Translator: Maria K. Reviewer: Denise RQ
Many of you probably know this news,
that about two months back,
we reached a major milestone in science: gravitational waves were discovered.
The next day, most of the major newspapers across the world,
the front page was covered by this news.
So why is it so important? As we go along, it will become clearer.
But this is just the beginning.
Let's start the story.
Einstein's theory of general relativity
is a relativistic theory of gravity,
which has so far passed all the experimental tests
with flying colors.
That theory describes gravity in a geometric way,
which means that...
Consider a flat, stretchable surface,
And you put two massive balls on that.
You know if the balls are moving,
they will follow the geometry of the sheet.
But on the other hand,
the balls themselves are going to influence the geometry.
So, it is like a couple sort of system,
where the geometry is telling matter how to move,
and matter is telling geometry how to curve.
This was Wheeler's one-line explanation for general relativity.
The general relativity comes up with exciting predictions.
In 1916, Einstein himself predicted gravitational waves;
that when two very massive objects go around each other like this,
they create ripples in this fabric of space-time.
In the example I gave you, this fabric was a two-dimensional surface.
But for general relativity,
it is a three-spatial and one-time-dimensional surface,
which is like a four-dimensional space-time.
Not very easy to imagine.
But if you can hypothetically imagine that,
there will be ripples in that space-time fabric,
which would propagate at the speed of light.
So what happened was that about a billion years ago,
more than a billion years ago,
two massive black holes which were about 30 solar masses each,
came close, they were going around each other,
they were emitting these ripples that are the gravitational waves,
which carried the energy.
To balance for that, they had to come closer,
and they started emitting even more energy
which led to an in-spiraling motion;
in the figure, you can see that the first part,
where they're going around each other and coming close,
where the amplitude and frequency both are increasing with time.
At some point, they come so close that they will touch each other,
and there will be a very violent and marginal phase,
which are the most energetic events in the universe.
That takes place forming a final black hole,
which leans down like a bell.
That signal which was predicted by general relativity was observed
in two detectors in the U.S., two LIGO detectors,
which are 3,000 kilometers away, but almost at the same time,
giving us a confidence that we detected gravitational waves.
As I said, this is just the beginning of the story.
How were they detected?
The ripples in space-time
are sort of, again, like a--
What they do to the space-time fabric is to stretch and compress.
It is like a rubber sheet, through which you are pulling and pushing.
There, if you put a ruler
- so consider a ruler which you use for measurement,
and it is made of rubber -
and you are stretching and compressing,
the distance between two points changes,
even though it will still read the same numbers.
If we use something called Michelson interferometer,
which has two arms,
a laser beam is sent through a beam splitter
which covers both the arms and comes back
and it basically gives you
the difference in arm length in these two in the interferometer.
Michelson interferometer is one of the most precise
distance measurement device for these sort of measurements.
But the thing is
that the differential signal
Is proportional to the arm length.
So if the detector is big, you get a bigger signal.
How big?
On Earth, you cannot make too big detectors,
but you can make about few kilometers detectors,
because otherwise,
the Earth's curvature comes into play, it becomes difficult to align meters.
So LIGO detectors,
which are the most sensitive detectors for gravitational waves,
which have operated on Earth, have four-kilometer arms.
You can see one of these illustrations.
They can measure distances,
which are like 1,000th of the size of a proton.
They are so accurate.
These detectors detected the signal.
What is the big deal about it?
I mean, why we are making so much fuss?
The thing is that many people compare this to the Galileo's observation
with the first telescope which he made.
As you know, from history,
Galileo's observations brought breakthroughs in science.
We know that it was a crucial test to prove that the Sun
is at the center of the Solar System and the planets are going around it.
Today, we cannot think about any other possibilities,
but it was a big thing there
and as you know, it has a significant effect
on our day-to-day life, as well as the society as a whole.
That's why, when LIGO. With this detection of gravitational waves
Is starting a new era in astronomy,
people are highlighting it.
I mean, have we exhausted all the knowledge
that was available in gravitational waves?
The answer is no.
It is just the beginning.
It is a very important milestone.
What is there to see now?
Gravitational waves are carrying a lot of information.
In fact, they are telling a long story even about these black holes.
We just don't have enough ears right now to listen to all those.
We need many more detectors in order to decode
all the information data engraved in these gravitational waves signals.
For instance, right now, we cannot even say accurately
where on the sky this event happened, what was the distance.
Because we have only two detectors.
If we make more detectors, then, these sort of possibilities will be there.
We would be able to accurately locate the source on the sky.
Again, why is that so important?
Because we will be able to then observe the same source
with multiple electromagnetic telescopes,
which will be fantastic,
because we'll have a full coherent story
of the most violent events in the universe.
Then what?
Again, right now, with LIGO and these ground-based detectors,
we will be seeing only a small part
of the whole gravity-wave spectrum.
It is like for example...
What we see is a very small part of the electromagnetic spectrum.
Gravitational waves are there in every frequency.
So, consequently, LIGO is covering the audible range,
basically around 10 hertz to maybe a kilohertz.
If we go to space, for example, we can go to the lower frequencies,
which would be very useful for seeing super-massive black holes.
They are at lower frequencies.
Then we may be able to see the radiation that is coming right after Big Bang.
Now again, I mean, the way I am saying it,
it may look like that we are just talking about some details.
But the thing is that, when we are working very hard,
trying to write scientific papers, etc.,
we feel like there are these missing links,
maybe there are these details which are missing,
and we are trying to fill up those details.
But on a relax day,
when we spend a lot of time staring at the walls,
we feel - at least I feel it -
that 95% of the constituents of the universe are unknown.
There's a phase in the very early universe
- it's called inflation,
when the universe was a fraction of a second old -
when a significant part of the universe's expansion
happened during that phase.
How do we know that?
There is no direct observational evidence.
We need to introduce that phase in order to explain the observations.
Then, there are these very precise clocks in the universe,
which can even challenge the atomic clocks.
They're called pulsars.
We don't know the properties of the material that the pulsars
- basically, we think they are spinning neutron stars -
are made of.
Then, there are many other questions.
For example, is general relativity the right theory of gravity?
I told you that it has passed all the tests with flying colors so far.
But science progresses by challenging.
Just imagine, as early as maybe 1800s;
Newton's law was very accurate in predicting several things;
the day-to-day life observations could be explained very well
with Newton's laws of motion.
But then, now we know, that that law is not complete.
It cannot explain things which move at high speeds.
For example, you cannot accelerate bodies as you like.
You cannot break the velocity of light barrier.
The faster a body is moving, the harder it is to accelerate.
That way, we want to see whether gravitational waves
also can tell us something more about general relativity.
Can it tell us when general relativity breaks?
Then we will be able to probably come up with new theories of gravity.
There are these whole lot of questions,
and all these answers - probably at least partially -
can be captured by performing experiments in different frequency bands.
These are not fantasies.
All these experiments which are shown in this slide,
the cosmic microwave background, pulsar timing arrays, and so on.
- I am not getting into all those -
but those are real experiments which are being built right now.
They will probably start gathering data a little bit later.
Then of course, in the LIGO band,
there can be a detector in India.
It is being planned and hopefully, it will also come up in a few years.
So there are exciting times ahead.
When we are talking about these unknowns
like what we would be able to get from new observations,
we also try to see what else can we learn from gravitational waves.
One of the things is
- you probably see this news also many times in the newspaper -
that people are finding many, many planets outside the Solar System.
They are called exoplanets.
We have so far detected a few thousand exoplanets.
But the estimate is that there are probably
more than 100 billion exoplanets in our Galaxy.
That is like a factor, I think, of 100 millions,
I cannot even connect the zeroes; 100 million extrapolation.
That can give you a little bit of discomfort.
Can we test this?
Can we really count the number of planets that are there in our Galaxy?
My students Anirban and Shilpa calculated
how much radiation is coming out from each of these planets,
that is very little because planets are much much lighter than a star
and a star is much lighter than big black holes.
But it turns out, if the number is so large,
the total gravitational wave energy that it is going to emit,
it is going to be quite significant.
It is not as significant
as the current detectors are going to detect.
But there may be a day in the future, when technologies improve,
that we may be able to count these planets directly.
We cannot do it in any other way.
With electromagnetic observations, we are limited to a small volume.
We cannot go beyond that
because measurements of planets with electromagnetic observation
is not very easy.
Gravitational waves have this property
that they interact very weakly with matter,
which makes them very difficult to detect, but on the other hand,
they can travel a long distance without getting distorted,
so that preserve information for a long time.
So maybe one day, this will become a reality.
And this is just one example.
Maybe people will come up with many, many different ideas
to find different information in the gravitational waves.
Like I said, one of the key things would be to detect the signal
that is coming right after Big Bang.
It will be a smoking gun test of this phase called cosmic inflation.
Before I end,
I want to tell you something very interesting.
As I said, this is just the beginning.
Gravitational wave astronomy
was born two months back,
when the results were announced.
And the journey has begun.
This journey will be long.
As you know, in electromagnetic astronomy after Galileo,
400 years have passed, and we have done wonders in astronomy.
It has completely changed the way we look at things.
If you see what are the current astronomy projects
in electromagnetic astronomy,
you will see there are many big projects which are coming up.
If you see what they are trying to aim, they are things like these:
how fast the universe is accelerating?
How exactly it is expanding, in other words.
Then, things like:
are there black holes in the centers of the galaxies?
Then, you may say,
"What are the detailed features in the cosmic microwave background?"
People are trying to create arrays with pulsars,
to have more accurate measurements of gravitational waves.
But the interesting thing to observe is that 100 years ago,
we didn't even have a clue about all these objects.
That means forget about looking for the details involved in these objects.
We did not even know that these objects exist.
All these things are serendipituos discoveries
that the universe is expanding,
that there is this cosmic microwave background,
that there are pulsars.
People just came to know all these things when they were observing.
So now when I am trying to predict that what we are going to see
with gravitational waves, maybe it is too much of an extrapolation,
maybe we don't even know
what we are going to see with gravitational waves.
As I said, this journey has just begun.
We have certain expectations; we think that we are going to see certain things.
Probably that will be true in the beginning of the journey.
But as we progress, probably we will be able to see things
which we cannot even imagine right now.
Thank you.
(Applause)