know, in fact, things are much more jiggly than that.

• 2:49Regina Barber?

  Yeah, I mean, I like to think of spacetime being viscous, maybe like the stuff that like kids make at home with cornstarch. Some call it flubber.

• 2:57Speaker 03

  Flubber. I don't think I've ever made flubber. I mean, I think of it more like Jell-O.

• 3:01Regina Barber?

  Okay.

• 3:01Speaker 03

  Right? Like sort of jiggly, jiggly. And, you know, you can poke it and it jiggles. And in the universe, when you have extreme things, like big things that go boom, that can send out shock waves through spacetime itself. And those shock waves or jiggles, like those are gravitational waves. And they, they propagate out, you know, from the smash up, sort of like ripples in a pond. You know, you throw the pebble in and it disturbs things and the, and the ripples come out. And I was talking to Max Essi, he's at Columbia University. And he was telling me, you know, these waves, they don't just move through, you know, stuff outside of us, like the earth. I mean, they actually go through us, like our own bodies.

• 3:39Speaker 04

  So the effect of a gravitational wave is to stretch and squeeze distances as that gravitational wave goes through. So at one, at one moment, it makes me taller and thinner. The next moment, it makes me shorter and fatter.

• 3:53Speaker 03

  But we don't notice, of course.

• 3:55Regina Barber?

  Yeah, because that effect is like so small, right?

• 3:58Speaker 03

  Yeah. I mean, to detect this really small effect, researchers had to build these massive instruments. So there's these facilities in Washington State. There's one in Louisiana. And basically, you know, they work by sending lasers down these long pipes. I mean, I'm talking like two and a half miles long.

• 4:14Regina Barber?

  Yeah.

• 4:15Speaker 03

  And, you know, if a gravitational wave rolls through stretching space, these facilities can detect it. I mean, they detect a tiny, tiny change in the distance traveled by the lasers.

• 4:27Regina Barber?

  So a tiny, tiny change, like how tiny are we talking about?

• 4:30Speaker 03

  Like a fraction of the width of a subatomic particle.

• 4:34Regina Barber?

  Wow, that is so small.

• 4:37Regina Barber?

  Today on the show, 10 years of detecting gravitational waves. We plunge into the extreme cosmic events that produce these waves and what scientists are learning about them. You're listening to Short Wave, the science podcast from NPR.

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• 6:15Regina Barber?

  Okay, now, let's get into this time machine. Let's go back 10 years, back to the chirp. What was the violent event that made those particular gravitational waves, those first ones that were ever detected?

• 6:27Speaker 03

  So that was two black holes coming together. So each of these black holes was about 30 times the mass of our sun, and they basically circled together and circled closer and closer and closer. And then, looop, they finally merged into one.

• 6:41Regina Barber?

  Yep.

• 6:41Speaker 03

  A new black hole.

• 6:42Regina Barber?

  Yeah.

• 6:42Speaker 03

  And so this event took place over a billion years ago, like 1.3 billion years ago. And it took that long for the gravitational waves to reach Earth and trigger the detectors.

• 6:52Regina Barber?

  That's so cool. You know, black holes, they're hard to study, right? Because they're basically invisible. Their gravity sucks in everything that's close to them, including light. And I remember when I was in grad school, professors said, "Light was the only thing we astronomers use to study the universe." So these gravitational wave detectors, they're completely different. They're a completely different tool.

• 7:15Speaker 03

  Exactly. Some people have compared it to listening to the universe.

• 7:18Regina Barber?

  Yeah.

• 7:19Speaker 03

  Instead of looking at it.

• 7:20Regina Barber?

  I like that.

• 7:20Speaker 03

  You know, I mean, looking at it with telescopes has been the mainstay of astronomy since the days of Galileo, right? So hundreds of years, they've been using telescopes to look at light. And this was a huge, huge change.

• 7:31Regina Barber?

  Yeah. So going into this, like, when they were building these detectors, what kind of extreme cosmic events were astronomers listening for? What did they think they would find?

• 7:42Speaker 03

  So I asked Gabriela Gonzalez that. She's a gravitational wave researcher at Louisiana State University, who was working on this project, you know, for a long time. And I asked her, you know, "What did you expect when these things were being built? These detectors, you know, what would they catch?" And she said, "Well, they thought they would probably hear collisions between two neutron stars." So neutron stars are these super-dense stars that are kind of like small. They're the size of a city, but they're, you know, a star, a massive star.

• 8:09Regina Barber?

  Yeah.

• 8:09Speaker 03

  Pairs of black holes that orbited each other? Like, that wasn't the focus.

• 8:14Speaker 05

  Wow. We knew very little about them. We didn't really expect to see them, certainly not so soon. But since then, it's almost the only thing we have seen.

• 8:24Speaker 03

  Over the last 10 years, they've detected hundreds of pairs of black holes merging.

• 8:28Regina Barber?

  Wow, that is so many black holes.

• 8:31Speaker 03

  Yeah. I mean, there have been a few occasions when the gravitational waves came from something else, right?

• 8:36Regina Barber?

  I do remember that, yeah.

• 8:37Speaker 03

  There was this one beautiful collision between two neutron stars and... They sensed it and were actually able to point telescopes in the right direction to capture the light from that event. So that one they actually saw, that was a truly historic and spectacular observation. But it's mostly been black holes.

• 8:55Speaker 05

  We have seen so many black hole mergers. We're learning so much about them that sometimes I feel tempted to call this black hole astronomy rather than gravitational wave astronomy.

• 9:06Speaker 03

  At this point, they're detecting a black hole merger every three days, like every couple of days.

• 9:10Regina Barber?

  Going from not seeing anything from over a decade to this, it's so amazing. So what have they learned?

• 9:16Speaker 03

  Well, like, let's just look at one example, okay? So here's a gravitational wave detection from earlier this year. It came in on January 14th, 2025, right? And this is what the wave sounded like when they converted it to sound waves.

• 9:35Regina Barber?

  This sounds a little deeper, like more whooshing instead of chirping. So is that two black holes coming together like the first example that was played earlier?

• 9:42Speaker 03

  Yeah. And when I was talking to this researcher at Caltech, Katerina Hatzioanu, here's what she said.

• 9:48Speaker 06

  It's actually quite interesting. If you look at the signal, it looks very similar to the black holes that created the first signal 10 years ago. They're about 30 times the mass of the sun, and they are about 1.3 billion light years away from Earth.

• 10:05Speaker 03

  But she says the detectors have been substantially upgraded over the years. So now the signal is much clearer. In fact, that was the clearest signal to date. And that meant they could do all kinds of tests for basic theories about black holes.

• 10:18Speaker 06

  So one of the tests that we could do with this signal is test the nature of the final black hole and confirm that it looks exactly like the spinning black hole of general activity.

• 10:28Regina Barber?

  And I'm assuming that it did?

• 10:30Speaker 03

  Absolutely. It was just like they thought it should be, which, you know, always makes scientists happy. They were also able to test out an idea that famous physicist Stephen Hawking came up with, all the way back in 1971.

• 10:43Speaker 06

  Which says that the event horizon of a black hole, the region beyond which nothing can escape from the black hole, only grows with time.

• 10:51Speaker 03

  So the basic idea is that even though things are happening in a black hole merger that would seem to go against that, the surface area of the black hole is always got to get bigger.

• 11:02Regina Barber?

  Okay, so did that turn out to be true when they tested it?

• 11:05Speaker 03

  Yep, so the initial black holes combined had a total surface area that was like roughly the size of Oregon.

• 11:11Regina Barber?

  Okay.

• 11:12Speaker 03

  And then they were able to tease out from the data the surface area of the final black hole that was created when they merged, and it was like way bigger. It was like roughly the size of California.

• 11:23Regina Barber?

  That's so weird. I mean, it is too bad that Stephen Hawking wasn't around to see it, though.

• 11:28Speaker 03

  Yeah, I mean, he was alive when the first gravitational wave detection happened, and he actually asked a member of the research team if they could use gravitational waves to test his idea about the area of black holes.

• 11:39Regina Barber?

  Oh, wow.

• 11:40Speaker 03

  But back then, they couldn't because the data was just too cluttered up. And when I was talking to Max Esee at Columbia, he was noting that back in the 1970s, when Stephen Hawking came up with this one, like, people weren't even totally sure that black holes were real.

• 11:55Regina Barber?

  Yeah.

• 11:55Speaker 03

  Okay? They were just coming up with all of these ideas using just kind of like theory and pure math.

• 12:02Speaker 04

  All of these ideas that people thought up in the 70s, thinking it was just idle speculation. Now, they are manifested in actual data. We see these things happening almost exactly as predicted.

• 12:16Speaker 03

  He told me, you know, it would like really blow Albert Einstein's minds to know that they're able to catch gravitational waves from colliding black holes every few days. But that is what is happening.

• 12:26Regina Barber?

  That is what's happening after like decades of work and, you know, over a billion dollars to create these giant detectors, and you've got hundreds of scientists all over the world working on this, too.

• 12:36Speaker 03

  Yeah, and you know, the work never stops, right? Because researchers already have plans for even bigger, more sensitive detectors. One of them, they call it the Cosmic Explorer, and so that one would send lasers down pipes that are more than 20 miles long.

• 12:50Regina Barber?

  What?

• 12:50Speaker 03

  Obviously, they need to get funding and support, right? So that's not easy these days with the Trump administration trying to cut a lot of basic science. But, you know, there's also new projects going on in Europe, so who knows? I mean, 10 years from now, they could be seeing things that they're not even imagining right now.

• 13:07Regina Barber?

  And, and Nell, I hope you come back and tell us all about it.

• 13:10Speaker 03

  Well, if we are still around, you know it would be my pleasure.

• 13:16Regina Barber?

  This episode was produced by Hannah Chin. It was edited by Amina Khan. Nell Greenfield Boyce and Tyler Jones checked the facts. Jimmy Keely was the audio engineer. Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Regina Barber. Thank you for listening to Short Wave from NPR.

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