Barry Barish: On the Shoulders of Giants

[Music] I'd now like to introduce today's on the shoulders of giants speaker so it's my pleasure to introduce Barry bearish professor Barry Barris is an experimental physicist at Caltech who has been closely involved in some of the most important physics experiments of modern history in 1994 professor barish became the principal investigator for LIGO laser interferometer gravitational-wave Observatory that you'll be hearing more about leading the ambitious project that went on to observe gravitational waves this past September professor bearish became LIGO director in 1997 in the same year founded the LIGO scientific collaboration an international collective of over a thousand scientists who are dedicated to the study of gravitational waves and recently professor Barris has turned his efforts to another ambitious project directing the international linear collider global design effort from 2006 to 2013 and with that please join me in welcoming professor Barry barish [Applause] I'm an experimental physicist and what I'm gonna try to do today is try to show you by one case example which is basically gravity leading up to gravitational waves and how science works in physics as an interplay between the experiment and theory how it makes wrong turns we're maybe personalities come in and so I'm going to trace the history as a case example up to and including gravitational waves with a hint at what maybe comes next for us if I get through all this so we're gonna start with Newton but before we start with Newton to make sure I get to it basically this is the reason why several of us are here which is a picture of a computer a computer simulation of what it what fits an observation that we made last fall and reported last February of two black holes going around each other and merging into one object and that's what we observed so what what is that two black holes a black hole is when there's been a gravitational collapse of something of a large star for example giving such intense gravity that nothing can get back out so the simplest explanation of a black hole is that's what it is the most important thing to realize is that it's incredibly small the size of each of these black holes is about the size of metropolitan New York but it weighs 30 times the mass of our Sun so that's why the gravity is so far is incredibly strong two of them are going around each other much like the earth around the Sun only they're equal and mass pretty much and slowly they're losing energy and merged into each other as you saw in that in that picture while they're doing that in order to merge into each other they're losing energy and that that energy it comes out in the form of gravitational waves which 1.3 billion years later we've detected so this thing happened 1.3 billion years ago we got our technology good enough to act see it when it came through the earth last September so I'm gonna come back to that we're gonna lead up to him I'm gonna start with Newton since this this talk is even labeled after a famous saying of Newton's and where he started with gravity how it led to Einsteins gravity and then to us both experimentally and theoretically so I think we all know Newton's theory of gravity is probably one of the most successful theories in science it lasted more than 200 years and explained the awful lot he really recognized as you kind of see in this picture what happens when you have two massive objects separated from each other he he somehow put together a connection before we knew it that when the Apple falls from the tree and the moon goes around the earth that they're they're governed by the same theoretical reason gravity and so that's the word universal gravity that he developed and he made a formula which you'd all seen in school that is that the force between two massive objects is a product of those two masses it's inversely proportional to the square the distance you separate them and this governed by some constant G which I'll talk about in a minute he did more he actually in doing this he did the calculus and proved that you could take the distance between these two and just take them as if all the mass was at the center of each one so it's not some complicated thing that you have to look where you talk to each mass is but you can take the center he proved that if you had an inverse square law like this that it would lead to elliptical orbits around the Sun which was a thing that then if it's going to be the planets or the moon you have to have and that all led to something that he then published eventually in the Principia that's the clean picture just like when you read our paper the picture you get is that a hundred years ago Einstein predicted gravitational waves a hundred years later we detected them the actual story has more complexity than that and even this one does it turns out that that's what we know about Newton but the problem is actually more complicated there was another very good scientist at that time the in Britain at the Royal Society named Robert Hooke and he was a physicist and we know him mostly for kind of his freshman physics law which is shown on the right that is if you have a spring and you put a weight on it and stretch it it'll stretch twice as far if you put twice as much weight on there or twice as much force on it but he also was a very broad scientist he got very enamored with the microscope and he studied a lot of geological things rocks and things but he got interested also in old fossils and he studied old fossils and actually was one of the first people that came up with the conclusion that there was biological evolution so Hooke was actually a pretty sound scientist however he ran into Newton he claimed at one point this is before the Principia was published that he invented the inverse square law and that he had written a letter to Newton which nobody's ever seen and it became a dispute between them Newton when he wrote the Principia then he said he wouldn't publish the Principia because of this dispute when he wrote it finally and published it there's no mention of Hooke anywhere in the Principia which is kind of a shame but that's the fact so it just starts with a confusion which I'm going to try to emanate the second thing that happened is that we talked about this formula here but we didn't talk about gee the strength of the interaction that took a hundred years to figure it out what G was and I'll show you how that's done but also there was no sense of what caused this force this is an empirical formula but no one conjectured what caused the Apple default of the earth there were all kinds of ideas that you read about for example planets around the Sun which are about wrong that the Sun had some sort of magnetic field that basically did it which isn't the reason so basically this is an empirical formula and the next problem was to determine the strength of this and whether it was Universal and that was done a hundred years later by a Cavendish and he did in a very clever experiment he built what we call torsion balance and that's a long wire and on the end of the wires is a rod and he put two lead weights on the end of the rod and then he did all the careful work of twisting it letting it go how much he had to strain it to twist it to calibrate it all and then he brought up to heavy weight heavy lead weights next to it and measured how far it moved and when he did that he then could determine the strength in the laboratory of this constant gene and he got an answer which I have here six point seven five ten to the minus eleven and the units that we use interestingly he was almost exactly right you can see I put just for reference what the number is now and so this is impressive and that was kind of the experiment coming into it the next thing that happened is celestial mechanics and this is kind of an interesting story about how science proceeds first of all this theory was incredibly successful it a vine of Newtons it explained everything from the orbits of the planets which I talked about already to the tides or even the escape velocity from the earth so everything you do with gravity on a macroscopic sail scale and in two hundred years or so that was only one place where it didn't work and that was the orbit of our smallest planet or a planet closest to the Sun mercury which has a orbit around the Sun about every 88 days and as it gets near the Sun it gets perturbed by the presence of all the other planets that are around and if you calculate if you calculated it using Newton's formula you didn't quite get the right answer so the answer that was gotten was 500 and can't read this 532 arc seconds per century from Newton's theory and the actual measurements which were good we're 575 so there's a small discrepancy that existed and for some people maybe that is at least part of the motivation are part of the success of Einstein's theory which came along later that's not quite true either there's another guy named Ferrier who was a mathematician and did celestial mechanics and he basically made celestial mechanics a success as a field and respected by doing the following thing he basically used the formulas of Kepler and Newton he calculated the orbit of Uranus and found that there were discrepancies in that orbit from those those theories those formulas and he predicted then that it was because there was a missing planet Neptune which is the name that he gave it that it was missing and he predicted them where it would be because otherwise it wouldn't fix the problem so he predicted where it would be the story goes that then he sent a letter to a astronomer in in Germany in Berlin and it took five days for the letter to get there he took him to account those five days he knew how long it would take the astronomer got the letter this is the story at least the astronomer got the letter he then looked up at the sky and the place that that was predicted by Ferrier and within one degree he found Neptune and at least the part of the story is true is that he predicted Neptune I don't know with all the colors true but this made celestial mechanics a let's say a viable subject he went on he's the guy that saw that mercury had an orbit that wasn't quite right so he tried the same idea on Mercury so whether the Newton's theory is right or not he said okay it's gonna be just like Neptune and Uranus and there's something missing he called that Vulcan and he said it will either be a small planet or maybe a series of small objects that were between mercury and the Sun he predicted what they were he gave him the name and they've never been found they've been looked for even by NASA in recent years but they've never been seen of course they were reinvented for Star Trek so those of you so that we know so that's the that's basically where the situation was when Einstein came along there was not a great case that Newton's theory didn't work in fact this in fact they didn't find Vulcan but maybe they were experiments weren't good enough at that point so some people think that that was the motivation Einstein had developed the theory of special relativity in 1905 and he extended it to include accelerations which is eventually gravity and ten years after his three papers that most of us in physics idolized kind of in 1905 he came out with the theory of his theory of gravity and that theory of gravity forget the formula in detail that theory of gravity basically had the feature that it unified all of space in space-time one one universal or one word space-time of course my laptop doesn't understand that are mr. gates because it puts little waves under space-time when I typed it in so oh you still have to teach him that space-time as a word anyway Einstein came up with this theory and in addition if he had the right answer for Mercury but that's not an enormous triumph obviously yeah but it was a good start that guy gave the right answer for Mercury around the Sun and when we come out with a new theory of nature one of its ingredients of course is to explain things that haven't been explained before but that can maybe be done non uniquely you can make many theories that will explain something that you didn't know before in this case the theory was good enough to predict some new things and the thing that it predicted that's most famous was the bending of light or the calculation of exactly how much light would Bend if it went near a massive object Einstein's theory has a not as the as the thing that causes gravity is a curvature space-time and so if you get near a massive object it doesn't matter whether you have something of mass going by or something that's not mad like photons going by they're gonna follow the path of the distortions of space-time and curve as they go by so he calculated the curvature that it would have by the way it also curves in Newton's theory but not as much he he calculated the curvature that it would have and that was followed up by Arthur Eddington who in 1919 went to the southern hemisphere and an expedition and looked at the bending of light through an eclipse of the Sun where you could see galaxies moved behind and he got the right answer are basically he got the same answer as Newton as Einstein had predicted this was a great triumph it's the thing it is the single thing that they dine Stein famous worldwide that hit newspapers all over the world and basically not his wonderful articles in 1905 but the bending of light was what captured the public in in about 1920 I've looked at that data from Eddington which other people probably have now and let's say the requirements of what would be convincing to the scientific community were a lot different than when we did LIGO our big issue in seeing gravitational waves was to produce what seems to be the standard you have to do which is what's called five signal that's that it's so in significantly probable that it happened by some statistical fluctuation that you can ignore that it has to be that you did something wrong in the case of in the case of Eddington this was nowhere near five Sigma by any stretch of the imagination it but it was right and it was repeated a couple years later and of course that is history so it was right that same feature of the bending of light can actually be seen if you go to the southern hemisphere and some of you probably have seen it and that's called the Einstein cross and it's quasar image that appears around us by central glow from a nearby galaxy and it's visible and looking like this cross and that feature so you can see with your naked eyes but for astronomers this idea that you basically can have a curvature around objects means you can look at objects that are dark and see the curvature around them and it's become kind of a cottage industry for for astronomers what's called gravitational lensing so that basically came out of this whole thing I don't know if that convinces you that Einstein's theory has anything to do with with the real world but this one will and that is all of you maybe not as much in New York as we do in Los Angeles but you drive around your car using GPS and GPS relies on general relativity I'll explain that to you uh-huh maybe I use a picture of somebody walking just because it's New York so if you wanted to walk and guide yourself around then you have this picture so what is this the reason why general relativity matters for GPS and is a good illustration that GPS really Ben Stein theory of general relativity is relevant to all of us first these satellites are going fast they're going 24,000 miles an hour and we have a something we learn as soon as we learn special relativity and that is moving clocks move more slowly so that's true we applied special relativity which is something that's well established and if we do that we find that we have to make a correction I'll tell you how big the correction is but how much it matters in a minute a correction of minus seven microseconds per day so just because of special relativity the fact that they're going fast the actual clocks are going to go slower and we have to correct for it somehow of course you can put something right in the satellites to do that but that's not the whole answer there's actually a bigger correction that comes from general relativity and that's the fact that the satellites are have about a quarter of the gravitational field that we have here on the surface of the earth because they're way out and because they have less gravitational feel they have less space time orbit around them and in that case clocks move faster and in fact if you calculate it with general relativity they gained 45 microseconds a day so the total is the difference between those two where the GPS correction is 38 microseconds a day does that matter yes the accuracy that you need to put us on the road is about 30 nanoseconds in order to be 10 meter resolution to be on a road you have to have something like 30 nanosecond resolution which is a thousand times better than the 30 microseconds a day or if you divide by the number of minutes in a day every minute or so you're going to wander off the road if you don't make the correction so now we need that correction and so you should believe that general relativity matters and actually works this is a regime of general relativity that's less interesting for us as physicists it's what we call the weak field limit and there it works it works in other ways but this is kind of the most dramatic I think for those of us that walk around the streets or drive our cars we're interested in what I'll get to later in Lego as creating a laboratory where we can study general relativity where the theory is most interesting and most in question and that is when the fields are incredibly strong like in these black holes and that's why studying black holes become so interesting to us in terms of testing and understanding general relativity okay so Einstein then had put this theory out of general relativity and a year later he predicted gravitational waves he did this not rigorously but in analogy in a sense to electrodynamics where we have electromagnetic waves and the similarity in the equations and so he predicted gravitational waves in exactly 100 years ago or in the winter of 1916 and he wrote a paper and that paper was badly flawed but it's the paper we refer to when we say that a hundred years ago Einstein predicted gravitational waves that paper has a series of errors in it but one of them is a even a factor of two and the magnitude of the gravitational waves the idea that there were gravitational waves was right you wrote a follow-up paper two years later in 1918 and in that paper he fixed the errors fixed the error made it a factor of two it's still not a rigorous derivation of gravitational waves but it has an important element it basically defines the source so the source of gravitational waves he shows in that paper is is from a quadrupole moment or a quadrupole formula and to remind you when we have electromagnetic waves which we compared with electromagnetic waves comes from what we call a dipole moment if you have a plus charge and a minus charge and you wiggle them to each other they emit electromagnetic waves depending on how fast we do it we have microwaves or infrared or ultraviolet or whatever are radio waves same thing here we have a quadrupole moment and said that's for instead of a dipole moment in order to make gravitational waves and so we know how to produce them if we wanted to and and again how fast you do it will depend on what the frequency is of those waves just like electromagnetic waves they're basically produced I'll come to in a second they're produced I want to bring one one other point in first the gravitational waves he also shows travel with the speed of light so the fundamental difference that we that matters kind of conceptually between Newton and Einstein is that Newton's theory when the Apple Falls is that you have what we call instantaneous action at a distance so wherever we are when it happens we detected immediately there's nothing in between well in Einstein's theory we produce a messenger if its gravitational waves that travels at the same speed as he has in the problem which we call the speed of light but it's basically the speed in the problem and basically then it takes in our case for the example I showed in the beginning 1.3 billion you for the signal because it was distant to get to us that we're gonna show in a little bit and so Einstein has the propagation the propagation is not however like the propagation of light waves which is particles it's actual distortion of space and time okay the story doesn't end there as I said the stories become quite contorted Einstein moved to the US and in 1933 from Germany wrote two famous papers Einstein Podolsky and Rosen which was a famous paper about quantum mechanics and a second one about with Einstein and Rosen that had to do with what we call wormholes now he didn't call it that and then he wrote a third paper in 1936 with Rosen that third paper was entitled do gravitational waves exist you might guess from the title that he had conclude at almost twenty years later that they don't exist after he had proposed them earlier and he submitted this paper to Physical Review this is the man that received the paper John Tate was the editor of Physical Review at that time physical reviews our main journal as certainly at that time John Tate then took the paper and he sent it for peer review and we know now from the records at physical review that he sent it to this guy Howard Percy Robertson Howard person Robertson was also a general relativist he was on sabbatical from Princeton and was at Caltech my institution on sabbatical when he got the the paper to review and you can see on the second line as einstein-rosen and the dates they came and he sent it back he actually found the mistake in the paper I had to do with coordinate system the fact that Einstein used a single coordinate system to cover all of space-time which is very difficult and found singularities that he thought basically were false and were the reason that they were assuming there were gravitational waves but there weren't and Robertson reform in a different coordinate system a cylindrical coordinate system and showed that the singularity went away did his review sent it to mr. Tate who was the editor of his Rev and mr. Tate then sent a letter which I've seen but I'm not duplicating here back to Einstein that was very mild he basically said the part I took in quotes that he would be glad to have Einstein's reaction to the referees comments and criticism nothing about whether he published it or not Einstein's response was this and he actually wrote a written response and that written response which he wrote in German but it's translated was dear sir we mr. Rosen who had gone back to the Soviet Union by this time and I had sent you our manuscript for publication but had not authorized you to show it to specialists before it's printed I see no reason to address the in any case erroneous comments of your anonymous expert on the basis of this incident I prefer to publish the paper elsewhere respectfully Einstein there's a PS saying that Rosen agrees with it even though he's gone back to the Soviet Union so he went on just as he said here and he sent it to a more obscure journal it was actually the journal for the Franklin Institute in in Pennsylvania in those days we didn't have kind of a high-tech way that we published now and the publication's had to be set up and printed and read and that took months and so this was percolating through the system at the Franklin Institute they accepted the article as was written which was exactly like the one sent to Physical Review in the meantime Rosen had gone back to the Soviet Union and Einstein had a new assistant who was a early important cosmologists Leopold infill and Infeld then was important in the final resolution so the sabbatical of Robertson although these are anonymous reviewers so nobody knows that it what happens to the reviews that are done return to Princeton and when he returned to Princeton he had no idea what had happened in this but he happened to be friend mister in film so in after he met in filled they in talking physics Infeld he mentioned to Infeld that he didn't believe this conclusion and Infeld showed him the proof that he had of Einstein's and Rosen's idea and Robertson Robertson continued to then basically show him where the error was so in Phil went back and discussed this with Einstein and Einstein responded saying he'd already found the error in the meantime the article came back from the Franklin Institute got rewritten it changed the title changed the substance and basically if you read the first sentence it says basically that he had rigorous solutions of the cylindrical gravitational wave and the present semanas paper so it became a paper that carried the the proof of gravitational waves further so he wasn't embarrassed by this he's never officially come out against gravitational waves however Einstein never published anything again are in gravitational waves and he never published anything again in Physical Review to be fair peer review is a system that was invented not long before that it's probably true he had moved to the US in 1933 that he had never experienced peer review so in his articles published in Germany he probably never had peer review his two famous papers that I mentioned as far as we can tell both were accepted by the editor at that point who had that he could send these out to peer review on discretion at that point and so the two articles the one with Podolsky and Rosen and the one on wormholes that he published weren't reviewed so this is probably the first review that he ever experienced so there's his side of the story as well but anyway all's well that ends well the next thing is as an experimentalist we like to control all the variables so if we do a measurement that's hard or easy or whatever we do you really want to have everything in your control so that you can understand all the aspects so the source the ends and in our case the analogy here is to go back to Hertz in the 1880s who demonstrated for the first time electromagnetic waves and he did this in a laboratory by starting with a source of making electromagnetic waves going far enough away so that you could see the wave nature and prove there were electromagnetic waves and this is a famous experiment if you wanted to do this in the laboratory for gravitational waves what would it take to do that and why don't we do that so let me just give you a an imaginary experiment so just like these two objects that go around each other that we're going to talk about later black holes we could sit in our laboratory and make something similar by making a big barbell and putting a lot of weight on each end and rotating it just like that and it's going to emit gravitational radiation so I've put in numbers here just to give you a feeling of what that will take you can ignore the little formula it just tell us what the strength of the whole effect is and so I've put kind of unreasonable but trying to make the the numbers larger unreasonable numbers in if we took masses of a thousand kilograms for the two masses separated them by a metre spun at a thousand times a second get out of the lab because it's pretty dangerous and go away about three hundred meters you actually want to go much further than that to see the wave nature but I've tried to make the effect as big as we can then so we spin it then the effect is and just remember the number 10 to the minus 35 so this is the fractional change that's created by a gravitational wave the numbers that I'm going to show you from the object that we that we've detected are more like 10 to the minus 21 14 orders of magnitude that's 0 0 14 times larger than this and we can barely detect it as you can see I spent years developing the to do this so we're nowhere near the ability to do what an experimental Mentalist once which is to carry the whole problem from start to finish inside our laboratory so instead we're forced to look out in the universe of course that in this case turns out to be very fortunate because what we see out in the universe turns out to be so interesting in its own right so we don't don't just a measure gravitational wave so here's the same numbers just for completeness for the object that we saw that I'm going to talk about that gives something like 10 to the minus 21 again their weight their mass is the roughly 30 times the mass of the Sun there's their radius is 100 kilometer 100 kilometers and the frequency where we detected is roughly a hundred Hertz and if we do that then and the distance away we wish we've measured if we do that we get a strength of something like 10 to the minus 21 about 14 orders of magnitude stronger than this barbell has just been so that's why we go out into space okay so we're looking then driven this way for Astrophysical sources they can give us signals in the detectors that we're I'll talk about now a little bit that we bill so what is the effect the effect is that if a gravitational wave comes it distorts space-time in a way shown in this picture at the frequency that it's coming through you or me or our detectors it'll make in one direction stretch and there at the same time a squash in the other direction and then go back and forth at the frequency of the gravitational wave so in this case it's once a second or so but and we try to measure that this is just on the on that on this side just the picture of that statically but it basically goes back and forth and that our problem then is to make an instrument if we want to detect these that can measure this difference in the two directions fortunately it's one of the best kinds of instruments we have what's called an interferometer so we want to measure this with interferometer from the metrics techniques but that's not where the problem so the problem of doing it experimentally began with a man named Joe Weber it began in the 1960s and he had the idea that you could take a very sensitive way to measure it would be to take a great big bar of aluminum in this case instrumented so that any changes in its shape could be detected easily and it could be detected even if they're very very small and if a gravitational wave came through it would distort the distorted and we know that the amount of distortion or what happens in a big object like this is it has a resonant frequency if you hit it it rings at some frequency so the advantage and good thing is that it has a lot of sensitivity near its resonance frequency the bad thing is it doesn't have very much width if you get off the resident frequency you can't see anything very much I won't have any sensitivity and the instrument that we're going to talk about in a while though interferometer we have a broadband ability to see so that's an improvement but the concept was is and he pursued this he was at the University of Maryland he had been at Princeton before and had come up with the idea he never detected gravitational waves I'll show you that but we have a long legacy of what he's done other than in addition to basically beginning the field experimentally so Joe Weber really was the pioneer that began the experimental efforts to see gravitational waves but if we look at LIGO which I'm going to come to in a while we've actually benefited from some of the techniques that he invented I have three listed here the sensitivity calculation and noise analysis that he did is similar to what we do that is understand what it is that gives you signals besides the thing you're looking for that he did a coincidence for background rejection just what we do we have two detectors one in Louisiana one in Washington and we asked that they be coincident within the time resolution of the gravitational waves and lastly he even explored how to measure what the backgrounds were by looking at things that had the same time and things that don't have the same timing all three of those are basically legacies that we've inherited and use as part of the basic scheme for detecting gravitational waves even though the technique is quite different the sensitivity of the bars as I said is limited by their narrow width and that practical size that you can make it I showed you it's a fractional change in in stretching or squashing and so the bigger you make it the better and you can only make a bar so big this effort though to do bars went for a long ways because clever people decided you can make it better by making by cooling it and until we got LIGO working in the early part of this century these bars were running as as one of the main search elements to look for gravitational waves now they're in the in the storehouse there's another thing that came from him and that is his student Robert forward was one of the first people in the u.s. to actually look at the idea of using interferometers he worked with Webber and then he went to Hughes research labs in California and actually built a little interferometer so he had that history this idea was then picked up by my colleague Ray Weiss who developed the the real ability to what it would take to make a real interferometer that would be sensitive enough a few years later so Webber though unfortunately as great as he was in bringing all this was a much better technologist and visionary than he was as a scientist in 1969 he published and even though in Steiner couldn't he published in Physical Review the discovery of gravitational waves and this is the figure from that paper so the first this is his first discovery and that's what he said is he saw a blip one at argon lab near Chicago and one at University of Maryland this proved to be wrong and unfortunately this happened more than once in his career so he somewhat spoiled his reputation by having wrong science results which is devastating as a scientist despite the fact that he made such an incredible impact on the field we have another way that gravitational waves have been observed and that is indirectly so if you look at the picture here we have two objects these are called neutron stars one going around the other and again in this elliptical orbit they're going to radiate some gravitational radiation this was Taylor and Hulse and they were studying this fast rotating one on the Left here which was rotating at 17 times a second notice that it had a modulation of about once every eight hours and that was due to the fact that there were a pair once they just got decided just once they discovered there was a pair then in subsequent years it was the period of the long-lived one the long period the eight hour period are almost eight hour period was measured over a period of time and as it radiates radiates gravitational radiation that period gets a little faster and a little faster and in a million years or so we would detect it in LIGO but at this point the data that they had is published is shown on the picture on the on the right and in that picture you see a bunch of dots and the vertical scale it's years I'm sorry in the vertical scale its seconds and that's how many seconds faster or shorter the period got and on the bottom scale is years so over about 20 years the actual period of this almost eight hour period got shorter by about 20 seconds and they measured that very accurately as you can see by all those little dots the line that's shown on top of the dots is the line that's calculated from general relativity so knowing all the features of that system which they had measured independently you can calculate the line it's not a fit to the impressive thing about this to me is it's not a fit to the data it's a line drawn from the parameters that they have which is unbelievingly impressive that you can actually calculate independently and it falls right on the line so this is a very strong indication that gravitational waves and caused this effect if we wanted to measure this system this way we'd wait about a million years and it would be in our frequency Bend okay so now the direct detection and I'm going to do this in a kind of qualitative way we have the same problem what we want to do is see the squashing and stretching of free masses as they move through time so I've drawn here the technique that we have superpose on a circle of free masses imagine masses that are free to move as they're there so a an interferometer measures basically the time it takes for light to go down the arms Paris on ttle and the light going vertical we basically send the light and both split the light from a laser send it in both directions bring it back and if it comes back at the same time we can already n things so they cancel each other but if one arm gets a little longer than the other because of gravitational wave came through then they won't pencil at that time and we measure that in time evolution the details of how that's really done which is very important and very clever and very difficult were explained by Ray Weiss my colleague yesterday in a lecture and you just have to go on to the website here and I don't whenever they put it up and see how the interferometry really works to do this I don't have time to do that today but it stretches and squashes like this so that's basically what we're doing is changing the length of the arms and then measuring the signal that we get we have to have free masses for two reasons one is we want free masses like the picture conceptually but we also want to isolate the masses from the earth itself and we do that by hanging the masses from wires and it's a pendulum basically and it wants to move and shake itself move itself and has a natural frequency but that frequency is lower than the frequency band we work in so we're able to have them hang from a wire use all kinds of isolation and make them as isolated from the ground as possible let me just show you in a little video that I didn't make this idea just to emphasize that light comes from from a source which is a goes through a mirror gets split goes out the two arms comes back almost cancels but if any light goes to the receivers then we see that light this is just showing the same idea with waves themselves because the light comes in different waves and how they come back and they cancel are nearly canceled and then what we see in the detector so that's the basic idea and I said it as a as a more quantitative explanation just go to race talk when it gets posted and he does it in nice detail okay so how small is this change that we want to measure that's the technique and you can learn more about the technique from Ray but how small is that we ultimately want to go to something like ten to the minus nineteen meters we talk about 10 to the minus 18 but it's 10 to the minus 19 is kind of our goal and so how small is that let me remind you how small it is a meter we all know how big a meter is it's basically that long the human hair some of us have some left is about a hundred microns in width so that's ten thousand times smaller than a meter okay we can all handle that the wavelength of light like the light in the laser beam is a hundred times smaller than that that's basically a mite one micron the atomic diameter now getting two things are a little harder for us to visualize but the atomic diameter is ten thousand times smaller than that and that's ten to the minus ten meters the proton getting into something that's basic and using particle accelerators for example it's a size of 10 to the minus 15 meters that's a hundred thousand times smaller than the atom and we want to go a factor of ten thousand smaller than that so that's the magic and maybe the reason people thought this would never work but it does so that's the goal and you have to do a lot better than the simple picture I did drew of the interferometer to make it that good so we built these instruments they're large for the reason that I said the amount of effect depends on how big you make it so the largest practical size the time we built this was to make the arms the two arms on the interferometer about four kilometers long so they're four kilometers long this picture is LIGO in Livingston Louisiana and you'll notice the terrain it's all it's a commercial forest and along the way we build it is to build up build it up on a berm to get it high enough above the surface so it doesn't flood and so we build it to the 500-year floodplain that's about 15 feet up and in Louisiana add another meter for safety so we don't get flooded in the lifetime with LIGO and in doing that it makes we borrow the dirt from a channel which immediately fills with water and that's what you see on the right side and then alligators fish and everything else you can imagine so that's a terrain we live with in Louisiana there's no bedrock so trying to make a stable physical structure is very difficult it's basically floating on water the second LIGO Observatory which is identical for our purpose is totally different from the standpoint of this picture and reality it's on high desert it's in Hanford Washington so it's basically on sand or it's you have to go very very deep to find any water and yet the two instruments we make and their sensitivity are basically identical how we try to keep them identical so that's the two instruments we have to in order for confidence like it was done like Webber was trying to do but also to tell where this gravitational wave when we detect it comes from so once in Hanford Washington once in Livingston Louisiana there are 3,000 kilometers apart at the speed of light if we have a signal or gravitational wave that first goes through living Sun and then goes to Hanford and goes at the speed of light which is a speed of gravitational waves it'll take ten milliseconds to go from one to the other if we go the other direction it'll take ten milliseconds to go from Hanford to Livingston and if we happen to have a gravitational source that was directly overhead between the two they would come at exactly the same time so that's the scheme we ask that they both happen and we tell something about the direction by the difference in the to the event that well I'll talk about or show you it was six point nine milliseconds different coming first in Livingston and then to Hanford Washington and I'll talk about what we use that for in a minute this is just some more of the internal workings of of LIGO it has a great big vacuum pipe yet I think it's the largest high vacuum system in the world it's 1.2 meter diameter pipes a total of 16 kilometres of vacuum in the two sites and these are the kind of working parts of like Oh big chambers where we put the optics and and mirrors to guide the laser beam and these have a lot of ports and things because we have a lot of test equipment and side beams and all kinds of things that we do which I'm not going to talk about today so what limits us so we can make it big enough we can calculate that we're going to try to do as well as I said but in reality we get limited by things that get in our way and these are the most common ones first if the vacuum isn't good enough and that's why we make very high vacuum we get residual scattering off the vacuum itself so the light goes down scatters off the vacuum you can imagine a lot of ways you can get in trouble with that you can have particles that scatter then off-the-wall photons and back have a different path length and are out of time and so forth so we need to keep the scattering down as much as we can second as much as we think of a laser beam has very very stable it's not stable enough for us so we do a tremendous amount of work to stabilize the laser in its wavelength and in its amplitude that's the second thing we have to do to keep from being able to do this the third is that we have to isolate ourselves well enough from the ground this is a huge problem we basically are living on the earth where the ground shakes a lot and we do that in two ways as I'll show you the next one is that we were at room temperature and at room temperature protons and molecules move around in any substance so the mirrors that we have are made out of molecules and they move around something we call Brownian motion and lastly there's more subtle things that we like to increase the light level as high as we can to get the best measurements but if we do that we have other problems that come in in this case what I've shown here is something we call quantum noise and that is as we increase the light level I'm making more more powerful lasers we produce a pressure on the test masses themselves that we have to worry about if we put that all together we get a curve that looks like this now what's a look at and this is the shaded area this is an old picture so it's not the advanced LIGO but it shows the concept if we look in the shaded region that's the highest level that we're limited by something all the lines below are the things that we try to control and make sure they're below and we're eventually limited by three things at the lowest Center at the lowest level on the left very steep is seismic noise that's the shaking of the earth at the highest and we're limited by what's called shot noise which is really fundamentally how many photons you have so it's photon statistics and in the middle we're limited by this thermal noise the fact that we're working at room temperature it's a nasty problem for experimentalist because we don't have one problem that we have to deal with we have three different ones the total sensitivity region might look familiar to you it's essentially the same as the year so the audio but so we talked about being in the audio band we basically have ears that respond to tens of Hertz and go up to thousands of Hertz just like this does and it's really for the same physical reasons we live on the earth that's where the earth is quietest and so our laboratory for LIGO is on the earth and although we're not dealing with audio things we have the same basic things that limit us so we work in what's called the audio band and and it comes from the fact that that's the laboratory that we have which is here on earth so that's our sensitivity and it looks like this curve here this is a real curve now lines that go up you can ignore those are little resonances that we have in our system and just ignore them for now if you want to learn about them in more detail we can talk the the colored the colored lines are the data from our earlier measurements that we were we didn't detect gravitational waves and then we went through a big rebuilding program to improve in three directions to improve the power of the laser to be better at high frequencies I said it's the number of photons or how fast you can sample at the low frequencies to do better seismic isolation and the middle levels to make bigger and better test masses and suspension systems to try to control that that region we did those improvements and completed them about I don't know a year and a half or so ago and technically these are just some pictures I'll just show you quickly the laser that we have only message to take away here from here is this a lot more complicated than the little one I would use as a pointer if I didn't have glass up here and it has all these stabilizing elements in it the mirrors themselves are fantastically wonderful objects these are made out of few silica they're a twenty centimeters thick 35 centimeters or 34 centimeters across they're weigh 40 kilograms so they're big things the optical surface is that the best the best we can make and they're coated so that they they basically tell the eye they look perfectly clear but they're coated so that they reflect at the wavelength of our laser which is 1024 nanometers this is the suspension system as I say we hang from wires we do it in a series of four steps and this is done so that the bottom layout level is as quiet as we can make it we do all the controls and higher levels and lastly and you can't see this very well in the picture we have a very complicated system of trying to isolate ourselves from the ground both passively by using basically very advanced shock absorbers like you have in your car but more recently by doing it with feedback system that does it does it actively that is by measuring the motion just like sound things that you were using your ears on an airplane drowned out the sound in the background we correct for the motion now combination of those has given us the improvements that we needed to make this measurement so the curves that I showed before were the top curve here and we started improving too with the physical improvements we made on our way to get to this very bottom line and we stopped in between because of the fact that as we increase the sensitivity say a factor of two that's means that we look out into the universe a factor of two further and the number of galaxies stars and so forth in the universe as we go out a factor of two furthers our factor of eight bigger because it goes as the volume so having increased our our sensitivity a factor of three at the high frequencies and where we put in active isolation at the low frequencies it's even more we were able to make this measurement so we turn the apparatus on last September this is a familiar curve that people have seen one is from LIGO Livingston on the left the one on the right is from Hanford Observatory and what they show is there are seven milliseconds apart what they show is something that's getting bigger and higher frequency and that's the motion as it's going in it's going faster around the around each other the two objects and then they merge and they merge at this merging and finally ring down a little bit and that's the picture which I'll show you what we calculate so that drawn through there then in the last is a line which comes from our best fit using general relativity we now look at that this is the picture that we have going backwards now from what looks like the data the top is the physical picture of the different parts of the graph we see that picture here that is basically the picture that is extracted from the raw data where the frequency gets higher and higher the signal gets bigger and bigger these are the two objects getting closer and closer together these very compact objects and then merging and then there's a little bit of ring down which we're not able to detect very well at this point we fit all that with general relativity and we're able to tell the masses of the two objects were able to tell the amount of energy that was released into gravitational radiation about three times the mass of our Sun the equivalent of the mass of our Sun radiated away and other things about it how far away it is and so forth if you look at the bottom picture here to me it's very revealing of what's happening first look at the the black curve on the top and you see that the separate it's the scale on the right which is in units of what we call Schwarzschild radii but that's about a hundred kilometers each they start in our band about 400 kilometers apart and when they're finally merging they're only about a hundred kilometers which is their size and that's what you saw and this all happens in about a quarter of a second or less the other curve going up is one that's astounding these things although it didn't look like that in the first movie that I showed you are highly relativistic they're going at the beginning at 30% the speed of light and by the end when they're merging they're up to about 60% of the speed of light so it's a tremendously energetic system and fast system we compare the times when it arrived I kept saying they're seven milliseconds apart if you remember how you tell where something is from using triangulation on a boat or something this should tell us a ring in the sky basically and if we had a third detector which we will have soon in Italy then you can do better you can tell two points in two positions in the sky what I show here is how well we actually are able to tell the detection we can tell it comes from the southern hemisphere and it's this banana shaped thing that's for us not very good it's about 500 square degrees but it's not the whole ring and the reefs it isn't the whole ring is that the true detectors don't have spherical acceptance they have an acceptance that depends whether you're overhead or inside and so forth and they're separated from each other about 16 degrees on the Earth's surface and so looking at how big the signal is in Livingston compared to Hanford we're actually able to rule out that pretty much rule out the Northern Hemisphere and so this is what we end up with it's very important to us to be get this localization much better because we're now able to for the first time look at this guy with gravitational waves and you'd like to see that whatever you see is it also seen in some sort of electromagnetic radiation whether it's visible or infrared or some other object and of course this huge swath is not good enough so the way we're going to do that is to add other detectors there's one that's been built and will soon be hopefully in Italy French Italian collaboration near Pisa and it should be operational hopefully by the end of this year and that will give us much better resolution which I'm not going to show you but much better and finally we've been approved by the Indian government to put a third interferometer we we are contributing the interferometer and them the infrastructure in India and that should be finished in about five years so that will in the end end give us much better position we've just opened and that's why this is so important and exciting a totally different way of looking at the sky we know there's a lot of what we don't know is probably the most important that there's things that give gravitational signals that we don't see electromagnetically in fact this very first signal that we see this 30 solar mass black holes weren't known to be there by any electromagnetic experiments that have been done so that's already new the known sources that we expect to see I've just listed here as the possible targets that we're going to be looking looking for in the next few years on the upper right I show bursts that's like the collapse of a star supernova or a gamma burst all these things that are seen optically but the information that we can get if we add information gravitationally and give us great insight for example into the collapse of a star the collapse of a star is a gravitational the physics is gravitational the signal that's seen optically is kind of a shock wave that comes out and so to really understand it the gravitational signals should be added to the other so we hope to see the collapse of a star gamma-ray sources eventually as are in our sensitivity improves what I call continuous sources in the lower-right that's spinning spinning objects that aren't totally spherically symmetric examples are what we call pulsars or neutron stars and if they're young enough they're probably not so so spherically symmetric and we look for those as a continuous wave at some frequency that they're rotating at the lower left is a dream we're not able to do that very well in the LIGO bandwidth but that's to see signals from the early universe lastly we're going to improve the technologies in LIGO I showed you we're only partway to where we've designed to go we'll finish that in the next few years we know how to go beyond that and we'll keep improving we gain as a cube with every improvement we make but there's other improvements that can go beyond that there's a detector being made in Japan now that has two improvements that will be important in second or third generation detect in the sense that like Oh advanced LIGO second generation I would call this two-and-a-half generations that is it includes an advantage of being deep underground but in deep underground there's much less seismic noise to worry about things don't shake and they're making it an attempt that we looked at but gave up on in our time scale before and that is cooling this system cooling the mirrors so that we don't they don't have the thermal noise that that we have there's also been look both in the US but primarily in Europe so far that how to make a detector on the Earth's surface that can go well beyond LIGO and in Europe through a study they've done one that would be deep underground ten kilometers instead of instead of four kilometers a triangle which helps you do the polarization which exists that I didn't talk about that it's cryogenic and that it has two Coptic o configurations one that will work at very low frequencies and one that would look at it work at much higher frequencies we're looking at some alternatives to this but the point is that this we're not at the end of the road at all we can develop this technology over the coming years on the Earth's surface to do much better than we're doing now that's not the end of the story we know how to do that but just like in astronomy where the big one of the big gains in the last decades has been the ability to look at different wavelengths looking at optical looking at astronomical things that happened but looking at them in the infrared and the ultraviolet in the visible and different wavelengths has been able to pull together the dynamics similar to that gravitational waves are going to be at different frequencies we're working at the very highest frequencies which means we're working where things take milliseconds to happen those are very violent things that only take milliseconds and we've seen one that's this object coming together if we want to look at and there's many possibilities at things that take longer timescales then if it's minutes or hours we go in space you can't do that for the reasons that I said on the earth we go in space there's a program to go into space it's called Lisa the Europeans are supporting it it was supported by NASA but because of budget problems that was pulled back we're hoping that it'll be put back into the NASA program possibly stimulated some by our success but that's a very important way to get to longer timescales yet longer time scales are done by pulsar timing on an array of timing very accurately where the timing gets affected by the passage of gravitational waves and that can get to years and decades and finally we can look at the experiments that look at the early universe signals billions of years away and look at the effect of what they measure by the presence of gravitational waves that affects the patterns that they look at or the polarizations that they see so the future in our mind is really exciting and it'll take a long time but opens up a totally new area of physics and astronomy and with that I'll close thanks [Applause] [Music] [Applause] [Music] [Applause] [Music]

Channel: World Science Festival

Views: 31,732

Rating: 4.8723402 out of 5

Keywords: Barry Barish, nobel prize, nobel, On the Shoulders of Giants, Isaac Newton, Laser Interferometer Gravitational Wave Observatory, LIGO, gravitational waves, black holes, Einstein, Interferometer, New York City, NYC, world science festival, full program, World, Science, Festival, 2016

Id: ReUaTI5gI80

Length: 66min 19sec (3979 seconds)

Published: Thu Aug 25 2016