NASA's Curious Universe - The Invisible World of Gravitational Waves

Episode Date: February 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 easiest way I can explain a gravitational wave is, you know, many people have seen these pictures of the idea that space time is kind of like this rubber sheet. You have this idea that massive objects like stars and planets, or especially black holes, can deform that sheet of space time. 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. This is NASA's curious universe. Our universe is a wild and wonderful place. I'm your host, Patty Boyd, and in this podcast, NASA is your tour guide. 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. 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. 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. them. By using huge and highly sensitive detectors, we can sense some of these waves and learn more about the objects sending them out. 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-wurping world of gravitational waves. My name is Dr. Ira Thorpe, and I study gravitational waves. 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. Gravitation waves gives us a chance to break out of the electromagnetic spectrum altogether

and have it 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 ways. 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. 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 bounce the ball, the ripples would be really strong near the middle and get weaker and weaker as you move toward the edge of the sheet. sheet. That's what happens when any object interacts with what we call space time, the dimensions we experience of time and physical 3D space. If something with enough mass moves or explodes, its gravity warps the sheet of space time around it and causes a ripple effect. 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, really, really massive. And even though we can't feel it, we are washing these waves from different space objects all the time. 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, we wouldn't ever notice them.

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 released scientists have ever discovered wasn't in the form of a visually bright star. It was in gravitational waves. 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 big, are mergers of black holes, and all the energy comes out in gravitational waves. The amount of energy release 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. Not only can these waves be really strong, they're always really fast, just like light. In fact, gravitational waves travel at the speed of light.

light, 186,000 miles per second. 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. I'm Dr. Judy Rackson. I'm an astrophysicist. I study gamma-reberst 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. A gravitational wave is a ripple in space-time itself that is caused by massive objects doing something that has some asymmetry to it. 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. 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. The study of gravitational waves is part of a field called multi-messinger 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. We talk about gravitational waves sometimes instead of seeing it as hearing it, it's not really sound, but it is something that has frequencies, like sound has frequencies.

Different masses of objects cause those different sizes of those ripples in space time. Supermassive black holes in the centers of galaxies that in spiral, those cause longer frequency of gravitation. gravitational waves. Smaller objects like stellar mass black holes or neutron stars have the shorter frequency gravitational waves. 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, really, really dense objects we're talking about here. 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. 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. on the planet. 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. And then if you play audio that someone's recorded in a rainforest, now you're like, oh, you know, I hear these insects, I hear these birds, I hear this jaguar. Without your hearing,

no matter how good your sight is, you're going to miss those things. Of course, without your sight, you're not going to see all the trees and everything else. And you put the two things together and you get this complete understanding of what's going on that maybe add some smells and such as well. That's what we're trying to do, is add another sense to our toolkit for understanding of the universe. Observing gravitational waves is a relatively new technique in the world of astronomy. But physicists have been theorizing about gravity and space time before they even knew about the supermassive objects like black holes.

In fact, I'm relatively sure you'll recognize the first person to suggest the existence of ripples in space time. The origin of gravitational waves from a theoretical understanding goes back to Einstein about 100 years ago. 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. That was back in the 60s, and then especially into the 70s,

that people started working in earnest to build gravitational wave detectors. And it wasn't until 2015 that we found he actually detected the first gravitational wave directly with the LIGO instrument. 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. 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 supermassive objects extremely far away. Not only does the LIGO instrument have two arms, but there are two facilities, each with 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 is in Hanford, Washington. The instruments work kind of like an antenna. You can collect different frequencies based on how big or small the antenna itself is.

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 space time. When we talk about the term space time, and this is when I talk about it, the essential difference that came out of Einstein's work is that prior to that, we think of space time as like 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. When we talk about dynamic space time, which is how we understand gravity to work, the space

time 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. 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. 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. 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. I think the average scientist works with the same number of dimensions as everybody else. We are four-dimensional beings, right? Meaning the three of space and time. That's what we work with? So what would it feel like to experience a strong gravitational wave firsthand? Well, it turns out the name gives us a bit of a clue.

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. 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. If you were in an environment where there is 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. There's no chance of this happening anywhere near us. Even if one happened in our galaxy, fascinating, but it's unlikely. 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 a strong gravity that is pulling you apart and squishing you back together. 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. 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. 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. 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. 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 spacecraft connected by long, long lasers.

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, that's 2.5 million kilometers between each spacecraft. Just as a reminder, LIGO is on the ground,

and its arms are 4 kilometers, or about 2.5 miles long. Lisa's arms will be out in space, 2.5 million kilometers, or over 1.5 million miles long. long. 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. This is a really fun area of astrophysics to think about, supermassive objects messing with the normal routine

of space and time. And there are lots of plans like the 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 and laboratories, but we can't test this. We don't possess the ability to harness that much energy

to make gravitational waves in the lab. 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 orbiting each of, 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.

People have proposed maybe some advanced alien civilization 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 base 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. 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. 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. But more than that, it opens the door for a deeper understanding of physics and important truths about how our universe behaves. 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. 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. 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. This is NASA's Curious Universe. This episode was written and produced by Christina Dana. Our executive producer is Katie Konins. The Curious Universe team includes Maddie Arnold and Michaela Sosby, with support from Christian Elliott. Our theme song was composed by Matt Russo and Andrew Santaguit of System Sounds. Special thanks to Amber Strawn, Barb Mattson, and Claire Andrioli.

If you liked this episode, please let us know by leaving us a review, tweeting about the show and tagging at 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 to NASA-curiousuniverse

at mail.nassah.gov. Go to nassah.gov slash curious universe for more information. Does anything we've talked about today have anything to do with time travel? Time travel is a fun construct for science fiction. We do travel in time. We travel forward. 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 to the speed of light, like the rules are all very different.