What Does a Black Hole Collision Sound Like?

Summary

Short Wave – What Does a Black Hole Collision Sound Like?

Episode Summary

Podcast: Short Wave (NPR)
Air Date: September 17, 2025
Hosts: Regina Barber, Nell Greenfield Boyce
Featured Guests: Max EC (Columbia University), Gabriela Gonzalez (Louisiana State University), Katerina Hatzioannou (Caltech)

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1. Overview of the Episode's Main Theme

This episode marks the 10th anniversary of a historic moment in physics—the first detection of gravitational waves, or as the hosts put it, "the chirp heard around the world." The episode explores what gravitational waves are, how scientists detect them, the cosmic events that produce them (like black hole collisions), and what we’ve learned in the decade since their discovery. The hosts use humor and creative analogies to make complex concepts approachable, highlighting both the science and the wonder of listening to the universe.

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2. Key Discussion Points and Insights

a. What Are Gravitational Waves? (00:20–04:38)

• Gravitational waves are ripples in space-time caused by the universe’s most violent events—collisions of black holes or neutron stars.
• Historical context: Predicted by Albert Einstein over a century ago, but he believed they'd be impossible to detect.
• Creative analogies:
  • Regina describes them as “a ripple in reality itself.”
  • Nell likens space-time to “jello,” “jiggly jiggly”—disturbed by cosmic events, like ripples from a pebble in a pond.
• Max EC: Explains the minuscule but real impact on our bodies as gravitational waves stretch and squeeze distances.

  “So at one moment it makes me taller and thinner, the next moment it makes me shorter and fatter.” (03:47)

b. The Search and Scientific Breakthrough (04:38–06:42)

• LIGO (Laser Interferometer Gravitational Wave Observatory) was built in 1999, but it took over a decade to get results.
• Detection requires recognizing changes in distance smaller than a “fraction of the width of a subatomic particle.” (04:38)
• The breakthrough came September 14, 2015.

  Nell: “It was a huge, huge deal. And pretty soon after some of the key people won the Nobel Prize.” (02:07)

c. The First Chirp: Black Hole Collision (06:05–07:20)

• The first detected event was two black holes, each about 30 times the sun’s mass, merging over 1.3 billion years ago.
• Light vs. waves: Traditional astronomy relies on light; gravitational wave detectors allow us to "listen" to the universe for the first time.

  Nell: “Some people have compared it to listening to the universe instead of looking at it.” (07:04)

d. Expecting Neutron Stars, Finding Black Holes (07:20–08:44)

• Scientists initially expected to find neutron star collisions, but black hole mergers were the main surprises.

  Gabriela Gonzalez: “Pairs of black holes that orbited each other, like, that wasn't the focus… But since then, it's almost the only thing we have seen.” (08:03)

• To date, hundreds of black hole mergers have been detected—one every 3 days.

  Nell: “At this point, they're detecting a black hole merger every three days.” (08:55)

e. A Decade of Discovery: What Have We Learned? (09:05–10:51)

• Improved detectors yield clearer signals; the clearest to date was detected January 14, 2025.
• Allows rigorous tests of black hole physics, such as checking if merged black holes follow predictions from general relativity and Stephen Hawking’s theories.

  Gabriela Gonzalez: “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 relativity.” (10:07)

• Hawking's 1971 claim—that the surface area of a black hole's event horizon only grows—has been confirmed.

  Nell: “So the initial black holes combined had a total surface area that was, like, roughly the size of Oregon… The final black hole… was, like, way bigger. It was, like, roughly the size of California.” (10:54–11:01)

f. Scientific Progress and Future Possibilities (11:44–12:56)

• Theoretical ideas from the ‘70s (once “idle speculation”) are now proven by data.

  Max EC: “All of these ideas that people thought up in the 70s thinking it was just idle speculation, now they are manifested in actual data.” (11:51)

• Major investments: Decades of work, $1 billion+ in funding, and hundreds of scientists worldwide.
• Next steps: Plans for even more sensitive detectors—like the “cosmic explorer” with 20+ mile-long laser pipes.

  Nell: “10 years from now, they could be seeing things that they're not even imagining right now.” (12:56)

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3. Notable Quotes & Memorable Moments

• Max EC (03:47–04:01):

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

• Gabriela Gonzalez (08:03):

  “Pairs of black holes that orbited each other, like, that wasn't the focus. 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.”

• Gabriela Gonzalez (08:44):

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

• Max EC (11:51):

  "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."

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4. Timestamps for Important Segments

• 00:20 – Anniversary of the first gravitational wave “chirp,” basics of gravitational waves
• 03:47 – The physical effect of a gravitational wave on humans, explained by Max EC
• 06:05 – Story of the first detected gravitational wave and what caused it
• 07:04 – The shift from “looking” at the universe to “listening”
• 08:03 – Unexpected prevalence of black hole collisions (Gabriela Gonzalez)
• 09:24 – Playing a recent gravitational wave signal and scientific advances in clarity/testing
• 10:07 – Testing Stephen Hawking’s black hole “surface area” law
• 11:51 – How once-theoretical ideas have now been confirmed by data (Max EC)
• 12:56 – The future: plans for bigger, better detectors

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5. Tone and Language

The hosts maintain a conversational, witty, and enthusiastic tone, using vivid metaphors ("ripples in reality," "space-time jello") to demystify the science. There is an undercurrent of awe at how far the field has come—from theorizing ripples in space-time, to hearing black holes collide across the universe, to planning ever-bigger observatories.

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6. Conclusion

This celebratory episode showcases the thrill of scientific discovery: how a century-old prediction became a real, measurable phenomenon, how black holes went from theory to frequent observation, and how every new “chirp” confirms or challenges what we know about the cosmos. If “black hole astronomy” is a real, booming field—as the scientists josh—Short Wave captures both the magnitude of this scientific leap and the delight of listening to the universe's most profound events.

Transcript

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[Regina Barber]

Everything is better with a podcast, and we know we're probably your favorite, but we also know you have room in your heart for others. That's why NPR is launching podclub. It's a newsletter for podcast fans by podcast fans. Subscribing is free, and it means you'll get fresh recommendations from real human beings in your inbox every Friday. The link to sign up is in our show notes [e-mail] podclub. You're listening to Shortwave from NPR. Hey, Shortwavers, 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. Did you hear it? I'll play it for you again. 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. And the existence of these waves was first predicted by Albert Einstein over a century ago.

[Nell Greenfield Boyce]

Yeah. But he thought no one would ever detect them.

[Regina Barber]

That's NPR science correspondent Nell Greenfield Boyce. Hey, Nell.

[Nell Greenfield Boyce]

Hey, there.

[Regina 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 cause we were kind of nearby and they'd give talks and they were so excited that they would be detecting gravitational waves very soon. But it wasn't soon. It took over a decade.

[Nell Greenfield Boyce]

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

[Regina Barber]

So long.

[Nell Greenfield Boyce]

It was amazing, actually, that the National Science foundation, which funded this, kept it going and eventually, you know, they detected them on September 14, 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.

[Regina 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 think is the best way of explaining, like, what a gravitational wave really is, Huh?

[Nell Greenfield Boyce]

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, the moon is solid. But, you know, in fact, things are much more jiggly than that.

[Regina Barber]

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

[Nell Greenfield Boyce]

Flubber? I don't think I've ever made flubber. I mean, I think of it more like jello, 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 shockwaves through space time itself, and those shock waves or jiggles, like those are gravitational waves, and they propagate out from the smash up, sort of like ripples in a pond. You know, you throw the pebble in and it disturbs things and the ripples come out. And I was talking to Max ec, 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.

[Max EC]

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

[Nell Greenfield Boyce]

But we don't notice, of course.

[Regina Barber]

Yeah. Because that effect is, like, so small, right?

[Nell Greenfield Boyce]

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. 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.

[Regina Barber]

So a tiny, tiny change. Like how tiny are we talking about?

[Nell Greenfield Boyce]

Like a fraction of the width of a subatomic particle.

[Regina Barber]

Wow, that is so small. 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 shortwave, the science podcast from npr.

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[Regina 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?

[Nell Greenfield Boyce]

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 loop. They finally merged into one, a new black hole. 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.

[Regina Barber]

You know, black holes, they're hard to study. Right. Because they're basically invisible. Their gravity, you know, 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.

[Nell Greenfield Boyce]

Exactly. Some people have compared it to listening to the universe instead of looking at it.

[Regina Barber]

I like that.

[Nell Greenfield Boyce]

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.

[Regina Barber]

Yeah. So, 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?

[Nell Greenfield Boyce]

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.

[Regina Barber]

Yeah.

[Nell Greenfield Boyce]

Pairs of black holes that orbited each other like, that wasn't the focus. Wow.

[Gabriela Gonzalez]

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.

[Nell Greenfield Boyce]

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

[Regina Barber]

Wow, that is so many black holes.

[Nell Greenfield Boyce]

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

[Gabriela Gonzalez]

Right.

[Regina Barber]

I do remember that.

[Nell Greenfield Boyce]

Yeah. 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.

[Gabriela Gonzalez]

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

[Nell Greenfield Boyce]

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

[Regina Barber]

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

[Nell Greenfield Boyce]

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 14, 2025. Right. And this is what the wave sounded like when they converted it to sound waves.

[Regina 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?

[Nell Greenfield Boyce]

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

[Gabriela Gonzalez]

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.

[Nell Greenfield Boyce]

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.

[Gabriela Gonzalez]

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.

[Regina Barber]

And I'm assuming that it did.

[Nell Greenfield Boyce]

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, which says that the event.

[Gabriela Gonzalez]

Horizon of a black hole, the region beyond which nothing can escape from the black hole, only grows with time.

[Nell Greenfield Boyce]

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. Wow.

[Regina Barber]

Okay. So did that turn out to be true when they tested it?

[Nell Greenfield Boyce]

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

[Regina Barber]

Okay.

[Nell Greenfield Boyce]

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.

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That's so weird.

[Regina Barber]

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

[Nell Greenfield Boyce]

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. But back then, they couldn't because the data was just too cluttered up. And when I was Talking to Max E.C. 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.

[Regina Barber]

Yeah.

[Nell Greenfield Boyce]

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

[Max EC]

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.

[Nell Greenfield Boyce]

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

[Regina 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 got hundreds of scientists all over the world working on this too.

[Nell Greenfield Boyce]

Yeah. And, you know, the work never stops 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. What 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.

[Regina Barber]

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

[Nell Greenfield Boyce]

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

[Regina Barber]

This episode was produced by Hannah Chin. It was edited by Amina Khan, Nell Greenfield Boyce and Tyler Jones. Check the facts. Jimmy Keeley 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 shortwave from NPR.

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