The Technical Challenges of Measuring Gravitational Waves – Rana Adhikari of LIGO
Here’s a cool video made from their measurements.
Craig Cannon [00:00:00] – Hey this is Craig Cannon and you’re listening to Y Combinator’s podcast. Today’s episode is with Rana Adhikari. He is a professor of physics at Caltech and one of the members of the LIGO team, who were the first to measure gravitational waves. Rana and I met at the YC Research conference. Shout out to Michael Nielsen, thanks for the intro. While there, Rana gave a talk about LIGO and their effort to parse all the data their collecting. He was actually looking for help and they still are looking for help. If you’re interested, you can reach out to Rana on Twitter @ranaxadhikari. I’ll link that up in the show notes. Just two quick announcements before we get going. The first is that YC is going on a fall tour where we’re going be doing a bunch of office hours and Q&A sessions all over the world. That’s at blog.ycombinator.com. The second is that YC applications are open for the winter 2018 batch. That link is ycombinator.com/apply. Alright, here we go. What is LIGO?
Rana Adhikari [00:01:00] – LIGO is a huge project aimed at being able to take the bending of space that we think is happening all the time and turn it into some kind of a signal that we can use and measure. It’s a transducer. It’s like having a voltage meter or microphone but it’s like a microphone for space.
Craig Cannon [00:01:22] – Can you do the beginner’s explanation of what the device actually is?
Rana Adhikari [00:01:27] – We now know based upon Einstein’s relativity theory from a hundred years ago that there isn’t a real force of gravity like there is a force between magnets or between charges or things like that, but instead the way that gravity works is that it curves space. It’s a lot like imagining what happens when you’re jumping up and down on your bed, and somebody else is jumping on their bed I guess. Because if this happens to you all the time and this is something you’re familiar with but, whoever is heaviest makes the big impression in the bed and whoever is littler has to account, what kind of analogy is this? Whoever’s littler has to account for the depression in the middle of the bed and adjust their jumping accordingly, and you tend to slide into the biggest dimple in the bed. Those who do trampolines, they’re jumping up and down on their bed, understand how you’re well-trained and understand how gravity works. So to detect this on the earth is incredibly hard. And people have long ago measured the curvature of the space due to the earth and due to the moon, and of the planets, and that sort of thing. And so we well understood that in fact space is curved but we had no evidence to support the idea that the curvature of space could travel through space as waves. The detection of gravitational waves for decades had been debated in the scientific community. People thought it was just imaginary, that it was waves of thought, people called it, because they thought it was just waves of mathematics. It was just some equation he wrote down but didn’t make any sense, just kind of a nonsense thing. Some decades ago people realized it was real and then some crazy people said, “Hey let’s try to measure this.” Even though it was millions of times, factors of millions not possible. But luckily, it’s just something I’m not capable of, but through a combination of optimism and courage and not knowing the right answers to several equations, they were able to start up the field, and start to look for these things. I think if they had known how tough it would be or that it was going to take 55 years to have success, probably no one would have started. Here in the modern times this is the way we do it is we use the tool of laser and interferometry which is for those of you who are interferometer afficionados, it is a Michelson-type interferometer with a lot of extra stuff added onto it. For those who are not, the concept is simple. It just has to do with interference. You take a laser, like a laser pointer, but much more expensive, and therefore much more stable.
Craig Cannon [00:04:36] – Is it a billion dollars now into the project?
Rana Adhikari [00:04:38] – Yeah, roughly. The laser itself is cheaper. Probably you could do the whole thing with a $100,000 laser. That’s about the laser cost. You split it in two and you send it in two separate directions, and then when the waves come back, they interfere with each other. And you look at differences in that interference to tell you the difference in how long it took for one beam to go one way, and the other beam to go the other way. The way I said it was really careful there because there’s a lot of confusion about the idea of, these are waves and spaces is bending, and everything is shrinking, and how come the light’s not shrinking, and so on. We don’t really know. There’s no real difference between the ideas of space and time warping. It could be space warping or time warping but the only thing that we really know is what we measure. And that’s the mantra of the true empirical person. We sent out the light and the light comes back and interferes, and the pattern changes. And that tells us something about effectively the delay that the light’s on. And it could be that the space-time curved so that the light took longer to get there. But you could also imagine that there was a change in the time in one path as opposed to the other instead of the space but it’s a mixture of space and time. So it sort of depends on your viewpoint. But this warping of space-time is what’s measured. We turn it into a real signal by putting the interference of the two beams onto a usual photo detector that’s like a solar cell that you would use. And so it turns light into electricity. And that’s the whole thing.
Craig Cannon [00:06:32] – And then you’ve measured it by looking at waves?
Rana Adhikari [00:06:37] – The whole measurement is right there in the electrical signal out of the photo cell.
Craig Cannon [00:06:41] – Okay, and so then, how do you go about converting it to, I have not seen a bunch of these, sounds of things you’ve measured, how do you go about that process?
Rana Adhikari [00:06:52] – Yeah it’s the same as like an electrical instrument. If you have an electrical guitar. When you play whatever you’re playing on there, it generates an electrical current in the pickup coils of the guitar. And that signal comes out through a little cable and goes into an amplifier, and then that directly makes sound, and the same for us. So the photo detector detects the signal which turns into electricity and then we take that electricity and we drive a speaker, and then it makes sound. I can understand why you’re asking that. That seems a little weird. How is it that the wave from outer space can directly get turned into a signal in a speaker? I think this is a whole another topic to discuss, it happens to be that the waves that we are detecting and the waves which are easiest to detect are exactly in the human audio band. The waves that you and I can hear with our ears, that’s the whole frequency range for gravitational waves that we can detect. Gravitational waves happen at all frequencies. But they’re really loud right in this band. And so our detectors aim for this band because we expected the audio band would be a good place. But our technology also happens to be only capable of detecting things in this band for a lot of technical reasons which I can tell you about. That happens to be the case. It is a little weird, but the signals directly make sound.
Craig Cannon [00:08:34] – What I wanted to talk about then, I saw in one of your talks, I guess it was from last year, you were talking about creating new kinds of mirrors to focus, rather the mirrors that you had created focus about the vibrations to certain parts of mirrors, but you were working on creating new mirrors to detect other things.
Rana Adhikari [00:08:53] – Yeah, exactly. When I first heard about it, it was, I was going to say when I first heard about it, I thought maybe it was unbelievable but I have to say, when I first heard about this project, I don’t think I understood enough to even understand that it was impossible. I never went through this disbelief-belief, I just kind of slowly merged into it. But a lot of people with good reasons say it’s impossible to make these measurements. And the reason is, if you imagine zooming in to where the black holes are in outer space, they’re a billion light years ago, a few billion light years ago. And the universe is only 13 billion light years, 13 billion years old. It’s a good fraction of the size of the known universe away. And if you get close to these black holes, as they’re merging and eating each other, the amount that the space is warping is enough so that, like, this water glass, it would be shattered but it would be stretched if it was stretchable, like this. It would be a huge stretching. But the wave as it propagates to us like waves do, they get it anyway, so imagine the stretching has a lot of energy in it but as it spreads out, the amount of energy has to be conserved as it propagates, and so the amplitude of the wave as it comes to us gets reduced like one divided by the distance. The energy in the wave goes one divided by the distance squared, like any other kind of radiation. We measure the amplitude instead of the power. We measure directly the stretch rather than the heat or something like that. We’re able to look a lot deeper into the universe than you would naively expect because our signal only decays one over the distance instead of the distance squared.
Craig Cannon [00:11:01] – Okay.
Rana Adhikari [00:11:03] – The wave comes to us and by the time it gets to us, because it’s billions of light years, the squeezing and stretching is much less than 500%. It’s more like a part in 10 to the 21 or 22. So that means if you have a, the whole earth for example is about 10,000 kilometers in size, and so the whole earth will be only stretching by about 100th of a micron. I don’t even know how to imagine that.
Craig Cannon [00:11:43] – What’s a micron?
Rana Adhikari [00:11:44] – Micron is like the wavelength of light. The diameter of what little hair I have here is about a hundred microns. It’s 10,000 times smaller than the width of this hair.
Craig Cannon [00:11:56] – It’s how much the entire earth would stretch when it was hit with a gravitational wave that you initially measured?
Rana Adhikari [00:12:03] – Yeah.
Craig Cannon [00:12:04] – Which was a large one.
Rana Adhikari [00:12:05] – Which is a very large one. We’ve not seen anything of that size since then, since that first one. That’s such a tiny distance. Our detectors are big. They’re four kilometers so they’re not 10,000 unfortunately. If I was in charge, I would drill through the center of the earth, and there would be mirrors. I would have put a big L in the center of the earth, and then there would be mirrors in both ends.
Craig Cannon [00:12:30] – That would be ideal.
Rana Adhikari [00:12:32] – That would be ideal and that’s what we would use. But I’m not yet in charge of everything.
Craig Cannon [00:12:36] – But then, okay, so to divert a little bit, can you explain how the Fabry-PÃ©rot interferometer works? Because it’s four kilometers but it’s also kind of like bounce back and forth at the same time, right? So effectively it’s actually longer. The laser travels a longer distance.
Rana Adhikari [00:12:54] – Yeah that’s right. What’s it like? The Fabry-PÃ©rot cavity means rather than just send the laser beam down, we send it into a thing which has two mirrors into it, and just like, I don’t know if everybody does this but, when I was a kid I wondered, here’s this bathroom mirror and what if, I can see my reflection, and then if I go into the fun house during halloween, I can see multiple reflections, and what if you put two mirrors together and you set a flashlight and you took it out, would it bounce around infinity times and explode the mirrors? I’ve always tried, I would wonder about it then. It turns out no, sadly. You cannot destroy the universe based on two mirrors together.
Craig Cannon [00:13:42] – One kid didn’t figure it out.
Rana Adhikari [00:13:44] – Yeah and the reason is at each mirror’s surface, a little less than 100% of light gets reflected. Some of it gets turned into heat or things like that. And so the Fabry-PÃ©rot optical resonators is just two mirrors facing each other. And one of them has a finite transmission. It’s set up so that, let’s say, 1% of the light comes out and 99% goes in, or 99% gets reflected and 1% goes through. That’s the 100%. And so when you set it up at first, you have these two mirrors and you put in the beam from this side, let’s say. And only a little bit gets in. And so that little bit gets in and starts bouncing around. But by the time it comes back, you’re already putting in more laser light. You’re constantly putting in more laser night. And that builds up constructively with the electromagnetic waves you’re sending in. And slowly the power builds up in the system just through this little leakage. And it builds up until the point where the amount coming out is about the same as the amount going in. And at that point you have, let’s say, a few hundred times more laser power in this system than if you had just sent in a single beam. And that’s not so challenging, it’s easily doable. And it gives you basically a factor of 200 extra sensitivity than you would get.
Craig Cannon [00:15:10] – So the extra power, the laser, generates more sensitivity?
Rana Adhikari [00:15:14] – Yeah it’s just like you said it though, it’s effectively like the laser bounces a bunch of times. And so you can imagine, here’s this space-time which is curved, so now the laser has to travel through this curved path, and so it’s a little bit longer of a distance, and when it goes down and comes back, it picks up a little bit of extra phase shift, it’s just a delay. And now that’s through one round trip. And if you do 200 round trips, you get 200 times the phase shift. And so that’s what we do.
Craig Cannon [00:15:45] – But does that net out 200 times the noise as well?
Rana Adhikari [00:15:51] – Yeah. The more times you go around and the more you pick up the signal from the mirror motion, and so the signals’ noise trying to get below the fluctuations of the mirror, don’t actually get better by building up more power in the system. And in fact it can even go the wrong way. At some point the quantum mechanical fluctuations and the number of photons that are in your system becomes so large that the mirrors are shaking. The more power you have, it just gets worse. There’s a limit to how much power you want to put in there before you get into trouble.
Craig Cannon [00:16:34] – Okay.
Rana Adhikari [00:16:35] – But, so you might reasonably ask why do you do it then if it doesn’t help? It helps just in building up the signal. If you’re limited by the fluctuations of the mirror motion due to the environment or something like that, then more power is not any good. But most of the time, in most cases, when people are doing precise measurements with lasers, they’re limited not by the mirror motion but by the noise due to the fact that the, they have a finite number of laser photons. If you just take a laser and put it onto a photo cell, and you listen to it with your speakers which you can do, it sounds kind of like a hiss. And that hiss is because quantum mechanically, the energy of the light is sort of is in discrete packets which we call photons. If you have a one-watt laser or something like that, you have some many billions and billions of photons, you end up with a hiss level of noise due to the basically the quantum nature of the light that you can’t get beyond. And increasing the laser power builds up your signal. If you double the laser power, you double your signal, but your noise due to these random photon fluctuations, only goes up, like, the square root of that power. So you win a little bit for that particular noise.
Craig Cannon [00:18:08] – And why not just have a super powerful laser?
Rana Adhikari [00:18:12] – We do have super powerful lasers.
Craig Cannon [00:18:14] – Okay but you need it to be even stronger?
Rana Adhikari [00:18:19] – Yeah. It is tough to build a super powerful laser. I’m trying to think of how to put this in context. People who have worked with these lasers will understand why we do it this way. If you have, these days if you have $50,000 in your pocket, you can go on to the internet and buy yourself a scientific research grade laser that’s a couple of watts and will work fine for you. We end up with a 200-watt laser. And there’s no such, you can’t buy anything like that these days. You can buy lasers that have that much power but their frequency is not very stable. Our laser is used as the meter stick for doing the measurement. It’s like each wave of the laser is one tick on the meter stick. And if you have an unstable laser, that means these little waves, instead of being very precise, are jiggling all over the place. It would be like having a meter stick where the tick marks are kind of dancing around. You can’t use it to measure anything.
Craig Cannon [00:19:25] – Right, and it’s so precise, it’s like just so I don’t mistake it, the first, the two-black-hole measurement, how much did it move?
Rana Adhikari [00:19:35] – About 10 to the minus 18 meters which means one billionth of about the size of an atom. It’s small.
Craig Cannon [00:19:46] – So yeah you can’t do that.
Rana Adhikari [00:19:47] – You can’t do that, so, there’s no laser in the world which is good enough to measure this. And so we take the best laser in the world that we can find and then we stabilize it and make it about ten million times more stable than what you can buy. It’s kind of just barely good enough. And we’re going to have to do better if we want to do better.
Craig Cannon [00:20:09] – Yeah so maybe that makes sense. What changed between LIGO and Advanced LIGO? What is a shift in the machining? What did you do?
Rana Adhikari [00:20:19] – We got a bunch of new younger people who are really smart. That is by far the biggest effect. Miraculously, they showed up just at the right time. They showed up for graduate school in 2010 or 2011 right when we happened to need them. Without knowing about the timing, they just showed up. And they finished some classes and then they decided to ship out and live at these remote sites in Louisiana and Washington. They just figured it all out. Day in and day out, they just figured out every problem and solved it.
Craig Cannon [00:21:04] – What are some concrete examples?
Rana Adhikari [00:21:07] – I mean there were some engineering changes in going into this new detector which a was 10 times more powerful laser. And we isolated the mirrors a lot better from the environment. And the mirrors are much higher quality. They’re super beautiful and much heavier and really good. There’s a bunch of these technical things which were changed. And each one of them on their own worked really well because the engineers who were constructing them did a great job. The problem was when trying to put it all together. These things just never reveal themselves when you’re sitting in a little room and designing your widget. You put it in a suitcase and carry it on the airplane and try to bolt it on to the four-kilometer billion-dollar machine, and then it’s just like total disaster.
Craig Cannon [00:21:55] – When I first heard you talked, you were talking about, to deer getting close to the tube and all these things. How do you isolate?
Rana Adhikari [00:22:04] – These things just don’t matter at all here in our labs at Caltech. You measure one thing or two things but the full problem of putting it all together, and what’s the problem that only shows up when you have hundreds of kilowatts of laser power and giant mirrors, and it’s all running together. For example, I would say and it’s still one of the toughest problems, and it’s not completely solved, is it has to do with the interaction between the laser beam and the mirrors. Normally when you think about these things, you say, “Well the laser beam goes out there and it bounces off that thing and it comes back and that’s all there is to it.” And maybe the mirror is shaking around so that’s a problem. But in fact there is so much laser power, it’s weird o think about but, there’s so much laser power when we hit the mirror, it moves the mirror. And the mirror, to imagine it, is about this big. And it is 40 kilograms which is, is it like a 100 pounds?
Craig Cannon [00:23:09] – 80 something, but yeah.
Rana Adhikari [00:23:10] – It’s the number of pounds, who knows what pounds mean. It’s 40 kilograms which is heavy, it’s like a little person. In the previous LIGO 20 years ago, I would just pick up a mirror, and would carry it in and put it in. There’s no longer any of this like, you all pick this thing up and carry it around.
Craig Cannon [00:23:29] – Bring it over in your truck.
Rana Adhikari [00:23:31] – Way too expensive and way too heavy. But the super heavy thing were hanging from handmade glass fibers which are super thin. And it’s sitting there and swaying around. And when the laser power hits it, it just moves. And even more annoyingly, when the mirror moves a little bit like this, the laser beam at the other end, four kilometers away, yeah, it moves, and so then that mirror twists a little bit like this. Now the reflective beam moves a little bit. These two things are talking to each other through this. The pressure from the radiation, the laser light, and that’s super annoying. It’s a not a thing that you can test if you’re in a tabletop. You have to put the whole thing together. You can simulate and calculate it as we did. But when you put it together it’s a lot more trouble than expected. And that took a long time to solve. It’s still not solved, it kind of works but, when we start increasing the laser power, a lot of these interactions happen which are really troublesome. But luckily we have a fresh stream of new people coming in to grad school. We hope they remain as good as the people we’ve had so far. If that doesn’t work out, we’re in trouble.
Craig Cannon [00:24:56] – What do you suspect will be the changes that suspend the mirrors in a way that the laser doesn’t move them?
Rana Adhikari [00:25:04] – Oh they’re just going to. I don’t think we have any way of doing it. What we’re doing right now, we have just a sophisticated feedback control system that we measure the light beams which leak out of the system and a bunch of places. We detect it and then we have a system of something like 20 feedback loops which put forces on the mirrors to try to keep it aligned, to keep this from being such a problem. The trouble is we’re trying to detect gravitational waves which are tiny and so when you imagine like this, let’s imagine like this is the mirror, this little thing which is about how long the gravitational wave lasts like this. Now we’re trying to control the pointing of this thing because it’s getting stirred on by the beam and so I have my feedback control like this trying to hold it on. But you can’t do this.
Craig Cannon [00:26:09] – It stirs the whole thing.
Rana Adhikari [00:26:09] – Yeah it just screws up the whole thing if you’re applying too much feedback because you mask the signal that you’re trying to detect. We’re in a place where we would like to figure out how to better optimize our feedback controls so that they don’t mask the gravitational wave signal so much. And luckily there is a community which thinks about how to optimize control systems and they’ve been a great help to us but we’re at the limit of what I understand and so I’m looking for someone who knows more than me to help us improve this situation better. I hope some of our modern learning techniques and signal processing techniques can be used for this.
Craig Cannon [00:27:01] – I wanted to talk to you about that as well. What techniques are you applying right now to the data and what do you hope to apply in the future?
Rana Adhikari [00:27:10] – There’s a whole menu of things. Basically everything that we can find. We just read a lot of things on the internet. Everything that sounds clever we want to use it. Since 10 years ago or more, we have been trying every single kind of linear subtraction. So we have I don’t know, let’s say, tens of thousands of sensors which are measuring the environment, and the motion, and these feedback controls, and all kinds of things like this. And we take each one of those and we compute the optimal wiener filter which is the optimal filter that you can apply to the sensor signal and subtract it from the data. And so we use it. In some cases we calculate the filter and then directly drive the mirrors, so that in hardware we remove the noise before it’s actually made. We do that with a lot of things. We remove by about a factor of a hundred, some large noise sources that way in the hardware. And we do it in a hardware because they would mix in some sort of non-linear way back into the data. It’s better to clean up the data in the hardware, in the analog because there’s no dynamic range limit in analog. There’s no number of bits more or less. It’s atoms so it’s a lot of bits. Once we get into the data, there’s still more we can do. We do some more linear subtraction of the noise. And we’re able to improve the data a little bit by factors of a few. But now we’ve reached the limit of what you can do with linear noise subtraction. And we need some better ideas on how to do the next thing. And the next thing involves non-linear regression. One of the things I’m working on right now is how to take basically a huge data set, it depends on who you’re talking to, when you say huge data set it doesn’t really, huge data set for me but not a huge data set for a lot of other people I guess. We have thousands of signals which are 16-bit and recorded at 16-kilohertz. Some of these signals, not all of them, will combine in some sort of, via some sort of non-linear function and show up in our main data stream where we like to look for the gravitational waves. For example it might be like the cosine of one signal times another signal plus another signal. And then that whole thing squared or cubed or something like that. It’s not super strange. It’s the kind of thing you can imagine doing on a laptop. But it’s a little tough to search through the full space of sensors which we haven’t done yet. But I think a lot of the things that are masking the lowest frequency gravitational waves which come from the biggest black holes, that data can be cleaned up quite a lot if we were to come up with better techniques of doing it. And I think it’s all doable. I personally haven’t found the algorithms to do it but we’re working on it.
Craig Cannon [00:30:41] – Because there have been three now in the past 10 to 12 years, right?
Rana Adhikari [00:30:46] – Right, there was one in September 15th of 2015, and then there was one on Christmas day of that year, in the evening around nine o’clock. Then for almost a year we shut down to improve a lot of things in our detector from most of 2016 basically. We turned back on November or December of last year. And then we had another detection in January.
Craig Cannon [00:31:16] – But do you suspect there have been many more that you just can’t…
Rana Adhikari [00:31:20] – Yeah and I think in our data, probably we could double the number of signals we have right now. That’s my guess. It could be much more. And the reason is, if you imagine, this is my only prop, this works for everything. If you imagine this lip is a black hole here, and I do this if I can hold it right, and I hold this part, alright, this is not cooperating, yeah there you go, it rings a little bit, and that ringing frequency has to do with something complicated about the vibration of this thing and the water that’s in it. The black hole however is a really, really simple animal. The mathematics are really complicated or depends on who you ask.
Craig Cannon [00:32:17] – It seems pretty complicated.
Rana Adhikari [00:32:19] – Yeah I would say. It’s more challenging. This is about as challenging as the mathematics for the black hole. The physics are stranger. Anyway the black hole that’s just sitting there and not spinning. You can easily compute the frequency at which it rings. And it has to do with the amount of time it takes light to travel around the border. So once we know the size of the black hole, if you’re looking at it, or if you know its mass, you can compute the frequency at which it’s going to ring. So it’s sitting here and let’s imagine I throw like this glass into the black hole. As this glass gets really close to it, the black hole horizon will perturbed a little bit like this as it swallows this new piece of mass. And that perturbation immediately settles down in this little wave that travels around the edge of the black hole. And then that gets radiated out. That radiation is what we detect. So when the two big black holes merge together, the same thing happens, they form a bigger black hole. But now since the thing’s bigger, the frequency of the ringing will be bigger just like if I made this cup two times bigger, this frequency would be two times lower. So the bigger something is, the resonant frequency is lower.
Craig Cannon [00:33:35] – Okay.
Rana Adhikari [00:33:36] – So the biggest black holes we can’t find right now because this kind of technical noise feedback and the vibrations form the environment are bigger than the fundamental quantum physics limits of measurement. And all of that data is being measured by other sensors, microphones, and things like that. It’s a data science job right now to figure out how to take thousands of standard sensors that you can buy off the shelf. And mix that data somehow with the gravitational wave data stream in a smart way that removes this kind of foreground noise and allows you to find the deeper signals.
Craig Cannon [00:34:20] – But it’s all modeled? Because that’s what I was wondering, when I saw the first announcement of the two black holes, I was wondering, like, “Oh have they just been looking for this pattern of waves or one of this pattern the whole time?” And so that you kind of have like a guidebook, like, okay if we see this it means that.
Rana Adhikari [00:34:40] – There’s a bunch of different variables which characterize the black holes. I mean it may sound simple but they can be spinning, and depending on their orbits and that sort of thing. There’s a lot of parameters, maybe several parameters that go into it. But in the end it’s just some parameter so we might have a five or 10 dimensional waveform space that we search through. But it is just a big catalog of waveforms, little wavelet looking things. And so for all these signals that can possibly come from black holes, we think we can search for them just by comparing with a known template.
Craig Cannon [00:35:16] – Really and so that’s just the wavelength and the amplitude?
Rana Adhikari [00:35:22] – Well the frequency evolves as the signal comes in. So when they’re far away, when they’re first born, maybe they’re a million years before merging. So they’re far apart and they just spin together like this and as they get closer, eventually there’s little and then they popped together. But we know what that frequency evolution is based on their masses and how they start.
Craig Cannon [00:35:46] – Do you think in addition to paying more attention to all the other measurements that you’re doing, there’s going to be a hardware innovation. What happens next?
Rana Adhikari [00:36:01] – Hopefully someone will watch this podcast and then say, “Have I got the solution for you.” And I’ll just get a piece of, someone will send me a link to their GitHub and then I will have the whole answer. And then we’ll break this thing wide open this year or next year. Then we’ll be back more at the fundamental limits. You asked before about this mirror, new mirrors, that sort of thing. Again using the universal prop here, so with this, if I do it, so that rings for about a second, and that has a frequency of 500 hertz. And so that means the energy stays pretty well localized in the vibration of the glass and doesn’t go someplace else. And this is, I don’t even think mine, it’s something I found in this hallway. But it’s an okay piece of glass. But it’s not meant for, I don’t know, it’s not meant for scientific purposes. The mirrors we have are more like, they store the energy better, something like, about 10,000 times longer. If I were to ping one of those, those would last for hours. It would just keep ringing and ringing. And that has to do with, that tells you a little bit about how well you can measure the motion of that mirror using lasers. And the reason for it is because of the motion of the atoms and the thermal energy in the system. When you come to it, if you’ve removed every other noise from the outside world, and you just have a thing sitting there, because it’s sitting at a finite temperature, its molecules are bouncing around like this and it’s shaking. That’s just sort of a thermo dynamic limit that you can’t get past. But the question is, is there a pattern to the way that they’re moving or are they just moving randomly? If you have something like this, an empty old glass, it’s pretty much moving randomly, except for there’ll be a lot of energy in the different harmonics and tones that you could make. If you measure this thing, you would notice that there’ll be a lot of oscillations that have a certain frequency.
Craig Cannon [00:38:23] – Is that how you can measure something, actually rather the laser is also affected by the gravitational wave.
Rana Adhikari [00:38:32] – It is yeah.
Craig Cannon [00:38:34] – So is the resonance of the mirror, that’s the reason why you can measure it? Because you can shoot it after and it’s still resonating and you pick it up?
Rana Adhikari [00:38:43] – No, it’s just that we just ignore the frequencies at which the mirror resonates. So the mirror resonates at a few specific tones. You can think about it like waves on a string. If you have a guitar or violin depending upon how you fret it, I don’t understand violins, anyway intimates with frets I understand, so depending on where you fret a guitar string, it plays a different tone. And it just has to do with the length of the wire and the tension. The same for the mirror, the mirror depends on something like how heavy it is and how fast sound travels in the mirror. And so if you have a mirror that’s really, really pure, and all of that thermo dynamic energy is focused on just a few frequencies, and so it’s just sitting there and if you shoot it with a laser or with a very expensive system, you can hear the thermo dynamic vibrations of the molecules in there. And in fact that’s what we hear most of the time. And so if you’ve listened to the LIGO data stream, there are these high frequency ringing going on all the time. And it’s all the mirrors just constantly vibrating thermally. We just don’t look for gravitational wave that does very specific frequencies. They’re very narrow. So it’s just like doing the removal of like the power line harmonics, we have all of that, so we have to remove it. So if you have a hum filter as you do when you record music for example, you’d do the same thing. We have hum filters that remove all the lines that are happening.
Craig Cannon [00:40:26] – It’s kind of like why you’re measuring everything, right? It’s like if you can detect it, then you take it out, and that’s therefore not a gravitational wave.
Rana Adhikari [00:40:33] – Yeah.
Craig Cannon [00:40:34] – Do you suspect that you’re picking up anything that you can’t even define right now? Are you lumping things in as gravitational waves that might not even be? That’s unlikely?
Rana Adhikari [00:40:47] – That’s unlikely. I mean it’s always a worry. We’re really paranoid about that kind of thing. Because you’d just hate to be like the boy who cried wolf, say, I have gravitational wave and then you find out six months later that it’s, nah, just a misbehaving refrigerator that was located…
Craig Cannon [00:41:05] – We bought the same fridge.
Rana Adhikari [00:41:08] – I’m sure we did. I’m sure there’s something going on like that. And so those problems we’ve been finding for decades, things like we bought the same electronics board from the same manufacturer, and it has a crystal that happens to radiate at a certain frequency. And those two things are kind of synchronous at two different places. And once in a while you’ll get three crystals beating with each other which will produce something in the audiogram. There’s thousands of stories like that which we all forget because you do it, you find it, and then you take the thing out, and you smash it with a hammer. You have a party because you found some terrible thing.
Craig Cannon [00:41:51] – Do you have a wiki of known bugs? It’s like chronicling all these stuff?
Rana Adhikari [00:41:57] – No. I think it’s just stories. Just mostly stories.
Craig Cannon [00:42:03] – Because that’s like most of the job, it sounds like.
Rana Adhikari [00:42:05] – It is, it is.
Craig Cannon [00:42:07] – You’re just like find anything, get rid of it.
Rana Adhikari [00:42:08] – All the time, all the time. And it’s just, it’s gotten to the point where we all feel like we’re telling UFO stories because we’ve already found all the easy things years ago. And now the things that are limiting us are the weirdest mechanisms. And you come back, you spend the day working late, it’s like 2:00 a.m. and everybody kind of comes back together and they’re like, “What happened? What’d you find?” Then you start telling this crazy story and you say, “It seems like if I stand in this part of the room, and then she stands over there, and then we turn the mirror like this, this kind of hooting, screeching noise happens.” And everyone’s like, “You’re crazy.” There’s no science in that, it sounds like a superstition. I don’t know what to tell you. It’s 2:00 a.m., we’ve been working on this all day but this is what happens. And it’s alright. Let’s go get some sleep and think about this. And all the problems are of that sort now.
Craig Cannon [00:43:03] – Okay.
Rana Adhikari [00:43:05] – But luckily we have people who are obsessed about these kinds of problems and they’re going at it and finding them and solving them. Last summer we found another one of these problems that was like, it reminded me of some kind of sci-fi horror movie. Trying to think of what it is, it’s like, there’s this really bad movies from the 80s called They Live.
Craig Cannon [00:43:39] – I haven’t seen it.
Rana Adhikari [00:43:42] – Yeah they have two professional wrestlers, I think the actors in it, anyway, it’s terribly bad. But at one point one of the guys says, “You need to put on these glasses “‘cuz if you put on these glasses “you can see who the aliens are.” They mostly look like people, and you put on, and so the guy puts it on and then he starts looking around and he’s like all of his friends and everybody he knows, he’s just like he doesn’t want to know, it’s too much. We found a problem like that last year which is that the light which bounces off of our mirrors mostly keeps going back and forth, and these optical resonators that we have, it’s something like a few parts per million of the light shoots off in some other direction. And then it shoots off, hits some unknown thing, and then some few parts per million comes back and then interferes with the main system. So then, it’s something like, there’s basically an infinity, it’s like a disco ball, you can imagine like a disco ball is lit, and then the light beams go everywhere, that’s an extreme case. But at the part per million level, our mirrors as wonderful as they are, are acting like disco balls. And so little bits of light are heading off in all directions. And when the light comes back from those places, it has picked up a little of the vibration from whatever thing it hit. And so finally at some level, a bit we’re measuring the acoustical vibrations of the entire eight kilometers of metal tubing of our system because there’s a little bit of light hitting those things are coming back and then now what?
Craig Cannon [00:45:28] – I mean that’s kind of like the rub of, you create this perfect sealed systems but now it’s sealed. And so you just throw a marble around, it’s just going.
Rana Adhikari [00:45:37] – Yeah it just keeps going. We’ve got to take care of it. We’ve been here at Caltech and also MIT, we’ve been thinking about what to do about this. And so we’ve come up with some designs on what to do and basically we’ve taken some of these substances which are the blackest, darkest things you’ve ever seen. And we’re going to put them in our system to block these places where the light beam goes.
Craig Cannon [00:46:07] – How do you put it?
Rana Adhikari [00:46:08] – We’re going to open up the vacuum system and walk inside. We’re going to put on full clean room suits and walk inside and put these things in.
Craig Cannon [00:46:15] – Where?
Rana Adhikari [00:46:17] – All over, every place that we can find. I never want to see this again.
Craig Cannon [00:46:23] – What kind of substance is it? I don’t fully understand.
Rana Adhikari [00:46:27] – Like my shirt, before washing, was very black, now it’s kind of gray. Are you asking why are things black? That’s a good question.
Craig Cannon [00:46:38] – You can answer that though. It’s like spinal tap.
Rana Adhikari [00:46:44] – It’s an honest scientific question.
Craig Cannon [00:46:46] – No, is it like a paint that you’re putting on stuff? That’s what I’m asking.
Rana Adhikari [00:46:50] – Yeah it’s a bunch of different, so each, these are kinds of trivia questions that I know and then I wonder what have I done with my life that I know the answer to these things. In the array of different blackening things, there are the things that you think are black, which are black sort of to your eye, but then you shoot a huge laser at it and you use a really sensitive detector to sense it, and then you find that it’s not so black, it’s really gray. And so there’s a bunch of garbage you can buy online which says that it’s the best blah, blah, blah, blah. Most of it is junk. We have a couple of engineers and a building over here who have been exhaustively and carefully looking at every single thing that is promised to be black on the internet. And then it’s like a myth busters episode over there. And then finally you have come down to a few different solutions. And so some of them are, I don’t think I could accurately describe all of them but some of them are black like a, it’s basically a glass like this but colored glass like a, I’m missing the word for it, they have it in churches, what do you call it?
Craig Cannon [00:48:04] – Oh, stained glass.
Rana Adhikari [00:48:05] – Stained glass, yeah. You have glass and then when you’re making the glass, you put some other stuff in it and it comes out a different color. And so you can make, I don’t know what I’m doing, I’m sure they don’t do this, but I don’t know how to make glass so I don’t understand, I imagine it’s like this, you have this molten glass, and then you pick up magic pixie dust, and you put in some stuff, and it’s perfectly absorptive for the wavelength of the laser you’re looking, that you care about. Red stained glass for instance lets through red light but it absorbs green. So red stained glass is really good if you have a green laser. We have an infrared laser which is, if you imagine this is the whole rainbow from purple, violet to red, then our laser is sort of over here so it’s, yeah anyway, imagine the rainbow and there it is, it has a wavelength of one micron. And so for it, some of the things that look black don’t work. But if you have a special kind of welder’s glass, it really works. Welder’s glass is good at absorbing pretty much everything with a longer wavelength and green. That’s one of the best materials to use for black. And then you can also get these so-called nanotube things, vantablack, I don’t remember all the various black names but they’re all trademarked. So there’s a bunch of stuff which is essentially, you know how you can get lost in the forest, it’s like that for light beams also. So if you take a thing and you put a bunch of spaghetti-looking nanotube things, then the light goes in and bounces around like a hundred million times. So it dissolves energy.
Craig Cannon [00:49:51] – And so then, do you also expect you’re going to build more interferometers that are longer?
Rana Adhikari [00:50:00] – Yeah, to clean it up.
Craig Cannon [00:50:03] – Do you suspect the next version is eight kilometers?
Rana Adhikari [00:50:12] – It is a good question. That’s a really interesting question about if longer, we’ll clean it up. I have to get back to it. That’s probably a few days of computing for me. Yeah indeed if we make the interferometers longer, like 10 times longer, that’s dramatically good. I mean it would cost a lot of money but that would take us from being able to measure things which are sort of I would say, with the current systems, as big as they are, if we put in our best technical hacks into them that we can imagine, we could maybe get to the place where the universe was about 1/5 or 1/6 of its current age. So we could look back something like 10 billion years into the past which is pretty great. But if we build systems which are 10 times bigger, it’s hard to do anything better than just make this system bigger. So the bigger you make it, the bigger the signal gets. A lot of people have thought about the idea of making a 40-kilometer system which you can put, there are several places in the US for example which have–
Craig Cannon [00:51:37] – Each arm is 40k?
Rana Adhikari [00:51:39] – So big open spaces which are unused. And if we could find a place like that and get the finding to build something like that, it would be traumatic, not traumatic, I hope not traumatic, dramatic.
Craig Cannon [00:51:53] – We’ll see if the laser gets out?
Rana Adhikari [00:51:56] – It would be dramatic, it would be wonderful. We’d be able to find signals from basically all the way back.
Craig Cannon [00:52:03] – Really?
Rana Adhikari [00:52:05] – We would find the first stars in the universe and when they were collapsing if they exist which I think they do. But it would be so dramatic, we’d be able to measure things like how did space-time evolve from those early times? And did the universe start from different number of spatial dimensions and that sort of unpacked? Has it expanded and become three dimensional? Did it start different? Did it go through a phase where like an extra dimension came up and then collapsed again? And who knows? We’d also like to know does gravity travel through the three dimensional space or is it something like there’s another spatial dimension which only gravity can see. And so something from that far back into the universe may have, I don’t know how to draw this.
Craig Cannon [00:52:59] – The cup can’t do it.
Rana Adhikari [00:53:00] – No, no, the cup can do it. Watch this. So imagine that we’re living on the surface of this cup, and this is effectively our three dimensional universe, now if I empty the cup, then when I go like this, the signal has to travel around the border of the cup, but because there’s water inside, when I go down here, some of the vibration gets into the water and comes out this other side. And so there’s sort of this boundary on which we’re used to everything taking place which is the three dimensions that we’re familiar with. But there could be a fourth dimension which is something like this bulk, the inner part. And in that dimension, gravity could travel faster. So it’ll look like it’s going faster than the speed of light. But that kind of stuff…
Craig Cannon [00:53:52] – This is kind of blowing my mind. I’m trying to figure out…
Rana Adhikari [00:53:56] – It sounds like there’s just a crazy person who you found on the streets, who’s just telling you stuff about other dimensions.
Craig Cannon [00:54:01] – Just think about it, man. 10 billion years ago, man.
Rana Adhikari [00:54:03] – 10 billion years ago there’s other dimensions and there are half dimensions, and maybe we unfolded from a flower. It’s all in the table I’ll tell you. In the late 90s when I was starting grad school, everything felt like it was pretty much wrapped up. The word on the street was like, “Well thanks for showing up but we’ve got this all wrapped up now.” And everything makes sense. And the universe is exactly like we predicted it. We have a few loose ends to tie up. And this is the same thing that people were saying in the late 1890s also. They said we got it all, we figured it out, we got magnets, we got electric fields.
Craig Cannon [00:54:46] – Telescope.
Rana Adhikari [00:54:47] – That’s all there is. There’s nothing else out there. And then there was this weird quantum thing, there was some data but they’re like, “That’s not real, that’s just some nonsense, it’s going to go away.” And we’re back into that period of now where everything’s back on the table. The universe is so strange and so far inexplicable that if you have got an idea that’s a crazy idea, then your crazy idea is just as good as my crazy idea. And let’s put it to a test. If it’s a hypothesis which is testable, we ought to test it.
Craig Cannon [00:55:21] – Does it make sense to build an interferometer in space like people have been talking about?
Rana Adhikari [00:55:27] – Yeah of course.
Craig Cannon [00:55:29] – I know it sounds cool.
Rana Adhikari [00:55:31] – Yeah that’s a plus. That’s a plus that it sounds cool. It makes sense for a lot of reasons I would say. On the ground, kind of we’re limited to measure things that have a signal frequency which is more than five or 10 hertz or something like that. We can go a little bit below the human audio band but not much. And the reason for that is that the earth is just vibrating all the time. And you talked about these animals before. The animals are going to be a problem. The clouds are a problem. I mean eventually the gravity from the clouds, and the gravity from beavers and humming birds and whatever, I mean, who knows what is out there? Washington state, there’s not a lot of animals out there but there’s tumbleweed. And those things are fierce. If you have never been chased by a tumbleweed mini tornado, then you’re lucky. And in Louisiana there’s a lot of animals. And you could build bigger buildings but eventually the gravitational fluctuations from the dirt, from the air, clouds, I mean eventually, there’s just too much gravity fluctuation on the earth. We just can’t get past it. So we can remove every other kind of noise but we can’t go putting vibration sensors in all the clouds or something like that.
Craig Cannon [00:56:59] – Not yet.
Rana Adhikari [00:57:00] – We’re getting to the Baron Munchausen kind of crazy level. So we could go to the moon but the moon’s not all that quiet. And the near earth orbit is not really that good for vibrations, and it’s not a place you want to put a stable system. So to have a space interferometer, it’s got to be on a far out kind of orbit that you can get to with things like SpaceX’s Falcon Heavy. So there’s a project called Lisa, which is aimed to launch in 16 years, 16-17 years from now. And that will put a triangular interferometer in space which is several interferometers. And that will measure gravitational waves at around the milli hertz. So super low frequency. At that level there’s almost no vibration out there, and they should be able to measure things all over the universe in super, super hi fi. We’re measuring things with the signal to noise ratio of tens, and we hope to get to a signal to noise ratio of thousands which is really good. But they would be hundreds of times better than that. And it would be like a, it depends if you’re a real connoisseur of violin or cello or some of these things, different musicians have a different finger signature a little bit, so when they’re playing, you can hear things like the way that, their finger moves on the bow, or the way that their finger moves on the string, like a little bit of friction. So you can hear these little tiny…
Craig Cannon [00:58:50] – Slide?
Rana Adhikari [00:58:51] – Yeah a little bit of the slide. But you hear these little things which change the character of the music. If you leave one of these instruments sitting for too long, some of these instruments like to be played, like warm up a little bit, the wood warms up in them, it becomes more like a warm instrument. If you listen to it on a, I don’t know, like a 80s cassette deck with a walkman, you’re never going to hear that stuff. You need to have a full hi fi system and some good cans, then you really feel it, you hear it. And these are the kinds of the things in the live performance that you’ll never get otherwise. I’ve been to the Berlin Philharmonic. That’s what I’m talking about. If you want to understand about why we need better gravitational wave detectors, you go and you sit there in like row five or 10, and these are some of the best musicians in the world. And they’ll pieces that you know but you’ve never heard like that. And there’s no recording that’s ever going to do it because you feel it in your chest, and you feel it all over your body, the sound and it’s a kind of richness that there’s no way to record. And that’s the kind of feeling that we want to get from what’s happening out in space. And for that we need a exquisite hi-fi system to get these little things. And it’s not just for the pleasure of “Oh look that black hole did exactly what we predicted.” It’s more for, we like to find out where the laws of physics break down and where something new pops up. If we want to find out other extra dimensions and new kinds of particles and, is space and time really just an illusion? And there’s really a microscopic graininess to empty space.
Craig Cannon [01:00:48] – That’s kind of the underlying question ‘coz I like, I mean, you’d said so many reasons before, but I wonder what the pitch was in the beginning, like why? Why this? Obviously it’s quest for knowledge straight. Where those the concrete answers that people gave? Why are you doing this? Why make it bigger? What do you do?
Rana Adhikari [01:01:10] – Yeah I think for everybody there’s a different reason for it. There’s a whole spectrum of reasons. I always tell people the think that I’m most interested in which is I think gravity, we have never been able to use it as a real probe of what’s going on in the universe. What’s the universe made of? What is all this stuff? Why is space empty? Why is space so stiff? Why are there quantum fluctuations in empty space? And how come the universe ended up looking the way it does? Why are the galaxy so far apart? How come there are galaxies? Why don’t we just have a bunch of planets…
Craig Cannon [01:01:47] – Just floating around.
Rana Adhikari [01:01:47] – Yeah, how come planets are smaller than some? There’s an endless number of questions about the whole, not really the stuff in our universe but the structure of everything and why did they end up like this? It could have been any number of things. And then we don’t even know what’s space is made of. What’s empty space? It sounds like a question that’s stupid and doesn’t have any meaning to it but, imagine like two sturgeon floating around in the water, they’re like a hundred years old, and they’ve gotten used to it. They don’t really ask anymore what is water but we know we can take it out and look at a microscope, and it’s got flagellums and all kinds of stuff and it’s made up of H2O which we can study. It has a real microscopic character which is important. And we need to understand it. And for those fish it doesn’t really matter. It just seems like a continuum. It’s just everything is that stuff. But there is a whole deep structure to it. And space may be like that which is this whole, it’slike opening the curtain on the real universe and what’s really going on. What is the structure that we’re living on? It could just be this weird framework, a way that we’ve never imagined. I think, totally legitimate question is so what? And let’s say we’ve revealed the true structure of space-time and it’s like a bunch of leprechauns down there building space or who knows? Some crazy thing.
Craig Cannon [01:03:27] – It’s just black paint.
Rana Adhikari [01:03:29] – And then, yeah, and then, so what? What does that do for me? Is that going to relieve the traffic in LA? No, probably not. So, really, really, it’s a curiosity driven research. We’re trying to figure out how to find out, reveal the unknown and what’s going on. These big science projects are really expensive. And a lot of people are involved and they work at it. And then you really wonder, so what? They’ve found out stuff like there’s this particle or not that particle, or this star did something one billion years before, six billion years old, not five billion, or whatever. I think if that really was all that there is to it, we could certainly make the legitimate argument that, look, our society has got a lot of problems that we need to solve, and then how much of our resources are we going to put into pure curiosity driven basic research which doesn’t have any kind of finite timeline pay-off. We got real things we want to solve here, there are people going hungry, what are we going to do? And I would say to that, when you look at the history in the last 100 years, why has wealth increase and standards of living increase all over the world? And the reason for it is that people have been investing a lot in basic science for hundreds of years. And the reason that the US became the leader in this is the government said, soon after World War II, that we’ve got to be serious, and put our money into this because there is really a huge pay-off. “And we don’t really care what you’re researching, just do something, I mean, find something that excites you and do it and do it really well. And if you’re interested in engineering and science and technology, we are going to support that because we’ve shown decade after decade that it’s a hugely profitable pay-off investment-wise.” It pays off in gadgets and learning, and wealth for the country in the long term. It never fails. You’ll always be funding something which turns out to be a dumb idea, and then, okay, so it doesn’t work out but to find a really great idea, you might have to test out 99 dumb ideas. And you might not understand why they’re bad until you do it.
Craig Cannon [01:06:12] – But relative to the outcome, I think it makes a lot of sense. We have a couple of questions from Twitter so we can transition into those. Denis Norton asks, “What would happen to earth if there is a black hole merger closer to home than the three detected? Say, where Sagittarius A star is now.”
Rana Adhikari [01:06:39] – That’s not close enough to really do anything to us. But you can imagine it even being closer. If it’s too close, it just eats the earth. But there’s some range of distances in which other things could happen. So you can imagine for example it being out by, oh I’d say like at the next star system like Alpha Centauri or something like that. We can compute it. Like the ones we detected were at let’s say, several hundred million light years, and Alpha Centauri is only four light years at least. It would be stronger by that factor of a hundred million. It sounds like a lot, it is a lot. But that means that that motion of 10 to the minus 18 meters would have been 10 to the minus 10 meters which for us would have been like a hardware destroying level of signal.
Craig Cannon [01:07:46] – Really?
Rana Adhikari [01:07:47] – We would have just had electronics overload and saturated. We would have just ignored it because we would have said it’s way to strong.
Craig Cannon [01:07:54] – The levels are too much, it can’t be right.
Rana Adhikari [01:07:57] – But it would not have done anything like disrupt the tides or knock the moon out of orbit or anything like that. It would have to be extremely close for something like that.
Craig Cannon [01:08:08] – So close that we might even already…
Rana Adhikari [01:08:11] – We would see it with our optical telescopes.
Craig Cannon [01:08:13] – Yeah, sure, sure.
Rana Adhikari [01:08:14] – For it to hurt us.
Craig Cannon [01:08:15] – Yeah. And it also would have been, would have happened already, right?
Rana Adhikari [01:08:20] – Yeah. Well you could imagine, when we’re trying to think of all the nightmare disaster scenarios, let’s say a pair of black holes gets formed in some weird three-way encounter by cluster, and then it gets shot out, and it’s traveling at like a million meters per second, so it’s like 1% the speed of light, and it’s shooting at us, and it’s coming at us from some strange direction, so that we don’t see it ‘cuz it’s included by something else, I don’t know what that would be, I can’t simulate the whole solar system in my head, so I couldn’t figure out the answer, but let’s say it’s coming from out of the plane, and then eventually we see it, it’s like including some stars, and so I’m thinking of this Hollywood movie that we would make based on this idea, so it’s coming to us, and the binary is doing this as it’s traveling, it’s going to merge right when it hits the solar system somehow, I don’t know what we would do to stop this, I don’t know what that would be but, we can compute something like that. And then that thing would really be bad because it would stretch space by, like I was saying, like hundreds of percent, right when it get close to us, and the black holes themselves would be about the size of LA. So they would be effectively like tiny pinpricks but, might be like 50 or 100 kilometers in size. I don’t think the earth would get destroyed. However so I can’t say this with a high confidence, but, the earth, again, using this, the earth is a physically resonant system. And this thing has a quality factor of a few thousand, meaning it’s like a few thousand oscillations when I ring it, that’s why it lasts for a second. So the earth is like that also, except for the vibration frequency is about 30 millihertz, once per 30 seconds. If the binary black hole pair came cruising through our solar system, and right when it was coming through if it happened to be going once per 30 seconds, it could excite the acoustic modes of the earth.
Craig Cannon [01:10:46] – That would be bad.
Rana Adhikari [01:10:49] – It would likely be bigger than the 9.5 earthquake in Chile which happened in the early 60s and kept the earth ringing for months. I don’t know what would happen exactly but I can imagine if we had a 10 times bigger than the world’s biggest earthquake, it could hurt us in terms of earthquakes and tsunamis.
Craig Cannon [01:11:08] – Or we would find out what the inside of a black hole looks like if it got too close.
Rana Adhikari [01:11:12] – Yeah. Alright that’s an interesting question and one I should compute. I will answer on Twitter.
Craig Cannon [01:11:19] – Perfect, okay, so we got one more then. This is from Margin Collector, “Is the current method for detecting gravitational waves the best idea out there or only the best practical way given the tech?”
Rana Adhikari [01:11:36] – It is not the best idea out there in a number of ways. I mean one way is making things longer as people said but what I gather from this question, I’m reading between the lines here but, there’s a good Elon Musk story where I think his analogy is like, we take New York City in the 1800s and it’s all horses, and then we ask what’s going to happen to the output of all the horses once New York City scales 20 million. You can’t just scale everything by saying we’ll have like more horses and we’ll have so many more street sweepers or something, eventually you shift to a new technology like cars. It’s the same. The normal answer I think people would give to are we using the best technology is oh no, next year we’re going to use double the laser power and a mirror that’s even double…
Craig Cannon [01:12:32] – Make it longer.
Rana Adhikari [01:12:34] – Like in the Simpsons’ they said “Eventually these humans will make a board with a nail so big through it that they will destroy themselves.” It’s not the right way to go if you ask in hundred years from now, will people still build Michelson laser interferometers and do the same thing? I have a hard time believing that’s true. And one of the exciting possibilities out there, so there are ideas with using acoustic detectors, and space detectors, and using the timing of signals in space and so on. But in this frequency band, in the audio frequency band, me and some of the people who think about the quantum mechanics of this kind of protector have been thinking about how far are we, if you think about the pure mathematics of how information is propagated through space-time, what’s the information carrying capability in terms of number of bits of space. You have this much space and how many bits can you send before space collapses on itself. And like for fiber optics, you have a limit to the number of bits you can send which depends on your modulation bandwidth, and the amount of laser power you can put in the fiber. And eventually if you put too much laser power in there, you get stimulated, Brillouin scattering from the glass, and so on, you just have a kind of bandwidth limit there which is pretty high, it’s plenty for you too. But still there’s a limit. And we have been thinking about the same kind of thing. Why aren’t we doing better? Or where is all the signal to noise ratio going? When you think about the wave coming from outer space, we think probably the quantum fluctuations of space-time itself are probably at the Planck scale which is 10 to the minus 34 meters. And the signal like I said is around 10 to the minus 18 or 19 or something. So there’s a signal to noise ratio of 10 to the 14 or 15 there. And I’m telling you we’re only getting tens. So there’s 10 to the 13 and the signal to noise is lost from converting from space-time to laser light. That doesn’t seem a good thing. Since that’s the biggest chunk of where we’re losing it, we should be doing something better to transduce the space-time curvature into an electrical signal. It might be that light is not the best thing. But even with light we can do a lot better than what we’re doing. And not just by making things heavier and doubling this or switching colors or something. There’s an idea which is around, which is called the coherent quantum feedback. It takes this problem, I would guess, I would say of the pressure from the light, moving the mirrors around and turning it into an advantage. Like I described before, the beam pushes this thing and then this thing pushes back, and that changes the light. You can take this instability and essentially turning it into a system where quantum mechanically, the mirror laser system has positive feedback. A lot like a, like an audio system. If you’ve heard when musicians practice sometimes, it can be bad. So right when people have feedback, their standing too close to the–
Craig Cannon [01:15:57] – Like two mics next to each other.
Rana Adhikari [01:15:59] – Yeah. But as we know from Jimi Hendrix, feedback can also be a wonderful thing. And he turned it into a fantastic thing from just an annoyance. So we’d like to do the same thing. So we’d like to take this mechanical optical instability that comes from the laser system interacting with itself, and turn the entire four kilometer plus four kilometer L-shaped thing into a unstable feedback system, so when the space-time fluctuation comes in, it excites this instability in our system, and then we detect the signals in a much stronger way. aRather than think about it like the laser light goes and measures the space, and comes back, it’s almost like we have this eight kilometer L-shaped laser tuning fork that picks up the space-time signal.
Craig Cannon [01:16:50] – Right, so it’s optimized for that one particular length. It goes wild when it sees, hears something.
Rana Adhikari [01:16:56] – Yeah. You can optimize it for a single frequency but the thing we’ve been thinking about just in the last month or two, is how to make it optimum for a wide band. So we want to make a wide band unstable system.
Craig Cannon [01:17:16] – To be determined.
Rana Adhikari [01:17:17] – Yeah. I think we have got 95% of the problem solved.
Craig Cannon [01:17:24] – Oh wow.
Rana Adhikari [01:17:25] – On paper but still. Our aim is to try to build something like this this year. And once we figure out how to do it and…
Craig Cannon [01:17:36] – Just like a scale model you mean?
Rana Adhikari [01:17:40] – At the Caltech campus we have a 40-meter size system. And it is a 1/100th scale of the real LIGO detector. And we want to build this in so we have little mirrors and little lasers. But they seem big to us but they’re really little and we’re going to build up this instability and see how sensitive it can become.
Craig Cannon [01:17:57] – Very cool, okay, cool, thanks, man. Alright thanks for listening. As always, we’re posting the video and transcript at blog.ycombinator.com. This time we’re also posting a pretty cool video of two black holes colliding, which is technically not a video of the black holes colliding. It’s more of a composite from the data turned into a video. Regardless, it’s pretty cool. That’s at blog.ycombinator.com. Please remember to rate and subscribe to the show. Okay, see ya next time.