cc  >> Welcome.  I'm Nicki Denison with the  Wisconsin Alumni Association.  It is my pleasure to introduce  our speaker here today.  Dr. Sebastian Heinz is an  assistant professor in the  Astronomy Department here  at UW Madison.  And his research focuses on the  observable properties of black  holes and the impact growing  black holes have an their  environment and the universe at  large.  He's currently working with his  grad students and postdocs on  the interaction of relativistic  jets black holes with their  environment and on the analysis  of x-ray emissions on black  holes and neutron stars.  Before joining the faculty here,  Dr. Heinz was a postdoc at MIT  working in the Kavli Institute  for Astrophysics and Space  Research and a postdoc at the  Max Planck Institute for  Astrophysics in -- Germany.  He received his Ph.D.  in 2000 at the Astrophysics and  Planetary Science Department at  the University of Colorado,  Boulder.  Please welcome  Dr. Sebastian Heinz. 

 [APPLAUSE]  >> Thank you, very much  for the introduction.  I hope you can all hear me.  I will talk to you today about  the current understanding and  the current efforts of observing  and studying black holes as we  can observe them in the  universe.  This is distinct from the  mathematical studies  of black holes as constructs  of general--  This is really more of an  observational science.  Nonetheless, as everything in  astronomy, the objects we're  studying are still millions to  billions light years away from  where we are.  Which makes them somewhat  abstract.  I'll try to bridge that gap a  little bit and introduce you to  what the current status of this  science is and some of the  things that we're doing here at  Wisconsin.  So let me give you a brief  outline of what I'll be talking  about.  I'll talk about briefly what the  classical concept of a black  hole was when it was first  thought of in the 17th and 18th  century when people just  realized that light actually had  a particle characteristic.  You could describe light as  particles. 

 Then I will describe the modern  ideas of what we think black  holes are that go back to  Einstein.  And I'll talk about using  gravity, the gravitational  force, to power the universe,  and then I'll show some examples  of how we actually find black  holes.  And I'll conclude, hopefully,  with a brief introduction as to  what we're actually studying  directly, these so-called jets  and black holes.  And just as a reminder and as a  notice, I will leave a signup  sheet here.  We're planning electro series in  spring 2009 at UW Space Place  which is the public outreach arm  of the Astronomy Department that  is specifically geared towards  making black holes and gravity  understandable for a broad  audience.  So if you're interested in that,  please come talk to me  afterwards.  Okay, so let me begin with a  brief description of what the  first ideas were that people had  about black holes.  So Newton predicts that massive  objects have an escape speed.  What does that mean?  If you throw up a ball, if I  were to throw up my remote  control here it would usually  fall back down.  This is an experience you've  probably all had before in your  life. 

 This is something you can relate  to.  Now if you throw the ball higher  it will take longer for the ball  to fall back down, or for my  remote.  But the question is, what  happens if you throw the ball  really, really hard?  We all know that if you have a  very powerful engine strapped to  a rocket then that rocket can  fly into space and never come  back.  So there it goes very slowly.  [LAUGHTER]  So if you throw the ball  fast enough it will just keep  going and it will not come back.  And if you throw the ball faster  than what's called the escape  speed it will just do that.  And the escape speed for the  earth is about 25,000 miles per  hour.  So if you throw the ball faster  than that it will just keep  going.  Now the escape speed is  different from object to object.  It depends on the mass of the  object and it depends on the  size of the object. 

 If you recall physics, probably  high school physics, Newton's  Law of Gravity tells you a more  massive object has a more  powerful gravitational force.  And it's a stronger pull.  The more mass, the more  gravitational pull.  So you would imagine that a more  massive object would take a  faster speed for that object to  keep going to infinity and not  come back.  At the same time, if you make  the object smaller it also has a  stronger gravitational pull.  As you know, the gravitational  force on an object goes up if  you go closer and closer to the  object.  That's why we don't feel much of  the force of the moon on earth.  But when you're standing on the  moon you're actually standing on  the moon and being bound by the  gravity of the moon.  So making an object very small  or making an object very massive  means that you're increasing the  escape speed, and it's harder  for you to throw the object so  fast that it never comes back.  For the moon the escape speed is  smaller than for the earth  because it's less massive. 

 It's only 6,000 miles per hour.  And for the sun the escape speed  is very fast.  It's 1.4 million miles per hour.  So even if you were to throw  your object fast enough to  escape from the earth, it might  not escape from the sun.  So taking, putting that together  with a classical concept of  light that emerged in the 17th  century, people were able to  derive basically, to predict the  existence of black holes.  So Remer, in 1676, used  Jupiter's moon as a clock to  time the speed of light.  And he measured the speed of  light to the finite.  The speed of light is about  300,000 kilometers per second.  Very fast.  And Newton realized that light  can be described as a particle,  and, if that is the case, you  would assume that that particle  has a mass and that particle  mass feels the force of gravity  so if light travels at a speed  of 300,000 kilometers per  second, it has a mass, it feels  the force of gravity, you would  imagine that if you make an  object massive enough and small  enough its escape speed would  become so large that a light  particle would not be able to  escape it. 

 Laplace, among others, realized  this in the 18th century  and predicted that if you make  such a small heavy object,  light would be bound  and it could not escape.  And if it holds on to all the  light that would usually be  escaping from such an object, it  would make the object dark.  And such an object would be a  black hole.  So that was the classical  concept of a black hole.  But, alas, people quickly  realized that that idea didn't  hold much sway.  So, first of all, such a black  hole is quite different from  what we envision a black hole to  be today.  First of all, because back then  people didn't realize that the  speed of light is the fastest  you can go.  And so if you throw something  with twice the speed of light  from an object that would just  be a black hole with an escape  speed for the speed of light,  you could still throw that  object and it would never  return.  Furthermore, a light particle  could almost travel to infinity  and then it would fall back down  to the black hole, and so in  principle you could see that  black hole if you were very far  away.  But Maxwell really realizing  that we can describe  electromagnetism and light as a  theory of continua and waves,  you can describe lights as  waves, as massless particles. 

 Realized that light particles,  or photons as we call them, have  no mass and as such probably  would not feel the force of  gravity.  There was no consistent way to  describe the photon that would  feel the force of gravity when  Maxwell came up with this theory  of electromagnetism.  So the idea of a black hole  disappeared until a little  more recently.  So in order to arrive at the  modern picture of a black hole,  we have to start with the year  1905.  Which is really a miraculous  year for physics in which  Einstein published four  fundamental papers.  One of these fundamental papers,  which you've probably heard  about, was the paper that  derived the Theory of Special  Relativity.  So in that theory, Einstein  realized that space and time are  interwoven.  They make a fabric.  They cannot be separated from  each other.  You can't look at just time or  just space separate from each  other. 

 They interfere with each other.  And so he came up with the idea  that space and time are  interwoven and he coupled that  with the idea that nothing goes  faster than the speed of light.  If you make that postulate which  was experimentally proven, and  you make the assumption that all  the laws of nature should be the  same no matter which frame  you're in.  Whether you're traveling or  whether you're stopping, that  all the laws of nature should be  the same for every observer in  the universe.  Which is a very basic  assumption.  You want the universe to be  beautiful.  You want the same laws to apply  to everybody.  It's really a statement of  equality.  Everybody is subject to the same  laws of nature.  If you couple those things  together you get the Theory of  Special Relativity. 

 But in 1905, Einstein could not  yet marry this Theory of Special  Relativity to gravity.  He did not have a theory, a  self-consistent theory, for how  to make relativity and gravity  work together.  So it took him 10 years and a  couple of moves between  different countries to get to  the point where he could  actually come up with this  theory of gravity that describes  everything.  So Einstein realized that  gravity is a force and a force  causes an acceleration.  So the first postulate he said  was well, acceleration and  feeling a force are really the  same thing.  So you all feel the acceleration  of gravity but you also feel a  force of your chair that's  holding you up where you are  right now.  If you were to take that chair  away, we would all fall and that  would be very unpleasant.  But luckily you have the chair  in the way, but you still feel  the force of gravity so that  means you're experiencing an  acceleration.  And so Einstein made that  comparison between somebody who  is accelerated and somebody who  is feeling a force.  So he could describe gravity  basically just as an  acceleration. 

 And that made it a lot easier to  combine it with this Theory of  Special Relativity.  The next thing he realized is  that if you are an accelerated  observer, if you were sitting in  an elevator that has its cable  cut loose, and is just dropping  down very rapidly and it's  accelerating.  The longer you wait, the faster  you go.  A light beam that's entering  your accelerated elevator will  follow a curved path from where  you are.  At first it will go in straight  but at the end when you're  falling very fast, the light  will be deflected upward just  because your speed has to be  superimposed on the speed of  light.  So to you it looks like the  photon of the light path is  curved.  So that's what Einstein realized  and he said if gravity is equal  to acceleration and acceleration  can curve a light path then  gravity should be able to curve  a light path.  So that was his fundamental  insight.  And the last step he had to take  was to realize that light always  travels on the straightest path  possible.  If I were to shine my laser beam  at the end of the room,  back there which you can see  if you turn around, it goes in  a straight path. 

 That's the straightest path  possible.  But the straightest path  possible is not always a  straight path, right?  And for example, in the  elevator, the straightest path  possible is curved.  And that curvature is due to the  acceleration of the elevator.  But in principle it could just  as well be due to the affect of  gravity.  So he realized that when you add  gravity to the picture, you have  to let go of the idea of a space  that is completely flat where  all lines can be straight.  So Einstein realized that  gravity can curve light and it  can curve space itself.  So that is the Theory of General  Relativity that describes how  gravity curves space-time.  Just like a falling elevator.  I recommend that you shouldn't  try that experiment at home but  you can try to envision it.  So how does that work?  And how does that lead to the  concept of a black hole?  Well, we just said mass curves  space. 

 So I don't if you can see this,  here is a little piece of space  fabric, space-time fabric that's  flat.  There's no mass here whatsoever  and there's a straight line  right through it.  If you add a little bit of mass,  it will curve that space-time  fabric.  Can you see?  And the light path is no longer  the straight line.  It actually has to follow that  fabric.  It has to stay on the fabric of  space-time.  So it's curved.  You see that light beam is  following the curvature of  space-time.  Mass curves space, that's what  Einstein says.  Now if we have more mass it  should curve space more, right?  Just like more gravity causes  more acceleration. 

 And so the question is how much  is too much?  Can we add so much mass that  something fundamental happens to  space-time.  And the fact is we can.  Just to illustrate to you that  this isn't just a theorist  concept of how the universe  works and some crazy idea about  gravity and space-time that's  floating around in somebody's  head, this is an observation of  gravity curving space-time.  And what you see here is a bunch  of fuzzy objects.  These were taken with the Hubble  Space Telescope which is  currently waiting for repair.  But you see here a number of  galaxies.  You see these fuzzy blobs here,  these are all galaxies.  And these galaxies together form  a galaxy cluster.  And a galaxy cluster is the most  massive bound object that we  know in the universe.  So this is a really massive  object and, based on what I just  said, it should curve space-time  a lot. 

 And it does.  What you also see here are these  little arcs here.  There are some arcs here which  probably are very difficult to  see because the light is not  turned down all the way.  But there is a series of  arc-like rings that you see  around these galaxies.  And these arc-like rings are  galaxies far, far in the  background.  Way behind this galaxy cluster.  Twice as far away as this galaxy  cluster.  And the galaxy cluster is half  the way across the universe  almost.  And we see that being stretched  out and turned into rings  because the light has to travel  around this galaxy cluster and  as it does that the galaxies get  stretched out.  And we can perfectly well  describe these rings as the  effect of the mass of the galaxy  cluster on curving space-time. 

 So we can actually observe it.  So what happens when you add too  much mass?  Something's got to go.  So after Einstein published his  Theory of Relativity in 1915  Karl Schwarzschild, in the same  year, published a paper that  presented the solution of a  stellar object, a round object  to Einstein's equations.  And so he basically found the  solution for a star.  It works fine for normal stars  but when you add a lot of mass  or you make the star very, very  small, things go wrong in a  certain way.  And, as I just said, this if you  think back to this little ripple  in curved space-time, if you  make that object deep enough, at  some point the well that is  being imposed on space-time here  is vertical.  The lines become vertical at a  certain point.  And when the lines become  vertical, the light will keep  going down forever and it will  never come back out.  So if you curve space-time so  much that the light just keeps  going down into that well, into  that hole that's being dragged  into the space-time fabric, the  light is trapped.  And this is exactly what a black  hole is. 

 So if you make that well deep  enough either adding a lot of  mass or by making the mass very,  very small, you curve space-time  so much that light has to just  keep going inward.  It will just keep going inward  never coming back out.  And this particular size scale  where this object becomes what  we call a black hole is called  the Schwarzschild radius.  Unfortunately, Karl  Schwarzschild died a year after  he published his theory  otherwise we probably would have  seen an emergence of black hole  science very quickly.  But people were very, very  skeptical for a very long time  about this idea.  Many people had serious doubts  about this idea.  But as we know today we have  good evidence that this is  actually going on.  So just to give you an idea of  how big an object like that is,  the Schwarzschild radius for the  sun is about three kilometers.  If you take the sun and squeeze  it into a ball that has a two  mile radius, it will become a  black hole.  For the earth, the Schwarzschild  radius is one centimeter. 

 Take the earth, squeeze it into  a ball the size of one  centimeter, the size of a pea,  it will become a black hole.  Somebody asked me before the  lecture if it's possible to  create a black hole in the lab.  It's difficult to squeeze the  earth into a one centimeter  ball.  So we're relatively safe.  Nonetheless, it is in principle  possible.  So Einstein's Theory of  Relativity predicts black holes.  Here is just a couple of  examples of what happens when  you have strong gravity acting  on space-time.  Strong gravity makes clocks go  slower.  It just has to do with the fact  that you curve space-time so  much that time gets stretched  out.  This is something that's  actually been measured. 

 If you use your GPS receiver in  your car's navigation system, it  has a correction for the  generaltivistic time dilation in  it.  It will not work otherwise.  So that's been tested very  nicely.  Distances are stretched out.  Light rays are curved.  So these are the effects that  Einstein predicts, and we've  actually observed these effects.  So another way of phrasing this  existence of a surface within  which things just have to keep  traveling downward because the  slope of the space-time  curvature is so steep that  things can't come back out is  what we call a horizon.  Something that has, an event  horizon, something that has the  surface inside which nothing,  not even light, can come back  out is what we call a black  hole.  And this is very different from  other objects that are extreme  like neutron stars.  Neutron stars do not have this  horizon. 

 If you're standing on top of a  neutron star, if you're strong  enough you can jump back off.  Very difficult but possible.  In principle there are three  fundamental quantities that we  need to describe black holes.  We need the mass of the black  hole to describe it, we need  it's rotation because black  holes, as it turns out, can  rotate and, in principle, we  would need to characterize its  charge.  Now as it turns out in  astrophysics, almost nothing is  really charged.  No large-scale objects in  astronomy are charged.  So we often just drop that, but  that leaves us with two  fundamental quantities that  describe a black hole, its  rotation and its mass.  So here's just an example of  what it looks like when a black  hole rotates.  So let me now go to the science  of black hole astrophysics.  What are the objects that we can  actually see. 

 So there are two kinds of black  holes that we have direct  evidence for.  There's what we call stellar  mass black holes, and then there  are what we call supermassive  black holes.  And these are distinct not  because they are different black  holes but just because they're  masses are vastly different.  Oops.  That is not good.  I think that's better.  Okay, let's go back.  Okay.  Yes.  So there are two kinds and  they're only distinguished by  their vastly different masses. 

 Stellar mass black holes, as the  name suggests, have about the  mass of the sun or a few times  the mass of the sun.  Maybe 10-20 times the mass of  the sun.  And they are very common.  I'll show you a couple of  examples.  We see them all over inside our  galaxy.  And then there are supermassive  black holes and supermassive  black holes have much, much  larger masses.  They have masses of a few  million to a few billion times  the mass of the sun.  And there's a clear gap between  those two kinds of populations  of black holes.  We're looking for objects that  fill that gap but we have not  found any yet.  And we have some candidates but  for now let's just assume we  have two different kinds of  different populations of black  holes. 

 So these black holes are just  the remnants of stars.  And as stellar remnants, they  have the same properties that  many other stars have.  So that means that many of them  come in companions.  They are binaries, is what we  call them.  Each star has a companion star  that orbits with it in a binary  sort of tango.  Probably more like a waltz  really because they're mostly  just turning.  But most stars have these  companions and when one of those  stars becomes a black hole, it  will just become a black hole  with a companion star.  Unless something violent happens  but let's not go there.  So in that case you a have a  regular star and then you have a  black hole in the very, very  center here.  And I'll explain to you what all  the other stuff is that you see  in the image. 

 In the centers of galaxies we  find these supermassive black  holes.  It looks like every  self-respecting galaxy has one  of these really big black holes  in its center.  And we can find a lot of  evidence for those black holes,  we just have no idea how they  formed.  So this is a fundamental  question of black hole  astrophysics that we're still  struggling to address right now.  And how they formed we don't  know.  We know how they grow, but we  don't know where they come from  in the beginning.  So the question is, if black  holes are black, if they swallow  all the light that otherwise  come from the object, how could  you ever hope to see one?  So you might have some ideas of  how to do it.  Who has an idea?  >> Look for the companion star.  >> You can look for the  companion star.  And then you look for the effect  of the black hole on the  companion star, for example. 

 And the effect that you would  look for is somewhat visible  here in this animation, it's the  effect of gravity.  So a black hole has a lot of  mass.  So it must have an effect of  gravity of than companion star.  It has the mass of the sun so if  you put a companion star in  orbit around that black hole it  should do what the planets do in  orbit around the sun.  It should just orbit that star.  And so you can look for the  gravitational influence on the  black on the gas surrounding it.  Most easily you can look at the  stars.  And so you've seen this movie  before, I can play it again.  So Kepler's Law tells you that  gravity makes the planets orbit  the sun on roughly circular but  really elliptical orbits.  And Kepler's Law describes this  orbit very well. 

 You can measure the mass of the  central star by measuring the  size of the orbit and the time  it takes for the planet to go  around the sun.  We can do this for other stars  and we can find extra solar  planets that way.  But in principle, there is no  reason you can't do this with a  black hole.  You can just look at how long  the star takes to go around the  black hole and measure the mass  of the black hole and infer that  there is something really  massive and heavy in the center  that is otherwise dark.  Or you can look, for example,  for the deflection of light  around the black hole.  So there is something really  dark and then you look for the  deflection of light around that  really dark object.  That's very difficult.  You have to know where you're  looking.  You can't look at the entire  universe and look for things  that look like little arcs.  But measuring the orbits of  things is possible. 

 So what you see here is a movie  of observations of the center of  our own galaxy.  And there's a star cluster in  the center of our own galaxy and  each of those fuzzy blobs is a  star.  Now these have been put in by  hand, these are not really  observations.  But all the observations are  are predictions of stars and  somebody put a little fuzzy blob  and made a movie out of it.  But what you see here are these  stars are on these elliptical  orbits.  This one is really nice.  You see that it goes around some  object.  It goes really fast here and  then goes slow.  And there's another one that  comes in here and now goes  really fast and goes away.  These are all ellipses. 

 You can measure the size of the  ellipse and the time it takes  for the star to go around the  ellipse and people have done  this now for about 16 years.  And you can measure the mass of  the object that is right  where that red cross is,  and figure out what it is.  Well, this object is very dark,  but it has a mass about  3 million times the mass  of the sun.  And it's very small.  It's less than the size of the  earth's orbit around the sun.  Less than when we call one  astronomical unit.  So something really massive and  really small.  And the most reasonable  interpretation of this is a  black hole.  All other possible  interpretations people have come  up with a lot more pathological  than a black hole.  The black hole explanation for  the galactic center object is  really the most rational that we  have today. 

 Yeah?  >> The object before it was a  black hole and was visible was  larger and then over time gets  compressed and then at some  point becomes a black hole?  >> So the question was whether  the object was once larger and  luminous and then evolved into a  black hole.  And yes, in principle that's how  we envision that process to  happen.  As I said, this is a  supermassive black hole.  It has three millions times the  mass of the sun so we don't  really know where it came from.  But the stuff that makes the  black hole was once outside of  the black hole, and we could  have seen it and, in fact, we  see it in a lot of cases.  And I'll show you a couple of  examples.  But this is the galactic center  black hole.  We have a black hole of --  masses in our galactic center.  It's about 24,000 light years  away.  So it's in a safe distance I  would say. 

 It's a very dim object.  It's only a hundred times  brighter than the sun which is  not very bright at all.  For something that's 3 million  times more massive.  It turns out that this is the  blackest black hole that we know  to date.  It's the dimmest black hole that  we have good evidence for.  Just to show you, and this is  really not visible and I  apologize for that.  This is an x-ray image off the  galaxy.  The galactic center really.  The center region around that  black hole, and the little white  circle here is the region where  that black hole is located.  And let me just blow up the  circle and show you where it is. 

 Well, it's difficult to see but  there is a little dark spot in  here where there isn't a lot of  bright emission and that's where  the black hole is.  Yes, there was a question.  >> With a mass like that, that  implies that black holes are  perhaps the most -- and get  bigger because things fall into  them?  >> So the question was are black  holes growing by swallowing  other stuff, for example, stars  and other gas.  And that's exactly what's  happening.  Black holes swallow material and  in the process they grow.  They just add more mass.  There's two ways that they with  accrete.  They can accrete gas or stars  but the can also accrete black  holes.  And both things happen in the  universe.  So this is an example of a  really black, black hole. 

 Now as I said, the galactic  center has one but every  self-respecting galaxy has one  too.  And by self-respecting galaxy I  mean a galaxy that is bright  enough and big enough that it  has what we call a bulge.  That thing that looks like a big  fuzzy sphere in the middle.  No reason to really go into the  details here except to say that  if we look at any of those  galaxies that have one of those  bulges and we measure the mass  of the bulge which is really  shown on the bottom here and we  measure the mass of the black  hole they're nicely correlated.  So bigger galaxies have bigger  black holes.  This is a relatively recent  result.  And that means there is some  link between the growth of  galaxies and the growth of black  holes.  And this is a really active  field of study that we're trying  to understand this link that  couples the growth of black  holes and the growth of  galaxies.  So these black holes, at least  the red ones, have been found by  looking for stellar orbits or  looking for influence through  gravity of the black hole on  stars.  This is really hard. 

 You have to know where you're  looking.  And you have to look really  close because you have to be  able to see how the stars move.  And it takes a long time.  And we don't really have a lot  of time.  We don't have thousands of  years.  We're all impatient so we want  to look for something else.  So the question is, are there  black holes that are actually  not black?  And it turns out there are.  And so this brings us to the  topic of accretion.  Accretion is what we call when  black holes grow by accreting by  swallowing gas around them.  That's what we call accretion. 

 And probably the simplest way to  explain this is to look again at  these stellar mass black holes  that are in binary stars.  So you have a black hole and you  have a star.  So the star is this big ad the  black hole is this big.  So the black hole and the star  orbit each other like that and  in the process the star gives  off a wind like the sun gives  off a wind and the black hole  actually through its gravity can  suction off gas from that star.  So this is shown here, in an  artist's rendition really,  obviously this is not an  observation.  If we had really, really big  telescopes we might be able to  see something like this.  But not today.  So here's a black hole in the  center, here's a star and the  black hole sucks gas from the  surface of the star.  And that gas is pummeling onto  the black hole.  The black hole is trying to  swallow that gas but it can't  just swallow it like that  because the gas has to first  spiral in. 

 So when you pull the plug on the  drain in your bathtub, the water  doesn't just all fall out.  It has to actually go through  the bathtub and it has to get  there first and as the bathtub  gets shallower and shallower you  notice that the water starts to  spiral.  And that spiraling is what we  call conservation of --  momentum.  It's what happens when you spin  on the ice and you pull your  arms in and you spin faster.  At some point you fall, that's  what happens when the gas falls  into a black hole.  So it has to spin.  It has to spiral first.  It settles into what we call and  accretion disk, a spinning disk  of gas around the black hole.  The same thing happened when the  sun formed and the residue of  that spinning disk is the solar  system.  This disk formation isn't  something that's just special to  black holes, we see it all over  astronomy. 

 Now as it goes further in, the  gas has to speed up.  If you think about the fact that  when you pull your arms in you  have to speed up, that's the  same thing that happens to the  gas.  The planets, Mercury goes a lot  faster than the earth, for  example.  So the closer in you get to the  black hole, the faster you have  to rotate.  Now that means that gas closer  in goes faster than gas further  out.  So that means that there is  friction between the layers of  gas in that disk.  And that friction, the gas  rubbing against other gas that's  going a different velocity is  going to heat up the gas.  So this friction heats the gas a  lot.  And if you heat something a lot  it starts to shine.  And if you heat something more  it starts to shine even  brighter, and this is exactly  what happens to black holes. 

 So here is a movie of a  simulation that a colleague of  mine did to simulate one of  those disks around a black hole.  So there is a black hole in the  center and this movie actually  takes into account all of the  effects of gravity on how the  light travels from the black  hole to you.  This is why this side is  brighter.  But you really see the stuff  going around a lot faster in the  interior and the yellow regions  are closer into the black hole.  At some point there is no more  emission.  What happens there?  [inaudible]  That's when it's inside  the black hole, right.  So there's no more emission  once the stuff goes in.  But if it's still outside the  horizon, it spirals and heats up  more and more and becomes  brighter and brighter.  So the inner regions closest to  the black hole are the ones that  are brightest.  Yes?  >> Is there some known benchmark  number as far as how large an  object is relative to it's  Schwarzschild radius?  It might be one-third, typically  one-half, typically it's .95,  whatever. 

 >> Black holes?  >> Black holes.  >> Black holes are, so the  question was is there a typical  benchmark of how big black holes  are compared to their  Schwarzschild radius.  So I should back off one second  here and say that black holes  have a different Schwarzschild  radius depending on how fast  they're rotating.  They have a different, what we  call, horizon size depending on  whether they rotate or not.  But a black hole is always  defined just by its horizon  size.  So a black hole always has the  same size for a given mass and  spin.  What happens inside is a  completely different question.  But it's a question that we  can't answer today because we  don't have the theories for  that.  >> So when we cease to see  anything, that means it's part  of the black hole?  >> Yes.  Basically. 

 It's a little more complicated  but it's not really worth  getting into too much.  If you measure the size of this,  you can measure the size of a  black hole.  Right?  Now as I said, that's a  simulation.  We have no observations that  come anywhere close to resolving  this.  Black holes are tiny.  That means they're tiny on the  sky; we can't really see them.  Here's another, now this is not  a simulation this is an artist  rendition.  NASA is very good at producing  these.  So here's the companion star and  there's a little black hole in  the center and you see this  stream of gas being sucked off  the companion star and it's  settling into what you will see  in a second is this disk here.  And it's going around the black  hole just to illustrate how this  is working. 

 Now you see this thing here,  this fuzzy stuff that's coming  out which we'll talk about at  the end.  So friction heats up the gas and  the principle here is you have a  very deep potential well, you  drop a very, very steep flight  of stairs or you go down a very  steep slide, when you get to the  bottom you're very fast.  You have a lot of energy.  And that energy gets -- away in  large part in a black hole.  So really this power, the power  that you get to drive this  radiation to make the black hole  so bright is the power of  gravity.  The question is, how powerful is  gravity?  Well, let's make a simple  comparison.  Einstein tells you that there's  an equivalence between mass and  energy.  You've all seen this formula,  right?  The energy in a certain mass M  is just EMC squared.  So the question is, how  efficient is gravity in  releasing some of that energy to  use?  Well, gasoline powered cars if  you use a Prius maybe it has 45  miles per gallon.  So that means you get one gram  of energy for every 2,000 tons  of fuel. 

 A very small faction of the mass  actually gets converted to  energy.  Only one gram for 2,000 tons of  fuel.  Well, we can do better than  that.  So nuclear power, if you were to  have a nuclear powered car it  would have about 200 million  miles per gallon in gas mileage.  It would have other problems  probably but in principle you  could get there.  [LAUGHTER]  And so one gram of energy  is released for about  two kilograms of fuel.  Now what about black holes?  Black holes reach a gas mileage  of about 45 billion miles per  gallon.  That is if you had a black hole  powered car you wouldn't have  any energy problems for at least  a long time.  So one gram of energy is  released for every 10 grams of  fuel.  So 10% of the rest mass is  actually released as energy by a  black hole. 

 This is very powerful.  So actually finding black holes  is easy then.  Because you just have to look  for the brightest things in the  universe because they're so good  at radiating.  And that's what we do.  So here is an x-ray image of  what you see when you take a  very powerful, very expensive  x-ray telescope in this case  take the -- x-ray telescope and  you just stare into empty space.  Now they didn't just pick any  random direction to stare and  they picked a direction where  there was nothing else.  You just pick a direction where  you don't see any other  foreground objects.  No galaxies, no galaxy clusters,  no stars, no planets, no  nothing.  And you just look into space and  this is what you see.  You see all these little dots  here. 

 Every single dot is a black  hole.  And every single of these dots  is a black hole more than  halfway across the universe.  Very far away.  In the early stages, when the  universe just formed, when the  stars formed, when the galaxies  formed, and because I just said  galaxies and black holes form  together when black holes  formed.  So you just look and the  brightest things you see in the  x-rays at least are the black  holes.  So why look in the x-rays?  Well, I just said accretion  disks are really, really hot and  when you have something really  hot you have to look at very  high energies so the x-rays are  where you want to look.  These are what we call quasars.  They're supermassive black holes  that grow by swallowing stuff,  and they're really bright.  And they can easily outshine an  entire galaxy.  No problem. 

 Now, as I said there are two  kinds of black holes so you can  just also look at the stellar  mass black holes which I said  have companions that orbit  around them.  Rather than just looking for the  gravitational interaction  between the two, you just look  for the radiation from the black  hole.  And so there aren't just black  holes that make that, there are  also neutron stars that have  basically the same qualities as  black holes.  So it's a little difficult to  distinguished the two that's why  it's a little safer to look at  the super massive black hole and  say well, it's only really a  supermassive black hole.  For a stellar mass object it  could be a neutron star.  But in this object you have an  orbit, you have this normal star  orbit.  It transfers its gas to the  black hole, it makes it really  shine very brightly and it  becomes the brightest object in  x-rays for a little while.  That's why we call these things  x-ray binaries because they're  so bright in the x-rays.  This is again an image by the --  x-ray telescope of the galactic  center.  The region where the  supermassive black hole in the  center of our galaxy is  somewhere here. 

 These bright things are actually  stellar mass black holes in  orbit around the center of the  galaxy.  With the companion star that  transfers mass onto the black  hole.  And these things can be up to  100,000 times brighter than the  sun.  So I said before that there is  actual evidence to spinning  black holes and this is what it  is.  So here is calculation of what  accretions look like around a  black hole that's not spinning  and one that's spinning.  As I said, if you increase the  spin of the black hole the size  of its horizon changes.  It becomes smaller.  So stuff can go further in  closer to a black hole, if you  increase the spin of the black  hole stuff goes closer.  If stuff goes closer it goes  even faster, it becomes even  hotter and that's exactly what  you observe.  Unfortunately, I think this is  basically impossible to see. 

 Can you see this?  Nobody can see it?  I'll show you the spectrum.  I'll just show you, I won't show  the you spectrum.  These points here are a spectrum  in the x-rays.  So you take an observation in  the x-rays and you plug every  single photon that you get from  that object and you plot its  energy.  You see that there's this bump  here.  This bump is due to the fact  that there's iron in that  accretion disk.  Very hot iron and that iron  emits and emission line.  Emission lines are pretty common  when you look at fluorescent  lights.  Fluorescent light is all  emission lines.  This is a fluorescence line  except it's from really hot iron  around a black hole and not from  a fluorescence light bulb. 

 That's the only difference.  And it's really, really broad.  And when you measure the breadth  of the line, how broad it is,  it's about half the speed of  light.  That's what it takes to make  that line so broad.  So there's something going  really fast and the only thing  or the most reasonable thing for  that thing is a black hole.  A spinning black hole really.  Because you have to get so close  in.  Okay, so last topic I want to  talk about is a topic dear to my  heart is a topic called, I'm  going to talk about a class of  objects that we call  relativistic jets.  What is a relativistic jet?  Well, this is a relativistic  jet.  And I could spend a thousand  words to describe this picture. 

 You know a picture is worth a  thousand words.  What you see is this elongated  structure here, this is  a radial image of what we call  a radial galaxy.  Now the size of the image  just to give you a perspective,  is very large.  This is the size of the milky  way compared to the size of this  image.  So there's a very large  structure in radio emission that  you see.  Very, very large.  You see these fuzzy blobs here  of hot gas.  This is what we call --  emission.  These are relativistic particles  in field that fill space.  Now they all come from the  stream of gas that's traveling  in this direction here and this  direction here. 

 Where you can actually see stuff  travel down that path.  And very close to the speed of  light and the gas that's glowing  flowing down that path is  filling up these big balloons of  very hot gas.  Now as I said a picture is worth  of a thousand words but it's  better to show you a movie.  So this movie is about a  thousand pictures so you can do  the math and figure out how many  words it would take to describe  it.  So this is a simulation that we  did of this particular jet.  And so the jet is injected into  a large reservoir of gas outside  going in these two directions  and it just interacts with the  gas and it blows up these  balloons and we calculate the  emission.  It looks pretty much the same  way that the radio image looks  like and we can analyze it in  more detail, and it nicely  describes the features that we  observe.  Now, how do we think this thing  is made?  Well, we know how it's made.  The computer, it took about 500  CPU years to calculate, but the  actual time it takes for this  thing to evolve is a few tens of  millions of years because it is  a very large object and can  really only go at the speed of  light.  No faster than that so it takes  a long time to inflate. 

 And it's coming from a black  hole that's about a few billion  solar masses.  We can actually measure the mass  of the black hole on this object  that I showed you before.  And it's is few billion solar  masses.  And there's a stream of hot gas  that's coming away from this  object at very close to the  speed of light that's inflating  these enormous bubbles.  Now, this isn't the only object  that does that.  There are many, many other  objects that do that.  So this is a gallery of radio  images of these jets.  You see many, many different  kinds of jets.  You see these long features  here.  Here is actually two black holes  that make two different jets in  orbit in two separate galaxies. 

 Here is one that's kind of cork  screwy.  Many different morphologies.  Some are very, very straight.  This is a rather common  phenomenon.  Many black holes actually seem  to make these jets.  So here is an artist rendition  which is almost invisible at the  current light setting.  But you see there's an accretion  disk, there's a black hole and  somehow makes this jet.  When you showed you this  artist's movie of how an x-ray  binary accretes, it has this  magic jet coming out.  And that's pretty much exactly  what it is.  It's magic. 

 We don't really understand how  it's formed.  We just know it's there.  These things are quite extreme.  They move up to 99.9%  the speed of light.  Very fast.  Faster than pretty much anything  we can actually see moving in  bulk.  It's very powerful.  This jet I showed you the image  of, this radio image of that  jet, is about 10 trillion times  more powerful than the sun.  More powerful than all the stars  in the galaxy together, easily.  And that's why it makes such a  big hole. 

 It's a huge ball of hot gas  that's being inflated by this  black hole.  And because it is so fast and so  energetic it must come from very  close to the black hole.  We know that much.  We have some theories as to how  these things are formed and I'll  show you an image of how we  think this might be happening.  And they're basically made  exclusively by black holes, and  we think they're made by  spinning black holes, in fact.  I have a picture of how that  works.  I should probably skip this  over.  Just to show you how extreme  they are, they look like they're  moving faster than the speed of  light.  You see this thing is moving,  and I think you can't read this,  but it's moving 21 light years  in seven years.  So this is a timed sequence of  this jet. 

 And you see this thing here,  this blob is moving this  distance here that it's moved  from here out to here is 21  light years.  In seven years.  That means it's moving 3 times  the speed of light.  I said before, Einstein tells  you nothing can move faster than  the speed of light.  Something's wrong here, right?  So let's look at this.  Let's say this blob is this  thing here.  It's actually a trick a little  trick of nature that's being  played on you here.  So this blob is traveling in  this direction.  And it's sent out a little  photon when it started.  And the photon has just arrived  here. 

 And it's sending out another  photon when it gets to this  point here.  The first photon comes from  here; the second photon comes  from here.  So it has traveled from here to  here when it sends out the  second photon.  On the sky this is 21 light  years and the difference between  the arrival times between those  two photons is only seven years.  To you it looks like it traveled  21 light years in seven years  moving three times the speed of  light.  But what really happened is it  moved with a speed very close to  the speed light but very close  to your line of sight.  It's moving almost towards you  and what really happened is that  it traveled 80 light years in a  time of 90 light years.  So there's all these interesting  tricks of nature that you have  to sort of sort out if you want  to understand how fast these  things really move.  So when you say we see something  that's moving three times the  speed light it turn us out it's  only moving at .9 times the  speed of light or maybe 99% the  speed of light.  So, as I said before, gravity is  quite powerful. 

 If you have gravity powered car,  you can get trillions of miles  per gallon.  The question is what does it do  for you?  Well, if you add up all the  black holes in the universe,  which we can, we have a good  sense of how many black holes  there are because we know how  many galaxies there are and we  know how big a black hole there  is in every galaxy.  Because galaxy mass and black  hole mass correlate.  So we just add up all the  galaxies and add up all the  black holes in the galaxies and  say, how much mass is there in  black holes today.  And say 10% of that mass can get  converted to radiation.  That radiation is quite  powerful.  About 1% of the mass we  calculated gets converted into  these powerful jets.  We can also measure that.  And these jets, they don't just  radiate.  They actually interact with the  stuff they run into. 

 There's an explosion, basically,  and that explosion must heat the  gas it's running into so  radiation can go through space  without doing much until it hits  your eye or a telescope.  But that jet has to stop  somewhere and when it stops it  must heat and deposit all its  energy into that gas.  So 1% of the mass in black holes  is released.  And 1% of that mass is enough to  heat every single particle in  the universe to about 3 million  degrees.  So a very small fraction of the  mass in the universe is actually  in black holes,  about 1-millionth.  But that mass has enough  influence to heat every single  particle in the universe  to 3 million degrees.  Now black hole jets are very  common.  You see them in many objects.  Supermassive black holes.  Stellar mass black holes. 

 We actually see them in gamma  ray bursts.  And we see them in neutron  stars.  So this isn't some freak of  nature that is happening here.  We actually see them in many,  many different environments.  Let me show one example that I  didn't show yet.  Another kind of black hole, also  stellar mass black hole.  Gamma ray bursts.  These are flashes of gamma ray  radiation we see with gamma ray  telescopes.  The first set of gamma ray  telescopes are actually  satellites that were developed  to test the test ban treaty, the  space test ban treaty in the  '60s.  And they found flashes of gamma  ray bursts. 

 Which is exactly what they  expecting from Russian tests,  nuclear tests in space.  Luckily people weren't stupid.  They actually did the  calculation and realized they  were coming from outside of the  solar system.  This is what they look like  on the sky.  They are uniformly distributed.  They last for about 100 seconds  and they come from further than  halfway across the universe.  And that makes them the  brightest objects in the  universe ever.  Since the big bang.  The big bang was brighter,  but the big bang was special.  So gamma ray bursts are really  bright, and they flicker really  fast. 

 And that tells us that they're  black holes.  And they're actually jets from  black holes that are pointing  directly at you.  They're traveling so fast,  99.9% the speed of light, that  they're really, really bright.  And they flicker so fast just  because they're coming towards  you directly.  So this is exciting.  Just to quickly sketch out how  we think these jets are created.  They're created by spinning  black holes.  And it's nothing fancy.  If you have something spinning  and you put a magnetic field  into it, can you extract power.  Wind does it. 

 Turbines do it.  Nuclear power plants do it.  Black holes do it.  The way it works is the spinning  black hole takes that fuel line  and twists it up like a rubber  band and converts energy from  its rotation into energy that  flows along that fuel line.  And basically like a cork it  just spews out stuff that's  sitting on those fuel lines,  a lot faster than a cork  of course, but in principle  the same thing is happening.  So we believe that spinning  black holes wind up so much that  they can create these super fast  outflows that we observe as  these relativistic jets.  So let me stop here and put up  my summary.  Black holes are predicted by  Einstein's Theory of Relativity.  But they're not some crazy  construct.  We actually observe objects that  are best explained and most  simply explained as black holes. 

 Gravity is really a very  powerful force.  It's the most efficient force  that we have to actually make  and convert energy.  Jets and accretion disks are  objects that we observe  for many, many different  black holes.  And they're very easy to see and  they're very powerful probes  to study.  And black hole power is so  strong that it can heat every  single particle in the universe  to a few million degrees.  Thank you.  (applause) jets black holes with their