Thank. You very much. Thanks. For having me here this is this is very exciting, for me. It's. Very, exciting for my kids, who are excited that this will appear on YouTube now dad is a celebrity. Anyway. I, as. Chandra, said I, have, done a lot of work on neutron. Stars on radio pulsars, and on, black holes. Recently. We've done a lot of work on fast radio bursts, which has been a topic of some excitement, it's appeared on the. Cover of nature a couple of times and. I, play, a very, very small role in the event horizon telescope, collaboration. Which. Produced, this iconic, image of the black hole in m87, if, we have time we'll get to touching on those topics a little bit but, what I really want to talk to you guys about is gravitational. Waves and how, we are building a galaxy, scale gravitational. Wave detector. So. With that let's. Get started all. Right I like. To start with this image of someone. Poking. Their head behind the, curtain, a little bit and. It conveys to me this idea of the world being knowable, right, this is a this, is an engraving. That is of dubious. Provenance, I, guess but it somehow conveys, this idea of peeking, behind the curtain and figuring. Out how the universe, works so. For the start I want you guys to take away maybe three, ideas. The, first of these the most important, thing is that the world is knowable, it, mint, have been this way but, this idea that the scientific, method with observations. Inference, modeling, and prediction, can, give, us real knowledge, that. Is really important, particularly in our current climate it seems, like this is an idea that needs to be emphasized, more, and more the. Second big thing I want you guys to take away is this idea of gravitational. Waves and how, they are offering, a new window, on the universe, so, unlike, all. The, other wavelengths. That we think of which are all finally, forms of light radio. Waves x-rays gamma, rays these are all forms, of light but. Gravitational. Waves are a completely, different new, Messenger and we've only just begun, to open this window on the universe so, it's a tremendously, exciting time I will, tell you a little bit about how we've done. This and what we've seen so far, and then, the final thing is a, thing that calls Sagan used to emphasize. Over. And over again that, extraordinary, claims require extraordinary, evidence, and, so, for, example with fast radio bursts the very first thing that people jump to was oh maybe it's evidence, for aliens, and it's very very, very very probably. Not aliens, right, it is just not extraordinary. Claims require extraordinary evidence, and, the scientific, method is built so that we, can get to these we, can build evidence. And, chains of causality, and eventually, get real knowledge, about the world we live in ok, so. This. Talk is a little bit back there's a lot of different.
Things. That I'll tell you about which could be their own talks on their own but. We'll see how it goes all, right so, the first thing I want to start off with is this discovery, of radio, pulsars, and, this. Is a picture, of one of my scientific heroes Jocelyn. Bell she's. Standing in front of the telescope it doesn't look much like a telescope, it's basically, an array, of these long wavelength, dipole, antennas. This. Is the radio telescope, that she. Instrumented. Up by hand, and she, was a PhD student working, on this project to, actually study. Interplanetary. Scintillation. And while. She was going through the output of the chart recorders, by, hand. Miles. And miles of chart recording, because, she was a good scientist, and because she really, really knew her instrument, this is actually a picture of the original chart, recording, she, noticed this bit of scruff, and this. Bit of scruff did not look to her trained eye like any of the other bits of scruff on here instead. This looked interesting, and it, came back every, day at the period not of the Earth's rotation. Which is 24 hours but, at the period of rotation of the sky around Earth which is 23, hours and 56 minutes and, so, you could tell that okay, she could tell that okay this is something in the sky, what. Could it be that was producing, these little bursts, of radio waves as it, went, through the telescope. Haha, joking, but, maybe not really they named it lgm1. For. Little green man, joking. Bad and. Then, they found le GM too and then they found LG m3 and it became fairly clear, that it was not in fact anything to do with aliens but it was some, natural, phenomenon. Today, we understand, that these pulsars, are actually, neutron, stars they, are the tiny dense, little remnants. Left behind, when, massive, stars explode and, when, these massive stars explode they, spew out the elements, that have been synthesized, in them into. The, Atmos. Into the space and then, what may be left behind in certain cases is this, incredibly. Massive, incredibly. Dense, condensed. Ember of a star, for. Scale here it is on the San Francisco, Bay it's about 10, kilometers, in radius and. It's incredibly, massive it's about one-and-a-half times the mass of our Sun squeezed. Down into that tiny area, now, neutron stars are a completely. Fascinating, topic we can and we do in fact teach an entire college courses about them and, they make for these great stories their magnetic fields are strong enough that if you replace, the moon with a neutron, star it would erase the credit cards in your pocket, that kind of magnetic fields we are talking about and, a teaspoon of neutron star matter weighs about as much as Mount Everest does so. It's. Absolutely. Exotic. Form of matter what's. Interesting about them and for the purposes, of this talk we won't really go into neutron stars at all except. To say that. We. Understand. That, because these are extremely, tiny extremely. Magnetic, stars and because they are spinning on their axis, they, have this beam of radio emission that is spun around and, it each, time it's three across our line of sight we, get a blip, of radio.
Emission, Okay. So, we don't quite perfectly. Understand, what produces. This, beam of radio emission but we do understand, why the pulse, because. They're spinning like a top it's this lighthouse model, and so that's what this looks like, each time the beam sweeps around you, see a little blip of radio emission and that's. Great okay. This. Rotation rate, is extremely. Extremely stable, in some, cases and in some cases it is stable enough that, it can actually rival. Some of our atomic clocks now, these are radio waves but if you took this radio pulse and converted, it into sound if. You did that here's, what one might sound like. Each, time, you hear that click, that's. Basically, a, rotation. Of this, massive. Neutron, star and that, beam of radio, waves being swept across our line of sight now that was, a pretty sedate, radio. Pulsar this, is BA 833. - 45 it's the Vela pulsar it's one of the millisecond, pulsars. And. Again. Each of those clicks is one. One-and-a-half. Times the mass of our Sun spinning. Once on its axis, right. The, fastest, ones are actually spinning faster, than a kitchen blender they. Are really, really fast. Ok. Great let's, hold, that thought because this could be its own talk but. Let's set aside these. Celestial, clocks and now let's talk a little bit about gravity. So. What is gravity, well, Isaac Newton while he was sitting under his apple tree this is one of the famous, descendants. Of that apple tree and the apple supposedly, falls on his head and he, realizes, that it's not just the Apple falling to earth but the earth falling to the Apple as well every. Particle in the universe attracts. Every other particle right. So that was Newton's model, of gravity. And it's an incredibly, successful model. It's a model that's successful enough for us to precisely, navigate. Spacecraft, all the way across the solar system. But. Then Einstein comes, along and he, comes up with a different, conception, of gravity. This. Is where I put up an equation for, general relativity and, half of you fall asleep immediately so. We won't do that instead I have a little picture, that shows you what Einstein model, looks like Einsteins. Model, says that matter. Tells. Space-time, how to curve and. Then. Space-time, tells matter how to move, okay. This is basically what the equations of GR tell you that, matter, without. The presence of mass is this uniform, thing but, then you place mass in it and like a rubber sheet it bends and, then. Other masses, move in this bent rubber sheet and those, are the gravitational, trajectories, so, that sort the summary. This. Of course was. An incredible. Incredibly. Different, conception, compared to Newton's conception, of every. Particle in the universe attracting. Every other particle, and so it led to entertaining magazine, covers like this was Einstein, wrong right was, he wrong, and, the, consensus. Is this. Is a theory, that has been tested, over and over and, over again and so far it passes, with flying colors, now, that doesn't mean it's right in fact we know that Einstein's. Description, of the universe is not complete, because. That description, is not reconcilable. Yet, with, quantum mechanics which is the other incredibly. Successful, theory of nature that we have that, describes things on very small scales however. It is a very very good description, it's been tested extensively and. Now, if you think about this description, of space, like a rubber sheet that, stretches, in the presence of mass you, immediately say ok maybe it can have ripples. Maybe. It can have ripples in space-time itself, and those, are gravitational. Waves so. Einsteins conception, implies, the. Existence of gravitational waves and this is what might happen if a gravitational wave were to come out from behind the screen and towards, you you. Would have a ring of particles. Stretching. Squeezed stretch. And squeezed and there's. Two different modes, in which this stretching and squeezing can happen so, there is a plus mode and a cross mode, ok, so those are the two modes of gravitational. Waves now ok new gravitational, waves really exist well, here, now to, go back to those neutron, stars that I talked about before we. Can time some of these neutron stars with exquisite, precision and, some. Of these neutron stars are in binary systems, so we can actually, watch those binary system, orbits, change because. Of the ticking clock sometimes. Moving away from us sometimes, moving towards us we, can trace the orbit and here. Is what the traced orbit, the, orbital, this. Is basically one of the parameters, of the binary orbit the shift of the periastron and you, can see that it changes, over time as the years tick away and. This. Line that's drawn through here is basically. The. Prediction, from general relativity, you.
Can See that it is a spectacularly. Successful prediction. It is, just an absolutely. Astonishing, fit, to, what the observations, show right. And so, as a result of this discovery, of this system, Hulse. And Taylor won the Nobel Prize in 1993, because. They showed that the reason, for this shift is that these neutron stars were radiating, gravitational. Waves and spiraling. In towards each other they were losing energy by the radiation, of gravitational, waves so. This demonstrated. That, the gravitational waves must in fact exist, and be real and be exactly, like. Einstein. Had predicted but, this was still an, indirect. Detection of gravitational waves, because. We're detecting, the effect of the radiation of gravitational, waves can, we detect them directly, well, people went ahead and have done very, very complex, numerical, simulations, of what that would look like I'm gonna show you here a picture of this gravitational. Wave in spiral, and. Merger, simulation. In this, case the two blue dots, are neutron stars but. All the way until the merger, basically. The wave form looks the same as it would look if it was something more massive, than a neutron star and that would of course be a black hole after. They. Get close enough for the neutron stars to interact, tidally, with each other things, look different but in the meantime this. Here down here is what the gravitational, wave waveform looks like you can see these ripples, being emitted and then, as they, spiral in towards each other the, ripples get bigger. And they, also start, getting closer and closer to each other so, you have this chirp, more. And more and now they're beginning to interact, idly, and then, this is the merger, of the system and then, what's left behind is a ring down as space-time. Stabilizes. After, the formation of this more massive, thing from, the molder of two neutron stars, this. Is the output of a, simulation. There. Are people who work on numerical, relativity simulations. And they do this kind of thing and that's great. This was their. Credit up here. Can. We detect them directly, well. The. Big problem, in detecting, the stretching, and squeezing of space itself, you, can't just hold your hold up a meter stick and detect that because. Your meter stick is also going to stretch and squeeze. Right. So it becomes a really difficult conceptual, question, how are you going to detect that Einstein. Tells us that there's one quantity, that, does not change and that is the speed of light. Right. So, you can use that and what you can do is you, can build a detector. Where, you have two arms and each, of those arms, are going to shine a laser down, and, they're.
Going To reflect off of mirrors at the two ends and then they're going to come back and you're, going to compare their time of flight down those two arms with each other and. Now. If a gravitational wave passes by one. Of your arms will squeeze. A little bit the other arm will stretch, a little bit and, the. Light will reflect and come back a little earlier, on the squeeze side compared, to the light going down the stretch side and so. You should be able to see that, the crests, and troughs of, the wavelength of the light no longer line up and instead, you get a shift in this interference, pattern right. So you build one of these detectors, and then, you realize that what, we are talking about the. Stretching, and squeezing is much, smaller than the size of an atomic nucleus, across, the length of these arms, of these detectors. Okay. And so, when you in the lab ordered pizza and the pizza delivery truck, shows up that's enough, rumbling, in the ground to, produce a signature, that is going to swamp your gravitational, wave signature, so, how are you going to tell this apart from all the false positives, of seismic rumbling and tides and wings and everything else you. Have to have a second, copy of the detector, and then. A signal is Astrophysical. And real only, if it is seen at both of the detectors, and so, these are the two LIGO detectors one, in Hanford, Washington and. One in Livingston, Louisiana, there's, also the Virgo detector, in Europe and then there are other gravitational. Wave detectors, be built around the world but. This event. Gravitational. Wave event 15 or 914 on the, 14th of September, 2015, is the, first direct. Detection, of gravitational waves. And you, can see here the, wave form at Hanford, the. Wave form at Livingston Louisiana and, then. Below in the comparison, to the theoretical, models, for what it should look like if two, massive, black holes basically. Merge, with each other and emit, a burst of gravitational, waves so, this is a twofer, it tells, you it's a direct, detection of gravitational waves, you've seen it from the stretching and squeezing of the arms of your detector, and also. It is a demonstration, that, black holes must exist, because. This is what the wave form is predicted, for these black holes right, so, now we've done we've. Gone a fair, amount and there, are lots of these gravitational, wave events, that have been detected, including. Massive. Black holes merging with each other and then, GW. 1708, 17, which is the first detected, binary. Neutron, star merger, so just, like the binary neutron star that hull sem talar detected. Which were losing energy by in spiraling, towards. Each other except. Millions. Of years further along in their lifespan when they actually merge, and produce. A burst of gravitational, waves and that, was GW 1708, 17, we. Estimate that maybe a third of practicing, astronomers, around the world are on people is related to this one event it. Was just this absolutely, incredible. Bonanza, because unlike. Black hole mergers neutron star mergers put out a lot of stuff and so, it was not just the gravitational, wave astronomers, but the electromagnetic, astronomers, who could get in on the act as well we, now know of all of these masses, in the stellar graveyard from, detections, by.
LIGO, And Virgo, we, know of binary neutron, star mergers, we, know of black holes that have merged to form larger, black holes and, if. You look at this table it tops out at maybe about 80 100. Times the mass of our Sun these, are definitely black holes. But. Can. We go larger. Can. We go to, supermassive, black, holes, right. Well. Let's. Back up a step and. Let's. Look at some pictures of the sky for a second, this. Fuzzy, blob in the middle of this image is a picture it, at optical, wavelengths of. Cigna. See an, optical, wavelength it looks like a fairly unremarkable. Galaxy. This, fuzzy little blob at, radio. Wavelengths, it. Is one of the strongest, radio sources in the sky okay. It is this absolutely, incredibly. Bright thing where, there's a bright. Hot dot at the center and then, it's putting out these. Gigantic. Jets, of radio emission, okay. What. Could it be this. Is the composite multi-wavelength. Image, and this image should tell you why modern, astronomers. Are so insistent, that we need telescopes, at different, wavelengths this. White patch left behind in the middle is the optical, image the. Blue is the, x-ray, image which is showing you the hot gas in this. Cluster, of galaxies, and then, the reddish color in this image is the radio image that I showed you and together. They paint a very different picture, than just the optical, picture showed, right. It shows, you that at the center of this galaxy there's a supermassive. Black hole that is feeding on gas and then shooting, out these Jets, of particles into. The intergalactic, medium right. It's an incredibly, energetic, system, and we, understand, that the supermassive, black hole that is powering the system is about, two-and-a-half billion. Times. The mass of our Sun so. We've gone from a hundred times the mass of our Sun to, two-and-a-half billion, times, the mass of our Sun and that's what we're talking about when we talk about a supermassive, black hole, where. Did the supermassive, black hole come from. Let's. Take a quick look at the, Virgo cluster this. Is a cluster of galaxies, it's about six and a half million, light-years away and if, we zoom in on one of them this, giant, elliptical, galaxy is, Messier, 87, m87. It's a large. Well-known galaxy. If you. Zoom in on it at optical, wavelengths with the hubble space telescope you, see that the inside has, this optically, jet it's a jet of material ejected. At very high velocities. It. Looks one-sided, because this side is actually aimed towards, us and therefore Pierce brighter the, other side is aimed away, from us and therefore it's fainter. If. You look at it at radio wavelengths it looks, very very different you have this huge, large-scale, jet like structure, at radio wavelengths and, then. You can say okay and we, zoom in so, this little touch which in this image appears burnt out at radio wavelengths is, what. We are zooming, in on in this image and now, you can zoom in further onto the jet and you, can zoom in even, closer.
In And you can see that at its center there is this blob. Right. And this, blob when, you image it with a telescope, that is the size of the entire Earth and, you, image it with high enough resolution and, a tine of frequencies, you, basically get this picture which. Is the, shadow of a supermassive, black hole, in the center of m87 this, is the event horizon telescope, collaboration. Put this image out last. Year and it was an instantly, iconic, image what. It tells us is that there's a five billion, solar mass black hole, at the center of this galaxy and it's surrounded, by gas in this accretion disk which is moving at two million miles per hour these are just mind-boggling, numbers, they're not just astronomically. Large numbers there may be large. Enough numbers in the realm of economics, now like, there are economically. Large numbers. Okay. So where did these supermassive, black holes come from. We. Understand, that galaxies. Grow. By merging with each other we, understand, that in the distant universe mass, assembled. In this hierarchical, fashion where, little galaxies, merge with big galaxies, and produce, even bigger galaxies, there, are lots and lots of these images, iconic. Images, that are where. You can see different. Stages, in the mergers of these galaxies, with each other right. And there's, lots of these okay. If you, zoom in on the Centers, of these, merging, galaxies. You. Can find these bright, hot, dots, and these, bright hard dots are the massive, black holes at the center of these massive, galaxies, which, presumably, like, the galaxies, merge and get bigger, presumably. These black holes are also merging, and getting, bigger over time right. We, have instances, where we can see these black holes supermassive. Black holes with their jets locked. In a dance with each other as they spiral inwards, and as, they spiral inwards, they should be losing energy by. Emitting gravitational, waves. And, eventually. They, should be merging so. That's how supermassive, black holes are born and now, we can put the story together and we can say okay so. We have supermassive. Black holes merging, can, we detect gravitational, waves from. Those. Well. What are the timescales the. Mergers, of these supermassive, black holes basically, take, millions. Of years or, longer. And in. Fact the in spiral, phase the final in spiral phase takes years, and then, there is a ringing phase that takes may be of the order of a year over, which you have dispersed. So. A year time scale means, that, the wavelength of the gravitational, waves that's emitted because gravitational. Waves travel at the speed of light takes. A year which means it's light year long wavelength. So. Now to. Detect gravitational waves, that are a light, year law we. Need detectors, which have arms which are off the scale of light years and so. Now we need to make a detector, that is the size of our entire galaxy in order. To be able to detect these, very low frequency, very long wavelength, gravitational. Waves how. Are we going to do that and this, is where of course we. Bring the, first part of the talk back into the picture we say okay instead.
Of Having arms, like the LIGO and Virgo detectors, what, if we, replace, these arms, by, the lines, of sight to, these radio pulsars, in the sky that have very, very stable, clocks. Right. And as these, very very, stable clocks, tick, and a. Gravitational. Wave passes, through, them. The. Line of sight from us to the Pulsar is going to stretch and squeeze. Stretch. And squeeze. Over a period of years but. It is going to do that and so, the ticks of our clock are going to get closer together and further, apart and if. We can time these. Radio, pulsars, to, very, high, precision. The. Gravitational, wave strains we are talking about from the mergers, of these supermassive, black holes are of the order of one part in 10 to the 15th. Right. This, is actually substantially, larger, than what we talked about with the LIGO detectors because. There we were talking about kilometer, long things moving. By the size, of an atomic nucleus this, is actually a larger, string than that but. That means that it's a change in length of the order of meters, over. Distances. Of light-years and, so. We have to be able to time these neutron, stars to a precision, which corresponds. To knowing, where. They are to. A precision, of meters, over, a distance of light-years. And. That, means that, we need timing, of these pulsars, to a precision of tens of nanoseconds. Over. Long periods, of time and so. This of course is, an incredible, challenge and when, we successfully. Do this, just. Like we have different. Wavelengths, of light so that there are x-rays, there are ultra violet rays optical. Infrared, and radio wavelengths we. Will end up with gravitational. Waves that are detected, from the ground of the, order of hundreds, of cycles per second, gravitational. Waves that will be eventually detected. When we fly detectors, like Lisa in space and then, gravitational. Waves that we will be able to detect with these pulsar, timing, arrays which, are coming from the mergers, of supermassive, black holes these, are entirely, new, windows, on the universe, that are being opened and the, first of these this now, exists. And I have shown you that we are successfully. Doing this, there's. Much. More that is going to happen on this, front, again. A reminder these, gravitational, waves are not, light almost. Every, other way of sensing the universe, that we have is a form, of light there. Are very few other messengers, some, of you know about neutrino, astronomy and, that's a different messenger, gravitational. Waves are a completely. Different messenger. On the, universe. The. Collaboration, that I'm part of is the nano graph collaboration. And I should emphasize that everything, I've shown you so far is. Work. By large, collaborations. Not just of senior, people but lots and lots of students here's a picture of the nano graph collaboration. Meeting. In front of the Green Bank radio telescope which, is one of the radio telescopes, that we use here. Are lots, of our student, meeting pictures, and. What, we are doing basically is, using, these telescopes, which are some of the largest radio telescopes, in the world the, earth sea Observatory the Green Bank radio telescope and, every. Year we are timing more, of these neutron stars so. What you're seeing in this graph is basically. Each of these dots represents, also that we're timing, over, the years and, so. Every, year we add more, pulsars, to our detector, that's like adding more arms, of our. Gravitational, wave interferometer. And one, of the things that we are looking for is we, are looking for a correlated. Change, in the, timing, of these bolts ours we are looking for a change where all the, pulsars, in this part of the sky appear to speed up a little bit and all. The pulsars in that part of the sky appear to slow down a little bit and vice.
Versa And that, will tell us that, it's not something that's happening with, the neutron star it's, not something that's happening as a propagation, effect, but, instead it is the stretching and squeezing of space itself, that, is causing this correlated. Change in the timing, of these neutron stars and so. That's sort of our ongoing, enterprise. It's, not just us in the United States who are doing this we have international, partners, all over the world this is a global project. With. Lots of telescopes, around the world lots, of people working, on detecting, these gravitational, waves at low, frequencies. I have. One technical, slide that I thought I'd throw in just to show you where things stand okay. This. Is basically, our. Limit. On, gravitational. Waves that we have detected, so far with eleven years of timing, pulsars, we have not yet detected gravitational, waves however. We. Have theoretical. Models, for the mergers, of these supermassive, black holes, -, in order to get to the universe that it looks like today and theoretical. Models, say okay, we should be expecting, gravitational, waves if we're optimistic, in this, green band and you. Can see that, our upper limit which is the solid black line has. Already dug into that green band and sort of ruled it out because if in, fact the. Optimistic, prediction, was correct we, should have already detected green we should have already detected those gravitational waves the, orange bang shows what we think is a moderate, scenario, the, blue band shows what is the most pessimistic scenario. And the, black line is where we were with eleven years of data we. Have we are now working on our 15-year data release and. We hope that this line will get pushed substantially. Further down, eventually. We know that supermassive, black holes exist, and so, either we should be detecting, this background of, gravitational, waves or. We. Should be detecting, gravitational, waves. From the in spiral, of an individual. Source on the. Right hand side this is a plot of what our individual. Source limit, looks like we're looking for the continuous, waves from. The in spiral, of a pair of supermassive, black holes okay. The. Blue line is sort of our sky average, sensitivity. The, red line is our sensitivity, at the most sensitive, part of the detector, on the sky just. Because of the way, our pulsars, are distributed, we're more sensitive in some directions, compared, to other directions and then, the purple dots are a simulation, of the universe. They. Are not the real universe they're our simulation, based on the. Reality, but we don't know what the gravitational, wave universe, looks like that's, what we're trying to see the, Pope will not show what it would have looked like and it says that in one simulation, of the universe if there, was the right source, in the most sensitive part of our detector we should have already seen, we. Have not. But. That, curve, gets lower every, time we do our data release and eventually. We should be able to detect individual. Gravitational. Wave sources, this. Is what, our dataset, looks like now as I said every. Year our dataset, gets larger, because we add more pulsars, and the, pulsar we are timing with time for longer durations. This, was our five-year data release this was our nine-year data release as I said we are now working on our 15-year data release all our data immediately, becomes public, at data dot nano graph org, should, you want to play with it it's all right there. Again. This is something where, lots. Of people have and we are actually very proud of the fact that. Completely. Independent. Groups of scientists, have taken our data and done interesting astrophysics. With it that, is that is something that makes us very very happy indeed. Okay. So, where. We are on, this, is we, are now approaching a detection of the gravitational wave background.
The. Black line was where we were in 2018. We, expect by 2030s. Which will have dug in to. The spectrum, where realistically, sources, must exist, and. This is opening, complementary. Windows, on the gravitational, wave spectrum, with, LIGO, which has already working, and Lisa, which does not exist yet except on paper but will eventually fly, in space and fill, in the gap between us okay. So that's where we stand with gravitational, waves as, I said every year we are looking for more pulsars, and we are adding to our detector, and while, we build this detector. Interesting. Things pop up right. I, already, showed, you Jocelyn, Bell and her discovery, of radio pulsars, which was this little bit of scruff, in the radio sky that. She went back to over and over and realized that it was a radio, pulsar there's, another famous signal, like this which. Is which, goes by the name of the WoW signal. This, is just a one-off signal, that was detected, in 1977. At. The, big ear telescope, where, basically. The telescope, if this is a primitive system back then it. Prints, out either nothing, or one to show the level of noise right. And it's, usually, nothing or one and then the, occasional twos, or threes maybe. Fours, and then, suddenly it goes into five. Ju. Q, e, this. Massive. Signal, that comes through and it. Gets circled, in a red pen and wow. It's. Famous as the vowel signal it's never been redirected as I said extraordinary, claims require extraordinary, evidence. This, our best, guess now is that it is some form of interference, it, is very, very probably, not aliens. We. Don't know until it's reproduced, we will never know, similar. To this is, this idea of these fast radio bursts, so the fast radio bursts, are these, millisecond. Flashes. Of radio waves that, seem to pop off all over the sky and because. They're radio waves and the space is not a perfect. Vacuum space, is a better vacuum than anything we can produce on earth it's. A pretty, darn good vacuum but, it's not a perfect, vacuum and so. The radio waves don't travel at precisely, the speed of light in a vacuum they, travel slightly, slower depending on the frequency and so, you end up with these swept, pulses. Where. The higher frequencies, arrive slightly. Earlier the, lower frequencies, arrive slightly, later and that tells you that it's propagated. Through a huge, amount of space to get to us and, there. Are these millisecond. Flashes, of radio waves the initial, detection, was at the Parkes radio telescope. Which. Became famous, this. Is a very very famous detection. Of this fast radio bursts. There's. There's, lots of stories to tell here but I'm just going to move along and tell you about the one that I have worked the most on that, is close, to my heart it. Is this completely, unimpressive. Looking, little patch here this. Is a fast radio bursts that was detected, at the Arecibo, radio telescope you. Barely, even see, it it's. Barely, there, this, gives you a little bit of a sense of the data processing, challenge, that we face because here I have highlighted, the, one section, of the data where, the fast radio bursts I already know is existing. If. On, this scale the. Actual observation, data straight will stretch for miles. And. We just have to go through that we, effectively, take lots and lots of survey data and I like to describe that as it's not just looking for a needle in a haystack it's, looking for a needle in a haystack that is full of needles because, there's lots and lots of interfering, signals, that have to be figured, out and discarded, in order to find one with the correct characteristics. For these fast radio bursts, this, is one of those challenges. Where some, of you should be thinking, oh machine learning I wonder if it could help it, can help and I am cheered.
I Would be glad to talk to some of you about what we can do and what we have done and what we should be doing just. In case some of you are interested in talking to me about it. But. This was a fast radio bursts and so what. Produces. That we don't know, we. Have lots of good ideas in fact we have so many good ideas that there were many more ideas for what these things could be then there were examples, of these things, not. Joking there, really were many more models than there were theories the. Bulk of them consisted, of things that explode things that go boom which, produce these flashes, in. What is what has gone down among, our collaboration. As one of the most. Amazing email, subject lines ever a graduate, student Paul Schultz sent out this email to our collaboration, saying a minor, point of interest, about. This, verse that Laura Spitler had discovered before and the minor point of interest was that this particular direction, in the sky was producing, multiple, bursts, and it. Was a repeating, source now, at one stroke the fact that it's a repeating source tells you that it cannot, be a cataclysmic. Event it has, to be something where, the engine, survives, the process, and comes. Back and produces, more of them okay. The. Other thing it tells you, is. Well. If. It's a repeating, source then. Unlike, every, other fast, radio bursts till that point we, know where to go fishing, for more of them and so. The large circles, here are the detection beams at the Arecibo, radio telescope those, are the beams in which it was detected, and you can see that Arecibo, even though it's a large radio, telescope, it's a single dish telescope, it, has a fairly large patch within which there are lots of sources and, we. Don't know which of these is the, source that's emitting, this fast radio bursts. What. We can do though and it's, this dispatch of skies full of hundreds, of these sources what. You can do is you can go to the Jansky. Very Large Array, which, is an interferometer, and that lets you make a more, detailed map, of the sky with. Much higher resolution and, then, you can go and you can run it like a movie camera and you can make, images. Of the sky at 200, frames per second, a terabyte. Of data every, hour and you can keep taking this data which, makes them very very. Anxious because that's. Not how this instrument, is meant to be used. But. This. Is really exciting you've got to give us some time and so, we actually were successful, in getting a lot of time at this very large array, it's. A whole other story a whole other talk but, basically, in one of, these millisecond. Frames of the sky we finally, caught one of these fast radio bursts, and so, what you're seeing here is what the sky looks like in a five millisecond. Image there's, just nothing there except for this very very bright fast radio bursts popping off and now, you can go back and go to the whole image of the sky and you can say okay so where was the source it's, right there so. That source, is emitting. This, fast radio bursts and then you, can basically go. And you can study what. That source is and strikingly. UB. This is a picture of the Gemini Observatory where, we went to get. Deeper, images, at optical, wavelengths of the sky and we, could find that it was this tiny, little, fuzzy. Blob, a puny. Worf galaxies unlike, all the other galaxies, that I've shown you so far which, was responsible, for producing this fast radio bursts. We. Can measure how, far, away it is because, the light from that fast from, that host galaxy, is redshifted, as the universe, expands, the. Spectral, lines from. That galaxy appear. At different wavelengths so, that's a Doppler shift just like you get when you have a car. Honking, moving. By you. That. Shift, that's the Doppler, shift and just like that these wavelengths of light are, redshifted, so we can infer, how far away that galaxy, is it turns out to be 3 billion, light years away and then, in other work that I won't have time to tell you about we, have figured out that it comes it's embedded, in a region, of extremely, high magnetic, fields so this is this completely, mysterious, little thing there.
Are Lots of questions about fast radio bursts that we cannot answer yet we, have lots of theories as I said we have we, until, recently had more theories, than we have examples, now we have more examples. There's. Lots of possibilities we're. Pretty, pretty, sure that it's not aliens we, don't yet understand, whether all of them repeat whether they're cosmological. Where the nearby ones are but, what I will leave you with is this idea that there are new telescopes, new instruments, and there's lots of ongoing, work we, need to find more of these bursts, there are very, large, data sets that we're trying to mine to look for these things and. Then once we find more of them we can use them to probe the universe and, find out more about both, what they are and what, makes up the universe around, us and I'll stop there thank you very much for your patience. So you mentioned earlier that things like you. Know pairs. Of neutron stars or pairs of black holes lose. Energy to gravitational, waves so does this imply that one, can convert energy into. Gravitational. Waves somehow. Yes. I mean gravitational, waves carry, energy, these. Ripples, in space-time they're just carrying away energy, and so, as they carry away energy because, energy can't, be created or, destroyed it, just is converted, from one form to another what, you're doing is you're converting the kinetic energy, of the orbit, into. Gravitational. Wave energy, and therefore, the orbit must shrink as. It. Loses energy right. And so. With. Supermassive, black holes merging, we think that initially it's just things. Like gas drag, and in, three-body interactions. Where you have for, example dense. Clusters, of stars, and inside it these black. Holes are spiraling. In towards each other and then, a star gets too close and it gets ejected at very high speed, it picks, up that speed and therefore the, orbit shrinks a little bit eventually. You run out of those stars then. There's gas drag where as they. Spiral in there's gas drag the gas gets heated up eventually. That gas is all accreted, onto those supermassive, black holes and, then. What's left behind is these two, supermassive. Black holes orbiting, each other, with, nothing to break them and that. Used to be called the final parsec, problem, where the last parsec, we don't know how they would actually get around to merging with each other we, believe that we have resolved, the final parsec problem, that is no longer a problem in these simulations, and. Then as they merge they emit gravitational waves, to carry away energy. Hi. Thanks. For the talk so. You, mentioned, that you use. The. Shrinking. And expanding of, space to detect these rotational. Waves right so.
Why. Not just use the gravitational, pull on the objects. That's, that's, a good question, and. So. The initial, idea, of how. To detect, gravitational waves, there, were all of these detector, models, like there were these bar detectors. Where, the ball would be deflected, a little bit. Be. Running to problems, I mean first of all. It's. Incredibly, hard to get something to hold still. Well, enough, so. That you can actually measure, the miniscule, deflections, that we're talking about even. With LIGO, people give entire talks about just, the arrangement, of lasers. And mirrors at Lyle the mirrors at the ends of these tubes, are suspended. So that they're completely, decoupled, from everything else because remember if they were connected to the detector. Arms themselves, all, of the vibrations, would go through them so, you have to decouple them from all of the environments, somehow and so, then really most. Of the other ways you can think of will run into the problem. That your meter stick also shrinks. And expands, as space, stretches, and experience. So. You need a meter stick that, is independent. Of the stretching, and squeezing of space and really, the only way we've come up with that's a reliable, way to do that is to, compare, two different directions to different meter sticks against each other and then, for the gravitational waves I was talking about which are very very low frequency, gravitational, waves with very very long wavelengths, you need to do this over light your skills. Thank. You so it's not impossible but it's just not reliable, enough it's not technically, feasible in, fact I mean I would say it is technically, impossible it. Is it conceptually, it is not but, technically I think it is impossible yeah thank you yeah. You. Mentioned the. Fast radio bursts that repeated, yes how frequently, did it repeat that's, a great question, because. The. Initial detection. Was in 2012, or well, it was earlier than that but we analyzed the data and found it in 20 the detection. Was in 2012 even, though, we found it two years later in analysis, of the data and then every year we'd go back to the telescope and we take some more of evasions and we'd, see nothing and we'd go back and see take some more data and we'd see nothing until, we finally started seeing it so, the. Repetition, is sporadic, and, completely, unpredictable, so, far with. A sensitive, enough, telescope, it looks like when. It is active, this particular source, we, can find maybe. One, burst, every couple, of hours of, observation. Time, but. That's very source, dependent, and that's very telescope, dependent, if you have a more sensitive telescope. You can find weaker bursts. So. Yeah, we've, looked very hard for some kind of PR city, in the repetition because, that's immediately. The obvious, thing we jumped you is could, this be some kind of rotation. Model that we can fit to it if not a rotating, neutron star, couldn't be things, orbiting, around each other could, it be something like that so far we have not been able to find any piata city in the repetition it, is sporadic hey. Thanks. A lot for coming today I have a question regarding. The precision. Of measurements, you're working, with.
Sorry. For my ignorance in advanced duty. Gravitational. Waves participate. In the interference, and if, they do how, would that affect the, precision. Of the measurements, you're working with and won't, it. Produce. Any false alarms, while, you are working on detection of the supermassive. Black holes the, question is whether the, gravitational. Waves are participating, in the. Interference. Okay. I. Did, not go into the technical details here but that's actually a very interesting question. Because. For. These gravitational, waves there's two terms we, call them the earth term and the pulsar term, the. Earth is sitting, at, the. Middle of this detector, and so, the gravitational wave, hits, the earth and causes. Effectively. The, space to imagine, it stretches, a little bit and so. Then all of the, pulsars. In this part of the sky will look like they've speeded. Up a little bit all the in that part of the sky looks like they've slowed down a little bit and then. There is a term which is for pulsar, which is when the gravitational, wave hits one of the pulsars, that. Pulsar, the line of sight between it and the earth is stretched a little bit and therefore, that pulsar, speeds up or slows down but not the rest of them, right. So effectively, what you're saying there is I think is how. It works except the term that we are more sensitive to is the earth term because that's correlated across all of the pulsars some will speed up some will slow down the, pulsar term is a poor pulsar, thing and that's really, hard because you can never. Really be sure that, you've detected a gravitational, wave effect, as opposed, to something, along, the line of sight some, refractive, effects some electromagnetic, effect, or some weird. Thing in the Pulsar itself, so, as far as we understand it our initial, detection is going to be a detection of the earth term the earth bobbing, up and down in the, gravitational, wave background, effectively. Except, it's not bobbing up and down it's bobbing in 3d. Space in a fourth dimension. Yes. But. Will, detect, that before, we detect individual, atoms, so. Maybe that gets to the question you're asking thank. You. Thanks, for the very interesting talk and. This may be a knife question, but it seems like all of the detectors. That you mentioned whether it's like over go or the pulsar based detection it's, predicated, on the assumption that the speed of light is constant and that's what you're using to detect, gravitational waves yeah so I guess, why. Is that. Absolute. Theory, that. That's that's absolutely correct, but remember. The. Existence. Of gravitational waves, is a consequence. Of Einstein's, theory. Of relativity and general relativity. And according. To that theory the speed of light is the constant so in that sense it is it is a closed loop now, there are there have been, periodically these, theories of tired light or what if the speed of light was changing as, a function of time so. Far the theory gr. Is tested, extensively, on. Many, many many different, scales right, and it seems to hold up real. Well to, the precision that we can attain so far, if. It turns out not to hold I think that will be tremendously. Exciting, to us this. Is why people still, continue, to propose tests, of GR when, we any time we find a binary pulsar the first thing we do is okay can we use this to test gr, and push the precession one more step forward that's. Becoming increasingly, hard now because gr, is tested so well it seems, to work on scales.
All The way from GPS satellites, orbiting, the Earth to, these distant, universe things so so far we have no grounds to question, it but. We are testing, we are not willing to take that on faith because anyone, who shows that. This ray does not work you, bet they're going to go for that that is going to be really, really good news for them that, is for the rest of us maybe you like okay lots of work to be done but that that's what gets us excited right. I mean this is something that I think people get wrong about scientists, in general we are we. Are the opposite, of invested. In our theories, the, way to get ahead is to show that your favorite theory is wrong because that is guaranteed to, push the state of that forward we. Are the opposite, of invested, in it we want to show that things are wrong, but. Yeah so far it holds out thank. You hello. Thank, you for coming um. Can. You use, gravitational. Waves to determine, anything, about the source such. As like, what element, the source is made of or anything. Like, that matter. That's. That's a great question, you wouldn't. Probably use, it to determine, what the source is made of but what you would be able to do what, we are doing in fact right now with the LIGO detection, is we. Can tell whether. Like. What it was that was merging so, for example, the waveforms, look different, depending on whether they are black holes merging with each other or neutron, stars merging with each other neutron stars as, they merge with each other, there are tidal effects, black. Holes do not show those tidal effects and so the wave form simulated, are different, we, are. Not yet convinced, that we've seen a merger, of a neutron star with a black hole that. Will produce tidal. Stretching of the Transtar but not of the black hole there are simulations, of what those waveforms should look like so. Yes very much so the waveforms. Tell, you what, was going on at the merger it's a whole new window a whole new way of looking, at the, universe. You.
2020-02-16