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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
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)
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Weekly talking 2016 年 2 月 14 日 に公開

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