WSU: Gravitational Waves | Einstein’s Astrophysical Messengers with Gabriela González

thank you thank you all for coming i really appreciate people coming on sunday to university to lectures so i wanted to tell you about gravitational waves these are the prediction of einstein's theory of relativity something that we are celebrating the 100th anniversary this year but also how to measure them and what we can learn from these gravitational waves as as we call them astrophysical messengers so the plan i have for the lectures are first to talk about eyes and gravity i know you've been hearing about these and perhaps you heard everything i'm going to say but i can't resist i mean that's what we are all here that we all have different ways of visualizing this uh and we like talking about it then i'll talk about the sources of gravitational wave what kinds of gravitation and what kind of stars and astrophysical systems produce gravitational waves and then how to detect them which is the difficult part that's going to be the hardest part and then how we want to do what we call multi-messenger astronomy with these gravitational waves so let's start then with the first part einstein's gravity uh i know that you are perhaps experts at this already after this world science festival with so many events on gravity but let me tell you a bit more about that first what you learned in school newton's gravity you probably thought that this was canonical it was it was a standard theory of gravity we learned it as a perhaps a bit boring class but it was revolutionary at his time newton's theory was actually a revolution in unifying theories he had this one theory to explain how the forces between masses could predict not only how the apple falls to the earth and how objects move around in in billiard balls and objects that we see every day but the same theory could explain how the earth went around the sun how the moon went around the earth and the motion of all the planets that were seen at the time that was the first unified theory then came maxwell and now we're still looking for more and more unified theories but that was the first one that was revolutionary and his formula was actually very simple it said that the force between two masses is proportional to the product of the two masses and inversely proportional to the square of the distance so it explains everything that every motion that people knew about the time of course that there is a lot more to physics and motion but that was the theory we had einstein came along and for different reasons he came up with this theory of general relativity which is very very different than newton's theory it's not a formula that tells you about the force between two masses in fact he says that there is no such force and the reason uh one of the reasons he was looking for this is that in his theory of special relativity he had postulated this universal speed limit the speed of light was the maximum speed that any object could have and newton was saying that if you had two masses the force between those two masses was instantaneous if one mass moved the other one would know immediately and that violated his speed limit so that was one of the reasons why he was looking for this theory and this beautiful theory says that what you have is not forces between masses but masses living in a dynamical space time and the masses move and change that space time and the masses can tell how the space time is changing and then follow what seems like the shortest path so einstein's vision and let me play this nice movie again is that if you have a mass it it doesn't feel the force of the other mass what it feels is the stretching and squeezing of the space space-time around it and then if you have another mass it will go around it just like newton's theory predicted but not because there is an instantaneous force but because it's following the shortest path as seen by the grid of the space-time so it was revolutionary again in a different way so the essentially what the theory says is that when the masses move they move they change the space-time they wrinkle the space-time fabric and then that is what make other masses move and that happens at the finite speed that happens at the speed of light so now there is no more violation of the speed of the speed limit now the math is complicated and i'm only going to put this formula here but not going to the math details that are actually very very thick books that tell you how to do this but essentially what he did was to put the description of the geometry of space-time in something that we call the einstein tensor so it's a four by four matrix four because it has the three space dimensions plus time dimension but all mixed up space time is all the same so this tensor describes space time the geometry of space time and this tensor on the other side describes the mass and energy that is in the space time so this is how mass determines geometry and how geometry determines how the masses move now the problem with this theory and he realized this very early on is that the constants in here are newton's constant talking about gravity and the speed of light that is the constant in the theory of relativity and what this says is that masses usual masses produce distortions of geometry that are very very very very small so it does explain after a lot of math you can explain same things that newton did like the up how the apple falls and how the earth moves around the sun those are not small effects but those are very small effects in the curvature of space-time the curvature of space-time is mostly flat except near very very strong masses or masses moving very very fast now einstein after writing his theory was looking for predictions that could be measured to see whether the well i don't think he doubted his theory was right but he wanted to prove to others that his theory was right so there were some predictions that actually people realized from the beginning uh and some he liked and some he didn't one of the predictions was that the universe expands this uh when you put a spherical symmetry in this uh spherical if you put some dust with spherical symmetry it tells you that the universe is not static in fact it tells you that because mass is only attractive it is compressing but it could have started expanding and then compressing in any case it was not a static universe and he didn't like that in fact he added a constant to his uh to this tensor in here to make the universe static to make it stop moving he didn't like this expansion theory but then we realized but we i wasn't there at the time but observers measured that the universe was actually expanding all the galaxies were getting further away from our galaxy so he took that constant away and said that that has had been his biggest blunder and now we think that we have to put it back with a different sign and with a very very small value but that's another story so the the story of the universe starting with a big bang and expanding is a prediction of this theory it also predicts that there are black holes in fact the first exact solution to einstein's equations was not done by einstein it was done by schwarzschild during the war he was in the trenches doing the math on a small notebook and he realized that if you have a star a spherical star and you want to solve the equations for space-time you find that there is a singularity at the center and that singularity was what was called black hole and einstein didn't like it many people didn't like it but now we believe that black holes exist and that's one of the things that we hope to measure distortions in space-time from as i will tell you later the one prediction that he really liked and was the one that made everybody believe in in the theory at the time and and his theory appeared in the first page of the new york times was that light bends when it goes through a big mass like the sun so if you see a star when the sun is closed then you would see that a different place than if the sun is not there and that you could test with an eclipse and that's what was done in 1919 and that is what appeared in the first page of the new york times and made einstein very very famous another prediction that's actually very interesting and very very practical is that clocks run at different rates different heights so if you are at the top of a mountain or at the bottom or near the surface of the earth and you have an atomic clock with you which is the most precise clock you can have it will have different rates you can't have synchronized clocks if you are in different gravitational fields and that is very important not only because it's well it's a prediction and a test of general relativity it's something you have to take into account if you use clocks to measure distances and you probably are all relying on your gps to get here to get to many places gps would not work if we didn't have to if we didn't apply the corrections needed because of this asynchronism of clocks at different distances from the earth general relativity is weak it has weak predictions but some of those are very very important at least for people who get lost even with gps like i do so go back to einstein gravity the one prediction that i like the best and the one i want to tell you about is gravitational waves this is a very different prediction than the newton's theory if you have two stars or two objects like the sun and the earth newton says that there's going to be circular motion around the center of mass and that's going to go on forever einstein says that the two objects the two stars are going around each other because they disturb the space-time and they follow these judasics but in moving they also produce these other distortions of space-time that travel away from the system and they carry energy away and if they carry energy away then the end the binary system has less energy and that means that the stars get closer and closer together that is very very different than newton's theory and it's all about these gravitational waves carrying energy away now it took about 40 years or 50 years to realize that gravitational waves could actually carry energy most people even einstein thought that gravitational waves might be well perhaps you chose a coordinate system that makes it look like a wave but you can make it go away if you choose some other coordinate system the physics is all the same independent of the coordinate system that you choose so perhaps gravitational waves are just a mathematical artifact but they are not they do carry energy away and that's why they can be measured and we know that is true because that effect has been measured and i'll tell you more about that so that's einstein my version of einstein's theory it does predict it has many predictions because of this in dynamical interaction of matter with space-time and what i'm going to tell you first is what kind of matter produces these gravitational waves and then i'm going to tell you how we can detect those disturbances of space-time sources of gravitational waves are according to the theory any matter any non-spherical matter that is accelerated so in principle i am producing gravitational waves moving my arms in here i'm not spherical i don't want to be but and in moving i am accelerating i'm not moving at a constant velocity in any reference system so i am producing gravitational waves it turns out that if you take quantum theory in into account i may not be i may not be moving fast enough to produce a quanta of gravity so perhaps i'm not moving space-time but according to the classical theory of gravity anything that moves produces gravitational waves but what einstein realized in the very beginning independent of whether gravitational waves were real or not is that most ordinary mass produces disturbances that are tiny tiny assets as of unmeasurable he thought so what we have realized since then is that if you think about astrophysical sources where you have very compact objects or very cataclysmic events then those big masses in small radia moving at large velocities can produce detectable gravitational waves and those are the sources i'm going to tell you about first one let me not skip one is the early universe itself in the early universe at the very very beginning there was matter in the universe the same amount of matter that we have now matter and energy has been constant but it wasn't completely homogeneous it had quantum fluctuations in that and those are the ones that much later on produced our galaxies and the clamping of matter that we see now but in the expansion of the universe those fluctuations of matter produced gravitational waves because they were moving they were changing the space time that was itself getting uh expanding since the big bang so we want to see those early gravitational waves it's like a rumble it's the same kind of rumble that we call cosmic microwave background except that that rumble is the electromagnetic rumble that was produced by those photons getting free in the early universe and traveling to us and they were so stretched that they are now microwaves there are there is a similar kind of rumble produced much earlier this is the microwave background produced later in the universe much earlier there were these quantum fluctuations that produced this gravitational rumble that we want to detect now there are several ways of trying to detect this the one that's most promising is the one looking at the imprint of these gravitational waves on this microwave background and you might have heard about bicep two news in the last year or so they had evidence of strong fluctuations in the polarization of the microvolt background that was thought to be originated in these gravitational waves it turns out that it's not easy to see whether it's dust would produce dust in the way from this early universe to us would produce the these changes in polarization or the gravitational wave background but there are many experiments trying to do this they are getting more and more precise with time so that's going to be coming soon so that's one kind of gravitational wave produced in the early universe like a rumble the other you probably know that stars often explode well not that often it happens about once every 100 years in our galaxy but they explode in supernova there are different kinds of supernovas some of these supernovas are caused by a core collapse the of the mass so the the stars have nuclear fuel that is producing all the light all the electromagnetic light that we see from the stars but then they uh they also have mass that's compressing the stars so sometimes the nuclear explosions balance the pressure to get compressed but then as they lose the fuel then the gravity begins to win until it core collapses the core collapses into a very very compact start and that bounces back and produces these beautiful images that we have in the electromagnetic spectrum but at the very center there is a very compact star left and that can either be a neutron star which is made of elementary not atoms but the elementary particles in atoms that's why we call it neutron star and we know very little about how that behaves in the earth but that's the most compact kind of stars that we have it's like having the mass of the sun compressed in a radius of about the size of manhattan imagine manhattan having the mass of the sun that is what the neutron star would be that's how compact it is but sometimes if there is enough mass and there is enough compression something else happened there's a black hole happening so if there is too much compression in there instead of a neutron star we get a black hole so this is what how we know how many neutron stars and black holes that might be in the universe because we know that there are lots of supernova in the universe that explosion if it's not perfectly symmetrical if it's not perfectly spherical will also produce gravitational waves those gravitational waves will be like blips very difficult to detect but still blips that we could detect in in detectors like our ligo detectors i'll tell you about those neutron stars that are left by the supernova are actually very very interesting objects they first of all have an electromagnetic beam like a jet it's not a jet that only happens once and goes away it keeps it keeps moving around it's actually the star rotates but the axis of this beam is different than the axis of rotation just like the north and south magnetic poles are different than the earth north and south geographic poles so the same thing happens with a neutron star which means that these radio beams are mostly radio beams pulsate if we are happy enough to be in the to be in the way to detect these radio beams so sometimes we are looking at it at the neutral start from here and we see a radio bleep that comes periodically every time than the beam points to us and these these radio blips are very very very precise in fact they are almost as precise or sometimes more precise than atomic clocks you we tend to think as stars as very messy systems these are actually very very precise systems there are we can see these pulsars not only from the radio beams we can also see sometimes the jets sometimes we can see the nebula around and that is because they happen with a supernova this is an image of the crab pulsar and nebula that was created by a supernova now the fact that these beams that these neutron stars emit these radio beams that are so precise is something that we use also to detect gravitational waves and i'll tell you more about that later i'm building suspense here so how many neutron stars are there in the universe well we kind of we can try to estimate that from the number of supernova explosions at the rapper galaxy we also know how many pulsars there are so we have observed about 2 000 pulsars in our galaxy most radio pools are summer x-rays among gamma rays but there are about a hundred million neutron stars perhaps a billion neutron stars in our galaxy and the gravitation and the stars might produce gravitational waves of a constant frequency related to the rotational frequency of the star if they are not perfectly symmetric now with the supernova explosion that i showed you it's easy to think that it might not be perfectly spherical and in fact this ferricity is amazing it's a part in a hundred part in a thousand for supernova explosion for these stars they are the smoothest objects known in the universe i told you that a neutral star of the mass of the sun would have the size of manhattan the the mountain that you would expect on that neutron star of the size of manhattan would be this high so imagine this vertical object the size of manhattan with this high of a mountain that's how smooth neutron stars are nature is amazing it never stops to amaze me so there are they are not they are very spherical but not perfectly spherical if they are not perfectly spherical then theory predicts that they will emit gravitational waves this would not be a rumble this would not be a bleep this would be a constant sine wave in the universe in this fabric of the space-time produced by the neutral star so yet a different gravitational wave oh by the way um in the previous one if you look at einstein at home you can help us look for those sine waves in the universe so write it up or google it einstein at home.org now these stars are often in binary systems about five percent of all kinds of stars are not alone during binary so not every star finds a partner like most of us do but most but only about five percent so they're lonely objects but still i was telling you that there are a hundred million neutron stars in the in the galaxy five percent of those are a lot of those in binary systems and in those binary systems often both stars end up as neutron stars or a neutron star and black hole or two black holes these binary systems like i told you earlier they would circle around each other they would produce these gravitational waves that carry energy away because the system is getting closer and closer together then the gravitational wave is going to have a higher and higher frequency and it's going to get larger and larger because the stars are getting closer together so the gravitational wave that we expect from these signals is not a constant sine wave it's not a bleep it's a chirp it's like bird singing and i'm a very bad singer so i won't try it but it's like it's a bird that's singing with a higher frequency and amplitude see i'm not very good at that that's about as good as it gets so those are the ones we want to detect those are our best target for ground-based interferometers ligo detectors that i work with now this is also our best evidence that gravitational waves exist because there's been at least two and we hope there are more systems binary systems in our galaxy where we can see the radio blips from the stars so we know how far apart they are and we have seen them getting closer and closer together the first binary system that was that was found was found by hurls by taylor and his graduate student working in arecibo with what i i suspect was a very very boring thesis he had to measure very small very small disturbances in this radio blips to see whether they were due to a companion to the to the pulsar that was producing the blips and he discovered in that doing that very hard work uh the first binary system but when they kept watching the binary system they all the stars kept getting closer and closer together and you may not see the the x-axis in here the y-axis is the distance something proportional to the distance between the stars the x-axis is years and this started in the 70s and this is in the 2000s so these are 30 years of data showing that these stars are getting closer together just like the theory predicts because the gravitational waves are carrying energy so some people say that we have seen gravitational waves we have certainly seen the effect of gravitational waves and that was worth a nobel prize in 93 already and actually uh hurls went on to work in industry and do other things but taylor still works looking for these stars and measuring distances between stars he's he's amazing so now i want to tell you about how to measure these gravitational waves how do we detect them i first need to tell you that they have not yet been detected we have been looking for these waves for tens of years the first detectors were built in the 60s there were other kinds of trying to see distortions in space-time since the 60s we have not yet detected gravitational waves but i told you that they exist so it's not that they don't exist it's that the theory tells us that the disturbances in space-time are very very very small and very difficult to detect how difficult to detect they are well i told you the best target source we have are these binary systems getting closer together the gravitational wave will be most powerful just before the merger these mergers happen about once per 10 000 years in our galaxy so that means that if we want to see one per year we need to look at tens of thousands of galaxies and we have not been able to do that we have not had detectors as sensitive now that tells you the rate let me tell you about the amplitude a binary system merging in the vertical cluster let's say that's a cluster of galaxies whatever milky way leaves and that happens once every 100 years or so that merger would produce a change in distance near earth that would take the sun and the earth apart by one atomic diameter that's a part in 10 to the 21 so it would change the distance between you and me in a part in 10 to the 21. that's the calculation that make the einsteins think that these gravitational waves were irrelevant you could never measure them he was wrong we have not measured them yet but we know there are many different ways of measuring them i told you about many different sources of gravitational waves from the early universe to supernova explosions to mergers of black holes and neutron stars those gravitational waves not only have different signatures but they have also different frequencies so here i have a plot of the amplitude of those gravitational waves i told you that these binary neutron star mergers would produce a part in 10 to the 21 distortion in distances that's about here but there are these early universe gravitational waves at much much lower frequencies would be much larger but that doesn't mean they're easier to detect we are trying to detect those early gravity early universe gravitational waves with the imprint in the polarization of the cosmic microwave background that's what you heard those are the news you heard about bicep again those seem to be have caused by dust but there are lots of experiments trying to do the same and it's just a matter of more sensitivity that will come in a few years i told you about pulsars being very precise clocks you can use uh the the radio beams coming so precisely from these neutron stars that are in different parts of the galaxy as ways of measuring the distance between the pulsar and the earth and if there is a gravitational wave going through the galaxy it would change those timing the timing of those radio beams and we would notice that so that's called pulsar timing and there are arrays of doing that there are there's an array in the in north america and array in europe and rain australia and they they would be looking at the primordial background but also at many many of these mergers of black holes that would produce a rumble because they all mixed together i'll tell you more about ground-based interferometers this is using an interferometer that is a laser beam going in two different ways and then using the measurement of the timing of the beam with respect to that one measuring the interference in the way back to measure distortions of the distance of this distance with respect to that distance we have those detectors working at this position at a few hundred hertz and that would allow us to see these measures of neutron stars and mergers of neutron stars and black holes supernova explosions in our galaxy the black holes in here are what we call small black holes black holes that have a few times the mass of our sun to about a few hundred times the mass of our side but you probably all know that there are big big black holes especially at the center of the galaxies we now believe that almost every galaxy if not every galaxy at the center has a big black hole our galaxy at the center of it as we know has a black hole of about three million solar masses that's huge and you know how we know about that using newton's law we use newton's law to look at stars that are orbiting around that object at the center of the galaxy and that's how we know the mass of that object at the center of the galaxy and we know that it's about three million solar masses we know that it's in this small space so it's so compact it has to be a black hole we don't see the black hole but we see what goes around it but we also know that galaxies merge we have seen evidence of galaxies bursting we have caught one in the act of merging those mergers of galaxies are mergers of black holes of binary systems of black holes and those could be measured by a space interferometer putting this kind of device in space so you put three masses satellites and then you take a laser to measure the distance between those satellites and that way you would measure the distortion in that distance produced by a gravitational wave that's a project that's not flying it's actually gone up and down from the books for the for a long time we like to call it lisa it's now being considered by the european space agency so this tells you that there are lots of ways of looking for gravitational waves and there are lots and lots of people doing it and of course there's a lot a lot of money invested in this much of it your money this is your tax dollars at work i think a good at good work so how do we detect these gravitational waves with an interferometer you might know that the interferometer that michelson and morley used to detect the velocity of ether that they didn't detect and that's why we know that ether doesn't exist so it all goes back to relativity experiments but the interferometer does is use a beam splitter to split the laser in two parts and then use mirrors at the ends to measure the light coming back and then when the light comes back it splits again in two so there's half a beam going there have been going back when this beam is coming back there's half there half going back half going here so at the output you can tell more or less light depending on whether these waves this wave and that wave are one on top of each other and one canceling the other that's why we use interference to measure the difference in distance between this distance and that distance and gravitational waves if you do the math have a character that's called quadrupolar meaning that it changes distances in this funny way of making circles look like ellipses it doesn't make circles larger and smaller it doesn't change distances that way it makes a circle convert into ellipse back and forth and that's exactly what we would measure with an interferometer like this now i told you that what we want to measure is a part in 10 to the 21 actually better than that for this distance so how long do we have to make this interferometer to make this effect measurable well i told you that if we make the interferometer as big as the distance between the sun and the earth we can measure an atomic diameter diameter is easy to measure but not on that scale we need to make it smaller so we can put it in space that's what i told you about lisa or we can put it on the ground if we put it on the ground then you have to talk about kilometer scale and that's what we have with ligo ligo is a u.s national project handled by caltech and mit that has built two observatories one in hanford washington another in livingston louisiana very close to where i live so this is my favorite one and you can go to google maps these are actually google maps of those and see them they are big enough to be seen in google maps they are four kilometers long now we don't use the metric system often enough i think you all know how much four kilometers is but it's more than a mile and a half this is a huge distance and even then we have to achieve precision of a part intent a part in a thousand of a proton diameter so here is when usually the question comes you cannot measure a proton diameter let alone a part in a thousand of a proton diameter so how you do that well we use lots and lots of technology and tricks one of those tricks to avoid measuring um nuclear nuclear um vibrations is to use laser beams this laser hits the mirror and averages the position on the mirror the mirror is this big and the laser beam when it gets to the mirror is this big so we are averaging this the position of these many atoms on the front of the mirror so we can do that in fact we have done that we have operated these detectors with that precision apart in a thousand in proton diameters until 2010 and since 2010 we have built advanced ligo detectors that are 10 times better and we have just finished installing those detectors this year we actually had a dedication of those detectors in hanford washington a couple of weeks ago and what's advanced about advanced lego well these initial detectors i told you about when we achieved this precision of a part in a thousand of a proton diameter would let us see coalescences of binary neutron stars in the virgo cluster but i told you those happen once every 50 hundred years so we took data for a couple of years we didn't see anything but we weren't expecting to see anything actually i was expecting that we were going to be lucky but but we had to be very lucky and we weren't but we we kind of expected that so we were already working we had already been working for decades on technology to improve that initial reach and this advanced ligo detectors that like i said have been finished being installed but they're not yet at the sensitivity we want will be ten times more sensitive if they are ten times more sensitive then we see a volume that's a thousand times larger so we see a thousand times more galaxies we see a thousand times more binaries coalescing and once we reach that sensitivity we expect to see tens of events per year perhaps hundreds perhaps a few actually after astronomers have their uncertainties in the order of magnitude so it's very difficult to predict how many we will see but once we begin saying once we begin seeing those we will know what is the rate of these binary measures we will know how many binary systems how many neutron stars how many black holes are out there what masses they have how much they spin we will know a lot more about that universe and that's what's so exciting about this let me tell you that i will have a short data taking run later this year these detectors are already at three times the sensitivity of initial ligo that is not yet enough to to make us think that we could have a detection but perhaps we'll be lucky but then from this year on we'll begin taking data once a year for a few months with better and better sensitivity until we get to these 10 times and i think that before we get to that sensitivity we'll see the first gravitational waves and then the second and then the third so the future is very very close and very exciting now this technology is very complicated and let me show you what we have in here so the laser it's a high power laser that's how we went from initial to advance is using one of the reasons we did is using a much higher power laser it's a very established laser you can see that these are crystals so you you need to wear body suits it's actually not very comfortable to work in this in these environments because you have to be always so clean so careful uh the mirrors are not mirrors bolted to a table they are 40 kilogram masses crystals also few silica mirrors that are hanging from another mirror that are hanging from blades that are hanging from blades that are hanging from an active seismic isolation system and we need all of that to make sure that we don't confuse our gravitational waves with the ground which is rambling all the time and making our mirror move so we isolate our mirror from the ground motion through all these pendulums you may not know but pendulums are very good seismic isolators and that's why we use quadruple pendulums but we also use ways to reduce seismic noise measuring it and canceling it we do all of that the laser bouncing back and forth from these mirrors travel in vacuum tubes those four kilometer detectors i told you is the laser traveling in a beam tube that's about a meter in diameter four kilometers long and this is ultra high vacuum so this is the largest vacuum system in volume in the world and we there are two of those on top of that and then there are lots of other auxiliary optics that we need to have to know what all the beams are and how and and what how they move what they are doing we have mirrors looking at the transmitted light we have mirrors looking at the output of the interferometer and cleaning it with something that we call a mod cleaner so the technology here is amazing and it's the engineering in here is amazing we have not just physicists looking at the data and designing the interferometer but we have optical engineers electrical engineers mechanical engineers all talking to each other putting all of this together it's an amazing experiment and of course that's what i like the best about it so what would we do with this we will detect gravitational waves what can we do with them well of course we will be very happy detecting these disturbances of dynamical space time here in earth that that to me is just mind-boggling but that's not worth the hundreds of millions of dollars that we have spent in building these detectors what is worth is that we are going to be doing astronomy that will tell us not only what are black holes and neutron stars and unknown things and supernovas out there in the universe in our galaxy and other galaxies but we will also be able to learn from this by putting together the information we get from the electromagnetic spectrum that we receive from these systems and the gravitational waves which have never been seen before it's in putting these things together how we learn about the universe and you might have seen this is one of my favorite nasa pictures it's very old but still still one of my favorites this is what our milky way looks like this is more or less what you would look like in a very very dark night actually the best way to see the milky way is from the southern hemisphere that's one of the things i miss i am from argentina and i have to say the night sky from argentina is a lot nicer than from louisiana now of course it's difficult to see it if you are in a bright city but anyway that's what you would see in the optical spectrum but if you look at gamma rays or x-rays or infrared or atomic lines it's very different and that's how we know what is happening where at what energies and what is changing some of these features are changing some are constant in sun in time some are blipping some are not that is how we do astronomy today is using many different windows gravitational waves will be a new window to the universe this so we have an event here and that's an astrophysical event and we will be we want to see gamma rays neutrinos gravitational waves cosmic rays gum x-rays we want to see all of these different windows to the same event using different instruments that is what's being called multi-messenger astronomy even without ligo because you can see things in x-rays and gamma rays and so on but now with gravitational waves we'll be seeing a completely different universe there gamma ray bursts are the main example of these things you might have heard about gamma ray birth they were first detected by military satellites that looked for found they weren't looking for that but they found these blips in the gamma ray spectrum uh uh when they were looking for nuclear explosions it turns out that they knew these weren't nuclear explosions they couldn't tell other people what they were doing but they finally did and then there were other experiments our satellites built to to look for these gamma rays they found them they came from all over the sky they come in two kinds short and long shorter than two seconds longer than two seconds and we think now we have theories although not quite proven yet that the long gamma ray bursts happen when we have supernova explosions that is core collapsed supernova i was telling you that end up producing a compact object like a neutron star with a jet and that explosion is what produces the gamma ray burst that we see and that's longish it's longer than two seconds sometimes 10 seconds long but there are shorter ones and those shorter ones we think are produced by the merger of binary systems in which one or both of the stars are neutron stars if they are both black holes they don't produce they don't have matter enough electrons to produce to produce gamma rays so one or both of these stars have to be in a neutron star but when they merge they produce they have this explosion when they produce the black hole at the end and they always produce a black hole at the end and that again that would emit gamma rays which is what we see so these gamma ray bursts the name tells you that they are best detected in the gamma ray spectrum but we see them now that we have been looking at these for a long time we see them in many different wavelengths whether they are produced by a binary merger or by a core collapsed supernova they end up with a black hole with an accretion disk or a magnetar which is a neutron star with a very very high magnetic field and then we see not only the gamma rays which are the prompt emission mission at the same time that we would see gravitational waves but we also see an afterglow that is seen in the optical in the x-ray in the radio in the optical spectrum within hours in the x-ray within days in the radio within months so these are all the different electromagnetic windows to to that event if we have gravitational waves we would be able to measure the amount of mass in this system the spin of the system the macroscopic quantities of the system that we cannot tell from all of these electromagnetic signals so this is what we call multi-messenger astronomy now to do this you not only need to detect gravitational waves you need to know that those gravitational waves are coming from the same event that's producing the gamma rays or the electromagnetic waves how do you know where where the gravitational wave is coming from well that is more difficult than you think if you think about ligo as a telescope then you might imagine that the telescope is looking at some direction in the sky that is not the case ligo as an interferometer is more like a microphone it can measure gravitational waves that are coming almost from anywhere there are only a few directions where the gravitational wave can come from that would not produce a signal but from most directions it would produce a signal with different amplitudes what that means is that if you detect a signal in the detector you don't know where it's coming from so you need what do you do when you want to have good sound you have more microphones you do stereo that's what we do so we have two detectors in the us that's not quite enough but we collaborate with another detector in italy and with three we can do triangulation so we know that these gravitational waves travel at the speed of light well that's a theory but once we detect them we can test that but because they travel at the finite velocity then there will be a timing difference a difference in the time of arrival of these of these waves and from that time difference we can tell where the gravitational wave is coming from now the localization we would have is is quite poor at least for astronomical standards but the idea is that once we know more or less a region it comes from then we look in that region with satellites in x-ray gamma ray with telescopes on in earth like lsst which is a big survey telescope that's being built and then we find coincidences now that sounds more difficult than it sounds easier than it sounds it's actually a difficult problem that we are working on but we are getting ready to do it now i told you that we have a network of three detectors those are the two advanced lego detectors in the us and advanced virgo in europe we also have a fourth detector in germany that's called geo 600 that is part of the ligo collaboration however that one is 600 meters long so it doesn't have as much sensitivity it's not competitive to this it is the one where we develop a lot of the technology that then we use in advanced lego detector and more importantly is the only one that's running now when the other advanced detectors are getting prepared to run later so if there's a supernova in the galaxy we don't want to be all off at the same time geo is in what we call astro watch time keeping keeping watch in case there's a supernova but those are not the only three there's a detector being installed in cagra that's an underground cryogenic detector in the camioca mine where the super k detector is they're looking for neutrinos and we are also there there used to be two ligo detectors two initial lego detectors in the hanford observatory that we were planning to have two advanced ligo detectors in hanford but like i told you for triangulation it's better if we have it somewhere else so india is preparing to build an observatory to host this third detector and with that we can get a lot better localization if we have three detectors the two lego detectors and virgo and we want to know how a signal is localized we know that if it is coming from this direction the actual signal is coming from this direction the uncertainty is very very big if it's coming from this election it's kind of small in this scale but it's huge for astronomical standards it's tens of square degrees and we have some blind spots there are some directions from which we wouldn't see gravitational waves if we have four detectors if we have this lego india detector these uncertainty ellipses become a lot smaller that means our localization becomes smaller and there are no more blind spots no more red crosses in there so that's where we want to go and that's where i end my talk thank you [Applause]

Channel: World Science U

Views: 1,495

Rating: 4.8857141 out of 5

Keywords: Gabriela Gonzalez, gravitational waves, LIGO, Astronomy, gravitational wave arrives to Earth, LIGO detectors, general relativity, ripples in spacetime, black hole collisions, black hole, massive astronomical event, World Science U, WSU, Brian Greene, World, Science, festival, New York City, 2020

Id: tec1G7YGYek

Length: 56min 26sec (3386 seconds)

Published: Thu Sep 03 2020