Daniel and Jorge Explain the UniverseHave we seen more or fewer gravitational waves than expected?
Daniel and Katie talk about what physicists expected when they turned on a new kind of ear for listening to the Universe.
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Hey, Daniel, has the Large Hadron Collider found fewer or more particles than expected?
I guess it depends on what people expected. There were some physicists who thought we'd find zero, and some expected hundreds of particles.
Hundreds that seems like a lot.
Well, you know, in the sixties we had this era called the particle Zoo. Basically every time you turned on the collider you found something new. This time, unfortunately, we've only found the one particle.
Why is it so hard to know how many particles you're going to find? Don't you guys kind of have an idea of what you're doing?
You know, research is exploration in the universe is full of surprises.
Maybe it should be full of physicists who are also psychics.
Maybe if we stuck physicists in the collider, they would turn into psychics.
Have you ever tried sticking your head in the beam, Daniel? Did you get any premonitions?
No?
But if any listeners out there want to volunteer.
Write to me, there's going to be so many volunteers, the same volunteers who want to go set up the Mars colony.
I don't know if getting your brain fried makes you more or less likely to volunteer to go to outer space. Hi, I'm Daniel. I'm a particle physicist and a professor you see Irvine, and I definitely do not have the authorization to put your brain in the beam.
Hi, I am Katie. I am not a particle physicist. I am more interested, well not more interested, but more familiar with the animal sides of things. And I've put my head in a few beams and I feel great.
Well.
I have no comment about how you turn down. Perhaps we should ask your parents. But on a parent thing note, my wife is a biologist and I have sometimes come home to discover that she's included our children in her experiments.
Ah, the classic sort of using your own child as the labrat.
We had to at some point lay down very strict guidelines about how many parents have to say yes before we could take samples from our children.
She's doing some tongue scrapings or did she build like a maze for them?
No, our children are not the subjects of psychological experiments. It's more like collecting samples to get data. So it's totally passive and she was not really breaking any moral thresholds. But I did feel like, hey, we should have a conversation before you do any kind of experiments on our children, because you never know.
Just ask permission before you do a cheek swap. That's my philosophy.
And welcome to the podcast. Daniel and Jorge explain the universe, in which we explore all the sorts of surprises there are out there in the universe, the things we anticipate and the things we do not anticipate, including coming home to our wives experimenting on our children. My guest and normal co host, Jorge can't be with us today, but I'm very pleased to have with us Katie Golden. Katie, thanks very much for joining us again.
Yeah, and I have a good idea and maybe should start having kids so that I can experiment on them. This is this is an interesting idea because right now I have a dog and the only experiments I can do are treat based behavioral studies such as who is a good girl? Are you the good girl?
Well, you know, to some extent, every step you make, every dicis you make as a parent, you are kind of experimenting on your kids. You're saying, hmm, let me try this parenting technique and see if they grow up to be a serial killer or a Nobel Prize winner or both.
It seems like the child is the one putting you in the skinner box, and you are just trying to modify your behavior such that the child is happy.
Well. Like parenting, research is exploration, which means there are always surprises. You know, when we land a rover on Mars and drive around to see what's behind those rocks, we don't know what we're gonna find. When we train our telescopes on distant galaxies, we always find something new. The same is true and we turn on a particle collider trying to smash particles to create new, higher energies than have existed since the early moments of the universe. Research is exploration, which means that there are no guarantees about what you're going to learn. There's always something surprising out there to be discovered, which is one of the joys of research and exploration and also one of its frustrations.
I mean, it sounds like your interest in research is maybe the same as your interest in parenting. Like new things happen, little surprises happen. Sometimes they're great and sometimes they're very frustrating.
I do sometimes feel a sort of love for a really nice result that I spent a lot of time building, Like, oh wow, look at this little paper. Go out there into the world. I hope it doesn't get crushed.
You're just cradling Matt Lab like a little baby.
But there are a lot of other real connections between parenting and research, such as mentoring students. Just like every child needs a different kind of parenting, every student also needs a different kind of guidance. Some of them need almost no guidance and some of them need a lot of hand holding. Fortunately I haven't had to do much like actual disciplining when it comes to my students. But you know, it's a relationship for each individual one.
You have to sit in like the knotty corner if you describe a particle as a wave or a wave as a particle.
And so when physicists are not at home at making parenting mistakes, we are at work trying to understand the nature of the universe the same way that you are. And for thousands of years we had basically only one way to gather knowledge about the far flung corners of the universe. That was electromagnetic radiation. Light in all of its different flavors and kinds, from high energy gamma rays to long wavelength radio waves would come to the Earth and tell us what was going on in those other galaxies, the stars burning bright and creating those photons which would travel across the cosmos to us. But of course there are other ways to get information about what's happening in space. Sometimes actual particles made of matter and will fall to the Earth and we will gather them up cosmic rays and neutrinos and all sorts of stuff. But a few years ago we pioneered a brand new way to listen to the universe to collect information about what's happening, to be able to see and hear kinds of things we had never seen and heard before.
A kind of form of intergalactic semaphore.
In some sense, it kind of was a few years ago we made the fantastical reel and developed the technology to be able to listen to gravitational waves to detect these tiny ripples in space itself. Today, on the episode, I want to look back to that time and think about what we expected to hear when we first turned on this new kind of eyeball or earball, or whatever biological analogy you want to make for new technology.
I mean, I'm very interested because I've had ears for pretty much my whole life, and I have never heard gravity before.
So well, when the scientists finished building this apparatus, they didn't know what they were going to hear. And so today on the episode, we're going to answer the question have we seen fewer or more gravitational waves than we expected?
Now, this is the thing is, this is my first time learning about what a gravitational wave is, so I had no expectations how many we would see, given that this is my first time hearing about them at all.
Well, for me, it's one of those really amazing moments when you create a new channel between the universe and humanity and you allow nature to speak to us and you get to ask a question and get an answer. There are very few moments like that when you really are hearing something from nature and answer to a question, especially in a new channel you've never opened up before. Because It creates the possibility for great surprises because the universe is often very different from the way that we thought it was, and the only way to make those discoveries is to explore it, is to gather this information. So I was wondering what people thought about this sort of history of gravitational waves. Was it a surprise that we saw some have we seen more than we expected? Are the gravitational waves that we have seen the kind that we expected? Or is there something we about them? So I went out there into the Internet and asked our army of volunteers to give me their uneducated opinion on this question, which helps me understand what you listeners might have in your heads. So thanks very much to all the volunteers. If you'd like to participate for a future episode, please don't be shy. Write to me too questions at Danielandjorge dot com. So before you hear these answers, think to yourself for a moment. Have we seen fewer or more gravitational waves than we expected? Here's what people had to say.
Maybe fewer because I know waves and particles, you know, they you know, act like one another. So if there's a gravitational wave there have to be like a particle. Maybe we've seen a lot of waves without the particles. Maybe I'm going to say as much as we expected, but we want to see more.
I think we've seen more gravitational waves than we expected. I remember correctly. I think LEGO detected the first one in twenty seventeen, and as far as I know, there's also been a few detections since then, so I think we've actually been able to discover more than we were expecting.
I know that the LIGO has been able to detect some gravitational waves, but it's a really hard task. So if it could have better detection equipment, I think we would be able to see even more detections.
I would say that we have seen more because when we first before we first detected gravitational waves, I don't know if we were even expecting to see any. So since we detected some, I would say more.
I'm not sure how many gravitational waves we expected. I think we may have discovered more than we thought we would, but I'm not sure if it's more than what we expected.
I have a question, Daniel. Is it pronounced lego orlego?
I think it's pronounced ligo, and this also one in Italy called rugo.
I see, so I can't say lego, my ligo.
You can say that all you like, and you can even build your own lego out of legos, I believe.
What does logos stand for?
Lego stands for laser interferometer gravitational wave observatory where the W there is sort of suppressed. Otherwise it'd be like lig wo.
That would be a little bit too like French. I guess sounding I don't know.
Lig who bro sounds a little big California to me. Well, we've talked about gravitational waves a couple of times in the podcast, but maybe not everybody is really comfortable with this idea because frankly, it's a pretty weird concept and it's something that Albert Einstein predicted but also predicted we would never ever be able to detect. So it's worth taking them and to remind ourselves what are gravitational waves exactly?
Yeah, because we're talking about like lig wo BRO and gravitational waves. Is this something I could surf on? Is this something I could see? Is it something where if I jump during a gravitational wave, I feel myself get pulled down? Or Earth faster. I'm ready to learn, because this is an entirely new concept to me.
Gravitational waves are one of the amazing predictions of general relativity, and one of the coolest things about them is that they are waves, waves like we see in other things, meaning that they follow the same mathematical formula. It's incredible to me that the same sort of phenomenon appears in sound, and in water and in light, and also in the ripples of space itself. So that's what gravitational waves are. They're like updating information about gravity. They're wiggles in space.
That sounds like a kid's program, wiggles in space. I guess I'm trying to wrap my head around the idea of what is a wiggle in space? How does space wiggle?
Yeah, before we can answer how to space wiggle, we have to understand a little bit about what it means for space to bend at all, and they will put that together to understanding wiggles and space bending is the fundamental concept of general relativity. It tells us that the force that we feel of gravity, the reason that you are held to the Earth and the Earth is going around the Sun, is not actually a force at all. It's an apparent force, something we describe as a force because we don't really understand what's going on. And apparent forces are not something weird or magical. We experience them all the time. If you're in a Merry Go Round with your friends and somebody spins it, you feel this force outwards. You feel like somebody's trying to throw you off the Merry go Round, But there is no force there. Nobody is pushing you outwards. It's just a consequence of the fact that you're spinning. There's this accelerated frame of reference. You want to keep going in the direction that you were going, and that looks to somebody on the Merry Go Round as if you're being pushed, you know. Or if you try to throw a ball from one side of the Merry go Round to the other, it wouldn't move in what looks to you like a straight line because the Merry Go Round is spinning. That's the concept of an apparent force. It's not a true force. It's something that comes out of the properties of the system that we're in. And so gravity is also an apparent force. Space itself curves, and when we say space bending we're talking about the relative distances between two points. So as the Earth, for example, moves through space, the space in front of it is curved, and so it moves through what it seems to be the natural path, which follows that curvature. But to us it looks like somebody is bending it because we can't see that curvature of space directly.
So when I like jump from the top of the stairs down a few steps, it feels just like I'm falling. It's this normal, you know, I'm falling apart, am I just traveling through sort of the path of space that has been created by the gravity of Earth.
That's exactly right. Anytime you move according to gravity, you're in free fall. You're just following the curves of space. So for example, you get an airplane, you fly it pretty high, and you throw out a tennis ball. What happens to that tennis ball. Well, Newton would say it's accelerated by the force of gravity towards the center of the Earth. And Einstein would say, no, no, it's just following the curvature of space. It's just moving with space.
It's interesting that the curvature of space often like winds up at the pavement and then I hurt myself. But good job, space. Why couldn't you wind up always at a nice soft mattress.
And it's really a role reversal because Newton says, oh, that tennis ball is being accelerated, right, it's being pulled down towards the center of the Earth. And Einstein says, no, it's not. It's just free falling with gravity. And in fact, if you have an accelerometer or something which measures whether you're getting pushed or pulled on that tennis ball, then it won't notice any acceleration. It doesn't feel like it's being accelerated, because it's not. The acceleration comes when you splat on the pavement and the pavement accelerates you very rapidly upwards. Right, that's the acceleration. So Einstein reverses that because the.
Pavement is just in the way of the way that space is now shaped for you, right, because you're following this this sort of path carved out for you by gravity, and it just so happens that the pavement's right there in your way.
Einstein says, if you jump off a building, you're not accelerating as you fall. You only accelerate when you hit the ground, accelerate very rapidly and destructively upwards. Newton says that if you jump off a building, yes, you're accelerated down towards the center of the Earth. So it's two very different pictures about how gravity works. And Einstein's picture is beautiful because it tells us that space itself has this feature which was invisible to us until now. Right, we can't see the curvature directly. It's not like a road you're following you can see it curving ahead of you. It's invisible to us. So it looks like there's this weird, mysterious force acting on things and changing their paths, when really things are just naturally following the invisible curvature of space.
So before we actually had the abilities to see something like a gravitational wave, how did we know that Einstein was probably right and that Newton was probably wrong.
Well, Einstein makes a very different prediction from Newton in some cases. In many cases they're totally exactly the same. But for example, Einstein predicts that photon, that light itself can be bent by gravity. Newton would say, well, photons have no mass, so there's no gravitational effect. But Einstein would say that space is curved and photons follow that curvature, so photons can get bent around heavy things. So he made this prediction for what we would see in eclipse as light got bent around those heavy objects, and he was right and Newton was wrong.
So now I understand that space can basically bend, and this gravity is basically this bending of space. I just follow this path of space. So if I think about it, it's like I'm like on a sheet and someone's angling the sheet downwards, and I slide down the sheet, And so if you get a gravity wave, are you just sort of like wiggling that sheet a little bit.
It's very tempting to use the sheet analogy, but I find the kind of problematic because the sheet is two dimensional, and now you're sliding down, which implies some sort of gravity in the third dimension. What's really happening is that you're changing the relative distances between things to make one path shorter and one path longer. But you're right that we can put this conception together to understand wiggling because the source of that bending is mass. It's not like space is just bent willy nilly. Here and there it's bent around mass. So the Sun bends space around it. Now, what happens if you move the sun You sort of shift the Sun over a meter, Well, the bending of space has to follow, right, but that doesn't happen instantaneously. So if you shift the Sun over suddenly by one meter, then the bending of space propagates outwards. It's like a ripple in space. Now do the wiggles where you move the Sun one meter to the right and then one meter to the left, and you know, then do the hokey poke and turn it all around. If you're going back and forth, then those ripples are constantly being generated. And would you get are wiggles in space? That's a gravitational wave.
So if wiggles in space are caused by the movement of large masses, or I guess it would be caused by any mass size, right, would you just have a smaller wiggle for a smaller mass.
Yes. Gravity is super duper weak, and so while everything generates gravitational waves when it accelerates, only more massive things can generate gravitational waves that we have any chance of detecting, which is one reason why when we started out we looked for gravitational waves from huge black holes spiraling around each other super duper fast.
So given that for any kind of like gravitational wave that we would observe, it would have to be from something pretty.
Big, right, It have to be from something really big and not too far away.
So given that, I would think that it might be kind of rare, because I don't know how many masses of sort of wiggly bodies are too close to Earth. But you know, so, my guess, now that I understand what a gravity wave is, would be that we would only see them every so often.
Well, that was exactly the question they didn't know the answer to. They were building this new technology which could for the first time listen to these things. What they didn't know was is the universe noisy in this new spectrum? Or is it totally quiet? Right? Is there anything out there to here? They didn't know the answer because we don't understand the astrophysics of black holes. How often are they spiraling into each other and making this sound that we just built a new microphone for? That was the fundamental question.
Well, I'm going to say it happens once a month. That's my production as a very very educated person. But when we will take a quick break, I'll do a little back of the envelope math see if I got that right, and then maybe you can tell me how often we've actually seen these gravity waves.
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So we are back. I drew this very demonstrative tech tac toe on the back of this envelope, and I think it will lead you to some pretty interesting revelations about gravity wells. But maybe I should actually ask the particle physicist how many of these gravitational waves have we actually seen.
It's amazing to me that we've seen any. You know, Einstein predicted that these things existed, but he was very skeptical that we could ever see them because gravitational waves are so weak. I mean, the effect they have on matter is really really tiny. You take a ruler, for example, and a gravitational wave passing through it, which shrink it by one part in ten to the twenty or one part inten to the twenty one, which seemed so impossible to measure, and it took decades before anybody even really tried. We know now about the success of Lego and Virgo in twenty sixteen, but well before that people were thinking about the possibility, and there was some pretty colorful history about early attempts.
Yeah. I mean, I can't even wrap my head around how you measure a gravitational wave.
So what you're trying to do is see space wiggle back and forth. And so for example, if you have two objects at a fixed distance and a gravitational wave passes through it, what you'll see is those things get slightly closed and then further, and then closer and then further. So if you have some very precise ruler which doesn't shift also with the gravitational waves, right, it's like rigid where it uses laser beams, as we'll talk about later. Then if you could measure their positions super duper precisely and you have a way to avoid them otherwise wiggling, then you can detect gravitational waves. That's the key.
Sounds like you need someone with incredibly steady hands.
The first person is sort of take this seriously. It was a guy named Joseph Weber and in the late sixties he built this big rod made of aluminum, these huge aluminum cylinders that could vibrate a resonant frequency. And his idea was that gravitational waves passing through it would cause these things to resonate. And he attached these piezoelectric sensors to them to make them super duper sensitive to tiny, tiny wiggles in these cylinders, and so these are called Weber bars. And he actually made claims of discovering I mean, he saw wiggles in these things and he thought, I'm seeing gravitational waves. Now, at the time, this was not taken very seriously because nobody thought gravitational waves were observable at all, and so the idea that this guy had seen them was like, very very outlandish. Another problem was that nobody could reproduce his results. I mean, people were excited about it in principle, and a few other people tried building similar devices to see if they could see what he had seen. But nobody else ever saw the blips that Webber had seen.
So this is kind of the equivalent of someone saying, hey, I just held a seance and found a cure for cancer, exactly.
I had sort of a polarizing impact on the field of gravitational wave of astronomy. It made some people feel like, oh my gosh, this feel is poisoned. It's filled with Charlatan's And you know, there was some really colorful and heated exchanges at conferences. You know. In June nineteen seventy four, this physicist Garwin aggressively confronted Weber with a claim that they had found a mistake in his computer program that analyzed his data and said that Weber's model was insane quote, because the universe would convert all of its energy into gravitational radiation in fifty million years or so if one were really detecting what Joe Webber was detecting. So this got pretty hot and heavy, and there were like letters written back and forth and physics today. This is like a real physics feud.
I mean, it's very funny from the outside looking in that this kind of you know, very technical thing of detecting gravity waves would cause basically a physics riot, people turning over cars.
Well, it was an outlandish claim, right, and so while people are excited to believe it, also scientists are skeptical and they got to be persuaded. And you're going to make a claim like this, it's got to be rock solid. And Weber continued to claim until his death that his gravitational wave discoveries were real. And so, on one hand, nobody believes that he saw gravitational waves, and he sort of gave the whole field a bad name. On the other hand, he was kind of a pioneer. He had the courage to try something out there, something weird, something crazy, something nobody even thought was possible at all, and in that sense did inspire the next generation of gravitational wave astronomers, who in the end did figure it out. There's a quote from a famous physicist, Wheeler whose Fineman's grad school advisor, and he says, quote, no one else had the courage to look for gravitational waves until Weber showed that it was within the realm of the possible, So you got to give where's some props for like cracking this field open? Even if his claims of discovery never were born out.
It's definitely sort of a situation where nobody wants to be the first idiot to try something, and then once someone is the first idiot, it's like, well, at least I won't be as much of an idiot as that guy, so I get to try it this time. But that first idiot is often very brave, and as long as they don't die doing it by eating the wrong berries, they are I think heroes.
Yeah, well, you know, just be those berries to your dog or your kid or whoever else you're willing to experiment. So this whole field is facing two big questions. One is is it technically possible to see gravitational waves?
Right?
Like can we build a device that can detect these things? And the second is are there any gravitational waves at all? And you know, now we are seeing gravitational waves from black holes, But remember that back in the sixties and seventies, the whole idea of black holes being real was kind of new. You know, black holes were another prediction of general relativity, which for a long time people thought, nah, that can't actually exist in the universe. It wasn't until we found bright radio sources from the center of the galaxy and saw compact objects we couldn't otherwise explain. The whole idea of black holes went from crazy land to realistic, and so this whole field is facing these two challenges simultaneously. But Weber did inspire a bunch of other folks who had ideas for how to build gravitational wave detectors using lasers. One of the problems with Weber is that his device was just too I mean, gravitational waves are super duper weak, and so if you need to see a tiny increase in the length of something, that's easier to do if the something is really really big, bigger than like your basement laboratory. You want something which is like tens of meters or even kilometers long, so the effect of the gravitational wave is larger.
I mean, it's sort of the blue whale model of physics, where if your prey is very very tiny, it actually helps to have a really really big net so that you can catch a whole bunch of them. The bigger the mouth, the easier it is to find the tiny things exactly.
And so a bunch of folks that Countech developed these ideas for using lasers. The idea is instead of building like a bar that's a kilometer long, just having to use an interferometer, which means you don't have an actual physical device. You now measuring the distance between two mirrors, for example, by shooting a laser beam back and forth. And essentially what you're doing is you're counting the number of times the laser has wiggled on its journey. Because light has a certain wavelength, as you actually split the beam and send it in two different directions, and so when those two beams come back, you interfere them, and if they wiggle a different number of wavelengths on their journey, then it changes the pattern of light that comes out in a way that you're very very sensitive to.
Oh interesting, So when you split a beam right and neither of them changes when they rejoin, would they sort of go back to the original state that they were in when they had split exactly?
If they both take exactly the same length journey, then when they come back they're the same part of their wiggle and so they add back up to be their original beam. If instead one of them has like gone a half wavelength further, then now it's wiggling down when the other one's wiggling up, and they would cancel out, they would destructively interfere. So by seeing that interference pattern, you can tell very precisely how far that photon has traveled.
That's really clever. But you got a whole the mirrors like really stable, right.
Exactly, And that is the whole challenge is stabilizing these mirrors against like a truck driving down the highway or you know, a fly flying by, or seismic noise.
From the under underground. I want to go underground.
So they developed this prototypic Caltech in their early eighties with these laser arms that were like forty meters long. Actually visited this thing when I went to Caltech to think about going to grad school there, and I remember thinking, this is a crazy experiment that's never going to work. And boy was I wrong, And I'm very happy to be wrong about it. That experiment improved and principle that you could do this laser interferometry. Though there were all sorts of like crazy cost overruns and all sorts of shenanigans and the sort of how they actually got hundreds of millions of dollars to build Lego. In nineteen ninety four, NSF funded it at two hundred and seventy million dollars, this huge, scaled up version of the Caltech project with arms that were kilometers long. It's an amazing story of like the right person at the right time who happened to be really into this despite like crazy cost overruns and mismanagement in the original project. But somehow the NSEF did it. It was the biggest project they've ever funded in their history.
That person who wrote the grant deserves some kind of award because writing a grant where it's like, now, we don't know if these things exist, and it's very unlikely we would ever be able to detect them. However, money exactly.
There's a guy the NSF, Isaacson, who said, quote, it should never have been built. There's a couple of maniacs running around with no signal having ever been discovered, talking about pushing vacuum technology and laser technology and material technology and seismic isolation and feedback systems. Orders of magnitude beyond the current state of the art, using materials that hadn't been invented yet.
Grant writers who get their grants like rejected for a reasonable proposal just snapping their pencils in half in rage right.
Now, yes, exactly, and so what kudos to them? At the time, a lot of astronomers were furious because this was a huge chunk of money which could have gone to other projects, and this seemed like a real boondoggle and if it didn't see anything, it was going to kill the whole field. So it's very controversial decision at the time. You know, now, of course it's one Nobel Prizes that were all grateful for it, but at the time it was a really really big risk for the NSF.
It's like the saving Private Ryan of the physics world.
So the first iteration of the experiment Ligo ran from two thousand and two to twenty ten, and they didn't see any gravitational waves, and you know, they could have. They were sensitive to gravitational waves, but they weren't as good at like tamping down the noise and made everything really crisp and clean as The next version of Logo was the one that actually made the discoveries. So for them to have seen something, the gravitational waves would have to be like an elephant stampede. It was really more about the developed in the technology than actually discovering anything.
Two elephant shaped black holes crashing into each other.
So they made that work and it meant a lot of stuff along the way, and then they shut down to build advanced LEGO, which meant like reducing the noise even further. Because the quieter you can make the environment for your lasers, the smaller the wiggle you can actually detect and be confident in, which means you can hear fainter signals, which means you can hear signals from further out in the universe, so you're sensitive out to like larger distances.
Now, as a podcaster, which is basically the same complexity as being a part of CULL physicist, I always struggle to get the sound quality good. I mean, right now, I'm traveling, so I'm actually in a closet trying to reduce the amount of bouncing of audio waves. So how do you do this though with something like this, Because you can't just put a bunch of foam around it. We're stuck on earth. Well we're not, but for this we are. And you have all this movement of the earth, how do you and like you can't even going underground, you still have I would assume some seismic movement underground. So what do you do to cancel that out?
So they have a lot of different tricks they use. One is that they put it on a pendulum. They basically balance the whole thing on a string, which helps decouple it from the motion of the earth. But then they actually add another pendulum to that one, and another pendulum, so they have like four pendulum stages, so that you can like shove the top of this thing and the mirror itself will hardly move. On top of that, they have like active servos to reduce seismic noise, which is sort of like noise canceling headphones, you know. They detect wiggles in the earth, and then they unwiggle the mirror and sort of to move it the opposite direction to prevent the mirror itself from moving.
Now, have they tried plugging in a chicken brain? Because chickens and other birds are really good at stabilized their heads. You can grab a chicken, buy its body and move it around and its head stays perfectly stable. So it sounds like servos are just basically little robotic chicken brains.
Yeah, that's actually the internal name they used in the experiment is robotic chickenhead. You know, that's how they referred too. So they rebuilt this thing. It's much more powerful, it's much quieter, and now they're sensitive to gravitational waves from a much larger part of the universe, or just quieter gravitational waves. So this is like the end of twenty fifteen. It's in September, and they're turning this thing on and you know, nobody knows what they're going to hear. And incredibly, what they heard on September fourteenth, just before eleven in the morning was a huge, perfect gravitational wave. It's basically like the first few days after they turned this thing on, they heard a perfect one. It was so nice that people thought they were being fooled. And the experiment had actually done test runs where they like injected fake gravitational waves into the data to see if people know them, you know, just to like figure out if their systems were working to keep people honest. So everybody thought, oh, this must be one of those test ones ha ha ha. But it wasn't. It was real.
Wow, that's amazing. That's like stepping outside and suddenly seeing like a new planet just like waving at you across the sky.
Or it must have been something like you know, the development of vision. There were like hundreds of millions of years before anything on Earth, any kind of life, was sensitive to photons, right, and then once we opened those first primordial eyeballs, we're like, whoa, there's a lot to see out here.
That little flatworm was probably wigging out, like what is going on? I don't even have a brain yet, and yet my mind is blown. This sounds really incredible, Daniel. If only we could hear from someone who's actually studied.
This, and fortunately we can. What. I had a fun conversation with cosmologist Angelika van Son who told us all about why it was the surprise to hear their first gravitational wave, how many we have seen, and what we have learned about how many black holes are out there.
Well, why don't we take a quick break and when we get back, I think people would like to hear from the expert.
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Okay, we're back and now we're going to hear from a real expert on gravitational waves. Here's my interview with cosmologist Angelika van Son. So it's my great pleasure to introduce the program. Leka van Son a cosmologist at Harvard working on black holes and gravitational waves, and who recently wrote a paper with the phrase monstrous black holes in the title Nika, Welcome to the program.
Thank you, thank you, Daniel, very happy to be here.
So we have some basic questions about gravitational wave astronomy, especially the first days. I remember when the first discoveries were made. Everybody seemed sort of surprised, like they had just turned this machine on and all of a sudden they were seeing a signal. Why was it such a surprise to everybody.
Okay, so there's multiple aspects here. So first of all, they did just turn a machine on, But you have to take in you account that the history behind us goes back much further, Like this was already advanced Lego that they were running, and so they've been already trying to detect black holes for like twenty five years. But inherently black holes are just very rare. So about one in every ten thousand stars forms a black hole, and that's just forming one black hole. What we were seeing with Lego Virgo, with gravitational waves was two black holes smashing in together, forming a newer, more massive black hole. So this brings us actually to the second part of the problem. Which is what's called in the literature the separation problem. So gravity is actually a very weak force, and so to merge two black holes within the age of our universe, which within a huple time, we have to put the black holes very close together on a very very short orbit. However, the massive stars that come before these black holes, that are their progenitors, they are much bigger than a small orbit. They're like a thousand times or one hundred thousand times is big. And so this is what we call the separation problem, which basically asks the question, how can we get two stars to form two black holes and such short orbits when they cannot have worn that pain. So this is basically the whole field of trying to understand how binary black hole mergers form. And this is exactly where all the uncertainties come from. But before the first detection, we thought this was very rare. This never happens most of the times stars they don't form black holes right at the right location to form merging black holes.
So, just to make sure I understand, you're saying that in order to make a black hole, a star has to be sort of already anomalously big, and that also to make a pair of black holes that will merge. They have to be close together, and it's hard to understand how you get two really big stars so close together. So they formed two black holes which can end up merging.
Almost the thing is indeed, these two black holes have to be really close. But the stars, throughout their life they change in their size, So throughout their life before they die and form a black hole at one point, they will be really big stars. When they get older, they swell up, just like humans tend to do. When they get older, they become much bigger, and so they become so big that they wouldn't fit next to each other anymore. And what would happen. What we expect would happen is that these stars would merge as a stellar merger. But that didn't happen because we formed two black holes. So in some way, these stars managed to lose some of their weights, to exchange some of their angular momentum, and to put the stars on different orbits than they started. So they might have started out very wide, but then exchanged masks between each other to finally end up as as very much smaller orbit but much more compact, forming two black holes.
I see, so we thought this sort of arrangement, this particular dance was pretty rare, and it would be hard to see black hole mergers because this configuration didn't happen very often in.
The universe exactly. And this really shows how little we actually know about binary interactions, which is this exchange of mass between stars and massive stars in general, because massive stars it was not a very popular field before gravitational waves came around. If you came to your supervisor or advisor twenty years ago and you'd said, hey, I want to go and study stars, they'd probably say, wow, stars, that's all figured out. We know that by now, we don't have to do anything anymore. Except then they figured out in twenty twelve that most stars don't live alone. They actually live in pairs in these binaries, which can then form binary black holes. And we also found out that massive stars are why different from low mass stars, And we don't have any idea how these high mass stars live their life and die.
What are the mysteries there? I mean, they are hotter, they burn brighter, so they shorter lived. What are the questions people are still asking?
This hotter and brighter part is actually what makes it so difficult to study these stars while they're alive, because basically they live fast and die young, and as I mentioned before, they're intrinsically rare. Most stars that form will form as lower mass stars, and only a few stars will form a very small fraction of all stars will form as high mass stars, where with high mass I now mean something of about twenty times as massive as our sun, because that's what you need to form a black hole. But because they're so intrinsically rare and so short lived, we only have a handful of stars observed while alive of the order of a fifty to one hundred solar masses, so that solar mass is this unit that we use for how massive stars are and black holes. And even when you study them, they're often shrouded in all these clouds of dust literal dusts that they've expelled themselves, making it very hard to actually observe any of the internal properties or if any of the properties of these stars. So we don't know how big they get, we don't know how much of their fuel they will use, we don't know if they're spinning, we don't know how strong their winds are. And that's just for single massive stars. Then if you make two massive stars, it's double the trouble. And so we don't know when they interact with each other, when they interact, how it proceeds, like is it a stable or an unstable interaction? So there's a lot of question marks there, and we actually hope that gravitational waves can help us shed some light on the line life of these stars because black holes are basically foss sales of massive stars, and so we can use them to learn something about stars that lived long ago.
All right, So then take us back to September twenty fifteen. They just turned on advanced lego. Nobody ever seen a gravitational wave with this history of like Joseph Webber and all of his claims, the field was sort of not the most exciting one, maybe not the one people thought would yield the discovery. And then they turned this thing on and they see a signal within days or weeks of their new telescope opening this eyeball to the universe. What does that mean about our understanding of these massive stars and how often binary black holes form? Did we totally undershoot it? Were there some people who were predicting we would see something, or was it a surprise to everybody.
Yeah, basically before that, we indeed definitely undershot the expectation. Our population models expected much lower rate of binary black hole and black hole, nutron star, etc. And indeed, it was right on the first date that they turned the detector on they got this beautiful signal that was so beautiful that people thought they were still looking at a test signal and someone forgotten to put the test off. But yeah, we heavily undershot with the rates. But as I mentioned, the theory of massive stellar evolution is still very uncertain. There's lots of things we need to learn about how these stars live their life, and this causes the main uncertainty in how we predict what the rates of these events will be. And so currently we're still at the stage where our models have a rates prediction that varies over multiple orders of magnitude. So the rates of Lego Virgo, the rates that we see today are within that error. But we also can heavily overshoot or heavily undershoot depending on what parameters we adopt in our models, and so peace by piece, as we are getting more information about these gravitational waves, we can constrain parts of our models, and then not just using the rate, right, but also using the masses, their spins, their mass ratios, all the properties that we can get our hands on, and using that we can slowly understand our models better and constrain them as well.
I see, so we've measured now a bunch of gravitational waves. We were surprised to see them so quickly, which means we undershot them. And now we sort of adjusted all of our estimates up to match what we've observed. Can we then make any predictions? Are we just fitting these rates to the data or do we have any other alternative way to make these predictions?
Now?
Do we have like a deeper understanding of the internal mechanisms of the massive stars, any other handle on this?
Yeah, so definitely what we were trying to do is approach us from multiple sides, right, We're trying to make these predictions from our knowledge up right, So we start with what we know about stars. We say, okay, we expect to see this many black holes, and then some of their models actually overshoot. Some of those models undershoot in the rate that we observed. But to make a full picture of massive stars and black holes and gravitational waves, we should use all the information that we have, so we can also tailor other transient events such as supernovae, which happens when a star dies, so that's a step before you form a merging black hole or any black hole. And the supernova rate is also something that we've observed through normal electromagnetic waves. But ideally we want to match all these different rates at the same time, which will give us the stronger constraints on which of our models make some sense and which of our models don't make sense.
Interesting And so now we have this little sample like roughly one hundred or so gravitational waves from these mergers, what are their feature in there? Like we can measure the masses. Are there surprises in the masses of these black holes that we're seeing, or they're like clumps or gaps or things in there that we didn't expect.
This is one hundred percent the way that we're going. The distribution of masses contains more information than just the rate, and at this moment we've seen two features, certainly in the mass distribution of merging binary black holes. We've seen one feature at higher masses, which has received by far the most attention and has been really in the news a lot, and that is because this feature has been this kind of bump in the mass distribution has been linked to something that we call parents stability supernova, and that's a type of supernova that basically causes the most massive stars that would form the most massive black holes instead of forming a black hole, they will die premature, so they will go into supernova premature living they haven't finished all their burning cycles yet, and this causes them to explode without forming any black hole at all. And so theory predicted that single stars or massive stars couldn't form black holes with masses above about forty five solar masses, and that there should be a sort of bump in the distribution right below that gap. And so the bump that we saw in the mass distribution at thirty five solar masses, people were really excited because they thought, oh, that's that bump from the parent's stability. So that means that that is the limit where normal stars can form black holes. But we don't see a gap above this bump. We still see black holes. So basically these black holes shouldn't exist, but yet they are there. And so this intrigued a lot of people in trying to figure out how can you form black holes above this kind of cutoff mass from massive stars above this parents stability limits, and so that excited a lot of people coming up with all kinds of wild ideas going from bosone stars to just galaxy clusters where you have second order or second generation black holes. But more recent work actually points towards that bump not being related to the parents stability. So I think at this point we don't know what the bump is caused by, and the real bump, the real parents stability limit, should be somewhere around sixty solar masses instead of thirty solar masses where we see it now, And so I'm quite excited for the next run to see if we will discover an extra feature at sixty solar masses. I see there we will exactly, and I want to understand what the bump at thirty five is doing as well. And then there's also the low mass bump. So that bump actually is also really interesting because at the moment we're not really sure if it's a bump or if it's just a continuous line. And this bump is very much related to something that we've called the neutron star black hole mass gap, which was actually thought of twenty years ago. Twenty years ago, people were looking at black holes through X ray observations, so that's you can only see black holes through X ray observations if they are creating mass, And they were looking at these black holes that were treating mass, and they said, hey, all of these black holes are significantly more massive than nutron stars. So there's a gap between the most massive nutron star and the least massive black hole that we can form. And since people came up with this observation twenty years ago, it has been heavily debated over the full twenty years. But the gravitational wave detections that are now rolling in are providing us with a new opportunity to measure if this gap is real. And there's a few observations that are already contradicting this gap between neutron stars and black holes, saying that, oh, maybe there isn't a gap between nutron stars and black holes, which tells us a lot about how nutron stars and black holes.
Form, So why would there be a gap.
So that's a good question. This was actually a case where the theory came after the observation, so people came up with theories to explain the observation and X ray binaries saying that maybe supernova for nutron stars are just very different than supernova for black holes. Whereas, if you have a supernova for a nutron star that forms a nutron star, you managed to blow all of the outer layers away and you only keep the core nutron start that's in the inner bit of your star. Whereas if you become slightly more massive and you have a core that will form a black hole, then the explosion is no longer strong enough to really throw away all the outer layers of the star, and part of these outer layers fall back onto the proto black proteo compact objects. And so these extra layers add on an extra few solar masses, bumping the mass up to by a few solar masses and thereby creating a gap in a random mass distribution.
Wow, fascinating. So what are you looking forward to seeing in this next run of Advanced Lego and Virgo and Carga, the one that's just starting.
Yeah, So the most exciting observations are always the crazy outliers, because things that are outliers are things that we don't expect, so they mean that we are to readjust our theory or figure out something new, which as a theorist I love to do. I mean, something super fun would be something that contradicts both mask gaps. So something that is in the lower and this upper mask gap at the same time. That's something wild. I don't expect that to happen, which is exactly why you're so exciting, but.
It sounds like you're secretly hoping for it, exactly.
I always like when things have to be thrown over and we can start at the drawing table again.
Those are the most fun moments in history.
Absolutely exactly. So what I'm more realistically excited about is, as I just mentioned, at the low masses, I would like to know if that's really truly empty or not. Is there a gap between neutron stars and black holes or not, And is there another feature at about sixty solar masses or not. These are things that we can probably hopefully see in the next oh for a run. But also I've been talking a lot about the masses, but I'd be very excited if we got a certain observation of a system with a lot of eccentricity or with a lot of spin, because those parameters are more difficult to measure, So we kind of need a golden event in order to be able to make a proper observation there. So I would love if we have one or two golden events in the next run so that we can say something about these spins and eccentricities.
What makes an event golden is the orientation of the system relative to Earth or something.
Yeah, and it's location basically, So it's golden if it has a very high signals to noise ratio. And you can do that by being very loud and very nearby. So loud events are more massive events, and so you can also do things if they're more easily distinguished things if they're quind of extreme. Right, So if you have a very extreme mass ratio, it's quite nearby, it's easier to constrain what that is.
Extreme mass ratio means like a big black hole and a small black hole merging.
Yes, sounds a very big black hole at a very small black hole.
Cool. And what do you think are the prospects for these future facilities like the Einstein telescope or Lisa Do you think we'll see those things in our lifetime or in your career.
Yeah, I really really hope. So because I'm extremely excited about third generator. We call all these new facilities, we call them third generation detectors. We're currently we're kind of at second generation, and they're going to be wildly exciting because, first of all, they're going to give us thousands of golden events, so we're going to be able to measure everything to extreme precision. Secondly, because these facilities like Cosmic Explorer and the Einstein Telescope, they'll be much bigger, like ten times bigger than the experiments that we have today. They will allow us to see the very early universe, or how we would call it, the very high redshift. Especially they would see black hole mergers at red shift one hundred. And to bubble your mind about what rech Off one hundred means, if the universe were condensed into one year, then the Earth was born in August and humanity came around in the last I don't know, twenty minutes or so, then red shift one hundred would be ten am on January first, so that would be right at the beginning of the universe, and the first stars we expect to be born at January third, so later than where we could throbe the first merging black holes. Now, if you followed along a little bit, you might be thinking, wait, how are you forming a black hole without a star, and that'd be good question. We don't know, but there is a theory that you can form black holes without stars, and we call them primordial black holes, and these black holes that form out of the fluctuations of the birth of our universe. So it's very science fiction kind of spacey, but we would be able to detect these with next generation detectors. So there's just an endless possibility of things that we're going to see. Also, there's, of course, you have to keep in mind that we're seeing a whole new force of nature. Gravity is not electromagnetic lights, which we normally see. So there's just going to be a whole lot of unknown unknowns of things that we can't even predict that what they are, but we're probably going to see them. And then the third thing is that now all I've been talking about is earth based or ground based detectors, so detectors or gravitational waves that we put on the Earth. But if you really want to detect very different systems, you have to go to very different wavelengths, and you have to go to space because gravitational waves work similar to so if you want to see a more massive thing, you have to look at lower frequencies, just as if a very big bell will ring at lower frequencies than a very small bell. So big black holes low frequencies, just like big bells low frequencies, and the big frequencies that we want to go to are of the order of the size of the Earth. So LISA is this next generation space based experiment where we basically have three satellites following the Earth shooting lasers at each other and working as a gravitational wave detector like that, and they will be able to see super massive black holes. So now we're seeing black holes with thirty times the mass of the Sun. Then we will see black holes with a million times the mass of the Sun.
So the biggest black holes make the longest wavelengths gravitational waves, and to see them, we need a bigger detector to capture those long wavelengths.
Exactly awesome.
Well, that's it's like a lot of fun. Do you feel like in thirty years when graduate students have like, you know, thousands and thousands of golden events on their laptops. You're going to be like in my day, we were lucky to get just one. I wrote my whole thesis on two.
When the first it's actually no black holes was out there. Yeah, and they're going to be okay, hooomer or whatever.
Yeah. Well I feel that way because I share with undergraduates these data samples from the large ade On collider that have like tens of thousands of top quarks in them. And I literally wrote my PhD on four top quark events. So that's literally my experience. But it's wonderful. It's progress, right, I'm glad that everybody gets to marinate in all this data and hope we all find surprises in it.
Yeah, it will be a very exciting time. I'm looking forward to it the next thirty years.
So wonderful. Well, thanks very much for telling us about this exciting field, all the surprises in the past and some of the surprises we can look forward to in the future. Really appreciate your time.
Yeah, thank you very much for having me.
I mean, you could have instead of that amazing interview just ask me to guess a bunch of stuff. But you know that's fine. It's fine too to actually listen to someone who has studied this.
I love how you can hear in her voice the joy at these discoveries, you know, hearing these discoveries from the universe, and also the excitement for the future. There's so many things we still have to learn about gravitational waves, the kind of things that we might learn from future gravitational wave experiments, these crazy plans they have to build future detectors that are much bigger or that are floating in space. It's exciting to think about all the things that we might learn.
I mean, it really does sound like we have now discovered a new secret language of the universe.
We are definitely building out our capacity to listen to the secrets of the universe, to eavesdrop on the universe and violate its price.
That's a creepy way to put it, but I like it.
Maybe we need to get the universe's consent, or at least the universe's parents consent before we do more.
Experiments, before we swab the universe's cheeks. We got to get consent.
So there you go, folks. The answer is that we have discovered a lot more gravitational ways than anybody dared hope. As soon as we turned on this new kind of ear to the universe, we heard all sorts of stuff going on out there, and it hasn't quieted down ever since. So fortunately, the universe is a very noisy place when it comes to gravity.
Like whoa dude, cow banga serfs up on those gravitational waves.
Bra all right, thanks very much Katie for going on this tour with us of the history of gravitational wave astronomy.
Thanks for having me, Bro, and.
Thanks everybody for listening. Tune in next time. 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. When you pop a piece of cheese into your mouth, you're probably not thinking about the environmental impact, but the people in the dairy industry are. That's why they're working hard every day to find new ways to reduce waste, conserve natural resources, and drive down greenhouse gas emissions. House US dairy tackling greenhouse gases many farms use anaerobic digestors to turn the methane from manure into renewable energy that can power farms, towns, and electric cars. Visit you as dairy dot COM's Last sustainability to learn more.
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