Modern WisdomThe Secret World Of Black Holes - Dr Becky Smethurst
EVERY SPOKEN WORD
110 min read · 22,498 words- 0:00 – 0:23
Intro
- BSDr Becky Smethurst
People ask me questions like, "What's on the other side of a black hole?" Which doesn't make any sense when you realize 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. (wind blows)
- CWChris Williamson
What is your
- 0:23 – 9:19
What’s Wrong with the Name ‘Black Holes?’
- CWChris Williamson
problem with the name black holes?
- BSDr Becky Smethurst
(laughs) 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. (laughs) 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 realize 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 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, it, I mean, I'm sure a lot of listeners could probably come up with their own (laughs) 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. (laughs)
- CWChris Williamson
Who was the guy that called them black holes? What was the story with him?
- BSDr Becky Smethurst
Yeah. So it comes from a, a bit of a harrowing part of history, actually. Um, are you familiar with the Black Hole of Calcutta?
- CWChris Williamson
No.
- BSDr Becky Smethurst
No. So the Black Hole of Calcutta is a prison cell, um, in an old Fort William in Calcutta in India. And, um, 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, uh, 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, um, Robert Dicke in the '60s, who was studying what then were known as gravitationally completely collapsed objects, or GCCOs, which, again, is a term I'm quite glad that didn't stick. Um, 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, as like, almost like brevity and advertising value essentially was how it was described by the physicist, uh, called Wheeler as well. And, uh, essentially, he compared the crush of matter down to the, 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 lots of memorials still in Calcutta in India today, they think that the prison got its name from the astronomical object, but in fact, it's the other way around, which I think people don't realize.
- CWChris 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-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... side onto it that, uh, what's on the other side, you look at it from behind. What- what is it like as you were, i- if you were to orbit around it? Is it a sphere?
- BSDr Becky Smethurst
Yeah, it's a sphere, is probably the best way of putting it. So, I mean, they 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?
- CWChris Williamson
More of the sun, yeah.
- BSDr Becky Smethurst
So, (laughs) more of the sun, yeah. So it's the same with black holes, and if you could sort of orbit one very safely and, 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, so 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.
- CWChris Williamson
If it's a-
- BSDr Becky Smethurst
That's sort of a, a known law of physics.
- CWChris Williamson
... if it's a dark sphere, what's your problem with the word black then?
- BSDr Becky Smethurst
(laughs) 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. 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, super massive black holes that we think live in the center of every single galaxy, every island of stars in the universe of s- say it's tr- trillion stars, there's a super massive black hole in the middle that's, say, a million to a billion times the mass of the sun. 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 super massive 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 (laughs) essentially astronomers dubbed them, uh, 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, realized that these points of light that we were seeing in the X-ray, they were actually entire galaxies. But it took the Hubble Space Telescope being launched to realize 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 super massive black hole in the very center of them, and yet we didn't know what they were at the time when they were first spotted.
- CWChris Williamson
Do super massive black holes and normal black holes, do they spin? Or is that-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... neutron stars that have a spin and do some wild stuff?
- BSDr Becky Smethurst
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 i- is kinda like the, the baby sibling of a black hole, or for all the Pokemon fans out there-
- CWChris Williamson
The black hole that never was.
- BSDr Becky Smethurst
(laughs) Yeah, not quite, yeah. The, for the Pokemon fans out there, a, a neutron star is a Pikachu to a black hole's Raichu, right? It's the, (laughs) 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 s- 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, sort of like rotational energy to make them spin faster as well. Um, 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 center of our Milky Way that was released back in May of this year, 2022?
- CWChris Williamson
No. Is that a James Webb thing or not?
- BSDr Becky Smethurst
(laughs) No, this is the Event Horizon Telescope. So to jog your memory, it's, looks like a big orange donut. And I feel like 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. (laughs) 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-"
- CWChris Williamson
Because it's moving.
- BSDr Becky Smethurst
"... nice, crisp picture." Exactly,
- 9:19 – 16:37
The Formation of Black Holes
- BSDr Becky Smethurst
yeah.
- CWChris 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-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... it's no longer got the energy to continue to support or to stop gravity from pushing it in.
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
But another one is a neutron star which is pretty big star, condenses-
- BSDr Becky Smethurst
Yeah.
- CWChris Williamson
... down, but then accumulates more matter over time, just as when the formations of planets and stars originally happened-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... it's just gravity is bringing stuff in. That must be a very interesting way to form a black hole because it's-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... the, there has to be a point at which the, 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.
- BSDr Becky Smethurst
Exactly that, yeah. And so that limit that you were just talking about is called the Tolman-Oppenheimer-Volkoff Limit. Uh, so Oppenheimer might be a name that some people recognize from the Manhattan Project, nuclear bombs. The reason he was brought onto that project was because he was a nuclear physicist studying neutrons and neutron stars. Um, 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, if you will, formed in that way. Um, 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, 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 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 3.115, you know, getting at that is, is quite difficult. Um, and you can get at 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 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?
- CWChris Williamson
What is the form of matter that's in a black hole then? If a neutron star-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... 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, what, it, that can't be as dense as it could be because there's something denser than that, and that's a black hole.
- BSDr Becky Smethurst
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 that's one of those frustrating things as a scientist. Like, we're taught to ans- 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 'cause we don't have the ability to, to make anything that dense. But also you'd never be able to observe it 'cause 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, which I think if you remember the film Mean Girls with Lindsay Lohan, the answer to that is the limit does not exist. (laughs) Um, 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.
- CWChris Williamson
Given the fact that there are varying sizes from black hole to super massive black hole-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... it would suggest that they're... i- it can't be, uh, uniform, that they're ha- the variation in the size of the black holes-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... is due to something that's going on. So the infinitesimally small might be slightly less infinitesimally small-
- BSDr Becky Smethurst
(laughs)
- CWChris Williamson
... in bigger black holes? I don't know.
- BSDr Becky Smethurst
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 math still describes them as this singularity where everything 10 ma- 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 gonna want. But under the laws of physics as we understand them, we're probably never gonna 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, y- you say like, you have this obviously distribution of these masses because we do see this sort of distribution of what I call normal black holes, your 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 super massive 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, like, gap almost, because if you think about, well, how did super massive black holes become so super massive if the only process we know to create a black hole comes from a supernova which forms something like, you know, that's star mass, like 10 times the mass of the sun? You've gotta then grow that to become super massive. So you'd think if these black holes were growing all the time to become super masses 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. Um, has all the growth already happened of super massive 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.
- 16:37 – 26:43
Supermassive Black Holes
- CWChris Williamson
Your best models at the moment suggest that there is a super massive black hole at the center of every galaxy. Are there any galaxies that don't have super massive black holes in the middle?
- BSDr Becky Smethurst
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 a 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 of stars that are much denser and there's sort of gas around it if you crack out a camera or a camera phone or something like that that you can snap it with. Um, and something like that there's still debate about, okay, well maybe do they have these intermediate mass black holes in the center 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 m- maybe that's something where we'll find them as well, but there are some candidates where we observe the galaxy and the orbit of the stars suggest there is nothing in the center. In which case well, h- h- why and how? (laughs) How did they almost escape, uh, getting a super massive black hole? And if it- that can happen then how did all the other galaxies that we see having super massive black holes in the center 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 super massive 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 'cause we didn't quite know what they were, at least super massive black holes were. And it was only sort of with the observations of the center of the Milky Way and the stars orbiting around the very, very center that we were looked at with the, the Keck space telescopes of Mauna Kea in Hawaii. They look in the infrared so they can sort of peer through the dust that's towards the center 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, 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 of light all over the sky. But super massive 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 (laughs) want 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 super massive black hole. And since you've finally been able to get a, a picture of one with the Event Horizon Telescope as well, it's confirmed the idea that it is one super massive black hole.
- CWChris Williamson
Super massive black holes in the middle of almost all galaxies. 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-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
Or is it the fact that black holes end up at the center of galaxies and the rest of the gravity from that matter just holds itself together?
- BSDr Becky Smethurst
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 collapsed into a black hole, and then stars sort of were 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 center 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 center of the solar system, 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, and the galaxy always outweighs the super massive black hole in the center. 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 disc of gas just slowly fueling the black hole over, you know, billions of years. 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 center, 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? (laughs) 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 gonna fall into the sun, uh, I mean, you don't need to panic about falling into a black hole either.
- CWChris Williamson
What is the biggest black hole that's been found so far?
- BSDr Becky Smethurst
So it's got a rather, uh, uninspiring name, the biggest black hole found. It's called TON 618, you know, recommendations for what we can rename it are much appreciated. (laughs) Um, and the super massive black hole at the center of that galaxy is close to 70 billion times the mass of the sun, which w- we basically had to give it a new name. It had to become ultra massive black hole at that point because that is just i- it- 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.
- CWChris Williamson
Why would there be a maximum mass?
- BSDr Becky Smethurst
Because it ... Yeah. So it's because of this idea of self-gravity again. If you've got this disc of gas spiraling 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 disc of gas around the black hole and it will never fall into the black hole in the future.
- CWChris Williamson
Would that mean that you would end up with a band of no man's land in between the-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... material and the black hole?
- BSDr Becky Smethurst
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 p- 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, is by what we call accretion, which is from this gas that's spiraling around the black hole. And essentially what you've gotta do is you've gotta 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 the cue ball stops and the other one goes flying. So, if that kind of collision happens where, where one of these molecules stops, all of a sudden that, that disrupts that orbit and it can fall in because that pull of gravity from the black hole will 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 w- we're reaching this epoch of the maximum mass black holes could, could grow to in the universe with finding this TON 618, uh, super massive black h- 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 (laughs) that we're living through the era that black holes could be reaching their maximum.
- CWChris 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?
- BSDr Becky Smethurst
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 super massive black hole in the center. Was there some interaction in the galaxy that sort of pooled on all the gas and stars in it that happened to send gas tumbling towards the center? 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 not recording it in the super massive black hole yet. Maybe in TON 618 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 15.8 billion years to evolve, right? There's a lot that can happen in that time.
- 26:43 – 29:32
What Happens When Two Black Holes Meet?
- BSDr Becky Smethurst
- CWChris Williamson
You've just mentioned there that two black holes could meet.
- BSDr Becky Smethurst
Mm.
- CWChris Williamson
Two super massive black holes could meet. What happens then? What's, what's that event like?
- BSDr Becky Smethurst
We don't know. We've never observed an event like that, so we do have, uh, the LIGO experiment, uh, and the VIRGO experiment on Earth right now that are detecting gravitational waves from, you know, the, the stellar mass black holes that form from supernova. Those are in our own galaxy that are merging together. So, I mean, by gravitational wave, if we picture, um, 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 center 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 (laughs) , you can imagine the trampoline is, is obviously under a lot of stress. And so when two black holes are coming together and merging, they're spiraling 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 super massive black hole, it's a different frequency of wave that you get, 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 (laughs) . 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, uh, 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 stable point, just like what the James Webb Space Telescope's currently at as well. Um, 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 (laughs) . We can say, "This is what happens when two super massive black holes merge," but you can imagine it's probably a very cataclysmic event when it does happen.
- 29:32 – 34:27
The Latter Stages of the Universe
- BSDr Becky Smethurst
- CWChris 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-
- BSDr Becky Smethurst
Wow.
- CWChris Williamson
... it, it, it talks a lot about the different stages that the universe will go through and that-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... a period of just black holes will take up an awful lot of the future-
- BSDr Becky Smethurst
(laughs) .
- CWChris Williamson
... of the universe. And then very, very slowly due to Hawking radiation?
- BSDr Becky Smethurst
Yeah. (laughs)
- CWChris Williamson
Yes. Uh, they will evaporate over time.
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
How long... Why is it that black holes lose their mass? How long does it take? What's Hawking radiation?
- BSDr Becky Smethurst
Yeah. (laughs)
- CWChris Williamson
Please.
- BSDr Becky Smethurst
I mean, you've really gone for literally the most difficult questions we, we could ever ask, right? So obviously, this, this, um-... sort of situation that you would have for the end of the universe, it being black holes and them having enough time to evaporate, means that the universe is probably just gonna 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 gonna happen. We have some sort of ways of testing which one that would be, but sort of on the fence still. Um, 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 then was a black hole, they could indeed evaporate, but it would take, I mean, a super massive black hole would probably be like a Google number of years, right? 10 to the power of 100, which is an in- incredibly large number. And Hawking radiation is a, 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, and essentially there's a, a law of physics that says entropy cannot decrease. And 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 gonna happen is gonna 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 warm the air around it and everything will get more disordered essentially, rather than having it hot here and cold here, it will all be sort of mixed about. And so Haw- Hawking was obsessed with this idea of, it, this was a law of physics that had been known for a very long time, for hundreds of years. It was a 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 organizing 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 in trying to reconcile the theory around black holes and thermodynamics. 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. And 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. The thing is, it's incredibly unlikely that ha- that happens. The same with anything in quantum mechanics, right? There is a, almost this very strange probabilistic s- 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. I- in quantum mechanics, that is true. That is a true statement. And so 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 13.8 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 from 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. I like to have the observational evidence before, you know, sort of saying this is a real thing. So Hawking radiation is still a, a hypothetical concept.
- 34:27 – 43:38
The Universe’s Speed Limit
- BSDr Becky Smethurst
- CWChris Williamson
I learned a little while ago something interesting-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... 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-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... upper bound of the speed that the universe moves at at its maximum velocity is determined by light. Is that correct?
- BSDr Becky Smethurst
Exactly. That's correct. Yeah. It just so-
- CWChris Williamson
Two for two.
- BSDr Becky Smethurst
... happens to be that light moves at the o- the speed limit of what we have in physics. Um, and it comes from Ein- 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 know, 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 is true for a car, right? You push the accelerator, you burn more fuel, you put more energy in, you start to go faster. 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 momentum still increases, it means your mass increases. So as you travel fast at 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? (laughs) You get this, this limit where nothing can go faster than the speed of light.
- CWChris Williamson
It just gets heavier.
- BSDr Becky Smethurst
... just gets heavier, yeah.
- CWChris Williamson
That's interesting. Okay.
- BSDr Becky Smethurst
(laughs)
- CWChris Williamson
What is the Schwarzschild radius?
- BSDr Becky Smethurst
Mm-hmm. 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 Schwarzchild was, uh, I mean, an incredible physicist, so, uh, I mean, Einstein powered his Theory of General Relativity smack bang in the middle of World War I, in 1916, and Schwarzschild was on the, uh, one of the fronts, uh, on 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, uh, about a year or so after Einstein put out his Theory of General Relativity. 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."
- CWChris Williamson
In the trenches.
- BSDr Becky Smethurst
And w- in the trenches, yeah. And unfortunately, then d- literally after that, you know, passed away very young, I think 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. (smacks lips) And he was sort of doing it in terms of like a, a star, right? It w- it was sort of if you have a spherical object and you work in what's called, um, 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, uh, how big's your circle, how far round your circle are you, and how far round the other side of the circle (laughs) are you, essentially. 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, uh, spherical coordinates instead. But what it defines is that, what people didn't realize at the time, 'cause this was back in, you know, the, the, the 1910s, right? And black holes weren't really even theoretical curiosities until sort of the '50s and '60s. Um, what people realized 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.
- CWChris Williamson
Is it not a paradox a little bit that nothing can move faster than the speed of light-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... but that something can have enough gravitational pull to be stronger than the speed of light?
- BSDr Becky Smethurst
I don't think it's a paradox necessarily. No, 'cause 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, if you have to be traveling faster than that speed limit of the universe. Don't think it's a paradox necessarily. I just think it's a really interesting quirk of nature.
- CWChris Williamson
Yes. I guess it depends on how you see the movement of light versus the effect of gravity, 'cause I-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... got it in my head that it's kind of like somebody running on a treadmill, you know?
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
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. But it's-
- BSDr Becky Smethurst
No, not necessarily.
- CWChris Williamson
... it seems that that's not the way that it works. You know, with regards to gravity-
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
... if you were... I, I'm pretty sure that this is a common thought experiment. If the sun was to disappear like that-
- 43:38 – 50:48
Can Black Hole’s Form without a Neutron Star?
- CWChris Williamson
Do black holes ever form without a neutron star? Is it possible that-
- BSDr Becky Smethurst
Mm.
- CWChris Williamson
... sufficient m- 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?
- BSDr Becky Smethurst
(laughs) 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 their 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 bec- 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 times the mass of the sun, that same process'll happen in terms of you have the supernova that throws ... Since you get this, like, rebound of all the outer layers that bounce back off, 'cause 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. And that might be-
- CWChris Williamson
Is that a direct collapse event?
- BSDr Becky Smethurst
... a possibility. 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, uh, sort of trifecta of the graveyard of stars basically, white dwarf, neutron star, black hole, and it all depends on 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 m- the majority of stars are actually not alone like the sun. They're gonna be in binary pairs, especially heavier stars. They, heavier stars form where there's more gas, so with more gas, you're gonna get more stars and they end up in these, not just binary systems, tertiary, quadr- quadrenary, whatever the, the word is to describe it where you have three, four, five, up to seven or eight stars all orbiting each other in this intra- 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 s- 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 other stars and use it to grow until it becomes a neutron star. 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 Ia. 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-
- CWChris Williamson
Ah.
- BSDr Becky Smethurst
... a neutron star.
- CWChris Williamson
Of course. Yes.
- BSDr Becky Smethurst
Mm-hmm. 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 c- and 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 has this been expanding for? And you work out how old the age of the universe is.
- CWChris Williamson
It seems like everything astronomically is moving toward black holes in that case.
- BSDr Becky Smethurst
(laughs) I guess so, yeah. In the fact that if you just keep adding more mass, eventually y- it'll, it, everything will become a black hole. And it, 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 all the, like, it's 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. But in other cases, the expansion means that galaxies are m- majority of cases, most galaxies are getting further apart.
- CWChris Williamson
What's the best image that we've got of a black hole?
- BSDr Becky Smethurst
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 res- re- was released, this giant orange donut. (laughs) Um, and then we had our own super massive black hole in the center of the Milky Way released in May this year. And they are incredible to look at because I mean, I had... W- I mean, this is what got me into space in the first place, but I was eight years old, I remember I got this book for a 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, the Voyager crafts in the '70s and '80s or even the Hubble Space Telescope. And then you got onto stars and galaxies and black holes. 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'd 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 realized 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, uh, you know, be the case that we could actually see that for ourselves. And what I think's so powerful about that image, you know, you've got the blackness of empty space around the outside and then you've got this incredibly hot gas, this plasma that's swirling around the black hole. Bright orange it's colored, 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. I- and comparing the, the sort of the two black is, is, is, is insane when you, when you think about how different and yet similar, similar color but so different in, uh, in nature almost as well, i- of what those two things are. And, uh, I mean, I get, I get goosebumps every time I see it when I remember that and, and really think about it. Stop and think about it.
- 50:48 – 55:01
What’s Next After the James Webb Telescope?
- CWChris Williamson
James Webb Telescope recently went up.
- BSDr Becky Smethurst
Mm-hmm.
- CWChris Williamson
Some incredibly interesting and awe-inspiring images coming from that.
- BSDr Becky Smethurst
Mm.
- CWChris Williamson
What's next as one of the experimentalist observer-ists-
- BSDr Becky Smethurst
(laughs)
- CWChris Williamson
...that's not just theoretically writing this stuff down?
- BSDr Becky Smethurst
Mm.
- CWChris Williamson
James Webb's been planned for what, decades now? Three decades? Something like that? Maybe even more.
- BSDr Becky Smethurst
Yeah. It was originally five years. I think it's now 20 years is the sort of promised time, but could be longer if-
- CWChris Williamson
Right.
- BSDr Becky Smethurst
...it doesn't need to use as much fuel, essentially.
- CWChris Williamson
So, what is next in terms of-
- BSDr Becky Smethurst
Mm.
- CWChris Williamson
...telescopes or y- we've already mentioned about this, a gravitationally stable triad of mirrors-
- BSDr Becky Smethurst
(laughs)
- CWChris Williamson
...that are gonna follow, follow the universe around like a little tail. Um, w- what-
- BSDr Becky Smethurst
Yeah.
- CWChris Williamson
What else is there that's coming up?
- BSDr Becky Smethurst
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 gonna have such a huge impact is something called the Extremely Large Telescope, the ELT. That is actually its name. Um, it is extremely large. It's 30 meters across. So, James Webb, we think of that being very large, it's 6.5 meters across, so maybe a, you know, a two-story house. But with 30 meter mirror to collect the light, a- as much light as it can and focus it down, is, is... I mean, it's just incredible to think of how big that would be, you know, a sort of mansion-sized telescope almost. And that's gonna give us better resolution than the Hubble Space Telescope but from the ground, not from space where you're not looking through atmosphere that distorts everything. Um, and so that's gonna 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 te- 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. And so we know how much hydrogen or oxygen or nitrogen there exists in a galaxy and, and how that relates to how many stars are forming or how many stars are dying at that peri- at that, that time. And you can do it in every single little segment of that galaxy, which e- the amount of detail we're gonna have from that, the kind of resolution we're gonna 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, uh, one in, uh, South Africa and one in Australia. Uh, 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. 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 30 meters across, but a square kilometer across 'cause as you get to longer and longer wavelengths of light, so, uh, 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 sh- tells us usually where there's like magnetic fields and stuff like that in space. And, uh, and also hydrogen gas as well, uh, also has its 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 the, 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 gonna have with a, a telescope that is a square kilometer across (laughs) is g- just gonna be insane.
- 55:01 – 56:01
Where to Find Dr Smethurst
- CWChris Williamson
Dr. Becky Smethurst, ladies and gentlemen. If people want to keep up to date with the stuff that you do and s- everything online, where should they go?
- BSDr Becky Smethurst
Uh, so I'm on YouTube, uh, Dr. Becky is the channel name, and I chat about basically what we've been chatting about now, um, sort of like the questions that you're not able to Google because all you're gonna get back is, you know, just research papers that are written from, you know, by my colleagues for my, you know, my colleagues in the, uh, w- 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. Um, but also, you know, Twitter, Instagram, and, uh, from my books as well.
- CWChris Williamson
Becky, I appreciate you. Thank you. What's happening people? Thank you very much for tuning in. If you enjoyed that episode then press here for a selection of the best clips from the podcast over the last few weeks, and don't forget to subscribe. Peace.
Episode duration: 56:01
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