1,300 million years ago,
there were two huge black holes that were dancing tango.
They were, while dancing, forming waves in space-time,
deformations of space-time.
And they were getting closer and closer, turning faster and faster,
almost at the speed of light,
until, embracing each other, they formed a single black hole,
60 times the mass of our Sun,
with a radius of 200 km.
And gravitational waves took the message of that embrace,
at the speed of light, to the rest of the Universe.
This, sounding like science fiction, a Hollywood movie, was true.
And we know it was true
because we measured those gravitational waves,
last year in 2015.
The story of this discovery is also pretty long.
When this event happened 1,300 millions years ago
in a faraway galaxy,
on Earth, the first multicellular organisms were just appearing.
Life, humanity, societies grew, advanced,
and a little over 100 years ago,
in 1915,
Einstein published his theory of general relativity.
It's a theory of gravity.
Relativity and gravity don't sound like the same thing, but they are.
His theory says that two masses attract each other,
two black holes, or the Earth and the Sun,
attract each other, not because, as we're taught at school,
there's a gravitational force,
but because all masses, according to this theory,
deform space-time,
just as we do when we lie down on a mattress.
When we, a physical mass, lie down on a bed,
we deform the mattress, making a depression.
And if someone else lies down on the bed, they will roll closer to us.
That is the force of gravity as Einstein imagined.
The Sun deforms space-time.
The Earth does not feel an instant gravitational force,
but it sees the space-time curvature,
and the Earth spins around the Sun as we know it to do.
What is space-time?
We have to imagine space-time as a grid,
not a two dimensional grid, but a three dimensional one,
the three spatial dimensions that we can measure with rulers,
with time, with clocks.
A grid of rulers and clocks.
Four dimensions, and all of them are connected.
That is the space-time that gets deformed according to Einstein's theory.
The theory is pretty complicated,
but what matters with theories are the predictions they make,
and the checking of those predictions
to prove them right or wrong experimentally,
so as to know whether to believe the theory or not.
And Einstein's theory makes many predictions,
all of them very crazy,
quite incredible.
One, for example,
the first to be verified a few years later in 1919,
is that light does not travel in a straight line
but deflects when it passes close to a mass,
it deflects a little bit, by a certain amount.
And that was verified in 1919,
and the theory was rather better believed.
But it also made other predictions.
For example, it being a theory of space and time,
that clocks are not always synchronized.
If we start, me and you,
with atomic clocks synchronized to the millionth of a second here,
but then you go and hike up the Aconcagua,
our clocks will no longer be synchronized.
Yours will be always ahead of mine.
And why is that?
It's because space-time is dynamic,
and your clock is further from the Earth than mine.
Distances and gravity change time too.
Another prediction was this prediction of gravitational waves.
Since masses distort space-time,
and masses are moving,
those little ripples in space-time are also moving,
and they are traveling at the speed of light.
And what they do, is to take distances and stretch and shrink them,
stretch and shrink them, in proportion to distance.
But when Einstein,
or any of the other scientists that followed him,
calculated how much the distances distorted,
it was very, very little.
He even wrote that this would probably never be measured.
And many people thought he was right,
that this was one of those predictions that cannot be measured.
As I said, it is a long story.
In the 70s, people thought that it indeed could be measured.
There are some instruments
that are widely used in physics and engineering
to measure distances very precisely,
which are called interferometers.
Interferometers because they use light, and the interference of light.
We take a ray of light,
split it in two with a half-silvered mirror,
reflect them back with mirrors,
and when the two rays arrive back, they interfere.
The waves interfere destructively,
they destroy each other, and there is no resulting light,
if this distance and this distance are the same.
But if this distance gets shorter and this one gets longer,
or the other way round,
then the interference out here is not totally destructive,
and we can see a little light, or not.
So, by measuring with a photodetector how much resulting light there is,
we can measure the difference in distance between this distance and that distance.
It looks simple enough to measure distances,
and it is used a lot,
but what's the amount we have to measure?
That is the big question.
The theory predicts that, resulting from these black holes,
the distance between the Earth and the Sun
changed by an atomic diameter.
And all smaller distances by less than that.
But those scientists in the 70s, starting with scientists at MIT,
said this can be measured.
If we construct interferometers 4 km long
in a vacuum with hanging mirrors,
we could get to measure differences between this 4 km and that 4 km
of a millionth of a proton.
Many laughed; others didn't.
Many other people started believing this.
The national science agency of the United States
bet on this in the 90s.
From the 70s to the 90s,
when these interferometers began to be constructed,
two interferometers, two LIGOs,
one in the state of Washington,
and one in the state of Louisiana - quite close to where I live -
3,000 km apart.
They were completed in the 2000s.
First generation technology was operated and it went well.
It didn't discover gravitational waves,
but it was known that the technology had to be made more advanced.
In 2010, the second generation of this technology began to be installed.
It worked.
It works - more or less.
In 2015, we said that we should start taking a look at what is happening,
in spite of needing to do more work on the detectors.
In 2015, we started collecting data with these two detectors.
And in September,
on September 14th, 2015,
these photodetectors told us
that, 3,000 km away, we had signals indicating this gravitational wave.
We couldn't believe it.
And in December it happened again.
Listen.
(Sounds)
That sound seemed impressive to us.
We marveled.
It left us speechless and dancing afterwards.
This is the sound of the Universe.
This is the music of the Universe.
We felt that from that moment -
Before that we were looking at the Universe
with electromagnetic waves and telescopes and observatories,
and now we were listening to it,
with gravitational waves.
We had added another sense.
And from then on,
we have not only been working to measure more gravitational waves,
but we have also been talking about this.
And many people ask me:
"And what are gravitational waves good for?"
And I say, "And what are gravitational waves good for?
What is astrophysics good for? What is science good for?"
Ah, that we know. We all know what science is good for.
All the technological advances that we use in communication,
in transport, in medicine, all that is based on science,
we all know.
But gravitational waves, astrophysics?
Actually the path science takes, is very long.
It starts with basic theories relating to how the Universe works,
and then, they are applied.
First they test to see if the theories are good or bad.
After that, ways in which they can be applied are discovered,
in general, in other areas of physics or chemistry.
Engineers take hold of it all to build precision instruments.
And finally, sometimes, new useful technologies appear.
If Einstein had been asked in 1915: "What is your theory good for, sir?"
He would have said: "To better understand the Universe,
to explain gravity.
What else does a theory need to be good for?"
Yet, today, many of you are going to use the theory of relativity
when leaving here
if you are going somewhere you don't know.
Because GPS needs the theory of relativity.
It needs many other things,
but if it doesn't take into account that the clocks in the GPS satellites
and ours, our clock, our little device,
are desynchronized
because they are at different distances from Earth,
the GPS will lead us to the wrong place.
And I, that uses it all the time, would get lost.
So the theory of relativity, almost 100 years later,
has practical applications.
And so it is with everything.
Science takes a lot of time
in producing useful applications and technologies,
and many people working with many different abilities.
If we think what technologies we wouldn't have today,
what wouldn't have been invented
if basic effects in physics, in chemistry, in mathematics,
had not been studied a 100 years ago,
there would be a lot of technologies that would not exist today.
The laser, GPS, medicines, medical applications.
A whole load of technologies.
The challenge I put to you
is to imagine
when you hear about a discovery like the one of gravitational waves
or any other discovery in astronomy, in physics, in mathematics,
that show up in the newspapers many times a year,
when you hear about those discoveries,
what technologies will there be in a 100 years
that will use these theories?
That is the challenge for you.
Thank you.
(Applause)