which says that if everyone in China jumps up in the air all together,

then the Earth will be rocked off its axis.

Now, believe me, I've done the calculations, and I can say

that the Earth's axis is perfectly safe.

Although, as someone who grew up in Britain in the 1980's,

the words 'Michael Fish' and 'hurricane' do spring to mind.

Nevertheless, even a single person, if they jump up in the air,

can, so to speak, make the Earth move.

The trouble is, you don't make it move very much.

So let's suppose we could make a measurement,

not so much about jumping scientists shaking the Earth,

but a measurement so precise

that it could tell us something about the change and the shape of space itself

produced by an exploding star halfway across the galaxy.

That really does sound like science fiction,

but in fact such a machine already exists.

It's called a laser interferometer,

and it's one of the most sophisticated scientific instruments we've ever built.

And in a few years time

we're confident it's going to open up for us

a whole new way of looking at the universe called gravitational-wave astronomy.

Now gravitational waves are not the same thing as light;

they're not part of the spectrum of light that we call the electromagnetic spectrum,

stretching all the way from radio waves to gamma rays.

We've already got lots of different types of light,

and over the last 60 years or so,

we've got really rather good at probing the universe

with all those different kinds of light.

Whether it's building a giant radio telescope on the surface

or putting a gamma ray observatory out in space,

we've used these different windows in the cosmos

to tell us some quite amazing things about how our universe works.

We've probed the birth and the death of stars.

We've explored the hearts of galaxies.

We've even started to find planets like the Earth going around other stars.

But the gravitational wave spectrum will be completely different.

It will give us a window in the universe

into some of the most violent and energetic events in the cosmos:

exploding stars, colliding black holes, maybe even the Big Bang itself.

Now, what will we learn

from the gravitational wave window on the universe?

Well, maybe the most exciting thing is the things we don't know about yet,

the so-called unknown unknowns,

the things that we don't even know we don't know yet.

It's going to take a few more years but we are almost there.

Now, before we talk about gravitational waves,

let's have a think about gravity.

There's another urban myth which I'm sure everyone has heard of,

the one about the apple falling on Isaac Newton's head.

Now, I'm not really sure if there was any genuine fruit involved in that,

but wherever he got his inspiration from, Newton came up with a very clever idea.

Because he worked out that he could use the same physical law

to describe both an apple falling from a tree

or the Moon orbiting the Earth.

And he called this his universal law of gravity.

And it basically says that everything in the cosmos attracts everything else.

It's a beautiful theory and it's also very practically useful.

It lets us do all sorts of useful things in our modern world

and has done for more than 300 years.

It lets us fly aircraft halfway round the world,

it lets fly a rocket to the Moon and back.

But there is a problem with Newton's law of gravity, a philosophical problem.

On a very fundamental level it doesn't really make sense,

because Newton says there's a force between the Earth and the Moon.

Well, how does the Moon know it's supposed to orbit the Earth?

How does the force actually get from the Earth to the Moon?

This was a problem which no less than Albert Einstein puzzled over

in the early years of the 20th century.

And Einstein came up with a truly remarkable answer.

Now, Albert Einstein was probably the first celebrity scientist.

Even though he died in 1955,

in 1999, the editors of Time magazine voted him the person of the 20th century.

Although I should mention there was a public vote on the website

and they went for Elvis Presley.

(Laughter)

Now I'm as big a fan of the King's music as anyone,

but I still have to go with the editor's decision here.

In fact I even have my own action figure of Einstein at the university.

(Laughter)

So what exactly did Einstein do, if he was the person of the 20th century?

Well, what he did, was make us rethink what gravity really is.

In Einstein's picture, gravity isn't so much a force

between the Earth and the Moon or apples and trees,

instead it was a curving or a bending of space and time themselves.

So a good metaphor here

is to think of the Earth sitting on a stretched sheet of rubber,

like a trampoline.

The mass of the Earth, the very great mass of the Earth,

will bend that rubber sheet a lot,

and then you don't really need

to have the Moon anymore feeling a force reaching out from the Earth.

The Moon just follows the natural curves and bends

of space and time around the Earth.

In fact, Einstein also said

that we should no longer really think of space and time as separate things,

so you hear people talk about the fabric of space-time.

What Einstein said was, that gravity is a curving, a bending of space-time.

Or as another physicist, John Wheeler, put it rather neatly:

'Space-time tells matter how to move, and matter tells space-time how to curve.'

Now, all that sounds very grand and fundamental

about the nature of the universe,

but it's got a lot of practical applications as well.

Down here on the Earth, in the Earth's feeble gravity,

there's a very remarkable prediction of Einstein's theory,

which you probably have never noticed before.

Did you know for example

that clocks run more slowly on the surface of the Earth

than high above the Earth,

because the gravitational field is stronger.

You might remember that scene in the movie

'Mission Impossible Ghost Protocol',

when Tom Cruise is scaling

the Burj Khalifa, the world's tallest building.

But even when he was 800 metres above the ground,

Tom's watch, I'm sure he was too busy to notice,

but Tom's watch would only be running a few billionths of a second faster

than it would have done down at ground level.

So what's a few billionths of a second between friends?

Well, that's actually enough to make a difference

to the Global Positioning System.

The GPS satellites, their data has to be adjusted

for time running faster at the altitude of the satellites.

And that's a whopping 40 microseconds a day.

Now the radio signals and microwave signals from those satellites

can travel about 10 kilometres in 40 microseconds.

So just think how bad your SatNav would be,

if it were only good to 10 kilometres.

We'd all get lost pretty damn quick.

So Einstein's theory of gravity, his General Theory of Relativity,

really does have everyday practical effects on our daily lives.

But it's out there in deep space where you really see it to the max.

In fact, if gravity is all about bending space-time,

we can do a kind of thought experiment.

We can imagine that if you could put enough matter into a small enough space,

eventually you would bend space-time so much

that even light couldn't escape the clutches of gravity.

You've got yourself a black hole.

Now black holes were imagined around the time of Einstein.

In fact, in 1916, just after Einstein had published his theory,

there was a wonderful paper written by a young scientist,

who was at the front in the First World War at the time,

Karl Schwarzschild.

And it sets out the theory of a black hole.

Black holes really do sound as if they belong in the realms of science fiction.

But we do think that black holes actually exist,

and that for even light to escape from a black hole

truly would be Mission Impossible.

We find black holes in the remnants of exploded stars,

we even seem to find them in supermassive form

in the hearts of virtually every galaxy in the universe.

Imagine you could take a black hole and move it close to the speed of light.

That would shake up space-time a lot,

like dropping a cannonball on that fabric of a trampoline.

It would send ripples spreading out,

and those ripples are what we call gravitational waves.

So gravitational waves would be produced by things like black holes,

or their slightly less extreme gravitational cousins

called neutron stars.

And if you could get two of them to collide together

close to the speed of light,

that would really make some waves.

That's what we're looking for

as we embark on this new field of gravitational-wave astronomy.

If only it were that easy.

That's the plan, but to do it is tough,

because even though the gravitational waves

shake up space-time colossally where the black holes are,

just like ripples in a pond, if they spread out through the universe,

they get weaker and weaker.

By the time they arrive at the Earth,

the shaking of space-time that we're trying to measure

is roughly speaking about a millionth of a millionth of a millionth of a metre.

That's pretty tough to measure.

So how do you do it?

Well, at the risk of sounding like one of those Las Vegas magic shows,

it's all done with mirrors and lasers.

What you do, is you take a laser beam, you shine that laser beam at a mirror,

you split it into two beams that go at right angles to each other,

bounce them off a mirror, recombine them,

and then have a look at what you've got.

If the two beams have travelled exactly the same distance,

then what you get back is the beams in perfect step with each other.

They're light waves just like all those other forms of light,

so the wave trains will be matched up.

But if they've travelled a different distance,

they'll be out of step with each other, they'll interfere with each other -

we call this phenomenon interference,

so that's why these things are called laser interferometers.

So a laser interferometer is a cool thing to have

if you want to try and catch a gravitational wave.

But remember they're incredibly minute signals,

so it's going to be a huge engineering challenge to build one.

So Einstein said that when a gravitational wave goes by,

it will stretch and squeeze the space-time in our vicinity,

but by this incredibly tiny amount.

So we're trying to use the laser beam and its interference pattern

to tell us if a gravitational wave has gone past.

But you've really got to scale up the experiment and go large.

And that is where LIGO comes in.

LIGO stands for Laser Interferometer Gravitational-Wave Observatory.

And it's the most ambitious and sophisticated

scientific project ever undertaken by the National Science Foundation in the US.

In fact, there are two LIGO's.

There's one in Louisiana and there's another one in Washington State.

And together with two other interferometers,

one called GEO in Germany and Virgo in Italy,

this is our early warning system for gravitational waves.

Now, they're built in quite remote locations, LIGO,

and I think the locals don't really get what they're for.

One of my LIGO colleagues was flying over the Livingston site

and a fellow passenger on the flight was looking down at the detector and said,

'I have a theory what that's for.

It's actually a secret government time machine.'

He wasn't quite sure how to respond,

but well he sort of said, 'OK then, why the L-shape?'

And she said, 'Ah, they have to come back again.'

(Laughter)

Time travel really is science fiction,

but finding gravitational waves, we very much hope,

in a few years time, will be science fact.

Now it is tough.

All those tiny, tiny effects we're trying to measure