Nobel Prize for Black Holes - Sixty Symbols

well we're in radio nottingham as you can see and uh they've today announced the the nobel prize for physics so in a nutshell it's it's gone for black holes it's the only way i could describe it and in fact the wording of the citation is kind of interesting so let me just read it out it says the nobel prize in physics 2020 was divided one half awarded to roger penrose for the discovery that black hole formation is a robust prediction of the general theory of relativity the other half jointly to reinhard genzel and andrea getz for the discovery of a supermassive compact object at the center of our galaxy and you'll notice it sort of skates around saying anything there being about there being a massive black hole at the center of our galaxy because technically no one has really proved that that's what it is it's just all you know no one can think of anything else that it could possibly be at this point the two astronomers they you know been looking at the black hole the center of our galaxy basically what they they were looking at they managed to establish that there was a really compact object there four million solar masses in basically a region the size of our solar system or even less and you know the remarkable astronomical observations but the guy i want to talk about is the other half of the prize which is uh sir roger penrose because i'm a theorist and he's a theorist and so obviously we share a lot of interest um and the work that he did and the reason that uh penrose got the prize uh was really he basically sort of learned taught something about the structure of black holes and what lurks inside them and what lurks inside a black hole is this dreaded singularity this is this region of sort of uh infinitely strong gravitational field infinite uh space-time curvature that horrible word infinity that basically that these this horrible monster at the center of a black hole exist so i would say it's you know that 1965 paper that remarkable paper which showed that these singularities are for real and it really shocked the world of physics everybody was stunned by it no one saw it they thought these singularities were just a an artifact of symmetry they didn't think they were real penrose showed that they were real general relativity generally really did predict them and then you know hawking then took that on and worked with penrose as well and they showed that you get these singularities you know in a sort of at the big bang either started the universe or maybe at the end of the universe it wasn't just inside black holes so the whole story sort of sort of blossomed into something and the other thing it tells you these singularities tell you is that general relativity is not a complete theory because it can't describe those singularities it's outside even though even though it predicts they exist and penrose showed us that you know you follow your nose and follow general relativity you're going to end up in this place with these dreaded singularities actually it can't really describe it and that's why we know we need more than general relativity we need a quantum theory of gravity whatever that might be string theory or whatever so it also tells it also sort of spelled the demise of general relativity we know it's not the full story just one of the other wonderful things about what penrose did this is a great piece of it it's a fabulous piece of work as i said it's probably the most important uh paper in in gravity between einstein's original papers and his in 1965 but you don't really see nobel prizes in the past given for these kind of theory papers um no one's actually gone into a black hole and spotted a singularity that hasn't happened and yet they've given him this prize and i think it marks a bit of a change for um for theorists and potential nobel prize winners and i think it's a great thing i absolutely applaud it so uh long may this continue and maybe is uh once again uh sir roger's gonna be a bit of a trailblazer for everybody else well so it's really all about showing that there's a very massive object at the center of the galaxy and in principle showing measuring the mass of something showing that there is a massive object somewhere is reasonably straightforward in astronomy in that for example we know that the sun's at the center of the solar system not just because you know it glows in the sky but actually because it holds all the planets in orbit around it and so actually even if the sun weren't giving out any light at all we'd be able to say there's a massive object in the center of the solar system and actually we'd even be able to say what its mass was just by studying the orbit of the earth and the other planets around it so in principle you can sort of do that make that kind of measurement for anything which is gravitationally bound together and in essence that's what they've done for the center of the milky way in that they've been looking at the stars orbiting around the center of the milky way and showing that the orbits they're following imply that there's a very massive object at the center of the milky way it sounds very simple the way i say it there of course in practice it's a huge amount more complicated and in part that's because so it's part in part it's because you're looking at something which is pretty small in that the whole of the region that they're looking at where those things are all orbiting around are all within an arc second or so and that's the scale on which if you just take a picture of a star with a telescope from the ground typically the atmosphere blurs out the image to about an arc second so actually all those stars packed in with those central arc seconds if you just took a naive picture of the center of the galaxy with the telescope they will be blurred together and you wouldn't actually be able to see anything at all and follow their motion the other complication is that the center of the milky way is a very difficult place to see because between us and the center of the galaxy there's a huge amount of merc there's this stuff that astronomers refer to as dust it's sort of this smoky material which obscures our view of the center of the galaxy which essentially means that very little of the light coming from the center of the galaxy gets to us if you look in the optical part of the spectrum it's something like you know one photon in 10 billion emitted near the center of the galaxy actually gets to it so there's no chance of actually seeing it the solution to both of these problems actually came along sort of in the 1990s when um astronomers started first making images decent images in the infrared part of the spectrum so it's sort of technology driven that we started getting the the detectors that allow us to make images uh in the infrared part of the spectrum where previously we'd really only been able to make optical images and the infrared has two big advantages firstly that that merck is more or less transparent so instead of sort of one photon in 10 billion getting through it's something like one photon in five gets through so still a fair amount of it gets obscured by the dust between us and the center of the galaxy but actually enough gets through that you can start making images the other big advantage of the infrared is it's more straightforward to take out those blurring effects of the atmosphere to sharpen up the images and so a lot of the the breakthroughs in this area have come because adaptive optics these techniques that astronomers use to take out the blurring effects of the atmosphere work well in the infrared part of the spectrum so they've really been able to sharpen up those images so instead of seeing those 20 or 30 stars near the center of the galaxy all blurred together into a single splodge they really can make pictures of the individual stars and that has enabled them to monitor those stars and actually literally see them orbiting around the center of the milky way apparently around nothing there's nothing very much there well there's a radio source there so there's something there that we thought was a bit suspicious and there's something that gives out radio waves um but there's not there's no obvious star there or anything bright there and these stars near the center of the galaxy are all following these nice elliptical orbits just like the planets around the sun whizzing around something which doesn't appear to be there and now they've been able to observe them for long enough that some of those stars they've actually seen them go around their complete elliptical orbits now that gives you a very strong constraint on okay i can actually measure i know what the pull of gravity has to be to be making things orbit around the center of the galaxy at that speed and that's how they come up with this number of a supermassive object in that central region of the milky way of a few million times the mass of the sun so it's called the singularity the black hole singularity where penrose entered the game was he wrote this paper in in a in 1965 which basically shocked the world of physics and basically what penrose showed was that this singularity was likely to was there okay that it was predicted by general relativity so what happened previously is people like oppenheimer and schneider had been looking at things like sort of collapsing uh you know spherically symmetric collapsing sort of clouds of dust and shells and things like that and in these very symmetric scenarios they were seeing it as that those those symmetric sort of distributions of matter collapsed in that indeed you would form a black hole and at the center of that black hole would be one of these dreaded singularities and people were like all right we're not taking that seriously that's it's just because you were considering a situation with an awful lot of symmetry that's the only reason that weird singularity that weird infinity that madness is there but what penrose showed was it wasn't really at account of this symmetry it would always be there okay that this was really an artifact of general relativity that black holes do contain these singularities okay he proved these singularity theorems and it was much more general it didn't require symmetry it was extremely general statement all you had to say was basically as long as i have effectively positive energy if i'm thinking about matter that's made up of positive energy then i'm guaranteed then it collapses to form black hole i'm guaranteed to get one of these singularities which basically is just an end of space or an end of time it's one of those things that kind of crept up on the field in that you know back in the 1960s people started discovering these things called quasars which were these incredibly bright distant objects which were varying on time scales of days or at least you know weeks and months as well um which tells you they have to be very small right for something to vary very quickly you know if you actually had a whole galaxy's worth of stars even if you found some way of switching them all off and on because of the amount of time it takes light to travel just from one side of the galaxy to the other you wouldn't see the whole thing switching on and off very quickly so when you see something varying on time scales of say a few months that tells you it can only be a few light months across so these quasars were varying on very short time scales which told you they were very small and they were also incredibly bright and this combination of things people were casting around trying to figure out could they be super massive stars or various other ideas were thrown around but the the idea that one out is that had to be a very very massive object and material was falling into it and you were seeing the kind of the energy that was liberated as the material fell into it and that would both give you a lot of energy because if the thing's very compact the pull of gravity is very strong so the stuff falls in extremely fast and you have a lot of energy but it's all in a very small region so it varies very quickly so that was you know as i say back in the 1960s 1970s people were starting to think about it in the 1970s this radio source at the center of the milky way sagittarius a star which is where this black hole is located was found and that got people thinking well maybe there's one in the middle of the milky way as well with the advent of the hubble space telescope in the 1990s we were for the first time able to actually start um resolving regions close enough to the center of other galaxies to see actually yes stars are whizzing around extremely fast there as well so it's one of those things that kind of crept up on us a bit at the time um but this is sort of you know this is the the the best measurement we have the ultimate measurement and actually at some level you know knowing it's there in our own galaxy tells us quite you know quite how commonplace this is that even an ordinary galaxy like ours has a massive black hole in the middle of it what did penrose actually show he showed that that the space time wasn't complete inside the black hole and what he said was wasn't geodesically complete now what's a geodesic geodesics are the sort of shortest paths in space time they're the paths that a particle will take if it's if it's not accelerating through space time it'll follow a shortest path now assuming you were immortal and you could in principle live for as long as time could exist you would just follow geodesics forever okay you just go on and on and on and on and on right what penrose showed was if you fall into a black hole and you follow one of these geodesics eventually the geodesic itself will grind to a halt and think about what that really means that means there's an end to either time or space there's actually an end to time or space so even though you might think you're immortal nothing can destroy you well the end of time can destroy you so if you're falling into one of these black holes you're following one of these geodesics depending on the nature of the geodesic you will start to feel very strong gravitational fields you'll start to feel yourself being ripped apart by the tides of gravity literally you know the atoms in your body being torn apart and then you know as you sort of beg for mercy you know even if you're capable of begging because your ass will be torn apart as you beg for mercy what you'll find is is that you'll be given that mercy because time itself or possibly space but in the case of um depends on the nature of the black hole but time itself will end okay so even though you think you're immortal well it doesn't matter because there is no more time time itself stops and that's what penrose showed that these things are really lurking there inside black holes we talked about these stars orbiting around very close to the center of the milky way they're still you know so this one particular star s2 which is the one that's gone closest to the center and that's one that they've now followed around a full orbit and in fact it got so close you remember if you think about elliptical orbits uh like comets in the solar system for example when they're kind of in their outskirts of their orbits they travel very slowly and then they whiz past the sun and then back out again it's just the same story here the stars hang around and then when they're close to the black hole they whizz past it and then slow down when they're traveling away from it the one that went closest to the um to the black hole at the center of the milky way was traveling at about two and a half percent of the speed of light so it was actually a decent fraction of the speed of light and got sufficiently coast there's all sorts of interesting things you can do with it for example it was sufficiently close to the black hole that the light escaping from it not only had to escape from the star but had to escape from the gravitational field of the black hole as well and so they were able to measure that gravitational redshift the energy that the light lost as it was escaping from the region of the black hole but even in these cases we're still you know we're still the the distance i think it was a hundred or so astronomical units from the the putative black hole in the center of the milky way um so sort of a hundred times the distance of the earth to the sun which is about a thousand or so times this thing called the short shield radius which at some level is sort of the size of the black hole it's that point of no return where even light can't escape so the sort of the size of the black hole was a thousand times smaller than the scale that this star was probing so we can't actually say we can say within a thousand times the size of a black hole was the mass of a black hole but for it to actually be definitely a black hole we have to be able to say you know within one times the size of the black hole there's that much mass but actually we can only say it's within a thousand times that region there is nothing else that anyone could think of that would pack that much mass into that smaller region that wasn't a black hole but that's why they sort of hedged their bets that tiny bit in the sanitation by just saying this there's the huge amount of mass in this very small region rather than saying there is a black hole there i mean the mass of the of whatever is there is incredibly accurately determined i have to look up the number now but it says i think the best measurement of it is it's 4.15 plus or minus 0.01 million solar masses so that's an incredible you know by astronomical standards usually in astronomy we're happy if we get something within a factor of two right so actually you know measuring something to that level of precision is incredible that they know very accurately how much master is there they just don't know for sure that it's all compressed within a region small enough that it has to be a black hole um so as i say it almost certainly is there's nothing else that anyone could think of that would explain it but they haven't in that sense seen a black hole in the same way that for example the event horizon telescope was able to actually image very close to a black hole itself they're actually looking at a slightly larger region which is telling you how much mass is there and it's all very concentrated but it doesn't in itself tell you for certain that there's a black hole there they were very much in competition with each other so there were two groups in the world so guetz working at ucla ucla in california was using the keck telescopes in hawaii and genzyl was working in germany and using the european southern observatory telescopes in chile um so they really were sort of in competition and it's one of those sort of healthy competitions i don't think it was you know it wasn't they weren't um unpleasant towards each other it's just one of those things that was an exciting thing that sort of occurred to both of them at the same time they both started working on it at more or less the same time um and they've sort of both been competing to get the results out which is both healthy and that it's sort of competition it drives us on to do things better um but also it's sort of healthy to have that cross-check because if it was only one person in the world was claiming they'd found something you know you always think you know could they possibly have made a mistake somewhere along the line so it's good to have two completely different groups using different instrumentation and potentially different techniques for analyzing their data and so on both arriving at essentially the same conclusion we have one corner of the penrose institute and we had the first um sort of ignore inaugural workshop of the penrose institute took place right here in nottingham a couple of years ago and he was here for an extended period and drank and we had a cup of coffee in the in the library there and taught for you know a long time about physics he also gave us some lectures which were fabulous and and the remarkable thing about it was this is a guy who's well into his 80s and these lectures were going on for like two hours and his stamina was was astonishing i first met him sort of well it's probably about 10-15 years ago now and i was a postdoc in oxford and and he was he was actually retired by that point um but there's this this idea he'd had in in a sort of late 60s early 70s twisters suddenly came back into fashion and and he came out of um retirement to give us all some lectures on it and uh what i remember about those lectures was i mean they were fabulous but what i remember most about them was how good he was at drawing pictures he would draw these pictures in the lectures and they were just awesome it's like oh great he's not just a great mathematician a great physicist he's great he's a great artist as well so it seems so that was one thing that stuck my mind about about those early days of meeting roger penrose but obviously he's a legend of physics on maths and it's been a privilege to have been in nottingham and it's great that he's won the nobel prize that poses a few problems i'm sure you can imagine one of them is the fact that as you tip the mirror through the night to look at different angles of the sky gravity starts to affect the mirror and deform it ever so slightly so what you do is you have something called active optics which are see all those little pistons there there's 150 actuators basically little pistons that can push and pull

Channel: Sixty Symbols

Views: 119,343

Keywords: sixtysymbols, nobel prize, black holes

Id: 8Qrj2LZAM-w

Length: 18min 58sec (1138 seconds)

Published: Tue Oct 20 2020

Reddit Comments

[u/ijmacd]

Already watched it this morning. It was a good video and had a nice feel to it split between the park and professor Merrifield's new webcam.

Everytime I see a post in this sub though, I do find myself wondering if there's a u/BradyHaran posting in the sub r/JeffDujon