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Black holes with Sarah Gallagher


Uploaded on February 4, 2022

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Black holes with Sarah Gallagher

2022-02-04 – A black hole is created after the death of a very massive star. In this video, learn more about black holes with Sarah Gallagher, astrophysicist and Science Advisor to the President of the Canadian Space Agency. (Credit: Canadian Space Agency)

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Sara Gallagher: Hi, everyone.  It gives me great pleasure to be here today to talk to you about black holes. 

So, I’m going to tell you about supermassive black holes, and I call them the biggest blowhards in the universe. So, I’ll let you -- and I don’t know how to translate that appropriately in French, so I would appreciate some feedback on that later.  So, I will tell you a bit about them.

So, first, what is a black hole?  So, you’ve probably seen images something like this.  These are artists’ pictures of what a black hole looks like.  And, of course, it’s black and it does something weird to space time, but these are artists’ pictures.  And when I describe black holes, when I define them, I think it’s more helpful to think about it in terms of escaping from a black hole and talking about escape speed.

So, we have different objects in the universe.  We work at the Space Agency.  We are very familiar with rockets escaping the gravitational pull of the Earth, it has to be going 11 kilometres per second, straight up from the surface of the Earth in order to escape.

Now, if we have a bigger planet, something like Jupiter, you have to be going quite a bit faster in order to escape from the gravity of Jupiter, something like 60 kilometres per second.  From the Sun, you have to be going 10 times faster, 620 kilometres per second in order to escape from the gravitational pull of the Sun.

But a black hole, the gravitational speed -- the speed that’s required to escape from a black hole is the speed of light, 300,000 kilometres per second, and nothing can go faster than the speed of light.  So once you cross that threshold, once you have something that is so massive, that has such intense gravity that it requires that you travel faster than the speed of light to escape it, you have a black hole because nothing can travel faster than the speed of light.

So black holes, in some sense, are extremely, extremely exotic, but in another sense, they’re really, really simple.  We can think of them, in some sense, as being bald.  They have three numbers that you can use to describe them.  We call this the no-hair theorem in physics, and they have no hair because they have no distinguishing features.  So, think about something like the Earth.  If you want to describe the Earth, think about how much information you would have to share.  You would have to talk about its mass and its shape.  It’s not even spherical.  You’d have to talk about how much water there is and where it is, and clouds, and all of these different complicated things about the Earth in order to describe it.  But a black hole, you only need three numbers, and you can completely and uniquely describe a black hole.  You need to know how massive it is, how fast it’s spinning and if it has an electrical charge.

And in fact, black holes that actually exist in the universe can mostly be described with two numbers, just the mass and just the spin, because something that exists in the universe and has a charge will normally attract the opposite charge and become neutral.  So, two numbers can completely describe a black hole, and that’s it; that’s all you need.  So, in some sense, they’re beautiful, simple objects, even though they’re very exotic compared to what we’re usually used to.

So, I always get asked: “What happens if you fall into a black hole?”  I actually got asked that this morning when I was getting my coffee by the lady at the cashier stand.  So, what happens when you fall into a black hole?  I usually answer this by saying “I don’t know.  It could be leprechauns and unicorns.”  We have no information about what happens once you fall into a black hole.  That piece of the universe is inaccessible to us, because we learn about the universe; we communicate with each other ultimately through lights, and light cannot escape from a black hole, which means we have no information about what happens once you cross that horizon.  We call it the event horizon because we cannot see what happens past it.  We know that you can grow a black hole, so you add to the mass when something falls into it.  We know that you can spin it up, so that can change.  But other than that, we have no information about what happens when something crosses that threshold, and we also have no information about what it is anymore.  The property of whatever falls into a black hole, what elements it’s made out of, if it’s square or round, all of that information becomes inaccessible to us once it crosses the event horizon.  So, they’re very exotic objects.

So, if we think about how dense something has to be in order to be a black hole, we can take something like the Earth, which from our point of view is a very substantial, large object.  If we wanted to turn the Earth into a black hole, we would have to take the entire mass of the Earth and compress it down to the size of a sugar cube.  So, in order to make a black hole out of the Earth, it would have to be compressed down to the size of the sugar cube.  It would still have the mass that it has today.  It would still keep the spin that it has today, the rotation of the Earth, but it would be the size of a sugar cube.

So, the Earth is not going to turn into a black hole.  This is -- among the things we need to worry about, this is not one of them.  But what about the Sun?  What if the Sun turned into a black hole?  So now we take something the size of the Sun and we compress it down to a black hole, and the Sun would be approximately -- would have a radius of three kilometres across, and so I would put a map of where we are right now -- there we are at the Canadian Space Agency, and that blue circle represents the size of the black hole -- of the event horizon of the black hole if we took the sun and we compressed it down into a black hole.  So, very, very tiny compared to the size it is today, so just to give you an extent of what would happen.

Now, what would happen to us if the Sun were compressed down into a black hole?  So that’s something I also get asked all the time.  Because especially if you see movies and how black holes are rendered in those sorts of things, you think that something turns into a black hole and it automatically starts sucking everything in.  But that’s actually not what would happen.  So, if the Sun turned into a black hole tomorrow, which again is nothing anyone needs to worry about, but let’s follow this thought experiment, what would happen to the Earth?  Okay, it would be dark and cold, and that would certainly be bad for our future.  It would definitely cure climate change -- well, the global warming part of it.  But it’s -- but it would not affect the orbit of the Earth at all.

So, here is a picture of our solar system with the Sun as a black hole, and you can see  -- I don’t know what the source of light is supposed to be here, but you can see that the orbit of the planets would stay the same because we are far enough away from the Sun that our orbit, the Earth’s orbit, would not be affected if the Sun turned into a black hole tomorrow.

And so, the takeaway point from this is that black holes don’t suck.  So, first of all, they are awesome, objectively, but they also don’t just suck things in, so, in general, when you -- as long as you are far enough away from something.

So, then the next question is -- and I’ve told you about the nature of black holes, and the next question is okay, so they don’t give off any light.  We know that the black hole itself is invisible, so how do we find them?  So, I need a volunteer.  I need a star.  Who is willing to be a star?  It won’t hurt, I promise.  Come on!  Come on up!  Fantastic!  Okay.  So how do we find a black hole?  Nice to meet you, Mr. Star.  Okay, you’re a star.  You’re a superstar.  Okay.  Can you put it on your head?  Okay, fantastic.  Come on over here.

Now, I know it may not be obvious, but I am a black hole and you cannot see me.  I am completely invisible.  This is our star.  Can you introduce yourself?

Syd: I’m Syd.

Sarah Gallagher: Syd!  Syd is our star.  Okay, Syd.  So, I am a black hole.  He is a star.  We’re going to hold hands, and I want you to orbit around me.  Okay.  I’m invisible.  You can’t see me, right?  Can you see Syd moving?  Yes.

Syd: This is fun.

Sarah Gallagher: I don’t want you to get dizzy.  Thank you very much, Syd.  Let’s thank Syd for being a fantastic star.


Sarah Gallagher: Thank you.

So that’s how you find a black hole.  You don’t find the black hole itself.  You look at the things around it and you see how it is affecting the things around it, things like stars, if it has a partner star, or gas and dust that’s around it.  The black hole -- the gravity of the black hole affects the area around it, even though we can’t see it, and that’s how we find it.

So, I promised you supermassive black holes, and I’m going to deliver.  So, where is the nearest supermassive black hole?  Does anyone know?  Centre of the galaxy, absolutely right.  The centre of our galaxy has a supermassive black hole.  Our galaxy is called the Milky Way.  The image on the right there is a fish-eye view of our Milky Way.  If you go to a dark site, in the summertime in particular, it’s easier to see.  Go to a dark site and you can see our Milky Way galaxy.  That’s the galaxy that we live in.  From our point of view, we often get a view that looks kind of like that view on the right-hand side, but if we were able to take a spaceship, which is not within our current capabilities, anyone human, and fly above our galaxy and look down on it, it would look something like that galaxy on the left.  Our galaxy is a spiral galaxy, and we’re about two-thirds of the way from the centre of our galaxy and the disk.  So, there we are.  I’ve marked our spot on the map.  And in the centre of our galaxy, in the centre of every massive galaxy is a supermassive black hole.  And this black hole, supermassive black hole, is the one that is the best known.  Here it is; this is a star map.  Our supermassive black hole is called Sagittarius A*, for weird historic reasons.  It’s called Sagittarius, though, because it’s in the constellation of Sagittarius.  It’s called A because it’s -- that’s the first radio source that was found in that constellation, and it’s called star because it’s actually not that radio source; it’s the one next to it.  So that’s why it got the little asterisk.  But it turns out that that’s where the supermassive black hole in the centre of our galaxy is, and Sagittarius is a summer constellation.  So, we can get the best views in the summer because we’re looking towards the centre of our galaxy.

So, this is animation which shows how we have measured the mass of the supermassive black hole in the centre of our galaxy.  Now, this is not -- I know you’ve seen all -- you’ve all gotten spoiled by the beautiful animations that come out of Communications here.  They didn’t have that awesome team, but what this represents is really remarkable.  This represents three decades of work of two separate teams working in the northern hemisphere and the southern hemisphere and going and looking at all of the stars right around the centre of our galaxy one year, every year, going every year, measuring the positions of all of those stars, and then watching how those stars moved over the course of decades.  And with the motions of all of those individual stars -- that’s what’s represented there -- they can measure very, very accurately the position and the mass of the supermassive black hole in the centre of our galaxy.  And what you can see is that there’s no bright thing right there in the centre of our galaxy, and that’s because our supermassive black hole, though it’s ours and though we love it and we understand it very well, better than any other supermassive black hole in the universe, is actually kind of boring.  So, it’s not really doing much.  It’s just kind of hanging out.  There’s stars moving around it.  It burps and fizzles every once in a while, but nothing dramatic.  And it’s also, on the scale of supermassive black holes, kind of wimpy.

So, supermassive black holes can range from about one million times the mass of our sun to about 10 billion times the mass of our sun, and this one is about 4 million times the mass of our sun, so on the low end of supermassive, but still, it squeaks in over the top, so we’ll count it.

So, let me tell you where the nearest really, really big black hole is.  And this is in the giant galaxy M-87.  So, this is a galaxy.  You can see right away this looks really different than the image of the spiral galaxy that I showed you before.  The colour is different.  The shape is different.  It looks very, very smooth.  So, this is a giant galaxy, many thousands of times more massive than our galaxy, and in the centre of it, it’s got a really, really big black hole.  Here, this shows the map of where M-87 is located.  It’s in the Virgo constellation.  It’s part of the Virgo cluster, which is a cluster of thousands of galaxies that all live together.  And what’s remarkable, when you start looking at galaxies like this, is you can see it looks kind of boring.  This is with optical light, the light we can see with our eyes.  But if instead we look with radio light, this is what we see.  Oh, sorry, this is X-ray light.  So, this is from the Chandra X-ray Observatory.  It’s a space observatory that’s taking an X-ray picture of this galaxy.  X-rays are capturing light, capturing gas, a picture of gas that’s million of degrees hot, and what you can see is M-87 looks way cooler in X-rays than it does in optical light.  We’re able to see these extraordinary shapes there, so it looks bright in the centre.  So, that tells us something interesting is in the centre.  And then you can see that there are these sort of squiggly patterns coming out from it, and that’s energy that’s ultimately generated in the centre of that galaxy by the supermassive black hole that lives there and is being shot out into the galaxy at very, very, very velocities, and it’s disturbing all of that million-degree gas.  It’s invisible in optical light because the stars don’t -- it doesn’t bother the stars, but it bothers the gas, and the gas we can see with X-ray images.

Recently, last year, there was an image that was taken of the supermassive black hole in the centre of M-87 by what’s called the Event Horizon Telescope.  And it’s not very often that I get to include an astronomical image that was fixed by XKCD, which is a fantastic web comic, if you don’t know it.  So, I’m quite fortunate about this.  And this shows you the image, which looks kind of like a lopsided doughnut, not nearly as symmetric as a proper Montreal bagel, but you can see the lopsided doughnut there, and this is an image that was taken with the event horizon telescope.  Now, just to give you a sense of scale -- so I’m going to go back to the image I showed you before.  So, from end to end, the squiggly bits right there of the jet, which is the X-ray emitting gas that’s been shot out from the centre of the galaxy, that is about a million light-years across, a few million light-years across.  So, it takes a million years for light to travel from one end of that to the other.

Now, in contrast, this is an image where you can see the circle right there is the orbit of Pluto.  So, Pluto is a couple of light-hours across.  The distance is not quite light-hours, but light -- well, close to a light-hour.  Okay.  So, remember, millions of light-years, which is the size of that entire galaxy, and this black hole right here is about a few light-hours.  This is the inner part of the black hole.  The black hole is in the centre.  The black hole itself is invisible, but this image is -- this is an image taken of the gas, the hot gas that’s around that black hole.  This was taken by what’s called the Event Horizon Telescope, which is one of the most beautiful experiments that’s been done recently, from my point of view.  This was an experiment where you had hundreds of astronomers from around the world and computer scientists and engineers, and lots of other talented people who took eight telescopes and they pointed them all -- eight telescopes around the world, from Chile, Hawaii, Europe, eight telescopes around the world, pointed them at M-87 at exactly the same time.  The weather was clear at all of those places, which never happens, and they observed this galaxy, and then they spent months and months and months then taking all that data and putting it together and making this amazing image.  So, the image itself is not that fancy looking, but what it represents is really, first of all, a beautiful story of international collaboration and ambition and just, I mean, initiative that’s fantastic, but also just this technical challenge that they overcame in order to take this picture of the inner part of that supermassive black hole.

So, M-87 is a big black hole.  That’s supermassive by anybody’s standards, 10 billion times the mass of our sun.  But again, I have very high standards.  It’s still kind of boring because it’s not really doing that much.  It’s got that cool jet and it’s in this really big galaxy, and there’s this lovely image, but compared to other black holes, it’s really not doing that much.  But let me just step back and show you another image.

So, this is another image where if you look at the radio light, so the radio light comes from electrons that are moving close to the speed of light.  They emit radio light, and that’s what that -- the reddish light looks there.  The bluish light there is from the X-ray light, and you can see that as you keep adding more information from more different kinds of telescopes, you get a different picture of what’s going on.  But it’s still not that exciting compared to the kind of black holes I study.

So, let me move on to those.  So, you might say, “How do you grow a black hole?”  And there’s more than one way to grow a black hole.  So, one way to grow a black hole is to take two black holes and put them together.  So, you have to have some sort of setting where that is actually going to happen.

And recently -- now, think about this.  If you have two black holes and you want to put them together, they both are invisible if they’re by themselves, if they don’t have a star living around them.  They’re both invisible.  So, how do you find two black holes that you’re going to put together.
And let me show you a video which represents two black holes that are moving together.  So, this is a computer simulation.  You can see the two black holes right there, the black holes.  The surface underneath represents how they are distorting space time.  So, a black hole is an intense pull of gravity, and it distorts the space time around it.  Black holes, if they’re close enough together, they will merge through a process that’s called gravitational radiation, and that’s what this video represents.  You can see those black holes are getting closer and closer together, and as they get closer and closer together, watch what they do to the space time around them.  So, the time is slowing down right here, and you can see that what they’re doing, as they -- it’s called the in-spiral, that last little bit where they’re coming together.  They are disturbing the space time continuum so intensely that they are actually sending a ripple of energy out into the rest of the universe that carries energy away.  You start with two black holes, and when they merge, the mass is a little bit less than the sum of their two masses because some of that mass gets converted to energy, which is sent out into the universe.

And what’s really amazing is that we can actually measure those ripples in space time, and that is the result of the experiment, which is called LIGO, which is the Laser Interferometer Gravitational Observatory, which is actually measuring the ripples in space time that are sent out by two black holes that come together.  And the way it works is that they have two arms which are each four kilometres long, and there is a laser that is sent out from the centre into each arm, and it’s measuring the distance along those arms incredibly precisely.  And if a ripple of space time passes over the observatory, those distances are going to change a little bit in a characteristic way by a fraction of the radius of a proton, by an incredibly, incredibly tiny amount.  This is my second favourite beautiful experiment because it is so hard, and they did such a beautiful job.  And if you see a characteristic ripple in the distance between those two arms, then that lets you know that there has been a gravitational wave that has passed through all of the Earth but was measured by these two observatories.  And that happened with -- and this observatory has now measured a few events.  Those graphs on the bottom measure the ripple in space time between those two different -- between the two different arms, and the way it -- you can see that it starts as a little sort of slow ripple, and then it becomes faster and bigger, and that’s a very characteristic ripple that allows you to actually tell the mass of the black holes that merged together, how far away they were and what you wind up with at the end.

So, I’m going to play the sound of that for you, because it is -- it gives you a better sense, I think, of what it is.  And these are called -- they call it a chirp, and this is just representing that ripple in sound so we can hear what it is.  Just let me play it.  So that’s called a chirp.  Let me play it again because I love this.  And so, what that is, you can hear that it sounds -- it’s low.  It’s a lower sound, and it’s quieter, and then it goes up in pitch and it gets louder.  And so that’s the characteristic pattern of two black holes that are merging, and this is how you actually find black holes that are merging.  So that’s one way that you can grow black holes, by merging two black holes together.

But there’s another way you can grow black holes, and this is my favourite way of growing black holes, and this is if you have a black hole in the centre of a galaxy, now, the black hole itself is invisible, but if there is gas near the black hole, it can basically be pulled in.  If it’s in a dense disk, it can be pulled into the black hole.  As it’s pulled into the black hole, it gets hotter and hotter.  It gives off light.  The light carries away energy, and that allows that gas to fall into the black hole.

And this is what we call a quasar.  So, a quasar is a black hole that’s growing very rapidly.  We don’t see these in local galaxies.  We have to look far away in order to see quasars because that’s when the big black holes in galaxies grew, when the universe was much younger.  So, we have to look farther away, which is back in time, to see those black holes growing.

Now, what’s remarkable about a quasar is that accretion disk is a few -- say a light-year across.  So that distance is basically the size of our solar system if we include all of the big things on the outside of our solar system.  But something like a quasar can outshine all of the trillions of stars in a galaxy by a thousand times.  So, think about that.  You have something about the size of our solar system.  It’s got a supermassive black hole, say a billion times the mass of our Sun, and it is outshining the trillions of stars in a galaxy by a thousand times.  So, these are incredible, amazing objects in the universe.  And one thing that they can do, which is what I do my research on, is they can actually blow really, really, really fast winds.  So that light, which is so powerful that I told you is in the accretion disk from the gas that’s falling in, it has to give off light to shed its energy so it can fall into the black hole so it can grow, that light is so powerful that it can blow really, really fast winds.

Now, I already told you that black holes don’t suck, and in this case, what’s happening is the material is falling into the black hole.  Only material that’s quite close to it will feel that intense gravity from the black hole, which allows it to fall in.  But I like to think of black holes like this, as being kind of like the Cookie Monster.  Everybody know the Cookie Monster?  Yeah?  Okay.  If you don’t, come see me later and we’ll go over it, because he’s very important.  Okay.  So, I think black holes are kind of like the Cookie Monster.  So, first of all, there’s the Cookie Monster.  You know the Cookie Monster really, really likes cookies, right?  And you can see it’s sort of got a black hole right there where all the cookies go down.  But the Cookie Monster is a total slob.  And so, he’s eating cookies, but there’s probably as much cookie that goes all over the place as actually winds up in his belly.  And black holes are kind of like that too.  So, a black hole that’s actively growing that is giving off a tonne of light, everything that gets near the black hole doesn’t wind up in the belly of the black hole.  It doesn’t wind up actually falling into the black hole.  Some of it goes flying off into space.

And this is the same type of thing that we see with something like a solar sail.  So, if you have a solar sail, and I imagine you have heard of this as an idea for how we might travel to other solar systems.  You would have something that would catch light, catch the momentum from light from the Sun, and that light actually packs a punch.  It has momentum.  It can push on material like a solar sail, and it can accelerate it and it can make it go faster.  And that’s exactly how windy quasars work as well.  So, the light is so intense that it can blow really, really fast winds, and this is -- these are the types of objects that I study for my research.  These winds are really fantastic because, first of all, they carry a huge amount of energy.  So, these winds can be up to tens of thousands of kilometres per second fast.  That is, by any objective standard, really fast.  So those winds can carry a huge amount of energy away from the black hole.  So, you have material that’s falling into the black hole.  It’s causing it to grow its mass and to spin up, but you also have material that’s being flung out into space at really high velocities which carries a tremendous amount of energy.  And in fact, we think that these winds might be so energetic that they might be able to actually strip all the material from a galaxy.

Now, our Milky Way galaxy is currently making new stars.  We have gas and dust in our Milky Way.  It’s making new stars.  There are certain things, if you go out in the winter and you look at Orion’s sword, there’s a red star that you can see that’s actually a stellar nursery, where new stars are being born in our galaxy.  And galaxies that are still making stars usually look like spiral galaxies.

But if you have a growing black hole in the centre of the galaxy that’s making stars and that black hole gets really going and can have really, really fast winds, there can be enough energy in those winds to strip all the gas from the galaxy and completely shut down star formation in the galaxy.  And if we go and we look out in the universe, we see galaxies like that, where nothing is going on.  I showed you one earlier, M-87.  And so it could be the case that when you have an actively growing black hole with a really, really fast wind, you can take something that looks like a beautiful spiral galaxy, strip out all the gas and dust, turn off star formation, and you can wind up with something like what you see on the right, an old red and dead galaxy like we see, like M-87, where we have a supermassive black hole that was observed with the Event Horizon Telescope.

So, black holes can be really important even though they’re quite small, quite tiny.  So, remember, only, say, a light-year across for the black hole in something that’s a million light-years across for the galaxy that it lives in, and yet that relatively small thing can have enough energy to actually really affect the big galaxy that it lives in.  And this is why I think these windy black holes—one of the reasons that I think they’re so interesting and exciting to study.

So, I’m just going to take you through and remind you that we started with what is a black hole and how we can think about it.  How we find them in the local universe and elsewhere, and then also what effect they can have on the huge environments in which they live, and how they can actually have a really profound effect on galaxies.

So, I would like to thank you so much.  This is my blowhard.  So, there you go.  So, thank you so much for your time today.



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