Q&A: The Physics of Black Holes - with Chris Impey

[Music] and was I somebody on this side as well I think holes created however black holes created wormholes okay so it's a good question cuz I didn't talk about wormholes you good good catch I shouldn't mention wormholes so wormholes are a hypothetical situation where the black hole that is you know something we can't inspect pinched off part of the universe somehow connects to another part of the universe and maybe matter and radiation come out of the other end that would be a white hole so a mirror image so the question then becomes could wormholes exist and how would we know and the answer is interesting because it's theoretical physicists have written papers and even books about this and in relativity wormholes are possible it once you've created a tunnel in space-time and pinched off a piece of space-time it's possible that it could connect to another part of the universe and and provide you almost like a shortcut between those parts of the universe so yes it's possible in principle how would you know of course really you don't ever know by going right by taking the journey now the sort of dampener on the enthusiasm is that for a universe where that happens the space-time has to be kind of convoluted and interesting otherwise it's unlikely that distant regions can connect together you know like you have a big grumble cheat and two pieces of the sheet could be close to each other and you make a tunnel between them our universe looks to be very vanilla very smooth space-time except for black holes in very special places the universe does not seem to be ultra curvy and convoluted and connected so people are pessimistic that wormholes actually exist in our universe but they could it's possible so this question over here and then we'll take one from the gallery as well sort of a two-part question really is are galaxies created with a black hole and why don't galaxies aren't while they're destroyed by the press of a black hole okay so both good questions so the question of are they created with the black holes are sort of chicken and egg which came first the black hole this the galaxy or its central black hole is a very cutting-edge research question and we hope that the James Webb Space Telescope the successor to Hubble to be launched in 2021 will answer that question to do to answer that question we have to look back to the first few hundred million years after the Big Bang so pretty far back in time beyond what Hubble can do because that's when we think the first galaxies formed and so at the moment two hypotheses are on the table one that the first things to form were tiny galaxies maybe a million times the mass of the Sun and within them tiny black holes 1050 a hundred times the mass of Sun formed and then the things just grew together so the galaxies really came first but maybe slightly more probable from theory and simulations is that in the early universe a generation of massive stars formed the first things the first thing is to crystallize out of gravity were actually stars and in those simulations you can form 200 300 400 times the mass of the Sun black holes before you have any galaxies and then the galaxies accrete around them and that would be really interesting too and those two predictions are different enough that if you have a telescope that can look back that far you should be able to see which of those two things happen so people are very optimistic that James Webb will answer that one and then the second part of the question of why does the that black hole not destroy the galaxies or what's that relationship it was partly answered by the fact that the black hole in a sense in any galaxy is point 1,000th of its mass and also it's very tiny I mean the black hole in the center of our galaxy to take an example you know isn't smaller than the solar system and the galaxy is a hundred thousand light years across so the rest of the galaxy just doesn't even know about the black hole it's just inconsequential so that's really why they don't have an impact on the whole galaxy I'm just gonna take a question from the gallery and then someone from the left afterwards hello you shared a slide of dark matter with that five times the amount of bear any matter what's the current thinking behind black hole formation and dark Matic the so much dark matter Vance a galaxy using massive gravitational effects right so it's a very good question is this question what's the relationship between black holes and dark matter I mean black holes are dark they're dark matter of some kind and we know there's a ton of dark matter out there which Dwarfs all the normal stuff stars and stars and galaxies by a factor of six could it be black holes so people have asked and not fully but mostly answered that question so the first part of the question you can answer that's easy to answer was answer decades ago is what if there aren't what if there are a lot of black holes out there you know between galaxies even and the answer is normal dead star black holes the thing that happens before they die is there a supernova goes off and supernova is a spectacular event releases a lot of radiation shock heated gas so it's sort of like you know if you leave your house for vacation and you're you know your teenage children swear they're not gonna have a crazy party but they do have a crazy party you know they can swear blind they didn't have a crazy party but you're gonna see the mess in your house so you will know there was a crazy party so black holes there's a crazy party that precedes the creation of black wholeness a supernova and so we know that there weren't enough black holes to form the dark matter because we're to see the residue of the supernova events that preceded them so that's the basic answer the little door the little window that's left open or ajar on dark matter as black holes is as possible there are micro black holes small black holes black holes the mass of a tennis ball or an atom or or an asteroid so so smaller than a star and smaller than a planet there's a little window still permitted where if such black holes formed in abundance early in the universe essentially from the Big Bang that they could be the dark matter if they were too small then how Kings theory says they would have evaporated before now and so they're already gone turned into radiation but if they're massive enough you know grain of sand up to golf ball-sized they could still be out there and and that hasn't formally been ruled out so you know most astronomers and physicists don't think that's really what dark matter is but it's not been ruled out so we have to be keep an open mind jenny was a gentleman just a nice cue and I'll take another question this side and then we'll we'll come to you in a second listen to when you said that black holes are actually small components of the universe compared to all the dark matter and dark energy that's out there material so I was just wondering about is it possible that towards the end of the universe's life there will be so so many black holes they'll just sort of eat up the whole universe that could happen except for the fact that the universe is accelerating in its expansion so the the expansion of the universe that Hubble measured a century ago we now know to be accelerating because of dark energy we don't understand dark energy but we can measure the fact that the rate at which galaxies are moving apart is accelerating so galaxies are going to be completely isolated in the far future so we can forget about black holes roaming around between galaxies and sucking up material there won't be any material it's just a ultra incredible vacuum so then we're just asking the question about galaxies themselves and then it's sort of the answer to the previous question a black hole that's a point one percent of the mass of a galaxy like the black hole in the center of our Milky Way it's not gonna eat the galaxies if the Sun overnight turned into a black hole so someone some evil intelligent genius civilization like the ones I was talking about at the end you know came and squeezed the Sun down the three kilometers turned it into a black hole while we were sleeping on the earth would just continue in its orbit at this at the hundred million kilometers 150 million kilometers you just need Newton's theory general relativity doesn't apply so the distances are such that the black holes are not going to consume everything of course if that evil intelligent civilization did that you know we we'd all die because the radiation would be gone and we'd learn about it eight minutes later - too bad but the gravity situation would not be a threat so belief is another question up in the gallery and then we'll come to the question in the center so you were saying how basically when matter is compressed into this small enough volume it will create a black hole how do you compress the matter how would you do it outside of even say a supernova so I mean nature makes it happen in a dead star and the supercomputers are now good enough to really understand how that works so the very impressive calculations of the inevitability of black holes by this gravitational collapse was done by Oppenheimer amazing calculations done before computers really just theory the simulations with supercomputers just kind of nail it and basically once you have a star core where all the fusion sources are exhausted so you've reached iron you've made you fused all the way up to iron and you get no more energy after iron the most stable element then gravity takes over and if you just run the math of the gravitational collapse it forms a state of matter that eventually becomes denser than nuclear matter pure nuclear matter is a neutron star that's the second most dense object in the universe and we know they exist - we know of hundreds or thousands of those but if the mass is sufficient then even normal even nuclear matter where the protons or the neutrons are sitting next to each other like eggs in a crate is overcome by gravity and at that point you push matter into itself and create what is formerly a black hole so I mean that's what the theory says that's what simulations say and if you have enough mass in a small region of space big or large big or small it'll happen the big black holes in the big black holes it happens in a very similar way they probably don't form in one dramatic event they may form as I mentioned in answer to the previous question there may be a seed black hole that was at the beginning of all galaxies that's a few hundred times the mass of the Sun and it's just been gradually getting more and more massive as it heats gas and stars over billions of years so it's a pretty gradual process so but the initial seed would have formed in the same way by a supernova core collapse and the initial black hole we've had a young man in the middle question about a trillion years that all the stars would die out and become black holes and then you said black holes evaporate so there will be nothing Mike am I kind of depressing you I hate you depress young people you've got your whole life ahead of you we're talking about trillions of years yes so what you described is about right the caveat I didn't put it in perspective the stars that die and leave behind black holes our tiny fraction of all stars they're essentially for every star like this for every hundred stars like the Sun there's one black hole mass star and there are a hundred thousand red dwarfs or very low mass stars so around the universe most stars will be low mass stars that will die as white dwarfs those are just cooling Amber's but they will fade to black eventually the Sun will die as a white dwarf too and it's just this tiny fraction of massive stars that will die as black holes and of course they're black and they're radiation their evaporation process is incredibly slow compared to the the gradual death of stars where their radiation is lost but I guess my point was that it's falsely sentimental to get attached to stars as lightbulbs and energy sources in the sky because the the biggest source of energy in the universe is gravity and once you just get over being attached to a star or it's life then you have tons of you all energy you need to do all the cool things you want to do forever so you know there's plenty of energy out there so if you could just pass the microphone forward we've just got a question over left here hey somewhere in your talk you mentioned that the size of a black hole in the center of a galaxy is correlated with its mass and my question is first what does it mean and a related question is could that be an artifact of how the size of the black holes is measured in the first place because you mentioned do you measure them by looking at the trajectories of these stars which would then kind of depend on their mass right so that's a good question and and I fall back on the fact and should always remind everyone of the fact that that image I showed of m87 that's the first time we've ever measured with the telescope the event horizon size all the all the normal the smaller black holes that were detected starting in the sixties seventies eighties we've never measured their event horizons so this is this image was spectacular because we've never actually measured the size and the mass of any black hole until this one so it's pretty important otherwise yes we're relying on theory and the theory of black holes and how they form suggests it's what the theory says and we've seen nothing to say that it's wrong that the mass of a black hole that it's the size of the event horizon is linearly proportional to the mass so a star that's ten billion times the mass of the Sun is about a billion times bigger than a star that's ten times the mass of the Sun and and that was what I made seven looks like now the implication of that is interesting because if you do the simple math of density density is God if you want to do the math right in front of you but it's you if you think about an object that gets bigger linearly with its mass that actually means that its density is going down with its mass because the density goes with volume right and that's the cube of the size and that's why the density of big black holes inside their van dries and gets lower and lower as the black hole gets bigger and that's why this retching force as you fall into a big black hole gets less and you can survive it so if you work out the density of the seven billion solar masses of material inside that m87 black hole the density is not much different than the arrow-breathing it's not a bizarre super extreme state of matter that's amazing in its own right whereas the density of mass inside a normal stellar-mass black hole is of course more dense than nuclear matter it's incredibly dense so that's an interesting consequence of the fact that mass and radius go linearly with black holes and we do have evidence m87 is now a data point to show that that calculation is correct so if you could just pass the microphone along to that person there we've got a question in the middle a question upstairs there we'll have a question there and then we'll see if we can squeeze them any more before we have to go thank you question the middle first yes this is following the invitation to a space stupid questions but the conventional view of space-time is the rubber sheet with the ball bearings if you like sitting on it stretching it what I don't really understand is with a black hole is it actually tearing a hole in that sheet in space-time was it merely like a very very dense object which is because of its accumulation of matter and its density prevents light from leaving it but it's still there as enormous mass sitting there in the universe on that rubber sheet orbit very depressed rubber sheet I think it's you know given that we're talking analogies it's more appropriate or accurate the second to use a second analogy it's it's not that you've created a hole in space-time or it a rupture per say it's really just that you've created a concentration of energy and mass such that space-time is so distorted that that region is invisible from any other part of the space-time so it's it's pinched off if you like and of course it's even harder to imagine when it exists in three dimensions like this room what does it mean to have pinched something off within three dimensions I mean rubber sheets or two dimensions and we can see them and understand what that looks like so formally it just means that there's a little piece of the space-time the size of the event horizon is all which is invisible from your view and really you can ask an answer the question of what's going on inside it there's no way to diagnose it and that raises the wormhole question you know maybe does connect to somewhere else in space-time but again you can't answer the question without doing an experiment so the the conceptualization of the event horizon is is difficult and the singularity is even more difficult because the singularity is is a that's why that's why I chose the title of my book and Einstein I didn't even mention this and should have but Einstein I did not think that black holes existed that they were real he thought they were artifacts of the theory and he was keying off the singularity which he knew was a prediction of the theory back then physicists have got used to the singularity but as I mentioned even Hawking recognized that is a serious problem for the theory that the theory is incomplete so the singularity is the worst problem conceptually and physically than the vent horizon itself and nobody's made that go away it's a problem we need a quantum theory of gravity which of course is a holy grail of physics to come up with that or there a number of reasons you know string theory there's there's a convergence of theoretical physicists who are hoping to unify quantum theory and gravity theory and it hasn't happened yet and out of that unification we will probably understand black holes a lot better than we do now and we won't have to use these crutches of analogies so change their question upstairs sorry what's exactly inside the black holes what matter I mean Newton's photons what's going on there can we simulate this or think about it and the other half of question is can Koby destroyed by a different way than the radiation for conciliation somehow I know maybe you send a positive charge to it a lot of charge or something else yeah I'll come back to that because the question of the electric charge is interesting people have wondered whether black holes actually have charge most matter in the universe is neutral and since black holes are made of stuff that fell in that's neutral it's unlikely they have an electrical charge but if they did it actually leads to some interesting consequences but as for what is the state of the stuff that's inside the event horizon how do we describe it how do we understand it I mean the simple answer is we don't know because I mean it sounds like a cop-out but it's just what you can say as a scientist being careful to say what you can say and not say what you can say the black hole is defined by its mass and its event horizon size and that's all you get to send its spin its angular momentum and that's all you get to say any other detail of what of how to describe what's inside that space is it's not said by the theory if there doesn't tell you how to describe it so the the black hole as I mentioned in a flip way the black hole could have been made by a dead star collapsing or a bunch of people aggregating all their lost socks in a whole galaxy and pushing them into a region of space or by people who just hated knowledge the throwing all the books they ever had into the black hole and all the encyclopedias they were had and the black hole would look the same now the physics way of describing this is that black holes have very high entropy because if you remember your high school physics which I'm sure you all had Boltzmann defined entropy is the number of equivalent microscopic states that equation is on his tombstone in Vienna which I which I've also visited you know I make a little sideline of visiting the tombs of famous physicists it's not that weird but anyway so a black hole has enormous an enormous number of equivalent microscopic states because it could have formed in an essentially an infinite number of ways and it would look just the same it would have it would have a size and that's all you could say so that means black holes have very high entropy or disorder that's the mathematical definition of entropy and the number to put a number on that a black hole the mass of the Sun has a hundred million times the entropy of the actual Sun so they are sinks of entropy too which is a way of describing the ignorance of what's inside them and how to describe it which is what your question was so father spacetime is looming very large but I think we can bend him just a little for one final question affirmatively if black holes are spewing out squeezes and if white holes are spewing out light and radiation then surely black holes are white holes at the same time at being black holes so the white whole concept is interesting because in the in the original framing of black holes and white holes to black the the black hole was the exit point of space-time from our view and a white hole was where that matter or energy emerged in a different part of the universe and as I mentioned that possibly could happen it's not ruled out it's not general relativity doesn't say it can't happen it's just it in the universe we live in it's unlikely that it does happen so white holes in that sense don't seem to exist but we do of course see this situation where black holes are extremely active and although at the middle of these galaxies the black hole itself is mute to what's inside it it is black it's an event horizon you can't see in it no information can get out the phenomenal gravity nearby is is creating this particle acceleration is heating up matter to millions or billions of Kelvin and it's radiating across the electromagnetic spectrum and it's self white it's not just white it's x-rays and gamma rays too so we don't have white holes in that tunnel in space-time sense in the universe but we have black holes that are white-hot because there's such phenomenal gravity engines and in an aside just since I've been talking about what intelligent civilizations will do before they era where black holes are evaporating you would definitely want to situate yourself near one of these accreting black holes in the center of a galaxy because it's an incredibly efficient gravitational engine you can get energy out of that like I said a hundred times more efficiently than a star and it's a huge object so you know we imagine if there's intelligent life in the future of the universe they'll they'll huddle near the heat and energy of accreting black holes and and have a wonderful old time well just reminds me saying a huge thank you to Chris for an amazing talk thanks so much [Applause]

Channel: The Royal Institution

Views: 39,819

Rating: 4.9118328 out of 5

Keywords: Ri, Royal Institution, black holes, Q&A, lecture, chris impey

Id: 9X8YAv3kErI

Length: 25min 25sec (1525 seconds)

Published: Wed Aug 21 2019