When Two Stars Collide

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This is the BBC.

Hello, I'm Brian Cox.

I'm Robert 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 a hexadecimal joke, Zeb.

That's a typical thing for this audience to do.

Some of them deliberately laughed as if they understood what you said.

It's base 16, isn't it?

So it's 116 and 11.

15 of BF.

Yep, 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

Spandout Ballet.

Sadly, this is not about the science of Spandout Ballet because the producer said we couldn't do that.

Whereas 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,

oh that's that's someone I'm happy with that.

Someone

uncertain.

Today's show is called When Two Stars Collide and it turns out that the answer is

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 we've used spanned our ballet in the intro, 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 Niels Anderson, 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 Darby and I'm the Emeritus Professor of Cosmology, Archaeology and Biology at the Combined Oxford and 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 doing some incredibly

important signers.

And that's why you won the Nobel Prize.

Yada, yada, yada.

Let's move on.

All right.

My favourite one was really

shouting in our ear when he was, I'm sorry, Bazaldrin, I've got to stop you there because we've got to go over to canine, a metal dog who's going to ask a stupid question.

Yes, and that was my favourite.

This the fictional robot dogs always win.

So let's start off with you, Sheila.

Just to, 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 traveling waves, are gravitational waves.

And what general relativity does is mathematically describe that, but to us it feels actually like tiny changes in the direction of gravity here on Earth that have traveled 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, you know, going around across the universe, these are occurring.

So gravitational waves are passing through us now, gravitational waves from many different directions.

That's right.

We're 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 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 to 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 of

what is happening in the garden, what has he done now?

Just at the moment, just before impact.

And so yeah, that that's the way that's how fast it goes.

That's an incredible thing.

how quick it is that's 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 and was there any doubt as well about their existence yeah you could say that I think I so the first 50 years were doubt and the second 50 years were trial and 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 criticized by a colleague, so he said, Oh, 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, I'm sorry, that's literally the point of you being here.

And you cannot do that midway through.

See, I always like the fun thing about rulers for these things is that I'm 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 in the expansion?

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 is a great one, which is rather that 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 tail in the head.

Well, I was going to say, you're not going to get out from Niels.

He's already said that he's not going to.

So you go first and then blame me.

There we go.

Okay.

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 should stretch that ring, stretch the circle, so it stretches out into an ellipse in one direction, and it's squished at 90 degrees to that.

So it looks a bit like a rugby ball 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 light's 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 lights travelled a shorter distance, the waves could add up again so you have a peak in a trough that cancel one another out and you'd see a dark spot.

And literally when we look at where the light 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, in the middle.

They are, they are very big.

The paths that each the distance that each light beam has to travel, as you see, is 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 I say, you've got your two L-shaped tubes, you've got your mirrors at the very end of them, you've send anything 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-camera tube expand or lengthen or will shrink?

Okay, so the motions of the mirrors at the end are about a few times 10 to the minus 18 meters.

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...

It's a thousandth the nucleus of a atom.

Yeah, I think it's actually a thousandth the size of a proton.

Yeah, it's tiny.

Four kilometers and a bit.

And a bit.

I think Niels 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 that the wave goes through, so the light stretches stretches and the mirrors stretch and everything stretches so why doesn't it all just cancel out?

That's a great question.

Okay 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.

Because I think I saw Kip Thorne describe, who got the Nobel Prize, the theorist who really drove this, who said 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 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 they it's clear that we're more more comfortable thinking about measuring things in in space than as you know time being messed about with and us aging faster and slower than tiny little bits just because a wave passes through and and things like that.

So it's clear we're we're much more comfortable thinking in in in space than in time.

But that's really true that now, because the waves are coming through this room now, so we are aging at different rates.

That's a way to think about it as the waves go through.

I'm really aging at a different rate.

I'm a year younger than you, and I look like your dad.

And it's not fair.

Computational waves.

Yeah.

I knew it was physics' fault.

I used to blame biology, but I'm not anymore.

And 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

it goes ping a lot?

I mean it does.

That's a technical answer.

It does.

Many times when the instruments are running, the data is 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 US for the LIGO instruments.

One's in the northwest and 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, 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.

So you've got two of these L-shaped tubes, and they act as a kind of a check on each other.

So, when events happen at the same time or within a certain amount of time, that might be significant, I think.

But the events themselves, I mean,

are they just instantaneous?

I mean,

if a giant event, and we'll talk about, I'll presume what the events are that trigger this thing, is when they happen, is it not like a series of waves coming at us?

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?

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.

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 it 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.

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 those 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 he 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 sixties.

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, you know, 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.

You're 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 is 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, right?

Okay, I'm dragging the ladder up behind me here, lads.

It sounds nice, though, doesn't it?

It's pleasant enough.

It raises a joke.

Can I just, because it is the one thing, and this is not a criticism, although it'll sound instantly like a criticism, is it 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 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 star, 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 magnets in the universe.

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, we're 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.

That's right, that's right.

And

in terms of the collision of two neutron stars, this seems to have had, if anything, capture people's imagination even more than the black holes, possibly because of the element of bling, which

the idea of now understanding where gold itself is created the and platinum as well is that right platinum as well many of the heavy pretty much every all the material matter or the atoms heavier than than carbon so this event is spectacular for many reasons it was many many firsts in 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 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 goal.

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.

It even made that.

There's so much gold.

I could 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 could understand this physics of it.

I think people are fine with the explosions.

I think the explosions also work as a way of getting this idea across.

Yeah.

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.

It is.

And the gold that's now plays in your headphone jack socket and will never be returned to the universe again.

That, you know, it's all in leaf form now.

But what is it?

You said there's two swimming pools worth of gold in total on the planet or something, though.

It's something like that, isn't it?

Well, one of them's in his garden.

There's a lot of money in it.

Speptic 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.

Sounded bleak the way he said that.

Five billion.

It was just 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, Lather, right?

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 Line fails even with no, say in cosmology that it does fail at some point.

We know with quantum physics that it does fail, so we know that we don't know exactly how, but we know that it 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, 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.

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 a clearer to get to get a real picture that's a great thing isn't it Dari an LHC with stars it's that's amazing and also it's it's it's how you tune it it there was a danger that given that you got a result in two days right the uh from it that actually it was too sensitive and they were constantly hearing things banging on repeatedly like is there a danger still like that like that this thing is just going to hear too much?

Um, we waited 50 years for that first one, so I don't believe we're going to be having you know too much.

So it's fantastic to now have those signals.

With our detectors.

We're up to like five events farther.

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're 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.

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 they 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 was saying.

I mean, 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 a more of a theoretical guy, but the notion of actually building a machine that precise, I can't can't build a Lego house that precise and the piece that laid out in grids

so no it was it was no I'm I'm I'm step back and go well well done I mean because just the notion of how the stuff you have to eliminate there they 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 gonna walk all the way down to check

and

let's just 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 that of a position.

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 Centuri 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 we'll 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.

It's not fair.

It's a little bit cruel, isn't it?

Yeah, I think all of us there.

Mike Williams suggested one, which is stars with Judge Clyde, and he said, Betelgeuse, Betelgees, and Pollock, so we can have at last have a star called Boll.

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 and in agony, we'll still just be observing you kind of like that, won't we?

You'll be on the edge in our face.

You'd never see me fall in, would you?

No, no.

I'd just be red-shifted, wouldn't I, on the event horizon?

And if it was big enough, I wouldn't get a stretch, would I?

So you'd just see my image just fading away.

I'd be there forever.

I'd be eternal.

Drawn the humour out of that situation, haven't you?

I've just got one for you because you can do an impression of this.

Brian Blessed.

I want to go to where.

And Brian May.

Can't do it.

Because the legendary tar riffs would be heard throughout the multiverse.

You have to sing Bohemian Rhapsodies, Brian Blessed.

That's the way to do it.

Go on.

Mother to kill the man!

I had to tell him.

He was looking at me weirdly.

So, do you do impressions, Dara?

I don't do impressions, no, I don't.

I can't even do an impression of you.

I've worked with you for 10 years, and I occasionally just go billions.

And I can't do it at all.

But Robin does a very good impression of you.

Oh, I don't.

I can't really do that.

It's like a campaign.

It is.

Some of the universe is really shiny, but some of it isn't.

It's dark energy, and no one knows why.

Anyway, so

the

anyway, while we've been off air, we have received lots of questions inquiring about the nature of space-time and existence.

And this week's question on space-time and existence is from William Minns, age 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 freefall.

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, that free fall is nothing's happening.

You're not accelerating, are you?

You're absolutely floating, minding your own business, essentially.

And 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 at such a space really is unfortunate for me.

Anyway William I think the main thing is she broke the scales.

So uh thank you very much for that.

Would you like to uh add anything to William's question Shino are you quite no 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.

Yes, it's hassles.

Nothing to do with gravity at all.

Because they were all over the shop.

That's made it thank you.

I hope that's helpful enough with it.

It is also true, though, that the scales do curb space-time, don't they?

A little bit.

Yeah.

Very small bits.

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 producing it.

The great thing about this is it's going out in January when loads of people had already given up on the diets they meant to do.

So they are looking for 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.

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.

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

that we've got a whole eight series worth of just us.

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.

Suffs, the new musical has made Tony award-winning history on Broadway.

We demand to be heard.

Winner, best score.

We demand to be seen.

Winner, best book.

We demand to be quality.

It's a theatrical masterpiece that's thrilling, inspiring, dazzlingly entertaining, and unquestionably the most emotionally stirring musical this season.

Suffs, playing the Orpheum Theater, October 22nd through November 9th.

Tickets at BroadwaySF.com.