okay some last thoughts about ligo and the observ observations of uh uh gravitational waves I want to talk about one particular Source gravitational wave 170817 that's just the date it occurred on 2017 August the 17th and uh just a little bit about plans for a future uh Observatory designed to detect the mergers of super massive black so this is where we left off here's the first observation of a merging Black Hole uh made by the ligo observatories in 2015 um and uh from this data you can determine what the mass of the two objects is what the final mass of the combined black hole is uh and uh therefore how much mass was lost between the two initial black holes and the final Black Hole uh and in this case it's about three solar masses have disappeared in this process where did they go uh well that's the energy carried away by the gravitational waves has to come from somewhere and so from eals mc^2 the uh merged black hole is actually less massive than the black holes uh the two black holes that started out you can figure out how much total energy that is uh turns out to be 5 x 10 47 jewles and most of it is emitted in the very last orbit in the final orbit before the merger which lasts about one millisecond so during that orbit the average Luminosity is uh 10 to the three times greater than that because most of this emission is is emitted in 1 1000th of a second that's 5 time 10 50 Watts not quite at the maximum but of course it's growing all the way through that millisecond so by the time it gets to the merger moment it's really quite close uh to the expected uh amount of uh Luminosity that is supposed to be emitted by this uh by objects of this kind and in that moment it outshone uh the whole rest of the universe really quite a remarkable thing uh so remarkable that the very next year they awarded the Nobel Prize in physics for this most of the time it waits five years 10 years 20 years uh in Chandra Sear's case 50 years uh before the community felt confident enough in the result to award the Nobel Prize uh the ligo result was so compelling so convincing and so obviously important uh that they awarded the prize the very next year to these three individuals whose biographies Cor respond approximately uh to uh the cases I gave you in the uh quiz on Wednesday uh Riner Weiss had the original idea that uh laser interferometry might be a good way to detect gravitational waves and worked his whole career on improving uh the technology uh Barry barish was the leader of the team for 15 crucial years uh when it went from a sort of wild idea to an actual team of thousands of people with hundreds of millions of dollars to actually produce this uh remarkable uh piece of equipment and Kip Thorne here was the theorist in charge uh who worked out exactly what would happen he and his students uh when uh black holes collided and wrote computer programs uh that generated detailed simulations of what you ought to see which indeed corresponded to uh what was observed uh and so uh this happened as I say uh really very quickly uh and since then uh the discoveries have gone on a pace uh there was uh another one in December of that year and then one in January uh of the following they shut down for about a year uh in order to make a set of improvements and after that the discoveries came even faster uh and there was a whole uh series of things discovered in 2017 then they shut down for another year uh and now they're back uh more sensitive than ever this is the most recent uh uh representation of the Lio catalog that I could find look at them all all of these black hole mergers have been observed and there are now two neutron star observers uh uh mergers that have been observed as well uh so I want to talk a little bit about this event uh uh that took place August 1 that was observed August 17 2017 uh a double neutron star merger uh because that has some remarkable astronomical consequences so the first thing to say about this is that by 2017 they had a third Observatory online in Italy called the Vergo Observatory and so there were two ligo observatories in the United States plus the Virgo observatory in in Italy and because they had three they could localize exactly where the gravitational waves were coming from much more accurately than had been done before see in these cases you know there was a whole stripe across the entire sky where the thing could have been but by the time they saw this one and another one a few days before which was a couple of black holes uh it was much more quick much more localized uh you knew what part of the sky it came from the other interesting thing that happened essentially right away uh was that um a burst of gamma rays this is El very high energy electromagnetic radiation was observed essentially simultaneously from roughly the same part of the sky this is observed by a NASA mission called phy which is an orbiting gamma ray Observatory and uh here is uh what they observed in the gravitational waves this is the frequency as a function of time and it goes from below 100 up to about 400 that's 400 cycles per second over the course of six seconds or so and then a couple of seconds later there's this little burst of gamma rays uh now so the nice thing about this is that uh this this takes about this is about 10 seconds of time here uh and so you can uh uh show it as a kind of 10-c long movie furthermore uh the gravitational wave frequency if you translate it into um sound waves is in the range of frequencies the human ear can hear uh those of you who are musicians will know about a440 tuning what that means is that the a has a frequency of 440 Cycles a second uh that would be right up at the top of this plot and so you can hear uh if you were to translate these frequencies and the uh and the volume that they represent you can hear this at as uh an actual uh sound so they've made a a a kind of realtime uh simulation of what this sounds like they also put a little ding at the end when the uh gamma rays show up so uh let me go to the YouTube here and uh uh play that for you hang on just a

second all right see what we can so here is the ligo and the uh fairy results move myself

[Music]

over wait for

it

I think that's kind of cool the way you can translate this uh into sound and this is an astronomical observation that you can actually hear in real time um so this uh uh has a lot of consequences first of all as I said You observe this burst of gamma rays at almost precisely the same time two seconds after the uh peak of the the gravitational waves uh and um another thing you can that was quickly discovered is that there was Optical light coming from this thing as well since they knew where it was in the sky they looked with telescopes on ordinary optical telescopes a day later here's a little Galaxy in that same part of the sky and when they looked one day later on August the 18th they saw uh an additional source that hadn't been there before in the outskirts of this galaxy so they could see Optical light as well as gamma rays uh in fact they could see electromagnetic radiation in every possible wavelength radio infrared ultraviolet x-rays gamma rays and in the very first week uh uh uh when this was announced 85 different papers were published about observations and interpreting interpretations of this object and when they finally put all the data together in one paper uh that paper had over 4,000 authors essentially every major observatory in space on the ground uh uh observed this particular object um so what was it why was this so interesting why was everybody so excited about it first of all the gravitational waves themselves indicated that the masses of these things were appropriate masses for two neutron stars between about 1.2 and 1.6 times the mass of the sun

bursts of Gamay short second long bursts of gamas had been previously observed this was a known phenomenon and had already it had already been suggested that these were created by neutron star mergers and the idea was that uh these things would merge together and they would uh for a brief amount of time create a black hole in the Middle with an accretion disc of neutron matter around it uh and as that Neutron matter sort of spiraled into the to the Black Hole uh an enormous uh jet of gamma rays would be emitted uh from the pole sort of analogous to the radio waves uh that are emitted in x-ray binaries so this was already uh a theory out there and of course now that the gamma the gravitational waves indicated two neutron stars were playing a role here this confirmed what people had already suggested the short Gamay bursts were on the physics side uh what this demonstrates is that gravity waves really do travel at the same speed as light Einstein predicted this from the basic theory in 1916 and here we have an object which turns out to be 130 million light years away we could tell because we know what galaxy it CES from and there are various ways you can measure the distances to galaxies and uh it emits gamma rays and gravitational waves and those arrive within two seconds of each other which means that the G the gravity waves and the gamma rays are moving at the same speed to within two seconds divided by 130 million years they've traveled for 130 million years they arrive within two seconds of each other these are things that are going at the same speed so this is a dramatic confirmation of one of the original predictions of Einstein as he thought of uh general relativity now they don't arrive at precisely the same time there is this 2cond delay it's actually around 1.7 seconds uh and this again was already predicted because it takes a little while for the black hole to form and the disc around it to start to accrete uh and uh in so doing generate this uh beam of gamma ray so that too uh was understood from prediction that had already been made about what these kinds of things might look like um the ultraviolet Optical and infrared light in this system which was tracked very carefully uh comes from material that starts out in this in this disc but doesn't make it onto the black hole and get spun out because of the centrifugal force instead there's so much energy around that uh uh all of this material can actually build up to heavy elements heavier than iron you'll remember the problem with he elements heavier than iron is that they generate energy when they split apart so to create them from smaller from lighter elements you have to put energy in uh and so but there's a huge amount of energy floating around in this event and so this is how you create the heavy elements the the trans feric elements the elements heavier than iron uh and uh there's predictions of how much of what kind of elements in this particular case there's a lot of neutron Rich elements remember this is made out of two neutron stars some of the neutrons turned back into protons and electrons but there's a lot of neutrons floating around the place so you tend to make neutron-rich isotopes of elements these are radioactive and the radioactive decay shows up in the x-rays That's What You observe when you look at this with an x-ray telescope You observe it over many months as the radioactive decay from Neutron Rich Isotopes uh and you create lots and lots of he heavy elements in the right proportion to each other to explain the observed abundances of the heavy elements that you see in stars and on the earth you know how much rubidium is there how much gold is there how much lead is there which elements are there a lot of which elements are there not a lot of and it turns out that the theory of how uh uh the this excess neutron star matter recombines to form ordinary elements as it's ejected away from the neutron star uh uh and as observed through the X-ray x-rays from the radioactive decay of certain isotopes of this and confirmed by that explains what kinds of heavy elements are observed in the universe and and there's a lot of it from this single event the predic prediction and expectation is that over a whole earth mass of gold for example was predicted gold is a fairly common uh heavy element and uh this uh single event uh is supposed to have produced over one Earth mass of gold and it spews it out into the interstellar medium uh which then creates the next generation of stars and planets and that's why plan Planet uh that are created uh at some point actually have gold in them and also silver and lead and all the other heavy elements that's why they have them at all so this is a great object first of all it demonstrates a basic prediction of Relativity theory that gravitational waves travel at the speed of light second it confirms a strong hypothesis from relativistic astrophysics that short gamma ray bursts a phenomenon which had been observed before are in fact merging neutron stars finally it solves a key problem in Galactic astronomy how do you explain the distribution of different heavy elements uh and uh so with a single object uh it does all three of these different kinds of Science and I think that's worth 4,000 astronomers spending a certain amount of time uh establishing so where are we going with gravitational wave astronomy uh we're going to get detailed information on colliding compact objects they're going to be lots and lots and lots of these things discovered uh there'll be hundreds of them within a few years there are some other sources as the uh uh instruments get more sensitive that you expect to see uh Galactic white dwarf systems for example these are systems of orbiting white dwarfs or the white dwarf and the neutron star that I talked about before uh that are in our own Galaxy they don't produce as much gravitational wave radiation uh as the mergers but they're much much much closer you ought to be able at some point to start picking them up uh nearby supern noi these are not periodic but uh one sort of great uh uh implosion of matter will generate a burst of waves you might be able to see those uh you might be able to see the remnants of primordial black holes black holes created not from stars and galaxies but in the earliest stages of the universe uh they are hypothesized to exist there's at the moment no evidence of them whatsoever this is the kind of thing we might discover from gravitational waves and perhaps most importantly like any other new kind of astronomy like radio astronomy or x-ray astronomy or anything else probably the most interesting sources of gravitational waves will be the ones that we don't expect that we didn't predict to happen uh and that suddenly turn up uh and turned out to indicate some totally new kind of phenomena so we have that to look forward to as I said before uh gravitational wave astronomy is now in about the same situation that x-ray astronomy and radio astronomy were in the 1960s uh it works we've seen celestial objects we've identified the first celestial objects and uh all the new phenoma uh that are likely to show up in any new uh kind of radiation we observed uh are still to come in the future so we have that to look forward to there is one major prediction that uh uh uh is going to be quite important which is that we should also see colliding super massive black holes remember the way you build the active galaxies is you merge you do successive mergers of increasingly massive black holes all the way up to a billion solar masses uh and so these should also be observable uh the thing is that because they're so much more massive it takes them more time to go around in their orbit right before they touch so the frequency is much lower instead of a 100 times per second it's a 100 or more seconds per orbit uh so the frequency is much lower and at those frequencies 100 seconds a thousand seconds uh you can't have an observatory on the earth because the seismic activity of the Earth the sort of General sort of minor earthquak that are always taking place uh would drown out they they those seismic activities also take place at these hundreds of seconds uh time scales and they would totally drown out anything you could see from space So the plan is to put a laser interferometer uh that would detect gravitational waves in space and here's the configuration this is the sun the Earth and the moon and you put three of these things uh in a little triangle you want three so that you can localize the event um at a distance of some number of millions of kilometers from each other and you sign little lasers back and forth which keep track of exactly how far apart these things are and as the gravitational waves roll over them uh you see changes in the separation of the satellite so this is a thing called Lisa the laser interfer ometry space antenna there are detailed plans for this uh and we're hoping that something along these lines will be launched in the 2030s and so this is a look forward uh to uh what we hope to be able to get out of uh gravitational wave astrophysics uh 10 to 20 years from now so that's where we're going uh and it's I think one of the uh most exciting new areas in astrophysics and will remain so uh for decades to

come