The Infinite Monkey Cage - When Two Stars Collide

Episode Date: January 8, 2018

When Two Stars CollideBrian Cox and Robin Ince are joined on stage by comedian Dara O'Briain, Professor Sheila Rowan of Glasgow University and Professor Nils Andersson of Southampton University to look at last summer's spectacular discovery of gravitational waves from two colliding neutron stars. The observation of this huge cosmic event not only confirmed one of Einstein's great predictions, some 100 years ago, but also revealed the source of gold in our universe. Brian, Robin and guests look at how this momentous discovery brought together nearly 1/3 of the world's astronomers and astrophysicists as they raced to point their telescopes at the collision, but also confirmed the presence of gravitational waves, first predicted in Einstein's theory of general relativity back in 1915. They also discover why the source of our heavier elements such as gold and platinum has been so difficult to prove, until now. Producer: Alexandra FeachemThe Infinite Monkey Cage book "How to Build A Universe (Part 1)" is out now and available to buy from all the usual places.

In our new podcast, Nature Answers, rural stories from a changing planet, we are traveling with you to Uganda and Ghana to meet the people on the front lines of climate change. We will share stories of how they are thriving using lessons learned from nature. And good news, it is working. Learn more by listening to Nature Answers wherever you get your podcasts. This is the BBC. Hello, I'm Brian Cox. I'm Robin Ince and welcome to the first episode of the 17th series of The Infinite Monkey Cage. Or in hexadecimal, 1-1.

Right, so that's... Yeah, don't laugh at hexadecimal, one, one. Right, so that's... Yeah. Don't laugh at a hexadecimal joke. That was a typical thing for this audience to do. Some of them deliberately laughed as if they understood what you said. It's based 16, isn't it? So it's one, 16, and one, one. 15 would be F. Yeah, well, that saved a lot of time.

Anyway, what I loved is one man down there went, Yes. Yes! So, anyway, today's show is all about... Spandau Ballet. Sadly, this is not about the science of Spandau Ballet because the producer said we couldn't do that. It's a part of the new BBC's public service remit to bring artificial balance to everything.

In this case, the airwaves by placating new romantics. Yeah, well, it's worse than that. They've started using synth players from 1990s pop bands. Anyway... LAUGHTER Oh, that's... Some of them happy with that. Some want... Uncertain. Today's show is called When Two Stars Collide,

and it turns out that the answer is... Gold! Stop doing that. It's not about gold. It's about this year's Nobel Prize-winning discovery of gravitational waves, ripples in space and time predicted by Einstein a century ago and first detected by the LIGO experiment in 2015.

The gravitational waves were caused by the collision of two black holes. In 2017, LIGO announced that they had also detected gravitational waves from the collision of two neutron stars and subsequent optical observations of the collision revealed that large amounts of Was created in that collision This is the first time if you spanned our ballet in the and it's probably the last, so enjoy this level of show business. Anyway, to help us discuss gravitational waves, neutron stars, and possibly the wavelengths of Tartan on top of the pops in 1982, if we have the time, we are joined by three physics graduates,

and they are... Hello, I'm Nils Andersson, Professor of Applied Mathematics from the University of Southampton, and the cosmological event I'd most like to observe is neutron stars crashing into black holes. And I'm Professor Sheila Rowan. I'm Professor of Experimental Physics at the University of Glasgow, and the cosmological event I'd most like to observe

is the Big Bang itself. I'm Dara Bain. I'm the Emeritus Professor of Cosmology, Archaeology and Biology at the combined... from many universities. And it's been, you know, a heady time for me. So, yeah. And this is our panel. Yay! Can I just ask you, by the way, to do your popular stargazing catchphrase? Unfortunately, it's a bit overcast tonight. You know, we don't say that as often as you think.

My catchphrase is usually, well, we're out of time. And, well, we can't get to that. Or, saying to some incredibly important scientists, and that's why you won the Nobel Prize, yadda, yadda, yadda. Let's move on, all right? My favourite one was really shouting in our ear when he was, I'm sorry, Buzz Aldrin, I've got to stop you there because we've got to go over to K-9,

a metal dog who's going to ask a stupid question. Yes. And that was my favourite. The fictional robot dogs always win. So let's start off with you, Sheila. I suppose we should define, as we're going to be talking about gravitational waves, what is a gravitational wave? Gravitational waves, mathematically, are a prediction of

Einstein's theory of general relativity. But that probably doesn't leave people feeling very enlightened as to what a gravitational wave is. But we're quite lucky in that Einstein's theory, general relativity, describes how space, space-time and mass interact with one another. So how do gravitational waves fit in that? Well, if you imagine our universe, first of all, as empty, it's an empty space. In fact, imagine it as a flat, empty sheet,

flat, empty rubber sheet. There's nothing there, no stars, no mass. And then we come along and we add a star to this universe. So we put a star on our flat rubber sheet, and the mass of that star causes the sheet to curve. In Einstein's picture, that curvature caused by the mass, we can think of as gravity. And mathematically, that's what general relativity says.

So we've got the curvature of our universe, the space of our universe caused by mass. Say that mass moves. Say it's a star and it's got the end of its life, it's run out of fuel, its core has collapsed down suddenly, the stars exploded. That mass on our rubber sheet suddenly moves.

It changes the curvature around it, and in fact it sets up ripples that travel out across that rubber sheet. So what we have there is changes in the curvature of space. Those waves, those travelling waves are gravitational waves and what general relativity does is mathematically describe that but to us it fuels actually like tiny changes

in the direction of gravity here on Earth that have travelled across the universe to us from exploding stars or colliding black holes, massive astrophysical events far out in the universe. Because these events are, I presume, going across the universe, these are occurring.

So gravitational waves are passing through us now, gravitational waves from many different directions. That's right. We are all the time being bathed in these gravitational waves. And physically, that has a meaning. If you think back to our rubber sheet analogy, as that sheet was curved, it was stretched. As those waves rippled across the rubber sheet,

they were stretching and squashing the fabric of space itself. So as we sit here on an Earth and are bathed in these gravitational signals coming in from the cosmos, the whole time we, the Earth, the room, everything around us is actually being stretched and squashed just a little bit. Fortunately, just a little bit. Otherwise, we'd notice, but it's a tiny, tiny effect,

and that's the effect that we've been searching to measure, actually, for about the past 40 to 50 years. Dara, have you tried to ever explain such complicated issues on a... On a television... Sorry, on a broadcast. Actually, last week, I was in Belfast discussing Brexit, so, yeah. And it was something we'd actually... Couldn't get any resolution of, whereas this, I think we've got it nailed. It is... And I'm sure my esteemed colleagues will back me up on this

as an analogy. Robin, if you're having any difficulty, imagine a trampoline, you're a parent yourself, imagine a trampoline, imagine a child running on a trampoline and imagine getting a second child to run towards that first child. And a gravitational wave is what your wife senses in the house.

What is happening in the garden? What has he done now? Just at the moment, just before impact. And so, yeah, that's how fast it goes. That's an incredible thing, how quick it is that your wife goes, what, from the house, just as you're arranging this experiment. Niels, Einstein first predicted gravitational waves about a century ago, not long after the publication of General Relativity. So why has it taken so long, a century, for us to detect them?

Was there any doubt as well about their existence? Yeah, you could say that. I think, so the first 50 years were doubt, and the second 50 years were trial and failure, I think, and then finally after about 100 years, in fact, almost exactly 100 years, success. So if you look at the first 50 years, basically, people couldn't agree. Einstein himself famously wrote a paper at some point quite a few decades after the prediction

where he first wrote the paper saying, oh, gravitational waves, no, they don't exist. And then he was criticised by a colleague, so he said, oh, yes, of course, gravitational waves exist. And then in the final version of the paper, he said, oh maybe. So I think that's one of the classic hedging your bets kind of in science things.

The reality is it was not until the late 1950s that people started agreeing that yes, these things did exist and there was no doubt we should be able to catch them in theory. But, as Sheila said, they are absolutely minuscule. And so to build a ruler, if you like, to measure this stretching and squeezing of space and time is an astonishing experiment.

And I'm glad, as a theorist, I don't have to worry about actually doing these things. That's Sheila's job. And so the next, well, from the late 60s until 2015, was essentially trying to build this ruler. And don't ask me how it works, because that's Sheila's job. I'm sorry, that's literally the point of you being here. And you cannot do that midway through.

See, I always think the fun thing about rulers for these things is that we're going to build a ruler to measure the expansion in space. Oh, dear, the ruler has expanded as well. How do you separate out the fact that you and the ruler have also expanded at the same rate as the thing you're measuring the expansion of? It's a great question, actually. Perhaps you could describe, first of all,

what this experiment looks like, and then answer Dara's question, which's a great question, actually. Perhaps you could describe, first of all, what this experiment looks like and then answer Dara's question, which is a great one, which is why the rulers move as well. Sure. It's a very good question and not an easy one to answer, but I'll give it a go for you, but you might need to lie down in a darkened room for a bit afterwards with a wet towel around

your head. Well, I was going to say, you're not going to get any help from Niels. He's already said that he's not going to answer. I see. So you go first and then blame me. There we go. So what do these instruments look like? Well, it turns out general relativity makes a very specific prediction about how space should be stretched and squashed, how space-time should be stretched and squashed.

And it says that imagine you've got a ring of particles, not just two objects separated, but a ring of particles. A gravitational wave passing through it Mae'n dweud, dyna, dydych chi'n cael rhing o particlau, nid dim ond ddwy bwyllgau'n gadael, ond rhing o particlau. Bydd y rhing y byddai'n trechio'r rhing, y cercle, i'w trechio i allan i fyny mewn un arall, ac mae'n cael ei gwneud yn 90 o gwmpas, felly mae'n edrych fel botl rygbi. Ac yna mae'n osolu'n ôl ac yn ôl, y rhing o particlau. instead. And then it oscillates back and forward, this ring of particles. So we build instruments, what we call interferometers, and I can talk about what they are, to measure how two objects move. We take light, and we use light as our sensing device here. We take light from a laser, we split the light beam into two, and we effectively have a half-silvered mirror to do that.

A bit more complicated than that, but that's the principle. Splits the light into two, and that light travels out at right angles to one another. Two perpendicular paths, travels out, bounces off mirrors, and those mirrors are our markers in space. We've put them down, carefully positioned them, isolated them from all other things we can think of that could make them move. We bounce the light beams off those mirrors, let the light travel back and that lights a wave so the wave travels out along the arms, bounces off the mirrors, comes back, and adds up again at the beam splitter. And depending on how far the wave has travelled in each arm, when it bounces off the mirrors and comes back,

you can think of it as a bit like the same as a water wave. If the wave comes back with two peaks in the light, they'll add up and give a bright spot. If, on the other hand, a gravitational wave has passed by, stretched out one of those arms so the light has had to travel further, squashed the other arm so the light's travelled a shorter distance, the waves could add up again so you have a peak and a trough that cancel one another out and you'd see a dark spot. And literally, when we look at where the two light beams add up again, we measure the intensity, the brightness of the light spot there to see has a gravitational wave passed by and shaken the mirrors? Has it moved the mirrors?

So that's the principle that we use, and we're sensing the brightness of that light spot to see have mirrors in our instrument been shaken, been disturbed by a gravitational wave passing by? And these are four kilometres long, aren't they? They are. They are very big. The path, the distance that each light beam has to travel, as you see, it goes out four kilometres,

bounces off a mirror and comes all the way back again. And again, that's built into the way that gravitational waves work. The further apart the objects are, whose separation we're trying to measure, the more they move. Again, it's built into general relativity, that effect. So the bigger we make our instruments, the more sensitive they can be.

So these are huge devices. So you've got your two L-shaped tubes, you've got your mirrors at the very end of them, you've got something pinging along four kilometres down, all the way down, it bounces back. In the event of there being a gravitational wave that passes through it, by how much will a four-kilometre tube expand or lengthen

or will shrink? OK, so the motions of the mirrors at the end are about a few times 10 to the minus 18 metres. So, not very much is the answer, and that's why it took us 50 years to get to the point of being able to measure those, and to give you some scale, a human hair is about

100 microns, 100 times 10 to the minus 6, and a nucleus of an atom is about I think 10 to the minus 15. So it's a tiny, tiny... So it's a thousandth the nucleus of an atom is about, I think, 10 to the minus 15. So it's a tiny, tiny... So it's a thousandth the nucleus of an atom. Yeah, I think it's actually a thousandth the size of a proton. Yeah, it's tiny.

Four calendars and a bit. And a bit. I think Neil should answer the second half of Dara's question because he said he didn't want to, which is why... How is it that the ruler itself, the positions of these mirrors, how is it that the whole thing doesn't stretch? Because you expect the wave goes through,

so the light stretches and the mirrors stretch and everything stretches, so why doesn't it all just cancel out? That's a great question. OK, so it's a great question. And so the answer is, yes, it is true that the ruler does stretch, but when you make the argument, what you're not explaining is that in Einstein's theory, you have married space to time. So when I describe the ruler stretching and squeezing,

I'm talking about space. So if I do this calculation in space and time, then I see the difference. So you need... Now, it's absolutely crucial to explain how the detectors really work, that you look at the space and time problem, not just the space squeezing, but also how time changes as the wave comes through.

So that's the part we don't like to talk about. I saw Kip Thorne, who got the Nobel Prize, the theorist who really drove this, he described gravitational waves as a storm in time, which I thought was a very poetic way of doing it. But that's the interesting thing, isn't it? Because it's almost easier to picture or to understand a stretch and squash of space.

But we're talking about these waves making time pass at a different rate as well from a particular point of view. Yeah, I think that's right. I think it's clear that we're more comfortable thinking about measuring things in space than, you know, time being messed about with us ageing faster and slower in tiny little bits

just because a wave passes through and things like that. So it's clear. We're much more comfortable thinking in space than in time. But that's really true, because the waves are coming through this room now, so we are ageing at different rates.

That's a way to think about it, as the waves go through. I'm really ageing at a different rate. I'm a year younger than you, and I look like your dad. And it's not fair. It's gravitational waves. I knew it was physics' fault. I used to blame biology, but I'm not anymore. How often does the machine go ping?

If it can happen and it's like a thousandth of a proton, does it not constantly give you...? So, it goes ping a lot. I mean, it does. That's a technical answer. It does. When the instruments are running, the data's searched all the time, searched in real time, to see.

And we've got two instruments that are quite widely separated. They're in different parts of the U.S. for the LIGO instruments. One's in the northwest in Hanford in Washington State. The other one's in Louisiana in the south. So they're widely separated. And there's an automatic search to see, oh, has something happened in each of these instruments? The data's compared. And that happens in an online database, happens automatically,

and many times a day there'll be kind of false alarms. The system will look and say, oh, something looked like it moved in both of these detectors, but then there are other checks done to see, was somebody walking around in the room, could there have been another event, was the system operating properly, was the laser pointing in the way it should, so there are lots of automatic checks done to rule out the false alarms. And eventually, and quite quickly in fact, in real time, once, you know, and quite quickly, in fact, in real time,

once the system has determined nothing, you know, really obvious was wrong, the system will then contact a person who will then... A human will come and intervene, look at all the information and see if it's interesting enough to start to tell their colleagues about it.

So you've got two of these L-shaped tubes and they act as a kind of a check on each other. They do. So when events happen at the same time or within a certain amount of time, that might be significant. But the events themselves, I mean, are they

just instantaneous? I mean, if a giant event, and we'll talk about, I'm presuming, what the events are that trigger this thing, when they happen, is it not like a series of waves coming at it? Is it not a large thing? Or is it just, it just passes through as just one ping? So it depends what's produced these gravitational waves far out in the universe. And the first signal that we detected came from two black holes

that had been circling round one another. They'd got caught in one another's gravity. They were circling round, orbiting one another, getting closer and closer together, faster and faster, until they eventually smashed into one another and made a new black hole. And the bit that our instruments were sensitive to were the last few wobbles, the last few cycles of those black holes causing space-time to vibrate before they smashed into one another, and then that signal, what we call a chirp, was sent out across the universe

and it travelled for 1.3 billion years across the universe before it arrived with us here on Earth and flashed through our detectors in less than a second. Yeah, wasn't it about two days after you'd turned it on as well? It was. Which just seems extremely fortunate, given that it took 1.3 billion years to cross the universe. It would be no exaggeration to say it took us by surprise.

Yeah. It's true, actually. Our instruments were up and running. They'd been being commissioned for a couple of months. So we'd been working on them, getting them into a state of working. And they were working. But we'd set an official date by which we would declare that we were taking science data.

But we were ready, and fortunately we were, because two days or so before that official date, the signal came in. And it was a big surprise to everyone. Neil, I wanted to ask you, because you work in calculating what these things do. I mean, it's remarkable to me that a theory that's 100 years old can describe nature to that level of precision,

and something that Einstein could not have imagined, that black holes existed, in fact, when he wrote down the theory. I mean, it's a remarkable achievement for theoretical physics, isn't it? Absolutely. And if you imagine the fact that this is a pure construction of Einstein's mind, he didn't have no experiments, he didn't really have, he had some tests, you know, the solar system, motion of planets and so on, but he really didn't have any conception of the kind of things we're talking about.

Gravitational waves he didn't quite believe in, more or less. The black holes, well, again, people didn't believe in properly until the 60s. There was no evidence whatsoever until X-ray observations and so on in the 60s. And so it is indeed astonishing that this theory passes every single test. And this is not...

It's kept on passing tests for this first century and it seems to be doing brilliantly, which is... I mean, it makes us jealous, I guess. Is it the... It's often described as the most beautiful of physical theories. I mean, is it really... You said it came out of his mind. Are you going to ask me if I think it's beautiful? Yes. Well, I have to say yes, I guess.

My job relies on this. I mean, no, I think it's... Beauty is in the eye of the beholder. I work on it. I think it's nice. Ish. That is not a quote to put on the book cover. I think it's nice. Don't forget the ish.

Well, the ish is there, of course, because it is remarkable how well general relativity has done, but it doesn't answer all the questions we have in theoretical physics. There's got to be a point at which relativity breaks because it doesn't join up with quantum mechanics. And so it is absolutely beautiful and it does keep passing these tests, but one day it will be very exciting when it doesn't. Well, as a theoretical physicist, I guess I would be very disappointed if physics sort of came to an end

and we had all the answers to all the questions. I mean, what would we do with ourselves? We'd have to go on comedy programmes all the time. Whoa, whoa, whoa, whoa, whoa. OK, I'm dragging the ladder behind me here, lads. That sounds nice, though, doesn't it? It's pleasant. It's totally raised the tone. Can I just... Because it is...

This is not a criticism, although it will sound instantly like a criticism. It's sort of a blunt instrument in the sense that it registers something happening but doesn't tell you what it was or where it happened. And then you rely on somebody else then to step in and do that. I think there's an important thing we haven't mentioned, which is the use of the theory to check the stuff you're looking for.

Because this instrument is so noisy, because of all these different sources of disturbances that Sheila talked about, it's absolutely essential that you have an idea of what you're looking for. And so in parallel with this development of the instruments has been the development of supercomputer simulations to figure out what happens when two black holes crash together, what should we expect to see. And so that comparison of the simulation data with the

observations gave confidence in this is what we're seeing. Now in August we saw the neutron star collision. So could you talk through what that was, what that event was, perhaps introducing neutron stars first, and then the question would be why that was so interesting and important. Okay, so neutron stars is the other typical end point of stellar evolution. So a heavy star either becomes a black hole, which is a very simple object, even though the theory is complicated, or a neutron star,

which is probably the most complicated object we can think of, because what you do is you take an object that weighs a little bit more than the sun, squash it down to 10 kilometers. You have the strongest magnetic field we can imagine. So it's the strongest magnet in the universe. It spins around faster than a kitchen blender.

It has the hottest superconductor we know. So that's pretty much all the physics we think we don't understand on speed. And so two of these guys crashed together, and that was what was discovered in August. We were lucky in that we had a third instrument also operating, the advanced Virgo detector in Europe was operating. And a signal came in that this time, from looking at the way the wobbles were happening,

the way space-time was vibrating, the frequency of those wobbles, we could tell it wasn't two black holes. Instead, it was consistent with two neutron stars spiralling in and smashing into one another. And we could tell that from the gravitational wave signal. Because we had a third instrument operating, a third observatory, the Virgo Observatory, we could also get good information

about where this collision had happened in the sky and very quickly were able to ask our colleagues with telescopes to point and look at that point of the sky and what they saw was extraordinary. And smashing those stars together gave out a spectacular set of optical and other wavelengths of light that I think makes it

probably one of the most studied events in recent history with telescopes. So essentially it's an instrument which isn't like a telescope. You can't scan the skies with it. It's more like Alexa. It sits there waiting for you to say something to it and then it turns the lights

off or whatever it does. It's like an always on microphone just waiting to hear something occur and then if you line them up the right way then people can just all pile on to have a look at stuff where it's happening. In terms of the collision of two neutron stars, this seems to have had

if anything captured people's imagination even more than the black holes possibly because of the element of two neutron stars, this seems to have had, if anything, captured people's imagination even more than the black holes, possibly because of the element of bling, which, you know, the idea of now understanding where gold itself is created. And platinum as well, is that right? Platinum is many of the heavy, pretty much every, all the material matter or the atoms heavier than carbon. So this event is spectacular for many reasons. It was many, many firsts in astronomy, and it was also totally unexpected.

It was totally unexpected because even though we know these things should happen, we expected that the outflow of matter would lead to the production of heavy elements like gold. There was no... I think there was no one on the planet that expected we would actually see all of those things the first time around. As late as December, I think, last year, there was a meeting where I know people were talking, oh, this will be decades away, don't worry about this now, don't fret, it won't happen, before we retire or something like that.

And then it all came in one go. It was close enough that the optical astronomers could see this afterglow that faded away over days, which is the signature of the production of elements like gold. The radio astronomers could catch it. It's astonishing that across the whole range of electromagnetic radiation from radio, infrared, X-ray, as Sheila said,

we'd see one thing, the first event. Just to say, wasn't it many times the mass of the Earth's worth of gold? This is a lot of gold. It's a lot of gold. A lot of gold. You told me earlier it was on the cover of the Financial Times. There's so much gold. I only understand this idea if I could sell it.

If I could sell it or I could gather it and made in it. That's the only way I can understand this physics. I think people are fine with the explosions. I think the explosions also work as a way of getting this idea across. It's quite an exotic idea, though, that the gold in your jewellery was made, most likely, in a neutron star collision before the solar system formed,

I think that's quite a powerful idea. And the gold that's now placed in your headphone jack socket and will never be returned to the universe again, it's all in leaf form now. But you said there's two swimming pools worth of gold in total on the planet or something? Something like that, isn't it? Well, one of them's in his garden. There's pools worth of gold in total on the planet or something? There's something like that, isn't it? Well, one of them's in his garden.

There's a lot of money in it. Sexy cosmology really pays, apparently. A lot more than pop music. Who would have thought? That's such a bizarre thing. We've not got long left, but I wanted to move into the future. That sounded bleak, the way you said that. Five billion.

I thought it was just for the show, just so you know the end times are here. Anyway... Yeah, and a note about your gold. You can't take it with you, lads, all right? You've not got long left. But in terms of this rich sort of mine of data that we've now got, what are we hoping for?

Are we hoping that Einstein's theory fails to describe some of these signals that we're seeing? I think we're hoping for several things. We're not necessarily hoping that Einstein fails, even with no, say, in cosmology that he does fail at some point. We know with quantum physics that he does fail, so we know that. We don't know exactly how, but we know that he fails.

With these events, I think we're more interested in questions like, are the black holes that we're seeing really the black holes that we are predicting? We're interested in questions like, how do we get this outflow of matter that leads to the gold, etc.? We're interested

in questions like, how do we form these explosions? How do we get explosions? And we're interested in questions like, do we form these explosions? How do we get explosions? And we're interested in questions like, okay, what happens if you take a star and you squeeze it down to 10 kilometers? What kind of stuff do you get? So this is kind of our version of the Large Hadron Collider. We don't have the luxury of tweaking and smashing things together at will. We just have to wait, and hopefully the universe will oblige, and in this case it did. And we're hoping to answer all these questions. hopefully the universe will oblige, and in this case it did.

And we're hoping to answer all these questions. And the problem, of course, they all come together in a sort of intertwined puzzle kind of thing. And so with enough events, we're hoping to be able to start laying this puzzle in the clear to get a real picture. That's a great thing, isn't it, Darren? An LHC with stars. That's amazing.

And also, it's how you tune it. There was a danger that, given that you got a result in two days, that actually it was too sensitive and they were constantly hearing things, banging on repeatedly. Is there a danger still of that, that this thing is just going to hear too much?

We waited 50 years for that first one, so I don't believe we're going to be having too much. It's fantastic to now have those signals. We're up to, like, five events? That's right. So with our current instruments for black hole collisions, we're sensitive to about one a month, but we do not even yet have the detectors operating at their full sensitivity.

Still about another factor of three to go. So at the moment, in fact, the observatories are off. They're being tweaked, made a bit better, sensitivity improved, and we'll spend actually nearly a year doing that because it's really worth it. Once they get to design sensitivity, this generation should detect about a black hole collision a day. But that's because as we make them more sensitive, we're sensitive to signals from further out in the cosmos. So we're encompassing signals from further away.

And we are already figuring out how we could make these instruments another factor of 10 more sensitive. And that gives us 10 cubed in terms of the volume of the universe we could sense so many more sources and there's different science that you can do

once you start to have that large number of collisions you can start to use them whether it's black holes colliding or neutron stars colliding, to give you a different way to measure the expansion rate of our universe using gravitational signals, a gravitational probe of how that's happening, rather than using light as our probe of how the universe is expanding,

and that's very exciting. It's a whole new type of astronomy, presumably. It is. Again, the puzzles that we're talking about, some of them, this is going to give us a completely different handle on them. It is a truly different tool to study the cosmos out there. And for the first time,

we've been bathed in these signals for the history of mankind, but for the first time we're now able to sense them. Dara, when you hear these kind of advances of our understanding, is there a little bit of you that goes, oh, maybe I should have kept with physics and not gone into show business? Do you think I haven't got the patience to come up with

an idea and then go, you'll probably be dead by the time we've invented the machine that's required? You've very much answered your own question. If you know what I'm saying. I would have been happy to answer, but really you've supplied everything there. You've gone quite the journey. No, I think it's enormously impressive, but I think it was the kind of thing,

I was more of a theoretical guy, but the notion of actually building a machine that precise, I can't build a Lego house that precise, and the piece are laid out in grids. So, no, I'm set back and go, well done. I mean, because just the notion of the stuff you have to eliminate there. Mine would be all over the place. I'd have got bored of the tube long before I stuck

any old mirror in. Oh, no one's going to walk all the way down to check. Let's just say they don't exist, will we? Let's just constantly just go, and just let it go, because frankly, I couldn't be bothered working out how to do this. So, no, well done them. That is amazing

level of precision. We have, we asked the audience a question and this week the audience's question was which stars would you like to see collide and why? And the first one is Proxima Centauri and Wolf 359 two red dwarfs colliding is bound

to be lively. And this is a lovely kind of answer because when our producer came up with this question, she went, I imagine everyone will confuse it for stars as in celebrities and do a funny joke about that, but not realising our audience will be very specific and will go with actual stars and the possible astronomical results. Except for Daniel Fordham, who has said, Robin Ince and Justin Bieber.

So, Robin can finally have decent hair. Not fair, is it? It's a bit cruel, isn't it, Dara and Neil? Yeah, I think all of us there. Mike Williams suggested one, which is stars were judged Clyde, and he said Beetlejuice, Beetlejuice, and Pollux, so we can at last have a star called... I'm not going to read the end of that.

Brian Cox and a black hole to see what his hair looks like when stretched out. But will we actually see that, or will you still just be kind of to our, while you are being stretched in agony, we'll still just be observing you kind of like that, won't we? You'll be on the edge in our observations. You'd never see me fall in,

would you? No. I'd just be red-shifted, wouldn't I, on the event horizon. And if it was big enough, I wouldn't get stretched, would I? So you'd never see me fall in, would you? No. I'd just be red-shifted, wouldn't I, on the event horizon. And if it was big enough, I wouldn't get stretched, would I? So you'd just see my image just fading away. I'd be there forever. I'd be eternal. We've really drawn the humour out of that situation. I've just got one for you, because you can do an impression of this. Oh.

Brian Blessed. Ah! I want to go to where? And Brian May. Can't do him. Because the legendary tar riffs would be heard throughout the multiverse. You have to sing Bohemian Rhapsody's Brian Blessed. That's the way to do it. Go on.

Mother, just tell the man I had to kill him. He was looking at me weirdly. Do you do impressions, Dara? I don't do impressions, no. I can't even do an impression of you. I've worked with you for ten years. I occasionally just go, billions! And I can't do it at all. Robin does a very good impression. No, I don't. I can't really do you. It's like a camp Orville. It is.

LAUGHTER Some of the universe is really shiny, but some of it isn't. It's dark energy and no-one knows why. Oh. Anyway, so... Anyway, while we've been off air, we have received lots of questions enquiring about the nature of space, time and existence.

And this week's question on space, time and existence is from William Minns, aged 12. And he says, Dear Brian, my mum dropped the kitchen scales. Since then, we've been unable to weigh accurately. Is this a fault in the scales or did mum dent space-time? Yours, William. Genuine letter. It's a good question, actually, of what happens to scales in free fall.

This is the basis of general relativity, isn't it? That's right, but you have to warn the audience before we talk about this, right, because of health and safety, I reckon. So, basically, Einstein got his idea for one of the fundamental things of general relativity called the equivalence principle by allegedly seeing

a window cleaner drop off a ledge from a building opposite and he drew the conclusion that if you took the bathroom scales and jumped out the window and tried to measure how much you weigh as you're falling you would weigh nothing but no, don't do that

but actually the idea is of course, free fall, is you're not accelerating, are you? You're absolutely floating, minding your own business, essentially. It's the ground that's the problem. I mean, it's a bad way to mourn, to grieve with somebody when you go to the window cleaner's wife and go, I'm so sorry that the ground accelerated towards your husband as it's been. It's unfortunate for him.

Anyway, William, I think the main thing is she broke the scales. So thank you very much for that. Would you like to add anything to William's question, Shona, or you? Not only to point out that gravity is not to blame and it's the electromagnetic force when you come into contact with the ground that really causes

the problems. Excellent. That has sorted that out. That has. Nothing to do with gravity at all. Thank you, because they were all over the shop. I hope that's helped you a lot, William. But it is also true, though, that the scales do curb space-time, don't they? A little bit. Yeah.

A very small bit. And you would get gravitational waves, wouldn't you, from when the scales hit the ground? You would, as long as they didn't do it in a perfectly spherical fashion. As long as there was some smashing involved, then gravitational waves would be produced. The great thing about this is it's going out in January, when loads of people have already given up on the diets they meant to do.

So they are looking for an alibi to smash scales, and we have given it to them. So they are looking for an alibi to smash scales, and we have given it to them. Thank you very much for listening, and next week it's the secret life of birds. Goodbye! In the infinite monkey cage. Till now, nice again. The Monkey Cage! Do you want that one, Ash, again? Well, Adam Rutherford, that was a marvellous episode of The Infinite Monkey Cage, wasn't it? It was, Hannah Fry.

Not necessarily the best ones, because I think the best ones are the ones that you were on. I like the ones that you were on. I like the ones that you were on. Yes, but if you enjoyed those episodes of The Infinite Monkey Cage that I, Adam Rutherford and you, Hannah Fry, were on, it turns out... Hello. ...that we've got a whole eight series worth of just us.

Yes, we do. The Curious Cases of Rutherford and Fry, our very own science podcast in which we investigate your questions. Questions like, does Kate Bush have a secret sonic weapon that she's trying to use to kill all of humanity? We did answer that question. What about what would happen to Hannah if we threw her into a black hole? Specifically me. I wasn't particularly happy about that episode.

That's The Curious Cases of Rutherford and Fry, which you can download from your podcast providers. In our new podcast, Nature Answers, rural stories from a changing planet, we are travelling with you to Uganda and Ghana to meet the people on the front lines of climate change. We will share stories of how they are thriving using lessons learned from nature. And good news, it is working. Learn more by listening to Nature Answers wherever you get your podcast.