There's a classic urban myth

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

could be swamped by the local effects
of disturbances from shaking the ground;

not because of out there in the universe,

but because of very much more
mundane phenomena here on Earth.

So what you've got to do,
is put your mirrors

on very complex suspension systems

that push against the limits
of materials technology.

And even the buffeting of the air
in the laser beam

could swamp our signal,

so we have to send
the lasers back and forth

in the most ultra-high vacuum system
anywhere on Earth,

only one trillionth of the atmospheric
pressure that we're breathing here today.

So put all that together,
spend a few hundred million dollars,

and hope you're going to find
some gravitational waves,

but it takes a lot of scientists to do it.

So at Glasgow we're part
of the LIGO scientific collaboration.

More than 900 scientists
and engineers around the world

looking for gravitational waves.

Now we haven't found any yet,

but having multiple detectors,
it's not just a 'buy one, get one free',

It's because if you detect a signal in
both detectors, both LIGO detectors,

that helps to convince you
you've really got something.

And if you see it in Virgo
and GEO as well, all the better.

So very soon we're going to have
a global network of advanced detectors

because the LIGO's aren't quite
sensitive enough to do the job yet.

But we're giving them more heavy mirrors,

more powerful lasers,
better suspension systems,

and we expect by about 2016

that we'll have a network of advanced
gravitational-wave interferometers

looking for gravitational waves.

Now how long will we have
to wait to get a signal?

We don't really know,
but based on what we do know,

we don't think it should be more
than a few months.

In fact, at a conference last year,

a group of us in Poland
tried to come up with a figure, a date,

of when we expect to see one.

Now our tongues were
a little bit in our cheeks

when we predicted
the date of January 1st, 2017.

I did point out there probably
wouldn't be very many people

at work in Glasgow that day.

(Laughter)

However gravitational waves are coming.

We stand on the brink of opening
this new window on the universe

and it's a very exciting time
to be an astrophysicist.

Thank you very much.

(Applause)