Allan Adams: Can Gravitational Waves Tell Us How The Universe Began? In 2015, scientists first detected gravitational waves— ripples in space caused by massive disturbances. Allan Adams says this discovery helps answer some of our biggest questions about the universe.
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Allan Adams: Can Gravitational Waves Tell Us How The Universe Began?

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Allan Adams: Can Gravitational Waves Tell Us How The Universe Began?

Allan Adams: Can Gravitational Waves Tell Us How The Universe Began?

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It's the TED Radio Hour from NPR. I'm Guy Raz.

So you ready to hear it?

HENRY: Yeah. Yeah, sure.

RAZ: Five years ago, when my oldest son was 3, he made a brief appearance on this show looking at the stars through our telescope. It was an episode we called Peering Into Space, and recently, I played it back for him.


HENRY: Oh, now I want to see - oh, I see Orion's belt.

RAZ: Where?

HENRY: The star.

RAZ: Where's Orion's belt?

HENRY: There's one star there.

RAZ: Oh, yeah.

HENRY: And another star there.

RAZ: Oh, yeah, yeah, there it is.

HENRY: One, two, three.

RAZ: On that episode, we explored what it is about space that ignites our curiosity.


RAZ: What's your favorite star?

HENRY: Polaris and Sirius are my favorite stars. Maybe Sirius is Polaris' neighbor.

RAZ: Whatever happened to that telescope, by the way?

HENRY: I think I got - I think the legs broke.

RAZ: You mean the tripod?

HENRY: Yeah. I think it got smashed in the closet.

BRAM: Well, mommy threw it away, I think. I saw her secretly throwing it away.

RAZ: That last voice was my younger son, by the way. And as you can tell, our family stargazing has waned a little bit over the past five years. But in that same time, there has been an explosion of new information about our solar system, our galaxy and our universe.

Do you still think space is cool?

HENRY: Yeah.

RAZ: What's cool about space to you?

HENRY: Aliens. I hope they exist.

RAZ: Yeah. Would you ever want to meet an alien?

BRAM: Yes. We would trade baseball cards. We'd have a lemonade stand. We would be best friends.

RAZ: Wow.

So on the show today, we're going to pick up where we left off - exploring the vastness of space, finding new areas of discovery and revisiting what we thought we knew about the universe.


ALLAN ADAMS: The last few years have completely changed what's possible and viewed as possible in our understanding of the universe on large scales, absolutely.

RAZ: This is Allan Adams. He's a theoretical physicist.

ADAMS: And I spent the past 20 years studying gravitational fluids and quantum mechanics, and I teach at MIT.

RAZ: And Allan's really interested in gravitational waves, a phenomenon that was entirely theoretical until just a few years ago.

ADAMS: So Einstein developed the theory of relativity, which is our modern theory of gravity and how it works. And built into the equations of relativity are exactly the gravitational waves that we're talking about.

RAZ: A gravitational wave is basically a ripple in space caused by a massive disturbance, and they're a really big deal because they allow us to see back in time and even unlock some of the mysteries of the origins of our universe. So for example, one huge collision that was detected in 2017 - it was a collision that happened 130 million years ago, far outside of our galaxy.

ADAMS: Out there in the darkness, there are these two really wonderfully strange objects. Each of them is a failed star, a star that has lived its life and blown off its outer core and collapsed, leaving behind only an incredibly dense mass of neutrons. And the key thing here is that when they start out as a, you know, an honest-to-God star, these are huge, young, big bucks of stars just really ravenously, you know, eating through their nuclear fuel.

And then, by the end of their life, they're these dying hulks, these little wrecks - but not just any dying wreck. They're incredibly dense. So as they get smaller and smaller and spend off the last remaining little ounces of whatever's left, they collapse down and form a neutron star with nothing but neutrons and nothing to cook. And so there are these final coals of the fire of an earlier star.

RAZ: So these are the embers of two stars hanging out 130 million years ago in the galaxy, and what happened?

ADAMS: These two coals find each other and orbit. And as they orbit each other, they slowly, slowly get closer and closer, and the reason they get closer itself is really cool. They get closer and closer because they're moving through something - space itself. And as they move through space, they send off ripples in that space. And those ripples, we call gravitational waves. They spread out like waves in a pond if you throw a rock in. And as they spread out, they carry away energy just like a wave in a pond carries away energy, and because that energy is going away, those two neutron stars fall closer and closer and closer together.

So as those two get closer and closer and start revolving around each other faster and faster, with every revolution, they start pushing off, sending off more waves like someone putting their hand through a pond. And those waves spread out. And as they go faster and faster - so the two neutron stars go around each other faster and faster - the waves that they send out get bigger, and bigger, and stronger and stronger, until finally, the two neutron stars collide.


ADAMS: And what happens is one of the most spectacular events we know in the universe. A huge set of waves is launched out into space in every direction, and it will continue moving at the speed of light all the way across the cosmos. And nothing - nothing - can stop the gravitational wave.

RAZ: Wow.

ADAMS: So that collision happened 130 million years ago. The waves from it didn't reach Earth until August 17, 2017.

RAZ: Wow.

Now, the technology that even allowed that wave to be detected here on earth is still brand-new. Scientists only discovered the very first gravitational wave two years before, in 2015. Allan Adams picks up that story from the TED stage.


ADAMS: Let me give you a sense of the time scale at work here. One-point-three billion years ago, Earth had just managed to evolve multicellular life. Since then, Earth has made and evolved corals, fish, plants, dinosaurs, people and even the Internet. And about 25 years ago, a particularly audacious set of people decided that it would be really neat to build a giant laser detector with which to search for the gravitational waves from things like colliding black holes.

Now, most people thought they were nuts, but enough people realized that they were brilliant nuts that the U.S. National Science Foundation decided to fund their crazy idea. So after decades of development, construction, and imagination and breathtaking amount of hard work, they built their detector called LIGO, The Laser Interferometer Gravitational-Wave Observatory. In early September of 2015, LIGO turned on for a final test run while they sorted out a few lingering details. And on September 14 of 2015, just days after the detector had gone live, the gravitational waves from those colliding black holes passed through the Earth, and they passed through you and me, and they passed through the detector.


RAZ: Wow, so it took 1.3 billion years for that wave from these two black holes to reach us here on Earth. And what happened? Did it do anything to Earth? Did it change anything?

ADAMS: Well, very literally, every one of us - everyone who's listening right now - stretched just a little tiny bit in one direction and contracted just a little tiny bit in the other direction.

RAZ: Yeah.

ADAMS: ...And did that back and forth a few times. But it was so small that nothing - nothing - on Earth - no instrument that we've ever built could possibly measure the effect it had on you.

RAZ: But obviously, the LIGO detectors, which I guess are, like, these, like, 3-mile-long tubes in the middle of - one's in Louisiana, and one's in Washington state, right?

ADAMS: Right.

RAZ: Those detectors did pick up on that teeny wave.

ADAMS: Exactly. That teeny-tiny motion told us a tremendous amount about the collision. It told us how many objects there were - two. It told us how heavy each of them was. It told us how much matter was totally destroyed and turned into the ripples in space and time that spread out and finally hit our detector. It told us how far away it was.

RAZ: So presumably, now that we have this technology to detect gravitational waves, are we just experiencing them all the time? Like, presumably, these huge, massive events are happening, you know, at least once a week, once a month in distant space billions years ago.

ADAMS: Yeah. Presumably, yes. So the LIGO collaboration has been overwhelmed in dealing with the data that they have. Part of what's so amazing about this whole story is no one expected to find anything. No one expected to detect gravitational waves. And already in the years since, we've detected event after event and learned unbelievable things about the universe, way more than I think anyone really expected. This is the bit that really brings me to tears. The binary neutron star event, where two binary neutron stars collided - not only could we learn how heavy they were, we also learned, independent of every other measurement that's ever been made about cosmology - we were able to measure the Hubble constant and put constraints on the acceleration of the universe.

RAZ: Wait a minute. You're talking about this idea that the universe is constantly expanding.

ADAMS: Yeah, and you can test that by looking at the data from LIGO. You can test the rate of expansion and how it's evolved on cosmological scales by listening to the black holes.

RAZ: Because Hubble's theory was that the expansion was happening, but it was slowing down. But now we actually think that the expansion is happening, but it's speeding up.

ADAMS: Exactly. And the reason we believe that for the past many years is because we've been able to establish, through long effort and lots of detailed observation, distances to galaxies and then measure the light coming from distant galaxies during the explosion of stars.

RAZ: Yeah.

ADAMS: What's amazing to me about the neutron star event is that just by looking at that one event and the gravitational waves coming off of it, we've got almost as accurate a prediction of the rate of the acceleration of the universe. It's a truly astonishing thing.

RAZ: So through gravitational waves, we can absolutely confirm affirmatively that the universe is expanding at a faster and faster and faster pace every moment.

ADAMS: I'm a scientist. I never say absolutely. But with that said, gravitational waves have now given us a completely independent measure of the acceleration of the universe, telling us that the universe is expanding faster and faster. And it's a utterly completely different measurement than we've ever used to make that prediction before, and it agrees beautifully. And in a few years, with just a little more data, I'm sure the gravitational wave evidence is going to be by far the strongest evidence for the rate of acceleration of the universe.


RAZ: When we come back, Allan explains how gravitational waves can not only teach us about past events in the universe but even about the very first event - the Big Bang. On the show today, we're Peering Deeper Into Space. I'm Guy Raz, and you're listening to the TED Radio Hour from NPR.


RAZ: It's the TED Radio Hour from NPR. I'm Guy Raz. And on the show today, we're Peering Deeper Into Space. And as physicist Allan Adams was saying just before the break, the detection of gravitational waves by LIGO is helping us observe the universe in an entirely new way. Here's Allan again on the TED stage.


ADAMS: That's the lasting importance of LIGO. It's a way that lets us hear the universe and hear the invisible. And there's a lot out there that we can't see. The Big Bang - I would love to be able to explore the first few moments of the universe, but we'll never see them because the Big Bang itself is obscured by its own afterglow. With gravitational waves, we should be able to see all the way back to the beginning. Our challenge now is to be as audacious as possible.

Thanks to LIGO, we know how to build exquisite detectors that can listen to the universe, to the rustle and the chirp of the cosmos. Our job is to dream up and build new observatories - a whole new generation of observatories - on the ground, in space. I mean, what could be more glorious than listening to the Big Bang itself?

RAZ: Wait, wait - gravitational waves could show us the Big Bang.

ADAMS: Oh, yes, yeah.

RAZ: It could happen.

ADAMS: It could absolutely happen. The early universe started with a big bang. We've all heard the term. And in that big bang, lots and lots of the elements were produced - hydrogen and helium filling up the universe. And hot hydrogen glows, and it absorbs light. So what that means is we'll never see the Big Bang itself because the universe was so hot that it was glowing like a candle. And as a result, we can't see past it. We can't see through it. So the universe is opaque to us from 100,000 years back to beginning. But gravitational waves go through everything. They go straight through the glowing hot gas of the early universe.

RAZ: Oh, wow, yeah.

ADAMS: And so what they let us do is see back way past that barrier in time and let us, in principle, touch back to the very earliest moments of everything around us.

RAZ: You know what, Allan? Some people hear gravitational waves, and they're like, you know - you know, like, what's the big deal? Because it's - it doesn't feel real to all of us, right? So what does this even tell us about where we come from or where we're going?

ADAMS: Oh, my God, it tells you all of the most important things in the world. So to start it tells you where the universe is going. It's going to expand and expand and expand and get really cold and lonely and big and empty. Yeah, that's really horrible. It also tells you that that's not going to happen for an extraordinarily long time. So don't worry about it too much. That's a good thing. It tells you that everything around you came from a big bang and then stars cooking up lots of stuff, like carbon and oxygen and nitrogen and all the things that make up the food you eat, except for all the metals and the trace stuff, which came from the collision of two neutron stars, which is completely insane because think about this - where you came from is not Iowa.

Where you came from is a star exploding, creating all sorts of elements, having them collapse back into another star. And then it explodes, and it creates more elements, and they fall into another star, but this one turned into a neutron star - turned into a big lump of nothing but neutrons - collided with another one and shot out a huge set of waves. And that is absolutely staggeringly cool. How can we ask why is this important? What else could possibly be more important than understanding where we come from?

RAZ: Yeah. You're really intense - phew.

ADAMS: (Laughter).

RAZ: I get it, though. I'm pretty blown away, too.

ADAMS: Good. It is freaking amazing.

RAZ: That's theoretical physicist Allan Adams. You can find his full talk at Hey, and, by the way, do you think any gravitational waves passed through us during the interview?

ADAMS: An uncountable infinity of gravitational waves have passed through our bodies as we've carried on this interview. Every time you wave your hand, you're creating gravitational waves. They're insignificantly tiny, but they're there in the same way that there are always ripples on the surface of the ocean. The ocean is never still. The universe is never quiet.


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