What Does a Black Hole Look Like: How We Got Our First Picture

good evening everyone my name is Andrew frack Noi I'm the emeritus professor of astronomy here at Foothill College and it's a great pleasure to welcome everyone here in the Smithwick theater and everyone hearing and listening on the web to this lecture in the 20th annual Silicon Valley astronomy lecture series this series of free public lectures is co-sponsored by four organizations the Foothill College astronomy program NASA Ames Research Center one of the premier NASA centers around the country the Astronomical Society of the Pacific a venerable organization doing public outreach in astronomy and the SETI the search for extraterrestrial intelligence Institute in Mountain View were delighted to have their support for this series tonight's speaker is one of our favorites I think this is his third time right talking to the series dr. Elliott quartered dr. quartered is professor of astronomy in physics at the University of California Berkeley and the director of the theoretical astrophysics center there he's an astrophysics theorist who works on a wide range of problems at the forefront of astrophysics from stars to black holes to how galaxies form he's received a number of national awards for his research and is also a highly regarded teacher and public lecturer as you will see this is his third talk in the series and were delighted to welcome him talking about what does a black hole look like how we got our first picture of a black hole ladies and gentlemen dr. Elliott quarter thanks Andy it's great to be here be back again and telling you about some of the really exciting results that have just come out in the last year the picture that's basically the entire focus of this talk is the real one on the lower left here this is a picture of a black hole the region around a black hole that was taken in April 2017 and then released actually this spring so in April 2019 the observations were released and really what I want to do over the course of this talk is try to explain to you what we've learned from this image how it was taken why it looks like it does why is part of it bright part of it not as bright was there a gap in the middle where there isn't any light coming etcetera the subject of this talk black holes is one of the most common objects for science fiction really since the 1960s and 1970s when black holes really became part of modern astrophysics and physics and began to be studied in great detail they've also become part of science fiction there are many many Star Trek episodes where the enterprise pierces its way through the event horizon or something crazy like that there's this Disney movie from 1979 about black holes the most recent I would say modern version of black holes featuring in the movies is the movie interstellar from 2014 this actually I think stands out among all of the science fiction movies that have been done about black holes are really on other topics and that the original screenplay and one of the executive producers for interstellar was Kip Thorne who's shown on the right there and Kip is a Nobel Prize winning physicist at Caltech so I actually know some decent amount about black holes black holes also show up in slightly unexpected places this is a graphic novel about teenagers who were cranky and just affected in Seattle I don't really know what it has to do about black holes but but it makes for a good image just to give you a sense of how much a part of popular culture black holes are if you go to Amazon and you type in black holes in this particular case I got one hundred one thousand eight hundred and forty-four results this was a while ago I think there's a lot more now this is actually the graphic novel that I showed you the image of a second ago this is the Disney movie this is a nice book by Neil Tyson about black holes and it goes on and on there's many many popular books movies TV shows on this theme slightly unexpectedly there's also health and personal care products related to black holes which are mostly in the area of liposuction and things like that okay so really I could go on and on and give you an entire talk about how black holes manifest themselves in science fiction but instead I want to turn really to the science focus of this talk and what I want to do is I want to try to first explain to you a little bit about what black holes are how we as physicists think about them and then how do we actually find black holes out in the universe how did we end up taking this remarkable image which really is showing us what happens incredibly close to what we call the event horizon of the black hole which is the point of no return inside of which everything gets sucked into the central object so I'll start with this kind of make sure we're all on the same page understand what we're talking about when we talk about black holes and that get into how we actually took this picture so before we talk about black holes I want to talk about something that's a little bit simpler which is the Sun shown here or the planet earth the one we're on the story about black holes is really a story about gravity it's a story about gravity really winning at the end of the day over every other force we know of in the universe causing an object to collapse to form a black hole but in our everyday experience in a star like the Sun or a planet like the earth gravity doesn't actually win there is a sort of happy balance gravity is always pulling inwards it's what keeps us here on earth it's what keeps the earth together at what's keeps the Sun together so gravity is this sort of relentless force pulling in on all matter in the universe but in something like the Sun there's a counterbalancing force there's something pushing out stopping the object from collapsing in on itself and in the case of the Sun it's basically that the Sun is a big hot ball of gas the temperature inside the Sun is millions of degrees it's kept hot by nuclear fusion of hydrogen into helium at the center of the Sun which will keep the Sun nice and hot in its interior for billions and billions of years and able to fight off this inward gravitational pull of gravity now this happy balance between gravity pulling in and something else pressure in the case of the Sun pushing out that happy balance is one that we know at least in our everyday experience it's one that can be overcome right the example of this that you know about is that we've been able to successfully launch rockets and satellites off of the earth and escape the gravitational clutches of the earth we've launched Voyager all the way out of the solar system and we've launched many satellites into the outer part of the solar system in astronomy we talked about how hard it is to escape the gravitational clutches of an object by asking the question of if you were to launch something throw something off the surface of the object how fast would you need to throw it in order to escape the gravitational clutches of the object and we call that the escape speed or the escape velocity of the object if I were to throw a ball off the surface of the earth and not throw it very fast it would go up and it would come crashing back down if I were to throw it a little faster would go up further come crashing back down if I threw it just right fast enough in the case of the earth that's about 27,000 miles an hour or about 11 kilometers a second then that ball would actually be able to leave the earth entirely and travel out into the rest of the solar system but the strength of gravity depends on how much stuff there is how much mass there is it depends on how large something is gravity is stronger when things are closer together so if you have a smaller object in general with the same amount of mass you'd have a stronger gravitational pull so every object in the universe every object in the solar system for example has a different escape speed a different speed that you'd need to throw something launch a rocket throw a ball to escape the gravitational clutches of the object so for the earth it's 27,000 miles an hour 11 kilometers a second the Sun is much more massive three hundred thousand times more massive than the earth and as a result it has a much higher escape speed so all of those rockets that were so extraordinarily proud of being able to escape the gravitational clutches of the earth they would come crashing back down on the surface of the Sun because the sun's escape speed is something more like a million miles an hour or six hundred kilometers a second the physics I'm describing to you here this is physics that was known in the time of Newton this is really some of the major conceptual advances that Newton taught us in terms of gravity and motion a little bit later we also learned that even light actually travels at a finite speed so when I press on the laser pointer and it goes and it bounces off the wall and then it bounces to your eyes it actually takes a certain amount of time for that to happen because light doesn't travel instantaneously it's very fast it happens in a tiny tiny tiny fraction of a second because light travels very very quickly but it does take a certain amount of time for to travel the speed of light it turns out is about 300,000 kilometers a second or a billion miles an hour or 1 foot every billion of a second every nanosecond so that's an extraordinarily fast speed much faster than speeds that we're familiar with in our everyday experience which is why in practice we don't really have an intuition for the fact that light has a finite speed but this is something that one can measure in various experiments the speed of light is actually taken to be a constant a universal constant and Einstein's theories of relativity and the first notion of black holes in physics actually arose in the late 1700s and it arose basically independently by two scientists Michelle and Laplace Laplace is a very famous mathematician Michele was actually a geologist who did a lot of work on earthquakes and what they did is they basically just combined these two ideas that I've just told you every object has an escape speed the speed you need to escape its gravitational clutches and even light has a finite speed and they speculated what would happen if you had a star that was so small that the speed needed to escape its gravitational pull was larger than the speed of light then not even light would be able to escape the surface of that object and so they speculated that there might be stars out there dark stars that we couldn't see that wouldn't emit any light because their gravity was so strong that not even light could escape the gravitational clutches of the object and they actually went one step further they estimated how large would such an object have to be the strength of gravity depends on the mass of an object and its size and so depending on how much stuff you're talking about the mass of the object you need a different size to have the escape speed than the speed of light for an object that has a mass like the mass of the earth you would need to shrink the entire earth down into a size that's smaller than a baseball in order to reach these extreme conditions where not even light could escape the gravitational clutches of the object the Sun weighs 300,000 times more than the earth it's a larger mass you would need to take the Sun and shrink it down to about the size of a city a few miles big in order for gravity to be so strong that not even light could escape and this size the size needed for the escape speed to be about the speed of light this is what we now call the event horizon of a black hole so this idea was sort of out there in the late 1700s but honestly not much happened until the early 1900s so 130 years or so later and the reason is that Michelle and Laplace basically pulled a little bit of a swindle when they made this argument and the reason is that the theories that they were using newton's theories don't really apply to light in the way they were using them and they don't apply when gravity is so strong that can it can affect light itself and so we needed to wait for new theories of physics that could apply in these extraordinarily extreme conditions where gravity is this strong and that theory is the theories that Einstein developed his theories of relativity the special theory of relativity and the general theory of relativity which were developed in the early 1900's and the basic idea of a black hole in Einstein's theory of relativity and this is really our modern understanding of black holes the basic idea I would say is really captured by the one I told you here there's a lot of interesting things that are not captured by this idea but the core concept is there one sort of wrinkle that shows up in Einsteins way of thinking about is that in Einstein's theory it turns out there's no pressure in the universe large enough to counteract gravity if you make the object smaller than this size where the escape speed is larger than the speed of light if an object is that small gravity wins out over all other forces in the universe causing the object to collapse entirely in on itself all of the matter we think collapses to the very center the little red dot in the image there we don't exactly know what goes on right there at the center and I can come back to that and questions if people are interested the event horizon this radius that we estimated following Michelle and Laplace a few miles for something that has the mass the Sun in Einstein's theory this is really the point of no return if you're outside the event horizon you can get out if you can travel nearly the speed of light but if you're inside the event horizon nothing can get out and the reason nothing can get out is that according to Einstein's theories of relativity nothing can travel faster than the speed of light and so if gravity is so strong that not even light can get out then nothing can get out and so normally when we talk about the radius of an object the radius of the Sun the radius of the earth I think you know what I mean when I talk about the radius of the earth it's the thing we stand on ok it's not the podium but it's the crust of the earth that you stand on where the material transitions from being a solid to a gas there's stuff there you're standing on it right that's the radius of the earth black holes are weird and that there's actually nothing there at the event horizon the radius of a black hole is really defined as this point of no-return inside of that region everything is forced to fall into the center outside of the event horizon in principle if you can travel at nearly the speed of light you can escape out to the rest of the universe and so the radius of a black hole is really defined in terms of how material can trap or how information can be communicated this idea that once you're inside the event horizon everything gets sucked into the center this leads to what I think is the single biggest misconception about black holes that are out there in popular culture you know largely because of science fiction and that's that you should really go home tonight and have horrible nightmares about being sucked into a black hole because everything gets sucked into the event horizon and falls into the center that's not true okay black holes are not actually a cosmic vacuum cleaner I've tried to emphasize this by using every keynote tool at my disposal including one I didn't realize until last night the bounce future the key point here is that when you're far away from a black hole way far away from the event horizon then Newton basically got it right gravity is the same for a black hole as it would be for any other object there that has the same mass so just like you don't need to have nightmares that we're gonna fall into the Sun tomorrow you don't need to have nightmares that there's some black hole out there that we're necessarily gonna get sucked into because far away from the black hole gravity is essentially what Newton predicted so to highlight this if you got rid of the Sun and you put a black hole in its place and the black hole had the same mass as the Sun then gravity would be the same and the orbit of everything in the solar system the planets the moon the asteroids would be the same lots of bad things would happen but all of the bad things would be because we rely on the heat of the Sun to have life here on earth okay gravity wouldn't change so it's only when you're close to the event horizon that gravity is fundamentally different and all these interesting things happen and what's so amazing about this picture I showed you at the beginning of the talk and that we'll come back to is that that's a picture of the region very close to the event horizon where all the crazy things okay so now let's move to try to actually find some black holes and study them that's not so easy that's a real picture of the night sky there are places on the sky that looked dark where there doesn't seem to be much light coming from those are most of those regions are not black holes they're just regions where there isn't any aren't any stars or in this case there's actually a bunch of junk that's blocking the light of all the background stars so finding black holes is more challenging than just looking up at the sky and finding a place where there isn't any light coming from we have to be a little bit more clever and there's actually different ways of finding black holes astronomers have been studying black holes in detail now for 40 or 50 years we have different techniques for finding them the one I'm going to focus on is the one that's actually at first going to sound completely paradoxical to you and it's that black holes objects that are defined by the fact that they cannot on their own produce light black holes are actually the source of the brightest sources of light we know of in the universe that doesn't register so let me show you a couple of examples this is the galaxy m87 that's the galaxy there you might not be able to tell that's about a trillion stars right there from the very center of that galaxy there's actually material that's being flung out at nearly the speed of light in this linear feature you see here called a jet that jet lights up producing light that we can see and we think that that material is flung out that's material that almost got into the event horizon but before it made its way and it got flung back out again come back to how that happens so that's material that almost made it down to the black hole that's at the very center of that galaxy but it got kicked back out again at nearly the speed of light this is another example on the left here you see what actually sort of looks like a star it looks like two bright source of light but if then astronomers go and they very very very carefully subtract off that bright point source of light you see some faint fuzz around it that faint foot fuzz is the light from about a trillion star there's a single region at the center of that galaxy a region no bigger than our solar system that is producing more light than all of the trillions of stars in that galaxy and we think both of these amazing phenomena are caused by big black holes that are at the center of these galaxies so how is it that objects that are defined really by not producing light can somehow be associated with the brightest sources of light and the universities are called quasars or active galactic nuclei these incredibly bright sources of light in an extraordinarily small region less than the solar system inside what we think it is is all associated with gas falling into the black hole so this is not a real image see the caveats this is an artist's conception sort of schematic diagram of what the region around a black hole might look like I'll show you some real computer calculations of what it looks like in a second that's supposed to be the black hole this is material swirling around the black hole as the material swirls around the black hole some of its spirals into the black hole falls into the event horizon is lost forever some of it is flung out and one of those jets that you see in a real image here on the left okay so what's special about black holes we think this process of gas spiraling on to a central object this is a process known as accretion in astronomy this happens all over the place I do a lot of my research on this accretion process you can have gas spiraling into a normal star you can have gas spiraling into a black hole you can have gas spiraling into a galaxy what's special about black holes is that gravity is so strong that when gas is spiraling into a black hole it moves it nearly the speed of light and material as it's spiraling in moving at nearly the speed of light that material gets extraordinarily hot and hot material produces a lot of light the Sun is much brighter than the earth the Sun is much brighter than Jupiter because the Sun is much hotter than Jupiter so why does this get hot it's not because of nuclear fusion that's what keeps the Sun high instead it's actually something that you're even more familiar with in everyday experience its friction okay it's that different parts of the material as it's spiraling in are moving at different speeds so what happens when you rub your hands together your hands are moving relative to each other at different speeds they get warm how do boy scout how our Boy Scouts taught to make fire you rub sticks you create some friction that generates heat exactly the same principle is operating in this gas spiraling into the black hole the difference is that the gas is moving at nearly the speed of light so when you have friction and stuff is moving so fast it generates a huge amount of heat which produces this extraordinarily bright source of light so that's the basic way that black holes are actually able to power extraordinarily bright sources of light it's the gas spiraling into the black hole that ultimately produces the light that we see once the gas falls through the event horizon none of the light can get out so the light that we see is coming from outside the event horizon these objects gas spiraling into black holes were discovered in the 1960s and 1970s soon after they were discovered astronomers really embarked on a quest which was to try to look closer and closer to the event horizon to look closer and close closer to this central region where the gas was actually falling through the point of no return so where do you look we actually know about a lot of black holes now how do you pick which one is the most promising one if what you want to do is take a picture of what it looks like close to the event horizon to figure that out it's basically a matter of perspective what you want is you want a black hole that has a very large mass and the reason you want that is because then the event horizon is bigger remember the event horizon for a black hole that has the mass of the Sun is bigger than the event horizon for a black hole that has the mass of the earth so you want a black hole that has a large mass so the event horizon is a big size but you also want it to be as close to us as possible so that it looks big on the sky right that's what this perspective image is showing the Eiffel Tower is much further away than the person's fingers but they look the same angle they look the same size on the sky because one is much closer than the other so what we care about an astronomy isn't the actual physical size of the object if we want to take a picture of it what we care about is how big is its angle on the sky so we want simultaneously big black hole and as close to us as possible it turns out there two black holes that we know of that are the most promising one is the black hole at the center of our galaxy which I'm not going to talk about because the picture that hasn't been taken yet that will be coming out I think this year instead the one I'm going to talk about is the black hole at the center of the galaxy m87 so this is again a picture of the galaxy m87 it doesn't really look exciting it's a trillion stars it's something like tens of thousands of light-years across it has a very big black hole at its center one of the biggest we know of the black hole weighs about six billion times the mass of the Sun this galaxy is about 55 million light away just to give you a sense of scale these days that's actually pretty close by strong any standards how big is the event horizon so the angle okay the angle on the sky that the event horizon subtends is about 1 billion of a degree so let me try to give you a sense that's really a small angle so let me give you a sense of that so remember a circle is 360 degrees so take one of those degrees okay and divide it by a billion that's the angle we're trying to measure for comparison the moon this is the full picture the full moon actually rising over the hills near where I live the full moon is about half a degree on the sky so what we want to do is we want to take a picture where we could see something that's equivalent to seeing something on the moon that's about a billion times smaller than the moon so it turns out that a dime is about a billion times smaller than the moon so the challenge taking the picture of what's happening near the event horizon of this blue black hole the challenge is analogous to the challenge of taking a picture of a dime on the moon not on the moon you're on earth okay no cheating you're on earth looking at the moon taking a picture of a dime on the moon and checking whether or not the dime was twenty seventeen twenty sixteen so that's really hard so thankfully we have some awesome telescopes one that you've probably heard of is the Hubble Space Telescope there's probably taken many of the most famous pictures in astronomy this is called the pillars of creation one of Hubble's most famous images of where new stars are forming this is two galaxies that are colliding with each other so you might think okay Hubble has taken these amazing pictures maybe this is the telescope to use to take a picture of what's happening near the event horizon of the black hole it turns out that Hubble isn't up to this not because of some current inadequacy of the Hubble Space Telescope although it's sort of is it's just not big enough but really it's some laws of physics it turns out and this is a bit of a detail so don't worry about this if it goes over your head but the angle that you can measure with the telescope depends on the size of the telescope the bigger the telescope is the smaller the angle that you can measure using a telescope and this is related to a law of physics called the diffraction limit or the fact that light gets bent as it travels through telescopes it turns out that the Hubble Space Telescope is too small by a factor of 10,000 to take the picture we want to take so that's bad okay because forget about budgets right it's just totally impractical to build technologically a telescope 10,000 times bigger than the Hubble Space Telescope so how do we do this it turns out there's a trick the trick was originally developed in the 1940s and the modern version of the trick was developed in the 1970s the trick is that you actually combine the light very very carefully how you combine the light is tricky but you combine the light from telescopes that are far apart from each other and then the light thinks that it's all been captured by one super big telescope so the light comes into this telescope the light comes into this telescope and you have to record all of the details of the light as it comes in exactly when the light came in for those who know this light is an electric field goes up and down up and down as it comes to the telescope you need to know exactly when the light wave was up when it was down when it was up when it was down so you need really good clocks at the telescopes to know exactly when the light came in so you then freeze the light like Han Solo in Star Wars right you freeze it by recording it onto a hard drive and then in your leisure you Dex the hard drives together to a common place and you combine the light later on a computer so this is the technique of interferometry and it allows you to create much better pictures than you could make with just one of these telescopes alone a lot of the work on interferometry I was actually done at Berkeley in the 1970s Hat Creek California which is in Northern California is where a lot of the pioneering experiments were done this is the some of the early version of the experiments that they did these are the two telescopes they combine this is actually a tractor that they use to pull the telescope's around to get them closer together or further apart the modern version of this experiment where you take these telescopes in different places we now separate them by a lot more than you know thousand feet or so we put them all over the earth and we combine the light from all over the earth so the modern version of this is the event horizon telescope this consists of telescopes in Hawaii Arizona Mexico Spain Chile the South Pole South Poles actually a great place for astronomy it's not a great place for astronomers but it's a great place for astronomy they're now putting a telescope in Greenland to enhance the capabilities of the telescope and then all of these different telescopes record the light and the way I described to you and then you later on a computer combine all of the light from those different telescopes to create a picture that's much better than what either of these telescopes could do on their own and the big technological breakthrough what really made the picture of the black hole possible was the completion of a new telescope in Chile called Alma which is a very large telescope so it can gather up a lot of light and it made the telescope the event horizon telescope it made it basically a much more powerful telescope than it was before this team the team that does these observations is about 200 people at sixty universities and research institutions in about 20 countries I'm actually not a part of the team so my expertise is in the theoretical modeling so I make predictions for what these types of observations hopefully will see and try to interpret the observations when they come out but this is a huge team and the results that came out as a testament to the hard work of many many many people so this is the image that they took they took the data in April 2017 the image was released in spring of this year I was actually in Princeton for a few months on sabbatical we all gathered in the auditorium to watch the press conference live and this is one of the I think highlights certainly of my scientific career literally getting goosebumps seeing that image come up on the on the projector for the first time so just to make sure we're clear what we're looking at here the color here my my eight-year-old niece is in the audience ask me why is it orange which is good question okay the orange is not anything related to color here orange yellow white here means bright lots of light the redder here means less light and black means very very little left so really the color is encoding the brightness of the light so to tell you what this means and how we interpret it I'm going to take you through what the theoretical predictions were and how we made those predictions prior to this observation being made and then we'll come back to the observations themselves and I do that in part because I'm a theorist so I like to brag about when the theoretical predictions were pretty good in advance of the observations and this is one of those cases so one of the ways that we do to predict is we actually do computer calculations that describe this process of gas spiraling into a black hole getting really hot because of friction and producing a huge amount of light and this is an example of one of those computer simulations that my group is done so the black thing here is you guessed it that's the black hole the other stuff you're seeing the color now is encoding how much gas is there at a given place in this computer simulation what you're actually seeing is basically a doughnut so think of this as a slice through a doughnut okay the computer simulation is a full three-dimensional doughnut but that's hard to visualize so we're looking at a slice through the doughnut why there's lots of gas there blue there's less gas green there's very little gas and the computer simulation I'll show you just as a function of time as time goes on gas spirals into the black hole some of it falls through the event horizon you can see some of it looks like it's getting flung out that's material being launched into one of these Jets that I showed you the image of where the material doesn't quite make it into the event horizon but instead is flung out at nearly the speed of life these computer simulations tell us where the gas is around the black hole how much gas is there how fast it's moving how hot it is and with that information we can calculate how much light is produced at a given region around the black hole then we can ask the question if we were looking at this computer simulation from far away what would it look like what would an image of the light from the gas spiraling into the black hole in this computer simulation look like to a distant observer and in this particular case it looks like this so again the color here is basically showing you information about how bright the light is so yellow white is very bright light black is very little light and so on in between this is one simulation and one image we can do another simulation and make a prediction and it will like this some differences but you notice some similar features we can do another one looks like that so you'll and I could keep going we've done many of these other groups have done many of these you'll notice that depending on the exact simulation the exact time that I make this mock observation it looks a little bit different but there's some very broad features that are present in all of these pictures there's always one side is brighter than the other and there's an absence of light from the central region and more light in a sort of ring around the outside so why is that so once I explain that to you all basically I've explained to you the key features of the actual image that the event horizon telescope took there's two key things about light you need to know to make sense of this image what you're familiar with in everyday experience is that if you have a light bulb and you look at the light bulb from different sides it basically looks the same it puts out the same amount of light in every direction amazingly that's not true if the light bulb is moving really really really fast in particular if the light bulb or whatever is producing light the gas spiraling into the black hole if the light bulb is moving in this direction at close to the speed of light then it turns out that if you look at it here it looks way brighter than if you look at it from this side or from this side or from this and the way we kind of describe this is we say that the light is beamed in the direction of motion of the source of life the light bulb in this case so this may sound weird but you're actually familiar with a very related piece of physics from your everyday experience [Music] [Applause] so that's a car horn driving past you [Music] [Applause] the pitch changes okay it goes from higher pitch to lower pitch at some point why is that that's because the car is coming towards you and then at some point it reaches you and then it starts going away from you and the properties of the sound that you hear are different when the car is coming towards you and when it's going away from you that's what we call the Doppler effect in physics and what I'm describing to you here is basically a slightly more sophisticated version of the same basic idea that the properties that you hear or see depend on how fast the object is moving relative to so why is the image brighter on one side than the other well remember I described to you that how is this light produced in the first place it's produced because there's this swirling disk of material around the black hole moving at nearly the speed of light and half of that disk is going to be coming towards you like this the other side's going to be going away from this side is brighter because that is the side that happens to be coming towards us when I created this mock observation this side is dimmer because that's the side that's moving away from us at nearly the speed of look so that difference in brightness on the two sides is fundamentally due to the fact that the material the gas spiraling into the black hole is moving very close to the speed of light it's moving so fast that the light produced by that gas preferentially goes in the direction of motion which creates this asymmetry one side brighter than the other okay the second bit of physics about light I need to describe to you is that the gravity of an object can actually deflect the path of light now this is true certainly for a rock right let's go back to our example we throw a rock up it comes back down that's gravity deflecting the motion of the object the same thing is true for light gravity actually changes how light moves so this is an example where you have the Sun here the earth here and a star here the light seems to be going on a line that would take it over there but because of the gravity of the Sun as the light passes close to the Sun it gets it gets tugged towards the Sun a little bit and that deflects it towards the earth and so it's path is bent a little bit by the gravity of the Sun and so when we look up at the sky where we actually would see that stars over here even though the star is actually there this is an effect known as gravitational lensing this was first detected actually in basically the experiment I'm showing you here the bending of light by the Sun is one of the famous experiments that confirmed Einstein's theory of general relativity this deflection of light by the Sun now this also happens if you have light passing by a black hole the difference is that the gravity of the Sun is pretty puny compared to the gravity of the black hole so the gravity of a black hole can dramatically change how light travels so to show you this this is a these are actual calculations showing you light coming in the black region is the black hole light is coming in each of these rays follows the path that light would travel so here light comes in and is bent by actually almost 45 degrees that's pretty good this light ray is a little bit closer to the black hole it gets bent more this one's a little bit closer still it gets bent more at some point the light gets bent so it basically starts going out the direction it came from so it basically the light gets turned around almost like a mirror and it goes back out the direction it came from this case is even more remarkable the light comes in sort of orbits around and then goes back out again so light traveling close to a black hole its path is dramatically altered by the gravity of the black hole if the light rays are too close to the black hole the light actually gets absorbed by the black hole so this light is trying its best to avoid the black hole and go past the black hole right it's not headed straight towards the event horizon here that's the event horizon it seems like it should be ok but the bending of light by gravity causes this light to actually bend in and get absorbed by the event horizon of the black hole so this is gravitational lensing bending of light by gravity of objects and it applies to any object in the universe it's just in the case of black holes those are the objects with the strongest gravity and so they have the most dramatic effect on light that's near that so in this image the reason that you don't see much light from the center ok is because that's the region where light is doing this most of the light there is gas close to the event horizon there but most of that light as it tries to get out is bent so dramatically by the gravity of the black hole that it ends up falling into the black hole so you literally end up seeing a black deficit of light because of the strong gravitational lensing and bending of light by the black hole the light that does manage to make its way out ends up showing up in this sort of ring like structure that you see here and that you saw in the other theoretical predictions that I showed so what's truly astonishing then about this real observational picture okay that the event horizon telescope took is that it confirmed these basic predictions of Einstein's theories of physics about what happens close to a black hole material is moving at nearly the speed of light one sides a bit brighter than the other and very close to the black hole the bending of light is so extreme that light produced outside the event horizon actually ends up being bent and most of that light ends up falling into the event and that's why there's this deficit of emission and so in sort of one fell swoop these observations tested our understanding of how gas behaves and how light behaves very close to the event horizon of a black hole really an amazing set of results one thing you'll notice is that my theoretically predicted images shown here they seem like they have more detail there's finer structure than in the actual observations which look a little blurry err that's basically a limitation of how hard this experiment is this is as good as the telescope can currently do so to illustrate that for you I'll show you a picture from one of the papers that the event horizon telescope team did they did a bunch of the type of computer simulations that I showed you they actually made about 60,000 of these mock images that I showed you three of okay I didn't show you all 60,000 I showed you only three so these are three examples from their paper and then they said what would that look like if this were reality and we were looking at it with the telescope with the current technical capabilities that the telescope had and this is what it would look like so the fact that the observations are a bit blurrier than the theoretical models is because the telescopes basically are limited the furthest they can be apart is the size of the earth astronomers are if any you know they're very entrepreneurial so there's now ideas about putting these telescopes in space and so as time goes on the observations are going to get better and better and better and my guess is that on some timescale 10-15 years we'll start having images which actually look more like this as the observational capabilities continue to improve this is sort of like the first picture we had of the surface of Mars right which lacked all of the amazing exquisite detail of river valleys and everything that we can see today so this is really just the beginning not the end of this scientific enterprise ok so I'll end there and happy to take any questions that you have thank you like are we looking at the accretion disk edge-on or at a different angle first part of my question the second one is that image should not be taken as a qualitative appearance of what the accretion disk looks like mathematically it's more complicated right right so the question is if we go back to this schematic there's this disk of material spiraling in and what it looks like depends a bit on the orientation of that disk relative to us and so in this particular case we actually think we have an understanding of what the angle that we're looking at it is because we know the direction that the jet is coming out of the black hole that's a measured thing we're looking at something like 20 degrees off-axis but in one big difference between reality and this particular artist's conception is we think that in the gas spiraling into the black hole in the galaxy m87 it's not a thin pancake like it looks like in this picture it's much more like a big puffy doughnut and so that means it's a little bit less sensitive to the exact angle that you're looking at thank you yep another question yes could you please go back to the actual image yes you should have that on speed dial for this I think now based on your diagram the light that goes that gets bent into the black hole is going to look dark but it's bending so in my my understanding is that the black spot in the middle of this image therefore represents an area of space wider than the event horizon itself and I was wondering if you could tell me how big the event horizon is on that image right there is it the radius good question so this diameter is indeed bigger than the event horizon this diameter is related to there's a special place it turns out around a black hole where if you send a light ray out it would travel in a circle and come back to where it started okay that has a radius that's something like about about five times there so bigger than the event horizon good question thank you yep you had the equation up there that showed wavelength divided by the size of the telescope yep so in a size of telescope is determine around the location two different locations on the earth so if you had the same locations instead of using a radio telescope used an optical telescope now you're dealing with much smaller wavelengths higher energy right wouldn't that be a better solution so the question is could you do even better than we've done by combining not radio telescopes which is what this does the actual observations were taken at radio wavelengths about a millimeter or so in wavelength but if you went to a shorter wavelength okay so the wavelength of light is shorter which would be the optical or the infrared or the x-ray or something like that in principle you could get a much better image there's two issues with that the biggest one is that it's very very technically challenging to do interferometry at shorter wavelengths because that shorter wavelength it's really it's much much harder to measure accurately when the light arrived at the telescope to the precision you need to know whether the electric field was up or down or up or down so technically what I'm saying is for those who know this it's it's hard to measure the phase of the light accurately enough optical or infrared wavelength so that's that that is very very challenging and in particular it this has never been done there has been interferometry at optical or infrared wavelengths with telescopes separated by like 100 meters but not separated across the earth that's never been done the other problem it does slightly more practical problem but it is a relevant one is that it turns out that the two black holes that are the biggest on the sky m87 in the centre of our galaxy this is an amazing coincidence they turn out to emit most of their light in the millimeter which is just complete blind luck basically so they're not as bright in the optical or the infrared which means it would be sort of a harder experiment to do for that reason as well good question thank you yep with this technique can we image the dimes on the moon or maybe some can we image the solar system bodies like Phobos Deimos maybe we can look have a closer look yeah great question so can we use this technique to actually image objects in the solar system to create high-resolution images of objects in the solar system the challenge with this technique is it needs bright sources that are actually relatively small so if you tried to look at something like the moon with this technique you'd be looking at this tiny patch on the surface of the Moon a dime in size and it would be incredibly faint and it would be completely hopeless so this technique I think actually doesn't work particularly well for objects in the solar system you can use it on other astronomical objects but you need sources that are very very bright very bright and somewhat small you want an object whose size not that much smaller than the sort of angular resolution of the telescope so good question yep yes I my question relates to interferometry and I was curious what is what is it that you need to synchronize so I see the clock's up there is it you alluded to it I think a couple questions back is that you only need to synchronize the peaks and valleys of the waves themselves so it doesn't really matter when each telescope captures those images you do need to know when they catch the images the key thing is you need to be able to accurately measure the exact phase of the light the peaks and valleys of the of the light as it hits the telescope and then how much data do need to capture how long do you need to capture light for origins and do all those stations do that at the same time right so the dishes do it at the same time they do it basically at the same time and they actually took data for something like four or five hours and part of the reason you're doing that is because you know these are distant sources they're not that bright I mean they're they're bright but they're not that bright so it takes a while to get enough light to see you also use the fact that the earth is moving rotating in four or five hours that turns out to help you take make a somewhat better image because it's moving exactly where the telescope's are relative to the source on the sky and that helps you make a better image so the kind of I would say dream vision now is the idea of having a couple of satellites in orbit orbiting around and you orbit every 90 minutes and so you orbit you know three or four times you take all this data you've essentially filled in an incredibly detailed image of the object that's that's one direction that the field might end up going question this one's related to time and as something is crossing the event horizon after and before that you see in all the movies about the dilation up to the event horizon but after it's cross the event horizon does that dilation just continue till you get to the singularity at the bottom of what happens inside right so the question is asking about sort of the passage of time which turns out as material is falling into the black hole as measured by us far away it seems like time is sort of slowing down as the object falls in so for instance if you had a laser pointer in every second as I don't want to do it as Andy falls into the black hole every second as he's pressing the laser pointer sending the signal out to me what I would measure far away's as he gets closer and closer to the event horizon the signals would get further and further apart not one second one and a half two three four etc as he falls through the event horizon when you get inside the event horizon it actually gets a bit more complicated everything falls into the horizon sorry everything falls into the singularity the there the time dilation switches character so it doesn't really continue in the same way because it's already gotten when you hit the event horizon it's gotten as extreme as it can in the way it was doing it and it sort of changes character and how time dilation actually happens and there's a sense in which the time and space sort of switch character once you're inside the horizon it's that's a slightly technical sense in which that's true but that's the that's actually the change that happens as you go through the event rods I'm uh say I want to hear you may say more about your last answer if you can even though it wasn't one of the questions I plan to ask because you've left me in a state of mystery which will help me fall asleep very nicely deny but okay you're welcome yeah but anyway yeah but let me ask my questions but if you do have more to say about time and space or switching character I'd love to hear it but when you were talking about most of the energy we observed from the region of black hole coming from the friction of the differential speed to the matter being accreted into the disk or into the black hole I also had the impression that black holes would have a very strong magnetic field or outside the event horizon and that if you accelerate charged particles through magnetic field you get a lot of energy that way roughly how much of what we see is because of that right so I I gave you the friction answer and analogy and that glosses over a lot of details so what is actually the source of the light that we see here this is a process of producing light that's called synchrotron radiation and that's when you have an electron spiraling around a magnetic field the electron is very hot it's spiraling very quickly around the magnetic field and it produces a bright source of light so that actually is the process that is producing the light somewhat related to what you were alluding to what gets what makes the electrons hot in the first place hot enough to produce that radiation is the friction that I was referring to last question I was killed I could I could throw things at people who ask multiple questions well I'll take that advisements I'm falling to sleep thinking about the mysterious answer the question before mine but but if the Sun were suddenly to turn into a black hole and there's no more line I've got to think there'd be a lot of material close enough to the Sun instead of being blown out toward us because of the solar wind would be sitting there and then drawn back into the Sun and accelerated to high speeds how much white light would we get and for about how long so that's a great question I've made that analogy about replacing the Sun with the black hole many times nobody's ever asked that question so I don't know the answer to that I would that's a calculable thing but I don't know the answer off the top of my head good question all right this is a question I've had for a little while I don't know much about and it's not related to taking pictures of black holes but it's about a particle physics and it's about disproving like did you like the idea of a graviton as a particle that exists to intermediate like the force between two masses so if gravitons exists and if they behave like electromagnetic radiation in that they exchange photons doesn't that mean that black holes should not be able to exert any gravitational force because gravity like graviton so would not be able to leave the event horizon so like right so yeah just prove that specific idea so let me give a little bit of context for the question so our understanding of modern forces like gravity electromagnetism is that at the quantum mechanical level they can be understood as being produced by the exchange of particles so the electric force which is actually what's stopping us you know stopping my hand from going through the podium is the electric force that is associated with the exchange of photons gravity is associated with the exchange of a particle that's called a graviton that's the quantum mechanical particle associated with gravity we don't really have a complete quantum mechanical understanding of black holes that would require a quantum theory of gravity that we don't have but in the versions in the theories that we do have the resolution of your question is that the gravitons are produced outside just outside the event horizon they're what are called virtual particles some of which are produced outside the event horizon some fall in some escape to a distant observer sort of mediating the force so they don't you're you're right they can't come from inside the event horizon in a classical sense and the resolution of that is you can his thinking of them as being produced just outside the horizon um really quick follow-up question yeah no okay sorry okay could you say a little bit more about mechanisms linking the accretion disk or doughnut to the jet so the right the question is what is the relationship in this again very schematic thing between the accretion disk and the jet the the basic mechanism is as came up in a previous question is actually magnetic fields so we think what's going on in in practice in real accretion disks and real jets is that this disk of material has magnetic fields in it those magnetic fields are sort of going out of the disk and the jet material is material that's accelerated out along those magnetic fields so the magnetic field is sort of the conduit where matter gets matter and energy gets transported out away from the black hole mediated by the magnetic field in your presentation there was just one little diagram showing something labeled a singularity which might to some might be considered an abhorrent notion one of the question is touched on this yeah so so what is this thing called a singularity is it is it just a punt because we don't know the physics that's going on there or has it really been worked out theoretically what attributes does it have what kind of the particle structure does it have oh and what happens if black holes collide right so the question is about this singularity here so the prediction of Einstein's theory on Stein's theory of relativity makes an interesting and somewhat surprising prediction it predicts that all of the matter in the event erotic falls through the event horizon if the black hole is not spinning okay which is the diagram here all of the matter ends up at a point at the center so it predicts that the material gets arbitrarily close together if it's all at a point it gets arbitrarily close together and we think that can't be the complete answer so this is an interesting case where the theory sort of predicts its own breakdown because it predicts something that we don't think in reality is likely to happen this is where a lot of the interest in black holes from a physics perspective comes from is trying to understand what really happens at that singularity because that's one of the few places in nature where you have lots of mass so gravity is important but it's in a really small region so our theories of they're really small which are the quantum theories are important so understanding what happens there we think requires a quantum theory of gravity which we don't have so there's not a complete description of what's really going on at the center until we have a better more complete quantum understanding of gravity and a quantum understanding of black holes so that's that's the somewhat unsatisfying but but actual state of things right now there's a huge amount of interesting work sort of in physics departments trying to understand this for the most part for an astronomer we think that that's not very important for understanding what we see when we think about black holes in the universe because that's all happening inside the event horizon and we can't see inside the event horizon anyway so it's very interesting conceptually very important for physics but not important for understanding sort of the astronomy of black holes so that's the distinction I would make three more okay so I was wondering how long do black holes last and when or if they do end what do they turn into yeah great question so in Einstein's theory of relativity which is our most complete standing of black holes black holes live forever black holes in Einstein's theory can have any mass and they live forever they just sit there if they're by themselves they just sit there you know looking well not like that like if there's nothing there there's no gas they'll just sort of be an isolated black hole wouldn't look like anything particularly interesting but it would last forever and it's more complicated when people try to understand quantum theories of black holes in those theories there's something called Hawking radiation which is that black holes actually do lose a little bit of mass very very very slowly over time so in principle black holes like this billion solar mass black hole at the center of m87 in principle if you wait long enough that black hole if our quantum guesses about how black holes behavior correct that black hole might eventually disappear that will take much much much much longer than the current age of the universe to happen what would be left behind is an incredibly active area of debate whether there is anything left behind whether there are some fundamental particle left behind and I think the jury is really still out on that question because that again gets back to this you have to understand the last phases in the quantum mechanical life of this black hole and we don't really understand the quantum quantum mechanical nature of black holes completely enough to answer that question okay this is sort of a repeat a little bit I mean the universe seems to have originated in a Big Bang and if the Big Bang came from a very small place would have have to have been a black hole and if so how can a black hole that the previous question was how did how would a black hole ever explode so they're right so there are two instances we know of in the universe where there seem to be according to our current theories these singularities where matter is incredibly dense incredibly close together we need some quantum gravity theory to understand what's going on one is the center of black holes and the other is you rewind the clock of the expansion of the universe back to the beginning of the history of the universe at the Big Bang but that doesn't mean that the even though both have singularities that doesn't mean that the Big Bang actually was a black hole or were in inside a black hole in some sense in fact in the expanding universe you can ask the question at any point in the history of the universe you can draw a circle and ask how much mass is in that region and is that region then larger or smaller than the event horizon and the answer is that there's never a black holding so there's a singularity at the beginning but that doesn't imply that there's so much mass that there's actually an event horizon so there's a singularity but not a black hole or an event horizon in the sense that we've been talking about how could that be I mean it's there's there's more mass there than in any of these black holes right so why don't we talk about this afterwards so again if you the the models allow you to calculate at any point in the history of the universe how much material is there at a given place so you can just plop down a sphere and ask in this sphere do I have enough mass to collapse to form a black hole or not and the answer is no so really yeah so in just a uniform expansion of the universe you don't have the conditions needed to produce black hole oh yeah that doesn't again we don't understand the singularity so exactly at the very very very earliest time what happened is still up in the air but for every time after that we actually can't address that question yep hi thank you for the talk I'm interested in knowing if it was difficult to get astronomers from different countries to kind of collaborate together on this project and if there are any future projects that will require the collaboration of the international community so that's a that's a great question that's like a politics of big science questions more about sociology it's a really it is a really good question and the answer is is actually very hard and the challenge is that each of these telescopes decides who gets to do the observations based on a peer-reviewed proposal process so if you want tells time on Alma or the plateau they are to telescope or the submillimetre array or whatever you have to write a proposal to that telescope and ask to have the time and then they usually tell you you get this day you get that day and so what this takes is an incredibly orchestrated process of putting together a coherent proposal to all of these facilities telling them that you need the observations on exactly the same time because remember the light has to be recorded at the same time because we need to kind of reconstruct what the light was doing at every telescope at the same time so it's actually quite challenging there's also other logistical things which are more mundane but turn out to be a huge pain also like weather right because you want the weather to be pretty good at most of these facilities at the same time and that's not necessarily going to happen so for instance they had data a second year where the data just wasn't as good because the weather was not good at some of the sites so this is both sociologically non-trivial and practically non-trivial for a variety of reasons so yeah great question you

Channel: SVAstronomyLectures

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Keywords: astronomy, science, astrophysics, science news, black holes, black hole, Eliot Quataert, M87, M87 black hole, Event Horizon Telescope, astronomy news, black hole photograph, black hole image

Id: EoJGSKGBrHU

Length: 77min 26sec (4646 seconds)

Published: Tue Jan 28 2020