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There's a classic urban myth

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

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then the Earth
will be rocked off its axis.

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

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that the Earth's axis is perfectly safe.

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

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

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

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can, so to speak, make the Earth move.

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

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So let's suppose we could
make a measurement,

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not so much about jumping scientists
shaking the Earth,

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but a measurement so precise

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

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produced by an exploding star
halfway across the galaxy.

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That really does sound
like science fiction,

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but in fact such a machine already exists.

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It's called a laser interferometer,

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

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And in a few years time

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

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

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Now gravitational waves are not
the same thing as light;

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

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stretching all the way from
radio waves to gamma rays.

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

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and over the last 60 years or so,

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we've got really rather good
at probing the universe

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with all those different kinds of light.

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

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or putting a gamma ray
observatory out in space,

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we've used these different
windows in the cosmos

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

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

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We've explored the hearts of galaxies.

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

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But the gravitational wave spectrum
will be completely different.

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It will give us a window in the universe

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

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

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Now, what will we learn

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from the gravitational wave window
on the universe?

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

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the so-called unknown unknowns,

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

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

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Now, before we talk
about gravitational waves,

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let's have a think about gravity.

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

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

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

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

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

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to describe both
an apple falling from a tree

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or the Moon orbiting the Earth.

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And he called this
his universal law of gravity.

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

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

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

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and has done for more than 300 years.

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It lets us fly aircraft
halfway round the world,

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it lets fly a rocket to the Moon and back.

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

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

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

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

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

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

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in the early years of the 20th century.

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And Einstein came up
with a truly remarkable answer.

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Now, Albert Einstein was probably
the first celebrity scientist.

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Even though he died in 1955,

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

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

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and they went for Elvis Presley.

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(Laughter)

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

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

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

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(Laughter)

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

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

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

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between the Earth and the Moon
or apples and trees,

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

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So a good metaphor here

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

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like a trampoline.

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

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will bend that rubber sheet a lot,

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and then you don't really need

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

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The Moon just follows
the natural curves and bends

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of space and time around the Earth.

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In fact, Einstein also said

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

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

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

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

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

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Now, all that sounds
very grand and fundamental

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about the nature of the universe,

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

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

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

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which you probably
have never noticed before.

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Did you know for example

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that clocks run more slowly
on the surface of the Earth

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than high above the Earth,

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because the gravitational
field is stronger.

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You might remember
that scene in the movie

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'Mission Impossible Ghost Protocol',

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when Tom Cruise is scaling

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the Burj Khalifa,
the world's tallest building.

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But even when he was
800 metres above the ground,

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

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

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than it would have done
down at ground level.

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

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Well, that's actually enough
to make a difference

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to the Global Positioning System.

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The GPS satellites,
their data has to be adjusted

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for time running faster
at the altitude of the satellites.

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And that's a whopping
40 microseconds a day.

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Now the radio signals and
microwave signals from those satellites

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can travel about 10 kilometres
in 40 microseconds.

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So just think how bad
your SatNav would be,

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if it were only good to 10 kilometres.

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We'd all get lost pretty damn quick.

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

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really does have everyday
practical effects on our daily lives.

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

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

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we can do a kind of thought experiment.

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

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eventually you would bend
space-time so much

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

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You've got yourself a black hole.

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Now black holes were imagined
around the time of Einstein.

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

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there was a wonderful paper
written by a young scientist,

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

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Karl Schwarzschild.

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And it sets out
the theory of a black hole.

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

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But we do think that
black holes actually exist,

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and that for even light
to escape from a black hole

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truly would be Mission Impossible.

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We find black holes
in the remnants of exploded stars,

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we even seem to find
them in supermassive form

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in the hearts of virtually
every galaxy in the universe.

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

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That would shake up space-time a lot,

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like dropping a cannonball
on that fabric of a trampoline.

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It would send ripples spreading out,

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and those ripples are
what we call gravitational waves.

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

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or their slightly less extreme
gravitational cousins

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called neutron stars.

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And if you could get two of them
to collide together

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close to the speed of light,

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that would really make some waves.

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That's what we're looking for

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

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If only it were that easy.

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

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because even though
the gravitational waves

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

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

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they get weaker and weaker.

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By the time they arrive at the Earth,

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

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

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That's pretty tough to measure.

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So how do you do it?

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

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it's all done with mirrors and lasers.

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

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

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bounce them off a mirror,
recombine them,

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

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If the two beams have travelled
exactly the same distance,

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

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

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so the wave trains will be matched up.

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But if they've travelled
a different distance,

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

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we call this phenomenon interference,

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

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So a laser interferometer
is a cool thing to have

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if you want to try and
catch a gravitational wave.

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But remember they're
incredibly minute signals,

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

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So Einstein said that
when a gravitational wave goes by,

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

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but by this incredibly tiny amount.

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

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to tell us if a gravitational wave
has gone past.

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

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And that is where LIGO comes in.

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LIGO stands for Laser Interferometer
Gravitational-Wave Observatory.

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And it's the most ambitious
and sophisticated

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

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In fact, there are two LIGO's.

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

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And together with
two other interferometers,

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one called GEO in Germany
and Virgo in Italy,

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this is our early warning system
for gravitational waves.

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

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

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One of my LIGO colleagues
was flying over the Livingston site

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

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'I have a theory what that's for.

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It's actually a secret
government time machine.'

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He wasn't quite sure
how to respond,

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

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

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(Laughter)

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Time travel really is science fiction,

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but finding gravitational waves,
we very much hope,

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in a few years time, will be science fact.

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Now it is tough.

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

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

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not because of out there in the universe,

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

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

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on very complex suspension systems

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that push against the limits
of materials technology.

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And even the buffeting of the air
in the laser beam

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could swamp our signal,

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so we have to send
the lasers back and forth

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

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

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

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

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but it takes a lot of scientists to do it.

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

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More than 900 scientists
and engineers around the world

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looking for gravitational waves.

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Now we haven't found any yet,

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

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

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

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

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

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

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But we're giving them more heavy mirrors,

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more powerful lasers,
better suspension systems,

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and we expect by about 2016

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

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looking for gravitational waves.

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

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

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

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In fact, at a conference last year,

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

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of when we expect to see one.

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Now our tongues were
a little bit in our cheeks

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

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

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at work in Glasgow that day.

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(Laughter)

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However gravitational waves are coming.

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

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

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Thank you very much.

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(Applause)