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The Invisible World of Gravitational Waves

Season 5Episode 2Feb 28, 2023

Information about the universe is all around us. But there’s more than meets the eye! Gravitational waves are the invisible ripples in spacetime caused by supermassive interstellar activity. Join astrophysicists Ira Thorpe and Judy Racusin on an exploration of how NASA studies these unseen bends in time and space.

The cover art display for the NASA's Curious Universe podcast.

NASA's Curious Universe

Introducing NASA’s Curious Universe:

Our universe is a wild and wonderful place. Join NASA astronauts, scientists and engineers on a new adventure each episode — all you need is your curiosity. Explore the lifesaving systems of space suits, break through the sound barrier, and search for life among the stars. First-time space explorers welcome.

Episode Description:

Information about the universe is all around us. But there’s more than meets the eye! Gravitational waves are the invisible ripples in spacetime caused by supermassive interstellar activity. Join astrophysicists Ira Thorpe and Judy Racusin on an exploration of how NASA studies these unseen bends in time and space.


Ira Thorpe

The easiest way I can explain a gravitational wave…

[Song: Gravitational Waves Instrumental by Green]

Ira Thorpe

…Is, many people have seen these pictures of the idea that space time is kind of like this rubber sheet. And you have this idea that massive objects like stars and planets, or especially black holes, can deform that sheet of space time.

Ira Thorpe

And a gravitational wave is a ripple in that sheet. You can imagine if you bounced up and down a star, you can get a ripple that travels across that medium. That ripple travels with the speed of light, it carries energy, it carries momentum. And it can carry information about the objects that produced it. We want to build a gravitational wave detector so that we can read those waves, detect those waves, use them to understand the objects that produced them.

[Theme Song: Curiosity Theme by SYSTEM Sounds]

HOST PADI BOYD: This is NASA’s Curious Universe. Our universe is a wild and wonderful place. I’m your host Padi Boyd and in this podcast, NASA is your tour guide.

HOST PADI BOYD: We have so many ways of understanding our universe. Telescopes on Earth and in space bring us incredible pictures, provide information about the composition of faraway planets and galaxies, and even clue us in to information the human eye can’t see.

HOST PADI BOYD: But the electromagnetic light we capture with a telescope isn’t the only information the universe is providing us! There are so many ways to “sense,” or understand, the universe, including through a relatively new discovery called gravitational waves.

[Song: Internals Underscore by Coon and Goebel]

HOST PADI BOYD: Gravitational waves are traveling ripples in time and space. The ones we can detect here on Earth are caused by the gravity of really, really big objects! Things like neutron stars, black holes, and orbiting binary stars send out far-reaching waves of gravity, affecting how space and time behave around them. By using huge and highly sensitive detectors, we can sense some of these waves and learn more about the objects sending them out.

HOST PADI BOYD: You can’t see a gravitational wave, and the ones that reach us aren’t strong enough for us to feel or experience on our own. But we are at the very beginning of learning what they can teach us about our origins and the universe around us. So today we’re going to join two astrophysicists on their journey into the invisible, time-warping world of gravitational waves.

Ira Thorpe

My name is Dr. Ira Thorpe and I study gravitational waves.

[Song: Breaking Underscore by Coon and Geobel]

Ira Thorpe

With the exception of the planets that we can travel to, maybe some cosmic rays, and interstellar meteors and such that might have come to us, all the other information we can get about most of the universe is coming from light which is traveling to us.

Ira Thorpe

Gravitational waves gives us a chance to break out of the electromagnetic spectrum altogether, and have a different what we call messenger, a different form of information, and it gives us different kinds of information. Some of the information that’s really difficult to get electromagnetically is easy to get through gravitational waves.

HOST PADI BOYD: The way an object interacts with space and time around it can tell us so much about its properties! But understanding these changes, which we can’t see or feel, can be tricky. So we create different ways to represent and conceptualize this important piece of information.

HOST PADI BOYD: You might have seen computer generated images of gravitational waves. Imagine a big sheet pulled really tightly. If you put a heavy object, like a bowling ball, in the middle of that sheet, it would dip and curve, causing effects not only where the ball has landed but out to each edge. And if you bounced the ball…

[[Bouncing ball sound]]

HOST PADI BOYD:…The ripples would be really strong near the middle and get weaker and weaker as you move toward the edge of the sheet.

HOST PADI BOYD: That’s what happens when any object interacts with what we call “spacetime” — the dimensions we experience of time and physical, “3-d” space. If something with enough mass moves, or explodes, its gravity warps the “sheet” of spacetime around it and causes a ripple effect.

HOST PADI BOYD: The objects we’re talking about have to be really massive in order for our instruments to sense the difference it makes in how we experience time or space… really, massive!

[Song: Facts Underscore by Coon and Goebel]

HOST PADI BOYD: And even though we can’t feel it, we are awash in these waves from different space objects all the time.

Ira Thorpe

The Earth is constantly bathed in gravitational waves. You have to have an incredible amount of mass and energy moving at really high speeds in order to make an appreciable gravitational wave. In principle, something like the Earth going around the Sun makes gravitational waves, but they’re so weak, you wouldn’t ever notice them.

HOST PADI BOYD: We don’t notice these effects from the space objects around us, but that doesn’t mean all gravitational waves are weak. In fact, the strongest energy release scientists have ever discovered wasn’t in the form of a visually bright star, it was in gravitational waves.

Ira Thorpe

It’s only when you get something like a black hole that you can produce these ripples. And once you do, they carry a lot of energy. The most energetic single events since the Big Bang are mergers of black holes, and all the energy comes out in gravitational waves. The amount of energy released per unit time at the very end of that burst is bigger than any high energy event that we ever see with our telescopes. Brighter than the supernova, brighter than a gamma ray burst, but none of that energy comes out as light. It’s all coming out as gravitational waves.

HOST PADI BOYD: Not only can these waves be really strong, they are always really fast, just like light. In fact, gravitational waves travel at the speed of light: 186,000 miles per second!

HOST PADI BOYD: If you want to study these fast-moving, space-rippling waves, you should probably aim your interest at cosmic objects whose gravity is really strong.

[Song: Spotter Underscore by Coon and Goebel]

Judy Racusin

I’m Dr. Judy Racusin. I’m an astrophysicist. I study gamma ray bursts, primarily, this is my specialty. These are the most energetic explosions in the universe. And I work on current and future missions that we use to study the highest energy form of light.

Judy Racusin

A gravitational wave is a ripple in spacetime itself that is caused by massive objects, doing something that has some asymmetry to it.

HOST PADI BOYD: Asymmetry means something that isn’t symmetrical, or isn’t perfectly balanced. A single spinning star isn’t likely to cause a big gravitational wave by itself. As soon as you add an orbiting companion, you get an asymmetry.

Judy Racusin

Maybe it’s a really dense star with a bump on it, or two black holes or two neutron stars that are in-spiraling towards each other.

HOST PADI BOYD: The study of gravitational waves is part of a field called “multi-messenger astronomy.” Scientists are interested in unlocking all the secrets of the cosmos that we can. For a long time, we just studied light from distant objects with our telescopes. But light is no longer the only “messenger” bringing us information from distant objects. We can now use other tools to sense the universe.

[Song: Pigments Underscore by Coon and Goebel]

Judy Racusin

We talk about gravitational waves sometimes instead of seeing it as hearing it. It’s not really, you know, sound, but it is something that has frequencies like sound has frequencies.

Judy Racusin

Different masses of objects cause those different sizes of those ripples in spacetime. Supermassive black holes in the centers of galaxies that in-spiral, those cause longer frequency of gravitational waves. Smaller objects, like stellar mass black holes or neutron stars have the shorter frequency gravitational waves.

HOST PADI BOYD: Keep in mind, those “smaller” stellar mass black holes are still about 10 times more massive than our sun. And the supermassive black holes Judy mentioned can be millions or billions of times more massive than the sun. These are really, really, dense objects we’re talking about here.

HOST PADI BOYD: Studying gravitational waves is crucial to building upon our understanding of the universe. This newly discovered type of information scientists get from these invisible waves can help us gather a fuller, more detailed picture of our place in space.

[Song: Circles of Life Underscore by James Carlin Baker]

Ira Thorpe

The universe is incredibly rich in terms of the amount of light and radiation and such that it’s bringing to us. And yet, there’s this entire other hidden part of the universe that has been with us the entire time we’ve been on the planet.

Ira Thorpe

To give you an example, you can show a person a picture of a jungle. And you say, ‘Well, what do you see?’ and you see all kinds of plants and such, but it’s basically all green.

Ira Thorpe

And then if you play audio that someone’s recorded in a rainforest. Now you’re like, ‘Oh, I, you know, I hear these insects, I hear these birds, I hear this jaguar.’

[[Jungle sounds: insects buzzing, birds singing, jaguar roars]]

Ira Thorpe

Without your hearing, no matter how good your sight is, you’re gonna miss those things. Of course without your sight, you’re not gonna see all the trees and everything else.

Ira Thorpe

And you put the two things together, and you get this complete understanding of what’s going on, then maybe add some smells and such as well.

Ira Thorpe

That’s what we’re trying to do is add another sense to our toolkit for understanding the universe.

[[Jungle sounds end]]

HOST PADI BOYD: Observing gravitational waves is a relatively new technique in the world of astronomy. But physicists have been theorizing about gravity and spacetime before they even knew about the super massive objects like black holes. In fact, I’m “relatively” sure you’ll recognize the first person to suggest the existence of ripples in spacetime.

[Song: Report Underscore by Coon and Goebel]

Ira Thorpe

The origin of gravitational waves from a theoretical understanding goes back to Einstein, about 100 years ago.

Ira Thorpe

He famously, in 1915, wrote down the theory of general relativity. It wasn’t until decades later, when astronomers started to understand that things like black holes might exist, when people started to develop technologies like lasers, and computer chips, that people started to get serious about, well maybe we could actually detect them.

Ira Thorpe

That was back in the 60s, and then especially into the 70s, that people started working in earnest to build gravitational wave detectors.

Ira Thorpe

And it wasn’t until 2015, that we finally actually detected the first gravitational wave directly with the LIGO instrument.

HOST PADI BOYD: The LIGO instrument is run by the National Science Foundation. Its name, LIGO, stands for “Laser Interferometer Gravitational Wave Observatory.” The instrument itself is a huge L-shaped structure. Each of the arms is nearly four kilometers, or two and a half miles, long!

HOST PADI BOYD: This incredible machine works to detect super minute changes in space and time. It uses lasers and mirrors to determine if space around it is stretching or contracting in tiny increments because of super massive objects extremely far away.

HOST PADI BOYD: Not only does the LIGO instrument have two arms, but there are two facilities, each with their own instrument, working together for even more sensitive detection of changes across a much larger area of Earth’s surface.One is in Livingston, Louisiana, and the other in Hanford, Washington.

HOST PADI BOYD: The instruments work kind of like an antenna. You can collect different frequencies based on how big or small the antenna itself is.

HOST PADI BOYD: Einstein’s theory and the new discoveries with the LIGO instruments not only solidified gravitational waves as a new way to sense the universe, it allowed for a new understanding of how our dimensions work together. A new understanding of spacetime!

Ira Thorpe

When we talk about the term, spacetime, and this is when I talk about it, the essential difference that came out of Einstein’s work is that prior to that, we’d think of spacetime as an empty framework in which physics happens. In which the universe does stuff. It’s like the box in a theater where the actors are running around. It just sort of sits there. And it’s the place where the action happens.

Ira Thorpe

When we talk about dynamic spacetime, which is how we understand gravity to work, the spacetime is involved in the physics, it’s a fundamental component of it as well. That, to me, is what we mean by spacetime, It’s not a rigid framework where the action happens, it’s part of the action.

Ira Thorpe

Dimensions of space, you know, three dimensions of space plus the time dimension. And as Einstein showed in his work, those things are related to one another in kind of interesting ways.

HOST PADI BOYD: There are whole books written about Einstein’s theory of relativity, but to put it simply it showed that gravity is not just a force but a field that can distort time and space.

HOST PADI BOYD: This is pretty advanced physics done by astrophysicists who have done lots of homework. But Ira, Judy and their colleagues are just like us. They put their shoes on one foot at a time, in precisely the same amount of dimensions as you and me.

Ira Thorpe

I think the average NASA scientist works with the same number of dimensions as everybody else, we are four dimensional beings, right? Meaning that three of space and time, that’s what we work with.

[Song: Ants Instrumental by Taylor Welsh]

HOST PADI BOYD: So what would it feel like to experience a strong gravitational wave first-hand? Well it turns out the name gives us a bit of a clue.

Ira Thorpe

Gravitational waves manifest themselves the same way that other gravitational effects do, which is through tides. You can think of them as producing a tidal effect. And this is what they do to our detectors, our detectors just have to be very sensitive to pick it up.

Ira Thorpe

So what a tidal effect means is you basically have a different gravitational pull on different parts of the same object. A tidal effect is your head being pulled a little harder than your feet and so it’s sort of an effective stretch. What happens when a gravitational wave passes by is on one direction, you get a stretch, and on the opposite direction, or the perpendicular direction, you get a squish, and then those oscillate back and forth.

Judy Racusin

If you were in an environment where there was a strong gravitational wave, you are going to have bigger problems because you are next to black holes or neutron stars with intense radiation, not to mention the explosive energy that will fry you immediately.

Judy Racusin

There’s no chance of this happening anywhere near us. Even if one happened in our galaxy, it’d be fascinating, but it’s unlikely.

Judy Racusin

You would experience kind of the effects that you would I guess have around a black hole. Space would be stretching and time would be stretching and contracting. It’s just strong gravity that is pulling you apart and squishing you back together.

HOST PADI BOYD: In order to learn more about these far off but fascinating phenomena, Ira is working on a future mission called LISA, being led by the European Space Agency. LISA is a gravitational wave detector that won’t reside here on Earth but in the vast expanse beyond our atmosphere instead!

Ira Thorpe

So we want to do gravitational wave detection from space, but not just because going to space is cool, right? Going to space is hard.

[Song: River Run Underscore by Elias Ramani]

Ira Thorpe

It’s much easier to have your detector on the ground, and to be able to go diagnose it and adjust it and improve it.

Ira Thorpe

The reason we want to go to space is because we can make the detector much, much bigger. And by making it much, much bigger, we can actually access different kinds of gravitational waves, different wavelengths.

HOST PADI BOYD: Like LIGO, LISA will have arms that work together to sense the environment around them. But instead of physical arms rooted on the ground, LISA will consist of three orbiting space craft connected by long, LONG lasers!

Ira Thorpe

There’s three individual spacecraft. They connect to one another with these laser links. So they shoot lasers back and forth between the three satellites in this triangle. The arm lengths of this particular mission, the Laser Interferometer Space Antenna, or LISA, it’s two and a half million kilometers between each spacecraft.

HOST PADI BOYD: Just as a reminder, LIGO is on the ground, and its arms are 4 kilometers, or around two and a half miles long. LISA’s arms will be out in space, two and a half million kilometers — or over 1.5 million miles — long.

Ira Thorpe

And that works out, and this is just coincidence, I promise, that if you were to draw that around the Sun, the Sun fits just perfectly right inside it. So that’s how big this instrument will be, it’ll be something the size that the sun could literally fit inside.

HOST PADI BOYD: This is a really fun area of astrophysics to think about — super massive objects messing with the normal routine of space and time. And there are lots of plans, like LISA mission, to expand our understanding of gravitational waves. But like a lot of things here at NASA, we have to be ready and wait for the universe to send information our way. We can test all kinds of things on Earth in laboratories, but we can’t test this.

Ira Thorpe

We don’t possess the ability to harness that much energy, to make gravitational waves in the lab.


Ira Thorpe

The first source detected by LIGO was a pair of black holes, each weighing roughly 30 times the mass of our Sun, and they’re orbiting each other many times a second, hundreds of times a second. So if you just sort of picture that in your mind, there’s something that weighs 30 times the mass of the Sun, and it’s going around another one of those things, as fast as your kitchen blender, then you understand why we can’t build that on the Earth.

Ira Thorpe

People have proposed maybe some advanced alien civilization that could produce gravitational waves, and it would be a way for them to announce their presence. I’m a little skeptical about how that would actually be but it’s an interesting idea. Maybe we will see unexpected signals. In fact, I hope we see unexpected signals, with the ground-based and the space-based detectors that we have built and are working to build. I think most of them will teach us about our universe from an astrophysical, cosmological kind of standpoint. But maybe we’ll learn something really unprecedented and unexpected. That’s another reason why we do the work.

HOST PADI BOYD: Even if we never hear a gravitational wave beacon from another civilization, it’s still important for us to follow these theories and find out more about how things work.

[Song: Cosmological Underscore by Elias Ramani]

HOST PADI BOYD: Gravitational waves can tell us so much about the universe — it can show how different objects interact, explain strange phenomena, and help us better understand how our universe is expanding.

HOST PADI BOYD: But more than that, it opens the door for a deeper understanding of physics — and important truths about how our universe behaves.

Judy Racusin

Like anything in astrophysics, we want to know how the universe works. How stars and galaxies and planets evolve over time, I mean, it tells us something fundamental about where we came from, about the history of our galaxy, you know, our solar system. It’s also just learning about the fundamental physics, like how does physics work? And gravitational waves are a unique and different way to view the universe. This field is just at its beginning. And I think there’s a lot of exciting science that’s going to happen in the next few years.

HOST PADI BOYD: This area of study is ripe with new knowledge and, as an astrophysicist myself, I cannot wait to see what we discover next. We just have to be ready and keep our eyes and ears — and gravitational wave detectors — open to what the universe has in store.

Judy Racusin

Any observatory you go into you propose certain science you’re going to do but there’s always things you learn that you never expected. You won’t see those things if you don’t look.

[Song: Curiosity Outro by SYSTEM Sounds]

HOST PADI BOYD: This is NASA’s Curious Universe. This episode was written and produced by Christina Dana. Our executive producer is Katie Konans. The Curious Universe team includes Maddie Arnold and Micheala Sosby, with support from Christian Elliott.

HOST PADI BOYD: Our theme song was composed by Matt Russo and Andrew Santaguida of SYSTEM Sounds.

HOST PADI BOYD: Special thanks to Amber Straughn, Barb Mattson, and Claire Andreoli.

HOST PADI BOYD: If you liked this episode, please let us know by leaving us a review, tweeting about the show and tagging @NASA, or sharing NASA’s Curious Universe with a friend. Still curious about NASA? You can send us questions about this episode or a previous one, and we’ll try to track down the answers. You can email a voice recording or send a written note Go to for more information.

Producer Christina Dana

Does anything we’ve talked about today have anything to do with time travel?

Judy Racusin

Time travel is a fun construct for science fiction. We do travel in time, we travel forward. Um, whether or not you can go backwards is something in the realm of science fiction, or theorists who are well beyond what I do. Time can move faster in a dense gravitational field. Or if you’re accelerating. If you’re traveling close to the speed of light, like, the rules are all very different.