WSU: 100 Years of Gravitational Waves with Rai Weiss

hi I'm ray Weiss and thank you for having me let me start by saying the following the I'm gonna give you now the outline of my talk it's fundamentally that I'm gonna say a little bit about the history of this field because it's not known that well I mean gravitational waves themselves have had a very complex history and you'll I'll describe some of that to you but then I will try to also explain some of the technology and then I want to talk about the big discovery that has been made and at the end I'd like to talk a little bit about the future of the field so there's an awful lot I want to cram into this thing and I'm a little worried about that I've done too much but let's get started so I got to learn all these different things let's see if this does what yeah well I mean these are slides which are the complicated things they're gonna be many of them and they are the history of what the field is now the point to various things in it so that I can keep you abreast of where we are in the story of gravitational waves but I'm gonna start with Albert Albert Einstein and last year we had a big celebration in 920 and 2015 we celebrated the 1915 discovery by Albert Einstein of the field equations and that was a huge celebration all over the world and what it was was the beginning of a new theory of gravitation I mean all of us have been in high schools and probably in college had learned about Newtonian gravity Newtonian gravity was a theory I won't go into any of it I just want to say what's the difference between Newtonian gravity and Einstein's gravity Newtonian gravity is fundamentally a theory where things and you know of its then got hold you down the ground so does Einstein's theory but what it is it's a theory that has a force the force is proportional to the masses of the two objects and it gets smaller the bigger the distances between the objects it doesn't talk about information traveling between things in gravity which that's something when you begin to think of Einstein in 1905 he was already thinking about that you had to worry about things having a limiting speed and information even in a gravitational field has to be limiting it can't go faster than the velocity of light that something that probably Newton didn't really know it couldn't have known and so anything that came after that after then had to include the idea that there was a finite speed of propagation now what Einstein Theory did and it developed out of special relativity is the fact that it was a theory that did not did not have a gravitational force it was a theory in which space gets deformed by mass and then the when that affirmation has taken place the mass is the things that are in it move because of the deformations in other words may space then tells mass how to move around so it's a completely different idea it's a geometric idea geometrization of gravitation and that was the big thing that Einstein did and I want to show you now immediately get to the notion of gravitational waves and in 1916 Einstein wrote his first paper on it so that's a year later than the big discovery of the field equations in nineteen eighteen year-old another paper where he corrected his errors but the most important thing is in 1916 already he described that there are certain things you didn't describe this properly that was the part that he didn't do that the sources of the waves he knew they would be accelerated masses in that 1916 paper turned out any kind of motion would have made gravitational waves but it turns out that it's only non the non spherical part of the motion that makes gravitational waves and in his initial paper he he didn't distinguish those and that's profound but and it was a wonderful physics era but let's leave ago the thing is he described completely properly already in that paper was that they will propagate at the speed of light these waves and they are transfers waves now I'll show you an example of that in a second in this picture that's right here and there they are strange in space as you'll see they are both tension and compression and what you're gonna see in this picture which I'll show in a minute is a way that's coming at you or going away from you and it is a thing as you see of a it has a very special property and here that's why they had to move the computer so I could be able to initiate this little motion for you I hope I can yeah okay now what you're seeing there's a little Fernet ik but nevertheless it has that I want to walk you through this well the red dot is pretty much where you're standing and the and emotion as of you have laid out in space and you'll notice something interesting about this picture this is a picture of time varying but constant strain across the space and this ray the constant strain is the strain is that remember what the strain is it's the ratio of the separation of two points well that's the denominator and the numerator is the amount of change of separation so it's Delta L divided by L and so what you're seeing here is a motion that's very large on these outer points but but quite small on the inner points and then you notice another thing about it its expansion in one direction while simultaneously contraction in the other dimension that's perpendicular to it and that keeps oscillating and that's what a wave looks like and that's what I want you to imagine through the whole talk on the giving or anybody who gives a talk about gravitation ways because it's fundamental ok so strain it has a strain amplitude is the field amplitude and it's a constant strain and the plane wave okay so now but I just want to talk about the equations and they're interesting equations this first equation relates that this H by the way is this H is the strain it's the change in side this separation divided by the separation that's the field quantity so from now on H is Delta L over L it turns out that this relationship is the thing that relates the amount of duty for the first derivative for this a time derivative the strain that at the time change of the strain squared with the intensity in the way what you would call the power in the way the amount of power that's the number of watts if you want to call it or herbs in case that's what I use here it's a CGS units urged per centimeter squared per second that is in the wave and you'll see this it's a very simple relations very same you have a very similar relation between the field quantities and the intensity but this one has a very complicated and terrible thing it's this quant this context constant right here and it's over here in units and CGS units but it's a god-awful huge number a monstrous number and that says it takes if you want to have a tiny bit of strain even derivative you forget about the derivative strain it turns out it outs to it an enormous amount of energy that you need or it carries an enormous amount of energy I prefer to turn this thing around and look at how much for a little how much of a little bit of strain how much energy do you have to put out there to make that strain and what you're finding out is right off and this is what Einstein found out also right away even though he had the wrong formalism to begin with that it takes a tremendous amount of energy to distort space a little bit that's the thing that makes it so hard to detect these waves it's also makes it very hard to make the waves okay and so for example this is the formula d screwed up in the 2016 paper but it's called the quadrupole formula and it relates the amount of power that's radiated to the parameters of the source I won't go into all of this but the important thing is this is key you will get a very similar relationship in electricity and magnetism if you did the following thing and you it's called quadrupole radiation and even in a.m. the radiation we always teach students about and we experience it's called dipole radiation it's radiation from accelerating charges that are positive and negative you have both positive and negative charges and that gives you a net field but it turns out suppose you did the same problem all over again using positive charge and positive charge it says most of the radiation field cancels and the only reason you see any field at all is because when a source is wiggling or it's turning around like that you don't see the two charges exactly at the same time that's the reason there's radiation at all and that's also true in gravity so now here's the thing that's so Debye what you want to impress on you I went through all of this development by the way what I'm about to tell you you can do for yourself and this formula is the only one I wish you would look at hard because I'll tell you what isn't this is if you want to estimate for yourself how big is H if you just want to put some parameters in and this is these they're very lovely things in here what it is if you want to from a Sun system that is moving and we'll get to it and it has a mass M this is the Newtonian constant of gravity G which he'll any units you like you have to use them compatibly and that's the distance you are away from the source and this is the velocity of light squared so this quantity GM divided by R and C squared that is a dimensionless quantity and that's fundamental in gravitation that's the thing that you use that quantity GM divided by RC squared is the quantity used to decide whether you want to use Einstein's theory or Newton's theory because what we live in right here right now when you put the Earth's mass in and the Earth's distance and the thing that's holding us to the ground that number in this thing although you are suffering when you have to climb a mountain its zilch that number is ten to the minus ten it's a huge and it's a tiny number it only becomes a big number when you get to some things which are outrageous which are like black holes and we'll talk about more about that that number gets close to one at the event horizon of a black hole and this ratio of the velocity of the source squared to the velocity of light squared is the thing that's the thing that is doing the radiating in effect okay so if you want to just I'd taken some examples and let's the thing that I sign who we love trains as you probably when you read nine Stein's history that's the sort of thing a lot of his gigantic Dokken experiments involve trains let's do this now the cannon he must have made this calculation for himself because in that twenty sixteen paper he made a very interesting statement he said this thing this gravitational waves which he had just discovered in the in the theory will never have an effect in physics and he said it very bluntly he says this is gonna be so small it never has able to never have an effect anywhere and this is what he must have done I've asked the people who do the science papers of the Einstein papers to see if they can find these two calculations somewhere in his papers they have not found them I mean these were back-of-the-envelope calculations he must have done himself once he got to their having this theory and this is for example two trains colliding I mean so you put some numbers in it's easy to do this once you have that estimating formula for you say the mass is about a hundred thousand kilograms the velocity of the trains had before they collide it probably was 100 kilometers per hour maybe a little faster and the whole collision because you have to know the length of train it's only the local mode is about a third of a second that's pretty fast and now you want to separate this from Newtonian gravity because you want to be out in the radiation zone you want to be out far enough so you are actually seeing the radiation field not just the Newtonian field that's still there the thing that you learned in high school so you have to go out 300 kilometres away and what the H value you get for that is infinitesimal it's 10 to the minus 42 and Einstein must have made that number himself and that was in his mind measuring a thing all over a meter you're talking about 10 to the minus 42 meters that's I'm even now hopeless so that's this is the biggest thing you could have thought of in 1916 that man could have made so you look at as four astronomical things and you say well what are binary stars with people knowing about and they probably said I know that people's soul binary stars and they let's say so one solar mass you stick that into here and you say that the orbital period is a day for these things to go around each other so that tells you the velocity that you wind up with and and they say how far away is it well let's put in the center of the galaxy although people didn't know what how big the galaxy was in those days they could make it 300 light-years away but okay so still however you do it you would wind up with an H that in astronomical but for an astronomical thing with big masses is about 10 to the minus 23 that's a tiny number again but that's getting on to things that we can now think of measuring it was not possible to do in in 1916 at all it was inconceivable but what you would now do is and what Einstein was hoping I would think is could you from a telescope see that change energy in other words how what would happen if it's losing energy to gravitational waves could you see that orbit contract that was seeing later on in our centuries I'll get back to in our century and if you put the numbers in and calculate what the loss of energy is divided into the total energy that system has is it takes 10 to minus 13 years for a system like that to lose about all half of its energy or one ovary of its energy so if you're looking for 10% change with a telescope of the distance between these two guys you'd have to wait 10 to the 12 years or so and Einstein I think did I'm sure he made a number like this so that's where that whole statement came from this was hopeless and that's why I made these pictures that are the history of the field so the very first thing is I the reason and I'll give you now the the basis of these pictures is that I've made things or I was made so that things that are blue our theory developments in theory from 1900 to 1960 yeah well there are two more of these slides so you got bear with me and then the things that are green or observations things that have changed it because of observation things that happen in astronomy and then finally the technology and I won't walk why we cannot go walk you through all of this that's crazy that's for you to look at a couple of places that are kind of interesting and some of the faces are interesting so this history from 1900 here is sort of the here's when the theory the theory became you know let's published an effect and then right away and then the idea of looking for black holes with part of the theory that was short healed we'll get back to that but here is the first guy he's the guy who made I'm Stein famous that's Albert Eddington sir Alfred Eddington and what he did is he made Einstein famous by doing a cliffs expedition to see the bending of light but he also did a study of relative you know got myself in a trouble right away and he said look supposing he knew about gravitational waves and he found two things that formula I showed you which it's not important is coordinate dependent depends on what coordinate system you use you don't get the same answer from that for every observer that was very disturbing already that's why it's called a pseudo tensor and the other thing is he wrote have noticed that when he did the calculations himself for two stars going around each other they stars gained energy somehow which sort of not to know you're not allowed I mean not losing energy to gravitational waves and then on top of that gaining energy he had a terrible time with it so he thought this was all fraudulent in a way and he called it that the gravitational made waves move with a speed of thought that's what he thought and that fact is the whole book written about this by Daniel kenefick it's an interesting book if you're interested in the history of science this is gonna run out oh well maybe not okay so that's a bad beginning but and then what happened is Einstein himself began to doubt this whole business in 1936 or so he wrote a paper which he tried to get published which he did get published I won't go into this wrest an interesting story on its own and with with Nathan Rosen and they came to the conclusion that gravitational waves are exact solutions of the field equation didn't exist so they're all right through another poll on the whole idea and then the whole field got more and more complicated as life went on the thing that next happened is that we lots of technology things that began became very important yeah sort of the technology of electronics became important but there is a period right in here which Fineman described later on as a period when this theory became mathematics and nobody in physics pay the attention to it anymore and that's absolutely true because what happened was that there was not any physics new physics coming out of it and so you know what happened was a this threshold or the important change that happened was in 1960 when it turns out that there was a conference in 1957 right in here when there was a conference at Chapel Hill which then was this in fact it started by Leo Goldberg and it began to discuss gravitational waves as a reality and people began to see that they were real and that was a thing that then convinced Johnny wheeler down here and Joe Weber up there to do an experiment in fact they together and began to think how would you actually try to measure gravitational waves and that was the real beginning of doing things but it also got into trouble as you probably know because what happened is that your Webber designed that great bar up there his idea was that a gravitational wave would come along stretch that bar just in those motions that I showed you in the way and that that would set the bar ringing and that ringing would then persist and he could make a measurement with with pzt detectors that are around the bar and it turns out that he in 1969 said he had discovered gravitational waves the whole world and I mean there were groups 12 groups in the world easily Europe United States started doing these experiments they saw nothing absolutely nothing and in fact the whole field got discredited because of that and so what then happened is that in about I fact I started thinking about it then and in about 1972 I a lot of others people also had beginning like young to think about it especially the Russians for example Justin Stein he was a guy working in Russia in 1962 came up with this idea himself but then it got lost we never knew about it it came up again and I in the 1970s and the idea was not to use bars but rather could you do it by timing light between masses that idea could you just time light and then see if the space between the masses got changed by the gravitational waves and see the time change that was the basic idea gacchanti and had that he had it one way and servos have also had and so the thing was right at that point and I'll come back to the slide again another whole technique was developed and that was it was thought about so one way to do this is to actually this is the sort of way you think about it you write down the the metric which is the tightly dot this is a space between space and time interval between the arrival and the receipt of a light wave at some place and you need this and here's the metric that a gravitational wave does it stretches space in one dimension as you'll see it stretches and shrinks it in another and then there's another polarization as you'll see in a minute which is I turned at 45 degrees to the first one and they do it does exactly the same again so that's the metric that's used and now here's the experiment and that I can display playing quite easily what you do is this there's a good docking experiment a la is done and what you do is you do something you can't do what you do is you put a clock in a mass here a very good clock in another very good clock in a mass there and then you just do the experiment of sending light from one to the other and you calculate the new metric that's it that's the metric that's the you have the piece that's the space that's always there that's called the Minkowski metric and then you add to it this thing which is the gravitational wave metric which is now time dependent and has this strain in front of it that's that new thing that age and that multiplies and what the way you do the calculation I won't go any deeper than that if you want to it for yourself the tricky thing here is to say the masses don't move it's space between them that changes that's the way to think about it so the coordinate distance between the masses doesn't change that's this and that and the time that's kept by clocks in this particular representation the time the coordinate time kept by clocks is the same as proper time by the clocks there's nothing fancy going on so the time does not get changed and you just it's a very simple equation you can solve it for the case of H being small which always happens and also let's make the gravitational waves not move so quickly that they change a lot between the time the light goes from here to there so that's the second condition and then you solve for the inferred separation between the two masses just by looking and timing in the light and lo and behold you get an answer which is that Delta L over L that's the distance between those two is indeed H divided by two so that was the basic idea I taught that in a course because that's the only way I could understand it and put a Sheldon until Weber got into trouble and here's a whole bunch of people who then worked on this thing here is the basic idea the idea was and now you want to implement this you'd want to see how could you make a Gedanken experiment into a real experiment and that was done here in an audit drawing that was done in 1972 or so and the idea was effectively they use an interferometer in other words send light in and the light comes in from here gets split and then it gets bounced back and forth and I'll talk about that a little more and it goes gets bounced back and forth and then you use tricks that people before us had already done you then measure the time it takes light to go between here and there and between here and there and compare the two that gets rid of the very accurate clock you have to do and then what happens is you very carefully look and see as you hold these masses you don't let them move you actually use a servo system which was Bob Nikki's idea we'll get back back to that you don't let them move you what you do is you look at the force you need to keep the light the same time in other words use the the equate and use the force that you're applying on the masses so they don't move as a signal - and that gives you a tremendous amount of control over the system and we use that all over LIGO it's used probably a hundred places in LIGO that concept so and that's the way you can do precision experiments that was first known but then they were at MIT we started this and then it turns out that very rapidly people who had in fact there's a bit of group that did it first MIT we were slow but this got around to people and the group that did I think the most spectacular work in the beginning of this was a German group it's a much plonk and here they are and what they were working the reason why they were ready for this they were thinking what should they do next they had made a big bar just like Weber and they were thinking what should be the next thing they should do and they were they decided not to go with the flow and make a cryogenic bar which is a cold bar which had less noise they decided to adopt this idea and they developed many of the practical things that we needed to do this then it turns out that group influence this group which was a group in Glasgow in that there also had been working on bar and many of these people I think they're all no the only one died is this one all of these people are dead except yeah they're all dead except the reader good no no I said it wrong Billings is still alive he's a hundred or so okay but but the Scotch group there is Ron Ron River is sort of alive and he and these three are still very much in the business okay and they in fact Ron river then move to Caltech and we'll get to that in a minute so they are the responsible people for getting this idea into something that became real and here is the idea that so now here's why I want you to be able to have these slides I've done something here which we people don't normally do I actually show you how it works okay it's a little dip I'm not gonna walk through all of it summer but you're gonna have to invent it for yourself but what here's a basic Michelson interferometer here is that distance mass there's another distance mass here's the gravitational wave coming down on this and here is a blazer and here is a beam splitter that's a device that splits the light and remember what we're trying to do we're trying to keep the light I'll get to all these arrows in a minute we're trying to keep the light so that the time the light spends in here and the time the light spends in there think of it as being equal equal and very well equal because it turns out when you recombine the lights and there's a side which is called the anti-symmetric side of a beam splitter where the two fields that come from the two arms cancel each other there they come in and and then and the field flips want from from the reflection on this side you'll learn that in it's Ordinary enm there always is going to be a side of a beam splitter where you have a phase inversion of the electric field that hits it on reflection and another side where there is no phase reflection so that's called the anti-symmetric port of the interferometer that's called the symmetric port of the interferometer that's important to know later thinking about this and so what happens is you make it so the time spent in here by the light and that in there is the same then if it's truly the same the lights cancel will cancel at the photo detector so the deadest place is dark there's no light there and now what happens all the light goes back to the laser that's where it goes that's the symmetric side and you can see this with some of the arrows we'll get to that in a minute but now comes the tricky bit the tricky bit is that okay you've got no light going the photo detector a gravitational wave comes along and it stretches this arm a little bit and shrinks that one let's say and that's and then what it does it's disturbed the condition that there would be no light coming here anymore the cancellation isn't perfect anymore and so now there is light at the photo detector so in the most simple-minded explanation of this you're just detecting more light and that time varying amounts of light at the photo detector if you have set this up the way I've described but I want to give you a more elegant way of thinking about it because the real development requires the more elegance and what's really going on is and it's the same except it's one step more subtle and that's why I want you to look at the slide I'm not gonna be able to explain all of this but now let's look at these arrows that I've drawn in here and that's a little dangerous of me I've never done this with people before so here is the carrier this is the light that's coming out of laser is this big red thing the big that's the amount of light and it comes into the unifrom and hits this mirror and you can see that the propagation direction is the purple stuff and you can see here there when it goes down this way and coming out of the laser there's only the carrier once it hits the mirror or has gone through this space but let's say it hits the mirror that's an easier way to think about it comes back from the mirror and it has two side bands on it these the mirror remember is being driven by the by the gravitational wave which is wiggling at a certain frequency at the frequency of the gravitational wave and that causes in the carrier to be reflected from this that's certainly there you can see that but it also makes side bands on the light that are at the frequency of the carrier plus and minus the frequency of the gravitational wave and side bands are things that's one way to think about the Doppler shift effectively that's coming from the motion of the mirror if you want to think of it that way I like to think of it more of this space getting doing the same thing and that's all the way there sometime you thought about it also so now remember that this business is anti-symmetric it turns out here that the same thing is happening in this arm in hits this mirror but now look at the the upper sideband has a positive sign as it comes down here but the upper sideband here has a negative sign remember this side is being stretched when this one is being compressed and so the two side bands have opted a sign they come to this point they come to an ad together and lo and behold what happens is the two side bands add up because of this negative sign on the reflection and they the side bands do not get cancelled as they go to the photo detector I hope you understood this if you haven't you should study the picture a little bit that's why I gave you the picture because the next picture is the way we actually do it so that's the basic idea the gravitational waves I show up at the photodetector but the carrier does not a little bit of the carrier does so you can detect it but let's go on okay so now here's the actual detector that we made and this is the detector that made that this is the format of the detector that made the detection it's the same idea but now you can see it some extra mirrors in this thing and many of these ideas come from the groups that I showed you before okay so what you have is you know here's the basic structure so I can remind you the other picture here's the beam splitter there is the distant mirror on one there is a distant mirror on the other and then and now two things have been added we've added a mirror into each arm so the light goes back and forth it bounces back and forth that's okay that's improves the sensitivity it makes the sidebands bigger because you hit hit the mirrors more often or do the same space over and over again and then what happens is you notice that there's another mirror here which wasn't there before that mirror is called the power recycling mirror remember all the light that comes back out of this if the times and here are equal all the light goes back to the photo detector if there's no light going to the photo detector and that you've capitalized on that you reflect that back into the interferometer effectively and so that no light goes to the laser and disturbs it and what you've done is you've made it so you've increased and made a cavity out of this entire interferometer so now the light intensity is much larger than it was just from the laser it could be a hundred times larger than just what came out of the laser that's called power recycling and that we did that in the initial detector you'll see that when we get to actually the curves that come with it and the detector that actually made the detection had another mirror this one right here was called a signal recycling mirror that's a harder one to explain but now that you know about sidebands it's not so difficult because what you're doing with that is you're reflecting not the carrier because there's no carrier here anymore that's been cancelled because you've made that time equal to that time and there's no carrier coming here the side bands have been doubled and now you take and put this in a place where you can increase and make the whole system resonant for the side bands so you can build up the signal and the side bands and that's the another piece of elegance that's why it's called signal recycling so all these mirrors are used to make the system work and that's the one you need a little bit of carrier and that's what we do we do we make at the time difference a little different in these a little bit a little bit we don't make it exactly equal and opposite we put a little bit of extra time in one so that you do get a little bit of carrier at this so you can demodulate that and get it at the photo detector so that's the basic idea of the interferometer okay and you can ask me questions about later but that's as much as I can do without all those arrows you can follow the arrows and see what I told you but you can't do it on real time you're gonna have to do it when you look at the slide so here then is a noise budget for this this is now what are the things that disturb the measurement and the reason we did all this tricks with the optics is so we can get enough sensitivity so that you can get a measurement and know what I haven't told you we can get into that regime that Einstein thought was impossible namely I'll be very blunt with you this detection we made that we made was at 10 to minus twenty one strain that's the boo black holes you hear about the end of my talk the best they did was make a strain of 10 to the minus 21 so what you see here is a somewhat different curve this is now frequency at the bottom this is frequency frequency of the gravitational wave and what you see here is H but not H directly it's a spectrum of H well I'm sorry for that and what to sell its H divided within units of strain per square root of Hertz query would have bandwidth so for example if you want to get 10 minus twenty one here is a hundred Hertz and here is a noise curve which I'll get to describing in a minute and let's say at this point here which is sort of the most sensitive part of the detector you're at 10 to minus 23 in those units you multiply by the square root of bandwidth and you're dealing with a noise that's about 10 to the minus 22 H in other words of an RMS strain of about 10 to minus 22 for this system so now what are the components and that's what I wanted to show you here are the things that come and disturb it what what makes it so that you can see this this is the the sensitive region is right here and this is for the first detector and we will show you at the end for the for the better detector that made the detection it's not very different it's just better but the ideas are the same so in here this thing what's called shot noise on this side and called radiation pressure on that side is the what's call we call the quantum noise in other words this is the thing that would lead to the same thing in this system as the heisenberg microscope does when you study quantum mechanics okay so in other words here is how well you can measure the phase and here is the fact that you have photons hitting the mass and pushing the masses around that makes noise that actually physically pushes the masses around and I if I have time I'll show you that now so as you increase the power into the system into the laser power into the system this gets smaller but this gets larger the the radiation pressure noise so there is an optimum and then there are other noises these the one that's most troublesome to us of course a seismic noise that's the noise of the earth shaking you live in a place right here where the Earth's is shaking by about a few probably here in this building by about 10 microns which is sort of the size of your well yeah one third the size of your here that's much shaking going on it's it certainly in a city building and so here we are at 10 the mine that's ten to the minus let's call it ten to the minus five meters and what we're asking for is 10 to minus 18 meters so we got to do a hell of a job to get down to this so that's the one of our biggest problems you'll see how we do that in a minute then there is the fact that everything is a room temperature and even though the mirror and everything is beautifully hanging and suspended it's still the fact that at 300 degrees there are excitations in the Ming that are called thermal excitations just like people see under a microscope thing called Brownian motion and so the mirrors do that too and so here is the noise from that and then those are the big terms and here's another piece that's the thermal noise and then finally here is sort of why we had to I'd cost so much to build like though it's because you had to get rid of the gas that's in those long four kilometer tubes I didn't tell you that that's why the tubes have been made four kilometers because we're measuring a strain and remember a strain is Delta L over L if you can make L very large Delta L gets progressively larger and that's where the measurement fits well that's where the measurement technique hits the reality how is small at Delta L can you measure not the L how small is Delta L that you can measure so you make the thing longer and that way you win right away and that this is all right I should have said this is all for a four kilometer system already and so now here's a remaining that's the noise that made things expensive you need to evacuate that and then here is a noise which is the reason why and I'll explain that to you in a second why one wants if you want to prove this whole system and go to lower frequencies you want to build this in space and this is a noise which is not so obvious it's a noise that although anomie gets quickly say you can do a very good job of getting rid of seismic noise because you have a wonderful reference it's called the inertial frame Newton told us all about that you can always measure your motion with respect to the inertial frame and you use that as a way of getting rid of the noise of the Earth's accelerations but you can't get rid of this noise the noise that there are waves running in the earth and they are things that cause the acceleration but they also causing compressions of the earth and so for example if that lectern is a mirror and here is the seismic wave coming along and it changes the density here that mirror gets pulled to the earth to this side because of that change of density and those make gravity gradients we call them and that's this thing right here and that you can't get away from you can't shield that so we're stuck and their ideas we have but where this is fundamentally one of the limits and you don't have that in space because there's no medium that's carrying acoustic waves like that so this is the basic year this is the basic geometry of the noise curves okay and that's what the basic idea of the experiment was to get everything down to that point I don't think I have time for this slide if you want in the questions I will show the slide and explain it this is the noise if you really go in detail and try to explain what the radiation pressure noise and the phase noise comes from from the quantum theory I'll just say this much about it to entice you it is the quantum fluctuations in the radiation field that are part of that and that make the quantum noise something that you have to take into account when people are thinking of ways of getting around that so the next big step in this thing in the in this thing was in now where ganda up to 1960 and I think this is petering out again what happens is that then what we're thinking of is here here's 1970 when not all oldest business I just described you get started but the next big venture was to try to figure out how you would make such a thing large enough so that you could make detection and so a study was done that's called the NSF report which was then a thing which said how much my it cost and we and how would you do it to make a thing that was instead of a tabletop thing was as big as a four kilometer system and that's this thing turned from small science into big science and there was no way out we had to do it there was no way to get the sensitivity unless you made it big enough and so then what happened is that right here there was a study done the NSF got started on it they were very cooperative in doing this but then we didn't know internally the people of us who were running this thing and that's driver and and they shipped thorne and myself how to do it that's fundamentally the problem we just were not clever enough to make a collaboration that worked well enough and so what happened is that then here then is where we were we we had we had people study this thing a group studied this people who are not in the field who gave it a very very high recommendations but they said get yourself a single director and that was done the first director of the project it was a Rabi vote and he pulled a bunch of screaming physicists together and got them going together we wrote a very good proposal to the NSF and that got the project going but it really didn't get it going what happens is we had a terrible time getting it through the science board and everything else and eventually we got a real leader which is Barry and he had done this kind of thing before he knew enough physics he knew how to do it and that we are eternally grateful to him for this and then there are other directors that followed and so I hear then I think the last thing of these slides I'll show you then is that here's the time and this timeline that's now run over this long time here's where the detection was made okay so it's time to talk about the detection I think that's the next thing I hope well the next thing is really a network and what happens is that we will develop this network what you will see and I'll get back to this at the end of the talk again is that in order to find out where the sources are on the sky once you see something you don't you can't point these detectors they they they you have to do it by timing the timing between detectors that's the way you do it and I'll get to that again when we I mean I tell you about the discovery but you have to have a network of detectors around the world and what now is happening is that here there was the two that were built in the United States these big four kilometer systems one in Louisiana another one in Lewis in Washington state very soon after that there was built one in Italy which was a collaboration of the collaboration of the Italian open a French group and then and then a group in Scotland joined it and that is another detect that sits in near Pisa and in Italy and those three detectors have made runs and you'll see what they look like but unfortunately we'll get to that this detector was not running when the big day when the big discovery was made that because they were fixing things we'll get back to that and then there are new detectors being built one in in Japan that's in in the Kamioka mine and that's a place where they they hope they can get around some of those gravity gradients and then there's a detector that has a long history which we'll get to but it's me that was a third detector Lyle had in it's in it's in its Hanford Site we had two detectors in the same envelope and the thought was given by Jay marks and others who was this this is the third detector of the third third director of the site of the project - why not give it to the Indians or maybe the Australians to get more spatial distance between so you have more separations and you could do a better job of pointing and so that's and that's will happen now LIGO the LIGO India thing is going to happen all right so now let me quickly show you how the progression of the sensitivity of this work and now we're getting into the deets on what more into the details what you see that top curve is the red curve is the very first run that we made where is virgos first one and what you're seeing in this thing is frequency it's very much like the theoretical plot we had frequency versus strain and so you have an idea here is 10 to the minus 25 in strain pollute Hertz the next thing up is 10 to minus 24 and you can see that there what's now is superposed on this is real data which is the fuzzy stuff that's up there and then projections and the very first thing you see is a the top curve is a red curve that's the best that Virgo ever made and more there that's why they're rebuilding the next one down is the the curve that LIGO had for many years and we looked with that with that purple curve and we saw nothing and we saw a very good nothing meaning we really didn't see anything and no know that there's a significance of that statement a lot of people see things it's bad if you see something because that's affects a lot of trouble if it isn't real and that was we we saw nothing we means we did the experiment well we that's I didn't I Oh verse stated it we saw nothing we shouldn't have seen anything okay until and it turns out that saying technology later was applied and that's the green curve which is the one that is the advanced LIGO curve and that is a real curve that is in fact the performance of the detector that made the detection you're going to hear about in a few minutes now the curves that follow you can see something already in that curve that turquoise curve which is below the green one is where we ought to be with all the things we have is so we know we have a mystery noise that worries us especially at low frequencies we don't have right about there if you go up to the turquoise curve you see there's a gap between the green and the turquoise one so there's gaps but there's an enormous improvement that was made for between the purple and the green especially at low frequencies that's the region in which the detection was made but maybe have more to go with the stuff that's already there here's what we particularly ought to get when we get done with this thing and hear ideas I'll talk about at the end which are improvements that are quite different yet and improvements on the whole idea okay so that's sort of the history of the performance of this I think what were the big changes that were made where I think I may just say it and this is one thing that may be useful to people in the room and one of the big changes that was made to improve the ground ground noise isolation and what that does is we use seismometers to measure the ground noise feedback on it just like Bob Dicke taught us to do and makes it so that you can null out the ground motion by sort of feedback motions and it's the same thing as people when they have noise canceling noise cancelling headphones it's the same idea so okay now let's get to the detection that's actually quite intricate topic and what are the criteria for that we detect something these are important things to know in order in order to a sort of sense whether we really saw something and it's something with very much on our minds and that is did we see something at both sites and then at each site that's important we see the same thing in within 10 milliseconds at each site the other thing is that you have all sorts of sensors which sense the environment and they those signals whatever you see that you think is a gravitational wave should not be seen in these these are sensors of the environment around you and furthermore there's something like that hundred thousand signals that come out of the detector that are measuring things like how well the mirrors are pointing how well the laser is that stabilizing satellite and there should be no disturbing signals in those either when you think you've made a detection and so that's all part that people who did more junior experiments earlier on couldn't do this is one of the reasons they've turned into big science there's a very well instrumented system that can do this and here in fact is the discovery and here is this this is what we saw there's time on this axis you can see where it is it's about 0.2 second maybe 5 0.3 to 0.45 seconds here is a thing we saw at Hanford and you saw this motion and in the output of the interferometer you saw in the blue you see the same thing at Livingston and here there superposed on each other but displaced in time by about 7 milliseconds in other words this signal that we see here is a signal a came from the south it hit Livingston first then it hit Hanford within about seven milliseconds so it is something that's going close to the velocity of light we think it's the velocity of light but that's not a precise thing that comes an end result then superpose on this is now the width calculation once you do this you see these signals you can calculate a mass you get that from different parts of that you can calculate s you can estimate some of the parameters of the source and you can then make a model of it and this is the model and the model can be done in two ways one way it's done analytically which is this grey line that's behind there and the other one is a line which is done for numerical relativity which is using computers Einstein equations on the computers and this now is the residual the difference between the computer calculation and this data right and that's sort of white and that's true at both sides now the thing that you'll see in a minute is that this is a curve that has several of oscillations out of growing in amplitude will to explain to you what they are in a minute but they have been filtered by a filter that looks like this this is frequency and this is the filter this is one and it's here so it's cutting out this filters like a bass control on your audio set and a treble control on your audio set and you're killing the treble and you're killing the bass and then we did that deliberately because you want to be able to see the signal this is a signal we see directly without all sorts of fancy signal processing so that was very important to us that because we put it through a filter like that and that was something we didn't expect and at the bottom is the same thing in frequency in other words this is frequency and this is the time and this is the frequency content of the wave you see how it goes it goes from about 30 Hertz up to about middle C and that's true at both sides so that was the our big discovery and we would do knocked our eyes out I mean we didn't expect something so big right away and here is the explanation of it once you do the modeling and you fit it you see two black holes they weigh 30 masses each about and they are going around each other there's a sort of an exploration of the wave form they get closer they get faster at this point they're getting so close that they're about to merge and then there's a new black hole formed and we did not see this as well as we would like this little piece right here and hope in others on other detections we will see it and that is the ring down of the space at as the event horizon forms around the new big black hole space rings a little bit and the reason why is that it takes velocity of light times for the event horizon to form properly it's not if it was formed absolutely smack on so the black hole and the collision was actually sadly symmetric you wouldn't get this but you do because it coming in off access a little bit so here's a sort of a Wow a number that goes with this here is the this is the time again for this collision and this is the here is the relative velocity and velocities of light and that's you can see that in the green curve here it's going in this is blast ease of light so here are two things the size of 30 times the mass of the Sun going about well 1.6 is the velocity of light that's sort of amazing and then here's their separation that's given on this axis here and they're getting closer and closer that was really an enormous discovery and it's what course made quite a stir let me show you how we detected it the way we did it is we do it two ways we did it by cross correlating the two detectors the Livingston detector with the Hanford detect I just cross correlating them and that's what's shown on the on the right and you can see what that cross correlation looks like here you can see the cross correlation product squared you can see it that one signal that's the lower one the red ones moving toward the blue one and when they coincide you'll see a significant signal noise improvement or though in single improvement there it is but that was the one way but that's not the best way but already with that you could sell that we had a detection here doing that same process on other here signal noise and here's the and this is the number of events we saw as a function of the signal noise and here is the this event it's way out here it's a very rare event it's out a place where it's funny to have I mean it says it's a hundred million ten million years before you see this the apparatus itself generating such a thing I'm being a little loose there but it's of that order the thing was better seen in something called match filtering which is where you actually once you know a little about it you have a whole set of filters with different masses and different spins and different distances and so forth but mostly spins and and and masses and you've play them through it's a very expensive thing for computation and you play them through the system and you see when is a signal that you have optimized against the theoretical waveform and that's what this is and you can see it's a it gives you a different statistic and it's the way actually the parameters were found you can see that again it's now on the bottom of that when they when these things collide the theoretical one which has no noise on it whereas the other one has the instrument annoys you see a much more well-defined signal and that's in fact how all the parameters that we we published we're measured this way and you can get the false alarm rate by so assuming that every little time interval of the instrument all the time I was running is an independent Institute is an independent measurement over the times that are longer than the waveform that you see there so that's in fact how we get the the false alarm rate and here are some curves for that I don't think I want to go through all of it but it shows something interesting here sort of a distribution for the masses of the two initial stars I come together they peak at thirty five and thirty this one here is just the Ru what's the distance away and it turns out it's a distance away plotted against the inclination angle of the orbit and where and that's of course changes the amplitude you can once you know this small once you know what you're looking at the theory gives you a very good model and that they're effectively testing the theory along with this measurement and you find that the thing is about something like four hundred megaparsecs away is at one point two to 1.3 billion light-years away and it has an aspect that's close to 90 degrees it's almost face on to you and then here is the thing which is quite disappointing but at least the best we could do with only two detectors where in the sky is this and you see this great big banana and that banana sort of is better and we set people on the looking we sent telegrams out to tell people where it might be here is the attempt that was made by people with electromagnetic instruments we gave them a warning where these are and they've tried to find something they found nothing and this is the patches over which they look in the same this is a map of the sky and a funny projection but it's a it's a it's a declination and and our angle and they saw nothing on the other hand the neutrinos here's the from the South Pole the ice pole experiment found detections but here's the error bar that that we gave them this is a different projection on the sky and they saw nothing at that time there's one group of people the thought they did see something and that's the Fermi the Fermi telescope the gamma-ray telescope and I'm not sure that is a convenient as a trustworthy thing so once we have this we can look for other things and I thought that was the idea here's a whole list of the kinds of sources that we in fact expect that and this is the thing that's different over those hundred years between now and when Einstein first thought of it people began to find black holes they found neutron stars but certainly that's what we saw we had really been planning on finding neutron star binaries that was the thing that was actually we had been planning on we're probably going to see those because we know about them we didn't know how we knew how strong this would be a black hole but we didn't know how many there were that was the big thing that was discovered and then here are the other things that these things which have very low do the duty cycles supernovae for example be a wonderful source to find they don't happen but once every in a hundred years in our own galaxy we haven't yet wait till the next one and then this business of black hole normal modes which is that ringing that I told you about we haven't the MIDI seen that and then there various things for example there's a very active search going on every time you run to look for things which are constant signals a pulsar if we know about pulsar as we know where they are we know their frequencies and we know their theory if period derivatives and what they are you're looking for pulsar that has been squashed by its magnetic field so it looks like a wobbling football and the reason for that is that the magnetic field of the of the pulsar and the spin axis of the mic don't necessarily coincide it's exactly the same as with the earth the Earth's magnetic field and it's spin axis don't coincide either and the thing that's looked for over and over again and would be extremely exciting to find would be something that comes as a cosmological background it's I mean unless all the models are wrong it's that's unexpected but what's certainly going to be seen is just what are the foreground sources average over many okay this then is again that curve which I just want to quickly say there is ways to go we're hope that with this instrument get to an H value and this with about the the blue here are ideas that people have for what you can still do and make the instrument better in the the four kilometer system that we now have and here ideas that both in Europe and in the United States have for making the next one called the third-generation inspector and these are huge advances and they could take this whole field into cosmology and that's something to look forward to and in fact here's a very beautiful picture of showing you what localization does this is having more just having more detectives here's sort of the error bars on the sky from a neutron star binary with a single noise of about eight and this is the noise you have on the error in position over the sky for that if you have Hanford Livingston that's those two of the LIGO detectors and Virgo actually running that would be already quite wonderful and if you now add to that India which is you get something that looks like that so that becomes to be something that you could then you would give to astronomers and say please look in that era bar right there and see what we see if you see something that's there so here then is sort of the real future the real future is that here is now the gravitational wave frequency and here is H now H as an RMS value and here's us the movie we've nominated section and we're at frequencies that are around 10 to the 4 Hertz down to about 12 10 Hertz I hope we get to 10 Hertz so there we are at this point here is a space and has a complete this is if you put the Lisa detector up you will cover a completely different bandwidth different sources again black holes and the most important you also see white dwarf binaries in our own galaxy and this then is between minutes to 4 minutes to hours then there is another technique which is called pulsar timing which is just looking at a bunch of pulsars all in our own galaxy and watching how the how they were the the pulse rate changes as a function of time after the gravitational wave moves through our galaxy and that's something which was very promising technique people have been working out for a while I expect that this is going to give very interesting results but this is the one that's nearest to my heart that's only because of my background this is trying to look at cosmic at the background radiation that comes from the actually the the the moment when the universe got created that's a very exciting thing it looks at the cosmic background as a to look at the polarization the polarization of the thing that is induced in the cosmic background by gravitational waves that come from the beginning and let me leave you at that this is the summary anyway so we've made a detection on earth of gravitational waves that's now known we have a consistency with the Einstein field equations which is sort of amazing in the sense not that we did such a price of precise measurement that over the size of the strength of gravity gravity we now know that Einsteins field equations are good from strengths of 10 to minus 16 to about one that's a fantastic range you know and and then we know now that the universe can has contains but black holes that collide we didn't know that beforehand and there is now a pretty good guess now there are more black holes than we thought and now what we have done by this is we've opened the field of what we really wanted to do in the beginning which is gravitational wave astronomy thanks [Applause]

Channel: World Science U

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Keywords: Rai Weiss, Gravitational Waves, LIGO, Cosmology, Astrology, Laser Interferometer Gravitational-Wave Observatory, spacetime, speed of light, Albert Einstein, observational astronomy, white dwarfs, neutron stars, black holes, Kip Thorne, general theory of relativity, Brian Greene, World Science U, University, science unplugged, New York City, NYC, Physics, Stephen Hawking, Quantum Mechanics, General Relativity, black hole, WSU

Id: qsB_EbgjSmY

Length: 54min 52sec (3292 seconds)

Published: Wed Jul 22 2020