 Welcome to the translation. So this is our last astronomy talk after we had a lot about pictures. And now we are doing it differently with sound, with gravitational waves. And someone who is really an expert is Benjamin Knispel. And his favorite stars are neutron stars. And he already discovered a few of them. And he has been doing research in gravitational waves. And you are the best to tell us how to do that, Benjamin. Thank you very much for the introduction. Moin from Hannover. And I will give you an overview of gravitational wave astronomy, who has read the abstract, abstract knows that it's a current topic. It started about six years ago. And until then astronomy was done with only, in quotes, with telescopes. So we looked into the universe, captured electromagnetic waves and other parts. And it was really, so to speak, a silent movie. And what we do with gravitational waves is we add sound to astronomy or to give us sound where we cannot see anything. And what we can do with that, I want to explain to you a little bit and show you how these instruments work and what we can learn about the universe from that. If it's about gravitational waves, so maybe I heard about that, but what is it really? And of course, it's an important point that we understand that in the beginning. Gravitational waves are vibrations in space and time and a consequence from the general theory of relativity. So the general relativity, you can play, explain it with food and jello. And of course, the green jello, because it's the only delicious one. And what we know from the theory of relativity from Einstein from 1915 is that space and time work a bit like this jello, like space and time. And in every day we experience them as static things. The space seems to remain the same, but in very big scales and isn't so. And in 1915 Einstein figured that out with his theory of relativity and we really have to look at four-dimensional space-time. And you cannot really picture that, but in three dimensions you can imagine it's a bit like this jello. In space and time or space-time, this changes if there are masses. With our jello, it's quite obvious. If I add an apple to it, put an apple on it, it changes the shape of this jello around the apple. And that's exactly what the theory of relativity said. Masses change space and time in their vicinity. Or mathematically speaking, it changes the geometry of space-time. So space and time are curved. With jello, it's quite obvious that something is curved. But what we see as a consequence is what we used to call gravity because everything follows the shortest path in space-time and if the geometry changes, then the shortest wave is a different one. And there are different shortest waves when masses are there or if they're not there, and that's just simple Einsteinian gravity. And what also happens, as Einstein found out, if masses are accelerating then the space-time itself is also vibrating. And if masses are moving accelerated, then the entire space-time starts vibrating and these vibrations move at the speed of light through space and time and we call them gravitational waves. These gravitational waves give us a new sense for experiencing the universe and they are a bit like listening in the universe like sound waves in the air. Gravitational waves are vibrations of space-time caused by accelerating masses and they expand at the speed of light and these vibrations in space and time they can really move through space without any limitations and every mass that is accelerating we will see that it creates that and we will see that we need big masses in fact to be able to see that but if masses move accelerated then it creates vibrations in space and time and without any obstructions they come here and arrive here and we could in theory detect them if they come from inside the Earth and we can really detect them here. As I said, accelerated masses and if I shake my fist I create gravitational waves but in practice, signals that we can detect can be only created by very big masses so what are the kinds of gravitational waves that we can see? One, there are pairs of black holes, binary black hole systems it's a very small, very massive object. If we have two of them they circle around each other and this is an accelerated movement and they create gravitational waves and it could be a binary system of neutron stars and I explained a little bit about neutron stars or other very compact objects and they can also create gravitational waves or we can have a black hole and a neutron star circling each other this also creates gravitational waves and all these three kinds in the upper row we have seen some of these and detected these and they can be neutron stars that are perfectly round and rotate around their own axis and supernova explosions in our own galaxies we know that they create gravitational waves but we haven't seen them yet maybe they are too rare that we could see them regularly and then there are the three question marks there can be other sources of which we don't know anything and that would be really cool if we could see a signal and this is real and we have really detected it but we don't know what it is so that's where science is really exciting and now I talk about the main actors here which will occur in our talk one is neutron stars, neutron stars and black holes create in these stellar explosions if stars that are heavier than our sun at the end of their life collapse in their core they create a heavy ion core and it becomes so dense that matter cannot remain stable anymore and collapses further and if it's big enough it creates a neutron star and you can see this artist's impression here is the blue-white sphere comparing it to the city of Hannover there isn't really a neutron star of course because then the earth would have gone away but the neutron star is about the size of Hannover it's about 1.5 times as heavy as the sun or maybe twice as heavy as the sun and it is in a very small volume it's only 20 kilometers in diameter which means the density of the neutron star is as dense as an atomic nucleus and because all the empty space in normal matter it has been squished together and that happens when a neutron star is created and we know that some of them rotate up to 700 times per second this is much faster than a mixer or a blender in our kitchen so because they have already extreme magnetic fields so I really love these objects, they are among my favorite stars because there is very matter in a very extreme state and our universe gives us these things that really we can't create in the lab and if this neutron star, it's stable but if more matter is added to it from another star in the vicinity and then there's much more then you cannot create physical pressure from the inside so the matter collapses even further and according to this theory of relativity it collapses to a single point that is called a singularity from a very simple black hole that is not rotating a black hole has very few properties it has the properties in one point and then there's an event horizon that's the distance beyond which you cannot escape anymore you need it to go faster than the speed of light to escape and the event horizon separates the universe into two areas outside and inside and if I'm beyond the event horizon I cannot escape because everything that goes into it vanishes a black hole has a size this is the so-called Schwarzschild radius it's one of the two formulae in this talk this R is the Schwarzschild radius can be calculated from the gravitational constant G and the speed of light squared and the mass of the black hole so the heavier the bigger but they only have really three properties they have a mass if I know the mass then I know a lot about the black hole and they have a spin, an angular momentum they rotate around their own axis no matter the falls into it may rotate and in theory they have an electric charge but in nature that doesn't really happen because the star that collapses into a black hole is electrically neutral so black holes are really boring you have only three numbers mass, charge and spin and whatever falls into the black hole and in the end you can describe it with only three numbers these are the numbers that we the things that we can observe and we can really measure things about these gravitational waves above black holes but black holes are black we cannot see them and we don't have a lot of means to detect them except by gravitational waves so neutron stars are very small and even if they are hot they are not very bright maybe I just can't see it and I cannot really find out a lot of them just from the light alone so now back to our gravitational wave and back to the shaking gelol so what are the effects that this will have obviously it doesn't have anything to do with the taste but rather with the physical attributes and properties that we can take a look at so we wanna take a look at the actual effects and the stuff that they do so imagine you're in a vacuum and you're free falling and you construct a circle made out of small masses and it's gonna levitate or fall in that circular form and if a wave goes perpendicular to the screen so perpendicular to the thing then it would this is an exaggerated image of what would happen it would extend and be squished in horizontal and vertical directions this is the effect that the gravitational effects have and this is the thing that we have to measure so these dimensions are obviously exaggerated enormously and in general these changes in the length are a very a very miniscule effect but the longer the the areas that I observed the bigger the longitudinal change will be so the relative change of lengths for the strongest gravitational waves that we can expect is 10 to the minus 21 so of any given length a billion of a thousand and this means that the orbit of the earth would change by one atom so this means that we would need to measure the distance of the earth the circle traveling around the sun up to one atom exact which is obviously ludicrous so this is for me that it will look at and it has it's the so-called quadrupole momentum on the right side and what makes it this constant or this this coefficient so small is the one over the speed of life to the fourth which is a very large number so whatever you're gonna have in here is gonna be tiny so we need an enormous amount of energy in order to have any noticeable distance and it also changes with the distance from the source the further I am away and the harder to detect so what we need are these so-called laser interferometers and send the laser through the divider and it's gonna be reflected by a mirror on both of these beams and it goes back into wider and then it's going to be overlaid again and depending on the transition of the phase or in the change of the phase due to the different lengths of the axis I can see if they extinguish themselves or I get added back up basically measuring the length of the distance that I've run by the change in the luminosity of the laser so of these laser interferometers there are pretty much five around the whole world that are being used in use right now there's one that is underground so I don't have a pretty photo like this they all have the same type of construction they just have a very long running or pipe or distance that the light can run along and then go back so the Geo 600 is the one on the left on the top that's the one that our institute is running it's in Germany itself of Hanover the focus is developing the technology it's always been like that because we are the smallest of these types of installations with only 2.2 kilometers. The next one is next to Pisa so remember the longer it is or the bigger it is the more sensitive the whole construction is the better I can measure and the Virgo is in Japan and the LIGO one is in Livingston so to give you a short idea of what they will measure the relative change in length is the 10 to the minus 21 you can always get it slower or you can get it lower if you look at something smaller so the total change of length is 10 to the 18 minus 18 it's a small fraction of an atom so obviously that's how to measure and we need a lot of technical tricks in order to see something but the question is how can we achieve an even higher sensitivity and at the very end these installations that we have are currently very sensitive to frequencies in the audio range so I can actually legitimately claim that this is somewhat similar to a microphone for a gravitational wave I can hear them in the truest sense of the word I can actually hear what they sound like these detectors now listen to the cosmos and they're obviously very sensitive this is the one example these are the spectra that we measure one of the last campaigns of our measurement that was prematurely ended what you can see is there is a frequency of the this is a fairly chunk of the audio spectrum and you look at the blue and the red ones those at once on the bottom on the y-axis you can see the density of a gravitational wave and the higher up you go the deeper is the we need to get this the curve down because the higher up you are the more you get background noise and the strongest gravitational waves that we can expect would roughly be at the level or below of that green line and the frequencies commonly in the medium frequency ranges so about the 1000 Hz range you can see there is a pretty decent distance at that frequency between the green line and the measured spectrum so there is something you can measure decent signal to noise ratio the campaign is a lot of measurements with some breaks in between during which we try to improve the detectors and the measurements and we have started measuring 2015 we have done three campaigns since then and the last one was ending in March this year so we have measured a lot and the question is what have we measured and learned from this is the masses of the stellar graveyard those are the remains of some stars that exploded and that we measured these are all the ones that we measured so the the distance between the axes the heaviest ones in the middle are over 100 times the mass of the sun there is a smaller dot on the bottom that is about 60 or 80 masses of the sun those are two black holes that are orbiting each other and are merging into one so in the middle at around 60 solar masses about that is 100 and they merge to one of about 160 solar masses this is a typical signal that we can see of these merging black holes we have seen 90 so we have lots of these merging black holes that is the main objects that we see and you can see orange spheres there that is lower masses, they are the so-called neutron stars they have just maybe about two solar masses the neutron star merges merging to something, probably a black hole but we don't know exactly and there is also somewhere there were merges of neutron stars and black holes and the red and blue are black holes that we have seen directly, the black and yellow ones and now I will summarize shortly so Einstein was wrong because during his lifetime he doubted that his solution to the relativity theory of relativity, that black holes would exist in nature, he wrote papers and he said no, the nature will not allow the creation of these objects but we see things that behave like black holes so he wasn't right, but of course on the other hand he was right because his theory described gravitational waves and we know that his theory predicted the gravitational waves exactly which is a bit boring because one was hoping that one might something new but the properties of black holes can be measured directly, for example the mass and the spin in some cases and we can see maybe what's the typical population of black holes spinning, what are the relations of the masses and this is what we know about the entire population of black holes and how that works I want to show a few examples about that and the signals have these names like GW 150914 and that's just the name of the gravitational waves from the year 2015 in the ninth months and in the 14th day before it was found on the 14th of September 2015 was the very first signal that we detected and what the detectors really measured this is only very little adjusted and edited data we can see that was about 0.2 seconds of data was read from one LIGO detector and on the right side in blue from the other LIGO detector and the y-axis shows the strength of the signal in the strain in units of 10 to the minus 21 at 0.3 seconds this wave starts developing and it rises in amplitude and in frequency to about 0.43 seconds and then it stops so if you listen to that it works and this is the typical sound from merging objects they circle each other and get closer and closer and this gravitational wave is emitted and then they merge to a single object that doesn't emit gravitational waves anymore and if you get an identical signal of both detectors and the red wave has been time shifted and mirrored and if they get the same you can see that it's probably a real signal in this wave you can deduce what had happened and we know that two black holes of about 30 solar masses were merging and we can demonstrate that and there's an online tool where you can test it you can for example what the sensitivity is and you can make change the data the input parameters and you can change the sliders here on the left is the total mass and on the right is the distance and if you think you have found a good match to these two curves and then you can see the total mass around 64 solar masses and sensitivity is 1.3 billion light years and you can find a good match of these two curves of course we have some noise from the detectors there and there's a discrepancy between the theoretical and the measured curve but you can very simply detect the properties and the spin may still change the curves and so on but you might get the impression that nothing much happens here because the signal is so tiny it's 10 to the minus 21 so one thousandth of an atomic nucleus it's a shift in length but we can look at the energy of this event and from this paper you can look at the mass of the first black hole and the mass of the second black hole and what assume a relativity is correct so what is the mass of the final black hole and you can see around three solar masses are missing so of course they are not missing but they have been radiated out as gravitational waves which means three solar masses according to E equals MC squared have completely been converted into gravitational waves in these 0.2 seconds and it is the event with the highest luminosity in the entire universe and the peak makes 50 times as much power as all stars in the universe but except for our detectors it is completely invisible and we can also see where it came from because we have two detectors like we here have a sense of direction with two ears we can see roughly where it came from but another question to which we found answers is where does the gold in the universe come from so gold is very important culturally and economically and one thought for has thought for a long time that elements like gold and platinum as I have circled here basically have been created in the merger of Newton stars this is just the periodic table of the elements and in the beginning there were only the blue ones helium and hydrogen and then in massive stars they create the light yellow parts but most of the heavy elements are created in the merger of Newton stars and so far it was only a theory but if we look at merging Newton stars we can verify it and merging Newton stars do several things they create gravitational waves and they make a gamma ray burst and then there is an explosion cloud that cools down because of the radioactive decay and if you wait a little bit you can see the afterglow of the shock wave of the gases but if you really could observe that in all the details one could really see if if it really is about the creation of heavy elements and that is exactly what we can do and what we did and we have one gravitational wave signal in 2017 detected by LIGO and Virgo where we found gravitational waves that really said these were the merger of two Newton stars and was there a gamma ray burst that accompanied it and indeed there was the merger of the Newton stars and 1.7 seconds later there was a detection of a gamma ray from the Fermi satellite in an Earth orbit it could be just chance but we can make a stellar chart and what we can see is the Earth is in the middle and this is the entire sky that we can see from the Earth and from the data of LIGO and Virgo there is this little green cucumber where it says LIGO-Virgo and somewhere here in that region the Newton star merger took place and the Fermi observation of another gamma ray satellite is where the light blue and dark blue overlap and this overlaps perfectly with the gravitational wave area so this gamma ray burst was with high probability from this Newton star merger and the little delay isn't caused by gravitational waves being superluminal but that the event takes a while to develop and look for galaxies we can search for galaxies that have shown up after this explosion and after 11 hours this succeeds this is what you see on the right that is about 130 million light years away it's a new sun popped up that is marked with the cross here and if you now look at the explosion the cloud that is left by the explosion then this is colored in a way that would look like so if you look on the left side this is a nova that you see on the left this is an archive image you don't see it yet and this is before the explosion this is where it would be and now if you look at the time lapse on the right it shows up this explosion and it shows up it becomes less luminous it becomes more red it cools down and you can now fall this for a longer time and after about 11 days you cannot see it anymore on the right you see the spectra so the energy distribution or the luminosity across the spectrum on the left you have the visible spectrum on the right you are in the deep infrared and you can see there are obviously some peaks for example between 0.8 and 1.2 these happen because you have the presence of heavy metals like platinum that absorbs certain frequencies what you can do together with some models that we have that gold, platinum and a few other elements originate in these in these neutron stars that are merging most likely those are just the remainder of the beginning of the cosmos so at the end a small overview of some highlights obviously I cannot discuss all the 19 signals that we have but some that are very prominent for example from a certain date there was another neutron star merging with surprisingly heavy components in total this could be an early suggestion or hint that there are neutron stars that are heavier than we know or are aware of so we'll observe that some more results up until April 2019 the black holes that we saw usually had a very similar set of masses we were actually starting to expect that this is always the same and this was the first time that there was a big difference between the masses of the two black holes that were merging and another thing that's very special here is that there wasn't just one frequency of the gravitational waves but overtones that you would expect but you haven't seen before and then in 2018 we saw a medium size black hole about 23 times the mass of the sun and it merged with a 9 times smaller object it could be a very light black hole or maybe a neutron star we're not entirely sure yet but we're sure there's going to be more of these types of signals in the future and then maybe we can figure out what the other type is that it merged with and in 2019 again we saw the birth of a medium sized black hole medium sized anything that is below about solar masses so it's still quite heavy but between 100 and 100,000 masses we weren't exactly sure if they actually exist and here we have conclusive proof that it exists and that it was a merger of two smaller black holes and just one thing you can participate if you're interested in this and at home it's a distributed voluntary computational project you donate your computer rather the time that your computer is running while you're not sitting at it in order to search for a neutron star and there are different searches the most important one is the gravitational wave search it's but they're also searching for radio waves gamma ray bursts by doing that we found 80 more new neutron stars that we didn't know about before which is quite a sizable portion of the one that we know about and if you want to join you can go to einsteinathome.org it's mostly open source you can check the source code we're also happy if you find improvements that you want to suggest if you come from the other side for the observing there's an app called CHERP that you see below it shows you life when there's a new gravitational wave that has been observed and you can for example see the afterglow of merging neutron stars but starting you might even be able to get push notifications to your phone so that you maybe if you want to get up at night and then observe the event as it's happening so I'm already just saying thank you and open up for questions and I want to thank you for the very understandable and very interesting talk very well explained there's a lot of questions and so the first we saw the detection of the sound so you can if you normally do this you can see something about the shape of the room that the sound originated in is that the same from gravitational waves okay if I hear something I can hear how big the room is what the room is like I don't know that light is deflected by masses like this is called gravitational lensing effect and that is also true for gravitational waves for example if between me and my gravitational wave source there's a heavy object between it then the gravitational wave will go around the object on both sides and maybe use different times and you can figure out the mass of the object in the middle and for gravitational waves we cannot yet do that because we only find that maybe every five days or so but in principle from this direct line of sight we can figure out something about the line of sight and we can also figure out something about the shape of space time itself for example we can figure out how fast the universe expands this is interesting for cosmology and it's interesting for figuring out what the universe really is like something new and we can figure out the Hubble constant and because there's still a huge uncertainty about that but it's not really a big surprise so nothing surprisingly wrong has come out from this but also nothing else really how many properties does the neutron star have in addition to those of the black so we really don't know what the neutron star is like inside the density is really about as dense as atomic nuclei but it changes inside and maybe a centimeter or a millimeter at the surface is an atmosphere with lots of electrons and there's an outer crust maybe there's heavy nuclei but the further you go into the middle there's mostly the neutron soup that's why it's called neutron stars because really only neutrons survive this really but maybe if there's exotic matter like quark matter or something like that inside we really don't know so in the end to describe a neutron star there are lots of state equations if there's a pressure like this then it has this diameter and this mass but this is unknown and we really don't know and if you really if you will there are millions of parameters that you can tweak but if we know from the gravitational waves the exact mass and so and so then it has been deformed by tidal forces from its partner then we know something about these state equations and in a few cases we have done that at our institute they have measured how big it has been probably at of course there are error bars but there are lots of additional parameters because we really don't know but it's still matter it's not a black hole is there a way to theoretically gain energy from gravitational waves? yes in theory because they interact a little bit otherwise we couldn't detect them but the coupling to matter is very very weak so it's not really practicable you can see how much effort we have to detect these and yeah they leave a little bit of energy in the earth but I don't know it's single digit joules maybe something like that so so better just do some more research with so yeah it's for the research but for energy sources but in science fiction maybe maybe something like that but they can also then create gravitational waves maybe I have another question about the measurements there are theoretical maximum to the precision that you measure? yeah so in the end if you make the legs of the measuring too long and if you but what you really do you have a resonator it's a trap for light or a stall for light maybe which it goes back and forth a few thousand or a hundred times and then you can increase the precision but if you make it too long but if you make it too long and then while the light is still bouncing around then the gravitational wave is already through and has already gone through an entire phase so you can't really measure it so you can increase it but there's also like in electrodynamics you really need an antenna that is roughly in the size of the wavelength of the gravitational waves that I want to measure or maybe shorter but not a lot longer than the wavelength because you can get maybe interference so but you can measure lower frequencies if it's really huge so then you need really huge detectors it may be in the 2030s where you can get a detected detector in space where you have lower frequencies and longer lengths okay this already answers the next question about the wavelengths so the laser is just over a thousand nanometers okay how can you prevent that minimal changes in the position of the reflectors and the lasers change the measurement so the mirrors are decoupled from the seismic events the high vacuum and it's damped and actively and passively and on a triple pendulum and then there's a pendulum suspended from the roof and another mass and another pendulum and three triple or quadruple pendulars really decouple the detector from the movements of the earth the detector of several million and so what we are really interested in is really very very very still and doesn't move okay so a nice engineering solution but some made a remark about the ART is it compatible quantum theory and where does it break down yeah so that's one of the problems in the black hole we have this singularity in a single point that contradicts the quantum theory and you can't really have finite mass in an infinitely small point and so it conflicts with relativity and there are other things that happen at the event horizon with destruction of information which also contradicts theory of relativity sorry contradicts quantum theory but maybe we can see in black holes we can see the first contradictions to relativity we haven't seen it yet but that doesn't really mean anything because we can't measure precisely enough but maybe in the next years or decades maybe we can find something okay so the next question is are there any more again the knowledge that you expect to come next years so there's a huge there's a team worldwide several thousand people working on this on the engineering and on different aspects but we continue doing that we're doing more astronomy together with other astronomers something that we have done very well in this one case where we saw something about this neutron star this will happen more regularly there's one big breakthrough would be continuous gravitational waves with an uneven neutron star that rotates that would be a great lever to learn something about neutron stars and that would really be a breakthrough because we don't know a lot about that yet or we could see the signals and we see some deviations that cannot be explained by relativity and then we would really have another good lever to modify maybe relativity but it's just speculation at this point and maybe we can find signals from very early in the universe maybe with a space detector Lisa where we can see the reverberations of the Big Bang maybe something that would be great very cool that would be nice okay I have one last question before we go into the extended Q&A I actually assumed there's quite a few more so black holes are very rare so why is it that coincidentally some happen to upon each other yeah they don't really meet by chance they have been a binary stellar system before that and maybe one of them goes supernova and becomes a black hole or they overlap each other so but in the end they create into they evolve into two black holes circling each other or it may also be individual black holes in a very dense environment for example a and maybe one star is being kicked out and then two others then still circle each other and or merge each other but we don't really know how these are created but maybe in the future we will know more about that okay I'm parked my interest so anyone who's interested you now know where to work on and thank you for your time