2020 Nobel Lectures in Physics

nobel laureates in physics and chemistry laureates in economic sciences members of the academy ladies and gentlemen on behalf of the royal swedish academy of sciences i wish you all welcome to the 2020 nobel lectures in physics and chemistry and the lecture of this various rick spank prize in economic sciences in memory of alfred nobel the foresight of the great alfred nobel in 1895 to devote his fortune to five international prizes reflects his own international experiences and activities it is now 120 years since the first nobel prizes were awarded this year is very different from the past several years a pandemic is haunting humankind restricting our possibilities to meet and to interact the nobel lectures are delivered separately in different places fortunately thanks to the many discoveries in science and inventions the lectures can nevertheless reach across the globe this year is indeed seeing science and scientists from many disciplines on the center stage of the news feed scientific knowledge is high in demand and has increased our understanding of the kovid 19 virus sarskov ii at an astonishing pace pandemics and epidemics have plagued humankind on numerous occasions during the recent history of our species if it hadn't been for the progress of science with the discoveries of bacteria and viruses and the development of vaccines and other medical treatments and interventions our predicament would have been even worse science is our hope our rescue the curiosity of humans our urge to explore and investigate is a hallmark of our species research also requires determination and stringency and often advanced technologies the roads to discovery can be long and winding and filled with blocks but sometimes progress is made with a giant leap occasionally offering surprises and open on opening phenomenal and wonderful possibilities the results can be enriching astounding and magnificent however in some contexts we have witnessed political leaders and others who have questioned well-established scientific findings be that regarding global warming or the fact of evolution or the benefits of vaccines instead some leaders and other people are indulging in wishful thinking and spreading disinformation yet when their own lives are at risk they have relied completely on and benefited from evidence-based health care we have seen political leaders promoting unsubstantiated remedies even distributing bogus medicines to their citizens alfred nobel has been quoted to have said that second to agriculture the largest industry in his time was hamburg alfred nobel acknowledged the importance of scientific discoveries inventions and improvements the nobel week is both a celebration and a tribute to science and research we will today have the privilege and honor to listen to the laureates who have greatly increased our knowledge and made a difference to the way we think this year's lauriers illustrate like so many laureates before them the broad versatility of research a sophisticated theoretical prediction has been demonstrated to be factually correct by measurements using advanced instruments bringing fundamental insights about the complex world we're living in and the forces of the cosmos detailed understanding of a bacterial defense system a kind of immunity allows it to be tamed and harnessed for practical use with exquisite precision and economical interactions can be described through understanding of human behavior and reasoning and optimized from various points of view we surely have interesting lectures ahead of us i now invite my colleague in the academy of sciences professor ariel gubar who is a member of the nobel committee for physics to introduce the laureates in physics once again welcome everybody to this year's nobel prize and the economy prize lectures this year's nobel prize in physics celebrates one of the great triumphs of mankind's thinking and exploration the formation of black holes was predicted on theoretical grounds these are objects that by definition cannot be observed directly astronomers took up the challenge and thanks to great innovations and by pushing the boundaries of technology were able to show firm evidence for their existence this bizarre regions of space-time where gravity is so strong that even light is trapped have captivated the interest of both scientists and laymen alike among physicists many resisted the idea that such beasts could arise in the universe as they involve a so-called gravitational singularity at their center a point where gravity becomes infinitely strong albert einstein was arguably the most notable skeptic simply put the notion shared by many of the sharpest minds of the first half of the 20th century was that it would take unphysically perfect symmetry for collapsing matter to become a black hole thus in a real universe natural imperfections would prevent such objects from ever forming this year's nobel laureates have drastically changed that perception roger penrose receiving one half of this year's nobel prize used ingenious mathematical tools to demonstrate that the formation of black holes is in fact a robust prediction of the theory of general relativity regardless of the geometry of the mass being pulled gravitationally the other half of the prize is shared by reinhard gensel and andrea guess for their spectacular detective work examining the orbits of stars around the milky way center with exquisite precision circumventing formidable observational challenges to disclose the nature of the invisible object in the galactic center harboring over 4 million times more mass than our sun with that i would like to invite our first speaker roger penrose to talk about black holes cosmology and space-time singularities roger penrose earned his phd from cambridge in 1957 in the late 50s and in the 60s he held positions in prestigious institutions in the us and in the uk and in 1973 he became the rose bowl professor of mathematics at the university of oxford turning into emeritus in 1998 there are numerous mathematical and physical findings that carry his name including penrose tilings parents hawking singularity theorems and of course penrose diagrams is also exceptionally gifted as a popularizer of science and has written several best-selling books please join me in welcoming sir roger penrose in 1908 hermann minkowski introduced the idea of space-time which was a four-dimensional space which encapsulated pretty well all of einstein's 1905 theory of special relativity at first einstein didn't like the idea very much he thought it was mathematical so history or something but then he picked up on it and it was central to his generalization to his general theory of relativity now in the first picture i've imagined three axes for a three-dimensional space and then we can move on to introduce the time axis to see our axes four axis four dimensional space now the important most important thing of this is to represent the speed of light here we have a light ray and this we want to see it so that it doesn't sort of lie on the floor so we want to have units so that it can be seen as maybe 45 degrees or some reasonable angle so that you have your space and time unit so they're comparable now here we have the nalcone which represents the directions of all the null rays so the light rays in all directions are represented by this cone it's very important in fact we don't really need the light ray there because we've got them all in the cone we don't need the axes so the important thing is this null cone now in general relativity you see there's a null cone at each point representing the local speed of light but the cones can be more or less all over the place now you can imagine a point and the light rays coming out of that point and that's the light cone if you like the narcons can be tangent to it wherever it goes but you can see at the back at the top and right hand side where the light rays start to cross each other and this sort of thing makes it like cones complicated but it's important for what i i'm going to talk about later that you understand these things you certainly are going to get crossovers crossing points caustics and things like that and they're the central feature of what i'm going to discuss okay now let's consider the following picture in this picture we see basically the oppenheimer snyder collapse of a dust cloud to what we now call a black hole this was in 1939 when they studied a collapse of what they called a dust cloud this has no pressure what what you call dust is simply a fluid or something with no pressure and the thing was spherically symmetrical so the fact that you fell inwards and it focused itself into the central point and you see as you move up the picture you see this singularity in the middle where the dust cloud gets itself focused to and you find that since the density goes infinite the space-time curvature becomes infinite and this is what's called a singularity now this was known and at the time that quasars were discovered people do people started to wonder whether there wasn't something like the oppenheimer schneider collapse involved now i wasn't aware of this openstack it's schneider paper in the i think it was in 1958 when i went to a lecture in london in king's college london my good friend and mentor dennis shaw drove me there he said it would be interesting to me and this was a talk describing how you get through what was then thought of as as the schwarzschild singularity now the top part of this picture is what finkelstein described you see schwarzschild shortly after einstein introduced his general theory of relativity solved the equations for a spherically symmetrical body now he also solved it sort of for the interior of the body but that wasn't a very realistic model wasn't important so much what was important was the solution for the exterior of the body spherically symmetrical vacuum now the thing about this is if you imagine squashing the body down smaller and smaller and smaller you get to a point which is called the schwarzschild singularity often it was called that because the equations all go crazy and things go infinite and people used to think this was a singularity which means you simply have some physical nastiness which you can't extend beyond but the model as i'm showing you here well at least the top part of it was described to me by david finkelstein at king's college where he gave a talk in in i think it was 1958 and i came away thinking gosh you've got this singularity still in the middle where you got right rid of the one on the outside but you still have that one in the middle so i wondered whether there was a theorem or something which showed that whatever you did if you had it complicated irregular in some way you would still get a singularity i had no idea of how you might prove such a thing so i started to think to myself what do i know about general relativity that maybe other people don't know and possibly this will be helpful to me to do something that people aren't familiar with what i settled on was two components spinners now here we have in the next picture a picture on the right-hand side of a two-spinner i learned really about these things from the great physicist paul dirac um i think it was earlier in the same year when he gave he sort of deviated from his normal course and talked about two spinners he he was famous for discovering the equation for the electron but these use four spinners four component spinners and you can break them down into these two spinners and direct was well aware of that and i was familiar with the idea but i simply didn't understand them and iraq's lecture made it absolutely clear to me and so i began thinking okay this is something comply to general relativity and maybe it will give me an insight that perhaps isn't familiar to people in the picture you see now on the left hand side you see the celestial fear sphere and that's the different directions in which the two spinner can point in the two spinners it points along the light cone but it's also got a little flag attached to it i won't go into that at the moment but it was important to have an understanding of the geometry of two spinners now the important thing mainly for me was that you can understand the curvature much better in two spinners in particular the part of the curvature which is called the vile curvature now in this picture you see the curvature splits into two parts one is called the richie curvature and that is what matter directly influences einstein's theory tells you that the matter density the energy density the pressure and all that stuff tells you what the richie tensor is directly but what's left that's ten components what's left is another 10 components which describe the way the gravitational field behaves so that the free gravitational field in the gravitational wave say is described by the vial curvature and you see it gives you a distortion in the field of vision so somebody's looking back and you see the focusing effect due to the richie curvature and then you see the effect due to the distortion due to the vowel curvature now the vowel curvature satisfied very nice equation when you rise in two spinners and it really attracted me very much but getting used to how light roads behaved it was something that i felt at home with and how they focused and how caustics behaved and crossing surfaces behaved and all that sort of thing so i got familiar with all that but uh at the time this was when the quasars were being seen this was in 1962 63 64 that sort of time when these bodies which were producing enormous amounts of energy and they seemed to be very small so they had a an energy which was i forget 100 times an entire galaxy but yet they seem to vary in a few wows or days or something like that which meant they had to be very small and they had to be very big to emit such energy very massive to such energy so how could all that energy be squashed into that small volume and people started to speculate speculate on whether something like the oppenheimer schneider model might be relevant but then you think just collapses radially inwards and doesn't give you anything so if you want to have radiation coming out you need to have at least a quadrupole structure or something complicated not just a radial collapse so i started thinking about it partly uh at the instigation of john wheeler and at the time there was a paper by two russians lyft kalatnikov which seemed to have proved that in general you would not get singularities and only very special situations so if you had a generic collapse it would just swish around and come swirling out again so i started to worry about this problem i had a look at their paper i didn't notice the mistake there was a mistake in the paper i didn't see that but what i did see was the methods they were using i didn't feel altogether convincing and that it was worth trying to see whether you could get singularities in a generic situation and i remember walking in the woods and trying to think about this and i came to the conclusion that it couldn't be a local thing it had to be something non-local some kind of criterion which tells you the point of no return in some sense is being reached and i then devised the idea of a trapped surface the picture you see here is the picture that appeared in my paper a paper that i published in 1965 i gave a talk at king's college london in 1964 about it and uh the argument was that if you have a collapse which is generic that you might still have problems even though it wouldn't be focused right into the central central region and i developed this idea of a trapped surface you can see that's this little ring in the sort of middle of the picture you have to imagine that it's not a ring because i'm only depicting two dimensions and the whole thing should be four dimensional and that ring is really a two-dimensional surface like a sphere but you can imagine it might be distorted not like that now what is a trap surface you have to imagine that there's a flash of light emitted all along that surface here on the top left you see a picture of a little surface and you imagine suppose there's a flash of light occurring on that surface and if it's convex well on the concave side the flash will be converging on the convex side it will be diverging and that will be the normal thing for a surface now on the top right you have something which is sort of you might imagine that it can converge on both sides on the bottom you see a much more general situation which doesn't depend on any kind of curvature you get this in flat space the intersection of two paths like cones you get this property of a locally trapped surface now the trouble is that when you have a surface like this which is global all the way around such as in the in the in the picture that i have on the uh the central ring which is really a two surface and i'm imagining that a flash of light occurring on that it's converging on both sides and that's what a trap surface is and what i was able to show that if you have energy which is locally positive and you look at the behavior of the future of that surface that you must run into a contradiction that is to say you get a singularity so this was a proof in general circumstances that you couldn't have irregularity or something swishing around in a complicated way and come swirling out again you would always get a singularity and this was the central theorem that i proved which eventually seems to have won me the nobel prize okay now let's think about the universe as a whole now you see stephen hawking was not that talk but dennis sharma got me to give a repeat talk in cambridge and stephen was at that talk in cambridge and we held quite a long chat afterwards about the methods i was using george ellis was there also and uh steven picked up on it very quickly and developed these ideas by generalizing my arguments to apply to a cosmonaut cosmological model i won't go into the details of this but the main point was that you could also show using the arguments that he developed and also we got together eventually and wrote a paper together uh which more or less encompassed the things that we've done before and this showed on the under very general circumstances in the past now if you have some kind of diverging ray some kind of expansion in the universe then you couldn't avoid having this singular state in the very early universe now this was fine that you seem to prove these singularities but i remember being very puzzled by why cosmologists didn't study all the different kinds of singularity you could have because they're very common there are many many solutions known to during relativists where you get very complicated singularities and i remember this was an occasion when i think i was at princeton and we were about to go to a conference at stevens institute in hoboken and we used to drive up in several cars i noticed in the car in the back of one of these cars was jim peebles this was last year's nobel prize winner in physics this was long before that of course but um i noticed he was there and i asked him i said why don't you cosmologists ever consider all these complicated possible singularities that you could have and you just look at this simple case why don't you look at these other cases and he looked at me and says because the universe is not like that and i thought my gosh it's not is it i presume that he was looking at the microwave background radiation which is very uniform over the whole sky and it tells you that the universe really is very uniform so it tells me that there's something very strange about these singularities that the big bang singularity is utterly different from the kinds that you see in the future in the collapses in black holes and i was very puzzled by this and since everybody seemed to think that the solution of the singularity problem was the that you had to combine general relativity with quantum mechanics you had to find a quantum gravity theory uh it must be a very peculiar quantum gravity theory which is grossly asymmetrical in time which gives you a theory which makes the singularities quite different in the past and the future well i had this view for quite a long time until i think i started to worry more about well well i thought worried about the singularities in relation to the entropy in the universe i'll come to that in a minute but first of all let me talk about what the universe is actually like according to what people think you see what they think is that the big bang was not just like this and if you have a magnifying glass you can have a good look at it there's something called inflation now inflation according to the theory is another exponential expansion which took place with a tiny tiny tiny fraction 10 to the minus 30 something or other second of a second and that is supposed to have taken place this exponential expansion and it was supposed to smooth out the universe and do various other things to do with the microwave background make it look as though scale and variance and things like that so it had role to play but i didn't see that it would scale up you smooth out the universe well the reasoning i was using is suppose if you imagine a collapsing universe so here we have a collapsing universe and this collapsing universe suppose we put the infilton field which is supposed to give you inflation well it just doesn't do anything if you have not irregularities black holes or congealing together the picture would look more like this a great mess with your vowel curvature diverging like mad in then you imagine well that is much more likely thing and you can work out how much more likely it is something like the probability of finding this is 10 to the 10 to the 100 something 10 to the power of 120 or something that you get that more likely than the isometrical model that we seem to see at least in the big bang but in the big crunch if you have the universe collapsing into a mess like this this is what you get but the question is why wasn't it like this in the past is this a very strange kind of quantum gravity theory which is what i thought for a while now another this is connected with another problem which is the problem of the second law of thermodynamics now let me describe the picture of the second law of thermodynamics people often talk about a gas in the box so the top three pictures on the left hand side we see a gas in the tucked up into the corner in a little box you open the box and it spreads out through the through the big box if you like so as the gas spreads out it gets more and more uniform the entropy or the randomness increases in accordance with the second law now that's what you you see that the right hand side of this picture you see matter which is very very uniform in the beginning which would be a maximum entropy state in fact the macro background where you see it it looks like a maximum entropy state so there's something very funny going on how can it start off at a maximum but then when you think about gravity it works the opposite way because gravity is uniformly attractive so here i have a picture on the left of a lot of stars running around and then they time to clump and then they finally get black holes and the entropy goes shooting up enormously by the bekenstein hawking formula which tells you what the entropy in a black hole is and it completely and utterly dominates the entropy in the universe right now almost the entire entropy in our current universe is in black holes by an enormous factor so we see that what's special about the universe is that the gravitational degrees of freedom were not activated somehow and i tend to postulate just that maybe quantum gravity tells us in some mysterious theory that somehow the vile curvature had to be zero in a past singularity and it could be infinite in the future singularity just sort of waving hands around and no theory which tells you that and i thought that for a long time but then after i became persuaded and i think i want to go back now look at this picture that the universe this was a another nobel prize saw palmetto mutter and and schmidt and rhys discovered the distant supernovae stars seemed to be accelerating away from us and i had to be persuaded of that uh and when i came around to believe it i had some wrong reason for disbelieving it but when i came around to believing it i thought gosh this means this is interesting it tells you that the future infinity is space-like now let's talk about infinity you see people think how do you talk about infinity well now you aisha has a very nice way of describing infinity this is a conformal representation of what's called hyperbolic geometry don't worry about the geometry but the boundary of this represents infinity now it's a conformal picture that means that angles are preserved but sizes can be big or small and the the fish as they get closer to the edge they don't really realize that they're small according to them they're the same size as the ones in the middle so this is a conformal represent representation you can squash or stretch as long as you don't alter the the small shapes and that's that's conformal geometry now the importance of control conformal geometry i'll come to in a minute but let me think about the light cone again the space-time structure means not not just the lycone you need to know these little surfaces within it which are the surfaces of equal time from the origin point so if you have some light rays now these aren't right here so you have two massive particles and i'm having two different ones going at different speeds going through the central point and the ticks of a clock at the top you see a little clocks these and the clock registers the time as the as the light as the world line of that particle intersects the these various bowl shaped surfaces the first stick the second take the first stick and by the two formula at the bottom we have the two most famous formula of 20th century physics and size equals m3 squared and max planck's equals h nu you put the two together and you see that energy and frequency are equivalent from the planck and energy and mass equivalent from einstein put the two together and you get that mass and frequency are equivalent which tells you that a massive particle is a very perfect clock so this is where you get the metric structure but what about light rays but you see the light ray doesn't even hit the surfaces so a light ray doesn't register the passage of time at all and here we have we'll forget about the bar the light ray the light cone or the null cone is itself gives you the structure of space time so if you only got now cones and not the scale then you have a good picture of infinity and in fact you can do this and not only you can you have a picture of infinity but you can stretch out the big bang as well and this is you see i was trying to say as a sort of criterion that the big bang or initial singularities ought to have vanishing vial curvature my student paul todd had a better way of doing it to say okay let's say that it's extendable to something you see you could imagine the escher picture you could extend the little fish to beyond the bounding circle to something outside they don't experience that but you could imagine there was a world continuing beyond infinity and here i imagine there might be a world continuing beyond infinity and there might be a world continuing before the big bang now is this physic does this make physical sense it makes geometrical sense but what about physical sense well you see the big the remote future is very rarefied very cold and the density is very very low when you scale things to squash it to make it a finite boundary this makes the energies go up and the temperature go up and if you stretch out the big bang that makes the energy go down and the temperature go down and you might imagine that physically they are similar so would they match that's a possibility you need the space-like nature of infinity and that comes from the cosmological constant the space-like nature of the big bang comes about automatically you also get something more if you do what i'm now going to suggest which is to imagine not just that you have a finite boundary the two ends but you continue our big bang was the continuation of something before and our remote future is a continuous will continue to something beyond it now does this make physical sense which it only really makes physical sense if you've got massless things around what about the remote future well the main things running around will be photons and they're massless so let's say there's something more you've got mass as well i have to sort of suggest that the mass does fade out eventually but let's say it's dominated by the photons what about the big bang well it's the opposite reason here the einstein z equals m c squared you'll find that the energy in the mass is pretty well completely dominated by the energy in the motion so the kinetic energy when the mass when the motions get so big the energy gets so great then it's almost entirely in the kinetic energy and not in the mass so you might as well consider that the mass is zero and the physics relevant to the two ends is the physics of zero massless particles in fact in the remote future the photons that's maxwell's equations maxwell's equations don't even note notice the metric they are conformally invariant and in fact you can consider that you could actually if you have electromagnetic things or masses things or gravitational wave things they could get across from one side to the other so i can imagine that the crossing over from one side to the other makes sense if you are you know if your physics is sufficiently conformal and you're not worrying too much about the mass okay now i'm going to now finish this talk by talking about two classes of observation the first one was an idea i had about maybe you would see in the previous eon if it's like ours i'm calling the different sections the different eons so if i go back to this picture our eon is this initial cylinder it's not really a cylinder you see the back there's some curly stuff it may not close up at the back so i don't mind whether the universe is closed spatially or open now here i have the join from one to the next and you might imagine light rays getting cross but what else might get across well here the previous eon i'm calling all those different what we previously called the universe was our eon there was an eon prior to ours and eon after ours and so on now the big bang of our eon was the remote future of the previous eon now there might have been in that previous eon black holes supermassive black holes running each into each other as they run into each other they would emit gravitational wave signals these would get through and make a signal that we might observe in our eon moreover if you imagine what the big really super duper massive black holes which ultimately swallow clusters of galaxies they will be the result of many many supermassive black hole collisions and they will be maybe one two three four five six several of them so you might see not just a signal of one of these but several of them now what you would expect to see according to this theory would be rings in the in the cosmic microwave background where the temperature is slightly lower uh variance as you go around or higher or lower temperature but my colleague vaheguru zidane who looked at this he looked at concentric sets of at least three rings of low variance uh low temperature variance and he plotted these things out and then the next picture i'm going to show you which is rather remarkable to me he didn't select these things from the color which is the temperature he didn't select it for that at all he selected just for the low variance and these are plotted out in the planck data centers of three at least three concentric low variance rings now whatever you believe the origin of the signal is what is very remarkable is the extreme anisotropy of the picture or inhomogeneity of the apparent universe here on the lower right you see a very large collection of red points now in the color coding and in the theory i won't go into the details here the red means distant so these would according to the theory be a collection of very distant supermassive black holes clumped together in some super duper cluster on the top right you see a bluish region that would be closer to us within our past like cone so you probably could see that of uh something which again some not quite so big and super duper but pretty super duper and a little on the lower slightly lower left you see something intermediate the red ones would be distant according to my theory the blue ones but what's remarkable is not just their clumping they're clumped together in where you see them but they're clumped together in the color which means according to the theory in the distance so if you don't believe my theory you have to think of another explanation for this but it's very much at odds with the current view where the inflation is supposed to stretch things out and flatten everything out another possible observation of events in the previous eon would be the evaporation of supermassive black holes according to stephen hawking's hawking evaporation where the entire mass ultimately it may take something like 10 to 100 years or more for a supermassive black hole finally to disappear in form of radiation now this radiation would get through into the next eon so here i have a picture taken from a paper written by christophe meisner pavlo nirovsky daniel ann and myself in the monthly notices of royal astronomical society and we see the crossover is the lowest horizontal line in the picture and the vertical line just meeting it is the world's line of a supermassive black hole which finally evaporates right at the crossover almost into the next eon really squashed up into that little point it spreads out through 380 000 years until you finally see it when it reaches the last scattering or decoupling surface and according to the work of james peeble and his colleagues you have a good knowledge of what happens in that previous 380 thousand years but what we actually see is the spread of that point to about eight times the diameter moon so this would be a heated point which is hotter in the middle by about something like 15 times it could be as much as that times the background variations in temperature the normal variations and we seem to see in both the wmap and planck satellite data these points in the planck data we see it with a confidence in something like 99.98 confidence level so this is a very common very strong signal these points are there of the five strongest points in the planck data these are also seen as exactly the same places in the wmap data there's another one in wrap which is seeing the same point in plank data so these six points i think are pretty i have pretty great confidence that they are genuine hawking points if not i think somebody else will have to come up with an explanation for this effect and explain how we can have a confidence of the level we seem to see on the basis of some other theory thank you very much we're going to listen to reinhard gensel talk about the amazing story behind the study of the compact object in the galactic center a 40 year journey reinhard gensel earned his pa earned his phd in physics and astronomy from the university of bound in 1978. he held a postdoctoral position at the harvard smithsonian center of astrophysics followed by a miller fellowship at the university of california in berkeley where he became full professor in 1985. in 1986 he was appointed director of the max planck institute for extraterrestrial physics in garching reinhard genzel has been spearheading the high spatial resolution imaging and spectroscopy studies of the motions of stars surrounding the sagittarius a radio source in the milky way center using the european southern observatory facilities in chile as we are about to hear a fascinating and still evolving story professor genzel the floor is yours ladies and gentlemen dear colleagues it's a great honor and a pleasure to be here and speak to you about the research we have done in trying to establish that in the center of our nearest galaxy the milky way is an object which is a massive black hole of four million solar masses beyond any reasonable doubt i've called this the 40-year journey it's been a long time many people were involved experimental work has been ongoing for these decades to improve the sensitivity the accuracy of positional measurements and the precision of data to come where we are now and when we can say with some certainty what i will tell you many people have been involved in my own group people before me mentors but also many students and postdocs i'm very very glad to have been able to work with such a great crowd of people and i also would like to emphasize the great work of our team in california who have been competing with us but in the end of course we have come up with the same basic result which only strengthens the overall believability of the result now over these decades of course there were several phases so i'll give you this talk in five phases first the motivation and then four phases within the program which i participated in now let's look at the backdrop of course we know that the theory of general relativity is 100 years old and the mathematical refinements went on for a number of decades ending in the 60s with the so-called kerr space-time metric and the word of penrose on the ergosphere and others roger penrose being the one who is being honored with the prize this year as well so this theoretical foundation gave us the idea the perspective that there are objects which one would call black holes which basically uh you know this are not communicating from the insides with the outside but have tremendous can have tremendous impact on the gravitational field the space time surrounding it and therefore influence their environment now this was our theory until the 60s when several things happened x-ray astronomers started observing very intensely x-ray emitting binary stars some of which we now believe are stellar black holes and that's the most important thing for this talk the quasars were discovered initially by radio astronomers and then optical astronomers discovered that these objects in optical images look like stars yet in their spectra there's evidence that spectrum lines which are in the laboratory very well known are shifted by such an amount as to make these objects very distant many hundreds of millions of light years away such that what looks like a fairly uh faint star really must be a interme tremendously luminous object this one the first one has a thousand times the luminosity of our entire milky way all coming from the central light here or so theorists thinking about this then found out that it was basically impossible to explain the radiation produced in these quasars by normal fusion like in stars but and that's the surprising thing when material falls into spirals into a massive black hole before the material disappears in the event horizon and therefore is you know escapes from our line of from from our view it can release enormous amount of energy uh due to the gravitational energy lost and and that can be up to forty percent uh m c squared so surprisingly paradoxically therefore uh the model of massive black hole paradigms for the explanation of of quasars was invented but of course how would you prove that prove that in the sense that you cannot only observe phenomena like high energy radiation or these spectacular radio jets of relativistic plasma emanating for many of the quasars this is indirect evidence for a very energetic object but not proof in the sense of actually showing that there's a gravitational potential as compact as you would have to have for a massive black hole so how would you prove that in a very famous paper in 1971 lyndon bell and reese then proposed that whilst it's impossible to measure you know effects of gravity in distant quasars if all galaxies would have central massive black holes just most of them not very active in terms of accretion well then you could use nearby galaxies in particular also the center of our milky way so that's how our milky way center became center stage and it will evolve from there to to a lavatory so let's take a trip from the outer parts of the milky way into our disc galaxy towards the center now you see immediately in optical light there are these dark patches that means that there's a lot of dust between the stars which actually prevents us from seeing into the center of the milky way so to see there we have to switch to longer wavelengths infrared radial or shorter wavelength x-rays this is an infrared image which we can now make with modern telescopes which shows us the innermost region of our milky way being a very dense star cluster this is shown here in the blue region and in the center of this star cluster as you will see in a second radio astronomers found very intense radio emission in particular a very compact radio source surrounding that radio emission in green here is a gas very dense neutral gas as well as ionized gas the so-called mini spiral in ionized gas in fact that was the first path towards looking whether there is a central mass basically what one does is to look whether this gas is orbiting and it look from the velocities what the mass would be so indeed a group around child sounds at the university of california where the students started looking at this ionized gas stream is here if you see how the velocities change along this stream or here and the numbers here give you the mass which would be required if this would be a free falling if you like parabolic or elliptical orbit and the numbers are two three four five millions of solar masses so that was the first evidence that there was a lot of mass there more mass than the stars would allow then the next stage was the radio source many years have been used to refine the measurement on the radio source on this diagram where you have wavelength versus size of the source you can see the shorter wavelength you go to the smaller the source becomes and spectacularly when you go to the millimeter waves here's one millimeter the size of the source is 10 micro arc seconds to give you a feeling that's about the size of a euro coin on the moon that's how compact the radio emission is and in fact that would be the event horizon size of a four million solar mass black hole so the combination of this mass being large and this size being small was the first evidence in the 1980s that there might be something in the center of our milky way so here on the left side one could take the various measurements here and see that this mass was between one and four million solar masses not very accurately initially maybe 50 percent or so on the on the right side you see as a function of distance from the center the mass and here you see that uh within about a parsec or so uh there seems to be a constant mass that's an indication for a concentrated central mass but still at a very large number of event horizon radia a million but it was the first evidence how to go from there well you want to go further in and you want to use indications which are more stable and more easy to interpret than gas which can be pushed around by forces other than gravity and these are stars and so in the 90s two groups one in europe using telescopes in chile and one group at in california using the keg telescope starting using new developed infrared imaging detectors for the space telescope to build cameras which allowed very short exposures the trick is that when you take a picture in the optical or in the infrared then of course the earth atmosphere will blur the image such that you cannot make a very high position measurement to measure the motions of stars on the sky you want to have very accurate positional measurements over time and see whether they change so this was the first technique both groups used the speckle techniques as you see here you make short exposures and freeze the motions of the earth's atmosphere and then in post-processing you basically stuck add the images to get about a five times improvement in the angular sharpness of the images over what you can do with normal detection the second technique which had to be developed is to get spectra because the emotions on the sky give you two motions two and the doppler motions along the line of sight which is the third you can get through the spectra from lines in in the stellar in the stellar spectra for that we developed what is now called the integral field unit basically a normal spectrometer just disperses the light in one direction and doesn't allow you to image but by using an image slicer you can basically generate a huge long slit which decodes the two dimensions and then you bring it through a spectrograph which disperses the light and then in the end in the computer you can put put that back together so using these techniques a few years later we had sharp images of the central light years and we could look for changes on the sky of their positions here's an example the green cross is the position of secretaries a star that radio source which is so compact this is one light month this circle and you can see from time to time the time from one year to the next in three different colors uh the position of this star very near star clearly changes if you look up then that's a motion of in excess of a 1 000 kilometers per second something which you see not in the solar system so these stars are visiting around at a speed of more than 100 times the earth around the sun so that's already a qualitative indication of a strong mass and is it a point mass well okay kepler's laws tell you that if you have a central mass and objects are moving around it and the further away you are the slower and indeed when you look here the innermost stars move obviously very fast while the other ones here you know do not show this prism effect so that's kepler's laws in color so to speak and indeed when you make this more quantitatively you can derive than the mean speed of the stars as a function of distance from the radio source that's what this is and this tail here is indeed the first quantitative indication that indeed there is a four million solar mass or three million solar mass object in the center and further out when this flattens that is the the effect of the star cluster surrounding it so that was the second phase that was until 98 with stellar motions making more precise estimates of the stellar mass now you see we are you know at the level of about a little little over ten percent and a factor of uh five or so inwards so it's both a more precise measurement of the mass as well as concentrating that mass on a smaller region so by that time it was already fairly difficult to construct reasonable alternatives so the path was this could be a black hole but now we have to exclude alternative options of other configurations by going closer and closer and measuring more and more precisely well how could you go from this phase onward well here we are measuring the basically the average of the motions now we would like to see actually orbital motion direct orbital motion of individual stars that's much more precise but mostly in astronomy these kinds of motions can only be detected over many hundreds of years that's why nature came in and helped us and gave us some stars which were not supposed to be there namely very very close on solar system scales and those indeed move fast enough that we can see motions so by the early 2000s both groups were working on eight meter class telescopes the biggest telescopes we have now and we had both equipped these telescopes with what is called adaptive optics so instead of the speckle imaging we are now correcting the wavefront before we detect it and make sharp imagery right away that gives us a lot of sensitivity and high precision imaging by 2000 andrea's group saw the first from straight la straight motion accelerations so that gave us already hope that maybe we could see these accelerations for a number of stars in particular this one here had one of the possible solutions you see the three possible solutions which was an orbit of only about 20 years or so and indeed two years later both groups saw that this star whose orbit you now see highly elliptical orbit approached the galactic center to about 17 light hours moved there at about 7 600 kilometers per second tremendously fast and on a distance scales of about three times the neptune's orbital uh radius so if you put this into kepler's laws that gives you again four million solar masses the same four million solar masses we had before but now concentrated on the scale of only 17 light hours now this is only one of the stars we followed as you see in the movie down below in fact we have partial orbits uh for about 30 40 stars yet this is the best one so at the end of this stage um which is uh you know a few years ago we had ever more precise measurements of the mass and what's more important because of this close approach we could exclude that the mass is many more extended than about 100 times the event horizon size so by now by that stage the exclusion principle was very powerful you could exclude all kinds of hypothetical situations including what we would call dark astrophysical clusters like for instance a conglomerate of neutron stars a million neutron stars or or or stellar black holes or something like this so there's very very little lift it's a million four million solar mass black hole and perhaps this thing here a so-called boson star so here we are we have already very good evidence do we stop are we finished no we are not a now we are we are looking at this as a laboratory not anymore to say is there a compact mass we are certain of that now but now we want to know hey is it really a black hole in the sense of general relativity and is general relativity the proper theory also in these extreme environments which have not been tested for that we have to still make better measurements so by 2004 we talked to iso the european southern observatory and proposed to build a new instrument a new instrument which would allow us to combine the light of all four eight meter telescopes into a so-called interferometric telescope very well known from radio techniques of over 100 meter diameter so our 100 over 8 meters is a factor greater than 10 so actually it's you know more than an order of magnitude improvement in in in resolution so we built this instrument we call gravity which brings the light together from all the four meter telescopes it's underneath here uh you have to make all kinds of corrections etc which are all in tunnels underneath this device and has been built by by the european southern observatory why will we build this new instrument which had three milliarcs second resolution and a astrometric position coming close to the event horizon scale the star in the meantime was during its orbital period and was coming back and we were already so that in 2018 when the next period occurred we could use the interferometer now to use this high precision tool to really look very very closely by the way you can see that here you see the star with in the in the very close to parry and you see the black hole itself i'll come back to this later so the experiment in fact is now actually conceptually very simple you measure directly on the sky the separation between what you think is the center of mass and the object here near perry we could see the motion every day previously we had to wait for a month or two and we had a very precise definition of the orbital parameters of this ellipse and thereby a high precision on the mass and now we can begin to look for effects of general relativity in fact the first one is the fact that when light from the star comes to us when it's very close to the massive black hole it has to climb out of this potential well which is uh basically you leads to a red shift to a loss of energy and here relative to a newton orbit is in fact this red shift which we have measured in comparison to what you would predict from general relativity which is the blue scale so that was already the first triumph and the first evidence that indeed gr holds very important this was uh you know confirmed by uh the ucla group one year later and the first step the next one is to actually show that the equivalence principle holds in particular in this particular form the position in variance and that we can do because in the spectra of the stars we have different atoms here's helium and here's hydrogen and here's helium so we have different atomic clocks and according to einstein's theory the results should not depend on anything but the gravitation shouldn't come in that they are different atomic clocks and indeed the results we get from the different tracers agree to within a few percent the next effect expected is the fact that while in newton's theory a planet orbiting the sun on an orbit would hold its position in the in in the plane if there is no other uh nor other object nearby but in uh general relativity that's not the case there is a procession of the orbit which depends on the the distance the star comes to the object and the mass and so for the star s2 relative to a newton orbit and setting an angle equals to zero up here at upper apple near parry this position is is is maximum you see basically in in x-coordinate here this is a kink or in angle it's it's it's a change by about 11 12 arc minutes per orbital period so that's the prediction and that's what we saw with gravity okay the different measurements in x so you see clearly this kink and here you can see the change in in angle and indeed exactly as predicted by general relativity so that's that's very very important because it shows us that indeed gr holds also in these extreme environments which have not yet had not yet been been tested now how how close can we come to the center would it be possible to actually probe the region right around the event horizon well actually in the early 2000s the x-ray astronomers and then we discovered variability variable emission in the infrared in that x-rays from the black hole itself continuously variable emission and the theorists in particular broderick and lerp had proposed that what you see here are accelerated energized electrons which basically are formed in this hot plasma right around the black hole as the gas is orbiting thereby forming hot spots for you know a few tens of minutes if so you should you might be able to see the motions of these hot spots so indeed in 2018 on three occasions the black hole got brighter than its normal state by about a factor of 100 or so and we could see indeed these motions on the scale of you know about five times the event horizon size or six to ten times the gravitational radius with orbital periods of 40 to 50 minutes uh inferred from this this means the velocity is our speed of light which is exactly what you would expect if these hot spots would move in in keplerian motion if you like another thing here is that this this emission is polarized so that tells us that magnetic fields are present and from the properties of the change of the direction of the magnetic field with time which we can infer from these data we we find that the b fields is likely colloidal so along the axis of the black hole and not in the orbital axis of the motion that's surprising and probably indicates that the galactic center has a very very strong magnetic field this will be very important for the future to to to do research further in order to understand this strange but highly exciting environment around the black hole so here we are after phase four periods happened now as you see with gravity in particular our accuracy of measuring the mass is better than one percent in fact the statistical uncertainty is 0.3 percent so that's a precision measurement something which you can do not very often in in astronomy and we can penetrate now basically from the sphere of influence where we showed the first measurements in phase one all the way inwards by six orders of magnitude close to the event horizon size are we done well it's very likely that this object we are looking at indeed is a black hole we don't know its spin yet and we would have to know that in order to begin to understand whether the current matrix holds and then we would have to test whether the quadrupole moment of this object fulfills what we call the no hair theorem if we know that then we would in fact have the final result that gr is indeed correct at the scale of the event horizon which is the closest we can come that we do not have yet but that's where we want to go so after these 40 years i would say the galactic center has been extremely helpful to astronomy and to physics two groups have used this to push ever further and many questions remain or go beyond what we have now but we now have evidence indeed that these objects which were theoretically discussed by roger penrose and others actually are realized in nature thank you very much it is my great pleasure to introduce our third 2020 nobel laureate in physics professor andrea guezz the title of a talk is from the possibility to the certainty of a supermassive black hole andrea guess earned her phd in physics in 1992 from the california institute of technology the following years she held a hubble postdoctoral research fellowship at the university of arizona professor guess has been at the university of california in los angeles since 1994 and became full professor in the year 2000 in 2014 she became the head of the galactic center orbits initiative at ucla gia guess has been pushing the frontiers of imaging at near infrared wavelengths using the keck telescopes in hawaii her masterful use of laser-guided adaptive optics allowed her to make detailed mapping of the orbital motions of resolved stars close to the galactic center the ultimate goal was to uncover the nature of the supermassive object lurking in the innermost part of the milky way professor guess we are excited to hear about your fascinating discovery i'm so grateful to have the opportunity to speak in this forum before i begin i'd like to dedicate this talk to two people who were very important in my development as a scientist but are no longer with us first my father gilbert guess who taught me to be curious about the world of numbers and to find their delight and second my phd advisor gary neugebauer who taught me the importance of pay of paying attention to data above all else the work that is being recognized is a 25-year long project that i began when i first started my faculty position at ucla ucla was my dream job because it gave me access to the newly constructed keck telescopes which are the largest telescopes in the world and i was interested in developing new ways of using this telescope to study black holes i like to begin my lectures with the question how do you observe something you can't see this is an essential question if you want to find and study black holes because black holes are objects whose pull of gravity is so intense that nothing can escape them not even light the story that i would like to tell you today is how we've been able to find a supermassive black hole at the center of our own galaxy and this has provided us with the best evidence yet that these exotic objects really do exist but it's also provided us with a wonderful laboratory for studying the fundamental physics of black holes and in particular how black holes warp space-time and what role black holes play in the formation and evolution of galaxies while the black holes are relatively exotic objects and require pretty exotic physics to properly describe them the way i want you to think of a black hole today is an object whose mass is confined to zero volume so despite the fact that i'm going to talk about objects that are super massive and i'll get back to what that means there they have no finite size so what does this mean this means that if we think about density which is mass divided by volume density goes to infinity and in physics anytime you have a number going to infinity this is known as a singularity which is equivalent to a giant red arrow that says you don't have your description of the physical world quite right here so in fact this is what makes black holes so interesting because it points the way forward to understanding a deeper way of thinking about physics today we don't know how to make the world of general relativity which describes gravity near highly gravitating objects so that's what einstein is so famous for work together with the study of quantum mechanics which is a study of things that are very small and black holes are both they have tremendous gravity and they're infinitesimally small so when we understand what a black hole is we'll be able to presumably make these two fields work together but in the meantime fortunately there is an abstract size that we can think about that's quite important to the studies of black holes and this is known as the short shield radius and this is the size that if you can compress an object down to its short shield radius and in fact every object has a short shield radius gravity will overcome all other known forces forcing the object to become the infinitesimally small object of a black hole so if we think about the earth and we could figure out how to squeeze it down to the size of a sugar cube it would become a black hole now fortunately the short shield radius is actually quite simple to figure out it just scales with mass so if we were to look at a more massive object like the sun and scale it down to ucla's campus i couldn't help but put ucla in there it would become a black hole so this notion is important because it not only defines what a black hole is but it gives us a way of demonstrating or proving that a black hole exists we need to show that there's some mass confined to at least within its short shield radius to claim a black hole so in astrophysics there are two kinds of black holes that we talk about and in fact they're a wonderful example of the interplay the different ways that science proceeds so in the first kind that i'm going to call the ordinary mass black holes as if there could be such a thing as an ordinary black hole are black holes that were thought from theoretical work that thought about what would happen at the end of a very massive star's life so a star that starts off with more than 30 times the mass of the sun it's thought at the end of its life it's going to become a supernova and what we're seeing here is that the outer layers explode off into interstellar space and all sorts of exciting and really important things happen here but for the point of view of this talk what's interesting is what happens at the middle in the middle gravity overcomes all other known forces and is expected to become a black hole that has roughly 10 times the mass of the sun so this was identified from theory and then subsequently verified by observations so we have wonderful evidence today of the existence of these stellar mass black holes now the story of the supermassive black holes is quite different and by supermassive i mean a million to a billion times the mass of the sun this is a story which is led observationally and um observations of galaxies and in this picture almost every object in the galaxy is a galaxy only one object in here is actually a star within our own own galaxy so let's focus on the top left corner here this is a galaxy so this is a very different scale than the last picture that we were looking at we were looking at a single star here we're looking at a galaxy that has a hundred billion stars so it's a really different scale and there's actually nothing in this picture that suggests the existence of a supermassive black hole because what this picture is is that taken at optical wavelengths which is what your eye detects so you're sensitive mostly to starlight so the idea of supermassive black holes residing at the heart of these galaxies came from radio pictures radio pictures of a tiny class of galaxies known as active galactic nuclei and the name says it all we think the nuclei or the center is very active of the galaxy and so this is a very different picture so at radio wavelengths so somewhat akin to where your cell phone works and you no longer see the starlight but rather you see these jets of emission emanating out from the center the whole galaxy is roughly um in the in a modest scale at the center so these very energetic jets combined with i the fact that the emission from the center of the star or sorry center of the galaxy looks unlike anything emitted by stars or gas led people to suggest that perhaps it's a supermassive black hole that's driving all this energetic phenomena and in particular it's a black hole that is dining or feasting or accreting a lot of material from its surrounding so these you could say are like galaxies that are having a thanksgiving feast now this led people to think about the possibility that perhaps all galaxies harbor supermassive black holes at their centers but these are rather stealth black holes or rather black holes on a diet because they don't have the material around them to light them up because remember black holes themselves don't emit any emission and in fact this galaxy which is the one we were zooming in in the the galaxy picture looks very much like what our galaxy would look like if we could get it outside it and look back in fact our solar system is roughly halfway out so galaxies in general are flattened disk-like structures so if we're going to ask this question of do all galaxies harbor supermassive black holes or really more profoundly do these supermassive black holes exist our galaxy is simply the best place to look because it's the closest example of the center of a galaxy we'll ever have to study it's a hundred times closer than the next closest galaxy now so the pro in studying our own galaxy is that it's close the center of our own galaxy the con because in life usually every time you get a pro you get a con um is that um we're in this flattened disc-like structure we're about halfway out and we're looking through the galaxy to the center so this is a picture of our own galaxy if you could get out to a dark site this is actually taken from the side of mauna kea and what you see is the galaxy the plane of the galaxy it's lit up with stars so you see it as a band of white light or that's why it gets its name the milky way but in the middle of it you see a dark lane and that dark lane is dust and dust comes is like smog in la it's very good at absorbing optical light so we can't see through the center of the galaxy through the galaxy at optical light so if we think about um the ability of light to penetrate from the center of the galaxy to us optical light what our eye detects only one out of every 10 billion photons or light packets made except from the center of the galaxy to us in contrast if we go just a slightly long word of what your eye detects to the infrared which is where your tv remote control works one out of every 10 photons makes it to us so in fact you can see or perceive what's happening at the center of the galaxy if you go to infrared wavelengths so a key to this work has actually been the rapid development of infrared technology over the last 30 years so if we want to prove most directly the existence of a supermassive black hole at the center of our galaxy the most direct way that we can do this is by watching how stars move so stars move under the influence of gravity of whatever's inside their orbit so their orbit described by measuring how fast it goes around and the size of the orbit you can figure out how much mass is inside the orbit this is just due to gravity it's for the same reason that planets orbit the sun so these stars orbiting this the the black hole or whatever is inside its orbit for the very same reason so if you want to show that there's a lot of mass confined to a very small volume it's the stars that are as close to the heart of the galaxy that are the keys so in a sense we're inward bound in order to carry out this experiment and this actually tells you very much why i was so interested in getting access to the keck telescopes so as i said before the keck telescopes are the largest telescopes in the world and we characterize telescopes by the width or the diameter rather of the mirror the collecting element and these telescopes there are two of them each one has a mirror of 10 meters so to give you a sense of scale this is equivalent to the width of a tennis court now the campaign promise of large telescopes are twofold one is that with a large telescope you should be able to see things that are very faint you can think of it as a light bucket the larger your telescope aperture is the more light that you can collect in a given amount of time so for a typical astronomer you're interested in seeing things that are faint which is equivalent to things far away and this is effectively a history lesson in many cases because it takes light a finite amount of time to travel to us now what i'm interested in is actually the second campaign promise which is the idea that large telescopes the larger the telescope the finer the detail that you can see this is called angular resolution and the analogy i like to make here is it's like the painting technique of pointillism and the closer you get to the painting the better your ability to resolve those individual dots so that as you go closer to the painting you're improving the angular scale of those dots now in astronomy the equivalent is by getting a bigger telescope you should be able to see smaller structures out in space now the challenge or why this second campaign promise has been so difficult to achieve is the earth's atmosphere the earth's atmosphere is great for us it allows us to survive here on earth but it is a total headache for astrophysicists because it distorts our images of astronomical objects so this is an animation of data that we took early on of the center of the galaxy and there are five bright stars that are seen or in this picture and if there were no atmosphere the structure of each star would be equivalent to the smallest structure that you're seeing here and rock solid as opposed to something that looks more akin to a bug splat pattern and it's actually rather profound if you think about this because the light that we're collecting here came from the center of the galaxy it actually took this light 26 000 years to travel from the center of the galaxy to us and it's in the last 30 microseconds that it hits the top of our atmosphere and becomes completely distorted so um i'm interested in ways of overcoming this this problem because otherwise you can't get to the promised resolution of a telescope and over the last 25 years there's been a tremendous revolution in the ways that we've done this so this is an experiment that as you'll see in a moment is benefited by time just the duration of how long you've been making the measurements but it's also benefited tremendously from the advancements in technology which has improved our ability to make these measurements in the beginning we use something fairly straightforward and simple which required lots of computational power but very little hardware and then in the last 10 years we've gone on to a much more sophisticated hardware-based solution and in fact this is nicely connected to the military because it turns out the military cares about seeing both up and down through the atmosphere so they were developing techniques as well so in the early 90s there was a huge leap forward in our ability to correct for the distorting effects of the earth's atmosphere this new technique of adaptive optics the way i like to describe adaptive optics or even the problem of the atmosphere you can think of the problem of the atmosphere in two ways one is as a river that's turbulent and moving by you quickly and the equivalent of seeing our astronomical objects is like seeing a pebble at the bottom of the river and that moving river makes that object look distorted because of the turbulence in the river so our job is to take that turbulence out of our um or the distortion that's caused by the turbulence out so another way to think about this is like a circus funhouse mirror that's warped and makes if if you only look at one of them makes you look um distorted so the goal of adaptive optics is to put in a mirror that has the exact opposite shape or the conjugate shape to take out the distorting effects of the earth's atmosphere and make you look flat again and of course you have to do this at really rapid speeds to keep up with turbulence in the earth's atmosphere so typically this is a system that's running at about a thousand times a second now before we were looking at a little inset screen or an inset box to what was happening at the center of the galaxy so um we're only looking at that little central box to see what happens to the stars there now we actually have a much bigger field of view and we're tracking thousands and thousands of stars but if we zoom into this scale you can see what's going on and at this point it's probably um helpful um to think back um to the beginning of this experiment which was the early 1990s to realize the skepticism with which people held this health this experiment because in fact the first time we put in this experiment proposed to use the telescope it was turned down it was turned down because people didn't believe our techniques those early techniques would work and even if they did we wouldn't see stars and even if they did we wouldn't see their motion so i have to say this is one of my favorite movies to watch so on the left is what we did for the first 10 years that was the computationally intensive but hardware simple approach um it didn't give us much it gave us an image at a single wavelength but it told us that there were stars there and you can probably find my favorite star in the galaxy its name is s02 it probably needs a better name but for the moment it's an it's a name that actually helps us identify where in the field it is now the next 10 years of adaptive optics intro um made the observations much more powerful now we can take measurements images that are much deeper so you can see far more stars you can position these stars far more accurately and you can take images at a at a number of different wavelengths and you can take spectra for the first time and that spectra those spectra are important for a number of reasons one is that for the first time you get to measure the motion along your line of sight so this movie shows you how things are moving in the plane of the sky but you're missing the third dimension so spectroscopy allows you to figure out how things are moving along the line of sight and for the first time actually allows you to explore the astrophysics of these objects in other words to ask what kind of stars are we looking at and i'll return to that in a moment now the next animation i'm going to show you is a fourth of the real estate i mean this is a ridiculously small scale if you recall back to the original animation without any of these techniques it's one blurry image over the scale of this image in other words without adaptive optics or speckle imaging your resolution would be the size of this box so we're going down to a fourth of the scale and that's necessary because things are moving so fast and i never get tired of looking at this animation either it really describes so this is no longer the direct data the last images were real data there was nothing enhanced about them it was just the data that we get shown now this is a animation that shows how the brightest stars that we're measuring not all of them are moving over the last 25 years so this is definitely an experiment that requires a tremendous amount of patience in this animation when a star is detected in an image it's trailed by a dashed line so you'll see that stars appear and they're not appearing because they're getting brighter but rather that your sensitivity your adaptive optics has allowed us to see stars that the speckle imaging experiment didn't allow us to measure and then when it's trailed with a solid line we finally have the instrumentation and methodology that's allowing us to measure the spectroscopy of that particular star so you also you can see that there's a time evolution in our ability to get spectra as well okay so this is just getting better with time the other thing that i'd like to take a pause to just acknowledge is that when these experiments um both those done by my group and the those done by the max planck group no one envisioned orbits we were just thinking about velocities so remember that even the idea of measuring speeds how fast individual stars are moving on the plane of the sky was already a challenging enough concept back in the in the early 90s so the idea there was just that stars closer to the center would be moving faster on average so you're just measuring setting out to measure little arcs so little line segments so that was the first phase of the experiment and yet people in the early um in the late 90s by the time these experiments were being published we're questioning and suggesting that there could be all sorts of other things pushing these um stars around so we kept going and the next thing you can kind of see from this animation is that things are going to deviate from a straight line so then in the um in about 2000 you could measure accelerations and the minute you get accelerations you understand how exciting this project is going to be because you can foresee at that point that the orbital periods are going to be shorter than a human lifetime and here again just consider that it takes the solar system 200 million years to get around the center of the galaxy and here we're talking about things that are happening on a decade time scale so my favorite star there is going around every 16 years and what is that star i mean now we have the most direct so the early velocity ideas was an indirect sort of statistical approach so sort of phase one accelerations were phase two and then the orbits were really the important phase three um with phase three um you should you see an orbit that's going around every 16 years and what does that tell us well before we started this experiment we knew that there was four million times the mass of the sun inside a region that's depicted here with a a circle and at this point you're well outside the short shield radius but what this experiment has done is to show that in fact there's 4 million times the mass of the sun inside a volume that's 10 million times smaller and that volume corresponds to the scale of the solar system so there are a couple of things to point out here if we think about our own solar system there's one sun worth of material there effectively now we have the same region that you have four million times the mass of the sun crammed into that region and this has actually increased the evidence for a supermassive black hole by a factor of 10 million i mean think about anything that you could move forward or increase that you'd like more of in this case this has been a dramatic increase which has really moved us from the possibility of a supermassive black hole to a certainty that there's a supermassive black hole at the center of our galaxy and not only at the center of our galaxy but really of any other galaxy in the universe all right so this is really what is being recognized with this prize but there's so much more that's um that's happened with this work this work which has really given us an ability to see the center of the galaxy in a way that's not that really hasn't been possible before so i want to just very briefly touch upon two things one is our ability to understand the role that supermassive black holes play in the formation and evolution of galaxies when the work was first being um announced in these various stages the thinking about this question was very different it was really the chicken or the egg which came first the galaxy or the black hole so this is like the chicken or the egg problem and in fact we had ideas for how to get either one of these scenarios to work but today what we recognize as more and more candidates supermassive black holes are emerging that that's just the wrong framework in fact the mass of the black hole seems to correlate with the mass of the central part of the galaxy and called the bulge and the scale of the these two objects are so vastly different that we recognize today that whatever form one had to result in the formation and the other and that over time there has to be a feedback mechanism between the two now um one of the my favorite aspects of this project is that it has allowed us not only to address the question that we set out to address but it has raised more questions that it has answered and this animation just shows you touches upon a few we anticipate and in fact it was an early argument against the existence of a supermassive black hole that there should be no young stars and there were some young stars hanging around the outside of that really big circle before which was one of the arguments against a black a supermassive black hole at the center of our own galaxy and today we recognize that there is a supermassive black hole and the stars that we were tracking right at the center are young okay so i like to call that problem the paradox of youth so we have a lot to think about and as you can see in this end actually what i should say is in this animation the young stars are the aqua blue so they are actually the most dominant part of the population that we can see is the young stars which we predict should not be there and in this animation as it pulls back you can see a plane which is providing us with a very important hint as to how those stars arrived um to be where they are we actually think um that there is um was an episode of star formation which probably made our galaxy look a lot like those active galactic nuclei that i mentioned to begin with in our galaxy's history but that was a millions of years ago the other prediction is that there should be lots of old stars old stars have lots of time to congregate or settle into the middle to congregate around the most massive object in the in the system and that's the black hole and the old stars are the orange things and in fact there's a dearth of old stars so that's sort of paradox number two where are the old stars and then the third thing that i find the most interesting is the thing that we didn't even think to make predictions for are these objects that are being tidally torn apart as they go through um closest approach to the black hole these are that have been driven to merge by the supermassive black hole and we think this actually might illuminate important processes that connect what's happening at the center of the galaxy with the gravitational wave detections that have been recently observed with with um with ligo now um the last but not least i want to turn my attention to um using the center of the galaxy as a test or a laboratory for understanding how gravity works in this extreme environment so we know that supermassive black holes ultimately represent the breakdown of our understanding of how gravity works because we don't know how to make quantum mechanics and general relativity work together and the depiction that you often see in science museums is to flatten out three dimensions of space into two and then to use the third dimension as a um to depict time so as you approach a black hole you get this mixture of space and time which is one of the predictions of of relativity and these orbits of stars are providing us with the first direct test of how gravity works near a supermass black hole and there are two tests that are now possible one you can look at how photons are affected by their climbing out of the gravitational potential well they have to give up some energy and in fact as this star goes through closest approach so2 that was actually seen so this effect is actually best seen at closest approach so that was seen in 2018 and i'm going to call that phase number four and then the next thing is the procession of the peri-apps which is how the object itself moves through space time and time and this is best seen at apple apps i should say for both of these you need to see the orbit go all the way around once in other words in order to get a seat at the table for doing these next two projects you have to have the measurements for the first complete orbit which is 16 years so there's a lot of patience to get to this next phase but the signatures are are emerging now before i conclude i'd like to acknowledge that all this work has been done in collaboration with a number of people and in particular with my long-term collaborators eric becklin and mark morris who are shown on the left and also included in this picture is my first graduate student beth klein so this started as a small project with a very with three collaborators today it's a much larger team today we're a collaboration of uh 30 people and along the way you know over 25 years we've had four or five generations of students who have been trained to do all sorts of wonderful things they've gone on to academia to industry to the financial world to all sorts of amazing jobs that this kind of training opens the door to all of this work has been done with support from a large number of organizations over the last 25 years i in particular want to acknowledge the backbone of the support which comes from ucla and the national science foundation and then along the way we've just we've been so fortunate to be supported by those who support early faculty work with the sloan foundation the packard foundation and the macarthur foundation and then more recently we've been supported by the keck foundation the moore foundation and the heising simons foundation and last but not least i'd like to recognize the role of the galactic centers group star society group in particular the chair of our board arthur levine and his wife lauren likeman who have been just amazing supporters of this work for the last few years so let me conclude with if nothing else i hope i've convinced you that we have remarkable evidence for the existence of a supermassive black hole this evidence has increased our understanding of the presence of a black hole by a factor of 10 million which has moved the idea of black holes from a possibility to a certainty and the revolution and technology that has come with this has opened more questions than answers so there's more work to be done and thank you for your attention you

Channel: Nobel Prize

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Id: DWF1uNb9Q1Q

Length: 101min 46sec (6106 seconds)

Published: Tue Dec 08 2020