Black Holes | Scientific Controversies

so both priya and fariel are experts professors theoreticians of black holes this is really for all three of us actually really central to all of our work in very different ways um and and it's it's the kind of situation where each of us has such a different focus despite that we all work on black holes we have a lot to learn from each other so i think this is going to be really fun i mean if we were sitting on a couch together we'd be able to ask each other a lot of questions and that's kind of what we're going to do tonight so i wanted to um show some of these visuals this i'll try to distinguish what are animations and what are um real simulations so this is an animation it is not an actual observation what was it should i duck yeah you should die no it's okay so this is sort of a classic visualization of a black hole so i just wanted to start in the beginning you know what is a black hole we all have these sort of ways of explaining them fariel what what do you usually say when somebody asks you what is a black hole so i say it's a dense object it's the densest object that we know in the universe we know of several types there are some that form from dead stars when they run out of fuel and there are some that are huge they are at the centers of galaxies and they formed over the basically the lifetime of the universe so one of the things is when people say they're they're enormous they're actually physically small right so exactly that's a really big misunderstanding that i've run into a lot i mean a black hole is physically incredibly small in distance yeah so the way you make something dense is you take a lot of stuff mass and you squeeze it into something very very small so the actual volume they take up is tiny but there's just so much mass in there like we're talking about it for some black holes 10 billion times the mass of our sun and it's in a tiny tiny center of the of a galaxy so if it was a black hole the mass of the sun how big would it be it would be about three kilometers if you had to squeeze all the matter in the sun to make it have the i mean the idea here right is what the compactness really does it tells you how intense the gravity is going to be so how much of an influence it's going to exert on objects right around it so black if we took the sun or you had an evil genius who figured out a way to collapse the sun to a black hole it would fit in manhattan oh absolutely yeah oh much smaller yeah it would fit in central park yeah yeah about right i mean we're very new york centric yeah yeah so so i guess one of the things i would want to ask is how did this discovery happen like why are they so bizarre so they're very dense but why are they dark right this is the the classic conundrum and kind of the history of it priya i know you've thought in your book a lot about the history of science in general but of black holes i mean um i think these are incredibly enigmatic objects uh partly because of these totally bizarre sort of properties they have so density and how tightly they're packed and how intense our gravity is but also they're kind of interesting because they were discovered not as real objects in the universe the history is that they were found as a mathematical solution an exact solution to einstein's field equations that actually describe all of space and time and um the evolution of everything in the einstein didn't believe it yeah einstein yeah and he you know um he did he thought that the field equations were so complex that there wouldn't be an exact solution and lo and behold you know a few months after he presented the theory in 1915 um carl swashchild found a solution so i think one of the reasons that historically as a scientific idea black holes are very interesting because they started out as a merely mathematical entity with bizarre properties and the correspondence to real objects in the universe took a very long time to establish right so so schwarzschild one of the great anecdotes is that he was allegedly and maybe apocryphally calculating ballistic trajectories during the war world war one while he's on the russian front i think that's true yeah yeah it is true but i don't think it was actually in the trenches but i like that part i tried that too it's good because when my graduate students can't get any work done i'm like are you in the goddamn trenches of world war one calculating like ballistic trajectories so he's like reading the proceedings of the prussian academy of sciences as you do right yeah right why are you at war and he says something to einstein they were friends i guess i hadn't really realized that until recently and he said something like i've had the pleasure to wander through the land of your ideas you know and very quickly comes up with the solution einstein believes the solution but fario why doesn't he believe that they're physically real so they didn't think that things that dense and so dense that they would actually lose their surface and collapse into a singularity so something infinitely dense would actually happen um in real in the real universe so of course he believed the math and it's a very simple very elegant solution and funny enough it's not just einstein but sir arthur eddington too one of the pioneer figures of general relativity so einstein's theory the first experimental verification he didn't believe it either yeah so they were it's hard to believe i mean i i can't crush this clicker you know it's not easy to crush things atomic forces resist collapse so it's sensible actually to think that it would be impossible yeah exactly you go through layers of crushing before you can get to oh wait there is no force that can stop this anymore and it's just going to keep on collapsing into this infinite thing do you think that if there's a mathematical solution that nature will always figure out a way to do it no i don't not always i do i don't there's a multiverse i mean anything can happen i mean if i thought you said it i mean when you said nature you meant i thought an instantiation here that we could go measure or see well okay so let's talk about that let's talk about how did it happen that you go from einstein's very reasonable response which is that this is beautiful math i'll help you get this published but nature will protect us from their formation to actually theorizing before they're observed like oppenheimer um theorizing again very involved with the war effort that it would happen like how do you get to that point but we should maybe talk about collapse in a second but but isn't it kind of fascinating that nature did find a way and what was the way exactly right um i think what is really fascinating is how um from this mathematical solution with simulations that were done during world war ii for other reasons understanding detonations um there was a realization that you could have explosions that looked like the end states of stars that could then eventually give you a multiplicity of fates so if you're a very massive star then you could end up with an object that looks rather like a black hole that keeps crushing down you left with a very dense compact object but as if you start out with a star that is not as massive our sun like our sun you would get a completely different fate so you would get you know a neutron star you get a a white dwarf um at the end of um stellar evolution so this sort of you know this tussle between gravity and the energy of fusion that keeps the centers of stars going it's sort of that tussle and when gravity wins that tussle and that was what the equations showed them when they were able to actually compute them in great precision they were able to see that there is a point where gravity basically takes over and as peril mentioned at that point there's no it's a point of no return there's no other force that can actually withstand the crushing of gravity so it is kind of fascinating they're studying nuclear physics because they're in a race for the nuclear weaponry and the same equations that they're used to understand nuclear physics are so amoral right that they also apply to this super abstract concept of the emergence of stars and their lives and deaths and and so it's really linked to the war effort that they made the discoveries i mean oppenheimer's first paper was published in 1939 right during the poland's advance i mean the nazi advance on poland and was kind of lost in obscurity so interestingly he got the neutron star solution wrong you know he um they calculated what could hold up a neutron star so black holes one step before a black hole forms so it still has a surface but it's not the form of matter that we are familiar with on earth and they asked what could hold up such a star and they got that part wrong so they said oh neutron stars wouldn't exist in nature this is the open eymer volkov paper do you think it's okay to get things wrong occasionally oh it's it's fun to get things wrong i mean my first failed cosmology made me so happy when i realized i was wrong i thought we lived inside the sphere of the earth all right we were trying to figure out that yeah i was so excited when i learned we lived on the surface of the earth it was like way more interesting than my idea um so tell me why they're black why black holes are are dark so um the classic way we explain it is that when gravity is so strong it pulls everything towards it and because light has mass it also pulls light towards it so basically as you're headed towards a black hole there are if you're far away there are exits you can take if you're oriented in the right way but as you get closer and closer especially to that part that's called the event horizon that that distance um then there is nothing you can do you can't run backwards and light can't run backwards even at its at the speed that it goes in so that part becomes like an infinite vacuum everything can go in but nothing can come out so and it just looks black i think about it slightly differently um uh and sort of you know if you think of you know when you're launching rockets from the surface of the earth for example right you sort of have to launch them at about 11 sort of kilometers per second it's like 30 times the speed of sound that's why we need rockets right to boost and so you can think of the black hole as a kind of entity from which the escape speed to actually launch something off um is greater than the speed of light so that sort of automatically shows you that not even light can escape and therefore they'll be dark so here we have again this is an animation it's not real we've never taken a picture of a black hole it's important that people know that okay we'll get there nobody's ever uh released the press report about that correct image which they will any minute now but so here was this was a visualization of the black hole basically casting a shadow i mean that's effectively what it does right it casts a shadow right on the sky because any light that comes near it falls in and why does it look so smeary priya like this right so um you know there when you think about the fate of light rays that graze close to sort of the sacred boundary of the event horizon what happens is that some of these light rays that will cross basically are lost there are some that are going to skirt and they're sort of going to be in dante's limbo forever right and that's sort of the photon ring so that's sort of what you see and then there's stuff because of the intense bending of light because the shape of space is so dramatically modified right around the black hole that you can see if you will the back of your head so you see the sort of smearing when you see the sort of the top where you see so if you were standing at the right location and you had a flashlight and the light from the flashlight took an orbit it could bounce off the back of your head and come back to your eyes and you'd look at the back of your head i've always wanted to get a better image of my hair like a lining i could use it as a method to like check my hair before going on stage yeah it is a way to see things behind you like actually that outer circle that you see it's called the einstein ring it's made up made up of light that's entirely behind the black hole and it's what we call gravitationally lensed basically bent towards you i mean like the back of your head so yeah well you can't hydrate in a black hole if you're hiding behind a black hole you can't hide back it gets lensed around so yeah if you're i always say this if you're like in a space battle like don't try to hide behind the black hole it's like a completely it's not a good spot um so i actually have this is different than a cartoony animation this is actually a mathematical model by andrew hamilton as a physicist in colorado so it's a completely mathematical simulation of what it would be like if you were an astronaut and you were orbiting a black hole and so the reason why the ring is swirling it's really because of your own orbit right so you're seeing our own galaxy behind the black hole being lensed now we were talking about it as an object and i kind of wanted to take issue with that so far he said it's a very dense object but right here at the event horizon there's nothing right right i mean so it's actually empty so that's what we i mean it's a singularity well not not i don't want to start with it's an infinite singularity so yeah technically that's true it doesn't have a surface it's not really an object the way we know other objects yeah so if you walked into the shadow of the event horizon what would you experience actually not much so the title force would bother you but otherwise you're just falling in right no one can hear you scream well not even light can escape right exactly there you are and and yeah no one can see if your hair is being torn apart either so you'll be fine yeah so i'll have one consolation yeah right exactly so those are these slow down right for your time would slow down and but you wouldn't experience you wouldn't experience it but your friend your poor friend who was waiting outside for you so you korea you are waiting for me for coffee somewhere back at the earth and i'm falling through the event horizon it could be like several million years yeah yeah we will we would think that not much is happening to you you're kind of frozen in that horizon and you're like what are you talking about i'm going straight in and you're going really fast you're evolving really quickly yeah so these are two myths that i love to dispel one is that black holes are objects they're actually nothing they're sort of regions of space they're places as much as anything and also that you know black holes are dark they're only dark from the outside that's right they can be very bright on the inside so why how does that happen well i mean this simulation actually shows that right i mean you're starting to see light coming from all all areas i mean all directions of the black hole here yeah so you're inside the black this i think astronaut in this simulation is already inside the black hole and so they're collecting light from the whole galaxy so for them it's remarkably bright for the micro second they have to live so the only thing we do know for sure is like death by black hole is assured right absolutely exactly so i think you know a lot of the intriguing questions though jenna right they still remain like we don't really understand we can talk about simulate what's going to happen to an astronaut or whatever but we also know that when you go into a black hole it's unclear what's happening to the information about the properties of the person who's falling into the black hole right do you lose that information as you know right that's much debated right now and there's a really nice analogy that i think um is sort of fun to think about and you know it's not mine it's uh stephen hawkings which is you know if you think of an encyclopedia britannica right and you look up you want to look up the capital of india and it's new delhi you look it up then you put the encyclopedia britannica in a box and then you burn it right and all the ashes are in that box nothing's left the box right so in principle the information that new delhi is the capital of india is still in there just that we don't know how it's stored anymore and we don't know how to retrieve it either right i mean we we we know when it is the encyclopedia that it's printed paper blah blah blah but so i think there are this is a big raging question by the way like you know how is this information actually stored and how can it if at all can it be retrieved i think and people have gone back and forth on it right jenna i mean stephen himself stephen would make many bets saying that the information was lost then he would concede the bets then people would tell him why did you concede you conceded too early so yeah it's been going it's been going back and forth since 1974 probably so it's kind of messy what happens when you fall in so it is just so sorry i would say it is actually one of the controversies surrounding black holes what happens to information that priya was talking about um in fact we were we've been talking about this point of no return the event horizon there are also a lot of people who work on general relativity that argue that that horizon is not there so the singularity that infinite density at the center forms but the horizon never forms and we call that the naked singularity as you know yeah so things would look differently if there was a naked singularity versus a horizon and on top of that even if there is a horizon and we know there is also quantum mechanics going on there how does it look does the information get stored on the skin and it is in principle retrieval even if retrievable even if we don't know how to or does it truly get lost i mean i would say that's one of the main controversies so in the context of what we're describing just to orient people about the difference between the event horizon and singularity we have you know this is again just a simulation just a cartoon of um of a stellar explosion of a supernova explosion so this was the work that oppenheimer and wheeler were doing where they understood that stars were basically thermonuclear bombs and that at the end of the life cycle of a heavy enough star it would explode and the core that remained if that was big enough it would collapse under its own weight and then the idea was that when it collapsed small enough so here you might have something 10 times the mass of the sun and 60 kilometers across that's really small when it collapses it forms the event horizon right the event horizon like you were saying priya's when light can't escape but then what happens to the material of the star what happens to it it's it's still energy and we don't know in what form but it doesn't stay at the surface anymore the event horizon isn't the surface of a snow a thing no so it just keeps falling it keeps falling it keeps falling see the event horizon is almost like an archaeological fossil left behind in some sense and then it keeps falling and then we don't know it could be crushed down the we say that oh a black hole 10 times the mass the sun is 60 kilometers across but the matter that formed it might be what tiny like the size of a grain of sand yeah absolutely and that's the singularity the singularity is different than this big shadow yeah yeah and indeed i mean the event horizon is not marked with a flag saying now you have arrived at the event horizon i mean it's as we were saying to somebody falling in they wouldn't know it might be like stepping in the shadow of a tree yeah just totally benign they don't know that they haven't but the peculiar thing about the black hole is this kind of combination this kind of boundary that then encloses the singularity and the fact that singularity is a real annoying thing because all known laws of physics kind of break down in our trying to understand what's going on there right so at some level i mean if one wants to be poetic right it's sort of the limits of our knowledge basically it's sort of you can think of this thing of the event horizon that's encasing the singularity as kind of you know the boundary between no ability and unknowability in a way right it sort of um it protects us from whatever is inside there yeah so because no information can leave the black hole because nothing can travel fast in the speed of light could we know from the outside the difference between there being a singularity or there being a new universe blowing out from the singularity or there being a quantum remnant can we know or does that like you say that boundary of knowledge mean we're forever on the other side of that well mathematically you can come up with solutions that will help you figure out the structure right around the singularity so mathematically you can postulate that what the singularity does is sport you on to another universe right to a another region of space-time entirely from which we are causally disconnected right our future cannot be affected by what's inside the black hole it's over we can affect it right but it can't affect us yeah and it depends also a little bit on how different that object is how how big those fluctuations are one of the things that we're working on right now we being the community is can we find observational signatures that something turned truly into a black hole in the sense that we understand it here like einstein's theory sense or something else like there are quantum fluctuations at the horizon scales or it's a naked singularity so you know they they can look different um in appearance or in how fast the signals take to travel around it so if we saw some periodic phenomenon for example but it depends very much on how big those differences are so let's talk about what we can see because we've described them as massive but but spatially small so this is a animation just showing how tiny they are right in the scheme of the galaxy if that's our milky way it's a little marble at several thousand light years distance we will never be able to see such a thing never i mean a stellar mass black hole we'll talk about the other ones so i mean it's kind of an extraordinary even oppenheimer and wheeler who coined the term black hole in the 60s um thought it has a darker history oh yes tell us that it's good yeah so in the 1700s that the term originated black hole originated well before there was any idea of these objects and it turns out that it's the name of an infamous prison in india where the local nawab imprisoned the mercenary soldiers from the east india company and left them overnight basically they didn't make it out so it was a point of no return and so it became sort of used in vernacular and then later in um you know in the 60s wheeler used that term so it was the black hole of calcutta really and that was what he repurposed to describe the story differently maybe we have to merge our histories you know what did they say history is very difficult to predict right um so what i heard was that wheeler was giving a lecture he kept saying the end point of complete gravitational collapse which he got tired of saying repeatedly and i i heard it was above tom's restaurant on broadway by columbia um where the goddard space center is now and apparently someone shouted from the background how about black hole now i don't know i think marcia bertucciac claims that okay you don't believe it maybe the person shouting it out was referring to the black hole of calcutta well the black hole of calcutta i mean was a thing because you know it's there in mason and dixon in bichon it's there in edgar allen poe's poetry so you know that term has sort of been around so we keep throwing the name wheeler around john wheeler was the great american relativist student of einstein's theory who really trained the first generation of relativists richard feynman kip thorne who just recently won the nobel prize so many people were trained by wheeler in general relativity and so he he was this incredibly formative figure in the history of this subject i mean and a subject which was very considered very esoteric and mathematical till really the late 60s when we started finding these objects that appeared to be powered by growing black holes quasars right so this is crazy to actually have this as another animation if black holes are so tiny so dark a little shadow dark against a dark sky why would we have any insane aspiration to actually see them um veril but so um before the stuff around the black hole gets to this event horizon and is no longer part of our observable universe they actually become really hot really fast the speed with which they circle the black hole becomes very high and when that happens they give out copious amounts of light and anywhere from radio wavelengths of meter long wavelengths to x-rays to gamma rays so we have a lot of different ways of understanding as matter is for example a star losing its envelope or just gas clouds or whatever happens to be in the path as it's falling into the black hole and feeding it it becomes very very luminous and it's that that in-falling part on its way to the black hole that we are able to dying gasps of matter dying gasps yeah um but like in that previous cartoon it was it's like eating the neighboring star like it's cotton candy right it's just pulling out yeah because it's gravity is so strong that it's not going to let anything just wander by you know like it's it is going to attract it towards itself and if it has a little bit of a swirl then it's going to make this what we call the accretion disk it's just going to make its way down this toilet bowl and in the meantime luckily it's just going to give out a lot of light that we can study and learn about black holes and you mentioned quasars priya which are yeah so we believe that some of quasars are some of the brightest objects in the universe they're sort of cosmic lighthouses and we believe they're so bright because they really are um objects which are feeding very very rapidly so the sort of feasting black holes where a lot of matter is siphoning in and you know and we're talking about uh matter at the rate of 10 of the mass of our sun gobbled per second right so that's really huge feeding rate and that glows very very brightly and we see these cosmic beacons out to very large distances in the universe they sort of almost out to when the universe was a very small fraction of its current age and so these objects the amount of energy that you get out of them they're so bright that the only way we think they can be powered just by doing the physics and the math and the numbers is that they have to be powered by creation in fact donald lyndon bell at cambridge was one who figured out that from the energetics from the amount of energy that it had to be matter flowing into a black hole and it turns out then you can work out how big the black hole has to be to be such a good sucker right and it turns out that they have to be super massive that's just a technical word for something that has to be well above a million times the mass of our sun and you know and we thought lo and behold right that these are sort of rare objects they're very bright but it turns out that when black holes are not feasting they're fasting like the one in the center of our galaxy and most other nearby galaxies uh when they're not feeding they're still there sitting maybe feeding at a very low rate so they're not going to sort of blaze like a quasar they're going to be this little fiddly thing sitting in there but their intense gravity is still sensed by the stars right around the black hole so it was it i mean here we have this history of a mathematical solution nuclear physics exploding dead stars how do you get from 10 times the mass of the sun maybe even 20 or 30 times the mass of sun to a million or a billion i mean wasn't that a huge surprise so earlier in the universe the stars may have been more massive and the clusters that they formed may have been more massive than what we are seeing today and that helps a little bit in the sense that you can then form slightly bigger black holes to begin with are they formed through stellar collapse are you saying either collapse of individual stars or collapse of entire star clusters we don't know the answer to that i have a pet idea i know everybody has a pet idea and you know we can priya's uh priya's hoping to test her pet idea yeah we are right with the next generation of x-ray telescope and then and james wade space telescope we are trying to see where these first black holes formed and how big they are they were but it must have been for the people who first discovered quasars okay the energetics as you're saying for you mathematically you would think oh wow the only thing that could really do that would be a gravitational object incredibly dense etc um but the numerology works out nicely but but physically was like mind-blowing oh absolutely and i think that you know you know that the end states of stars as you just showed in that very nice animation so you have this explosion you're left with a little black hole right so the pathway from going from there to something that's a million times four million times the mass of the sun that's in the center of our own galaxy you know there's a lot of overfeeding that needs to be happening you know for such a hole to be uh black hole to grow so we believe that um one possibility is that you just are you have episodes of overfeeding or you can start out making black holes really obese from the get-go so instead of forming a little piddly black hole that's the end state of the lifetime of a massive star maybe what you can do is have an entire early galaxy before it forms stars when it's all gassy kind of siphon in gas driven by some sort of instability you know like when you pull the plug in your bathtub and the water goes in a vortex really fast something akin to that with the gas in the very early universe then you can make these seeds that are ten thousand times hundred thousand times the mass of the sun then growing it by you know 10 times 20 times how do you grow it by 10 or 20 times collisions so super massive before we talk about that let me just say either way it's difficult right i mean this is a truly unsolved problem if you start with something i mean even if you say okay the stars were more massive maybe there were clusters of them i'm starting out with something a hundred or thousand times the mass of of the sun you have to feed it a lot because in seven eight hundred million years which sounds a lot to us but it's really not much in the beginning of the universe they're already at a billion times the mass of the sun the quasars these are right the brightest things that we saw at very high redshifts meaning early in the universe so that is difficult and what priya is describing the alternative so get them obese from the get-go like just make a healthy baby and then you don't have to grow it as much that's also difficult because things like to just break up we call it fragmentation right i mean one of the reasons why we don't have very massive stars right now is because they just break up as they are forming all i'm saying is it's an unsolved problem these are open questions right so we're waiting for data from you know upcoming x-ray television emphasize how um populous they are right how i mean it's not like oh there are a few quasars out there there are these obscure rare objects we're trying to explain we think that there are super massive black holes in essentially more or less every galaxy in the universe so that's hundreds of billions of galaxies just in the region we can see right and each one of them seems to have a super massive black hole right the brightest most massive ones might be rare but i mean if it's not 10 billion or a billion times the mass of the sun it's still there it's right right you know and there's a family right there's a hole if you take our galaxy you have one that's a few million times in the center but then you have it's littered with black holes smaller black holes the end states of all the stars right generations of stars that have formed so they are just galaxies are basically littered with black holes tiny black holes intermediate mass black holes and the supermassive that manages to sit down in the center i think what has been intriguing though wouldn't you say in the last uh decade or so is that the mass of the black hole the supermassive one um you know oofy as it is in the center right it's still tiny compared to the mass of the stories in a galaxy it's a millionth of the mass of all the stars but now we believe and we understand that these little black holes sitting there during their feasting and fasting actually shape the entire properties of galaxies over cosmic time they are sort of like pistons that turn on and off star formation they seem to do a lot they have like outside influence how do they like turn star formation on and off like so star formation you're saying if i can connect the dots is crucial to understanding the world in which we live because we need to live on a star of a particular size or our sun is you know modest we don't have to have too many stars near us so we're not under chaotic conditions and we have basically the dna of the universe i'm sorry it's the dna of the universe yeah star formation i mean right so we have this nice habitable lovely little planet on the wings of the you know milky way galaxy a safe 27 light years from the supermassive black hole and you're saying effectively that to have those conditions for habitability in life might actually be traceable back to the supermassive black hole i mean i think the chain from life is kind of a long set of ifs and chains there but what we do know for galaxies to look the way that they do now to have the kind of star formation the ages of stars where the stars are located in a galaxy the distribution what shape is are are there ellipses are there spirals like our own that must have something to do that is shaped like i believe so what like the black hole powering quasars and jets regulates so it heats up the gas it heats up the gas and it blows some stuff out it grows regulatory ranges matter it rearranges matter even though it occupies a tiny tiny place it rearranges matter in galaxies out to very very large scales we know that now we see evidence for jets and so on and but we also know that what is critical for star formation is you need cold clumps of gas to get denser and denser and then they eventually light up and form stars so any process that prevents cooling that prevents star formation that injects energy and heat like the black holes do is going to scupper star formation so we believe that there are these sort of regulated cycles yeah so jenna you were asking um how do we know that there are black holes if they are dark and we were talking about how how bright the gas gets and how energetic the gas gets on routes to the black hole well it basically blasts radiation and i mean just wins powerful jets and winds to the surrounding gas and so we think that it forms a self-regulating cycle so a lot of matter will fall in it will create this very energetic environment where it's just blasting its the surroundings with light and jets and whatever so that will stop the gas from coming in it will stop star formation and then it will quiet down and then start all over again so and so i mean there are people like me who also believe that this is also you know part of the story is uh the way a black hole will also eventually stunt its own growth because what it will do it will evacuate all the gas that feeds it right so the hand that feeds has been cut off now and so we believe that all of these processes are intricate they are sort of um they dovetail with each other and that is the explanation for many sort of correlations we see between the properties of black holes and the galaxies i mean there must be a reason everyone has a black hole at its core i mean it can't just be right so i guess it's one of these hierarchical questions did the black hole come before the galaxy did the galaxy get shaped by the black hole but farielle i'm wondering if you would go as far to say that you can't imagine a planetary system or a solar system like ours were it not for the black hole i am not sure about that i mean clearly the way that histories of galaxies happened were were shaped by black holes um but could it have happened in some other way so once you form the black hole explaining why it's at the center of the galaxy is pretty easy because it's the most massive thing and it will just gravitationally fall to the center so and then we're talking about how influential it is to its surroundings and how it can go through many many cycles affecting how much star formation happens etc and during all of this actually one of the things that's happening is stars are forming heavy metals in their bellies and they through those supernova explosions that you were talking about and we talked about oppenheimer etc they're returning it back into the galaxy so they're enriching it with all i mean even oxygen and carbon and everything else let alone you know more more massive um or uh higher elements in the periodic table so that cycle is absolutely necessary to form planets uh planetary systems but you know i think it would uh i think i would actually put myself out there to say that you know you probably need um a fasting black hole in order to acquire one in the center of a galaxy to enable uh planetary systems to survive and enable uh them to eventually you know after there are many ifs right in this sort of the so-called drake equation of you know all the factors that uh to give you life and then intelligent life sentient our supermassive black hole sajstar which we'll talk about actually in a second is essentially quiescent absolutely essentially it was probably a quasar there's probably another galaxy five billion light years so it's probably a quasar in ages ago right so there's a galaxy five billion light years away that's looking at us seeing it when it was a quasar that's right that's right that's certainly true so um our own galactic center black hole which we call sage star lovingly um because it's in the direction of the constellation sagittarius it is sword so you can find where the center of our galaxy is by looking for sagittarius and um that one is kind of wimpy so i'm wondering if it never had a true quasar phase i mean clearly it's it ate a lot at some point but not i mean it's only like criticizing your grandfather billion what you're like criticizing your ancestors so maybe it's just a modest black hole it's it it said it grew to four million times the mass of the sun and kind of stopped there so can you tell me about these images which i took from your work actually sure so these are different ways in which our galactic center black hole sagittarius a star might look if we were to take a picture of it and why is there an uncertainty there well we're talking about swirling gas we're talking about how it emits light etc but as we come closer and closer to the event horizon we don't know exactly how that gas is being transported into the black hole so if it remains in a accretion disk like we were talking about it would look like something on the left so in order to see the shadow you need a light behind the shadow right you need some contrast you can only see a shadow against light and i just yeah we're just going to emphasize again we've never been able to take a picture of any black hole like when i show you these animations that's a picture i mean but we have indirect you know indirectly we collect light but we've never taken a resolvable picture so yeah what would it take so this black hole we said was 4 million times the mass of the sun it's 27 000 light years away and so it it's like if you were to move that would be like resolving a piece of fruit on the moon well we like to say a donut because it all also should look a little bit like a doughnut more like a half eaten doughnut because as gas is swirling the part that's coming towards you is brighter and the part receding from you is dimmer so that's why it looks like this half crescent on the left rather than a full circle um so this half-eaten doughnut if you put it on the moon and try to take a picture with your camera that's basically the technology that we need and you you would say that's completely crazy um we will never be able to do that but actually this is something we've been working on for about 20 years we have now grown to a collaboration of about 200 people and what we've done is since no single telescope no single dish is going to be big enough we hooked up different telescopes across the globe in order to have the resolving power to be able to take a picture of sagittarius it's basically a telescope the size of the earth yeah yeah that's what that's what you're creating a virtual telescope do you think this will succeed priya do you think this will succeed she's on the collaboration that's right she's biased no i i was i was giggling in between because the person who had this crazy idea and has worked tirelessly to make this happen was um at mit with us he was actually a co-graduate student with janna shep doleman right so that was not true but um but that part was true he's been working tirelessly but the idea doesn't come from him but i mean making the um you know coming up with the network putting things together to make uh the entire earth be basically one radio dish one large radio dish right he was pretty instrumental in doing that i think that i'm very optimistic i think this is the best chance we have this is the black hole that is nearest to us and if we can pull this off anywhere in the universe it's here and i think we have the technology we have the capability so let me go a step further um in 2017 we took the first what we call the full array observations meaning all the telescopes that are critical for this effort were functional um they pointed to not just our galactic center black holes rj star but also our nearby black hole m87 a more massive one a more massive so farther away but a thousand times more massive so it also looks as big as anything can in the sky so we have the data uh in 2018 we repeated our observing campaign we're all waiting for the press release we're working very hard so i just took a break to come here and i'm gonna go right back get back to work okay um so andromeda also is a is a nearby galley which one's andromeda is how big is that i have no idea i feel like it's a billion andromeda no it's not it's uh less massive it's about it's like 10 times more massive but the galaxy itself is bigger yeah and so we're on a collision course with andromeda right we will merge with this so there's two spiral galaxies they look like they're frozen and the time scale in which we can perceive it but over the next well it's the only object in the universe that's actually falling towards us right everything else in the universe is like moving away so in a few billion years our our supermassive black holes will merge yes and i mean that's got to be also part of the theory of how they grow that's right and i think that you know you're right that you know part of the larger picture that we have for how galaxies assemble over cosmic time and cosmic history is it's by collisions of galaxies so little galaxies form first then they collide with each other and all their components collide right and we now know that galaxies are primarily made of dark matter which is this other enigmatic thing and then we have stars and you have the black hole in the center they all kind of crash into each other and this is one way that we think might also be important in how to build the mass up of black holes especially early on in the universe the crash rate is pretty high in the early universe because at later times the universe is expanding and it's harder to find things to crash into for galaxies to find each other to crash into it our milky way galaxy which um is a collection of 100 billion stars probably has experienced previous collisions with other galaxies yeah we think that it has you know statistically speaking so this is sort of work that we do on building models like possible life histories right within this context and we think that the milky way probably had three major on average between three to five major big time collisions many small collisions with things that were much much smaller than itself um well i could talk about that forever but i wanted to ask farielle about the big announcement today so i don't know if people heard but the european southern observatory i stole this um next animation from them had a press release early this morning u.s time afternoon in europe about observations of sage star this black hole that we're discussing yes this is another very exciting experiment that's been ongoing so this is a set of four infrared telescopes that are in close proximity they are located in chile and they've been following the stars around our that that orbit around here we're honing into the center of our galaxy so so this is this is actual images i mean this is not a simulation okay so right now we switched to a simulation and that's the donuts that we were talking about sort of and so what you're seeing here are is the star cluster that is orbiting around the galactic center black hole and that's real data those ellipses are real tracks of about 27 stars yeah so reconstructed tracks from real data that's certainly true and one of them um the s2 star comes really close to the black hole so um this the gravity experiment is what it's called the um the set of four telescopes has high enough resolution that it was able to follow the track of this s2 star extremely well over a long period of time so for the first time it detected a redshift uh meaning the wavelength of light that is emitted from the star changed a little bit to to the red as it climbed out of the gravitational well of the black hole the stretching due to the gravity basically absolutely gravitational redshift so you know it's it's orbiting around this black hole getting really close and it's emitting light and that light has to climb out of that gravitational well to reach us on earth to reach our telescopes so it was the detection of the gravitational redshift that small change in the wavelength of light that is it first of all it tells us the mass of the black hole with incredible precision and that mass is okay for i mean i'll you don't have to give me all the decimal points yeah i mean i i'm i'm trying to think yeah four million times the mass of the sun but i don't remember how it's four million times the mass with sun but it probably would fit inside our solar system absolutely it wouldn't be four million times bigger than our solar system oh yeah come out to the radius of one of the outer planets it's 12 million kilometers across which is small which is small and i think it sounds big but i think the major feat here is that this star came so close to the black hole that this is the point where we can see the most intense gravitational effect of the black hole yeah so that's the you know the the great sort of awesome thing here that it comes it you know it's it's not as close as uh as we would uh think it would be right i mean it's 1500 times the event horizon or something right it's like 1500 times yeah so uh further it didn't get torn apart it didn't get torn apart and it didn't get swagger it didn't get swallowed so yes indeed it has done this passage before and it survives intact and at the point of closest approach which as priya is saying is not even that close but still gravity wise it's very strong so right we can see that redshift and what what this experiment does differently is that they have such sharp eyes that they can follow it and take little spectra as it's going around so right and then when you're yeah when you move away from the point of closest contact with the black hole right then you see the change so and they're able to track it further on as it's moving away yeah so we're not seeing the shadow of the black hole we're not taking a picture of the black hole but we're watching stuff orbit something dark further out right further out but it's something dark it's orbiting something we can't see it's not luminous and we assess that the mass of the object must be enormous four million times the mass of the sun from the movement but we realize it must be very very spatially small that's right so that's what i mean when we say it's right and i think that you know the kind of picture that we're trying to build up right we are trying to build up sort of um the sort of the event horizon telescope uh observations are going to tell us about right around the event horizon and this is for the same black hole at the center of our galaxy 1500 times further away so we've really kind of pinned this guy basically we know where it is we know how massive it is so the question is figuring out what are the impacts of this black hole on at various distances so even 1500 times or so outside the event horizon right the gravity impacts right and it's affecting um this record so this is actually sorry to interrupt no please this is great news for the event horizon telescope because um one of the reasons we want to take a picture is now coming back full circle we want to understand if the black holes predicted by einstein's theory of gravity general relativity are actually what we think they are and in order to do that test we need to know the mass of the black hole that we are looking at and the better we know it the more precise our test is so this was excellent news for the event horizon telescope too now it's a there's no wiggle room basically either that half doughnut with a shadow in the middle is there and it's as big as predicted by the theory or it's not so um i know the headlines were you know general relativity tested confirmed although we as we know that you can never really fully confirm theory but you know tests are confirmed by this experiment but isn't there a part of you that's a little disappointed like don't we all secretly kind of want to see something that that's we're wrong about it's like you said we love being wrong yeah like being wrong would be so exciting if it wasn't coming back over and over again with the theory of relativity the reason i wasn't too disappointed with this one is because it tested only one aspect of general relativity which is the equivalence principle it doesn't actually tell you how strong the gravity should be um so there's still work to do don't worry so i just want to give us a little um global perspective this is a digital atlas made by the american museum of natural history i like to show it a lot with the reuben museum it's actually placing everything that we know in our galaxy as accurately as it can based on real observations so this is not a cartoon here we're going into our milky way galaxy and you see if you've ever seen the milky way the reason why it looks like this beautiful riff on the sky is because we're in the galaxy we see it edge on and here you're coming to our humble star you kind of turn off the light on it and we're coming to our little solar system and i just want to give a little bit of perspective that we do all of this from that little rock right i mean we've never gone beyond the moon i mean going to the moon was extraordinary but we've never gone beyond the moon and we do all of this right but we managed to send voyager one and two beyond the solar system which is amazing which is incredible so voyager which was launched in the 70s when did it go beyond the solar system 2012 i think voyager 1 actually crossed the heliopause and we think boys are two another couple of years go voyager yeah i know you know when uh sagan put that golden disc the um record on the you know which was a sort of representation of humanity you know cultural selection um people were upset that he was indicating where we were located as though aliens were going to find this golden record and then like hunt us down you know but it's been what 30 some years and voyager is just outside of this oh so like if you found the record you could like there we are we're like right there so it's not the biggest threat um but this is quite extraordinary so just to give people a sense of the scale how many black holes do you think there are in the universe you've already sort of toyed with this concept but an assessment of how many black holes there are in the universe i think that's a very difficult question i mean it's difficult to answer even if you ask how many black holes are in our galaxy so let's start with our galaxy there's one okay so one one one big one right one big one three on that um and we don't think there are any other big ones that we haven't seen they could be if they were wandering around so theoretical work suggests that if you could have another supermassive one or maybe one that's a slightly lighter cousin sort of you know wandering around in the outskirts of our galaxy where there's not a whole lot of gas not a whole lot going on you could accommodate a few the theoretical work like our simulations now show us that you could have a few wanderers yeah it would have to be smaller it would have to be far out otherwise it just spirals in and comes to the center of the galaxy and merges with the existing one so um as far as the stellar mass ones things that are about five times the mass of the sun above we know for a fact that there are 26 i think is what we are up to right now um in our own galaxy so when i say a confirmed black hole what i mean is we have really good data of things orbiting it it's dark what we have detected though is x-ray radiation from um these little black holes and we found a swarm of them really right and down the inner regions they're pretty faint and pretty challenging tens of thousands possible yeah yeah so again i'm i'm distinguishing between things that we've done precise measurements on and that's literally handfuls and then we can infer through other means like the swarm around the around our galactic center that is in the tens of thousands um and then there must be many more that just don't have a companion so they are not glowing and we don't know that they're there if we add up all the other galaxies i think it would be extremely conservative extremely conservative to say a trillion sure but i mean i think you know it could be more like the trillion is impressive however they don't punch their weight in terms of the total mass in stars in the universe or dark matter in the universe they're like a tiny tiny fraction you know much less than a hundredth of a percent or something in the overall mass inventory of stuff so yes the number as i said earlier on i think the universe is probably littered with black holes of all kinds but and but they don't actually contribute significantly to the overall mass inventory so but they have outsized influence so let's just in our last kinds of thoughts just for the last few minutes um why aren't we falling into the supermassive black hole at the center of our galaxy or will we ever will we ever is there is a question sorry will we ever fall into the supermassive black hole at the center of our galaxy no we're very far away never no i think unless you know of course we are very very far away so yes but of course we will i mean you would have a very you'd need a very baroque condition you'd need no you're changing your gravitational waves we can call it on a one-way trip if you're really interested but otherwise we're not going there so here's the argument okay what's your argument my argument is that um like anything that orbits a black hole we admit gravitational waves which is this energy that we give back to space-time causing ripples and space-time so our orbit decays granted incredibly slightly very yes we take an eternity it will take a look in eternity no not in eternity it'll take a lot longer than the past the future is a lot but our son is going to die out in fine sure i'm not saying are we going to be alive right i'm not asking if you and i are going to be having that coffee that's right but i think it is absolutely a fact that everything in the universe in the far far future it's true it's a number i can't even estimate like the universe is 14 billion years old and the future is is staggeringly longer than that there's a bigger there's a bigger uncertainty right which is this overall stretching of space that dark energy is engendering right that if it turns out that the accelerating expansion of the universe we've seen a glimpse of it we know that it happens it kicks in about you know a few billion years you know for um in the universe we know it starts to dominate but if that really continues relentlessly then you know the fate of the universe is different right so i agree with that so it's not guaranteed if the universe expansion gets more severe or if it doesn't decay but if it does decay or if it doesn't get incredibly more severe so that we have what you're referring to as a big rip where the galaxy itself gets torn apart we must everything that can will fall into black holes that is the ultimate fate of everything in the universe unless it gets it's contingent on other macro properties of the universe that we don't yet know yeah another unsolved problem like acceleration i will give you that gravitationally bound objects will remain gravitationally bound even in a big rip scenario but i don't think they will all fall into a black hole they will form isolated regions that that where basically gravitational radiation just doesn't play a role they're just too far apart little island universes yeah yeah i'm sorry i'm not we're not an isolated island universe from our black hole we orbit it we are actually in orbit around it so we are actually definitely emitting gravitational radiation because of our orbit so our orbit is getting smaller it just takes us i don't know however long to make one orbit and it's really really small but if that's left alone there is one and only one fade for this for this like a closed box if you will right so yes fair enough but i think the question is you know how i'm going to get them to relent [Laughter] but it is true there could be these other factors like like the dark energy and the expansion of the universe but it is interesting or this right you you get so isolated that the time scale is so long that you're no longer talking about things you can assess but it is an interesting at least possible fate that the in that black holes are the ultimate death state of the entire universe and here's where i want to get to the really weird stuff what i call the hard stuff oh that was nothing which is speculate away jenna suppose we have to get to the stuff we don't know right that's the fun part no i'm totally for it and i'm not joking because i think look black holes came out of this whole einstein's theory right you one can argue was a creative leap of imagination that he took yeah right and then all the stuff that was in the realm of unreal became real right so hey you never know you might be coming up with something i plan on being around too yeah check it out so let's say we have this very far future of black holes hypothetically um what then is that the end state is that really truly the death state i mean you were talking about black hole evaporation so we're just getting to the idea that black holes are dark they emit no light they reflect no light nothing escapes and then hawking comes along right and says in this very subtle very slow chapter seven brief history of time remember it's so heavily yeah he he realized that quantum mechanics allowed for the possibility this very subtle possibility that the black hole would actually evaporate now it's incredibly incredibly slow it is bigger so the longer it takes so you know what i thought of maybe it's a contest between whether hawking radiation is more efficient or gravitational wave emission is more efficient we think hawking radiation must happen i mean that in itself isn't that weird um so basically the hawking radiation and evaporation is just the idea that you know empty space vacuum is really not empty it's bustling with particles and anti-particles that are zooming in and out of existence right and so if you are right around a black hole you could imagine a scenario where basically one of this particle anti-particle pair gets sucked into the black hole the black i mean with less energy is extracted if you will is lost the black hole will shrink in its size a little bit and this other guy the partner would actually leave and so we would detect that right or we would detect these pairs that then become energy gamma ray radiation very energetic light that we that we would see later on the only problem with this right is that any sort of um reasonable size black hole our friends the guys we've been talking about the real guys the evaporation time scale is just so so yeah yeah but remember what she was talking about before the the time i'm saying exactly gravitational wave a mission to fall into the black hole so janna's two eternities may actually kind of you know fight off each other it will be a comparison of two eternities yeah um well i love this i just um i i wanted to open it up to a q a and but before i do i just wanted to ask fariel and then you priya um what's the thing you're most excited about on the horizon for asked for your area your specific area like your real like when you go home what are you going to lose sleep over pun intended or no pun intended on the horizon oh see what i did there not intended sadly i am most excited about the event horizon telescope and um this is this one where the whole earth is one telescope looking at the telescope two years of data on it and we are trying to reconstruct the images and analyze them to understand what happens in the horizon yeah and priya so i'm i'm most excited about the possibility with the james webb space telescope for getting at the origin of the first black holes to sort of really figure out how the first black holes came to be and also this whole picture of their growth history right but you know guys it's like delayed gratification my patience is being tested as the james webb deadline is sort of moving along but i think we're pretty close to ratifying either validating or invalidating a lot of our conceptions of how black holes form grow and evolve over cosmic time so i think it's super exciting and i'm excited about the event horizon of course i mean anyway i mean i'm excited about every black hole discovery so this morning when i saw this i was like oh wow this is cool i hope we get to talk about it excellent lovely it's a great note to end on let's thank our guests before we open to the q a [Applause] um we're only going to do a few minutes of questions there are mics running around the room if you have a mic in your hand you can just try us we can't see very well out in the audience if there are questions who has a mic hi can you hear me yeah i can't see you if it can hear you yeah okay um so my question is does every star collapse into a black hole and how does that change how many black holes there are it seems like at least in my mind that there'd be more and more black holes forming so i'm wondering if it's sort of a consistent number or if that changes over time sure oh i can take this no not every star forms a black hole so our sun for example is pretty small um and it's going to end its life as a white dwarf it is going to run out of fuel it's going to expand at first and then contract and it's no longer going to generate energy so it's going to form a dense core but it's not it's not actually that dense and um the dead form of stars like our sun is is going to be a white dwarf a little bit more massive so somewhere between maybe eight times and 20 times the mass of our sun they're going to still go through this contraction phase but they're going to go past the white dwarf stage into neutron stars even denser cores the ones that are the size of manhattan let's say and anything even more massive than that uh when it dies and runs out of fuel it's going to collapse into a black hole right so so the number of black holes are growing yes um because new stars are being formed they live out their life they die they either have a supernova explosion if they're massive enough or um after that yeah if i may add you know when stars are born they're born with a range of natal birth masses so that's the thing so this is a thing called the initial mass function stars are born with a range of masses so each one will live out its life and end its life in one of these three fates basically yeah the smallest star would be a fraction of the mass of our sun and the most massive one not forming nowadays would be probably 500 times the mass of the sun right and you know and there are also going to be some things that form in the universe that just failed to ignite basically that failed stars basically you have those brown dwarfs they're also sort of lurking around um i've heard a number as big as one percent of all the stars in the galaxy will ultimately end up as black holes that's about right if you take so that's about 10 billion the mass cut off is and add up everything that's towards yeah about that 10 billion stars at the end of the day that's a lot that's why i was saying it to be huge number 10 times 100 billion galaxies it's actually a huge number but right priya less than the rest of this stuff um anyone else have a mic please uh yes i do and this is a question for you uh something that you mentioned early on in an offhand manner and i i believe i heard you say something about a black hole blowing out a new universe i'm just wondering if you can clarify if there's significance to that she's the one to ask that one so um we don't know how to predict what happens when all of the stuff of the star that forms the black hole continues to collapse to basically microscopic scales so imagine all the material of a star being in a microscopic scale but one of the conjectures was from some of the very early relativists that you could actually sew together space times it's not a prediction it's just asking could i make it this way this is um so you're just saying that basically we don't have a quantum level understanding of gravity right we don't yet understand quantum gravity so without quantum gravity we can't make a prediction we can't say oh i can follow from the initial state to the final state but you could make a quilt and just ask does a quilt actually is it possible to sew the quilt together and so people have tried mathematically to sew a big bang onto the interior of a black hole and it can be done so so it's again it's not a prediction it's just saying there's some continuity that's conceivable and what that would mean is that even though the black hole is only say the size of central park on the outside it could be as big as an entire universe on the inside you know it's like doctor who's tardis you know it's much bigger on the inside than it is on the outside um and that's just one of these extraordinary things that warping space time allows you to do a lot of people don't think it's viable for all kinds of complex reasons but it always recurs you know in the imagination yeah no and i think you're absolutely right that you know that kind of grand unification of gravity with quantum mechanics has remained a pipe dream still right yeah and and one of the interesting um issues that we didn't have a chance to get anywhere near is um whether or not we'll discover that gravity is kind of an illusion when we understand quantum gravity that it's not fundamental and doesn't exist but that's a whole other cycon right we'll do that another time um is there another uh mike out there um so if i'm understanding this correctly um the black holes are kind of eating everything around it and then emitting energy so just think i mean i might be oversimplifying probably but is that sort of its purpose if you could say the black holes had a purpose to eat the stuff so that it could emit so i would say stuff is emitting light and energy on its way to the black hole but once it's in there other than this mine minuscule um hawking radiation that we were talking about at the end it's actually staying in there um so that's that's just the first part of what you are asking do black holes have a purpose i don't know i mean i don't know i mean um do cosmic objects have intention um we i don't know but but i think that you know we think of black holes basically as you know one of these other one of the sort of triumvariate the trinity of dark entities uh in the universe right dark matter dark energy and black holes are basically these sort of bizarre entities that don't interact with light in our usual ways so we can just think of them as you know a phobic to sort of electromagnetism if you will right um so i don't know i don't think i want to say any more about sort of purpose their purpose seems to be that they um want to defy interacting with any of the other known forces right other than gravity yeah so i mean maybe they turn out to be inevitable given our physical laws oh totally i think they yeah that's quite clear i i would not call it a purpose but it's just the way that our the physical laws in our universe and maybe there are many more out there that work differently were black holes turn out to be inevitable and that's why they are ubiquitous and that's why they are so interesting and you know at the center of many phenomena when we do think about quantum stories we think of black holes as fundamental particles of gravity and that's extraordinary like there's no the chair is not a fundamental particle a star a neutron star nothing else in the universe except i mean at that large scale at the macroscopic scale except possibly black holes meaning there was a lot of discussion about whether or not we would actually make them the large hadron collider when we smashed particles together that they would be like any other product a fundamental particle that could that would just emerge that would just emerge and would hawking evaporate safely away fingers crossed so there was there was a lot of concern about whether the large hadron collider was safe right but i think well not a lot that's not reasonable because you know how much energy you're putting in and you know that you can't get out more than you're putting in so it was a bit of a hype it was a bit of a but i think that you know coming back to sort of i think your question was meant to be somewhat philosophical right and i think that's where i want to circle back to what jana said that you know the big bang we believe was also a singularity right it might be a different kind of singularity from the interior of a black hole we don't know but you know sort of in terms of a coming full circle there there's a lot of uncertainty about exactly how this whole thing started the thing being the entire universe and similarly we don't know how things end right so um maybe we're inside a black hole maybe we're the big bang inside the black boy um i think we only have time for one more question i know it's getting warm in here and people want to go to the garden and come to the book table outside but i i'd love to grab more question if someone has the mic um i was gonna ask something about entropy but more broadly are there any ways in which black holes have challenged our preconceptions about the universe that we had before studying them or our predictions that we have actually oh um are there any ways in which black holes have challenged our preconceptions about the universe or our predictions for the way the universe will grow and develop so have black holes changed our predictions or challenged or challenged our predictions for the i think you said how grown and developed so the history and future i'll interpret that i think um i think i might have mentioned this earlier on ray that the we never imagined that they would have this kind of outsized influence given the scales that we've been talking about even for supermassive black holes in the center of galaxies they seem to be shaping the overall fate of galaxies via the kind of modulation of star formation in galaxies this is something that has really changed we never thought that they would play such an important role in galaxy formation we just thought they were sort of sitting ducks you know they're sort of sitting there they happen to be there but it now turns out that they are likely playing a very active role so that has been sort of one thing that has shifted in the last decade with data oh i think that's extraordinary and it is very recent and it's and it you know once when you say past 10 years that means there's a long buildup of of sort of dispute and uncertainty and now it's starting to really calculations yeah new instruments were needed to you know new instruments had to be invented and data had to be taken and and i think i think i think jana and ferial both kind of alluded to this earlier one of the interesting things about black holes is that we've gotten a really rich picture of them when we have patched together data from many different wavelengths right many different windows into the universe including gravitational waves now so this whole sort of patchwork of the energy that's coming out in visible light in x-rays gamma rays infrared everything put together it's just phenomenal how much we know now right yeah and just i do find it really chilling this idea that black holes might have sculpted the universe on the largest scales i mean that is totally extraordinary and and unanticipated from where we started but the conversation and the history of black holes curiosity right mathematical lovely but nonetheless mathematical curiosity you know and then the elegance of math kind of forcing its way onto nature again and that does seem to be kind of an extraordinary history of that um uh so i know i said it was the last question but i was talking and i can't be the last one to talk so i'm going to get one more question then we're out of here hi yeah you've talked a lot about black holes in terms of space matter and uh and i was wondering what happens to your perception of time as you approach a black hole and what happens to your perception of time after the event horizon after the event horizon's particularly interesting yeah right so i mean we believe that what really happens um once you cross the event horizon is that the role of space and time actually get flipped so remember we talk about this sort of four-dimensional sheet called space-time which is sort of um the way in which we describe everything that happens in the universe and we need to sort of bring time in there because time is a relative thing that you measure only with respect to something else and so it takes you know light eight minutes to reach us from the sun and if the sun shuts off we won't know for eight minutes right so to take that into account sort of cosmic distances and the finite speed of light einstein realized that time has to be intrinsically combined with space to give you a description of not just the shape of space but of all motion and of all gravity so once you cross the event horizon we believe structurally space and time flip right and so in the mathematical equations what that means what do i mean by time and space where mathematical equations they actually swap where they appear in the equation singularity is in your future yes yes it's not a it's always naively as the center of the black hole but in reality if you're falling in it seems to you like it's your future it's your future it's totally inevitable as the next second is for the next second yes and in fact when we were defining black holes we kind of alluded to that right i mean um if you're just passing from a distance it's fine you can determine your future but if you're getting close to the black hole then your future is tilted toward i mean that's that's the kind of terminology that we use then your future is so great i mean i think i think janna's book has a really nice description of this if i may add black hole blues and i think carlo ravelli recently had a book called in order of time and i think these are the two places where i've seen a very nice actually lyrical description of what's really happening what a beautiful note to end on a plus note um really lovely to have you both you're both friends of mine it's such a pleasure to see you again please let's welcome them and thank them for coming you are free to escape

Channel: Pioneer Works

Views: 10,922

Keywords: pioneer works, science studios, scientific controversies, event horizon, event horizon telescope, black holes, janna levin, science, spacetime, stephen hawking, theoretical physics, quantum physics, supermassive black hole's, black hole event horizon, Messier 87, M87, Sagittarius A*, milky way, how do black holes form, event horizon telescope explained, Priyamvada Natarajan, Feryal Özel, astrophysics, astronomy, physics, cosmos, dark matter, dark energy, black hole experts

Id: oVi9FqmcZBM

Length: 83min 16sec (4996 seconds)

Published: Thu Oct 15 2020