3. Mirror, Mirror

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You're about to listen to a brand new episode of Curious Cases.

Shows are going to be released weekly, wherever you get your podcasts.

But if you're in the UK, you can listen to the latest episodes first on BBC Sounds.

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I'm Hannah Fry.

And I'm Dara O'Brien.

And this is Curious Cases.

The show where we take your quirkiest questions, your crunchiest conundrums, and then we solve them.

With the power of science.

I mean, do we always solve them?

I mean, the hit rate's pretty low.

But it is with science.

It is with science.

Hannah, what is our question this week?

We've got one in from a young curio this week.

Here we go.

Have a listen to this.

My name's Kobe.

I am nine years old and I live in the Isle of Wight.

My question is, what's the shiniest thing in the world slash universe?

Right, two things to say about that.

One, absolutely loving that he's

saying his punctuation out loud.

World slash universe.

Really into that, Kobe.

Secondly, shiny thing.

I mean, that could mean lots of different stuff.

Yeah, it does, actually.

My head always goes to diamonds, shiny things, particularly loose, by which I mean not stuck in a ring, ring, like just on a cloth.

Like a bag of diamonds.

Like a bag of diamonds, like whatever.

Have you been in a room with a bag of diamonds?

I've been in a room with a bag of diamonds.

Have you not been in a room with a diamond?

Dari, I mean, I know you're a big deal.

Yeah, do you know what I mean?

Like, whatever.

I like my liquidity to be portable.

Portable.

I know you say something.

And diverse.

No, I did buy a diamond once, and they go and they show you the diamonds.

And they say, how about this?

And they...

And they genuinely are incredible.

They're the best shiny thing, but they're not necessarily the most shiny thing.

Look, we could do an entire episode on diamonds, because I sort of think diamonds are nonsense.

Okay, well, park the diamonds idea.

Park the diamonds, okay.

The fact that he said universe as well, hello, Kobe.

The fact that he said universe as well makes you think that

we might get distracted down into into just

stars.

Like shining stars.

Yeah, yeah, yeah.

So he didn't say brightest, he said shiniest.

Yes, nor did he say luminous.

No, this is shiniest, which I think has to mean that if you shine something onto it, it's how much it reflects back.

So so most reflective.

Mirrors, both mirrors.

What are you gonna do half an hour on mirrors which is that sounds absolutely useful because we've booked two guests who know all about them

oh did you book because i booked two diamond merchants ah okay

my two regular diamond merchants incidentally if you'd like to ask us a question about diamonds um you can send in your questions to curious cases at bbc.co.uk yeah i might be in rotterdam where i do most of my diamond merchants

but i'll i'll come back for it specially

well luckily we've two experts on mirrors in with us first we have stuart reed who's a professor at the the University of Strathclyde.

And we also have Dr.

Felix Flicker, who's a theoretical physicist from the University of Bristol.

Now, okay, Felix, what colour are mirrors?

Well, that's a very good question.

Thank you.

I've been wondering this for a while.

I've been confused about this since I was a child.

I had this written on a folder at school.

I'd wondered about this all the time.

What colour is a mirror?

I kept thinking about it.

The best I could come up with as an answer was, well, what sound is a wall?

Because, I mean, a wall is kind of like a sound mirror, and you don't really think a wall has a sound, right?

And in some sense, you don't think a mirror has a colour.

But then you also think a mirror is kind of silver, right?

Well, exactly.

If it's a physical object, surely it has to have a colour.

Well, okay,

I've been given here a couple of mirrors to find this out.

Now,

you know, I am a theoretical physicist, as you just said.

This is an experiment.

And this is an experiment, and famously.

Immediately going to drop the mirror because it'll smash.

Shall we try it out?

We've got two mirrors.

Let's do it, let's do it.

And we're going to try and point them towards each other and have a look.

Why do we need two?

Well, I think what we're going to do is try and set up a load of reflections between the mirrors so the light's going to bounce back and forth a load of times.

And

if there's any sort of colour coming from, it's going to be essentially enhanced by this process, right?

Because if you look at it once, maybe it's a bit silvery.

Maybe if you look straight on, it looks kind of like there's no colour.

But if you do it loads of times, maybe you'll start to see the colour coming out.

Does that make sense?

Yeah, I think so.

Because of course, if you're looking in a mirror, it's just reflecting the exact colours that are appearing back at you.

So it doesn't have a colour.

Well, you'd like to think that's the case, but also I think most mirrors like you'd find in your house aren't actually perfect.

Okay, come on in, so do this.

Okay, so let's give it a go.

All All right.

Okay, so what if I if I turn around?

So you've got a mirror in front of you and a mirror behind you basically.

Yeah it's sort of like the setup that you would have at hairdressers if you were looking at the back of your head.

I'm glad you're telling this to me like I have any

how does the back of my head look like it looks as bald as the front of your head.

Okay, I look progressively sicker as I go through.

So it's sort of like mirror infinity.

That's what we've created.

Yes.

Yeah, yeah, yeah, yeah.

When you say sicker, sicker in the, is there a colour attached to this?

I would say paler and greener.

Just to go down and down and down.

It is, yeah.

I think sicker is the best way to describe it, but yeah, slightly greener, darker, isn't it?

Yeah, definitely darker.

Okay.

So what does that mean in terms of the physics of it?

Okay, so these normal bathroom type mirrors that we have here, they're not reflecting perfectly, and clearly the more reflections we get, the greener the light appears to be.

So I'd say probably the reason for this in these cases is that these mirrors

They have a reflective bit on the back, but then they have some glass on the front protecting that bit.

And the glass is essentially green.

Like, if you look through enough glass, you'd see light through it would be green.

So we're kind of seeing through loads of different bits of glass here, because it's bouncing back and forth, and every time it goes through some more glass.

It gets a bit more green.

Gets a bit more green.

Okay, but we have an answer then.

So mirrors are green.

A bit green.

But

you also kind of think they're a bit silver, don't you?

When you look at it at an angle, you kind of think that's silver.

And it is reflecting off metal, and in a sense, you could say metal is silver.

But I admit I'm still confused about this point.

I'm not really convinced they're silver.

I think they're shiny, a little bit green.

Okay, one thing we can say though, if the image is getting darker and darker and darker as you go further off into sort of the infinite reflection,

that must mean that less light is being reflected each time, presumably.

Yeah, you're losing some every time, exactly.

And do we have an idea of what per s how much light sort of a standard bathroom mirror will reflect?

Between about 60 and 90%.

Okay,

which is shiny, but not that shiny.

Yeah, and you can see how that would degrade over over time.

However, can I ask a more fundamental question?

Please do.

What is it?

It's a piece of glass.

So what have you added to the piece of glass that makes it a mirror?

Well, you've added a bit of metal behind it.

That's the main bit.

So the reflecting bit really is the metal on the back.

So that could be...

Yeah,

the glass isn't the mirror.

The glass isn't the mirror, no.

What's the glass doing?

Well, it's protecting the metal.

In olden times, you would make mirrors out of just polishing metal, right?

But then the surface would degrade with time, it would tarnish.

So silver makes a good mirror, but silver gets a kind of grotty surface after a while and goes black, and then it becomes a bit of a rubbish mirror.

So the glass is stopping that happening for example.

So the glass is only there to protect the metal behind us.

What is tell me more about the physics of what's actually happening?

So like a packet of light comes in, hits this reflective surface.

What is different down at the subatomic level?

I mean the simplest way to think of it is you know light is an electromagnetic wave and it's got an electric field waggling up and down and a magnetic field waggling side to side, for example.

And when it comes in towards a metal, the electric field's waggling up and down, and it finds a load of electrons that can conduct, right, because metals are good conductors.

So, this waggling electric field causes the electrons in the metal to waggle up and down, and a waggling electron up and down generates an electromagnetic field again.

So, it generates light that the light comes in, wobbles the electrons, and that generates light that goes back out again.

So, that's why metals make good mirrors generally, because they're good conductors, electrical conductors.

That's so interesting.

There are loose electrons basically

in the metal, That's right.

And they

can absorb, shake around and then emit.

But emit it at exactly the right angle.

It comes off, bouncing off comes in at forty-five goes off at forty-five.

Yes, that's what you'd expect.

There's a complicated explanation which is you stick it into Maxwell's equations that tell you how light and electrons interact and you f you find that it it does it at exactly the right angle.

But I mean essentially it's conservation of momentum I suppose.

Light has momentum.

If it comes in at a certain angle then when it bounces off you'd like to conserve momentum in much the way that when you throw a ball at the floor at an angle it bounces off and it should go at roughly the same angle in the opposite direction.

Does that make sense?

It makes perfect sense, absolutely.

It feels slightly miraculous that it does at exactly the same amount so that we can see something coherently coming off it.

That's it's definitely one of those things where it's like innately, of course I know that reflections are possible, but once you really start thinking about it it feels like a magic trick.

Yeah it does, it does, and an incredibly lucky event that this is how it works and we can shave or whatever, check the back of our heads

in a salon.

I'm going to say, have they got salons anymore?

Yeah, the only ones I go to are the ones that say in the French accent.

I know a good mirror.

In fact, there's seven of them.

The Giant Magellan Telescope.

which is currently being built in Chile in the Atacama Desert.

A lot of great astronomy stuff is done in the Atacama Desert because it's incredibly clear sky and incredibly cloudless.

And so there's seven giant mirrors that they have there.

They're each 8.4 meters in diameter.

And they've been polished for two years.

By one person.

One guy goes in every day, starts at 8 o'clock,

clocks in, clocks in, takes another little bit, takes that little cloth, rub, rub, rub, rub, rub, rub, rub.

Eventually uses the cloth up, buys another cloth.

And they'd love it to have done faster, but that's how long it takes.

That's who he says.

Hey, look if you want to do science properly.

You know what I mean?

Like whatever.

Why do they have that many mirrors?

Why are they that big?

They're that big to gather light and then they reflect all into a point.

So this basically has seven huge mirrors and they're all focused into one point.

So it's not in it dissimilar to if you went to a telescope shop and bought a telescope and bought a reflecting telescope.

It does the same thing.

It all comes in a big wide aperture and then bounce and then they all get pushed into one point.

But this is just doing it on a mammoth scale with a really shiny mirror that one guy did for two years and now is sitting in the desert.

It actually hasn't been built yet, but it would be sitting in the desert and you're going to go, he must be going, oh, it's covered in dust now.

Can I go in and have a look at it?

Like, this is a bit they may presumably have thought of this, and there's a mechanism by which you would hope so.

But we've done this before, like, we stuck mirrors in space.

The Hubble is a mirror, the um, James Webb uses mirrors at the uh, and they're incredibly precise mirrors, unbelievably precise mirrors, some of that are suctioned into place.

The reason I had to repair the Hubble is because one of the parts of the mirrors wasn't quite aligned, and so they set an entire mission up to replace that one part of the mirror.

Like, yeah, so it's astonishing how precise we can get get these.

More than we need in the bathroom, more, subtly more, subtly more.

Thing is, is that actually that mirrors with like a metallic reflective surface on the back and then glass on top is not the only way to shinily reflect light because there is actually a different kind of mirror altogether,

some that you can find in a quite familiar place.

So, here is engineer Quinn Sanford, who is talking about a kind of super shiny film which is called the Enhanced Specular Reflector, produced by his company 3M.

So Enhanced Specular Reflector is a multi-layer optical film.

It's about the thickness of a piece of paper.

It typically goes into mobile devices, smartphones, tablets, on the back side of an L C D backlight.

Inside of a L C D backlight is LEDs that run on one edge.

Every bit of light that's created by that LED, we want to get to the viewer.

The more efficient the display is, the less battery power is required.

A typical bathroom mirror could be anywhere from 60 to 90% reflective.

Our ESR is 99%,

so there is very little loss.

It's made up of hundreds of tiny layers, just different types of plastics with a different index of refraction.

Each of those layers help reflect a different portion of the visible spectrum.

There's about 600 layers.

So you can imagine how thick each of those layers are.

We're talking tens of nanometers.

But if you were able to actually remove one layer, it would look like a thin, clear piece of plastic.

Okay, so we actually have some of that here that my producer has handed me.

Well Stuart, you could describe this to me.

Yeah, so we work with these types of materials and it's a strange concept sometimes to get your head around because these are completely transparent materials that don't have a particular colour and but when they're very very thin they can interact with this wave property of light.

So we heard Felix talking about the wavelength of light and so you can set up these very thin layers to reflect those waves in such a way that they can add up or combine in reflection, but they tried to cancel each other if they tried to propagate further into the material.

And then, so you get a very strong reflection, you see a nice, a nice mirror surface.

I mean, these

reflective films are really impressive because it just looks like a thin kind of acetate.

I'm showing my age there, a thin plastic sheet, but it's a beautiful bathroom mirror.

I am just peeling off the backing of this, and that is, I mean, that's very, very reflective.

That is astonishingly reflective so i think it is a magic trick you have loads and loads of layers of see-through stuff that eventually

and how light is that in your hand extraordinary i think i haven't even taken off the both bits of backing anyway and that is 99 reflective weirdly watching you handle it is if you've ever seen the film terminator 2 it has that effect of watching you're watching essentially like a liquid of like a you're absolutely right it's like it's like watching reality through a liquid yeah it is it's incredibly thin incredibly light, and unbelievably reflective.

And there's a sheet of this, presumably the size of my phone screen, within the phone.

Something like this, or this film is in everybody's phone.

All of these different layers, these sort of invisible layers, that have different refractive indexes, what does that actually mean in terms of the physics of it?

So

it's useful to think about the wall analogy that Felix started us with.

When we think about sound bouncing off a wall, why does that happen?

When sound moves through the air, it encounters something with a greater density, like the wall, you get a reflection of that sound.

And light has a similar effect.

It's a wave, it travels through materials.

When it passes from a material with one density to another density, so higher to lower, lower to higher, then you're going to get a reflection at that interface.

And so, what this film does very cleverly is lots of tiny little layers which have different densities, different refractive indexes, which basically is telling you that the speed of light through those layers is different

in each material.

And at each interface, you get a reflection.

And if you can tune those really cleverly, you can reflect all the different colours.

The analogy, which often people refer to as oil on water, I mean, oil is not rainbow colour.

If you have a tub of oil, you'll struggle to see, but then when it becomes very thin, you get this interference effect, and it starts interfering with the visible wavelengths of light, and you see those oil colours.

And all you're doing with these reflective films is very precisely controlling the thickness of each of these layers of different material so that you reflect the colours that you want.

And in this case, they've tried to make it so it reflects as much of the visible spectrum as possible.

Extraordinarily clever.

But hang on a second, Felix.

You were talking about metals and the sort of electromagnetic properties of metals.

That's plastic.

Right.

What's going on with the electrons in that?

Not much.

So this isn't working by the same mechanism as the metals.

It's totally different.

As Stuart was saying, this is like a structural property.

So really you have a load of things that reflect somewhat well, you know, each one's doing okay, but the trick is that you get interference between the light reflecting off all these different mirrors into the different layers,

each of which isn't particularly good, but together they add up in a powerful way.

So the way to think of it, I think, is

you can think of like water waves, and if you send a water wave along, it's got these

peaks and troughs.

And if you send another water wave along, maybe they meet head-on, where two peaks meet where they're both going up at the same time that will be basically twice as high and similarly when they both go down at the same time that's twice as low.

On the other hand you can do it so that when one's going up the other one's going down and then they'll kind of cancel out.

So light is doing that.

You can think of light as a wave and you're getting the light reflecting from these different layers and you can essentially they tune it the thickness of these layers so that you get the two waves going up at the same time.

But then they do it not with two, but three, four, five, and then hundreds and you get this brilliant reflection effect amazing it's an incredible thing i mean and playing with it which is what you do of course because we're not sticking into phones or being useful engineers with this the uh is like holding your own funhouse mirror it is yeah because it's it's incredibly clear and and then all of a sudden yeah and you'll go ah look at that's really weird and suddenly it'll go straight and you have an amazingly clear shot of your own eyes staring at the chuck and it's actually kind of frightening it's fun fun fun oh that's who i am now really sure if you put glass on top of that would it be noticeably more reflective than these bathroom mirrors?

Well, you can give it a go, don't you?

Like, if you mounted this in your bathroom, for example, would it be brighter than the bathroom mirror that you have at the moment?

I think it probably should be.

Yeah, because it'll bounce more light out.

You might do even better if you put a kind of normal mirror behind it as well, because it is actually letting some light through.

Mirror layers, that's what I'm going for.

I'm stealing the rest of that envelope.

That's phenomenal stuff.

And if you can get that and the guy who polishes the one in chili to come around once a week and just give it a once over,

this is the one.

I mean, because it is, it's striking actually when you're used to looking at 60 to 90% of a bathroom mirror, or if you've ever looked into an antique mirror and they're slightly duller, when you look into this, it is astonishingly bright, as well as clarity.

Just the amount of light you're getting back is really noticeable.

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Is that the best that we can do then?

99%?

I think we can do a bit better.

I mean, there are other places where we need really, really precise

mirrors.

I mean, you know LIGO, don't you?

Yeah.

LIGO is the,

okay, we have to, from first principles.

Gravitational wave detector.

Gravitational wave detector, right?

Which is big L-shaped tubes.

I guess they say L-shaped.

There are two of them.

Two of them.

I've never been to CF.

No, and it's one of those kind of buckets things.

Not that they do tours as such, but I will contrive a documentary.

So they're 90 degrees to each other, these two tubes, and there's lasers fired along the tubes.

and they need to be reflected back again incredibly precisely because every so often the universe sends this whoa whoop kind of like

bit that one of them warps one of them changes or extends lengths because of gravitational waves yeah so my understanding this is you have you have one laser, it splits.

I'm looking to you, Felix.

You can correct us here.

You have one laser that splits, goes off down a tunnel, the other tunnel is at exactly right angles to it, gets bounced off a mirror, comes back, and then it's like your water analogy.

Ordinarily, these two halves of the laser should cancel each other out.

But if there is a,

I can see when I do the sound of the thing going like a water, like a stone dropping in a pool, and you see the wave, you know, the surface vibrating.

It's like that.

But space-time.

But space-time.

Imperceptible to us, space-time.

But when that happens to space-time, one of the arms will stretch, one of them will shrink, and so the two bits of laser won't perfectly cancel each other out.

Yeah, so that's a good way to look at it, yeah.

But the mirrors, I mean, we're talking seriously reflective mirrors.

Also, should we point out that the two tunnels that we're firing them down are four kilometers long?

Are they?

Yeah.

But the mirrors for these are insane.

In fact, here's the lady responsible for this.

I'd like her to tell us a bit more.

So my name is Gary Lynn Billingsley and I manage the optical sciences group here at LIGO

and I get to work with some of the most beautiful mirrors in the world.

So LIGO is set up to measure gravitational wave radiation.

Now these mirrors are ideally free-floating in space.

And so as a gravitational wave comes along, space-time changes a little bit and the mirrors appear to be closer or further apart.

And the mirrors have to be highly reflective because this light needs to bounce back and forth across this four kilometers, you know, over a hundred times.

So it needs to be a nearly perfect reflection.

And our mirrors are designed for a single color of light, 1064 nanometers.

So this is just beyond the visible, what you can see, it's slightly into the infrared.

But they're designed to reflect all but maybe five parts per million.

So that means 0.00005

of the light can leak through.

We couldn't use them at all if they were 95% reflective.

Just all the leak, you would shoot some light into the cavity and it would all leak out within a bouncer, I don't know, five, however many, and you'd have nothing.

I mean, that's that's pretty perfect.

Apparently about $2 million to produce, 40 kilograms in weight.

Shud, how do you make these?

Yeah so these they start off with a lump of glass but it's a very ultra-pure glass and on the front surface we deposit actually in a vacuum at the atomic level multiple layers of material which have these high and low index different refractive index and we have usually around 30 or so layers of material and and then after that that's about four or five microns thick so still you know a very very thin layer but you can build up an optical interference effect that gets you what we call five nines reflectivity so 99.999%

reflective albeit as was said at one wavelength or one colour of light but yeah far more reflective than your bathroom mirror and then that allows you to to bounce light back and forward between mirrors many many times and then look for these tiny stretching and squeezing of space-time that Einstein predicted.

I've seen pictures pictures of them though and they're huge slabs of glass.

I know they're very precision glass and but still we talked about how you want to get away from having glass.

Glass isn't the thing that's doing the reflecting.

So why the need for so much glass for these?

That's a slightly more abstract answer to that because

from an optical point of view, when we just think about reflecting the light off this object, then of course you don't need a big thick piece of glass.

But as was mentioned by Gary Lynn, we really want to have free-floating objects in space, you know, not affected by gravity here on Earth or any disturbances.

The trap bodies pass.

You don't want

it to be off.

And so you end up being forced from a mechanical point of view, wanting to have a very heavy mirror suspended on very, very thin wires so that in the horizontal direction, you know, where that mirror would swing as a pendulum, it is essentially soft.

So the mirror behaves like it's free-falling, albeit it's not free-falling in the vertical direction because you're holding it with very thin wires.

So for other reasons, you actually want the mirror to be very, very heavy.

And another reason for this is that there's a principle in physics called the Heisenberg uncertainty principle.

Many people have heard of this

when you think about trying to measure something at a very, very small level.

Heisenberg said that you try to measure it so precisely at some point you end up pushing on it.

And so you try to measure the position, but you end up imparting momentum.

And there's this uncertainty.

And actually, that also affects us with these mirrors.

and to get around that you just need to make the mirror heavier.

So these 40 kilogram mirrors behave also like quantum particles

and so their position uncertainty is related to their mass.

And so there's several reasons why we want them to be heavy, but not in terms of the light that's being reflected off them.

Yeah, it's more the fact that you're doing the experiment here for one thing rather than where you'd ideally be doing it floating out somewhere in space.

Okay, this sounds like the five nines mirror in LIGO.

I mean that that sounds like a pretty good candidate for the shiniest thing in the universe, I would say.

Can I put in a proviso, though?

Because it only reflects a certain part of light.

So cheating.

Yeah, if you were to stand in front of it and just look at it, would you see anything?

So it usually looks either a pinky color or a green color, depending on the angle you're looking at it.

But you never see something that looks like a perfect silver mirror.

Rubbish for rubbish for large.

I'm dismissing that from the game.

Also, $2 million each sorry.

And 40 kilograms.

Come on.

What's going on here?

If you've got any candidates then, if we're still looking for the shiniest thing in the universe.

Okay, it's a bit out there.

Are you happy with an out there, answer?

Hey, have you heard about the conversation we've been having?

Okay, my answer as to what the shiniest thing in the universe is, is a pint of beer.

Right.

Or a glass of water would do as well.

Go on.

Okay, so if you look in, a glass of water is probably easiest if it's a bit cold and you hold it.

Have you ever noticed you see your fingerprints very nicely on the inside?

Okay, so you're seeing kind of the ridges of your fingerprints.

But the bits where you're not seeing the fingerprint, you're just getting a perfect mirror reflecting back to you.

So what's happening there is a process called total internal reflection.

So the light is heading through the glass.

When it tries to leave the glass to exit the glass back into the air,

it can get reflected back essentially perfectly.

Now if there weren't air on the outside, it were in the vacuum of space, then this reflection would be actually perfect, according to theoretical physics.

But wait, how does the light know that there's air on the other side to travel to?

Well that's a very good question.

I mean okay it's the same answer that why do I see the ridges of my fingerprints in this process.

How does it know the fingerprints are there?

So that's it's a very interesting phenomenon.

You can explain it using

quantum mechanical tunneling.

So you can say that the light tunnels quantum mechanically into your the ridges of your fingers and so then you you lose those bits of light but the bits where there were the kind of troughs in in your fingerprints, it doesn't do that because they're already so far away the light can't tunnel.

So then it reflects back.

And in the vacuum of space, it would be more than five nines.

It would be truly perfect.

Here's what I'm hearing about that explanation: something, something, something magic.

Yeah.

I'm hearing something, something, something pint in space.

That's sort of all I got.

And I

see myself raising a pint up and going, see that?

Total internal reflection.

This happened to all of us.

Yeah, but we've all totally internally reflected while while holding a point at some stage.

But it feels that is, while theoretically the one,

it feels like that's not going to really work.

Abe, because our caller is nine years old and not in space.

So there are many contextual issues that aren't going to work for that yet.

You said it's like magic.

I think, you know, asking these questions of theoretical physicists, it's like asking a genie for wishes.

You know, you get what you ask for, but it's kind of useless.

But, okay, I was thinking about this.

You'd say, well, okay, if this is true, if this thing is really perfect, why don't we put that to some use?

And the answer is we do this put this to some use.

We use this total internal reflection all the time in

optical fibers, so fiber optics.

Because you're sending this beam of light often many kilometers and it's bouncing around inside, not coming out.

Exactly.

So you're sending, it's always at this very shallow angle to the surface.

So that's what you want.

And it's bouncing around and it bounces essentially perfectly off the surface of the fiber optic cable.

You think of it as like glass.

You do lose some of the signal, but that's from travelling through the material on the inside not from the reflections those bits are really perfect so there we go we found the answer it's slightly less satisfying than i was expecting the shiniest thing in the universe the inside of a fiber optic cable oh yeah fair enough all right we'll give you that

i still think there are more romantic ones than that but gentlemen thank you

you rejected the pint of beer in space dialysis i just felt that like i mean i mean of all the things you've got in the on the international space station there isn't like kind of a patio area where you can sit out and have a pipe and go to you earth I can't wait to get back.

It doesn't work that way.

No, indeed.

No, absolutely.

Well, thank you both for your contributions.

Absolutely extraordinary discussion.

Felix Micker and Stuart Reed.

I think the real answer then is going back to this shiny stuff, the 99.9%.

And of all light, not of just a very specific...

Sheeting.

Yeah, and isn't 40 kilograms in weight.

Absolutely.

And it's genuinely quite brilliant.

Yeah.

And do we have a spare one?

We do have a spare one.

Should we send it to Kobe?

Let's send it to Kobe.

And we're not saying that everyone will get a free gift when we ask a question, but do send them into curiouscases at bbc.co.uk.

Do you know what else I'll do?

I get a guy in Chile, he'll Polish off.

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