Pod Insights Modern Wisdom The Secret World Of Black Holes - Dr Becky Smethurst About this episode Dr Becky Smethurst is an astrophysicist, author, YouTuber and a Junior Research Fellow at the University of Oxford.
Black holes are the weirdest, densest, most mysterious objects in the universe. However they're not black, and they're not holes. In fact pretty much everything you think you know about them is probably wrong.
Expect to learn why galaxies don't actually orbit black holes, why the biggest black hole in the universe needed an entirely different name, whether black holes can form without neutron stars, what happens when two black holes collide, why nothing can go faster than the speed of light and much more...
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#physics #blackholes #space
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00:00 Intro
00:23 What’s Wrong with the Name ‘Black Holes?’
09:19 The Formation of Black Holes
16:37 Supermassive Black Holes
26:43 What Happens When Two Black Holes Meet?
29:32 The Latter Stages of the Universe
34:27 The Universe’s Speed Limit
43:38 Can Black Hole’s Form without a Neutron Star?
50:48 What’s Next After the James Webb Telescope?
55:01 Where to Find Dr Smethurst
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Transcript 10,967 words · 5 speakers
Speaker names marked ? are inferred from speaking time and the show’s known hosts — they are not voice-verified.
0:00 Speaker 00 People ask me questions like, what's on the other side of a black hole? Which doesn't make any sense when you realise that black holes aren't holes. They are 3D objects that were once stars that have just been crushed down until they are so, so dense that the gravity is so strong that nothing can escape from them anymore, not even light.
0:22 Chris Williamson? What is your problem with the name black holes?
0:27 Speaker 00 This is the thing, this is something I've spent my entire life like trying to understand. And yet I hate the name for them. I hate it so much. I don't think there's any other words in physics that have caused more misconceptions than black holes, because they are neither black nor are they holes, black holes. And this is, this is what bugs me about them. So, you know, people picture a black hole as this physical hole in space that stuff is like lost down. You know, people ask me questions like, what's on the other side of a black hole? Which doesn't make any sense when you realise that black holes aren't holes.
1:07 Speaker 00 They are 3D objects that were once stars that have just been crushed down until they are so, so dense that the gravity is so strong that nothing can escape it from them anymore, not even light. They're almost like prisons for light. And I often say that a better name for them was probably dark star. But even that's not, not perfect either. So, I mean, I'm sure a lot of listeners could probably come up with their own better names for them. But black hole especially is one that just, if I could go back and be like, no, let's not call it that. Back to the sort of 60s and 70s, I would.
1:47 Chris Williamson? Who was the guy that called them black holes? What was the story with him?
1:52 Speaker 00 Yeah. So it comes from a bit of a harrowing part of history, actually. Are you familiar with the Black Hole of Calcutta?
1:59 Chris Williamson? No.
2:00 Speaker 00 No. So the Black Hole of Calcutta is a prison cell in Old Fort William in Calcutta in India. And there is this sort of tale from history where British soldiers were imprisoned in this prison cell, which was the size of three double beds. And there was about 70 soldiers imprisoned overnight in this one very cramped, tight space. And historians sort of claims estimate, you know, that around about 20 of them actually survived that one single night after being imprisoned in this such incredibly cramped quarters. And there was a physicist called Robert Dickey in the 60s, who was studying
2:42 Speaker 00 what then were known as gravitationally completely collapsed objects, or GCCOs, which again, as a term, I'm quite glad that didn't stick. And him and his family used to say that, you know, if something was lost in their house, oh, it's gone to the Black Hole of Calcutta. And so he started to use this phrase, you know, in his academic talks as sort of, you know, it's like, almost like brevity and advertising value, essentially was how it was described by another physicist called Wheeler as well. And essentially, he compared the crush of matter down to the crush of these soldiers in this prison cell as well, in the Black Hole of Calcutta. And I think when people hear about the Black Hole of Calcutta, there's loads of memorials still in Calcutta and India today, they think
3:26 Speaker 00 that the prison got its name from the astronomical objects. But in fact, it's the other way around, which I think people don't realise.
3:34 Chris Williamson? What would happen as you move around a black hole? So I think when people think about a black hole, they think about kind of a two dimensional object. And if you were to go side on to it, that what's on the other side, you look at it from behind, what is it like as you were, if you were to orbit around it? Is it a sphere?
3:51 Speaker 00 Yeah, it's a sphere is probably the best way of putting it. So, I mean, there were once stars, stars are spherical. So it's like saying, you know, what's on the other side of the sun? It's just, you're still looking at the sun, you know, so more of the sun. Yeah. So it's the same with black holes. And if you could sort of orbit one very safely and see what was going on, what you would essentially see is this sphere of darkness where no light was coming from it at all. You were receiving no information, nothing. And the edge of that sphere is what we call the event horizon. People might have heard that term before, this sort of point of no return, where nothing can escape the black hole anymore, because you can't travel faster than the speed of light. That's sort of a, of no law of physics.
4:34 Chris Williamson? If it's a dark sphere, what's your problem with the word black then?
4:37 Speaker 00 So the other issue that I have with black is that not all black holes are dark. In fact, black holes are some of the brightest objects in the entire universe. They light up like Christmas trees. Um, and that's because as gas comes into a black hole and spirals around it, beyond the event horizon, where we can still see that gas, it gets accelerated to huge speeds by the gravity of the black hole. And it gets hot. And just like, you know, if you stick an iron poker in a, in a forge fire, right? And the iron starts to glow because it gets hot. This gas starts to glow in X-rays and UV light and even visible light that you can see with your own eyes as well.
5:17 Speaker 00 And so most black holes from, you know, what we call stellar mass black holes that are formed in supernova when stars die and we find them across our entire galaxy. You know, there's probably millions to billions of black holes in our entire galaxy right now that are somewhere around like 10 times the mass of the sun, something like that. They all light up so that you see them peppering the night sky. If you look at the night sky with, say, an X-ray telescope, they light up and you see them there. But not only that, supermassive black holes that we think live in the center of every single galaxy, every island of stars in the universe of trillions of stars. There's this supermassive black hole in the middle that's, say, a million to a billion times the mass of the sun.
6:02 Speaker 00 Those things can have so much gas around them that they're so bright that they can outshine all the stars in their galaxies. To the point where before we launched the Hubble Space Telescope, we detected the X-ray light from these supermassive black holes and didn't know what they were. And we essentially dubbed them, I say we, I wasn't around at this point, but essentially astronomers dubbed them at quasi-stellar objects, quasi-stellar, looks a bit like a star. And that eventually got shortened to Quasar. And when the Hubble Space Telescope got launched, realised that these points of light that we were seeing in the X-ray, they were actually entire galaxies. But it took
6:43 Speaker 00 the Hubble Space Telescope being launched to realise there are entire galaxies, billions of light years away, that we weren't able to see the starlight from, but we could see the X-ray light from the quasar, the growing supermassive black hole in the very centre of them. And yet, we didn't know what they were at the time when they were first spotted.
7:02 Chris Williamson? Do supermassive black holes, normal black holes, do they spin? Or is that neutron stars that have a spin and do some wild stuff?
7:10 Speaker 00 So neutron stars have a spin, yes, and they do do wild stuff. And if they have magnetic fields, they can send off these big beams of radiation like a lighthouse. And I say a neutron star, it's kind of like the baby sibling of a black hole, or for all the Pokémon fans out there.
7:26 Chris Williamson? The black hole that never was.
7:27 Speaker 00 Yeah, not quite, yeah. The Pokémon fans out there, a neutron star is a Pikachu to a black hole's Ritu, right? It's the, a black hole is the next stage of evolution for, for a neutron star. If you add enough stuff to a neutron star, it eventually becomes a black hole. And so, yes, black holes do spin, because they start life as either stars or neutron stars. If the star wasn't quite heavy enough to become a black hole, but that neutron star could grow and grow until it did become one. And you really can't shake off that spin that stars have. So the sun is spinning, neutron stars are spinning, and black holes also remain spinning as well. And also, as you add more stuff to them, and that comes in swirling around them, that adds more of what we call angular momentum,
8:11 Speaker 00 sort of like rotational energy to make them spin faster as well. So a spinning black hole is definitely something that we see in the universe when we observe them. And that was really clear in, did you see the Event Horizon Telescope image of the black hole in the centre of our Milky Way that was released back in May of this year, 2022?
8:32 Chris Williamson? No, is that a James Webb thing or not?
8:35 Speaker 00 No, this is the Event Horizon Telescope. So to jog your memory, it's, it looks like a big orange donut. I feel like if people go, oh yeah, the big orange donut image when I say this. And that image was was really quite blurred. And the reason it was blurred was because, you know, we were taking an image for a couple of hours, trying to collect as much light as possible. You know, imagine sort of like a night mode shot on your phone. You do 10 seconds, right? To let in all the light. This was a few hours. And in that time, the black hole had been spinning. And so the material around it that you were capturing the image of was moving. And so you end up with a slightly, a slightly blurred orange donut image. And that was one of the questions I got when that image came out was, why is it blurred? Like, surely we should be able to get a nice crisp picture. Exactly, yeah.
9:19 Chris Williamson? So you have the opportunity, there's multiple ways that a black hole could form. You have a large star that collapses in on itself when it's no longer got the energy to continue to support or to stop gravity from pushing it in. But another one is a neutron star, which is a pretty big star, condenses it down, but then accumulates more matter over time, just as when the formations of planets and stars originally happened. It's just gravity is bringing stuff in. That must be a very interesting way to form a black hole. Because it's the, there has to be a point at which is it, it's the straw that broke the camel's back. It's the piece of matter that created the black hole from the neutron star. That seems like quite a unique way to create one.
10:02 Speaker 00 Exactly that. Yeah. So that limit that you were just talking about is called the Tolman Oppenheimer Volkov limit. So Oppenheimer might be a name that some people recognize from the Manhattan Project, nuclear bombs. The reason he was brought on to that project was because he was a nuclear physicist studying neutrons, neutron stars. And so that limit is actually incredibly informative. If you can measure what that is, like, what's the maximum mass of a neutron star? Or the minimum mass of a black hole, he will formed in that way. And there's loads of different ways you can get at it. You can survey the neutron star population and say, let's see if we can observe all the neutron stars. How big are they? How massive? The sort of edge of that distribution will tell us. But also, we've recently started detecting what's known as gravitational waves.
10:50 Speaker 00 Um, which are literal ripples in space itself from this cataclysmic merger of say, two neutron stars that can also form a black hole. And again, that kind of a limit of when do you cross over that point? When have you added too much matter in the straw that's broken the camel's back to make it a black hole? And we think that's around three-ish times the mass of the sun, but drilling that down to is it, is it 3.2? Is it 3.1? Is it 3.15? You know, getting at that is, is quite difficult. Um, and you can get it in terms of like modeling what neutrons do and how neutrons behave and, and literally how can they be arranged in this almost perfect crystal? Like a neutron star is essentially this perfect
11:34 Speaker 00 crystal of neutrons arranged as tightly as they can go. You know, the physics of our understanding of neutrons that we can actually look at in the lab at how they behave. We can then work out, okay, mathematically, what would be the limit? And is that model right? Or is there some bit of physics that we've forgotten here that we get from our observations?
11:52 Chris Williamson? What is the form of matter that's in a black hole then? If a neutron star is neutrons pushed together in this perfect form of a crystal, which is as dense as we could possibly imagine theoretically as we could, what would, that can't be as dense as it could be because there's something denser than that and that's a black hole.
12:10 Speaker 00 Exactly. And unfortunately, that's one of those questions that we don't know. And under the laws of physics as we understand them, we will probably never know. And it's one of those frustrating things as a scientist, like we're taught to ask so many questions and it's like the one thing that we almost can't answer. So inside that event horizon, where it's, it's sort of that, that shroud that you're no longer receiving any light. So there's no information. So you have no idea what's going on inside that sphere of darkness essentially. There could be some exotic form of matter that we don't know that exists yet because we could never recreate that on earth in a lab because we don't have the ability to, to make anything that dense. But also you'd never be able to observe it
12:54 Speaker 00 because you could never get light from it because it would be this form of matter that was so dense as to create this event horizon in a black hole. Or it could be that there is no form of matter that can resist collapsing down completely. And you end up with how we describe a black hole mathematically, which is a singularity. So that might be a phrase that people have heard of before. Essentially means all your matter is compressed into an infinitely dense, infinitesimally small point that's undefinable in space in terms of how strong the gravity is there. Mathematically, what happens is you end up trying to divide by zero,
13:34 Speaker 00 which I think if you remember the film Mean Girls with Lindsay Lohan, the answer to that is the limit does not exist. So it's something that you can't do mathematically is divide by zero. You end up essentially going to infinity if you're trying to divide by zero. And so that's how we describe them mathematically. But whether that's actually truly the physics of what's going on inside that shroud of darkness in the event horizon, we don't know. I would love to know, but we don't.
14:03 Chris Williamson? Given the fact that there are varying sizes from black hole to supermassive black hole, it would suggest that that it can't be the uniform, that the variation in the size of the black holes is due to something that's going on. So they're infinitesimally small, might be slightly less infinitesimally small and bigger black holes. I don't know.
14:27 Speaker 00 No, not necessarily. So even if a black hole is 10 times the mass of the sun or 10 million times the mass of the sun, the maths still describes them as this singularity, where everything 10 times the mass of the sun or 10 million times the mass of the sun is compressed into this infinitesimally small point. But again, that's the maths. It's not necessarily an observation that we've made in terms of astronomy or astrophysics. And I am an observer, I use telescopes. So really, that's, that's what I'm always going to want. But under the laws of physics, if we understand them, we're probably never going to be able to observe that. So we do have to fall back on the maths necessarily of how we describe them. But it's interesting, you, you say like you have this obviously distribution of these masses because.
15:09 Speaker 00 We do see this sort of distribution of what I call normal black holes, you piddly ones that sit in the galaxy, you know, formed from the death of stars, 10 times the mass of the sun up to say, say 100 or so times the mass of the sun. And then supermassive black holes, which go from a million up to tens of billion times the mass of the sun. And we just have this dearth of things in the middle. We don't, we've never found anything between 100 to a million or so times the mass of the sun. They're called intermediate mass black holes and they're considered almost like the missing piece of the puzzle. Like sort of like the, the missing gap almost, because if you think about, well, how did supermassive black holes become so supermassive? If the only process we know to create a black hole comes from a supernova,
15:53 Speaker 00 which forms something like, you know, that star mass, like 10 times the mass of the sun, you've got to then grow that to become supermassive. So you'd think if these black holes were growing all the time to become supermassive, you'd see something in the middle, but we don't. So it's a very interesting problem in astrophysics about why we don't see things of that mass. Has all the growth already happened of supermassive black holes yet? But then why don't we see them in the early universe as well? And that's one question we're hoping maybe James, the James Webb Space Telescope will be able to help answer as well as it looks back further and further to fainter things at greater distance, which are also, the light has taken longer to get to us. So we're seeing the universe as it was billions of years ago.
16:36 Chris Williamson? Your best models at the moment suggest that there is a supermassive black hole at the centre of every galaxy. Are there any galaxies that don't have supermassive black holes in the middle?
16:47 Speaker 00 Yeah. So the working hypothesis is that every single one does. There are a few candidates where there is speculation about whether there might not be. And that is usually in the case of what we call dwarf galaxies. So for those in, who've been to the southern hemisphere or who live in the southern hemisphere, you'll know what I'm talking about when I say the Large Magellanic Cloud, which is the little dwarf galaxy to the Milky Way that's in orbit around us. And you can see it from the southern hemisphere. It's this little patch on the sky, stars that are much denser and there's sort of gas around it. If you crack a camera or a camera phone or something like that, you can snap it with.
17:22 Speaker 00 And something like that, there's still debate about, okay, well, maybe do they have these intermediate mass black holes in the centre? Because we know that galaxy mass and black hole mass are correlated. So if they're smaller in size and mass, they should have smaller black holes. So perhaps maybe that's something where we'll find them as well. But there are some candidates where we've observed the galaxy and the orbit of the stars suggests there is nothing in the centre.
17:47 Speaker 00 In which case, well, why? And how? How did they almost escape getting a supermassive black hole? And if that can happen, then how did all the other galaxies that we see having supermassive black holes in the centre end up with one? There's lots of different questions. And I think people are surprised when we say there is actually that much that we don't know still. But black hole studies and supermassive black hole studies especially is still such a young science that's only been going for probably about 20 years or so. I mean, back in the 90s, black holes were still called massive dark objects because we didn't quite know what they were, what these supermassive black holes were. And it was only sort of with the observations of the centre of the Milky Way and the stars
18:30 Speaker 00 orbiting around the very, very centre that we were looked at with the Keck Space Telescope on Mauna Kea in Hawaii. They look in the infrared, so they can sort of peer through the dust that's towards the centre of the Milky Way and see the actual positions of the stars and track them over like 15 years. One of these stars has made a whole orbit in that time in just about 12 years, I think. And from that, you can then work out, well, how fast they're orbiting the thing in the middle, you can work out, okay, how big is the thing in the middle? And you work out that it's about four million times the mass of the sun in an area smaller than the solar system. And for a while, people were like, maybe it's a swarm of black holes. Because these smaller black holes were known, they'd been seen as these little X-ray points
19:11 Speaker 00 of light all over the sky, but supermassive black holes weren't thought to be real necessarily. So people considered the idea of a swarm of black holes, which I almost wish was real. I just thought this sort of like beehive of black holes almost just swarming around. But in fact, that would be really unstable. You'd have things sort of like slingshotting around each other and being pinged out all the time. And so instead, this idea of, yeah, a supermassive black hole, and since you've finally been able to get a picture of one with the Event Horizon Telescope as well, it's confirmed the idea that it is one supermassive black hole.
19:44 Chris Williamson? Supermassive black holes in the middle of almost all galaxies.
19:50 Chris Williamson? All of the galaxies are around the point in the middle where the black hole is. Does that mean that the black hole is what's holding the galaxy together? Does the, or is it the fact that black holes end up at the centre of galaxies? And the rest of the gravity from that matter just holds itself together?
20:09 Speaker 00 Yeah, so that's a really big question that we have in astrophysics, like what comes first? The galaxy or the black hole? It's kind of like the astrophysics chicken or the egg, essentially. Like, do you have a galaxy of stars? One goes supernova, become a black hole, happens to sink to the middle and the whole thing forms around it? Or do you have like a cloud of gas in the early universe that just directly collapses into a black hole? And then stars sort of shepherded and form around that? That's also one of the questions we're hoping that the James Webb Space Telescope will answer, because it's, you know, been designed to look back to the first stars and first galaxies forming. But that idea of taking a black hole out of the centre of the galaxy, would the galaxy fly apart? Because if you did the same thing in the solar system, if you took the sun out of the centre of the solar system,
20:52 Speaker 00 the sun is 99.99% of all the mass in the solar system. Take it out and all the planets would just fly off. The whole thing would completely disperse and fall apart. In a galaxy, the black hole is not even 1% of the entire mass of the galaxy. The galaxy always outweighs the supermassive black hole in the centre. And so if you removed it, nothing would actually happen to the galaxy. It would actually hold itself together under what's known as self-gravity. And when you actually stop to think about this, this is the reason we have a galaxy of stars in the first place, and not just a big disk of gas just slowly fuelling the black hole over, you know, billions of years.
21:36 Speaker 00 Because essentially what it means is that the gravity... That is in this region of space that we're in, in terms of the sun, the gravity is sort of stronger, like, holding the earth around the sun than it is the gravity pulling towards the black hole in the centre, or even holding the sun together in the first place. And so that self-gravity of the galaxy is enough to to stop everything from falling into the black hole. And it's why I tell people, like, you don't need to worry, right? Everything just orbits black holes and will happily continue to do that in the same way that the earth will happily orbit the sun. Like, as long as you don't panic, you know, at night that the earth is going to fall into the sun. I mean, you don't need to panic about falling into a black hole either.
22:16 Chris Williamson? What is the biggest black hole that's been found so far?
22:20 Speaker 00 So it's got a rather uninspiring name, the biggest black hole found. It's called T O N six one eight. You know, recommendations for what we can rename it are much appreciated. And the supermassive black hole at the centre of that galaxy is close to 70 billion times the mass of the sun, which we basically had to give it a new name. It had to become ultra massive black hole at that point, because that is just right on the edge of, of, you know, what we think is possible. And actually reaching the point that we think might be the maximum mass that a black hole can grow to at all.
22:57 Chris Williamson? Why would there be a maximum mass?
22:59 Speaker 00 Yeah. So it's because of this idea of self-gravity again. If you've got this disk of gas spiralling around the black hole, then at some point, as the as the black hole grows, you sort of push out what's called the the innermost stable circular orbit. And so that's essentially the last point that you can get something orbiting safely before it, you know, it can't hold itself up and it will fall into the black hole and it will spiral in. And essentially what happens is that that moves so far beyond the event horizon that you can get a disk of gas around the black hole and it will never. Fall into the black hole.
23:38 Chris Williamson? Would that mean that you would end up with a band of no man's land in between the material and the black hole?
23:45 Speaker 00 Exactly. Yeah, you would have this no man's land where if something did happen to fall into there, like say there was a, you know, a passing gas cloud or, you know, you chucked an asteroid in there or something like that, then if it hit no man's land, yes, it would fall into the black hole. Or if it merged with another black hole that came in, then it could grow. But the usual way that black holes grow is by what we call accretion, which is from this gas that's spiraling around the black hole. And essentially, what you've got to do is you've got to remove energy from that gas. So if you think about it in terms of like molecules with energy that are all pinging around and they've got enough energy to keep them on an orbit. But if they collide with another molecule, it's like a game of pool or snooker. You know, you hit the cue ball into another ball and
24:27 Speaker 00 the cue ball stops and the other one goes flying. So if that kind of collision happens where one of these molecules stops, all of a sudden that disrupts that orbit and it can fall in because that pull of gravity from the black hole then pull it in. But if you've got a case where you've got this no man's land in the middle, where even if it lost enough energy, it still wouldn't fall into the black hole. Because it's far enough away that it could always still escape, even if it had, you know, lost a lot of energy in a collision or something like that, which is amazing to think of that I think we could be living through this era of really, almost like reaching this epoch of the maximum mass black holes could grow to in the universe with finding this TON 618, uh, supermassive
25:12 Speaker 00 black, oh, sorry, ultra massive black hole at the center of this galaxy. I mean, it kind of gives me goosebumps. Like, I don't know whether to be excited or disappointed that we're living through the era that black holes could be reaching their maximum.
25:24 Chris Williamson? Why is IC 1101 not the universe, which has got the biggest black hole at the center of it, given that it's the biggest galaxy that we've found?
25:36 Speaker 00 So it's interesting because there is the correlation there. So if you plot galaxy mass and you plot black hole mass, the two are correlated, but it's not a perfect correlation. There's always some scatter that depends on what's happened in the universe's history in terms of to that galaxy. Has it merged with other galaxies? In which case it's probably merged its supermassive black hole in the centre. Was there some interaction in the galaxy that sort of pulled on all the gas and stars in it that happened to send gas tumbling towards the centre? That kind of thing. Every galaxy has an individual history. So perhaps for that one, you know, maybe it's merged a lot of times, but the black holes haven't merged yet. So maybe we're recording more mass, but in in terms of stars, but we're
26:21 Speaker 00 not recording in the supermassive black hole yet. Maybe in T O N six one eight, it just got really lucky in terms of the galaxy managed to feed it by itself to grow it that big, even if the galaxy didn't get any bigger as well. So there's all sorts of different reasons that just very complex histories. You know, these things have had thirteen point eight billion years to evolve, right? There's a lot that can happen in that time.
26:44 Chris Williamson? You've just mentioned there that two black holes could meet, two supermassive black holes could meet. What happens then? What's what's that event like?
26:53 Speaker 00 We don't know. We've never observed an event like that. So we do have the LIGO experiment and the Virgo experiment on Earth right now that are detecting gravitational waves from, you know, the the stellar mass black holes that are formed from supernova. Those are in our own galaxy that are merging together. So I mean by gravitational wave, if we picture sort of gravity like how Einstein did, where he said, you know, if you have mass, it curves space. So imagine taking like a football, chucking it on the centre of a trampoline, and then, you know, like rolling like a ping pong ball around it, that would be sort of the equivalent of what mass does to space. And if you imagine taking that football and having two footballs and bouncing them on that trampoline, you imagine the trampoline is is obviously under a lot of stress.
27:38 Speaker 00 And so when two black holes are coming together and merging, they're spiralling around each other at incredible speeds and curving and uncurving space as they go past to extreme amounts. And so they send ripples out into space when this happens. And the biggest of those ripples happens when they finally come together and merge. And we actually are able to detect that here on the ground on Earth. We literally watch the distance between two mirrors where you've got a laser firing back and forth, measuring the distance between them very, very accurately, change by, you know, less than the width of an atom. We see that and we can detect that, okay, well, something just merged in the universe over there. With a supermassive black hole, it's a different frequency of wave that you get.
28:21 Speaker 00 And so what we have to do is we have to build bigger and bigger detectors. That distance between the two mirrors has to increase for us to be able to detect that, to the point where you would need it bigger than the entire Earth, which is insane, right? But there is a plan to do that. There is a plan. It's called the LISA Observatory and we're hoping to maybe launch it in sort of the 2030s, 2040s, probably more likely. And it would literally be these three sort of spacecraft in space in a triangle that would sort of trail the Earth in its orbit, kept in one of these what's known as sort of a gravitationally
29:02 Speaker 00 stable point, just like what the James Webb Space Telescope is currently at as well. And they would stay there and they would essentially have this, this system of firing a laser back and forth to detect the distance between the two mirrors. And if it changes, you know, a gravitational wave has gone past. So hopefully I can answer your question, maybe in 2040 or so, we can say this is what happens when two supermassive black holes merge. But you can imagine it's probably a very cataclysmic event when it does happen.
29:32 Chris Williamson? I read the five ages or the five stages of the universe, which was a book recommended to me ages ago and I had to get it on self print on Amazon. Anyway, it talks a lot about the different stages that the universe will go through and that a period of just black holes will take up an awful lot of the future of the universe. And then very, very slowly due to Hawking radiation.
30:02 Speaker 00 Yeah.
30:02 Chris Williamson? Yes, they will evaporate over time. How long, why is it that black holes lose their mass? How long does it take while talking radiation?
30:15 Speaker 00 I mean, you've really gone for literally the most difficult questions we could ever ask, right? So obviously this, this sort of situation that you would have for the end of the universe of being black holes and them having enough time to evaporate means that the universe is probably just going to keep expanding forever or reaches a happy medium and stops. The other option is that it starts to contract again. We still don't know quite know which one of those is actually going to happen. We have some sort of ways of testing which one that would be, but sort of on the fence still. But if it was to expand forever and yes, everything eventually might become part of a black hole as stars start to die off and slowly accrete stuff around them. If everything there was a black hole, they could indeed evaporate, but it would take, I mean, a supermassive black hole would
30:59 Speaker 00 probably be like a Google number of years, right? 10 to the power of 100, which is an incredibly large number. And Hawking radiation is a strange thing that Hawking worked on because he was obsessed with this idea of entropy. I don't know if you've ever heard this term in physics before. Essentially, there's a law of physics that says entropy cannot decrease. Entropy is almost like a measure of the disorder of the universe. But really, I mean, what it means is that the most likely thing that's going to happen is going to happen. So like if I had a nice warm mug of tea in front of me right now, that's a nice ordered system because all of the heat is in the tea and the air around it is cooler. But we know what will happen is that the heat will disperse and it will
31:41 Speaker 00 warm the air around it and everything will get more disordered essentially rather than having a hot here and cold here. It will all be sort of mixed about. And so Hawking was obsessed with this idea of, this was a law of physics that had been known for a very long time, for hundreds of years. It was the law of thermodynamics, as it's called. And black holes seem to break this. Because if you're trapping matter and energy and light in a black hole, you are removing disorder from the universe and making it more ordered, almost like someone organising things and being like, this all lives here and this all lives here. You're making something so much more ordered. And so he was obsessed with this idea and trying to reconcile the theory around black holes and thermodynamics.
32:23 Speaker 00 And eventually, he, he sort of came up with a very theoretical description that goes into very much of the quantum mechanics and what's going on in terms of like vacuum energy of the universe. But essentially, if you form a black hole, you disrupt some of the quantum signatures that are going through space. So these are sort of very tiny, sort of the quantum realm, we're talking about the physics of the very, very small sort of particles, what energy does space itself actually has. So by creating a black hole, you disrupt that. And from the surface, you then could have some of that disrupted energy escaping as Hawking radiation. And so this would mean that actually you are putting disorder back into the universe in that respect.
33:08 Speaker 00 The thing is, it's incredibly unlikely that that happens. The same with anything in quantum mechanics, right? There is a, almost this very strange probabilistic aspect to it that says, you know, like the edge of the table that's in front of me right now, there is a very vanishingly small probability that that edge of that table is not there because the atoms that make it up could be halfway across the universe. In quantum mechanics, that is true, is a true statement. And so
33:34 Speaker 00 Hawking radiation, while the maths checks out, the time it would take for that to happen is ridiculously long. Even what we call a primordial black hole that might have formed in the early universe, that might be, you know, less than the mass of the earth even. That still wouldn't have had enough time to evaporate by now in thirteen point eight billion years and put its energy back out into space. Um, and it's never something we've ever observed either. We should be able to observe it. It would be given off in what's known as gamma rays, the highest energy form of light. And we do have gamma ray telescopes looking at the sky like supernova and things like this give off gamma rays. But we've never spotted anything that looks like Hawking radiation before. And like I said before, I use telescopes. I'm an observer.
34:17 Speaker 00 I like to have the observational evidence before, you know, sort of saying this is a real thing. So Hawking radiation is still a hypothetical concept.
34:28 Chris Williamson? I learned a little while ago, something interesting about the speed of light. So nothing can go faster than the speed of light is something that everybody hears about. I heard, correct me if I'm wrong, that there is a speed limit in the universe and that the speed of light happens to go at that speed limit, not that the upper bound of the speed that the universe moves at at its maximum velocity is determined by light. Is that correct?
34:56 Speaker 00 Exactly. That's correct. Yeah. It just so happens to be that light moves at the, the speed limit of what we have in physics. Um, and it comes from Einstein, I mean Einstein's theory of general relativity and special relativity, you know, marries this beautifully and shows why this is the case. But essentially, as you get closer to the speed of light, things start to act very differently than what they do, you know, here on like earth in what we call non-relativistic speeds, normal everyday speeds. Like if you are running somewhere, you're jogging pace, maybe you start to put more energy in, you increase the speed that you jog at. And that's sort of, you know, the same truth for a car, right? You push the accelerator, you burn more fuel, you put more energy in, you start to go faster.
35:36 Speaker 00 As you approach the speed of light, you put more energy in and all of a sudden it doesn't start increasing your velocity. It starts to increase your momentum instead and your momentum is sort of like how difficult it is to stop you moving. So we have momentum on the ground here as well and we calculate that as mass times velocity. But if it's not the velocity that increases then, but the momentum still increases, it means your mass increases. So as you travel faster, the speed of light, you you put more energy in, you don't get any faster. Instead, your mass just keeps going up and up and up and up and up to infinity. And so there's almost, again, this limit does not exist, right? You get this this limit where nothing can go faster than the speed of light.
36:21 Chris Williamson? It just gets heavier.
36:23 Speaker 00 Just gets heavier, yeah.
36:25 Chris Williamson? That's interesting. Okay. What is the Schwarzschild radius?
36:31 Speaker 00 The Schwarzschild radius is the event horizon, essentially. So that that sphere around the black hole where you don't get any information from. And Schwarzschild was, I mean, an incredible physicist. So, I mean, Einstein put out his theory of general relativity smack bang in the middle of World War One in 1916. And Schwarzschild was on the one of the fronts, one of the German fronts, I think it was the Eastern Front. And he shouldn't have been there. He volunteered, essentially. He was too old at the time, but he was like, no, I'll still volunteer, despite having a lot of medical problems as well. And he unfortunately died about a year or so after Einstein put out his theory of general relativity.
37:16 Speaker 00 In that time, though, he was fighting on the Eastern Front and wrote three, maybe four physics papers on Einstein's theory of general relativity, wrote a lot of letters back and forth to Einstein, being like, I found this solution.
37:27 Chris Williamson? In the trenches.
37:29 Speaker 00 In the trenches. Yeah. And unfortunately, then, and literally after that, you know, passed away very young, I think it would have been in his early 40s. And essentially what he derived was this idea of what happens if your mass is all sort of contained in this sort of sphere. And he was sort of doing it in terms of like a star, right? It was sort of if you have a spherical object and you work in what's called, you know, spherical coordinates. So you work in instead of X, Y, Z, so up, down, forward, back, left, right, you work in sort of how big's your circle? How far round your circle are you? And how far round the other side of the circle are you essentially?
38:11 Speaker 00 And he worked it out for those kind of coordinates, which Einstein hadn't done. And if you look at that solution, you get this, what we call a singularity, where essentially R equals zero, the very middle, you cannot define the strength of gravity at that point. And it's what's been come to known as the singularity. But there's another singularity that occurs, which is at the Schwarzschild radius, which depends on how heavy the thing is, which for a normal star, the Schwarzschild radius is well in with that inside the star. It's nothing to worry about, you know, whatever. Because it's not a real singularity like the one in the middle. It just comes about because of the fact that you work in, you know, these spherical coordinates instead.
38:53 Speaker 00 But what it defines is that what people didn't realise at the time, because this was back in, you know, the nineteen tens, right? And black holes weren't really even theoretical curiosities until the fifties and sixties. What people realised eventually was that that Schwarzschild radius was the event horizon, that point of no return, where you can't literally see anything over the horizon anymore.
39:18 Chris Williamson? Is it not a paradox, a little bit, that nothing can move faster than the speed of light, but that something can have enough gravitational pull to be stronger than the speed of light?
39:30 Speaker 00 I don't think it's a paradox necessarily. No, because a paradox would suggest that there is something broken in there. But it would just suggest that there's this law of physics that, you know, says nothing can escape from it if you have to be travelling faster than that speed limit of the universe. I think it's a paradox necessarily. I just think it's a really interesting quirk of nature.
39:52 Chris Williamson? Yes, I guess it depends on how you see. The movement of light versus the effect of gravity. Cause I got it in my head that it's kind of like somebody running on a treadmill. You know, and it's like, in order for something to not be able to go that way, something has to go in the opposite direction at quicker than that speed.
40:11 Speaker 00 No, not necessarily.
40:12 Chris Williamson? It seems that that's not the way that it works. You know, with regards to gravity, if you were, I'm pretty sure that this is a common thought experiment. If the sun was to disappear like that, how long would it be before the gravity effect hit Earth?
40:27 Speaker 00 Eight minutes, the same as light.
40:29 Chris Williamson? Speed of light, right, okay. So gravity travels at the speed of light as well.
40:33 Speaker 00 Yes, exactly. And that's something Einstein's theory of general relativity explained as well. And it's the speed that say those gravity waves that we were talking about before, in terms of these LIGO experiments that detect gravitational waves, that's the speed they also travel at. So it's very interesting that we didn't know that for sure. It was a prediction of Einstein's general relativity. But when LIGO detected the merger of two neutron stars, which obviously we can see still because they're not quite heavy enough yet to trap light, there was both a gravitational wave and a burst of light from it. Which was astronomy's worst kept secret at the time, when it happened a few years ago. Because literally every telescope in the world dropped what they have taken and moved to look at this thing to record
41:16 Speaker 00 it as soon as they'd had the alert that this had happened. To the point where, you know, we have Twitter bots these days as well. That's like, the Hubble Space Telescope is now looking at, you know, such and such a star for this person. And this happened and we were like, that person works on neutron star collisions. And like the next day, it was like, for such and such a person, that person works on LIGO. You know, immediately like there was all these cogs turning if everyone knew what was going on. And obviously, you know, you have, everyone has mates at telescopes and stuff. So eventually sort of word crept around that that's what had happened. So we all knew it was coming to know that, you know, have it confirmed or not, whether gravity traveled at sort of the same speed as light did.
41:56 Chris Williamson? I like the idea of a VIP guest list for working out what's going on in the universe. And if you're on the right, if you're in the right WhatsApp chat, then you get to find out when cool stuff's gonna happen before everybody else.
42:07 Speaker 00 Yeah, or if you just happen to be like scheduled to use that telescope that night, then, you know, doing your own sort of stuff. And then someone comes in and goes, no, sorry, we just need an hour to look at this thing.
42:18 Chris Williamson? We're busy this evening, we've got to focus it somewhere else.
42:21 Speaker 00 So.
42:23 Chris Williamson? You've said earlier on that there is potentially an upper bound, that black holes could potentially become full. Does that, does that mean that the event horizon creeps out then? Does the event horizon become bigger as a black hole increases in size?
42:38 Speaker 00 Yeah, so the event horizon or the Schwarzschild radius is related directly to the mass of the black hole. So when we say the size of the black hole, we mean the size across, we mean like the diameter almost of the event horizon. And when we're talking about the mass, we're talking about how heavy is it? And the two are directly related. And basically, what the Schwarzschild radius is, is an equation to tell you how big is that diameter of the event horizon, given how big your black hole is. And it's basically just depends on how fast is the speed of light, how strong is gravity? Boom, there's your radius for when light can't escape from it anymore. And it's a really simple equation. And I just, I mean, I would encourage anyone to Google Schwarzschild radius right now and put your own weight in
43:21 Speaker 00 there. And you can work out what size of a black hole you would be. I've done it many a time, and it's quite fun that you're like, I am naught point naught, naught, naught, naught, naught, like many, many zeros, one meter size black hole, if I could squish myself down into a black hole.
43:38 Chris Williamson? Do black holes ever form without a neutron star? Is it possible that sufficient material would be able to come together? Or if it did that, would it form a star that would then live a life that would then collapse in on itself that would then become a black hole?
43:54 Speaker 00 It just all depends on how big the star is that's lived before. So something like the sun won't make a black hole, it will make what's called a white dwarf, which is again, the baby sibling to a neutron star. Something heavier than the sun, let's say three times to 10 times the mass of the sun will become a neutron star at the end of the life. It'll go supernova, throw out all the outer layers. So the majority of the mass, say like seven or eight times the mass of the sun, just gets swirled out back into space. And that's how we end up with these beautiful nebula that we see, these big round nebula. And at the middle, you'll leave behind the core. And if it's three to 10 times the mass of the sun that's the star beforehand, it will be, it will be a neutron star, that core. But if the star's heavier than say 10 times the mass of the sun, up to let's say 100, 200
44:39 Speaker 00 times the mass of the sun, that same process will happen in terms of you have this supernova that throws, then you get this like rebound of all the outer layers that bounce back off because they're very light hydrogen gas with a very heavy core and the core is what collapses down then into the black hole. There are some stars that we have seen there one day and gone the next and have speculated whether that those stars have just skipped supernova entirely and directly collapsed down into a black hole.
45:08 Chris Williamson? Is that a direct collapse event?
45:11 Speaker 00 Mm hmm. Exactly. Yeah, exactly that. So that's sort of what you end, what you end up with, you end up with this sort of trifecta of the graveyard of stars, basically white dwarf, neutron star, black hole, and it all depends how big the star was in the first place, essentially. But as we said before, you can add mass to your neutron star. You can keep adding stuff to it and it could become a black hole. And we actually see this happening because the majority of stars are actually not alone like the sun. They're going to be in binary pairs, especially heavier stars. They heavier stars form where there's more gas. So with more gas, you're going to get more stars and they end up in these, not just binary systems, tertiary, quadrinary, whatever the word is to describe it, where you have three, four,
45:56 Speaker 00 five, up to seven or eight stars, all orbiting each other in this intricate pattern. So you can imagine if one of them goes supernova and becomes a black hole, which also the biggest stars die first. They burn through their fuel quickest because they have to burn more fuel to resist the larger crush of gravity because they're so big. And so you have all these other stars with a black hole then. And obviously the black hole can then start to steal material off other stars. Same can happen if you've got a neutron star. It can pull material off other stars and use it to grow and become a black hole. Same is true for a white dwarf. It can pull material off of the stars and use it to grow until it becomes a neutron star.
46:39 Speaker 00 And when that happens, when a white dwarf reaches that upper limit as well of its mass, we have another type of supernova. A supernova called a type one A and they be always go off at the exact same brightness across the entire universe. Because it's this limit to how big a white dwarf can be before it becomes a neutron star.
46:57 Chris Williamson? Of course, yes.
46:58 Speaker 00 Um. And you might have heard of these things called standard, these are standard candles, which are these things that we can use to calculate distances. Because if they're always the same brightness from how bright they appear, we can work out how far away they are. And then cut, then say, okay, this galaxy is at this distance, we now know, and it's got this red shift. This stretching of its light due to the expansion of space. And then we can work out, okay, well, how fast is space expanding? Rewind time, how long it has been expanding for? And you work out hold the age of the universe is.
47:29 Chris Williamson? It seems like everything astronomically is moving toward black holes in that case.
47:37 Speaker 00 I guess so, yeah. And the fact that if you just keep adding more mass eventually, it'll, everything will become a black hole. And as we said before, if, if stars continue to die and you get left behind with all of these things and everything starts to merge and come together over time, then yeah, you would end up with a black hole. It's sort of almost like the inevitable end to all matter unless you've got something resisting that crush of gravity down. And the expansion of the universe could be one of those things, right? That's literally moving matter away. From, or that likes moving galaxies at least away from other things. So it's obviously galaxies are held together by their local gravity, but between galaxies. Sometimes the pull of gravity is strong enough that it'll bring two of them together, like the Milky Way and Andromeda will merge in, say, two billion years.
48:22 Speaker 00 But in other cases, the expansion means that galaxies are majority of cases, most galaxies are getting further apart.
48:29 Chris Williamson? What's the best image that we've got of a black hole?
48:34 Speaker 00 It's definitely the Event Horizon Telescope images, um, in terms of we have this one from the Galaxy Messier 87 that was the first one was, was released, this giant orange doughnut. And then we had our own supermassive black hole in the centre of the Milky Way, released in May this year. And they are incredible to look at because, I mean, I had, I mean, this is what got me into space in the first place, but it's eight years old. I remember I got this book for birthday or Christmas or something, and it was just a fact file about all the things in space. So it went through all the solar system objects, you know, and they had all the beautiful images from, say, the Voyager craft in the seventies and eighties, or even the Hubble Space Telescope. And then you got on to stars and galaxies and black holes.
49:13 Speaker 00 And I remember the black hole page, I was almost annoyed at because on every other page, there's this beautiful image from a telescope. And on the black hole page, it was like artist's impression. And I sort of resigned myself at an early age to be like, yeah, we'll never get an image of a black hole because, you know, they're, they're black and there's nothing to see and whatever. And as I got further into my sort of education, doing an undergrad degree in astrophysics and my PhD and everything. I realised, actually, no, it could be a possibility if we could take an image of that gas swirling around the black hole and capture that darkness in the middle. And that's essentially what they did. And when, when I saw that image for the first time, I remember getting just goosebumps because I never thought that that could ever,
49:56 Speaker 00 you know, be the case that we could actually see that for ourselves. And what I think so powerful about that image, you know, you've got the blackness of empty space around the outside. Then you've got this incredibly hot gas plasma that's swirling around the black hole, bright orange, it's coloured, you know, so that humans can sort of detect where it's brighter and fainter. And then you've got the blackness in the middle. And you think about the black on the outside is nothingness and the black on the inside is everything.
50:29 Speaker 00 Comparing the sort of the two black is insane when you think about how different and yet similar similar color but so different in in nature almost as well of what those two things are. And I mean, I get I get kish bumps every time I see it when I remember that and really think about it, stop and think about it.
50:48 Chris Williamson? James Webb telescope recently went up some incredibly interesting and or inspiring images coming from that. What's next as one of the experimentalist observerists that's not just theoretically writing this stuff down? James Webb's been planned for what decades now, three decades, something like that, maybe even more?
51:12 Speaker 00 Yeah, it was originally five years. I think it's now 20 years is the sort of promised time, but could be longer if it doesn't need to use as much fuel, essentially.
51:21 Chris Williamson? So what is next in terms of telescopes? Or we've already mentioned about this gravitationally stable triad of mirrors that are going to follow follow the universe around like a little tail. What else is there?
51:40 Speaker 00 So after the James Webb Space Telescope, there's so many more telescopes coming and so many things to get excited for. One I'm particularly excited for because in my research field, it's really going to have such a huge impact is something called the Extremely Large Telescope, the ELT, that is actually its name. It is extremely large. It's 30 meters across. So James Webb, we think of that being very large, 6.5 meters across. So maybe, you know, a two story house, but with 30 meter mirror to collect the light, as much light as it can and focus it down. Is, I mean, it's just incredible to think of how big that would be, you know, sort of mansion sized telescope almost. And that's
52:21 Speaker 00 going to give us better resolution in the Hubble Space Telescope, but from the ground, not from space where you're not looking through atmosphere that disorts everything. And so that's going to be an incredible thing because not only will that telescope be able to take, say, a single image of, say, a galaxy, what it will do is almost like mosaic a galaxy up. Into little segments and observe all of those simultaneously. But not just take an image from each one of those segments. It will split the light through a prism into its rainbow, into its spectrum of light, and make a trace of how much light of every color you're receiving. And that's critically important because elements in the universe give off specific colors like a fingerprint.
53:04 Speaker 00 And so we know how much hydrogen or oxygen or nitrogen there exists in a galaxy. And how that relates to how many stars are forming or how many stars are dying at that period, at that, that time. And you can do it in every single little segment of that galaxy. Which, the amount of detail we're going to have from that, the kind of resolution we're going to have in terms of, you know, Hubble resolution, but from the ground and being able to do this, this piecing together is something I'm really excited for. Um, and then you've also got massive radio telescopes being built as well. There's something called the Square Kilometer Array being built, one in South Africa and one in Australia. The ELT that I just talked about is in Chile, in the Atacama Desert. But these, uh, Square Kilometer Array really is what it says on the tin.
53:47 Speaker 00 It's an array of radio antennae spread across a square kilometer. And you combine them so that they become one telescope that is a square kilometer. So not like thirty meters across, but a square kilometer across. Because as you get to longer and longer wavelengths of light, so the wavelength being sort of the distance between the peaks of light. With radio light, for example, which is the light we use to communicate, you know, we send radio signals, we send TV signals using, sort of encoded on radio light, because the wavelengths are meters, tens of meters long. And we can detect that light from space as well. And it tells us usually where there's like magnetic fields and stuff like that in space. And, uh, and also hydrogen gas as well.
54:30 Speaker 00 Uh, it also has this resonance in radio light too. So this fact that, you know, as you get to larger, longer wavelengths of light, you need a larger telescope to detect it. This is why we have to keep going to these square kilometers. But it also means your resolution is incredible as well, that, as in the thing, the size of the thing you can actually resolve on the sky just gets smaller and smaller, the bigger your telescope gets as well. So the, I mean, the resolution we're going to have with a telescope that is a square kilometer across is just gonna be insane.
55:00 Chris Williamson? Dr. Becky Smethurst, ladies and gentlemen. If people want to keep up to date with the stuff that you do and everything online, where should they go?
55:08 Speaker 00 So I'm on YouTube, Dr. Becky is the channel name and I chat about, basically what we've been chatting about now, sort of like the questions that you're not able to Google, because all you're going to get back is, you know, just research papers that are written from, you know, by my colleagues, or by, you know, my colleagues and the general public just don't have a hope of, you know, going through all of that language and jargon that we use. So it's sort of like I act as a translator, if you will, on the channel. We chat about what's new in space news and things like that. But also, you know, Twitter, Instagram, and from my books as well.
55:40 Speaker 02 Becky, I appreciate you. Thank you. Mr. President, my company informed me of the explosion on our rig on October 2nd. Causing massive water displacement and triggering a tsunami. If y'all heap the blame on me. Tom Cruise is.
55:53 Speaker 03 Digger got us into this mess. And Digger's gonna dig us up. From the director of Birdman and The Revenant.
55:59 Speaker 04 What would happen if I stopped going? You stall, you freeze to death, that's what.
56:03 Speaker 03 With great power comes great deniability.
56:06 Speaker 04 I'm not the first then.
56:07 Speaker 03 Digger. See the trailer online. Only in theaters October 2nd. Rated R. Under 17.9 and middle without parent.
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