Translator: Alessandro Brizzi Reviewer: Maria K.
What is invisibility?
Why does it surprise us if we think something is invisible?
Perhaps it's because so much of our understanding of the world
comes to us through our eyes, through light.
You even mean, sometimes, if you say, "I see," that you mean, "I understand."
And when we don't understand what light is telling us, we're confused
and, perhaps, even frightened.
But what I want to tell you about today
is how we have developed some tools, and some materials,
that you can use together to make things invisible.
So, what is the challenge of invisibility?
To paint something black is not enough, that's the first thing to say.
Conventional stealth, as used to make aircraft invisible to radar
is just making them black.
But if I take a black object, and I have it in a bright room,
then I know it's there because it has a shadow.
So, if you want to make something invisible,
like Peter Pan, you've got to lose your shadow.
So, let me explain how we try to do that through an analogy with water.
If you put a stone in a stream, the water naturally flows around the [stone],
and the flow of water will close up behind the stone,
so that somebody downstream of the stone
will see smoothly flowing water and will not know the stone is there.
In the stream, the stone is effectively invisible.
If you could make light do that --
and light of course does not naturally flow around objects --
if you make light do that, then the object it was flowing around, would be invisible.
And that's our strategy: to make something invisible
through that technique.
Now, somebody who had a lot to say about light was Albert Einstein;
and we are going to use some of his work to inspire us how to create invisibility.
Now, he had a theory of gravity
which was revolutionary.
And he said gravity acts through changing the properties of space.
Einstein regarded space as a material, almost as if it was made of rubber
and very massive objects could stretch or compress that rubber,
and if space was distorted, then objects moving through it
no longer travelled in a straight line,
and that's how he explained the motion of the planets in his theory.
But also, if space is distorted and light moves through it,
then light also must not move in a straight line.
And this was a very controversial aspect of his theory
because Newton said that gravity acts between two massive objects.
But what Einstein was saying: "Only one of the objects needs to be massive."
And light is a mass-less object, but, according to Einstein,
is still affected by gravity.
And it was Arthur Stanley Eddington's experiment
of seeing the bending of light around the Sun, during a solar eclipse
that is one of the foundation stones
of Einstein's general theory of relativity.
These days, astronomers routinely see what they call "gravitational lensing."
This is a picture from the Hubble Telescope,
and it's a picture of a red galaxy.
Behind that galaxy is another second galaxy, which is emitting light,
and the light, as it comes near the first galaxy,
is bent in its gravitational field; it's flowing like water,
and that's what's producing this ring of light around the first galaxy.
It's light that has been bent,
gravitationally lensed by the [first] galaxy.
Now, we don't have to go to the trouble of finding a galaxy,
or black holes, or something very massive, to bend light.
Because what Einstein also showed was that the effects produced
by bending or stretching space
are equivalent to a material with a certain refractive index.
So you could use all this fancy metric technology that Einstein had,
you could say what sort of space would do this bending,
and then you say, "Well, hey, I don't need to create a space like that.
I just need a material with a refractive index which is related
to the space."
But we're going to use Einstein's thoughts as our design process.
Well, we all know that materials do refract light,
and here's a block of glass bending light,
but it does so at its surface, very, very sharply.
But we don't want to do that, we want to do something more sophisticated:
we're going to have to make the light bend continuously, rather than suddenly,
and that means not a sudden change in the refractive index,
but a smooth change.
And that's going to be difficult,
and I'll explain how we get around some of those difficulties in a moment.
And so, how do we design a cloak?
Well, we start with a sphere -- and inside that sphere is just ordinary space --
and inside that sphere I'm going to create a cloak,
and inside the cloak a hiding space, where I'm going to hide something
that will be invisible.
So, I go inside the sphere and --
space is made of rubber, you can stretch it --
and I make a hole in this rubber space,
and I push the space into an annulus contained within the sphere
and create inside that that hole: a space where I can hide something.
And I push all the space into this annulus, clear it out.
Now, if light happens to be going through the sphere,
the light behaves as if it's embedded in the rubber,
so that when you push the rubber space away, you take the light with it,
you clear all the light out of that central region;
so now no light gets there, and anything in there cannot be seen
because no light can reach it.
Conversely, it cannot send any light out either.
But, although we've stretched space
and done severe damage to it inside this sphere,
and moved the light around considerably,
what we haven't done is interfere with space outside that sphere.
And if you don't move the space, then you don't move the light,
which is stuck inside it.
And so you achiev this objective of creating no shadow
because the light outside doesn't know what's going on inside the sphere.
There's no change.
And therefore, if you try to look at the sphere,
in which all this nonsense is going on, you would actually see through it.
You would see what was behind the sphere.
And so what is happening is that, just as we wished,
the light is moving in a smooth curve, flowing like water.
And then we can say, "Well, what are the properties of this compressed space?
What is the effective refractive index
that Einstein's theory would say would have the same effect?"
And that is the material property we want.
Unfortunately, we're not done at that stage,
because in nature, we very rarely find the materials we need for a cloak.
There are exceptions which I'll come to,
but at this stage we have to introduce new materials,
they're called metamaterials, and they work like this:
So an ordinary material, the way it refracts light is due to its chemistry.
So that if you wish to change the properties of glass,
you put some lead in it, you make lead crystal,
and that is great for making really sparkly wine glasses.
But there's another way you can change the properties of glass:
you can make very, very tiny holes in it.
They have to be very tiny because if they're big, you'd see them,
and you would know there are holes there, but if very tiny,
the effect of the holes is to make the light less dense,
to make the glass less dense, excuse me.
And that makes it less refracting.
So you can actually change the properties of glass by making these tiny holes;
much less than the wavelength, so you can't see them,
but they do alterate refracting properties.
I'll give you an extreme example of that:
If you take silver, you can make a plate of silver,
and if you polish it very, very smoothly, it makes one of the best mirrors
that there can be.
It's very highly reflecting.
Do something else to it: cut it up into little nano-spheres,
make a colloid out of it, and it's the blackest substance known.
You've probably seen some of it.
Have you ever seen a black and white photographic negative?
The black stuff is silver colloid.
So just by changing the structure -- it's still silver in there --
just by changing the structure you can have this extreme effect
on the way it reacts to light.
And it's this extra dimension of design of changing the structure,
which we use in metamaterials,
enables us to have access to all sort of remarkable properties,
only some of which I have time to talk about today, in invisibility.
So, using this design tool, which Einstein is presented with,
and these metamaterials, which we have developed here in London,
David Smith, at Duke University,
who has been a very intimate part of this story,
built a cloak in 2006.
His cloak was for radar waves, making objects invisible to radars,
but the principles are the same for visible light.
But I shall pass lightly over David's cloak,
because I'm sure what you'd rather hear about is invisibility to light,
as we do it.
And a similar strategy was followed
by Shuang Zhang's team in Birmingham
working with Jensen Li.
And what they chose to do was to hide something on a mirror
by making a little tunnel in the mirror.
So they lifted the surface of the mirror
and they made a tent, or a tunnel, in which you could hide something.
And of course, if that's all you do, you know that it's a hiding place
because those little facets of the tunnel will be very visible from outside.
So, their cloak was designed to have the function
of making the mirror look as though it was flat.
So although the mirror has a kink in it and you're hiding something,
if you look at it through the cloak from outside, it should appear flat.
And how did they do that design?
Well, this slide shows you how they did it.
So once again, you start with some ordinary space,
it's contained in a triangle which sits on a mirror;
and then you lift the surface of the mirror,
to make your tunnel, or tent,
and as you do so, you compress the space into the triangle.
Now, instead of ordinary space, you've got two regions of compressed space.
And then you say -- you have got some compressed space --
what's the material property, what's the refractive index
that reproduces the effect of that compressed space?
And it turns out in this case
that if you get those angles of the triangles just right,
you can find a natural material
and that natural material is called calcite.
And if you have contacts in China, you can get huge crystals of calcite
and this cloak is about ten centimetres across.
And on the right-hand side there you see, I'm afraid, a rather smudgy picture
of the Birmingham calcite cloak.
So, they had a rather ingenious way
of explaining how their cloak works:
so they looked at the alphabet through the mirror.
Now, if you didn't cloak this tent, because the mirror has a kink in it,
you couldn't see the middle bit of the alphabet.
So on the left, you see a picture of the alphabet
viewed through the mirror, without the cloak.
And as you can read, the middle bit is missing.
And then, they put the calcite crystals in place,
they should make the mirror look flat,
you should be able to see all the alphabet, and you can.
H, I, J, K, L.
And by the way, there's some controversy about whether cloaks can cloak all colors
at the same time.
This is a real photograph taken in white light.
So you can just see little fringes of red
on the end, but it's a broadband cloak.
Now before I finish, I want to tell you about some aspects of this work,
which I think are equally important.
We've touched the hem of the great,
we've borrowed some of Einstein's theories,
and that adds some sophistication to our work, I think.
We've also had a lot of fun,
because designing a cloak interests lot of people,
and I have the chance to talk to audiences like this.
But also, at the end of the day,
these ideas, these design tools,
these materials, have practical applications.
I have just time to tell you about three of them very briefly.
One of them is being worked on here
in the Electrical Engineering Department, by Richard Syms,
and is a way of making magnetic resonance imaging scans more efficient.
He has designed a wire,
which, instead of transporting electricity, transports magnetism.
Of course, magnetic resonance imaging works through a magnetic signal,
so that's a way of making MRI more efficient.
Another application, which is leading to a start-up company funded by Bill Gates
in Seattle,
is replacing satellite communication receivers,
which, at the moment, consist of a rather big dish, 30 centimeters across,
because you need to point that dish
at the satellite and the dish has to be steerable,
so it's heavy, it's expensive, it takes power.
And you can replace all that with a metamaterial sheet,
which is technical name, it's phased array,
that you can point at the sky without any mechanical movement.
You can change where it's pointing at electronically.
It's cheap, metamaterials are cheap, they're just bits of copper and insulator.
It's light, and you can plug it into the USB port of your laptop.
So I think that will revolutionise satellite communications;
and Toyota already have a contract with a company,
to put these phased array metamaterials in the roof of their cars,
so that they can have continuous real-time communication with their cars.
The final application I'll talk about is terahertz radiation.
Terahertz radiation is increasingly important:
they use it in airport security,
it's also used in collision avoidance radar,
it's a central feature of the Google self-steering car.
And to control that radiation, an ideal material is metamaterials.
So we've interacted with some
very, very great science through Einstein,
we've had a lot of fun,
and hopefully, through these companies, will pay our way in the future.
That is an ideal research project vote for me.
(Laughter)
Thank you.
(Applause)