WSU: Exploring the Warped Universe with Nergis Mavalvala

so hi everybody welcome it's a pleasure to be here with you all um so uh even with that lovely introduction about squeeze light and optical cooling today i'm going to tell you about something that's even cooler and that's uh the detection of gravitational waves now i want to start off by just having you pay a little attention to this title slide because there's a lot of important information uh in there so the first thing that i want you to notice is the number 100 years so we're exploring the warped universe so we're going to go on a journey today that takes us to those parts of the universe that are inherently dark we can't see light from them and also very warped and actually quite violent so if you're a little queasy with those things you might want to leave all right now the other thing i want you to notice is it's a 100 year journey so there's going to be a little bit of a journey with to do that all of the things i'm going to talk to you about which was the detection of gravitational waves with these detectors that we call ligo which is the laser interferometer gravitational wave observatory uh was done in a big collaboration so i'm just a messenger this was the ligo and virgo collaborations of many hundred scientists and finally i also want to point out that a small little round logo on on on the bottom right is the national science foundation they funded this whole thing over many decades so this is also a story in public funding but now let me step back to the fun part and that is to tell you that on the front pages of virtually every newspaper in any language that you would you choose in any country the headlines on february 12 2016 was about the first direct detection of gravitational waves and so today what i want to do is i want to take those headline stories and unpack them a little what was the story behind the headlines and what did we really do okay so the first thing that i want to do is step back a little bit and ask how do we know about most of the things in the universe when we look out into the sky what do we see well it started with the ancients we looked at things with our naked eyes and then when we got better at building instruments and eventually we got so good at that that we could look at some of the most spectacular objects in the universe and here is an example this is a supernova remnant and this is one of my most favorite pictures in astronomy maybe my second most favorite after the detection of gravitational waves now but what this picture shows out is it's actually a composite made up of three different wavelengths of light this is an object called cassiopeia a it is what's left over after several hundred years after a star died so a star very much like our own sun exploded and why did it explode because it ran out of nuclear fuel and so it kind of got crunched under its own self-gravity and as this process happened it threw off a bit of material and that's all the gas and dust you see in different colors and those different colors are different wavelengths of light the reddish colors are infrared light so that's the color of light that our own eyes can't see snake eyes can but ours can't then the green yellows are optical which is what our eyes see and then the blue colors are actually x-ray so very energetic photons and what you see is that as these ejecta of gas and dust were blown off by the star the blues went out the farthest because of the most energetic okay now if you pay very close attention at the very center of this object you'll see a small blue dot does everybody see it yes what is that it's blue because it only shows up when you look at this object with an x-ray telescope cassiopeia a it is a new star a kind of star called a neutron star and it is the star that's born when this parent star that was like our own son died and this neutron star is kind of remarkable it has about the mass of our sun but it has a radius that's 10 kilometers so actually about the size of of the width of manhattan okay so imagine that our own sun its radius 700 000 kilometers this object same mass 10 kilometers so it has a lot of gravity in it and that's my point when stars like our own sun die they've produced these very dense compact stars called neutron stars if this parent star that exploded had been heavier if it had been three to ten times the mass of our sun then instead of a neutron star its gravity would have been so much larger that it would have kept shrinking until it became a black hole and that's how black holes are born they're actually born out of stars like our own star dies if it's light it forms a neutron star if it's a bit heavier it forms a cousin which is a black hole and so that's the process that we see when we look out in the sky with different colors of light now this black hole this is actually not a real picture the first one's a real picture taken with telescopes this one is actually an artist rendition of a black hole and in this picture what you see is all this swirl of gas and dust as it's orbiting the black hole and of course the stuff that gets too close to the black hole will get sucked in but there's a whole bunch of stuff around the black hole that's just being swirled around as the black hole spins and this is the way that we collect evidence for black holes until recently we look out into the sky we look for objects like this where the gas and dust around the black hole starts glowing and usually in the x-ray you can even see this gas and dust flickering and the frequency at which it flickers tells you the frequency at which the black hole is spinning around its axis so that's another thing we learn about black holes they can spin but until recently we have not not been able to answer the question of what do black holes really look like how might we observe them because intrinsically unless they're surrounded by gas and dust which some are they're invisible to light light doesn't escape a black hole that's its definition it's a star that's so that has so much gravity packed into such a small volume that even light can't escape right so this brings me to a new type of messenger a gravitational wave and that's gravity's messenger so if we want to understand what a gravitational wave is we should really think about what gravity is and the first person who really gave serious thought to gravity and formed a very uh fine theory of it was isaac newton and so in the 17th century isaac newton had this fantastically successful universal law of gravitational it was called universal because it could explain why apples fell from trees but it could also explain why moons orbited planets and perhaps and eventually why the planets orbit the sun and it's actually quite simple if you look at the equation that newton coined it it says as a force if you have two masses and they have a mass m1 and m2 so two masses and they're separated by some distance r then they feel a gravitational force mutually and that force is proportional to their masses and inversely proportional to the square of the distance we all learned this in our early early physics classes and it's a beautiful theory now newton himself worried about something he worried about this idea of action at a distance what's that well how can this mass here and this mass there know about each other what is the the the way that information is exchanged between them and that never got solved in newton's time in fact didn't get solved until several hundred years later 300 years later more or less and that was with the work of our next great hero of gravity and that's albert einstein now einstein was pretty radical when it came to gravity he kind of told us to throw out the idea of force gravity is not a force einstein said to us gravity is geometry what does that mean well so einstein's version of gravity is that when you have some massive object sitting out in empty space that massive object will warp that region of empty space very much like if you put a bowling ball in the center of a cushion the bowling ball will curve the the cushion and if you put a playing marble at the edge of the cushion the marble must fall into the bowling ball that was the way he described gravity it's curvature of space simon in fact he actually also was also able to codify it in a very beautiful equation and that's this equation that relates on the one side how how um energy is is contained in a system and on the other side how the geometry is contained in the system so this equation it looks really lovely and sweet but it is actually really a horrendously difficult equation to solve and in fact it has taken almost a century to get even the first sort of handles on exact solutions of this equation in the case of black holes so that's einstein's idea of gravity now einstein added one more piece to to this in his general theory of relativity he asked the question of what happens if the if the massive object isn't just sitting around just sitting still what if it's accelerating what what if it's oscillating or vibrating what happens then well then his picture was that space-time must ripple very much like if you threw a rock in the middle of a quiet pond ripples spread out from the rock same thing happens if you take a massive object and you you accelerate it or bob it around and that's what this next video shows space time is this flat grid if the star is oscillating up and down you see that this flat grid of space time is actually forms ripples and these ripples spread out and travel away from the source just like the ripples on the surface of a pond would travel away after you dropped the rock and that was einstein's picture of gravitational waves these are ripples of space-time itself this is of course a very simplified picture because it's it's a two-dimensional representation of a theory and which is actually four dimensional because there's three dimensions of space and also a dimension of time but this is a nice picture to carry if you're wondering what the gravitational wave is it's really just a ripple of space-time and it behaves very much like you would think of other waves they actually take carry away energy and they have frequencies associated with them or wavelengths and all of those things okay all right so if you wanted to do astrophysics with this gra these messengers of gravity what must you know you must know a few things about the properties of these these lights of these of these waves rather so ordinarily when we look out into the universe we use light that's what we that's what i started my talk with that beautiful picture of cassiopeia eh and light is created when you accelerate charge so if you just took an electron out of your pocket and you put it on a spring and you accelerate it it will radiate electromagnetic waves which is light now very much in the same way in a loose analogy gravitational waves are irradiated when you accelerate mass okay so that's the first thing you want to know now it turns out that char uh charges are pretty light and as a result you can accelerate them to very very high uh frequencies and as a result you can have objects that have you know that wig cause light waves with uh with very short wavelengths and that allows us to form images or pretty pictures so whenever you look out into the sky with light the very first thing you do is you see some beautiful picture of an object and then you dig deeper and you understand the the physical process is going on now it turns out with gravitational waves if you're trying to oscillate something that has the mass of a sun or or or bigger it doesn't want to oscillate very fast so gravitational waves are intrinsically low frequency uh waves and in fact their wavelengths are are are very long it can be you know kilometers to hundreds of kilometers and as a result you don't really form these pretty images but in fact you form waveforms and what are waveforms waveforms are a way of of mapping the bumps and wiggles of the space time itself as a function of frequency and that's what that picture in the box shows we call the amplitude of the gravitational wave a strain and we'll come to why that's useful as a function of time and in fact because some of the objects that we are looking for with our detectors here on the earth oscillate at the frequencies that belong in the human audio band we encode them sometimes even into pretty sound so instead of pretty you can think about this way with light telescopes you make pretty pictures with gravitational wave detectors you can make pretty sounds okay now there's a couple of things if you're an astronomer that are very important about gravitational waves versus light light it turns out is an extremely friendly creature every time a photon meets matter it interacts with it it gets absorbed it gets scattered it gets dispersed by matter gravitational waves on the other other hand are extremely aloof they simply they interact extremely weakly with matter and as a result if you're if you're an astronomer and you point your telescope on an object and you see the light from it you have to work pretty hard to decide if the light has somehow been changed along the way because it met some other object as it was traveling from its original source gravitational waves you don't have to worry about that at all they pass through everything more or less unchanged so you don't have to worry about what's between you and and and the source and the the way to think about this as a nice analogy if you want to carry it away if you are light you're like it's like going to a party with an extrovert and you're ready to go home and you say all right let's go and then your extrovert friend meets someone chats a bit meet someone else chats up and it takes you an hour to get out of the party and by the time you get out of the party you might not even get out the front door you might go through a side door or something else gravitational waves are exactly the opposite they're like going to the same party with an introvert you say i'm ready to leave and you're lucky they'll say goodbye and thank you to the host and you're out the front door very little interaction so that's the the power of these these one of the powerful things about these objects is that they're they're actually carry information without much change from when they were generated at the source now just as as a reminder light waves because the oscillations are pretty fast those are happening at 100 megahertz or faster so you know and gravitational waves because the oscillations can't be very fast they're usually about 10 kilohertz or slower now if you want to do gravitational wave astrophysics how do you construct a source of gravitational waves well the basic ingredients are you need lots of mass and that's why you we think usually of things like neutron stars and black holes lots of mass come compacted into small volume and you also need rapid accelerations which means you need the these objects to be in orbit or you need explosions or collisions and so that sort of brings me to my starting point when i said we'd be sort of in the warped and violent part of the universe a warp because you have a lot of of gravitational pull in those regions where you have neutron stars and black holes and violent because they're usually doing something rather explosive or or that involves a lot of acceleration so some sources are colliding neutron stars or black hole pairs you take these you know pairs of neutron stars or black holes and as they orbit each other they lose energy to gravitational waves and eventually they get faster and faster onto each other and collide you can also get gravitational waves from just neutron stars that's a spin about their own axis that causes that spinning causes them to become slightly football shaped or have other deformations on them and those can be those can generate gravitational waves supernovae that's a very nice source of gravitational waves because uh of course we know that this mass is being blown off these ejecta are are are being accelerated to actually uh you know very high accelerations and then it turns out there's a source that is futuristic because we you know the detectors are not yet good enough for it but you can look back to the big bang itself so now i want to remind you of that of that um property of gravitational waves that they're aloof so when the universe was very young right after the big bang it was hot and dense and it was so hot and dense that the light the photons didn't escape they were trapped inside the the hot dense plasma they were they were the extrovert who couldn't get out of the party and the universe had to expand and cool before the photons could escape and that happened when the universe was about 400 000 years old so when we look back at the very beginning of the universe with light our information comes from 400 000 years after the universe was born in the big bang as as we think of it now gravitational waves on the other hand also present in the very early universe but they've been streaming out to us right from the beginning because they are not tracked by the by the hot dense plasma so if you want to look out to the very earliest moments after the big bang light doesn't do it for you and you have to have another messenger and gravitational waves would be one such messenger and then of course what source that we don't really i can't say much about is that since we haven't really this is only the very beginning of how we are uh able to detect gravitational waves uh from uh you know cosmic objects there may be unknown worlds there that we would discover so i won't speculate but we expect that to be true here's a movie that shows this process this is a simulation of two black holes that are orbiting each other and eventually they'll collide and give off gravitational waves these black holes are sitting in a star and gas and dust fields so they can they can orbit about each other and you can see that orbit because of the way that they change the properties of the gas dust and light around them and so the the reason why these black holes are getting closer to each other is that they're radiating gravitational waves the energy carried away by those gravitational waves comes from the orbit and as a result these star these black holes have to get closer and closer to each other until they finally collide and form one quiescent black hole notice the ring of light around the black hole that actually is the bending of light this is light that's actually directly behind the black hole that's been curved around it so that's the process by which one can get gravitational radiation from a pair of black holes they're in in an orbit about each other and typically what we are able to observe is usually the the very last moments before they crash into each other okay now einstein was very ambivalent about this whole thing he formulated actually a very complete mathematical framework and theory to describe gravity and he did this between 1915 and 1918 his original paper is on general relativity actually had some mistakes which he eventually uh corrected in a 1918 paper and in 1918 schwarzschild actually looked at einstein's equations and said look sitting in here sitting in the math is a type of star that has so much mass and such a small volume that even light cannot escape their gravitational pull and in those early years those were called dark stars if the word black hole hadn't been coined as yet and einstein did not like these uh dark stars at all and in fact he actually vacillated about whether gravitational waves even exist and in 1937 he actually uh um uh submitted a retraction saying gravitational waves don't exist and then in the same year he retracted the retraction so he was actually you know really pretty uh pretty torn about about whether they were there and then after that after that retraction of the of the retraction he never again published on gravitational waves to the end of his life so he had this relationship with them in his original 1918 16 paper rather he actually dismissed them as saying well they're so incredibly weak that they have no practical purpose whatsoever ever so that was the dismissal so he had a kind of an ambivalent relationship with them now doubts and controversy actually not just einsteins but the whole scientific communities started to subside off in the in the late 1950s and um and later but this is as an important lesson in in this little story which is in the end none of this matters unless you can ask is this what nature does you know so experiment and observation have the final say always and so evidence started to mount slowly but surely over over the last century so neutron stars were first proposed in 1934 as again as a theoretical object as the end product of an ordinary star as it collapses rotating neutron stars were which are called pulsars were proposed in 1967 and they were first observed in that same year the first black holes weren't observed until 1971 and in fact those first black holes also had a lot of debate around them and in fact there was a famous debate between stephen hawking and kip thorne that wasn't resolved until 1990 and the debate was that the bet was is this object which was called cygnus x1 the first black hole uh um discovered was it even a black hole and so the debate rage we didn't really have a definitive story about black holes now in 1974 house and taylor two two astronomers found a pulsar that was part of a binary system a pulsar is a neutron star but it's a kind of slightly special neutron star in that because of the very strong magnetic fields of of the of pulsars the light that they emit is not isotropic so if you look at our sun any side you look at it from it's emitting light pulsars don't do that pulsars have very beamed light so they're like lighthouses they're spinning about their own axis and every every time the pulsar beam comes across your line of sight you can register a light pulse and that's how they discovered the system hulson taylor and they found in fact that it was part of a binary it had another companion neutron star and what they did was they measured so here's the data they measured look at this this plot this plot goes from 1974 when the system was first observed and it's it's gone out to 2005 and on the vertical axis what it is is just the the the size of the orbit so here's a pair of neutron stars that's orbiting each other and indeed if they they should be the orbit should be shrinking they should be getting closer to each other as the um as they emit gravitational waves and as they get closer to each other the the period should be shrinking it takes less time for them to orbit each other and that's what the data shows that's the the dots the data points show that in fact the orbit does decay they are getting closer to each other and then the solid line in that in that curve is one of the one of the great triumphs of observational um you know astronomy as which is that they measured all the parameters of this binary neutron star system and then they asked general relativity is this the right answer for the rate at which the orbit should shrink if it's gravitational waves and the answer is exactly yes you can see that the line lies on the dot so this was considered a a great triumph of uh and and the first sort of indirect evidence for uh gravitational waves and it earned them the nobel prize in 1993. so now we have this this this mounting evidence now einstein's ambivalence was justified the first observational evidence for neutron stars and black holes did not come in his lifetime he died in 1955 and those things started to happen in the late 60s and early 70s but one of the most remarkable things about him and his theory in general relativity theory is the general relativity made theory made very firm predictions about gravity space-time and black holes so he didn't like black holes he was not clear what he thought of gravitational waves but what was inescapable was that his theory could predict them pretty well okay and so here's a movie in which i'll show you two black holes and you can see that they're orbiting each other and at the bottom you'll see a waveform that's basically the the way that space-time distorts as a function of time as these two black holes orbit each other and get closer and closer to each other the blue parts of the plane are just our sort of flat space time and the greener and yellower parts are where spacetime is very very curved by the black holes and what you see is that as the black holes orbit and lose energy and get closer to each other the gravitational their gravitational pull gets stronger and the space time around them gets more warped and in fact the movie will freeze at the moment that the horizons the the putative edges of these two black holes will touch and that's where we get the the largest amplitude of the wave if you look at the waveform at the bottom there it is you can see spacetime is horribly distorted and then these two black holes form into one larger black hole and the whole system sort of just kind of wobbles a little and then becomes quiescent and that's the process by which uh which these gravitational waves are are generated and then they travel off into the distant universe okay and this simulation is also an an and a solution of einstein's field equations for a pair of black holes that are merging now listen to the sound [Music] okay that is the sound of two neutron stars or black holes colliding so all that that sound is is we take a waveform like the one that's shown here uh and and we encode it onto a loudspeaker we might do some filtering to get it in the right into the right uh uh frequency band for that that's pleasing to our ears but that's called a chirp it starts off with a kind of a hum these two stars are far apart and they're just orbiting and then as they get closer and closer the hum gets louder and the pitch gets faster they're going their frequency increases because they're going faster around each other until they collide when you get the pop right so that's called the chirp and this was the the sort of now in this in this mid 60s and early 70s there was a bit a great deal of activity around this idea because neutron stars had binary neutron stars had been observed halsey taylor system was there a black hole had been observed and so professor kip thorne at caltech was one of the people who was sort of leading the effort to understand theoretically how you could use einstein's theory to predict these waveforms you know what would the signal from such systems look like because this was this was the work that he was doing back back then and has done ever since now let me just do a quick recap of gravitational waves so we can we know what we need to get to the next part which is how do we detect them all right so we know they're predicted by einstein's theory of general relativity they are ripples of space-time and what they do is they stretch and compress the space-time itself now what does that mean that simply means that if a gravitational wave goes through some region of space it makes that region of space stretch and shrink like so now the change in distance that these gravitational waves caused by the stretching and shrinking of space is proportional to the amplitude of the wave which we call h and it's also proportional to the size of the space-time region so here's a way to think about this oh my okay imagine you have a region of you have a ring of particles on a circle of radius with a l if a gravitational wave comes through it will cause that circle to become an ellipse and the amount by which it stretches the circle in one direction and shrinks it in the other is an amount delta l delta l is proportional to the original distance l and the amplitude of the wave now one of the things that that uh was also came out of the the work of of of kip thorn and others was you could you could actually put an amplitude on this wave if you take a pair of neutron stars and they're orbiting each other to at the very end of their orbit before they collide and you put it in a galaxy not too far from our own they would have an amplitude of 10 to the minus 21. so now we're armed with some numbers that are that actually are going to make us feel a bit depressed so you have this id you have the amount by which a space-time region changes if it has length l it will ch it will it will you'll see changes of delta l proportional to h times l so here's the numbers now the amplitude of the wave is 10 to the minus 21. imagine that wave went through me i'm an object of order a meter then my dimensions would change by 10 to the minus 21 meters it's an awfully small number it's actually so small that it's it's almost laughable right it's meters i mean so let me put a scale on that if you had an an atom an atom has about the size of 10 to the minus 10 meters if you look at the nucleus of the atom it's a it's a hundred thousand times smaller that has a size of about 10 to the minus 15 meters and this amount by which i would be changing as this gravitational wave went through me is a million times smaller than the nucleus of an atom so you pretty absurd okay so that doesn't mean we shouldn't try to detect these gravitational waves and so we do so let's think a little bit about how one might measure gravitational waves now there's a property of gravitational waves that i i i told you which is that as they go through a region of space time they shrink and stretch the space time so here is a simple concept for how you might do it imagine you had a laser and some distance away from that laser you had a mirror and it's a good mirror it reflects the laser light back at you and you had a good clock then all you do is you measure how long it took the light to travel to the laser and back to the mirror sorry and back to you if a gravitational wave went through that region of space-time that distance would change the light travel time would change and your clock would register a slightly different time done that's the concept that's the principle of the measurement now it turns out for even reasonably long distances between the laser and the mirror we do not have clocks that are good enough to make this measurement the clocks are just not precise enough by several orders of magnitude so even though the idea is is is simple and sound it doesn't work practically so in practicality what we do is we build an interferometer instead so you take the same laser but now instead of shining it at a single mirror you split the laser beam into two parts one goes north south and the other one goes east west and reflects off of two mirrors those light beams come back to the object in the center which is a beam splitter and you can measure the interference if a gravitational wave comes through that region of space-time one arm of the interferometer will get longer the other one will get shorter and the light will travel at different uh uh it will take different amounts of time to travel through now why is this measurement so much better than just using a clock well it's it's an it's an important principle in measurement if you're using a clock you need a clock that's absolutely good whereas here this is a relative measurement you can think of it as using one arm of the interferometer as a reference for the other arm then your clock doesn't have to be uh have as as as good of precision so that's good that's the principle now what do you need to do well you need to make mirrors that are very very still remember the gravitational wave is going to change those space time distances by a ridiculously small amount everything else on the planet wants to move the mirrors by more than the gravitational wave does so that's the first thing you have to do the second thing is so you have to do a lot of vibration isolation and thermal control now imagine that you did that and you did that really well that doesn't help you very much if you can't measure small distances and that's where the laser light comes in that's our meter stick so the laser light is the way that we probe the mirror positions that's what's telling us where the mirror is relative to uh to the laser and relative to the other mirrors okay now this is professor ray weiss and he was the one of the other heroes of the story of ligo he was the first person to think about a practical way to use these interferometers to uh to observe gravitational waves and this was this he was at mit and this was again in the late 60s and early 70s it's a very vibrant time for this because of the discovery of black holes and pulsars and binary neutron stars and so on so what he did was he he was at the time actually an atomic physicist and in 1960 the laser had just been invented so he knew lasers he actually liked them he worked with them and then he had this other idea which is well if you're stuck on you know the change in length delta l being the amplitude of the wave which is given by nature and the length of your of your detector then let's just make it long how long could you make it well he came up with the idea of making it four kilometers long so 4 000 meters and that's a nice number because that's kind of the longest path on the earth where a laser beam can travel in a straight line before you actually have to tunnel because of the curvature of the earth so that was that was the number he came up with and then if you make it long enough then instead of having to measure this you know 10 to the minus 21 meters over my distance you are you have to make a measurement of 10 to the minus 18 meters so that's only a thousand times smaller than a proton and he looked at that and he said not so bad we'll try it okay so that's how this thing was born he wrote up the complete theory for how the complete design for how this could be done uh in between 1968 and 1972. okay and now there was born a very bold experiment that's the laser interferometer gravitational wave observatory or ligo ligo comprises two detectors there's one in louisiana and one in washington state they're l-shaped and they're four kilometers long as as weiss originally had proposed and they're capable of measuring changes in in mirror separation at the level of 10 to the minus 18 and eventually 10 to the minus 19 meters okay so that's what they are uh a quick tour here's a ligo uh uh um detector this is the one in louisiana you can tell because it's in this in this in this leafy green um uh forest and you the uh in the center you have this the corner station of where the laser lives and then the going out in in you know for four kilometers in an l-shape are uh two beam tubes along which the laser light runs and reflects off of mirrors at the end and comes back here is what the the beam tube itself looks like so you can see that it's a it's pretty big it's a stainless steel tube about 1.2 meters in diameter running for four kilometers and it's covered by this uh by a concrete housing and that concrete housing actually has had some utility um so here what what happened was this is now at our washington observatory and this patrol car came flying over the dune and didn't notice that there was this four kilometer long barrier in the middle of the desert so um you know that was you know the driver i think had had a broken rib but there was not much more damage than that um then if you go into the observatory into say the central uh station where the laser is you'll see objects like these these are vacuum chambers every one of these holds about one mirror of the interferometer and they're pretty big if i were to stand beside one of these the top of my head would be just under that lowest row of viewports those little round circles over there so why so big well every mirror of the interferometer has to sit on a lot of vibration isolation and the vibration isolation systems look like objects like this so this you can see the the the uh the copper colored objects are springs and the springs are are loaded down by masses which is the stainless steel cylinders and this should remind you of things like shop shock absorbers in your car so this is a this is passive isolation where you simply rely on the properties of a spring mass system to give you isolation from from the motion that drives it and then you can also have active isolation systems where you measure the motion and you cancel it out now each mirror of the interferometer looks like something like this so the very bottom is a nice beautiful glass object it's about 35 centimeters in diameter so quite big weighs 40 kilograms and this mirror is hanging as a pendulum now it turns out that pendulums have pretty nice filters for motion if you take a pendulum and and you and you uh move it's the point where the string attaches and you look at what the plumb bob is doing at very low frequencies when you move the string the plumb bob will just go with it it's like a it's like a rigid body they're attached above the natural frequency of the pendulum if you move the the string very fast the plumb bob at the bottom will move hardly at all and that's a nice way for filtering because you can have a lot of motion here but your mirror at the bottom doesn't move very much and so that's and in this case it's these mirrors are actually hanging not just from one pendulum but from four layers of pendulum so if you go from the bottom mirror to the the the object above it that's the another stage of pendulum then you have this flat steel uh object that's another stage of pendulum and finally you have the flexures on the top which is another stage of pendulum and every stage gives you more and more filtering of this motion so that's how we do the vibration isolation there's that's the laser it's uh the ligo uses a pretty a pretty powerful laser it's a 200 watt laser it's an in near infrared laser and to just give you a comparison if you take a typical laser pointer like the ones that in in in these gizmos they should emit about a milliwatt so when you have a 200 watt laser that's a lot of laser power okay and this is the control room from which the whole experiment gets gets controlled now the sensitivity of ligo is is measured in in plots like these on the on this uh plot in the horizontal axis you see frequency notice something very important about the frequency it goes from 10 hertz to 10 kilohertz which is kind of the human audio band and so these instruments are most sensitive it's at the same frequencies that that our ears work now they weren't designed because we want to hear things they were designed because it turns out that that's the frequencies at which we can detect colliding neutron stars and black holes so so that's the and these curves over here ligo is built in phases the red curve is the design sensitivity of the first phase of ligo it's the curve that that ray weiss had had proposed back in 1972 very close to that and then the blue curve and the green curve are the sensitivity of ligo in 2007 and in 2010. now with that first phase of ligo we didn't see anything we looked out into the sky and we saw no signals that correspond to cosmic objects and that drove us to design and build a next phase of ligo called advanced ligo and that's given by this black curve here and so in this black curve is basically the target sensitivity of this second phase of ligo which we call advanced ligo and to go from the original initial ligo to the advanced ligo curve we essentially had to do better seismic isolation at those very low frequencies where the vibrations of the earth were bothering us so that's where where the improvements there were were gained now intermediate frequencies in the region between sort of 50 and 150 hertz we had to use better materials because we were limited by the thermal vibrations of of the the mirrors and the wires from which they were hanging and then finally at the highest frequencies we had to use more laser power and when you increase the laser power you actually can improve the sensitivity of the measurement and so that's how those those things were were done now i just want to remind everybody that weiss calculated this red curve back in 72 and notice when it when we actually achieved that curve i'm sorry the red curve in in 1972 we achieved those curves in 2007 and then a little bit better in 2010. so this has been a very long process okay now why do we want these curves to do this to get better every time you make the curve lower you've done a better you you've got a more sensitive instrument the reason is it's just like if your telescope improves now most of you will recognize that this is this blobby looking object is saturn now if you get a better telescope you might see saturn like this now if you get a telescope that flies by saturn on a satellite you would see this and that's really the game we're playing here too we're simply just making our sense our instruments more and more sensitive okay now ligo is not the only gravitational wave detectors we have we all have a global network of detectors there's two in the u.s which i've already appointed to you which are in washington state and louisiana they're four kilometers long then there's also a a detector in italy which is a french italian collaboration it's a three kilometer long detector called virgo there's a 600 meter long detector in germany a british german collaboration and then in japan under construction not yet operational is kagura three kilometers long and recently approved is a uh is a proposal to put a a ligo four kilometer long ligo detector in india and then finally there's also proposals to have satellite-based space detectors and that's called lisa which is the laser interferometer space antenna and lisa can have arm lengths that are 5 million kilometers long so because i mean real estate in space is abundant and cheap so now we come to the event that we detected so i've told you what gravitational waves are i've told you how we can go about measuring them with interferometers but that have lots of good vibration isolation and and other important things and now i want to tell you what happened on september 14 2015. the advanced ligo detectors both the ones in louisiana and washington were on the air and they each registered a signal and it turned out to be on a signal from a binary uh a black hole collision so here's what happened so here's an animation here's the two black holes they're orbiting each other and they collided and gravitational waves were emitted and those gravitational waves made a long journey across the universe and eventually passed through our very own planet earth and as they came through the earth they caused small vibrations now you'll see the earth jiggling like jello here but please pay attention this is this effect that's greatly greatly greatly exaggerated the waves hit our louisiana observatory first they were coming from the south and then seven milliseconds later they we detected it in washington so the waves came from the south deposited teeny tiny amount of energy into the interferometer uh into the detector and we detected that in louisiana seven milliseconds later in washington and that signal was detected using the interferometer so here you see the a little simulation of how that was done there's a laser light and the laser light at the detector itself becomes brighter or darker depending on the relative lengths of the two paths that the laser light takes and that's simply because it's a question of whether the peaks of the uh and troughs of the of the two light beams line up with each other or they don't then you get either constructive or destructive interference and that was how the signal was detected as brighter or dimmer so that's the the principle by which the gravitational wave signal is is converted to an optical signal all right and this is what the signal looked like these two signals have been so the the the washington which is called the hanford data signal was time shifted by seven milliseconds six point nine milliseconds to be precise so that they line up with each other both detectors saw the signals and you can see that it actually has some of the properties that that i have described in the waveforms before the maximum amplitude of the signal the strain or the amplitude of the wave was 10 to the minus 21 and that corresponds to a change in distance or displacement of our mirrors of about four atometers or four times ten to the minus eighteen meters okay this is what it sounds like if we encode it into sound that's that's the original sound and that's just sped up by a factor of two so our ears can register it better and what you can see is it's it's a it's a classic chirp starts at low frequency low amplitude frequency grows amplitude glows grows and then when the two two objects collide you you hurt you're a thud or a thump or or or a a clap and so that was what was what what uh those signals look like now what does this signal tell us about the source well here is the the actual reconstruction of the signal and from the frequency and the way the frequency changes as a function of time we can extract the masses of the two objects that were were were orbiting each other from the amplitude and the way that the amplitude evolves we can actually extract how far the the objects were and we can also extract the inclination angle that simply means the plane of the orbit was it along our line of sight or directly normal or something in between and then at the very end where you see the signal sort of uh decay from that last part of the waveform uh from the frequency and the time it takes for it to decay we can tell what the mass and spin of the final black hole was so there's a lot of information encoded in in these in these waveforms and remarkably all of that information is embedded there by einstein's field equations it's all there in in the theory so now we are ready to reveal the story of the of the two black holes that that were observed once upon a time 1.3 billion years ago there were two black holes they were rather big black holes actually what we learned was they were about 30 times more massive than our own sun which was a bit of a surprise we didn't expect this class of stellar mass black holes to be that heavy and in fact we don't really know how nature might form them the two black holes danced in orbit about each other and they emitted gravitational waves exactly as einstein instructed them to this made them get ever closer to each other and orbit ever faster and at the moment that they collided they were actually moving at about half the speed of light so you've got to wrap your head around this these are objects that are 30 times more massive than our sun they're about 150 kilometers apart and they're whipping about each other at half the speed of light so it's an extremely relativistic system the black holes merged and they formed a bigger black hole and of course they gave off the spectacular storm of gravitational waves which we recorded 1.3 billion years later in our detector now the newly formed black hole was not as massive as its parents the newly formed black hole was missing three solar masses of of mass or energy and this tells us that three times the mass of our sun was converted into gravitational wave energy now what that translates into is that for that brief instant for for those few hundred milliseconds as these two black holes were colliding more energy was released than all the shining stars emit in the universe so it's a very very energetic uh explosion or collision if you will now even though it was so energetic the measurement we made was an atmeter it was a few atometers and that's the the amazing thing that all this energy is given off but very little of the deposits in our detector they did not live happily ever after we learned that too but i do want to point out something that the two parent black holes died but a new black hole was formed they gave birth to a a bigger black hole and so the cycle of of black hole life continues okay now why all the excitement so i'm going to wrap up and try to give you some perspective on why has this been why was all those headlines around so the the first thing was this was the first direct detection of gravitational waves einstein predicted them a hundred years earlier they were you know he himself was unsure whether there they were not here we were directly seeing the ripples of space-time this is also the first direct observation of a binary black hole system we've never seen black holes crash into each other like this before and we saw this in real time those bumps and wiggles in our signal were the motion of two black holes colliding it was also the first test of einstein's general relativity theory in the strong field limit now what does that mean strong field just simply means in regions of space where gravity is very strong why is that important so black holes would be such a space why is that so important let me remind you of newton's theory of gravity there were newton's theory was extremely successful but it had a failure that was known you know quite quickly uh and a few you know quite a while before einstein came along newton's theory could not correctly uh explain the orbit of mercury the planet mercury what's special about mercury it's closest to the sun so it feels it's in a region of of space-time in our own solar system where gravity is the strongest and newton's theory was failing there it was one of the early triumphs of general relativity was that einstein explained the orbit of mercury so there's no reason for us to believe that general relativity theory is the correct theory when gravity gets very strong we've never been able to test that before we were with this observation and so far so good it's working and then finally for me personally as someone who's really worked all of my career on the the gravitational wave instruments themselves the machine works and it works with atmeter precision so this is another amazing thing about precision measurement so that's all why it was also exciting but the real excitement is that we have never before looked out into the universe and seen the universe with gravitational waves and so it's inevitable that we're going to see new things every time we've turned on a new type of sense whether it's a new color of light looking it uh out in the universe we've discovered things we didn't know were there and that's sure to happen here too in in in in time okay now this didn't just excite um you know scientists it also had some some effect in uh in the in the mainstream so this is a photograph that was taken in a new york city subway in march so shortly after the announcement and it claims that it's easier to detect gravitational waves than to find an apartment in new york city with a good closet now i might argue with that but you know new yorkers maybe would argue back okay but it was kind of remarkable to see something like this it has sort of touched you know mainstream society and culture as well so what's next well we're working on improving the advanced ligo detectors and walking our way down towards the final design sensitivity and beyond and that's what those curves show the green curve shows what we had going in 2010 the red curve shows what the sensitivity of the instruments during this observing run in in 2015 when we made the first detection the blue curve shows where we're heading that's the ultimate design sensitivity of advanced ligo and then the science curve shows that we're already thinking of ideas to do better than that sensitivity we're also waiting for these partner observatories of the global network to come on the air and that surely we hope will give us more sources new sources unknown sources to detect so i'm going to leave you with this one final thought 400 years ago galileo pointed the first telescope into the sky it was a one and a half inch mirror diameter a refractor and he observed that the moon has craters and mountains he observed the phases of venus he observed the the the rings of saturn and it took us almost 300 years to go from those early one-inch telescopes to a 100-inch telescope so that is mount wilson 100-inch telescope it took another 100 years to put 100-inch telescope into space and now we are building these 25-meter class telescopes here on the earth at the same time as this was going on we also learned to to look out into the sky with colors of light that we ourselves can't see we've put out infrared telescopes we've put out gamma ray telescopes we've put out x-ray telescopes we put out radio telescopes and all of those things have revealed to us uh you know all that our universe is made of and the gravitational wave sky is the same the gravitational waves span the same 20 orders of magnitude and frequency or wavelength whichever you like to think about and we've just touched the very first observations with the terrestrial detectors which are the fastest systems these neutron stars are black holes that are whipping about each other and colliding in time we should see supernovae this way if you go to those are happening sort of in the 100 hertz region that's where the best sensitivity are in sort of the millihertz region you can think about space detectors like lisa and those will observe different kinds of objects those will observe super massive black holes that are are at the centers of galaxies for example if you get to the nano hertz region you start to you can use pulsar timing arrays to observe gravitational waves in those frequency range and then if you get to the time scale of the age of the universe you can actually observe gravitational waves with these extremely long wavelengths by measuring the polarization of the cosmic microwave background and these are all techniques that are about to explode you know we've started off with the terrestrial detectors in the next 20 years these methods will all start filling our information about what the gravitational wave sky looks like so i hope you're excited because this is a very exciting time thank you

Channel: World Science U

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Keywords: gravitational waves, LIGO, Laser Interferometer Gravitational-Wave Observatory, detection of a gravitational wave, laser interferometry, What are Gravitational waves?, groundbreaking discovery, speed of light, Albert Einstein, spacetime, Gravity wave, World Science U, WSU, World, Science, Festival, Brian Greene, Nergis Mavalvala

Id: YJb01o7LlLA

Length: 56min 56sec (3416 seconds)

Published: Tue Aug 18 2020