Translator: Maria K. Reviewer: Denise RQ
If you look at the night sky -
you walk away from the city,
there are no clouds,
and your eyesight is really, really sharp,
you may be able to see an image of stars and galaxies like this one.
This is actually a deep field image from the Hubble Space Telescope,
which has been orbiting around Earth since the 90s.
Tremendous progress has been made
since Galileo first pointed a telescope to the sky.
We now use radio waves, infrared lights, visible lights, x-rays, gamma rays,
to know what we do about the Universe around us.
And so we can zoom in and look at the structure of galaxies,
and we know that the Universe - it's the sky out there -
is not a quiet place at all.
There are violent events that are happening all the time.
Stars collide, stars explode,
there are black holes that suck everything that's around them.
There are, however, still many questions that we don't have answers to.
We don't know what is all that matter that composes the Universe.
We know that there are black holes, but we don't know exactly what they are.
We don't know exactly how stars explode.
So, there are still many unknowns,
including what happened at the very beginning of the Big Bang.
New problems that we're going to have to find new ways and techniques to solve.
And this is where my research comes in.
Gravity is at the core of the theory of general relativity.
General relativity was formulated by Einstein 100 years ago -
so last year was the anniversary -
to explain gravity.
Einstein was not happy with the concept of gravity as Newton introduced it.
He was not happy with just saying
there is a force between any two objects,
so the Sun attracts the Earth,
and that's why the Earth is rotating around the Sun.
He wasn't happy that there is no -
classically we didn't know why -
what is the messenger
that carries information
of how planets and stars attract other bodies.
So, Einstein had a new vision, a new way to explain gravity,
that's not in terms of forces but in terms of the geometry of space.
So, according to general relativity,
two objects are not attracting each other because of a mysterious force,
but because they are moving in a space that is not flat - it's curved.
So according to Einstein -
this is how Einstein would see the Universe
in a two-dimensional representation.
So the Universe is not flat,
it is curved by heavy objects like stars, heavy planets,
all the way down to a black hole.
So you see in this image that the planets are producing some curvature in it.
And so the reason why the Earth is moving around the Sun
is because it's moving on a curved surface.
If you look very carefully all the way to the back,
there is what looks like like a funnel.
That's a black hole in general relativity.
It's a bending of space-time
that is so deep that it almost pulls, it almost punches a hole,
in the fabric of space-time.
So that's why matter and light cannot come out of it,
because they're moving down this funnel.
So that's how black holes are introduced in general relativity.
If we now think about two heavy objects, like two black holes -
so in this representation you see those two whitish blobs.
Pretend they're black and call them black holes.
And think about these two objects orbiting around each other,
like in this animation.
So as the two objects are moving around each other,
they change the way they are curving space-time around themselves.
So, as they do that, they produce these ripples,
these waves that you see in this animation that propagate outwards.
It's a little bit -
think about when you throw a rock in a pond.
You are disturbing the surface of the pond,
you're producing waves, and the waves propagate outwards.
The same thing is happening to space here
as these two heavy objects, these two -
again, let's call them black holes -
are rotating around each other
and are producing ripples, waves, that propagate away.
Space is a very stiff medium,
so it takes a lot of energy to perturb it and produce these waves.
That's why, when Einstein, 100 years ago, calculated it -
it's the back of the envelope calculation of how strong an effect it is -
he decided it was going to be impossible to ever measure it,
it's going to be too hard.
Well, he was wrong on that one.
On February, 11, this year on February, 11,
the science team that I am privileged to be a member of
announced the first direct measurement, the first discovery,
of gravitational waves.
They were produced by the collision of two black holes.
I am going to show you here an animation.
This is a computer simulation of what happened 1.3 billion years ago.
What you see is the effect of two black holes
that are rotating around each other.
As they do that,
they bend the way the light coming from the stars behind it propagates,
and so you see this effect,
this motion of what looks like the motion of the stars.
That's not something we can see.
Now you see the two black holes have merged,
and we are left with a single black hole.
We cannot see the light coming from that,
but what we can see
is the gravitational wave that was produced
as these two black holes inspiraled and then eventually collapsed.
And so these green streaks in this video
are the gravitational wave that was produced.
It goes further and further
until the moment the two black holes merged.
Then you see the ripple that propagates away.
That's the last moment, the moment of the merger,
and that's where most of the energy was produced.
Then it started propagating through the Universe, through space,
in modified space and time.
Fast forward 1.3 billion years, and it reached Earth.
It reached Earth on September, 14, 2015.
As you can see, that's the effect that the gravitational wave had on Earth
as it came by.
That wave produced a wobble in Earth.
That wobble is actually a very exaggerated effect.
I didn't feel it, you guys did not feel it.
The only way to sense something like that
is to have a super-accurate instrument.
Just to give you a sense of the scale of this,
what was seen produced,
so the echo of that event that took place 1.3 billion years ago,
it was equivalent of a change in the distance
between Earth and Alpha Centauri -
which is the closest star -
by the thickness of a hair.
So it was that small an effect.
So measuring this is what Einstein thought was not going to be possible,
but it was possible to do it thanks to this detector that's called a LIGO.
LIGO stands for Laser Interferometer Gravitational Wave Observatory.
What you see here is an aerial picture of the detector,
that's in Washington State, in the deserts in Washington State.
We have a similar detector installed in Louisiana,
so closer to here.
And this is an interferometer whose arms, the length of those arms,
is four kilometers.
I am going to show you, in an animation, the operating principle for a LIGO.
The basic idea is to use lasers, a laser interferometer,
to measure the difference in the change between two mirrors
that are four kilometers apart.
The idea is that we get a laser beam,
we send it against the semi-splitting mirror,
we split it in two.
The light travels in the two arms,
gets reflected, comes back, recombines,
and then we set things up in such a way
that the light coming from the two arms interferes destructively,
so we get no light coming out, they cancel each other.
As the gravitational wave comes through, it moves the mirrors,
it changes the distance between the mirror and the beam splitter,
and it does so alternatively, in one arm and in the other one.
And as it does that,
you see that the phase between the two light beams changes,
and so we get on that screen over there,
we get a little bit of light that comes and goes:
it pulsates with the frequency of the gravitational wave.
So by monitoring that spot very carefully,
we can measure changes in the length of those four-kilometer arms,
and then we use that to track the passage of a gravitational wave.
We expect that that happens
both in Louisiana and in Washington State in the same way,
and that's how we recognize it's a gravitational wave.
Just to give you an idea of how small an effect this is,
and why this is so difficult,
I am going to show you here
how the effect that we're measuring compares with the scale of an atom.
So, as I said, those arms are four kilometers long.
The level of the changes, the movement of those mirrors,
is 4x10⁻¹⁸ [minus 18] meters.
So how small is that?
This is just to give you a scale,
it's the representation of a hydrogen atom.
The red trace is the electron,
the blue is the proton, which is the nucleus of the hydrogen.
Now we're going to zoom in.
Yes, here we go.
So this is the electron moving around the nucleus.
The size of that atom is 10⁻¹⁰ [minus 10] meters.
The nucleus is 10⁻¹⁵ [minus 15] meters,
and on we go,
and the effect we measure is of the order of 4x10⁻¹⁸ [minus 18] meters.
It's that small, it's that small a measurement.
That's something that requires a high level of engineering and planning
in order to be able to do that measurement.
So that's why this discovery of gravitational waves is actually -
what's most impressive to me
is that this is really something that builds from a vision,
and it developed all the way to the collaborative work of 1,000 people,
to its realization.
So, 100 years ago, Einstein first set this vision
that we can use geometry to explain gravity.
He had this crazy idea of a core of space-time,
and a space that ripples.
He just saw that, and he used math to write it down,
to make the prediction of gravitational waves.
It was about 60 years later that those three gentlemen,
Rai Weiss, Ron Drever, and Kip Thorne,
took that idea, that concept of something that to Einstein was non-measurable,
and actually came up with a design and an idea for a detector.
So, in 1972, Rai Weiss, with his historical paper,
was the first one to formulate the idea of using laser interferometry,
and these techniques that I described to you,
as a way to detect gravitational waves.
Ron Drever built the first prototype,
a much smaller version of the detector that I showed you before,
and came up with some of the techniques that are still used in a LIGO itself.
Kip Thorne was the theorist who really advocated the possibility
of using astrophysical sources to measure gravitational waves.
He did all the calculations of what kind of signals we can expect,
and theorized ways to solve some of the many challenges
that a LIGO has to face.
Because, to make such an accurate measurement,
we need to have very precise instruments
that can find lots of possible unknown sources.
So in the middle row, you see pictures of the two detectors,
the LIGOs in Louisiana and in Washington State.
And in the middle - I apologize the pictures are a little small -
are some of the snapshots
of how complicated the instrument actually really is.
To realize those, we needed collaboration.
So in this corner picture over there,
you see some of the logos of the members of the collaboration.
I am a part of the LIGO scientific collaboration,
which includes about 1,000 members across 80 institutions in 16 countries.
This is now a worldwide effort.
There was a vision, there was the potential for a realization,
but then, in order to bring this to completion,
we had to really have a cooperative effort,
brought together by many brilliant scientists and engineers
that had to learn how to work together
towards this common and very challenging goal.
That's part of the beauty of this discovery.
Beyond the discovery itself
is also how these 1,000 people
had to learn how to work together towards a common goal.
And then, in the lower corner, you see an image of that first detection.
So we call this event GW - for gravitational wave,
150914 - that's the date, September, 14, 2015.
What you see, those traces that you see there, those there are,
those are, three lines, they are overlapped,
so the more wiggly one, the blue and the - sorry, the green and the red,
those are, that is the data that was measured
in the two detectors in Louisiana and in Washington.
Those are simply overlapped and shifted by a little bit.
So that signal happened with the same feature, the same wiggles,
within seven milliseconds,
which is consistent with the fact
that this signal came from the lower hemisphere
and reached the two detectors seven milliseconds apart.
And then, on top of it, is the prediction of general relativity.
So this signal that we saw
is matching very, very well with the prediction of general relativity
for a signal that's coming from the inspiral,
the merger and the event, or relaxation, of those two black holes,
this cataclysmic event that took place 1.3 billion years ago and then reached us.
So I want to play for you what this would sound like
if we take that signal and plug it into a speaker.
Now, I want to be clear,
gravitational waves are not sound waves,
so we're not really listening with our ear.
However, this stretching and squeezing in space-time
is something that we can put into a speaker and listen to it.
So you are going to now hear the final yelp of those two black holes
that 1.3 billion years ago collided.
And you are going to hear it four times.
First as it sounded,
and then shifted in frequency so you can hear it a little better.
And then I am going to do it again.
(Gravitational wave sounds)
So, what you just heard is the final cries of these two black holes as they merged.
If you think about it, this is phenomenal.
We have just opened a new way
of listening to the sounds of the Universe.
This is like having a new sense, a new way to perceive the Universe.
It's like the first time Galileo pointed his telescope.
It's a new window that we have opened on the Universe.
And this is what's most exciting to me.
We have confirmed the theory of general relativity,
we have confirmed that gravitational waves exist,
we have also discovered that black holes
that are about 30 times the size of the Sun exist.
We were not really sure about that,
they were only theorized.
And now we know that we can use these signals
to understand more about the properties of black holes, for instance.
We're going to use them to do a new type of astrophysics
that we call gravitational wave astrophysics.
We have a new messenger
that's carrying information from the motion of matter
that happened a billion years ago.
This event took place as life on Earth was just starting to form.
1.3 billion years later, we could just hear it,
we're now listening to it.
So this is a momentous window opening.
It's a new opportunity.
The tracks have been laid,
thanks to the work of those people who had the vision,
and the thousand scientists who made it possible and realized it.
And now that the tracks are laid,
I am looking forward [to] what arrives [for] all of us.
Thank you.
(Applause)