Darkness Visible: Shedding New Light on Black Holes

so thank you for being here this evening this conversation about black holes and you know you'll find online that Einstein himself is often quoted as having said that black holes are what happens when God divides by zero he never said that he couldn't have the phrase black holes was coined about 10 years after Einstein died but poetically that's actually a fairly good way of describing what it is that's gonna be the focus here tonight because dividing by zero is a it's a very extreme mathematical operation right and black holes are a very extreme physical object in the universe and what we'd like to do here tonight is of course you know start gently get everybody up to speed and then take us right to the frontier of understanding these real gargantuan monsters of the universe now to get there your number of ways that you can think about black holes but one nice way in to the subject is to think about something that's much more familiar thinking about escape velocity let me describe what I mean through a couple of sequences here so imagine that you are on the surface of the earth so let's sort of zoom down right to our planet and we ask ourselves if we fire a projectile from the surface of the earth upward what will happen and we all pretty much know what will happen right so if we take a cannon and we fire a cannonball at a fairly modest speed it's going to go up it's going to come back down if we fire it with a somewhat larger speed it'll go up higher but still it'll come back down but finally if we launch it with the right speed it will go up and it will just barely escape the gravitational pull of the earth and it will go off into space and that speed required for that to happen is what we call escape velocity at the surface of the earth now what is the escape velocity at the surface of the earth yeah if someone actually said it was 11.2 km/s thank you Wow a gold star in the back there about 11 kilometers per second is what you needed to surface the earth but here's the question if you were to look at a different planet one that's bigger than the earth what will happen well again you can pretty much picture what will happen if it's bigger it's more massive you're going to need a bigger cannon to fire that cannonball with higher speed because the escape velocity will go up it'll be larger than it is at the surface of the earth but of course if you have that big cannon you actually fire it the Cannonball will go up and again if its speed is bigger than escape velocity it will be able to get away but now I want you to think about something a little bit less familiar imagine that this cannon is not firing cannon balls but it's firing balls of light photons now light goes really quickly right I mean light speed what does this be the light above example yeah everybody uses different units which is nice 671 million miles per hour 300 million comments per second right and meters per second I should say so at that high speed of course light will easily be able to escape and be able to go off into space but here's the interesting thought experiment and this is a thought experiment that goes way back this is an experiment that this fella over here John Michell this is in the 1700s right long time ago he asked the following question he said look what if you were to imagine looking at say a star like the Sun now clearly the escape velocity at the surface of the Sun is much less than the speed of light so certainly all the light that the Sun emits easily gets away but just as the escape velocity of a planet goes up if you make it bigger more massive he asked well the same should happen for a star so let's imagine making the star bigger where the escape velocity goes up now if it's still less than the speed of light the light will get away but he asked what would happen if you made the star so big that the escape velocity at its surface would be bigger than the speed of light right in that case he imagined that if you made that gigantically massive star light could not get away the escape velocity would be bigger than the speed of light and if light doesn't get away the star would go dark a Dark Star now this is again in the 1700s right so he is thinking purely in a Newtonian framework that's the only description of gravity that we had back then so the natural question is is this musing of John Mitchell right this theologian this natural philosopher from the 18th century does this idea have relevance when you start to think about gravity in the manner that was given to us by this fellow over here Albert Einstein because I think as we all know in the early years of the 20th century Einstein rethought our understanding of gravity and he gave us the general theory of relativity in which gravity is now thought of in a completely new way not the Newtonian description gravity is thought of as warps and curves in the fabric of space and time so Einstein takes this idea this new way of thinking about gravity he writes his famous paper on the general theory of relativity this is in 1915 his paper becomes widely circulated and indeed about a year later 1916 on the Russian front there's a German astronomer mathematician named Karl Schwarzschild and he's out there in the trenches charged with calculating artillery trajectories and somehow just by coincidence what happens is Einsteins paper just kind of goes by he grabs ahold of it and he gets so captivated by Einsteins ideas that he forgets about artillery trajectories and starts to calculate with general relativity he finds that if you have a spherical body that you crush down to a very small size according to Einstein's math the warp in the fabric of space would be so extreme that nothing can pull away not even light can pull away so it's now a modern day version if you will of what John Michell had imagined an object that goes black because light cannot pull away from it so roughly speaking it would be as if you had a flashlight near the edge of one of these objects and when you turn on the light instead of a light going off into space like it's pulled down into the hole into the black hole this is the modern-day version of what a black hole would be now the term black hole it turned out this was coined on 112 Street and Broadway I'm not joking at the Goddard Institute of Space Studies on 112 in Broadway John Wheeler was given a talk and this way of describing these dark stars came up and wheeler pushed this idea he popularized this a he advanced our understanding of it but this is where the term black hole comes from now this of course is a sort of cartoon version that gets at the basic idea for those that want to see it a little bit more precisely here's really what goes on near the vicinity of a black hole and if you don't understand this it doesn't matter we can put down a spacetime diagram if you remember from high school this is where we have time on this vertical axis and you set off a beam of light that fills out a cone so-called light cone and what happens is the geometry of space and time is so distorted by a black hole that beyond what's called the event horizon the direction of time and space is so twisted that as light propagates it cannot get out of the edge it cannot go beyond the event horizon of the black hole and that's why no light can get out that's why the black hole is black now natural question is okay these are interesting ideas but how in the world would one of these objects come to be and people began to think about this idea for a long time 30s 40s 50s and let me give you one possible scenario by which the kind of object we're looking at a black hole would form and for that we can imagine that we have a large star like a red giant to support its incredible weight this star has nuclear processes taking place in the core that generate heat and light and pressure that props the star up but sooner or later the star uses up all of its nuclear fuel and at that point it can't support its own weight so it begins to implode and as it implodes it gets hotter and denser finally setting off an explosion that ripples through the star and when the explosion reaches the surface of the star it causes the outer layers to explode and what remains if the star was big enough to begin with is a tiny core a dense core that can no longer support its own weight at all and it will collapse all the way down into one of these objects these black holes that's the idea of how these objects could form and what we'd like to do here tonight is explore our current thinking about these objects are they real how would we actually see them and can we get any insight into what happens inside of these spectacular objects and to do that we are going to bring out some experts who spent their careers examining these very questions and let's get to them right now so our first guest is one of the world's leading experts in observational astrophysics who heads UCLA's galactic center group best known for a groundbreaking insights on the center of our galaxy she's the winner of among other things the Crawford prize in astronomy from the Royal Swedish Academy of Science Anna MacArthur Fellow please welcome Andre agos all right also joining us is an astronomer at the harvard-smithsonian Center for Astrophysics who leads an international collaborative project called the event horizon telescope whose goal is to image the edge the event horizon of a black hole so please welcome Shep dollar mark [Music] all right so thank you both for joining us here tonight let me just begin with sort of one general question so people have been thinking about this idea of black holes for a long time as I said all the way back in the 1700s and a lot of research has been done thousands of pages of calculations do you think that there are really black holes out there or our theorist imaginations overworked I think it's pretty clear that there are black holes out there of course I'm a little biased since you spent your life trying to observe them yes that's it think about the fact that there are two kinds of black holes yes black holes that you were just talking about the ones that come from the lifespan of stars and then the supermassive black holes that we think are at the center of the galaxy and those are the ones that you've actually been studying in some detail so that we're gonna get to those in just a moment but Shep your general view is more or less the same or do you think there's a chance that it's a red herring that these black things are not really out there oh no it's beyond a shadow of a doubt I really think they exist I mean there's all these lines of evidence you know we see these terrifying engines the Centers of galaxies that spew out these Jets on either side of them and the only thing that can power them are supermassive black holes so everything is pointing to the fact that these really do exist good so I'm glad you're saying that because had you both said no I don't know what we do with the rest of our time here today but that's great so so Andreea your work as I understand it has been focused on the center of the Milky Way galaxy so first of all just just give us some sense of what you think is residing in the center of our galaxy and then we'll try to look at the evidence that led you to come to that conclusion so we are pretty convinced that there's a supermassive black hole at the center of our galaxy and how do you go Superman when we say supermassive we mean a million - well in the case of our own galaxy four million times the mass of the Sun and in terms of these really big ones that are at the center of galaxies that's on the low end because we think about things that are a million to a billion times the mass of the Sun and for a million solar mass block like how like for the Sun let's maybe we start simple if the Sun was turned into a black hole how what would its radius be it would be about the size of college campus no it depends which college you're talking about good so so a couple kilometers across for the one in the center of the galaxy how big do we think it is it's about ten times the size of a sudden so about ten mil 10 million kilometers Wow so it's it's it's a big object but it still follows the same basic pattern that it has an edge and event horizon and all the standard lore would apply to it it's just on a bigger scale right it just scales simply with math math yeah so so so what evidence do you have that it is a black hole can you sort of take us through that yeah so to prove that there's a black hole directly what you want to do is you want to you want to show that there's a lot of mass inside a small volume or inside a small region in particular you'd like to show that it's confined within its short sheild radius that you were just talking to our latest is is the radius for a given mass where if you can crush the mass within that radius it will naturally turn into a black hole right and so that's what we're the size that we're talking about because of course the black hole itself is infinitesimally small so this this isn't that the abstract size and so our job just clarify that so when you say that you're talking about when the black hole forms the matter crushes down to a small size yeah so the idea of the Schwarzschild radius or is that no light can escape it as we were just talking about but it's also true that once you get the mass to that scale gravity will overcome all other known forces and there's nothing that can stop the collapse of the object so from a scientific point of view once you've shown that a mass is within its Schwarzschild radius you have come up with for the proof of a black hole so from the point of view of somebody who's hunting for black holes your job is to show that there's some amount of mass inside a small volume so the way we've approached it at the center of the galaxy is to look for the stars that are at the heart of the galaxy and to develop techniques that allow us to not only see the stars that are that close but that allow us to observe how they go around the center so if you want to find the center of the galaxy you can look up in the night sky and find the constellation of Sagittarius it's the teapot and the teapot pours into the center of the galaxy all right that's your road is so convenient and if you look up at the night sky not in of course New York but in a place that you can actually see the night sky you can see the Milky Way and the Milky Way is that band of white light that comes from the Stars but there's also a lack of light which is from all the dust so you can't actually see the center of the galaxy at wavelengths at your eye detects so a key to the work that we've done is to use infrared technology so looking at light that is just long word of what your eye detects maybe we're a TV remote control works yep and that allows us to see the stars that are at the center of the galaxy and we'll have you found and we've found that they go or one that we can see them which is rather amazing and that they go around the center of the galaxy quite fast so those my favorite star in the galaxy it's name is so2 goes around every time again so2 okay probably needs a better name yeah it's real catchy so if you have a better name we'll leave it to the audience to figure that out but you can use actually Newton's laws of physics to show that if you go around every 16 years and you measure the size of the orbit which is about the size of our solar system that shows that there's about 4 million times the mass of the Sun inside this incredibly small region and and to give you a sense of the change of our knowledge over our understanding of what resides at the center of the galaxy we've increased the density of dark matter by a factor of ten million compared to what was known before artwork so in a sense we've we've advanced the case for the existence of supermassive black holes by that amount may think about anything in your life that you'd like more of and being able to get ten million times more of right and that's what's happened at the center right so the basic argument as I understand it is you're tracking these stars and their motion can only be explained if there is a black hole of that mass residing in the center of the galaxy you're basically weighing this what's at the center there was this actual data this is the real thing so we've looked at two versions of it one is the flat version which was show uh was playing just a moment ago and and it showed my favorite star and this is actually a bigger view of the data that we took that we've taken over the last and I can't believe I'm saying this twenty-five years have you been involved with it maybe I shouldn't ask oh yeah this is my been since the beginning this is I mean this is my baby it is so in fact it's uh it's interesting to reflect back because when I first proposed this experiment when I first got my job at UCLA I thought I had a good idea it was actually turned down they said the technique wouldn't work and even if it did we wouldn't see stars and even if we saw stars we wouldn't see them move so it was a lotta no no no's right and in fact we were asking to do a project that was only three years long just to see that stars were moving fast no one anticipated that they were moving they would move so fast how fast is fast just to give it oh oh like three million miles an hour so they're they're hauling and it's and it's rather remarkable that we can measure something on a human time scale and so as this project has gone on and maybe we can talk a little later about you know what it takes the technology has changed so much that it's enabled us to do more and more sophisticated kinds of work so this three dimensional animation actually shows the kinds of stars that we see at the center of the galaxy and almost every single prediction for what we should see near the black hole is inconsistent with the observations what does that mean that means it's job security you're trying to figure it out yeah but but it's not inconsistent yet with say the general relativistic prediction no so there's both the physics side of this work where you're trying to ask physics questions like do supermassive black holes exist how does gravity work near a supermassive black hole so the where we are today is that we can definitive or at least in my opinion we can definitively say that the supermassive black hole exists and then the where we are actually we are so in the middle of this can we test Einstein's theory of general relativity and that is what your you're doing now if I understand like we're actually a special time at this very moment right yes we're in such a special moment I can't believe I'm actually sitting here as opposed to being in Hawaii thank you for taking the time with but tell us tell us what's going on so the reason I'm so excited and we've been preparing for this for years so we've been doing wait there's no chance you're gonna miss like the special moment but I have grad students so we've been using the Keck telescopes which are pictured here for 25 years and watching the star that goes around every 16 years and if you want to test Einstein's theory of general relativity near supermassive black hole with these stars what you have to do is first make your first go around that gives you a baseline of what what part of space these stars are probing and that's 16 years right there that's 16 years right there my various moments in my life around the star and then what you want to do is you want to catch it the next time it goes through closest approach and that next time was the year 2018 so I've been thinking 2018 or bust for a number of years so we're in it we're in this season and for us there really is a season because the earth goes around the Sun so and because we're looking at infrared light you're gonna hear from Shep there are different kinds of light so Shep doesn't care [Laughter] because the earth goes around the Sun there's only a part of the year that we can see the center of the galaxy at infrared wavelengths so for me we can see it from roughly March to roughly October so there was the beginning of the season and through these roughly six months this star is experiencing incredible accelerations and is experiencing the most extreme forms of gravity as it makes its closest approach so actually there are three key moments one that happened April 10th one that happened roughly last week and one that's happening in September that are going to nail down this experiment so it is it's an exciting moment for us and to see the signal emerge from the data it's it's just a treat so if I understand correctly have a prediction based on the general theory of relativity what the trajectory should be well here we have to be a little bit careful because there's a series of kinds of experiments that you part me there's a series of tests of gravity one is how the the light from the star makes it from the star to us in other words how it escapes the curvature of space-time that's actually the first thing that we're getting at this summer the next thing is then how the object itself moves through space-time which is actually should emerge over the next few years so again and if you keep going and of course that's what we want to do you can actually measure the spin of the black hole so this experiment just keeps getting better right and and it's particularly interesting I gather because you know most people think that Einstein's general relativity has been confirmed but is that the right way of thinking about it well it's uh you know it's one of the gravity is one of the four fundamental forces but oddly enough it's the least tested of those forces so it's been tested in some regimes but it's never been tested near a supermassive black hole and as some say it's a supermassive black hole or black holes and generals represent the breakdown of this theory so what you want to do is you want to get as close as possible to the Hat point where you actually know that theory is no longer holding up and I think we have to have today all sorts of pieces of evidence that says this theory is fraying at the edges so we just push that frontier forward by a large amount in a direction that hasn't been explored before so in principle you could find the first concrete evidence that we need to go beyond Einsteins ideas to really describe what's going on I mean the best of all worlds that would be the outcome of 16 plus years of observation well the outcome is actually just figuring out what's really happening your the black hole whatever it is yeah yeah yeah it's spectacular so Shep you you were also in the business of looking at black holes there's no business like black hole biz no business like back over there and so you're going about in a different way so we're hearing about infrared light as the probe being used to in andreas work you're using what radio yeah we're using radio waves it turns out that black holes in a paradox of their own gravity are some of the brightest things in the sky right and that's because of a very simple construct all the matte all the gas and dust is trying to get into a very small region so it heats up to hundreds of billions of degrees around Sagittarius a star the supermassive black hole in the center of our galaxy and it radiates in infrared that Andrea looks at and also radio waves even little bit in x-rays so if you want to look at a black hole you can come at it from many many different angles no no critical to both of what you're saying is you're not really looking at the black hole you're looking at its effect on its environment right so you can't actually see it per se exactly what happens in the black hole stays in the black hole yeah let's just get that out of the way right now but what we do is we tease around the edges right so in that cannonball analogy you had before light was leaving the black hole but it also orbits around the black hole just think about that for a minute light orbits something right and goes around in a circle and Einstein 100 years ago when he came up with this general theory of relative those equations show that you should see the silhouette of light around the black hole and that's because of these light orbits around so we look at it and you see light moving around the black hole and it gets brighter on one side on the other side you see something that should be about five times the source or radii across right you know how how big it should be so the event horizon is here you're looking at five times that distance exactly exactly so you never see inside the black hole but you see outside and that shows us the geometry of space-time when you see something like this when you see this shadow feature you're really looking at the deepest puncture in space-time that we can imagine right and the ground it's actual have you is this what you guys I mean can you share with us what you guys have seen this is the event horizon telescope I gather you're talking right so they so the question is if you wanted to zoom in by orders of many many thousands and see what was happening right the edge of a black hole you'd have to go way beyond where aundrea sees her stars and go much closer and those stars are about a thousand times farther out than this silhouette that you're seeing here right and if we could measure the size of the silhouette in the shape we test Einstein's theory of gravity right at the edge of the black hole right now how are you doing that there is a handful of radio telescopes I gather yeah so to see something this small these are the smallest objects in the known universe right black holes are tiny and to see them you need magnifying power and as it is with all telescopes the bigger the telescope the more magnifying power you have now we can't make a one huge telescope that sees radio waves what we do is we install atomic clocks had multiple radio dishes around the world we record data and then we play it back at a central facility and we create a virtual telescope as big as the earth ourselves if there's as big as the earth itself these are some of the people who who work on the on these various places how many teams are there well we have eight geographic locations right now we're going to nine and then ten the year after next and when you stitch together all of these telescopes you wind up getting a virtual dish that's the size of the earth that is exactly tuned it turns out to image the supermassive black hole in the center of our galaxy and we've just taken some of the first data from this event horizon telescope one year ago and we're crunching on the data now and what have you what do you found I can't tell you oh come on no it turns out that if you if you really want to make one of these images it takes a long time to calibrate the data it's all about the details a lot of people think that we just turn this telescope on and we'll see something immediately but it's you know we're nerds at heart right and we just love to get to the telescopes and find all these details and it turns out that you really have to run down every single one of these possible sources of contamination the data before you can be sure that you've seen this kind of shadow silhouette yeah you know we were able to actually talk to a couple members of the of the team who slipped us actually some of the data I hope you don't mind if we if we show it here you show the show the this is actually I'm understanding this is the the most precise image ever of a black hole do we do we have that can we bring the lights down and show this is that is that doable yeah so so yeah all right there there there it is very good yeah so think that's good not for us thank you no so um so uh you can look the real nervous there for a second yeah so so when will you be releasing I saw an article just a couple of days ago which is kind of a teaser it seemed like for a release of data that's coming up is that soon and no it'll probably be in the first part of the 2019 2019 yeah because right now we're crunching the data you know we know that the event horizon telescope worked so what we did was we also looked at quasar supermassive black holes that are really far away where basically point sources and everything seems to have worked perfectly technically on the telescope so so we know that all the systems are a go and then we turned all the telescope swivel to look at Sagittarius a star the supermassive black hole in the center of our galaxy and and we think everything is working fine there but we're still crunching on the data so and and the data I saw some article that had come from the South Pole and you had to wait for the the winter to clear to fly that was that the kind or yeah that's going on or well so that the whole point as Andrea said if you want to test Einstein's theory you've got to go to the most extreme points in the universe you've got to go to the ultimate proving ground which is the edge of a black hole and we have to go to some pretty extreme places ourselves right so we have teams that go down to the South Pole we go to the tops of extinct volcanoes where there are radio dishes that do the work that we want to do we go to Hawaii Mauna Kea high desert plains and Chile and you go up to these sites and it's really a bit of a labor of love because all these teams go there they work their hearts out they capture data in this technique that we use this event horizon telescope technique is really the ultimate and delayed gratification all right go see here's what you do it so what when Andre goes to our telescope it's pretty straightforward from conceptually the light bounces off a paraboloid it goes to the focus that's it right you get what you want right there what happens with us is the light hits one of our dishes it's stored through high-speed instrumentation that we've built over the last decade on hard disks the same kind of hard disk that you would get in your computer and they stay there until they're brought back on an airplane because nothing beats the bandwidth of a 747 filled with distress nothing ok even when I'm walking down the hallway with two of these disks you know I'm beating the fastest internet in the world ok and we bring them back together and the operation in this supercomputer that we use is equivalent to light bouncing off of a perfectly shaped parabola joining in coherence alright so we play it back together and we adjust it back and forth until we get it just right and that effectively turns the earth into a parabola if you think about that so all of this data has to come back and if it's at the South Pole it's in a deep freeze all right so we've got to wait six months just to get that data back right all right so that's one of the delays that we've been faced not just now you have done some simulations of what you anticipate emerging from the data looking at the magnetic field and vicinity of a black hole can you take us through some of the things that you anticipate emerging from the study I think we have some things up Serena so what you're seeing here is the best guess we have from your high speed simulations of what you would see if you had infinite illusion goggles right so you wind up seeing this this shadow feature the circular feature with some jets leaving from the North and South Poles and that's because there are magnetic fields right around the boundary of the black hole there's relativistic particles they're orbiting these magnetic fields and they're releasing something called synchrotron emission which is kind of a characteristic radio emission you get from these kinds of sources and it's so bright in that synchrotron emission that it shines out from the deepest part of the gravity well right so you so think about it everything has to go right it's a Goldilocks situation you have to be able to see through the Earth's atmosphere and radio waves can do that you've got to be able to see through the distance between the earth and the galactic center radio waves can summarize that just to give people a sense of it so it's about 25,000 light-years away good right so it's you know this black hole is not threatening to us we observe it it's nice so these radio waves can go all the way to the you know from the black hole but then we're not done yet because it has to go through the hot gas swirling around the black hole right and then it has to go all the way down into the gravity well so it's a Goldilocks situation because we meet all those criterion with radio waves and it turns out that the earth happens to be just the right size so that when you look at radio waves with a wavelength of one millimeter they're perfectly tuned to take the picture of Sagittarius a star that's it with an earth-sized dish it was just a resolution yeah so sometimes nature throws us all these curve balls you know what we can't do this or this is hard this is one case where everything's falling into place and so we really think we have a good shot of taking a the first image of a black hole and and do you have a chance as well of finding a deviation from the general theory of relativity can this be viewed as another extreme testing ground what we're looking it's never a good idea to bet against Einstein okay I don't make it a point in my career to do it you know but it is a trust but verify situation okay I mean he's a he was a very smart guy let's just put it that way but every theory needs to be tested well when you say he's a very smart guy that true but he wasn't a great fan of this idea of black holes at all right I mean he kind of didn't think they were real well well this out he was a little bit off on that one right okay so he had one bad moment but but this really speaks to this kind of golden age that Dondre was talking about with the event horizon telescope where the observations are being made by the Keck were really in this discovery space for black holes and we could be at the moment when we can start to answer these questions like do black holes exist was Einstein right at the very black hole boundary I mean you know you showed short child in the trenches in World War one yeah you know and he died later that year actually so this was his last big discovery and he wrote down the Schwarzschild metric and he gave what the decay my space outside of black the shape of space outside the black hole and and now we're kind of engaging in this handshake across 100 years where we're kind of completing this circuit and we're saying you know you made these intense predictions and we're just at the point now where we might be able to test them right and that is extraordinary and and it speaks to the fact that science is not linear we don't go from point A to point B we don't say we're gonna March and test this it's very erratic and and that's why Einstein felt black holes might not exist it took a hundred years for them to become part of our lexicon right you're part of the reality of our everyday conversation now do you do do either of you or both of you as you're working on your observational projects do you have pet ideas or pet theories about what the next phase beyond Einstein might be or do you just basically just go forward into the data the observations and and that's the only thing that's really driving you or do you have a an idea of what might be the next phase of this understanding of gravity well well let me just take this one second I'm a real I'm a realist I'm kind of a craftsman at heart like I like to go to the tops of mountains and make observations I like to see what's around the black hole right I can understand and wrap my brain around the light that's coming outside the event horizon what's inside the event horizon you know that's a question that is hard to even ask you know little Otis asked it I didn't order to answer it I did it's easy to wonder out ya know but but but for example one thing we're looking at with the event horizon telescope is to see if that silhouette is not round ah what if it's distorted okay and if it's distorted then we have some framework of understanding how general relativity itself might be violated to give us those strange shapes so we're betting on Einstein we're betting it's going to be circular but if it's not we have some ways of understanding what might make it non circular yeah so we're winding down to the end of our section but Andre I want to ask you a question which is how do you proceed in an era when you might be going beyond ensign when do you know that you're right when when do you know that things have coalesced to the point that you're willing to make a statement of that sort interesting an important question that we're really the thick of because if you see things that don't make sense you you don't have a context you're you're kind of air and you're exposed and you have to convince yourself that what you're seeing is physics as opposed to experimental error right err err err so III think there's a an interesting philosophical point here about how do you convince yourself you've got the right answer yeah I mean there was an interesting case with bicep2 oh yes a few years ago where some have said that they so knew what they were looking for that they were biased in assessing what the dad was telling them it's a classic thing called confirmation bias so and and this kind of work where we have such a respect for Einstein and his ideas that we go in with a premise that it must be correct so it is interesting in terms of how you actually design your team's work to avoid getting a result you believe to be true and allowing your team to really trust what the data is telling you so I think that's a that that's certainly my goal as a scientist to get to that point where you're really just listening or paying attention to the information that might be unexpected but to try to remove your your desire to have any particular answer be it Einstein right or be it Einstein wrong yep just to be open to whatever the answer truly is right yeah well I should say you know all of us revere Einstein but at the same time how thrilling would be if either or both you find evidence that we do need to go beyond the insights that he gave us 100 years ago so we wish you well and we'll have you back a year or two maybe then you'll be able to give us some insight into what you guys have found so everybody please join me in thanking our great guys Jeff : oh thank you very much in 1916 this is year after Einstein wrote his paper on the general theory of relativity Einstein continued to think about the theoretical ideas and he wrote a paper which we have here where he was thinking about the possibility that if space and time can warp and curve then it might be the case that space and time can also ripple right the image to have in mind is think about trampoline right a trampoline you put something heavy in the middle it has a nice curvature to it but if you have kids that are jumping all around on the trampoline the shape doesn't stay nice and static it ripples it vibrates so he was wondering whether it might be the case that space itself might be able to undergo these kinds of ripples these kinds of vibrations and this would be known as a gravitational wave now interestingly he wrote a first paper in 1916 in 1918 he corrected an error in the 1916 paper and it continued to struggle with whether or not he actually believed that gravitational waves were a prediction of the general theory of relativity and he worked on this by himself he worked on it with collaborators Nathan Rosen being one of them and a couple years after this paper where they expressed some confidence that gravitational waves were real Rosen writes another paper where he basically says that he thinks they're all just a mathematical artifact and I think many historians think that Einstein himself kind of had that view that the really weren't these ripples in the fabric of space and yet by the 1960s when the mathematical methods had been refined where we could really look at Einstein's equations and extract from them the actual physical predictions with certainty it became clear that gravitational waves were a prediction of Einstein's theory which would mean and that if you had say two objects like two neutron stars rotating around they would so disturb the environment that they send out this train of gravitational waves and that would mean in principle you could detect these because downstream if you're in the wake of one of these gravitational waves you will experience this kind of an effect of stretching and squeezing and stretching and squeezing now I should say this animation is not to scale when you actually do the calculation you find that for a typical Astrophysical phenomenon you'd find that the stretching and squeezing would be less than an atomic diameter so the question is how could you ever measure something so fine and yet as we'll discuss now exactly that kind of a measurement has been achieved and so to talk about that let's turn to our next guest and let me make sure that I'm able to introduce her appropriately and I believe I can do that now okay so joining us to take a closer look at gravitational wave has the lead astrophysicist in the LIGO scientific collaboration she is a distinguished professor of physics and astronomy at Northwestern please join me in welcoming Vicki Calogero [Music] so thank you for being here thank you and we'd like to get some insight into how gravitation waves have been detected and LIGO is the facility so what is LIGO Stanford LIGO like or is the double acronym actually it stands for laser interferometer gravitational-wave Observatory and laser itself is an acronym so it's a double acronym and what's that acronym laser is such engrained in my head I don't remember the full acronym and I think occasion by stimulated emission of radiation but I'm not sure all right so okay so here's our detector here's our our telescope our gravitational wave telescope it's a new type of telescope it doesn't look like your traditional telescope does it so it has this corner station where you can see it there can we run that again yeah thank you yes and and then it has these tubes and it's like tubes of a regular telescope except these are on the ground and there are two of them and from the corner station which should down lasers in a triangle and they travel down these four kilometer tubes vacuum tubes and they reach the end they bounce off mirrors and they come back and they we study the light the laser light that comes together and through the interference pattern of the laser light we can tell whether the mirrors at the end of those arms are being shaken in these squeezed and stretched squeezed and stretch motion that gravitational waves are supposed to affect space and time but space is what we can think about easier and you can actually measure shaking by atomic distances in fact it's even smaller than that it's it's a smaller than one thousandth of the nucleus on the scale of the four kilometers it's the most accurate measurement we have ever achieved humans have ever achieved anywhere in any field of science or an engineering now how do you know that the shaking is a gravitational wave and like not someone just kicking the equipment there's a lot of shaking going on everywhere and that was the reason why we didn't just build one of these detectors we built two detectors one in Louisiana State and one in Washington really far away from one another because if the shaking is happening and it's affecting the one detector and it's coincident I'm sorry and it is earth-based then it's very hard to reproduce the exact kind of shaking the exact kind of squeezing and stretching and have it happen in two different locations independent locations so far away so having coincidence as we call it at the same time with exactly the same squeezing and stretching pattern it was extremely important so we needed to be able to claim such a an unprecedented claim that we detected gravitational waves it was really important to have observations of the exact same signal at the same time in two different independent locations and this in this first happiness was the first achieved in 2015 got the world's a 40 well the world didn't know on September 14 2015 some of us did and that was a life-changing day for the hundreds of scientists and engineers who are members of the LIGO scientific collaboration the world found out On February 11th 2016 when we made the first an announcement and and so what was found so what was found is that basically two black holes one in orbit around the other were disturbing space-time not very close to us not at the center of our galaxy but actually over a billion light years away at some other galaxy and the two black holes were coming together because of the emission of gravitational waves they were disturbing space-time around them generating these ripples that you talked about earlier and these ripples were travelling for over a billion years at the speed of light and on September 14 they came approached the earth from the south they hit our Louisiana detector first and seven about seven millisecond laters they hit our Washington State detector second and that's what you expected is that how long we take light to travel that's naughty so there has to be a finite delay because of course gravitational waves just like light doesn't travel instantaneously it takes incidentally next time so the two black holes as they were coming together in their orbit losing energy because of gravitational wave emission at the end they merged like in this other movie as well they emerged and they I'll explain the sound in a minute because it's not self-explanatory yes not really not everybody gets it well I did a version of this on Stephen Colbert show and his interpretation is God Bugs Bunny that that's how he should say Stephen Colbert is a northwestern undergraduate look at that and we have talked about the graduation of discovery so so the two black holes are coming together and eventually they have nowhere to go they're coming and they're they're basically physically touching except there is no actual surface there is no hard surface that is that imaginary surface where light can't escape and the two black holes merge into a single bigger black hole they form a single black hole and then that single bigger black hole settles and the disturbance stops and the gravitational wave signal stops so it's a finite transient signal there is this whole turmoil in space-time and by the time you form the single signal whole the whole thing ends now we talk about the actual signature as shaking these devices in Washington and Louisiana by less than an atomic diameter yeah but what was that signal like when the black holes actually collided way out there a billion um I don't have that particular number for you but it does scale as one over the distance so if we take 1.3 for that first detection if you take 1.3 billion light-years away it was that many times bigger when it was generated so I'm calculating yes so that would be about 50 times the energy output of every star in the observable you so so that's so the basic lesson though is it's a huge huge explosion or gravitational wave energy for a very small amount of time so the signal was 0.2 seconds and for that 2 seconds when it that lasted within our the frequency range that LIGO the LIGO detectors are sensitive to so you should think you heard about electromagnetic waves and they come in different frequencies our optical is the optical frequency range infrared radio waves etc these are all different frequencies of the same type of wave electromagnetic waves gravitational waves have frequencies themselves so like God can't see every single gravitational wave out there can only see frequencies between about 20 Hertz to about now maybe hundreds of Hertz let's say 700 of Hertz at best right so in that frequency range this first binary black hole collision lasted about 0.2 seconds right in that short amount of time the collision and energy generated in gravitational waves outshined by a factor of 50 all the light generated by all the stars in the whole visible universe not visible by eye but in the whole universe that we know often we could ever detect and it just dilutes as it travels hey that's right yes so so you know I've you know been asked about this is such an amazing discovery that you and your team I mean it's just it's fantastic but how I'm watching can we look at the data because I don't want to ask you a question yeah so can you actually go back to the sound by the way that that's what I'm gonna come back right now so can you bring up the actual data of the of the two colliding black holes do we have that up there great yes can you bring up can you raise the volumes I can't really hear it so that's the tiny signature that you're talking so you can just hold it up there if you would so here's my a few things please go ahead well my general question is how can that that little tiny window of data give you so much insight into what the source was yeah so that so I the reason is that this scribble that we can see on the screen and the banana that you can see on the screen carries a lot of information okay and I'll take a few you know maybe a minute or two to explain it first let me start by explaining the significance of the sound okay I don't want anybody in this room to have any misunderstanding gravitational waves are not sound waves okay so don't go away telling anybody that gravitational waves are sound waves however we can take the frequencies remember I said something about 20 Hertz to a few hundred Hertz if you know something about music and the frequency of sounds that are really sensitive to that's about the range of the frequencies of sound that our ear is sensitive to so we can take the gravitational wave frequencies and pretend it's a sound it's not a sound but pretend it's a sound and convert it into sound and say what would the gravitational wave sound like if it were a sound and this is what it sounds like it sounds like a birds chirp which is not quite what you heard but that's how it came out my mouth but can't see one question on that one question that so it's laudable to be clear on that but just to also because if our eardrums were able to vibrate via the gravitational wave influence if if them that is what we would kind of if there is one distinction okay if I may that it's not a transverse wave and Excise so sound waves are transverse waves so the oscillation is along the direction of propagation gravitational waves and electromagnetic waves oscillate perpendicular to the direction of propagation right and that kind of has it's important for how it affects our geology okay so if we go back to the image up there the oscillation that we measure how the mirrors at the end of that l-shape telescope are being displaced are basically recorded by the scribble we can see at the bottom of each banana the two graphs are what we measured at the two different detectors a Hanford in Washington Livingston in Louisiana so we had the signal was detected independently in the two detectors at the same time with only a slight time delay as I said and what you see in the signal the real data is the scribble so we see that the oscillation of the mirrors increase as time progressed the duration the axis under that scribble is not shown but the duration is those point two seconds the amplitude went up as you can see and eventually I'm sorry I'm following my vision but I should you know it's in your direction it's this way it went up and then it went down and petered off that's when the two black holes came together formed a stable black hole and then there were no more gravitational waves all right so that's one thing the amplitude went up the sound you hear becomes louder before it dies off the second thing is that the peaks of each oscillation come closer and closer together that means the frequency of the oscillation is increasing and that's the pitch of the sound is becoming higher and higher so that's the chirp great so that's what we call a gravitational wave chirp the banana you see is another way of the exact same phenomenon the banana gets brighter that's the amplitude of the stretching becomes stronger and stronger and the frequency again if I may turn my back momentarily to you the frequency goes up as a function of time and that's how you get that curvature and the closer the two black holes are coming together the faster the frequency goes up until it dies off because you form the single black hole right all right now now I'm gonna finish by coming back to your initial question what we measure is the amplitude of the wave the frequency of the wave and the fact that that frequency of the gravitational wave is changing as a function of time so we're measuring a frequency derivative these three pieces of information are encoding the masses of the two black holes and how far away this system was so that's how by studying that progression that scribble as I call it and comparing it to templates we have from Einstein's theory to talk about it tell us about those what are the templates the templates are basically if we take if we take on Stein's theory of general relativity and ask ourselves if two black holes are at some distance and we follow space-time and we solve with general activity equations the change in space-time as the two black holes are moving as the eggbeater is messing up space-time around them we can calculate what is the amplitude the frequency and the frequency pollution with time of the gravitational waves being produced calculate with computers so supercomputers in fact because this calculation this simulation is very very hard tough to do and you need supercomputers to do it and some of these simulations may run even four months at the time so but but we have we the Royal way this is actually not my own personal work but relativities have been able to do these simulations for about a little over 10 years now so we can create templates of these kinds of signals and when we correlate them with our data from the detector we can then find the best fitting template that then tells us that that particular signal came from a pair of black holes that is that far away in the masses in this particular case was about 20 or 30 solar masses plus or minus of course there's always errors in every measurement we make so that that was a twenty fifth twenty twenty sixteen so there have been discoveries since he give us a sense of what's been going on the last couple years yes so since that first one that shook our world and and honestly it shook it shook humanity if I may say because we on that day of the announcement we were media people at all the universities did the count not ourselves but on that one day we were on front covers more than 900 newspaper front covers across the whole world almost as much as trauma okay yeah so then after that up until now we have announced another five or so more collisions of black holes that we have detected in our data and another third gravitational wave detector in Italy has joined operations so now there is more confirmation of more of these events independent confirmation and now we're discovering a population of binary black holes in the universe now you've also gonna be on black exactly and that has made again that sort of them the second most significant detection which is two neutron stars details for the neutron star yes and we're gonna we're gonna I'm gonna tell you what a neutron studies and and have you all in New York look at the Chicago skyline since northwestern is in Chicago so so this is actually this is to scale unlike his movie so this is the Chicago skyline so a big city you know about ten miles across and the shadow you see hanging above the Chicago skyline is the edge of a neutron star to scale now a neutron star is the death remnant of a star that may be ten times the mass of a Sun we end up forming when it runs out of nuclear fuel so it will form it will be about the the death remnant will be about one-and-a-half times the mass of the Sun and it will be about as big as a big city downtown okay that's roughly the scale you can imagine how big the circle is thankfully no neutron star is hanging above Chicago now as we speak or above New York City but that gives you some sense so and our second biggest discovery which reached us on August 17 2017 so we're approaching the one-year anniversary soon I was right around the clip that was another memorable day of course for many of us so it was just a few days before the Eclipse now it used to be that the big event of that month was gonna be the Eclipse and if this had not happened I would remember the date of the Eclipse but right now I forgot the day thank you because of what I remember is August 17th so two neutron stars came together in a similar fashion the the to neutral cells came together and we got another banana a we this time we had three detectors so you see again Hanford Livingston and we had the Italian detector operating as well and we saw the two bananas in the two detectors the scale you see the signal duration is much longer it's no longer just a fraction of a second what you see on the screen is about 30 seconds but actually in our data this is what you see visually here but in our data we extracted the the signal lasted a hundred and forty seconds so a couple of minutes a really long signal which tells us that the masses that came together in this death spiral were actually much smaller right so about an one and a half solar mass as opposed to thirty years as opposed to certainly so the lower the mass the longer the signal the longer this death spiral lasts now the third detector doesn't show a banana partly because the third detector is not as sensitive and partly because it was at a very special location on the sky that they said detector didn't have good visibility let's say and then and then spectacular things happened after the collision of the two neutron stars unlike two black holes which come together in peace form a single black hole and nothing else happens after that two neutron stars actually give us a whole set of fireworks in electromagnetic waves so you could actually not only see them in gravitational waves you can see them yes you can see them in light real light and that started a whole other type of astronomy multi messenger astronomy multi messenger meaning two types of waves came out of the same source gravitational waves and electromagnetic waves and what what have people learned from the neutron star collision there's a lot of talk about new ways of thinking about nuclear astrophysics yes so we learned a couple of different things so first of all in the electric learn things up from this multi messenger character the source so one thing is that the first thing we saw in electromagnetic waves was a gamma ray signal this is the highest frequency electromagnetic waves we can detect and we knew gamma ray short bursts of gamma rays existed we had detected them for many years since the late 60s but and we had hypothesized that maybe mergers of neutron stars were responsible for them but we had no proof the proof came from the gravitational waves because only in the gravitational waves we can measure masses so the measurement that chirped the banana tells us that it was two neutron stars that collided so we've associated with for the first time we had proved that two neutron stars collide can give you a gamma-ray burst burst so that multi messengered combination proved the origin of short gamma-ray bursts are due to neutron star collisions that was one big the second was that the two neutron stars came together and as the neutrons collided they actually formed the heaviest elements we know on earth so elements like gold and platinum a lot heavier than iron we of course know they exist we have them on our planet we have them you know on our rings and earrings or whatever my wedding ring so I used to have one of those she does it's a bit of a sore point but let's move on okay so so we know they exist but but actually astrophysically or physically we didn't know for sure how they are formed they are not formed in the Centers of stars that we knew and through nuclear reactions so a hypothesis was maybe there formed in neutron star collisions with this one event from August 17th we got experimental proof through with the electromagnetic waves and gravitational waves telling us two new trousers collided we have now proved that gold was formed at this one event and therefore we solve that mystery as well that's fantastic congratulations well I take credit not just myself but there's a whole collaboration of hundreds of scientists absolutely so we're reaching the end of our segment but there's one other question or maybe one and a half questions so have you received any data that doesn't fit the templates or more generally have how can you imagine testing Einstein's general relativity in this extreme environment using gravitational waves we always try to test Einstein yep okay we push that frontier he made I mean his theory must break down at some point because it involves the singularity at the center of the black hole and we don't like that that's what the Masters break exactly that's where the division by zero doesn't make sense so somewhere general activity has to break down with gravitational wave observations we are observing black holes moving at half the speed of light 60% of the speed of light this is the strongest regime of gravity we have ever probed with anything nothing else has probed that regime so so far we have not seen anything in the gravitational wave data that disagrees with general relativity but we keep pushing that frontier and maybe one day we'll see something so it's always at the back of our minds and would you say that sort of would be the culmination of this enterprise - to push the understanding of gravity to the next place the physicists in me might say that so that's always something we try to keep testing that theory the astrophysicist in me wants to know about these black holes and neutron stars how do they form what's their masses how can nature form these pairs of black holes and neutron stars in such high numbers that we see so many of them right so this is what we're after on the astrophysics side well good luck with all those fantastically interesting projects and everybody please join me in thanking Vicki Calogero [Applause] and talking about talking about black holes of course Stephen Hawking is the great scientist who comes to mind and this to think all of you no doubt know sadly Stephen Hawking passed away this year so I'd like to spend a little bit of time thinking about what it is that Hawking told us about black holes what were the insights that he gave us and what puzzles emerged from his work that we are still struggling to resolve and probably will continue to struggle with for some time and toward that end as a little bit of background we need to have some understanding of a concept that everybody is familiar with at some level or another but let's use this opportunity to get us all in the same place and that's the concept of entropy so entropy is a word that's freely used in the culture we often use it to describe a certain amount of going from order to disorder that's going from low entropy to high entropy that's sort of the intuition that the culture has about this word so let's start with that very basic way of thinking about entropy and a good example would be let's say I take my book I throw it up in the air it starts very ordered all the pages were in complete numerical order but as they come down it's overwhelmingly likely that they will not ant land in numerical order they'll land in a highly disordered state and that gives you some sense of what it means to go from an ordered arrangement to a highly disordered arrangement going from low entropy to higher entropy now of course that's just a sort of fanciful everyday example if we had more toward a physics example we could imagine having a box of gas and imagine it just has a small number of atoms in there and imagine that they're all neatly arranged in this nice cubic lattice this would be a very ordered or low entropy configuration of the gas like the pages all in numerical order now contrast that with a box of gas that has a whole lot of atoms a whole lot of particles are all bouncing around in a completely haphazard way this would be a very disordered state a high entropy state and another way of thinking about the distinction between low entropy and high entropy which will be use of in just a moment is the amount of information that would be required to describe these two configurations because look on the left you don't need a lot of data it's a cubic lattice you've got 3 by 3 by 3 atoms filling out that lattice and if I give you the separation of them say 10 centimeters that basically describes the configuration on the left you don't need a lot of information to describe a system that has low entropy but let's say I wanted to describe the configuration on the right well I basically have to tell you where each and every particle is located that's the only way I could describe and look at all the information I need to describe that configuration so the idea is low entropy not a lot of information high entropy takes a lot of information that's hidden in this configuration of particles and that's kind of the distinction between order disorder low in to behind your P low information high information now of course I'm talking in loose language so that we can all get a feel for the critical ideas but you can make this all quite precise and this gentleman over here you know who this is who's this Ludwig Boltzmann that's right and this is his his famous tombstone and you see that formula on the top of his tombstone and that is the formula that makes these statements precise this is a formula that you learn if you take a course in statistical mechanics thermodynamics S stands for entropy K is Boltzmann's constant and the rest of it has to do with the number of configuration of the ingredients of the particles that make up the system and the famous second law of thermodynamics makes use of this formula and again it's something that we're quite familiar with in the culture that things typically go as I said from low entropy to high that's the natural progression of a system which means that this quantity s tends to increase over time the entropy later on is almost always greater than the entropy earlier on in the evolution of a system and what we'd like to do now is have this idea come into contact with the idea of black holes and we'll see that there's a certain kind of tension that arises it's a very fruitful tension that will take our understanding further as we try to come to grips with it and to help us go on that journey we're going to now bring out the donner professor of science and the department of physics at Harvard whose primary area of research is string theory he's received numerous awards and recognitions for his work including the Dirac Medal and the breakthrough prize please welcome qumran bhava [Music] now I should say that I have known this gentleman for over thirty years we were I guess both postdocs at Harvard in the 1980s you you got a job at Harvard as faculty but that's okay we're still were still you're still a good you're doing very well today thank you and and I should say my most exciting intellectual adventures in string theory have been the papers that we have done together so there's a lot of fun with you Brian thank you so it's been an honor to work with you so thank you so so let's let's now get into this subject here we heard a little bit about entropy and about the second law that entropy increases over time but in the 1970s people like bekenstein and Hawking and I think we have some pictures of these folks that can come up on the screen they began to worry about how the second law of thermodynamics would behave in the environment of a black low so you give us a sense of what they were puzzling about so parts of the puzzle was the objects they knew falls into the black holes but they cannot come out so objects that are falling in have entropy just like what Brian already described you have some information about where they are how they are but once you fall into the black hole then you don't have anymore access to that information so what happened to that information so that seems like you lost entropy entropy went down and that's against what Boltzmann said so can I give a concrete example of that so I think we have a little silly example if I took my wallet it has information it has entropy I throw it into the black hole and you're asking what happened to that entropy did it disappear from the universe exactly so that was the kind of question that they were struggling to understand how could this be consistent right so they began thinking about this and then there are a few ideas that they were thinking about and in particular the properties of the horizon of a black hole the edge the edge the event horizon that without the edge in which beyond which you can no lie can escape they found it had some interesting properties and in particular they began studying what is the area what is the area of this edge and how does it change and in particular what happens if you throw objects into this yep black hole what happens with this area yeah and one of the surprising things I found was that when you throw objects in this area cannot stay put and in fact increases yep so they began to think about okay what could this mean about this what what this could be related in some way compensated by the fact that you're losing something the entropy could it be somehow compensated by the fact that this area is growing so they begin to ask the question could the area of this event horizon be in any way related to the lost entropy so we actually can unpack that so let's just go through a couple of little examples here so this is the standard second law of thermodynamics the entropy grows over time let's just take the two boxes that I started with to illustrate what you're describing if we take those two boxes and we put them together what happens to the entry it goes up right Boltzmann's right so we have the combined entropy is greater than the individuals entropy now you're describing something that doesn't have to do with gas in a box you're talking about the geometry of a black hole so for example they studied what happens if you study one black hole with one area of event horizon and another black hole with a different area and you bring them together you merge them and you get a new black hole with a new area and they found that the area of the new formed black hole is bigger than the sum of the two areas just like the entropy had that property the entropy resulting on the box when you combine them was bigger than the individual entropy so the entropy sounded very similar to the area or area sounded very similar to the entropy so they began asking could it be that the area and entropy are proportional to one another for black holes and indeed they found a formula that we can bring up here so entropy is the area and the proportionality constant is a little bit complicated before to you right because you don't really have to worry too much about that this number on the right hand side but the basic lesson is that they're guessing they're they're suggesting that the area of the event horizon is somehow holding on to the entropy holding on to the information in some sense that's inside the black hole now you gave an interesting thought experiment about how to test that you were talking about throwing something in and and watching the area and it just so happens then we have a little little segment on that here so why don't we go through that so here's a so take us so what are we seeing here and then we'll do your little thought experiment yeah I guess we are seeing a black hole a red black hole yes exactly with the event horizon of it you can see that the idea that the information is proportional to the area with some proportionality constant if each individual element of the information is like one square there we just divided this sphere into this area so these represent bits of information on that surface of that black hole so those little squares can roughly speaking me thought of as Planck areas these in the Planck length that describe sort of the smallest length that makes sense in a in a quantum theory of gravity squares of the size 10 to the minus 33 centimeters or so if you want to tend 7/10 or my 30 days squared yeah each size of the squares yep so you basically count the number of those little squares that are on the event horizon and that's the amount of entropy it has so if we now talk if this is true and we were to take a particle that sort of carried one unit of entropy what would we expect to happen if this is true we expect this area to grow exactly by one square so let's let's sort of do that we have this is actually a real real film of space here so so let's get our our particle going in to this guy over here oh and it's and it's kind of interesting stroboscopic effect there but so so let's good so the one on the left was the original black hold the one on the right is a larger one after a particle has fallen in so if we take all those little squares and we smear them out on the larger black hole what we'd expect to happen is something like that exactly so one one more square so we throw a sort of one particle in that carries one unit of disorder and the black hole eats it it grows a little bit and in fact you can calculate that the amount by which it will expand can accommodate one more of these little tiles and that makes this a consistent way of thinking about black holes so so this was like an important happening how important was was this kind of an insight well it was the amazing insight because it was the first time where Einstein's theory of relativity was combined with quantum effects and that is what Hawking used to actually argue that the entropy of a black hole has something to do with the area and so this combination of quantum theory and Einstein's theory was quite novel and this was in early seventies mid seventies so is this the end of this story when it comes to entropy and black holes like what what what issues does this leave over well actually the problem was that Hawking's argument showed that there must be information but on the other hand the horizon of a black hole is featureless so you don't see these point squares or anything so there seems to be no room for information to be stored on it so in fact if you use Einstein's equations and solve it you find there is exactly one solution for a spherical symmetric black hole one solution means it has zero entropy it's exactly ordered state there's no choice one piece of information one piece of information and that is inconsistent with what Hawking found when he combined Einstein's theory of quantum mechanics where he found a huge amount of information and the question is where is this huge amount of information stored is it the shape of the horizon or is it what aspect of black hole encodes the microscopic scarred structure of the black hole that was open when Hawking discovered this fact so so for Hawking's idea to work let's just go back to my little example of sort of throwing the wallet in so so in that little example I threw the wallet and we said where did the information go so so the new idea if it's true would be I take my wallet and I throw it in and rather than it like disappearing the idea roughly speaking would be the information that is in my wallet the configuration of all the atoms and molecules it goes toward the black hole and we want it to be the case of that information kind of like smears itself out on the outside and is encoded in sort of bits ones and zeros that's a picture that's the picture and principle we're this true I could actually extract that information in principle and use it to rebuild anything that thrillin in this case it would be my wallet so that's yeah that's if the information is not lost that information is there that's what we would expect right yes that's the hope and the puzzle is is that out is that actually true and how do you actually see this information where is it stored or how do you account for the microstates yes of the black hole or the entropy of the black hole and that's where you come in well me and my colleagues in string theory began studying these questions more seriously with the advent of duality symmetries in string theory so first exactly where to drink there I should say a few words about string theory so string theory is is the framework where we believe combines Einstein's theory of relativity with quantum theory in a consistent framework and as part of it it demands that the entities making of the matter is not are not just point particles but entities like strings or membranes or extended objects have to play a key role and so that's one aspect of string theories a little if you want to take us through this is just a little example what string theory might look like so you got a piece of matter and we're gonna dive into it and then you're saying that if we go sufficiently you go to the beginning inside you'll see the atoms first and you know inside the atoms you see the the nucleus and the nucleus you see well here I guess the protons and neutrons okay and you go inside them see the Cork's and once you see the course you can go inside them and if you zoom in that's the further you might see actually that these featureless quarks that you think are point particle actually they are tiny strings vibrating strings of some sort and that's the kind of a picture that we expect to be true in the context of string theory that point particles are point like because we are not zoomed in sufficiently close to them and if you zoom in enough they will have features like strings how big with those we don't know exactly but it could be like 10 to the minus 30 centimeters somewhat close to the Planck scale that we discussed already in the context of the areas of the horizon of the black hole so that's the basic feature of string theory but as one of the aspects of string theory that was originally one of the negative features was that string theory predicted that the spatial dimension of the universe is not three and that was manifested in contradiction with the fact that we think we live in three spatial dimensions we have three spatial dimensions string theory seemed to demand that we have nine spatial dimensions and so this was a contradiction originally and physicists thought how could this possibly work and one resolution was oh these extra six extra dimensions that we have are so tiny like little circles at every point in space let's imagine six dimensions at each point six types of circles which are so tiny that you cannot see so the idea would be the macroscopic dimensions are only three the big ones are three but the tiny ones are six and the tiny ones are hard to see unless you really zoom in and that's difficult to do with the energies that's available to us in our experiments today so actually I don't know if you guys can can grab it but there is a little video that we had tacked on there with the skyscrapers if you can bring it up to to show this idea of extra dimensions great if you can't get to it then that's okay too but the idea would be that you've got these these big dimensions that we know about these extra dimensions and then how do you relate this to this idea of black hole so this was actually a negative feature for string theory so string theorists were kind of struggling yeah we have these six extra dimensions it's a little embarrassing it's tiny you don't see them and then other people said sure sure yes yes so this was the state-of-the-art till mid 90s and where we didn't know what to do with these extra dimensions they were there just to make the theory consistent and we were hiding it away in tiny little things and then came the question about the black holes and the black hole had the opposite problem the problem with the black hole was we were missing information of the black holes where were they what were that information it turned out that these two problems cancelled each other out and so this is the problem that was very are they in where are the ingredients or the degrees of freedom that constitutes black hole ended up in the internal spatial tiny spatial things that I was talking about so extra dimensions were helpful in finding the resolution and perhaps we can see there yeah so can we actually skip the next one and go one beyond where we can see the perfect thank you so as you can see here the the red doughnut shaped object there is to represent these tiny internal dimensions and the blue sheet that you see represents the macroscopic three macroscopic dimensions and so the idea there is that if you take a string or a membrane or one of these extended objects and wrap it around the cycles on these external tiny spaces like a torus or like a circle or doughnut or whatnot shape you have it will create a mass at the point where you're wrapping around that circle and the more it wraps around the more mass is going to be constituting there and the more its gonna shape and distort the geometry of space around it and it is enough of it it's going to create this warped space which is what we call the black hole so the black hole itself can be viewed as these strings or membranes wrapped around these extra cycles of this tiny space and so then that if the question is what accounts for the degrees of freedom of the black hole it are these are precisely these strings or membranes wrapped around these extra dimensions how many are there how many degrees of freedom are there what are they entropy gets translated to how many ways these extra strings or membranes can wrap around these extra dimensions and there are many ways and that accounted for the entropy of the black hole so with anteye anteye straw meter and I did actually computation of how much entropy is there in these wrappings of the string around a around the external around these internal dimensions of string theory and we found exact match with the prediction that Pickens Stein and Hawking had made about the entropy of these black holes so just to quickly unpack a little bit so so basically you found a new way of describing black holes in string theory which makes use of the extra curled up dimensions the red part of this image that we find here and you're saying that by wrapping these strings or ingredients of string theory around the extra dimensions you can create a warping in space that looks just like the black hole that we've been talking about for many years before string theory even was on anybody's mind but in this description you can do a direct count of the amount of information necessary to describe the situation and bang on it matches what Hawking's exact said yes so so this gives us insight into one possible way of describing the internal structure of a black hole which in many ways I think the community of uses is one of the most important pieces of data that string theory may be going in the right direction we don't have experimental support for these ideas but here's a mathematical piece of experimental support if you will that it matches ideas that have been on the table for 25-30 years so so this isn't a key step forward but now let's turn to some of the puzzles that still yes remain and and part of the puzzles surround something that's tightly related to what we've been talking about which is something called Hawking radiation if you just tell us a bit about what Hawking radiation is so Hawking after discovering the properties of having information the backhaul and the entropy he also noticed that the black holes actually are not quite black and what happens is that if we go to the next perhaps yet PowerPoint so if you if you go near get up to the next one if you go near the event horizon of the black hole yep so what happens is that you get pairs of particles created out of vacuum these are quantum fluctuations the quantum fluctuation is always creating particles and antiparticles in pairs on typically they go in and out without doing anything but if they are near the event horizon one of them perchance could go towards the inside of the black hole and then one of them could go outside and the mum which goes inside has no way of getting out so stuck there but the one going outside can leave and go far away from it all the way to infinity and that looks like a radiation from the viewpoint of some of the outside so in other words from the viewpoint of some of the outside the black hole is actually radiating energy as it radiates energy it loses mass and it shrinks and shrinks and shrinks until after a while the black hole totally disappears where there's no more energy left to emit so once the once the black hole disappears the question is what happens to all that information that went into the black hole what happened to that area was talking about where is that information gone so this was a question that Hawking basically posed and this was the probably the slide previous to this one yeah we can go back if you wait two slides if you would so if you if you saw that two slides actually you know further back sorry two is the wrong number go back to the court of Hawking if you would so so he had this amazing coat if you if you perhaps you recall the discussion between Einstein and Bohr that Einstein did not like the probabilistic aspect of quantum mechanics and so he said God does not play dice and Niels Bohr responded by saying to Einstein stop telling god what to do and hawking added his own wrinkle on this he said well what happens if you throw a dice inside the black hole and the black hole after white disappears and we did disappears the dice so and then he said not only does God throw dice he sometimes throws it throws them where they cannot be seen and then you made a little little cake so yeah we had we had the lucky occasion of having Stephen Hawking as our guest at home a couple years ago when he was visiting Harvard and we made the cake in the form of a black hole with the dice and the book his famous book which is actually made of is a cake and it's actually gluten-free because he was allergic so so so Steven had had his cake and it could either - yeah so in that in that picture you see that there are two two dice hanging there floating towards the black hole and it's going in and that's the issue of the information puzzle you lose the information of what what is the dice roll and that's what's called the information puzzle because if the dice rolls some number and the black hole evaporates and disappears we did this appears the information about what the role of the dice was and that's called the information puzzle which we still are struggling to understand even today now most people in in the field for a long time have anticipated that somehow the information does get out with that radiation that we saw coming off of the black hole now for a while Stephen Hawking said that the information would would not come and I mean that quote was quite serious he was saying that we have to rethink quantum mechanics because the information does not come back out he then changed his mind on that yes so part of the reason for this is the discovery of a more detailed version on what is called holography in other words this this began with the work already back in the in the long time ago but became more precise with the work of war mother Senna where it was discovered hub that you can describe a gravitational system precisely using objects which are much simpler to understand and in particular in that system you could prove mathematically rigorously there's no way you can lose information so therefore indirectly there was a proof that whatever happens to black holes the information should get out so that part was kind of beginning to be established using these kind of arguments in string theory but the wrinkle is that even though there's this mathematical proof that this should happen we still did not know exactly how and some people thought maybe the information gets out with this radiation as you were saying right very gradually and very tiny bit at a time but gradually builds off to give you information out and this is what in the early 2000s I believe Stephen agreed that this is the resolution but I still think and we I think the community still believes we haven't really understood how this happens exactly so the information puzzle about how this information comes out is still one of the big mysteries of black holes and we don't really understand it so do you think it's possible that we will one day conclude that the original view of Hawking was right that the information does not get out is that still on the table in your view well I think it sounds much less likely given that we think we understand a dual description another description of these black holes so it seems like that seems much less likely now but you know we can never say we know everything about the subject that's our current understanding it because of course evolve but but even even with that understanding it's not our understanding is not sufficiently detailed to convince us of this of how it happens now a lot of our discussion here has been focusing on what happens at the edge of a black lot the event horizon or black hole that it may be where the information stored it may be where the entropy is stored we haven't said much about what happens at the center right can we just spend a little time talking about that well one of the things that happens in the center of the black hole is that there's this singularity this infinity that Brian discussed already this is this division by zero analogy and we don't believe that's physical we believe that's just the phantom of our equation it's just the mathematical thing that comes out and that means that our description breaks down not that there is an infinity now quickly the infinity could be physically that you've got all this mass crushed to zero size and that size is not physically up for us there's nothing smaller than planks length which is about 10 to minus 33 saying there's the notion of space breaks down anything smaller than that so there cannot be an infinity in physics we don't believe so so we don't understand what is happening at that at that at that in that way and the understanding of this singularity we are not at all close to understanding what's happening there we know that there should be a good description but we don't have it and why that's important is that well we want to know what's happening inside the black holes of course we can send somebody and find out what happens to that infinity but that would not be a fair thing and even if we did that even if we said well we'd send some creature not not us that robot or something the robot cannot come back to tell us what happened or what they saw so because nothing can scape from black holes so it won't be useful so we have to figure that without going inside the black hole what's going on there and that's still we have no not only we don't have observational data we won't we don't have enough theoretical understanding and we think it's important not just for the black holes but it turns out that those infinities that we see if you try to pass through those infinities to the other side so to speak it turns out it looks very much like the infinities we see at the beginning of our own universe it is as if our universe is emerging from inside one of these black holes along those lines and don't lose your train of thought there oftentimes people think of the center of the black hole as a location in space the center is that the right way no actually it's like in a given time it happens at a given time and that's because the nature of space on time changes as you enter the black hole but we need cross the event horizon of the black so one of the reasons why you can't get out is because you can't stop time from ticking forward exactly so going forward in forward in time it's like going closer and closer to distinct there and then at certain time you hit the singularity that's the time it's at the given time is that the singularity could be the end of time in some sense it could be but we don't believe that and then so what you would say what's the after that and that looks very much like what could be a new universe or something so so we are interested in understanding the resolution to the issue of singularity of black holes and I think it's very exciting we know that black holes as we discussed already in this program there are a huge amount of evidence that they exist so they are there we cannot say we're just imagining so what is that what is that singularity what are we learning from it it's one of the most beautiful mysterious objects I think in the universe and hopefully by some experiments and maybe by some theoretical understanding we'll have some progress in that so what's your guess I mean if we come back and we invite you back further I don't know the 2025 world science festival can you give us the answer to what happens at the center of a black hole maybe you inviting 20 55 million 20 50 all right good good prediction a safe prediction please join me in thanking Qumran Vasa thank you thank you very much fantastic good night everybody thank you [Applause] [Music]

Channel: World Science Festival

Views: 581,781

Rating: 4.7132187 out of 5

Keywords: Darkness Visible: Shedding New Light on Black Holes, gravitational waves, black holes, quantum mechanics, singularity, Event Horizon, radio telescopes, astronomy, general relativity, gravity, Brian Greene, Shep Doeleman, Andrea Ghez, Vicky Kalogera, New York City, NYC, world science festival, World, Science, Festival, 2018, Big Ideas Series, Breakthrough Prize

Id: 3JE_KMfuEWk

Length: 106min 30sec (6390 seconds)

Published: Sat Jun 02 2018