0:01

BBC Sounds Music

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Radio Podcasts You're

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about to listen to a brand new episode of

0:07

Curious Cases. Shows are going to be

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released weekly, wherever you get your podcasts, but

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if you're in the UK, you can listen

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to the latest episodes first on BBC Sounds. I'm

0:19

Hannah Fry. And I'm Dara O 'Brien. And

0:21

this is Curious Cases. The show where

0:23

we take your quirkiest questions, your

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crunchiest conundrums, and then we solve them with

0:27

the power of science. I mean, do we always solve

0:29

them? I mean, the hit rate's pretty

0:31

low. But it is with science. It

0:33

is with science. Today

0:38

on Curious Cases, we're on the hunt for

0:40

things we cannot see. Perfect topic for

0:42

radio. Isn't it? Isn't it? Yeah, it is

0:45

an invisible medium on which we were

0:47

discussing invisibility. People do say there that radio paints the

0:49

best pictures. It'll have to do a

0:51

hell of a job in this one. It's going

0:53

to paint. It's who we need to radio to

0:55

paint a picture of things that are literally unseeable.

0:57

Yeah. This is not our fault, by the way.

0:59

This is the fault of a listener called Dylan.

1:03

Hello, my name's Dylan and I'm

1:05

from Epsom. I would like

1:07

to know if anything in the

1:09

universe is truly invisible and if

1:11

so what and how? I think

1:13

that is a spectacularly good question. I think there's

1:15

going to be hardcore physics here. What I will

1:17

say, though, is as a general trend, I think

1:20

using the word fault to

1:22

people who are sending in questions

1:24

is... Oh, I'm sorry. Is that

1:26

the wrong tone? That

1:29

might preclude people from wanting to send them

1:31

in. Please send in your questions. I won't

1:33

blame you for it if it makes the

1:35

show technical or difficult or filled with science.

1:37

You're right. That is actually the whole point,

1:39

isn't it? Oh, double down. Let's go.

1:41

Let's go on to who can anger us

1:43

the most. In

1:46

fact, if you want something invisible, my

1:48

ire, my just, until it erupts, it's there

1:50

bubbling away and oh my god. Absolutely.

1:52

How are we going to, what do we

1:54

have to answer? The

1:56

disdain, the disdain that you have for

1:58

Dylan. No, no, disdain for Dylan.

2:00

That's a genuinely brilliant question. And he's cut

2:03

to the top of a really, really interesting topic,

2:05

you know? And we're going to keep it

2:07

physics. It's not going to be about, you know,

2:09

But can we truly see love? It

2:12

could be. I mean, who knows. Maybe

2:14

these physicists are going to come in and

2:16

say, yes, there is something truly

2:19

invisible. And it is. You,

2:21

the Nordic spirit of Kufufalha, who

2:23

comes out every winter into the

2:25

winter landscape and arranges the snow

2:27

on the trees. If that happened,

2:29

if one of the physicists went

2:31

suddenly rather than be amazing. Or the

2:33

other way, if we went that way, when they came

2:35

in with a sheet of paper about black holes, you

2:37

know, and they've done all the study in this and

2:39

we're going, but one is, what about, you know, just

2:42

happiness? Is that my measure? I

2:44

mean, look, I think the chances of this

2:46

episode going in that direction are low, but not

2:48

zero. Okay. In

2:51

the studio with us to reveal the

2:54

hidden universe, we have Andrew Ponson, professor

2:56

of physics at Durham University. And Matthew

2:58

Botwell, the public astronomer at the University

3:00

of Cambridge, and author of The Invisible

3:02

Universe. I mean, sounds like you're the

3:04

man Just handed over to you. Simple

3:07

question starts off, then what do we mean by

3:09

invisible? Well... not one

3:11

of these words that you need to

3:14

sort of break down the etymology, right?

3:16

I mean, invisible just means not visible,

3:18

anything you can't see with your I

3:20

guess. I mean, in this studio there

3:22

are invisible Wi -Fi signals and there's invisible

3:24

heat radiation bouncing around, and yeah, we

3:26

live in this whole invisible world. Okay,

3:28

so why is air invisible? I

3:30

mean, it's still made of matter. Well, it's

3:33

just because so light waves that we see

3:35

with our eyes, the optical light waves can

3:37

just travel through it like pretty unimpeded. If

3:39

we could see with different wavelength eyes,

3:41

then air would be very, very visible. Like

3:44

if you could see with infrared eyes, for example,

3:46

then the atmosphere is a brick wall. Like

3:48

the atmosphere is completely opaque in the

3:51

infrared. So if we had infrared vision,

3:53

the sky would be black. Who figured this

3:55

out for us? Who

3:57

noticed these spectrum outside of our vision?

4:00

Well, it's a long scientific detective

4:02

story. I think the infrared was

4:04

discovered by a guy called William

4:06

Herschel, very famous astronomer, discovered all

4:08

kinds of stuff, including the planet Uranus. He

4:11

was playing with the spectra you get from sunlight,

4:13

you know, during the famous Isaac Newton light through

4:15

a prison thing. And he noticed that beyond the

4:17

red end of the spectrum, there was some sort

4:19

of heat, and so he was like, ah, there

4:21

must be some sort of heat. You

4:23

know, there's some invisible aspect to sunlight that's

4:25

heating this thermometer up. Can you repeat this?

4:27

Okay, so you know how sometimes you get

4:29

like, like... through the window there's a bit

4:31

of a rainbow on the that it casts

4:34

can you do this yourself can you like

4:36

put a nice a good thermometer you know

4:38

I've never tried it I think you probably

4:40

could right I mean he was able to

4:42

do it in like the 1770s so with

4:44

it with a state -of -the -art thermometer I would

4:46

guess beyond the red of a rainbow on

4:48

your on the floor from whatever glass thing

4:51

and stick it into that empty space It'll

4:53

start heating up. Yes, you will notice a tick upwards

4:55

and heat because there is some infrared light being, you

4:57

know, diffracted by the prism that your eyes can't see

4:59

but your thermometer can pick up. I have an

5:01

infrared camera with me. Do you want to have a

5:04

play with it? just looks like a phone, but

5:06

it's not. It does. Yeah, it's an infrared

5:08

camera. Oh, you can actually see, sorry, you have

5:10

a very red nose, by the way. Do you know what? Every

5:13

time. Every time. I

5:16

mean, don't be all rude off about it. I know, it

5:18

is. Oh, no, actually, sorry, I'm

5:20

not correct. You all have very red

5:22

nose. Does infrared pass

5:24

through materials? It depends on the

5:26

material, right? So, yeah, it can travel through

5:28

sort of the air around us reasonably easily.

5:31

It can't travel through sort of 100 miles

5:33

of air in the same way that optical

5:35

light can. Yeah. And so this is seeing

5:37

a wavelength through about 10. microns, about 10

5:39

micrometers, and that wavelength just not come

5:41

through the atmosphere at all. But

5:43

yeah, there's something thin, like a plastic bag or

5:46

something. So if we were to take, for example, and

5:48

thank you for building up to that, if you

5:50

were to build a series of glasses that contain either

5:52

ice or water, so different temperatures, obviously, same thing

5:54

at different temperatures. You would be able to see them

5:56

from behind plastic, yes. Thank you very, much. Sorry,

5:58

just before we move on, just before we move on,

6:00

I'd just like to tell you that you are

6:02

red all over, apart from the tip of your nose. Wow.

6:06

Old Johnny Cold nose ever. That's

6:08

interesting. Between us, we make

6:10

one warm person. OK,

6:14

so we have five glasses of water

6:16

which are being lowered behind, I think,

6:18

what looks like a bin bag, very

6:20

scientific piece of equipment. And

6:22

with the camera, we should be able to see which

6:24

glasses are warm and which glasses are cold. So

6:26

obviously, optically, we can't see anything at all. We can't

6:29

see the glasses at all, exactly. But

6:31

can we hold up the camera then? We can. Shall

6:33

we do this together, Hannah? Let's do it. Oh

6:35

look, oh my gosh, that's incredibly

6:37

obvious. So there are, you

6:39

can see all five glasses

6:41

appearing on the infrared image.

6:44

I mean, very clearly, two

6:46

of them are bright orange

6:48

with sort of a yellow glow

6:50

around them. And then the other three

6:52

are like these dark blue

6:54

spots. So okay, it's

6:57

the ones, so from left to right,

6:59

it's ice, hot, hot, ice,

7:01

ice. Correct. As

7:03

if by magic. As if by magic. But all the

7:05

things that we have in the strong image in our

7:07

head about how light travels, that notion

7:09

of an up and down wave travelling along,

7:11

it's all the same for infrared. Absolutely,

7:13

yes. It's the same for any kind of

7:15

electromagnetic waves. So from X -rays all the

7:17

way down to radio waves, with everything

7:19

in between, microwaves, infrared, optical light, it's all

7:21

electromagnetic waves, right? Like the optical part

7:23

of the spectrum that our eyes are sensitive

7:25

to is quite narrow. If you put

7:27

it in terms of a piano keyboard, if

7:30

we see one octave, On

7:32

a piano keyboard how many octaves are on that

7:34

keyboard? Yeah, one octave is right because yeah

7:36

It's about a factor of two in wavelength right

7:38

so a middle C up to a C

7:40

one octave higher is about a factor of two

7:42

in wavelength and we see Visually about a

7:44

factor of two in wavelength right so the entire

7:46

at the you know the all

7:48

the light or all the electromagnetic radiation coming

7:50

down from the universe is something like

7:52

65 octaves from the highest energy gamma rays

7:54

all the way down to the lowest

7:56

energy radio waves it's about 65 octaves so

7:59

about eight or nine grand pianos side

8:01

to side compared to our vision which

8:03

sees one little tiny octave in

8:05

the middle we're useless yeah we see

8:07

almost nothing well then so almost

8:09

everything is invisible to us and most

8:11

lights we don't see And that's

8:14

just an evolutionary trick. That's just the

8:16

way we have developed. Yeah, absolutely.

8:18

I mean, the key thing is that

8:20

the sun emits light of particular

8:22

colours. And generally speaking, most

8:24

of the energy from the sun

8:26

does come out around about the

8:28

kind of wavelengths that we are

8:30

sensitive to, that our eyes are

8:32

sensitive to, and also that the

8:34

atmosphere lets those wavelengths through. So...

8:37

sensitive to the things that are easiest

8:39

to see, if you like. I think

8:41

we've got to be careful about the

8:43

definition that we're using here then. Because,

8:45

okay, if it's like invisible as in

8:47

you just can't see it with your

8:49

eyes, that feels like a kind of

8:51

not quite the right definition here. So

8:53

maybe we just need to say the

8:55

invisible is stuff that you can't detect. Yeah,

8:59

I mean that would be that would

9:01

be a perfectly good definition. I mean

9:03

the trouble is that then you start

9:05

coming into the question of what does

9:07

it mean to detect something? So there

9:09

are loads of examples where people sort

9:11

of detect things in very indirect ways.

9:13

I mean if we go to astronomy,

9:15

the discovery of the planet Neptune. for

9:17

example, was not done by pointing a

9:19

telescope at the sky and seeing that

9:21

Neptune was there first. It was actually

9:23

done by measuring the way that other

9:25

planets were orbiting around within our solar

9:27

system and going, hang on a second,

9:29

something doesn't look right here. It looks

9:31

like there's another planet that we've never

9:33

seen that's pulling the planets that we

9:36

can see around. And

9:38

that turned out to be absolutely right. In

9:40

the end, a telescope was developed that was sensitive

9:42

enough. If you pointed it in the right

9:44

direction, you could find Neptune for real. But at

9:46

first, do you want to call it a

9:48

detection or not? I don't know. Is it a

9:50

detection? Is it a deduction? You

9:53

start to get into language questions here.

9:55

But there was one point where people thought

9:57

there was an invisible planet. Well,

9:59

yeah. I mean, they thought there appears

10:01

to be a planet. there, but we

10:03

just can't see it directly. I'm stretching

10:05

the definitions of invisible here. Okay,

10:07

here's what I want to know.

10:10

If you if you pop up

10:12

to space, little spaceship, and then

10:14

you start having a look around

10:16

using those kind of invisible lights,

10:18

as it were, do you see

10:20

different things? yet 100 % you

10:22

see a completely different universe if you look

10:24

at the sky with this invisible light but if

10:26

you look in the like the far infrared

10:28

for example so like super long infrared wavelengths just

10:31

before they become microwaves and radio waves and

10:33

you look at our Milky Way galaxy instead of

10:35

the you know you can listen to this

10:37

might be able to picture the Milky Way galaxy

10:39

that they've seen sort of on hanging on

10:41

walls and so on In the

10:43

far infrared, the Milky Way galaxy looks

10:45

enormously puffed up and dramatic and volatile,

10:47

like the huge loops and sheets and

10:49

whirls of dust being thrown up out

10:51

of the plane that we can't see

10:53

with our eyes. It looks completely incredible.

10:55

You see this completely alien universe when

10:57

you look in any other wavelength, but what

11:00

we're familiar with. And we have sent up

11:03

All manner of different things. Now that we've known,

11:05

now that the blinkers are off and we

11:07

realise there's a lot more going on than we

11:09

could possibly see. We've sent up telescopes in

11:12

x -rays, in infrared, in ultraviolet. We

11:14

are constantly getting images of all these different.

11:16

Yes, to all of this. Yeah, we've

11:18

sent up, I think basically if you can

11:20

name it, we have sent up a

11:22

telescope that can see it. What's an x

11:24

-ray of the universe looks like? Oh, it

11:26

looks amazing. You see these, like, the

11:28

galaxy looks very dotty, and each little dot

11:30

is a black hole. Isn't the whole

11:32

point of black holes the... see them. Well

11:34

yeah, so black holes are called black

11:37

holes because the gravity is enough to swallow

11:39

light, right? So there's what we call

11:41

the event horizon, the outside edge of a

11:43

black hole, and anything that goes past

11:45

that even light is swallowed. But

11:47

we can see black holes because

11:49

before things fall past the event horizon,

11:51

they sort of swirl around the

11:54

cosmic plug hole a bit. Black

11:56

holes are quite messy eaters, so anything

11:58

that falls in sort of gets all

12:00

kind of churned up into this what

12:02

we call an accretion disk, this sort

12:04

of swirling maelstrom of stuff being gobbled

12:06

up by the black hole and because

12:08

the gravity of the black hole is

12:10

so strong that swirling maelstrom goes incredibly

12:12

fast and gets incredibly hot and at

12:14

those temperatures you glow with x -rays

12:16

so if you take an x -ray of

12:18

the sky you can see black holes

12:20

glowing it's uh it's incredible I like

12:22

that Can

12:28

we get back then to what

12:30

we can do with visual light in

12:32

terms of making things possibly appear

12:34

invisible? Yeah, because if stuff can like

12:36

become visible and then become invisible

12:38

in space, can you do that on

12:40

Earth? Can you like trick stuff

12:42

into becoming invisible? Yeah, I

12:45

mean not in quite the same way,

12:47

but there's a lot of interest you

12:49

can imagine from military circles in particular

12:51

in trying to make things that you

12:53

would normally be able to see perfectly

12:55

clearly. invisible and by designing special types

12:57

of materials people are making steps towards

13:00

being able to do that like a

13:02

sort of invisibility cloak that you can

13:04

put around things. It's very early days

13:06

as far as we know unless there's

13:08

any kind of very secret technology that

13:10

nobody's told us about but as far

13:12

as we know it's very early days

13:14

very difficult to actually do this but

13:17

people are making developments in that direction.

13:19

But Matt you have an example of

13:21

something that would trick the eye. So

13:23

in fact I have a a very neat piece of plastic

13:25

here. It's a sort translucent playing card type

13:27

thing and it's going to make a pen

13:29

disappear. An actual invisibility cloak. Yeah, exactly. And I

13:31

think the word trick is right. It is

13:33

literally a magic trick you can buy. So

13:36

the trick is called Lubor's Lens. I think that's

13:38

how it's pronounced. But it's what's called a Fresnel Lens.

13:41

So there are tiny little scores. I'm

13:43

holding up a sort of translucent playing

13:45

card sized thing of material. And there

13:47

are these tiny little scores in it

13:49

which bends the light around an object

13:51

and it's sort of direction sensitive. So

13:53

it will let you see vertical things

13:55

well horizontal things become almost invisible. Go

13:57

on give it a go. Okay so

13:59

if you come over so you've laid

14:01

out pens basically in a kind of

14:03

a grid formation. Exactly. Yeah. So we

14:06

have three vertical pens and one horizontal

14:08

pen, and if you hold up the

14:10

lens like so, the vertical pens are

14:12

unchanged, and the horizontal thing just completely

14:14

disappears. And if you rotate it

14:16

90 degrees... I make the vertical disappear.

14:18

You get the opposite effect. It's very

14:20

surreal, isn't it? It really works. I want

14:22

to see it! Oh

14:24

my gosh, that is...

14:27

It's uncanny how well

14:29

it works. Wow. Wow,

14:32

you can make your own finger disappear. That's

14:34

cool. That is genuinely really

14:36

impressive. As long as the

14:39

military want to hide pencils from

14:41

their adversaries. You could attack

14:43

people if you stage. horizontal

14:45

the entire way, and then we're

14:47

against a vertical grid. That's

14:50

remarkable how effective that is, yeah. I

14:52

mean, yes, obviously in incredibly specific circumstances. So

14:55

it bends, sorry, explain to me again, it

14:57

bends the light. So it takes the light from

14:59

the, the sort of the edges and redirects

15:01

them towards the middle, because your eyes are expecting

15:03

to see this horizontal pencil in the middle,

15:05

but where the light would be coming, you actually

15:07

see the light from the edges that have

15:09

been redirected. Which is why you need the vertical

15:11

light, because it gives you a sense of,

15:13

you can see this, a similar strip. Exactly. If

15:15

you take the vertical things away, you

15:17

just sort of see a smudgy blur and

15:19

the illusion doesn't really work at all. Well

15:21

actually interesting, there is in fact a whole

15:23

area of physics that's dedicated to making special

15:25

materials called metamaterials that bend light in fancy

15:27

ways like you showed us there, but could

15:29

one day produce a better invisibility shield? Here's

15:31

Dr Mitch Kenny from the University of Nottingham

15:33

to tell us more. When

15:38

everybody thinks about invisibility their mind

15:40

goes straight to Harry Potter So essentially

15:42

what he's got is a magic

15:44

sheet that he can wrap around himself

15:46

and what the sheet will do

15:48

is allow light to come from behind

15:50

him through the cloak into your

15:53

eyes and vice versa and There's a

15:55

field of physics called metamaterials and

15:57

these metamaterials are hoping to achieve something

15:59

very, very similar. If you were

16:01

to place an object in the middle

16:03

of these metamaterials, you could guide

16:05

light around them. It's a bit like

16:07

water flowing around a post or

16:09

something like that. It's all about the

16:11

flow of waves and recombining the

16:13

waves on the other side so that

16:15

what you see is the thing

16:17

that's behind the object. You don't actually

16:19

see the object itself, it's cloaked. So...

16:22

A lot of the time the building

16:24

blocks used for invisibility cloaks consist of

16:26

small metal C shaped rings with a

16:28

little opening on one side. They're probably

16:30

about one or two microns in size.

16:33

A thousandths of a millimeter I guess

16:35

you would you would say. When the

16:37

light gets absorbed because of these properties

16:39

of these metal rings you can reshape

16:41

how light leaves so you can control

16:43

which direction it leaves and how much

16:45

of the light is also absorbed. To

16:47

my knowledge no invisibility cloaks have been

16:49

made for visible light yet. So the

16:51

best that we've sort of done at

16:53

the moment is a little block that

16:56

is a few inches across or you

16:58

know maybe a few centimetres and what

17:00

would happen is that it only works

17:02

from one direction. That is

17:04

the problem. It works for one wavelength or sort

17:06

of one colour you could say of light,

17:08

not a colour that we can actually see though.

17:10

So you would actually see the metamaterial just

17:12

sat there when you'd be wondering what's that there

17:14

for. So

17:17

we'd see the cloak, we wouldn't see

17:23

It's really far from perfect. The military

17:25

application thus far is you can have

17:27

very, very skinny, one -dimensional soldiers or really,

17:29

really tiny ones that are a couple

17:32

of inches across and approaching from only

17:34

one direction as long as they were

17:36

not visible in the visible light spectrum

17:38

from the first place. So I mean,

17:40

I think in the long term, there's

17:42

a hope that you'll be able to

17:44

do this visible light. The reason that

17:46

it's a bit easier to do for

17:49

this sort of longer wavelength lights and

17:51

so infrared. or radio waves is because

17:53

that basically the way that you are

17:55

redirecting the light is instead of using

17:57

the properties of atoms and molecules, which

17:59

is how light normally gets manipulated in

18:01

everyday life, you're actually trying to create

18:03

little structures that are on a similar

18:06

scale to the wave itself. And those

18:08

little structures are the things that kind

18:10

of redirect the light and get it

18:12

to do crazy things. So

18:14

you need to be able to fabricate

18:16

those structures, you know, physically make them

18:18

on a similar scale to the size

18:20

of the wave that you're trying to

18:22

redirect. So if that's radio waves, no

18:24

problem. You know, you can easily make

18:26

something sort of centimeters meters scale. But

18:28

if you're talking about visible light where

18:31

it's actually more like a millionth of

18:33

a meter, the thing that you're trying

18:35

to redirect, your structures also need to

18:37

be a millionth of a meter or

18:39

smaller. And you need to fabricate, obviously,

18:41

lots and lots and lots of those

18:43

structures in a very precise way across

18:45

something that's really big if you want

18:47

to shield something really big. So that's

18:49

where the technological challenge. But I think

18:51

in terms of the sort of basic

18:53

physics of it that the cloaking could

18:55

be applied even even to visible light

18:57

I had say my go -to is not

18:59

how you put on invisibility my go

19:01

-to is James Bond who had a

19:03

car that took a picture of the

19:05

opposite side and then projected the picture

19:07

on the near side. So that's what

19:10

made the car look invisible. That

19:12

could only ever work from one angle,

19:14

though. Whereas these metamaterials, at least in principle,

19:16

you can engineer them to work from

19:18

multiple angles. And of course, that's really important

19:20

if you really want to cloak convincingly.

19:22

I did see a very good Halloween costume

19:24

that was based on that premise, though,

19:26

of like two phones, one strapped to the

19:28

front of someone's stomach, one strapped to

19:30

the back. And they were basically video calling

19:32

each other so that you could it

19:34

appeared as though you could see through this

19:36

person's stomach. Oh, that's very smart. Isn't

19:38

it? Yeah. I mean, do you

19:40

want to get competitive about Halloween costumes? Which I totally

19:42

do. okay,

19:45

Van. But so we're not close. Which

19:47

is, I know, is probably what Dylan

19:49

is really asking. We're not close to

19:51

creating something that'll allow you to cloak

19:53

yourself. No. Okay,

19:58

well, let's go back to the universe

20:00

thing because I'll be honest with you Dylan.

20:02

It's not looking good for you If

20:04

we've got all of these different telescopes though,

20:06

right, you know infrared and x -ray and

20:08

microwave and so on and You're seeing

20:11

in inverted commas all of this stuff out

20:13

there in the universe Then is anything

20:15

really this invisible? Well, yes. So

20:17

over the last sort of century

20:19

or so astronomers have come to

20:21

this sort of conclusion that Actually,

20:23

most of the universe is completely

20:25

invisible at every single wavelength. There

20:28

is this very strange stuff that we call

20:30

dark matter. And you shouldn't

20:32

be tricked into thinking that we know what it

20:34

is. Astronomers use the word dark to mean I

20:36

don't really understand what's going on here. But

20:39

there is some sort of, the universe

20:41

is mostly made of some sort of

20:43

strange invisible substance that is truly genuinely

20:45

invisible. It doesn't shine at any wavelength

20:47

at all. Not on the piano, or

20:49

the however many grand room. Exactly, entirely.

20:51

It is not playing a note on

20:53

the piano. OK. And so how do

20:55

we know it's there? Well, it's

20:57

so what Andrew was saying before actually about

20:59

discovery of Neptune I think is a

21:01

nice key up for it So we didn't

21:03

see Neptune itself We noticed Uranus behaving

21:05

strangely and that clued us in that there

21:08

was this mysterious planet pulling Uranus around And

21:10

we discover dark matter in a similar

21:12

way. When we look at how galaxies move

21:14

around the universe, whether it's

21:17

sort of galaxies orbiting inside clusters

21:19

or even individual galaxies spinning

21:21

around, everything is moving as

21:23

if there is a lot of invisible

21:25

stuff around. Like we're seeing the gravitational

21:27

footprints, if you like, of this weird

21:29

invisible stuff. And we just can't see

21:31

it at any wavelength. And that's

21:33

not the only one of these things. I

21:36

mean, dark matter... solve all of the

21:38

unusual behavior that we've seen in the universe.

21:40

It only partially solves this. Well, yes,

21:42

there is a lot of weird stuff going

21:45

on in the universe, you're right. So

21:47

dark matter on sort of quote -unquote small

21:49

scales as big as a galaxy, dark matter

21:51

appears to be around and sticking stuff

21:53

together and making everything move around bizarrely. But

21:55

when we look on the biggest scales

21:57

of all, look out, you know, the other

21:59

side of the universe and we measure

22:01

how fast the universe is expanding, it

22:03

seems to be speeding up. which no one really

22:06

understands why. So we know we've known the universe

22:08

is growing for about a hundred years now. Then

22:10

about about 20, 25 years ago, astronomers

22:12

did an experiment to measure exactly how

22:14

fast the universe was expanding. But then

22:16

the answer was it's speeding up like

22:18

something has its foot on the accelerator

22:20

pedal of the universe. There's some sort

22:22

of energy pushing the universe apart faster

22:24

and faster and faster. We have no

22:27

idea what it is. We call it

22:29

dark energy, but. And again, it doesn't

22:31

play any notes from the piano, it

22:33

doesn't register on any of the notes.

22:35

No, the only evidence we have of

22:37

it is the fact that we can

22:39

see the universe speeding up. Right, this

22:41

don't matter stuff. Is it in the

22:43

room with us? Yeah, absolutely. It

22:46

almost certainly is just suffusing the

22:48

entire universe. It's not very much

22:50

of it in the room with

22:52

us, because although there's a lot

22:54

of it in the universe at

22:56

large, here on Earth and in

22:58

the solar system in general, there's

23:00

an awful lot of normal matter.

23:03

So the normal matter vastly outweighs the

23:05

dark matter in this room to

23:07

the point where the dark matter is

23:09

just entirely negligible in this room.

23:11

It's only when you look out to

23:13

the scale of say galaxies where

23:15

in terms of the normal matter there

23:17

are sort of vast empty spaces

23:19

and the dark matter is sort of

23:21

able to just fill up. those

23:23

spaces and that's how in

23:25

the end dark matter comes

23:27

to outweigh regular matter across

23:30

the universe. But yes, in

23:32

the universe at large, it's

23:34

roughly speaking 5 % of the

23:36

entire universe is sort of regular

23:38

material that we know and love

23:40

from here. Of which we can

23:42

only see one octave of 60

23:44

something. We're pathetic. But

23:47

we're not pathetic because we figured these

23:49

things out. And I said, I mean,

23:51

that's the amazing thing. the human mind

23:53

of course is that we can reach

23:56

beyond just what our senses tell us.

23:58

It's five percent exists on the on

24:00

the scale on the piano okay how

24:02

much of the universe exists as Dark

24:04

Matter? About 25 that they're abouts. And

24:06

then the rest of it possibly is

24:08

Dark Energy. Yeah exactly yeah. And it's

24:10

not by the way that there is

24:12

a sort of ghost other universe where

24:14

they exist and do radio shows where

24:16

they go where they describe us as

24:18

Dark Matter and Almost

24:20

certainly not, because everything that we

24:22

know about dark matter shows that

24:24

it behaves in a very simple

24:26

way. So although it's very mysterious,

24:28

in some sense it's very simple,

24:30

that it only seems to experience

24:32

one force and that is gravity.

24:34

It generates gravity and it feels

24:36

gravity and so it can get

24:38

pulled around and it can pull

24:40

us around through gravity. But in

24:42

order to have life and discussions

24:44

in a studio, you need other

24:46

forces and especially electromagnetism, which is

24:48

the thing that enables... to have

24:50

basically molecules and atoms and in

24:52

fact light and radio waves and

24:54

all the things that we've been

24:56

talking about. It's because it doesn't

24:58

experience electromagnetism that it is so

25:00

so hard to pin down but

25:02

that also means there isn't a

25:04

sort of shadow universe with alive

25:06

creatures that are wondering what we

25:08

are. Hard on though because a

25:10

few minutes ago you were saying you

25:12

had only a vague understanding of five

25:14

percent of the entire How do you

25:17

know that there aren't other forces that

25:19

they're experiencing? Maybe they think that we're

25:21

only experiencing simple forces. Yeah, I mean,

25:23

I think the remarkable thing about dark

25:25

matter is it's been very predictive as

25:27

an idea. So although we haven't captured

25:29

it, we haven't seen it directly. It

25:32

has made astonishingly accurate predictions.

25:34

So for example, it's made

25:36

predictions about the cosmic microwave

25:38

background light. This is the

25:40

sort leftover glow from the

25:42

Big Bang made very, very precise predictions

25:45

about what you want to see there

25:47

because of the way that Dark Matter

25:49

would have pulled that around. And all

25:51

of these are based on one assumption,

25:53

which is that Dark Matter is as

25:55

boring as can be, that it just

25:57

experiences gravity and no other forces. The

26:00

moment that you try introducing any other

26:02

forces into the picture with Dark Matter,

26:04

it suddenly starts behaving in a different

26:06

way and it messes up all of

26:08

those predictions that were so successful. So,

26:10

although we haven't captured it and we

26:12

haven't seen it directly, it's an astonishingly

26:15

predictive and therefore successful But

26:17

if it experiences gravity, would it not

26:19

all pull together? We're not clump, like

26:21

planets. Could there be planets of dark

26:23

matter? I mean, no. It does clump,

26:25

but there can't be planets. And so

26:27

it clumps because of gravity, as you

26:30

say. But if you want to create

26:32

a planet, you need to do more

26:34

than just clump. You really need to

26:36

compress. A planet is

26:38

so much denser than the universe

26:40

around it. It's just mind

26:42

bogglingly dense compared to space at

26:44

large. So you not only need to

26:46

have gravity sort of pulling stuff

26:49

together, you also need to have a

26:51

way to actually start sticking bits

26:53

of things together. For that, you need

26:55

the other forces. So gravity will

26:57

get you so far, but it will not get you down

26:59

to the scale of a planet. Do

27:01

you think Dylan expected the conversation to go

27:03

in this direction? think so. And

27:05

I think the overall conclusion is Dylan,

27:07

we can only see 160 -something of

27:09

5 % of the universe stop trying

27:12

to lose more. Also,

27:14

the main thing is, what are we

27:16

regarding as invisible? Because we can detect the

27:18

behavior of a lot of things, like

27:21

whatever. Similarly, if you did actually have a

27:23

Harry Potter invisibility cloak, you'd still be

27:25

able to smell the teenage children who live

27:27

in a boarding school as they walked

27:29

around. There is still a residue of actual

27:31

other stuff going on here that you

27:33

would spot. So I think we can safely

27:35

say it's not happening. It's not a

27:37

perfect thing. going to go with the note.

27:40

Well, thank you very much to our

27:42

guests, Matthew Bothwell and Andrew Ponds. I

27:46

mean, so no is the answer to

27:49

Dylan. Nothing is truly invisible. Well, it kind

27:51

of depends how you're defining it, isn't

27:53

it? Yet another curious case without a positive

27:55

answer. Yeah, but I mean, but we

27:57

can take comfort in the fact that if

27:59

we were recording this 200 years ago,

28:01

we would have seen a far more general

28:03

universe and it also wouldn't have gone

28:05

anywhere because we hadn't discovered radio wave. Excuse

28:10

me, the town crier would have had

28:12

an absolute feel to this one. But

28:15

also, they didn't know that they lived

28:17

in a dolly universe. They thought they were

28:19

like the kings. They were like, we

28:21

know everything there. We've seen colours from red

28:23

all the way to violet. We know

28:26

all of the universe. But no, it turns

28:28

out there's a way more going on.

28:30

Will Dylan be happy with that though? Do

28:32

you know what? I think any The

28:34

simple seeming question that ends with dark energy

28:36

is all right by me. He wants

28:38

to make things disappear. Should we just send

28:41

him the... Oh, that's a good idea.

28:43

We'll send you the little lens thing. Yeah,

28:45

there you go. So you can pretend,

28:47

yeah. In the very specific conditions of a

28:49

horizontal pencil against some vertical pencils, yes,

28:52

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