Hearts of Darkness: Black Holes in Space

good evening everyone my name is Andrew frack no I'm the Astronomy instructor at Foothill College in Los Altos Hills California and it's a great pleasure to welcome everyone here in the auditorium and all those listening on the web to this lecture in the 11th annual Silicon Valley astronomy lecture series at Foothill College each time we present a noted astronomer sharing the excitement of astronomy with the public here and on the web tonight's lecture and like all the lectures is co-sponsored by NASA's Ames Research Center the Astronomical Society of the Pacific the SETI Institute search for extraterrestrial intelligence Institute in Mountain View and the Foothill College astronomy program tonight's speaker is dr. Alex Filippenko a professor at the University of California at Berkeley dr. filipenko specializes in things that go bang whether its stars or other explosive phenomena including the universe itself that's his bailiwick in the study of the universe he's a distinguished research professor in astrophysics at Berkeley but in addition to all his good work in science he is also one of the foremost popularizers of astronomy in the world today he's been at this lecture series several times and were delighted to welcome him back we got the idea for this appearance because I was sent by the teaching company which records great professors all around the world a tape called black holes explain which is dr. Filippenko doing in many hours what he's going to summarize briefly tonight explaining all the fun aspects of black holes it's called black holes explained and we were hoping that the tapes would go on sale just as he's coming to talk to us but they're not on sale now and he urges you to wait until they are on sale so he's very nice about this and you can just go to the teaching company website he'll have the website for you it's his second course for the teaching company and these courses are delightful they're Illustrated introductions to the most exciting aspects of modern astronomy so this one is called black holes explain doctor Filippenko was recently elected to the National Academy of Sciences to honor his research work but in addition he's been elected seven times the most a popular professor at Berkeley and a few years ago he was chosen by the Carnegie Endowment for higher education as the professor of the year for the whole country in universities around the country as part of the work that he does he writes a textbook in astronomy he records these courses and year after year he gives the most popular introductory course at Berkeley in recognition of this work he was recently chosen for a very special award the Astronomical Society of the Pacific which is a hundred and twenty year old organization working in the interface between science and education has recently instituted a special award called the Emmons award which recognizes a lifetime of contributions to higher education in astronomy and I'm delighted to report that dr. Filippenko is the 2010 Emmons Award winner this is the first public announcement of this award so we've asked him to earn this award by coming tonight and talking to us about one of his favorite subjects and I think a subject that is a favorite of the public of science fiction writers and of astronomers of black holes so it is both a professional privilege and a personal pleasure to introduce to you one of the great speakers in astronomy dr. Alex Filippenko well thank you very much Andy for that very kind introduction what Andy didn't tell you was that a few years ago he was I believe the very first recipient of the ASP Emmons award is that correct Andy the second oh well in that case it doesn't count no he's the second so I think we should give a round of applause to Andy and moreover a few years ago he was designated by the Carnegie case people as the California professor of the year so anyway he's he's no slouch in public education in astronomy either and I'm delighted to be once again part of this lecture series that Andy so tirelessly puts together and as he said I like to talk about my favorite subjects I've spoken about exploding stars hear about the expansion of the universe and other topics and today I'll take tell you about black holes one of the greatest topics in in modern astronomy so you might wonder well why study black holes well there are many reasons but here's one reason from a speech by former President Bill Clinton look there are so many more questions yet to be answered and so I wonder are we alone in the universe what causes gamma-ray bursts what's in those black holes anyway right if the President of the United States in his Science and Technology Policy address mentions black holes then surely every educated American and even every educated person in the world should should learn something about them so there you go I'll tell you today what we think is inside black holes a point of infinite or nearly infinite density called a singularity where all the material that went into the black hole is concentrated so bill is here is Bill Clinton here not just any old bill but Bill Clinton well sorry we'll have to send him the web link and he said that he can learn what black holes are anyway so more seriously a black hole is a region of space where matter is concentrated into such a small volume that the local gravitational field is sufficiently strong to trap everything nothing can escape not even light so if light can't escape it doesn't get reflected doesn't get transmitted you know doesn't get emitted then the thing appears black and so we call it a black hole and here's one of my prize-winning photographs of a black hole there it is right there my normal price is ten thousand dollars but for you friends of astronomy five grand each special price or if you want to do it more cheaply you can just keep the lens cover on your camera on or if you're making a PowerPoint or keynote presentation just just choose a black background so I just I just gave away my trade secret and have lost out on the opportunity of becoming really rich but anyway there it is you could say abandon hope all ye who enter here because as I said nothing can escape so if you choose to wander into one of these things or if you do so by accident you're never going to get out again so if you choose it for example as a PhD thesis you'd better explore the black hole from outside its boundary and not too close I'll tell you about that later anyway black holes are part of pop culture they of course appear in science fiction and things like that we're getting a hum cool okay I won't use this one this okay so that means I'm tethered more here but that's okay you don't want me to wander around but that's alright I'll stay here usually I like to wander a bit but anyway it appears in pop culture quite a lot and here you can see of course some movies Disney's flick from 1979 black hole a journey that begins where everything ends right and then here's another one event horizon infinite space infinite terror now the event horizon is the boundary of a black hole but from what I can tell this movie actually has really nothing to do with black holes but they know that if they choose black hole theme it will suck people in and people will be attracted to it and maybe they'll get more of a viewing audience then here's a black hole Charles burns graphic novel which is sort of a compilation of his comic strips titled black hole and again it really has nothing to do with black holes it's about some kids in Seattle that contract some terrible alien disease that turns them into zombies or something but anyway it attracts people more examples of pop culture greeting cards this card is light another black hole starts to form and wouldn't you know it right in SIDS room now as I will show you tonight there's only certain conditions under which black holes can form rest assured they don't form in SIDS room or in any of your rooms don't worry about this but it's funny though the dog the clock everything's being sucked in where might you find black holes in nature well here in the Bay Area we have an excellent example of a black hole in the Oakland Raiders a fan section there's a there's a section with particularly rabid Oakland Raiders fans and they call themselves the the black hole or the black hole zone and I think it's because anyone who happens to be wandering by this region of rabid foaming at the mouth fans will get sucked in inexorably by these fans never to emerge again perhaps that's why they call themselves the black hole you know anyway not really but it's amazing how much it appears in pop culture as a result so as you can imagine they're pretty fierce places they suck you in and they hold on fiercely and never let go so based on that idea we have a saying what happens in black holes stays in black holes okay yes some of you are familiar with the saying about Vegas all right well if you google black hole you'll find that there's something like 36 even up to now 40 million hits some of the first few websites are really quite good the the wiki article is good and there's a Hubbell site which has lots of information about black holes so there's a lot of actually very good websites about black holes as you start going down in the list some of the websites start becoming perhaps not quite as reputable and you might want to watch out you know not everything on the internet is true but there is a lot of good stuff about black holes on the Internet there's also some ads look at that here black hole prices slashed today only free UPS service for you okay I don't know maybe this is somewhat liposuction company or something like that I'm not I'm not sure anyway there's a lot of good stuff out there as well as of course this set of twelve lectures that that Andy so generously mention all right so let's go back to this definition that I gave and explore it a little bit more carefully let's go back and use a purely Newtonian argument suggesting that there might be at least in principle objects whose local gravity is so strong that nothing not even light can escape so this goes all the way back to Newton he himself did not make these arguments but in the late 18th century several other people did john mitchell and pierre de la semaine de Laplace made these arguments they they said the following here we are on earth and let's say we hold an apple and the gravitational attraction between the earth m1 and the apple m2 and by the way this is the only equation you'll see tonight I believe is proportional to the product of their masses and inversely proportional to the square of the distance between them so the earth pulls on the Apple bringing it down now I can toss this Apple up and it comes down and if I had eaten my Wheaties this morning and if there weren't the technical problem of a ceiling I could in principle throw this Apple sufficiently fast that although it's forever slowing down it never comes to a complete stop and certainly never reverses its motion that speed is known as the escape speed from the surface of the earth happens to be about 11 kilometers per second about seven miles per second all right now suppose I were to take the Earth's mass and squish it down to half of its current radius and I remain on the new compressed surface of this earth so here's my Apple here the two masses have stayed the same but the distance between the Apple and the center of the earth the center of mass is now half of what it used to be so 1 over 1/2 squared is for the force on the Apple is now 4 times greater that means I have to throw the Apple faster in order for it to escape not four times as fast it's a bit more complicated but nevertheless a faster speed the escape velocity increases so now imagine compressing the earth even more the escape velocity at the new surface will increase even more and you can imagine then compressing the earth so much that the escape velocity reaches or even exceeds the speed of light now at the time of John Mitchell's work and Laplace's work they didn't know that nothing can go faster than light but at least they could say that gee you could in principle have an object from which material or at least from which light can't escape and it now turns out that we know that material objects can't escape either because none of them can exceed the speed of light and they suggested that dark stars might in fact exist from which light doesn't escape now it turns out you'd have to compress the earth to the size of a walnut in order for the local speed to reach the local escape speed to reach the speed of light and that's hard to do there's no cosmic vises that are going to do that but at least in principle you can imagine squishing the earth down this way to such a point where the scape velocity reaches or exceeds the speed of light and that then would be an object from which light can't escape and when we bring in relativity and the idea that nothing can go faster than light that means nothing can escape and you have a black hole now to do it right you need not a Newtonian argument but you need an argument enveloped in encompassing all of general relativity and the curvature of space and time the presence of mass and that's what Einstein did and in that case an object forms this dimple a warp in space and time around it but as you make the object denser and denser and denser the warp becomes progressively stronger the sides becomes steeper and steeper until eventually you compress it so much that the sides of the warp become effectively vertical and nothing can escape in a sense because in trying to climb out of this well they use up all their energy that's one way of thinking of even light trying to escape from a vertical well it maybe does so but it loses all its energy in the process and so there's nothing coming out it ceases to exist and so it really can't escape or it goes around in circles like that if it's not going trying to go go up the sides like that but in any case that would then be a black hole a region of extreme space-time curvature in Einstein's general relativistic formulation of the physics but even with the Newtonian argument some people have the idea that there might be such objects and Einstein proved and his disciples and other people studying general relativity proved that in principle such a mathematical object could exist that doesn't mean that nature chooses to make such objects okay there's a difference between being possible in principle and actually finding examples in nature of such a phenomenon well how might nature makes such a object I said that there are no cosmic vices that would compress the earth to the size of a walnut but there might be something that can compress a star to the proper size to make it a black hole and one such mechanism is exploding stars now most exploding stars don't form black holes but some we think may in fact form a black hole in the central region and the remaining stuff expands outwards it explodes and in particular the types of exploding stars that we think are best at producing black holes or the so called gamma-ray bursts if you look at gamma rays up at the sky using gamma-ray satellites you find that roughly once a day somewhere in the sky there's an intense burst of gamma rays high-energy electromagnetic radiation and we now think that those intense bursts of gamma rays are associated with the birth of black holes or the growth of black holes when a black hole eats another star and here's the idea in some cases massive stars are rotating sufficiently quickly and are sufficiently massive such that the collapsing core at the end of the life forms a black hole and all the energy in this material here can get channeled through two oppositely directed Jets they go pummeling out of the star forming these high-speed jets and if our line of sight happens to be along the direction of one of those two Jets then we see a truly brilliant flash whereas if we're viewing the explosion and by the way the rest of the star explodes even though the middle imploded but if you're viewing the explosion from the side you see a more normal supernova only a few billion times the brightness of the Sun rather than you know trillions of times as bright at a Sun as you get when you watch this thing down the down the axis of of this beam so that's what we think gamma-ray bursts are they're the birth of a black hole following the collapse of the core of an exceptionally massive star that's rotating at just the right rate and has other properties to allow the formation of these too rapidly moving beams of particles and light but in the middle of this exploding star you get the birth of a black hole also when two neutron stars which are usually the things leftover when a star explodes if you've got two neutron stars orbiting one another sufficiently closely then those two neutron stars can merge together and form a black hole and that's thought to be the reason for some gamma-ray bursts as well or maybe you have a neutron star being eaten by black hole that would be the rapid growth of a black hole and it can produce a gamma-ray burst as well so it's the gamma-ray burst that we think are associated with the birth or growth of what we call stellar-mass black holes black holes that are basically as massive as massive stars are and the agent that compresses the material rather than being a mechanical vise it's just gravity gravity causes a collapse of the central region or a merging of these two neutron stars and they form a black hole producing a gamma-ray burst so that's one way we think nature can produce black holes the other way is by concentrating a large amount of material in the central part of a galaxy now a galaxy a gigantic collection of hundreds of billions of stars maybe a hundred thousand light-years across but in the central region in some cases in fact we think in many or most big galaxies their forms a central concentration of gas that then undergoes gravitational collapse forming a black hole not just a puny stellar-mass black hole having ten or twenty times the mass of the Sun but rather what's called a supermassive black hole forming or having masses of millions or even billions of times the mass of the Sun and I'll show you observational evidence for both types of black holes in due course so how do we find these things okay here's you know this is how we expect that they're produced but how do we actually find them if they're black well you could just take photographs of random parts of the sky and look for arrows and where you see arrows there's uh there's black holes right there right well obviously it has to be a bit harder than that because we give degrees for this kind of research and if it were that easy we we wouldn't give degrees blackness in space could simply just be an absence of stars and galaxies it doesn't necessarily mean that there's a black hole there here's a Sidney Harris cartoon based on this idea it's black and it looks like a hole I'd say it's a black hole well again it's it's not quite that simple the absence of light alone not guarantee that you've got a black hole so how do we find them well we rely on their gravitational influence on their surroundings it turns out that many stars in fact one could say even most stars are not solitary but they are parts of binary systems they are gravitationally bound together they orbit their common center of mass and you might not even know this if you're just looking at a star because it looks like a solitary star but if you examine it through a telescope you might see two stars there but even then you might see only one star but if you take its spectrum then you can tell that there are two stars you take the light call it star light sorry I stole this from either Andy's book or my book and this is a spectrum of the Sun but you could have star light coming in from this apparently single star and you spread the light out into a rainbow or the component colors or wavelengths and you can see these dark lines here which are due to chemical elements in the atmosphere of the star absorbing certain wavelengths and normally there's only one set of lines like this these are due to sodium these down here are due to calcium and so on keeps on knocking over well I knock over my water and then the gravity does the rest okay but if you take the spectrum of the star over the course of time you might notice the following and interestingly enough yeah my my animation is gone oh well it's just gone see that we have this problem before but oh well no big deal what you see are two sets of lines and they go back and forth and back and forth as these two objects orbit one another because it's kind of like the oh now I've completely messed it up right to err is human to really foul up requires a computer oh gosh good I got it too advanced okay let me try it again now if you watch this thing from say over there and that's what my little animation showed then at times this star is moving towards you so the absorption lines that it produces are blue shifted and this one is moving away from you so the absorption lines are redshifted and then this one becomes blue-shifted and that one becomes redshifted light experiences a similar phenomenon as the audible Doppler effect that you've all heard when a siren goes past it goes here like that that's because the waves are squished as the object is coming toward you and spread apart as it's moving away now if you hear a siren going yah-yah-yah it doesn't mean that the driver is going in circular motion and is drunk or something like that it just means that that's the natural town of the of the siren but you can still hear a generally high-pitched ER morph into a low-pitched ER as the thing you know passes you and and so that's the audible Doppler effect but we see this in stars as well so if you look at the spectrum of of this binary star you can see two sets of lines going back and forth and you can tell them that it's a binary star now suppose one of the two stars has turned into a black hole it's gravity is still there right I mean still acting gravitationally on its companion and they're - they are still orbiting their common center of mass but this one won't produce any absorption lines so you'll see one set of absorption lines going back and forth back and forth and I'm sorry my little animation doesn't seem to be showing on my screen it was when we prepared the talk but oh well and so when you have one set of lines going back and forth you can figure out how massive the object must be that's causing the visible star to undergo circular motion and if that mass is very large greater than say three or five or ten times the mass of the Sun yet you see no visible evidence for the star that's pulling on the visible star then by the process of elimination it becomes reasonable to suggest that that's a black hole basically because we haven't thought of anything else it could possibly be and we've found no other objects in the universe that could have so much mass crammed into such a small volume yet not be visible in any way in other words the black hole becomes the conservative conclusion any other conclusion would be more radical all right so that then becomes a good black hole candidate all right well but which stars should you monitor there are millions of stars billions of stars in our galaxy there's not enough telescope time to take sequentially spectra of more than a handful of these things looking for you know a single set of lines zapping back and forth you need a clue which ones are likely to be the ones that are orbiting black holes because most stars are not orbiting black holes well a clue comes from x-ray wavelengths let me move this annoying error out of the way if you have two stars in orbit around one another and one of the stars is stealing material from its companion and if that star is very compact let's say it's a neutron star which is a very compact star or let's say it's a black hole then the material falling in robbing all the gas particles and dust particles are rubbing against their neighbors and they're falling in a very strong gravitational field because there's a lot of mass in a small volume there they're falling fast they hit against each other they heat up to very high temperatures okay and they emit x-rays now most stars do not emit x-rays profuse ly our Sun emits some x-rays but it mitts a heck of a lot more visible radiation but these stars then would be emitting x-rays and this accretion disk that forms is clumpy it's not uniform and when clumps of material undergoing instabilities and fall toward the compact object then they emit a burst of x-rays and so not only is the star emitting x-rays but there might be a gigantic burst of x-rays it becomes what's called an x-ray nova now to produce such large quantities of x-rays you need a very compact object they're either a neutron star or a black hole so those stars that become x-ray nova are good candidates for looking for a black hole because there's either a neutrons or maybe a very compact white dwarf but even white dwarfs generally don't produce a lot of x-rays or you have a black hole so there are satellites like the Chandra x-ray Observatory flying around above Earth's atmosphere that study the sky at x-ray wavelengths okay and they find stars that undergo these x-ray outbursts and then the rest of us get really excited ooh there's an x-ray nova maybe that's a star orbiting a black hole so we start monitoring it and it brightened not only at x-ray wavelengths but at ultraviolet and visible infrared as well so you have to wait until the Nova dies down that is you have to wait until the secretion disk dies down and becomes faded so that you can then see the light of the relatively normal star otherwise the light is dominated by the by this outburst from the accretion disk once you see the light from the relatively normal star you can take a sequence of spectra see whether the absorption lines are moving back and forth and determine the mass of the object that's pulling on your unseen companion I'm sorry that's pulling on the visible companion so here's one of these objects it's it was found by a Japanese x-ray satellite called Ginga in the 1980s and this number here tells you roughly where it is in the sky it was an x-ray Nova my team then waited patiently until the x-rays and ultraviolet and optical light all died down until finally we saw what we thought was just the normal quiescent visible star we then used one of the two 10 meter Keck telescopes in Hawaii to take a sequence of spectra of that visible star 13 spectra over the course of one glorious summer night and you know the Keck mirror was able to gather enough light from this faint star to allow us to actually see these weak little absorption lines and to see that they're actually moving back and forth back and forth there is something pulling on that visible star and the Keck telescopes are wonderful to use I love them here's Fred Chaffee a former director of the Keck Observatory sitting in the hole in the primary mirror showing you the scale 10 metres of the Keck telescopes now usually he wasn't there while we were taking data this is just a PR shot because the extra light gathering power provided by the eye of a human is dwarfed by the collecting area of this light year but anyway it just shows you really how big these wonderful light buckets are and when we measured these absorption lines going back and forth using the Doppler formula we could convert the wavelengths or colors into the speed that must be causing those shifts in wavelength and again I wish I had my little demo here for you but I don't so that's the way it goes and we found that if we plot this speed measured from the absorption lines so called radial velocity because the Doppler effect tells you how quickly something is going toward you or away from you not how quickly it's going transverse to your line of sight but anyway if we measure the radial velocity as a function of time we found that well for a while the visible star was moving away from us at 520 kilometers per second four hours later it was moving toward us at 528 second another four hours later it's again moving away from us and the points in between beautifully are fit by a sine wave a sinusoid which is sort of the hallmark of circular motion so the visible star is in circular motion around something else more precisely around the center of mass between it and something else but whatever and from the speed and the orbital period you can use Newton's laws to figure out the mass of the object that's pulling on the visible star and it turns out to be at least five times the mass of the Sun and more likely once you take into account the inclination of the system eight or nine times the mass of the Sun so here's an object that's eight or nine times the mass of the Sun pulling on the visible star yet there's no evidence for any light being emitted by this massive object normal stars that are eight or nine times the mass of the Sun are incredibly luminous so it's not a normal star it's also not a collection of little sun-like stars in a small volume because we would see them as well and it can't be a neutron star or a white dwarf because they have a maximum mass limit which cannot be exceeded of roughly one and a half solar masses for white dwarfs roughly three solar masses four neutron stars so this becomes a very good black hole candidate dynamically it looks like there's a black hole there that is it's influencing its surroundings in a way consistent with that expected of a black hole so then I'm a very happy camper when we got data like this and this shows you the real reason we build observatories in Hawaii I like swimming in the ocean as much as any of you and I personally find Northern California waters to be a bit too cold without a wetsuit and to me a wetsuit is a big pain to put on so but there are good scientific reasons for building these things in Hawaii as well okay our team has found a bunch of these types of objects this one shows in fact the visible star bound to what we believe is the most massive stellar-mass black hole ever found and that one is 30 solar masses typically the ones that have been found are 15 or below 10 5 7 8 9 that kind of thing this one is 30 solar masses or at least 23 okay so that's the most massive one and you might say okay well that all sounds good and we now have several dozen cases of these stellar-mass black holes and binary systems but do we really know that they are black holes have we seen anything go beyond the so-called event horizon the boundary of a black hole beyond which nothing can escape from okay and the evidence that we have seen this at least tentative evidence is the following there are these binary systems where the mass calculation suggests that there's a neutron star and there are other binary systems where the mass calculations suggest that you have a black hole in the cases where we have a neutron star the material that's being accreted eventually falls clonk oops sorry about that well why am i apologizing to a podium anyway um I do that sometimes here the material the falling apples you know they're picking up speed and they fall onto a surface and they liberate their kinetic energy their energy of motion when they hit that surface that makes this system glow quite a lot all right so the glow not just from the accretion disk but from the material hitting the actual surface of the star if instead you have a black hole then the material never hits a surface and so there isn't the extra liberated kinetic energy right because the falling apples go past the event horizon of the black hole and end up in the singularity somewhere and so there isn't that extra radiated energy and what we found is that the black hole x-ray binaries appear to be darker they're less luminous than the neutron star x-ray binaries or x-ray novi so that's some indirect evidence that stuff is actually passing beyond an event horizon additional evidence comes from an object called Cygnus x1 now this is an artist's rendition but the data suggests that a blob that we saw falling into the putative black hole in Cygnus x1 faded and became redder for a reason I'll tell you later consistent with what's expected if it's entering or falling into the gravitational field of a black hole and eventually it just faded completely we lost sight of this blob of gas completely as though it went behind or beyond end of that horizon so you know we haven't grabbed one we haven't stuck one in our labs yet fortunately right that would be like SIDS room but we have pretty darn good evidence that there really are stellar-mass black holes out there let me now tell you about the supermassive black holes those are the ones thought to form in the central regions of galaxies the idea is this in the central region of a galaxy you have lots of stars they gravitationally pull on one another causing them to move just like stars in binary systems pull on one another causing each other to move without a black hole the amount of mass causing the motion of each individual star is just some amount causing those individual stars to move by some amount if in addition you have the mass associated with a supermassive black hole millions or billions of times the mass of the Sun then you have an extra speed these these stars in order to successfully stabili orbit the black hole they have to move more quickly you can just figure that out from Newton's laws of motion okay and so with a black hole there's the gravity not only of the regular stars but of the black hole pulling on the surrounding stars causing their motions to be greater than in the absence of the black hole so what you want to do is look in the central regions of galaxies monitor the motions of stars either individually which we can do for the middle of our own galaxy or collectively which we do when we look at other galaxies that are very far away and see whether the stars are moving unusually quickly if they are then that could be the signature of a black hole so our own galaxy is the one of which we have the clearest view not at optical wavelengths such as in this photograph because there's a bunch of what I call Galactic smog interstellar gas and dust that actually blocks our view of the center of our own galaxy at visible wavelengths much like on a foggy day you can't see buildings that are a few blocks away but you can still hear your favorite radio station right so radio waves can pass through this interstellar matter much more easily than optical light and infrared light can do it pretty easily as well not quite as easily as radio waves but certainly more easily than the a light so when we look at the galactic center at infrared wavelengths we can actually see individual stars here's a ground-based image of the central region and you see that in the central tiny volume there's a bunch of blurry things the blur is caused by the Earth's atmosphere if we turn on a technique called adaptive optics on our largest telescopes then we get much much greater clarity those of you who are avid amateur astronomers will know what I mean by one arcsecond you can see the size of this of the stars here look much smaller than one arc second whereas here they're comparable to one arc second so with adaptive optics over a small region of the sky you get clarity rivaling that or even superior to that of the Hubble Space Telescope with the Keck 10-meter telescopes we get four times the clarity admittedly over a small region of the sky but nevertheless four times the clarity of the Hubble that doesn't mean I'm a Hubble basher I love Hubble I'm a Hubble hugger but Hubble is not the only way to get clear images of limited regions of the sky okay and so my colleagues Reinhard genzel at Berkeley and at the Max Planck Institute in Germany and aundrea Gaz at UCLA have had for years now two decades in fact two independent teams that have been monitoring the motion of stars in the central region of our galaxy and what they find is the following the year is given up here you see these stars go zoom zoom I mean I love to watch these things right white watches watch this one here this one goes zoom zoom money and these are the data points and here's the the Newtonian orbital fit to the data now these stars went very quickly around an unseen object you might say well there's a Red Cross there but we stuck that in in fact there's a black hole there and a black hole is an excellent place to have a Red Cross medical station because if you get too close as some of these stars surely will someday they will get torn apart I'll talk about that in a few minutes anyway when you when you look at these data you find that the only thing that's consistent with the observed motions of these stars is something that has a mass of four million times the mass of the Sun within a volume no bigger than our solar system that is out to Pluto or Uranus let's say or Neptune since Pluto doesn't really count anymore right anyway um well it does but it's just not a planet anyway and I and I agree with Michael Brown so you can lynch me as well but I didn't do it so don't lynch me okay so uh anyway it's a large amount of mass within a volume no bigger than that of the solar system this in fact is the very best evidence for black holes in other words this is even better by a long shot actually than the evidence provided among the x-ray novi that I just discussed this is really it's very very difficult to explain this in any way other than with a black hole because there's no way you can cram that much material into such a small volume basically okay and this has been done now not only for own our own galaxy but for other galaxies as well notably with the Hubble Space Telescope and also with the Keck telescopes with adaptive optics you can do this you then see collectively the motions of a bunch of stars in other galaxies and so the constraints aren't quite as good as with the data provided by individual stars in our own galaxy nevertheless the case is quite compelling in galaxies such as m87 there's a gigantic mass within a very small volume right in the middle and I always said and Andy and other astronomers always said that it's three billion solar masses but recent new calculations and models by astronomers at the University of Texas suggest that we may have underestimated the mass of this monster by a factor of two or even two-and-a-half this is still a bit controversial but it might be a six or seven billion solar mass object within a very small volume again probably a black hole here's the sombrero galaxy Messier 104 very beautiful galaxy it has a 700 million solar mass dark object in the middle again you can tell from the collective motions of the stars our neighbor the Andromeda galaxy much like our own galaxy in many ways well it too has a central black hole but it is blessed with a much more massive black hole than our own Milky Way our own Milky Way got cheated in a sense not that size matters necessarily but you know in that drama two galaxies there's a 140 million solar mass black hole in our own seemingly similar galaxy the Milky Way there's only a four million solar mass black hole and we're not quite sure why there's this difference between our galaxy and Andromeda in the very central region in fact a correlation has been found between the size of the elliptical galaxy or the bulgy part of a spiral galaxy link go back a couple of slides there's the Bulge of Andromeda there's the disc in spiral arms of Andromeda here's the Bulge of the sombrero galaxy here's the disc of the sombrero galaxy m87 is only a bulge in a sense it's what's called an elliptical galaxy well the curious thing is is that the size of a black hole in the center of a galaxy appears to be strongly correlated not with the overall size of the galaxy as a whole if you include the disc but rather with the size and compactness of the Bulge region the big bulge galaxies especially those where a lot of stars are crammed into a relatively small central region of that bulge so by big I mean massive and by crunched-up I mean a lot of stars are near the central region those those galaxies have the biggest black holes and galaxies with smaller bulges and bulges that aren't as compressed toward the central region they tend to have smaller black holes and in fact there are some disk galaxies some spiral galaxies that have virtually no bulge and they have almost no evidence for a black hole I mean I won't say that there's no black hole there I should have put that in quotes except that someone else's slow there might be a 1,000 solar mass black hole there but we can't tell the data aren't good enough to tell us whether there's no black hole there are only a 1000 or 10,000 solar mass black hole but virtually none compared to millions or billions right and we don't quite yet understand this correlation but clearly it's telling us something about galaxy formation and black hole formation the two seem to go together whatever it is that draws in and causes a lot of stars to form in a bulge also allows the process to go even farther some of the material about half a percent of the material collapses all the way down to the center and forms a black hole and those galaxies that form big bulges form big black holes and the little bulges have only little black holes but bulge formation and evolution and supermassive black hole formation are intimately tied together and are telling us something important about the formation and evolution of galaxies and we're still trying to figure that out now there's been additional evidence for black holes in the central regions of galaxies and this evidence goes back even several decades you look at galaxies and most of them have a central concentration of light but some have in addition to the normal starlight a very bright central region a nucleus that's brighter than it normally would have been and these so-called active galaxies show a lot of evidence that the process by which the nucleus is anomalously bright has nothing to do with normal stars it's not a bunch of supernova explosions as much as I'd like it to be perhaps it's rather thought to be black holes a giant black hole swallowing material from the surroundings and we see many of these objects here's one where it's clearly the bright central region of a galaxy and in fact quasars are an extreme example of this where in some cases you don't even see the galaxy very easily but the bright central region is being powered by some sort of a process that's very efficient in producing radiated energy and material falling into a black hole in an accretion disk is a very efficient way to radiate energy because the particles rub against one another and they heat up and they radiate electromagnetic radiation at an efficiency that's at least ten times higher than the efficiency of nuclear energy and nuclear energy is thousands even tens of tens of thousands of times more efficient than than chemical and energy like burning and this process can even give rise to relativistic Jets that is Jets of very rapidly moving material kind of similar to what I showed you for the stellar case of a gamma-ray burst what we think is happening is there's a supermassive black hole with an accretion disk around it and through a combination of either forming a nozzle because of the natural thickness of this accretion disk or perhaps through the action of magnetic fields magnetic fields can also channel charged particles as they do in the case of pulsars many of you know that pulsar czar rapidly rotating highly magnetized neutron stars anyway by a combination of the disk and magnetic fields particles can be accelerated along the axis of rotation of the black hole and that's what forms these relativistic Jets that we see now many newspaper articles will say that astronomers have seen material coming out from a supermassive black hole in a quasar or in an active galaxy that's not quite right what we see is material coming out from the vicinity of the black hole the particles are energized and pushed out along this axis in the vicinity of the black hole no particles are actually escaping from within the event horizon itself okay but in loose talk in in articles they often just are kind of glib about this and they get it a bit wrong and as I said quasars which were first identified in the 1960s are the extreme example of a supermassive black hole accreting a lot of material maybe like one to ten solar masses of gas per year and some of that gas becomes very hot glows and causes the quasar to far outshine the 100 billion normal stars in the rest of the galaxy and sir Martin Rees and a few other people Donald Bell in the mid-1960s quickly came to the conclusion that quasars are probably powered by this process again by the process of elimination but now we actually have measurements of gas orbiting in the vicinity of a quasar or stars orbiting in the middle of galaxies that used to be quasars and we have more direct evidence that the black holes really are there but they were suspected to be present even four decades ago when quasars and active galaxies were being studied so it's interesting that we now have a lot of evidence for the existence of black holes the stellar mass variety the supermassive variety I'm Stein himself didn't think that black holes exist they think that they were a mathematical curiosity or he thought that they were a mathematical curiosity of his laws of general relativity of his equations but he didn't think Nature has any way of actually compressing material to such a small volume so he thought it was just a mathematical curiosity here he is perhaps sad that one of the most bizarre predictions of his theory doesn't have an actual counterpart in nature but Einstein died in 1955 I believe around then before the evidence for black holes became available what would his reaction be if he were alive right now confronted with the evidence that I've just presented to you observational evidence for the existence of black holes his reaction might be something like this okay and really in the case of our Milky Way galaxy were virtually certain it's nearly 100% I mean I I I'd almost bet my life I'm not quite at that stage because a life is a very precious thing but I'd certainly bet my house that there's a black hole in the middle of our galaxy and that's an easy thing to do because I rent so but I'd um you know I bet I bet my cat that I love very much but you know anyway we really think black holes do exist all right few more things and then we'll do Q&A but let me first dispel some popular myths about black holes the first is related to this idea that black holes are these cosmic vacuum cleaners and they can form just anywhere not only in Cid's room but in Darin Bell Skees room as well suddenly through forces not yet fully understood they are in belts keys apart and it became the center of a new black hole and started sucking everything in I hope I've convinced you that black holes form in nature only under very special circumstances you need something to compress material into a very small volume usually that something is gravity we don't know of any other vices that will do this so don't worry be happy' a black hole isn't about to devour your house okay the other misconception is that black holes that do exist and we really do think that they exist are these really dangerous things that go sucking up everything around and in fact many of you have seen me on some shows that I would say overly sensationalize astronomy perhaps to make it more interesting to those who are not already interested you are the converted you're already to some degree interested in astronomy but you know it's the ratings game on TV if people flip the channel and watch the football game or sumo wrestling or whatever then you know these science documentaries won't get approved for future seasons and so that's why they sometimes overly sensationalize things but they have sometimes made it sound like black holes are these really terrible things that are about to devour the universe and in particular suppose our Sun were to turn into a into a black hole would it become this cosmic vacuum cleaner you know that would suck in everything including us and all the other planets in the solar system absolutely not we here on earth are on a stable orbit around our Sun gravitationally we don't care whether our Sun is the Sun or a black hole of equal mass now from the perspective of life we do care the Sun gives us light and heat and all that but from a gravitational perspective we really don't care could have been a black hole from our distance from that from the Sun Newtonian gravity is basically just like Einstein yin gravity we're not going to get sucked in so don't worry first of all our Sun is not going to turn into a black hole it's not massive enough it'll turn into a white dwarf and secondly you know even if it were to turn into a black hole we wouldn't get sucked in all right so only if a black hole becomes really quite close to the solar system do we have anything to worry about there is an interesting aspect of black holes however and this is not a myth it's called tidal forces if you're near a black hole especially near a small black hole so called stellar mass black hole then the force on your feet is significantly greater than the force on your head because your feet are closer to the center of the black hole than your head is so the force big F is bigger than little F and your feet get pulled toward the black hole more than your head does that means you get stretched apart like this especially near a low mass black hole near a massive black hole that horizon whose radius is proportional to the mass might be you know billions of kilometers instead of just three kilometers which is which is what it would be for a one solar mass black hole or thirty kilometers for a tensile or mass black hole my height is about two meters compared to 30 kilometers that might seem small but nevertheless it's significant and my feet would get pulled a lot more than my head but if I were near a supermassive black hole my two meter height is really quite insignificant compared to the billion kilometer radius of the event horizon of the black hole so this stretch effect is smaller but if you're near a black hole and you get closer and closer the stretching gets more and more and the official term that we get that we give to this process is spaghettification okay you get turned into a piece of human spaghetti so if you're a PhD thesis student and you want to study the environment of a black hole choose wisely choose a supermassive black hole like in the middle of our galaxy not a stellar mass black hole because from a tidal perspective a stellar mass black hole is much more dangerous than a supermassive black hole because you get tidally disrupted outside the that horizon of a stellar mass black hole whereas you don't necessarily get tidally disrupted depending on your distance in the environment of a supermassive black hole so so choose wisely now suppose you were to throw your mortal enemy toward a black hole what would happen so let's say your yota okay and you're throwing Darth Vader maybe I should have chosen Luke you know and he would say but Luke I'm your father and Luke sit round to hell with you anyway anyway so um you throw him into a black hole because you just you know you're trying to rid the galaxy in the universe of this terrible evil what would you actually see well first of all you would see Vader's clock slowed down that's a relativistic effect from your perspective it would appear that Vader never crosses this boundary the event horizon of the black hole because it would take an infinite amount of time from your perspective the only to my knowledge practical application today of Einstein's general theory of relativity special relativity and quantum mechanics has many practical applications but anyway all right so that's what you would see so you synchronize two clocks far from the black hole four hours might have passed you know near a black hole two hours might have passed and the closer you are to this so-called event horizon the slower is is the apparent running of the clock evader until he reaches the event horizon at which point from our perspective time his time comes to an end so you won't have the pleasure of seeing him cross the event horizon now suppose Vader goes close to the black hole and then turns on some really powerful retro rockets and escapes before crossing them at horizon Vader will come back not younger than he was when he first went to the black hole but younger relative Toyota or Luke who aged at a more rapid rate being far from the black hole so Luke and Yoda age more quickly than Vader aged when he was close to the black hole so this is a way of jumping into the future without aging very much now you don't read as many books and see as many movies you know as the people on earth did because in your frame of reference very little time went by but you propel yourself into the future and this is a physically legitimate form of time-travel there's nothing theoretical about what I've said that's wrong to our knowledge and indeed GPS clocks have verified this on a much much smaller scale the practical difficulty of getting close to a black hole and turning on some retro rockets so you don't get sucked in might be difficult but but in principle this form of trying travel into the future is is is a physical reality so if he escapes before crossing the horizon he will have aged less than you and your actual lifespan is not affected but this is a way of jumping into the future finally black holes aren't entirely black it turns out they can evaporate and you can learn about this in Hawking's book a rather dense book by the way I like to say that this is the most purchased but least read through completion book in the history of mankind and people buy it and they understand the first couple of chapters if they're lucky and then it really gets kind of hairy but you know you put it on your coffee table and visitors come for dinner and they say wow your read Hawking's book you know you must be really intelligent you know yes yes I understand all of it and in fact very few people do but in this book you can find his description of the evaporation and the basic idea is the following there's different ways of thinking about it but one idea is that in the vacuum of space even here in the air of this room though it has nothing to do with the air itself could be a vacuum particles and antiparticles flit into and out of existence all the time so they're just forming for a short time and then they disappear this is actually a consequence of the Heisenberg uncertainty principle and usually for example electrons and positrons just they appear and then a short time later they disappear and this can happen outside the black hole and it can happen inside the black hole or it can happen outside the black hole then both of them go into the black hole but occasionally the pair can form and one of the particles it doesn't matter whether it's the matter or the antimatter the electron or the positron can go into the black hole leaving the other one to escape and from our perspective from a distance if you look at the equations to us it looks like the one that went in went in with a negative energy a negative mass if this one goes in with a negative mass that decreases the mass of the black hole a little tiny bit and exactly compensates for the amount of mass carried by the other partner particle which can escape out to infinity this is a quantum mechanical evaporation process it's one of the first steps into our ultimate desire to unify the two great pillars of modern physics quantum physics on very small scales and general relativity on very large scales they work very very well in their domains of applicability but when you try to bring them together they're at war with one another and so string theory is one idea for you know melding these two theories together we have no fully self consistent theory of everything yet but generically it seems that any successful theory of this sort will have the possibility of black hole evaporation and that's really really cool that's a great triumph on Hawking spark we've never observed this process it's negligible for a stellar mass black hole or a supermassive black hole there accreting faster than they evaporate nevertheless in principle they are evaporating and if there were little little tiny black holes in existence shortly after the birth of the universe then right now they might be evaporating and producing bursts of gamma rays we've seen bursts of gamma rays gamma ray bursts unfortunately for Hawking they're quite obviously not evaporating black holes but we still think that the evaporation process may well be real now this is a pretty abstract concept I teach it to my introductory astronomy non-science majors at at Cal to help them enjoy and remember this idea I dress up as a black hole this happens to be the lecture the day of Halloween or the immediately preceding day if Halloween happens to be on a Saturday or a Thursday or a Tuesday because I teach Monday Wednesday Friday I have an alien who's being spaghettified by the black hole if I punch a little button there he says take me to your leader ha ha ha ha ha anyway many people think I look like the Unabomber those of you from a decade or so ago will remember who that was but anyway I don't mean to be a Unabomber I just have these glasses that I can turn on and off and they glow and so more to the point I have bags of celestial II themed can be attached to me Mars bars Milky Way bars orbit gum Eclipse gum very important to have celestial II themed candy and after having described this rather bizarre quantum mechanical evaporation process I then illustrate it by throwing the candy out to the crowd in other words I impersonate an evaporating black hole in this process students have come back to me 20 years later saying you know I didn't learn a darn thing in your class well I hope they don't tell that to me ah but the one thing I remember is that black holes can evaporate through this Hawking process and anyway it's negligible except for miniature black holes but but that's what what what we think happens now I've heard Hawking give lectures where he says well that since the material eventually comes out anyway you shouldn't even fear being gobbled up by a black hole but well I don't want to say he's crazy because these lectures are available to the general public and Hawking might watch it but I would say that that's a misrepresentation of what you might want to do if you value your health because you will be spaghettified on your way in you will temporarily be compressed to nearly infinite density in the singularity and once you evaporate you will just be a whole bunch of subatomic particles in photons you will not retain your existence you know as an defrag Nuria Alex Filippenko or whoever the information in the particles of which you consisted is still there in some weird way and Leonard Susskind talked about that I believe last year or two years ago in one of the Silicon Valley lectures but it's not exactly assembled in the same way so I would suggest that you read Hawking's book take most of what he has seriously but do not throw yourself into a black hole expecting to be evaporated away and retain your identity you won't okay really abandon hope forever all ye who enter here so as a summary I'll just leave this up physicists have said black holes could exist it's the ultimate victory of gravity over all other forces observational astronomers have now found that black holes do exist a big one in the middle of each galaxy nearly at least each galaxy with a bulge stellar mass ones and even some tentative evidence that I didn't have time to talk about but you know for so-called intermediate-mass black holes in the centres of certain kinds of globular clusters and stuff and black holes are detected indirectly through their gravitational influence on their surroundings and that's the take-home message one of the most bizarre predictions in all of theoretical physics has many many known examples in the universe and appears to be a rather common and if you want to learn more there's that course that unfortunately is not on sale right now it when it is on sale it will only be 40 bucks which for 12 lectures is a deal the course that is on sale is my giant one the biggest course that the teaching company has ever produced 96 lectures normally 800 bucks but through tomorrow oops I spelled Tim is tomorrow has two M's right sorry about that is no oh good all right a cosmic ray hit my brain it has two R's anyway through tomorrow it's on sale for just two hundred and thirty dollars and there is scattered through these lectures a lot of information on black holes where I talk about stars where I talk about galaxies where I talk about evaporation but it's sort of scattered around but there's lots of other astronomy as well okay so anyway um I hope there's time for questions and thank you for being so attentive in letting let me overstep my bounds we have time for questions and we invite people who have questions to line up it to two microphones right in the middle of the auditorium in front of the railing dr Filippenko has agreed to answer them we'd like to thank you in the meantime for a wonderfully illuminating lecture and we encourage all of you who do need to leave to take your sheets with you we don't want to leave things here in the auditorium so we'll wait a minute for the order for the people who need to leave to go and I encourage you if you have a question to line up in front of one of the two microphones and then if you'll just recognize one at a time we can go from there that's right sure I'll be glad to do that and those who want to make a graceful exit or welcome to do so now I by the way took a bit longer than I normally would in part because I know that this is a rather sophisticated audience I was right from the show of hands many if not most of you have been to previous lectures in the Silicon Valley lecture series and so you already know quite a bit of astronomy so I thought I'd tell you more than I tell say you know the Rotary Club during one of their breakfast or something like that okay let's most of the people are out now there's the noise level of sufficiently down so why don't I start with the gentleman over here on this side great what what is the smallest theoretically possible like least mass black hole okay so I'm I'm glad I don't have to repeat the questions right because that's the purpose of microphone so the smallest theoretical master up for a black hole is not known there could be processes early in the universe density fluctuations that produced rather small ones and let me go back two or three slides here if you have black holes that are about the mass of a mountain and Mount Everest is about 10 to the 15 grams or so those are the ones that should be in the process of evaporating in their final stages right now less massive ones would have already evaporated more massive ones aren't yet at the final burst and I forgot to say that this evaporation process accelerates and so the final stages are a burst B because we have not found any obvious evaporating black holes it suggests to quite a few of us that nature hasn't found a way even shortly after the Big Bang of actually compressing small density fluctuations into these little miniature black holes but there's nothing in principle that prevents this and in fact if we had a big enough vise what's your name Joe Joe we could squeeze you Joe and you would form a black hole now the the radius your radius if we were to make you a black hole would be comparable to the actual radius of a proton ten to the minus thirteen centimeters Wow yeah I you know proton is yay big and I exaggerate a lot okay so and and in fact the Large Hadron Collider might produce miniature black holes the exciting thing is is that under normal theory of gravitation the energy of the Large Hadron Collider is not sufficiently big to form a little black hole but if there are extra dimensions into which gravity can leak then it turns out to be easier to form a black hole on small scales because gravity is stronger than you thought it was so maybe they'll produce black holes and give evidence for extra dimensions so I would say astrophysically the smallest ones we know of are three solar masses but normal three solar mass stars will not typically form black holes they'll typically form white dwarfs and to get even a three solar mass black hole you have to start out with a initially much more massive star so great question then over there you mentioned that most if not all galaxies have a central black hole is there any theoretical consensus as to whether the stars come first to form the black hole or the black yeah do the stars come first or the you know it's a chicken or a egg you know we actually don't know we think it's a coevolution of the stars and the black hole but what we do know is that the highest redshift that is the most distant quasars we're seeing back when the universe was only seven eight nine percent of its present age some of those quasars are so luminous that they already have to have a several billion solar mass black hole there so either it formed first or when it was forming stars were also for me and we have we have some evidence that the first stars formed when the universe was only 300 million years old that's even farther back so so I think the answer to the question is there were some stars already but the big black holes were beginning to form and my best guess is that most of the star formation in the Bulge and most of the black hole growth were roughly contemporaneous but that's one of the big questions that's still outstanding yes over here if you have a binary system consisting of two black holes that are spiraling in on each other and eventually collide with each other could that be an observable event yeah so that's a very interesting question it won't be electromagnetically observable unless those black holes each have an accretion disk around them and there's some normal matter rubbing against other matter but if you have to bear black holes there will be no emission of electromagnetic radiation what there will be is the copious emission of gravity waves ripples in the actual fabric the shape of space-time and physicists are now trying to find gravity waves they've built a number of detectors LIGO the laser interferometric gravitational wave observatory and it hasn't detected anything yet but now super an advanced LIGO are in the final stages of completion and we expect that if there are you know merging black holes in the next say ten years right there's no guarantee that there will be one but if there is one we expect to detect it and that'll be the opening up of a brand-new window on the universe gravitational waves which are distinctly different from any form of electromagnetic radiation and that'll be a nobel prize for sure and an opening more importantly of a new area of astrophysics gravitational wave astronomy over there that her your your colleague dr. Michio Kaku is in town a few weeks ago my colleague who dr. Michio Kaku who okok cocky yeah yeah yeah so he's at the State University of New York City University oh yeah so yeah and he was he was talking about his book physics of the impossible and one things he's talking about was wormholes and time travel through wormholes yeah not time travel to travel aren't wormholes somehow connected or tied with black holes or yeah so related if you look at the mathematics of general relativity the black hole appears to be connected with another black hole either in our universe elsewhere or perhaps even connected with a different bubble of you know space which we call other universes I mean you know when I was a kid the joke was defined universe and give three examples now we actually think that there may well be other universes okay well the connecting so called einstein-rosen bridge is popularly called a wormhole and and I describe this in considerable detail by the way in this course not to put in any more plugs for it but I do it appears as though you could traverse it especially in the case of a rotating black hole in the case of a non rotating one but the wormhole only exists for a split second of time so you actually can't traverse it but in the case of a rotating black hole mathematically it looks like you can traverse it and this idea has been used in Sagan's you know novel contact and things like that right the problem is that in any real black hole the equations are not the idealized equations that give rise to that mathematical solution the equations rather suggest that either the black hole the wormhole will scrunch down and you need some sort of exotic anti gravitating material to keep the throat open or as my colleague at the University of Colorado Andrew Hamilton has shown there's or helped show a number of people work on this problem there's something called a mass inflation instability where a whole bunch of material and radiation collects up at the inner of the two event horizons it turns out rotating black holes have two not one event horizon and near that inner event horizon a whole bunch of radiation builds up and it fries anything that happens to traverse that region so you get cooked you you don't actually make it through alive and moreover if there were won't wormholes that are traversable and are being traversed then why haven't we ever found in the universe what's called a white hole that is a region of space from which matter gushes but no matter can enter that's the opposite of a black hole into which stuff can go but no matter can exit quasars briefly when they were first found were thought you know to be maybe white holes but then the tailed studies of them showed quickly that they are not white holes so we've you know we've explored much of the universe and these things shouldn't be subtle lots of radiation and matter should be gushing out and we've never found anything like it so I think that basically says that that these wormholes are not reversible or or perhaps even they don't exist they're just a mathematical idealized solution but true nature is more messy than that but it is an interesting idea the trouble with the wormholes is that if they're truly traversable then you run into the problem of causality that is you could come back to a point in time before you were even born and prevent your parents from ever meeting for example I won't say kill them that's the usual thing kill your grandfather but let's say let's be kinder more gentler nation and just say that we're going to prevent your grandparents or parents from ever meeting and so you would never have been born so then how could you make the trip which allowed you to prevent your parents from meeting right these violations of causality are taken very seriously by physicists and the traversable wormholes inexorably lead you to these kinds of difficulties great question yeah I'm the guy that threw Darth Vader into the black you threw Darth Vader II okay and I was really very disappointed that he didn't go in he just stayed there sorry so that frustrated me and I got the next baddest guy and I threw him in yeah and and then he got stuck that's right they all got stuck this this this is not just frustrated but I got kind of mad and so then I started getting all the bad guys I could find and there are a lot of them and I threw them in one at a time pretty soon aren't I going to have kind of a mess they're in the black hole so everything from our perspective that has ever tried to fall into a black hole including the star that collapsed to form the black hole has never made it inside everything from our perspective in this infinitesimally thin membrane and indeed Kip Thorne at Caltech developed a mathematical formalism known as the membrane paradigm and I don't need to tell you that I discussed it in this set of lectures okay ah so it's this thin membrane okay which contains all of the material that ever got thrown into a black hole and in a sense this allows you from our perspective to even understand the evaporation because as leonard susskind discussed what's really evaporating is all this really hot material on the boundary of the black hole it was never even really inside from our perspective so it didn't even need to really escape yeah so it's just a mess it's a hot mess in an infinitesimally thin membrane just above a Planck length above the event horizon yep question over there question with respect to the slide in which you explained evaporation of a black hole my question is if a black hole is perfectly symmetric would it ever evaporate because by virtue of your definition I figured just think a little bit more loudly I know by rote you have audio described on the evaporation of the black hole slide you said that if for suppose I'm saying an antimatter or a matter enters the black hole right it's sort of tilts the balance and in its favor but yeah that slide there if the black hole were perfectly symmetric then you should expect as much antimatter turned her yeah yeah okay so so this is something I didn't explain quite clearly enough and I thank you for asking the question first of all let me tell my standard joke about matter and antimatter it doesn't matter which we when we called matter or antimatter we could have called the antimatter matter and then the matter would have been antimatter and it doesn't matter they're perfectly symmetric okay now from our perspective there was this matter antimatter asymmetry and for every billion proton antiproton pairs early in the universe there was one extra proton and that's one of the magical things that led to our existence okay I say magical not because I believe in well I mean you know what I mean it just it's one of the one of the sentence of nature that that led to our existence okay however it doesn't matter which one you call matter and anti-matter regardless of whether the positron goes in or the electron goes in it's in either case from our perspective if you look at the equations stuff going in with negative mass negative energy indeed even when you form pairs both of them have positive mass positive energy it's not that the positron has a negative mass it has all the same properties as the electron including mass except for as its charge and there might be some quantum mechanical spin that might be opposite to I'm not sure but the mass is definitely positive okay so these are both positive masses and regardless of which one goes in it appears as a negative mass from our perspective and that goes back to it's related to the fact that mathematically the spatial and temporal coordinates space and time from our perspective mathematically reverse their meaning inside a black hole not that we can see inside a black hole but from the mathematics that you write down of a black hole what we think of X and T outside the black hole become T and X inside and that's in a subtle way related to this issue that from our perspective it goes in with negative mass but if you don't like that perspective and I admit it's kind of weird sounding then just go back to my membrane paradigm nothing from our perspective has ever across the horizon and so you've got a hot object there and hot objects radiate particles in light you can think of the black hole evaporation in those terms if you don't like this another way to think about it is what's called quantum mechanical tunneling there's a particle here and it tunnels its way out that's similar to radioactive decay you might have a uranium nucleus where it's it's it's largely stable but a particle inside has this energy barrier that it can't quite hop over but because it has this wave function scribing it part of the wavefunction penetrates this energy barrier and the particle can suddenly find itself again magically but it's quantum mechanics outside of the nucleus okay yeah cool yeah we have to announce that only the people standing now ah well why don't you are not sure so I'm sorry but because of all the volume of questions we have to say that only the people standing now are going to get a chance to ask questions in public because we have to be respectful of the ending time of the program so if you're standing now stay in line and you can ask your question but beyond that we're not gonna have any more right okay that sounds good and in part my answers are long because every single question has been really quite thoughtful and interesting so far yes is there a way to predict when a black hole will explode and have we seen any that actually did that yeah so we've never seen a black hole explode otherwise you know Hawking would have would have would have received a Nobel Prize by now the only way we know of in which they can theoretically explode is through this evaporation process which accelerates near the end so the smaller and smaller the mass the red greater is the rate of evaporation until finally you have a truly powerful burst of gamma rays but not any of the known gamma ray bursts if you know what I mean okay but there's no other way we know that a black hole can disintegrate so they don't explode the way stars do and there's no way to predict it unless you know the mass if you know the mass you can calculate the evaporation rate but then you have to also take into account the rate at which the black hole is swallowing surrounding material so you have to have a very good census of what's near it and how close is it and what orbits does it have so we have no way of right now of predicting when any particular black hole will will evaporate but all the stellar-mass black holes and supermassive black holes won't evaporate for a minimum of 10 to the 60th or 10 to the 80th power years I think it's 10 to the 80th power for stellar-mass black holes and 10 to the 100 power so a Google for supermassive black holes yeah over there now I was learning if there is a way in which black holes conform with dark matter and is there any way to that is there anyone who has theorized on this obviously it might be difficult to observe it yeah that's a great question black holes forming out of dark matter so first the simple answer in a sense black holes are a form of dark matter right they are a percentage of the dark matter we know that's out there they are objects that gravitate Yetter invisible or very difficult to see okay so they are a form of dark matter they're not the major form of dark matter the major form of dark matter is thought to be weakly interacting massive particles wimps and they're little particles leftover from the earliest moments of the of the Big Bang it's a little bit annoying that we haven't yet definitively detected a wimp but they are thought by most physicists to exist but because they are weakly interacting unlike gas which can collide and then radiates its energy through electromagnetic radiation the weakly interacting particles rarely collide and have no way to dissipate their energy so the weakly interacting particles form an extended halo around our galaxy they go out much farther than the stars and gas in our galaxy I mean there's some even in this room but mostly they're in this big halo and there's no way to for them to dissipate to get rid of their energy so there's no way for them to clump down into a small enough volume to ever collapse to form a black hole so the so-called nonbaryonic dark matter won't form black holes but baryons protons and neutrons in the form of massive stars cannon do form black holes and become part of the celebrated dark matter of the universe great question yes hi there my original question was about wormholes but your comment on have any waves brought something else up so I read just recently the novel rendezvous with Rama and about the ship moving without using rockets so to speak right so in terms of gravity waves is it a wood is there a way once this field opens up to be able to use gravity waves for humans to yeah so probably not because the gravity wave is a ripple going through space created by motion of two closely orbiting stars or emerging white you know black holes or something like that right so the creatures you know light-years away can't capture that gravity wave it's just a ripple moving through space so as far as I can tell the answer would be no but maybe I'm just old and conservative and don't have a very vivid imagination you know it's sort of like some people postulate that the dark energy which is accelerating the expansion of the universe in which I've discussed at a previous lecture here some years ago some people think it can be utilized to Harn you know to to effectively achieve warp drive and I just filmed with the History Channel an episode for season five of the universe okay and unfortunately they you know well I mean I think they're hopefully they will voice enough caveats to this but they decided to include this possibility that dark energy could be somehow harnessed to propel objects at speeds faster than the speed of light not that they're moving through space faster than light but what you do is you end up compressing space in front of it and expanding space behind it if you don't do that to space it's okay but that effectively makes this you know Alcubierre you know type drive that zips you along and and and and that's it's completely fictional right now so you know Kaku likes these kinds of things and you know it's great it's it's great to think about these things and to consider the extremes of physics because this is how we can learn right by considering physics in the most extreme environments and I completely applaud that but I would not applaud making it seem to the general public that we have any shred of evidence that this kind of thing can be achieved right now or any time in the future and I have very little control of what they actually end up saying in these public TV programs and I just hope that they'll use the quotes where I said this is an interesting idea but it almost certainly can never be made to work and similarly harnessing gravity waves with my limited brainpower I don't see how that can be but I'm not saying it's completely impossible but nor would I want to give the impression that this is just around the corner and probably Kaku didn't give that impression I hope he didn't and I like I say I applaud considering the physics of these things as long as you don't mislead people into thinking that this is you know right around the corner or wormhole travel or things like that ok ok I'm trying to be very careful here and what I said yeah ok question over there yes in the far future we would be able to harness energy from black holes in the far future will we be able to harness energy from black holes ok so that's something that actually can be done it doesn't present the theoretical impossibility or high improbability of some of these things that I was just now addressing which Kaku covered in his book the physics of the impossible what he really means it's not completely impossible it's just highly improbable the the tapping of the energy of a black hole we've actually seen nature do even though we haven't done it yet the accretion disk around a spinning black hole actually taps some of the rotational energy of the black hole and in fact we've seen this because some accretion discs we think appear anomalously bright because in fact they're tapping some of the energy of the rotating black hole so Nature has done this unlike for example creating a warp driver traversal of a wormhole we actually have some evidence that Nature has done this even if we're wrong in those particular cases there's no new reason nature shouldn't do this in some cases will we ever do this you know fat chance but but it's nowhere near as improbable as these other things I was just discussing like traversing a wormhole in other words there's nothing that's physically against the the laws of physics right now unlike say the violation of causality which would be a real problem okay if I went back in time and killed my parents now in my class and in my various lectures I discussed the possibility of an advanced civilization that has two problems they have a lot of garbage and they don't have enough energy so here's how they solve their problem they park themselves around a rotating black hole they send garbage trucks into this region where space-time is being dragged at an incredible rate it's called the Ergo sphere of a black hole they dump their garbage at precisely the right moment that then gives a kick to the garbage truck because the material the garbage got dumped into the black hole at precisely the right moment and the truck itself then gets ejected from the Ergo sphere with the greater kinetic energy than it had coming in the trucks then go heading back to the civilization having been flung this way kind of like the gravitational effect that you know has been used in propelling Voyager and other spacecraft to Saturn this gravitational that's called a slingshot effect but that's a bad term it's also called what's called gravity assist yeah thank you um so then these these garbage trucks hit a garbage truck mill this is very much like a windmill or or a water turbine but garbage trucks hit these blades they make them rotate what you've got is a circuit some coiled wire inside a magnetic field that causes a current to flow through the wire and that lights up your light bulbs so they've solved their energy problem and their garbage problem simultaneously okay now you don't want to throw too much into the black hole without moving your civilization comfortably farther out right because otherwise it will envelop you okay here then yeah yes young man well if you just jump into the black hole just when it's turned into gas if you talk if you jump into the black hole just at the last stages of evaporation is that what you mean yeah it wouldn't be a good idea ah because you would be subjected to enormous ly strong high-energy particles and waves gamma rays and other electromagnetic radiation that would fry you young man so don't do it not that there's much danger because we've never found a microscopic evaporating you know black hole but but don't do it even if you find one thank you for the question question over there now another young man good I love to see this um well what about white holes and could they be feeding on well if black holes suck things in maybe they're launching it out of you know all universes but into this strange sort of a place outside all universes and that's where white holes feed on the things that that are out there and they just so that's a very good idea you caught what I was saying that stuff going into a black hole at least theoretically might traverse the wormhole and come out of a white hole so what you're saying is suppose all the white holes are in other universes and that's why we don't see any of them but by the same token then wouldn't we expect black holes in those universes to have wormholes connecting up with ours and spewing matter out of white holes within our universe you'd have to postulate a very special universe our universe that only has black holes connecting up to other universes but no black holes in those other universes connecting up to ours and that just seems improbable yeah well what if there is just you know like white hoes just way way out there there were a lot of detective a right there there ejecting all the stuff that the black hole at the other end of you know ever accreted ever swallowed and you know from the materials perspective that's falling in by the way it does cross the event horizon clocks are running at the normal rate blah blah blah so these things should be very bright they shouldn't be subtle they should be extremely obvious and with today's powerful arsenal of telescopes most of us think that bright things all the bright things the universe of have pretty much been discovered so that's you know it's not impossible but we think it's improbable yes this with the two black holes forming but what if two black holes are a black hole formed and started sucking in more stars to get the energy like the neutral stars and it formed with another black hole is it possible that it could just keep growing or is there like any limit to it yeah but black holes can definitely keep growing in fact the black hole in the middle of every galaxy that's active is growing growing at a rate of you know maybe a solar mass per year now if it's already a billion solar masses that means that it'll take another billion years for it to double its mass nevertheless that's not you know I mean the universe is 10 billion 14 billion years old so there's that's how we think black holes grew from small humble beginnings in the middle of little galaxies to bigger and bigger things as more and more material got accreted forming a bigger and bigger bulge of these galaxies so they definitely grow so the possible that they could just keep on sucking more and more yeah so then you know are we then in any danger a nice comfortable twenty seven thousand light years away from the center of the Milky Way we're not in it in any great danger we're in a stable orbit around the center of the Milky Way we're very very far away the event horizon of the black hole in the middle of our galaxy is only twelve million kilometers it's three kilometers for every solar mass and I it's four million solar masses so so twelve million kilometres sounds like a lot but that's a less than a tenth of the distance from the earth to the Sun so the black hole in the middle of our galaxy is small even if there happened to be a gravitational interaction of our Sun and our solar system with another star gravitationally flinging our solar system toward the general vicinity of the center of our galaxy you'd have to have pretty good aim for us to be aimed sufficiently precisely at this physically very small object a black hole is very small okay they're easy to miss so so you know it's not impossible that our solar system will someday be eaten but it's highly improbable and it's one of these things that the TV shows want us to play up that we might get eaten by a black hole but it's a it's a bit of a it's a bit unfortunate because people then think that there were in grave danger of being eaten by a black hole and we're not it's not a physical impossibility but there are many other things to worry about that are much more germane to your everyday life question over there if what what would it be like if a black hole waters were just sucking the earth like would the earth be specific yeah the earth would be spaghettified if earth were going into a stellar mass black hole if it were being sucked into a supermassive black hole then the earth would get spaghettified eventually but not outside the that horizon not outside the boundary so it's just a matter of time when do you want to get spaghettified if you really do just cert decide to make a journey into a black hole you're going to get spaghettified eventually if you choose to remain outside the black hole and not get spaghettified then choose a supermassive black hole where the tidal forces are much much less than those near a stellar mass black all but yes the earth would get ripped apart everything gets ripped apart or including all all humans living yeah yeah living tissue would easily get ripped apart much more easily than an iron you know rot or something like that because then you have to get into the complicated stuff you know strength of materials different materials have different strengths and it's much easier to pull a human apart than it is to pull an iron rod apart and so I actually I must admit have not done the calculation of whether an iron rod would get pulled apart outside the horizon of a tensile or mass black hole an iron rod might survive but you living tissue and flesh would not survive if you're studying the black hole and you want to write a PhD thesis about it the iron rod would eventually get destroyed if it goes inside everything even kryptonite if it were to exist would get destroyed inside the black hole but outside the event horizon it depends on the size of the black hole and on the strength of the materials if you're the consistency of souffle you get ripped apart much more easily than if you're an iron rod question-there yeah okay so um I don't know if like you are you're talking about a guy or their gang spaghettified and um I wasn't even listening but I started listening then but uh it rebelled yeah so if you got into a black hole and you don't get ripped apart like so you just don't you don't just get ripped apart what if um you stay in there can you stay yeah so that's a good question suppose you don't get ripped apart your brain cells don't all just disintegrate and all that can you stay there arbitrarily long the answer is no no matter how powerful your rockets and no matter which direction you aim them in you can only prolong your life by a certain maximum amount and it's a very small amount now you can take bad trajectories that make you die in the singularity more quickly but even the best possible trajectory leads to a very very short life and I believe that for the supermassive black hole in the middle of our galaxy it's of order one minute and for stellar-mass black holes it's it's a small fraction of a second so you have a maximum time to live regardless of your trajectory and then even if you were to survive or not get spaghettified you would then get crushed into this singularity a point in classical general relativity which has infinite density because by that I mean a finite amount of matter gets compressed into a mathematical point now a mathematical point is a point of zero volume so some nonzero amount of matter divided by zero volume is an infinite density this is where we think classical general relativity breaks down whenever yeah whenever we've considered very small things in the history of physics especially in 20th century physics that's what it was all about we find that things are not point like even particles aren't point like they have some fuzziness to have some size that's where quantum mechanics comes in so presumably when we develop finally eventually a truly self-consistent quantum theory of gravity be it string theory or quantum loop gravity or some other thing we will find that the singularity is very very small very very dense but not infinitesimally small and not infinitely dense but listen between you and me whether I'm compressed to infinite density or for all practical purposes infinite density hardly matters and that'll happen within a very short finite amount of time once you enter that event horizon question over there Oh more young ones that's great is there anything behind a black hole oh cool so a black hole especially a non-rotating one it's basically a spherical object you look at it from any direction it looks spherical the bending of space that I tried to illustrate near the beginning of my lecture is a bending into a fourth spatial dimension which has mathematical reality you can write equations that that have this dimension as part of the equations but it's physically inaccessible so it bends off into some direction and we can't see it it's like a balloon think of your universe being a two-dimensional balloon just the surface you can wander around on the surface and let's say the laws of physics prevent you from going into the balloon right inside but I could I could take my finger and I could push on the balloon and make it Bend into the inside but the creatures on the surface constrained by the laws of physics to only physically access the surface they can only mathematically conceive of this other dimension okay but that's a dimension into which a black hole curved space so really there's there's nothing behind a black hole in other words just take a spherical ball and you know whatever you are there's something behind it but it's nothing special right I mean if there were a black hole between me and you you would be behind the black hole from my perspective and I would be the black hole from your perspective now you may be a fine person and I think that I'm an okay guy but it's not a particularly exciting thing when you're behind the black hole from my perspective and I'm the behind the black hole from your perspective so that's the sense in which I say that there's nothing special behind a black hole you see does that make some sense cool ok and then the other young one there good most people dry black holes as a circle and are they a circle or a sphere it's a sphere so we draw them as circles just because of the confines of the two-dimensional plane of the blackboard or the you know projector screen or whatever but yeah they're they're spherical regions for a non rotating black hole for sure and even a rotating black hole is is a sphere but around it has this weird kind of elliptical torus shaped region the Ergo sphere from which you can tap the energy ok anyway yeah there's fears and the bending occur to a different dimension and we just draw them as circles for simplicity cool and then a question right there I believe you might be the last one okay you've been talking about leaving about two which to me seemed contradictory ideas on the one hand black big black holes like at the center of our galaxies I have been growing in size as they acquire several million solar masses and and I guess an even bigger galaxies even billions of solar masses and so there are event horizons have been growing in size but on the other hand you said that things falling into the black hole gets stuck just outside the event horizon right so if those things can't pass through the event horizon can't the event horizon grow past them or somehow the event horizon pushing things on its skin outward yeah I see you mean good good question this is the essence of relativity people in and creatures in different frames of reference see nature unfolding in different ways and they're both right and you see this even in special relativity you know a particle zipping along through your lab frame that normally sitting on a desk would decay into other particles it lasts a long time and you say yes it lasts a long time before decaying because it's clocks are running slowly the particle itself from its own frame of reference says no I don't last a long time your lab is incredibly short and that's how I got from one side of the lab to the other whereas I and my frame of reference say that it got from one side to the other by by living a long time so so in the case of the black holes from our perspective outside the black hole nothing ever crosses from the perspective of the infalling material that's a different frame of reference it can and does cross the horizon so then going back to your conundrum what about the membrane well as stuff accretes on to this membrane the membrane radius grows it just expands like a balloon you're going like that and it just grows and it never passes up the material in other words it continually pushes out the material that that constitutes the membrane star is falling into our mass button that's right from our perspective and you lost the microphone there but what the gentleman said was that from our perspective the material of which the stars that are being swallowed by the black hole consists which I claim is on this membrane as the black hole is growing that material is being pushed out to greater and greater radii that's exactly right that's what's happening in our frame of reference yeah and it's it's hard to bend one's mind around these ideas of relativity but to the degree that we've measured experimental effects at for example low speeds when testing special relativity and this has been done with clocks and airplanes and stuff clocks run more slowly when they're traveling at a high speed relative to us and to the degree that we've measured the effects of general relativity in weak gravitational fields example GPS stronger gravitational field there are things called binary neutron stars pulsars which orbit one another and they're emitting gravity waves and that changes their orbit and we've seen changes in those orbits that are exactly matching up with the predictions of the equations to that degree we've tested relativity and though we have not tested it in the what's called the strong field approximation of of black holes we have no reason to doubt it based on what we've tested so far but we do not know it to be absolutely true in science you never know anything to be absolutely true the way you do in a mathematical proof all you have is a progressively better more complete this that accounts for things that are seen and correctly predicts so far unobserved phenomena which then experimentalists in observers try to find and when you do confirm the predictions that's another feather in your hat but you still don't know whether this is ultimate reality it's just our best description of what we consider to be reality okay well you've been given great thank you very much and thank you as I knew would be the case the the questions were really outstanding so congratulate yourselves on coming so with the answer yeah

Channel: SVAstronomyLectures

Views: 96,389

Rating: 4.522059 out of 5

Keywords: Astronomy, space, cosmos, black holes, physics, astrophysics, Einstein, supermassive black holes, general relativity, general theory of relativity, space warps, time machines, time and relativity, science, science fiction, event horizon, relativity, cosmic physics, Alex Filippenko, astronomer, Filippenko, gravity, stars, stellar evolution

Id: 4tiAOldypLk

Length: 116min 12sec (6972 seconds)

Published: Wed Apr 10 2013