Daniel and Kelly’s Extraordinary Universe - Can we use the galaxy as a gravitational wave detector?

Episode Date: August 3, 2021

Daniel explains how pulsars can be used as a galaxy-sized physics experiment. Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

This is an I-Heart podcast. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, everything changed. There's been a bombing at the TWA terminal. Just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, terrorism. Listen to the new season of Law and Order Criminal Justice System

On the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, luckily, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend's been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want or gone.

Hold up. Isn't that against school policy? That seems inappropriate. Maybe find out how it ends by listening to the OK Storytime podcast and the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Do we really need another podcast with a condescending finance brof trying to tell us how to spend our own money? No thank you. Instead, check out Brown Ambition. Each week, I, your host, Mandy Money, gives you real talk, real advice with a heavy dose of eye. I feel uses, like on Fridays when I take your questions for the BAQA. Whether you're trying to invest for your future, navigate a toxic workplace, I got you. Listen to Brown Ambition on the IHeart Radio app, Apple Podcast, or wherever you get your podcast.

I'm Dr. Scott Barry Kaufman, host of the psychology podcast. Here's a clip from an upcoming conversation about how to be a better you. When you think about emotion regulation, you're not going to choose an adaptive strategy, which is more effortful. to use unless you think there's a good outcome avoidance is easier ignoring is easier denials easier complex problem solving takes effort listen to the psychology podcast on the iheart radio app apple podcasts or wherever you get your podcasts the deepest questions about the nature of the universe have answers answers that are out there right now waiting to be discovered. The nature of dark matter, the secret to quantum gravity,

the mystery of dark energy. The answers lie in wait for us. But not just the answers. Also, the clues to reveal them. Right now, the clue to unravel these mysteries is out there. Probably the information we need to crack the code is washing over us in the form of messages from space we don't yet know how to decode. Think about how much we have learned from listening carefully to the sky, answers to questions that previous generations didn't even know to ask. So that makes us wonder, of course, what answers are arriving here on Earth right now,

waiting for someone clever enough to know how to listen? Hi, I'm Daniel. I'm a particle physicist and I desperately want to hear the answers to questions about the universe. I know that these answers are out there and I know that humans will figure them out. In 50 years, in 100 years, in a thousand years, people will know the answers to deep questions about the universe that we are totally perplexed by. Little children will read books explaining to them secrets that it takes Nobel Prize winning discoveries to uncover about the universe. They will hear about them. They will be bored. They will throw a tantrum. I am desperate to read those books. I am desperate to know those things about the universe. I know that the answers are out

there. I know there are ways to figure them out. But science moves slowly and steadily with brief flashes of insight, sometimes revealing the nature of the universe. And we just have to wait for it to happen. But not just wait, we can also push it forward. So welcome to the podcast, Daniel and Jorge, Explain the Universe, a production of IHeart Radio, in which we do our best to push it forward by encouraging your curiosity about science, by encouraging everybody's curiosity about science, by asking the big questions about the universe and thinking about how we might possibly answer them. On this podcast, we talk about everything in the universe, the tiniest little particles,

the most super massive black holes and all the signals we are receiving from these cosmic and tiny objects that are telling us those secrets of the universe. My friend and co-host Jorge can't be here today to join us, so I'm on my own telling you about all the things we can learn from the universe if we just knew how to listen. And there's no shortage of questions about the universe we'd like answers to.

Of course, there are the known unknowns, the things that we know we don't know, and we also know how to figure them out. There are so many things in science where we know exactly what it is we need to do. We know exactly what question we want to answer and we just haven't done it yet, either because of time or expertise or, frankly, just money. Think about all we could accomplish if we poured more money into science. We know how to send rovers to other planets.

We know how to send satellites to land on the moons of Jupiter. We know how to build big space telescopes. We can do these things, and we know that if we did them, we would essentially just be buying scientific knowledge. We know that the answers to some of the questions we have about whether there are life in the oceans on those moons and what is going on in the deepest, darkest reaches of the universe. Those answers are out there waiting for us if we would just buy them. We are like children walking around in a scientific candy shop, keeping all of our money in our pockets. and not purchasing those tasty, tasty, tasty, delicious scientific treats. So that's why on this program and everywhere in my life,

I'm always advocating for increasing the spending to government agencies. We could buy so much knowledge about the universe. We could crack open some of these mysteries if we just spent a few more pennies. All right, maybe not pennies, maybe a few more millions of dollars. But on the scale of government spending, it really is pennies. But in the face of those budget realities, we have to get clever. And one of my favorite things that astrophysicists do is that they don't try to build experiments themselves. They just go out and look for them.

Like in particle physics, if I want to know what happens when I smash two protons together, then I say, let's go build an accelerator that does just that, that smashes the particles together and see what comes out. We are creating the conditions that we want to study. Astrophysicists don't usually have that luxury. For example, if you're curious about what happened to be a smash, mash two galaxies together, well, you can't go out and build a galaxy-sized collider. Fortunately, however, the universe is vast and it is crazy, and it is filled with all sorts

of amazing stuff, including if you look hard enough, almost every experiment you would want to do, including colliding galaxies. The same thing goes for colliding things like black holes. Who doesn't want to shoot one black hole at another one to see what happens? Does one black hole eat the other one? Do they eat each other? somehow, is it some crazy cosmic dance? We now know, of course, that when you do that, you emit enormous quantities of energy in gravitational waves. We've actually seen these things using gravitational wave detectors. So while we didn't have to build the experiment to shoot black holes at each other, we did have to build the experiments to see the gravitational

waves. But what if we didn't have to build the experiment and we didn't have to build the detector? What if nature set it all up for us? What if all of all we had to do was listen. And so today on the podcast, we'll be talking about just that, a crazy new idea that might just work. So on today's program, we'll be answering the question. Can we use the entire galaxy as a gravitational wave detector? I know that sounds preposterous, but astrophysicists like to think big. And when they do, sometimes they make it work. So I was wondering how much people had heard about this topic already if this was something everybody was talking about or only people in the physics community. So as usual, I asked for volunteers out there on the internet to answer random physics questions with no preparation.

It gives me an idea for what people out there already know and what they think about when they hear this question. If you'd like to participate for future episodes of the podcast, please don't be shy. It's easy. It's fun. We love hearing from you. Please just write to me to questions at. Danielanhorpe.com. So think for a moment. Do you have an idea for how you could use the entire galaxy to detect gravitational waves? Here's what our volunteers had to say. I don't know. I have no idea.

I don't know how detectors like LIGO measure gravitational waves. I don't know the mechanism. But objects in our galaxy are quite spread out. And compared to the size of the galaxy, each of them is quite small. So I don't think our galaxy could be an effective gravitational wave detector. I don't know. The ones on Earth, I believe, used lasers, and obviously there are known distance apart. I sort of seen lectures one time about using dust or hydrogen gas or something in the universe, whether that's gravitational waves or something else. I don't know. I'm not quite sure how you do that one. Yeah, if you think about how they measure gravity waves, you're essentially like measuring the separation between two mirrors in one direction versus another direction.

That's what the LIGO observatory does. So you get this oscillation signal. So I imagine you can do something similar with the entire galaxy. You can just look at the Milky Way galaxy and see if things get squished in one direction relative to another for like super massive gravitational waves. Like maybe you can look at the red shift at one end versus the blue shift at the other and see if there's a difference that sort of correlates. as you move around.

Yes, yes, we can. We just have to observe how gravitational waves are affecting other bodies in the galaxy. I don't see why you couldn't. In fact, that would be probably a good way to measure and detect gravitational waves. So if you used an entire galaxy, let's say you're one edge of the galaxy versus the opposite edge of the galaxy and you were using the same principle, you should be able to pick up the movement of a gravitational wave through that galaxy. Since you're asking that question, I would say, yes, you can use the whole galaxy as a gravitational wave detector.

If you have large gravitational waves traveling through the galaxy, it will impact some stars before others and we'll increase and decrease the length, the distance between two far apart stars quite significantly. So it's probably a very good gravitational wave detector. All right. So our listeners are smart. They know something about gravitational waves and they have the idea that the gravitational waves

must be somehow affecting things in the galaxy and then we could see that effect somehow. But nobody quite figured out exactly how we see those effects. There's ideas there about red shifts and blue shifts and optical lensing and ripples against things, but it's a tricky topic. And we're going to dig into it and explain to you exactly how to use the entire galaxy as your physics experiment. But first, let's just remind ourselves what we're talking about observing. What we want to see out there, what we're trying to study, what we're using the entire galaxy as a detector of, are these crazy things called gravitational waves.

What is a gravitational wave anyway? Well, you know what a wave is? A wave is when something moves up and down. Like you put your hand in your bathtub and you slap the water and you get a wave of the water, right? The water goes down and then goes up and then it goes down and then goes up. Or you see waves in the ocean here in Southern California and you can surf them. You might wonder like, well, why are there waves? Why does the water in the bathtub go down in waves rather than all going down at once?

And that's because of a really important property in physics called locality. When your hand hits the water, it only affects the water that it touches. It doesn't affect the water on the other side of the bathtub or in bathtubs all over the universe, right? Physics is local and information takes time to propagate. So the water on the other side of the bathtub doesn't know that you've hit the water on this side until it gets that information until the wave arrives there. It's the same story if, for example, you pluck a guitar string, right? Which part of the string starts to vibrate first? Well, the part that you plucked and the rest then moves as that information moves down the string.

So then why do you get a wave? Because the part that you pluck moves down and then it comes back up. And then it goes back down and then it comes back up. And the wave are those ripples moving down the string. If information propagated instantaneously, the whole string would move at once. But the reason it doesn't, the reason you get a wave is because information doesn't propagate instantaneously. So let's get back to gravity. gravitational waves are the waves in space itself, right?

How do you make waves in space? What does that even mean? Well, remember that we know now that space is not just like the backdrop. It's not just like where things happen in the universe. It's a thing in and of itself. It has properties. It can do things that emptiness or nothing this can do. For example, it can bend.

If you put a big mass in space, what happens? Well, space bends around that mass. It changes the curvature of space. And a lot of you are probably thinking like a bowling ball on a rubber sheet, right? And that's a helpful analogy to sort of shake your mind out of the idea that space is nothingness, that it can have curvature. But it's also confusing because it's suggesting that space is curving in some other dimension. In that example, space is a two-dimensional rubber sheet and it's curving in some third

dimension because the bowling ball is pulling it down, right? Well, that's not what happens in our universe. Our space is already three-dimensional. And when it curves, what we mean is that it changes the relative distance between two points. It means that things that might have seemed further away are now closer because space is sort of scrunched. And that's why, for example, photons bend around massive objects because photons always take the shortest path available to them. And that shortest path looks like a curve if you aren't aware of the bending of space. And we have no way to detect the bending of space. We can't see it.

We can't feel it other than watching things trace it out. So that's why the earth moves in a circle around the sun because the space is bent in a way that makes that its most natural motion. All right. So space can bend, but how does it ripple? How do you get a wave in space? Well, the same way you get a wave in your bathtub. Say, for example, you deleted the sun from our sun.

solar system. What would happen? Well, you guys know that gravity doesn't move instantaneously, so we wouldn't notice for eight minutes. That's a gravitational wave. That's information about the shape of space propagating through space itself. Just like when you plug a guitar string, the information that the string was plucked moves down the string. If you delete the sun, the information that space is no longer bent moves through space, making it flat rather than curved. And if you have two black holes, for example, orbiting each other, then they are making huge gravitational waves because anything that has mass and accelerates makes a gravitational wave. It's changing the curvature of space through time. And that's what creates gravitational waves.

And so black holes, for example, orbiting each other and eventually collapsing into a single mega black hole, they send out these ripples as they orbit each other. They're changing the shape of space around them because they're very massive and they're bending space. and the way they're doing is changing because they are orbiting each other. So that's where those gravitational waves come from. That's where the ripples in space come from. And so how do you detect the ripples in space and time? I said earlier, then we can't see or feel the bending of space.

All we can do is detect the change in relative distances. If the space between me and you contracts, then the distance between us contracts. You might think, well, hold on a second. If we have a ruler between us and space between us contract, Won't the ruler also contract? There's a chance of that. So to avoid that, we build rulers out of light.

We send light pulses back and forth. Because if the space between me and you gets smaller, then light will cover that distance more quickly. And if you have a mirror, I can shoot my laser at your mirror and measure how long it takes for that light to come back. And so if a gravitational wave passes between you and me, and I'm doing that all the time, I'll notice because all of a sudden the space between us will be shorter

and then longer, as the gravitational wave passes. And this is not a theoretical idea. This is real. This is something we have actually seen. People have built these detectors. They're called LIGO and Virgo. And they have these mirrors underground.

And the mirrors are very, very stable. They try not to shake them at all because any shake in those mirrors makes it impossible to see gravitational waves, which, after all, look a lot like the shaking of the mirrors. So they have these mirrors hyper-stabilized. They're hanging from cords and those cords are attached to. something else which is buffered, which is attached to something else, which is protected from shaking. It's like nine layers of protection against any sort of activity. Trucks driving by or people slamming screen doors or any kind of thing that would make a signal that looks like a

gravitational wave. The first one we saw was in 2016. It won a Nobel Prize. And now we've seen more than a dozen of these things from black holes nearby that have collided and sent these pulses. And we were kind of surprised by how common these things were. So now you might be thinking, great. We have a gravitational wave detector. We've seen these things. Why would we need to build a gravitational wave detector at the size of the galaxy? Well, gravitational waves come in lots of different colors, basically. Just the same way light has lots of different frequencies, light is electromagnetic radiation, and it can have a frequency that puts it in the visible, so it has different colors, or it can have very long frequencies, like down in the radio, or it can have very short frequencies

and be an x-ray or a gamma ray. So all of these are different. kinds of electromagnetic radiation, and we need different kind of instruments to see them. You can't see the same thing with an optical telescope that you can see with a radio antenna or an x-ray telescope. Well, the same thing is true for gravitational waves. Gravitational waves come in all different frequencies. Space can ripple at lots of different frequencies, and very high frequency ripples are different from very low frequency ripples. And the kind of things that we can see with LIGO are in a sort of narrow range of gravitational waves. It was designed to see gravitational waves from solar mass black holes that were colliding with each other. So it's sort of only sensitive

to that little spectrum. Imagine if we had only ever built optical telescopes and we couldn't look at the sky in the UV or in the x-ray or in the radio. We would be missing a huge slice of the picture. So what we need to do are build gravitational wave detectors that can detect very different frequencies of gravitational waves so we can listen to the messages from the universe and learn all of its crazy secrets. So we'll talk about how to build gravitational wave detectors that can detect very, very low frequency gravitational waves and tell us all about what's going on in the early universe and the collisions of supermassive black holes. But first, let's take a quick break. December 29th, 1975, LaGuardia Airport.

The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, at 6.33 p.m., everything changed. There's been a bombing at the TWA terminal. Apparently, the explosion actually impelled metal, glass. The injured were being loaded into ambulances, just a chaotic, chaotic, chaotic. In its wake, a new kind of enemy emerged, and it was here to stay. Terrorism. Law and Order Criminal Justice System is back.

In season two, we're turning our focus to a threat that hides in plain sight. That's harder to predict and even harder to stop. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Well, wait a minute, Sam, maybe her boyfriend's just looking for extra credit. Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot.

He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone. Now, hold up. Isn't that against school policy? That sounds totally inappropriate. Well, according to this person, this is her boyfriend's four. former professor, and they're the same age. And it's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him?

Well, he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person's boyfriend really cheated with his professor or not? To hear the explosive finale, listen to the OK Storytime podcast on the Iheart radio app, Apple Podcasts, or wherever you get your podcast. Your entire identity has been fabricated. Your beloved brother goes missing without a trace. You discover the depths of your mother's illness, the way it has echoed and reverberated throughout your life, impacting your very legacy.

Hi, I'm Danny Shapiro. And these are just a few of the profound and powerful stories I'll be mining on our 12th season of Family Secrets. With over 37 million downloads, we continue to be moved and inspired by our guests and their courageously told stories. I can't wait to share 10 powerful new episodes with you, stories of tangled up identities, concealed truths, and the way in which family secrets almost always need to be told. I hope you'll join me and my extraordinary guests for this new season of Family Secrets.

Listen to Family Secrets Season 12 on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Hola, it's Honey German, and my podcast, Grasasas Come Again is back. This season, we're going even deeper into the world of music and entertainment with raw and honest conversations with some of your favorite Latin artists and celebrities.

You didn't have to audition? No, I didn't audition. I haven't auditioned in like over 25 years. Oh, wow. That's a real G-talk right there. Oh, yeah. We've got some of the biggest actors, musicians, content creators, and culture shifters

sharing their real stories of failure and success. You were destined to be a star. We talk all about what's viral and trending with a little bit of chisement, a lot of laughs, and those amazing vibras you've come to expect. And, of course, we'll explore deeper topics dealing with identity, struggles, and all the issues affecting our Latin community. You feel like you get a little whitewash because you have to do the code switching? I won't say whitewash because at the end of the day, you know, I'm me.

But the whole pretending and code, you know, it takes a toll on you. Listen to the new season of Grasasas Come Again as part of My Cultura podcast network on the IHartRadio app, Apple Podcasts, or wherever you get your podcast. All right, we're back and we are talking about building a gravitational wave detector the size of the galaxy, actually made out of the galaxy. And we reminded ourselves what gravitational waves are and how they have been seen so far by a gravitational wave of. observatories on Earth that use delicate mirrors balanced underground miles apart to detect very, very small deviations in the distances between those mirrors. These deviations are like one part in 10 to the 20. It's an extraordinary experimental accomplishment that they can do these

things. It took a huge amount of work to make those mirrors insensitive to all sorts of things that would shake them that would look like the gravitational waves. When they finally did see them, it was a really nice, very clear signature because they knew exactly what they were looking for. They knew what kind of gravitational waves black holes should make as they fall into each other. What happens is that the black holes start slowly moving towards each other. As they get closer and closer in, they start spiraling faster and faster. So the frequency of the gravitational wave increases as the black holes get closer together. And so they call this like a chirp because it goes faster, faster, faster, faster, faster and higher, higher, higher, higher, higher, higher, higher.

And that's as high as my voice will go. So they knew sort of what they were looking for. They did all these numerical relativity calculations to figure out just what it looks for. But you know, those black holes were generating gravitational waves long before we saw them. It took years for these black holes to actually merge. What we saw was just the last little bit as the frequency moved into a range that LIGO could see it. LIGO was designed to see gravitational waves from black hole mergers, but only the last few seconds. of them, right? There were years probably of gravitational waves that we couldn't see. So, why can

LIGO not see gravitational waves that are longer? The problem is seismic noise. The Earth itself is shaking. We live on the surface of the Earth, which is part of the crust, and the crust is always sliding. And that makes it very difficult to see little ripples in space and time. We can see them if the ripples are fast enough, sort of faster than the Earth typically shakes. But anything at a lower frequency, the seismic noise, the shaking of the earth itself makes basically impossible to see those things. The earth is shaking more loudly than those gravitational waves. And it's not just the frequency, it's because of the amplitude also. The intensity of the gravitational wave signal gets stronger as you get near the end of the black hole merger. As the black holes are

getting closer and closer together, the gravitational waves get stronger. So the gravitational waves from the early part of the story, we're missing because they're at longer frequency that our detectors can't see over the seismic noise of the Earth, and they're much quieter, which makes it harder for us to see them. So how do you see these longer frequency gravitational waves then? Well, if the problem is that you're buried in the Earth, one idea is don't be buried in the Earth. Take it to space, right?

So one science fiction sounding project that's actually very real is a project called Lisa, which is a laser interferometer in space. It takes the same concept of having mirrors where you're bouncing lasers back and forth and it puts it out there in space. That's much more technologically difficult and expensive, of course. But it does solve this problem of the earth background noise. There is no seismic noise out there in space. So Lisa would be much more powerful, much more sensitive and able to hear gravitational waves

at longer frequencies. But again, that's expensive and that's far off in the future. And so until then, people are thinking, do we need to build our own gravitational wave detectors or can we find one already existing in the galaxy? Can we use the galaxy itself as a gravitational wave detector? And the answer, of course, obviously is yes because if we're doing a whole episode about it, I wouldn't get to this point of the episode and then just say, nope, goodbye, see you later. The way we do it is use an ocean of very precise clocks that naturally exist in the galaxy.

Of course, I'm talking about pulsars. Pulsars are the end point of a star. You know, the star forms when gas and dust swirl together and compactify and eventually get dense enough that fusion happens, then hangs out for a few billion years, burning all of that fuel, pushing back against gravity, preventing it from collapse. But eventually, that fuel gives out, and it can no longer provide the heat and the radiation to prevent gravity from compacting it even further. And depending on the mass of the star, it can end up in various scenarios.

It might turn into a black hole if it's very massive. It might turn into a white dwarf, basically a hot lump of stuff if it's not that massive. In the middle is a category of objects called neutron stars. Here there's enough gravity to compactify it to squeeze it down really, really dense. We're talking about a significant fraction of the mass of the sun in an area like the size of Los Angeles. It's incredibly dense. It's incredibly weird matter also. It's called a neutron star because it's been taken and squeezed.

so much that the electrons and the protons in the atoms are squeezed together and turn into neutrons usually it goes the other way you have a neutron hanging out it turns into a proton and an electron but here because the pressure that's basically been reversed and you get an object which is mostly neutrons and in some really weird intense state in addition these things are spinning really really fast because they have all the angular momentum of the original stuff that made them but now they're a really small space. And because angular momentum is conserved, you can't just get rid of it. It doesn't just disappear. Then it has to spin faster as it gets smaller. Just like a figure skater pulling in her arms, as you get more compact, you need a higher velocity to match the smaller radius to have

exactly the same angular momentum. All right, so we have a spinning object, a neutron star. Some fraction of these have also really powerful magnetic fields. And those magnetic fields operate on the particles on the surface of the neutron star and can generate beams of energy. They push the protons and the neutrons and they generate these massive beams of energy which follow the magnetic fields. So you have this spinning object with a very powerful magnetic field with a beam of energy coming out the top and the bottom, the magnetic north and the magnetic south. What happens if the spin of this object is not aligned with the magnetic axis. What if the beam is not shooting straight up, so it's always going in the same direction, but sort of off to the side.

a little bit, then what happens is that that beam sweeps around. It points in a different direction, right? Imagine holding a flashlight and spinning around. If you're holding it straight up, the flashlight doesn't change as you spin. But if you're holding it to the side, then you're going to be blinding different people as you spin around, right? That's what a pulsar is. A very intense beam of light, pointing sort of out sideways so that as it sweeps around, that beam passes over different things. And from Earth, we see these things when that beam passes us. So there's a lot of pulsars out there in the galaxy that we can't see because their beam never passes us. But the ones where the beam does sweep over the earth, we see that as pulses.

We get a pulse every time it sweeps by. The incredible thing is that they are very, very regular. Here you have an object of incredible mass, trillions of tons of stuff, spinning at very high speeds up to like hundreds of hertz, right? Like an incredible amount of stuff spinning many times per second and doing it very regularly. It's not like every 0.2 seconds and then every 0.3 seconds and every 0.4 seconds. These things are more precise than some atomic clocks. They're like the most precise natural clocks out there we have found. And the universe is filled with them.

They are all over the galaxy. So you might imagine then how we might be able to use them to measure the distortion of space. If you are on Earth and you're surrounded by a bunch of pulsars, and you've been watching these pulsars for a while so you know them. You know how long it takes between pulses. for a given pulsar, then what you could do is see if that changes. Think about what happens as a gravitational wave passes over the Earth. It changes the distance between us and those pulsars. What that means is that the pulses would take longer or shorter amounts of time to arrive here

on Earth. So if you know how often the pulses should be arriving and you see a deviation, you see a residual from what you expect, then that means something happened. The distance between you and that pulsar has changed. And so a while ago, people figured out how to use all of these pulsars, these precise clocks, to calculate what would happen if a gravitational wave passed us. And it wouldn't affect all pulsars the same way, right? Because gravitational waves have this sort of quadrupole effect. They squeeze in one direction at the same time they're pulling in another direction. So we can't look at an individual pulsar and say, oh, there was a gravitation wave. What we need to do is have a whole network of pulsars, have them all around us in every direction so that a gravitational wave

has a very distinct signature. So it looks different from other random weird blips we might see or changes in our instrument or anything else that might affect the timing but isn't due to gravitational waves. This is a classic trick in experimental physics is to make the thing you're looking for look unique so that when you see it, you know you saw it. And so there were a couple of folks named hellings and downs and they did this analysis and they showed what would happen if a gravitational wave passed over the earth and between us and a whole network of pulsars and what would happen is a predictable pattern in the way that the pulses arrive on earth you can google this and check it out if you're interested in learning more details but there's a particular signature we would expect to

see in the pulses from pulsars in the timing of those pulsars arriving here on earth Earth if a gravitational wave passed. And remember that we're not targeting fast gravitational waves, the ones that LIGO can see. Those are things where it's like 100 hertz. The frequency there is fast ripples in space and time. We're interested in slow ripples in space and time. We're interested in very long gravitational waves. We're interested in like the beginnings of black holes coming together. And not just little itty-bitty black holes like the ones that LIGO has seen. We're interested in super massive black holes, right? Because we think that those black holes also combined. We talked earlier about galaxy colliders, shooting one galaxy at another. Well, that actually

happens in the universe. I don't know who's controlling it or if anybody ever is, but galaxies do merge. We see evidence for this in lots of galaxies. We can tell that some galaxies have recently undergone a merger because they're sort of chaotic. And we can see other galaxies that have had mergers billions of years ago. We think that the Milky Way, for example, has remnants of other galaxies that it's eaten. So if galaxies have supermassive black holes at their center, then what happens when two galaxies merge? What happens when one eats another one? What you get is the merger of supermassive black holes. These things are black holes, not like just a little bit bigger than our sun. These things have masses like 10 million or sometimes billions of times the mass of

our sun. It's staggering. It's hard to even get your mind around. Now imagine two of them and they're coming together and they're eating each other. They're forming one huge grandma black hole, right? Well, that is going to emit a lot of gravitational waves. And in the very beginning, the very early part of that, while the galaxies are still merging, while those black holes are just beginning their dance, there's going to be very low frequency, long gravitational waves that take a long time to propagate in a long time to measure as they move through the universe. And so that's what a pulsar array could be sensitive to. It could see gravitational waves from the collisions of supermassive black holes, from the beginning stages of those collisions, while the two galaxies are still beginning

to form together. And we also don't understand the size of supermassive black holes. We know that there's a relationship between galaxies and supermassive black holes that typically the larger the black hole, the larger of the galaxy. But we don't understand how these supermassive black holes got so big. We look back in the very early universe and we see that there are already black holes like a billion times the mass of the sun only a billion years into the history of the universe. And in our calculations, that's just not enough time to make that big a black hole. It's a deep mystery how these supermassive black holes got so super massive. And so one way to figure this out is to see them merging, is to understand what happens when these two things combine, is to look at

the early parts and say, oh, okay, this came from two slightly smaller black holes or maybe three or maybe something else entirely is going on. That's why we are desperate to listen to these messages and to understand what's going on with these very low frequency black holes. So we talked about how to listen to low-frequency black holes by building a system of pulsars all across the galaxy and watching as the signals from those pulsars shift in frequency as a gravitational wave passes. And we talked about what might be generating those gravitational waves. I want to tell you all about an experiment that claims to maybe have seen some of these low-frequency gravitational waves by using a pulsar array. But first, let's take another break.

December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys. Then, at 6.33 p.m., everything changed. There's been a bombing at the TWA terminal. Apparently, the explosion actually impelled metal, glass. The injured were being loaded into. ambulances, just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, and it was here to stay.

Terrorism. Law and Order Criminal Justice System is back. In Season 2, we're turning our focus to a threat that hides in plain sight. That's harder to predict and even harder to stop. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple podcasts or wherever you get your podcasts. My boyfriend's professor is way too friendly and now I'm seriously suspicious. Oh, wait a minute, Sam.

Maybe her boyfriend's just looking for extra credit. Well, Dakota, it's back to school week on the OK Storytime podcast, so we'll find out soon. This person writes, my boyfriend has been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now, he's insisting we get to know each other, but I just want her gone. Now, hold up. Isn't that against school policy? sounds totally inappropriate. Well, according to this person, this is her boyfriend's

former professor and they're the same age. It's even more likely that they're cheating. He insists there's nothing between them. I mean, do you believe him? Well, he's certainly trying to get this person to believe him because he now wants them both to meet. So, do we find out if this person's boyfriend really cheated with his professor or not? To hear the explosive finale, listen to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcast. A foot washed up, a shoe with some bones in it. They had no idea who it was. Most everything was burned up pretty good from the fire that not a whole lot was salvageable.

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I was just like, ah, gotcha. On America's Crime Lab, we'll learn about victims and survivors, and you'll meet the team behind the scenes at Othrum, the Houston Lab that takes on the most hopeless cases, to finally solve the unsolvable. Listen to America's Crime Lab

on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. I had this, like, overwhelming sensation that I had to call it right. And I just hit call, said, you know, hey, I'm Jacob Schick. I'm the CEO of One Tribe Foundation, and I just wanted to call on and let her know. There's a lot of people battling some of the very same things you're battling. And there is help out there.

The Good Stuff Podcast, Season 2, takes a deep look into One Tribe Foundation, a non-profit fighting suicide in the veteran community. September is National Suicide Prevention Month, so join host Jacob and Ashley Schick as they bring you to the front lines of One Tribe's mission. I was married to a combat army veteran. He actually took his own life to suicide. One tribe saved my life twice. There's a lot of love that flows through this place and it's sincere. Now it's a personal mission. Don't want to have to go to any more funerals, you know.

I got blown up on a React mission. I ended up having amputation below the knee of my right leg and a traumatic brain injury because I landed on my head. Welcome to Season 2 of the Good Stuff. Listen to the Good Stuff podcast on the Iheart Radio app, Apple Podcast, or wherever you get your podcast. All right, we're back and we are talking about using the entire galaxy as a gravitational wave detector. We reminded ourselves that gravitational waves are these ripples in space and time. Sometimes they're generated when two small black holes merge become a larger black hole,

but they can also be generated by supermassive black holes as galaxies merge and their central masses do a dance to find out who's going to be in charge of the new galaxy. And you can use pulsars to watch these things happen. Pulsars are very regular clocks that send us pulses at a very precise intervals. And as a gravitational wave passes between us and them, shortening or extending the distance between us and them, it can change the frequency of which those pulses arrive and give us a clue that a gravitational wave may have passed us.

And this is not a brand new idea, which means people have been doing this for a while now. There's a group called nanograv, that's been doing it for the last 15 years. They have a set of about 45 pulsars that they've been listening to very regularly. They pick them and they watch them with radio telescopes and they observe the frequency at which these pulses arrive here on Earth. And after 12 and a half years, they think they see something interesting.

They see something which they can't explain. They see deviations in the patterns of these pulsars, right? And that's exactly what you would expect to see if there was a gravitation. wave. You'd expect that the pulsars wouldn't be sending you their pulses at the very precise atomic clock level calibrated pulses that we're used to, but that there would be these deviations. Now, nanograv is not his own experiment. It's, of course, using pulsars that are already out there in the universe, and it uses telescopes that already exist on Earth. For example, the Green Bank Observatory that we once talked about in the center of the radio quiet zone in the United States where you're not

allowed to own a telephone or turn on your microwave. They used the Erescebo radio telescope before its unfortunate collapse, and they use all of these things together to try to monitor all of these pulsars. Now, in January of 2021, they released their preliminary results. And what they see is not consistent with no gravitational waves, right? It's not what you would expect if everything was normal. Unfortunately, it's also not consistent with gravitational waves. We talked about how if there were gravitational waves, you would expect to see sort of a regular pattern. You would see pulsars in one direction from Earth looking closer to you and pulsars in another direction looking further because gravitational waves squeeze space in one direction and lengthen it

in another direction. So that's not what they see. What they see can't be explained by gravitational waves, but it also can't be explained by anything we know. And that's exciting, right? Because every time you open up a new kind of eyeball or build a new kind of ear to listen to the universe's messages, we hear a surprise because sometimes we go out there trying to answer one question and we get evidence to answer another one, one we didn't even know existed. We all remember stories of accidental discoveries. In fact, pulsars themselves were an accidental discovery. Somebody was out there looking to study quasars in the distant universe and hear their radio messages

and accidentally discovered pulsars. So it would be pretty funny if Pulsars then in turn gave us clues about something else in the universe we didn't even know to look for. There are several of these groups doing these studies watching Pulsar. It takes a while because we're talking about very low frequency events. We're talking about gravitational waves that could take years, decades, centuries to propagate across the universe. Not that they're moving slowly, but that their frequency is very, very long. So the information moves quickly, but the ripples in space. themselves are moving at a very slow speed just like you can have light traveling at the speed

of light having very low frequency waves like radio and the kind of things they can look for are not just super massive black hole collisions although that is super fascinating we're also interested in general in what is the gravitational signal out there we recently did an episode about the cosmic gravitational background because we suspect that the universe is filled with these low frequency gravitational waves. We know that everything that has mass and accelerates creates gravitational waves. That means that as the earth goes around the sun, it generates gravitational waves. It means that every time you run to the store to get a pint of ice cream, you generate gravitational waves. And so there should be gravitational waves everywhere. There should be sort of hard to make

out. It's not like we can pick out individual things unless there are very dramatic events like two nearby black holes colliding. But in general, there should be sort of like a low level hum of gravitational waves in the universe. Some of them from inspiring supermassive black holes and some of them from neutron stars being formed or other black holes being created or supernovas should be generating gravitational waves. It should be everywhere.

So we should be able to sort of pick up this low frequency gravitational waves as it sort of sloshes through the universe. And if we see something in those low frequency gravitational waves that we don't expect, we might learn something new about the universe. For example, we said that one way to generate low-frequency gravitational waves that you could detect with a galaxy-sized pulsar array come from in spiraling black holes. Well, that might be true.

What it might be that the way these black holes merge, these supermassive black holes merge, is different from what we expect. There might be something else going on. And that might help us understand how they get so big and how galaxies form. Because when black holes pull each other together, mostly what's going on, is the force of gravity that dominates everything. But black holes have other properties as well, right? Black holes can spin because when something falls into a black hole doesn't lose its angular momentum. So if something falls into a black hole with angular momentum, then the black hole itself

has to spin. Angular momentum doesn't go away. Same way, electric charge doesn't go away. If you have a black hole which is electrically neutral and you throw an electron into it, what happens? Well, now you have a black hole with a charge. So there's a famous theorem called the note. hair theorem that tells us that black holes can have only those properties, mass, spin, and charge. And any other information about what's going on inside the black hole is hidden from you. And that's not because you can't give it a charge. It's because it's sort of unstable that the processes going on there will seek to balance it out.

If, for example, you throw a quark into a black hole. Well, a cork feels the strong nuclear force. It's a colored object where color is the equivalent of electric charge for the strong nuclear force. What happens if you throw that into a black hole? But there's so much energy there that it will pull other quarks out of the vacuum and eventually balance itself out. That's why most things around us don't have a strong nuclear charge because those charges are inherently unstable. So that's something that's going on with black holes. And black holes are able to neutralize all those forces except for spin, charge, and mass. But what if there are other forces out there? We know that there's a lot

we don't know yet about the universe. We know that there are huge questions that are unanswered. There might be entirely new forces out there. What if, for example, dark matter feels a force? Not a force that we're familiar with, but a force that only dark matter can feel with itself. Imagine if there was like a dark photon out there that interacted with dark matter particles that had a dark charge. In that case, it might be that supermassive black holes have more than just spin and mass

an electric charge, they might also have a dark charge. In which case, that could affect the way that these supermassive black holes fall into each other. It could have a powerful force that changes the way they're interacting and how fast they're falling into each other. And that could change the way these gravitational waves look. These very low frequency gravitational waves as they're beginning their dance would look different if there are different forces in play because it would change the frequency. Remember that the frequency of the gravitational wave is determined. by how fast the black holes are moving around each other, which depends entirely on the forces between them. So we could use these gravitational waves as a probe to look for new physics,

new beyond the standard model things that we do not yet understand. So while it would be exciting to see gravitational waves and have them be exactly what we expect, have them be just the kind of gravitational waves we expect from neutron stars and supernovas and in spiraling supermassive black holes. It might be even more exciting if these pulsar arrays detect gravitational waves that we don't understand that need new explanations, that need new ideas, because there are clues that there are things going on out there in the universe that we don't know about. And in the end, that's the biggest goal. That's the reason we listen to the night sky. That's the reason we do science because we want to find something new. We want to gain a broader understanding of what's out there

in the universe. We want to break the cognitive shackles of being. here on earth and be creatures of the universe. We want to understand everything that's out there and we want to use all of our tools to find it. Unfortunately, we are trapped here on the earth and can only use the signals that get here. But at the very least, we should pay careful attention to those signals. So even if they are little hints from pulsar timings, that something weird is going on in the space between us and the pulsars, revealing that something else weird is going on between supermassive black holes as they dance. These are the kind of clues that we need. to unravel these subtle little stories that tell us the deepest secrets of the universe.

So write to your politicians and tell them we should fund more science because we want to learn more about the universe. But until then, we will come up with clever and cheaper ways to listen to those signals from the universe and hope to unravel those mysteries. Thanks for listening to this crazy story of ingenuity and creativity in astrophysics. Stay tuned. And if you have a topic you would like to hear us talk about, please don't be shy, or if you have any question at all about physics or something you read,

I answer all my emails, so write to us to questions at danielanhorpe.com. Can't wait to hear from you. Thanks, everybody. Thanks for listening, and remember that Daniel and Jorge Explain the Universe is a production of iHeartRadio. For more podcasts from iHeartRadio, visit the iHeartRadio app, Apple Podcasts, or wherever you listen to your favorite shows. December 29th, 1975, LaGuardia Airport. The holiday rush, parents hauling luggage, kids gripping their new Christmas toys.

Then, everything changed. There's been a bombing at the TWA terminal, just a chaotic, chaotic scene. In its wake, a new kind of enemy emerged, terrorism. Listen to the new season of Law and Order Criminal Justice System on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. My boyfriend's professor is way too friendly, and now I'm seriously suspicious. Wait a minute, Sam. Maybe her boyfriend's just looking for extra credit. Well, Dakota, luckily, it's back to school week on the OK Storytime podcast, so we'll find out soon.

This person writes, my boyfriend's been hanging out with his young professor a lot. He doesn't think it's a problem, but I don't trust her. Now he's insisting we get to know each other, but I just want her gone. Hold up. Isn't that against school policy? That seems inappropriate. Maybe find out how it ends by listening to the OK Storytime podcast on the IHeart Radio app, Apple Podcasts, or wherever you get your podcasts. Do we really need another podcast with a condescending finance brof trying to tell us how to spend our own money? No, thank you. Instead, check out Brown Ambition.

Each week, I, your host, Mandy Money, gives you real talk, real advice with a heavy dose of I feel uses. Like on Fridays, when I take your questions for the BAQA, whether you're trying to invest for your future, navigate a toxic workplace, I got you. Listen to Brown Ambition on the IHeart Radio app, Apple Podcast, or wherever you get your podcast. This is an IHeart podcast.