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  • 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