[Music] and it's a great pleasure to be here a building in a room full of history full of wonderful talks about science my accent will not tell you but I am a Brit I was born here born in Edinburgh and I went I was an undergrad at Imperial College and then a PhD student in Edinburgh and then I left because I'm an astronomer and it might have escaped your attention but there's a lot of clouds in the UK and so I went to Arizona where the skies are clear about 300 and I don't make you feel bad about 310 days in the year we have blue skies and then dark skies at night so I've been there ever since for my whole career and I want to talk about a subject obviously of great interest generally and topically because it's in the news pretty much every week and there's been very much in the news in the last month or so black holes and the book that I decided I needed to write based on black holes it's my eighth book if you care for other topics I've written about cosmology and a strobe eye ology I've written a book about teaching cosmology to Tibetan monks and a program started by the Dalai Lama that's a slightly strange book and a science fiction novel somewhere in there too tonight I'm talking about black holes and I guess the message up front is going to be that they're just as exciting and enigmatic as you might have thought they were and hoped they were but we've learned a lot about them and and so it's at that wonderful cutting edge of science murray gell-mann who was a Nobel Prize winner in physics once said research is what I do and I don't know what I'm doing and you can just smile and say well if you have a Nobel Prize you can say anything and get away with it what he meant was that science and and this subverts the the bad archetype or stereotype about science and scientists which nobody in this room of course adheres to but out there in the in the polis in the public place it's that science is sort of cut-and-dried facts and you know it ends up being a little dull gomen was saying that science is the edge between what we know and what we don't know and the implication of that is that you might be wrong at any point the implication is of great uncertainty it's a very dynamic thing to be at the edge of knowledge and that's why research is fun that's why I do research and that's why black holes are still fun because we're at that cutting edge is everywhere with black holes there are things we know things we don't know things we thought we knew that turned out to be wrong and and so on so it's it's a fun subject in that way we're gonna meet to the fathers of black holes well known to everyone of course Einstein and Hawking and I'm going to start by pointing out that the concept of a black hole actually predates general relativity and Einstein by over a hundred years John Mitchell who is an amateur astronomer and physicist there were no professional astronomers in the early in the late 18th century he was a clergyman he thought deeply about physics and philosophy and mathematics and he imagined a star that was large enough or massive enough that the escape velocity that every object with gravity has equal the speed of light and logically that's a Dark Star now you can't formally understand black holes with Newton's theory of gravity which is all he had available to him but it's an interesting step in a direction of imagining dark stars in the universe things that boy like cannot escape and so again a hundred years before the theory that really lets you understand them the idea is out there so that's the very earliest inkling of black holes of course the real understanding of black holes starts with relativity and it starts with Einstein he's owned by the popular culture he's he's become a creature you know of culture rather than of physics and astronomy at in his day clearly the most famous scientist and still one of the greatest scientists in history instantly recognizable very elevated and public esteem he was offered and turned down the presidency of the State of Israel he was you know he was celebrated in many ways and with good reason so like we can't talk about black holes without talking about relativity your sophisticated audience so I think you I can hate you with the full horror of it which is ten second order partial coupled differential equations which I will ask you to solve and we'll take we'll take answers here before you before you're allowed out of the room and that even for physics students gives them the sweats the night sweats so I took a general relativity course it was not that much fun I'm not a mathematical physicist but relativity is conceptual so let me give you the conceptual understanding of relativity and it starts with an awareness of Einstein based on a coincidence physicists hate coincidences when there's something that's a coincidence either numerical or physical it means we don't understand something the coincidence the Einstein was struck by it was the fact that the inertial mass of an object that's its resistance to a change in its motion if I push something it doesn't want to be pushed and we can imagine this is a perfectly smooth surface like ice it will still resist a change in its motion gravity is not really involved here it's keeping the book on the table but it's not resisting the motion sideways that's the inertial mass the gravitational mass is the mass that leads it to fall in the Earth's gravity those are really quite different things conceptually and yet numerically those masses are identical not known with this precision but now to one part and a thousand trillion that begs for an explanation Einstein conceptualized this with a little thought experiment he said well if you imagine these two situations straighted there's no way you can distinguish these experimentally someone in a spaceship that's being accelerated with no windows they can't see their situation accelerated 9.8 meters per second per second into space and then someone in a stationary in a lift on the Earth's surface again they can't see out and objects being dropped experiments done you couldn't tell a difference one situation clearly involves gravity the lift sitting there on the floor held down by the Earth's gravity and the other no gravity at all deep space there doesn't have to be any object there it's a rocket he also realized that these two situations are indistinguishable a one is fairly innocuous it's an astronaut floating around in deep space inside a spaceship in the space station you would see the same thing and there's no gravity involved here and deep space there might be no object anywhere near it's just zero gravity this are just floating things the other is much more ominous the cable on the lift is broken the person is plunging to their death and they're floating around in the lift and Einstein say this that he was said this is the happiest thought of his life this awareness that these two situations are indistinguishable so general relativity is a a statement that there's nothing special about acceleration due to gravity relative to any other force like chemical rockets or something like that when obviously those things are totally different that simple awareness led him down a road that led to a mathematical theory of gravity based on a very different concept of gravity the concept of curved space-time Newton's gravity theory was based on objects that exerted forces or over a distance and the fact that this happened in the vacuum of space apparently instantaneously was a little puzzling and when Newton was asked about this he said I don't I frame no hypothesis he didn't actually know there was some deep philosophical underpinning of his theory that were still mysterious it worked you can calculate it it's how we send people to the moon and how we launch rockets still Einstein got rid of the idea of forces in action at a distance in the vacuum of space and how does that work and he said no mass and energy Bend space and time he'd already hyphenated mass and energy with equals mc-squared and he hyphenated space and time in general relativity and he said general relativity those horrid ten partial differential equations are just equations that relate mass and energy to space and time the density of mass and energy to the curvature of space and time in three dimensions and it's a very different idea of gravity and it works it's been tested and it works and I'll show you some of the tests some of the early tests in the 1950s involve very subtle experiments because these were done on the gravity of the earth the earth is a weak gravity object black holes that we're gonna talk most of the time about our intense gravity objects so doing these tests is really difficult but it was done these were done in the 50s and 60s and general ativy implies that a clock a timekeeping piece run slower in strong gravity than weak gravity and that extends to the earth and so in principle and in theory inin general relativity a clock here runs very slightly slower than a clock here now this was tested in the 1950s with atomic clocks flown in high-altitude planes compared to an identical atomic clock on the ground and the timekeeping was slightly different clocks are now so good optical switch clocks are so good that we can measure time different working differently in 1 meter in the lab so and the precision of this experiment you can see is one part in 10 to the 19 incredible physics experiment another experiment done also decades ago shows that gravity photons struggle as they you can think of them struggling it's very an anthropocentric to do that but they lose energy and that's a redshift so losing energy for light is stretching the wavelength is redder it's called the gravitational redshift and that's also been measured many many times in many different ways so general relativity by the 1960s had been measured tested in all manner of situations of wheat gravity and had passed every test with flying colors and that's still a true statement the most dramatic demonstration of generality comes in astronomy when we take pictures of the sky and watch light being bent by space so if space-time is curved by objects by mass and energy then light essentially follows the curvature of space-time and so it undulates it moves it bends it can be focused by an object just like a lens like an optical lens it could be a lens of mass and in the hubble space telescope this is just one of now thousands of pictures that have been taken that show that the cluster the bright bunch of galaxies in the middle is a cluster at about three billion light years and the little arcs of light that you can see are arranged roughly in concentric circles around the center of mass of the cluster are much more distant galaxies five seven eight billion light years away and their light has been sheared and amplified actually and obviously bent by the intervening cluster so this is an a lens of mass bending light dramatic demonstration that needed imaging from space to see initially now we can do it from the ground as well techniques are better from the ground to shake sharp images and literally thousands of these pictures have been taken so mass bends light unequivocally so what are some of the other differences conceptually between Newtonian gravity and Einstein's gravity because if we're gonna believe general relativity we have to believe these differences here's a here's a beautiful experiment that was done very difficult experiment done in the 90s it took almost twenty years to get the result but in Newton's theory as I've mentioned space-time is the base and time are not coupled space and time are different things and really intuitively that's what it feels like space is the stuff around us and objects fill space and time is something that flows and never goes backwards and we experience that they do not seem related so to hyphenate them as audacious just in principle so in Newton's theory a spinning object or a gyroscope on a satellite around a spinning object because that's the experiment we're going to talk about doesn't really care about its larger situation the space and the time they don't care about this they're in Newton's theory space was infinite and flat and time was Infinite and smooth and never changed but in Einstein's theory these things are now coupled in interesting and subtle ways and in principle these are testable and in this particular situation of the earth the earth will bend space-time but very subtly because it's not a big object it's not a very dense object either and so in this two dimensional analogy we can imagine it like the surface of a sheet being bent or distorted space-time around the earth by the gravity of the earth but something else is going on the earth is spinning and that actually leads to a second effect which is the twisting of the space-time contours just think of it like a vortex like a whirlpool and in principle both of those phenomena the twisting of the space-time contours and the bending of space-time by the earth are detectable and the probe used in this one satellite experiment called gravity probe B involved a gyroscope and gyroscopes are always supposed to point locked on one direction in deep space in Newton's theory that would not change as the satellite orbits the earth in Einstein's theory the gyroscope is subtly tugged by the curvature of space-time and the twisting of the space-time contours and you can measure it in principle so what does it take to make a black hole here's a physical ization of it you would have to crush something like the Sun which is million and a half kilometers in diameter down to the size of a small town three kilometers and then it would formerly be a black hole it would have an escape velocity that was the speed of light and nothing could escape because light is the fastest there is but the same principle is true for any object nature does not impose a limit on the size or mass of a black hole so if you could take the earth and crush it down to you know golf ball-sized then formerly it would be a black hole you could take a rock and crush it down to proton size and it will be a black hole so nature in theory and in principle can make black holes of any mass and any size and the question for astronomers for scientists being empirical is which of these does nature make which actually exist in the universe the first type of black hole to be proven to exist the type that was anticipated from the theory within a few decades of the theory is the kind of object left behind when a massive star dies this is not the fate of the Sun because when the Sun dies our having lost some of its outer envelope the 2/3 of its mass that remains will crunch down to a white dwarf which is a cooling carbon rich amber which is incredibly dense millions of times denser than the Sun now but formally not the density of a black hole but a star that started its life 10 times the mass of the Sun will lose some mass along the way and then its core when all fusion stops and the fusion remembers the only thing that keeps a star puffed up so the Sun is only the size it is because of a balance between pressure from fusion reactions and gravity in well when that equation is broken because there's no energy from fusion gravity will win and so the inexorable victory of gravity and a massive star in theory leads to a black hole because there's no force to resist compression to that dense state the theory the calculation of this so the basis for it is from general relativity and Einstein's theory the actual calculation emerged from mostly Robert Robert Oppenheimer and Hans bethe in the 1930s and it was essentially a byproduct of Oppenheimer's work on the Manhattan Project figuring out super dense states of matter that are how we generate bombs and fusion and and you use those same calculations to show that logically a massive star should have no force that could turning into a black hole so from the late 1930s early 40s the prediction was there there should be dead stars that are black holes go look for them black holes in the theory are very simple they're incredibly simple objects they are characterized by an event horizon which is not a physical boundary it's an information membrane it marks the difference between places we can see in the universe and a part of the universe that's sequestered off that's hidden from our view forever that nothing can escape from and we cannot see inside no information can escape and that has a particular size that scales linearly with the mass of the object and in the theory the black hole also has something a little more monstrous a singularity so if you calculate in general relativity there's a cusp of density that's infinite at the center and that's a singularity and that's a problem anytime in physics you get an infinity coming out of your calculation for physical quantity it means you don't understand something and Einstein or actually Hawking has said famously this means that black holes contain the seeds of their own demise so everyone from Einstein through Hawking to the present day is aware that the theory of black holes is incomplete because singularities are nonsensical they're also impossible to inspect because they lie inside the event horizon so all we can do is speculate but the theory has a problem just because they're predicted the other property of a black hole is spent because the stars that form black holes are spinning as they collapse they spin faster angular momentum conserved so we would anticipate all black holes in the universe or spinning and probably very fast so black holes have mass they have size they have angular momentum and that's it very simple objects and in 1969 Cygnus x1 the brightest x-ray source in the sky and the constellation of Cygnus was shown with very clever and quite complicated observations to be a binary star system where one member of the binary pair was a and star supergiant star and the other was a black hole sucking material off the giant star and that material glows brightly so brightly and so hot that it emits x-rays enormous amount of x-rays so the irony of black holes is that yes an isolated black hole is black by definition is invisible by definition so how do you find them and sir you don't look for an isolated black hole they could be all over the place we won't find them most stars are in binary or multiple systems so it's anticipated it's normal that they're gonna be binary stars where one is massive enough to but die as a black hole and the other is not a black hole and that if they're in a tight binary orbit the black hole will pull material off the other the normal star heat that material up and the heating of that material will be the telltale that there's a compact object and then you measure the orbital properties of the binary and by Kepler's laws don't need general relativity and if the mass is sufficient and if the dead start has to be a black hole in the intervening half-century we still only have 50 examples of black holes that are like this it's not many it's a pretty thin haul for half a century of work so it's hard to find black holes the nearest ones are hundreds of light years away so we can rule out right away the popular notion in the culture that they're gonna eat everything and they're nearby and their danger and a threat they're quite rare because only a tiny number tiny fraction of stars are massive enough to die that way so the nearest examples are going to be far hundreds of light-years away but we do know examples and so black holes too most astronomers are confirmed as real Astrophysical objects and then we come to the contributions of Hawking so sadly the bard of gravity and black holes is lost to us just last year being in London on sabbatical actually now I'm here for six months I got to visit his grave and of course many other illustrious scientists in Westminster Abbey and what did Hawking add to the conversation about black holes a lot much of this work was done when he is quite young when he was a graduate student or a postdoc his singular contribution was the prediction that black holes have another property beyond the three I mention and that last property is temperature how can a black hole have a temperature how can anything escape a black hole hawking saw that there was a very clever mechanism in in physics and in the lab it's known that spontaneously from the vacuum from a pure vacuum of space particle/anti-particle pairs can appear and disappear that's allowed by Heisenberg's uncertainty principle so you can steal energy from the vacuum as long as you give it back very quickly if that happens near the event horizon of a black hole there's a finite chance that one member the pair will be lost inside the black hole the other will escape and in the aggregate that is a net loss of either mass and mass and energy as we know are equivalent or energy and the radiation from the black hole is called Hawking radiation it's because this is a very subtle phenomenon it's a very subtle temperature the Hawking radiation for a black hole that's a dead star is about a billionth of a Kelvin this is completely unmeasurable in astronomy and may never be measurable unfortunately Hawking was kind of sad about this because he realized Nobel prizes are only awarded for discoveries not for theories and he knew that if Hawking radiation were ever detected he was a lock for a Nobel Prize but the truth is it's very very hard to detect and the corresponding the concomitant effect that black holes are slowly losing mass is black hole evaporation and that's also an incredibly subtle effect ass black hole that's left when a massive star dies we'll take 10 to the power 68 one was 68 zero years to fully evaporate by Hawking radiation and again no way astronomers can ever measure that probably ever I mean certainly not now and maybe not even in principle so but this is a contribution to black hole theory that has various implications and they're all important obviously the fact that black holes evaporate is important it means they're not eternal are going to eventually disappear and a logical question is what becomes of the information that was lost in a black hole and so one of the consequences of Hawking's theorizing was something called the information paradox and there are different ways to frame it but it's a big issue and this is a totally current issue they're probably 50 or 60 papers a year written right now on information paradox and related issues and it's something very simple to describe a black hole is mute to all of the things that went into it so a black hole that was made of a dead star looks exactly the same and has the same simple monolithic properties as a black hole that's made of all that all the odd socks that humans or other civilizations ever lost or made of cats or made of whatever it doesn't matter and so black holes have lost information what you could toss books you can make it in a black hole out of books and encyclopedias and you'd never give that information out what happens to the information so this has become the information paradox it's a problem because a premise of quantum theory is that information is preserved in microscopic interactions so when you have particle interactions in the lab at a subatomic scale they can run forward or backward in time they can involve matter or antimatter and the information in the interaction is always preserved its premise of quantum physics this premise is violated by black holes overall and it's definitely violated at the event horizon and this is essentially a statement that the information paradox is a statement of the fact that our gravity theory and our quantum theory don't play well together they are not compatible theories and we've known this for many years this is just a particular example of it and it's led to all sorts of speculations of how you may preserve maybe the information is preserved because as we'll see when things fall onto a black hole their time slows down asymptotically infinitely so maybe the information is preserved as matter falls slowly onto a black hole like a hologram and it's coded on to the event horizon if we could somehow extract it which we can maybe it's destroyed in something called a firewall so there are many theoretical ideas like I said dozens and dozens of papers a year written about this all unresolved issues in theoretical physics and in science that black holes still have plenty of juice left in them as theoretical objects of study meanwhile astronomers were busy trying to find out whether nature made other types of black holes and nature does nature make some amazingly big black holes here's a example of how black holes have been found over a range of actually a factor of a billion in mass incredible and probably the most dramatic evidence and I would say the best evidence for any black hole in the universe better than any of those nearby black holes and binary stars the first ones to be discovered the best evidence for any black hole is the object of the center of our galaxy so what you're seeing here now is it's not a simulation or you know cartoon like animation these are real three-dimensional orbits of stars near the center of our own galaxy the Milky Way 26,000 light-years away and each of these stars is a test particle that's probing the mass at the center and they're moving really fast this data expands about two decades this was very hard data to obtain it was impossible until the early 1990s to get data this this good close to the center each star is testing and measuring the mass of the center of our galaxy and if you do them close enough to the center they're showing that the galaxy has four million times the mass of the Sun crushed or contained in an incredibly small space such that it must be a black hole and like I said because the black hole is diagnosed by each of these stars and their orbits the evidence for this black hole is better than any black hole in the universe there is no doubt that there's a four million solar mass black hole right in the middle of our galaxy very exciting meanwhile the Hubble Space Telescope was working and through the 80s and 90s in in observations and run as dramatic they don't prove with on with on the shadow of a doubt the black hole you drop the slit of a spectrograph on the left over a galaxy and on the right the colored lines show the Doppler shift of stars near the center of the galaxy and you can see the strong blue and red deviations close to the center those high stellar velocities are also probes of that central mass of that galaxy and when you do the math nearby galaxies have black holes too and every galaxy that was studied through a period of twenty years with the Hubble has a black hole so black holes are not special to our galaxy why should they be that would violate the Copernican principle they're found in every galaxy that we've studied so far but these galaxies are quiet they're sanders are not especially active or bright so if these black holes are doing anything they're they're probably not eating a lot of material they're not consuming material they don't have active accretion so another a puzzle raised by this discovery was if every galaxy has a black hole and there's tons of stuff for them to be accreting that as we've seen in binary star systems when that gas has sucked in it gets really bright and emits x-rays and optical ways and radio waves and everything why aren't these black holes doing that too and the answer seems to be that they're only active about 1% of the time so every galaxy has a black hole but each one is only switched on or bright or actively accreting material 1% of the time and in between it seems that they blow out enough material that they sort of starved themselves and that material has to accumulate in the center of a galaxy and trigger a new episode of activity that's the guess we don't get to stare at a galaxy and watch it evolve and see them switch on and off you do it with statistics and in these studies it was shown that the center the central black hole in a galaxy scales very nicely and beautifully actually over orders of magnitude with the mass of the old stars in a galaxy in the Milky Way that would be the Bulge component the center part of our star distribution so that's so somehow the galaxy knows about the galaxy the black hole knows about the galaxy they it's it's in and if you can zoom in on these these are radio images and the galaxies in this case was about the size of the central dot and it's an it's sending that huge Jets of plasma hundreds of thousands of light-years a couple of million light-years actually out into space if we could zoom in close enough we'd see there was a spinning black hole it was accreting material around its equator in a scaled up version of those binary star systems and it was emitting Jets of plasma very very close to the speed of light out of the poles and sending it deep into intergalactic space and radio astronomers made these pictures starting 40 years ago and now we've seen many examples and this is what the black hole looks like when it is active when it's doing something dramatic as opposed to the one in our galaxy which is pretty quiet and pretty dark and pretty quiescent the information that's recent and very exciting from within the last month is of course the fact that we now can see the black hole so the first image of a black hole was made with an array of radio telescopes essentially treating the earth as a giant radio telescope and so getting the angular resolution the sharpness of imaging is if you had a telescope that was ten thousand kilometers across by combining information from radio dishes across the planet one of those telescopes was at my observatory and some of the lead investigators in the event horizon project or at my university so I've heard about this and I said I saw the images you know sworn to secrecy like a few other people before this image was put out just a few weeks ago and this is the m87 galaxy Messier 87 and this is a direct image of the event horizon the dark circle is the event horizon the glowing ring is the gas around the black hole that's being heated up by gravity and energy basically the asymmetry is because the black hole is rotating and the gas on the lower part is coming towards us and it's Doppler booster it's radiation is boosted by the Doppler effect to be brighter and the radiation on the top is moving away for the top part is moving away from us and is dimmer and all of this is understood by relativity and here's a zoom in so this shows the biggest scale of m87 which is sending jets way out into space like that other example i showed and then you zoom in on the right-hand side it's closer and closer and closer until you get to a scale that's about a light a few light weeks and that's when you see the black hole and this data has already shown some amazing things there's things that it can't do but it's a simple image it doesn't look that impressive but it's also it's already been used to show what the spin and the orientation of the black hole is because you can run simulations and models and see whether the models fit the ring that you observed and its asymmetry and its brightness and these models have also measured the mass to be six and a half billion times the mass of the Sun so remember that's a billion times more than the mass of those stellar black holes that were the first to be predicted and the first to be measured it's a spectacular black hole and cuz even astronomers get blase about big numbers I just want to remind you how spectacular it is but first the fact that black holes do not overwhelm the universe if you make a pie chart of the universe dark matter dark energy dominate that's another talk I mean that's that's some best research I work on - and it's the biggest question in cosmology what are those two things but black holes if you can see our 5,000 of a percent of the universe so as spectacular as they are there are a little minor component of the universe each galaxy has a black hole that's roughly 0.1% of its mass so it doesn't they don't dominate the budget of the universe in mass but they are spectacular because when they are accreting when they are chewing in material there are superb gravity engines that's the way we would refer to them they can convert mass into radiant energy with 40% efficiency by which I mean mc-squared 40% efficient the Sun and all stars are less than 1% efficient so big black holes are dozens nearly a hundred times more efficient gravitational energy engines than stars are and remember the Sun is a hundred million times more efficient than our energy sources on the earth so humans are pretty pathetic in the scale of the universe and how we get energy this is the this is the way you would get energy incredibly efficient and so they're spectacular for that reason and we're studying the m87 black hole just to put it in context is this compares it to the size of the solar system and our most distant message in a bottle tossed outside the solar system so this is a black hole that's you know has this mass of a small galaxy seven billion times the mass of the Sun squashed into a region not much bigger than the solar system and the event horizon at the edge of the dark circle that event because of the angular momentum that event horizon is probably moving at eighty percent of the speed of life so just imagine something bad size and that mass spinning at eighty percent of the speed of life and what is nine what is seven and a half what is seven billion solar masses even me well let's just step up the scales of black hole mass because again astronomers maybe we do it too we talk about billions and billions and we just get used to those numbers but you need to digest what that means to put seven billion stars worth of stuff into the solar system or something not much bigger so here's a sort of modest size black hole we start with and then we'll scale up a couple of times to get to the big one so this is what we would call an intermediate mass black hole the kind we found in the edge of the solar at the edge of the Milky Way galaxy maybe the size of a planet instead of the size of a small town [Music] so that's about a thousand times the mass to the Sun like I said nature makes black holes of all these different masses but what about the big one now we're not using m87 we're using a different big black hole but there are a number of billion solar mass black holes now [Music] and while you're seeing it how does this happen we think that the black holes at the center of galaxies must have formed around the time the galaxies started and they grew together so over 14 billion years black holes and galaxies in the universe have grown it didn't all happen this quickly it took 14 billion years so it could happen here could have it could happen gradually yeah that's that's 20 bets that is the record the record black hole is about 20 billion times the mass of the Sun so the other new discovery not as new is that image of the black hole you just saw but only a couple of years old and very spectacular is as if we needed any more absolute proof that black holes exist LIGO the laser interferometer gravitational observatory showed that they exist and when this signal was measured a couple of years ago it's called a chirp signal because it's a if you translate into frequency it's in the mid audible range basically like the middle part of a piano and a crescendo and these signals detected with a seven millisecond time difference which reflected the difference in time of a gravitational wave crossing between the two LIGO sites represent the merger of two black holes that are each a few dozen times the mass of the Sun a spectacular event that opened up a new window onto the universe and again because these signals can really only come from black holes combining there's no other way to explain it so again unequivocally black holes exist what LIGO is doing in cartoon form was is to vacuum cylinders each five kilometers long very very good almost as good as the best lab vacuum lasers bouncing along each tube and when a gravitational wave which flexes space-time remember space-time is invisible it's nothing and yet in a gravitational wave predicted by general relativity space-time flexes in all the ways it can it flexes sideways and longitudinally it flexes in all three dimensions and that means anything in the space like a physical object is flexing too but even the vacuum is flexing space-time out there beyond between galaxies and so the instrument is flexing and if you make the two arms orthogonal they will each Mather measure different components of the flexure and so what LIGO is doing by bouncing lasers up and down the arms and trying to see the way the arms are differently squeezed by a gravitational wave in an interferometer is it's measuring this tiny space-time distortion by tiny so this is an instrument that's miles across that's measuring a space-time distortion that is smaller than a proton that was the experiment it's amazing that it could succeed but it did succeed this is what you would see if you could be close to two big black holes not big there are a few dozen times the mass of the Sun so these are dead star black holes we can't get close enough to black holes to see what happens when they merge but this is what it would look like and all these distortions of the surrounding stars are that gravitational lensing effect so this is a pretty accurate simulation of what would happen what LIGO is looking at is not the visible picture of that which we couldn't make anyway LIGO is looking at gravitational space-time ripples and this is a visualization of those essentially invisible waves as the black holes merge that little crescendo was a cacophony of gravitational waves here sent out into the universe traveling at the speed of light for over a billion years to reach us and be measured by that instrument so it's an amazing experiment they've since detected another eight binary black hole mergers and when LIGO comes back online in less than a year its sensitivity should be at the level where every week where it's gonna get old hat every week they'll be talking about a new black hole merger or neutron stars merging or neutron stars merging with black holes when any of the to compact types of objects in the universe merge they send out a torrent of gravity ways and LIGO can detect them but what about the big black holes well LIGO doing its incredible job which resulted in the Nobel Prize being awarded to that the architects of that experiment just two years ago has got given juice to some very ambitious experiments to measure gravity waves in space so if you think of a black hole as a physical object that that oscillates and rings like a bell or an organ pipe then bigger black holes will ring with lower frequencies than small black holes and it's linear with the mass LIGO can only detect the ringing and merger and oscillation and gravity ripples of small black holes the big ones at the Centers of galaxies are millions or billions of times bigger so their oscillations are millions or billions of times slower that means one of their space-time ripples might take months or years or decades and there's no way you can do that from the earth you don't have the sensitivity and so you see to the in the middle and to the left and this diagram people are conceiving of space versions of LIGO satellite versions in the super quiet super steel environment of space that will be able to detect the massive black holes of the universe merging and combining and growing from the Big Bang til now incredible experiment and if LIGO had not succeeded these projects would never have had a chance of funding they're so audacious so ambitious so difficult but like Oh having succeeded there is now budget the Europeans have put serious money these are multi-billion dollar multi-billion euro missions and they are going to be funded so is very exciting science for about 10 or 15 years from now which will look at how big black holes have grown in the universe and combined because astronomers think that galaxies in the universe grew by initially being small and then gradually merging and combining to form bigger galaxies and logically if every galaxy has a black hole then when the galaxies combined and merged the black holes combined and merged and grew that way by combination by addition and like these face experiments will actually show that happening they'll also possibly show which came first the galaxies or the small seed black hole that formed and we don't know the answer to that that's an open question believe me I've been waiting a long time for someone like you to record this moment thank you doctor then I'm ready ready to embark on man's greatest journey certainly his riskiest the risk is incidental compared to the possibility to possess the great truth of the unknown there long-cherished laws of nature simply do not apply they vanish and life life life forever so now at the end of my talk I want to address two issues that concern people are relevant to black holes they're just out there and it's nice to know about them but what about black holes and mortality the issue of creating black holes black holes and death well it's it's known and been written many times that if we any of us fell into a normal stellar-mass black hole you'd be spaghettified and that's a pretty unpleasant fate because that's not like a Stretch Armstrong doll it gets pulled from the heatin feet in the head that's being stretched at the level of muscles tendons molecules probably an excruciatingly ly painful way to die but if you work it out any black hole more than a thousand times the mass of the Sun of which there are plenty the gravity is strong but the stretching force is not as strong and so above that mass he'd probably feel a little queasy but you would not be ripped apart so yes you can survive falling into the bigger black holes in the universe in principle and so it becomes a question could it happen could it be done it would be a bizarre experiment because as seen from afar you would appear to slow asymptotically your life would be redshifted and you would never actually reach the event horizon the people who went with you to watch you falling would get bored and go home because your time would be running slowly asymptotically and infinitely slowly at the event horizon however to you you would fall in just on a straight trajectory through the event horizon to an unknown fate bizarre difference in perception of the observer and the person falling into the black hole wouldn't it be nice to test that of course you still wouldn't be able to get the information out because you've passed through the event horizon so this idea of surviving a black hole if it's big enough is interesting this is what the journey would look like so this is not again a cartoon or anything this is a general relativity simulation or den relativity calculation showing on the left you can see the spiral trajectory towards the event horizon which is the red boundary and this is what it would look like from your spaceship as you were falling in let's make it a big black hole so you're gonna survive this and all of these distortions are all of the the hot gas that's near the black hole and the accretion zone this is what it would look like all caused by gravity and finally as you hit the event horizon the universe's lost you you would be you lose view of the rest of the universe [Music] it's no way no real way to sum up the experience but if this is what the physics would suggest wouldn't that be an amazing experiment so I'm gonna leave you with a thought I hope I've reassured you the world is not going to be destroyed we do not have the technology to make black holes large or small but they exist in nature and I hope I've tantalized you with the possibility that black holes are objects of true inspection and discovery in some distant future where we do have the ability to travel through space thousands of light years okay that's not around the corner then the experiment can be done and it does in some bizarre way offer immortality to the person who falls in because if they do that experiment you go with your buddies and you have it's you know we're all gonna die so why not do it by falling into a black hole you go out with all your friends and your family you have an incredible party in a safe orbit far from the black hole you go into the black hole and a nice spaceship with a big bubble dome and you wear your best clothes you make sure you're looking pretty nice and you fall towards the event horizon and then you probably time your salya Tory final wave because your time will slow down to zero and to be frozen in your final wave as seen by all the people that came out and once they've drunk all the beer and they've got bored they then go home and a memorialized on the event horizon of the black hole that's pretty cool okay well what about the rest of the time of the universe so rather than making black holes objects of fear and death let me offer them as hopes for sustenance for life and immortality in the future of the universe now this is a future far beyond us in about a trillion years all the stars in the universe will be dead don't get sad about that don't get all sentimental about stars you know whatever they're just stars you know you can get your energy lots of different ways you don't need a fusion reactor sitting there in the sky the star the cycle of star birth and death it's cause stars to form through the history of the Milky Way will eventually be broken because the stars will all form stellar remnants white dwarfs neutron stars black holes and there won't be enough gas to make new stars and the lowest mass stars will die last the red dwarfs but eventually all the lights will go out like a big rheostat turned down not on in our galaxy but on every galaxy in the universe so the universe will go dark in a trillion years and eventually according to physics normal matter will decay solar systems will evaporate the planets will spin off into interstellar space galaxies will evaporate the stars will spin off into intergalactic space and the universe will turn into a thin uniform gruel this is the victory of entropy the second law of thermodynamics the exception is black holes they will be the last concrete objects in the universe and so advanced civilizations of the very very far future will naturally use black holes to run their lives and their civilizations for their set there will be the stars of the far future and in the intermediate future between a trillion years 10 to the 12 and about 10 to the 50 years stay with me on this these are long timescales it will be easy to extract energy from the black hole by judiciously dropping in probes and retrieving their energy or angular momentum so essentially what you're doing is tapping the gravity rotational energy of the black hole and so in this intermediate long term future civilizations around black holes because that's where there that's where the gravity power is we'll be able to run their civilizations and and run their wireless internet and whatever else they do using gravity rotational energy of black holes but eventually the black holes will spin down and so in the far future they're left with a much more subtle form of energy and that's in a universe where everything is a thin uniform super low temperature gruel of positrons electrons and super low energy photons the energy remaining will be the Hawking radiation of the black hole and so an enterprising civilization will build a Dyson Sphere and energy trapping sphere to capture that feeble Hawking radiation and in logical terms the last black hole to survive will be the biggest black hole and the number on that is about ten to the hundred years that's how long it'll take the biggest black hole in the universe to finally evaporate and that is all she wrote when that happens and so the last image I'll leave you with is of a super advanced super far in the future civilization that's huddled around the last black hole in the universe warming their hands by what's essentially a kilowatt of power from the Hawking radiation it's not much but they'll be efficient with it and they look at each other and they say you know ten to the hundred years we gave it a good run thank you [Music]