The Supermassive Podcast - 54: Q&A - Black Hole Special

Episode Date: July 3, 2024

Have black holes existed since the beginning of the Universe? Could we produce a black hole in the lab? Could we use gravitational waves to probe beyond the event horizon?  It's a black hole Q&A as Izzie put your questions to Becky and Robert. Plus, author and broadcaster, Marcus Chown, runs through a potted history of discovering black holes. His latest book on the same topic, A Crack In Everything, is out now: https://www.bloomsbury.com/uk/crack-in-everything-9781804544303/ The Supermassive Podcast is a Boffin Media production by Izzie Clarke and Richard Hollingham. 

They are just dark stars that we can't see because we can't get light from them. We've only explored their existence for just over 100 years. We do define a black hole's size as the diameter of the event horizon. Hello and welcome to the Supermassive podcast from the Royal Astronomical Society with me, science journalist Izzy Clark and astrophysicist Dr. Becky Smethurst. It's been a while since we did one of these, but who's ready for a good old Q&A episode? And this time Izzy has been very kind to me because she's made it all about my favourite topic of black holes.

Yeah, to be honest, we've got quite a lot of questions on black holes um and quite a lot of them are quite difficult so i'm that was a lot that was a lot of quite yeah yep yep british sentence quite quite quite i've uh sort of been watching them grow with the super massive mailbox i was like it is time it is time that we do a black hole q a because this is this is only ever just going to grow um so along with all of the questions that are coming up in this episode we're also going to be bringing you a potted history of discovering black holes so becky are you ready no i was born ready is i'm so excited and obviously dr robert massey from the royal astronomical society is with us too now don't worry robert i'm going to throw the easier ones your way i mean i mean i

know people can hear the sigh of relief the relaxation in my body at hearing that um you know but it's interesting to see the number of questions on this grow at the rate of a super massive black people are absolutely fascinated are you saying the supermassive mailbox is accreting questions on black holes? It is! Accreting questions about black holes. I blame Becky for this. Honestly, yes. I don't know if this is either going to help

reduce the number of questions about black holes or just increase them and they're just going to come thicker and faster. Yep, yep, yep. Okay, so Becky, can you start us off very simply what is a black hole sure so first of all black holes are not holes i feel like i have to say this a lot right if you're picturing like an emptiness in space like something you can fall down like a well like don't don't picture that like i want you to picture them instead like mountains of matter of material right because a black hole is where you've got

so much stuff in one place that the gravity is so strong that nothing can escape it right not even light that travels at the fastest speed there is around about 300 000 kilometers per second you know if we think about like the earth for right, we can't all jump off the earth because earth's gravity is pulling us back down, but rockets can escape because they can go faster than the earth's escape velocity, right? But black holes escape velocity, nothing's got a chance, right? So everything is crushed down into it. And the only way that we know to form a black hole is when a star runs out of fuel in its core and there's like no process pushing back outwards against gravity anymore, right? Because when a star is, you know, happily turning hydrogen into helium,

right, there's some energy pushing outwards against gravity and that's what supports the star. But then once the fuel runs out, there's nothing to resist that crush of gravity inwards anymore. And all that material gets squished and squished down. Now, when that happens, you know, if you start with a star that isn't quite heavy enough, you'll get what's called a neutron star. So that's where you squish the matter down. And then all of a sudden, like, you've merged the protons and electrons in atoms to become neutrons as tightly packed as they can go that are incredibly dense right and it's the sort of neutrons resisting being pushed together any further that's now resisting gravity crushing down anymore like they're the size of cities but the same mass of stars right this is how dense that they are. But if you have enough mass, if you

have enough material there, that crushdown will continue. It will be like, I don't care about you neutrons. Like there's enough here that gravity's winning, right? And then is when you get something that's so dense that the escape velocity in that region of space, right, goes above the speed of light and everything that's there gets trapped there, right? And I think I've explained that because I think it helps people to understand black holes better, right? They are just like the next evolutionary step of stars, right? Star, neutron star, black hole, you know? They are just dark stars that we can't see because we can't get light from them, right? No light, no information from them beyond at least

this region that we call the event horizon, which is this region that we call the black hole where, you know, you would have to be traveling faster than the speed of light. And beyond that, like, because we can't get any information from it, we don't know what they look like, right? It could be that material just keeps collapsing down and collapsing down under gravity until it's all squished down into an infinitely small point that we call a singularity. That's how we describe it with like the maths or the equations. Or it could be that this neutron star has turned into some exotic form of matter, some evolutionary stage beyond neutrons that we don't know of that lurks like beyond that event horizon. We just, we don't know, right? Because we can't get any information from them. And that's what makes them so fascinating to me and to so many others as well.

The fact that like by the very laws of physics themselves, we cannot know what's beyond the event horizon. And so to study black holes, that means we do have to get a little bit clever to work things out about them. maybe that's convinced people that black holes are the best things in astrophysics and that's why we're doing this q a i don't know but that is essentially what a black hole is i think like the statement we have to be a bit clever is just like the understatement of the year you have to absolutely flip everything on its head work out the most like ridiculous scenarios and see, does it match up? Does the maths work out?

They're just such a head scratcher. Okay, so shall we move on to questions from our listeners? So Robert, Polish Nerd wants to know, have black holes existed since the beginning of the universe? It's good to start with an easy question there, Polish nerd. So, I mean, quite possibly, or at least very soon afterwards. So, the idea is that there are or there were what we call primordial black holes that form from pockets of hot, dense material that was found from the first second after the Big Bang. Now, if those were dense enough locally, then they could have collapsed into black holes.

There might have been a whole range of sizes. Whether any of these are still around today, we just we don't know for sure. So the smallest ones evaporate due to an effect that was first suggested by Stephen Hawking called Hawking radiation, whereby they lose mass over time. And after 13 billion years, the smallest ones certainly would have done for them over the history of the universe but if there are bigger ones say the size of asteroids planets stars certainly uh then they should still be around and so they would have been around for the whole history or pretty much the whole history of the universe but we haven't actually found any of them for sure just yet and that's why we don't know whether they can actually form like that either which then stops us thinking about how black holes actually form in the early universe.

And it's very frustrating and people would really like to find one. Okay. And there's this follow up actually for you, Becky, which is from Matt in Australia, who says, why didn't the overwhelmingly density of matter in the early, rapidly expanding universe create a black hole? That's a really good question matt and it's a question that you know i get a lot i think i think astrophysicists in general do essentially the jury's still out a little bit because that very early sort of first moments of the universe's history are a little bit unknowable to us but we think it's because that space itself started

expanding at this exponential rate that perfectly balanced matter with energy, so that basically the universe wasn't dense enough to collapse into a black hole. Now we do describe the universe as starting as a singularity with everything crushed into one space, which is very similar to how I just described a black hole, right? And so there are some people that have considered whether that singularity is different to the singularities we find in black holes and instead is something known as a white hole. So the opposite of a black hole, essentially, in every single way, where instead of all the matter cannot escape from it, all the matter is escaping away from it. But as I said, our current laws of physics, they break down earlier than 10 to the minus 43 seconds in the universe's history. All right. So before that time, which is a zero with a point and then 42 zeros and then a one seconds into the universe's history.

Like none of our laws of physics work at that point. Like gravity breaks down, like magnetism breaks down. It's strong for everything breaks down. at that point like gravity breaks down like to magnetism breaks down it's strong for everything breaks down so it's hard for us to make any predictions with our mathematics or study it or draw any sort of conclusions of what goes on there at all so we think we have some idea but without the physics to definitely confirm that that's what's going on we're not sure but the fact that we're all here suggests that obviously that didn't happen and we probably have some idea at least why okay okay so black holes might have been around since the beginning of the universe but we've only explored their existence for just over

100 years writer and broadcaster marcus chown has condensed this history of their discovery in his latest book a crack in everything so it's over to him for where their discovery all began. Well, the first realistic discussion was during the First World War. So it came from a man called Karl Schwarzschild. He was the director of the Berlin Observatory at the outbreak of the First World War. And he immediately joined the German army. Even though he was 40, he didn't have to join because he was Jewish and anti-Semitism was on the rise, and he wanted to show he was a kind of, you know, patriotic German. And he ended up running a weather station in Belgium, then he calculated shell trajectories, and he ended up at a place called Mole House, which was the only part of the Western Front that was mountainous. And he was on leave in November 1915. And he attended one of the four lectures given by Einstein, where Einstein

presented his new theory of gravity, which supplanted Newton's. And he was completely captivated by this theory, went back to the Western front, and he did something which Einstein thought was impossible. So Newton's theory of gravity has one formula to describe gravity, but Einstein's has 10. And in Einstein's theory, gravity is actually the curvature or warpage of space. So in order to say anything useful, you need to find the curvature of space, which is gravity, around a realistic body. Einstein thought it was impossible because he thought, how do I solve or manipulate these 10 formulae? And incredibly, Schwarzschild achieved the impossible. And he found the curved space, the valley of space-time around something like a star. And he later realised

that if the star became more and more compressed as it shrank the gravity would become more intense the valley would would become more steep and eventually would become a bottomless pit from which nothing not even light could escape so it'd be shut off from the universe it'd be black a black hole and how did he manage to do that you know obviously if Einstein thought this was impossible how does what shall go and try and test that, find that out. Well, that's a fantastic question because Einstein's theory is amazingly complex. Oddly enough, as early as 1900, which was about 14, 15 years earlier, he just speculated on whether space might be curved.

In order to understand this, he'd got himself up to speed on the geometry of curved space, which was developed by Ryman and Gauss in the 18th century. So he just happened to be the right person in the right time and he knew the mathematical machinery which Einstein had used. Gosh, that's incredible. So we had that in, you know, 1915, 1916. And then how does the field then progress from that point? Well first of all Einstein was quite upset that the solution was found because gravitational collapse would continue until the mass of an object became infinitely dense what we call a singularity and that tells you that there is a flaw in the theory. It's broken. You've stretched it beyond a point where it's useful.

So to learn within weeks of presenting his theory that it was actually broken was upsetting to him. And he never really believed in black holes. And anyone who thought about this thought there's got to be some other force. A star runs out of fuel in its core, no longer generates the heat to push outwards and oppose gravity, so the star starts collapsing. There's got to be some force that stops it. In the 1920s, we got quantum theory, which is a theory of the microscopic world of atoms and their constituents, and it did look like a component of quantum theory called the Pali exclusion principle,

this stops electrons being on top of each other. It was thought that that would actually stop collapse. But then a 19 year old, Subramanian Chandrasekhar, was on a ship coming from Bombay to England to take up a post in Cambridge. And he applied quantum theory to stars. And he realised one thing that no one had actually realised, that quantum theory is not sufficient, you have to use relativity. And remember, the force which is opposing gravity is squeeze electrons, they don't like being squeezed together, so they effectively push outwards, like raindrops drumming on a roof. But Einstein's theory says that you cannot drum faster than the speed of light,

because that's the ultimate limit. So they are constricted. And Chandrasekhar realised that there was a mass, what we call the Chandrasekhar limit, 1.5 times the mass of the Sun. If a star was bigger than that, even quantum theory couldn't save you. To just take you on, very, very little interest in black holes, really, for the next half of a century.

And the main reason was, stupidly, people thought black holes would be black and small against the black of space. How are we going to spot them? So even people like Robert Oppenheimer, who investigated gravitational collapse, even he, you know, the man who built the atomic bomb, thought that. Already the story is like a who's who of scientists from the history books. So how does that then lead us to our first discovery of a black hole? So in 1971, the story goes to the Royal Greenwich Observatory, which was at the

time was in a 15th century castle, Hurstmonceau in East Sussex. We've been there, the Supermassive podcast actually recorded an episode from Hurstmonceau. So I was so excited to see this name come up in the story. The story is that Paul Murdin had come back from America where he'd been a graduate student in New York, and he wanted to make a name for himself. And he came to the conclusion that if something emitted x-rays, it's bound to be interesting because x-rays are emitted by matter at millions of degrees. Well, in the early 1960s, the first X-ray telescopes were launched on sounding rockets. They went up into the air, came back down again. And in a minute or so, they were above the obscuring atmosphere. They found these X-ray sources and they were terribly

crude. So they could only say which constellation. And there was one in the constellation of Cygnus called Cygnus X-1. When Merding came to the Royal Greenwich Observatory, Uhuru had just been launched and this was the first X-ray satellite. And he noticed when he saw the catalogue that the box showing the location of Cygnus X-1 had shrunk. So the idea, he thought, was did it have a companion? Was material being sucked off this super giant star swirling like water down a plug hole onto a compact object and friction heating it to millions of degrees and as luck would have it Merdin was sharing an octagonal turret room office with Louise Webster a person who's been completely written out the history of science but who turns out to be the co-discoverer of black holes and

she was working on a project to measure the speeds of stars in the Milky Way so he said to her could you could you find out whether this this object this blue super giant is orbiting anything she looked to find out whether there was a periodic change in the velocity of the star and sure enough there was every 5.6 days this massive star was being whirled around something but when they looked at the photographic plates there was nothing there amazing and so how long were they studying this and exploring this when when does it go from you know an idea a curiosity into you know what we think we've got a black hole well they sat down in their little

office and they used newton's law of gravity and they got formulae from the from the library and they worked out what how massive would this invisible object be to be whirling this star around so fast and they came to the conclusion it had to be more than four times the mass of the sun possibly more than six times the mass of the sun we now know it's four times the mass of the sun, possibly more than six times the mass of the sun. We now know it's 15 times the mass of the sun. But this object was much bigger than both a white dwarf and a neutron star. So it could only be a black hole. But eventually they managed to publish it in, I think, 1971.

So we have Paul Murdin and Louise Webster, both pretty much forgotten. And certainly Louise Webster is forgotten. She was in Australia and she went back to Australia and she had a liver disease and had one of the first liver transplants, successful ones in Australia, but then died at 49 of cancer. So if I can do anything with this book, I want to get the credit for Louise Webster's being the co-discoverer of black holes. Thank you to Marcus. And we've put a link to his book, which I really recommend in the show notes. And we're going to have part two coming up later on in the show. Let's jump back into the questions.

Becky Hudson-Andley asks, is the spin of a black hole a similar property to the spin or rotational speed of planets and stars if so can it be measured so let's set things straight here what do we mean by spin yeah so spin is just rotation rate so the earth spins every 24 hours the sun spins every 26 days or so and yes a black hole spin is the same thing. And that's because black holes formed from stars collapsing down. So they inherit essentially the spin of the star that formed them. But if you think about the fact that, okay, the matter has been crushed down until it's incredibly dense, the spin will go up. So is if you can picture like an ice skater, right, spinning on the spot,

like a pirouette, but then they pull their arms in so they get denser, right? And they spin faster and faster and faster. It's incredible when you watch them do it because they can spin so incredibly fast. It's amazing. So things like neutron stars and black holes, they all spin incredibly, incredibly fast. When we talk about black hole spin though we we don't tend to put like an absolute number on it like they spin every however many seconds or whatever like neutron stars we tend to talk about it in terms of like maximally spinning and minimally spinning black holes this is normalized between like zero and one so you'll hear people being like oh my gosh this black hole's got a spin of 0.95 because it's like 95 of the maximum spin it can have and that's essentially thinking about like the mass of

the black hole and then the radius of what the event horizon is and therefore like what speed the event horizon would be spinning because obviously if this is spinning then it's also spinning on like causing space-time right issues and stuff like that so it's all about like in terms of like what the spin rate would be if you had something at the event horizon and that would be the speed of light and that would be like the maximum spinning rate that it can get to yeah okay fair enough uh robert matt p on instagram asks under what circumstances could we theoretically produce a black hole in the lab could we theoretically produce a black hole in the lab? Yeah, that's a good question, isn't it?

Well, Matt, there are people actually doing not black holes in the labs, but analogues for them. And one really interesting one that was publicised a couple of months ago is at King's College London, where they're using liquid helium, so down to four degrees above absolute zero, about minus 270 degrees C, to try and test how space around the black hole moves. So, you know, really intriguing stuff. And you can see on the King's College website, there's a nature paper associated with it, too. But of course, that is just a simulation. That's not you know, that's not a real black hole. It's just an analog. So in theory, you could do this for real and you could create a teeny black hole without much risk because it would probably evaporate very quickly it'd be very very small indeed

and there were scare stories when the large hadron collider started up about this yeah i was gonna even a lawsuit there was a guy in the u.s attempted to sue the u.s government because the suggestion was there's going to be a black hole that would swallow the world and you know obviously that didn't happen and when they at it, what they realized was that the energies of the Large Hadron Collider were less than natural cosmic ray ones. So particles from space coming in much higher energy than the LHC, and they don't create black holes as far as we can tell. The problem is you need a vast amount more energy than the LHC, probably anyway, to do this, unless there's something really exotic happening in their ideas that if you have many many extra additional dimensions in space that it might be possible to

do it at lower energies but we don't see any evidence for that so i think the answer is theoretically we could do it with an enormous amount of energy but at the moment you know we don't think we have access to anything like enough energy to do that and if we had done it then we'd have seen them decaying in places like the lhc so it's probably really hard to do that and if we had done it then we'd have seen them decaying in places like the LHC so it's probably really hard to do but in theory yes we could that would be pretty godlike I think if you start making black hole yeah certainly certainly that's my new goal in life I think it's an aspiration definitely by the time you retire have your own black hole become godlike and Becky Willie McDonald has emailed us.

He says, I've been listening to your podcast since the beginning and it keeps me amused while driving my work van. One thing that has been going around my head for a while is this. As black holes are an object within the fabric of space and time and gravitational waves are an effect which travel through the fabric of space-time, do they pass through black holes? So as we become more proficient at deciphering information relayed by gravitational waves, is it possible that we could use gravitational waves to probe the event horizon and discover what may lie behind it? This is a great question,

really. And I hope you're listening right now in your work van to this question as well. And you've got a big smile on your face. But the short answer is yes. I mean, this is why we can even detect gravitational waves from the merger of two black holes in the first place. Because if you think about it, they come from within the event horizon and they propagate outwards. They go where light waves cannot. This is why there was so much excitement when they were first discovered, because it allows us to probe regions of space-time that are completely unknowable to us if we just look with light, right? So, I mean, if you define the black hole as anything inside the event horizon itself, which I do, and many of my astrophysics colleagues do, then yeah, they can travel through black holes.

Now, if you're talking about, you know, gravitational waves from another object passing through another black hole, I guess we do also get that in a merger of two black holes, where you have like interference of the gravitational waves from the two black holes messing with each other. And people are really just starting to model those very complex effects now and trying to test with, you the gravitational wave data that we that we do have that it's kind of there but it's not quite at the resolution that we would particularly like but like improved data with like the next generation of gravitational wave detectors will help even more with this as well so that we might have a hope of being like oh you know there was a

gravitational wave merger here and know what happened exactly when this wave passed through the event horizon of this other one next to it, you know. Maybe we'll be able to isolate that sometime in the future. This is the Supermassive Podcast with me, astrophysicist Dr. Becky Smithhurst and science journalist Izzy Clark. Now, I want to take a break from the questions on black holes for a moment because we've had this question from Lee Beresford. Don't worry, Becky, you get to talk about black holes even more in a moment. But first, Lee's question.

He says, Hi all, absolutely love the pod. Please make it weekly. We get that request a lot. I'm not sure if it's possible. Anyway, I find it hard to accept dark matter as an explanation for gravitational anomalies. Anyway, I find it hard to accept dark matter as an explanation for gravitational anomalies. However, what I do accept is that people who are a lot smarter than me

have worked on this forever. And thus I accept it's more of an issue of me wrapping my non-physics brain around it rather than it being wrong. So my question is, are there any theories or facts in astrophysics that any of you just couldn't accept as true at first or even any that you still just can't wrap your head around now but begrudgingly accept as it's been accepted by others with a more thorough understanding thanks for doing what you do lee i love this lee and if it helps your understanding of dark matter just quickly first think about how there are some elements that aren't magnetic. So like some metals that aren't magnetic, so they don't interact with the magnetic part of the electromagnetic light wave, right? So why is it so weird that there could be matter that doesn't interact with any part of the

electromagnetic light wave, right? That makes me, you know, wrap my head around dark matter a little bit easier. As for theories that I took ages to get my head around when I was learning them, I really struggle, still do to this day with quantum mechanics. You know, this idea that the edge of the table, the desk that I'm currently sat at, like might not be here, right? There's a very, very small probability in quantum mechanics that it's somewhere else. And it just never made sense to me and i remember trying to have a conversation with like a physics and philosophy student once at university which just made my head hurt even more thinking about this oh my gosh they are fascinating people anyone that did physics and

philosophy i was just like what yeah i didn't even know that was possible. Yeah. So good. I know. I had also a psychology student friend when I was doing my DPhil, my PhD, who was studying the psychology of does time actually go faster when you're having fun? Like from a physics perspective of like time dilation and stuff like that, which I thought was amazing. Anyway, quantum mechanics is just one of those things

that I think I just accept what the maths tells us, but like logically still my brain can't quite accept. But I think that's true possibly for many of my colleagues. And while I'm remiss to quote Feynman, he did famously say, you know, nobody understands quantum mechanics. Yeah, totally. I think the idea of something also being in like two states at once until you look at it as well and I always think of that as yeah the thing that gets me to that point of just accepting that is it's like if you flip a coin it is both heads and tails whilst it's in the air and then when you look at it it's either heads or tails and that is my entry level into it i'm like yeah

it's only when you catch it yeah yeah yeah like if you're doing that thing where you flip it and catch it on the back of your hand right like it's heads and tails while it's landing and it's only you intervening yes that makes it be a head or a tail yeah yeah that's a good good analogy we have to think of poor shredding yeah always what about you robert is there anything that you know makes you scratch your head a little bit yeah oh far too many things really i was thinking earlier i was talking about you know the fact that black holes it might be possible to create black holes with many if we there are many more dimensions in space well try getting your head around the idea that there's an 11 dimensional universe you know seriously how you? Our brains just are not programmed to

intuitively understand it. Again, it's an example of something where the maths might work for it, but, you know, intuitively, no way. Quantum mechanics, definitely as well. And also concepts like dark energy, right? You know, the idea that there's this stuff that we don't know what it is that's driving the expansion of the universe. And again, dropping out, dropping out of the equations that we use to understand it. So there's lots of concepts in astronomy, I think, where that's driving the expansion of the universe and again dropping out dropping out of the equations that we use to understand it so there's lots of concepts in astronomy i think where i wouldn't say they're an article of faith they're rational you can sit there you can say okay i can see how you need this but the idea that you intuitively understand it is a massive massive struggle

uh you know and how could it not be really yeah um and i think for me i mean i think it feels a bit more simplified and some of the stuff that we've been talking about already but when it comes to looking at like referencing the speed of which different things are traveling my starting point is my brain always takes me to a very simplistic model of if I am driving on the motorway and I'm going in one direction at one speed and the cars are going in the opposite direction at a different speed and and how that appears to you and that helps me sort of picture jumping between different reference frames to sort of then move into the things of like relativity and travel and different speech and things like that but I have to start with

I am on the motorway and I am driving in one direction from motorways let's perhaps go back to black holes and here's part two of our history of discovering black holes with marcus chown let's move this on into the timeline then of black holes and studying them and we get to a point where we understand that there is a supermassive black hole in almost every galaxy so when do we see that change? What is that shift? Well, in the 1990s, the Hubble Space Telescope was launched, initially with the telescopic equivalent of a squint, and it cost $800 million to fix it. But it was able, with its sharp eyesight, it was above the blurring effect of the atmosphere, it was able to look into the heart of nearby galaxies.

And in every single one, it saw that the look into the heart of nearby galaxies. And in every single one, it saw that the stars in the very centre were whirling around far too fast. In every single one, they were in the grip of the gravity of a very massive object. So we then realised there's actually one in every galaxy. So the reason we only see 1% of galaxies are active is not because 1% of galaxies contain supermassive black holes. It's just that in only 1% of galaxies are active is not because 1% of galaxies contain supermassive black holes. It's just that in only 1% of galaxies are those supermassive black holes active. So in other words,

in only 1% of them, are they getting any fuel? They've only been fed. In the 99%, they've run out of ripped apart stars and gas. And so that's why they're dormant. And of course, there's one in the center of

our own galaxy well exactly and i think that is something that does blow people's minds a little bit when you really start to think about these things and operate on such a big level it's like oh my goodness okay but why is our special you know is it helpful to our galaxy in any way? Well, one of the big surprises, I should just step back here, is that these supermassive black holes have an effect on their galaxies. Now, I need to tell you how small they are. So if you imagine a bacterium compared to London, that's the size of a supermassive black hole compared to its parent galaxy now would you think that the bacterium could control the street plan orchestrate the street plan of london well you wouldn't would you no no i wouldn't but we discovered all these correlations between the the back hole and its

parent galaxy so for instance in every galaxy we see that the supermassive black hole is one two hundredth the mass of the stars in the central bulge of the galaxy so this is implying there's an intimate connection between the supermassive black hole and the galaxy but getting back to your question there's something weird about the supermassive black hole in our galaxy it's a tiddler so if we look at andromeda which is the nearest big galaxy very similar to ours it's supermassive black hole is 50 times bigger than ours, as is a thousandth the size of many of the supermassive black holes we see. So there's something really, really unusual.

It's only 4.2 million times the mass of the Sun. But it's almost certain that we're having this conversation because the supermassive black hole in our galaxy is small because in the bigger uh the galaxy is the bigger holes um all the raw material stars was blown away and they only got one of one generation of stars in our galaxy our supermassive black hole did not blow away the gas so there'd be multiple generations of stars the sun is a third generation of stars. The Sun is a third generation of stars. And in each successive generation, more heavy elements like carbon and oxygen, all the things that are necessary to make planets and to make life formed. So the big galaxies with the big supermassive black holes

are probably lifeless deserts. And there's a reason why we find ourselves in a galaxy with a tiddler of a supermassive black hole. We then bring it into this century and we have the Event Horizon Telescope, which has given us like the first image or we've had two images of black holes now. What do you think is next for our understanding of black holes? Are there any themes that come up in the book that are going to push that next frontier well of course if we can if we can image them better with the event horizon telescope we can maybe test Einstein's theory of gravity we can probably create movies of material falling into

the center of the black hole but I think most of the information is going to come from gravitational waves so the incredible thing about that discovery is that the black holes that we found merging, and we've now found about 100 of these events, the black holes are far bigger than anything we expected. We think black holes form in supernova explosions. So a star explodes, and paradoxically, its core implodes to form a black hole. But 90% of the material is blown away into space. Okay, so if we're now seeing black holes merging, which are about, some of them, almost 100 times the mass of the sun, that would mean that the star, if it had been formed from a supernova, would be 1000 times the mass of the sun. Well, that's impossible. We don't see stars like that. They don't exist. So that's telling us that probably the black holes we're seeing emerging,

each have merged results of mergers before. So this merger process is much more common than we thought. And we're seeing black holes in a range of mass which we didn't think would exist because supernovae have this thing called a pair production supernova. When the star is massive enough it becomes so hot in its interior it blows itself apart without leaving a black hole so there's a mass range where you shouldn't see black holes but we see we're seeing black holes there so again that's the great thing about science you know it's getting the unexpected result

thank you to marcus chown and as i said we'll put a link to his book a crack and everything in the show notes back to our listener questions and Becky Nick who states that they're an Irish listener in the Netherlands has sent us an email he says hi everybody I've gone through a bit of an obsession with your podcast Dr Becky's YouTube and most recently your books. On the topic of primordial black holes and their potential sizes, Dr. Becky's book compares one to the size of a tennis ball, including a nice black circle to show the size. My rather important question is, what if a creature, say a dog, tried to chase this black tennis hole and catch it in her mouth? tried to chase this black tennis hole and catch it in her mouth what would happen is the event horizon much farther out than the nine centimeters would the event horizon fit in her mouth at what point would the danger toy start to impact her and how i really want to explain this to my dog

toothless so that she knows what to look out for should she stumble across one in the wild photo of said floof is attached many thanks like it's brilliant nick also toothless is a great name for a dog let's put that out there so i i mean i did say this earlier in the podcast but i'll say it again it's just that we do define a black hole's size as like the diameter of the event horizon so nine centimeters there that i put in my book brief history of black holes if you don't mind the little plug is is like the diameter of the event horizon right so yes technically if a dog can fit a tennis ball in its mouth it could

fit this you know primordial black hole that i was talking about in the book we think there might be a primordial black hole at the edge of the solar system that could be in orbit like a planet that's making things at the edge that go a bit weird but it's still a hypothesis you know it's not been proven yet so yes technically the dog will be able to hold it in its mouth i don't know who's throwing this black hole tennis ball to them in this scenario because uh you wouldn't be able to hold it in your hand never mind have the dog catch it sadly because neither of you would be able to escape its pull you'd start feeling the pull millions of kilometers out in terms of top speeds that you can run and stuff if there was nothing

else around you for example you know that was had a stronger gravity being close by so um i think i think i would just tell your dog to just permanently avoid anything that looks like it. Like there's nothing there. To be honest, I don't think a dog would be that interested in it anyway, because, you know, it's nothingness. They'd probably be like, oh, a squirrel. Yeah, exactly. Okay, here's hoping Toothless isn't in any danger anytime soon.

Robert Eric Moi asks, what is the smallest known black hole and how small can they be? Well, Eric, in the present day, the ones that form from stars seem to have a mass of a few suns. So it'd be three or four times the mass of the sun. And you don't seem to be able to make them any smaller than that because then you end up with a neutron star instead. And they all formed as a result of supernova explosions. So in a supernova explosion, most of a very massive star is ejected into space, but the core left behind collapses into the black hole. And those progenitor stars maybe have at least 10 to 15 times the mass of the sun. So that also means they have much shorter lives than our star and tens of millions of years, and they're tens of thousands of times brighter. But there

might be really tiny black holes created in the early universe as we mentioned before and some of those could still be around so for example if you had one the mass of an asteroid it might be the size of an atom which i found a quite a mind-bending concept the idea that there's these teeny tiny black holes just plowing through space completely impossible to detect i'm guessing and then but there they might be but we haven't found any yet yeah the one i was talking about before the one that nick asked about that nine centimeter diameter one that would be about five times the mass of the earth and that just shows how compact they are really doesn't it taking yeah taking five years yeah there is a

really nice equation that just relates like the the event horizon size to the mass of whatever you have and of course as soon as i learned that equation at university i uh immediately calculated what my event horizon would be it's very small it's very very small i encourage everyone to do that that's so good um but though vicky if you could go out and just find us the smallest black hole possible, that would be great. Thanks. Yeah. Okay, cool. I'll at least poke my colleagues who are working on this too.

Okay. Becky, Jake Pierce has sent us a message. He says, was much more dense, was it possible for stars to be so massive that when they triggered nuclear fusion, the clouds of pure hydrogen gas kept feeding the star until the force of gravity was so intense it turned the core of the star into a black hole. This would then mean the black hole would be force-fed and could explain how there are black holes much bigger than expected in the early universe. Love podcast thanks jake so we do think that stars were able to form much larger in the early universe and we also think that stars at

the end of their life could actually skip supernova entirely and just collapse straight down to a black hole we think that could be possible thanks to some observations we've made in our own galaxy of just stars like winking out so yes we think technically that could happen in the early universe and maybe make a black hole a few hundred times the mass of the sun as opposed to like the say 10 to 50 maybe pushing up to 100 times the mass of the sun that you can get sort of in our galaxy at the moment the thing is that's still not big enough to explain i think what you're mentioning here about how there's black holes much bigger than expected in the early universe. Those are the

super massive black holes that we're seeing that are like millions to billions of times the mass of the sun. And we're not entirely sure how they've gotten that big that quickly. And we don't think that can come from just like a star forming alone. So the big idea at the minute is actually something called direct collapse black holes, where these clouds of pure hydrogen gas that you were talking about in the early universe, they actually just skip making the stars entirely and they collapse down into black holes that are say 10,000 to 100,000 times the mass of the sun, and then they can grow from there into like the supermassive black holes. jwst is actively searching for

signatures of this happening in the early universe so keep your fingers crossed that somebody spots something but essentially the way we think it happens is that if you have say two clouds of hydrogen gas next to each other and one of them collapses down first and makes some stars and then those stars shine and they give out energy and that energy hits into the glass cloud next to them right that's separate and hasn't formed stars yet and just keeps heating it and heating it and heating it and this heat essentially you know adds this energy to the molecules so they move around more and they resist that collapse down of gravity into say a star but if the you know gas cloud keeps

growing because it was very gas rich in the early universe then eventually you can reach the point where it does get dense enough that all of a sudden air gravity will collapse it down into a black hole so that's what we think maybe is happening simulations have suggested that's possible and it could explain the supermassive black holes we're seeing in the early universe but we still haven't quite found evidence for that actually happening just yet oh thanks for that becky i think i need to go back to lee's question where this i mean a lot of the stuff that we've covered in this episode is just things that i have to accept i'm like yes okay sure becky says so okay fine sure but i think it's just things that I have to accept I'm like yes okay sure Becky says so okay fine

sure but I think it's fun to think about these things though because like we've been talking about primordial black holes you've been talking about direct collapse black holes but at the end of the day the only way that we know to make a black hole is from a supernova so we have all of these things that were like these should exist these should exist but how how are they being made right and I think that's what I love about this area of research still is there's so much that we don't know still um and that's what makes it so exciting to me yeah so that actually does bring us to the end of our listener questions but like just from your point of view what are some of the big questions that you would love to get to the bottom of when it comes to black holes?

Yeah, I mean, for me, obviously the big question is what I like to work on, which is how supermassive black holes grow. If direct collapse black holes exist, then you can make something 100,000 times the mass of the sun. How do you then take that to a million or billion times the mass of the sun? Like you'd think, okay okay black holes are just hoovers so they just hoover up you know whatever's in the region around them but the thing is space is really big so the regions around them are a tiny patch of actual space there's not actually that much material in them so somehow you have to keep funneling material down to these black holes and the one way that we thought that happened was when two galaxies

merge together and if they both have supermassive black holes at the center then that'll sort of like scramble everything up and just send everything tumbling to the middle where the two supermassive black holes will merge and then they'll also grow with whatever tumbled to the middle and we've seen really tight correlations between like the mass of the supermassive black hole and then like the mass of bulges of stars that are made in the centers of galaxies and mergers so we were like oh it's obviously that then but then my work's focusing on like okay if you take a galaxy that's never merged with anything else before how big is its supermassive black hole you're like oh that's also millions to billions of

times the mass of the sun how did they get like that and so we still don't really know and we still don't know the limits of black hole growth properly. Like we think there is a limit to how fast they grow, but then we see black holes greater than that limit. So, you know, there's a lot of confusion still around black holes. And there's, like I said, so much that we still don't know. And we have to be really clever about what observations we can make to try and piece all of this out and build up the jigsaw very very slowly between you know hundreds of people around the world that work on this yeah amazing it's brilliant well thank you to everyone who has sent in a question uh for this episode and robert so

can you wrap things up what can we see in the night sky this month? Any black holes? Well, theoretically. Okay, so I'll come to that. So look, July is still a month of hopefully warm weather, but short nights. But by the end, they're getting a bit longer and it gets properly dark once again, even from the UK. So look, you've got the summer stars stretching across the horizon, not least the Milky Way. If you wait till a bit later in the night and there's no moon. Absolutely fantastic view of the inside view of our galaxy. Stretches across some Sagittarius in the south, which is the direction of the galactic centre and its supermassive black hole, even if you can't see it, up through the overhead point and down to the northern horizon. And Scorpius, the Scorpion's

there to the right of that with its brightest star, the beautiful red supergiant Antares. You've also got the Summer Triangle, which is the three bright stars, Altair, Deneb and Vega. And Deneb is actually in Cygnus, the constellation of the Swan, which looks like a sort of cross. And it happens if you've got the reference charts and you know what you're looking for and you've got a decent size telescope. It has the black hole V404 Cygni and the flashes, the material falling into that sometimes generates flashes that are bright enough to be seen by amateur telescopes. I've never done it. It sounds a really intriguing thing to do. And I guess there are people imaging it, but it is incredible that, you know, there are

one or two of these things you can actually see as an amateur astronomer. Also, if you're going on holiday, then these constellations in the summer sky generally is higher in the sky further south. So you get the view of Sagittarius and Scorpius, generally is higher in the sky further south so you get the view of Sagittarius and Scorpius the best views in the southern hemisphere but even if you're going down to the Mediterranean I always think you know just take a pair of binoculars with you because if you do go somewhere where it's even slightly darker or you know you go away from your resort you're on some dark Mediterranean island then just enjoy the view of those rich fields of stars it's you know it's a really stunning sight and a lovely thing to do with a drink on a warm summer evening the amount of times i've been on holiday and been

like i wish i had my binoculars like yeah no but like not just for astronomy but also like because you're like oh i think there's like a whale out to sea and like i can't tell and i want my binoculars or look at that like incredible colorful bird down there like what you know i want to see it so take your binoculars and you won't you won't regret it you know put them in your hand luggage take the weight up i don't care agree and they are just so versatile the thing is for anybody I want to see it. So take your binoculars and you won't regret it, you know. Put them in your hand luggage. Take the weight off. I totally agree. And they are just so versatile.

The thing is for anybody starting, you know, just wants to have with a casual interest in astronomy, looking at the sky, that they can do so many things. That's why we always say get those first, even before a telescope, you know. So also I mentioned the fact planets are starting to come back a bit now. You generally have to be up reasonably late in the night, but you can in the morning sky or early in the morning, rather, if you get up before dawn, then you can see planets like Mars and Jupiter. And actually on the 30th and 31st of July is a nice photogenic event where the two planets are next to the crescent moon. So that's worth being up for. It'll still be about five o'clock

in the morning in the UK, but it's a good thing to do. And finally, thinking back to our last episode, it is the major lunar standstill year. So what that means is that the moon is quite often going to be very low in the sky in the summer months. And that makes for really nice photos too. So if you imagine the moon, you know, just hanging above a hill in the landscape. So maybe look out for the full moon on the 20th of July in particular for that. You'll see it'll look big because of the moon illusion. The moon always looks bigger when it's on the horizon. But it'll also be really photogenic too.

Right, well, that's it for this episode. How did you find it, Robert? Sigh of relief. I am indeed relieved. Yeah, it's true. We're just very glad to have Becky doing this. Well, we're going to be back in a few weeks with another bonus episode

where we're going to be bringing back Book Club and checking in with JWST and after that our next main episode is a deep dive into Venus because we've done all of the gas giants and then Richard and I realized that we haven't properly covered Venus before so uh it's about time we do that yeah definitely and thank you to everybody who sent in questions we have a growing pile as we said so please make sure to keep adding to it please do accrete the uh supermassive mailbox some more if you if you have a burning question for the team you can email it to podcast at ras.ac.uk or you can find us on instagram at supermassivepod is you're really good over there aren't you at

putting out a call for questions each month reminding people our episode's gonna be on this send us any questions so make sure you follow us on instagram to keep up and of course if you send in any questions we'll try and cover them in a future episode but until then everybody happy stargazing