What is a pulsar? How can we use them to map gravitational waves? What kind of technology does it take? I discuss these questions and more in today’s Ask a Spaceman!

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EPISODE TRANSCRIPTION (AUTO-GENERATED)

If there's something you can bet on in life that will guarantee a win, it's betting on the cleverness of a scientist. There's this perception that scientists are uncreative, unimaginative, just focused on numbers and methodology. And but this couldn't be further from the truth, folks. Science is hard. It's really hard making predictions, testing against observations, developing hypotheses. This is all hard and requires an insane amount of creative work like like think of, think of quantum mechanics and superposition and spooky action at a distance and and all the weirdness of quantum mechanics. Do you think an uncreative person or a group of uncreative people came up with quantum mechanics with how nuts it is? No. It takes a truly imaginative creative person to come up with quantum mechanics and then to test these ideas.

If someone just walked up to you and say, By the way, by the way, I think there's fusion happening in the core of the sun. First off, that's a crazy thought. That's a crazy, creative, ingenious idea. And then how do you test that? How do you know if that's true? How do you know if that's accurate or not? Think of how creative you must be in order to come up with a way to determine if the sun is fusing hydrogen into helium in its core or not. How do you test that? How do you figure that out? That requires an enormous amount of creativity and cleverness as another example. Today's topic. The pulsar timing array. We've got a problem here. Gravitational waves. Einstein developed the concept of the theory of general relativity, an astounding leap of imagination and insight into the natural workings of our world.

I've said it before, and I'll say it again. When Einstein developed special relativity in 19 05, there were a few other people that were sniffing along similar lines and probably would have come up with it. If Einstein had never existed, we would still have special relativity. We probably still would have gotten there relatively quickly with general relativity. It's a that's a lot bigger barrier that it was so out of nowhere. It was so clever and so smart and so ingenious that you have to make a really, really strong A like you have to dig really far in order to make a case that we would have general relativity like now, 100 years later, we probably would have, but it would have taken a lot longer and it would have come from a completely different direction. As far as I'm aware, there is only one other human being on the planet that it was even coming close to developing general relativity and and he was having.

And that was David Hilbert, famous mathematician who and he was having conversations with Einstein so it didn't exist in a vacuum. In outgrowth of general relativity is the existence of gravitational waves, these tiny little ripples in space and they're amazing. And and just the fact of the, uh, concept that gravity can support waves is really out there. Like just lean back and think about that. Close your eyes and think about the fact that gravity can make waves. Think of everything you know about waves and all the waves you've experienced in your life. Sound waves, water waves, slinky waves. I'm running out of examples, but you get the point now. Apply everything you know about waves to gravity, and gravity has waves. If someone just said that out of the blue, you would think they were crazy, but you're listening to a science podcast. And so you assume it's somewhat true. Gravity has waves.

That's weird. And one of the hardest things about gravitational waves is how weak they are. In fact, for decades we assumed that they would be impossible to detect that they are so weak that they you just couldn't do it. Even Einstein himself is like, Yeah, gravity makes waves. I figured this out. Yay! Me, I guess. But these are so small. This is just a fun little side. Interesting commentary on general relativity. But these are so weak we're not actually gonna measure them. But it's just a fun consequence. Isn't that neat? A half century later, we started seriously working on it. And then a quarter century century later, we actually discovered and measured real gravitational waves. And now, to detect gravitational waves. I've talked about LIGO and gravitational waves in General LIGO itself the Laser Gravitational Wave observatory, already insanely clever in their experimental setup and how they find gravitational waves.

But it has its limitations. Yes, LIGO is detecting gravitational waves emitted by merging black holes all the time. But it does have its limitations. It has its limitations in frequency. So there's all sorts of things in the universe that make gravitational waves. Technically, you flapping your arm around is generating gravitational waves. But understandably, those are not going to be very strong. And understandably, those are probably not going to be detectable. When massive, super energetic, super strong gravitational things happen in the universe. They generate a lot of gravitational waves, still incredibly weak but now detectable. Even the strongest gravitational waves washing over the earth right now can barely move an atom across its own width. That's the kind of strength of gravitational wave we're talking about these waves of gravity itself.

What kind of events generate gravitational waves? Well, merging black holes, supernova, the inflation that occurred in the early universe. We We're talking big stuff here, folks and LIGO can sense a lot of things, but it does have a very, very narrow window of frequency. It is almost like, uh, like a telescope or like visible light. We know that there's a broad spectrum of electromagnetic radiation, but our eyeballs can only see a small fraction of it. And there are all sorts of interesting processes throughout the universe that are generating all those different kinds of electromagnetic radiation. But our eyeballs can only see a very, very narrow band of frequencies. Of all the electromagnetic radiation. We can't see the radio. We can't see the infrared. We can't see the x-ray. We can't see. The gamma ray LIGO is really, really good at measuring the high frequency, relatively high frequency stuff, the things that are loud and brief.

And this is a consequence of its experimental setup. Because it's sitting there, you've got the weights hanging and and I encourage you to go back to the episodes on gravitational waves so you can learn more about LIGO because it's such a cool experiment. Uh, but it's waiting for these. These giant, massive things that are are hanging off of wires. And then, as the gravitational goes by the distances between those these two things, these masses changes and they're bouncing wings. There's back and forth, and it's super cool. The best way I can describe, like a bunch of lasers bouncing back and forth across things that are hanging. And then the gravitational wave passes through, and it changes the distance between those hanging masses and then by bouncing the lasers. You can measure that. But there's all sorts of other things that are constantly making those masses move around like a truck driving by on the freeway, someone sipping their coffee in the next room. That's how sensitive these detectors are. So LIGO look works best when it's something brief and loud.

When it's just a bang. Bang, There it is. You can see it. It pops up above the background. It has a really hard time hearing low frequency gravitational waves, things that are that was my impression of a low frequency gravitational wave. I hope you appreciated it. LIGO has a really hard time hearing those because it's working so hard to filter out stuff in the background of the trucks driving by and the people getting their coffee in little micro earthquakes that are happening near the detector all the time. It's working so hard. Uh, it it it just tunes it out. It can't hear that low and slow stuff. It can only hear the short and high pitched stuff so it can't give us access. LIGO and other ground based gravitational wave observatories can't give us access to a lot of gravitational wave events they can hear. LIGO can hear merging black holes. It can hear merging neutron stars. It can hear neutron stars merging with black holes, and then that's about it.

There's a lot more to the gravitational wave universe out there. For example, it can't hear giant black holes merging supermassive black holes. It can't hear them merge. It can't hear material in spirals where a smaller black hole gets gobbled up by a giant black hole, it can't hear the primordial gravitational waves left over from the early universe. These are all generating gravitational waves and big gravitational waves, for sure, but at a very, very low frequency. Imagine when when two black holes merge normal size, stellar mass black holes merge. The whole event is over and done with in less than a second, like it's done. But when two super massive black holes merge, it takes years or centuries for this process to play out. And as these giant behemoths are swirling around each other, they're stirring up space time. They're generating the gravitational waves, but it's over a much, much longer time scale because the just the objects are bigger and the orbits are bigger and just LIGO is going to miss it.

So if we want to explore more of the gravitational wave universe, you need something that's sensitive to these super low frequencies. And in order to capture really low frequencies, you need something really big that you can watch for a really long time. It's like the difference between visible light and radio visible light. I can use my eyeballs and and my irises are not very, very large, and they can capture a lot of visible light. But if I want to capture radio, I need a big antenna. I need a larger device to hear that low frequency emission, and I need to pay attention to it for a really long time. Someone flashes a light at me. I can hear it right away, but someone flashes radio. It takes a little bit longer to make that detection. So what can we use? What can we build or use to find low frequency gravitational waves? Well, what about pulsars? I know, I know. Hear me out. Pulsars are a certain kind of neutron star. Neutron stars are what happens when massive stars die but aren't big enough to collapse into black holes.

neutron stars are crazy. Some of the craziest objects in the universe. I love pulsars. They weigh several times the mass of the sun. They are no bigger than a typical city. And they rotate really, really, really fast up to several 1000 times a minute. Now, pulsars are made of almost entirely neutrons. They are essentially city sized atomic nuclei hanging out in the universe like it's no big deal. But they're not entirely neutral. They don't. They aren't 100% neutrons. They do have some protons mixed into there. They do have some electrons on their surface. And because they're spinning really, really quickly, you take electric charges and you spin them really quickly. You generate a magnetic field. This is how the earth gets a magnetic field. This is how the sun gets a magnetic field. This is how Jupiter gets a magnetic field. This is how neutron stars get a magnetic field. Now, these magnetic fields are a little bit stronger than the earth's, like a million or a billion or a quadrillion times stronger than the Earth's magnetic field.

They are very powerful magnetic fields and due to insert complicated physics. Here you end up with a situation where the magnetic fields drive the release of radiation out of the magnetic pulse like beams, just like a giant laser beam. I know it's not technically a laser, but I just imagine it at like as a laser. It's like a giant beam of light punching out of the North Pole and then punching out of the south pole of a of a neutron star. Imagine if the Earth out of its magnetic North pole was shooting a giant laser into space. That's basically what you get. Now this neutron star is spinning, but its geographic North Pole, the axis of its rotation, is never gonna be exactly lined up with the magnetic North Pole. Why should it be? That would be an extremely lucky coincidence. This even on the Earth. Our magnetic North pole is not the same as our geographic North Pole. So it's the same situation just more so, and what you get is this giant beam of radiation that's punching out of the magnetic north pole.

But because the magnetic north pole isn't lined up with the spin axis, it wobbles. This beam of radiation just draws these circles through space just I can't imagine what it would be like to see it in person. It would just be just be insane to be up close and personal up. A assuming I could live while I watch it. It's crazy to think about it. Most pulsars we don't see because what gives them the name pulsars. What makes pulsars a special kind of neutron star is when this beam of light this lighthouse flashes over the earth when it's making its circle. So it's drawing these circles on the sky, and and if the earth just happens to line up on that circle that it's drawing. When it passes over us, we see a flash, and then it circles back around and we get another flash, another flash and another flash. We see pulses of radiation. We see pulsars. If it doesn't line up with the earth, we just see a normal neutron star, even though it's doing the same thing.

It's just not lined up, luckily enough to to hit our patch of space. But before I continue, I need to take a quick break for a word from our sponsor. Better help. Mental health is so important, and you know I'm a firm, firm advocate for mental health. You you take care of your body. You go to the doctor when things are a little off, or just do regular checkups with your doctor. You should also take care of your mind. I know a lot of you tune in to this show to just escape and relax and have your mind blown. Well, maybe you should have your mind helped a little, too. I've gotten a lot out of therapy, and I'm not ashamed to admit it. I think it is a powerful tool for everyday life, and that's where better help comes in. Better help is online therapy. It's like a podcast where you get to do a lot of the talking. That's pretty cool. And someone's there to listen up. Real professional over the video, over phone, even live chat only sessions. It's more affordable than in person therapy.

You can be matched with the therapist in under 48 hours. This is a powerful tool for your everyday life, and I seriously encourage you. Even if you don't think anything is wrong, you will be surprised at how much therapy can help ask a spaceman listeners get 10% off of their first month at better help dot com slash spaceman. That's better. HE LP dot com slash spaceman. And the cool thing about pulsar is that they're clocks. These pulsations are pretty damn accurate. They they don't change very much over time. In fact, when the Voyager probes were launched, they included this golden record and on the Golden Record was, uh, our location of the solar system relative to other known pulsars with the the pulsation frequency of those pulsars marked. So supposedly an alien civilization would encounter the Golden record and then look at it and say, OK, what are they trying to tell us? Oh, I see.

These must be pulsars, I. I suppose they would make that logical leap and then they would because they're spacefaring civilization. They could go out and they would know all the pulsars and they would recognize them. And they say, Oh, yeah, yeah, that's pulsar over there. And that's pulsar that that one over there and they would recognize each pulsar by its unique rotation frequency and then reconstruct where the earth is and then invade us or something. I don't know what the ultimate plan of the Golden Record was, that's how accurate they are. That's how reliable they are. But if we want to use them for gravitational waves, and I know I haven't said how yet, but we're getting there, uh, they're they're they're not good enough. They're really good clocks, but they're not accurate enough. They do slow down over time. Very, very tiny slowdowns, but it's measurable. We we now have the technology and the sensitivity to measure those slowdowns. Sometimes the pulsars have, like star quakes where their crusts realign and they snap back into place and they release the amount of energy. And there are these glitches that will change the rotation frequency of the pulsar.

We can measure that now. Uh, all this, yes, is making the Golden Record slightly less useful than we hoped it would be. But there's a certain kind of pulsar that is better than the others. We call them the millisecond pulsars. We we think the millisecond pulsars and then we they get their name because they pulse on the order of milliseconds. We see a flash every few milliseconds. That is thousands of times per minute. These are the fastest known pulsars. We think they spin up because they've been recycled. We think that they were a normal pulsar, a normal neutron star minding its own business slowed down over the eons but then had a companion star that donated material onto it. And it's like pushing a kid on a merry go round and you're just pushing and pushing and pushing. And then they barf. That's a millisecond pulsar. They have been re rotated. They have been recycled, and now they mean business. They are spinning so quickly that they are much more accurate than a normal pulse are. They are able to maintain their rotation speed much, much better than a normal pulsar.

Because they're going so fast, nothing can slow it down. There are no glitches. There's no slow down there. There's no friction like these. Things are hanging out for a really, really long time, for example, to show you just how accurate these things are. There's one pulsar PSRB 1937 plus 21. Its pulsation speed is 1.5578064688197945 milliseconds with an uncertainty of that number of plus or minus 0.0000000000000004 The measurement of that pulsar and its pulsation rate, how quickly it spins is accurate to 10 to the minus 18 seconds, folks, that is the level of accuracy of an atomic clock. You can't get that without patreon patreon dot com slash PM Sutter. It is how you support the show, and I am extremely grateful for every contribution that's patreon dot com slash P MS U TT ER these These are nature's atomic clocks just hanging out in the universe, spinning once a millisecond and maintaining that rotation rate to an accuracy of 10 to the minus 18 seconds.

That is crazy. Why am I talking about pulsars when I should be talking about gravitational waves? Well, here's the clever bit, because millisecond pulsars are far away and they're accurate. They become gravitational wave detectors. Imagine a super super simple universe. There's us far away. There's a pulsar. And then somewhere over here, there's, I don't know, two supermassive black holes merging together. The two supermassive black holes are merging. They're releasing gravitational waves. The gravitational waves wash over us and wash over the pulsar, and they wash all through the space between us and the millisecond pulsar. What would happen as that gravitational wave washes over us and the pulsar, the distance between us and the pulsar would shrink and expand. As that wave sloshes back and forth, we get a little bit closer to the pulsar and a little bit further from the pulsar, a little bit closer to the pulsar, a little bit farther from the pulsar, when we're a little bit closer.

The light from that pulsar would arrive a little bit sooner because it doesn't have to travel as far as the space between us has literally shrunk. But that's what it means to be a wave of gravity. You are changing distances. We are now closer to the pulsar, so the pulse that flash of light from the pulsar it doesn't have to travel as far so it gets here a little bit sooner. Then, on the opposite end, when the wave passes or the next phase of the wave passes, we get stretched out. We're now a little bit further away from the pulsar, so the next pulse gets a little bit delayed. Now you can calculate, you know, gravitational waves are super weak. This isn't a huge effect. You can calculate how, how much the the pulsar timing would change. And it's around, like 10 or 20 nanoseconds, that's it. And these gravitational waves. When I say low frequency, you remember the the merging black holes with LIGO, and it's over and done with in a second. With these merging supermassive black holes, the waves are sloshing back and forth over the course of months.

So one month you'd see a pulse or set of pulses arriving 20 nanoseconds or or whatever earlier. And then a few months later, the pulses would arrive 20 nanoseconds later than average. You measure this over months and you're measuring nanosecond tens of nanosecond delays. But because millisecond pulsars are so dang accurate, nature has built an atomic clock for you. They are accurate enough to potentially detect this that if you want your pulsar long enough and a gravitational wave sloshes over you, between you and the pulsar, you can measure that 20 nanosecond delay because the pulsar itself is so accurate. The timing of the pulsar itself doesn't change. And so the only change left is from the gravitational wave. In principle, you can do this at any frequency. You could use them to find high frequency gravitational wave events, but you would need to measure it so often like you would need to measure the pulsation like every second, which we just can't do.

We don't have the technology to monitor enough pulsar that often. So that's why it's best to look at these pulsars and for low frequency stuff, because we can observe a whole network of pulsars like once a week or so, and capture these shifts. Now our universe is more complicated than just us a pulsar and one pair of merging black holes. Our universe is swamped in gravitational waves from all sorts of events. It's not just one merging black hole over here. There's one over here and another over here. There's one that went off a billion years ago. There's one over here that went off 2 billion years ago. There's one that's going, uh, going on right now. They're all happening together, and all their gravitational waves get mixed up, so this gets confusing. How do you untangle? How do I look at one pulsar and figure out the gravitational wave that is generating a shift in that one pulsar's timing. Because of the changing distance between us and the pulsar.

The answer is, you don't Here's another little bit of cleverness. Instead, you look for correlations. Instead of watching one pulsar, you watch a network of them. If a supermassive black hole merges with another one over here and its gravitational wave passes throughout the Milky Way galaxy, it will change the distance to multiple pulsars, sometimes delayed in time, sometimes at the same time. But if you're watching all those pulsars, then there should be matches where there's a shift in the position. A shift in the distance, a shift in the arrival time of the next pulse of one pulsar over here and one over here because the same gravitational wave changed our day distance to both of them. We should see these kinds of correlations. We should see these kinds of connections. We should be able to look at multiple pulsars and they'll be affected by the same gravitational wave event, and then another gravitational wave event happens over here, and it affects a different set of pulsars, so by looking at an entire network of pulsars, we can look at not individual events, but we can look at the general sloshing of gravitational waves that are constantly happening in our galaxy.

You don't get to see an individual event unless you're extremely lucky by, but by observing lots of pulsar lots of times you do get to see the background of these events, and you can tell about the statistics. You can say how often big mergers happen. You can say what masses are typically involved in those mergers of supermassive black holes. It's It's like looking out at the ocean, and it's completely dark. There's no moon, there are no stars. It's pitch black, and you have a bunch of buoys scattered across the ocean, and each buoy has a light on it. And if you look at one buoy, you can watch it bob up and down as the waves underneath it pass by. And if you look at another buoy, you can see that light bobbing up and down as the waves pass by underneath it. If the same wave passes through both buoys, you'll see a correlation.

They'll rise at the same time, and fall at the same time, or if the wave is passing from one side to another, you'll see one buoy wave go up and then down, and then a little bit later, the next buoy goes up. But if the wave comes from, say behind them, then both buoys will come up at the same time and both go down at the same time. And you can imagine 100 buoys scattered across the ocean. What's generating the waves? Well, a storm beyond the horizon. A passing boat that you can't see all sorts of sources are generating the waves in the ocean, and you can't see the individual source. But you can tell when the buoys are moving together. And if you watch every single time and you pay attention every single time. Uh-huh the buoys are moving together. That means they shared the same wave. That means there's an event back there by behind beyond the horizon, shrouded in darkness that I can't see. But I can tell that event happened, and every time I see correlated events, every time I see buoys moving together synchronous, I can start to build up statistics.

I can keep counting this and I can get a sense if I watch this over the course of months. I can tell when the seasons change when storms are coming, when there's a hurricane, how frequently those hurricanes happen, what their intensities are. I can learn about the deeper ocean and the source of those waves just by looking at correlations in the behaviors of buoys just by watching when the lights move. At the same time. These are the pulsar timing arrays. The arrays are not arrays of telescopes, but arrays of pulsars. They are a set of clocks in the universe in our galaxy that we use and that we watch and when they pulsation shifts in a correlated way when more than one pulsar shifts, we can use that to reconstruct a merger of supermassive black holes that happened billions of years ago. How crazy is that? How clever is that? These experiments have been going on for decades.

There's a few of them. The Nano Grav, the North American Nano Hertz Observatory for gravitational waves. There's the European pulsar timing array, the Indian Pulsar timing array. Nano grav recently announced the hint of a detection at the time of the recording of this episode. They've seen something they they believe it's a signal they'd see a definite shift in the pulsar timings. But it's not correlated. It just looks random. Uh, so they're not exactly what they what they saw. Like they are seeing changes in the timings of pulsars. But it's not that key correlation, so they can't pin it on a gravitational wave yet. But the pulsar timings themselves do seem to change. They don't know yet. We don't know yet if it's a hint of an unexpected gravitational wave background that we weren't prepared for. Or it's something about pulsars that we didn't understand before. We don't know nature has given us this potentially powerful clock an atomic clock in space, and by seeing and carefully studying how they change, how these clocks change, we might be able to reconstruct the history of black hole mergers or the consumption of material by giant black holes.

This background of gravitational waves are constantly sloshing over us like waves in the ocean on a dark night. We don't know yet if there's a signal we don't know yet. If there is an observation, we're seeing changes in the buoys but not in a way we expected. We're not exactly sure what that means, but I'm sure some clever astronomers will figure it out. Thank you to at Unplugged Wire on Twitter, John F on email and Daniel Kay on Facebook for the questions that led to today's episode. And, of course, thank you to my top patreon contributors and all my patreon contributors. Honestly, all of you are awesome, but especially Justin G, Chris L Barbeque Duncan M, Coy D, Justin Z, Nate H and F Nalla. Aaron S, Scott M, Rob H, Justin Lewis and Paul G, John W Alexis Aaron J, Jennifer M, Gilbert M, Joshua Bob HW, John S and Thomas D That's patreon dot com slash PM Sutter and hey, I love all the questions I get. Hashtag ask us Spaceman.

Ask us spaceman at gmail dot com. Go to the website. Ask us spaceman dot com. Follow me on social media. I'm at Paul Mats Sutter on all channels and you can use the hashtag. Ask a spaceman on those channels and I will see you next time for more complete knowledge of time and space

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