Black Holes and the Fundamental Laws of Physics - with Jerome Gauntlett

[Music] black holes are structures in the fabric of space and time itself and there are amongst the most extraordinary objects that we know that exists in the universe despite the mathematical complexity in describing black holes and the myriad ways in which black holes can form in the universe we also know that they they possess a very striking simplicity which is one of the things I wanted to try and describe to you tonight the modern conception of black holes began in 2015 sorry 1950 and it took many years before the properties of black holes were properly understood and longer again until we have found evidence that black holes exists but we do know black holes exists and in 2015 there was a dramatic confirmation of the existence of black holes the first direct detection of black holes by the measurement of the gravitational waves emitted by a system of two black holes this was a system of two black holes that were orbiting each other they coalesced to form a single black hole in that cataclysmic explosion when they form that single black hole they emitted gravitational radiation that gravitational waves moved through the universe and was measured by the like our experiment in 2015 that was the first direct detection of black holes it was the first direct detection of a binary system of black holes before then we weren't sure that they existed and was also the first direct detection of gravitational waves so in short that was a truly spectacular discovery that's something I want to tell you about in tonight's lecture in telling the story of black holes we'll start with the beginning of graph the story of gravity or the modern story of gravity which is Isaac Newton and starting from Isaac Newton will move to general relativity Einstein's theory of gravity which is our best theory of gravity and it's the arena where most of the discussion of black holes will have tonight but we know that general relativity is not the final story it's not the most fundamental story of gravity we know there's a new chapter and the new chapter must include quantum theory so there is some way in which general activity must be subsumed in a deeper more fundamental structure and I want to explain why black holes and the properties of black holes are the key theoretical laboratory for making that next next step forward in this search for fundamental physics so I want to begin our journey with in 1687 which is when Isaac Newton formulated his theory of gravity and along with his laws of mechanics that comprises really the beginning of modern fundamental physics a Newton thought a lot about how things form so for example this Apple famously he thought about how apples fall and he realized that everything falls in a very similar way and in fact in exactly the same way so that Apple Falls as I've just demonstrated this ball simile falls in the same way and if I take this ball and impart a small transverse motion to it it's still falling to the earth in the same way the Apple was but because of the transverse motion I gave it the actual orbit was was slightly different a Newton thought about this a lot and from these simple observations he made the breathtaking leap that the moon is also falling to the earth in exactly the same way the Apple was falling to the earth or this ball was falling to the earth and he did he came up with an ingenious thought experiment which hopefully I can convince you following Newton of why that's the case so here is a a picture that you drew and it's a picture of a planet I'm not quite sure why it has all these fishes in it but in any case he has a planet with a little volcano a little mountain at the top of it and he imagined standing on top of that hill and throwing objects in fact he thought of having a cannon and firing cannon balls from it so starting from from the point V he fires a cannon and if it was thrown at a small velocity the Cannonball would fall to the earth and end up at the point D if you threw it a little further he'll do it end up sorry a little faster it would end up at the point E but in both cases just like the ball I demonstrated a few moments ago it's it's falling to the earth all the time faster still it would go up to point F and then G and if you took at the surface of the earth and I took that ball and I was able to throw that ball at 7.9 kilometers a second which in middlee I couldn't throw but if it could be thrown then it would keep falling to the earth the whole time but it would never actually touch the earth it would come back and make the orbit which was the final line that Newton drew so Newton understood that all of these orbits these clothes motion of particles like the moon are actually objects that are falling so from simple thought experiments of apples he made this leap to the cosmos and it was a brilliant but simple inside but Newton didn't stop there he made this mathematically precise and in Newton's theory of gravity he said that all objects or masses feel a gravitational force the there's a gravitational force between the Apple and the earth and we perceive that as the motion of the Apple falling towards the Earth there's also a gravitational force between the earth and the moon and we perceive that because of the transverse motion that the moon stays in a stable orbit Newton's theory is is magnificent and a held sway for over 200 years this was our deepest most fundamental understanding of gravity it predicted the motion of all of the planets in the solar system and it actually predicted the existence of a new planet it's a planet Neptune so by studying the motion of Uranus and its using the mathematics of his theory people realized that it almost worked that there were some small deviations and those deviations thought would be accounted for in Newton's theory if there was another planet just further in a further orbit just outside of the orbit of Neptune and the theory was precise enough that you could predict what astronomers predicted exactly where that planet should be and was found so Newton's theory was quite spectacular but there's something very odd about Newton's theory as well and the odd thing is that the force the gravitational force that Newton invoked is instantaneously communicated so with this Apple if I move the Apple the force between the earth and the Apple is instantaneously communicated it's not doesn't take it as a little split second for it to be communicated its instantaneous and this troubled Newton instantaneous action at a distance and I want to put up this fragment of a ever letter that Isaac Newton wrote to Bentley in 1692 Bentley was the master of Trinity College in Cambridge and I was actually fortunate enough to see this this volume at the Wren library just a couple of months ago and actually I found it very moving to see Newton's handwriting and I felt a physicality a connection spanning us across these centuries that's separating us in time in this letter somewhere towards the bottom you can says something quite extraordinary so I've highlighted it here it says that gravity should be innate inherent and essential to matter so that one might body may act upon another at a distance through a vacuum without the mediation of anything else is to me so great an absurdity that I believe no man who has in philosophical matters any competent confident Faculty of thinking can ever fall into it so this action of distance is disturbing him at this very deep fundamental level and it's worth registering this for a moment so at the very foundations of his theory he's he's troubled but nevertheless despite those troubles and those obstacles he's been able to build his theory and the theory as of as I was just saying it's truly magnificent it's not a hodgepodge theory it's a great theory and it's a little template I think that how often progress can happen in theoretical physics one doesn't necessarily have the big picture in the complete picture and there can be troubling aspects in your understanding but it's the creative act to be able to pierce through the void and come through and create some new body of knowledge even though you you have at the back of your mind some deep reservations that this is not the final story and that things will change the next chapter began in nineteen hundred and five nineteen revived ensign came up and felt a special theory of relativity and one of the aspects of a special theory of relativity is that Einstein Einstein discovers it is that the universe has a speed limit and the speed limit is 300,000 kilometers per second that's the speed of light at this point Newton theory which has just been happily existing as the most fundamental understanding of gravity suddenly ran into a contradiction or a paradox if forces instantaneously communicated in his theory and Einstein says that nothing goes faster than the speed of light something has to give in a remarkably short amount of time Einstein himself was the one who worked out what this set the next step was 1915 Einstein developed his general theory of relativity and he swept away completely the notion that gravity is a force there is no force of gravity in Einstein's theory of relativity instead gravity is the manifestation of the curvature of space and time space-time so we can draw some pictures to get some into intuition and understanding of what this means so imagine that space is a rubber sheet and you placed a heavy object in the rubber sheet like a shop put it would curve the rubber sheet much like in the picture of the earth curving the space in the picture that you can see so the Earth's curving space around it and now the moon is orbiting the Earth not because of force pulling on it but just like a marble rolling on that curved rubber sheet if there was no friction that ball the moon the marble would just roll around on that rubber sheet indefinitely so the slogan of general relativity is that matter curves space and time and curved space-time informs how matter moves it's important to understand emphasize that general relativity doesn't prove inés Newton's theory is wrong that's not the way science progresses what Newt what Einstein did or what his theory does is subsumes Newton's theory into a deeper more profound structure and if you take the domain in which Newton's theory was developed essentially the solar system then Einstein's theory if your as a very good approximation does reduce to Newton's theory so Einstein's theory incorporates Newton's theory that extends it it makes new predictions and one of the most stunning predictions it makes is that there exists black holes two or three weeks after Einstein wrote down these field equations cosh wall shield who was a German scientist who was in the Prussian front a soldier on the Prussian front and a few weeks after Einstein wrote down these field equations he found the first black hole solution a black hole is a region of space and time in which once you go inside the black hole you can never escape the boundary of the black hole is called the event horizon so if we imagine we're in a spaceship and we cross the event horizon go from the outside to the inside as a black hole once past the event horizon no matter how you fire rockets you're always going to stay inside the black hole similarly if you shine a light beam you shine a torch or any other light beam from inside the black hole it necessarily stays inside the black hole and won't ever emanate past the boundary of the black hole the event horizon so since light can't come from inside to the outside that black they don't emit any any light one of the ways in which black holes can form is through in the final stages of evolution of stars an evolution of star is a battle it's a battle on the one hand between the matter wanting to collapse in on itself and the pressure caused by the thermonuclear reactions happening deep inside the core of this on push it holding that that pressure at the the force of gravity at bay so for most of the evolution of the star those two forces using that Newtonian language the force of gravity pulling the matter in and the pressure pushing it out are in equilibrium but as the nuclear material is exhausted the nuclear fuel is exhausted inside the star in the saiga star star if the star is big enough it will collapse in on itself and a supernova will happen the matter will collapse in and then explode off and again if if the star is big enough there will be a core of matter inside the interior of the star that will keep collapsing inner on on on itself and will form a black hole one of the most extraordinary facts about general civet e and these black holes is despite the complexity of what I've just said of how a black hole can form after all the star could be 20 solar masses 50 solar masses you think of the complexity of the supernova explosion how much matter is ejected and so on and so forth the final black hole that is created is specified by two numbers its mass and its spin all the other information about the black hole sir all the other information about how the black hole was formed is lost and you just left with those two numbers actually to be more precise there's three numbers and exactly three so now this is a fact the third number is the electric charge but in practice the electric charge and Astrophysical black holes is going to be extremely small or non-existent because charge matter will neutralize the black hole very quickly but the mathematics of relativity says specified by three and in practice it'll be just the mass and the spin which will determine the properties of the black hole so that's what I wanted to mention that's what I mentioned at the beginning of the lecture that black holes in general acidity extremely simple if black holes are black how do we detect them here is I quickly add an artist's impression of a star and a black hole it's a companion system a binary system and the black hole is is spinning and you can see that it stripped off or it is continuing to strip off matter from the star and as it strips off the matter because the black hole is rotating that matter gets swirled around the black hole and starts to form heat arc and form a plasma in fact that plasma as it generates magnetic fields it also shoots up these Jets along the axis of of the direction which it's spinning this plasma reaches very high temperatures and starts to emit x-rays and we the first indirect detection of black holes came from measuring the x-rays produced by black holes as in this picture we now know that there's many black holes in the universe by these indirect tests and in particular there's a supermassive black hole at the center of the galaxy a galaxy the Milky Way it's in the constellation of Sagittarius and it weighs 3 or 4 million times the mass of our Sun in fact most galaxies in the universe we think has a similar sized black hole or even bigger there's another very beautiful piece of indirect evidence the existence of this black hole in the center of our galaxy so here is a picture of I'm gonna run a little video and it's a video of the motion of stars right at the center of our galaxy and it was taken over 14 years and the Red Cross is the place where we think the supermassive black hole exists and there's no light being emitted from that region so the date was taken from 1992 to 2006 and let's have a look at it so one thing I should quickly add this is not a simulation often you see simulations but this is real data this is real observations of individual stars around the center of our galaxy and if I just stopped it about here you see we have these nice elliptical orbits of these stars going around this central object and by studying the detailed shape of those orbits and the time taken for the stars to go around a complete orbit we can use the mathematics of general divet II to deduce what the mass of the object is of their orbiting and that's three to four million times the mass of our Sun and in addition if I just take it a little bit forward there's two there's two orbits that get extremely close to this central object but don't hit whatever it is so we know the size as well as the mass and we know of no other object that can weigh three to four million times the mass of the Sun and squeeze into that amount of space except a black hole so we believe that there is a black hole at the center of our galaxy so these are the indirect ways to check the existence of black holes let me come on to the direct way in 1916 just one year after Einstein formulated his theory of generality he realized that gravity the general ativy predicts the gravitational waves exists as matter moves through space and time it forms gravitational waves and the picture you can have in your mind is that as you move your hand through the surface of a pond you form ripples on the surface of the pond similarly as matter moves through space and time it forms gravitational waves in fact as we're moving around we're forming gravitational waves however these gravitational waves of the motion of ourselves are extremely tiny the gravitational waves caused by the earth orbiting the Sun are also extremely tiny there as the Earth orbits the Sun it's emitting gravitational waves in all directions and you can calculate what the total amount of power is from all these gravity waves so imagine you gathered them all together and to how how powerful is that that would be powerful enough just to light up a light bulb so even though these theory these are existing the Sun and the earth is only producing these tiny tiny little ripples in space and time there's a reason for why these are so small and the reason is is that gravity is an extremely weak force so I'm going to use the Newtonian language because it's convenient here we're not used to thinking of gravity as being weak for example if we jump we fall to the ground and it feels like gravity is pulling us down with a strong force but we can do a very simple experiment which compares gravity and out the strength of gravity to another force and it's a very simple experiment but it's an important experiment and involves a magnet and a paperclip and obviously just like the beginning of the lecture if we drop the paperclip it falls to earth under the influence of gravity but I can compare the gravitational force of the whole of the planet earth on that paperclip to the electromagnetic force on this tiny little magnet and we know what will happen it holds it up and it's a simple observation but it's a profound one it tells us that gravity is extremely weak it's just that when all the other forces are not around gravity is the only one left and it dictates what's going on so gravitation gravity is weak gravitational waves are very small if we're going to have any hope of detecting gravitational waves we're going to have to look for a cataclysmic explosion and the cataclysmic explosion is the coalescence of two black holes so we imagine we have a system of two black holes perhaps formed like from the picture we had before one went supernova formed the black hole and then the companion star later also formed a supernova and turned into a black hole and those black holes were rotating they're spinning on their own axis but they're also rotating around each other because of the simplicity of black holes specified by their mass and their spin we can calculate how much gravitational waves would be emitted by this system if every black hole was different and a highly complicated individual object we wouldn't be able to do this but that simplicity of black holes allows us to calculate the gravity waves that are produced and here is a prediction from general tivity of the gravity waves that are produced so we're imagining the two black holes orbiting each other they're emitting gravitational waves as they emit the gravitational waves they lose energy and they get closer and closer to each other and they eventually in spiral and form a single black or so on this on this graph on the vertical axis I've got the strength of the gravitational waves and on the horizontal axis it's a time scale and you'll notice the time scale is in roughly tenths of seconds so we're looking at the very final part of the coalescence of these black holes for millions of years they'll be actually rotating around each other producing producing gravitational waves at a constant frequency like on the left-hand side of the graph as they get closer in the final in spiral and coalescence the frequency of the gravitational wave goes up and the amplitude goes up they form a single black hole and then the Freak the strength of gravitational waves dies away in fact this frequency of these gravitational waves being produced in this process of almost the same as audio frequency so people talk about the gravitational wave signals using the analogy of sound so the frequency is constant and then as they coalesce it goes up in frequency and louder and then it suddenly rings down so it's called a chirp and ringed down so be something like aw that's a sound version of that graph if you don't like looking at graphs the point here is that simplicity of those black holes and relativity allows us to determine that signal so we know have a prediction from from Einstein's theory of general relativity of what gravity waves are produced gravitational waves are produced by these systems how do we measure them the technique is laser interferometry and being a theorist I love the fact that I can now play around with some equipment here and hopefully not break it what I brought in is a laser interferometer and maybe if we can just have yeah the picture there let me just switch this on it should just start warming up in a second okay right so what have we got here I've got it in a black box it's not very dangerous but it's a little bit dangerous so you can't see it but you can see from above what's happening we have the laser here and it's emitting a laser beam straight out the end here and this little object here is a beam splitter so half of the light gets reflected up here to a mirror which then gets reflected back and then will go straight forward and the other half will go through to this mirror and then come back to this the splitter and then go off in this direction so there's a coherent laser light beam that split into two beams and then is sent down in this axis so let's have a look what happens and if we can have the lights down there are the two beams I mentioned and something interesting happens if we manipulate them so that they're sitting on top of each other this is the phenomenon of interference light is a wave and just like water waves on a pool on the surface of a pond or something like that interferes light wave also interferes the point about this setup is that this interference pattern is very sensitive and if you change the geometry a little bit fire a sound wave then you see that the interference pattern changes this is exactly the principle that's being used in the LIGO experiment to detect gravitational waves taking laser beams interfering them and then see when the gravitational wave comes in that the interference pattern has moved so kind of the lights back up again thank you and quia behalf of the slides back as well thank you so here is here is a photograph of two photographs of two detectors one in Hanford in Washington and the upper left-hand side of the United States and the other one in Livingston in Louie gianna down in the lower right and the pictures above them are pictures of the two jet LIGO detectors and you can see that there's two long arms and down each of those arms which are actually four kilometers long a laser beam is being sent as being sent and there's a big mirror at the end of one arm there's a big mirror at the other end of another arm and the lights coming back and it's being interfered much in the same way we just demonstrated here at that central detection facility at the apex of the two arms if a gravitational wave comes through this system it tends to squeeze one of the arms makes a little bit shorter it makes the other arm longer or vice-versa the gravitational waves that were caused by the two black holes colliding that were observed in 2015 changed the length of these arms these four kilometer arms by one thousandth the size of an atomic nucleus this is truly breathtaking engineering all vibrations for example me hitting the table just like a few moment moments ago would completely swamp this signal all seismic vibrations or vibrations of machinery in the laboratory etc all of that has to be shielded away by elaborate and sophisticated engineering and once that's happened you have this perfectly stable system if a gravitational wave comes in it produces a signal and you're waiting for the movement of a thousandth of the size of the diameter over four kilometers one of the key aspects of this experiment is that there should be to the distance between the two detectors is about three thousand kilometers so given that gravitational waves move at the speed of light the time taken for the gravitational the gravitational wave will hit one of the detectors first and then I'll hit the second one a little bit later and a little bit later is exactly quantifiable it's about 10 milliseconds so if a gravitational wave signal has come in the two signals when you set when you move them on top of each other separated by 10 milliseconds should be the same and in the lower right hand of the figure of the the image that is actually the real data that was published by the LIGO experiment saying we have detected gravitational waves now it's true that there's a lot of noise there but the main feature that I want you to pick up on is that they are exactly in phase especially towards the latter part of the event they're exactly in phase you can see that constant frequency process increasing amplitude to chirp and in the ring down so this was as I said before this was the first direct detection of black holes the first direct detection of gravitational waves and the first direct detection of a binary system of black holes so if you think about it for a moment it's very hard to know you that you have a binary system of black holes a black hole and a star you look for x-rays and you also see the light coming from a star but if you have just two black holes they might have been producing x-rays from some remnant accretion discs around them but you wouldn't know if that's one black hole or two black holes so this was the first director to have evidence that there are binary systems of black holes in the universe I've blown your mind once hopefully about the engineering feat I want to blow your mind again the mass by studying that signal and comparing it to the theoretical observations predictions from general relativity we know that the two mass the masses the two black holes were 36 times the mass of our Sun and 29 times the mass of our Sun in fact before before this experiment we didn't know such black holes of that mass existent they coalesced to form a black hole which weighs 62 times the mass of our Sun so there's one equation in this talk and here it is 36 plus 29 minus 62 and that's three so after they coalesced there was three times the mass of the Sun of matter that was ejected into gravitational waves and the amount of energy that is you use the famous equals MC squared equation of of Einstein and that was released in a tenth of a second which was that final chirp on our figure on the graph that I showed before the amount of power that was produced in that moment was ten three times ten to the forty nine watts even if you have facility with numbers you're a mathematician or you're a physicist when you get to numbers like this ten to the forty nine that's one with 49 zeroes following it you lose intuition this is a huge number so you can try and think about what's a powerful object that you know about in the universe you might begin on earth with powerful objects nuclear reactors for example forget that let's go to the Sun let's think of the song for a moment all of the power from the Sun reaching us about 90 million miles away driving all of life on Earth that power of that the total power emitted by the Sun is about 10 to the 24 watts one followed by 24 zeroes so this is much much more powerful than all of the power emitted by the Sun in fact it's Morse more power emitted by all of the stars not just in our galaxy but in all of the galaxies in the universe so when I mentioned cataclysmic explosion earlier this simplicity of space-time just the mass and the spin swirling around each other that rearrangement of space and time into a single black hole is truly terra clinic and that's despite the fact that gravity's is weak in the sense that I was describing it before we also know that the event happened about 1.3 billion light years away that means the gravitational waves were emitted about 1.3 billion years ago and then arrived at Earth in 2015 and I find that kind of poetic 1.3 billion years ago earth was had just evolved from single-cell organisms to multicellular organisms and we wait a little bit longer and multicellular organisms become fish and reptiles dinosaurs come dinosaurs go mammals come evolved humans evolved Isaac Newton comes Einstein comes we start building this detector and I say we because it is we it is humanity who does this we build this detector and finally when it's ready to go it has its upgrade we switch it on and actually very soon after the upgrade was switched on bang the signal was there coming towards us I also want to emphasize at this point that gravitational wave observations that we've had and there's been three more since then there was just one two days ago people may have noticed that in the news it's not just verifying that Einstein was right it's not verifying that our black holes really do exist confirming what we knew that's all true but the really exciting thing is it's a new type of astronomy that's being created we now have a new window and looking at the universe not through electric electromagnetic radiation like in the telescopes using but by gravitational waves and we're going to learn more about black holes and their properties and their distribution and we probably are going to learn things that are brand-new that we haven't thought about before as well so gravity way gravitational wave astronomy and observations over the next 10 20 30 40 years is going to be an extremely exciting field in science or fundamental science okay so let's let's leave the observations and let's go back to some theoretical ponderings let's return to the question about what happens inside a black hole so the two figures here are Stephen Hawking of course and Roger Penrose who in the 60s and early 70s really pioneered and lay the foundations for understanding that the true properties of black holes and in particular what happens deep inside the black hole if you cross into the event horizon as I said before you're never coming out past the event horizon in fact if you crossed a big enough black hole you wouldn't even notice that anything in anything was different you would feel exactly the same as if I stood on this table and jumped off the table so I'd been freefall for a moment that sense of freefall would be exactly that what you would feel if the black hole is big enough as you cross the event horizon as time went on you would then find actually you'd be squeezed and stretched it's called spaghettification you'd be squeezed and stretched and eventually you would confront what's inside the black hole is spacetime singularity and the spacetime singularity is inevitable and the picture I've drawn doesn't capture a key feature of space-time singularities and the key feature is based on singularities inside black holes is that there are singularities in time and not in space if a singularity was a point in space let's imagine it was in the center of this room you have this strong sense that you should be able to avoid it if you fired your rockets surely I could just move away from the spacetime singularity why do I have to hit the spacetime singularity that's true and that's because space time singularities and black holes are not of that form the spacetime singularity is a singularity in time and you should think of a moment in time in the future that you can't avoid we can't avoid next Tuesday and that's exactly not roughly exactly how you can't avoid the spacetime singularity in a black hole and it's this change of grammar that's actually important a spacetime singularity happens you go inside the event horizon and the spacetime singularity will inevitably happen for the biggest black holes that we know of it's of the order of a hundred hours inside the black hole the center of our galaxy is about three hours oh I think oh maybe it's one hour it's roughly that interestingly the longest you have before the singularity happens is just to relax you just freefall and that's the longest you've got any other motion you do if you switch on your rockets you will make sure the singularity happens quicker I think there's a life lesson there somewhere but I'm not sure what it is but it's interesting I think it's not emphasized enough that one more level of that analogy with Tuesday is firing your rockets trying to avoid the spacetime singularity would be exactly analogous to saying I'm gonna hop in my car and I'm not sure which direction you're don't drive in but you just try and drive away from next Tuesday it's meaningless and similarly that's exactly what's happening inside the black hole so what is the spacetime singularity so a spacetime singularity is where the laws of generality break down so general relativity is no longer valid at this base time singularity and at first hearing this that sounds like a very bad thing it sounds like Jen relativity is no longer I mean somehow a flawed theory it can't tell us what we what we need to know about what would happen inside a deep inside a black hole but in fact that's completely the wrong way of looking about looking at it what the space-time singularities are telling us is that there's an opportunity for deeper insight into the fundamental laws of physics we know that general relativity is not the theory of everything after all it doesn't describe any of the quantum forces which we'll be discussing in a moment so there must be a bigger theoretical framework deeper fundamental physics beyond generality and watch the result of Stephen Hawking and Roger Penrose says is that black holes are a great place to think about where you should go beyond our existing knowledge black holes aren't the only only place that space-time singularities exists they also exist at the very beginning of the Big Bang so if you look at all the galaxies today they're all expanding away from each other if you run the history of the universe backwards in time those galaxies will be getting closer and closer and closer and about 14 billion years ago all of those galaxies would be on top of each other and that would be the Big Bang and that moment of the Big Bang in the context of Einstein's theory of general relativity is a spacetime singularity so another place to think about fundamental physics beyond general relativity is the very beginning of the Big Bang or the very early part of the universe up until now I've just been discussing just describing gravity but I now honor to introduce another player into the game which is the quantum world all of fundamental physics rests on two pillars on the one hand Einstein's theory of general relativity describing gravity and on the other the quantum world which describes all of elementary particle physics electrons protons neutrons quarks all of the elementary particles that are observed in the particle accelerators including the Large Hadron Collider are described by the quantum world and there's something called the standard model of particle physics which encapsulates our deepest understanding of elementary particles in their interactions both of these fundamental theories are extremely accurate and describe nature with exquisite precision and beauty but they're mathematically inconsistent and that's a paradox when you first hear it but it has a very simple explanation when were I've been describing gravity I've literally been forgetting about the quantum world and we can do that because all of the quantum forces for example the electromagnetic force is one of those forces are neutralized over the scales that I've been to Scott describing so the only force that's important using the Newtonian language of gravity of on the scales of black holes and galaxies and so on is gravity itself and we use Einstein's theory of general relativity and forget about the quantum world similarly when people do experiments in Geneva at the Large Hadron Collider they understand and appreciate that the gravitational interactions are extremely tiny compared to the interactions they're looking at think of the paperclip in the magnet and they again pretend that gravity doesn't exist at all they just pretend it's not there so in both domains we're approximating what's going on the quantum world we approximate by saying there's no gravity and in these descriptions of black holes and gravitational waves and so on we approximate by saying the quantum oil doesn't exist but nevertheless we would like to do better than that we would like to put these both together into a cohesive logical framework and the question remains then is how do we unify these two great pillars of knowledge so in the rest of the talk I want to just give you some flavor why we think black holes are the key laboratory or a key laboratory for that next stage of unification in fundamental physics I can just tell you one thing about quantum theory to motivate what I want to say in the quantum theory in quantum theory the vacuum empty space and time is actually not as empty as you might think it's actually a way of thinking about it it's a sea of virtual particles and antiparticles coming into existence out of nothing literally nothing existing for a little while and then annihilating away to nothing so in this picture I've got a red a red arrow might represent an electron coming into being with its antiparticle a positron they exist for a short amount of time and then they annihilate away and we call these particles virtual because they're not directly detected however they can be indirectly detected or the effects can be indirectly detected and they can be measured and we know that these are real so our conception of what the vacuum is in quantum theory is changed from our naive picture Stephen Hawking asked the great question in essence what happens to these processes in the presence of a black hole now Stephen Hawking did not have the theory of quantum gravity this is a work in progress but he's taking one component of the quantum theory and taking a component of general relativity a black hole and fusing them together to obtain some insight and by doing a specific calculation he showed that the following happens away from the black hole these processes processes these virtual processes continue much as they were happening as if there wasn't a black hole but near the event horizon occasionally one of these particles can be stripped off one of the virtual particle because it can be stripped off it goes inside the event horizon and the singularity will happen for the other particle can escape away from the black hole and in this process the particle that escapes turns into a real particle so the net effect of this process is the black hole with this virtual processes happening all over the all over the place is such that effectively the black hole is emitting particles and it's not just milling particles in any old way it's emitting them in a very precise way it's emitting them as if it was a hot object with the temperature and this is Stephen Hawking's famous Hawking radiation there's more the properties of black holes obey a series of laws which have the same form as the law laws governing thermal systems a thermal system is any system involving heat so think of a steam engine there's very precise laws that were developed in the nineteenth century to describe the physics of steam engines and any objects involving heat and these laws have a great universality there are another great achievement in the quest for funder understanding fundamental physics they explained for example if you pump up a bicycle tire the valve will always get hot if you release the valve and the air comes rushing out the valve will get cold if it's a hot sunny day and you're in your kitchen and you think you'd like to be cooled down a bit you might be tempted to open the fridge door and the cold air comes out but these laws that I'm describing would say if you do that over all the room will heat the temperature of the room will increase because the heat being produced at the back of the fridge will always be bigger than the cold air coming out of the fridge or the reduction of temperature so these laws they basically govern everything in everyday life with anything to do with heat it's quite remarkable that black holes are BAE these same kinds of laws these laws of thermal systems were developed in the mid 19th century before atoms were universally accepted as fundamental building blocks of matter when the atomic theory came along people realize that it explained where these thermal laws came from the thermal laws have a microscopic explanation so for example take a hot cup of tea it has heat and as a temperature but we know from an from a from the point of view atoms that can be explained in terms of the individual motions of the atoms so heat is this disorder motion of the atoms that's what it is in terms of this atomistic picture and for example the temperature of the fatigue is determined by the average speed of of these atoms so the atomic picture provided a deeper more profound understanding of where of what these thermal laws were and that's kind of a status of black hole thermodynamics or thermal physics at the moment we know that they obey a series of laws but we don't have the underlying picture of atoms of space and time which explains those laws so that's one of the big outstanding question what is the microscopic description and understanding of these thermal laws what happens when the black hole emits its particles more and more particles eventually it'll reduce to nothing and remember I said in general relativity black holes are very simple that no matter how they were formed from all the matter others of the collapsing star material in the star they were then specified by the mass and the charge mass than they are spin if the black hole then evaporates producing all of this thermal radiation it seems like we have a process in which information has turned into pure thermal radiation and that goes against the grain of quantum theory that's a major outstanding question what happens to the black hole as it fully evaporates a variation of that question is the singularity inside the black hole will be revealed when the black hole evaporates so what is the nature of the singularity and will is that these new atoms of space and time tell us what that is and how it's resolved this is the cutting edge of research into black hole physics today so I just wanted to end the talk with some comments about string theory so string theory is an attempt to unify quantum with Einstein's theory of general relativity and the fundamental idea of string theory is extremely simple the idea is this if you zoomed in to sub into thee into the atom you'd find a halo of electrons surrounding a nucleus of protons and neutrons if you zoomed into the protons and neutrons you would find quarks if you zoom further in onto the quarks we don't know what you would find but we think you'll find something some deeper structure but don't go next step next step go all the way to the end and what is the end point of that process and the EM process its postulated is the fundamental building blocks are strings these are one-dimensional filaments of energy which would have no further substructure and the one of the most appealing things of strings is that the different vibrations of the string would correspond to different elementary particles so the one string if it's vibrating just like a violin string one note would correspond to an electron another note would correspond to a quark another note to a photon a particle of light and also another vibration would correspond to a graviton a quantum particle of gravity so we don't know yet if string theory is correct but research in string theory or a large branch of research is trying to address these questions of the thermal properties of black holes to have a deeper understanding of what black holes are as well as to get evidence that string theory is actually on the right track and it is the theory did unifies quantum theory and gravity so in tonight's lecture I wanted to tell you several things the first was that black holes are really cool there are structures in the fabric of space and time itself they have this beautiful simplicity and they really are amongst the most extraordinary objects that exist in the universe black holes are real without indirect evidence that black holes are real for a long time but we now have this beautiful direct detection of black holes that was made by the LIGO experiments in 2015 and then three times since then and this detection of gravitational waves is precising in a new era of astronomy this is not the end of a story like many developments in physics as one story ends and you want opens up and the new one that's opening up is this new era of astronomy using gravitational waves for example one of the things that we're going to do we include humanity in the way there is put satellites up in space and they're going to be spent separated a million kilometres apart and we're going to look for the gravitational waves that much change the length of that million kilometres by a thousandth for size of a diameter of a proton it's breathtaking what we are thinking about doing and is plaint being planned to be done in the next 20 30 40 years so spike holes are real and finally black holes are the way forward by thinking about quantum theory we know that our thermal objects we know there are space-time singularities where we need a quantum theory of gravity to tell us what's going on and black holes and thinking about their properties is the arena in which we think that we can get the most insight in that next step the last point I wanted to make was a recurring theme that's gone throughout the talk is that often that there's very long time scales between developments both in theory the of fundamental theoretical physics over over the scale of hundreds of years sometimes it's punctuated by trapper development but often that's over the scale of a hundred years and also experimental verification predictions come up with new theories but we sometimes have to wait ten twenty thirty again up to 100 years before we get evidence and confirm the predictions but that's the nature of the activity we are undertaking and even though that progress can be slow the clearest message that comes through is that progress does get made we do make inexorable progress in a planning a deeper understanding of the fundamental laws of physics and I think that's surely one of the great intellectual and cultural achievements of all of humanity thank you so this is actually it'll be another talk it's actually another of the deep clues to how to unify quantum theory and gravity

Channel: The Royal Institution

Views: 1,210,640

Rating: 4.515708 out of 5

Keywords: Ri, Royal Institution, black holes, lecture, physics, cosmology, theoretical physics, laws of physics

Id: laKp1XeEF74

Length: 62min 34sec (3754 seconds)

Published: Thu Oct 26 2017

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