The Invisible World of Gravitational Waves

01The 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.

02You have this idea that massive objects like stars and planets, or especially black holes, can deform that sheet of space-time.

03And a gravitational wave is a ripple in that sheet.

04You can imagine if you bounced up and down a star, you can get a ripple that travels across that medium.

05That ripple travels with the speed of light, it carries energy, it carries momentum, and it can carry information about the objects that produced it.

06We 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.

07This is NASA's Curious Universe.

08Our Universe is a wild and wonderful place.

09I'm your host, Patti Boyd, and in this podcast, NASA is your tour guide.

10We have so many ways of understanding our universe.

11Telescopes on Earth and in space bring us incredible pictures, provide information about the composition of faraway planets and galaxies, and even clue us into information the human eye can't see.

12But the electromagnetic light we capture with a telescope isn't the only information the Universe is providing us.

13There are so many ways to sense or understand the universe, including through a relatively new discovery called gravitational waves.

14Gravitational waves are traveling ripples in time and space.

15The ones we can detect here on Earth are caused by the gravity of really, really big objects.

16Things like neutron stars, black holes, and orbiting binary stars send out far-reaching waves of gravity, affecting how space and time behave around them.

17By using huge and highly sensitive detectors, we can sense some of these waves and learn more about the objects sending them out.

18You can't see a gravitational wave.

19And the ones that reach us aren't strong enough for us to feel or experience on our own.

20But we are at the very beginning of learning what they can teach us about our origins and the universe around us.

21So today we're going to join two astrophysicists on their journey into the invisible, time-warping world of gravitational waves.

22My name is Dr.

23Ira Thorpe, and I study gravitational waves.

24With 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.

25Gravitation 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.

26And it gives us different kinds of information.

27Some of the information that's really difficult to get electromagnetically is easy to get through gravitational waves.

28The way an object interacts with space and time around it can tell us so much about its properties.

29But understanding these changes, which we can't see or feel, can be tricky.

30So we create different ways to represent and conceptualize this important piece of information.

31You might have seen computer-generated images of gravitational waves.

32Imagine a big sheet pulled really tightly.

33If 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.

34And 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.

35That's what happens when any object interacts with what we call spacetime, the dimensions we experience of time and physical 3D space.

36If something with enough mass moves or explodes, its gravity warps the sheet of spacetime around it and causes a ripple effect.

37The 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.

38Really, really, really massive.

39And even though we can't feel it, we are awash in these waves from different space objects all the time.

40The Earth is constantly bathed in gravitational waves.

41You have to have an incredible amount of mass and energy moving at really high speeds in order to make an appreciable gravitational wave.

42In principle, something like the Earth going around the Sun makes gravitational waves, but they're so weak we wouldn't ever notice them.

43We don't notice these effects from the space objects around us, but that doesn't mean all gravitational waves are weak.

44In fact, the strongest energy release scientists have ever discovered wasn't in the form of a visually bright star.

45It was in gravitational waves.

46It'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.

47The most energetic single events since the Big Bang are mergers of black holes, and all the energy comes out in gravitational waves.

48The 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.

49Brighter than the supernova, brighter than a gamma-ray burst, but none of that energy comes out as light.

50It's all coming out as gravitational waves.

51Not only can these waves be really strong, they're always really fast, just like light.

52In fact, gravitational waves travel at the speed of light, 186,000 miles per second.

53If you want to study these fast-moving, space-rippling waves, you should probably aim your interest at cosmic objects whose gravity is really strong.

54I'm Dr.

55Judy Rackson.

56I'm an astrophysicist.

57I study gamma-ray bursts primarily.

58This is my specialty.

59These 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.

60A gravitational wave is a ripple in spacetime itself that is caused by massive objects doing something that has some asymmetry to it.

61Asymmetry means something that isn't symmetrical or isn't perfectly balanced.

62A single spinning star isn't likely to cause a big gravitational wave by itself.

63As soon as you add an orbiting companion, you get an asymmetry.

64Maybe 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.

65The study of gravitational waves is part of a field called multi-messenger astronomy.

66Scientists are interested in unlocking all the secrets of the cosmos that we can.

67For a long time, we just studied light from distant objects with our telescopes.

68But light is no longer the only messenger bringing us information from distant objects.

69We can now use other tools to sense the universe.

70We talk about gravitational waves sometimes instead of seeing it as hearing it.

71It's not really, you know, sound, but it is something that has frequencies, like sound has frequencies.

72Different masses of objects cause those different sizes of those ripples in spacetime.

73Supermassive black holes in the centers of galaxies that end spiral, those cause longer frequency of gravitational waves.

74Smaller objects like stellar mass black holes or neutron stars have the shorter frequency of gravitational waves.

75Keep in mind, those smaller stellar mass black holes are still about 10 times more massive than our sun.

76And the supermassive black holes Judy mentioned can be millions or billions of times more massive than the sun.

77These are really, really, really dense objects we're talking about here.

78Studying gravitational waves is crucial to building upon our understanding of the universe.

79This 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.

80The 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.

81To give you an example, you can show a person a picture of a jungle.

82And you say, well, what do you see?

83And you see all kinds of plants and such, but it's basically all green.

84And 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.

85Without your hearing, no matter how good your sight is, you're going to miss those things.

86Of course, without your sight, you're not going to see all the trees and everything else.

87And you put the two things together and you get this complete understanding of what's going on.

88And then maybe add some smells and such as well.

89That's what we're trying to do, is add another sense to our toolkit for understanding the universe.

90Observing gravitational waves is a relatively new technique in the world of astronomy.

91But physicists have been theorizing about gravity and space-time before they even knew about the supermassive objects like black holes.

92In fact, I'm relatively sure you'll recognize the first person to suggest the existence of ripples in space-time.

93The origin of gravitational waves from a theoretical understanding goes back to Einstein about 100 years ago.

94He famously, in 1915, wrote down the theory of general relativity.

95It 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.

96That was back in the 60s and then especially into the 70s that people started working in earnest to build gravitational wave detectors.

97And it wasn't until 2015 that we found they actually detected the first gravitational wave directly with the LIGO instrument.

98The LIGO instrument is run by the National Science Foundation.

99Its name, LIGO, stands for Laser Interferometer Gravitational Wave Observatory.

100The instrument itself is a huge L-shaped structure.

101Each of the arms is nearly 4 kilometers, or 2.5 miles, long.

102This incredible machine works to detect super minute changes in space and time.

103It uses lasers and mirrors to determine if space around it is stretching or contracting in tiny increments because of supermassive objects extremely far away.

104Not 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.

105One is in Livingston, Louisiana, and the other is in Hanford, Washington.

106The instruments work kind of like an antenna.

107You can collect different frequencies based on how big or small the antenna itself is.

108Einstein'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.

109When 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.

110It's like the box in a theater where the actors are running around.

111It just sort of sits there, and it's the place where the action happens.

112When we talk about dynamic space-time, which is how we understand gravity to work, the space-time is involved in the physics.

113It's a fundamental component of it as well.

114That, to me, is what we mean by space-time.

115It's not a rigid framework where the action happens.

116It's part of the action.

117Dimensions 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.

118There are whole books written about Einstein's theory of relativity.

119But to put it simply, it showed that gravity is not just a force, but a field that can distort time and space.

120This is pretty advanced physics, done by astrophysicists who have done lots of homework.

121But Ira, Judy, and their colleagues are just like us.

122They put their shoes on one foot at a time, in precisely the same amount of dimensions as you and me.

123I think the average NASA scientist works with the same number of dimensions as everybody else.

124We are four-dimensional beings, right?

125Meaning the three of space and time.

126That's what we work with.

127So what would it feel like to experience a strong gravitational wave firsthand?

128Well, it turns out the name gives us a bit of a clue.

129Gravitational waves manifest themselves the same way that other gravitational effects do, which is through tides.

130You can think of them as producing a tidal effect.

131And this is what they do to our detectors.

132Our detectors just have to be very sensitive to pick it up.

133So what a tidal effect means is you basically have a different gravitational pull on different parts of the same object.

134A tidal effect is your head being pulled a little harder than your feet, and so it's sort of an effective stretch.

135What 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.

136If you were in an environment where there was a strong gravitational wave, you were 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.

137There's no chance of this happening anywhere near us.

138Even if one happened in our galaxy, it'd be fascinating, but it's unlikely.

139You would experience kind of the effects that you would, I guess, have around a black hole.

140Space would be stretching, and time would be stretching and contracting.

141It's just strong gravity that is pulling you apart and squishing you back together.

142In order to learn more about these far-off but fascinating phenomena, Aira is working on a future mission called LISA, being led by the European Space Agency.

143LISA is a gravitational wave detector that won't reside here on Earth, but in the vast expanse beyond our atmosphere instead.

144So we want to do gravitational wave detection from space, but not just because going to space is cool, right?

145Going to space is hard.

146It's much easier to have your detector on the ground and to be able to go diagnose it and adjust it and improve it.

147The 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.

148Like LIGO, LISA will have arms that work together to sense the environment around them.

149But instead of physical arms rooted on the ground, LISA will consist of three orbiting spacecraft connected by long, long lasers.

150There's three individual spacecraft.

151They connect to one another with these laser links, so they shoot lasers back and forth between the three satellites in this triangle.

152The arm lengths of this particular mission, the Laser Interferometer Space Antenna, or LISA, it's 2.5 million kilometers between each spacecraft.

153Just as a reminder, LIGO is on the ground, and its arms are 4 kilometers, or about 2.5 miles long.

154LISA's arms will be out in space 2.5 million kilometers, or over 1.5 million miles long.

155And 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.

156So that's how big this instrument will be.

157It'll be something the size that the sun could literally fit inside.

158This is a really fun area of astrophysics to think about.

159Super massive objects messing with the normal routine of space and time.

160And there are lots of plans, like the LISA mission, to expand our understanding of gravitational waves.

161But like a lot of things here at NASA, we have to be ready and wait for the universe to send information our way.

162We can test all kinds of things on Earth in laboratories, but we can't test this.

163We don't possess the ability to harness that much energy to make gravitational waves in the lab.

164The 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.

165So 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.

166Then you understand why we can't build that on the Earth.

167People have proposed maybe some advanced alien civilization could produce gravitational waves, and it would be a way for them to announce their presence.

168I'm a little skeptical about how that would actually be, but it's an interesting idea.

169Maybe we will see unexpected signals.

170In 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.

171I 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.

172And that's another reason why we do the work.

173Even 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.

174Gravitational waves can tell us so much about the universe.

175It can show how different objects interact, explain strange phenomena, and help us better understand how our universe is expanding.

176But more than that, it opens the door for a deeper understanding of physics and important truths about how our universe behaves.

177Like anything in astrophysics, we want to know how the universe works, how stars and galaxies and planets evolve over time.

178I mean, it tells us something fundamental about where we came from, about the history of our galaxy, you know, our solar system.

179It's also just learning about the fundamental physics, like how does physics work?

180And gravitational waves are a unique and different way to view the universe.

181This 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.

182This area of study is ripe with new knowledge.

183And as an astrophysicist myself, I cannot wait to see what we discover next.

184Next, we just have to be ready and keep our eyes and ears and gravitational wave detectors open to what the universe has in store.

185Any observatory you go into, you propose certain science you're going to do, but there's always things you learn that you never expected.

186You won't see those things if you don't look.

187This is NASA's Curious Universe.

188This episode was written and produced by Christina Dana.

189Our executive producer is Katie Konins.

190The Curious Universe team includes Maddie Arnold and Michaela Sosby, with support from Christian Elliott.

191Our theme song was composed by Matt Russo and Andrew Santaguida of System Sounds.

192Special thanks to Amber Straughn, Barb Mattson, and Claire Andreoli.

193If 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.

194Still curious about NASA?

195You can send us questions about this episode or a previous one, and we'll try to track down the answers.

196You can email a voice recording or send a written note to NASA-CuriousUniverse at mail.nasa.gov.

197Go to nasa.gov slash curiousuniverse for more information.

198Does anything we've talked about today have anything to do with time travel?

199Time travel is a fun construct for science fiction.

200We do travel in time.

201We travel forward.

202Whether or not you can go backwards is something in the realm of science fiction or theorists who are well beyond what I do.

203Time can move faster in a dense gravitational field.

204Or if you're accelerating, if you're traveling close to the speed of light, like, the rules are all very different.

205I don't know.

206I don't know.

207I don't know.

208I don't know.

209I don't know.

210I don't know.

211I don't know.

212I don't know.

213I don't know.

214I don't know.

215I don't know.

216I don't know.

217I don't know.

218I don't know.

219I don't know.

220I don't know.

221I don't know.

222I don't know.

223I don't know.